Walter M. Duncan

Emerging digital micromirror device (DMD) applications

For the past six years, Digital Light Processing technology from Texas Instruments has made significant inroads in the projection display market. With products enabling the world’s smallest data and video projectors, HDTVs, and digital cinema, DLP technology is extremely powerful and flexible. At the heart of these display solutions is Texas Instruments Digital Micromirror Device (DMD), a semiconductor-based “light switch” array of thousands of individually addressable, tiltable, mirror-pixels. With success of the DMD as a spatial light modulator for projector applications, dozens of new applications are now being enabled by general-use DMD products that are recently available to developers. The same light switching speed and “on-off” (contrast) ratio that have resulted in superior projector performance, along with the capability of operation outside the visible spectrum, make the DMD very attractive for many applications, including volumetric display, holographic data storage, lithography, scientific instrumentation, and medical imaging. This paper presents an overview of past and future DMD performance in the context of new DMD applications, cites several examples of emerging products, and describes the DMD components and tools now available to developers.

Fast, high-resolution atomic force microscopy using a quartz tuning fork as actuator and sensor

We report a new method of achieving tip–sample distance regulation in an atomic force microscope (AFM). A piezoelectric quartz tuning fork serves as both actuator and sensor of tip–sample interactions, allowing tip–sample distance regulation without the use of a diode laser or dither piezo. Such a tuning fork has a high spring constant so a dither amplitude of only 0.1 nm may be used to perform AFM measurements. Tuning-fork feedback is shown to operate at a noise level as low as that of a cantilever-based AFM. Using phase-locked-loop control to track excursions in the resonant frequency of a 32 kHz tuning fork, images are acquired at scan rates which are fast enough for routine AFM measurements. Magnetic force microscopy using tuning-fork feedback is demonstrated.

Insitu spectral ellipsometry for real-time measurement and control

Abstract Described in this work is a spectral ellipsometer designed and built specifically for adaptive control in single wafer processing. This instrument is capable of precisely measuring thicknesses and compositions of multilayer structures in situ and in real time. Utilizing phase modulation, multichannel detection and digital signal processing techniques, this ellipsometer is capable of acquiring spectra in less than 75 ms. Efficient algorithms have also been developed for extracting layer parameters (thicknesses and composition) from measured spectra of multilayer stacks in one second or less. This spectral ellipsometer has been applied to on-line monitoring of wafer properties during etch in a single-wafer, remote microwave plasma chamber. Of interest has been etching of Si3N4, SiO2, Si3N4 and SiO2 mixtures, polycrystalline Si, and multilayer combinations of these materials. Film thicknesses and etch rates are updated during etch such that interfaces can be anticipated and endpoints determined. Spectral ellipsometry allows overdetermination of variables in multilayer stacks, hence, structure models and layer parameters can be verified using statistical methods. Spectral information also allows self-calibration of incidence angles.

Photoluminescence and x-ray properties of heteroepitaxial gallium arsenide on silicon

We report a study of the residual impurities and defects in heteroepitaxial GaAs films grown by molecular‐beam epitaxy on 〈100〉 Si substrates. Low‐temperature photoluminescence measurements are used to identify residual impurity and deep defect levels in unintentionally doped heteroepitaxial GaAs and compared to homoepitaxial GaAs grown under similar conditions. Scanning Lang x‐ray measurements demonstrate that the heteroepitaxial layers are under biaxial tensile stress in the surface parallel direction. The presence of internal tensile stress is also corroborated by double crystal x‐ray rocking curve measurement which shows tetragonal compression in the surface perpendicular direction. This is also the first reported use of interferometric techniques for studying photoluminescence properties of a wide‐gap semiconductor in the near infrared region.

In situ spectral ellipsometry for real-time thickness measurement : Etching multilayer stacks

A polarization modulated spectroscopic ellipsometer is used in situ to measure the thicknesses of films in real time during semiconductor plasma etch processing. Utilizing the speed of phase modulation multichannel detection, and digital signal processing techniques, this ellipsometer is capable of acquiring spectral data in less than 75 ms. Efficient algorithms were developed for determining layer thicknesses from the measured spectra of multilayer film stacks in real time with a typical solution time of a few seconds. The measured thicknesses and etch rates are used to anticipate interfaces in multiple layer stacks and control process end points. The ability of the spectral ellipsometer to measure multilayer stacks during etching is demonstrated by the etching of a stack consisting of silicon nitride, polycrystalline silicon, and silicon dioxide. Such stacks are commonly used as masks for field oxidation for electrical isolation in memory device fabrication. An isotropic plasma etch is used to remove this film in a single‐wafer process environment.

Dynamic optical filtering in DWDM systems using the DMD

Summary form only given. We describe applications of the Texas Instruments Digital Micromirror Device (DMD) as a high efficiency spatial light modulator for dynamic optical filtering and switching in Dense Wavelength Division Multiplexed (DWDM) optical networks. Whereas the DMD has found wide acceptance as a spatial light modulator in video display applications, the present paper describes applications of the DMD to DWDM signal processing. The DMD is shown in this work to behave as a 2-dimensional switched blazed gating when modulating coherent light. In addition, the efficiency of fiber coupling will be shown to depend on the amplitude of the overlap integral between the modulated field and the mode of the output. Hence, the efficiency of the fiber coupling depends not only on the amplitude of the fields, but on how well they are matched in phase. The DMD is very suitable to digital processing of optical networking signals where it can be used as a series of parallel optical switches (e.g. 400-l/spl times/2 switches). A very useful optical platform combines the DMD with a dispersive element such as a gating or a prism. DWDM signals are first dispersed and impinge onto the DMD, which is then used as a reflective adaptive slit. The DMD selectively routes specific wavelengths back into one of two optical paths or ports. Using this general configuration of the DMD as an adaptive optical slit, we have demonstrated several network functions including digital optical equalization, optical add drop multiplexing and optical performance monitoring. In this presentation, we will describe these applications of the DMD as well as describe system performance attributes (insertion loss, polarization dependent loss, group delay and power penalty) as they pertain to the DMD structure and DMD based optical signal processing.

Spectral ellipsometry on patterned wafers

Ellipsometry has seen only limited application to the post-deposition pattern etch process despite the fact that physical parameters such as groove width, depth and pitch are as critical to product performance as the more basic thin film parameters traditionally analyzed using ellipsometry. This paper presents initial theoretical results pertaining to modeling the reflectance from a 1D etched pattern on a semiconductor substrate. To analyze the samples effects upon the incident beam polarization, we formulate the zeroth-order reflection coefficients for the orthogonal p and s polarization states and construct models of ellipsometric parameters (Psi and Delta) for a rectangular-groove surface pattern, emphasizing the effect of groove geometry upon these quantities.

DLP™ switched blaze grating; the heart of optical signal processing

We have developed an approach for processing communication signals in the optical domain using a DLP digital mirror array driven by a Digital Signal Processor (DSP). In optical communication systems, modulation rates of 10 GB/s and above are common, hence, direct processing of Dense Wavelength Division Multiplexed (DWDM) optical signals without undergoing Optical to Electrical conversion has become a key requirement for cost effective deployment of dynamic optical networks. This work will discuss primarily applications of Optical Signal Processing (OSP) to coherent DWDM signals. Optical Signal Processing has also found applications in spectroscopy, microscopy, sensing, optical correlation, and testing.

Fabrication and High-Temperature Characteristics of Ion-Implanted GaAs Bipolar Transistors and Ring-Oscillators

GaAs bipolar transistors and ring-oscillators were fabricated by ion implantation into VPE structures. The transistor and circuit performance was tested between 25°C and 400°C. Leakage currents determine the useful temperature range. Present GaAs circuits fail at approximately 390°C due to the metallization technology.

Real-time diagnostics of II-VI molecular beam epitaxy by spectral ellipsometry

Spectral ellipsometry has been applied to in situ diagnostics of Hg1−xCdxTe molecular beam epitaxial growth. Spectral ellipsometry provides surface, composition, and film thickness properties of thin film electronic materials in real-time during growth and processing. This study is discussed in two parts. In the first part, the surface state properties of Hg1−xCdxTe and the substrate material Cd1−xZnxTe were studied under ultrahigh vacuum (UHV) conditions. By measuring the spectral ellipsometric quantities Ψ and Δ versus temperature under UHV conditions, the presence and desorption temperatures of surface species have been determined. Tellurium stabilized Cd1−xZnxTe surfaces were not observed to exhibit any apparent surface changes during heating under these conditions. Oxidized Cd1−xZnxTe surfaces, however, exhibit discontinuities in Ψ and Δ indica-tive of desorption of a surface species in the 300 °C range. Tellurium stabilized Hg1−xCdxTe surfaces also exhibit discontinuities in Ψ and Δ occurring at abo...