GB2060876A

GB2060876A - Testing photomasks

Info

Publication number: GB2060876A
Application number: GB8033396A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1979-10-16
Filing date: 1980-10-16
Publication date: 1981-05-07
Also published as: SU1142733A1; FR2468100A1; GB2060876B; DD146500A1; JPS56142533A; DE3037467A1; FR2468100B1

Abstract

An arrangement for testing a photomask (2) for defects, includes two identical lens systems which produce respective images of portions of the photomask on rows of photosensors (15, 16). Signals from the photosensors are compared to indicate defects in the photomask. Two further rows of photodetectors (13, 14) are provided for determining positional errors of the image formed on the photosensor row (16), and a pair of glass plates (11, 12) which are rotatable about orthogonal axes, are used to shift the image on the row (16) to correct for the positional errors. <IMAGE>

Description

SPECIFICATION Arrangement for testing photomasks This invention relates to an arrangement for testing photomasks.

In the manufacture of semi-conductor elements, freedom from defects in the photomasks used is of great importance for the level of the yield. Photomask defects can be produced in the manufacturing process of the mask. Also, in use, the masks are subject to wear which, in later use, leads to defective components.

A mask-testing arrangement which works rapidly and fault-free, is therefore important for the detection of wear phenomena and defects.

Only statistical data on the state of the photomasks can be obtained with the aid of the method of visual inspection of the masks under the microscope. Various opto-electronic methods for testing samples of photomasks have also been developed. The functioning of these arrangements is based on holography or on matched filters.

Arrangements are likewise known in which the sample is swept in grid scanning by means of coherent light.

A fundamentally different approach to the problem of automatically detecting photomask defects, is based on comparing two individual images of the photomask, which is moved on a coordinate table, by means of two lens systems and two photosensor rows as opticalelectronic transducers and electronically forming the difference. In this arrangement, the lens systems and the photosensor rows are adjusted to a standard distance of the individual images which are to be compared, except for a residual error. This residual error, which represents a machine error, and the positional error of the individual images of the photomask generate an error signal, that is to say a pseudo-error, in the defect control of the photomasks.

It is an aim of the invention to eliminate the pseudo-errors which are caused, during the defect control of photomasks, by the positional errors of the individual images of the photomasks and by the machine error of the instrument used for the control.

An object of the invention is an arrangement for the testing of photomasks, wherein the pseudo-errors are reduced by compensating for the positional error of the individual images of the photomaks in the image plane, and for the machine errors.

In accordance with the invention, this object is achieved in an arrangement for the automatic testing of photomasks for semiconductor components when, in front of each photosensor row, one further photosensor row is arranged parallel and at the same distance, in order to detect defects of the photomask, and when two light-transparent plane-parallel plates each of which is pivotable about an axis at right angles to the optical axis and to the axis of rotation of the other plate, are associated, one behind the other, with at least one lens system in the optical path between the lens system and the associated photosensor rows.

The compensation of the positional errors of the individual images of the photomask is achieved by obtaining information of the positional error of the individual images in the xand y-coordinate, with reference to the structures of the individual images, by means of two additional photosensor rows and a system of plane plates which, in the x- and y-coordinates, that is to say in the image plane, act as a correction element which is controlled via an electronic evaluation of the information of the positional error of the additional photosensor rows and electronically controlled actuating members in such a way that the positional errors are compensated during the examination.

The invention will be explained in more detail below by reference to the diagrammatic representation of an exemplary embodiment thereof given in the single Figure of the accompanying drawing.

The Figure shows the arrangement according to the invention for reducing pseudo-errors in the comparison of photo-lithographic structures, the arrangement consisting of a light source 1, an optical beam divider system 1' and the object 2 which is to be examined and is located above the former. The lens systems 3 and 4 are located below the object to be examined, that is to say the photomask 2, typically mounted on a coordinate table, not shown, capable of moving the photomask in two orthogonal directions mutually perpendicular to the light paths from beam divider 1'.

The lens systems 3 and 4 are followed by the optical components comprising prisms 5, 6 and mirrors 7, 8 which direct optical paths 1 7 and 1 8 to respective lens systems 9 and 1 0.

After the lens system and in front of photosensor rows 14 and 16, plane transparent plates 11 and 1 2 are located in the optical path 17. In the optical path 18, the lens system 9 is followed by the photosensor rows 13 and 15.

Starting from the light source 1 and optical beam divider means 1', an image of a defined part of two individual images of the photomask 2 is formed in the lens systems 3 and 4.

The optical imaging path 1 8 leaving the lens system 3 is projected by means of a prism 5 and the mirror 7 to the lens system 9 which illuminates both the photosensor row 1 3 which is used for detecting the positional errors, and the photosensor row 1 5 which is used for detecting the defects of the photostencil 2.

The optical imaging path 1 7 leaving the lens system 4 is projected by means of the prism 6 and the mirror 8 onto the lens system 1 0. This lens system 10 is followed in the optical path 1 7 by the two plane plates 11 and 1 2, of which the plane plate 11 is pivotable about the X-axis, and the plane plate 1 2 is pivotable about the Y-axis, of an x-yplane normal to the optical path 1 7. The plates 11, 1 2 can thus be rotated to achieve a positional shift of the image falling on the photosensor rows 14, 16.It is thus possible by means of the plates 11, 1 2 to correct the positional error of all or part of an individual image of the photomask 2 by the information, obtained via the photosensor row 14 or 1 3 and 14, on the positional error of a part of the individual image of the photomask 2. The corrected optical image path 1 7 passes to the photosensor row 1 6 for the examination of defects in the photostencil 2.

Many modifications falling within the scope of the claimed invention are possible, and for example the single light source 1 and beam divider 11 could be replaced by two light sources each for a respective one of the paths 1 7, 1 8. Furthermore, it will be appreciated that the plates 11, 1 2 can be driven automatically by servo motors to correct for the positional errors, in dependence upon an electrical control signal derived from the output of the photosensors 13, 14.

Claims

1. An arrangement for testing a photomask for a semiconductor device, comprising a light source, first and second lens systems of substantially the same optical properties and having respective optical paths arranged for producing respective images of portions of a photomask, first and second arrays of photosensors responsive to said images respectively, means responsive to electrical signals from said arrays and arranged to produce a signal which is a function of defects in the photomask, third and fourth arrays of photosensors responsive to said images respectively, and positional error compensating means arranged in one of said light paths for producing a positional shift of one of said images in dependence upon electrical signals produced by said third and fourth photosensor arrays.

2. An arrangement according to claim 1 wherein said positional error compensating means includes first and second plane transparent plates disposed in one of said light paths, the plates being rotatable about transverse axes for producing said positional shift of the image.

3. An arrangement according to claim 1 or 2 and including the photomask, the photomask being mounted in a guide movable selectively in the plane of the photomask.

4. An arrangement according to any preceding claim, wherein said third and fourth arrays are disposed the same distance from the respective images to which they are responsive.

5. An arrangement for automatically testing photomasks for semi-conductor elements, consisting of at least one light source, of two independent lens systems, of identical optical properties each for forming an individual image of the photomask on a respective photosensor row, for detecting defects in the photomask, and a device for moving the photomask, which is to be examined, in at least two directions parallel to the photomask, wherein, in front of each photosensor row, one further photosensor row is arranged parallel and at the same distance, and two light-transparent plane-parallel plates each of which is pivotable about an axis at right angles to the optical axis and to the axis of rotation of the other plate, are associated, one behind the other, with at least one lens system in the optical path between the lens system and the associated photosensor rows.

6. An arrangement for testing a photomask, substantially as hereinbefore described with reference to the accompanying drawing.