Ion-beam technology. w. K. Hofker. Depth distribution of boron ions. Experimental methods

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1 320 Philips tech. Rev. 39, ,1980; No.11 Ion-beam technology w. K. Hofker One of the applications of ion-beam technology is in the manufacture of transistors, integrated circuits - ICs, 'chips' - and semiconductor detectors. In this application the ions from the beam are 'implanted' in the surface layer of semiconductor material [1]. An ion beam can also be used to analyse surface layers. An important application here is the analysis of implanted layers. The information obtained in this way can be used to improve the implantation process. Ionbeam technology is therefore important in manufacturing and research. The Philips group at IKO started to do experiments on ion implantation in Silicon was doped with ions of phosphorus and lithium to make semiconductor detectors. Although this work on semiconductor detectors was a logical continuation of earlier work by the group [2] it was soon clear that ion implantation could be very useful in the manufacture of other electronie components, such as transistors, and the work was therefore expanded to cover such activities. Today IC technology is inconceivable without ion irnplantation: with other methods it is almost impossible to obtain the desired electrical characteristics to the same high accuracy. Consequently, there was an increasingly close cooperation with other IC groups within Philips, and this was one of the reasons why it was decided to transfer the Amsterdam group to Eindhoven. At present the group -is working mainly on special implantation technologies, such as implan-, tation at high ion energy and large dose; these new technologies require both fundamental and applied research. In this work it is also important to be able to obtain certain chemical properties, as well as electrical ones. Experimental methods As we noted earlier, ion beams are also used for investigating doped semiconductor material. The material can be bombarded with light ions, e.g. of hydrogen or helium. When they collide with atoms, there is back-scattering. Measuring the energy and number of the back-scattered ions at a given angle to the surface gives information about the composition and structure of the surface layer of the material Dr Ir W. K. Hofker is with Phifips Research Laboratories, Amsterdam Department. (High-Energy Ion Scattering: HElS; Rutherford Back- Scattering: RBS). It is also possible to bombard the material with ions and 'peel away' the surface layer by layer, i.e. by 'sputtering'; the ions released are then analysed with a mass spectrometer (Secondary-Ion Mass Spectrometry: SIMS). Finally, ion beams can be used to measure depth profiles by means of nuclear reactions. In the earliest work on ion implantation the ions were supplied by a mass separator from IKO [3] (its acceleration voltage was 100 kv). Fig. 1 shows a cutaway drawing of the machine. This equipment was later replaced by a machine for high-energy implantations (acceleration voltage 500 kv) and one for large-dose implantations (ion current 2 ma) [1]. HElS analyses of implanted materials were performed with the aid of a Van de Graaff accelerator (3 MV) from the State University of Utrecht, in cooperation with the FOM (the Organization for Fundamental Research on Matter) Institute for Atomic and Molecular Physics in Amsterdam, where the necessary test equipment was already available. Since 1978 the group has been using its own instruments and equipment, which were housed at IKO until the group moved to Eindhoven. Some experimental results Depth distribution of boron ions When ions are implanted in a material such as silicon, the crystallattice is damaged, and this causes changes in the electrical behaviour. The damage can be repaired, however, by annealing. One of the things [1] W. K. Hofker and J. Politiek, Ion implantation in semiconductors, Philips tech. Rev. 39, 1-14, This contains a description of the implantation equipment and the HElS and SIMS methods. [2] See for example the articles by W. K. Hofker in this issue: Geiger-Müller counters, page 296, and Semiconductor detectors, page 298. [a] K. J. van Oostrum and J. H. Dijkstra, Nucl. Instr. Meth. 29, 231, [4] W. K. Hofker, H. W. Werner, D. P. Oosthoek and H. A. M. de Grefte, Radiation Elf. 17, 83, [6] W. K. Hofker, H: W. Werner, D. P. Oosthoek and H. A. M. de Grefte, in: B. 1. Crowder (ed.), Ion implantation in semiconductors and other materials, page 133; Plenum Press, New York [6] W. K. Hofker, Philips Res. Repts Suppl. 1975, No. 8. [7] W. K. Hofker, H. W. Werner, D. P. Oosthoek and N. J. Koeman. in: S. Namba (ed.), Ion implantation in semiconductors (Proc. 4th Int. Conf., Osaka 1974), page 201; Plenum Press, New York 1975.

2 Philips tech. Rev. 39, No. 11 ION-BEAM TECHNOLOGY 321 we have investigated is the effect of such a treatment on the depth distribution of boron ions. It was for such studies that we, in cooperation with H. W. Werner and H. A. M. de Grefte of Philips Research Laboratories, Eindhoven, developed the SIMS method [4]. In this method the mass spectrometer ditions for which the implantation and annealing will produce the desired electrical properties. SIMS is a test method much in use today and the analytical description has been taken up by many working in IC technology. The depth distribution of boron ions was also Fig. 1. Cutaway drawing of a mass separator from IKO that was used for ion implantation in an earlier phase of the work. 1 ion source, A 100 kv accelerator tube, M analysing magnet, C chamber in which the ions are collected. is set to the boron line and the boron ion current is recorded as a function of time. The sputter rate is also measured. When the instrument has been calibrated, the boron concentration can be derived as a function of the depth from the measured results (fig. 2). We were also able to give an analytical description of the measured variation with depth [5] [6]. From the experimental results it was possible to specify the confound to be affected by the implantation of other ions before the annealing [7]. We implanted various kinds of ions in silicon that had been homogeneously doped with boron (at a concentration of 1.3 X boron ions/ern"). The doses were selected in such a way that the implanted areas in the silicon crystal became amorphous. After the annealing (at 800 C) we found peaks in the originally uniform depth distribution of

3 322 w. K. HOFKER Philips tech. Rev. 39, No cm" CB l d 1.6pm Fig. 2. Depth distributions determined by SIMS of boron (concentration CB as a function of depth d) implanted in polycrystalline silicon with a dose of 1016 ions/ems and energies of 30, 100, 300 and 800 kev. The points correspond to the measured values. The solid curves represent an analytical description of the measured results. (The analytical description was obtained [6] [6] by determining the four statistical moments: mean value, variance, skewness and kurtosis for the measured points for each implantation energy and introducing these into a Pearson IV distribution.) The figure shows how good the agreement is. The depth profile for anyone implantation energy can thus be calculated readily from a few measured profiles. the boron ions (fig. 3). The location of the peaks coincided exactly with the points where the amorphous layer was in contact with the unchanged crystalline material. The redistribution of the boron that we have observed may therefore be due to the considerable local mechanical stresses [6]. In double implantations, which are often used in IC technology, the implantation profiles mayalso have an adverse effect on one another [8]. Damage caused by implantation Even after annealing, some damage may still remain, usually in the form of dislocations in the crystal. These can be studied with an electron microscope. Our research has shown that the occurrence of dislocations, e.g. after the implantation of antimony in silicon, is closely dependent on the temperature of the silicon during the implantation, even though it may be annealed later at a much higher temperature. For example, a dislocation network will be formed (fig. 4) if the temperature of the silicon during the antimony implantation is 150 C and the silicon is annealed later (8 hours at C). However, no dislocation network is formed if implantation at a temperature twenty degrees higher or lower is followed by a similar annealing procedure. Closer investigation has revealed that the network is due. to lattice defects and mechanical stresses in the lattice, caused by the implanted ions. The combined effect of lattice defects and stresses is greatest at 150 C. Oxidation properties Oxidation processes - which often take place at a temperature of 1000 C - can play an important part in the manufacture of integrated circuits. It is well known that the presence of a thin layer of SiaN4 on the silicon inhibits the oxidation of the silicon. We recently carried out a study [9] to find out whether the oxidation processes can be regulated locally by implanting nitrogen in the silicon. A relatively small dose of nitrogen (3 X ions/ern") appeared to be sufficient to inhibit oxidation for an hour at 1000 C. 2.5 l1u It 1111 II IIII '111 Ipl IIII }Il l III \ I _, I f J~ 2.5't- fii Il ~ '11 Ju' ~" 111 "" ,I ~I, I. 1.3 If l I I , ~, ~ T I' \: keV 28 5;+ 70 kev -\. J\ 1\. 84Kr+ 300keV I I'. 11 " \ / "._.'.., 1\ 1/ \1\,,,.,,~ '-. t3r r'~-----'~ , u V o flm..d Fig. 3. Boron concentratien CB as a function of depth d after a silicon substrate, uniformly doped with boron, has been implanted with either nitrogen, oxygen, silicon or krypton, and has then been annealed for 40 minutes at 800 oe. The implantation energy for nitrogen, oxygen and silicon is 70 kev and for krypton it is 300 kev. The implantation dose is 1016 ions/erne. <

4 Philips tech. Rev. 39, No. I1 ION-BEAM TECHNOLOGY 323 Fig, 4. A photomicrograph made with a transmission electron microscope of a silicon substrate implanted with antimony (energy 150 key, dose 1016 ionsycrn") and annealed for 8 hours at 1100 "C. During the implantation the icmperature of the subsirare was 150 C. n -d 8ooor--o, ~0----_,m ,n-m keV -E Fig. 5. Nitrogen distribution before (0) and after (6) annealing for an hour at 1000 C in a silicon substrate implanted with nitrogen (dose 5 x iona/cm>, energy 50 key). The measurements were made with a resonant (p,ay) nuclear reaction. The horizontal axis gives the proton energy E and the vertical axis the number of gamma quanta n observed at this energy. The vertical dashed line indicates the position of the surface. The depth d is plotted horizontally across the top. To obtain a clearer understanding of the inhibiting mechanism we analysed the depth distribution of the nitrogen, making use of a resonant nuclear reaction: if a 15N nucleus is..struck by a proton (hydrogen nucleus) of suitable energy, a carbon nucleus is formed, accompanied by the emission of a helium nucleus and gamma radiation; if the energy of the proton deviates only slightly this process will not take place. The incident protons lose energy when they pass through the material, so that the depth at which the protons have the correct energy for this nuclear reaction is associated with the energy with which the protons strike the material. We implanted 15N in silicon, bombarded the material with protons and recorded the intensity of the resultant gamma radiation as a function of the energy of the incident protons (jig. 5). The figure shows that after annealing for an hour at 1000 C nitrogen accumulated just below the surface. Further investigations showed that the accumulation of nitrogen at the surface is maintained during oxidation, while the nitrogen located deeper in the material gradually disappears. We assume that the inhibition of oxidation is due to formation of an Si 3 N 4 monolayer at the surface. During the oxidation silicon-nitrogen bonds are broken and replaced by silicon-oxygen bonds. Because this is a slow process, only very little SiOz is formed. The Si 3 N 4 layer is continuously replenished by diffusion of nitrogen from the supply located just below the surface - it is this that inhibits the oxidation - until the supply has been used up. Oxidation of the silicon will then proceed much more rapidly. Oxidation can therefore be inhibited locally by implanting nitrogen at the desired locations, with the aid of a mask. This procedure will soon be used in the manufacture of CM OS integrated circuits (CMOS = Complementary Metal-Oxide Semiconductor) [10], One of the electrodes (the 'gate') is oxidized - the oxide layer acts as an insulator - while the other electrodes (the 'source' and 'drain') do not become oxidized at the same time because of the implanted nitrogen. This technology can be used to produce a CMOS circuit of half the area of one made without implanted nitrogen: its potential is considerable. [8] W. K. Hofker, H. W. Werner, D. P. Oosthoek and N. J. Koeman, in: G. Carter, J. S. Colligon and W. A. Grant (ed.), Applicarions of ion beams to materials 1975 (Inst. Phys. Conf. Ser. No. 28, 1976), page 13. [9] W. J. M. J. Josquin and Y. Tarnrninga, unpublished. [10] B. B. M. Brandt, W. Stemmaier and A. J. Strachan, Philips tech. Rev. 34, 19, 1974.

5 324 Philips tech. Rev. 39, No. 11 Microscope photograph of a proton-bombarded double hetero-junction AlGaAs laser (CQLIO) mounted on a copper heat sink. The dimensions of the crystal are 250 x 300 x 80 urn, The proton bombardment is made in such a way that the damage produced can be used to define the active region in the lateral direction. One advantage of this method is that by selecting the correct energy the damaged area can be very accurately located at the desired depth. The photodiode in the base stabiltzes the output of the laser by feedback to the laserpower supply.