Physical evaporation is one of  the oldest methods of depositing metal films.  Aluminum and gold are heated to the point of vaporization, and then evaporate to form to a thin film covering the surface of the silicon wafer.  All film deposition takes place under vacuum or very controlled atmosphere.  The degrees of vacuum and units is shown below:

    1 atm = 760 mm = 760 torr = 760,000  mm Hg
              = 29.92 in Hg = 14.7 lb/in2 = 14.7 p.s.i
              = 1,013,250 dynes/cm2

    1 torr = 1mm Hg
    1 millitorr = 1 mm Hg

Degrees of vacuum:
 

low vacuum 760 to 25 mm Hg
Rough vacuum 760 to 1 mm Hg
High vacuum 10-3 to 10-6 mm Hg
Very high vacuum 10-6 to 10-9 mm Hg
Ultra-high vacuum < 10-9 mm Hg = vacuum in space
Kinetic Gas Theory
 
The ideal gas law can describe the behavior of gases under vacuum.  Pressure P, volume V, and temperature T of one mole of a gas are related by

                    PV = Nav kT                    Nav=Avogadro's #, k=Boltzman constant

The concentration of gas molecules is given by
 
                    n = Nav/V = kT/P

the rate of formation of a surface layer is determined by the impinging molecules if 100% stick

F = P/Ö (2Õ mkT) (molecules/cm2 - sec)

where m is the mass of the molecule.  This can be reduced to

F = 2.63 x 1020P/Ö (MT)

P = pressure in Pa
M = molecular weight

The time required to form a monolayer on the surface is given by

T = NS/F = NSÖ (2Õ mkT)/P

where Ns is the number of molecules/cm2 in the layer.  Pressure and temperature also determine another important film-deposition parameter called the mean free path.

l = kT/(Ö 2)Õ Pd2    where d is the diameter of the gas molecule at room temperature and using an approximate size of the molecule.

l = (L in cm) = 5/Pressure( in microns)
 

Vacuum Processing Equipment

Most thin films are deposited under reduced pressure conditions.  Mechanical pumps and absorption pumps are used in the rough pressure range.

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