David B. Saint John

Electrical properties of plasma enhanced chemical vapor deposition a-Si:H and a-Si1−xCx:H for microbolometer applications

Electrical properties for resistive microbolometer sensor materials including resistivity, temperature coefficient of resistance (TCR), and normalized Hooge parameter were explored in n-type a-Si:H and a-Si1−xCx:H prepared by plasma enhanced chemical vapor deposition. The complex dielectric function spectra (e = e1 + ie2) and structure were measured by spectroscopic ellipsometry. Two-dimensional drift-diffusion simulations were used to understand the band-tail slope dependency of TCR and 1/f noise.

pH-Controlled Selective Etching of Al2O3 over ZnO

We describe pH-controlled selective etching of atomic layer deposition (ALD) Al2O3 over ZnO. Film thickness as a function of etch exposure was measured by spectroscopic ellipsometry. We find that alkaline aqueous solutions with pH between about 9 and 12 will etch Al2O3 at useful rate with minimal attack of ZnO. Highly selective etching of Al2O3 over ZnO (selectivity >400:1) and an Al2O3 etch rate of ∼50 nm/min can be obtained using a pH 12 etch solution at 60 °C.

High temperature coefficient of resistance molybdenum oxide and nickel oxide thin films for microbolometer applications

Abstract. Molybdenum oxide (MoOx) and nickel oxide (NiOx) thin films were deposited by reactive biased target ion beam deposition. MoOx thin film resistivity varied from 3 to 2000  Ω·cm with a temperature coefficient of resistance (TCR) from −1.7% to −3.2%/K, and NiOx thin film resistivity varied from 1 to 300  Ω·cm with a TCR from −2.2% to −3.3%/K, both easily controlled by varying the oxygen partial pressure. Biased target ion beam deposited high TCR MoOx and NiOx thin films are polycrystalline semiconductors and have good stability in air. Compared with commonly used vanadium oxide thin films, MoOx or NiOx thin films offer improved process control for resistive temperature sensors.

Microstructural evolution in si 1−x Ge x :H thin films for photovoltaic applications

The growth of hydrogenated silicon germanium alloy (Si1−xGex:H) films by plasma enhanced chemical vapor deposition (PECVD) has been investigated using real time spectroscopic ellipsometry (RTSE) to understand the effect of incorporated Ge on the relationship between the evolution of microcrystallinity and the optical properties, the latter in the form of the complex dielectric function spectra (ε = ε1 + iε2). The motivation is to explore the variations in the growth and properties of microcrystalline alloy films that arise due to Ge incorporation in materials suitable for integration into thin film Si:H based photovoltaic devices. Variations in the microcrystal evolution and the optical properties of microcrystalline silicon germanium (μc-Si1−xGex:H) are extracted from films that initially nucleate microcrystallites from the amorphous phase at a thickness near 100–200 Å for alloy films spanning from Si:H to Ge:H. Although an increase in absorption is observed for the alloys, low Ge content films do not show the critical point features characteristic of crystalline Ge. Transmission electron micrographs (TEMs), the microstructural evolution obtained from RTSE, and a conical growth model for microcrystallites have been used to identify the average microcrystallite nucleation density and cone half angle. Monotonic decreases in the cone half angle with increasing Ge content are observed, indicating a reduced difference between the growth rates of the amorphous and microcrystalline phases with higher Ge incorporation. Also the nucleation density is lower in the alloy films with higher incorporation of Ge, which is consistent with a weaker tendency for microcrystallite formation expected on a more disordered alloy substrate.

Evaluation of 1/f noise in prospective IR imaging thin films

Vanadium oxide (VOx) and hydrogenated silicon germanium (SixGe1-x) are the two predominant thin film material systems used as the active layer in resistive infrared imaging. Thin films of VOx used in microbolometers have a resistivity typically between 0.1 and 1 Ω-cm with a temperature coefficient of resistance, |TCR| between 1.4%/K to 2.4%/K, while SixGe1-x:H thin films have a resistivity between 200-4,000 Ω-cm with a |TCR| between 2.9%/K to 3.9%/K. Future devices may require higher TCR materials, however, higher TCR is loosely associated with higher resistivity and therefore also with high noise. This work compares 1/f noise of high resistivity VOx and Ge:H thin films having |TCR| < 3.6%/K. The high TCR thin films of VOx were found to be amorphous while, depending on the deposition conditions, the Ge:H thin films were either amorphous or mixed phase of amorphous + nanocrystalline. Evaluation of these VOx and Ge:H thin films indicates a prospects for a superior process-property relation of 1/f noise in Ge:H thin films in comparison with thin films of VOx.

Nickel oxide and molybdenum oxide thin films for infrared imaging prepared by biased target ion-beam deposition

Vanadium oxide (VOx) thin films have been intensively used as sensing materials for microbolometers. VOx thin films have good bolometric properties such as low resistivity, high negative temperature coefficient of resistivity (TCR) and low 1/f noise. However, the processing controllability of VOx fabrication is difficult due to the multiple valence states of vanadium. In this study, metal oxides such as nickel oxide (NiOx) and molybdenum oxide (MoOx) thin films have been investigated as possible new microbolometer sensing materials with improved process controllability. Nickel oxide and molybdenum oxide thin films were prepared by reactive sputtering of nickel and molybdenum metal targets in a biased target ion beam deposition tool. In this deposition system, the Ar+ ion energy (typically lower than 25 eV) and the target bias voltage can be independently controlled since ions are remotely generated. A residual gas analyzer (RGA) is used to precisely control the oxygen partial pressure. A real-time spectroscopic ellipsometry is used to monitor the evolution of microstructure and properties of deposited oxides during growth and post-deposition. The properties of deposited oxide thin films depend on processing parameters. The resistivity of the NiOx thin films is in the range of 0.5 to approximately 100 ohm-cm with a TCR from -2%/K to -3.3%/K, where the resistivity of MoOx is between 3 and 2000 ohm-cm with TCR from -2.1%/K to -3.2%/K. We also report on the thermal stability of these deposited oxide thin films.

Open Source 3D Scanning and Printing for Design Capture and Realization

Traditionally, the complexities and costs associated with design conceptualization (e.g., 3D scanning) and design realization (e.g., 3D printing) have limited the diversity of individuals capable of participating in the process to individuals/entities with advanced technical backgrounds or substantial financial resources. The authors of this work propose a methodology that utilizes low cost hardware and open source software to make the capture, reuse and management of design knowledge more accessible to the general public.3D scanners are digital tools that facilitate the conversion of physical object information into the digital space through multiple image capture techniques. 3D scanners have the potential to revolutionize design conceptualization in society by enabling individuals to seamlessly transform physical representations of objects into a digital 3D rendered version. The 3D rendered version can then be manipulated using existing 3D CAD tools (e.g., SolidWorks) and subsequently printed using a 3D printer.Design realization via 3D printers (e.g., RepRaps) is becoming an integral aspect of the engineering design process. While the conceptualization of designs (e.g. CAD models) helps designers visually experience potential candidate designs, product prototypes that can actually be touched and manipulated add an important ‘feedback’ dimension to the engineering design process. This scan-edit-print approach to design conceptualization and realization will enable designers collaborating in online environments to work towards achieving a common design by providing them with tools and techniques.A case study is presented that demonstrates the feasibility of the scan (knowledge capture), edit (knowledge reuse) and print (knowledge management) approach to design using low cost hardware and open source software.© 2014 ASME

Toward Metamodels for Composable and Reusable Additive Manufacturing Process Models

Though the advanced manufacturing capabilities offered by additive manufacturing (AM) have been known for several decades, industry adoption of AM technologies has been relatively slow. Recent advances in modeling and simulation of AM processes and materials are providing new insights to help overcome some of the barriers that have hindered adoption. However, these models and simulations are often application specific, and few are developed in an easily reusable manner. Variations are compounded because many models are developed as independent or proprietary efforts, and input and output definitions have not been standardized. To further realize the potential benefits of modeling and simulation advancements, including predictive modeling and closed-loop control, more coordinated efforts must be undertaken. In this paper, we advocate a more harmonized approach to model development, through classification and metamodeling that will support model composability, reusability, and integration. We review several types of AM models and use direct metal powder bed fusion characteristics to provide illustrative examples of the proposed classification and metamodel approach. We describe how a coordinated approach can be used to extend modeling capabilities by promoting model composability. As part of future work, a framework is envisioned to realize a more coherent strategy for model development and deployment.Copyright

Optical characterization of structurally graded Si 1−x Ge x :H thin films

High efficiency thin film silicon solar cells consist of multiple junctions with hydrogenated amorphous silicon, silicon germanium alloys, and nanocrystalline silicon (nc-Si:H) absorbers. Uniformity over large areas is challenging for nc-Si:H and an accurate method of mapping material quality via a technique like ex situ spectroscopic ellipsometry (SE) is desirable. In situ, real time SE (RTSE) measurements during growth show material evolves from amorphous to nanocrystalline, which complicates the analysis of single SE measurements. Information from RTSE has been applied to develop procedures to accurately extract the thickness at which nanocrystallites initially appear and coalesce from single SE measurements.

Infrared dielectric functions of hydrogenated amorphous silicon thin films determined by spectroscopic ellipsometry

Amorphous hydrogenated silicon (a-Si:H) thin films have found use in photovoltaic, transistor, and microbolometer applications. Routine optical metrology of a-Si:H is generally performed in the visible range but is not directly sensitive to hydrogen bonding. Infrared spectroscopic ellipsometry (IR-SE) allows a direct measurement of the relative absorption strength of various hydrogen-related modes, giving some insight into the hydrogen content and relative disorder of films. IR-SE is used here to develop a parameterization of ε=ε1+iε2 for several thin (<; 300 nm) hydrogenated amorphous germanium (a-Ge:H) and a-Si:H films deposited onto silicon nitride or titanium-coated crystalline silicon substrates.