Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2001-10-11
2004-04-27
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
C356S237400, C356S237500
Reexamination Certificate
active
06727987
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image pickup apparatus having a high resolution and a high signal-to-noise (S/N) ratio using a confocal optical system.
Further, the present invention relates to a photomask defect inspection system having such an image pickup apparatus.
2. Description of the Related Art
In the process of production of a semiconductor device, a large number of photomasks formed with various different patterns are used. A desired device is manufactured by repeating the steps of pattern transfer and etching on a semi-conductor wafer. When there is a defect in the photomask serving as the master for pattern transfer due to foreign matter etc., an accurate pattern cannot be projected on the semi-conductor wafer and defects end up occurring there. Therefore, there has been a strong demand for improvement of a defect inspection system for detecting a defect of a photomask.
In the past, in a photomask defect inspection system, the photomask to be inspected has been scanned at a high speed by light beam, the light transmitted through or the light reflected from the photomask has been received by a one-dimensional image sensor, and output signals from the image sensor are compared with data stored in a database or compared output signals with each other to detect the presence of foreign matters and the detects of a light blocking pattern.
In an another defect inspection system, the light transmitted through or the light reflected from the photomask is received by a two-dimensional CCD camera, and output signals from the light receiving elements of the CCD camera have been compared with data stored in a database or compared with each other to detect a defect.
Along with the higher integration and higher density of LSIs, the patterns of photomasks have also become finer. There has therefore been a strong demand to further raise the resolution of a photomask defect inspection system and obtain a defect detection signal of a high S/N ratio. The method of receiving the light from the above photomask by a one-dimensional image sensor has the advantage of giving a relatively high resolution since the confocality is maintained. The charge stored in a light receiving element of the image sensor, however, is proportional to the time of irradiation of the illumination light, that is, the time of accumulation of the charge. In the method of high speed scanning using a light beam, the time during which the light from the photomask strikes the image sensor is short, so the amount of charge stored in a light receiving element becomes smaller and there are limits in regard to the S/N ratio of the defect detection signal.
On the other hand, in the method of receiving light from a photomask by a two-dimensional CCD camera, the illumination time can be made relatively longer, so a good characteristic is obtained with respect to the S/N ratio. When picking up a reflected image or transmitted image of the photomask by a two-dimensional CCD camera, flare, glare, and other stray light ends up striking the light receiving elements, there is a limit to the resolution, and there is a limit to the inspection for defects of a fine pattern.
Further, to optically inspect for defects of a fine pattern, it is desirable to use light of a short wavelength, that is, ultraviolet light, as the illumination light. With ultraviolet light, however, since the absorption by the optical elements is large and the sensitivity of the photodiodes is low, there is the disadvantage of difficulty of obtaining a sufficient detection sensitivity with a defect inspection system of the related art. Further, among the types of defects are defects due to the deposition of foreign matter and pattern defects derived from the failure of accurate formation of a chrome light blocking pattern. If these types of defects can be discerned, the applications for defect inspection systems can be further increased.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an image pickup apparatus and photomask defect inspection system able to pick up the image of a sample at a higher resolution than a defect inspection system of the related art and obtain an output signal of a higher S/N ratio.
Another object of the present invention is to provide a defect inspection system able to simultaneously inspect for a defect by transmitted light and inspect for a defect by reflected light and able to discern the properties and details of the detected defect.
According to a first aspect of the present invention, there is provided an image pickup apparatus provided with a sample stage for moving a sample whose image is to be picked up in a first direction; an illumination light source for projecting illumination light on the sample; a first spatial filter arranged between the illumination light source and sample and having a plurality of slits formed at a predetermined pitch along the first direction and extending in a second direction perpendicular to the first direction; an image sensor for receiving reflected light or transmitted light from a sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a second spatial filter arranged between the sample stage and the second image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; an objective lens arranged between the sample stage and second spatial filter and forming an image of transmitted light or reflected light from the sample on the image sensor via the slits of the second spatial filter; and a drive control circuit for controlling the charge transfer speed of the image sensors and speed of movement of the sample stage; the first and second spatial filters being arranged so that light emitted from the slits of the first spatial filter strikes the sample and the image sensor through the slits of the second spatial filter; the speed of movement of the sample stage and the charge transfer speed of the image sensor being set so that the time interval during which the sample moves from the position which the illumination light passing through an i−th slit of the first spatial filter strikes to the position which the illumination light passing through the adjoining i+1st slit strikes and the time interval during which the charges stored in the line of light receiving elements, of the image sensor, which the light passing through the i−th slit of the second spatial filter strikes are transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other.
In the present invention, since the illumination light is projected as lines of illumination light through a spatial filter having a plurality of slits extending in a direction perpendicular to a direction of movement of the sample and the transmitted light or reflected light from the sample is received at an image sensor through a spatial filter having a plurality of slits extending in a direction perpendicular to the direction of movement of the sample in the same way, a line confocal optical system is formed and as a result it is possible to pick up an image greatly reduced in flare, glare, and other stray light and having a high resolution.
Further, since the speed of movement of the sample and the line transfer speed of the image sensor are made to correspond to each other, that is, since the speed of movement of the sample stage and charge transfer speed of the image sensor are set so that the time during which the sample moves from the position which the illumination light passing through an i−th slit of the first spatial filter strikes to the position which t
Barth Vincent P
Lasertec Corporation
Oliff & Berridg,e PLC
Rosenberger Richard A.
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