Nano- and micro-crystalline diamond film structuring with electron beam lithography mask

Direct current plasma enhanced chemical vapor deposition (CVD) was employed to create polycrystalline diamond films from CH4/H2 gaseous mixture at 98 mbar pressure and various substrate temperatures between 720 °C and 960 °C. The Si chips with patterns of periodic masked and open seeded zones were used as substrates. The mask free seeded areas evolved into polycrystalline diamond films after CVD process. The diamond crystallites of the films featured single crystal ordering individually with distinct cubic (100) or octahedral (111) facets on the film surfaces. Notably, specific growth conditions were determined for obtaining diamond films composed of the crystallites of nanometre and micrometre scale. These conditions are differing from those observed for non-pattern-prepared Si substrates. The nano-crystalline diamonds emerged within the 4.5–5 A current range, with growth conditions involving 3% CH4/H2 mixture at 98 mbar. The micro-crystalline diamonds (MCDs) predominantly characterized by well-developed rectangular (100) crystal faces on the film surface were successfully grown with current settings of 5.5–6 A, under 3% CH4/H2 mixture at 98 mbar. Furthermore, MCDs characterized by entirely crystalline (111) diamond faces forming CVD film surface were attained within a growth parameter range of 4.5–5.8 A, employing 3% CH4/H2 mixture for certain samples, or alternatively, utilizing 5 A with a 1.5% CH4/H2 mixture for others. Upon thorough evaluation, it was established that SiO2, TiO2, and Cr masks are well-suited materials for the planar patterning of both nano- and micro-crystalline diamond films, and the bottom-up approach can pave the way for the production of diamond planar structures through CVD, facilitated by electron beam lithography (EBL).


Introduction
Diamonds comprising colour centres are known as one of the best candidates for quantum sensing.By utilizing optically accessible quantum states of nitrogen-vacancy (NV) colour centres using optically detected magnetic resonance, sensing of electrical current and magnetic field [1], temperature [2], pressure [3], reactive oxygen species (ROS) [4] and pH [5] have been demonstrated.However, the lack of a simple, reproducible, and standardized technology for fabricating diamond-based device structures, along with the shortage in fluorescence collection efficiency, limits the usage of this type of sensing in real-world.
In this communication we will show a diamond planar structure fabrication through chemical vapor deposition Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
(CVD) growth on a foreign substrate pre-patterned by electron beam lithography (EBL).We will compare the different mask materials considered to assess their impact in preventing diamond growth and their influence on the diamond films deposition.
The fabrication of diamonds with colour centres by CVD offers a lot of advantages [6] for quantum applications.The CVD method allows fabrication of large-scale diamond films composed of micron-sized monocrystals of controllable needle shape and procures precise control of colour centres distribution in diamond during synthesis [7].
The main advantages of CVD are high purity, low amount of noisy defects, high coherence times of colour centres combined with its potential compatibility to well-developed lithography based nano-and micro-electronic technologies.It can provide an optimal combination of precise colour centre positioning allowing nano-metric spatial resolution with long coherence times and high sensitivity [8].

Materials and methods
10 mm × 20 mm p-type boron doped single-side polished Si substrates with a thickness of 525 ± 10 μm were seeded with diamond powder that was about 1 μm in size (scheme 1(a) and (b)) using common approach [9].Seeded substrate was spin coated with a positive tone polymethyl methacrylate (PMMA) resist (scheme 1(c)), and EBL unit by RAITH VISTEC EBPG 5000+ (100 keV, 50 MHz writing frequency) was used to pattern the surface of coated substrate (scheme 1(d)) at 300 μA beam current.Development in mixture of methyl isobutyl ketone and isopropanol (MIBK/IPA) revealed the patterns (scheme 1(e)).Evaporator Lesker Lab 18 and Leybold Univex 300 evaporators were used to deposit masking materials (scheme 1(f)).We used LESKER pallets to evaporate masking materials as known to provide smooth and amorphous films with good adhesion to the substrates.Following 24 h lift-off process in acetone, the substrate surfaces patterned with periodic strips of veiled and seeded zones (scheme 1(g)).Seeded zones-initiated diamond growth, whilst veiled areas prevented sp 3 carbon from developing.
Periodic parallel lines were chosen as masking pattern.Each suppressing line has 20 μm width (red lines in scheme 1(d) and grey lines in schemes 1(g) and (h)) while gaps in which diamond material further grows are 10 μm (black lines in scheme 1(g) and green in scheme 1(h)) giving period of 30 μm.Thickness of masking lines is defined by thickness of evaporated layer and was 500 nm in all presented samples (grey lines in schemes 1(g) and (h)).
A direct current plasma enhanced CVD (DC PECVD) system was used for the diamond film synthesis.Substrates were placed on the anode (bottom electrode of 50 mm diameter) in the reactor.The discharge current of the system was carefully set to operate within the range of 4.5-6.5 A, enabling the controlled synthesis of diamond films, containing both nano-crystalline diamonds (NCDs) and micro-crystalline diamonds (MCDs).Used setup as well as deposition methodology are described in supplementary materials and in [10][11][12].
For all processes involved in synthesizing the diamond films, a mixture consisting of 3% CH 4 /H 2 was employed.The substrate temperature was in the range from 720 °C to 960 °C, depending on the applied current and pressure in the reactor.The pressure was maintained at 98 mbar for all experiments described here.Duration of the synthesis for all processes was one hour.Standard seeded Si substrates were placed during each CVD procedure together with prepared patterned substrates.The comparison of deposits produced on the standard and patterned substrates was used to determine the impact of the masks on the forming material.For each of the diamond materials, independently, the suitability of masking materials for structured growth of diamond through suppressing was examined.If application requires lower synthesis temperature, microwave type of plasma enhanced CVD (MW PECVD) could be promising.It was successfully implemented for the nanodiamonds of 100 nm size growth at 250 C [13].However, PL properties of such diamond may differ significantly.
The LEO GEMINI 1550 (Zeiss) microscope was used to characterize the morphology with electron beam acceleration voltage of 10 kV.For imaging, an In Lens detector was used.We used the RENISHAW in Via Raman spectrometer unit for optical characterization.The spectrometer operated in both Raman and photoluminescence modes, and we acquired all presented spectra using an excitation wavelength of 514.5 nm.

Masking materials
Nineteen materials were considered as candidates for selective diamond growth, with the detailed list provided in table 1 of supplementary material.From this extensive selection, five materials, namely-Au, Cu, Cr, SiO 2 , and TiO 2 -were singled out for deployment as masking materials.The selection at this stage was based on their adaptability to diamond CVD synthesis conditions, possibility to prevent carbide formation that significantly influence heteroepitaxial diamond growth [14].

Structured nanocrystalline diamonds (NCDs)
NCDs were primarily found in agglomerates of diamond crystals.Due to small size of the nanocrystallites their facets are not developed.These films were produced using current values ranging from 4.5 to 5 A when Si substrates were subjected to treatments involving Cu, SiO 2 , and TiO 2 coatings.
When considering all types of the suppressing masks, both TiO 2 and SiO 2 masks (figures 1(b), (c) and (h), (i)), yielded the most favourable results for the synthesis of structured NCD films.However, it is essential to acknowledge that while some processes involving Cu masks did result in the growth of NCD films, the quality of the lines was notably inferior, rendering them unsuitable for high-quality  diamond structuring (please refer to the supplementary material for further details).

Synthesis of structured microcrystalline diamonds (MCDs)
MCDs composed of single crystalline diamonds (SCDs) having their well-developed 〈100〉 rectangular faces on film upper surface were obtained with SiO 2 and TiO 2 masks (figures 1(e), (f), and (k), (l)) with current range of 5.5-6 A. However, Cr mask in turn demonstrated an interesting feature (figures 1(n), (o)).Unlike the typical MCDs with SCDs having 〈100〉 crystal-oriented faces on their surface, under the same parameters, the diamond film synthesized on the substrate with the Cr mask contained on their surface 〈111〉 faces of the diamond crystals.(Compare figures 1(l) and (o)).
It is worth noting that we also achieved MCDs with SCDs having 〈111〉 crystal-oriented faces when using Au mask, although this required significant adjustments to the process parameters.SEM images illustrating substrates with these structures can be found in the supplementary material.
The observed effects of mask material on crystal orientation may be due to redistribution of plasma current, which depends on local substrate conductivity.Alternatively, variations in local substrate temperature may affect the crystallographic orientation of the grown diamond [8].
Figure 2 presents characteristic SEM images of diamond films grown on patterned substrates.They include MCD and NCD material alternating with masking pattern.Due to much higher secondary electron emission efficiency diamond material looks much brighter in comparison with the masking coatings.Some bright spots can be found also on dark lines.They correspond to diamond crystals also grown in masked region.Amount of these imperfections may be used to characterize quality of the masking with coatings suppressing diamond nucleation but not excluding it perfectly.Thus, based on SEM images in figure 2 the performance of used masking process can be evaluated qualitatively.Their quantitative analysis is presented below.

Suitability of masking materials
The effectiveness of the sp 3 carbon formation suppressing masks was checked after CVD process by evaluation of pattern defects presence.Defects, in this context, are the diamond particles located within the masked regions.The values of defect densities were calculated to quantitatively analyse the suppression quality.The calculations are based on obtained scanning electron microscopy (SEM) images.According to the analysis of about 20 SEM images for each process, the defects are distributed homogeneously over the substrate.To calculate defects densities, we counted the number of defects in SEM image of 100 × 100 μm 2 area, obtained numbers were divided by analyzed area (10 4 μm 2 ) and recalculated to defects per square millimetre units.SiO 2 , and Cr masking showed exceptionally good lines (figures 3(a), and (c)) with less densities of 1800 defects per square millimetre and 1600 defects per square millimetre.Cu masking demonstrated standard quality lines but due to melting experienced during CVD process and poor lift-off process (refer to supplementary material) it seems not suitable.These limitations can be avoided, perhaps, through optimization of lithography procedures and synthesis parameters.To avoid melting for example, substrate temperature should be lower than in standard process.However, detailed analysis of features of material synthesized in processes with significantly different parameters from the standard is a subject for separate research and is out of scope of this work.
A plasma current range of 4.5-5.8A yielded perfectly looking diamond film lines (figures 3(a), (b), and (c)) with a lower defect density of 14 800 defects per square millimetre.When we increased the plasma current to 6.2 A, the wellstructured diamond film lines applying SiO 2 mask with a density of 42 600 defects per square millimetre were obtained.However, a further increase to 6.5 A in plasma current led to a deterioration in diamond film structuring, resulting in a higher defect density of 73 300 defects per square millimetre.Pushing the plasma current to 7 A produced exceedingly poor diamond lines, characterized by diamond agglomerations and deep surface irregularities on masked areas on the Si substrate, with a calculated defect density of 83 700 defects per square millimetre.
SiO 2 , TiO 2 , and Cr emerged as effective masks, potentially due to their ability to hinder diamond growth.This hindrance may be attributed to their propensity, under the specified conditions, to form chemical compounds or interfere with the chemical reactions essential for diamond nucleation and growth within the CVD chamber.It therefore conceivable that reactions with the carbon-containing gases during the CVD process, disrupted the formation of diamond [15].

Optical characterization of synthesized films
Raman spectra (figure 4) of CVD films exhibited a significant peak at approximately 1332 cm −1 , characteristic of crystalline diamond phase [8].This peak relates to first-order Raman scattering within diamond lattice associated with the vibrational motion of sp 3 -bonds, the fundamental building blocks of a typical crystalline diamond structure [16].
Bands at 1140 cm −1 and 1470 cm −1 served as clear indicators of nanocrystalline diamonds, which are diamond particles of 5-2 nm in size.These bands are known to become more pronounced as the size of diamond crystallites decreases from tens to few nanometers [17].Presence of these bands in spectra obtained for MCD film is associated with the nanometric diamond crystals formed together with microcrystals due to secondary nucleation.
Additional Raman bands were observed at 1350 cm −1 and 1580 cm −1 , both associated with non-diamond carbon features.The 1350 cm −1 line commonly called the D band, is closely linked to phonons originating from the K point of the Brillouin zone, resulting from resonant Raman scattering through a double resonance mechanism that involves two successive electron transitions with varying energies and momenta [18].
Concurrently, the Raman line at 1580 cm −1 , commonly known as the G-line, represents a distinct feature related to a firstorder phonon mode connected to the stretching of sp 2 -bonds.This signifies a well-structured configuration stemming from inplane C-C atom vibrations [18].Notably, this characteristic is also commonly observed in multiwall carbon nanotubes and other materials with graphite type (sp 2 ) atomic bonding [16].
Other Raman bands at 520 cm −1 and 960 cm −1 are attributed to first and second order scattering in Si substrate material [19].
It is worth noting that all the samples exhibit a strong photoluminescent (PL) signal centred at approximately 575 nm zero phonon line, corresponding to defects involving nitrogen vacancies in their neutral state.The recorded values are consistent with previous research findings [20][21][22].Variations in the positions of these peaks can be attributed to factors such as temperature fluctuations and internal mechanical stress arising from differences in thermal expansion coefficients between the diamond, substrate, and the inclusion of graphitic phases [23][24][25].

Conclusion
In conclusion, our experimental findings indicate that Cr mask effectively impedes diamond growth on Si substrates across a wide range of synthesis parameters.Both SiO 2 and TiO 2 masks demonstrate reliable performance throughout the various synthesis processes.The occasional presence of diamond particles on covered lines is likely attributed to some mask imperfections arising from the high roughness of the Si substrate after mechanical seeding.Enhancing mask efficiency can be achieved by implementing smaller or nanometric particles for seeding.
The films synthesized on masked Si substrates exhibited the expected photoluminescence of nitrogen-vacancy colour centres, rendering them suitable for applications in magnetic sensing.Additionally, incorporating microwave wires on masked lines can facilitate efficient delivery of microwave irradiation to NVs within the diamond structure.
SEM images revealed that the Cr mask induces a transition from (100) to (111) diamond growth across all utilized parameters, while SiO 2 and TiO 2 mask-synthesized material exhibited subtle distinctions compared to the unmasked Si substrate.These variances likely result from localized parameter shifts within the masked regions due to changes in conductivity induced by dielectric SiO 2 and TiO 2 masks.
Our controlled initiation of growth sites using thin layers of TiO 2 , SiO 2 , and Cr masks in a bottom-up approach has allowed us to successfully cultivate diamond films housing nitrogen-vacancy centres.These structured diamond materials can assume specific and regular shapes, limited by the resolution of electron beam lithography, thereby paving the way for the potential development of bottom-up diamond planar structures nanofabrication, and holding significant promise for advancements in quantum technologies.