Deposition

Materials include wires (Al, Cu, and W) and dielectrics.

There are four major methods used for deposition

1. growth
2. Chemical Vapor Deposition (CVD)
3. spin-on
4. Physical Vapor Deposition (PVD)

The first two are chemical processes. The last two are transport processes.

Quality depends on environment control (temperature, pressure) and purity of materials.

There are a number of gasses used in semiconductor production: Argon, Oxygen, Helium, and Nitrogen are obtained from the air. Nitrogen is distributed through a pipeline in Silicon Valley.

The following characteristics of films are important

1. film stress
2. film adhesion
3. film composition
4. film compatibility
5. film conformality
6. ease of deposition

Oxidation

SiO₂ is formed when an oxidant is introduced as gas or water vapor. The oxidant supplies oxygen. The silicon comes from the substrate itself. Vapor is introduced at 800-1000 degrees C. The rate of oxide formation depends on temperature and oxide availability.

Thickness control is essential as are resulting properties

1. dielectric constant
2. dielectric strength
3. low impurity level

CVD

Silicon nitride is deposited via CVD. Combination of silene (SiH₂Cl₂) and ammonia (NH₃) form silicon nitride (Si₃N₄). Ammonia is provided in abundance. Process is controlled by other active ingredients. Carrier gasses are used to dilute active materials.

Nitride uses a low pressure furnace. At high temperatures the bonds break and allow ions to react. A temperature profile in the furnace adjusts uniformity.

Doped polysilicon is used to control resistivity, reduce contact resistance, and reduce electromigration.

Silane diluted by nitrogen is introduced at 620 degrees C. Need to be concerned with high vertical sidewalls and corner pockets.

Nitride sidewall spaces use CVD process conducted at moderate pressure and relatively low temperature 400 deg C.

Annealing at high temperature with lamps.

TEOS/ozone producce BPSG (Boro-phosphate silicate glass).

Planarization is used to smooth surface and reduce required depth of focus for lithography.

Spin-on process

Chemicals spread over spining wafer, then baked and annealed.

Chemical - mechananical polish sometimes used for planarization

Tungsten plugs are used as contacts and vias.

Sputtering

Target material is placed near wafer. Large electric field ionizes gas and target material spreads everywhere.

Tungsten is deposited via CVD.

Aluminum is sputtered on and then etched.

Once Aluminum is deposited, temperature is an issue. High temperatures will melt Al.

Plasma etching and CVD - RF is used to drive chemical reaction.

New Materials

New materials are constantly being investigated. Cu is being used instead of Al. New insulation materials may be carbon-based or polymer based.

Maintained by John Loomis, last updated 17 Nov 2003