Process Information
Raith 150 E-Beam Lithography System: raith
E-beam Lithography Resist Processing
Please note that there are new ebeam resist processes (below) and new ebeam resist handling procedures in place. See Resist List for complete list of resists and developers used in the lab.
This machine is capable of routinely producing 3:1 aspect ratios in a high resolution, high contrast resist, with a minimum feature size of roughly 50 nm. For example, if you want 50 nm features, plan on using resist no thicker than 150 nm; alternatively, if your process requires 0.30 µm resist, do not expect feature sizes below 100 nm. For greater process latitude, try to use lower aspect ratios. For sparse patterns with PMMA resist, it may be possible to make aspect ratios as high as 5:1.
In order to get features to print at their design size, a sizing may be specified in ELSA. Most positive resists require a sizing in the range of -0.04 to -0.08 depending on resist type, contrast, thickness, and development. Note that features equal to or smaller than the sizing get shrunk to nothing and will be deleted from your pattern. Although specifying a sizing will help linewidths in most cases, it will also result in an (apparent) increase in the proximity effect on some patterns. Negative resists generally require little or no sizing. Sizing may also be used to compensate for other effects such as etch bias. The resist thickness varies as the square root of the spin speed. However, the resist thickness is highly sensitive to the solvent concentration; ie diluting the resist by a factor of two will reduce the thickness by an amount much greater than a factor of two.
PMMA
PMMA is an ultra-high resolution, high current positive resist used for nanolithography. It has poor sensitivity and poor dry etch resistance. It sticks well to almost any surface, lasts forever, is not white light sensitive, and gives very reproducible results. There are several flavors of PMMA recipes that have been characterized at SNF:
Single Layer 2% 950K MW PMMA - Anisole <PDF>
Single Layer 5% 495K MW PMMA - Anisole <PDF>
Bilayer PMMA Resist Process for Liftoff
Single Layer 2% 950K MW PMMA - Chlorobenzene <PDF>
Single Layer 5% 495K MW PMMA - Chlorobenzene <PDF>
ZEP is a very high resolution positive resist that like PMMA is simple to use and gives reproducible results. Compared to PMMA, it has an advantage of being 3 times faster and has good dry etch resistance. It has the disadvantages of poor adhesion (requires HMDS) and undercut profiles. This may be good for lift-off, however.
Although we post process notes for chlorobenzene-based ZEP520, please understand that we now discourage use of chlorobenzene-based resists in the lab. Instead, we now recommend the much safer, anisole-based ZEP520. Here are the spin-speed curves for anisole-based ZEP520.
APEX-E
Apex-E is a high performance, chemically amplified DUV positive resist that works well as an E-beam resist as well. It is extremely fast, has excellent dry etch resistance, and moderate resolution (0.15 µm). It's drawbacks are its high speed, which requires a large pixel spacing of .04 µm to keep the writing speed under 100Mhz, and sensitivity to airborne contamination. In order to get reproducible results, the resist should be spun, written, and developed all within a few hours. APEX-E is also very sensitive to contamination from underlying films on the wafer which may make it unusable in certain situations.
HMDS prime
Spin
PAB: 90°C 1 minute
Expose at 3 - 6 µC pattern dose
large area clearing dose is 2.5µC
PEB: 85°C 1 minute
Develop in MF319 1 minute (standard SVG track develop program)
Rinse: DI water
SAL-601
SAL-601 is a high sensitivity chemically amplified negative resist. It has very good resolution (sub 0.1 µm), high contrast, and moderate dry etch selectivity. The main drawbacks are scumming and bridging between features, particularly in dense patterns; poor adhesion (requires HMDS); a very short shelf life (hours on the wafer, days in a bottle at room temperature, months when refrigerated). Reproducibility is not great due to aging effects and sensitivity to bake time and temperature as well as minute levels of airborne contaminants. Because of the scum problem, this resist is best suited for isolated features. Since this is a negative resist, the wedge pattern does not work well. Try looking for color changes using wedge5, which requires a 0.04 µm pixel spacing to keep under 100 Mhz. SAL-601, like APEX-E, is also sensitive to the chemistry of the underlying film. In particular, nitride may reduce its sensitivity by 30%.
HMDS prime
spin
PAB: 90°C for 10 min.
expose at 5-15 µC/cm2 (doses below 12µC may require 0.04 µm pixel spacing)
PEB: 115°C for 1 min on hotplate
Develop for 2 - 5 min in MF322 (try a develop time 2X the dose to clear)
May require a light RIE descum.
SNR-200
This resist has found to not have the scumming problems that SAL has and people are resolving smaller features. The following is a SNR recipe:
Singe 150°C for 30 minutes
HMDS prime (not critical)
Spin at 7.5KRPM for .5µm layer
Pre-bake at hotplate 120°C for 2 minutes
Expose. Do an exposure matrix here or start with 6.5µC.
Post exposure bake at 110°C for 2 minutes. (this is a critical step - do not vary time or temperature)
Develop for 20 sec in MF CD-14.
UVN2
UVN2 is high sensitivity chemically amplified negative photoresist with a wide process window overlap and PED stability > 2hours.
Singe 150°C for 30 minutes
HMDS prime
Spin at 7.5KRPM for 30 seconds = 4200A thickness
Pre-bake at hotplate 110°C for 1 minutes
Expose. 4.5 to 15 uC
Post exposure bake at 95°C for 1 minutes. (this is a critical step - do not vary time or temperature)
Develop for 30 sec in MF CD-26 developer.
UVN30
UVN30 is high sensitivity chemically amplified negative photoresist with a wide process window overlap and PED stability > 2hours. This resist is better at fine line-space resolution without scumming.
Singe 150°C for 30 minutes
HMDS prime
Spin at 7.5KRPM for 30 seconds =3800A thickness (or 4K =5300A)
Pre-bake at hotplate 140°C for 90 seconds
Expose. 4.5 to 15 uC (simular to UVN2)
Post exposure bake at 130°C for 40 secnds. (this is a critical step - do not vary time or temperature)
Develop for 30 sec in MF 702 developer, Normality 0.21(recommended by Shipley).
or Develop for 45 sec in MF 322 developer, Normality close to 0.21.
or Develop for (?30 sec) in MF CD-26 developer, Normality 0.26 (used by Cornell).
UV5
This resist has good plasma etch resistance (better than APEX-E). Resolution performance: For .5um thick resist: 0.2 um lines and spaces, 0.1um contact holes and 0.1um isolated lines.
Singe 150°C for 30 minutes
HMDS prime
Spin at 5KRPM for 30 seconds = 5000A thickness
Pre-bake at hotplate 130°C for 1 minutes
Expose. 10 to 20 uC
Post exposure bake at 115°C for 90 seconds. (The PEB delay stability has been shown to be greater than 90 minutes up to 2 hours.)
Develop for 45 sec in LDD26W developer
Hard Bake at 145C for 3 minutes
E-beam Resist Handling Procedure:
Revision 2.3 October 28, 2003
James W. Conway
Electron Beam Lithography Resist often are more moisture sensitive and reactive than the Novalak based resist systems employed in optical lithography. They will require more care and diligence in their handling to achieve a quality process result, minimize contamination, and to achieve uniform film thickness across your wafers.
Our standard E-beam resist we use here at SNF is 5% 495K Molecular Weight PMMA, Poly Methyl Methacrylate, but these guidelines are applicable to all E-beam resist handled here in the Lab. The 2% 950 K MW PMMA is our high resolution resist.
Please review the Optical Lithography manual coating processes for additional info.
1. Cleanliness of process benches, application equipment, wafer chucks and wafers:
-Always wipe down the process bench surfaces with IPA or water using lint-free cloth. This aids in removing contamination sources and particulate debris from the deck surfaces. Be sure to maintain the proper chemical handling methods, as some benches do not allow solvents to be used in them. Ask a staff member to help you.
-Deck and Spinner interior surfaces must be covered with Aluminum Foil to minimize particles and traces of materials from previous processing operations.
-Solvent clean your syringes and wafer chuck in Acetone and IPA at the Solvent Bench, blow off these items with the N2 Blow off gun, and allow to dry an addition minute or two on lint-free before moving back to the spinner area.
- All wafers you cover with resist should have been cleaned using standard wafer cleaning methods, depending on your contamination level dependency. Clean and semi-clean wafers should undergo the Std. Pre-diffusion Wafer Cleaning process.
2. Deposition of Resist Material:
The Headway Spinner in the Photolithography area is our normal PMMA spinning tool. It has very reproducible spin speed control and will achieve as good as 1% thickness uniformity across a 100 mm Si wafer.
- Completely cover the interior of the bowl with aluminum Foil. NO solvents or resist material should ever be introduced to the drain of this bench whatsoever.
- You should run several dummy wafers before starting in order to set and confirm the spin speed and set time accurately to the process specifications.
- It is important to maintain cleanliness and minimize exposure to air of both the resist in the storage bottles and the material to be dispensed onto your wafers.
To reduce cost and minimize waste of resist material you should use either clean BD Type II Glass, or Plastic PE/PP syringes, and PTFE filters to transfer and dispense your resist material on to your wafers. I recommend the 10 ml size syringes. Plastic syringes and filters are available in the SNF stock room. BD Type II syringes are available at Bio-stores. Small chips coating can employ the 2.5 ml syringes.
- Direct from a wide mouth bottle using the syringe without the Luhr lock filter, fill the syringe to the fill line. Invert the syringe end up, push to plunger bleed off any extra air bubbles and place the filter onto the tip securely. Prime the filter to remove bubbles, and allow a very small volume of resist (~1ml) to flow out of the syringe. This can go directly into the foil in the base of the spinner. (But never allow any material to go down the drain, as it should all be covered with Al foil.)
3. Spin Deposition of Resist Material:
- Carefully place and center your wafer onto the Wafer Chuck, and confirm the vacuum chuck securely holds your wafer. Close the cover of the Spinner if open.
-Blow off your wafer with N2 Gun to displace particles.
-Dispense 2 – 4 ml of resist material covering approximately 1/3 of the wafer surface and without delay immediately activate the spinner by pressing gently on the pedal on the floor. Allow the spinning operation to complete. Stand back! If a wafer flies off the spinner simply press the rear section of the pedal to stop the spinning operation.
- Carefully remove the wafer with tweezers, grasping the wafer perpendicular to the cleavage planes adjacent the flat, and place on either the hotplate or into a quartz wafer boat for the oven soft bake.
-Repeat with your next wafer(s) until you are finished.
4. PLEASE: Clean up the Headway Spinner area after each use:
Remove all resist and Al Foil from the spinner bowl and wipe down the edges and deck surfaces removing all traces of resist material. This waste is placed into a polyethylene bag and label with a hazardous waste label. Place into the waste receptacle. Replace the foil covering all surfaces of the spinner bowl.
Wipe down the deck leaving a clean area ready for the next Lab member’s use.
5. Soft Baking of Resist Materials:
Soft Bake to remove most of the solvents from the resist and to form a continuous polymer layer of resist across your wafers. You must fully cure your films before exposure in order to not contaminate the UHV vacuum system of the EBL systems. This is very important!
Some of our PMMA processes can be performed on a Hotplate, other processes are better soft baked in a N2 purged Oven. The Blue M Oven using Process No. 1 at 170 degrees Celsius for 2 hours is the standard bake process for Std. 5% 495K MW PMMA. Hot plate bakes can also be employed to soft bake your wafers.
Carefully remove the wafers from the Quartz oven boat and place them immediately into a clean Polypropylene wafer carrier box. Be sure the new box is labeled with your Name, CORL login, Process Lot Number, and Process Name and Date.
6. Optical Microscope Inspection:
Many defects and artifacts of the spin process can be observed by eye such as Comets and Spin irregularities. But you should also examine the films using an optical microscope at both 5 – 10 X and at 200 – 500 X magnification. Seek out and count the number of particles and defects you find in 20 fields of view and note the counts in your lab notebook. This takes only a minute or two but yields good information.
This will help you later to identify the best wafers for your work and to monitor the cleanliness of the wafers you are processing.
You did inspect your wafers after the cleaning process – right!!
7. Store your wafers before exposure in a clean dry location preferably w/ N2 purge.
If you run into problems any SNF staff member is happy to assist you! -- JWC
