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Silicon Wafer in Production:
Type and Specification
By
Assoc. Prof. Dr. Uda Hashim
School of Microelectronic Engineering
Silicon Wafers: Basic unit
• Silicon Wafers Basic processing unit
•150, 200, 300 mm disk, 0.5 mm thick
• Newest ones 300 mm (12 inches)
• Typical process 25 - 1000 wafers/run
• Each wafer: 100 - 1000's of microchips (die)
• Wafer cost $10 - $100's
• 200 mm wafer weight 0.040 Kg
• Typical processing costs $1200/wafer (200 mm)
• Typical processed wafer value $11,000(all
products, modest yield)
• Value/Mass of processed wafer $275,000/Kg
Conversion of Raw Sand into Metallurgical Grade Silicon
Comparison of Cz and Fz growing
Methods
Comparison of Material Properties
and requirements for VLSI
Current common Si Wafer Sizes
•Since 1994 common sizes:
• 200 mm (8 inch) state of art fabrication
• 150 mm (6 inch) most 2nd level to front line fabs
• 125 mm (5 inch) Bastard size (only a few facilities)
• 100 mm (4 inch) Smallest production wafers: research
• 75 mm (3 inch) Obsolete size: still used in research
•(special order: more expensive than 4 inch)
• 300 mm (12 inch) now in some production
• Basically need to rebuild entire fab to change wafer
size
General Wafer Specs
• Orientation type
– <100>, <111>
• Crystal growth type
– Cz
– Fz (Float Zone)
• Substrate Type
–
–
–
–
–
Si substrate
Ge substrate
GaAs
InP
SiGe
Si Wafer Types in Production
• Reclaim (Dummy) wafer
• Test wafer
• Prime wafer
– Si Wafer
– SOI Wafer
Reclaimed Silicon Wafers
•As larger diameter silicon wafers were being developed, pricing for the larger
diameter wafers increased. In order to provide cost savings to customers, the
wafer reclaim process was developed.
•Reclaiming wafers is a process in which, the wafers are stripped of films, metals
or other contaminants, cleaned and then polished to a customer’s specification.
This allows customers to get three and four uses out of one wafer, instead of
having to buy new wafers each time.
•This service has become especially popular since the introduction of 300 mm
wafers. Considering 300 mm wafers can cost anywhere from $600 each, many
customers wanted to get the most usage as possible out of each wafer.
•Different types of reclaim can be provided depending on the condition of the
wafers. The customer specifies how clean and flat the wafers need to be after the
reclaim process is finished.
•The wafers can also be sorted into specific groups for resistivity, thickness, type
and dopant or any specification that is of particular interest to you or your project.
Test Wafer
•Test Silicon wafers are ideal for running equipment tests, research and
development, experiments and many other applications.
•We supply very high quality test grade wafers and sometimes there is very little
difference between a test grade and prime grade wafer.
•Test wafers are manufactured in all diameters.
Furnace Grade
•Furnace grade wafers have very tightly controlled electrical characteristics.
Typically they are used for high temperature applications or thin film depositions.
• Furnace grade wafers display very tightly controlled Oxygen content, resistivity,
radial resistivity gradient (RRG) and carbon content.
•The wafers are also doped very evenly.
•All of these factors help films grow uniformly without slip lines or stress fractures.
Lithography Grade
•Lithography grade wafers are used for lithography or photolithography
applications that require wafers to have very tightly controlled metrology.
•Litho wafers have a very tight site flatness specification that provides little
variation in thickness from site to site making for a very flat wafer.
Mechanical Grade
– Mechanical grade wafers are usually 300 mm wafers.
Although the specifications can be applied to any
diameter wafer, the term “mechanical grade” most
often refers to a 300 mm specification.
– Mechanical grade wafers are also referred to as
handling wafers. Mechanical grade specifications
aren’t quite as stringent as most other wafer grades.
Example of mechanical grade specifications. :
• Diameter: 300 +/- 0.5 mm
Type/Dopant: P/Boron
Orientation: {100} +/- 1º
Growth Method: Cz
Resistivity: 1-100 Ω-cm
Thickness: 750-800 μm
Front Surface: Polished
Back Surface: Polished
Notch: SEMI Standard
Particle Grade
Particle grade wafers typically refer to 300 mm wafers and have a minimal amount of
contaminants, or particles. Particles are measured at various sizes, 0.09 μm, 0.12 μm,
0.16 μm, 0.20 and 0.30 are common specifications. The number of particles at a certain
size is limited to a predetermined number, 10, 20, 50, etc. The specification is written:
≤50@≥0.12 μm. This means there are less than or equal to 50 particles greater than
0.12 μm in size on the entire wafer. Considering one human hair is approximately 1 μm
thick, these particles are very small and cannot be seen with the naked eye. The
smallest particle that can be seen with the naked eye is 0.5 μm.
Particle Grade wafers are considered to be very clean. Depending on the user’s
application, there are a couple of specifications that can be considered particle grade.
Here are a couple of particle grade specification.
Diameter: 300 +/- 0.5 mm
Type/Dopant: P/Boron
Orientation: {100} +/- 1º
Growth Method: Cz
Resistivity: 1-30 Ω-cm
Radial Resistivity Gradient (RRG): <=10%
Oxygen Concentration: <=30 ppma
Carbon Concentration: <= 1 ppma
Surface Metals: <=1xE10
Thickness: 750-800 μm
TTV: <=2 μm
GBIR: <=5 μm
STIR: <0.25 μm
Bow/Warp: <= 40 μm
Front Surface: Polished
Particle Count: <=50@>=0.09 μm
Back Surface: Polished
Prime wafer
– Prime Silicon wafers are high quality wafers. While they have
many different applications, they are most often used as a
surface to build electronic devices on, such as microchips for a
laptop computer. Prime wafers have tightly controlled resistivity,
metals, flatness and particle counts and often times customers
have very unique specifications.
– Some prime wafers have an epitaxial layer grown onto the wafer
to provide an even smoother surface for building devices or
depositing films. The “Epi” layer is doped separately, so the layer
can have its own specific electrical characteristics.
Silicon On Insulator (SOI) Wafers
•The technology for SOI wafers has been around for more than 20 years.
However it was never seen as a cost-effective method for manufacturing
semiconductors until recently.
•One challenge chip manufacturers are constantly encountering is heat.
When electronic devices built on top of wafers operate, they give off heat;
excessive heat causes the device to malfunction.
•A number of different solutions have been developed, but, as the space
between circuits grows smaller and smaller, more power is running
through microchips then ever before. One solution is the SOI wafer.
•There are two types of SOI wafers. Thin film SOI wafers have a device
layer <1.5 μm and thick film wafers have a device layer >1.5 μm.
Example Wafer Specification
Wafer Specs
Growth
Dia.
Type
Dopant
Orientation
Resistivity
Thickne
ss
Surface
Grade
Cz
100
N
ANTIMONY
<100>
0.005-0.02
500-550
P/E
PRIME
Cz
100
N
PHOS
<100>
1 to 20
300-350
P/E
PRIME
Cz
100
N
PHOS
<100>
1 to 20
475-575
P/E
TEST
Cz
100
N
PHOS
<100>
1 to 20
375-425
P/E
PRIME
Cz
100
N
PHOS
<110>
1 to 20
500-550
P/E
PRIME
Cz
100
P
BORON
<100>
1 to 20
475-575
P/E
TEST
Cz
100
P
BORON
<100>
1 to 20
500-550
P/E
PRIME
Cz
100
P
BORON
<111>
1 to 20
500-550
P/E
PRIME
Cz
100
P
BORON
<111>
1 to 20
475-575
P/E
TEST
Method