Test Mask for Determining Alignment of an Automatic IC Mask Testing Apparatus

The present invention is related to apparatus for automatically testing masks such as photomasks utilized in the manufacture of semiconductor devices and specifically is related to a test mask for determining the alignment of such an apparatus.

The manufacture of large scale integrated circuit chips requires extremely complex equipment and stringent procedures in order to maintain production losses at an acceptable level. Such losses are frequently due to defects in one or more of the various masks used in the manufacturing process. These defects are usually in the nature of a clear spot in an area which should be opaque or an opaque spot in an area which should be clear. As circuit complexity increases and device geometries become smaller, the effect of photomask quality upon final yield becomes increasingly significant.

Visual inspection of these photomasks is quite time consuming and yields mixed results due to inherent subjectivity and varying operator skill levels. As integrated circuits become larger and more complex, such visual inspection becomes extremely difficult, if not impossible. The need for automated inspection systems was recognized resulting in the development of several such systems. Typical of these automated inspection systems is the Mask Analysis system disclosed in U.S. Pat. No. 4,218,142 which issued Aug. 19, 1980 to Kryger et al. Kryger et al. utilizes a laser driven scanning spot which scans a particular chip area of the mask comparing the signal received from the scanning device with a data base containing standard data representing an ideal mask.

A more modern and widely used automated inspection system is the KLA-101 series automatic photomask inspection system which is manufactured by KLA Instruments Corp. of Santa Clara, California. The KLA-101 machine, which utilizes an image scanning system, simultaneously scans two different die areas on the same mask. All die areas on a given mask should contain identical patterns. If a difference is detected between two die areas, comparison with a third will determine which die area is defective. Alignment of the machine with the photomask to be inspected is accomplished manually by visually aligning each of the two sets of optics on their respective die areas. Identical features of the two die areas are selected so that the optics are imaging nominally identical patterns. As the photomask is scanned by the system, the scanning signal is gated to exclude signals generated while the scanner is outside of the die area. The die area is commonly referred to within the industry as the care area.

Occasionally, the KLA machine will become misaligned and scan slightly outside of the care area thereby giving erroneous indications of defects and failing to detect actual defects occurring within the care area. Heretofore, there has been no reliable and simple way for an operator to determine whether or not the machine is scanning outside of the desired care area. The present invention permits an operator to easily make this determination by means of a novel test mask.

The present invention includes a mask for determining whether or not an automatic integrated circuit (IC) mask testing apparatus is scanning within a desired care area. The mask comprises a substrate having a major surface and a plurality of identical care areas defined on the surface in substantially parallel rows. Each of the care areas is surrounded by a peripheral region on the surface which is adjacent an edge of the care area and extends outwardly therefrom. Portions of the peripheral regions have first optical properties and other portions have second optical properties so that for a pair of care areas, each portion of the peripheral region of one of the pair having first optical properties spatially corresponds to a portion of the peripheral region of the other of the pair having second optical properties.

FIG. 1 is a plan view of a test mask utilizing the teachings of the present invention; and

FIG. 2 is an enlarged view of a portion of FIG. 1 indicated at 2, showing a number of care areas and their surrounding peripheral regions.

There is shown in FIG. 1 a test mask 8 having a rectangular shaped substrate 10, which may be made, for example, of glass, having a layer 14 of an optically opaque material, such as chromium, deposited thereon. By selectively removing specific portions of the layer 14, using techniques that are well known in the art, multiple micro-miniature patterns 16, or care areas, comprising transparent and opaque regions can be formed.

While the present example is set forth in terms of optically opaque regions and transparent or non-opaque regions, these terms, opaque and non-opaque, are more generically identified by the terms first optical properties and second optical properties respectively. The terms first and second optical properties are intended to include electromagnetic radiation within the approximate range of, and including, infrared through X-ray. It will be pointed out later in this specification some other possible specific examples of the first and second optical properties which may be utilized in the practice of the present invention.

As best seen in FIG. 1, the care areas 16 are arranged in spaced columns 18 and spaced rows 20 which substantially correspond to the die areas on a wafer, not shown. While the care areas 16 are in identical proportion to their corresponding die areas on a wafer, the peripheral regions 24 which surround each care area 16, see FIG. 2, are also in identical proportion to the streets between the die areas on a wafer, not shown. Note that, in FIG. 2, each care area 16 is defined by four phantom lines 26 which represent the outer limits of the care area 16 and, therefore, the corresponding die area on a wafer.

As shown in FIG. 2, the portions 30 of the peripheral regions 24 between the care areas 16 in alternate columns 18 as well as the vertical portions 32 on one side of the column 18 are opaque. All other portions 34 of the peripheral regions 24 are transparent or non-opaque. The significance of this opaque and non-opaque characterization will be explained below.

Each care area 16, which is otherwise opaque, includes a non-opaque feature 28 which is identical in all respects to all other features 28, all features 28 being identically positional within their respective care areas 16. The features 28 are used to visually align the two sets of optics of the KLA-101 automatic photomask inspection system, not shown, so that the optics are imaging nominally identical patterns on two different care areas.

As mentioned in BACKGROUND OF THE INVENTION, the scanning signal of the KLA-101 is gated to exclude signals generated while the scanner is outside of the care areas 16. Therefore, while scanning remains within the limits of the care areas 16, as represented by the gated scanning signals, the KLA-101 will not detect errors because the two different care areas 16 being scanned are both opaque and have no detectable differences. Those skilled in the art will understand that similar results would be obtained were the care areas 16 transparent, however, this would increase the likelihood of erroneous error indications due to possible presence of dust or other foreign particles on one of the transparent care areas 16. Should the scanning signals generated while the scanner is outside of the care area not be excluded due to misalignment of the machine, and the peripheral regions 24 being erroneously scanned were of opposite first and second optical properties, that is, one was opaque and the other non-opaque, the KLA-101 would indicate continuous detection of errors while scanning the peripheral regions 24. This would provide a valid indication to the operator that the KLA-101 was misaligned and scanning outside of the nominal care area 16.

The key to effecting such operation of the KLA-101 is to assure that the portions of the peripheral regions 24 being erroneously scanned have opposite first and second optical properties. That is, where the portion of the peripheral region 24 of one care area 16 being scanned by the one optical system is of one optical property, the spatially corresponding portion of the peripheral region 24 of the other care area 16 being scanned by the other optical system is of the other optical property. By way of example, assume that one optical system of the KLA-101 is aligned on the care area 16A and the other is aligned on the care area 16B of FIG. 2. Since both optical systems are imaging nominally identical patterns on the two care areas 16A and 16B, when the one is scanning the leftmost edge 36 of the care area 16A, the other one is scanning the leftmost edge 38 of the care area 16B resulting in no detected error. However, as the scan crosses the edges 36 and 38 into the peripheral regions 24, one optical system detects the non-opaque portion 34 while the other optical system detects the spatially corresponding portion 32 which is opaque, thereby indicating a detected error. While the scan continues in the peripheral region 24, detected errors are continuously indicated. Similarly, should the scan cross into the portions of the peripheral area 24 above, below, or to the right of the care areas 16A and 16B, as viewed in FIG. 2, the two optical system will encounter portions 30, 32, and 34 which are of opposite first and second optical properties and thereby indicate detected errors.

While a specific arrangement of opaque portions 30 and 32 and non-opaque portions 34 of the peripheral regions 24 is shown in FIG. 2 and described herein, numerous other arrangements may be used in the practice of the present invention. Again, the key requirement are that the spatially corresponding portions of the peripheral regions 24 being scanned are of opposite first and second optical properties. All such arrangements are deemed to be within the spirit and scope of the present invention.

Similar results are obtainable where the first and second optical properties of the portions 30, 32, and 34 allow or block reflection of radiation instead of transmission or non-transmission of radiation as set forth above. For example, the first optical property may reflect radiation of one wavelength while the second optical property may absorb that wavelength. The automatic photomask testing machine would then indicate an error whenever it failed to detect the specific wavelength of radiation on both scanners. In another example, the first optical property may absorb substantially all wavelengths of radiation, while the second optical property reflects substantially all wavelengths of radiation. In these two examples the KLA-101 would be arranged to detect reflected radiation instead of transmitted radiation.