Thin film measurement solutions: Hardware, software, applications

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1 Thin film measurement solutions: Hardware, software, applications We develop and manufacture wide range of optical thin-film metrology instruments from high-precision sophisticated ellipsometry and reflectometry s (spectroscopic, fast single wavelength and Imaging) for research and production applications to low-budget reflectometers for variety of applications. Our TFCompanion data analysis software is the most sophisticated and user-friendly application that supports reflectance, transmittance, ellipsometry and imaging analysis. The modular design of hardware and software enables to customize and configure each to specific application requirements without additional cost, in most cases. Optical measurements are indirect. Selection of measurement methods, configuration to meet measurement requirements are not trivial. We have extensive experience in using thin-film measurement technique and we do rigorous quantitative simulation of the performance on specific application to reliably optimize configuration and specification requirements. System Description Characteristics PicoProbe series A family of BME (photoelastic modulator based) ellipsometer s: SWEXY single wavelength ellipsometer SEXY spectroscopic ellipsometer Supports TIRE, SPR and RDS mode of operation PicoScan series TR-Probe series Imaging ellipsometer That includes BAM, TIRE, SPR, RDS support Spectroscopic reflectometry and/or transmittance in wide spectral range (VUV-NIR) Unparalleled precision and fast measurement for in-situ and exsitu applications. Full VASE capabilities. Up to 15 µm spot measurements Versatile research with spatial resolution up to 1 µm. Production for full 200mm wafer imaging (ultradense mapping) High-precision s (up to 0.01% precision). M-Probe series Spectroscopic reflectometry and/or transmittance s with fiber optics probe Spectral range: UV-NIR In-situ measurement. Wide thickness range (100A-100 µm) Ellipsometry has more powerful measurement capabilities as compared with Reflectometry or Transmittance, for example: Simultaneous measurement of thickness and optical properties of the filmstack layers; Sub-Angstrom sensitivity to thickness variations Measurement of inhomogeneous layers PicoProbe series ellipsometers offer unparallel precision and reliability. In-situ and ex-situ, R&D lab and production environment s are available. One limitation of the conventional ellipsometry is that measurement is performed in one spot. Frequently, there is a need to map the surface of the sample. Low density mapping does not present a problem but in many cases of non-uniform samples a dense map is required. As a mapping density requirements increase it is becoming impractical to use conventional approach. Another limitation is a spatial resolution of the measurement. Phone Fax

2 These problems are resolved by use of Imaging Ellipsometry that enables simultaneous/parallel measurement of the surface area in ~2 million points with resolution up to diffraction limit (~1um). The size of the measured area depends on magnification/spatial resolution. For example, one can measure full 200mm wafer in few seconds with ~ 200um resolution. PicoScan series Imaging takes Imaging ellipsometry to the new level - it is the first truly qualitative Imaging ellipsometry. However, Imaging Ellipsometry has its trade-offs as well: spectroscopic measurements can be taken only sequentially. It may be relatively slow for production environment. Spectral Reflectometry/transmittance is a more simple, fast and low cost technique. It looks attractive for production-monitoring applications that require dense mapping of the wafers and fast throughput or in-situ deposition monitoring. Combining Reflectometry with Ellipsometry or Reflectometry with Transmittance (in case of transparent substrates) or both with Ellipsometry yield excellent result on many complex multilayer applications. Oblique incidence polarized reflectance is another approach that offer good trade-off between measurement speed and performance. TR-Probe series are custom made to most rigorous specification requirement with repeatability up to 0.01% and wide spectral range from 120nm-1700nm. TR-Probe s are build on special request and for OEM customers. The majority of applications require simple, precise and low-cost that allows quick reflectance/transmittance measurement and analysis of the thin film structure. M-Probe series is a real workhorse in the R&D lab and production environment. It is available in UV- IR range and can be used in in-situ, ex-situ configuration. It is compact, affordable, and easy to use and include everything for quick measurement and data analysis. Phone Fax

3 Ellipsometer s: Instrument PicoProbe SWEXY System Type, description BME- PM Single wavelength Wavelength range 632.8nm (He-Ne laser) other laser wavelengths available Time 10ms Spot size* -3mm -15µm (MS option) Precision (σ) on 150A SiO Å (1 s integration time) Comments The best for measurement monolayers (in-situ and kinetic measurement) PicoProbe SEXY PicoScan IM-2000 PicoScan IM-2000MW PicoScan IM-2000S PicoScan IM-2000MWF BME- PM Spectroscopic ellipsometer (wavelength scanning) Modified RCE One wavelength Research imaging Modified RCE Several wavelengths Research imaging Modified RCE Spectroscopic Imaging Modified RCE Several wavelengths Large area imaging nm (UVV) nm (Vis) nm(UNI) nm (BB) One HI-LEDs Wavelength selected in ( nm) Six HI-LEDs Wavelengths in ( nm) range Extra wavelengths available nm (continuous scanning) Six HI-LEDs Wavelengths in ( nm) range Extra wavelengths available 10ms/pt 5s per 1200 x 1600 pixel Image 5s per 1200 x 1600 pixel Image 5s per 1200 x 1600 pixel Image/per wavelength 5s per 1200 x 1600 pixel Image -15µm (MS option) Depend on magnificat ion Depend on magnificat ion Depend on magnificat ion 200x 200mm 0.01 Å The best research spectroscopic Reflection/transmitta nce and in-situ modes 0.1 Å Up to 1 µm spatial resolution 0.1 Å Up to 1 µm spatial resolution 0.1 Å Up to 1 µm spatial resolution 0.1A 200 µm spatial resolution. Perfect for full 200mm wafer imaging no edge exclusion * Microspot option is available on all PicoProbe s. Spot size of 15µm allows measurement in 40x40 µm box Phone Fax

4 Goniometer choices. Several goniometer options are available for all s: 1. Precision table (computer controlled) 2. Light table (computer controlled) 3. A goniometer with up to 5 fixed angle positions (manually changed) 4. No goniometer or stand (in-situ of integration option) Precision table. The Precision Table employs a substantial casting and large diameter bearings to ensure very high rigidity. Angular resolution is better than 0.01º over 360º range. The Precision Table measures angular position using an optical encoder driven by a belt connected to the arm assembly. Smart stepping motors with embedded micro controllers are used to position the arms as well as the rotating-optical mounts, providing concurrent micro stepping (producing higher resolution than full step size) and smooth acceleration and deceleration (allowing heavier arm loading). Light table. The Light Table has sufficient rigidity & loading capacity to provide a low cost alternative to the Precision Table and can be used in many applications for both PicoProbe and the PicoScan s (Light table is not recommended for high magnification PicoScan s). Unlike the Precision Table, where the optical encoder is driven by a belt directly from the arm assembly, the Light Table relies on the precision of the mechanics to translate the motor movement into arm movement. This translation is affected by the load on the arm, and the repeatability will vary to some degree in practice. Angular resolution is better than 0.05º over 180º range. Goniometer Resolution, deg Angle range Configuration Comments (Optical angle of incidence)* Precision Table < Vertical setup Embedded Optical encoder Light Table < Vertical setup Direct step motor drive * angle >180 indicate transmittance measurement mode Precision Table with PicoProbe SWEXY Light Table with PicoProbe SWEXY Phone Fax

5 p Ellipsometry is a non-destructive and highly sensitive optical measurement technique used to analyze thin-layer stacks. It is based on the observation that polarized light reflected from the surface with thin layer is changing polarization state, e.g. linearly polarized light will, in general case, change to elliptically polarized hence, the name ellipsometry. This change of polarization state can be related to optical thickness of the layer(s). To quantify the change in polarization state one measures r / r s value, where rp, r s are reflectances of light polarized inplane of incidence and perpendicular (senkrecht) to the plane of incidence. The main equation of ellipsometry is expressed in the following form: rp r= Re( ) Im( ) tan i X 2 Re( ); 2 r = r + i r = Ψ e or = r Y = Im( r); ρ = r ; s ρ 1+ ρ, Ψ are frequently referred to as ellipsometry angles and X,Y as ellipsometry parameters. They are directly related using following equations: X = sin 2Ψcos ; Y = sin 2Ψsin Ψ, X are sensitive to polarization contrast, while,y are primarily sensitive to phase difference. There are several measurement configurations that are being used for ellipsometry measurement in instruments currently available on the market: null-ellipsometry, RPE/RAE (rotating-polarizer or rotating-analyzer configuration, RCE (rotating-compensator configuration), BME (birefringent modulator configuration). 1). Null ellipsometry. Polarizer-Sample-Compensator-Analyzer (PSCA) manual was first used by Prof. Paul Drude in The principle of operation is very intuitive and straightforward: Compensator (optical retarder) azimuth is adjusted to convert reflected elliptically polarized light to linear polarization ( compensate the phase change due to reflection from sample), Analyzer azimuth is adjusted to get null signal (no light). Zone averaging allows to remove instrumentation artifacts and improve accuracy. The can be easily automated: since measurement at null position is very difficult, one can take several measurements at off-null positions and find actual null position by mathematical fit. High accuracy can be achieved with null-ellipsometry but the measurements are slow and high light intensity is required. 2). RPE/RAE is the first photometric ellipsometer (1979, Aspnes & Studna) and the most widely used configuration for spectroscopic measurements. The does not have a compensator (wavelength dependent element), and is based on the observation that 1 st and 2 nd harmonics (of polarizer rotation frequency) in the measured signal can be easily related to ellipsometry angles: I() t = I0(1+ αcos2a+ β sin2 A), where A is angular frequency of analyzer rotation α = 0.5cos 2 Ψ ; β = cos sin 2Ψ (polarizer azimuth=45deg.) It works well for spectroscopic measurements. However, it has dead spots in the areas of ~0 deg. or 180 deg. Fig. 1 shows sensitivity of α, β parameters to SiO2 oxide thickness: sharp drop in sensitivity near 0 and cycle thickness shows the shortcomings of this. This limitation normally requires making measurements in the wide spectrum and selecting good regions (Fig. 2,3) or the use of additional optical element - adjustable retarder. 3). RCE configuration largely eliminates the problems associated with the RPE/RAE s. Even though compensator retardation is wavelength-dependent, a special measurement algorithm that uses Sin and Cos components of the 2 nd and 4 th harmonics of the signal allows

6 substantial compensation of the chromatism. This can be achieved because only two out of four measured quantities are needed to determine Delta, Psi ellipsometric angles: I( t) = I (1 + a cos 2C + b sin 2C + a cos 4C+ b sin 4 C) a b a b = sinδ sin 2Ψsin = 0 = 0.5(1 cos δ ) sin 2Ψcos = 0.5(1 cos δ ) cos 2Ψ A= 45deg, P = 45deg C- rotational frequency of the compensator. Since retardation phase is changing ~ 360 deg over nm range there will be regions of low sensitivity where RCE performs like RAE (Fig. 4,5) 4). BME configuration uses a birefringent modulator as a modulation element. Like RCE, it is a phase-modulation technique and modulation amplitude is wavelength dependent. However, the birefringent modulator amplitude can be controlled, while the regular retarder phase (in a RCE ) is fixed. This wavelength adjustment of the modulation amplitude can completely compensate for chromatism. There are several types of birefringent modulator. The two types that are frequently used are photoelastic modulator (PM) and liquid-crystal modulator (LC). a. PM uses piezoelectically generated standing acoustic wave that induces birefringence in otherwise isotropic material (i.e.fused quartz); consequently it can be used in a very wide spectral range. The modulation frequency, that depends on the length of the modulator and cannot be changed arbitrarily, is ~ 50kHz. b. LC modulators are frequently either a combination of LC polarizer and a regular compensator or a LC retarder. In order to compensate for retardation chromatism, one needs to change modulation amplitude. LC transmittance is limited to visible range. But LC modulator has one advantage it has a low modulation frequency that can be arbitrarily changed. This low frequency modulation makes it easy to implement multiplexing or parallel measurement of all spectral range. Although parallel measurement increase throughput, there is no compensation for chromatism; in this case, performance of the cannot be better than RCE configuration. PicoProbe series ellipsometers are using a special three-piece photoelastic modulator (PEM); this design allows active feedback control of the modulation amplitude to assure high long-term stability and unprecedented precision. PicoProbe ellipsometer directly measures X, Y ellipsometric parameters. Following equations describe the measured signal: I( t) = I + Xsin( Asin ωt) + Y cos( Asin ωt), 0 I I ω ω 2 J1( A) I ; Y 0 2 J2( A) I0 X = = where A, ω amplitude and frequency of PEM modulator, I ω is a signal harmonic at ω and J n a Bessel function of n th order. Using the lock-in amplifier (narrow-band detectors) in the 50kHz and 100kHz low-noise regions yields exceptional performance and allows to measure X,Y values directly. PicoProbe spectroscopic ellipsometers are working in wavelength scanning mode and takes full advantage of PEM s wavelength dependent amplitude adjustment. In this configuration, PicoProbe is using scanning monochromator and PMT detector. In some cases, the measurement speed is important e.g. in-situ measurement proprietary patented approach allows implementing fully parallel measurement using spectrometer and CCD detector. Single wavelength picometer is also available for fast and extremely precise in-situ measurements.

7 System Advantages Disadvantages Comments Null ellipsometer RAE/RPE RCE BME-LCD modulator BME- PM modulator High-precision (zone averaging removes atic errors) Easy parallel SE measurement implementation (no dispersion elements) Easy parallel SE implementation Good sensitivity for very thin films Better than RAE/RPE in all cases Low-frequency modulation (easy SE parallel measurement) Fast measurement Adjustable retardation (better than RCE in scanning mode) High frequency low noise. Very fast measurement Adjustable retardation high sensitivity (no gaps in spectrum) High sensitivity to very thin films The best SWE in-situ Table. Summary of different measurement s. Very slow measurement Need high light intensity Poor sensitivity at ~0 and 180 deg. (very thin films), gaps in spectrum. Need adjustable compensator to improve accuracy (slow down and only scanning in this case) Complex calibration and corrections for broadband use Low sensitivity regions and gaps in spectrum Fundamentally limited spectral range (~ nm) Need higher light intensity (modulator assembly transmission) SE scanning mode. High stability PEM (active feedback control, etc.) Impractical for SE measurements Modified RCE is used in PicoScan BME-PM is used in PicoProbe

8 Fig.1 RAE : Sensitivity of α,β coefficients to thickness (SiO2/Si at 70 deg., nm) as a function of thickness. Sensitivity drops sharply at very thin film or cycle thickness (these points correspond to ~180 deg) Fig. 2,3. RAE : Thin film measurement (10A SiO2/Si at nm, 70 deg.) is possible only in UV region. Sensitivity drops in visible range as ->180 deg). Fig. 4 RCE coefficients sensitivity to oxide thickness as a function of thickness (632.8 nm, 70 deg). There is a good sensitivity for very thin oxide and at cycle thickness (compare to Fig. 1 of RAE performance) Fig. 5 RCE coefficients sensitivity to oxide thickness as a function of wavelength (500A SiO2/Si). Several areas of low sensitivity due, in part, to compensator chromatism.

9 Fig. 6 PME : Sensitivity of X,Y parameters to thickness in PME as a function of the thickness (632.8 nm, 70 deg). Excellent sensitivity for very thin films, no gaps in all thickness range