Patent Number: 055241310
Section: summary

FIELD OF THE INVENTION AND RELATED ART The present invention relates to an alignment apparatus for aligning a mask having a semiconductor chip pattern and a semiconductor wafer in a predetermined positional relationship, more particularly to a semiconductor chip manufacturing SOR (synchrotron orbital radiation) X-ray exposure apparatus wherein after the mask and the semiconductor wafer are aligned, a resist on the semiconductor wafer is exposed to the orbital radiation rays (SOR X-rays) in the form of a pattern corresponding to the pattern of the semiconductor chip on the mask. Lithography using X-rays for semiconductor chip manufacturing has been noted as a fine lithographic technique to realize high density semiconductor chips and was proposed by Spears and Smith in "Electron Lett. Vol. 8, No. 4: P.102, 1972" in 1972. Since, however, there has not been a small size and high power X-ray source, it has been difficult to install a semiconductor chip manufacturing X-ray exposure apparatus using X-ray lithography in the semiconductor manufacturing plant. In addition, it has not met the needs for mass-production, because of low throughput and the like. Recently, however, a very small size SOR ring has been developed, capable of providing high power X-rays with the use of a normal conductor or super conductor magnet. Therefore, the X-ray source is no longer a major problem. Referring first to FIG. 2, in the X-ray exposure apparatus, pattern exposure is effected in a so-called proximity exposure method. The mask 2 comprises a substrate 201 having a thickness of several microns with high X-ray transmissibity, an absorber material 202 having a high X-ray absorptivity formed into a pattern to be printed on the wafer and a supporting frame 203 for supporting the substrate 201. The mask 2 is opposed to the wafer 3, which is the workpiece, with a predetermined gap (approximately several tens of microns) with precision and stability. The X-rays 1 are applied to the semiconductor wafer 3 through the mask 2, so that the resist 5 applied on the wafer 3 is exposed to the pattern determined by the absorber material 202, by which the pattern is transferred onto the wafer 3. As for the pattern transfer process, there have been proposed a scanning exposure system shown in FIG. 3A, scanning mirror exposure system shown in FIG. 3B and a whole surface exposure system shown in FIG. 3C. The scanning exposure system and the whole surface exposure system have been proposed in "J. Vac. Sci. Technol. B1 (4) 1984, p.1271" and in "IBM Research Report RC 8220, 1980", respectively Referring to FIG. 3A, the scanning exposure system is such that after the mask 2 and the wafer 3 are aligned in a predetermined positional relation, the mask 2 and the wafer 3 are moved as a unit in a direction indicated by an arrow relative to the X-rays 1 in the form of a sheet beam produced by the SOR source 4 to transfer the pattern of the mask 2 onto a predetermined area of the wafer 3. In the scanning mirror exposure system shown in FIG. 3B, after the mask 2 and the wafer 3 are aligned in a predetermined relation, a mirror 301 disposed between the SOR source 4 and the mask 2 is swung in the direction indicated by an arrow to scan the exposure area (the entire area of the mask pattern to be transferred) with the X-rays 1 from the SOR source 4, thus transferring the pattern. In the whole surface exposure system shown in FIG. 3C, a mirror 302 having a convex reflecting surface is disposed between the SOR source 4 and the mask 2 to diverge the X-rays 1 from the SOR source 4. The diverged X-rays 1 are projected simultaneously onto the entire exposure area. In this manner, the mask pattern is transferred onto the wafer 2 after alignment therebetween. E. S. Piller proposes in "JT Applied Physics, Vol. 47, No. 12, p. 5450" that the mask 2 and the wafer 3 are disposed within a predetermined closed ambience in an X-ray exposure apparatus, and then the pattern exposure is carried out. It is also proposed therein that the mask 2 and the wafer 3 are disposed in a He gas ambience from the standpoint of temperature. Furthermore, common inventors have proposed in Japanese Laid-Open Patent Application Publication 178625/1985 that in an X-ray exposure apparatus using an X-ray tube, a state apparatus, mask and wafer transportation apparatus, an alignment apparatus or the like are disposed in a plurality of closed ambiences. Referring to FIGS. 4A and 4B, the apparatus disclosed in the above-mentioned Japanese Laid-Open Patent Application will be described. The wafer is contained in a cassette, which is contained in turn in a wafer loading cassette accommodating chamber 402. The exposed wafers are contained in a cassette in a wafer unloading cassette accommodating chamber 403. The mask having a pattern to be transferred is contained in a cassette, which is in turn contained in a mask cassette accommodating chamber 404. The mask and the wafer are aligned with each other by an electron beam device 411 shown in FIG. 4B in the main chamber 401, and thereafter they are shifted as a unit into an illumination chamber 405, where the pattern exposure is effected with the use of the radiation source, that is, the X-ray rube 410. Between the chambers, shut-off valves are disposed between respective chambers to maintain the ambiences thereof independently from each other, although FIG. 4B shows only the shut off valve 409 between the illumination chamber 405 and the main chamber 401. An SOR X-ray exposure apparatus is proposed in "Proceeding of SPIE, Vol 448, 1983, p 104", for example. FIG. 5 shows this apparatus schematically. This apparatus includes a so-called vertical stage which is movable in the vertical direction. The pattern exposure to the X-rays 1 is performed with the mask 2 and the wafer 3 being supported on the stage. SUMMARY OF THE INVENTION Taking as an example, a dynamic random access memory, which will hereinafter be called "DRAM" is recently a standard of the density of the semiconductor chip or device, 64 MB DRAM requires the line width of 0.3-0.4 micron, and more than 100 MB DRAM requires 0.25 microns of line width, and therefore, an exposure apparatus capable of pattern transfer of this dimension. However, no exposure apparatus capable of such a fine pattern exposure as not more than 0.25 micron has been made practical. In order to accomplish such an exposure apparatus, it is necessary to establish (1) ambience control for stably maintaining the mask and the wafer, (2) removal of contamination such as dust, (3) X-ray exposure for uniformly exposing the pattern transfer area, (4) an alignment with precision on the order of 1/100 micron and the resolution on the order of 1/1000 micron and others. Accordingly, it is a principal object of the present invention to provide an exposure apparatus which executes key processes in the semiconductor device manufacturing, capable of manufacturing semiconductor devices of 64 MB or higher density. It is another object of the present invention to provide an SOR X-ray exposure apparatus which uses X-rays from the SOR source and which can transfer a fine pattern on the mask onto a resist on a semiconductor wafer. It is a further object of the present invention to satisfy the requirements of the above-described (1)-(4) to accomplish a practical X-ray exposure apparatus. In order to realize manufacturing of the semiconductor devices having a pattern including lines of approximately 0.25 microns width, very high precision is required for the exposure apparatus. Table 1 shows various factors of the registration accuracy which is the major item of accuracy required for the semiconductor exposure apparatus and allocations thereof. TABLE 1 ______________________________________ (unit: micron) Items Reg. Precision ______________________________________ Mask manufacturing error 0.025 Wafer processing error 0.025 Stage accuracy 0.025 Alignment accuracy 0.025 Optical system 0.025 Other errors in exp. apparatus 0.025 Registration accuracy 0.06 ______________________________________ The "other errors in the exposure apparatus" in the above Table contains deformation or strains of the mask and the wafer due to heat. As will be understood from Table 1, the deformation of the mask and the wafer permitted in the exposure apparatus, the deformation permitted during the exposure, are 0.01 micron at maximum. Referring to FIGS. 6A and 6B, aspects of a typical SOR source 4 will be described. In FIG. 6A, the SOR source 4 is schematically shown as an orbit of electrons. The X-rays 1 are produced by brmsstrahlung of electrons accelerated to an extent of a relativistic velocity. The X-rays thus produced have a power profile which is Gaussian, in the vertical direction V with a divergence angle of several mRad. at maximum and with substantial uniformity in the horizontal direction H over a length open to the orbit of the electrons. The profile is generally trapezoidal. This profile is the one detected by an X-ray detector 1551 shown in FIG. 15 for example, at an exposure position where the mask and the wafer are present during exposure. A wavelength used in the SOR X-ray lithography according to the present invention is within a range of 5-15 angstroms. The electron energy accelerated to the relativistic velocity is approximately 0.5-1.0 GeV. FIG. 6B shows spectral strength characteristics for the respective wavelengths at various points on a path of the exposure beam, more particularly, a position immediately before the mirror, the position immediately after the mirror, a surface of the Be window, a mask surface and a resist surface in the order from the SOR source 4, when the X-ray exposure apparatus is constituted by disposing the mirror for reflecting the X-rays and the Be window between the SOR source 4 and the mask. It is understood from this Figure, 5-15 angstrom wavelength range is preferable in SOR X-ray lithography. From the same, it is understood that the SOR has continuous spectral characteristics. FIG. 7A shows a mask using inorganic material for the substrate 201, and FIG. 7B shows the same using organic film. In FIG. 7A, the substrate 201 joined to the frame 203 is constituted by a silicon wafer, and a silicon nitride, for example, is laminated in thin film in a pattern area 701 in which an absorber material pattern is to be formed. Or, the silicon wafer is laminated by etching. In this case, the thickness of the substrate in the pattern area 701 is approximately 2 microns. In the example of FIG. 7B, an organic film having a thickness of 20-10 microns is bonded to the frame 203. An example of the material of the organic film, there is polyimide. Referring back to FIG. 2 showing the proximity exposure system, the distance between the mask and the wafer during the exposure, which will hereinafter be called "proximity gap", is approximately 10-50 microns. Table 2 shows temperature rise of the silicon nitride mask of FIG. 7A and a polyimide mask of FIG. 7B when the ambience in the space between the mask and the wafer is vacuum, air or He. TABLE 2 ______________________________________ (Unit: degree) Ambient Medium Silicon Nitride Mask Polyimide Mask ______________________________________ Vacuum .sctn.60 .sctn.60 Air 1.17 1.18 He 0.343 0.351 ______________________________________ The data of the above Table is on the basis of the X-ray power of 120 mW/cm on the mask surface and on the thermal emissivity of 0.5 in the vacuum. In view of the facts that the sensitivity of the resist material at present is approximately several tens mJoule--one hundred mJoule/cm.sup.2 and that the transmissivity of the mask substrate to the X-rays is approximately 50%, the above data are appropriate. As will be apparent from the Table, in order to prevent thermal strain, it is preferable that the space between the mask and the wafer is filled with He gas. Therefore, in the present invention, the alignment between the mask and the wafer and subsequent pattern exposure are performed while the mask and the wafer are within a He ambience chamber. By containing the mask and the wafer in the chamber, the problems of contamination, such as dust, can be solved. Further, in the apparatus of the present invention, the temperature, pressure and purity of the gas functioning as a thermal conduction medium in the chamber are controlled with high precision so as to stabilize the X-ray transmissivity in this ambience, as in a vacuum. Referring to FIGS. 8A, 8B, 9A and 9B, the differences between the whole surface exposure system and the scanning exposure system (scanning mirror exposure system) will be described. FIG. 8A illustrates the whole surface exposure system wherein the X-rays 1 are applied simultaneously to the entirety of the pattern area 701. FIG. 8B illustrates strains of the mask resulting from this system. FIG. 9A illustrates the scanning exposure system wherein the X-rays 1 in the form of a sheet beam scan the pattern area 701 sequentially. FIG. 9B shows the mask strains or deformations resulting from this system. The X-rays produced from the SOR have a very small divergence in the vertical direction, and therefore, the half peak width of the X-ray power profile is only approximately 10 mm even if the exposure position is away from the SOR emitting point by a distance of 10 m. Therefore, in order to reduce the exposure period of the scanning exposure system (the same in the scanning mirror exposure system) to such an extent as being comparable to that of the whole surface exposure system, the strength of the X-ray applied to the mask should be several times that of the whole surface exposure system. This increases the mask strain. Table 3 shows the temperature rise and the mask deformation in the whole surface exposure system as compared with those in the scanning mirror exposure system. TABLE 3 ______________________________________ (Unit: degree, micron) Silicon Nitride Mask Polyimide Mask Exposure Temp. Max. Temp. Max. System Rise Strain Rise Strain ______________________________________ Scanning mirror 0.5 Hz 1.34 0.016 1.40 0.094 8 Hz 1.22 0.015 1.22 0.082 1000 Hz 0.343 0.0084 0.351 0.046 Whole 0.343 0.0084 0.351 0.046 Surface Exp. ______________________________________ The data of this Table are based on the material of the wafer chuck 1807 (FIG. 18) being alumina (Al) having a thickness of 0.5 mm under the condition that the back surface temperature is constant. With respect to the scanning mirror exposure system, the mirror swing is taken as a parameter. As will be understood from the above Table, the mask strain can be made under the tolerable level (0.01 micron) only by the whole surface exposure system and the scanning mirror exposure system, the latter being possible only when the mirror is swung at a high frequency. Also, it is understood that a usual polyimide film is not usable as a mask substrate. However, in consideration of the situation wherein the mirror 301 (FIG. 3B) is located in a high vacuum ambience of approximately 10.sup.-9 Torr., it is very difficult to realize that the mirror is swung at a high frequency over several tens of Hertz. Further, the difficulty is more significant in the scanning exposure system wherein the mask and the wafer are moved together. Therefore, the present invention employs the whole surface exposure system. In the exposure apparatus, the accuracy of the pattern line width is required to be 5-10% of the minimum width of the line to be transferred. In the exposure apparatus capable of exposing 0.25 micron of the minimum line width, the pattern line width accuracy is 0.012 micron. This means that the uniformity of the X-ray illuminance over the entire exposure area (pattern area 701) or the uniformity of the amount of the exposure over the entire exposure area is required to be approximately +2.5%. On the other hand, the strength profile of the SOR X-rays, as described hereinbefore, is in the form of the Gaussian function in the vertical plane, and therefore, it is not easy to profile the uniformity of approximately .+-.2.5% in the amount of the exposure over the entire exposure area in the whole surface exposure system. If the illuminance is made uniform over the entire exposure area, it cannot be avoided to use only the central portion of the Gaussian distribution, with the result that the efficiency of the X-ray energy is low. The present invention, adopts the exposure system shown in FIGS. 10A and 10B so as to solve those problems. In this system, the X-rays from the emitting point 1001 in the SOR source are incident on the X-ray mirror 1002 at a low glancing angle, and the X-rays diverged by the mirror 1002 are applied on the mask. The mirror 1002 is designed such that the minimum illuminance of the X-ray strength profile in the exposure area is maximum under the set conditions of the exposure apparatus. FIG. 11 shows the X-ray strength profile in the exposure area in this example. As will be understood, the illuminance is significantly different at the central area and the marginal areas of the exposure area. In this system, the illumination distribution can include .+-.10% difference in the exposure area. However, in the exposure system, the non-uniformity of the illuminance is corrected by the shutter mechanism 1003 during the exposure, so that the uniform exposure is effected over the entire exposure area. The shutter mechanism will be briefly described in conjunction with FIG. 10B. A steel belt 1009 is trained between a driving drum 1009 and an idler drum 1001 and is provided with a rectangular aperture 1012 having a leading edge 1004 and a trailing edge 1005. The Y axis is perpendicular to the optical axis of the SOR X-ray and is substantially vertical. A t-axis represents time. A curve 1006 indicates movement of the leading edge 1004, and a curve 1007 indicates the movement of the trailing edge 1005. The shutter mechanism drives the driving drum 1009 so that the exposure period .DELTA.T(y) at each point on the Y axis is different corresponding to the illuminance profile shown in FIG. 11, by which the amount of exposure (=exposure period.times.illuminance) is uniform over the entirety of the exposure area. In the exposure apparatus (FIG. 5) disclosed in the above-mentioned "Proceeding of SPIE, Vol. 448, 1983, p 104" the path of the X-ray from SOR source is once stopped by a Be window, and thereafter, the wafer is exposed to the X-ray through the wafer in the air. FIG. 12 shows the dependency of the thermal conductivity to pressure for air and He. It will be understood from this Figure that air has a lower thermal conductivity than He under the same pressure. Therefore, in consideration of the strain of the mask described with respect to Tables 2 and 3, it is difficult to achieve the object of the present invention by exposure in air. In consideration of this, the present invention adopts exposure in the closed He ambience for which the temperature, pressure and purity are controlled with high precision. In addition, in order to meet the SOR X-ray source, the conveying mechanism and the exposure stage are of vertical type, and the mask and the wafer are conveyed within a completely closed ambience. By this, in the SOR X-ray exposure apparatus, a high throughput and reduction of influence by particles (dust) and contamination are achieved. Furthermore, in the present invention, a wafer stage is controlled in 6 axes (X, Y, Z, .THETA. (=.omega.z, .omega.x, .omega.y)), and the mask stage is controlled in the .THETA. axis only, so as to achieve the high accuracy of the mask stage, in view of (1) that the optical axis of the X-ray from the SOR source hardly changes, (2) that the accuracy of the mask is sufficient as described hereinbefore and (3) that in the SOR X-ray exposure, the stability in the relation between the optical axis of the X-ray from the SOR source and the mask is most important. In the present invention, the mask stage is rotatable about the .THETA. direction in order to align the orientation of the mask with the movement direction in X and Y axes of the wafer which is step-and-repeat-exposed. In order to accomplish this mask alignment with great precision, a reference mark is provided on the wafer stage, and the mask alignment is effected using the reference mark. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.