Deep Ultraviolet Laser Metrology for Semiconductor
Photolithography
M. L. Dowell, C. L. Cromer, R. W. Leonhardt, T. R. Scott

Sources and Detectors Group, Optoelectronics Division
National Institute of Standards and Technology, Boulder, Colorado 80303-3328

Recent improvements in calibration procedures have led to reductions in overall uncertainties of laser power and

energy calibrations to +1%.
response.

We plan to extend these services to include laser dose measurements and angular
Deviations from cosine behavior for angular response can introduce measurement errors when dose

meters, calibrated with parallel laser beams, are employed in stepper systems. These measurement errors will become
important as variable numerical aperture systems become commonplace. Current and future laser measurement

services at 193 and 248 nm will be reviewed.

INTRODUCTION

Several years ago, the Optoelectronics Division with
Semiconductor Manufacturing Technology (SEMATECH)
support, developed primary standard laser calorimeters for
248 nm excimer laser power and energy measurements
[1].Our efforts are now focused on addressing the
demands for 193 nm measurement capability as quickly as
possible. At the same time, we are adding a laser dose,
ie., energy density, capability to our measurement
services. Accurate measurements of laser dose are crucial
to the development of new mask and resist materials at
248 and 193 nm. Furthermore, transfer standards for dose
measurement will be critical to ensure high accuracy to
the end user. We will be working with industrial partners
to improve transfer standard capabiliti

Since the first draft of the National Technology
Roadmap for Semiconductors in 1992, the semiconductor
industry has made an organized, concentrated effort to
move toward smaller critical dimensions. As a result,
there has been a shift toward shorter laser wavelengths in
the optical lithography process. Ultraviolet (UV) lasers,
specifically KrF (248 nm) and ArF (193 nm) excimer
lasers, will be the preferred sources for high resolution
optical lithography stepper systems until the future shift to
nonoptical lithography for the fabrication of devices with
features smaller than 100 nm. Therefore, UV laser
metrology is crucial for the development of new optical
lithography processes and tools.

LASER CALORIMETRY

Primary Standard Calorimeters
The Optoelectronics Division at NIST has designed and
built several laser calorimeters for accurate measurements

of laser power and energy [2].Current NIST laser
measurement services are listed in Laser calorimetry.
Briefly, a laser calorimeter operates as follows [3]: laser
radiation is injected into a thermally isolated cavity.
Upon absorption, the electromagnetic radiation is
converted into thermal energy. The corresponding
temperature rise is precisely measured with thermal
sensors, such as thermocouples. An electrical heater is
used to calibrate the calorimeter’s temperature response
by injecting equivalent, known electrical energy into the
cavity.

Table 1. NIST laser measurement services.

Laser Power and Energy Calibrations
Laser Wavelength Range
cw ‘Argon Ion 488 and Thw-1W
514 nm
HeNe 633 nm TW -20 mw
Diode 830 nm 100 wW -20 mw
Nd:YAG 1064 nm 100 pW -450 W
1319 nm 100 tW - 10 mw
HeNe 1523 nm 100 pW - 1 mW
CO, 10.6 pm 1 pW - 1 kW
Pulsed KrF Excimer 248 nm TnJ - 200 milpulse
50 nW-9 W
AMF Excimer 193 nm. .
Nd:YAG 1064 nm 1-50 mi/pulse
10° - 10 nJ/pulse
peak power 10 nW - 100 pw

*Available September 1998

The localized temperature rise is related to the volume
of absorbing material over which the laser energy is
deposited. In order to avoid damage due to the short laser
pulses, we use two volume absorbers in the calorimeter’s
cavity to safely absorb the incident radiation (see Figure
1). The volume absorber materials for both UV primary
standards are UV absorbing glasses, chosen for durability
for the wavelengths of interest.

CP449, Characterization and Metrology for ULSI Technology: 1998 International Conference
edited by D. G. Seiler, A. C. Diebold, W. M. Bullis, T. J. Shaffner, R. McDonald, and E. J. Walters
1998 The American Institute of Physics 1-56396-753-7/98/$15.00

539


Using this technique, NIST has developed a primary
standard ultraviolet-laser calorimeter (QUV) for use at
248 nm, which can provide accurate measurements of UV
laser power and energy within +1%. Additional primary
standards for use at 193 nm are now under construction
and estimated to be completed in FY 98 (see Figure 1).
After extensive testing, an appropriate volume absorber
has been selected for the 193 nm standards.

‘Thermocouple
leads

LZZZZZLLLA

LLL

Heater leads

Electrical Heater

Figure 1. 193 nm laser calorimeter (under construction).

NIST Laser Measurement System

Laser Power and Energy Measurements

A schematic drawing of the NIST UV_ laser
measurement system is shown in Figure 2. The incident
laser beam is split with a 2° wedged fused silica beam
splitter. The beam splitter ratio, Bg, is determined from a
series of energy measurements using the two primary
standards, QUV-1 and QUV-2. The value for this ratio,

B= (transmitted energy)/(reflected energy), qd)

is approximately 25. Then, the detector under test is
substituted for one of the standards and the beam splitter
ratio Bpyr is measured. The detector calibration factor K
is determined from the ratio of these two beam splitter
values,

K = Byyr/Bo - Q)

With judicious selection of transmitted and reflected
beams, this system can span a four decade range in laser
power and energy [4].

— Device Under Test
1 KrF Excimer er Laser|

SiO,
Beam Splitter

Data Acquisition
System

= =
Voltmeter, Scanner regan, *
& Power Supply

Figure 2. NIST UV measurement system for laser power and
energy measurements at 248 nm, QUV-1 and QUV-2 are the
primary standard calorimeters. A 2° wedged, fused silica (SiOz)
beam splitter is used as both a monitor and an attenuator for
laser power and energy measurements.

Dose and Cosine Response Measurements

As the microelectronics industry moves toward smaller
critical dimensions, process tolerances will shrink and
accurate measurements of laser energy will play a crucial
role in maintaining good quality control. Accurate dose
measurements are important because small area
photodiode detectors are widely used to monitor the laser
energy that is deposited at the wafer plane. Furthermore,
absolute dose measurements are important for the
development of new mask and resist materials at 248 and
193 nm, since lower dose requirements can translate
directly into higher throughput and extend the lifetime of
a stepper’s optical components as well. The selection and
calibration of transfer and working standards for UV dose
measurements, and ensuring high accuracy for the end
user, will be difficult. In particular, detector damage
issues related to long term UV exposure have yet to be
fully investigated. We will be working with industrial
partners to investigate new transfer and working
standards.

One difficulty associated with dose calibrations is
cosine, or angular response. Typically, dose meters are
calibrated using a parallel laser beam with a uniform
energy density. However, with the move toward high
numerical aperture (N.A.) stepper systems, significant
measurement errors can be introduced because the cosine
response of dose meters can vary wildiy (see Figure 3).
The deviation from a true cosine response can be due to
the angular response of a dose meter’s constituent parts,
the optical diffuser, the UV filter, and the UV photodiode
detector (see Figure 4). An example is shown in Figure 3,
where the difference in relative angular response between
two i-line (365 nm) meters that are used in a stepper with

540


off-axis illumination and N.A.=0.5 was approximately
40%.

The development of the calibration facility will evolve
in two stages: (1) a 193 nm primary standard calorimeter
will be developed and implemented for the calibration of
laser energy meters, (2) a measurement system, including
beam shaping optics with calibrated apertures, will be
established for the direct calibration of dose meters that
are used in wafer plane measurements. The techniques
and instrumentation developed will also be used to add to
the capability for dose meter calibrations to the existing
248 nm calibration facility which currently supports only
large area detectors that capture the entire beam. In
addition, a cosine response measurement capability will be
added in order to ensure an accurate calibration transfer
for meters that are used in high N.A. systems.

12 1
a Motera
2 10 = sis |
ao t | ;
oO \
% os \ | |
2 : ° | |
Boa L Pe —
% | iq
° Ina=os IN
| “4 =

hal

Angle of Incidence (degrees)

Figure 3. Angular dependence of i-line (365 nm) probes [5]. The
arrow indicates the relative response at a numerical aperture
(N.A.) of 0.5. As the plot indicates, a significant measurement
error can be introduced due to the imperfect cosine response of a
dose meter. This offset would be unaccounted for if the meters
were calibrated using a parallel beam, but intended for use in a
stepper with off-axis illumination.

Aperture

Diffuser, > :
Ss << Filter

Photodiode

Figure 4, A typical UV detector for dose measurements at the
wafer plane.

15 30 45 6 ac 90

CONCLUSIONS

With the move towards critical dimensions of 0.2-4m or
smaller, the dose exposure tolerances are expected to
decrease precipitously. We anticipate that this will
translate into a need for accurate, absolute dose
measurements at the +1-2% level [6]. Existing UV
transfer standards and calibration methods are unable to
meet this need. Therefore, improved transfer standards
and techniques will be required to ensure accurate
dissemination of the measurements to industry, and to
facilitate accuracy improvement activities such as
intercomparisons with calibration laboratories and industry
round robins. NIST is working toward meeting these
requirements by offering laser power and energy
measurement services at 248 nm. Laser power and energy
measurement services at 193 nm will be available to the
general public during the latter part of 1998. Progress has
been made towards establishing a calibration facility for
dose meters. NIST solicits from end users and equipment
manufacturers input that would aid in the identification of
new transfer standards and measurement needs.

ACKNOWLEDGEMENTS

We thank SEMATECH for their ongoing support of this
program.
Keywords: Laser metrology, dose control, transfer standards,
ultraviolet detectors

Contribution of the National Institute of Standards and
Technology; not subject to copyright.

REFERENCES

1, Leonhardt, R. W. and Scott, T. R., Integrated Circuit
Metrology, Inspection, and Process Control IX, Proc.
SPIE 2439, 448-459 (1995).

2. West, E. D., Case, W. E., Rasmussen, A. L., and
Schmidt, L. B., Journal of Research of the National
Bureau of Standards 76A, (1972).

3. West, E. D., and Churney, K. L., Journal of Applied
Physics 41, 2705-2712 (1970).

4. Beers, Y., National Bureau of Standards Monograph
146 (1974).

5. Cromer, C. L. and Bridges, J. M., “NIST
Characterization of !-Line Exposure Meters,”
SEMATECH Technology Transfer Document, No.
91040516A-ENG (1991).

6. Cromer, C. L., Lucatorto, T. B., O’Brian, T. R., and
Walhout, M, Solid State Technology, 75 (April 1996).

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