Electron beam resist

Other investigations dealt with straight-chain molecules (oi-tricosenoic acid) in which the penultimate and final carbon atoms at the hydrophobic end are connected by a double bond [91, 92]. The material does not polymerize as rapidly as those described before when irradiated by UV light, however, but it is readily polymerized when bombarded with an electron beam. It was thus thought to be an optimal material for the fabrication of electron beam resists. 

Although these polymers have iaadequate stabiUty and processibiUty for most plastics appHcations, the abiUty to undergo scission back to the gaseous monomers has afforded some utility ia fabrication of electron-beam resists for photoUthography. Polybutene sulfone (251) and polyhexene sulfone (252) have been developed for this small-volume but high value appHcation. 

The pursuit of further miniaturization of electronic circuits has made submicrometer resolution Hthography a cmcial element in future computer engineering. LB films have long been considered potential candidates for resist appHcations, because conventional spin-coated photoresist materials have large pinhole densities and variations of thickness. In contrast, LB films are two-dimensional, layered, crystalline soHds that provide high control of film thickness and are impermeable to plasma down to a thickness of 40 nm (46). The electron beam polymerization of CO-tricosenoic acid monolayers has been mentioned. Another monomeric amphiphile used in an attempt to develop electron-beam-resist materials is a-octadecylacryUc acid (8). 

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 .

Methods 1 and 3 have been utilized in dry developed resist systems. To our knowledge, there are no resist systems commercially available that depend on post-exposure treatment other than the post-curing effect in negative electron beam resists mentioned earlier. Since such systems are still largely in the research phase we will not discuss them here but rather refer the reader to the literature for more detailed descriptions (44-50). 

Note 2 A resist material that is optimized for use with ultraviolet or visible light, an electron beam, an ion beam, or X-rays is called a photoresist (see [2], p. 307), electron-beam resist, ion-beam resist, or X-ray resist, respectively. 

Electron beam resist has been a key material for mask fabrication in the semiconductor industry. EB and x-ray lithography have recently attracted much attention as not only a next-generation lithography in the semiconductor industry but also a nanofabrication tool... 

The size and scope of the symposium have changed over the years. In 1973 only seven papers were presented, all of which reported on electron-beam resists. The number of papers rose to 17, then to 20, and then to 24, but the number describing electron-beam resists has dropped steadily in this symposium, electron-beam resists constituted only a minority of the papers. This broadening of the topical base for the symposium indicates the level to which chemistry penetrates the electronics industry. 

These vinyl systems were chosen also because they function as high-resolution electron beam resists and deep UV resists at X<300 nm. 

In the course of our research on organic metals, we discovered that certain of these materials can function as electron-beam resists for high resolution lithography with a combination of unique features that have no parallel among conventional resist materials. ... 

Figure 12. Aperture exposure for multi-layer siloxane electron beam resist...

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. 

The progress of technology for the high-resolution fabrication of semiconductor and magnetic bubble devices has required sub-micron exposure techniques such as electron beam x-ray and deep UV. Although a number of papers have been published on electron beam resists, reaction mechanisms of electron resists are still largely unknown since few studies on reactive intermediates by means of direct measurements have been done in order to elucidate the reaction mechanisms. 

A Novel Technique for Determining Radiation Chemical Yields of Negative Electron-Beam Resists... 

Bowden, M. J. "Energy Absorption in Polymeric Electron Beam Resists" private communication. 

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 ... 

Positive-Working Electron-Beam Resists Based on Maleic Anhydride Copolymers... 

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