# Ideal Gases

As a beginning it is helpful to recall some aspects of dealing with gases and vapors.

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

**PV = NRT**

P = pressure (Pascal)

V = volume (m

^{3})

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

Pressure is often also reported in other units, most frequently Torr (which are millimeters of mercury), and standard atmospheres.

## USEFUL CONVERSIONS:

1 atm = 760 Torr = 101,000 Pa

1 Pa = 7.6 milliT = 7.6 mT

1 cubic meter = 1000 liters = 10

^{6}cm

^{3}

K = C + 273

1 mole = 22.4 liters at "STP" (0 C, 1 atmosphere)

1 liter = 0.045 moles at STP

1 cm3 = 4.5x10

^{-5}moles @ STP

1 cm3 = 6.4x10

^{-8}moles @ 1 Torr, 23 C

1 A/minute = 1.6E-10*(molar density)

moles/(cm2 second)

1 SLPM = 7.4x10

^{-4}moles/second

1 sccm = 7.4x10

^{-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:

In most CVD applications absolute temperature varies modestly (factors of 2 to 3) whereas pressure varies tremendously (factors of 1000)=> large volume expansions occur! A few cubic centimeters of input gas at atmospheric pressure can become liters of gas at chamber operating pressure.

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