Photolithography is the process of transferring a pattern from a mask to the substrate. The goal is to expose parts of the wafer to the subsequent processing step while protecting the rest of the wafer. This is done through a sequence of steps: application of photoresist, selective UV exposure through a mask, and developing which removes exposed photoresist from the desired regions.
SAFETY:
This step involves exposure to a number of organic solvents that are all dangerous if you get in contact with them. The MSDS sheets for all of these chemicals can be found in the yellow MSDS binder outside of Meyer 425. These solvents have a high vapor pressure and should be handled exclusively in the fume hood. Materials that are contaminated with photoresist are disposed of in the solid waste containers. TCE is a halogenated organic, and the developer is essentially sodium hydroxide and is disposed with bases.
This steps are all performed in the cleanroom where gloves, labcoats, eye protection, and booties are standard apparel. A face shield should be worn throughout the spin coat procedure. The samples rotate at very high velocities (several thousand RPM), and it is imperative to place the lid and draw the hood shield down during this operation.
The UV light produced by the lamp can cause erythema of the skin (similar to sunburn), conjunctivitis and possible retinal burn thatcould result in blindness. Though the operator is protected from direct exposure to UV light it is recommended that the operator does not look at the mask/wafer assembly at the time of the exposure; indirect UV light may also harm the eye retina.
PROCEDURE:
I. Degreasing Procedure
The degreasing procedure is used to remove any contaminants and drive off any moisture in order to improve uniform spreading and adhesion.
A. Using tweezers, agitate wafer in acetone solution for approximately 30 seconds.
B. Take wafer out of acetone solution using the tweezers. Next, place the wafer into the
solution of trichloroethylene (TCE). Again agitate wafer for approximately 30 seconds.
C. Using tweezers, take wafer out of TCE solution and place the wafer back into the
solution of acetone. Agitate wafer for 30 seconds.
D. Lastly, remove the wafer from the acetone and place wafer in the methanol solution.
Agitate for 30 seconds.
*NOTE: IT IS IMPORTANT TO FOLLOW THE ABOVE STEPS IN THE ORDER PRESENTED. TRICHLOROETHYLENE
AND METHANOL ARE NOT MISCIBLE IN ONE ANOTHER.
E. Verify that the hotplate is between the temperature of 215-235 ºF. Place the wafer on
the hotplate using the tweezers to rid the wafer of any water. Keep wafer on hotplate and
begin preparing for the Spin-Coat procedure.
F. Hotplate will be used later in the lithography procedure. Hotplate temperature should
be kept between 215-235 ºF.
II. Spin-Coat Procedure
This step leaves a uniform coating of photoresist approximately 1.5 microns thick across the wafer.
A. To begin, turn on the vacuum pump power switch. (Power switch is located on the
power cord of the vacuum pump.)
B. Turn the Spin-Coater "Power" switch on.
C. Press the "Control" button. Red light above "Control" button should
illuminate.
D. Next, set the spin cycle times and spin speeds.
E. Set "Speed I" at the ~ 500 RPM by adjusting 'clock' position.
F. Set "Timer I" for 9 seconds. Step 1 clears excess resist.
G. Set "Speed II" at desired coating speed( depending on thickness) 'clock'
position using the inside tick marks.
H. Set "Timer II" 40 seconds.
I. Place and center a test wafer onto the spin chuck.
I. Once the wafer is centered, press "Vacuum" button to initiate vacuum. Again,
check to see that the red light above "Vacuum" is illuminated.
J. Without applying photoresist put the lid on the spin-coater, push the 'Start' button,
and draw down the hood shield. Verify that the spin times and speeds are at the desired
values. Adjust controls and repeat until the values are satisfactory. The Spin Coater is
now ready to be used.
K. Press the "Vacuum" button (red light should turn off), remove the dummy wafer
and using tweezers, take wafer off of the hotplate and center the wafer onto the spin
chuck.
L. Using a disposable pipette dispense enough resist to coat ~ 90% of the wafer, push the
"Start" button, place the lid on the Spin-Coater and pull down the hood shield.
M. Allow the Spin Coater to complete its entire cycle. Once both the "Timer I"
and "Timer II" lights turn off, lift the hood shield.
N. Wafer is now ready for the bake procedure. Press the "Vacuum" button (red
light should turn off) to release the wafer.
O. Dispense of pipette in 'Solid Waste' container. Wipe up any spilled photoresist with
Kimwipes and dispose of similarly. If your gloves were contaminated with photoresist
dispose and put on new gloves.
III. Softbake Procedure
The softbake step drives off solvents, improves adhesions, and anneals away stress that was introduced during the spin process.
A. Verify that hotplate is still operating between 215-235 ºF (100-115 ºC).
B. Remove wafer from the vacuum chuck and place directly on hotplate using tweezers. Bake
wafer for approximately 45 seconds.
C. Using tweezers, remove the wafer from the hotplate. Wafer is now ready to be exposed
using the Mask Aligner.
IV. Wafer Exposure Procedure
The resist is exposed to UV light in order to dissolve (positive) or cure (negative) the photoresist.
A. Turn on the EMS Photolithography Main Power.
B. Clamp appropriate mask (mask-side down) in holder.
C. Raise mask and place sample in place.
D. Rotate table and align sample as desired using X, Y, and Z micrometers..
E. Using the toggle switches set the exposure time (in seconds).
F. Rotate table to expose position.
G. Press the 'EXPOSE' button to illuminate.
H. After exposure is complete rotate table and remove sample.
I. Develop the wafer.
In this step the exposed(positive) or unexposed (negative) photoresist is removed to leave the desired mask pattern.
A. If new developer solution is needed: mix one(1) part NaOH developer solution with
five (5) parts DI water (50 ml:250 ml), otherwise use prepared solution.
B. Place wafer on a teflon carrier, immerse in developer, and gently agitate the wafer for
the desired time to remove all of the exposed photoresist.
C. Rinse the wafer off completely in DI water for 1 minute.
D. Inspect the wafer under the microscope, if it appears acceptable proceed to Hardbake.
VI. Hardbake
Like the softbake this step is used to further remove solvents, improve adhesion, and increase the etch resistance of the resist.
A. Verify that hotplate is still operating between 215-235 o
F (100-115 o C).
B. Place wafer directly on hotplate using tweezers. Bake wafer for approximately 45
seconds. Using tweezers, remove the wafer from the hotplate.
C. Done. Proceed to etching step.