Radiant energy – Irradiation of objects or material – Ion or electron beam irradiation
Reexamination Certificate
2000-05-09
2003-04-01
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Ion or electron beam irradiation
C250S492300, C250S492220, C250S492230, C250S398000, C250S398000, C250S307000
Reexamination Certificate
active
06541784
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electron beam drawing apparatus for drawing a circuit pattern or the like on a semiconductor wafer by using an electron beam and a drawing method using an electron beam.
BACKGROUND OF THE INVENTION
An electron beam drawing apparatus has a great task for improvement in accuracy. Taking several examples, process conditions, proximity effect, blur of an electron beam (error caused by Coulomb effect, an electron beam deflector), accuracy of a mechanism system (positioning accuracy in moving a wafer and a mask) or the like influences on the accuracy.
Among the above-described, the proximity effect is an error caused by whether a drawing pattern is dense or sparse. For example, this is a phenomenon in which in the case of an isolated pattern having a wide interval between patterns with respect to a total of a region of a wafer for drawing, a line width thereof is slender and in the case of a pattern referred to as an overall pattern having a wide area, that is, having a high density with respect to a total of a region, thickening is produced. In recent years, resolution of the above-described problem becomes important owing to a necessity of miniaturization of a line width referred to as design rule for a pattern drawn on a wafer.
As a resolution of a problem in which a pattern is drawn differently from a design value in this way, there is provided an exposure dose determining method for executing proximity effect correction by using an area density map or an exposure dose map disclosed in a literature, F. Murai et al, “Fast proximity effect correction method using a pattern area density map”, J. Vac. Sci. Technol. B 10(6) November/December 1992, pp. 3072-3076, Japanese Patent Laid-Open No. 03-225816, Japanese Patent Laid-Open No. 08-213315, Japanese Patent Laid-Open No. 10-229047, U.S. Pat. Nos. 5,149,975 and 5,278,421.
This is a method in which a mesh in a rectangular shape having a certain size is assumed, an area density of a pattern is calculated for each mesh, a map representing a change in the area density of a total of a drawing region is referred to as an area density map or an exposure dose map, and an exposure dose is determined in accordance with the size of the area density to thereby draw the pattern. For example, in the case of the above-described isolated pattern, the area density is small and therefore, the exposure dose is increased whereas in the case of a pattern having a high area density, the exposure dose is reduced.
Further, according to the method, drawing operations of virtual drawing for forming the area density map and actual drawing are carried out. According to the virtual drawing, a calculation is executed up to deflection control of an electron beam with no irradiation on a wafer with the electron beam. Thereby, the area density map is formed and an actual exposure dose is calculated based on data of the area density map to execute actual drawing.
An explanation will be given of the above-described conventional method in reference to FIG.
5
and FIG.
6
.
FIG. 5
is a functional block diagram representing a constitution of proximity effect correction according to the prior art and
FIG. 6
is a flowchart showing its procedure and the explanation will be given as follows.
(1) Drawing for Forming an Area Density Map
(a) Start of Proximity Effect Correction Function
In
FIG. 5
, there exists data for each shot (a pattern diagram which an actual electron beam can draw by one time exposure) subjected to diagram decomposition (decomposition of pattern data into the pattern which the actual electron beam can draw) at a preceding stage (not illustrated) in an input unit
1
and the following processing is executed for each shot.
(b) Step 1: Start (Input of Data)
Initial shot data is inputted to the input unit
1
in
FIG. 5
(block
201
in
FIG. 6
) and is transmitted to an area density map forming unit
2
.
(c) Step 2: Forming an Area Density Map
In the area density map forming unit
2
, an area value of a region included in mesh of shot is calculated (block
202
in
FIG. 6
) and is cumulatively added to an area value of the same mesh (block
203
in FIG.
6
). When there is next shot (block
204
in FIG.
6
), the next shot is further inputted (block
205
in FIG.
6
). The area value of the mesh is previously determined and accordingly, a ratio thereof to the area value of the shot is defined as an area density per mesh. Similar processing is executed for the next shot, the area density per mesh p(x) is calculated, an area density of a total of a drawing reason is mapped and processings of all the shots are finished (block
204
in
FIG. 6
) and an area density map of the area density p(x) is finished in an area density map memory
3
(block
206
in FIG.
6
).
(d) Step 3: Smoothing Processing
Next, the area density map p(x) stored in the area density map memory
3
is read and smoothing is executed for the read map by using a smoothing unit
4
(block
207
in FIG.
6
). In this case, the reason of executing smoothing is as follows. Originally, the role of the proximity effect correction resides in executing a correction by simulating back-scattering in which an electron beam is spread in a resist of a wafer by shot and the smoothing unit is a unit for simulating the back-scattering.
Generally, as shown by the above-described literature, the back-scattering can be approximated by a Gaussian distribution and accordingly, the simulation can be carried out by adding a filter such as a Gaussian filter.
After the above-described smoothing operation, the smoothed data is stored again in the area density map memory
3
. This operation is repeatedly executed, simulation of the back-scattering is finished (block
208
in
FIG. 6
) and an area density map constituted by data of the smoothed area density Q
0
(x) is finished.
(2) Actual Drawing
An exposure dose is calculated based on the area density map of the area density Q
0
(x) finished in the above-described processing.
(a) Step 4: Start of Actual Drawing (Reading of Data)
Similarly to (1) drawing for forming an area density map, data of the same initial drawing pattern is inputted from the input unit
1
(block
210
in
FIG. 6
) and is transmitted to the area density map forming unit
2
.
(b) Step 5: Area Density Calculation for Each Shot
In the area density map forming unit
2
, an address in the area density map memory
3
is calculated, and a value of the area density Q
0
(x) for each shot is calculated from the area density map memory
3
based on the address (block
211
in FIG.
6
).
(c) Step 6: Exposure Dose Conversion Processing
Next, an exposure dose ratio [(1+&eegr;)/{1+2&eegr;Q
0
(x)}] for each shot which is a coefficient in consideration of both of forward-scattering energy and back-scattering energy is calculated based on the area density Q
0
(x) smoothed by simulating the back-scattering (block
212
in FIG.
6
). Here, notation &eegr; designates a reflection coefficient representing a ratio of the back-scattering energy to the forward-scattering energy. The reflection coefficient &eegr; is varied by influence of resist or process and accordingly, the reflection coefficient &eegr; must be determined for a material of forming a pattern of a wafer and individual steps.
By using the exposure dose ratio, an exposure dose I(x) for each shot is given by the following equation (1)
I
(
x
)=
I
50%
(
x
)·(1+&eegr;)/{1+2
&eegr;Q
0
(
x
)} (1)
where I
50%
(x) is an optimum exposure dose with respect to a pattern of 50%.
Further, a value of the calculated exposure dose I(x) is transmitted to an output unit
7
via an exposure amount converting unit
5
and calculation
6
(block
213
in FIG.
6
).
When there is next shot (block
214
in FIG.
6
), the next shot is further inputted (block
215
in FIG.
6
), Step 4 through Step 6 are repeatedly executed and the proximity effect correction is finished by finishing all the shots (block
214
in FIG.
6
).
According to the convention
Kawano Hajime
Yoda Haruo
Anderson Bruce
Hashmi Zia R.
Kenyon & Kenyon
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