Atomic layer deposition of HfO2 – nucleation, growth and structure development of thin films
Date
2011-07-19
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Abstract
Õhukesed tahkismaterjalide kihid ehk kiled, mille paksus ulatub paarist nanomeetrist (10-9 m) kuni mõnesaja nanomeetrini, leiavad laialdast kasutust elektroonikas, optikas ja mikromehaanika seadistes, aga samuti näiteks pindaktiivsetes ja pindu kaitsvates katetes. Selliseid kilematerjale või nendel põhinevaid mitmekihilisi struktuure saab valmistada erinevate meetoditega, kuid üheks perspektiivikamaks on peetud aatomkihtsadestamist. See meetod võimaldab saada ühtlase paksusega kilesid väga erineva pinnaprofiiliga alusmaterjalidel, mistõttu selle meetodi võimalike rakenduste valdkond on lai. Sõltuvalt sadetusprotsessi parameetritest on aatomkihtsadestamise abil võimalik valmistada erinevate omadustega (nt. parema või halvema elektrijuhtivusega, suurema või väiksema murdumisnäitajaga) kilematerjale. Konkreetseid rakendusi silmas pidades on materjali sobivate omaduste tagamine aga äärmiselt oluline.
Käesolevas töös uuriti hafniumdioksiidi (HfO2) aatomkihtsadestamist ning selles protsessis saadud kilede omadusi sõltuvuses sadestusprotsessi parameetritest. Kasutati mitmeid lähteainete kombinatsioone (HfCl4-H2O, HfI4-H2O ja HfI4-O2) ning laia sadestustemperatuuride (180–750 oC) ja kilepaksuste (0–320 nm) vahemikku, et uurida HfO2 kasvukiiruse sõltuvust kile alusmaterjalist, paksusest, kristallstruktuurist ja pinnakaredusest. Näidati, et lisaks aluse temperatuurile mõjutas kilede kasvu ja kristallstruktuuri kandegaasi voog ja rõhk kasvukambris. Tehti kindlaks kilede optiliste omaduste sõltuvus kilede kristallilisusest, tihedusest ja topograafiast. Selgus, et mida tugevam on kristalliitide eelisorientatsioon kiledes, seda suurem on murdumisnäitaja. Kilede pinnakaredus sõltub nende struktuurist ja avaldab tugevat mõju optilistele kadudele. Kõrgematel sadestustemperatuuridel on kilede kasv ränialustele HfCl4-põhise protsessi algfaasis äärmiselt ebaühtlane. Õhukeste kilede karedus on sellest tingituna suur, mistõttu tuleks nendel temperatuuridel eelistada HfI4 põhinevat aatomkihtsadestamist või kasutada kaheastmelisi protsesse, milles madalal temperatuuril sadestatud siirdekiht tagab ühtlase kasvu ka kõrgetel temperatuuridel. Madalamatel sadestustemperatuuridel on mõlema metallilähteaine puhul võimalik saada siledama pinnaga kilesid, kuid kilede amorfsust suurendavate ja optilisi omadusi kahjustavate jääklisandite eemaldamiseks tuleb nendel temperatuuridel kasutada suhteliselt suuri hapniku lähteaine doose.
Various thin layers of solid state materials, often referred as thin films, are widely used in electronics, optics, micro-mechanics as well as catalytic and protective coatings. Depending on certain applications the thicknesses of the films may vary from few nanometers (10-9 m) to couple of hundreds nanometers. Single- or multilayer thin film structures can be prepared by various techniques. However, using atomic layer deposition (ALD), uniform films can be deposited with a precision at sub-nanometer level also on complex three-dimensionally profiled surfaces. Therefore this method has emerged as a potential solution for fabrication of device with nanometer-level sizes. When employing this technique one can control film properties (electrical conductivity, refractive index etc.) by choosing proper deposition parameters. As certain applications require defined material properties that are determined already in the fabrication process, exact understanding of the deposition process parameters on the material quality is crucial. In this thesis, ALD of technologically highly important hafnium dioxide (HfO2) and influence of ALD process parameters on the properties of HfO2 thin film is thoroughly investigated. Several precursor combinations (HfCl4-H2O, HfI4-H2O ja HfI4-O2) and wide ranges of growth temperatures (180–750 oC) and film thicknesses (0–320 nm) were used to study the dependence of HfO2 growth rate on the substrate material as well as crystal structure and surface roughness of growing film. It was shown that besides other process parameters, the carrier gas flow rate and the pressure in the growth zone influenced the growth rate. Optical properties depended on crystallinity, density and topography of the films, whereas higher refractive index was obtained for films showing preferential orientation of crystallites in a certain direction. Optical transmission of the films was influenced by the surface roughness and material inhomogeneity that were, in turn, related to the crystal structure of the films. At elevated growth temperatures, HfCl4-based processes yielded highly non-uniform HfO2 films with rough surfaces. Therefore, HfI4-based processes are more advantageous at these temperatures. Another solution is to use two-temperature growth process, where uniform seed layer deposited at lower temperature enhances uniformity of deposition at higher temperature. Generally, at lower growth temperatures smoother and more uniform films can be obtained but the purity and density of these films are low. In two-temperature processes, by contrast, relatively high uniformity together with high overall purity, density and refractive index of the films can be obtained.
Various thin layers of solid state materials, often referred as thin films, are widely used in electronics, optics, micro-mechanics as well as catalytic and protective coatings. Depending on certain applications the thicknesses of the films may vary from few nanometers (10-9 m) to couple of hundreds nanometers. Single- or multilayer thin film structures can be prepared by various techniques. However, using atomic layer deposition (ALD), uniform films can be deposited with a precision at sub-nanometer level also on complex three-dimensionally profiled surfaces. Therefore this method has emerged as a potential solution for fabrication of device with nanometer-level sizes. When employing this technique one can control film properties (electrical conductivity, refractive index etc.) by choosing proper deposition parameters. As certain applications require defined material properties that are determined already in the fabrication process, exact understanding of the deposition process parameters on the material quality is crucial. In this thesis, ALD of technologically highly important hafnium dioxide (HfO2) and influence of ALD process parameters on the properties of HfO2 thin film is thoroughly investigated. Several precursor combinations (HfCl4-H2O, HfI4-H2O ja HfI4-O2) and wide ranges of growth temperatures (180–750 oC) and film thicknesses (0–320 nm) were used to study the dependence of HfO2 growth rate on the substrate material as well as crystal structure and surface roughness of growing film. It was shown that besides other process parameters, the carrier gas flow rate and the pressure in the growth zone influenced the growth rate. Optical properties depended on crystallinity, density and topography of the films, whereas higher refractive index was obtained for films showing preferential orientation of crystallites in a certain direction. Optical transmission of the films was influenced by the surface roughness and material inhomogeneity that were, in turn, related to the crystal structure of the films. At elevated growth temperatures, HfCl4-based processes yielded highly non-uniform HfO2 films with rough surfaces. Therefore, HfI4-based processes are more advantageous at these temperatures. Another solution is to use two-temperature growth process, where uniform seed layer deposited at lower temperature enhances uniformity of deposition at higher temperature. Generally, at lower growth temperatures smoother and more uniform films can be obtained but the purity and density of these films are low. In two-temperature processes, by contrast, relatively high uniformity together with high overall purity, density and refractive index of the films can be obtained.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
hafniumiühendid, oksiidid, aatomkihtsadestamine, õhukesed kiled, füüsikalised omadused, hafnium compounds, oxides, atomic layer deposition, thin films, physical properties