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    Micro Electro Mechanical Systems (MEMS)

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    Neethu Krishna K
    This document discusses various microelectromechanical systems (MEMS) fabrication techniques. It describes bulk micromachining which involves removing silicon substrate material through etching to form 3D structures. Wet etching uses liquid chemicals while dry etching uses gases. It also describes surface micromachining which builds structures layer by layer on a substrate through thin film deposition and etching of sacrificial layers. Other topics covered include lithography techniques like photolithography and grayscale lithography for patterning, as well as deposition methods like chemical vapor deposition and physical vapor deposition.

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    © All Rights Reserved
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    Micro Electro Mechanical Systems (MEMS)

    Uploaded by

    Neethu Krishna K
    87 views44 pages
    This document discusses various microelectromechanical systems (MEMS) fabrication techniques. It describes bulk micromachining which involves removing silicon substrate material through etching to form 3D structures. Wet etching uses liquid chemicals while dry etching uses gases. It also describes surface micromachining which builds structures layer by layer on a substrate through thin film deposition and etching of sacrificial layers. Other topics covered include lithography techniques like photolithography and grayscale lithography for patterning, as well as deposition methods like chemical vapor deposition and physical vapor deposition.

    Original Description:

    MEMS-MODULE-II-PPT

    Original Title

    MODULE-II

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    This document discusses various microelectromechanical systems (MEMS) fabrication techniques. It describes bulk micromachining which involves removing silicon substrate material through etching to form 3D structures. Wet etching uses liquid chemicals while dry etching uses gases. It also describes surface micromachining which builds structures layer by layer on a substrate through thin film deposition and etching of sacrificial layers. Other topics covered include lithography techniques like photolithography and grayscale lithography for patterning, as well as deposition methods like chemical vapor deposition and physical vapor deposition.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    87 views44 pages

    Micro Electro Mechanical Systems (MEMS)

    Uploaded by

    Neethu Krishna K
    This document discusses various microelectromechanical systems (MEMS) fabrication techniques. It describes bulk micromachining which involves removing silicon substrate material through etching to form 3D structures. Wet etching uses liquid chemicals while dry etching uses gases. It also describes surface micromachining which builds structures layer by layer on a substrate through thin film deposition and etching of sacrificial layers. Other topics covered include lithography techniques like photolithography and grayscale lithography for patterning, as well as deposition methods like chemical vapor deposition and physical vapor deposition.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 44

    MT 09 702

    MICRO ELECTRO MECHANICAL


    SYSTEMS
    (MEMS)

    Module-II (14 hours)


    FABRICATION TECHNOLOGY
    Etching: wet and dry etching methods, selective etching,
    directional etching
    Deposition methods : Physical and chemical vapour
    deposition method
    Surface micromachining bulk micro machining,
    advanced surface micromachining: LIGA, and DRIE.
    electro plating, electroless plating and electro deposition
    thin film deposition
    Lithography technique: shadow masking, grey scale
    lithography.

    1. BULK MICROMACHINING

    Bulk micromachining is a process that defines


    structures by selectively removing /etching a substrate
    In MEMS fabrication bulk micromachining uses the
    entire thickness of the silicon wafer (or substrate) to
    form microsystem structures that can result in high
    aspect
    ratios.
    In
    bulk
    micromachining
    monocrystalline silicon wafers are selectively etched
    to form 3-D MEMS devices.
    Bulk micromachining is used to remove relatively
    large amounts of a silicon substrate.
    3

    Bulk Micromachining Process


    Bulk etch is a subtractive process in which
    the silicon substrate is selectively
    removed.
    Specific etchants are chosen that remove
    substrate material either isotropic (same
    etch rate in all directions) or anisotropic
    (etch rate is different for lateral and
    vertical direction).
    Anisotropic etching depends on the
    planes and orientations of the crystal
    [(100), (110),(111) planes]
    4

    Etch Rate:
    How fast the material is removed in the etch
    process
    Etch Rate = Thickness before etch - Thickness
    after etch
    Etch time
    Selectivity:
    Ratio of the etch rates between the different
    materials
    = Etch Rate of Material 1
    Etch Rate of Material 2

    ETCHING
    This excess material and the material in unwanted
    places must be removed. The removal has to be done
    carefully & slowly, as otherwise material may also
    be removed from places where it is actually needed.
    A fast removal of material may be called dissolution,
    while the slow removal that is used in IC
    manufacturing is called etching.
    Two types of removal techniques are employed in
    this. (1) Removal by using liquids, called wet
    etching (2) removal by using gases, called dry
    etching
    7

    WET ETCHING
    In this method, a solution which can etch the material
    will be kept in a tank and the wafer will be dipped in the
    tank for a given time, at a given temperature. The excess
    material will be etched (removed) from the wafer.

    10 or 20 or 25 wafers may be etched at the same time.


    After etching, the wafers will be taken out and rinsed
    in water and dried by spinning.
    For almost all materials, there is some solvent which
    can etch it.

    The wet etching process involves:


    Transport of reactants to the surface
    Surface reaction
    Transport of products from the surface

    10

    In any method, if the wafer is etched for too long, it will


    remove too much material. If we want to completely remove
    the material, then it is fine. Otherwise, it will-over etch (fig:1).
    Similarly, if we etch for short time, it may not remove to the
    desired level and it is called under etch.
    Both over etch and under etch are undesirable. Ideal etching
    should be done (fig:2)

    11

    Anisotropic etchants: (silicon substrates)

    Potassium Hydroxide (KOH)


    Ethylene Diamine Pyrocathechol (EDP)
    Tetramethyl Ammonium Hydroxide (TMAH)
    Sodium Hydroxide (NaOH)
    N2H4H2O (Hydrazine)

    Isotropic etchants : (SiO2, Si3N4 etc..)


    HNO3 (Nitric acid)
    CH3COOH (Acetic acid)
    Hydrofluoric acid (HF)
    etchant acid+H2O
    (1:1)
    12

    DRY ETCHING
    Material removal reactions occur in the
    gas phase.
    Two types:
    Plasma etching
    Deep reactive ion etching (DRIE)

    13

    Plasma Etching
    Plasma is one of the four fundamental
    states of matter.
    Plasma is a stream of positive charge
    carrying ions with large no: of electrons
    and diluted inert carrier gas such as
    argon.
    Generation of plasma is by cont.
    application of high voltage electric charge
    / RF source (13.56MHz).
    Generated in low Pr. environment
    /vacuum
    14

    15

    16

    Drawback of plasma etching

    The etched walls remain at a wide angle


    () to its depth.
    Cavity angle () is critical in many MEMS
    structures
    For e.g.: the comb electrodes of micro
    grippers should be parallel each other.
    Aim = 0

    17

    Deep Reactive Ion


    etching (DRIE)

    Use nCF2 (fluropolymers)

    18

    Bulk Micromachining Components

    The following MEMS components possible only


    through the use of bulk micromachining processes.
    sensors including micro pressure sensors, cantilever
    arrays, and accelerometers
    Nozzles
    Microfluidic channels
    Needle Arrays
    Probes
    Membranes
    Chambers

    19

    2. SURFACE
    MICROMACHINING
    This technique builds microstructure

    by adding materials layer by layer on


    top of the substrate.
    Sacrificial component, structural
    component
    SiO2 , PSG (phospho silicate glass)
    Polysilicon,metals
    2-20um
    microcantilever beam
    20

    Surface Micromachining Process

    21

    22

    General process
    Etching
    of
    sacrificia
    l layer

    Depositi
    on of
    sacrificia
    l layer

    Depositi
    on of
    structur
    al layer
    23

    DEPOSITION METHODS
    1) CHEMICAL VAPOUR DEPOSITION
    Depositing thin films over the surface of substrates and other
    MEMS and microsystem components is a common and
    necessary practice in micromachining.
    CVD involves convective heat and mass transfer as well as
    diffusion with chemical reactions at the substrate surfaces.
    Working principle of CVD
    Flow of a gas with diffused reactants over a hot substrate
    surface.
    Carrier gas
    Due to surface temp, chemical reactions happens between the
    reactants and surface which leads to formation of film
    By products are vented.
    3 types
    Silicon dioxide
    Silicon nitride
    { chemical equations in the text book}
    Polycrystalline silicon

    24

    25

    26

    2) PHYSICAL VAPOUR DEPOSITION


    SPUTTERING
    Used to deposit thin metallic film (thickness of 10-10 m)
    Sputtering process is carried out with plasma under very low pressure
    and low temp
    Plasma is made of positively charged gas ion
    The positive ions of the metal in an inert argon gas carrier bombard the
    surface of the target at high velocity
    This causes the metal ions to evaporate
    This metal vapor is then led to the substrate surface and is deposited
    after condensation

    27

    There are three basic steps to a PVD process:


    The material to be deposited (the source) is converted into vapor
    The vapor is transported across a low pressure region from the source
    to the substrate
    The vapor condenses on the substrate to form the desired thin film.
    Sputtering Process
    The basic sputtering process includes the following steps:
    The substrate is placed in a chamber with the source material (target)
    The chamber is evacuated to the desired process pressure (usually a
    high vacuum).
    An inert gas (such as argon) is introduced.
    A plasma is generated using a RF power source. This causes the gas to
    ionize.
    The ions accelerate toward the target
    The high-energy ions bombard the target causing target atoms to break
    off as a vapor

    28

    29

    LIGA
    Lithographie ( Lithography) Galvanoformung ( Electroforming) Abformung
    ( Molding)
    Non silicon based microstructure
    Using X-ray (high penetration power)
    example:
    Desired product : microthin wall metal tube of square cross section
    Taking substrate
    Depositing thick film of photoresist material
    Photoresist material : polymethylmethacylate (PMMA)
    Then placing the desired mask made of silicon nitride having thin film of
    gold for blocking X-ray transmission
    The deep x-ray dissolves the exposed area
    Later electroplating is done (nickel) to produce the tubular product of the
    desired thickness
    Then removing the photoresist material (PMMA) by oxygen plasma or 30

    31

    32

    Substrate material of LIGA :


    Base plate
    Electrical conductor- bcoz electrical conduction of the substrate is
    necessary in order to facilitate electroplating which is a part of the LIGA
    process
    Austenite steel, silicon wafers with thin titanium top layer, copper plated
    with gold, titanium, nickel
    Photo resist material :
    Must be sensitive to x-ray radiation
    Must have high resolution
    High resistance to wet and dry etching
    Thermal stability (1400c)
    The unexposed resist must be absolutely insoluble during development
    It must exhibit very good adhesion to the substrate during electroplating
    Advantages of LIGA:
    High aspect ratio
    Flexible microstructure configurations and geometry is possible
    The only technique that allows the production of metallic microstructure
    The best manufacturing process for mass production

    33

    ELECTROPLATING & ELECTROLESS PLATING


    Notes given at the time of lecture

    LITHOGRAPHY
    It is a general name given to processes used to transfer patterns on to a
    substrate to define structures that make up devices
    Optical lithography: Uses light
    Electron Beam lithography: Uses electrons
    Ion beam lithography: Uses energetic ions to bombard and pattern
    surfaces
    Soft lithography: Uses mechanical contact indentation to transfer
    patterns

    34

    PHOTOLITHOGRAPHY

    35

    GRAYSCALE LITHOGRAPHY
    Grayscale lithography is a process for fabricating complex 3D structures in
    photosensitive polymers (photoresists).
    As opposed to standard binary lithography (where photoresist is exposed
    to either all or none of the light), grayscale lithography allows for
    controlling the intensity of light in each region.
    This results in the ability to vary the depth at which the photoresist is
    developed.
    Curved structures such as ramps, domes, and microfluidic devices that
    would otherwise be impossible or too time intensive to fabricate can
    readily be produced.

    36

    Grayscale lithography changes exposure dose locally to develop the 3D


    structure in the photoresist. So differential exposure dose make differential
    depth of exposed photoresist across the surface.
    This is because the photoactive compound is absorbing the ultraviolet light
    energy as it travels in the depth of the photoresist. By using chrome-onglass (COG) masks, which induces diffraction, the ultraviolet intensity can
    be modulated.
    gray-scale lithography is using diffraction to modulate intensity of the
    ultraviolet
    diffraction is the bending, spreading and interference of waves when they
    pass by an
    obstruction or through a gap
    if wave source is passing through single slit, which is narrower than
    wavelength of wave,
    bending & spreading of waves to pass around obstacle form semicircular
    ripple. This is the diffraction pattern, and it is approximately equally strong
    37

    38

    The intensity passing through the mask is dependent on the fill area
    of each pitch.
    For example, if the mask is designed with square pixels and a set
    pitch between pixels as is shown in, then the intensity depends on
    the percentage of the opaque area for each pitch area.
    In this case the pitch is chosen to be below the resolution of the
    projection system so that the distance between each pixel remains
    below resolution. So, the pixel size can be modified to modulate
    directly the intensity passing through the objective lens.

    Another method to change the intensity is to keep the size of the


    pixel constant and change only the pitch, or it is possible to change
    both the size and the pitch.

    Actually, first method, changing only pixel size is easy to design the
    mask.

    39

    40

    SHADOW MASK LITHOGRAPHY


    Stencil lithography is a novel method of fabricating nanometer scale
    patterns using nanostencils, stencils (shadow mask) with nanometer size
    apertures.
    It is a resist-less, simple, parallel nanolithography process, and it does
    not involve any heat or chemical treatment of the substrates (unlike
    resist-based techniques).
    Stencil lithography was first reported in a scientific journal as a microstructuring technique by S. Gray and P. K. Weimer in 1959.
    They used long stretched metallic wires as shadow masks during metal
    deposition. Various materials can be used as membranes, such as
    metals, Si and polymers.
    Today the stencil apertures can be scaled down to sub-micrometer size
    at full 4" wafer scale. This is called a nanostencil.
    Nano-scale stencil apertures have been fabricated using laser
    41
    interference lithography (LIL), electron beam lithography, and focused

    Process
    Several process are available using stencil lithography: material
    deposition and etching, as well as implantation of ions. Different stencil
    requirements are necessary for the various processes, e. g. an extra etchresistant layer on the backside of the stencil for etching (if the membrane
    material is sensitive to the etching process) or a conductive layer on the
    backside of the stencil for ion implantation.
    Deposition
    The main deposition method used with stencil lithography is physical
    vapor deposition. This includes thermal and electron beam physical vapor
    deposition, molecular beam epitaxy, sputtering, and pulsed laser
    deposition. The more directional the material flux is, the more accurate
    the pattern is transferred from the stencil to the substrate.
    Etching
    Reactive ion etching is based on ionized, accelerated particles that etch
    both chemically and physically the substrate. The stencil in this case is
    used as a hard mask, protecting the covered regions of the substrate,
    while allowing the substrate under the stencil apertures to be etched.

    42

    Ion implantation
    Here the thickness of the membrane has to be smaller than the
    penetration length of the ions in the membrane material. The ions will
    then implant only under the stencil apertures, into the substrate.
    Challenges
    Despite it being a versatile technique, there are still several challenges
    to be addressed by stencil lithography. During deposition through the
    stencil, material is deposited not only on the substrate through the
    apertures but also on the stencil backside, including around and inside
    the apertures. This reduces the effective aperture size by an amount
    proportional to the deposited material, leading ultimately to aperture
    clogging.
    The accuracy of the pattern transfer from the stencil to the substrate
    depends on many parameters. The material diffusion on the substrate
    (as a function of temperature, material type, evaporation angle) and the
    geometrical setup of the evaporation are the main factors. Both lead to
    an enlargement of the initial pattern, called blurring.

    43

    44

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