Micro-electro-mechanical systems fast fabrication by selective thick polysilicon growth in epitaxial reactor

A thick layer selective polysilicon growth technique has been developed for micro-electro-mechanical systems (MEMS) fabrication. It allows fast MEMS fabrication without using silicon on insulator wafers or deep ICPRIE etching. The fabrication technique is based on two main steps: a first seed layer of polysilicon is deposited and patterned; the second step consists in the selective growth of this layer in an epitaxial reactor. The first part of this work is devoted to the optimisation of growth parameters. Afterwards, this technique is applied for fabrication of different kinds of actuators, involving films several microns thick with good mechanical properties such as a low mechanical stress and a low roughness of the polysilicon film surface. Furthermore, thermal actuator prototypes were fabricated by using this technique;showing good mechanical properties and high reliability.     

Sub-Micro-Gravity In-Plane Accelerometers With Reduced Capacitive Gaps and Extra Seismic Mass

This paper presents a robust fabrication technique for manufacturing ultrasensitive micromechanical capacitive accelerometers in thick silicon-on-insulator substrates. The inertial mass of the sensor is significantly increased by keeping the full thickness of the handle layer attached to the top layer proof mass. High-aspect-ratio capacitive sense gaps are fabricated by depositing a layer of polysilicon on the sidewalls of low aspect- ratio trenches etched in silicon. Using this method, requirements on trench etching are relaxed, whereas the performance is preserved through the gap reduction technique. Therefore, this process flow can potentially enable accelerometers with capacitive gap aspect-ratio values of greater than 40:1, not easily realizable using conventional dry etching equipment. Also, no wet-etching step is involved in this process which in turn facilitates the fabrication of very sensitive motion sensors that utilize very compliant mechanical structures. Sub-micro-gravity in-plane accelerometers are fabricated and tested with measured sensitivity of 35 pF/g, bias instability of 8 mug, and footprint of <0.5 cm².     

Characterization of selective polysilicon deposition for MEMS resonator tuning

Variations in micromachining processes cause submicron differences in the size of MEMS devices, which leads to frequency scatter in resonators. A new method of compensating for fabrication process variations is to add material to MEMS structures by the selective deposition of polysilicon. It is performed by electrically heating the MEMS in a 25/spl deg/C silane environment to activate the local decomposition of the gas. On a (1.0/spl times/1.5/spl times/100) /spl mu/m/sup 3/, clamped-clamped, polysilicon beam, at a power dissipation of 2.38 mW (peak temperature of 699/spl deg/C), a new layer of polysilicon (up to 1 /spl mu/m thick) was deposited in 10 min. The deposition rate was three times faster than conventional LPCVD rates for polysilicon. When selective polysilicon deposition (SPD) was applied to the frequency tuning of specially-designed, comb-drive resonators, a correlation was found between the change in resonant frequency and the length of the newly deposited material (the hotspot) on the resonator's suspension beams. A second correlation linked the length of the hotspot to the magnitude of the power fluctuation during the deposition trial. The mechanisms for changing resonant frequency by the SPD process include increasing mass and stiffness and altering residual stress. The effects of localized heating are presented. The experiments and simulations in this work yield guidelines for tuning resonators to a target frequency.     

Power delivery and locomotion of untethered microactuators

The ability for a device to locomote freely on a surface requires the ability to deliver power in a way that does not restrain the device's motion. This paper presents a MEMS actuator that operates free of any physically restraining tethers. We show how a capacitive coupling can be used to deliver power to untethered MEMS devices, independently of the position and orientation of those devices. Then, we provide a simple mechanical release process for detaching these MEMS devices from the fabrication substrate once chemical processing is complete. To produce these untethered microactuators in a batch-compatible manner while leveraging existing MEMS infrastructure, we have devised a novel postprocessing sequence for a standard MEMS multiproject wafer process. Through the use of this sequence, we show how to add, post hoc , a layer of dielectric between two previously deposited polysilicon films. We have demonstrated the effectiveness of these techniques through the successful fabrication and operation of untethered scratch drive actuators. Locomotion of these actuators is controlled by frequency modulation, and the devices achieve maximum speeds of over 1.5 mm/s.     

Reliability design of thermally actuated MEMS switches based on V-shape beams

This paper presents the design and fabrication of the thermally actuated MEMS switches based on out-of-plane V-beams. The purpose of this research is to analyze the mechanical response of a V-thermal actuator fabricated from aluminum in order to improve the accuracy in response and to increase the switch lifetime. The actuation of this kind of switches is based on the thermal displacement of the mobile electrode under thermal load that is generated when the actuation voltage is applied. It can be used either as a capacitive switch or as a metal-to-metal one. The displacement of the mobile electrode for a given temperature is analytically calculated and validated both numerically and experimentally. Experimental investigations are performed on a macro-scale sample using a 3D digital image correlation measuring system, a heating source and a thermal camera for temperature monitoring. The first fabrication steps of the MEMS switch based on the V-beam thermal actuator are presented. The out-of-plane V-beams thermal MEMS switches can be monolithically integrated in RF applications.

     

PECVD-SiOxNy films for large area self-sustained grids applications

In this work we study the structural properties and mechanical stress of silicon oxynitride (SiO_xN_y) films obtained by plasma enhanced chemical vapor deposition (PECVD) technique at low temperatures (320 °C) and report the feasibility of using this material for the fabrication of large area self-sustained grids. The films were obtained at different deposition conditions, varying the gas flow ratio between the precursor gases (N₂O and SiH₄) and maintaining all the other deposition parameters constant. The films were characterized by ellipsometry, by Fourier transform infrared (FT-IR) spectroscopy and by optically levered laser technique to measure the total mechanical stress. The results demonstrate that for appropriated deposition conditions, it is possible to obtain SiO_xN_y with very low mechanical stress, a necessary condition for the fabrication of mechanically stable thick films (up to 10 μm). Since this material (SiO_xN_y) is very resistant to KOH wet chemical etching it can be utilized to fabricate, by silicon substrate bulk micromachining, very large self-sustained grids and membranes, with areas up to 1 cm² and with thickness in the 2–6 μm range. These results allied with the compatibility of the PECVD SiO_xN_y films deposition with the standard silicon based microelectronic processing technology makes this material promising for micro electro mechanical system (MEMS) fabrication.     

Fabrication of a freestanding micro mechanical structure using electroplated thick metal with a HAR SU-8 mold

In this thesis, fabrication technology of a freestanding micro mechanical structure using electroplated thick metal with a high-aspect-ratio SU-8 mold was studied. A cost-effective fabrication process using electroplating with the SU-8 mold was developed without expensive equipment and materials such as deep reactive-ion etching (DRIE) or a silicon-on-insulator (SOI) wafer. The process factors and methods for the removal of SU-8 were studied as a key technique of the thick metal micro mechanical structure. A novel method that removes cross-linked SU-8 completely without leaving remnants of the resist or altering the electroplated microstructure was utilized. The experimental data pertaining to the relationship between the geometric features and the parameters of the removal process are summarized. Based on the established SU-8 removal process, an electroplated nickel comb structure with high-aspect-ratio SU-8 mold was fabricated in a cost-effective manner. In addition, a freestanding micro mechanical structure without a sacrificial layer was successfully realized. The in-plane free movements of the released freestanding structure are demonstrated by electromagnetic actuation. This research implies that various types of MEMS devices can be developed at a low-cost with design flexibility.     

选择性化学机械抛光制作微拱形结构

为克服平面微机械结构无法释放残余应力、刚度小、跨度小,平面牺牲层工艺成拱出现非光滑台阶状边缘而造成器件失效或能量泄漏的缺点,改进现有成拱工艺在器件尺寸、结构稳定性及可靠性等方面的不足,提出一种用选择性化学机械抛光技术制作微拱形结构的方法。该方法是在化学机械抛光过程中,加入对牺牲层材料和硅材料抛光速度具有差异性的选择性抛光液,在牺牲层和硅材料的边界处利用滑动摩擦过程中的物理和化学作用,在牺牲层处形成连续平滑的拱形凸起,最后在其上制作微拱形结构。实验结果证实:微拱形结构有一定的曲率,拱起高度约为3.5μm,跨度大于100μm,微拱形表面光滑且为连续的曲面。该方法可为MEMS传感器、微型压电驱动器、薄膜体声波谐振器及滤波器的微拱形结构的制作提供参考。     

Asymmetric surface roughness measurements and meniscus modeling of polysilicon surface micromachined flaps

Polycrystalline silicon (polysilicon) films are primary structural materials for microelectromechanical systems (MEMS). Due to relatively high compliance, large surface-to-volume ratio, and small separation distances, micromachined polysilicon structures are susceptible to surface forces which can result in adhesive failures. Since these forces depend on surface properties especially surface roughness, three types of microhinged flaps were fabricated to characterize their roughness and adhesive meniscus properties. The flaps enabled access to both the top and bottom surfaces of the structural polysilicon layers. Roughness measurements using an atomic force microscope revealed that MEMS surfaces primarily exhibit non-Gaussian surface height distributions, and for the release procedures studied, the bottom surface of the structural layers was significantly smoother and prone to higher adhesion compared to the top surface. A non-symmetric surface roughness model using the Pearson system of frequency curves was coupled with a capillary meniscus adhesion model to analyze the effects of surface roughness parameters (root-mean-square, skewness, and kurtosis), relative humidity, and surface contact angle on the interfacial adhesion energy. Using the measured roughness properties of the flaps, four different surface pairs were simulated and compared to investigate their effects on capillary adhesion. It was found that since the base polysilicon layer (poly0) was rougher than the base silicon nitride and the structural layer on poly0 was also rougher than that on silicon nitride, depositing MEMS devices on poly0 layer rather than directly on silicon nitride will reduce the adhesion energy.     

Integrated self-assembling and holding technique applied to a 3-D MEMS variable optical attenuator

The application of polysilicon/gold bimorph stress-induced curved beams for three-dimensional self-assembly of MEMS devices is reported. The mechanical principle behind this self-assembling procedure is presented and comparison with current assembling methods are made. With this self-assembling technique, no postprocessing is required. A free-space optical MEMS device in the form of a variable optical attenuator (VOA) has been fabricated and self-assembled using this technique. The angular elevation of the self-assembled structures and the attenuation characteristics of the optical MEMS device are reported. The VOA has a measured dynamic attenuation range of 44 dB at 1.55 /spl mu/m optical wavelength. The bending of the bimorph beams is also temperature controllable, and the thermal behavior of the beams is also reported.     

Fusion bonding of rough surfaces with polishing technique for silicon micromachining

 Surface roughness is one of the crucial factors in silicon fusion bonding. Due to the enhanced surface roughness, it is almost impossible to bond wafers after KOH etching. This also applies when wafers are heavily doped, have a thick LPCVD silicon nitride layer on top or have a LPCVD polysilicon layer of poor quality. It has been demonstrated that these wafers bond spontaneously after a very brief chemical mechanical polishing step. An adhesion parameter, that comprises of both the mechanical and chemical properties of the surface, is introduced when discussing the influence of surface roughness on the bondability. Fusion bonding, combined with a polishing technique, will broaden the applications of bonding techniques in silicon micromachining. Received 30 October 1996/Accepted: 14 November 1996     

The fabrication of all-diamond packaging panels with built-in interconnects for wireless integrated microsystems

To explore polycrystalline diamond (poly-C) as a packaging material for wireless integrated microsystems (WIMS), a new fabrication technology has been developed to fabricate thick WIMS packaging panels with built-in interconnects. An ultrafast poly-C growth technique, used in this study, involves electrophoresis seeding and filling of dry-etched Si channels by undoped poly-C followed by removal of Si. A second layer of highly B-doped poly-C, which acts as a built-in interconnect, is deposited on the backside of undoped poly-C layer. The lowest resistivity values demonstrated on control samples are in the range from 0.003 to 0.31 /spl Omega/-cm. The results show that, by increasing the poly-C growth areas through the use of 2-/spl mu/m-wide Si channels, the poly-C growth time can be reduced by a factor in the range from 2.75 to 10.5 depending upon the aspect ratio of Si channels. The poly-C packaging technology, which is expected to provide new structures/concepts in MEMS/WIMS packaging, is being reported for the first time.     

Porous polycrystalline silicon: a new material for MEMS

A new technique for the fabrication of thin patterned layers of porous polycrystalline silicon (polysilicon) and surface micromachined structures is presented. First, a multilayer structure of polysilicon between two layers of low-stress silicon nitride is prepared on a wafer of silicon. Electrochemical anodization with an external cathode takes place in an RF solution. A window in the outer nitride layer provides contact between the polysilicon and the HF solution; the polysilicon layer contacts the substrate through openings in the lower silicon nitride layer (remote from the upper windows). Porous polysilicon growth in the lateral direction is found at rates as high as 15 μm min^-1 in 12M (25%, wgt) HF to be controlled by surface-reaction kinetics. A change in morphology occurs when either the anodic potential is raised or the HF concentration is decreased, causing the polysilicon to be electropolished. The etch front advances proportionally to the square root of time as expected for a mass-transport-controlled process. Similar behavior is observed in HF anodic reactions of single-crystal silicon. Dissolution of the polysilicon layer is confirmed using profilometry and scanning electron microscopy. Enclosed cavities (chambers surrounded by porous plugs) are formed by alternating between pore formation and uniform dissolution. Porous polysilicon also forms over a broad-area layer of polycrystalline silicon that has been deposited without overcoating the silicon wafer with a thin film of silicon nitride. The resulting porous layer may be useful for gas-absorption purposes in ultrasonic sensors     

Microrivets for MEMS packaging: concept, fabrication, and strengthtesting

Microriveting is introduced as a novel and alternative joining technique to package MEMS devices. In contrast to the existing methods, mostly surface bonding, the reported technique joins two wafer pieces together by riveting, a mechanical joining means. Advantages include wafer joining at room temperature and low voltage, and relaxed requirements for surface preparation. The microrivets, which hold a cap-base wafer pair together, are formed by filling rivet holes through electroplating. The cap wafer has a recess to house the MEMS devices and also has through-holes to serve as rivet molds. The seed layer on the base wafer becomes the base of the rivet. The process requires only simple mechanical clamping of the wafer pair during riveting, compared with the more involved procedures needed for wafer bonding. Directionality of electroplating in an electric field is what makes this process simple and robust. Strength testing is carried out to evaluate the joining with microrivets. Different modes of rivet failure under different loading conditions are identified and investigated. Effective strength between 7 and 11 MPa was measured under normal loading with nickel microrivets. Joining strengths comparable to conventional wafer bonding processes, ease of fabrication with repeatability, and compatibility with batch fabrication show that microriveting is a feasible technique to join wafers for MEMS packaging, especially when hermetic sealing is not essential     

Solder bonding with a buffer layer for MOEMS packaging using induction heating

In this paper, the selective induction heating technology is applied to glass–glass and glass–silicon solder bonding for MOEMS (optical MEMS) packaging. The Ni bumping with a buffer layer is successful to release the thermal stress for avoiding delamination. The Au wetting layer must be thick enough to prevent from being solved entirely into Sn, and it will improve bonding strength. The bonding specimens are soaked into 25°C water and placed into 85°C/85% RH oven, respectively. No moisture penetrates into the cavity after 1 day in both test conditions. In the test condition of 125°C leakage-test liquid (Galden HS260), no bubble is observed. The lowest bonding strength is 3 MPa.

     

Low-temperature bonding of thick-film polysilicon for microelectromechanical system (MEMS)

Polysilicon thick films have been found to be an irreplaceable option in various sensors and other microelectromechanical system (MEMS)-designs. Polysilicon is also a prospective option for replacing single-crystal silicon in customized silicon-on-insulator-substrates. Due to the nature of polysilicon, bonding for MEMS-purposes has so far concentrated on anodic bonding, which has drawbacks for instance in terms of process duration and thermal load. The objective of this work is to develop low-temperature direct bonding for various polysilicon films. Polysilicon films were grown at varying temperatures and pressures with and without boron doping. The films were polished by chemical–mechanical polishing and cleaned. Surface qualities were studied by atomic-force-microscope before bonding. Wafers were then activated with argon plasma and bonded to oxidized silicon, quartz and glass. Bonding quality was evaluated with scanning-acoustic-microscope, the crack-opening-method and HF-etching. Scanning-electron-microscopy was used to investigate film and interface quality. This development has led to a new kind of polishing process, where several microns of polysilicon are removed still leaving surface direct bondable. This is accomplished by a dedicated and effectively planarizing polishing process. Spontaneous bonding took place and good bonding quality was achieved after annealing at 200°C.     

MEMS milliactuator for hard-disk-drive tracking servo

This paper describes the design, fabrication, and operational characteristics of a MEMS milliactuator designed for servo tracking in a hard-disk drive (HDD). The actuator is designed to increase the bandwidth of an HDD tracking servo and pack more recording tracks on a disk. An Invar (low thermal expansion metal) electrode position process was developed to meet the thermal stability requirement. The electroplated Invar's thermal coefficient of expansion is as low as 6.3×10^-6/K, which is almost half of that of pure nickel. For the plating mold pattern definition, a high-aspect-ratio polymer etching technique was developed. A high-aspect-ratio structure line-and-gap definition is required to achieve both a high directional stiffness ratio and electrode efficiency for the actuator. The etching technique described can etch through a thick (<40 μm) polymer layer with an aspect ratio of 16:1 at an etch rate of <2 μm/min. Low-cost/high-volume manufacturing is achievable by this batch fabrication technique. A milliactuator was fabricated and assembled with a suspension and a slider weighted at around 2 mg. The slider was successfully driven by the milliactuator while the slider was flying on a spinning disk. The operational characteristics (frequency response) of the in-flight milliactuator were measured, and the results indicate that the actuator is suitable for high-bandwidth HDD servo-tracking applications     

Low-stress fabrication of 3D polymer free standing structures using lamination of photosensitive films

Free-standing microstructures such as cantilevers, membranes or microchannels are building blocks of microfluidic systems and MEMS. As a complement to silicon, the large family of polymers offers many opportunities for micro and nanotechnologies. Their low temperature processing and the planarizing properties of many resists is a definitive advantage for system integration, paving the way to complete lab-on-chips. In this article, we investigate a fabrication process of polymeric free standing structures based on the lamination of SU-8, a thick epoxy photoresist. Our motivation is the hybrid integration of polymer microfluidic or MEMS components with silicon chips (e.g., integrated circuits or sensors). Compared to rigid substrates used in more conventional SU-8/SU-8 bonding process, the flexible photosensitive films used within this lamination technique allows a more homogeneous and reliable bonding at low pressure and temperature, and a 3D fabrication with an excellent level-to-level alignment. A parametric optimization of the lamination process is presented. The fabrication of a leakage-free 3D microfluidic network is demonstrated by stacking up to five layers. A polyethylene terephtalate layer has been employed to easily release the SU-8 devices. We show that this release layer also significantly decrease the curvature of the substrate by 32% and the related residual stress in a 100 μm SU-8 layer by at least 10%. Finally, we briefly describe the hybrid integration of a silicon sensor in a microfluidic network as a direct application of our lamination process to the fabrication of lab-on-chips.

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A self-aligned fabrication method of dual comb drive using multilayers SOI process for optical MEMS applications

A novel method for fabricating a self-aligned electrostatic dual comb drive using a multi-layer SOI process is developed. The present method utilizes four aligned masks, greatly simplify the existing SOI-MEMS fabrication methods in manufacturing optical MEMS devices. Here, the actuating structure consists of fixed combs and moving combs that are composed of single crystal silicon, nitride and polysilicon. One mask is used to provide a deep etching to etch polysilicon, nitride and single crystal silicon respectively. The nitride separates polysilicon and single crystal silicon and provides an additional dielectric for the purpose of producing bi- directional motion upon applying electrostatic forces. A dual comb drive actuator with optical structures was fabricated with the developed process. The actuator is capable of motion 250 nm downward and 480 nm upward with 30 V applied voltage at 4 kHz frequency. The dynamic characteristics of the first and the second resonant frequency of the dual comb-drive actuator are 10.5 kHz and 23 kHz respectively. Experimental results indicated that the measured data agreed well with simulation results using the ANSOFT Maxwell^® 2D field simulator, ANSYS^® and Coventor Ware^®.     

Electrostatic micro torsion mirrors for an optical switch matrix

We have developed a new type of compact optical switch using silicon micromachining technique. Torsion mirrors (300 μm×600 μm) supported by thin polysilicon beams (16 μm wide, 320 μm long, and 0.4 μm thick) are arranged in a 2×2 matrix (total size 3 mm×5 mm, t 0.3 mm). The mirrors are independently attracted by electrostatic force of applied bias voltage to redirect the incident light in a free space. Using collimated beam fibers for optical coupling, we obtained small insertion loss (⩽-7.66 dB), considering the length of a light path (⩾10 mm), a large switching contrast (⩾60 dB), and small crosstalk (⩽-60 dB). The fabrication yield was higher than 80% thanks to the newly developed releasing technique that used a silicon oxide diaphragm as an etch-stop layer and as a mechanical support in the process. Holding voltage (⩽50 V) was lower than the voltage to attract the mirror (100~150 V) because of the hysteresis of angle-voltage characteristic of electrostatic operation