Optical Technology Division

Technical Highlights

• NIST Rapid Thermal Processing Temperature Measurement Facility. This project is developing advanced methodologies for making accurate temperature measurements in rapid thermal processing (RTP) tools using radiation thermometers (RT). To achieve the goal of ±2 °C, major emphasis was placed in three areas this year. In order to establish the temperature scale in the tool using a calibration wafer, the radiation thermometers were compared with the NIST thin-film thermocouples (TFTC). When the differences of the RT and TFTC temperatures were plotted against the TFTC temperature for two separate data sets, the results indicated a variation of about ±1 °C between the two data sets. The radiation in the RTP production tools was characterized to explain any differences between radiation and contact temperature scales. Characterization of the radiation environment inside the RTP chamber included three types of modeling with progressively more complex effects. Results from the three models have shown sufficient agreement for validation cases. Actual cases will be further analyzed with these models. (R. Saunders, B. Tsai, and D. DeWitt)
• High Heat Flux Sensors Calibration Competence Program. A major development was the successful commissioning and validation of a spherical blackbody facility to calibrate heat flux sensors up to 100 kW/m² at radiant source temperatures of about 1300 K. Extensive experimental studies on Schmidt-Boelter and Gardon heat flux sensors were conducted to investigate the absolute and transfer calibration techniques. The results of the study demonstrated that, for successful application of the absolute technique, it is necessary to account for the heat transfer effects at the sensor surface due to induced flow effects of the hot gas from the furnace. However, the transfer calibration technique presently in use at NIST showed good agreement in calibration over a wide range of convective heat transfer conditions at the sensor surface and also with intercomparison calibration results from the 25 mm Variable Temperature Blackbody primary facility. The success of the transfer calibration technique using the new spherical blackbody suggests its use in future calibration services. (B. Tsai, M. Annageri, and R. Saunders)
• Absolute Detector-Based Spectral Irradiance and Radiance Temperature Scale. The present NIST spectral irradiance scale and radiance temperature scale is based upon the freezing point of gold at 1337.33 K, although the output of a 1000 W FEL spectral irradiance lamp sold as a calibrated source is more closely approximated by the output of a nearly 3000 K high-temperature blackbody (HTBB). The new, proposed scale uses broad-band, filtered, silicon detectors calibrated for absolute spectral response derived from the HACR to determine the radiance temperature of a 3000 K graphite blackbody. The spectral irradiance of FEL lamps used as working standards can be directly assigned by comparison to the 3000 K (±1 K) blackbody, and the final uncertainties in the spectral irradiance of issued FEL lamps will be reduced by the new, more direct scale assignment. Preliminary radiance temperature assignments of the HTBB using the old source-based scale and the new detector-based scale are in agreement to <0.5 K at 2500 K and higher temperatures. Work is in progress to issue FEL lamps using the new detector-based scale. Since the filter radiometers are stable over time, the radiance temperature scale can be maintained on the filter radiometers as opposed to maintaining the series of blackbody sources. Recent results show agreement between the absolute detector-based radiance temperature and the current radiance temperature scale to ≤ 0.5 K from 2200 K to 2800 K. This will provide the basis for the radiation temperature scale's reference to a detector base. Work is in progress to extend the comparison to the gold freezing point (1337.33 K). The results will be presented at TEMPMEKO '99. (H. Yoon, C. Johnson, and R. Saunders).
• Integrating Sphere System for Absolute Infrared Reflectance, Transmittance, and Absorptance. A new device and method has been developed for characterizing the optical properties of materials and components in the infrared from 1 µm to 19 µm. The system employs a custom integrating sphere mounted on motorized rotation stages, and is connected to a Fourier transform spectrophotometer. All types of samples can be characterized, but for those that do not scatter light absolute transmittance, reflectance, and absorptance can be determined with high accuracy. The sphere-based absolute method has a number of advantages over other methods commonly used. Primarily, the sphere system is insensitive to beam alteration effects due to attributes of the sample under test.
  
  The integrating sphere system is used as the FTIR Spectrophotometry Laboratory's "reference instrument" in support of both internal Division and NIST projects as well as external programs and customers. For example, it has been used to characterize mirrors in support of the SIRTF (Space Infrared Telescope Facility) and SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) programs. (L. Hanssen)

Absolute Radiometer Using High-Temperature Superconductors. Absolute electrical substitution radiometers have been used for several years at liquid-helium temperature by standards laboratories and others for high-accuracy implementation of radiometric scales. Room temperature versions, more convenient though often lacking in sensitivity and accuracy, have also been available for many years. Now a collaboration of NIST scientists from the Optical Technology Division in Gaithersburg and the Electromagnetic Technology Division in Boulder have built and demonstrated a radiometer that strikes a useful compromise between accuracy and convenience by requiring only liquid-nitrogen cooling yet providing adequate sensitivity and accuracy for many calibration needs. This High-Critical-Temperature Space-Based Active Cavity Radiometer (High-Tc SACR) uses extremely sensitive temperature sensors made from thin film YBa2Cu3O7, a high-Tc superconductor. By operating the sensors on the resistive edge of the superconducting phase transition at a temperature near 90 K, unprecedented sensitivity for an active cavity radiometer at these temperatures was achieved. For example, an absolute flux measurement of 10 microwatts can be made with a noise-limited 1-σ uncertainty below 0.2%. This level, though not nearly as low as can be obtained with a liquid-helium-cooled device, is acceptable for a wide market of calibration labs that seek easier, cheaper ways of making accurate optical power measurements. The device is shown in figure 1. (J. Rice)

Figure 1. Section view of the high-Tc SACR (Tc = critical temperature; SACR = space-based active cavity radiometer).

• TASSII. A team, led by the Naval Research Laboratory (NRL) and including NIST's Optical Technology Division staff, was selected by NASA for the Phase B study of the Total Solar Irradiance Monitor (TSIM). This satellite, scheduled to be launched in 2001 and have a five-year lifetime, will orbit the earth at about 600 km altitude and stare at the sun. The major objective is to monitor the solar irradiance (power per unit area) reaching the top of the Earth's atmosphere. Though once thought of as a constant, the solar irradiance has been measured to vary by about 0.1% over the past two decades by instruments on previous satellites. This variability is considered a major enough forcing element of climate variability that NASA plans to continue monitoring this with small satellites. The instrument being designed by the NRL/NIST team, called the Total and Spectral Solar Irradiance Investigation (TASSII), utilizes the latest absolute radiometer expertise contributed by the NIST scientists to achieve unprecedented precision. Furthermore, the planned calibration of TASSII instruments directly against NIST radiometric standards will enable unprecedented accuracy of the solar irradiance measurements. This will be the first time a NIST absolute radiometer is used to measure the solar constant from space. (S. Lorentz)

Theory of X-ray and Optical Absorption Spectra of Solids. The Optical Technology Division has developed theoretical and computational techniques to predict optical properties of materials from first principles. Work over the last year has included computing the complex dielectric constant of several technologically important semiconductors and insulators and the calculation of x-ray absorption near-edge structure. The critical "new" ingredient of such calculations, now included in much more detail than previously, is the interaction between the electron and hole that are created as a pair during absorption. The techniques that have facilitated computing the above optical properties will continue to be generalized and extended for treating other spectral regions and types of experimental data. Results for silicon are shown in figure 2. (E.L. Shirley). Figure 2. Imaginary part of dielectric constant (ε2) vs. photon energy as measured (dashed) and computed (solid), with previous results in top panel and recent results in bottom panel.

Magneto-Raman Spectroscopy of Novel Condensed Matter Systems. The current effort on magneto-Raman spectroscopy concentrates on the investigation of charge, spin, and lattice excitations in novel materials that are at the forefront of condensed-matter physics research. These studies are conducted over a wide range of temperatures (4 K to 350 K) and magnetic fields (up to 8 Tesla). The materials of current interest are based on transition-metal oxides, which manifest technologically important physical properties such as high-temperature superconductivity in copper-oxides, colossal magnetoresistance (CMR) in manganese-oxides, and ferroelectricity in lead-strontium titanates. This work is done in conjunction with the Materials Research Science and Engineering Center at the University of Maryland. In addition, our collaborators from Argonne National Laboratory and Nagoya University, Japan, provide high-quality single-crystals. Figure 3. Raman spectra of the layered manganite LaSr2Mn2O7 showing the activated peaks for T <Tco. Applying a magnetic field H=7 Tesla melts the charge-ordered state.

Much of the present work is focused on layered manganites, La_n-nx Sr_1+nx Mn_n O_3n+1, since they could provide new insight towards understanding the unconventional, normal-state properties of the cuprates. With the layered manganites, the effects of dimensionality (by varying n, the number of MnO₂ layers) and doping (by changing x) on polaron formation and charge and spin dynamics are explored. Figure 3 shows results for one of the LaSr₂Mn₂O₇ samples. As an example, our observations of Raman signatures of local symmetry-breaking due to polarons and real-space ordering of charges have important implications on models of the CMR phenomenon in the manganites. On a more applied front, a major obstacle in integrating non-volatile ferroelectric memories with silicon-based technologies is the loss of electric polarization upon hydrogen annealing of Pb(Zr,Ti)O₃ thin-films. Using Raman spectroscopy, we have demonstrated that hydrogen is incorporated at interstitial tetragonal sites which prevents the Ti ion from switching. (D. Romero and V. Podobedov)

• Short Course in Photometry and Colorimetry. The Optical Technology Division hosted the first NIST Photometry Short Course on September 10th and 11th at NIST, Gaithersburg, with 19 participants from industry and academia. The number of participants had to be limited due to the experimental sessions provided in the course. The course addressed the need for education and training for photometry engineers and technicians in industry that had been identified recently by CORM, the Lamp Testing Engineer's Conference (LTEC), and other metrology groups within industry. The course was mainly aimed at customers of NIST photometric calibrations and, more widely, technicians and engineers engaged in photometry work in industry. The course covered fundamentals in photometry, radiometry, colorimetry, uncertainty analysis, quality systems, and practical aspects of measurements of luminous flux, luminous intensity, illuminance, luminance, color temperature, and chromaticity of light sources. The course consisted of lectures by NIST photometry experts and an invited lecturer, Dr. G. Sauter from PTB, Germany. Based on its success and future demand, the short course will be offered again next year with an extended schedule. (Y. Ohno, S. Brown, M. Navarro, Y. Zong, and S. Bruce)

• New 2.5 m Integrating Sphere for a Novel Luminous Flux Calibration Method. A new 2.5 m integrating sphere has been built and installed in the NIST photometry laboratory. This sphere replaces the previous 2 m integrating sphere used for many years. The new sphere, designed by the Division's photometry experts, is equipped with a beam scanner and an external source and features a capability for the detector-based measurement of total luminous flux of light sources. The beam scanner, consisting of a small, tungsten-lamp beam source and two rotation stages, allows automatic measurement of the spatial nonuniformity of the sphere responsivity over the entire sphere wall. The external source, consisting of a stable tungsten-halogen source and an aperture/photometer wheel, allows precise calibration of the sphere responsivity based on the illuminance of the introduced light measured by a reference photometer.

  This new method makes possible the absolute measurement of total luminous flux using an integrating sphere rather than a costly, large goniophotometer. The Division is helping BIPM (International Bureau of Weights and Measures) to adapt this method to maintaining the lumen in their laboratory also. This new NIST facility now provides calibration of luminous flux of lamps, with no need for traditional working standard lamps and with reduced uncertainty of measurements. (Y. Ohno)

• Short Course on Ultraviolet Radiometry for Lithography. There was a Short Course presented at the SPIE 23^rd Annual Meeting held in Santa Clara, CA on Ultraviolet Source and Detector Radiometry in Semiconductor Production Microlithography. The course was taught by R. Gupta (Div. 844), J. Burnett (Div. 842), and M. Dowell (Div. 815). The course, geared to the semiconductor industry, included an introduction to the basic concepts of optical radiometry in the deep and vacuum UV, and the applications of UV radiometry to current and future semiconductor lithography tools. The NIST instructors also outlined procedures for maintaining traceability to high-accuracy national radiometric standards. A wide variety of U.S. and Japanese semiconductor industry representatives attended, including those involved in stepper design and manufacturing, laser applications, lithography dose metrology, lithography source designers, and optical engineering. (R. Gupta)

• New ACR-based Monochromator System for an Improved Near IR Scale. We have developed a detector calibration system by using a monochromator system and an ACR (absolute cryogenic radiometer) for the near IR from 900 nm to 1600 nm. With the high accuracy of an ACR, the system achieves detector calibration to an uncertainty of less than 0.5 % in most of this region. We have already measured the responsivity of several InGaAs photodiodes and pyroelectric detectors. These detectors will be used as working standards for the SCF (spectral comparator facility) to reduce the uncertainties in this region for NIST's near IR detector calibration service. Further improvement of this ACR-based calibration system is underway for near UV calibration. (P. Shaw)

• Polarized Light Scattering from Dielectric Layers. The polarization of light scattered by a material can help to identify the cause of that scatter. For a single interface, for example, particles above the surface, defects below the surface, and roughness of the interface each have unique polarization signatures. The application of polarized light scatter measurements was also found useful for characterizing the properties of surfaces with multiple interfaces. Theoretical calculations, the results of which are shown in figure 4, demonstrate that defects located at different positions in a dielectric layer on a silicon substrate yield different scattered light polarizations, while roughness at the two interfaces can be distinguished. Furthermore, the polarization can be used to determine the correlation between roughness at two interfaces. Bidirectional ellipsometry measurements were carried out with an air/SiO₂/Si correlated pair of interfaces and yielded results that were in reasonable agreement with the theoretical predictions.
  
  
  

  Figure 4. Results of theoretical calculations for the degree of linear polarization, P_L, and the principal axis of polarization, η, as functions of out-of-plane scattering angle for scattering from roughness and defects associated with a 1 µm thick SiO₂ film grown on silicon. The incident light is assumed to be p-polarized, and the incident and scattering polar angles are 45°.

  These measurements and models help the semiconductor industry detect and identify particulate contamination and defects on silicon wafers. By understanding the light scattered by residual roughness of interfaces, inspection tool manufacturers can increase the sensitivity of their production line inspection tools to these defects. These improvements will allow smaller features to be created reliably on semiconductor wafers. (T. Germer)

• Polymer Blends Studied by NSOM. Thin polymer films are becoming ever more important in optoelectronic and electronic devices. Understanding and characterizing the structure and dynamics of thin films, and especially of thin-film polymer-blends, present challenges associated with the small size of typical features in these films. We are using near-field scanning optical microscopy to meet these challenges. Changes in structure with annealing and with changes in the concentration of conducting polymer have been observed for features as small as a few hundred nanometers. The different optical properties of the two components make detection by either transmission or fluorescence contrast possible, and both mechanisms have been used simultaneously to characterize these materials. An example of the data for a polystyrene/ polythiophene blend is shown in figure 5. (L. Goldner and J. Hwang)

Figure 5. Topographic (left) and fluorescence (right) images of a polystyrene/polythiophene film. Note that the topography is correlated with, but not the same as, the domain structure of the film, which is measured more directly by the fluorescence images. Here, the polythiophene is fluorescent.

Vibrationally Resolved Sum-Frequency Generation Studies of Self-Assembled Monolayers Using Broad-Bandwidth Infrared Pulses. Vibrationally-resolved sum-frequency generation (SFG) spectroscopy is a powerful probe of the molecular order at surfaces and interfaces. In most SFG studies, a narrow-bandwidth IR pulse mixes with a narrow-bandwidth visible pulse through a χ(2) process at an interface to produce SFG that is not spectrally analyzed. The sum frequency (SF) spectrum is laboriously acquired by scanning the wavelength of the IR pulse over the vibrational resonances. We developed an alternate approach that obtains SFG spectra with short acquisition times and without wavelength tuning. A nominally 100 fs commercial laser system generates broad-bandwidth (BB) IR pulses that are mixed with narrow-bandwidth visible pulses at the interface of interest. The SF light is collected and dispersed in a spectrograph where a CCD detects all the SF light (spectral range: 600 cm1) in parallel. This BB SFG approach rapidly produces excellent spectra and in principle, permits ultrafast time resolution. Figure 6. SFG spectrum of an Au film covered with (a) a SAM of d-ODT, (b) a SAM of ODT, and (c) the ratio b/a, the ODT spectrum. Traces (a) and (b) are offset vertically; a line indicating zero signal is shown for each. The spectra were recorded in 60 s.

In collaboration with scientists from the Surface Science and Biotechnology Divisions of CSTL, we demonstrated the BB SFG approach in studies of self-assembled monolayers (SAMs) of normal and deuterated octadecanethiol (ODT, d-ODT) on a Au film. The SFG spectrum in figure 6 shows the high signal-to-noise achieved in only 60 s of data acquisition. The spectrum is dominated by the 3 CH stretch features associated with the terminal methyl (CH₃) groups of the SAMs. Features at 2890 and 2930 cm^-1 due to methylene (CH₂) groups show disorder from the desired all-trans configuration in this sample. SFG spectra of important molecules such as ubiquinone (CoQ) and integral proteins in biomimetic or biological membranes are being studied. (J. Stephenson with L. Richter and T. Petralli-Mallow, CSTL.)

[Figure 7 and the paragraph explaining the work have been removed.]

• Realization of an IR Spectral Radiant Power Response Scale on a Cryogenic Bolometer. The Optical Technology Division has developed an IR radiant power response scale for the spectral range from 2 µm to 20 µm for use with a new IR detector comparator facility to provide calibration of spectral radiant power response. The scale operates at much higher sensitivity (input power levels are 10 µW to 20 pW) than typical existing IR scales, while using a detector with a flat spectral response over the entire spectral range. The scale has been realized on a cryogenic bolometer, which accounts for the improved sensitivity relative to detector scales based on pyroelectric detectors. The response of this bolometer has been determined by multiple ties to a primary standard, the NIST High Accuracy Cryogenic Radiometer (HACR). The uncertainty over the entire spectral region is typically better than 0.25%, with the typical uncertainty of an individual measurement made with the bolometer being ~0.8%. This IR scale and the associated IR detector calibration facility have been documented in the recently published NIST Special Publication 250-42. (A. Migdall and G. Eppeldauer).
• Laboratory Measurements of the Atmospheric Continuum Absorption by Molecular Oxygen. Laboratory measurements of the molecular oxygen, collision-induced, continuum absorption have been undertaken in the mid and near infrared to provide accurate quantitative data for atmospheric modeling. It has recently been suggested that continuum absorption in the near-infrared to ultraviolet by molecular oxygen may be responsible, in part, for the 10 W m^-2 to 30 W m^-2 discrepancy between general circulation models and measurements of the solar atmospheric absorption. Moreover, continuum absorption in the mid-infrared affects the retrieval of NO₂, H₂O, and aerosol concentration profiles by limb-sounding satellite radiometers, such as the High Resolution Dynamics Limb Sounder (HIRDLS), scheduled for launch aboard the CHEM-1 EOS platform in 2002. The laboratory measurements are made with a Fourier-transform spectrometer coupled to a 2 m long variable-temperature, White-type absorption cell. To mimic the integrated product of ρ ²L along the optical path L, where ρ is the atmospheric density, we multiple-pass the light through the sample to achieve a long-optical-pathlength of 84 m and pressurize the sample to obtain densities up to 10 times the ground-level atmospheric density at 296 K. The figure shows the absorption spectrum obtained for pure O₂ at 296 K at a density 7.5 times larger than the density of O₂ at STP. The sharp spectral features on top of the broad continuum absorption are due to the v=0 - 0 component of the a ¹Δ_g-X ³Σ_g^- magnetic dipole band of molecular O₂. The present measurements show that the previous best continuum profiles are in error by 25 % for air as a collision partner and by a factor of 3 for nitrogen as a collision partner. (G.T. Fraser, W.J. Lafferty, C.L. Lugez, and B. Maté)
• High Resolution UV Spectroscopic Methods to Probe Chemical Reactions at-or-near Threshold. We have completed construction of a high resolution UV laser spectrometer capable of resolving, at full rotational state resolution, the structure and dynamics of large molecules and molecular complexes in electronically excited states. Molecules are prepared both rotationally and vibrationally cold by seeding in a cw molecular beam. One meter downstream of the expansion, the molecular beam is crossed by a frequency doubled cw dye laser. The doubled light (~300 nm) is generated in an external ring build-up cavity with 5 to 10% conversion efficiency of the injected 1 Watt of fundamental power. The laser induced fluorescence is detected using a spatially selective light collection assembly coupled to a photon counting/photomultiplier detection system. The sub-Doppler resolving power with narrow band UV excitation (<1 MHz) and the spatial resolved fluorescence filtering is nearly at 1 part in 10⁹. Bolometric detection of metastable states and microwave double resonance experiments are also possible. The rotationally resolved fluorescence excitation spectrum of the electronic origin of 1-fluoronaphthalene is shown in figure 9. As a demonstration of the frequency resolution and control of this spectrometer, a single rotational line taken from the most congested part of the central a-type Q-branch is shown in the bottom panel. Current research projects include the fully quantum-state resolved reactions and near threshold chemistry induced by the absorption of MW, IR and/or UV light. Examples are proton transfer reactions in acid-base molecular complexes and oxygen atom/ozone reactions with vibrationally and/or electronically excited molecules in crossed molecular beams. (D.F. Plusquellic and R.D. Suenram).