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

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

http://iopscience.iop.org/article/10.1088/0022-3727/40/20/S19/pdf

Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications

The majority of microelectromechanical system (MEMS) devices must be combined with integrated circuits (ICs) for operation in larger electronic systems. While MEMS transducers sense or control phys ...

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Integrating MEMS and ICs

Over the past four decades, the field of silicon piezoresistive pressure sensors has undergone a major revolution in terms of design methodology and fabrication processes. Cutting edge fabrication technologies have resulted in improved precision in key factors like dimensions of diaphragm and placement of piezoresistors. Considering the unique nature of each sensor and the trade-offs in design, it is not feasible to follow a standard design approach. Thus, it is useful to derive the specific design from a number of important factors to arrive at the `ideal' design. In this paper, we critically review and analyze the various design considerations and principles for silicon piezoresistive pressure sensor. We also report the effect of these considerations on the sensor output taking help of various CAD tools. Keeping in view the accuracy of state-of-the-art fabrication tools and the stringent demands of the present day market, it has become important to include many of these design aspects. Modelling using analytical expressions for thin plates has also been looked into as it gives a quick guideline and estimation of critical parameters before detailed finite element method analysis. Wherever possible, fabrication imperfections and their effects have been discussed. Dependency of piezoresistive coefficients on temperature and doping concentration, the effect of clamping condition of diaphragms and fabrication using wet bulk micromachining is also analyzed. Silicon-on-insulator based sensors along with innovative design strategies, and future trends have also been discussed. This paper will serve as a quick and comprehensive guide for pressure sensor developers.

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Erratum to: Design principles and considerations for the `ideal' silicon piezoresistive pressure sensor: a focused review

The majority of microelectromechanical system (MEMS) devices must be combined with integrated circuits (ICs) for operation in larger electronic systems. While MEMS transducers sense or control physical, optical or chemical quantities, ICs typically provide functionalities related to the signals of these transducers, such as analog-to-digital conversion, amplification, filtering and information processing as well as communication between the MEMS transducer and the outside world. Thus, the vast majority of commercial MEMS products, such as accelerometers, gyroscopes and micro-mirror arrays, are integrated and packaged together with ICs. There are a variety of possible methods of integrating and packaging MEMS and IC components, and the technology of choice strongly depends on the device, the field of application and the commercial requirements. In this review paper, traditional as well as innovative and emerging approaches to MEMS and IC integration are reviewed. These include approaches based on the hybrid integration of multiple chips (multi-chip solutions) as well as system-on-chip solutions based on wafer-level monolithic integration and heterogeneous integration techniques. These are important technological building blocks for the More-Than-Moore paradigm described in the International Technology Roadmap for Semiconductors. In this paper, the various approaches are categorized in a coherent manner, their merits are discussed, and suitable application areas and implementations are critically investigated. The implications of the different MEMS and IC integration approaches for packaging, testing and final system costs are reviewed.

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Micro-Electro-Mechanical Systems (MEMS) are sensing the environmental conditions and give input to electronic control systems MEMS are miniature systems which usually combine tiny mechanical structures with electronic circuits Typical MEMS structures have a size of a few micrometers MEMS sensors make system reactions to human needs more intelligent, precise, and at much faster reaction rates than humanly possible Today MEMS sensors can be found in nearly every motor vehicle, smart phone or laptop Due to continuous product innovations, the sensors find their way into more and more applications in automotive and consumer electronics According to IHS iSuppli an amount of 43 billion micromechanical sensors were sold in 2011 with an impressive increase to 98 billion sensors in 2015 — a growth rate of 23% per year These growth rates are only possible with continuous efforts to improve the performance and to decrease the size, power consumption and costs of the sensors Historicall, these improvements have lead to the breakthrough to industrial mass production for MEMS This was achieved over the last 20 years by intense and successful development activities combined with the experience of volume production of billions of sensors Since then, MEMS technology has enabled accelerometers, gyroscopes, geomagnetic sensors, barometric pressure sensors, inkjet nozzles or micro-mirrors for Digital-Light Processing (DLP) projection systems Driving future applications in consumer electronics will be Human-Machine Interfaces (HMI), Augmented Reality (AR) and in-door navigation

MEMS for automotive and consumer electronics

We report a new surface micromachining technology to fabricate monocrystalline silicon membranes covering a vacuum cavity for applications like piezoresistive pressure sensors. The main process steps are: (i) local anodic etching of layered porous silicon with different porosities, (ii) thermal rearrangement of the porous silicon, and (iii) epitaxial growth of the silicon membrane layer. In contrast to conventional bulk micromachining the new technology has the benefit of a considerable freedom in the design of mono-crystalline silicon membranes. The membrane geometry is only determined by the porous region. Further, the new fabrication method is fully CMOS compatible. In fact, except for anodic etching, all process steps are part of a standard mixed signal IC production line. Various aspects of the used key process steps are discussed, particularly with regard to the oxygen and fluorine desorption during the porous silicon annealing. A piezoresistive pressure sensor with integrated ASIC based on the new fabrication method is demonstrated.

A novel micromachining process for the fabrication of monocrystalline Si-membranes using porous silicon

The invention relates to a method for producing a micromechanical sensor element, which can be configured with a monolithic integrated construction and is capable of the capacitive detection of a physical variable. In addition to said production method, the invention also relates to a micromechanical device containing a sensor element of this type, such as a pressure sensor or an acceleration sensor.

Micromechanical capacitive sensor element

One of the first MEMS products - the pressure sensor - has still room for innovation. We report a completely new pressure sensor generation based on a novel surface micromachining technology. Using porous silicon the membrane fabrication can be monolithically integrated with high synergy in an analog/digital semiconductor process suited for high volume production in an IC-fabrication facility. Only two mask layers and one electrochemical etching step are inserted at the beginning of a standard IC-process to transform the epitaxial silicon layer from the electronic process into a monocrystalline membrane with a vacuum cavity under it.

Next generation pressure sensors in surface micromachining technology

Micro-Electro-Mechanical Systems (MEMS) are sensing the environmental conditions and give input to electronic control systems. MEMS are miniature systems which usually combine tiny mechanical structures with electronic circuits. Typical MEMS structures have a size of a few micrometers. MEMS sensors make system reactions to human needs more intelligent, precise, and at much faster reaction rates than humanly possible. Today MEMS sensors can be found in nearly every motor vehicle, smart phone or laptop. Due to continuous product innovations, the sensors find their way into more and more applications in automotive and consumer electronics. According to IHS iSuppli an amount of 4.3 billion micromechanical sensors were sold in 2011 with an impressive increase to 9.8 billion sensors in 2015 - a growth rate of 23% per year! These growth rates are only possible with continuous efforts to improve the performance and to decrease the size, power consumption and costs of the sensors.

Stefan Finkbeiner

Papers

MEMS for automotive and consumer electronics

A novel micromachining process for the fabrication of monocrystalline Si-membranes using porous silicon

Micromechanical capacitive sensor element

Next generation pressure sensors in surface micromachining technology

MEMS for automotive and consumer electronics