| Daniel M. Dobkin |
High Density Plasma Deposition: Overview
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HDP Film Requirements:
In our discussion of high
density plasma oxide films, we saw that in order to obtain
the desired results of high deposition rate, and good gap filling
and planarity (and thus high sputter rate), we need to build a
reactor that provides very high plasma densities (exceeding 1010
electrons/cm3). High sputter yield also requires that
we accelerate ions in the sheath to several hundred volts, so
we need a high plasma potential (at least relative to the wafer).
We'd like this all to happen at a well-controlled temperature,
much less than 400 C for intermetal dielectric applications, and
we want to have well-controlled stoichiometry throughout the deposition.
These requirements force us to move to radically different reactor
designs from the showerhead plasma:
- much lower pressures (a few mTorr) are needed to minimize
scattering in the sheath and obtain high-energy, directional
ions for sputtering; low pressure also helps us achieve good
uniformity at high gas flows
- at these low pressures, showerhead reactors cannot achieve
high plasma density [energetic sheath electrons pass right through
the plasma]; instead, we use more exotic approaches such as electron-cyclotron resonance
or inductive excitation.
Such methods often place severe demands on reactor materials
and design
- the requirement for high ion flux at high ion energy means
that hundreds or even thousands of watts of power are dissipated
on the wafer, which is in a vacuum environment where heat transport
due to conduction and convection
is negligible. With radiative equilibrium alone, the wafer
will rapidly heat up to above 600 C. Some means of cooling the
wafer is needed; generally, the approach adopted is to dispense
several Torr of helium on the back of the wafer. This gas would
cause the wafer to float away from the chuck, so a clamping mechanism
is needed, but a mechanical clamp generates particles and disturbs
the plasma. An electrostatic chuck is the solution, but involves
more non-trivial material and procedural challenges.
- process control is of great importance: if the RF potential
is turned on before the silane gas reaches the chamber, the edges
of the metallization will be rapidly sputtered, causing dimensional
loss and leakage from metal sputtered onto the oxide surface
between the lines. If the silane is turned on before the RF potential
is applied, the initial layers will not be sputtered and will
have poor gap fill properties, perhaps making it impossible to
fill a high-aspect-ratio gap. "Run/vent" plumbing configurations
(in which gas from the mass flow controllers is directed into
the exhaust until needed, and then immediately directed into
the chamber with valves immediately adjacent to the gas dispense)
can be useful.
Thus, there are many technological challenges to fabricating
a working, practical HDP reactor. In the remainder of the discussion
we will look in some more detail at the challenges and their solutions.
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