SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 1:PHOTOLITHOGRAPHY
SEMICONDUCTOR PROCESS TECHNOLOGY
LAB MODULE 1: PHOTOLITHOGRAPHY
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SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 1:PHOTOLITHOGRAPHY
THEORY
Introduction
Photolithography is the patterning process that transfers the design pattern from the mask or
reticle to the photoresist on the wafer surface. By all means, photolithography is a technique that
a pattern is transferred from mask to wafer surface using light. It is the most crucial step in IC
fabrication, since the device and circuit designs are transferred to the wafer by either etch or ion
implantation through the pattern defined on the photoresist on the wafers surface by the
photolithography process.
The concept of photolithography is simple. A light sensitive photoresist is spun onto wafer
forming a thin layer on the surface. Photoresist are the photosensitive materials used to
temporarily coat the wafer and to transfer the optical image of the chip design on the mask or
reticle to the wafer surface. The resist is then selectively exposed by shining light through a mask
which contains the pattern information for the particular layer being fabricated. The resist is then
developed which completes the pattern transfer from the mask to the wafer.
Photolithography is the core of the IC manufacturing process flow, as shown in Figure 1. From
the bare wafer to the bonding pad etch and photoresist strip, the simplest MOS-based IC chip
needs five photolithography process or mask steps, and an advanced IC chip may take more than
30 mask steps. The IC processing is very time consuming: even with a 24-hour-a-day, nonstop
work schedule, it takes six to eight weeks from bare wafers to finished wafers. The
photolitography process takes about up to 50% of the total processing time.
Figure 1: IC Fabrication Process Flow
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MODULE 1:PHOTOLITHOGRAPHY
Photoresist Types
Photoresists is a light sensitive material similar to the coating on a regular photographic film.
Photoresist can be classified as positive or negative, depending on how it responds to radiation.
For positive resists, the exposed regions become more soluble and more easily removed in the
development process. In positive resist, the transferred pattern will be exactly the same as the
mask disregard the mask is dark field or clear field mask. For negative resists, the exposed
regions become less soluble and the patterns formed in the negative resist are the reverse of the
mask pattern.
Exposure to light will cause changes in its structure and properties. The term describing this
change is photosolubilization. There are four basic ingredients in photoresist: polymers, solvents,
sensitizers and additives. The ingredients that contribute the photosensitive properties to the
photoresist are special light and energy sensitive polymers. The most commonly used resists are
designed to react to ultra violet or laser source are called optical resists. In negative resist, the
polymer change from unpolymerized to polymerized after exposure to a light or energy source.
Polymerization can also happen when the resist is exposed to heat or normal light. Thus, to
prevent accidental exposure, photomasking areas processing resist use yellow filters or yellow
lighting. The photosolubilized part of the resist can be removed by a solvent in the development
process. In positive resist, the light will change the resist to be unpolymerized and be removed
during photoresist development process. Changing or switching resist type requires changing the
polarity of the mask or reticles.
Masks
Mask used in IC manufacturing are usually reduction reticles. The first step in mask making is to
use a computer aided design (CAD) system in which designers can completely describe the
circuit patterns electrically. The digital data produced by the CAD system then drives a pattern
generator, which is an electron beam lithographic system that transfers the patterns directly to
electron-sensitized mask. The mask consists of a fused silica substrate covered with a chromium
layer. The circuit pattern is transferred to the electron sensitized layer (electron resist), which is
transferred once more into the underlying chromium layer for the finished mask. The patterns on
a mask represent one level of an IC design.
There are several photomask types that can be classified as transparent and opaque substrates.
Transparent substrates are quartz; expansion glass and soda lime glass while opaque materials are
chrome, emulsion and iron oxide are used to block the lights during exposure. Two polarities of
masks are commonly described as light field and dark field. Light field are having most clear
region.
Photolithography Process
As in the previous experiment, student should be able to understand the theory and concept in
photolithography process. In summary, photolithography process which consists of 10 steps that
can be simplified in Figure 2:
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Surface/ wafer Preparation
Photoresist Coating
Alignment and Exposure
Soft bake
Photoresist Development
Hard bake
Etching
Visual Inspection
Photoresist Stripping
Visual Inspection
Figure 2: Basic Photolitography Process Steps
Before wafer is coated with photoresist, it will undergo dehydration bake to drive off water. Lack
of adhesion of photoresist to many film surfaces is commonly encountered problem in silicon
processing. Then it may require adhesion promoter of hexamethyldisilazane (HMDS) to treat the
wafer. Then, photoresist typically in liquid form will be coated to the wafer using static or
dynamic dispense and spin coating at optimized spin speed. This depends on speed and viscosity.
Then, wafer is soft baked to drive off solvents and to improve adhesion. Then wafer will be
exposed and develop before visual inspection.
Figure 3: The difference in negative and photoresist development.
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MODULE 1:PHOTOLITHOGRAPHY
Optical Exposure System
There are basically two optical exposure methods: shadow printing and projection printing.
Shadow printing may have the mask and wafer in direct contact with one another (as in contact
printing) or in close proximity (as in proximity printing). The intimate contact between resists and
mask provides a resolution of approximately 1µm. However, contact printing can damage the
surface of both mask and wafer. To minimize mask damage, the proximity exposure method is
used. It similar to the contact printing, except there is a small gap (10-50µm) between the mask
and the wafer during exposure. The small gap results in optical diffraction at feature edges on the
photomask when light passes by the edges of an opaque mask feature, fringes are formed and
some light penetrates into the shadow region. As a result, the resolution is degraded to the 2 to
5µm range.
To avoid mask damage problem associated with shadow printing, projection printing exposure
tools have been developed to project an image of the mask patterns onto a resist-coated wafer
many centimeters away from the mask. To increase resolution, only a small portion of the mask is
exposed at a time.
Photoresist Application
The Single Wafer Spin Processor is used to apply films and photoresist to various sizes and types
of wafers. The spin processor housing has been precisely machined from solid natural
Polypropylene (NPP). The material, which does not degrade or generate particles, is nearly
impervious to all chemical attack. The bowl-shaped interior forces fluid downward where it is
routed directly to the rear drain. This spin processor is capable to handle 4” wafer.
After a substrate is loaded on to the chuck, vacuum hold-down is engaged from the side mounted
control panel and the lid closed, a preprogrammed process is selected and then initiated by only a
few keystrokes. An interlock prevents rotation or valve actuation without the vacuum switch
being engaged and the door closed.
Process Characterization
1. Thickness Characterization - Spectrophotometers
Light is a form of energy. White light is really a bundle of rays, each with different energies.
When the rays interfere through the transparent film, the result is a ray of one color, one
wavelength and one energy level. Our eyes will interpret the energy as color.
In a spectrophotometer, monochromatic light in the ultraviolet range is reflected off the sample
and analyzed by the photocell. To ensure the accuracy, readings are made at different conditions.
The conditions are changed by either using monochromatic light (to change wavelength), or
changing the angle of the wafer to beam. Spectrophotometers specifically designed for use in
semiconductor technology have onboard computers to alter the measurement conditions and
calculate the film thickness.
2. Visual Inspection - High power microscope
The metallurgical microscope is the workhorse tool of surface inspection. In metallurgical
microscope, the light is passed down to the non transparent sample through the microscope
objective. The light reflects off the sample surface and transmitted back up through the optics to
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the eyepieces. With the white light illumination, the picture in the field of view exhibits the
surface colors, which identify particular components on the wafer. Use of filter will change the
surface color. Obviously, a microscope inspection procedure is used to judge surface and layer
quality and (in masking) pattern alignment.
A typical fabrication microscope is fitted with 10X or 15X eyepieces and a range of objectives
from 10 to 100X. Increasing the total viewing power (eyepiece power times the objective power)
reduces the field of view.
3. Linewidth Measurement (CD control and resist profile)
A number of techniques are currently available to perform such feature size measurement. The
most commonly used tecgnique is based on scanning electron microscope (SEM). Besides
linewidth data, SEM also provides data about the resist profile. Figure 4 shows some of the
important information provided by SEM on the CD control.
Figure 4: Critical Dimension
4. CD Uniformity
The uniformity of the CD measurement is routinely measured during process development and
for statistical process control (SPC) monitoring.
The more measurement points 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 5
illustrate the standard mapping patterns for 5-point, 9-point and 49-point measurements
respectively.
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Figure 5: 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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SEMICONDUCTOR PROCESS TECHNOLOGY
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Experiment 1: Photolithography
Objective
In this experiment, student will carry out a basic process steps involved in photolithography. At
the end of this experiment, students shall be able:
1. To describe the basic principle of the photolithography process.
2. To explain the principle of masking and pattern transfer process.
Equipment / Chemicals
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
xi.
xii.
Photoresist
Acetone
Developer
DI Water
Spinner
Hot Plate
Mask Aligner System
Optical Microscope
Wet Process Bench
Timer
Spectrophotometer
High Power and Low Power Microscope
Characterization/Testing
1. Resist Thickness
2. Optical observation – window profile, defect and particle
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SEMICONDUCTOR PROCESS TECHNOLOGY
Photolithography Process Run Card
Group:
Lot Number:
Name:
Orientation:
Size:
Resistivity:
Lot Start Date:
LP#
Equipment
PR Coat
1
PRCoater
MODULE 1:PHOTOLITHOGRAPHY
Thickness:
Planner:
Process/Recipe
Time
Out
1. Load wafer on the wafer
chuck.
2. Drop the photoresist onto the
wafer surface.
3. Spin the photoresist.
RU : 850rpm
t : 5 secs
SS : _________rpm
t : ______secs
RD : 0rpm
t : 5 secs
4. Unload the wafer.
PR Thick
2
SPM
Soft Bake
3
HP
1. Measure photoresist thickness.
1. Place the wafer on oven plate
2. Bake the photoresist.
T :90oC
t : 90 secs
3. Remove wafer from oven
plate.
4. Cool the wafer.
Align and Expose
4
MA
1. Load wafer into wafer chuck.
2. Align the wafer.
X, Y, Z
3. Expose the photoresist.
t : ______ secs
Light Intensity : 0.002mW/cm2
4. Unload the wafer.
PR Develop
5
PR Dev
INSEP
Date:
Substrate Type:
Start Wafer Quantity:
Authorized by:
1. Immerse exposed wafer into
developer to develop the
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Data
out
Remarks
SEMICONDUCTOR PROCESS TECHNOLOGY
MODULE 1:PHOTOLITHOGRAPHY
photoresist.
t : _____ secs
2. Rinse with DI water.
t : 15 secs
3. Spin dry
t : 15 secs
4. Inspect the wafer.
Hard Bake
6
HP
1. Place the wafer on oven plate
2. Bake the photoresist.
T :90oC
t : 60 secs
3. Remove wafer from oven
plate.
4. Cool the wafer.
Visual Inspection
7
HOM
1. Observe etch profile.
Results and Discussion
1. Sketch window profile after etches.
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2. What type of resist used in this experiment?
3. From your understanding, what is the main advantage of contact exposure method over
projection exposure method?
4. What type of aligner that you used in this experiment?
5. What type of solvent is used to strip the photoresist?
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6. Explain why the expose area of the photoresist is removed after development.
7. From your experiment, summarize photolithography process in detail.
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