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Background Statement for SEMI Draft Document 4783
NEW STANDARD: GUIDE FOR THE HANDLING OF RETICLES AND
OTHER EXTREMELY ELECTROSTATIC SENSITIVE (EES) ITEMS
WITHIN SPECIALLY DESIGNATED AREAS
NOTICE: This background statement is not part of the balloted item. It is provided solely to assist the recipient in
reaching an informed decision based on the rationale of the activity that preceded the creation of this document.
NOTICE: Recipients of this document are invited to submit, with their comments, notification of any relevant
patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this
context, “patented technology” is defined as technology for which a patent has issued or has been applied for. In the
latter case, only publicly available information on the contents of the patent application is to be provided.
Background Statement
Among users and manufacturers of semiconductors, MEMS devices and flat panel displays, the effects of
electrostatic surface charge are well known. Charged surfaces attract particles (electrostatic attraction or ESA) and
increase the defect rate. Charged products are sometimes difficult to handle and cause equipment jamming or
breakage. Finally, electrostatic discharge (ESD) damages products and reticles, as well as causing numerous
equipment malfunctions.
Static control methods have been employed by semiconductor and equipment manufacturers to reduce the effects of
static charge while handling product or reticles. But static charge problems continue to occur due to methods and
materials used to construct the cleanroom as well as activities within the cleanroom itself. SEMI has issued
E78:Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) in Production
Equipment and E129-0706:Guide to Assess and Control Electrostatic Charge in a Semiconductor Manufacturing
Facility to address electrostatic issues that occur within the equipment and manufacturing facility.
This document is a guide for establishing optimum electrostatic compatibility of the handling environment for
reticles and other items that are extremely electrostatic sensitive (EES) to electrostatic charge, voltage and electric
field. This guide is complementary to SEMI E78 and E129 and is intended to improve the protection of the most
electrostatic damage-susceptible items. For the purposes of this document, extremely electrostatic sensitive (EES)
items are those that are affected by any combination of electrostatic charge, electrostatic voltage or electric field.
This document can be used as a guide for equipment manufacturers during the design and testing of their equipment
and by those who either use or produce reticles and other EES items.
Process technology used in the manufacture of semiconductors and electronic devices continues to achieve increases
in active feature density and device complexity. With increased levels of integration, longer interconnects and
smaller conductor separations, sensitivity to field-related problems increases.
This document provides
recommendations for addressing the problem of damage through closer examination of electric field as a supplement
to existing static charge mitigation techniques.
This document define principles for handling reticles and other EES items within a specially designated controlled
environment and recommends appropriate levels of electric field to maintain within that environment. This
document presents recommendations about grounding and material selection that may conflict with established
methods of electrostatic charge control and should be applied only within a specially designated and clearly
identified area.
Wherever this document makes reference to reticles, this should be regarded as an example of an EES item that has
been studied in depth and which is being used for illustration purposes. The principles being discussed may also be
relevant to other items that exhibit extreme electrostatic sensitivity so the guidance should not be regarded as
exclusive to reticles and reticle handling. Such items include small geometry device structures on wafers, charged
device model (CDM) sensitivity of packaged devices in back end processing, and device structures on glass in flat
panel display manufacturing.
i
Review and Adjudication Information
Group:
Date:
Time & Timezone:
Location:
City, State/Country:
Leader(s):
Standards Staff:
Task Force Review
ESD/ESC Task Force
2011/07/12
1330-1630 PDT
San Francisco Marriott Marquis
San Francisco, CA
Arnie Steinman (Electronics Workshop)
[email protected]
Ian McLeod (SEMI NA)
408.943.6996
[email protected]
Committee Adjudication
NA Metrics Committee
2011/07/13
1500-1800 PDT
San Francisco Marriott Marquis
San Francisco, CA
David Bouldin (Fab Consulting)
Mark Frankfurth (Cymer)
Ian McLeod (SEMI NA)
408.943.6996
[email protected]
This meeting’s details are subject to change, and additional review sessions may be scheduled if necessary. Contact
the task force leaders or Standards staff for confirmation.
Telephone and web information will be distributed to interested parties as the meeting date approaches. If you will
not be able to attend these meetings in person but would like to participate by telephone/web, please contact
Standards staff.
ii
Semiconductor Equipment and Materials International
3081 Zanker Road
San Jose, CA 95134-2127
Phone:408.943.6900 Fax: 408.943.7943
DRAFT
SEMI Draft Document 4783
New Standard: Guide For The Handling Of Reticles and Other
Extremely Electrostatic Sensitive (EES) Items Within Specially
Designated Areas
1 Purpose
1.1 The purpose of this document is to minimize the negative impact on productivity caused by static charge and
electric fields in semiconductor manufacturing equipment and facilities. It is a guide for establishing optimum
electrostatic compatibility of the handling environment for reticles and other items that are extremely electrostatic
sensitive (EES) to electrostatic charge, voltage and electric field. This guide is complementary to SEMI E78 and
E129 and is intended to improve the protection of the most electrostatic damage-susceptible items.
NOTE 1: For the purposes of this document, extremely electrostatic sensitive (EES) items are those that are affected by any
combination of electrostatic charge, electrostatic voltage or electric field.
1.2 This document can be used as a guide for equipment manufacturers during the design and testing of their
equipment and by those who either use or produce reticles and other EES items.
1.3 Process technology used in the manufacture of semiconductors and electronic devices continues to achieve
increases in active feature density and device complexity. With increased levels of integration, longer interconnects
and smaller conductor separations, sensitivity to field-related problems increases. This document provides
recommendations for addressing the problem of damage through closer examination of electric field as a supplement
to existing static charge mitigation techniques.
2 Scope
2.1 The scope of this document is limited to the definition of principles for handling reticles and other EES items
within a specially designated controlled environment and recommendation of an appropriate level of electric field to
maintain within that environment.
2.1.1 This document presents recommendations about grounding and material selection that conflict with
established methods of electrostatic charge control such as those defined in ANSI/ESD S20.20, so the guidance
given here should be applied only within a specially designated and clearly identified area.
2.2 This document references SEMI E78, SEMI E129, SEMI E43, and other methods of measuring electrostatic
parameters. The set of documents is complementary, providing guidance on managing different aspects of
electrostatic risk under a wide range of conditions.
2.3 While this document makes frequent reference to electrostatic fields and effects, for the purposes of this guide
this should also be considered to include alternating, variable and transient electric and electromagnetic fields. All
such fields can potentially generate electrical stress within an EES item.
2.4 Wherever this document makes reference to reticles, this should be regarded as an example of an EES item that
has been studied in depth and which is being used for illustration purposes. The principles being discussed may also
be relevant to other items that exhibit extreme electrostatic sensitivity so the guidance should not be regarded as
exclusive to reticles and reticle handling. Such items include small geometry device structures on wafers, charged
device model (CDM) sensitivity of packaged devices in back end processing, and device structures on glass in flat
panel display manufacturing.
NOTICE: This standard does not purport to address safety issues, if any, associated with its use. It is the
responsibility of the users of this standard to establish appropriate safety and health practices and determine the
applicability of regulatory or other limitations prior to use.
3 Limitations
3.1 General — This guide contains general recommendations.
3.1.1 Specific field related problems or certain devices may require / allow different levels of electric field than are
recommended in this document.
3.1.2 The calculation of electric field induction involves some simplification, but the effects of such simplification
will be within the typical measurement accuracy of accepted field measurement techniques.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other
reproduction and/or distribution without the prior written consent of SEMI is prohibited.
Page 1
Doc. 4783  SEMI
LETTER BALLOT
Document Number: 4783
Date: 2010/04/20
Semiconductor Equipment and Materials International
3081 Zanker Road
San Jose, CA 95134-2127
Phone:408.943.6900 Fax: 408.943.7943
DRAFT
4 Referenced Standards and Documents
4.1 SEMI Standards
SEMI E43 — Guide For Measuring Static Charge On Objects And Surfaces
SEMI E78 — Guide To Assess And Control Electrostatic Discharge (ESD) And Electrostatic Attraction (ESA) For
Equipment
SEMI E129 — Guide To Assess And Control Electrostatic Charge In A Semiconductor Manufacturing Facility
4.2 ESD Association Standards and Advisories1
ANSI/ESD STM3.1— Ionization
ANSI/ESD SP3.3 — Periodic Verification of Air Ionizers
ESD TR3.0-02-05 — Selection and Acceptance of Air Ionizers
ANSI/ESD S20.20 — Development Of An Electrostatic Discharge Control Program For Protection Of Electrical
And Electronic Parts, Assemblies And Equipment (Excluding Electrically Initiated Explosive Devices)
4.3 Other Documents
International Technology Roadmap for Semiconductors – ITRS2
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 Definitions
5.1.1 carrier — a device for holding wafers, dies, packaged integrated circuits (ICs), or reticles for various
processing steps in semiconductor manufacturing. [SEMI E78]
5.1.2 extremely electrostatic sensitive (EES) — any item that is very highly susceptible to degradation or
malfunction caused by electrostatic charge, voltage or field, even when handled under conditions that would
normally be classified as “ESD controlled”.
5.1.3 electromagnetic interference (EMI) — any electrical signal in the non-ionizing (sub-optical) portion of the
electromagnetic spectrum with the potential to cause an undesired response in electronic equipment. [SEMI E78]
5.1.4 electrostatic compatibility — charge control adequate to allow the manufacturing of products and the interequipment transfer of products, reticles, and carriers without electrostatic problems [SEMI E129].
5.1.5 electric field induced migration (EFM) — the movement of normally stationary atoms or molecules on a
surface as a consequence of the presence of an electric field.
NOTE 2: Related Information 1 describes the characteristics of EFM in chrome-on-glass reticles.
5.1.6 electrostatic discharge (ESD) — the rapid spontaneous transfer of static charge induced by a high electrostatic
field. [SEMI E78]
5.1.7 minienvironment — a localized environment created to isolate product from contamination and people.
[SEMI E78]
5.1.8 product — any item intended to become a functional semiconductor device. [SEMI E78]
5.1.9 carrier — a device for holding wafers, dies, packaged integrated circuits (ICs), or reticles for various
processing steps in semiconductor manufacturing.
5.1.10 EES minienvironment carrier — a transport method for extremely electrostatic sensitive (EES) devices that
excludes electric fields by surrounding the device with a Faraday Cage (i.e., a conductive enclosure).
5.2 Acronyms used in this Standard
AFM — Atomic Force Microscope
ANSI — American National Standards Institute
1
Electrostatic Discharge Association, 7900 Turin Road, Building 3, Suite 2, Rome, NY 13440-2069, USA. Telephone: 315.339.6937; Fax:
315.339.6793, http://www.esda.org
2
ITRS Global Communication Center, SEMATECH, 2706 Montopolis Drive, Austin, TX 78741, USA; http://public.itrs.net
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other
reproduction and/or distribution without the prior written consent of SEMI is prohibited.
Page 2
Doc. 4783  SEMI
LETTER BALLOT
Document Number: 4783
Date: 2010/04/20
Semiconductor Equipment and Materials International
3081 Zanker Road
San Jose, CA 95134-2127
Phone:408.943.6900 Fax: 408.943.7943
DRAFT
CD — Critical Dimension (referring to reticles)
EFM — Electric Field-induced Migration
EMI — Electromagnetic Interference
ESD — Electrostatic Discharge
ESDS — Electrostatic Discharge Sensitive
ITRS — International Technology Roadmap for Semiconductors
SMIF — Standard Mechanical Interface
6 Considerations About Assessing The Risk From Electric Field
NOTE 3: Appendix 1 of this document contains a detailed treatment of the interaction between an electric field and a fieldsensitive object (in this example, a reticle).
NOTE 4: Related Information 1 describes the methods that were used for determining the recommended electric field level for
reticles. A similar approach may be adopted to determine the field sensitivity of other EES items.
6.1 Electric fields cause a number of undesirable effects in electronic device manufacturing environments.
6.1.1 Field induction can affect EES items such as reticles or flat panel displays without any physical contact
between the source of the electric field and the sensitive item.
6.1.2 Field induction can cause damage within such EES items without any transfer of static charge to or from them.
6.1.3 Electric fields that do not cause ESD may still be a hazard to small device structures.
6.1.4 In some circumstances, the damage caused by exposure to electric fields can develop continuously over an
extended period of time, resulting in gradual deterioration and eventual failure of the affected item.
6.2 Measurements of parameters such as electric field are difficult to make.
6.2.1 The nature of an object (insulator, conductor or in most cases a mixture of both), its geometry, its
surroundings and the measuring equipment itself are only a few of the factors affecting the accuracy of an electric
field measurement.
6.2.2 The levels of electric field that can cause progressive damage in EES items such as reticles may be lower than
it is practical to measure with hand-held apparatus.
6.2.3 It is not possible to measure the internal electric field within an item such as a reticle or a packaged device.
This internal field may be the result of static charge generated on its surface or fields through which it may pass
when it is moving through a piece of equipment. However, sensor devices that can be handled in the same manner as
the item of interest and that can record the electric field exposure that the item might experience are becoming
available.
6.2.4 A field measurement made at a particular location and time may not be representative of the field that will be
present in the same location at another time, or in another apparently identical location.
6.2.5 Electric fields may be transient in nature and may leave no permanent evidence that they have been present.
6.2.6 Not all field measurement equipment is sensitive to rapidly changing or transient fields.
6.3 It is difficult to relate the measurement of an electrostatic quantity like electric field to the effect it will have on
a sensitive item.
6.3.1 Tests conducted with one sensitive item may not be representative of the effects that will be produced in
another apparently similar item.
6.3.2 Almost every reticle in use has a unique conductor pattern and variations in the pattern of conductors and
insulating spaces alter the field induction that will take place upon exposure to an electric field.
6.4 Due to the variable nature of reticle designs and the inability to quantify the field induction that takes place at
different places in a reticle, it is not possible to establish definitive damage thresholds for production reticles.
Therefore, guidance values for electric field exposure have been determined following experimentation with
specially designed test reticles.
NOTE 5: Refer to Related Information 1 for details of the methods used and technical references.
6.5 It will be impossible to define electric field levels that guarantee field-related problems are totally eliminated.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other
reproduction and/or distribution without the prior written consent of SEMI is prohibited.
Page 3
Doc. 4783  SEMI
LETTER BALLOT
Document Number: 4783
Date: 2010/04/20
Semiconductor Equipment and Materials International
3081 Zanker Road
San Jose, CA 95134-2127
Phone:408.943.6900 Fax: 408.943.7943
DRAFT
6.5.1 Due to the progressive deterioration of a reticle that can be caused by electric fields and the possibility of
similar continuous damage effects in other field-sensitive items, no particular value of electric field can be
considered as “safe”. A weak electric field that is present for a long period may possibly cause more significant
damage than a strong electric field that is present for a shorter time.
6.6 Damage may be caused by internal or external electric fields.
6.6.1 Internal electric fields can be caused by an electrically isolated part of an object becoming charged
electrostatically while another part remains uncharged. The epoxy encapsulation of a packaged device may be
tribocharged during handling and this can create an electric field that emanates from the device.
6.6.1.1 If the leads of such a charged device are now connected to ground, a balancing charge attracted by the
charge on the epoxy encapsulation will flow onto the leads. Externally, the object may appear to have been
neutralized by connection to ground but internal charge separation and hence an internal electric field will still exist.
6.6.1.2 The strength of the internal electric field and hence the risk of field-induced damage will have been
increased by grounding, even though the device may now appear to be electrically neutral and hence safe.
6.6.2 External electric fields that can penetrate a field-sensitive object may be generated in many ways but electric
field interacts with a field-sensitive object in exactly the same way regardless of its source.
6.6.2.1 An electric field coming from static charge on an insulating surface is equivalent to the same strength of
electric field originating from a conductor connected to a power supply, in terms of the damage it can induce.
6.6.2.2 All sources of electric field are not, however, equivalent in terms of how they can be treated.
6.6.2.2.1 Electric fields from static charges present on insulating surfaces or isolated conductive objects can be
eliminated by using air ionization, but fields emanating from powered sources cannot be eliminated in this way.
6.6.2.2.2 Fields from powered sources can only be eliminated by shielding them with a grounded conductive
enclosure (Faraday Cage). The ability of a Faraday Cage to shield alternating or rapidly varying fields depends on
both the conductivity and continuity of the material used for its construction and the rate of change of the field. A
metal Faraday Cage protects against field penetration from any source.
6.6.3 Field perturbation will occur whenever conductors are introduced into an electric field. Such field
perturbation can result in a greater electric field strength being present within a field-sensitive object, without there
being any change in the charge state or voltage of the field source.
6.6.3.1 A field sensitive object is itself likely to perturb an externally produced electric field and may cause the
local field strength to increase to much higher levels than would be measured in the absence of the object.
6.6.3.2 Similarly, the field strength within a field-sensitive object can be increased by changing its position relative
to other conductive surfaces such as equipment panels, robot arms and workbenches.
6.6.3.3 Such field perturbations may not be intuitively obvious. For example, the placement of a conductive ring
around a planar array of isolated conductors (as in the case of a chrome border around the pattern area of a reticle)
can cause the direction of the electric field between the isolated structures to reverse. The movement of any similar
conductive structure (such as a ring-shaped carrier frame) close to a field sensitive item could cause significant field
perturbation and consequential damage.
6.7 Field induction is virtually instantaneous. It can take only picoseconds for an electric field to induce damage in
a field-sensitive object such as a reticle, so even transient electric fields and perturbations caused by the movement
of tools and equipment during material handling can be sufficient to cause damage. Such transient fields may not be
detectable unless specialist high-bandwidth monitoring equipment is used.
7 Considerations For Assessing The Risk From Electrostatic Discharge (ESD)
7.1 When an ESD event happens during the handling of a charged, packaged device, there may be a very rapid
transfer of a balancing charge onto the device leads. There may be no neutralization of the original charge on the
device package material. When a charged object is connected to ground, even through a dissipative connection, the
charge transfer that takes place delivers a balancing charge onto the device, not a neutralizing charge.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other
reproduction and/or distribution without the prior written consent of SEMI is prohibited.
Page 4
Doc. 4783  SEMI
LETTER BALLOT
Document Number: 4783
Date: 2010/04/20
Semiconductor Equipment and Materials International
3081 Zanker Road
San Jose, CA 95134-2127
Phone:408.943.6900 Fax: 408.943.7943
DRAFT
LETTER BALLOT
Document Number: 4783
Date: 2010/04/20
Figure 1
Charging during packaged device handling
7.2 Since it is virtually impossible to eliminate the in-process charging of devices that takes place inside tools,
measures should be taken to protect the devices even in the charged condition.
7.3 It is also important to recognize that measures that have been put in place to reduce the generation of ESD
events in general - often to quite justifiably avoid tool lock-ups and particle attraction - can actually increase the risk
of damage to a sensitive device.
7.4 This document explains that ESD avoidance and sensitive device protection do not always require the same
methods and that grounding is not always the preferred method.
7.4.1 If a charged EES device is connected to ground through a dissipative contact, there may not be a damaging
discharge at the moment of grounding as a controlled, balancing charge flows slowly onto the device pins. In normal
ESD-prevention terms, there is no ESD event on contact. However, the gradual transfer of a balancing charge to the
circuitry can raise the potential difference across an internal circuit junction to the critical point, causing internal
damage. This happens as a direct consequence of connecting the pins to ground and allowing static charge elsewhere
on the device to draw a balancing charge into the circuitry. The device can now appear to be macroscopically
neutralized. There will have been no ESD event at the moment of grounding and no event registered on any ESD
sensors, but the device may have been damaged.
8 Apparatus
8.1 Electrostatic charge measurement — For measuring the charge generated on product, reticles or carriers as
defined in SEMI E78, the Faraday Cup test method is shown in Figure 1 and is described in more detail in SEMI
E43. However, when using this method to measure charge on EES items inaccurate results may be obtained due to
the presence of internal electric fields as described previously (refer to ¶6.6.1 to ¶6.6.1.2).
8.1.1 Insulators and complex EES items are capable of simultaneously being charged with both polarities of static
charge. Measurement with a Faraday cup will indicate the net excess charge on an object, not the amount of charge
separation within it.
8.1.2 If a tribocharged item being tested has been grounded at any time during its movement into the Faraday Cup,
for example by the use of an equipotential bonding scheme for ESD management, a balancing charge may have been
placed on it which will nullify the charge measurement.
8.1.3 An electrostatic fieldmeter or voltmeter may be capable of indicating whether this condition exists, but this
will not always be possible.
8.2 Electric field measurement — The instrument used for making electrostatic field measurements is known as an
electrostatic fieldmeter. Instructions concerning its use should be obtained from the instrument manufacturer and
SEMI E43. Typically, an electrostatic fieldmeter measures at a distance of 2.54 cm (1 inch).
8.2.1 The measurement configuration shown in Figure 2 illustrates the effect of the instrument on the measurement.
In most cases the presence of the fieldmeter will increase both the flux from the charged surface and the
convergence of the electric field lines. The fieldmeter will generally indicate a higher value of electric field than
would be present without the fieldmeter.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other
reproduction and/or distribution without the prior written consent of SEMI is prohibited.
Page 5
Doc. 4783  SEMI
Semiconductor Equipment and Materials International
3081 Zanker Road
San Jose, CA 95134-2127
Phone:408.943.6900 Fax: 408.943.7943
DRAFT
8.2.2 An electrostatic voltmeter can be used as an alternative to the fieldmeter measurement. For small objects or
surface areas, an electrostatic voltmeter is appropriate. Refer to E43 for further information.
8.2.3 Under appropriate conditions, electrostatic voltmeters exhibit a high degree of accuracy and stability that is
independent of the distance from the charged object. The electrostatic voltmeter probe can be located very close to a
charged surface without arc-over, and it is able to resolve the field from a small charged object.
8.2.4 For measuring transient electric fields a high bandwidth electrostatic voltmeter with an output to a computer
or a storage oscilloscope may be needed.
Isolated
Inner Cup
Electrometer
Shielding
Outer Cup
In
Ground
Faraday
Cup
Figure 2
Faraday Cup Charge Measurement
Electric Field Lines
1999
Electrostatic
Fieldmeter
(v olts/cm)
2.54 cm
(1 inch)
+ + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + +
Charged
Surface
Charged
Surface
Figure 3
Electrostatic Field Measurement
8.2.5 For recording the electric field within the restricted handling environment of equipment, carriers and process
chambers where there is insufficient access for hand-held devices or where the presence of the probe itself would
significantly alter the measurement, customized sensor devices may be required that can take the place of the EES
item under consideration.
8.2.5.1 The electric field configuration in the presence of such a sensor device will be as close as possible to the
field configuration with the EES item present. Such a device can sample virtually all the environments through
which the EES item will pass and can record process- or handling-induced charging. It can also record the field
exposure duration as well as the field strength. Therefore, this is the most suitable way of assessing electric field risk
for EES items.
9 Identification of EES Classified Zones
9.1 Owing to the different principles employed for EES item handling and the conflict with conventional ESD
precautionary handling methods which use equipotential bonding (grounding) schemes, it is recommended that
zones where EES items are handled be segregated and clearly marked.
9.1.1 The symbol for identification of an EES item, handling zone or equipment that is compatible with EES
handling techniques is shown in figure 3.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other
reproduction and/or distribution without the prior written consent of SEMI is prohibited.
Page 6
Doc. 4783  SEMI
LETTER BALLOT
Document Number: 4783
Date: 2010/04/20
Semiconductor Equipment and Materials International
3081 Zanker Road
San Jose, CA 95134-2127
Phone:408.943.6900 Fax: 408.943.7943
DRAFT
Figure 4
Symbol for identifying an EES item, compatible handling zone or equipment
10 Principles for EES Classified Zones
10.1 The primary objective is to maintain an EES item in the absence of any static charge or electric field.There are
two different aspects of protection for EES items – charge management and field management.
10.2 Charge Management
10.2.1 The safety of charge management by grounding, even through a resistive contact, will depend on the nature
of the circuitry within the sensitive item, how rapidly excess charge flows to ground and the route it takes.
Grounding may also cause internal electric fields to increase as described previously. For these reasons, grounding
of the EES item may need to be avoided, when possible, within an EES classified handling zone.
10.2.2 Within the EES zone other methods of charge management, such as air ionization, are employed as an
alternative to grounding. When charge cannot be safely neutralized, changes to handling methods may be needed to
assure that charge generation does not occur. Materials that contact EES devices may need to be selected to
minimize triboelectric charge generation.
10.2.3 Air ionization is inherently capable of neutralizing charge at a rate that is unlikely to cause damage, so it can
be used in place of grounding.
10.2.3.1 In any application of air ionization it is necessary to assure that the ionized air reaches the charged surface.
Air ionization operates relatively slowly, and sufficient time must be allowed for the ionization system to work.
10.2.3.2 Air ion streams respond to the presence of static charge by reacting to the electric field that is produced by
the charge, so it is important to recognize that ionizers do not actually prevent the generation of static charge or
electric field.
10.2.3.3 Ionizers supply positive and negative ions to neutralize either polarity of static charge. Ionizers must be
carefully balanced to avoid generating a charge on isolated conductive objects. Ionizers that are incorrectly balanced
or badly maintained can create a risk rather than reducing the risk. Refer to ESD Association documents ANSI/ESD
S3.1, ANSI/ESD SP3.3, and ESD TR3.0-02-05 for more information regarding the use and testing of air ionization.
10.2.3.4 Regular monitoring of ionizer efficiency and balance is required for the effective operation of an EES
handling zone. The required frequency of checks will depend on many factors including the inherent stability of the
ionizers being used, so it cannot be prescribed in this document. Operators of EES zones must satisfy themselves
that auditing intervals and monitoring methods are adequate to maintain the effective operation of ionizers.
10.2.4 Field mitigation
10.2.4.1 Field reduction
10.2.4.1.1 Avoiding the accumulation of static charge should minimize electric fields. This is best done through the
careful selection of materials and by employing air ionization as described previously.
10.2.4.1.2 Corona-type air ionizers work by generating strong electric fields at emitter tips. Care should be taken to
ensure that any electric fields produced by such air ionizers cannot reach an EES item. Alpha or x-ray ionizers
produce no strong fields so may be more suitable for use close to EES items
10.2.4.2 Shielding
10.2.4.2.1 Ionizers cannot neutralize the electric fields that are generated by powered systems, so wherever power is
used steps must be taken to minimize stray electric or electromagnetic fields by using shielding.
10.2.4.2.2 Shielding with metal is recommended to achieve the best electric and electromagnetic field reduction
around powered systems and great care should be taken when making joints in the shield to ensure that high
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frequency fields cannot escape. Methods and materials that are suitable for EMI shielding purposes are preferred for
shielding in an EES zone.
10.2.4.2.3 The only fail-safe means of protecting an EES item from externally generated electric and
electromagnetic fields is to enclose it within a fully conductive Faraday Cage. Such a field-protective, EES
minienvironment carrier should be used to house EES items whenever possible.
10.2.4.2.4 Shielding efficiency is dependent on the conductivity and density of the material that is used to make the
shield. Inherently “conductive” or metallized plastics and even metal wire meshes may not be sufficient to achieve
complete field shielding at all frequencies. The most suitable shielding material to use is sheet or machined metal,
with metal-to-metal connections being made between any separate parts. It should be noted that additional shielding
may increase the risk of field perturbation as described below.
10.2.4.3 Field perturbation
10.2.4.3.1 Changing the proximity of an EES item to any conductive surface or other objects will change the field
configuration and field strength that the EES item may experience. Refer to Appendix A1.4.1 for an example of
placing a reticle in a grounded enclosure. To keep field perturbation effects to a minimum, EES items should be kept
as far as possible from conductive surfaces and other objects.
10.2.4.3.2 It is recommended that EES items be handled with inherently insulating (i.e. field-transparent) end
effectors or tools so that any electric fields that may be present in the handling environment or created by the items
themselves being tribocharged are not perturbed. For example, adding an insulating surface layer or contact pad to
an otherwise conductive end effector is not equivalent to a fully insulating end effector because the body of the tool
is not field-transparent and hence will cause field perturbation.
10.2.4.3.3 If it is not possible to implement fully insulating end effectors in equipment that handles EES items, it
may be necessary to adjust the timing of the handling sequences. If air ionization is used it must have sufficient time
to neutralize any static charge that may have been generated on the item or its surroundings before a grounded end
effector approaches the EES item to move it.
NOTE 6: Refer to Related Information 2 for an example of the effect of a grounded end effector on the field of a charged reticle.
10.2.4.3.4 An EES item may itself be tribocharged during handling and this may generate an internal electric field.
Placing such a charged item inside a Faraday Cage would result in field perturbation that could increase the risk of
damage. Therefore all previously described precautions must be followed to neutralize static charge and ensure that
the use of a Faraday Cage does not itself increase the risk of the EES item being damaged. Refer to Appendix
A1.4.1.
11 Interfacing EES Zones And Carriers To Other Zones
11.1 When transferring an EES item from an EES zone to an ESDS handling zone where equipotential bonding is
used and where the item will be either purposely grounded or closely approached by other grounded objects, excess
static charge must be neutralized before transfer takes place. Hence, ionization is recommended at all transfer points,
especially where EES items are being placed into and removed from EES minienvironment carriers.
11.2 When EES items are within an EES minienvironment carrier, the handling of that carrier should comply with
external handling standards as follows.
11.2.1 If the outside environment is designated as an ESDS handling zone, ESDS norms should apply, e.g. the
minienvironment carrier should be connected to an equipotential bonding point when it is placed on a load port for
opening.
11.2.2 If an EES compatible minienvironment carrier is being handled outside a controlled zone, no special
handling precautions for electrostatic protection are required.
11.3 Some equipment in a facility may be designated EES compatible while other equipment may not. Whenever an
EES compatible minienvironment carrier is used with non-EES compatible equipment, ESDS handling practice
should be adopted.
11.3.1 Such interoperability allows facilities to upgrade their handling systems only where it is considered
necessary or appropriate to do so. Thus the adoption of EES handling or the use of EES compatible
minienvironment carriers does not necessitate a complete change of the established handling methods, ESD
precautions, certification programs or operator training in the wider facility.
11.3.2 Likewise the use of EES compatible minienvironment carriers within an ESD protected area does not
degrade the electrostatic protection rating of that area towards the sensitive objects being handled. Since grounding
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of ESDS items has been shown to increase the risk of them being damaged rather than to reduce it, the adoption of
EES compatible carriers is an improvement to ESD protection zones.
11.3.3 Hence existing certification schemes and auditing practices are not altered by the adoption of EES handling,
EES is merely appended to them.
11.3.4 However, the use of EES compatible minienvironment carriers may change the requirement for certain ESD
precautions in parts of the facility or may alter the specification that is needed. For example, if reticles were
previously transported in non-EES compatible minienvironment carriers or were handled bare, air ionizers
throughout the area would have been required. When using EES compatible minienvironment carriers, some of this
ionization might not be necessary. Depending on site conditions and requirements ceiling ionizers may still be
needed to control EMI. They will not be required to protect the EES items now protected by the EES compatible
minienvironment carriers. Hence the specifications for ionizer location and performance may be reduced.
12 Guide Specifications For EES Zones
Table 1 Suggested Parameters For Use Within An EES Classification Zone
Parameter
Value
Notes
Ambient electric field
< 500V/m
Value is based on EFM risk in chrome on glass reticles
Field recovery time
< 60 sec
Field levels should be recoverable to the allowable ambient level
(<500 V/m) within this time of a “normal transient stress” event
such as the ending of a manufacturing process step.
Maximum transient field
5000V/m
Transient stress should be present for no longer than 1 second
before field recovery process starts.
#1 Ambient electric field – see Related Information 1 ¶ R1-2.4.11.
12.1 The levels in Table 1 are appropriate for the handling of chrome on glass reticles. Electrostatic fields may also
damage other specialized components such as compound semiconductors, flat panel displays or magneto-resistive
(MR) disk-drive heads. Safe handling these devices may need to use different levels.
12.1.1 The levels in Table 1 are measured with an electrostatic fieldmeter, or other field measuring devices. The
measurement instrument should have sufficient response time to allow measurement of transients. Refer to
paragraph 8.2.
12.2 In all cases it is desirable to achieve the minimum possible field strength, static charge levels and recovery
times from transient events. The lower the stress that is present and the shorter its duration, the longer an EES item
may remain within the environment without suffering potential damage.
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APPENDIX 1
COMPUTER SIMULATION OF ELECTRIC FIELD INTERACTIONS WITH
RETICLES
NOTICE: The material in this appendix is an official part of SEMI XXXXXX and was approved by full letter
ballot procedures on XXXXXX.
A1-1 Introduction
A1-1.1 Electrostatic damage is most commonly caused by rapid charge transfer, through a spark (ESD event).
A1-1.2 Charge moves in reaction to the electrostatic force of attraction or repulsion exerted on it by another charge.
This force is represented by the concept of an electric field – the field strength indicates the magnitude of the force
that a unit charge would experience at that point and the direction of the field line is the direction of the force.
A1-1.3 Hence, for charge to move there must be an electric field present to exert the force that moves it. Without an
electric field present, charge will not move.
A1-1.4 Computer simulation can produce a “map” of the electric field configuration around any object, so it can
indicate when there will be a risk of ESD or other field-related damage.
A1-2 How Electric Field Causes ESD Damage
A1-2.1 Figure A1-1 shows schematically the stages of field induction when an electric field (voltage gradient)
passes through a reticle.
A1-2.1.1 Figure A1-1a is the state of a uniform electric field created by two vertical electrodes at different
potentials before a reticle is introduced. The voltage changes linearly from one electrode to the other.
A1-2.1.2 When the reticle is introduced as shown in figure A1-1b, each isolated conductor comes to an intermediate
potential that is induced by the external electric field.
A1-2.1.2.1 Electrons are forced to move within the conductors under the influence of the external electric field.
This charge displacement continues until it produces a balancing electric field that cancels out the external electric
field. Thus, at equilibrium there is no net electric field present within each conductor.
A1-2.1.2.2 The electric field is seen to have been amplified in the gaps between the isolated reticle structures. Any
array of isolated conductors will function as an electric field amplifier in this way. The degree of field amplification
depends on the length and relative separation of the conductors and the orientation of the array with respect to the
external electric field. Typically, a reticle can amplify the ambient field strength by up to 1000x.
A1-2.1.2.3 Such an amplified electric field may be sufficient to initiate a discharge between the structures. After
discharge has taken place, the charge redistribution within the reticle will be as shown in figure A1-1c. Charge has
been displaced internally and there is now no electric field remaining between the reticle structures.
A1-2.1.2.4 When the external electric field is removed as shown in figure A1-1d, each of the isolated reticle
structures has a charge imbalance – the structures have been charged by field induction. Note that in this condition
there has been no charge transfer to or from the reticle by any external source, only the charge within the reticle has
been forced to move.
A1-2.1.2.5 When the external field is removed, the displaced charge within the reticle creates an internal electric
field that is exactly opposite to the previously applied field. This causes further discharges as the displaced charge
returns to its original location.
A1-2.1.2.6 After the field induction cycle, the reticle will have been damaged twice as the internal charge has been
forced to move in two opposite directions. The reticle has remained electrically neutral throughout the process and
no charge has been added to or taken from it.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
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+
–
+
–
+
–
+
–
–
+
–
+
–
+
a)
+ + +
+
+
+
–
b)
–
–– –
–
+ +
+
+
+
+
–
–
–– –
–
+
c)
d)
Figure A1-1
Schematic representation of the field-induced damage process in a reticle (the graph represents voltage as a
function of position in the plane of the conductors).
1. Uniform electric field between two charged plates before the reticle is present
2. Reticle introduced. Charge within the conductors moves in response to the field. The field is zero within the
conductors, and thus is amplified in the gaps between the conductors.
3. Discharges have occurred between the conductors and charge is now redistributed, which removes the internal
field. The conductors have now been charged by field induction even though no charge has been transferred to
or from the reticle.
4. When the external field is removed, the redistributed charge creates its own field. More discharges will occur as
the charge returns to its original location.
A1-3 Details Of The Simulations
A1-3.1 Computer simulation of electric field interactions with isolated conductors on an insulating support has been
carried out using commercially available software (e.g., Opera 2D). Voltage contours (lines of equal potential) are
plotted around the simulated objects.
A1-3.1.1 Two-dimensional rather than three-dimensional analysis has been used to simplify the modeling. Such two
dimensional models consider that the structures extend to infinity in front of and behind the plane being modeled.
Since electric field strength is greatest at edges and points that are reduced by this simplification, this means that the
simulations slightly understate the true field strength that would be present around a three-dimensional object.
A1-3.1.2 While these limitations of two-dimensional simulation do affect the results, the differences between the
calculated values in the simulations and the true values in a three-dimensional structure will be small enough to not
significantly affect the conclusions that can be drawn from the simulations.
A1-3.2 Structures simulated
A1-3.2.1 Reticle in cross-section
Figure A1-2
Structure used to represent a reticle in cross-section
A1-3.2.1.1 A reticle in cross-section is simulated as a block of insulating material representing the glass substrate,
supporting 100 isolated conductive lines representing the image area of a reticle.
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A1-3.2.1.2 On either side of the 100 isolated conductors are two wider strips of conductor representing the chrome
border. When these are used to represent a continuous chrome field around the image area, these two strips are
constrained to be at the average potential of the reticle. When a reticle without a continuous chrome border is being
simulated, these two strips are allowed to “float” independently.
A1-3.2.2 Reticle environment
A1-3.2.2.1 To represent free space conditions the model is given floating boundary conditions. Such simulations
represent the situation with no other field-perturbing objects in close proximity to the simulated object.
A1-3.2.2.2 When a uniform electric field is to be applied to an object in free space, the two side boundaries are
given specific voltages that create a uniform electric field between them. The simulated structure is allowed to
“float” within this field to determine the induced potentials and field configurations that would result.
A1-3.2.2.3 When a non-uniform electric field is to be simulated to represent static charge on a nearby object, that
object is given a fixed potential and a grounded plane is introduced that represents a work surface, equipment wall
or other such surface. This defines an electric field with a known strength and initial configuration prior to the
insertion of the object to be simulated.
A1-3.2.2.3.1 Fixing the potential of an insulating or isolated charged object is physically incorrect, since its
potential will vary with its separation from ground according to the relationship Q=CV. However, the inaccuracy
introduced by this simplification of the model is small and does not invalidate the conclusions drawn, which are
comparative.
A1-4 Simulated Scenarios
A1-4.1 Static charge on a reticle
A1-4.1.1 Since a reticle is normally handled by contacting the chrome border that surrounds the image area and
which is outside the pellicle enclosure, this is where static charge is most likely to be deposited. A reticle that has
5kV on the chrome border is simulated in figure A1-3.
a)
b)
Figure A1-3
5kV on the chrome border of a reticle
a) Reticle in free space – potential difference within reticle 80mV
b) Reticle in large grounded box – potential difference within reticle 3kV
(the lines are equipotentials, or “voltage contours”)
A1-4.1.1.1 Figure A1-3a shows the voltage contours surrounding the reticle in free space conditions, meaning there
are no nearby objects that will affect the field emanating from the charge on the reticle. The entire reticle is at almost
the same potential, with there being only 80 millivolts of potential difference between the edge and the central part
of the image area.
A1-4.1.1.2 Figure A1-3b shows the same reticle enclosed in a “large” grounded box. The presence of the grounded
enclosure perturbs the field conditions around the reticle, resulting in 3kV of potential difference induced between
the central region of the reticle and the chrome border.
A1-4.1.1.3 It is seen that the proximity of a grounded surface is a factor that strongly affects the risk of damage to a
field-sensitive object like a reticle if it is charged. Voltage alone is not hazardous, since there will be no charge
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movement within the reticle as a direct result of it (figure A1-3a). When a nearby object disturbs the field
conditions, this can perturb the situation and create a high risk of field-induced damage (figure A1-3b).
A1-4.1.1.4 The high field strength in the reticle is produced long before the grounded object (in this case the box)
makes contact and provides a path to ground through which the charge on the reticle might be removed. This shows
that attempting to discharge an EES object by grounding is highly likely to damage it, regardless of whether or not
the path to ground is resistive.
A1-4.2 The effect of a “guard ring”
A1-4.2.1 The chrome border around the edge of a reticle combined with an insulating channel between the chrome
border and the image area is sometimes referred to as the “guard ring”. This is often considered to act as a
protective structure, reducing the risk of ESD damage to the sensitive image area of the reticle.
A1-4.2.2 The presence of a continuous chrome border around the image area of a reticle indeed has a significant
effect on the penetration of electric field, as shown in figure A1-4, but the effect is not simply to attenuate the field.
E
E
E
E
E
E
E
E
Figure A1-4
The effect of the chrome border (guard ring) on field induction in a reticle
a) Voltage contours as they would be without a chrome border present
b) Voltage contours with a chrome border surrounding the image area
A1-4.2.2.1 Figure A1-4a shows the voltage contours as they would be if a chrome border was not present on the
reticle and the reticle was placed in a uniform horizontal electric field. The voltage contours are seen to pass
uniformly through the structure, inducing a constant potential gradient from one side of the reticle to the other, as
shown in figure A1-1.
A1-4.2.2.2 Figure A1-4b shows the situation with a continuous chrome border around the image area. The chrome
border comes to the average potential of the reticle and the presence of this conductive ring at this average potential
strongly disturbs the electric field. The field bends around and its direction is actually reversed at the points where it
passes through the pattern area on the way to the chrome border (as indicated by the overlaid arrows).
A1-4.2.2.3 The effect of the chrome border can be seen clearly in figure A1-5, where the induced voltages from the
simulations of figure A1-4 are plotted. It can be seen that for this particular orientation there is indeed an
attenuating effect on the average field strength within the reticle due to the presence of the “guard ring”.
A1-4.2.2.4 However, the reticle features at the edge of the image area closest to the guard ring experience a field
strength that is just as high as it would have been without the guard ring present – the key difference is that the
direction of the potential gradient is reversed.
A1-4.2.2.5 The field induction characteristic illustrated in figure A1-5 explains the “ring of fire” distribution pattern
for much of the ESD damage that is seen in reticles. Reticles with significant amounts of ESD damage frequently
have a high concentration of damage sites in close proximity to the edge of the pattern. This has often been
explained as being due to static charge that is deposited on the chrome border from a non-grounded handling tool,
which then jumps into the image area by means of a spark.
A1-4.2.2.6 The considerations of figures A1-3 and A1-4 show the true reason for the concentration of ESD events
in this area. Field induction causes the local field strength to be highest here. Damage would occur here due to field
induction whether charge was placed on the chrome border or not. As figure A1-4 shows, attempting to remove any
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charge from here with a grounded handling tool would probably induce damage in the reticle long before electrical
contact could be made.
Figure A1-5
Induced potential and potential gradient in a reticle with and without a guard ring (sloping bars indicate
potential gradient at the edges near the guard ring )
A1-4.3 Insulating versus conductive supports
A1-4.3.1 It is common practice to handle ESD sensitive items with grounded tools and end effectors, which is
referred to as “equipotential bonding”. The objective behind this is to maintain all objects at the same electrical
potential so that when they contact each other during handling procedures static discharges will not occur.
A1-4.3.2 However, it is quite easy for items to be tribocharged during handling or processing, so they may
unavoidably develop a static charge.
A1-4.3.3 A high resistance to ground (106 – 109 Ω) at the point of contact is commonly prescribed so that if any
static charge does flow to or from the ESD sensitive item during handling, the current and hence the power
dissipated will be low.
A1-4.3.4 While this practice does indeed reduce the likelihood of a static discharge between a tool and the object
that it is handling, it has an unfortunate effect on the field induction that will take place within a field sensitive item
as has been shown in previous examples.
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b)
Figure A1-6
Field induction with a) conductive / static dissipative or b) insulating supports
(the lines are equipotentials, or “voltage contours”)
A1-4.3.5 Figure A1-6 represents the field induction pattern in a reticle that is resting on supports, for example in a
reticle pod. The base of the pod defines the ground plane in the simulation, since equipment load ports are always
grounded and pod bases are always conductive or static dissipative. A high voltage is simulated immediately above
the reticle, representing static charge that may be present on the pod handle or on an operator’s gloved hand.
A1-4.3.5.1 Figure A1-6a shows the field configuration if the reticle is resting on conductive or static dissipative
supports that are connected to ground. Figure A1-6b shows the same situation except that the reticle is supported by
fully insulating (i.e. field-transparent) support structures. The potential differences present between the structures in
the reticle image area are several times higher when the chrome border is grounded through the supports than when
it is allowed to “float” to an intermediate potential along with the other isolated reticle structures.
A1-4.3.5.2 It has been confirmed experimentally that field-induced damage inside a reticle pod is worse if the
reticle is grounded on the supports as shown in figure A1-6
NOTE 1:See reference by Rudack, Pendley, Gagnon and Levit in Related Information 1.
A1-4.3.6 Figure A1-7 shows a similar situation to figure A1-6 but the field source is now displaced to one side,
representing static charge on an operator’s gloved hand while loading a reticle onto a support structure using a
reticle pick.
A1-4.3.6.1 Figure A1-7a has conductive reticle supports, while figure A1-7b has fully insulating supports. The
scenario is almost identical to that of figure A1-6 except that the consequences of field distortion by the chrome
border / guard ring in the asymmetric field coming from the side of the reticle are shown.
a)
b)
Figure A1-7
Asymmetric field induction during reticle loading onto
a) conductive / static dissipative supports and b) insulating supports.
The arrows represent the direction and magnitude of the field at each edge of the reticle.
(the lines are equipotentials, or “voltage contours”)
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A1-4.3.6.2 In figure A1-7a the highest potential gradient induced in the reticle is at the point nearest to the source of
the field, which can be seen by the density of the voltage contours at that point. By contrast, the highest potential
gradient in figure A1-7b where the reticle is allowed to “float” on insulating supports is on the side of the reticle
furthest from the source of the field.
A1-4.3.6.3 The direction of the field as it passes through the features in the image area close to the chrome border is
also seen to be different in the two cases, as represented by the arrows above the figures. This has a very significant
implication for the loading of a reticle onto conductive supports; as the reticle approaches but does not yet make
contact with the supports the field configuration will be similar to that in A1-7b. As soon as the reticle makes
electrical contact with the supports the chrome border immediately comes to ground potential and the electric field
configuration switches to that of A1-7a.
A1-4.3.6.4 As soon as the reticle is grounded by the supports, the potential differences in the reticle reverse and
increase. The damage caused will be significantly worse in case a) than in case b) and will involve sequential
discharges taking place in two different directions.
A1-5 Conclusion
A1-5.1 In all situations studied, grounding a reticle has been found to increase the risk of field induced damage.
A1-5.2 Under no circumstances would grounding ever reduce the damage sustained by a field sensitive item like a
reticle.
A1-5.3 Hence, it has been demonstrated that while equipotential bonding does indeed help to prevent external ESD
events between an object and its handling means, if the object being handled is field-sensitive, grounding creates a
much greater risk of damaging the object itself.
NOTE 2:The use of field-transparent (i.e. fully insulating) handling arms and tools to avoid field perturbation effects around EES
items necessitates the use of air ionization.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
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Document Number: 4783
Date: 2010/04/20
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RELATED INFORMATION 1
DEVELOPING NEW RECOMMENDATIONS FOR THE ELECTRIC FIELD
EXPOSURE OF RETICLES
NOTICE: The material contained in this related information is not an official part of SEMI XXXXXX and is not
intended to modify or supersede the guide in any way. These notes are provided as a source of information to aid in
the application of the guide and are to be considered reference material. Determination of the suitability of the
material is solely the responsibility of the user.
Contributed by Gavin Rider, PhD. E-mail: [email protected]
R1-1 Previous Guidance
R1-1.1 All guidance values published in SEMI Standards and the ITRS prior to publication of this document have
been based on managing ESD risk or contamination rates. ESD risks for reticles were evaluated by exposing
production reticles to calibrated electric fields and also by stressing specially designed test reticles in the same way.
R1-1.2 Since those guidance values were developed further research has shown that a physically different damage
mechanism (EFM) was responsible for some of the reticle damage that had previously been attributed to ESD. EFM
causes the continuous CD degradation of certain features in chrome on glass reticles at induced potential differences
well below the onset threshold for an ESD event.
R1-1.3 Since EFM involves a completely different physical process to ESD, it was apparent that guidance designed
to protect against ESD would not necessarily be effective at protecting against EFM.
R1-1.4 The following experimental quantification of EFM was conducted to establish accurate thresholds and rates
for reticle damage below the ESD threshold, to find out more about the physics of EFM and to confirm whether or
not ESD was involved. A more complete description and treatment of the data can be found in the references.
R1-2 Experimental Quantification Of EFM Risk In Reticles
R1-2.1 Stress testing.
R1-2.1.1 Previous field induction experiments had been conducted with “Canary” test reticles but those
experiments did not produce quantifiable data about the local electric field level at the point of damage. It was
decided to apply voltages directly to reticle structures and thereby to create the local electric field directly.
R1-2.1.2 This approach would allow fully calibrated and reproducible stress testing of the structures and accurate
timing of the stress duration.
R1-2.1.3 To enable direct comparison with the earlier field-induced damage data a new test reticle was designed
which had similar electrode structures having an almost identical local electric field configuration at the places
where damage would be produced. The structure of the test reticle is shown in figure R1-1.
Figure R1-1
Design of the EFM test reticle.
There are 8 columns of test cells in groups of 5 x 5, with line widths from 1µm to 10µm. The width of each
spur line is the same as its spacing from the chrome border. A blank cell with no spur line is positioned at the
intersection of the borders around the 5x5 groups of cells, to monitor surface conditions across the reticle.
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R1-2.1.4 Voltage was applied to individual test structures in a systematic way to apply calibrated electrical stress
over the range 1V to 100V for different periods of time.
R1-2.1.5 Voltage was also scanned continuously from 0V to 100V and the current flowing was recorded so that
current/voltage characteristics of the test cells could be produced.
R1-2.1.6 The tests were conducted with positive and negative polarity.
R1-2.2 Results
R1-2.2.1 After the stress testing was completed, the test cells were imaged using optical microscopy and atomic
force microscopy.
R1-2.2.2 Line scans were conducted using a Critical Dimension Atomic Force Microscope (CD-AFM) to determine
accurately the critical dimension (CD) variation produced.
R1-2.2.3 The current-voltage measurements indicated that current flowing on the reticle surface had a highly nonohmic characteristic, with the current increasing nonlinearly with increasing field strength at the positively biased
electrode. This indicated that field-generated positive charge carriers play a role in the surface conduction.
R1-2.2.4 The CD-AFM line scans as shown in figure R1-2 revealed progressive alteration of the left chrome line
edge topography starting with the formation of a “meniscus” at the point where the chrome meets the quartz. As the
stress duration and stress voltage increase so the extent and character of the CD variation change.
7a
a
b7b
7c
c
100n
m
7d
d
1µm
Figure R1-2
Line profiles of 1µm test cells:
a) Spur stressed at –100V for 300 seconds
b) Spur stressed at –100V for 15 seconds
c) Spur stressed at –50V for 15 seconds
d) Unstressed.
R1-2.2.5 The rate of line edge modification at 50V was measured at 3nm per second. At 100V the rate of edge
modification of the chrome line increased to over 6nm per second.
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R1-2.2.6 “Photo-realistic” AFM images of the damaged test structures were also produced, revealing the full extent
of the chrome line modification and many details about it that were not apparent from the CD-AFM line topography
profiles. An example is shown in figure R1-3.
Figure R1-3
Composite atomic force microscope image of a 1-micron test structure
that had been stressed at 100V for 250 seconds.
R1-2.3 Characteristics of EFM
R1-2.3.1 The AFM images were overlaid with electric field intensity contours produced by computer simulation to
indicate the local field strength that would have been present at different parts of the structure during the experiment.
R1-2.3.2 This indicated that the field strength at the chrome line edge corresponding to the onset of meniscus
formation is 2 x 106 V/m, with the rate of line edge modification rising continuously until a field strength of
approximately 2 x 107 V/m is reached, at which point the line edge spreading rate appears to reach a maximum.
R1-2.3.3 The characteristics of the material migration pattern suggest that the formation of a meniscus at the base of
the line is due to the field-initiated (but not field-driven) migration of chrome atoms (designated as EFM type 1).
R1-2.3.4 At slightly higher local field strength the chrome atoms become ionized at the meniscus edge, separate
from the chrome line and migrate across the quartz surface (designated as EFM type 2). This migrating material is
responsible for the formation of the small droplets that are seen alongside the line in figure R1-3.
R1-2.3.5 Higher magnification images revealed the presence of a surface film of migrated material on the quartz
surface against the opposite chrome line, indicating that the migration range of the chrome ions in the region with
greatest field strength was over 1 micron.
R1-2.3.6 The shape and surface topography of the chrome line was significantly modified, indicating that chrome
atoms migrate on the chrome as well as on the quartz as a result of field induced stress.
R1-2.4 Post-experimental analysis
R1-2.4.1 The experimental data showed directly how reticle damage varied as a function of feature spacing, applied
voltage and time.
R1-2.4.2 Computer simulations were conducted to correlate the observations with the local field strength that was
present when the damage was being produced.
R1-2.4.3 The simulations were then extended to explore how field-induced electrical stress would vary as a
function of feature size and separation. This allows the effect of field induction in future generations of reticles to be
predicted and related to the experimental data that have been obtained with larger feature sizes and spacing.
R1-2.4.4 Figure R1-4 shows how the induced voltage between adjacent features varies as a function of conductor
separation. Figure R1-5 shows how the local electric field strength between adjacent features varies as a function of
conductor separation. For example, this is representative of the field induction trend that might exist in a sequence of
metallization-layer reticles, where the conductor length remains constant but the feature width and separation is
decreased from one product generation to the next.
R1-2.4.5 The ESD and EFM damage thresholds that have been determined experimentally are shown as dotted lines.
Under the simulated conditions the induced voltage in the reticle would be well below that needed to initiate an ESD
event, but the local electric field strength would be at least 50x higher than the threshold for EFM.
R1-2.4.6 The simulation clearly indicates that as reticle feature separations continue to decrease it becomes
increasingly difficult to induce sufficient voltage for an ESD event, but much easier to induce EFM.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
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Date: 2010/04/20
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Figure R1-4
2D finite element analysis of field induction in a constant electric field. Voltage between two coaxial metal
lines are plotted as a function of their separation.
Figure R1-5
2D finite element analysis of field induction in a constant electric field. Local field strength between two
coaxial metal lines are plotted as a function of their separation.
R1-2.4.7 Further simulation was conducted to calculate the induced potentials and local field strength that might
exist between reticle features under conditions equivalent to the electric field guidance values in SEMI E78 and the
ITRS.
R1-2.4.8 The result was similar to figures R1-4 and R1-5, following the same trend. While there would be
insufficient induced voltage between two adjacent lines to cause an ESD event, the local electric field was well over
the EFM threshold. Under the simulated conditions, damage equivalent to that shown in figure R1-3 would take
place. This indicates that while the previous electric field guidance values for reticles are appropriate for managing
ESD risk, they are unsafe when considering the risk of EFM.
This is a draft document of the SEMI International Standards program. No material on this page is to be construed as an offi cial or adopted standard. Permission is granted to
reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other
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R1-2.4.9 Since EFM in reticles is known to have been causing yield loss in semiconductor production for over a
decade and because the analysis indicates that the trend will be towards greater field induction problems with each
new reticle generation, it is necessary to update the guidance for the electric field exposure of reticles.
R1-2.4.10 Using similar models to those used for the previous simulations, it was calculated how much external
electric field would be needed to initiate EFM between two isolated reticle lines. This indicated that the EFM
threshold could be reached with an ambient field strength in the range 100-1000 V/m, depending on the feature
layout and other conditions such as field orientation.
R1-2.4.11 Based on this consideration and taking into account the practicalities of achieving such low field levels in
manufacturing environments and being able to measure them, it was proposed to recommend a single level of 500
V/m of electric field as a new guidance value. It must be emphasized that the levels of field induction in real
structures will vary significantly and may be more extreme than the situations simulated, so the electric field
guidance value cannot be guaranteed to be safe. No amount of electric field exposure should ever be considered safe
for a reticle.
R1-3 References And Related Sources Of Information
Rider, G., Kalkur, T. S., “Experimental Quantification Of Reticle Electrostatic Damage Below The Threshold For
ESD”, Proceedings of SPIE: Metrology, Inspection, and Process Control for Microlithography XXII, vol 6922-73,
(2008).
Rider, G., “Electric Field-Induced Progressive CD Degradation In Reticles”, SPIE Photomasks, 7122-14, (2008)
Rider, G., “EFM – A Pernicious Threat To Reticles – Exposed”, Future Fab 25, pp 67-73 (2008). www.futurefab.com
Sebald, T., “Don’t Kill Canaries!: Introducing A New Test Device To Assess The Electrostatic Risk Potential To
Photomasks.” SPIE Photomask Technology, 7122-15 (2008).
Chubb, J., “Measuring The Shielding Performance Of Materials”, Proceedings of IEEE-IAS, pp 662-665, (2000)
Englisch, A., van Hasselt, K., Tissier, M., Wang, K. C.; “CANARY: A High-Sensitivity ESD Test Reticle Design
To Evaluate Potential Risks In Wafer Fabs”, SPIE Photomask Technology XIX, vol 3873, pp 886-892, (1999).
Rudack, Levit and Williams, “Mask Damage by Electrostatic Discharge: A Reticle Printability Evaluation”,
Proceedings of SPIE – Optical Microlithography XV, vol 4691-2, pp 1340 –1347, (2002)
Wallash, A., Levit, L., “Electrical Breakdown And ESD Phenomena For Devices With Nanometer-To-Micron
Gaps” Proceedings of SPIE Vol. 4980, pp. 87-96, (2003)
Rudack, Pendley, Gagnon and Levit, “Induced ESD Damage on Photomasks: A Reticle Evaluation”, Proceedings of
SPIE BACUS 23, Vol 5256, pp 1136-1142, (2003).
Strong, F., Skinner, J., Tien, N, “Electrical Discharge Across Micrometer-Scale Gaps For Planar MEMS Structures
In Air At Atmospheric Pressure”, J. Micromech. Microeng. 18 (2008) 075025 (11pp)
Tchikoulaeva, A., Holfeld, A., Arend, M., Foca, E., “ACLV Degradation: Root Cause Analysis and Effective
Monitoring Strategy”, Proceedings of SPIE Photomask and Next-Generation Lithography Mask Technology XV,
Vol 7028 (2008)
Bruley, J. et al, "Cr Migration on 193nm Binary Photomasks", Proc. SPIE Metrology, Inspection, and Process
Control for Microlithography XXIII, Vol 7272 (2009).
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