Abstract:
The resist to be used for the method of this invention in producing a semiconductor device is patterned by a procedure which comprises the steps of disposing in the direction of a semiconductor wafer a first mask having circuit patterns repeatedly formed at a plurality of positions, then shielding those of said plurality of circuit patterns which overlap the edge of the semiconductor wafer with a blind to an extent such that the remaining circuit patterns are not shielded, exposing a resist overlying the semiconductor wafer by using the first mask held in a state partially shielded by the blind, projecting light through a second mask provided with a light passing pattern defined by a shielding film to an area of the resist to which the edge of the blind is transferred, and developing the resist.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for the production of semiconductor devices. 
     2. Description of the Prior Art 
     The process of producing a semiconductor integrated circuit includes the operation of forming a film on a semiconductor wafer and patterning the film by photolithography at several steps. 
     For the purpose of forming a plurality of identical semiconductor integrated circuits on one semiconductor wafer, the photolithography adopts the method of step and repeat reduction-type projection printing. The projection printing method utilizes a reticule having a plurality of identical circuit patterns arrayed adjacently thereon, to effect simultaneous exposure of a resist to the plurality of circuit patterns. 
     The semiconductor wafer, after having these semiconductor integrated circuits formed thereon, is divided into individual chips, each bearing one of the semiconductor integrated circuits. 
     However, as illustrated in FIG. 1A, since a semiconductor wafer  101  has a circular shape and, one unit circuit pattern forming area on a reticule  102  has a rectangular shape, a certain proportion of the circuit patterns  103   a  to  103   d  on the reticule  102  used during the exposure inevitably protrude from the semiconductor wafer  101  during the exposure (the hatched area in the diagram). The circuit patterns  103   d  which are formed near the circumference of the semiconductor wafer  101  and contain a missing portion will be referred to hereinafter as “rejectable circuit patterns” or “rejectable chips.” 
     Also, as the puddle developing method, inwhich a liquid developer is piled on a semiconductor wafer, is generally adopted as the means to develop an exposed resist, the circumferential region of the semiconductor wafer is insufficiently developed. This inevitably gives rise to abnormally shaped resist patterns. 
     When the abnormally shaped resist patterns such as this exist near the circumference edge of the semiconductor wafer, films which are patterned using this resist pattern as a mask, are also abnormally shaped. Problems can occur, for example during the formation of storage electrodes on DRAM, when a hydrofluoric acid treatment is done to remove the silicon oxide film from beneath a polycrystalline silicon film, which forms the storage electrode. The polycrystalline silicon that is not connected to the substrate in the abnormally patterned area floats off and becomes attached as a particle in the normal pattern area, forming a factor causing reduced yields. 
     As a way of removing such abnormal patterns, a method of projecting light onto the whole area destined to produce rejectable chips after the exposure may be adopted as disclose in JP 07142309A. This method, however, is not as effective in preventing the occurrence of abnormal patterns as expected because it produces no change in the fact that the development is insufficient in the area of rejectable chips. 
     To cope with this problem, a method has been adopted of omitting the projection of the circuit patterns  103   a  to  103   d  of the reticule  105  of FIG. 1B at the positions where the circuit patterns overlap the circumference edge of the semiconductor wafer  101 , i.e. the position which correspond to the hatched area of FIG.  1 A. 
     However if the exposure to light is omitted from the area in which the circumference edge of the semiconductor wafer overlaps at least one of the circuit patterns on the reticule, a wasted area of a width which covers one to three circuit patterns arises near the circumference edge of the semiconductor wafer. This causes a problem of lowered yield. 
     The practice of forming only one circuit pattern on a reticule and using this reticule for the purpose of exposure to light is also an option. However, this causes a large increase in the number of shots of light involved during the treatment of exposure to light, and inevitably results in a decrease in the throughput. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide a method for the production of semiconductor devices, which incorporates therein a patterning step which is capable of repressing the occurrence of particles and also improves the throughput and the yield during the treatment of exposure to light. 
     In this invention, after a plurality of circuit patterns in a reticule (exposure mask) are projected onto a resist on a semiconductor wafer, with the exposure of the circuit patterns which overlap with the edge of the wafer being blocked by a blind, a separate mask is used to selectively expose the parts in the resist to which the edge of the blind was projected, and to remove these parts. 
     As a result, the exposure of the ineffective circuit patterns which overlap with the edge of the wafer is prevented, the formation of abnormal resist patterns which occurs around the circumference of a semiconductor wafer is avoided, and the occurrence of abnormal resist patterns which occur as a result of the blurred focus where the edge of the blinds are projected is also prevented. It is therefore possible to greatly suppress the occurrence of particles, which arise from these abnormal resist patterns. 
     Further, since only those of the plurality of circuit patterns formed in the reticule which do not overlap the edge of the semiconductor wafer are exposed to light, the number of available circuit patterns that are formed on the semiconductor wafer does not need to be reduced. As a result, the yield is improved because the number of available chips per semiconductor wafer is increased. 
     There is no reduction of throughput because the reticule to be used has a plurality of circuit patterns formed therein. 
     The reticule which is used for erasing the edge of a blind transferred to the resist may be formed in the same reticule that contains the circuit patterns, or may be formed as a separate entity. 
     When a plurality of circuit patterns are to be arranged repeatedly in a lengthwise and a crosswise direction in one reticule, this reticule is provided with lengthwise oblong or crosswise oblong light passing patterns for erasing the latent image or the edge of the blind in the resist. In this case, the aperture area of the lens can be effectively utilized by locating the light passing pattern in the area surrounding the circuit patterns area and near the center of the circuit patterns area. 
     Further, the lengthwise oblong light passing pattern is given a greater length than one side of a circuit pattern in the lengthwise direction, and the crosswise oblong light passing pattern given a greater length than another side of the circuit pattern in the crosswise direction. In this arrangement, it is possible to leave no space between the projections of lights on line of the latent image of the edge of the blind, when the part of the resist which was exposed at the edge of the blind is split into several parts for the exposure. 
     The available exposure area of the lens can be utilized to the utmost by positioning the light passing pattern close to the center of the circuit patterns area. The formulation of the program at the stage of designing the circuit patterns on the reticule is facilitated if the light passing pattern is symmetrical. 
     It becomes possible to obviate the necessity of changing reticules and avoid a decline in throughput arising from re-alignment, if the circuit patterns and the light passing patterns for the erasure of the latent image of the blind edge are formed on one and the same reticule. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1A depicts the results of the exposure to light by the conventional method of light exposure and FIG. 1B is a plan view schematically illustrating a reticule in popular use; 
     FIG. 2 is a structural diagram of a device for light exposure to be used in an embodiment of this invention; 
     FIG. 3 is a plan view illustrating one example of a blind in the device for light exposure shown in FIG. 2; 
     FIG. 4 is a plan view illustrating the first example of a reticule to be used in an embodiment of this invention; 
     FIG. 5A is a plan view illustrating the area on a semiconductor wafer which is exposed to light by the reticule shown in FIG. 4, and FIG. 5B is a cross section view of the semiconductor wafer; 
     FIG. 6A is a plan view illustrating the first example of the positional relationship between the reticule as shown in FIG. 4, and the blind shown in FIG. 3, and FIG. 6B is a plan view schematically illustrating the latent pattern image resulting from the exposure using a reticule shielded from light by a blind as shown in FIG. 6A; 
     FIG. 7A is a plan view illustrating the second example of the positional relationship between the reticule and the blind shown in FIG. 4, and FIG. 7B is a plan view schematically illustrating the latent pattern image resulting from exposure using a reticule shielded from light by a blind as shown in FIG. 7A; 
     FIG. 8A is a plan view illustrating the third example of the positional relationship between the reticule and the blind shown in FIG. 4, and FIG. 8B is a plan view schematically illustrating the latent pattern image resulting from exposure using a reticule shielded from light by a blind as shown in FIG. 8A; 
     FIG. 9A is a plan view illustrating the fourth example of the positional relationship between the reticule and the blind shown in FIG. 4, and FIG. 9B is a plan view schematically illustrating the latent pattern image resulting from exposure using a reticule shielded from light by a blind as shown in FIG. 9A; 
     FIG.  10 A and FIG. 10B are plan views respectively illustrating the second example and the third example of the reticule to be used in an embodiment of this invention; 
     FIG. 11 is a plan view illustrating the fourth example of the reticule to be used in an embodiment of this invention; 
     FIG. 12 is a plan view illustrating the fifth example of the reticule to be used in an embodiment of this invention; 
     FIG. 13 is a plan view illustrating the sixth example of the reticule to be used in an embodiment of this invention; and 
     FIG. 14 is a plan view illustrating the seventh example of the reticule to be used in an embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the preferred embodiments of this invention will be described below with reference to the accompanying drawings. 
     FIG. 2 illustrates one example of the exposure device to be used for the exposure of a resist in the production of a semiconductor device according to an embodiment of this invention. 
     In this exposure device, a blind  3 , a mirror  4 , an upper lens system  5 , and a projection reduction part  6  are positioned sequentially in the order mentioned, from a light source  1  toward a wafer stage  2 . A reticule (exposure mask)  7 , which will be specifically described herein below, is interposed between the upper lens system  5  and the projection reduction part  6 . The circuit patterns formed in the circuit pattern areas of the reticule are projected onto the resist R on the semiconductor wafer W. 
     The blind  3  is provided with a plurality of shielding plates  3   a  to  3   d  for example as illustrated in FIG. 3, and it is possible to form an opening part  3   e  of a necessary shape, by appropriately combining these shielding plates  3   a  to  3   d.  The size and the position of the opening part  3   e  can be varied by moving these shielding plates  3   a  to  3   d  individually in the lengthwise and the crosswise direction. 
     The reticule  7  is provided with a light-passing substrate (a quartz substrate, for example)  7   s,  which is having a polygonal shape, for example, as illustrated in FIG.  4 . On this light-passing substrate  7   s,  a first through a fourth rectangular circuit pattern area  7   a  to  7   d  are positioned adjacently. A rectangular assembly area  7   e  formed by the assemblage of these circuit pattern areas  7   a  to  7   d  is enclosed by a shielding frame  7   f,  made of chromium. Outside each of two pairs of adjoining sides in the four sides of the assembly area  7   e,  a first slit pattern (light passing pattern)  7   g  and a second slit pattern (light passing pattern)  7   h  are formed parallel to the sides mentioned above, and within the shielding frame  7   f.  The first slit pattern  7   g  is about 0.2 mm to 1.0 mm (specifically 0.5 mm, for example) longer than the side of the circuit pattern  7   a  lying parallel thereto and the second slit pattern  7   h  is about 0.2 mm to 1.0 mm (specifically 0.5 mm, for example) longer than the side of the circuit pattern  7   d  lying parallel thereto. The numerical value in this range of 0.2 mm to 1.0 mm may be equal to the width of the scribe line, for example. The scribe line is a line which is severed when the plurality of circuit patterns  7   a  to  7   d  are split into chips. 
     Although not specifically illustrated in the diagram, the patterns for, wiring, capacitor cell plates, or gate electrodes for the MOS transistor, for example, are formed inside the circuit pattern areas  7   a  to  7   d  of the reticule  7  using a chromium film, i.e. the same material as the shielding frame  7   f.    
     The size of the area of the reticule  7  is 150 mm×150 mm, for example, the size of the area of the circuit pattern areas  7   a  to  7   d  is about 120 mm×120 mm at most, and the gaps G 1  and G 2  of the first and the second slit pattern  7   a  and  7   h  are 2 mm each. The circuit patterns formed in the circuit pattern areas  7   a  to  7   d  are projected onto the resist R on the semiconductor wafer W, and reduced to ⅕ of their original size. The reticule  7  constructed as described above is interposed between the upper lens system  5  and the reduction projection part  6  of the exposure device and the circuit pattern areas  7   a  to  7   d  of the reticule  7  are repeatedly projected onto the resist R on the semiconductor wafer W as illustrated in FIG.  5 A. 
     The rectangular parts enclosed within the solid lines illustrated in FIG. 5A do not overlap the circumference of the semiconductor wafer W. These are the areas onto which all the circuit pattern areas  7   a  to  7   d  shown in FIG. 4 are simultaneously projected. 
     For the sake of the exposure to these areas, as shown in FIG. 6A, the shape and the position of the opening part  3   e  of the blind  3  are set such that the light passing the blind  3  is allowed to pass exclusively through all the circuit pattern areas  7   a  to  7   d  of the reticule  7 . As a result, the latent image of the pattern described on the resist R by one projection of light forms a plan view as is illustrated in FIG. 6B, with the result that the latent images of the plurality of circuit patterns  10  will be formed simultaneously in an adjoining manner. The plurality of circuit patterns  10  shown in FIG. 6B are repeatedly projected onto the resist R by the motion of the wafer stage  2  in either or both of the X and Y directions. 
     In contrast, the circuit patterns in the area to which not all of the circuit patterns on the reticule  7  is transferred, namely the hatched area shown in FIG. 5A, are projected as follows. 
     First, when the circuit pattern areas  7   a,    7   b,  and  7   d,  i.e. some of the circuit pattern areas on the reticule  7 , would overlap with the circumference of the semiconductor wafer W during the course of exposure, the opening part  3   e  or the blind  3  is reshaped by adjusting the positions of the shielding plates  3   a  to  3   d  of the blind  3  as illustrated in FIG.  7 A. The position, size, and shape of the opening formed are such that the light is only projected onto the circuit pattern area  7   c,  which does not overlap the edge of the semiconductor wafer W, and peripheral area thereof. In this case, the opening part  3   e  essentially blocks the passing of light through the other circuit pattern areas  7   a,    7   b,  and  7   d.    
     Therefore, only available circuit patterns, with no missing part, are exposed on the resist R. 
     Consequently, the latent images of circuit patterns  11  which are formed on the resist R assume a profile as shown in FIG.  7 B. Along the sides of the circuit pattern area  7   c  which are not defined by the shielding frame  7   f,  the edges of the circuit pattern areas  7   a,    7   b,  and  7   d  are projected in part, as severed by the blind  3 . The circuit patterns which are consequently exposed only partially to light are referred to as rejectable circuit patterns  12 . 
     On the resist R which has been exposed to the part severed by the blind  3 , a blur of the focal point occurs in the part to which the edge of the blind  3  is projected. In other words, when the circuit patterns  7   a,    7   b,  and  7   d,  i.e. the ineffective circuit patterns on the reticule  7 , are shielded by the blind  3 , the relevant edge part of the blind  3  is projected onto the resist R. Since the edge part of the blind  3  is not focussed on the resist R, it produces a blur of the focal point and forms a cause for the occurrence of abnormal patterns and consequently for the occurrence of the particles which were described in the paragraphs introducing the prior art. Therefore, the part of the resist with the blurred focal point is removed by projecting light through the first and the second slit patterns,  7   g  and  7   h,  onto the area in which the edge part of the blind  3  appears. The edge of the blind  3  under discussion will occasionally be referred to hereinafter as “blind edge.” 
     The following additional exposure or double exposure is carried out to remove the rejectable circuit pattern  12  from the area surrounding the normally exposed circuit pattern  11 . 
     By moving each the shielding plates  3   a  to  3   d  of the blind  3 , the opening part  3   e  of the blind  3  is formed in a position, size, and shape such that the light is projected only through the second slit pattern  7   h  on the reticule  7 , as shown in FIG.  8 A. The positional correspondence between the opening part  3   e  and the rejectable circuit pattern  12  is attained by moving the wafer stage  2 . 
     Then, in the exposure device, the light which has passed through the second slit pattern  7   h  is projected onto the part of rejectable circuit pattern  12  which adjoins the available circuit pattern  11  in the X direction. Consequently, the latent pattern image as shown in FIG. 7B is formed by completely erasing the X direction part of the rejectable circuit pattern  12  shown in FIG. 8B, to which the blind edge is transferred. 
     As a result, the edge of the available circuit pattern  11  on the X direction side is sharpened as illustrated in FIG. 8B and, at the same time, the edge of the blind existing on the X direction side is erased and a latent image  13  of a fine light exposure pattern is formed. 
     Subsequently, by moving the shielding plates  3   a  to  3   d  of the blind  3  to the position illustrated in FIG. 9A, the opening part  3   e  of the blind  3  is formed in a position, size, and shape such that the light is projected only through the first slit pattern  7   g  on the reticule  7 . The positional correspondence between the opening part  3   e  and the rejectable circuit pattern  12  is attained by moving the wafer stage  2 . 
     Then, the light which passes through the first slit pattern  7   g  is projected onto the part adjoining the available circuit pattern in the Y direction, to which the edge of the blind  3  is transferred. Consequently, a latent pattern image as illustrated in FIG. 9B is formed by completely erasing the blind edge which exists on the Y direction side of the available circuit pattern  11 . As a result, the edge of the available circuit pattern  11  on the Y direction side is sharpened and, at the same time, the transferred part of the edge of the blind  3  in the Y direction is erased and a latent image  14  of a fine light exposure pattern is formed. 
     Since the first and second slit patterns  7   g  and  7   h,  are both given a slightly greater length than the parallel sides of the circuit patterns  7   a  to  7   d  on the reticule  7 , the edge parts of the first and the second slit pattern  7   g  and  7   h  inevitably overlap each other when these slit patterns  7   g  and  7   h  are projected onto the resist R. 
     The exposure treatment described above is based on the fact that an abnormal pattern due to a blur of the focal point occurs in the part to which the edge of the blind  3  is projected when some of the plurality of circuit patterns  7   a  to  7   d  on the reticule  7  are covered by the blind  3 . This fact is not disclosed in JP-A-07-142,309. 
     The latent circuit pattern images as are shown defined by solid lines and broken lines in FIG. 5A, are formed after completion of the step of repeatedly exposing the circuit pattern onto the resist using the reticule and the step of erasing the transferred part of the blind edge by the exposure to light. Consequently, circuit patterns which would overlap with the edge of the semiconductor wafer W are not exposed to light. Therefore, when the resist R is subjected to puddle development after the exposure described above, the resist R remains attached in the area lying along the edge of the semiconductor wafer W and, at the same time, the edges of the available circuit patterns formed near the edge thereof appear in a shape unblurred shape. 
     After the development process such as is described above, is completed, a plurality of semiconductor integrated circuits are formed by a step of patterning a film  20  beneath a pattern R 0 , of the resist R, and by other steps which follow as shown in FIG.  5 B. 
     Next, other examples of the reticule will be describer below. 
     The number of circuit patterns to be formed in the reticule is not limited to four as illustrated in FIG.  4 . The double exposure (peripheral exposure) by the slit pattern described above can be utilized wherever the number of circuit patterns is two or more, as illustrated in FIG. 10A to FIG.  14 . 
     First, when the reticule to be used has a plurality of circuit pattern areas  21  arrayed in series in just one direction, as illustrated in FIG.  10 A and FIG. 10B, it suffices to provide within the shielding frame  22 , at least one slit pattern  23  parallel to the boundary of the adjoining circuit pattern areas  21 . This is because the edge of the blind  3  is projected near the boundary of the adjoining circuit pattern areas  21 . 
     When the reticule to be adopted is so constructed as to partition two circuit pattern areas  24  with a partition band  25  as illustrated in FIG. 11, a U-shaped auxiliary exposure pattern  26  may be provided instead of the slit pattern. In this case, all the edges of the four sides of the available circuit pattern area  24  are sharpened because the partition band  25  and the shielding frame  27  are transferred with a just focus. 
     The rejectable circuit pattern which is partially exposed, therefore, has the blind edge thereof enclosed within the transferred part of the partition band  25  and the U-shaped auxiliary pattern  26 , as indicated by a broken line in FIG.  11 . Light is therefore projected onto this enclosed area. 
     Incidentally, when a plurality of circuit patterns are positioned repeatedly in just one direction on the reticule, the light passing parts of the slit patterns and the U-shaped auxiliary patterns read only be of the same length as the bordering sides of the circuit patterns. The blind edge can be erased by projecting the light through these light passing parts. 
     When the reticule to be adopted has a plurality of circuit patterns  30  formed in both the X and the Y direction as shown in FIG. 12 to FIG. 14, it becomes necessary to form within the shielding frame  33 , at least two slit patterns,  31  and  32 , which are respectively parallel to the X direction and the Y direction. These slit patterns  31  and  32  are preferably positioned close to the center of the assembly area  34  for assembling the circuit patterns  30 . This is because, the available exposure area on the stepper  2 , the exposure device shown in FIG. 2, has a circular shape, for example, of about 22 mm in diameter. By positioning the slit patterns  31  and  32  to the center and with a narrow gap as shown in FIG. 13, therefore, the circuit patterns  30  can be position with the largest possible size in the available exposure area  35 . 
     The preferred embodiment, as depicted above, has the slits and U-shaped auxiliary patterns for projecting the light onto the part of the resist to which the blind edge is transferred on the same reticle as the circuit patterns. However, they may be formed on different reticules. Also, in the method described above, the erasure of the blind edge was done after the exposure of the circuit pattern, but the order of these operations may be reversed. 
     Furthermore, it is preferable that both the slit patterns and the circuit patterns are only projected in positions where they do not protrude from the semiconductor wafer. 
     It is possible that a length of the light passing pattern has a length equal to the sum of the length of one side of each of the circuit patterns. 
     In this invention, as described above, after a plurality of circuit patterns in a reticule are projected onto a resist on a semiconductor wafer, with the exposure of the circuit patterns which overlap with the edge of the wafer being blocked by a blind, a separate mask is used to selectively expose the parts on the wafer to which the edge of the blind was projected, and to remove these parts. 
     As a result, the exposure of the ineffective circuit patterns which overlap with the edge of the wafer is prevented, the formation of abnormal resist patterns which occurs around the circumference of a semiconductor wafer is avoided, and the occurrence of abnormal resist patterns which occur as a result of the blurred focus where the edge of the blinds are projected is also prevented. It is therefore possible to greatly suppress the occurrence of particles, which arise from these abnormal resist patterns. 
     Further, since only those of the plurality of circuit patterns formed in the reticule which do not overlap with the edge of the semiconductor wafer are exposed to light, the number of available circuit patterns that are formed on the semiconductor wafer does not need to be reduced, making it possible to increase the yield. 
     There is no reduction in throughput because the reticule to be used has a plurality of circuit patterns formed therein. 
     The necessity for changing reticules is obviated and the decline of the throughput due to re-positioning is avoided because the circuit patterns and the patterns to be used for the erasure of the blind edge are formed on one and the same reticule.