Shane T. Todd

A multi-degree-of-freedom micromirror utilizing inverted-series-connected bimorph actuators

An ove lm ulti-degree-of-freedom electrothermal micromirror design that uses thermal inverted-series-connected (ISC) bimorph actuators is presented. The ISC bimorph actuators eliminate problems observed in previous designs that use single bimorph actuators. The micromirror can operate in one-dimensional (1D) piston-mode and two-dimensional (2D) tilt-mode. Analytical models for the piston-mode and tilt-mode actuations are shown and are compared to FEM simulation results. The device was fabricated using the AMI 1.5 µ mC MOS (complimentary metal-oxide semiconductor) process followed by a post-CMOS micromachining process for device release. The displacement versus temperature of the micromirror was measured experimentally over a range of temperatures and compared to analytical and FEM simulation results .E xperimental results showed that the micromirror displaced by 56 µ ma t an applied temperature 150 ◦ C. The integrated polysilicon heaters were open-circuited during the post-CMOS microfabrication. The failure was caused by pinholes in the metal layers that allowed etchants to attack the polysilicon layer during post-CMOS microfabrication.

An Electrothermomechanical Lumped Element Model of an Electrothermal Bimorph Actuator

This paper reports a simple electrothermomechanical lumped element model (ETM-LEM) that describes the behavior of an electrothermal bimorph actuator. The ETM-LEM is developed by integrating an electrothermal LEM of a heater with a thermomechanical LEM of a bimorph actuator. This new LEM uses only one power source in both the electrical and thermal domains. The LEM provides a simple and accurate way of relating the output mechanical response of a bimorph actuator to the electrical inputs. The model shows that the tip angular rotation of the bimorph actuator is linearly proportional to its average temperature change. The LEM predicts a linear relationship between both the average temperature change and bimorph tip angular rotation versus voltage when operated above a certain voltage. The LEM is used to predict the rotation angle of a fabricated electrothermal bimorph micromirror in response to the electrical inputs and produces results that agree with finite element model simulations and experimental data within 15% for all measured parameters.

Optimization of Hybrid Silicon Microring Lasers

In this paper, we review the recent optimization work in hybrid silicon microring lasers. Device structure and fabrication procedures are discussed first, followed by two major improvements in carrier injection and thermal management. A simple, well-controlled quantum well undercut leads to about 20% enhancement of injection efficiency. A silicon-on-diamond (SOD) substrate is demonstrated to fundamentally solve the device heating issue for silicon-on-insulator (SOI)-based active devices. SOD rib waveguides show low propagation loss of 0.74 dB/cm, which is 0.2 dB/cm higher than SOI counterparts. More than 22× reduction of device heating in hybrid silicon microring lasers is projected by simulations.

Fabrication of Silicon-on-Diamond Substrate and Low-Loss Optical Waveguides

A simple, complementary metal-oxide-semiconductor (CMOS)-compatible fabrication technique is developed to demonstrate a robust silicon-on-diamond (SOD) substrate. The 2.5 μm × 700 nm waveguides with a spiral structure are fabricated on both SOD and conventional silicon-on-insulator (SOI) substrates, showing 0.74- and 0.54-dB/cm propagation loss, respectively. Finite element method (FEM) thermal simulation shows over 3× improvement in heat dissipation on a hybrid silicon laser structure when Si substrate is not a limiting factor.

Steady-state 1D electrothermal modeling of an electrothermal transducer

Electrothermal models that describe the steady-state electrothermal behavior of a general electrothermal transducer have been developed using the one-dimensional heat transport equation. Compared to previously reported electrothermal transducer models, these models produce simpler equations for the temperature change versus an electrical input. Models of the transducer temperature distribution are derived using various thermal conditions such as surface convection and temperature-dependent electrical resistivity. The models are made for a general electrothermal transducer by assuming that the transducer is attached to arbitrary thermal resistances at its boundaries. Critical thermal parameters of the transducer—such as the position of maximum temperature, maximum temperature change and average temperature change—are derived from the models. It is shown that the average temperature change versus applied power and voltage relationships of the simpler models always agree with the more accurate model within factors of 12/π2 and respectively. It is also shown that the temperature change of an electrothermal transducer is approximately linear with respect to applied voltage when actuated in a certain voltage range. The models are compared to FEM simulations and experimental results of an electrothermal micromirror.

An electrothermally-actuated, dual-mode micromirror for large bi-directional scanning

We report a novel single-crystal-silicon (SCS)-based micromirror that can perform large bi-directional scans and can also generate large piston motion. The micromirror rotates over /spl plusmn/15/spl deg/ at less than 6 V dc voltage, and over /spl plusmn/43/spl deg/ (i.e., >170/spl deg/ optical scan angle) at its resonance of 2.4 kHz. A maximum vertical piston motion of 200 /spl mu/m is also achieved with this 700 /spl mu/m-by-320 /spl mu/m device. A circuit model has been developed for electrothermal behavioral simulation and design optimization. The simulation results using the circuit model match the experimental data to within 8%.

Fabrication, Modeling, and Characterization of High-Aspect-Ratio Coplanar Waveguide

A silicon micromachining process has been developed to fabricate high-aspect-ratio coplanar waveguide (hicoplanar). A new closed-form analytical transmission line model that is valid for both conventional coplanar waveguide (CPW) and hicoplanar is introduced and compared with simulations and experimental results. The major novelties of the model are that it includes the effect of the conductor height on the output parameters and combines Wheelers inductance rule and perturbation theory to predict the line resistance. Transmission lines with characteristic impedances of 18-25 Ω have been fabricated on high-resistivity silicon. Attenuation was measured to be 2.4-3.4 dB/cm at 30 GHz before silicon removal and 1.7-2.4 dB/cm at 30 GHz after silicon removal. The analytical model is compared with Ansoft High Frequency Structure Simulator simulations and experimental data and shows excellent agreement.

A Novel Micromachining Process Using DRIE, Thermal Oxidation, Electroplating, and Planarization to Create High Aspect Ratio Coplanar Waveguides

A micromachining process has been developed to create high aspect ratio coplanar waveguides (HARCs). The process creates tall Si mesas using deep reactive-ion etching and converts them into solid SiO2 mesas using thermal oxidation. Tall Au conductors are deposited using electroplating and planarized using lapping and chemical-mechanical planarization. The solid SiO2 mesas form the dielectric gap between the tall Au conductors, resulting in HARCs with a planar surface. The tall conductor sidewalls created from the high aspect ratio process reduce the transmission line resistance, which allows the lines to have lower loss at low impedances compared to conventional transmission lines. Transmission lines with characteristic impedances of 16-21 ¿ have been fabricated on high-resistivity Si. Transmission line characteristics were measured from 1 to 50 GHz and showed an attenuation of 1.0-1.4 dB/cm at 10 GHz. Measurements were compared to HFSS simulations and showed reasonable agreement over the frequency range.

High aspect ratio CPW fabricated using silicon bulk micromachining with substrate removal

A silicon micromachining process has been developed to fabricate high aspect ratio CPW (HARC). The micromachining process combines Si DRIE, thermal oxidation, electroplating, and planarization to create tall CPW with Au conductors and SiO2 dielectrics. Si is removed underneath the transmission lines to minimize the field interaction with the substrate which virtually eliminates substrate loss. It is shown that HARC has better loss and isolation compared to microstrip and conventional CPW. Transmission lines with characteristic impedances of 18 – 25 Ω have been fabricated on high resistivity Si. Attenuation was measured to be 2.4 – 3.4 dB/cm at 30 GHz before Si removal and 1.7 – 2.4 dB/cm at 30 GHz after Si removal, demonstrating a substantial improvement in loss due to Si removal. Applications of HARC include CMOS MMICs and hybrid microwave circuits.

Single-Crystal Silicon Based Electrothermal MEMS Mirrors for Biomedical Imaging Applications

Micromirrors have extensive applications in optical switches, displays and many other areas. This chapter is dedicated to introducing a new class of single-crystal silicon (SCS) based micromirrors for biomedical imaging applications particularly for optical coherence tomography (OCT). The SCS micromirrors include one-dimensional (1-D) and two-dimensional (2-D) scanning micromirrors and large-vertical-displacement (LVD) phase-only micromirrors. All the 1-D and 2-D micromirrors are 1 mm by 1 mm in size and coated with aluminum for high reflectivity in broad band. All the micromirrors have flat surface due to the thick SCS supporting layer. The measured static rotation angles are more than 30° at less than 15-V drive voltages, and the resonant frequencies are in order of 1 kHz. A large 200-µm static piston motion has been achieved with the LVD micromirror which has a size of only 0.7 mm by 0.32 mm. Design, simulation, modeling, fabrication and experimental results of these thermally-actuated devices are discussed in detail in this chapter. This first section will introduce the operation of OCT and various existing MEMS mirror techniques.