TimeDomain CVD, Inc.

Review: Ideal Gases

Most Gases and vapors in CVD are for practical purposes "ideal":

PV = NRT

P = pressure (Pascal)

V = volume (m3)

N = number of (gram)moles

R = universal gas constant, 8.3 Joules/mole K

T = absolute temperature (K)

Gas flows are usually measured and reported as:

standard liters per minute (SLPM) or
standard cubic centimeters per minute (sccm)

both measure gas volume at 0 C, 1 atmosphere: these are measures of MOLAR flow

USEFUL CONVERSIONS:

1 atm = 760 Torr = 101,000 Pa
1 Pa = 7.6 mT
1 cubic meter = 1000 liters = 1E6 cm3
K = C + 273
1 mole = 22.4 liters at "STP" (0 C, 1 atmosphere)
1 liter = 0.045 moles at STP
1 cm3 = 4.5E-5 moles @ STP
1 cm3 = 6.4E-8 moles @ 1 Torr, 23 C
1 A/minute = 1.6E-10*(molar density)
moles/(cm2 second)
1 SLPM = 7.4E-4 moles/second
1 sccm = 7.4E-7 moles/second

 

k = 1.38E-23 J/°C
R = N(Avogadro)¥k
= 8.3 J/ mole °C
= 8,300 J/kg-mole °C
= 8.2x10-2 liter atm/mole °C
= 62 torr liter/ mole °C
an important consequence of the ideal gas law for practical CVD reactors:

volume flow increases tremendously for the same molar flow (i.e. same slpm) at lower pressures

 

 

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