Silicon Micromachining Research Articles

Terahertz Diode Arrays and Differential Probes based on Heterogeneous Integration and Silicon Micromachining

Due to the technological needs of the radio astronomy and remote sensing scientific communities, as well as emerging applications in the areas of imaging, security, and broadband communications, terahertz and submillimeter-wave electronics continues to be an area of growth and increasing interest for academic researchers, government laboratories, and industry. The recent establishment of a commercial infrastructure for test and measurement instrumentation in this spectral region has fueled this growth and the emergence of CMOS as a submillimeter-wave technology has greatly expanded access to this spectral region by providing circuit designers with a platform for realizing terahertz circuits without need for specialized fabrication facilities or processes. The continued emergence of new terahertz devices has created a need for improved approaches to packaging, integration, and measurement tools for diagnostics and characterization in this portion of the spectrum. This paper focuses on progress in two parallel efforts aimed at addressing these needs: (1) the development of a direct-contact probe technology for on-wafer measurement of differential scattering-parameters in the WR-5.1 (140—220 GHz) and WR-3.4 (220—330 GHz) frequency bands, and (2) the development of processing technologies for realizing highly-integrated submillimeter-wave diode-based quasi-optical arrays, including phase modulators and sideband generators, that are based on heterogeneous integration of III-V semiconductors onto thin silicon membranes as a support substrate. The foundation for these efforts is micromachining and processing of silicon, allowing the fabrication of mechanically-robust and low-loss membrane carriers that can support and interconnect terahertz devices as well as directly interface them to surrounding circuitry. Examples of heterogeneous integration onto silicon as an approach to packaging and interfacing terahertz components that are detailed in this paper include development of differential micromachined wafer probes for in situ measurements of devices and circuits in the 140—330 GHz region. The probe design concept includes an integrated on-chip balun and matching network for terminating common-mode signals that may be generated by the DUT. The design methodology and initial measurement results for this probe will be presented. In addition, an example of heterogeneous integration/packaging of a submillimeter-wave frequency sideband generator array for phase modulation at 1.6 THz will be discussed. The sideband generator design incorporates 100 planar varactor diodes integrated into an array of bowtie antennas on a common substrate. Performance of the array as a phase shifter is described and the application of a new quasi-vertical diode fabrication process that consists of transfer of GaAs epitaxy to thin silicon support substrates will be discussed as an approach for implementing optimized arrays in the terahertz frequency range.

Lumped Electromechanical Modeling of Capacitive Micromachined Ultrasonic Transducers

Conventional plane piston lead zirconium titanate (PZT) based transducers perform poorly for audio speakers and as capacitance receivers due to the lack of proper matching layer materials. Conventional designs have relatively large gaps, 50-100 μm, and use the air in the gap as the restoring force of the vibrating electrode. With the help of silicon micromachining, Capacitive Micromachined Ultrasonic Transducers (CMUTs) are defined. The gaps are made to be as small as 500Å, and the restoring force of the vibrating electrode is the stiffness of the electrode itself. In this paper, a single CMUT element is represented by the lumped electromechanical model. It is realized with comparison of the experimental and simulated outcomes that a single spring constant value is not sufficient to faithfully describe the entire region of operation of CMUT. The search results in two optimized values of spring constants, one fits well to describe the low bias voltage region and the other the collapse voltage regime, where the membrane touches the substrate and is unable to produce vibrations. The membrane displacement behavior under static bias is analyzed. It is found that the displacement at the centre of the membrane is a function of not only the bias but depends highly on the membrane structural geometries and physical characteristics. CMUT membrane’s displacement can be optimized with a change in the above which is a necessary condition for obtaining the required sensitivity. Numerical calculations are done with the help of MATLAB and Finite Element Method (FEM) simulations are aimed with help of PZFlex.

Force Feedback Control System Dedicated for Robin Heart Surgical Robot

3D contact force sensors were developed and integrated in a demonstration system for testing the feasibility of their application in minimal invasive surgery (MIS). Piezoresistive MEMS based vectorial force sensors were designed and fabricated by 3D silicon micromachining technology and packaged according to their proposed transducer and sensor applications. In this work we demonstrated the integrability and functional applicability of the 3D force sensors in MIS robotic systems to improve their flexibility and reliability by providing real-time force and tactile information during the operation.The final goal is to integrate the new subsystems in the Robin Heart surgery robot of FRK. [1] Three different functions are targeted: 1. Micro-joystick actuator to be integrated in the hilt of the laparoscope to easily control robotic movement during operation. 2. Force sensor inside the laparoscopic jaw to provide feedback to the surgeon by measuring the grasping strength and 3. 3D force/tactile sensor which facilitates palpation for tissue diagnostics during operation. In this paper we demonstrate a feasibility study regarding these proposed applications.

Selected problems in IR and UV laser micromachining of Si and GaAs in submillimeter scale

Results of laser micromachining of silicon and gallium arsenide in submillimeter scale were compared. Studies were performed using a laser beam with IR and UV wavelengths and pulses of femto- and nano-second duration. Investigation of effectiveness of micromachining with different parameters of laser beams was presented. Quality of surface and edges was estimated by means of an optical microscope and an electron microscope. Studies of surface profiles were performed using a confocal microscope. Results of micromachining were with good accordance with computer modeling.

Silicon Micromachined Device Testing by Infrared Low-Coherence Reflectometry

With the development of silicon micromachining technologies, non-contact measurement techniques for in-depth non-destructive inspection of layered and microstructured samples are becoming increasingly relevant. In this paper, we apply optical low-coherence reflectometry (OLCR) to detect the optical path between the interfaces of several silicon devices with characteristic distance in the range 3–17 $\mu \text{m}$ . The implemented configuration is based on a fiberoptic Michelson interferometer that exploits infrared broadband radiation in the wavelength range of 1.2–1.7 $\mu \text{m}$ , with coherence length shorter than 2 $\mu \text{m}$ , for performing spot tomographic measurements. OLCR enabled out-of-plane measurements on a MEMS linear accelerometer and in-plane measurements on vertical periodic silicon/air microstructures. The optical distance between hidden interfaces was found well in agreement with the design parameters. [2015-0108]

Potential Stable Low-Permeation Rate Standard Based on Micro-machined Silicon.

Silicon wafers with micro-machined holes were evaluated for use as low-permeation-rate standard artifacts. Accuracy, stability, and reliability were assessed. Two independent experimental techniques for evaluating permeation were used: chilled mirror hygrometer and mass loss. The wafers exhibited a well-defined linear relationship between hole area and resultant water partial pressure for both techniques, although the mass loss curve exhibited a constant vertical offset from the hygrometer curve, attributed to water loss through the O-ring seal. In contrast to polymer permeation standards, Si wafers provided long-term reproducible permeation rates. However, they were also highly fragile, with most of them cracking during the course of the investigation.

High temperature and current density induced degradation of multi-layer graphene

We present evidence of moderate current density, when accompanied with high temperature, promoting migration of foreign atoms on the surface of multi-layer graphene. Our in situ transmission electron microscope experiments show migration of silicon atoms at temperatures above 800 °C and current density around 4.2 × 107 A/cm2. Originating from the micro-machined silicon structures that clamp the freestanding specimen, the atoms are observed to react with the carbon atoms in the multi-layer graphene to produce silicon carbide at temperatures of 900–1000 °C. In the absence of electrical current, there is no migration of silicon and only pyrolysis of polymeric residue is observed.

Silicon micromachining in 25 wt% TMAH without and with surfactant concentrations ranging from ppb to ppm

Tetramethylammonium hydroxide (TMAH) is one of the most commonly used anisotropic etchants in silicon wet bulk micromachining. It has outstanding etching characteristics such as compatibility with semiconductor processes and provides high etch selectivity between silicon and silicon dioxide. In this work, we have investigated the effect of surfactant concentration ranging from 20 ppb to 1000 ppm on the etching characteristics of 25 wt% TMAH for bulk micromachining of Si{100} and Si{110}. Triton X-100 with formula C14H22O(C2H4O)n, where n = 9---10, is used as surfactant. The major objective of this work is to determine the surfactant concentration at which the etch rate, etched surface morphology and convex corner undercutting are significantly affected. Undercutting length and etched depth are measured using 3D scanning optical microscope. Surface morphology of etched silicon samples are inspected using scanning electron microscope. The surfactant concentration below 100 ppb does not affect etching characteristics significantly, while the effect of surfactant concentration above 100 ppb is substantial on the undercutting at convex corners on Si{100} surface, etch rate and etched surface roughness of Si{110}. The etch rate and etched surface morphology of Si{100} are not influenced considerably when the surfactant is added into 25 wt% TMAH.

Design and Analysis of a Micromechanical Three-Component Force Sensor for Characterizing and Quantifying Surface Roughness

Abstract Roughness, which can represent the trade-off between manufacturing cost and performance of mechanical components, is a critical predictor of cracks, corrosion and fatigue damage. In order to measure polished or super-finished surfaces, a novel touch probe based on three-component force sensor for characterizing and quantifying surface roughness is proposed by using silicon micromachining technology. The sensor design is based on a cross-beam structure, which ensures that the system possesses high sensitivity and low coupling. The results show that the proposed sensor possesses high sensitivity, low coupling error, and temperature compensation function. The proposed system can be used to investigate micromechanical structures with nanometer accuracy.

Fabrication of 3-D Silicon Microneedles Using a Single-Step DRIE Process

Fabrication of 3-D microstructures is one of the most challenging aspects of silicon micromachining. In this paper, we present a novel microfabrication method using one single-step deep reactive ion etching process with gray-scale e-beam lithography mask that offers deeply etched (>350- $\mu \text{m}$ deep) dual-angle 3-D microneedles with control over the height and shape of the structures. Moreover, we found that the shape of the e-beam lithography patterns can determine the general configuration and features of the final etched microneedles, and that the etching process parameters have the most impact on the microneedles’ shape, such as size and vertical base angle. Large arrays of $20\times 20$ microneedles with height uniformity of better than 3% are fabricated. [2014-0209]

Out-of-plane electret-based MEMS energy harvester with the combined nonlinear effect from electrostatic force and a mechanical elastic stopper

This paper presents the fabrication, modeling and characterization of an out-of-plane electret-based vibrational energy harvester (e-VEH) that has both positive and negative charged electret plates integrated into a single seismic mass system. Strong electrostatic spring-softening effect is induced due to the electric field provided by the double-charged electret plates. An elastic stopper is introduced for reliability concern by limiting the motion of seismic mass and meanwhile serves as a functional element to realize spring-hardening effect. The device has an overall volume of about 0.14 cm3 and is fabricated based on MEMS compatible silicon micromachining technology. When subject to weak excitations, the device exhibits an approximately linear frequency response but changes to a significantly larger broadband when strongly excited due to the combined nonlinear effect from electrostatic force and a mechanical elastic stopper. At a high excitation level of 0.48 g, the experimental results show that the device has 3 dB bandwidths of 3.7 Hz for frequency-up sweep and 2.8 Hz for frequency-down sweep, respectively, which demonstrate a large enhancement compared to the linear response (1.3 Hz). An optimal output power of 0.95 μW is also achieved with a low resonance of 95 Hz. This corresponds to a normalized power density of 37.4 μW cm−3 g−2.

A study on co-axial water-jet assisted fiber laser grooving of silicon

Potential of fiber laser grooving in silicon has been investigated with continuous wave beam and modulated beam of micro-second durations. Though silicon shows considerably high transparency for the utilized laser wavelength (1.07μm), due to the increase of absorption coefficient with temperature its micro-machining is feasible with good precision by laser. A co-axial water-jet has been employed for ejecting out the molten silicon effectively and simultaneously for rapid cooling. This in turn, overcomes the limitation of micro-machining of silicon with laser pulses of relatively longer durations and alleviates the oxidation problem also. It has been observed that with increasing water-jet speed at constant laser parameters, spatter and recast are reduced. Grooves of controlled depth in 14–520μm range, free from micro-cracks and thermal damage could be obtained by controlling the laser and process parameters. Compared to argon-jet assisted laser grooving this technique produced grooves of relatively better quality in terms of lesser micro-cracks, spatter and thermal damage.

Silicon‐based, low‐g microelectromechanical systems inertial switch for linear acceleration sensing application

Most of the microelectromechanical systems inertial switches developed in recent years are intended for shock and impact sensing above 40 g. These switches are fabricated based on non-silicon surface micromachining with multiple steps of electroplating. In this reported work, a silicon-based low-g inertial switch typically used for linear acceleration sensing was conceived, designed and fabricated. The developed inertial switch consists of a high volume proof mass and low stiffness spiral spring, and is fabricated in a specially designed double-buried layer silicon-on-insulator wafer, with standard silicon micromachining. The measurement results show that the threshold value is about 7.42 g and the stiffness is about 1.5 N/m, in accordance with the finite element method calculation.

MEMS probes for on-wafer RF microwave characterization of future microelectronics: design, fabrication and characterization

This article presents microelectromechanical system (MEMS) ground-signal-ground (GSG) probes based on silicon-on-insulator (SOI) technology for on-wafer microwave characterization of radio-frequency (RF) microelectronics. The probe is designed using optimized coplanar waveguide structures with the aim of ensuring a low-contact resistance between the probe and the pads of the device under test (DUT). The probes are batch fabricated using SOI substrates and employ a simple silicon micromachining process. The probes have a pitch of 4.5 µm with miniaturized dimensions for a DUT pad area with a similar size. Electrical (dc) measurements show that the fabricated probe has a low-contact resistance (~0.02 Ω) on gold pads. Excellent extracted RF performances of the probe are observed up to 30 GHz, showing an insertion loss better than 2.2 dB and return loss better than 20 dB over the frequency range. An ageing study shows the probes are capable of forming this dc contact for over 6000 contact cycles. The preliminary result of the repeatability of on-wafer one-port measurements with the miniaturized probe shows a consistent RF performance maintained through several contacts. The data indicates that the proposed MEMS probe is suitable for the high-frequency characterization of integrated nanoscale devices having reduced pad dimensions.

A comprehensive review on convex and concave corners in silicon bulk micromachining based on anisotropic wet chemical etching

Wet anisotropic etching based silicon micromachining is an important technique to fabricate freestanding (e.g. cantilever) and fixed (e.g. cavity) structures on different orientation silicon wafers for various applications in microelectromechanical systems (MEMS). {111} planes are the slowest etch rate plane in all kinds of anisotropic etchants and therefore, a prolonged etching always leads to the appearance of {111} facets at the sidewalls of the fabricated structures. In wet anisotropic etching, undercutting occurs at the extruded corners and the curved edges of the mask patterns on the wafer surface. The rate of undercutting depends upon the type of etchant and the shape of mask edges and corners. Furthermore, the undercutting takes place at the straight edges if they do not contain {111} planes. {100} and {110} silicon wafers are most widely used in MEMS as well as microelectronics fabrication. This paper reviews the fabrication techniques of convex corner on {100} and {110} silicon wafers using anisotropic wet chemical etching. Fabrication methods are classified mainly into two major categories: corner compensation method and two-steps etching technique. In corner compensation method, extra mask pattern is added at the corner. Due to extra geometry, etching is delayed at the convex corner and hence the technique relies on time delayed etching. The shape and size of the compensating design strongly depends on the type of etchant, etching depth and the orientation of wafer surface. In this paper, various kinds of compensating designs published so far are discussed. Two-step etching method is employed for the fabrication of perfect convex corners. Since the perfectly sharp convex corner is formed by the intersection of {111} planes, each step of etching defines one of the facets of convex corners. In this method, two different ways are employed to perform the etching process and therefore can be subdivided into two parts. In one case, lithography step is performed after the first step of etching, while in the second case, all lithography steps are carried out before the etching process, but local oxidation of silicon (LOCOS) process is done after the first step of etching. The pros and cons of all techniques are discussed.

Surface and subsurface characterisation in micro-milling of monocrystalline silicon

Abstract This paper presents an experimental investigation on surface and subsurface characterisation of micro-machined single-crystal silicon with (100) orientation. Full immersion slot milling was conducted using solid cubic boron nitride (CBN) and diamond-coated fine grain tungsten carbide micro-end mills with a uniform tool diameter of 0.5 mm. The micro-machining experiments were carried out on an ultra-precision micro-machining centre. Formal design of experiments (DoE) techniques were applied to design and analysis of the machining process. Surface roughness, edge chipping formation and subsurface residual stress under varying machining conditions were characterised using white light interferometry, SEM and Raman microspectroscopy. Tens of nanometre-level surface roughness can be achieved under the certain machining conditions, and influences of variation of cutting parameters including cutting speeds, feedrate and axial depth of cut on surface roughness were analysed using analysis of variance (ANOVA) method. Raman microspectroscopy studies show that compressive subsurface residual stress and amorphous phase transformation were observed on most of the micro-machined subsurface, which provides evidence of ductile mode cutting. Surface and subsurface characterisation studies show that the primary material removal mode is ductile or partial ductile using lower feedrate for both tools, and diamond-coated tools can produce better surface quality. Silicon brain implants were fabricated with good dimensional accuracy and edge quality using the optimised machining conditions, which demonstrated that micro-milling is an effective process for fabrication of silicon components at a few tens to a few hundreds of micron scale.

Silicon Micromachined Waveguide Quadrature-Hybrid Coupler at Terahertz Frequency Band

A waveguide quadrature-hybrid coupler operating from 350 to 410 GHz is designed and fabricated using the deep reactive ion etching (DRIE) silicon micromachining technique. The four-port branch-line coupling configuration with five branches is used as the basic structure of the coupler. The size of the coupler is about 6.3 mm × 6.3 mm × 1 mm, which is much smaller than the flange used in the vector network analyzer (VNA). Because it is difficult to measure all the characteristics of a four-port coupler with a small size using the two-port VNA with big-size flanges, four testing topologies with different bended input/output waveguide ports are designed for measurements. The terahertz (THz) wedged-type absorbing material is adopted as the replacement of the conventional waveguide matching load because the size of the conventional waveguide matching load is too large to use. Measured results of the proposed coupler are in good agreement with simulated results, indicating that the average insertion loss is 0.9 dB and the isolation is better than 20 dB. Effects of the DRIE process on the coupler’s performance are discussed in detail.

芯片级铷原子气室的制备

The fabrication processes of chip-scale alkali atom vapor cells are designed, which include silicon micromachining, filling arrays of miniaturized cells with rubidium azide, hermetic packaging using anodic bonding technology, and photodecomposition of rubidium azide. The improved anodic bonding technology is demonstrated and used to improve the gastight bonding of alkali atom vapor cells to the glass/silicon/glass bonding structure. After the anodic bonding of the glass and silicon wafer is finished, an Al metal film with a certain thickness is sputtered at a certain angle at the back (i.e., the glass side) and side of the glass/silicon-bonded structure. Therefore, the electrical contact is led into the silicon wafer when a second anodic bonding is performed, and a high-quality glass/silicon/glass bonding structure is realized. Such anodic structures have great bond strength and gastight bonding of as low as 2.5×10-8Pa·m3·s-1. After gastight sealing, the cells are irradiated with ultraviolet, causing the azide to photodissociate into pure rubidium and nitrogen in situ. Rubidium optical absorption spectrum indicates that rubidium atoms are effectively sealed in the cells. It is confirmed that the process for the fabrication of the chip-scale alkali atom vapor cells is simple and feasible.

Reliability of potassium ion electret in silicon oxide for vibrational energy harvester applications

In this paper, we report on the long-term reliability of potassium ion electret included in a thermally grown silicon oxide. The electret in this work is used in a microelectromechanical systems (MEMS) energy harvester to generate electrical current from mechanical vibration. A spring-mass system similar to a comb-drive electrostatic actuator is developed by silicon micromachining, and the surface is oxidized by wet-oxidation through a potassium hydroxide bubbler, thereby including potassium atoms at a high concentration. The potassium is then electrically polarized by an applied voltage of 150 V at 650 °C for 5 min. Degradation of the stored polarization potential is monitored in a vacuum of 1 × 10−3 Pa at elevated temperatures of 350, 400, and 450 °C. The time needed to cause a −1 dB decay of the potential is used as the lifetime of the electret, and the Arrhenius extrapolation plot suggested a life time of more than 400 years at 25 °C.

A high performance micro-pressure sensor based on a double-ended quartz tuning fork and silicon diaphragm in atmospheric packaging

A resonant micro-pressure sensor based on a double-ended quartz tuning fork (DEQTF) and bossed silicon diaphragm in atmospheric packaging is presented. To achieve vacuum-free packaging with a high quality factor, the DEQTF is designed to resonate in an anti-phase vibration mode in a plane that is under the effect of slide-film damping. The feasibility is demonstrated by theoretical analysis and a finite element simulation. The dimensions of the DEQTF and diaphragm are optimized in accordance with the principles of improving sensitivity and minimizing energy dissipation. The sensor chip is fabricated using quartz and silicon micromachining technologies, and simply packaged in a stainless steel shell with standard atmosphere. The experimental setup is established for the calibration, where an additional sensor prototype without a pressure port is introduced as a frequency reference. By detecting the frequency difference of the tested sensor and reference sensor, the influences of environmental factors such as temperature and shocks on measuring accuracy are eliminated effectively. Under the action of a self-excitation circuit, static performance is obtained. The sensitivity of the sensor is 299 kHz kPa−1 in the operating range of 0–10 kPa at room temperature. Testing results shows a nonlinearity of 0.0278%FS, a hysteresis of 0.0207%FS and a repeatability of 0.0375%FS. The results indicate that the proposed sensor has favorable features, which provides a cost-effective and high-performance approach for low pressure measurement.