Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1995-02-14
2002-11-26
Chang, Andrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S891000, C430S005000
Reexamination Certificate
active
06487018
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aperture and an optical device using it, and more particularly, the present invention aims at improving a structure of the aperture and thus a function which is required for the optical device.
2. Description of the Background Art
Recently, various optical devices have been used for manufacturing of a semiconductor device. A structure of these optical devices includes a light source for emitting a light beam, a collimator lens for making a light beam parallel, an aperture for shaping a light beam passing through the collimator lens into a prescribed shape, and an objective lens for condensing a light beam passing through the aperture.
An optical device with the above-described structure includes, for example, a photomask defect repairing apparatus for repairing a defect found in a photomask, and a scanning critical dimension measurement apparatus for measuring the dimension of a pattern which is formed on a semiconductor substrate. The photomask defect repairing apparatus for repairing a defect which has been found in a photomask will now be described.;
Referring to
FIG. 11
, in a photomask
200
, a metal thin film
208
which includes a light-transmitting portion
206
and a light-shielding portion
204
is formed on a transparent glass substrate
202
. About 10 to 20 kinds of photomasks
200
each of which has a different pattern are required to manufacture a single semiconductor device. If a defect exists in these photomasks, the defect also is transferred to a wafer, resulting in poor quality and reduced yield of the semiconductor device.
A defect which could be found in photomask
200
can be classified into two types: a remaining defect
210
and a pinhole defect
220
. In order to repair pinhole defect
220
, a carbon-type film
224
is usually deposited and filled in pinhole defect portion
220
by an FIB assist deposition system.
In order to repair remaining defect portion
210
, a laser beam
222
is collected and directed to remaining defect portion
210
and the metal thin film is removed. A device using YAG (Yttrium, Aluminum, Gahnite) laser has been practically available. Higher resolution has been achieved by use of second harmonics (wavelength of 0.53 &mgr;m) of YAG laser, and a mask defect repairing apparatus using the second harmonics, therefore, has been used increasingly.
The basic structure of this mask defect repairing apparatus is disclosed in Japanese Patent Laying-Open No. 60-132325. Referring to
FIG. 12
, the structure of the mask defect repairing apparatus
250
which has been disclosed in Japanese Patent Laying-Open No.60-132325 includes a light source (Nd. YAG laser)
252
for emitting a light beam
254
, a collimator lens
256
for making light beam
254
parallel, a variable rectangular aperture
258
for shaping laser beam
254
passing through collimator lens
256
into a prescribed shape, and an objective lens
260
for condensing laser beam
254
passing through aperture
258
into a defective portion
264
on a photomask
262
.
Referring to
FIG. 13
, aperture
258
used in this mask defect repairing apparatus
250
is constituted by an opening
268
for shaping laser beam
254
into a prescribed rectangular shape, and a light-shielding portion
266
for intercepting a laser beam.
The structure of a critical dimension measurement apparatus
300
will now be described with reference to FIG.
14
. Critical dimension measurement apparatus
300
includes a light source
302
for emitting a laser beam
301
a,
a collimator lens
304
for making laser beam
301
a
parallel laser beam
301
b,
an aperture
306
for shaping laser beam
301
b
into a laser beam
301
c
having a prescribed rectangular shape, and an objective lens
310
for condensing laser beam
301
c
into a surface of a wafer
314
. This wafer
314
includes a pattern
314
a
with a prescribed shape, which is made of conductive material, insulating material or the like, formed on a substrate
314
b.
A laser beam is reflected from wafer
314
and then from a semi-transparent mirror
308
, and enters a detector
312
.
Width d of pattern
314
a
is measured, referring, for example, to
FIG. 15
, when laser beam
301
d
is moved with respect to pattern
314
a
in the direction shown by an arrow and change in intensity between light beams
301
d
reflected from substrate
314
b
and from pattern
314
a
is recognized by detector
312
.
Principle of measurement of width d of pattern
314
a
will now be described with reference to FIG.
16
. In
FIG. 16
, abscissa indicates a ratio of half-width W of a scanning beam (laser beam) to width d of a pattern, and ordinate indicates intensity of a signal indicated by a reflected light beam which is received by a detector. Resolution of the critical dimension measurement apparatus will now be described with reference to FIG.
16
. Resolution is usually represented by a W/d value that is obtained just before the decrease of intensity of a reflected light beam. For example, pattern width d can be measured, if two edge portions of the pattern can be recognized. Therefore, as understood from
FIG. 16
, pattern width d can be measured so long as the W/d value is 2.0 or less. This is explained in more detail with reference to
FIGS. 17-21
.
FIG. 17
shows change in signal intensity when a scanning beam moves across a substrate and W/d is 0.5. By representing intensities of signals received from substrate
314
b
and pattern
314
a
as A and B (A>B), respectively, the signal intensity becomes (A+B)/2 when the central point of the scanning beam comes to an edge portion of pattern
314
a.
Therefore, pattern width d can be measured if signal intensity of (A+B)/2 can be recognized.
FIG. 18
shows change in signal intensity when a scanning beam moves across a substrate and W/d is 1.0. In this case, pattern width d can be measured by recognizing signal intensity of (A+B)/2, as shown in FIG.
17
.
FIG. 19
shows change in signal intensity when a scanning beam moves across a substrate and W/d is 2.0. In this case, pattern width d can be easily measured, since the range in which signal intensity is (A+B)/2 will correspond to pattern width d.
FIG. 20
shows change in signal intensity when a scanning beam moves across a substrate and W/d is 2.5. In this case, the range in which signal intensity is (A+B)/2 cannot be recognized. Therefore, pattern width d cannot be measured.
From above description, it is to be understood that patten width d cannot be measured when W/d is more than 2.
Recently, with miniaturization of a semiconductor device, improvement in degree of integration and in function of the semiconductor device has been desired, resulting in requirement of further miniaturization of an element which is formed in a semiconductor device. Therefore, optical devices such as a pattern defect repairing apparatus and a critical dimension measurement apparatus which can deal with the miniaturization have been required.
In these optical devices, spot, that is, half-width of a laser beam must be reduced in order to deal with miniaturization of a pattern. Reduction in half-width can be achieved by reducing an opening width of an aperture. Half-width can be reduced when the opening width of the aperture is larger than a wavelength of light, while diffraction of light might introduce a problem when the opening width of the aperture becomes smaller with miniaturization of a pattern. This is illustrated in
FIGS. 21A
to
21
D. Light has such a clear amplitude as shown in
FIG. 21B
right after passing through the aperture. Light projected onto a wafer, however, has an amplitude that is widened at its bottom, such as shown in FIG.
21
C, because of diffraction of light. Therefore, light on the wafer has an intensity which is not sharp as shown in
FIG. 21D
, resulting in increase in half-width.
It is known that light passing through the aperture has the corners rounded with radius of curvature of about the wavelength of light because of the above-descr
Chiba Akira
Mori Masayoshi
Chang Andrey
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
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