Embedded benzocyclobutene in silicon: An integrated fabrication process for electrical and thermal isolation in MEMS

This paper reports a novel fabrication process to develop planarized isolated islands of benzocyclobutene (BCB) polymer embedded in a silicon substrate. Embedded BCB in silicon (EBiS) can be used as an alternative to silicon dioxide in fabrication of electrostatic micromotors, microgenerators, and other microelectromechanical devices. EBiS takes advantage of the low dielectric constant and thermal conductivity of BCB polymers to develop electrical and thermal isolation integrated in silicon. The process involves conventional microfabrication techniques such as photolithography, deep reactive ion etching, and chemical mechanical planarization (CMP). We have characterized CMP of BCB polymers in detail since CMP is a key step in EBiS process. Atomic force microscopy (AFM) and elipsometry of blanket BCB films before and after CMP show that higher polishing down force pressure and speed lead to higher removal rate at the expense of higher surface roughness, non-uniformity, and scratch density. This is expected since BCB is a softer material compared to inorganic films such as silicon dioxide. We have observed that as the cure temperature of BCB increases beyond 200 °C, the CMP removal rate decreases drastically. The results from optical microscopy, scanning electron microscopy, and optical profilometry show excellent planarized surfaces on the EBiS islands. An average step height reduction of more than 95% was achieved after two BCB deposition and three CMP steps.     

Design, Fabrication, and Characterization of a Rotary Micromotor Supported on Microball Bearings

We report the design, fabrication, and characterization of a rotary micromotor supported on microball bearings. This is the first demonstration of a rotary micromachine with a robust mechanical support provided by microball-bearing technology. A six-phase bottom-drive variable-capacitance micromotor (Phi = 14 mm) is designed and simulated using the finite-element (FE) method. The stator and the rotor are fabricated separately on silicon substrates and assembled with the microballs. Three layers of low-k benzocyclobutene polymer, two layers of gold, and a silicon microball housing are fabricated on the stator. Microball housing and salient structures (poles) are etched in the rotor and are coated with a silicon carbide film that reduces the friction without which the operation was not possible. A top angular velocity of 517 r/min, corresponding to the linear tip velocity of 324 mm/s, is measured at plusmn150-V and 800-Hz excitation. This is 44 times higher than the velocity previously demonstrated for linear micromotors supported on the microball bearings. A noncontact method is developed to extract the torque and the bearing coefficient of friction through dynamic response measurements. The torque is indirectly measured to be -5.62 plusmn 0.5 muN ldr m at plusmn150-V excitation which is comparable with the FE simulation results predicting -6.75 muN ldr m. The maximum output mechanical power at plusmn150 V and 517 r/min was calculated to be 307 muW. The bearing coefficient of friction is measured to be 0.02 plusmn 0.002 which is in good agreement with the previously reported values. The rotary micromotor developed in this paper is a platform technology for centrifugal micropumps used for fuel-delivery and cooling applications.     

Electrical characterization of benzocyclobutene polymers for electric micromachines

An approach using interdigitated capacitors for electrical characterization of CYCLOTENE, a spin-on low-k benzocyclobutene (BCB)-based polymer is introduced and the effect of moisture uptake is investigated. The dielectric constant of CYCLOTENE is extracted from capacitance measurements with a systematic error less than 0.1%, giving an average value of 2.49 with a standard deviation of 1.5%. The dielectric constant increases by 1.2% after a humidity stress of 85% RH at 85/spl deg/C. The I-V characteristics of CYCLOTENE show a dependency of breakdown strength and leakage current on the geometrical dimensions of the device under test. A breakdown strength of 225V//spl mu/m and 320 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a leakage current of a few to tens of pA are measured. The I-V characteristics degrade drastically after the humidity stress, showing a breakdown strength of 100 V//spl mu/m and 180 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a maximum increase in the leakage current as large as one order of magnitude. The maximum performance and long-term reliability of an electric micromachine are adversely affected by the degradation of the breakdown voltage and the leakage current after moisture absorption. It is expected, however, that the electrical efficiency is improved using BCB-based polymers with negligible dependency on moisture absorption.     

A Novel Benzocyclobutene-Based Device for Studying the Dynamics of Heat Transfer During the Nucleation Process

A novel microelectromechanical device has been developed to study the details of the heat transfer mechanisms involved at the nucleation site for the nucleate boiling process. This device enables quantifying the magnitude, time period of activation, and specific areas of influence of different mechanisms of heat transfer from the surface with a resolution several times greater than previously reported. This is achieved through the use of an array of embedded temperature sensors within a carefully designed dual-layer (silicon and benzocyclobutene) wall which allows for the accurate calculation of local heat flux, circumventing difficulties encountered when using existing methods. The sensors are radially distributed around the nucleation site. Heat is supplied to the wall by a thin film heater fabricated on the outer nonwetted surface. Single bubbles are generated at the center of the array while the temperatures and the bubble images are recorded with a sampling frequency of 8 kHz. The temperature data provided the necessary thermal boundary conditions to numerically calculate the surface heat flux with an unprecedented radial resolution of 22-40 mum. Fabrication, characterization, and the ability of the developed device to elucidate the heat transfer aspects of the nucleation process are demonstrated.