Lateral Patterning: Electron Beam Lithography and Bonding

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Lateral PatterningElectron Beam Lithography
and Bonding

• Sanja Hadzialic

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Outline

• Lateral Patterning
• Patterning Schemes
• Direct Writing Methods
• Lithography Methods
• Electron Beam Lithography (EBL)
• Introduction
• Resists
• Electron Optics
• Electron-Solid Interactions
• Proximity Effects
• Bonding
• Ball Bonding
• Wedge Bonding

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Lateral Patterning Patterning Schemes
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Direct Writing Methods

• Focused Ion Beam Writing
• Ions are implanted in the substrate
• Can localize electrons and make resistive regions
• Used for local doping of the sample
• Scanning Probe Lithography
• Moving single atoms
• Material deposition from the tip on the substrate
• Scratching of the surface
• Local oxidation (depletion of 2DEG)

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Lateral Patterning Lithography methods

• Optical Lithography
• The illumination of the resist by UV-light
  through a mask
• The mask can be a quartz plate coated with a thin
  chromium film
• Contact illumination the mask in contact with
  the resist
• Projection illumination the mask pattern is
  transferred via lenses
• Large throughput
• One mask can be used to make many patterns
• Masks are expensive
• The resolution is limited by the wavelength
  (smallest feature ?/2)

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Lateral Patterning Lithography methods

• X-Ray Lithography
• The wavelengths are so small that the diffraction
  no longer limits the lithographic resolution
• The problem with short wavelengths (high energy)
  is the mask
• Difficult to construct any type of optic systems
• Requires either a custom built x-ray source and
  stepper or access to a syncrotron storage ring to
  do the exposure.

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Electron Beam Lithography (EBL)

• The Technique
• A beam of electrons is scanned across a surface
  covered with a resist film, thus depositing
  energy in the desired pattern in the resist film
• Attributes
• Capable of very high resolution, almost to the
  atomic level
• Flexible, works on a variety of materials and an
  almost infinite number of patterns
• Slow - one or more orders of magnitude slower
  than optical lithography
• Expensive and complicated the equipment can
  cost many millions of dollars and requires
  frequent service to stay properly maintained

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Electron Beam Lithography (EBL)

• Applications
• Support of the integrated circuit industry
• Mask making
• Direct write for advanced prototyping of
  integrated circuits
• Manufacture of small volume specialty products
  (GaAs integrated circuits, optical waveguides)
• Research into the scaling limits of integrated
  circuits
• Study of quantum effects and other novel
  phenomena at very small dimensions
• Aharanov Bohm effect
• Ballistic electron effects
• Quantization of electron energy levels
• Single electron transistors

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EBL Resists

• Usually polymers dissolved in a liquid solvent.
  The resist is dropped onto the substrate, which
  is then spun at 1000 to 6000 rpm to form a
  coating
• After baking out the casting solvent, electron
  exposure modifies the resist, leaving it either
  more soluble (positive) or less soluble
  (negative) in the developer
• The pattern in the resist is transferred to the
  substrate either through an etching process
  (plasma or wet chemical) or by liftoff of material

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EBL Resists

• The sensitivity of the resist
• Positive resist the point at which all of the
  resist is removed
• Negative resist the point at which some of the
  resist remains
• Negative resist is faster
• The contrast
• A measure of the ability of the resist to
  distinguish between light and dark portions of
  the mask. It is defined as
• which is the slope of the line.

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EBL Resists

• Charge dissipation
• Substrate charging causes considerable distortion
  when patterning insulators and may contribute
  significantly to overlayer errors even on
  semiconductors
• Solved by evaporating a thin metal film on top of
  the resist which is removed before resist
  development.
• PMMA (polymethyl methacrylate)
• The most used EBL positive resist
• One of the highest resist resolutions available

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EBL Electron Optics

• EBL column
• Electron source
• Two or more lenses
• Beam deflector
• Beam blanker for turning the beam on/off
• Stigmator for astigmatism correction
• Apertures for helping to define the beam
• Alignment systems for beam centering
• Detector system for assisting with focusing and
  locating marks on the sample

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EBL Electron Optics

• Electron Sources
• Thermionic source
• A conducting material is heated to the point
  where the electrons have sufficient energy to
  overcome the work function barrier of the
  conductor
• Field emission source
• Applying an electric field sufficiently strong
  that the electrons can tunnel through the barrier
• Desired source qualities
• High intensity (brightness)
• High uniformity
• Small spot size
• Good stability
• Long life

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EBL Electron Optics
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EBL Electron Optics

• Electron Lenses
• Electrostatic
• Magnetic
• Not as good as optical lenses
• Spherical aberration
• The outer zone of the lens focuses more strongly
  than the inner zone
• Chromatic aberration
• Electrons at slightly different energies get
  focused at different image planes
• Electrostatic lenses have worse aberrations than
  magnetic lenses

Magnetic lense
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• Other Electron Optical Elements
• Apertures
• Small holes through which the beam passes on its
  way down the collumn in order to shape the beam.
• Electron beam deflection
• Used to scan the beam across the surface of the
  sample
• Can be done both electrostatically and
  magnetically
• Magnetic deflection causes less distortion, but
  electrostatic deflection is faster.
• Beam blanking (turning the beam on/off)
• Usually accomplished with a pair of plates set up
  as a simple electrostatic deflector
• Stigmators
• Special type of lens used to correct astigmatism,
  where the beam focuses in different directions at
  different lens settings the shape of a nominally
  round beam becomes oblong. Can be both magnetic
  and electrostatic.

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• The resolution
• Virtual source size dV divided by the
  demagnification M-1 of the column gives the
  diameter
• Spherical aberrations limit the beam diameter to
• where Cs is the spherical aberration
  coefficient of the final lens and a is the
  convergence half-angle at the target.
• Chromatic aberrations limit the beam diameter to
• where Cc is the chromatic aberration
  coefficient, DV is the energy spread of the
  electrons and Vb is the beam voltage
• The wavelength of the electrons gives a
  diffraction limit of resolution
• The theoretical beam size is given by

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EBL Electron Solid Interactions

• Forward scattering
• As the electrons penetrate the resist a fraction
  of them of them undergoes small angle scattering
  events.
• This effect can be minimized by using the
  thinnest resist possible
• Can be used to tailor the resist sidewall profile
  (liftoff)
• Backscattering
• In the substrate the electrons experience large
  angle scattering and some of them return back to
  the resist at a significant distance from the
  incident beam, causing additional resist exposure
  (proximity effect)

• Secondary electrons
• As the electrons slow down much of their energy
  is dissipated in the form of secondary electrons.
  Their range in the resist is only a few
  nanometers, so they contribute little to the
  proximity effect. Instead the net result is an
  effective widening of the beam by roughly 10 nm.

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EBL Electron Solid Interactions

• The forward scattering of electrons in the resist
  leads to overcut resist profile after
  illumination, which is useful in liftoff processes

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EBL Proximity Effects

• The scattering of the electron beam results in
  pattern specific linewidth variations called the
  proximity effect.
• A narrow line between two large exposed areas may
  receive so many scattered electrons that it can
  develop away (in positive resists)
• A small feature may lose so much of its dose due
  to scattering that it develops incompletely

SEM micrograph of a positive resist pattern on
silicon exposed with a 20 kV electron beam
demonstrates the proximity effect, where small
isolated exposed areas receive less dose
relative to larger or more densely exposed areas.

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EBL Proximity Effect Avoidance

• Dose adjustment
• Works well if the pattern has uniform density and
  linewidths
• Multilevel resist
• A thin top level is sensitive to the electrons
  and the pattern developed in it is transferred to
  the underlying layer by dry etching
• Additional process complexity
• Higher beam voltages
• Minimize forward scattering, but can increase
  backscattering
• Low beam energies (electron range smaller than
  the minimum feature size)
• Must have thin resist layer
• Electrons more difficult to focus and more
  sensitive to stray electric and magnetic fields

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EBL Proximity Effect Correction

• Dose modulation
• The dose received by the individual shapes is
  corrected so that the shape prints at its correct
  size
• Shape-to-shape interactions are computationally
  very time-consuming
• Pattern biasing
• The extra dose that the dense patterns receive is
  compensated by slightly reducing their size
• GHOST
• The inverse tone of the pattern is written
• with a defocused beam designed to mimic
• the shape of the backscatter distribution.

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Wirebonding

• The wire, usually made of gold or aluminum, is
  used to establish electrical connection from the
  semiconductor chip (in the microscopic world) to
  the external device leads (the macroscopic world)
• Two versions of bonding
• Ball bonding
• Wedge bonding

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Wirebonding Ball-bonding

• The thin gold wire is melted with a small
  electrical spark forming a metal ball at the end
  of the wire
• The capillary from which the wire is fed presses
  the wire into the bonding pad on the chip
• The capillary lifts and moves to the connection
  where it descends and attaches the wire to the
  pad
• The wire is pulled back so it breaks close to the
  bonding area
• Commonly referred to as Ball-Wedge-bonding

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Wirebonding Wedge-bonding

• The wire is pressed into the semiconductor pad
  horizontally and attaches in that position
• The wedge tool moves to the connection pad
  position where, identically, the wire is attached
  horizontally
• The wire is pulled back in order to break after
  the attachment bond area
• Directional bonding
• Smaller wire loop
• Can provide closer bonding (relevant for smaller
  devices)

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Wirebonding

• There are three major wirebonding processes
• Thermocompression
• Applying a relatively high force with the formed
  wire ball into the pre-heated chip or substrate
• Ball-bonding
• Ultrasonic
• Applying a relatively smaller force pressing the
  wire to the chip at room temperature and applying
  vibrational energy in form of ultrasounds
• Reliability and bond strength not as good as
  thermosonic bonding
• Wedge-bonding
• Thermosonic
• Most common method
• Temperature around 100-150 oC, combines pressure
  and ultrasonic energy to form the bond
• Suitable for both ball-bonding and wedge-bonding

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Wirebonding