Source: http://www.google.com/patents/US7142294?dq=5,815,488
Timestamp: 2014-03-11 21:41:23
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Patent US7142294 - Method and apparatus for detecting defects - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsAn image of a sample that has high contrast both in large and fine pattern parts is acquired by using an optical system for coaxial bright field epi-illumination, forming the optical image of the sample with various transmission ratio of 0-th order diffracted light that is reflected regularly from the...http://www.google.com/patents/US7142294?utm_source=gb-gplus-sharePatent US7142294 - Method and apparatus for detecting defectsAdvanced Patent SearchPublication numberUS7142294 B2Publication typeGrantApplication numberUS 10/020,977Publication dateNov 28, 2006Filing dateDec 19, 2001Priority dateDec 20, 2000Fee statusPaidAlso published asUS7440092, US20020089664, US20070064225Publication number020977, 10020977, US 7142294 B2, US 7142294B2, US-B2-7142294, US7142294 B2, US7142294B2InventorsYukihiro Shibata, Shunji MaedaOriginal AssigneeHitachi, Ltd.Export CitationBiBTeX, EndNote, RefManPatent Citations (10), Referenced by (7), Classifications (11), Legal Events (2) External Links: USPTO, USPTO Assignment, EspacenetMethod and apparatus for detecting defectsUS 7142294 B2Abstract An image of a sample that has high contrast both in large and fine pattern parts is acquired by using an optical system for coaxial bright field epi-illumination, forming the optical image of the sample with various transmission ratio of 0-th order diffracted light that is reflected regularly from the sample, and capturing the image by an image sensor. Optical conditioning is automatically set and in a short time by detecting a plurality of optical images of the sample under various conditions for the transmission ratio of the 0-th order diffracted light, evaluating quality of the detected images, and determining the transmission ratio of the 0-th order diffracted light showing the maximum defect detection sensitivity.
repeatedly obtaining image signals of a same portion of a sample by imaging said sample through an optical system by changing optical conditions;
analyzing said repeatedly obtained image signals and selecting plural optical conditions which decrease a difference of contrast in the image signal among segments corresponding to a plurality of regions on said sample;
obtaining image signals of said sample under said selected plural optical conditions by imaging said sample with said optical system;
evaluating images obtained under said selected plural optical conditions to adjust optical conditions for inspection; and
detecting a defect of said sample by processing image signals of the sample under said adjusted optical conditions;
wherein in the step of evaluating, said images are evaluated so as to determine an inspection threshold, which is greater than a maximum contrast difference among false defects detected at the step of obtaining image signals and with which a maximum number of true defects can be detected.
2. A method for detecting a defect according to claim 1, wherein the changing of optical conditions includes selecting different transmission ratios of 0-th order diffracted light included in entire light generated by illumination and reflected from said sample.
3. A method for detecting a defect according to claim 2, wherein the adjusting of the transmission ratio of said 0-th order diffracted light is performed by utilizing a spatial filter that is positioned on or near a Fourier transform plane of said sample and that reduces the transmission ratio of the 0-th order diffracted light.
4. A method for detecting a defect, comprising the steps of:
illuminating a sample through an optical system;
repeatedly obtaining a plurality of image signals of a same portion of said sample through said optical system by changing optical conditions included in entire light generated by said illumination and reflected from said sample and imaging said sample;
selecting plural optical conditions for which defect detection sensitivity is increased by analyzing said repeatedly obtained plurality of image signals having the changed optical conditions;
evaluating images obtained under the selected plural optical conditions and setting the optical conditions for inspection in accordance with the evaluation of the images which includes determining an inspection threshold, which is greater than a maximum contrast difference among false defects detection at the step of repeatedly obtaining image signals and with which a maximum number of true defects can be detected;
obtaining the image signals by imaging said sample with said optical system while scanning said sample in a viewing field of said optical system under said set optical conditions; and
detecting a defect of said sample by using the image captured under said set optical conditions.
5. A method for detecting a defect, according to claim 4, wherein the sample is illuminated through an objective lens, and the changing of optical conditions includes providing different transmission ratios of 0-th order diffracted light through said objective lens by changing the transmission ratio of the 0-th order light included in entire light generated by said illumination and reflected from said sample and imaging said sample.
6. An apparatus for detecting a defect, comprising:
a stage for loading a sample;
an illuminating system which illuminates the sample loaded on said stage through an objective lens;
an image detecting unit which forms an optical image of said sample illuminated by said illuminating system and detects said optical image with a sensor to output the image signals of said sample;
an image processing unit which processes said image signal output from said image detecting unit to detect defects of said sample; and
a control unit which controls said image detecting unit to repeatedly detect the optical image of said sample by changing optical conditions, and controls said image processing unit to analyze said repeatedly detected image signals and to select plural optical conditions which decrease a difference of contrast in the image signal among segments corresponding to a plurality of regions on said sample, to evaluate images obtained under the selected plural optical conditions and to determine the optical conditions including an inspection threshold, which is greater than a maximum contrast difference among false defects detected and with which a maximum number of true defects can be detected, which are utilized for inspection so as to decrease a difference in contrast in an image signal among segments corresponding to a plurality of regions on said sample.
7. An apparatus for detecting a defect according to claim 6, further comprising a contrast calculating unit which calculates contrast in the image signals of said sample.
8. A method for detecting a defect, comprising the steps of:
repeatedly obtaining image signals of a same area of a sample by imaging said sample by changing optical conditions;
analyzing said repeatedly obtained image signals and selecting plural optical conditions which modify a contrast in the image signal;
obtaining image signals of said sample under said selected plural optical conditions by imaging said sample with an inspection system;
evaluating images under said selected plural optical conditions to adjust optical conditions for inspection including an inspection threshold, which is greater than a maximum contrast difference among false defects detected at the step of obtaining and with which a maximum number of true defects can be detected; and
detecting a defect of said sample by processing the image signals of the sample obtained through said inspection system under said adjusted optical conditions.
9. A method according to claim 8, wherein said optical conditions include a polarization sate of a light which illuminates said sample in the step of obtaining.
10. An apparatus for detecting a defect, comprising:
an imaging unit which repeatedly obtains image signals of a same area of a sample by imaging said sample by changing optical conditions;
an analyzing unit which analyzes said repeatedly obtained image signals and selects plural optical conditions which modify a contrast in the image signal;
said imaging unit obtaining image signals of said sample under said plural optical conditions;
an evaluating unit which evaluates images obtained under the selected plural optical conditions and which adjusts optical conditions for inspections based on the evaluation of the image which includes determining an inspection threshold, which is greater than a maximum contrast difference among false defects detected and with which a maximum number of true defects can be detected; and
a detecting unit which detects a defect of said sample by processing the image signals of the sample obtained through an inspection system under said adjusted optical conditions.
11. An apparatus according to claim 10, wherein said optical conditions include a polarization state of a light which illuminates said sample.
DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is shown in FIG. 5. A sample (a wafer) 1 contained in a cassette 41 is transported to a Z-stage 10, a θ-stage 11, an X-stage 12 and a Y-stage 13 by a wafer transporting robot 40 of the system 45. The wafer 1 that has been transported to any one of the stages is moved into a field of view of a sub-optical system 20 having low magnification for detection in an entire chip area to detect an image of the entire chip area. Then, the chip image being divided into a peripheral circuit 2 a 1, a logic part 2 a 2, a memory part 2 a 3 and the like, is captured by a camera 21 in the sub-optical system 20. This image is transferred to an image processing section 30. This image is stored in a data server 31. The system is configured so that this image may be shown on a display of an operating computer 35 in this inspection apparatus. Therefore, the operating computer 35 can select a region (a peripheral circuit part 2 a 1, a logic part 2 a 2, a memory part 2 a 3 and the like) to acquire an image for conditioning the transmission ratio (Ib/Ia) of the 0-th order diffracted light, on the display. The sub-optical system 20 for detection in an entire chip area is provided a polarizing conditions adjusting section 201 which comprises a PBS and a half wave plate or a quarter wave plate, and an objective lens 2.
FIG. 3 shows an example of an image obtained from the die region of the representative chip 2 a by conventional bright field detection. Considering distribution of detected light quantity (intensity) in a range A�A of the detected image, it can be found that the peripheral circuit part 2 a 1 having the large pattern width and the low density shows high pattern modulation M1. The memory part 2 a 3 having the fine pattern width and the high density is generally detected darkly and has low modulation M3. In addition, M2 shows modulation in the logic part 2 a 2. Such generally dark detection of the memory part 2 a 3 results from reduction of ratios of the 0-th order light and the higher order diffracted light captured by an objective lens. In a defect inspection, a difference between images of adjacent dies is firstly acquired by difference image calculating section 64, and then points having values beyond a threshold are determined as defects by defect determining section 65. Therefore, inspection sensitivity is reduced in a region (part) having low modulation of the detected image. Thus, in order to have uniform defect detection sensitivity, it is desirable that the modulation (contrast) is equal in the entire die region irrespective of the pattern width and the pattern density.
Since every wafer to be inspected varies in a pattern width and a pattern density, it is necessary to condition the detection ratio of the 0-th order diffracted light (regular reflected light) for the defect inspection in advance. FIG. 6 shows a flowchart of such inspection. A wafer to be inspected is loaded on the stage 10�13 into an inspection apparatus (S61). Then information about a die arrangement in the wafer is registered to the operating computer 35 or the data server 31 (S62). Then, coordinates of an inspection area in a die are registered to the operating computer 35 or the data server 31 (S63). Then, a region to acquire an image for conditioning the transmission (detection) ratio of the 0-th order diffracted light is selected on the display of the operating computer 35 (S64).
The series of varying transmission ratios of the 0-th order diffracted light is narrowed down to a plurality of transmission ratios having relatively higher evaluation values (S69). Then, a test inspection is performed with sensitivity including false defects by using the optical system 15 (S70). And defects to be detected are classified as true or false defects by the image processing section 30 (S71). Images of the true and false defects parts are detected for each of the plurality of narrowed-down detection ratios of the 0-th order diffracted light by the optical system 15 and difference images for each detection ratio are computed by the image processing section 30 (S72). Then, the maximum contrast difference of the false defects part Nmax is determined for each transmission ratio of the 0-th order diffracted light by the image processing section 30 (S73). Further, for each transmission ratio of the 0-th order light, when an inspection threshold is determined by adding a constant α to the Nmax, the number of the detectable true defects is calculated by the image processing section 30 (S74). The transmission ratio of the 0-th order diffracted light with which the maximum number of the true defects can be detected is set as a condition of the actual inspection for the region selected by step S64 (S75). Then, setting the inspection threshold to [Nmax+α], the test inspection is performed for the particular region (S76), and then, if desirable sensitivity is satisfied (S77), the conditioning procedure is completed (S78). Hereinafter, the actual inspection will be performed with the conditioned sensitivity under the conditions in that the transmission ratio of the 0-th order diffracted light has been adjusted for each of a peripheral circuit part 2 a 1, a logic part 2 a 2, a memory part 2 a 3 and the like in a die (chip).
Next, the maximum contrast difference in divided regions is described. A detected image is divided into regions of a predetermined size, the minimum and the maximum contrast value are acquired for each divided region, and then the difference between the minimum and the maximum values is calculated as the maximum contrast difference. The maximum contrast difference values are calculated for all divided regions and absolute values of the maximum contrast difference values are summed. As an example of how to divide a detected image into regions, a region of 3 pixels�3 pixels in X, Y coordinates of the image may be defined as one segment. Since the one segment contains brightness information for 9 pixels, a contrast difference value for the one segment can be acquired by determining a difference between the maximum and the minimum value for such 9 pixels. Considering the fact that the contrast difference value for the one segment corresponds to contrast of the pattern image of such segment, the larger the maximum contrast difference value is, the more advantageous the value is for defect inspection. Therefore, since the larger summation value of the maximum contrast difference values is also more advantageous for defect inspection, it is contemplated to select the inspection conditions for the transmission ratios of the 0-th order diffracted light that have the larger summation value as candidate conditions for actual inspection. Here, it is to be noted that the maximum contrast difference value indicated here does not include variations other than wafer pattern information, such as a sampling error of an image, variations of partial illumination distribution and the like.
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