Suspended Microstructures Research Articles

A novel fabrication method for suspended high-aspect-ratio microstructures**This paper was orally presented in the Transducers'05, Seoul, Korea (paper ID: 3B1.3).

Suspended high-aspect-ratio structures (suspended HARS) are widely used for MEMS devices such as micro-gyroscopes, micro-accelerometers, optical switches and so on. Various fabrication methods, such as SOI, SCREAM, AIM, SBM and BELST processes, were proposed to fabricate HARS. However, these methods focus on the fabrication of suspended microstructures with relatively small widths of trench opening (e.g. less than 10 µm). In this paper, we propose a novel process for fabricating very high-aspect-ratio suspended structures with large widths of trench opening using photoresist as an etching mask. By enhancing the microtrenching effect, we can easily release the suspended structure without thoroughly removing the floor polymer inside the trenches for the cases with a relatively small trench aspect ratio. All the process steps can be integrated into a single-run single-mask ICP–RIE process, which effectively reduces the process complexity and fabrication cost. We also discuss the phenomenon of corner erosion, which results in the undesired etching of silicon structures during the structure-releasing step. By using the proposed process, 100 µm thick suspended structures with the trench aspect ratio of about 20 are demonstrated. Also, the proposed process can be used to fabricate devices for applications which require large in-plane displacement.

Wet-etch release process for silicon-micromachined structures using polystyrene microspheres for improved yield

One of the final steps in fabricating microelectromechanical devices often involves a liquid-etch release process. Capillary forces during the liquid evaporation stage after the wet-etch process can pull two surfaces together resulting in adhesion of suspended microstructures to the supporting substrate. This release-related adhesion can greatly reduce yields. In this paper, we present a wet-etch release method that uses polystyrene microspheres in the final rinse liquid. The polystyrene microspheres act as physical barriers between the substrate and suspended microstructures during the final liquid evaporation phase. A plasma-ashing process is utilized to completely remove the polystyrene microspheres from the microstructure surfaces. Using this process, release yields >90% were achieved, as compared to yields of 20-50% when the polystyrene microspheres were not included. This release process is inexpensive, easy to implement, and is effective for both single-crystal and polycrystalline silicon MEMS devices.

Stress characteristics of suspended porous silicon microstructures on silicon

The mechanical properties of porous silicon (PS) microstructures on crystalline silicon (c-Si) were investigated. Micro-Raman spectroscopy was applied to probe stresses in PS micro-hotplates in the form of cantilevers. Important information was obtained for the stress distribution across cantilevers supported at one edge. The structural stability of thermally treated PS after long time exposure in air was also investigated. The results can be used for optimization of microsensors based on the PS microstructures.

Fabrication and characterization of amorphous Si films by PECVD for MEMS

Thick and smooth amorphous Si film of 2 µm without hillocks has been obtained at low temperature of 300 °C by plasma-enhanced chemical vapor deposition (PECVD) technology. In comparison with conventional sputtering deposition, PECVD-deposited thick amorphous Si film has better adhesion to Si or oxide substrate without cracking or peeling. The film quality depends on the following process parameters: RF power, frequency mode and gas flow ratio as well as substrate material. The deposition rate increases with the RF power for both RF modes of 380 kHz and 13.56 MHz, and higher deposition rate together with lower compressive stress occurs at high frequencies of 13.56 MHz. Amorphous Si film without hillocks is formed on the 100 nm oxide/Si(100) substrate while some hillocks appear on the top of the amorphous Si film deposited on the crystalline Si(100) substrate. Good quality of amorphous Si at low temperature is very important for the fabrication of MEMS devices. Fabrication of suspended MEMS microstructure and sensor array has been demonstrated using the smooth amorphous Si films as sacrificial layer in surface micromachining.

An approach to fabricating microstructures that incorporate circuits using a post-CMOS process

This study proposes a method of fabricating microstructures that incorporate circuits using a standard CMOS (complementary metal oxide semiconductor) process and a post-process. The post-process has two main steps. One uses a photoresist (PR) mask to protect the bonding pads, the circuits and the unneeded etched regions in the chip. The other step employs CF4/O2 RIE (reactive ion etching) and SF6/O2 RIE to etch the sacrificial layers to release the suspended microstructures. A 5.8 GHz low-noise amplifier (LNA) was employed to verify whether the post-process affects the performance of the circuits or not. The test results indicated that the performance of the LNA was the same before and after the post-process, proving that the post-process did not change the functionality of the circuits. Hence, the advantage of the post-process is that it does not damage the bonding pads, the passivation layers and the circuits, which are protected by the PR mask.

Deposition and Characterization of Low-Stress Plasma Enhanced Chemical Vapor Deposition of Tetraethoxysilane Oxide for Micro-Electro-Mechanical-Systems Applications

Micro-electro-mechanical systems (MEMS) often use insulated suspended microstructures such as cantilever beams, bridges and membranes for supporting the functional active part of devices. Moreover, these structures are themselves functional devices. In this paper we report a comprehensive study of low-stress plasma-enhanced chemical vapor deposition (PECVD) tetraethoxysilane (TEOS) oxide with respect to the relationship of its deposition process parameters with its properties after deposition such as stress, deposition rate and etching rate in buffered oxide etchant (BOE) or N2H4 silicon etchant. The effect of annealing on the oxide stress is also studied. Low-stress, low-temperature oxides which are resistant to N2H4 silicon micromachining etching were obtained and demonstrated in the fabrication of long and thin cantilever beams. The same technology can also be used for the deposition of thick oxides as a sacrificial layer in MEMS applications.

Compensation of temperature effects on the pull-in voltage of microstructures

The pull-in voltage of a suspended microstructure has been investigated for use as on-chip voltage reference in a compatible MEMS-IC process. Pull-in is detected using capacitive displacement measurement. The stability is affected by an initial parasitic charge build-up and a temperature sensitivity of −149 μV/K. A burn-in procedure is required to minimize the first effect. The temperature coefficient is compensated for by applying additional temperature dependent electrostatic energy to the microstructure. Devices fabricated in an epi-poly process and designed for a nominal pull-in voltage at 5 V have a measured value at 4.7424 V. Drift becomes negligible after 120 h of operation. The temperature reproducibility is within the resolution of the readout at 100 μV over a temperature range between 20 and 60 °C.

DESIGN AND FABRICATION OF SIO2 MICROMECHANICAL STRUCTURES INSIDE ANISOTROPICALLY ETCHED CAVITY

In the present work, we report a novel process for fabricating suspended silicon dioxide micro-structures inside the anisotropically etched cavity formed in (100) silicon wafers. Using this new process, the moving parts are recessed in the substrate which can be packaged and handled easily compared to non-recessed structures. In the fabrication process, spin coated liquid photoresist is used. There is a common problem of thinning of photoresist on convex edges of the cavity during photolithography steps. As a result, silicon dioxide is etched away from these edges. During subsequent anisotropic etching of silicon to release the microstructures, the unprotected convex edges are also etched resulting in failure of the process. To overcome this problem, two different methods are used. In the first method, high viscosity positive photoresist (Shipley STR-1045) is used and the multiple coatings are carried out to cover the convex edges adequately. This protects the oxide on convex edges in buffered hydrofluoric acid. In the second method, standard low viscosity positive resist (Shipley 1400-27) and negative resist (E. Merck N60) are used and the convex edges are diffused heavily with boron (P+). Recessed microstructures have been successfully fabricated using both these process.

Powder blasting patterning technology for microfabrication of complex suspended structures in glass

We present a powder blasting microfabrication method and realize with it complex three-dimensional and monolithic suspended microstructures in glass. We also compare the usual metal contact mask to an alternative photosensitive flexopolymer. At normal incidence of the powder beam, the obtained profiles are similar for both methods and both the metal contact mask and the flexopolymer erosion selectivity with respect to glass is ∼50. By controlling the underetching induced by oblique powder blasting, suspended structures are realized, using a metal contact mask directly into the substrate. These structures can be realized in a single oblique erosion step or by a two-step normal/oblique erosion process.

Fabrication of microsuspensions on silicon substrate

A new technique to remove the silicon from beneath a large structure, by micromachining for making a suspended microstructure both for thermal, as well as electromagnetic, isolations on a complementary metal-oxide-semiconductor (CMOS) chip is reported. Conventional methods require two step front-side etching, an isotropic step, followed by an anisotropic etching step. An alternative technique is based on the backside etching process, which requires extra masks, and processing steps. In order to keep the postprocessing steps to a minimum, a simple technique has been developed that exploits the front-side anisotropic etching to create both undercuttings as well as deep etching in one single step. This method uses the gate oxide and polysilicon layer in CMOS technology as the sacrificial layer for initiating the undercutting needed to make a freestanding microstructure. The microsuspension thickness, width and length of 2 μm, 150 μm×150 μm, respectively, are made out of low-pressure chemical vapor deposition oxide and have been fabricated.

Fabrication of single crystal Si cantilevers using a dry release process and application in a capacitive-type humidity sensor

A process is presented for the fabrication of suspended silicon microstructures, such as cantilevers and open type membranes, constituting the movable plate of a capacitor at a small distance over the silicon substrate. The process relies on the silicon fusion bonding of two wafers and features dry release of the suspended silicon structures at the final stage of the process, as a step to prevent stiction. With the addition of a hydrophilic polymer film on the Si cantilevers, a capacitive type humidity sensor is fabricated; the moisture-dependent bending of the microcantilever reflects on the measured capacitance between the microcantilever and substrate.

Fabrication of GaN suspended microstructures

We report on a versatile processing technology for the fabrication of micro-electromechanical systems in gallium nitride (GaN). This technology, which is an extension of photo-electrochemical etching, allows for the controlled and rapid undercutting of p-GaN epilayers. The control is achieved through the use of opaque metal masks to prevent etching in designated areas, while the high lateral etch rates are achieved by biasing the sample relative to the solution. For GaN microchannel structures processed in this way, undercutting rates in excess of 30 μm/min have been attained. We propose two mechanisms to account for these high etch rates.

Microbeams with Electronically Controlled High Thermal Impedance

In some MEMS (Micro-Electro-Mechanical Systems) applications, a tradeoff much be reached between the mechanical strength of a suspended microstructure and the thermal losses through the support beams. This is typically the case of suspended thermal MEMS, a major domain of application of CMOS-compatible bulk-micromachining technologies. This paper illustrates how suspended MEMS can be strengthened by means of additional support beams which can have a very high thermal impedance, thus having a very small impact in the thermal behavior of the microstructure. The high thermal impedance of a support beam is achieved through self-heating: an electronic control monitors the temperature drop and heats up the beam to reduce the heat flow. The control electronics of a beam is implemented using a single high-gain stage with auto-zeroing. A High Thermal Impedance Beam (HTIB) can be considered as a new MEMS design cell. We illustrate the use of this cell in the design of an Electro-Thermal Converter with long time constant, requiring several HTIB cells which can share the same control electronics. A single low-frequency high-gain stage is used, achieving 60 dB DC gain and DC offset and broadband noise below 100 μV, which is suitable for the ETC application.

Focused ion beams and silicon-on-insulator - a novel approach toMEMS

We report on the fabrication of microaccelerometer structures inbonded silicon-on-insulator material employing a focused ion beam technique toform the readout gap. The silicon-on-insulator starting material allows thesimple fabrication of suspended microstructures with the advantages ofsingle-crystal silicon. Focused ion beam machining is essential for this kindof modification without a photolithographic resist mask. In particular, forthe milling of narrow trenches at oblique angles an ion beam has advantagesover other etching techniques. We show that milling times can be held shortenough to consider this process for manufacturing devices. We also demonstrateother novel applications of focused ion beams for prototyping MEMS.

A sticking model of suspended polysilicon microstructure including residual stress gradient and postrelease temperature

A sticking (stiction) model for a cantilevered beam is derived. This model includes the effect of the bending moment, which stems from stress gradient along the vertical direction of structural polysilicon, and the temperature during the release process. The bending moment due to the stress gradient will play an important role in evaluating antisticking efficiency since liquid tension and surface energy of microstructures tend to become smaller by newly developed antisticking techniques. The effects of stress gradient and temperature were analyzed and verified with surface-micromachined polysilicon cantilevers. By modifying the substrate polysilicon with grain-hole formation technique, the effects of residual stress gradient in polysilicon on stiction could be observed in the condition of low work of adhesion.

InP-based micromachined Mach-Zehnder interferometer stress sensors

In the field of III–V-based compounds, new functionalities can be reached by integrating micromechanical structures with electro-optical functions, in order to fabricate Micro Opto Electro Mechanical Systems (MOEMS). A possible application is an InP-based integrated optical stress sensor. Such a system is based on a partly suspended waveguide that can be strained under the effect of an external stress. The photoelastic effect induces a phase shift that can be converted into an intensity shift of the signal if the device is configurated as a Mach-Zehnder interferometer. This system can be integrated monolithically with the optical source and the photodetector. The mechanical, photoelastic and optical properties of this structure has been simulated in order to configurate the alloy composition of the epitaxial layers and the geometry of the device. Micromachining processes have been developed in order to realize InP-based suspended microstructures by sacrificial layer etching and bulk micromachining. Preliminary results showed that the optical behaviour of the waveguides is close to the theoretical analysis. Characterisation of the complete interferometer is underway in our group.

Polysilicon integrated microsystems: technologies and applications

Co-fabrication of polysilicon microstructures with CMOS electronics enables monolithic inertial sensors to be fabricated. Correlations are well established between the deposition, doping, and annealing conditions of LPCVD polysilicon and its mechanical properties, such as residual strain, strain gradient, and Young's modulus. Surface passivations for alleviating stiction have been demonstrated recently that greatly reduce the tendency for polysilicon microstructures to adhere to adjacent surfaces when dried after release by wet etching and rinsing, or when brought into contact due to mechanical shock during use. Sigma-delta control strategies are attractive for linearizing closed-loop sensors and are well suited to implementation in CMOS. Basic design principles for sense elements and electromechanical actuation in a single structural layer of polysilicon have emerged rapidly in the past several years. Monolithic polysilicon integrated sensors for the X, Y, and Z components of linear acceleration, angular rate, and angular acceleration have been demonstrated using the BiMEMS process of Analog Devices, Inc. Mechanical suspensions, electrostatic actuators and capacitive pickoffs, and interface- and control-circuit building blocks are all portable to alternative integrated technologies that share the basic characteristics of thin, laminar suspended microstructures.

Electrothermal responses of lineshape microstructures

Electrothermal responses of attached and suspended lineshape microstructures fabricated by surface micromachining are investigated. A three-dimensional electrothermal model has been established. This model is based on an axial, one-dimensional electrothermal analysis and a heat-conduction shape factor that represents heat transfer perpendicular to the axial direction. Experimental devices have been built by the surface-micromachining process and a 2 μm gap is constructed for suspended microstructures. Electrical power is passed through these microstructures to characterize the electrothermal responses. Bubble-formation experiments which use local electrical heating to generate micro thermal bubbles in a working liquid have been tested by using these microstructures and the experimental results are consistent with the theoretical model.

Multimode digital control of a suspended polysilicon microstructure

Digital control of a suspended 350 /spl mu/m/spl times/380 /spl mu/m/spl times/1.6 /spl mu/m-thick surface-micromachined polysilicon plate is demonstrated in three degrees of freedom, with application to multimode accelerometers, vibratory rate gyroscopes, and actively positioned micromirrors. Plate displacement about the 2.2 /spl mu/m nominal position above the substrate is measured with shielded capacitive sensors connected to CMOS buffer circuits fabricated adjacent to the microstructure. Four micromechanical sigma-delta loops are used to control eight electrostatic actuators that drive the plate vertically (z) and in out-of-plane rotation (/spl theta/ and /spl phi/). Resonant frequencies are 2.7 kHz for the /spl theta/ rotational mode and 3.7 kHz for both z and /spl phi/ modes. The system is evaluated using a mixed mechanical/electromechanical/circuit simulation in SPICE. Closed-loop transient simulation of a 150-Hz square-wave position input signal is in good agreement with experimental results. Squeeze-film damping limits the plate slew rate to 0.83 mm/s in air. Position is controlled to within /spl plusmn/25 mm, being limited by quantization noise at the 50 kHz sampling rate.

Fabrication of integrated micromachined electron guns

We present a novel process for the fabrication of silicon tip emitters with self-aligned planar tungsten gate electrodes formed by low-temperature selective chemical vapor deposition. Emitters with tip radii of less than 20 nm and gate apertures with diameters of 0.5 μm have been fabricated using this process. The planar nature of the process allows the integration of multiple additional planar tungsten electrodes to produce an entire micromachined electron gun on a single silicon substrate. The gate electrode is self-aligned. Misalignment of the lens electrodes, typically less than 0.25 μm, can be corrected using integrated microelectromechanical actuators or multipole electrodes. Fabrication of multiple levels of complex suspended tungsten microstructures is demonstrated using a multiple level planar tungsten process. Test structures consisting of three levels of suspended tungsten honeycomb microstructures (1.5 μm tungsten beam width, 5 μm aperture diameter) and suspended microstages with integrated actuators are shown.