• Overview
• Resist Process Library
• Resist Spin Speed Curves

Overview

Any e-beam exposure is highly dependant 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

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	170°C hotplate, 15 min., non-critical. Must be 150 < T < 200 degrees, for at least 10 minutes. May also be oven baked at 170°C 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 120°C. 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	170°C hotplate for 15 min., non-critical. Must be 150 < T < 200 degrees, for at least 10 minutes. May also be oven baked at 170°C 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 120°C. 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 @ 170°C 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	110°C vacuum hotplate (Brewer) for 2 minutes.
Expose	Dose around 80 uC/cm2 at 100 kV; 10% of PMMA dose requirement.
Post-Bake	95°C 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	170°C 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	170°C 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	130°C 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	130°C 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://www18.cnf.cornell.edu/spiebook/toc.htm

Resist Sources:

NEB-31:

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

ZEP-7000A:

• Nagase California Corp., 710 Lakeway, Suite 135, Sunnyvale, CA 94086
• Ted Weber / Yuko Loveall, 408-773-0700, http://www.zeon.co.jp/products/imagelec1.html

UV-5:

• Shipley Co., 455 Forest St., Marlborough, MA 01752, (508) 229-7251, http://www.shipley.com/
• Mark Wirzbicki, Senior Sales Rep, mwirzbicki@shipley.com, http://www.shipley.com/

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:

• MicroChem Corp., 1254 Chestnut St., Newton, MA 02164-1418, (617) 965-5511
• Rob Hardman / Myracle Williamson, http://www.microchem.com/
• Arch Chemicals, Inc., 80 Circuit Dr., North Kingston, RI, 02852, 401-435-2613, http://www.archmicro.com/