Progress in the production and modification of PVDF membranes

Piezoresistive sensors are among the earliest micromachined silicon devices. The need for smaller, less expensive, higher performance sensors helped drive early micromachining technology, a precursor to microsystems or microelectromechanical systems (MEMS). The effect of stress on doped silicon and germanium has been known since the work of Smith at Bell Laboratories in 1954. Since then, researchers have extensively reported on microscale, piezoresistive strain gauges, pressure sensors, accelerometers, and cantilever force/displacement sensors, including many commercially successful devices. In this paper, we review the history of piezoresistance, its physics and related fabrication techniques. We also discuss electrical noise in piezoresistors, device examples and design considerations, and alternative materials. This paper provides a comprehensive overview of integrated piezoresistor technology with an introduction to the physics of piezoresistivity, process and material selection and design guidance useful to researchers and device engineers.

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Review: Semiconductor Piezoresistance for Microsystems

Piezoresistive cantilevers are limited by two major noise sources: Johnson noise, which is independent of frequency, and conductance fluctuation noise, which has a 1/f spectrum. The 1/f fluctuations of piezoresistive cantilevers are shown to vary inversely with the total number of carriers in the piezoresistor, as formulated by Hooge in 1969. Therefore, while 1/f noise is reduced for large heavily doped cantilevers, sensitivity considerations favor thin lightly doped cantilevers. Balancing these conflicting constraints produces optima for many design and processing parameters. For a cantilever with specified spring constant and bandwidth requirements, optima are identified for the beam thickness and length, and it is shown that the legs should be between 1/3 and 2/3 of the total length with a doping depth that is 1/3 of the beam thickness. Additionally, an optimal doping concentration is identified as a function of the cantilever volume and the measurement bandwidth. Annealing reduces 1/f noise, but causes a loss in sensitivity due to dopant diffusion, and an optimal anneal is computed with a typical diffusion length 10/sup -8/ cm. The analysis, methods, and some of the conclusions of this paper are also applicable to other types of piezoresistive sensors.

1/f noise considerations for the design and process optimization of piezoresistive cantilevers

We report the development of an artificial hair cell (AHC) sensor with design inspired by biological hair cells. The sensor consists of a silicon cantilever beam with a high-aspect-ratio cilium attached at the distal end. Sensing is based on silicon piezoresistive strain gauge at the base of the cantilever. The cilium is made of photodefinable SU-8 epoxy and can be up to 700-mum tall. In this paper, we focus on flow-sensing applications. We have characterized the performance of the AHC sensor both in water and in air. For underwater applications, we have characterized the sensor under two flow conditions: steady-state laminar flow (dc flow) and oscillatory flow (ac flow). The detection limit of the sensor under ac flow in water is experimentally established to be below 1 mm/s. A best case angular resolution of 2.16deg is also achieved for the sensor's yaw response in air.

Design and Characterization of Artificial Haircell Sensor for Flow Sensing With Ultrahigh Velocity and Angular Sensitivity

1/f noise is a fluctuation in the conductance of semiconductors and metals. This noise could be a fluctuation in the number of free electrons or in their mobility. Many experimental studies have proved that the 1/f noise in homogeneous samples is a fluctuation in the mobility. There is a reliable empirical relation for the power density of the mobility noise. Theoretical models for mobility noise have not had much influence in the discussions. The theoretical position for number fluctuations is quite the opposite. The generally accepted McWhorter model is simple; however, very few experimental studies definitely prove number fluctuations and exclude uncorrelated mobility fluctuations. There is an extensive literature on noise in MOSTs. Submicrometer MOSTs are notorious for their high low-frequency noise. Almost all studies start from the McWhorter generation-recombination (GR) model. The discussion is confused by the many complications of the MOSTs, such as mobility degradation, surface and contacts effects, size of the samples, etc. An additional problem is that the real 1/f noise is often mixed up with other types of low-frequency noise, such as burst and GR noises. A critical discussion is given for the Kirton and Uren model. Analytical expressions are presented for the results of their computer simulations. This explains their surprising discovery that the density of McWhorter states may be ldquosubstantiallyrdquo different from the usual 1/ tau distribution.

What Do We Certainly Know About $\hbox{1}/f$ Noise in MOSTs?

The conductivity fluctuations in 1‐MeV, boron‐implanted layers in silicon have been investigated at room temperature with the anneal temperature as a parameter. The 1/ f  noise parameter α has been calculated from conductance, Hall voltage, and noise experiments. Annealing of the implants causes, in addition to an increase in the number of electrically active impurities, a decrease in the 1/ f  noise parameter α by a factor of at least 50. The α value is proportional to eΔE/kTan, with an annealing activation energy ΔE=1.1 eV in a temperature range of 722 K <Tan<1022 K. The annealing affects the quality of the crystal, the scattering mechanism, the temperature dependence of the mobility, and the value of α. The better the crystal, the lower the 1/ f  noise parameter α.

S. Oosterhoff

Papers

Annealing of ion‐implanted resistors reduces the 1/ f noise