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Background: To achieve high performance Micro Electro Mechanical Systems (MEMS) it is important to overcome the drawbacks of the planar nature of standard IC and micromachining fabrication techniques. Therefore a variety of techniques based on out-of-plane rotation or folding of silicon microstructures (like Japanese Origami) have been proposed during the last ten years. An overview of such techniques is presented in the Lic. Eng. thesis by Thorbjörn Ebefors. The main drawbacks of most of these techniques are that they results in relatively fragile structures which requires time and cost consuming manual assembling steps.

To achieve self-assembled 3-D silicon structures useful both for dynamic and static application (such as 3-D sensors) we have developed a novel and simple technology for making robust, small radius micro-joints (or micro-hinges) based on polyimide. Polyimide is an excellent micro-joint material because of its flexibility (Youngs modulus, E=2-3 GPa), high thermal shrinkage (up to 60%) and high thermal expansion (effective approx. =140 ppm/°C).

The polyimide joint principle – Static mode (for 3-D sensors)

The principle for the static mode of a four V-groove polyimide joint.

The technique for bending 3-D structures out of the wafer plane is based on thermal shrinkage of polyimide in V-grooves, as illustrated in the figure bellow. The absolute contraction length of the polyimide is larger at the top of the V-groove than at the bottom ( · a > · b) which results in a rotation of the free standing structure out of the wafer plane. Different curing temperatures gives different polyimide shrinkage and different static bending angles, . One therefore has a large freedom to chose bending angle by varying the number of V-grooves in the joint and/or by varying the curing temperature.

The static mode of the polyimide joint has been used to realize miniaturized three dimensional triple hot-wire sensors for measurement in turbulent gas flows.

To keep the out-of plane rotated structure in place we also use a self-assembling interlocking mechanism as shown the figure bellow. It is not necessary to use an interlocking arrangement but it gives a simpler fabrication which improves the accuracy in the bending angle.

A self-assembled interlocking mechanism.

Dynamic mode (for actuators)

The dynamic behavior of a four V-groove polyimide joint. The movement is obtained by electrical heating and thermal expansion (·T) of the polyimide which is larger at the top of the V-groove than at the bottom (a>b).

The rather high thermal expansion of the polyimide together with a current through an integrated heater in the joint is used to achieve a dynamic movement of the structure. By use of a current through an integrated heater local power dissipation in the joint is achieved which makes it possible to control the bending angle of each individual structure. The figure bellow illustrates the dynamic mode of the polyimide joint.

The dynamic mode of the polyimide joint has been used to realize different kinds of micro-motion systems (e.g micro-conveyers and walking micro-robots).

Project sponsor

• wedish Research Council for Engineering Sciences (TFR) and Swedish Foundation for Strategic Research (SSF)

Project members

Thorbjörn Ebefors

Edvard Kälvesten

Göran Stemme