New Developments in Deep Ultraviolet Laser Metrology for
Photolithography

MLL. Dowell, C.L. Cromer, R.D. Jones, D.A. Keenan, and T.R. Scott

National Institute of Standards and Technology
M/S 815.01; 325 Broadway; Boulder, CO 80303-3328 USA

Abstract. Current and future laser measurement services at 157, 193 and 248 nm will be reviewed. Laser power and
energy measurements at 193 nm will be presented; electrical calibration issues will be reviewed. We report an overall

calibration uncertainty of laser
ultraviolet materials also will be discussed.

INTRODUCTION

Increasing information-technology requirements
have yielded a strong demand for faster logic circuits
and higher density memory chips. This demand has led
to the introduction of deep ultraviolet (DUV) laser-
based lithographic tools for semiconductor manu-
facturing. These tools, which employ KrF (248 nm) and
ArF (193 nm) excimer lasers, have led to an increased
demand for accurate laser measurements at the DUV
laser wavelengths. As a result, the National Institute of
Standards and Technology (NIST), with SEMATECH
support, has developed primary standard calorimeters
for excimer laser power and energy measurements at
both 193 and 248 nm.

UV LASER MEASUREMENTS

There are a number of laser measurements that are
important for both tool development and performance.
Measurements of a laser’s power and energy are used to
help stabilize the source’s pulse energy and to optimize
laser dose at the wafer plane. Optical materials
characterization measurements are used in tool design
and modeling.

Laser Power and Energy

At this time, NIST is the only national laboratory in
the world to offer excimer laser power and energy
calibration services. Primary standards and associated
measurement systems have been developed to support
measurements with KrF and ArF excimer lasers. An
157 nm primary standard and measurement system is

power and energy meters of less than 2 %. Characterization measurements for

under development. The KrF primary standards and
measurement system at 248 nm has been discussed in
detail elsewhere [1,2]. Therefore, we limit this dis-
cussion to the 193 nm primary standards and
measurement services.

Primary Standards

The design of the 193 nm primary standard is simi-
lar to that of earlier NIST pulsed-laser calorimeters {1-
3}. A schematic drawing of the 193 nm primary
standard is shown in Figure 1. Optical energy captured
within the cavity is converted into thermal energy.
Twenty pairs of thermocouples, connected in series,
record the temperature difference between the cavity
and a surrounding constant-temperature jacket. The
calorimeter’s thermal response is accurately calibrated
by injecting a fixed amount of electrical energy into the
cavity through an electrical heater attached to the rear
of the cavity (Figure 2). A plot of the electrical
calibration factor as a function of injected energy is
shown in Figure 3.

Thermocouples

Thermocouple a
Leads = 1
| Volume At
Heater Leads | ‘olume Absorbers
= rT a |
Cavity
Electrical
Heater
Figure 1. Schematic of 193 nm Calorimeter.

CP550, Characterization and Metrology for ULSI Technology: 2000 International Conference,
edited by D. G. Seiler, A. C. Diebold. T. J. Shaffner, R. McDonald. W. M. Bullis, P. J. Smith, and E. M. Secula
2001 American Institute of Physics 1-56396-967-X

361


The amount of optical energy E injected into the
calorimeter is determined by solving the function,

E=K{V,-V, +e[’ (V-v. a]. et)

where K is the electrical calibration factor and € is the
calorimeter’s cooling constant. The calorimeter’s
cooling constant is determined by fitting both the rating
periods data with a single exponential function. During
the rating period, the cavity cooling constant is
dominant and the calorimeter thermal response may be
characterized by a single exponential coefficient €.

Calibration Method

A diagram of the NIST laser measurement system is
shown in Figure 4. A pair of 193 nm primary standard
calorimeters is used in conjunction with a specially
designed calibration measurement system consisting of
a nitrogen-purged enclosure; a 2° wedged, fused silica
beamsplitter, and beam-shaping optics. The incident
laser beam is split by the beamsplitter. The beamsplitter

a ay _
1 Y N 4
08 4
E06 4
> 04 >
02 BZ 4
00 Pera o ood ib staal
0 100 200 300
t(s)
Figure 2. Series Thermocouple Voltage Response versus

time. In this example, 11.66 J of optical energy was injected
into the calorimeter over a 49.96 s interval (gray shaded
region). The hatched regions represent the rating periods
before and after energy was injected into the calorimeter. The
calorimeter responses to injections of optical and electrical
energy are equivalent.

90 T root a
89 — + Nsec 4
588
A
537
< 86
85
84 desi tiiitiiitiiitir
2 4 6 8 10 12 4
Energy (J)
Figure 3. Electrical Calibration Factor Versus Injected

Energy for Two Injection Times. The electrical calibration
factor K was determined by injecting a fixed amount of
electrical energy into the calorimeter during a given interval
and solving for K in Equation (1) (Figure 2). The calibration
factor is relatively insensitive to injection time.

362

ratio, Bg, is determined from a series of energy
measurements using the two primary standards,
QDUV-1 and QDUV-2. The value for this ratio,

Bg = (transmitted energy)/(reflected energy), (2)

is approximately 19 (Figure 5). Then, the detector
under test is substituted for one of the standards and
the beamsplitter ratio Bpyr is measured. The detector
calibration factor x is determined from the ratio of
these two beamsplitter values,

K = Byyr/Bg- (3)

With judicious selection of transmitted and reflected
beams, this system can span a range of four decades in
laser power and energy [4]. Calibration results are
shown in Figure 6.

—

[ | Device Under Test
(— ArF Excimer Laser | fi

SiO.
Beam Splitter
in

\

x a
Voltmeter, Scanner
& Power Supply

Figure 4. NIST UV measurement system for 193 nm laser
power and energy measurements. QDUV-1 and QDUV-2 are
the primary standard calorimeters. A 2° wedged, fused silica
(SiO.) beamsplitter is used for monitoring the power and
providing attenuation. The entire system is placed within a
nitrogen-purged enclosure. This 193 nm system is similar to
the 248 nm measurement system, although the 248 nm
measurements are performed in air rather than nitrogen.

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Figure 5. Beamsplitter Ratio Bg. These results are derived
from data that were taken under three distinct operating
conditions. The mean value of these data sets is 18.984, with
a standard deviation of 1.62%. Beamsplitter measurements
are performed before and after each set of calibration
measurements to ensure consistent calibration results.


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Figure 6. Calibration Results. The detector under test, a

25 mm diameter pyroelectric detector, was calibrated at .wo
incident laser powers, 10 and 100 mW. For each power
setting, a series of calibration runs were performed. The
calibration factor x (solid and dashed lines) was determined
by computing the mean value of the individual runs. The
overall uncertainty of the calibration, including Type A and
Type B contributions, is less than 2 % {1}.

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Figure7. Excimer Laser Transmittance Measurement
System. Transmittance measurement services are available as
a special test.

Optical Materials Characterization

Optical material characterization measurements, such as
transmittance and birefringence, are important for tool
development and performance as well. In 1999, at the
request of SEMATECH, we established an 193 nm
laser transmittance measurement service to resolve
discrepancies between laser- and lamp-based
transmittance measurements. (Figure 7.) At short
wavelengths, a surface cleaning process occurs when
samples are exposed to intense laser light. Therefore, it
is important, particularly in an exposure too] which may

363

contain as many as 30 optical elements, to measure the
transmittance of the individual elements in the same
manner in which they will be used.

Birefringence can be an inherent property of the
material, or it can be introduced during the
manufacturing process or by applying mechanical
stress, such as clamping a mask. In particular, spatial
variations across phase-shifting masks can lead to
spatial variations across exposed wafers. Current
measurements of  stress-optical coefficients and
birefringence are made in the visible at helium-neon
laser wavelengths. We have initiated a two-pronged
effort to provide UV birefringence measurements and
to develop a two-dimensional imaging polarimeter.
This effort will allow us to pinpoint spatial
nonuniformities in photomasks and to provide absolute
retardance measurements at the appropriate UV
wavelengths.

CONCLUSIONS

NIST excimer laser measurement services in support of
DUYV optical lithography have recently been extended
to include ArF excimer laser power and energy
measurements at 193 nm as well as optical materials
characterization measurements. Efforts are underway to
extend these services to include F, excimer lasers at
157 nm as well.

ACKNOWLEDGEMENTS

We thank SEMATECH for their ongoing support of
this program.

REFERENCES

1 Leonhardt, R. W. and Scott, T. R., Integrated Circuit
Metrology, Inspection, and Process Control IX, Proc.
SPIE 2439, 448-459 (1995); Leonhardt, R.W.,
“Calibration Service for Laser Power and Energy at 248
nm.”, NIST Technical Note 1394, Washington; US
Government Printing Office, 1998.

Dowell, M. L., Cromer, C. L., Leonhardt, R. W., and
Scott, T. R., “Deep Ultraviolet Laser Metrology for
Semiconductor Photolithography” in Characterization
and Metrology for ULSI Technology: 1998 International
Conference, edited by D. G. Seiler et al., AIP Conference
Proceedings 449, New York; American Institute of
Physics, 1998, pp. 539-541.

West, E. D., Case, W. E., Rasmussen, A. L., and Schmidt,
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Beers, Y., National Bureau of Standards Monograph
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