P. Räcke, R. Wunderlich, J.W. Gerlach, J. Meijer, D. Spemann
New Journal of Physics 22 (2020) 083028
DOI: 10.1088/1367-2630/aba0e6

Ion implantation is not only an established technique for semiconductor industry and research, but also a key method enabling ground-breaking research in novel quantum technologies like quantum sensing and computing, e.g. based on donors in silicon or the nitrogen-vacancy-(NV)-centre in diamond. For the realisation of quantum computing, a main challenge is the scalability of the qubit fabrication. For ion implantation, this means combining a high throughput with spatially precise placement of a counted number of single dopants or defects is crucial. For this purpose, a focussed ion beam (FIB) based ion implanter equipped with an electron beam ion source (EBIS) able to produce highly charged ions was set up. The ion optical system was optimised and beam spot sizes below 200 nm achieved, showing that nanoscale ion implantation using an EBIS is feasible. As an example of its utilisation, we demonstrate the direct writing of nitrogen-vacancy centres in diamond using mask-less irradiation with focussed Ar⁸⁺ ions with sub-micron three-dimensional placement accuracy.

P. Räcke, R. Staacke, J.W. Gerlach, J. Meijer, D. Spemann
J. Phys. D: Appl. Phys. 52 (2019) 305103 (5pp)
DOI: 10.1088/1361-6463/ab1d04

Image charge detection (ICD) is a technique that can be used to non-perturbatively measure the velocity, charge, mass and other properties of moving charged particles. Image charge detection can also be used as a non-perturbative pre-detection approach for deterministic ion implantation. Using low energy ion bunches as a model system for highly charged single ions, we experimentally studied the error and detection rates of an image charge detector setup. The probability density functions of the signal amplitudes in the Fourier spectrum can be modelled with a generalised gamma distribution to predict error and detection rates. It is shown that the false positive error rate can be minimised at the cost of detection rate, but this does not impair the fidelity of a deterministic implantation process - this is a specific advantage of the pre-detection approach where the ion detection is independent from the implantation itself. Independent of the ion species, at a signal-to-noise ratio of 2, a false positive error rate of 0.1% is achieved, while the detection rate is about 22%. A detection rate of 22% means that 22% of all ions moving through the detector are detected and passed on to the implant site whereas the remaining 78% of the ions are automatically discarded by a beam blanker.

P. Räcke, D. Spemann, J.W. Gerlach, B. Rauschenbach, J. Meijer
Scientific Reports 8 (2018) 9781
DOI: 10.1038/s41598-018-28167-6

Deterministic ion implantation enables the placement of single counted impurity atoms into a host material where they can be functionalized and used as quantum objects, for example, in quantum sensing or quantum computing. The ability to control the exact number of atoms implanted into a specific location is a prerequisite for the fabrication of quantum devices that rely on a set of individual quantum objects like qubits. The detection of each implanted ion can be done before the implantation itself (pre-detection) or during the implantation process (post-detection) exploiting secondary effects from the ion stopping in the host material. The latter approach, therefore, depends on the specific properties of the host material, whereas a pre-detection approach does not.
Here, a concept for detection of charged particles in a single fly-by, e.g. within an ion optical system for deterministic implantation, is presented. It is based on recording the image charge signal of ions moving through a detector, comprising a set of cylindrical electrodes. This work describes theoretical and practical aspects of image charge detection (ICD) and detector design and its application in the context of real time ion detection. It is shown how false positive detections are excluded reliably, although the signal-to-noise ratio is far too low for time-domain analysis. This is achieved by applying a signal threshold detection scheme in the frequency domain, which - complemented by the development of specialised low-noise preamplifier electronics - will be the key to developing single ion image charge detection for deterministic implantation.

Jürgen W. Gerlach, Jan Meijer, Sébastian Pezzagna, Bernd Rauschenbach, Stephan Rauschenbach, Daniel Spemann, (2018) (EP16723276.8), EPA
Patent specification

This patent details a setup for the detection of a single, charged particle, e.g. an ion, in a single pass through a set of electrodes such that the trajectory of the particle is practically undisturbed. The latter is a prerequisite for an ion optical image formation that allows for a lateral placement accuracy of the ion at the nanoscale. The detection approach relies on a regular arrangement of electrodes in which the passing ion influences an image charge. This image charge can be amplified and results in an alternating signal of specific frequency that can be measured. This approach is the foundation for the single ion detection required for the deterministic ion implantation developed at the Institute.

S.G. Robson, P. Räcke, A.M. Jakob, N. Collins, H.R. Firgau, V. Schmitt, V. Mourik, A. Morello, E. Mayes, D. Spemann, and D.N. Jamieson
Physical Review Applied 18, 034037 (2022)
DOI: 10.1103/PhysRevApplied.18.034037

When phosphorus or other donors are introduced into high-purity silicon by means of single ion implantation, they form a promising quantum bit (qubit) system for the development of a scalable quantum computer. However, what is needed for scalability is a method for deterministic ion implantation. For this purpose, our collaboration partners from the CQC2T of the School of Physics, University of Melbourne are developing dedicated silicon substrates to detect each individual ion during implantation using IBIC (Ion Beam Induced Charge).

This paper reports on the results of a collaborative work of CQC2T members and colleagues from IOM. Here, the feasibility of single ion detection via IBIC is explored by implanting low-energy ions into silicon devices featuring a 60 × 60 μm² sensitive area and an ultrathin 3.2-nm gate oxide—capable of hosting large-scale donor arrays. An IBIC-mapping system consisting of a modified focused-ion-beam machine and ultralow-noise ion-detection electronics is employed to demonstrate the capability for evaluating the device response characteristics to shallowly implanted 12-keV 1 H₂⁺ ions. Despite the weak internal electric field, near-unity charge collection efficiency is obtained from the entire sensitive area. The approach is then adapted to perform deterministic implantation of a few thousand 24-keV ⁴⁰Ar²⁺ ions into a predefined microvolume, without any additional collimation. Despite the reduced ionization from the heavier ion species, a fluence-independent detection confidence of ≥99.99% is obtained. Our system thus represents not only a method for mapping the near-surface electrical landscape of electronic devices but also a framework toward mask-free prototyping of large-scale donor arrays in silicon.