Optics: measuring and testing – Infrared and ultraviolet
Reexamination Certificate
2001-04-26
2003-10-21
Juba, John (Department: 2872)
Optics: measuring and testing
Infrared and ultraviolet
C250S365000, C250S472100, C250S483100, C372S032000
Reexamination Certificate
active
06636297
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a wavelength measuring apparatus for measuring wavelength characteristics of a laser beam oscillating from a vacuum ultraviolet laser.
BACKGROUND OF THE INVENTION
Conventionally there is known a vacuum ultraviolet laser emitting a laser beam
11
having a wavelength of approx. 20 nm to 200 nm referred to as vacuum ultraviolet region such as, for example, ArF lasers (193 nm) and F2 lasers (157 nm).
This type of vacuum ultraviolet laser is mainly used for precision processing such as laser lithography or the like. To favorably perform the precision processing, it is necessary to mount a wavelength selecting element on the vacuum ultraviolet laser so as to stabilize a center wavelength of the laser beam applied to an object to be processed and to narrow a spectral width of the wavelength (it is referred to as narrowing a laser beam band).
Furthermore, to preferably perform the precision processing, the center wavelength and the spectral width (hereinafter, generally referred to as wavelength characteristics) of the above laser beam need to be limited within a predetermined allowable range. For this purpose, the wavelength characteristics of the laser beam
11
having the narrowed band must accurately be measured and controlled on the basis of the measured values.
Referring to
FIG. 5
, there is shown a configurational view of an F2 laser unit having a wavelength measuring apparatus related to a prior art.
In
FIG. 5
, the F2 laser unit
1
comprises a laser chamber
2
enclosing laser gas and emitting a laser beam
11
by causing an electric discharge inside, a band narrowing unit
10
for narrowing a band of the laser beam
11
emitted from the laser chamber
2
, a wavelength measuring apparatus
3
for measuring wavelength characteristics of the laser beam
11
, and a wavelength controller
4
for controlling the wavelength characteristics of the laser beam
11
whose band has been narrowed so as to be limited within an allowable range with being electrically connected to the wavelength measuring apparatus
3
and the band narrowing unit
10
.
The laser chamber
2
encloses laser gas such as, for example, fluorine (F2) and helium (He) at a predetermined pressure ratio. A pair of discharge electrodes (not shown) are installed in a predetermined position inside the laser chamber
2
and the laser beam
11
is caused to oscillate by applying a high voltage between the discharge electrodes.
A rear window
9
at a rear end (left-handed in the drawing) of the laser chamber
2
transmits the oscillating laser beam
11
and then the laser beam is incident on the band narrowing unit
10
arranged externally at the back of the laser chamber
2
. Inside the band narrowing unit
10
, an etalon, a grating, or other wavelength selecting elements (not shown) are arranged in predetermined positions to narrow the band of the laser beam
11
.
The laser beam
11
whose band has been narrowed passes through the laser chamber
2
and penetrates through a front window
7
at a front end of the laser chamber
2
, and then a part of it partially penetrates through a front mirror
8
arranged externally ahead of the laser chamber
2
to be emitted to the outside.
At this point, a beam splitter
12
is arranged on an optical axis of the laser beam
11
in order to measure the wavelength characteristics of the emitted laser beam
11
. The laser beam
11
is partially reflected downward by the beam splitter
12
to generate a sample beam
11
A and it is incident on the wavelength measuring apparatus
3
, so that its wavelength characteristics are measured.
The wavelength measuring apparatus
3
comprises a diffuser panel
24
for diffusing the sample beam
11
A, a monitor etalon
25
for generating an interference pattern
29
corresponding to the wavelength characteristics of the diffused sample beam
11
A, a first imaging lens
27
for imaging this interference pattern
29
, a pattern detector
17
(for example, a line sensor) for measuring an intensity distribution of the imaged interference pattern
29
, and an arithmetic unit
28
for calculating the wavelength characteristics of the sample beam
11
A on the basis of an output from the pattern detector
17
.
This arithmetic unit
28
transmits the wavelength characteristics of the calculated sample beam
11
A to the wavelength controller
4
. The wavelength controller
4
outputs a command signal to the band narrowing unit
10
on the basis of the wavelength characteristics and controls the band narrowing unit
10
so that the wavelength characteristics of the laser beam
11
are limited within a predetermined range. This feedback control enables the wavelength characteristics of the laser beam
11
to be controlled.
The prior art set forth in the above, however, has problems described below.
In other words, the interference pattern
29
has the same wavelength as for the laser beam
11
. Light in the vacuum ultraviolet region has a short wavelength and receives very large energy of photons inversely proportional to a wavelength. Therefore, the pattern detector
17
is damaged by the energy of photons of the incident interference pattern
29
, by which the wavelength characteristics cannot be measured accurately.
In addition, this causes the measured values of the wavelength characteristics of the laser beam
11
to be inaccurate, which results in a fluctuation of the wavelength characteristics of the laser beam
11
controlled by the wavelength controller
4
on the basis of the measured values. It further fluctuates the wavelength characteristics of the laser beam
11
applied to an object to be processed, which results in a fluctuation of a focal position of the laser beam
11
inside a processing machine which is not shown and causes a precision processing failure.
Furthermore, the wavelength characteristics are measured at all times during processing and a signal for halting the processing is outputted to the processing machine when the wavelength characteristics deviate from a predetermined range. In this condition, the inaccurate measurements of the wavelength characteristics causes the processing to be halted in spite of favorable wavelength characteristics or to be continued in spite of poor wavelength characteristics.
DISCLOSURE OF THE INVENTION
The present invention has been provided in view of the above problems. It is an object of the present invention to provide an apparatus for measuring a wavelength of a vacuum ultraviolet laser, capable of accurately measuring wavelength characteristics of the laser beam.
To achieve the above object, in accordance with a first aspect of the present invention, there is provided a vacuum ultraviolet laser wavelength measuring apparatus having spectral means for generating an optical pattern corresponding to wavelength characteristics of an incident laser beam and measuring wavelength characteristics of a laser beam in a vacuum ultraviolet region oscillating from a vacuum ultraviolet laser on the basis of the optical pattern, comprising: a fluorescent screen for generating a fluorescent pattern having an intensity distribution corresponding to an intensity distribution of the incident optical pattern, a pattern detector for measuring the intensity distribution of the fluorescent pattern generated from the fluorescent screen, and an arithmetic unit for calculating the wavelength characteristics of the laser beam on the basis of the intensity distribution of the measured fluorescent pattern.
With these features, an interference pattern or other optical pattern generated by the spectral means is caused to be incident on the fluorescent screen, the intensity distribution of the fluorescent pattern generated from the fluorescent screen is measured by the pattern detector, and the wavelength characteristics of the laser beam are calculated by the arithmetic unit on the basis of the measured values.
As set forth in the above, the wavelength characteristics of the laser beam can be measured without causing the laser beam to be directly incident on the pat
Enami Tatsuo
Nagai Shinji
Wakabayashi Osamu
Armstrong Westerman & Hattori, LLP
Gigaphoton Inc.
Juba John
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