Positive resist poly

Many papers have been published on positive electron-beam resists. These resists are mostly polymers which are degraded upon electron-beam irradiation. The resulting lower molecular weight polymer in the exposed area can be selectively removed by a solvent under certain developing conditions. The development is accomplished by the difference in the rate of dissolution between the exposed and unexposed areas, which is a function of the molecular weight of the polymer. Recently, Willson and his co-workers reported the new type of positive resist, poly(phthalaldehyde), the exposure of which in the presence of certain cationic photoinitiators resulted in the spontaneous formation of a relief image without any development step (/). 

Another class of "chain scission" positive resists is the poly(olefin sulfones). These polymers are alternating copolymers of an olefin and sulfur dioxide. The relatively weak C-S bond is readily cleaved upon irradiation and several sensitive resists have been developed based on this chemistry (49,50). One of these materials, poly(butene-l sulfone) (PBS) has been made commercially available for mask making. PBS exhibits an e-beam sensitivity of 1.6 pC cm-2 at 20 kV and 0.25 pm resolution. 

Resist solutions of o-cresol novolac-siloxane copolymers were prepared as 15 w/v % solutions of the polymer in 2-methoxyethyl acetate using 20 wt % (based on polymer) of the positive sensitizer. Poly(hydroxystyrene) and 2-methyl resorcinol copolymers were spun into films from 2-methyl tetrahydrofuran. Solutions were filtered through successive 1.0, 0.5 and 0.2 pm filters and stored in... 

Poly(methyl methacrylate) (PMMA) is a classical one-component, positive resist system. PMMA is a single, homogeneous material that combines the properties of excellent film-forming characteristics, resistance to chemical etchants and intrinsic radiation sensitivity. 

Figure 34. The Polystyrenes. Polystyrene is a low sensitivity negative resist, poly (a-methylstyrene) is a low sensitivity positive resist and the para-substituted analogs listed are all sensitive negative resists.

Positive Resists for Deep UV. Resists based on poly(methyl methacrylate) (PMMA) have been widely used (20,21). The spectral absorption of a variety of methacrylates is similar, showing absorption below 260 nm with a peak at... 

Poly(p-formyloxystyrene) was also subjected to quantitative sensitivity analysis using a calibrated multidensity resolution mask. A plot of the normalized thickness remaining as a function of dose is provided in Figure 11. The resist exhibits a sensitivity of approximately 70 mJ/cm2 in the deep UV and has a contrast ( ) comparable to that of the classical diazonaphtoquinone-novolac positive resists that are commonly employed in semiconductor manufacturing. 

A second olefin copolymer with a more promising structure is that with isobutylene (2-methylpropene-l). Poly isobutylene itself is a chain-scissioning polymer which has been studied often. It is not much used as a positive resist despite its G(s) of 1.5 to 5 (21) because its Tg is so low, about -60°C. 

The majority of organosilicon polymers crosslink efficiently upon e-beam exposure (5) and thus behave as negative-tone resists. Poly(alkenylsilane sulfone) s, on the other hand, have been found to degrade easily upon e-beam exposure, which places them in a small class of positive... 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

Poly/tetrafluoro-chloropropyl methacrylate/ (PFCPM) prepared by solution free radical polymerization was tested as a potential candidate for electron positive resist. 

Bowden and his coworkers(j).) proposed a new type of positive electron beam resist which consists of an alkali-soluble novolac and polymeric dissolution inhibitor. The positive working mechanism of this new type positive resist( NPR ) is similar to that for the conventional positive photoresist 10). It was also found that poly(2-methylpentene-l sulfone)( PMPS ) is good as a polymeric dissolution inhibitor for NPR(lil). In addition, it was clarified that one of the difficulties with NPR is phase separation in the resist films(10)(n). 

---