Overview of MEMS - The Art of MEMS

MEMS stands for Micro Electromechanical Systems. MEMS technology is related to microelectronics technology and the integrated circuit industry - there is large overlap of materials and processing techniques. MEMS stands for a novel class of fabrication technology, as well as a class of devices and products. As fabrication technology, MEMS and micromachining can realize precision three dimensional micro structures. Important characteristics of a MEMS device include:

1. Small sizes: optical lithography is used to define features that are as small as 10 nm. Small sizes sometimes lead to favorable scaling of performance characteristics, such as resonant frequency, response speed, sensitivity, etc.
2. Rich performance: transduction blocks and circuits are combined to provide superior performance and signal quality.
3. Low cost: MEMS can potentially achieve very low cost due to bulk fabrication techniques. The cost advantage is especially obvious for large volume products such as consumer electronics (CE).

Famous MEMS products with large sales volume ($ 1 billion) include: 
- Ink jet printer head chip for dispensing tiny droplets of inks for printing documents, photos, and chemical reagents;
- Digital light processor (DLP) chips for large format TV and movie theaters; each chip consists of millions of tiny mirrors that can rotate to reflect light.
- Accelerometers for automotive crash sensing and air-bag deployment;
- Gyros (angular rotation rate sensors) for motion-sensing games such as Nintendo Wii and mobile phone gaming.

Applications and Market:

MEMS is a vibrant and sophisticated field. MEMS technology is used to make sensors (devices that measure physical and chemical characteristics), actuators (devices that produce displacement, force, or torque), or structures. Historically, MEMS technology has enabled such products as ink jet printing, automotive airbag sensors, automotive tire pressure sensors, medical pressure sensors, digital light processors, acoustic sensors, accelerometers, and gyroscopes. The market associated with MEMS OEM product is approximately $8b in 2010. MEMS applications in areas such as biomedical research, medicine and health, drug discovery, therapeutics, and optical communications remains strong. MEMS products is also entering other areas, such as cameras and cell phones (microphones, lens, display). Meanwhile, there is also a growing industry serving the needs of MEMS (equipment, design tools, packaging, foundry). With increasing ability to design and manufacture MEMS products, it is expected that the function, performance, and cost of emerging MEMS products will be competitive in many future applications areas. 

MEMS has been a technology searching for "killer applications".  Over the years, MEMS technology has found big applications in automotive industry, biomedical industry, printing and display industry, and portable electronics. Today, MEMS can be found in at least the following products:
Consumer electronics products (smart phones, iPhone, iPad, gaming devices (Wii)), touch displays, home printers, theaters (projectors), Automobiles (measuring pressure, collision, bearing), hospital equipments, patient monitoring (blood and cardiac pressure sensors), computer hard disk drives, communication devices, military equipment, industrial equipments (flow and pressure sensors), and robots.  Link to top ten famous MEMS products  

Fabrication and Manufacturing

Production of a MEMS device consists of two phases - phase 1 involves the fabrication of structures and elements on the silicon wafer, and phase 2 involves packaging and assembly of individual silicon chips. The assembled chip products are mounted on PC Boards for insertion into electronics systems.

Tour-de-force Case Study: MEMS Gyro

A tour-of-force demonstration of MEMS technology is today's 3-axis digital gyroscopes embedded in handheld electronics (such as iPhones). Needlessly to say, the sensor must have small size, since the electronics product itself is extremely lightweight and compact. The sensor must incorporate accurate three-axis rotation sensing for capturing motion. The chip itself must be small (example: less than 20 mm2). Due to the small sizes, the sensor must be able to pick up extremely small signals caused by rotation. Further, the cost of the sensor must be extremely low (~$1) to be competitive. MEMS products can satisfy such needs since processing is highly parallel and automated. Without such a sensor, iPhone and other smart sensors would lose a crucial interface device. The sensor must be designed such that the device's characteristics does not change with temperature and environmental moisture. The sensor must also compensate for linear acceleration. All these feats are results of integrated circuitry and logic. 

Apple iPhone 4G, released in 2010, employs a gyroscope chip manufactured as L3G4200D (STMicroelectronics). The STMicroelectronics product L3G4200D is made of silicon. The excitation and sensing are both based on electrostatics. The L3G4200D uses hybrid MEMS-MOS chip integration, as opposed to monolithic integration. The capacitive drive and sensing is based on comb drive design, with the combs made by Deep reactive ion etching poineered by Bosch Gmbh.The price is $ 8.92 for a minimal of 250 from US distributors (Sept 24, 2010). Both the accelerometer and the gyros produce digital output, therefore eliminating the need to have a digitization circuitry outside of the system. The same iPhone also uses a three-axis accelerometer (LIS302DL), also from STMicroelectronics. It is believed that using two chips from the same company in one product offers advantages both for STMicroelectronics and Apple. The price for LIS302DL is $3.96 for a quantity of 250 (lower price for large volume) from US distributor (9/24/2010). STMicroelectronics gains large volume of production, and can provide larger discount and uniform electronics interface to Apple engineers.

The MEMS commercial space is becoming increasingly competitives, with companies busy distinguishing their products with patented design, manufacturing, and packaging technologies. The three axis gyroscope is truly a high-tech marvel. As of September 2010, only one other company, InvenSense, offers three axis gyro products. The InvenSense ITG3200, a three-axis gyro, can be purchased for $8 for a quantity of 250 on the company's own website. However, InvenSense does not have product offerings in the accelerometer space. Analog Devices, a company that has pioneered the MEMS accelerometer product based on surface micromachining, does offer three-axis accelerometer.  The ADXL 345 device is quoted for $11.96 for an order of 250, and $3.7 for 10,000 (9/24/2010). However, Analog Devices does not offer three axis gyros (9/24/2010).  Read here for a detailed case perspective on MEMS motion sensors.

Three pillars of MEMS.

A strong MEMS developer must have three pillars of essential knowledge - design, materials, and fabrication process.  Simply put, materials determines the mechanical and electrical properties and, as a result, influence the design. Fabrication process provide different available geometries and materials combinations, and therefore affects the design. The materials and fabrication process are certainly intricately involved. (See a further discussion page on the three pillars of MEMS: design, materials, and fabrication process).

Many MEMS devices are multidisciplinary and multiphysics. A MEMS pressure sensor for patient care requires engineering and medical knowledge. To develop a low cost MEMS rate sensor requires knowledge of packaging, materials, circuitry, and mechanical design. The MEMS field is very exciting and yet challenging.

Overall, to successfully develop MEMS requires knowledge in electronics, mechanics, mechanical design, materials, microfabrication, and domain-specific application field know-how. The textook Foundations of MEMS introduces the three pillars of MEMS device development: design (modeling), materials, and fabrication.  These elements are interlinked and have complex relationship to the performance and cost of devices. 

Design of MEMS is increasingly accomplished with computational tools and models. Designing include physical simulation and mask layout. There are companies and products dedicated to these processes.

MEMS history milestones and important people

A lot of visionary, imaginative and hardworking people shaped the industry. This include academic researchers, industry pioneers, and funding advocates.

Academic pioneers (James Angell, Stanford/Richard Muller, UC Berkeley/Stephen Senturia, MIT/Ken Wise, Michigan/Roger Howe, Berkeley) and more 
Industry pioneers (Larry Hornbeck, TI DLP/Kurt Petersen, Janusz Brezyk, Joe Lemon Nova Sensors, Cepheid, and SiTime)/ Steve Nasiri, InvenSense) and more 
Funding supporters (Ken Gabriel, DARPA; Rajinder Khosla, NSF; Noel McDonald, University of California at Santa Barbara; )

MEMS commercialization milestones (Graphic history of MEMS)

1978 Hewlett Packard (HP) begins inkjet research in 1978
1987 Nova Sensors ships $5 blood pressure sensors.
1977-1993: Larry Hornbeck at Texas Instruments develops DLP display technology with team, with first shipment in 1993;
1987-1993: Richie Payne at Analog Devices, together with team, develops surface micromachined accelerometers, with first shipment to Saab in 1993;
2003-2009: InvenSense was founded to make three axis gyros by Steve Nasiri, with $100M revenue in 2009;
August 18, 2009: Knowles acoustics ships 1 billionth Sisonic MEMS microphone. (Shipment in 2002=0)

Challenges for the MEMS Field

Apart from normal challenges faced by almost any high tech industry (patents, technology, materials, design, investment), the MEMS field faces challenges that the IC industry does not need to be bothered.  These include:
1. MEMS devices and processes are non-standard.  Every product requires a different design, fabrication process, and manufacturing technique. As such, there is a fragmentation of the field that makes it difficult to realize the economy of scale. Hence MEMS for large volume CE products have been especially successful in recent years (since 2000).
2.An MEMS chip typically contains moving parts that must (1) not be disturbed by particles and contacts and (2) be kept under constant environment (humidity, temperature, pressure). These dictate that a MEMS chip must be encapsulated. The encapsulation process lowers the yield, is often proprietary, and contributes to the cost of MEMS devices.

However, there is no doubt that the ingenuity of engineers in the MEMS field is beating the odds on many fronts. Today MEMS systems are smallers, environmentally staple, and amendable to mass production. MEMS are now made in commercially viable foundries. Further, MEMS and CMOS circuits are integrated in more ways than before. Although MEMS is still 1/40th the size of the CMOS industry, its growth outpaces the circuit industry. Many circuit companies are now interested in integrating MEMS elements in their products. With the continued development and science discovery in portable consumer electronics, intelligent systems, medical and health care innovation, and automotive industry, there is a bright future for the MEMS field ahead.

Copyright 2010, MEMSCentral and Chang Liu