Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 2
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.5.4 Autodoping3.5.5 Pattern Shift3.5.6 Low-Temperature Epi SiliconReferences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .666666666872747677777780818283. 84858889. 904. THERMAL CVD OF METALLIC CONDUCTORS4.1 Introduction4.2 Refractory Metal Silicides4.2.1 Tungsten Sil icide4.2.2 Molybdenum Silicide4.2.3 Tantalum Silicide4.2.4 Titanium Silicide4.3 Tungsten4.3.1 Blanket Tungsten4.3.2 Selective Tungsten4.4 Aluminum92929494100100103103103106114Contents117References5.
PLASMA-ENHANCED CVD5.1 Introduction5.2 Silicon Nitride5.3 Silicon Dioxide and Oxynitrides5.4 Polysilicon5.5 Epitaxial Silicon5.6 Refractory Metals and Silicides5.6.1 Tungsten5.6.2 Molybdenum5.6.3 Tantalum5.6.4 Titanium5.7 AluminumReferences. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . .6. PRODUCTION CVD REACTOR SYSTEMS6.1 Introduction6.2 Low-Temperature Silicon Dioxide Reactors6.3 Hot Tube, Low Pressure, Thermal Systems6.4 Epitaxial Silicon Reactors6.5 Plasma-Enhanced Systems6.6 New Conceptsxi1191191201311361371391391421441461481481501501511561581651691706.6.1 Hot Wall Cross-Flow Reactor6.6.2 Cold-Wall Thermal Systems170References. . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1747. FI LM EVALUATION TECHNIQUES7.1 Introduction7.2 Physical Measurements7.2.1 Thickness7.2.2 Stress7.2.3 Sheet Resistance7.2.4 Visible Defects7.2.5 Morphology-SEMITEM7.3 Chemical Measurements7.3.1 Refractive Index-Ellipsometry7.3.2 X-Ray Spectroscopy7.3.3 Dopant Distribution7.3.4 Infrared Spectroscopy7.3.5 Surface Spectroscopy7.3.5.1 ESCA7.3.5.2 Auger7.3.5.3 SiMS7.3.5.4 RBS7.3.6 Hydrogen Concentration EvaluationReferencesINDEX<••••••••••••••1751751751751821841881891901901901911931971972012022072092122131Fundamentals of Thermal CVD1.1 INTRODUCTIONChemical vapor deposition (CVD) is a process where one or more gaseousspecies react on a solid surface and one of the reaction products is a solid phasematerial.
For example, consider the pyrolysis of silane (Si H4 ) on a hot surface.When a silane molecule strikes a surface, it can either be reflected or adsorbed.If it is adsorbed, and the temperature is high enough to promote its decomposition, it may decompose into Si and H2 with the latter going back into the gasphase. The silicon left behind can build up as a thin solid film. Similar reactionsoccur where two compounds adsorb onto a surface and react there, leaving behind a solid phase.
If all products of the surface reaction are gaseous, the processis called heterogeneous gas phase catalysis. Much research has been done inthis field which will eventually be useful in understanding CVD.The several steps that must occur in every CVD reaction are as follows:(1) Transport of reacting gaseous species to the surface.(2) Adsorption, or chemisorption, of the species on the surface.(3) Heterogeneous surface reaction catalyzed by the surface.(4) Desorption of gaseous reaction products.(5) Transport of reaction products away from the surface.In all CVD processes, we are dealing with the change from one state (i.e.,the initial, low-temperature reactant gases) to a later one (i.e., the final statewith some solid phase and product gases) in time.
Since any practical commercial process must be completed quickly, the rate with which one proceeds fromthe initial to the final state is important. This rate will depend on chemicalkinetics (reaction rates) and fluid dynamic transport phenomena. Therefore,in order to clearly understand CVD processes, we will not only examine chemical thermodynamics (Section 1.2), but also kinetics and transport (Section 1.3).2Chemical Vapor Deposition for MicroelectronicsTwo types of CVD systems can be considered. One is a closed system intowhich a finite quantity of reactant gas is introduced, such as shown in Figures1A and 1 B. Initially, the silane (Si H4 ) is introduced at a low temperature (To).Silane will then diffuse to the hot wall through a concentration gradient layer,adsorb on the walls, dissociate there and leave solid silicon behind while H2diffuses back into the gas.
After a finite time, an equilibrium is reached whereno more silicon is deposited.FINAL STATEINITIAL STATESi (SOLID)I '--",-~-~-..--.--.l1(8)(A)Figure 1: Closed CVD system.The second type is the open system. Actually, this is the most commonsystem used, and the one we will be primarily studying in this text. We dealhere with a flowing system such as shown schematically in Figure 2.REACTANT - -.....SiH 4 (To )- - . SiH 4- - . H2Si (SOLID)----..r----PRODUCTSSiH 4 , H2Figure 2: Open flow CVD system.In this situation, a film is grown on the hot surface (T w), and its thickness will increase without Iimit as long as fresh reactants are provided and products can be removed. The gas state will be in quasiequilibrium far from the hotsurface and in a strongly nonequilibrium condition close to it.
The change fromone to the other will occur across a boundary layer where temperature, velocity,and species concentration vary rapidly. The behavior of this boundary layerwill be det~rmined by gas transport properties such as viscosity, thermal conductivity, as well as gas-phase kinetics and diffusion coefficients. So, even ifthe kinetics at the surface are very fast, we must deal with quasiequilibriumphenomena where gas conditions vary rapidly over short distances.Fundamentals of Thermal CVD3In the analysis of CVD reactions, It IS Important to recognize the ratesof the various processes. The slowest rate will be controlling, and which oneis the slowest or fastest can depend on gas as well as surface conditions. Forexample, surface reactions may be fast at high surface temperatures.
In thiscase, the CVD process will tend to be limited by the rate at which reactantscan get to the surface or products leave it. For this situation, the fluid dynamicboundary layer phenomena will govern the deposition rate. On the other hand,at low pressures diffusion is very rapid and the rate at which surface reactionsproceed will tend to govern the deposition rate. Alternatively, low surfacetemperatures will have low reaction rates, and this will govern no matter howmuch material diffuses to the surface.The essential issues that one is concerned with in all CVD processes are:(1) Nature of solid deposit given particular reactants.(2) Rate of deposition of solid film.(3) Uniform ity of deposition over extended surfaces.(4) Morphology of solid film.In the remainder of this chapter, some of the basic ideas governing theseissues will be covered.1.2 CHEMICAL EQUILIBRIUMAlthough CVD processes inherently involve rapid changes, it is useful to examine the lirniting case of long reaction times for insights into the nature of thefilms that can be deposited.
To do this, we examine the final equilibrium statefor the reactions of interest, which will depend on the initial reactant gas composition and the final pressure and temperature.The problem we are addressing here is: what is the gas phase compositionof a mixture of gases under specified conditions of pressure and temperature,where as much time as is necessary is allowed for the gases to equilibrate? Ifthere is a change of phase as one proceeds from one equilibrium state to another(i.e., solid silicon film forming on the container walls), then this has to be accounted for as well.1.2.1 Law of Mass ActionHistorically, the state of reaction at chemical equilibrium was evaluatedfor fairly simple reactions, with only a few species, from the "Law of MassAction." 1 In recent years, high-temperature reactions, including many possiblespecies (as many as 20 or more), have become of interest and newer techniquessuitable for numerical solution on high-speed digital computers have been developed.
2 Initially, we will discuss chemical equilibrium from the vantage point1Iof the "Law of Mass Action. It states that the rate at which a chemical reaction proceeds is proportional to the "active" masses of the reacting substances.The active mass for a mixture of ideal gases is the number density of each react-4Chemical Vapor Deposition for Microelectronicsing species, or for a given temperature, it can be represented by its partialpressure.Consider a typical reaction,Then thellIILaw of Mass Action states that(1 )where p is the partial pressure, v is the stoichiometric coefficient, and Kp is theequilibrium constant which is a function of the temperature alone.As a simple example of how this relation may be used to establish theequilibrium composition of reacting gases, consider the dissociation of nitrogentetroxide when its tenlperature is increased from room temperature to someelevated value.Equation (1) gives(2)but we also know that(3)PNO+ PN 0224pwhere p is the total pressure.
Equations (2) and (3) can be solved for PNO and.(2PN 2 0 4 In terms of Kp T) and p.For reactions in which the number of molecules do not change during thereaction, the amount of each reactant decomposed at equilibrium will be independent of the total pressure. Consider the reactionIn this case(4)Fundamentals of Thermal CVD5and we know(5)Pifthen Equations (4) and (5) become(6)and(7)pSolving Equations (6) and (7) for(8)PNO=PNO=PNO + 2PN2givesK~. P2 + I~Now, assume we start with A moles of N2 plus 02' We then heat this mixtureto temperature T, at which point x molecules of each N2 and O2 have reactedto form NO.
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