Novel Processing for Diamond Quantum Technologies

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

State of the art bulk single crystal diamond material e.g. very low impurity contamination, low structural defect density,
isotopically engineered, has been realised but many quantum technologies demand near surface nitrogen-vacancy (NV)
centres "on demand" with reproducible properties approaching the best that can be achieved when positioned in the bulk of
the diamond. NV centres can be grown-in, produced by irradiation damage plus annealing where by pre-existing
substitutional nitrogen captures a mobile vacancy, or produced by nitrogen ion implantation plus annealing. In the bulk
(typically 10 mirco-m from the surface) NV centres can have exceptional quantum properties but these are typically
significantly degraded close to a surface due to subsurface damage (e.g. micro-cracks, trapped charge, parasitic spins,
strain) produced by polishing. Even though diamond can be mechanically polished (e.g. with small diamond particles
embedded in a cast iron wheel) with low surface roughness (< 1 nm), the sub-surface regions are damaged. As grown
surfaces are not sufficiently flat for many applications but current "polishing" technology significantly impairs the
performance of near surface NV centres so a new technology that can be easily transferred to production is required.
This project focuses on the implementation and optimisation of chemical mechanical polishing (CMP) of single crystal
diamond to achieve both low (<< 1 nm) surface roughness and low surface damage such that the touted supreme and
exploitable quantum properties of near surface (< 50 nm) NV centres are fully realised. Moreover, this work will facilitate the
routine production of very thin (< 30 mirco-m) large area single crystal diamond plates with optimised surface finishes.
There is considerable demand for this material for a variety of photonic and quantum technologies; here CMP is an
enabler.
The research programme will provide irrefutable evidence that CMP of diamond is a quantum technology enabler.
Traditional polishing techniques such as standard resin bond and scaife polishing are known to produce different damage
profiles that affect the near surface NV centres. ICP etching is currently the standard tool used to make quantum devices
out of diamond but is also known to impart damage to the diamond. Hence, studies on NVs near to a variety of differently
polished, etched and as-grown surfaces, as well as those in the bulk will be employed to bench-mark against the CMP
process.
Characterisation of near surface NV centres will be achieved using (i) high resolution single centre confocal
photoluminescence microscopy which enables statistical analysis of the properties of NV's e.g. probability of production,
stability, sensitivity to surface termination etc., with respect to distance from the top surface and surface processing
methodology (e.g. CMP, etching, termination etc.) and (ii) optically detected magnetic resonance, using state of the art high
through-put equipment under ambient conditions and at cryogenic temperatures. This provides information on the allimportant
spin lifetime of individual centres as a function of depth, surface preparation and processing. There is expected to be a significant statistical variation of the NVs properties due to other uncontrolled parameters on the nanoscale (i.e.
location of 13C, substitutional nitrogen, dislocations, surface spins, surface charges), hence the statistical analysis is
essential.
Near surface NV centres will be implanted (and annealed) after and before the CMP polishing process using low energy ion
implantation (via collaboration, at no cost to program, with Jan Meijer, Leipzig University) and fully characterised. Low
energy ion implantation provides the ultimate control in NV positioning ideal for device manufacture, and is an ideal way to
demonstrate that CMP can significantly improve the yield of useful near surface implanted NV centres.

Planned Impact

Since the discovery that the nitrogen-vacancy (NV) defect in diamond offers up easily measurable quantum properties,
even at room temperature, there have been numerous demonstrations exploiting NV defects in applications ranging from
magnetic and electric field sensing to quantum repeaters and quantum computing. The potential impact of this research
ranges from new diagnostic tools for healthcare and transformative imaging methodologies for materials and life sciences,
all the way through to secure communications and quantum simulators.
These diamond based quantum technologies will be game changing in many technology sectors over a 5-30 year time
scale and their development will become a multi-million pound industry that could benefit the UK economy over the same
period. We will all benefit from new capabilities, especially in the healthcare where advances will improve the health and
quality of life of our ageing population, and communications domains essential for a thriving modern economy. It is crucially
important is that the UK is at forefront of the development and exploitation of these novel technologies. Delivering the
benefits of new industries, new jobs, etc. is at least as important as the R&D. To ensure that the UK reaps the benefit it is
essential facilitate links between the materials producer, Element Six (E6), academics developing the technology and
exploiter industries who know the potential markets. For example, UK companies manufacturing, developing and selling
magnetic resonance and spectroscopic equipment will benefit through targeted collaborative R&D on diamond based
magnetometry leading to innovative and accelerated product development. Established businesses must evolve and will
only be sustained through continued innovation. Furthermore, we envisage that SMEs will have a significant role to play in
pioneering the new diamond based quantum technologies. In all cases transformation of scientific demonstrators into new
technologies requires the scalable production of "quantum grade diamond", with low surface roughness, low surface
damage and near surface high performance NV centres on demand. The bulk material exists, but to achieve all of the
above the team will develop Chemical Mechanical Polishing techniques and characterise the material to show that high
performance NV centres can be produced with high yield near (down to a few nm) the surface.
The new Quantum Technology Hubs, and many Centres for Doctoral Training (CDT), especially the Diamond Science and
Technology CDT, will be near term beneficiaries of the work since the material will enable R&D and technology
demonstration. Working closely with Quantum Technology Hubs and CDTs will facilitate new partnerships with industry and
develop a stronger technology pull.
For young researchers who make use of the material produced it will not only benefit their immediate research but the
exposure to cutting-edge technologies and methodologies will ensure they are attractive to employers in both academic
and industrial environments.
The new material gives the UK a research advantage that will promote international collaboration and development since
visitors will want to make use of the capability. New collaborations will be established, research income leveraged into the
UK and through the expertise drawn inwards the UK research base strengthened.

Publications

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Description The overall objective of this project was to build on the current high purity "quantum" materials produced by E6 by delivering a material with low surface damage, in a scalable way, which will allow the diamond to contain near surface diamond defects with 'good' and repeatable quantum properties.
To do this the team established a new type of diamond polishing facility and subsequently developed novel chemical mechanical polishing (CMP) and plasma etching techniques to remove diamond material without introducing subsurface damage. In addition, novel characterisation techniques were established to evaluate the quality of diamond materials processed using the novel techniques developed under this project.
The project succesfully demonstrated that diamond can be plasma etched with reproducible uniformity and polished to a high level of smoothness using CMP technology.
It is anticipated that, following further process development and refinement under follow-on project TSB 102676, the new diamond processessing techniques will open up a range of quantum applications from nano-magnetic field imaging to quantum repeaters. The outputs from this project will accelerate access a $25m market with a potential ROI of 565%.

More specifically, under WP2 of "Novel Processing for Diamond Quantum Technologies" (NPDQT), E6 set up and developed a new diamond polishing facility and have developed new processing techniques. E6 are now routinely polishing diamond using CMP processes at the GIC.
As part of WP4 of NPDQT has greatly accelerated Warwick's ability to characterise material containing isolated defects, both optically and in the crucial area of magnetic resonance. It is now possible to routinely optically locate and address single atomic defects within the diamond lattice, including near-surface defects within 80 nm of the diamond surface. Working with national UK facilities we have characterised defects intentionally implanted at depths of 400, 200, and 80 nm from the diamond surface. Measurements of the implanted defects have reinforced the need for advanced surface processing of diamond for technological applications: for the shallowest defects, over an order of magnitude was gained in the critical metric of spin lifetime following the etching protocols developed as part of this project, when compared with mechanically polished material (42 +/- 5 µs for developed protocol c.f. 2.7+/-0.4 µs for mechanically polished material). The achieved lifetimes have yet to reach the target (>50 µs) but future developments on CMP-processed surfaces are expected to meet this. It should be noted that we have not required cryogenic measurements to achieve these lifetimes: this is important as the need for cryogenic apparatus would be a major barrier to several potential technological applications of this material.

As part of WP5, the development of etching recipes using known chemistries e.g. Argon/Chlorine and Oxygen, suitable for etching diamond material has been realised. Etching facilities at Warwick are now set-up for diamond processing, allowing the removal of material in a controlled and well-defined way. Procedures developed as part of this project can now be utilised by any future projects where diamond processing is required.
Through the use of Argon/Chlorine chemistries, it was found that feature depths of microns could be etched into material surfaces without significantly increasing the surface roughness. Oxygen based etches were capable of removing material at three times the rate of any chlorine based processes but came with a marked increase in surface roughness. However, when combined with CMP, oxygen etch processing of surfaces is expected to be a viable route for completely removing subsurface damage from mechanically polished diamond material while maintaining a low surface roughness. This is of great importance for any quantum characterisation of the material.
Under WP6, processed material surfaces were analysed using interferometric techniques, providing all important topography data of material surfaces both before and after processing. The rapid acquisition time and high resolution (0.1 nm) of this technique was found to be well suited for analysing etched and CMP processed material, making it invaluable during this project. As such, this technique has now been adopted as the standard protocol for all future material surface characterisation and will greatly benefit the follow-on project (TSB 102676).
Exploitation Route Although this project demonstrated the feasibility of novel processing technology to diamond, we were unable to establish an optimised technology. To allow the commercial exploitation of the CMP technology we need to demonstrate a robust and commercially attractive CMP alternative to existing (production) polishing processes. To assist the route to exploitation, the consortium will continue to work closely on the development of the CMP processes for quantum applications (using dedicated resources at the E6 GIC) under the follow-on project (TSB 102676).

We anticipate that the results are going to impact on the work being undertaking in the NQIT Hub, the Diamond Science and Technology CDT and beyond. The CMP capability will undoubtedly underpin future research in both quantum and traditional sensor development.
Sectors Digital/Communication/Information Technologies (including Software),Electronics

 
Description Our participation in this project has generated several new developments and process improvements, and as such the time dedicated to this project has been thoroughly worthwhile. Within the project, we have: • Developed a robust technique for the exfoliation and recovery of very thin diamond films • Compared the effects of different types of diamond surface preparation methods on the quantum properties of isolated point defects. Crucially, this includes a comparison between mechanical polishing, ICP etching and chemical mechanical polishing, where we have recorded dramatic lifetime improvements for each step (3 us, 42 us, and 116 us, respectively) • Improved our methodology and equipment for the measurement of quantum properties, including dramatically increasing our throughput, with typical setup times reduced from >5 hours to <10 minutes • Re-established and subsequently significantly improved reactive ion plasma processing and high-resolution photolithography of diamond at Warwick Our primary goals were to compare different surface preparation methods and to develop a thin plate generation process: we have therefore achieved what we set out to achieve and the commercial potential of the new processes are being evaluated by Element Six.
Sector Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Description Technology Strategy Board -CR&D PROPOSAL
Amount £281,000 (GBP)
Funding ID TSB102676 
Organisation Innovate UK 
Sector Public
Country United Kingdom
Start 09/2016 
End 08/2017
 
Description Element Six Ltd (UK) 
Organisation De Beers Group
Department Element Six
Country Luxembourg 
Sector Private 
PI Contribution Understanding of defect incorporation in CVD diamond
Collaborator Contribution Provision of samples
Impact Further research on defects in diamond
Start Year 2014