Photolithography Procedure

INTRODUCTION:

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. Like many solvents in this category, exposing yourself to TCE is considered a cancer risk, so be sure to use gloves and keep it in the hood. The developer is essentially sodium hydroxide and is disposed with bases.

These steps are all performed in the cleanroom where gloves, labcoats, eye protection, hair nets and booties (you should have put on one set when you entered MH425, and a second set before entering the clean room) are standard apparel. Saftey goggles should be worn throughout the spin coat procedure. The samples rotate at very high velocities (several thousand RPM), and it is imperative to close the spinner lid during this operation.

The UV light produced by the lamp can cause erythema of the skin (similar to sunburn), conjunctivitis and possible retinal burn that could 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 organic 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. (Note, we have found agitation actually increases particulate on the samples when the cleaning solution contains particulate form prior use. These are often shards of Si from cleaving. It is really better to hold the sample in the solutions without agitation.)
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 100-115 ºC. The hotplate controller should have been pre-adjusted to the proper temperature. If it doesn't seem to be at the right temperature, see the lab coordinator or teaching assistant to have them set it. Place the wafer on the hotplate using plastic tweezers. This will rid the wafer of all but a monlayer of water. Note, your wafer will tend to slide across the hot plate on the cushion of evaporating solvent and can easily land on the bench and need re-cleaning. If you set the wafer between the two posts on the hotplate, it will tend to run into the posts and be stopped. 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 100-115 ºC.

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. Following the procedures for programming the spin coater, set it for a two step spin process. The first spin speed should be 500RPM with an acceleration of 100RPM/s and a time of 9 seconds. The second spin speed should be your desired coating speed which will depend on the thickness you want. Typically it will be from 2000-5000RPM. The acceleration will be 1000RPM/s and it should last for 40 seconds.
D. Place and center a test wafer onto the spin chuck.
E. Using a disposable pipette dispense enough resist to coat ~ 90% of the wafer. Typically there is a contaminated beaker in the fume hood that you can use to hold the pipette between dispenses. Try not to use a new beaker, once they are contaminated they can be hard to clean and often need to be thrown away.
F. Run the spin cycle following the instructions to operate the spin coater which are given below the programming instructions.
G. Allow the spin coater to complete its entire cycle. It will beep at the end. Remove your wafer from the chuck.Wafer is now ready for the bake procedure.
H. When all samples are coated, 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 100-115 ºC.
B. Place wafer directly on hotplate using plastic tweezers. Bake wafer for approximately 45 seconds.
C. Using plastic tweezers, remove the wafer from the hotplate. Wafer is now ready to be exposed using the Mask Aligner.

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.

V. Develop Procedure

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. Alternatively, the wafer can be clamped in the plastic tweezers during immersion.
C. The length of time in the developer will be determined by process conditions. After the development time, rinse the wafer off completely in DI water for 1 minute. Typically, during the development step, you will see the pattern form as red photoresist leaves the surface and enters solution. Leaving the wafer in the developer too long after the pattern has formed can result in loss of the pattern. Removing too soon can lead to an incomplete pattern. Inspection of patterns for sharpness or thin regions/pin holes can be used to find an optimal time.
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 100-115 ºC.
B. Place wafer directly on hotplate using plastic tweezers. Bake wafer for approximately 45 seconds. Using tweezers, remove the wafer from the hotplate.
C. Done. Proceed to next step.