JP2012068103A - Wafer defect detection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of improving accuracy in detecting a light point defect on a wafer, and classifying the light point defect into a crystal defect or an adhered foreign substance.SOLUTION: In a wafer defect detection method, a surface of a wafer is irradiated with perpendicular irradiation light in a perpendicular direction thereto, and a first size of a light point defect on the irradiated surface is obtained based on detected intensity of scattering light that scatters in a high angle range, in scattering light from the irradiated surface. The surface of the wafer is also irradiated by oblique irradiation light in a direction oblique thereto, and a second size of the light point defect on the irradiated surface is obtained based on detected intensity of scattering light that scatters in a low angle range, in scattering light from the irradiated surface. When classifying the light point defect into a crystal defect and an adhered foreign substance based on a ratio of the first size to the second size and a previously obtained threshold value, wavelength of the perpendicular irradiation light and wavelength of the oblique irradiation light are set to be different.

Description

  The present invention relates to a method for detecting defects on the surface of a wafer, and more particularly to a method for distinguishing and detecting stacking faults and crystal defects such as pits on an epitaxial wafer and foreign particles such as particles.

  Conventionally, epitaxial wafers are used as substrates for transistors, diodes, MOS type and bipolar type ICs. An epitaxial wafer is obtained by growing an epitaxial film on a polished wafer, and has excellent characteristics such as high surface flatness and low metal contamination.

  By the way, in recent years, with the miniaturization of semiconductor devices, the influence of crystal defects and foreign matters on a wafer serving as a substrate on the yield of products is increasing. On the surface of the epitaxial wafer, there are crystal defects peculiar to the epitaxial wafer such as stacking faults and pits and adhered foreign substances such as particles. Among them, crystal defects increase the leakage current of the P / N junction and degrade the gate oxide characteristics of the MOS device. Therefore, in order to improve the quality of the wafer, it is important to distinguish and detect crystal defects and adhered foreign substances existing on the surface, clarify their origin, and reflect them in the manufacturing conditions of the wafer.

  As a conventional technique for precisely evaluating the quality of the wafer surface, the surface of the wafer to be inspected is irradiated with laser light, the intensity of the scattered laser light is detected by a particle inspection machine, and the value of the detected scattering intensity is used. A method of detecting crystal defects such as stacking faults and pits and adhering foreign substances such as particles as bright point defects (LPD) is known.

For example, in Patent Document 1, a silicon wafer to be inspected is irradiated with inspection light from a direction perpendicular or oblique to the surface of the wafer, and a high angle and a low angle with respect to the surface of the wafer. It describes a method for distinguishing and detecting scratches and foreign matter on the wafer surface from light scattered in each direction.
However, in the technique of Patent Document 1, since only incident light from one direction is used, there is a problem that the accuracy of classifying scratches and foreign matters is low.

  Therefore, in Non-Patent Document 1, two incident systems (normal incidence and oblique incidence) and two detection systems (high angle direction scattering and low angle direction scattering) are prepared, and four detection channels obtained from the combination thereof. LPD detected on the epitaxial wafer from vertical incidence / high angle scatter (hereinafter referred to as “DNN channel”) and oblique incidence / low angle scatter (hereinafter referred to as “DWO channel”). Has been proposed for classifying the above into stacking faults and particles.

JP-T-2004-524538

A. Chen et al. "Advanced inspection methods for detection and classification of killer substrate defects", Proc. of SPIE, 2008, Vol. 7140, 71400W-1-10

However, when the surface of the epitaxial wafer is inspected by the method of Non-Patent Document 1, LPD having a relatively large size can be accurately classified into stacking faults and particles, but relatively small LPD of 0.15 μm or less. Left a problem where classification was still difficult.
Therefore, an object of the present invention is to provide a method for improving the accuracy of classifying the detected LPD on the epitaxial wafer into crystal defects and adhered foreign substances.

  The inventors diligently studied ways to solve the above problems. That is, paying attention to the wavelength of the light used when detecting the defect, the wavelength was changed and the correlation between the detection size of the crystal defect and the adhering foreign matter and the wavelength was investigated in detail. As a result, it has been found that the change rate of the detection size with respect to the change in the wavelength of the light applied to the wafer differs between the crystal defect and the attached foreign matter. Therefore, in order to improve the accuracy of classifying the detected LPD on the wafer into crystal defects and adhered foreign substances, it is effective to change the wavelength of light used in the detection of the DNN channel and DWO channel. The headline and the present invention have been completed.

  That is, the defect detection method for a wafer according to the present invention is based on the detection intensity of scattered light scattered in a high angle direction among the scattered light from the irradiated surface of the vertical irradiation light irradiated in the direction perpendicular to the surface of the wafer. The first size of the bright spot defect on the irradiated surface is obtained, and the scattered light scattered in the low angle direction among the scattered light from the irradiated surface of the oblique irradiated light incident on the irradiated surface in the oblique direction. A second size of the bright spot defect is obtained from the detected intensity, and the bright spot defect is classified into a crystal defect and an adhering foreign substance based on a ratio of the first size to the second size and a predetermined threshold value. In doing so, the wavelength of the vertically incident light and the wavelength of the obliquely incident light are made different.

  In the defect detection method for a wafer of the present invention, it is preferable that the wavelength of the perpendicular incident light is larger than the wavelength of the oblique incident light.

  In the defect detection method for a wafer of the present invention, the crystal defect is one or both of a stacking fault and a pit.

  In the wafer defect detection method of the present invention, the wafer is an epitaxial wafer.

  According to the present invention, it is possible to improve the accuracy of detecting LPD on a wafer and classifying it into crystal defects and attached foreign substances.

It is a figure which shows the defect detection apparatus used for this invention. It is a figure explaining the method of classifying the detected LPD into a crystal defect and an adhesion foreign material. It is a figure which shows the size of the defect by a DNN channel by the conventional method, and a DWO channel. FIG. 6 is a diagram showing the size of defects due to the DNN channel and the DWO channel according to the method of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a defect detection apparatus used in the present invention. The apparatus 1 includes two types of incident systems and two types of detection systems. The apparatus 1 irradiates the surface of the wafer W to be inspected rotated by the motor 24 with the intensity of the scattered light. Crystal defects and adhering foreign matter existing on the surface of the wafer W are detected as LPD. The incident system includes normal incident light 11 (Normal) incident in a direction perpendicular to the surface of the wafer W and oblique incident light 12 (Oblique) incident in an oblique direction. The detection system includes a high angle scattered light detector 22 (Narrow) that detects light scattered in a relatively narrow angle range in the high angle direction with respect to the surface of the wafer W, and a relatively wide angle in the low angle direction. And a low-angle scattered light detector 23 (Wide) for detecting light scattered in the range.
In the present invention, the high angle direction means a direction in an angle range of 6 to 20 degrees from the surface vertical direction of the wafer W, and the low angle direction means a direction in an angle range of 25 to 72 degrees. .
Further, the oblique direction means a direction in an angle range of 65 to 85 degrees from the surface vertical direction of the wafer W, and normal incidence means a direction in an angle range of 0 to 20 degrees from the surface vertical direction. The actual angle setting depends on the device manufacturer / model.

Here, the normal incident light 11 is reflected by the reflecting plate 31 and irradiated perpendicularly to the surface of the wafer W to be inspected. Of the light scattered on the wafer surface, the light scattered in the high angle direction with respect to the surface of the wafer W is collected by the condenser lens 33 and then reflected by the reflecting plate 34 to detect the high angle scattered light. It is detected by the device 23. The light scattered in the low angle direction with respect to the surface of the wafer W is collected by the condenser 21 and then detected by the low angle scattered light detector 22.
On the other hand, the oblique incident light 12 is configured to be irradiated from the oblique direction to the surface of the wafer W by the reflecting plate 32. The detection process of the scattered light is the case of the above-described normal incident light 11. It is the same.

  Accordingly, a channel (DNN channel) for detecting light scattered in a high angle direction by irradiating the surface of the wafer W with vertical incident light from the combination of these two incident systems and two detection systems, vertical A channel for detecting light scattered in a low angle direction by irradiating incident light (hereinafter referred to as “DWN channel”), a channel for detecting light scattered in a high angle direction by irradiating oblique incident light (hereinafter referred to as “DWN channel”) , Referred to as “DNO channel”), and four detection channels that detect obliquely incident light and scatter light in a low angle direction (DWO channel). By measuring the intensity of the scattered light in these four detection channels, LPD existing on the wafer W to be inspected is detected to determine its size, and the LPD is classified into crystal defects and adhered foreign substances.

  The size of the LPD is determined from the intensity of scattered light in the above four detection channels. That is, spherical standard particles such as polystyrene latex (PSL) having a known size are arranged on the wafer, and the incident light irradiated on the surface of the wafer is scattered by the standard particles for each of the four channels. A correlation between the intensity of light and the size of the standard particles is obtained in advance, and is determined by applying the intensity of light scattered by crystal defects or adhering foreign matter existing on the wafer W to be inspected to the correlation.

  It is known that the intensity of scattered light detected by the four channels (that is, the detection size of LPD) varies depending on the type of defect and attached foreign matter. That is, for crystal defects having a low aspect ratio such as stacking faults and pits, incident light is scattered in a specific direction, whereas attached foreign matters such as particles scatter incident light in all directions. Specifically, for crystal defects having a low aspect ratio such as stacking faults and pits, the signal intensity of the scattered light by the normal incident light 11 is much higher than that of the oblique incident light 12, whereas the particles have such There is no trend. Further, the normal incident light 11 scattered by the crystal defects having a low aspect ratio is mainly scattered in the high angle direction. Further, with respect to oblique incidence, both of the crystal defects and particles having a low aspect ratio are scattered in the low angle direction. Due to these properties, the detected LPD is attached to the crystal defect based on the size of the LPN detected by the DNN channel, that is, the normal incident light / high angle scattering, and the DWO channel, that is, the oblique incident light / low angle scattering. It can be classified as a foreign object.

  However, as described above, when classified into low-aspect-ratio crystal defects and adhering foreign substances based on the detection size of the DNN channel and the DWO channel by the method of Non-Patent Document 1, the above-mentioned classification is performed particularly for relatively small LPDs of 0.15 μm or less. Is inaccurate.

  Therefore, the inventors paid attention to the wavelength of incident light used in the DNN channel and the DWO channel, and investigated the correlation between the defect type (crystal defect and attached foreign matter) and the detected size in detail by changing the wavelength. As a result, it was found that the rate of change in size with respect to the change in the wavelength of light differs between the crystal defect and the attached foreign matter. In other words, for adhering foreign matter such as particles, the detected size does not change much even if the wavelength of incident light changes, whereas for crystal defects of low aspect ratio such as stacking faults and pits, incident light It became clear that the detected size changes when the wavelength is changed. Hereinafter, the details of the experiment that obtained the above knowledge will be described.

First, an epitaxial wafer was prepared by growing a 4 μm p-type epitaxial film on the surface of a 300 mm p-type silicon wafer. Using this particle inspection machine (SP1 and SP2 of KLA-Tencor) for this epitaxial wafer, crystal defects and adhered foreign substances existing on the wafer surface in the DNN channel and DWO channel are detected as LPD, and the size is obtained. It was. Here, both 0.488 μm (SP1) and 0.355 μm (SP2) were used as the wavelengths of vertically incident light and obliquely incident light.
Next, for each of the LPDs detected by the particle inspection machine on the same epitaxial wafer, the scanning electron microscope (SEM) is used to cause the LPD to be a crystal defect or an attached foreign matter. I identified it.

  As a result of analyzing the correlation between the detected size in the DNN channel and the DWO channel and the wavelength of incident light with respect to the identified crystal defect and attached foreign matter, the crystal defect and attached foreign matter are The change rate of different detection sizes was shown. That is, when the wavelength of the incident light is changed from 0.355 μm to 0.488 μm, the ratio of the detected size when the wavelength is 0.488 μm with respect to the detected size when the wavelength is 0.355 μm for the adhering foreign matter is the DWO channel : 0.90 to 1.10, DNN channel: 0.95 to 1.15. For crystal defects having a low aspect ratio such as stacking faults and pits, the ratio of the detected sizes is DWO channel: 1.10. 1.20, DNN channel: 1.30 to 1.50. As a result, foreign particles such as particles do not change the detection size of the LPD so much even if the wavelength of incident light is changed, whereas crystal defects such as stacking faults detect LPD when the wavelength of incident light is increased. It shows an increase in size.

  This is because, as described above, the determination of the size of the LPD in the particle inspection machine is based on the standard particle of the sphere, and the detected size is incident on the adhering foreign matter such as a particle whose shape is relatively close to a sphere. Although it does not depend on the wavelength of light, crystal defects such as stacking faults and pits have a shape far from a spherical shape, so that the detection size shows the wavelength dependency of incident light. Underlying this is the difference in the properties of light depending on the wavelength, such that straightness becomes stronger at shorter wavelengths, while wavefront is maintained and isotropically scattered at longer wavelengths. This property changes sequentially from the infrared having a wavelength of 800 nm or more to the soft X-ray having a wavelength of 10 nm or less. In the DNN channel, the size is smaller when the wavelength is 0.355 μm (SP2) than when the wavelength is 0.488 μm (SP1). This is also due to stacking faults on the wafer surface and crystal defects having a low aspect ratio such as pits. When the scattered light has a wavelength of 0.355 μm (SP2) than the case of a wavelength of 0.488 μm (SP1), the straightness at that wavelength is stronger. This is because there are many components that leak.

  Based on the knowledge based on the above experimental results, the inventors classify LPD detected on the epitaxial wafer into crystal defects and adhering foreign substances by making the wavelength of incident light different between the DNN channel and the DWO channel. It was found that the accuracy can be improved.

Hereinafter, a method for distinguishing and detecting stacking faults and crystal defects such as pits on the wafer W to be inspected and adhering foreign matters such as particles by the method of the present invention will be described.
First, while rotating the wafer W to be inspected by the motor 24, the surface of the wafer W is irradiated with the vertical incident light 11 having a predetermined wavelength and scanned, and the LPN is detected in the DNN channel to obtain its size.

  Next, the same wafer W is scanned by irradiating obliquely incident light 12 in the same manner as in the case of the normal incident light 11 described above, and LPD is detected in the DWO channel to determine its size. At that time, it is important that the wavelength of the obliquely incident light 12 is different from the wavelength of the vertically incident light 11. Since the same wafer W is used for the inspection, the detection size in the DWO channel can be obtained at each position where the LPD is detected by irradiating the normal incident light 11.

  Subsequently, for each LPD detected on the wafer W, the ratio of the detection size in the DNN channel to the detection size in the DWO channel is obtained, and if the detection size ratio is equal to or greater than a predetermined threshold, the crystal defect is less than the threshold. In this case, it is classified as a foreign object. When the detection size in the DNN channel is plotted on the vertical axis and the detection size in the DWO channel is plotted on the horizontal axis, it is as shown in FIG. The threshold value is a slope of a straight line passing through the origin, which classifies the detected LPD into crystal defects and attached foreign substances.

  Here, the threshold value for classifying the detected LPD into a crystal defect and an adhering foreign substance is obtained in advance as follows. That is, for a predetermined wafer, the surface of the wafer is inspected using the normal incident light 11 and the oblique incident light 12, and the detected sizes of the detected LPD in the DNN channel and the DWO channel are obtained. At that time, the wavelengths of the vertically incident light 11 and the obliquely incident light 12 are set to be the same as those used in the above inspection.

Next, for each of the detected LPDs, the same predetermined wafer is inspected by SEM or the like to classify whether the LPD is a crystal defect or an adhering foreign substance.
Subsequently, similarly to FIG. 2A, the detection size in the DNN channel is plotted on the vertical axis, and the detection size in the DWO channel is plotted on the horizontal axis. On the obtained graph, a straight line passing through the origin is drawn so as to classify the LPD in which the crystal defect or the adhered foreign substance is specified with the highest accuracy. The slope of this straight line serves as a detection size ratio threshold for classifying the LPD into crystal defects and adhering foreign substances.

The wavelength of incident light used in the method of the present invention is 10 to 800 nm. Preferably, it is 150-800 nm.
The ratio of the wavelength of the normal incident light 11 used in the DNN mode to the wavelength of the oblique incident light 12 used in the DWO mode is not particularly limited, but is preferably 1.1 to 5 from the viewpoint of classification accuracy. .

When the vertical incident light 11 and the oblique incident light 12 are irradiated to the wafer W for scanning and the LPD on the wafer W is detected, the oblique incident light 12 is first irradiated to detect the LPD, and then the vertical incident light is detected. Needless to say, LPD may be detected by irradiating light 11.
In addition, each position on the wafer W may be configured to be inspected by irradiating the oblique incident light 12 after being inspected by irradiating the normal incident light 11 and then to repeat the above inspection by shifting the position.

  As described above, by using the threshold value of the detection size ratio obtained in advance, the detected LPD is accurately converted into crystal defects and adhered foreign substances from the ratio of the detection size in the DNN channel to the detection size in the DWO channel. Can be classified.

  In the present invention, the effect of changing the wavelength of incident light between the DNN channel and the DWO channel will be described with reference to FIG. As described above, adhered foreign matters such as particles have a small rate of change in the size of the detected LPD even when the wavelength of incident light is changed, whereas crystal defects such as stacking faults change the wavelength of incident light. When the increase is made, the rate of change in the size of the detected LPD is large. Accordingly, in FIG. 2A, when the wavelength of incident light in the DNN channel plotted on the vertical axis is different from the wavelength of incident light in the DWO channel plotted on the horizontal axis, as shown in FIG. Compared with the case of the same wavelength, the plot due to crystal defects shifts in the vertical direction. As a result, when the same detection size ratio threshold is used, it is possible to improve the accuracy of classifying LPDs into crystal defects and attached foreign substances. At that time, if the wavelength of the incident light in the DNN channel is made larger than the wavelength of the incident light in the DWO channel, the plot of LPD due to crystal defects such as stacking faults and pits shifts upward, Therefore, it is preferable to make the wavelength of vertically incident light in the DNN channel larger than the wavelength of obliquely incident light in the DWO channel.

  Thus, the defect detection method of the present invention can detect the LPD on the epitaxial wafer and improve the accuracy of classifying the LPD into a crystal defect and an attached foreign substance.

Examples of the present invention will be described below.
(Comparative example)
Using the particle counter (SP2 of KLA-Tencor), the method described in Non-Patent Document 1, that is, on the wafer using light of the same wavelength (0.355 nm) in both the DNN channel and the DWO channel LPD was detected. As the wafer, five epitaxial wafers obtained by growing a 4 μm epitaxial film on a 300 mm p-type silicon wafer were used. FIG. 3 shows a plot of the detected size of the detected LPD on the DNN channel on the vertical axis and the detected size on the DWO channel on the horizontal axis. In FIG. 3, only the LPD having a size of 0.15 μm or less is plotted. Here, as described above, the threshold value of the detection size ratio in the DNN channel and the DWO channel used to separate the crystal defect and the attached foreign matter is 1.625. As a result, among the detected LPDs, 5 points were classified as crystal defects and 8 points as adhered foreign substances.
Next, for each LPD detected by the particle inspection machine, it was specified by SEM whether the detected LPD was a crystal defect or an attached foreign substance. As a result, among those classified as crystal defects by the particle inspection machine, there were actually 4 crystal defects, and 9 were actually adhered foreign substances.

(Invention example)
With respect to the wafer used in the comparative example, LPD on the wafer was detected using the method of the present invention, that is, using light of different wavelengths in the DNN channel and the DWO channel, and classified into crystal defects and adhered foreign substances. At that time, two types of particle counters (KLA-Tencor SP1 and SP2) were used, and the wavelengths of light used were 0.488 μm (SP1, DNN channel) and 0.355 μm (SP2, DWO channel), respectively. It was. FIG. 4 shows a plot of the detected size of the detected LPD on the DNN channel on the vertical axis and the detected size on the DWO channel on the horizontal axis. In FIG. 4, only the LPD having a size of 0.15 μm or less is plotted. Here, 2.125 was used as the threshold value of the detection size ratio between the DNN channel and the DWO channel for separating crystal defects and adhered foreign substances. As a result, among the detected LPDs, 4 points were classified as crystal defects and the remaining 9 points were classified as adhered foreign substances.
Next, each LPD detected by the particle inspection machine was compared with the result of specifying the LPD as a crystal defect and an adhering foreign substance using the SEM in the comparative example. As a result, among those classified as crystal defects by the particle inspection machine, there were actually 4 crystal defects, and 9 were actually adhered foreign substances.
FIG. 4 shows that the wafer defect detection method according to the present invention can classify relatively small LPDs of 0.15 μm or less into crystal defects and adhered foreign substances with high accuracy. It can also be seen that the classification accuracy is improved as compared with the comparative example.

DESCRIPTION OF SYMBOLS 1 Defect detection apparatus 11 Normal incident light 12 Oblique incident light 21 Condenser 22 Low angle scattered light detector 23 High angle scattered light detector 24 Motor 31, 32, 34 Reflector 33 Condensing lens W Wafer

Claims (4)

The first size of the bright spot defect on the irradiated surface from the detected intensity of the scattered light scattered in the high angle direction among the scattered light from the irradiated surface of the vertical irradiated light irradiated in the direction perpendicular to the surface of the wafer. The second size of the bright spot defect is determined from the detected intensity of the scattered light scattered in the low angle direction among the scattered light from the irradiated surface of the oblique irradiated light incident on the irradiated surface in the oblique direction. And classifying the bright spot defects into crystal defects and adhering foreign substances based on a ratio of the first size to the second size and a predetermined threshold value,
A defect detection method for a wafer, wherein the wavelength of the perpendicular incident light and the wavelength of the oblique incident light are made different.   2. The wafer defect detection method according to claim 1, wherein the wavelength of the perpendicular incident light is larger than the wavelength of the oblique incident light.   The wafer defect detection method according to claim 1, wherein the crystal defect is one or both of a stacking fault and a pit.   The said wafer is an epitaxial wafer, The defect detection method of the wafer as described in any one of Claims 1-3 characterized by the above-mentioned.

Images