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    Ion Beam Machining (IBM)

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    Ion beam machining (IBM) is closely associated

    with the phenomenon of 'sputtering', first

    reported by Grove in1852

    Ion beam machining takes place in a vacuumchamber, with charged atoms (ions) fired from

    an ion source towards a target by means of an

    accelerating voltage

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

    [1] Process Principle

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    [1] Process Principle (Contd)

    The surface atoms are dislodged (sputtering)

    by incident bombarding ions

    Ions are formed by knocking of electrons from

    the atoms and are accelerated in an electric

    fieldwhich collide with the work surface

    Transfer of kinetic energy takes place on the

    work surface to finally dislodge its surface atoms

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    [2] IBM System

    An ion beam machine has three main

    components:

    1. Plasma Source which generates the ions

    2. Extraction Grids for removing the ions from

    the plasma, and accelerating them towards

    the substrate (or the specimen)

    3. Tablefor holding the specimen

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    Fig.-1: Main features of ion beam machining (Melliar-Smith, 1976)

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    [2.1] Plasma source

    A heated filament, usually tungsten, acts as

    the cathode, from which electrons areacceleratedby means of a high voltage above

    1 kV, towards the anode

    During the passage of the electrons from the

    cathode to the anode, they interact with argon

    atoms in the plasma source

    The following reaction then occurs:

    2Ar e Ar e

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    [2.2] Extraction Grids

    The grids are normally made of two or three arrays of

    perforated sheets of carbon or molybdenum The outer grid is usually kept at ground potential,

    which is a more negative level than that of the anode.

    The second grid is held at a negative potential belowtheground value

    A third grid, which is maintained at the anode potential,

    is sometimes placed between the plasma and the

    electron suppressor grid

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    [2.3] Substrate mounting

    When the ions have been removed from the

    source, they drift in a field-free region to thecomponent/ specimen/ substrate which is to be

    machined or milled

    The specimen is usually mounted on a water-cooled table which can be tilted through an angle

    of 0 to 090

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    [3] Collision MechanismBy Stuart (1983): when an ion strikes the

    surface of a material it usually collides with anatom there

    Primary Collision

    Secondary Collision If the incident ion strikes the surface obliquely,

    the ejection is very likely to result from the

    primary collision between the incident ion andthe surface atom

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    Fig. 3: Ion (shaded) bombardment at normal incidence to

    surface (Stuart, 1983)

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    Mechanism continued

    Spencer and Schmidt (1972) explain material

    removal in terms of the transfer of momentumfrom the incident ions to atoms on the surface

    of the material

    The higher the energy of the incident ion, themore deeply this cascading effect occurs into

    the material

    Yield: Number of Atoms Removed per Incident

    Ion

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    Fig. 4: Collision (mass of ion less than that of atom)Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

    S d S h idt (1972) d S kh

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    Spencer and Schmidt (1972) and Somekh

    (1976) confirmed that the yield depends on:

    Material being treated

    Type of Atoms and their Energy

    Angle of Incidence Gas Pressure

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    The Current Associated with the Extraction of the Ion may be

    calculated from Childslaw:

    3/ 2 1/ 2 2

    0( / )(2* / ) ( / )eI V q q m d l

    To achieve the highest current,

    Lowest Spacing between the Grids

    Grids should have the Largest number of Holes of the Smallest Size

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

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    Fig.-7: Variation of yield with ion energy(Spencer and Schmidt, 1972)

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

    Applications

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    Applications

    Smoothing

    Ion Beam texturing

    Ion beam Cleaning Shaping, polishing and thinning by IBM

    Ion milling

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

    4) Widely used for:

    Cleaning -Removing surface contamination

    Polishing glass -Not to crystalline

    Etching -Study of micro structure wire dies

    Micromachining -Integrated circkt, bearings

    Sputter deposition -Atomic deposition i.e. thin film deposition

    Material Applications

    Fair: Al, Steel, Super alloys, Titanium, Refractory, Plastics and Glass

    Good: Ceramics

    Advantages

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    Advantages

    1) Process is almost universal.

    2) No chemical reagents etchants

    3) No under cutting

    4) Etching rates controllable

    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

    Disadvantages

    1) Relatively expensive

    2) Slow etching rates

    3) Little possibility of some thermal or reaction damage

    [7] SUMMARY of PROCESS CAPABILITIES and OPERATIONAL CHARACTERISTICS of IBM PROCES

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    Thermal Type AMPs: Ion Beam Machining (IBM) Dr. Neelesh K. Jain

    Type Capability/Characteristics Common Value/Range

    FinishingCapabilities

    Surface Roughness [CLA in m] 0.2 0.8

    Dimensional Tolerance or Accuracy [ m] 50 125

    Minimum Corner Radii (mm) 2 m

    Minimum Overcut (mm)

    Minimum Surface Damage(m) Chemical Damage NoMechanical Damage No

    Thermal Damage 0.005 m

    DrillingCapabilities

    Hole Diameter (mm) Fair for Micro-holes (d < 0.03 mm) andgood for small holes (0.15 < d