Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2002-01-31
2004-02-03
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S397000
Reexamination Certificate
active
06686591
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a technical method that an electronic beam transmitting through a mask is detected by means of a detector, in which a plurality of elements is aligned in a plurality of lines, as well as transferred synchronously with movement of the mask, so that it would be achieved to inspect a mask with high resolution at high speed. The invention also relates to a technical method that, in scanning in a straight line at right angles to a direction of the plural elements of a CCD sensor with an electronic beam illuminating the mask, an electronic beam is scanned in a straight line with the extremely high accuracy without zigzag scan when a stage with an easy structure is used.
DESCRIPTION OF THE RELATED ART
Conventionally, inspection of a mask (reticle mask) for exposing an LSI circuit pattern or such on a wafer is carried out by illuminating a mask entirely by light, measuring the strength of the light transmitting through the mask so as to obtain its image, and then, referring to a circuit diagram. In the inspection by means of light, resolution is limited by a wavelength of the light, and the current highest resolution is around 0.1 mm.
On the other hand, a scan type of electronic microscope (referred to SEM, hereinafter) is used for scanning an electronic beam narrowed down and radiated on a circuit pattern of a wafer after exposure and development with a mask so that a second electron, which is generated in the above scan, is detected to be used for inspecting a detailed structure of the above circuit pattern. A thin film forming a pattern in a stencil mask, however, has a thickness of around 2 to 20 mm, and when it is assumed that a minimum size of an opening portion of the pattern be 0.4 mm, an aspect ratio would be 5 to 50, which is extremely large compared with that of a usual structure of a surface of a wafer. Thus, the second electron generated in the opening portion cannot be taken to the outside even when the usual SEM is used to radiate an electronic beam narrowed down for scanning, and it causes a problem that the inside of the above opening cannot be inspected practically.
In the above-mentioned conventional SEM for inspecting a wafer, an electronic beam narrowed down is radiated while scan (so-called raster-scan) is performed respectively in the X and Y directions, and the second electron generated in the scan is detected so that a second electronic image would be displayed on a screen. Therefore, a time necessary to obtain whole signals (images) in a certain area on a wafer is: the number of whole pixels in a certain area X sampling time, wherein the sampling time is usually around 0.13 ms. When it is assumed that detection sensitivity be 30 nm, a time necessary to inspect 1 cm
2
of a sample surface is:
0.13
ms×
1
cm
2
/(50
nm
)=5,200 seconds=86 minutes=1 hour and 26 minutes.
In the above calculation, a time necessary for inspection such as a time for moving a stage, for example, is entirely omitted, while it takes much more time practically. Accordingly, there is also a basic problem that such a raster-scan type of apparatus is not suitable for practical use since it needs about one and half hours to inspect 1 cm
2
of a sample.
In order to solve the problems, an object of the invention is to achieve inspection of a mask with high resolution at high speed by detecting an electronic beam transmitting through a mask by means of a detector, in which a plurality of elements is aligned in a plurality of lines, as well as transferring an image signal by the above detector synchronously with movement of the mask, so that high resolution due to a short wavelength of the electronic beam would be effectively utilized and that an image signal is transferred at right angles to a line of the detector synchronously with same time detection of pixels in a direction of the line.
Another object of the invention is to form an image scanned in a straight line with the extremely high accuracy without zigzag scan when a stage with an easy structure is used, by illuminating a sample (mask) by an electronic beam while scanning in a straight line at right angles to a direction of plural elements of a CCD sensor, detecting a moving and shifting amount in a direction of right angles to a direction of the scan, and correcting an electronic beam by deflection or by means of a sample moving mechanism, so that the scan of the sample (mask) can be performed in a straight line with respect to an electronic beam with extremely high accuracy.
DISCLOSURE OF THE INVENTION
In the invention, it is arranged that, as shown in
FIG. 1
, an electronic beam emitted from an electron gun
3
illuminates/a mask
1
through a condenser lens
4
, an objective lens
5
forms an image of the electronic beam having transmitted through the mask
1
, a CCD
9
, which is a detector, detects an image transduced into light by a transducer
7
here, a controlling device
15
controls a stage
2
to move the mask
1
and synchronously transfers an image signal at right angles to a line of the CCD
9
, and the image signal outputted from the CCD
9
is displayed or recorded.
In the above arrangement, it is repeated that the controlling device
15
controls the stage
2
to move the mask
1
at a predetermined width in a certain direction or in a direction adverse to the certain direction, so that the whole surface of the mask
1
can be scanned.
It is also arranged that the controlling device
15
would control the stage
2
to move the mask
1
continuously.
Further, it is arranged that an amount of an X ray would be reduced by disposing before a CCD, which is a detector, a board that cuts off the X ray and that an optical image passes through, or by forming on the CCD, which is a detector, by means of a lens an optical image that is transduced from an image of an electronic beam by an electronic beam-light transducer disposed obliquely.
It is also arranged that a location of the mask fixed on a sample table would be measured by means of a laser interference measuring instrument so that the mask would be moved.
Moreover, it is arranged in the invention, as shown in
FIG. 10
, that an electronic beam emitted and focused by an electron gun
1
and a focusing lens
3
illuminates a sample (mask)
4
, a transmitted electronic beam or an electronic beam reflected (reversed) at a surface of the sample (mask)
4
, to which a minus voltage not shown in the drawings is applied, is formed into an image on an objective lens
6
, and a CCD sensor
14
detects the formed electronic image directly or an optical image having been transduced into light by a fluorescent screen
10
. Under this condition, a stage
5
is scanned in a straight line at almost right angles to a direction of a plurality of elements of the CCD sensor
14
while a moving and shifting amount at right angles to a direction of the scan is detected, and thereby, a controlling signal is outputted to a moving mechanism of a deflector
8
or a sample
4
so as to correct the moving and shifting amount.
In the above arrangement, a plurality of pixels is disposed as the CCD sensor
14
in a straight line with respect to the formed image so as to form a detector, in which an electric charge is transferred and accumulated in a direction of right angles adjacently to the plurality of pixels disposed in a straight line synchronously with the scan of an electronic beam and in which a line or a plurality of lines of the plurality of pixels is provided in a straight line.
It is also arranged that a CCD sensor comprising a plurality of detecting elements in a straight line for detecting an electronic image directly, or a CCD sensor comprising a plurality of detecting elements in a straight line for forming and detecting an optical image having been transduced from an electronic image would be used as a detector.
Furthermore, it is arranged that a rotating device for making a direction of the scan almost same in a direction of right angles to the plurality of pixels in a straight line of the CCD sen
Anazawa Norimichi
Ito Minoru
Holon Co., LTD
Nguyen Kiet T.
Staas & Halsey , LLP
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