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Overview

Any e-beam exposure is highly dependent upon processing and the substrate.  This information is provided as a starting point and will required experimentation to optimize things for your work. Some of the information contained here is reprinted from resist data sheets from the manufacturer. Often, more information is available when you need to develop a specific resist process to use.

Many of the chemicals involved in resist processing can be extremely dangerous. Solvents such as acetone are highly flammable and explosive. Chlorinated solvents such as and methylene chloride are carcinogenic. Potassium hydroxide and TMAH developers are bases with a Ph of 14. Skin contact with these chemicals is immediately dangerous, and eye contact will quite possibly result in loss of sight. Mask processing involves use of highly dangerous strong acids.

BEFORE you can do any resist processing, you must be familiar with the chemicals you will be using, and know and respect the dangers of them.

Resist Process Library

Resist
Tone
Resolution
Contrast
Etch Resistance
Thickness
Shelf Life
Film Life
Sensitive To White Light
PMMA
Positive
Very High
Low
Poor
Many dilutions
Long @ RT
Long
No
P(MMA-MAA)
Positive
Low
Low
Poor
Many dilutions
Long @ RT
Long
No
NEB-31
Negative
Very High
High
Good
Several Dilutions
Long @ RT
Short
Yes
EBR-9
Positive
Low
Low
Poor
Single Dilution
Long @ RT
Long
No
ZEP
Positive
Very High
High
Good
Several Dilutions
Long @ RT
Short
Yes
UV-5
Positive
High
High
Good
Several Dilutions
Long @ RT
Short
Yes

 

PMMA Resist

Poly(methyl methacrylate) (PMMA) is far and away the most popular e-beam resist, offering extremely high-resolution, ease of handling, excellent film characteristics, and wide process latitude.

One of PMMA's primary attributes is its simplicity: PMMA polymer dissolved in a solvent (Anisole safe solvent). Exposure causes scission of the polymer chains. The exposed (lighter molecular weight) resist is then developed in a solvent developer.

Characteristics:

  • Positive tone
  • Very high resolution, low contrast
  • Poor dry etch resistance
  • Several dilutions available, allowing a wide range of resist thickness
  • No shelf life or film life issues
  • Not sensitive to white light
  • Developer mixtures can be adjusted to control contrast and profile

Basic Processing:

Surface Preparation In general, no surface preparation (aside from normal cleaning) is necessary. Excellent adhesion to most surfaces.
Spin Speed 1000-5000 rpm, 60 sec. (100-1000 nm)
Pre-bake 170C hotplate, 15 min., non-critical. Must be 150 < T < 200 degrees, for at least 10 minutes. May also be oven baked at 170C for 1 hour.
Expose Dose around 800 uC/cm2 at 100 kV.
Develop

For low resolution features:  MIBK:IPA 1:1, 1-2 minutes. For Higher resolution features: MIBK:IPA 1:3, 1-2 minutes

Rinse With IPA
Dry By spinning or dry N2
Post-Bake Not normally necessary. Flow can begin as low as 120C. Does not seem to noticeably improve adhesion or etch resistance.
Descum Light! (But necessary for good liftoff and clean etching.) PMMA etches very fast in oxygen. In an oxygen RIE, descum times are short, around 5 sec. In a barrel asher, times can be around 1 minute, but beware! Do not preheat the PMMA. Removal rates increase dramatically with temperature.
Stripping Most solvents, including methylene chloride and acetone will strip PMMA, as will NMP (Remover 1165). It is removed very well by strong bases (KOH), and by acid normally hostile to organics, such as NanoStrip. Oxygen plasmas etch PMMA very well.

 

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Copolymer P(MMA-MAA)

Copolymer, P(MMA-MAA), offers a higher sensitivity than PMMA, (thus can be exposed at a lower dose, thus faster), with a tradeoff in contrast.

It is most useful in Bi-level resists with PMMA, to produce undercut profiles useful in liftoff processing.

Characteristics:

  • Positive tone
  • Low resolution, low contrast
  • Poor dry etch resistance
  • Several dilutions available, allowing a wide range of resist thickness
  • No shelf life or film life issues
  • Not sensitive to white light
  • Developer mixtures can be adjusted to control contrast and profile

Basic Processing:

Surface Preparation In general, no surface preparation (aside from normal cleaning) is necessary. Excellent adhesion to most surfaces.
Spin Speed 1000-5000 rpm, 60 sec. (100-1000 nm)
Pre-bake 170C hotplate for 15 min., non-critical. Must be 150 < T < 200 degrees, for at least 10 minutes. May also be oven baked at 170C for 1 hour.
Expose Dose around 150 - 200 uC/cm2 at 100 kV.
Develop 1:1 MIBK:IPA, 1-2 minutes. (1:3 MIBK:IPA is an option, offering higher contrast, but lower sensitivity ie. higher dose.)
Rinse With IPA
Dry By spinning or dry N2
Post-Bake Not normally necessary. Flow can begin as low as 120C. Does not seem to noticeably improve adhesion or etch resistance.
Descum Light! (But necessary for good liftoff and clean etching.) PMMA etches very fast in oxygen. In an oxygen RIE, descum times are short, around 5 sec. In a barrel asher, times can be around 1 minute, but beware! Do not preheat the PMMA. Removal rates increase dramatically with temperature.
Stripping Most solvents, including methylene chloride and acetone will strip PMMA, as will NMP (Remover 1165). It is removed very well by strong bases (KOH), and by acid normally hostile to organics, such as NanoStrip. Oxygen plasmas etch PMMA very well.

 

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NEB-31

High resolution chemically amplified negative resist with high sensitivity and contrast

Characteristics:

  • Negative tone
  • Very high resolution (40 nm demonstrated), high contrast
  • Dry etch resistance comparable to most photo resists
  • Several dilutions available, allowing a wide range of resist thickness
  • No shelf life issues for resist solution if stored at room temperature
  • Film life issues
  • Sensitive to white light

Basic Processing:

Surface Preparation In general, no surface preparation (aside from normal cleaning) is necessary. Excellent adhesion to most surfaces. For metals, particularly noble metals, dehydration bake @ 170C for 15 minutes and apply P2 liquid prime or HMDS vapor prime.
Spin Speed 1000-5000 rpm, 60 sec. (100-1000 nm) Coated samples may be stored up to 2 weeks prior to exposure.
Pre-bake 110C vacuum hotplate (Brewer) for 2 minutes.
Expose Dose around 80 uC/cm2 at 100 kV; 10% of PMMA dose requirement.
Post-Bake 95C vacuum hotplate (Brewer) for 4 minutes - PEB should occur within 24 hours of exposure.
Develop MF-321; 10 seconds / 100nm resist thickness.
Rinse DI water
Dry By spinning or dry N2
Descum RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 5 sec. or: Descum in barrel etcher, 0.6 Torr of oxygen, 150W, 1 min.
Stripping Remover 1165 overnight @ RT, or 1165 @ 70(bath in PG room) for ~ 30 minutes. O2 plasma etches NEB very well.

 

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Toray EBR-9 Resist

EBR-9 is a fast, medium resolution positive resist used mostly for mask masking.

Characteristics:

  • Positive tone
  • 500 nm best resolution
  • Poor dry etch resistance
  • For masks, normally applied at 3000 rpm / 320nm thick
  • Long shelf life for resist solution
  • No film life issues
  • Not sensitive to white light
  • Developer mixtures can be adjusted to control contrast and profile

Basic Processing:

Surface Preparation In general, no surface preparation (aside from normal cleaning) is necessary. Excellent adhesion to most surfaces.
Spin Speed 3000 rpm, 60 sec. (320 nm)
Pre-bake 170C oven, 1 hr. Non-critical. Must be 170 < T < 180 degrees, for at least 30 minutes. May also be hot-plate baked.
Expose Dose around 30 uC/cm2 at 100 kV.
Develop 3:1 MIBK:IPA, 4 minutes. (Note that this is not 1:3 MIBK:IPA ! )
Rinse With IPA
Dry By spinning or dry N2
Descum RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 5 sec. or: Descum in barrel etcher, 0.6 Torr of oxygen, 150W, 1 min.
(Cr Etch for mask plate) Etch with Transene Cr etchant, ~1.5 min
Stripping Most solvents, including methylene chloride and acetone will strip EBR-9, as will NMP (Remover 1165). It is removed very well by strong bases (KOH), and by acid normally hostile to organics, such as NanoStrip. RIE in oxygen. Do not use a barrel etcher. RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 3 min.

 

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ZEP SERIES  

The ZEP series encompasses positive-tone, chemically amplified electron beam resists with high resolution and excellent dry-etching resistance for device fabrication. The series is ideally suited to the creation of photo masks and X-ray masks as well as ultra-fine processing.

Characteristics:

  • Positive tone
  • Resolution at least 20nm
  • Dry etch resistance comparable to most photo resists
  • Film Life
  • Wide process margin

Basic Processing:

Surface Preparation In general, no surface preparation (aside from normal cleaning) is necessary. Excellent adhesion to most surfaces.
Spin Speed 1000-5000 rpm, 60 sec. (100-1000 nm)
Pre-bake 170C hotplate, 2 minutes
Expose 10 - 20% the dose requirement of PMMA
Develop Solvent develop depending on resist
Rinse With IPA
Dry By spinning or dry N2
Post-Bake Not normally necessary.
Descum RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 5 sec. or: Descum in barrel etcher, 0.6 Torr of oxygen, 150W, 1 min.
Stripping Remover 1165 overnight @ RT, or 1165 @ 70(bath in PG room) for (30 minutes. O2 plasma etches NEB very well. Remove residual resist with oxygen RIE: 30 sccm O2, 30 mTorr total pressure, 0.25 W/cm2, 5 min.

 

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UV-5 Photoresist

High resolution chemically amplified DUV positive resist with high sensitivity and contrast

Characteristics:

  • Positive tone
  • Resolution at least 150 nm
  • Excellent dry etch resistance
  • Several dilutions available, allowing a wide range of resist thickness
  • No shelf life issues for resist stored at room temperature
  • Film life issues
  • Sensitive to white light
  • Wide process margin

Basic Processing:

Surface Preparation Plasma clean Si wafer in Branson barrel etcher; Process 3 - 1000W O2 for 3 minutes followed by HMDS P-20 liquid prime. Cover wafer with primer puddle and leave for 1 minute prior to spinning at any speed for > 30 seconds. Proceed to spin on resist immediately.
Spin Speed 1000-5000 rpm, 60 sec. (100-1000 nm)
Pre-bake 130C vacuum hotplate (Brewer), 60 seconds. Exposure should occur within 24 hours of pre-bake.
Expose Dose 80 uC/cm2 at 100 kV; about 10% of PMMA dose.
Post-Bake 130C vacuum hotplate (Brewer), 60 seconds. PEB should occur within 90 minutes of exposure.
Develop CD-26 for 45 - 90 seconds.
Rinse DI water
Dry By spinning or dry N2
Descum RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2, 5 sec. or: Descum in barrel etcher, 0.6 Torr of oxygen, 150W, 1 min.
Stripping Remover 1165 overnight @ RT, or 1165 @ 70(bath in PG room) for ~ 30 minutes. O2 plasma etches NEB very well.

 

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Further Reading:

SPIE Handbook of Microlithography, Microlithography, and Microfabrication - Chapter 2 - Editor: P. Rai-Choudhry, http://www.cnf.cornell.edu/cnf_spietoc.html

Resist Sources:

NEB-31:
  • SUMIKA Electronic Materials, Inc., Phoenix, AZ
  • Customer service 602-659-2590

ZEP-7000A:

UV-5:

EBR-9:

  • Toray Marketing and Sales, 411 Borel Ave., Suite 520, San Mateo, CA, 94402
  • Yuri Okazaki, 650-524-2731, y.okazaki@toray.tomac.com

PMMA and P(MMA-MAA) Copolymer:

 



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