Overview

Birck supplies a selection of photoresists for both optical and electron beam lithography. Resists that are not supplied may be purchased for use by individual research groups. The chemical mechanism for each resist may be slightly different, so be sure to verify the appropriate process for each resist.

Optical Lithography:

BNC Supplied:

• AZ1518: Positive photoresist, thicknesses from 1-4 μm
• AZ9260: Positive photoresist, thicknesses from 5-20 μm

Used at facility (not supplied):

• SU-8: Negative epoxy resist
  
  

Electron Beam Lithography:

BNC Supplied:

• PMMA: Positive resist, BNC supplies:
  • 950: A2, A4, A6, A8, A10

  • 495: A2, A4

• XR1541 (HSQ): Negative resist

Used at facility (not supplied):

• ZEP520A: Positive resist.

Optical Photoresists

Composition

The AZ positive resists consist of a resin (Novolak), a photoactive compound or PAC (a DNQ compound), and a solvent (PGMEA).

General Properties of AZ/Ti Photoresists

Via MicroChemicals:

AZ1518

Properties:

AZ1518 is optimized for adhesion and stability in wet etching. It has broad photosensitivity from 310-440 nm, a minimum resolution of 0.8-1.2 µm, and is optimized for thicknesses of 1.5-3 µm (films thicker than 3 µm may begin to show "bubbles", a result of the nitrogen released during exposure that cannot escape the top of the film).  It has a softening point at 110 °C, and will begin to formed rounded structures at and beyond this temperature. Beyond 120 °C, the resist will react with oxygen, become brownish, and cracks will begin to form. At 170-180 °C, the resist will crosslink. It is stable in HF/BOE and other acids, though strong acids (Sulfuric, Nitric) and bases will dissolve it. Dry etching performance will be limited by the thermal stability and rounding temperature.

Process:

Starting from a clean, hydrophobic substrate surface, the resist should be processed:

1. Spin Coating: AZ 1518 will reach a thickness of ~1.8 µm when spun at 4000 RPM.
   
   1. Resist dispense: Dispense resist in center of wafer/sample in puddle.

   2. Spin recipe: It has been found (for 4" wafers) that coverage is best with a direct spin to 4000 RPM without a dispersal step. The spin recipe would then look like:

      Step	Ramp (s)	RPM	Dwell (s)
      0	0	0	0
      1	2.0	4000	40
      2	2.0	0	0

      For other thicknesses, consult the spin speed chart, via AZ (for a static dispense on 6" wafers):
      

2. Hotplate Prebake: 100 °C for 110 s, or 110 °C for 55 s. (Manufacturer recommends 100 °C for 60 s, but the cleanroom hotplates seem to run somewhat cold.)
3. Expose: For a silicon substrate (other materials will have a different reflectivity and require different dosages),
   
   1. MJB3: 18 s.
   2. MA6: 15 s.
4. Development: For bath development, mild agitation is recommended. Either AZ 340 or MF26A are recommended, with development in AZ Developer or AZ 400K also possible.
   
   1. NaOH based: 1:5 solution of AZ 340:H₂O for 50-60 s.
   2. KOH based: AZ 400K for ?.
   3. TMAH based: Undiluted MF26A for 30 s, ±10 s depending on substrate reflectivity and resist thickness.
5. Postbake: No postbake required for most processes, especially if used for metallization or liftoff.
   1. Postbake will increase the chemical and thermal stability of the resist for wet or dry etching. In those cases:
      Hotplate Postbake: 115 °C for 60 s.
6. Removal: Acetone, Remover PG (or other NMP based stripper), or PRS 2000. Postbaked (above 120 °C resist will require substantially more time to fully remove, and may require heated NMP or Nanostrip for removal.

PGMEA (as AZ EBR Solvent or another brand name) can be used for edge bead removal or dilution.

References:

• AZ 1500 Series Technical Datasheet (Merck GmbH)
• Microchemicals Website: AZ1518
• AZ 1500 Series Positive Photoresist (AZ)
• AZ 1500 Photoresist Data Package (AZ)
• AZ 1500 Standard Photoresists (Clariant GmbH)

AZ9260

For thick (>5 µm) films of AZ9260, the required exposure time will depend on the photoresist thickness.

References:

• MicroChem: AZ9200
• Clariant: AZ9200
• AZ Electronic Materials: AZ9260 Photoresist
• UMN: Thick AZ9260 - Double Coat
• CMI: AZ9260 Parameters

AZ 5214

LOR3B

LOR3B is a mixture of the solvents Cyclopentanone and 1-Methoxy-2-propanol, with a 'Polyaliphatic imide copolymer' as the polymer. Edge beads can be removed with EBR PG, and it can be lifted off with Remover PG.

References:

• MicroChem: PMGI and LOR
• MicroChem Datasheet: PMGI and LOR
• MSDS: LOR B Series
  
  

Electron Beam Photoresists

PMMA

BNC supplies a variety of PMMA blends. The PMMA specification (e.g. 950 PMMA A4) indicates that the average molecular weight of the dissolved molecules (950 = 950,000 mw, 495 = 495,000 mw). The "A" indicates anisole is the primary solvent, which replaced older chlorobenzene blends. The number after the A indicates the weight percentage of polymethylmethacrylate dissolved. 

Currently stocked varieties:

• 950: A2, A4, A6, A8, A10

• 495: A2, A4

Properties

• Positive tone.
• Refractive Index: 1.49-1.52 at 632.8nm.
  Pros:
• Good adhesion to most substrates
• Very high resolution (down to 10 nm).
• Long shelf life and spun film life
• Insensitive to white light
• Resistant to water, IPA/Methanol, TMAH based developers, and dilute acids for short periods
  Cons:
• Poor etch resistance for dry etching
• Low contrast
• Attacked by acetone, HF, Piranha.

Manufacturer recommended process

(summarized from Microchem PMMA Data Sheet)

1. Solvent clean of substrate.
2. Dispense 5 - 8 mL for a 150 mm wafer.
3. Ramp to 500 RPM for 5 s OR let sit without rotation for 10 s.
4. Quick ramp to spin speed, holding for 45 s.
5. Prebake on hot plate at 180 C for 60-90 s (Note: longer times will not negatively affect PMMA. Too cool of a bake causes burning/bubbling of resist in metallization chambers, and may gradually contaminate the chamber).
6. Expose with dose between 50-500 μC/cm² depending on equipment and polymer.
7. Development: For high resolution, 1:3 MIBK to IPA for 60-120 s. Rinse in IPA or DI water immediately following develop to prevent scumming. Blow dry.
8. (Optional) Postbake: 100 C hot plate for 60-90 s. Note that PMMA will reflow above 125 C.
9. Removal: Will generally be removed by common positive PR strippers, including acetone. Thorough removal can be accomplished with Remover PG at 50-60 C.

Notes

Sensitivity of PMMA depends on the concentration, developer used, and accelerating voltage of the exposure (e.g. Rooks 2002) . At 100 kV for our PMMA, developed in 1:3 MIBK/IPA, 700 μC/cm² may be a good dose for 2D features (Hoole 1997), with small isolated lines requiring a dose >3000 μC/cm² .

PMMA References:

• Microchem product page
• Microchem PMMA FAQ
• Microchem PMMA Data Sheet
• Microchem technical reference list
• Optical constants of PMMA 950

PMMA Processes/Common Issues

Burning/Bubbling in evaporators

PMMA burning/bubbling may be seen in metallization, particularly the E-beam evaporators. This is seen commonly as a result of insufficient pre-bake of the spun PMMA film before exposure. The recommended is 180 C for 60-90 s. If it's below that temperature, insufficient pre-bake temperature is likely the issue. If you're already baking at that temperature and time, it may be an issue with placement on the hotplate or poor contact.

If it's neither of those, it could be thermals in the machine, as PMMA is further crosslinked in the evaporators due to X-rays. However, this would likely be more of a cracking than burning.

If the sample was properly baked and thermal contact is good, it may be an issue with the evaporator. A crack in the crucible can cause thermal shorting.

Mechanisms

• Photolithography with polymethyl methacrylate (PMMA)
  
• Dose influence on the PMMA e-resist for the development of high-aspect ratio and reproducible sub-micrometric structures by electron beam lithography
  

Cold Development

• Optimal temperature for development of poly(methylmethacrylate)
• Low stress development of poly(methylmethacrylate) for high aspect ratio structures
• Sub-10 nm electron beam lithography using cold development of poly(methylmethacrylate)
• The effects of molecular weight on the exposure characteristics of poly(methylmethacrylate) developed at low temperatures
  

XR1541 (HSQ)

Properties

ZEP520A

ZEP520 References:

• ZEP520A Technical Report - Zeon Chemicals

References

MicroChemicals Photoresist references:

General Properties of AZ/Ti Photoresists

Action of DNQ photoresists:

Diazonapthquinone-based Resists, Ralph Dammel, 1993.

"i-line Resists", Henderson Group, GA Tech