JP4905029B2 - 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 observe a scattered image of a cross section of an object to be measured without using an expensive two-dimensional stage, and can also observe the position and size of a scatterer existing in the object to be measured. .

  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 scattered image of the cross-section of the object to be measured can be observed without using an expensive two-dimensional stage. The object is to provide a light scattering observation apparatus capable of observing the position and size of an existing scatterer.

That is, the invention according to claim 1
Assuming a light scattering observation device that irradiates a laser beam from a laser light source onto a measurement object, images the laser beam emitted from the measurement object with a CCD camera, and observes a scatterer present in the measurement object,
The object to be measured is composed of a sapphire ingot before wafer processing, and a half-wave plate, a galvano mirror and a reflection mirror for rotating the direction of linearly polarized light are arranged on the optical path between the laser light source and the sapphire ingot. In addition, the laser beam transmitted through the half-wave plate is scanned in a plane by the galvanometer mirror and the reflection mirror, and sapphire is obtained by a CCD camera having an automatic focus adjustment function disposed 90 degrees directly above the sapphire ingot. A scattered image of the ingot cross-section is observed, and the polarization of the laser beam applied to the sapphire ingot is selectively switched between horizontal polarization and vertical polarization by the half-wave plate. Therefore, the microbubbles present in the sapphire ingot are detected by incident horizontal polarization, and the vertical polarization is detected. Wherein the bubbles and the edge dislocation due to a large portion of the strain of large than microbubbles present in the sapphire ingot is adapted to be detected by incident.

The invention according to claim 2
On the premise of the light scattering observation apparatus according to the invention of claim 1 ,
The laser beam transmitted through the half-wave plate is scanned in a plane by a galvanometer mirror and a reflection mirror so that a scattered image of a sapphire ingot section scanned perpendicularly to the central axis of the sapphire ingot is observed. It is characterized by becoming.

Next, the invention according to claim 3 is
On the premise of the light scattering observation apparatus according to the invention of claim 1 or 2 ,
The sapphire ingot is moved by a vertical movement stage with an encoder, and a scattered image of a sapphire ingot cross section at a different position in the vertical direction is observed,
The invention according to claim 4
On the premise of the light scattering observation apparatus according to the invention of claim 3 ,
The image information of the scattered image in each cross section of the sapphire ingot and the horizontal position information and vertical position information in each cross section are stored, and the position of the scatterer existing in the sapphire ingot by image processing based on each stored information And the size is specified,
The invention according to claim 5
Based on the light scattering observation apparatus according to the invention of claim 1, 2, 3, or 4 ,
The sapphire ingot is a sapphire ingot before wafer processing applied to an epitaxial growth substrate.

According to the light scattering observation apparatus according to the invention of claim 1 ,
Since the laser beam that has passed through the half-wave plate is scanned in a plane by a galvanometer mirror and a reflection mirror, it has an effect of allowing observation of a scattered image of a cross-section of the sapphire ingot that is a measurement object without using a two-dimensional stage .

In particular, according to the light scattering observation apparatus according to the invention of claims 3 to 4 ,
A sapphire ingot , which is a measurement object , is used without using a two-dimensional stage because a measurement object is moved in a vertical direction by a vertical movement stage with an encoder and a scattered image of a cross section of the measurement object at different positions in the vertical direction is observed. This has the effect of observing the position and size of the scatterers existing inside.

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

  First, as shown in FIG. 1, the light scattering observation apparatus according to the present invention includes a laser light source 1 and a half-wave plate 2 disposed on the optical path between the laser light source 1 and the object to be measured 5. The galvanometer mirror 3 and the reflection mirror 4, the object to be measured 5 are mounted, the vertical moving stage 6 with an encoder for moving the object to be measured 5 in the vertical direction, and the automatic focus adjustment function and the cross section of the object to be measured 5 The CCD camera 7 that captures the scattered image and the computer 8 that stores and processes image information of each cross section of the object to be measured 5 and position information of each cross section in the horizontal direction and the vertical direction are configured. ing.

  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, passes through the half-wave plate 2, and enters the galvanometer mirror 3. The laser beam thus scanned is horizontally scanned, and further, the laser beam incident on the reflection mirror 4 is vertically incident on the object to be measured 5 mounted on the vertical moving stage 6 and the cross section of the object to be measured 5 is scanned by the laser beam. The In other words, the surface scanned by the laser beam is perpendicular to the central axis of the DUT 5. Then, a scattered image of the cross section of the object to be measured 5 is taken by the CCD camera 7 having an automatic focus adjustment function disposed 90 degrees directly above the object to be measured 5, and image information of the cross section, horizontal position information, and The position information in the vertical direction is stored in the memory of the computer 8 respectively. Next, the vertical movement stage 6 moves up or down in the vertical direction to move the object 5 to be measured to a predetermined position, and repeats the above-described photographing process to obtain image information of the cross section of the object 5 to be measured in the ascending or descending position. The position information in the horizontal direction and the position information in the vertical direction are stored, and thereafter, these steps are repeated a plurality of times, so that the image information of each cross section, the position information in the horizontal direction and the position information in the vertical direction in the entire object to be measured 5 are obtained. Each is stored in memory. Then, image processing is performed based on each stored information, and the position and size of the scatterer existing in the object to be measured are specified. The galvanometer mirror 3 can set the scanning frequency and the deflection angle of the laser beam by a 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, a laser beam is randomly scanned in a horizontal plane by one galvanometer mirror, and scattered light from a scatterer such as a defect existing in the measurement object is measured. The image is taken with a CCD camera placed directly above the image, and the measurement position is moved in the vertical direction by the vertical movement stage, and the above photographing process is repeated, and the image information of each cross section and the horizontal position information in the entire object to be measured In addition, the position information in the vertical direction is stored in the computer memory. If image processing is performed based on these pieces of information, the position and size of a scatterer such as a defect present in the object to be measured can be accurately known.

  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。

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 light emitted from the laser light source 1 passes through the half-wave plate 2 and is converted into horizontal polarization or vertical polarization. The mirror 3 scans in the horizontal direction at a frequency of 200 Hz. The distance between the reflection mirror 4 and the object 5 to be measured 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. However, if there are scatterers or defects in the crystal, they remain as protrusions or pits on the surface of the wafer. It is known that the GaN defects are transferred as they are. 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 this light scattering observation apparatus, a red laser having an output of 300 mW and a wavelength of 670 nm is applied. Further, since it is necessary to scan the entire ingot with a 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.

  Then, the control system of the galvanometer mirror was adjusted so that the horizontal amplitude of the galvanometer mirror 3 would be 80 mm so that the laser beam scans the entire cross section of the sapphire ingot.

  Hereinafter, images of scattered images in a sapphire ingot cross-section photographed with vertically polarized light incident on the ingot are shown in FIGS. It is a photograph taken with the camera exposed at F5 and the exposure time of 2 seconds. The value of Z represents the distance from the bottom surface of the ingot.

  The following is confirmed from the obtained image.

First, from the light scattering photographs of FIGS. 4 and 5, a large scatterer is not observed at the position of Z = 5 mm, but it is confirmed that a large scatterer is present from the position of Z = 20 mm. It is confirmed from the light scattering photograph diagrams of FIGS. 6 and 7 that this large scatterer lump is located in a height range of approximately ± 5 mm with Z = 25 mm as the center. The cross-sectional area of the lump of scatterers at this position (the area of the horizontal cross section at the position of Z = 25 mm) is found to be 2.45 cm ² by performing image processing.

  In this example, the image of the scattered image was stored while changing the value of Z in the height direction in the sapphire ingot at intervals of 5 mm. However, if the interval is set to an arbitrarily smaller value, the scattering of the scatterer It becomes possible to know the detailed size.

  By using this light scattering observation apparatus, when a wafer is cut out from a sapphire ingot, it is possible to exclude in advance a portion that has a defect and cannot be used as an epitaxial growth substrate. For this reason, it can contribute to cost reduction and shortening of processing time.

  According to the light scattering observation apparatus according to the present invention, it is possible to observe the position and size of the scatterer existing in the object to be measured without using an expensive two-dimensional stage. 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 light-scattering photograph figure of the sapphire ingot cross section in the position of 5 mm from the ingot bottom face image | photographed using the light-scattering observation apparatus which concerns on this invention. The light-scattering photograph figure of the sapphire ingot cross section in the position of 20 mm from the ingot bottom face image | photographed using the light-scattering observation apparatus which concerns on this invention. The light-scattering photograph figure of the sapphire ingot cross section in the position of 25 mm from the ingot bottom face image | photographed using the light-scattering observation apparatus which concerns on this invention. The light-scattering photograph figure of the sapphire ingot cross section in the position of 30 mm from the ingot bottom face image | photographed using the light-scattering observation apparatus which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 1/2 wavelength plate 3 Galvanometer mirror 4 Reflecting mirror 5 Object to be measured 6 Vertical movement stage 7 CCD camera 8 Computer

Claims (5)

In a light scattering observation apparatus for irradiating a measurement object with a laser beam from a laser light source, photographing a laser beam emitted from the measurement object with a CCD camera and observing a scatterer present in the measurement object,
The object to be measured is composed of a sapphire ingot before wafer processing, and a half-wave plate, a galvano mirror and a reflection mirror for rotating the direction of linearly polarized light are arranged on the optical path between the laser light source and the sapphire ingot. In addition, the laser beam transmitted through the half-wave plate is scanned in a plane by the galvanometer mirror and the reflection mirror, and sapphire is obtained by a CCD camera having an automatic focus adjustment function disposed 90 degrees directly above the sapphire ingot. A scattered image of the ingot cross-section is observed, and the polarization of the laser beam applied to the sapphire ingot is selectively switched between horizontal polarization and vertical polarization by the half-wave plate. Therefore, the microbubbles present in the sapphire ingot are detected by incident horizontal polarization, and the vertical polarization is detected. Light scattering observation apparatus characterized by large bubbles and the edge dislocation due to a large portion of the strain other than microbubbles present in the sapphire ingot is adapted to be detected by incident. The laser beam transmitted through the half-wave plate is scanned in a plane by a galvanometer mirror and a reflection mirror so that a scattered image of a sapphire ingot section scanned perpendicularly to the central axis of the sapphire ingot is observed. The light scattering observation apparatus according to claim 1, wherein: 3. The light according to claim 1, wherein the sapphire ingot is moved by a vertical moving stage with an encoder so that a scattered image of a cross section of the sapphire ingot at different positions in the vertical direction is observed. Scattering observation device. The image information of the scattered image in each cross section of the sapphire ingot and the horizontal position information and vertical position information in each cross section are stored, and the position of the scatterer existing in the sapphire ingot by image processing based on each stored information The light scattering observation apparatus according to claim 3 , wherein the size is specified. The sapphire ingot, light scattering observation apparatus according to claim 1, 2, 3 or 4, characterized in that a sapphire ingot before wafer processing to be applied to the epitaxial growth substrate.

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