Presentation on theme: "Sputtering Processes for Thin Film Deposition"— Presentation transcript:
1Sputtering Processes for Thin Film Deposition Suiqiong LiELEC 7730Instructor Dr. Y. Tzeng
2Sputtering zinc oxide thin film OutlineFundamentals of sputteringTechniques of sputteringDC sputteringRF sputteringMagnetron Sputter DepositionReactive sputteringSputtering zinc oxide thin film
3QuestionsCompare with DC sputtering, what are the advantages of RF sputtering?What parameters would affect sputter yield?
4The Mechanisms of Sputtering Interactions of ions with surfacesThe ion impact may set up a series of collisions between atoms of the target, possibly leading to the ejection of some of these atoms. This ejection process is known as sputtering.
5Sputtering Deposition Process Ions are accelerated into targetSome of the surface atoms are sputtered off of the target.These sputtered atoms “flow” across the chamber to where they are deposited
6Sputter Yield Sputtering process is characterized by sputter yield (S) S values are typically in the range of 0.01 and 4 and increase with the mass of metals and energy of the sputtering gas.matlb.kjist.ac.kr/~master/lecture/TFT/PVD2(14).ppt
7Sputter Yield (Cont.)The sputter yield depends on: (a) the energy of the incident ions; (b) the masses of the ions and target atoms; (c) the binding energy of atoms in the solid and (d) the incident angle of ions.60o – 70o
8Sputtering Alloy Targets composition of alloy in film is approximately the same as alloy in target (unlike evaporation)slow diffusion mixing in solids (sputtering)target reaches steady statesurface composition balances sputter yield
9Advantages of sputter deposition Elements, alloys and compounds can be sputtered and deposited.The sputtering target provides a stable, long-lived vaporization source.In some configurations, reactive deposition can be easily accomplished using reactive gaseous species that are activated in plasma.The source and substrate can be spaced close together.The sputter deposition chamber can have a small volume.
10Disadvantages of Sputter Deposition Sputtering rates are low compared to those that can be attained in thermal evaporation.Sputtering targets are often expensive and material use may be poor.Most of the energy incident on the target becomes heat, which must be removed.In reactive sputter deposition, the gas composition must be carefully controlled to prevent poisoning the sputtering target.
13DC Sputtering The simplest sputtering technology E (e-) < 2eV - no ionization, elastic collisions onlyE (e-) > 2eV - inelastic collisions add energy to Arionization (highest energy process, ~15eV)Note: mass (e-)/mass( Ar) ~ 10-5energy transfer smalle- gain energy via elastic collisions until E>15eV for ionization#ions ~ #neutrals ~ 3 x mT
14DC Sputtering (Cont.)Light e- pulled towards walls faster than ions, leaving slightly more ions in glow regionLight e- move away from cathode faster than ions, leading to a large field, high acceleration of ions into cathodehigh-E ions (10keV to 1 MeV) knock target material loose resulting plume of neutralsnew electrons from impact reactions replenish the plasma
15Operating Pressure for DC sputtering Operating pressure limitations are imposed by the requirement of both the glow discharge and of film deposition.Optimum deposition rate around 100 mTorrMilton Ohring, Materis Science of Thin Film, second Edition, P208
16Parameters for DC Sputtering Sputter voltagetypically -2 to -5 kVSubstrate Bias Voltagesubstrate is being bombarded by electrons and ions from target and plasmasputtering film while you depositneutral atoms deposit independentlyput negative bias on the substrate to control thiscan significantly change film propertiesDeposition ratechanges with Ar pressureincreases with sputter yieldusually increases with high voltage
17RF Sputtering DC sputtering - what about dielectrics? in DC systems, positive charge builds up on the cathode (target) need 1012 volts to sputter insulators !!avoid charge build up by alternating potentialRF sputtering
18Target Self-BiasDue to the different masses, electrons can be accelerated to oscillate with the RF, while ions are to heavy and react only on the averages generated in the RF system.It behaves like a DC target where positive ion-bombardment sputters away atoms for subsequent.Milton Ohring, Materis Science of Thin Film, second Edition, P214
19RF Sputtering Typically 13.56 MHz is used frequencies less than about 50 kHzelectrons and ions in plasma are mobileboth follow the switching of the anode and cathodebasically DC sputtering of both surfacesfrequencies above about 50 kHzions (heavy) can no longer follow the switchingenough electrons to ionize gases(5~30MHz)Typically MHz is used
20Electrode-Size Effects Both electrodes in RF sputtering system should sputter.cause a contamination in the sputtered film.The ratio of the voltage across the sheath at the small capacitively coupled electrode(Vc) to that across the large directly coupled electrode(Vd) including substrate and chamber walls, etc is given byThe fourth- power dependence means large Ad is very effective in raising the target sheath potential while minimizing ion bombardment of grounded fixtures.substrate and chamber make a very large electrode - so not much sputtering of substrate
21Advantages of RF Sputtering It works well with insulating targetsHigh efficiencyeasier to keep plasma going → can operate at lower Ar pressures (1-15 mTorr) → fewer gas collisions → more line of sight deposition
22Magnetron Sputter Deposition Use with DC or RFHigh sputtering efficiencyincrease ionization of ArWhy? Higher sputter rates at lower Ar pressures (down to 0.5 mTorr)fewer gas collisions - more line of sightHow ? increase probability of electrons striking Arincrease electron path lengthuse electric and magnetic fields
23Magnetron Sputtering Principle This technology uses powerful magnets to confine the “glow discharge” plasma to the region closest to the target plate. That vastly improves the deposition rate by maintaining a higher density of ions, which makes the electron/gas molecule collision process much more efficient.
24Advantages of Magnetron Sputtering High deposition rateReducing electron bombardment of substrateExtending the operating vacuum rangeability to operate at lower pressuresThe most widely commercially practiced sputtering method
25Parameters for Magnetron Sputtering Deposition pressure : 10-3 to 0.1 Pa ( 10-5 to 10-3 torr)Deposition rate : 0.2 ~ 2-6 m/min (10 times higher than conventional sputtering)Deposition temperature : 100 to 150 oC
26Disadvantages for Magnetron Sputtering An erosion track in the targethis leads to poor efficiency of sputtering yield versus target volume compared to non-magnetron sputteringNonuniform removal of particles from target result in nonuniform films on substrate
27Reactive SputteringSputtering metallic target in the presence of a reactive gas mixed with inert gas (Ar)• A mixture of inert +reactive gases used for sputteringoxides – Al2O3, SiO2, Ta2O5 (O2)nitrides – TaN, TiN, Si3N4 (N2, NH3)carbides – TiC, WC, SiC (CH4, C2H4, C3H8)
28Reactive Sputtering (Cont.) chemical reaction takes place on substrate and targetcan poison target if chemical reactions are faster than sputter rateadjust reactive gas flow to get good stoichiometry without incorporating excess gas into film
29Reactive Magnetron Sputtering Zinc Oxide Thin films Zinc oxide is one of the most interesting II/IV compound semiconductors It has been investigated extensively because of its interesting electrical, optical and piezoelectric propertiesReactive sputtering is the best technique for Zinc Oxide deposition.R. Ondo-Ndong , F. Pascal-Delannoy, A. Boyer, A. Giani, A. Foucaran, Materials Science and Engineering B97 (2003) 68 /73
30Process ParametersThe physical properties of ZnO films are influenced not only by the deposition techniques, but also by process parameters.The film deposited on high temperature substrate will show finer uniform grains and smoother surfaceWalter Water, Sheng-Yuan Chu*, Materials Letters 55 (2002) 67– 72
31Process ParametersQuality of the film dependents on deposition conditions, such as substrate temperature, deposition power, deposition pressure and argon–oxygen flow.Walter Water, Sheng-Yuan Chu*, Materials Letters 55 (2002) 67– 72
32ConclusionSputter deposition, also known as physical vapor deposition, or PVD, is a widely used technique for depositing thin metal layers on semiconductor wafers.The range of applications of sputtering and the variations of the basic process, is extremely wide.
33Questions Answer It works well with insulating targets; High efficiencyThe sputter yield depends on: (a) the energy of the incident ions; (b) the masses of the ions and target atoms; (c) the binding energy of atoms in the solid and (d) the incident angle of ions.