Sensitivity of PMMA

The deep-UV sensitivity of PMMA, >1 Jem-2 (40), is also far from adequate to allow its use as a production scale UV resist. This is primarily due to the lack of... 

Relative sensitivities of PMMA sensitized by various sensitizers were measured using practical conditions. The results are shown in Table 3. 

Figure 9. Spectral sensitivities of PMMA and sensitized PMMA ( ) PMMA (tH) PMMA + p-tert-bvtyl benzoic acid (A) PMMA -)- 2,4-di-tert-butylphenol (Q) PMMA -)- 2,4,6-tri-tert-butylphenol...

Attempts to improve the sensitivity of PMMA through synthesis of analogs while preserving its attractive processing characteristics occupied resist chemists for several years during the early 70 s and research in the area of acrylate radiation chemistry continues to produce new results. The first electron beam resist used in device manufacturing can be considered to... 

Attempts to improve the DUV sensitivity of PMMA have spawned a variety of new DUV resist materials. Notable among these are copolymers of methyl methacrylate and indenone (80) which are reported to provide positive-tone resist function at 20 to 60 mJicnP in the DUV and copolymers of methyl methacrylate and 3-oximino-2-butanone (81). The latter materials provide a substantial increase in sensitivity over PMMA and are capable of 1 micron resolution. 

MONOCHROMATIC SENSITIVITY OF PMMA AT 220 nm USING MIBK AS THE STANDARD DEVELOPER IS 420 mj/cm2... 

Several approaches have been used to enhance the sensitivity of PMMA, (45) either by introducing chemical and/or steric configurations which tend to weaken the main chain stability of the polymer (Table V), or by substitution on the quarternary carbon with polar electronegative substituents (Table VI) which also has the effect of weakening the main chain. The -substituents are believed to enhance the capture of secondary electrons followed by resonance dissociation (57). 

There is some reason to expect that conversion of the anhydride to a half-ester might reduce the sensitivity of the copolymers. Hiraoka (10) determined the relative sensitivities of PMMA, PMA (polymethacrylic acid) and PMA AN (polymethacrylic anhydride) by measuring the gaseous products (CO, C02, and H2) given off when these polymers were exposed to electron beam radiation of 2.5 keV at 297 °K. He found that the G values (number of chemical events produced per 100 eV of absorbed radiation) for the removal of side groups are 2.0, 7.4 and 16 for PMMA, PMA and PMA AN, respectively. Anderson (11) found a similar relative order of sensitivity. For copolymers of methylmethacryate with 25% dimethylitaconate, 25% monomethyl itaconate or 25% itaconic acid (or anhydride) the G(s) values were 1, 2, 3, respectively. For the copolymer of alpha-methylstyrene and monomethyl maleate, on the other hand, we find an increase in sensitivity by a factor of 2.5 over the corresponding anhydride as described below. 

In summary, the side-chain radical of the structure -COOCH2 is the direct precursor and plays a key role in the radiation-induced scission of PMMA main-chain. Therefore, the main-chain scission can be suppressed by inhibiting the formation of the side-chain radical or by killing it with an adequate scavenger. On the contrary, the enhancement of the formation of the side-chain radical will be a guiding principle to increase the sensitivity of PMMA as an electron-beam resist. 

Poly(methylmethacrylate), (PMMA), is one resist which is especially favored by researchers due to its high resolution and contrast. Linewidths as small as 100 A have been produced with an extremely high dose of x-rays (10 J/cm ) using PMMA. However, even PMMA s normal sensitivity of 600-1000 mJ/cnr is too slow for commercial use. There have been many attempts to improve the sensitivity of PMMA to electron beam irradiation mainly by copolymerization (8). In the current paper, we describe an alternative approach whereby a sensitive polymer is physically blended with PMMA to increase its sensitivity and yet maintain the good film qualities associated with PMMA as a positive x-ray resist. 

MIBK used in the uncapped process dissolves the interfacial layer. However, attenuation of DUV light due to the novolac resin in the interfacial layer, coupled with the intrinsic, low sensitivity of PMMA developed in MIBK (see Table 3.6), demands prolonged blanket DUV exposure. This high dose of DUV leads to excessive cross-linking and photooxidation of the novolac resist (53, 166-169) and, consequently, to incomplete removal of the top resist during the development of PMMA in MIBK. Lin et al. (85) reported that a soak in methanol/water (1 1) prior to the MIBK development... 

The ultimate test of usefulness is, of course, the lithographic performance of a resist. The often-quoted sensitivity of PMMA of 50 pC/cm2 assumes conventional developing conditions and is measured by competitive dissolution rates of exposed and unexposed films. A secondary measure of sensitivity for positive-working resists is the G(s) value, the yield of chain scissions per 100 e.v. of absorbed energy. In this case, mea-... 

One of the identified causes of the low sensitivity of PMMA to DUV radiation is the low absorption coefficient of the material in this region of the spectrum and the poor match of its absorption spectrum with the emission of the Xe-Hg lamp. Incorporating acyloximino groups into the base polymer has been reported to overcome this deficiency and significantly enhance the photospeed of the resist (Scheme 7.19). The a-keto-oxime chromophore, first introduced by Delzenne et al., has a strong broad absorption band in the DUV, centered around 225 nm it is photolabile and is relatively thermally stable.The incorporation... 

As indicated above, the introduction of electron-withdrawing groups is one of the strategies for improving the sensitivity of PMMA-type resists. This is best exemplified by fluorination in the ester moiety of PMMA, which has produced many useful electron-beam resists, such as poly(perfluorobutyl methacrylate) (XXII),... 

Other strategies that have been reported for improving the sensitivity of PMMA resists include the introduction of substituents in the ester part of the PMMA and copolymerization with methacrylic acid, with acrylonitrile, and with methacrylic anhydride. In particular, Moreau et al. have described a resist based on the terpolymer of methacrylic acid, methacrylic anhydride, and methylmethacrylate (XXVI) that has demonstrated significantly faster speed than other resists based on PMMA, while maintaining desirable properties. 

Table 7.4 provides an overview of the radiation sensitivity of PMMA and its derivatives as DUV, as electron-beam, and as x-ray resists. Despite their excellent resolution and much improved sensitivity, PMMA-type resists suffer from poor etch resistance, necessitating the search for alternative resist materials for lithographic imaging applications. 

Table 7.4 Lithographic sensitivity of PMMA and derivatives. (Adapted with permission from Ref. 170. 1989 John Wiley Sons.)...

Most polymers that are positive resists tend to depolymerize via a monomer-unzipping action when degraded, and poly(methyl methacrylate) (PMMA) is typical of this type. Unfortunately, the sensitivity of PMMA to electron beam radiation is low, and in an attempt to improve this feature, PMMA derivatives have been prepared by replacing the a-methyl group with more polar electron-withdrawing substituents, e.g.. Cl, CN, and CF3, to assist electron capture (Figure 16.5). Modification of the... 

One way to increase the sensitivity of PMMA in electron-beam lithography is to weaken the main chain stability of the polymer. Substitution either on the quaternary carbon (using polar electronegative substituents) or in the side chain may do this. Some examples are given in Table 6.4. 

PMMA has been the most studied of the e-beam positive resist polymers. Images approaching 200 A have been resolved in PMMA. In addition, PMMA has excellent coating and development properties for resist applications. However, the low sensitivity of PMMA (8 x 10 Ccm at 20 keV) and the poor resistance to fluorine plasmas have minimized its use in practical applications other than mask making. 

The sensitivity of PMMA stability to various foreign units in the chain is due to several effects. Firstly, there may be blocking of the long unzipping reaction at the foreign unit, which reduces the... 

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