Surface Micromachining

Transcription

1 Surface Micromachining

2 Outline Introduction Material often used in surface micromachining Material selection criteria in surface micromachining Case study: Fabrication of electrostatic motor Major issues in sacrificial layer etching Accelerating the etching process Anti-stiction methods 3D assembly of surface-micromachined structures Surface micromachining foundry process: MUMPS

3 Introduction Surface micromachining creates movable (vertical, horizontal or rotational) thin-film microstructures that reside near and above the surfaces of a substrate.

4 Sacrificial Layer Etching Process Sacrificial layer etching is the core of surface micromachining. Sacrificial layer etching utilizes lateral etching to completely undercut the sacrificial layer underneath the structure layer. The structure layer then can have horizontal, vertical or rotational motion.

5 Materials Frequently Used in Surface Micromachining Silicon-based materials: polycrystalline silicon (poly), SiO, SiN Deposited with low-pressure chemical vapor deposition (LPCVD)

6 Materials Frequently Used in Surface Micromachining Polycrystalline silicon (poly) Structural material, etched with F-based plasma SiH 4 = Si + 2H 2 (580~620 o C) Intrinsic stress: tensile or compressive Can be in-situ doped during deposition Adjusting resistivity or intrinsic stress Silicon Nitride (SiN) Structural material, etched with F-based plasma SiH 4 + NH 3 = SiN + H 2 (~800 o C) Intrinsic stress: tensile or compressive Can be adjusted by changing gas mixture ratio Silicon Oxide (SiO) Sacrificial, etched with HF SiH 4 + NO 2 = SiO 2 + H 2 + N 2 (~500 o C) (also called LTO) Can be deposited on top of metal structures for passivation and protection Intrinsic stress: compressive Can be in-situ doped during deposition PSG: phosphorous-silicate glass for higher etch rate in HF

7 Materials Frequently Used in Surface Micromachining Metals Gold. Copper, aluminum, nickel Low-temperature deposition: evaporation, sputtering, electroplating Etch with wet acidic solutions Polymers Photoresisit, polyimide, parylene, SU-8 resist. Low-temperature deposition: spin coating, vapor coating. Remove with solvents or etch with oxygen plasma Metal-polymer combination can be used to achieve lowtemperature surface micromachining process Friendly to integrated circuit components Use metal as structural layer and polymer as sacrificial layer Use polymer as structural layer and metal as sacrificial layer

8 Structural-Sacrificial Materials Selection Criteria The deposition of the structural material on top of the sacrificial material must not cause the sacrificial layer to melt, dissolve, crack, disintegrate, or becomes unstable or destroyed in any other ways (step c). The method used for patterning the structural layer must not attack the sacrificial layer and any existing layers on a substrate (step d). The method used for removing the sacrificial layer must not attack, dissolve, or destroy the structural layer and the substrate (step e and f).

9 Structural-Sacrificial Materials Selection Criteria Temperature and achievable thickness of material deposition Intrinsic stress of the structural layer material Long-term stability of structural layer material Sacrificial layer etch rate and selectivity Smoothness and cross-sectional profile Conformal or non-conformal deposition Cost of material and processing

10 Example

11 Micro Motor Fabrication Process 1 st Pass Si PSG Poly

12 Acceleration of Sacrificial Etching For large planar structures, the sacrificial layer etching usually takes very long time. The etch rate is limited by the diffusion of etchants into the small gap between the planar structure and the substrate. Increasing the etching solution concentration helps, but is still limited by the diffusion. Available sacrificial layer material is not always fast-etch material Deploying small through-holes (etch holes) on the planar structure can significantly speed-up the sacrificial layer etching. The trade-off is that the etch holes would affect the mechanical, electrical, optical properties of the planar structure.

13 Stiction in Surface Micromachinining Suspended structure Liquid layer Substrate Suspended structure Substrate Suspended structure Substrate Sacrificial layer etching usually is performed in wet chemical solutions. After the etching and rinse, a liquid (usually water) layer is trapped in the small gap between the substrate and the suspended structure. As the liquid layer is drying, the surface tension force will drag suspended structure toward the substrate until getting into contact. The worst scenario is the suspended structure permanently bonds to the substrate surface. The bonding force is related to the area of the suspended structure. Larger structure is more likely to have stiction.

14 Anti-stiction Methods Releasing stuck structures using ultrasonic vibration. Using additional support to prevent the suspended structure from touching the substrate surface. Reducing the surface tension force Replace water with liquid having smaller surface tension (alcohol) before drying Treat the surface of the substrate and the suspended structure with special chemical to form hydrophobic surface Replace water with liquid having zero surface tension before drying

15 Anti-stiction Methods Using additional support to prevent the suspended structure from touching the substrate surface. photoresisit

16 Supercritical CO 2 drying Anti-stiction Methods Soak the device in ethanol or methanol. Put the device into the high pressure chamber and send in LCO 2. Liquid phase (1): 800psi/20 o C Having surface tension Apply heating Supercritical phase (2): >1100psi/>32 o C No surface tension Stop heating and discharge CO 2 from the chamber Gas phase (3): pressure drop to ATM and temperature kept >32 o C

17 3D Assembly of Surface Micromachined Structures Surface micromachining usually generates planar structures (2D flaps). These 2D flaps could be moved out-of-plane using a number of methods to form 3D structures (micro assembly). Serial assembly: Piece by piece and device by device Using robot and micro manipulator (Zyvex) Low efficiency Parallel (self) assembly: Assembling all devices on substrate simultaneously Using integrated micro actuators High efficiency Configuration of the 2D flaps With micro hinges Without micro hinges

18 Micro Hinged Structures Etch hole The main flap can be rotated out of substrate surface around the hinge. Needs two structural layer and two sacrificial layer Rely on conformal deposition process

19 Self-assembly of Micro Hinged Structures Permalloy Wu, Proc. IEEE, 1997 Use magnetic force Use electrostatic force

20 Self-assembly of Planar Structures without Hinges Plastic Deformation Magnetic Assembly (PDMA) Magnetic material piece can be removed after PDMA.

21 Self-assembly of Planar Structures without Hinges Self-Assembly using surface tension force PR Solder Thermal stability is an issue. Syms, JMEMS, 2003

22 Self-assembly of Planar Structures without Hinges Self-Assembly using surface tension force

23 Surface Micromachining Foundry Process Multi-user MEMS Process 3 polysilicon surface micromachining process One process for different designs PolyMUMPS, SOIMUMPS, MetalMUMPS

24 Poly-MUMPS Examples

25 MUMPS Design Rules Overlapped region

26 Micro Motor Fabrication Process 2 nd Pass Si PSG Poly SiN

27 Micro Motor Fabrication Process 3 rd Pass Si PSG Poly SiN