Micro-Electro-Mechanical Systems

40

Micro-Electro-Mechanical Systems

More than meets the eye

Micro-electro-mechanical systems (MEMS) or Microsystems

tech-nology (MST) is a fascinating and exciting field which significantly

contributes to build a bridge between science and society. Physical

properties and material characteristics are translated into structures

and devices that can have a large positive impact on people’s lives.

Silicon micromachining is largely responsible for the expansion of

sensors and actuators into more complex systems and into areas

not traditionally related to microelectronics, such as medicine,

biol-ogy and transportation. The shift to 3D microstructures has not

only added a physical third dimension to silicon planar technology,

but it also added a third dimension in terms of functionality and

applications.

Author: Prof.dr.ir. Pasqualina M. Sarro, TU Delft

MEMS Technology

MEMS or Microsystems Technology gen-erally refers to design, technology and fab-rication efforts aimed at combining elec-tronic functions with mechanical, optical, thermal and others and that employ min-iaturisation in order to achieve high com-plexity in a small space. Microsystems are thus intelligent microscale machines that combine sensors and actuators, me-chanical structures and electronics to sense information from the environment and react to it. These tiny systems are or will soon be present in many industrial and consumer products and will have a huge impact on the way we live, play and work.

This technology has experienced about two decades of evolution, mainly driven by a few key applications. It is likely to drive the next phase of the information revolution, as microelectronics has driven the first phase.

The continuous advances in silicon-based integrated circuit (IC) technology, both in term of processing and equipment, have definitely contributed to the microsys-tems developments. At the same time the enormous growth in microsystems appli-cations has stimulated the development of dedicated equipment and generated a larger knowledge of material and struc-ture characteristics, especially in the me-chanical area, which have been of great help to the IC world.

Microsystems technology has a strong multidisciplinary character. Integration across several disciplines takes place. Next to physics and engineering, the ba-sic disciplines of microelectronics, we find that chemistry and biology are becoming more and more relevant as new materials and phenomena play a major role in the development of new microsystems. As movable or flexible parts are often essen-tial components of the system, the role of mechanics or rather micromechanics

is by far larger than it ever was in con-ventional microelectronics. Although the broad range of expertise and know-how this field requires might make the path to problem solving and product develop-ment more difficult, it can also be seen as enrichment in the engineering world.

Miniaturisation

Another key aspect is miniaturisation. Miniaturisation is necessary to achieve an increased functionality on a small scale; to utilize particular effects and phenom-ena that are of no specific relevance at the macroscale level; to increase performance in order to make new areas of application possible and to interface the nanoworld. A key process is the three-dimensional (3D) machining of semiconductor mate-rials, leading to the miniaturised struc-tures constituting the sensing, actuating or other functional parts. The main pro-cesses are bulk micromachining and sur-face micromachining.

Bulk micromachining covers all tech-niques that remove significant amounts of the substrate (or bulk) material and the bulk is part of the micromachined structure. This microstructuring of the substrate is often done to form structures that can physically move, such as floating membranes or cantilever beams. Other types of structures that can be realised by bulk micromachining are wafer-through holes, often used for through wafers in-terconnects in chip stacks, and very deep cavities or channels to form microwells or reservoirs for biochemical applications. The substrate (generally silicon) can be removed using a variety of methods and techniques. In addition to a number of processes using wet (or liquid) etchants, techniques using etchants in vapor and

Maxwell11.2 March 2008

41

plasma state (generally referred to as dry) are available.

Surface micromachining is a quite differ-ent technology, which involves the depo-sition of thin films on the wafer surface and the selective removal of one or more of these layers to leave freestanding struc-tures, such as membranes, cantilever beams, bridges and rotating structures. The basic principle of surface microma-chining shows two types of layers, the sacrificial layer and the mechanical layer. The sacrificial layer is removed during subsequent processing to leave the free-standing mechanical structure.

Other processes or techniques relevant to microsystems fabrication are wafer-to-wa-fer bonding techniques, 3D-lithography and some other high aspect ratio tech-niques like LIGA and Laser Machining. The choice of the 3D micromachining technology depends strongly on the appli-cation field and design of the product to be manufactured. The use of techniques compatible with standard semiconduc-tor processing is often preferred as this enables batch fabrication, potential cost efficiency and system integration. This is also the approached followed at DIMES, the Delft Institute of Microsystems and Nanoelectronics of the Delft University.

The power of a small world

There are several examples that illustrate the “power of the small world”, i.e. the re-alisation of applications, components and systems that would not be available, or would not be as functional, as light, or as small as they are now thanks to MEMS. An area where microsystems technology has made a big difference is transporta-tion. Not only vehicles are equipped with an increasing amount of MEMS devices, creating smart cars, planes, boats; but this technology is also used in efforts to monitor highways, roads and bridges, en-abling smart roads and safer skies.

How-ever, it has been the automotive industry, thus the car, which has played a key role in promoting this technology and devel-opment. One of the biggest commercial applications is the accelerometer for air bags. We are observing a complete tran-sition from the mechanically driven au-tomobile system to a mechanically based but ICT-driven system. Microsystems technology is indispensable for fulfilling this ambition.

Biomedical sensors enable the reliable generation of essential physiologic infor-mation required to provide therapeutic, di-agnostic and monitoring care to patients. One example related to chronic cardiac disease is the pacemaker. This small ap-paratus, which has been implanted in more than two million people worldwide and a true life saver, is based on a sensing and actuating unit, signal processing and a power source. In the newest generation of pacemaker-defibrillators a MEMS

_Q

ExAMPLES oF MicRoMAcHiNED STRUcTURES

42

accelerometer capable of tracking change in motion, in this case the heart beat, sends the information to the processing unit and the proper amount of electrical shock to restore the natural rhythm of the heart is delivered through the pacing lead.

Microsurgery or minimally invasive sur-gery, an almost unknown notion a few years ago, is also an area where progress has been remarkable, and tangible results are seen everyday. Smart medical devices can be realised by embedding MEMS-based sensors at the most effective place on or within the device.

MEMS can also be quite relevant to wire-less communication by providing more function and power with smaller parts. Microsystems for RF applications, also known as RF MEMS, cover a large variety of devices, such as micro-switches, tun-able capacitors, micromachined induc-tors, micromachined antennas, micro-transmission lines and micro-mechanical resonators. Manufactured by conventional or novel 3D microstructuring techniques, they offer increased performance, such as lower power consumption, lower losses, higher linearity and higher Q factors.

Autonomous microsystems:

more intelligence in a small

space

What are the challenges that lie ahead and which are the areas that the MEMS community will focus on? It is clear that the future generation of microsystems will have to satisfy quite challenging de-mands. These systems have to be capable of self-regulation and wireless commu-nication, should be compact in size and operate at low power, often in harsh en-vironments.

An integrated autonomous microsystem needs to contain several basic functional modules to interact with its environment. It should be able to sense the

perturba-tion in an environment (hearing or sight), as well as to actuate perturbation to the environment for response (motion). It also needs to communicate with other microsystems and with a central point to establish collective and coordinated func-tions. Many of these functions can be po-tentially realised by integrating memory/ microprocessors with MEMS in a power-efficient manner (system-on-a-chip). The effective three-dimensional microstruc-turing offered by MEMS together with the introduction of new materials into micro-electronics will be essential for this future generation of integrated microsystems. Applications can include any measure-ment requiremeasure-ments in remote locations or where access is difficult, from monitor-ing weather patterns to trackmonitor-ing human movement. Next to space applications, where reliability, power consumption and size are very critical, a large applica-tion area can be envisioned in implant-able sensors for medical applications and home monitoring systems for the elderly. A more generic discussion addresses the approach to follow in order to pursue fur-ther miniaturisation: Top down or Bot-tom up? The Top down method focuses on downscaling to miniaturise devices and systems to the nanometer scale. In order to achieve this, novel micromachin-ing technologies are required. The Bot-tom up approach relies on rather different technologies and materials to generate novel miniature systems. Atoms and molecules are integrated to form devices, shaping the system atom by atom. In-spired by biology, where the precise (self) organisation of molecules enables the many functions carried out in living cells, scientists have started to explore means to build molecular machines, wires and other devices in a very precise manner, by stacking individual atoms or molecules These two approaches can also be com-bined to create a new converging technol-ogy, thus further increasing future

per-spectives. We are definitely at the point where both approaches can be used to cre-ate new systems, devices and even new materials.

Finally, although remaining on the micron or submicron scale, MEMS will continue to be of significant importance for nano-technology and nanoscience. In fact, mi-crosensors, microactuators or mechanical microstructures are often the unavoidable bridge between the atom or molecular structure and the macro-world. Moreover specific tools that are indispensable for building molecular systems or investigat-ing phenomena at the atomic level heav-ily rely on MEMS technology.

conclusions

MEMS research activities continue to fo-cus on innovative technological processes and material findings to create the nec-essary environment to address the many challenges in areas presently recognised to be of strategic importance and of high so-cietal relevance. In particular, increasing the autonomous and wireless character of microsystems will be intensively pur-sued. Mostly a top down approach will be followed to realise the truly 3D micro-structures, essential to many microsys-tems. Research on silicon related/com-patible materials such as SiC, polymers and alternative metals to further expand application areas or improve system per-formance will be carried out as well. The potential of the bottom-up approach will be increasingly explored as it can offer more possibilities and proper solutions in new areas as well as stimulate new devel-opments.

Whatever the approach followed or the ap-plication area targeted, research in MEMS technology will continue to provide fur-ther developments in many scientific dis-ciplines and at the same time to give a tangible contribution to society.

_A