Defect control in silicon crystal growth and waferprocessing

Robert Falster

MEMC Electronic Materials SpANovara, Italy

Abstract

Accurate control of the defectivity of silicon crystals and wafers is a subject of immenseimportance to both the silicon and IC industries. Exploding costs of wafer developmentand production as well as the processing of 300mm wafers means that predictive defectengineering is now, more than ever a requirement for both industries. There is littlescope any more for iterative approaches to these problems. It is simply too expensive.Where ever possible generic – as opposed to application specific or tailored - waferproducts suitable for a wide variety of demanding applications must be developed inorder meet cost targets. This paper reviews recent developments in the understandingseveral aspects of defect control in silicon crystal growth and wafer processing which areof particular relevance to 300mm silicon products and processes. Among the subjectscovered are the problems of intrinsic point defect concentration and reaction control inthe growth crystals including effects of impurities and the uses of vacancy concentrationprofiles installed into silicon wafer in order to achieve ideal oxygen precipitationperformance. The importance of accurate modeling of defect dynamics is stressed.Finally, the requirement of dealing with significantly higher levels of mechanical stress in300 mm processing has led to a new appreciation of the role played by oxygen in thelocking of dislocations and the dynamics of wafer hardening during processing. Thesedevelopments are reviewed briefly.

Introduction

Many aspects of the silicon industry have changed over the past few years. Quite a fewof the changes have been a result of two simple facts. There has been a huge increase inboth the costs of developing new (and in particular 300mm) silicon products and the costsof testing new silicon products by users. Control of defectivity has always been andremains a critical aspect of silicon product design. In the past much of this was done on amore or less empirical basis with a close coupling of product development to individualapplication. This is changing as more positive control and specification and moreuniversal defect solutions are sought. The major issues remain the control of the intrinsicpoint defects in crystal growth and the behavior of oxygen in silicon wafers during waferprocessing.

There are two central general problems associated with defectivity in conventionalsilicon wafers which have plagued the silicon industry in many ways over many years.One relates to difficulties in specification and the other to the generally complexinteraction between material and process. Both are of these problems are equallyimportant and each has outward-rippling implications of their own.

Defectivity specification has been a highly problematic aspect of the silicon industry formany years. For example, specifying the oxygen concentration for a given conventionalsilicon wafer order obviously does not accurately describe or predict the oxygen behaviorof any specified lot of wafers in any specific application. Likewise, specifying suchthings as “flow pattern defect” or “COP” density does not accurately describe the state ofagglomerated point defects – the other main defectivity issue in silicon technology.Simple specifications, and their accompanying roadmap goals for conventional silicon,do not guarantee that – in general, let alone for the specific wafer group in question - thiswill be sufficient to meet the needs of this, that or the other technology node. It hascreated huge problems for product road map development.

Metrology and sampling issues are only part of the problem. The second major problemof defectivity lies in the fact that there is a strong coupling between the various defectformation mechanisms the ultimate performance of the material in specific applications.The reason why oxygen concentration alone doesn’t accurately portray the oxygenperformance of a silicon order is that oxygen concentration (the one thing which isrelatively easily determined) is but a small part of the complex phenomena which controlthe state (the actual important feature) of oxygen in a silicon wafer. Similar statementscan be made of the state of agglomerated intrinsic point defects where such things as sizedistribution are important.

An upshot of this state of affairs is that this produces a dilemma in product design. It isunclear what the target is. Recent solutions such as micro-defect free PerfectSiliconTM

and Magic Denuded Zone® (MDZ®) materials offer a pathway out of this dilemma.Important to their development has been an improved understanding of the behavior ofintrinsic point defects and their interactions with impurities. Some of this is brieflyreviewed here. This understanding has greatly improved our ability to design and specifymaterial and to develop cost effective processes to manufacture them. An importantaspect of this has been a significant improvement in our ability to model the complexthermal fields and point defect dynamics of silicon crystal growth.

Intrinsic point defects in silicon crystal growth

Figure 1 shows schematic illustrations of the space of the various important features ofvacancies and their reactions in CZ silicon [1]. Illustrated are solubility concentrationversus the conditions under which voids (COPs) are formed their binding to oxygen andsubsequent sharp reduction in mobility and the space where vacancies and oxygen join

forces to dramatically alter oxygen clustering (and subsequent oxygen precipitation)behavior. With such diagrams many of the processes important to defect control can bevisualized.

Mapped onto the space are two simple illustrations of vacancy defect reaction paths.Figure 1 illustrates the close coupling between the problem of simultaneously controllingvoid number density and size and oxygen precipitation behaviour. The initial vacancyconcentration is controlled by conditions near the growth interface [2], the void densityby conditions near the nucleation temperature (itself a function of melt interfaceconditions) [3] and the void size, which depends on the cooling conditions just below thevoid nucleation temperature (and also depends on void density and initial vacancyconcentration) [4]. Oxygen behavior is then furthermore complicated by details ofcooling at lower temperatures and the oxygen concentration itself. Process changes atany of these stages in the crystal growth process can have a large impact on thesubsequent behaviour of oxygen.

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Figure 1. Schematic illustrations of vacancy reactions and concentration pathsduring the cooling of crystal during growth. Illustrated are cases of crystalvacancy type crystals grown with sufficiently large vacancy concentrationsnucleate voids. In (a) the cooling rates near the void nucleationtemperature are slow enough to result in the consumption of free, grown-in, vacancies to low enough levels so as not to cause oxygen precipitationenhancement. (b) illustrates the case of more rapid cooling in this phasewith the result of strong vacancy-enhanced oxygen precipitation.

Techniques for the production of micro-defect free silicon have been developed over thepast several years [1,5,6]. They involve processes which control and manage the intrinsicpoint defect concentrations throughout the crystal growth process such that critical super-saturations of either vacancies or silicon self-interstitials are never reached in temperatureranges where micro-defect formation (voids and dislocation loops, respectively) is a risk.Much has been learned through the intense engineering efforts of these past years

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resulting in great strides in the efficiency and cost effectiveness of such processes.Processes for 300mm PerfectSilicon have been developed.

A truly micro-defect free wafer (as opposed to a merely controlled defect) producteffectively side-steps the question of what degree of defectivity is acceptable in a givenapplication. It is also very simple to specify. As long as the “Perfect” criterion is met,there is no question of hard-to-determine-effectively micro-defect density or sizedistributions. There is no question of the suitability of such material (from the micro-defect point of view) in advanced applications.

Importantly, the simplicity and completeness of the specification (“perfect”) implies thatit is the material and not the process which is specified. This is an important fact and onewhich is central to a maturing product such as silicon. Perfect Silicon is not a process; itis a result. It is a result which can, in principle, be reached by a variety of paths with avariety of hardware solutions. It is an orthogonal solution to the application in the sensethat, independent of the exact details of the approach used to achieve it, the well-defineddefect-free result insures that there is no impact of the micro-defectivity of the wafer – orindeed process variation (so long as the result “perfect” is maintained) - on the resultantyield or performance of the IC produced by it. No other crystal approach can achieve thisclarity in specification and ultimate simplicity of use - for now and in the long term.

The engineering of vacancy concentration profiles in silicon wafers andthe control of oxygen precipitation

The problem of oxygen precipitation control in silicon has been an important arearesearch in silicon technology for over 20 years. Since it was first recognized thatoxygen precipitates could act a gettering sites for fast diffusing and harmful transitionmetal contamination [7], the use of oxygen precipitates has played an important role in acontamaination management schemes throughout the IC industry. Such systems arecalled Internal Gettering (IG) systems. It has, however, turned out to be quite a difficultproblem from a practical perpective. The gettering part is largely easy [8]; the difficultpart lies in the precise control of oxygen precipitation behavior to insure effectivegettering without harmful side effects - in every wafer. In particular, a firm grasp of thenucleation processes has proved illusive [9]. The result of this has been often less thanideal and reliable control of oxygen precipitation performance in practice.

In general, the distribution of oxygen precipitation achieved in an ensemble of siliconwafers depends strongly on a close coupling of the oxygen content, the details of thecrystal growth processes and the details of the application to which the wafers aresubmitted [10,11]. Many, often complicated and expensive, approaches have beendeveloped over the years to manage these coupled complications and achieve the desiredgettering effect without side effects. These include such wafer pre-treatments as the so-called “Hi-Low-High” treatments in which oxygen is first out-diffused at hightemperatures (to create a low oxygen content surface layer) followed by a generally long,low temperature treatment to (re-) nucleate oxygen clusters followed by another high

temperature treatment to grow them into precipitates. Other approaches have includedattempts to very narrowly specify oxygen concentration and crystal growth process oreven the segments of the crystal from which wafers should be taken for specificapplication.

The MDZ® wafer is a serious departure from previous attempts at solving theproblem of oxygen precipitation control. The RTP family of processes which produceMDZ® wafers result in a radical change of the wafer’s material properties. At its core isthe enormous effect that vacancies have on the control of the nucleation processes ofoxygen in silicon [12]. The process which produces the MDZ® wafer installs a usefulvacancy concentration profile (or template) into a silicon wafer which subsequently takesover the control the wafer’s oxygen precipitation behavior from all of the difficult factorsimportant in conventional silicon: crystal growth, IC application, and even oxygenconcentration itself. An MDZ® wafer is a wafer which is programmed through the RTPtreatment to behave in a well defined, ideal manner in any application, sweeping aside anentire raft of technological difficulties.

An illustration of the huge affect that vacancies have on oxygen precipitationbehavior is shown in Fig. 2 in which the dependence of the resulting oxygen precipitatedensity on vacancy concentration is shown. The core concept of the MDZ® wafer is toutilize this very strong dependence and engineer a profile of vacancies into a siliconwafer. The very steep, switch-like dependence of precipitate density on vacancyconcentration means that a profile of vacancy concentration rising from the surface andgoing through the threshold value produces a rather sharply layered structure with ahighly precipitating bulk underneath a non-precipitating surface layer. The threshold forthis layered design lies at a vacancy concentration of about 1012 cm-3. Figure 3schematically illustrates the design of such a wafer and compares it to a conventionaloxygen – out diffusion approach to the problem of forming a denuded zone.

Vacancies may be introduced into silicon wafers at high temperatures by a number ofdifferent mechanisms. Two examples are nitridation [15,16] or through simple hightemperature Frenkel pair generation [17]. Generating large concentrations of vacanciesin wafers at high temperatures is not a difficult task at all. The problem lies in fightingthe tendency of the wafer return to equilibrium during the cooling of the wafer andkeeping them in the wafer in sufficiently large concentrations to be useful. This is whereRTP comes in.

The simplest procedure for installing a useful profile of vacancies in a silicon wafer reliessolely on Frenkel pair generation and the close proximity (relative to vacancy diffusionlengths) of the two wafer surfaces. Heating a thin wafer to a high temperature T resultsin the rapid equilibration of vacancy-interstitial system. First, Frenkel pairs – vacanciesand self-interstitials in equal amounts - are produced. This - very fast – reaction leads to arecombination-generation equilibrium. The product CiCv of the two concentrationsacquires the equilibrium value Ci*Cv*, with the concentration of both equal to(Ci*Cv*)1/2. Were the sample to be cooled at this point under the condition of equalconcentration, the vacancies and interstitials would merely mutually annihilate each other

completely in the reverse process of their generation resulting in no vacancyconcentration enhancement by the time the samples reach room temperature.

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Figure 2. Oxygen precipitate densities produced following test heat-treatments(800°C 4 hours + 1000°C 16 hours) as a function of wafer vacancyconcentration. Vacancy concentration was determined by platinumdiffusion experiments [13,14].

This is averted by the next stage of the process: equilibration. Both Ci and Cv willapproach their equilibrium values, Ci*and Cv*, due to exchange with the wafer surface(considered as ideal sink/source of point defects). This coupled process is controlledmainly by diffusion of self-interstitials which are the faster diffusers – the twoconcentrations being comparable. The total time to achieve the complete equilibrium in astandard wafer (ca. 700 :m thick) was found to be extremely short, less than severalseconds at 1250oC. The speed of the equilibration is, in fact, a measure of interstitialdiffusivity and implies that the interstitial diffusivity is high, on the order of 2.5x10-

4cm2/s.

After equilibration, the vacancies become the dominant species since Cv*>Ci*. Onsubsequent cooling the point defects quickly recombine, only now, some of the vacanciessurvive. If the equilibration effect had not occurred, and vacancies and interstitialsremained in equal concentrations, they would recombine equally with each other leavingno excess concentration of either species. When equilibration is reached, and positionsremote from the wafer surfaces the majority vacancies consume the minority interstitialsuntil there are no more to consume leaving behind an excess concentration of surviving

vacancies which is equal to the initial concentration difference at the anneal temperature,∆C= Cv*-Ci*.

Figure 3. A schematic illustration of the difference between conventional methodsof installing denuded zones (DZ) in silicon wafers via oxygen out-diffusion and renucleation and a new method based on the installation oftailored vacancy concentration profiles.

The proximity of the wafer surfaces during cooling adds another component to theprocess. In addition to recombination, vacancies will out-diffuse to the wafer surfaceswhere the local (equilibrium) vacancy concentration is rapidly decreasing. However, ifthe cooling rate is fast enough – in the range of about 40 to 100K/s (readily accessiblethrough using RTP) - the middle of the wafer will be not affected by vacancy out-diffusion, and the vacancy species will be present there in the concentration ∆C. Theregions near the surfaces will be strongly affected however. At lower temperatures theprofile is effectively frozen-in by the binding of vacancies to oxygen which becomescomplete by about 900 oC [18]. At this point the vacancies convert from their relativelymobile state (free mono-vacancies) to their relatively immobile state, VO2. The profilingof the vacancy concentration through out-diffusion achieved in a given RTP heattreatment is largely the result of the cooling conditions of the wafer above thistemperature.

In the present discussion, the most important thing is that the near-surface regions of aquenched wafer are depleted of vacancies, by vacancy out-diffusion during the coolingstage of RTP. In the near-surface zones the vacancy concentration is below Cv*, andoxygen precipitation is suppressed in a practical sense as a result of prohibitively longincubation times (the tabula rasa effect). The rapid increase in temperature during the

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ramp-up to the process temperature serves to dissolve all pre-existing oxygen clusters. Inthe middle of a wafer the precipitation is strong, due to the presence of vacancies inconcentration over Cv*. Such a precipitation profile is precisely what is required for idealIG. The width of DZ (precipitation-denuded zone) is easily controlled by the cooling rate.Faster cooling rates mean a shorter vacancy diffusion length, and thus a narrower DZ. Ifthe cooling rate is too slow, such as occurs in conventional furnace annealing, the hightemperature vacancy concentration profile will be allowed to fully relax throughout thesample thickness to its equilibrium value near the binding temperature, which is wellbelow the threshold for precipitation enhancement. At this point the entire waferthickness becomes of the tabula rasa type.

Installing a vacancy concentration profile which rises from the wafer surface into thebulk of the wafer crossing the critical concentration Cv* at some desired depth is the coreof the concept behind the MDZ® wafer. The installed vacancies have full control of theoxygen precipitation behavior of the wafer. An example of a depth distribution of oxygenprecipitates produced by vacancy concentration control is shown in Fig 4.

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Figure 4 (a) Depth profiles of platinum diffusion profiles (~ vacancy concentration)measured at 730 and 800°C, calculated vacancy concentrations, andmeasured oxygen precipitate densities in an RTP treated sample processedat 1250°C. The oxygen precipitate density axis is scaled to correspond tothe vacancy-precipitate density calibration and (b) an etched cross sectionof a silicon wafer containing such a profile following a precipitation heattreatment (800°C 4 hours + 1000°C 16 hours) showing the depth profile ofoxygen precipitates resulting from an RTP-installed vacancy concentrationprofile. The bulk density of precipitates in this example is 1 x 1010 cm-3.

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Largely because of the speed of the vacancy-interstitial recombination reaction and therapid diffusivities of both vacancies and interstitials (with that of the vacancies beingconveniently lower than that of the interstitials) the process which produces an MDZ®

wafer is very rapid indeed. It is accomplished in several seconds compared to thetypically many hours required of the conventional oxygen-based approach. The denudedzones produced are also in general larger than conventional DZ. Larger denuded zonesinsure no risk of precipitation induced side effects and complete gettering even in thereduced “thermal budget” processes. Since gettering proceeds by precipitation drivenundercooling, it happens during cooling. The length of process time is essentiallyirrelevant. Important technologically is the fact that the structure developed isindependent of oxygen concentration, crystal growth process and to a large extent thedetails of the subsequent thermal processing. This last result is due to the consumption ofthe vacancies during the very rapid initial nucleation processes. Any subsequentnucleation is then again subject to the “normal” prohibitively long incubationrequirements which exist in essentially all practical situations.

In the simplest case, the shape of the profile which produces the MDZ® wafer iscontrolled by two parameters. The first is the soak temperature, Tp, with controls thequenched-in vacancy concentration at the center of the wafer (= {Cv*(Tp) – Ci*(Tp)}) andhence the bulk oxygen precipitate density. The second is the cooling rate controlling thedepth of the DZ and, at slower rates, eventually the vacancy concentration (and with it,resulting precipitate density) at the center of the wafer. Fig 5 illustrates the cooling rateeffect.

Figure 5. Schematic illustration of cooling rate effect.

The minimum Tp for an effective straightforward RTP treatment corresponds to Cv*(Tp) –Ci*(Tp) /1012 cm-3, the threshold value. This happens around 1150°C. The quenched-invacancy concentration (and with it bulk precipitate density) rises with increasing Tp. Thebulk precipitate density reaches about 1010 cm-3 by about 1250°C.

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In RTP treatments, the concentrations of intrinsic point defects in silicon wafers can bedynamically and very rapidly altered means other than temperature and cooling rate.Ambient is also very important in these technologies. Examples include interstitialinjection from the wafer surfaces during oxidation, and vacancy enhancements due tonitridation effects. One example is the complete elimination of the effect when oxygenambient concentrations exceed about 2000 ppma [19].

Figure 6. Oxygen precipitation behavior (BMD) sampling for MDZ® wafers (filledcircles) and normal (open circles) precipitation distributions. BMD standsfor Bulk Microdefect Density, in this case equal to the oxygen precipitatedensity.

There are many advantages to using a RTP-vacancy based system to control oxygenprecipitation behavior in silicon wafers: simplicity, copy-exactability, ease-of-use andreliability of results. Figure 6 shows just one example of the degree of control such aprocess results in. In this plot, random examples of precipitation behavior (4 hours800°C + 16 hours 1000°C) for various sampled crystals over a long period of time isshown. The performance of MDZ® wafers is compared to conventional uncontrolledmaterial as a function of oxygen concentration. The width of the signal of the MDZ®

wafers is roughly equal to the counting error. The data of conventional wafers span 5orders of magnitude, essentially the entire range of possible values.

Oxygen of the locking of dislocations

The requirement of dealing with significantly higher levels of mechanical stress in 300mm processing has led to a new appreciation of the role played by oxygen in the lockingof dislocations and the dynamics of wafer hardening during processing. Recently a largeamount of detail relating to the dynamics of oxygen locking of dislocations has been

worked out [20,21]. The increased hardness due to the attachment of oxygen todislocations is a highly dynamic effect with the size of the oxygen induced unlockingstress a strong function of oxygen concentration, time, temperature and path takenthrough a high temperature treatment. Examples of the time dependence of the increaseof locking stress during an isothermal anneal at 650C with applied stress at 550C forthree different oxygen concentrations is shown in Fig 7.

Successful models for the transport and binding of oxygen to dislocations have beendeveloped [22]. These have led to models which can simulate the instantaneousresistance to applied stresses of a wafer during arbitrary process cycles. It is expectedthat such new capability will lead to improvements in slip resistant wafer/process design.

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Acknowledgements

Acknowledgements are due many people for their contributions over the years Some ofthese include Vladimir Voronkov, Joe Holzer, Daniela Gambaro, Max Olmo of MEMCElectronic Materials; Peter Pichler of the Fraunhofer Instititut, Erlangen; SemihSenkader, Armando Giannattasio and Peter Wilshaw of Oxford University; Harold Korband Paolo Mutti.

References

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States Patent 5,919,302, Jul. 6, 1999.[6] R Falster and VV Voronkov, Materials Science and Engineering B73 (2000) pp.

87ff.[7] TY Tan, EE Gardner and WK Rice, Appl. Phys. Lett. 30, 175 (1977).[8] R Falster, Semiconductor Fabtech, 13th Edition, p. 187-193, 2001.[9] F Kelton, R Falster, D Gambaro, M Olmo, M Cornara and PF Wei, J Appl Phys,

85, 8097 (1999).[10] Y Shinamuki, H Furuya, I Suzuki and K Maurai, Jpn. J. Appl. Phys., 24, 1595

(1985).[11] G Fraundorf, P Fraundorf, RA Craven, RA Moody and RW Shaw, J.

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and M. Jacob, Solid State Phenomena, Vols, 57-58, 129 (1997).[16] M. Jacob, P Pichler, H. Ryssel, R. Falster, M. Cornara, D. Gambaro, M. Olmo and

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