Photomask and pattern forming method employing the same
A semitransparent phase shifting mask has, in the periphery of a pattern element area, a light shielding portion which is formed by a semitransparent phase shifting portion and a transparent portion with the optimal size combination. A pattern is formed employing the semitransparent phase shifting mask.
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The present invention relates to a photomask which is used to manufacture a semiconductor device and the like, and more particularly to a photomask which has been subjected to a processing of shifting a phase of exposure light beams and a pattern forming method employing the same.
Along with an increase of the integration scale for semiconductor devices, sizes of patterns for forming constituent elements of the devices become fine, and size equal to or smaller than the critical resolution of a projection aligner are required. As a method of fulfilling such a request, in JP-B-62-50811 published on Oct. 27, 1987, and corresponding to JP-A-57-62052 (laid open on Apr. 14, 1982) for example, a photomask is employed in which a transparent film for shifting a phase of exposure light beams is provided on one of transparent portions on the opposite sides sandwiching an opaque portion, and thus the resolution of a pattern is exceptionally improved.
In the above-mentioned prior art, a phase shifter needs to be arranged in one of the transparent portions adjacent to each other, and for the arrangement of the phase shifter in the complicated element pattern, high trial and error is necessarily required. Thus, there is required considerable labor. In addition, since the number of processes of manufacturing a photomask is doubled as compared with the prior art, the reduction in yield and the increase in cost become problems.
Those problems can be settled by employing a semitransparent phase shifting mask in which a semi-transparent portion and a transparent portion are provided, and a little quantity of light beams passed through the semitransparent portion is phase-inverted with respect to light beams having passed through the transparent portion. With respect to this point, the description will hereinbelow be given with reference to the accompanying drawings.
In this method, the light beams the intensity of which is equal to or lower than the sensitivity of a photoresist, to which the pattern of the mask is to be transferred are made to pass through the semitransparent film so that the light beams which have passed through the semitransparent film is phase-inverted with respect to the light beams which have passed through the transparent portion, and thus, the contrast of the pattern is improved. As a result, it is possible to improve the resolution of an aligner for transferring the mask pattern. The basic principle of the semitransparent phase shifting mask is described in D. C. Flanders et al.: “Spatial period division—A new technique for exposing submicrometer-linewidth periodic and quasi-periodic patterns” J. Vac. Sci. Technol., 16(6), November/December pp 1949 to 1952 (1979), U.S. Pat. Nos. 4,360,586 and 4,890,309 and JP-A-4-136854 (laid open on May 11, 1992).
In the lithography process in which the above-mentioned semitransparent phase shifting mask is employed, in the normal exposed area, the good pattern formation can be performed. However, it has been made clear by the investigations made by the present inventors that since in the actual exposure of the wafer, the mask pattern is repeatedly transferred by the step and repeat, the light beams which have leaked from the semitransparent area, which is located outside the periphery of the actual pattern element corresponding to an active region of a substrate, leak out to the adjacent exposed area, and thus this is an obstacle to the good pattern formation.
It is therefore an object of the present invention to provide a photomask by which a good pattern can be obtained even in the case of an exposure, in which a mask pattern is repeatedly transferred by the step and repeat, and a pattern forming method employing the same.
According to one aspect of the present invention, the above-mentioned object can be attained by effectively making a light-shielding or opaque area of a semitransparent phase shift mask which is located outside the periphery of a pattern element formation area of the semitransparent phase shifting mask.
The light shielding portion in the semitransparent phase shifting mask is formed by processing a semitransparent film to a pattern having a width equal to or lower than the resolution. The reason of adopting such a method is that if a light shielding film is newly formed as the light shielding portion, this will result in an increase of the number of processes of forming the mask. Incidentally, by optimizing the area ratio of the semitransparent portion to the transparent portion, it is possible to further effectively form the light shielding portion.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
When both a semitransparent phase shifting pattern and a transparent pattern are arranged with the same size and at a pitch equal to or lower than the critical resolution, a pattern image can be erased. But, in this case, the resulting uniform light intensity does not become zero. This reason is that since there is a difference between the quantity of light beams having passed through the semitransparent phase shifting portion and that of light beams having passed through the transparent portion, the function of cancelling those light beams each other due to the phase inversion effect is not efficiently performed.
Then, when the ratio of the area of the semitransparent phase shifting portion to that of the transparent portion is adjusted in accordance with a set transmittance of the semitransparent phase shifting portion, it is made clear that the light intensity can be zero. By using this pattern for an area of a to-be-exposed water surface, which may be otherwise undesirably double-exposed by the step and repeat by an aligner, it is possible to prevent the double exposure on the wafer, and a pattern of constituent elements as desired can be formed. Therefore, since the present photomask is made up of only semitransparent phase shifting portions and transparent portions, there is no need to newly form a light shielding film for the formation of the light shielding portion, and thus, the process of forming the photomask can be simplified. Incidentally, the above-mentioned light shielding portion is applicable to the formation of a light shielding portion in a pattern element region of a substrate. In this case, since the ratio of the transmittance of the transparent portion to that of the light shielding portion can be made large, it is possible to increase the tolerance for the variation of the quantity of light beams required for the exposure.
As for the materials used for the formation of the semitransparent phase shifting portion, a lamination film of a semitransparent metal film (made of chromium, titanium or the like) or a silicide film (e.g., a molybdenum silicide film) and a silicon oxide film for the phase shift, or a single layer film such as a metal oxide film (e.g., a chromium oxide film) or metal nitride film (e.g., a chromium nitride film) may be employed. In the case where a single layer film such as a chromium oxide film or a chromium nitride film is employed, since the refractive index thereof is larger than that of the silicon oxide film, the film can be thinned. As a result, since the influence of the light diffraction can be reduced, this single layer film is suitable for the formation of a fine pattern.
Embodiment 1 A first embodiment of the present invention will hereinafter be described in detail.
P=α·λNA,
where NA represents a numerical aperture of a projection lens, λ represents a wavelength of the exposure light beams, and α represents a coefficient. In this connection, on the basis of the experiments made by the present inventors, it is desirable that the coefficient α is set to a value equal to or larger than 0.8. However, the optimal value of α is not limited thereto or thereby because the optimal value of α depends on the characteristics of the illuminating system, the pattern configuration and the like. A width 12 of the transparent pattern 10 influences largely the formation of a dark portion.
α=β·{square root}{square root over (T)},
where T represents a transmittance of the semitransparent phase shifting portion, and β represents a coefficient. The allowable intensity of the projected light beams is variable depending on intended purposes. In the case of preventing exposure of a photoresist due to a double exposure, the allowable intensity of the projected light beams may be set to an intensity which is about one-half the intensity of light having passed through the semitransparent phase shifting portion. However, in the case of preventing a double exposure of a dark portion with a fine pattern containing portion, the change in the size of the fine pattern needs to be reduced as much as possible, and thus it is desirable that the allowable intensity of the projected light beams is set to a value equal to or lower than 0.05. The value of β in this case is in the range of about 0.5 to about 2.0. Then, the area 6 of
A second embodiment of the present invention will hereinafter be described with reference to
As a result of using this photomask in order to manufacture the 64 Mbits-DRAM, the double exposure in the periphery of the chip can be perfectly prevented, and thus the good device can be manufactured. In addition, in the case where the pattern element area 5 is formed by one chip, or the photomask having the dark portion is applied to devices other than DRAM, the same effects can be obtained.
Further, the description will hereinbelow be given with respect to an example in which the dark portion of the present invention is arranged in the periphery of a window pattern which is used to align the mask position with reference to
Hereinbelow, an example will be shown in which a semiconductor device is manufactured according to the present invention.
As set forth hereinabove, according to the present invention, by forming the semitransparent phase shifting portion and the transparent portion with the optimal size combination, even if a light-shielding film is not newly formed, the effective dark portion can be formed. In addition, without increasing in the number of processes of forming the mask, the semitransparent phase shifting mask which is useful in practical use can be produced. Further, as a result of manufacturing the semiconductor device by using the photomask of the present invention, it is possible to form the pattern in which the effects inherent in the semitransparent phase shifting mask are sufficiently utilized, without any problem in the double exposure portion, and also it is possible to realize the reduction of the device area.
It is further understood by those skilled in the art that the foregoing description is a preferred embodiment of the disclosed device and that various changed and modifications may be made in the invention without departing from the spirit and scope thereof.
Claims
1-9. (canceled)
10. A method of manufacturing a dynamic random access memory, comprising the steps of:
- preparing a phase shifting mask including (a) an element forming area having a semitransparent phase shifting film, and (b) a light shielding area provided at a peripheral edge of said element forming area and serving to make an intensity of light having passed through said light shielding area smaller than an intensity of light having passed through said semitransparent phase shifting film, as measured on a to-be-exposed photoresist film, and
- transferring a hole pattern formed at said element forming area of said phase shifting mask onto said photoresist film, by using a projection exposure apparatus.
11. A method for manufacturing a dynamic random access memory according to claim 10, wherein said light shielding area includes a semitransparent phase shifting pattern having a semitransparent phase shifting portion and a transparent portion.
12. A method for manufacturing a dynamic random access memory according to claim 11, wherein a ratio a of an area of said transparent portion to an area of said phase shifting portion is defined by a α=β{square root}T, where T is the transmittance of said semitransparent phase shifting portion, and β is a value falling within a range of 0.5≦β≦2.0.
13. A method for manufacturing a dynamic random access memory according to claim 10, wherein said peripheral edge is double-exposed in said transferring step.
14. A method for manufacturing a dynamic random access memory according to claim 10, wherein an alignment pattern is provided at said peripheral edge.
15. A method for manufacturing a dynamic random access memory, comprising the steps of:
- preparing a phase shifting mask including (a) an element forming area having a first semitransparent phase shifting film having a transmittance to exposure light of not more than 25%, and (b) a light shielding area provided at a peripheral edge of said element forming area and serving to make an intensity of light having passed through said light shielding area smaller than an intensity of light having passed through said semitransparent phase shifting film, as measured on a to-be-exposed photoresist film,
- preparing a semiconductor substrate on which said to-be-exposed photoresist film is formed, and
- transferring a hole pattern formed at said element forming area of said phase shifting mask onto said photoresist film by using a projection exposure apparatus.
16. A method for manufacturing a dynamic random access memory according to claim 15, wherein said light shielding area includes a semitransparent phase shifting pattern having a semitransparent phase shifting portion and a transparent portion.
17. A method for manufacturing a dynamic random access memory according to claim 16, wherein a ratio α of an area of said transparent portion to an area of said phase shifting portion is defined by α=β{square root}T, where T is the transmittance of said semitransparent phase shifting portion, and β is a value falling within a range of 0.5 ≦β≦2.0.
18. A method for manufacturing a dynamic random access memory according to claim 15, wherein said peripheral edge is double-exposed in said transferring step.
19. A method for manufacturing a dynamic random access memory according to claim 15, wherein an alignment pattern is provided at said peripheral edge.
Type: Application
Filed: Mar 16, 2005
Publication Date: Jul 21, 2005
Applicant:
Inventors: Norio Hasegawa (Tokyo), Fumio Murai (Tokyo), Katsuya Hayano (Hachioji-shi)
Application Number: 11/080,511