MEMS Technology

Knowles' Micro Electro-Mechanical Systems (MEMS) technology platform brings you smaller product design possibilities and related cost advantages, including silicon-based, surface mount manufacturing.

Traditional uses of silicon involve creating pathways for electricity within components such as integrated circuits. In contrast, MEMS transforms silicon into mechanically moving parts. During the past decade, this process has become useful in an increasing number of industries. For example, the automotive market uses MEMS accelerometers to sense crashes and deploy airbags.

Our research in MEMS led to the introduction of the world's first surface mount microphone in 2001 to 2002, the SiSonic™ series. We have since helped major brands design breakthrough cell phone and consumer electronics products, while eliminating the cost of traditional offline ECM microphone subassemblies. 

MicroElectroMechanical Systems (MEMS) and Microrobotics research spans design methodologies, physical investigations and manufacturing techniques involving various microsensors, microactuators and other microsystems. Selected applications include inertial sensor suites for control and guidance, miniature wall-climbing robots using micro/nano-fiber adhesives, arrayed MEMS probe manipulators for tip-based nanomanufacturing, gas chemical sensor arrays for early warning systems, and ultra-compliant neural probes for brain-computer interfaces.

MEMS and Microrobotics RI faculty have developed strong multidisciplinary research programs on integrated MEMS design and fabrication (Fedder), micro and nanorobotic systems (Sitti), and implantable medical microsystems (Fedder, Weiss). This research links the RI to many departments throughout the University through active collaborations with faculty from across CIT and MCS as well as with Pitt and UPMC. These RI faculty members also have joint appointments in other entities. Expanded research in micro-inertial navigation technologies (Kelly and Fedder) is also planned to target new DOD initiatives.

The future for research in MEMS and microrobotics remains bright, with trends toward use of the technology to access and manipulate nano-scale phenomena as well as to enable emerging biomedical applications. RI faculty are taking leads in these areas in collaboration with faculty throughout the University. One of the challenges faced is to merge systems across macro-, micro-, and nano-scales. CMU's planned Nano/Bio/Energy Building (2014 expected date of completion) for nano- and biotechnologies will facilitate efforts in this area by providing state-of-the-art shared infrastructure for nano/microfabrication. In addition, a budding collaboration in multi-scale factory automation (Hollis, Fedder) is leveraging renewed efforts in reconfigurable, high-precision minifactories (Hollis).

There are several different broad categories of MEMS technologies, outlined below:

Bulk Micromachining:

Bulk micromachining is a fabrication technique which builds mechanical elements by starting with a silicon wafer, and then etching away unwanted parts, and being left with useful mechanical devices.  Typically, the wafer is photo patterned, leaving a protective layer on the parts of the wafer that you want to keep. The wafer is then submersed into a liquid etchant, like potassium hydroxide, which eats away any exposed silicon.  This is a relatively simple and inexpensive fabrication technology, and is well suited for applications which do not require much complexity, and which are price sensitive.

Today, almost all pressure sensors are built with Bulk Micromachining.  Bulk Micromachined pressure sensors offer several advantages over traditional pressure sensors.  They cost less, are highly reliable, manufacturable, and there is very good repeatability between devices.

All new cars on the market today have several micromachined pressure sensors, typically used to measure manifold pressure in the engine.

The small size and high reliability of micromachined pressure sensors make them ideal for a variety of medical applications as well.

Surface Micromachining:

While Bulk micromachining creates devices by etching into a wafer, Surface Micromachining builds devices up from the wafer layer-by-layer.

A typical Surface Micromachining process is a repetitive sequence of depositing thin films on a wafer, photopatterning the films, and then etching the patterns into the films. In order to create moving, functioning machines, these layers are alternating thin films of a structural material (typically silicon) and a sacrificial material (typically silicon dioxide).  The structural material will form the mechanical elements, and the sacrificial material creates the gaps and spaces between the mechanical elements. At the end of the process, the sacrificial material is removed, and the  structural elements are left free to move and function.

For the case of the structural level being silicon, and the sacrificial material being silicon dioxide, the final "release" process is performed by placing the wafer in Hydrofluoric Acid. The Hydrofluoric Acid quickly etches away the silicon dioxide, while leaving the silicon undisturbed.

The wafers are typically then sawn into individual chips, and the chips packaged in an appropriate manner for the given application.

Surface Micromachining requires more fabrication steps than Bulk Micromachining, and hence is more expensive.  It is able to create much more complicated devices, capable of sophisticated functionality. Surface Micromachining is suitable for applications requiring more sophisticated mechanical elements.

LIGA:

LIGA is a technology which creates small, but relatively high aspect ratio devices using x-ray lithography. The process typically starts with a sheet of PMMA. The PMMA is covered with a photomask, and then exposed to high energy x-rays.  The mask allows parts of the PMMA to be exposed to the x-rays, while protecting other parts.  The PMMA is then placed in a suitable etchant to remove the exposed areas, resulting in extremely precise, microscopic mechanical elements.

LIGA is a relatively inexpensive fabrication technology, and suitable for applications requiring higher aspect ratio devices than what is achievable in Surface Micromachining.

Deep Reactive Ion Etching:

Deep reactive ion etching is a type of Bulk Micromachining which etches mechanical elements into a silicon wafer. Unlike traditional Bulk Micromachining, which uses a wet chemical etch, Deep Reactive Ion Etching micromachining uses  a plasma etch to create features.  This allows greater flexibility in the etch profiles, enabling a wider array of mechanical elements. The fabrication tools needed to perform Deep Reactive Ion etching are somewhat expensive, to this technology is typically more expensive than traditional Bulk Micromachining based on wet etching.

Integrated MEMS Technologies

Since MEMS devices are created with the same tools used to create integrated circuits, in some cases it is actually possible to fabricate Micromachines and Microelectronics on the same piece of silicon.  Fabricating machines and transistors side by side enables machines that can have intelligence.  A number of exciting products are already taking advantage of this capability.

The micro mechanical device embedded with electronics/electrical system fabricated through a mix of integrated circuit manufacturing and micro-machining process where material is shaped by etching away micro layers is called Micro Electro Mechanical System (MEMS). The intelligent electronic system part is integrated in the same way of IC device fabrication. The most popular material used for MEMS is Silicon for it's semiconductor , physical and commercial properties.

Micro-Electro-Mechanical Systems consists of mechanical elements, sensors, actuators, and electrical and electronics devices on a common silicon substrate.

The sensors in MEMS gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. The advantages of semiconductor IC manufacturing such as low cost mass production, reliability are also integral to MEMS devices.
The size of MEMS sub-components is in the range of 1 to 100 micrometers and the size of MEMS device itself measure in the range of 20 micrometers to a millimeter.

Some of the advantages of MEMS devices are,

1. Very small size, mass, volume
2. Very low power consumption
3. Low cost
4. Easy to integrate into systems or modify
5. Small thermal constant
6. Can be highly resistant to vibration, shock and radiation
7. Batch fabricated in large arrays
8. Improved thermal expansion tolerance
9. Parallelism

Typical Applications:

There are plenty of applications for MEMS. As a breakthrough technology, MEMS is building synergy between previously unrelated fields such as biology and microelectronics, many new MEMS and Nanotechnology applications will emerge, expanding beyond that which is currently identified or known.

MEMS technology finds applications in the below general domains

Automotive domain:
1. Airbag Systems
2. Vehicle Security Systems
3. Intertial Brake Lights
4. Headlight Leveling
5. Rollover Detection
6. Automatic Door Locks
7. Active Suspension

Consumer domain:
1. Appliances
2. Sports Training Devices
3. Computer Peripherals
4. Car and Personal Navigation Devices
5. Active Subwoofers

Industrial domain:
1. Earthquake Detection and Gas Shutoff
2. Machine Health
3. Shock and Tilt Sensing

Military:
1. Tanks
2. Planes
3. Equipment for Soldiers

Biotechnology:

1. Polymerase Chain Reaction (PCR) microsystems for DNA amplification and identification
2. Micromachined Scanning Tunneling Microscopes (STMs)
3. Biochips for detection of hazardous chemical and biological agents
4. Microsystems for high-throughput drug screening and selection
5. Bio-MEMS in medical and health related technologies from Lab-On-Chip to biosensor & chemosensor.

The commercial applications include:

1. Inkjet printers, which use piezo-electrics or thermal bubble ejection to deposit ink on paper.
2. Accelerometers in modern cars for a large number of purposes including airbag deployment in    collisions.
3. Accelerometers in consumer electronics devices such as game controllers, personal media players     / cell phones and a number of Digital Cameras.
4. In PCs to park the hard disk head when free-fall is detected, to prevent damage and data loss.
5. MEMS gyroscopes used in modern cars and other applications to detect yaw; e.g. to deploy a roll     over bar or trigger dynamic stability control.
6. Silicon pressure sensors e.g. car tire pressure sensors, and disposable blood pressure sensors.
7. Displays e.g. the DMD chip in a projector based on DLP technology has on its surface several      hundred thousand micromirrors.
8. Optical switching technology, which is, used for switching technology and alignment for data      communications.
9. Interferometric modulator display (IMOD) applications in consumer electronics (primarily displays for     mobile devices).
10. Improved performance from inductors and capacitors due the advent of the RF-MEMS technology

MEMS devices:

Few examples of real MEMS products are,

1. Adaptive Optics for Ophthalmic Applications
2. Optical Cross Connects
3. Air Bag Accelerometers
4. Pressure Sensors
5. Mirror Arrays for Televisions and Displays
6. High Performance Steerable Micromirrors
7. RF MEMS Devices
8. Disposable Medical Devices
9. High Force, High Displacement Electrostatic Actuators
10. MEMS Devices for Secure Communications

MEMS devices used in Space exploration field include:

1. Accelerometers and gyroscopes for inertial navigation
2. Pressure sensors
3. RF switches and tunable filters for communication
4. Tunable mirror arrays for adaptive optics
5. Micro-power sources and turbines
6. Propulsion and attitude control
7. Bio-reactors and Bio-sensors, Microfluidics
8. Thermal control
9. Atomic clocks