Patent Application
20240309554
This disclosure relates to a single crystal diamond components, in particular those that include quantum spin defects, and methods for producing the same.
Point defects in synthetic diamond material, particularly quantum spin defects and/or optically active defects, have been proposed for use in various imaging, sensing, and processing applications including: luminescent tags; magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; spin resonance imaging devices for magnetic resonance imaging (MRI); and quantum information processing devices such as for quantum communication and computing.
Many point defects have been studied in synthetic diamond material including: silicon containing defects such as silicon-vacancy defects (Si—V), silicon di-vacancy defects (Si—V2), silicon-vacancy-hydrogen defects (Si—V:H), silicon di-vacancy hydrogen defects (S—V2:H); nickel containing defect; chromium containing defects; and nitrogen containing defects such as nitrogen-vacancy defects (N—V), di-nitrogen vacancy defects (N—V—N), and nitrogen-vacancy-hydrogen defects (N—V—H). These defects are typically found in a neutral charge state or in a negative charge state. It will be noted that these point defects extend over more than one crystal lattice point. The term point defect as used herein is intended to encompass such defects but not include larger cluster defects, such as those extending over ten or more lattice points, or extended defects such as dislocations which may extend over many lattice points.
It has been found that certain defects are particularly useful for sensing and quantum processing applications when in their negative charge state. For example, the negatively charged nitrogen-vacancy defect (NV−) in synthetic diamond material has attracted a lot of interest as a useful quantum spin defect because it has several desirable features including:
The NV− defect in diamond consists of a substitutional nitrogen atom adjacent to a carbon vacancy. Its two unpaired electrons form a spin triplet in the electronic ground state (3A), the degenerate ms=±1 sublevels being separated from the ms=0 level by 2.87 GHz. The electronic structure of the NV− defect is such that the ms=0 sublevel exhibits a high fluorescence rate when optically pumped. In contrast, when the defect is excited in the ms=±1 levels, it exhibits a higher probability to cross over to the non-radiative singlet state (1A) followed by a subsequent relaxation into ms=0. As a result, the spin state can be optically read out, the ms=0 state being “bright” and the ms=±1 states being dark. When an external magnetic field is applied, the degeneracy of the spin sublevels ms=±1 is broken via Zeeman splitting. This causes the resonance lines to split depending on the applied magnetic field magnitude and its direction. This dependency can be used for magnetometry by probing the resonant spin transitions using microwaves (MW) and using optically detected magnetic resonance (ODMR) spectroscopy to measure the magnitude and optionally direction of the applied magnetic field.
NV− defect in synthetic diamond material can be formed in a number of different ways including:
As described in WO 2015/071487, formation of NV− defects by nitrogen ion implantation and annealing (optionally including a vacancy generating irradiation step pre- or post-ion implantation) can be advantageous because NV− defects in synthetic diamond material used in applications such as nano-magnetometry, wide-field magnetometry, and quantum processing applications typically need to be close to the surface of the synthetic diamond material (within a few nm) and ion implantation is a useful method of providing near surface NV− defects;
A problem with the formation of near surface NV− defects in synthetic diamond materials via nitrogen ion implantation and annealing is that to date such near surface NV− defects exhibit a shorter spin coherence time than native NV− defects found in the bulk of high purity single crystal CVD diamond material such as the single crystal CVD diamond materials described in WO01/096633, WO2010/010344, and WO2010/010352.
A further problem with forming near surface NV− centres by nitrogen ion implantation is that is that the surfaces are typically not smooth, and so unsuitable for device fabrication. Surfaces can be improved by mechanical polishing but this can remove near surface NV− centres and reduce the decoherence time, T2 of the NV− centres. Currently, the best solution has been to use Inductively Coupled Plasma (ICP) etching, but there is evidence that this too causes damage to the surface and the NV− centres.
It is an object of the present invention to provide a single crystal CVD diamond component with a combination of a polished surface and spin centres close to the polished surface with a high T2 value, and to provide a method for producing such a diamond component.
According to a first aspect, there is provided a single crystal CVD diamond component comprising a surface, wherein at least a portion of the surface has been processed by chemical mechanical polishing, CMP, and a layer of quantum spin defects, said layer of quantum spin defects being disposed within 500 nm of the surface.
As an option, at least a portion of the surface has been further processed by inductively coupled plasma, ICP, etching.
As an option, at least a portion of the surface has been further processed by mechanical polishing.
Exemplary types of quantum spin defects are selected from any of silicon containing defects, nickel containing defects, chromium containing defects, germanium containing defects, tin containing defects, and nitrogen containing defects.
Optionally, the quantum spin defects are negatively charged nitrogen-vacancy defects, NV−.
As an option, a concentration of quantum spin defects is selected from any of equal to or greater than: 1×1013 defects/cm3; 1×1014 defects/cm3; 1×1015 defects/cm3; 1×1016 defects/cm3; 1×1017 defects/cm3; and 1×1018 defects/cm3.
As a further option, a concentration of quantum spin defects is selected from any of equal to or less than: 4×1018 defects/cm3; 2×1018 defects/cm3; 1×1018 defects/cm3; 1×1017 defects/cm3; and 1×1016 defects/cm3.
Optionally, the quantum spin defects have a Hahn-echo decoherence time T2 equal to or greater than 10 μs, 50 μs, 100 μs, 300 μs, 600 μs, 1 ms, 10 ms, 100 ms, or 500 ms.
As an option, the single crystal CVD diamond component surface has a surface roughness Ra of no more than 5 nm, 2 nm, 1 nm, or 0.5 nm.
The single crystal CVD diamond component optionally comprises a further layer having a single substitutional nitrogen concentration of no more than 300 ppb, 200 ppb, 100 ppb, 80 ppb, 60 ppb, 40 ppb, 20 ppb, 10 ppb, 5 ppb, or 1 ppb, said layer being disposed distal to the surface relative to the layer of quantum spin defects.
The layer of quantum spin defects is optionally disposed within 500 nm, 200 nm, 100 nm, 50 nm, 30 nm, 10 nm, or 5 nm of the surface.
The thickness of the layer of quantum spin defects is optionally no more than 200 nm, 100 nm, 50 nm, 30 nm, 10 nm or 5 nm.
As an option, the single crystal CVD diamond component has at least one lateral dimension of at least 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 4.5 mm, or 5 mm.
According to a second aspect, there is provided a device comprising the single crystal CVD diamond component described above in the first aspect.
According to a third aspect, there is provided a method of fabricating the single crystal CVD diamond component described above in the first aspect, the method comprising providing a single crystal CVD diamond having a surface and processing at least a portion of the surface using chemical mechanical polishing, CMP, such that the single crystal CVD diamond comprises a layer of quantum spin defects, said layer of quantum spin defects being disposed within 500 nm of the surface.
The method optionally further comprises, prior to processing at least a portion of the surface using chemical mechanical polishing, processing the portion of the surface using inductively coupled plasma, ICP, etching.
The method optionally further comprises, prior to processing at least a portion of the surface using chemical mechanical polishing, processing the portion of the surface using mechanical polishing.
In an optional embodiment, the method further comprises implanting nitrogen into the surface of the single crystal CVD diamond and annealing the single crystal CVD diamond to cause migration of vacancy and/or nitrogen defects within the single crystal CVD diamond and formation of nitrogen-vacancy defects from the implanted nitrogen and the vacancy defects, such that the implanting and annealing form the layer of quantum spin defects disposed within 500 nm of the surface. As a further option, the single crystal CVD diamond, prior to implanting and annealing, has a single substitutional nitrogen concentration of no more than 300 ppb, 200 ppb, 100 ppb, 80 ppb, 60 ppb, 40 ppb, 20 ppb, 10 ppb, 5 ppb, or 1 ppb.
In an alternative optional embodiment, the method further comprises, prior to processing at least a portion of the surface using chemical mechanical polishing, providing the single crystal CVD diamond having a quantum spin defect concentration selected from any of equal to or greater than: 1×1013 defects/cm3; 1×1014 defects/cm3; 1×1015 defects/cm3; 1×1016 defects/cm3; 1×1017 defects/cm3; and 1×1018 defects/cm3. As a further option, the method comprises, prior to processing at least a portion of the surface using chemical mechanical polishing, providing the single crystal CVD diamond having a quantum spin defect concentration selected from any of equal to or less than: 4×1018 defects/cm3; 2×1018 defects/cm3; 1×1018 defects/cm3; 1×1017 defects/cm3; and 1×1016 defects/cm3.
The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:
Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity.
The present inventors have determined that a diamond surface can be greatly improved with near surface quantum spin defects by a combination of mechanical polishing, followed by ICP plasma etching to remove polishing damage. Critically, a final step of Chemical Mechanical Polishing (CMP) is applied to the surface to remove etch damage and produce a high-quality finish. This may be performed on diamond that already has quantum spin defects present, or the diamond can undergo an implantation technique such as that described in WO 2015/071487 at the polished surface. This gives a significantly improved decoherence time T2. A long T2 (e.g. greater than 50 μs) enables high resolution sensing and control operations. The T2 value decreases dramatically where mechanical surface and subsurface damage is present, and hence T2 values can provide a proxy for estimating subsurface damage.
CMP is a process in which a chemical slurry is applied to a surface to alter its surface bonding via chemical techniques and therefore form a softer phase of material. The mechanical component is the application of abrasive particles, typically on a polishing wheel, to remove the softer phase of material from the surface being polished. The advantage of this technique over regular mechanical polishing is that abrasive particles that are softer than the bulk diamond can be used. This ensures that there is less surface and subsurface damage using CMP than using regular mechanical polishing.
It is advantageous to have quantum spin defects in proximity to the surface so that they are readily accessible for end applications such that they can be place in optical structures or are close to the surface for sensing applications.
Examples of quantum spin defects include silicon containing defects, nickel containing defects, chromium containing defects, germanium containing defects, tin containing defects and nitrogen containing defects. The following description focuses on the quantum spin defect being a negatively charged nitrogen-vacancy (NV−) defect, but the skilled person will understand that the same techniques may be used for any type of quantum spin defect.
A diamond sample was provided consisting of single crystal CVD diamond with a very low nitrogen concentration, as described in WO01/096633.
The diamond of example was mechanically polished using a standard polishing technique to remove a depth of 80 nm of material. The diamond subsequently underwent nitrogen ion implantation at less than 150 keV, implanting to a depth of less than 100 nm, followed by irradiation and annealing to form near-surface NV− centres. Annealing was carried out for 16 hours at 800° C., followed by 1200° C. for 2 hours, as described in WO2012/090662
A diamond sample was mechanically polished in the same way as the diamond sample of example 1. The diamond then underwent ICP plasma etching under the conditions described in Mildren and Rabeau, Optical Engineering of Diamond, Wiley-VCH 2013 page 130 by 2 μm. The diamond subsequently underwent ion implantation followed by irradiation and annealing to form near-surface NV− centres in the same way as described in example 1.
A diamond sample was mechanically polished and ICP etched in the same way as the diamond sample of example 2. The diamond was then further processed using CMP, using a Logitech Tribo CMP system. The diamond subsequently underwent ion implantation followed by irradiation and annealing to form near-surface NV− centres in the same way as described in example 1.
It can be seen that, as is known, the step of ICP etching after polishing had a significant effect on the T2 value. However, it was surprising to see that the subsequent step of CMP processing had a further significant effect, increasing the value of T2 by a factor of almost 3 compared to the sample that had not undergone CMP.
It was subsequently found that the equipment used imposed artificial limits on the measured T2 values. As longer and longer T2 values are measured, equipment stability and stray magnetic fields make the measurement more difficult. The T2 value may be, for example, 1 μs, but the measurement time may be hours, such that there is interference from the environment which limits the T2 values that are recorded. Example 3 was measured once again using modified equipment to take into account environmental factors. Curve (a) shows the original measured values for Example 3, curve (b) shows the measured value using the modified equipment, leading to a T2 value of 550 μs. This is comparable to the values measured in WO 2015/071487 where no surface processing was applied.
The techniques described above may be used on any single crystal CVD diamond that includes quantum spin defects. Alternatively, the techniques above may be used on a diamond material that has negligible quantum spin defects, and the quantum spin defects are subsequently introduced to the CMP process surface.
The exact implantation depth and concentration of nitrogen will depend on the required characteristics of the diamond component in an end application. Typically, the nitrogen is implanted into the as-grown growth face of the single crystal CVD diamond layer to a depth of no more than 1 μm, 500 nm, 100 nm, 50 nm, 30 nm, 10 nm or 5 nm. Typically, the implantation dose will be at least 105 N/cm2, 106 N/cm2, 107 N/cm2, 108 N/cm2, 109 N/cm2, 1010 N/cm2, or 1011 N/cm2 and/or no more than 1014 N/cm2 or 1013 N/cm2. In certain circumstances for reasons of yield, it can be desirable to control the temperature of the diamond material during implantation, for example by heating or cooling the sample during implantation.
Note that it is not necessary to create a source of nitrogen and vacancies in the single crystal CVD diamond material at the same location as the desired end location of nitrogen-vacancy defects which are to be formed within the single crystal CVD diamond. In fact, it can be desirable to implant nitrogen and/or create vacancy defects at a location within the single crystal CVD diamond which is removed from the desired end location of nitrogen-vacancy defects which are to be formed within the single crystal CVD diamond. This is because implantation and irradiation creates damage within the diamond crystal structure which, if located near nitrogen-vacancy defects, can detrimentally affect the properties of the nitrogen-vacancy defects. Even using an annealing process to remove damage caused by implantation and irradiation some residual defects will remain which can adversely affect properties of the nitrogen-vacancy defects such as decoherence time T2. For example, it can be desirable to implant nitrogen at a different location within the diamond material to that which is irradiated to form vacancy defects. The material may then be heated to cause diffusion of the vacancy defects to the implanted nitrogen defects to form nitrogen-vacancy defects while minimizing crystal damage in the region where the nitrogen-vacancy defects are formed. Furthermore, charge donor defects, such as single substitutional nitrogen, which donate charge to nitrogen-vacancy defects to form NV− defects may also be separated from nitrogen-vacancy defects within the diamond material as described in WO2012/152617.
Note also that vacancies can be introduced by irradiating the single crystal CVD diamond either before or after the ion implantation step. It is known that nitrogen-vacancy defects form at around 800° C. As such, the annealing comprises an annealing step at a temperature in a range 700 to 900° C. for at least 2 hours, 4 hours, 6 hours, or 8 hours. It has also been suggested that treatment at a higher temperature can be advantageous for removing various paramagnetic defects to increase the decoherence time of NV− spin defects. Accordingly, the annealing may comprise a further annealing step at a temperature in a range 1150° C. to 1550° C. for at least 2 hours, 4 hours, 6 hours, or 8 hours. For example, the further annealing step may be performed at a temperature of at least 1200° C., 1300° C., or 1350° C. and/or a temperature of no more than 1500° C., 1450° C., or 1400° C. In addition, prior to the aforementioned annealing steps, an initial annealing step may be performed at a temperature in a range 350° C. to 450° C. for at least 2 hours, 4 hours, 6 hours, or 8 hours.
Of course, CMP could be used on an as-grown surface, but in order to remove sufficient material to get a smooth, flat surface, the process may take many days. In many applications it is therefore optimal to remove surface material using a technique such as mechanical polishing or ICP etching, as described above, followed by CMP processing. The total depth of surface material that may be removed by the combined processing techniques may be at least 10 nm, at least 50 nm, at least 100 nm, at least 1 μm, at least 5 μm, or at least 10 μm. The advantage of finishing the process with CMP is that subsurface damage that affects the T2 values of the spin defects is minimised.
While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appending claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2109750.6 | Jul 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/068565 | 7/5/2022 | WO |
