Single Crystal Silicon Structures Research Articles

A Device for Noncontact Determination of the Photosensitivity Distribution on Areas of n+–p(n)–p+-Type Silicon Structures

A device is described that allows one to measure the photosensitivity contrast over the area of n+–p(n)–p+ silicon structures without contacts. The structure is placed between the capacitor plates and is locally illuminated on one side by two intensity-modulated lasers. The laser wavelengths are 1064 and 808 nm. The radiations of the first and second lasers are absorbed in the volume of the base region and only near its illuminated surface, respectively. The local photosensitivity is determined by the ratio of the amplitudes of the modulations, at which the total variable photo-emf vanishes. This compensation allows one to avoid an error that is associated with shunting of the illuminated area of the structure by the rest of it due to currents in the n+ and p+ layers. The photosensitivity contrasts measured by the proposed compensation method and the standard method based on short-circuit current measurements were compared on n+–p–p+ single-crystal silicon structures. The difference was no more than 6%, which is consistent with the calculations.

The Effects of Structure Thickness, Air Gap Thickness and Silicon Type on the Performance of a Horizontal Electrothermal MEMS Microgripper

The ongoing development of microelectromechanical systems (MEMS) over the past decades has made possible the achievement of high-precision micromanipulation within the micromanufacturing, microassembly and biomedical fields. This paper presents different design variants of a horizontal electrothermally actuated MEMS microgripper that are developed as microsystems to micromanipulate and study the deformability properties of human red blood cells (RBCs). The presented microgripper design variants are all based on the U-shape ‘hot and cold arm’ actuator configuration, and are fabricated using the commercially available Multi-User MEMS Processes (MUMPs®) that are produced by MEMSCAP, Inc. (Durham, NC, USA) and that include both surface micromachined (PolyMUMPs™) and silicon-on-insulator (SOIMUMPs™) MEMS fabrication technologies. The studied microgripper design variants have the same in-plane geometry, with their main differences arising from the thickness of the fabricated structures, the consequent air gap separation between the structure and the substrate surface, as well as the intrinsic nature of the silicon material used. These factors are all inherent characteristics of the specific fabrication technologies used. PolyMUMPs™ utilises polycrystalline silicon structures that are composed of two free-standing, independently stackable structural layers, enabling the user to achieve structure thicknesses of 1.5 μm, 2 μm and 3.5 μm, respectively, whereas SOIMUMPs™ utilises a 25 μm thick single crystal silicon structure having only one free-standing structural layer. The microgripper design variants are presented and compared in this work to investigate the effect of their differences on the temperature distribution and the achieved end-effector displacement. These design variants were analytically studied, as well as numerically modelled using finite element analysis where coupled electrothermomechanical simulations were carried out in CoventorWare® (Version 10, Coventor, Inc., Cary, NC, USA). Experimental results for the microgrippers’ actuation under atmospheric pressure were obtained via optical microscopy studies for the PolyMUMPs™ structures, and they were found to be conforming with the predictions of the analytical and numerical models. The focus of this work is to identify which one of the studied design variants best optimises the microgripper’s electrothermomechanical performance in terms of a sufficient lateral tip displacement, minimum out-of-plane displacement at the arm tips and good heat transfer to limit the temperature at the cell gripping zone, as required for the deformability study of RBCs.

Thermoreflectance-based in-depth stress distribution measurement technique for single-crystal silicon structures

In this paper, we suggest a new stress measurement technique based on the thermoreflectance method for the estimation of the in-depth stress distribution of fabricated silicon devices. Changing the modulated intensity of a heating laser beam of the frequency-domain thermoreflectance method (FD-TRM) can vary the estimation depth optionally. We developed a measurement system on the basis of the FD-TRM and demonstrated in-depth stress measurement for a single-crystal silicon (SCS) sample. The result measured at a modulation frequency of 3 MHz showed the phase distribution of the TR signal corresponding to the stress distribution determined by 632-nm-excited Raman spectroscopy. In addition, it was found that the phase distribution changed depending on the modulation frequency. The FD-TRM can be a powerful technique for estimating the in-depth stress distribution of silicon materials.

Design and fabrication of a silicon probe for surface profilers

A micromachining probe for surface profilers is reported in this paper. This probe employs single crystal silicon double-ended tuning forks (DETFs) as the sensitive unit to achieve high resolution and frequency read-out. The frequency shift of the DETFs caused by the induced axial stress is directly proportional to the undulation of the measured surface. Moreover, one-stage and two-stage microleverage amplification mechanisms are respectively implemented to improve the probe performance. The silicon on glass (SOG) process which can provide high aspect ratio single crystal silicon structures has been adopted to enhance the performance of the probes. The testing results indicate that the probe with two-stage mechanical amplifiers has a frequency-displacement sensitivity of 257.1 Hz/μm with a nominal frequency 27.3 kHz at room temperature in atmosphere, which is in close agreement with the simulation results.

Polymer as the protecting passivaton layer in fabricating suspended SCS structures in both anisotropic and isotropic etching

This paper presents a rapid bulk micromachining process named polymer passivation layer for suspended structures etching by using a polymer as a protecting passivation layer at both anisotropic and isotropic etching steps. Without using silicon-dioxide (SiO2) deposition or boron doping as a protection layer at the releasing step, the proposed method can fabricate suspended single-crystal silicon structures in an inductively coupled plasma reactive ion etching chamber directly, which would simplify the fabrication process and save fabrication time. The current study systematically investigates critical fabrication parameters to verify the feasibility of the proposed method, and discusses the polymer passivation time and removal time of a polymer at the base of a substrate at four different opening gaps of 5, 10, 30 and 50 µm with the 30 µm deep trench to establish suitable recipes for fabricating suspended microstructures. It is also shown that the proposed method can fabricate not only the suspended microstructures with the same thickness, but also suspended microstructures with different thicknesses, as well as in sub-micro scale.

Silicon probe for micromachined surface profilers

A single-crystal silicon probe for micromechanical surface profilers is presented, which uses a double-ended tuning fork (DETF) resonator as the displacement-sensing element. In addition, a two-stage micro-leverage mechanism for force amplification is introduced to increase the overall sensitivity. The frequency shift of the DETF caused by the induced axial stress is directly proportional to the displacement input. The probe was fabricated through a standard silicon-on-glass process, which can realise high aspect ratio single-crystal silicon structures. The experimental results indicated that the probe had a nominal resonant frequency of 54.5 kHz under atmosphere at room temperature. The scale factor and linearity of the probe with the input range of 0–10 µm were evaluated to be 359.7 Hz/µm and 6.3%, respectively. The measured scale factor shows good agreement with the simulated value of 330 Hz/µm using ANSYSTM.

Fracture fabrication of precision single-crystal silicon surfaces for MEMS devices

Processes for creating complementary and smooth surfaces by deliberately fracturing a weakened portion of a larger single-crystal silicon structure are described. The structure was designed to refine and concentrate an externally applied load pre-fracture and to provide precision guidance post-fracture. Under the right conditions, material is not ejected from the fracture zone, and the extreme brittleness of crystalline silicon produces complementary fracture surfaces; “closed” post-fracture gaps of 20–30 nm are typical. The effects of lithographic, focused ion beam, and anisotropically etched notches as well as crystal orientation and specimen thickness were studied, and a method was developed for fabricating smooth surfaces perpendicular to the wafer plane: 10 μm square specimens oriented with the (1 1 0) plane fully notched by anisotropic etching (KOH) and fractured within a structure optimized to apply pure tension during fracture. While developing the fabrication process for a variable capacitor device, it was found that the high temperature processing associated with the thermal oxidation necessary for growing the actuator’s dielectric layer blunted the anisotropically etched notches. This blunting manifested itself as material ejecting fractures and occurred even when the notches were covered with a nitride diffusion barrier. The blunting was overcome by employing the actuator’s thermal oxide dielectric layer as a mask for etching the notch after all high temperature processing was complete, indicating it was likely due to diffusional smoothing and that high temperature processes may be useful for strengthening anisotropically etched structures.

Aging of MEMS – Correlation of Mechanical and Structural Properties

Related to the dramatically smaller volume of microelectromecanical systems (MEMS), new methods in testing and qualification are needed. In single crystal silicon (SCSi) based devices, stress and loading in operation introduces defects during the MEMS life time and increases the risk of failure. Reliability studies on potential failure sources have an impact on MEMS design and are essential to assure the long term functioning of the device. In this paper, a comprehensive approach is proposed for assessing the resistance of SCSi structures under conditions of accelerated aging. Defects introduced by DRIE, thermal annealing, dicing and bonding and also the device environment (radiations, temperature) influence the crystalline perfection and have a direct impact on the mechanical properties of MEMS and their aging behavior. Defects and deformations are analyzed using High Resolution X-ray Diffraction Methods (HRXRD) and supporting simulations are done by Finite Element Method (FEM). MEMS reliability is assessed. Preliminary results indicate a decrease in lifetime and strength of SCSi devices with defect density enhanced by irradiation.

Edge luminescence of single-crystal silicon modulated by voltage variation on the p-n junction

The output power and quantum efficiency of the room-temperature quasi-stationary edge photoluminescence (PL) in a single-crystal silicon structure cut from a high-efficiency solar cell have been studied as functions of the power of exciting laser radiation with a wavelength of 658 nm at variable direct current and reverse-bias voltage on the p-n junction. It was found that the direct current passage led to a significant increase in the output power and quantum efficiency of the quasi-stationary PL and a decrease in the threshold power of PL excitation. The application of a reverse bias voltage produced the opposite effects. Possible physical factors responsible for these phenomena are considered.

An X band RF MEMS switch based on silicon-on-glass architecture

Communication systems such as those used on satellite platforms demand high performance from individual components that make up the various systems and sub-systems. Switching and routing of RF signals between various modules is a routine and critical operation that determines the overall efficiency of the entire system. In this paper, we present the design and fabrication aspects of a direct contact RF MEMS switch designed to operate in the X band (8–12 GHz) with a target insertion of about 0·5 dB and isolation better than 30 dB. The actuation voltage is expected to be around 50 V. The die size is designed to be 3 mm (H) × 3 mm(W) × 2 mm(H). The switch is built from a low residual stress device layer of a highly conducting (0·005 Ohms-cm) silicon on insulator (SOI) wafer. After subsequent lithographic steps, the wafer is bonded to a Pyrex glass wafer which has been previously patterned with gold transmission lines and pull in electrodes. Being built from a single crystal silicon structure, the mechanical robustness of the actuator is much greater than the those in similar membrane-based devices. A 6 mask fabrication process utilizing Deep Reactive Ion Etching to achieve high aspect ratio stiction free structures was developed and implemented. Devices from the first fabrication run are being analysed in our laboratory.

Free-standing <inline-formula><math display="inline" overflow="scroll"><msub><mi mathvariant="normal">C</mi><mn>60</mn></msub></math></inline-formula> nanowire fabricated using <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">Xe</mi><msub><mi mathvariant="normal">F</mi><mn>2</mn></msub></mrow></math></inline-formula> sacrificial dry etching

We report a fabrication of doubly supported free-standing buckminsterfullerene (C60) nanowires onto single crystal silicon microstructures in order to demonstrate an integration of carbon nanomaterials to surface-micromachined microelectromechanical devices. By irradiating vacuum-deposited C60 film with an electron beam, polymerized C60 nanowires were patterned. Free-standing structures of the C60 nanowires were obtained by sacrificial etching of underlying silicon structures using XeF2 gas. The supporting 5-µm thick silicon structure was released using vapor hydrofluoric acid. The C60 nanowire of 2-µm width, 25-µm length, and 40-nm thickniss connected to a movable single crystal silicon structure was successfully fabricated.

Life Time Predictions through X-Ray Defect Analysis of MEMS Devices

In single crystal silicon (SCSi) MEMS devices, crystalline imperfection is recognized to favor failure. A DRIE etched SCSi structure was built to study the crystal strain profile in dependence of the SCSi deformation by applying a mechanical force. High resolution X-ray diffraction methods such as the rocking curve method and reciprocal space mapping were used to determine the strain as well as the defect concentration in the crystal. The investigations also include the numerical simulation of deformations.

Growth of “New Form” of Polycrystalline Silicon Thin Films Synthesized by Hot Wire Chemical Vapor Deposition

AbstractIn this work, we report on next-generation hot wire chemical vapor deposition technique, we call it ceramics hot-wire CVD. Using a new concept of rectangular ceramics filament holder and “confinement of thermal radiation from the filament”, a “new form” of polycrystalline silicon thin films has been developed at low temperature (˜ 250°C). The grains are found to be symmetrically distributed in array along the parallel lines, in (111) direction. On the surface of individual grains, “five-fold” and “six-fold” symmetries have been observed and we suspect that we developed “buckyball” type “giant silicon molecular solids” with different crystalline silicon lattice other than standard single-crystal silicon structure. We observed rarely found “icosaderal” symmetry in silicon thin films. This hypothesis has been supported by multiple Raman active transverse optical modes and the crystallographic structure analyzed by X-ray diffraction.

Raman monitoring laser-induced phase transformation in microcrystalline silicon thin films prepared by PECVD

The results of a study of phase transformation in hydrogenated microcrystalline silicon thin films prepared from hydrogen-diluted silane and exposed to an Ar+ laser light are presented. Through Raman spectroscopy, it was determined that the induced phase transformation is interrelated with silane dilution in hydrogen used as a reactive gas in the preparation of the films and, therefore, depends on the grain size and crystalline fraction of the material. Dark conductivity, transmission electronic microscopy (TEM) and atomic-force microscopy (AFM) data have been used to corroborate the Raman measurements. The results show that the sequence of phase transformation in microcrystalline thin films presents different characteristics from those, which are produced in single-crystal silicon structures. The occurrence of a Raman peak at 350 cm−1 is attributed to the formation of the Si-XII phase. A thermal model to explain the phase transformation is proposed. To our knowledge, neither the presence of the phase Si-XII nor the phase transformation process in microcrystalline silicon thin films has been reported before.

High aspect-ratio combined poly and single-crystal silicon (HARPSS) MEMS technology

This paper presents a single-wafer high aspect-ratio micromachining technology capable of simultaneously producing tens to hundreds of micrometers thick electrically isolated poly and single-crystal silicon microstructures. High aspect-ratio polysilicon structures are created by refilling hundreds of micrometers deep trenches with polysilicon deposited over a sacrificial oxide layer. Thick single-crystal silicon structures are released from the substrate through the front side of the wafer by means of a combined directional and isotropic silicon dry etch and are protected on the sides by refilled trenches. This process is capable of producing electrically isolated polysilicon and silicon electrodes as tall as the main body structure with various size capacitive air gaps ranging from submicrometer to tens of micrometers. Using bent-beam strain sensors, residual stress in 80-/spl mu/m-thick 4-/spl mu/m-wide trench-refilled vertical polysilicon beams fabricated in this technology has been measured to be virtually zero. 300-/spl mu/m-long 80-/spl mu/m-thick polysilicon clamped-clamped beam micromechanical resonators have shown quality factors as high as 85 000 in vacuum. The all-silicon feature of this technology improves long-term stability and temperature sensitivity, while fabrication of large-area vertical pickoff electrodes with submicrometer gap spacing will increase the sensitivity of micro-electromechanical devices by orders of magnitude.

Mesa-supported, Single-crystal Microstructures Fabricated by the Surface/Bulk Micromachining Process

In fabricating microelectromechanical systems (MEMS), bulk micromachining using (100) and (110) single crystal silicon and surface micromachining using polycrystalline silicon are most commonly used. However, both micromachining methods have drawbacks, and fabricating actuating or sensing microdevices using single crystal silicon has been an active research topic in recent years. The surface/bulk micromachining (SBM) process using (111) silicon, developed by us previously, allows fabricating released structures in single crystal silicon. This paper extends the SBM process to fabricate released structures that are anchored to the substrate via mesa islands. This allows fabricating folded-type comb drive resonators. With the extension of the SBM process developed in this paper, any microstructure fabricated by the one structural polysilicon surface micromachining technique can also be fabricated in single crystal silicon.

Mechanical behavior of a μm-sized single crystal silicon structure with sharp notches

The mechanical behavior of μm-sized single crystal silicon structures is investigated in this paper. A new apparatus for microsample tensile testing is used to determine the mechanical properties of single crystal silicon microbars. Force and elongation are measured independently with high accuracy. Elastic constants as well as critical loads are determined. Failure of the sample occurs at the extremities of the microbar at a sharp notch. The stress field in the notch tip vicinity is therefore analyzed to characterize the load capacity of the structure. For this purpose, finite element computations are combined with approximate analytic solutions based on the Stroh formalism. Both the case of plane stress and two-dimensional displacements are considered. Comparison with the FEM solution shows that the plane stress assumption is better, leading to the determination of the critical stress intensity factor F cr for this case. Because F cr is dependent on the geometry, a new scalar failure criterion—i.e. a critical radius R cr—is introduced and derived from the calculated stress field at the notch. It is based on a comparison of stored elastic energy and surface energy at the critical location. Because it is a scalar criterion, it can be applied to single crystal microstructures with notches of arbitrary notch angle. For the case considered, experiments yield R cr = 0.8 nm. The proposed criterion can be considered as a step towards the definition of a design rule for sharp notches in single crystal structures, which is required for the optimized design of micromechanical devices.

Single step electrochemical etching in ammonium fluoride

This paper presents a new technique of micromachining using single step electrochemical etching in an ammonium fluoride based etchant. This etching technology is to fabricate 3-D structures in single crystal silicon by a combination of anisotropic and isotropic etching mode. The etch rate and morphology of the etched surface are investigated for the etch parameters (etchant concentration, current density). Optimization of these parameters makes it possible to make free standing beams. Using the ammonium fluoride as the etchant, aluminium survives the etching.

Fabrication of free standing structure using single step electrochemical etching in hydrofluoric acid

This paper presents a new technique of micromachining using single step electrochemical etching in hydrofluoric acid (HF). The electrochemical etching in HF is known as a technique for porous silicon formation. This etching technique is applied to fabricate 3-D structures in single crystal silicon by a combination of anisotropic and isotropic modes. The diameter of the pore, or the width of the trench, can be controlled by the current density. First, vertical walls are formed and after desired depth is obtained, current density is increased by adjusting the light intensity. The width of the trenches is increased under the structures without affecting the width of existing trenches. The connection of the trenches can be achieved and free standing beams obtained with only one mask. The free standing beams with height, width and length of 40 μm, 2 μm and 250 μm, respectively, have been made of single crystal silicon.

The surface/bulk micromachining (SBM) process: a new method for fabricating 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-/spl mu/m-thick arbitrarily-shaped single crystal silicon structures, including comb-drive resonators, at 5-, 30-, and 100-/spl mu/m 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.