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CHEMICAL VAPOR DEPOSITION

CHEMICAL VAPOR DEPOSITION 14.13

A commercial CVD tool is a complex piece of equipment capable of processing up to 100 wafers
per hour (or more) and costing millions of dollars. It may consist of multiple CVD modules or CVD
modules combined with other processes like etch or PVD. In addition to the components discussed
here, there will be control systems to run the machine and sensors to monitor its state, as well as a
computer for interacting with the operator. There may also be metrology equipment for measuring
the thickness and uniformity of the film just deposited and reporting any issues back to a central
computer in the fab.

14.4 PRECOATING AND CLEANING

Why Precoat. After a CVD chamber has been opened for maintenance or cleaning, it must be run
for some time before the deposition rate, thickness uniformity, and other film properties stabilize.
The time required varies with the process, but is typically in the range of one to ten wafers worth of
deposition. It is difficult to determine why CVD processes exhibit this transient behavior upon startup.
It is likely that the maintenance or cleaning leave residual chemicals behind that poison the reaction.
Once these are consumed, or buried, the process then runs at its normal rate.

Methods of Cleaning. CVD chambers are cleaned with plasma processes. Frequently fluorine con-
taining molecules such at NF , CF , and C F are used. The resulting F and F species react with the
3 4 2 6 2
CVD film that has deposited on the chamber walls and platen and form volatile compounds that are
then pumped away. For films that don’t contain a volatile fluorine containing compound other clean
gases must be used. For instance, chlorine is effective at removing Al films whereas F is not.
Unfortunately, some materials don’t readily form volatile compounds. Copper, for instance, is very
difficult to remove by plasma cleaning.
In situ plasma cleaning can, over time, wear chamber components out leading to frequent main-
tenance and additional expense. The ion bombardment that accompanies the plasma process rough-
ens surfaces. Additionally AlF layers can grow on aluminum parts. These changes can lead to drift
3
of uniformity or deposition rate. Because of this, CVD tools frequently use remote plasma for clean-
ing. Inductively coupled or microwave high-density plasma is generated upstream of the chamber
and the F radicals are fed into the chamber. The very short-lived ions are lost to wall recombination
before the gas reaches the chamber. The result is a purely chemical clean with no ion bombardment.

14.5 TROUBLESHOOTING

CVD processes are complex and it is not uncommon for things to go wrong. Once a process goes
out of spec, a good understanding of CVD fundamentals, combined with a systematic approach, will
help identify and resolve the problem. Unfortunately it is human instinct to draw hasty (and poor)
conclusions from incomplete data. (See Dorner for a fascinating discussion of this trait). 17 Some
common problems along with possible causes are listed next.

14.5.1 Particles
A well-designed CVD system will have excellent particle performance. However, particle problems
do crop up from time to time.
Gas-Phase Nucleation. Gas-phase nucleation occurs when the process gases react in the gas
phase and begin growing particles. This will typically occur when there has been a process change
that promotes gas-phase reactions. These changes include increasing the process pressure, reac-
tant concentrations, and temperature. Another possible cause is changing from a light carrier gas

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