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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 6: METALIZATION
SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 6: METALIZATION PROCESS
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SEMICONDUCTOR PROCESS TECHNOLOGY
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THEORY
Introduction
A variety of conductors is applied in IC manufacturing. Metals with high conductivity are widely
used for interconnections forming microelectronics circuits. Metalization is an adding process
that deposits metal layers on the wafer surface.
Metals such as copper and aluminum are good conductors, widely used to make conducting lines
to transport electrical power and signals. On the IC chip, miniature metal lines connect millions
of transistors made on the surface of semiconductor substrate.
The requirements for metalization are low resistivity for low power consumption and high IC
speed, smooth surface for high resolution patterning process, high resistance to electromigration
to achieve high chip reliability, and also low film stress for good adhesion to underlying
substrate. Other requirements are stable stable mechanical and electrical properties during
subsequent processing, good corrosion resistance, and relative receptivity to deposit and etch.
Although copper has lower resistivity than aluminum, technical difficulties such as adhesion,
diffusion problems, and difficulties with the dry etching, etc., have hampered copper application
in IC multilevel interconnection. Aluminum is better suit for this purpose with its compatibility
with silicon dioxide. Aluminum interconnections have dominated metalization applications since
the beginning of the semiconductor industry.
Figure 1 below shows the cross-section of standard 0.5um CMOS process with 3 level of
aluminum metalization.
M3
M2
M1
Figure 1: Multi-level aluminum interconnection
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A disadvantage of aluminum as the metalization material is its low melting point temperature
(600 ºC) and the low Al-Si eutectic temperature (577 ºC). These restrict the maximum wafer
processing temperature once the aluminum layer has been deposited. Also, upon exposure to
oxygen, aluminum readily forms a native thin oxide on its surface, even at ambient temperature.
The presence of such oxide layer can increase the contact resistance of the aluminum layer. It can
also inhibit the sputtering of an aluminum target or etching of aluminum thin film, resulting in
process difficulties.
Deposition Process Technology
Aluminum Deposition by CVD Technique
The aluminum CVD process has been in research and development for a long time to replace
tungsten plug and reduce the interconnection resistance. Aluminum can be deposited at a
relatively low temperature with aluminum organic compound, such as dimethylaluminum hydride
(Al(CH3)2H, DMAH and tri-isobutyl-aluminum (Al(C4H7)3, TIBA). They can decompose and
deposit aluminum in a thermal process in a vacuum chamber. DMAH chemistry seems more
promising. At about 350 ºC, DMAH dissociated and deposits aluminum. The chemical reaction
can be expressed as;
Al(CH3)2H → Al + volatile organics
Figure 2 shows the standard CVD process chamber for aluminum depostion.
Figure 2: Aluminum CVD process chamber
CVD aluminum process has very good hole-fill capability. However, it is difficult to deposit AlCu alloy with a CVD process. Furthermore, aluminum CVD film is of poorer quality and higher
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resistivity compared to aluminum PVD film. Therefore, aluminum CVD film is normally did not
found wide applications in global interconnections.
Aluminum Deposition by PVD Technique
PVD stands for physical vapor deposition. It works by vaporizing the solid materials, either by
heating or by sputtering, and recondensing the vapor on the substrate surface to form the solid
thin film. PVD plays an important role in the metalization of processes in smiconductor
manufacturing.
The comparison of PVD and CVD process is illustrated in Table 1;
Pre-cursor
Process mechanism
Film characteristics
CVD
Gas source
Chemical reaction
Good step coverage
Conformal film
Hazardous by products
PVD
Solid source
Physical
Poor step coverage
Lower impurity concentration
Lower resistivity
Cheaper process
No by product
Table 1: Metal CVD and PVD process comparison
1. PVD by Evaporation Process
In the early years of IC manufacturing, only aluminum was used for metalization process, and
thermal evaporation was widely used for aluminum deposition. Later, the electron beam
evaporator was developed to deposit higher-purity metal films.
A schematic of the thermal evaporation system is illustrated in Figure 3. During the process, the
system needs to be under high vacuum (about 10-6 Torr), to minimize the residue oxygen and
moisture. These residual can react with aluminum and form high resistivity aluminum oxide that
would increase the film sheet resistance.
Flowing a large amount of electric current through the aluminum charge heats it up by resistive
heating (P = I2R). Aluminum starts to vaporize in the vacuum chamber. When aluminum vapor
reaches the wafer surface, it recondenses and forms a thin layer of aluminum film on the surface.
In a filament evaporation system as shown in Figure 4, a shutter mechanism is usually placed
between the filament and the wafers. At the beginning of the deposition process, the filament is
heated to just above the metal melting point to melt all the metal charge while the shutter is
closed. After the temperature is stabilize, and volatile impurities are driven away from the charge
by heat, the current ramps up to raise the temperature and evaporate the metal. The shutter is
opened, which allows the metal vapor to emit, reach the wafer surface, condense there, and
deposit metal thin film on the surface.
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Figure 3: Schematic of thermal evaporator
Figure 4: Schematic of filament evaporator
For the thermal evaporation deposition process, the deposition rate of aluminum is mainly related
to the heating power, which is controlled by the elctric current. The higher the current, the higher
the deposition rate. Normally, aluminum deposited with a thermal evaporator always has a trace
amount of sodium, high enough to shift the threshold voltage of MOS transistor and affect the
device reliability. It also associated with a low deposition rate and poor step coverage.
More advanced evaporation technique, called electron beam evaporator is developed to produce a
better performance aluminum film. In this technique, a beam of electron, typically with energy
about 10 keV and current up to several amperes, is directed at the metal in a water-cooled
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crucible in a vacuum chamber and heats the metal to the evaporation temperature. Figure 5 shows
schematically e-beam evaporator system.
Figure 5: Schematic of e-beam evaporator
2. PVD by Sputtering Process
Sputtering deposition is the most commonly used PVD process for metalization in the IC
industry. It involves energetic ion bombardments, which physically dislodge atoms or molecules
from the solid metal surface called target, and redeposit them on the substrate surface to form a
thin metal film.
Figure 6: Schematic of sputtering chamber
Figure 6 shows a standard DC magnetron sputtering chamber. When the electric power is applied
between the two electrodes under low pressure, a free electron is accelerated by the electric field,
continuously gaining energy from the electric field. When it collides with a neutral Ar atom, one
of its orbitting electrons can become a free electron. This is called an ionization collision, which
generates free electron and a positively charged argon ion. The free electrons repeat this process
to generate more free electrons and ions, while other electrons and ions are constantly lost from
collision with electrodes and chamber walls and electron ion recombinations. When their
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generation rate equals their losing rate, the steady state is reached and a stable plasma is
generated.
While the negatively charged electrons are accelerated to the positive bias electrode called the
anode, the positively charged argon ions are accelerated towards a negatively biased cathode
plate, usually called the target. The target plate is usually made from the same metal the fab
desires to to be deposited on the wafer. When these energetic argon ions hit the target surface,
atoms of the target materials are physically removed from the surface by the momentum transfer
from the impaction ions and are thrown into the vacuum in the form of metal vapor.
Eventually, some of them reach the wafer surface, adsorb on the surface and become so-called
adatoms. The adatoms migrate on the wafer surface until they find the nucleation sites or a
sticking place and rest there. Other adatoms recondense around the nucleation sites to form
grains, which are in the single crystal structure. When the grains grow and meet with other grains,
they form a continuous poly-crystalline metal thin film on the wafer surface. The border between
grains is called a grain boundary, The grain size mainly determined by the surface mobility which
is related to many other factors such as wafer temperature, substrate surface condition, chamber
base line pressure, and final annealing temperature.
Normally, higher temperature results in higher surface mobility and larger-sized grains. The grain
size has a strong effect on the film reflectivity and film sheet resistance. The metal film with
larger grain size has less grain boundary to scatter electron flow, thefore lower resistivity.
Aluminum Thin-Film Process Characterization
Conducting films usually have polycrystalline structure. The conductivity and reflectivity of a
metal are related to the grain size – normally larger grain size has higher conductivity and lower
reflectivity. These properties are related to the deposition process. Routine reflectivity and sheet
resistance measurements monitor their value change, which provides information about the drift
of the process condition to keep process under control.
1. Thickness Measurement
Metal films such as aluminum, copper, titanium silicide and titanium nitride are opaque films;
therefore optical based measurement techniques such as spectrophotometer commonly used for
dielectric thin-film measurement cannot be used to measure metal film thickness. A destructive
process normally is required to precisely measure the actual metal film thickness, either by SEM
(scanning electron microscope) cross section or by measuring the step height with a profilometer
after removing part of the deposited film.
For SEM measurement, the test wafer needs to be cut after metal deposition and the cut samples
put on the stage of the tool. Energetic electron beams scan across the sample, and the
bombardment causes a secondary electron emission from the sample. Since different materials
have different generation rates of secondary electron emission, by measuring the intensity of the
secondary electron emission, SEM can precisely measure the metal film thickness from its image.
However, it is expensive, destructive, time consuming, and it is hard to measure the uniformity of
the film across the whole wafer.
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A faster and cheaper technique is by using a profilometer measurement, but it needs a patterned
film. The patterned film is put in a stylus profilometer which can sense and record the
microscopic surface profile with a stylus probe. Figure 7 illustrate this measurement technique.
Figure 7: Schematic of Profilometer
2. Electrical Resistivity
Electrical resistivity (also known as specific electrical resistance) is a measure indicating how
strongly a material opposes the flow of electric current; if the resistivity of the material is small,
that means that material is effective to carry electrons.
The resistivity of a material is usually denoted by the lower-case Greek letter rho (ρ) and is given
by RS/l, where R is the resistance of a uniform specimen of the material, having a length l and a
cross-section area S. The units of ρ are ohm meters. Its reciprocal quantity is electrical
conductivity. Also the resistivity is the magnitude of the electric field divided by the magnitude of
the current density.
In general, electrical resistivity of metals increases with temperature, while the resistivity
of semiconductors decreases with temperature. As the temperature of a metal is reduced, the
resistance usually reduces until it reaches a constant value, known as the residual
resistivity. This value depends not only on the type of metal, but on its purity and thermal
history. Some materials lose all electrical resistivity at sufficiently low temperatures; this
effect is known as superconductivity.
3. Sheet Resistance Measurement
Sheet resistance is one of the most important characteristics of the conducting materials,
especially for conducting films. It is commonly used to monitor the conducting thin film
deposition process and deposition chamber performance. For conducting film with known
conductivity, the sheet resistance measurement is widely used to determine the film thickness,
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since it is much faster than the actual thickness measurement. Resistivity is one of the most
fundamental properties of a material. For a conducting thin-film, the resistance can be calculated
by the product of film sheet resistance and the film thickness.
Sheet resistance (Rs) is a defined parameter. The four-point probe is one of the most commonly
used measurement tools, which measures voltages and currents and calculates the sheet
resistance.
For a conducting line as shown in Figure 8, the resistance can be calculated as;
R=ρL/A
Where R is the resistance, ρ is the resistivity of the conductor, L is the length of the conducting
line, and A is the area of line cross section.
Figure 8: A conducting line
If the wire is rectangle, as in Figure 9, the area of the cross section is simply changes to the
product of the width and thickness (w x t). The line resistance can be expressed as;
R=ρL/wt
Figure 9: A rectangular conducting wire
For a square sheet, the length is equal to the width, L = w, thus they cancel each other. Therefore,
the resistance od a square conducting sheet, defined as sheet resistance, can be expressed as;
Rs = ρ / t
The unit of the sheet resistance is ohms per square (Ω / □). The square symbol is only to denote
that the value is the resistance of a square; the size of the square does not matter.
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4 .Four-point Probe Thickness Measurement
The four-point probe measurement is used to measure the resistivity of wafers and crystals. The
formula relating sheet resistance to the voltage and current
R s  4.53
V
I
Where 4.53 is a constant that arises from the probe spacing. The Rs is called sheet resistance of
the electrical quantity measured on a thin layer and the units of ohms per square (Ω).
The thickness of uniform conducting layers on insulating layer can be determined using fourpoint probe. Since the resistivity is a constant for pure material such as aluminum, the sheet
resistance measurement is actually a measurement of the film thickness.
T  s
Where
Material
R
T = layer thickness
s = resistivity
Rs = sheet resistance
Resistivity (Ohm-meters)
Silver
1.59 x 10-8
Copper
1.7 x 10-8
Gold
2.44 x 10-8
Aluminum
2.82 x 10-8
Tungsten
5.6 x 10-8
Table 1: Resistivity Value for Certain Material
A four point probe is the most commonly used tool to measure sheet resistance. A certain amount
of current is applied between two of the pins, and the voltage is measured between the other two
pins as illustrated in Figure 10.
Figure 10: Four Point Probe Measurement
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The sheet resistance equals the ration of the voltage to the current and multiplies a constant,
which depends on the pins being used.
Since four point probes make direct contact with the film, they are only used on test wafers for
the process development, qualification and control. Sheet resistance can provide important
information about the resistance of the film, which is affected by the film thickness, grain size,
alloy concentration, and the impurities.
In the early stage of process development, the relationship between sheet resistance and yield is
normally established. Therefore, it is always closely monitored in the IC fabrication process.
5. Film Reflectivity Measurement
Reflectivity is an important property of metal thin films. For a stable metalization process, the
reflectivity of the deposited film should be keep near a constant. The change of film reflectivity
during the process indicates a process drift. Reflectivity is a function of the film grain size and
surface smoothness, and needs to be controlled. Normally, the larger the grain size, the lower the
reflectivity.
Reflectivity is critical film parameter during the photolithography process because it can cause a
standing wave effect due to the interference between incoming light and reflecting light. This can
affect the photolithography resolution by creating wavy groves on the sidewall of the photoresist
stack from the periodic overexposure and underexposure. Anti-reflective coating layer (ARC) is
required for the metal patterning process, especially for aluminum patterning, because it has very
high reflectivity (~ 200% relative to silicon).
Reflectivity can be measured by focusing a light beam on the film surface and measuring the
intensity of the reflected beam. Reflectivity measurement results usually use the relative value to
silicon. It is normally easy, quick and non-destructive measurement, and frequently performed in
the metal bays in semiconductor fab.
6. Process Uniformity
The uniformity of the thickness, sheet resistance and reflectivity are routinely measured during
process development and for statistical process control (SPC) monitoring.
The more measurement point are taken, the more accurate is the analysis. However, more
measurement points need longer measurement time, which means lower throughput and higher
cost.
The 49-point, 3σ standard deviation nonuniformity is the most common definition for process
qualification in the semiconductor industry. For production wafer, less time consuming 5-point
and 9-point measurement are commonly used for process control and monitoring. Figure 11
illustrate the standard mapping patterns for 5-point, 9-point and 49-point measurements
respectively.
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Figure 11: Mapping Patterns of Uniformity Measurement
The most widely used non-uniformity measurement is based on the equation, called Max-Min
Uniformity;
Non-uniformity (%) = (Max Value – Min Value) / 2 x average
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Experiment: Metallization Process
Objective:
In this experiment, students will carry out a thin film deposition method using Physical Vapour
Deposition. At the end of this experiment, students shall be able:
1. To determine sheet resistance on aluminum deposited wafer
2. To determine thickness of different aluminum dimensions.
Equipment / Chemicals
i.
Steam (H20)
ii.
Gas N2 and O2
iii.
Quartz boat
iv.
Quartz Rod
v.
Timer
vi.
Wet Oxidation Furnace, WFM
vii.
Spectrophotometer, SPM
viii.
Four point probe, FPP
ix.
High Power Optical Microscope, HOM
Characterization/Testing
1. Thickness measurement
2. Sheet resistance
3. Particle/defect/ profile window inspection
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Process Run Card
Process Flow Run Card
Group:
Name:
Lot Number:
Exp No:
Orientation:
Size:
Resistivity:
Lot Start Date:
LP#
Equipment
Thickness:
Planner:
Process/Recipe
Time
Out
Aluminum Deposition
1
1. 1. Take two wafers.
PVD
2. Scribe
lightly
on
the
backside of the wafer.
3. Then, cut the aluminum foil.
Al Size = 1” × 1”, 2” × 2”,
3” × 3”,
4. Clean aluminum foil with
acetone.
5. Fold the aluminum and
ready to run the aluminum
deposition.
6. Insert 2 wafers A and B
namely
and
properly
on
clamp
the
it
wafer
hanger.
7. Run
the
aluminum
evaporator according to the
Aluminum
Standard
Evaporator
Operating
Procedure.
8. Repeat step 4 to 8 for
different aluminum foil size.
9. Take wafer A for sheet
resistance
measurement.
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INSEP
Date:
Substrate Type:
Start Wafer Quantity:
Authorized by:
Data
out
Remarks
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Take at least 5 readings at
different point.
10. Take wafer B for step
height measurement.
Aluminum Masking
2
1. Load wafer B on the wafer
chuck.
2. Drop the photoresist onto the
wafer B surface.
Resist quantity : 3ml
3. Spin the photoresist.
Ramp up : 900 rpm
t : 6 secs
Spin speed : 3000 rpm
t : 15 secs
Ramp down : 0
t : 2 secs
4. Unload the wafer.
SPM
5. Measure photoresist thickness.
HP
6. Place the wafer on hot plate
7. Softbake the wafer.
T : 90oC
t : 90 secs
8. Remove wafer from hot plate.
4. Cool the wafer.
Aluminum Pattern Transfer
3
1. Load wafer B into mask
MA
aligner wafer chuck.
2. Align the wafer B.
3. Expose the photoresist.
t : 90 secs
Light Intensity : 0.002mW/cm2
4. Unload the wafer.
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Aluminum Pattern Transfer
4
1. Immerse exposed wafer B into
WB2
developer to develop the
photoresist.
t : 15 secs
2. Rinse with DI water.
3. Spin dry
T :90oC
t : 60 secs
4. Hardbake the wafer
t : 15 secs
5. Inspect the wafer.
Aluminum Etch
5
WB2
1. Prepare chemical for etching
process.
2. Immerse the wafer B into
ALUM Etchant solution.
Etch time : 15 secs
3. Rinse with DI water.
4. Spin dry.
t : 15 secs
5. Inspect the wafers.
Resist Strip with Acetone
6
WB2
1. Load wafers B into wafer
chuck.
2. Start the spinner.
Ramp up : 3000 rpm
t : 5 secs
Spin speed : 6000 rpm
t : 15 secs
Ramp down : 0 rpm
t : 2 secs
3. Rinse with Acetone to strip
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resists.
4. Rinse with DI water.
5. Spin dry
t : 15 secs
6. Unload the wafer.
7. Inspect the wafer.
8. Measure aluminum thickness
9. Measure sheet resistance.
Result and Discussion
1. Calculate Aluminum thickness based on sheet resistance value.
2. Describe the principle of the evaporation process.
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3. Can you measure the aluminium film using spectrophotometer? If not, please explain
why? What is the best method to measure the aluminium film thickness?
4. What are the parameters affecting the deposition rate. Which parameter you think the
most contributing to the deposition rate.
5. Explain the difference between evaporation and sputtering technique.
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