Photo mask

Abstract: A photo mask and method for manufacturing the same increase the capacitance of a capacitor by improving the proximity effect of the mask pattern. The photo mask includes a transparent substrate, an opaque mask pattern (11;15;20) for defining an optical transmission area on the substrate, and an optical transmittance control film pattern (10;14;17&18) for suppressing proximity effect in the optical transmission area. The proximity effect is suppressed by forming an optical transmittance control film pattern in the transmission area between the individual portions of the opaque mask pattern, so that the mask pattern shape can be exactly transferred onto a substrate. Thus, capacitor surface area is increased thereby improving the reliability of manufacture of a semiconductor device.

Description: The present invention relates to a semiconductor photolithography technique, and has particular application in providing a photo mask which can increase the capacitance of a capacitor in a semiconductor memory device by improving a proximity effect of a mask pattern. Generally, a unit cell of a semiconductor memory device can largely be divided into a transistor portion and a capacitor portion. The transistor uses a MOS structure and the capacitor uses a stack structure where several material layers are sequentially formed. Especially, for a semiconductor memory device having a high packing density, a unit cell having one transistor has been used. FIG. 1 of the accompanying drawings is a schematic view showing an example of a unit cell having one transistor in a semiconductor memory device. Referring to FIG. 1, a capacitor C is connected between the gate and source of a MOS transistor. In a semiconductor memory device, especially DRAMs, it is highly desirable for the capacitance of capacitor C to be large, since several performance characteristics thereof are improved given a larger capacitance. The capacitance (C) of a parallel-plate capacitor, such as that used in semiconductor memory devices, is calculated by the well-known formula C = ε(A/d) , where ε denotes a dielectric constant, A denotes the surface area of a plate, and d denotes the distance (gap) between the plates. Thus, in order to increase the capacitance of the parallel-plate capacitor, a material having a larger dielectric constant is used, the distance (gap) between the plates is minimized, or the surface area of the plate is increased. However, the number of practical dielectric materials is limited and their dielectric constants are fixed. On the other hand, decreasing the gap size greatly increases the chance of punch-through, thereby degrading the characteristics of the device. Accordingly, research has largely concentrated on ways to increase the surface area of the plates (electrodes) of the capacitor. It is necessary to transfer a pattern so as to form a source, drain and capacitor of a semiconductor memory device on a semiconductor substrate. Accordingly, the use of a photolithography technique using a photo mask is indispensable in semiconductor manufacturing processing. Generally, the photolithography technique can largely be divided into two processes. A first process is of forming a photoresist pattern on a layer using a photo mask and a second process is of etching an under-layer (formed under the photoresist pattern) using the photoresist pattern as a mask and then removing the photoresist pattern. Here, the first process will be explained in more detail with reference to FIG. 2A and FIG. 2B of the accompanying drawings. FIG. 2A is a plan view of a photo mask for transferring a pattern to form a capacitor on a semiconductor substrate. Referring to FIG. 2A, a rectangular opaque mask pattern 1 is formed regularly, a reference numeral 2 denotes a space existing between the edges of the opaque mask patterns 1 at intersections of the thus-created margins thereof. In a practical semiconductor device, a process for transferring a photo mask having the opaque mask pattern 1 onto a semiconductor substrate is used. FIG. 2B is a plan view of a photoresist pattern 3 when an opaque mask pattern is transferred onto a semiconductor substrate using the photo mask shown in FIG. 2A. Referring to FIG. 2B, when an opaque mask pattern 1 is transferred on a semiconductor substrate using a photo mask as shown in FIG. 2A, a proximity effect takes place between the edge portions of the opaque mask pattern 1. The proximity effect is due to a diffraction of an incident ray (e.g., ultraviolet light) on a photo mask. When the ultraviolet rays pass between the opaque mask patterns 1, the shape of the photoresist pattern 3 is undesirably rounded by a diffraction of the rays. Also, the distance (reference numeral 4) between edges of the photoresist patterns is made longer due to the proximity effect. Specially, when the photoresist pattern 3 as an etching mask is applied a layer for forming a capacitor on a semiconductor substrate, the etched shape of the layer is also rounded. As a result, an area, i.e., capacitor area ("A" of above formula), is reduced thereby to decrease the cell capacitance of a semiconductor memory device. In order to explain the proximity effect in more detail, the results of a SOLID (Simulation of Optical Lithography In three Dimensions) simulator are shown in FIG. 4 and FIG. 5 of the accompanying drawings, wherein an exposing and developing process simulation based on a conventional photo mask as shown in FIG. 3 of the accompanying drawings is performed. The simulation is performed using a deep ultraviolet (DUV) stepper at a wavelength (λ) of 0.248µm and a numerical aperture (NA) of 0.45. FIG. 3 is a layout view showing a conventional photo mask. Here, a rectangular opaque mask pattern 7 for selectively blocking light is formed on a substrate, and the area where the opaque mask pattern 7 does not block the light is an optical transmission area. The dimensions of the rectangular opaque mask pattern 7 are as follows. The width 31 and length 33 of each portion of rectangular opaque mask pattern 7 are 0.42µm and 1.18µm, respectively, and a spacing 32 (X-axis) and a spacing 34 (y-axis) between the opaque mask patterns are 0.28µm and 0.22µm, respectively. FIG. 4 is a plan view showing an aerial image created by the simulation when a mask pattern is transferred on a substrate using the photo mask of FIG. 3. Referring to FIG. 4, the contour lines around the point of a reference character 5a are much denser than in other areas. These densely formed contour lines correspond to an optical transmission area between the edges of opaque mask patterns 7 (FIG. 3). Especially, in the optical transmission area between opaque mask patterns 7, light is gathered when the mask pattern is transferred, to thereby exhibit the maximum degree of the proximity effect. FIG. 5 is a graph representing the simulated light intensity along line a-a' of FIG. 4. Here, we note that the light intensity along line a-a' is not uniform, and is high at the portions indicated by reference numeral 5 (corresponding to reference numeral 5a of FIG. 4) by a proximity effect. As a result, when a conventional mask as shown in FIG. 2 is used in a photolithography process, a rounded photoresist pattern is formed on the semiconductor substrate by a proximity effect. Therefore, when a layer for forming a cell capacitor is etched using the rounded pattern, capacitor unit area is reduced, thereby decreasing the cell capacitance of a manufactured semiconductor memory device. EP-A-0,529,338 and EP-A-0,551,621 each disclose a photo mask which includes a phase shifting mask for shifting the phase of transmitted light. It is an object of the present invention to provide a photo mask which can improve a rounding phenomenon due to a proximity effect on an exposed photoresist pattern formed on a semiconductor wafer. According to the present invention there is provided a photo mask for transferring a mask pattern onto a semiconductor wafer, comprising: a transparent substrate ; and, an opaque mask pattern formed on the substrate, wherein the opaque mask pattern defines an optical transmission area between edges of portions of the opaque mask pattern which selectively block light, characterised in that the photo mask further comprises an optical transmittance control film pattern which suppresses a proximity effect within the optical transmission area wherein the opaque mask pattern forms a two-dimensional array of masking portions having equal sized rectangular shapes spaced apart by regular intervals such that horizontally adjacent masking portions are separated by columnar optical transmission areas, and vertically adjacent masking portions are separated by row-wise optical transmission areas, and wherein said optical transmittance control film pattern is formed at the portions of the optical transmission area where the columnar optical transmission areas cross the row-wise optical transmission areas. The optical transmittance control film is preferably formed of a material selected from the group consisting of spin-on-glass (SOG), photoresist (PR), silicon oxide (SiO 2 ), aluminum (Al), silicon nitride (Si 3 N 4 ), a polysilicon, titanium (Ti), titanium nitride (TiN), chromium (Cr) and tungsten (W). In a photo mask in accordance with the present invention, since a proximity effect is suppressed by forming an optical transmittance control film pattern in an intersection area of the transmission area and in areas adjacent thereto, the shape of a mask pattern is exactly transferred onto a substrate, to thereby increase capacitor area. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a schematic view showing an example of a unit cell having one transistor in a semiconductor memory device; FIG. 2A is a plan view of a photo mask for transferring a pattern to form a capacitor on a semiconductor substrate; FIG. 2B is a plan view of the photoresist pattern when an opaque mask pattern is transferred onto a semiconductor substrate using the photo mask shown in FIG. 2A; FIG. 3 is a layout view showing a conventional photo mask; FIG. 4 is a plan view showing an aerial image created by a simulation, when a mask pattern is transferred onto a substrate using the photo mask shown in FIG. 3; FIG. 5 is a graph representing the light intensity along line a-a' of FIG. 4; FIG. 6 is a layout view of a photo mask according to a first embodiment of the present invention; FIG. 7 is a plan view showing an aerial image created by a simulation, when a mask pattern is transferred onto a substrate using the photo mask shown in FIG. 6; FIG. 8 is a graph representing the light intensity along line b-b' of FIG. 7; FIG. 9 is a layout view of a photo mask according to a second embodiment of the present invention; FIG. 10 is a plan view showing an aerial image created by a simulation, when a mask pattern is transferred onto a substrate using the photo mask shown in FIG. 9; FIG. 11 is a graph representing the light intensity along line c-c' of FIG. 10; FIG. 12 is a layout view of a photo mask according to the a third embodiment of the present invention; FIG. 13 is a plan view showing an aerial image created by a simulation, when a mask pattern is transferred onto a substrate using the photo mask shown in FIG. 12; FIG. 14 is a graph representing the light intensity along line d-d' of FIG. 13; FIG. 15A to FIG. 15D are sectional views illustrating a method for manufacturing a photo mask according to the first embodiment of the present invention; FIG. 16A to FIG. 16D are sectional views illustrating a method for manufacturing a photo mask according to the second embodiment of the present invention; and FIG. 17A to FIG. 17D are sectional views illustrating a method for manufacturing a photo mask according to the third embodiment of the present invention. When an exposing and developing process is simulated using a photo mask according to the present invention, the result of a SOLID (Simulation of Optical Lithography In three Dimensions) simulator is illustrated for the following embodiments. The simulation is performed using a deep ultraviolet (DUV) stepper at a wavelength (λ) of 0.248µm and a numerical aperture (NA) of 0.45. Also, an optical transmission rate of optical transmittance control film is controlled as 50%. Embodiment 1 FIG. 6 is a layout view of a photo mask according to a first embodiment of the present invention. Referring to FIG. 6, a rectangular opaque mask pattern 11 for defining an optical transmission area is formed periodically on a mask substrate (not shown). Also, an optical transmittance control film pattern 10 is formed in an intersection area of the optical transmission area between the opaque mask pattern 11 and in the areas adjacent thereto. In more detail, the optical transmittance control film pattern 10 is formed in an area defined by edges of the opaque mask pattern 11. Also, the optical transmittance control film pattern 10 is formed in a parallel direction (Y-axis) with respect to the longer side of the rectangular opaque mask pattern. Especially, when a mask pattern is transferred onto a substrate, the optical transmittance control film pattern 10 maintains a uniform light intensity, thereby suppressing a proximity effect. The size, shape and layout of the optical transmittance control film pattern 10 can be variably controlled. In the first embodiment, the width and length of the optical transmittance control film pattern 10 are 0.28µm and 0.66µm, respectively. The width and length of the opaque mask pattern 11 are 0.42µm and 1.18µm, respectively. Also, the spacings between the opaque mask patterns 11 are 0.28µm (X-axis) and 0.22µm (y-axis), respectively. FIG. 7 is a plan view showing an aerial image created by a simulation, when a mask pattern is transferred onto a substrate using the photo mask shown in FIG. 6. Referring to FIG. 7, the contour lines around a reference character 13a are less dense than those in FIG. 4. Areas in which the contour lines are not densely formed correspond to an optical transmission area between opaque mask patterns 11 shown in FIG. 6. Especially, in an optical transmission area between the opaque mask patterns 11 of FIG. 6, light is not gathered when a mask pattern is transferred, thereby suppressing a proximity effect. FIG. 8 is a graph representing the light intensity along line b-b' of FIG. 7. Here, the light intensity along line b-b' is uniform since a light intensity (reference numeral 13) is very low as compared with reference numeral 5 of FIG. 5. As a result, the proximity effect is suppressed when a photoresist pattern is formed using a photo mask according to the present invention. Embodiment 2 FIG. 9 is a layout view of a photo mask according to the second embodiment of the present invention. Referring to FIG. 9, a rectangular opaque mask pattern 15 for defining an optical transmission area is formed periodically on a mask substrate (not shown). Also, an optical transmittance control film pattern 14 is formed in an intersection area of the optical transmission area between the opaque mask patterns 15. In more detail, the optical transmittance control film pattern 15 is formed in an area defined by edges of the opaque mask pattern 14. Especially, when a mask pattern is transferred onto a substrate, the optical transmittance control film pattern 15 maintains a uniform light intensity, thereby suppressing a proximity effect. Also, the optical transmittance control film pattern 14 can be formed in a part of the intersection area of the optical transmission area between the opaque mask patterns 15. At this time, the proximity effect can be suppressed by controlling the transmission rate of the optical transmittance control film pattern 14. In the second embodiment, the width and length of the optical transmittance control film pattern 14 are 0.28µm and 0.22µm, respectively. The dimensions (width and length) of the opaque mask pattern 15 and the length between the opaque mask pattern 15 are the same as those of the first embodiment. FIG. 10 is a plan view showing an aerial image created by a simulation, when a mask pattern is transferred onto a substrate using the photo mask shown in FIG. 9. Referring to FIG. 10, the contour lines around a reference character 16a are less dense than those formed in FIG. 4. Those areas where the contour lines are not densely formed correspond to an optical transmission area between an opaque mask patterns 15 as shown in FIG. 9. Especially, in an optical transmission area between the opaque mask patterns 15, light is not gathered when a mask pattern is transferred, thereby suppressing a proximity effect. FIG. 11 is a graph representing the light intensity along line c-c' of FIG. 10. Here, the light intensity along line c-c' of FIG. 10 is relatively uniform since the light intensity at reference numeral 16 is much lower than that at reference numeral 5 of FIG. 5. As a result, a proximity effect is suppressed when a photoresist pattern is formed using a photo mask according to the present invention. Embodiment 3 FIG. 12 is a layout view of a photo mask according to the third embodiment of the present invention. Referring to FIG. 12, a rectangular opaque mask pattern 20 for defining an optical transmission area is formed periodically on a mask substrate (not shown). Also, a first and second optical transmittance control film pattern 17 and 18 are formed in an intersection area of the optical transmission area between the opaque mask pattern 20 and in the areas adjacent thereto. In more detail, the first optical transmittance control film pattern 17 is formed parallel to the longer side of the rectangular opaque mask pattern 20, and the second optical transmittance control film pattern 18 is formed perpendicular to the longer side of the rectangular opaque mask pattern 20. Especially, when a mask pattern is transferred onto a substrate, the first and second optical transmittance control film patterns 17 and 18 maintain a uniform light intensity, thereby suppressing a proximity effect. In the third embodiment, the width and length of the first optical transmittance control film pattern 17 are 0.28µm and 0.42µm, respectively, and the width and length of the second optical transmittance control film pattern 18 are 0.48µm and 0.22µm, respectively. The dimensions (width and length) of the opaque mask pattern 20 and the length between the opaque mask patterns 20 are the same as those of the first embodiment. FIG. 13 is a plan view showing an aerial image created by a simulation, when a mask pattern is transferred onto a substrate using the photo mask shown in FIG. 12. Referring to FIG. 13, contour lines around a reference character 19a are less dense than those formed in FIG. 4. Areas where the contour lines are not densely formed correspond to an optical transmission area between an opaque mask patterns 20 as shown in FIG. 12. Especially, in an optical transmission area between an opaque mask patterns 20, light is not gathered when a mask pattern is transferred, thereby suppressing a proximity effect. FIG. 14 is a graph representing the light intensity along line d-d' of FIG. 13. Here, the light intensity along line d-d' of FIG. 13 is very uniform since the light intensity at reference numeral 19 is much lower than that at reference numeral 5 of FIG. 5. As a result, the proximity effect is suppressed when a photoresist pattern is formed using a photo mask according to the present invention. Hereinafter, a method for manufacturing a photo mask illustrated by the first embodiment of the present invention will be explained by the following embodiments. Embodiment 1 FIG. 15A to FIG. 15D are sectional views illustrating a method for manufacturing a photo mask according to the first embodiment of the present invention. FIG. 15A shows a step of forming an opaque mask pattern 101 on a transparent substrate 100. In more detail, an opaque material layer is formed on a substrate 100, a light-blocking material is used as the opaque material layer. In the present embodiment, a chromium layer is used as the opaque material layer. Then, a photoresist layer (not shown) is formed on the opaque material layer and patterned to form a photoresist pattern (not shown) by exposing using an electron beam and developing process. Then, the opaque material is etched to form an opaque mask pattern 101 using the photoresist pattern as an etching mask. Thereafter, the photoresist pattern is removed. FIG. 15B shows a step of forming an optical transmittance control film 102 over the entire surface of the substrate 101. Referring to FIG. 15B, an optical transmittance control film 102 is formed on an opaque mask pattern 101 and a substrate 100. The optical transmittance control film 102 is formed of spin-on-glass (SOG), photoresist (PR), silicon oxide (SiO 2 ), aluminum (Al), silicon nitride (Si 3 N 4 ), a polysilicon, titanium (Ti), titanium nitride (TiN), chromium (Cr) or tungsten (W). FIG. 15C shows a step of forming a photoresist pattern (103a, 103b) for patterning the optical transmittance control film 102. Referring to FIG. 15C, a photoresist layer is formed on the optical transmittance control film 102 and patterned to form a photoresist pattern (103a, 103b) by exposing using an electron beam and developing process. The photoresist pattern (103a, 103b) is formed between the individual portions of the opaque mask pattern 101. FIG. 15D shows a step of forming a transmittance control film pattern (102a, 102b) between the opaque mask patterns 101. Referring to FIG. 15D, the transmittance control film 102 is etched to form a transmittance control film pattern (102a, 102b) using the photoresist pattern (103a, 103b) as an etching mask. Thereafter, the photoresist pattern is removed and a manufacturing process of a photo mask according to the present invention is completed. Embodiment 2 FIG. 16A to FIG. 16D are sectional views illustrating a method for manufacturing a photo mask according to the second embodiment of the present invention FIG. 16A shows a step of forming an opaque mask pattern 101 and etch stopping film 104 on a transparent substrate 100. In more detail, an etch stopping film is formed on an opaque mask pattern 101 and a substrate 100, the etching stopping film is used as an etch stop when etching an optical transmittance control film by subsequent process. The etch stopping film 104 is spin-on-glass, silicon oxide or silicon nitride when the optical transmittance control film is a metal, e.g., aluminum or chromium. The formation of an opaque mask pattern 101 is the same as that of the first embodiment. FIG. 16B is a step of forming an optical transmittance control film 105 on the etch stopping film 104. Referring to FIG. 16B, an optical transmittance control film 105 is formed on the etch stopping film 104. The optical transmittance control film 105 is formed of spin-on-glass (SOG), photoresist (PR), silicon oxide (SiO 2 ), aluminum (Al), silicon nitride (Si 3 N 4 ), a polysilicon, titanium (Ti), titanium nitride (TiN), chromium (Cr) or tungsten (W). FIG. 16C shows a step of forming a photoresist pattern (106a, 106b) for patterning the optical transmittance control film 105 104. Referring to FIG. 16C, a photoresist layer is formed on the optical transmittance control film 105 and patterned the photoresist layer to form a photoresist pattern (106a, 106b) by exposing using an electron beam and developing process. The photoresist pattern (106a, 106b) is only formed between the opaque mask pattern 101. FIG. 16D shows a step of forming a transmittance control film pattern (105a, 105b) between the opaque mask pattern 101. Referring to FIG. 16D, the transmittance control film 105 is etched to form a transmittance control film pattern (105a, 105b) using the photoresist pattern (106a, 106b) as an etching mask. Thereafter, the photoresist pattern is removed and the manufacturing process of a photo mask according to the present invention is completed. Embodiment 3 FIG. 17A to FIG. 17D are sectional views illustrating a method for manufacturing a photo mask according to the third embodiment of the present invention. FIG. 17A shows a step of forming an photoresist layer 107 on an opaque mask pattern 101 and transparent substrate 100. In more detail, an photoresist layer is formed on an opaque mask pattern 101 and a substrate 100. The formation of an opaque mask pattern 101 is the same as those of the first and second embodiments. FIG. 17B shows a step of forming a photoresist pattern (107a, 107b, 107c). Referring to FIG. 17B, a photoresist layer is patterned to form a photoresist pattern (107a, 107b, 107c) by exposing using an electron beam and developing process. The photoresist pattern (106a, 106b) is only formed on the opaque mask pattern 101. FIG. 17C shows a step of forming an optical transmittance control film 108 over the resultant material. Referring to FIG. 15C, an optical transmittance control film 108 is formed over the entire surface of the resultant material. The material used as the optical transmittance control film 108 is the same as those of the first and second embodiments. FIG. 17D shows a step of forming a transmittance control film pattern (108a, 108b) between the opaque mask pattern 101. Referring to FIG. 17D, the transmittance control film 108 is etched to form a transmittance control film pattern (108a, 108b) using a lift-off process. If so, the manufacturing process of a photo mask according to the present invention is completed. According to a photo mask of the present invention, since a proximity effect is suppressed by forming an optical transmittance control film pattern in an intersection area of the transmission area between the opaque mask pattern and in the areas adjacent thereto, the shape of a mask pattern is exactly transferred onto a substrate, thereby increasing a capacitor area and reliability of a semiconductor device. In addition, when a mask pattern is exactly transferred onto a substrate, an effective use of the substrate and fine pattern formation according to the high integration of a semiconductor device are possible. It is to be understood that changes and variations may be made without departing from the scope of the present invention.

Claims: A photo mask for transferring a mask pattern onto a semiconductor wafer, comprising: a transparent substrate (100); and, an opaque mask pattern (11;15;20) formed on the substrate, wherein the opaque mask pattern defines an optical transmission area between edges of portions of the opaque mask pattern which selectively block light, characterised in that the photo mask further comprises an optical transmittance control film pattern (10;14;17,18) which suppresses a proximity effect within the optical transmission area wherein the opaque mask pattern (11;15;20) forms a two-dimensional array of masking portions having equal sized rectangular shapes spaced apart by regular intervals such that horizontally adjacent masking portions are separated by columnar optical transmission areas, and vertically adjacent masking portions are separated by row-wise optical transmission areas, and wherein said optical transmittance control film pattern is formed at the portions of the optical transmission area where the columnar optical transmission areas cross the row-wise optical transmission areas. A photo mask according to claim 1, wherein the optical transmittance control film pattern (10;14;17,18) is formed of a material selected from the group consisting of spin-on-glass (SOG), photoresist (PR), silicon oxide (SiO 2 ), aluminum (Al), silicon nitride (Si 3 N 4 ), polysilicon, titanium (Ti), titanium nitride (TiN), chromium (Cr) and tungsten (W). A photo mask according to claim 1 or 2, in which the optical transmittance control film pattern (10;14;17,18) overlies respective optical transmission areas of the substrate. A photo mask according to claim 3, wherein the optical transmittance control film pattern (10) extends into the columnar optical transmission areas between said horizontally adjacent masking portions. A photo mask according to claim 4, wherein the optical transmittance control film pattern (18) extends into the row-wise optical transmission areas between said vertically adjacent masking portions.

Inventors: HAN WOO-SUNG, SOHN CHANG-JIN, HAN, WOO-SUNG, SOHN, CHANG-JIN
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1995
Main classification: G03F1
IPC codes: G03F7, G03F1, H01L21