Godin:Protocols/Lift-Off Resist

Safety

 * Safety goggles
 * Nitrile gloves
 * Lab coat
 * Fume hood

Materials
Microscope Slide Cleaning


 * 8 microscope slides
 * Acetone
 * Isopropanol
 * Ethanol
 * Microscope slide holder
 * 3 300 mL bottles
 * Godin:Protocols/Sonicator
 * Tweezers

Pattern Photoresist


 * TMAH
 * Toluene
 * Acetone
 * Ethanol
 * Distilled water in small dish
 * Shipley S1813 Photoresist
 * 2 hot plates
 * Magnetic stirrer, 2 cm
 * Aluminum foil
 * Scotch tape
 * 300 mL beaker
 * Glass dish
 * Spill tray
 * 1 ml pipette and rubber bulb
 * Precision pipette for >= 1 mL
 * Positive photomask
 * Godin:Protocols/Spin coater
 * Godin:Protocols/Mask aligner

Description
Lift-off is a technique used to pattern metallic films using photolithography. In this procedure, Shipley S1813 photoresist is used in conjunction with toluene to create micro-sized features on glass slides. A negative mask is used, so as to expose the glass where electrodes are to be placed. These steps are prior to depositing a metallic layer.

Microscope Slide Cleaning

 * 5 min sonicate Acetone / IPA / EtOH + N2 dry

Cleaning the microscope slides is an essential first step to the lift-off process. Start by inserting 8 microscope slides in the microscope slide holder, and filling it with acetone. Place the slide holder in the sonicator for 5 minutes, then pour the used acetone into a 300 mL bottle for future use. Repeat this step for isopropanol (IPA) and ethanol (EtOH). Once complete, enter the clean room. Use the tweezers to remove each slide, and blow dry with high-purity nitrogen. Visually inspect each slide. Do not touch the ethanol with your gloves, as the latter will slowly dissolve and taint your slides. If any impurities remain, dip the slides in ethanol and dry them. If any are irreparably scratched or damaged, discard. Store in carrying case.

Pattern Photoresist

 * Setup

This step shall be completed completely within the clean room, so use appropriate gowning. Wrap a hot plate with aluminum foil, and cover with glass top also wrapped in aluminum foil. Place on counter. Place a 2nd hotplate in the fume hood, also covered in aluminum foil.

Place glass dishes in the spill tray, and place within the fume hood. Fill the 1st glass dish with toluene, the 2nd with distilled water. Fill a beaker with 185 mL of distilled water, then use the precision pipette to add 15 mL of TMAH to the beaker, and place on the hot plate in the fume hood. Add a stirring stick, and mix at 400 RPM.

Wrap the top of the spin coater with aluminum foil, and make sure it is clean. Turn on the power, vacuum and nitrogen feed. Set the program to L, or so as to have the following:

10s @ 500 RPM, ACL = 015 30s @ 4000 RPM, ACL = 010 20s @ 200 RPM, ACL = 005

Turn on the ventilation switch, and open the shutter leading to the mask aligner, making sure to reach a flow level of 0.2. Turn on the mask aligner main power, and then the lamp. Set timer to 16 seconds.

Place your photomask ink-side down under the glass filter using tape. Make sure the glass is securely in place by applying the vacuum. Also, place a silicon wafer as a target, and vacuum it down securely in place. Your coated slides will later be placed on the silicon wafer. Use an unclean glass slide (practice one) to determine correct positioning of the mask/slide, so as to have it centered. We want illumination to be uniform on the sample. Make sure the mask can be lifted all the way without hitting the 4 corner clamps. This may take some adjustment. Once satisfied, delineate the slide's position with tape. It will serve as a guide later on.


 * 10 min @ 110°C

Place slides on hot plate at 110°C for 10 minutes, then let cool 2 min. This step removes any adsorbed humidity from the surface of the slides. The idea is to use the lid and aluminum foil as an oven; heating uniformity is important. This will become more important later when we heat the coated slides. Remove the slides from the hot plate using tweezers, and place on a clean wipe. Carefully stick two pieces of tape so as to completely cover the bottom of each slide, with a little bit of tape protruding from each end. Set the hot plate to 90°C, this will be useful 2 steps down...


 * Spin coat

Place a slide on the spin coater (slide attachment), tape side down, and vacuum in place. The idea of the tape is to protect the underside as the top gets coated with photoresist (PR). Open the small opaque PR container, and use the 1ml pipette and bulb to disperse about 1.5 mL of PR on the slide. Make sure it's entirely coated. Close lid, run program L. This will create a film of about 1.4 micron thickness. Remove vacuum, remove slide and pull off tape. It will likely leave residue, which you can remove using ethanol and cleaning wipes.


 * Prebake 60s @ 90°C

Place slide on hot plate, and bake for 60s. Remove with tweezers, place on aluminum foil for 2 min cooling. Make sure to cover it, to prevent unnecessary light exposure.


 * UV Expose 16s

Place slide on silicon wafer using tape alignment previously determined. Close lid, make final positioning adjustments, then slide into the light. Make sure you have eyewear protection.


 * Toluene soak 60s + N2 dry

Place slide in toluene dish for 1 minute. In this step, the top of the PR is being chemically modified to develop slower than resist underneath (top is less soluble). This leads to undercut features favorable to lift-off. Dry with high-purity nitrogen.


 * Postbake 15s @ 90°C

Place slide on hot plate, and bake for 15s. Remove with tweezers, place on aluminum foil for 1 min cooling.


 * Develop in 15ml TMAH / 185ml H2O + N2 dry

Place slide in the TMAH solution, and develop 1 minute. This step can be extended or reduced in length, depending on solution concentration (especially if multiple slides have used the same solution). If necessary, increase the TMAH concentration (maybe 1-2 mL of TMAH) so as to get 1 minute development time. Do not heat solution, but leave the mixing on. Make sure it's not hitting the sample. After 1 minute, rinse in water dish and shake gently, then lightly in distilled water under tap.


 * Microscope verification

It's important to check microscopic features under the microscope. If PR remains, further development may be required.