We have repeatedly mentioned that the desire to use thin beams of ions or electrons in lithography is due to the impossibility of achieving submicron resolution with visible light (λ = 400–600 nm or light in the near ultraviolet region (λ ≥ 300 nm). Electron or ion “optics” can compress corpuscular beams to submicron sizes and is the principal means of obtaining a “submicron” pattern by exposing a material via a scanning corpuscular ray (beam). At the same time, diffraction makes it difficult to obtain submicron beams of light or to transfer a submicron mask image by illuminating it with a plane light wave (at a wavelength in the visible spectrum).

The ultimate goal of lithography is to produce a resist mask that will serve for the local processing of a lower structural layer. Figure 6-1 shows the cross section of part of a resist mask. The length of the base, W 2, is taken as the mask width, since the mask covers the surface of the structural layer in this space, preserving the W2-wide portion of the surface when processing through the resist mask. The width W 2 W 2 with the width WM of a dark band on the mask-photostencil pattern (in a 1:1 scale or an n-fold reduction). The algebraic difference ΔW = W 2 -WM can be called distortion. The amount of distortion and the other mask parameters depend to large degree on the properties of the optical system that transfers the photomask image onto the surface of the resist film as a two-dimensional intensity distribution I(x, y). Second, the size of W 2 is a function of the resist material parameters and the resist layer processing parameters. Uncontrolled variations in the resist film parameters and its processing modes will produce variations in W 2 even when the light I(x, y) is invariant. Ordinarily, we assume that the relative error in linewidth reproduction in a resist mask cannot exceed 10%. As a result, we must face the problem of revealing those factors that cause deviations in W 2 and of defining the necessary accuracy with which these factors must be controlled when resist processing procedures, excluding exposure, are performed.

In this chapter we will examine the issues in obtaining bright sources of electrons and forming electron beams of submicron cross section. Thermionic emission and field emission are the usual methods of obtaining electron beams having a brightness on the order of 106 A/cm2-steradian. The first sections of this chapter discuss the physics of these processes (Sections 1–5). Subsequent sections discuss electron optics as a method of forming a beam having a submicron cross section; in this presentation we will in part follow the sources.(1–8)

Ion beams are used for very diverse purposes in electronics and can be divided into three categories: 1) micromilling by removing material, 2) controlling changes in the properties of a material by doping, and 3) analyzing the structure and composition of materials. Figure 3-1 shows the physical processes that occur when an ion beam E < 1 MeV interacts with a solid target and how they are used in electronic technology and diagnostics. The parameters of the beams used in technology and diagnostics work are shown in Table 3-1, and the energy and dose ranges of the ion beams are shown pictorially in Fig. 3-2.(2) Thus, the physical theory presented in this chapter may be thought of as the theoretical foundation for not only ion-beam lithography, but for ion-beam technology and diagnostics as well. As can be seen, the energies of the beams used in electronics range from a few keV to 100 MeV and the doses are 109-1019 ions/cm2. A surface is machined by sputtering the material with low-energy ions. A controlled change in a material’s properties is accomplished by implanting fixed amounts of impurities in the material. Familiar examples of changing the properties of a semiconductor material are the creation of “buried layers,” isolating transistor regions in an integrated circuit, and altering the charge in the SiO2 oxide of a SiO2-Si structure in order to change the threshold voltage of a field-effect transistor, etc.

The opinion that optical lithography has exhausted all of its possibilities for improving resolution and is incapable of encroaching into the submicron region—that only electron-beam, ion-beam, and x-ray lithography can be used in this region—has been expressed several times in the last ten years. Recently however, this opinion has been revised and experts now believe that optical lithography via shadow and projection systems with stepwise align and expose (steppers) can provide the resolution and reproduce elements having minimal sizes of 0.25–0.5 μm.(1–3) Thus, optical lithography can be classified with microlithography methods, and, when this is done, x-ray lithography becomes the successor to optical lithography for printing elements having sizes less than 0.2 μm.(4,5)

A new resist composition based on PMMA-MAA copolymer with antracene additive was exposed by a single pulse of KrFexcimer laser (X =248 nm). Its contrast was found to exceed significantly the limit determined by light absorbtion. Studies of the development kinetics of the resist and corresponding theoretical reasoning justify the assumption of a thermal activated process (boiling of the residual dissolver).

An investigation was made of the spatial coherence of excimer lasers in different resonator geometries. Lasing with a spectral width of 5 nm was obtained in a scheme using a grating in the autocollimation regime and a four-prism telescope. A discussion is given of the questions involved in employing excimer lasers having a narrow laser emission line in photolithographic projection systems.

Александр Михайлович Прохоров (К семидесятилетию со дня рождения), Александров А.П., Боровик-Романов А.С., Валиев К.А., Велихов Е.П., Вонсовский С.В., Гинзбург В.Л., Зельдович Я.Б., Кадомцев Б.Б., Котельников В.А.

The characteristic features of the use of pulsed ultraviolet lasers in photolithography were studied, including the high pulse power and monochromaticity of laser radiation. An experimental study was made of the photolithographic process carried out using a pulsed nitrogen laser emitting at λ = 337 nm. Structures of (1.2–1.3)λ size were formed in FP-617 photoresist.

Семен Александрович Альтшулер (К семидесятилетию со дня рождения), Башкиров Ш.Ш., Боровик-Романов А.С., Валиев К.А., Гинзбург В.Л., Зарипов М.М., Кочелаев Б.И., Прохоров А.М., Теплов М.А., Фабелинский И.Л.