SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography
Section 2.7 Resists: 2.7.2 Positive Resists
2.7.2 Positive Resists
In the simplest positive resists, electron irradiation breaks polymer backbone bonds, leaving fragments of lower molecular weight. A solvent developer selectively washes away the lower molecular weight fragments, thus forming a positive tone pattern in the resist film.
Polymethyl methacrylate (PMMA) was one of the first materials developed for e-beam lithography. [133-134] It is the standard positive e-beam resist and remains one of the highest resolution resists available. PMMA is usually purchased in two high molecular weight forms (496 K or 950 K) in a casting solvent such as chlorobenzene or anisole. PMMA is spun onto the substrate and baked at 170C to 200C for 1 to 2 hours. Electron beam exposure breaks the polymer into fragments that are dissolved preferentially by a developer such as MIBK. MIBK alone is too strong a developer and removes some of the unexposed resist. Therefore, the developer is usually diluted by mixing in a weaker developer such as IPA. A mixture of 1 part MIBK to 3 parts IPA produces very high contrast  but low sensitivity. By making the developer stronger, say, 1:1 MIBK:IPA, the sensitivity is improved significantly with only a small loss of contrast.
The sensitivity of PMMA also scales roughly with electron acceleration voltage, with the critical dose at 50 kV being roughly twice that of exposures at 25 kV. Fortunately, electron guns are proportionally brighter at higher energies, providing twice the current in the same spot size at 50 kV. When using 50 kV electrons and 1:3 MIBK:IPA developer, the critical dose is around 350 uC/cm2. Most positive resists will show a bias of 20 to 150 nm (i.e. a hole in the resist will be larger than the electron beam size), depending on the resist type, thickness, and contrast and development conditions and beam voltage.
When exposed to more than 10 times the optimal positive dose, PMMA will crosslink, forming a negative resist. It is simple to see this effect after having exposed one spot for an extended time (for instance, when focusing on a mark). The center of the spot will be crosslinked, leaving resist on the substrate, while the surrounding area is exposed positively and is washed away. In its positive mode, PMMA has an intrinsic resolution of less than 10 nm.  In negative mode, the resolution is at least 50 nm. By exposing PMMA (or any resist) on a thin membrane, the exposure due to secondary electrons can be greatly reduced and the process latitude thereby increased. PMMA has poor resistance to plasma etching, compared to novolac-based photoresists. Nevertheless, it has been used successfully as a mask for the etching of silicon nitride  and silicon dioxide,  with 1:1 etch selectivity. PMMA also makes a very effective mask for chemically assisted ion beam etching of GaAs and AlGaAs. 
EXAMPLE PROCESS: PMMA POSITIVE EXPOSURE AND LIFTOFF
- Start with 496K PMMA, 4% solids in chlorobenzene. Pour resist onto a Si wafer and spin at 2500 rpm for 40 to 60 seconds.
- Bake in an oven or on a hotplate at 180 C for 1 h. Thickness after baking: 300 nm.
- Expose in e-beam system at 50 kV, with doses between 300 and 500 uC/cm2. (Other accelerating voltages may be used. The dose scales roughly with the voltage.)
- Develop for 1 min in 1:3 MIBK:IPA. Rinse in IPA. Blow dry with nitrogen.
- Optional descum in a barrel etcher: 150W, 0.6 Torr O2.
- Mount in evaporator and pump down to 210-6 Torr.
- Evaporate 10 nm Cr, then 100 nm Au.
- Remove from evaporator, soak sample in methelyne chloride for ~10 min.
Agitate substrate and methylene chloride with an ultrasonic cleaner for ~1 min to complete the liftoff. Rinse in IPA. Blow dry. 
EBR-9 is an acrylate-based resist, poly(2,2,2-trifluoroethyl--chloroacrylate),  sold by Toray Inc.  This resist is 10 times faster than PMMA, ~10 C/cm2 at 20 kV. Its resolution is unfortunately more than 10 times worse than that of PMMA, ~0.2 m. EBR-9 excels for mask writing applications, not because of its speed (PBS is faster) but because of its long shelf life, lack of swelling in developer, and large process latitude.
EXAMPLE PROCESS: EBR-9 POSITIVE MASK PLATE
- Starting with plate purchased with a coating of EBR-9, skip to step 5. Starting with a mask plate purchased with a coating of photoresist, soak mask plate in acetone > 10 min to remove the photoresist. Rinse in isopropanol, blow dry.
- Clean the plate with RIE in oxygen. Do not use a barrel etcher. RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 2 min
- Immediately spin EBR-9, 4 krpm, 1 min 400 nm
- Bake at 170 to 180 C oven for 1 h.
- Expose with e-beam, 50 kV, 25 C/cm2 Make sure the plate is well grounded. (Other accelerating voltages may be used. The dose scales roughly with the voltage.)
- Develop for 4 min in 3:1 MIBK:IPA, rinse in IPA, blow dry in nitrogen
- Descum -- important. Same as step 2 above, for only 5 s.
- If this is a Cr plate, etch with Transene Cr etchant, ~1.5 min.
- If this is a MoSi plate, then RIE etch: 0.05 Torr total pressure, 0.05 W/cm2, 16 sccm SF6 4.2 sccm CF4, 1 min.
- Plasma clean to remove resist: same as step 2 above, for 3 min.
Poly(butene-1-sulfone) is a common high-speed positive resist used widely for mask plate patterning. For high-volume mask plate production, the sensitivity of 1 to 2 C/cm2 is a significant advantage over other positive resists. However, the processing of PBS is difficult and the only advantage is the speed of exposure. Plates must be spray developed at a tightly controlled temperature and humidity.  Contrast is poor, with ~2. For small to medium scale mask production, the time required for plate processing can make PBS slower than some photoresists.  (See Sect. 22.214.171.124.)
EXAMPLE PROCESS: PBS POSITIVE MASK PLATE
- Start with plates spun with PBS. 
- Expose, 25 kV, 1.0 to 1.6 C/cm2 (Other accelerating voltages may be used. The dose will be different.)
- Spray develop, 101 C, humidity 301%, in MIAK (5-methyl-2-hexanone) : 2-pentanone 3:1  ~30 s.
- Rinse in MIAK:2-propanol 3:2, 10 C. Spin dry under nitrogen.
- Inspect pattern, repeat steps 3 and 4 as necessary.
- Descum in a barrel etcher, 150 W, 0.6 Torr O2, 0.5 min.
- Bake to harden resist, 30 min 120 C. Heat and cool slowly.
- Etch chrome in wet etch from Transene or Cyantek (acetic acid and ceric ammonium nitrate) ~1 min. Rinse in water. Blow or spin dry.
- Strip PBS with RIE in O2 or by soaking in acetone. (rinse in IPA, blow dry).
A relative newcomer to e-beam lithography is ZEP-520 from Nippon Zeon Co.  ZEP consists of a copolymer of -chloromethacrylate and -methylstyrene. Sensitivity at 25 kV is between 15 and 30 C/cm2, an order of magnitude faster than PMMA and comparable to the speed of EBR-9. Unlike EBR-9, the resolution of ZEP is very high -- close to that of PMMA. ZEP has about the same contrast as PMMA. Lines of width 10 nm with pitch 50 nm have been fabricated with this resist. [149-150] The etch resistance of ZEP in CF4 RIE is around 2.5 times better than that of PMMA but is still less than that of novolac-based photoresists. ZEP is reported to have a long shelf life.  One disadvantage in using this resist is that (like PMMA) its sensitivity to electrons makes it difficult to inspect with a SEM. Resist lines shift and swell under high magnification SEM viewing, so it is necessary to judge the resolution of the resist by inspecting the etched patterns.
EXAMPLE PROCESS: ZEP PATTERNING OF SiO2 HOLES
- Prepare oxidized Si wafer. Spin ZEP-520 at 5 krpm for thickness 300 nm.
- Bake at 170 C, 2 min.
- Expose at 25 kV, 15 to 30 C/cm2 (Other accelerating voltages may be used. The dose will be different.)
- Develop in xylene:p-dioxane (20:1) for 2 min. Blow dry.
- Descum in barrel etcher, 0.6 Torr of oxygen, 150W, 1 min.
- Etch oxide in 4 min intervals (to avoid resist flow) 15 mTorr total pressure, 42 sccm CF4, 5 sccm H2, 0.03 W/cm2; oxide etches at ~15 nm/min.
- Remove residual resist with oxygen RIE: 30 sccm O2, 30 mTorr total pressure, 0.25 W/cm2, 5 min.
126.96.36.199 Photoresists as e-beam resists
Most photoresists can be exposed by e-beam, although the chemistry is quite different from that of UV exposure.  Because electrons cause both positive exposure and cross-linking at the same time, a photoresist film exposed with electrons must be developed with a strong developer for "positive" behavior, or, the same film can be blanket-exposed with UV light and then developed in a weak developer for "negative" behavior. One of the best photoresists for positive e-beam exposure is AZ5206.   This resist has sensitivity around 6 uC/cm2, contrast =4, and good etch resistance. With resolution around 0.25 um and very simple processing, AZ5206 is one of the best alternatives for high-speed mask production.
EXAMPLE PROCESS: AZ5206 POSITIVE MASK PLATE
- Soak mask plate in acetone > 10 min to remove the original photoresist. Rinse in isopropanol, blow dry.
- Clean the plate with RIE in oxygen. Do not use a barrel etcher. RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 5 min.
- Immediately spin AZ5206, 3 krpm.
- Bake at 80 C for 30 min.
- Expose with e-beam, 10 kV, 6 C/cm2, Make sure the plate is well grounded. (Other accelerating voltages may be used, but the dose will be different.)
- Develop for 60 s in KLK PPD 401 developer. Rinse in water.
- Descum - important Same as step 2 above, for only 5 seconds, Or use a barrel etcher, 0.6 Torr oxygen, 150W, 1 min.
- If this is a Cr plate, etch with Transene Cr etchant, ~1.5 min. If this is a MoSi plate, then RIE etch: 0.05 Torr total pressure, 0.05 W/cm2, 16 sccm SF6, 4.2 sccm CF4,1 min.
- Plasma clean to remove resist: same as step 2 above, for 3 min.
Other UV sensitive resists used for e-beam include EBR900  from Toray,  (8 uC/cm2 at 20 kV), the chemically amplified resist ARCH  from OCG,  (8-16 uC/cm2 at 50 kV), and the deep-UV resists UVIII and UVN from Shipley. [156-157] The latest offerings from Shipley have been optimized for DUV (248 nm) exposure, and have higher resolution than that of AZ5206. The use of DUV resists allows exposure by both photons and electrons in the same film, thereby reducing e-beam exposure time.
Next Sub-Section: 2.7.3 Negative Resists
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