Patent Number: 052272680
Section: description

DETAILED DESCRIPTION OF THE INVENTION The X-ray mask of the subject invention shall be explained, referring to the figures. FIG. 1A through FIG. 1F are production process cross-section figures of the X-ray mask of one embodiment of the subject invention. In FIG. 1A through FIG. 1F, 2 is the silicon nitride film, 3 is the tantalum film and 3a the first alignment pattern, while 6 are the alignment marks using photosensitive resin. The X-ray mask of the subject embodiment, as shown in its cross-section in FIG. 1E, has formed on the surface on the 2 micrometer thick SiN film 2, which is the X-ray permeable film, a circuitry pattern 3b and first alignment pattern 3a composed of 0.7 micrometer thick tantalum film which is the X-ray absorbant pattern. Further, via the SiN pattern 2, there are formed on the back surface of this SiN film 2 second alignment pattern 6 of photosensitive resin having the same pattern as and opposing the first alignment pattern 3a. As against the prior art X-ray mask, by forming the second alignment pattern on the laser beam side as well, the laser beam emitted from the alignment optical system does not pass through the SiN film 2 but is directly diffracted by the second alignment pattern 6 so that the attentuation of the laser beam strength is quite small. We shall now explain simply the method of manufacturing the X-ray mask of the subject embodiment, based on the manufacturing process cross-sections in FIG. 1A through FIG. 1F. As shown in FIG. 1A, a prior art X-ray mask 11 (FIG. 4) consisting of silicon support framework 1, the SiN film 2 (which is to become the X-ray permeable film) and the tantalum film 3 (which is to become the X-ray absorbant pattern) is formed. 3a is the prior art alignment pattern and 3b is the circuitry pattern. Next, on the silicon nitride film 2 a uniform coat of photosensitive material 4 (positive photoresist, for instance) is deposited using a spinner, etc. At this time, the film thickness of the photosensitive material 4 is so set to maximize the diffraction efficiency which is governed by the structure of the alignment optical system. At this point, an explanation will be given for the setting of the thickness of the photosensitive material 4, referring to FIG. 2 and FIG. 3. FIG. 2 is a cross-section figure of a lamella type diffraction grating. As shown in FIG. 2, if the incoming laser beam's wavelength is .lambda., the incoming angle is .alpha., the diffraction angle is .beta., the pitch of the diffraction grating is d, and the height of the diffraction grating is h, the diffraction efficiency .eta. can be expressed by the following expression from the lamella diffraction grating's theory: EQU .eta.=(400/m.sup.2 .pi..sup.2) . cos.sup.2 [(.delta.'+m .pi.) / 2 ] Where, .delta.'=2.pi.h/.lambda. (cos .alpha.+cos .beta.) m .lambda.=d (Sin .alpha.-Sin .beta.) and where m is the order of the diffracted beam. The diffraction efficiency .eta. changes cyclically depending on the height h of the diffraction grating and has maximum and minimum values. FIG. 3 shows the relationship when a helium neon laser (.lambda.=633 nm) is used as the laser beam with a diffraction grating with pitch d=4 micrometers and the incoming angle is set at approximately 10 degrees, between the diffraction efficiency .eta. of the zero order diffracted beam, the diffraction efficiency .eta. of the first order diffracted beam and the height h of the diffraction grating. Since the first order diffraction beam is usually used as the alignment beam, from FIG. 3 we can see that by setting the height h of the diffraction grating at 0.16 micrometer, 0.48 micrometer, etc., a maximum diffraction efficiency .eta. of approximately 40% is obtained. The height h of the diffraction grating thus obtained will be the film thickness of the photosensitive material 4. In this manner the height of the diffraction grating can be freely selected to increase the diffraction efficiency, and, since alignment marks are formed on the laser beam side of the SiN film the attenuation of the alignment beam is small, and a sufficiently strong diffracted beam can be obtained. Next, as shown in FIG. 1C, by using a light 5 to which the photosensitive material 4 is sensitive (for example, a light such as a mercury lamp) the entire surface is exposed at once using as the mask the tantalum film 3 which will be the X-ray absorbant pattern. Since the thickness of the silicon nitride film 2 which will be the X-ray permeable film is usually around 2 micrometers, the method becomes the same as contact exposure and the X-ray absorbant pattern will be accurately and equimultiply transferred to the photosensitive material 4 on the reverse side. Naturally, the first alignment pattern 3a composed of the first X-ray absorbant pattern will also be transferred without slippage in position. In this way, a second alignment pattern 6 and a second LSI circuit pattern are transferred to the opposite positions corresponding to the first alignment pattern 3a and the first LSI circuit pattern 3b respectively as illustrated in FIG. 1D through a development process. Then, before application of baking, another light exposure is applied to the unnecessary pattern of the second LSI circuit pattern only such that the second alignment pattern 6 is left, as illustrated in FIG. 1E, after development and baking. As a matter of course, instead of going through development immediately after the first entire surface exposure performed from the first main surface side of the x-ray permeable film 2 and by going through development after an exposure from the second main surface side of the x-ray permeable film 2 with the second alignment pattern 6 being masked, the second alignment pattern 6 only can be transferred at a time as shown in FIG. 1E skipping the stage of FIG. 1D. In this manner, according to the embodiment's X-ray mask, by using the second alignment pattern 6 as beam refracting beam elements, a sufficiently strong refracted beam is created when illuminated by the laser beam and accordingly a high resolution power can be obtained. That is to say, this X-ray mask would have second alignment pattern 6 having an optimized form to maximize the diffraction efficiency versus the alignment optical system. Thus a sufficient alignment signal strength is obtained, making possible the realization of a high alignment accuracy. Also, the process of forming the second alignment pattern 6 can be done just by uniformly coating the photosensitive material 4 and by exposing the entire surface at once. This does not require specialized equipment and is very easy to accomplish. Moreover, in the transfer of the X-ray absorbant pattern, as it is the same in principle as contact exposure, the pattern position distortion accompanying the transfer of the X-ray absorbant pattern is at such a low level as to be virtually insignificant and has no effect at all as a factor in alignment errors. Accordingly, as a X-ray exposure mask requiring high precision alignment it is possible to realize a high performance X-ray mask provided with second alignment pattern having sufficiently high optical characteristics. The X-ray mask of the subject invention, by forming the second alignment pattern for the mask on the other surface of the X-ray permeable film through equimultiple transfer using the first alignment patterns for the mask and substrate and the self-adjustment method, can obtain a sufficiently strong alignment signal. And, it offers great improvement in alignment accuracy and it also results in superior alignment accuracy and it also results in superior industrial productivity. Also, since the second alignment pattern are also formed on the laser beam source side with a height which increases the diffraction efficiency, a sufficiently strong alignment signal is obtained. Furthermore by using the X-ray mask of the subject invention to expose to the semiconductor wafer the circuitry pattern formed over the mask, there will be virtually no positional slippage between the mask and the wafer so that the desired circuitry pattern can also be easily exposed.