Positive electron beam resists

A variety of techniques have been used in the present work to establish the relative sensitivity of positive electron-beam resists made from copolymers of maleic anhydride (Table I). The term sensitivity is used rather loosely at times. In the most practical sense, sensitivity is a comparative measure of the speed with which an exposure can be made. Thus, the exposure conditions, film thickness, developing solvent and temperature may be involved. Most often, the contrast curve is invoked as a more-or-less objective measure of sensitivity. The dose needed to allow removal of exposed film without removing more than about 70% of the unexposed film can be a measure of sensitivity. The initial film thickness and the developing conditions still must be specified so that this measure is not, strictly speaking, an intrinsic property of the polymeric material. 

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 (/). 

In this article we will describe two different types of positive electron-beam resists, which were briefly reported in our previous communications (2,3). One is the homopolymer or copolymer with methyl methacrylate and a-substituted benzyl methacrylate, which forms methacrylic acid units in the polymer chain on exposure to an electron-beam and can be developed by using an alkaline solution developer. In this case, the structural change in the side group of the polymer effectively alters the solubility properties of the exposed polymer, and excellent contrast between the exposed and unexposed areas is obtained. The other is a self developing polyaldehyde resist, which is depolymerized into a volatile monomer upon electron-beam exposure. The sensitivity was extremely high without using any sensitizer. 

Positive Electron-beam Resist of Poly (a-substituted Benzyl Methacrylate). The electron-beam resist behaviors of poly(a-substituted benzyl methacrylate)s are given in Table III. When the exposed resist films were developed with a mixture of MIBK and IPA, the sensitivities of these polymers were on the order of 10-4 C/cm2. When a dilute solution of sodium methoxide in methanol was used as a developer, the sensitivity was enhanced as compared with the former case, and increased with an increase in the bulkiness of the ester group of the polymer except for poly(a,a-diphenylethyl methacrylate). 

Novolac- or phenolic resin-based resists usually show no pattern deformation induced by swelling during development in aqueous alkaline solution. Examples of such resists are naphtho-quinonediazide/novolac positive photoresists, novolac-based positive electron-beam resist (NPR) (1), and azide/phenolic negative deep-UV resist (MRS) (2). Iwayanagi et al.(2) reported that the development of MRS proceeds in the same manner as the etching process. This resist, consisting of a deep-UV sensitive azide and phenolic resis matrix, is also sensitive to electron-beams. This paper deals with the development mechanism of non-swelling MRS and its electron-beam exposure characteristics. 

Materials Synthesis and Characterization. In addition to the requirements of etching resistance, sensitivity, solubility and high glass transition temperature (Tg), one of the criteria used in designing both a negative and positive electron-beam resist system was synthetic simplicity. The trimethylsilylmethyl appendage allows the incorporation of silicon into polymeric resists without adverse synthetic complications. Standard free radical or condensation polymerization techniques can be employed with appropriately substituted monomers that are readily available. 

Poly(butene-l sulfone) (PBS), a sensitive, positive, electron beam resist, is highly sensitive to 185-nm radiation (Table 3.4) (9). However, PBS does not absorb above 200 nm, and the sensitization has not been successful. Incorporation of pendant aromatic rings into the polysulfone structure extends the photosensitivity to the DUV and mid-UV regions (72). Himics and Ross (73) reported that carbonyl-containing poly(olefin sulfones) such as poly(5-hexen-2-one sulfone) are sensitive to UV-induced degradation and... 

Since polymers with tetrasubsltuted centers are difficult to synthesize, two main types of positive resists have been dlsclosed. Polymethylmethacrylate r polymethyl Isopropenyl ketone hich undergo side chain elimination and subsequent main chain fracture have been used as positive electron beam resists (cf. 

From Table XI, it can be seen that polymers have spectrum of doses necessary to obtain maximum readout efficiency. We will examine in the following example the imaging of a positive electron beam resist as a function of dose and molecular weight-solubility changes. 

A positive electron beam resist image is developed by immersion in a solvent which dissolves the exposed region at a rate (Sf)which is faster (approx. lOX) than the unexposed rate (Si. The rate of dissolution of a linear polymer is related to its molecular weight by the Uberreiter function (26) ... 

Novolac Based Positive Electron Beam Resist Containing a Polymeric Dissolution Inhibitor... 

A novolac-based positive electron beam resist has been investigated for use in direct device... 

A cresol novolac resin has been synthesized which exhibits a much greater dissolution-inhibiting effect than in various commercially available novolac or phenolic resins. Using this resin, a positive electron beam resist was prepared and its exposure characteristics were examined. A tetramethylammonium hydroxide aqueous solution was used as the developer. The sensitivity reaches 3xl0 6 C/cm2 without post-exposure baking. It was found that the sensitivity to double exposure was much higher than that to single exposure with the same total dose. A similar phenomenon was also... 

However conventional positive electron beam resists like PMMA(4) or PBS(5.) do not have excellent dry etching resistance. The electron beam sensitivities of these positive resists primarily result from radiation-induced degradation of polymer main chains. If the main chain bonding force of these polymers is weakened in order to improve sensitivities, the dry etching resistances of these polymers will decrease. In such cases, sensitivity to electron beam exposure and dry etching resistance are in a trade-off relationship. 

Conventional photoresists which consist of alkali-soluble novolac resins and photoactive dissolution inhibitors(2.) have excellent dry etching resistance. Hatzakis and his colleagues( .) have investigated such positive photoresists as positive electron beam resists. However, sensitivity to electron beam exposure was not so good(2xl0 5 C/cm2 at 20 kV). 

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). 

Name of a positive electron beam resist (ketal resist system)... 

The depolymerization mechanism from the polymer end has been recently revisited in the design of positive electron beam resists. 2-Phenylallyl-termi-nated poly(a-methylstyrene) was prepared by living anionic polymerization, which exhibited a significantly lower depolymerization temperature on TGA than the H-terminated counterpart [340]. The 2-phenylallyl-terminated polymer depolymerized completely when treated with n-BuLi in THF at room temperature. A single-component resist (without PAG) formulated with the 2-phenylallyl-terminated poly(a-methylstyrene) demonstrated a higher e-beam sensitivity (500 pC/cm2 at 20 keV) than the one based on the H-terminated polymer when developed with methanol/methyl isobutyl ketone (2/3 vol/vol) [340]. However, the sensitivity of the non-catalyzed single-component system... 

S. Matsuda, et al., Thermally reacted poly(methacrylamide) as a positive electron beam resist, Polym. Eng. Sci. 17, 410 (1977). 

Bowden and L.F. Thompson, A new family of positive electron beam resists poly(olefin sulfones), J. Electrochem. Soc. 120, 1722 (1973) Poly(styrene sulfone) A sensitive ion millable positive electron beam resist, J. Electrochem. Soc. 121 1620 (1974). 

Bowden and L.F. Thompson, Electron irradiation of poly(olefin sulfones) Application to electron beam resists, J. Electrochem. Soc. 120, 1722 (1973) Poly(Styrene sulfone) A sensitive ion millahle positive electron beam resist, J. Electrochem Soc. 121, 1620 (1974) D.R. McKean, U.P. Schaedeli, and S.A. MacDonald, Acid photogeneration from sulfonium salts in solid polymer matrices, J. Polym. Set Polym. Chem. Ed. 27, 3927 (1989) D.R. McKean, U.P. Schaedeli, P.H. Kasai, and S.A. MacDonald, The effect of polymer structure on the efficiency of acid generation from triarylsulfonium salts, J. Polym. Sci. Polym. Chem. Ed. 29, 309 (1991). 

Concerning the design of positive electron-beam resists [228,229], anionic polymerization was used to introduce 2-phenylallylgroups at the end of poly(a-MeSt) chains. 

---