November/December 2011
Web Exclusive

Making Sense of MEMS

By Jesse Bonfeld

web ex1Micro Electro Mechanical Systems (MEMS) describe a type of device or sensor as well as a manufacturing process. MEMS sensors incorporate tiny devices with miniaturized mechanical structures, typically ranging from 1-100 μm (about the thickness of a human hair), while 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 forefront in recent years with the automotive industry's wide-scale adoption of MEMS sensors and the growing use of accelerometers and gyroscopes in consumer electronics. Perhaps the most well-known consumer electronics incorporating MEMS motion sensors are leading smart phones, digital cameras, and gaming consoles/controllers.

Micromachines on the rise

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. This process, known as bulk micromachining, was commercialized during the late 1970s and early 1980s, but a number of other etching and micromachining concepts and techniques have since been developed.

webex2

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/defense 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, using standardized processes, when in actual fact, they remain largely a one-product, one-process business. A number of companies develop and produce MEMS devices themselves and are defined as integrated device manufacturers (IDMs), 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.

Shape of sensors to come

Advances in MEMS technologies and techniques mean manufacturers are now able to produce capable MEMS sensors and devices, but quite a few of these sensors and devices cannot be installed directly into an end application because they cannot survive the rigors of final assembly. Conversely, conventional sensors can survive just about any assembly process and any application, but they are often perceived as being too big and too expensive. Therefore, the challenge for the manufacturers of MEMS sensors for use 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 specialist manufacturers moving forward must envision and understand to realize 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 end users can use directly.

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 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 chemical-biological (chem-bio) sensors. Governments worldwide have been investing heavily in R&D, driven primarily by the heightened threat a chemical or biological attack poses. Chem-bio sensors respond to changes in their chemical/biological environment and convert this response to 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, Calif., officials recently proposed to regulate the sensors on its buildings in order to detect such agents, and last year, the U.S. Army demonstrated the feasibility of a sensor network to improve situational awareness and reaction time in the field during chemical or biological incidents.

The U.S. 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 center 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 his 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 it may even combine a global positioning system (GPS) 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 biodefense and home diagnostic markets. Remember, however, 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 (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 THE AUTHOR

Jesse Bonfeld is the vice president of business development for Sherborne Sensors and manages the company's activities in North America. Bonfeld has more than 27 years of experience in the semiconductor, medical, aerospace and defense industries. Bonfeld holds Bachelor of Science degrees in Biochemistry and Chemical Engineering from the University of Arizona.

MEMS fabrication techniques

Jesse Bonfeld of Sherborne Sensors provides a summary of the common techniques used in Micro Electro Mechanical Systems (MEMS) fabrication.

  • Bulk micromachining - whereby the bulk of the silicon (Si) substrate is etched away to leave behind the desired micromechanical elements
  • Wafer bonding - permits an Si substrate (aka 'wafer') to be attached to another substrate, typically Si or glass, to construct more complex 3D microstructures, such as microvalves and micropumps
  • Surface micromachining - where the structures are built on top of the substrate and not inside of it, enabling fabrication of multi-component, integrated micromechanical structures not possible using bulk micromachining
  • Micromolding - a process using molds to define the deposition of the structural layer, and enabling the manufacture of high aspect ratio 3D microstructures in a variety of materials, such as ceramics, glasses, metals, and polymers
  • LIGA - a micromolding process that combines extremely thick-film resists (>1 mm thick) and high-energy x-ray lithography, to enable the manufacture of high aspect ratio 3D microstructures in a wide variety of materials
  • High aspect ratio micromachining (HAR) - combines aspects of surface and bulk micromachining to allow for silicon structures with extremely high aspect ratios through thick layers of silicon (hundreds of nanometers, up to hundreds of micrometers)

Sensor types

  • An electronic sensor is any device that uses electricity to sense a change in something, and then through a voltage change, sends a signal to a device that captures this information (e.g., a thermostat that controls heat). 
  • A mechanical sensor measures a property through mechanical means, although the measurement itself may be collected electronically. An example of a mechanical sensor is the common thermometer, which operates via a fluid that expands and contracts depending on the temperature it is exposed to.
  • A microactuator is a device that accepts a data signal as an input and then performs an action based on that signal as an output.
  • MEMS devices (also known as 'microsystems' in Europe and 'micromachines' in Japan) can encompass any product incorporating a component manufactured using the MEMS process (and might comprise thousands of MEMS elements).
  • A MEMS sensor is manufactured using a MEMS process and measures a physical or electrical input. It combines electrical and mechanical components into or on top of a single chip (i.e., it is an electro-mechanical sensor) whereby electrical elements on the chip process data, while the mechanical elements act in response to that data.
  • A microsensor is a sensor that has at least one physical dimension at the sub-millimetre level. Not all microsensors, however, incorporate MEMS components, a good example being bio-chemical (bio-chem) microsensors.