MEMS' World - Thermal actuation

All solid materials are sensitive to the temperature variations. They have a tendancy to get larger, depending on what we call their thermal expansion.
We can use this property in MEMS devices design to actuate a device by generating heat with the Joule's law. The use of these phenomenons is hardly usable at macroscopic level. But on a few microns large actuator, everything is different, and thermal heating is both easy and efficient in terms of power consumption.
This page intends to provide an introduction to these thermal microactuators.

The Joule's law

The Joule's law tells that a material crossed by an electrical current will get some thermal energy. This is the result of the collision between the flowing electrons and the atoms making the material. So, using Joule's law is as easy as making currents flowing through what will be the heater!

Thermal expansion

The nature of heat is in the vibration of atoms and electrons making the material. The more an atom is vibrating, the more it is hot and will excite atoms around it.
Now consider all atoms from a material part are strongly vibrating. They will have a tendancy to push each other away to get enough place as their vibration amplitude is large. This means that the whole part will get bigger!
This phenomenon is the thermal expansion!
Most of time, thermal expansion is rather weak in solid states. Polymers have amongst the highest thermal expansion, metals are second, and ceramics are the less expansive materials. So the question is: How do we get a large displacement with a thermal excitation?

Amplifying thermal expansion

There are two ways of amplifying the rather small thermal expansion displacement to a usable displacement relative to the device size, and thus, making an efficient thermal actuator. The first one is purely geometric. The second one involves a more complicated fabrication (i talk about MEMS! See microfabrication page to understand what i mean by "more complicated"!).

Let's think about what we have, and what we need:

To make a large displacement microactuator, we must rather use the difference between two displacement rather than the absolute displacement. So our goal is to design mobile parts so that the same electrical current flow will make one expanding more than the other! You still don't see? Fortunately, people have found solutions for us before!!

Asymmetric thermal arm

If a current flowing through a structure meets different resistivity levels, the heating caculated with Joule's law will be higher where the resistance is higher. So, building structures with non constant width make different resistivity levels!

One of the most well known thermal actuator in the MEMS's field is the asymmetric arm:

Asymmetric arm microactuator moving due to current flowing through them
Schematics of the asymmetric arm. The thinnest arm get hotter than the large one while being heated, so the difference of thermal expansion makes the structure move to the right.

The principle is very simple: if the current flows through three resistive parts being the mobile part, with two long paths and one short linking them, current will flow through them with approximatively the same heat distribution. If one of the path is thinner than the other, it will become hotter than the two other ones! So its expansion will be higher, and you can easily imagine what happen! The thinner part create a mechanical force pushing the structure in the direction thin to large arms. This is an amplification of a thermal expansion, enough to be used for thermal actuator design!

Bilayers structures

The other way of getting such an amplification is in material properties.
As we've said, thermal expansion is a property particular to each material. So two different materials will have two different thermal expansion. There we are: if you heat a structure made of two different materials, and you heat it, the difference in thermal expansion will give a large displacement microactuator!

In microtechnology, it is very hard to make a composite structure in the plane, but it is common to have out-of-plane layers stack. So the bilayer structures are made of materials being deposited one on the other. If the bottom material have a higher thermal expansion than the top one, heating the whole structure will make it bend up! In the opposite case, the structure will have a tendancy to bend in the substrate direction.
The sample process fabrication is a bilayer thermal actuator!

Though the technique is very functional, there is another point to care about with bilayers structures: residual stress. This point is introduced in the solid mechanics section. Residual stress can make the structure being bent during releasing, so at neutral state. This is sometimes annoying, sometimes useful, depending on the targeted application.

In the following example, residual stress makes the microactuator naturally bending up at neutral state. If you heat the top part, its thermal expansion will make the whole cantilever bend down.

Thermally excited bilayer thermal actuator bending down
Schematics of a bilayer cantilever thermal actuator under heating.
The yellow part has a larger thermal expansion. Heating it make the whole cantilever bend down.

You can find real examples of thermal actuators in my PhD project section.