Front-to-backside alignment using resist-patterned etch control andone etching step

A processing technique that aligns features on the front side of a wafer to those on its backside has been developed for bulk micromachining. A 30 μm-square and 1.6 μm-thick diaphragm serves as an alignment pattern. At the same time that the alignment diaphragm is made, much thicker, large-area diaphragms can be partially etched using `mesh' masking patterns in these areas. The mesh-masking technique exploits the etch-rate differences between (100) and (111) planes to control the depths reached by etch pits in selected areas. The large partially etched diaphragms (2 to 3 mm², roughly 100 μm thick) are sufficiently robust to survive subsequent IC-processing steps in a silicon-foundry environment. The thin alignment diaphragm can be processed through these steps because of its very small area. The partially etched diaphragms can be reduced to useful thicknesses in a final etch step after the circuits have been fabricated     

Improvement of the performance of microphones with a silicon nitride diaphragm and backplate

The performance of a single-wafer fabricated silicon condenser microphone has been improved by increasing the stress and the acoustic hole density of the backplate and by decreasing the diaphragm thickness. The best microphones show a sensitivity of 5.0 mV Pa⁻¹, which corresponds to an open-circuit sensitivity of 10 mV Pa⁻¹ for a microphone capacitance of 6.6 pF. The measured frequency response is flat within ±2 dB from 100 Hz to 14 kHz, which is better than the requirements for a hearing-aid microphone. The operating voltage of these microphones is only 5.0 V, which is about 60% of the collapse voltage. The measured noise level of the microphones is 30 dBA SPL, which is approximately as low as required for a hearing-aid microphone ( <29.5 dBA SPL).     

Fabrication of thick Si resonators with a frontside-releaseetch-diffusion process

A frontside-release etch-diffusion process has been developed to create released single-crystalline Si microstructures without the need for wafer bonding. This frontside-release process is simple and requires only a single mask. A deep dry etch in an electron cyclotron resonance source is used to define the structures, followed by a short boron diffusion to convert them to p⁺⁺ Si. A short etch in ethylenediamine pyrocatechol (EDP) is then used to undercut and release the structures from the frontside of the Si wafer. The structures are isolated from the substrate using a reverse-biased p⁺⁺/n junction. Since the structures have a high aspect ratio, beams longer than 1 mm can be released without sticking to the substrate, and thick resonators are flat with no bending due to stresses. Resonant microstructures with thicknesses ranging from 10 to 55 μm thick have been fabricated using this process and their resonant frequency has been measured. For typical clamped-clamped beam resonators that were 24 μm thick, 5 μm wide, and 400 μm long, with 2-μm comb gaps, a resonant frequency of 90.6 kHz and a quality factor of 362 were measured in air     

Silicon micromachining using in situ DC microplasmas

This paper reports on the generation of spatially confined plasmas and their application to silicon etching. The etching is performed using SF₆ gas and dc power applied between thin-film electrodes patterned on the silicon wafer to be etched. The electrodes also serve as a mask for the etching. The typical operating pressure and power density are in the range of 1-20 Torr and 1-10 W/cm², respectively. The plasma confinement can be varied from <100 μm to >1 cm by varying the electrode area, operating pressure, and power. High power densities can be achieved at moderate currents because the electrode areas are small. Etch rates of 4-17 μm/min., which enable through-wafer etching and varying degrees of anisotropy, have been achieved. The etch rate increases with power density, whereas the etch rate per unit power density increases with operating pressure. Scaling effects are explored for varying sized mask openings. Plasma resistance measurements and electric field modeling are used to provide an initial assessment of the microplasmas     

Fabrication of semiconducting YBaCuO surface-micromachinedbolometer arrays

Thermal infrared detectors require thermal isolation to permit the infrared-sensitive material to integrate the incident photon energy and thereby obtain high responsivity and detectivity. This paper describes the fabrication of semiconducting YBaCuO microbolometer arrays into thermal isolation structures by employing Si surface-micromachining techniques. An isotropic HF:HNO₃ etch was used to remove the underlying Si substrate from the front-side of the wafer and suspend SiO ₂ membranes into 1×10 pixel-array structures. The infrared-sensitive material, YBaCuO, was subsequently deposited onto the thermal isolation structures and patterned to form the detector arrays. The high-temperature coefficient of resistance and low noise of semiconducting YBaCuO at room temperature is attractive for uncooled infrared detection. The fabrication process was conducted entirely at room temperature. In this manner, infrared detectors are fabricated in a process that is compatible with CMOS technology to allow for the integration with on-chip signal processing circuitry. The end result is low-cost infrared-detector arrays for night vision in a variety of applications including transportation and security. Preliminary results show a temperature coefficient of resistance above 3%, voltage responsivity close to 10⁴ V/W, and detectivity over 10⁷ cm·Hz^1/2/W at a bias current of 0.79 μA     

Released Si microstructures fabricated by deep etching and shallowdiffusion

A novel etch-diffusion process is developed for fabricating high-aspect-ratio Si structures for microsensors. This is accomplished by first dry etching narrow gap Si microstructures using an electron cyclotron resonance (ECR) source, followed by a shallow B diffusion to fully convert the etched microstructures to p⁺⁺ layer. Microstructures up to 40 μm deep with 2-μm-wide gaps were etched with a Cl₂ plasma generated using the ECR source. Vertical profile and smooth morphology were obtained at low pressure. A shallow B diffusion at 1175°C for 5.5 h. was then carried out to convert the 40-μm-thick resonant elements to p⁺⁺ layer. A second dry etching step was used to remove the thin p⁺⁺ layer around the bottom of the resonant elements, followed by bonding to glass and selective wet etch. Released high-aspect-ratio Si microsensors with thicknesses of 35 μm have been demonstrated. At atmospheric pressure, only 5 V_dc driving voltage is needed for 2.5 μm vibration amplitude, which is less than the 10 V_dc required to drive 12-μm-thick resonators fabricated by conventional dissolved wafer process     

Fabrication of Flexible Transducer Arrays With Through-Wafer Electrical Interconnects Based on Trench Refilling With PDMS

Flexible transducer arrays are desired to wrap around catheter tips for side-looking intravascular ultrasound imaging. We present a technique for constructing flexible capacitive micromachined ultrasonic transducer (CMUT) arrays by forming polymer-filled deep trenches in a silicon substrate. First, we etch deep trenches between the bottom electrodes of CMUT elements on a prime silicon wafer using deep reactive ion etching. Second, we fusion-bond a silicon-on-insulator (SOI) wafer to the prime silicon wafer. Once the silicon handle and buried oxide layers are removed from the back side of the SOI wafer, the remaining thin silicon device layer acts as a movable membrane and top electrode. Third, we fill the deep trenches with polydimethylsiloxane, and thin the wafer down from the back side. The 16 by 16 flexible 2-D arrays presented in this paper have a trench width that varies between 6 and 20 ; the trench depth is 150 ; the membrane thickness is 1.83 ; and the final substrate thickness is 150 . We demonstrate the flexibility of the substrate by wrapping it around a needle tip with a radius of 450 (less than catheter size of 3 French). Measurements in air validate the functionality of the arrays. The 250- by 250- transducer elements have a capacitance of 2.29 to 2.67 pF, and a resonant frequency of 5.0 to 4.3 MHz, for dc bias voltages ranging from 70 to 100 V.     

Micromachined flat-walled valveless diffuser pumps

The first valveless diffuser pump fabricated using the latest technology in deep reactive ion etching (DRIE) is presented. The pump was fabricated in a two-mask micromachining process in a silicon wafer polished on both sides, anodically bonded to a glass wafer. Pump chambers and diffuser elements were etched in the silicon wafer using DRIE, while inlet and outlet holes are etched using an anisotropic etch. The DRIE etch resulted in rectangular diffuser cross sections. Results are presented on pumps with different diffuser dimensions in terms of diffuser neck width, length, and angle. The maximum pump pressure is 7.6 m H₂O (74 kPa), and the maximum pump flow is 2.3 ml/min for water     

Design and fabrication of silicon condenser microphone usingcorrugated diaphragm technique

A novel silicon condenser microphone with a corrugated diaphragm has been proposed, designed, fabricated and tested. The microphone is fabricated on a single wafer by use of silicon anisotropic etching and sacrificial layer etching techniques, so that no bonding techniques are required. The introduction of corrugations has greatly increased the mechanical sensitivities of the microphone diaphragms due to the reduction of the initial stress in the thin films, For the purpose of further decreasing the thin film stress, composite diaphragms consisting of multilayer (polySi/Si_xN_y/polySi) materials have been fabricated, reducing the initial stress to a much lower level of about 70 MPa in tension. Three types of corrugation placements and several corrugation depths in a diaphragm area of 1 mm² have been designed and fabricated. Microphones with flat frequency response between 100 Hz and 8~16 kHz and open-circuit sensitivities as high as 8.1~14.2 mV/Pa under the bias voltages of 10~25 V have been fabricated in a reproducible way. The experimental results proved that the corrugation technique is promising for silicon condenser microphone     

A method to etch undoped silicon cantilever beams

A simple method has been designed to etch cantilever beams oriented in the <100> direction on (100) silicon wafers without back-etching, heavily doped boron etch stop, or anodic oxidation etch stop. The scheme requires only two levels of masking. Silicon dioxide and evaporated gold film are used as passivation materials. Anisotropic etching is performed in a sodium hydroxide bath. Silicon cantilevers with background doping concentration levels and having vertical edges are produced     

Capacitive microphone with a surface micromachined backplate usingelectroplating technology

A technology for surface micromachining of free-standing metal microstructures using metal electrodeposition on a sacrificial photoresist layer has been applied to a condenser microphone. Electroplating technology has been used to implement a suspended and perforated 15-μm-thick microstructure in copper, which serves as backplate electrode in the condenser microphone. The 1.8×1.8 mm ² large microphone diaphragm is in monocrystalline silicon and is fabricated with anisotropic etching of the substrate wafer. The realized prototypes have a measured sensitivity of 1.4 mV/Pa using a bias voltage of 28 V. The bandwidth is limited by an anti-resonance at 14 kHz which is due to the semi-rigid backplate. The resonance behavior of the backplate structure has been analyzed with finite element modeling with results in good agreement with measured data     

Fabrication of curved sub-micron Si structures by a combination of anisotropic etching using surfactant-added TMAH solution and FIB direct-drawn mask

Silicon anisotropic wet etching is applied for fabricating round-shaped micro-structures in a size range of sub-microns. In this work, we demonstrate that arbitrary 2-D mask patterns having curved profile can be successfully transferred to deep-etched cavity profiles on a Si {100} wafer. The sub-micron mask is directly drawn on the Si wafer by irradiating focused ion beam to the wafer surface. Anisotropy in etch rate of Si using tetra-methyl ammonium hydroxide solution was modified and controlled by adding a surfactant Triton X-100 to the solution. Etched profile was conformal to etch mask patterns having smooth curvatures.     

Silicon-based microelectrodes for neurophysiology fabricated usinga gold metallization/nitride passivation system

The multimicroelectrode probe (microprobe) is a device used in neurophysiology to record signals from nerve cells. Microprobes typically have a number of gold recording sites supported on a narrow cantilever beam which is inserted into the tissue. Conducting tracks connect the recording sites to bonding pads on the body of the device. The metallization is insulated, except at the recording sites and bonding pads, by a passivation layer. Boron etch stop techniques can be used to produce narrow cantilever beams upon which recording sites are situated. Previously, polysilicon interconnects were used on microprobes fabricated using boron etch stop techniques, with gold inlaid onto the recording sites using a lift-off technique. This meant that mechanical jigging was required before the final shaping of the probes in potassium hydroxide (or other etch) to prevent the etch from attacking the polysilicon conductors beneath the inlaid gold. The process reported here incorporates a gold metallization layer, in conjunction with a plasma-enhanced chemical vapor deposition (PECVD) nitride passivation layer. Since both these materials etch very slowly in potassium hydroxide, no mechanical jigging, or other steps, need to be taken to protect the front of the wafer during the shaping stage. This simplifies the fabrication of these devices     

Strongly buckled square micromachined membranes

The buckling of compressively prestressed square membranes with built-in edges is investigated experimentally and analyzed theoretically. The buckling depends weakly on Poisson's ratio and essentially is a function of the reduced prestrain, ε¯₀ =ε₀a²/h², where ε₀ is the physical prestrain, a is the width, and h is the thickness of the membrane. As ε¯₀ becomes increasingly negative, the membrane undergoes two symmetry breaking buckling transitions. Beyond the first transition occurring at ε¯_cr1, the buckling profile has all the reflection and rotation symmetries of a square. The reflection symmetries are lost through a second instability transition at ε¯_cr2. The bifurcation points, ε¯_cr1 and ε¯_cr2, and buckling profiles were calculated using analytical energy minimization and nonlinear finite-element simulation. Both methods agree. The buckling of micromachined plasma-enhanced chemical vapor deposition silicon nitride membranes on a silicon wafer is interpreted in terms of the theoretical results. Good matching between measured and calculated buckling profiles is found. The extracted strain values are consistent irrespective of the size and buckling mode of the membranes. From the average strain across the wafer ε₀=-3.50×10^-4 and complementary wafer curvature measurements, a Young's modulus of 130 GPa is deduced. Methods for the straightforward extraction of ε₀ from experimental center deflections of buckled square membranes are described     

Towards a versatile DRIE: silicon pit structures combined with electrochemical etch stop

A novel approach for fabricating low-pitch arrays of silicon membranes on standard CMOS wafers by combining deep-reactive ion etching (DRIE) and electrochemical etching (ECE) techniques is presented. These techniques have been used to fabricate membrane-based sensors and sensor arrays featuring different membrane sizes on a single wafer with a well defined etch stop. The described procedure is particularly useful in cases when the usage of SOI wafers is not an option. The combination of a grid-like mask pattern featuring uniform-size etch openings for the DRIE process with a reliable ECE technique allowed to fabricate silicon membranes with sizes ranging from 0.01 mm/sup 2/ to 2.2 mm/sup 2/. The development of this new method has been motivated by the need to design a compact n-well-based calorimetric sensor array, where the use of a standard ECE technique would have significantly increased the overall size of the device.     

压阻式微型压力传感器微型 化限制因素的研究

本文初步探讨了对压阻式传感器的微型化起限制作用的多种因素:根据(100)晶面硅片各向异性腐蚀的特点,得出了器件芯片最小尺寸与硅片厚度的关系;根据方膜和矩形膜上应力分布曲线,得出了在一定的图形位置偏差下压力灵敏度与硅膜几何尺寸的关系曲线;讨论了硅片厚度均匀性与力敏电阻尺寸对器件总尺寸的限制作用,比较了全桥结构和横向压阻X型结构对器件几何尺寸的要求。最后介绍了两种实用的微型压力传感器设计与其主要技术参数。     

The role of fabrication techniques on the performance of widely tunable micromachined capacitors

Coventorware 2001.1^® is used to identify key vertical dimensions for the low voltage operation of a two-gap widely tunable capacitor. Using identical masking stages, two varieties of tunable capacitor are presented. Nickel structures are demonstrated which have a tuning ratio of 5.1:1 from an initial capacitance of 0.7 pF. Gold devices exhibit a tuning ratio of 7.3:1 from an initial capacitance of 1.5 pF. These are the most widely tunable devices reported to date.     

Sub-10 cm3 interferometric accelerometer with nano-gresolution

A high-resolution accelerometer with a bulk-micromachined silicon proof mass and an interferometric position sensor was developed for measuring vibratory accelerations. The interferometer consists of interdigitated ringers that are alternately attached to the proof mass and support substrate. Illuminating the fingers with coherent light generates a series of diffracted beams. The intensity of a given beam depends on the out-of-plane separation between the proof mass fingers and support fingers. Proof masses with mechanical resonances ranging from 80 Hz to 1 kHz were fabricated with a two mask process involving two deep reactive ion etches, an oxide etch stop, and a polyimide protective layer. The structures were packaged with a laser diode and photodiode into 8.6-cm³ acrylic housings. The 80-Hz resonant proof mass has a noise equivalent acceleration of 40 ng/rt Hz and a dynamic range of 85 dB at 40 Hz     

Slotted capacitive microphone with sputtered aluminum diaphragm and photoresist sacrificial layer

In this paper, we have fabricated a new microphone using aluminum (Al) slotted perforated diaphragm and back plate electrode, and photoresist (AZ1500) sacrificial layer on silicon wafer. The novelty of this method relies on aluminum diaphragm includes some slots to reduce the effect of residual stress and stiffness of diaphragm for increasing the microphone sensitivity. The acoustic holes are made on diaphragm to reduce the air damping, and avoid the disadvantages of non standard silicon processing for making back chamber and holes in back plate, which are more complex and expensive. Photoresist sacrificial layer is easy to deposition by spin coater and also easy to release by acetone. Moreover, acetone has a high selectivity to resist compared to silicon oxide and Al, thus it completely removes sacrificial resist without incurring significant damage silicon oxide and Al. The measured zero bias capacitance is 17.5 pF, and its pull-in voltage is 25 V. The microphone has been tested with external amplifier and speaker, the external amplifier was able to detect the sound waves from microphone on speaker and oscilloscope. The maximum amplitude of output speech signal of amplifier is 45 mV, and the maximum output of MEMS microphone is 1.125 μV.     

High-resolution high-voltage sensor based on SAW

A surface acoustic wave (SAW)-based high-voltage sensor is described. The sensor consists of a SAW oscillator fabricated on a 10 mm × 10 mm 128° rotated Y-cut, X-propagating LiNbO₃ substrate. The voltage is applied to electrodes on the substrate, and the resulting electric field changes the propagation time of the SAW. The propagation time is directly related to the output frequency of the SAW oscillator. The high-voltage sensor offers a small-sized high-voltage measurement device with several attractive features: a high resolution (better than 0.2 V up to 2.4 kV, better than 0.4 V for higher voltages), a large range (−10 to +10 kV), a high input impedance (> 10¹³ ω) and a low input capacitance (< 10 pF). The sensitivity amounts to 16 Hz V⁻¹.