U.S. patent application number 09/301301 was filed with the patent office on 2002-01-03 for method and apparatus for accurately compensating both long and short term fluctuations in the refractive index of air in an interferometer.
Invention is credited to DE GROOT, PETER J..
Application Number | 20020001086 09/301301 |
Document ID | / |
Family ID | 23162792 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001086 |
Kind Code |
A1 |
DE GROOT, PETER J. |
January 3, 2002 |
METHOD AND APPARATUS FOR ACCURATELY COMPENSATING BOTH LONG AND
SHORT TERM FLUCTUATIONS IN THE REFRACTIVE INDEX OF AIR IN AN
INTERFEROMETER
Abstract
Methods and apparatus that combine dispersion interferometry
with refractometry to compensate for refractive index fluctuations
in the measurement path of a dispersion interferometer over both
short and long time periods. Dispersion and refractometry data are
weighted over appropriate time intervals, and means and methods are
also provided for initializing .GAMMA., the inverse dispersive
power, so that the dispersion and refractometry data are self
consistent. A refractometer is placed in close proximity to the
measurement path of the dispersion interferometer to experience
substantially the same air flow and act as a surrogate for
obtaining information about the index of refraction.
Inventors: |
DE GROOT, PETER J.;
(MIDDLETOWN, CT) |
Correspondence
Address: |
FRANCIS J CAUFIELD
6 APOLLO CIRCLE
LEXINGTON
KY
024217025
|
Family ID: |
23162792 |
Appl. No.: |
09/301301 |
Filed: |
April 28, 1999 |
Current U.S.
Class: |
356/486 ;
356/517 |
Current CPC
Class: |
G01B 9/0207 20130101;
G03F 7/70883 20130101; G03F 7/70775 20130101; G01B 2290/60
20130101 |
Class at
Publication: |
356/486 ;
356/517 |
International
Class: |
G01B 009/02 |
Claims
What is claimed is:
1. An interferometric apparatus comprising: a source for generating
at least two beams having different wavelengths; interferometer
means adapted for receiving said beams and having a measurement
path, said interferometer means being arranged to employ said beams
such that at least a first one of said beams at a first wavelength
is used for measuring the displacement of an object along said
measurement path and to generate short-term information about the
local refractive index of air directly within the measurement path
using the inverse dispersive power .GAMMA. or its equivalent; means
proximate said measurement path for measuring the long-term
variations in refractive index of air directly for at least said
first wavelength; means for changing the physical path length of
said measurement path; initialization means for establishing an
initial value of the inverse dispersive power .GAMMA.; and
computational means for analyzing data about the short and long
term refractive index and for providing a calculated refractive
index along said measurement path, said calculated refractive index
incorporating both long and short-term fluctuations and employing
at least said initial value of said inverse disperse power
.GAMMA..
2. The interferometric apparatus of claim 1 wherein said
computational means further includes means for calculating the
physical length of the measurement path.
3. The interferometric apparatus of claim 1 wherein said
interferometric means comprises a separate displacement
interferometer operating at said first wavelength and a dispersion
interferometer operating at two wavelengths, one of which may be
the first wavelength.
4. The interferometric apparatus of claim 1 wherein said source
comprises a laser and second harmonic generation means for
frequency doubling the output of said laser to generate said two
beams.
5. The interferometric apparatus of claim 3 wherein said dispersion
interferometer further includes second harmonic generation means
for receiving one of said beams and generating another beam at a
third wavelength for generating dispersion information about the
measurement path.
6. The interferometric apparatus of claim 1 wherein said means for
measuring the long term refractive index of air comprises a
refractometer.
7. The interferometric apparatus of claim 6 wherein said
refractometer comprises two refractometers operating at two
different wavelengths to generate information about the inverse
dispersion power.
8. The interferometric apparatus of claim 1 wherein said means for
measuring the long term refractive index of air comprises a pair of
reference marks on a translation stage having a known physical
length between them, generating information relating to the optical
path length corresponding to the distance between the reference
marks, and determining the index based on the ratio of the optical
path length to the physical path length.
9. The interferometric apparatus of claim 1 wherein said
computational means is further adapted to monitor at least said
initial value of .GAMMA. and update it to a current value if it
changes by a predetermined amount where .GAMMA. is acquired using
index and dispersion data acquired during at least one change in
the length of the measurement path.
10. The interferometric apparatus of claim 1 further including a
microlithographic means operatively associated with said
interferometric apparatus for fabricating wafers, said
microlithographic means comprising: at least one stage for
supporting a wafer; an illumination system for imaging spatially
patterned radiation onto the wafer; and a positioning system for
adjusting the position of said at least one stage relative to the
imaged radiation; wherein said interferometric apparatus is adapted
to measure the position of the wafer relative to the imaged
radiation.
11. The interferometric apparatus of claim 1 further including a
microlithographic means operatively associated with said
interferometric apparatus for use in fabricating integrated
circuits on a wafer, said microlithographic means comprising: at
least one stage for supporting a wafer; an illumination system
including a radiation source, a mask, a positioning system, a lens
assembly, and predetermined portions of said interferometric
apparatus, said microlithographic means being operative such that
the source directs radiation through said mask to produce spatially
patterned radiation, said positioning system adjusts the position
of said mask relative to radiation from said source, said lens
assembly images said spatially patterned radiation onto the wafer,
and said interferometric apparatus measures the position of said
mask relative to said radiation from said source.
12. The interferometric apparatus of claim 1 further including
microlithographic apparatus operatively associated with said
interferometric apparatus for fabricating integrated circuits
comprising first and second components, said first and second
components being moveable relative to one another and said
interferometric apparatus, said first and second components being
connected with said first and second measurement legs,
respectively, moving in concert therewith, such that said
interferometric apparatus measures the position of said first
component relative to said second component.
13. The interferometric apparatus of claim 1 further including a
beam writing system operatively associated with said
interferometric apparatus for use in fabricating a lithography
mask, said beam writing system comprising: a source for providing a
write beam to pattern a substrate; at least one stage for
supporting a substrate; a beam directing assembly for delivering
said write beam to the substrate; and a positioning system for
positioning said at least one stage and said beam directing
assembly relative to one another, said interferometric apparatus
being adapted to measure the position of said at least one stage
relative to said beam directing assembly.
14. An interferometric method comprising the steps of: generating
at least two beams having different wavelengths; receiving said
beams in interferometer means having a measurement path, said
interferometer means being arranged to employ said beams such that
at least a first one of said beams at a first wavelength is used
for measuring the displacement of an object along said measurement
path and to generate short-term information about the local
refractive index of air directly within the measurement path using
the inverse dispersive power .GAMMA. or its equivalent; directly
measuring the long-term variations in refractive index of air for
at least said first wavelength at a location proximate said
measurement path; changing the physical path length of said
measurement path; establishing an initial value of the inverse
dispersive power .GAMMA.; and analyzing data about the short and
long term refractive index and providing a calculated refractive
index along said measurement path, said calculated refractive index
incorporating both long and short-term fluctuations and employing
at least said initial value of said inverse disperse power
.GAMMA..
15. The interferometric method of claim 14 wherein said step of
analyzing further includes the step of calculating the physical
length of the measurement path.
16. The interferometric method of claim 14 wherein said
interferometric means comprises a separate displacement
interferometer operating at said first wavelength and a dispersion
interferometer operating at two wavelengths, one of which may be
the first wavelength.
17. The interferometric method of claim 14 wherein said beams are
provided by a laser and second harmonic generation means for
frequency doubling the output of said laser to generate said two
beams.
18. The interferometric method of claim 16 wherein said dispersion
interferometer further includes second harmonic generation means
for receiving one of said beams and generating another beam at a
third wavelength for generating dispersion information about the
measurement path.
19. The interferometric method of claim 14 wherein the step of
measuring the long term refractive index of air employs a
refractometer.
20. The interferometric method of claim 19 wherein said
refractometer comprises two refractometers operating at two
different wavelengths to generate information about the inverse
dispersion power.
21. The interferometric method of claim 14 wherein said step of
measuring the long term refractive index of air employs a pair of
reference marks on a translation stage having a known physical
length between them, generating information relating to the optical
path length corresponding to the distance between the reference
marks, and determining the index based on the ratio of the optical
path length to the physical path length.
22. The interferometric method of claim 14 wherein said step of
analyzing further includes the step of monitoring at least said
initial value of .GAMMA. and updating it to a current value if it
changes by a predetermined amount where .GAMMA. is acquired using
index and dispersion data acquired during at least one change in
the length of the measurement path.
23. The interferometric method of claim 14 further including a
microlithographic steps for fabricating wafers, said
microlithographic steps comprising: at least one stage for
supporting a wafer on at least one wafer stage; imaging spatially
patterned radiation onto the wafer; and adjusting the position of
said at least one stage relative to the imaged radiation; wherein
said interferometric method measures the position of the wafer
relative to the imaged radiation.
24. The interferometric method of claim 14 further including a
microlithographic steps for use in fabricating integrated circuits
on a wafer, said microlithographic steps comprising: supporting a
wafer on at least one stage; providing an illumination system
including a radiation source, a mask, a positioning system, and a
lens assembly directing radiation through said mask to produce
spatially patterned radiation, adjusting the position of said mask
relative to radiation from said source, said lens assembly imaging
said spatially patterned radiation onto the wafer, and measuring
the position of said mask relative to said radiation from said
source.
25. The interferometric method of claim 14 further including
microlithographic steps for fabricating integrated circuits
comprising first and second components, said first and second
components being moveable relative to one another and an
interferometric apparatus, said first and second components being
connected with first and second measurement legs, respectively,
moving in concert therewith, such that said interferometric
apparatus measures the position of said first component relative to
said second component.
26. The interferometric method of claim 14 further including a beam
writing process for use in fabricating a lithography mask, said
beam writing process comprising the steps of: providing a write
beam to pattern a substrate; supporting a substrate onat least one
stage; delivering said write beam to the substrate; and positioning
said at least one stage and said beam relative to one another, said
interferometric process being adapted to measure the position of
said at least one stage relative to said beam directing
assembly.
27. An interferometric method comprising the steps of: storing a
self-consistent value of inverse dispersive power, .GAMMA. and
updating its value if the current value exceeds a predetermined
limit; measuring the refractive index for .lambda..sub.1 using a
refractometer located near a measurement path; determining a time
average for the refractive index generated over a characteristic
time period; measuring the optical path length of the measurement
path for wavelengths, .lambda..sub.1,2; calculating the local
index, N.sub.1.sup.2.lambda. using dispersion interferometry; time
averaging the local refractive index; calculating the fluctuation
the local index as the difference between the instantaneous value
and time averaged value; calculating the physical distance
corrected for atmospheric effects; testing for the difference
between the time averaged value of the local index and the time
averaged value of the index at the refractometer; and calibrating
.GAMMA. and updating its value should .GAMMA. change by said
predetermined value.
28. An interferometric method for calibrating the inverse
dispersive power fro subsequent use in calculating physical length
in the measurement path of an interferometer, said method
comprising the steps of: measuring the refractive index for
.lambda..sub.1 using a refractometer near the measurement path;
determining the time average of the refractive index near the
measurement path over a characteristic time T; moving an
interferometer stage between two positions; measuring the change in
optical path length of the measurement path for two wavelengths,
.lambda..sub.1,2; and calculating the inverse dispersive power,
.GAMMA..
Description
BACKGROUND OF THE INVENTION
[0001] This invention, in general, relates to interferometric
methods and apparatus for measuring linear and/or angular
displacements and, in particular, to apparatus and methods by which
fluctuations in the index of refraction in the measurement path of
a displacement measuring interferometer (DMI) can be accurately
compensated for in determinations of displacement.
[0002] High-precision displacement measuring interferometry (DMI)
depends on an accurate determination of the index of refraction, n,
in the measurement path. One way to determine n is to place sensors
in close proximity to the measurement path to monitor thermodynamic
properties such as pressure, temperature, and humidity, and then
use the values of those parameters together in well-known
expressions relating index of refraction to monitored properties
as, for example, Edln's equation with modern corrections for the
index of refraction of air (See "Recent advances in displacement
measuring interferometry", Bobroff, Norman, Measurement Science and
Technology, Vol. 4, Number 9, September 1993). If required, sensors
for detecting the composition of the gas in the measurement path
may also be employed to further refine the calculation of the index
of refraction. For example, CO.sub.2 sensors may be usefully
employed.
[0003] While few applications require greater absolute accuracy in
the calculated value of n than can be obtained using Edln's
equation in combination with environmental monitoring, all DMI
systems are extremely sensitive to index fluctuations after
initialization. This is particularly true for the case of a
microlithography tool where DMI metrology is an integral part of
the wafer and reticule positioning systems. Here, the most severe
requirements are placed on the repeatability and stability of the
DMI measurements for the purposes of accurate overlay.
[0004] In addition to providing continuous data free of
high-frequency noise, a microlithography DMI must be stable over
the entire time needed for a single wafer exposure, including
whatever time is needed for wafer alignment. For some measurements,
such as establishing the "baseline" metrology between
through-the-lens and off-axis alignment sensors, the interferometer
system must be stable for several hours. In both of these
situations, undetected changes in the index n can have serious
consequences. A typical target stability for DMI in the next
generation of steppers is 1 nm. The corresponding minimum allowable
fluctuation in index in the measurement path is therefore 10.sup.-9
over a 1-m distance within a bandwidth of 10.sup.-4 to 10.sup.2 Hz.
Detection of these fluctuations is presently beyond the capability
of environmental sensors.
[0005] Accordingly, there has been a great deal of interest in
compensation systems that deal with the problem of fluctuations in
the refractive index n for microlithography tools. One approach has
been the use of a refractometer, also called a wavelength tracker
or compensator. Such devices, which are commercially available, are
actually relative refractometers. If properly positioned in the
path of the forced air flow in a photolithography tool, the
information from a refractometer can be used to accurately
compensate for low frequency (e.g. 10.sup.-2 Hz) changes in
index.
[0006] Another approach to this problem has been the use of air
turbulence compensation systems (ATC), which are based on
dispersion interferometry. ATC systems use two widely-separated
wavelengths and rely on the wavelength dependence of index of
refraction. This wavelength dependence is characterized by the
inverse dispersive power .GAMMA., which is the ratio of the
refractivity at one wavelength to the difference in refractivity
between two wavelengths. Typical values of .GAMMA. for air are
between 15 and 75.
[0007] It is accordingly, a primary object of this invention to
provide apparatus and methods by which dispersion interferometry
may be combined with refractometry to compensate for both short and
long term index of refraction fluctuations that may occur in the
measurement path of an interferometer.
[0008] It is another object of this invention to provide apparatus
and methods by which the inverse dispersion power may be
initialized and monitored prior to displacement calculations.
[0009] It is yet another object of the invention to provide
apparatus and methods by which index of refraction may be
determined by using known physical lengths.
[0010] Other objects will in part be obvious and will in part
appear hereinafter when the following detailed description is read
in connection with the drawings.
SUMMARY OF THE INVENTION
[0011] The invention combines dispersion interferometry with
refractometry to compensate for refractive index fluctuations over
both short and long time periods. It, accordingly, includes a
method and means for weighting the dispersion and refractometry
data, as well as a method and means of initializing .GAMMA. so that
the dispersion and refractometry data are self consistent and can
be used to accurately calculate physical displacements.
[0012] The inventive apparatus comprises:
[0013] interferometer means employing at least two wavelengths at
least a first one of which is used for measuring the displacement
of an object along a measurement path and for detecting short-term
fluctuations in the refractive index of air directly within the
measurement path by means of the inverse dispersive power .GAMMA.
or its equivalent;
[0014] at least one refractometer means for measuring the long-term
variations in refractive index of air directly for at least the
first wavelength, placed as close as practicable to the measurement
path, and preferably within the path of any forced airflow directed
at the measurement path;
[0015] initialization means for establishing an initial value of
the inverse dispersive power .GAMMA. using refractometer and
dispersion data acquired during a change in the length of the
measurement path; and
[0016] computational means for analyzing data from the
refractometer and said dispersion interferometer and for providing
a calculated refractive index along said measurement path, said
calculated refractive index incorporating both long- and short-term
fluctuations and employing said initial value of said inverse
disperse power .GAMMA..
[0017] In another aspect the interferometer means may be in the
form of a separate displacement interferometer operating at the
first wavelength and a second dispersion interferometer operating
at two wavelengths, one of which may be the first wavelength.
[0018] Another aspect of the invention is a method comprising the
steps of:
[0019] storing a self-consistent value of inverse dispersive power,
.GAMMA.. Initially, this may be assumed and later updated as
significant changes in it occur;
[0020] measuring the refractive index for .lambda..sub.1 using a
refractometer located near the measurement path;
[0021] determining a time average for the refractive index
generated in step over a characteristic time period (Eq. 3)
[0022] measuring the optical path length of the measurement path
for wavelengths, .lambda..sub.1,2 (Eq. 2);
[0023] calculating the local index, N.sub.1.sup.2.lambda. using
dispersion interferometry (Eq. 6);
[0024] time averaging the local dispersion (Similar to Eq. 3);
[0025] calculating the fluctuation the local index as the
difference between the instantaneous value and time averaged value
(Eq. 4);
[0026] calculating the physical distance corrected for atmospheric
effects (Eq. 6; and
[0027] testing for the difference between the time averaged value
of the local index and the time averaged value of the index at the
refractometer;
[0028] calibrating .GAMMA. and updating its value should .GAMMA.
change significantly.
[0029] Another aspect of the invention relates to a method for
calibrating the inverse dispersive power comprising the steps
of:
[0030] measuring the refractive index for .lambda..sub.1 using a
refractometer near the measurement path;
[0031] determining the time average of the refractive index near
the measurement path over a characteristic time T;
[0032] moving the stage between two positions;
[0033] measuring the change in optical path length of the
measurement path for two wavelengths, .lambda..sub.1,2;
[0034] calculating the inverse dispersive power, .GAMMA., using Eq.
1; and
[0035] returning to the measurement path with the value for
.GAMMA..
[0036] In yet another aspect of the invention, the index is
determined by using a known physical distance between two reference
points instead of a refractometer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] The structure and operation of the invention, together with
other objects and advantages thereof, may best be understood by
reading the detailed description in conjunction with the drawings
wherein the invention's parts have an assigned reference numeral
that is used to identify them in all of the drawings in which they
appear and wherein:
[0038] FIG. 1 is a diagrammatic view of an interferometric system
of the invention;
[0039] FIG. 2 is a diagrammatic view of a refractometer that may be
employed in the apparatus of FIG. 1;
[0040] FIGS. 3a and 3b are flowcharts of steps of methods for
practicing the invention;
[0041] FIG. 3a representing the steps for making measurements
and
[0042] FIG. 3b the steps for initializing for the inverse
dispersive power, .GAMMA., to update values of .GAMMA. used in the
measurement steps;
[0043] FIG. 4 is a diagrammatic view of an alternative apparatus of
the invention employing a second harmonic generator (SHG)
dispersion interferometer;
[0044] FIG. 5 is a diagrammatic view of an embodiment of the
invention employing a dual-wavelength refractometer;
[0045] FIG. 6 is a diagrammatic view of an embodiment of the
invention employing alignment marks to initialize both refractive
index and inverse dispersive power.
[0046] FIGS. 7-9 relate to lithography and its application to
manufacturing integrated circuits wherein
[0047] FIG. 7 is a schematic drawing of a lithography exposure
system employing the interferometry system;
[0048] FIGS. 8 and 9 are flowcharts describing steps in
manufacturing integrated circuits; and
[0049] FIG. 10 is a schematic of a beam writing system employing
the interferometry system of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0050] This invention relates to interferometric apparatus and
methods by which short and long term index of refraction
fluctuations in the measurement path of a displacement measurement
interferometer may be compensated for in determining displacement.
The invention combines dispersion interferometry with refractometry
to compensate for refractive index fluctuations over both short and
long time periods.
[0051] Included are a method and means for weighting dispersion and
refractometry data, as well as a method and means of initializing
.GAMMA., the inverse dispersive power, so that the dispersion and
refractometry data are self consistent.
[0052] Referring now to FIG. 1, there is diagrammatically shown at
10 an interferometric system of the invention. System 10 comprises
interferometric apparatus 12 and a computer 14 that interfaces with
the interferometric apparatus 12 in a well-known manner via link 16
to exchange housekeeping and control signals. Resident in computer
14 is application software for implementing various algorithms that
are subsequently described and system software by which a user can
operate system 10 through the use of a graphical user interface
(GUI) and/or other input devices such as a keyboard or mouse.
[0053] Interferometric apparatus 12 preferably comprises a
two-wavelength source 18, with a first wavelength .lambda..sub.1
and a second wavelength .lambda..sub.2 that is phase locked to the
first wavelength .lambda..sub.1 by, e.g., means of frequency
doubling (SHG=second harmonic generation). Example wavelengths are
633 and 316 nm for .lambda..sub.1,2, respectively, as measured in a
vacuum. Source 18 generates a first beam 20 at .lambda..sub.1 and a
second beam 22 at .lambda..sub.2.
[0054] It will be appreciated by those skilled in the art that
beams 20 and 22 may be provided alternatively by a single laser
source emitting more than one wavelength, two laser sources of
differing wavelengths combined with sum-frequency generation or
difference-frequency generation, or any equivalent source
configuration capable of generating light beams of two or more
wavelengths.
[0055] A laser source, for example, can be a gas laser, e.g. a
HeNe, stabilized in any of a variety of conventional techniques
known to those skilled in the art, see for example, T. Baer et al.,
"Frequency Stabilization of a 0.633 .mu.m He--Ne-longitudinal
Zeeman Laser," Applied Optics, 19, 3173-3177 (1980); Burgwald et
al., U.S. Pat. No. 3,889,207, issued Jun. 10, 1975; and Sandstrom
et al., U.S. Pat. No. 3,662,279, issued May 9, 1972. Alternatively,
the laser can be a diode laser frequency stabilized by one of a
variety of conventional techniques known to those skilled in the
art, see, for example, T. Okoshi and K. Kikuchi, "Frequency
Stabilization of Semiconductor Lasers for Heterodyne-type Optical
Communication Systems," Electronic Letters, 16, 179-181 (1980) and
S. Yamaqguchi and M. Suzuki, "Simultaneous Stabilization of the
Frequency and Power of an AlGaAs Semiconductor Laser by Use of the
Optogalvanic Effect of Krypton," IEEE J. Quantum Electronics,
QE-19, 1514-1519 (1983).
[0056] Beam 20 is intercepted by a beamsplitter 24 that directs
part of it by reflection to a refractometer 26 while transmitting
the remainder of it to a mirror 28.
[0057] Refractometer 26 may be any of the common types known
variously as wavelength trackers or wavelength compensators. It may
be combined with environmental sensors (not shown), particularly
for initializing system 10 to an absolute index of refraction. FIG.
2 shows an example of refractometer 26, based on a type known as a
differential plane-mirror interferometer. As shown, refractometer
26 comprises an interferometer section 30 that directs portions of
beam 20 into a concentrically configured measurement cell 32 of
fixed length L. Cell 32 comprises an evacuated inner chamber 34
that is surrounded by an outer annular region 36 occupied by gas,
preferably air. End reflectors 40 and 42, respectively, are
provided to control the propagation of beam 20 as it travels
through cell 32. In operation, inner chamber 34 is assigned the
role of a reference leg and outer chamber 36 that of a surrogate
for the gas or air occupying measurement leg over which physical
distance is to be measured. The output of refractometer 26 is a
refractive index N.sub.1 for the wavelength .lambda..sub.1.
[0058] Mirror 28, as well as intercepting the portion of beam 20
transmitted by beamsplitter 24, intercepts beam 22 and directs it
and the remainder of beam 20 to a two-wavelength interferometer 44.
Interferometer 44 is adapted to both measure the distance between
it and an X-Y translation stage 46, which may form part of a
photolithographic apparatus for the fabrication of integrated
circuits or the like, and to provide information about the local
index of refraction, N.sub.1.sup.2.lambda., in the measurement
path. The measurement path is located in the intervening space
between interferometer 44 and X-Y translation stage 46. As can be
seen in FIG. 1, air is controllably flowed over the refractometer
26, i.e., its outer chamber 36 and over the measurement path of
interferometer 44. Refractometer 26 is preferably positioned close
enough to the measurement path of interferometer 44 so that it can
be assumed that the index of refraction of the air in both is
substantially the same, at least over measurement time intervals of
interest.
[0059] Interferometer 44 here is a dispersion type for which one of
the wavelengths, e.g., .zeta..sub.1, is also used for standard DMI.
Thus, the output of the 2-.lambda. interferometer 44 is the
interference phases .phi..sub.1,2 corresponding to
.lambda..sub.1,2, respectively. Interferometer 44 may be any of a
variety of well-known types including a common-wavelength
differential plane mirror interferometer (DPMI) with each
wavelength combined with a dichroic beamsplitter or a
two-wavelength dynamic interferometer of the type described in
commonly owned co-pending U.S. patent application Ser. No.
09/157,131 filed on Sep. 18, 1998, the contents of which are
incorporated herein by reference. System 10 is operated in the
manner set forth in the flowcharts of FIGS. 3a and 3b which will be
described in more detail later. FIG. 3a represents the measurement
mode of operation of system 10 and FIG. 3b the method for
initializing .GAMMA..
[0060] To initialize .GAMMA., X-Y stage 46 shown in FIG. 1 is moved
so as to change the physical length of the DMI measurement path
over as large a range as possible. The exact value of the
displacement need not be known. The move may be part of an
alignment procedure on a wafer (more on this later) or it may be an
independent motion prior to wafer alignment. The initialized,
self-consistent value of .GAMMA. is then 1 = N 1 - 1 N 1 L 1 ( L 2
- L 1 ) , where ( 1. ) L 1 , 2 = 1 , 2 ' 1 , 2 / 2 , ( 2. )
[0061] and .phi.'.sub.1,2 are the changes in the values of
.phi..sub.1,2 resulting from the displacement of the X-Y stage 46.
The quantity <N.sub.1>is the time average of the refractive
index N.sub.1 calculated from data supplied by the refractometer
26.
[0062] The time averaging indicated by the brackets <> may
be, for example, the sum of a series of previous measurements,
taken over a time period T considered to be sufficiently long to
average out air turbulence: 2 N = 1 T t - T t N t ( 3. )
[0063] Other appropriate means for calculating the time average
include recursive formulas that impart an exponential or other
functional dampening having a characteristic time T.
[0064] After initialization of .GAMMA., the 2-wavelength dispersion
interferometer provides a continuous measure of index fluctuations
.sub.1.sup.2.lambda. local to the measurement path: 3 N ~ 1 2 = N 1
2 - N 1 2 where ( 4. ) N 1 2 = L 1 L 1 - ( L 2 - L 1 ) . ( 5. )
[0065] and the brackets <> describe a time averaging over a
characteristic time constant T. The DMI measurement of the physical
(i.e. index corrected) displacement is 4 = L 1 N 1 + N ~ 1 2 ( 6.
)
[0066] This calculation is compensated for both long term and short
term fluctuations in the index of refraction in the measurement
path.
[0067] The foregoing calculations involve a characteristic time
constant T, which separates short term (T<t) and long term
measurements (T>t). Short-term measurements of the index of
refraction rely principally on dispersion interferometry, whereas
long-term measurements rely principally on the refractometer or
equivalent means shown in FIG. 1. Conceptually, T may be regarded
as the minimum time interval for which the time-integrated value
<N.sub.1> measured at the refractometer 26 correlates within
10.sup.-9 to the true time-integrated index of refraction in the
measurement path. Generally, the longer the time T, the better the
correlation. A quantitative approach to determining a quantitative
value of T is to consider the effect of pockets or cells of air of
various sizes moving with a velocity determined by the forced
airflow within the system. Suppose a sinusoidal fluctuation in
refractive index of amplitude A and a spatial period .LAMBDA.. Now
imagine a refractometer placed at a distance D<<.LAMBDA. from
the measurement path and let the air flow in the z direction at a
velocity .nu.. The resulting maximum error will be
E.apprxeq.2.pi.AD/.LAMBDA.. (7.)
[0068] Now, suppose that the objective of the compensation system
is to reduce refractive index errors by at least a factor of three.
This requirement translates to:
.LAMBDA.>6.pi.D. (8.)
[0069] For air moving at a velocity .nu., this corresponds to a
characteristic time
T=6.pi.D/.nu.. (9.)
[0070] For example, if the airflow is .nu.=0.5 m/s and the
refractometer is 0.5 m from the measurement path, then T is
approximately 20 s.
[0071] Referring now to FIGS. 3a and 3b, there are shown flowcharts
depicting the steps for practicing the method of the invention. As
seen, these are given in blocks 50-59 in FIG. 3a, which is for
measurement and in blocks 61-71 in FIG. 3b, which is for
calibrating .GAMMA.. In FIG. 3a, the measurement steps include:
[0072] (50) storing a self-consistent value of inverse dispersive
power, .GAMMA.. Initially, this may be assumed and later updated
from step (59) as significant changes in it occur;
[0073] (51) measuring the refractive index for .lambda..sub.1 using
a refractometer located near the measurement path;
[0074] (52) determining a time average for the refractive index
generated in step (50) over a characteristic time period (Eq.
3)
[0075] (53) measuring the optical path length of the measurement
path for wavelengths, .lambda..sub.1,2 (Eq. 2);
[0076] (54) calculating the local index, N.sub.1.sup.2.lambda.
using dispersion interferometry (Eq. 5);
[0077] (55) time averaging the local dispersion (Similar to Eq.
3);
[0078] (56) calculating the fluctuation the local index as the
difference between the instantaneous value and time averaged value
(Eq. 4);
[0079] (57) calculating the physical distance corrected for
atmospheric effects (Eq. 6); and
[0080] (58) testing for the difference between the time averaged
value of the local index and the time averaged value of the index
at the refractometer; and
[0081] (59) calibrating .GAMMA. and updating its value to block
(50) should .GAMMA. change significantly.
[0082] Referring now to FIG. 3b, the steps for calibrating .GAMMA.
are seen to comprise:
[0083] (61) measuring the refractive index for .lambda..sub.1 using
a refractometer near the measurement path;
[0084] (63) determining the time average of the refractive index
near the measurement path over a characteristic time T;
[0085] (65) moving the stage between two positions;
[0086] (67) measuring the change in optical path length of the
measurement path for two wavelengths, .lambda..sub.1,2;
[0087] (69) calculating the inverse dispersive power, .GAMMA.,
using Eq. 1; and
[0088] (71) returning to the measurement path with the value for
.GAMMA..
[0089] Various benefits flow from the invention compared with the
prior art. With respect to a simple refractometer, the invention
provides superior compensation for short-term fluctuations in
refractive index within the measurement beam that cannot be
detected with a refractometer alone.
[0090] With respect to an ACT system, the invention provides
superior long-term compensation with respect to dispersion
interferometry by: (1) providing a self-consistent .GAMMA. that is
more accurate than a calculated value; and (2) providing long-term
stability in the compensation in the presence of changes in
atmospheric composition.
[0091] With respect to compensation systems that rely on dispersion
for the computation of .GAMMA., the invention provides lower cost
and potentially more accurate long-term compensation by relying on
refractometry for refractive index measurements over long time
periods. With the inventive apparatus, the dispersion
interferometers need only be stable over a period of approximately
one minute or even less, rather than several hours. This greatly
relaxes requirements on the optics, mechanics, electronics and
fiber coupling.
[0092] As those skilled in the art will appreciate, there are many
possible variants of the invention without departing from its
essential teachings.
[0093] For example, shown in FIG. 4 is an alternate apparatus of
the invention designated generally at 60. Apparatus 60 employs two
independent lasers, 62 and 64, operating at wavelengths
.lambda..sub.1,2, respectively, to generate beams 66 and 68,
respectively. A second-harmonic generation (SHG) interferometer 69
of the type, for example, described in U.S. Pat. No. 4,948,254 by
Ishida, is provided to receive beam 68 and generate an additional
beam 70 at .lambda..sub.3. As before, a refractometer 74 receives
beam 66 at .lambda..sub.1 and the displacement of an X-Y stage 72
is to be measured. The operating principles of this embodiment are
substantially the same as for the prior embodiment. However, in
this case, the dispersion calculation involves predicting the index
of refraction at a first wavelength .lambda..sub.1 using a second
and third wavelength .lambda..sub.2, 3 that may be very different
from the first. The initialization equation changes to 5 ' = N 1 -
1 N 1 L 1 ( L 3 - L 2 ) , ( 10. )
[0094] and the refractive index calculations change to 6 n 1 = N 1
+ N 1 2 - N 1 2 ( 11. ) N 1 2 = 1 + ' ( L 3 - L 2 ) N 1 L 1 . ( 12.
)
[0095] Note that .GAMMA.' is no longer the inverse dispersive power
in the usual sense, but serves an analogous function in the
calculations.
[0096] In another embodiment shown in FIG. 5, the simple,
single-wavelength refractometer 26 shown in FIG. 1 has been
replaced by a form of .GAMMA.-meter, shown symbolically in FIG. 5
as a dual-wavelength refractometer 80 with other parts bearing the
same numerical identifiers as their counterparts in FIG. 1. This
embodiment is capable of tracking slow changes in the value of
.GAMMA. without having to re-initialize the system with a stage
motion. However, for long-term changes in refractive index, this
embodiment continues to rely on the refractometry data alone,
rather than on dispersion interferometry.
[0097] In yet another embodiment shown in FIG. 6, there is no
refractometer at all. Instead, reliance is made on alignment marks
92 carried on X-Y translation stage 46 (again parts in common with
FIG. 1 bear the same numerical identification here) and an
alignment sensor 90 for locating these marks. The basic assumption
of this embodiment is that the physical path length .chi..sub.0
between the marks 92 is either perfectly known or is a suitable
reference length for all subsequent measurements. After a
displacement between the marks 92, the initial index of refraction
is determined from
N.sub.1.sup.0=L.sub.1/.chi..sub.0, (13.)
[0098] where L.sub.1 is the measured optical path for wavelength
.lambda..sub.1. The self-consistent .GAMMA. is 7 = ( N 1 0 - 1 ) 0
( L 2 - L 1 ) . ( 14. )
[0099] All subsequent measurements of a physical displacement .chi.
use
.chi.=L.sub.1/n.sub.1, (15.)
where 8 n 1 = 1 + ( L 2 + L 1 ) 0 . ( 16. )
[0100] This embodiment has the advantage of the greatest simplicity
in hardware; however, it may not be suitable for applications which
require very long term measurement stability independent of the
stability in the physical location of the alignment marks.
[0101] The interferometry systems described above can be especially
useful in lithography applications associated with their X-Y stages
and used for fabricating large scale integrated circuits such as
computer chips and the like. Lithography is the key technology
driver for the semiconductor manufacturing industry. Overlay
improvement is one of the five most difficult challenges down to
and below 100 nm line widths (design rules), see for example the
Semiconductor Industry Roadmap, p82 (1997). Overlay depends
directly on the performance, i.e. accuracy and precision, of the
distance measuring interferometers used to position the wafer and
reticle (or mask) stages. Since a lithography tool may produce
$50-100 M/year of product, the economic value from improved
performance distance measuring interferometers is substantial. Each
1% increase in yield of the lithography tool results in
approximately $1 M/year economic benefit to the integrated circuit
manufacturer and substantial competitive advantage to the
lithography tool vendor.
[0102] The function of a lithography tool is to direct spatially
patterned radiation onto a photoresist-coated wafer. The process
involves determining which location of the wafer is to receive the
radiation (alignment) and applying the radiation to the photoresist
at that location (exposure).
[0103] To properly position the wafer, the wafer includes alignment
marks on the wafer that can be measured by dedicated sensors. The
measured positions of the alignment marks define the location of
the wafer within the tool. This information, along with a
specification of the desired patterning of the wafer surface,
guides the alignment of the wafer relative to the spatially
patterned radiation. Based on such information, a translatable
stage supporting the photoresist-coated wafer moves the wafer such
that the radiation will expose the correct location of the
wafer.
[0104] During exposure, a radiation source illuminates a patterned
reticle, which scatters the radiation to produce the spatially
patterned radiation. The reticle is also referred to as a mask, and
these terms are used interchangeably below. In the case of
reduction lithography, a reduction lens collects the scattered
radiation and forms a reduced image of the reticle pattern.
Alternatively, in the case of proximity printing, the scattered
radiation propagates a small distance (typically on the order of
microns) before contacting the wafer to produce a 1:1 image of the
reticle pattern. The radiation initiates photo-chemical processes
in the photoresist that convert the radiation pattern into a latent
image within the photoresist.
[0105] The interferometry systems described above are important
components of the positioning mechanisms that control the position
of the wafer and reticle, and register the reticle image on the
wafer.
[0106] In general, the lithography system, also referred to as an
exposure system, typically includes an illumination system and a
wafer positioning system. The illumination system includes a
radiation source for providing radiation such as ultraviolet,
visible, x-ray, electron, or ion radiation, and a reticle or mask
for imparting the pattern to the radiation, thereby generating the
spatially patterned radiation. In addition, for the case of
reduction lithography, the illumination system can include a lens
assembly for imaging the spatially patterned radiation onto the
wafer. The imaged radiation exposes photoresist coated onto the
wafer. The illumination system also includes a mask stage for
supporting the mask and a positioning system for adjusting the
position of the mask stage relative to the radiation directed
through the mask. The wafer positioning system includes a wafer
stage for supporting the wafer and a positioning system for
adjusting the position of the wafer stage relative to the imaged
radiation. Fabrication of integrated circuits can include multiple
exposing steps. For a general reference on lithography, see, for
example, J. R. Sheats and B. W. Smith, in Microlithography: Science
and Technology (Marcel Dekker, Inc., New York, 1998), the contents
of which are incorporated herein by reference.
[0107] The interferometry systems described above can be used to
precisely measure the positions of each of the wafer stage and mask
stage relative to other components of the exposure system, such as
the lens assembly, radiation source, or support structure. In such
cases, the interferometry system can be attached to a stationary
structure and the measurement object attached to a movable element
such as one of the mask and wafer stages. Alternatively, the
situation can be reversed, with the interferometry system attached
to a movable object and the measurement object attached to a
stationary object.
[0108] More generally, the interferometry systems can be used to
measure the position of any one component of the exposure system
relative to any other component of the exposure system in which the
interferometry system is attached, or supported by one of the
components and the measurement object is attached, or is supported
by the other of the components.
[0109] An example of a lithography scanner 100 using an
interferometry system 126 is shown in FIG. 7. The interferometry
system is used to precisely measure the position of a wafer within
an exposure system. Here, stage 122 is used to position the wafer
relative to an exposure station. Scanner 100 comprises a frame 102,
which carries other support structures and various components
carried on those structures. An exposure base 104 has mounted on
top of it a lens housing 106 atop of which is mounted a reticle or
mask stage 116 used to support a reticle or mask. A positioning
system for positioning the mask relative to the exposure station is
indicated schematically by element 117. Positioning system 117 can
include, e.g., piezoelectric transducer elements and corresponding
control electronics. Although, it is not included in this described
embodiment, one or more of the interferometry systems described
above can also be used to precisely measure the position of the
mask stage as well as other moveable elements whose position must
be accurately monitored in processes for fabricating lithographic
structures (see supra Sheats and Smith Microlithography: Science
and Technology).
[0110] Suspended below exposure base 104 is a support base 113 that
carries wafer stage 122. Stage 122 includes a plane mirror for
reflecting a measurement beam 154 directed to the stage by
interferometry system 126. A positioning system for positioning
stage 122 relative to interferometry system 126 is indicated
schematically by element 119. Positioning system 119 can include,
e.g., piezoelectric transducer elements and corresponding control
electronics. The measurement beam reflects back to the
interferometry system, which is mounted on exposure base 104. The
interferometry system can be any of the embodiments described
previously.
[0111] During operation, a radiation beam 110, e.g., an ultraviolet
(UV) beam from a UV laser (not shown), passes through a beam
shaping optics assembly 112 and travels downward after reflecting
from mirror 14. Thereafter, the radiation beam passes through a
mask (not shown) carried by mask stage 116. The mask (not shown) is
imaged onto a wafer (not shown) on wafer stage 122 via a lens
assembly 108 carried in a lens housing 106. Base 104 and the
various components supported by it are isolated from environmental
vibrations by a damping system depicted by spring 120.
[0112] In other embodiments of the lithographic scanner, one or
more of the interferometry systems described previously can be used
to measure distance along multiple axes and angles associated for
example with, but not limited to, the wafer and reticle (or mask)
stages. Also, rather than a UV laser beam, other beams can be used
to expose the wafer including, e.g., x-ray beams, electron beams,
ion beams, and visible optical beams.
[0113] In addition, the lithographic scanner can include a column
reference in which interferometry system 126 directs the reference
beam to lens housing 106 or some other structure that directs the
radiation beam rather than a reference path internal to the
interferometry system. The interference signal produced by
interferometry system 126 when combining measurement beam 154
reflected from stage 122 and the reference beam reflected from lens
housing 106 indicates changes in the position of the stage relative
to the radiation beam. Furthermore, in other embodiments the
interferometry system 126 can be positioned to measure changes in
the position of reticle (or mask) stage 116 or other movable
components of the scanner system. Finally, the interferometry
systems can be used in a similar fashion with lithography systems
involving steppers, in addition to, or rather than, scanners.
[0114] As is well known in the art, lithography is a critical part
of manufacturing methods for making semiconducting devices. For
example, U.S. Pat. No. 5,483,343 outlines steps for such
manufacturing methods. These steps are described below with
reference to FIGS. 8 and 9. FIG. 8 is a flowchart of the sequence
of manufacturing a semiconductor device such as a semiconductor
chip (e.g. IC or LSI), a liquid crystal panel or a CCD. Step 251 is
a design process for designing the circuit of a semiconductor
device. Step 252 is a process for manufacturing a mask on the basis
of the circuit pattern design. Step 253 is a process for
manufacturing a wafer by using a material such as silicon.
[0115] Step 254 is a wafer process which is called a pre-process
wherein, by using the so prepared mask and wafer, circuits are
formed on the wafer through lithography. Step 255 is an assembling
step, which is called a post-process wherein the wafer processed by
step 254 is formed into semiconductor chips. This step includes
assembling (dicing and bonding) and packaging (chip sealing). Step
256 is an inspection step wherein operability check, durability
check, and so on of the semiconductor devices produced by step 255
are carried out. With these processes, semiconductor devices are
finished and they are shipped (step 257).
[0116] FIG. 9 is a flowchart showing details of the wafer process.
Step 261 is an oxidation process for oxidizing the surface of a
wafer. Step 262 is a CVD process for forming an insulating film on
the wafer surface. Step 263 is an electrode forming process for
forming electrodes on the wafer by vapor deposition. Step 264 is an
ion implanting process for implanting ions to the wafer. Step 265
is a photoresist process for applying a photoresist (photosensitive
material) to the wafer. Step 266 is an exposure process for
printing, by exposure, the circuit pattern of the mask on the wafer
through the exposure apparatus described above. Step 267 is a
developing process for developing the exposed wafer. Step 268 is an
etching process for removing portions other than the developed
photoresist image. Step 269 is a photoresist separation process for
separating the photoresist material remaining on the wafer after
being subjected to the etching process. By repeating these
processes, circuit patterns are formed and superimposed on the
wafer.
[0117] The interferometry systems described above can also be used
in other applications in which the relative position of an object
needs to be measured precisely. For example, in applications in
which a write beam such as a laser, x-ray, ion, or electron beam,
marks a pattern onto a substrate as either the substrate or beam
moves, the interferometry systems can be used to measure the
relative movement between the substrate and write beam.
[0118] As an example, a schematic of a beam writing system 300 is
shown in FIG. 10 A source 310 generates a write beam 312, and a
beam focusing assembly 314 directs the radiation beam to a
substrate 316 supported by a movable stage 318. To determine the
relative position of the stage, an interferometry system 320
directs a reference beam 322 to a mirror 324 mounted on beam
focusing assembly 314 and a measurement beam 326 to a mirror 328
mounted on stage 318. Interferometry system 320 can be any of the
interferometry systems described previously. Changes in the
position measured by the interferometry system correspond to
changes in the relative position of write beam 312 on substrate
316. Interferometry system 320 sends a measurement signal 332 to
controller 330 that is indicative of the relative position of write
beam 312 on substrate 316. Controller 330 sends an output signal
334 to a base 336 that supports and positions stage 318. In
addition, controller 330 sends a signal 338 to source 310 to vary
the intensity of, or block, write beam 312 so that the write beam
contacts the substrate with an intensity sufficient to cause
photophysical or photochemical change only at selected positions of
the substrate. Furthermore, in some embodiments, controller 330 can
cause beam focusing assembly 314 to scan the write beam over a
region of the substrate, e.g., using signal 344. As a result,
controller 330 directs the other components of the system to
pattern the substrate. The patterning is typically based on an
electronic design pattern stored in the controller. In some
applications the write beam patterns a photoresist coated on the
substrate and in other applications the write beam directly
patterns, e.g., etches, the substrate.
[0119] An important application of such a system is the fabrication
of masks and reticles used in the lithography methods described
previously. For example, to fabricate a lithography mask an
electron beam can be used to pattern a chromium-coated glass
substrate. In such cases where the write beam is an electron beam,
the beam writing system encloses the electron beam path in a
vacuum. Also, in cases where the write beam is, e.g., an electron
or ion beam, the beam focusing assembly includes electric field
generators such as quadrapole lenses for focusing and directing the
charged particles onto the substrate under vacuum. In other cases
where the write beam is a radiation beam, e.g., x-ray, UV, or
visible radiation, the beam focusing assembly includes
corresponding optics for focusing and directing the radiation to
the substrate.
[0120] Yet other changes may be made to the invention. For example,
it may be desirable in certain applications to monitor the
refractive index of the gas contained on both the reference and in
the measurement legs of the interferometer. Examples include the
well-known column reference style of interferometer, in which the
reference leg comprises a target optic placed at one position
within a mechanical system, and the measurement leg comprises a
target optic placed at a different position within the same
mechanical system. Another example relates to application of the
measurement of small angles, for which both the measurement and
reference beams impinge upon the same target optic but at a small
physical offset, thereby providing a sensitive measure of the
angular orientation of the target optic. In addition, it will be
appreciated that a separate displacement interferometer may be
employed with a separate dispersion interferometer. These
applications and configurations are well known to those skilled in
the art and the necessary modifications are intended to be within
the scope of the invention.
* * * * *