Micro Electro-Mechanical Systems (MEMS) are the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through micro fabrication technology.
While the electronics are fabricated using integrated circuit (IC) process sequences (e.g. CMOS, Bipolar, or BICMOS processes).
The micromechanical components are fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
MEMS technology is based on a number of tools and methodologies, which are used to form small structures with dimensions in the micrometer scale (one millionth of a meter).
There are three basic building blocks in MEMS technology:
Topics covered in this snack-sized chapter:
- Which are able to deposit thin films of material on a substrate.
- To apply patterned mask on top of the films by photolithographic imaging.
A MEMS process is usually a structured sequence of these operations to form actual devices:
MEMS deposition technology can be classified in two groups:
- To etch the films selectively to the mask.
1. Depositions that happen because of a Chemical reaction:
- Chemical Vapor Deposition (CVD),
2. Depositions that happen because of a Physical phenomenon:
Many Micro/ Nano-electronic devices that are currently manufactured require the deposition of thin films through the aid of chemical vapor deposition (CVD).
Chemical Vapor Deposition is a widely used method for depositing thin films of a large variety of materials.
CVD processes and systems are based on four major factors:
CVD depends on the availability of a volatile chemical which can be converted by some reaction into the desired solid film.
Reactors and processes are designed in order to limit the reaction to a particular place and time within the chamber
(typically on the substrate).
In CVD process, the substrate is exposed to one or more volatile precursors, which react and/ or decompose on the substrate surface to produce the desired deposit.
Volatile byproducts are also produced, which are removed by gas flow through the reaction chamber.
These byproducts or particles can fall onto the substrates, coat the chamber walls, and/or clog exhaust openings.
- Physical Vapor Deposition (PVD),
Sequence of reaction steps in a CVD Process
The two most important CVD Technologies in MEMS are the
- Plasma Enhanced CVD (PECVD)
Low Pressure CVD (LPCVD)
The LPCVD process produces layers with excellent uniformity of thickness and material characteristics.
The main problems with the process are the high deposition temperature (higher than 600°C) and relatively slow deposition rate.
LPCVD systems deposit films on both sides of at least 25 wafers at a time.
Plasma Enhanced CVD
The PECVD process can operate at lower temperature (down to).
Extra energy is supplied to the gas molecules by the plasma in the reactor.
Most PECVD deposition systems can only deposit the material on one side of the wafers on to wafers at a time.
It is simply oxidation of the substrate surface in an oxygen rich atmosphere.
The temperature is raised to to to speed up the process.
The growth of the film is spurned by diffusion of oxygen into the substrate, which results in the film growing downwards into the substrate.
As the thickness of the oxidized layer increases, the diffusion of oxygen to the substrate becomes more difficult leading to a parabolic relationship between film thickness and oxidation time for films thicker than 100nm.
This is the classical process used to form silicon dioxide on a silicon substrate.
Wafer oxidation furnace to form silicon dioxide on a silicon substrate is shown in figure below:
The physical vapor deposition technique is based on the formation of vapor of the material to be deposited as a thin film.
The material in solid form is either heated until evaporation (thermal) or sputtered by ions (sputtering).
In the last, ions are generated by a plasma discharge usually within an inert gas (argon).
It is also possible to bombard the sample with an ion beam from an external ion source. This allows varying the energy and intensity of ions.
The two most important PVD technologies in MEMS are
In evaporation the substrate is placed inside a vacuum chamber, in which a source of the material to be deposited is also located.
The source material is then heated to the point where it starts to boil and evaporate.
The vacuum is required to allow the molecules to evaporate freely in the chamber, and they subsequently condense on all surfaces.
There are two evaporation technologies:
In e-beam evaporation, an electron beam is aimed at the source material causing local heating and evaporation.
A filament produces a beam of electrons
that is directed by a magnetic field onto the material being evaporated.
The magnetic field focuses the beam and aligns it to the proper location.
A schematic diagram of an e-beam evaporation system is shown in figure below:
In resistive evaporation, material is heated by passing an AC high current through a boat, most often tungsten due to its high melting temperature (3410°C).
In sputtering, material is released from the source at much lower temperature than evaporation.
The substrate is placed in a vacuum chamber with the source material, named as target.
Target at a negative potential is bombarded by positive Argon ions.
The ions are accelerated towards the surface of the target, causing atoms of the source material to break off from
the target in vapor form and condense on all surfaces including the substrate.
A schematic diagram of sputtering system is shown in figure below in which sputtering is performed:
In this process, the material to be deposited is dissolved in liquid form in a solvent.
The material can be applied to the substrate by spraying or spinning.
Once the solvent is evaporated, a thin film of the material remains on the substrate.
In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films previously deposited and/ or the substrate itself.
There are two types of etching:
In wet etching, the material is dissolved when immersed in a chemical solution.
In dry etching, the material is sputtered or dissolved using reactive ions or a vapor phase etchant.
Nanoelectromechanical systems (NEMS) are devices integrating electrical and mechanical functionality on the Nano scale.
Nano electro-mechanical systems are similar to MEMS but much smaller.