Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2000-11-15
2003-07-22
Lee, John R. (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492210, C250S492220
Reexamination Certificate
active
06597001
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of electron-beam exposure and a mask as well as an electron-beam exposure system used therein, and more particularly to an electron-beam exposure method of segmented mask-pattern transfer type that is employed to manufacture a semiconductor device and especially suited for the proximity effect correction and a mask as well as an electron-beam exposure system used therein.
2. Description of the Related Art
In the electron-beam exposure that is performed in the step of lithography for manufacturing a semiconductor device, the proximity effect caused by scattered electrons within a substrate and its coating resist layer strongly affects the linewidth accuracy of projection patterns. For instance, in closely spaced line and space patterns, electrons that enter into an exposed section may be severely scattered (back-scattering) within a substrate, and a resist in an adjacent unexposed section may be subjected to exposure (background exposure) by such back-scattering electrons. As a result, edge sections and central section of one pattern become displaying different distributions of deposited energy, as shown in
FIG. 4
, and a prescribed pattern that is set at an appropriate threshold level of energy becomes unobtainable when the resist is developed (particularly in edge sections). This highlights the fact that the proximity effect correction is one of the essential techniques in the art.
As the actual method of the proximity effect correction, there are known the dose compensation method in which, at the time of the pattern exposure, the optimum dose is appropriately chosen depending on the dose of background exposure and the GHOST exposure method wherein correction exposure is made so as to bring the dose of the background exposure to a constant level in all regions where pattern exposure is carried out.
In the cell projection method and the variable-shaped beam exposure method both of which are currently widely used methods of electron-beam exposure, in order to make the proximity effect correction according to the dose compensation method, the self-consistent method using the exposure intensity distribution (EID) function, the pattern density method or the like has been presently employed, any of which requires complicated calculations. In consequence, a lengthy time is required for the data processing, and besides for every different pattern to transfer, another set of complicated calculations of this sort must be made.
The GHOST method is a technique in which, after the primary pattern (the positive pattern) for exposure is subjected to exposure, weak correction exposure (GHOST exposure) is performed with the beam that is formed by defocusing the inverse pattern of the positive pattern over the back-scattering range, and thereby the proximity effect that may be brought about through back-scattering of the incident electrons for the positive pattern exposure is corrected.
FIG. 5
is a diagram in explaining the principle of proximity effect correction according to the offset GHOST method that is a sort of the GHOST method, which shows schematically the distribution of deposited energy by the electron-beam exposure. FIG.
5
(
a
) presents the distribution of deposited energy with the primary pattern of line and space (1/1) and FIG.
5
(
b
), the distribution of deposited energy by the correction exposure with the beam that is formed by defocusing the inverse pattern over the back-scattering range, while FIG.
5
(
c
) illustrates the distribution of deposited energy in the case the correction exposure to provide such a distribution of deposited energy as shown in FIG.
5
(
b
) is applied to the exposed region of FIG.
5
(
a
). In the drawings, the energy of forward-scattering electrons is set to be
1
, and &eegr; and &bgr;b represent the back-scattering coefficient and the back-scattering range, respectively. By making the proximity effect correction according to the GHOST method, the dose of background exposure can be brought to a constant level as shown in FIG.
5
(
c
). Consequently, the distribution of deposited energy can become uniform throughout and the linewidth accuracy of the pattern, improved.
However, to apply the GHOST method of this sort to the cell projection lithography method or the variable-shaped beam exposure method, the exposure intensity must be none the less calculated using the EID function or the like. In addition, since complicated calculations are necessary for formation of the inverse pattern, considerable time is required for data processing. The projection of the inverse pattern obtained in this way also takes time. These factors all contribute to marked reduction of the throughput
Meanwhile, as a novel method of electron-beam exposure to replace the cell projection lithography method and the variable-shaped beam exposure method, an electron-beam exposure method of segmented mask-pattern transfer type has been recently proposed. This electron-beam exposure method of segmented mask-pattern transfer type is a method wherein a prescribed primary pattern for exposure is segmented into a plurality of divisions and every said divisions is subjected to exposure one by one till the whole of this prescribed primary pattern is transferred. Although the prescribed primary pattern is segmented into a plurality of divisions, this electron-beam exposure method of segmented mask-pattern transfer type uses a mask onto which the whole segmented portions of the prescribed pattern of one chip are formed in all. In this respect, the electron-beam exposure method of segmented mask-pattern transfer type is altogether different from the variable-shaped beam exposure method wherein a pattern that is to be formed is not actually formed onto the mask but processed as soft data or the cell projection lithography method which employs a mask onto which only repeated parts of a prescribed pattern is formed.
This electron-beam exposure method of segmented mask-pattern transfer type is explained well in the section of the prior art in Japanese Patent Application Laid-out No. 176720/1999 with reference to
FIG. 2
in the publication. On the basis of this description, the electron-beam exposure method of segmented mask-pattern transfer type is described below.
FIG. 6
is a schematic view in explaining the electron-beam exposure method of segmented mask-pattern transfer type. In
FIG. 6
, referential numeral
100
indicates a mask;
100
a
, a division on the mask;
100
b
, a demarcation region between divisions
100
a
;
110
, a substrate coated with a resist, such as a wafer;
110
a
, a region for one die (one chip) on the substrate
110
;
110
b
, a region for projection on the substrate
110
, each corresponding to a division
100
a
; AX, an optical axis of an optical system of charged particle beam; EB, a charged particle beam and CO, a crossover point of the optical system of charged particle beam.
On the mask
100
, being separated by a demarcation region
100
b
without a pattern, there are present numerous divisions
100
a
each of which is provided, on a membrane, a pattern to be transferred onto the substrate
110
. Further, a support structure in the form of a grid is set over the demarcation region
100
b
, protecting the membrane thermally and mechanically. The mask
100
herein is a scattering membrane mask wherein, on a membrane, for example, a silicon nitride film with a thickness of 100 nm or so, there are formed electron-beam scatterer patterns made of, for example, tungsten with a thickness of 50 nm or so. This scattering membrane mask is the mask used mainly for the electron-beam exposure method of scattering-angle limiting type (referred to as “SAL type” hereinafter) and the exposure method herein is assumed to be the SAL type.
Every division
100
a
is provided with one of segmented patterns which the pattern that is to be transferred onto a region
110
a
for one die on the substrate
110
is segmented into, and every segmented pattern is transferred onto the subst
Kobinata Hideo
Yamashita Hiroshi
Fernandez Kalimah
Lee John R.
NEC Electronics Corporation
Young & Thompson
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