U.S. patent application number 09/737181 was filed with the patent office on 2001-08-16 for double pass double etalon spectrometer.
Invention is credited to Buck, Jesse D., Ershov, Alexander I., Smith, Scott T..
Application Number | 20010013933 09/737181 |
Document ID | / |
Family ID | 27057823 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013933 |
Kind Code |
A1 |
Smith, Scott T. ; et
al. |
August 16, 2001 |
Double pass double etalon spectrometer
Abstract
A first double pass etalon based spectrometer. In a preferred
embodiment a second etalon matched to the first double pass etalon
is used to produce extremely precise fringe data. Spectral
components of a diffused beam are angularly separated as they are
transmitted through an etalon. A retroreflector reflects the
transmitted components back through the etalon. Twice transmitted
spectral components are directed through a second etalon and
focused onto a light detector which in a preferred embodiment is a
photo diode array. The spectrometer is very compact producing the
extremely precise fringe data permitting bandwidth measurements
with precision needed for microlithography for both
.DELTA..lambda..sub.FWHM and .DELTA..lambda..sub.95%.
Inventors: |
Smith, Scott T.; (San Diego,
CA) ; Ershov, Alexander I.; (San Diego, CA) ;
Buck, Jesse D.; (San Marcos, CA) |
Correspondence
Address: |
John R. Ross
Cymer, Inc.- Legal Department-MS/1-2A
16750 Via Del Campo Court
San Diego
CA
92127-1712
US
|
Family ID: |
27057823 |
Appl. No.: |
09/737181 |
Filed: |
December 14, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09737181 |
Dec 14, 2000 |
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09245134 |
Feb 4, 1999 |
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6243170 |
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09737181 |
Dec 14, 2000 |
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09513324 |
Feb 25, 2000 |
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Current U.S.
Class: |
356/454 ;
356/519 |
Current CPC
Class: |
G01J 3/26 20130101 |
Class at
Publication: |
356/454 ;
356/519 |
International
Class: |
G01B 009/02 |
Claims
What is claimed is:
1. A double pass, double etalon based spectrometer for making
spectral measurements of a beam, said spectrometer comprising: A) a
diffusing optic for directing light in said beam in a very large
number of directions, to produce a diffuse beam; B) a first etalon
positioned in said diffuse beam and configured to transmit portions
of said beam to produce a once transmitted beam having angularly
separated spectral components; C) a retro-reflecting optic
positioned to reflect at least a portion of said once transmitted
beam back through said etalon to produce a twice transmitted beam
having angularly separated spectral components; D) a second etalon;
E) a focusing optic; F) a light detector; G) a reflecting optic
positioned to reflect at least a portion of said twice reflected
beam through said second etalon, said focusing optic onto said
light detector wherein spectral components of said beam are
detected by said light detector.
2. A spectrometer as in claim 1 wherein said retro-reflector is a
hollow retro-reflector.
3. A spectrometer as in claim 1 wherein said retro-reflector is a
hollow rectangular prism.
4. A spectrometer as in claim 3 wherein said hollow prism is
comprised of two mirrors coated to reflect light within a selected
wavelength range.
5. A spectrometer as in claim 1 wherein said reflecting optic is a
mirror positioned to reflect said twice transmitted beam but permit
transmittal of at least a portion of said diffuse beam to said
etalon.
6. A spectrometer as in claim 3 wherein said diffuse beam defines a
primary polarization direction said reflecting optic is a
polarizing beam splitter and said hollow prism is comprised of two
intersecting reflected plates defining a cross-line at their
intersection, said two plates being positioned at a 90 degree angle
relative to each other and the cross-line of the said plates is
positioned at an angle of about 45 degrees to the primary
polarization direction of said diffuse beam.
7. A spectrometer as in claim 6 wherein said hollow prism is
comprised of two mirrors coated to reflect light within a selected
range of wavelengths.
8. A spectrometer as in claim 1 wherein said light detector is a
detector array.
9. A spectrometer as in claim 8 wherein said detector array is a
linear photo diode array.
10. A spectrometer as in claim 1 and further comprising a spatial
filter positioned between said diffusing optic and said etalon.
11. A spectrometer as in claim 10 wherein said spatial filter
comprises a slit and a first lens for focusing at least a portion
of said diffuse beam through said slit.
12. A spectrometer as in claim 11 and further comprising a second
lens for collimating diffuse lights passing through said slit.
13. A spectrometer as in claim 1 wherein said second etalon is
aligned to a precise integer ratio with said first etalon.
14. A spectrometer as in claim 13 wherein said ratio is 5.
15. A spectrometer as in claim 14 wherein said first etalon has a
transmission function of about 10 and said second etalon has a
transmission function of about 2.
Description
[0001] This invention relates to spectrometers and especially to
etalon based spectrometers. This is a continuation-in-part of Ser.
No. 09/245,134, filed Feb. 4, 1999 and Ser. No. 09/513,324, filed
Feb. 25, 2000.
BACKGROUND OF THE INVENTION
[0002] Etalon based spectrometers are well known devices for
measuring the intensity of light in a beam as a function of
wavelength. FIG. 1 shows the features of a prior art etalon
spectrometer used for measurement of wavelength and bandwidth of a
laser beam 16. The beam is diffused by diffuser 2 so that rays
propagating in a very large number of angles illuminate etalon 4.
FIG. 1 shows a single ray 20 being reflected many times within the
etalon gap between surfaces 8A and 8B which are coated to reflect
about 90%. Spectral components which are transmitted through the
etalon are focused by lens 14 onto photo diode array 12. Photo
diode array 12 registers a fringe pattern 15 which can be read
using electronic data acquisition boards (not shown). The
transmission or reflection of light incident on an etalon such as
that depicted is well understood and depends on the design of the
etalon, particularly the reflectance of the two reflecting
surfaces.
[0003] Etalon spectrometers are widely used to measure the spectrum
of lasers. A particularly important use of etalon spectrometers is
to measure the bandwidth of line narrowed excimer lasers such as
the line narrowed KrF excimer laser. These lasers are used, for
example, as light sources for deep-UV microlithography. There are
two spectral characteristics of these lasers which are very
important for microlithography applications. These are the spectral
bandwidth of the laser measured at 50 percent of the peak
intensity, called its full width-half maximum band width
(abbreviated .DELTA..lambda..sub.FWHM), and the spectral bandwidth,
which contains 95% of laser energy called the 95% integral
bandwidth (abbreviated .DELTA..lambda..sub.95%). It is very
important that the laser is always operating within specifications
during microlithography chip manufacturing because spectral
broadening would cause blurring of the integrated circuits being
printed on silicon wafers which will result in yield problems.
Therefore, it is very important to provide continuous monitoring
capabilities for the laser spectrum.
[0004] The prior art etalon spectrometer is capable of accurately
measuring .DELTA..lambda..sub.FWHM values, and is currently used
for this purpose in production microlithography lasers, such as
manufactured by CYMER, Inc. (San Diego, Calif.). However, prior art
etalon spectrometers are not very suitable for accurately measuring
.DELTA..lambda..sub.95% values. Typical production quality KrF
excimer lasers should have a .DELTA..lambda..sub.FWHM of about 0.6
pm and .DELTA..lambda..sub.95% of about 2 pm, if operating
properly.
[0005] FIG. 2 shows the calculated slit function spectrum of a
typical prior art etalon having a free spectral range (FSR) of 5 pm
and a coefficient of finesse (finesse) of 38. (The terms FSR and
finesse are defined and explained in a variety of optic texts such
as OPTICS by Eugene Hecht/Alfred Zajae published by Addison-Wesley,
Reading, Mass.) The slit function spectrum of FIG. 2 can be derived
from one of the peaks of fringe pattern 15. The calculation graphed
in FIG. 2 assumes that the light illuminating the etalon is
monochomatic (i.e., an infinitely narrow bandwidth). If such an
etalon is used to measure the bandwidth of a laser beam, the slit
function bandwidth of the etalon is a source of error and
contributes to uncertainty or error in the measurement. The
calculated FWHM bandwidth for this prior art etalon is 0.13 pm and
the 95% integral bandwidth for the etalon is about 1.5 pm.
[0006] For the etalon to accurately measure spectrum of a real
laser, the slit function bandwidth of the etalon itself should be
substantially smaller than the laser bandwidth. While this
condition is satisfied for .DELTA..lambda..sub.FWHM measurements,
where etalon slit function FWHM of 0.13 pm is substantially smaller
than typical laser .DELTA..lambda..sub.FWHM of about 0.6 pm, the
same is not true for .DELTA..lambda..sub.95% measurements, where
etalon slit function bandwidth of about 1.5 pm is a substantial
fraction of the expected laser bandwidth of about 2 pm.
[0007] Therefore, if the prior art etalon spectrometer with the
FIG. 2 slit function is used to measure .DELTA..lambda..sub.95%, a
complicated numerical analysis is needed to deconvolve the real
.DELTA..lambda..sub.95% value. Such analysis is prone to errors and
ambiguous results, so no reliable .DELTA..lambda..sub.95%
information is available during the microlithography process. As a
result, a laser can go out of specification unnoticed. This can
lead to very expensive yield problems and should be avoided.
[0008] Another way of accurately measuring laser spectrum is to use
a high resolution grating spectrometers. These instruments can
provide accurate spectral measurement including accurate
.DELTA..lambda..sub.95% measurements, but are very bulky and
expensive. These instruments are successfully used in the
laboratory but are not well suited for production line
microlithography use.
[0009] What is needed is a compact spectrometer, capable of
accurate measurement of both .DELTA..lambda..sub.FWHM and
.DELTA..lambda..sub.95%, which can be built as a part of internal
laser diagnostic set, so that it can be used in the field during
the microlithography process.
SUMMARY OF THE INVENTION
[0010] The present invention provides a first double pass etalon
based spectrometer. In a preferred embodiment a second etalon
matched to the first double pass etalon is used to produce
extremely precise fringe data. Spectral components of a diffused
beam are angularly separated as they are transmitted through an
etalon. A retroreflector reflects the transmitted components back
through the etalon. Twice transmitted spectral components are
directed through a second etalon and focused onto a light detector
which in a preferred embodiment is a photo diode array. The
spectrometer is very compact producing the extremely precise fringe
data permitting bandwidth measurements with precision needed for
microlithography for both .DELTA..lambda..sub.FWHM and
.DELTA..lambda..sub.95%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a drawing of a prior art spectrometer.
[0012] FIG. 2 is a graph of the slit function of a prior art etalon
spectrometer.
[0013] FIG. 3 is a drawing of a first preferred embodiment of the
present invention.
[0014] FIG. 4 shows the spectrum of a frequency doubled Ar--ion
laser beam measured with the FIG. 3 embodiment.
[0015] FIG. 5. is a drawing of the second preferred embodiment of
the present invention.
[0016] FIG. 6. shows the effect of a very small etalon plate
mismatch.
[0017] FIG. 7 shows a fringe pattern on photo diode array.
[0018] FIG. 8 is a drawing of the third preferred embodiment of the
present invention.
[0019] FIG. 9 is a drawing of a polarization rotating hollow
prism.
[0020] FIG. 10 is a drawing of a fourth preferred embodiment of the
present invention.
[0021] FIGS. 11 and 12 are test results from the fourth preferred
embodiment.
[0022] FIGS. 13 and 14 show a comparison of test results of a MPME
spectrometer with a grating spectrometer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Preferred Embodiment
[0023] FIG. 3 shows a preferred embodiment of the present
invention. A laser beam 16, whose size is reduced three times using
telescope 32 illuminates diffuser 34. Light scattered from diffuser
34 illuminates etalon 25. Hollow retro-reflector 38 is used to
return the beam back to the etalon for the second pass. Each
component of the beam is reflected at exactly or almost exactly
180-degrees for a second pass through etalon 25 but there are small
displacements of the reflected components. These small displacement
permit the use of a 45-degree mirror 40 which reflects beam
components after double-passes through the etalon, but allows
passage of a sufficient portion of the incoming beam. The reflected
beam components are focused by lens 42 with a 1 meter focal length
onto linear photo diode array (PDA) 44, where a fringe pattern 49
is detected. The preferred PDA is a 2048 element,
14.mu..times.14.mu.array available from supplier such as EG&G,
Inc., Sunnyvale, Calif.
[0024] The fringe pattern 49 consists of multiple peaks located in
the same position as the peaks 15 of prior art etalon of FIG. 15.
The difference though is that the peaks of the etalon of the
present invention more closely match the actual laser spectrum
because of improved resolution of etalon spectrometer in this
double pass configuration.
[0025] Prototype double pass etalon spectrometers based on the FIG.
3 design have been assembled and tested by Applicant with excellent
results.
[0026] FIG. 4 shows a spectrum (recorded with PDA 44) of the beam
from a frequency doubled Ar--ion cw laser which emits light at
248.25 with an extremely narrow spectral bandwidth of only about
0.003 (FWHM). (The light spectrum from this laser is narrow enough
to be considered monochomatic for purposes of testing etalons with
bandwidths in the ranges greater than 0.1 pm.) The FWHM bandwidth
recorded by PDA 44 (as shown in FIG. 3) was about 0.12 pm with a 95
percent integral value of 0.33 pm. The theoretical values for two
perfect etalons in series under the above conditions would be 0.09
pm (FWHM) and 0.25 (95% integral). These results show that
bandwidth resolutions in the range of 0.1 pm for FWHM and about 0.3
for the 95% integral are obtainable with the FIG. 3 double pass
etalon spectrometer.
[0027] Applicant has compared a typical spectrum of a
microlithography KrF laser measured with a high resolution grating
spectrometer and a compact spectrometer of the present invention.
The grating spectrometer had a slit function of about 0.12 pm at
FWHM level and was manufactured by CYMER, Inc. with offices in San
Diego, Calif. for the purpose of testing KrF excimer lasers. Very
good agreement between the results obtained with etalon
spectrometer and grating spectrometer were obtained. The FWHM value
of the laser bandwidth was 0.65 pm and 0.62 pm as measured by
double pass etalon of the present invention and grating
spectrometers respectively, while 95% integrated value of the
bandwidth is 1.67 pm and 1.70 pm as measured by double pass etalon
spectrometer of the present invention and high resolution grating
spectrometer, respectively.
[0028] The double pass etalon spectrometer of the present invention
creates a fringe pattern similar to conventional etalon
spectrometer the differences being that the fringes correspond to
actual spectrum more closely and are less convolved by etalon
resolution. Therefore, any of the well known techniques can be used
to analyze the fringe pattern. This etalon spectrometer can also be
used in combination with a relatively low resolution grating
spectrometer to render accurate center wavelength measurements.
Such applications are described, for example, in U.S. Pat. Nos.
5,025,445 and 5,450,207. The etalon spectrometer alone cannot do
the absolute wavelength measurements; therefore, a method to
calibrate the etalon data is needed. A plurality of center
wavelengths can be obtained from the etalon separated exactly by
the free spectral range (FSR) of the etalon. In the preferred
embodiment, the FSR is about 5 pm. In order to be able to determine
the correct center wavelength value using the precise etalon data,
a low resolution grating spectrometer may be used as explained in
U.S. Pat. No. 5,025,445. The resolution of that grating
spectrometer can be about half of the FSR, therefore, a unique
center wavelength value can be chosen. A preferred calibration
technique is described in U.S. Pat. No. 5,450,207.
Second Preferred Embodiment
[0029] A second preferred embodiment of the present invention is
shown in FIG. 5.
[0030] A laser beam 16 illuminates diffuser 34 after being reduced
in size by telescope 32 in a way similar to the first embodiment.
In the second embodiment, however, a spatial filter 52 is used to
select a portion of diffused light. The spatial filter 52 consists
of two lenses 54 and 58 with a focal length of 10 cm each,
separated by a distance, twice the focal length, or about 20 cm. An
aluminum aperture 56 with a diameter of about 0.1 cm is placed at
the focus of lens 54. The purpose of spatial filter 52 is to select
a fan of rays coming from a diffuser within an angle (spatial
frequencies) of about 0.01 steradian. This filtered fan of rays is
incident on beam splitter 46. Beam splitter 46 is a partially
reflecting mirror, which transmits about 50% of the light and
reflects the rest away (not shown). The portion of diffused light
62, which passes through beam splitter 46, illuminates etalon 25.
Light passing through etalon 25 is reflected by hollow
retroreflector 70 which returns the beam back to the etalon for the
second pass. This reflected portion of the beam is shown as 64.
Each component of the beam is reflected at exactly or almost
exactly 180-degrees for the second pass through the etalon 25.
[0031] About 50% of the portion of the beam 64 passing through the
etalon 25 is reflected by beam splitter 46. This reflected portion
is focused by lens 42 having a 1 meter focal length onto linear PDA
array 44, where a fringe pattern is detected.
[0032] In this embodiment, etalon 25 is tilted at a small angle of
about 0.01 rad. relative to the axis of the incoming fan beam
transmitted through spatial filter 52. As a result, there are two
sets of fringes registered by PDA 44. On one side of the PDA there
is a fringe set 45B depicted on FIG. 5 which is a set of fringes
created by beams passing through etalon 25 two times. On the other
side of PDA 44, however, there is a different set of fringes. These
fringes 45A also depicted on FIG. 5 are created by a portion of
original beam 62 reflected by etalon 25 on the first pass. This set
of reflection fringes look different than fringe set 45B. Fringes
45A are dips, while fringes 45B are peaks. Because intensity of the
beam after double pass through the etalon might be significantly
smaller, then intensity of the beam reflected from the etalon, an
optional light reducing filter 48 can be placed over the portion of
the PDA, where fringe set 45A is formed. This optional filter 48
can be a neutral density filter with transmission of about 30%.
FIG. 7 shows the relative orientation of fringes 45A and 45B on PDA
44. If there were no spatial filter 52 and etalon 25 had not have
been tilted, both reflection fringes 45A and double transmission
fringes 45B would have been created as concentric circles in the
plane of PDA 44. The diameters of these circles for fringes 45A and
45B would have been exactly the same, therefore they would tend to
cancel each other. The purpose of spatial filter 52 is to limit
light available for fringes so that only a portion of circle 45 is
formed. The purpose of tilting the etalon is to separate the
portions of the circle 45 created by reflected (45A) and
transmitted (45B) beams as is shown in FIG. 7. Therefore, PDA 44
detects portions of both reflected and transmitted circles but the
array detects the left side of one circle and the right side of the
other circle.
[0033] Because positions of the dips 45A are exactly at the same
place where peaks of the conventional etalon spectrometer should
be, the double pass etalon spectrometer is capable of all
measurements the conventional etalon spectrometer is capable of.
Thus, fringes 45B can be used to analyze spectrum shape, while the
actual wavelength is determined by a diameter of fringe 45 (FIG. 7)
together with calibration data. In a conventional etalon
spectrometer, this diameter is determined as a distance between
peaks 15A and 15B (FIG. 1). In the etalon of the present invention,
this diameter is determined as a distance between peaks 45B and
corresponding dips 45A (FIG. 5). Therefore, all the prior art
techniques for determining the spectral shape and center wavelength
information can be used with the etalon of the present invention as
well.
Third Preferred Embodiment
[0034] A third embodiment of the present invention is shown in FIG.
8.
[0035] This embodiment is just like the second referred embodiment
shown in FIG. 5 except:
[0036] 1) 45 degree 50% beam splitting mirror is replaced with a
polarizing beam splitter 47. This beam splitter is arranged to
maximum transmit the predominant polarization of the laser light
and reflect the other polarization.
[0037] 2) hollow retroreflector 70 is replaced with a hollow prism
72. This prism is created by two reflected rectangular mirrors 72A
and 72B coated with material which efficiently reflect the laser
light. The line 72C (shown in FIG. 9 described below) at which
plane 72A and 72B cross is aligned at an angle of about 45 degrees
to the polarization of the beam 62 transmitted through the
polarizing beam splitter 47.
[0038] FIG. 9 shows the prism 72 as seen along direction of the
beam 62 (FIG. 8). FIG. 9 shows the orientation of the polarization
162 of the beam 62 (shown in FIG. 8) relative to line 72C, as well
as polarization 164 of the reflected light. This polarization 164
is rotated by 90 degrees after being reflected back by hollow prism
72. Going back to FIG. 8, this back reflected light goes through
the etalon 37 for the second time just like in the second
embodiment (FIG. 5). Because of its 90 degrees rotated
polarization, this double pass beam 64 will now be reflected by
polarizing beam splitter 47 and focused by lens 42 onto PDA array
44 to form fringe patterns 45A and 45B similar to the second
embodiment (FIG. 5).
[0039] In the present embodiment, though, the amplitude of the
signal detected by PDA 44 is higher because of higher throughput,
provided by polarizing beam splitter 47 as compared with 50%
reflector 46 in the second embodiment (FIG. 5). The fringe pattern
in this embodiment, however, is similar to that of the second
embodiment, therefore, all the same techniques for measuring
spectrum shape and center wavelength can be used with this
embodiment as well. Many lasers, such as excimer KrF lasers used
for microlithography, produce highly polarized light, therefore,
the use of this third embodiment will significantly (up to 2-3
times) increase the signal. The preferred polarizing beam splitter
47 will have about 90% transmission for one polarization and more
than 97% reflection for the other.
[0040] The reader should appreciate that all three embodiments have
the advantage in that the light goes twice through the same etalon.
Therefore, assuming the reflecting surfaces are very parallel,
essentially the same spacing between the plates of the etalon is
guaranteed for both passes.
[0041] In any series etalon spectrometer arrangement precise
matching of etalon spacing is extremely important. For 248 nm light
a spacing difference between the plates of the etalon as small as
6.33 nm ({fraction (1/100)} of the 633 nm frequency doubled,
Ar--ion laser wavelength) substantially destroys the resolution of
the spectrometer. The results of such spacing difference is shown
in FIG. 6, the heavy curve showing the results of the 6.33 nm
spacing difference and the light curve representing the two etalon
spectrum with perfectly matched spacings. It is extremely difficult
to have two etalons with spacings (which are typically in the range
of 1-15 mm) matched to 6 nm. With a one etalon as in the etalon
spectrometer of the present invention, the gap for two passes
remains the same as long as the etalon is of good quality with
precisely parallel reflecting surfaces. The Applicant has tested
first and second embodiments with great success.
Fourth Preferred Embodiment
[0042] A fourth preferred embodiment of the present invention is
shown in FIG. 10. This embodiment is substantially the same as that
shown in FIG. 5 and described above as the first preferred
embodiment except that a second etalon has been added to the
system. The double pass etalon is designated as 25A and the second
etalon which is a single pass etalon is designated as 25B. In this
embodiment, reducing telescope 32 is a 3:1 reducing telescope,
diffuser 34 is a small defractive diffuser, double pass etalon 25A
is a 10 pm etalon, retroreflector 38 is a hollow corner cube,
etalon 25B is a 2 pm etalon and lens 42 is a 1.5 m lens. Spacial
filter selects a fan or rays and removes any zero order component
which could distort the spectrum. Fold mirrors (not shown) permits
the entire system to fit on a 2 foot by 1 foot optical
breadboard.
[0043] To align the transmission functions of the two etalons to a
precise ratio of 5 required etalon tuning. The 2 pm etalon was an
air spaced etalon and was pressure tuned to match the 10 pm etalon.
The pressure tuning was accomplished by enclosing the etalon in a
sealed housing and connecting it to an adjustable bellows by an air
line. By compressing the bellows the air pressure and therefore the
FSR of the etalon could be adjusted. It was found that the
temperature stability of the laboratory used was not adequate for
the solid 10 pm etalon. To increase the stability of the 10 pm
etalon, a temperature control assembly was built. The assembly was
composed of a resistive heating element, a resistive temperature
sensor, and a temperature control module.
[0044] Testing of the impulse response of the multi-pass,
multi-etalon (MPME) spectrometer was conducted by using the FreD
laser as a monochromatic source. A 1024 element, Hamamatsu, linear
photo-diode array (PDA) and a digitizing oscilloscope recorded the
fringe pattern produced by the spectrometer. In addition to the
fringe image, a dark level image was recorded. Each of the two
files represented the average of 60 frames of the PDA running at a
line rate of about 90 hz. The integration time of each frame was
approximately 10.5 ms. The recorded images were processed by
subtracting the dark level image from the fringe image and fitting
a base line to the resulting data. The resulting data was then
graphed in FIG. 11. FIG. 12 shows a magnified view of the
noise.
[0045] The experimental transmission function recorded matches
closely with the modeled function for the MPME spectrometer. The
slightly superior FWHM performance of the spectrometer over the
modeled value may have been produced by a higher finesse value of
one or both of the etalons than the modeled value of 30.
[0046] The preferred embodiment of the spectrometer would have both
etalons to be of the air spaced type and to have the tuning
accomplished by pressure tuning the 10 pm etalon. This would
increase the temperature stability of the spectrometer and
eliminate the need for pressure tuning the 2 pm etalon. By tuning
the 10 pm etalon, the variability of the position of the first
fringe would also be reduced. Since the FSR of an etalon bounds the
position of a fringe with regards to wavelength, tuning the 10 pm
etalon would set the variability to 2 pm. With lower fringe
position variability, a more consistent integration range for
calculating the integrated 95% width can be maintained for all
wavelengths.
[0047] It should also be noted that the linear PDA used is limiting
the resolution of the spectrometer. The 25 micron size pixels were
comparatively large to the width of the fringe pattern produced. To
alleviate this problem the size of the fringe pattern was magnified
by using a longer 1.5 meter focal length lens. At this
magnification the diameter of the first fringe when positioned 5 pm
from the center is too large to be imaged by the detector. The
entire PDA only covers a radial section of the fringe pattern that
translates to about 12 pm in wavelength space. Even at this maximum
magnification the FWHM of the fringe is 3 pixels or less. Therefore
to utilize the full resolution of this spectrometer design would
require a linear array with at least 2048 pixels.
[0048] The practical limitations of this type of design sets the
maximum ratio between etalon gaps at about 10. When the gap ratio
of two etalons exceed 10, the proximity of the first subsidiary
peak to the designed pass band causes a substantial increase in the
integrated 95% width. Using this gap ratio guideline and the
additional constraint of a minimum FSR of 10 pm, sets the highest
resolution multi-pass, multi-etalon spectrometer to a device using
a 10 pm etalon and 1 pm etalon. The theoretical transmission
function of such a device with an estimated finesse value of 30 for
both etalons produces a FWHM of 0.03 pm and an integrated 95% of
0.15 pm.
Fifth Preferred Embodiment
[0049] In a fifth embodiment built and tested by Applicant is one
in which a 2 pm single pass etalon was combined with a 20 pm double
pass etalon and the MPME system was used to measure bandwidth of a
KrF laser beam and the results were compared with similar results
using a state of the art precision grating based spectrometer. Raw
results are shown in FIGS. 13 and 14. FIG. 14 is a magnified
portion of FIG. 13. These graphs show the much more compact MPME
has substantially higher resolution than the grating
spectrometer.
[0050] Persons skilled in the art can recognize that other
modifications can be done without departing from the spirit of the
present invention. For example, a scanning exit slit and a
photometer can be used instead of photo diode array if fast
response is not a requirement. This slit-photometer assembly can
scan the area of the PDA and measure the light intensity at
different wavelengths. The second single pass etalon could be
combined with double pass etalon arrangement shown in FIGS. 3 and 8
as well as the one shown in FIG. 5. Also, the features described in
the fourth and fifth embodiments could be combined with the
teachings of patent application Ser. No. 09/513,325, filed Feb. 25,
2000 which is incorporated herein by reference. Therefore, the
invention is only to be limited by the claims and their legal
equivalents.
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