Materials Challenges for the MEMS Revolution

Reducing the size of an engineered system is a challenge often faced by designers and nowhere is this more evident than in the field of electronics. The first digital computer known as the Electronic Numerical Intergrator and Computer (ENIAC) was an enormous system constructed from over 18,000 vacuum tubes, 1,500 relays, and hundreds of thousands of resistors, capacitors, and inductors. ENIAC weighed 30 tons and required 1800 ft of floor space, all to provide a computational capability that was rapidly eclipsed by personal computers and even hand-held calculators. Soon after ENIAC was completed, researchers began to investigate ways to build computers with smaller, faster, and more capable devices. Computer speeds increased markedly but they were still dependent upon components that were relatively large and bulky. The invention of the integrated circuit and the utilization of lithography enabled numerous discrete devices to be combined and interconnected with one another on a single chip. These technologies also brought about true mass production on a scale not seen before, dramatically lowering the unit cost of devices. Lithographic processes utilize a printed mask that locally protects the conductive top layer of the circuit board or device. The board or device with the mask applied to its surface is then immersed in an acid that attacks and removes (etches) all surfaces not protected by the mask. The mask is later removed with a suitable solvent. The result of this etching process is a network of electrically conductive traces that form the circuitry formerly provided by wires. As a result, electronic equipment has generally became smaller and faster, and requires far less time to manufacture. Lithographic techniques have improved dramatically over the past forty years. For instance, during the late 60’s, linewidths of 5 microns were being produced experimentally. Today’s linewidths are over 90% smaller, dramatically reducing the size of electronic devices. Additionally, many different types of materials can be etched, not just the conductive top layer of a circuit board. Researchers now realize that the same processes used to fabricate electronic devices may also be employed to miniaturize and mass produce mechanical systems. By integrating these miniature mechanical devices with existing electronic capabilities on the same chip, a new class of devices known as MicroElectroMechanical systems or MEMS is emerging.