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Model-based Analysis and Design of Atomic Layer Deposition Processes

Holmqvist, Anders LU (2013)
Abstract (Swedish)
Popular Abstract in Swedish

Tunna skikt av olika material har en framträdande roll inom både vetenskap och teknik idag. Det bakomliggande intresset för denna typ av ytbeläggningar är främst möjligheten att kombinera egenskaper hos de tunna filmerna med övriga egenskaper från det underliggande materialet för att därigenom erhålla förbättrade kemiska, mekaniska eller elektriska egenskaper. Därutöver kan helt nya fenomen uppstå när tjockleken skalas ner, vilket tillämpas inom elektronikindustrin. Idag finns ytbeläggningar av olika slag i nästan all materialteknologi och exempel på tillämpningsområden är: solceller, mikroelektronik, batterier, bränsleceller, sensorer, optik, datalagring, telekommunikation, och många fler. I... (More)
Popular Abstract in Swedish

Tunna skikt av olika material har en framträdande roll inom både vetenskap och teknik idag. Det bakomliggande intresset för denna typ av ytbeläggningar är främst möjligheten att kombinera egenskaper hos de tunna filmerna med övriga egenskaper från det underliggande materialet för att därigenom erhålla förbättrade kemiska, mekaniska eller elektriska egenskaper. Därutöver kan helt nya fenomen uppstå när tjockleken skalas ner, vilket tillämpas inom elektronikindustrin. Idag finns ytbeläggningar av olika slag i nästan all materialteknologi och exempel på tillämpningsområden är: solceller, mikroelektronik, batterier, bränsleceller, sensorer, optik, datalagring, telekommunikation, och många fler. I takt med att skiktmaterialen blir tunnare och tunnare kommer de kemiska framställningsmetoderna i förgrunden. Denna utveckling har medfört att en mängd olika metoder har framtagits för att på ett effektivt sätt framställa de tunna skikten. Dessa metoder har också fördelen att jämna skikt med samma egenskaper kan framställas på ytor med mer komplicerad geometrisk form, däribland på insidan av tunna kapillärer, som endast är ett tiotals nanometer i diameter.





Av särskilt intresse är de metoder som utnyttjar gaser som utgångsmaterial och som vid en kemisk reaktion ger fasta material. Dessa metoder innebär sålunda att en reaktionsgas leds in i en reaktor och på eller i närheten av den yta som ska ytbeläggas sker den kemiska reaktionen som slutligen resulterar i den fasta beläggningen. Utnyttjande av kemin för att framställa ytbeläggningar ger möjligheter att på ett ytterst finstämt sätt styra deponeringsprocessen med skikttjocklekar ner till enskilda atomlager. Faktum kvarstår dock att den fundamentala bakomliggande transporten av reaktionsgas till den yta som skall ges en ytbeläggning är ännu inte fullständigt kartlagd för dessa processer. Utvecklingen inom detta område hämmas också av bristande kunskap kring de kemiska reaktioner som sker på ytan och hur detta kopplar till den struktur/egenskaper materialen får.



Denna avhandling beskriver teoretiska och experimentella studier av en beläggningsteknik för tunna filmer kallad ``Atomic Layer Deposition'' (ALD), vilket kan översättas med atomlagerdeponering. Själva beläggningstekniken ALD baseras på att högreaktiva tillväxtspecier tillförs sekventiellt med täta intervall till reaktionskammaren under vakuumförhållanden. Genom att åtskilja tillförseln av tillväxtspecier undviks oönskade gasfasreaktioner och under dessa förhållanden adsorberas bara ett monolager av reaktionsgasen. När ett monolager finns på ytan fortskrider en kemisk reaktion mellan detta monolager och den reaktionsgas som tillförs i nästkommande puls. På detta sätt är det möjligt att bygga upp ett material monolager för monolager genom att periodiskt alternera och upprepa tillförseln av reaktionsgas.



Det primära syftet med avhandlingen har varit att studera och karakterisera den komplexa interaktionen mellan gasdynamiken som uppstår under tillförseln av tillväxtspecier och deras reaktionsvägar på tillväxtytan. Dessa fenomen har beskrivits matematiskt för att fysikaliskt modellera ALD reaktorn. För att förbättra modellens predektiva förmåga har de reaktionskinetiska parametrarna, som bestämmer tillväxthastigheten, skattats genom att anpassa modellen till en serie experiment. Den experimentellt verifierade modellen användes därefter för att simulera hur förhållandena i reaktorn påverkar filmtillväxten. Ett viktigt resultat av detta är möjligheten att optimera pulskaraktäristiken och styrsignalerna till deponeringsprocessen för att maximera tillväxthastigheten, minimera materialåtgången samt säkerställa att kvalitetskraven för materialegenskaperna uppfylls. Härigenom står det klart att matematisk modellering i kombination med numeriska optimeringsalgoritmer utgör ett kraftfullt verktyg för analys och desig av ALD processer.



Den utvecklade modellbaserade metodiken som presenteras i denna avhandling är av stort intresse för materialindustrin där resurs- och produktionseffektiviteten till stor del driver teknikutvecklingen. Energibesparingar, materialhushållning samt behovet av mer effektiva och flexibla materialframställningsprocesser för både befintliga och kommande användningsområden är exempel på drivkrafter. I perspektivet processutveckling står simuleringsteknik för ett betydelsefullt hjälpmedel vid övergången till större produktionsenheter och volymer samt för att studera effekter av ändrade produktionsförutsättningar. Härigenom är det övergripande målet med denna avhandling att simuleringsramverk för beräkning, styrning och kontroll av materialegenskaper och framställningsprocesser blir tillgängligt för alla tillverkare. (Less)
Abstract
Atomic layer deposition (ALD) is a thin-film manufacturing process in which the growth surface is exposed to non-overlapping alternating injections of gas-phase chemical precursor species separated by intermediate purge periods to prevent gas-phase reactions. ALD is characterized by sequential self-terminating heterogeneous reactions between highly reactive gas-phase precursor species and surface-bound species which, when allowed sufficient conditions to reach saturation, results in highly conformal films, on both planar and topographically complex structures. ALD has already emerged as the prime candidate for depositing ultra-thin layers with high conformality in semiconductor manufacturing. With recent advances in current technologies,... (More)
Atomic layer deposition (ALD) is a thin-film manufacturing process in which the growth surface is exposed to non-overlapping alternating injections of gas-phase chemical precursor species separated by intermediate purge periods to prevent gas-phase reactions. ALD is characterized by sequential self-terminating heterogeneous reactions between highly reactive gas-phase precursor species and surface-bound species which, when allowed sufficient conditions to reach saturation, results in highly conformal films, on both planar and topographically complex structures. ALD has already emerged as the prime candidate for depositing ultra-thin layers with high conformality in semiconductor manufacturing. With recent advances in current technologies, novel applications of ALD are expanding beyond semiconductor processing in several emerging areas, such as surface passivation layers in crystalline silicon solar cells, buffer layers in CuIn_{1-x}Ga_{x}Se_{2} (CIGS) solar cells, and diffusion barrier layers in organic light-emitting diodes and thin-film photovoltaics. This trend brings with it a growing necessity for high-throughput and low-cost production techniques.





This thesis describes the modeling of the ALD process with temporally separated precursor pulsing, which is a special modification of the chemical vapor deposition (CVD) technique, with which it shares a number of phenomenological characteristics. In particular, both deposition processes are inherently nonlinear and time-dependent, and mathematical model components describing the precursor thermophysical properties, the underlying reactor-scale mass transport of the gas-phase precursor and the deposition surface reaction are strongly coupled. Thus, the integration of physical and chemical phenomena over multiple time and length scales is a fundamental requirement for the understanding and modeling of the complete reactor system. However, what distinguishes ALD from CVD is that the steady-state deposition rate in CVD does not exist in ALD. The deposition rate of ALD depends strongly on the dynamic composition of the growth surface and the local precursor partial pressure in the vicinity of the active surface, which both change continuously through each exposure and purge period. The completely dynamic nature of the ALD process adds considerably to the difficulty of developing simulators, as the entire process cycle must be modeled due to the complex interdependence between the sequential ALD half-reactions, such that the reactivity in one half-cycle is influenced by that in the half-cycle preceding it. One of the essential advantages of ALD, on the other hand is that its self-terminating nature enables uniform coating of large-surface-area substrates, thus providing an easier pathway for process scale-up compared to CVD.



In the work presented in this thesis, a physically-based dynamic model of the ALD process was developed, based on a laboratory-scale, continuous cross-flow ALD reactor system (F-120 manufactured by ASM Microchemistry Ltd.) equipped with a quartz crystal microbalance for in situ deposition measurements. The mathematical model of the low-volume, continuous cross-flow ALD reactor with temporally separated precursor pulsing comprises components that describe reactor-scale gas-phase dynamics and surface state dynamics to accurately characterize the continuous, cyclic ALD reactor operation that is described by limit-cycle dynamic solutions. The model is coupled to a heterogeneous surface reaction model based on estimated kinetic parameters from ex situ and in situ deposition measurements. The heterogeneous gas--surface reactions mean that there will be a net mass consumption at the growth surface, and the total gas-phase mass flux of species at the growth surface is balanced by the net consumption rate per unit area. Likewise, the accumulated mass resulting from the epitaxial film growth, governed by the adsorption/chemisorption and surface reaction of precursor species, was conveniently expressed in terms of the spatial and temporal evolution of the fractional concentrations of surface states for each half-reaction. In this way, the film growth per cycle (GPC) and its relative uniformity were unambiguously defined.



The work described in this thesis was motivated by the predictive capabilities of physically based ALD process models, as such models can be used in the design of novel reactors, the optimization of deposition conditions, and in the scale-up of laboratory thin-film process. The process model, oriented towards optimization and control, was validated experimentally, to ensure that it could adequately predict the spatially dependent film thickness profile and provide statistically reliable least-square estimates of the parameters involved in the heterogeneous gas--surface reaction mechanism that governs the thin film growth of ZnO from Zn(C_{2}H_{5})_{2} and H_{2}O precursors. However, the general formalism of the model allows it to be used to simulate other ALD process chemistries and more complex (e.g., multi-wafer) reactor systems, thereby providing a framework for model-based process design and dynamic optimization studies, as well as controller development.



The primary contribution of this work is the solution strategy developed for the dynamic ALD process model, to consider the limit-cycle solutions that describe steady (but periodic) operation of the reactor system, in conjunction with numerical solvers for limit-cycle-constraint, multi-objective optimization, dynamic optimization, and dynamic parameter estimation problems. The utility of the model-based framework developed was demonstrated by a study of constrained multi-objective optimization of the incommensurable process objectives of limit-cycle deposition rate and overall precursor conversion, subject to a set of operational constraints on the uniformity of cross-substrate film thickness and the duration of the post-precursor purge. Additionally, limit-cycle dynamic optimization targeting precursor utilization was also demonstrated in a scale up-analysis of the laboratory-scale reactor, while assuring that identical deposition profiles were obtained in the scaled-up system. In these studies, the optimal solutions obtained revealed the mechanistic dependence of the process operating parameters on the proposed optimization and constraint specifications, and reduced the design space of the ALD process to a feasible set of design alternatives. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Prof. Adomaitis, Raymond A., Department of Chemical and Biomolecular Engineering, Institute for Systems Research, University of Maryland, College Park, MD 20742, USA
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Atomic layer deposition, Heterogeneous reaction kinetics, Limit-cycle dynamics, Parameter estimation, Multi-objective optimization, Dynamic optimization, Scale-up analysis
pages
321 pages
publisher
Lund University (Media-Tryck)
defense location
Lecture hall K:B at the Center for Chemistry and Chemical Engineering, Getingevägen 60, Lund University Faculty of Engineering
defense date
2013-12-06 13:15
ISBN
978-91-7473-741-7
978-91-7473-740-0
language
English
LU publication?
yes
id
15e0874d-dee5-4871-9b25-b7192e0ffc5e (old id 4144913)
date added to LUP
2013-11-13 10:34:40
date last changed
2016-09-19 08:45:05
@phdthesis{15e0874d-dee5-4871-9b25-b7192e0ffc5e,
  abstract     = {Atomic layer deposition (ALD) is a thin-film manufacturing process in which the growth surface is exposed to non-overlapping alternating injections of gas-phase chemical precursor species separated by intermediate purge periods to prevent gas-phase reactions. ALD is characterized by sequential self-terminating heterogeneous reactions between highly reactive gas-phase precursor species and surface-bound species which, when allowed sufficient conditions to reach saturation, results in highly conformal films, on both planar and topographically complex structures. ALD has already emerged as the prime candidate for depositing ultra-thin layers with high conformality in semiconductor manufacturing. With recent advances in current technologies, novel applications of ALD are expanding beyond semiconductor processing in several emerging areas, such as surface passivation layers in crystalline silicon solar cells, buffer layers in CuIn_{1-x}Ga_{x}Se_{2} (CIGS) solar cells, and diffusion barrier layers in organic light-emitting diodes and thin-film photovoltaics. This trend brings with it a growing necessity for high-throughput and low-cost production techniques.<br/><br>
<br/><br>
<br/><br>
This thesis describes the modeling of the ALD process with temporally separated precursor pulsing, which is a special modification of the chemical vapor deposition (CVD) technique, with which it shares a number of phenomenological characteristics. In particular, both deposition processes are inherently nonlinear and time-dependent, and mathematical model components describing the precursor thermophysical properties, the underlying reactor-scale mass transport of the gas-phase precursor and the deposition surface reaction are strongly coupled. Thus, the integration of physical and chemical phenomena over multiple time and length scales is a fundamental requirement for the understanding and modeling of the complete reactor system. However, what distinguishes ALD from CVD is that the steady-state deposition rate in CVD does not exist in ALD. The deposition rate of ALD depends strongly on the dynamic composition of the growth surface and the local precursor partial pressure in the vicinity of the active surface, which both change continuously through each exposure and purge period. The completely dynamic nature of the ALD process adds considerably to the difficulty of developing simulators, as the entire process cycle must be modeled due to the complex interdependence between the sequential ALD half-reactions, such that the reactivity in one half-cycle is influenced by that in the half-cycle preceding it. One of the essential advantages of ALD, on the other hand is that its self-terminating nature enables uniform coating of large-surface-area substrates, thus providing an easier pathway for process scale-up compared to CVD.<br/><br>
<br/><br>
In the work presented in this thesis, a physically-based dynamic model of the ALD process was developed, based on a laboratory-scale, continuous cross-flow ALD reactor system (F-120 manufactured by ASM Microchemistry Ltd.) equipped with a quartz crystal microbalance for in situ deposition measurements. The mathematical model of the low-volume, continuous cross-flow ALD reactor with temporally separated precursor pulsing comprises components that describe reactor-scale gas-phase dynamics and surface state dynamics to accurately characterize the continuous, cyclic ALD reactor operation that is described by limit-cycle dynamic solutions. The model is coupled to a heterogeneous surface reaction model based on estimated kinetic parameters from ex situ and in situ deposition measurements. The heterogeneous gas--surface reactions mean that there will be a net mass consumption at the growth surface, and the total gas-phase mass flux of species at the growth surface is balanced by the net consumption rate per unit area. Likewise, the accumulated mass resulting from the epitaxial film growth, governed by the adsorption/chemisorption and surface reaction of precursor species, was conveniently expressed in terms of the spatial and temporal evolution of the fractional concentrations of surface states for each half-reaction. In this way, the film growth per cycle (GPC) and its relative uniformity were unambiguously defined.<br/><br>
<br/><br>
The work described in this thesis was motivated by the predictive capabilities of physically based ALD process models, as such models can be used in the design of novel reactors, the optimization of deposition conditions, and in the scale-up of laboratory thin-film process. The process model, oriented towards optimization and control, was validated experimentally, to ensure that it could adequately predict the spatially dependent film thickness profile and provide statistically reliable least-square estimates of the parameters involved in the heterogeneous gas--surface reaction mechanism that governs the thin film growth of ZnO from Zn(C_{2}H_{5})_{2} and H_{2}O precursors. However, the general formalism of the model allows it to be used to simulate other ALD process chemistries and more complex (e.g., multi-wafer) reactor systems, thereby providing a framework for model-based process design and dynamic optimization studies, as well as controller development.<br/><br>
<br/><br>
The primary contribution of this work is the solution strategy developed for the dynamic ALD process model, to consider the limit-cycle solutions that describe steady (but periodic) operation of the reactor system, in conjunction with numerical solvers for limit-cycle-constraint, multi-objective optimization, dynamic optimization, and dynamic parameter estimation problems. The utility of the model-based framework developed was demonstrated by a study of constrained multi-objective optimization of the incommensurable process objectives of limit-cycle deposition rate and overall precursor conversion, subject to a set of operational constraints on the uniformity of cross-substrate film thickness and the duration of the post-precursor purge. Additionally, limit-cycle dynamic optimization targeting precursor utilization was also demonstrated in a scale up-analysis of the laboratory-scale reactor, while assuring that identical deposition profiles were obtained in the scaled-up system. In these studies, the optimal solutions obtained revealed the mechanistic dependence of the process operating parameters on the proposed optimization and constraint specifications, and reduced the design space of the ALD process to a feasible set of design alternatives.},
  author       = {Holmqvist, Anders},
  isbn         = {978-91-7473-741-7},
  keyword      = {Atomic layer deposition,Heterogeneous reaction kinetics,Limit-cycle dynamics,Parameter estimation,Multi-objective optimization,Dynamic optimization,Scale-up analysis},
  language     = {eng},
  pages        = {321},
  publisher    = {Lund University (Media-Tryck)},
  school       = {Lund University},
  title        = {Model-based Analysis and Design of Atomic Layer Deposition Processes},
  year         = {2013},
}