M. Weiser

An in-beam-line low-level system for nuclear reaction γ-rays

Abstract The applicability of nuclear γ-ray reactions to the impurity analysis of solids is generally limited by background radiation. In order to significantly improve the sensitivity of this method, low-level counting techniques have to be adopted. To this end a multiple shield system has been integrated into the beam line of the EN tandem accelerator. In this way, the background level as seen by a 10″×10″ NaI detector has effectively been reduced by a factor of about 100 over the energy region of 0.5–20 MeV. Thus, the detection limit for the 15 N reaction with 1 H has been improved from about 0.1–1 at.% to about 10–100 at.ppm for normal and low-level operating conditions, respectively. A number of applications will be discussed and first results will be presented.

Ion-implanted Si pn-junction detectors with ultrathin windows

Abstract Nuclear radiation detectors have been fabricated by using ion implantation and the Si planar processing technology. While ion dose and energy were kept constant at values of 5 × 10 14 B/cm 2 at 12.5 keV and 2.5 × 10 15 P/cm 2 at 30 keV, the annealing temperature has been varied between 500 and 1000°C. The Si p + -n dead layer, or detector window, has been measured as a function of the annealing state by using the technique of tilting the detector surface with respect to an incident α-particle beam. The resulting energy losses of the monoenergetic particle beam have been measured with standard high-resolution nuclear spectrometric techniques. The result is that annealing beyond 800°C is necessary to reduce the window thickness from about 0.2 μm to well below 0.1 μm, the concomitant deterioration in detector current being negligible.

A multisegment annular Si-detector system for RBS analysis

Abstract In order to increase the signal rate of Rutherford backscattering spectrometry 2 × 12 detectors have been integrated on a single 3 in. Si wafer, covering two solid angles of ΔΩ = 2π sin θ Δθ ∼ 100 msr > each, where θ is the backscattering angle. Each data line is processed separately, then digitized and stored, and finally handled by an on-line computer to yield a single backscattering spectrum. The capacity of this system will be demonstrated by several applications to typical ion beam analysis problems, where for example low concentration levels have to be detected or high statistical precision is mandatory. The examples include four-moments analysis of deep implants and trace impurity detection in semiconductor specimens. Several approaches of solving the pulse pileup problem will be demonstrated and further steps for improving the detection limit into the sub-at. ppm regime will be discussed.

Depth distributions of megaelectronvolt 14N implanted into various solids at elevated fluences

Abstract Nitrogen was implanted at an energy of 1 and 1.5 MeV up to concentrations of typically 0.1–1 at.% into various metals, semiconductors and polymers. The depth profiles were analysed by means of nuclear reaction analysis techniques with thermal neutrons and protons, and by secondary ion mass spectrometry, and compared with theoretical predictions (TRIM Monte Carlo code with Ziegler-Biersack-Littmark electronic stopping power). For the metals and semiconductors, reasonable agreement was found, whereas nitrogen in polymers redistributes after implantation essentially according to the depth profile of electronic excitation and ionization. The 14 N profiling shows that, as well as the implanted ions, a considerable background of natural nitrogen exists in concentrations of up to 0.1 at.%.

Implanted boron profiles in silicon

Abstract 10 B and 11 B implants into amorphous Si, with energies ranging from 50 keV to 2 MeV and 10 keV to 1 MeV respectively, were profiled by the nuclear reactions 10 B(n, α) 7 Li and 11 B(p, γ) 12 C. The projected range R p and straggling ΔR p agree within a few percent with recent calculations due to Ziegler, Biersack and Littmark (ZBL). These results show that the ZBL electronic stopping power is adequate to reproduce range parameters resulting from MeV implantations.

Trace-impurity detection by Rutherford backscattering and nuclear resonance reactions

Abstract The use of a multisegment annular Si detector together with a multiline electronic processing system has led to an improvement of the detection limit in Rutherford backscattering spectrometry by about two orders of magnitude as compared to the standard single-line arrangement. With regard to nuclear reaction analysis, using particle/γ-ray resonance processes, similar large enhancements have been realized by an effective background-suppression system eliminating environmental and cosmic radiation. The present state of both experimental facilities will be demonstrated and compared with other trace-element detection techniques.

Experimental and calculated range moments of deep implants

Abstract The first four range moments of deep implants, ranging to micrometre depths, have been determined by nuclear reaction and Rutherford backscattering analysis for a few selected ion species in amorphous semiconductors. In order to achieve reasonable accuracy for the higher moments, considerable efforts have been undertaken to obtain sufficient statistical precision over the total range distribution. In the nuclear reaction analysis, we have made use of efficient background suppression by using an in-line low level system. In the Rutherford backscattering measurements, a multisegment annular silicon detector system has been used to fulfil the statistical requirements. The experimental results are compared with TRIM calculations, and the degree of agreement for the different moments has been evaluated. In general, some systems show an overall agreement of better than 10% even for the higher moments; other systems, however, show deviations up to 30%, especially for the first moment. These results will be discussed in terms of basic ion-solid interaction mechanisms.

Range parameters of deep ion implants in group IV semiconductors

Abstract Ions of selected elements of the periodic system were implanted into amorphous layers of C, Si and Ge with specific energies of up to 1 MeV/amu. The resulting profiles were measured with specific nuclear reactions ( A 1 A 2 ) and Rutherford backscattering ( A 1 > A 2 ) up to depths of 10 μm. In all cases, the first and second range moments of the concentration profiles were determined as a function of implantation energy, whereas both the next two higher moments could be derived only in those cases where the complete ion distribution was measurable down to the surface position. Here, for a few typical examples, also lateral distributions, by measuring implants with varied angle of incidence, were obtained and analyzed with respect to the first two even lateral moments. The results of a detailed comparison of the range parameters are discussed in terms of scaling properties within the framework of LSS theory. As regards the projected ranges predicted by TRIM calculations, significant deviations from our experimental values occur within the interval of + 20% and − 60%. These discrepancies are attributed to the electronic stopping powers used by the TRIM program.

A 3-MV Pelletron for materials research at Heidelberg

Abstract We describe an accelerator laboratory under construction for materials research. A 3-MV tandem of the Pelletron type, obtained recently, is planned to feed three beam lines for materials analysis and modification. The purpose of the experimental arrangements will be explained and the performance of the accelerator described.

High energy Li implanted profiles in silicon

Abstract 6Li and 7Li implants into amorphous Si with energies ranging from 4.5 keV up to 2 MeV were profiled by the 6 Li(n ,α) 3 H and 7 Li(p ,γ) 8 Be nuclear reactions, respectively. The projected range Rp agrees within a few percent with recent calculations of Ziegler, Biersack and Littmark. However significant differences are found for the range stragglings ΔRp.