Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-07-25
2004-03-23
Mathews, Alan (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S071000, C355S053000, C359S558000
Reexamination Certificate
active
06710855
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed generally to an exposure method and an exposure apparatus for use to form a pattern of a semiconductor integrated circuit, or a liquid crystal device, or the like, and more particularly, to a projection exposure method and a projection exposure apparatus which are employed in a lithography process for liquid crystal elements and semiconductor memory cells having regular hyperfine patterns.
2. Related Background Art
A method of transferring mask patterns on a substrate typically by the photolithography method is adopted in manufacturing semiconductor memories and liquid crystal elements. In this case, the illumination light such as ultra-violet rays for exposure strikes on the substrate having its surface formed with a photosensitive resist layer through a mask formed with the mask patterns. The mask patterns are thereby photo-transferred on the substrate. In a projection exposure apparatus (for example, a stepper), the image of a circuit pattern drawn on the mask so as to be transferred is projected on the surface of the substrate (wafer) via a projection optical system so as to be imaged.
The typical hyperfine mask patterns of the semiconductor memory and the liquid crystal element can be conceived as regular grating patterns arrayed vertically or horizontally at equal spacings. Formed, in other words, in the densest pattern region in this type of mask patterns are the grating patterns in which equally-spaced transparent lines and opaque lines, formable on the substrate, for attaining the minimum line width are arrayed alternately in X and/or Y directions. On the other hand, the patterns having a relatively moderate degree of fineness are formed in other regions. In any case, the oblique patterns are exceptional.
A typical material for the photosensitive resist exhibits a non-linear photosensitive property. A chemical variation thereof quickly advances on giving an acceptance quantity greater than a certain level. If smaller than this level, however, no chemical variation advances. Hence, there exists a background wherein if a difference in light quantity between a light portion and a shade portion is sufficiently secured with respect to a mask pattern projected image on the substrate, a desired resist image according to the mask patterns can be obtained even when a boundary contrast between the light portion and the shade portion is somewhat low.
In recent years, a projection exposure apparatus such as a stepper, etc. for transferring the mask pattern on the substrate by reductive projection has been often employed with a hyperfiner pattern construction of the semiconductor memory and the liquid crystal element. Special ultra-violet rays having a shorter wavelength and narrower wavelength distributing width are employed as illumination light for exposure. The reason why the wavelength distribution width is herein narrowed lies in a purpose for eliminating a deterioration in quantity of the projected image due to a chromatic aberration of the projection optical system of the projection exposure apparatus. The reason why the shorter wavelength is selected lies in a purpose for improving the contrast of the projected image. Shortening of the wavelength of the illumination light induces a limit in terms of constraints of lens materials and resist materials in addition to the fact that no appropriate light source exists for the much hyperfiner mask patterns required, e.g., for the projection exposure of line widths on the submicron order. This is the real situation.
In the hyperfine mask patterns, a required value of the pattern resolution line width is approximate to the wavelength of the illumination light. Hence, it is impossible to ignore influences of diffracted light generated when the illumination light penetrates the mask patterns. It is also difficult to secure a sufficient light-and-shade contrast of the mask pattern projected image on the substrate. In particular, the light-and-shade contrast at the pattern line edges remarkably declines.
More specifically, respective diffracted light components, a 0th-order diffracted light component, (±) primary diffracted light components and those greater than (±) secondary diffracted light components that are generated at respective points on the mask patterns due to the illumination light incident on the mask from above pass through the projection optical system. These light components are converged again at the respective points on the substrate conjugate these points, thereby forming the image. However, the (±) primary diffracted light components and those larger than the (±) secondary diffracted light components have a much larger diffraction angle than that of the 0th-order diffracted light component with respect to the hyperfiner mask patterns and are therefore incident on the substrate at a shallower angle. As a result, a focal depth of the projected image outstandingly decreases. This causes a problem in that a sufficient exposure energy can not be supplied only to some portions corresponding to a part of thickness of the resist layer.
It is therefore required to selectively use the exposure light source having a shorter wavelength or the projection optical system having a larger numerical aperture in order to transfer the hyperfiner patterns. As a matter of course, an attempt for optimizing both of the wavelength and the numerical aperture can be also considered. Proposed in Japanese Patent Publication No. 62-50811 was a so-called phase shift reticle in which a phase of the transmitted light from a specific portion among the transmissive portions of reticle circuit patterns deviates by &pgr; from a phase of the transmitted light from other transmissive portions. When using this phase shift reticle, the patterns which are hyperfiner than in the prior art are transferable.
In the conventional exposure apparatus, however, it is presently difficult to provide the illumination light source with a shorter wavelength (e.g., 200 nm or under) than the present one for the reason that there exists no appropriate optical material usable for the transmission optical member.
The numerical aperture of the projection optical system is already approximate to the theoretical limit at the present time, and a much larger numerical aperture can not be probably expected.
Even if the much larger numerical aperture than at present is attainable, a focal depth expressed by ±&lgr;/2NA
2
is abruptly reduced with an increase of the numerical aperture. There becomes conspicuous the problem that the focal depth needed for an actual use becomes smaller and smaller. On the other hand, a good number of problems inherent in the phase shift reticle, wherein the costs increase with more complicated manufacturing steps thereof, and the inspecting and modifying methods are not yet established.
In an irradiation optical system for irradiating the reticle with light, an optical integrator such as a fly-eye type optical integrator (a fly-eye lens) and a fiber is used so as to uniform the distribution of the intensities of irradiation light with which the surface of the reticle is irradiated. In order to make the aforesaid intensity distribution uniform optimally, a structure which employs the fly-eye lens is arranged in such a manner that the reticle-side focal surface (the emission side) and the surface of the reticle (the surface on which the pattern is formed) hold a substantially Fourier transform relationship. Also the focal surface adjacent to the reticle and the focal surface adjacent to the light source (the incidental side) hold the Fourier transform relationship. Therefore, the surface of the reticle, on which the pattern is formed, and the focal surface of the fly-eye lens adjacent to the light source (correctly, the focal surface of each lens of the fly-eye lens adjacent to the light source) hold an image formative relationship (conjugated relationship). As a result of this, irradiation light beams from respective optical eleme
Mathews Alan
Miles & Stockbridge P.C.
Nikon Corporation
LandOfFree
Projection exposure apparatus and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Projection exposure apparatus and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Projection exposure apparatus and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3258818