Patent Application
20070146707
The subject application claims benefit of the earlier filing date of Japanese Patent Application (JPA) No. 2005-373900, filed on Dec. 27, 2005, the entire contents of which are incorporated herein by reference.
The present invention relates generally to workpiece pattern inspection technologies and, more particularly, to a method and apparatus for inspecting circuit patterns of workpieces including, but not limited to, wafers of highly integrated semiconductor devices and liquid crystal display (LCD) panels or photomasks, called reticles, adapted for use in the manufacture thereof. This invention also relates to a workpiece which is tested by the pattern inspection method and apparatus and to a management method of the same.
Prior known pattern inspection apparatus is typically designed to perform inspection by comparing together optical images of patterns formed on workpieces, such as reticles, which images are captured at specified magnification, or alternatively by comparing this optical pattern image to a reference image that is obtained from design data. An example of this approach is disclosed, for example, in Published Unexamined Japanese Patent Application No. 8-76359. Currently available pattern inspection methodology employs several techniques, one of which is die-to-die (DD) inspection, and another of which is die-to-database (DB) inspection. The DD inspection is for comparing together optical images which are gained from identical patterns on the same reticle at different locations. The DB inspection is a method having the steps of preparing from the reticle design data a reference image with much similarity to an optical image as drawn on a reticle and for comparing an optical image to this reference image to thereby inspect a reticle pattern for defects. With either one of these inspection methods for use in pattern inspection apparatus, a target workpiece is mounted on a stage, which is driven to move whereby a beam of light scans a top surface of the workpiece so that pattern inspection is performed. A light source and its associated illumination optics are used to irradiate and guide the light beam to fall onto the workpiece surface. Light that passed through or was reflected from the workpiece is focused onto a photosensor via optics. An optical image picked up by the sensor is then sent forth to a comparator circuit as measurement data. This comparator circuit compares optical images together or an optical image to the reference image in accordance with appropriate algorithm after having performed position alignment of these images. If mismatch or inconsistency is found therebetween, then the pattern being inspected is determined to be defective. Unfortunately, this pattern inspection method is encountered with difficulties in adequately detecting deterioration or “corruption” of reticle images.
It is therefore an object of this invention to provide a technique for inspecting image deterioration of a workpiece to be inspected.
It is another object of the invention to provide a technique for promptly detecting degradation of the image of a workpiece being inspected.
It is a further object of the invention to provide a method and apparatus capable of detecting workpiece image deterioration and also a workpiece obtained thereby along with a management method thereof.
In accordance with one aspect of the invention, a pattern inspection apparatus includes an optical image acquisition unit operable to gain the optical image of a workpiece to be inspected, a fiducial image storage memory device which stores therein the optical image of the workpiece as a fiducial image, a test image storage memory device for receiving after acquisition of the fiducial image another optical image of the workpiece obtained by the optical image acquisition unit and for storing therein this optical image as an image to be tested, and a comparison processing unit for comparison between the fiducial image and the image to be tested. The fiducial image of the workpiece as read out of the fiducial image storage memory device is compared to the to-be-tested image of the workpiece as read out of the test image storage memory device.
In accordance with another aspect of the invention, a pattern inspection method includes the steps of gaining as an image to be tested an optical image of a workpiece being inspected, and comparing the test image to an optical image of the workpiece as has been previously acquired as a fiducial image.
In accordance with further aspect of the invention, a pattern inspection method includes the steps of gaining as an image to be tested an optical image of a workpiece being inspected, and comparing the test image to an optical image of the workpiece as has been previously acquired as a fiducial image, wherein the test image is gained through calibration of an optical image acquisition unit to a state at the time the fiducial image was gained by said optical image acquisition unit.
In accordance with a further aspect of the invention, a workpiece is provided, which was obtained by comparison of a fiducial image of a workpiece under inspection as obtained in past and a test image as acquired during inspection of the workpiece.
In accordance with another further aspect of the invention, a workpiece management method is provided which includes the steps of acquiring an optical image of a workpiece under testing as an image to be tested, and comparing the test image to an optical image of the workpiece which has been acquired previously.
An explanation will now be given of the pattern inspection of a workpiece to be tested, such as a reticle, in accordance with an embodiment of the invention.
A pattern inspection technique embodying the invention is arranged in principle to gain an optical image of a workpiece with maximal excellence in quality and then save it as a fiducial or “standard” image. Thereafter, capture, as an image to be inspected, an optical image with the possibility that the workpiece being tested becomes inferior in quality—i.e., deteriorable. Then, compare the to-be-inspected image to the fiducial image for inspecting the former image to thereby determine its deterioration state. Embodiments of the invention as will be described below relate to a method and apparatus for inspecting workpiece pattern image deterioration along with a workpiece obtained through the inspection and a management method of workpieces to be inspected.
The fiducial image is an optical image which has been acquired in the past from a workpiece to be inspected. The fiducial image is the one that was acquired prior to deterioration inspection of the optical image—for example, an optical image of the maximal excellence in the manufacture of the workpiece being tested, at the time of product delivery inspection in manufacturing facility or in the event of initial inspection at the time of product acceptance check. Inspection conditions and test results at the time of acquisition of the fiducial image are stored in any given memory device which stores therein calibration data. During deterioration inspection, these inspection conditions or test results stored are used without change or, alternatively, are used in a way pursuant to a degree of deterioration and/or an inspection method used.
A reticle pattern image to be inspected is an optical image that is captured from the same workpiece being inspected after having obtained the fiducial image. An example of the to-be-inspected image is an optical image with the possibility that the image deteriorates due to the usage of the to-be-tested workpiece. Note that the optical image is an image that was acquired at an optical image acquisition unit. Although an explanation will be given under an assumption that the to-be-tested workpiece is a reticle, the workpiece may alternatively be any other types of ones having a surface on which a pattern image is formed, such as for example photomasks, semiconductor wafers or equivalents thereto.
It is very likely that the reticle image deterioration is caused by the so-called “growing defects” on a reticle, which takes place due to ArF and F2 as exposure-use light source modules become shorter in wavelength. In particular, regarding the image deterioration, there are problems which follow. First, at the time a device obtained by exposure begins to have appreciable problems, the reticle deterioration is progressed too extensively, resulting in defects being no longer sufficiently removable even when cleaning is applied thereto. Second, such defects generate probabilistically and thus it is hardly possible to identify defect generation portions. Third, it is deemed effective to perform early defect detection and early-time cleaning to thereby remove the “core” that will give rise to deterioration sooner or later. Fourth, reflection inspection becomes important in view of the fact that the defect generation is due to workpiece surface degradation. Fifth, this deterioration inspection is carried out recurrently, so it is inevitable to shorten the length of an inspection time period and a preparation time. For example, it is a must to provide an ability to urgently perform inspection whenever a deterioration risk is found. Consequently a need is felt to detect as early as possible the occurrence of low-glade or “minor” deterioration by comparison to the initial state in accordance with an accumulated exposure number—for example, the inspection is first performed at an increased length of intervals, and thereafter done at gradually shortened intervals. This problem is readily occurrable especially in the manufacture of highly integrated semiconductor memory devices or central processing units (CPUs), which are relatively large in exposure number per reticle. Additionally the inspection using such time difference is preferably designed to employ an advanced scheme for comparison between images being processed (i.e., simulation images) by use of a large capacity of storage devices.
Referring to
Turning to
The pattern inspection apparatus 1 also includes an input unit (not depicted) for accepting entry of data and/or commands from an operator and an output unit (not shown) for output of inspection results while the data memory 47 stores design pattern data whereas the program memory 48 stores an inspection program(s). The input unit is arranged by a keyboard, a pointing device known as the “mouse,” a light pen called a stylus, or a floppy diskette drive (FDD). The output unit may be a display device and/or a printer machine.
The pattern inspection apparatus 1 offers calibration capabilities. Upon acquisition of a fiducial or “base” image used for initial inspection, obtain calibration-use data indicative of a state or else of the pattern inspection apparatus 1. Then, store it in an appropriate memory device, such as a calibration data storage memory device 61. During deterioration testing, the calibration data obtained by the pattern inspection apparatus 1 is used to perform calibration, thereby enabling adjustment or “recovery” to the state at the time of initial inspection.
The optical image acquisition unit 3 acquires the optical image of a reticle 2, which is mounted on the XYθ table 34. The XYθ table 34 may be a three-axis (X-Y-θ). manipulator that is movable in the X and Y directions and also rotatable by an angle θ under control of the table controller 41, which operates in response to receipt of a command from the CPU 40. Table drive/control for movement in the X direction is made by the X motor 343; table motion in Y direction is by Y motor 344. Rotation by angle θ is done by θ motor 342. Examples of these X, Y and θ motors are servo motors or stepper motors of the known type. The coordinates of a present position of the XYθ table 34 are measured, for example, by the laser-assisted length measurement system 341 so that its output is sent to the position unit 46. This unit generates at its output a table position coordinate data signal, which is fed back to the table controller 41.
The reticle 2 is automatically transferred to and mounted on the XYθ table 34 by auto-loader (not shown) and, after inspection, automatically unloaded thereby. A light source 31 and its associated light irradiation unit are located over XYθ table 34. The light source 31 emits light, which is guided by a condenser lens assembly to fall onto the reticle 2 as a focused light beam. At a location underlying the reticle 2, a signal detection unit is placed, which includes a magnifying optical lens assembly 32 and a photodiode (PD) array 33. The light that passed through reticle 2 is focused by the magnifier optics 32 onto a photosensitive surface of the PD array 33. The optics 32 is subjected to automatic focus adjustment by a focus adjuster device (not shown), which includes piezoelectric elements or the like. This focus adjuster is operation-controlled by an auto-focus control circuit (not shown), which is connected to the CPU 40. The focus adjustment may alternatively be monitored by a separately provided observation scope. The PD array 33 for use as a photoelectric conversion unit may be a linear array of multiple photosensors or an area sensor having a matrix of rows and columns of PDs. While letting the XYθ table 34 move continuously, PD array 33 detects a measurement signal corresponding to a sensed image of the. reticle 2's region of interest, which is under inspection.
This measurement signal is converted by the sensor circuit 35 into digital data, which is then passed to the buffer memory 36 as the data of the optical image sensed. The buffer memory 36 may be an ensemble of more than two semiconductor memory modules. The data as output from buffer memory 36 is sent to the comparison processing unit 5. An example of the image data is eight (8) bits of signless data indicative of the brightness of each picture element or “pixel.” The pattern inspection apparatus 1 of this type is typically operable to read the pattern data out of the PD array 33 in a way synchronized with a clock frequency of about 10 to 30 megahertz (MHz), get them lined up to provide an adequate form of data, and handle as raster-scanned two-dimensional (2D) image data.
Turning to
Respective sets of measured pattern data of the reticle strips 21 as output from the sensor circuit 35 are sent to the comparison processor unit 5 along with the output data of position unit 46 indicative of a present position of the reticle 2 on the XYθ table 34. An optical image to be compared is cut into partial areas of an appropriate size; for example, regions each having a matrix of 512 by 512 pixels. Although the optical image is captured here by using the light that passed through the reticle 2, similar results are obtainable by use of reflection light, scattered light, polarized scatter light, or polarized transmission light. In particular, in the case of a reticle 2 with possible surface deterioration, appreciable effects are attainable by using images of light as reflected at the reticle surface. To sense these image light rays, the image acquisitor 3 has a prior known capturing mechanism that obtains images of light, such as reflected light, scattered light, polarized scatter light, polarized transmission light or else. Very importantly, the optical image acquisitor 3 has calibration functionality for permitting acquisition of calibration-use data (correction data) when obtaining a fiducial image at the time of initial inspection, which data will be stored in a given memory device, such as a calibration data storage memory 61 or else. The optical image acquisitor 3 is also operable to use the calibration data obtained by the calibration function to perform adjustment through calibration to the state in the initial inspection event during deterioration testing.
A reference image is an image that was prepared to have much similarity to the optical image by execution of various conversion processes from the design data of the reticle 2. The reference image is preparable, for example, by the expander 43 and referencer 44 shown in
An internal configuration of the comparison processor unit 5 is shown in
The comparison processor 5 is configurable to have a plurality of built-in parallel processing modules. This parallel processor 6 functions to perform more than two processing tasks at a time. Parallel processor 6 includes, but not limited to, a comparator having DD comparator 51 and DB comparator 52, correction processor 53, defect analyzer 54, image manipulator 55, low-resolution converter 57, multi-test data creator 58, difference memory 59, shared memory 60, and calibration data storage memory 61, which are rendered operative to do tasks in a parallel way.
The DD comparator 51 performs comparison between optical pattern images of the reticle 2 as obtained by the optical image acquisitor 3—for example, compares a pattern image under inspection to the fiducial image of optical image. The DB comparator 52 compares an optical image to reference image. An image indicative of a difference between the DB comparison-obtained optical image and the reference image is stored in the difference memory 59. The DD comparison and DB comparison are capable of detecting variations in light transmissivity, foreign matter attached, precise edge positions and ultra-small changes in intensity, thereby sensing reticle deterioration, if any. Additionally the comparison processor 5 performs comparison of ultrasmall shapes, such as contact holes, while performing setup of a margin pursuant to the materiality of graphics and the sensitivity of a region of interest in conformity with graphic features, thereby offering more accurate and definite deterioration testing capabilities. The multi-test data creator 58 is designed to create at least one of the transmissivity, light intensity, line width and edge roughness and also perform comparison thereof. Preferably the multi-test data creator 58 has a plurality of testing functions of the transmissivity, light amount, line width and edge roughness. Thus more precise testing is enabled.
The correction processor unit 53 performs the testing of photomasks of the type having non-exposed regions, such as pellicle-added masks, which unit is designed to do testing even for anti-exposure mask areas which are free from deterioration without doubt. The corrector 53 generates at its output a calibration image to be later used during testing in future, which is stored in a memory device, such as the calibration data memory 61. Then, compare the calibration image of non-exposure region(s) to a uniformly deteriorated portion(s) due to exposure, thereby determining through computation an exact level of due-to-exposure deterioration; next, correct or “amend” the fiducial image so that it has a uniform deterioration level. The computation of such uniform exposure-caused deterioration level is achievable by maximum sensitivity comparison techniques using the calibration data.
The image manipulation unit 55 performs image manipulation in a such a way as to enable accurate comparison of the fiducial image and an image being inspected. Image manipulator 55 offers executability of a variety of types of image manipulation tasks including, but not limited to, distortion and expansion/shrink plus wobbling along with SIM processing. The SIM processing is a simulation process, such as image resolution conversion, production of a synthesis image by combining together a plurality of images, emphasized image creation, transferable processing, etc. The image manipulation is executable by the data processor unit 4 or alternatively at the comparison processor 5.
To ameliorate or “cure” the reticle's deterioration, wash and rinse the reticle of interest. Then, an image of the washed and rechecked reticle is stored in the calibration data storage memory 61, together with testing conditions and test result data. For example, the initial test image and the retested image along with the test conditions at that time and test result data are saved together in the fiducial image storage memory device. Whereby, it is possible to additionally perform the testing of reticle deterioration simultaneously, such as pattern thinning due to the washing. It is also possible to determine whether the reticle washing is appropriate or not in the aftertime.
A process of
An exemplary processing procedure in the initial inspection and deterioration testing events is shown in
An exemplary deterioration testing technique using the parallel processing unit 6 is shown in
During deterioration testing, an optical image of the reticle pattern to be tested is captured by the optical image acquisitor 3 and is then divided by the image distributor 56 into subareas. Read the saved fiducial image out of the fiducial image memory 45 and then divide it into an equal number of subareas. The reticle subareas and the fiducial image subareas are distributed to a set of parallel processors 6-1 to 6-n in a way such that a reticle subarea and its corresponding fiducial image subarea are passed to a parallel processor 6i (where., “i” is an integer 1, 2, . . . , or n). Respective reticle/fiducial-image subarea pairs are compared together at parallel processors 6-1 to 6-n at a time—namely, in a parallel fashion—to thereby inspect the reticle for deterioration.
An exemplary reticle pattern deterioration inspection method using the parallel processing subunits 6-1 to 6-n is shown in
It should be noted that in Embodiment 3, an optical image of high resolution is acquired at the optical image acquisitor 3 by using the process of
Also note that in any one of the processes of
Additionally in Embodiment 3, the image obtained as the fiducial image is “decomposed” into partial images, which are passed to the parallel processors 6-1 to 6-n for being applied the image manipulation, such as SIM processing, followed by storage of resultant image data in the local fiducial image memories 62. Examples of such stored images are light-intensity distribution images, developed images, and transferred images. Next, let an image obtained as the to-be-tested image during inspection be divided by image distributor 56 into partial images, which are then applied the image manipulation, such as SIM processing, at the parallel processors 6-1 to 6-n in a parallel way. Then compare the partial images of the to-be-tested image thus manipulated to corresponding parts of the fiducial image being stored in the local fiducial image memories 62, thereby performing the deterioration inspection.
Turning to
While the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. For example, the embodiments having parallel processor units are modifiable and alterable to have the applicability to pattern inspection machinery with the lack of such parallel processors.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-373900 | Dec 2005 | JP | national |
