1. Field of the Invention
The present invention relates to the field of optical lithography. More particularly, the present invention relates to an apparatus and a method for producing an interference pattern having a variable periodicity.
2. Description of the Related Art
Optical lithography is used extensively in industry for translating variations in the intensity of a beam of light into chemical or mechanical features on the surface of a material object, such as for fabricating microscopic electronic circuits. The optical patterns used for fabricating the microscopic circuits are most commonly generated by lenses that produce a magnified image of an object bearing a pattern that is desired to be reproduced lithographically. This conventional approach, however, becomes increasingly difficult to implement as the desired features become finer.
An alternative approach for generating patterns is by using interferometric lithography, in which optical patterns are generated by constructive and destructive interference of optical fields of uniform beams of light. FIG. 1 shows the basic geometry for generating an interferometric pattern. Two light beams k.sub.1 and k.sub.2 having the same wavelength .lambda. and propagating in respectively different directions intersect at a surface S of a material object so that the normal n to surface S is coplanar with the respective directions of light beams k.sub.1 and k.sub.2 and makes an angle .theta. with each of light beams k.sub.1 and k.sub.2. Under these conditions, the optical intensity along surface S varies, or is modulated, sinusoidally with period .LAMBDA. as EQU .LAMBDA.=.lambda./2 sin .theta.. (1)
For a fixed optical wavelength .lambda., .LAMBDA. can be adjusted to have any value greater than .lambda./2 by varying .theta. between 90.degree. to 0.degree..
The periodic modulation of the optical intensity .LAMBDA. can be used for producing a periodic structure on surface S through well-known techniques of optical lithography. The ability to produce features as small as .lambda./2 is one of the principal attractions for interferometric lithography because patterns having small periodicity, i.e., small feature size, have significant technical importance, such as in the fabrication of electronic circuits. Patterns more complicated than a simple periodic structure can be generated, in principle, through multiple lithographic steps, such as disclosed by S. Zaidi et al., J. Vac. Sci. Tech. B11, p. 658, 1993, and by U.S. Pat. No. 5,415,835 to Brueck et al. Accordingly, each step of a multiple lithographic technique relies on the interference phenomenon shown in FIG. 1.
At the heart of any apparatus for interferometric lithography is an interferometer, that is, the device that generates two optical beams in the configuration shown by FIG. 1. A clear interference pattern can only arise when the interfering beams have a stable, well-defined phase. To achieve this, light beams k.sub.1 and k.sub.2 are derived from a single incoming beam by either a reflecting surface or a refracting surface of limited extent, or by a partially reflecting surface. A useful interferometer must satisfy the following requirements:
(1) The interferometer must be mechanically stable because fluctuations on the order of a fraction of an optical wavelength in the beam paths will degrade the interference pattern; PA1 (2) The angle .theta. is desirably adjustable so that the period .LAMBDA. can be varied; and PA1 (3) The lengths of the optical paths traversed by beams k.sub.1 and k.sub.2 are desired to be equal so that a light source of limited coherence length can be used.
When the paths of beams k.sub.1 and k.sub.2 are related by reflection in a plane of symmetry, then condition (3) is met and, moreover, the phase difference of the interfering beams vanishes in the plane of symmetry. Thus, the plane of symmetry serves as a reference plane that allows the registration of multiple exposures, as is required to generate complex patterns using multiple lithographic techniques.
Interferometers can be classified into one of two types according to how the single incident beam is split into two interfering beams. The first type, referred to as "amplitude division" splits the incident beam by a partially reflecting surface. The second type, referred to as "wavefront division" splits the incident beam by two distinct surfaces that intercept different parts of the incident beam.
Variations of the Mach-Zehnder interferometer, in which adjustable mirrors are used to direct the resulting beams onto the surface S with the correct angles, belong to the first class of interferometers. Nevertheless, the mirrors introduce the potential for misalignment and instability, which is contrary to the first requirement listed above. A more recent variation, the achromatic grating interferometer disclosed by A. Yeh et al., Appl. Opt. 31, 4540, (1992), is stable and symmetric, but has no possibility for adjusting .theta..
In the second class of interferometers, that is, interferometers that rely on wavefront divisions, the incident beam is required to have a relatively large spatial extent, even though only a small region of the substrate is desired to be illuminated. Expanding the incident beam reduces the optical intensity, which is undesirable for lithography. The simple one-mirror Lloyd interferometer disclosed by S. Tolansky, An Introduction to Interferometry, Longmans, Green and Co., 1955, is adjustable and mechanically stable, but is highly asymmetric and requires that the incident beam have approximately the same dimension as the entire substrate, which may be much larger than the area that is to be illuminated. Lastly, the Fresnel biprism disclosed by S. Tolansky, supra, is symmetric, simple and mechanically stable, but in practice the biprism is not well-suited for generation of variable periodicity patterns, even though adjustment of .theta. is possible in principle.
What is needed is a mechanically stable, symmetric monolithic device that splits a beam of light into two beams that intersect at an adjustable angle .theta., thereby producing an interference pattern having a variable periodicity, while also providing equal length optical paths for the split beams so that a light source of short coherence length can be used.