Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-06-30
2004-11-02
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100
Reexamination Certificate
active
06812473
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a transfer mask (or a reticle) to be used in a lithography technique for manufacturing a semiconductor device or the like by using a charged particle beam or an electron beam, a mask blank (or a mask forming substrate), and a method for manufacturing the mask.
In order to form a wiring pattern or the like, there has been utilized the lithography technique. As the wiring pattern becomes the finer, the optical lithography technique or a general technique has found it the more difficult to form the pattern. For this difficulty, there has been investigated an exposure technique which uses a charged particle beam such as an electron beam or an ion beam or a short-wavelength beam such as an X-ray for the finer structure.
Of these, the electron beam drawing technique has transferred from the point beam drawing method at the initial stage to the variable shape drawing method for drawing by changing the size and shape of a square beam. After this, from the standpoint of improving the pattern precision and shortening the drawing time period, there has been proposed a partial batch drawing method by which a portion of a pattern is partially drawn in a batch through a mask such that the drawing is repeated. Subsequent to this partial batch drawing method, moreover, a new electron projection system (or the SCALPEL system) was proposed about eight years ago by S. D. Berger and others. After this, there have been made various proposals such as a similar drawing system (or the PREVAIL system) or a transfer mask (or reticle) structure to be applied to those drawing system, and their manufacturing methods. For example, U.S. Pat. No. 5,466,904 relates to the PREVAIL system, as invented by H. C. Pfeiffer and others. This PREVAIL system will be briefly explained. First of all, there is prepared a stencil mask in which a through-hole (or aperture) pattern is formed at each small region in predetermined size and arrangement. The small regions of the stencil mask are irradiated with the charged particle beam to shape the beam according to the through-hole pattern. An exposure substrate having a photosensitive material is irradiated with the shaped beam through an optical system so that the through-hole pattern is transferred in a reduced scale to the exposure substrate. Moreover, the device pattern is formed by jointing the predetermined pattern, as formed separately on the stencil mask, over the exposure substrate.
The transfer mask, as proposed for the PREVAIL system of this kind, has such a stencil type mask for its main structure that the pattern portion is composed of non-shielded through holes. This transfer mask is disclosed in Unexamined Published Japanese Patent Applications Nos. 10-261584 and 10-260523, for example. In the stencil type mask, the deflection of the pattern region is reduced by separating and reinforcing it from the back side with a strut (or bridge) structure. By this, it is intended to improve the pattern positioning precision.
As the mask for the SCALPEL system, on the other hand, there has been mainly proposed a scattering mask (or reticle) than the stencil mask. This scattering mask is disclosed, for example, in a publication (pp. 153, of APPL, PHYS. LETTERS 57(2) (1990), EDITED BY S. D. BERGER & J. M. GIBSON) or Unexamined Published Japanese Patent Applications Nos. 10-261584 and 10-321495. In the mask structure, according to these publications, there is formed over a membrane (or a self-sustaining thin film) of SiN or the like a heavy metal layer, over which a desired pattern is formed. Both these layers are irradiated with the electron beam, but the electron scattering degree is different depending upon whether the electron beam scattering body is present or not. By making use of this difference in the electron scattering degree, a beam contrast on the wafer is obtained to transfer the pattern in a reduced scale.
In the aforementioned exposure system, the high resolution featuring the charged particle ray can be satisfied to form a finer pattern than 0.1 micron. This exposure system is enabled to Improve the throughput in the manufacture of the device by enlarging the shot size drastically, as compared with the partial batch drawing method. When the maximum shot size over the exposure substrate is enlarged from 5 micron to 250 micron, for example, a throughput of 30 sheets/hour or more can be obtained with the minimum line width of 0.08 micron and for an eight-inch substrate. This exposure system has a highly practical system having an ability to produce a general-purpose device.
Thus, there have been made the various proposals including the proposal of the aforementioned new exposure system, the proposal relating to the transfer mask (or reticle) to be applied to that system, and proposal relating to a method for manufacturing the mask. However, the various mask structures thus proposed have several problems from the standpoint of practice. These problems will be cursorily reviewed in the following.
The types of the masks proposed heretofore are coarsely divided into two kinds. The first type is the stencil mask having a pattern of through holes. The second type is the scattering mask in which an electron beam scattering body made of a heavy metal is formed over a thin film transmission layer having a thickness of 100 to 200 nm. In addition, there is a proposal of the mask of the reflection type, the description of which will be omitted. The representative structures of the first and second types are shown in
FIGS. 1 and 4
, respectively.
As shown in
FIG. 1
, the transfer pattern portion of the stencil mask has through holes
1
. It follows that there is little energy loss of the drawing electrons. Because of the pattern of a high aspect ratio, on the other hand, there is a problem in the pattern sizing precision. Because of the through holes, there is a problem on the mechanical strength of the mask. For these solutions, there is known the technique for improving the working precision and enhancing the mask strength by making the pattern region (or the thin film portion) as thin as possible (e.g., to 2 micron) and by forming struts (or a bridge) (although not shown) on the mask back side for supporting the pattern region (or the pattern field).
Where the transfer pattern has the through holes, however, the ring-shaped (or donut-shaped) pattern or the like cannot be formed in a completely independent form. A solution for this case is disclosed on pp. 210 of Solid State Technology, September (1984), edited by H. Bohlen and others. According to this method, there is prepared a complementary mask for assembling desired structural component patterns so that the pattern is formed by overlaying the complementary patterns. According to this method, however, at least two times as many as masks are required, and the shot number of exposures are increased to invite a drastic elongation of the exposure time period. This lowers the processing ability owned by the exposure system. Another demerit is that a proper pattern division is required for each device pattern. Moreover, a new problem occurs if the pattern region (or the thin film portion) is thinned to improve the working precision (or the pattern sizing precision).
The transfer pattern portion of the stencil mask has through holes. The pattern to be formed at this time raises no special problem if it is only contact holes (
FIG. 2A
) or a short line pattern (FIG.
2
B), as shown in
FIGS. 2A and 2B
(in which the black portions indicate the through holes). For conveniences of the element pattern design, however, a pattern supporting portion
4
may be a cantilever pattern (as will be called the “leaf pattern”), as shown in
FIGS. 3A and 3B
(in which the black portions individually indicate the through holes
1
).
in this case, the leaf pattern causes a deflection displacement in the longitudinal direction (i.e., in the normal to the mask face) in dependence upon the conditions. In the fine pattern or the line pattern (having an L & S ratio of 1:1) having a hig
Gill Erin-Michael
Hoya Corporation
Lee John R.
LandOfFree
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