PE2-4.4 Micro Sensors

What this is and why it exists

Make a sensor small enough and it can be built on a chip, in quantity, for very little. That is why a phone knows which way up it is.

Something else changes on the way down, and it is the theme of this topic. The physics does not scale evenly.

The vocabulary

• Micro sensor — a sensor with features measured in micrometres, built by chip processes.
• Proof mass — the small mass whose movement a micro accelerometer measures.
• Capacitive readout — sensing movement as a change in capacitance between fixed and moving parts.
• Surface force — a force that scales with area, such as friction or surface tension.
• Volume force — a force that scales with volume, such as weight or inertia.
• Selectivity — responding to the wanted substance and not to everything else.
• Microfluidics — moving and measuring fluids in channels of microscopic width.

The mental model

Start with the scaling argument, because everything else follows from it.

Take a device and shrink every dimension by a factor of ten. Areas fall by a hundred. Volumes fall by a thousand. So forces depending on volume, such as weight and inertia, fall far faster. Forces depending on area, such as friction, adhesion and surface tension, do not.

The consequence is that the world at small scale is dominated by different forces. Gravity becomes almost irrelevant and stickiness becomes a serious design problem. Two polished surfaces that touch may not come apart. Some principles that work well at ordinary sizes stop working at all, and others that are useless at ordinary sizes become excellent.

That is why a micro sensor is not merely a small sensor. It is often a different design reaching the same measurement.

The accelerometer is the example worth understanding in detail, and it is the seismic transducer of the previous topic made microscopic. A small proof mass is suspended on thin springs etched from the same silicon. Accelerate the whole device and the mass lags behind, so its position relative to the frame changes.

That movement is read as a capacitance. Fingers on the moving mass interleave with fixed fingers, forming capacitors. When the mass moves, one gap narrows and the other widens, so one capacitance rises while the other falls. Measuring the difference gives the displacement, and it cancels effects that change both.

The whole device is a few hundred micrometres across and is made by the same processes as a chip. Its electronics sit beside it on the same die. That is why it costs almost nothing and appears in every phone.

Force and pressure sensors shrink in the same way. They become a thin membrane whose deflection is read capacitively, or by strain gauges built into it. Position and speed sensors let a mechanism be instrumented without adding meaningful mass. That is part of the point, because a large sensor changes what it is attached to.

Chemical and biological sensors are where the field meets medicine, and they have a harder problem than the others. Detecting a substance is not difficult. Detecting only that substance, in a sample full of other things, is. The selectivity problem is harder than the sensing problem, and that is what limits the field rather than any question of sensitivity.

Temperature sensors at this scale are straightforward and very widely integrated. They are frequently on the same die as the sensor they are correcting, which is the neatest way to compensate for drift.

Flow sensors in channels a hair's width across close the topic. Microfluidics needs them, and it is the application area growing fastest. Moving very small volumes of liquid accurately is what a laboratory on a chip requires.

What you should now be able to explain or do

State the scaling argument and say why surface forces come to dominate. Explain why a micro sensor is often a different design rather than a smaller one. Describe a micro accelerometer completely, including how its movement is read. Say why differential capacitive readout is used. Explain why selectivity, not sensitivity, limits chemical and biological micro sensors.

Check yourself