PE2-5.4 Microsystems Design

What this is and why it exists

Design here is unusual in one respect that matters more than any technique. The device and the process that makes it are designed together.

In circuit design you draw what you want and a foundry builds it. Here, what can be built constrains what you may draw, and far more tightly.

The vocabulary

• Process design — designing the sequence of fabrication steps, as a design activity of its own.
• Thermal budget — the total heating a wafer can receive before earlier steps are damaged.
• Stiffness — how much force is needed to deflect a structure by a given amount.
• Resonant frequency — the frequency at which a structure naturally vibrates.
• Finite element method — dividing a shape into small pieces and solving numerically.
• Mesh — the division of the shape into those pieces.
• Boundary condition — how the model says the structure is held and loaded.
• Piezoresistor — a resistor whose value changes when it is strained.
• Diaphragm — a thin membrane that deflects under pressure.

The mental model

Start with the constraint that shapes everything. Every process step limits the ones after it. Anything requiring high temperature must happen before the metal is deposited. Anything needing a flat surface must happen before a structure is released. A shape that cannot be etched cannot be drawn.

So the process sequence is designed alongside the device, and the thermal budget is one of the hard limits. That makes design here a negotiation between what you want and what the fabrication will allow, which is unfamiliar to anyone coming from circuit design.

Mechanical design is the part an electronics graduate finds least familiar and should spend most time on. Three quantities carry most of it.

Stiffness decides how far a structure moves for a given force, and so how sensitive the device is. A softer structure moves more and gives a larger signal. It is also more fragile and more easily disturbed by shock.

Resonance decides how fast the device can respond, and where it must not be operated. Every mechanical structure has frequencies at which it vibrates readily. Below resonance the device follows its input; near resonance it exaggerates; above it, it cannot keep up. Damping controls how sharp that peak is, and in a small device the surrounding air provides much of it.

Stress decides whether the structure survives. Stress concentrates where a shape changes abruptly, so a sharp inside corner is where a device breaks. Rounding those corners is one of the cheapest improvements available.

Analytical formulas exist for simple shapes such as a uniform beam or a circular diaphragm, and they are worth using for a first estimate. They run out quickly once the geometry is real, which is where numerical analysis earns its place. Divide the shape into many small elements, apply the loads and supports, and solve for the whole.

The important habit with that method is knowing what it assumes. It solves the model you gave it, not the device you intend to build. A wrong boundary condition, saying a beam is rigidly clamped when it is really on a compliant support, gives a confident and wrong answer. A mesh too coarse at a stress concentration underestimates the peak stress badly. So check a numerical result against a simple analytical case first, where you can compute the answer yourself.

The pressure sensor is the worked example that ties the module together. Following one device end to end is the point of the whole thing.

The principle is a thin diaphragm that deflects under pressure. Deflection produces strain in the diaphragm, and resistors built into the silicon change value when strained. Four of them in a bridge give a readable output that largely cancels temperature effects, exactly as in the sensor module.

Every design decision is then visible. The diaphragm's thickness and area set the sensitivity and the burst pressure, pulling against each other. The resistors go where the strain is largest, at the edge and at the centre. Their orientation matters too, because the effect depends on direction relative to the crystal. The cavity behind the diaphragm decides whether the sensor reads absolute pressure or a difference. And the process must produce that thickness repeatably, or the sensitivity varies from device to device.

What you should now be able to explain or do

Say why the process and the device are designed together, and give two examples of a process constraint. Relate stiffness, resonance and stress to sensitivity, speed and survival. Say what the finite element method assumes and name two ways it can be trusted wrongly. Describe a pressure sensor from principle to layout. Say what decides the diaphragm's dimensions and where the resistors go.

Check yourself