High Aspect Ratio Microelectromechanical Systems Research Articles

Influence of fabrication tolerances on performance characteristics of a MEMS gyroscope

Fabrication of high aspect ratio microelectromechanical systems (MEMS) like comb drive structures are mostly done by DRIE (Deep Reactive Ion Etching) process. Bosch process is a type of DRIE which involves alternate repetition of plasma etching and side wall passivation to achieve deep vertical structures. A continued plasma etching may lead to tapered wall due to some lateral etching. Although, a straight and smooth wall is expected due to combined alternate processes of etching and passivation, very often the resulting walls are having surface undulations called as scalloping. So, due to slanting, scalloping and combined slanting and scalloping defects the working and performance of a MEMS device get affected. MEMS gyrocope is one such widely comercialized device, which is required to measure angular rotation wth certain accuracy. We take a simple single-axis MEMS vibratory gyroscope to study the effect on these defects on its dynamics performance. The study presented in the paper may be useful for including undesirable effects in designing MEMS vibratory gyroscopes.

Electrostrictive microelectromechanical fibres and textiles

Microelectromechanical systems (MEMS) enable many modern-day technologies, including actuators, motion sensors, drug delivery systems, projection displays, etc. Currently, MEMS fabrication techniques are primarily based on silicon micromachining processes, resulting in rigid and low aspect ratio structures. In this study, we report on the discovery of MEMS functionality in fibres, thereby opening a path towards flexible, high-aspect ratio, and textile MEMS. The method used for generating these MEMS fibres leverages a preform-to-fibre thermal drawing process, in which the MEMS architecture and materials are embedded into a preform and drawn into kilometers of microstructured multimaterial fibre devices. The fibre MEMS functionality is enabled by an electrostrictive P(VDF-TrFE-CFE) ferrorelaxor terpolymer layer running the entire length of the fibre. Several modes of operation are investigated, including thickness-mode actuation with over 8% strain at 25 MV m−1, bending-mode actuation due to asymmetric positioning of the electrostrictive layer, and resonant fibre vibration modes tunable under AC-driving conditions.

Novel microelectromechanical systems image reversal fabrication process based on robust SU-8 masking layers

This paper discusses a novel fabrication process that uses a combination of negative and positive photoresists with positive tone photomasks, resulting in masking layers suitable for bulk micromachining high-aspect ratio microelectromechanical systems (MEMS) devices. Mi- croChem's negative photoresist Nano TM SU-8 and Clariant's image rever- sal photoresist AZ 5214E are utilized, along with a barrier layer, to effec- tively convert a positive photomask into a negative image. This technique utilizes standard photolithography chemicals, equipment, and processes, and opens the door for creating complementary MEMS structures without added fabrication delay and cost. Furthermore, the SU-8 masking layer is robust enough to withstand aggressive etch chemistries needed for fabrication research and development, bulk micromachining high-aspect ratio MEMS structures in silicon substrates, etc. This processing tech- nique was successfully demonstrated by translating a positive photomask to an SU-8 layer that was then utilized as an etching mask for a series of trenches that were micromachined into a silicon substrate. In addition, whereas the SU-8 mask would normally be left in place after process- ing, a technique utilizing Rohm and Haas Microposit TM S1818 as a re- lease layer has been developed so that the SU-8 masking material can be removed post-etching. C 2011 Society of Photo-Optical Instrumentation Engineers

A resonant accelerometer with two-stage microleverage mechanisms fabricated by SOI-MEMS technology

We present the design, fabrication, and testing of a push-pull differential resonant accelerometer with double-ended-tuning-fork (DETF) as the inertial force sensor. The accelerometer is fabricated with the silicon-on-insulator microelectromechanical systems (MEMS) technology that bridges surface micromachining and bulk micromachining by integrating the 50-/spl mu/m-thick high-aspect ratio MEMS structure with the standard circuit foundry process. Two DETF resonators serve as the force sensor measuring the acceleration through a frequency shift caused by the inertial force acting as axial loading. Two-stage microleverage mechanisms with an amplification factor of 80 are designed for force amplification to increase the overall sensitivity to 160 Hz/g, which is confirmed by the experimental value of 158 Hz/g. Trans-resistance amplifiers are designed and integrated on the same chip for output signal amplification and processing. The 50-/spl mu/m thickness of the high-aspect ratio MEMS structure has no effect on the amplification factor of the mechanism but contributes to a greater capacitance force; therefore, the resonator can be actuated by a much lower ac voltage comparing to the 2-/spl mu/m-thick DETF resonators. The testing results agree with the designed sensitivity for static acceleration.

Advanced plasma processing combined with trench isolation technology for fabrication and fast prototyping of high aspect ratio MEMS in standard silicon wafers

A bulk micromachining technology for fabrication of micro electro mechanical systems (MEMS) in a standard silicon wafer is presented. A fabrication process, suitable for full integration with on-chip electronics, employs advanced plasma processing to etch, passivate and release micromechanical structures in a single plasma system, and vertical trench isolation to obtain electrical isolation between the released components. Distinct electrical domains can be defined even on movable parts. The sophisticated electrical isolation between high-aspect-ratio single-crystal silicon (SCS) components allows simplification of the fabrication and improvement of the performance of existing devices and design of entirely new MEMS. The presented technology is an attractive platform for both fabrication and rapid prototyping of MEMS. This is due to a short processing time, a large freedom of design, high process flexibility and low-cost of the starting SCS substrate relative to SOI substrates. Several example microstructures demonstrating the capabilities of this technology have been successfully fabricated.

High-resolution x-ray masks for high aspect ratio microelectromechanical systems applications

The advanced requirements of bio-MEMS and MOEMS, i.e., low sidewall surface roughness, submicron critical dimension, and high aspect ratio, necessitate the use of an intermediate mask and a soft x-ray lithography process to fabricate working x-ray masks that are suitable for deep x-ray lithography. Intermediate masks consist of 2 to 2.5-µm gold patterns on membranes/substrates that are highly transparent to x-ray radiation, whereas working masks possess greater than 5 µm of gold patterns. In this work, 1-µm silicon nitride membranes are produced by a low pressure chemical vapor deposition (LPCVD) process on both the front and backside of 100 prime grade wafers and anisotropic wet etch through silicon nitride etch masks. E-beam lithography is used to pattern 0.8- to 3-µm-thick resist layers with submicron resolution. In the case of the 3-µm resist layers, the features are electroplated with approximately 2 µm of gold to form an intermediate mask. The 0.8-µm-thick layers are electroplated with gold up to a thickness of 0.6 µm and form initial masks, which are in turn used in a soft x-ray lithographical process to make intermediate masks. The process of building a high-resolution intermediate x-ray mask, directly by e-beam patterning a 3 µm layer of e-beam resist, followed by gold electroplating, is found to be viable but requires the use of a high energy (>100 keV) e-beam writer. The stability of the resist pattern during soft x-ray lithography (SXRL) by use of an initial mask is found to be problematic. Double-side lithography and gold electroplating, can effectively reduce the aspect ratio of the mask pattern, eliminates the problems associated with the use of an initial mask to fabricate intermediate x-ray masks.

Metrology study of structural transfer accuracy in fabrication of high-aspect-ratio microelectromechanical systems: From optical mask to polished electroplated parts

This article focuses on some fabrication issues and characterization at various steps of the LIGA [German acronym for Lithorgraphy, Galvanoplasty, and molding (Abformung)] process, in particular on the acuity of pattern transfer through the various steps of the process, from the optical mask to planarized electroplated parts for basic LIGA processing (single level). Metrology data on the accuracy of the LIGA process as currently applied in the high-aspect-ratio microelectromechanical systems (HI-MEMS) Alliance at various stages of the process are presented and the sources of process biases are investigated. Some potential areas for improvement with respect to dimensional control will also be highlighted.