METHOD FOR ALIGNING FIRST AND SECOND OBJECTS RELATIVE TO EACH OTHER, AND APPARATUS FOR PRACTICING THIS METHOD

Abstract: In a method for aligning first and second objects (2, 4) relative to each other, according to this inven­ tion, the first and second objects (2, 4) are arranged opposite to each other, and are aligned in a direction perpendicular to their opposing direction. A grating pattern (2l) is formed, as an alignment mark, on the first object (2), and a checkerboard-like grating pattern (22) is formed, also as an alignment mark, on the second object (4). A light beam emitted from an alignment light source (ll) is radiated onto the checkerboard-like grating pattern (22) of the second object (4). The light beam diffracted by the checkerboard-like grating pattern (22) is guided onto the grating pattern (2l) of the first object (2). The light beam diffracted by the grating pattern (2l) of the first object (2) is detected by a detector. Since the light beam emitted from the light source (ll) is diffracted by the checkerboard-like grating pattern (22), a relative position of the first and second objects (2, 4) can be detected, irrespective of the distance therebetween. The first and second objects (2, 4) are accurately aligned, based on the detection result. This invention can be applied to a method for aligning a mask (2) and a wafer (4) when a circuit pattern pre-formed on the mask (2) is to be transferred onto the wafer (4).

Description: The present invention relates to a method for aligning first and second objects relative to each other, and, more particularly, to a method for aligning a mask and a wafer when a circuit pattern is to be transferred onto a wafer, in the process of manufacturing a semiconductor device. Furthermore, the present invention relates to an apparatus for aligning first and second objects and, more specifically, to an apparatus for aligning a mask and a wafer. In the process of manufacturing a semiconductor device such as an LSI, an optical projection/exposure apparatus is used for transferring a circuit pattern onto a wafer. Using this apparatus, a circuit pattern pre-formed on a mask is reduced in scale and then transferred onto a wafer by a optical projection system and a mercury lamp. Before the circuit pattern is transferred, however, the mask and the wafer must be precisely aligned (mask alignment). More specifically, the mask and the wafer must be aligned in a direction perpendicular to the direction of light projection onto the circuit pattern. As examples of mask alignment methods, the TTL (through-the-lens) method and the off-axis method are known. When using the TTL method, a light beam from a projection optical system is projection onto diffraction gratings formed on each of a mask and a wafer, in order to detect the relative position of the mask and the wafer. In contrast, when using the off-axis method, an off-axis microscope, instead of the projection optical system, is used to align the mask with the wafer. The mask and the wafer are individually aligned with the off-axis microscope, outside the projection range of the circuit pattern. Then, the mask and the wafer, which are aligned with each other, are moved to within the projection range. For this reason, the throughput of the off-axis method is higher, using the off-axis method than when the TTL method is used. However, the off-axis method involves many error factors, and high-precision alignment is hard to attain. For this reason, the TTL method is the most widely-used method for aligning the mask and the wafer. One TTL mask alignment method, which involves completely overlapping two grating patterns, is disclosed in the article, G, Dubroeucq, l980, ME,W.R.Trutna.Jr., l984 SPI. When this method is employed, laser beams are radiated onto grating patterns respectively formed on a mask and a wafer, and diffracted beams are incident on a photodetector, so as to be converted to an electrical signal. The electrical signal is processed by a processor, and the relative position of the mask and the wafer is detected. The mask and the wafer are aligned based on the detection result. As is described above, when using the TTL method, laser beams is radiated through a projection lens of the projection optical system. The projection lens is arranged to minimize all aberrations with respect to a predetermined exposure wavelength of a mercury lamp. For this reason, when light which has a wavelength other than the exposure wavelength is radiated through the projection lens, chromatic aberration appears. The relative position of the mask and the wafer is detected, in a state wherein the distance between the mask and the wafer corresponds to a defocused state. In this case, sufficient detection sensitivity cannot be obtained, and reproducibility is also poor. Therefore, an error may occur in the alignment of the mask and the wafer. When the difference between two wavelengths becomes large, an aberration-correction means becomes necessary, in order to correct the optical path length. We acknowledge the disclosure in US patent number 4 631 416, in particular in Figure 1 thereof, of a method of detecting the degree of registration between a one-dimensional grating on a mask and a two-dimensional pattern on a wafer, by detecting the amplitude of a selected diffraction order of a laser beam. This forms the basis of the pre-characterizing portions of Claims 1 and 13 below. It is an object of the present invention to provide a method and apparatus for precisely aligning first and second objects relative to each other, irrespective of the distance between them, for example when the second object is a wafer and a circuit pattern is to be transferred onto the wafer. Accordingly, the invention provides a method as defined in Claim 1 below, and apparatus as defined in Claim 13 below. The first and second objects may be respectively a mask and a semiconductor wafer. In the preferred embodiment, when a circuit pattern pre-formed on the mask is to be transferred onto the wafer, the mask and the wafer are arranged to oppose each other along an axis, and are aligned in a direction perpendicular to that axis. A grating pattern, serving as an alignment mark, is formed on the mask, and a checkerboard grating pattern, also serving as an alignment mark, is formed on the wafer. Light emitted from an alignment light source is radiated onto a checkerboard grating pattern on the wafer. Then, light diffracted by the checkerboard grating pattern is guided onto the grating pattern of the mask. Light diffracted by the grating pattern of the mask is detected by detection means. Since the light emitted from the light source is diffracted by the checkerboard grating pattern, the relative position of the mask and the wafer is detected, irrespective of the distance therebetween. The mask and the wafer are accurately aligned based on the detection result. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figure 1 is a perspective view schematically illustrating a reduction projection/exposure apparatus in an embodiment wherein an alignment method of the present invention is applied to the reduction projection/exposure apparatus; Figure 2 is a plan view of a grating pattern formed on the mask; Figure 3 is a plan view of a checkerboard grating pattern formed on the wafer; Figure 4 is a perspective view schematically illustrating the structure of a spatial filtering mirror; Fig. 5 is a plan view of a diffraction image formed by a beam diffracted by a grating pattern of a wafer; Fig. 6 is a graph showing the relationship between the intensity of double diffraction light and the displacement of a mask and a wafer with respect to their setting positions; Fig. 7 is an illustration showing the focusing state of a diffracted beam incident on a mask surface; Fig. 8 is a block diagram of an apparatus for performing signal-sync detection processing, using vibrational plane-parallel; Fig. 9 is a signal waveform chart showing the relationship between a sync-detection output and the displacement of a mask and a wafer; Fig. 10 is a perspective view schematically illustrating a reduction projection/exposure apparatus in a second embodiment wherein an alignment method of the present invention is applied to the reduction projection/exposure apparatus; Fig. 11 is a perspective view which schematically illustrates the relationship between light reflected by a grating pattern of a wafer, and a reflection mirror; Figs. 12A to 12C are front views schematically illustrating a reduction projection/exposure apparatus according to a modification of the first embodiment of the present invention; Fig. 13 is a view showing a modification of a grating pattern; Fig. 14A is a sectional view showing a third embodiment, wherein an alignment method of the present invention is applied to an X-ray exposure apparatus; and Fig. 14B is a graph of output characteristics, showing the detection result of an alignment between the mask and the wafer, in the third embodiment. As is shown in Fig. 1, light source 1, for emitting an exposure light beam, is arranged in a reduction projection/exposure apparatus. Light source 1, comprises, for example, a mercury lamp. Light emitted from light source 1 is shielded by shutter 6, when it is not required. Mask 2, on which a circuit pattern is pre-formed, is arranged in the apparatus, and wafer 4, onto which the circuit pattern is transferred, is arranged in the apparatus. Wafer 4 is supported by wafer table 5 which is movable in a predetermined direction. As a circuit pattern projection optical system, condenser lens 7 is interposed between shutter 6 and mask 2, and demagnetization projection lens 3 is interposed between mask 2 and wafer 4. More specifically, the circuit pattern formed on the mask is reduced in scale and projected by projection lens 3. When shutter 6 is opened, and light emitted from the mercury lamp is radiated, the circuit pattern is transferred onto wafer 4. In this exposure apparatus, optical system 10 for aligning mask 2 and wafer 4, is arranged. A one-dimensional grating pattern, serving as an alignment mark, is formed on mask 2 as is shown in Fig. 2. A checkerboard-like grating pattern is formed on wafer 4, as can be seen in Fig. 3. Optical system 10 comprises laser 11 for emitting laser beams having good coherence. Optical system 10 further comprises spatial filtering mirror 12, deflector 14, and reflection mirror 15. Spatial filtering mirror 12 is constituted by transmission area 12-1 and reflection area 12-2, as is shown in Fig. 4. Filtering mirror 12 is a dichroic mirror which transmits the total amount of exposure light, when the circuit pattern is exposed. Filtering mirror 12 may be moved outside an exposure range, when the circuit pattern is exposed. In addition, photodetector 16 is arranged for receiving light diffracted by the grating pattern, and converting it into an electrical signal. Processor 17 processes the electrical signal. A method for aligning mask 2 and wafer 4, using the projection/exposure apparatus, will now be described. Laser beam 24 emitted from laser 11 is reflected by spatial filtering mirror 12, and is incident on grating pattern 22 of wafer 4, through projection lens 3. The diffracted beam reflected by grating pattern 22 is focused on grating pattern 21 of mask 2, via projection lens 3, spatial filtering mirror 12, and deflector 14. Grating pattern 21 is a checkerboard-like pattern, as can be seen in Fig. 3. For this reason, diffracted beams on a Fourier transfer plane of projection lens 3 are two-dimensionally diffracted beams. Fig. 5 shows diffracted beams of the 0th and ± first orders. The intensities of diffracted beams of the (±1,0)th and (0,±1)th orders are zero, and diffracted beams of the (0,0)th and (±1,±1)th orders have predetermined intensities. When these two-dimensionally diffracted beams 23 pass through spatial filtering mirror 12, mirror 12 shields diffracted beams of the (-1,-1)th, (1,-1)th and (0,0)th orders. The diffracted beams of the (±1,0)th and (0,±1)th orders have an intensity of zero. Therefore, the diffracted beams transmitted through spatial filtering mirror 12 are only those of the (1,1)th and (-1,1)th orders. These diffracted beams are incident on grating pattern 21 of mask 2, via deflector 14. The diffracted beams of the (1,1)th and (-1,1)th orders are again diffracted when they pass through grating pattern 21. The double-diffracted beams are detected by photodetector 16, and are converted to an electrical signal. The electrical signal is supplied to processor 17, to be processed, whereby the relative positions of mask 2 and wafer 4 are detected. In this manner, the double-diffracted beams which pass through grating pattern 21 provide positional information of grating patterns 21 and 22, i.e., relative position information. Thus, when the intensity of the double-diffracted beams is detected, this positional information can be obtained. For example, Fig. 6 shows the relationship between the intensity of the double-diffracted beams of the 0th order and displacements of the mask and the wafer with respect to their setting positions. More specifically, when position correction is performed to maximize the intensity of the diffracted beams of the 0th order, wafer table 5 is moved, so that mask 2 and wafer 4 are aligned. When employing a alignment method using a two-dimensional diffraction grating disclosed in the article G. Dubroeacq, the intensity of the double-diffracted beams of the 0th order, passing through a mask, can be given by the following formula I D , based on theoretical analysis: I D = Io × [(1/2)⁴ + (1/π)²·cos(πλZ/P²)·cos(2πX/P) + 4(1/π)·cos²(2πX/P)] where Io is the amount of light determined by the diffraction efficiency, P is the pitch of a diffraction grating, λ is the wavelength of an alignment light beam, X is the relative position of mask 2 and wafer 4, and Z is the amount of defocus. As can be seen from the above formula, I D varies greatly in accordance with a change in distance, in the Z direction, between the wafer and the mask. Using the alignment method of the present invention where a checkerboard-like diffraction grating is employed, the intensity of the double-diffracted beams of the 0th order can be given by the following formula Ic: Ic = Io × (2/π²)² × 2[l + cos(2π2X/P)] In this manner, since the above formula does not include the Z term, Ic does not vary in accordance with a change in distance, between the wafer and the mask, in the Z direction, i.e., a change in the amount of defocus Z. More specifically, two diffracted beams of (-1,1)th and (1,1)th orders from wafer 4 are focused on mask 2, as is shown in Fig. 7. Even if the focusing positions correspond to fl or f2 in Fig. 7, due to chromatic aberration of projection lens 3, an alignment-detection signal will not be changed. Therefore, the relative positions of the mask and the wafer can be detected, irrespective of any chromatic aberration which occurs when an alignment beam is radiated through projection lens 3. For this reason, detection errors can be prevented from occurring. As can be seen from the above formula, according to the method of this embodiment, a change in diffracted beam intensity Ic, with respect to detection position X, is represented by a cyclic function of (2X/P), i.e., two cycles appear per pitch. In the conventional method, a change in diffracted beam intensity Ip, with respect to detection position X, is represented by a cyclic function of (X/P), i.e., one cycle appears per pitch. For this reason, in this embodiment, a detection sensitivity twice that of the conventional method can be obtained, using the same size of grating patterns. The grating pattern has a limited pattern size with regard to miniaturization. For this reason, any improvement in detection sensitivity can be very advantageous with respect to high-precision alignment of the mask and the wafer. As regards the method of this embodiment, the wavelength of the alignment light and the exposure wavelength may be different from each other. A resist film on wafer 4 can be protected from being exposed during alignment. Thus, almost no alignment light is absorbed in the resist film. Since the grating pattern is not exposed, it can be easily protected. As has been described above, Fig. 6 shows the relationship between the intensity of the double-diffracted beams of the 0th order and displacements of the mask and the wafer, with respect to their setting positions. More specifically, when position correction is performed to maximize the intensity of the double-diffracted beams of the 0th order, mask 2 and wafer 4 are aligned. In this case, this correction means must detect the peak of a cycle function shown in Fig. 6. However, it is, in general, difficult to detect the peak of the cycle function. Thus, as a method for processing a detection signal, a detection method wherein alignment light beams are vibrated at a constant frequency to modulate a signal and the frequency is sync-detected, is proposed (this sync detection method is described in detail in U.S.P. No. 681,491 to the present inventors). In this embodiment, as a vibration mechanism, plane-parallel 14 is vibrated, as is shown in Fig. 8. Plane-parallel 14 is arranged between mask 2 and wafer 4. An alignment beam reflected by wafer 4 is changed in synchronism with vibration of plane-parallel 14. A signal obtained by photodetector 16 is a modulation signal synchronous with this vibration. An output signal from photodetector 16 is sync-detected by sync detector 32, with reference to a reference signal from oscillator 33 which serves as an oscillation source for vibration plane-parallel 14. An output, following sync-detection processing, is obtained as an S-shaped curve with respect to a positional displacement. Since the detection output is represented by a line crossing zero displacement, the mask and the wafer are aligned with very high precision. A second embodiment of the present invention will now be described, with reference to Fig. 10. Differences between this embodiment and the previous embodiment are that spatial filtering mirror 12 is replaced by half mirror 18, and reflection mirrors 13-1 and 13-2 are arranged midway along the optical path of optical system 10. As is shown enlarged in Fig. 11, only beams of the (1,1)th and (-1,1)th orders of diffracted beams 23, reflected by grating pattern 22, are reflected by mirrors 13-1 and 13-2. Other arrangements are the same as in the previous embodiment. Grating patterns 21 and 22, respectively formed on mask 2 and wafer 4, have the same patterns as shown in Figs. 2 and 3. With this arrangement, substantially the same alignment as in the above embodiment can be performed. More specifically, half mirror 18 allows all the diffracted beams, which are reflected by wafer 4 and which are incident on projection lens 3, to pass therethrough. Thereafter, some of diffracted beams, for example, beams of the (1,1)th and (-1,1)th orders, are reflected by mirrors 13-1 and 13-2. For this reason, an image is focused on mask 2 by the interference of two beams, in the same manner as in the above embodiment. For this reason, alignment of the mask and the wafer can be performed irrespective of the defocus state. Therefore, with this embodiment, the same effect as in the above embodiment can be obtained, and the mask and the wafer can be aligned with high precision. The present invention is not limited to the methods of the above embodiments. For example, in the first embodiment, a simple half mirror can be used instead of the spatial filtering mirror. In this case, all the diffracted beams, which are reflected by wafer 4 and which are incident on projection lens 3, are incident on the grating pattern on mask 2. Thus, the interference of two beams will not occur. The defocus state of mask 2 and wafer 4 thus influences the detection sensitivity. However, two cycles of change appear per pitch in detection position X of the diffracted beam intensity, as in the above embodiments. Therefore, the alignment precision can be improved to twice that in the conventional method. As is shown in Fig. 12, the following various methods can be adopted to a means for illuminating the grating pattern of a wafer. In Fig. 12A, an alignment beam which is transmitted, through mask 2, and then through half mirror 18, illuminates the grating pattern of the wafer via projection lens 3. In Fig. 12B, an alignment beam is incident, from the bottom of half mirror 18, and illuminates the grating pattern of the wafer via mirror 13-2 and projection lens 3. In Fig. 12C, an alignment beam is incident from the top of half mirror 18, and illuminates the grating pattern on the wafer, along an optical path directly opposite to that in a detection mode. Although these methods provide illumination in different ways, their output characteristics of detection signals are essentially the same. In addition, various other illumination methods can be employed. In the above embodiment, a means for vibrating a plane-parallel to vibrate the position of a beam, is adopted as a signal modulation means. Alternatively, a means for vibrating a vibration mirror can be adopted. A means for vibrating a mask or wafer, in a horizontal direction, can be used for vibrating the position of a beam. A means for alternately illuminating two sets of grating patterns having different phases, can also be adopted as a signal modulation means. In the above embodiments, a grating pattern formed on a mask is a one-dimensional diffraction grating. The grating pattern of the mask may be a checkerboard-like pattern, as is shown in Fig. 3, or it may be a two-dimensional diffraction grating. In the above embodiments, a checkerboard grating pattern on a wafer is in the form of a combination of squares, as is also shown in Fig. 3. However, the grating pattern of a wafer can be a checkerboard pattern in the form of a combination of rectangles, as shown in Figure 13. The present invention can be applied not only to the process of alignment between a mask and a wafer, but can also be applied to the process of aligning between two objects. In this latter case, an optical system need not be interposed between the two objects. For example, as is shown in Fig. 14A, in an X-ray exposure apparatus, one-dimensional grating pattern 21 is formed on first object 2, and checkerboard-like grating pattern 22 is formed on second object 4. Light emitted from laser 11 is radiated onto grating pattern 21, and the light diffracted thereby is incident on checkerboard-like grating pattern 22. Light that is reflected and diffracted by pattern 22 is incident on photodetector 16, through grating pattern 21. The light detected by photodetector 16 is converted to an electrical signal, and the signal is processed by a processor. This detection result is indicated by M of Fig. 14B. In Fig. 14B, a detection result, indicating that a two-dimensional grating pattern has been formed on a second object, as in a conventional detection method, is indicated by N. In this manner, the conventional detection method is influenced by distance d between mask 2 and wafer 4. Therefore, a detection error is present. In contrast to this, in this embodiment, irrespective of distance d , a detection error will not occur, and the relative positions of a mask and a wafer can be detected with high precision, thereby enabling them to be very precisely aligned.

Claims: A method for mutually aligning first and second mutually opposed objects in an alignment direction perpendicular to an axis along which the objects oppose each other, comprising the steps of: forming a diffraction grating pattern (21), as an alignment mark, on said first object (2); forming a two-dimensional diffraction grating pattern (22), as an alignment mark, on said second object (4); directing a light beam, emitted from a light source (11), onto said two-dimensional grating pattern (22) on said second object (4); transferring the diffracted light beam, diffracted at said two-dimensional grating pattern (22), onto said grating pattern (21) on said first object (2); and detecting the diffracted light beam resulting from transmission through said grating pattern (21) of the first object (2) and adjusting the relative position of said first and second objects (2, 4) in the alignment direction in accordance with the intensity of the detected diffraction beam; characterized in that: the two-dimensional grating pattern (22) is a checkerboard pattern having square or rectangular areas. A method according to Claim 1, in which the first and second objects are respectively a mask (2) and a semiconductor wafer (4). A method according to Claim 2, characterized in that an optical system (3) which allows the diffracted beam to pass therethrough is arranged between said mask (2) and said wafer (4). A method according to Claim 2, characterized in that in the step of transferring the diffracted beam, the beam diffracted by checkerboard grating pattern (22) is selected by a spatial filtering mirror (12) to be guided onto said grating pattern (21) of said mask (2). A method according to Claim 3 or 4, characterized in that said optical system (3), arranged between said mask (2) and said wafer (4) for allowing the diffracted beam to pass therethrough, is an optical projection system (3) which is also used for transferring a circuit pattern from the mask (2) onto the semiconductor wafer (4). A method according to Claim 5, comprising the further step of transferring an electronic circuit pattern from the mask (2) onto said wafer (4), in which the wavelength of the beam for aligning said mask (2) and said wafer (4) is different from the wavelength in said optical projection system (3) used for transferring the circuit pattern. A method according to any preceding claim, characterized in that said light source used in the step of directing a light beam is a laser (11). A method according to Claim 2 or any preceding claim appendant to Claim 2, characterized in that said checkerboard grating pattern (22) formed on said wafer (4) is of a reflection type. A method according to Claim 4, characterized in that, in the step of detecting the diffracted beam, the intensity of a double-diffracted beam of the 0th order diffracted by this grating pattern (21) is detected. A method according to any preceding claim, characterized in that said checkerboard grating pattern (22) is formed by combining a plurality of squares. A method according to any of Claims 1 to 8, characterized in that said checkerboard grating pattern (22) is formed by combining a plurality of rectangles. A method according to Claim 1 or 2, comprising the steps of; transferring the light beam transmitted through said grating pattern (21) of said first object (2), to said checkerboard grating pattern (22) of said second object (22); and transferring the-light beam diffracted and reflected by said chackerboard grating pattern (22), to said grating pattern (21) of said first object (2). Apparatus for mutually aligning first and second mutually opposed objects (2, 4) in an alignment direction perpendicular to an axis along which the objects oppose each other, comprising: a diffraction grating pattern (21) formed, as an alignment mark, on said first object (2); a two-dimensional diffraction grating pattern (22) formed, as an alignment mark, on said second object (4); a light source (11) for emitting a beam for detection; and detection means (10) for detecting the beam which is emitted from said light source (11) and which is diffracted at said grating pattern (21) of said first object (2) and said two-diamensional grating pattern (22) of said second object (4) and adjusting means for adjusting the relative position of said first and second objects (2, 4) in the alignment direction in accordance with the intensity of the detected diffracted beam; characterized in that the two-dimensional grating pattern (22) is a checkerboard pattern having square or rectangular areas. Apparatus according to Claim 13, in which the first and second objects are respectively a mask (2) and a semiconductor wafer (4). Apparatus according to Claim 14, characterized in that an optical system (3) for allowing the diffracted beam to pass therethrough is arranged between said mask (2) and said wafer (4). Apparatus according to Claim 14 or 15, characterized by further comprising means for selecting the beam diffracted by said checkerboard grating pattern (22) of said wafer (4). Apparatus according to Claim 16, characterized in that said means for selecting the beam diffracted by said checkerboard grating pattern (22) of said wafer (4) commprises a spatial filtering mirror (12). Apparatus according to Claim 16, characterized in that said means for selecting the beam diffracted by said checkerboard grating pattern of said wafer comprises a half mirror (18) and at least one reflection mirror (13-1, 13-2). Apparatus according to Claim 15, 16, 17 or 18, characterized in that said optical system, arranged between said mask (2) and said wafer (4), for allowing the diffracted beam to pass therethrough, is an optical projection system (3) capable of use for transferring an electronic circuit pattern onto the wafer (4). Apparatus accordng to any of Claims 13 to 19, characterized in that said light source is a laser (11). Apparatus according to any of Claims 13 to 20, characterized in that said grating pattern (22) formed on said wafer (4) is of a reflection type. Apparatus according to any of Claims 13 to 21, characterized in that said checkerboard grating pattern (22) is formed by combining a plurality of squares. Apparatus according to any of Claims 13 to 21, characterized in that said checkerboard grating pattern (22) is formed by combining a plurality of rectangles.

Inventors: SHIMOZONO, HIROAKI, TABATA, MITSUO C/O PATENT DIVISION, TOJO, TORU C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA, TOKYO KOGAKU KIKAI KABUSHIKI KAISHA
Application year: 1987
Main classification: G03B41
IPC codes: H01L21, G03F9