Optical Technology Division

Technical Highlights

• SURF UV Radiometry Facility. The Optical Technology Division and the Electron and Optical Physics Division have obtained first results from a new UV radiometry facility at the Synchrotron Ultraviolet Radiation Facility (SURF II). Synchrotron radiation in combination with a monochromator provides a broadly tunable source of radiation that is especially useful for UV radiometry. The facility combines a high-throughput normal-incidence monochromator with an absolute cryogenic radiometer (ACR), optimized for UV measurements, to provide absolute detector-based radiometric calibrations (of absolute spectral responsivity) in the spectral range from 125 nm to 320 nm.

  The optical system delivers several microwatts of radiation with a bandwidth of 1 nm in the wavelength range from 125 nm to 320 nm. The facility has been used to characterize the absolute spectral responsivity of a wide variety of quantum detectors, e.g., Si-based UV diodes, PtSi, GaN and diamond detectors. The spatial uniformity of the detectors has also been measured by rastering the synchrotron beam across the detector area. The facility can be easily adapted to other spectroradiometric measurements including UV transmittance and reflectance of optical materials.

  The new radiometric beamline has also been used to measure the absolute internal quantum efficiencies (number of electron-hole pairs formed for every photon absorbed) in photodiodes and the change in the quantum efficiencies resulting from damage by UV irradiation. (See cover illustration.) This unique capability enables detailed studies of the damage mechanisms responsible for photodiode degradation in the UV. (J.L. Dehmer, G. Eppeldauer, K. Lykke, and P-S. Shaw)
• New Measurement Service Improves Aircraft Safety. The Optical Technology Division has introduced a new measurement service to address an urgent need in aviation safety. Aircraft are equipped with flashing "anticollision" lights to provide a visual warning to other pilots of an aircraft’s location. However, a lack of visibility of some lights has been cited by the National Transportation Safety Board (NTSB) as a contributing factor in airliner crashes, including the USAir crash at LAX in 1991. The problem has received wide media attention, including an investigative report by NBC Dateline last summer.
  
  To help solve the problem, NIST worked with the FAA, which specifies the requirements for the effective intensities of the anticollision lights and enforces their maintenance on all commercial aircraft. Prior to NIST’s involvement, there had been large variations in measurements due to the absence of standard procedures and calibration standards. Now NIST has established flashing-light photometric standards and offers a new measurement service for calibrating photometers for both white and red anticollision lights. The flashing-light photometric unit (lx s) has been realized based on the NIST detector-based candela. The relative expanded uncertainty of the NIST flashing-light standards in this measurement is estimated to be 0.6 %. This work was reported recently to the aviation industry at an SPIE symposium. (Y. Ohno)
• Deep-UV Index-of-Refraction Measurements for Photolithography. The Optical Technology Division and the Atomic Physics Division have completed initial measurements of the index of refraction of fused silica near 193 nm. These measurements are critical in the race to develop photolithographic wafer steppers for future-generation integrated circuit manufacturing. This collaborative project with MIT Lincoln Laboratory and SEMATECH seeks to develop the infrastructure required to use 193 nm excimer-laser emission to form 0.18 µm feature sizes for products such as gigabit memory chips. These results keep industry on track to meet the Semiconductor Industry Association roadmap target date of 2001 for commercial production of these chips.

  The NIST researchers have made high accuracy, temperature- and wavelength-dependent index-of-refraction measurements on optical materials that are suitable for 193 nm photolithography. Design engineers need this accurate data to achieve the exacting performance required of the photolithography tools. To meet the immediate need, we upgraded a precision refractometer, including precisely characterized UV line sources, to enable minimum-deviation angle, refractive-index measurements that are accurate to 1 part in 10⁵ with a temperature control of 0.1 °C. For the longer term and for shorter wavelengths, we are developing interferometric methods capable of even higher accuracy. (R. Gupta)

• Infrared Transfer-Standard Detectors Developed. Infrared detectors are now available that can provide NIST traceability for radiometric applications in the 2.5 µm to 30 µm wavelength range. The detectors have a blocked impurity band design employing arsenic-doped silicon. They were developed at the NIST Low Background Infrared Calibration Facility (LBIR) in conjunction with Rockwell Corporation and funded by the Ballistic Missile Defense Organization. Operating at 12 K, these detectors are unique in their spectral range; they have a high degree of spatial uniformity and ultra-low noise. These detectors meet the requirements of the aerospace industry and other government agencies to perform radiometric calibrations of satellite sensors for a wide range of needs from environmental remote sensing to military applications. (S. Lorentz)

Temperature Measurement for Rapid Thermal Processing. During the production of semiconductor devices, the surface of a silicon wafer becomes covered with increasingly complex multilayer patterns. These patterns can cause localized changes in absorption and emission of heat during thermal processing, resulting in temperature non-uniformities and difficulty in accurately measuring the wafer temperature. The Optical Technology Division, in collaboration with the Process Measurements Division of CSTL, has initiated a project to develop methods to measure silicon wafer temperatures in a rapid thermal processing (RTP) environment to ±2 °C accuracy in a 600 °C to 1000 °C range. The project will establish reliable contact and optical thermometry traceable to NIST temperature standards. This tracks the Semiconductor Industry Association Roadmap goals for RTP thermometry. (B.K. Tsai)

Figure 1. Rapid thermal processing chamber.

• Development of a New Infrared Source. The Optical Technology Division and the Atomic Physics Division have collaborated to develop a new, brighter infrared source. The new source yields better signal-to-noise ratios, and therefore higher accuracy infrared measurements. The source is a stabilized argon arc, which has been characterized in the spectral range of 1 µm to 20 µm. Its radiance was calibrated and found to be approximately equal over much of this range to that of a 10,000 K blackbody. A high-resolution spectrum taken with a FTIR instrument shows mostly line emission below 5 µm, and pure continuum emission between 5 µm and 20 µm. The stability and geometrical properties of the radiance were determined, as well as its dependence on pressure and current. This source is now being used in calibrating IR detectors, as well as in projects aimed at advancing IR measurements and technology. (A.L. Migdall)
• Short Course in Radiation Thermometry. On May 6-8, 1997 the Optical Technology Division and the ASTM Committee E20.02 on Radiation Thermometry inaugurated a new Short Course on Temperature Measurement by Radiation Thermometry. It was designed by NIST experts and by Dr. David DeWitt, professor at Purdue University and editor of Theory and Practice of Radiation Thermometry. It was conducted in the new Facility for Advanced Radiometric Calibrations (FARCAL).

  The course consists of lectures covering the fundamentals of radiometry and temperature measurement, complimented with hands-on, skill-building, problem-solving laboratory experiments. During these exercises the participants learn ASTM voluntary industry standard test methods. They gain practical laboratory experience using commercial radiometers and blackbody sources that are loaned to the ASTM committee, and learn first hand about the treatment of the measurement equation and proper uncertainty analysis.

  The enrollment for the course was limited so that each laboratory instructor could concentrate on at most four students. Fourteen participants enrolled in and completed the course, including representatives from federal agencies such as NASA, the Navy, Los Alamos National Laboratory, and NIST; and industries located in Colorado, California, Oregon, and Ohio. Based on its success, the course will be offered again next year. (C. Johnson)
• Sensitive Diagnostics of Narrow-band Infrared Filters. Infrared filters transparent only over a small range of wavelengths are used in applications such as two-color optical pyrometers, filter radiometers, and military guidance and infrared seeker systems. Small "leakage" bands with transmittance of 10^-5 to 10^-4 are often present in these filters in wavelength regions where they are nominally opaque. This leakage is of critical importance in applications where an infrared sensor must be shielded from unwanted radiation.

Figure 2. Transmittance of a narrow band-pass filter.

The Optical Technology Division has developed capability to measure the out-of-band rejection of narrow-band infrared transmittance filters down to a transmittance level of 10-6, over a wavelength range from 2 µm to 20 µm, at temperatures from 10 K to 300 K. To allow the out-of-band transmittance to be determined with high sensitivity, the measurements are performed using an FTIR spectrometer and a set of high- and low-pass optical filters to block the main transmittance band of the filters under test. Temperature-dependent out-of-band rejection measurements of a set of narrow band filters for the Hypersonic Aircraft Launch Option (HALO) program of the Ballistic Missile Defense Organization have recently been performed. Measurements for various sets of filters used for the HALO program will continue over the next few years. (S. Kaplan)

New Instrument for Calibrating Optical-Density SRMs (Step Tablets). Measurements of optical transmission density are important for the medical, photographic, and graphic arts industries, as well as non-destructive testing of a variety of materials. Hundreds of Standard Reference Material units for transmission density, in the form of both x-ray and photographic step tablet films, were available and sold from NIST until a few years ago. The Optical Technology Division has recently completed development of a new instrument for measuring transmission densities of step tablets, using the diffuse influx method. It is designed to comply with the industrially important ANSI/ISO standards for transmission density measurement. The basis of the instrument is a temperature-controlled silicon photodiode with amplifier electronics capable of measuring signals spanning seven orders of magnitude, allowing transmission densities as high as six to be determined. The instrument is designed to automatically measure many tablets using computerized data acquisition and control of all of the components involved in the measurement, enabling reliably, fast, and routine measurement of many tablets. Tablets measured on the new instrument are now available from NIST as Standard Reference Materials. (T. Early)

Figure 3. Step tablet measurement instrument.

• Infrared Arrays for FTIR Microscopy. Infrared spectroscopy has long been used to identify chemical species and their environments. Molecules of different species have characteristic spectral absorption features in the 2.5 µm to 11 µm wavelength region, which provides distinctive "fingerprints" for their identity. Coupled with microscopy, spatially resolved infrared spectroscopy is a versatile measurement tool with applications including materials and film characterization, biological research, and medical diagnosis. However, the application of infrared microscopy has been limited by the brightness of infrared sources and by the use of single-element infrared detectors, which necessitate slow rastering of the probed areas to generate images.

  Figure 4. Infrared image of a mouse brain, with bright and dark areas showing relative levels of lipids and proteins indicative of disease.

  In a collaborative effort, the Optical Technology Division and NIH have demonstrated rapid, sensitive spectral imaging in the infrared-fingerprint region by attaching NIST's mercury-cadmium-telluride (MCT) infrared detector array to NIH's step-scan Fourier transform infrared microscope. The NIST MCT array is a 256×256-pixel focal-plane detector array, originally designed for DoD projects but now available for civilian applications. An interferometer with a glow-bar source step its moving mirror through successive positions while the MCT array obtains background-corrected infrared images. In this way, 65,536 time-domain interferograms are obtained each corresponding to a specific spatial position of the sample. Fourier transforming this massive data set (~100 MB) yields spectrally sensitive images with 10 µm spatial resolution and 16 cm-1 spectral resolution over the entire 2.5 µm to 11 µm spectral region. The NIST/NIH team obtained chemical images of inhomogeneous polymer and lipid samples, laminates, and brain tissue, illustrating a wide range of industrial and biomedical applications for this new imaging technique. (E. Heilweil)

• New Invention Aids Semiconductor Manufacturing. Optical scattering is used in the semiconductor industry to measure microroughness and to detect particulate contaminants and subsurface defects on silicon wafers. As the feature sizes in modern integrated circuits continue to shrink, ever-stricter demands are being made on instruments to detect smaller and smaller contaminant particles. One important issue that limits the sensitivity of such instruments is that light scattering due to the silicon wafer microroughness can obscure or overwhelm the scattering due to particles. This problem is widely recognized. The Semiconductor Industry Association (SIA) Roadmap declares that detection of particles with diameters less than 0.1 µm is a potential "show stopper."

  The Optical Technology Division is pursuing a novel measurement technique that can solve this problem. The new method, called bi-directional ellipsometry, showed that light scattered by microroughness has a characteristic, well-defined polarization for each scattering direction. Other scattering sources, such as particle contaminants and subsurface defects, scatter with different characteristic polarizations. The discovery of polarization signatures enabled the design of a microroughness-blind hemispherical optical-scatter-measuring instrument. A provisional patent application has been filed on this invention, which addresses the issue of measuring nanoscale particles on silicon wafers. However, it is expected that bi-directional ellipsometry also will be a powerful technique for identifying and characterizing defects in optical components, data storage materials, and film coatings. (T. Germer and C. Asmail)

  Figure 5. The signature pattern of polarization of light scattered from a microrough surface, for p-polarized light incident at 45°.

• Precision Chemistry of Oxygen Reactions. A new oxygen-atom source developed by the Optical Technology Division enables difficult measurements of the chemistry of oxygen reactions. The new source uses laser-induced dissociation of ozone, followed by efficient quenching of reactive species in collisions, to produce a molecular-beam pulse of oxygen atoms that are purely in the ground electronic state. The chemistry of these atoms plays essential roles in combustion, propulsion, and atmospheric systems, but they have not been well studied in the past because available oxygen-atom sources typically produce highly energetic species that mask the reactions of the ground-state atoms.

  The energies of the oxygen source and target molecules can be tuned to induce near-threshold, single-collision reactions. Lasers are used to optically manipulate the reactants to produce highly controlled reaction conditions, enabling the study of the molecular forces that govern the basic chemical processes. Example targets are molecules such as hydrogen, water, methane, and silane, which play critical roles in environmental and industrial processes.

  The data developed by the NIST apparatus will help to improve and validate theoretical models of chemical processes that involve oxygen, including quantum-mechanical models of the most elementary reactions, climate-change predictive models, and industrial-process models. Data on ground-state oxygen-atom interactions are also needed to understand and control the interactions of spacecraft in low earth orbit, such as certain communication and instrumentation satellites, the Space Shuttle, and the Space Station, as well as hypersonic aircraft. (M. Casassa, D. Plusquellic, and J. Stephenson)

  Figure 6. Laser-induced fluorescence spectrum of the OH(ν=1,N=5) products of the oxygen-silane reaction. The absorption line shows a distribution of Doppler shifts indicative of the kinetic energy released in the reaction.

• New Infrared Beamline Installed on NIST Storage Ring. The Optical Technology Division and Electron and Optical Physics Division have developed a new facility for infrared spectroscopy and microscopy at the NIST Synchrotron Ultraviolet Radiation Facility (SURF II). The facility was completed and initial tests were performed prior the shut down of SURF II in FY97 for upgrade to SURF III. These data will guide beamline modifications to be undertaken during FY98.

  The NIST storage ring can provide two to three orders of magnitude more brightness for infrared microscopy than conventional black-body sources. This additional flux-per-unit-area will enable high-resolution spectroscopy with spatial resolution near the diffraction limit, and analysis of samples with extremely low concentrations of absorbers.

  The infrared radiation emitted from the storage ring is transported to a Fourier transform (FTIR) spectrometer that is coupled to an infrared microscope. These instruments are housed in a clean room that affords an environment suitable for advanced spectromicroscopy research. Once SURF III is operational, the infrared spectromicroscopy facility will be available to internal and external users for microscopic chemical analyses of a variety of samples from industry, the forensic community, and researchers at NIST. (A. Hight Walker)



Figure 7. IR beamline and IR spectromicroscopy laboratory at SURF.

• Rotational Linestrengths and Self-Pressure-Broadening Coefficients for O₂. The 1.27 µm a¹ Δ_g-X³ Σ^-_g, v=0-0 band of O₂ is observed in atmospheric absorption against the solar background and in emission in the twilight airglow. The electronically excited a¹ Δ_g O₂ responsible for the airglow is produced mainly by solar photolysis of O₃ via the Hartley bands, which has lead to efforts to determine O₃ concentrations in the mesosphere and thermosphere by satellite monitoring of the 1.27 µm emission. The Solar Mesosphere Explorer (SME) satellite and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite follow this approach. However, accurate spectroscopic parameters are needed in order to use the 1.27 µm band for atmospheric modeling and sensing. For altitudes between 30 km to 75 km, a 15 % error in the Einstein A coefficient for spontaneous emission by a¹ Δ_g O₂ gives a similar error in the O₃ concentration, and recent experiments suggest errors as large as a factor of two.

  We have undertaken accurate laboratory measurements of the rotational line strengths and self-pressure-broadening coefficients for the 1.27 µm band of O2 using a long-path-length, high-pressure, White cell and a high-resolution, Fourier transform infrared spectrometer. Our analysis determines an Einstein A coefficient for 1.27 µm emission by a1 Δg O2 of 2.226(50) × 10-4 s-1 (uncertainties are one standard deviation), which agrees to within ~15 % with the nearly 30 year old measurement of Badger et al. The present measurement is a factor of 1.5 larger than the most recent laboratory value of Hsu et al., as corrected by Mlynczak and Nesbitt. Our results together with recent atmospheric studies of the band demonstrate that the Hsu et al. measurement is in error. (G.T. Fraser and W.J. Lafferty)

  Figure 8. Observed (top) and simulated spectra of the 1.27 µm band of O2 .

• Magneto-Raman Spectroscopy of Solid State Materials. In a new program in Raman spectroscopy of materials, attention is focused on high-T_C superconductors, giant magnetoresistance (GMR) materials, and thin films. Studies are performed with and without the presence of a magnetic field (H_max = 8 tesla) and with sample temperatures ranging from 4 K to 325 K. High temperature superconductivity and GMR are manifestations of strongly correlated electrons in a solid medium. For these phenomena there are several problems whose solutions stand at the center of current research. Our studies have been devoted to the Raman scattering by phonons, free carriers, and polarons.

  Figure 9. Raman spectra of an undoped LaMnO3 single crystal at room temperature. Different scattering geometries a(yy), b(xy), c(zz), and d(xz) reveal the Ag, B1g, Ag, and B2g modes, respectively.

  Specific systems that we have investigated include Yttrium-Barium-Cuprate (YBCO), Lanthanum Manganate (LaMnO₃), alkaline-earth-doped Lanthanum Manganates (La_1-xM_xMnO₃ with x=0, 0.1, 0.2, and 0.3, and M=Sr, Ca) and Manganites, (La_1.2Sr_1.8Mn₂O₇), Copper-Germanium-Oxide (CuGeO₃) and thin films of AlN and GaN. We plan to extend these investigations to other materials and dopings, and also to thin films of ferroelectric materials. Much of the work is done in collaboration with staff of the University of Maryland, Howard University and others. (A. Weber)

• Femtosecond Far-Infrared (THz) Spectroscopy. The discovery that ultrashort pulses of THz radiation are produced when femtosecond optical pulses impinge upon biased semiconductor antennas or oriented semiconductor crystals has attracted widespread interest. These THz generators coupled to interferometers and bolometer detectors, or optically gated antenna/crystal detectors, directly yield intensity, phase, and refractive-index information of absorbing materials in the THz (>50 fm) frequency range. Pulsed THz radiation with gated detection also eliminates ambient background signals and thus yields superior signal-to-noise over conventional FTIR techniques. Fourier transforms of detected THz pulse electric fields show that the spectral coverage is broadband in the 1 THz to 30 THz range. While this technology has great potential for remote sensing, imaging, and process-control applications, the characteristics of the THz devices are poorly understood.

  Figure 10. Temperature dependent output of an ultrafast THz-emitting antenna constructed on semi-insulating GaAs.

  The Division has initiated an effort in pulsed-THz spectroscopy to (1) develop measurement methods for characterizing THz pulses, (2) to study semiconductor electronic processes which govern the THz output spectrum and power, and (3) to develop measurement applications for THz pulses. For example, to better understand THz generation when GaAs and low-temperature-grown GaAs (LT-GaAs) antennas are used, temperature-dependent measurements were made which revealed a four-fold power increase as the generator temperature is lowered from room temperature, followed by an abrupt drop in output power below 90 K. Bulk GaAs excited electronic properties are inadequate to account for this effect - it is believed to arise from the time-dependent mobility of carriers in the large (KV/cm) DC-biased fields of the antennas. (E. Heilweil and A. Markelz)