Spectrometer

Details Spectrometer Spectrometer

2001-03-08

2003-06-03

Chang, Audrey (Department: 2872)

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

C356S326000, C356S333000, C356S334000

Reexamination Certificate

active

06573989

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spectrometer which is used to measure a spectrum of a light beam supplied from a light source, in particular, a spectrometer that can obtain, from a spectrum including specified wavelength components, fine information (information of a spectral component in a relatively narrow range in the vicinity of a peak of the spectrum) at high accuracy and coarse information (information of a spectral component in a relatively wide range around the peak, in which information of a foot part of the spectrum is important).
2. Description of a Related Art
A spectrometer disclosed in Japanese patent application publication JP-A-11-132848 is known as an optical instrument which is used to measure a spectrum of a light beam supplied from a light source.
FIG. 10
shows constitution of the spectrometer.
The spectrometer as shown in
FIG. 10
has slit board
100
, collimator lens
101
, beam splitter
102
, diffraction grating
103
, mirror
104
, magnifier lens
105
and line sensor
106
.
In the spectrometer, a light beam supplied from a light source passes through slit board
100
and is changed into a parallel light beam by collimator lens
101
. In beam splitter
102
, the parallel light beam is divided into a reflected light beam which travels toward collimator lens
101
and a transmitted light beam which travels toward diffraction grating
103
. The transmitted light beam is incident into diffraction grating
103
, and a part of the transmitted light beam is diffracted toward beam splitter
102
as a first diffraction light beam (a single pass beam).
A part of the single pass beam is reflected by beam splitter
102
toward a direction shifted by a slight angle from the optical axis of collimator lens
101
. On the other hand, the rest of the single pass beam passes through beam splitter
102
.
The single pass beam reflected by beam splitter
102
is incident into diffraction grating
103
again, and a part of the incident light beam is diffracted as a second diffraction light beam (a double pass beam) from diffraction grating
103
to beam splitter
102
. A number of times of diffraction of the double pass beam is one more than that of the single pass beam, so that a dispersion value (a difference in wavelengths which corresponds to an interval of adjacent two channels of line sensor
106
) of the double pass beam is small. Therefore, the double pass beam realizes high resolving power.
And then, a part of the double pass beam is reflected by beam splitter
102
toward a direction shifted by a slight angle from the optical axis of collimator lens
101
. On the other hand, the rest of the double pass beam passes through beam splitter
102
.
By the way, when an excimer laser source is used as a light source, according to performance of the excimer laser source and so on, a useless component may appear in a foot part of the spectrum which is distant from a peak of the spectrum, and a blur may arise by chromatic aberration of the optical system. Therefore, in measurement of a spectrum of a light source, it is necessary to carry out not only measurement in the vicinity of a peak of the spectrum with higher resolving power (fine measurement) but also measurement in a foot part of the spectrum with lower resolving power (coarse measurement).
In the conventional spectrometer as shown in
FIG. 10
, line sensor
106
can detect either a single pass beam or a double pass beam in general.
FIG. 10
shows the case where line sensor
106
detects a single pass beam.
Therefore, two spectrometers which are different in a value of resolving power are prepared. At first, measurement of a spectrum of an excimer laser beam is carried out by using a spectrometer for the single pass beam, which has lower resolving power, so as to obtain coarse information of the spectrum. Next, the spectrometer for the single pass beam is replaced with a spectrometer for the double pass beam, which has higher resolving power, and measurement of the spectrum is carried out so as to obtain fine information of the spectrum. Further, a compound spectrum of the excimer laser beam is obtained by compounding the coarse information and the fine information together under a suitable processing.
As another way, two diffraction gratings which are different in the number of groove lines are prepared. At first, measurement of a spectrum of an excimer laser beam is carried out by installing a diffraction grating for the single pass beam to the body of the apparatus so as to obtain coarse information of the spectrum. Next, the diffraction grating for the single pass beam is replaced with a diffraction grating for the double pass beam, which is different from the diffraction grating for the single pass beam in the number of groove lines, and measurement of the spectrum is carried out so as to obtain fine information of the spectrum. Further, a compound spectrum of the excimer laser beam is obtained by compounding the coarse information and the fine information together under a suitable processing.
In the spectrometer disclosed in JP-A-11-132848, however, the following problems cause when the coarse information and the fine information in the spectrum of the excimer laser beam is obtained.
(a) The cost becomes higher because two spectrometers or two diffraction gratings need to be prepared for the single pass beam and the double pass beam.
(b) The process of measuring a spectrum becomes more complicated because one spectrometer or diffraction grating for the single pass beam needs to be replaced with the other for the double pass beam.
Therefore, it is proposed to enlarge a size of the line sensor in the longitudinal direction so as to focus the single pass beam and the double pass beam on channels of the line sensor (different channels are used for the single pass beam and for the double pass beam).
However, according to the above-mentioned proposal, either the single pass beam or the double pass beam is focused on a channel in the edge portion of the line sensor. As a result, the inaccurate detection result due to image shift may be obtained.
Further, it is also proposed to rotate a diffraction grating adequately so as to focus the single pass beam or the double pass beam on channels of a line sensor. That is, the single pass beam is focused on channels of the line sensor by rotating diffraction grating
103
from the Littrow arrangement, in which an incident angle is equal to an output angle, by a predetermined slight angle. Next, the double pass beam is focused on channels of the line sensor by rotating diffraction grating
103
from the Littrow arrangement by a predetermined slight angle (which is different from that in detecting a single pass beam).
However, according to the above-mentioned proposal, there is a problem that it is practically difficult to realize high resolving power without enlarging a size of the apparatus.
The resolving power of a spectrometer according to the above-mentioned proposal (which will be explained with referring to
FIG. 10
in the following) becomes higher as a dispersion value in the line sensor becomes smaller. The dispersion value is defined by the following expression:
disp=
sw/
(
f·
angDisp)   (1)
Where each symbol represents the following value.
disp: a dispersion value
sw (=swd/mag): a one-to-one conversion size of the line sensor
swd: a size of the line sensor
mag: a magnification rate of a magnifier lens
f: a focal length of a collimator lens
angDisp: an angular dispersion value
Furthermore, an angular dispersion value in the expression (1) is given by the following expressions:
angDisp1
=m
/(
d
·cos &bgr;)   (2)
angDisp2=2
m
/(
d
·cos &bgr;)   (3)
Where each symbol represents the following value.
angDisp1: an angular dispersion value of the single pass beam
angDisp2: an angular dispersion value of the double pass beam
m: an order of diffraction
d: an interval of groove lines of a diffraction grating
&bgr;: an output angle from a diffraction grating

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