Page 305 - A Practical Guide from Design Planning to Manufacturing

Semiconductor Manufacturing 275

The temperature and relative pressure of the reactant gases deter-
mine the rate of deposition. CVD can be carried out at normal atmos-
pheric pressure (APCVD) and can achieve high deposition rates at
relatively low temperatures. However, the flow of the reactant gases
must be very carefully controlled. Because of the high pressure, the
reactant chemicals diffuse more slowly through the chamber, and it is
easy to have a higher concentration in one part of the chamber than the
other. This will lead to different rates of deposition and different film
thickness on different wafers.
Performing CVD at low pressures (LPCVD) produces higher-quality
films and makes the flow of gases less of a concern, but higher temper-
atures must be used to achieve the same deposition rate. To achieve high
deposition rates at low pressures and low temperatures, many chambers
use plasma-enhanced CVD (PECVD). In this process, microwaves are
used to strip electrons off the reactant gas molecules, turning the gas
into plasma. In this state, reactions proceed much more quickly even at
low temperatures. PECVD is used commonly enough that silicon diox-
ide deposited in this fashion is often referred to as plasma oxide (PO).
The biggest benefit of CVD over sputtering is that it provides excel-
lent step coverage and gap fill. Deposition will occur at the same rate
on every surface regardless of its orientation, so filling deep via holes is
not a problem. The biggest disadvantage is that it is not practical to
deposit some materials by CVD. To use CVD, a suitable chemical reaction
must be found, which produces the desired deposition material at a suffi-
ciently low temperature, without any unsuitable by-products. Some reac-
tants may be too expensive to manufacture. Some reactions may occur too
slowly at temperatures low enough to not damage already deposited layers.
Some by-products contaminate the wafers or are too dangerous to work
with safely at a reasonable cost. By-products like gaseous HCl and HF are
extremely corrosive and toxic, and H is explosive. Handling these chem-
2
icals safely adds significantly to the cost of CVD.
A second disadvantage of CVD is difficulty in depositing mixtures of
materials. Aluminum is commonly used as a conducting layer, but sili-
con tends to diffuse into aluminum wires, causing the aluminum to
spike into the silicon surface and potentially short shallow diffusion
layers to the well in which they sit. A common solution is to add 1 per-
cent silicon to the aluminum material. Another problem with aluminum
is poor electromigration. The aluminum atoms move easily out of regions
of high current. Adding 0.5 percent copper can dramatically improve the
reliability of aluminum wires. When sputtering, all that is needed is a
target with the desired mix of materials. Depositing such a mixture
with CVD would require multiple simultaneous reactions proceeding at
precisely controlled rates. As a result, the preferred method for deposit-
ing alloys is sputtering.