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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.

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.

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.

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.

There are places in this paper where the transducer convection thermal resistance is incorrectly noted as RTC. Wherever RTC is mentioned in the paper it should be replaced with RCA, which was originally defined as the transducer convection thermal resistance in the last line of the first column on p 2266. This error occurs in equations (9), (15), (18) and (21). It also occurs in the second line of the first column on p 2267.

This paper reports a novel large vertical displacement (LVD) microactuator that can generate large piston motion and bi-directional scanning at low driving voltage. A LVD micromirror device has been fabricated by using a unique deep reactive ion etch (DRIE) post-CMOS micromachining process that simultaneously provides thin-film and single-crystal silicon microstructures. The bimorph actuation structure is composed of aluminum and silicon dioxide with an embedded polysilicon thermal resistor. With a size of only 0.7 mm × 0.32 mm, the LVD micromirror demonstrated a vertical displacement of 0.2 mm at 6 V dc. This device can also be used to perform bi-directional rotational scanning through the use of two bimorph actuators. The micromirror rotates over ±15 ◦ at less than 6 V dc, and over ±43 ◦ (i.e., >170 ◦ optical scan angle) at its resonant frequency of 2.6 kHz.

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.

Low-loss optical rib waveguides (2.5 μm × 700 nm) have been fabricated on a silicon-on-diamond (SOD) substrate. 0.74 dB/cm propagation loss is measured. Benefits to heat dissipation for active photonic devices are studied.

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 Wheeler's 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 micromachining process has been developed to create high impedance and low loss high aspect ratio coplanar waveguide (HARC) on low resistivity silicon. The process uses silicon DRIE to create an array of tall mesas that are spaced with a precise pitch. The silicon mesa array is then merged into a single solid SiO2 mesa using thermal oxidation. The solid SiO2 mesa creates a wide dielectric for use in high impedance HARC. The complete fabrication process includes DRIE, thermal oxidation, electroplating, planarization, and substrate removal to create HARC on low resistivity silicon with a planar surface. A high impedance HARC has been fabricated on silicon using this method. Measurements show that silicon substrate removal increases the line impedance from ∼ 20 Ohms to 57 Ohms, reduces effective dielectric constant from ∼ 6 to 2, and reduces attenuation constant from ∼ 33 dB/cm to 4 dB/cm @ 30 GHz. Measurements are compared to an analytical model derived for HARC.

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 SiO 2 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.

A micromachining process has been developed to fabricate high aspect ratio CPW. The tall conductor sidewall created from the high aspect ratio process reduces the resistance per length of the transmission line which lowers the attenuation. The micromachining process combines Si DRIE, thermal oxidation, electroplating, and planarization to create tall CPW with Au conductors and SiO2 dielectrics. Transmission lines with characteristic impedances of 16 - 21 � have been fabricated on high resistivity Si. Transmission line characteristics were measured from 1 - 50 GHz and showed propagation loss of 1.1 - 1.3 dB/cm at 10 GHz.

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.

A new electrothermal micromirror with large piston and tilt motions was proposed and fabricated. Large initial tilt angles and optical path shifting of previously fabricated thermal micro mirrors were eliminated by employing inverted-series-connected (ISC) bimorph actuators

We have developed an analytical model that describes the steady-state thermal behavior of a 1-D electrothermal bimorph MEMS micromirror. The steady-state 1-D heat transport equation is used to solve for the temperature distribution of the device upon actuation. Three models are developed using different thermal conditions on the device. The models consider heat dissipation from conduction and convection and the temperature dependence of the actuator electrical resistor. The temperature distribution equation of each model is analyzed to find critical thermal parameters such as the position of maximum temperature, maximum temperature, average temperature, and equivalent thermal resistance. The simplest model, called the Case 1 model, is used to develop an electrothermal lumped element model that uses a single thermal power source. In the Case 1 model, it is shown that a parameter called the “balancing factor” predicts where the maximum temperature is located, the distribution of power flow, and the division of thermal resistances. The analytical models are compared to FEM simulations and agree within 20% for all of the actuation ranges and thermal conditions tested.

This paper reports a novel large-vertical-displacement (LVD) microactuator that can generate large piston motion at low driving voltage. A LVD micromirror device has been fabricated by using a unique DRIE CMOS-MEMS process that can simultaneously provide thin-film and single-crystal silicon microstructures. With only a 0.7 mm by 0.32 mm sized device, the LVD micromirror demonstrated a vertical displacement of 0.2 mm at an actuation voltage of 6 V d.c. This device can also perform bidirectional rotational scanning through the use of two bimorph actuators. The micromirror can rotate ± 20q, and has negligible initial tilt angle and high scanning speeds (~2 kHz).

This paper reports a 1mm-by-1mm single-crystal silicon (SCS)-based aluminum-coated micromirror (Fig. 1) that rotates 36° at a 7.5 mA d.c. current. Large, flat micromirrors with large scanning angles are required by applications such as medical imaging and interferometer systems. SCS micromirrors have been explored because of the manufacturability and flatness of SCS-based microstructures [1,2,3]. A previous SCS micromirror rotated 17° at 15 mA (d.c.) [3]. However, the buckling of the employed bimorph mesh structure resulted in a discontinuity in the angle versus current curve, which limited the usable scanning range to about 5°. From FEM simulation (Coventorware) we found that the orientation of the embedded poly-silicon resistor significantly changes the temperature behavior of the micromirror. Fig. 2 illustrates the difference between a previous design and the new design. The new design (Fig. 2(c)) has a transversely oriented serpentine poly-silicon resistor to obtain uniform heating along the transverse direction. This results in a continuous response curve (Fig. 2), and an even larger rotation angle. The micromirror is fabricated using deep reactive-ion-etch (DRIE) CMOS-MEMS processing [4].

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%.