JP5585438B2 - Wafer defect detection method - Google Patents

Description

  The present invention relates to a method for detecting defects on the surface of a wafer, and more particularly to a method for detecting defects in a wafer that suppresses erroneous detection by reducing the influence of noise during detection.

  2. Description of the Related Art In recent years, with the miniaturization of semiconductor devices, the effects of crystal defects and attached foreign substances (hereinafter collectively referred to as “defects”) on a substrate have increased. Therefore, high crystallinity and cleanliness are increasingly required for the surface of a silicon wafer as a substrate. In order to meet these requirements, it is necessary to identify defects present on the surface and reflect them in the manufacturing conditions of the wafer. For this purpose, it is important to develop a technology that can correctly detect defects existing on the wafer surface.

As such a technique, the surface of the wafer to be inspected is irradiated with laser light, the intensity of the scattered laser light is detected as a signal light by a particle inspection machine, and the defect is detected using the intensity of the signal light as a bright spot defect ( There is known a detection method as Light Point Defects (hereinafter referred to as “LPD”). That is, using standard particles with a known size, the correlation between the intensity of the light scattered by the standard particles and the size of the standard particles obtained by irradiating the light incident on the wafer surface is obtained in advance, and the detected signal The quality of the wafer surface is evaluated by counting (counting) the number of times the light intensity (that is, the size of the LPD) exceeds a predetermined threshold.

  In general, signal light detected by a particle inspection machine includes noise, and there are light originating from the apparatus and light originating from the roughness of the wafer surface. The signal detected by the particle inspection machine is detected by superimposing such noise on the signal derived from the defect on the wafer, so if the defect actually exists but not detected, or conversely It may be detected even though it does not exist. Therefore, in order to correctly evaluate the quality of the wafer, it is important to reduce the influence of such noise and suppress false detection.

  Various methods for removing such noise have been proposed so far, but a method of reducing the influence of noise by increasing the integration time of the obtained scattered light with respect to the detection signal is general ( For example, see Patent Document 1).

JP 2005-526239 A

However, in the method described in Patent Document 1, when the signal strength of the defect is about the same as the noise strength, there is a problem that the noise cannot be sufficiently reduced even if the integration time is increased, and erroneous detection still occurs. .
Therefore, an object of the present invention is to provide a way to reduce the influence of noise and suppress erroneous detection of defects on the wafer.

  As a result of earnestly examining ways to solve the above problems, the inventor repeatedly inspected the same wafer under detection conditions in which the number of LPDs detected by one inspection of the wafer is equal to or less than a reference value. At this time, it was found that LPDs detected twice or more at the same position are not affected by noise and can be regarded as being caused by defects, and the present invention has been completed.

  That is, in the wafer defect detection method of the present invention, when the irradiation light is scanned over the entire surface of the wafer and the defects on the wafer surface are detected as bright spot defects by the scattering intensity of the irradiation light, Is set to an initial value, and the number of the bright spot defects is detected under the initial value. If the detected number is equal to or less than a reference value, the initial value is set as a threshold value, while the detected number exceeds the reference value. In this case, the detection of the bright spot defects is repeated by increasing the initial value, and the initial value when the number of bright spot defects is equal to or less than the reference value is set as a threshold value. The detection of the number of point defects is repeated, and the number and position of the bright spot defects that are detected at least twice at the same position of the wafer, and the scattering intensity of the irradiation light on the wafer surface is equal to or higher than the threshold value. The number of defects in the wafer and Those characterized by determining the position.

In the defect detection method for a wafer according to the present invention, the number of repetitions of the detection of the number of bright spot defects under the threshold value is two or more.

  Further, in the wafer defect detection method of the present invention, the reference value is 830 when the wafer diameter is 200 mm, and 1000 when the wafer diameter is 300 mm, In the case of 450 mm, the number is 1250.

  According to the present invention, since the influence of noise can be reduced by repeating detection processing under appropriate processing conditions, it is possible to suppress erroneous detection of defects on the wafer.

It is a figure which shows the defect detection apparatus used in the defect detection method of this invention. It is a figure which shows the relationship between the average haze value of a wafer at the time of test | inspecting a wafer 20 times, and the count number of the position where LPD was detected 1-3 times. It is a figure which shows the relationship between the average haze value of a wafer when the threshold value is raised and the wafer is inspected 20 times and the count number of the position where LPD is detected 1-3 times.

Hereinafter, embodiments of the present invention will be described in detail.
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. A defect present on the surface of the wafer W is detected as LPD. The incident system includes vertical incident light 11 (Normal) incident in a direction perpendicular to the surface of the wafer W and oblique incident light 12 (Oblique) incident from an oblique direction. On the other hand, 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. Yes.
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 defect detection apparatus 1 is a dark-field defect detection apparatus. However, the defect detection apparatus and its configuration used in the present invention perform binarization processing of a detection signal based on a set intensity threshold value and perform LPD in the wafer surface. There is no particular limitation as long as it is a device that measures the coordinates and the number of these.

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 for detecting light scattered in a high angle direction by irradiating the surface of the wafer W with normal incident light from the combination of these two incident systems and two detection systems (hereinafter referred to as “DNN channel”). ), A channel for detecting light scattered in a low angle direction by irradiating normal incident light (hereinafter referred to as “DWN channel”), and light scattered in a high angle direction by irradiating oblique incident light. 4 detection channels (hereinafter referred to as “DNO channel”) and light detection channels (hereinafter referred to as “DWO channels”) that irradiate obliquely incident light and scatter light scattered in a low angle direction. Exists. By measuring the intensity of scattered light in these four detection channels, LPD present on the wafer W to be inspected is detected.

However, as described above, the signal light detected by the defect detection apparatus 1 includes noise, and when the intensity of the defect signal and the noise is approximately the same, the conventional method of extending the integration time. Then, the influence of noise cannot be reduced sufficiently.
Therefore, the wafer defect detection method of the present invention repeatedly inspects the same wafer under the detection condition that the number of LPDs detected by one inspection of the wafer is equal to or less than the reference value, and 2 at the same position. An LPD detected more than once is regarded as being caused by a defect and is counted.

ここで、ｘは変数、ｍは平均値、σは標準偏差である。 Here, it will be explained that the repeated inspection of the wafer to be inspected in the wafer defect inspection method of the present invention is more effective in reducing the influence of noise than the conventional method.
First, assuming that a signal detected by the defect inspection apparatus 1 is D (x), a signal caused by a defect is S (x), and a signal caused by noise is P (x),
D (x) = S (x) + P (x) (1)
It can be expressed as. Here, it may be considered that noise is randomly generated with respect to time and position on the wafer and enters the detection signal D (x) stochastically, and the occurrence probability follows a normal distribution. That is,

Here, x is a variable, m is an average value, and σ is a standard deviation.

となる。ここで、

である。この累積度数関数は、ニュートン・コーツの式等の様々な数値積分法や、簡便にはＭｉｃｒｏｓｏｆｔ Ｅｘｃｅｌ（登録商標）のＮＯＲＭＳＤＩＳＴ関数を用いて容易に計算することができる。 Now, assuming that the threshold value of the detection signal intensity (that is, the defect size threshold value) is k, the noise intensity exceeds the threshold value k and the LPD at a point where no defect exists (that is, S (x) = 0). The probability of erroneous detection that is detected as is given by 1−f (k), where f (x) is the cumulative frequency function of P (x). Therefore, in the method of multiplying the integration time by N times according to the conventional method, the probability n _c (k) that noise appears in the detection signal is

It becomes. here,

It is. This cumulative frequency function can be easily calculated using various numerical integration methods such as Newton-Cotes equation, or simply the NORMSDIST function of Microsoft Excel (registered trademark).

となる。 On the other hand, in the case of the method of the present invention, that is, the method of detecting defects by repeating inspection N times on the same wafer, the noise appearance probability n _p (k) is:

It becomes.

Here, the magnitude relationship between n _c (k) and n _p (k) is specifically examined. Considering the case of inspecting a wafer to be inspected by the defect detection apparatus 1, the wafer diameter to be inspected: 300 mm Laser beam width: 50 μm laser light is irradiated onto a wafer having a wafer diameter: 300 mm and scanned. Suppose that defects on the wafer are detected. At this time, the distance over which the light scans on the wafer is 1,413,235,500 μm, and if 50 μm × 50 μm is one sector, the total number n of sectors on the wafer is 28,264,710.

By multiplying the total number of sectors n by the above-mentioned n _c (k) and n _p (k), it is possible to estimate the number of erroneous detections of defects when a wafer is inspected by the conventional defect detection method and the defect detection method of the present invention, respectively. it can. Table 1 shows the number of false detections obtained by taking m = 0, σ = 1, and k = 2.0 as an example. Here, the number of repetitions means the number of inspections of the same wafer repeatedly in the defect detection method of the present invention, and in the conventional defect detection method, means the multiple of the integration time with respect to the standard integration time. doing.

  As is apparent from this table, in the conventional defect detection method, the number of false detections is less than 1 by multiplying the integration time by 8 times the standard. However, the method of the present invention can be performed by repeating inspection five times. The number of false detections is less than 1. In the conventional defect inspection method, increasing the integration time to N times the standard is equivalent in time to inspecting the same wafer N times in the present invention. Therefore, the defect detection method of the present invention is conventional. It can be seen that noise can be reduced more efficiently than the above method.

  This tendency is similarly seen regardless of the threshold value, and when the threshold value is small, that is, when the defect signal and the noise intensity are close to each other, the difference in reduction effect between the present invention and the conventional method becomes large. As shown in Table 2, when k = 1.2, the number of false detections is less than 1 in the eighth time in the method of the present invention, but it is 21st in the conventional method.

  As described above, the defect inspection method of the present invention can effectively reduce the influence of noise and is effective in suppressing erroneous detection, as compared with the conventional method in which the integration time is increased by the above-described repeated inspection. I understand.

  However, if the inspection is repeated N times and all N times are detected at the same position, assuming that there is a defect at that position, for example, a defect exists at a certain position, and N-1 times are detected. However, if the scattering intensity (that is, the size of the defect) does not exceed the threshold value due to the noise only once, it is determined that there is no defect at that position, and the number of defects on the wafer is too small. Evaluate. We have made further investigations on how to solve this problem. As a result, the inventors have found that after repeating the above inspection, when LPD is detected twice or more at the same position on the wafer, the position may be regarded as having a defect. It is. Below, the experimental results that led to this finding will be described.

  FIG. 2 shows the number of positions at which LPD is detected 1 to 3 times at the same position on the wafer as the average haze value of each wafer when the surface inspection is repeated 20 times for 10 300 mm diameter wafers. It is a figure which shows the relationship. This detection was performed with a beam having a scan width of 50 μm. Here, (a) and (b) show the results for the DWO and DNO modes, respectively, and the thresholds for detecting LPD were (a) 32 nm and (b) 51 nm, respectively. Further, in this figure, the crosses indicate the number of LPDs on the wafer when the surface of the wafer is inspected only once.

  2A, it can be seen that the tendency for the average haze value differs between the count number of the position detected only once and the count number of the position detected twice or more. That is, the count number of the position detected only once is constant until the average haze value is 0.06 ppm but increases exponentially when it exceeds 0.06 ppm, whereas it is detected twice or more. The position count is constant up to 0.1 ppm and increases beyond 0.1. The average haze value on the horizontal axis represents the roughness of the wafer surface. As the value increases, the roughness of the wafer surface increases and the intensity of noise superimposed on the scattered light also increases. Yes. Therefore, the number of counts of the positions detected 1 to 3 times increases as the average haze value increases because the number of false detections due to the increase in noise intensity increases.

  Moreover, the count number of the position detected twice or more shows the tendency which is substantially flat irrespective of an average haze value, when an average haze value is 0.1 ppm or less. This indicates that the count number of the position detected twice or more is not affected by noise. That is, it indicates that a defect may exist at a position detected twice or more. This is because the detection signals are all signals caused by noise and do not include signals caused by defects, or if the signals caused by defects are sufficiently small compared to noise and can be ignored. This is because the number of positions detected due to the number of positions depends on the average haze value when the positions exist at a density below a certain level and should not show a flat tendency.

  The above tendency is the same for the measurement result in the DNO mode shown in FIG. That is, the count number of the positions detected 1 to 3 times is constant until the average haze value is 0.015 ppm, but tends to increase when it exceeds 0.015 ppm. Further, the count number of the position detected twice and three times does not depend on the haze value up to 0.015 ppm.

  In the result of the DWO mode shown in FIG. 2A and the result of the DNO mode shown in FIG. 2B, the average haze values at which the counts of the positions detected twice or more increase are 0.1 ppm and 0, respectively. Although it is different from .015 ppm, the number of LPDs detected by one inspection in a wafer having the average haze value is 1000 or less in all cases. That is, when the same wafer is repeatedly inspected using the threshold value that the number of detected LPDs is 1000 or less by one inspection, and is detected twice or more at the same position May be regarded as having a defect at that position.

  On the other hand, for a wafer in which more than 1000 LPDs are detected in one inspection by a beam having a scan width of 50 μm, the number of counts detected at least twice tends to be flat with respect to the average haze value. Absent. This is because the positions detected due to noise are present at a certain density or higher, and erroneous detection occurs. Therefore, in FIG. 2, for three wafers in which the number of LPDs detected in one inspection exceeds 1000, the threshold is set to 33 nm for (a) DWO mode, and to 52 nm for DNO mode in (b). After each increase, the wafer was inspected once. As a result, the number of detected LPDs for all three wafers was 1000 or less. Therefore, 19 inspections were repeated continuously. The obtained results are shown in FIG. In (b) of this figure, the 2 count and 3 count numbers are 0 when the average haze value is 0.0174 ppm, and the 3 count number is also 0 when 0.0206 ppm. As is apparent from this figure, the number of LPDs detected in one inspection becomes 1000 or less, and in FIG. 2, the same tendency as in the case of wafers in which the number of LPDs detected in one inspection is 1000 or less. Is seen. As described above, for a wafer in which more than 1000 LPDs are detected in one inspection, the threshold value is increased until the number of LPDs detected in one inspection is 1000 or less, and then the inspection is repeatedly performed. I understand that

  The above results show the case where the wafer is inspected 20 times, but the wafer may be repeatedly inspected twice or more. The upper limit of the number of repetitions is not particularly limited, but is preferably 2 from the viewpoint of productivity.

From the above experimental results, it can be seen that the detection of defects on the wafer as described below can reduce the influence of noise and suppress false detection. That is, an initial value is set for the scattering intensity, and the number of LPDs is detected under this initial value. If the detected number is equal to or less than the reference value, the initial value is set as a threshold value for defect detection. On the other hand, when the detected number exceeds the reference value, the initial value is increased and the LPD detection is repeated, and the initial value when the number of LPDs is equal to or less than the reference value is set as a threshold for defect detection.
Subsequently, the number of LPDs is repeatedly detected under the threshold value, and the number and position count of LPDs in which the scattering intensity of the irradiation light on the wafer surface is equal to or greater than the threshold value and detected twice or more at the same position on the wafer. To do. Hereinafter, each step of the wafer defect detection method of the present invention will be described.

  First, a method for setting an initial value of an intensity threshold for detecting a defect will be described. Although an example of the initial value setting method will be described below, the present invention is not limited to this. The wafer to be inspected by this patent is a high-quality wafer whose total number of detected LPDs is about 100 or less in a 300 mm wafer surface, for example. Here, in the inspection conditions in which the total number of LPDs is about 100, a condition in which a sufficient margin is provided for noise of a normal detection signal is employed. Starting from this normal condition and lowering the intensity threshold, the total number of LPDs suddenly increases from the vicinity where the noise reaches the threshold. Therefore, the intensity threshold may be adjusted to about 1000 while watching the increase.

  Under this set initial value, the wafer to be inspected is inspected only once with a beam having a scan width of 50 μm, and the number of LPDs is detected. Here, it is determined whether or not the number of detected LPDs exceeds a reference value. Based on the above experimental results, the reference value is preferably 830 when the wafer to be inspected is 200 mm in diameter, 1000 when the wafer is 300 mm in diameter, and 1250 when the wafer is 450 mm in diameter. If the number of detections is equal to or less than the reference value, the initial value is set as a threshold value. On the other hand, when the detected number exceeds the reference value, the initial value is increased by an appropriate value, for example, 1 nm, and then the wafer inspection is repeated, and the initial value when the number of detected LPDs is equal to or less than the reference value. Is a threshold value.

Next, the number and positions of LPDs in which the wafer inspection is repeatedly performed under the above threshold value, the scattering intensity of the irradiation light on the wafer surface is equal to or higher than the threshold value, and detected twice or more at the same position of the wafer. Thus, the number and position of defects on the wafer are determined.
Here, the number of repetitions described above is two or more.

  Thus, when detecting defects on the wafer, it is possible to reduce the influence of noise and suppress false detection.

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 (3)

Scanning irradiation light over the entire surface of the wafer, and detecting defects on the surface of the wafer as bright spot defects by the scattering intensity of the irradiation light,
Setting an initial value for the scattering intensity, and detecting the number of bright spot defects under the initial value,
If the detected number is less than or equal to a reference value, the initial value is set as a threshold value.If the detected number exceeds the reference value, the initial value is increased and the detection of the bright spot defect is repeated, and the bright spot defect is detected. The initial value when the number falls below the reference value is set as a threshold,
Subsequently, the number of the bright spot defects is repeatedly detected under the threshold value, and the scattering intensity of the irradiation light on the wafer surface is equal to or higher than the threshold value and is detected twice or more at the same position of the wafer. A method for detecting a defect in a wafer, comprising determining the number and position of defects in the wafer based on the number and position of bright spot defects. The wafer defect detection method according to claim 1, wherein the number of repetitions of the detection of the number of bright spot defects under the threshold is two or more. The reference value is 830 when the diameter of the wafer is 200 mm, 1000 when the diameter of the wafer is 300 mm , and 1 250 when the diameter of the wafer is 450 mm. The wafer defect detection method according to claim 1, wherein:

Images