State various deposition techniques. Explain in brief the technique of PVD for MEMS device Fabrication. Also define step coverage and shadowing.
7 Answers

- physical deposition techniques :

  • Thermal evaporation

  • E - beam evaporation

  • Sputtering

- Chemical deposition techniques :

  • Electrochemical deposition

  • Electroless plating

- Physical vapor deposition (pvd) :

- Chemical vapor deposition (cvd) :




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.

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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 = $\frac{F_{reacted}}{F_{incident}}$

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

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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.

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where, P = vapor pressure

K = Boltzmann's constant

T = temperature in K

d = diameter of the gas molecule

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

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