# Impurity analysis of synthetic diamond for electronics and quantum physics

Estelle Loire, Rémi Gillet, Mohamed Bouras, François Jomard, M.A. Pinault-Thaury

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Estelle Loire, Rémi Gillet, Mohamed Bouras, François Jomard, M.A. Pinault-Thaury. Impurity analysis of synthetic diamond for electronics and quantum physics. Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures, 2025, 43, pp.054001. 10.1116/6.0004607. hal-05008695v2

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Impurity analysis of synthetic diamond for
electronics and quantum physics

Estelle Loire¹, Rémi Gillet¹, Mohamed Elhachmi Bouras¹, François Jomard¹
and Marie-Amandine Pinault-Thaury ¹,a)

¹Université Paris-Saclay, UVSQ, CNRS, GEMaC, 78000 Versailles, France

a) Electronic mail: marie-amandine.pinault-thaury@uvsq.fr

Diamond is an ultra-wide band-gap semiconductor with exceptional physical properties
making it highly suitable for various advanced applications. However, precise doping
control remains a challenge, particularly for n-type conductivity due to doping
asymmetry. Additionally, in quantum applications, phosphorus donors enhance NV
center coherence times, requiring precise impurity level control. This work presents a
secondary ion mass spectrometry (SIMS) study on impurities analysis in diamond,
focusing on hydrogen, boron, carbon isotope, nitrogen and phosphorus. Using four
implanted standards and two lab-grown samples, we optimized SIMS conditions to
improve detection limits and isotopic ratio measurements. We measured a ¹³C/¹²C
isotopic ratio of ~ 0.004% in a ¹³C-depleted diamond layer. At low mass resolution, we
showed detection limits of up to 1.5 x 10¹⁶ and 3.0 x 10¹⁵ at/cm³ for boron and phosphorus
respectively. For hydrogen, the increase in sputtering rate enabled us to move from 4.7 x
10¹⁸ to 6.3 x 10¹⁷ at/cm³. High mass resolution settings enabled reach phosphorus
detection limits above 3.0x10¹⁴ at/cm³. Concerning nitrogen, we use an ultra-pure lab-
grown layer to achieve the measurement of a detection limit of 2.3 x 10¹⁶ at/cm³, thanks
to the raster size reduction. These results provide crucial insights into impurities
<figure><img src="image_3.png" /></figure>

characterization in diamond, supporting advancements in diamond-based electronic and
quantum applications.

# I. INTRODUCTION

Diamond is a precious and rare stone that presents exceptional physical
properties. With a band-gap of 5.47 eV at room temperature (RT) ¹ and compare to other
wide band-gap semiconductors (as SiC, GaN), diamond benefits of a higher breakdown
electric field (2x10⁷ V/cm) ², higher intrinsic carrier mobilities (> 3200cm²/V.s for holes
and in the range of 2400–4500 cm²/V.s for electrons ³⁻⁵ at RT) and a higher thermal
conductivity (24W/cm.K at RT ⁶, 5 times that of copper). Moreover, this material is
radiation hard ⁷ and chemically inert. Therefore, diamond is considered as the ultimate
ultra-wide band-gap semiconductor.

Diamond can be synthesized either by high pressure (5-10 GPa) and high
temperature (1200 – 2200 °C) (HPHT) process that reproduces the conditions created on
Earth, or micro-wave plasma chemical vapor deposition (MPCVD) at low pressure (< 1
GPa). The MPCVD method allows a more suitable control of impurities for several
research fields.

In the field of power electronics, mastering both n-type and p-type doping is a key
issue to make bipolar devices⁸. This doping is performing by incorporation of
substitutional impurities such as phosphorus for n-type and boron for p-type ⁹,¹⁰. The
<figure><img src="image_4.png" /></figure>

expected concentration range extends from $10^{13}$ to $10^{21}$ at/cm$^3$, form intrinsic to metallic material.

In quantum physics, the electron spin plays a significant role through the spin manipulation of the nitrogen-vacancy (NV) center $^{11}$. Recently, Herbschleb et al. $^{12}$ show that phosphorus donors in the diamond crystal extend the coherence times of NV centers. These breakthroughs have been made possible by an overall improvement in the quality of CVD diamond and, in particular, a reduction of defects that generate spin interference such as carbon isotope $^{13}$C. To obtain these results, the phosphorus doping range was between $3 \times 10^{15}$ and $1 \times 10^{17}$ at/cm$^3$ and the nitrogen content was generally lower than $2 \times 10^{18}$ at/cm$^3$ $^{13}$.

For these applications, a high control of diamond growth is therefore expected. This includes knowledge of the impact of growth conditions on impurity levels. In this work, we report on secondary ion mass spectrometry (SIMS) study performed on diamond. We detail the SIMS analysis conditions we use for the analysis of impurities ($^{1}$H, $^{11}$B, $^{13}$C, $^{14}$N and $^{31}$P) to overcome inherent challenges of measurements due to the material itself and the analysis environment.

## II. EXPERIMENTAL

SIMS analysis has been performed (see ref. 14 for details on SIMS technique) on different samples under our conventional conditions used for diamond.

### A. Diamond samples

(100) oriented 3x3 mm$^2$ diamond plates are currently commercially available with just a few specifications regarding contaminations. Depending on the synthesis method
<figure><img src="image_5.png" /></figure>

used, one can order HPHT Ib plates that are known to contain high level of nitrogen (between $2 \times 10^{17}$ and $2 \times 10^{19}$ at/cm$^3$, i.e. 1-100 ppm, from Sumitomo supplier) or CVD IIa plates from which the nitrogen content should be much lower ($< 2 \times 10^{17}$ at/cm$^3$, i.e. $< 1$ ppm, from Element Six supplier). Sometimes, further specifications are given like boron contamination of CVD plates: lower than $9 \times 10^{15}$ at/cm$^3$ (i.e. 0.05 ppm) from Element Six supplier. In SIMS analysis, standards are commonly used to transform raw data into quantified data. In our case, we used HPHT Ib (100) diamond plates to fabricate hydrogen, boron and phosphorus implanted standards while a CVD IIa (100) diamond plate is used to make a nitrogen standard. Ion implantation allows to introduce elements into these plates with precise depth and concentration control. The peak implantation depth is around 200 nm from the surface, with a maximum concentration of $\sim 10^{19}$ at/cm$^3$ (i.e. $\sim 100$ ppm). The values of implantation energy and fluence used by the Ion Beam Services company in Peynier (France) are reported in Table I.

In addition to the four diamond standards, we studied two diamond samples by taking advantage of our lab capability in diamond synthesis. We grew an ultra-pure layer (UPL) and an isotopic layer (IsoL) on Sumitomo HPHT Ib (100) diamond plates that have been previously polished to get a miscut angle of $\sim 3.5^\circ$ $^{15}$. Both layers were grown by MPCVD in two separate growth reactors. The UPL sample was synthetized in a Plassys Bestek BJS150 commercial reactor (bell jar type $^{16}$ with natural methane ($^{13}C$/($^{12}C$ = 1.1%)), while the IsoL sample was realized in our home-made reactor equipped with a metallic plasma chamber $^{17}$ and with purified methane ($^{13}C$/($^{12}C$ < 1.1%). Table II summarized the growth conditions used for those two diamond samples.
<figure><img src="image_6.png" /></figure>

<table><caption>TABLE I. Implantation energy and fluence used to make standards from commercially available diamond plates.</caption><thead><tr><th rowspan="2">Implantation parameters</th><th colspan="4">Chemical elements</th></tr><tr><th><sup>1</sup>H</th><th><sup>11</sup>B</th><th><sup>14</sup>N</th><th><sup>31</sup>P</th></tr></thead><tbody><tr><td>Energy (keV)</td><td>120</td><td>180</td><td>180</td><td>180</td></tr><tr><td>Fluence (at/cm<sup>2</sup>)</td><td>3x10<sup>15</sup></td><td>3x10<sup>14</sup></td><td>3x10<sup>14</sup></td><td>3x10<sup>14</sup></td></tr></tbody></table>

<table><caption>TABLE II. Growth conditions used to synthetize, by MPCVD into two separate reactors, an ultra-pure diamond layer (UPL) and an isotopic layer (IsoL).</caption><thead><tr><th rowspan="2">Growth conditions</th><th colspan="2">Sample</th></tr><tr><th>UPL (A46)</th><th>IsoL (D330)</th></tr></thead><tbody><tr><td>Carbon precursor</td><td>Natural methane<br>(<sup>13</sup>C/<sup>12</sup>C = 1.10%)</td><td>Isotopic methane<br>(<sup>13</sup>C/<sup>12</sup>C &lt; 0.01%)</td></tr><tr><td>Phosphorus precursor</td><td>Tertiarybutylphosphine<br>(TBP: C<sub>4</sub>H<sub>11</sub>P)</td><td>--</td></tr><tr><td>Temperature (°C)</td><td>~ 940</td><td>~ 1100</td></tr><tr><td>Pressure (mbar)</td><td>140</td><td>75</td></tr><tr><td>[C]/[H<sub>2</sub>] (%)</td><td>5</td><td>1.8</td></tr><tr><td>Total gas flow (sccm)</td><td>250</td><td>400</td></tr><tr><td>Micro-wave power (W)</td><td>3100</td><td>230</td></tr><tr><td>[P]/[C] (ppm)</td><td>40</td><td>--</td></tr><tr><td>Duration (h)</td><td>5</td><td>5</td></tr><tr><td>Growth rate (µm/h)</td><td>14.6</td><td>5.9</td></tr><tr><td>Thickness (µm)</td><td>~ 73</td><td>~ 29.7</td></tr></tbody></table>
<figure><img src="image_7.png" /></figure>

## B. SIMS analysis

We use a magnetic sector mass spectrometer that allows to perform analysis in dynamic mode (CAMECA IMS7f, Gennevilliers - France). Our analyzer is dedicated to materials (metals, oxides, semiconductors, ...) that do not require any specific preparation. We are used to performing doping profiles of several impurities in once because the analysis creates craters, and the sample surface must be preserved for future physical characterizations.

### 1. Our conventional diamond conditions

The analysis chamber is kept under high vacuum (~2x10⁻¹⁰ - 6x10⁻¹⁰ mbar). As the SIMS technique is mostly used in diamond to detect impurities, analyses are carried out using parameters that provide high sensitivity. We employed the Cs⁺/M⁻ configuration: positive primary ions with a Cs⁺ source and detection of negative secondary ions of mass M¹⁸. The energy of the Cs⁺ primary beam is set to 10 keV. Secondary ions are detected in the negative mode by biasing the sample to -5kV, leading to an interaction impact energy of the primary ions of 15 keV and an incidence angle of ~23° with respect to the normal of the sample.

For routine analyses, the raster size is 150x150 µm². The analyzed zone is restricted to a diameter of 33 µm and an electronic gate is set to 70% to avoid the crater edge effects. The measurements were done using peak jumping of the magnetic field and the duration of each counting cycle does not exceed 30 secondes. The depth of the resulting SIMS crater is measured with a Dektak8 step-meter from Veeco-Bruker (Wissembourg – France). Samples are gold-coated (35-50 nm) to avoid possible charging
<figure><img src="image_8.png" /></figure>

effects. By using a 60 nA primary intensity, the sputtering rate, SR, of around 0.4 nm/s in diamond matrix is reached. Note that we deliberately limit the primary current to 60 nA to maintain a correctly focused primary beam.

## 2. Detected ions and their masses

In this work, we focused on the detection of hydrogen, boron, carbon, nitrogen and phosphorus. With zero electron affinity, nitrogen give no negative ion $ ^{14}N^{-} $. The most efficient way to detect nitrogen is then to collect a molecular ion consisting of a nitrogen atom and a atom of the matrix. Knowing that carbon is the only matrix element in diamond, the $ ^{14}N^{12}C^{-} $ molecular ion is used to detect nitrogen with a good sensitivity $ ^{19} $. The corresponding detected mass, M, is close to 26 amu.

Unlike nitrogen, the other elements of interest have non-zero electron affinity and give negative ions. The $ ^{1}H^{-} $, $ ^{11}B^{-} $, $ ^{12}C^{-} $, $ ^{13}C^{-} $ and $ ^{31}P^{-} $ single ions are then used to detect hydrogen, boron and phosphorus. The corresponding detected masses are respectively around 1 amu, 11 amu, 12 amu, 13 amu and 31 amu.

## 3. Mass resolution

In mass spectrometry, resolution is a measure of the ability to distinguish two peaks of slightly different mass-to-charge ratios $ \Delta M $, in a mass spectrum. With low mass resolution (LRM) analysis ($ M/\Delta M = 400 $), compounds are discriminated by their nominal mass, which can lead to false positives in the case of isobaric components. To solve this relative lack of specificity, high resolution mass spectrometry (HRM) is necessary. While hydrogen and boron are not affected by mass interferences at LMR, the 26 amu signal for
nitrogen contains $^{14}$N$^{12}$C$^{-}$ (26.003 amu) with mass interferences as $^{13}$C$_2$$^{-}$ (26.007 amu), and $^{12}$C$^{13}$C$^1$H$^{-}$ (26.011 amu) and $^{12}$C$_2$H$_2$$^{-}$ (26.016 amu). In the case of phosphorus, the 31 amu signal contains $^{31}$P$^{-}$ (30.973 amu) with mass interferences of $^{12}$C$^{19}$F$^{-}$ (30.998 amu) and $^{12}$C$^1$H$_3$$^{16}$O$^{-}$ (31.018 amu). Finally, the 13 amu signal contains $^{13}$C$^{-}$ (13.003 amu) with mass interferences as $^{12}$C$^1$H$^{-}$ (13.008 amu). As a result, $^{12}$C$^{14}$N$^{-}$, $^{31}$P$^{-}$ and $^{13}$C$^{-}$ masses are then not clearly distinguishable from neighboring masses at LMR.

To work at high mass resolution (HMR), the widths of the spectrometer's entrance and exit slits are reduced. to get M/ΔM close to a value that allows a clear identification of ion masses. Fig. 1 presents the mass spectrum around the 13 mass with a HMR of ~3000. We observe that the $^{13}$C$^{-}$ signal is clearly separate from the closest signal of $^{12}$C$^1$H$^{-}$. In the case of nitrogen and phosphorus, the mass resolution has to be adjusted at ~7500 $^{19}$ and ~1600 $^{20}$, respectively.

<figure><img src="image_9.png" /><figcaption>FIG. 1. Mass spectra around 13 amu at high mass resolution (M/ΔM ~3000) where $^{13}$C$^{-}$ ions are clearly separate from neighboring ions (as $^{12}$C$^1$H$^{-}$).</figcaption></figure>

We have applied our standard SIMS conditions for diamond analysis using either LMR settings (usually used to ensure a high level of transmission and a maximum of
<figure><img src="image_10.png" /></figure>

sensitivity) or HMR settings. It is worth noting that the use of HMR settings induces lower transmission results in lower transmission, thereby reducing signal intensities in SIMS analysis.

### III. RESULTS AND DISCUSSION

#### A. Carbon isotopic abundance

Figure 2 (a) shows the depth profiles performed under HMR-SIMS conditions around 13 amu on the IsoL sample. We observe the ¹²C⁻, ¹²C¹H⁻ and ¹³C⁻ signals throughout the CVD diamond layer and part of the HPHT diamond substrate. We voluntarily performed the analysis to get signals from both, the layer and the substrate. In the case of the IsoL sample, this explains analysis time is over nearly 80 000 s (more than 22h) as shown on the top axis of Fig.2 (a). For such a long analysis time, we choose to regularly control the magnetic field. In practice, we select a mass that presents high intensity and is constant over the sample volume (as a matrix element) to check the magnetic field stability. Each analyzed mass is then recalibrated according to the recorded drift. In our case, we use the ¹²C mass and a check every 20 cycles (~460 s). The evolution of the magnetic field over the analysis is presented in Fig.2 (b). We observed that the variation of the magnetic field does not exceed 0.006 mT around the average value of 328.415 mT, meaning a variation of only 0.00183%.

The thickness of the CVD layer can be estimated from the depth profile. The time value at the end of the analysis, $t_{total}$, corresponds to the time required to sputter the crater. The measurement of the crater depth (in nanometer, nm), $p_{crater}$, allows to determine the sputtering rate as $SR = p_{crater}/t_{total}$ in nm/s, and then to convert the time axis (top axis on
<figure><img src="image_11.png" /></figure>

This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.11676/0004607

Fig.2) into depth axis (bottom axis on Fig.2 (a)). As the CVD layer is grown with methane having a low carbon isotopic ratio ($^{13}$C/$^{12}$C < 0.01%, as provided by the supplier), a change in the $^{13}$C$^-$ signal corresponds to the interface between the CVD layer and the HPHT substrate that is expected to present natural isotopic ratio of carbon (i.e. 1.1%). As a result, we see from Fig.2 (a) that the CVD layer thickness is ~29.7 µm.

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>
FIG. 2. (a) Depth profiles of the masses around 13 amu obtained using our HMR-SIMS conditions for ¹³C⁻ analysis on our IsoL sample; (b) Evolution of the magnetic field during the analysis with an average value of 328.415 mT and a variation of ± 0.006 mT.

From Fig.2 (a), it is possible to calculate the carbon isotopic abundance of the CVD layer Indeed, it is the ratio of the ¹³C⁻ and ¹²C⁻ signals: I₁₃C<sup>CVD</sup>/I₁₂C<sup>CVD</sup> ~ 0.0004%. This value is much more precise than the one given by the methane supplier: ¹³C/¹²C < 0.01%. Even if this difference between gas and bulk purity is still under investigation, we can conclude that the isotopic abundance of ¹²C in our IsoL sample is ~99.9996%.

## B. Boron and phosphorus detection limits

The depth profile of the boron implanted standard obtained in LMR-SIMS conditions is shown in Figure 3. With ion implants, it is possible to quantify the species of interest and to measure the detection limit ¹⁴. Knowing the implantation fluence, Φ<sub>implant</sub>, it is possible to determine the relative sensitivity factor (RSF, in at/cm³) value. By recording the signals (in cps) of the matrix element, I<sub>matrix</sub>, and the element of interest, I<sub>element</sub>, in the implanted reference, the RSF<sub>element/matrix</sub> is given by

$$RSF_{element/matrix} = \Phi_{implant} * \frac{I_{matrix}}{Sr * \int_{0}^{t} I_{element} dt} \quad \text{in at/cm}^3 \quad (1)$$

The definite integral between 0 and t is indeed the area under the intensity curve of the element of interest. To convert the intensity (right axis in Fig.3) in concentration (left axis in Fig.3), the following equation is then used:

$$[element] = RSF_{element} * \frac{I_{element}}{I_{matrix}} \quad \text{in at/cm}^3 \quad (2)$$
<figure><img src="image_14.png" /></figure>

The profile tail of the implanted reference gives the value of the detection limit DL, of the element of interest <sup>14</sup>As. As a result, the DL of boron with our LMR-SIMS conditions is 1.5 ± 0.5 x 10<sup>16</sup> at/cm<sup>3</sup> (85 ppb). The uncertainty of the DL is evaluated to 20% <sup>14</sup>As.

<figure><img src="image_15.png" /><figcaption>FIG. 3. Depth profile of our boron implanted standard obtained using our LMR-SIMS conventional conditions.</figcaption></figure>

In the case of phosphorus analysis, we shown in previous work <sup>20</sup> that LMR-SIMS conditions allow the measurement of phosphorus content in diamond above 4.0x10<sup>15</sup> at/cm<sup>3</sup> (23 ppb). Moreover, applying HMR conditions allows the measurement of one decade lower, meaning starting from 4.0x10<sup>14</sup> at/cm<sup>3</sup> (2.3 ppb). Figure 4 shows the depth profile of the phosphorus implanted standard with the DL of 3.0x10<sup>15</sup> at/cm<sup>3</sup> (17 ppb) at LMR (Fig.3 (a)) and 3.0x10<sup>14</sup> at/cm<sup>3</sup> (1.7 ppb) at HMR (Fig.3 (b)). While boron detection is not affected by any mass interference, it is worth noting that the DL of phosphorus in diamond is limited by unexpected and unidentified interfering species having the exact
<figure><img src="image_16.png" /></figure>

phosphorus mass²⁰. This contamination could come from previous phosphorus contamination of the immersion lens by other materials containing phosphorus in their matrix (e.g. InP) which cannot get rid of.

<figure><img src="image_17.png" /><figcaption>Fig.4. Depth profiles of the carbon and the phosphorus content obtained by analyzing the phosphorus-implanted standard with our conventional SIMS diamond conditions with (a) LMR and (b) HMR settings.</figcaption></figure>
<figure><img src="image_18.png" /></figure>

C. Case of hydrogen

Figure 5 (a) presents the depth profile of hydrogen for the hydrogen-implanted
standard obtained in our LMR-SIMS conditions. The DL of hydrogen is then thus
measured at 4.7 x 10¹⁸ at/cm³ (~27 ppm). However, hydrogen element is commonly
referred to residual gas elements as nitrogen. Analysis of this element requires special
attention because its presence in the vacuum system results in redeposition on the region
of interest during analysis. There are several methods to improve residual gas detection
limits. With an ultra-high vacuum analysis chamber, we choose to increase a sputtering
rate that will reduce the impact of redeposition. For that purpose, the raster is reduced to
50 x 50 µm² and the diameter of the analysis area down to 8 µm (see Fig.5 (b)). The
sputtering rate is then ~2.7 nm/s and the DL of hydrogen reduced to 6.3 x 10¹⁷ at/cm³ (~4
ppm).

We note that reducing the raster size by a factor of 3 induces an improvement of the DL by the same factor. This improvement could be due to the suppression of OH surface contamination as shown in the work of A. Stephant et al. 21
<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>FIG. 5. Depth profiles of the hydrogen implanted standard obtained using LMR-SIMS conditions (a) with our usual raster size and analyzed area, (b) with a 50x50 µm² raster size and a 8 µm diameter of analyzed area.</figcaption></figure>

**D. Focus on nitrogen detection limit**

In figure 6 (a), we present the raw profile of the nitrogen-implanted standard obtained in our HMR-SIMS conditions. At the end of the acquisition, the area swept by
<figure><img src="image_22.png" /></figure>

the primary beam is restricted while the primary current and the size of the analysis area are kept constant. This leads to an increase of SR and is called the raster reduction method. Depending on the evolution of the intensity of the element of interest, we can distinguish 3 cases. (1) In the first case, the intensity increases with the raster reduction and within the same proportion than SR. This means that the signal comes from the sample (as the matrix element). (2) In the second case, the intensity stays constant while the raster is reduced. This indicates that the signal is not coming from the sample and gives the background level of the analysis. (3) Finally, the last case corresponds to an increase of the intensity but in a smaller proportion than SR. The signal is then a mix from both previous cases.

From Fig.6 (a), the profile tail of the nitrogen-implanted standard presents an increase of the intensities with the raster reduction size. The ¹⁴N¹²C⁻ signal increases by a factor of 1.6 while the ¹²C⁻ signal increases by a factor of 2.2. This indicates that we are in case (3): the ¹⁴N¹²C⁻ signal is a mix between the background signal and the nitrogen contamination of the commercial CVD diamond plate. As a result, the tail profile of our nitrogen-implanted standard cannot be used to evaluate the DL of nitrogen in diamond during our analysis.
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1167/6.0004607

<figure><img src="image_23.png" /><figcaption>Fig.6. Raw profiles get with our HMR conditions with both ending with a raster reduction size applied to (a) the nitrogen-implanted standard and (b) our UPL sample.</figcaption></figure>

We analyzed our UPL diamond sample using the HMR-SIMS conditions and applying a raster reduction size at the end of the analysis (see Fig.6 (b)). We observe, as the raster size is reduced, that the matrix signal increases while the nitrogen related signal stays constant. This means that we are in case (2): the ¹⁴N¹²C⁻ signal is at the background
<figure><img src="image_24.png" /></figure>

level. As a result, we can determine the DL of our analysis by applying equation (2). The DL values are then 4.6x10¹⁶ at/cm³ with the initial raster size and 2.3x10¹⁶ at/cm³ with the smaller raster size (here, 125x125 µm²) while applying our HMR settings for nitrogen detection.

## IV. SUMMARY AND CONCLUSIONS

We studied four implanted standards and two lab-made diamond samples by using our SIMS diamond conditions routine: Cs⁺/M⁻ configuration, 15 kV impact energy, 60 nA primary beam, 150x150 µm² raster size with 33 µm diameter of analyzed area, ~0.4 nm/s sputtering rate. By adjusting the mass resolution to the analyzed species, we show our ability to measure a carbon isotopic ratio of ¹³C/¹²C = 0.0004% in a diamond layer grown with ¹³C-depleted methane precursor gas and refine the precision given by the supplier (¹³C/¹²C < 0.01%). Moreover, we evaluate the detection limits at low mass resolution of hydrogen, boron and phosphorus to 4.7 x 10¹⁸, 1.5 x 10¹⁶ and 3.0 x 10¹⁵ at/cm³, respectively. For hydrogen, by using higher sputtering rate (~2.7 nm/s), the detection limit can be enhanced to 6.3 x 10¹⁷ at/cm³. In the case of phosphorus, high mass resolution settings allow to reach 3.0 x 10¹⁴ at/cm³. Knowing the SIMS detection limits makes it possible to assess the reliability of the results, to qualify diamond samples, to ensure the performance of electronic or quantum diamond devices, and to avoid misinterpretations due to insufficient sensitivity.

We also suited nitrogen detection in diamond. We evidenced that our nitrogen-implanted standard cannot be used for the detection limit determination because of the contamination level of the initial commercial CVD IIa diamond plate. However, we
<figure><img src="image_25.png" /></figure>

benefit of a lab-made ultra-pure diamond layer that allowed us to measure the detection limit to 4.6 x 10¹⁶ at/cm³ with our routine HMR-SIMS conditions. This value is lowering by a factor two by reducing the raster size to 125 x 125 µm² (and so, increasing the sputtering rate).

In further work, it might be interesting to use electronic grade diamond plates to realize ion-implanted standards. Indeed, the specifications of such plates give an impurity level around or below 10¹⁴ at/cm³ (~0.6 ppb). The challenge would be to find dedicated SIMS settings to reach the lowest possible detection limits for each impurity presented in this study.

## ACKNOWLEDGMENTS

The authors would like to thank the French National Research Agency (ANR) for its assistance in funding SIMS analyses and diamond growth via the MOVeToDiam (ANR-17-CE05-0019) and LAPIN113 (ANR-20-CE05-0032) projects. The authors also thank CNRS Physique that provides funding for precursors through the Tremplin 2020 project.

## AUTHOR DECLARATIONS

The authors have no conflicts to disclose.

## DATA AVAILABILITY

Data are available at 10.5281/zenodo.15096061.

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