Positive X-ray resists

One method for obtaining a high masking speed and resolution with X-ray lithography is use of highly sensitive positive resists. This paper reports some investigations on such sensitive positive X-ray resists. 

Positive X ray Resists. PMMA is one of the best-known positive X-ray resists, although it lacks sufficient sensitivity to be of practical use (122,123). One attempt to increase the sensitivity of methacrylate polymers is the incorporation of more-reactive groups that, upon exposure, produce large amounts of volatile products (124). Possibly, dissolution is enhanced in the exposed resist as a result of gas-induced microporosity. Incorporation of metals such as Tl or Cs into MMA-MAA copolymers (123) or fluorine atoms... 

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. 

The sensitivity of this system is 1.5 mj/cm. Highly sensitive positive X-ray resists are not really well adapted at this time since they do not possess all the required sensitivity and masking properties. However, they offer interesting advantages for inclnding inorganic atoms which could be transformed into oxide (e.g., silica). The X-ray masking technique itself suffers from problems such as the difficulty of mask manufacture. 

As a general rule, the sensitivity of conventional electron beam resists is not sufficient for economic throughput in an x-ray lithographic system. This is particularly true of positive electron resists such as PMMA, the most widely used x-ray resist for experimental purposes, whose sensitivity of >500 mJ/cm2 is some 100 times too slow for practical application. Even PBS only shows a sensitivity of 94 mJ/cm2 to PdLa x-rays. Consequently, the major research effort has concentrated on negative resists because of their higher inherent sensitivity. 

One feature which is still lacking in x-ray resists is a highly sensitive positive resist. It seems unlikely that a breakthrough will be achieved using traditional chemical approaches although recent work with plasma-developable systems is encouraging (see later section). 

Recently it was disclosed in a Japanese patent that the copolymers of hexanal with propanal, butanal and isobutanal could be used as self-developing X-ray resists of 200 — 400 mJ/cm2 sensitivity (32). Our poly(ethanal-co-butanal) showed the sensitivity of 30 mJ/cm2 on the exposure to X-ray radiation without requiring a wet development process (Table VIII). Other copolymers also functioned as a positive self developing X-ray resist. 

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). 

The last vinyl system investigated is PCMS (7). This resist has H at the alpha-position, which is well known to establish the e-beam or x-ray resist tone as negative (1). Like other negative e-beam resists such as PS and PMFA, PCMS resist is also dry-process compatible, that is, has an etch rate ratioCl.0. The PCMS resist is... 

Conventional Photoresists. PE rate ratio values for several positive photoresists are also included in this study (see Table II), because several of these novolac resin containing formulations also function as positive e-beam and x-ray resists. Generally speaking, these formulations are more dry-process compatible than most of the vinyl systems (see also ref.2). This is due primarily to the aromatic nature of the novolak resins in the photoresists. Thus, the photoresist PE rate ratio data is close in value to those of the aromatic vinyl and negative behaving polymers. 

Photochemical and Radiation Sensitive Resists Table 3 Some Positive-working X-Ray Resist Polymers... 

Resists used to define circuit patterns are radiation-sensitive and may be either positive- or negative-working. As a result of the fine lines, there has been movement away from optical Hthography and iato the mid- or deep-uv regioas. Developmeatal work has also beea focused oa electroa beam, x-ray, and ion-beam exposure devices and resists (9,10). 

Thus it can be seen that X-ray sensitivity for a positive resist... 

A 0.4 m thick SPP layer was exposed to X-rays followed by a flood exposure using near UV radiation. The resist was then dip-developed in a 0.8 wt% TMAH solution for 60 s at 25 °C. We used two x-ray exposure systems to evaluate the characteristics of the SPP resist. One is SR-114 which has a source composed of a molybdenum rotating anode with a 0.54 nm Mo-La characteristic line. The exposure was carried out in air. The other has a synchrotron radiation source with a central wavelength of 0.7 nm (KEK Photon Factory Beam Line, BL-1B). The exposure was carried out in vacuum (<10-4 Pa). A positive resist, FBM-G,15) was used as a standard, because its sensitivity only weakly depends on the ambient. 

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