Doped Single-crystal Silicon Research Articles

Effect of overheat and direct flow loading on the MEMS bistable flow sensor

We present the findings from an experimental study of a MEMS flow sensor in which an initially curved, double-clamped bistable microbeam is the primary sensing element. Our research explores how the overheat ratio, direct flow loading, and turbulence-induced vibration affect the sequential snap-through (ST) buckling and snap-back (SB) release of an electrostatically actuated beam heated by an electric current. The sensor is fabricated from highly doped single-crystal silicon using a silicon-on-insulator (SOI) wafer. Positioned at the chip’s edge, the microbeam is exposed to airflow, enabling concurrent dynamic response measurements with a laser Doppler vibrometer and a video camera.Our research demonstrates that the overheat ratio can be significantly lower for this sensing principle than conventional thermal sensing elements, pointing to the potential for substantial energy savings. We also emphasize the significant impact of flow angles and vibrations on the critical ST and SB voltages, which are vital for the flow sensor’s output. Additionally, we introduce the first direct experimental observation of the beam profile’s time history during the snap-through/snap-back transition.The potential impact of this research lies in developing more robust MEMS flow sensors with enhanced sensitivity and a better understanding of their response to environmental factors, which could have broader applications in fields such as aerospace, environmental monitoring, and industrial process control.

A ScAlN Piezoelectric High-Frequency Acoustic Pressure-Gradient MEMS Vector Hydrophone With Large Bandwidth

In this letter, a scandium-doped aluminum nitride (Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {Al}}_{1-x}\text{N}$ </tex-math></inline-formula> , x = 9.5%) piezoelectric high-frequency MEMS vector hydrophone with large operation bandwidth is presented, which measures acoustic pressure gradient underwater. The hydrophone sensor consists of four groups of sensing cell arrays. Each sensing cell is composed of a hexagonal sensing diaphragm made up of a piezoelectric stack of ScAlN piezoelectric thin-film sandwiched by Mo and highly doped single-crystal silicon, with a top and a bottom oxide film surrounding the piezoelectric stack. The hydrophone sensor can operate in not only omnidirectional receive mode, but also dipole receive mode, depending on the signal processing algorithm used for the four array groups. The vector hydrophone operates in the frequency range of 20 kHz to 160 kHz, which is specially designed to monitor the dolphin calls. The measured maximum receive sensitivity is -175 dB (re: 1 V/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Pa) in the dipole receive mode at 160 kHz and −169±2 dB (re: 1 V/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Pa) in the omnidirectional receive mode in the operation bandwidth, respectively. The reported vector hydrophone also presents a smooth directivity pattern at 100 kHz. The experiments show the reported MEMS vector hydrophone has good potential in conservation of dolphins and other underwater detection at high frequency between 20 kHz to 160 kHz.

High-Q energy trapping of temperature-stable shear waves with Lamé cross-sectional polarization in a single crystal silicon waveguide

A multi-port electrostatically driven silicon acoustic cavity is implemented that efficiently traps the energy of a temperature-stable eigen-mode with Lamé cross-sectional polarization. Dispersive behavior of propagating and evanescent guided waves in a ⟨100⟩-aligned single crystal silicon waveguide is used to engineer the acoustic energy distribution of a specific shear eigen-mode that is well known for its low temperature sensitivity when implemented in doped single crystal silicon. Such an acoustic energy trapping in the central region of the acoustic cavity geometry and far from substrate obviates the need for narrow tethers that are conventionally used for non-destructive and high quality factor (Q) energy suspension in MEMS resonators; therefore, the acoustically engineered waveguide can simultaneously serve as in-situ self-oven by passing large uniformly distributed DC currents through its body and without any concern about perturbing the mode shape or deforming narrow supports. Such a stable thermo-structural performance besides large turnover temperatures than can be realized in Lamé eigen-modes make this device suitable for implementation of ultra-stable oven-controlled oscillators. 78 MHz prototypes implemented in arsenic-doped single crystal silicon substrates with different resistivity are transduced by in- and out-of-plane narrow-gap capacitive ports, showing high Q of ∼43k. The low resistivity device shows an overall temperature-induced frequency drift of 200 ppm over the range of −20 °C to 80 °C, which is ∼15× smaller compared to overall frequency drift measured for the similar yet high resistivity device in the same temperature range. Furthermore, a frequency tuning of ∼2100 ppm is achieved in high resistivity device by passing 45 mA DC current through its body. Continuous operation of the device under such a self-ovenizing current over 10 days did not induce frequency instability or degradation in Q.

High temperature setup for measurements of Seebeck coefficient and electrical resistivity of thin films using inductive heating.

We have developed an automated setup for simultaneous measurement of Seebeck coefficient S(T) and electrical resistivity ρ(T) of thin film samples from room temperature to ∼650 °C. S and ρ are extracted from current-voltage (I-V) measurements obtained using a semiconductor parameter analyzer and temperature measurements obtained using commercial thermocouples. The slope and the x-axis intercept of the I-V characteristics represent the sample conductance G and the Seebeck voltage, respectively. The measured G(T) can be scaled to ρ(T) by the geometry factor obtained from the room temperature resistivity measurement of the film. The setup uses resistive or inductive heating to control the temperature and temperature gradient on the sample. Inductive heating is achieved with steel plates that surround the test area and a water cooled copper pipe coil underneath that generates an AC magnetic field. The measurements can be performed using resistive heating only or inductive heating only, or a combination of both depending on the desired heating ranges. Inductive heating provides a more uniform heating of the test area, does not require contacts to the sample holder, can be used up to the Curie temperature of the particular magnetic material, and the temperature gradients can be adjusted by the relative positions of the coil and sample. Example results obtained for low doped single-crystal silicon with inductive heating only and with resistive heating only are presented.

Correlating Microscale Thermal Conductivity of Heavily-Doped Silicon With Simultaneous Measurements of Stress

The functioning and performance of today’s integrated circuits and sensors are highly affected by the thermal properties of microscale silicon structures. Due to the well known size effect, the thermal properties of microscale silicon structures are not the same as those of the bulk silicon. Furthermore, stress/strain inside microscale silicon structures can significantly affect their thermal properties. This article presents the first thermal conductivity measurements of a microscale silicon structure under applied compressive stress at 350 K. Atomic force microscope (AFM) cantilevers made of doped single-crystal silicon were used as samples. A resistance temperature detector (RTD) heater attached to another RTD sensor was used as the heating module, which was mounted onto a nanoindentation test platform. This integrated system applied compressive load to the cantilever in the longitudinal direction while supplying heat. The thermal conductivity of the cantilevers was calculated using steady state heat conduction equation. The result shows that the measured thermal conductivity varies from 110 W/(m·K) to 140 W/(m·K) as compressive strain varies from 0.1% to 0.3%. Thermal conductivity is shown to increase with increase in compressive strain. These results match with the published simulation values. The measured thermal conductivity and stress values vary in the similar manner as a function of applied strain.

Lifetime Studies of Electrothermal Bent-Beam Actuators in Single-Crystal Silicon and Polysilicon

Microsystems using electrothermal bent-beam microactuators have been demonstrated for a variety of applications including optical attenuators, RF switches, and micro positioners, thus creating a need for information on the longevity of these devices. This paper reports on the dc and pulse mode lifetime testing of this class of actuators constructed using polysilicon and p/sup ++/ doped single crystal silicon. The relative temperature profile along the top surface of an actuator is experimentally verified by scanning probe microscopy. Displacement measurements are used to explore links between aging behavior and the design variables and operating conditions. At low power levels (which result in average operating temperatures of 300-400/spl deg/C) both polysilicon and p/sup ++/ Si devices provide continuous dc operation for >1400 min, in air without change in amplitude. While some types of p/sup ++/ Si devices show monotonic loss of amplitude in pulse tests, others have been operated up to 30 million cycles without degradation. The displacement for polysilicon actuators can either increase or decrease depending on the geometry of the device and operating conditions, both of which are related to temperature and stress of the structural members. Polysilicon grain transformations are observed over extended operation at high temperatures. Performance changes are correlated to material properties using SEM and TEM images.

Thermal conduction in ultrathin pure and doped single-crystal silicon layers at high temperatures

This work presents the in-plane thermal-conductivity data for pure as well as boron-doped (1.6×1021∕cm3), arsenic-doped (2.3×1020∕cm3), and phosphorus-doped (2.3×1020∕cm3) silicon layers of thickness 30nm in the temperature range of 300–450K. The steady-state Joule heating and electrical resistance thermometry are used to measure the lateral thermal conductivity of suspended silicon layers. Thermal-conductivity data for pure and doped single-crystalline thin silicon layers can be interpreted using thermal-conductivity integral in relaxation-time approximation that accounts for phonon-boundary and phonon-impurity scatterings. No additional fitting parameters are used in this work in contrast with previous studies that required an unusually large phonon-impurity scattering coefficient to fit the thermal-conductivity data for bulk doped silicon to the predictions of the thermal-conductivity integral in relaxation-time approximation.

Thermal conduction in doped single-crystal silicon films

This work measures the thermal conductivities along free-standing silicon layers doped with boron and phosphorus at concentrations ranging from 1×1017 to 3×1019 cm−3 at temperatures between 15 and 300 K. The impurity concentrations are measured using secondary ion mass spectroscopy (SIMS) and the thermal conductivity data are interpreted using phonon transport theory accounting for scattering on impurities, free electrons, and the layer boundaries. Phonon-boundary scattering in the 3-μm-thick layers reduces the thermal conductivity of the layers at low temperatures regardless of the level of impurity concentration. The present data suggest that unintentional impurities may have strongly reduced the conductivities reported previously for bulk samples, for which impurity concentrations were determined from the electrical resistivity rather than from SIMS data. This work illustrates the combined effects of phonon interactions with impurities, free electrons, and material interfaces, which can be particularly important in semiconductor devices.

Thin-beam bulk micromachining based on RIE and xenon difluoride silicon etching

A new process for fabricating thin mechanical beam structures from single-crystal silicon has been developed. Lateral and vertical dimensions of the beam can be precisely defined. The beam is positioned in the middle of a silicon wafer at exactly equal distances from both sides. The beam design is not limited by crystal orientations of silicon. The silicon beam structure is essentially stress free because the whole structure is made of uniformly doped single-crystal silicon. This thin-beam process offers significantly expanded design freedom to bulk silicon micromachining. Additionally, a silicon dioxide structure with very high aspect ratio has been fabricated with a similar technique.

Growth Mechanism of Hydrogen Clusters

ABSTRACTIt is demonstrated that the exposure of polycrystalline silicon (poly-Si) to monatomic hydrogen results in the formation of H clusters. These H stabilized platelets appear in the near-surface region ( 100 nm) and are predominantly oriented along {111} crystallographic planes. Platelet concentrations of ≈5×1015, 1.5×1016-cm−3, and 2.4×1017 cm−3 were observed in nominally undoped poly-Si, phosphorous doped poly-Si (P=1017 cm−3), and phosphorous doped single crystal silicon (P&gt;3×1018 cm−3), respectively. Results obtained on doped c-Si demonstrate that platelet generation occurs only at Fermi-level positions of Ec -EF &lt; 0.4 eV.

Micromechanical and tribological characterization of doped single-crystal silicon and polysilicon films for microelectromechanical systems devices

Microelectromechanical systems (MEMS) devices are made of doped single-crystal silicon, LPCVD polysilicon films, and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Micromechanical and tribological characterization of p-type (lightly boron-doped) single-crystal silicon (referred to as “undoped”), p+-type (boron doped) single-crystal silicon, polysilicon bulk, and n+-type (phosphorous doped) LPCVD polysilicon films have been carried out. Hardness, elastic modulus, and scratch resistance of these materials were measured by nanoindentation and microscratching using a nanoindenter. Friction and wear properties were measured using an accelerated ball-on-flat tribometer. It is found that the undoped silicon and polysilicon bulk as well as n+-type polysilicon film exhibit higher hardness and elastic modulus than the p+-type silicon. The polysilicon bulk and n+-type polysilicon film exhibit the lowest friction and highest resistance to scratch and wear followed by the undoped silicon and with the poorest behavior of the p+-type silicon. During scratching, the p+-type silicon deforms like a ductile metal.

Microtribological studies of doped single-crystal silicon and polysilicon films for MEMS devices

Microelectromechanical systems (MEMS) devices are made of doped single-crystal silicon, LPCVD polysilicon films and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Atomic force microscopy/friction force microscopy (AFM/FFM) have been used to study surface roughness, friction, scratching/wear and indentation hardness, and Young's modulus of elasticity of p-type (lightly boron-doped) single-crystal silicon (referred to as ‘undoped’), p +-type (boron-doped) single-crystal silicon, polysilicon, and n +-type (phosphorus-doped) LPCVD polysilicon films. Undoped silicon samples are very smooth (r.m.s. = 0.08 nm). Surface-roughness values for p +-type silicon and lapped polysilicon bulk samples are comparable. As-deposited polysilicon films are rougher than undoped and doped single-crystal silicon, unlapped and lapped polysilicon bulk samples. Microscale friction measurements show that the friction values for undoped silicon, p +-type silicon and lapped polysilicon bulk samples are comparable. Polysilicon films exhibit the highest coefficient of friction as compared to other samples, due to the roughness effect. Macroscale friction values for undoped silicon and polysilicon bulk are comparable with those of p +-type silicon sample having the highest value and polysilicon films having the lowest values. The microscale friction values for all samples are lower than the macroscale friction values as there is less ploughing contribution in microscale friction measurements. Local variation in microscale friction is found generally to correspond to the local surface slope. Directionality in the friction is generally observed on the microscale which results from the surface preparation and anisotropy in surface roughness. Microscratching/wear studies indicate that scratch and wear resistance of p +-type silicon is poor as compared to other samples used in this study. Nanoindentation studies indicate that the p +-type silicon sample exhibits lowest hardness and Young's modulus as compared to the other samples, which is responsible for high macrofriction and poor microscratch and microwear behavior of the p +-type silicon sample. Microtribological behavior and nanohardness and Young's modulus values for polysilicon bulk and n +-type polysilicon film are comparable.

The formation of titanium silicide by arsenic ion beam mixing and rapid thermal annealing

The reaction of titanium thin films with n+ doped single crystal silicon induced by As+ ion beam irradiation and subsequent rapid thermal annealing has been studied. Arsenic doping in the silicon substrate retards the thermal growth of silicide resulting in an incomplete conversion of the titanium into titanium disilicide during low temperature (∼600 °C) rapid thermal annealing under a nitrogen ambient. High energy As+ion beam irradiation (90–170 keV) prior to the rapid thermal annealing leads to intermixing of the titanium and silicon which, at sufficiently high energy and dosage, results in complete conversion of titanium to a silicide phase. Subsequent rapid thermal annealing of the ion beam irradiated samples induces further uptake of silicon into the silicide phase leading to the formation of the stable C-54 titanium disilicide phase. However, very little additional titanium consumption occurs during the thermal annealing. The silicide formed by the ion beam mixing process shows superior sheet resistance uniformity and has a smoother morphology compared to thermally grown silicide. The uniformity of the silicidation reaction by ion beam mixing also leads to superior n+p junction characteristics.