METHOD OF FABRICATING A PHOTOMASK PATTERN

Abstract: A method of fabricating a photomask pattern (210), hav­ ing a matrix array of unit patterns (201) on a photomask (200), involves two exposure modes. In a first exposure mode select­ ed matrix locations (211 to 214) are exposed to an optical image for forming a unit pattern. In a second exposure mode the other matrix locations are exposed to the optical image in a regular ordered sequence by step-and-repeat printing (D, E, F, G, H). Each unit pattern has a vernier pattern. Vernier pat­ terns of adjacent unit patterns overlap (230) to provide for measurement of shear. The employment of the two exposure modes allows accumulated printing shear to be measured from overlapping verniers patterns provided at locations (e, f, g, h) where unit patterns formed in the first and second modes re­ spectively are adjacent.

Description: The present invention relates to a method of fabricating a photomask pattern for example for use in the fabrication of a semiconductor device such as an integrated circuit (IC) or a large scale integrated circuit (LSI). A photomask is made of a base plate of silica glass on which a metal layer and a photoresist layer are sputtered and coated as shown in Fig. 1. Fig. 1 gives a schematic cross-sectional view of a photomask 200: a metal layer 2 of chromium is sputtered on a glass plate 1 and photoresist layer 3 is coated on the metal layer 2. A photomask pattern is fabricated by processes involving exposing the photomask to an optical image, developing and etching the photomask. Fig. 2 gives a schematic plan view of a photomask 200 on which a photomask pattern 210 has been printed. The photomask pattern 210 consists of plurality of unit patterns 201 separated by scribe lines 220. Each unit pattern 201 is of the same shape and size, and corresponds for example to a pattern for a semiconductor die: in this case the photomask pattern 210 is for mass production of semiconductor devices. Fig. 3 schematically illustrates a printing process for forming a photomask pattern. Each unit pattern 201 is printed by exposing the photomask 200 to an optical image of a reticle pattern 101 on a reticle 100 by means of an optical system 400. The reticle pattern 101 is a mother pattern for example for a semiconductor die and the size of the reticle pattern is as large as 10 times of that of an exposed unit pattern 201 on the photomask, to provide for high accuracy printing. Thus, the optical system 400 has a lens 401 having a reduction factor of one tenth. 402 is a light source of the optical system. The unit patterns 201 are printed in turn, one by one, shifting the photomask 200 in X and Y directions with respect to fixed optical system 400. This is called "step-and-repeat printing". Apparatus for step-and-repeat printing will be explained in more detail with reference to Fig. 4. Fig. 4 is a schematic block diagram of an example of a step-and-repeat printing apparatus. In Fig. 4, a photomask 200 is mounted on a stage 500 which is shifted in X and Y directions under control of a stage controller 800. The sequence of the step-and-repeat printing (the sequence in which unit patterns are printed on the photomask) is determined in advance and stored in a memory 600. Data in the memory 600 is read out and amplified by an amplifier 700 and goes to the stage controller 800. The data fed to the stage controller 800 controls stage movement and also controls a light controller 900 which controls on-off switching of the light source 402 of optical system 400 as shown in Fig. 3 in synchronism with the step-and-repeat movement of the stage 500. Fig. 5 is a schematic diagram illustrating the sequence of step-and-repeat printing as previously employed, showing an arrangment of unit patterns in a matrix having rows and columns on a photomask pattern area of a photomask. The step-and-repeat printing is performed (successive exposures of the photomask to the reticle pattern, to form successive unit patterns, are performed) so that the exposure of unit patterns advances from a leftmost matrix element 11 to matrix elements 12, 13, ... sequentially in turn in a rightwards direction in Fig. 5 along a first row of the matrix, as shown by arrow A in Fig. 5. When exposure of unit patterns in the first row is finished, exposure is shifted down and proceeds to a rightmost matrix element 16 in the second row of the matrix and then advances leftwards as shown by arrow B. Thus, exposure of unit patterns advances through the matrix in a zigzag pattern as shown by arrows A, B, C,... with a succession of step-and-repeat exposures effected with equal pitches "P" in row and column directions. Semiconductor circuitry tends to be complicated, and a plurality of photomasks are required to fabricate a semiconductor device (IC) from a wafer. The position of a unit pattern on one photomask must properly coincide with the position of a unit pattern on another photomask when they are overlapped to make up the pattern of the IC device. Therefore, the unit patterns on a photomask, especially the positions of the unit patterns, must be carefully inspected after printing of the photomask, to ensure that the photomask is satisfactory for use in semiconductor device (IC) fabrication. The inspection is performed by observing through a microscope vernier patterns printed together with each unit pattern on the photomask. Fig. 6 shows an example of a vernier pattern, a cross-shaped vernier pattern, which has four branches-top, bottom, left and right. Each branch is like a comb in form. The numbers of teeth in top and bottom branches differ by one, and the numbers of teeth in left and right branches differ by one. This difference of one tooth between branches provides for vernier measurement when branches with different tooth numbers are combined. In the reticle pattern used to form unit patterns mother patterns for vernier patterns are provided at its four corners so that the numbers of teeth at top and bottom and left and right branches become opposite in cross-shaped vernier patterns placed diagonally to each other. For example, in two vernier patterns at diagonally opposite corners of a unit pattern, the numbers of teeth in opposite branches of one vernier pattern are transposed with respect to the numbers of teeth in the corresponding opposite branches of the other vernier pattern. Vernier patterns of adjacent unit patterns formed on a photomask overlap each other when they are printed on the photomask. Fig. 7 schematically illustrates this situation. For example, a pattern 230 is formed by overlapping vernier patterns of unit patterns 202, 203, 204, and 205. This overlapping of vernier patterns allows shear of unit patterns which are printed adjacent to each other to be measured by observing pattern 230. (That is, displacement or departure from regular printing pitch, hence incorrect unit pattern printing, e.g. incorrect unit pattern positioning, can be measured.) This kind of pattern 230 will be called an "overlapped vernier pattern" hereinafter. As there can be as many as several thousand unit patterns on a photomask in some cases, it is in practice impossible to measure shear for all the overlapped vernier patterns. However, it is well known in the art of inspection that inspection only at several points on a photomask is sufficient. For example, inspection of four points each placed near a middle part of a respective one of the four sides of the photomask pattern as shown by points a, b, c, and d in Fig. 5 is sufficient inspection of positions of unit patterns on the photomask pattern. Thus, in the previous method of fabricating a photomask pattern, step-and-repeat printing as explained above is effected to print all the unit patterns on the photomask, and vernier patterns are inspected at chosen locations on the photomask. According to the present invention there is provided a method of fabricating a photomask pattern, comprising an array of unit patterns, on a photomask pattern area of a photomask plate, each unit pattern having a vernier pattern such that when the photomask pattern is fabricated vernier patterns of respective adjacent unit patterns register with one another so as to provide for vernier measurement of shear, comprising 2 exposure modes where in one exposure mode, exposing at least one selected array location to an optical image for forming a unit pattern, and in another exposure mode, exposing the other array locations one by one in turn to the optical image for forming the unit pattern, in a regular ordered sequence, such that registering vernier patterns of adjacent unit patterns respectively provided by exposures in the one and other exposure modes provide for measurement of accumulated shear which occurs, primarily in the regular ordered sequence of exposures of the other exposure mode. The present invention overcomes problems which have been determined to occur with the previous method of fabricating a photomask pattern as explained above. First, to enable a better understanding of the present invention, problems determined to occur with the previous method of photomask pattern fabrication as described with reference to Figs. 1 to 7 will be explained. 1. Printing shear accumulates as unit pattern exposures are advanced in steps along a row of unit patterns, for example as shown by the arrows A, B, or C in Fig. 5. 2. The shear accumulation arises from electrical and mechanical causes which depend on features of the printing apparatus used, and at unpredictable intervals the shear may accumulate to unacceptibly high values as illustrated in Fig. 8. Fig. 8 is a graph in which the abscissa t represents time at which a photomask pattern exposure is carried out, and the ordinate s represent a maximum amount of shear measured for an exposure at the relevent time. Absolute values of shear are represented by s, obtained by measuring the shear in a photomask pattern. Broken line R indicates an allowable maximum value of shear (displacement) from ideal positioning of a unit pattern. The curve M shows actually occurring amounts of shear. In the Figure, t₁, t₂ ..., t₉ show points in time. If it is assumed that the exposure of the whole unit patterns (if it is assumed that exposure of all the unit patterns on a photomask pattern) is done in a time interval from t₁ to t₂, or from t₂ to t₃, ..., or from t₈ to t₉, exposures performed in the intervals from t₁ to t₂, t₂ to t₃, t₆ to t₇, t₇ to t₈ and t₈ to t₉ (in relation to which respective indications "0" are given) result in good printing (shear less than allowable maximum), but exposures performed in the time intervals from t₃ to t₄, t₄ to t₅ or t₅ to t₆ (in relation to which respective indications "x" are given) result in bad printing (shear more than allowable maximum). The time interval required for the exposure (for the exposure of all unit patterns on a photomask pattern) is several tens of minutes, and time intervals in which shear begins to exceed the allowable maximum amount R occur with a frequency of several house. 3. It is impossible to detect such printing shear which exceeds the allowable maximum R by observing and measuring vernier patterns with the previous method of photomask pattern fabrication, because each unit pattern has an amount of accumulated shear which will be almost equal to the amounts of accumulated shear suffered by adjacent unit patterns. Examination of overlapped vernier patterns (formed by the vernier patterns of adjacent unit patterns) will reveal only relative shear of the adjacent unit patterns, which will normally be small. Zero deviation (deviation from zero shear) of the vernier does not show absolute zero deviation. 4. Absolute amounts of shear can be measured using a laser measuring system. However it takes a great deal of time to carry out such measurements, for example time is needed for optical setting of the system. Therefore, whilst laser measurement is used in the laboratory it is not practical for use in day-to-day manufacture. Vernier pattern measurement is still used in actual fabrication of photomask patterns. An embodiment of the present invention provides a method of fabricating a photomask pattern having a plurality of unit patterns, corresponding for example to semiconductor dies, such that accumulated printing shear, which occurs in the course of step-and-repeat exposures in the process for printing the unit patterns, and which has been impossible to detect by previous methods suitable for actual manufacture (rather than merely laboratory methods), can be measured using vernier patterns belonging to the unit patterns. Briefly, in an embodiment of the present invention, two exposure modes are used in the printing process for printing the unit patterns on a photomask. First, in a process for printing unit patterns in a matrix having rows and columns (each unit pattern corresponding to a matrix element), some selected matrix elements placed as far as possible from one another in the photomask pattern area are exposed to the optical image of a unit pattern (with associated vernier patterns). Second, after the above exposures, exposures of the optical image are carried out in a step-and-repeat fashion along each row. That is, exposures to form unit patterns are advanced one by one continuously through the matrix elements along a row of a matrix, then proceed to a next row and so on in a zigzag pattern row by row from end to end of the photomask pattern area. With exposures carried out in first or second stages or modes as set forth above, the difference between the shear of a unit pattern exposed in the first stage and that of a neighboring unit pattern exposed in the second stage appears very largely because of shear accumulation during the second exposure stage. Thus, employing an embodiment of the present invention for photomask pattern fabrication, accumulated printing shear becomes detectable using the vernier patterns. As semiconductor devices become more complicated, many photomask are required to fabricate such semiconductor devices on a wafer. By applying an embodiment of the present invention to the fabrication of each photomask, inspection of each photomask more certainly enables the suitability of the photomask to be determined, which contributes greatly to reductions in time required for semiconductor device fabrication and in the cost of semiconductor devices. An embodiment of the present invention also facilitates increases in reliability of semiconductor devices. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a schematic cross-sectional view of a photomask, Fig. 2 is schematic plan view of a photomask with unit patterns thereon, Fig. 3 is a schematic perspective diagram illustrating fabrication of a photomask pattern on a photomask, Fig. 4 is a schematic block diagram of apparatus for photomask pattern fabrication illustrating elements controlling devices shown in Fig. 3, Fig. 5 is a schematic diagram illustrating the sequence of printing of unit patterns in a matrix in a photomask pattern area of a photomask as previously effected, Fig. 6 schematically illustrates a cross-shaped vernier pattern used for measurement of shear of unit patterns in a photomask pattern, Fig. 7 is a schematic plan view diagram showing unit patterns printed in a photomask pattern area of a photomask with overlapped vernier patterns, Fig. 8 is a graph illustrating time variation of accumulated shear as occurs with apparatus for photomask printing, and Fig. 9 is a schematic diagram for explanation of sequence of printing of unit patterns in a matrix in a photomask pattern area of a photomask in accordance with the present invention. Fig. 9 schematically illustrates a photomask pattern to which a method of fabrication embodying the present invention is applied. The Figure illustrates a matrix corresponding to the arrangement of a plurality of unit patterns on a photomask pattern area of a photomask 200. Parts in the Figure divided by horizontal and vertical lines, having a checked pattern, are matrix elements corresponding to respective unit patterns. Each matrix element is to be exposed to an optical image for forming a unit pattern including four vernier patterns placed at respective corners of the unit pattern. Exposures are effected in the following two exposure stages or modes: first, exposures are respectively carried out on selected matrix elements 211, 212, 213, and 214, located at middle parts of respective ones of the four side edges of the area to be occupied by a photomask pattern 210 (the photomask pattern area); These exposures will be called the "first exposure mode" hereinafter; and second, exposures are carried out for other matrix elements by step-and-repeat exposures along successive rows of the matrix, row by row in a zigzag pattern as shown by arrows D, E, F, G, and H Fig. 9 (matrix elements exposed in the first exposure mode are not exposed in the second exposure mode); These exposures will be called the "second exposure mode" hereinafter. The first an second exposure modes are effected using printing apparatus as shown in Fig. 4 for example using a suitable program for controlling the exposure modes previously stored in the memory 600. After printing, the positions of the unit patterns in the photomask pattern 210 are inspected by observing overlapped vernier patterns at selected points as shown by points such as e, f, g, and h in Fig. 9. Overlapped vernier patterns at these points are formed by the overlap of unit patterns formed in different exposure modes, i.e. in the first and second exposure modes, and accumulated shear can be certainly detected. This is because the vernier patterns printed in the first exposure mode are printed in quite a different time slot from vernier patterns printed in the second exposure mode. That is, in the first exposure mode, there is (substantially) no accumulated shear in the selected unit patterns 211, 212, 213 and 214. On the other hand, in the second exposure mode, shear accumulates in the course of the step-and-repeat procedure used. The exposures effected in the first exposure mode need not always be made at middle parts of the side edges of the photomask pattern area. They can be effected at locations spaced far apart, e.g. as far as possible apart, from one another in the photomask pattern area. To make the detection of accumulated shear in the photomask pattern possible, it is only required that overlapped vernier patterns should be provided by vernier patterns printed in different exposure modes. The first and second exposure modes can be carried out in reverse order (i.e. second mode before first mode). If unit pattern exposures were effected in a random order instead of in an ordered step-and-repeat sequence along rows, accumulated shear could be detected and there would be no need to provide two exposure modes as explained above. However, random exposure is not practically applicable because it would require a great deal of complicated work and time to provide a suitable exposure control program, and further, great care would have to be taken to avoid occurrence of soft errors on photomasks. This would cause an increase in fabrication cost and would reduce reliability of photomask fabrication. Consequently, the use of two exposure modes as explained above, utilizing step-and-repeat exposures along rows in the second exposure mode is more practical for photomask fabrication. An embodiment of the present invention provides a method for fabricating unit patterns each having the same shape and size, and each corresponding to a semiconductor die for example, on a photomask pattern area of a photomask. Designating a matrix having rows and columns for the unit patterns on the photomask pattern area, fabrication is effected with two exposure mode. In a first exposure mode, several selected matrix elements placed as far as possible from one another in the photomask pattern area are exposed to a unit pattern optical image. In the second exposure mode, matrix elements (except the elements exposed to be exposed in the first exposure mode) are exposed to the unit pattern optical image in turn matrix element row by matrix element row. The unit pattern includes vernier patterns which produce overlapped vernier patterns when the exposures of the first and the second exposure modes are effected, so, after all unit patterns have been printed on the photomask base plate, an inspection of positions of the unit patterns can be made by observing some of the overlapped vernier patterns to measure accumulated shear produced in the second exposure mode.

Claims: A method of fabricating a photomask pattern, comprising an array of unit patterns, on a photomask pattern area of a photomask plate, each unit pattern having a vernier pattern such that when the photomask pattern is fabricated vernier patterns of respective adjacent unit patterns register with one another so as to provide for vernier measurement of shear, comprising two exposure modes where in one exposure mode, exposing at least one selected array location to an optical image for forming a unit pattern, and in another exposure mode, exposing the other array locations one by one in turn to the optical image for forming the unit pattern, in a regular ordered sequence, such that registering vernier patterns of adjacent unit patterns respectively provided by exposures in the one and other exposure modes provide for measurement of accumulated shear which occurs, primarily in the regular ordered sequence of exposures of the other exposure mode. A method as claimed in claim 1, wherein, in the one exposure mode, a plurality of selected array locations, spaced apart from one another, are exposed to the said optical image. A method as claimed in claim 2, wherein the selected array locations are spaced as far as possible apart from one another within the photomask pattern area. A method as claimed in any preceding claim, wherein the array is a matrix array, with matrix elements, in rows and columns, corresponding to array locations, and wherein the said regular ordered sequence involves step-and-repeat shifting, element by element through each row of matrix elements, row by row. A method as claimed in claim 4, wherein the step-and-repeat goes from row to adjacent row in a zigzag pattern. A method as claimed in claim 4 or claim 5, wherein the selected array locations correspond to four positions on the matrix array, placed at the middle parts of the respective four side edges of the matrix array. A method as claimed in any preceding claim, wherein the said one exposure mode is carried out before the other exposure mode. A method as claimed in any preceding claim, wherein each unit pattern has four cross-shaped vernier patterns placed respectively at each of the four corners of the unit pattern.

Inventors: ARAIHARA, SATOSHI C/O FUJITSU LIMITED
Applicants: FUJITSU LIMITED
Application year: 1984
Main classification: G03B41
IPC codes: H01L21, G03F9, G03F1, G03F7