Fabrication methods based on wet etching process for the realization of silicon MEMS structures with new shapes

In this paper, fabrication methods are developed in order to realize the silicon microelectromechanical systems components with new shapes in {100} Si wafers. Fabrication process utilizes wet etching with a single step of photolithography. The silicon etching is carried out in complementary metal oxide semiconductor process compatible pure and surfactant Triton-X-100 [C₁₄H₂₂O(C₂H₄O] _n , n = 9–10) added tetramethylammonium hydroxide (TMAH) solutions. The fabricated structures are divided in two categories: fixed and freestanding. The fixed structures are realized in single oxidized silicon wafers, while freestanding are formed in silicon nitride-based silicon on insulator (SOI) wafers. The SOI wafers are prepared by bonding the oxidized and the nitride deposited wafers, followed by thinning and chemical mechanical polishing processes. The etching results such as {100} and {110}Si etch rates, undercutting at rounded concave and sharp convex corners and etched surface morphologies are measured in both pure and Triton added TMAH solutions. Different concentrations of TMAH are used to optimize the etching conditions for desired etched profiles.     

Wet etching of undercut sidewalls in {001}-silicon

The development of undercut sidewalls along 〈100〉 directions of {001}-silicon by a two-step orientation dependent wet etching process is described. Thereby the first step generates vertical {100}-sidewalls in a pure aqueous potassium hydroxide etchant (KOH). The following second step in an etchant consisting of KOH saturated with isopropanol (KOH:IPA) develops slowly etched {101}-faces with undercutting inclination. A principal calculation is given of the expected sidewall dimensions and a comparison is also done with experimental prepared sidewalls. The differences are small. An application is proposed where sidewalls prepared in this way have a function as bending spring or free standing bridge.     

Hydrogenation-Assisted Lateral Micromachining of (111) Silicon Wafers

Micromachining of (111) silicon wafers by means of a plasma hydrogenation and chemical etching sequence is achieved. Vertical etching is used to define the depth of the craters as well as the thickness of the final suspended silicon body. After protecting the 3-D structure by a thermally grown oxide, a hydrogenation step is used to remove the oxide layer from the bottom of the crater, allowing a lateral underetching. Final exposure of the processed silicon to a KOH solution, etches silicon in a lateral fashion and in the exposed places. A lateral aspect ratio of four to six has been achieved. The evolution of suspended structures on (111) wafers, suitable for sensor fabrication, is feasible without a need to a 3-D lithography. Using this technique suspended interdigital structures have been realized with a depth up to 70 $mu hbox{m}$. In addition, ultrathin fully suspended structures have been successfully fabricated. A preliminary capacitive accelerometer has been realized and tested on (111) substrate.$hfill$ [2008-0070]      

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     

Micromachined silicon bolometers as detectors of soft X-ray, ultraviolet, visible and infrared radiation

This paper presents the development of micromachined thin-film silicon microbolometers which can be used for detection of soft X-ray, UV, visible and infrared radiation. The detector structure is a 1 μm thick polysilicon/Si₃N₄ membrane suspended over a cavity. This structure has been obtained by anisotropic etching of silicon with a previously deposited polysilicon/Si₃N₄ sandwich. Alternatively, porous silicon has been used as the sacrificial layer. Devices have been characterized. Good values of the voltage responsivity and detectivity have been obtained.     

Fabrication and characterization of 3-Dimensional MOS transistor tip integrated micro cantilever

We fabricate and characterize a three-dimensional (3-D) MOS (metal–oxide–semiconductor) transistor tip integrated micro cantilever to measure the surface properties. The 3-D MOS transistor tip is fabricated on the front side end of the cantilever, and the cantilever itself works as a tip. These features make the device possible to investigate hard-detecting parts such as the deep trenches and the sidewalls of the structure. The MOS transistor tip has other advantages such as the high operation speed, the high sensitivity, and the reduction of the required equipments like the lock-in-amplifier. The MOS transistor tip is fabricated three-dimensionally, utilizing the lateral diffusion and the anisotropic wet etching with TMAH solution, since the etch rate of {211} plane is much higher than those of {100} or {111} planes. The gate area is formed by self-aligned technique, using crystallographic dependant wet etching. The well-known convex corner compensation pattern is used for the gate length control during the tip fabrication process. The characteristics of the fabricated device are measured with respect to the various electric signals and the results show the well-established detection properties.     

用于承载铷原子钟滤光泡的高品质微腔体制备

在芯片级铷原子钟中,需要微腔体来承载Rb—87滤光泡,为此,提出了一种用于制作高品质微腔体的新技术。为了获得光滑的腔体侧面和避免腐蚀过程中凸角处产生削角现象,研究中采用了超声腐蚀技术和凸角补偿技术。首先,分别在纯KOH溶液,并结合搅拌和超声等方法,对(100)硅片进行湿法腐蚀,并运用激光共聚焦扫描显微镜对腐蚀后的{111}表面进行粗糙度测量,表明运用超声腐蚀技术可以获得光滑的{111}腔体侧面。在此基础上,引入条形掩模凸角补偿方法进行微腔体腐蚀。实验结果表明:在80℃、质量分数为30%KOH、超声频率和功率分别为59 kHz和160 W的溶液中腐蚀,其{111}腐蚀表面粗糙度为0.117μm,同时条形的长度取1200μm时,可以获得平滑规整的微腔体。     

MEMS Vertical Probe Cards With Ultra Densely Arrayed Metal Probes for Wafer-Level IC Testing

We have developed a MEMS probe-card technology for wafer-level testing ICs with 1-D line-arrayed or 2-D area-arrayed dense pads layouts. With a novel metal MEMS fabrication technique, an area-arrayed tip matrix is realized with an ultradense tip pitch of $90 muhbox{m} times 196 mu hbox{m}$ for testing 2-D pad layout, and a 50-$muhbox{m}$ minimum pitch is also achieved in line-arrayed probe cards for testing line-on-center or line-on-perimeter wafers. By using the anisotropic etching properties of single-crystalline silicon, novel oblique concave cavities are formed as electroplating moulds for the area-arrayed microprobes. With the micromachined cavity moulds, the probes are firstly electroplated in a silicon wafer and further flip-chip packaged onto a low-temperature cofired ceramic board for signal feeding to an automatic testing equipment. The microprobes can be efficiently released using a silicon-loss technique with a lateral underneath etching. The measured material properties of the electroplated nickel and the Sn–Ag solder bump are promising for IC testing applications. Mechanical tests have verified that the microprobes can withstand a 65-mN probing force, while the tip displacement is 25 $muhbox{m}$, and can reliably work for more than 100 000 touchdowns. The electric test shows that the probe array can provide a low contact resistance of below 1 $Omega$, while the current leakage is only 150 pA at 3.3 V for adjacent probes.$hfill$[2008-0273]      

Tungsten-Based SOI Microhotplates for Smart Gas Sensors

This paper is concerned with the design, fabrication, and characterization of novel high-temperature silicon on insulator (SOI) microhotplates employing tungsten resistive heaters. Tungsten has a high operating temperature and good mechanical strength and is used as an interconnect in high temperature SOI-CMOS processes. These devices have been fabricated using a commercial SOI-CMOS process followed by a deep reactive ion etching (DRIE) back-etch step, offering low cost and circuit integration. In this paper, we report on the design of microhotplates with different diameters (560 and 300 $muhbox{m}$) together with 3-D electrothermal simulation in ANSYS, electrothermal characterization, and analytical analysis. Results show that these devices can operate at high temperatures (600 $^{circ}hbox{C}$ ) well beyond the typical junction temperatures of high temperature SOI ICs (225 $^{circ}hbox{C}$), have ultralow dc power consumption (12 mW at 600 $^{circ}hbox{C}$), fast transient time (as low as 2-ms rise time to 600 $^{circ}hbox{C}$), good thermal stability, and, more importantly, a high reproducibility both within a wafer and from wafer to wafer. We also report initial tests on the long-term stability of the tungsten heaters. We believe that this type of SOI microhotplate could be exploited commercially in fully integrated microcalorimetric or resistive gas sensors. $hfill$[2007-0275]      

Comparative study of spin coated and sputtered PMMA as an etch mask material for silicon micromachining

SiO₂ and Si₃N₄, are usually used to mask the selected portions during etching of silicon in anisotropic etchants like KOH but polymers are expected to be very good alternative to SiO₂ and Si₃N₄ as masking materials for MEMS applications. An adherent spin coated PMMA layer is reported to work as a mask material. It is a low temperature process, cheaper and films can be easily deposited and removed. One of the problems in its use is its adhesion to the substrate. Our previous experience in the field made us feel that sputtered PMMA will act as better mask because of its better adhesion to silicon. In the present article, a comparative study of spin coated PMMA with sputtered PMMA as an etch mask for silicon micromachining is reported. Structural and adhesive characteristics of the films are determined and compared with those available in the literature. These films deposited on silicon wafer were exposed to anisotropic etchant, KOH, to estimate the masking behavior. The maximum masking time of 32 min in 20 wt.% KOH at 80 °C was obtained for spin coated PMMA samples, which were prebaked at 90 °C. Masking time of sputter deposited PMMA films was found to be 300 min under similar conditions such as 20 wt.% KOH at 80 °C. This masking time is sufficient for fabrication of various MEMS structures, thus indicating candidature of sputtered PMMA as masking material. Various properties of the films are discussed and compared with the ones obtained through literature.     

Fabrication of metallic microstructures by electroplating usingdeep-etched silicon molds

A new technology is presented here to fabricate three-dimensional micromachined metal structures. The microstructures are manufactured by electroplating in deep-etched silicon structures followed by a separation from their mold. Up to 140-μm-deep silicon structures with vertical sidewalls are realized by an anisotropic plasma etching process producing the mold for electroplating. An etching gas mixture of SF_6s and CBrF₃ is used to achieve both an anisotropic etching behavior by protective film formation of CF₂ -radicals and high etching rates. The anisotropy is due to photoresist masking, which enhances the polymer formation. The vertical trenches are electroplated from the trench base filling the structures uniformly to the substrate surface. By avoiding overplating across the whole substrate the resulting structures are suitable for micromechanical devices. If needed, released microstructures from the silicon mold can be obtained by direct lift-off     

Surfactant Adsorption on Single-Crystal Silicon Surfaces in TMAH Solution: Orientation-Dependent Adsorption Detected by In Situ Infrared Spectroscopy

This paper focuses on two aspects, macroscopic and microscopic, of pure and surfactant-added tetramethylammonium hydroxide (TMAH) wet etching. The macroscopic aspects deal with the technological/engineering applications of pure and surfactant-added TMAH for the fabrication of microelectromechanical systems (MEMS). The microscopic view is focused on the in situ observation of the silicon surface during etching in pure and surfactant-added TMAH solutions using Fourier transform infrared (FT-IR) spectroscopy in the multiple internal reflection geometry. The latter is primarily aimed at investigating the causes behind the change in the orientation-dependent etching behavior of TMAH solution when the surfactant is added. Silicon prisms having two different orientations ({110} and {100}) were prepared for comparison of the amount of adsorbed surfactant using FT-IR. Stronger and weaker adsorptions were observed on {110} and {100}, respectively. Moreover, ellipsometric spectroscopy (ES) measurements of surfactant adsorption depending on the crystallographic orientation are also performed in order to gain further information about the differences in the silicon–surfactant interface for Si{100} and Si{110}. In this paper, we determine the differences in surfactant adsorption characteristics for Si{110} and Si{100} using FT-IR and ES measurements for the first time, focusing both on the mechanism and on the technological/engineering applications in MEMS. $hfill$[2009-0140]      

Mechanical strengthening of Si cantilever by chemical KOH etching and its surface analysis by TEM and AFM

Mechanical strengthening of a Si cantilever by applying KOH wet etching was investigated. Two kinds of Si cantilever specimens having the different crystallographic orientations of the sidewall surfaces, i.e., Si{100} and Si{110}, were fabricated from the same SOI wafer by a Bosch process. The typical height and pitch of the scalloping formed on the sidewall were 248 and 917 nm, respectively. A 50 % KOH (40 °C) chemical wet etching was applied to increase the fracture stress of the Si cantilever. The fracture stress in the both of Si{100} and Si{110} cantilevers increased with the advance of the etching. The obtained maximum fracture stress in Si{100} and Si{110} were 4.2 and 3.7 GPa, respectively. Sidewall surface of the cantilever was analyzed to investigate the mechanical strengthening of Si cantilever by wet etching. The etched surface crystalline was analyzed by the transmission electron microscope (TEM), and confirmed that the thickness of the affected flow layer was less than 10 nm from the obtained TEM image. Then the change of the surface roughness by the KOH etching was analyzed by the atomic force microscope. The surface was smoothened with the advance of the KOH etching. The roughness value of Ra in Si{100} and Si{110} decreased to 12.1 and 37.7 nm, respectively.     

Cryogenic Etching of Silicon: An Alternative Method for Fabrication of Vertical Microcantilever Master Molds

This paper examines the use of deep reactive ion etching of silicon with fluorine high-density plasmas at cryogenic temperatures to produce silicon master molds for vertical microcantilever arrays used for controlling substrate stiffness for culturing living cells. The resultant profiles achieved depend on the rate of deposition and etching of an $hbox{SiO}_{x}hbox{F}_{y}$ polymer, which serves as a passivation layer on the sidewalls of the etched structures in relation to areas that have not been passivated with the polymer. We look at how optimal tuning of two parameters, the $ hbox{O}_{2}$ flow rate and the capacitively coupled plasma power, determine the etch profile. All other pertinent parameters are kept constant. We examine the etch profiles produced using electron-beam resist as the main etch mask, with holes having diameters of 750 nm, 1 $muhbox{m}$ , and 2 $muhbox{m}$. $hfill$[2008-0317]      

A Lyapunov function of two-dimensional linear systems

In this note a Lyapunov function for a 2-D time invariant discrete linear system is introduced, using the 2-D system model given by Roesser [1]. The Lyapunov function may be used to investigate the asymptotic stability of the 2-D system. Previous work dealing with asymptotic stability of 2-D systems [3]-[5] is based upon the location of roots of the characteristic polynomial in the closed polydiskoverline{U}^{2}.     

New 3-D Structures Fabricated on Si (hkl) Substrates by Bulk Micromachining

The paper deals with the new 3D structures fabricated by the bulk micromachining of (110), (112), and (522) silicon substrates. The structures employ a specific arrangement of {111} planes on these substrates and are entirely bounded by these slowly etching planes. Design rules and complete structures of new seismic-mass systems, suspended on two or four beams, composed of the {111} planes, are presented. The beams supporting the masses are inclined toward the substrate at different angles, which can be adjusted by an appropriate selection of crystallographic orientation of the etched substrate. The structures seem to be interesting as structural components of multiaxes accelerometers. Slanted membranes fabricated by the double-sided etching of (112) and (552) substrates have also been presented. The structures utilize the {111} planes, inclined at a low angle toward the etched substrate, both as structural elements, as well as a natural etch stop. It can be claimed that the application of Si substrates with unconventional crystallographic orientations opens new possibilities in the micromachining of 3D structures.     

Polymeric Thermal Microactuator With Embedded Silicon Skeleton: Part II—Fabrication, Characterization, and Application for 2-DOF Microgripper

This paper presents the fabrication, characterization, and application of a novel silicon-polymer laterally stacked electrothermal microactuator. The actuator consists of a deep silicon skeleton structure with a thin-film aluminum heater on top and filled polymer in the trenches among the vertical silicon parts. The fabrication is based on deep reactive ion etching, aluminum sputtering, SU8 filling, and KOH etching. The actuator is 360 $muhbox{m}$ long, 125 $mu hbox{m}$ wide, and 30 $muhbox{m}$ thick. It generates a large in-plane forward motion up to 9 $muhbox{m}$ at a driving voltage of 2.5 V using low power consumption and low operating temperature. A novel 2-D microgripper based on four such forward actuators is introduced. The microgripper jaws can be moved along both the $x$- and $y$ -axes up to 17 and 11 $muhbox{m}$, respectively. The microgripper can grasp a microobject with a diameter from 6 to 40 $muhbox{m}$ . In addition, the proposed design is suitable for rotation of the clamped object both clockwise and counterclockwise. $hfill$[2007-0192]      

The realization of an integrated Mach-Zehnder waveguide immunosensor in silicon technology

We describe the realization of a symmetric integrated channel waveguide Mach-Zehnder sensor which uses the evanescent field to detect small refractive-index changes (Δn_min ≈ 1 × 10⁻⁴) near the guiding-layer surface. This guiding layer consists of ridge structures with a height of 3 nm and a width of 4 μm made in Si₃N₄. This layer has a thickness of 100 nm. The sensor device has been tested with glucose solutions of different bulk refractive indices. Results of a slab-model calculation are in good agreement with obtained experimental results. The feasibility of applying this sensor for immunosensing, detecting directly the binding of antigen to an antibody receptor surface, is shown with antibody-antigen binding experiments.     

The surface/bulk micromachining (SBM) process: a new method forfabricating released MEMS in single crystal silicon

This paper presents the surface/bulk micromachining (SBM) process to allow fabricating released microelectromechanical systems using bulk silicon. The process starts with a (111)-oriented silicon wafer. The structural patterns are defined using the reactive ion etching technique used in surface micromachining. Then the patterns, as well as sidewalls, are passivated with an oxide film, and bare silicon is exposed at desired areas. The exposed bare silicon is further reactive ion etched, which defines sacrificial gap dimensions. The final release is accomplished by undercutting the exposed bulk silicon sidewalls in aqueous alkaline etchants. Because {111} planes are used as etch stops, very clean structural surfaces can be obtained. Using the SBM process, 5-, 10-, and 100-μm-thick arbitrarily-shaped single crystal silicon structures, including comb-drive resonators, at 5-, 30-, and 100-μm sacrificial gaps, respectively, are fabricated. An electrostatic actuation method using p-n junction isolation is also developed in this paper, and it is applied to actuate comb-drive resonators. The leakage current and junction capacitance of the reversed-biased p-n junction diodes are also found to be sufficiently small for sensor applications. The developed SBM process is a plausible alternative to the existing micromachining methods in fabricating microsensors and microactuators, with the advantage of using single crystal silicon