Microsystems

A convenient model of a microsystem is that of a control system (figure 1); many proposed microsystems take this form. Microsensors detect changes in the parameter to be controlled, electronic control logic then operates microactuators based on information from the sensors, to bring the parameter to be controlled within the desired limits.

Figure 1.

An example of such a system would be to refresh the medium in a small cell culture dish. Sensors could detect changes in pH, pO2, or pCO2, and a micropump could deliver new culture medium from a reservoir as required. Not all devices need follow this control-system scheme. For instance an accelerometer designed to inflate an air-bag in the event of a car crash may not only incorporate a micromachined acceleration sensor, but also electronics to condition the signal and detect a rapid deceleration, and microactuators that put a force on the sensor allowing the device to be tested before the driver moves off.

Microsystems may be constructed from parts produced using different technologies on different substrates, connected together; i.e. a hybrid system. For example, a silicon chip would be used to implement control circuitry, whereas the actuators it controlled could be micromoulded in plastic, or electroplated metal (using the LIGA technique, perhaps). Alternatively, all components of a system could be constructed on a single substrate using one technology (a monolithic system). Hybrid systems have the advantages that the most appropriate technology for each component can be selected to optimise system performance. This will often lead to a shorter development time since microfabrication techniques for each component may already exist, and compromises will not have to be made to ensure that each component can be fabricated without damaging components already existing on the substrate. Monolithic devices will typically be more compact than hybrid devices, and more reliable (fewer interconnections that can go wrong, for example). Further, once the fabrication process has been developed, they can be manufactured more cheaply since less assembly is required.

A significant problem facing microengineers is that of assembling many microscopic components. Potential solutions include self assembling systems, and desk-top factories staffed by microrobots!

At some point many microsystems will have to interact with macroscopic systems. Often it may be that only a critical component of a system has to be microengineered, and supported by a complex system produced using more conventional engineering techniques. It is easy to underestimate the problems involved in mounting and packaging microdevices, and integrating them with macroscopic supporting systems.

The remainder of this document goes on to introduce two components of microsystems; microsensors and microactuators. Microsystem "intelligence", in the form of microelectronics is a well developed subject, and covered in many textbooks. The document is rounded out with some comments on passivation, packaging, and assembly of microsystems. Micromachining techniques by which various components may be manufactured are introduced in the companion document: "Introduction to Microengineering" (see the contents page).