IR-VASE®

J.A. Woollam Co., Inc.Ellipsometry Solutions

Accurate

Capabilities

Infrared Variable Angle Spectroscopic Ellipsometer

The IR-VASE® is the fi rst and only spectroscopic ellipsometer to combine the chemical sensitivity of FTIR spectroscopy with thin fi lm sensitivity of spectroscopic ellipsometry. The IR-VASE covers the wide spectral range from 1.7 to 30 microns (333 to 5900 wave-numbers). It is used to characterize both thin fi lms and bulk materials in research and industry. This rapidly growing technology is fi nding uses in the optical coatings, semiconductor, biological and chemical industries, as well as research labs.

The IR-VASE can Characterize:

• Bulk Substrates • Phonon absorption - crystalline materials • Optical constants (n and k , ε1 and ε2) • Surface and interfacial layers • Film thickness (single and multilayers) • Doping concentration (resistivity) • Material composition (alloy fraction) • Free carrier absorption • Chemical bonding - molecular vibrations • Anisotropy - uniaxial and biaxial

Rotating Compensator Technology

The IR-VASE utilizes exclusive measurement technology with a rotating compensator, which provides accurate ellipsometric measurements over the full range of Ψ and ∆ values. The exclusive advanced technology incorporated by the IR-VASE also offers unambiguous relative phase (Δ) measurements from 0˚ to 360˚, a patented calibration routine for the highest data quality, automated angle from 30˚ to 90˚ for optimum data on any material, and advanced measurement capabilities including anisotropy, Mueller-matrix, and depolarization data. These advances allow the IR-VASE to excel under non-ideal measurement conditions including non-uniformity, patterned regions, incomplete fi lm coverage, and double side polished substrates.

Advantages

Wide Spectral Range

Covers near to far infrared.1.7 to 30 microns (333 to 5900 wavenumbers)Resolution from 1cm-1 to 64cm-1

Non-destructive Characterization

The IR-VASE offers non-contact, non-destructive measurements of many different material properties. Measurements do not require vacuum, and can be used to study liquid/solid interfaces common in biology and chemistry applications.

No Baseline or Reference Sample Required

Ellipsometry is a self-referencing technique that does not require reference samples to maintain accuracy. Samples smaller than the beam diameter can be measured because the entire beam does not need to be collected.

Highly Accurate Measurement

Patented calibration and data acquisition procedures, provide accurate measurements of Ψ and Δ over the full range of the instrument. The IR-VASE can determine both n and k for materials over the entire spectral range from 1.7 to 30 microns without extrapolating data outside the measured range, as with a Kramers-Kronig analysis. Perfect for thin fi lms or bulk materials including dielectrics, semiconductors, polymers, and metals.

High Sensitivity to Ultra-thin Films

Spectroscopic ellipsometry data contain both “phase” and “amplitude” information from refl ected or transmitted light. The phase information from IR ellipsometry offers higher sensitivity to ultrathin fi lms than FTIR refl ection/absorbance, while retaining the sensitivity to chemical composition.

1.7μm,5900cm-1

30μm,333cm-1

Near Infrared Far InfraredMid Infrared

Pictured: Harrick FastIR™ Horizontal Attenuated Total Refl ection accessory mounted on the IR-VASE® sample stage.

Accessories

ATR Cell

The ATR cell combines the Harrick FastIR™

Horizontal Attenuated Total Refl ection accessory with the IR-VASE® to accurately measure the mid-infrared optical constants of liquids, pastes and gels. Because of the ATR confi guration, the required volume of material can be very small.

Cryostat

Temperature Range: 4.2 to 700 KelvinIncludes UHV chamber/cryostat, turbo pump, and temperature controller.* The Cryostat window material will limit the measurement spectral range.

Windows:

• Zinc Selenide (spectral range: 0.7 to 16 μm) Contact the Woollam Co. about other options

Angle: 70° (typical) (Different angles available upon request)

• Obtain the temperature-dependent dielectric functions of semiconductors and other materials, including the low temperature optical properties of fi lms and bulk materials.

• The cryostat can be installed and removed, which allows the user to switch between the standard sample stage and the cryostat.

Most materials have regions of transparency that are separated by absorbing regions, with one or more resonant absorptions (see fi gure for Acrylic fi lm). In amorphous and glassy materials, the absorption is caused by molecular vibrational resonances. For these disordered materials, the bond lengths and angles are distributed randomly, so these absorptions tend to have a Gaussian character (see fi gure for Silica). In highly-ordered materials, such as crystalline semiconductors, the vibrational resonances are no longer localized to individual molecules and they become lattice vibrations. These resonance absorptions tend to be strong Lorentzian line shapes with very narrow broadening (see Figure for crystalline SiC).

Information in the Infrared

Many of the features observed by Infrared Spectroscopic Ellipsometry are related to the following absorption mechanisms:

Vibrational Absorption

Vibrational absorption occurs when molecules and lattices resonate at infrared frequencies. These absorptions act as a fi ngerprint of the materials, as the frequencies which resonate depend on the types of bonds and weight of resonating atoms or molecules. Organic materials are the classic example of molecular vibrations, but vibrational absorptions also occur in dielectrics and semiconductors.

Acrylic Optical Constants

Wave Number (cm-1)0 1000 2000 3000 4000 5000

Inde

x of

Ref

ract

ion

(n)

Extinction Coefficient (k)

1.21.31.41.51.61.71.8

0.0

0.1

0.2

0.3

0.4

0.5

nk

SiC Optical Constants

Wavelength (μm)9 11 13 14 16

ε 1

ε2

-400

0

400

0

400

800ε1ε2

Wave Number (cm-1)800 900 1000 1100 1200 1300

0.0

1.0

2.0

3.0

4.0

5.0

6.0ε2 float glasssi-o stretch gaussian #1si-o stretch gaussian #2si-o stretch gaussian #3si-o stretch gaussian #4

This organic exhibits a large number of molecular vibrations relating to different bonds within the material.

Strong lattice vibration absorption of crystalline silicon carbide.

Absorption shape in silica glass can be modeled as a combination of various Si-O gaussian stretch vibrations.

Free Carrier Absorption Occur when free charge carriers (electrons and “holes”) are accelerated by the Infrared light electric fi eld. This is most common in metals, but can also be found in heavily doped semiconductors and conductive oxides such as Indium Tin Oxide (ITO). Free-carrier absorption is readily modeled with a Drude-oscillator, which can provide details of the material conductivity.

Wave Number (cm-1)0 1000 2000 3000 4000 5000

Extin

ctio

n C

oeffi

cien

t (k)

0306090

120150180

Gold

Doped SiliconIndium Tin Oxide

Electronic Transitions

There are a few semiconductors (such as HgCdTe) with bandgap energies in the infrared. Metals can also have interband transitions in the infrared. The line-shape for these absorptions depend on the electronic states in the material. The Gen-Osc layer provides the fl exibility to model all different absorptions in the infrared. An example Gen-Osc layer is shown in the fi gure to the right.

Comparison of the Free-carrier absorption for different conductive materials.

Gen-Osc layer with a large series of Gaussian and Lorentzian oscillator line-shapes used to match the infrared optical constants of an organic fi lm.

Applications

Figure 3

Chemical Composition via

Molecular Bond Vibrational Absorptions

Like standard FTIR spectroscopy, IR ellipsometry contains information about molecular bond via vibrational absorptions. Infrared absorption caused by these vibrations can be studied in bulk or thin fi lm materials. IR ellipsometry offers increased sensitivity over FTIR spectroscopy. It also obtains both n and k rather than just absorbance. Figures below show measured optical constants of a silicone thin fi lm with vibrational absorptions labeled.

Multilayer Characterization

Multilayer fi lms can be extensively studied with the wide spectral range and variable angle capability of the IR-VASE®. Multiple angles provide additional information by changing path length through each layer. The following results show sensitivity to 4 layers. Infrared optical contrast between similar materials allows measurement of each layer thickness.

Optical Coatings

Characterize thickness and IR index of single and multilayer fi lms. Bulk uncoated substrates. Infrared optical systems. AR, HR, single-layer and multilayer coatings. High-index and low-index.

Wave Number (cm-1)0 1000 2000 3000 4000 5000

Yin

deg

rees

0

20

40

60

80

Model Fit Exp E 55°Exp E 75°

Δin

deg

rees

0

100

200

300

SiO2 4.975μm MgF2-Al2O3 1.488μm ZnS 0.72μm Al2O3 0.014μm Silicon

Epitaxial Layers, Doping Concentration

and Doping Profiles

At infrared wavelengths, there can be optical contrast between epitaxial or implanted layers and under-lying substrate due to differences in free-carrier density. This gives IR-VASE® excellent sensitivity to epitaxial layer thickness and substrate doping concentration. It also has good sensitivity to carrier gradients at interfaces. Carrier profi les are in excellent agreement when comparing nondestructive IR-VASE® and destructive SIMS and Spreading Resistance Probe measurements.

T.E. Tiwald et al., Phys. Rev. B, 60 (1999) 11 464.

Phonon Structure (Compound Semiconductors)

The wide spectral range IR-VASE® is important for phonon absorption studies of compound semiconductors and other crystalline solids. Data on the left show phonon modes of a GaN / AlGaN laser structure, modeled to determine alloy ratios, doping concentrations, and fi lm quality.

M. Schubert et al., SPIE Vol. 4449-8 (2001)

Research

Specifications

Spectral Range 1.7μm to 30μm (333cm-1 to 5900cm-1)

System OverviewPatented rotating compensatorellipsometry, simultaneous data collection with Fourier Transform Method (FTIR). Vertical sample mounted with vacuum-assist for flexible measurement configurations.

Angle of Incidence26° to 90° Standard

Data Acquisition Rate1 to 30 minutes, typical (1 angle of incidence at 16cm-1 resolution)*Finer resolution will require longer time.

Publications

“Application of IR variable angle spectroscopic ellipsometry to the determination of free carrier concentration depth profi les” Authors: H. Arwin, A. Askendahl, P. Tengvall, D.W. Thompson, J.A. Woollam Thin Solid Films, 313-314, (1998) 661-666.

“Determination of the Mid-IR Optical Constants of Water and Lubricants Using IR Ellipsometry Combined with an ATR Cell” Authors: T. Tiwald, D. Thompson, J. A. Woollam, and S. V. Pepper Thin Solid Films, 313-314, (1998) 718-721.

“Carrier concentration and lattice absorption in bulk and epitaxial silicon carbide determined using infrared ellipsometry” Authors: T.E. Tiwald, J.A. Woollam, S. Zollner, J. Christiansen, R.B. Gregory, T. Wetteroth, S.R. Wilson, A.R. Powell Phys. Rev. B, 60, 16, (1999) 11464-11474.

“Infrared dielectric anisotropy and phonon modes of sapphire” Authors: M. Schubert, T.E. Ti-wald, C.M. Herzinger. Phys. Rev. B, 61, 12, (2000) 8187-8201.

“Free-carrier and phonon properties of n- and p-type hexagonal GaN fi lms measured by infrared ellipsometry” Authors: A. Kasic, M. Schubert, S. Einfeldt, D. Hommel, T.E. Tiwald Phys. Rev. B, 62, 11, (2000) 7365-7377.

“Infrared ellipsometry studies of thermal stability of protein monolayers and multilayers”Authors: H. Arwin, A. Askendahl, P. Tengvall, D.W. Thompson, J.A. Woollam physica status solidi (c), 5, 5, (2000) 1438-1441.

“Use of Molecular Vibrations to Analyze Very Thin Films with Infrared Ellipsometry” Authors: H. G. Tompkins, T. Tiwald, C. Bungay, and A. E. Hooper J. Phys. Chem. B, 108, 12, (2004) 3777-3780.

“Toward Perfect Antirefl ection Coatings. 3. Experimental Results Obtained with the Use of Reststrahlen Materials” Authors: J. A. Dobrowolski, Y. Guo, T. Tiwald, P. Ma, and D. PoitrasApplied Optics, 45, 7, (2005) 1555-1562.

Infrared

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