PE2-5.1 MEMS & Microsystems: Introduction

What this is and why it exists

Chip fabrication was invented to make circuits. This field turns the same processes to making machines: structures that bend, move, resonate or pump.

That one sentence is the definition, and holding it makes the rest of the subject coherent. Everything unusual here follows from the fact that something has to move.

The vocabulary

• MEMS — microelectromechanical systems. Mechanical structures made with chip processes.
• Microsystem — the complete device, including its electronics and packaging.
• Batch fabrication — making many devices at once on one wafer.
• Proof mass — a small mass whose movement carries the measurement.
• Stiction — two surfaces sticking together permanently after contact.
• Residual stress — stress locked into a deposited film, which distorts a structure once it is freed.
• Aspect ratio — the ratio of a feature's height to its width.

The mental model

The distinction from ordinary microelectronics is a small sentence with large consequences. In a chip, nothing moves except charge. Here, something moves physically.

That changes fabrication, because a structure has to be freed rather than left supported. It changes packaging, because a sealed box is often exactly wrong for a device that must feel pressure or be reached by a fluid. And it changes reliability, because moving parts wear, stick and fatigue in ways that circuits do not.

The multidisciplinary description of the field is real rather than promotional. You need mechanics for the structure, materials for the properties, electronics for the readout and often chemistry for the etching or the sensing. That is why the syllabus looks scattered. It is not scattered; it is what one device actually requires.

Miniaturisation pays for reasons worth naming rather than assuming.

Less material is used, so the cost per device is low. Batch fabrication means hundreds or thousands are made at once on one wafer, and the cost of the run is divided among all of them. Lower mass means faster response, because a small mass accelerates readily. And the readout electronics can sit on the same die, so the signal is amplified before it has a chance to pick up noise.

There are also two design problems that do not exist in ordinary chip design, and they ruin most first attempts.

Stiction is the first. At this scale surface forces dominate, so two surfaces brought into contact can stay in contact permanently. A structure that touches down during release, or during a shock, may never come back. Designs are shaped to prevent contact, and surfaces are treated to reduce adhesion.

Residual stress is the second. A film deposited on a substrate is usually left with internal stress. While the film is attached everywhere, nothing happens. Free it and it relaxes, so a beam curls upward or downward and a membrane buckles. The stress in a deposited layer is therefore a design parameter here, not a process detail.

The applications are the honest evidence for the field. Accelerometers, pressure sensors and inkjet printing heads ship in billions, and every phone contains several of these devices. That is a stronger case than any list of advantages.

What you should now be able to explain or do

State what distinguishes this field from ordinary microelectronics, and three consequences of that difference. Say why the subject draws on several fields at once. Name the four benefits of miniaturisation. Explain stiction and residual stress and say why each ruins a first design. Give applications that are shipping in volume.

Check yourself