0
2.3kviews
State various deposition techniques. Explain in brief the technique of PVD for MEMS device Fabrication. Also define step coverage and shadowing.
0
15views

- physical deposition techniques :

• Thermal evaporation

• E - beam evaporation

• Sputtering

- Chemical deposition techniques :

• Electrochemical deposition

• Electroless plating

- Physical vapor deposition (pvd) :

- Chemical vapor deposition (cvd) :

• APCVD

• LPCVD

• PECVD

Physical vapor deposition :

• Thin metallic films are needed for fulfilling some requirements of MEMS devices and systems.

• Some of these requirements fall in the category of biasing, signal propagation, signal receiving, etc

• These metallic films are often obtained using different physical vapor deposition techniques like thermal evaporation, electron beam evaporation and sputtering.

• All these techniques use some form of energy that leads to an evaporation of metal.

• The metal vapor then travels to a suitably located substrate and is deposited on it after condensation.

• The most commonly used physical vapor deposition technique is sputtering

• figure (a) shows a schematic representation of sputtering process.

fig (a) Schematic representation of sputtering process.

• This plasma can be created by using either high voltage dc source or RF source

• The ions present in the plasma bombard the surface of the target with very high velocities.

• The resultant momentum transfer causes the metal ions to evaporate

• Metal vapors reach the substrate surface and get deposited due to condensation.

Evaporation issues :

1. step coverage :

• Step coverage is poor line of sight and sticking coefficient $S_c \approx 1$.

• Heating can decreases $S_c$, but may change film properties.

• Therefore, due to poor step coverage, rarely used in IC fabrication.

sticking coefficient : SC = $\frac{F_{reacted}}{F_{incident}}$

a) The depositing species have a high $S_c$ so that they are deposited where they first strike

b) The depositing species have a low $S_c$, so that many are re-emitted and re-deposited ( or migrated ) elsewhere on the topography such as on the sidewalls.

• The step coverage of evaporated films is poor due to the directional nature of the evaporated material.

• Heating and rotating the substrates help with step coverage problem, but evaporation can not form continuous films for aspect ratios greater than 1

AR= $\frac{step \ height}{step \ width}$

a) Low adatom mobility and non rotated b) heated and rotated

Shadowing : Problem can occur in low pressure vacuum deposition in which the mean free path is large.

• MFP : distance a molecule travels in a straight line (in vaccum) before its velocity vector is randomized by a collision

• This equation represent the average velocity of the molecules times the average time between collisions.

where, P = vapor pressure

K = Boltzmann's constant

T = temperature in K

d = diameter of the gas molecule

$\frac{t_1}{t_2}$ = $\frac{cos\beta_1}{cos\beta_2} = 3$