Northern Ireland Semiconductor Research Centre

The membrane valve structure was fabricated using a combination of bulk and surface micromachining techniques. The valve comprises a silicon substrate with an etched aperture to form the inlet for gas or liquid. A surface micromachined polysilicon movable plate is located at the centre above the inlet port. The plate is supported by four flexible polysilicon arms anchored to the silicon substrate.

The valve operates in a passive mode with the applied pressure controlling the position of the valve plate. During forward pressure, fluid first passes through the inlet aperture on the silicon substrate, pushes the valve plate open, then flows through the narrow gap between the plate and the substrate and finally through passages around the supporting arms. When the pressure acts in the reverse direction, the polysilicon plate is pushed against the substrate, forming a seal and hence restricting the flow. A pressure drop across the central plate is required to shut the valve. At zero pressure difference, the valve plate will move to its resting position, which is slightly open.


Fluid flow through membrane valve	FIB cross-section

The ZYGO surface profile scan shows that the polysilicon valve deflect upwards when released from the substrate due to built-in compressive stress, of approximately 113 MPa, in the film. An automated measurement system was developed to determine flow rates of liquid through the valve under forward and reverse pressure. The flow rate of methanol was approximately 7 times greater than that of de-ionised water at a pressure of 11 kPa due to its lower viscosity and surface tension. The membrane valve operates successfully with negligible reverse flow, the maximum reverse flow rate being less than 3 % of the forward flow.


ZYGO Scan	Liquid Flow	Gas Flow

References

Bien DCS, Mitchell SJN and Gamble HS, �Fabrication and characterisation of a micromachined passive valve�, Journal of Micromechanics and Microengineering, vol. 13, pp557-562, Sept 2003.