Looking for single ion impacts in PMMA Victoria Millar David Hoxley Chris Pakes

1. Looking for single ion impacts in PMMA Victoria Millar David Hoxley Chris Pakes Steven Prawer Why? • Top down process utilises ion implantation for the placement of P ions in the silicon substrate. Ion implantation is essentially random. • A single ion resist would be useful for locating the impact sites of a random implant. OR • A single ion resist would allow electrical registration to be tested. • verification of step and repeat fabrication of a qubit array.

2. Candidate Resists • Ion irradiation is an established tool for the modification of the chemical structure and physical properties of materials. • Need a resist with sufficient sensitivity to demonstrate a measurable modification due to the passage of a single ion. • Candidate resists are : diamond-like carbon, polymethylmethacrylate (PMMA), monolayer films of C60 and self assembled monolayers of Cyclooctatetraene (COT) and octadecyltrichlorosilane (OTS). • Studies tend to be restricted either to the use of high energy heavy ions or require particular environmental conditions. • PMMA appears to be the most promising.

3. PMMA • Ion beam modification of PMMA takes the form of latent chemical damage caused by secondary electrons. • This damage results in an increase in solubility of the irradiated areas. • Subsequent development of the irradiated sites results in a hole in the PMMA situated above the implanted ion. 50nm PMMA substrate We implanted 50 nm thick PMMA with 4 MeV 4He ions in an array of spots with varying ion fluence from 109 to 1014 ions.cm-2.

4. Imaging of the samples was done with an AFM in intermittent- contact mode. AFM image (11 mm), obtained using a conventional Si cantilever, showing a region of PMMA irradiated with an ion fluence of ~109 ions.cm-2. b) topographic profile taken from an individual etched feature Difficulties in observing single ion features arise from the shape of the conventional cantilever determining the lateral resolution and capability of imaging high aspect ratio features.

5. Probes consisting of multi-walled nanotubes have been fabricated by mechanically attaching a bundle or a single walled nanotube to a conventional AFM probe. The geometry of a carbon nanotube with diameter 10 nm and length of 500 nm offers extremely good lateral resolution in imaging features of high aspect ratio.

6. Intermittent-contact AFM image (11 mm), obtained using a carbon nanotube probe, showing a region of PMMA irradiated with an ion fluence of ~109 ions.cm-2 and a topographic profile taken from an individual etched feature SUMMARY • AFM images clearly demonstrate that the material properties of PMMA can be modified in a controlled way by ion irradiation. • Identification of features that we ascribe to single ion impacts is facilitated by the use of carbon nanotube based images.

7. FUTURE WORK • Use PMMA to create and array of P ions • PMMA allows us to potentially examine the possibility of subsequently metalising the developed films to form self-aligned contacts above a single implanted ion. Au gates PMMA Si Si