X-ray resists

X-ray lithography is an extension of near-contact lithography for replication of 1 pm and smaller geometry. X-ray masks are difficult to fabricate with sufficient yields. Resolution in X-ray lithography will never do better than the resolution of the electron-beam exposures used to make the masks, since X-rays cannot be focused or deflected. 

It can be said that positive or negative resists developed for electron-beam lithography can be used for X-ray lithography. However, the sensitivity of conventional electron-beam resists is not sufficient from an economical point of view. 

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. 

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Figure 5. LRO-parameter S versus temperature as determined by X-ray diffraction and as calculated from resistivity measurement for CujoPtso (fit-parameter in eqn(3) A=0 7) ( ) X-rays, ( ) resistivity The curve is calculated with the Foumet model taking for the interaction energies W =720k and W2=1220k ...

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. 

Figure 44. A schematic representation of the plasma developed x-ray resist process. Exposure serves to covalenty bind the monomer (m) into the polymer matrix (p). Heating (fixing) drives out (volatilizes) the monomer except where it is "locked in place" by exposure. Plasma treatment converts the silicon to Si02 which retards the etch rate in the exposed areas through formation of a metallic oxide (MO) layer.

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. 

Basic and applied researches on charged particle and photon-induced reactions of polymers are surveyed. The basic parts are fundamentals of radiation effects on polymers and pulse radiolysis studies on polymers. The intermediate parts are a great diversity of radiation effects on polymers and reaction mechanisms of electron beam (EB) and x-ray resists. The applied parts are economic scale of utilization of radiation and industrial application of radiation to polymers. 

Onium salts have been widely used as an acid generator for photo-, EB, and x-ray resist. In addition, aromatic polymers such as novolak and polyhydroxystyrene have been often used as a base polymer for EB and x-ray resist. The reaction mechanisms in a typical resist system have been investigated by pulse radiolysis [43,52,77-88], SR exposure [79,80,83-85], and product analysis [88]. Figure 6 shows the acid-generation mechanisms induced by ionizing radiation in triphenylsulfonium triflate solution in acetonitrile. The yields of products from electron beam and KrF excimer laser irradiation of 10 mM triphenylsulfonium triflate solution in acetonitrile are shown in Fig. 7 to clarify the... 

INDUSTRIAL APPLICATION OF RADIATION TO POLYMERS 4.1. Electron Beam and X-Ray Resists... 

Several factors need to be kept in mind concerning x-ray resist design. Firstly, source design has mainly utilized electron bombardment of the target material to generate x-rays. Power output is limited by heat transfer considerations, and when account is also taken of the separation between source and exposure plane (wafer) as necessitated by geometrical considerations, it... 

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. 

X-ray Resists. The key issues in X-ray resists are source brightness, resist sensitivity, and mask quality. The method used to generate X-rays... 

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