LECTURES 9 AND 10 KEY IDEAS

Thin film deposition is a key technology in modern IC fabrication. A wide variety of systems are used in manufacturing for depositing specific thin films. Two main deposition methods are used today: 1. Chemical Vapor Deposition (CVD) including: APCVD, LPCVD, PECVD 2. Physical Vapor Deposition (PVD) including evaporation, sputter deposition

CHEMICAL VAPOR DEPOSITION (CVD)

CVD systems operate at elevated temperatures and depend on chemical reactions.

Steps involved in a CVD process: • Transport of reactants to the deposition region. • Transport of reactants through the boundary layer to the wafer surface. • Adsorption of reactants on the wafer surface. • Surface reactions, including: chemical decomposition or reaction, surface migration to attachment sites (kinks and ledges). • Desorption of byproducts. • Transport of byproducts through boundary layer. • Transport of byproducts away from the deposition region.

Deposition rate in CVD systems depends one of the two factors: • mass transport of reactants (in gas) to the surface (geometry and pressure sensitive) • surface reactions rates (temperature sensitive)

An important measure of any deposition process is its speed or the deposition rate (DR)

dx Fg In general: DR   where Fg is the flux of the molecules (or atoms) that form dt N f the growing film in the gas phase assuming that they are all incorporated in the film. If dx F only a fraction Y of them end up in the film DR   g Y where Y is called the dt N f sticking coefficient.

dx Cg kshg In CVD process DR   where Cg is the reactive molecules concentration dt N f ks  hg

(volume density) in gas, ks is the reaction rate and hg is gas transport factor of reactants through the boundary layer. Cg depends on the gas pressure while ks depends strongly (exponentially) on temperature. Nf is the molecular volume density of the solid film.

LPCVD (Low Pressure CVD) In LPCVD systems, mass transport is usually not rate limiting. This can lead to increased throughput as the geometry of the system is less critical than in APCVD and many wafers can be processed at the same time with good uniformity.

PECVD (Plasma Enhanced CVD) or PACVD (Plasma Assisted CVD ) Non-thermal energy of plasma enhances processes at lower temperatures. Plasma consists of electrons, ionized molecules, neutral molecules, neutral and ionized fragments of broken-up molecules, excited molecules and free radicals. Free radicals are electrically neutral species that have incomplete bonding and are extremely reactive. (e.g. SiO, SiH3, F) The result from the molecular fragmentation, the free radicals, and the ion bombardment is that the surface processes and deposition occur at much lower temperatures than in CVD systems without plasma.

PHYSICAL VAPOR DEPOSITION (PVD)

PVD uses mainly physical processes to deposit films.

Evaporation In evaporation, source material is heated in high vacuum chamber. (P < 10-5 torr) Line-of-sight deposition occurs because pressure is low – no collisions with gas molecules. Any element can be evaporated but some elements very slowly at practical temperatures. Difficult to evaporate alloys, a separate source for each element is usually required. Compounds may disintegrate at temperatures required for evaporation. Step coverage is poor (line of sight deposition).

Sputtering or Sputter Deposition Uses plasma to generate ions which are accelerated to the target, which is the source of the material being deposited. The ions sputter (eject) atoms from the target which then deposit on the substrate (wafer) to form the film. Higher pressures are used than in evaporation (1-100 mtorr). Sputtering is better for depositing alloys and compounds than evaporation. Rate of sputtering depends on the ion current on the target and on sputtering yield (Ys), defined as the number of atoms or molecules ejected from the target per incident ion. Ys is a function of the energy and mass of ions, and the target material.

CONFORMAL COVERAGE Topography coverage and filling holes and trenches are very important, especially as circuit elements continue to decrease. In PVD systems arrival angle distribution is very important in determining surface coverage. Shadowing can be very important. This is the biggest problem for evaporation, which is rarely used today in circuit manufacturing. Special geometries and rotating of the wafers are needed to elevate this problem. Sputtering is better in this respect but the best topography coverage is achieved by CVD. •