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NIST ' NisrUnited States Department of Commerce

Technology Administration

National Institute of Standards and Technology

N/ST Technical Note 1394

I

NIST Technical Note 1394

Calibration Service for Laser

Power and Energy at 248 nm

Rodney W. Leonhardt

Optoelectronics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

January 1 998

^^TES O*

U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary

TECHNOLOGY ADMINISTRATION, Gary R. Bachula, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Director
National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Techno!., Tech. Note 1394, 40 pages (January 1998)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1998

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
CONTENTS

1. INTRODUCTION 1

2. DESIGN PHILOSOPHY AND CONSIDERATIONS FOR THE 248 nm

CALIBRATION SYSTEM 2

2.1 Advantages of the QUV Calorimeter Reference Standard 2

2.2 Limitations of the QUV Calorimeter Calibration System 2

3. DESCRIPTION OF THE 248 nm CALIBRATION SYSTEM 3

3.1 Excimer Laser Source 4

3.2 Beam Steering and Collimating Optics 4

3.3 Multiple Reflection Beamsplitter Attenuator 5 3.4 Data Acquisition and Processing Electronics 8

4. QUV STANDARD CALORIMETERS 10

4.1 Temperature Sensing Thermopile 1

1

4.2 Absorbing Cavity Structure 1

1

4.2.1 Absorber Material 13

4.3 Calorimeter Output Analysis 13

4.4 Electrical Calibration of the QUV Calorimeters 15

5. CALIBRATION OF LASER ENERGY OR POWER METERS 17

5.1 Performance Capabilities of the 248 nm Calibration System 17

5.2 Calibration of Energy Measuring Instruments 18

5.3 Calibration of Power Reading Meters 18

5.4 Laser Meters for Calibration 19

6. MEASUREMENT UNCERTAINTY 20

6.1 QUV Reference Standard Calorimeter Uncertainty 21

6.2 Calibration System Measurement Uncertainty 23

7. MEASUREMENT ASSURANCE 25

7.1 Electrical Calibration History 25

7.2 Laser Beamsplitter Ratio Measurements 25

7.3 Check Standard Calibrations to Monitor the QUV System 25

7.4 Revising the Uncertainty Sources from Accumulated Data 27

8. FUTURE CALIBRATION SYSTEM CHANGES 27

9. REFERENCES 28

111
APPENDIX A. Sample Calibration Report 29

APPENDIX B. Calibration Procedure Outline 33

APPENDIX C. Shipping Instructions 34

TRADE NAME DISCLAIMER

Certain commercial components used in the calibration system are identified in this report in order

to adequately document the design. Such use and identification do not imply recommendation or endorsement by NIST, nor do they imply that the items are necessarily the best available for the

purpose.

IV
CALIBRATION SERVICE FOR LASER POWER AND ENERGY AT 248 nm

Rodney W. Leonhardt

National Institute of Standards and Technology

Boulder, Colorado 80303

This document describes the calibration service provided by the National Institute of

Standards and Technology (NIST) for laser power and energy at the excimer laser

wavelength of 248 nm. The service supports the calibration of laser-power meters

from 400 jiW to 7.5 W, and energy meters from 80 |iJ to 150 mJ per pulse. Typical

expanded measurement uncertainties range from 1.6 to 2 percent. The measurement

system, which includes a source laser, beam optics, and NIST reference standard

calorimeters, is reviewed. Theory and design considerations for the reference

calorimeters that correlate optical energy to NIST electrical standards are discussed.

Critical components of the calorimeters and beam delivery system are specified.

Measurement-system parameters and calibration procedures are examined. Types

of laser meters suitable for calibration are delineated. A detailed analysis of sources of error, estimates of uncertainty, and expanded uncertainty are presented.

KEY WORDS: beamsphtter; calorimeter; excimer laser; laser power; pulse energy

1. INTRODUCTION

The rapid growth in the commercial application of excimer lasers has resulted in a compelling need

for corresponding NIST standards and measurement services. Excimer lasers are now being used

in semiconductor photolithography, medical procedures, and a variety of materials-processing

applications.

Electromagnetic radiation from excimer lasers has several characteristics which make accurate power

or energy measurements difficult. A device to be used for measuring excimer laser power or energy

must accommodate electromagnetic energy having spatial and temporal coherence, high irradiance,

nonuniform intensity profiles, and photon energies high enough to cause fluorescence or exceed

strengths of chemical bonds.

NIST measurement of laser energy at the excimer wavelength of 248 nm is accomplished using a

pulsed-laser calorimeter. Reference standard calorimeters (QUV- 1 and QUV-2) were designed and

constructed at NIST to measure the total energy injected during a prescribed period of time [1][2].

Laser pulses are received by a cavity designed to absorb the energy and produce a corresponding

temperature change. The calorimeters are electrically calibrated and traceable to national-standard

electrical quantities.

A measurement system consisting of a reference standard calorimeter, source laser, beamsplitter and

collimation optics, and computer-controlled data acquisition instrumentation is used to calibrate laser power or energy meters. Two calorimeters were built both to allow evaluation of the beamsplitter

properties and for redundancy.

1
DESIGN PHILOSOPHY AND CONSIDERATIONS FOR THE 248 nm

CALIBRATION SYSTEM

Previous NIST research in laser calorimetry yielded the development of instruments capable of

measuring absolute total energy of pulsed radiation from Nd:YAG lasers operating at 1064 nm [2]. This Q-series design was selected for the excimer laser calorimeter due to its ability to absorb short

pulses of laser radiation without suffering damage. The design was adapted for pulsed 248 nm

excimer lasers by using a suitable absorbing material. The 248 nm version, or QUY calorimeter,

is the resulting device and serves as the reference standard for the calibration system. Physical

dimensions and principles of operation of the QUV calorimeters for excimer laser energy are essentially the same as for the Q-series model. As a result, the uncertainty assessment and

expanded uncertainty estimates are similar. High absolute accuracy and precision are the primary

goals of the 248 nm calibration system.

2.1 Advantages of the QUV Calorimeter Reference Standard

The design of the QUV calorimeters is based on the theory of isoperibol calorimetry [3], in which

the temperature of the energy absorber changes while the temperature of the surroundings remains

constant. The principal objective is to absorb all the incident laser radiation and convert the optical

energy to thermal energy. Isoperibol calorimeter/ has been thoroughly investigated for many years

and provides a mechanism for correlating optical energy to electrical energy. Energy measurements

by the QUV calorimeters are traceable to fundamental NIST standards by electrical-substitution

calibration. The expanded uncertainty for a QUV standard calorimeter is on the order of 0.8 percent,

which is consistent with other NIST laser standards.

The ability to withstand very high peak power laser pulses is also an essential benefit of the QUV

calorimeter design. Two calorimeters are needed for measurement of the beamsplitter ratios and

to simplify the system configurations necessary for practical calibrations.

2.2 Limitations of the QUV Calorimeter Calibration System

While the QUV calorimeter design provides a robust reference standard for 248 nm laser energy,

the limited range of input energy (0.6 to 15 J) of linear operation requires beam attenuation to extend

the useful measurement limits of a calibration system. Disadvantages of beamsplitter attenuation

techniques include an increase in the overall uncertainty of a measurement and a more complex

optical alignment.

Because of their design, the calorimeters are relatively slow, and are not easily portable or simple

to use. Consequently, the QUV measurement system is not used to calibrate laser sources directly

but primarily to calibrate more conveniently operated meters for laser power or energy.

Good beam collimation is also required for light to propagate through the QUV calorimeter aperture

tube and to allow total absorption of the laser pulse energy. Collimation of the laser beam requires

extra lenses and alignment procedures. Measurement uncertainty is increased due to the destructive

effect of ultraviolet light on beam optics and absorber surfaces. Consequently, more firequent
quality-control procedures such as measurements with laser beamsplitters are used to monitor any

changes in the ratio.

Lower uncertainties are possible if improvements in laser pulse stability and beamsplitter optics can be achieved. In the present design form, the estimated practical limit of uncertainty in the QUV

calibration system is about 1 to 1.6 percent.

3. DESCRIPTION OF THE 248 nm CALIBRATION SYSTEM

A simplified diagram of the calibration system is shown in Figure 1 . The principal subsystems of

the calibration system are:

A. Excimer laser: source of 248 nm radiation

B. Beam-steering and coUimating optics

C. Multiple-reflection beamsplitter/attenuator

D. Data-acquisition and processing electronics

E. QUV reference-standard calorimeter

A.

248 nm

excimer Laser

dielectric mirror

B.

beam steering and collimation optics

T

multiple reflection beamsplitter

lUT

(instrument under test) D.

Figure 1. System for calibrating laser power and energy at 248 nm.
All the instruments shown in the diagram except the excimer laser and the computer system are maintained in an acrylic resin enclosure. This enclosure maintains a thermal buffer from the rest of

the room, shielding the calorimeters from drafts and rapid variations in temperature of the building

heating/cooling system. Acrylic resin is opaque to ultraviolet wavelengths, which prevents reflected or scattered 248 nm light from leaving the measurement table.

3.1 Excimer Laser Source

The source of ultraviolet radiation is an excimer laser using KrF gas as the lasing medium and

operating at a wavelength of 248.4 ± 0.7 nm. The peak rated output energy of the laser is about 400

mJ/pulse. However, beam-steering mirror and lens losses, aperturing, and pulse stability consfraints

reduce this to 150 mJ maximum pulse energy. Pulse duration is approximately 30 to 35 ns, full width at half maximum (FWHM). This laser provides pulse rates of 0.1 to 50 pulses/s, but has been

modified to provide up to 100 pulses/s under very restricted conditions.

The intensity profile of the output beam is rectangular with approximate dimensions of 10 mm x 30 mm at a distance of 1 meter from the laser. The profile is nonuniform, with intensity variations

occurring with a quasi-interference pattern. This pattern is evident regardless of input aperture size. Corrosive etching of the antireflection coating from the laser output mirror by the ultraviolet

radiation is the likely cause of the interference pattern. The output beam is unpolarized and has a

spectral width of about 0.3 nm.

3.2 Beam Steering and Collimating Optics

The raw laser beam has relatively high angular divergence (> 3 mrad, full angle) and must be

collimated to function with the long beam paths required by the beamsplitter system and the

aperture tube of the QUV calorimeter. Different values of beam divergence in the horizontal and

vertical dimensions complicate this task, so the final degree of collimation is a compromise

sufficient to operate within the constraints of the physical layout. Figure 2 shows the detail of the

beam steering mirrors and collimating lenses.

dielectric

mirror

30 cm 38 cm

input

aperture

spatial

filter ^

focal length = 30.4 cm

focal length = 35.4 cm

Figure 2. Details of beam steering and collimation optics.
The input and spatial-filtering apertures are circular irises which can be adjusted to collimate and

provide varying beam sizes. The resulting beam irradiance appears circular, but the major portion

of the energy is still contained in a rectangular area within the profile. Lenses and apertures are mounted on an optical rail and allow fine tuning of the collimation, with the ability to expand the

beam slightly to reduce energy density.

Spatial filtering is implemented with a variable aperture near the common focal points of the lenses,

providing improved collimation. The method is simple but does not appreciably homogenize the

beam, so the interference pattern remains. Adjustments to the laser cavity mirrors to remove the

interference pattern result in a drastic drop in output energy. Beam-profile equipment necessary to measure the degree of nonuniformity in the intensity pattern is not presently available to us, so the

exact spatial distribution is unknown. However, the calorimeter response and beamsplitter ratios are relatively insensitive to the estimated levels of nonuniformity in the laser beam.

3.3 Multiple Reflection Beamsplitter Attenuator

NIST makes extensive use ofmultiple-reflection, wedged beamsplitters for attenuation in calibration

systems for laser power or energy. The theory and use of wedged beamsplitters have been well

documented [4][5]. A brief summary of beamsplitter application for the QUV system follows.

There are two principal advantages to using beamsplitters in laser measurements. If the two

detectors used with the beamsplitter both measure total energy, then laser stability is not important

since the detectors are monitoring the beam at the same time. Power measurements require a stable

laser, but it is a less critical issue when using the beamsplitter method than a substitution method.

Another important advantage is that the beamsplitter extends the range of the reference calorimeter

since the beamsplitter can function as a calibrated attenuator. A multiple-reflection beamsplitter (Figure 3) is a wedged, transparent optical component that has

highly polished fiat surfaces and is made of a well characterized material. The relative powers of

the emerging beams can be calculated by applying both Snell's and Fresnel's laws of reflection and

refraction for dielectric materials. This requires knowledge of the index of refractive, the wedge

angle, the angle of incidence, and the polarization of the incident laser beam. In the ultraviolet

wavelength region, fused silica is one of the few materials that can be used for this purpose. It has

been thoroughly studied at various wavelengths, and its dispersion equation is well documented [6].

There are different techniques to align the beamsplitter in the laser beam and produce a consistent

angle of incidence. The beamsplitter can be oriented so that the incident beam bisects the reflected

m = -1 and +1 beams. This orientation provides a small incident angle, but requires long path

lengths to get the necessary physical separation of the different beams. This method is less susceptible to polarization effects than methods with a large angle of incidence.

An alternate method provides the required beam separation by increasing the angle of incidence.

In this method, the transmitted m = +2 beam is retroreflected back on itself and through a small hole

in a card. The beamsplitter is then rotated until the m = +2 retroreflection is concentric w ith the

m = +3 beam. When this condition is achieved with a 248.4 nm wavelength laser beam, then the

angle of incidence is approximately 9.07°.
m = +3

m = +4

Figure 3. Multiple reflection wedged beamsplitter.

Material: SiO, , UV grade fused-silica

Wedge Angle: 2.0 ±0.1°

Wavelength: 248.4 ± 0.7 nm

Index of Refraction: 1.5083 ± 0.0004

Angle of Incidence: -9.07 ± 0.04°

Index of Refraction of Air: 1.00024 ± 0.00001

Attenuation factors for the ratio of useful "m" orders of the beamsplitter have been calculated for

fused silica at the wavelength of 248.4 nm and an incident angle of 9.07° are shown in Table 3.1. These values are calculated for comparison to experimental measurements of the beamsplitter ratio

using a 248.4 rmi laser and the QUV calorimeters. The calculated, theoretical ratios are used as guidehnes at this wavelength.

+1

Table 3.1. Theoretical beamsplitter ratios.

Beam A Beam B Ratio A/B Ratio A/B Ratio A/B % difference

vertical horizontal mixed ofV&H

(order, m=

)

(order, m=

)

polarization polarization polarization polarization

+1 24.01 ±0.12 24.75 ±0.12 24.38 ±0.12 3.05

+2 24.28 ±0.12 24.47 ±0.12 24.38 ±0.12 0.77

+2 583 ±3 606 ±3 594 ±3 3.82
The ratio of the power or energy in the transmitted beam to that in the reflected beam can be con- firmed for a particular beamsplitter at a specific laser wavelength by using the standard calorimeters

or two linear detectors to simultaneously measure two beams. A set of ratio measurements is made,

then the two detectors are interchanged, and the measurements are repeated. Then the square root of the product of the two ratio averages is calculated. Ideally this method cancels the uncertainty

of the calibrations of the two detectors from the measurement [7]. It is particularly important to measure the beamsplitter ratios periodically because even high purity UV-grade fused silica suffers

damage, resulting in a modification of optical properties, under high fluence 248 nm irradiation [8].

Beamsplitter ratios were measured on a fused-silica wedge at 248.4 nm; the results are shown in

Table 3.2. The measured values differ from theoretical ratios by almost 4 percent for the 0/+1 ratio

and about 1 percent for the +1/+2 ratio. Initial measurements gave a 0/+1 ratio within 2 percent of

the theoretical value. Measured values have shown a tendency to differ by 1-4 percent from the

theoretical value over the three-year period the QUV system has been in testing or operation,

possibly due to UV damage. However, the direction in which the ratio differs is not always the same. As a result, the measured ratios are considered to be more appropriate than theoretical values,

and the ratio obtained from current beamsplitter data is used to calibrate laser power or energy

meters.

Corroborating evidence for modification of the spitter ratio by UV exposure comes from ratio

measurements made on a second fused-silica beamsplitter (#2) which was purchased at the same

time and from the same manufacturer as the operational beamsplitter (#1). This splitter is kept m

storage as a reference. Preliminary measurements of laser beamsplitter ratios have been made on

this component, and the ratios are within 1 percent of the theoretical values.

Avoiding surface damage to the ftised-silica beamsplitter from high-energy UV photons requires that

surfaces be kept especially clean. Long-term effects from UV irradiation may change the index of

refraction, or produce damage centers. As a consequence, beamsplitter measurements are performed

either before or after every calibration procedure to monitor any change in the ratio. This detects

any shift in the beamsplitter ratios, which have been gradually changing with exposure.

The ratio 0/+2 is calculated from the measured ratios. The energy range of the calorimeters does not

allow direct measurement of ratios over 26 to 1.

Table 3.2. Measured beamsplitter ratios.

Beam A

(order, m= )

+1

Beam B Ratio AJB

(order, m=

)

mixed polarization

+1 25.3 ±0.3

+2 24.5 ±0.3

+2 620 ± 1
3.4 Data Acquisition and Processing Electronics

QUV calibration system control and data analysis are achieved with a computer using the IEEE-488

bus architecture. The interconnection schematic of the data-acquisition system is shown in Figure 4. A high-resolution nanovoltmeter measures the voltage of each QUV calorimeter thermopile.

Measurement of the voltage drop across the standard resistor and heater resistor during electrical

calibrations is performed with a separate digital voltmeter.

Calorimeter Nanovoltmeter: This nanovoltmeter is a 6V2 digit instrument capable of 1 nV resolution on its most sensitive range (3 mV). This instrument is used to measure all dc voltages generated

by a QUV calorimeter thermopile during a laser or electrical calibration. Each calorimeter is connected to an individual nanovoltmeter for this purpose. These instruments are calibrated annually

by the manufacturer and are traceable to NIST voltage standards.

Electrical Calibration Digital Voltmeter (DVM): The voltage and current values being used to electrically calibrate the QUV calorimeters is measured with a digital voltmeter. This instrument

is a 6V2 digit device and therefore has a resolution of 1 [xV on the 1 V range. Current through the

electrical calibration resistance heater is determined by measuring the voltage drop across the

standard resistor (see Figure 8). This DVM can also be used to measure the voltage output of the

lUT if it provides an analog output port. This device is calibrated annually by the manufacturer, and

traceable to NIST voltage standards.

Electrical Calibration Programmable Power Supply : Electrical calibrations are performed on the

QUV calorimeters by supplying power to the resistance heater with a programmable power supply.

The voltage usually applied to the heater is on the order of 1 to 6 V dc, at 10 to 70 mA of current. The calorimeter heater resistance is 133.25 ± 0.03 Q. The output of this power supply is measured

with the annually calibrated digital voltmeter (DVM).

Standard Resistor: Electrical calibration of the QUV calorimeters depends on current values

calculated from the measured voltage drop across the standard resistor. The standard resistor has

been calibrated by the NIST Electricity Division, and is checked annually with a resistance meter.

The meter which measures resistance is calibrated annually by the manufacturer and is traceable to NIST standards.

Injection Shutter System : The laser energy used in a measurement is controlled by a mechanical

shutter system consisting of the shutter and a driver unit. The computer controls the shutter, which

switches on the order of a few milliseconds. The shutter has two blades, which can operate to completely block or pass the laser radiation. This unit operates fast enough to allow a predetermined

number of pulses to pass at our typical excimer laser pulse-repetition rates.

Switch/Control I Jnit : A switching and control unit is used to sequentially supply measurements from

the various electronic equipment to the controlling computer. This unit can also be used to route an analog signal from a customer's meter to the digital voltmeter to record voltage readings during a

calibration.
Computer Controller: A computer controls the tests and the data-acquisition equipment. This

computer performs such operations as turning on and off the pulsing of the laser, opening and

closing the shutter, acquiring voltage data, processing the data, and displaying the results. All

communication to and from the instruments is carried out over the IEEE-488 bus, except for serial

communication with the excimer laser and the injection shutter. Commercial instrument-control

software is used along with a custom written program to acquire and analyze the data. The

acquisition-and-analysis program was written by an outside contractor with NIST collaboration and

then tested in the QUV laboratory.

Conriputer Controller

ing Excimer Laser

)rimeter 3.0 runn Calc

serial

Excimer

Laser

Serial Bus

IEEE-488 Bus

Injection

Shutter Switch/Control unit

analog

Digital

Voltmeter

I

Programmable

dc Power Supply

standard

Resistor

Nanovoltmeter Nanovoltmeter

QUV-1 QUV-2

Temp

Controller

Temp

Controller

Figure 4. QUV calibration system schematic.
4. QUV STANDARD CALORIMETERS

The principal objective of the QUV laser calorimeter is to absorb all the incident laser radiation and

convert the electromagnetic energy to thermal energy. The resulting temperature change is then used

to detemiine the amount of absorbed optical energy. Each excimer laser calorimeter (Figure 5)

consists of an absorbing cavity suspended inside a temperature-controlled jacket which provides a constant (with respect to time) temperature environment. This design is called an isoperibol

calorimeter.

The absorbing cavity is made of thin-walled copper whose outside surface has been gold plated.

Laser energy is predominantly absorbed by volume absorbing glass placed at the back of the cavity

structure. • The inside surface of the cavity is coated with a flat black paint to absorb any light

reflected or scattered from the glass. Because of their design, these instruments are essentially

energy-storage devices; consequently the calorimeter cavity continues to rise in temperature as energy is injected. It is this temperature change that is measured and correlated to energy absorbed.

J

r— Temp. Controlled Jacket

Foam Insulation

Foam Insulation

I

Thermocouples

Absorbing Glass ^^

.jT Calorimeter Case —^

Scale:

10 cm

Figure 5. QUV calorimeter cross section.

10
4.1 Temperature Sensing Thermopile

A thermopile measures the difference in temperature between the absorbing cavity and the constant- temperature jacket. Change in thermoelectric voltage output represents a measure of the absorbed

energy. An electrical heater wire in thermal contact with the cavity is attached near the location

where the laser radiation is primarily absorbed. This provides the means to calibrate the response

of the calorimeter in terms of electrical energy input. The calorimeters have been constructed to maximize the equivalence between electrical and laser energy inputs.

The cavity temperature is monitored by the thermopile, which consists of 20 hot junctions made of

Type-E thermocouple wires bonded to the gold-plated cavity surface. Junctions are bonded using

a thermally conducting epoxy which has excellent electrical insulating and chemical resistance

properties. The 20 cold, or reference, junctions are bonded to the temperature-controlled jacket

using the same epoxy. Each junction has a temperature sensitivity of about 50 /uV/°C, so the total thermopile has an approximate sensitivity of 1 mV/°C.

The electrical calibration factor for each QUV calorimeter is about 15.J/mV. Therefore, an energy

injection of 15 J will produce a maximum estimated temperature change in the cavity of approximately 1 °C. This temperature increase is small enough that the thermal coefficients (heat capacity

and thermal conductivity) remain relatively constant during the measurement process. Also, both

the radiative and convective heat transfers are nearly linear for this small temperature gradient.

4.2 Absorbing Cavity Structure

Since the calorimeters are intended to measure the energy from excimer laser sources, they must be

able to absorb the pulsed radiation without incurring thermal or photochemical damage. The induced

temperature increase for a given energy deposition is directly related to the volume of material

through which the energy is dispersed. Therefore, the larger the volume over which a given amount

of energy is absorbed, the smaller the corresponding temperature change. Black paint or other black

materials (known as surface absorbers) absorb the radiation within a small volume of material. The

resulting temperature changes can be large for this type of absorber. Pulsed laser radiation is deposited much more rapidly than the energy can diffuse away from the absorption region, and

ablation of the absorber may result.

These characteristics severely limit the use of surface absorbing materials with pulsed laser sources.

Instead, volume absorbing materials, such as colored glass filters, are used. As the laser radiation

propagates through these types of materials the power (or energy) density decreases exponentially

in the form

i(x) = /,^-p^ (1)

where Ig is the incident energy intensity, I(x) is the intensity within the material at a distance x normal to the glass surface, and B is the absorption coefficient for the material. For materials with

large B, energy is absorbed more rapidly with distance, and the resulting temperature increase at the

surface (where the absorbed intensity is maximum) of the material is larger. In materials with small

11
B, the energy is absorbed more slowly (with respect to x) and is thus spread through a larger volume.

Since the thickness of the absorbing material must be large enough that essentially all the energy gets

absorbed, smaller B materials require greater thicknesses. Heat transfer becomes a consideration

because in a thicker piece of glass the heat will take longer to conduct to the copper cavity. Thus,

a compromise between heat transfer and thermal damage must be made when selecting the

appropriate material and thickness.

The absorber material selected for the initial QUV calorimeters was ionically colored glass [9]. The

manufacturer's transmittance data of the colored glass were compared to values measured at NIST.

In the 248 nm wavelength region, the transmittances from both sources of data are in close agreement. Based on this information, the absorption coefficient calculated for the colored glass is 3.32/mm. Less than 0. 1 percent of the incident energy will be transmitted through the 2 mm thick

glass plate.

The filter is thin enough (2 mm) that the laser-generated heat can be conducted into the copper cavity

relatively quickly. As shown in Figure 6, two pieces of colored filter glass are bonded with thermally

conducting epoxy to the back of the cavity structure. The piece at the back of the cavity (angled at 60°) is 2 mm thick, and the piece at the top is 1 mm thick. The incident laser beam strikes the 2 mm

thick glass where most of the radiation travels through the glass and is absorbed. The radiation

reflected from the front surface of the 2 mm glass then strikes the 1 mm thick glass (at 30° angle of

incidence) where most of the remaining radiation is absorbed. The small amount reflected from the

front surface of the 1 mm glass strikes the absorbing black paint on the inside of the cavity.

Electrical

Heater

Figure 6. QUV cavity cross section.

12
To calculate the energy reflected from the surfaces of the glass absorbers, the refractive index of the

glass for the 248 nm excimer wavelength must be determined. This was done by performing a curve

fit to a Sellmeier equation using three data points provided by the glass manufacturer [10]. The

resulting curve fit fiimishes an index of refraction for UG 1 1 (at 248.4 nm) of ~1 .62. Fresnel's laws

of reflection were applied for dielectric materials at 30° angle of incidence, and yielded the front

surface reflectances shown in Table 4. 1

.

The case where the least energy is absorbed by the initial filter is for radiation linearly polarized in a plane perpendicular to the plane of incidence. In this case 7.9 percent of the incoming laser beam

energy will be reflected from the surface of the 2 mm glass and 7.9 percent of the remaining beam

will be reflected from the 1 mm glass. Accordingly, about 0.8 percent of the incident beam energy

strikes the black paint. However, for the QUV system the excimer laser is unpolarized; thus about

0.34 percent reaches the black paint, where it undergoes 95 percent absorption as specified by the

coating manufacturer. We estimate less than 0.02 percent of the original beam energy is not

absorbed by the cavity at this point. If the flat black paint has partial specular reflection, the light

again encounters absorption at another cavity wall. The geometry of the cavity allows a negligible

amount of incident radiation to scatter out. This same geometry was used on the Q-series calorimeters, and measurements on that device confirmed this conclusion [2].

Table 4.1. Front surface reflectance for colored glass filter at 248 nm (30° incidence).

Polarization Reflectance

Unpolarized 0.058 ± 0.004

Perpendicular 0.079 ±0.012

Parallel 0.037 ± 0.004

4.2.1 Absorber Material

Schott Glass UGl 1 filter glass was employed as the volume absorber [9]. A piece 2 mm thick was

used as the main absorber, and a 1 mm section as the secondary absorber. This glass has been

damage-tested with high intensity 248.4 nm laser pulses having a FWHM of about 30 to 35 ns. The

excimer beam was focused to increase the energy fluence above typical operating conditions. The

maximum energy fluence on the UGl 1 filter was approximately 200 mJ/cm" per pulse. No surface

damage was visually observed for pulse repetition rates of 1 and 10 Hz for 1 to 2 minute durations.

4.3 Calorimeter Output Analysis

The theory of isoperibol calorimeters [3] predicts that the temperature (voltage) output can be

expressed as an infinite series of exponential terms. The voltage output of the thermopile is used as the output variable and is assumed to be linearly related to temperature. At some time after energy

injection, the higher order terms become negligible, and the output can be described by a single

exponential equation,

13
[V(.t)-VJ = (V^-VJe -at (2)

V^ = voltage at an infinite time after energy input,

Vq = voltage at time tg, any time where the output is described by a single exponential, and a = cooling constant.

The output voltage of the thermopile in the calorimeter is sampled and stored with a computer

controlled data acquisition system. The resulting voltage-time information must then be processed

to determine the energy received by the calorimeter. Figure 7 shows a t3^ical voltage output of a QUV isoperibol calorimeter during a measurement.

700

600-

§ 400

Time (s)

Figure 7. Calorimeter output voltage as a function of time.

Energy (either electrical or laser) is deposited during the interval labeled "Energy Injection Period."

Voltages V, and Vj at times t, and t2 occur long enough after an energy injection that the output is described accurately by a single exponential decay factor.

Laser input can be a single pulse or a series of pulses which occur during the energy injection period.

Heat flow in the glass absorber and the thermal time constant of the cavity act to thermally integrate

the laser pulses. This allows the absorption of laser pulses to produce a response equivalent to a continuous injection of electrical energy.

14
According to the first law of thermodynamics (as apphed to this calonmeter), the energy absorbed

by the calorimeter is equal to the change in internal energy plus the heat transferred from the system

during a measurement. The change in internal energy is proportional to the quantity (V2- V,), and

the heat transfer value is proportional to the quantity

h

^^{V{t)-V2dt. (3) h

Theory [11] predicts that the output of such a device is related to the total energy absorbed by the

calorimeter by

h

E = K[iV^-V^) + aj[V(t)-VJdtl (4)

h

where E is the energy absorbed and K is a proportionality constant. The proportionality constant

(calibration factor) is actually a combination of a weighted heat-capacity and heat-transfer function

of the structure of the calorimeter cavity. The value of K is determined electrically by injecting

known energy and then using the voltage curve to solve eq (4). Unknown parameters a, V^, V,,

and V2 are found for each measurement run by performing a least-squares fit (using eq 2) to the

voltage data of the calorimeter in the regions near Vj and Vj.

4.4 Electrical Calibration of the QUV Calorimeters

The QUV reference standard calorimeters are electrically calibrated by dc substitution, which

provides traceability to electrical standards. The schematic in Figure 8 shows the basic configuration

for electrical calibration runs. During an electrical calibration run, the computer commands the

programmable power supply to turn on a predetermined voltage to the standard resistor and

calorimeter resistance heater. This voltage is read by the digital voltmeter. A known electrical

energy is dissipated by the heater, and the resulting cavity temperature change is monitored with the

thermopile signal output as measured by a nanovoltmeter. The voltage data are logged and

processed by the computer using algorithms based on calorimeter theory, as outlined in Section 4.3.

Electrical calibration runs are performed under control of the computer. Parameters are chosen by

the operator to control the energy and the injection time. Automatic repetition of runs is possible

with software control of the program Excimer Laser Calorimeter 3.0, allowing large amounts of data

to be taken without operator intervention. All equipment is turned on and allowed to warm-up for one hour and reach operating equilibrium before any calibration runs are realized.

15
Nanovoltmeter

Thermopile Output

QUV

cavity

Standard

Resistor

IEEE 488 bus Computer

Controller

Timer

c

oO

5

CO

Hr+

Programmable

dc Power

Supply ~

Digital Voltmeter

Figure 8. Electrical calibration configuration.

16
5. CALIBRATION OF LASER ENERGY OR POWER METERS

A typical set-up for calibrating laser energy or power meters is shown in Figure 1 . The lUT is positioned in the beam which produces the required test conditions. The QUV standard is located

in a beam which provides enough total energy for the calorimeter to operate within its measurement

range. All instruments are optically aligned and then allowed to thermally stabilize in the system

enclosure before calibration measurements are performed. Electronic instruments are powered on and allowed one hour warm-up to reach operating temperature.

5.1 Performance Capabilities of thie 248 nm Calibration System

This calibration system is designed to measure pulsed laser energy and average power at the excimer

wavelength of approximately 248 nm. Although the standard reference calorimeters measure total energy injected, the calibration system can be used to measure other laser-beam characteristics such

as average pulse energy or average power. In fact, these two measurement quantities are most often

requested by laser power or energy meter calibration customers.

The calibration system has the following capabilities in the main transmitted (m = 0) beam.

Laser Wavelength: 248.4 ± 0.7 nm

Laser Spectral Width: 0.3 nm

Beam Diameter (adjustable): Maximum = ~20 mm. Minimum = ~5 mm

Pulse Energy @ full open aperture: 100-150 mJ

Maximum Average Power @ full aperture: -7.5 W

Pulse Repetition Rate (PRR): 0.5-50 s"' Pulse Width: -30-35 ns QUV Calorimeter Total Energy Measurement Range: 0.6-15 J Fused-Silica Beamsplitter Ratios (approximate): 25, 620

The energy and power calibration ranges of the system are extended by using a wedged beamsplitter

with the QUV calorimeters. Attenuation is available to reduce the beam energy and allow

calibration of energy meters down to 1 mJ total energy. The system camiot achieve all powers or energies at all beam aperture diameters. Laser power/energy meters to be calibrated should be able

to operate within the ranges listed above. Specific calibration requirements must be discussed w ith NIST staff to determine whether the system can be configured to meet customer requirements.

17
5.2 Calibration of Energy Measuring Instruments

Before average pulse energy calibrations are made, the excimer laser is filled with KrF gas and

checked for stable pulse operation. Discharge control voltage is checked to see if it can maintain

a stable laser pulse output during one measurement run. Laser pulse energy can be monitored for

fluctuations with a sensor internal to the laser or with an external pyroelectric detector aligned in a low-energy beam. Laser output, to be considered stable (for this laser), has a pulse energy standard

deviation on the order of 0.7 to 1.5 percent. Laser pulse energy which drops or cannot be

maintained during a measurement run is an indication to the operator that -the laser needs to be

recharged with gas. For a meter which measures total energy the laser pulse stability is not critical since the beamsplitter allows both the lUT and the QUV calorimeter to simultaneously measure total energy.

Each measurement run of the system is controlled by the operator and the data- acquisition system.

Once the measurement run is started with the parameters set by the operator, the excimer laser

operation is started manually, and pulse amplitude is allowed to stabilize with the injection shutter

closed. Manual start/stop of the laser is required for safety reasons.

Data-acquisition instruments log the QUV thermopile voltage throughout the measurement run for

later processing. Two seconds before the shutter opens, the computer ramps the laser pulse rate up

to that required for the calibration. The shutter opens, allowing injection of laser energy. The lUT

logs the pulse energy with its own electronics or the operator manually records the display value,

depending on the configuration. After the shutter closes, the QUV voltages continue to be recorded

until the data processing is completed under software control. The computer displays the total energy measured by the QUV calorimeter, and this value is used along with the calibrated

beamsplitter value and lUT data to calculate a calibration factor. Enough runs are performed to achieve a good statistical sample and are processed according to NIST measurement and uncertainty

guidelines covered in Section 6.

5.3 Calibration of Power Reading Meters

Meters that measure average power are calibrated similarly to energy meters. Again, stable laser

operation is important and checked with the internal laser-pulse monitor or an external pyroelectric

detector. Stable laser operation is achieved through the power lock feature of the excimer laser,

which automatically adjusts the control voltage to compensate for pulse energy fluctuations. At

higher pulse repetition rates this feature does not always function properly. Then the control voltage can be locked to preset value by the operator, and the pulse energy is monitored with the internal

laser sensor to see whether it remains constant over the measurement run.

Additional precautions are taken to ensure valid calibration results when measuring average power

of a pulsed laser. Laser power meters usually have thermal sensing heads. These heads typically

have response times on the order of seconds to tens of seconds. To ensure that laser pulses are averaged, the laser should be run at a minimum of 10 Hz unless otherwise instructed by the customer.

18
The laser injection period, beamsplitter ratio, and total energy measured by the QUV calorimeter are used to calculate the average power incident on the lUT. The calibration factor is calculated from

the power meter data. Enough runs are performed to achieve a good statistical sample, and

processed according to NIST measurement and uncertainty guidelines covered in Section 6.

For more complete details on the measurement procedures and routines please see Appendix B and

the NIST Excimer Laser Calibrations - Technical Manual.

5.4 Laser Meters for Calibration

Meters suitable for calibrated measurement of excimer laser power or energy may be purchased from

several of the manufacturers listed in laser-industry directories and buyers' guides. Typically the

meters consist of a thermal or pyroelectric detector head coupled with signal processing and display

electronics. Semiconductor sensors carmot now be calibrated with the QUV system because

calibrated beamsplitter ratios that attenuate the laser pulse energy below the saturation energy of

these very sensitive detectors are not available.

Stand-alone sensor heads can be calibrated as well, but then the customer's readout device is not

included in the calibration measurement. This can lead to higher calibration uncertainties unless the

customer's display device and connecting cables are independently characterized.

For each calibration, the associated measurement uncertainty is analyzed, taking into account the

applicable conditions and data from the lUT. Total expanded uncertainties (as discussed in detail

in Section 6.2) in the calibration of 248 nm laser power and energy meters typically range from 1 .6 to 2 percent with the present system uncertainty assessment. A stand-alone thermal head measuring

average power can be calibrated to a similar uncertainty if a high-accuracy voltmeter can be used

for the readout and data recorded by the control computer. Average pulse energy calibrations of a

pyroelectric head with an oscilloscope readout are often in the 2.5 to 3 percent uncertainty range due

to the less accurate signal-measurement capabilities of typical waveform-recording devices.

19
6. MEASUREMENT UNCERTAINTY

The total uncertainty associated with a particular laser power or energy measurement is composed

of the individual uncertainties of the components of the system. The actual magnitude of each

component is unknown; otherwise the result could be correspondingly adjusted to eliminate the

error.

Uncertainty estimates for our laser energy measurements are assessed using the following guidelines

[12] [13] [14]. To establish the uncertainty limits, the sources of error are separated into Type A and

Type B components. Components of uncertainty arising from a random effect which are evaluated

by statistical methods are called Type A uncertainty. Components of uncertainty arising from a systematic effect which are evaluated by other means are called Type B uncertainty.

^-^^ ^ ' (5)

Type A uncertainties are assumed to be independent. The standard deviation S^ for each component

iswhere the Xj values represent the individual measurements, x is the mean of the measurements, and

N^ is the number of Xj values used for a particular component of Type A uncertainty. The standard

deviation of the mean is S/N/''', and the total standard deviation of the mean is [E(S//N^)]'''', where

the summation is carried out for all the Type A uncertainty components.

The Type B evaluation of standard uncertainty is derived from scientific judgement based on

previous measurement data, manufacturer's specifications, and any other relevant knowledge. For

the QUV calibration system, all Type B uncertainties are assumed to be independent and have

rectangular or uniform distributions (that is, each has an uniform probability of being within the

region ±6- and zero probability of being outside that region). If the distribution is rectangular, the

standard deviation o^ of each uncertainty component is equal tOj 6 /3 ', and the total standard

deviation is approximated by (So^ ) \ where the summation is performed over all Type B

uncertainty components.

The expanded uncertainty is determined by combining the Type A standard deviation of the mean

and the Type B standard deviation in quadrature and multiplying this result by a coverage factor of 2. The expanded uncertainty U is then

^ = 2 ^

The number of decimal places used in reporting the mean value of the measurements is determined

by expressing the expanded uncertainty to two significant digits.

20
6.1 QUV Reference Standard Calorimeter Uncertainty

The Type A evaluated uncertainty components for the QUV calorimeters are:

(1) Rlectrical Calihration : The calorimeters are calibrated by performing a large number of

electrical measurements to determine an average calibration factor. The imprecision is an

estimate of the error due to the scatter of individual measurements about the mean value.

Typically, a set of 20 electrical calibration runs at a specific energy and injection time

exhibits a standard deviation less than 0.1 percent. For calibration energies less than 1 J the

standard deviation in the electrical-calibration factor is slightly higher, but usually less than

0. 1 5 percent.

The electrical calibration uncertainty of the QUV calorimeters was evaluated over the entire

operation range. Measured energies ranged from 0.6 to 25 J, and injection durations from

10 to 180 s. A selection of 70 electrical calibration runs with widely varying energies and

injection times yielded an average electrical calibration factor with a standard deviation of

0.5 percent. Since it is not practical to maintain a comprehensive electrical-calibration

history at every energy and injection period, the uncertainty has been set at 0.5 percent. This

component is not a dominant factor and lowering it will not significantly decrease the

expanded uncertainty until other components can be reduced.

The Type B evaluated uncertainty components of these QUV calorimeters are:

(1) Inequivalence : This is the error due to the inequivalence between electrical and laser

energy deposition. For a given absorbed energy, the output of the calorimeter may be different for different distributions of heat. The QUV calorimeters are designed to force the

heat to flow in a manner which minimizes this inequivalence. The QUV calorimeters are of

the same design, geometry and physical dimensions as the previously evaluated Q-series

calorimeters. The Q-series inequivalence was derived from measurement intercomparisons

with the C-series national standard calorimeters at a wavelength of 1 .06 \im. The estimated

inequivalence from the measurements is 0.48 percent.

Preliminary experimental comparisons were accomplished by calibrating the QUV

calorimeters with the Q-series, pulsed 1.06 ^m system. At 1.06 )im, the absorption of the

UGl 1 glass is very close in value to that of the IR glass used by the Q-series calorimeters.

Thus, the QUV calorimeters should function equally well at this IR wavelength. Results

showed that calibration factors were very close when data from the 1 .06 |im laser calibration

system when compared to the electrical calibration factors obtained with the QUV system.

The calibration factor difference was 0.43 percent for QUV-1, and 0.52 percent for QUV-2,

providing further confirmation of the inequivalence estimate and calorimeter consistency.

21
(2) Absoiptivity : This error is due to laser radiation which is reflected (or scattered) back out

of the cavity. Escaping radiation does not contribute to heating the cavity and, consequently, causes an error in the measurement since it is assumed that all the incident electromagnetic

energy is converted to heat in the cavity. The cavity geometry and absorbent coatings minimize this error.

Backscatter measurements on the Q-series cavity were made at 1 .06 \xm and the amount

scattered was below detection. A conservative upper limit on this uncertainty was set at 0.

1

percent for the Q-series. The QUV cavity is the same geometry. By calculation, the energy

not absorbed in the multiple reflections in the cavity is less than 0.02 percent at 248.4 nm as shown in Section 4.2. If the black paint is more nearly specular or less absorbing than

specified, the cavity geometry still forces absorption of all but a small fraction of the laser

energy. Also, some of the UV photons which strike the black paint in the absorber cavity

may remove the paint instead of convert to thermal energy, but this has not been observed.

An excimer laser was not available to directly test the QUV cavities for scattering losses at the time of construction. A conservative upper limit on this uncertainty was set to 0.2

percent.

(3) Heater Leads : During the electrical calibrations some heat is generated in the copper wire

leads which connect to the heater coil. This heat is not deposited on the cavity and causes an

error in the calculation of the energy going to the heater. Lead resistance was measured as percentage of the total resistance heater and is <0.35 percent. A four-wire measurement

technique was used to determine this value.

(4) Electronics : This component is due to the uncertainties in the electronic equipment used.

These instruments include digital voltmeters and timing systems. The uncertainties in the

electronic instruments are typically much smaller than the other uncertainty components.

Manufacturers' stated accuracies and electrical tests were used to estimate an upper limit of

0.1 percent.

(5) Injection Timing : This component arises from uncertainty in the injection time interval

of electrical or laser energy to the calorimeter during a calibration run and will affect average

power calibrations. Finite switching times from the dc power supply result in an injection

time uncertainty of less than 0.05 percent for 50 s, which is the shortest injection time used

for average laser power calibrations. Longer injection times result in less uncertainty, and

a conservative upper limit of uncertainty was set to 0.1 percent.

The estimated combined uncertainty of the energies measured by the QUV calorimeters is summarized in Table 6.1. The uncertainty due to inequivalence value is based primarily on the

uncertainty assessment for Q-series calorimeters, which is expected to apply to these calorimeters

because of their inherent similarity.

22
Table 6. 1 . QUV calorimeter expanded uncertainty

Source Standard deviation (%)

Inequivalence (Type B) 0.48

Absorptivity (Type B) 0.20

Heater leads (Type B) 0.35

Electronics (Type B) 0.10

Injection timing (Type B) 0.10

Electrical calibrations (Type A) 0.50

Expanded uncertainty ± 0.75

6.2 Calibration System Measurement Uncertainty

The total uncertainty of a laser meter calibration must not only include the QUV uncertainties but

also contributions from the beamsplitter, laser instability, and other potential error sources, including

data from the lUT. The following sources of uncertainty were evaluated to complete the total measurement uncertainty.

(1) Laser stability : Laser stability directly impacts calibration measurements of average power

and average pulse energy. The QUV calorimeters measure total energy, and average power or pulse energy can be calculated from the total energy, the energy injection time, and the pulse

rate. Thus some uncertainty will result from the pulse-to-pulse instability of the laser.

Pulse-to-pulse energy stability was evaluated with the internal excimer laser pulse monitor and

data from a pyroelectric laser-energy meter. Instability of the pulse energy was about 1 to 1.5 percent, but the net effect on calibration factors was less than this due to averaging. Laser energy

measurements have shown standard deviations of 0.3 to 1 percent for a variety of meter types.

Average-power calibrations were also investigated, comparing results with stable laser operation

to measurements performed with laser-pulse instability as high as ±5 percent. The resulting

effect on average power calibrations showed less than 0.4 percent difference between unstable

and stable laser runs. Although laser pulse-to-pulse instability will be partially accounted for in

the determination of overall standard deviation, additional uncertainty is evaluated as a Type B

component, with an upper limit of 0.75 percent for stable laser operation.

(2) Beamsplitter ratio : A beamsplitter ratio is used in all calibration measurements for the

calculation of the laser energy incident on the lUT. The theoretical values of these ratios are summarized in Table 3.1. There is a 3 to 5 percent difference between the theoretical

beamsplitter ratios and those measured with the QUV calorimeters (Table 3.2). As mentioned

earlier, UV damage to the fused-silica wedge is expected to modify its transmission and

reflection properties. Thus, the measured ratios are considered to be more accurate, and the

calculated ratios are used only for guidance in the overall evaluation of beamsplitter ratios.

23
The beamsplitter uncertainty source has been evaluated with both Type A and B procedures.

Measurements of the beamsplitter ratios 0/+1 and +1/+2 have typical standard deviations of 0.3

to 0.6 percent for 30 measurement runs. The most recent ratio measurement data is used to determine the beamsplitter values and associated Type A uncertainty estimates. The Type B

uncertainty is included because using a laser to determine the ratio further exposes the

beamsplitter to intense UV radiation, possibly changing the properties as the measurement is being made. As we evaluate laser beamsplitter ratios from the last three years, we expect to lower this uncertainty estimate. This evaluation will also give an indication of how often

beamsplitter measurements must be made to maintain a lower Type B uncertainty estimate.

(3) Measurement data from the Instrument Under Test (IT IT) : Calibration measurements on the

lUT provide data which can be evaluated with statistical methods, giving Type A uncertainty

components. Results will lead to a mean and standard deviation used in the calculation of the

overall uncertainty of the measurements.

(4) Beam pathlength : Absorption or scattering of UV light by the constituents in air will cause

the relative pulse energy arriving at the QUV standard or the lUT to be dependent on pathlength.

Measurements made to quantify this effect show a small impact (<0.04 percent, or less than one

standard deviation) when the beam pathlength is changed by <40 cm. Calibrations are usually

performed under conditions where beam pathlengths differ by no more than ±2 cm (from those

used for beamsplitter calibrations) to reduce this effect. Occasionally, the required pulseenergies dictate a physical configuration where the pathlengths differ by as much as 25 cm. This

uncertainty is evaluated as a Type B component, with a magnitude of 0.02 percent.

The overall measurement uncertainties for the QUV calibration system operating at 248.4 nm are summarized in Table 6.2. Typical expanded uncertainty values for laser power or energy meters are

in the range of 1 .6 to 2 percent.

Table 6.2. Typical QUV measurement system uncertainties.

Type A TypeB

SOURCE s.(%) N SOURCE 6, (%)

Electrical calibration 0.50 70 Inequivalence 0.48

Beamsplitter cal. (0/+1) 0.30 30 Absorptivity 0.20

Beamsplitter cal. (+1/+2) 0.60 30 Heater leads 0.35

lUT cal. (typical) 0.2-1 10 Electronics

Injection timing

Beamsplitter

Laser instability

Beam pathlength

0.10

0.10

1.00

0.75

0.02

24
7. MEASUREMENT ASSURANCE

Confidence for high-quality performance and precision of the QUV system is achieved with

electrical calibrations, frequent measurement of laser beamsplitter ratios, calibration of two different

check standards, and periodic evaluation of sources of uncertainty.

7.1 Electrical Calibration History

Electrical calibrations are performed periodically to provide a history at injection times and energies

necessary for specific laser meter measurements. Additional electrical runs are also acquired at the

energies (0.6 and 15 J) used for laser beamsphtter ratio measurements. This provides a statistical history for tracking the stability of the QUV calorimeters. As shown in Table 7.1 the difference in

electrical calibration factors is less than 0.2 percent in the last three years.

Table 7.1. Electrical calibration factors (J/mV).

Old electrical cal factors (1994) New electrical cal factors (1997) % difference

QUV-1 (low energy) 15.20 ± 0.13 QUV-1 (low energy) 15.18 ± 0.13 0.13

QUV-1 (high energy) 15.23 ± 0.13 QUV-1 (high energy) 15.21 ± 0.13 0.07

QUV-2 (low energy) 15.87 ±0.13 QUV-2 (low energy) 15.86 ±0.13 0.06

QUV-2 (high energy) 15.85 ± 0.13 QUV-2 (high energy) 15.83 ± 0.13 0.13

7.2 Laser Beamsplitter Ratio Measurements

As discussed in Section 3.3, frequent ratio measurements are necessary because of potential UVinduced damage in the fused-silica beamsplitter. A small set (5 to 10 measurements) is taken before

or after each laser meter calibration to confirm that the specific ratio used has not significantly

shifted.

The comprehensive measurements outlined in Section 3.3, in which the calorimeters and

nanovoltmeters are physically interchanged to cancel offset errors, are administered periodically to obtain the beamsplitter ratios. The measurements are time consuming, and there is risk of damage

to the calorimeters from the physical movement, so the frequency with which these measurements

are executed is limited. Sets of ratio measurements are performed at least once a year. Based on

accumulated data, it is recommended that ratios be measured twice yearly in order to lower

uncertainty estimates for the beamsplitter ratios and to track ratios more completely. Uncertainty

assessment for these measurements is covered in detail in Section 6.

7.3 Check Standard Calibrations to Monitor the QUV System

Two check standards have been maintained in an attempt to evaluate and monitor calibration

precision using technologies independent and external to the QUV system. The check standards, one

25
to assess laser power and the other to measure pulse energy, were purchased from independent U.S.

companies.

The laser power meter (Check Standard #1) functions at excimer laser wavelengths with a very

durable UV absorber coating that can withstand 30 W of average power and pulses of 15 J/cml

Calibration factors for this meter have been very consistent since the inception of the QUV system,

as shown in Table 7.2.

Table 7.2. UV laser power, check standard (#1) calibration.

Date Cal factor Laser PRR Average power Expanded

(W/reading) (pulses/s) (W) uncertainty (%)

9/94 0.9375 10 unavailable 1.8

2/1/95 0.944 50-100 5-10 1.9

11/29/95 0.938 10 1.2 1.7

»> 0.927 5 0.6 1.7

5» 0.945 20 2.4 1.6

55 0.940 50 2.8 1.6

12/18/96 0.943 10 0.8-1.4 1.7

9/23/97 0.937 20 1.5 1.7

A pyroelectric, pulse-energy measuring detector (Check Standard #2) has been calibrated to provide

long-term data on calibrations of this nature. Presently, only an oscilloscope readout device has been

used during measurements, so the overall uncertainty of this type of calibration is greater than

desired for this purpose. More-precise readout devices are being evaluated for possible application

to improving the laser-pulse energy-calibration check standard. Calibration factors have been

consistent and within the uncertainty levels estimated during the short period over which

measurements have been recorded for this detector.

Table 7.3. UV laser pulse energy, check standard (#2) calibration.

Date Cal factor

(V/mJ)

Laser PRR

(pulses/s)

Average pulse

energy (mJ)

Expanded

uncertainty (%)

10/26/95

1/27/97

1.20 5 1 2.8

1.18 5 0.04 2.8

1.17 5-15 0.3 2.8

26
7.4 Revising the Uncertainty Sources from Accumulated Data

A substantial volume of data has been accumulated from electrical calibrations, laser beamsplitter

ratio measurements, and device calibrations. Estimates of system uncertainty for the beamsplitter

ratio, injection timing, laser instability, and the lUT are essentially under evaluation every time a meter is calibrated and adjusted accordingly. Calibration-system uncertainty components will be

evaluated annually with the past year's data and updated.

8. FUTURE CALIBRATION SYSTEM CHANGES

Inevitably, changes will be made to the QUV calorimeter-based 248 nm measurement system. The

software will hkely be converted from the current computer to a PC-based platform. Refinements

in the instrument control may take place, but will probably be minor.

Investigation and testing of UV-grade optical materials other than fused silica may allow

beamsplitters with smaller ratios to be used. Improvements in fused silica will likely provide more

damage-resistant beamsplitters which will be stable in their attenuation factors and allow lowered

uncertainties. A broadband-UV volume absorber is being investigated which may allow new

calorimeters to be built which work at the excimer wavelengths of 157 through 351 nm.

The majority of modifications in the near future will probably be minor. Changes will be updated

in the active documentation files for the system. While the details may no longer be completely

accurate, this report should still adequately describe the service.

Major changes such as a new calorimeter design or measurement system will require the preparation

of a new document.

ACKNOWLEDGMENTS

The development of the QUV calorimeters is based on the work of E. D. West and Douglas Franzen,

who designed the original Q-series instruments. Credit also goes to Thomas R. Scott for project

support and suggestions on data analysis. I am indebted to David J. Livigni and Robert Folsom for

assistance in the final construction of the calorimeters and calibration system. Thanks also to Jeff

Parker of Metric Systems for system software development. This work was supported by

SEMATECH, a consortium of U.S. semiconductor companies.

27
9. REFERENCES

[I] Leonhardt, R.W.; Scott, T.R.; Deep-UV Excimer Laser Measurements at NIST. Integrated

Circuit Metrology, Inspection, and Process Control IX: Marylyn H. Bennett, Editor, Proc. SPIE

2439: pp. 448-459; 1995.

[2] Franzen, D.L.; Schmidt, L.B.; Absolute Reference Calorimeter for Measuring High Power

Laser Pulses. Applied Optics: Vol.15, No. 12, pp. 3115-3122; Dec. 1976.

[3] West, E.D.; Chumey, K.L.; Theory of Isoperibol Calorimetry for Laser Power and Energy

Measurements. Journal ofApplied Physics: Vol. 41, No. 6: pp. 2705-2712; May 1970.

[4] Beers, Y.; The Theory of the Optical Wedge Beam Splitter. National Bureau ofStandards

(U.S.) Monograph 146: 1974. 26p.

[5] Danielson, B.L.; Measurement Procedures for the Optical Beam Splitter Attenuation Device

BA-1. National Bureau ofStandards (U.S.) Internal Report 77-858: 1977. 20p.

[6] Malitson, I.H.; Interspecimen Comparison of the Refractive Index of Fused Silica. Journal of

the Optical Society ofAmerica Vol. 55: 1205-1209; Oct. 1965.

[7] Young, M.; Measurements of Pulsed-Laser Power. National Bureau ofStandards Technical

Note 1010: 1979; pp. 21-23.

[8] Rothschild, M.; Optical Materials for Excimer Laser Application. Optics & Photonics News,

Vol 4, No. 5: May 1993; pp. 8-15.

[9] Schott UG-1 1 . We use trade names to specify the experimental procedure adequately and do

not imply endorsement by the National Institute of Standards and Technology. Similar products

by other manufacturers may work as well or better.

[10] Sellmeier equation, n^ = 1 + AA,V(A,^ - B) where n is the refractive index at wavelength A,, and A and B are constants. The data points for a curve fit were obtained from Schott Glass

Technologies Inc., Duryea, PA.

[II] West, E.D.; Data Analysis of Isoperibol Laser Calorimetry. National Bureau ofStandards

(U.S.) Technical Note 396: 1971; 34p.

[12] Taylor, B.N.; Kuyatt, C.E.; Guidelines for Evaluating and Expressing the Uncertainty of

NIST Measurement Results. National Institute ofStandards and Technology Technical Note

1297: 1994; 20p.

[13] Eisenhart, C; Ku, H.H.; Colle', R. Expression of the Uncertainties of Final Measurement

Results: Reprints. National Bureau ofStandards (U.S.) Special Publication 644: 1983.

[14] Wagner, S.R.; On the Quantitative Characterization of the Uncertainty of Experimental

Results m Metrology. PTB-Mitteilungen 89: February 1979; pp. 83-89.

28
APPENDIX A. Sample Calibration Report

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

ELECTRONICS & ELECTRICAL ENGINEERING LABORATORY

Boulder, Colorado 80303

REPORT OF CALIBRATION

LASER POWER and ENERGY METER

Manufacturer's Name

Meter Model ABC, Serial No. 123

Head Model XYZ, Serial No. 456

Submitted by:

Customer's, Name

Customer's Address

Calibration Summary

The laser power and energy meter was compared to the NIST standard calorimeters at a wavelength of 248

nm using a KrF excimer laser source. The laser beam incident on the detector had a circular cross section

with an approximate diameter of 1.5 cm, and the test instrument was positioned so that the beam was

centered on the absorbing surface. The power (or energy) impinging upon the test instrument was measured

using a calibrated beamsplitter and a standard calorimeter (see Figure 1). For each energy measurement, 80

pulses were deposited within a 1 second time interval and a total energy reading was recorded. For each

measurement of power, the radiation was deposited over a period of 180 seconds at a pulse rate of 40 Hz,

and the steady state power reading was recorded. The ambient temperature during these measurements was

approximately 23 °C and, before the measurements began, the test instrument was allowed to reach

equilibrium with the laboratory environment. The actual power (or energy) striking the test instrument was

calculated by multiplying the average power (or energy) measured with the reference standard by the

beamsplitter ratio. The calibration factor was then found by dividing the test instrument reading by the calculated incident power (or energy).

Table L Calibration Results (typical example)

Meter

Range

Setting

Nominal

Power

(Energy)

Input

No.

of

Measurements

Meter

Calibration

Factor

Standard

Deviation

Expanded

Uncertainty

1 W 100 mW 10 1.01 ReadingAV 0.67% ± 1.68%

1 J 250 mJ 10 1.04Reading/J 0.89% ± 1.69%

Page: 1 of 4

Test No.: xxxxxx

Date of Report: September 23, 1996

29
LASER POWER METER

Customer's Name

Meter Model ABC, Serial No. 123

Head Model XYZ, Serial No. 456

A summary of the measurements is given m Table I. If the readings of the test instrument are divided by

the appropriate calibration factor listed in the table, then the resulting values will agree with those of the NIST laser measurement standards.

Laser Shutter

T

Standard

Calorimeter

Computer

System

Figure 1 . Laser Measurement System.

Uncertainty Assessment

The uncertainty estimates for the NIST laser energy measurements are assessed using the following

guidelines. To establish the uncertainty limits, the error sources are separated into (1) Type A uncertainties,

whose magnitudes are obtained statistically from a series of measurements, and (2) Type B uncertainties,

whose magnitudes are determined by scientific judgement using all the relevant information available.

Page: 2 of 4

Test No.: xxxxxx

Date of Report: September 23, 1996

30
LASER POWER METER

Customer's Name

Meter Model ABC, Serial No. 123

Head Model XYZ, Serial No. 456

All the Type B uncertainty components are assumed to be independent and have rectangular or uniform

distributions (that is, each has an equal probability of being within the region, ±6|, and zero probability of

being outside that region). If the distribution is rectangular, the standard deviation, o^, for each Type B

uncertainty component is equal to 6/3'^' and the total standard deviation is approximated by (So/) ' where

the summation is performed over all Type B uncertainty components.

The Type A uncertainties are assumed to be independent and normally distributed and consequently the standard deviation, S^, for each component is

^ N -1

(1)

where the Xj values represent the individual measurements, x is the mean of the measurements, and N^ is the

number of Xj values used for a particular Type A uncertainty component. The standard deviation of the mean

is S/N/''', and the total standard deviation of the mean is [S(S//Nr)]'''', where the summation is carried out for

all the Type A uncertainty components.

The total uncertainty is determined by combining the Type A and Type B standard deviations in quadrature

and multiplying this result by a factor of 2. The expanded uncertainty, U, is then

u

\

(2)

The values used to calculate the expanded uncertainty are listed in Table II.

The number of decimal places used in reporting the mean value of the calibration factors listed in Table I were determined by expressing the total expanded uncertainty to two significant digits.

Page: 3 of 4

Test No.: xxxxxx

Date of Report: September 23, 1996

31
LASER POWER METER

Customer's Name

Meter Model ABC, Serial No. 123

Head Model XYZ, Serial No. 456

Table II. NIST Pulsed-UV Measurement Svstem I ncertainties

T\T)e A TypeB

SOURCE ^, N SOURCE S,

Electrical Calibration 0.50% 70 Inequivalence 0.48%

Beamsplitter Cal. 0.59% 30 Absorptivity 0.20%

Heater Leads 0.35%

Test Meter Cal. See Table I Electronics 0.10%

Injection Timing 0.10%

Beamsplitter 1.00%

Laser Instability 0.75%

Beam Pathlength 0.02%

For the Director,

National Institute of Standards

and Technology

Calibrated By:

Thomas R. Scott, Group Leader

Sources and Detectors Group

Optoelectronics Division

Rodney W. Leonhardt, Electronics Engineer

Sources and Detectors Group

Optoelectronics Division

Page: 4 of 4

Test No.: xxxxxx

Date of Report: September 23, 1996

Reference: P.O. No. #######

32
APPENDIX B. Calibration Procedure Outline

Instrument control and data analysis for the QUV calibration system are performed with custom

software running on the system computer. (This system software will eventually be changed to a PC platform which is more currently supported.) Operational procedures for the software are documented in a manual titled "Upgrade to Q-series laser calorimeter data acquisition system" and

kept on file in the calibration laboratory.

1

.

Ascertain the meter type and the desired calibration conditions fi^om the customer.

2. From the system performance specifications (outlined in Section 5.1), calculate whether the

requirements can be met.

3. Check the system output for those requirements, utilizing the calibrated lab transfer standard,

check standard #1.

4. If the power/energy levels and beam diameter can be adjusted to match customer

requirements, then the calibration can commence. At this point the customer should make

arrangements for payment and shipping with the Office of Measurement Services, NISTBoulder, (303) 497-3753.

5. Once the test meter arrives, unpack and set up the equipment to be calibrated in the QUV

enclosure. The customer should include all cables and connectors necessary to calibrate the

equipment. If NIST provides cables, then the customer should be notified that these differing

conditions may change the calibration factor. Allow the detector head and electronics to stabilize at room temperature overnight.

6. Turn on the QUV calibration system electronics; then the test meter should be activated and

allowed to warm up for at least one hour before calibration-quality measurements are made.

7. If necessary, perform electrical calibrations on a QUV calorimeter at an injection duration

and energy level to match the desired measurement conditions.

8. Turn on the excimer laser, recirculating cooling water pump, and the external cooling water

supply. When the laser is functional, use the laboratory transfer standard to set pulse energy

levels necessary for calibradons. Adjust the beam diameter and collimation to allow use w ith two calorimeters.

9. Measure laser beamsplitter ratios to confirm that the ratio has not changed.

33
10. Configure the QUV calibration system for the desired performance specifications, including

pulse energy, and beam diameter. Perform at least 8 to 10 calibration runs on the meter

under test.

11. Calculate calibration factors and measurement uncertainty using NIST statistical guidelines.

Prepare calibration report and return equipment with the original signed copy of the report to the Office of Measurement Services for return shipment.

APPENDIX C. Shipping Instructions

Meters should be shipped in well-padded foam or otherwise mechanical-shock insulated cases, appropriate for reshipment back to the customer. Operation instructions or instruction manuals

should he included, as well as the customer chosen set-up parametersfor instrumentfunctions. The

customer should include all cables and connectors that are necessary to calibrate the equipment as

specified.

The excimer laser source beam has a nonuniform intensity profile as well as a short (~35 ns) pulse

width. These two properties combine to produce laser pulses of very high peak-power which can damage absorber surfaces. Information on the damage threshold ratings of the sensor head should

be included with the information sent to NIST. Absorber material test slides are very helpful in

evaluating whether the excimer laser levels will damage a particular sensor head. Every effort is made to stay well below (5x to lOx below) the rated damage threshold, but the best way to be certain

is to expose the same absorber material under the test conditions.

34

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