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=====Photolithography=====
Lithography is done in one of two locations: the OSU Semi-cleanroom or CAMCOR at UO. Keep in mind the different tooling can affect the process. Further, some chemicals can negatively affect our 2D materials: WSe2 will etch in PG Remover.

To use the shared equipment in the OSU Semi-cleanrom. You can look through the big windows to see the impressive equipment. The lab is run by Chris Tasker, chris@eecs.oregonstate.edu. To use equipment in this lab you must be trained by someone in our group and then certified. Rick Presley is our main contact for certification, presley@engr.orst.edu.

The most important source of processing information is always the photoresist manufacture's data sheet see the box folder. For more technical advice you can also contact the photoresist company, [[http://www.microchem.com/|MicroChem]], (the company that distributes Shipley products). We have worked hard to get good/reliable recipes - some of the trials are [[fine tuning|documented]].

(Moved some [[old_photolith_recipes]])

===== General Photolithography Procedures =====


//(updated 2026)//

General notes:

When using a hotplate, you should not assume it is hot enough just because the indicator says it is. See below:
{{ ::cleanroom_hotplate.png?nolink|}}

When referring to temperatures, we will refer to the displayed temperature, but notice that these are not the same as the actual surface temperature. We have thermocouples which allow us to check the surface temperature in our lab and in the cleanroom drawer. Gus also keeps one at UO for spinning resist at CAMCOR.

=== (I) Preparing your substrate ===


We typically prebake our substrates to remove water from the surface before spinning them with resist. We have two temperatures that we use for this:
  * 180 C: bake for 2 minutes
  * 115 C: bake for 3 minutes

Pick whichever matches the next temperature you need for your resist soft bake to avoid having to wait for the hotplate to heat/cool.

=== (II) Spin-coating ===

Members of the lab have enjoyed using the Laurell spin coater. It is also very common across other labs. Request this spinner specifically when training with Rick.

We typically will use single or bilayer resist stacks. If you are working with bilayer resist, you will repeat this section for each photoresist applied and use the relevant parameters for each (see below)


  - Set you spin parameters in the spinner.
  - Blow off your substrates (do this for every layer if you don't have 2D materials on the substrate, I should link an image which shows why! -Gus)
  - Load your substrate onto the vacuum chuck. Use the largest chuck which does not cover more than ~80% of the bottom of the sample. This helps prevent vacuum leaks or resist being pulled into the vacuum.
  - Cleanly pipette resist onto center of chip. Immediately spin for 45 s.
  - Place chip on the center of preheated hotplate for requisite time.


== Processing Parameters: ==


**LOR3A:**

  * Spin speed: 4000 RPM, 1000 rpm/s ramp rate
  * Spin time: 45 s
  * Bake temperature: 180 C
  * Bake time: 4 minutes

The etch rate of the LOR is //highly// temperature dependent. See the data sheet for details.

**S1818/S1813:**

  * Spin speed: 4000 RPM
  * Spin time: 20 s
  * Bake temperature: 115 C
  * Bake time: 2 minutes

S18XX is a product line where the last two digits represent the nominal thickness of the resist (it still depends on spin parameters, see datasheet) S1818 is expected to be 1.8 μm for example. 

**Edge bead removal for the mask aligner:**

The edges of the chip will have built-up photoresist that is many times thicker than the rest of the chip. Edge bead removal is particularly important for fine features on small substrates, as edge beads prevent the chip from fully contacting the mask.

Try putting a tiny amount of Remover PG on the tip of a swab and wiping it on an alphawipe, then swab your edges at a 45 deg. angle to remove edge beads.

(II) Exposure

Depending on the tool you use, the process and logistics will be different. We currently lack good documentation for:
  * The mask aligner in MASC
  * The direct write tools in CAMCOR


===Cross contamination=== 
  * Be extremely careful when using CD-26 & MF-351 in the same lab. One drop of MF-351 in a gallon of CD-26 ruins the whole gallon! This problem was so bad that Shipley built a separate facility just to keep these away from each other.

===Photoresist removal===
Prepare a bath of acetone (for single layer resist) or PG Remover (for bilayer resist) in a large crystallizing dish. The dishes labelled '50x100' will do for five or fewer chips. Ensure that the dish is filled approximately half way with removal solvent; Not using enough solvent can result in a high concentration photoresist solution, which can leave residue behind on the chips. 

Place the chips in the bath and cover with a watch glass. Liftoff can take as little as 30 minutes or as long as several hours depending on the process and patterning. When checking the liftoff status, gently swirling the bath can help remove loose metal from the surface of the chips. If needed, it can also be helpful to place the dish in the sonicator for 5 minutes to remove stubborn metal and photoresist.

When liftoff is visibly complete, prepare a rinse bath of IPA (for single layer resist) or DDI water (for bilayer resist) in similar crystallization dishes. With bilayer resist, prepare three baths to be used consecutively; PG Remover is very stubborn and will not come off with the first rinse. With a wash bottle of the appropriate solvent (i.e. acetone or PG Remover) in hand, use tweezers to fish chips out one at a time. Do not let the chips dry in the air. Unless your chips have fragile materials (e.g. 2D flakes), aggressively spray them with fresh solvent using the wash bottle, spraying the whole chip over several times. This step has been shown to be important in removing lasting photoresist residue on the metal. When finished spraying, immediately place the chip in the rinse bath. If removing bilayer resist, repeat this process by spraying water and moving the chip from water bath to water bath. Once clean, these chips should become hydrophobic (i.e. water will bead on the surface). 

Finally, remove the chip from the rinse bath and (again, assuming there are no fragile materials on your chip) blow-dry the chip using N2. Examine your lithography under a microscope.

Matt has documented that hot PG remover leaves less PR residue than any other method we have tried. 

This liftoff procedure can still leave behind ~5 nm of photoresist residue on the substrate. If your device's tolerances allow for it, this residue can be cleaned using the O2 inductive plasma cleaner. To do this, first unseal the cleaner by turning the pump off. Place your chips on a blank glass slide and using tweezers place the slide in the center of the chamber. Reseal the chamber by replacing the lid and turning the pump back on. If closed, open the primary valve on the O2 tank.
 
When the chamber pressure reaches ~150 mTorr, open the O2 valve directly next to the plasma cleaner. This should flood the chamber with O2, and the pressure should spike and then settle around 5-7 Torr. Ensure the RF dial is set to "off" and turn the RF power switch on. Quickly close the O2 valve and watch the pressure drop. When the pressure reaches ~1 Torr, turn the RF dial to "high". As the pressure continues to drop, you should see the O2 plasma appear and get brighter. 30 seconds after seeing the first dim plasma, turn the RF dial to "off" and flip the RF power switch off. You can now re-open the chamber to remove your samples. Be sure to leave the chamber under vacuum when you leave.

Photoresist residue or other lithography defects can be optically invisible; it is recommended that you AFM your lithography as necessary. 

We are also currently working on developing a good process for cleaning up chips after lithography using AFM cleaning.