Wenge Zhang

Fabrication of SiCN ceramic MEMS using injectable polymer-precursor technique

In this paper, a novel and cost-effective technology for the fabrication of high-temperature MEMS based on injectable polymer-derived ceramics is described. Micro-molds are fabricated out of SU-8 photoresist using standard UV-photolithographic processes. Liquid-phase polymers are then cast into the molds and converted into monolithic, fully-dense ceramics by thermal decomposition. The resultant ceramic, based on the amorphous alloys of silicon, carbon and nitrogen, possess excellent mechanical and physical properties for high-temperature applications. This capability for micro-casting is demonstrated in the fabrication of simple single-layered, high aspect ratio SiCN microstructures. A polymer-based bonding technique for creating more complex three-dimensional structures is also presented.

Design and modeling of RF MEMS tunable capacitors using electro-thermal actuators

A series mounted MEMS tunable capacitor in a CPW line is reported. An electro-thermal actuator has been used for driving the top plate of the parallel plate capacitor. The MEMS structure is bonded on an alumina substrate using flip-chip technology so that the silicon on the backside of the MEMS can be removed to reduce the RF losses. The lumped-element model of the capacitor up to 40 GHz has been developed based on Y-parameters, which are derived from measured S-parameters. The measured Q-factor is 256 at 1 GHz for a 0.102 pF capacitor and C/sub max//C/sub min/ ratio of the capacitor is about 2:1.

The realization and design considerations of a flip-chip integrated MEMS tunable capacitor

Abstract Microelectromechanical systems (MEMS)-based radio frequency (RF) components are being developed for various microwave and millimeter-wave applications. Using standard foundry processes, it is possible to create very complex MEMS devices. However, most RF MEMS need to be fabricated using GaAs, ceramics, high resistivity silicon or other RF-compatible materials. Such fabrication techniques are not commonly used by the mainstream silicon-based MEMS manufacturing infrastructure. As a result, the complexities of these MEMS devices are very limited. What is needed is a way to utilize the existing cost effective foundry processes, but not sacrifice RF performance. Utilizing a flip-chip transfer process, a complex, foundry fabricated, MEMS tunable capacitor has been demonstrated that yields high quality RF performance ( Q ∼100 at 10 GHz, 1050 at 1 GHz). The transfer process is described, and its performance (control, success rate, etc.) is presented. Several major design considerations for implementing the tunable capacitor using flip-chip technology are presented, including warpage, actuator design, and structural rigidity. Using the transfer process and design considerations, there is an opportunity to integrate complex MEMS onto any RF compatible substrate without the silicon semiconductor effects. Thus, it is possible to manufacture complex MEMS cost-effectively for a new generation of RF MEMS with superior functionality.

Flip-chip assembly for Si-based RF MEMS

MEMS-based RF components are being developed for various microwave and millimeter-wave applications. However, most RF MEMS have to be fabricated using GaAs, ceramics, high-resistivity silicon or other RF-compatible materials; such fabrication techniques are not commonly used by mainstream silicon-based MEMS manufacturing infrastructure. As a result, the complexity of these MEMS is limited. Using flip-chip assembly and silicon removal techniques, there is an opportunity to integrate MEMS onto any RF compatible substrate without the silicon semiconductor effects. Thus, it is possible to manufacture complex MEMS cost-effectively for a new generation of RF MEMS with superior functionality, e.g. tunable capacitors, multi-way switches and arrays of hundreds of these or other RF components. This new technology is described with an emphasis on four issues: warpage, actuators, release and flip-chip bonding.

Smart phase-only micromirror array fabricated by standard CMOS process

Smart, phase-only modulation micromirror arrays have been implemented through a commercial CMOS service. The novel, 2-dimensional array of deflectable micromirrors with integrated CMOS switching circuits and piezoresistive deflection sensors on flexures is presented in this paper. The individual mirror pixels are capable of modulating light in the visible to near-infrared spectrum by piston-like movement of a trampoline-type suspended micromirror driven by thermal multi-morph actuators. A flip chip bonding technology is used to integrate the micromirror array with a microlens array to increase the optical fill factor of the hybrid system. Finite element analysis is used to model electro-thermo-mechanical behavior of the micromirror. A 2.5 mrad beam steering angle was successfully demonstrated.

RF and mechanical characterization of flip-chip interconnects in CPW circuits with underfill

RF characterization of flip-chip interconnects in CPW circuits with underfill has been investigated by measuring the scattering-parameters up to 40 GHz for GaAs coplanar waveguide (CPW) through line chips flip-chip mounted on alumina substrate with and without underfill epoxy. Fatigue life of flip-chip assemblies has been computed for different chip sizes and substrates. The results show feasibility of using underfill encapsulant in microwave/mm-wave frequency range.

The effect of underfill epoxy on warpage in flip-chip assemblies

The thermally-induced warpage of both a real flip-chip thermosonically bonded assembly and a simulated tri-layered assembly was investigated. It revealed the warpage of the assemblies was dominated by the forces applied by the underfill epoxy rather than the solder joints. The roles the underfill epoxy and solder joints played in causing warpage did not change even when the assembly had 196 solder joints under a 5.8 mm/spl times/5.8 mm chip. Mechanical properties of epoxy depend on the curing and the glass transition temperatures, and these characteristic temperatures clearly divide the warpage levels into two distinctive regions. When the maximum temperature the assembly was exposed to was less than the glass transition temperature (T/sub g/), the warpage of the assembly was characterized by the curing temperature. When the maximum temperature the assembly was exposed to was higher than T/sub g/, the warpage was characterized by T/sub g/ regardless of how high the temperature was. The distinctive deformation curves with sub-micron repeatability are reported for the first time. Depending upon the different characteristic temperatures of an assembly, e.g., 80/spl deg/C for curing and 130/spl deg/C for T/sub g/, the warpage and the Von Misses stress each could increase by as much as a factor of two. Such an increase could affect device reliability for RF packages and alignment for optoelectronic packages.

Thermosonic bonding of an optical transceiver based on an 8/spl times/8 vertical cavity surface emitting laser array

This paper reports the results of our thermosonic (T/S) flip-chip bonding process development for the assembly of a smart pixel array (SPA) using an 8/spl times/8 vertical cavity surface emitting laser (VCSEL) array. The introduction of ultrasonic energy into the flip-chip bonding process increases the speed of the assembly process while at the same time lowering the physical stresses (temperature and assembly force) applied to bond the components. Many empirical studies have shown that T/S flip-chip bonding is feasible, but there is a lack of detailed understanding of the effects of the ultrasonic energy on the bonding results. We are conducting experiments and developing models that will provide a sound understanding and a rational basis for T/S flip-chip bonding. In particular, we have addressed the problems of the impact of joint bump size, control of the assembly force, and the repeatability of the ultrasonic power. This report details our findings concerning the following aspects important to the development of T/S flip-chip bonding technology: (1) Computer modeling to guide the selection of design parameters and provide a basis to study the effects of the interaction of the critical design and process parameters on process yield. (2) Design of a new end effector for accurately applying and monitoring small assembly force. (3) Monitoring and controlling the impedance of the ultrasonic mechanical and electrical system in order to insure repeatable delivery of acoustic energy to the assembly.

MEMS designed for tunable capacitors

A new tunable capacitor based on a standard microelectromechanical systems (MEMS) technology has been demonstrated. Its unique feature was the use of thermal actuators as indirect drives to change air gap from 2 to 0.2 /spl mu/m for high-Q MM-wave capacitors. Such a drive scheme achieved a sub-/spl mu/m controllability. The insertion loss of a polysilicon MEMS capacitor was measured to be -4dB at 40 GHz. Such a loss would have been better than -1 dB if the polysilicon were coated with metal.

Quick prototyping of flip chip assembly with MEMS

This paper describes a process to transfer microelectromechanical systems (MEMS) devices to a secondary substrate using flip-chip thermosonic bonding. A standard wire-bonding machine was used to place /spl sim/100-/spl mu/m bumps on unreleased MEMS chiplets. The bumped chiplet was then flip-chip bonded to a secondary substrate containing a microwave coplanar waveguide (CPW). After bonding, the entire assembly was run through the MEMS release process, after which the MEMS host substrate was removed. The thermosonic bonding was a very reliable prototyping tool with a 100% bonding yield. The transfer process can be used with any MEMS that can be wire bonded. The process can also be applied to a variety of applications.