JP4692754B2 - Light scattering observation device - Google Patents

Description

  The present invention relates to a light scattering observation apparatus for irradiating a measurement object with a laser beam, photographing a laser beam emitted from the measurement object with a CCD camera, and observing a scatterer present in the measurement object. The present invention relates to an improvement in a light scattering observation apparatus that can measure the distribution of scatterers over the entire object to be measured in a short time.

  Conventionally, there has been known an internal defect observation method and apparatus for visualizing the defect by irradiating the inside of the object with light such as a laser beam and obtaining the scattered light to obtain information on the defect inside the object to be measured. Yes.

  An apparatus for observing such a defect is disclosed in Non-Patent Document 1, for example. In this apparatus, a laser beam condensed by a lens is incident from the side surface of the object to be measured. The laser beam is scattered by defects inside the object to be measured. The scattered light is emitted from the object to be measured, and the scattered light is photographed by a CCD camera arranged at a position orthogonal to the optical axis of the laser beam. Thereby, the defect inside the object to be measured is visualized and detected.

  As an apparatus for evaluating defects or the like of a semiconductor wafer, an apparatus in which a laser beam is obliquely incident on the surface of the object to be measured to detect defects and particles inside the object to be measured is disclosed in Patent Literature. 1 is disclosed. In this apparatus, the influence of reflected light is eliminated as much as possible by observing scattered light from a direction different from the reflection direction of the laser beam from the object to be measured. Further, the depth at which the laser beam enters the object to be measured can be adjusted by changing the wavelength of the laser beam or the temperature of the object to be measured.

  Further, as an apparatus for evaluating defects or the like of a semiconductor wafer, a laser beam is used to evaluate whether the scattering source of the scattered light is a particle or the like existing on the surface of the semiconductor wafer or a defect or the like existing inside. An apparatus using the p-polarized component and the s-polarized component is disclosed in Patent Document 2.

  By the way, in the measuring device disclosed in these patent documents, the range that can be measured at a time is only a linear portion irradiated with a laser beam, and when it is desired to know the distribution of defects of the entire object to be measured, The object to be measured is placed on a precise two-dimensional stage, and the two-dimensional stage is operated to scan the entire inside of the object to be measured with a laser beam, and each image is stored in the memory of a personal computer. The process of obtaining the whole image by combining the pixels was necessary.

Therefore, there is a problem that the measuring operation is complicated and the apparatus itself is expensive because an expensive two-dimensional stage is required.
JP-A-4-24541 Japanese Patent No. 2847458 Kazuo Moriya, Applied Physics, 55 (1986) 542

  The present invention has been made paying attention to such problems, and the problem is that the entire object to be measured is scanned with a laser beam without using expensive parts such as a two-dimensional stage, and the object to be measured is measured. An object of the present invention is to provide a light scattering observation apparatus capable of measuring the distribution of scatterers caused by defects inside an object in a short time.

That is, the invention according to claim 1
Assuming a light scattering observation device that irradiates a laser beam from a laser light source to the object to be measured, images the laser beam emitted from the object to be measured with a CCD camera, and observes a scatterer present in the object to be measured,
A half-wave plate, a first galvanometer mirror, and a second galvanometer mirror for rotating the direction of linearly polarized light are arranged on the optical path between the laser light source and the object to be measured , and half with the laser beam having transmitted through the wavelength plate is adapted to be irradiated to the whole of the reflected two-dimensionally scanned with the DUT by the first galvano mirror and the second galvano-mirror, wave plate of the half The polarization of the laser beam applied to the object to be measured is selectively switched between horizontal polarization and vertical polarization, and microbubbles existing in the object to be measured are detected by entering the horizontal polarization, and the vertical It is characterized in that large particles other than microbubbles existing in the object to be measured and strain are detected by entering polarized light .

The invention according to claim 2
On the premise of the light scattering observation apparatus according to the invention of claim 1,
A laser beam is irradiated two-dimensionally from the horizontal direction to the object to be measured, and the laser beam emitted from the object to be measured is photographed by a CCD camera disposed 90 degrees directly above the object to be measured. It is characterized by
The invention according to claim 3
On the premise of the light scattering observation apparatus according to the invention of claim 1 or 2,
The object to be measured is a sapphire ingot before wafer processing ,
The invention according to claim 4
On the premise of the light scattering observation apparatus according to the invention of claim 1, 2 or 3,
It is characterized in that the distribution of the scatterer of the entire object to be measured can be measured.

According to the light scattering observation apparatus according to the present invention,
Since the laser beam transmitted through the half-wave plate is reflected by the first galvanometer mirror and the second galvanometer mirror and scanned two-dimensionally, the whole object to be measured is irradiated. It is possible to measure the distribution of scatterers over the entire object to be measured without using a stage or the like.

  Hereinafter, the present invention will be specifically described with reference to the drawings.

First, the light scattering observation apparatus according to the present invention includes, for example, as shown in FIG. 1, a laser light source 1 and a half wavelength arranged on the optical path between the laser light source 1 and the object to be measured 5. The plate 2, the first galvanometer mirror 3 and the second galvanometer mirror 4, and the CCD camera 6 for photographing the laser beam emitted from the object to be measured 5 constitute the main part.

In this light scattering observation apparatus, the laser beam emitted from the laser light source 1 is converted into p-polarized light or s-polarized light by the half-wave plate 2. In addition, the laser beam transmitted through the half-wave plate 2 and incident on the first galvanometer mirror 3 is scanned in the horizontal direction, and the laser beam incident on the second galvanometer mirror 4 is vertical in addition to the horizontal direction. Thus, the entire object to be measured 5 is randomly scanned by the laser beam.

  The first galvanometer mirror 3 and the second galvanometer mirror 4 can independently set the scanning frequency and the deflection angle of the laser beam by the control system.

  Here, it is desirable that the laser beam emitted from the laser light source 1 is linearly polarized light. However, if it is not linearly polarized light, it may be converted into linearly polarized light by a polarizer. Then, by rotating the half-wave plate 2, it becomes possible to select p-polarized light and s-polarized light.

  By the way, when considering the polarization state of incident waves and scattered waves, the description method differs depending on the position of the observer, and hence the case where the electric field vector of the laser beam is horizontal with respect to the ground regardless of the position of the observer. This polarization state is called horizontal polarization, and the polarization state when the electric field vector of the laser beam is perpendicular to the ground is called vertical polarization.

  Then, when the scattered light when the horizontally polarized light is incident on an object to be measured such as single crystal sapphire is observed from directly above, it is possible to detect microbubbles existing in the object to be measured. In addition, when the scattered light when vertically polarized light is incident on the object to be measured is observed from directly above, it is possible to detect large bubbles present in the object to be measured and parts with large distortion due to edge dislocations. It becomes. As described above, in the measuring apparatus disclosed in Patent Document 1, Patent Document 2, etc., in order to measure the defect distribution of the entire object to be measured, the object to be measured is scanned using a precise two-dimensional stage. It was necessary to save the scattered image data.

However, according to the light scattering observation apparatus according to the present invention, lasers are obtained by two galvanometer mirrors without using an expensive two-dimensional stage or a computer for storing individual images and synthesizing the whole image. By randomly scanning the light beam two-dimensionally and overwriting the scattered light caused by the defects present in the object to be measured, for example, by appropriately selecting the exposure time of the CCD camera placed directly above the object to be measured. It becomes possible to visualize the distribution of defects.

Next, the measurement principle of the light scattering observation apparatus according to the present invention will be briefly described (see Non-Patent Document 1).

  First, in order to optically observe defects in the crystal, the dielectric constant needs to change around the defects.

  Here, the dielectric constant of the defective part is

この式をMaxwellの方程式に代入し、

Substituting this equation into Maxwell's equation,

この近似が有効であるためには、相対散乱強度が非常に小さいことが必要である。上式において、第１項は入射波を表すから散乱波は第２項となる。入射波として平面波

For this approximation to be effective, the relative scattering intensity must be very small. In the above equation, since the first term represents the incident wave, the scattered wave becomes the second term. Plane wave as incident wave

散乱因子は散乱波の振舞いを示す量で、散乱体の誘電率のFourier変換によって与えられる。誘電率は２階のテンソルであるから、散乱因子も、

The scattering factor is a quantity indicating the behavior of the scattered wave, and is given by the Fourier transform of the dielectric constant of the scatterer. Since the dielectric constant is a second-order tensor, the scattering factor is

と求まる。これが９０°散乱の基本式である。

It is obtained. This is the basic formula of 90 ° scattering.

  FIG. 2 shows the relationship between the coordinate axis of a crystal defect and the coordinate axis of an observation system.

となる。

It becomes.

  The relationship of the scattering vector in the case of scattering by uniform spherical particles is as shown in FIG.

  In the light scattering observation apparatus according to the present invention, when the horizontally polarized light is incident on the spherical microbubble in the object to be measured, the intensity of the scattered light incident on the CCD camera located 90 degrees directly above the object to be measured is Become the maximum. When vertically polarized light is incident on the microbubble, no scattering occurs.

In the light scattering observation apparatus according to the present invention, when vertically polarized light is incident on the object to be measured, the light is scattered by relatively large particles (sufficiently large compared with the wavelength of the laser beam) and refracted by the photoelastic effect. Scattering occurs due to a portion (distortion) in which the rate changes locally.

  The strain referred to here is caused by, for example, edge dislocations in single crystal sapphire or artificial quartz. The edge dislocations in sapphire will be briefly described below. Consider the case where light is incident on a sapphire c-plane wafer in parallel. When the sapphire crystal is regarded as a hexagonal crystal, edge dislocations extending in the a-axis direction are distributed along the c-plane. Scattering due to edge dislocations is observed only when the electric field vector of the incident wave is parallel to the c-axis.

  In summary, scattering when horizontal polarization is incident on an object to be measured such as single crystal sapphire is due to scattering by microbubbles and large scatterers, and scattering when vertical polarization is incident is other than microbubbles. Scattering by relatively large particles and scattering by the photoelastic effect due to strain.

  Hereinafter, the present invention will be described specifically by way of examples.

First, in the light scattering observation apparatus according to the present invention shown in FIG. 1, the laser beam emitted from the laser light source 1 passes through the half-wave plate 2 and is converted into horizontal polarization or vertical polarization, The first galvanometer mirror 3 scans in the horizontal direction at a frequency of 200 Hz, and the second galvanometer mirror 4 scans in the vertical direction at a frequency of 200 Hz. Note that the distance between the second galvanometer mirror 4 and the DUT 5 is 1 m.

  Here, a sapphire ingot having a diameter of 3 inches and a length of 70 mm is considered as an object to be measured. The sapphire single crystal is used as a substrate when epitaxially growing GaN which is a material of a blue LED. The sapphire substrate is cut out from the ingot and processed into a wafer, but if there are scatterers or defects in the crystal, they remain as protrusions or pits on the surface of the wafer, and in the case of GaN epi growth, defects in the sapphire substrate Is known to be transferred as a GaN defect. Therefore, in order to epitaxially grow good quality GaN, it is indispensable to use a sapphire wafer having no defects on the surface as a substrate. Since the cylindrical surface of the sapphire ingot of the object to be measured and the upper and lower end surfaces are lapped, the internal scattering cannot be observed even when the laser beam is irradiated. If the cylindrical surface and the upper and lower end surfaces are optically polished, scattering inside the ingot can be observed. However, since sapphire has the second highest hardness after diamond, optical polishing requires a great deal of time and money. In order to solve this problem, a sapphire ingot is used by being immersed in methylene iodide for refractive index matching. Since the refractive index of sapphire in the visible range is about 1.76 and the refractive index of methylene iodide is 1.74, it is possible to remove the contribution of scattering and reflection on the surface of sapphire.

  The laser light source used for scattering measurement preferably has a higher output in order to detect minute defects. In addition, since a wavelength in the visible region where the light receiving sensitivity of a CCD camera for capturing a scattered image is high is desirable, a green laser having an output of 300 mW and a wavelength of 532 nm was used. Further, since it is necessary to scan the entire ingot with the laser beam, the exposure time is required to be 1/200 second or more of one scanning period. In order to detect minute scattering, it is necessary to set the exposure time longer.

  Adjust the control system of the galvanometer mirror so that the horizontal amplitude of the first galvanometer mirror 3 is 80 mm and the vertical amplitude of the second galvanometer mirror 4 is 70 mm so that the laser beam scans the entire sapphire ingot. did.

And the measurement image (measurement image by horizontal polarization incidence and vertical polarization incidence) of the light scattering of the commercially available "Atlas" sapphire ingot measured using this light scattering observation apparatus is shown in FIG. This is a photograph taken with an exposure of the camera of F10 and an exposure time of 30 seconds.

From the scattered image of the horizontally polarized light shown in FIG. 4, it can be seen that the microbubbles are cloud-like and distributed throughout the ingot. It is presumed that the bright spots seen in both horizontal polarization and vertical polarization incidence will be large scatterers and their aggregates that are expected to be included. As can be seen from these photographs, the distribution of the scatterers in the entire ingot can be measured in a short time by using the light scattering observation apparatus according to the present invention.

  Moreover, the measurement image (measurement image by horizontal polarization incidence and vertical polarization incidence) of the light scattering of the commercially available "CS" sapphire ingot is shown in FIG.

  By changing the placement of the ingot, the position of a large scatterer can be identified, and when processing into a wafer, there is an advantage that a portion with few defects can be selected. Therefore, it greatly contributes to cost reduction and processing time reduction.

  According to the light scattering observation apparatus according to the present invention, the laser beam transmitted through the half-wave plate is reflected by the first galvanometer mirror and the second galvanometer mirror, and is two-dimensionally scanned, so that the entire object to be measured is obtained. Since irradiation is performed, it is possible to measure the distribution of the scatterers in the entire object to be measured in a short time without using a two-dimensional stage or the like. Accordingly, the present invention has industrial applicability for use in an apparatus for evaluating defects such as single crystal ingots and semiconductor wafers.

Explanatory drawing which shows schematic structure of the light-scattering observation apparatus which concerns on this invention. Explanatory drawing which shows the relationship between the coordinate axis of the defect in the crystal system which is a to-be-measured object, and the coordinate axis of an observation system. Explanatory drawing which shows the relationship of the scattering vector in the case of scattering by a uniform spherical particle. The measurement photograph figure of the light scattering of the commercial sapphire ingot measured by the horizontal polarization incidence and the vertical polarization incidence, respectively, using the light scattering observation apparatus according to the present invention. The measurement photograph figure of the light scattering of the other commercially available sapphire ingot measured by the horizontally polarized light incidence and the vertically polarized light incidence using the light scattering observation apparatus according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 1/2 wavelength plate 3 1st galvanometer mirror 4 2nd galvanometer mirror 5 Object to be measured 6 CCD camera

Claims (4)

In a light scattering observation apparatus for irradiating a measured object with a laser beam from a laser light source, photographing a laser beam emitted from the measured object with a CCD camera, and observing a scatterer present in the measured object,
A half-wave plate, a first galvanometer mirror, and a second galvanometer mirror for rotating the direction of linearly polarized light are arranged on the optical path between the laser light source and the object to be measured , and half with the laser beam having transmitted through the wavelength plate is adapted to be irradiated to the whole of the reflected two-dimensionally scanned with the DUT by the first galvano mirror and the second galvano-mirror, wave plate of the half The polarization of the laser beam applied to the object to be measured is selectively switched between horizontal polarization and vertical polarization, and microbubbles existing in the object to be measured are detected by entering the horizontal polarization, and the vertical A light scattering observation apparatus characterized in that large particles and strains other than microbubbles present in a measurement object are detected by applying polarized light.   A laser beam is irradiated two-dimensionally from the horizontal direction to the object to be measured, and the laser beam emitted from the object to be measured is photographed by a CCD camera disposed 90 degrees directly above the object to be measured. The light scattering observation apparatus according to claim 1. The light scattering observation apparatus according to claim 1, wherein the object to be measured is a sapphire ingot before wafer processing .   4. The light scattering observation apparatus according to claim 1, wherein the distribution of the scatterer over the entire object to be measured can be measured.

Images