Scott D. Habermehl

Fabrication techniques for low loss silicon nitride waveguides

Optical waveguide propagation loss due to sidewall roughness, material impurity and inhomogeneity has been the focus of many studies in fabricating planar lightwave circuits (PLCs) In this work, experiments were carried out to identify the best fabrication process for reducing propagation loss in single mode waveguides comprised of silicon nitride core and silicon dioxide cladding material. Sidewall roughness measurements were taken during the fabrication of waveguide devices for various processing conditions. Several fabrication techniques were explored to reduce the sidewall roughness and absorption in the waveguides. Improvements in waveguide quality were established by direct measurement of waveguide propagation loss. The lowest linear waveguide loss measured in these buried channel waveguides was 0.1 dB/cm at a wavelength of 1550 nm. This low propagation loss along with the large refractive index contrast between silicon nitride and silicon dioxide enables high density integration of photonic devices and small PLCs for a variety of applications in photonic sensing and communications.

On dielectric breakdown in silicon-rich silicon nitride thin films

Observations of dielectric breakdown in Si-rich silicon nitride indicate that it is initiated by threshold field trap ionization. The films exhibit the charge transport mechanism of Poole–Frenkel emission with a compositionally dependent ionization potential ranging from 0.58 to 1.22 eV. Similar to silicon oxynitride, the barrier lowering energy at the point of dielectric breakdown is correlated with within ∼2kT of the ionization potential, thus revealing a dual role for bulk traps in the film: regulating charge transport and retarding hot electron generation. Additionally, a semiempirical expression is developed that accurately predicts the compositional dependence of the breakdown field.

Total Dose Radiation Response of NROM-Style SOI Non-Volatile Memory Elements

For the first time, NROM-style nonvolatile memory elements were fabricated in SOI and irradiated. Total dose characterizations of these transistors indicate that this new style of memory can be functional to at least 500 krad (SiO2).

TDDB and Pulse-Breakdown Studies of Si-Rich

Antifuses are electronic devices that can be irreversibly converted from a high-resistance state to a low-resistance state. Thus, they are ideal candidates for one-time-programmable many-times-readable nonvolatile memories. In this paper, the reliability and the programming characteristics of Si-rich SiNx, antifuses have been studied using time-dependent dielectric breakdown and pulse-breakdown measurements on both single-device test structures and full read-only memories. Contrary to measurements on thick films in which the Poole-Frenkel barrier lowering dominates breakdown, these measurements on fully processed and integrated antifuses indicate that a Fowler-Nordheim-like mechanism governs both programming and long-term reliability.

\hbox{SiN}_{x}

The coefficient of thermal expansion (α) and biaxial Youngs modulus is determined by comparing the differential thermal stress induced in Si-rich silicon nitride thin films deposited on single-crystal Si and sapphire substrates. The amorphous films are deposited in mixtures of dichlorosilane and ammonia, by low pressure chemical vapor deposition, in a temperature range of 1050–1115 K. Temperature-dependent wafer curvature measurements are performed to determine the differential thermal stress, across a temperature range spanning 300–800 K. Observations indicate that both α and the biaxial modulus decrease as the silicon content in the films increases. The trend of reduction in α is consistent with the relative α values for the limiting-case compositions of cubic-Si3N4 and amorphous Si. The decrease in α is attributed to a reduction in anharmonicity associated with Si–Si bonds relative to Si–N bonds. The biaxial modulus is observed to be proportional to the inverse cube root of the amorphous Si volume fra...

Antifuses and Antifuse-Based ROMs

A number of important energy and defense-related applications would benefit from sensors capable of withstanding extreme temperatures (>300°C). Examples include sensors for automobile engines, gas turbines, nuclear and coal power plants, and petroleum and geothermal well drilling. Military applications, such as hypersonic flight research, would also benefit from sensors capable of 1000°C. Silicon carbide (SiC) has long been recognized as a promising material for harsh environment sensors and electronics because it has the highest mechanical strength of semiconductors with the exception of diamond and its upper temperature limit exceeds 2500°C, where it sublimates rather than melts. Yet today, many advanced SiC MEMS are limited to lower temperatures because they are made from SiC films deposited on silicon wafers. Other limitations arise from sensor transduction by measuring changes in capacitance or resistance, which require biasing or modulation schemes that can with- stand elevated temperatures. We are circumventing these issues by developing sensing structures directly on SiC wafers using SiC and piezoelectric aluminum nitride (AlN) thin films. SiC and AlN are a promising material combination due to their high thermal, electrical, and mechanical strength and closely matched coefficients of thermal expansion. AlN is also a non-ferroelectric piezoelectric material, enabling piezoelectric transduction at temperatures exceeding 1000°C. In this paper, the challenges of incorporating these two materials into a compatible MEMS fabrication process are presented. The current progress and initial measurements of the fabrication process are shown. The future direction and the need for further investigation of the material set are addressed.

Coefficient of thermal expansion and biaxial Young's modulus in Si-rich silicon nitride thin films

We report on the efforts at Sandia National Laboratories to develop high temperature capable microelectromechanical systems (MEMS). MEMS transducers are pervasive in todays culture, with examples found in cell phones, automobiles, gaming consoles, and televisions. There is currently a need for MEMS transducers that can operate in more harsh environments, such as automobile engines, gas turbines, nuclear and coal power plants, and petroleum and geothermal well drilling. Our development focuses on the coupling of silicon carbide (SiC) and aluminum nitride (AlN) thin films on SiC wafers to form a MEMS material set capable of temperatures beyond 1000°C. SiC is recognized as a promising material for high temperature capable MEMS transducers and electronics because it has the highest mechanical strength of semiconductors with the exception of diamond and its upper temperature limit exceeds 2500°C, where it sublimates rather than melts. Most transduction schemes in SiC are focused on measuring changes in capaci...