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課程名稱：微製造技術Microfabrication Technology

授課教師：王東安Lecture 5

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Lecture Outline

•Reading Campbell: Chapter 10•Today’s lecture

–Kinetic theory of gases–Gad flow and conductance–Pressure ranges and vacuum pumps–Vacuum seals and pressure measurement–DC glow discharge–RF discharge–High-density plasmas

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Prologue

•Some processes are run in vacuum chambers•Beneficial to review vacuum science.

–Fundamental physics of molecules and atoms invacuum chamber

–Equipment used to produce, contain and measurevacuums.

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Probability distribution of gasmolecule velocities

•Maxwell speeddistribution

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Gas kinetics•Root mean square velocity of gas molecule

mkT

vrms3

•Direction of thermal velocity is random•If there is no external force, average velocity is 0•If a pressure gradient is imposed, there will be a

net flow from high to low pressure. This inducedflow velocity is added to the much larger thermalvelocity.

•Gases at atmospheric pressure change velocity isdue to collisions.

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Gas kinetics•Average distance between collisions, mean free path , is

PdkT

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d: molecule diameter, P: chamber pressure, T: temperature inkelvin.

•Number of molecules that strikes a surface of unit area perunit time

kTmP

J n 2

2

m: mass of molecule

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Gas flow and conductance•Gas is much more compressible than liquid.•Throughput, Q (pressure-volume/time), is used to

describe amount of gas flowing through a system.

VdtdP

Q

mass density, V: volumeGas flow is often measured in terms of a standard volume, ie. a

volume that an equivalent amount of gas would occupy at 0℃and 1 atm of pressure.

One standard liter is the amount of gas that would occupy a liter at1 atm at 273K. Alternatively, one standard liter per minute is athroughput of 760 torr-liter per minute.

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Vacuum system

•Conductance C of avacuum component is

•Flowing a large amountof gas through avacuum system wilekeeping the chamberpressure close to thepump pressure requiresa vacuum system with avery large conductance.

21 PPQ

C

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Pressure control of Vacuum system•Volumetric flow rate of a pump cannot be

independently controlled.•Pressure in chamber can be set by

–adjust flow rate of gas in chamber–Insert a variable conductance valve in pumping line

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Pumping speed

•Pumping speed Sp = Q/Pp=dVp/dt•Pp: inlet pump pressure•A pump rated for 1000 l/min (slm), pump 1000

slm at 1 atm inlet pressure. If the inletpressure is 0.1 atm and the pumping speed atthis pressure remains 1000 l/min, maximumgas for that this same pump could accept is100 slm.

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Vacuum pump

•Vacuum regionsRough vacuum 0.1 –760 torrMedium 10-4~10-1

High 10-8~10-4

Ultrahigh <10-8

•most vacuum in semiconductor fabrication operate inrough or medium vacuum regime

•To ensure a pure chamber, they are pumped into highor ultrahigh vacuum before introducing process gases

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Rough vacuum pump

•Positive displacementpump: 3 steps, capture ofa volume of gas,compression of capturedvolume, gas expulsion

•Near end of stroke,second valve is opened,and gas is expelled tohigher pressure region.

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Rough and medium pump•Rotary vane

system•Oil is used to seal

vane and as alubricant to helpwith sliding, helpto cool pump

•Vacuum is about20 mtorr.

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Compression type pump

•Positive displacement, rotary vane•Potential for condensation of vapor, if partial

pressure of a gaseous vapor exceeds vaporpressure of corresponding liquid at the gastemp, it will begin to condense, form droplets,mix with pump oil, lead to corrosion

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High vacuum pump•Diffusion pump: heating

oil at bottom of pump.Oil vapor rise throughcenter stack and areejected through vents atvery high speeds. Strikecooled walls at top ofpump, condense, rundown the walls

•Gases are pumped bymomentum transferbetween vapor streamand gas molecules

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High vacuum pump•Turbomolecular pump: large number of

stages in series, each stage consists of a fanblade and a stationary blade (stator).

•These pumps work on the principle that gasmolecules can be given momentum in adesired direction by repeated collision with amoving solid surface. In a turbopump, arapidly spinning turbine rotor 'hits' gasmolecules from the inlet of the pumptowards the exhaust in order to create ormaintain a vacuum.

•Each state may have a modest compressionratio, but large number of stages, totalcompression ratio can be 109.

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Ultimate in purity pump-gas entrainment pump•Cryo pump: A cryopump traps gases and vapors by condensing them

on a cold surface•Over time, the surface eventually saturates with condensate and the

pumping speed gradually drops to zero. It will hold the trapped gasesas long as it remains cold, but it will not condense fresh gases fromleaks or backstreaming until it is regenerated. Saturation happensvery quickly in low vacuums, so cryopumps are usually only used inhigh or ultrahigh vacuum systems.

•Regeneration of a cryopump is theprocess of evaporating the trapped gases.This can be done by heating the wholechamber under vacuum to the highesttemperature allowed by the materials,allow time for outgassing products to beexhausted by the mechanical pumps, andthen cool and use the cryopump withoutbreaking the vacuum.

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Ultimate in purity pump-gas entrainment pump

•Sorption pump: operate by chemically orphysically absorbing gas molecules. Adsorbingmolecules on a very porous material which iscooled by a cryogen, typically liquid nitrogen.

•Operated by cooling them to liquid nitrogentemperature and allow gas molecules tophysically adsorb on pore walls.

•Can be recycled by heating them while under avacuum.

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Ultimate in purity pump-gas entrainment pump•A Titanium sublimation pump may be used as a component

of ultrahigh vacuum systems• It consists of a titanium filament through which a high current

(typically around 40 Amps) is passed periodically. Thiscurrent causes the filament to reach the sublimationtemperature of titanium, and hence the surrounding chamberwalls become coated with a thin film of clean titanium. Sinceclean titanium is very reactive, components of the residual gasin the chamber which collide with the chamber wall are morelike to react and to form a stable, solid product. Thus the gaspressure in the chamber is reduced.

•After some time, the titanium film will no longer be clean andhence the effectiveness of the pump is reduced. Therefore,after a certain time, the titanium filament should be heatedagain, and a new film of titanium re-deposited on the chamberwall.

•Sublimation: To transform directly from the solid to thegaseous state or from the gaseous to the solid state withoutbecoming a liquid

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Ultimate in purity pump-gas entrainment pump• sputter ion pump: ionize gases and uses a

large electric field, typically 3kV to 7kV,to accelerate them into a solid electrode.A cloud of electrons which ionizesincoming gas atoms and molecules issustained in a large magnetic field. Theions are embedded in the electrode andeffectively removed from the vacuumchamber, resulting a net pumping action.

• Ion pumps are commonly used in ultrahigh vacuum systems, as they can attainultimate pressures less than 10-11 mbar

• In contrast to other common vacuumpumps such as turbomolecular pumpsand diffusion pumps, ion pumps have nomoving parts and use no oil and aretherefore very clean and low-maintenance.

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O-rings

•Seal vacuum chamber forrough/medium vacuum

•Low cost and ease of reuse•Popular elastomer material: viton.•Viton is a brand of synthetic rubber

and fluoropolymer elastomer. Thename is a registered trademark ofDuPont

•A fluoropolymer is a fluorocarbonbased polymer with multiple strongcarbon–fluorine bonds

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O-rings•Viton: a vulcanized rubber•Vulcanization refers to a specific curing process of rubber

involving high heat and the addition of sulfur or otherequivalent curatives. It is a chemical process in whichpolymer molecules are linked to other polymer moleculesby atomic bridges composed of sulfur atoms or carbon tocarbon bonds. The end result is that the springy rubbermolecules become cross-linked. This makes the bulkmaterial harder, much more durable and also more resistantto chemical attack. It also makes the surface of the materialsmoother and prevents it from sticking to metal or plasticchemical catalysts.

•The process is named after Vulcan, Roman god of fire

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O-rings

•Two types of O-ring•Elastomer seals begin to leak at

pressures below 10-7 torr.•For high/ultrahigh vacuum, metal-

to-metal seals are used, sealingmaterial must be plasticallydeformed, sealing gaskets must bereplaced after every use and knifeedge must be protected

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Pressure gauge

•Most common in microelectronic applications–capacitance manometer–Thermal conductivity gauge–Ionization gauge

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Capacitance manometer

•Depend on pressure difference between thechamber and a reference volume, to cause amechanical deflection.

•Detect movement of a thin metal diaphragm•Used at pressure above 1 torr

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Thermal conductivity gauge

•Measure thermal conductivity of gas and from that,inferring a pressure

•Operate by passing a current through a wire andmeasure temperature of wire, wire temp. is kept lowto ensure most hear transfer is thermal conductionthrough the gas.

•Pirani gauge: resistance of wire is measured by aWheatstone bridge.

•Thermocouple gauge: thermocouple is used tomeasure wire temp. Thermocouple consists of twodissimilar metals and they produce a smalltemperature-dependent voltage.

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Pirani gauge•The Pirani gauge consists of a metal filament suspended in a

tube which is connected to the system whose vacuum is to bemeasured.

•The filament is connected to an electrical circuit from which,after calibration, a pressure reading may be taken.

• If the gas is at high pressure, gas molecules collide frequentlywith the filament and absorb energy from the filament whichresults in cooling of the filament.

•As the pressure of the gas molecules decreases the number ofgas molecules inside the chamber also goes down resulting infewer collisions with the filament. As a result the temperatureof the filament increases because of decreased cooling.

•Electrical resistance of a wire varies with temperature. Henceby studying the variation of the resistance of the wire we canpredict the vacuum surrounding the wire. This is the principleof Pirani Gauge. This gauge is used to measure the pressurebetween 0.5 torr to 10-4 torr.

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High/ultrahigh pressure gauge

•Pressures below 1 mtorr•Ionization gauge: operate by using electron

stream to ionize gas in the gauge and anelectrical field to collect ions. Ion currentproduced this way is a function of pressure inchamber.

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Glow discharge

•Glow discharge: light given off by plasma•Plasma can be used to crack molecules and so

drive some reaction chemistry•Plasma can be used to create and accelerate

ions.•Plasma is partially ionized gas.

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plasma•Assume inlet flow of gas contains molecule AB made

from atoms A and B.•Processes occur in glow discharge

–Dissociation e*+AB <-> A+B+e–Atomic ionization e*+A <-> A++e+e–Molecular ionization e*+AB<->AB++e+e–Atomic excitation e*+A<->A*+e–Molecular excitation e*+AB<->AB*+e

•* refers to a species whose energy is much larger thanthe ground state

•Dissociated atoms or molecular fragments are calledradicals.

•Radicals have an incomplete bonding state and areextremely reactive.

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plasma

•Ar or other elemental plasmas, no radicals.Ions are charged atoms or molecules such asA+ and AB+. May have more than one positivecharge or may even be negatively charged.

•Radicals may constitute 1% of total plasma,charged species may be less than 0.01%,energetic ionized species are possible.

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Parallel plate plasma reactor•A typical pressure

for a plasmaprocess is 1 torr.

•A high voltagesource (a chargedcapacitor) isconnected to thecircuit to startplasma.

•At 1 torr, voltagefor 10cm electrodespacing is 800V

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Parallel plate plasma reactor•Until arc is struck, gas will not conduct current since is acts

as an insulator.•If voltage is high enough, the field in reactor will exceed the

breakdown field of the gas, and a high voltage arc will flashbetween two electrodes.

•Arc creates a large numberof ions and free electrons.because the electric fieldin the chamber, electronswill be accelerated towardsthe positively charged anodeions will be accelerated towardsnegatively charged cathode

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Parallel plate plasma reactor

•Due to small mass, electrons will be accelerated muchmore rapidly than the ions. Ions travel across thetube and eventually strike cathode and release a cloudof secondary electrons from the cathode material.Theses electrons are accelerated back toward anode.If voltage across the electrodes is large enough, thesehigh-energy electrons collide inelastically withneutral atoms, create more ions.

•Process of secondary electron release and ion creationsustains plasma.

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Parallel plate plasma reactor•Positive/negative charge

density and electric fieldstrength

•Glow discharge gains its nameby: when a moderate-energyelectron scatters inelasticallyoff a neutral atom, it mayexcite a core level electron to ahigh-energy state. Whenelectron decays back to itsground state, it gives off theenergy in the form of visibleradiation. The light from theglow discharge arises becauseof this optical emission process.

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Parallel plate plasma reactor•Requirement for moderate-energy electrons precludes

optical emission near each of the electrodes. Theseregions are called dark spaces.

•3 dark spaces:–Crooke’s: most electrons have very low energy just above

cathode–Anode: anode is a sink for electrons and electron density

just above anode is too small–Faraday: electrons have been accelerated to very high

energies, lead to ionization and there are few electrons withenergies appropriate for emission

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plasma•Large electric field in Crooke’s dark space: ions

that drift and diffuse to edge of this region areaccelerated rapidly toward the cathode. If cathode iscovered with wafers or other materials of interest, wecan used this ion bombardment to drive variousprocesses.

•Width of the dark space depends on chamber pressure,at low pressures, mean free path of electronsincreases, and so the width of the dark spaceincreases, by controlling chamber pressure, one cancontrol the energy with which ions strike the surface,this effect limits dc plasmas to pressures greater thanabout 1 mtorr.

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RF discharge•Materials on electrodes might be insulators•Etching of SiO2: wafers with a layer SiO2 are

patterned and placed on a plasma electrode. Resistand SiO2 are insulators.

•As ions strike wafer surface, secondary electrons areejected, these layers become charged. Chargeaccumulates on surface and the field is reduced untilplasma is extinguished.

•To solve this problem, plasma can be driven by anac signal.

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RF discharge•Ac sources can be in radio frequency

(RF) range, commonly 13.56 MHz.•At low frequencies, plasma follows

excitation, width of dark spaces pulsewith the applied signal.

•When excitation is > 10kHz, slowions cannot follow the voltage change.Electrons are rapidly accelerated,during alternate half-cycles, electronsstrike surface of each electrode,giving both a net negative charge w.r.t.plasma. One dark space exists invicinity of each electrode.

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RF discharge•When excitation is > 10kHz, slow ions cannot follow the

voltage change. Electrons are rapidly accelerated, duringalternate half-cycles, electrons strike surface of each electrode,giving both a net negative charge w.r.t. plasma. One darkspace exists in vicinity of each electrode.

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RF discharge•Since plasma is conductive, voltage drop across glow

discharge is small. Due to electron depletion, large dcvoltage drops exist between plasma and electrodes

•For asymmetric chamber, A1, A2: electrode area4

1

2

2

1

AA

VV

To maximize voltage differencebetween plasma and the lowerelectrode, and therefore the ionbombardment energy on the lowerelectrode, it is desirable to increase thearea of the upper electrode, this can bedone by connecting upper electrode towalls of chamber.

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High-density plasmas (HDP)•High-density plasma: processes with ion

concentrations > 1011 cm-3.•Use transverse electric and magnetic fields to increase

distance that electrons travel in plasma, allowingfrequent collisions between electrons and atoms toboost radical and ion densities in plasma.

•Two types of HDP–Operate at traditional plasma frequencies: Helicon plasma

sources, inductively coupled plasma sources, planar coilsources.

–Operate at > 1 GHz: electron cyclotron resonance (ECR)

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HDP at traditional plasma frequency

•If a magnetic field is added to a plasma, mayinduce helical motion of electrons. Due tolarge mass, ions move with minor deflections

•Path of electrons is many times larger in thepresence of magnetic field, increaseopportunity for impact ionization, ion densityand free radical density will be large

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Magnetron (a HDP at traditional freq)•Secondary electrons being accelerated away

from cathode can actually be trapped bymagnetic field and return to cathode, where thecycle will repeat, the ion density become largeand ion bombardment of cathode increasesdramatically

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Inductively coupled plasma (ICP)•RF current through a coil to create a magnetic

field, voltage of coil is raised w.r.t. thesubstrate until a plasma is established

•Once plasma is struck, coil carries RF current,produce a large RF field

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ECR (a HDP at traditional freq)•Electron cyclotron resonance (ECR)•A perpendicular magnetic field along

with an alternating electric field•Electric field increases magnitude of

electron’s velocity•Magnetic field changes direction of

velocity• If freq of oscillation is the electron

cyclotron resonance freq eB/m,amount of deflection caused bymagnetic field is just enough to turnelectron by 1800 as the direction ofthe field changes sign, as a resultelectron will move in a circle(electron resonance)

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ECR

•Use of ECR dramatically increases the densityof ions and free radicals in the discharge.