Dateline: Febuary 4, 2001
The miniaturization of mechanical devices (MicroElectroMechanical Systems - MEMS) is gaining increasing attention due to numerous commercial applications. Clocks, accelerometers, ink jet printer heads, color projection displays, scanning probe microscopy, pressure, temperature, chemical and vibration sensors, light reflectors, switches, vehicle control, pacemakers and data storage are only a few examples. Each of these applications uses MEMS in a unique way, but they are all based on fundamental micromechanical components.
Two fundamental components in MEMS devices are actuators and cantilevers. A mechanical actuator is a device that can convert an electrical signal into mechanical motion. For instance, if one applies a voltage to a quartz crystal, it will change its size in a very precise and predictable way. Such an actuator (known as a piezoelectric material) is the heart of a digital watch. A cantilever is basically a small diving board. Cantilevers (and diving boards) bend when pressure is applied to them and oscillate in a way similar to a spring when given the chance (e.g. if they are "driven" at their resonance frequency by an actuator). Such a micromechanical component is what makes atomic force microscopy (See Image) sensitive enough to see single molecules.
The miniaturization of electronic components is more established than for mechanical components. Fabricating smaller diodes and transistors allows more circuits to be integrated onto a single chip. Combinations of basic components such as diodes and transistors results in new, larger scale components such as a logic gate, which can carry out a single step of logical computation. Over the years, the integration of these logic gates has increased from "small-scale integration" (SSI, less than 10 gates) to medium-scale integration (MSI, 10-100 gates) to large-scale integration (LSI, 100-5000 gates) to today's very-large scale integration (VLSI, with more than 5000 gates).
The first steps towards integrating mechanical systems into a single chip have been taken by researchers at IBM. The device known as the 'Millipede' is an array of cantilevers integrated on a single chip. Each cantilever is sensitive enough to read and write data as nanometer scale indentations in a polymer film, giving it a capacity of 400-500 Gb/in2. While a single cantilever would be to slow to read all of the pits, integrating them significantly increases the speed, making the "Millipede" a realistic data storage device. Numerous variations on this theme can be conceived. For instance, researchers at Northwestern University have demonstrated that coating the cantilevers with ink allows one to write nanoscale patterns, and have named the method Dip Pen Nanolithography. Since this method can be operated in a parallel fashion, it may not be long before arrays of AFM cantilevers coated with ink begin to lay down nanoelectronic circuits.
Since these mechanical systems are controlled electronically, they could conceptually be added as "drop-in" components in current chips. Indeed, most micro-machines are fabricated by the same semiconductor fabrication methods used to make computer chips. Since these methods are approaching the nanoscale, it is reasonable to expect that the basic units of both electronic and mechanical integration will eventually reach atomic and molecular dimensions. Since atoms and molecules are the fundamental building blocks of all matter, we can expect revolutionary capabilities from such NanoElectroMechanical Systems (NEMS), also known as nanomachines.
Acknowledgements: Thank you George Baggs for explaining the levels of integration for electronic components.
