Inspection method and apparatus for the inspection of either random or repeating patterns

• US 6,021,214 A
• Filed: 09/07/1995
• Issued: 02/01/2000
• Est. Priority Date: 09/30/1993
• Status: Expired due to Term

• Current Assignee: KLA Instruments, Corporation (KLA Corporation)
• Original Assignee: KLA Instruments, Corporation (KLA Corporation)
• Inventors: Lin, Jason Z., Tsai, Bin-Ming Ben, Evans, David M. W.
• Primary Examiner(s): Tran, Phuoc
• Application Number:
  US08/524,608
• Time in Patent Office: 1,608 Days
• Field of Search: 382/141, 382/149, 382/168, 382/170, 382/172
• US Class Current: 382/141
• CPC Class Codes: G03F 7/7065 Defects, e.g. optical inspection of patterned layer for defects
• CPC Class Codes: G06T 2207/20056 Discrete and fast Fourier transform, [DFT, FFT]
• CPC Class Codes: G06T 2207/30148 Semiconductor; IC; Wafer
• CPC Class Codes: G06T 7/0004 Industrial image inspection
• CPC Class Codes: G06T 7/37 using transform domain methods
• CPC Class Codes: G06V 10/431 Frequency domain transformation; Autocorrelation
• CPC Class Codes: G06V 10/751 Comparing pixel values or logical combinations thereof, or feature values having positional relevance, e.g. template matching

Abstract

The present invention is a hybrid technique for finding defects on digitized device images using a combination of spatial domain and frequency domain techniques. The two dimensional spectra of two images are found using Fourier like transforms. Any strong harmonics in the spectra are removed, using the same spectral filter on both spectra. The images are then aligned, transformed back to the spatial domain, and subtracted. The resulting spectrally-filtered difference image is thresholded and analyzed for defects. Use of the hybrid technique of the present invention to process digitized images results in the highest-performance and most flexible defect detection system. It is the best performer on both array and random devices, and it can cope with problems such as shading variations and the dark-bright problem that no other technique can address. The hybrid technique of the present invention also uses frequency domain techniques to align the images with fewer errors than spatial domain techniques of similar or lesser complexity. Further, the relative offsets of the pairs of images are determined by frequency domain techniques--and this method may be the most accurate and the least expensive.

Claims

1. In an inspection system for detecting defects in devices of any type having either random, repeating or mixed patterns within the same device, said inspection system includes first and second fields of view each positioned along a different optic axis, the first field of view creates a first spatial domain pixel map of a portion of a first substrate pattern;

and said second field of view provides a second spatial domain pixel map against which said first substrate pattern is to be checked from either a pre-stored pixel map of the desired pattern, a scaled drawing of the desired pattern, a repeating pattern within the same device on said first substrate as in said first field of view, a pattern within a different device on said first substrate as in said first field of view, or a second substrate pattern;

a method for detecting defects in said first substrate pattern in said first field of view comprising the steps of;

a. generating a residual spatial domain pixel map by subtracting one of said first and second spatial domain pixel maps from the other of said first and second spatial domain pixel maps;

b. determining the pixel gray level histogram of said residual spatial domain pixel map of step a.;

c. fitting the histogram of step b. to an exponential distribution function;

d. determining a threshold amplitude that is above the expected noise level and below the expected defect level from said exponential distribution function of step c.; and

e. thresholding said residual spatial domain pixel map of step a. with the determined threshold from step d. to identify all of those values that exceed said threshold as defects in said first spatial domain pixel map.

2. In an inspection system for detecting defects in devices of any type having either random, repeating or mixed patterns within the same device, said inspection system includes first and second fields of view each positioned along a different optic axis, the first field of view creates a first spatial domain pixel map of a portion of a first substrate pattern;

and said second field of view provides a second spatial domain pixel map against which said first substrate pattern is to be checked from either a pre-stored pixel map of the desired pattern, a scaled drawing of the desired pattern, a repeating pattern within the same device on said first substrate as in said first field of view, a pattern within a different device on said first substrate as in said first field of view, or a second substrate pattern;

a method for detecting defects in said first substrate pattern in said first field of view comprising the steps of;

a. generating a residual spatial domain pixel map by removing the repeating pattern from said first spatial domain pixel map;

b. determining the pixel gray level histogram of said residual spatial domain pixel map of step a.;

c. fitting the histogram of step b. to an exponential distribution function;

d. determining a threshold amplitude that is above the expected noise level and below the expected defect level from said exponential distribution function of step c.; and

e. thresholding said residual spatial domain pixel map of step a. with the determined threshold from step d. to identify all of those values that exceed said threshold as defects in said first spatial domain pixel map.

3. An inspection system for detecting defects in devices of any type having either random, repeating or mixed patterns within the same device comprising:

a processor for controlling the operation of said inspection system;

first and second fields of view means each under the control of said processor and each positioned along a different optic axis, the first field of view for creating a first spatial domain pixel map of a first substrate pattern, and said second field of view for providing a second spatial domain pixel map against which said first substrate pattern is to be checked from either a pre-stored pixel map of the desired pattern, a scaled drawing of the desired pattern, a repeating pattern within the same device on said first substrate as in said first field of view, a pattern within a different device on said first substrate as in said first field of view, or a second substrate pattern;

generation means for generating a residual spatial domain pixel map by subtracting one of said first and second spatial domain pixel maps from the other of said first and second spatial domain pixel maps;

histogram means for determining the pixel gray level histogram of said residual spatial domain pixel map of said generation means;

fitting means for fitting the histogram of the histogram means to an exponential distribution function;

selection means for determining a threshold amplitude that is above the expected noise level and below the expected defect level from said exponential distribution function of said fitting means; and

thresholding means for thresholding said residual spatial domain pixel map with the determined threshold to identify all of those values that exceed said threshold as defects in said first spatial domain pixel map.

4. An inspection system for detecting defects in devices of any type having either random, repeating or mixed patterns within the same device comprising:

a processor for controlling the operation of said inspection system;

first and second fields of view means each under the control of said processor and each positioned along a different optic axis, the first field of view for creating a first spatial domain pixel map of a first substrate pattern, and said second field of view for providing a second spatial domain pixel map against which said first substrate pattern is to be checked from either a pre-stored pixel map of the desired pattern, a scaled drawing of the desired pattern, a repeating pattern within the same device on said first substrate as in said first field of view, a pattern within a different device on said first substrate as in said first field of view, or a second substrate pattern;

generation means for generating a residual spatial domain pixel map by removing the repeating pattern from said first spatial domain pixel map;

histogram means for determining the pixel gray level histogram of said residual spatial domain pixel map of said generation means;

fitting means for fitting the histogram of the histogram means to an exponential distribution function;

selection means for determining a threshold amplitude that is above the expected noise level and below the expected defect level from said exponential distribution function of said fitting means; and

thresholding means for thresholding said residual spatial domain pixel map with the determined threshold to identify all of those values that exceed said threshold as defects in said first spatial domain pixel map.