Chit Sheet

8/12/2019 Chit Sheet

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eat that drives a chemical reaction in which OH- radicals from the NaOH oxidizehe silicon. The SiO2 particles abrade the oxide away. Slurry consistsf nano- particles (10nm SiO2 or Al2O3) and chemicals (NaOH).

Why do we want SOI wafer? parasitic-- ; speed++ Better isolations between devices: prevent latchup Si Photonics latch-upSOI Wafer Manufacturing SIMOX (Separation by IMplantation of Oxygen)

Wafer bonding/Smart CutSIMOX was invented earlier. Wafer bonding is more popular now.1. Silicon crystal structure and defects.. Czochralski (CZ) single crystal growth.. Float zone single crystal growth and doping.. Wafer fabrication.. Measurement methods.rystal growth, wafer fabrication and basicroperties of silicon wafers

Hot probe measurement of carrier typeThe hot probe technique is used to determine the type of dopant in a wafer. It relies on the generation of the Seebeck voltage (open circuit), i.e. the diffusion f free carriers (electrons or holes) as a result of a temperature gradient.The sign of the voltage tells the carrier type. Alternatively, one can measure the current direction (short circuit). The current that flows due to the majority carrier is given by n is thermoelect ric power, negative for electrons, positive for holes.lectrical measurement techniques four point probehe four point probe method measures the resistance/resistivity of a wafer.Using values of carrier mobility, one can calculates the carrier concentration.

Use four points (rather than two) to eliminate the effect of contact resistance. =(e ) 1, ismobility, n is carrier density.Why can four point probe eliminate contact resistance? No current on voltageprobes.Hall voltage (VH) measurementilicon is placed inside a magnetic field and the resultant motion of charge will experience a force perpendicular to the electric field. F=qv n equilibrium this results in a transverse potential difference known as the Halloltage VH. (such that F=qE=qVH/w=qvB, i.e. forces are balanced, so VH=vBw)he magnitude and sign of the Hall voltage leads directly to the carrier concentration,

ype, and mobility (if the resistivity is known).urrent density j=I/tw=E/ =qnv,VH=vBw=(I/twqn) Bw=IB/ tqn.

Hall coefficient RH VHt/ BI=1/ qn = H, H is Hall mobility (we know that =1/ qn . is slightly lower than conductivity mobility, due to more scattering when Hall measurement of carrier type & mobilityhickness of the slab is t (along B direction), width is

w (along VH direction). VH is measured, and used toalculate RH. Then carrieroncentration p, n= 1/ qRH.on implantation: gas ionized, ions accelerated by a high electric field(damage to lattice), andinjected Thermallyctivated (600-1100C)Ion: P, As, Sb,B, In, O: Dose: 1011 - 1018 cm-2 ; Ionenergy: 1 - 400 keV ;

Uniformity andreproducibility: 1% ;Temperature: room temperature; Ion flux: 1012-1014 cm-2s-1Dose ( ) = # of atoms/cm2.; Concentration (C) = # of atoms/cm3.

ld: diffusion -bulk gas source top of surface, or pre-deposited chemical source on wafer surface.Advantage:Not limited by solid solubility; doping uniformity; Flexibility, control in peak depth and spread range- breadth(lateraliffusion) of spread but as long as they are small as deep implant not possible, precise control on Dose;ow-temperature process (can use photoresist as mask but

masking materials can be knocked into the wafer creating unwanted impurities,r even destroying the quality of the interface.)W ide selection of masking materials, e.g. photoresist, oxide, poly-Si, metalLess sensitive to surface cleaning procedures.Very fast (6" wafer can take as little as 6 seconds for a low and moderate dose), slow for high doze values.;Complex profiles can be achieved by multi-energy implants.he ion implanter forms a simple electrical circuit.By monitoring the current in the circuit (or by a monitoring circuit with Faraday cups), significant accuracy in the implse can be maintained. ; High dose introduction is not limited to solid solubility limit values.

Disadvantage: Done in high vacuum, it is a very clean process step.; Very expensive equipment ( $1M or more). ;At high dose values, throughput is less than diffusion (chemical source pre-deposition on surface). ; Ions damage the semiconductor lattice. Not all can be corrected bynnealing.; Very shallow and very deep doping are difficult or impossible. ; Masking materials can be knocked into the wafer creating unwanted impurities,r even destroying the quality of the interface. During high temperature steps after implant (most commonly an activation anneal),

he implanted impurities will begin to diffuse, broadening the implantation profile. End of Range EOR lattice damagewhich might evolve from simple point defect into complexdislocations TED is the biggest single problem with ion implantation chematic of Ion Implanter because it leads to huge enhancements in dopant diffusivity and difficulty in achieving shallow junctions. Ion source: operates at a high voltage and convert the electrically neutral dopant atoms in

he gas phase into plasma ions and undesired species. Somesources: Arsine,Phosphine, Diborane, Solid can be sputtered in special ion sources.

Mass separator: a magnet bend the ion beam through right angle, and select the desiredmpurity ion and purge undesired species. Selected ion passes through an aperture.Accelerator: add energy to beam up to 5MeV. (contained, to shield possible x-ray).Scanning system: x and y axis deflection plates are used to scan the beam across the wafero produce uniform implantation of desired dose. The beam is bended to prevent theeutral particles from hitting the target .on Current will give idea of no of doped ions in the sample.ON EXTRACTIONositive ions are attracted to the exit side of the source chamber, which is biased at a largeegative potential with respect to the filament.Filament emits electrons, which are accelerated to gain enough energy.The electrons collide with the molecules or atoms, and ionize them.


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The ions are extracted, rough-focused, then travel toward the magnetic analyzer.extraction)elect Implantation Specie:

Only one mass will have exactly the correct radius of curvature to exit the source through the slit.Dopant gas containing desired species,BF3, BCl3, B2H6, PH3, AsH3, AsF5. If using solid orliquid source, they need toe heated to vaporize . Plasma provides positive ions,(B11)+, BF2+, (P31)+, (P31)++.

DrawbackNEUTRAL TRAP

There may be ions that collided with other ions in the beam, undergoing a charge exchange.Neutrals are highly undesirable since they will not be deflected in electrostatic end-stationcanning mechanisms and so do not follow the beam, but instead strike a beam stop.

Neutrals are not deflected. The ions are sufficiently deflected by the plates to continue to travel down the tube.o avoid this problem, most systems areequipped with a bend, i.e. parallel plates of anlectrostatic deflection system. Beam scanning good for low dose

or high dose: Mechanical Scanning but drawbacks Excellent wafer coolingneeded. Substantial load/unload time.15 - 25 wafers /disc. Excellent throughput for high dose implants.High current ( 20mA) Scan speed adjustment to insure uniform dose.ractical implantation dosimetry (dose measurement)

Q*t=Dose*Cup Areawo Positive Polarity Faraday C up removes Secondary Electron Effectopant Impurity Profile Approximated by Gaussian Distribution for first orderpproximation.Rp= projected range, is a function of ionnergy and mass, and atomic numberf impurity as well as target material.Delta Rp =straggle = standard deviation.Np = peak concentration at x=Rp.Dose Q= N(x)dx=((2PI)^1/2)Np( Delta Rp)n textbook, C is used for concentration, toeplace N used here.

Mi and Mt are the masses for incident and target ion.unction depth in Si Junction formation by impuritymplantation. Two pn junctions are formed at xj1 and xj2. mplant into Si already doped atNB. .g. implantP into Bdoped Si.: n-type doping;B: p-type.

oping profiles other than the simple Gaussian are required.mplantations far away from the surface and for reasonable short characteristiciffusion lengths, the new profile can be approximatedby: ometimes Softwares used to curve out implantation distributionimulation software: SRIM - The Stopping and Range of Ions in Matter

How are drain and self aligned source formed today?mplant only certain part of the wafer by a mask such that its Rp lies within the mask material.

Unfortunately lateral spread still happens; In order to act as an efficient mask, thehickness of the mask should be large enough that the tail of the implant profile inhe silicon should not significantly alter the doping concentration.

How thick mask is needed?Rule ofthumb: good masking thickness; Xm = Rp + 4.3Rp, C(x=Xm)/C(x=Rp); order equal to 10-4.;Better way: using simulation software.Mask Material ; Photoresist is commonly used as implant mask. Resist may flow or be baked. 1.Outgassing: Heavily implanted resist layers often break apart organicmolecules have a hardened carbonized Layer post H2 released near surface difficult to remove later on. The outgassing can raise the pressure in the endtation sufficiently to cause neutralization of ion beam through impact with the H2 molecules, resulting in significant dose rate errors.

Resist damage at high implant currents --------------- BF2 + implant at 80A in Varian 400 without a water cooled chuck (water cool can reduce the problem)31Mask material thicknessateral scatteringhe implanted ion also scatter laterally around the impact point, which can alsoe approximated by Gaussian distribution with transverse straggleRT. -D formulation of implantation profile

Monte Carlo simulation of many ions into SiOccur when ion velocity is parallel to amajor crystal orientation.

ome ions may travel considerable distances with little energy loss. 2D Formulation of Implantation Prof

Once in a channel, ion will continue in that direction, making many glancingnternal collisions that are nearly elastic (their stopping is then dominatedy electronic drag only), until it comes to rest or finally dechannels. Theatter may be result of a crystal defect or impurity.OOKING INTO Si AT DIFFERENT ORIENTATION ------- Occur when ion velocity is parallel to a major crystal orientation.ome ions may travel considerable distances with little energy loss.

Once in a channel, ion will continue in that direction, making many glancingnternal collisions that are nearly elastic (their stopping is then dominatedy electronic drag only), until it comes to rest or finally dechannels. Theatter may be result of a crystal defect or impurity. Impurity Dis due to this effect


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Methods to reduce channeling effect 1. thin screen oxide which is amorphous is often used, or make Si layer amorphised by exceeding critical implantose causing some randomization of incident beam before it enters the lattice.

Most IC implantation is done off axis.. A typical tilt angle is 7o.. Destroy the lattice before implantation

High dose Si+ implantation to convert the surface layer intomorphous Si. After exceeding critical implant dose. Heavy atoms

will form critical implant dose early. Implantation of desired dopanton substrate interactionhe ions are stopped at random positions,

mostly not in crystallinetes, so not active as dopant

need anneal to active them).

on implantation energy loss mechanisms Nuclear stopping Energy Loss, crystalline Siubstrate damaged by collision. Electronic stopping energy Loss, electronic excitation creates heat.Total energy loss is the sum of the two processes.Nuclear stopping power: Coulomb scattering (does most damage)(dominates at low velocity)As fast particle have less interaction time with the scattering nucleus, the nuclear energy loss

small at very high energies/velocity.hus the nuclear energy loss tends to dominant towards the end of the range when ion has

ost much of its energy and where nuclear collisions produced most of the damage.ight ion slowed down at the end of range

Heavy ion is slow (v M1/2), more loss at the start of movement.here are also inelastic collisions that transfer energy.lectronic stopping power(dominates at high velocity) (does less damage)lso Coulomb interactions, but inelasticonlocal: ion experiences drag due to free or polarizable electrons.ocal: passing ion causes internal electronic transitions => energy and moment transferecause electrons can follow fields up to opticalfrequencies (velocities of 105 m/s - 100mes faster than phonons), electronic losses dominate at higheron velocities.lectronic and nuclear stopping power

OR damage

Damage at end of range (EOR) nuclear damagehe main disadvantage of ion implantation is the production of

attice damage (vacancies and interstitials, or V/I) which mayvolve from simple point defects into complexislocations or voids.olution to Problem? Most damage can be repaired by annealing. However, annealing cycles of 30 min at close to000oC can cause considerable spreading of the implant by diffusion.7(Si)Si-> SiInterstitial + Si Vacancy

Damage to the lattice: target atom displacementnergetic incident ions collide with target atoms, leading to their displacement.he result is an interstitial atom and a vacancy, V-I pair = Frankel defect.he displaced atoms may have energy high enough to further displace other

arget atoms along its path.Ed is the minimum energyrequired to break four covalentonds and dislodge a lattice atom.

Number of displaced target atomsd is called threshold energyr displacement energy (for Si, Ed 15eV). or example, 30keV As ion will lead to roughly 1000 displacedtoms.(More is the damage due to nuclear energy loss). The number will be less for 30keV light ions, whose energy is mainly lost by electronic stopping.(Electronic lossaamage) For light ion (lighter than target), small energy transfer to target atom for each collision, generate few displaced target atoms, and ioncatted at large angle. Low density non-overlapping damage, but over large area with a saw-tooth shape.For heavy ion, large energy transfer for each collision, small scatle. The displaced atom can further displace other target atoms. Small range, large damage density over small volume.

Annealing 900oC, 30min: repair damage and activate dopants. After implantation, we need an annealing step, usually under Ar, N2 or vacuum.Restore silicon lattice to its perfect crystalline state - silicon atoms can move back into lattice sites at these temperatures.

ut dopants into Si substitution sites for electrical activation - nearly the entire implanted dose becomes electrically active except for impurity concentrations >>10^19/cmRestore the electron and hole mobility now that the lattice becomes perfect again.Annealing and Solid Phase epitaxy (SPE): Restoring crystal structure-- when substrate has been rendered amorphous, the crystallinity is repaired bySPE, where crystal refosing the underlying undamaged substrate as a template. Most of impurities are incorporated into the growing lattice .f the substrate is amorphous, it can re-grow by solid state epitaxy(SPE).

Due to the high activation energiesrequired to annihilate defects ( 5eV), it is often easier to regrow the crystal from an amorphous layer viaSPE activation energy 2.3eVin Silicon) than it is to anneal out defects. Thus, two schemes for implants areused: Implant above the critical dose and use low temperaturenneal to regrow material. Implant below the critical dose and use high temperature anneal to get rid of defects.

ansient enhanced diffusion (TED). Stable defects formation near a/c-amorphous/crystalline interfaceut, the tail of the damage beyond the a/c (amorphous/crystalline) interface can nucleate stable, secondary defects (defects caused directly by implanted ion are primary ), and cause transient enhanced diffusion (TED).TED is the result of interstitial damage from the implant enhancing the dopant diffusion for a brief transient period.is anomalous diffusion, because profiles can diffuse more at low temperatures than at high temperatures for the same Dt. TED is the biggest single problem with ion

mplantation because it leads to huge enhancements in dopant diffusivity and difficulty in achieving shallow junctions. Physically based understanding of TED has ledo the methods to control it (rapid thermal annealing, or RTA).


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ntrinsic diffusivity Di; E.g. at 1000oC, ni=7.141018/cm3.ntrinsic: impurity concentration NA, ND < ni

Note that ni is quite high at typical diffusion temperatures, so "intrinsic" actually applies under many conditions.olve TED by Rapid thermal processing/annealing

Dopants can diffuse during high temperature anneal (activation energy 34eV)Activation energies of defects annihilation (~5eV) > the activation of diffusion. Thatmeans annihilation speed increases faster with temperature. To minimize this unwanted diffusion, high temperature short time annealing isreferred. Rapid ThermalProcessing (RTP) or Rapid Thermal Anneal (RTA) is used. RTA is extremely important for shallow junction devices.

Rapid heatingsource: high power laserelectron beamhigh intensity

Wet oxide: thick


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g is mass transfer coefficient in cm/sec.odels to explain the excess growth of thin oxidation n electric field exists across oxide that enhances diffusion (if diffusing species is O2it is not.) during early states of oxidation.hin micro-channels in oxide aid in the movement of O2 to the Si surface. Mismatch in thermal expansion coefficients of oxide and Si causes stress in oxided this stress may enhance the diffusivity of the oxidizing species.But NONE of these mechanism gained wide acceptance. The exact mechanism is still unknown.

r Xo>>7nm, Massoud modelproaches DG model.xidation rises with temp as well as pressure

pendence on crystal orientationxidation rate depends on the availability of reactiones on the silicon substrate.

xidation on the crystalane occurs at a higher ratecause there are a highermber of surface atoms/chemical bonds than the which is in turn greater than plane.

S0 is a constant, roughly proportional to the number of available Si bonds for reaction per unit area.nterface reaction rate constant Ks (cm/sec) depends on crystal orientation.o the liner grown rate B/A depends on crystal orientation. (B/A)111= 1.68 (B/A)100he parabolic rate constant B is NOT dependent on crystal orientation. With Cl addition, dopants of Si in high conc. and Temperature both B/A and B increases

The effect is particularly important at lower temperatures and for thinner oxides, and is more important for N+ doping than P+ doThe oxidation rate depends on:The Cs dopant concentration in SiO2 for diffusion controlled oxidation (B dominates).The C0 dopant concentration at Si surface for reaction controlled oxidation (B/A dominates).

-- Peculiarly this effect dont keep on increasing but decreases for high temperature and/orlong time oxidation when oxide become thick.

Phosphorous piles up at Si surface, leads to more vacancies in Si (oxidation process needspace/vacancies), enhances oxidation rate in the reaction controlled regime (increase ks).In this case Linear rate coefficient B/A increases rapidly for surface doping levels greater than 1020/cm3. Parabolic rate coefficient B shows only modest increases.

Effect of Diffusion on Doping Boron: B segregates into oxide, weakens SiO2 bond structure, increases oxidant diffusivity D. B increases rapidlyWSi is used??

Qf : Qf: Qf=3: 2 : 1 This is one of the reasons why IC uses Si. The other reason is that the mobility is high

Two dimensional oxidation( More bonds more oxidation, more radius more oxidation)Oxidation is slower for convex or concave corners. Concave corner is even slower than convex corner. The smaller the curvature radius is, the slower. More serious for low temperature oxidation, no effect for high temperature

1200oC (when oxide can flow).

Stress due to Volume Expansion(Causes more of Activation Energy Requirement.)Stress due to volume expansion (we know when Si oxidizes, it expands). As the oxidegrows, the newly formed oxide pushes out the old oxide whichrearranges itself through viscous flow. Stress occurs typically on curvedsurfaces.

Oxidation Rate after Chlorine AdditionAddition of Cl (1-3%) bearing species to oxidation ambient leadsto: faster oxide gBoth B/A and B increases,because bond energy of Si-O is 4.25eV, Si-Cl is 0.5eV, so Cl2

reacts with Si first to form Si-Cl, which then reacts with O2 to form SiO2. Cl2 is a kind of catalyst.Cleaner oxide, less metallic contamination, sinceCl is a metal getter. Most heavy metal atoms react with Cl to form volatile metal c


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erface trapped charge (Qit): located at Si/SiO2 interface. Energy of those interface states are within the band gap, so Qit can be +, - or neutral, depending on bias voltthose states can be filled with e- orh+) result of structural defects due to the oxidation process, metallic impurities, and bond-breaking (dangling bond Si).

has the same origin as Qf(incomplete oxidation) , so high Qf always means high Qit. Low oC H2 (forming gas, 10% H2 + 90%N2) anneal a t 400-450oC afterallization process effectively neutralizes most interface-trapped charges- Qit--xed oxide charge (Qf ): positive charge located within 3nm of Si/SiO2 interface. incomp. Oxidn; small for high oC; Cl++ causes Qf--; fast cooling small Q f; 111>: Qf: Qf=3: 2 : 1; Qf cannot be charged or discharged easily (unchanged during device operation), and it is reproducible/predictable.xide trapped charges (Qot): associated with defects in the SiO2. may be positive or negative, due to holes or electrons being trapped in the bulk of the oxide.is caused by broken Si-O, Si-H or Si-OH bonds, due to ionization irradiation and other energetic processes during evaporation (generate x-ray for e-beamporation), sputtering (plasma), RIE and ionimplantation. They can be annealed out by low-temperature (300oC) treatment in H2 or inert gas.00oC dry oxidation improves oxide structure and make it less susceptible toirradiation. Oxide can also be protected from irradiation by covering with Al2O3 and Si 3N

that are resistant to irradiation. It received more attention in recent years because, as the device shrinks, the electric field within the oxide is increahigh field may cause electrons toinject to the traps in the oxide (charge trapping), which shifts the threshold voltage.obile ionic charges (Qm): result from contamination from Na or other alkali ions, K, Li, - ions and heavy metals originate from processing materials, chemicals,

ient, or handling. It can be anywhere in the oxide layer, can move at high temperature or bias voltage.an shift MOSFET threshold voltage and cause device stability p Common techniques employed to minimizeQm include: eaning the furnace tube in a chlorine ambientddition of cl-containing gas during oxidationttering with phospho-silicate glass(PSG) to replace quartz tubesing masking layers such as Si-3N4 to prevent the contaminants from getting into the oxide.was a big problem in 1960s, nowadays no longer a serious issue.at was why Si MOSFET couldnt be made in the early days.

GREGATION COEFFICIENT : Two additional factors influence the redistribution process:he diffusivity of the impurity in the oxide (if large, the dopant can diffuseugh the oxide rapidly, thereby affecting the profile near the Si-SiO2 interface).he rate at which the interface moves with respect to the diffusion rate.

A DGmodel, 1micrometer/min possible)/poorfilm/popular in thin film deposition by nanofab. Involving easy lift off/non conformal

erage./SharonThermalEvaporator//little gas in film//large grain size//fewer grain orientation//poor adhesiongh sticking coefficient (at low T, adatom stays wherever it hits with limited surface migration), leading to poor conformal coverage/significant shadow.t this also makes evaporation the most popular thin film deposition for nanofabrication using liftoff process. Material source is heated to high temperature in vacuum< 10^-5 Torr) either by resistive,thermal or e-beam methods. High vacuum isrequired to minimize collisions of source atoms with background species (light of site dep

aterial is vapor transported to target in vacuum. Film quality is often not as good as sputtered film . The film thickness can be monitoredprecisely using a quartz balancis is necessary as the deposition is not reproducible (tiny change in T leads to large change of deposition rate. T is not monitored, power is.position rate is determined by emitted flux and by geometry of the source and wafer. For microfabrication R&D, evaporation is as important as sputter deposition.


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ation of deposit thickness Thickness Measurement : Quartz Crystal Micro-balance (QCM): (similar idea to quartz clock)artz is a piezoelectric material. With a high frequency AC voltage activation, the amplitude of vibration is maximum at resonancefrequency. s resonance frequency will shift when film is deposited on its surface. Thus by measuring frequency shift f, one can measure film thickness

h sub- accuracy. Variation of deposit thickness for large/multiple substrates has to rely ontz crystal micro-balance for thickness monitoring.

poration issues: alloy evaporationchiometrical problem ofevaporation: Compound material breaks down at high temperature. Each component hs different vapor pressure,efore different deposition rate, resulting in a film with different stoichiometry compared to the source.One solution is co-evaporationtwo e-beam guns).This again leads to shadow effect.

step coverage could be useful. Shadow/angle evaporation is routinely used for nano-electronics fabrication.

dow evaporation: for non conf ormal shape and easy lift off.pictures above show shadow-masks forbium rings containing a Josephsontion, prior to evaporation.metals are evaporated under differentes without breaking the vacuum.mask consists ofGermanium while theificial layer below the mask is made of

h temperature capable plastic (polyetherhone).

AD (glancing angle deposition): self assembly of filmle >80o, with (or without) substrate rotation.assembly mainly due to shadowing effect thatnifies the otherwise grain structures.

ttering (overview)utter is carried out in a self-sustained glowharge (plasma).not vacuum eavy inert gas is the major carrier, e.g. Ar.ctive chemical species (e.g. O2 toosit oxide) may be introduced tive sputtering

e ionized atoms bombard the target andse the transport of sputtered atoms ontosubstrate.ttering process can be run in DC or RF

de (insulator must be run in RF mode)or process parameters:peration pressure (~10-100mTorr)ower (few 100W)ubstrate bias (in addition to self-bias)ubstrate temperature (20-500oC)lectric materials can also be deposited, but are usually formed by sputteringals in O2 , N2, CH4 gases, forming AlN, Al2O3, TiN, TiCwide range of industrial products use sputtering: CD,LCD, computer disks, hardings for tools, metals on plastics.

vantages: ble to deposit a wide variety of metals, insulators and composites & dielectric.eplication of target composition in the deposited films.apable of in-situ cleaning prior to film deposition by reversing the potential on the electrodes .etter film quality dense and step coverage than evaporation, preferred deposition technique for micro-fabrication (semiconductor ustry).(nano fab thin film for R&D by evaporation)tter yield: the number of sputtered atoms per impinging ion. SY increases with ion Energy Ion energy, spuering yield s, 95% of incoming energy goes to target heating & secondary electron.be used as etching methodion impact may set up a series of collisions between atoms of the target, possibly leading to the ejection of some

hese atoms. This ejection process is known as sputtering.Here we are interested in sputter deposition. Of course sputter can also be used as an etching method (the subse etched will be the target), which is called sputter etching.tter increases with ion mass.

sadvantages: ubstrate damage due to bombardmentw rate of deposition but Ion energy, spuering yield h rate sputter processes need efficient cooling techniques to avoid target damage from overheating.


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LOY Better Compared to Evaporation where Alloy has stoichiometry problemike evaporation, composition of alloy in film is approximately the same as target.get NOT melted so no stoichimotery problem due to dif. In vapour pressure and dif. In deposition rate,val time, slow diffusion (no material flow) mixing.en target reaches steady state, surface composition balances sputter yield.

tter increases with ion mass.tter yield is a maximum for ions with full valence shells: noble gasses such

Ar, Kr, Xe have large yields.Dependence of sputter yield on ion masstter rate for Ag is higher than Cu, and Cu higher than Ta

Sputtering,RF Sputtering: Magnetron Sputtering

zation Eff: Low ;Ionization Eff: Medium ; Ionization Eff: High ; rge acc at cathode and anode ; non ionized collision insulatortrostatic plates control %e hitting and causing ionization, magnet control deposition as a result, trap e helical patternionization and sputtering efficiencies are increased significantly

m migration along surface also importantoms ejected from cathode escape with energies of 10 to 50 eV, which is 10-100 times the energy of evaporated atoms.s additional energy (together with bombardment by other ions) provides sputtered

ms with additional surface mobility for improved step coverage relative to evaporation.s additional energy also makes the deposited film denser - better film quality than evaporated film).position Rate: Low; Medium: Highosition rates increase by 10-100X, to 1 microm per minute.trons involved; neutral particle; trap e generated due to E and B

pendicular to each other, their Orbital motion of electrons increasesbability that they will collide with neutral species and create ions. osition parameters:cess pressure: compromise between the number of Ar ionsthe scattering of Ar ions with neutral Ar atoms

utter voltage: maximum yield, typical -2 to -5 kV, for high deposition rate.bstrate bias: control ion bombardment characteristics.bstrate temperature: modify deposited film properties.ays

When frequencies less than about 50kHz, just like medium pressure in DCtrons and ions in plasma are

bile o DC sputtering of both surfaces Whenfrequencies above about 50kHz,like high pressure in DChere in RF(heavy) can no longer follow the switching o Electrons can neutralize positivege build up.rate at moderate Pressure (RF: Capable of running in lower pressure5 mTorr)her chamber pressure:an-free path of an atom =4.8 0 3/P(torr) (cm). E.g. 0.1cmfor P=50mTorr.refore, as typically target-substrate separation is many cm, sputtered atoms have tohrough tens of collisions before reaching the substrate.s reduces deposition rate considerable materials are deposited onto chambers.many collisions also prevent ionization (reduce ion density and deposition rate).

wer chamber pressure:same power) higher ion energy that increases sputter yield/deposition rate.

fewer Ar ions to bombard the target for deposition, which reduces deposition rate.refore, there exist an optimum pressure (provided that such a pressure can sustainplasma) for maximum deposition rate.s optimum pressure depends on target-substrate configurations (their separation,et/substrate size).wer Ar pressure (to 0.5mTorr, can still sustain plasma) can be utilized sincezation efficiency is larger which can improve film quality as less argon will berporated into the film.

wback conducting materials only. positive charge builds up on the

ode (target) in DC sputtering so ionization is low (RF alternating V avoids charge buildupgood for insulators as well but neutral particles combine; ionization is medium)trap e so Ionization is maxntentionall wafer heating happens; Unintentionall wafer heating happens;ntentional wafer heating is reduced since the dense plasma is confined near theet and ion loss to the wafers is less.

d Heating and Cooling Mechanism easy in DC;( RF:cooling of insulating targets is difficult in RF systems). endence of deposition rate on chamber pressure

val angle can be tailored to some degreewever, when the mean free path of the target atom is much shorter than targetstrate separation, many collisions will occur, which broaden the arrival angleribution (in contrary to evaporation).


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w to get uniform conformal step coveragedes tilting and rotating substrate, step coverage can be further improved by: strate heating: improve step coverage due to surface diffusion, but may producecceptably large grains.ly bias towafers: increase bombardment by energetic ions, but it will also sputter the

osited material off the film and thus reduce deposition rate.msejected from cathode escape with energies of 10 to 50 eV, which is 10-100 times thegy of evaporated atoms.

s additional energy (together with bombardment by other ions) provides sputteredms with additional surface mobility for improved step coverage relative to evaporation.s additional energy also makes the deposited film denser - better filmquality than

porated film).

m migration along surface also important

ed laser deposition (PLD) r Beam

getstrate

me ma)

me generated by laser ablationh different tiny or micro-particles

Characteristics: produce the composition of the targetbricate multi-component multilayer structuresst response and well controlled deposition rate

vironmentally benign technique e of fs to ns with peak power highugh (hundreds of MW/cm2) to meltoil vaporize ablate the targetace material, to atoms, ions,trons, and clusters.b) (c) (d)

ure 5.1. Schematic laser-material interaction. (a)orption and heating; (b) Melting and flowing; (c)orization; (d)Plasma formation in front of theet. Under certain conditions the plasma canch from the target and propagate toward ther beam.D film growth stepsurce production of appropriate gasnsport of gas to surfacem thin films on the surface of a substrate by thermal decompositionor reaction of gaseous compounds

ually performed at high temperaturen be performed at various pressure and with assistance of plasma (PECVD) sorption of gas on substrateactions on substratensport of by-products away from substratees of CVD reactions Peculiarity of CVD ermal decompositiong) ---> A(s) +B(g) Si deposition from Silane at650oC: SiH4(g) Si(s) + 2H2(g)duction (using H2)(g) + H2(g) A(s) + HX(g)eposition at 300oC: WF6(g) + 3H2(g) W(s) + 6HF(g)

ida tion (using O2)(g) + O2(g) ---> AO(s) + [O]X(g)2 deposition from silane and oxygen at 450oC (lower temp than thermalation): SiH4(g) + O2(g) ---> SiO2(s) + 2H2(g)mpound formation (using NH3 or H2O)(g) +NH3(g) --> AN(s)+HX(g) or AX(g)+H2O(g)-->AO(s)+HX(g)osit wear resistant film (BN) at 1100oC: BF3(g) + NH3(g) --->BN(s) + 3HF(g)


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D processantages: gh growth rates possiblen deposit materials which are hard to evaporateod reproducibilityan grow epitaxial films (in this case also termed as vapor phaseaxy (VPE). For instance, MOCVD is also calledOMVPE.) nerally better film quality than PVD films.dvantages: gh process temperaturesmplex processesxic and corrosive gasses

m may not be pure (hydrogen incorporation).es of CVDVD (AtmosphericPressure CVD) (not very popular for nanofabrication), masssport limited growth rate, leading to non-uniform film thickness.VD, wafers have to be laidzontally side by side.

CVD (Low Pressure CVD)w deposition rate limited by surface reaction, so uniform film thickness (manyers stacked vertically facing each other; inAPCVD, wafers have to be laidzontally side by side.s pressures around 1-1000mTorr (lowerP => higher diffusivity of gas to substrate)tter film uniformity & step coverage and fewer defectscess temperature 500CQuality better than PECVD

ction at high temperaturection rate (determined byperature) limited deposition rate.

CVD (Plasma Enhanced CVD)sma helps to break up gas molecules: high reactivity, able to process at lower temperature and lower pressure (good for electronics on plastics).ction at low temperature, assisted by plasmaessure higher than in sputter deposition: more collision in gas phase, less ion bombardment on substraten run in RF plasmamode: avoid charge buildup for insulatorsm quality is poorer than LPCVD. ocess temperature around 100 - 400C.VD process parameterSubstrate temperaturentrol by external heater, very little heating from PECVD process Gas flowgher flow rates can increase deposition rate and uniformityPressure hanges the energy of ions reaching electrodesn change deposition ratereases pressure may lead to chemical reactions in the gasects also depend on gas concentration Power ects the number of electrons available for activation and the energy of those electronsreased power may lead to chemical reactions in gasreased power increases deposition rate Frequency (for PECVD) anges plasma characteristicsanges ion bombardment characteristics

D (Atomic layer deposition)

CVD (Metal-organic CVD, also called OMVPE - organo metallic VPE), epitaxial growthmany optoelectronic devices with III-V compounds for solar cells, lasers, LEDs,to-cathodes and quantum wells.

con nitride deposition Application:asks to prevent oxidation forLOCOS process nal passivation barrier for moisture and sodium contaminationch stop for Cu damascene processpular membrane material by Si backside through-wafer wet etch.mic layer deposition (ALD, break CVD into two steps)

milar in chemistry to chemical vapor deposition (CVD), except that the ALD reaction breaks the CVD reaction into two half-reactions, keeping the precursor materials sng the reaction. The precursor gas is introduced into the process chamber and produces a monolayer of gas on the wafer surface. A second precursoras is then introduced into the chamber reacting with the first precursor to produce a monolayer of film on the wafer surface.m growth is self-limited (monolayer adsorption/reaction each half-cycle), hence atomic layer thickness control of film grown can be obtained.at is, one layer per cycle; thus the resulting film thickness may be precisely controlled by the number of deposition cycles.o fundamental mechanisms : 1.o Chemi-sorption saturation process 2. Sequential surface chemical reaction processroduced in 1974 by Dr. Tuomo Suntola and co-workers in Finland to improve thequality of ZnS films used in electroluminescent displays.cently, it turned out that ALD also produces outstanding dielectric layers and attracts semiconductor industries for making High-K dielectric materials.antages and disadvantagesantages Stoichiometric films with large area uniformity and 3D conformality. Precise thickness control. Low temperature deposition possible. Gentle depositioncess for sensitive substrates. Each layer formed each cycle, controlled thickness.ALD: slowest, best step coverageadvantages Deposition rate slower than CVD. Number of different material that can be deposited is fair compared to MBE.


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axytaxy refers to the method of depositing a

nocrystalline film on a monocrystalline substrate. word epitaxy comes from theGreek for above (epi)

in an ordered manner (taxis): to arrange upon.toepitaxy(or Homoepitaxy): extension of the substratece by the overgrowth of a layer of identical material Si on Si orGaAs on GaAs) with no problem ofpatibility or mismatch.teroepitaxy: any two materials of different crystallinecture and orientation (e.g. (001) GaAs on (001) Si or

Si on Sapphire)

eroepitaxy conditions

eroepitaxy Conditions)(AlGaN && GaN) GaAs&Si strate must be physically and chemically inert to the growth environmentbeing prepared with a damage-free surface.emical compatibility between the materials to avoid compound formation or massiveolution of one layer by the other.t ched thermal expansion characteristics between the layer and substrate to avoid

ess stress upon cooling formaon of dislocaon at the interface, or even breakinghe structureched lace parameters between the layer and substrate not a serious

blem

VCVD MOCVD: metal-organic CVD = OMVPE: organo-metallic vapor phase epitaxyny materials that we wish to deposit have very low vapor pressures and thus arecult to transport via gases. solution is to chemically attach the metal(Ga, Al, Cu, etc) to an organic compound

has a very high vapor pressure.he organic-metal bond is very weak and can be broken via thermal means on wafer,ositing the metal with the high vapor pressure organic being pumped away.is the main workhorse for III-V semiconductor industry (semiconductor laser, LED,oelectronics).me MOCVD precursor gases:methyl-aluminum,liquid methyl-gallium,liquid ne AsH3, gas

methyl selenide,liquid methyl zinc,liquid

organometallic processantages: Highly flexible we can deposit semiconductors, metals, dielectricsdvantages: Highly toxic, very expensive source material, and environmentalosal costs are high.erial deposited:

V semiconductors AlGaAs, AlGaInP, AlGaN, AlGaP, GaAsP, GaAs, GaN, GaP, InAlAs,InAlP, b ,InGaN, GaInAlAs, GaInAlN, GaInAsN, GaInAsP, GaInAs, GaInP, InN,InP. I semiconductors - Zinc selenide (ZnSe), HgCdTe, ZnO, Zinc sulfide (ZnS)emiconductors - Si,Ge, strained silicon


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olecular-beam epitaxy (MBE) BE is an epitaxial process involving the reaction of one or more thermal beams of atomsmolecules with a crystalline surface under UHV conditions.

Precise control in both chemical composition and doping profiles. has a very low growth rate (e.g. forGaAs, a value of1m/hr is typical.)ingle-crystal multilayer structures with dimensions on the order of atomic layers can be

ade.

quid-phase epitaxy (LPE)PE involves the growth of epitaxial layers on crystalline substrate by direct precipitation from theliquid phase.

LPE, the substrate is held in contact with the supersaturated solution (As saturated solution of Ga). ool ing the arsenic saturated solution of gallium causes the arsenic to precipitate in the form ofGaAs.

ypical deposition rates for monocrystalline films range from 0.1 to 1m/min. PE in most cases is a very economic depositiontechniques, especially when up-scaled to mass-production


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hree types of solids - amorphous, polycrystalline, mono-crystalline (single-crystal).emiconductor devices and VLSI (very large scale integrated) circuitsrequire high-purity single-crystal semiconductors.Because: etter quality control

Higher mobility

morphous silicon is used in photovoltaic cells, electronic displays (large-area).olycrystalline silicon is used as a gate contact in MOSFETs (VLSI circuits).slocationacking Faultcipitate

oint defects

nt defects. a) Substitutional. b) Interstitial. c) Vacancy (Schottky defect). d) Frenkel-type defect (interstitial - vacancy pair).int defects dictate most dopant diffusion mechanisms, and thus determine the impurity profile.

u may want a dopant impurity to be on a substitutional site, but you may not want a heavy metal atom or otherwanted impurity to be on a substitutional site (harder to remove).pant atoms diffuse through the semiconductor faster as interstitials, but we

ed to place them in substitutional sites to make use of them (i.e. they act as dopants onlyhen in substitutional sites).

erstitials can be foreign unwanted impurities, intentionally introduced impurities, ormisplaced host atoms (an intrinsic defect, the self-interstitial).

int defects: interstitialne Defect: Dislocationslocation causes low yield strength of solids.slocation sources: by stress due to temperature gradient, due to agglomeration.acking : 2D defectecipitate: 3D defect

ectrical grade silicon (polycrystalline) EGSsically, the solid Si is first converted into a liquid form (SiHClfor purification, thennverted back into solid Si.th reactions occur at high temperatures.

etallurgical grade silicon is treated with hydrogen chloride to form trichlorosilane:+ 3HCl SiHCl

g) + H2(g) (use catalyst)HCl3 is liquid at room temperature, boiling point 32oC. Multiple distillation of the liquidmoves the unwanted impurities (99.9999% pure).e purified SiHCl3 is then used in a hydrogen reduction reaction to prepare thectronic grade Si (EGS):SiHCl

g) + H2(g) Si(s)+ 3HCl(g)is is the reverse reaction of the above reaction)

GS is the raw material for Si single crystal production.

xygen and carbon in CZ siliconThe CZ growth process inherently introduces O (fromO2 crucible) and C (from graphite

sceptor/supporter).ypically, CO 1018 cm-3 and CC 1016 cm-3.he O in CZ silicon often forms small SiO2 precipitatesthe Si crystal under normal processing conditions.ritical size is about 1nm: stable precipitates abovem, may shrink and disappear below 1nm. and these precipitates can actually be very useful:

ovide mechanical strength, internal gettering.

ost k0 values are


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C: high defect density ; HB: low defect densityZ: Impurity by C and O allowed FZ: not allowed impurityZ: large dia of wafer FZ: small diaZ: uniform radial distribution of charge FZ: not unitZ: normal electronics; HB: optoelectronics FZ: power electronics

oat-zone crystal growth: overview

For CZ-grown Si, impurities (O and C) can be introduced from the melt contacting the SiO2 crucible and fromaphite susceptor/supporter.his limits the resistivity to 20 cm, while int rinsic Si is 230kcm. hese float -zone grown crystals are more expensive and have very low oxygen and carbon.arrier concentrations down to 1011 atoms/cm3 have been achieved. is far less common, and is reserved for situations where oxygen and carbon impurities cannot be tolerated.

loat -zone does not allow as large Si wafers as CZ does and radial distribution of dopant in FZ wafer is not as uniform as in CZ wafer.ainly used in power electronics and THz applications. oat-zone crystal growth process Polycrystalline silicon is converted into single-crystal by zone heating (zone melting).he entire poly-Si rod from the EGS process is extracted as awhole. The rod is clamped at each end, with one end in contact with a single crystal seed.n RF heating coil induces eddy currents (power I) in the silicon, heating it beyond its melting point in the vicinity of thecoil. The "floating" melt zone is about 2cm wide/high.he seed crystal touches the melt zone and is pulled away, along with a solidifying Si boulelowing the seed. The crystalline direction follows that of the seed single crystal.imited to about a 4" wafer, as the melt zone will collapse - it is only held together byrface tension (and RF field levitation).ly-Si

Sielt is not held in antainer, it isoat, thus theme float zone.oat-zone: zone refiningopants/impurities prefer to stay in the liquid than in the solid. Thus, the impurities generally stay in the melt zone,d don't solidify in the boule. That is, segregation (and evaporation) of impuritiesthe melt zone help purify the Si further. One can "purify" FZ wafers further by successively passing the coil along the boule. The impurities thengregate towards the end of the boule. O f course, if neglecting impurity evaporation, the total amount of impurity is the same. Yet thepurity at the lower part is much lower than, and at the upper part approaches to, the original impurity concentration.hermal instability in the melt zone can cause micro-variations in composition and doping. Difficult in making a uniform dopant concentration.ping in FZ growthGas doping:Dopants are introduced in gaseous form during FZ growth.n-doping: PH3 (Phosphine), AsCl3p-doping: B2H6 (Diborane), BCl3od uniformity along the length of the boule.l doping:ill a small hole in the top of the EGS rod, and insert the dopant.If the dopant has a small segregation coefficient, most of it will be carried with thelt as it passes the length of the boule.Resulting in only a small non-uniformity.Ga and In doping work well this way.

ne refiningZone length is L. The rod has initial uniform impurity concentration of C0.he molten zone moves upwards by dx, the number of impurities in the liquid (=I) will changece some will be dissolved into the melting liquid at the top (=C0dx) and

me will be lost to the freezing solid on the bottom (=CSdx=k0CLdx).us: (assume cross-sectional area =1)


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