17-08-2012, 04:06 PM
Microelectronics Processing Chemical Vapor Deposition
CVD.ppt (Size: 1.22 MB / Downloads: 48)
CVD deposition
Chemical Vapor Deposition is the formation of a
non-volatile solid film on a substrate by the reaction
of vapor phase chemicals (reactants) that contain the
required constituents.
The reactant gases are introduced into a reaction
chamber and are decomposed and reacted at a heated
surface to form the thin film.
Plasma enhanced CVD system (PECVD)
As the thermal budget gets more and more constrained while more and more layers need to be added for multi-layer metallization, we want to come down with the temperature for the oxide ( or other) CVD processes.
One way for doing this is to supply the necessary energy for the chemical reaction by ionizing the gas, thus forming a plasma.
CVD of Si - Epitaxy
When SiH4 gas is used in a CVD reactor, a Si layer
is deposited on the wafer surface. The size of the
crystallites depends on the deposition temperature.
At high enough temperature, the ad-atoms have
enough kinetic energy to move on the surface and
align themselves with the underlying Si.
This is an epitaxial layer, and the process is called
Epitaxy instead of CVD.
At lower deposition temperatures, the layer is
poly-crystalline Si (consisting of small crystallites)
Tungsten (W) CVD
Ironically, W-CVD comes straight form nuclear power technology: High purity Uranium (chemical symbol U) is made by a CVD process using UF6 as the gas that decomposes at high temperature.
W is chemically very similar to U, so we use WF6 for W-CVD.
A CVD furnace, however, is not good enough anymore. W-CVD needed its own equipment, painfully (and expensively) developed a decade ago.
We will not go into details, however. CVD methods, although quite universally summarily described here, are all rather specialized and the furnace type reactor referred to here, is more an exception than the rule.
Advantages of CVD processes
CVD processes are ideally suited for depositing thin layers of materials on some substrate. In contrast to some other deposition processes which we will encounter later, CVD layers always follow the contours of the substrate: They are conformal to the substrate as shown below.
Disadvantages of CVD processes
The two most important ones (and the only ones we will address here) are:
They are not possible for some materials; there simply is no suitable chemical reaction.
They are generally not suitable for mixtures of materials.