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Chapter 4 : MEMS & NEMS

MEMS arrow_upward

  • 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:
    • Which are able to deposit thin films of material on a substrate.
    • To apply patterned mask on top of the films by photolithographic imaging.
    • To etch the films selectively to the mask.
  • A MEMS process is usually a structured sequence of these operations to form actual devices:
    • Deposition Processes,
    • Lithography,
    • Etching Processes.

    MEMS Thin Film Deposition Processes arrow_upward

  • MEMS deposition technology can be classified in two groups:
  • 1. Depositions that happen because of a Chemical reaction:

    • Chemical Vapor Deposition (CVD),
    • Thermal Oxidation.

    2. Depositions that happen because of a Physical phenomenon:

    • Physical Vapor Deposition (PVD),
    • Casting.
  • 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 (CVD) arrow_upward

  • 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:
    • Temperature,
    • Time,
    • Pressure,
    • Surface specificity.
  • 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.
  •  Sequence of reaction steps in a CVD Process

  • The two most important CVD Technologies in MEMS are the
    • Low Pressure CVD (LPCVD)
    • 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.

  • Thermal Oxidation arrow_upward

  • 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:

  • Physical Vapor Deposition arrow_upward

  • 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
    • Evaporation
    • Sputtering
  • 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:
    • E-beam evaporation
    • Resistive evaporation
  • 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).
  • Sputtering
  • 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:

  • Casting arrow_upward

  • 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.

  • Etching Process arrow_upward

  • 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:
    • Wet etching
    • Dry 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.

  • NEMS arrow_upward

  • 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.

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