Electron-beam resist sensitivity

Molecular Weight Dependence of Electron-Beam Resist Sensitivity... 

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... 

An early commercial interest in poly (olefin sulfones) was sparked by the low raw materials cost, but this interest waned when it became apparent that thermal instability is a general characteristic of this class of materials. In 1970 Brown and O Donnell reported that poly (butene-1-sulfone) is degraded by gamma radiation with a G(s) approaching 10, making it one of the most radiation-sensitive polymers known (38-39). The potential for use of this radiation sensitivity in the design of electron beam resists was quickly realized by several members of the electronics industry. Bell Laboratories, RCA, and IBM published studies demonstrating the potential of poly (alkene... 

C The Epoxy Resists. The first negative tone electron beam resist materials with useful sensitivity were based on utilizing the radiation chemistry of the oxirane or epoxy moiety. The most widely used of these materials, COP (Figure 32) is a copolymer of glycidyl methacrylate and ethyl acrylate and was developed at Bell Laboratories (43,44). COP has found wide applicability in the manufacturing of photomasks. The active element... 

Figure 3. Size of clear and opaque 2.0 fim features as a function of exposure dose for a negative electron beam resist. The dose Dp that results in the correct feature size is denoted as the "sensitivity .

It is important to note in this regard that there are practical limits to sensitivity. For example, an electron-beam resist with a sensitivity greater than 10-8 C/cm2 might well undergo thermal reactions at room temperature and would consequently be unsatisfactory because of an unacceptably short shelf-life of both the resist solution and spun films. The lower limit of sensitivity is governed by throughput considerations. Figure 4 illustrates the sensitivity... 

Figure 4. Sensitivity "window" for electron beam resists.

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. 

Electron beam resists to be used in direct wafer writing for submicron devices need significant improvement in sensitivity, resolution and dry etching durability. Multilayer resist (MLR) systems are now regarded as the most important technology to perform practical submicron lithography for VLSI fabrication (1-3). Many advantages in MLR compared with one layer resists (1LR) are listed here ... 

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. 

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). 

Vacuum Curing Effect. In the early stage of this work, we investigated a mixture of epoxy novolac and poly(p-vinyl phenol) (EP) to obtain an electron beam sensitive non-swelling resist. Epoxy novolac was chosen as the sensitizing component, because epoxy groups are known to be electron-beam-sensitive substituents (2). However, it is also known that electron beam resists... 

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. 

The high sensitivity of CMS as an electron beam resist is again due to the high efficiency for pair production of the two polymer radicals ( Pj and P2) at sites close to each other. 

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-1 sulfone (PBS) is a highly sensitive, high-resolution electron-beam resist (1-2) which is used primarily as a wet-etch mask in the fabrication of chrome photomasks. PBS has found little use as a dry-etch mask because of its lack of etch resistance in plasma environments (3-8). This primarily stems from the fact that PBS depolymerizes in such an environment which greatly enhances the rate of material loss from the film. Moreover, depolymerization is an activated process which causes the etching rate to be extremely temperature dependent. Previous work (3,7) has shown that the etch rate of PBS in fluorocarbon-based plasmas varies by orders of magnitude for temperature differentials of less than 30 C. 

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... 

Lin et al. (J77) replaced PMMA with IBM terpolymer resist (a terpol-ymer of MM A, methacrylic acid, and methacrylic anhydride developed as an electron beam resist), which has a higher thermal stability and a higher DUV sensitivity. 

The polymeric systems are usually composed of a polymer which Imparts the majority of physical properties and actinic additives. In simple systems such as curing films or electron beam resists, the polymer is also the radiation sensitive species. In most cases, the formulations behave simllarily in their response to high energy irradiation. Practically any polymer can be made radiation sensitive by direct exposure to ionizing energies or by formulation with additives such as free radical precursors. Thermally sensitive polymers are also likely to undergo a similar reaction when exposed. 

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). 

---