Pattern exposing method using phase shift and mask used therefore

Abstract: A pattern exposing method for forming a predetermined resist pattern (42) on a substrate (5). A resist layer (39) is exposed which is formed on the substrate (5) using a reticle (6) which includes a pattern for exposing a corresponding pattern on the resist layer by use of a phase shift of light transmitted through the reticle, the pattern of the reticle being defined by a phase shift layer (7) formed on a transparent reticle surface (2) and having first and second phase shift parts (7, 7A). The first phase shift part (7) has a width such that a closed ring pattern is exposed by the edge parts thereof, and the second phase shift part (7A) has a width narrower than that of the first phase shift part (7) so that patterns exposed by the edge parts thereof overlap in the form of a single line pattern. Then, the resist layer (39) is developed so that the predetermined resist pattern (42) is formed on the substrate (5).

Description: The present invention generally relates to pattern exposing methods and masks used therefore, and more particularly to a pattern exposing method which forms a resist pattern for the purpose of producing a semiconductor device and to a mask which is used for such a pattern exposing method. Recently, the operation speed and integration density of semiconductor devices have increased considerably, and the size of the semiconductor devices has been reduced accordingly. For this reason, there are demands to form a fine pattern exceeding a resolution limit of the conventional patterning apparatus using an optical system, that is, the resolution limit of an imaging optical system of a wafer stepper, for example. EP-A-0 395 425 discloses a mask comprising a transparent layer which is transparent with respect to a light which is used for an exposure, and a mask pattern layer which is formed on the transparent layer. At least a portion of the mask pattern layer is made up solely of a phase shift layer for transmitting the light, so that a phase shift 'occurs between a phase of the light transmitted through the phase shift layer and a phase of the light transmitted through a portion of the mask having no phase shift layer. A mask producing method as well as a pattern forming method using said mask are disclosed likewise. First, a description will be given of the general operation of a conventional demagnification projection exposure apparatus, by referring to Fig. 1. A light from a light source such as a mercury lamp is irradiated on a reticle 1. The reticle 1 has a reticle pattern in which a chromium (Cr) light blocking layer 3 is formed on a glass substrate 2, so as to expose a hole having a rectangular shape corresponding to the rectangular shape of the light blocking layer 3. The reticle pattern is reduced by a demagnification projection lens 4 and is imaged on a wafer 5 so as to expose a positive resist on the wafer 5. Fig. 2 is a diagram for explaining the light intensity of the exposure light on the positive resist. Fig. 2 (a) shows a partial cross section of the reticle 1. Fig. 2 (b) shows the light amplitude distribution on the positive resist of the wafer 5 for the case where the exposure is made using the reticle 1. Fig. 2 (c) shows the light intensity distribution on the positive resist of the wafer 5 for the case where the exposure is made using the reticle 1. When the exposure is made by the light transmitted through the reticle 1 having the rectangular light blocking layer 3, the light intensity distribution on the positive resist of the wafer 5 has a negative peak having a relatively gradual slope. Hence, it is impossible to form a fine pattern which has a narrow width using such a light intensity distribution. In order to form a fine pattern which has a narrow width, it is necessary to make the slope of the negative peak sharper than that of the light intensity distribution shown in Fig. 2 (c). For example, if the exposure light is an i-line having a wavelength of 0.365 µ m and the numerical aperture of the optical system is 0.50, the resolution limit is approximately 0.4 µ m. The slope of the negative peak of the light intensity distribution shown in Fig. 2 (c) is dependent on the resolution of the imaging optical system. The resolution of the imaging optical system is determined by the exposure wave length, the numerical aperture, the inconsistent performance of each individual itself and the like. Accordingly, when the conventional demagnification projection exposure apparatus is used, it is impossible to form a fine hole which exceeds the resolution limit of the imaging optical system, and there is a problem in that the conventional demagnification projection exposure apparatus cannot cope with the patterns of the semiconductor devices which are becoming finer as the integration density improved. In order to eliminate the above described problem, the so-called phase shift method has been proposed. According to the phase shift method, the phase of the light transmitted through the reticle is shifted by a phase shift layer, so as to improve the resolution and contrast of the exposed image on the resist. Fig. 3 shows the imaging optical system of the demagnification projection exposure apparatus and the reticle used when carrying out the phase shift method. In Fig. 3, those parts which are the same as those corresponding parts in Fig. 1 are designated by the same reference numerals, and a description thereof will be omitted. In Fig. 3, a phase shift reticle 6 is used in place of the reticle 1 shown in Fig. 1. The phase shift reticle 6 is made up of the glass substrate 2 and a phase shift layer 7 which is formed on the glass substrate 2. fig. 4 is a diagram for explaining the light intensity of the exposure light on the positive resist when the phase shift reticle 6 is used. Fig. 4 (a) shows a partial cross section of the phase shift reticle 6. The pattern which is to be exposed is formed by an edge part of the phase shift layers 7 which is formed on the glass substrate 2 of the phase shift reticle 6. A transmitted light 8' which has been passed through the glass substrate 2 and the phase shift layer 7 has a phase which is shifted by 180° (π) with respect to a transmitted light 8, which has passed through only the glass substrate 2. Fig. 4 (b) shows the light amplitude distribution on the resist of the wafer 5 when the exposure is made using the phase shift reticle 6. As shown, the light amplitude becomes zero at a position of the resist corresponding to the edge part of the phase shift layer 7, and the light amplitude sharply reverses on both sides of the edge part. Fig. 4 (c) shows the. light intensity distribution on the resist of the wafer 5 when the exposure is made using the phase shift reticle 6. Because the light intensity is proportional to the square of the light amplitude, the light intensity sharply becomes zero at a position of the resist corresponding to the edge part of phase shift layer 7. Accordingly, it is possible to form on the resist a fine line-and-space pattern which has satisfactory resolution and contrast. An example of a fine pattern formed by the phase shift method will be described with reference to Fig. 5. Fig. 5 (a) shows a plane view of the phase shift reticle 6 which has a square phase shift layer 7. Fig. 5 (b) shows a fine resist pattern 10 which is formed by the edge part of the phase shift layer 7 of the phase shift reticle 6 shown in Fig. 5 (a). As shown in Fig. 5 (b), the fine resist 10 is formed at the side part of the square, that is, along a part on the resist corresponding to the edge part of the phase shift layer 7. On the other hand, the phase shift layer 7 may be arranged in a checker board pattern in the plane view as shown in Fig. 6 (a). When the phase shift reticle 6 having the phase shift layer 7 shown in Fig. 6 (a) is used for the exposure, it is possible to form a resist pattern 10 which has edges with a fine contrast over a relatively large area as shown in Fig. 6 (b). However, when the phase shift reticle 6 described above is used for the exposure, it is impossible to pattern a cut part indicated by a dotted line in Fig. 5 (b). In other words, the edge part of the phase shift layer 7 inevitably takes a closed contour (or loop), and there is a problem in that the actual patterns of integrated circuits (ICs) cannot be formed using the phase shift reticle 6. Accordingly, in order to overcome this problem of the phase shift method, a modified method has been previously proposed to form the resist pattern using a phase shift reticle which has to kinds of phase shift layers as shown in Fig. 7. Fig. 7 (a) shows a plane view of the resist pattern which is to be formed. Fig. 7 (b) shows a plane view of the previously proposed phase shift reticle 6. This phase shift reticle 6 has the phase shift layer 7 for shifting the phase of the exposure light by 180° (π) at the edge part which is used for pattern forming, and a phase shift layer 12 for shifting the phase of the exposure light by 90° (π/2). This phase shift layer 12 is provided adjacent to the phase shift layer 7 at a part where no pattern forming is made, that is at a part where the pattern is to be cut. A region indicated by hatchings in Fig. 7 (b) is the region of the phase shift layer 7 which shifts the phase of the exposure light by 180°. On the other hand, a region indicated by dots in Fig. 7 (b) is the region of the phase shift layer 12 which shifts the phase of the exposure light by 90°. Unmarked regions other than the hatched and dotted regions are the regions of the glass substrate 2. By appropriately setting the exposure conditions or the developing condition of the resist, only the edge part formed by the phase shift layer and the glass substrate 2 as shown in Fig. 7 (c) are patterned. Other parts, that is, the edge part formed by the phase shift layers 7 and 12 and the edge part formed by the phase shift layer 12 and the glass substrate 2, are not patterned. The edge part formed by the phase shift layer 12 and the glass substrate 2 is indicated by a dotted line in Fig. 7 (c). However, this previously proposed modified phase shift method has a problem in that it is extremely difficult in actual practice to realize the phase shift reticle 6 having the phase shift layers 7 and 12 which are finely shaped as described. Accordingly, there are demands to realize a more feasible phase shift method which can form fine resist patterns and to realize a mask suitable for carrying out such a phase shift method. This object is achieved by the present invention (claims 1, 3) by providing a pattern exposing method for forming a predetermined resist pattern on a substrate, comprising the steps of (a) exposing a resist layer which is formed on the substrate using a reticle which includes a pattern for exposing a corresponding pattern on the resist layer by use of phase shift of light transmitted through the reticle, where the pattern of the reticle is defined by a phase shift layer formed on a transparent reticle substrate and having first and second phase shift parts, and (b) developing the resist layer so that the predetermined resist pattern is formed on the substrate, where the step (a) exposes the corresponding pattern on the resist layer using edge parts of the first and second phase shift parts, the first phase shift part has a width such that a closed ring pattern is exposed by the edge parts thereof, and the second phase shift part has a width narrower than that of the first shift part so that patterns exposed by the edge parts thereof overlap in the form of a single line pattern. According to the pattern exposing method of the present invention, it is possible to form extremely fine resist patterns having various shapes. Another object of the present invention is to provide a mask for use in exposing a predetermined pattern on a resist layer which is formed on a substrate using a shift in phase of light transmitted through the mask, comprising a transparent reticle substrate; and a phase shift layer formed on the transparent reticle substrate and having first and second phase shift parts, where the first and second phase shift parts have edge parts defining the predetermined pattern which. is exposed on the resist layer, the first phase shift part has a width such that a closed ring pattern is exposed by the edge parts thereof, and the second phase shift part has a width narrower than that of the first phase shift part so that patterns exposed bay the edge parts thereof overlap in the form of a single line pattern. According to the mask of the present invention, it is possible to expose extremely fine resist patterns having various shapes. The object is solved by the features of claim 1 concerning the method and by the features of claim 3 concerning the mask. Claim 2 concerns a further embodiment of the method while claims 4 and 5 concern improvements further embodiments of the mask. The features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. In the drawings, Fig. 1 generally shows an imaging optical system of a conventional demagnification projection exposure apparatus and a conventional reticle; Fig. 2 is a diagram for explaining an exposure light intensity on a resist using the conventional reticle; Fig. 3 generally shows an imaging optical system of a conventional demagnification projection exposure apparatus using a phase shift method and a conventional phase shift reticle; Fig. 4 is a diagram for explaining an exposure light intensity on a resist of the conventional phase shift reticle; Fig. 5 is a diagram for explaining a resist pattern formed by the conventional phase shift reticle; Fig. 6 is a diagram for explaining another resist pattern formed by the conventional phase shift reticle; Fig. 7 is a diagram for explaining a previously proposed modified phase shift method using a phase shift reticle having two kinds of phase shift layers; Fig. 8 is a diagram for explaining a first embodiment of the pattern exposing method according to the present invention; Fig. 9 is a diagram for explaining a second embodiment of the pattern exposing method according to the present invention; and Fig. 10 is a diagram for explaining a third embodiment of the pattern exposing method according to the present invention. A description will be given of a first embodiment of a pattern exposing method according to the present invention, by referring to Fig. 8. In this embodiment and the embodiments described hereafter, elements such as the wafer are. not shown, but the same reference numerals will be used within the previously described figures. This embodiment forms a T-shaped pattern on the wafer 5. First, a positive resist is formed on the wafer 5, and a phase shift reticle 6 having a phase shift layer (phase shift pattern) 7 as shown in a plane view of Fig. 8 (a) is used for the first exposure. The phase shift layer 7 of the phase shift reticle 6 includes a narrow part 7A. This narrow part 7A has a width such that the patterns generated by the edge part of the narrow part 7A overlap and form a single line pattern. The positive resit at parts other than the periphery of the phase shift layer 7 is exposed by the first exposure, and a resist pattern 42 shown in Fig. 8 (b) is formed on the wafer 5. Then, a reticle 14 having an opening 16 in a light blocking layer 18 thereof as shown in Fig. 8 (c) is used for the second exposure. After the second exposure, the positive resist is developed, thereby forming a T-shaped resist pattern 42a on the wafer 5 as shown in Fig. 8 (d). A resist pattern 42b which is indicated by a dotted line in Fig. 8 (d) and is exposed during the first exposure is completely removed after the second exposure and the developing process. The reticle 6 used in this embodiment forms an embodiment of a mask according to the present invention. This reticle 6 includes the phase shift pattern 7 which is used to form a narrow line pattern by the edge part thereof, and the narrow part 7A which is used to form a line pattern which is wider than that of the line pattern formed by the phase shift pattern 7. In other words, the phase shift pattern which uses the concept described above with reference to Fig. 2 and the phase shift pattern which uses the concept described above with reference to Fig. 4 coexist on the reticle 6. If the line width of the phase shift pattern 7 which has a relatively large area in Fig. 8 (a) were made narrow as in the case of the narrow part 7A, it would become unnecessary to form the T-shaped resist pattern by two independent exposures. However, the resolution of the pattern which is formed by the edge part of the phase shift patterns does not improve if the width of the phase shift pattern is narrow. This is the reason why the first and second exposures are made in this embodiment. For example, if the i-line is used as the exposure light and the numerical aperture NA of the optical system is 0.5, for example, the resolution of the pattern formed by the edge part of the phase shift pattern is 0.2 µm. However, the resolution of the phase shift pattern which is only 0.15 µm and narrow, for example, is only 0.35 µm. Even if the width of the phase shift pattern is further reduced, the contrast would only deteriorate and the pattern which should be black would become grey. Therefore, the use of this embodiment of the mask, that is, the reticle 6 shown in Fig. 8 (a), is extremely useful in forming fine resist patterns. Next, a description will be given of a second embodiment of the pattern exposing method according to the present invention, by referring to Fig. 9. This embodiment forms a piled brick pattern on the wafer 5. First, a negative resist is formed on the wafer 5, and a phase shift reticle 6 having phase shift layers (phase shift patterns) 7 as shown in a plane view of Fig. 9 (a) is used for the exposure. The phase shift layer 7 of the phase shift reticle 6 includes a narrow part 7A. This narrow part 7A has a width such that the patterns generated by the edge part of the narrow part 7A overlap and form a single line pattern. In addiction, an interval 7B is provided between a adjacent phase shift patterns 7 so that the patterns generated by the edge parts of the adjacent phase shift patterns 7 overlap and form a single line pattern. The resist at parts other than the periphery of the phase shift layer 7 is exposed by the exposure, and a resist pattern 42 shown in Fig. 9 (b) is formed on the wafer 5 after a developing process is carried out because the negative resist is used in this embodiment. The reticle 6 used in this embodiment also includes the phase shift pattern 7 which is used to form a narrow line pattern by 'the edge part thereof, and the narrow part 7A which is used to form a line pattern which is wider than that of the line pattern formed by the phase shift pattern 7. In other words, the phase shift pattern which uses the concept shown in Fig. 9 of EP 0 395 425 A2 and the phase shift pattern which uses the concept described above with reference to Fig. 4 coexist on the reticle 6. Next a description will be given of a third embodiment of the pattern exposing method according to the present invention, by referring to Fig. 10. This embodiment forms a curved piled brick pattern on the wafer 5. First, a negative resist is formed on the wafer 5, and a phase shift reticle 6 having phase shift layers (phase shift patterns) 7 as shown in a plane view of Fig. 10 (a) is used for the exposure. The phase shift layer 7 of the phase shift reticle 6 includes a narrow part 7A. This narrow part 7A has a width such that the patterns generated by the edge part of the narrow part 7A overlap and form a single line pattern. In addition, an interval 7B is provided between adjacent phase shift patterns 7 so that the patterns generated by the edge parts of the adjacent phase shift patterns 7 overlap and form a single line pattern. The resist at parts other then the periphery of the phase shift layer 7 is exposed by the exposure, and a resist pattern 42 shown in Fig. 10 (b) is formed on the wafer 5 after a developing process is carried out because the negative resist is used in this embodiment. The reticle 6 used in this embodiment also includes the phase shift pattern 7 which is used to form a narrow line pattern by the edge part thereof, and the narrow part 7A which is used to form a line pattern which is wider than that of the line pattern formed by the phase shift pattern 7. In other words, the phase shift pattern which uses the concept shown in Fig. 9 of EP 0 395 425 A2 and the phase shift pattern which uses the concept described above with reference to Fig. 4 coexist on the reticle 6. The resist patterns forme by the second and third embodiments are particularly suited for use during the production process of a dynamic random access memory (DRAM), because the DRAM has active regions having the patterns shown in Fig. 10 (b), for example. In the second and third embodiments, if the width of the narrow parts 7A and the interval 7B between the adjacent phase shift patterns 7 respectively are 0.15 µm, the i-line is used the exposure light and the numerical aperture NA of the optical system is 0.5, the width of the resist pattern 42 becomes 0.35 µm. The upper limit of the width of the narrow part 7A and the interval 7B in this case are 0.3 µm, and the width of the resist pattern 42 for this upper limit value is 0.5 µm. If the width of the narrow part 7A and the interval 7B are greater than 0.3 µm, two peaks would exist in the corresponding light intensity distribution and the intended pattern cannot be obtained by the phase shift. According to the first through third embodiments of the pattern exposing method the present invention and the embodiment of the mask according to the present invention, it is possible to form resist patterns having various kinds of shapes by use of the reticle 6 on which the phase shift pattern which uses the concept described above with reference to Fig. 2 and the phase shift pattern which uses the concept described above with reference to Fig. 4 coexist. Of course, in each of the embodiments described above, it is possible to selectively use positive and negative resists depending on the desired resist pattern which is to be formed on the wafer. In addition, the first and second exposures may use opposite types of resists depending on the resist pattern which is to be formed.

Claims: A pattern exposing method for forming a predetermined resist pattern (42) on a substrate (5), comprising the steps of (a) exposing a resist layer which is formed on said substrate (5) using a reticle (6) which includes a pattern for exposing a corresponding pattern on said resist layer by use of a phase shift of light transmitted through said reticle, said pattern of the reticle being defined by a phase shift layer (7) formed on a transparent reticle surface (2) and having first and second phase shift parts (7, 7A); and (b) developing said resist layer so that said predetermined resist pattern (42) is formed on said substrate (5), characterized in that said step (a) exposes said corresponding pattern on said resist layer using edge parts of said first and second phase shift parts (7, 7A), said first phase shift part (7) having a width in plane such that a closed ring pattern is exposed by the edge parts thereof, said second phase shift part (7A) having a width in plane narrower than that of the first phase shift part (7) so that patterns exposed by the edge parts thereof overlap in the form of a single line pattern (42). A pattern exposing method according to claim 1, characterized in that said step (a) uses a negative resist as said resist layer. A mask (6) for use in exposing a predetermined pattern (42) on a resist layer which is formed on a substrate (5) using a shift in phase of light transmitted through said mask, with said mask comprising: a transparent reticle substrate (2); and a phase shift layer (7) formed on said transparent reticle substrate (2) and having first and second phase shift parts (7, 7A) characterized in that said first and second phase shift parts (7, 7A) have edge parts defining said predetermined pattern (42) which is exposed on said resist layer; wherein said first phase shift part (7) has a width in plane such that a closed ring pattern is exposed by the edge parts thereof, and said second phase shift part (7A) has a width in plane narrower than that of said first phase shift part (7) so that patterns exposed by the edge part thereof overlap in the form of a single line pattern. A mask according to claim 3, characterized in that the width in plane of said second phase shift part (7A) is 0.3 µm at the maximum when an exposure light is an i-line and a numerical aperture of an optical system used for the exposure is 0.5. A mask according to claim 3 or 4, characterized in that an interval (7B) of two mutually adjacent first phase shift parts (7) is 0.3 µm at the maximum when an exposure light is an i-line and a numerical aperture of an optical system used for the exposure is 0.5.

Inventors: HARUKI TAMAE, KANAZAWA MASAO, NAKAGAWA KENJI, TABATA YASUKO, HARUKI, TAMAE, KANAZAWA, MASAO, NAKAGAWA, KENJI, TABATA, YASUKO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: G03F1
IPC codes: G03F1, G03F7