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Pulsed laser deposition (PLD), also sometimes referred aslaser evaporation, laser assisted deposition, laser ablationdeposition, and lasermolecular beamepitaxy (laser-MBE),

is an inexpensive, flexible, and user-friendly thin filmgrowth tech-nique [1]. Introduced in the 60-th [2], it attracted attention in the80-th due to the raised interest in the growth of high-temperaturesuperconducting oxides, such as YBa2Cu3O7 (YBCO). Here, itwas essential to achieve stoichiometric transfer at high oxygenpressure in order to synthesize high quality films. Even the first at-tempts of thin film fabrication of this complex oxide with PLDwere successful [3] making this technique extremely popular.Since then PLDwas used to growoxides [4], nitrides [5], chalco-genide glasses [6], andmetals [7]. Even nanocrystalline diamondand SiC films were deposited using PLD [8,9]. PLD alsowitnessed significant technical innovations and improvementsresulting in the development of advanced combinatorial PLD [10]and laser MBE [11] systems.

PLD, unlike many other deposition techniques, is based on thevery intuitive principles.A high-power pulsed laser is focused onthe target positioned in the vacuum chamber, as shown inFigure 1. The target is ablated after each pulse forming a plume

Figure 1. The schematics of the PLD system. PLD chamber is equippedwith the substrate heater and a carousel housing a number of targets.Laser beam is rastered over the target surface for better film uniformity.

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of atoms,molecules, and particulates directed towards the heatedsubstrate located a few centimeters away of it. The species con-dense at the substrate surface resulting in the film growth. Thistechnique requires only the use of a pulsed (Nd:YAG, ruby, or ex-cimer) laser and a basic 8” – 16” spherical or box vacuum cham-ber evacuated by the diffusion or turbomolecular pump. Ifepitaxial growth of multilayered structures is desired, chamber isequippedwith the rotatablemulti-target carousel (with three 2” orsix 1” targets) and a substrate heater. Since such system can bebuilt on a limited budget, PLD became extremely popular for ac-ademic research. In addition, a number of companies (AMBPTech, DCA Instruments, PVD Products Inc., Pascal TechnologiesInc., and SVTAssociates) recently entered themarket historicallydominated by a single pioneer company (Neocera Inc.) drivingthe prices of the turn-key systems, similar to shown in Figure 2,significantly down.PLD has a number of advantages when compared with other

deposition techniques. Similar to sputtering and e-beam evapo-ration, growth is performed in the vacuum environment mini-mizing unintentional impurity incorporation. Different compo-sitions of the same alloy as well as different materials can be de-posited in the same chamberwith the only necessity to switch thetargets. If there is a problem of cross-contamination, laser (themost expansive part of the system) can be shared between a fewdeposition chambers, as shown in Figure 3, just slightly increas-ing the total systemcost. Relatively high deposition rates, typically∼100sÅ/min, can be achievedwith the film thickness being con-trolled in real time by simply turning the laser on and off. There-fore, bothmulti-micrometer thick and a few nanometer thin filmscan be deposited. In addition, if the chamber is equipped with acarousel housing a number of targets,multilayered structures canbe grown without the need to break vacuum when changing be-tween materials.Nearly stoichiometric material transfer, due to the congruent

evaporation of the elements and compounds irrespective of theirevaporating points, is another important advantage of PLD.Thus,

film composition mimics composition of the target and complexalloys, such as superconducting (YBa2Cu3O7), ferroelectric(Ba0.6Sr0.4TiO3), and multiferroic (By2FeCrO6) oxides, can begrown without the need of individual atomic flux calibration. Inaddition, use of external energy source (laser) allows film growthboth in the high-vacuum conditions and in the mTorr pressurerange. If the proper pumping and exhaust system is available,corrosive gases (ozone), and reactive atmospheres (H2, H2S, andCH4) can be used.Clearly, PLD is one of the most flexible thin film growth

techniques.In spite of the sound simplicity, laser – target interaction and

material transfer are rather complicated processes. They can beseparated in a few steps, as shown in Figure 4. First, laser beamis focused onto the surface of the target. Due to the high power(∼ 1 J per pulse) and short duration (∼ 30 ns) of the pulse, surfaceof the target is rapidly heated above evaporation temperature.

Figure 2. The photograph of the PLD system built by PVD ProductsInc.

Figure 3.The schematics of the PLD systemwith the laser beam sharedbetween four deposition chambers.

Figure 4. The schematic diagram of the absorption and the ejectionprocesses during the PLD. A) Photo-absorption followed by rapidheating of the surface layer; B) Surface melting and vaporization; C)Formation and expansion of atomic plume;D) Plume expansion at lowpressures; E) Plume expansion at high pressures.

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highly energetic ions (100 eV-500 eV) and low energetic atoms(10–50 eV).The high-energy fraction expandsmuch faster reach-ing the substrate first.When the high-energy ions hit the substrate,they transfer kinetic energy activating diffusion of the surfaceatoms, implanting atoms into the substrate, and re-sputteringsubstrate atoms, as shown in Figure 5a. Next, re-sputteredsubstrate atoms collide with the stream of slower-expandingtarget atomswith lower kinetic energy.A high-temperature colli-sion region characterized by high plasma and particle densityforms above the substrate, as shown in Figure 5b. Processes ofthermalization, condensation, and cluster formation start in thisregion diminishing plasma density and finally dissolving the layer.Finally, target atoms with the lowest kinetic energy (∼ 10 eV)reach the substrate without interaction with the re-sputteredsubstrate atoms, as shown in Figure 5c.Therefore, film nucleation occurs under the heavy supersatu-

ration and the growth conditions are fare from the thermodynamicequilibrium. The process can be separated into three steps:

a)High-energy ion implantation, surface diffusion activation,and re-sputtering;

b)Cluster condensation from the collision zone;c)Adsorption of the low-energy ablated species.

These steps are repeated after each laser pulse.While the highdegree of supersaturation favors two-dimensional (2D) nucleationof highly dense and small clusters, high kinetic energy of arrivingspecies activates surfacemigration promoting layer-by-layer filmgrowth. Thus, films obtained by PLD are usually dense, exhibithigh degree of texture, and demonstrate good adhesion proper-ties. These properties are desirable both for the electronicmateri-als and optical or tribological coatings.In spite of these advantages, a few shortcomings stalled broad

industrial application of PLD. First, sub-surface boiling, expul-sion of the liquid layer by shock wave pressure recoil, andexfoliation lead to the target splashing and particulate deposition.These particulates vary in size frombelowhundred nanometers toa fewmicrometers. They greatly affect growth of the subsequentlayers, degrade electrical properties of the films, and produce filmswith the rough surface. Second, plasma plume has a very narrowangular distribution, which can be fitted by a cosnϕ curve (4 < n< 10 depending on the deposition pressure), resulting in thenon-uniformwafer coverage.Although thiswas not important foracademic researchwhere 5×5mm2 substrateswere routinely used,it made problematic industrial application where 4” wafer had tobe handled.However, these problems were mostly solved in the last years

by the joint efforts of a number of research groups.It was shown that particulates, which have much lower veloc-

ities than the atomic and ionic species, can be removed fromthe plume using amechanical velocity filter [12].More elaboratetechniques involving collisions between two plasma plumes(cross-beam PLD) [13], magnetic field filtering [14], andpositioning the substrate edge-on inside the plume (off-axisdeposition) instead of being directly perpendicular to it (on-axisdeposition) [15] were also developed.A few schemes were implicated to improve film uniformity.

Evaporating material forms a dense vapor layer above it. Laserbeam interacts with the evaporated material dissociating molec-ular species desorbed from the target. Photoionization (via thenon-resonant multiphoton processes) causes plasma formation.Once formed, plasma attenuates laser beamby inelastic free elec-tron scattering preventing further interaction between the laserbeam and the target.Due to the pressure differential, plasma expands from the tar-

get surface forming the ''plasma plume". Internal thermal and ion-ization energies are converted into the kinetic energy reaching afewhundred electron volts (eV).The kinetic energy of the speciesand the spatial distribution of the plume strongly depend on thechamber pressure. In general:

a) The plume is very narrow and forward directed at low pres-sures (10-5 - 10-4 Torr). The kinetic energy of the species ispreserved since almost no scatteringwith the background gasoccurs.

b) Splitting of the high energy ions from the less energeticspecies increases in the intermediate pressure range (1 - 10mTorr). The kinetic energy of the high energy ions is partiallyattenuated by themultiple collisionswith the background gas.

c)Diffusion-like expansion of the ablated material occurs athigher pressures (> 0.1 Torr). High energy ions loose energythrough the multiple scattering processes.

As was previously mentioned, plasma plume consists of both

Figure 5. The schematic diagram of the plasma plume-substrateinteraction

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Rastering of the laser beam over the rotating target, which wasperformed using a programmable kinematic mount installed onthe last mirror of the optical train, allowed deposition of the filmswith the uniform coverage over the large wafer areas [16]. Otherapproaches included tilting of the rotating target during the laserablation, off-axis deposition, and positioning of the target withthe center of the plasma plume being slightly offset from the cen-ter of the rotating substrate. In addition, a multi-beam PLD tech-nique produced films with a very low thickness deviation (∼ 5%)over the 4”wafers. In this case, three laser beamswere focused onthe same target and the improved uniformitywas achieved by theoptimized superposition of the generated plasma plumes, asshown in Figure 6 [17, 18].Multilayered coatings were deposited on the 4” wafers using

multi-beam PLD combined with the standard MBE into a novelhybrid beamdeposition [19,20].The growth systemwas equippedbothwith the effusion cells andmulti-target carousel, quarts crys-tal monitor for flux calibration, ellipsometer and Reflective HighEnergy Electron Diffraction (RHEED) for in-situmonitoring, asshown inFigure 7.Highly p-type dopedZnO,which is extremelydifficult to produce with other techniques, was also grown usingthis technique [21].While the ceramic target was used as a ZnOsource, effusion cell was used to deliverArsenic (p-type dopant)and RF-plasma source was used to efficiently increase the fluxdensity of available reactive oxygen.Hybrid beamdeposition, al-though lacking the simplicity of standard PLD, provides a real al-ternative to the conventional vacuum deposition techniques.Clearly, PLD has very exciting prospects and offers number of

advantages over traditional vacuum deposition techniques likechemical vapor deposition, sputtering and e-beam evaporation.Since the problems of particulate deposition and non-uniformwafer coverage are mostly solved, broader industrial applicationof PLD is expected

References

1. “Pulsed LaserDeposition of Thin Films” editedD. B. Chrisey andG.K. Hubler,Wiley-Interscience (1994).

2. H. M. Smith andA. F. Turner, Appl. 0pt., 4 147 (1965).3. D. Dijkkamp, T.Venkatesan, X. D.Wu, S.A. Shaheen, N. Jasrawi,Y.H. Min-Lee,W. L. McLean and M. Croft, Appl. Phys. Lett., 51, 619(1987).

4. T. Venkatesan, K. S. Harshavardhan, M. Strikovski, J. Kim in “ThinFilms and Heterostructures for Oxide Electronics” edited by S. B.Ogale, Springer, (2005).

5. G. Leggieri,A. P. Caricato, M. Fernandez, M.Martino, P. Mengucci,G. Barucca, Recent Research Developments in Applied Physics, 5,339 (2002).

6. M. Frumar, B. Frumarova, P. Nemec, T. Wagner, J. Jedelsky, M.Hrdlicka, J. Non-Crystalline Solids, 352, 544 (2006).

7. A. J. Francis and P.A. Salvador, J. Mat. Res., 22, 89 (2007).8. A. Keffous, K. Bourenane, M. Kechouane, N. Gabouze, T. Kerdja,Vacuum, 81, 632 (2007).

9. T. Hara, T. Yoshitake, T. Fukugawa, L. Y. Zhu, M. Itakura, N.Kuwano,Y. Tomokiyo, K. Nagayama, Diamond and Related Mate-rials, 13, 679 (2004).

10. M. Lippmaa, T. Koida, H. Minami, Z. W. Jin, M. Kawasaki, H.Koinuma,Appl. Surf. Sci., 189, 205 (2002).

11. H. Koinuma, M. Kawasaki, M. Yoshimoto, Mat. Res. Soc. Symp.Proc., 474, 303 (1997).

12. T.Yoshitake, G. Shiraishia, K. Nagayama,Appl. Surf. Sci., 197, 379(2002).

13. A. Tselev, A. Gorbunov, W. Pompe, Rev. Sci. Instrum., 72, 2665(2001).

14. R. Jordan, D. Cole, J. G. Lunney, Appl. Surf. Sci., 109, 403 (1997).15. Z. Trajanovic, L. Senapati, R.P. Sharma, T.Venkatesan, Appl. Phys.

Lett., 66, 2418 (1995).16. S. Boughaba,M. Islam, J. P.McCaffrey, G. I. Sproule,M. J. Graham,

Thin Solid Films, 371, 119 (2000).17. J.M. Lackner,W.Waldhauser, R. Ebner, B.Major, Surf. Coat. Tech-

nol., 188, 519 (2005).18. J. M. Lackner, Thin Solid Films, 494, 302 (2006)19. M. Panzner, R. Dietsch, Th. Holz, H. Mai, S. Vdlmar, Appl. Surf.

Sci., 96, 643 (1996).20. M. Panzner, R. Dietsch, Th. Holz, H. Mai, S. Vdlmar, B. Wehner,

Appl. Surf. Sci., 127, 451 (1997).21. Y. R. Ryu, T. S. Lee, H. W.White, J. Crystal Growth, 261, 4, 502(2004).

Figure 6. Principle of the multi-beam pulsed laser deposition approach(insert) and deviation of the deposition rate of TiCxN1−x over the de-posited area [17].

Figure 7. The schematics of the multi-beam PLD integrated with thecommercial MBE system [20].

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