IE990781A1 - "A method for improving focusing of an image to be projected onto a semiconductor wafer in a semiconductor manufacturing process" - Google Patents

Abstract

A method for improving focusing of an image of a reticle onto a surface (3) of a photoresist layer (4) of a wafer (5) comprises preparing a relationship graph (36) of the relationship between the pattern densities of the metal layers (37,38) beneath a metal layer (6) of which the photoresist layer (4) corresponds and corresponding offset distance values between optimum focusing positions of a stepper table (15) of the apparatus (1) and the actual focusing position of the stepper table (15) determined by a detector (33) which detects a reflected beam (30) of a focusing light beam (25) from a reflecting area (29) of the surface (3) for a plurality of wafers (5) of different pattern densities beneath the reflecting area (29). In the design of each reticle (2) from which an image is projected onto the surface (3) of the relevant photoresist layer (4) the pattern density beneath the reflecting area (29) is computed from the computer aided design system in which the wafer circuitry is designed. The offset distance value is read from the relationship graph (36) corresponding to the pattern density beneath the reflecting area (29), and the offset distance value is entered into the apparatus (1). Suitable software corrects the position of the stepper table (15) by the offset distance so that the entire photoresist layer (4) is within the depth of focus A of the apparatus (1). <Figures 1,4 & 5>

Description

”A method for improving focusing of an image to be projected onto a semiconductor wafer in a semiconductor manufacturing process” The present invention relates to a method for improving focusing of an image, in other words, an image of a mask or reticle of a metal or other layer onto a surface of a photo sensitive layer, such as a layer of photoresist laid down on the metal or other layer to be patterned of a semiconductor wafer in a photolithography apparatus.

In this specification the term “pattern density” is used to define the amount of a patterned layer per unit area remaining after the layer has been patterned. For example, in the case of a metal layer, the “pattern density” defines the amount of metal per unit area remaining after the metal layer has been patterned, and in the case of a metal layer could be referred to as the “circuit density”.

The patterning of metal and other layers of a semiconductor wafer is carried out by laying down a layer of photoresist material over the metal or other layer to be patterned. The wafer with the photoresist layer laid down is placed in a suitable photolithography apparatus in which an image of the desired pattern is projected from a mask or reticle onto the surface of the photoresist layer. Further processing of the wafer causes portions of the photoresist layer to be removed, depending on the image projected onto the surface of the photoresist layer, and depending on whether the photoresist layer is photo positive or photo negative. The wafer is then subjected to metal etching which results in areas of the metal layer not projected by photoresist being etched away. ---------·———-« INT ULyyu/οι In projecting the image of the mask or reticle onto the surface of the photoresist layer, it is important that the image is focused as sharply as possible on the photoresist layer, and not only on the surface of the photoresist layer, but also throughout the depth of the photoresist layer, since the entire photoresist layer should be subjected to the image throughout its depth. Photolithography apparatus is provided for carrying out the focusing of an image of a mask or reticle onto a photoresist layer. Such apparatus, in general comprises a table for receiving the wafer, and the table is moveable upwardly and downwardly by a suitable drive for locating the wafer with the photoresist layer in a position such that the photoresist layer is within the depth of focus of the apparatus throughout its depth. In general, the location of the surface of the photoresist layer of the wafer in the apparatus is determined by projecting a focusing beam at a relatively acute angle to the surface of the photoresist layer of the wafer, and a reflected beam from the area of the surface of the photoresist layer from which the focusing beam is reflected is monitored. The apparatus computes the position to which the wafer should be moved so that the photoresist layer lies within the depth of focus of the apparatus.

While such apparatus, in general, are reasonably adequate for determining the position of the wafer in the apparatus such that the photoresist layer lies within the depth of focus of the apparatus for photoresist layers which are relatively level, such as, for example, the photoresist layer corresponding to the first metal layer, such apparatus are relatively inaccurate when determining the position of the wafer such that the photoresist layers of the second and subsequent metal or other layers to be patterned are within the depth of focus. As the metal and other layers of a wafer are built up, the topography of the surface of the respective layers alters dramatically, and instead of the wafer presenting a relatively level photoresist surface onto which the image is to be projected, the surface of the respective photoresist layers as the wafer is built up become significantly uneven. The relative unevenness of the surfaces of the respective photoresist layers is largely determined by the variation in the pattern density of the layers which have already been patterned beneath the layer to be patterned corresponding to the photoresist layer onto which the image is to be projected. In other words, in areas above patterned layers where the pattern density is relatively high, the height of the surface of a photoresist layer will be relatively high by comparison to surfaces of the photoresist layer laid down on top of areas of patterned layers which are patterned with a low pattern density. The focusing beam of the photolithography apparatus only strikes a relatively small area of the photoresist surface, and thus the position at which the apparatus locates the wafer is such that the areas of the photoresist layer which are at a similar height to the area from which the focusing beam is reflected will be within the depth of focus. Other areas of the photoresist layer may not be within the depth of focus. Thus, if the area from which the focusing beam is reflected tends to be significantly higher than other areas of the photoresist surface, while the image will be accurately focused on the high parts of the photoresist layer, the image will be inadequately focused on the lower parts of the photoresist layer, and vice versa. Since the photolithography apparatus have a finite depth of focus, in other words, a finite range over which the projected image is of sufficient sharpness for accurately forming the image in the photoresist layer, should areas of the photoresist layer fall outside the finite depth of focus due to unevenness in the surface of the photoresist layer, the image will be inadequately projected into such areas, thus leading to significant inaccuracies in subsequent etching of the metal or other layer beneath the photoresist layer. As discussed above, this problem becomes progressively more serious as the layers are built up on a wafer.

There is therefore a need for a method for improving focusing of an image onto a photoresist layer of a semiconductor wafer onto which the image is to be projected in a photolithography apparatus.

The present invention is directed towards providing such a method: According to the invention there is provided a method for improving focusing of an image of a mask or reticle onto a photo sensitive layer of a semiconductor wafer onto which the image is to be projected in a photolithography apparatus, the method comprising the steps of deriving a relationship for the apparatus between the pattern density of the already patterned layer or layers beneath a reflecting area of the surface of the photo sensitive layer of wafers from which a focusing beam of the photolithography apparatus is to be reflected and an optimum focusing position for the wafers in the apparatus relative to an actual focusing position determined by the photolithography apparatus from the reflected focusing beam for different pattern densities of the already patterned layer or layers beneath the reflecting surface, and using the derived relationship for subsequently positioning wafers in the apparatus in the appropriate optimum focusing position in a production process. in one embodiment of the invention the derived relationship is provided in the form of a graph.

Preferably, the derived relationship is provided as a relationship of the offset distance between the optimum focusing position and the actual focusing position with the pattern density of the already patterned layer or layers.

In another embodiment of the invention a plurality of derived relationships are derived in respect of a plurality of respective photo sensitive layers for corresponding respective layers of wafers to be patterned for which the photolithography apparatus is adapted for projecting images thereonto.

In one embodiment of the invention the derived relationship is derived by computing the pattern densities of each already patterned layer beneath the reflecting areas of the photo sensitive layers for each layer to be patterned of a plurality of wafers of different pattern densities beneath the respective reflecting areas.

In another embodiment of the invention the derived relationship is derived for the photo sensitive layer of each layer to be patterned by placing the plurality of wafers in the photolithography apparatus as the processing of the wafers is completed to respective stages where the respective photo sensitive layers have been laid down for the respective layers to be patterned, and permitting the photolithography apparatus to determine the actual focusing positions for the photo sensitive layer of each layer to be patterned, and recording the actual focusing positions, moving the wafers from the respective actual focusing positions to corresponding optimum focusing positions, and recording the offset distances between the optimum focusing positions and the corresponding actual focusing positions for the photo sensitive layer of each layer to be patterned of each wafer, and recording the respective offset distances against the corresponding pattern densities beneath the reflecting areas for the corresponding layers to be patterned of the respective wafers. Preferably, the optimum focusing position for the photo sensitive layer of each layer to be patterned of each wafer is determined by observing the image focused on the surface of the photo sensitive layers corresponding to the respective layers to be patterned of each wafer. Advantageously, the images on the surfaces of the respective photo sensitive layers are observed through an electron microscope of the apparatus.

Ideally, the optimum focusing position for the photo sensitive layer of each layer to be patterned of each wafer is determined when the surface of the photo sensitive layer is within the depth of focus of optical apparatus of the photolithography apparatus.

In one embodiment of the invention the offset distance corresponding to each mask or reticle for use in the photolithography apparatus is determined from the derived relationship for the corresponding layer to be patterned by first computing the density of the already patterned layer or layers beneath the reflecting area, and then reading the offset distance from the derived relationship. In another embodiment of the invention the derived relationship graph is derived from the recorded offset distances and the corresponding densities of the already patterned layers.

Preferably, the offset distance is provided with each mask or reticle for use in a subsequent production process in the apparatus. Advantageously, the density of the already patterned layer or layers beneath the reflecting area corresponding to each layer to be patterned are computed using a computer aided design system in which the patterns for the respective layers of each wafer are designed.

In another embodiment of the invention before each production run of a batch of similar wafers the photolithography apparatus is set to position each wafer of the batch of wafers at the appropriate optimum focusing position for the layer to be IE99U7S1 patterned based on the corresponding offset distance from the actual focusing position which would be determined by the apparatus.

The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of conventional photolithography apparatus for projecting an image of a mask or a reticle onto a photoresist io layer of a semiconductor wafer, Fig. 2 is a diagrammatic representation of projection optics of the apparatus of Fig. 1, Fig. 3 is a plan view of a stepper field of a wafer, Fig. 4 is a transverse cross-sectional side elevationai view of a portion of the wafer of Fig. 3 on the line IV-IV of Fig. 3, and Fig. 5 is a graphical representation of a derived relationship for the apparatus of Fig. 1 between the pattern density of metal or other patterned layers beneath a photoresist layer and offset distances between an optimum focusing position of the wafer in the apparatus and an actual focusing position determined by the apparatus of Fig. 1.

Referring to the drawings there is schematically illustrated photolithography apparatus indicated generally by the reference numeral 1 for projecting an image of a reticle 2 onto a surface 3 of a photoresist layer 4 of a semiconductor wafer 5 for patterning the photoresist layer 4, for use in subsequently etching a metal or other layer 6 beneath the photoresist layer 4. The photolithography apparatus 1 is of the type which automatically positions the semiconductor wafer 5 so that the surface 3 of the photoresist layer 4 is within the depth of focus A about a focal plane 9 at which the image of the reticle 2 is focused by projection optics 10 of the apparatus 1. The invention provides a method for improving focusing of the image on the surface 3 of the photoresist layer 4 by the apparatus 1 so that at least the surface 3 of the photoresist layer 4 is located within the depth of focus A of the apparatus 1, and preferably, the entire photoresist layer 4 is within the depth of focus A, even allowing for an uneven surface topography of the surface 3, as for example is illustrated in Fig. 4.

Before describing the method according to the invention in detail, the apparatus 1 will first be briefly described. The apparatus 1 comprises a stepper table 15 for receiving the wafer 5 and for positioning the wafer 5 in the apparatus 1 with the photoresist layer 4 within the depth of focus A of the apparatus 1. The stepper table 15 also steps the wafer 5 in an X and Y-axis direction for stepping the image of a reticle 2 over the entire surface 3 of the photoresist layer 4. The X-axis direction is illustrated in Fig. 1, and as can be seen is in directions to the right and to the left of the stepper table 15. The Y-axis direction which is not illustrated in Fig. 1, but is illustrated in Fig. 3 is into and out of the page in Fig. 1. A drive mechanism (not shown) is provided for driving the stepper table 15 upwardly and downwardly in an Z-axis direction parallel to a focal axis 14 of the projection optics 10 for positioning the wafer 5 with the photoresist layer 4 within the depth of focus A of the apparatus 1. The focal plane 9 is illustrated in Fig. 2 and contains one focal point 17 of a lens 16 of the projection optics 10. The depth of focus A lies within upper and lower lines 19 and 20, respectively, illustrated in Fig. 2. The reticle 2 is located in the projection optics 10 above the lens 16.

A focusing light beam 25 is projected from a light source 28 onto the surface 3 of the photoresist layer 4 of the wafer 5 for determining the location of the surface 3 of the photoresist layer 4 for, in turn facilitating positioning the stepper table 15 in the Z10 axis direction with the photoresist layer 4 within the depth of focus A. The focusing light beam 25 is directed onto the surface 3 of the photoresist layer 4 and is reflected from a reflecting area 29 of the length L as a reflected beam 30, see Figs. 1 and 3. A light detector 33 detects the reflected beam 30, and suitable hardware and software in the apparatus 1 computes from the position at which the reflected beam 30 strikes the detector 33, the position of the surface 3 of the photoresist layer 4. Knowing the position of the surface 3 of the photoresist layer 4, the position of the focal plane 9 and the depth of focus A of the apparatus 1, the distance the stepper table 15 has to be moved in order that the wafer 5 is in a focusing position with the photoresist layer 4 within the depth of focus A can readily by computed by the hardware and software of the apparatus 1, and the stepper table 15 is moved in the Z-axis direction the appropriate distance for positioning the wafer 5 in the focusing position. In this way, the stepper table 15 is positioned so that the photoresist layer 4 at the reflecting area 29 at least, is located within the depth of focus A between the lines 19 and 20.

However, the reflecting area 29 is relatively small by comparison with the image field projected by the reticle 2, and as discussed above should the reflecting area 29 from which the focusing light beam 25 is reflected coincide with a relatively high area of the surface 3 of the photoresist layer 4, the wafer 5 will be positioned in a focusing position with the high area of the photoresist layer 4 within the depth of focus A of the apparatus 1, but lower areas of the photoresist iayer 4 may well fail outside the depth of focus A. For example, referring in particular to Fig. 4 should the focusing light beam 25 be reflected from the surface 3 of the photoresist layer 4 at a reflecting area 29a, which is at a high part of the photoresist layer 4, the stepper table 15 in response to the detector 33 positions the wafer 5 in a focusing position such that areas of the photoresist layer 4 which are at a level similar or substantially similar to that of the reflecting area 29a are within the depth of focus A. Thus, lower areas of the photoresist layer 4 such as the area within the dimension arrow D fall outside the depth of focus A. On the other hand, should the focusing light beam 25 be reflected from a reflecting area 29b on the surface 3 of the photoresist layer 4 while the area of the photoresist layer 4 within the dimension arrow D would fall within the depth of focus A, the majority of the higher areas of the photoresist layer 4 would fall outside the depth of focus A of the apparatus 1.

Such photolithography apparatus will be well known to those skilled in the art and further description of the apparatus should not be required for an understanding of the invention.

The method according to the invention for improving focusing of an image of a reticle on any photresist layer 4 irrespective of the topography of the surface 3 of the layer will now be described. According to the method of the invention a relationship is initially derived for the photolithography apparatus 1 between the pattern density of the already patterned layers beneath the reflecting area 29 of the photoresist iayer for each layer to be patterned, and an optimum focusing position for the wafer 5 relative to the actual focusing position determined by the apparatus 1 in response to the detector 33 and the reflected beam 30 for various pattern densities beneath the reflecting area 29. In other words, a set of derived relationships is prepared, one derived relationship for each patterned layer for which the apparatus 1 is capable of patterning, as will be described below. In this embodiment of the invention the derived relationship is derived between the layers which have already been patterned beneath the reflecting area 29 and the offset distance between the optimum focusing position for the wafer 5 and the actual focusing position for the wafer 5 as determined by the focusing apparatus in response to the detector 33 and the reflected beam 30. The optimum focusing position is a position of the wafer 5 in which all the photoresist layer 4 in the image field falls within the depth of focus A of the apparatus 1. Fig. 5 illustrates a graph 36 of the derived relationship for the photoresist layer 4 of the third patterned layer, namely, the third metal layer 6. The graph 36 is a plot of the offset distance values between the optimum focusing positions and the corresponding actual focusing positions of wafers against corresponding but different pattern densities of the already patterned layers beneath the layer to be patterned, in this case the third metal layer 6 under the reflecting area 29.

The relationship between the pattern densities and the corresponding offset distances is derived as follows. A plurality of wafers 5 are selected of different circuit designs, and which have different pattern densities beneath their reflecting areas 29. The selected wafers 5 are sequentially placed in the photolithography apparatus 1 as each wafer 5 is completed to a stage where a photoresist layer has been laid down on each metal layer or other layer to be patterned. The location of the reflecting area 29 at which the focusing light beam 25 will strike in an image field projected from the reticle 2 onto each photoresist layer of each wafer 5 is determined from the computer aided design system in which the circuitry for each wafer 5 is designed. The location of the reflecting area 29 can readily easily be determined by the computer aided design system, since the position of the reflecting area 29 will be known from the apparatus 1. In selecting the wafers 5 care is taken to ensure that the wafers provide a broad range of pattern densities beneath the reflecting areas 29. The pattern densities of the metal layers or other layers to be patterned beneath the metal or other layer which corresponds to the respective photoresist layers, the surface of which is to receive the projected image is computed in the area underneath the position of the reflecting area 29 for each photoresist layer of each wafer 5. The computed pattern densities for the respective photoresist layers of the respective wafers are recorded. As each photoresist layer is laid down on each wafer, the wafers are sequentially loaded into the apparatus 1 and the apparatus 1 is operated for automatically determining the focusing position for each wafer for the particular photoresist layer to which the wafer 5 has been built up, and the apparatus 1 is operated for moving the stepper table 15 to that focusing position which is referred to as the actual focusing position for the particular photoresist layer. The positions of the actual focusing positions for the respective photoresist layers of the respective wafers are recorded. Using an electron microscope (not shown) of the apparatus 1 the image projectedonto the respective surfaces of the photoresist layers 4 are visually inspected when the wafer has been moved into the respective actual focusing positions. In general, as discussed above parts of the surfaces 3 of the respective photoresist layers 4 will fall outside the depth of focus A of the apparatus 1, and in those areas the image projected by the reticle 2 will not be sharply defined. The apparatus 1 is then operated for moving the stepper table 15 with the wafer 5 thereon for improving the focusing of the image of the reticle 2 until the entire field of the image on the surface of the photoresist layer is within the depth of focus A, and a sharply defined image is to be found over the entire field of the image. This is the optimum focusing position for that photoresist layer 4 of the particular wafer 5 which corresponds to the particular pattern density beneath the reflecting area 29. The offset distance between the optimum focusing position and the actual focusing position is recorded, and plotted against the corresponding pattern density beneath the reflecting area 29 for the photoresist layer of the particular metal or other layer to be patterned. This procedure is carried out for the photoresist layer of each metal or other layer to be patterned of each wafer, and relationship graphs 36 are prepared, one graph 36 being prepared for the photoresist layer of each metal or other layer to be patterned.

The procedure for determining the graph 36 of Fig. 5 for the photoresist layer of the third metal layer for the apparatus 1 will be described with reference to the wafer of Fig. 4. The wafers 5 of different pattern densities beneath the reflecting area 29 are built up to the third metal layer 6, and the photoresist layer 4 is laid down on the third metal layer of each of the wafers. The position of the reflecting area 29 at which the focusing light beam will be reflected from the surface 3 of the photoresist layer 4 of each wafer 5 will have already been computed in the computer aided design system, and the pattern density of the first and second metal layers 37 and 38, respectively beneath the reflecting area 29 of each wafer 5 is computed also in the computer aided design system which designed the circuitry of the wafer 5. For example, should the reflecting area 29 be computed as falling at the position 29a of Fig. 4, the pattern density of the first and second metal layers 37 and 38 within the vertical lines 40 and 41 is recorded. The wafer 5 of Fig. 4 is then loaded into the apparatus 1, and the actual focusing position as determined by the apparatus 1 is recorded. The wafer of Fig. 4 is then moved in the Z-axis direction within the apparatus 1 to the optimum focusing position with the entire image field on the surface 3 of the wafer 5 within the depth of focus A. The position of the optimum focusing position is determined and the offset distance between the optimum focusing position and the actual focusing position is recorded for the photoresist layer 4 of the third metal layer of the wafer 5 and is plotted against the pattern density of the first and second layers 37 and 38 beneath the reflecting area 29a. This procedure is then carried out for each of the wafers 5 which have been built up to the photoresist layer of the third metal layer and the offset distances and pattern densities of the first and second metal layers are plotted to give the graph 36 of Fig. 5. As mentioned above corresponding graphs 36 for the first, second and other metal layers, for example, fourth, fifth and sixth metai layers as well as other layers to be patterned for the photolithography apparatus are also prepared.

As new designs of wafers are being prepared the location of the reflecting area corresponding to each reticle is computed by the computer aided design system.

The corresponding pattern densities beneath the corresponding reflecting area in respect of each reticle is also computed by the computer aided design system.

Knowing the relevant pattern densities beneath the reflecting area for a reticle allows the appropriate offset correction to be read from the appropriate graph 36 for the metai or other layer to be patterned corresponding to the photoresist layer onto which an image of the reticle is to be projected. The offset distance value can then be supplied with the reticle. Prior to each production run the photolithography apparatus 1 is set by inputting the offset distance value for the particular reticle so that the stepper table 15 positions the wafer in the optimum focusing position corresponding to that reticle by positioning the stepper table 15 at the appropriate offset distance from the actual focusing position which is determined by the photolithography apparatus 1. .

Claims (2)

1. A method for improving focusing of an image of a mask or reticle onto a photo sensitive layer of a semiconductor wafer onto which the image is to be projected in a photolithography apparatus, the method comprising the steps of 5 deriving a relationship for the apparatus between the pattern density of the already patterned layer or layers beneath a reflecting area of the surface of the photo sensitive layer of wafers from which a focusing beam of the photolithography apparatus is to be reflected and an optimum focusing position for the wafers in the apparatus relative to an actual focusing position determined by the photolithography 10 apparatus from the reflected focusing beam for different pattern densities of the already patterned layer or layers beneath the reflecting surface, and using the derived relationship for subsequently positioning wafers in the apparatus in the appropriate optimum focusing position in a production process. 15 2. A method as claimed in Claim 1 in which the derived relationship is provided in the form of a graph. 3. A method as claimed in Claim 1 or 2 in which the derived relationship is provided as a relationship of the offset distance between the optimum focusing 20 position and the actual focusing position with the pattern density of the already patterned layer or layers. 4. A method as claimed in any preceding claim in which a plurality of derived relationships are derived in respect of a plurality of respective photo sensitive layers 25 for corresponding respective layers of wafers to be patterned for which the photolithography apparatus is adapted for projecting images thereonto. 5. A method as claimed in any preceding claim in which the derived relationship is derived by computing the pattern densities of each already patterned layer beneath the reflecting areas of the photo sensitive layers for each layer to be 5 patterned of a plurality of wafers of different pattern densities beneath the respective reflecting areas. 6. A method as claimed in Claim 5 in which the derived relationship is derived for the photo sensitive layer of each layer to be patterned by placing the plurality of 10 wafers in the photolithography apparatus as the processing of the wafers is completed to respective stages where the respective photo sensitive layers have been laid down for the respective layers to be patterned, and permitting the photolithography apparatus to determine the actual focusing positions for the photo sensitive layer of each layer to be patterned, and recording the actual focusing 15 positions, moving the wafers from the respective actual focusing positions to corresponding optimum focusing positions, and recording the offset distances between the optimum focusing positions and the corresponding actual focusing positions for the photo sensitive layer of each layer to be patterned of each wafer, and recording the respective offset distances against the corresponding pattern 20 densities beneath the reflecting areas for the corresponding layers to be patterned of the respective wafers. 7. A method as claimed in Claim 6 in which the optimum focusing position for the photo sensitive layer of each layer to be patterned of each wafer is determined 25 by observing the image focused on the surface of the photo sensitive layers corresponding to the respective layers to be patterned of each wafer. 8. A method as claimed in Claim 6 or 7 in which the images on the surfaces of the respective photo sensitive layers are observed through an electron microscope of the apparatus. 9. A method as claimed in any of Claims 6 to 8 in which the optimum focusing position for the photo sensitive layer of each layer to be patterned of each wafer is determined when the surface of the photo sensitive layer is within the depth of focus of optical apparatus of the photolithography apparatus. io 10. A method as claimed in any preceding claim in which the offset distance corresponding to each mask or reticle for use in the photolithography apparatus is determined from the derived relationship for the corresponding layer to be patterned by first computing the density of the already patterned layer or layers beneath the 15 reflecting area, and then reading the offset distance from the derived relationship. 11. A method as claimed in Claim 10 in which the derived relationship graph is derived from the recorded offset distances and the corresponding densities of the already patterned layers. 12. A method as claimed in any preceding claim in which the offset distance is provided with each mask or reticle for use in a subsequent production process in the apparatus. 25 13. A method as claimed in any preceding claim in which the density of the already patterned layer or layers beneath the reflecting area corresponding to each layer to be patterned are computed using a computer aided design system in which the patterns for the respective layers of each wafer are designed. 14. A method as claimed in any preceding claim in which before each production 5 run of a batch of similar wafers the photolithography apparatus is set to position each wafer of the batch of wafers at the appropriate optimum focusing position for the layer to be patterned based on the corresponding offset distance from the actual focusing position which would be determined by the apparatus. 10 15. A method for improving focusing of an image of a mask or reticle onto a photo sensitive layer of a semiconductor wafer onto which the image is to be projected in a photolithography apparatus, the method being substantially as described herein with reference to and as illustrated in the accompanying drawings. F.F. GORMAN & CO. 1/2

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