JPH05187987A - Method and device for inspecting density distribution in substrate - Google Patents

Abstract

PURPOSE:To make it possible to detect scattering of density distribution in a base plate simply, speedily and accurately by hypothetically quantitating qualitative density distribution in the base plate with the duplicate use of a Schlieren method and polarization method (cross Nicol method). CONSTITUTION:If an inspection base plate 1 is arranged on the point of the solid line or the chained line in the Cassegrain optical system constituted of a concave mirror 22 having a hole for laser beam penetration and a convex mirror 23 put at the focus point and Schlieren method is performed, the density distribution in the base plate 1 is projected on a screen 14 like a silhouette. Then, a beam from a laser source 20 is polarized linearly with one of pair of Nicol prisms (polarizer) arranged on both sides of the base plate 1 and passed in the base plate 1, it is taken out of the other Nicol prism (analyzer) and projected on the screen 14. Other distribution information and phase change information which can not be confirmed, are obtained. By this, qualitative density distribution in the base plate can be hypothetically quantitated and the existence of distribution scattering is judged speedily and exactly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-contact method for inspecting the density distribution of a surface acoustic wave (hereinafter referred to as SAW) element substrate made of quartz, L _i N _b O ₃ or the like.

[0002]

2. Description of the Related Art It is an important factor to improve the product quality and the yield by inspecting the density distribution in the SAW element substrate in advance and detecting the distortion.

Therefore, conventionally, before processing the SAW element, the presence or absence of distortion of the substrate has been inspected non-destructively by visual inspection or by using an X-ray topograph or a light scattering tomograph.

Here, the X-ray topograph refers to irradiating a substrate with X-rays to visualize the internal structure based on the reflected light thereof, and the light scattering tomograph utilizes the scattering of light within the substrate. However, a tomographic image proportional to the scattering intensity is taken.

[0005]

Of the above inspection methods,
According to the X-ray topography, although the disorder of the crystal lattice in the substrate can be confirmed, there is a problem that the inspection takes several hours. In order to shorten the inspection time, a considerably high power radiation source is required, and the cost increases dramatically. Further, due to the nature of the topography, the information in the reflection on the substrate overlaps with each other, resulting in an averaged image.

Further, in the light scattering tomograph, in order to prevent surface scattering during inspection, it is necessary to optically polish the surface of the substrate to the extent that light can pass therethrough. Therefore, pretreatment for that purpose takes time and is expensive. There was a problem that a device was required.

Further, even in the case of visual inspection, the optical polishing of the substrate surface must be carried out precisely, and the preparation thereof is difficult. Further, since the inspection is performed by using both coherent and incoherent light and subtly changing the incident angle of light, there is a problem that it takes a lot of time.

The present invention has been made in view of the above problems, and an object thereof is to provide a method capable of detecting the presence or absence of variation in the density distribution in a substrate with high speed and high accuracy by a simple method. To do.

[0009]

The inspection method of the present invention comprises a Schlieren method for expressing the refractive index distribution of light transmitted through a substrate to be inspected by contrast of light and dark, and a phase velocity of light transmitted through the substrate to be inspected. A method that uses a polarization method that expresses changes by contrast of light and darkness or a color change in combination, and by performing either one of these Schlieren method and polarization method and then the other, a qualitative density distribution in the substrate Was quasi-quantified, and the presence or absence of the distribution variation was determined by this.

Further, the inspection apparatus of the present invention is an apparatus having an optical system for executing the Schlieren method for expressing the refractive index distribution of the light transmitted through the substrate to be inspected by the contrast of light and dark. In the case where a knife edge is arranged in the vicinity of the condensing part, a translucent spatial frequency filter having a predetermined pattern is arranged in the condensing part instead of the knife edge.

[0011]

The refractive index distribution in the substrate is imaged like a shadow by the Schlieren method, and the phase change information that cannot be seen by the naked eye is obtained by the polarization method. As a result, qualitative information is pseudo-quantified.

Further, by placing a spatial frequency filter in the condensing part of the optical system, when the light collected in the condensing part transmits the filter, information corresponding to a predetermined pattern is formed. Therefore, by processing this information together with the in-substrate density distribution information, it is possible to process it into higher-level information.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

Since the schlieren method and the polarization method are used together in the inspection method of the present invention, the principle of the schlieren method will be described first with reference to FIG.

FIG. 1 is a basic configuration diagram of an apparatus for executing the Schlieren method. Light from a light source 10 is converted into a light flux that travels in a certain direction by one of the Schlieren lenses 11, and the substrate 1 to be inspected has this light flux. Put it inside and let the light pass through. The luminous flux including the passing light is the other Schlieren lens 1
The light is converged on the second condensing portion (focal point), but at this time, when the internal density of the substrate 1 to be inspected is inhomogeneous, the passing light is bent according to the gradient of the refractive index. At this time, if the knife edge 13 is placed near the focus and the field of view is adjusted to an appropriate brightness, the portion of the knife edge 13 refracted in the direction approaching the blade becomes dark and the portion away from the blade becomes bright, whereby the substrate to be inspected. The density distribution within 1 is projected on the screen 14 as a contrast of light and dark like a shadow picture. By checking this projection state, it is possible to quickly and easily determine whether or not there is a variation in the density distribution within the substrate.

In the Schlieren method, the optical system generally has a long structure (several meters), but the optical system can be made compact by using a lens / mirror having a small F value used in a Schmidt camera or a Cassegrain telescope. Become. In the present embodiment, the Cassegrain type optical system having the configuration shown in FIG. 2 is assembled to achieve further compactness.

Comparing the structure of the Cassegrain system of FIG. 2 with the principle diagram of FIG. 1, the laser light source 20 and the spatial filter 21 are provided in the light source 10, and the concave mirror 22 having a hole through which the laser light passes and the focal position thereof. Convex mirror 2 placed on
3 corresponds to the pair of Schlieren lenses 11 and 12, respectively. The board 1 to be inspected may be arranged at the position shown by the alternate long and short dash line in addition to the position shown by the solid line in FIG.

Next, the polarization method will be described.

In this embodiment, a method generally called the crossed Nicol method is used. This method utilizes the property that the propagation state of light in the field differs depending on the phase distribution in the substrate and the phase velocity changes. For example, a pair of Nicols (Ni
col) The substrate to be inspected is placed between the prisms, the light emitted from the light source is linearly polarized by one Nicol prism (polarizer) and passed through the substrate, and then the other Nicol with the polarization axis crossed. It is taken out from a prism (analyzer) and projected onto a screen through an imaging lens or directly. A polaroid plate may be used instead of the Nicol prism.

According to this method, when the phase distribution in the substrate is uniform, the projection on the screen is uniformly dark, but when the density distribution is not uniform, it becomes elliptically polarized light due to the retardation of the optical path and on the screen. Since a contrast of light and darkness is generated in the projection of, and a color change is generated, it is possible to easily inspect the distribution state of the density in the substrate.

The pair of Nicol prisms are the same as the substrate 1 to be inspected in the system having the basic structure shown in FIG. 1 or 2.
The Schlieren method and the polarization method can be used in combination by arranging them on both sides of the incident side and the emitting side.

Therefore, in the present embodiment, the Schlieren method is performed with the polarization planes of these prisms being the same (paranicols), and the refractive index distribution in the substrate 1 to be inspected is first confirmed as a shadow pattern. Then, the crossed Nicols method is executed. The order of these may be reversed. The significance of using the crossed Nicols method together with the schlieren method is that the phase change information of the optical path, which cannot be obtained by the schlieren method, can be obtained by the crossed nicols method, and that the variation of the phase distribution that cannot be confirmed by the naked eye (shadow picture) can be confirmed. is there.

As a result, the qualitative density distribution in the substrate can be quasi-quantified, and the presence or absence of the variation in the distribution can be determined quickly and reliably.

By the way, the substrate used in the SAW device is usually a _{S i O 2, L i T} a O 3, L i N b O 3 or the like of the single crystal, they longer transmittance region (e.g. 80 In [%] transmission, infrared light having a wavelength of 4 to 5 [μm] is transmitted.

Therefore, in the above method, infrared light as well as visible light is used as the light source, and in the case of visual inspection, the infrared light visualization device is used before the projection on the screen 7 to give 0.4 to 0. The work of surface polishing of the inspected substrate 1 can be simplified as compared with the case where only visible light having a wavelength of 7 [μm] is used. This eliminates the need for an expensive optical polishing device, which was indispensable in the conventional method, and enables cost reduction. Moreover, the inspection time is further shortened.

In the apparatus having the structure shown in FIG. 1, various processing of image information can be optically realized by disposing a translucent spatial frequency filter in the condensing portion instead of the knife edge 13. For example, by forming a predetermined pattern on the spatial frequency filter, information corresponding to the pattern can be obtained when transmitting light. If this information is processed together with the information obtained by the Schlieren method and the crossed Nicols method (polarization method), pattern recognition of the distribution density in the substrate, contour extraction, and mathematical processing such as four arithmetic operations and differentiation are also possible. ..

[0027]

As described above in detail, in the present invention, the schlieren method and the polarization method are used in combination to inspect the density distribution in the substrate in a non-contact manner, so that the following effects are obtained.

(1) Since the qualitative density distribution in the substrate can be quantified in a pseudo manner, the presence or absence of distortion of the substrate can be promptly and reliably determined even by visual inspection.

(2) In the X-ray topograph and the light scattering tomograph, since the inside of the substrate is inspected depending on the incident method of electromagnetic waves, some information may be hidden depending on the incident direction, but according to the present invention. , Its effect disappears.

(3) It can be realized by a simple optical system, and since a cassegrain type optical system can also be assembled, it is excellent in versatility and can be downsized.

(4) Once the optical system is set, its density distribution can be confirmed without adjusting its position and simply by roughly inserting the substrate to be inspected into the optical path, which is extremely easy to handle. The line can be achieved. Further, since both the schlieren method and the polarization method can be automated and computer image processing can be performed, automatic measurement is possible.

(5) By arranging the spatial frequency filter in the condensing part of the optical system, not only the variation of the density distribution but also higher-level processing can be optically realized.

The present invention can be applied not only to the SAW element substrate but also to other types of substrates.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of a schlieren method execution apparatus constituting the present invention.

FIG. 2 is a basic configuration diagram of an apparatus that executes the Schlieren method using a Cassegrain type optical system.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Inspected substrate, 10 ... Light source, 11, 12 ... Schlieren lens, 13 ... Knife edge, 14 ... Screen, 2
0 ... Laser light source, 21 ... Spatial filter, 22
… Concave mirror, 23… Convex mirror.

Claims (3)

[Claims] 1. A Schlieren method for expressing a refractive index distribution of light transmitted through a substrate to be inspected with a contrast of light and dark, and a phase velocity change of light transmitted through a substrate to be inspected by contrast of light and dark or a color change. This is a method that uses the polarization method in combination, and by performing one of these Schlieren method and polarization method and then the other, the qualitative density distribution in the substrate is quasi-quantified and the distribution variation A method for inspecting a density distribution in a substrate, characterized in that the presence or absence is judged. 2. The method for inspecting the density distribution within a substrate according to claim 1, wherein visible light and infrared light are used as light sources for the schlieren method and the polarization method. 3. An inspection apparatus for executing a schlieren method for expressing a refractive index distribution of light transmitted through a substrate to be inspected by contrast of light and dark, wherein a knife edge is arranged in the vicinity of a condensing part of the optical system. In addition, in place of the knife edge, a translucent spatial frequency filter having a predetermined pattern is arranged in the condensing portion, and the in-substrate density distribution inspection device is characterized.