Millimeter-scale actuator with fiber-optic roller bearings

A technique combining semiconductor processing and fiber-optic technology has been developed to micromachine a 1-cm² silicon die that rolls on two wheels above a flat substrate. Each wheel consists of a glass capillary surrounding a fixed solid glass fiber axle. A 100-silicon die is anisotropically etched to create two variable width v-grooves. Each v-groove has a wide center section and two narrow ends, which is schematically illustrated as -==-. The capillary is free to rotate about the axle in the wide v-groove section while the axle is anodically bonded into the narrow v-groove ends. The gap between the die and the substrate is determined by the narrow v-groove width, fiber diameter, and capillary wall thickness. Several rolling die have been fabricated with 210-120 μm gaps. The coefficient of static friction (μ_S) has been investigated on several substrates as a function of the load on the die. Values for μ_S are compared to an unetched die with a silicon nitride coating. With loads ranging from 0-10 grams, the wheels reduce μ_S by more than 50% on borosilicate glass     

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     

Sealing of micromachined cavities using chemical vapor depositionmethods: characterization and optimization

This paper presents results of a systematic investigation to characterize the sealing of micromachined cavities using chemical vapor deposition (CVD) methods. We have designed and fabricated a large number and variety of surface-micromachined test structures with different etch-channel dimensions. Each cavity is then subjected to a number of sequential CVD deposition steps with incremental thickness until the cavity is successfully sealed. At etch deposition interval, the sealing status of every test structure is experimentally obtained and the percentage of structures that are sealed is recorded. Four CVD sealing materials have been incorporated in our studies: LPCVD silicon nitride, LPCVD polycrystalline silicon (polysilicon), LPCVD phosphosilicate glass (PSG), and PECVD silicon nitride. The minimum CVD deposition thickness that is required to successfully seal a microstructure is obtained for the first time. For a typical Type-1 test structure that has eight etch channels-each 10 μm long, 4 μm wide, and 0.42 μm tall-the minimum required thickness (normalized with respect to the height of etch channels) is 0.67 for LPCVD silicon nitride, 0.62 for LPCVD polysilicon, 4.5 for LPCVD PSG, and 5.2 for PECVD nitride. LPCVD silicon nitride and polysilicon are the most efficient sealing materials. Sealing results with respect to etch-channel dimensions (length and width) are evaluated (within the range of current design). When LPCVD silicon nitride is used as the sealing material, test structures with the longest (38 μm) and widest (16 μm) etch channels exhibit the highest probability of sealing. Cavities with a reduced number of etch channels seal more easily. For LPCVD PSG sealing, on the other hand, the sealing performance improves with decreasing width but is not affected by length of etch channels     

Piezoelectric microphone with on-chip CMOS circuits

An IC-processed piezoelectric microphone with on-chip, large-scale integrated (LSI) CMOS circuits has been designed, fabricated, and tested in a joint, interactive process between a commercial CMOS foundry and a university micromachining facility. The 2500×2500×3.5 μm ³ microphone has a piezoelectric ZnO layer on a supporting low-pressure chemical-vapor-deposited (LPCVD), silicon-rich, silicon nitride layer. The optimum residual-stress-compensation scheme for maximizing microphone sensitivity produces a slightly buckled microphone diaphragm. A model for the sensitivity dependence of device operation to residual stress is confirmed by applying external strain. The packaged microphone has a resonant frequency of 18 kHz, a quality factor Q≈40, and an unamplified sensitivity of 0.92 mV/Pa. Differential amplifiers provide 49 dB gain with 13 μV A-weighted noise at the input     

Silicon-processed microneedles

A combination of surface- and bulk-micromachining techniques is used to demonstrate the feasibility of fabricating microhypodermic needles. These microneedles, which may be built with on-board fluid pumps, have potential applications in the chemical and biomedical fields for localized chemical analysis, programmable drug-delivery systems, and very small, precise sampling of fluids. The microneedles are fabricated in 1, 3, and 6 mm lengths with fully enclosed channels formed of silicon nitride. The channels are 9 μm in height and have one of two widths, 30 or 50 μm. Access to the channels is provided at their shank and distal ends through 40-μm square apertures in the overlying silicon nitride layer. The microneedles are found to be intact and undamaged following repetitive insertion into and removal from animal-muscle tissue (porterhouse steak)     

Electrothermally activated paraffin microactuators

A new family of electrothermally activated microactuators that can provide both large displacements and forces, are simple to fabricate, and are easily integrated with a large variety of microelectronic and microfluidic components are presented. The actuators use the high volumetric expansion of a sealed, surface micromachined patch of paraffin heated near its melting point to deform a sealing diaphragm. Two types of actuators have been fabricated using a simple three mask fabrication process. The first device structure consists of a 9 μm thick circularly patterned paraffin layer ranging in diameter from 400 to 800 μm all covered with a 4-μm-thick metallized p-xylylene sealing diaphragm. All fabricated devices produced a 2.7-μm-peak center deflection, consistent with a simple first order theory. The second actuator structure uses a constrained volume reservoir that magnifies the diaphragm deflection producing consistently 3.2 μm center diaphragm deflection with a 3-μm-thick paraffin actuation layer. Microactuators were constructed on both glass and silicon substrates. The actuators fabricated on glass substrates used between 50-200 mW of electrical power with response times ranging between 30-50 ms. The response time for silicon devices was much faster (3-5 ms) at the expense of a larger electrical power (500-2000 mW)     

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     

A high-Tc superconductor bolometer on a silicon nitridemembrane

In this paper, we describe the design, fabrication, and performance of a high-T_c GdBa₂Cu₃O_7-δ superconductor bolometer positioned on a 2× 2-mm² 1-μm-thick silicon nitride membrane. The bolometer structure has an effective area of 0.64 mm² and was grown on a specially developed silicon-on-nitride (SON) layer. This layer was made by direct bonding of silicon nitride to silicon after chemical mechanical polishing. The operation temperature of the bolometer is 85 K. A thermal conductance G=3.3·10^-5 W/K with a time constant of 27 ms has been achieved. The electrical noise equivalent power (NEP) at 5 Hz is 3.7·10^-2 WHz^-1/2, which is very close to the theoretical phonon noise limit of 3.6·10^-12 WHz ^-1/2, meaning that the excess noise of the superconducting film is very low. This bolometer is comparable to other bolometers with respect to high electrical performance. Our investigations are now aimed at decreasing the NEP for 84-μm radiation by further reduction of G and adding an absorption layer to the detector. This bolometer is intended to be used as a detector in a Fabry-Perot (FP)-based satellite instrument designed for remote sensing of atmospheric hydroxyl     

A thermal inkjet printhead with a monolithically fabricated nozzleplate and self-aligned ink feed hole

A monolithic thermal inkjet printhead has been developed and demonstrated to operate successfully by combining monolithic growing of a nozzle plate on the silicon substrate and electrochemical etching of silicon for an ink feed hole. For the monolithic fabrication, a multiexposure and single development (MESD) technique and Ni electroplating are used to form cavities, orifices, and the nozzle plate. Electrochemical etching, as a back-end process, is applied to form an ink feed hole through the substrate, which is accurately aligned with the frontside pattern without any backside mask. The etch rate is nearly proportional to the current density up to 50 μm/min. Experiments with a 50-μm-diameter nozzle show ink ejection up to the operating frequency of 11 kHz with an average ink dot diameter of about 110 μm for 0.3-A, 5-μs current pulses     

The design and operation of a MEMS differential scanning nanocalorimeter for high-speed heat capacity measurements of ultrathin films

A MEMS sensor has been developed for use as a calorimetric cell in an ultra-sensitive, thin-film, differential scanning calorimetric technique. The sensor contains a freestanding, thin (30 nm to 1000 nm), low-stress silicon nitride membrane with lateral dimensions of a few millimeters. This membrane, along with a thin (50 nm) metallization layer, forms a calorimetric cell with an exceptionally small addenda. This small addenda creates a very sensitive calorimetric cell, able to make heat capacity measurements of nanometer-thick metal and polymer films. The sensor fabrication and various design considerations are discussed in detail. The calorimetric technique and examples of applications are described.     

Piezoelectric cantilever microphone and microspeaker

A micromachined piezoelectric cantilever transducer, which works both as a microphone and as a microspeaker, has been fabricated and tested. The 2000×2000×4.5 μm³ cantilever has a zinc oxide (ZnO) piezoelectric thin film on a supporting layer of low-pressure chemical-vapor-deposited (LPCVD) low-stress silicon nitride. A highlight of the fabrication process, which may also be relevant for other micromachined structures, is the technique for producing a flat, multilayer cantilever. The measured microphone sensitivity is fairly constant at 3 mV/μbar in the low frequency range and rises to 20 mV/μbar at the lowest resonant frequency of 890 Hz. The 3 mV/μbar sensitivity is the highest reported to date for a microphone with a micromachined diaphragm. When measured into a 2 cm³ coupler with 4 V(zero-peak) drive, the microspeaker output sound pressure level (SPL) is 75 dB at 890 Hz. It increases to approximately 100 dB SPL at 4.8 kHz with 6 V(zero-peak) drive. The measured microphone frequency response agrees well with the results of an ABAQUS simulation     

Multi-walled microchannels: free-standing porous silicon membranesfor use in μTAS

Electrochemically formed porous silicon (PS) can be released from the bulk silicon substrate by underetching at increased current density. Using this technique, two types of channels containing free-standing layers of PS were constructed, which were failed multi-walled microchannels (MW μCs). They can be used in devices like microsieves, microbatteries, and porous electrodes. Two types of MWμC were made: the “conventional” version, consisting of two or more coaxially constructed microchannels separated by a suspended PS membrane, and the buried variety, where a PS membrane is suspended halfway in an etched cavity surrounded by silicon nitride walls. The latter is more robust. The pore size of the PS was measured using transmission electron microscopy and field emission gun scanning electron microscopy (FEGSEM) and found to be of the order of 7 nm     

Sealing of adhesive bonded devices on wafer level

In this paper, we present a low temperature wafer-level encapsulation technique to hermetically seal adhesive bonded microsystem structures by cladding the adhesive with an additional diffusion barrier. Two wafers containing cavities for MEMS devices were bonded together using benzocyclobutene (BCB). The devices were sealed by a combined dicing and self-aligning etching technique and by finally coating the structures with evaporated gold or PECVD silicon nitride. The sealing layer was inspected visually by SEM and helium leak tests were carried out. Devices sealed with silicon nitride and with known damage of the sealing layer showed a helium leak rate of about 7–14 times higher than the background level. Devices of the same size without damage in the sealing layer had a leak rate of only 1.5 times higher than the background level. Experiments with evaporated gold as cladding layer revealed leaking cracks in the film even up to a gold thickness of 5 μm. The sealing technique with silicon nitride shows a significant improvement of the hermeticity properties of adhesive bonded cavities, making this bonding technique suitable for applications with certain demands on gas-tightness.     

Fabrication of ultrathin p++ silicon microstructuresusing ion implantation and boron etch-stop

This paper discusses the fabrication of submicron p⁺⁺ silicon microstructures for a number of MEMS applications using boron ion implantation, rapid thermal annealing, and boron etch-stop. To form these thin structures, the silicon is implanted with boron at an energy of 40 keV and doses of 5×10¹⁵ cm^-2 and 7×10¹⁵ cm^-2, which produce a peak concentration of more than 10²⁰ cm^-3, sufficient for achieving an effective etch-stop in ethylene diamine pyrocathecol. The thickness of the p⁺⁺ layer varies from 0.2 to 0.3 μm depending on the annealing time and temperature. SUPREM simulation has been used to determine optimum implantation and annealing conditions. A number of microstructures, including thin silicon diaphragms as large as 2 mm on a side and 0.2 μm thick, hot wire anemometers with a temperature coefficient of resistance of ~1600 ppm/°C, and piezoresistive sound detectors, have been fabricated with high reproducibility, uniformity, and yield     

Silicon condenser microphones with corrugated silicon oxide/nitride electret membranes

Corrugated electret membranes are used in a micromachined silicon microphone. The membranes consist of a permanently corona-charged double layer of silicon dioxide and silicon nitride, known to have excellent charge-storing properties. This electret can replace the external bias needed for condenser microphones. The well-known LOCOS technique—also combined with dry etching—is used for the first time to fabricate membranes with corrugation depths of several microns. The membrane thickness amounts to 600 nm.

The interferometrically measured center deflection is up to 40 nm/Pa for diaphragms with four corrugations of up to 3.3 μm depth and a size of A_M=1 mm². This high mechanical sensitivity limits the dynamic range to sound pressures below 50 Pa. The obtained mechanical sensitivities are in excellent agreement with the theory.

The most compliant corrugated diaphragms result in a microphone sensitivity of 2.9 mV/Pa, an equivalent noise level of 39 dB(A) and a total harmonic distortion below 1.7% at 28 Pa (123 dB SPL). The corrugation depth in the sensors has been only 1.3 μm. All sensors cover the whole audio and low ultrasonic range. The temperature coefficient is between 0.05 and 0.1 dB/K.     

Design and optimization of cantilever based piezoelectric micro power generator for cardiac pacemaker

Piezoelectric micro-power generator (PMPG) converts mechanical vibration energy into electric energy via piezoelectric effects. In cardiac pace makers, the use of PMPG eliminates the need for a traditional lithium iodide battery replacement. In this paper we design and optimize PMPG that is able to harvest the mechanical movement of the heart beat to be converted into usable electrical power in frequency range 1–1.7 Hz. Eight control parameters are selected: which are proof mass material, piezoelectric material, proof mass length, proof mass thickness, piezoelectric layer width, piezoelectric layer thickness, silicon nitride layer width, silicon nitride layer thickness. Orthogonal arrays of Taguchi method for these eight parameters mentioned with three levels and signal-to-noise (S/N) ratio, and ANOVA analysis is studied to determine the optimum design. COMSOL Multiphysics ver. 4.2 is used in 18 different simulations. The maximum output power and highest efficiency designed at 1.2 Hz is equivalent to 72 beat per min. Both Taguchi and ANOVA confirms the same results of determining the parameter of having the most influence on the generated output power at 1.2 Hz in descending order: which are piezoelectric material of PZT-5A, proof mass length of 5 mm, piezoelectric layer thickness of 30 µm, proof mass thickness of 4 mm, piezoelectric layer width of 0.12 mm, silicon nitride layer width of 0.16 mm, silicon nitride layer thickness of 30 µm, and proof mass material of aluminum. Eigen frequency analysis for the first six modes of operation for PMPG frequencies are: 1.2 HZ, 5.4 Hz, 6.9 Hz, 29,7 Hz, 694.8 Hz, 708.3 Hz. The first mode of operation is selected as operation mode and shows that 93 % of PMPG’s total displacement and output power was produced in the range of 1–1.4 Hz, therefore PMPG can work when the heart rate between 60 and 84 bpm. Transient analysis performed at 1.2 Hz reaches the steady state before the first 10 cycles with output power density of 23.13 µW/cm³, which is suitable for powering cardiac pace maker.     

Thick glass film technology for polysilicon surface micromachining

This paper explores the use of thick glass films as suitable alternatives to CVD oxide films for use as sacrificial, planarization, and passivation layers in polysilicon surface micro-machining processes. Such glasses can be spin-coated to produce films up to 20 μm thick in one step and to globally planarize the wafer surface, extending the overall mechanical design capability by enabling additional device structural complexity. Glass optical constants were determined, and the film quality was evaluated using SEM, EDS, XPS, and XRD. The films were found to have low intrinsic stresses and other characteristics desirable for sacrificial layer applications. A glass chemical-mechanical polishing process with 5300~Å/min removal rate and acceptable selectivity to polysilicon was developed, along with a wet etch chemistry that preferentially etches the film at 3.24 μm/min without affecting the silicon substrate or the structural polysilicon. The film was used to planarize up to 10-μm-tall topographies associated with surface micromachined features through spin-on and polish-back steps, and was in addition demonstrated to be a viable protective layer for silicon wafers during extended KOH etching in silicon bulk micro-machining processes. The glass has stable constituents that do not diffuse or contaminate either the substrate or the device features during the application and firing procedures     

A bulk silicon dissolved wafer process for microelectromechanicaldevices

A single-sided bulk silicon dissolved wafer process that has been used to fabricate several different micromechanical structures is described. It involves the simultaneous processing of a glass wafer and a silicon wafer, which are eventually bonded together electrostatically. The silicon wafer is then dissolved to leave heavily boron doped devices attached to the glass substrate. Overhanging features can be fabricated without additional masking steps. It is also possible to fabricate elements with thickness-to-width aspect ratios in excess of 10:1. Measurements of various kinds of laterally driven comb structures processed in this manner, some of which are intended for application in a scanning thermal profilometer, are described. They comprise shuttle masses supported by beams that are 160-360 μm long, 1-3 μm wide, and 3-10 μm thick. Some of the shuttles are mounted with probes that overhang the edge of the die by 250 μm. Resonant frequencies from 18 to 100 kHz and peak-to-peak displacements up to 18 μm have been measured     

Fabrication of silicon and oxide membranes over cavities usingion-cut layer transfer

Silicon and oxide membranes were fabricated using an ion-cut layer transfer process, which is suitable for sub-micron-thick membrane fabrication with good thickness uniformity and surface micro-roughness. After hydrogen ions were implanted into a silicon wafer, the implanted wafer was bonded to another wafer that has patterned cavities of various shapes and sizes. The bonded pair was then heated until hydrogen-induced silicon layer cleavage occurred along the implanted hydrogen peak concentration, resulting in the transfer of the silicon layer from one wafer to the other. Using this technique, we have been able to form sealed cavities and channels of various shapes and sizes up to 50-μm wide, with a 1.6-μm-thick silicon membrane. As a process variation, we have also fabricated silicon dioxide membranes for optically transparent applications     

Fabrication of silicon condenser microphones using single wafertechnology

A condenser microphone design that can be fabricated using the sacrificial layer technique is proposed and tested. The microphone backplate is a 1-μm plasma-enhanced chemical-vapor-deposited (PECVD) silicon nitride film with a high density of acoustic holes (120-525 holes/mm²), covered with a thin Ti/Au electrode. Microphones with a flat frequency response between 100 Hz and 14 kHz and a sensitivity of typically 1-2 mV/Pa have been fabricated in a reproducible way. These sensitivities can be achieved using a relatively low bias voltage of 6-16 V. The measured sensitivities and bandwidths are comparable to those of other silicon microphones with highly perforated backplates. The major advantage of the new microphone design is that it can be fabricated on a single wafer so that no bonding techniques are required