JP2664712B2 - Positioning method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a method for aligning two objects, and particularly a position suitable for alignment of a mask used for pattern transfer with a wafer. Regarding the matching method.

(Prior Art) In recent years, with the miniaturization of circuit patterns of semiconductor elements such as LSIs, optical projection exposure apparatuses having high resolution performance have been widely used as pattern transfer means.
When transfer is performed using this apparatus, it is necessary to align the mask and the wafer with high accuracy (mask alignment) prior to exposure.

As a method of performing the mask alignment, another optical system (off-axis microscope) different from the projection optical system is used to detect a pre-formed mark on the wafer to position the wafer, and thereafter the projection optical system is used. An off-axis system that moves to a predetermined position within the field of view with high precision and aligns with a mask that has been accurately positioned in advance, and an alignment mark formed in advance on the mask and wafer is detected through a projection optical system In addition, there is a TTL (Through The Lens) method for directly aligning a mask and a wafer.

The off-axis method has the advantage that the number of alignments is small, the time required for alignment is short, and the throughput (processing speed) is large. However, it is necessary to move the aligned wafer to the position to be transferred by an accurate distance, and an absolute length measurement system must be provided. Is difficult. Therefore, recently, in order to perform alignment with higher accuracy, a method of directly detecting and detecting a mark of a mask and a wafer through a projection optical system, such as a TTL method, has become effective.

As one of the alignment methods of the TTL method, a method of superimposing two grating marks (G. Dubroe
ucq, 1980, ME, WR Trutna. Jr. 1984, SPIE, etc.). This is because, as shown in FIG.
A light for alignment is made incident on the mask 81 and the wafer 83 which are formed so as to face each other with the projection lens 82 interposed therebetween, and the light diffracted by the two marks 81a and 83a is detected by the photoelectric detector 84. This is a method of detecting the overlapping state of the two grating marks 81a and 83a, that is, the relative position.

According to this method, the signal strength becomes maximum (or minimum) in a state where the two grating marks 81a and 83a overlap (or are shifted by half a pitch) as shown in FIG. Therefore, high-precision alignment can be performed by signal processing (for example, vibration-type synchronous detection processing or the like) that can accurately detect the maximum value (or minimum value).

By the way, using such a grating mark
The TTL alignment method has the following problems. That is, a normal projection lens is designed such that all aberrations are minimized only at the exposure wavelength, but there is chromatic aberration at other wavelengths. On the other hand, in the above-mentioned alignment method, a laser having good coherence is used as a light source, but since a mercury lamp is used as an exposure light source, it is difficult to make the wavelengths of the two light sources exactly the same. For this reason, the chromatic aberration cannot be ignored for the alignment light, and the position must be detected in a defocused state unlike the exposure wavelength at the imaging position relationship, that is, the focus position. Conventionally, when the position is detected in such a defocused state, sufficient detection sensitivity cannot be obtained and reproducibility is poor. Also, when the difference between the two wavelengths becomes large, aberration correction means such as correcting the optical path length is required, which complicates the apparatus and increases error factors. That is, since the wavelength of the alignment light cannot be made the same as the wavelength of the exposure light, there is a serious problem that the light is affected by the aberration of the projection lens.

In particular, a He-Ne laser (633 nm) is often used because the alignment light does not need to expose the resist, and the transfer exposure wavelength (λ) is generally R = Kλ.
Due to the relationship of / NA, the resolution R tends to be shorter in order to increase it. At present g-line (436 nm) is the mainstream, but i-line (3
65nm) and excimer laser (KrF 248nm,
ArF (193 nm) is expected.

For this reason, the above problem cannot be ignored. Conventionally, this problem of chromatic aberration has been absorbed by providing folding mirrors 85a and 85b in the middle as shown in FIG. However, as the wavelength difference increases, the structure of the folding mirror and the lens system becomes more complicated and impractical.
Here, in the figure, 86 is a He-Ne laser for alignment, 87 is a half mirror, and 88 is a reflection mirror.

On the other hand, pattern transfer for forming a semiconductor element is not performed only once, but is repeated every time an oxide film, an Al film, or the like is deposited on a wafer. The deposited film remains on the alignment mark (wafer mark) formed on the wafer without being removed by etching or the like after transfer. Therefore, an oxide film, an Al film, or the like is deposited in multiple layers on the wafer mark as the process proceeds. Then, the presence of the deposited film causes a reduction in the accuracy of the alignment using the wafer mark.

(Problems to be Solved by the Invention) As described above, conventionally, it has been difficult to align a mask / wafer with high accuracy due to a problem of chromatic aberration caused by a difference between an exposure wavelength and an alignment wavelength. Further, when a folding mirror is used as a solution to chromatic aberration, when the exposure wavelength and the alignment wavelength are largely different, the mirror interval needs to be largely separated, which is not practical. Also, as the process progresses, multiple layers are deposited on the wafer mark,
This has been a factor in lowering the positioning accuracy.

The present invention has been made in consideration of the above circumstances, and an object thereof is to accurately determine the relative position of two objects even when the wavelength of the light for alignment is different from the wavelength suitable for the optical system. It is another object of the present invention to provide a positioning method which can be easily realized without using a folding mirror or the like and can improve the positioning accuracy.

[Structure of the Invention] (Means for Solving the Problems) The gist of the present invention is that in a TTL alignment method using an alignment mark, even if light having a wavelength different from the wavelength suitable for the optical system is used as alignment light. Another object of the present invention is to improve a mark arrangement and an alignment light illumination method so that the influence of chromatic aberration of an optical system (projection lens) can be ignored. Another object of the present invention is to irradiate the film deposited on the wafer mark with exposure light so that the deposited film on the wafer mark can be removed in a subsequent etching step.

That is, prior to transferring the pattern formed on the mask onto the wafer (via a projection lens or as it is), the present invention uses the alignment marks provided on the mask and the wafer, respectively. In an alignment method for determining an optical relative positional shift amount and aligning a mask / wafer in accordance with the positional shift amount, the wafer is provided with a positioning wafer mark formed of a diffraction grating, and the mask is provided on the mask. A pair of mask marks for alignment consisting of a diffraction grating are provided at a predetermined distance from each other, and a window between the pair of mask marks can selectively irradiate a wafer mark with exposure light, or a pattern for forming a wafer mark. The mask mark is irradiated with alignment light having a wavelength different from that of the exposure light, and two diffracted lights are projected from the mask mark (projection light). Via's or directly) condensing on the wafer mark, to interfere, a method to obtain the detecting and relative displacement information of the marks reflected diffracted light from the wafer mark.

(Operation) According to the present invention, two diffracted lights from a pair of mask marks separated by a predetermined distance are condensed and interfered with a wafer mark, thereby completely eliminating the problem of a difference in imaging interval due to chromatic aberration. Alignment can be performed without it. Therefore,
It is not necessary to provide intermediate members such as a folding mirror and a lens which have been conventionally used for chromatic aberration correction, and it is possible to eliminate an error factor occurring therefrom. Further, since a folding mirror is not required, the structure is simple and the optical axis adjustment is easy. In particular, it is currently a problem with excimer laser steppers.
TTL alignment can be realized by this method.

Also, in the present invention, attention is paid to the fact that the exposure light corresponding to the position of the wafer mark passes through a portion located at the center of the pair of mask marks, and a window (or a new wafer mark formation) for exposing the wafer mark to this portion. Pattern). Therefore, in the pattern transfer after the alignment, the resist on the wafer mark is exposed, the deposited film on the wafer mark is removed by the etching process, and the wafer mark is exposed. Therefore,
It is possible to prevent the deposition film from being deposited on the wafer mark with the progress of the process, and it is possible to contribute to the improvement of the alignment accuracy.

(Examples) Hereinafter, details of the present invention will be described with reference to the illustrated examples.

FIG. 1 is a schematic structural view showing a reduction projection exposure apparatus used in the method of one embodiment of the present invention. Usually, the reduction projection exposure apparatus places a reticle (mask) 10 and a wafer 30 above and below a projection lens 20. The projection lens 20 is positioned such that the object image interval between the mask 10 and the wafer 30 is set so that the imaging relationship is maintained with respect to the exposure wavelength. In this state, the data light forms an image on the wafer 30 from the point C of the mask 10 through the projection lens 20. Conversely, light emitted from the wafer 30 forms an image on the mask 10 through the projection lens 20. But,
A wavelength different from the exposure wavelength as the alignment light, for example,
When a He-Ne laser or the like is used, this imaging relationship is disrupted,
The wafer image is formed on the virtual mask 10 'indicated by a two-dot chain line separated by the distance D. Usually, when this amount is small, correction is performed by a folding mirror or the like as shown in FIG. However, for example, an exposure wavelength of 248 nm and 633
When the wavelength difference is large, such as He-Ne laser light (alignment light) of nm, this amount becomes several meters, and it is not practical to correct the light with a folding mirror or the like.

Therefore, in this embodiment, this problem is solved by the method described below. That is, in FIG.
When the mark on the mask 10 'is viewed from above, the mark information comes with the light flux shown by the two-dot chain line. Therefore, for example, marks 11 and 12 (mask marks) are formed at two points A and B on the mask 10, and the marks 11 and 12 are irradiated with alignment light.
If the light beam emitted from the light beam has the mark position information coming from above the mask 10 ′, the light beam is condensed on the wafer 30. That is, the mark position information is created on the mask 10 as if it were on the mask 10 '.

Specifically, the A and B marks 11 and 12 are each set as a lattice mark, and the alignment light is irradiated as a spherical wave so as to be focused on the entrance pupil of the lens as shown in the figure. As a result, diffracted lights of the 0th order and ± 1st order are generated from the marks 11 and 12, respectively, but the 0th order enters the entrance pupil as it is. Mark 11
-1st order light from the mark 12 and + 1st order light from the mark 12 are condensed at one point on the wafer 30 through the projection lens 20 having respective positional information. At this point, the light beams interfere with each other to form a moire pattern (that is, a periodic pattern image depending on the angle θ).

In this case, as a design, the magnification β of the virtual mask position with respect to the wafer with respect to the alignment light of the projection lens 20 is obtained, and the distance D is similarly obtained. Therefore, the interval 2r between the marks 11 and 12 on the mask 10 can be obtained by 2r = 2Dtanβθ (1). Here, θ is the angle formed by the light beam condensed on the wafer 30. Further, the incident angle θ _{r of the} alignment light incident on the mask 10 is also determined by the lens constant of the projection lens 20. At this time, it is necessary to match the angles of the diffracted light emitted from the marks 11 and 12 of the A and B on the mask 10 as if they were light beams from the mask 10 '. This mark pitch P _r also can be obtained by _{sinθ r + sinβθ = nλ / P} r ... (2). Here, λ is the wavelength of the alignment light, and n is the diffraction order (± 1 in the figure).

On the wafer 30, a lattice mark 31 (wafer mark) corresponding to the moiré pattern is formed, for example, on a dicing line. Light diffracted again by this wafer mark 31,
For example, in the case of the drawing, the reflected and diffracted light returning vertically is extracted, guided by a mirror 41 and the like, and detected by a detector 42. In the detector 42, a light intensity corresponding to the lateral relative displacement between the wafer 30 and the mask 10 is obtained. With respect to this light intensity, a change in light intensity corresponding to the grating pitch of the mark 31 on the wafer 30 is obtained as shown by a double diffraction grating method or the like. In this method, each mark can be aligned without placing any structure between the mask 10 and the wafer 30, and a moire pattern is formed on the wafer 30 by interference of two light beams. Because of the formation, the image of the moire pattern does not move due to the vertical defocus of the wafer 30. Therefore, it has many features such that the presence of the defocus amount does not affect the alignment accuracy.

By performing signal processing by the signal processing circuit 43 based on the output from the detector 42, a position detection signal can be obtained. As signal processing at this time, for example, the wafer 30 is vibrated in the horizontal direction, the mask 10 is vibrated in the horizontal direction, or a part of the alignment light is modulated by a method such as moving a moire pattern image, and the frequency is synchronously detected. It is possible to do. Also, by changing the frequency of the left and right alignment light, creating the beat signal on the wafer,
It is also possible to measure by a so-called optical heterodyne method in which the relative displacement is known from the phase difference as compared with the reference beat signal.

A specific example of FIG. 1 is further shown in FIG. The alignment light enters the marks 11 and 12 on the mask 10 through the lens 45 and the condenser lens 46, for example. This light is lens 20
At the center of the entrance pupil (shown by a broken line). The ± first-order lights generated from the marks 11 and 12 pass through the entrance pupil and converge on the wafer mark 31 as two light beams. Wafer mark
Reference numeral 31 denotes a pattern such as a one-dimensional lattice shown in FIG. 3 (a), a secondary lattice shown in FIG. 3 (b), and a checkered lattice shown in FIG. 3 (c). These choices should be made in the best way depending on the use conditions of the transfer device. For example, in order to perform alignment using the alignment light during the transfer of the mask pattern, we use one of FIGS. 3 (b) and 3 (c) to reflect the reflected diffraction light two-dimensionally. And diffracted into
The mirror 41 is selected to be shifted outward by a mask pattern so as not to block the exposure illumination light. If necessary, the marks 11, 12 of the mask 10 can be formed as a two-dimensional lattice.

By the way, in such an alignment method, an oxide film, an Al film, or the like is deposited on the wafer 30 every time the transfer is repeated as described above, and the wafer mark 31 may be buried in these deposited films. Therefore, in this embodiment, a window 13 is provided at the center between the pair of mask marks 11 and 12, so that the wafer mark 31 is exposed by exposure light. That is, the exposure light corresponding to the position of the wafer mark 31 is applied to the pair of mask marks 11,1.
Focusing on passing through a portion located in the center of 2, a window 13 for exposing a wafer mark is provided in this portion. The window 13 is larger than the wafer mark 31 when the reduced image is formed by the projection lens 20, and is formed by removing a light shielding member such as chrome of the mask 30.

With such a configuration, when the alignment of the mask / wafer is performed using the alignment light, the diffracted light from the mask marks 11 and 12 is reflected on the wafer 3 as shown by the dashed line in FIG.
It is irradiated on the 0 mark 31. At this time, as shown in FIG. 5A, a first laminated film 51 such as an insulating film or an Al film is deposited on the wafer 30, and a resist (not shown) is applied thereon. Unlike the exposure light, the alignment light is selected to have a wavelength at which the resist is not exposed, so that the resist is not exposed during this alignment.

After the alignment is completed, the mask pattern is transferred onto the wafer 30 by irradiating the mask 10 with exposure light. At this time, the resist on the wafer mark 31 is also exposed through the window 13. Then, the resist is developed to form a resist pattern, and the deposited film 51 is etched using the resist pattern as a mask. However, since the resist on the wafer mark 31 has been exposed, the fifth
The deposited film 51 on the wafer mark 31 is also etched as shown in FIG. In the next process, the film deposited on the wafer again
52 is deposited, and the same alignment and transfer as described above are performed. At this time, the wafer mark 31 is displayed on the wafer mark 31 as shown in FIG.
As shown in (1), a new deposited film 52 is deposited. This state corresponds to the wafer mark as shown in FIG.
This is similar to the case where only one layer of the deposited film exists on 31, so that alignment can be performed with the same accuracy as above.

However, when the window 13 is not formed, the resist on the wafer mark 31 is not exposed at the time of exposure, so that the deposited film 51 remains even in the subsequent etching process. Therefore, in the next alignment and transfer steps, the deposited films 51 and 52 remain on the wafer mark 31 as shown in FIG. As described above, when the number of deposited films is increased and the film thickness is increased, errors such as the asymmetricity of marks due to deposition and the like are accumulated, and alignment accuracy is reduced. Further, since this deposited film is laminated every time the deposition of the film and the pattern transfer are repeated, the alignment accuracy decreases as the process proceeds. In this embodiment with a window, the wafer mark
Since only one layer of deposited film exists on 31, accurate alignment can always be performed regardless of the progress of the process.

The actual alignment wafer mark 31 is shown in FIG.
As shown in FIG. 6, a plurality of marks are required depending on the process, and accordingly, the mask marks 11 and 12 are also changed as shown in FIG.
It is necessary to prepare a large number of marks as shown in FIG. The figure shows a case of four mark groups, and marks formed on a plurality of masks are equivalently shown in the same drawing. The part shown by a broken line in FIG.
Window 13 at a position corresponding to A 'to DD'. Depending on whether or not the chromium in this portion is removed, the wafer mark 31 is irradiated with exposure light, and the resist can be removed by development. Therefore, it is necessary to increase the distance between the mask marks 11 and 12 so that the mask marks 11 and 12 and the window 13 do not buffer. This relationship is (N = 1). Here, 2l is the length of the wafer mark group, and β 'is the mask-wafer magnification of the exposure light (normally β' = 1/5 to 1/10). After determining the interval of 2r in this way, the mask grating pitch _Pr is determined by the equation (2).

With such a mark arrangement, the wafer mark group and the mask mark group do not interfere with each other, and
By selectively providing the window 13 through which the exposure light passes between 11 and 12, the wafer mark 31 can be preserved. As the resist, there are a positive resist and a negative resist, but a window on the reticle may be selected as necessary.

Thus, according to the present embodiment, two diffracted lights generated from the pair of mask marks 11 and 12 provided at positions separated by a predetermined distance are condensed and interfered with the wafer mark 31,
The alignment can be performed completely eliminating the problem of the difference in the imaging interval due to chromatic aberration. That is, the problems caused by the chromatic aberration of the projection lens 20 which have conventionally been a problem when the wavelength difference is large can be easily solved by the TTL alignment method using the alignment mark, and the mask 10 and the wafer 30 can be easily solved.
Can be detected very accurately, and accurate positioning can be performed. Accordingly, there is no need to provide intermediate members such as a folding mirror and a lens which have been conventionally used for chromatic aberration correction, and it is possible to eliminate an error factor caused thereby. Furthermore, since a folding mirror or the like is not required, the structure is simple and the optical axis adjustment is easy. In particular, it is currently a problem with excimer laser steppers. TTL alignment can be realized by this method.

Further, since the window 13 is provided at the center between the mask marks 11 and 12, the deposited film can be removed from the wafer mark 31 after the alignment, and the wafer mark 31 can be preserved. Therefore, high-precision alignment can always be performed irrespective of the number of transfers, and its usefulness is enormous.

The present invention is not limited to the embodiments described above. For example, a pattern for forming a wafer mark may be provided instead of the window. In this case, when exposing the mask pattern after the alignment, the wafer mark is exposed at a predetermined position (usually a dicing line) of the wafer, and the wafer mark is formed when the deposited film is removed by etching. Then, this mark can be used in the next alignment. Further, although a projection lens is used in the embodiment, the present invention can be applied to a transfer apparatus that does not use a projection lens, such as X-ray transfer. In this case, as shown in FIG. 7, marks 71 and 72 are provided on the X-ray mask 70, and a window 73 or a pattern 73 for forming a wafer mark is provided at the center between the marks 71 and 72. Then, diffracted light from the mask marks 71 and 72 is
Then, the reflected diffraction light from the wafer mark 31 may be detected by the sensor 74 or the like. The alignment illumination light was irradiated from above the condenser lens 46 in FIG.
Irradiation may be performed from directly above. In addition, the detector
Although the mirror 41 is provided between the mask and the wafer and guided to the mask 42, the mirror 41 may be guided from the upper surface of the mask 10. In addition, various modifications can be made without departing from the scope of the present invention.

[Effects of the Invention] As described above in detail, according to the present invention, even when the wavelength of the light for alignment is different from the wavelength suitable for the optical system, the present invention is not limited thereto.
The relative position of the two objects can be detected with high accuracy, and can be easily realized without using a folding mirror or the like, and the accuracy of alignment can be improved. Furthermore, by providing a window for exposing a wafer mark or a pattern for forming a wafer mark between a pair of mask marks, the wafer mark can be preserved, so that accurate alignment is always achieved regardless of the progress of the process. It can be carried out.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic configuration diagram showing a projection exposure apparatus used in a method of one embodiment of the present invention, FIG. 2 is a schematic configuration diagram showing a more specific version of FIG. 1, and FIG. FIG. 4 is a plan view showing an example of a mark on a wafer or a mask, FIG. 4 is a schematic diagram showing a relationship between a mask mark and a window and a wafer mark, and FIG. 5 is a cross section showing a state of a deposited film deposited on the wafer. FIG. 6, FIG. 6 is a plan view showing an example of a mark layout, FIG. 7 is a schematic view showing a modification of the present invention, and FIGS. 8 to 10 are diagrams for explaining problems in the related art. is there. 10 …… Mask, 11,12 …… Mask mark, 13 …… Window, 20
…… Projection lens, 30 …… Wafer, 31 …… Wafer mark,
41: Reflecting mirror, 42: Detector, 43: Signal processing circuit, 44: Phase shift mechanism, 45: Lens, 46: Condenser lens.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsuhiko Higashiki 1 Toshiba-cho, Komukai, Koyuki-ku, Kawasaki-shi, Kanagawa Prefecture (72) Inventor Toshikazu Yoshino 75-1 Hasunumacho, Itabashi-ku, Tokyo (56) References JP-A-60-66820 (JP, A) JP-A-61-73958 (JP, A) JP-A-63-14430 (JP, A)

Claims (3)

(57) [Claims] 1. Prior to transferring a pattern formed on a mask onto a wafer, using an alignment mark provided on each of the mask and the wafer, an optical relative positional shift amount of these marks is obtained. In a positioning method for positioning a mask / wafer in accordance with the amount of positional deviation, a wafer mark for positioning comprising a diffraction grating is provided on the wafer, and a pair of positioning marks comprising a diffraction grating is provided on the mask. And a window for selectively irradiating the wafer mark with exposure light, or a pattern for forming a wafer mark is provided at the center between the pair of mask marks, and a wavelength different from the exposure light is provided. Irradiating the mask mark with the alignment light of the above, and converging and diffracting two diffracted lights from the mask mark on the wafer mark,
A position alignment method comprising: detecting reflected diffraction light from a wafer mark to obtain relative position shift information of each mark. 2. Prior to projecting and exposing a pattern formed on a mask onto a wafer via a projection lens, an alignment mark provided on each of the mask and the wafer is used, and the optical relative positions of these marks are determined. Find the displacement amount,
In a positioning method for positioning a mask / wafer in accordance with the amount of displacement, the wafer is provided with a wafer mark for positioning made of a diffraction grating, and the mask is provided with a pair of positioning marks made of a diffraction grating. A mask mark is provided at a predetermined distance, and a window for selectively irradiating the wafer mark with exposure light or a pattern for wafer mark creation is provided at the center between the pair of mask marks, and a pattern having a wavelength different from the exposure light is provided. The mask mark is irradiated with alignment light, two diffracted lights from the mask mark are condensed and interfered on the wafer mark through the projection lens, and the reflected diffracted light from the wafer mark is detected to determine the relative position of each mark. A registration method characterized by obtaining displacement information. 3. The distance 2r between the pair of mask marks is: 3. The positioning method according to claim 1, wherein: Here, D is the difference between the distances between the object images when the exposure light and the alignment light are used (β, β ′ = 1, and D =
0), β is the magnification for the alignment light, β ′ is the magnification for the exposure light, λ is the wavelength of the alignment light, P is the grating pitch of the wafer mark, and 2l is the length of the wafer mark.