Metallization (Jaeger Chapter 6 Campbell Chapter 12)

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• Metallization (Jaeger Chapter 6 Campbell Chapter
  12)
• Contact to the different regions of the
  semiconductor forming a device and
  interconnection between different devices of an
  integrated circuit are needed to achieve the
  circuit functions expected. This is achieved by
  using thin layers of conductors deposited over
  the semiconductor surface but separated by
  necessary insulating layers.
• These metal layers may be carrying currents in
  the order of milliamperes. However because of
  their small cross-section area, the current
  density can be very high up to 105 Acm-2. It is
  essential that the I2R loss be reduced to
  minimum. The important properties of the
  conducting layers are
• High conductivity
• Low contact resistance with the semiconductor
  material (both n- and p-doped regions)
• Good adhesion to the insulating layer on the
  semiconductor surface (typically SiO2).
• Ease to pattern and etch
• Resistance to oxidation and corrosion
• Chemical and physical stability absence of
  physical changes and chemical reactions with Si,
  SiO2 and other materials forming the integrated
  circuit
• Ease to connect to circuits external to the
  integrated circuit through wire bonding or other
  bonding techniques.

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To date the most common conductor used in silicon
technology is aluminium. More recently heavily
doped polycrystalline silicon and metal silicides
are used to improve technology circuit
flexibility (multi-level metals reduced
resistance). Aluminium Evaporation Since the
beginning of the integrated circuit technology,
aluminium is used as the metal for contacting the
semiconductor and for interconnection. It started
with a single level wiring. The technology has
involved into five or even six levels (of
aluminium and other metals) of wirings. Traditiona
lly aluminium is deposited by vacuum evaporation
using a tungsten filament (I2R heating just like
the filament of a light bulb). This was
subsequently replaced by electron beam
evaporation (heating by e-beam bombardment). The
process consists of conversion of aluminium from
solid to vapour phase under vacuum by heating and
then allowing the vapour to condense onto the
surface of the silicon wafers which are held at
some distance away from the source of aluminium
vapour. As no chemical reaction is involved, the
process is often referred to as physical vapour
evaporation PVD.
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The diagram on the next slide shows typical
vacuum evaporation system used for aluminium
evaporation. It produces a vacuum of a few 10-7
torr (1 to 10 x10-5 Pa). The vacuum is produced
by two-stage vacuum pumping a rotary pump (also
known as the roughing pump) backing a high vacuum
pump in the form of a diffusion pump. This is a
batch process a batch of wafers are loaded
into the evaporation chamber, the chamber is
pumped down and the evaporation carried out and
the chamber open back to atmospheric pressure and
wafers unloaded. Large number of wafers per run
and fast vacuum pump down are essential for high
throughput. The wafers are not heated during
deposition, although they do get hot during
deposition.
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Wafers held here in chamber
Al source here
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In filament evaporation, a tungsten filament is
heated by passing a high dc current (up to 50 A)
during evaporation. Small loops of high purity
aluminium (better than .9999 purity) are hung on
the filament prior to the evaporation. When the
filament is heated up the aluminium melts and
wets (coats) the filament and eventually
vapourises. This method suffers from the fact
that the aluminium film can be contaminated by
impurities that are present in the filament and
evaporated together with the aluminium. The film
thickness that can be achieved in each run is
also limited because of the limited amount of
aluminium that can be attached to the filament.
Typical thickness is a 400 to 500 nm. Also the
filament has finite life typically two or three
evaporation runs. It is no longer used in
industry for production purposes. In
electron-beam evaporation, the filament is
replaced by a high energy electron beam (up to 10
to 20 keV) is directed at a crucible containing a
charge of aluminium. The bombardment of electrons
causes the aluminium to melt and evaporate.
Typically a magnetic field is used to steer the
electron beam so that the filament producing the
electron beam is hidden from the vapouring
aluminium and from the silicon wafers. X-rays
radiation is generated by the electron beam
hitting the aluminium. This can be damaging to
the silicon wafers (particularly MOS structures)
in that electron and hole traps can be generated.
Subsequent thermal annealing is needed to remove
these traps. There is usually a thickness monitor
to monitor thickness of film deposited as the
evaporation progresses.
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Due to the high vacuum, the evaporated atoms have
long mean-free path (i.e. they do not collide
with other atoms in their travel) and arrival of
the evaporated atoms at the wafer surface can be
considered as a line of sight problem. This
leads to shadowing by surface contour of the
wafer and may leads to poor step coverage if
there are steep features on the wafer. This is
illustrated by the diagram on the left. The
problem is overcome by mounting the wafers on a
planetary wafer holder which rotates continuously
during evaporation thus changing the direction of
arrival of the aluminium on each of the wafers
(see diagram on next slide).
Actual film deposition rate is controlled by the
evaporation rate of aluminium which is
proportional to the aluminium vapour pressure at
the temperature of the source aluminium and the
geometry of the evaporation system. In most
operation, the rate of deposition is in the order
of 0.1 to 1 micron per minute and the aluminium
is typically at 1100 to 1200 C (molten state).
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Planetary Wafer holder