The first commercial HDP oxide deposition
reactor designs were based on electron
cyclotron resonance plasmas. For example, the Lam Epic reactor looked
schematically something like this:
Although most modern HDP tools are based on inductive
plasma generation, this reactor design illustrates many of the
elements common to all HDP reactors:
- High density plasma source: in the
Epic reactor, an ECR plasma was created by illuminating a magnetized
region with high-power microwave radiation. At a magnetic field of
about 875 Gauss, the electron Larmor frequency is equal to the
microwave frequency of 2.45 GHz (another frequency used for economic
reasons: it is the microwave oven frequency). At low enough pressures,
electrons gain energy on each circuit around the field lines: the
field allows both confinement and efficient electron heating. ECR
sources can achieve densities around 1E12/cm2 at pressures of a few
mTorr (this is several percent ionization!).
- Big pumps: to deposit at high rates,
relatively large flows of silane (and thus oxygen and argon) are
required. To achieve a throughput of about 12-15 wafers per hour from
a module for a deposited film 0.75 micron thick, we might require a
deposition rate of about 5000 A/minute. About 14 sccm of silane are
required simply to provide enough silicon atoms to deposit this film
on the wafer surface, but the film deposits everywhere in the chamber.
A realistic flow might be 30 sccm of silane, with roughly twice as
much oxygen and some argon: perhaps 150 sccm total flow. That may not
seem like a lot, but at 5 mTorr it is a volumetric flow of 23,000
liters/minute or about 400 liters/second! The pumping capacity
required to remove the gas is even larger, since the pump ports and
gate valves have a finite conductance: several thousand liters per
second is needed for practical reactors. Fast pumping of hydrogen is
needed as well, since this gas is generated by the decomposition of
the silane. The only usable approach is to employ turbomolecular
pumps. Turbo pumps which can achieve these speeds are very large, very
heavy, and very expensive: tens of thousands of dollars for each unit.
- Wafer RF bias: The ECR plasma has a
very low plasma potential. In order to achieve significant sputtering
rates at the wafer, it is necessary to apply an RF bias, which
involves a separate power supply and matching network (but has the
nice feature that plasma density and ion energy are then independently
controllable). Several hundred volts of ion energy are required.
Recall that the current density is on the order of e.g. 10 mA/cm2, for
a total current of about 3 amperes to a 200 mm wafer. Typical RF bias
power for a 200 mm wafer is 500-1000 Watts, almost all of which is
dissipated onto the wafer surface.
- Wafer temperature control: At the low
pressures of operation of HDP reactors, very little heat will be
transported from the wafer by conduction or convection; recall from
our discussion of showerhead heat
transport that radiation will dominate at high temperature and low
pressure. To remove 500 Watts from a 200 mm wafer by radiation, the
wafer would need to achieve a temperature of about 500 C, much too
high for use in conjunction with Al metallization. Thus it is
necessary to monitor and control the wafer temperature. The control is
usually achieved by injecting a small flow of helium behind the wafer
to raise the pressure to a few 10's of Torr there; however, the
resulting force must be compensated to keep the wafer in position,
requiring the use of mechanical clamps or an electrostatic chuck.
- Gas injection: It is necessary to put
the silane into the chamber, and ideally to dispense it close to the
wafer so that efficiency of utilization is improved. For total gas
flows of 100-200 sccm in the typically large chambers employed in HDP,
transport is dominated by diffusion. The mean free path may be as
large as 1 cm, however: transport in local regions cannot be modeled
accurately using continuum techniques. Thus design of the injecting
apparatus is difficult, and it may also be necessary to compromise
efficiency to improve uniformity.
- Chamber clean: The combination of high
density plasma, low pressure, and high sputtering rates at the wafer
surface means that a lot of the silicon dioxide formed ends up on the
chamber walls. This buildup will spall off, creating particles, if it
becomes too thick. The Lam reactor had water-cooled walls to help
avoid temperature variations that would increase spalling.
Nevertheless, it is also necessary to remove the deposit from the
walls periodically, typically after 10-25 wafers. This may seem
straightforward: there's already a high density plasma, so if one
introduces some source of fluorine the oxide will etch. However, many
difficulties arise. Etching SiO2 requires either copious ion
bombardment or high fluxes of fluorine (i.e. high pressures, typically
several Torr). ECR sources don't operate well at these pressures, and
at low pressure the plasma potential is small so there's no wall
bombardment. Inductive sources operated at high pressure tend to
concentrate the power delivery very near the wall, which can lead to
localized heating of the dielectric liner ("dome") and
consequent erosion and cracking. Applied Materials has introduced a
separate microwave plasma
source remote from the chamber to provide fluorine for chamber
cleaning.
An inductive plasma realization, the Novellus SPEED reactor, is shown
schematically below:
In this case, the inductive coils are distributed over the ceiling to
provide improved plasma uniformity over the wafer surface, vs. a
solenoidal coil. However, in both cases the actual electron heating is
confined to within a skin depth
of the coils; rapid diffusion
at low pressure enables the plasma to fill the chamber.
Reactor Operation
The method of turning on the process is of particular importance in the
use of HDP reactors for intermetal dielectric deposition. Deposition and
sputtering both occur in the process. If RF power is applied to the chuck
before silane is present in the chamber, the underlying metallization will
be sputter-etched. This can lead to undesirable faceting of the metal
lines (reducing their cross-section, with as a consequence reduced
reliability and conductivity), and to electrical leakage between lines due
to the deposition of a thin metallic film on the lower insulator surface.
On the other hand, if silane is added to the reactor before the RF bias is
ramped up, the initial deposited oxide will have relatively poor
conformality, and may form a "lip" which prevents filling
high-aspect-ratio features. Therefore it is vital to carefully synchronize
power and gas flows. Placing gas flow controls a long distance from the
reactor is undesirable. One possible approach is to provide
"run/vent" valves at the reactor, which can immediately switch
the chamber inlet from argon to an argon/silane mixture with no change in
flow and minimal time delays.
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