//Surface micromachining
Benefits of surface micromachining are
I) spacer and structural specifications like thicknesses, can be smaller than 10 μm in size.
II) the footprint of micromachined device can be smaller than bulk wet-etched devices
II) surface microstructures have tolerance if we compared it to bulk wet-etched devices.
The main disadvantage is the weekness of surface microstructures to handling, particulates and condensation during manufacturing. Micromachining of Polysilicon is the most used form of micromachining surface.
Polysilicon has mechanical specifications like single silicon crystal
, which explains its popularity for MEMS.
Amain process steps, shown in Figure[5],begins with low-pressure chemical vapor…show more content… Usually, the layer is nitride silicon, which has less tensile stress than stoichiometric Si3N4. And thus clings to the substrate.
The spacer layer (b) is a 2 μm-thick layer of LPCVD phosphosilicate glass (PSG), which is patterned to form what will be structural anchors to the substrate (c).
The next process step is LPCVD of phosphorous-doped polysilicon forming a 2 μm-thick structural layer (d).
The polysilicon is dry etched to determine the microstructures and to present the underlying spacer layer (e).
A hydrofluoric acid (HF) etch step eliminates the spacer layer under and around the structure, getting it red of the substrate (f).
HF has a small but finite etch average for polysilicon and nitride silicon , so the wanted etch time for release must be limited. microstructures must have spaced-apart holes created into them to enable the released etch to remove the underlying…show more content… Microfluidic channels are created by etching channels into polymers and then bonding a glass or polymer overlay the channels. Channels are made in numerous polymer materials; no sacrificial material is needed.
As one example, Epon[SU-8] is an epoxy-based transparent ultraviolet (UV) sensitive negative photoresist, which has developed specifications with aspect rates of higher than(10:1) and thicknesses of greater than 200 μm. More complex 3-D filter structure can be created in [SU-8] by performing multiple exposures at different substrate tilt angles with respect to the UV source [7][8].
Microchannels are created through surface micromachining of any suitable structural material. The sacrificial material acts as a mold for the channel and is removed during the release etch. A process with two structural parylene layers has produced a variety of microfluidic channels,pumps and valves [6]. In this case, the sacrificial material is photoresist. In most of these processes, sealed cavities can be formed by depositing structural material across the cavity inlets after the release