Vacuum Systems

EE-527: MicroFabrication

Vacuum Systems

R. B. Darling / EE-527 / Winter 2013 Outline

• Vacuum principles • Vacuum pumps • Vacuum systems • Vacuum instrumentation • Vacuum materials and components

R. B. Darling / EE-527 / Winter 2013 Uses of Vacuum in Microfabrication

Rough Vacuum High Vacuum Ultra-High Vacuum wafer chucks evaporation surface analysis load locks ion implantation molecular beam epitaxy (MBE) sputtering reactive ion etching (RIE) low pressure chemical vapor deposition (LPCVD)

R. B. Darling / EE-527 / Winter 2013 Units of Pressure Measurement

• 1 atmosphere = – 760 mm Hg = 760 torr – 760,000 millitorr or microns – 29.9213 in. Hg – 14.6959 psi

– 1.01325 bar 760 mm Hg – 1013.25 millibar 33.93 ft H2O – 101,325 pascals (Pa)

– 407.189 in. H2O

– 33.9324 ft. H2O

1 Pascal = 1 N/m2 1 torr = 1 mm Hg 1 micron = 1 m Hg

R. B. Darling / EE-527 / Winter 2013 Vacuum Ranges

• Low or Rough Vacuum (LV) – 760 to 1 torr • Medium Vacuum (MV) –1 to 10-3 torr • High Vacuum (HV) –10-3 to 10-7 torr • Ultra-High Vacuum (UHV) –10-7 to 10-12 torr

R. B. Darling / EE-527 / Winter 2013 Partial Pressures of Gases in Air at STP

Gas Symbol Volume Percent Partial Pressure, Torr

Nitrogen N2 78 593 Oxygen O2 21 159 Argon Ar 0.93 7.1

Carbon Dioxide CO2 0.03 0.25 Neon Ne 0.0018 1.4 x 10-2 Helium He 0.0005 4.0 x 10-3 Krypton Kr 0.0001 8.7 x 10-4 -4 Hydrogen H2 0.00005 4.0 x 10 Xenon Xe 0.0000087 6.6 x 10-5

Water H2O Variable 5 to 50, typ.

Standard Temperature and Pressure (STP): 0C and 1 atm.

R. B. Darling / EE-527 / Winter 2013 Ideal Gas Law - 1

• V = volume of enclosure • N = number of molecules

• Nm = number of moles = N/NA • n = particle density = N/V • P = pressure • T = absolute temperature -23 • kB = Boltzmann’s constant = 1.381 x 10 J/K 23 • NA = Avogadro’s number = 6.022 x 10 particles/mole

• R = Gas constant = NAkB = 8.315 J/mole-K

PV  Nm RT

PV  NkBT

P  nkBT

R. B. Darling / EE-527 / Winter 2013 Ideal Gas Law - 2

• Historical Laws for ideal gases:

– Boyle’s Law: P1V1 = P2V2 at constant T

– Charles’ Law: V1/T1 = V2/T2 at constant P

– Gay-Lussac’s Law: V = V0(1 + T/273)

– Henry’s Law: Pi = Kixi (partial pressure of a gas above a liquid)

– Dalton’s Law: P = P1 + P2 + … + Pn (sum of partial pressures)

• Real gases are better described by van der Waals’ equation of state:  an2   P  V  nb  nRT  2   V  – an2 term accounts for intermolecular attraction – nb term accounts for intermolecular repulsion

R. B. Darling / EE-527 / Winter 2013 Kinetic Gas Theory  • Velocity of a molecule is v  vx xˆ  vy yˆ  vz zˆ

2 2 2 2 • Mean square velocity is v  vx  vy  vz

2 • Pressure exerted on a wall in the x-direction is Px  nmvx

2 2 • If velocities for all directions are distributed uniformly, v  3vx

• Thus, 1 2 1 2 3 P  3 nmv  nkBT 2 mv  2 kBT

• Each molecular DOF has an average excitation of kBT/2.

R. B. Darling / EE-527 / Winter 2013 Distribution Functions - 1

• Boltzmann’s postulates for an ideal gas: – The number of molecules with x-components of velocity in the 2 range of vx to vx + dvx is proportional to some function  of vx only:  dN dN dN vx  (v2 ) dv vy  (v2 ) dv vz  (v2 ) dv N x x N y y N z z – The distribution function for speed v must be the product of the individual and identical distribution functions for each velocity component:  dN  vx,vy,vz  (v2 ) dv dv dv  (v2 ) (v2 )(v2 ) dv dv dv N x y z x y z x y z

R. B. Darling / EE-527 / Winter 2013 Distribution Functions - 2

• A mathematical solution to the above equations has the

form of (A and vm are constants):

2 2 2 vx / vm (vx )  Ae

• Normalization of the distribution functions:

  2 2 dN  NAevx / vm dv  NA v2  N A  (v2 )1/ 2  vx  x m m     4 1 4v 2 2 3k T v2  v2 dN  ev / vm dv  3 v2  B  v  3 2 m N 0 0 vm  m 1/ 2 2  2kBT  dv dv dv  4v dv vm    x y z  m  R. B. Darling / EE-527 / Winter 2013 Distribution Functions - 3 • Normalized distribution function for a single velocity component (Gaussian):  1/2 1/ 2 2 2kT 2  m   mvx  B (v )    exp  vm   x     m  2kBT   2kBT   • Normalized distribution function for velocity magnitude (Gaussian):  1/2 3/ 2 2 2  m   mv  3kTB (v )    exp  vrms       m  2kBT   2kBT  

• Normalized distribution function for a randomly directed velocity (Maxwellian): 3/ 2 1/2   m   mv2  8kT 2   2   v  B (v )  4   v exp  a   2kBT   2kBT   m

R. B. Darling / EE-527 / Winter 2013 Distribution Functions - 4 Maxwellian Distribution of Randomly Oriented Velocities: (v2)

1/2 8kTB va    m

This is sometimes referred to as the average thermal velocity.

velocity, v 0 va

R. B. Darling / EE-527 / Winter 2013 Impingement Rates

• The number of molecules with a velocity from vx to vx + dvx 2 is dNvx = N(vx ) dvx. • A = area under consideration.

• Only those molecules within striking distance vxdt will hit the wall after dt seconds.

• The number of molecules with velocities from vx to vx + dvx impinging upon the wall per time dt is  1/ 2 N   k T  d 2 N  Av (v2 ) dv dt v (v2 ) dv  B vx x x x integrate:  x x x   V 0  2m 

1/ 2 dNi N  kBT  1/ 2     2mkBT P Adt V  2 m 

R. B. Darling / EE-527 / Winter 2013 Mean Free Path • MFP is the average distance a gas molecule travels before colliding with another gas molecule or the container walls. • It depends only on the gas density and the particle size. •  is the diameter of the particles • 2 is the cross-sectional area for hard-sphere collisions

V kT MFP   B NP2222  For common gases, {H2O, He, CO2, CH4, Ar, O2, N2, H2}, at T = 300 K:

5 x 10-3 torr-cm Mean Free Path (cm) = Pressure (torr)

R. B. Darling / EE-527 / Winter 2013 Viscosity of a Gas

• Viscous range: λ < D: – The viscosity is independent of gas pressure or density:

1/2 4mk T 0.499nmv 0.499 B a  3/2 2 • Free molecular range: λ > D: 1/2 Pmv m  fm P 42kTBB kT • Viscosity units: – 1 Pa-sec = 10 Poise (P); air at RTP has η = 1.78 x 10−4 P. –10−2 Poise = 1 cP; water at 25°C and 1 atm has η = 0.899 cP.

R. B. Darling / EE-527 / Winter 2013 Thermal Conductivity of a Gas • Conductive heat flow:

2 QT  W/m 

• Thermal conductivity: [W/m-°K] (in the viscous range) 1  95cc 4 vv

• cv = specific heat at constant volume

• cp = specific heat at constant pressure

• γ = cp / cv. • The thermal conductivity of a gas is directly proportional to the viscosity and is also independent of gas pressure or density, as long as λ < D = chamber size. R. B. Darling / EE-527 / Winter 2013 Surface Accommodation Coefficient • The accommodation coefficient, 0 < α < 1, is a measure of how well a gas molecule equilibrates to a surface’s temperature. • If α = 1, the kinetic energy of the recoiling gas molecule will be equal to that of the surface temperature. • The thermal accommodation coefficient between surfaces 1 and 2 will be     12 1212 • The accommodation coefficient depends upon the gas species and the local roughness of the surface. • Perfectly elastic collisions with a specular surface give a

zero accommodation coefficient. R. B. Darling / EE-527 / Winter 2013 Conductive Cooling in a Low Pressure Gas

• When the pressure is low enough that λ > D, gas molecules bounce between walls more frequently than they interact with each other. • The free molecular thermal conductivity is then

1/2 1 ccpvv cc pvk   B fm  88ccTpv cc pv mT • The conductive heat flow from surface 2 to surface 1, with

T2 > T1 is then cc v  QPTT  pv1 PTT W/m2  21fm  21 21  ccpv 8 T1

R. B. Darling / EE-527 / Winter 2013 Gas Flow - 1 • Viscous Flow – occurs for pressures greater than ~ 10-2 torr – gas molecules constantly collide with one another – collisions with each other are more frequent than wall collisions – gas behaves like a coherent, collective medium; it acts like a fluid – Knudsen number Kn = λ/D < 0.01. • Transition Flow – Knudsen number 0.01 < Kn < 1. • Free Molecular Flow – occurs for pressures less than ~ 10-2 torr – gas molecules travel for large distances between collisions – collisions with walls are more frequent than with each other – gas molecules fly independently of each other

– Knudsen number Kn = λ/D > 1. R. B. Darling / EE-527 / Winter 2013 Gas Flow - 2 • Pipe of radius r and length l: • Viscous Flow – Poiseuille’s equation:  r 4  r 4 Q  C (P  P )  P2  P2  (P  P )(P  P ) vis 2 1 16 l 2 1 16 l 2 1 2 1

• Free Molecular Flow – Knudsen’s equation: 1/ 2 2 r 3  8k T   B  Q  Cmol (P2  P1)    (P2  P1) 3 l   m  • Ohm’s Law for Gas Flow: Q = CP R. B. Darling / EE-527 / Winter 2013 Gas Throughput

•Q = PS • P = gas pressure in torr • S = pumping or leaking speed in liters/second (L/s) • Q = gas throughput in torr-liters/second (torr-L/s) – This is the quantity of gas moving through an orifice per unit time. • Q is directly related to the power needed to move the gas: – 1 Watt = 7.50 torr-L/sec = 1000 Pa-L/sec – PV is dimensionally equal to energy, so PS is power. • C = gas conductance in liters/second (L/s)

•Q = C(P2 − P1)

R. B. Darling / EE-527 / Winter 2013 Vapor Pressures of Gases

boiling point @ 760 Torr LN2 temperature, 77K

3

1 He

H2 -1 Ne N2 O2 -3

-5 H2O

Vapor Pressure, torr Vapor CO2 10 -7 NO Log -9

-11 1 3 10 30 100 300 1000 Temperature, °K

R. B. Darling / EE-527 / Winter 2013 Vacuum Pump Pressure Ranges

Ultra-High Vacuum High Vacuum Rough Vacuum

VENTURI PUMP ROTARY PISTON MECHANICAL PUMP DRY MECHANICAL PUMP ROTARY VANE MECHANICAL PUMP SORPTION PUMP BLOWER/BOOSTER PUMP LIQUID NITROGEN TRAP DIFFUSION PUMP CRYOPUMP TURBOPUMP ION PUMP TI SUBLIMATION PUMP

-12 -10 -8 -6 -4 -2 0 +2 Pressure in torr (units are exponents of ten) R. B. Darling / EE-527 / Winter 2013 Vacuum Pumps

• Two fundamental types: – Concentration pumps • Gas entering the inlet is compressed and expelled out an outlet • Can run continuously – Entrainment pumps • Gas entering the inlet is trapped inside (no outlet!) • Must be regenerated to empty the trapped gas

R. B. Darling / EE-527 / Winter 2013 Rotary Vane Mechanical Pumps - 1

Gas Ballast Inlet Outlet Valve

Electric Motor Oil Fill Oil Sight Glass

Oil Drain

Frame Oil-Filled Pump Body

R. B. Darling / EE-527 / Winter 2013 Rotary Vane Mechanical Pumps - 2

Discharge Valve Interstage Relief Valve

Gas Ballast Valve

Second Stage First Stage

R. B. Darling / EE-527 / Winter 2013 Rotary Vane Mechanical Pumps - 3

• Gases are removed by compressing them slightly above atmospheric pressure and then forcing them through a check valve. • The rotary vane modules are immersed in an oil bath. • The purpose of the oil is to: – cool the pump – lubricate the rotary vanes – provide a lip seal for the vanes – open the second stage exhaust valve at low inlet pressures • They are powered by an electric motor: – Belt drive: 250 to 400 rpm – Direct drive: 1725 rpm (most common type)

R. B. Darling / EE-527 / Winter 2013 Rotary Vane Mechanical Pumps - 4

• Potential Problems: – Oil must have low vapor pressure to achieve desired performance. • Water or dirt or impurities in the oil will raise the vapor pressure. – Backstreaming of the oil vapor can occur at low pressures. • This can be trapped in a molecular sieve filter. • This is most often responsible for the oily smell in a vacuum chamber. – Large gas loads can froth the oil and prevent sealing. • The gas ballast valve can be opened to allow any froth to settle. • Roughing valves should be opened slowly (feathered) to prevent this. – Belts can break on belt-drive pumps. • Direct drive pumps eliminate this problem. – High compression ratios can cause condensation of water vapor. • The gas ballast valve can be opened slightly to reduce the compression ratio. R. B. Darling / EE-527 / Winter 2013 Dry Mechanical Pumps • A fairly recent innovation in vacuum technology (mid 1990s). • These do not use any oil for sealing surfaces, so they produce no oil back streaming. Process gases do not contaminate the oil, since there is none. • Most are based upon two counter-rotating shafts with claws and hooks that compress the gas as it moves along the direction of the shafts. • The claws also obscure and reveal the inlet and outlet ports, so no mechanical valves are needed. • Most can discharge directly into atmospheric pressure. • Fairly expensive, most starting at about $10k+. • Generally the preferred type of pump for LPCVD and other processes which produce highly corrosive exhaust gases. • These can usually handle high temperatures and water condensation fairly well. • Typical pumping speeds are ~500 to 1000 L/min. R. B. Darling / EE-527 / Winter 2013 Hook and Claw Style Dry Mechanical Pumps

12 compression zone

expansion zone

hook and intake port exhaust port claw rotors 34

R. B. Darling / EE-527 / Winter 2013 Sorption Pumps - 1

R. B. Darling / EE-527 / Winter 2013 Sorption Pumps - 2

Neck Flange Blow-Out Plug

Aluminum Body with Internal Fins

Molecular Sieve Liquid Nitrogen Bath

Styrofoam LN2 Bucket

R. B. Darling / EE-527 / Winter 2013 Sorption Pumps - 3

• Gases are pumped by – Cryocondensation: gases freeze into solid phase on cold surfaces – Cryosorption: gases are trapped in a porous molecular sieve

• The vessel is cooled by immersion in liquid nitrogen (LN2) which reaches -196° C, or 77° K. • Pumping is completely oil free and has no moving parts.

• Each sorption pump requires about 2-3 gallons of LN2 and about 20 minutes to cool down. • Several sorption pumps are often combined on a manifold. • Pumps must be regenerated by heating to 250° C for 30 mins. to melt frost and degas the molecular sieve material.

R. B. Darling / EE-527 / Winter 2013 Venturi Pumps - 1

Low Pressure Venturi High Pressure Supply Air

Muffler Low Vacuum Suction

R. B. Darling / EE-527 / Winter 2013 Venturi Pumps - 2

• Bernoulli’s principle is used to pull vacuum from the pinched midsection of a flow restriction. • They are typically driven by ~60 psi clean dry air (CDA). • Venturi pumps can usually pump down a chamber from 760 torr to 60 torr. • They are completely oil free and have no moving parts. • They are instant on and off. • Venturi pumps can remove about 90% of the air in a chamber, greatly reducing the capacity requirements of other pumps. • Their main drawback is their noise; they usually need a muffler.

R. B. Darling / EE-527 / Winter 2013 Roots Blowers / Booster Pumps - 1

Inlet

Roots Blower Booster Pump

Outlet

Rotary Vane Mechanical Pump

R. B. Darling / EE-527 / Winter 2013 Roots Blowers / Booster Pumps - 2

Inlet

Figure-Eight Lobed Rotors

Outlet

R. B. Darling / EE-527 / Winter 2013 Roots Blowers / Booster Pumps - 3

• Precision shaped rotors mate to the housing and to each other to within only a few thousandths of an inch. • Rotors spin at 2500 to 3500 rpm. • Gears synchronize the rotors. • It is a high throughput, low compression pump that is used for moving large gas volumes. • Must be below 10 torr to operate. • “Windmills” at atmospheric pressure, creating much heat. • They require a mechanical foreline pump, either an oil- filled rotary vane or dry hook and claw mechanical pump.

R. B. Darling / EE-527 / Winter 2013 Diffusion Pumps - 1

Inlet

Water Cooling Lines

Outlet

Ejector Arm

Oil Fill and Drain Assembly

Electrical Plug

Oil Heater

R. B. Darling / EE-527 / Winter 2013 Diffusion Pumps - 2

Inlet

Multistage Jet Assembly Water Cooling Lines

Outlet

Ejector Arm

Oil Fill and Drain Assembly

Electrical Plug

Oil Heater

R. B. Darling / EE-527 / Winter 2013 Diffusion Pumps - 3

• Oil is vaporized and propelled downward by an internal boiler and multistage jet assembly. • Oil vapor reaches speeds of 750 mph or more (supersonic). • Oil vapor streams trap and compress gases into bottom of pump, which are then ejected out into the foreline arm. • Oil vapor is condensed on sides of pump body which are water cooled. • Can only operate at foreline pressures of ~100 millitorr or less. • A mechanical foreline pump is required for operation. • Multistage jet assembly is designed to fractionate the oil, using lighter weight fractions for higher vapor velocities. • Typically 300 - 2800 L/s pumping speeds. • Very high reliability pumps, since there are no moving parts. • Gravity collects oil in the base, so pumps must be mounted pointing upwards. R. B. Darling / EE-527 / Winter 2013 Diffusion Pumps - 4

• Potential Problems: – Backstreaming of oil vapor can occur if forepressure becomes too large. • Backstreaming occurs for pressures of 1 to 10 mTorr. • A cold cap on top of the multistage jet assembly helps to reduce this. • A liquid nitrogen filled cryotrap also helps to reduce this. • The maximum tolerable foreline pressure (critical fore pressure) must not be exceeded, or pump will “dump” or “blow-out”, sending oil up into the chamber. – Pump can overheat if cooling water fails • Most pumps have a thermal cutout switch. – Pumping requires low vapor pressure oil • Water, dirt, or other impurities will raise vapor pressure. • Only special oils are suitable for diffusion pump use.

R. B. Darling / EE-527 / Winter 2013 Diffusion Pump Oils

• Diffusion pump oils must have very low vapor pressure. • Types – Hydrocarbon oils • Apiezon A, B, C, Litton Oil, Convoil-20 – Silicone oils (most common for non-reacting gases) • DC-704, DC-705, Invoil 940 – Polyphenyl ethers • Santovac 5, Convalex 10 – Fatty esters • Octoil, Butyl Phthalate, Amoil, Invoil – Fluoroether polymers (“Teflon oils”) • Krytox, Fomblin

R. B. Darling / EE-527 / Winter 2013 Liquid Nitrogen Traps / Baffles - 1

Inlet Fill Vent Chilled Baffles

Liquid Nitrogen Reservoir

Outlet

R. B. Darling / EE-527 / Winter 2013 Liquid Nitrogen Traps / Baffles - 2

• Baffles and traps in the pumping lines can greatly help to reduce backstreaming: – low pressures in mechanical rough pumps (0.1 to 1.0 torr) – high pressures in diffusion pumps (1 to 100 millitorr) – most important within the “cross-over” region.

•LN2 cryotraps should not experience air pressure above 100 millitorr, or they will frost completely over. • Residual water in a cryotrap can be frozen and cause trap to break, causing catastrophic failure of vacuum system.

– Blow out any water vapor with dry N2 before filling with LN2.

•LN2 cryotraps require constant refilling. – Expensive, but autofill valves are available.

R. B. Darling / EE-527 / Winter 2013 Diffusion Pumped High Vacuum Bell Jar System

R. B. Darling / EE-527 / Winter 2013 Multiplexing the Rough Pump • A rough pump is used to: – Evacuate the process chamber (once upon loading) – Maintain foreline pressure for the diffusion pump to operate (constantly) • To same cost and complexity, a single rough pump is commonly used to provide both functions. • However, it cannot do both simultaneously, since the exposure of atmospheric pressure from the chamber would disrupt the foreline pressure. • One hard rule of operation: the foreline and rough valves must NEVER be open simultaneously! • With the diffusion pump running, as long as the gas load is light, the rough pump can be temporarily used to evacuate the process chamber and then immediately returned to maintaining the foreline pressure. • Some valve control panels take care of this as part of an automatic pump- down sequence. • For many laboratory vacuum systems, the user must manage this. R. B. Darling / EE-527 / Winter 2013 Turbomolecular Pumps - 1

Inlet

Rotors

Stators

Shaft Electrical Connection Foreline to Controller

High Speed Motor

R. B. Darling / EE-527 / Winter 2013 Turbomolecular Pumps - 2 • These are very clean mechanical compression pumps. • They use high speed rotational blades to impart velocity and direction to gas molecules. • They look a similar to a jet engine in construction, but they operate ONLY at mid- to high-vacuum pressures. • Typical motor speeds are 9,000 to 90,000 rpm! • 20 to 60 blades per disk • 10 to 40 compression stages per pump • Similar to a diffusion pump, each requires a constantly running mechanical foreline pump. • Typically pumping speeds are 100 to 800 L/sec. • They are ideal for hydrocarbon free applications.

R. B. Darling / EE-527 / Winter 2013 Turbomolecular Pumps - 3

• Their pumping speed is proportional to the rotor speed. • The compression ratio of the turbine establishes the base pressure. • The compression ratio is higher for higher molecular weights: 1/2 – Approximately: log10K = 1.5 (M) 1.5 –For H2, M = 1, so K = 10 = 30 = very small – For hydrocarbons, M = 100, so K = 1015 = very large

• The base pressure is usually limited by H2.

R. B. Darling / EE-527 / Winter 2013 Turbomolecular Pumps - 4

• Potential Problems: – Very high speed rotor blades have close-mating stator blades. • Slight imbalances can cause vibration and bearing wear problems. • A sudden blast of atmospheric pressure can bend the blades down, causing catastrophic failure, “crashing the pump.” – Lubrication of the high speed rotor is an engineering problem. • Circulating oil is most reliable, but pump must be right-side-up. • Grease-lubricated bearings are less reliable, but allow pump to be placed at any orientation. • Some fancier models use magnetic bearings. – Too high of a pressure will cause aerodynamic lift and drag. • A mechanical foreline pump must be used. • Aerodynamic lift can bend blades, causing catastrophic failure.

R. B. Darling / EE-527 / Winter 2013 Turbo Pumped High Vacuum Bell Jar System

R. B. Darling / EE-527 / Winter 2013 Cryopumps - 1

R. B. Darling / EE-527 / Winter 2013 Cryopumps - 2

Inlet

1st Stage Cryoarray

2nd Stage Cryoarray

Expander Module Relief Valve Regeneration Foreline Expander Motor Hydrogen Vapor Bulb Thermometer

Helium Line Connections

R. B. Darling / EE-527 / Winter 2013 Cryopumps - 3

• These use a closed-loop helium cryogenic refrigerator. • The primary parts are: – Compressor: uses He for its high heat capacity and low FP. – Expander: uses Joule-Thompson expansion of He gas for cooling – Cold Head: thermally insulated from pump bucket, 2+ stages • Gases are pumped by two processes:

– Cryocondensation (H2O, CO2, N2, O2, Ar, solvent vapors) • Gases are condensed into a solid phase on cryogenically cooled surfaces. (They become frost!)

– Cryosorption (H2, He, Ne) • Non-condensable gases are adsorbed onto surfaces of cryogenically cooled porous media, usually activated charcoal or zeolites. • Typical pumping speeds are 100 - 1000 L/s.

R. B. Darling / EE-527 / Winter 2013 Cryopumps - 4 • The first stage array operates at 50 to 80 K. – Primarily used for pumping water vapor and carbon dioxide. • The second stage array operates at 10 to 20 K. – Primarily used for pumping other condensable gases. • Activated charcoal in the second stage provides cryosorption. – Primarily used for pumping other non-condensable gases. – Charcoal and zeolites have about 8000 ft2/cm3 of surface area. • They offer completely oil free operation. • They can operate from any orientation. • They offer very clean vacuum with high pumping speed. • They have very high impulsive pumping capacity.

R. B. Darling / EE-527 / Winter 2013 Cryopumps - 5

• Potential Problems: – They must be regenerated to extract the trapped gases. • Allow to warm to room temperature (slow), or • Use a built-in heater to warm to 250 C and outgas (fast). • Regeneration takes the pump off-line for several hours. – The regeneration process can produce considerable pressure. • Pumps have a safety pressure relief valve on the bucket. – They must be started from below 100 millitorr. • They require the use a mechanical roughing pump to initially pump out the bucket, but once done, the rough pump is no longer needed.

R. B. Darling / EE-527 / Winter 2013 Cryopump Compressor Module

Power Supply

Coalescer

Adsorber

Compressor

Surge Volume

R. B. Darling / EE-527 / Winter 2013 Cryo Pumped High Vacuum Bell Jar System

R. B. Darling / EE-527 / Winter 2013 Titanium Sublimation Pumps - 1

Poppet Valve

Baffle

LN2 Cold Trap

Titanium Ball

Sump

R. B. Darling / EE-527 / Winter 2013 Titanium Sublimation Pumps - 2 • “TSP”; a type of “getter pump” • Titanium, which has been freshly evaporated onto the sides of a sump, will chemically combine with gas molecules. • Titanium sublimes from a heated source and evaporates to coat the walls of the sump. • Types of Ti sources: – 35 g Ti-ball; 750 W operating, 200 W standby – 15 g mini-Ti-ball; 380 W operating, 95 W standby – 4.5 g Ti filament; 380 W operating, zero standby Typical pumping speeds for freshly coated Ti surfaces (L/sec-in2):

H2 N2 O2 CO CO2 H2O 20°C: 20 30 60 60 50 20 -190°C: 65 65 70 70 60 90 R. B. Darling / EE-527 / Winter 2013 Non-Evaporable Getter Pumps

• These are also known as “NEG” pumps. • They usually employ a Zr-V-Fe alloy that is formed into a cartridge over a constantan strip heater. • These pump all of the time, until loaded with gas molecules. • They can be regenerated by heating to ~350°C for 30 mins. to degas the alloy. • They are very simple in construction and operation.

R. B. Darling / EE-527 / Winter 2013 Ion Pumps - 1 pump +V body power supply

multicell cylindrical anode array NS

cold cathode discharge

titanium cathodes external permanent magnet Diode Ion Pump

R. B. Darling / EE-527 / Winter 2013 Ion Pumps - 2 pump --V body power supply

multicell cylindrical anode array NS

cold cathode discharge

titanium cathode grids external permanent magnet

Triode Ion Pump

R. B. Darling / EE-527 / Winter 2013 Ion Pumps - 3

• Operation is based upon a rarefied gas electric discharge. – A high electric field can ionize a gas molecule, forming a free electron and a gas ion. – The free electron is collected by the anode, while the gas ion is collected by the cathode. – Fast electrons, accelerated by the E-field, will collide with and ionize other gas molecules. – A coaxial magnetic and electric field will produce spiral orbits for the free electrons; the larger paths greatly increase the ionization. – This creates a Penning cell which traps positive ions inside. – Higher ionization levels will sustain a cold cathode discharge. – Gas ions accelerated into the cathode can stick and therefore be pumped.

R. B. Darling / EE-527 / Winter 2013 Ion Pumps - 4

• The cathode plates are made of titanium (Ti). • Pumping mechanisms: – Incident gas ions may be implanted into the Ti cathode plates. – Incident gas ions may sputter Ti from the cathode plates into the cylindrical anode cells, thus providing additional getter pumping.

–H2 is directly absorbed by the fresh Ti surfaces. – Gas molecules may be trapped and buried by sputtered Ti. – The electric discharge cracks larger molecules into smaller ones that are more readily pumped. • Ion pumps must be started at 10-5 torr or less. • Intermediate pumping is usually provided by a sorption or a cryo pump.

R. B. Darling / EE-527 / Winter 2013 Ion Pumps - 5 • Diode pumps use a Ti plate as the cathode. • Triode pumps use a Ti screen as a grid electrode and the pump body as the cathode. • Typical triode pumps will operate for ~35,000 hours (about -6 4 years) at an inlet pressure of 10 torr of N2. • The ion pump current is proportional to the gas pressure in the pump, so this can be used as a pressure gauge. • Appendage ion pumps are often used to sustain high vacuum in long service devices such as microwave tubes. • Ion pumps can be rebuilt, but it requires cutting them open, replacing the consumed Ti elements, and re-welding them closed. That is still cheaper than buying a new one.

R. B. Darling / EE-527 / Winter 2013 Ion / Ti-Sub. Pumped Ultra-High Vacuum System

R. B. Darling / EE-527 / Winter 2013 Special Considerations for Ultra-High Vacuum Systems

• Achieving pressures below ~10-7 torr requires: – Extreme attention to the cleanliness of all surfaces – Elimination of all virtual leaks – Baking out of chamber to allow inside wall surfaces to desorb • Usually ~200-300C for 6-12 hours • Special baking blankets or heater tapes are used on the exterior of the chamber – Patience to achieve the base pressures • Can sometimes take >24 hours – UHV systems often have a load-lock system so that the main chamber does not need to come up to full atmospheric pressure to load and unload samples

R. B. Darling / EE-527 / Winter 2013 Low Vacuum Pumping Speed • At low vacuum, pumping is within the viscous regime. • The gas throughput is normally limited by the conductance of the pipe (of radius r and length ℓ) that connects the chamber (of volume V) to the pump.

•Pbp is the base pressure that can be obtained from the pump. dP  r 4 Q  V   P2  P2 dt 16 bp  4 dP r 2 • If Pbp ~0, then   P dt 16V 1  1  r 4t  • Which has the solution of P(t)      P(0) 16V  • The chamber pressure falls roughly as 1/t towards a base pressure of

Pbp with a rate set by the viscous conductance of the pipe. R. B. Darling / EE-527 / Winter 2013 High Vacuum Pumping Speed • At high vacuum, pumping involves free molecular flow. • The pumping speed is primarily engineered through the volume of the chamber V and the area of the pumping port A. Perfect high vacuum pumping is where any molecule that impinges upon the pumping port is exhausted from the chamber. This assumes no leaks!

• Combine the impingement rate with the ideal gas law: dN  2 mk T 1/ 2 P PV  Nk T Adt B B • Obtain: 1/ 2  1/ 2 dN A  kBT  N dP A  kBT  P  N     or  P     dt V  2 m  dt V  2 m  

• Which has the exponentially decaying solution:  1/ 2  t   t  V  2 m  N(t)  N(0)exp  or P(t)  P(0)exp  with        A  kBT 

R. B. Darling / EE-527 / Winter 2013 Real World Vacuum Pumping Speed

• The preceding two situations are idealized and do not include the effects of leaks or outgasing. • Ultimate chamber pressures are set by the balance between pumping rate and the rate of leaks and outgasing combined:

Qpump (P)  Qleaks  Qoutgasing (T ) • Both low and high vacuum pumping speeds are driven by the chamber pressure. As the chamber pressure falls, the pumping speed is reduced. • The outgassing rate is determined primarily by temperature. • High vacuum systems are usually “baked” at an elevated temperature to outgas surfaces more quickly. • The better chambers are usually stainless steel which can tolerate baking temperatures of up to 250-300°C.

R. B. Darling / EE-527 / Winter 2013 Base System Vacuum Pressure

• Base pressure for a vacuum system is where the pumping speed equals the leak and outgasing rates:

gas flow rate, Q (torr-L/sec)

pumping

base pressure achieved

leaks

outgasing

time, t (sec)

R. B. Darling / EE-527 / Winter 2013 Vacuum Gauge Pressure Ranges

Ultra-High Vacuum High Vacuum Rough Vacuum

BOURDON GAUGE THERMOCOUPLE GAUGE CONVECTION THERMOCOUPLE GAUGE PIRANI GAUGE CAPACITANCE MANOMETER

COLD CATHODE GAUGE IONIZATION GAUGE RES. NITROGEN ANALYZER RESIDUAL GAS ANALYZER

-12 -10 -8 -6 -4 -2 0 +2 Pressure in Torr (units are exponents of ten) R. B. Darling / EE-527 / Winter 2013 Bourdon Gauges - 1

Gear Box Expansion Tube

R. B. Darling / EE-527 / Winter 2013 Bourdon Gauges - 2

• Mechanical gas pressure flexes the Bourdon tube and causes the arc to unwind, which through a series of gears and levers moves a needle on the gauge’s face. • It is completely insensitive to the chemical composition of the gas. • It can be used for measuring positive pressure and vacuum. • Lower sensitivity for vacuum measurements is about 0.1 torr. • Gauges are precision instruments and can be damaged by mechanical shock.

R. B. Darling / EE-527 / Winter 2013 Capacitance Manometers - 1

Capacitor Metal Electrode Diaphragm

Stainless Steel Body

10 kHz oscillator and demodulator

R. B. Darling / EE-527 / Winter 2013 Capacitance Manometers - 2

• Mechanical gas pressure deforms a tensioned metal diaphragm. • An air gap capacitor is formed between the diaphragm and a set of fixed electrodes. • The capacitance thus varies with the pressure. • The capacitance is measured by a demodulation and amplifier circuit that makes a 10 kHz oscillator with the diaphragm capacitor. • Capacitance manometers are extremely linear and accurate. – Typically within 1% of full scale. • They are insensitive to the chemical composition of the gas.

R. B. Darling / EE-527 / Winter 2013 Capacitance Manometers - 3

• Can be used to measure pressure in all modes: – Gauge – Absolute – Differential • A single capacitance manometer can only read over 3-4 decades of pressure. • Capacitance manometers can be constructed to cover the range from atmospheric pressure down to ~10-5 torr by using diaphragms of differing stiffness. • Capacitance manometers are often used to accurately measure pressure in process reactors, and are often used in feedback control loops.

R. B. Darling / EE-527 / Winter 2013 Thermal Conductivity Pressure Gauges – 1

• Gas pressure can be indirectly measured from the rate of cooling that it provides to a heated filament. • Conductive heat flow in the free molecular range: cc v QPTT  pv1 PTT W/m2  21fm  21 21  ccpv 8 T1 1/2 cc k QPTT pv B 21 2 1 ccpv 8 mT1 • By applying a fixed electrical power as heat input, in equilibrium the filament temperature will rise to a value of

T2 to equal the above heat loss rate. (T2 – T1) will be inversely proportional to pressure P. R. B. Darling / EE-527 / Winter 2013 Thermal Conductivity Pressure Gauges – 2 • Heat transfer comes from radiation, conduction, and convection, which each dominate over different ranges:

Heat Flow [W/m2] convection

conduction

radiation

103 102 101 100 10-1 10-2 10-3 Knudsen Number = Kn = λ/D  1 kT Kn   B 22R. B. Darling / EE-527 / Winter 2013 D 22dnD00 dDP Thermal Conductivity Pressure Gauges – 3 • The response from a thermal conductivity pressure gauge depends upon the gas species through – The accommodation coefficient,

– The specific heats cp and cv, and – The mass of the molecules. • The response is best over the transition flow range, with Knudsen numbers in the range 0.01 < Kn < 10. • The heat flow is still described well by the free molecular flow equations. • This pressure range is particularly useful for monitoring the switch over from rough pumping to high vacuum pumping, e.g. in diffusion pumped systems. • The range is typically 1 – 1000 millitorr. R. B. Darling / EE-527 / Winter 2013 Thermocouple Gauges - 1

Thermocouple output:

VSTout K  Electrical power input: P  IR2 K-type elec 4-point Stainless heater and Heat flow balance: Steel Can thermocouple Pelec QPT21  fm  Chromel Afil Alumel Sensor transfer function: 2 SIRK Vout  Afil fm P

R. B. Darling / EE-527 / Winter 2013 Thermocouple Gauges - 2 • Electric current passed through a filament heats up the filament to a temperature that depends upon how fast the surrounding gas conducts the heat away. • The temperature is measured by a thermocouple, which is part of the filament assembly, and the temperature reading is converted into an approximate pressure on a meter. • Since different gases have different thermal conductivities, thermocouple gauges read differently for different gases. • Measurement range of about 1 to 1000 millitorr. • Very rugged, reliable, and inexpensive. • Convectron® thermocouple gauges make use of the convective cooling regime to extend the range to 1000 torr, (atmospheric pressure). R. B. Darling / EE-527 / Winter 2013 Pirani Gauges - 1

Stainless Steel Can

Tungsten Resistive Heaters

R. B. Darling / EE-527 / Winter 2013 Pirani Gauges - 2

• Similar to a TC gauge, an electrically heated filament takes on a temperature that depends upon the rate of heat loss to the surrounding gas. • The temperature of the filament is sensed by measuring the change in the resistance of the filament as it is heated. – For most metals, the TCR is about +200 ppm/°C. • Pirani gauges require a more sophisticated controller, but are more accurate and faster responding than a TC gauge. • Most use a Wheatstone bridge circuit to linearize the filament against a compensating filament that is held at atmospheric pressure. • Pirani gauges are also sensitive to the gas composition.

R. B. Darling / EE-527 / Winter 2013 Hot Filament Ionization Gauges - 1

Collector (0 V)

Glass Envelope

Filament Grid (+25 V) (+175 V)

R. B. Darling / EE-527 / Winter 2013 Hot Filament Ionization Gauges - 2

• Also know as “Bayerd-Alpert” gauges. • Electrons are thermionically emitted from a hot filament and then accelerated by a grid electrode. • The accelerated electrons will ionize any gas molecules in the vicinity of the grid, and the positively charged gas ion will contribute to a current through the collector electrode.

•IP = IE·S·P, where

–IP = positive ion current through collector electrode

–IE = electron emission current through filament – S = gauge sensitivity parameter – P = gas pressure

R. B. Darling / EE-527 / Winter 2013 Hot Filament Ionization Gauges - 3

• The ionization rate depends upon the gas species, so ion gauges are sensitive to the gas composition. • Accuracy is about 10% of full scale, when calibrated. • Ion gauges can work from 10-3 to 10-11 torr! • The lower pressure limit is set by soft x-ray emission from electrons striking the grid. • The hot filament requires some precautions: – Exposure to pressures above 10-3 torr will burn out the filament. – Most ion gauge controllers automatically turn off the filament when the collector current exceeds a maximum value. – The hot filament is an ignition source which can trigger explosions in the process chamber with combustible gases.

R. B. Darling / EE-527 / Winter 2013 Cold Cathode Ionization Gauges - 1

Permanent Cold Magnet Cathode Discharge S N

Stainless Cathode S N Steel Body (-2000 V) (0 V)

R. B. Darling / EE-527 / Winter 2013 Cold Cathode Ionization Gauges - 2

• A cold cathode “Penning” discharge replaces the hot filament for producing ionizing electrons. • Electrons orbit around the center post with long trajectories which are very effective in ionizing gas molecules. • The ionized gas molecules are collected by the negatively charged cathode, and the electric current is proportional to the gas pressure. Typically produce 1-5 Amps/torr. • These can operate from 10-2 to 10-8 torr. • More rugged than a hot filament ion gauge, but less accurate, typically only about 50% of full scale. • Cold cathode discharge is still a potential source of ignition for combustible process gases.

R. B. Darling / EE-527 / Winter 2013 Cleanliness Inside a Vacuum Chamber

-6 •At P = 10torr, T = 300 K, and m = 28.0 g/mole (N2): – there are 3.2 x 1010 molecules/cm3 – the mean free path is 5 x 103 cm – the impingement rate is 3.8 x 1014 molecules/cm2/sec • Thus, at 10-6 torr, a monolayer of molecules will deposit on any surface in about 1 second. •10-6 torr is equivalent to a purity of 1 ppb! – (Relative to atmospheric pressure at ~103 torr) • The matter in one fingerprint (1 cm x 1 cm x 20 m), when vaporized, will produce a pressure of 10-4 torr inside a 10 ft3 vacuum chamber! • Thus: ALWAYS WEAR GLOVES!!!

R. B. Darling / EE-527 / Winter 2013 Vacuum Materials - 1 • Stainless Steel – Type 304 SS is most common for chambers and flanges. • Easier to machine. • Easier to fusion weld. – Type 316 SS is most common for pipes, tubes, and fittings. • Copper – Use Oxygen-Free High-Conductivity (OFHC) alloy. • Used for electrical conductors. • Ceramics

–Alumina (Al2O3) is very common. • Used for electrical insulators. • Kovar – (54% Fe, 29% Ni, 17 % Co); used for glass-to-metal seals.

R. B. Darling / EE-527 / Winter 2013 Vacuum Materials - 2

•Elastomers – Buna-N • Inexpensive, good to 80°C, rather impermeable to He. – Viton • Outgasses very little, good to 150°C. – Polyimide

• Good to 200°C, stiffer than other elastomers, permeable to H2O vapor. – Silicones

• Can handle higher temperatures, but very permeable to H2O and He. –Teflon • Very inert, but exhibits cold flow plasticity, making it a poor seal. • Very permeable to He, good to 150°C.

R. B. Darling / EE-527 / Winter 2013 Vacuum Joining Techniques - 1

Internal continuous fusion welds are most commonly used for joining tubing, pipes, and chamber ports. For small bore tubing, external orbital welding must produce complete penetration welds.

R. B. Darling / EE-527 / Winter 2013 Vacuum Joining Techniques - 2

ASA flanges are a common standard that uses a captured O-ring to provide sealing.

R. B. Darling / EE-527 / Winter 2013 Vacuum Joining Techniques - 3

KF or “Quick-Flanges” are a common standard for rough or high vacuum plumbing. They utilize an O-ring supported against flat flanges with an internal centering ring. Compression is supplied by a tapered clamp and wingnut. No tools are needed.

R. B. Darling / EE-527 / Winter 2013 Vacuum Joining Techniques - 4

Metal-sealed, or “Con-Flat” flanges are used for ultra-high vacuum applications where elastomer sealed flanges would be too leaky. A knife edge on each flange bites into and compresses a copper gasket. The extremely high pressure of the knife edge causes the copper to deform to match the surfaces of both flanges. These flanges are bakeable up to 350°C.

R. B. Darling / EE-527 / Winter 2013 Things to Watch for in Vacuum Systems

• Real leaks • Virtual leaks • Water leaks • Oil contamination • Finger prints • Organic materials that outgas • Corrosion or surface oxidation which will outgas • Prior deposition coatings which will outgas – Many materials are incompatible with vacuum evaporation for this reason. Examples: Cd, Se, Te, Zn. • Any foreign matter

R. B. Darling / EE-527 / Winter 2013