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     

Bent-beam electrothermal actuators-Part I: Single beam and cascadeddevices

This paper describes electrothermal microactuators that generate rectilinear displacements and forces by leveraging deformations caused by localized thermal stresses. In one manifestation, an electric current is passed through a V-shaped beam anchored at both ends, and thermal expansion caused by joule heating pushes the apex outward. Analytical and finite element models of device performance are presented along with measured results of devices fabricated using electroplated Ni and p⁺⁺ Si as structural materials. A maskless process extension for incorporating thermal and electrical isolation is described. Nickel devices with 410-μm-long, 6-μm-wide, and 3-μm-thick beams demonstrate 10 μm static displacements at 79 mW input power; silicon devices with 800-μm-long, 13.9-μm-wide, and 3.7-μm-thick beams demonstrate 5 μm displacement at 180 mW input power. Cascaded silicon devices using three beams of similar dimensions offer comparable displacement with 50-60% savings in power consumption. The peak output forces generated are estimated to be in the range from 1 to 10 mN for the single beam devices and from 0.1 to 1 mN for the cascaded devices. Measured bandwidths are ≈700 Hz for both. The typical drive voltages used are ⩽12 V, permitting the use of standard electronic interfaces that are generally inadequate for electrostatic actuators     

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     

Thermally actuated microprobes for a new wafer probe card

A new type of MEMS microprobe was designed and fabricated which can be used for a nest generation wafer probe card. A prototype MEMS probe card consisting of an array of microprobes individually actuated by bimorph heating to make contact with the test chip was also fabricated. This probe card is called the CHIPP (Conformable, HIgh-Pin count, Programmable) card and can be designed to contact up to 800 I/O pads along the perimeter of a 1-cm² chip with a microprobe repeat distance of approximately 50 μm. Microprobes for a prototype CHIPP probe card have been fabricated with a variety of cantilever structures including Al-SiO₂, W-SiO₂ and Al-Si bimorphs, and with the resistive heater placed either inside or on the surface of the cantilever. Ohmic contacts between tips and bond pads were tested with contact resistance as low as 250 mΩ. The deflection efficiency varies from 5.23-9.6 μm/mW for cantilever lengths from 300-500 μm. The maximum reversible deflection is in the range of 280 μm. The measured resonant frequency is 8.16 kHz for a 50×500 μm device and 19.4 kHz for a 40×300 μm device. Heat loss for devices operating in air was found to be substantially higher than for vacuum operation with a heat loss ratio of about 2/1 for a heater inside the structure, and 4.25/1 for a structure with the heater as an outer layer of the cantilever     

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     

MgO sacrificial layer for micromachining uncooled Y-Ba-Cu-O IRmicrobolometers on Si3N4 bridges

This paper describes a new micromachining technique for fabrication of semiconducting yttrium barium copper oxide (YBCO) microbolometers using magnesium oxide (MgO) as the sacrificial layer. This type of bolometer can be operated at room temperature, providing a low-cost alternative for more expensive cryogenically cooled thermal detectors used for infrared (IR) imaging. The new micromachining techniques described here would enable the fabrication of YBCO IR focal plane array (FPA) with CMOS signal processing circuitry. Devices were fabricated by growing YBCO films on 4000-Å-thick suspended Si₃N₄ membranes measuring 40×40 μm² in area and extended over micromachined air gaps, which provide the low thermal conductivity that is required for high responsivity. The gap was created by etching an MgO sacrificial layer. This is the first example of using MgO in this type of application. The MgO sacrificial layer technique is fully CMOS compatible and presents no major fabrication challenges. Thermal conductivities achieved in vacuum with the Si₃N₄ suspended structures were on the order of 10^-7 W/K, yielding an effective thermal isolation for bolometer operation. Optical characterization has shown responsivity up to 60 kV/W and detectivity over 10⁸ cm.Hz^1/2/W to black-body IR radiation, indicating that this technology is a suitable candidate for low-cost thermal imaging     

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     

A simulation program for the sensitivity and linearity ofpiezoresistive pressure sensors

A simulation program is developed which is capable of calculating the output responses of piezoresistive pressure sensors as a function of pressure and temperature. Analytical models based on small and large deflection theories have been applied to predict the sensitivity and linearity of pressure sensors. Surface-micromachined diaphragms with square or circular shapes, fabricated by a low pressure chemical vapor deposition sealing process, are designed and tested to verify the program. They are made of polysilicon and have a standard width (diameter) of 100 μm and thickness from 1.5 to 2.2 μm. Various parameters of the piezoresistive sensing resistors, including length, orientation, and dopant concentration, have been derived and constructed on top of the diaphragms. For a 100-μm-wide 2-μm-thick square-shape pressure sensor, calculated and experimental results show that int sensitivity of 0.24 mV/V/(Ibf/in²) is achieved. Experimentally, non a maximum linearity error of ±0.1% full-scale span) is found out on a 100-μm-wide 2.2-μm-thick square-shape pressure sensor. Both sensitivity and linearity are characterized by the diaphragm thickness and the length of the sensing resistors     

A microfabricated electrochemical actuator for large displacements

A large-displacement electrochemical actuator was designed, fabricated, and tested. The large displacement is obtained by using a corrugated membrane made by physical vapor deposition of Parylene sandwiched with an intermediate layer of sputtered platinum. The layered structure is approximately 8-μm thick, with 26 grooves approximately 120-μm deep, and with a radial period of 350 μm. The electrochemical cell consists of platinum electrodes with a 1 M H₂ SO₄ solution. Hydrogen and oxygen gas is generated to displace the membrane. Although the actuator can operate at a voltage as low as 1.23 V, the experimentally determined efficiency of converting electrical energy to mechanical work is only 0.37%. The governing equations for the conservation of mass, momentum (equilibrium), energy, and the entropy generation rate were formulated assuming that the gas bubbles either nucleate without growth or grow without nucleation. For the nucleation case, simulations were performed for constant pressure isothermal actuation, and the average experimental efficiency was bounded by simulations with gas bubble radii between 1×10^-6 m and 1×10^-6 m. The predicted ratio of the power dissipated to the electrical power supplied is 1.37 for isothermal actuation     

Structural and mechanical properties of polycrystalline silicongermanium for micromachining applications

In this paper, we propose polycrystalline silicon germanium (poly SiGe) as a material suitable for MEMS applications. Films are prepared by chemical vapor deposition (CVD) at atmospheric pressure (AP) or reduced pressure (RP). The structure of the films is investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) for different deposition and annealing conditions. The stress in the as-grown and annealed layers is measured, and the correlation between stress and structural properties is discussed. It is demonstrated that by adjusting the deposition conditions, the stress of the as-grown material can be varied from -145 to 60 MPa. Examples of poly SiGe micromachined devices, prepared at 650°C, are presented. It is shown that by using as-grown poly SiGe, it is possible to realize surface-micromachined suspended membranes having 0.6-μm-wide and 50-μm-long supports. The effect of the average stress and stress gradient on the mechanical stability of surface-micromachined structures is illustrated. Finally, the strain in poly SiGe is measured, and it is found to vary, according to the deposition conditions from -6.88×10^-4 to 3.6×10^-1 These values are compared to those measured for APCVD poly Si     

6-MHz 2-N/m piezoresistive atomic-force microscope cantilevers withINCISIVE tips

Piezoresistive atomic force-microscope (AFM) cantilevers with lengths of 10 μm, displacement sensitivities of (ΔR/R)/A 1.1×10^-5, displacement resolutions of 2×10^-3 A/√Hz, mechanical response times of less than 90 ns, and stiffnesses of 2 N/m have been fabricated from a silicon-on-insulator (SOI) wafer using a novel frontside-only release process. To reduce mass, the cantilevers utilize novel inplane crystallographically defined silicon variable aspect-ratio (INCISIVE) tips with radius of curvature of 40 A. The cantilevers have been used in an experimental AFM data-storage system to read back data with an areal density of 10 Gb/cm ². Four-legged cantilevers with both imaging and thermomechanical surface modification capabilities have been used to write 2-Gb/cm² data at 50 kb/s on a spinning polycarbonate sample and to subsequently read the data. AFM imaging has been successfully demonstrated with the cantilevers. Some cantilever designs have sufficient displacement resolution to detect their own mechanical-thermal noise in air. The INCISIVE tips also have applications to other types of sensors     

A HARPSS polysilicon vibrating ring gyroscope

This paper presents the design, fabrication, and testing of an 80-μm-thick, 1.1 mm in diameter high aspect-ratio (20:1) polysilicon ring gyroscope (PRG). The vibrating ring gyroscope was fabricated through the high aspect-ratio combined poly and single-crystal silicon MEMS technology (HARPSS). This all-silicon single-wafer technology is capable of producing electrically isolated vertical electrodes as tall as the main body structure (50 to 100's (μm tall)) with various size air-gaps ranging from submicron to tens of microns. A detailed analysis has been performed to determine the overall sensitivity of the vibrating ring gyroscope and identify its scaling limits. An open-loop sensitivity of 200 μV/deg/s in a dynamic range of ±250 deg/s was measured under low vacuum level for a prototype device tested in hybrid format. The resolution for a PRG with a quality factor (Q) of 1200, drive amplitude of 0.15 μm, and sense node parasitic capacitances of 2 pF was measured to be less than 1 deg/s in 1 Hz bandwidth, limited by the noise from the circuitry. Elimination of the parasitic capacitances and improvement in the quality factor of the ring structure are expected to reduce the resolution to 0.01 deg/s/(Hz)^0.5     

Geometrical modulation-based interferometry for displacementsensing using optically coupled suspended waveguides

A geometrical modulation-based interferometry (GMI) for a displacement sensor is presented. The implementation of the GMI is based on the suspended optical waveguide displacement sensors (SOWDSs) technology. The interferometry effect of the GMI results from light propagating in geometrically modulated and mutually coupled suspended waveguides with an in-plane degree of freedom. The building block of the suspended waveguides is a single-crystal silicon (SCS) beam with superficial layers comprising a 0.6-μm-thick SiO₂, 0.4-μm-thick Si₃N₄, and 0.6-μm-thick SiO₂. The SCS beam is fabricated with a cross section of 1.6 μm×10 μm and may guide light with wavelength in the 1.3-1.5-μm range. The first SiO₂ layer serves as a buffer layer that allows light with wavelength in the 0.6-0.9-μm range to be guided in the Si₃N₄ layer. This paper discusses the theoretical consideration and the characterizations of a GMI displacement sensor     

Fabrication of metallic heat exchangers using sacrificial polymermandrils

This paper demonstrates the use of poly(dimethylsiloxane) (PDMS), polyurethane (PU), epoxy, and poly(methyl methacrylate) (PMMA) as mandrils to fabricate metallic heat exchangers having 300-700 μm internal channels. The mandrils were prepared using two soft lithographic techniques-replica molding and microembossing. To fabricate the heat exchangers, the polymeric mandrils were coated with a thin layer of metal by thermal evaporation or sputtering; this layer acted as the cathode for electrodeposition of a shell of nickel or copper that was 100 μm thick. The polymers were removed by burning them out at 400°C in air, or by dissolving them with a tetrahydrofuran solution of tetrabutylammonium fluoride. Studies of heat dissipation showed that the nickel heat exchangers with features that range in size from 150-750 μm have thermal resistances ranging from 0.07 to 0.12°^-2 C W^-1 cm at flow rates of water of ~20 L h^-1 and pressures of 8.6-83×10³ N m^-2     

Surface micromachined polyimide scanning thermocouple probes

This paper describes micromachined scanning thermocouple probes that exploit the low thermal conductivity and the high mechanical flexibility of polyimide as a structural material. They are surface micromachined using a low-temperature six-mask process suitable for appending to a CMOS fabrication sequence. The probes are 200-1000-μm long, 40-120-μm wide, and of varying thickness. They are assembled by a flip-over approach that eliminates the need for dissolving the substrate wafer or removing the probe from it. Temperature sensing is provided by thin-film Ni/W or chromel/alumel thermopiles embedded in the polyimide, which provide Seebeck coefficients of 22.5 and 37.5 μV/K per junction, respectively. Modeling results indicate that the low thermal conductivity of polyimide causes the temperature drop along the probe length to be much higher than with other candidate materials such as Si or SiO₂, which contributes to improved thermal isolation of the sample and higher temperature sensitivity of the probe. However, the response time of the probe is compromised, and the measured -3 dB bandwidth of the probes is ≈500 Hz. A sample scan is presented     

Thermally driven phase-change microactuation

This paper describes a microactuation scheme based on thermally driven liquid-vapor phase-change in a partially filled sealed cavity. A test structure for studying this system has been designed and fabricated. The cavity is 900 μm by 900 μm by 300 μm in size with a thin, 600 μm by 800 μm grid-shaped heater located on the floor of the cavity and elevated approximately 8 μm above it. The heater is composed of open diamond-shaped unit cells defined by 12-μm-wide, 3-μm-thick bulk-silicon beams, giving an overall electrical heater resistance of 3-10 Ω. Using methanol as the cavity fluid with partial filling, drive levels of 10 mW sustain a 1.2-Atm pressure rise within the cavity. Real-time measurements demonstrate a pressure response time on the order of 100 ms for an input power of 100 mW. Simulated pressure response calculations indicate the potential for an optimized response time on the order of 40 ms at this power level     

A merged process for thick single-crystal Si resonators and BiCMOScircuitry

A simple process has been developed which combines thick single-crystal Si micromechanical devices with a bipolar complimentary metal-oxide-semiconductor (BiCMOS) integrated circuit process. This merged process allows the integration of Si mechanical resonators as thick as 11 μm with any integrated circuit process with the addition of only a single masking step. The process does not require the use of Si on insulator wafers or any type of wafer bonding. The Si resonators were etched in an inductively coupled plasma source which allowed deep trenches to be fabricated with high aspect ratios and smooth sidewall surfaces. Clamped-clamped beam Si resonators that were 500 μm long, 5 μm wide, and 11 μm thick have been fabricated and tested. A typical resonator had a resonance frequency of 28.9 kHz and a maximum amplitude of vibration at resonance of 4.6 μm in air. The average measured resonance frequency across a 4-in-diameter Si wafer was within 0.5% of that predicted by theory. Working NMOS transistors were fabricated and tested on the same chip as the resonator with measured threshold voltages of 0.6 V and an output conductance of 2.0×10 ^-5 Ω^-1 for a gate voltage of 4 V     

Process yields for laser repair of aged, stiction-failed, MEMSdevices

Stiction, the adhesion of micromachined components to each other or the substrate, decreases production yields and operational lifetimes for MEMS devices. A recent study demonstrated the feasibility of using a Nd:YAG, 1063 nm laser to repair 2 μm thick polycrystalline silicon microcantilevers with lengths up to 1000 μm which had adhered to the substrate. This investigation examines the influence of sample age at the time of laser irradiation on the repair of stiction-failed MEMS structures. The operating conditions for the 1064 nm, Nd:YAG laser included a fluence of 70 mJ/cm², a repetition rate of 20 Hz, a pulse duration of 20 ns, and an exposure time of 60 s. For samples irradiated on the same day of release, repair yields were 100% for beams up to 500 μm in length. For 1000 μm long beams, the 10- and 30-μm-wide cantilevers had same-day repair yields of 56% and 71%, respectively. The experimental results show that increasing the amount of time the microstructures are adhered to the substrate before laser irradiation decreases the ability to repair stiction-failed devices with delays of longer than 1 month resulting in a decreased repair yield     

Silicon-micromachined gas chromatography system used to separateand detect ammonia and nitrogen dioxide. I. Design, fabrication, andintegration of the gas chromatography system

A miniature gas chromatography (GC) system was designed and developed using silicon micromachining and integrated circuit (IC) processing techniques. The micromachined gas chromatography (MMGC) system is composed of a miniature sample injector incorporating a 10-μm-long sample loop; a 0.9-m-long, rectangular-shaped (300 μm width and 10 μm height) capillary column coated with a 0.2-μm-thick copper phthalocyanine (CuPc) stationary phase, and a dual-detector scheme based upon a CuPc-coated chemiresistor and a 125-μm-diameter thermal conductivity detector (TCD) bead. Silicon micromachining was employed to fabricate the interface between the sample injector and the GC column, the GC column itself, and the dual-detector cavity. A novel processing technique was developed to sublime a homogeneous CuPc stationary-phase coating on the GC column walls. The complete MMGC system package is approximately 4 in, square and 100 mils (2.5 mm) thick, [96]     

Self-supporting uncooled infrared microbolometers with low-thermalmass

A new micromachined microbolometer array structure is presented that utilizes a self-supporting semiconducting yttrium barium copper oxide (Y-Ba-Cu-O) thin film thermometer. The Y-Ba-Cu-O thermometer is held above the substrate only by the electrode arms without the need of any underlying supporting membrane. This represents a significant improvement in the state-of-the-art for microbolometers by eliminating the thermal mass associated with the supporting membrane. The reduced thermal mass permits lowering the thermal conductance to the substrate to obtain increased responsivity or having a shorter thermal time constant to allow for higher frame rate camera. The simple structure does not suffer from warping problems associated with stress imbalances in multilayer microbolometer structures that utilize a supporting membrane such as Si₃N₄. Devices were fabricated by growing Y-Ba-Cu-O films on a conventional polyimide sacrificial layer mesa. Subsequent etching of the sacrificial layer provides the air gap that thermally isolates the microbolometer. Y-Ba-Cu-O possesses a relatively high temperature coefficient of resistance of 3.1%/K at room temperature. The 400-nm-thick Y-Ba-Cu-O film exhibited absorptivity of about 30%. The responsivity and detectivity approached 10⁴ V/W and 10⁸ cm Hz^1/2/W to filtered blackbody infrared (IR) radiation covering the 2.5 to 13.5 μm band. This extrapolates to noise equivalent temperature difference (NETD) less than 100 mK. The micromachining techniques employed are post-complementary metal-oxide-semiconductor (CMOS) compatible, allowing for the fabrication of focal plane arrays for IR cameras