Conductive interface layers

Since we found evidence for conductive interface layers, some literature-based intro-duction related with the topic is given below. Considering the chemical composition of the substrates containing Si, Al, Y and the CVD gas mixture (CH₄ + H₂), one can think about some candidates for the conductive phase, e.g. silicon carbide (SiC), pure Si or some unspecified amorphous or graphitic carbon phase. It is very unlikely that contamination with carbonized filament material (tungsten), normally present in the coating at the level of few ppm (by mass) [87], or some additional phases formed by metal oxides present in the substrate as sintering aids would cause such an effect.

5.3.2.1 Possible mechanisms

It is well known that carbide formation during the initial stages of CVD on substrate materials like Si, Mo, W and Ta precedes and accompanies the nucleation [85]. Some authors even claim that carbide can be considered as the glue, which enhances the

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adhesion between the substrate and the coating and making carbide formation nec-essary to obtain an adherent heteroepitaxial coating. It is supposed to concern also non-metals, including Si-containing compounds such as SiO₂ and Si₃N₄ [84]. Others claim that also an intermediate layer of diamond-like amorphous carbon or graphite can play the same role, namely, relief the stress at the interface, caused by lattice mismatch or contraction. The thickness of such interlayers can vary from several angstroms, up to a few micrometers in extreme cases (Ti, Mo), with typical value of a few nanometers [85]. Some theoretical models propose formation of two interlayers on the substrate, first a carbidic one (SiC) and then an amorphous one (DLC) [95].

Some proposed mechanisms of the carbide layer creation assume that β-SiC¹⁰ is formed from a thin layer of SiO₂, which inevitably covers the surface of Si₃N₄, others expect rather graded interlayers of SiC_xN_y [96]. In general, the literature concerning CVD on silicon nitride presents conclusions which are often unclear or contradictory and, in principle, there is no strong experimental evidence corroborating any of the models. A comprehensive summary of different approaches to the subject is given in [97], the general conclusions can be stated as follows:

• SiC can be formed on the interface at least for various process conditions and, when created, it strongly enhances adhesion of the coating [98]

• It is not clear whether an adherent coating can be produced without formation of a carbidic layer.

• Apparently, in some cases, the coating may grow from diamond ’seeds’ rather than from the substrate, producing a negligible chemical bond at the sub-strate/coating interface and resulting in solely mechanical bonding mechanism.

5.3.2.2 Thermodynamic calculations

Thermodynamic equilibrium calculations are a standard tool used to estimate the rate of possible reactions of the substrate surface with the gas phase species and are widely quoted in the literature, also in the papers concerning specifically N₂ -doped CVD [99, 100] or Si₃N₄ (one of our substrates [95]). When used properly, it can provide extremely helpful hints concerning the type of solid phases created and deposited during the CVD process. For this project, the analysis has been performed at Fraunhofer IKTS [101], using one of the commercially available codes (FACTSAGE with database FACT53 and SGSL, see Ref. [102]). It is worth mentioning that this type of modeling also has some intrinsic limitations, for instance, it does not take into account the real non-thermal property of the catalytically activated gas mixture;

it is only assumed that the hot vapor acts solely as supplier of reactive species and that no kinetic barriers exist (which is normally the case at temperatures at which the coating process takes place).

Table 5.3 presents results of the thermodynamic calculations and shows what phases are built on different type of substrates at 0.01 mbar pressure in the atmo-sphere consisting of 50% H₂and 50% CH₄(molar proportions). One can conclude that

10β-SiC has the cubic crystal structure.

5.3. Diamond coating 57

Substrate Temperature range [^◦C] Possible phases Si₃N₄ 500 – 850 Si₃N₄/ SiC, C Al₂O₃ 500 – 1000 Al₂O₃, Al₂CO, C MgAl₂O₄ 500 – 1000 MgAl₂O₄/Al₂O₃/C Y₃Al₅O₁₂ 500 – 1000 Y₃Al₅O₁₂, C, YAlO₃ 3Al₂O₃·2SiO₂ 500 – 950 SiC, Mullit; Al₂O₃, C

Table 5.3: Results of thermodynamic calculations [101].

in the normal HFCVD conditions, thus without the nitrogen admixture, conductive or semiconductive layers can be expected on Si₃N₄ and presumably on other silicates such as SiO₂ or 3Al₂O₃·2SiO₂ (mullit), which is also quite interesting, since commer-cially available Si₃N₄contains 6% Al₂O₃and 4% Y₂O₃, which form intergranular glass phase. A similar result for slightly different deposition conditions (p=40 mbar) has been presented by Buchkremmer-Hermanns [95]. Therefore it seems that – although nobody has ever experimentally proven the presence of SiC layer after conventional low pressure CVD on this class of substrates – their creation is presumably inevitable.

However, in different conditions, it does not have to be the case. As it was theoret-ically and experimentally demonstrated by Rozbicki [98] at atmospheric pressure (using combustion flame CVD) Si₃N₄ is thermodynamically stable up to 1350^◦C and a layer of β-SiC can be created only at temperatures exceeding this limit. On other substrates (Y-monosilicate, YAG and sapphire) carbides are not to be expected even in standard low pressure conditions. It is impossible to conclude, though, from equi-librium calculations, whether some kind of conductive graphite or amorphous carbon layer could be created or not.

The presence of nitrogen in the gas mixture during the deposition processes (per-formed for this work) completely changes the situation. Fig. 5.9 shows a phase diagram for a system of Si₃N₄, SiC and C at gas mixture with admixture of some N2 [101]. Nitrogen shifts the equilibrium in such a way, that the thickness of the hypothetical SiC layer would be reduced; the higher its partial pressure, the more difficult is the formation of the carbide. Therefore, varying the N₂ pressure within the range, where the CVD diamond deposition is possible might be a good means to avoid the conductive interlayer.

To conclude, according to thermodynamic calculations there is a way to vary the CVD process parameters, either changing the temperature and pressure or the com-position of the gas mixture, in such a way that conductive carbide layers are not created. However, there is no information about possible graphitic or amorphous phases. Besides, given the fact that virtually no experimental evidence unambigu-ously corroborating the model simulations has been published, it is not clear either, whether the equilibrium state analysis can fully describe the entire complexity of a CVD process, involving the physics and chemistry of plasma, surface effects, etc.

Nevertheless, it gives enough hope and motivation to study in detailed experimen-tal way the properties of CVD diamond coatings and optimize them for the nEDM chamber application.

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Figure 5.9: N₂ partial pressure as a function of temperature for SiC/Si₃N₄/C system [101].