Bi-Directional Electrothermal Microactuator

A thermo-force-driven, bi-directional electrothermal actuator with tapered Z-shaped beams, designed and simulated for fabrication via the PolyMUMPs (Polysilicon Multi-User MEMS Process) three-layer surface-micromachining flow. The research introduced a tapering optimization to improve vertical shuttle displacement; electrothermal simulations were conducted in ANSYS and validated through analytical structural calculations in MATLAB.

Design approach. The actuator uses Z-shaped actuation beams clamped between bonding-pad anchors that also act as electrical contacts. When current passes through the beams, Joule heating drives thermal expansion. The Z-geometry converts in-plane expansion into vertical mechanical motion of a rigid shuttle. Unlike traditional V-shape actuators, the Z-shape avoids buckling failure modes and supports bi-directional operation.

Key dimensions:

• Anchors: 20 × 20 μm
• Shuttle: 200 × 30 μm
• Longer beam arm: 400 × 10 μm
• Short arm: 200 × 10 μm
• Z-shape transition region: 30 × 10 μm

Tapering innovation. Beams were tapered from anchors toward the shuttle to improve thermal distribution and heat flux. Four tapered-width cases were studied: 10 μm, 9 μm, 8 μm, and 7 μm. Gradual tapering ensures more uniform temperature distribution across the beam, raising the effective expansion and resulting shuttle displacement.

Simulation methodology. Electrothermal simulations were carried out using ANSYS Steady-State Thermal with:

• Internal heat generation from upper beams driven by ~10 mA current
• Convective boundary conditions on surfaces exposed to air
• Anchors maintained at room temperature
• Steady-state initial conditions

Material properties (polysilicon):

• Thermal conductivity: 34 W/m·K
• Specific heat: 678 J/kg·K
• Density: 2320 kg/m³
• Thermal expansion coefficient: 2.6 × 10⁻⁶ /K

Performance results. Across the four tapered cases, vertical shuttle displacement scaled with tapering:

• 10 μm width: 10.49 μm displacement
• 9 μm width: 11.37 μm displacement
• 8 μm width: 12.22 μm displacement
• 7 μm width: 13.03 μm displacement

Compared to equivalent V-shape micro actuators, the untapered Z-shape already delivered ~50% better displacement; gradual tapering further improved thermal distribution, heat flux, and effective shuttle travel.

Fabrication. Designed for the PolyMUMPs three-layer polysilicon surface-micromachining flow:

1. Substrate preparation on a 1 mm silicon wafer
2. 2 μm polysilicon deposited via CVD
3. Photolithography to define the Z-shape beam pattern
4. Polysilicon etch to form beams
5. Sacrificial layer deposition (support during fab)
6. Sacrificial-layer etch to release suspended beams + shuttle
7. Metal deposition for anchor electrical contacts
8. Final etch to allow free beam motion

Fabrication challenges include maintaining precise tapered-beam dimensions, ensuring proper release of the suspended structures, preventing stiction during release, and creating reliable electrical connections at the anchors.

Applications. Bi-directional motion opens applications across precision positioning for optical components, micromanipulation tools for biomedical use, RF MEMS switches with improved reliability, and micro-robotics / autonomous microsystems.