Photolithography

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Photolithography

• D. Boolchandani
• Department of ECE
• Malaviya National Institute of Technology
• Jaipur

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Photolithography

• In a microelectronic circuit, all the circuit
  elements (resistors, diodes, transistors, etc.)
  are formed in the top surface of a wafer (usually
  silicon).
• These circuit elements are interconnected in a
  complex, controlled, patterned manner.
• Consider the simple case of a silicon p-n
  junction diode with electrical contacts to the p
  and n sides on the top surface of the wafer.

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Photolithography

• Silicon p-n junction diode with both electrical
  contacts on the top surface of the wafer

• Can you draw the diode symbol on this diagram?

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Photolithography

• In order to produce a microelectronic circuit,
  portions of a silicon wafer must be doped with
  donors and/or acceptors in a controlled,
  patterned manner.
• Holes or windows must be cut through insulating
  thin films in a controlled, patterned manner.
• Metal interconnections (thin film wires) must
  be formed in a controlled, patterned manner.
• The process by which patterns are transferred to
  the surface of a wafer is called photolithography.

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Photolithography

• Consider the fabrication of a silicon p-n
  junction diode with both electrical contacts on
  the top surface of the wafer

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Photolithography

• We start with a bare silicon wafer and oxidize
  it. (The bottom surface also gets oxidized, but
  well ignore that.)

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Photolithography

• We first need to open a window in the SiO2
  through which we can diffuse a donor dopant
  (e.g., P) to form the n-type region

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Photolithography

• The starting point for the photolithography
  process is a mask.
• A mask is a glass plate that is coated with an
  opaque thin film (often a metal thin film such as
  chromium).
• This metal film is patterned in the shape of the
  features we want to create on the wafer surface.

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Photolithography

• For our example, our mask could look like this

opaque metal,e.g.,Cr
Cross section
Top view
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Photolithography

• Recall that we start with a bare silicon wafer
  and oxidize it. (The bottom surface also gets
  oxidized, but well ignore that.)

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Photolithography

• The wafer is next coated with photoresist.
• Photoresist is a light-sensitive polymer.
• We will initially consider positive photoresist
  (more about what this means soon).
• Photoresist is usually spun on.
• For this step, the wafer is held onto a support
  chuck by a vacuum.
• Photoresist is typically applied in liquid form
  (dissolved in a solvent).
• The wafer is spun at high speed (1000 to 6000
  rpm) for 20 to 60 seconds to produce a thin,
  uniform film, typically 0.3 to 2.5 mm thick.

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Photolithography

• After coating with photoresist, the wafer looks
  like this

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Photolithography

• The wafer is baked at 70 to 90C (soft bake or
  pre-bake) to remove solvent from the photoresist
  and improve adhesion.

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Photolithography

• The mask is aligned (positioned) as desired on
  top of the wafer.

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Photolithography

• The photoresist is exposed through the mask
  with UV light. UV light breaks chemical bonds in
  the photoresist.

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Photolithography

• The photoresist is developed by immersing the
  wafer in a chemical solution that removes
  photoresist that has been exposed to UV light.

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Photolithography

• The wafer is baked again, but at a higher
  temperature (120 to 180C). This hard bake or
  post-bake hardens the photoresist.

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Photolithography

• The unprotected SiO2 is removed by etching in a
  chemical solution containing HF (hydrofluoric
  acid), or by dry etching in a gaseous plasma,
  containing CF4 , for example.

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Photolithography

• The photoresist has done its job and is now
  removed (stripped) in a liquid solvent (e.g.,
  acetone) or in a dry O2 plasma.

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Photolithography

• Phosphorous is next diffused through the window
  to form an n-type region. The SiO2 film
  blocks phosphorus diffusion outside the window.

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Photolithography

• Another photolithography step must be performed
  in order to open another window in the SiO2 so we
  can make an electrical contact to the p-type
  substrate from the top surface of the wafer.

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Photolithography

• The steps will not be shown in detail, but after
  photolithography, SiO2 etching, and photoresist
  stripping, the wafer structure is shown below.

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Photolithography

• The wafer surface is next coated with aluminum by
  evaporation or sputtering. The window outlines
  may still be visible.

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Photolithography

• Photolithography is used to pattern photoresist
  so as to protect the aluminum over the windows

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Photolithography

• What must the mask look like in order to pattern
  the aluminum film? Assume that were still using
  positive photoresist.

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Photolithography

• The aluminum is etched where it is not protected
  by photoresist. Wet or dry etchants can be used.

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Photolithography

• Then the photoresist is stripped.

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Photolithography

• The final step is to anneal (heat treat) the
  wafer at 450C in order to improve the
  electrical contact between the aluminum film and
  the underlying silicon.

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Photolithography

• So far we have only considered positive
  photoresists.
• For positive resists, the resist pattern on the
  wafer looks just like the pattern on the mask
• There are also negative photoresists.
• Ultraviolet light crosslinks negative resists,
  making them less soluble in a developer solution.
• For negative resists, the resist pattern on the
  wafer is the negative of the pattern on the mask.

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Photolithography

• In order to align a new pattern to a pattern
  already on the wafer, alignment marks are used.
• Various exposure systems
• Contact printing,
• Proximity printing,
• Projection printing, and
• Direct step-on-wafer (step-and-repeat
  projection).

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Photolithography

• A complete photolithography process (photoresist
  exposure tool developing process) can be
  characterized by the smallest (finest resolution)
  lines or windows that can be produced on a wafer.
• This dimension is called the minimum feature size
  or minimum linewidth.
• The limitations of optical lithography are a
  consequence of basic physics (diffraction).

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Photolithography

• For a single-wavelength projection
  photo-lithography system, the minimum feature
  size or minimum linewidth is given by the
  Rayleigh criterion

• l is the wavelength.
• NA is the numerical aperture, a measure of the
  light-collecting power of the projection lens.
• k depends on the photoresist properties and the
  quality of the optical system.

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Photolithography

• So how do we reduce wmin ?
• Reduce k.
• Reduce l.
• Increase NA.

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Photolithography

• Even for the best projection photolithography
  systems, NA is less than 0.8.
• The theoretical limit for k (the lowest value) is
  about 0.25.

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Photolithography

• Lenses with higher NA can produce smaller
  linewidths.
• This linewidth reduction comes at a price.
• The depth of focus decreases as NA increases.
• Depth of focus is the distance that the wafer can
  be moved relative to (closer to or farther from)
  the projection lens and still keep the image in
  focus on the wafer.

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Photolithography

• Depth of focus is given by

• Depth of focus decreases (bad) as l decreases.
• Depth of focus decreases (bad) as NA increases.

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Photolithography

• Numerous light sources are (and will be) used for
  optical lithography

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Photolithography

• Complex devices require the photolithography
  process to be carried out over 20 times.
• Þ over 20 mask levels
• Any dust on the wafer or mask can result in
  defects. Þ Cleanrooms are required for
  fabrication of complex devices.
• Even if defects occur in only 10 of the chips
  during each photolithography step, fewer than 50
  of the chips will be functional after a seven
  mask process is completed.
• How is this yield calculated?

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Photolithography

• Other lithographic techniques will play a role in
  the future.
• Electron beam lithography
• Ion beam lithography.
• X-ray lithography.