MEMS devices and processes

MEMS transducer 3a has cavity 8 in substrate 101. Membrane 1 overlies the cavity. A filter inhibits particle ingress into at least part of the cavity. In an aspect, filter layer 201 is formed on a lower surface of the substrate and patterned to form the filter. The filter may be at the opening of the cavity or recessed into the cavity, may be formed from silicon nitride, and may have a hydrophobic surface or coating. A perforated backplate 4 may be provided, and electrodes 2 and 3 coupled to the backplate and membrane, respectively, forming a capacitive sensor (e.g. microphone). Before forming the filter layer, the cavity may be filled with sacrificial material which may be made coplanar with the substrates lower surface or may be recessed. A sacrificial layer may be deposited beneath the filter layer and patterned, and the filter layer etched.