Si Bulk Micromachining Research Articles

A Monolithic Micromachined Thermocouple Probe With Electroplating Nickel for Micro-LED Inspection

This paper presents a pair of trapezoid microcantilever probes with a monolithic micromachined thermocouple for inspecting the electrical and thermal properties of Micro-LED. To meet testing requirements, one of the microcantilevers was designed as a single-layer Ni structure for electrical conduction, while the other one, which consists of n-type poly-Si, SiO₂ and Ni layers, was used for temperature sensing. Both microcantilevers were fabricated by using Si bulk micromachining, thin film deposition, and electroplating processes. Fabrication result shows the length of each probe is about 78 μm and 118 μm, and the thicknesses of the microcantilevers and probe tip are 7.2 μm and 5.5 μm. Experiment results depict that the thermocouple junction between Ni layer and poly-Si achieves a good ohmic contact, and the measured sheet resistances for poly-Si and electroplated Ni are 150 Ω /sq and 0.01 Ω /sq. This suggests that the induced thermoelectric voltage can be generated and detected in response to a temperature difference. Mechanical tests verified that the probes provide an available elastic deflection to guarantee that the probe tips can exactly contact the Micro-LED electrodes, with a low contact force. The thermoelectric characteristics of the device have been further confirmed and calibrated on the basis of the measurement results of a commercial mm-sized LED in cooperation with an infrared camera. For the subject of the Micro-LED chip with a square size of 75 μm, its electrical and thermal properties were successfully determined, as the measured chip temperature is 52 °C at the applied current of 100 μA. [2021-0053]

Fabrication of Area Effective Silicon Membrane By Frontend Bulk Micromachining for High Pixel Density Thermal Imager Array

The fabrication of Silicon membrane by frontend bulk micromachining process has been proven to be one of the simple and cost-effective technique, for fabricating microelectromechanical structures. In this paper, we propose novel geometric mask designs, to be used as a pixel for Microbolometer application, for achieving highly reliable and area-efficient microstructures by frontend Silicon (Si) bulk micromachining. In the proposed design, all the rectangular openings are aligned at 45⁰ with respect to the wafer primary flat (100) of the Silicon wafer, such that only <100> planes are exposed and etched during anisotropic etching. However rectangular openings lead to high silicon area consumption which makes the process unworthy. Therefore, we proposed different geometries to minimize area consumption for achieving the same dimension of suspended structure. The fabrication of these Silicon membranes, to be used as Microbolometer pixels were simulated using Intellisuite FABSIM based physical simulator by two different techniques, one involving only wet chemical etching (called Method I) and the other involving dry-cum-wet etching process (called Method II). The simulation result shows an 86% reduction in footprint by Method II over the basic mask design when compared with the Method I, thus making it suitable for high pixel density thermal imager array. Thermal imaging is an established technique to convert the invisible radiation pattern emitted by an object into visible images, without establishing contact with the object, for feature extraction and analysis. Though Infrared thermal imaging was first developed for military purposes, it later gained a wide application in various fields such as aerospace, agriculture, civil engineering, medicine, and veterinary and many more, which is left to the researcher's imagination to explore its diverse applications in near future. It can be applied in all fields where temperature differences could be used to assist in the evaluation, diagnosis, or analysis of a process or product. Thus from agriculture and food industry perspective, thermal imaging technology can have the following potential applications: predicting water stress in crops, planning irrigation scheduling, disease and pathogen detection in plants, predicting fruit yield, bruise detection in fruits and vegetables, evaluating the maturing of fruits, detection of foreign bodies in food material, grain storage & drying, etc. Finally, as a proof of concept, we fabricated a single-pixel Silicon membrane using the proposed Method I as per the process flow outlined in the paper. Both simulation and experimental result shows very good agreement on all process steps. The fabricated Silicon membrane which acts as a mechanical platform is monolithic, as it does not involve any sacrificial layer, thus making it highly robust and reliable, as a result, it can be used for diverse sensing applications. Figure 1

Bulk Micromachining of Si by Annealing-Driven Magnetically Guided Metal-Assisted Chemical Etching

Herein, the bulk micromachining of Si by a magnetically guided metal-assisted chemical etching (MACE) process is demonstrated. To improve the etching performance of Si, a trilayer metal catalyst (A...

Efficient thermal utilization in MEMS bulk micromachined pyroelectric infrared sensor using thermal oxide thin layer

Pyroelectric infrared sensors utilize interfacial polymer bonding, thick porous silica layer and Si3N4 thin film as conventional methods of reducing heat sinking with substrate. The thick polymer and porous silica layers cause additional enhancement in thermal time constant (?D) as well thermal mass (Cth) of pyroelectric sensor which reduces the rate of temperature change (dT/dt) and limits responsivity at lower frequencies. The SiO2 thin layer posses lower heat conductivity than Si3N4 thin films and often used in Si bulk micromachining as etch barrier. This paper proposes use of micromachined SiO2 thin membrane as thermal insulation layer in bulk micromachined pyroelectric infrared sensor. The study is supported by the comparative radiation heat transfer, finite element analysis and the practical thermal measurements on fabricated pyroelectric infrared sensor using non destructive thermal imaging method.

Micromachined multiple focused-ion-beam devices

Micromachined multiple focused-ion-beam devices were developed, which were composed of an array of ion sources, electrostatic extractors, and lenses to achieve concurrent processing of microdevices with high throughput. All of the parts of the devices were fabricated and packaged on the scale of a microchip, employing Si bulk micromachining technologies. Ionic liquids (ILs) were used to generate the ion species instead of conventional liquid metals such as Ga and In. Three types of testing devices were fabricated to comprehensively evaluate the system. The first, device 1, consisted of the emitter arrays and a biased Si target, whereby the etching characteristics of beams generated with several ILs were evaluated. The peaks of SiF+, SiF2+, SiF3+, and SiF4+, which provide evidence for reactive etching of Si, were obtained by in situ monitoring of the gas composition. Compared with a conventional Ga ion source, the ion beams emitted from the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide exhibited a 5.4-fold sputtering yield of 11.9 atom/ion at an acceleration voltage of 5 kV. In device 2, the electrostatic extractor and lens were mounted on the emitter array, and control of the divergence angle of the ion beam was confirmed. In device 3, which has a separated extractor on the electrostatic lens plate, an individual control of ion beams was demonstrated.

Etchant wettability in bulk micromachining of Si by metal-assisted chemical etching

Wet bulk micromachining of Si by metal-assisted chemical etching (MaCE) has successfully been demonstrated. Based on the mechanism of defective etching results from Ag and Au metal catalyst experiments, the wettability of etchant solution, in addition to metal type, has been found to have profound effect on the etching process. Addition of low surface tension co-solvent, ethanol in this work, into conventional etchant formulation has enabled complete wetting of etchant on surface, which prevents hydrogen bubble attachment on sample surface during the etching. The complete elimination of bubble attachment guarantees very uniform etch rate on all over the sample surface, and thus prevents premature fragmentation/rupture of catalyst metal layer. Under the optimized etching conditions, the MaCE could be done for up to 12h without any noticeable film rupture and thus etching defects. Thanks to very smooth surface of the etched patterns, conformal contact and direct bonding of elastomer on such surface has been easily accomplished. The method demonstrated here can pave the way for application of simple, low-cost MaCE process in the bulk micromachining of Si for various applications.

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.

Realizing Area efficient Silicon Micro Structures Using Only Front End Bulk Micromachining

Front-end bulk micro machining is one of the proven techniques of making suspended microstructures and is highly adapted due to its simple and cost effective way of fabricating the devices. In this paper we propose novel geometric mask designs for achieving area efficient microstructures by front-end Silicon (Si) bulk micromachining. In this work we adapt the geometric mask design having a microstructure between two rectangular openings. These openings are aligned at 45° to wafer prime flat of (100) silicon wafer and act as etch openings for front-end bulk micromachining. However rectangular openings lead to high silicon area consumption which makes the process unworthy. Therefore we proposed different geometries to minimize area consumption for achieving the same dimension of suspended structure. All these different geometries are simulated using Intellisuite FABSIM based physical simulator. We have observed more than 28 % reduction in foot print over recent literature and 80 % over the basic design used in this paper.

Mechanical characterization and modelling of Lorentz force based MEMS magnetic field sensors

In this work we present experimental results from dynamic and static tests on miniature magnetic field sensors which are based on Micro Electro Mechanical Systems (MEMS) technologies. These MEMS magnetometers were fabricated on SOI wafers using Si bulk micromachining techniques and they operate at the first resonant frequency under the action of the Lorentz force which arises when a current flows through them in the presence of an external magnetic field. Sensing is based on piezoresistive principles and high sensitivity is expected from devices that show high total quality factors Qtot. We investigate here the energy loss mechanisms and the temperature rise due to Joule heating effects in the resonators of the magnetometers by performing tests both in air and under vacuum conditions. Testing was performed using laser Doppler Vibrometry and white light interferometry. At each pressure different driving currents have been applied and Qtot was extracted. It is found that Qtot varies with pressure between two limiting values: a low one in air which was between 17 and 500 for the tested devices and a high one in vacuum which in the case of one of our devices was equal to 2800. The amplitude of the applied current is also affecting the Q value at a certain pressure due to the rise of thermal stress in the resonating structures. The sensitivity of the sensors in air was experimentally measured using a Helmholtz coil and an oscilloscope and values between 72mVT−1 and 513mVT−1 were obtained from the tested devices. We further attempt to estimate the temperature rise in the devices due to Joule heating effects by combining the topography scans which were experimentally obtained with results from thermomechanical analysis of the sensors using Finite Element Modelling.

A low-power, micromachined, double spiral hotplate for MEMS gas sensors

This paper presents the design, fabrication and reliability testing of a double spiral platinum-based MEMS hotplate for gas sensing applications. The structure of MEMS hotplate consists of a 0.7 µm-thick thermally grown SiO2 membrane of size 120 µm × 120 µm over which a double spiral platinum resistor is laid out. The hotplate membrane is supported by its four arms connected to the Si-substrate. The design and simulation of the hotplate structure was carried out using MEMS-CAD Tool COVENTORWARE. Based on the design, a double spiral platinum resistor of 103 Ω is fabricated on SiO2 membrane using lift-off technique. The platinum deposition is carried out using DC sputtering technique. Bulk micromachining of Si is done from front side of the structure to create the suspended SiO2 membrane. The temperature coefficient of resistance (TCR) of platinum is measured and found to be 2.19 × 10?3/ °C. The TCR value is used for temperature estimation of the hotplate. The test results show that the microhotplate consumes only 20 mW power when heated up to 500 °C. For reliability testing of fabricated structure, the hotplate is continuously operated at 300 °C for 1.8 h. Also, it can sustain at least 61 cycles pulse-mode operation at 530 °C with ultra-low resistance and temperature drifts. The structure can sustain a maximum temperature and current of 611 °C and 11.55 mA respectively without any damage.

Bulk micromachining of Si by metal-assisted chemical etching.

Bulk micromachining of Si is demonstrated by the well-known metal-assisted chemical etching (MaCE). Si microstructures, having lateral dimension from 5 μm up to millimeters, are successfully sculpted deeply into Si substrate, as deep as >100 μm. The key ingredient of this success is found to be the optimizations of catalyst metal type and its morphology. Combining the respective advantages of Ag and Au in the MaCE as a Ag/Au bilayer configuration leads to quite stable etch reaction upon a prolonged etch duration up to >5 h. Further, the permeable nature of the optimized Ag/Au bilayer metal catalyst enables the etching of pattern features having very large lateral dimension. Problems such as the generation of micro/nanostructures and chemical attacks on the top of pattern surface are successfully overcome by process optimizations such as post-partum sonication treatment and etchant formulation control. The method can also be successful to vertical micromachining of Si substrate having other crystal orientations than Si(100), such as Si(110) and Si(111). The simple, easy, and low-cost nature of present approach may be a great help in bulk micromachining of Si for various applications such as microelectromechanical system (MEMS), micro total analysis system (μTAS), and so forth.

A large-scan-angle piezoelectric MEMS optical scanner actuated by a Nb-doped PZT thin film

Resonant 1D microelectromechanical systems (MEMS) optical scanners actuated by piezoelectric unimorph actuators with a Nb-doped lead zirconate titanate (PNZT) thin film were developed for endoscopic optical coherence tomography (OCT) application. The MEMS scanners were designed as the resonance frequency was less than 125 Hz to obtain enough pixels per frame in OCT images. The device size was within 3.4 mm × 2.5 mm, which is compact enough to be installed in a side-imaging probe with 4 mm inner diameter. The fabrication process started with a silicon-on-insulator wafer, followed by PNZT deposition by the Rf sputtering and Si bulk micromachining process. The fabricated MEMS scanners showed maximum optical scan angles of 146° at 90 Hz, 148° at 124 Hz, 162° at 180 Hz, and 152° at 394 Hz at resonance in atmospheric pressure. Such wide scan angles were obtained by a drive voltage below 1.3 Vpp, ensuring intrinsic safety in in vivo uses. The scanner with the unpoled PNZT film showed three times as large a scan angle as that with a poled PZT films. A swept-source OCT system was constructed using the fabricated MEMS scanner, and cross-sectional images of a fingertip with image widths of 4.6 and 2.3 mm were acquired. In addition, a PNZT-based angle sensor was studied for feedback operation.

Prototype MEMS Capacitive Pressure Sensor Design and Manufacturing

This paper is intended to describe the design and manufacturing aspects of a simple micromachined capacitive pressure sensor working in the pressure range of 0-1000 mbar. 500 µm thick Borofloat® 33 glass and silicon wafers were used as substrates. The basic transducer structure consists of a rectangular silicon membrane as deformable electrode and a fix aluminum electrode formed on the glass surface. In order to determine the exact geometry of the silicon electrode structure numerical models and simulations were applied. The thin silicon membrane was fabricated by Si bulk micromachining, i.e. anisotropic alkaline etching with electrochemical etch-stop. The two wafers were bonded together at low temperature by anodic bonding. After bonding and dicing the wafers the pressure sensors were characterized mechanically and functionally also. Our results demonstrate the functional behavior of the manufactured sensor structures and provide excellent verification of the preliminary expectations based on theoretical calculations and electro-mechanical simulations.

A Monolithic Dual-Color Total-Internal-Reflection-Based Chip for Highly Sensitive and High-Resolution Dual-Fluorescence Imaging

We report a dual-color total-internal-reflection (TIR)-based chip that can generate two overlapping evanescent fields with different wavelengths for simultaneous imaging of two types of fluorophores. We derived a general relationship among the dimensions of the components of the chip to guarantee the overlap of two evanescent fields. Optical simulation results also confirm the generation and overlap of two evanescent fields. Using Si bulk micromachining and poly(dimethylsiloxane) (PDMS) casting, our fabrication method integrates all miniaturized optical components into one monolithic PDMS chip. Thus, assembly is unnecessary, and misalignment is avoided. Our PDMS chip can be employed with various sample delivery platforms, such as glass slide, flow cell, microchannel, etc. We first demonstrated the capability of the chip by imaging TIR fluorescent spots of a mixture of two fluorophores, namely, fluorescein and tetramethylrhodamine. We then employed the chip to observe the Brownian motion of a mixture of nile-red and dragon-green 500-nm microbeads. Our chip could potentially be integrated into a micro-total analysis system for highly sensitive and high-resolution dual-fluorescence imaging applications.

Three-dimensional surface sculpting of freestanding metal-composite nanomembranes

In this work we report structural functionalization of metal-composite nanomembranes through formation of micrometer-sized three-dimensional hollow self-supported shapes with tailorable ordering and dimensions. The basic idea is to use double-sided, two-stage etching of the sacrificial layer, first to define the preform for the nanomembrane sculpting, then to fully remove the sacrificial structure and release the nanomembrane. In our experiments we utilized 15–20 μm thick ‹100› Si diaphragms as sacrificial layers. Conventional KOH-based bulk micromachining was used to sculpt pyramidal holes within the diaphragms and modified radiofrequent sputtering to deposit a 5–20 nm Cr-based nanomembrane precursors. After the removal of sacrificial membranes the released freestanding nanomembranes had an aspect ratio up to 1000,000 and contained hollow micrometer-sized pyramidal shapes which do not collapse in spite of their large wall-to-side aspect ratio (>1000). Although the geometry of the sculpted shapes in our experiments is defined by the Si bulk micromachining, one may readily fabricate a number of other shapes, including hemispherical, ellipsoidal, etc., the sizes and shapes being limited by the particular microfabrication technique chosen for the diaphragm pre-forming. Some potential applications of 3D sculpted nanomembranes can be found in subwavelength diffractive optics, plasmonics, energy harvesting, microfluidics, biointerfaces, to name just a few.

Versatile microfluidic total internal reflection (TIR)-based devices: Application to microbeads velocity measurement and single molecule detection with upright and inverted microscope

A poly(dimethylsiloxane) (PDMS) chip for Total Internal Reflection (TIR)-based imaging and detection has been developed using Si bulk micromachining and PDMS casting. In this paper, we report the applications of the chip on both inverted and upright fluorescent microscopes and confirm that two types of sample delivery platforms, PDMS microchannel and glass microchannel, can be easily integrated depending on the magnification of an objective lens needed to visualize a sample. Although any device configuration can be achievable, here we performed two experiments to demonstrate the versatility of the microfluidic TIR-based devices. The first experiment was velocity measurement of Nile red microbeads with nominal diameter of 500 nm in a pressure-driven flow. The time-sequenced fluorescent images of microbeads, illuminated by an evanescent field, were cross-correlated by a Particle Image Velocimetry (PIV) program to obtain near-wall velocity field of the microbeads at various flow rates from 500 nl/min to 3000 nl/min. We then evaluated the capabilities of the device for Single Molecule Detection (SMD) of fluorescently labeled DNA molecules from 30 bp to 48.5 kbp and confirm that DNA molecules as short as 1105 bp were detectable. Our versatile, integrated device could provide low-cost and fast accessibility to Total Internal Reflection Fluorescent Microscopy (TIRFM) on both conventional upright and inverted microscopes. It could also be a useful component in a Micro-Total Analysis System (micro-TAS) to analyze nanoparticles or biomolecules near-wall transport or motion.

Fabrication of near-field optical probes using advanced functional thin films for MEMS and NEMS applications

Highly efficient atomic force microscopy (AFM) cantilevers and an near-field scanning optical microscopy (NSOM) aperture array were fabricated using the functional Si 3N 4 and SiO 2 thin films for the micro electro mechanical systems (MEMS) and NEMS applications. In order to generate the cantilevers and array, a silicon nitride (Si 3N 4) thin film with a stress below 100 MPa was deposited using NH 3 and SiCl 2H 2 gases with a relative ratio of 1:5 at 140 mTorr, and 835 °C by low pressure chemical vapor deposition (LPCVD). Cantilevers of 10–100 μm in length were fabricated with Si 3N 4 thin films. Photolithography and magnetically enhanced reactive ion etching (MERIE) techniques were employed for patterning the Si 3N 4 thin films and selective bulk etching of Si was carried out using a strong alkaline solution of tetramethylammoniumhyroxide (TMAH) to generate the Si 3N 4-based cantilevers with 3D shapes. In addition, we also successfully fabricated an array of SiO 2 apertures with sub-wavelength sizes as near-field optical probe in order to examine the possible light resonance-tunneling phenomenon. Initially, a (50 × 50) array with a dimension of (5 mm × 5 mm) was fabricated on a Si wafer followed by the V-groove formation using alkaline solution Si bulk micromachining technology. The size of the aperture on top of the pyramidal array was carefully controlled with an opening rate of ∼27 nm/min using HF solution diluted by a factor of 50 using water. The Al thin film was thermally evaporated on the (50 × 50) array pattern for the fabrication of apertures with sub-wavelength sizes.

CMOS integrated tactile sensor array by porous Si bulk micromachining

An integrated 8 × 8 element 3D force sensor array was developed for tactile sensing. The 300 × 300 μm 2 sensors are formed from mono-crystalline silicon membranes with spacing of 700 μm. The signals of the individual transducers are read out line-by-line via an on-chip CMOS decoder. The porous silicon bulk micromachining was implemented as one of the final steps of the n-well CMOS compatible processing.

High-Q micromachined three-dimensional integrated inductors for high-frequency applications

Three-dimensional micromachined inductors are fabricated on high-resistivity (10kΩcm) and low-resistivity (10Ωcm) Si substrates using a stressed metal technology. On high-resistivity Si substrate with low-k dielectric material (SU-8™), this technology achieves a quality factor Q of 75 for a 1nH inductor at frequencies around 4GHz and a self-resonant frequency fsr above 20GHz. Using Si bulk micromachining to etch away the low-resistivity Si substrate with a combination of deep reactive ion etching and tetramethyl ammonium hydroxide etching methods, a 1.2nH inductor achieves a peak quality factor Q of 140 at a frequency of 12GHz with a self-resonant frequency fsr above 40GHz. The dependence of high-frequency performance on the inductor’s variables, such as the number of turns, turn-to-turn gap, and substrate type, has been investigated. Excellent performance is achieved by removing the substrate due to the complete elimination of substrate losses and the reduction of the parasitic capacitance. This technology is simple and provides high performance integrated inductors on Si or compound-semiconductor platforms.

Synthesis of textured thin piezoelectric AlN films with a nonzero C-axis mean tilt for the fabrication of shear mode resonators

A method for the deposition of thin piezoelectric aluminum nitride (AlN) films with a nonzero c-axis mean tilt has been developed. The deposition is done in a standard reactive magnetron sputter deposition system without any hardware modifications. In essence, the method consists of a two-stage deposition process. The resulting film has a distinct tilted texture with the mean tilt of the c-axis varying roughly in the interval 28 to 32 degrees over the radius of the wafer excluding a small exclusion zone at the center of the latter. The mean tilt angle distribution over the wafer has a circular symmetry. A membrane-type shear mode thickness-excited thin film bulk acoustic resonator together with a micro-fluidic transport system has been subsequently fabricated using the two stage AlN deposition as well as standard bulk micro machining of Si. The resonator consisted of a 2-microm-thick AlN film with 200nm-thick Al top and bottom electrodes. The resonator was characterized with a network analyzer when operating in both air and water. The shear mode resonance frequency was about 1.6 GHz, the extracted device Q around 350, and the electromechanical coupling kt2 2% when the resonator was operated in air, whereas the latter two dropped down to 150 and 1.8%, respectively, when the resonator was operated in pure water.