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Making sense of MEMS

29 July 2011

MEMS technologies are the rising star of the sensors market. However, there are a number of misconceptions surrounding their capabilities, and conventional sensors continue to meet a much wider range of applications. Jesse Bonfeld traces the evolution of MEMS fabrication, describing the devices and their impact on the sensors market.

Micro Electro Mechanical Systems (MEMS) describes both a type of device or sensor, and a manufacturing process. MEMS sensors incorporate tiny devices with miniaturised mechanical structures typically ranging from 1-100μm (about the thickness of a human hair), whilst MEMS manufacturing processes provide an alternative to conventional macro-scale machining and assembly techniques.

Also known as ‘microsystems’ in Europe, and ‘micromachines’ in Japan, MEMS devices have come to the fore in recent years with the wide-scale adoption of MEMS motion sensors by the automotive industry, and the growing use of accelerometers and gyroscopes in consumer electronics. Perhaps the most well known consumer electronics incorporating MEMS motion sensors include a number of the leading smart phones, and gaming consoles/controllers.

Rise of the micromachines
MEMS sensors combine electrical and mechanical components into, or on top of, a single chip. In this way, MEMS sensors represent a continuum bridging electronic sensors at one end of the spectrum, and mechanical sensors at the other. The key criterion of a MEMS sensor, however, is that it typically contains some elements with mechanical functionality – stretch, deflect, spin, rotate or vibrate, for example.

MEMS development stems from the microelectronics industry, and combines and extends the conventional techniques developed for integrated circuit (IC) processing with MEMS-specific processes, to produce small mechanical structures measuring in the micrometer (one millionth of a metre) scale. As with IC fabrication, the majority of MEMS sensors are manufactured using a silicon (Si) wafer, whereby thin layers of materials are deposited on a Si substrate and then selectively etched away to leave microscopic 3D structures such as beams, diaphragms, gears, levers, or springs. This process, known as ‘bulk micromachining’, was commercialised during the late 1970s and early 1980s, but a number of other etching and micromachining concepts and techniques have since been developed.

The first micromachined pressure sensors – or ‘diffused’ sensors as they were originally known – were designed and manufactured by Kulite Semiconductor in the mid-1960s. Known as a ‘piezoresistive’ pressure sensor, or ‘silicon cell’, a pressure sensor consists of a micromachined silicon diaphragm with piezoresistive strain gauges diffused into it, fused to a silicon or glass backplate. The top-side of the diaphragm is exposed to the environment through a port, and deforms in reaction to a pressure differential across it. The extent of the diaphragm deformation is then converted to a representative electrical signal, which appears at the sensor output.

Of microsensors and MEMS
The history of Si pressure sensors is widely recognised as being representative of microsensor evolution. A microsensor is a sensor that has at least one physical dimension at the sub-millimetre level, and today can be used to measure or describe an environment or physical condition such as acceleration, altitude, force, pressure, or temperature.

Micromachining techniques have also enabled the development of micro-actuators, which are devices that accept a data signal as an input, and then perform an action based on that signal as an output. Examples include micro-valves for control of gas and liquid flows, optical switches and mirrors to redirect or modulate light beams, and micro-pumps to develop positive fluid pressures.

Advances in IC technology and MEMS fabrication processes have enabled commercial MEMS devices that integrate microsensors, micro-actuators and microelectronic ICs, to deliver perception and control of the physical environment. These devices, also known as ‘microsystems’ or ‘smart sensors’, are able to gather information from the environment by measuring mechanical, thermal, biological, chemical, optical, or magnetic phenomena. The IC then processes this information and directs the actuator(s) to respond by moving, positioning, regulating, pumping, or filtering. Any device or system can be deemed a MEMS device if it incorporates some form of MEMS-manufactured component. And there can be any number of MEMS devices within a particular microsystem – ranging from just a few, to several million.

Demand for MEMS devices was initially driven by the military. More recently, a maturing of the semiconductor manufacturing processes associated with the microchips used within personal computers, and the intersection with the huge requirement in the automotive and consumer electronics sectors, has propelled MEMS sensors into the mainstream. The key MEMS sensors today are accelerometers, gyroscopes, and pressure sensors.

Innovation and limitation
All too often, MEMS technologies are perceived as being all-encompassing solutions, when in actual fact, they remain a largely one product, one process business. A number of companies develop and produce MEMS devices themselves, and are defined as ‘IDMs’ (integrated device manufacturers), whereas some outsource production (fabless), and others operate both models. Much of the confusion in the market can be attributed to this diversity, and the way in which the various verticals subsequently interface make the MEMS market notoriously difficult to define.

At the point of fabrication, there are very few, if any, companies operating in the sensors market that offer MEMS together with another technology because of the high cost of market entry and the cost of packaging MEMS devices. Likewise, once a company has committed to manufacturing MEMS devices, it is difficult for that company to change focus, due to low margins, higher development costs and greater complexity. That said, MEMS does enable high-volume production, due to the batch fabrication techniques employed, resulting in very low costs for each single device.

It is also very rare for any MEMS manufacturer to provide products directly to end users. Given that MEMS sensors must interface with the external environment, the packaging of MEMS devices into a higher order assembly that can be used directly by end users adds an additional layer of complexity and calls for expertise and specialist manufacturing facilities. This market dynamic is akin to the semiconductors industry, whereby microchips are manufactured in bulk, packaged, and delivered to the manufacturers building commercial products (such as personal computers).

The shape of sensors to come
The advances in MEMS technologies and techniques means that manufacturers are now able to produce very capable MEMS sensors and devices, but many cannot be installed directly into an end application because they cannot survive the rigours of final assembly. Conversely, conventional sensors can survive just about any assembly process and any application, but are perceived as being too big and too expensive. Hence the challenge for the manufacturers of MEMS sensors that are to be used in commercial products is to take the MEMS price and form factor, and package it into something able to withstand harsh environments.

Indeed, it is this second level of packaging that must be envisioned and understood by specialist manufacturers looking for growth potential. Today, the majority of industry innovation and commercial opportunity is centred on the application of existing MEMS devices, in addition to new ways of packaging and integrating MEMS devices within a system that can be used directly by end users.

With the MEMS market returning to growth during 2010, the agile OEMs will be those that determine how to integrate conventional sensor fabrication technologies and performance capabilities with the emerging MEMS trends to overcome the limitations in material needs and processes. If the latter are addressed, then it is conceivable that all conventional manufacturing techniques and types of sensors will be replaced, but certainly not for the foreseeable future.

Jesse Bonfeld is with Sherborne Sensors

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