Final Project for Spring 2003-2004
Automatic Detection of Defects in Photomask Images

1. Introduction

At various stages in the IC manufacturing process defects need to be inspected to ensure process integrity. One key element of this is the detection of defects in photomasks (also referred to as “masks”), which are photographic quartz or glass plates that contain the circuit patterns used in the silicon-chip fabrication process. In order to have an IC production chain that has a sufficiently high yield, it is essential that any defects that arise in photomasks be detected, as the quality of the resulting ICs can only be as good as that of the photomask. In this sense, the quality of the photomask functions as one of several quality bottlenecks in the manufacturing process. A defect can be any flaw affecting the geometry of the resulting circuit. This includes chrome where it was not specified to be (spots, extensions, bridging, etc.) or undesired clear areas (pin holes, “mouse bites”, clear breaks, etc.). A defect can cause the circuit to either not function properly or at all. As no manufacturing process can ever be perfect, chip designers give a certain error tolerance to the fabrication plant and any defects that go beyond these specifications result in an unusable photomask.

Mask defect detection typically takes one of two forms:

·        Die-to-die (D:D) – This method compares two optical images from differed dice on the same mask, where the two dice are designed to have the same pattern. Since in most cases the probability of having the same defect occur at the same location in different dice is negligible, an agreement between the two images is considered to indicate a good pattern. A disagreement is therefore considered to be a defect.

·        Die-to-database (D:DB) – In this method, a captured optical image of the photomask is compared to a rendered image from the intended design pattern database, which functions as the ideal correct model. In this sense D:DB is similar to D:D except that the other die is actually a perfectly rendered target image.

 Figure 1: D:DB Inspection                                       Figure 2: D:D Inspection

2. Project Task

In this project, your goal is to develop an efficient automatic defect detection algorithm according to the D:DB method that can applied to photomask images that have been corrupted and distorted in various ways. As inputs to your algorithm you are given two images: one representing the database image (the template for a correct photomask) and a noisy and distorted image representing an actual photomask that you are inspecting for the presence of defects. Noise and distortion can take the form of warping, shifting, additive noise, linear filtering, rotating, shading, and resizing. The number of defects varies from case to case. Given these two images, your algorithm should produce as its output both the number of detected defects as well as the coordinates of the defects detected, if any. Each detected defect should be specified by its associated coordinates only once. Because defects will always be larger than a single pixel, your algorithm will be considered correct if it is able to return the coordinates of one (and only one) of the pixels associated with a given defect. Errors to be avoided are misses, false alarms, and multiple hits.

To assist you in developing your algorithms, 10 sets of training images are provided. Each set will consist of three images: the template (correct) database image, the distorted and noisy image to inspect, and an error mask image indicating in the frame of reference of the distorted image the location of pixels that are parts of defects. By using the error mask image you will both know the location of the defects in the training images and know what is considered to be a defect for this project. In all cases, defects will be clear and unambiguous.

While reliability of your algorithm is very important, computational efficiency should be considered as well. As described below, while accuracy will be the primary criterion by which you will be graded, in the cases of equally accurate algorithms performance time will be used as the tie-breaker.

3. Training Image Sets

The images below are a training set of "easy" images. These can be used to aid algorithm development. However, expect the test set to contain more difficult images.

4. Implementation Details

There are three major steps in the detection project:

(1).  Design and implement your routine in MATLAB based on the provided training image sets.

(2).  Test your routines with the training image sets by the evaluation program.

(3).  Performance of your routine will be tested with a test image set by the same evaluation program.

Each step is explained in detail below.

(1). Your Defect Detection Routine:

Format

Your main routine has to be in this format:  function defects = defect_detection(imgDB_filename, imgD_filename).

INPUT:   imgDB_filename and imgD_filename are filenames of the template database image and the image to be inspected in BMP format respectively. For instance, for the first training image set imgDB_filename and imgD_filename should be 'training_1_DB.bmp' and 'training_1_D.bmp' respectively.

OUTPUT:   a N-by-2 matrix named 'defects' containing the coordinates of each detected defect

-- N is the number of detects you’ve detected

-- defects(:,1) contains the detected vertical coordinates (row index of the image matrix)

-- defects(:,2) contains the detected horizontal coordinates (column index of the image matrix)

You can call other sub-routines under this main routine. The main routine serves as the interface with our evaluation program. In the final test with the test image set, we'll call your main routine in the evaluation program by defects = defect_detection(xxx, xxx).

Time-Limit

The time-limit for your routines is 10 minutes (when running on a ISE lab machine with most resource available). You should monitor the execution time and output the results by that time-limit. Our evaluation program can check if you are within the time-limit only after your routine is terminated. In other words, you need to make sure you finish your routine in time and return the proper result. 

You might want to start your routine with a rough estimation, and then check the time remaining. If there is a certain amount of time left, refine your estimation. Otherwise, output what you’ve got and terminate your routine. You can also down-sample the image to reduce the amount of computation required. However, this down-sampling process should be part of your own routine. When we test your algorithm, the test image set will be in the same resolution as the training image sets posted on the web.

(2). Evaluation Program (evaluate.m)

Format:

[detectScore, numHit, numRepeat, numFalsePositive, runTime] = ...

                        evaluate(imgDB_filename, imgD_filename, mask_filename)

INPUT:

1. imgDB_filename: filename of the template database image in BMP format

2. imgD_filename: filename of the image to be inspected in BMP format

3. mask_filename: filename of the mask image in BMP format

OUTPUT:

1. detectScore = numHit - numRepeat - numFalsePositive (from output 2,3,4)

2. numHit = number of defects successfully detected

3. numRepeat = number of defects repeatedly detected

4. numFalsePositive = number of cases where a non-defect is reported

5. runTime = run time of your defect detection routine

Example:

[detectScore, numHit, numRepeat, numFalsePositive, runTime] = ...

                        evaluate('training_1_DB.bmp', 'training_1_D.bmp', 'training_1_mask.bmp')

(3). Performance Criterion

The performance of your routine is judged by outputs of the evaluation program. There are two criterions:

(a). Detection Score:

    The detectScore from the evaluation program indicates your detection accuracy.

(b). Run-Time:

    You need to make sure your run-time is within the given time limit (10 minutes). Otherwise, we might need to terminate your routine without getting any result. In addition, while detection accuracy will be the primary criterion by which you will be graded, in the cases of equally accurate algorithms the run-time will be used as the tie-breaker.

5. Testing Image Sets and Final Evaluation Results

In evaluating your algorithms, a total of eight image sets were used. These were based on two different underlying circuit diagrams and varied with respect to the level of both the warping/distortion and additive noise that the images were subjected to. Projects are ranked below, listing the total score (maximum of 60 points) and the average run time. Note that projects were ranked first according to accuracy (reflected by the “program score”) and then ranked according to the average run time in the case of a tie. Although not used in the rankings, program report scores are also given below (maximum of 100 points). For the program score and average time metrics, “N/A” denotes the case of a student’s program failing to run during our tests.

SUID       Program Score  Average Time (sec.)  Report Score

5282348          55                    12.8                             90

5250843          55                    39.1                             87

5251589          55                    174.8                           86

5282208          53                    13.2                             92

5249821          52                    73.9                             90

5282347          51                    27.9                             95

3357936          50                    12.0                             85

5239433          50                    25.9                             89

5246123          50                    28.7                             95

5135197          50                    35.2                             93

5273677          48                    18.2                             85

5239460          47                    35.3                             90

5246741          46                    7.9                               78

5249584          46                    76.1                             88

4365672          45                    269.3                           84

5157803          42                    6.3                               85

5251106          41                    43.6                             88

5144745          41                    65.8                             86

4799482          40                    41.6                             92

5156136          40                    286.4                           78

5249039          40                    289.1                           84

5223781          40                    303.3                           86

5244480          39                    8.8                               84

5151400          38                    58.8                             81

5244703          37                    77.4                             86

5238838          36                    108.1                           91

5250977          35                    13.6                             84

5250783          35                    54.5                             90

5241126          35                    54.7                             70

5153329          35                    84.3                             82

5248972          34                    67.2                             87

5249709          34                    148.6                           89

5239486          31                    14.7                             93

5250957          31                    17.8                             87

4842597          30                    7.7                               88

5168071          29                    6.3                               72

5251160          28                    39.3                             84

4909958          28                    135.2                           84

5233415          27                    108.1                           88

4673745          27                    254.3                           81

5249012          26                    29.3                             92

5249022          26                    1042.4                         80

5248816          26                    1048.6                         83

5246803          25                    7.5                               79

4766630          24                    44.8                             85

5230896          23                    13.0                             84

5264635          23                    15.2                             79

5252465          23                    47.8                             86

5221975          21                    423.1                           90

5121837          20                    34.5                             83

5157887          18                    337.5                           80

5245903          17                    21.4                             79

4833109          15                    74.2                             85

5242600          15                    79.1                             85

5248194          13                    140.9                           83

5246064          10                    32.5                             95

5111139          12                    190.6                           83

5235104          11                    70.9                             80

5113339          10                    294.4                           87

5104960          08                    368.9                           74

4924262          07                    32.3                             86

5244057          07                    41.1                             87

5103372          06                    31.6                             91

5251323          01                    38.4                             82

5153772          01                    159.5                           80

5247553          01                    280.2                           82

4385035          0                      0.1                               78

3799236          0                      12.5                             90

5195553          0                      19.0                             77

5119910          0                      23.0                             74

5114544          0                      33.0                             77

5163081          0                      137.8                           88

5157767          0                      343.1                           82

5252124          0                      715.9                           74

5233395          N/A                 N/A                             80

4788030          N/A                 N/A                             0

4883724          N/A                 N/A                             77

4609897          N/A                 N/A                             0