ECE 6450 - Dr. Alan DoolittleGeorgia TechLecture 10Vacuum Technology and PlasmasReading:Chapter 10ECE 6450 - Dr. Alan DoolittleGeorgia Techλππ==12222dnkTdPIn order to understand deposition techniques such as evaporation, sputtering, , plasma processing, chemical vapor deposition, molecular beam epitaxy, etc…, we must first have a grasp of the manner in which vacuum and gases flow. Thus, we will begin with a discussion of vacuum physics and vacuum techniques. Definitions of Vacuum Regimes:1.) Rough Vacuum: ~0.1-760 torr (atmospheric pressure is 760 torr)2.) Medium Vacuum: ~ 0.1 to 10-4torr3.) High Vacuum: ~ 10-8to 10-4torr4.) Ultrahigh Vacuum: < 10-8torr2 modes of gas flow:1.) Viscous Flow regime: gas density (pressure) is high enough, many molecule-molecule collisions occur and dominate the flow process (one molecule “pushes” another). Collisions with walls play a secondary role in limiting the gas flow.2.) Molecular flow regime: gas density (pressure) is very low, few molecule-molecule collisions occur and molecule-chamber wall collisions dominate the flow process (molecules are held back by walls)The average distance between collisions (mean free path) is:Vacuum Science and Plasmaswhere d is the molecule diameter in meters, k=1.381e-23 J/K, and pressure, P, is in pascal (d~ 3 Angstroms for diatomic molecules).At room temperature, λ is 78 um for 1 torr (typical plasma process pressure) and 7.8e6 meters for 1e-11 torr (typical Molecular Beam Epitaxy systems).ECE 6450 - Dr. Alan DoolittleGeorgia TechThe Knudsen number (Kn) is used to distinguish between regimes. Kn(dimensionless number) is ratio of the mean free path to the characteristic dimension of the chamber (can be diameter of a pipe, or vacuum chamber). When Kn>1 then you are in the molecular flow regime. When Kn<0.01 you are in the viscous flow regime. In between, the flow characteristics are indeterminate. Vacuum pump systems are characterized by throughput, Q, which is a measure of the mass flow through a system and pumping speed. Units are pressure-volume/time such as:torr-liters/secondsccm=standard cubic centimeters per minute (or cubic centimeters @ 1 atmosphere (760 torr)/minute)slm=standard liters per minute (or liters @ 1 atmosphere (760 torr)/minute)Note: 1 standard liter is 1/22.4 moles of gas.Knudsen Numbers and Pumping TechnologyECE 6450 - Dr. Alan DoolittleGeorgia TechThroughput, Q=C(Pupstream–Pdownstream) where C is the conductance (we will use Liters/Sec unit) and…For Molecular flow:For a tube C=11.6(D3/L) where D is the tube diameter in cm, L is the tube length in cm and P is pressure in torr. Note for this case, C is independent of pressure. For an orifice (small hole with negligible length), C=11.6 πD2/4.For Viscous Flow:For a tube, C=180(D4/L)PaverageA more accurate means of characterizing gas flow in this regime is Q=K(P2upstream–P2downstream) where K is a constant related to conductance, C by the relationship K=C/(2 x Paverage) where Paverage=(Pupstream-Pdownstream).Proof:Conductance and Pumping Speed()()()()()()()⎟⎟⎠⎞⎜⎜⎝⎛=∴−=−⎟⎟⎠⎞⎜⎜⎝⎛=−+−=+−=−=−=averagedownstreamupstreamdownstreamupstreamaveragedownstreamdownstreamupstreamdownstreamupstreamupstreamaveragedownstreamupstreamdownstreamupstreamaverageaveragedownstreamupstreamaveragedownstreamupstreamPCKPPKQPPPCQPPPPPPCQPPPPPCQPPPPCQPPPCQ22212222222ECE 6450 - Dr. Alan DoolittleGeorgia TechLike electrical conductance, parallel conductance add while series conductance add in reciprocal form 1/Ct=1/C1 + 1/C2 + .... Pumps are specified in terms of pumping speed, S, (units volume/time) which is related to throughput as,S=Q/Pat the pump inlet. (Note: S is a function of P)A pump can be used with a tube to calculate the effective pumping speed at the end of the tube:pumpeffSCS /1/1/1+=Conductance and Pumping SpeedChamber @ pressure P1Pump with speed Spump@ pressure P2Gas InletQTube of Conductance CChamber @ pressure P1Pump with speed Spump@ pressure P2Gas InletQ2 Tubes of Conductance C1and C2Ct=C1+ C2Chamber @ pressure P1Pump with speed Spump@ pressure P2Gas InletQ2 Tubes of Conductance C1and C21/Ct=1/C1 + 1/C2ECE 6450 - Dr. Alan DoolittleGeorgia TechConductance and Pumping SpeedPump down time: (For Viscous Flow)The pressure after a pump is turned on is:Where P is pressure, Po is original pressure (for example 760 torr for atmospheric pressure), S is the pumping speed, V is the volume of the chamber and t is time.Thus, the time to reach a particular vacuum is:VStoePP−=⎟⎠⎞⎜⎝⎛=PPSVtolnExample: How long does it take to pump down a 50 liter chamber from atmosphere (760 torr) to 60 mtorr directly using a mechanical pump with a speed of 9 L/sec verses through a 0.5” diameter, 8 foot long tube using a pump.Directly:Through Tube:seconds52 torr0.06 torr760ln9L/sec50Lt =⎟⎠⎞⎜⎝⎛=In reality, the pumping speed of a pump and the conductance of the tube normally is not constant with pressure, resulting in a significant increase in the pump down time. You can see this by noticing the Conductance at 60 mTorr is 0.05 L/S While at 760 torr it is ~729 L/S. We can integrate or to simply get an estimate of the actual pump down time by breaking the calculation up into smaller pressure steps:()()seconds52 torr0.06 torr760ln9L/sec50Ltsec/9sec/9106.07605.054.2*1280.5x2.54180111C11S4tubingeff=⎟⎠⎞⎜⎝⎛=⇒≈++⎟⎟⎠⎞⎜⎜⎝⎛=+= LLxxSPtopPbotCtubingSef fPump down Time in step760 76 802.7101 8.900211 12.9355776 7.6 80.27101 8.092651 14.22647.6 0.76 8.027101 4.242878 27.134710.76 0.076 0.80271 0.736979 156.21780.076 0.06 0.130584 0.128717 91.82513Total: 302.3396 sec or 5.038994 minutesECE 6450 - Dr. Alan DoolittleGeorgia TechTypes of Pumps1.) Rough/medium vacuuma.) Piston pumps (not used much due to particle problems)b.) Rotary vane pumps (majority of cheap applications)c.) Dry pumps (no oil back streaming)d.) Add a Roots blower (similar to a supercharger on a drag racer) Increases the pressure on the primary pumps inlet by “pre-compressing” the gases. If kois the compression ratio of the roots blower, Srbis the pumping pumping speed of the RB, and Sp is the pumping speed of the pump, then the effective pumping speed of the combo is,porboprbeffSkSkSSS+=ECE 6450 - Dr. Alan DoolittleGeorgia TechTypes of Pumps2.) High and Ultrahigh Vacuuma.) Diffusion pump (not used much due to oil contamination)b.) Turbopump (oil, grease and oil free lubrication of