GB2367228A - Method for ascertaining the radiation dose for a layout - Google Patents

Abstract

A method for ascertaining the radiation dose for a layout in the context of shaped electron beam exposure, preferably for the purpose of correcting the "proximity effect" in a layout which comprises a plurality of exposure figures having different feature widths and feature spacings, comprises the steps of dividing the overall layout surface S<SB>1</SB> into a plurality of disjunct partial surfaces F<SB>x</SB> (x = 1 ... n) which differ in terms of feature widths and feature spacings, ascertaining evaluation criteria K<SB>x</SB> (x = 1 ... n) for the radiation dose to be directed onto the individual partial surfaces F<SB>x</SB>, assigning a corresponding evaluation criterion K<SB>x</SB> to each partial surface F<SB>x</SB> and performing the shaped-beam exposure in consideration of these evaluation criteria K<SB>x</SB>.

Description

METHOD FOR ASCERTAINING THE RADIATION DOSE FOR A LAYOUT

The invention refers to a method for ascertaining the radiation dose for a layout in the context of shaped electron beam exposure, preferably for the purpose of correcting the "proximity effect". In this context, the layout comprises a plurality of exposure figures having different feature widths and feature spacings.

One problem that must continually be solved in shaped electron beam exposure operations results from the so-called"proximity effect, "which originates from the undesirable phenomenon that each given point on the layout is influenced by radiation from its immediate vicinity. As a result, the layout points are generally not subjected to a homogeneous radiation dose, but rather to a"dose mountain"which distorts the layout that is being exposed.

The result is that relatively small and narrow areas that are exposed inside the layout receive too low an effective radiation dose, and during subsequent resist development these surfaces therefore become smaller and their edge definition is degraded. This is inconsistent with the constant effort in the microelectronics industry toward finer and finer layout features, since the necessary precision cannot be achieved in this fashion.

In previously known methods, an attempt is made to compensate for the proximity effect by determining the located that are affected by a lower effective dose so that a higher radiation dose can then be applied to them during exposure.

The CFA (conditional figure assignment) method, which is also referred to as the intraproximity correction method, has proven successful in practical use in this context. The CFA method differentiates and classifies individual exposure figures of which the layout is composed, and assigns a"dose index"to each figure. This index is equivalent to the exposure intensity. In other words, an evaluation of the exposure figures is made based, for example, on width, length, and periphery as evaluation criteria.

One substantial failing of the CFA method, however, is the fact that the environment of the exposure figures is not incorporated into the proximity correction. A sufficiently accurate correction of the proximity effect can be obtained with this procedure only for individual freestanding exposure figures of the layout.

In a complex layout in which many exposure figures touch one another and thus form a relatively large exposure surface, narrow internal figures receive a disproportionately high radiation dose. Exposing the figures on the outside of the layout using the same radiation dose then results in a layout distortion that causes a degradation in feature dimensional accuracy (critical dimension [CD] accuracy). As requirements in terms of feature resolution and accuracy increase, the CFA method is thus no longer suitable.

In particular now that microelectronic layouts are being produced using"OPC correction,"it is often possible to detect, on basic exposure figures, small corner rectangles with subcritical dimensions which consequently cause the aforementioned effects to an even greater degree when the layout tonal value is reversed.

In addition, the CFA method disadvantageously yields excessively high correction doses even in partial-coverage areas, which is typical primarily of line-and-space features.

Also known is the use of mathematical means that are utilized to eliminate the proximity effect completely or to compensate for the disadvantages of the CFA method. For this purpose, computer programs have been developed which attempt to generate effective radiation doses that are as uniform as possible for all layout points.

This is done by subdividing the overall layout into smaller and smaller exposure surfaces to which different dose values for application are then assigned. The greater the required accuracy, the more numerous and smaller the exposure figures into which the layout must be divided. This negates the advantages of the shaped-beam exposure method, however, and results in longer production times.

In addition, these computer programs must process a very large amount of information and, leaving aside the long calculation times, moreover require computing capacity that is not generally available. The use of such programs is therefore sensible during development phases or in the laboratory, but is not worthwhile in microelectronic prefabrication processes.

Proceeding from the above, it is the object of the invention to eliminate the disadvantageous mutual influence of adjacent exposure figures.

According to the present invention, it is provided in a method of the kind cited initially that the overall layout surface Si be divided into a plurality of disjunct partial surfaces Fx (x = 1... n)

which differ in terms of feature widths and feature spacings ; and furthermore that evaluation criteria Kx (x = 1... n) for the radiation dose to be directed onto the individual partial surfaces Fx be ascertained; that such an evaluation criterion Kx be assigned to each partial surface Fx ; and that the shaped-beam exposure be performed in consideration of these evaluation criteria Kx.

Definite improvements in quality have been obtained with this method. It is possible in this manner, for example, to produce complex mask layouts with substantially greater resolution and accuracy than was hitherto possible using the methods known in the existing art.

In a particularly preferred embodiment of the method according to the present invention, provision is made for a division of the overall layout surface S, into three disjunct partial surfaces F,, F,, and F,, partial surface F, being assigned an evaluation criterion K, for lowcoverage areas of the layout, partial surface F2 an evaluation criterion K2 for partial-coverage areas, and partial surface F3 an evaluation criterion K3 for full-coverage areas. The terms "low-coverage,""partial-coverage,"and"full-coverage"are used here for areas that differ in terms of the radiation dose with which these areas are processed.

In order to divide the overall layout surface into the disjunct partial surfaces F,, F2, and F3, provision is made according to the present invention for a data set Dz to be obtained from a data set D, corresponding to overall layout surface S, by sizing (a method known per se for edge shifting) using first-a/2 and then +a/2, and stored temporarily. The magnitude a in this context is a constant corresponding to the minimum feature width beyond which no further dose corrections are necessary. Sizing with a negative sign (e. g. -a/2) is to be understood in this context as edge shifts that are directed toward the inside of a surface or of an exposure figure.

Data set D2 corresponds to an auxiliary surface S2 that is later used to dissect partial surface F3 out of the overall layer surface. In this context, data sets D, and D are combined with a Boolean AND, thus yielding a data set D, that corresponds to partial surface Fs. This ensures that partial surface F3 contains only feature widths b that are larger than the predefined minimum feature width a. Partial surface F3 corresponds to those portions of the layout that are covered when exposed with the normal radiation dose. The term"normal radiation dose" is to be understood as the dose ordinarily used in the industry for an uncorrected exposure.

Data set Dg is stored for later use. The Boolean operation D, MINUS D3 is used to generate and temporarily store a data set 04, corresponding to an auxiliary surface S4, which contains only feature widths b that are smaller than the predefined minimum feature width a. Auxiliary surface S4 thus corresponds to the areas of the layout which contain both partial-coverage and low-coverage areas.

Data set D1 is then subjected once again to a sizing operation, first at +a/2, then at-a, and then at +a/2; this yields a data set Ds, corresponding to an auxiliary surface Ss, which is temporarily stored. Auxiliary surfaces S4 and Ss then yield the area which contains only feature widths b that are smaller than the minimum feature width a and in which the spacings c of the features are also smaller than the minimum feature width a. This is done by way of a Boolean operation on data sets D4 and Ds. The data set D6 thus obtained corresponds to partial surface F2, which now contains exclusively the partial-coverage areas.

Lastly, the Boolean MINUS operation applied to stored data sets D4 and D6 is used to ascertain a data set Dy, corresponding to partial surface FI'which is temporarily stored.

Partial surface FI contains only feature widths b which are smaller than the minimum feature width a but in which feature spacings c are larger than the minimum feature width a. Partial surface FI thus corresponds to the low-coverage areas.

Once partial surfaces F1, F2, and F3 have been defined in this fashion, evaluation criteria Ki, K, and K3 are assigned.

Evaluation criterion Ka, which is assigned to partial surface Fi, corresponds to the usual standard evaluation for exposure figures having feature widths b > a. In other words, a dose correction that results in a deviation from the normal radiation dose is not performed for the exposure figures within this portion of the layout.

Evaluation criterion Ksi'which is assigned to partial surface F,, is made according to the usual criteria for smaller, isolated exposure figures, and results in corresponding dose factors.

Evaluation criterion K2 assigned to partial surface F2 provides for attenuated dose factors as compared to evaluation criterion K1.

The invention will be explained in more detail below with reference to an exemplary embodiment. In the appended drawings : FIG. 1 shows an example of an overall layout surface; FIG. 2 shows an example of auxiliary surface S2 ; FIG. 3 shows an example of auxiliary surface S4 having partial-coverage and low coverage areas; FIG. 4 shows an example of auxiliary surface S, ; FIG. 5 shows an example of partial surface F2 having partial-coverage areas; FIG. 6 shows a depiction of partial surfaces F,, F2, and F3 in the layout ; FIG. 7 shows a depiction of partial surfaces Fi, F2, and F3 with the exposure figures drawn in; and FIG. 8 shows an assignment overview.

The method steps for dividing the overall or original layout into, for example, three partial surfaces F,, F2, and F, are performed as follows : Once a minimum feature width a (for example 3 micrometers) has been defined, first of all overall layout surface S, depicted in FIG. 1 is subjected to an action known as"sizing,""edge shifting, ""isotropic predistortion,"or"biasing." In a first step (sizing at-a/2), all the features intended for exposure that have a feature width b < a disappear. In FIG. 1 these are features 1, 2, 3, 4,5, and 6 on the left half of the Figure, and features 7,8, 9,10, and 11 on the right side.

Sizing at +a/2 is then used to restore the layout ; the result is that the features just mentioned, which correspond to the low-coverage and partial-coverage areas, are now absent, and an auxiliary surface S2 as shown, for example, in FIG. 2 is created.

A data set D corresponding to this auxiliary surface S2 is combined with the original layout by a Boolean AND operation (D, AND D,), as a result of which the full-coverage areas are remain behind from the original layout and are cleanly cut out. These are stored in a data set Os that corresponds to partial surface Fg. Partial surface F3 thus contains areas having features that, during subsequent exposure, will be subjected to the standard radiation dose (standard dose 1.0), since it contains only feature widths b > a.

The stored data set Dg is then used, by way of the Boolean operation D MINOUS 03, to ascertain the surfaces differing from the original layout, and these are stored in a data set D which defines an auxiliary surface S4 that contains all features with widths b < a.

FIG. 3 depicts auxiliary surface S4 that is left over as a result, containing the low-coverage and partial-coverage areas. The aforementioned features 1,3, 4, and 10 are low-coverage areas of this kind, and features 2,5, 6,7, 8,9, and 11 are referred to as partial-coverage areas of the layout.

In a subsequent method step, separation of the low-coverage and partial-coverage areas is then performed. For this purpose, firstly the partial-coverage and full-coverage areas need to be ascertained. This is done on the basis of original layout S, as follows : A sizing operation is performed, first at +a/2 and then at-a/2; the result is that feature 2 (classified as partial-coverage) and the array of features 5 (cf. FIG. 1) are each merged into larger areas which enclose these features. Following this, another sizing operation once again using-a/2 is performed, and then another sizing at +a/2 is performed for size restoration, in order to eliminate the individual narrow features; in the example, these are features 1,3, 4, and 10, which thus disappear. Advantageously, the two-a/2 sizing steps that are to be performed can be combined into a single-a sizing step.

The intermediate result obtained is auxiliary surface S, shown by way of example in FIG. 4; with this, by application of the Boolean function D4 AND D, to the stored data sets D, and D4, a data set Dg corresponding to partial surface F2 is obtained and stored.

If one considers, for example, rectangles 12 and 13 in FIG. 4 created in auxiliary surface Ss by merging, it is apparent that the AND operation has, in exactly the manner desired, extracted from auxiliary surface S4 feature 2 and feature 5, which derive from the original

layout (FIG. 1) and belong to the partial-coverage areas. FIG. 5 depicts all the features belong to partial surface Fi'which are stored in data set De. The Boolean operation D4 MINUS D6 is then used to subtract partial surface F, from auxiliary surface S4, thus removing from auxiliary surface S4 any partial-coverage areas still present. All that remains is therefore the low-coverage areas, in the form of individual narrow features, which lastly are stored in a data set 07 that describes partial surface F,.

The three partial surfaces F1,F2,F3 obtained in this fashion are depicted as follows in FIG. 6: Fi low-coverage b < a; c > a black F2 partial-coverage b < a ; c < a 1350 hatching F3 full-coverage b > a 450 hatching Once partial surfaces F,, Fz, F3 have been separated, they can then be subdivided further into exposure figures, and those exposure figures within partial surfaces F1'Fz'F3 can be individually subjected to suitable dose evaluations.

FIG. 7 shows the layout with the exposure figures drawn in. The exposure figures belonging to partial surface F3 receive no increase in dose; the normal exposure dose corresponding to a dose factor of 1.0 is assigned to them.

For the exposure figures present in partial surface Fl, on the other hand, a CFA evaluation is performed in known fashion, for example by continually increasing the dose factor as the figure width becomes smaller. Dose factor values in the range from 1.0 to 2.0 are sufficient for correcting these narrow, isolated exposure figures.

For the exposure figures that comprise partial surface F2, a CFA evaluation is preferably also performed, although (taking into account the same geometrical criteria corresponding to the figure width) a classification is made in such a way that attenuated dose factors (approximately in the range from 1.0 to 1.5) are used.

FIG. 8 shows, in summary form, the assignments of the full-coverage, partial-coverage, and low-coverage areas to partial surfaces F1,F2,F3 and to auxiliary surfaces S1, S2, S4, and S5, as well as the feature criteria.

Claims (7)

1. A method for ascertaining the radiation dose for a layout in the context of shaped electron beam exposure, preferably for the purpose of correcting the proximity effect, the layout comprising a plurality of exposure figures having different feature widths and feature spacings, wherein the overall layout surface Si is divided into a plurality of disjunct partial surfaces Fx (x

= 1... n) which differ in terms of feature widths and feature spacings ; - evaluation criteria K, (x = 1... n) for the radiation dose to be directed onto the individual partial surfaces Fx are ascertained and a corresponding evaluation criterion Kx is assigned to each partial surface Fx ; and

- the shaped-beam exposure is performed in consideration of these evaluation criteria Kx.

2. A method as claimed in claim 1, wherein a division of the overall layout surface S, is made into three disjunct partial surfaces F, F2, and F3, and partial surface Fi is assigned an evaluation criterion K, for low-coverage areas of the layout, partial surface F2 an evaluation criterion K2 for partial-coverage areas, and partial surface F3 an evaluation criterion K3 for full-coverage areas.

3. A method as claimed in claim 2, wherein - a data set D is obtained from a data set D corresponding to overall layout surface S, by sizing (edge shifting) using first-a/2 and then +a/2, a being a predefined minimum feature width, and is stored temporarily; - the Boolean operation D AND D2 is used to obtain a data set Ds which corresponds to partial surface F3 and which is stored; - the data set D is subjected to a sizing operation, first at +a/2, then at-a, and then at +a/2, yielding a data set Ds which is temporarily stored; - a data set D6 corresponding to partial surface F2 is obtained from the Boolean operation D AND Os and is stored; and - the Boolean operation D4 MINUS D6 is used to ascertain a data set Dy, corresponding to partial surface Fi, which is stored.

4. A method as claimed in any one of the preceding claims, wherein the evaluation criterion Kg corresponds to the usual standard evaluation for exposure figures having feature widths b > a.

5. A method as claimed in any one of the preceding claims, wherein the evaluation criterion K, is defined according to simple geometrical criteria for each exposure figure appropriately for smaller, isolated figure surfaces, preferably in such a way that the dose factor is continually increased for exposure figures whose feature widths become smaller.

6. A method as claimed in any one of the preceding claims, wherein the evaluation criterion K2 is defined as a function of the geometry of the exposure figures contained in the partial surface F2.

7. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.