Piezoresistive Silicon Research Articles

Eight-beam piezoresistive accelerometer fabricated by using a selective porous-silicon etching method

This paper presents the first experimental results of a piezoresistive silicon accelerometer with eight beams fabricated by a unique silicon micromachining technique using selective porous-silicon etching. This technique is capable of precisely constructing a microstructure without any lateral etching or undercutting. By the use of this eight-beam structure, the mechanical strength of the accelerometer can be highly improved due to smaller shear stress. The lower sensitivity of the sensor can be simply solved by combining the four output signals of the half-bridge. The effectiveness of the sensor is confirmed through an experiment in which the accelerometer is characterized.

Photolithographic packaging of silicon pressure sensors

The packaging technique presented here provides direct interaction between the sensing element and the physical or chemical variable to be measured as well as hermetic insulation and mechanical protection of the silicon sensor and its electrically active components. The sensors are embedded in a mechanically decoupling shell of silicone elastomer with low Young's modulus, leaving the sensing area free from covering. The process can be considered as a synthesis of conventional soft-mounting techniques and photolithographically controlled partial encapsulation. It has been applied to package piezoresistive silicon pressure sensors for use in humid environments and the results obtained show the suitability of the concept. Due to its modularity and unlike application-specific packaging methods, it can potentially be applied to a variety of microelectronic sensors.

Piezoresistive cantilever designed for torque magnetometry

New piezoresistive silicon cantilevers designed specifically for torque magnetometry on microscopic samples have been microfabricated and tested. These levers have been optimized to detect the torque in two directions corresponding to flexion and torsion. Torque resolution of ∼10−14 N m can be achieved depending on the operating mode. In one version an integrated loop allows an absolute calibration of the device with an accuracy of ∼1%. This loop can also be used to excite the lever mechanically. One application is the determination of the mass of nanogram samples by measuring the resonance frequency shift (nanobalance).

Fabrication of improved piezoresistive silicon cantilever probes for the atomic force microscope

This paper describes an improved design for a monolithic silicon atomic force microscope (AFM) probe using piezoresistive sensing. The probe is V shaped, with a sharp tip at the free end and two piezoresistors at the root, and is fabricated using silicon-on-insulator (SOI) starting material. The maximum sensitivity of the AFM probe is measured to be 4.0(± 0.1) × 10 −7 Å −1, which is larger than that of the previous parallel-arm piezoresistive AFM probe. The measured results are in reasonable agreement with the values predicted by theory. The minimum detectable force and minimum detectable deflection of the AFM probes are predicted to be 1.0 × 10 −10 N and 0.29 Å r.m.s., respectively, using a Wheatstone bridge arrangement biased at a voltage of ± 5 V and bandwidth of 10 Hz–1 kHz.

Micromachined silicon cantilever paddles with piezoresistive readout for flow sensing

A group of piezoresistive silicon cantilever paddles have been fabricated, each of which consists of a paddle square which is attached by two cantilever arms, and a piezoresistive sensor on the root surface of each arm. The process involves a combination of wet and dry etching techniques. The fabricated cantilever paddles are long and wide with a paddle square of . The measured sensitivity of the cantilever devices is , which is in good agreement with the values predicted by our derived formula. These cantilever paddles have potential application in flow sensors.

The Effect of Inorganic Thin Film Material Processing and Properties on Stress in Silicon Piezoresistive Pressure Sensors

AbstractSilicon bulk micromachined piezoresistive pressure sensors are sensitive to stresses caused by the application of inorganic thin films typically used for passivation purposes, and the change in stress that is caused by temperature changes in the operating environment of the sensor. Stress behavior over temperature is characterized for both thermal oxides grown on silicon at thicknesses from 0.18 μm to 0.36 μm, and PECVD silicon nitride films at thicknesses from 0.40 μm to 0.80 μn. Electrical parametric behavior is characterized for typical piezoresistive pressure sensors with these thin films deposited and patterned in several proposed passivation schemes. A finite element analysis is performed to predict how device parameters vary as a function of thin film patterning and properties. Correlations are drawn between model predictions, independent thin film behavior, and device performance.

Parallel atomic force microscopy using cantilevers with integrated piezoresistive sensors and integrated piezoelectric actuators

We have fabricated and operated two cantilevers in parallel in a new mode for imaging with the atomic force microscope (AFM). The cantilevers contain both an integrated piezoresistive silicon sensor and an integrated piezoelectric zinc oxide (ZnO) actuator. The integration of sensor and actuator on a single cantilever allows us to simultaneously record two independent AFM images in the constant force mode. The ZnO actuator provides over 4 μm of deflection at low frequencies (dc) and over 30 μm deflection at the first resonant frequency. The piezoresistive element is used to detect the strain and provide the feedback signal for the ZnO actuator.

Atomic force microscope lithography using amorphous silicon as a resist and advances in parallel operation

Lithography on (100) single-crystal silicon and amorphous silicon is performed by electric-field-enhanced local oxidation of silicon using an atomic force microscope (AFM). Amorphous silicon is used as a negative resist to pattern silicon oxide, silicon nitride, and selected metals. Amorphous silicon is used in conjunction with chromium to create a robust etch mask, and with titanium to create a positive AFM resist. All lithographies presented here were patterned in parallel by arrays of two piezoresistive silicon or two silicon-nitride cantilevers. Parallel arrays of five piezoresistive cantilevers were fabricated and used in imaging and lithographic applications. A 400 μm×100 μm parallel image is obtained in the time it would normally take to obtain a 100 μm×100 μm image. In our method of parallel operation, it is only possible to image and lithograph in modes that do not require feedback. In imaging, this limits the possible applications of the parallel AFM. During parallel lithography, discrepancies are seen between the tip in the feedback loop and those that are not. To overcome these differences it will be necessary to devise a system where each of the tips in the array are controlled by individual feedback loops.

Silicon piezoresistivity modelling: application to the simulation of MOSFETs

Abstract The piezoresistive effect in silicon has been extensively studied in this work. The Π11, Π2 and Π44 piezoresistive coefficients for both n-type and p-type silicon, respectively, have been analytically developed including the carrier transfer effect, the intravalley-intervalley scattering effects and the stress-induced effective mass variations. The contribution of the band-gap change due to the stress has been found to be important when the populations of electrons and holes are comparable. By introducing the quantum effect, the bulk modelling has been successfully extended to MOSFET devices. A good agreement between simulation and experiment has been achieved.

Characteristic analysis of a pressure sensor using the silicon piezoresistance effect for high-pressure measurements

The characteristics of output voltages and non-linearities of a silicon circular diaphragm pressure sensor using the piezoresistance effect are analysed for high-pressure measurements. The pressure sensor is analysed, using triaxial stresses and their corresponding piezoresistance coefficients, by a finite-element method (FEM). Radial and tangential stresses on the circular diaphragm, calculated by the FEM at a pressure of 50 MPa, shift by about 125 MPa to the compressive side from those calculated by plate theory of a fixed periphery. Moreover, the perpendicular stress (z stress) component to the diaphragm surface is not negligible at high pressure in either calculation. The results calculated by the FEM agree well with the experimental results. A pressure sensor for high-pressure measurements from 0 to 50 MPa is then designed and fabricated. It has a non-linearity within +or-0.1%.

Low pressure acoustic sensors for airborne sound with piezoresistive monocrystalline silicon and electrochemically etched diaphragms

Abstract In this paper experimental results and theoretical considerations of an acoustic silicon sensor for airborne sound, which is based on boron-implanted monocrystalline piezoresistors located on an electrochemically etched silicon diaphragm, are presented. The bandwidth of the sensor is nearly 20 kHz (full audio bandwidth), a maximum sensitivity of 80 μV Pa −1 (bias voltage, 8 V) was achieved and a lowest equivalent noise level of about 61 dB(A) was measured. The membrane thickness is about 1.3 μm; the membrane area amounts to 1 mm 2 . Furthermore, a good reproducibility and linearity could be obtained. The weak sensitivity can mainly be explained by a residual inherent tensile stress in the silicon-silicon dioxide-silicon nitride layer system of the diaphragm and by the large geometric dimensions of the resistors.

Small piezoresistive silicon microphones specially designed for the characterization of turbulent gas flows

Piezoresistive microphones have been designed and fabricated using surface micromachining technology. The acoustic sensors, which are specially designed for turbulence gas flow measurements, are fabricated with the very small diaphragm side lengths of 100 and 300 um. An expression for the dynamic pressure sensitivity is derived where the diaphragm bending forces, the tensile built-in stain force, as well as the fluid compressibility forces are considered. The acoustic sensitivity’s, which are in the range 0.3-0.9 u V/Pa for a supply voltage of 10 V, are compared with the calculated values, showing good agreement. The frequency response is falt up to 10 kHz within -+ 2dB. The ability to measure the smallest pressure eddies in a turbulent boundary layer is verified and compared for different diaphragm sizes.

Silicon sensors as process monitoring devices

Piezoresistive silicon based process monitoring devices (PMDs) employing flexible circuitry and substrates have been developed which are compatible with standard VLSI fabrication processes. These solid state sensors offer cost effective batch fabrication and a high degree of uniformity to produce either single elements or flexible conformable arrays. This is a state of the art report on emerging technology.

Piezoresistive accelerometer with overload protection and low cross-sensitivity

Abstract A piezoresistive silicon bridge-type accelerometer is described. The application of a standard bipolar process allows the mechanical structure to be realized by anisotropic wet etching in KOH with an electrochemical etchstop at selectively diffused n-type layers. A novel glass-bonding technology offers complete encapsulation of the sensor with silicon top and bottom caps, including wafer-level assembly and simultaneous overload protection. Optimization of the mechanical structure by finite-element modelling (FEM) with respect to the resonance frequency results in the application of transverse piezoresistors with excellent cross-sensitivity compensation. The sensor features a sensitivity of 40 μV/N g with a non-linearity of less than 0.5% up to 350 g, first resonance frequency of about 5 kHz, and cross-sensitivity in lateral directions of less than 0.2% with an overall chip size of 3.7 × 3.7 × 1.0 mm3.

Silicon Sensors as Process Monitoring Devices

Abstract Piezoresistive silicon based process monitoring devices (PMDs) employing flexible circuitry and substrates have been developed which are compatible with standard VLSI fabrication processes. These solid state sensors offer cost effective batch fabrication and a high degree of uniformity to produce either single elements or flexible conformable arrays. This is a state of the art report on emerging technology.

Silicon sensors as process monitoring devices

Abstract Piezoresistive silicon based process monitoring devices (PMDs) employing flexible circuitry and substrates have been developed which are compatible with standard VLSI fabrication processes. These solid state sensors offer cost effective batch fabrication and a high degree of uniformity to produce either single elements or flexible conformable arrays. This is a state of the art report on emerging technology.

A miniature piezoresistive catheter pressure sensor

This paper discusses some factors which affect the dimensional limits of miniature piezoresistive catheter pressure sensors, analyses the influence of silicon wafer thickness and piezoresistor area dimensions on the sensitivity of the sensor on the basis of the mechanism of anisotropic etching of (100) silicon and the stress-distribution curve of a rectangular silicon diaphragm, and presents the design, process and passivation technology of a miniature piezoresistive catheter pressure sensor. The size of the sensor chip is 1.0 mm × 2.5 mm, its sensitivity is about 100 μV/V kPa and its resonance frequency is more than 350 kHz. The processing technology is suitable for batch manufacture.

An array tactile sensor with piezoresistive single-crystal silicon diaphragm

This paper reports a 4 × 4 element array tactile sensor with ‘silicon box’ structure. The sensory elements of the tactile sensor are made of piezoresistive single-crystal silicon diaphragms. This sensor is fabricated by an SiSi direct bonding technique and one-side processing technology fully compatible with current MOS technology. The array tactile sensor and CMOS signal-processing circuits are integrated on the same chip. The structure, circuit, processing and experimental results of the device are described in detail.

Direct writing of piezoresistive silicon resistors using laser-induced CVD

Laser-induced chemical vapour deposition (LCVD), using a movable focused Ar + laser beam (1 W, 514.5 nm), was used for direct, maskless writing of strain sensitive silicon resistors for the fabrication of prototype pressure sensors on a silicon on sapphire (SOS) and sapphire substrates from the mixed ratio of 0,1% B 2 H 6 in the reactive gas SiH 4 . The strain sensitive resistors, approximately 300 μm long, were deposited, with in situ measurement of their resistance. The width of the deposited stripes were about 10 μm, and lines over 1 μm thick were written at speeds of 50 μm/s. The resistivity of the stripes was 7-10 mΩcm. The LCVD processing pressure was typically 100 mbar. In the case of silicon-on-sapphire (SOS) diaphragms (thickness of 330 μm) longitudinal gauge factors of 40-80 were observed when the direction of the resistors was parallel to the [110]-type epitaxial direction. The gauge factor of 13 was measured when the resistor was deposited parallel to the [100]-type direction. Polycrystalline deposition on the sapphire substrate (thickness of 530 μm) gave gauge factors of 31-48. Temperature coefficients of resistance (0 o C...70 o C) of silicon stripes were -500...-2200 (SOS) and -2300...-2900 ppm/ o C (sapphire), respectively

Characteristics of polysilicon resonant microbeams

Polysilicon resonant microbeams can be used as strain-sensitive elements to replace conventional silicon piezoresistors in precision sensor applications, such as pressure sensors and accelerometers. These elements are combined with conventional silicon diaphragms or flexures with a proof mass to convert pressure or acceleration directly into a frequency output. Vacuum-enclosed resonant microbeam elements 200 or 400 μm long, 45 μm wide and 1.8 μm thick have been fabricated using LPCVD mechanical-grade polysilicon at the University of Wisconsin. Q-values determined using gain/phase analysis are typically over 25 000. Lower Q-values are primarily the result of residual gas in the cavity. Closed-loop operation from −60 to 180°C using piezoresistive sensor and electrostatic drive has been achieved with automatic gain control (AGC) to prevent overdrive. The characteristic resonance frequencies of the beams have been measured, with 550 kHz, 1.2, 2.2 and 5.2 MHz being typical of the frequencies of the one-dimensional bending modes for the 200 μm length. These measurements of the multiple resonance frequencies of a single beam provide a means of testing mathematical models of the dynamic behavior as well as determining the residual beam stress. The one-dimensional (1D) differential equation of motion of a doubly clamped single-span beam with an axial load can be solved analytically for lateral natural frequencies and mode shapes. These 1D solutions have been verified by 3D finite-element methods. In addition, the finite-element models are used to identify both lateral and torsional modes. The closed-form solutions agree closely with the numerical results and the experimental data.