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Chapter 4 Radiation Damage and Annealing in Silicon after Ion Implantation

Jozsef Gyulaia, Kevin S. Jonesb, and Peter Petrika a Research Institute for Tech. Phys. and Matl. Sci., P.O.Box 49, H-1525

Budapest, Hungary gyulai@mfa.kfki.hu, petrik@mfa.kfki.hu

b Department of Materials Science and Engineering, University of Florida Gainesville, FL 32611-6400

kjones@eng.ufl.edu ABSTRACT

First, a simple microscopic scenario of implantation process is presented. This includes a discussion of primary defect formation from the crystalline state up to the amorphous state. Annealing strategies are discussed with an emphasis on understanding the effect of the annealing sequence on the defect reduction process. This section is intended to summarize how different annealing cycles (temperature as a function of time, T(t)) lead to different final states. The annealing section will complete this chapter through discussions of the solid phase epitaxial regrowth of implantation induced amorphous layers, formation of extended defects, location of these defects (most commonly dislocations) and their effect on the subsequent processing is determined by their origin. This understanding of the defect formation and annealing is used to better understand defects that commonly arise during device processing. Briefly, extreme low energy and molecular ions are dealt with. INTRODUCTION.......................................................................................................................................2 4-1 MICROSCOPIC PICTURE OF IMPLANTATION.........................................................................3 4-2 PRIMARY DEFECT PRODUCTION................................................................................................8

4-2-1 Elastic recoil, defect production rate ......................................................................................8 4-2-2 Indirect beam effects: Recoil ................................................................................................10 4-2-3 Implantation with molecular ions .........................................................................................11

4-3 FORMATION OF AMORPHOUS LAYERS ..................................................................................12 4-4 ANNEALING ......................................................................................................................................15

4-4-1 Annealing Strategies.............................................................................................................15 4-4-1-1 Annealing Strategies; post-implantation (adiabatic, isothermal) ......................................15 4-4-1-2 Annealing strategies: during-implantation (beam annealing, hot

implantation) .....................................................................................................................18 4-4-2 Thermal cycling (TC), thermal budget .................................................................................21

4-5 ELECTRICAL ACTIVATION .........................................................................................................23 4-5-1. Kinetics of activation...........................................................................................................23 4-5-2 Electrically active defects.....................................................................................................23

CONCLUSIONS AND OUTLOOK TO CHAPTER 5 ..........................................................................26

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INTRODUCTION

This chapter and the next one will summarize recent results in ion implantation damage and annealing that are helping ion implantation meet the demands of the rapidly advancing semiconductor processing market. Because ion implantation is a technique, where the primary, "as-implanted", state is far from equilibrium and there are ample of lattice defects, understanding the evolution of these defects and their effect on device processing is becoming essential to the manufacture of these devices.

The idea that after ion bombardment, a thermal treatment is necessary to bring the system to an equilibrium state is as old as the idea of ion implantation itself. However, the details of this "treatment" has been an area of active research active for over four decades and still new ideas emerge and deeper insight into the structure of matter is achieved. The increasing demand of semiconductor technology have broadened the arsenal of thermal treatments to include high-intensity light sources quoted as Rapid Thermal Annealing (RTA) or RTP ("P" for Processing) and different forms of radiation, such as lasers or particle beams.

The continuing demands for miniaturization bring in new complexity. First of all, the redistribution of the implanted species during this post-implantation treatment in the context of VLSI or ULSI demands is considered detrimental. Recent evidence that these defects have dramatic effects on the final device dimensions has led to ever increasing interest in their evolution. Furthermore, the lateral structure of the device causes new sources of structural problems, due to mechanical stresses and misfits. Needless to say at the end of the post-implantation anneal, the device should reach a state, equilibrium or not, which is stable enough for long-time operation at the desired operating temperature.

In coming nodes of integrated circuits, demands still immensely increase: production of extreme shallow junctions with low sheet resistance, possibly free from carrier scattering centers, high breakdown resistance, etc. Implantation doping is still the best choice, however, new ideas emerge.

It is a general statement that the study of ion implantation is a study of radiation damage effects and annealing. As for reviews, previous editions of this book by Ziegler1 2 3 4 5 are referred to. Conference proceedings contain also much information. First the proceedings of conferences called Ion-implantation in Semiconductors (Eisen and Chadderton6; Ruge and Graul7; Crowder8; Namba9; Chernow et al.10), later the Ion Beam Modification of Materials (Gyulai et al.11; Benenson et al. 12; Biasse et al.13; B.M. Ullrich14; S.U. Campisano et al.15; Namba16; Withrow and Poker17; Kalbitzer et al.18, J.S. Williams et al.19, etc.) was an important set of biannual conferences. The present series of conferences, called Ion Implantation: Equipment and Techniques, IIET, or later Ion Implantation Technology, IIT, (McKenna et al.20; Ryssel and Glawischnig21; Ziegler and Brown22; Current et al.23; Takagi24; Stephens et al.25; D.F. Downey et al.26, Coffa et al. 27, Larsen et al. 28, J. Matsuo et al. 29, H. Ryssel et al.30, Brown et al.31 ), are the closest topical conferences to this school. MRS and E-MRS conference proceedings edited by the Materials Research Society or the European Materials Research Society, also contain many relevant volumes, reviews and papers. Similar is true to editions of the Electrochemical Society, and occasionally to editions of SPIE.

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4-1 MICROSCOPIC PICTURE OF IMPLANTATION

Since the beginnings, the basic scenario of the implantation process has been reasonably well known. However, it was not until the mid-eighties, that progress in computing allowed detailed calculations, which confirmed many of the early ideas. Through these calculations, the events in individual cascades became visible and stages as it develops became clear.

The primary processes are statistical, thus, extremely complicated. However, there are some characteristic cases, which can be distinguished. A heavy target and light projectile will lead to a less dense damage cascade and, thus, to a defect structure, which differs very much from the other extreme, i.e. that of a heavy ion into a light target material. This latter scenario may even result in a short-living molten state in the "head" of the cascade (Kim et al.32). For a given ion mass, the role of electronic stopping increases as the ion mass increases changing the mechanism of damage accumulation.

In this chapter, emphasis will be given to different regimes of thermal annealing so that one better understands damage recovery including the so-called self-annealing during irradiation. Understanding amount of self annealing, often controlled by the implant temperature, is critical because the conditions inside and next to the layer, which undergoes implantation modification, is becoming a key factor in improving the reproducibility of the properties of implanted layers. This holds, especially, for very low and very high-energy (MeV) implants, which depend most critically on these circumstances.

Primary defects are the result of many simultaneous or consecutive processes including: * processes in and around a single cascade, including interactions between simultaneous and

overlapping sub-cascades, * superposition (maybe synergism) of processes produced by subsequent ions impinging on

or around previously damaged, but still "active" zones. For a description of ion stopping, both kinetic and thermodynamic treatments are possible. For

the kinetic approach, the TRIM of Biersack and Haggmark33and similar models, like that of Posselt (BOTE)34, are well-known examples that work well. An experimental verification of calculations is presented in Fig. 4-1, where final branching of a cascade is displayed as an AFM profile. The cascade was produced in muscovite-mica by 215 MeV Ne ions falling under normal direction to the surface. The AFM picture was taken along the surface of the mica. As another and popular approach is the use of molecular dynamics (MD), which led to impressive results since its early days, e.g., Rubia et al.35. The kinetic models, at least in their present form, neglect both thermodynamics and phase formation for high-dose implantation or for non-elemental targets, while the thermodynamic models do not understand effects such as cascade mixing which was explained by Sigmund and Gras-Marti36 and also ignore inelastic processes, like X-ray production. Later, reference will be made to the MD calculations, which take care of most of the problems. The goal is to handle basic questions of the as-implanted state, like substitutional or interstitial location of the implant or details of primary defect formation.

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Fig. 4-1. Final branching of a cascade, detected by AFM as roughening of the surface of mica. Primary ion was 215 MeV Ne impinging in the normal to the surface (Biro et al. 37).

In Fig. 4-2, history of an individual implantation event is summarized. For the as-implanted

state, it is very important in what state of the cascade region is, when the next ion impinges into the same area. ("same" in this chapter considers a cascade with a diameter of 2.10-6 cm.)

Fig. 4-2. Surface covered by individual implantation events using 10-8 s as active duration of the cascades and 10-6 cm for their diameter; PIB: implantation with Pulsed Ion Beams, FIB: Focused Ion Implantation, IBIEC: Ion Beam Induced Epitaxial Crystallization, IBIA: Ion Beam Induced Amorphization

In the case of "regular" implantation, ions hit the same area on a scale of seconds. This means

that damage accumulation for low temperature implantation takes place through independent events. However, for dose rates used in the so-called self-annealing regime where beam-assisted annealing, IBIEC or IBIA (separated by a critical temperature, Tcrit) operating, both kinetic and thermal effects are important, and they will dramatically change the pre-existing state. In the case of irradiation with focused ions (FIB), this interaction is even more explicit, and accordingly the annealing properties of the associated defects are quite different (Tamura et al.38). For Pulsed Ion Beams (PIB, e.g., Krafcsik et al. and Pogrebnjak et al.39), where in a single pulse of about 30 ns duration some 1012/cm2 ions are present with a typical energy of few hundred keV, conditions for macroscopic melting can readily be

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achieved. Here, the thermal picture is already very simple: one-dimensional heat flow conditions apply analogous to melting by a pulsed laser beam (Nastasi et al.40).

In a thermal picture, the cascade describes a state, when the lattice "temperature" still may differ from that of the electron cloud (ionization effects). This extreme state of the matter will relax by giving away its energy to the adjacent volume through collisions, damage (Frenkel pair, FP) diffusion and recombination. In case of thermal spikes ( review by Thompson41) the material can reach melting.

It is also useful to think on the thermal picture of the process based on the temperature dependent heat conduction (κ) and specific heat (C) and their ratio κ/C, called as thermal diffusivity (Glassbrenner and Slack42). These parameters give a link between the concept of local temperature distribution surrounding a single cascade and the macroscopic wafer temperature during implantation.

For example, it may not be just accidental that the marginal 20 m/s resolidification rate for amorphous growth of silicon from its melt produced by a pulsed laser (e.g. Cullis et al.43), roughly corresponds to 10-11 sec to grow one lattice plane. This value corresponds to the time, quoted by Kelly as "prompt thermal" regime44. It is also suggested that analogies inferred from the well-developed theories of Ion Beam Mixing can also serve for better understanding of damage production and defect engineering (see Section 4).

In the case of implantation of metals at low temperature, the events during ion beam mixing are reasonably well understood (Cascade Mixing, Vineyard 45, and also spike effects, Kim et al.37). Events at higher temperatures (thermally assisted regime) also present a more-or-less consistent picture (Akano et al.46).

Calculations of the cascade structure of single ions have been made using the molecular dynamics approach by Rubia et al.47. In addition, a rather simple Monte Carlo approach has been used for cascade structure and overlap effects in mixing by Rossi and Nastasi48 including the role of substrate temperature.

The thermal spike persists for picoseconds before the heat dissipates in the lattice. The

Fig. 4-3. Number of energetic atoms as a function of simulation time (ps) in Au after 12 keV Ar ion impact. Inset: The average temperature (K) of the thermal spike as a function of simulation time (ps). The first sharp peak is a short cascade period followed by the thermal spike (at 3-4 ps)

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dissipation of the thermal energy within the thermal spike can be described by Fourier's law J=-D grad T, where the diffusion constant D is given in terms of the thermal conductivity C, the specific heat c, and the mass density ρ by D=C/(c ρ) 49,. (see review by Averback and Rubia 50).

Modeling studies of low-energy ion impact into solids are giving insight to events important in scale-down. As an example, we are presenting MD calculations of thermal spikes produced with 12 keV Ar irradiation with on Au 51 using the PARCAS code52. In Fig. 4-3, the number of energetic particles is shown as a function of time in Au after 12 keV Ar+ irradiation obtained by a molecular dynamics simulation. An animation of the process can be seen in the web page of the authors53. The temperature of the spike is shown in the inset. In Au, the spike has an exceptionally long lifetime, (10-20 ps) as compared 3-6 ps in other metals. In Fig. 4-4, a cross-sectional view along the depth profile at 1 ps obtained by an MD simulation after 1 keV Ar ion bombardment of Pt(111).

For extreme low-energy implantation, MD-studies point on the fact that because of the thermal spike located close to the surface, detrimental effects, like roughening, enhanced sputtering, etc., may occur depending on the species involved.

Several conclusions from these ion beam mixing studies (with the assumption that C and κ are independent of temperature) have been made including the observations that

• Spike and Radiation Enhanced Diffusion (RED) effects are important and are often more important than ballistic effects,

• For high-density cascades a liquid-like behavior is possible. In case of silicon implantation and defect formation, there are important differences as

compared to metal calculations, for example: • implants in practice are at RT or higher, • Si is bonded covalently, thus defect formation and evolution processes are very different

Fig.4-4, Cross-sectional view of the thermal spike in Pt bombarded with 1 keV Ar+ at 1 ps obtained by molecular dynamics simulation.

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• heat conductivity (κ) of Si is excellent, but decreases about an order of magnitude in the temperature region of interest (100 to 600K, Fig. 4-5).

Figure 4-5. Heat conduction of silicon (Glassbrenner and Slack53)

Though the equation for heat conduction is too complicated to be solved for a volume of a real cascade, or at least for a volume containing it, solutions for simple cases may have some relevance to our problem. For example, for an instantaneous point source, the time (t) and κ or D occur always as a product (e.g. (1/κt) exp[-Z2/4κt]). This might allow for some speculations. The value κ and of D=D(T) are decreasing with increasing temperature. This will result in a time-delay in the homogenizing of the temperature contours more-or-less by the same factor. I.e., when implantation is done at higher substrate temperatures, defect reactions will have an extended time to run. This may be why thermal energies of only a few hundred °C (merely 0.2 to 0.4 Tm, Tm being the melting temperature) can have such a dramatic influence, for example the onset of IBIEC.

In a simple model of cascade cooling by heat conduction through a hemisphere which embeds all kinetic processes, Gyulai et al.54 have shown that a quantity of (Tm - Tcrit)Tm/Tcrit , for published values of Tcrit , correlates reasonably well with ∆Hρ/κm for six different semiconductors (and predicts Tcrit = 1200°C for SiC), where ∆H is the enthalpy of crystallization, ρ the density, and κm the heat conductivity at melt temperature. The slope of that graph gives for 100 nm diameter cascades a resolidification velocity of 20 m/s - without any fitting parameter. This gives a hint that the concept of a critical temperature, which separates crystalline/amorphous regrowth, is also valid for a volume around a single cascade. Thus, thermal balance there is a major controlling factor of FP recombination, thus, of crystallinity vs. amorphousness of the as-implanted state. Interestingly, the model predicts that in the final state of the cascade cooling, the smaller diameter of the still “hot” area results in a high resolidification velocity surpassing the 20 m/s margin. Thus, for each cascade, there will be a remnant amorphous core in agreement with finding and explanation of Cellini et al.55 from strain measurements on 400 keV N- implanted and annealed (up to 300°C) silicon.

To further illustrate the practical complexity of the problem, the formation and annealing of a real defect state after implantation (i.e. during wafer processing) is also influenced by

• native/process induced impurities in Si (not only in the surface layer!), • mechanical stresses, also from already existing lateral structures • heat flow (wafer contact) and radiation cooling,.

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4-2 PRIMARY DEFECT PRODUCTION

We call "primary" those defects, which are result of processes of defect generation/annihilation in the time scale of implantation kinetics, i.e., shorter than 10-8 s. This is often referred to as the "as- implanted" state. Certainly, there is an ambiguity in that, because there might be slower processes running within the time gap between 10-8 and hours or days after the implantation process, when characterization of the wafer or the next process step takes place. It is also true that, e.g., for low-temperature implantation, the "heating" of the wafer to room temperature is an annealing process in itself and the state of wafer being at room temperature still may be called as "as-implanted". "Secondary" defects will be dealt with in Chapter 5 and they are result of defect reactions during treatments, thermal or athermal, starting from the as-implanted state. 4-2-1 Elastic recoil, defect production rate

Mechanisms of the production of primary defects are described in details in the textbooks on ion implantation. For a more comprehensive discussion, the reader is referred e.g. to the book of H. Ryssel and I. Ruge5. Thus, here it is expected that the reader is familiar with concepts, like FPs, the Khinchin-Pease formula56, the concept of energy deposited into atomic (nuclear) processes by Brice57,58 (Fig. 4-6), which basically coincides with the primary damage distribution.

It is rule of thumb that the maximum of damage distribution lies closer to surface than dopant distribution itself. There is, however, one more recent result, which cannot be bypassed, as it has a large impact to many topics in this chapter, like the production of shallow junctions. It has been predicted by Mazzone using Monte Carlo calculations59 that because of the forward peaking nature of the momentum of an incoming ion the damage associated with the FPs alone should produce a vacancy-rich zone in the region extending from the surface down to about 0.8 Rp while between Rp and 2Rp, there should be an interstitial-rich zone. However, because of the non-conservative nature of the ion implantation process (i.e., atoms that want to occupy substitutional sites are being introduced into the crystal far in excess of the available unoccupied lattice sites), the interstitial component dominates the point defect distribution for low and medium energy implants (less than a few hundred keV). The only signs of any vacancy rich regions near the surface are for unannealed samples in which a small tensile strain has been seen by high-resolution x-ray rocking curve analysis60. Upon annealing the FP recombination process appears to be very efficient and thus the predominant point defects are interstitials due to the implanted ions. Their concentration is similar to the dose for self-implantation61 and with a concentration peak around the projected range of the implanted species for non-amorphizing implants.

Upon annealing these excess interstitials do not exist as free interstitials but instead coalesce into extended defects and clusters. With additional annealing, the dissolution of certain types of these defects can have a dramatic influence on dopant diffusion causing an effect called transient enhanced diffusion (TED) as will be discussed in Chapter 5.

There is another new aspect, which again will be described more in details also in Section 6 that is the defects produced by MeV ions. Here, the dominance of electronic stopping near the surface will change the defect evolution process. This range of implantation energies is still under intense investigation (e.g., Grob et al.62) as will be discussed.

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Figure 4-6. Energy deposited in nuclear processes (damage) profiles for B in Si, after Brice59

There are two important features. At low doses, electronic stopping results in point defects and small defect clusters. With increasing dose, the damage accumulation (amorphous layer) approaches the surface. Second, at 400K (and above) self-annealing occurs, which manifests itself as an "incubation" or delay in the onset of amorphization near the surface with increasing dose. In this model, the authors use the heat conduction equation to solve the energy relaxation. However, to compensate for the temperature dependence of thermal diffusivity, a value of D = 0.005 cm2/sec, suggested by Kelly63, was used. From model parameters it can be concluded that inter-cascade recombination of FPs is enhanced by implantation temperature, while their recombination probability to form clusters or via intra-cascade processes, is constant up to room temperature, RT.

In the work of Holland et al.64, the damage formation mechanism of 1.25 MeV Si+ in Si at 77K is different from room temperature studies. At this temperature the FPs maintain their spatial position inside the cascades well after quenching. Thus, the crystalline to amorphous (c/a) transition can occur as expected (Swanson et al.65), i.e., when the free energy of the defected region reaches that of the amorphous state.

This mechanism is supported by the fact that the size of the isolated amorphous regions has a value of 3 to 5 nm along the whole ion track. The damaging mechanism for RT implantation is different (Holland et al.66), where a homogeneous nucleation of the amorphous phase is observed as a result of the interaction of FPs.

The evolution of defect structures during implantation can be monitored by in-situ conductivity measurements, as demonstrated by Battaglia et al.67. They observed a decrease of approximately four orders of magnitude first which is followed by an improvement of two orders of magnitude back in conductivity as a function of dose. Implanted dose of 400 keV Si and 2.7 MeV Pt ranged from 109 to 1015 cm-2. The drop in conductivity for lower doses was attributed to production of charge compensating deep defects. The return of certain amount of conductivity is believed as a result of formation of extended defects and amorphous zones. A model was also presented to correlate defect structure with conductivity. There is, however, certain probability that current flow in conductivity measurement may influence details of the damage structure formed, as shown by results by Erokhin et al.68. This latter result joins earlier studies by different groups69 based on theoretical calculations of Morozov and Tetelbaum70 considering the effect of an external field of any kind during irradiation. Implantation of light elements, including hydrogen, is becoming increasingly important in a certain application in semiconductor technology. MeV He implants at room temperature produce only

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point defects and a few extended defects down to Rp (e.g., Kotai et al.71), thus type and distribution of defects can be well controlled. Golanski et al.72 have shown that the as-implanted morphology is set by the thermal stability of clusters and their growth is determined by the random diffusion of primary defects to these clusters. These defects can be used to reduce the lifetime of minority carriers allowing one to set the lifetime by varying the depth and the dose.

Details of defect aspects of MeV-energy implantation relevant to lifetime tailoring was re-viewed in, e.g., by Cheung73, and in papers in the Proceedings of E-MRS Spring Meeting, Symposium C: High Energy Implantation.74, and by Svensson et al.75. Though this application addresses only a small section of bipolar devices, namely, power devices, its feasibility, ease and, thus, its potential technical importance has been shown in practice.

Reduction of minority carrier lifetime in low carrier concentration materials is set basically by recombination centers in the space charge layer of the device. To date, industry practices, except for some laboratory scale development, use lengthy diffusion doping of with noble metals, as Pt, Au, to achieve the desired reduction of lifetime. These noble metal atoms are often called as "lifetime killers". The process is extremely sensitive because of proper control of concentration and depth by diffusion, especially, for fast interstitial diffusants is difficult. For smaller devices, this difficulty is even enhanced. Therefore, introduction of lifetime reducing recombination centers by light ion irradiation has a potential advantage. Problems with reproducibility are automatically eliminated using implantation. It is clear that light ions can be a preferred choice, because of deep penetration and that mostly point defects are generated, thus, device deteriorating effects of extended defects will not be encountered. Crucial is, of course, thermal stability of intentional defects, but this is also comparable with stability of recombination centers associated with distribution of the above-mentioned fast diffusants, as annealing characteristics of rather simple defects is known long since76.

As a major advantage, the technique allows late-in-process customisation, as light ion implantation can be the last step in wafer processing. MeV protons are energetic enough to penetrate films, even protective layers. Ion energy can be set to stop within active part of the device to produce desired defects with desired distribution. Batch of wafers can thus be processed by a standardized technology using low-doped silicon and the required catalogue data will be set by this final implantation. The technique is easy also in that sense that the necessary doses are very low, they range from 109 to 1010 ions/cm2 depending on ion species. Protons or helium ions suit the best, because their smaller damaging effect favors better dose control.

Point defect engineering using high energy ion bombardment can also be used as a method to inject vacancies near the surface region with excessive interstitials created near the end of the projected range. MeV Si implantation into a Si substrate can suppress boride-enhanced diffusion.77 This concept of boron diffusion control can be used as an approach to form ultra-shallow junctions. Junction depth of less than 10 nm (at 1x1017 cm-3 according to carrier concentration profiles) has been reported.78

4-2-2 Indirect beam effects: Recoil Recoil implantation is inherent with the implantation technique: forward peaking of the momentum causes surface atoms to move deeper leaving vacancies behind. This is why implantation is accompanied by volume changes (Gyulai et al.79). Defect aspects associated with recoils are manifold. For MeV energies, most of the effects associated with subcascades (intra-, and inter-cascade events) are essentially recoil effects (cf. 2.1).

In case of lower energies, oxygen from screen or native oxide will be recoiled into the near-

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surface area. Oxygen concentration can reach 1021 atoms/cm3 just below the surface, fortunately, the concentration rapidly decreases. Electrical characteristics of the near-surface region are dramatically influenced by recoiled oxygen. This can lead to a dependence of these near surface electrical characteristics on the pre-treatment of the surface and the implant conditions. In addition the recoil of oxygen into the crystal affects the regrowth of the amorphous layer as will be discussed in Section 5.

Another area recoil implantation is important is in the concentration of end of range dislocations that form below the amorphous crystalline interface. In this case, the recoil of atoms from the amorphous layer into the crystalline material appears help explain the high concentration of interstitials in the end of range defects.

In order to form ultra-shallow junctions, much effort was spent on the low energy recoil implantation, because of a generally accepted picture that a recoiled dopant profile will become deeper with higher implantation energy. However, it was found recently that 500 keV high energy recoil implantation produces a B profile shallower than that produced by lower energy (10 and 50 keV) implantation.80 10, 50, and 500 keV Si ions were implanted through a 0.4 nm B layer deposited previously by e-beam evaporation onto an n-type (100) Czochralski-grown Si wafer. When the incident Si energy is increased, recoil boron distribution in the surface region deviates significantly from an exponential decaying function expected for a typical recoil distribution. The higher the incident energy, the more rapidly the recoil boron distribution falls off. This is consistent with the fact that the cross-sections for large angle deflection become significant when the primary ion energy is increased, forming a sharp surface concentration profile within the first few hundred angstroms in the implanted direction. In addition to introducing boron into Si, high energy Si implantation also creates excess vacancies close to the silicon surface suppressing boride enhanced diffusion, allowing the formation of ultra-shallow junction in the 10 nm range.78

4-2-3 Implantation with molecular ions Molecular ions are gaining new attractiveness with device scale-down, because ion sources

and optics loose yield with lowering acceleration energy. Physically, molecular ions represent a condition where simultaneous cascades overlap partially both in time and in space. It is commonly accepted that a fast molecule hitting the surface falls apart to its constituents, each taking energy and momentum according to laws of conservation. As heavier species conserve more energy, the penetration depth of constituents will not be very different, usually nearly overlap. This mechanism results in an enhancement in damage production rate as compared to single ion implants. From reasons above, molecular ions are very popular since early days of implantation (Müller et al.81), because it was discovered quite early (see paragraph 5) that implantation amorphised layers anneal better and yield higher electrical activation than layers reordering from partly damaged ones. To date, two alternative practices coexist for formation of p+ contacts. One uses of BF2

+ ions, another one is known as ‘pre-amorphization’ or ‘dual implantation’ (see 4-4-3), where e.g., self-ion implantation is used to form an amorphous layer preceding the doping implantation. However, as no full agreement in industry practices was reached, one may think of other problems of using molecular ions. One is very general, as all practical molecular ions – including today’s decaborane – contain other ions which also get implanted and may play a detrimental role. E.g., the exact role of the fluorine in BF2

+ was never clearly pinned down. Most probable reason of some diverging results is that the final F-distribution and its amount, is very much dependent on later thermal treatments. The same worry might emerge when studying results with decaborane ions to form shallow pn-junctions, where the as-implanted state must contain more hydrogen than boron.

In order to realize ultra-shallow junctions (<60 nm), the scaling of the implant energy alone is

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not sufficient, because ion channeling during implantation, and/or TED of dopants during annealing start to dominate, resulting in deeper junctions. The enhanced damage of molecule or cluster implants allows amorphization even at moderate doses. The amorphous layer is desirable for shallow junctions, because it can reduce ion channeling and also inhibit the TED of dopants during subsequent annealing. The effective implantation energy of the dopant depends on the molecular weight of the implanted species. Ganguly et al.82 reported reduced junction depths for B-doping using BCl2

+ (molecular weight = 82) or BBr2

+ (171) instead of BF2+ (48). The effective B energy is diminished approximately by one

half for BCl2+ and one fourth for BBr2

+, compared to BF2+. This entails a reduction of the projected

range. The heavier species cause amorphization and reduction of channeling at lower doses. This enhanced damage can be a disadvantage when using sub-amorphizing BCl2

+ and BBr2+ implants: it

translates into higher residual defect density and a resulting degradation of the leakage current density. Furthermore, F out-diffusion triggers micro-cavities and, as a consequence, introduces trap states when using BF2 implantation at 20 keV and a dose of 2x1015 cm-2 with a subsequent excimer laser annealing. Implantation of low energy (<1 keV) B-ions followed by laser annealing with properly chosen fluence was shown to produce better result, with ultra-shallow profiles extending to a depth as low as 35 nm.83

Recently, another molecular ion implantation received attention in the thrust towards shallower junctions, because the most straightforward solution, i.e., to decrease ion energy using lower acceleration voltages, is technically difficult. At ultra-low energies, extraction and transport of ions at reasonably high currents become a critical problem. Consequently, great interest has been aroused by results demonstrating the formation of shallow junctions by implantation of cluster ions. One example is decaborane (B10H14). Although Agarwal et al.84 showed that hydrogen in decaborane molecular implantation does not contribute to transient enhanced diffusion (TED), some other effects, however, may be connected with the presence of hydrogen. Junction depth below 50 nm was reported using decaborane ion implantation and RTA.85

MD simulations comparing single boron (B1) implantation with B8 or B10 show that while for B1, vacancy-interstitial mechanism holds and knocked-on lattice atoms mostly remain tetragonal interstitials, for B8 and B10, large number of displacements occur as a consequence also of multiple collisions (Aoki et al.86). This damaged region is considered to be amorphised and appears as a box-like shape from the surface to a depth of 20 Ǻ for an implantation energy of 5 keV (500 eV/atom) for B10, which is comparable with the mean implant depth of 500 eV B atoms. Considering MD simulation of damage accumulation process of 5 keV B10 , the number of displacement increases more rapidly compared to 500 eV B1 impact, and the surface is well amorphised at the atomic dose of 1x1015 cm-2.87 4-3 FORMATION OF AMORPHOUS LAYERS

In this section the reader is expected to be familiar with basic defect formation models also treated e.g. by Ryssel and Ruge5, such as the modified Khinchin-Pease approach of Morehead and Crowder88. In these models, the amorphization takes place, when the number of displaced atoms in unit volume reaches the atomic concentration, i.e. all atoms are removed. A related term is the critical dose for amorphization by a dopant at a certain energy14. As a rule of thumb, for medium-to-heavy implants in silicon, this dose is a few times 1014 ions/cm2. It should be pointed out that in the implanted amorphous state,

* the short range order is believed to be conserved, however, there are UPS findings, which bring doubts on the conservation of sp3 -hybridization (Petö et al.89), at least near the

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surface, * the "onset" of amorphization depends on the detection method (optical methods vs.

Rutherford Backscattering, etc.). As mentioned, amorphization during irradiation starts, when the free energies of the defected

state and that of a-Si equalize (Swanson et al.90). With this in mind, the following mechanism was proposed by Holland et al.91 based on detailed experiments with silicon self-implants. First, the amorphization starts, when fluctuations in divacancy density around dislocation loops are reaching conditions of spontaneous c/a transition. Once the amorphous silicon starts to nucleate, a rapid increase of divacancy population occurs consistent with early work of Vook and Stein92. The process is connected with gettering of point defects at the amorphous regions, such as interstitial-type defects. For higher doses, interstitial (I) defects find sinks around small amorphous regions. Here, divacancies are generated, which lead to further amorphization. Below the critical dose, the damage accumulation is restrained, then comes a rapidly growing stage that leads to amorphization.

In an atomic picture of implantation, the as-implanted state is set by competition between instantaneous amorphizing effect of the beam and its self-annealing behavior, which can be of thermal (spike) and non-thermal origin.

This sensitive balance can be shifted into either direction depending on the purpose of ion beam modification. For example, if in Ion Beam Synthesis (Chapter 5) the crystallinity of the surface layer is to be preserved, low dose rates are recommended. On the contrary, if effective doping and activation is the concern, high dose rates would bring the surface quickly in an amorphous state. (Annealing of the layer is more effective once the amorphous state has been reached, Chapter 5).

Damage production, its rate, its forms, has always been in the center of implantation research because of its decisive importance on applications. In earlier editions of this book, this problem was reviewed more in details. However, a few practical comments on damage formation and amorphization will be dealt with. Already the first published data for the “critical dose for amorphization, Dcrit” by Morehead and Crowder78 for 200 keV boron, 40 keV phosphorus, arsenic, antimony, and bismuth, are still accepted as good values embedded in simulation programs like SUPREM, etc. (Fig. 4-7). The model for damaging (modified Khinchin-Pease) published in that paper is also a good approximation to determine Dcrit. The Dcrit depends, of course from relative masses of the ion and target, ion energy but most critically on implantation temperature, Timp. This is understandable when the temperature dependence of κ and the role of dynamic annealing is considered.

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Fig. 4-7. Logarithm of critical dose as a function of implantation temperature for 200 keV B+, and 40 keV P+, As+, Sb+, B+ implants in Si. Data are measured points, solid lines are calculated by their model (Morehead and Crowder78).

For RT implantation of heavy ions into silicon, i.e., where nuclear stopping is dominant, even a simple Khinchin-Pease57 approximation leads to reasonable results. First,

E/FN DD 2=

allows reasonable values for Dcrit. Here, ND is number of displaced atoms (after the cascade is over), FD is the energy deposited into nuclear processes, ED is threshold energy for an atom to remain displaced, again, after the cascade is over. For relatively heavy projectiles, FD can be expressed by the nuclear stopping power and ion range, Rp. For the above conditions, Rp is expressed by the nuclear stopping power, (dE/dx)n ,

( ) ( ) .nE

p dx/dE/Edx/dE/dER 0

0

0

== ∫

Thus, .E/EN DD 20=

Let Q denote the incident ion dose (in units of ions/cm2), then the surface density of displaced atoms is QND. Thus, the volume density of displaced atoms, Nd, is

( ) .E/dx/dEQN dnd 2=

The condition for full amorphization (every atom displaced), i.e., to calculate Dcrit from the above, is

,Sid nN =

nSi being the atomic density of silicon in, e.g., atoms/cm3. Value of Ed is typically 15 eV for semiconductors and insulators, 30 eV for metals. The nuclear stopping for the energies considered is

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around 100 to 2000 eV/nm. This gives, e.g., for 40 keV phosphorus (with (dE/dx)n = 500 eV/nm) Dcrit = 5.1014 cm-2 .

We refer to the TRIM program93 not only for data mentioned here, but the program is capable to calculate damage (vacancies) with fair approximation.

It is to be pointed out that the amorphous state plays an important role in implantation applications. The regrowth (Solid Phase Epitaxial Regrowth, SPEG) of an amorphous layer from an undamaged crystal plane underneath as a seed, is usually more perfect than annealing of a partially damaged layer. This will be discussed in detail in Chapter 5.

4-4 ANNEALING

4-4-1 Annealing Strategies

Because the implanted species is not substitutional upon implantation and the crystal can be severely damaged during implantation, a post-implantation annealing step is necessary. With shrinking device dimensions, the ability to avoid effect of defects (also of interfacial impurities) by simply driving "functional" interfaces (e.g. junctions) away from the defect location, has to be balanced with the need to reduce junction depths. This also holds for demands on transition regions between implanted and non-implanted layers. The expectation of minimum of atomic motions during technological processing brings in a new complexity to damage annealing and activation.

Another push is expected to come, if complex 3D-structures are being developed. One would expect growing demand to continue to reduce heat treatments, in an effort minimize dopant diffusion both vertically and laterally. Though the excellent heat conduction of silicon takes care of a rather quick thermalization in any case, heat and time cycles, often quoted to as the thermal budget of the process (Section 4.4), are extremely important.

Theoretically, an implantation at low temperatures with molecular ions is the most adiabatic annealing, but only in a volume of a single cascade. Pulsed beams of lasers or particles represent still adiabatic heating with huge quench rates. Focused ion beams and so-called beam annealing implan-tation belong to an in-between case. While RTA and furnace annealing (FA) have long enough duration in ramp-up—anneal—ramp-down processes that the bulk of the wafer is also heated up to the equilibrium temperature. Thus these methods belong to the isothermal approach. At this time, isothermal annealing is the method of choice for IC processing. In general most annealing can be divided into either annealing during implantation or annealing after implantation. In order to choose among these methods a better understanding of the differences between the methods is needed.

4-4-1-1 Annealing Strategies; post-implantation (adiabatic, isothermal)

The different annealing methods can be categorized through the * temperature X time cycle they produce, * and the volume in the wafer they heat up with its time and spatial dependence. According to this, the methods are adiabatic or isothermal, or somewhere in between. This

behavior is set by the usually radiative heat source, by the absorption depth (wavelength) and by the wafer temperature with its actual thermal conductivity, as discussed earlier. In addition, annealing can be treated as a special case in Defect dynamics. In this latter, extra defects which promote defect

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reactions, are introduced * isothermally (furnace, RTA), or * adiabatically (photon, electron, ion pulses), or * homogeneously (furnace), or * inhomogeneously ** in space (scan or full wafer) and/or in ** time. In the adiabatic case, the excitation can be 'athermal' followed by a thermalization

(equalization of electron and lattice temperature) in about 10-11 s. The end-state is a metastable state. Metastable states have certain instability characterized also by an activation energy. This activation energy must, however, be large compared with kT thermal energies (k being the Boltzmann constant). If this is not the case, practical applications are excluded. If one looks at how different technological steps with various activation energies became accepted in industrial applications, a condition of ∆E ≈ 20 kT seems to be a minimum to success, but a better value is around 100 kT.

The relaxation of a metastable state using implanted antimony in silicon and annealing with a pulsed laser, was demonstrated first by Revesz et al.94. The wafer was implanted to a level exceeding solid solubility and it was annealed with a laser pulse. After this treatment, all implanted atoms were electrically activated. This wafer was then subjected to an isochronal annealing sequence together with an as-implanted control specimen. Surface conductance of the activated wafer started gradually to decrease for temperatures over 600 °C and, finally, it met the increasing value of the control wafer at 1000 °C anneals. This shows that metastable state of a couple of tenths of an electron volt show degradation at relatively low temperatures. If the annealing process would be applied in devices, the structural relaxation would lead to an enhanced aging in device structures.

Adiabatic heating (heating in which there is a very high temperature gradient within the wafer) can be done with pulsed beams of lasers or of particles, like electrons or ions. The deposited energy can be high enough to cause melting of the surface layer, but with good control, it is also possible to do solid state processing. The temperature gradient can also be tailored. Using an irradiation with long-wavelength laser light or with penetrating electrons, a uniform heating (decreased thermal gradient) is possible. On the other extreme, large gradients can be achieved, if short pulses of nanosec or even picosec duration are used with shallow penetration in silicon, for example with UV light or heavy ions.

Recently, there has been an increase in interest in the use of adiabatic methods for processing. For example, the use of laser melting and regrowth for shallow junction formation (pulsed laser assisted deposition (PLAD)) is seriously being investigated. In this method the surface is melted in the presence of a doping gas and a thin heavily doped layer is produced. Again concerns over uniformity may limit its acceptance.

Though treatments converting the silicon surface into a liquid phase might also find uses, e.g., in lateral epitaxial growth with lasers, electron or ion beams or, simply, with strip heaters. We will focus only on techniques, which find practical application in today's silicon technology using ion implantation as a processing step. The time cycle possible during annealing covers many orders of magnitude

• lasers, e-beam or ion pulses, i.e., from below pico- to nanoseconds, thermalization • free running lasers, flash lamps, i.e., milliseconds • Rapid Thermal Annealing, RTA, i.e., seconds

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• Spike Annealing, an operation mode of an RTA, with short pulses and ~150 °C/s ramp-up rate,

• furnace annealing, i.e., minutes or more. As the time increases, the method shifts from being adiabatic to isothermal. Because of their

duration, and the heat conductivity of silicon, RTA and Furnace annealing are considered to be isothermal. Furnace annealing is still the method of choice by the silicon processing industry.

Lamp heaters or Rapid Thermal Annealing, RTA, systems have also became a common tool. Although their acceptance by industry has been tempered by concerns over temperature measurement and possibly uniformity. A possible advantage to RTA from a simple processing standpoint is that the equipment can serve not only annealing purposes, but it may have a controlled supply of process gases, too. This allows layer processing on a time scale from seconds to minutes, thus, we call then the equipment and the associated technique Rapid Thermal Processing, RTP. RTA systems have found more extensive application in the processing of GaAs based microelectronics because of the tendency of the GaAs wafer to degrade during extended annealing cycles. RTA may see increased use in the future in shallow junction formation as transient enhanced diffusion is reduced for RTA when compared to furnace annealing. This will be discussed more in Chapter 5.

A basic difference between FA and RTA is that the maximum of thermal radiation is at different wavelengths: ~2 µm vs ~0.8 µm. Photons with the latter wavelength are already well absorbed in the surface region and produce e-h pairs, or are differently reflected by films already on the wafer. Photonic effects on annealing are known since long95. Recent studies revisited the topic to explore possible applications. Fair and Li96 measured carrier deactivation to its equilibrium value in As implanted silicon as a consequence of generation by short wavelength photons. Time-temperature cycles were practically identical for the furnace and RTA used. The rapid deactivation in RTA caused the excess self-interstitials to contribute to the growth of end-of-range disorder, which, later, resulted in TED. In the furnace, however, strong deactivation caused carrier concentrations to fall below equilibrium values, but no TED was observed.

A mode of operation of certain RTA systems is the so-called "spike" mode, where ramp-up rate is ~150°C/s, anneal cycle is short. Spike anneal has certain advantages, as slower processes will not run in this mode.

From practical point of view, implant-RTA-FA tradeoffs to minimize, e.g., diffusion in shallow junctions, is a rather complicated procedure and, possibly, will never reach a state of a general recipe. A summary of parameters and tradeoffs was published recently by Murrell et al.97. Advantages of 'spike' annealing to simultaneously reduce Xj and Rs, thus, to get high activation in ultrashallow junctions, a boron implant with below 1 keV energy and short annealing, spike mode, are recommended.

Construction of RTA equipment is not simple. The measurement of the real and exact temperature under rapid changes and on a structured surface with different emissivities is not an easy task. Demands on response time also set basic construction principles. For example, the fastest response comes with blackbody radiation behind the wafer, but then the power consumption is immense. Built-in reflectors make cool-down cycles slower, but power can be reduced. If the ambient has to be controlled, the problem of windows must be addressed, as they also contribute to hysteresis. Finally the system should produce uniform heating for wafer diameters of 8 to 12 inches. For all these reasons furnace annealing continues to dominate as the preferred post-implantation processing step.

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4-4-1-2 Annealing strategies: during-implantation (beam annealing, hot implantation)

Beam annealing. The idea of self-annealing implantation is probably as old, as the technique itself. However, to understand beam annealing a thermal balance between the wafer and the beam must be kept under the utmost control. The newest machines are reliable in this respect. This is probably the reason that many detailed investigations of the problem are being conducted these days.

No extensive practical applications are known as yet known for beam annealing. In fact in most IC processing efforts are made to minimize the beam annealing by using various wafer cooling schemes. One reason for this is the increased damage done to the crystal below the amorphous/crystalline interface when beam annealing occurs. This will be discussed in greater detail in Chapter 5.

In beam annealing, as has already been mentioned, two basic phenomena compete: • Sample heating by the beam when wafer cooling is inadequate • Local heating inside the already implanted layer by new impinging ions. Temperature from the first comes from balance of radiative and conductive cooling vs

absorbed beam energy. The temperature from the second factor is less exact and may include many non-thermal processes, like Radiation Enhanced Diffusion, RED.

Methods to be reviewed are quoted often as Self Annealing Implantation, SAI, and Ion Beam Induced Epitaxial Crystallization, IBIEC, or Amorphization, IBIA. All these techniques involve similar physics. Namely, the outcome, again, depends on the balance between two simultaneously occurring effects

• amorphization by implant damage and • crystallization by beam heating. Thus, the beam annealing discussed here is a case of amorphous layer regrowth which is the

subject of Chapter 5. However, since its also a case of in situ annealing it will be presented in detail here, and amorphous layer regrowth via furnace annealing outside the implanter will be presented in the next Chapter. According to Berti et al.98, in case of SAI three stages can be found as implantation proceeds:

• amorphization stage, • beam assisted recrystallization, • post-annealing stage. Though the technique has "fortunate" features, e.g. the crystalline layer at the surface is

quickly amorphised, and thus the regrowth comes from the deeper amorphous/crystalline (a/c) interface, the devices produced by SAI were of moderate quality only.

IBIEC is a similar technique, but an amorphous layer exists before modifying implantation (Alestig et al.99), and the epitaxial growth is caused by the beam. The efficiency of IBIEC is connected with

• nuclear stopping and • generation and migration of point defects. The balance between crystallization and amorphization is set again by migration of the point

defects. If they reach a/c interface, epitaxial recrystallization will occur, if not, the amorphization will propagate (IBIA). The tailoring of the mobility of point defects is done by setting proper wafer temperature. Thus, growth rates can be comparable to that of with thermal growth (Chapter 5). In

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analogy with ion beam mixing, and as a consequence of additivity of the effects of the individual cascades, the annealing time during thermal regrowth is analogous to the ion fluence during beam annealing.

Basic findings, as summarized by Elliman et al.100 were, as follows: • a critical temperature (318°C) separates the regions of IBIEC and IBIA, • the activation energy for growth with MeV Ar+ ions ≈ 0.24 eV, • the grown layer thickness is proportional to ion fluence, • growth rate was at 318 °C: 6 nm/1016 Ne+/cm2. In an annealing study of IBIEC, one envisions the Tcrit as a dynamic consequence of • continuous amorphization by incoming ions, • formation of crystalline regions from point defect migration to the interface. There may be a correlation between Tcrit for IBIEC and Tcrit for implantation with molecular

ions. In a paper already cited55, the concept was applied for a volume around a single cascade, too. Hot Implantation is an annealing concept, which was around us with moderate success from

the beginnings. Considerations introduced earlier in this Chapter on consequences of the decrease of thermal conductivity of silicon with increasing temperature, allows atomic motion in the ion cascades to remain active on a longer time scale and reduces the tendency of the interstitials to form stable defects. Thus, implantation at elevated temperatures produces conditions, which favor defect migration, which, in turn, leads to enhanced crystallization. This is true to a certain extent for diffusion also, but the conditions there are more complex. Mostly used application of hot implantation is the production of buried SiO2 layers in silicon, the so-called SIMOX (Separation by Implanted Oxygen). Hot implantation was first used by Jaussaud et al.101

In the example below, the heating was a result of direct wafer heating and of the heating effect of the beam (dose rate was 0.2 µA/cm2). Aluminum was implanted into silicon at elevated temperatures (Holzer et al.102). Fig. 4-8 shows the basic results on channeled Rutherford Backscat-tering spectra. The crystalline quality of the silicon can be deduced from the curves

Figure 4-8. Channeled RBS spectra of 160 keV, 5x1015/cm2 implanted aluminum in silicon at different temperatures (Holzer et al.89).

Up to an implantation temperature of 600°C, the amorphization by the beam was the dominant

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process. A turn-over was found for higher temperatures leading to a perfect anneal at 1000 °C. It is to be noted that lamp heating and heating by increasing dose rate of the beam added up. Beam-induced crystallization is taking over for dose rates resulting in 600 °C wafer temperature.

In all of the discussed cases till now, hot implantation was dominated by or at least mixed with beam heating. Let us remember on considerations in Section 1 and 2. One may easily come to the conclusion that isothermal heating of the wafer can produce completely different phenomena in comparison with beam heating - even if a thermometer attached to the wafer shows the same reading for both cases.

Therefore, a special attention is given here to experiments performed on a specially designed heated stage at the Fraunhofer Institut für Integrierte Schaltungen, Erlangen. The stage is powerful enough to allow isothermal heating up to 1200°C without the beam on. Detailed experiments by Pichler et al.103 were performed under such conditions.

Thus, it was succeeded to avoid formation of extended defects for boron implants done between 880 to 1000°C. The species, as measured by SIMS, showed a marked uphill diffusion to the surface. The results were analyzed with existing diffusion models. It was shown, that only the pair diffusion theory (Mulvaney and Richardson104) can account for this kind of diffusion during irradiation. In Fig. 4-9, a set of boron SIMS profiles by Pichler et al.105 is shown. The decrease of boron penetration with increasing temperature is explained by the increased recombination rate of point defects leading to a negative activation energy of the effective diffusion coefficient.

Figure 4-9. SIMS profiles of 120 keV, 1015/cm2 boron implanted at different temperatures in silicon with a dose rate of 2.77 x 1011 /cm2sec (Pichler et al.104).

Under similar conditions, though with somewhat higher dose rate (5.55x1011 cm-2 s-1), arsenic

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was the other dopant which was investigated and found very different for implantation at elevated temperatures (Yu et al. 106). For implantation temperatures between 700 and 800°C, an elongated and very complex defect structure was found by TEM at a depth of about 3Rp. This defect layer behaved as a powerful sink for arsenic, as shown on SIMS profiles (Fig. 14-10).

However, an appreciable part of the gettered arsenic was electrically active (Fig. 4-11a-b). The extended defects disappeared for >800°C implantations and a layer with an electrical activity of >50% of the arsenic, and with an acceptable mobility of the carriers was produced. We do not suggest that hot implantation would be the right way of doing it, but these results, where the large redistribution of the dopant is also acceptable, may find applications.

Figure 4-11. SIMS profiles of implanted arsenic in silicon at 500, 700, 800, and 850°C. Note the accumulation on damage sites for 700°C (Yu et al.106).

4-4-2 Thermal cycling (TC), thermal budget This term is often used to characterize total thermal load during processing. In VLSI, ULSI

technologies, the thermal budget must be kept as low as possible to avoid deep dopant penetration by diffusion. This minimization is also expected to satisfy conditions for adequate dopant activation and adequate defect annealing. Figure 4-12 is a graph from Hill107, which can be used to illustrate TC. The time duration of annealing is shown vs process temperature. Calculated are diffused distances of boron at a given TC using a simple diffusion model [3√(Dt)]. Allowed temperature-time cycles are below those lines of acceptable junction deepening. This, of course, ignores transient enhanced diffusion (TED) which can dominate under certain conditions as will be discussed in Chapter 5. Thus, if the annealing is to be optimized one needs to also use a knowledge of defect generation during implantation to produce a structure which, upon annealing with the allowed thermal budget, will minimize the final defect density and TED.

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Figure 4-11. Carrier concentration (o) and mobility (∆) profiles for arsenic implanted silicon at 700 and 850°C (compare with total dopant concentration profiles on Fig. 4-11, Yu et al.106)

Figure 4-12. An optimization of thermal cycling by setting acceptable dopant redistribution, i.e., shift of junction depth, then demanding a possibly high electrical activity of the implant, the graph yields preferences for types of annealing, from furnace to thermal flux (after Hill107). A similar plot including damage annealing, too, will be presented in Chapter 5.

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4-4-3. Dual implantation doping, pre-amorphization The idea of a dual implantation and a double anneal, as proposed by Csepregi et al.114, is still under intense studies. The answer to this interest is the demand of shallower, i.e. now xj ≈ 60-70 nm deep junctions, as ion channeling, especially, for light dopant (boron) causes extra penetration with bad reproducibility. The dual (low+higher-temperature) SPE growth restores perfect crystallinity, and a doping implantation would be annealed at the same time and, if thermal budget is kept low, only small redistribution is expected. These features favor VLSI-ULSI demands. Already that first reordering experiment with implanted amorphous layers post-implanted with P, B or As resulted in acceptable electrical properties of the dopants. The experiment of Ishiwara et al.108 showed that in the above process implantation amorphization is simply a good option, but the technique works with post-implanted, UHV evaporated amorphous silicon, too. The interest from industries started, when Tsaur and Anderson demonstrated the technique on good-quality MOSFETs109. Soon after, the advantageous use of Ge instead of Si was proposed by Seidel110 is the most applied today (Section 7)

4-5 ELECTRICAL ACTIVATION

Implantation as a doping process suffers from its non-equilibrium nature. Concepts, like substitutional lattice position of the implant dopant atom or solid solubility may not apply here.

Depth dependence of dopant concentration, of defect type and distribution, clustering of implant and defects cause not only often uncontrolled defect reactions, but at the end, complicated charge compensation effects may occur resulting, e.g., in "deactivation" of the implant. Or, simply, the desired "full electrical activity" will not be reached. An "overkill" with implant dose is a practice, however, it was not up to the smaller and smaller devices that problems arising from presence of dopant atoms at irregular bonding states in the lattice, i.e. charge compensation by deep levels were studied in details. Since early days, the implantation community accepted that there are "off-limit" applications for the technique. These were applications, where good lifetime of minority carriers or high mobility (low carrier scattering), etc., were playing a crucial role, e.g., in analog HF devices. Despite successes in moving into new areas, what is new today that with shrinking device dimensions it is not easy to keep even "traditional" areas not to fall over those off-limit barriers.

4-5-1. Kinetics of activation In the very early editions of this book, phenomena connected with activation, like "reverse

annealing" were also discussed. This latter occurs for medium and high dose implants. During annealing, the substitutional fraction of the as-implanted atoms is first kicked out from lattice sites by the host silicon atoms, and it occurs at higher temperatures that dominant, or at least a useful fraction of the dopant will again occupy the lattice sites and act as donor or acceptor.

4-5-2 Electrically active defects

Characterization of electrically active defects applies standard techniques, like Spreading Resistance Probe, SRP, van der Pauw measurements, Deep Level Spectroscopies, DLS, lifetime

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testers, microwave, Elymat, etc. In many cases these techniques are destructive, but having excellent sensitivities, are still unavoidable. Electrically active defects are "lifetime killers", traps, SHR recombination centers, or may effect optical properties of the crystal and device. A comparison of dopant atom distribution measured, e.g., by SIMS, with the electrically active profile with SRP is one of the basic measurement preceding device characterization. In many cases, beveled etched, polished samples are used.

In a recent paper by Priolo et al. 111 reported on a detailed study of the dependence of the electrically active fraction of boron on native impurities of the silicon crystal and on vicinity of the surface. Dependence on native impurities was made upon a comparison of Czochralsky, CZ, float zone, FZ, and epi silicon. All were implanted with 5 – 160 keV boron ions and annealed identically (900 °C, 20 s) in RTP. The three substrate crystals differ only in their oxygen and/or carbon content*. The experiment answered old controversies in practice. Fig. 4-13 shows the electrically active fraction of boron for identical dose of 1x1013/cm2 and for different energies (represented by

Fig. 4-13. Electrically active fraction of boron for 5-160 keV implants represented with Rp for CZ and epi-Si (after Priolo et al. 111)

projected ranges in the figure). While boron in the epi-silicon was 100% active, activity in CZ-Si grew with ion energy up to 70% as maximum. Interaction of boron with O and C reduces electrically

Fig. 4-14. Electrical active fraction of 5 and 40 keV boron implanted with different doses into CZ- and epi-Si, annealing was at 900 and 1100°C, 20 s (after Priolo et al. 102)

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Impurity (/cm3) CZ FZ epi oxygen 1x1018 1x1016 <1x1015 carbon 1x1017 1x1017 <1x1015

active fraction through formation of, as was suggested, by interstitial boron/substitutional carbon and/or interstitial boron/interstitial oxygen pairs. Vacancies can contribute to dissolve the pairs and restore mobilities even in the presence of O and/or C. Vacancies, however, in the near surface region (~100 nm) can annihilate at the surface. Thus, despite their momentum generated abundance, the boron deactivation is even stronger there. Interesting to note that the thickness of surface region for this effect coincides with the effective trap-limited diffusion length of point defects in CZ-Si112 . Priolo et al. compare boron activity for different doses and energies in the three different substrates. Fig. 4-14 compares results with 5 and 40 keV versus epi and CZ-Si for 900 and 1100°C, 2m s anneals. For 40 keV, while in epi-Si the boron is fully active for all doses and anneals, the deactivation disappears with growing dose, and above 1014 /cm2, the two substrates behave identically. The picture is very different for 5 keV. Here, in epi-Si the deactivation enhances with dose the active fraction falling from 90% to 50%. This may be attributed to boron interstitial/substitutional pairing, which shows similarity to the effect observed by Stolk et al.113. The 5 keV implant in CZ-Si shows the same trap saturation behavior as the 40 keV implant pointing on difficulties to produce efficient shallow junctions in traditional material. Formation of B clusters during the early stages of annealing in the presence of a high interstitial concentration in 40 keV 2×1014 cm-2 B-implanted silicon was revealed by Pelaz et al.114 The dissolution of B clusters takes place very slowly with a quasi-equilibrium Si interstitial concentration.

For even lower energies, 500 eV, Schroer et al.115 demonstrated boron clustering visible as 'kinks' in SIMS profiles for different stages of annealing.

Electrical activation of ultra-shallow implants can be enhanced by carefully choosing the method of implantation (beam, plasma immersion, recoil), the implanted species (single ion, molecule, cluster or co-implantation with other species), the method of annealing (laser, e-beam, ion pulse, RTA, furnace), the annealing scheme, the ambient (oxidizing or non-oxidizing), or other techniques like preamorphization or high density current stressing. The latter is an effective method for the elimination of end-of-range defects in BF2

+-implanted SOI structure as shown by Lin et al.116 One of the most important technological problems is that during high temperature processing

TED of the dopant occurs. The TED process causes major redistribution of the dopant to depths well beyond the initial implant range. The dopant behavior during thermal processing is dependent upon the initial implant damage formed and the anneal condition employed. Although defect attraction to the surface during thermal processing reduces the defect concentration in the solid, with shrinking device dimensions the dopant concentration is raised in order to maintain a low sheet resistance. For doses normally used for device fabrication, the effects associated with cluster formation and dopant accumulation in the oxide can become significant for ultra-shallow profiles. The defects released from the dopant cluster at high temperature are able to contribute to enhanced diffusion. For ultra-low energy (3-0.25 keV B) implants investigated by Privitera et al.117, the complexity of the defect agglomerates formed by the energetic ions increases as the implant dose raises, due to the higher concentration of the doping impurities available to be trapped into the defect clusters. The electrical activation of ultra-low energy B implants with a dose of 1×1014 cm-2 is ruled by a thermal activation energy of 2 eV, whereas this value is 2.8 eV when the dose is increased to 1×1015 cm-2. This information helps to identify best conditions to maximize electrical activation, while minimizing diffusion.

The requirements needed to be achieved for future CMOS technology nodes, i.e. junction depth of 30 nm, sheet resistance lower than 500 Ohm/square and profile abruptness of 2 nm/decade,

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cannot be obtained by conventional RTA. Alternative procedures are needed to produce the required ultra-thin doped layers either by the development of the doping method, or the thermal process used for activation of the dopant. As an example, excimer laser annealing (ELA) was used for the dopant acticaton in ultra-low energy implanted Si. Junction depths below 50 nm can be formed using low energy ion implantation followed by ELA.,118 ELA provides very rapid thermal cycle (few tens of nanoseconds), leading to dopant activation with negligible diffusion. Using ELA, several problems of RTA can be solved including enhanced diffusion of B promoted by the dissolution of small aggregates of interstitial defects, and the trapping of B in defect clusters close to the surface, which needs high annealing temperatures to dissolve. Implantation schemes of BF2 at 20 keV and B below 1 keV were compared using different laser fluences controlling the melting depth.83 The approach of using BF2 appeared limited, as at high laser fluences, required to eliminate the residual defects induced by ion implantation, the formation of micro-cavities is triggered by the F out-diffusion and trap states are introduced. On the other hand, low energy B implantation and ELA is very promising, as ultra-shallow junctions (35 nm) with abrupt profiles have been formed in Si.

CONCLUSIONS AND OUTLOOK TO CHAPTER 5

This chapter was intended to summarize phenomena occurring during the implantation process. No or little emphasis was given to types and forms of the final defect structure forming during the annealing cycle. However, events influencing the as-implanted state were described in detail. The importance of that state was pointed out, as thermal treatment leads to transformation, coagulation and annihilation of as-implanted defects. Temperature-time cycles affect on different as-implanted states differently, thus, they lead to different final state. Defects thus produced and their defect reactions afterwards will be dealt with in the forthcoming Chapter, which is, to the authors’ intention, a direct continuation of the present one.

ACKNOWLEDGEMENTS Authors JG and PP acknowledge the partial support of OTKA Grant # T043704. PP also

acknowledges the “Janos Bolyai” postdoctoral fellowship of the Hungarian Academy of Sciences.

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