JPS6262047B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron beam exposure method, and more particularly to a method of forming a highly accurate electron beam exposure pattern by correcting the so-called proximity effect.
In pattern forming technology using electron beam exposure, correction of so-called proximity effect is essential in order to improve pattern accuracy. As is well known, the proximity effect is caused by electron beam scattering (forward scattering) in the resist layer coated on the exposed object and electron beam scattering (backward scattering) from the substrate, which is the exposed object. , is a phenomenon in which the resist pattern after drawing expands to a greater extent than the electron beam irradiation pattern. Especially when the interval between patterns becomes 2 μm or less, the result is significant distortion of the pattern shape, and the negative effect of reducing precision becomes noticeable. . The electron beam scattering intensity distribution in the resist due to this scattering is determined by the distance γ from the center of the beam irradiated from the outside.
As a function of the following formula (γ)=e−(γ/A)2+Be−(γ/C)2...
It is known that the first term is given by forward scattering, and the second term is given by backscatter. Note that in formula (1), A, B, and C are constants determined by conditions such as the thickness of the resist and the material of the substrate. The most common method for correcting the proximity effect is to set the optimal irradiation dose for each pattern in advance, taking into consideration the electron beam scattering intensity distribution, its shape, and the distance from adjacent patterns. Alternatively, the drawing pattern may be modified. In either case, the amount of correction must be determined in advance at the time of creating pattern data. When forming a pattern by directly drawing an electron beam on a wafer (direct exposure), it is necessary to maintain a large residual resist film thickness due to the processing process.

However, in the case of a negative resist, the residual film thickness becomes thinner when the irradiation dose is reduced, so when the proximity effect correction is performed by the irradiation dose correction, the residual film thickness differs depending on the pattern shape and spacing. Therefore, in the case of direct exposure, in order to maintain a large residual film thickness of the negative resist, it is essential to perform proximity effect correction by dimensional correction, which deforms the drawing pattern.

However, particularly in the method using dimension correction, the correction amount has conventionally been determined empirically based on experimental results.

This means that the required finer exposure pattern,
As complexity increases, it becomes extremely difficult to reliably correct for proximity effects.

Therefore, in order to quantitatively determine the amount of dimensional correction,
As shown in Figure 1, set appropriate sample points a, b, c, and d inside the edge of the pattern (specifically at x ₁ , x ₂ , x ₃ , and x ₄ ) for each pattern. A conceivable method is to calculate the influences from all other patterns using equation (1) above, and use simultaneous equations so that the exposure intensity at each sample point is constant.

However, as shown in equation (1), the electron beam scattering intensity distribution decreases exponentially as the distance increases, resulting in an equation that is strongly nonlinear with respect to distance. Therefore, with any size,
In many cases, a solution cannot be found for integrated circuit patterns, and it is difficult to accurately determine the amount of dimensional correction.

SUMMARY OF THE INVENTION In view of these problems, it is an object of the present invention to provide an electron beam exposure method that, although an approximate method, easily determines the amount of dimensional correction and obtains a highly accurate pattern.

The base of the present invention is, for example, when considering an adjacent pattern to be corrected as shown in FIG. a′,
If we focus on the slope at point b', we can see that when there is an influence of the proximity effect (Figure 2a, the slope is steep, and when the influence is small (Figure 2b), the slope is small and flat. Focusing on this, the amount of dimension correction is determined based on how much dimension correction (reduction) is necessary in order to make the gradient less than the threshold value using the gradient as a parameter.

Below, the procedure for setting the amount of dimensional correction according to the present invention will be explained in detail.

Figure 3 shows a pattern to explain this, where points a and b are sample points set on the sides of the exposure pattern, and point b is the actual electron beam drawing after dimension correction. Assume that it is on the edge of the pattern. The exposure intensities Fa and Fb at points a and b are determined by the following equation (2).

Fa=F(r ₁ )+F(r ₃ ) Fb=F(r ₂ )+F(r ₄ ) ......(2) Here, r _i (i=1 to 4) is point a from the center of each pattern. , b, and when considering the proximity effect, the exposure intensity at each point becomes a function of the distance from the surrounding drawing pattern, so it can be expressed in this way for convenience. However, to find exactly F(r _i ), f(r) given by equation (1) is
The exposure intensity at point a or b when a point in the pattern (a point where the distance to point a or b is r) is exposed with a minute beam of area S, and is integrated over the entire pattern. It is obtained by the following equation (3).

F(r _i )=∫ _s f(r) ds (3) The processing procedure for determining the dimensional correction amount for each unit pattern of the exposure pattern to be created and for drawing the exposure pattern is as follows.

(i) Set the initial value of the dimensional correction amount l.

(ii) Corresponding to the dimensional correction amount l, calculate Fa and Fb using equations (2) and (3), and calculate the exposure intensity ratio Fb/Fa.

(iii) Fb/Fa>ε (ε is a pre-given threshold,
(a constant amount determined once the exposure conditions are determined), then l
Update (increase) and repeat (i) and (ii). (still,
This is the case when the initial value of the dimension correction amount l is set to a sufficiently small value.On the other hand, if the initial value of l is set to a sufficiently large value and Fb/Fa<ε, then l is decreased. You may also take steps to update the information. ) (iv) Find l (=lo) at the time when Fb/Fa=ε.

(v) Draw the exposure pattern using the drawing pattern whose dimensions have been corrected using lo.

The above-described predetermined threshold value ε is a constant value that is determined once the exposure conditions such as the electron beam accelerating voltage, the type of resist, and the coating thickness are determined, and its derivation is performed by the following procedure.

That is, as for ε, the pattern dimensional error due to the proximity effect appears most strongly and is the most serious problem in pattern formation, as shown in Figure 2a, that is, when two patterns are separated by a minimum resolution limit gap. When they are close, Fb/Fa is used when drawing is done with a dimension correction amount lo that just gives the minimum resolution limit gap.

The specific calculation is as follows. That is, in the above case, by changing the size correction amount l for point b, the exposure intensity at point a and the exposure intensity at the middle of the two patterns can be changed.
It is determined based on equations (2) and (3), and the dimension lo is determined when both of these two exposure intensities become the development energy (the amount of electron beam irradiation that can dissolve the resist).
Then, when the dimension correction amount is lo, the exposure intensity ratio Fb/Fa at point a and point b is assumed to be ε.

Figure 4 is a diagram showing the relationship between the exposure intensity ratio and the dimension correction amount at each sample point when exposing such a close pattern.The vertical axis shows the exposure intensity ratio Fb/Fa.
is plotted with the dimensional correction amount l on the horizontal axis. From the same figure, the smaller the dimensional correction amount l, the more a
It is clear that the exposure intensity tends to increase due to the proximity effect at a point, and even for two patterns facing each other across the minimum resolution limit gap, when the correction amount is lo, the exposure intensity ratio Fb / Fa becomes ε, It becomes possible to form a pattern with a minimum resolution limit gap.

In the case of other adjacent patterns with different gap widths between the patterns, the distances between the opposing outer sides and inner sides of both patterns are different, which means that r ₃ and r ₄ change in equation (2). means. In such cases, the point on the outer side (a
The ratio Fb/Fa of the exposure intensity at a point (equivalent to point b) and a point on the inner side (equivalent to point b) is calculated, and the relationship between Fb/Fa and the correction amount l as shown in FIG. 4 is thus determined. Then, as the size correction amount for the inner side, a width l is selected such that the exposure intensity at a point on the opposing inner side and a point on the outer side of both patterns becomes ε. If the exposure intensity ratio does not reach ε even with a correction amount of 0, the correction amount is set to 0. According to this, although it is approximate,
It is possible to suppress dimensional errors due to the proximity effect to a certain level or less, and moreover, it is possible to accurately form a pattern with a minimum resolution limit gap, which generally requires the most severe dimensional accuracy. The relationship between the above exposure intensity ratio and dimensional correction amount is (2),
Since it can be uniquely determined from equation (3), calculations for creating exposure data are easy.

On the other hand, in order to accurately determine the amount of dimensional correction, it is necessary to determine the position of the opposing inner sides at which development energy is generated, and this position can be set in various ways independently of the amount of dimensional correction. Calculations are required to determine the exposure intensity. This generally requires a huge amount of calculation and is difficult to implement in practice.

In the method of the present invention, calculations for creating exposure data are relatively easy, and it is possible to obtain practically satisfactory dimensional correction results. In practice, the above-mentioned dimensional correction amount is determined at the time of creating the pattern data, and if the device is as shown in FIG.
The beam spot is stepped by driving the deflector 4, and irradiation is performed so as to cover a predetermined pattern. FIG. 5 is a conceptual diagram of the basic configuration of a typical electron beam exposure apparatus. Electron beam exposure equipment main body 1
The device has an electron gun 2, a converging electron lens system 3, and an XY deflector 4, and irradiates a thinly focused electron beam onto a resist-coated substrate and a sample 5, and the position of the electron beam spot on the sample 5 is determined by the electron beam. It is controlled by driving the XY deflector 4 with pattern data from the computer 6 via the DA converter 7 and amplifier 8. The electron beam is irradiated onto the sample 5 by a blanking device in response to a signal from the computer 6.

In addition, in the example, the case where the negative resist film thickness is thick is described, but when the resist film thickness is thin,
It goes without saying that even in the case of positive resist, a highly accurate pattern can be obtained by applying the method according to the present invention.

As described above, according to the present invention, the amount of pattern dimension correction can be easily found, and a highly accurate pattern can be obtained even when the resist film thickness is thick by proximity effect correction based on dimension correction.

[Brief explanation of the drawing]

FIG. 1 is a pattern for explaining the conventionally considered procedure for determining the amount of dimensional correction, FIGS. 2 to 4 are diagrams for explaining the method of the present invention, and FIG. , shows an example of the basic configuration of an electron beam exposure system.

Claims (1)

[Claims] 1 In an electron beam exposure method in which a predetermined exposure pattern is created by irradiating and drawing an electron beam onto a sample coated with resist, dimensional correction is performed on the design pattern of each unit pattern in the exposure pattern to be created. When performing electron beam lithography based on a lithographic pattern, the exposure intensity at opposing edges of the lithographic pattern is determined, and dimension correction is performed so that the exposure intensity ratio between the pattern edges is below a predetermined threshold. and set the edge,
An electron beam exposure method characterized by determining a drawing pattern that has been subjected to reduction correction.