# Synthesis and characterisation of NCD films on 10 x 10 mm² and deposition on 2 inch wafer using rotating substrate-holder set-up

Samuel Saada, Jean-Charles Arnault, Licinio Rocha, Bertrand Bazin, P. Bergonzo

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Samuel Saada, Jean-Charles Arnault, Licinio Rocha, Bertrand Bazin, P. Bergonzo. Synthesis and characterisation of NCD films on 10 x 10 mm² and deposition on 2 inch wafer using rotating substrate-holder set-up. Physica Status Solidi A (applications and materials science), 2008, 205 (9), pp.2121-2125. 10.1002/pssa.200879732. cea-05465531v3

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# Synthesis and characterisation of NCD films on 10 × 10 mm<fnref n="2" /> and deposition on 2 inch wafer using rotating substrate-holder set-up

S. Saada*,<fnref n="1" />, J. C. Arnault<fnref n="1" />, L. Rocha<fnref n="2" />, B. Bazin<fnref n="1" />, and P. Bergonzo<fnref n="1" />

## Abstract

In this study, we compare NCD films on 10 × 10 mm<fnref n="2" /> silicon substrates synthesised with high methane concentration and NCD films synthesised with prolonged bias during the growth step. Further, we performed in-situ sequential XPS analysis (carbon binding state) using an UHV surface analysis system connected to the MPCVD reactor. AFM and HRSEM were used to characterize films morphology. Using the same substrate holder, NCD films have been deposited on silicon 2 in wafers. To improve the homogeneity, a rotating substrate holder set-up enabling biasing and heating of the stage has been developed and coupled with computer control of the process for a better reproducibility. UV-interferometry was performed to map the film thickness on 2 inches and quantify its thickness uniformity. Considering the symmetry of the system, AFM measurements were performed along the radius of the wafer to evaluate the surface homogeneity and its smoothness. The thickness uniformity of a NCD film of 1.6 µm deposited on 2 inch wafer is under 10% and the RMS roughness comprised between 13 and 14 nm.

## 1 Introduction

NanoCrystalline Diamond (NCD) films are of great interest for many applications (MicroElectroMechanical Systems (MEMS), Surface Acoustic Wave devices (SAW), ...) [1–3] because they combine several attractive properties like high hardness, chemical inertness, excellent mechanical properties and smoothness. NCD films are defined by a size grain below 100 nm and could have columnar structure or nanograins stack up structure. The NCD films with columnar structure are obtained with high nucleation density (>1 × 10¹⁰ cm⁻<fnref n="2" />) associated with conventional selective growth conditions. These films are limited to a few hundred of nanometers thick for having grain sizes in the nanometer range. The nanograins stack up structure is obtained with a continuous nucleation during the growth step. If the grain size is in the range of 3–5 nm, we talk about UNCD films (UltraNanoCrystalline Diamond). Several methods have been developed to increase the secondary nucleation and include argon diluted methane (low hydrogen) plasma [4], negative bias voltage during deposition [5–7], nitrogen diluted methane plasma [8, 9], DC glow discharge plasma [10], high methane concentration hydrogen plasma [11, 12], etc.

The grain size of NCD film and the related properties not only depend on the grain size but also on the crystalline quality of the diamond phase and grain boundaries. As a result, there is a great variety of NCD films, with different morphologies and roughnesses, although many applications require films exhibiting low roughness surfaces and high thickness uniformity (SAW, MEMS). One approach to obtain smooth and thin NCD films is the use of high nucleation densities associated with the growth of nanometric grains (stack-up structure). In this study we compare the effect of the growth conditions of NCD films synthesised in the same reactor when associated with high nucleation density obtained by Bias Enhanced Nucleation (BEN). Moreover, for a relevant comparison, the duration of the synthesis has been tuned for all films to exhibit the same thickness whatever the growth conditions. At last, we characterize the NCD films using Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG-SEM).

<footnote n="1">¹ CEA, LIST, Diamond Sensor Laboratory, 91191 Gif sur Yvette, France</footnote>

<footnote n="2">² CEA, LIST, Laboratoire Capteurs et Architectures Electronique, 91191 Gif sur Yvette, France</footnote>

<footnote>* Corresponding author: e-mail [samuel.saada@cea.f](mailto:samuel.saada@cea.f)</footnote>
## 2 Experimental

The MPCVD reactor used for the BEN nucleation and growth of the layers is connected via UHV to a surface analysis chamber enabling XPS analyses without exposure to air. This unique system is based on a home-made design equipped with a controlled heater stage and providing a bias system for the BEN step. The MPCVD process is all computer controlled to improve the reproducibility. We recently developed a rotating substrate holder that enables subsequent improvement of the homogeneity of the grown layers. The substrate holder can be rotated at rev-rates up to 2 rpm during all the synthesis. A laser interferometry system ($\lambda$ = 670 nm) was implemented to allow the measurement of the film thickness in real time during growth. The XPS measurements are performed in the UHV analysis chamber with a monochromated Al Kα source ($h\nu$ = 1486.6 eV). The Au 4f 7/2 peak located at 84 eV was used to calibrate the binding energy scale. Samples are characterized using a Field Emission Gun Scanning Electron Microscopy (Hitachi S-4500 FEG-SEM) as well as an Atomic Force Microscopy (Molecular Imaging PicoLE). Mapping of the thickness of the films are after growth realised using UV-interferometry with a home-made system.

## 3 Results and discussion

### 3.1 Homogeneous BEN on 2 inch wafers

To compare the NCD films in terms of morphology and surface chemical analysis it is essential that the films start from substrates with identical initial nucleation densities as well as exhibit the same thickness after growth. In these conditions, the differences between the films are only linked to experimental growth conditions. The BEN pretreatment consists of the application of a negative voltage to the substrate holder during the first stages of H₂/CH₄ plasma exposure. During BEN, a sheath is formed just above the surface of the silicon substrate. Ions are accelerated in this sheath and induced diamond nucleation in some conditions which strongly depend on the experimental parameters (temperature, microwave power, etc.) but also on the reactor geometry. This is the reason why in the literature the experimental sets of parameters are so dispersed. The local electric field and the local current density near the surface are determined by the experimental parameters like pressure, microwave power, etc.

As it has been reported before [13] that the nucleation is correlated to the modified cathodic sheath, the reactor has been designed to provide a cathodic sheath which shape is the most homogeneous as possible over two inches. Nevertheless, a slight heterogeneity of the local temperature or the local microwave power density can be highly detrimental on the nucleation since it induces local current density variations which locally further amplify the nucleation heterogeneity. Indeed, when the nucleation occurs, the local current density increases because of diamond electronic emission and, as a consequence, the nucleation kinetics is much more intense in this area. As the total current is kept constant during the first minutes, it means that in other areas, the current density decreases and the nucleation kinetics slows down. Thus, a slight heterogeneity at the beginning of the BEN leads to a runaway nucleation. To avoid this phenomenon, we developed a solution which consists on a rotating substrate holder that enables to spread the heterogeneity owing to plasma dissymmetry and temperature surface all over the wafer to prevent nucleation disequilibria. Then, the BEN step has been optimised for high nucleation density with high homogeneity all over the wafer. The experimental conditions of the BEN parameters are the following: pressure of 30 hPa, microwave power of 1.1 kW, temperature of 1023 K, bias voltage of -307 V, 10% vol. CH₄ in H₂ and during 8 minutes. Figure 1 is a FEG-SEM image of the silicon substrate just after the BEN step.

The obtained nucleation density is probed at $3 \times 10^{10} \text{ cm}^{-2}$ and the covering rate is 6%. The nucleation density is homogeneous over the 2 inch wafer.
<figure><img src="image_2.png" /><figcaption>Figure 1. FEG-SEM micrograph just after the BEN step</figcaption></figure>

## 3.2 XPS analysis of 10 × 10 mm² NCD films

The UHV analysis system connected to the MPCVD reactor is compatible with 10 × 10 mm² samples mounted on a flat molybdenum support. To study the influence of the growth conditions using XPS surface analysis without air exposure, the 2 inch wafers were cut down to 10 × 10 mm² pieces. The same substrate holder has been used for the BEN pretreatment on 2 inch wafer and for the growth of 10 × 10 mm² samples. Four different growth conditions were studied on 10 × 10 mm² samples with constant initial nucleation densities of 3 × 10¹⁰ cm⁻². Sample A correspond to a reference for polycrystalline films obtained from selective growth, with a methane concentration of 0.6%. Then, samples B and C were grown at higher methane concentrations of 10% and 20%, respectively for continuous secondary nucleation during growth, all other parameters being kept constant. Finally, sample D was grown using a negative bias voltage of 307 V during all the growth step in order to induce continuously secondary nucleation under bias. The experimental parameters are reported in Table 1.

<table><caption>Table 1 MPCVD experimental parameters</caption><thead><tr><th></th><th>A</th><th>B</th><th>C</th><th>D</th></tr></thead><tbody><tr><td>Microwave power (W)</td><td>1100</td><td>1100</td><td>1100</td><td>1100</td></tr><tr><td>Pressure (hPa)</td><td>35</td><td>35</td><td>35</td><td>35</td></tr><tr><td>Temperature (K)</td><td>1023</td><td>1023</td><td>1023</td><td>1073</td></tr><tr><td>Bias voltage (V)</td><td>0</td><td>0</td><td>0</td><td>-307</td></tr><tr><td>Vol.% CH<sub>4</sub> in H<sub>2</sub></td><td>0.6</td><td>10</td><td>20</td><td>10</td></tr></tbody></table>

The growth temperature of sample D is significantly higher than that of the other samples because of the induced intense ion bombardment of the layer under the bias voltage. The estimation of the growth step duration is not a sufficient relevant criterium to enable to stop the experiment at exactly the same thicknesses. Indeed, the methane concentration is varied over a broad range from 0.6 to 20%. Alternatively, laser interferometry was used *in situ* to stop the growth step exactly when the probed film thickness was comprised between 200 and 250 nm. Then, the samples were transferred to the surface analysis chamber for XPS characterisation. No O 1s peak could be detected after growth (Fig. 2). This confirms the absence of oxygen contamination.
<figure><img src="image_3.png" /><figcaption>Figure 2. XPS spectra after the growth step</figcaption></figure>

The XPS analyses at carbon core level are reported in Fig. 3 for samples A, B, C and D. The broad C 1s peak suggests the presence of several components.

<figure><img src="image_4.png" /><figcaption>Figure 3. XPS C1s core level spectra. A) growth at 0.6% CH₄, B) 10% CH₄, C) 20% CH₄, D) 10% CH₄ using constant with bias at – 307 V. The relative areas determined from the C 1s spectra are indicated on the graphs</figcaption></figure>
For samples A to C, the methane concentration increases from 0.6 to 20%. The main component of the carbon core level at 284.6 eV is corresponding to sp3-carbon bonds, in agreement with the X-ray Electron Energy Loss Spectrum (XEELS) (exhibiting both surface and bulk diamond plasmons). A second peak is located at +0.4 eV. It could be attributed either to CHₓ bonds (x > 1) or to amorphous carbon. Its relative weight is increasing with the methane content: it represents 16% of the spectrum total area with 0.6% CH₄, 23% with 10% CH₄ and 27% with 20% CH₄. That suggests a lower quality of the diamond film for growth at high methane concentration. However, even at 20% of CH₄ in the gas phase, there is no detectable contribution of a graphitic phase which may appear at lower binding energy versus the sp3-carbon peak [14, 15].

For the sample which underwent a continuous BEN treatment during growth step, the carbon core level exhibits a broader peak. In addition to the two prior components, a new one is visible at lower binding energies (-0.4 eV from the diamond peak). It is assigned to sp2-carbon bonds. Moreover, the component at higher binding energy is located at a slightly higher energy (0.5 eV). It may be assigned to CHₓ bonds which have been reported at this binding energy shift [16]. Our recent HREELS study showed the formation of CHₓ (x > 1) weakly bonded surface or sub-surface species during BEN by subplantation mechanism [17].

### 3.3 Morphology of 10 × 10 mm² NCD films

The morphology of synthesised NCD films has been characterized by AFM in order to determine the influence of the growth conditions on the roughness. The AFM is driven in the tapping mode. AFM pictures related to samples A, B, C and D are reported in Fig. 4.

<figure><img src="image_5.png" /><figcaption>Figure 4. Atomic force microscopy images of NCD films (2 × 2 µm²): growth at a) 0.6% CH₄, b) 10% CH₄, c) 20% CH₄, d) 10% CH₄ with bias at -307 V</figcaption></figure>
The AFM image of film A is characteristic of polycrystalline thin films resulting from selective growth: the grain size is small but one can see some facets at the top of the film. The RMS roughness of film A is equal to 9 nm. The growth under 10% and 20% CH₄ leads to the formation of very small grains because of the intense secondary nucleation. The AFM images related to films B and C are quite similar and the RMS roughness values are respectively 10 nm and 9 nm. In the images presented, the observed deviation from a spherical shape of the grains for the sample C in comparison with sample B is characteristic of a probe tip wearing leading to a triangle like shape for the grains. Concerning the film D (grown under bias), AFM image shows that the grain size is larger and the topography is different from films B or C. The RMS roughness of film D is equal to 58 nm.

<figure><img src="image_6.png" /><figcaption>Figure 5. FEG-SEM micrographs of NCD films: growth at a) 0.6% CH₄, b) 10% CH₄, c) 20% CH₄, d) 10% CH₄ with bias at -307 V</figcaption></figure>

The FEG-SEM micrographs of NCD films are presented in Fig. 5. The faceted morphology of the films grown at 0.6% CH₄ is clear on micrograph a). For films B and C, the size of the grain is much smaller than for film A. Film D exhibits a particular morphology with clustering of nanograins at the top of the film. It shows that the applied bias strongly modified the film morphology.

## 3.4 NCD deposition on 2 inch wafer

Using the same substrate-holder, BEN pretreatment (identical to that developed in Section 3.1) followed by a growth step of 10 hours with the experimental parameters defined for sample B (Table 1) was performed on 2 inch wafers without (film E) and with rotation (film F) of the substrate holder during the growth step at 1.5 rpm. Then, the thickness map of the wafers was probed on 2500 points using UV interferometry connected to an XY motorised table. The refractive index of the material has been chosen equal to 2.5 for the determination of the thickness. Figure 6 shows the mapping of the thicknesses of the NCD films. Both films have quite the same average thickness: 1.7 µm for film E and 1.6 µm for film F. In the case of film E, if we do not take into account the outline of the wafer over 2 mm, then the minimum and maximum value of the thickness are 1.1 µm and 2.6 µm, respectively. The uniformity *U*% is defined by: $U\% = \frac{t_{max}-t_{min}}{t_{max}+t_{min}} \times 100$
where $t_{max}$ and $t_{min}$ are the maximum thickness and the minimum thickness is equal to 40% for film E. To improve the uniformity of the deposit, we compare the synthesis of film E with film F which has been elaborated exactly in the same experimental conditions except the rotation of the substrate holder. The comparison of the mapping (Fig. 6b) shows that film F is much more uniform and has only a slight radial heterogeneity. The maximum thickness of film F is 1.8 µm and the minimum thickness is 1.4 µm. So, the uniformity of film F is equal to 9%.

<figure><img src="image_7.png" /><figcaption>Figure 6. Thickness mapping of 2 inch wafers using UV interferometry. a) Thickness mapping of film E synthesised without rotation of the substrate holder. b) Thickness mapping of film F synthesised with rotation of the substrate holder</figcaption></figure>

The RMS roughness has been measured along the radius of the wafer because of the symmetry due to the rotation during the process. Three measurements have been done by AFM on area of 2 × 2 µm²: one at the centre, one at 13 mm from the center and one at 3 mm from the edge of the wafer. The three RMS roughnesses are included between 13 nm and 14 nm and the surface morphology is similar for each area. These values are slightly higher than the RMS roughness measured on film B (10 nm) probably because the thickness of the film is higher. However, the morphology of film F is very similar to film B (Fig. 7).

<figure><img src="image_8.png" /><figcaption>Figure 7. Atomic force microscopy image (2 × 2 µm²) of film F (centre of the wafer)</figcaption></figure>
## 4 Conclusion

Homogeneous BEN pretreatments have been developed on 2 inches silicon wafer using a rotating substrate holder set up equipped with a bias system and a heater stage. Nanodiamond films with different growth conditions were synthesized on $10 \times 10 \text{ mm}^2$ from the same 2 inches wafers with high nucleation densities ($3 \times 10^{10} \text{ cm}^{-2}$). Three samples are synthesized using different methane concentrations and one sample using prolonged bias during the growth step. The film thickness is *in situ* monitored during growth, therefore films have the same thickness (between 200 and 250 nm) as well as the same initial nucleation densities which is very important for the comparison of the present study to be relevant.

The XPS analysis was performed without air exposure to avoid any surface contamination. This point has been confirmed using XPS analyses where no oxygen signal has been detected. The NCD films obtained by increasing the methane concentration from 0.6 to 20% are composed of diamond and amorphous carbon and/or CHx bonds ($x > 1$). Determination of hydrogen concentrations in the subsurface area may help to discriminate between both hypotheses. However, there is no contribution of graphitic phases contrary to the film obtained with prolonged bias. The morphology of the film obtained at 0.6% of CH4 is characteristic of a polycrystalline film with selective growth but as the initial nucleation is very high, the RMS roughness remains low (9 nm). Films synthesised at 10 and 20% of CH4 show nanocrystalline morphology with a RMS around 10 nm. At last, when grown under prolonged bias, the films exhibit a higher RMS and a different morphology with aggregates of nanocrystals. The results obtained on $10 \times 10 \text{ mm}^2$ have been reported on 2 inches wafers using the same substrate holder and comparing the influence of the rotation of the substrate holder during the growth step. The resulting films have been characterized using interferometry for thickness mapping. The rotation of the substrate holder greatly improved the uniformity of the deposit and the RMS roughness remain low (14 nm) even for a NCD film of 1.6 µm.

## Acknowledgements

The authors wish to acknowledge Sylvain Foucquart for his technical support and for the fabrication of the rotating substrate-holder.

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