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Making sense of MEMS

MEMS technologies are the rising star in 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 of Sherborne Sensors examines the evolution of MEMS fabrication, Microsystems, and MEMS 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 1-100 µm (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 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 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 scale (one millionth of a meter).

As with IC fabrication, the majority of MEMS sensors are manufactured using a Silicon (Si) wafer, whereby thin layers of materials are deposited onto a Si base, and then selectively etched away to leave microscopic 3D structures such as beams, diaphragms, gears, levers, or springs [see Figure 1]. 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 [see box out].
Advances in IC technology and MEMS fabrication processes have enabled commercial MEMS devices that integrate microsensors, microactuators 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.

Demand for MEMS devices was initially driven by the government and military/defence sectors. 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 & limitation
All too often, MEMS technologies are perceived as being all-encompassing solutions made using standardized processes, 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 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, typically resulting in very low costs for each single device.

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 often 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 moving forward to realise 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 to package and integrate 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 MEMS will capture a larger percentage of the overall sensor market.

The rise of chem-bio
One area of intense industry focus over the past five years is that of chemical-biological (chem-bio) sensors. Governments worldwide have been investing heavily in R&D, driven primarily by the heightened threat posed by a chemical or biological attack. Chem-bio sensors respond to changes in their chemical/biological environment and convert this response into a signal that can be read.

Suitable for national security applications, chem-bio sensors are able to quickly and effectively detect dangerous agents in their immediate vicinity - including chemical, biological, nuclear and explosive materials. San Francisco officials recently proposed to regulate the sensors on its buildings in order to detect such agents and, last year, the US Army demonstrated the feasibility of a sensor network to improve situational awareness and reaction time in the field during chemical or biological incidents.

The US Army demonstration used military standard formatted Nuclear, Biological and Chemical (NBC) messages from a sensor located on the soldier, to pass information via machine-to-machine data exchange up to the operations centre to be validated. If a sensor was triggered or an incident occurred, the soldier received an automatic audio alert based on the NBC message type, and an icon appeared on their 'heads-up' display. The system displayed the areas that needed to be contained or avoided, and helped to plan egress routes and notify soldiers when the area was clear.

Further R&D will most likely see chem-bio sensors integrated into the smallest and most subtle of places, from an individual's clothing, to mobile phones. This will provide an instantaneous and automatic method of detection that can offer notifications of a chemical incident to the authorities, and may even combine GPS (global positioning system) to enable rapid location capabilities.

According to Frost & Sullivan, the biosensors market is expected to grow from $6.72 billion in 2009, to $14.42 billion in 2016 - driven largely by the biodefence and home diagnostic markets. However, it should be noted that in keeping with the diversity of the sensors market, a chemical sensor may only be deemed a 'biosensor' if it employs a biological element that detects chemicals (e.g. blood glucose testing, or screening for disease).

Chem-bio sensors add a new dimension to MEMS, in that they call for development of somewhat exotic microstructures, such as cylinders within cylinders or those that are semi-permeable. Moreover, the challenge of how to ensure they become pervasive is one the industry has still to address.


About Sherborne Sensors:
Sherborne Sensors is a global leader in the design, manufacture and supply of high-precision inclinometers, accelerometers, force transducers and load cells, rotary encoders, instrumentation and accessories for industrial, military and aerospace customers. Products offered under the Sherborne Sensors brand are renowned for their ultra-reliability and long-life precision within critical applications. Recent acquisition of synergistic technologies by Sherborne Sensors within our inclinometer and force and load cell offerings has allowed customers to benefit from expanded product lines, with added benefits of engineering support, global sales presence, repair, refurbishment and calibration services, stocking programs, and continuous product improvement.



For more information, please contact :-

Robin Butler
Sherborne Sensors Limited
1 Ringway Centre, Edison Road, Basingstoke RG21 6YH United Kingdom
Tel: +44 (0)1908 673 868
E-mail:
robin.butler@sherbornesensors.com
Web :
www.sherbornesensors.com

November 2011

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