JPS632324B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring the flatness of a substrate surface such as a silicon wafer, a GGG wafer, a printed circuit board, or a ceramic substrate.

In semiconductor integrated circuit manufacturing, projection alignment, in which a mask pattern is projected onto a wafer, has become mainstream. In the projection aligner, the flatness of the wafer on the wafer chuck affects the yield of fine pattern printing and is a problem.

To form a pattern on a silicon wafer,
A photoresist is applied onto a silicon wafer, and a photomask pattern is exposed onto the photoresist to expose the photoresist to light. This process is called photolithography, and is a contact method in which the mask and wafer are exposed in close contact.
There are various methods, such as a close-up method in which the mask and wafer are exposed with a distance of several μm to several tens of μm, and a projection method in which the pattern on the mask is projected onto the wafer.

At this time, if the flatness of the wafer surface is poor, it becomes difficult to uniformly expose a fine pattern over the entire surface of the wafer, so the wafer surface must be flat. This is particularly strongly required for projection type aligners as described above.

The present inventors have already invented an excellent device for flattening substrates such as wafers, and filed a patent application in 1983-
A patent application has been filed as No. 33882. It goes without saying that a device for detecting flatness is indispensable when planarizing a substrate using the device according to the invention or other planarization devices.

Conventionally, optical interference type, laser scan type,
Various types of devices are used, such as the optical micro type, capacitive detection type, overcurrent type, and air micro type, but none of them, except for the optical interference type, detect the flatness of the entire surface to be inspected. , only the flatness of the scanned part can be determined. Therefore, it is not suitable for knowing the highly accurate and detailed flatness of the entire surface to be inspected or the peak value within the entire surface. The optical interference type flatness detector can observe the flatness of the entire surface to be measured at once without contact, so it is ideal for determining detailed unevenness, true peak values, overall trends, etc. of the entire surface.

First, a conventional optical interference type flatness detector will be explained based on FIG.

A wafer 6 to be inspected is secured to a wafer chuck 11. A prism 5 is placed with a small distance from the surface of the wafer 6 to be inspected. Laser oscillator 1
The laser beam is expanded into a parallel beam by a beam expander consisting of a short focus lens 2, a pinhole 3, and a long focus large aperture lens 4, and is incident on a prism 5. This incident laser beam causes the wafer 6 to
The light reflected from the surface and the light reflected from the reference plane 7 of the prism 5 interfere, producing interference fringes. The interference fringes are projected onto ground glass 8 and detected by TV camera 9.
It is observed as a TV image 10.

These interference fringes serve as contour lines of the flatness of the wafer 6. Therefore, the flatness of the wafer 6 can be determined at any position by processing the TV image 10. Various methods have already been announced for this image processing, for example,
“Development in Semiconductor
Microlithography, SPIE vol.135, P.105~
110 (1978), and will not be specifically explained here.

Since the flatness measuring device of the present invention also applies the principle of optical interference type flatness detection, the principle of optical interference type flatness detection will be explained based on FIG. The A ray 12 and the B ray 13 are coherent lights, and have the same phase up to a ₁ and b ₁ . A part of the A ray 12 is caused by the reference plane 7.
a It is reflected at _two points. A part of the B ray 13 is on the reference plane 7.
It is transmitted through _one point b, reflected at _two points b on the wafer 6, and incident on _two points a. At this time, A ray 12 and B ray 13
Since there is a difference in optical path length between _{1 2} and _{1 2 2} , the two lights cause interference at _two points a. The interference condition can be easily determined as shown in equation (1).

d=Mλ/2・1/sinθ' (1) However, d: Pitch of interference fringes M: Positive integer λ: Wavelength of monochromatic light θ': Reflection angle From equation (1), from the reference plane 7 to Md The surface to be inspected that exists in the area is observed darkly. In this way, contour lines formed by dark lines are observed on the wafer 6 from the direction in which the light travels. Also, the contour line pitch changes depending on the reflection angle θ'14 on the wafer 6 surface. The reflection angle θ'14 is determined by the material of the prism 5 and the angle of incidence 15 on the reference plane.
The two sides 16 of the prism 5 in FIG. 1 are at right angles to the incident light to avoid loss of light.

As described above, the optical interference method has advantages that other methods do not have, but as the surface to be inspected becomes larger, the reference plane, water aperture lens, ground glass 8, etc. all become larger. The length of the beam expander also requires a long optical path length in order to obtain highly accurate coherent parallel light. or,
In order to avoid image distortion from TV input, the optical path length must be long. For this reason, it is difficult to incorporate it into a device such as an aligner, and even if flatness cannot be detected in detail, it is necessary to incorporate another scanning type flatness detector that can be made smaller.

An object of the present invention is to eliminate the drawbacks of the above-described optical interference type flatness detector and to provide a small optical interference type flatness detector that can be incorporated into a device such as an aligner.

The optical interference type flatness detector according to the present invention includes: (a) a flat transparent body fixedly arranged with a small gap from the surface to be measured; ,
means for irradiating monochromatic light over the entire vertical width with a limited width; (c) scanning means for moving the irradiation area to the entire horizontal length of the surface to be inspected; (d) receiving reflected and interference light of the irradiated light. (e) means for inputting a detection signal from the detection means and displaying an image based on the signal; and (f) moving the image display position of the image display means in synchronization with the scanning means. , synchronization means for making the image display of the image display means correspond to the entire surface to be inspected.

In a preferred embodiment of the flatness detector of the present invention, the illumination means, or when the illumination means includes a monochromatic light generation means and a reflection mirror, only the reflection mirror;
and a detection means are placed on the same table and moved by the scanning means.

In another preferred aspect of the flatness detector of the present invention, the detection means is fixed, and light refraction means causes the reflected and interference light whose light path moves in accordance with the scanning of the irradiation light to enter the detection means. is provided between the flat transparent body and the detection means. In this embodiment, the light refraction means may be made to move with the movement of the light path of the reflected and interference light.

In yet another preferred embodiment of the flatness detector of the present invention, the monochromatic light generating means to which the irradiation means is fixed and the flat transparent body attached to the rotating disk are parallel to and opposite each other in a parallel plane cross section. The monochromatic light from the monochromatic light generating means is reflected from the inner mirror and the outer mirror in this order, and is irradiated over the entire vertical width of the surface to be inspected with a limited width. The means is a rotating means of a rotating disk, and the reflected and interference light is reflected in the order of a separately provided reflecting mirror, the outer mirror and the inner mirror, and is made to enter the detecting means.

In short, the device of the present invention reduces the width of the incident light and scans the entire surface to be inspected, thereby reducing the size of the monochromatic parallel light generator and the reflected and interference light input optical system. This device uses flat glass with a low temperature to reduce the overall size of the detection device to the necessary minimum size, making it possible to incorporate it into devices such as aligners.

Hereinafter, the apparatus of the present invention will be explained based on drawings of embodiments. First, Fig. 3 a and b showing the first embodiment
I will explain based on. Figure a is a cross-sectional plan view taken along line A-A in figure b, and figure b is a longitudinal front view taken along line B-B in figure a. There are various means for obtaining monochromatic light, and any method may be used. In the following, an example using a laser oscillator, which is the most common type and can easily obtain powerful monochromatic light, will be explained. Lasers include He-Ne, but any laser can be used as long as it produces monochromatic light.

A transparent flat glass 21 is fixedly arranged with a small gap from the wafer 6 on the wafer chuck 11. A table 24 extending in the vertical direction of the wafer 6 and movable in the horizontal direction is provided above the flat glass 21, and a reflecting mirror 20 and a linear sensor 22 as a detection means are attached to both ends. A laser beam from the laser oscillator 1 is made into parallel light by a lens, and is incident on a flat glass 21 by a reflection mirror 20. The incident light causes interference between the reference plane 7 of the flat glass 21 and the wafer 6 surface, interference fringes are detected by the linear sensor 22, the detection signal of the linear sensor 22 is input to the image display device, and the frame 23 of the cathode ray tube is detected. One line of interference fringes is displayed within. The table 24 is the motor 2
5 in the width direction of the wafer 6,
The frame 23 is moved by a signal 26 synchronized with the rotation angle of the motor 25, and an image displayed on the cathode ray tube corresponding to the entire surface of the wafer 6 is obtained. Of course, measures are taken to prevent the image on the cathode ray tube from remaining until it is rescanned, so that an image corresponding to the entire surface of the wafer 6 can always be observed. Scanning does not need to be performed using the table 24 and motor 25 as in the embodiment, but any method that simultaneously moves the reflecting mirror 20 and linear sensor 22 may be used. In addition to the motor, it may be driven by an air cylinder, a magnetic force type, a link type, or the like. Furthermore, the detection does not need to be a linear sensor as long as it can detect one line.

Next, the major differences between the apparatus of the present invention and the conventional apparatus will be explained based on FIGS. 4a and 4b.

In Figure a showing the conventional device, the diameter D ₁ of the large diameter lens of the beam expander is determined by the formula (2) using the incident angle θ determined from the size L of the specimen and the pitch of the contour lines by formula (1). Seek.

D ₁ =Lsinθ (2) The prism height H ₁ is determined by equation (3).

H ₁ = Ltan θ (3) The length f ₁ of the expander depends on the flatness detection accuracy, but if the accuracy is ±0.5 μm or more, several times D ₁ is required.

On the other hand, in the device of the present invention shown in Fig. b, the prism 5 is not used but a flat glass 21, and the lens diameter is D ₂ and the height H ₂
are determined by equations (4) and (5), respectively.

D ₂ = Lsin θ′ (4) H ₂ = Lsin θ′ (5) The relationship between f ₂ and D ₂ is as shown in equation (6) since it is easier to manufacture a highly accurate lens when the lens diameter D ₂ is small.

f ₂ /D ₂ <f ₁ /D ₂ (6) When calculating the ratio M of the lens diameter and height between the conventional product and the product of the present invention, formulas (2), (4), and (3), (5) are obtained. It becomes more like this:

M=sinθ/sinθ' (7) It is clear from the law of refraction that θ' is smaller than θ. In other words, n/n'=sinθ'/sinθ where n' is the refractive index of glass, approximately 1.6, and n is the refractive index of the atmosphere, 1, so by substituting and transforming it, we get sinθ'=
It becomes sinθ/1.6. If 0゜＜θ, θ＜90゜ then θ′＜
θ
It is.

Now, when formula (7) is calculated with a pitch of 3 μm and a length of 100 mm, θ=49° and θ′=6.054°, so M=7. Furthermore, from equation (6), it can be seen that the optical system can be reduced in size to 1/7 or less.

In the case of the present invention, the surface accuracy of the flat glass 21 is stricter than that of the prism 5, but the flat glass 21
Since a high-precision surface can be obtained at a lower cost, this is not a problem.

Next, a second embodiment will be described based on FIGS. 5 and 6. In this embodiment, only the reflection mirror 20 of the irradiation means is moved by the scanning means, and the irradiation area of the illumination means, which spans the entire vertical width with a limited width, is moved to the entire length of the surface to be inspected. The reflected and interference light from the reference plane of the flat glass 21 and the wafer is refracted by the diffraction grating 32 of the light refracting means arranged obliquely to the optical path, and is reflected onto the magnifying surface 31 of the flat image input element 30 of the detecting means. is input. The pitch P of the diffraction grating 32 is easily calculated from the angle of the light to be refracted. Further, although FIG. 5 shows a transmission type diffraction grating 32, a reflection type diffraction grating may also be used. Since a large diffraction grating 32 is expensive, a small diffraction grating 33 may be moved along with the reflection mirror 20 along the position of the diffraction grating 32 in FIG. 5, as shown in FIG. In this case, the plane image input element 30
may use storage type.

Next, a third embodiment will be described based on FIGS. 7 and 8. Generally, the reciprocating scanning type generates vibration and has a slow scanning speed. On the other hand, this embodiment is a rotary scanning type device with little vibration and capable of high-speed scanning.

In this embodiment, monochromatic light generating means such as a laser oscillator 1 to which an irradiation means is fixed, and inner mirrors attached to a rotating disk 40 and facing each other in parallel with each other.
It consists of multiple pairs of MC and outer mira M. In the illustrated embodiment, MC1-M1, MC-M2, MC3-M
3. Four pairs of inner and outer mirrors, MC4-M4, are shown. The arrangement relationship of the laser oscillator 1, inner mirror MC, outer mirror M, and flat glass 21 is as follows:
The laser beam is reflected from the inner mirror MC and the outer mirror M in this order, and irradiates the entire vertical width of the wafer 6 with a limited width. Inside Mira MC
Since the outer mirror MC and the outer mirror M are parallel as seen in FIG. 7, the incident light on the inner mirror MC and the scanning laser beam 41 on the flat glass 21 are parallel in FIG.

First, assuming that the outer mirror M1 is in the position shown by the solid line in FIG.
1 position 42 is irradiated. As the rotary disk 40 is rotated by the motor 25 in the direction of the arrow, when the outer mirror M1 comes to the position M1' shown by the dotted chain line, the scanning laser beam 42 scans two positions 43, and then the outer mirror M2 scans two points. This is the M2″ position shown by the chain line,
3 positions 44 are scanned, outer mirror M2 becomes position M2, and 4 positions 45 are scanned. In this way, one rotation of the rotary disk 40 scans the mirrors four times in this embodiment, corresponding to the number of pairs of inner and outer mirrors.

The light irradiated onto the flat glass 21 is from the reference surface 7 and the wafer 6.
Interference fringe light is generated depending on the distance between the surfaces. Interference fringe light 4
8 is reflected by the reflection mirror 46, directly enters the outer mirror M1 or M2, M3, M4, is reflected by this, and is further reflected by the inner mirror MC1 or MC2, MC3, MC.
4 and is input to the linear sensor 22.
The linear sensor 22 is synchronized with the scanning of the laser beam, that is, the synchronization signal 47 synchronized with the rotation of the rotating disk 40.
By moving the image position based on the signal from the wafer 6, an interference fringe image of the entire surface of the wafer 6 can be obtained. When using this method, scanning is of a rotating type, the input/output system is fixed, and the same optical path is used for laser beam incidence and interference fringe detection, so scanning deviation does not occur and scanning accuracy is improved. Therefore, it is possible to detect it very stably and with high accuracy.

In the embodiments shown in FIGS. 3a and 3b and 5 and 6, if a sufficiently small device such as a semiconductor laser is used as the monochromatic light generating means, the table etc. can be directly scanned without using the reflecting mirror 20. A monochromatic light generating device can be mounted on the means.

Although the apparatus of the present invention is an optical interference type, it is basically a scanning type. The scanning method is used in other flatness detectors, but other flatness detectors scan the detector itself, and its vibrations directly affect accuracy, resulting in low accuracy. On the other hand, in the present invention, since the flat transparent body serving as the reference is fixed, the influence of vibration is small. Further, while other scanning methods cannot detect the flatness of the entire surface, the present invention detects the flatness of the entire surface. In particular, when a rotation scan method is used in the apparatus of the present invention, detection can be performed essentially at the input speed of a linear sensor or a planar input element synchronized with rotation, and detection can be performed at a sufficiently high speed.

With the present invention, the size of the optical interference type flatness detector, which has been a problem in the past, has been reduced to one-seventh of the optical system in a flatness detector for a 100 mmφ wafer, making it possible to incorporate it into devices such as aligners. I was able to do it.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a conventional optical interference type flatness detector, Fig. 2 is a principle diagram of an optical interference type flatness detector, and Fig. 3 shows an embodiment of the detector of the present invention.
Figure a is a cross-sectional plan view taken along arrow A-A in figure b.
Figure b is a longitudinal sectional front view taken along arrow B-B in figure a.
Figures a and b are diagrams explaining the differences between the present invention and a conventional device, Figure 5 is a schematic plan view of another embodiment of the detector of the present invention, and Figure 6 is a diagram of the light refraction means in the detector of Figure 5. Explanatory drawings of other embodiments, FIGS. 7 and 8 are schematic plan views and front views of still other embodiments. DESCRIPTION OF SYMBOLS 1... Laser oscillator, 4... Long focus large diameter lens, 5... Prism, 6... Wafer, 7... Reference plane, 8... Ground glass, 11... Wafer chuck, 20... Reflection mirror, 21... flat glass, 22
... Linear sensor, 23 ... Frame indicating one line,
24...Table, 25...Motor, 26,48
...Synchronization signal, 30...Detection element, 32, 33...
... Diffraction grating, 40 ... Rotating disk, 46 ... Reflection mirror, 47 ... Synchronization signal.

Claims (1)

[Scope of Claim] An apparatus for detecting the flatness of a surface, which includes: (a) a flat transparent body fixedly arranged with a small gap between the flat transparent body and the flat transparent body; The length and breadth of the inspection surface,
means for irradiating monochromatic light over the entire vertical width with a limited width; (c) scanning means for moving the irradiation area to the entire horizontal length of the surface to be inspected; (d) receiving reflected and interference light of the irradiated light. (e) means for inputting a detection signal from the detection means and displaying an image based on the signal; and (f) moving the image display position of the image display means in synchronization with the scanning means. A flatness detector comprising: synchronization means for making the image display of the image display means correspond to the entire surface to be inspected. 2. A patent claim characterized in that the illumination means, or when the illumination means consists of a monochromatic light generation means and a reflection mirror, only the reflection mirror and the detection means are placed on the same table and moved by the scanning means. The flatness detector in the first term of the range. 3. The detection means is fixed, and a light refraction means is provided between the flat plate transparent body and the detection means to make the reflected and interference light, whose light path moves according to the scanning of the irradiation light, enter the detection means. A flatness detector according to claim 1, characterized in that: 4. The flatness detector according to claim 3, wherein the light refraction means moves along with the movement of the optical path of the reflected and interference light. 5 Monochromatic light generating means to which the irradiation means is fixed, and an inner mirror and an outer mirror attached to a rotating disk and parallel and opposite to each other in a plane cross section parallel to the flat transparent body, and monochromatic light from the monochromatic light generating means. is reflected in the order of the inner mirror and the outer mirror, and is irradiated over the entire length of the test surface with a limited width, and the scanning means is a rotating means of a rotating disk,
The flatness detector according to claim 1, wherein the reflected and interference light is reflected in the order of a separately provided reflection mirror, the outer mirror and the inner mirror, and is made incident on the detection means. .