UDK621.3:(53+54+621 +66), ISSN0352-9045 Informacije MIDEM 29(1999)2, Ljubljana ION IMPLANTATION; A MODERN TOOL OF SURFACE ENGINEERING Jacek Jagieiski, Grzegorz Gawlik Institute of Electronic Materials Technology, Warszawa, Poland and Anton Zaiar, Miran Mozetič Institute of Surface Engineering and Optoelectronics, Ljubljana, Slovenia Keywords; surface engineering, modern mehods, ion implantation, PSIl, Plasma Source Ion implantation, ion doping, vacuum, thin films, ions, ion beams, kinetic energy, depth of penetration, collisions, irradiation damages, if^/I, Ion Mixing, high energy irradiation, LSS theory, Lindhard, Scharff and Schlott theory, PKA, Primary Knock-on Atoms, TRIM code Abstract: ion implantation is a doping technique which uses energetic ion beams as vector of mass transport. The atoms to be implanted (often called impurity atoms) are thus ionized, accelerated, selected and directed towards the target. Due to the high kinetic energy (typically tens to hundreds of keV) the accelerated ions penetrate the target up to the depths from tens to thousands of nanometers. This method allows one to dope virtually ail solids with any kind of atoms with high uniformity and up to very high concentrations. The basic principles, advantages and drawbacks as well as possible fields of applications of ion implantation technique are briefly presented. Ionska implantacija, sodobna metoda za obdelavo površin Ključne besede: obdelava površin, metode sodobne, implantacija ionov, PSIl implantacija ionov iz virov plazme, dopiranje ionov, vakuum, plasti tanke, ioni, curki ionov, energija kinetična, globina prodiranja, trki, poškodbe vsied obsevanja, IM mešanje ionsko, obsevanje visokoenergijsko, LSS teorija Lindhard, Scharff and Schiott, PKA satomi izbiti prvotno, TRIM koda Povzetek: Ionska implantacija je tehnika dopiranja, ki za prenos mase uporablja curke energetskih ionov. Atome, ki jih želimo implantirati, najprej ioniziramo, pospešimo, preberemo in usmerimo proti tarči. Zaradi njihove visoke energije (značilno desetine ali stotine keV) ioni prodrejo v tarčo do globine od nekaj deset do nekaj tisoč nanometrov. Metoda omogoča dopiranje praktično katerekoli trdne snovi s katerimikoli atomi do zelo visoke vsebnosti. Temeljne zakonitosti, prednosti in pomanjkljivosti, pa tudi možna področja uporabe te tehnike na kratko opisujemo v tem članku 1. Introduction; the history of ion implantation. The beginnings of ion implantation are related to the Manhattan Project. Searching for the efficient method of fission isotopes production the scientists tested also the technique of mass separation of accelerated ions in magnetic field. This method was found less efficient than the diffusional and centrifugal isotope enrichment, however, the interest was attracted by the modification of the properties of target on which ions were collected. The observation that accelerated atoms are not deposited on the target surface but penetrate into its crystalline structure becamed a basis of ion implantation. The rapid development of ion implantation in late sixties is mainly due to the emerging semiconductor industry. From technological point of view main characteristics of implantation such as: high uniformity, precision, repetability and low processing temperature resulted in the wide use of implantation for silicon doping. Till now this technique remains the basic doping method in silicon technology. In early seventeens the development in the construction of industrial ion implanters required for efficient doping of bipolar devices resulted in high current implanters delivering ion beams in milliampers range. This made possible high dose implantations what opened new areas of applications, namely the modification of composition of metals. Quite soon the research projects devoted to study the effects of ion implantation into metals allowed to determine the beneficial role of nitrogen implanted into ferritic steels. This in turn resulted in the construction of dedicated implanters without mass separation but equipped with specialized target chambers containing manipulators permitting the homogenous treatment of details of complicated shapes. The huge market for the improvement of mechanical properties of tools leaded to a rapid development of this particular application, however, till now ion implantation into metals is used only in niche domains, mainly biomedical or highly precise cutting and forming tools. Despite two main fields of applications, semiconductors and metals, ion implantation is also used for several specific purposes. Among them the fabrication of micrometer size membranes, optoelectronic and catalytic devices fabrication seems to be of particular interest. Recently, a new promising field of applications was identified, namely ion implantation into polymers. Low irradiation doses required and strong effects observed after implantation into polymers made this idea very promising. The above description deals with classical ion implantation, i.e. the line-of-sight processes carried out at energy range of tens to hundreds of keV. Recently numerous new techniques have emerged from ion implantation such as: ion beam mixing, plasma source ion implantation or high energy ion irradiation. These methods will be shortly presented later on. 2. Description of the metliod. 2.1 Interaction of energetic ions with solids Energetic ions when penetrate the solid target loose their energy in elastic collisions with target nuclei (so called nuclear stopping, Sn) and in inelastic collisions with the electrons (electronic stopping, Se) /1/. The stopping is defined as: S = dE/dx where E is ion energy and x the length measured along the ion path in solid (total range) or along the beam direction (projected range). The dependence of stopping versus ion velocity is presented in Fig. 1. The important feature of these dependencies is that for high velocity the ion losses its energy mainly in collisons with target electrons, whereas at the end of the path, when the ion energy decreases the collisions with target Fig. 1. Variations of nuclear stopping (Sn) and electronic stopping (Se) with the Ion velocity. Please note that both, stopping and energy, are expressed in normalized values (defined In the LSS theory /2 LSS/). nuclei dominate. The statistical character of the stopping process results in Gaussian-like distribution of implanted atoms, the determination of profile parameters was the first objective of the research on ion implantation. The first approach allowing the determination of stopping of ions in solid (hence the depth profile of implanted atoms) was developed by Lindhard, Scharff and Schlott /2/. The model, called from the authors names LSS theory, introduces normalized values of energy, range and standard deviation of atomic distribution. According to the LSS theory the depth distribution of implanted atoms can be described as: (X-BP) Where Cmax is maximum concentration of implanted atoms, Rp is a projected range of implanted ions, and ARp is a standard deviation of atomic distribution. Maximum concentration of implanted atoms, Cmax, can be calculated from the formula: C - D Where D is implantation dose (i.e. the number of ions implanted in surface unit of the target). The results of calculations were tabulated and can be used for the determination of the depth distribution parameters /3,4/. The development of Monte-Carlo numerical computer codes allowing the simulation of the processes occurring during the slowing down of the ions within the target made from them the generally used tool for the prediction of the effects of ion implantation. The best known and most widely used code is TRIM by Biersack and Ziegler /5/. The energy lost in inelastic collisions with electrons leads to the creation of ionized states which, in most cases, recombine rapidly with target electrons. Consequently, these collisions lead only to slowing down the ions but do not induce structural changes to the target. The only exceptions can be observed in weakly bonded insulators or in processes using extremely heavy and energetic ions (with energies exceeding hundreds of MeV /6/). On the other hand the energy transferred from the ion to the target nuclei in elastic collision may reach tens or even hundreds of keV what exceeds the binding energy of atom in crystalline structure by orders of magnitude. The elastic collisions lead thus to the displacement of target atoms from their initial positions, hence to the radiation damage creation /1, 2, 4/. The radiation damage is an intrinsic and very important characteristic of ion implantation as each ion may displace even thousands of atoms from their lattice sites /7/. When the hit atom (often denoted as PKA from Primary Knock-on Atom) received sufficiently high energy it may collide with other atoms creating so-called displacement cascade, i.e. local volume characterized by a high concentration of radiation defects. The example of TRIM calculation showing the cascade formed by 100 keV argon ion implanted into iron is presented in Fig. 2. The radiation defect depth distribution can also be extracted from TRIM simulations together with the distribution of the implanted atoms. An example presenting the results obtained for 50 keV nitrogen ions implanted into iron up to a dose of 1x10"!"^ atoms/cm^ is shown in Fig. 3. In order to understand better the specific character of ion implantation it is crucial to discuss in detail the processes occurring during the cascade evolution. The fast ion penetrating the target collides with few target atoms and is stopped after about lO"''^ second. Therefore most of radiation damage is created by displaced target atoms, not by incoming ion itself. The typical defect created by atomic collision is a simple Frenkel pair. The separation between displaced atom and its vacancy is usually small therefore most of the created defects recombine during the cascade evolution. Molecular dynamic simulations /8/ allowed to get a deeper insight into damage process. The concentration of defects during cascade evolution extracted from molecular dynamic simulation is presented in Fig. 4. Three stages can be noticed. The first one is collisional phase g TRIU - 1997 (97.B6) Ion Type = ftr < 4B amu) loti Energy = IBB keU Ion Angle = 8 degrees TARGET LAVERS Depth Density Fe lieaA 7.866 Ar Ar Fe Ion Conpleted-Backscattered Transmitted Long itud ina1 = Lateral ProJ= Radial Uac./'Ion = EMERGV LOSSCx: Ion i t ion UacN'^jic--■■■■"-■■0 PllOnOVIS 16( 99999) Ions -Ions = Range Straggle 431A 232A 188A 247A 276A 139A 1869.4 IONS RECOILS 23.96 32.18 8.14 3.52 8.58 39.77 HotKeys : Help,SB.F2,A.B.C.E,I,H,P.R,S.I X-Y Longitudinal Projection Sputtering= 2.688/Ion - Depth IIBBA Fig. 2. Results of TRIM calculations showing the formation of displacement cascade in Ar implanted Fe. The Ion path is shown as a solid line, the points correspond to displaced target atoms. One can note the formation of isolated cascades in the vicinit/ of primary knock-on atoms. 4e+22 E 3e+22 - E 2e+22 ~ V Nitrogen concentration Defect concentration c5Ä ■ 2e+25 - 1e+26 5e+24 L Oe+0 0 20 40 60 80 100 120 140 Depth (nm) Fig. 3. Depth distribution of Implanted atoms and of the defects created by 50 keV nitrogen implantation Into iron. The left vertical scale corresponds to the defect concentration and the right scale to the implanted atom concentration. (I) when ion collides with target nuclei. This stage takes about 10"''2 sec and is characterized by rapidly increasing concentration of defects. This is followed by a displacement phase (II) when the ion is already stopped and collisions are created only by displaced target atoms. During this stage the recombination of close Frenkel pairs took place leading to a rapid decrease of defect concentration. This stage lasts about 10"'''' sec. The last stage of cascade evolution is called cooling phase (III). All close Frenkel pairs disappeared already and the defect anihilation requires long range diffusion mechanisms. The average energy of displaced atoms drops below 1 eV, i.e. is in thermal energy range. The spatial evolution of defects in cascade is of great importance for the phase formation in implanted systems. The structures of vacancy clusters formed in tungsten irradiated with various ions are presented in Fig. 5 /9/. One can note the density of the vacancy cluster strongly increases with the mass of the incoming ion. The vacancy cluster is surrounded by the intersti-tial-rich zone. It is believed /10/ that the phases formed in ion implantation mainly depend on processes in central, vacancy rich region of cascade during its cooling phase. 0) X) E D Z 1' -III Eo =2.5 keV Eo = 600eV\ - / H S I '05' 1.0 Time (picoseconds i 1.5 Fig. 4. Evolution of radiation defect concentration upon time for cascade created by 2.5 keV and 0.6 keVW atoms In tungsten. . 30 keV ° -o „^O S< Vo, 30keV ■ Cr^ 4 Fig. 5. Spatial distribution of vacancies in cascades formed by various ions in tungsten (picture tat