Surface Micromachining Device in Microelectromechanical Systems
* Question
What are the three typical components of a surface micromachining device?
* Answer
A surface micromachining device is a type of microelectromechanical system (MEMS) fabricated by building mechanical and structural layers on top of a substrate (typically silicon).
The device is created through deposition, patterning, and selective etching of thin films.
Structurally, a surface micromachining device is composed of three typical components:
1. Structural Layer
Definition:
This is the main functional layer of the micromachined device — the part that forms mechanical elements such as beams, membranes, cantilevers, gears, or resonators.
Material Examples:
Polysilicon (most common)
Silicon nitride (Si₃N₄)
Silicon carbide (SiC)
Metals such as aluminum or nickel for specific applications
Function:
Provides mechanical strength and defines the moving or load-bearing parts of the device.
Determines the device’s mechanical, electrical, and optical properties.
2. Sacrificial Layer
Definition:
A temporary layer used during fabrication to define spaces or gaps beneath or between structural elements.
After patterning, this layer is removed (etched away) to release the structural layer and allow it to move freely.
Material Examples:
Silicon dioxide (SiO₂)
Phosphosilicate glass (PSG)
Polymers or other easily etchable materials
Function:
Acts as a spacer to create air gaps or cavities.
After etching, it leaves behind the movable microstructures (e.g., suspended beams or membranes).
Process:
Deposit sacrificial layer.
Deposit and pattern structural layer on top.
Selectively etch away sacrificial layer → structural layer becomes suspended.
3. Substrate (Base Layer)
Definition:
The foundation upon which all layers are built. It provides mechanical support and may also contain electrical interconnections or circuit elements.
Material Examples:
Silicon wafer (most common)
Glass or quartz for optical MEMS
Gallium arsenide (GaAs) for specialized applications
Function:
Provides mechanical stability for the device.
Can act as a ground plane or heat sink.
May contain integrated circuits (ICs) for control and signal processing in monolithic MEMS integration.
4. Summary of the Three Components
Component | Main Role | Typical Materials | Key Function |
Structural Layer | Forms movable or functional microstructures | Polysilicon, Si₃N₄, SiC | Defines the working part of the device |
Sacrificial Layer | Temporary layer removed to release structure | SiO₂, PSG | Creates gaps or free-moving parts |
Substrate | Mechanical base and support layer | Silicon, glass, quartz | Provides structural and electrical foundation |
5. Example: Polysilicon Micromechanical Beam
A classic surface micromachining process example:
Start with a silicon substrate.
Deposit a sacrificial SiO₂ layer to define the gap.
Deposit and pattern polysilicon as the structural layer.
Etch away the SiO₂ to release the polysilicon beam, leaving it suspended above the substrate.
This basic approach is used in devices such as:
MEMS accelerometers
Pressure sensors
Micro-mirrors
RF switches
Summary
The three typical components of a surface micromachining device are:
Structural Layer – the functional mechanical part.
Sacrificial Layer – a temporary spacer removed to free the structure.
Substrate – the supporting base for the entire device.
Together, these layers enable the fabrication of complex, movable microstructures on a chip using standard semiconductor processing techniques.