3.1.1. Optical Properties

In the case of thin solid films deposited for optical applications, the most significant tests comprise light transmission measurements in the UV-Vis range and ellipsometric examinations. Together, they supply the principal information concerning such optical parameters of the material as levels of light absorption or transmission as well as values of optical gap (*Eg*), index of refraction (*n*) and extinction coe fficient (*k*). In addition, they also allow one to assess film thickness (*d*) and surface roughness (*Sr*). Most of these parameters can be relatively easily correlated with the structure of the coatings. This is a reason both tests were selected as simple and fast criteria for the selection of an appropriate precursor of gradient films on one hand, and for the optimization of the deposition process on the other.

Light transmission measurements were carried out within the wavelength range of 200–1000 nm. The results for the coatings prepared from tetramethyldisilazane and from hexamethyldisilazane are presented in Figure 1, below. These coatings were deposited under conditions of di fferent composition of working gases in order to assess the e ffect of nitrogen-to-oxygen ratio on their optical parameters. The total flow rate of the gas mixture was constant and amounted to 20 sccm with the actual composition of that mixture being represented by the ratio of respective flow rates also expressed in sccm units.

An analysis of the results shows that all the coatings are characteristic of good optical properties revealed by an appropriate system of interference maxima. Independent of the type of precursor, films deposited under conditions of an excess of nitrogen in the reaction mixture exhibit sharper interference maxima compared to those prepared at the excess of oxygen. This is a consequence of the fact that silicon nitrides and carbonitrides are characterized by higher values of refractive index than those of the respective oxides.

A comparison of the spectra presented in Figure 1 allows one to conclude that the coatings deposited from tetramethyldisilazane are substantially thicker that those obtained from hexamethyldisilazane, since they exhibit a larger number of interference maxima. One should remember that the time of deposition was the same in all cases, and amounted to one minute. The thickness difference is a result of a substantially higher content of carbon incorporated into the coating prepared from TMDSN [26]. An increased content of carbon in the films deposited from this precursor is also revealed by their values of absorption threshold shifted to the infrared direction. The absorption threshold of the films is also dependent on the composition of the gas mixture—the films deposited at the excess of nitrogen are characteristic of a shift towards larger wavelengths (lower energy). In addition, on the contrary, the coatings prepared under conditions of oxygen excess exhibit an absorption threshold shift towards lower wavelengths and higher energy. It is interesting to note, however, that this tendency, generally shown by the films deposited from both precursors, is substantially stronger in the case of the coatings made from HMDSN.

**Figure 1.** UV-Vis transmission spectra of the coatings deposited from TMDSN (**a**) and HMDSN (**b**) at different N2/O2 proportions of the working gas mixture.

The effect described above finds its confirmation in the magnitudes of the optical gap *Eg* calculated for the respective coatings. For that purpose, the Tauc formalism was applied to the results of absorption measurements within the wavelength range of 200–1000 nm. The resulting *Eg* values for the coatings prepared from both precursors at different compositions of the working gas are presented in Table 3.

**Table 3.** The magnitudes of optical gap *Eg*, index of refraction n, roughness *Sr* and thickness d of the coatings deposited from different organosilicon precursor compounds under conditions of three different compositions of N2/O2 working gas.


In general, literature reports reveal higher magnitudes of optical gap of SiO2 coatings than those of silicon nitride [27–29]. As seen in Table 2, the *Eg* value for the films deposited from TMDSN under pure oxygen conditions amounts to 3.11 eV. This result is close enough to the literature data showing the highest values of optical gap being typical for silicon oxides and remaining in the range of 3.5–9.3 eV [30,31]. The coatings made of silicon nitride are characterized by lower magnitudes of optical gap. In our case, the *Eg* value for the films deposited from TMDSN in pure nitrogen equals 2.93 eV, which remains in agreemen<sup>t</sup> with the literature data, where values within the range of 2.4–4.75 eV are reported [32]. One has to remember that the coatings presented in this work contain substantial

amounts of carbon in their structure. Therefore, the respective values of silicon carbide should also be considered, and they are still lower than those of silicon nitride. Taking all the above arguments into account, one should state that a supplement of carbon lowers the magnitude of optical gap in both silicon oxide and silicon nitride coatings and the values recorded in this work remain well within the ranges reported in the literature [33–35].

An application of VASE spectroscopic ellipsometry allows one to determine the values of n and k coefficients as well as thickness and surface roughness of the material investigated. The respective data for the coatings deposited from both precursor compounds under different compositions of working gas are presented in Table 2, below. All the VASE measurements were conducted within the spectral range of 200–1000 nm, using three different angles of incidence of 65◦, 70◦ and 75◦. These values were selected in such a manner that they comprise the Brewster angle for the coatings produced. As a basis for the calculations, the Cauchy's model, often used for this type of materials [36–38], was assumed. An example of ψ angle measurement results together with the fitting of the above model for a coating prepared in pure oxygen is presented in Figure 2. When considering the relationship presented, one can observe a slight departure of the model from the experimental data within the 250–280 nm range, resulting from higher absorption in this range. Further studies have shown a responsibility for that absorption of carbon containing chemical moieties. Despite the above difference, the MSE fitting error is small and it amounts to 12.

**Figure 2.** Experimental and calculated values of tan Ψ for the coating obtained from HMDSN under pure oxygen conditions.

The results acquired from ellipsometric measurements conducted for the coatings deposited from both TMDSN and HMDSN at different oxygen concentrations are presented in Figure 3. They show unambiguously that, by changing the composition of the gaseous reaction mixture, one is able to deposit coatings characterized by low index of refraction when depositing in pure oxygen atmosphere and those of high index of refraction when using a pure nitrogen atmosphere. When, however, a mixture of oxygen and nitrogen is used as the working gas, then depending on their proportions, a coating characterized by a refractive index within the range of 2.31–1.99 for TMDSN precursor and within the range of 2.22–1.65 for HMDSN precursor is obtained. It was thus proven in these studies that, by using a gaseous reaction mixture of a changing composition, coatings of variable optical parameters can be obtained.

**Figure 3.** Dependence of optical parameters of the coatings deposited from TMDSN (**a**) and from HMDSN (**b**) on the composition of working gas mixture (oxygen–nitrogen).

The relationships presented above reveal high magnitudes of extinction coefficient of the coatings. They are still higher in the case of the films obtained under high nitrogen content conditions and lower for those deposited at high oxygen concentrations. In thin solid films, the extinction coefficient k usually depends on two factors. One is light absorption. As revealed by the UV-Vis transmission measurements, the materials under investigation primarily absorb in the 200–400 nm range, with substantially higher absorption exhibited by the coatings produced at higher nitrogen concentrations. This effect is a result of a presence of carbon in these films. Another important phenomenon is surface roughness. The rougher the surface is, the larger the fraction of radiation that undergoes scattering, thus affecting the magnitude of extinction coefficient. Ellipsometric measurements allow one to assess the roughness Sr of a given sample. The results obtained for the coatings under investigation are presented in Table 2 below. As seen in the table, surface roughness of the coatings deposited from TMDSN is far higher than that of the films obtained from HMDSN. This finding has been confirmed by surface topography examination with the help of AFM microscopy.

Ellipsometric measurements also provide data concerning sample thickness. The results presented in Table 2 show that, despite the fact that for all the samples deposition time amounts to 60 s, the coatings deposited from TMDSN are ca. two times thicker than those obtained from HMDSN. In both precursor molecules, there are two atoms of silicon. At the same time, there are four atoms of carbon in TMDSN and six atoms of carbon in HMDSN. The thickness and deposition rate differences result from the deposition mechanism, in which an important role is played by silyl radicals [39]. Due to the differences in bond energy, these radicals are much easier to form by a cleavage of Si-H bonds (*E* = 298 kJ/mole) than by a decomposition of Si-C bonds (*E* = 435 kJ/mole) [40]. A presence of the former bonds in a TMSDN molecule substantially enhances deposition, at the same time leading to an unfortunate incorporation of larger amounts of carbon.

From the results presented above, one can conclude that the optical quality of the coatings strongly depends on the composition of the reaction mixture. Those deposited in pure nitrogen atmosphere are characterized by high values of refractive index, amounting to 2.3 and to 2.2 for TMDSN and HMDSN, respectively. For samples deposited in pure oxygen, the same values amount to 1.99 and 1.65. In addition, the coatings deposited from TMDSN are of a lower optical quality than those obtained from HMDSN. An index of refraction of stoichiometric silica equals 1.45 [41]. To design and manufacture optical interference filters, one needs two materials: one of a high (*nH*) and one of a low (*nL*) value of refractive index. These two values should considerably differ from one another. In the case of the coatings deposited from TMDSN, that difference amounts to 0.32, and it is equal 0.57 for those obtained from HMDSN. In addition, the films deposited from TMDSN are characterized by higher absorption in the range of 200–400 nm.

#### 3.1.2. Surface Topography Studies

To project surface topography of the coatings, AFM microscopic observations were carried out. Surface images of the samples deposited from both precursor compounds at different compositions of working gas mixture are presented in Figure 4. The images presented reveal a relatively homogeneous structure stripped of large hills and other inequities.

**Figure 4.** Surface topography of the coatings: T-20 N2 (**a**), T-20 O2 (**b**), H-20 N2 (**c**), and H-20 O2 (**d**).

Based on the AFM observations, measurements of surface roughness have been performed, and their results, such as arithmetic mean of the profile departure from the average line (*Ra*) and mean squared deviation of the surface roughness (*Rq*), are presented in Table 4 below. As seen in the table, a coating becomes smoother and smoother with an increasing concentration of oxygen. This tendency was observed in all samples, independent of whether TMDSN or HMDSN was used as a precursor compound. In addition, slightly lower values of *Ra i Rq* parameters were observed in the case of the coatings deposited from HMDSN, which corresponds well with the results of optical roughness measurements performed with the help of ellipsometry.

**Table 4.** *Ra i Rq* surface roughness parameters of the coatings deposited from different organosilicon precursors at different N2/O2 composition of the working gas mixture.


Taking into account the results of optical measurements as well as those of surface topography, one can conclude that the coatings deposited from HMDSN precursor are characterized by better quality and higher homogeneity. It was due to these reasons that the further studies were confined

to the coatings obtained from HMDSN as a precursor more suitable for a successful design and manufacture of an interference optical filter with a gradient of refractive index.

3.1.3. Elemental Composition and Chemical Structure Studies Conducted with the Help of XPS Spectroscopy

XPS studies allowed for an assessment of qualitative as well as quantitative composition of the coatings deposited under conditions of different composition of the working gas mixture. An example of a wide scan XPS spectrum of a coating deposited in pure nitrogen atmosphere is shown in Figure 5, while Table 5 presents elemental composition of the films compared with that of the HMDSN precursor compound.

**Figure 5.** An example of a wide scan XPS spectrum of the H-20 N2 coating.

**Table 5.** Elemental composition of the films deposited from HMDSN at different composition of the working gas mixture as well as that of the HMDSN precursor compound.


An analysis of the above data reveals a close relationship between the elemental composition of the coatings and the N2/O2 proportion in the working atmosphere. The silicon content remains at a relatively stable level of 22.7–26.4 at.% for all the samples, with a weak increasing tendency with increasing concentration of oxygen in the working gas. A similar result was reported by Chang et al. [42], with a difference being a lack of reactive atmosphere. Both in their studies and in our work, the concentration of silicon in the films remains close to its content in the precursor compound.

The amount of nitrogen in HMDSN equals 11.1 at.%. In the coating deposited in pure nitrogen atmosphere, that content increases to 46 at.%. This result strongly indicates that, under plasma conditions, nitrogen becomes a reactive gas able to form chemical bonds with other elements. In the sample prepared in the 1:1 N2/O2 atmosphere, the content of nitrogen drops down to 20.5 at.%, while the coating deposited under pure oxygen conditions contains 5.8 at.% of nitrogen only. In the latter case, the entire amount of this element very likely originates from the precursor and its presence in the coating may be a result of secondary processes.

Oxygen is characterized by a high affinity towards silicon. This is a reason this element content in a coating deposited under pure nitrogen atmosphere still amounts to ca. 5 at.%. To a large extent, this is supposed to be surface bound element resulting from a reaction of atmospheric oxygen with the

coating surface after its exposure to the atmosphere. Following their etching, the oxygen content in the films, revealed by XPS measurements, drops down to 3.4 at.%, and this figure is entirely acceptable. In the coatings deposited under increasing oxygen concentration in the working gas, the content of oxygen increases dramatically to reach a value exceeding 54.4 at.%.

An interesting behavior is shown by carbon. Its content in the films is a subject of substantial variations, depending on the composition of the working gas mixture. Carbon content in the surface layer of a coating deposited under pure nitrogen conditions amounts to 25.8 at.%. When deposited in a mixed nitrogen/oxygen atmosphere, the coating contains 19.9 at.% carbon. Finally, for a coating produced in pure oxygen, C content further decreases to 13.3 at.%. This may be explained by a partial removal, under conditions of precursor fragmentation, of carbon in a form of its oxides CO2 and CO. The larger oxygen concentration in the gas mixture, the more e ffective is that process. When film deposition is performed in pure nitrogen, no oxides are formed and carbon is removed in a form of simple hydrocarbons, principally methane. However, this process is far less e fficient, which explains larger carbon concentrations in the coatings deposited under pure nitrogen conditions. In addition, it has to be noted that the content of carbon in the coatings, independent of whether deposited in the presence of oxygen or in the presence of nitrogen, is much lower that its concentration in the precursor compound. This e ffect points to a substantial fragmentation of the precursor molecules taking place in plasma, wherein the final content of carbon to a large extent results from secondary reactions.

Similar is the situation of nitrogen in the coatings investigated. In the case of materials deposited under pure nitrogen conditions, its content amounts to 46.5 at.%, and is over four times higher than that of the precursor compound. It convincingly indicates a reactive character of this element, thus forming Si-N bonds under plasma conditions applied. When, however, oxygen is used as the deposition process working atmosphere, the content of nitrogen in the resulting coatings deceases down to 5.8 at.%, a figure that is lower than that of the precursor compound.

Further characteristics of the coatings chemical structure is based on deconvolution of the Si 2p, N 1s and C 1s XPS spectral bands, therefore enabling a detailed analysis of chemical bonding present in that structure as a function of oxygen content in the working atmosphere. Such an analysis was performed for three di fferent samples: one deposited under pure nitrogen condition (H-20 N2, Figure 6), one obtained in an equimolar nitrogen/oxygen mixture (H-10 O2/10 N2, Figure 6) and one prepared in pure oxygen (H-20 O2, Figure 6).

Deconvolution of the Si 2p band points to a dominant role of a Si–N bond in the samples deposited under pure nitrogen conditions. Si–O bonds are also present, confirming oxygen a ffinity towards silicon. Finally, the spectrum also indicates the presence of Si–C bonding. In the coatings deposited under mixed N2/O2 atmosphere, the amount of Si–O bonds increases and a band characteristic for SiO2 appears, all that at the expense of decreasing contents of Si–N and Si–C bonds. When deposited under pure oxygen atmosphere, the coatings are characterized by dominant XPS bands corresponding to both Si–O and SiO2 chemical bonding.

Deconvolution process of the XPS N 1s band confirms a dominant role of a Si–N bond in the samples deposited under pure nitrogen conditions. Along an increasing amount of oxygen in the working atmosphere, the content of both C–N and N–O bonds in the coatings also rises. In fact, an increasing amount of N–O bonds, absent in the precursor molecule, points to plasma energetic conditions allowing for a substantial fragmentation of that molecule.

On the basis of XPS results one can state that, with an increasing concentration of oxygen in the working atmosphere, the amount of silicon–oxygen bonding (102.8 eV) in the coatings increases, and that of carbon–nitrogen bonding (399.1 eV) declines. In terms of optical properties of these materials, it indicates a drop of their index of refraction "*n*".

Deconvolution of the C 1s XPS band reveals a presence of the following carbon bonds: Si–C (283 eV), C–C (C–H) (284.8 eV), C–N (285.5 eV), as well as C–O (285.5 eV) and C=O (287.8 eV). Here, too, one can observe a decrease of the amount of Si–C and Si–N chemical bonds present in the coatings along an increasing concentration of oxygen in the working gas mixture. A decreasing content of carbon in these films makes an additional proof of carbon removal (in the form of carbon oxides) from an oxygen rich working atmosphere.

**Figure 6.** The Si 2p, N 1s and C 1s XPS narrow scans of films deposited from HMDSN precursor at different working gas compositions.

#### 3.1.4. Chemical Composition Studies by FTIR Spectroscopy

Structural studies were performed on selected samples of SiONC coatings deposited from HMDSN precursor at negative self-bias potential of −880 V and three different compositions of the working atmosphere: pure oxygen, 1:1 oxygen/nitrogen mixture and pure nitrogen. The respective FTIR spectra, recorded within the spectral range of 4000–400 cm<sup>−</sup>1, are presented in Figure 7. To enhance their clarity, the spectra have been divided into two ranges: 4000–1400 cm<sup>−</sup><sup>1</sup> (a) and 1300–400 cm<sup>−</sup><sup>1</sup> (b).

In the spectra presented in Figure 7a, characteristic bands corresponding to the following molecular vibrations can be identified:


#### • 1475–1440 cm<sup>−</sup>1—deformation vibrations of CH2 and CH3 groups [43,44].

It appears from the FTIR spectra that addition of nitrogen to the working atmosphere results in an enrichment of the coating structure with organic CH3 and CH2 groups originating from the precursor. In addition, a proportional increase of the amount of Si–H bond is also noted. A lowering of the C–H bond content in the films deposited in the oxygen atmosphere is interpreted as a result of a formation of volatile CO2 and H2O oxides in the course of the PE CVD process. These by-products are easily removed from the environment and they do not contribute to the process of deposition. As a result, the coating thickness drops down. In the case of N–H bonds, absorbing within the spectral range of 3440–3150 cm<sup>−</sup>1, their existence is also observed in the case of the samples deposited under pure nitrogen atmosphere. It is connected to the presence of nitrogen in the precursor molecule. Certainly, the amount N-H bonds in the coating structure increases with an increasing concentration of nitrogen in the reaction mixture. A shift of a 2200 cm<sup>−</sup><sup>1</sup> band, corresponding to the vibrations of C–N bonds towards higher wavenumbers, is also observed for the coatings deposited under oxygen atmosphere.

**Figure 7.** FTIR spectra of the coatings deposited at three different compositions of the working atmosphere and recorded within the range of 4000–1400 cm<sup>−</sup><sup>1</sup> (**a**), and 1300–400 cm<sup>−</sup><sup>1</sup> (**b**).

Within the spectral range of 1300–400 cm<sup>−</sup>1, shown in Figure 7b, several bands corresponding to chemical bonds of elements having significant effect on the coating optical properties have been separated. They were subjected to a detailed deconvolution procedure with the resulting surface areas of particular band components presented in Table 6. In the case of the coatings deposited under pure oxygen atmosphere, absorption bands corresponding to the following chemical moieties were selected: Si–O–(CHx) groups, as well as stoichiometric SiO2 and non-stoichiometric SiOx silicon oxide bonds. The strongest band, recorded at 1035 cm<sup>−</sup><sup>1</sup> and corresponding to stretching vibrations of stoichiometric silicon dioxide Si-O bonds, is accompanied by a substantial maximum at 445 cm<sup>−</sup>1, resulting from rocking vibrations of that group. A presence of such a large amount of oxygen in the coating is responsible for a low value of its refractive index equal 1.65. An increase of nitrogen content in the working environment results in an increasing number of Si–NH–Si bonds absorbing at 953 cm<sup>−</sup><sup>1</sup> as well as, typical for ceramic materials, Si–N bonds resonant with the radiation of the wavenumber of 830 cm<sup>−</sup>1. Additionally, Si–C bonds, typical for silicon carbide, are formed under these conditions. A presence of both these bond structures is responsible for an increase of the coatings refractive index up to 2.2. This means that, by a strict control of nitrogen content in the working N2/O2 atmosphere, one should be able to substantially affect chemical structure of the films and, therefore, to regulate, within a certain range, the magnitude of refractive index n of the resulting coating.


**Table 6.** Surface areas of particular components of a broad IR absorption band recorded at 1300–700 cm<sup>−</sup><sup>1</sup> for selected coatings deposited from HMDSN (key: *s*—stretching vibrations; *r*—rocking vibrations).

#### *3.2. Coatings Characterized by a Gradient of Refractive Index*

## 3.2.1. Optical Properties

The results presented thus far concerned coatings deposited under di fferent, but constant, proportions of nitrogen to oxygen in the working atmosphere. This also means a constant composition of these coatings. In the present chapter, results obtained for materials deposited under conditions of a variable composition of that atmosphere are reported. The composition of the working gas was changed every 5 s, with the subsequent O2/N2 proportions presented in Table 7. These proportions were selected in such a manner that in every case the total flow rate of the working gas was equal to 20 sccm. Such an arrangemen<sup>t</sup> resulted in a step-wise alteration of the coating chemical composition and, consequently, introduced a gradient of refractive index. Several sequences of a coating were produced, from a single nitrogen–oxygen sequence up to the most complex nitrogen–oxygen–nitrogen–oxygen–nitrogen sequence. The single nitrogen–oxygen sequence resulted in a film thickness of approximately 110 nm, while proportionally larger thickness values were acquired for the multiple sequences.

**Table 7.** Oxygen/nitrogen proportions applied in subsequent steps of a deposition of a gradient refractive index coating.


Profiles of refractive index and extinction coe fficient for particular coatings were determined with the help of spectroscopic ellipsometry. In the model applied for the calculations, respective values of these optical parameters obtained for di fferent (but constant) compositions of deposition working atmosphere, were used. The resulting profiles for a coating deposited under the working atmosphere changing from 100% nitrogen to 100% oxygen are presented in Figure 8 below.

On the basis of the models proposed, one can judge that the shape of the real profile does not substantially depart from that theoretically assumed. In the model, the magnitude of the refractive index varies in the range of 1.65–2.22, while ellipsometric data show respective values from the 1.65–2.38 range, with the course of changes remaining relatively close to that assumed.

In a similar way to the above data, the results of a two-fold sequence of a gradient coating deposition exhibit a satisfactory fitting of the proposed model. In this case, the refractive index was assumed to vary according to the model change of 2.25–1.65–2.25, while ellipsometric measurements showed that index to follow the 2.38–1.67–2.28 pattern. The di fference may indicate the contribution of residual oxygen, lowering the refractive index, in the deposition process.

**Figure 8.** Profiles of refractive index and extinction coefficient of a SiONC coating, deposited under conditions of variable nitrogen-to-oxygen proportions in the working atmosphere, as a function of the film thickness: theoretical model (**a**) and the real profile recorded by ellipsometric measurements (**b**).

A similar approach was applied in the case of the coating characterized by a four–fold nitrogen–oxygen–nitrogen–oxygen–nitrogen sequence. The profiles of theoretical and real values of refractive index of that coating are presented in Figure 9. In all instances, a good agreemen<sup>t</sup> between the model assumed and the measurement results have been observed, thus pointing to a high stability of the deposition process.

**Figure 9.** Model (**a**) and real profiles of refractive index and extinction coefficient changes (**b**) for a coating deposited in a N2–O2–N2–O2–N2 sequence.

Materials with a gradient of refractive index were also investigated by means of UV-Vis spectroscopy. Figure 10 presents transmission spectra of the coatings deposited with a one-fold, two-fold and fourfold change of the composition of working atmosphere.

It appears from Figure 10 that the effect of optical interference is recorded even in the case of the coatings deposited with a one-fold N–O change of working atmosphere. As far as materials produced with the two-fold N–O–N and four-fold N–O–N–O–N processes are concerned, an evident cutback of absorption at low wavelengths is observed in this case. What is important to note, is the fact that light interference takes place despite a lack of separate layers characterized by different magnitudes of refractive index. In fact, the effect develops from a gradient of refractive index. This finding constitutes proof of application potential of the coatings investigated in the technology of monolayer optical filters with a gradient of refractive index.

**Figure 10.** UV-Vis transmission spectra of the coatings deposited with a one-fold N–O, two-fold N–O–N and fourfold N–O–N–O–N change of working atmosphere.

#### 3.2.2. Elemental Composition Profiles of the Coatings

XPS studies of the coatings allow one to follow changes of elemental composition of these materials as a function of their thickness. Such elemental composition profiles of a coating deposited with a two-fold N–O–N change of working atmosphere are presented in Figure 11, below.

**Figure 11.** Elemental composition in-depth profiles of a coating deposited with a two-fold N–O–N change of working atmosphere.

It appears from the figure that a change of working gas composition, although having a significant impact on the chemical structure of the resulting material, does not affect the concentration of silicon in the coating, which continues to stay at the level of 24–28 at.% along the entire thickness of the sample. In contrast to that, the contents of the remaining elements are subjected to considerable changes. The amount of both nitrogen and oxygen in the coating is fundamentally dependent on the respective gas concentration in the working atmosphere. While being ca. 46 at.% for the best samples deposited under pure nitrogen conditions, the content of nitrogen drops down to 5.8 at.% in the case of a material obtained in pure oxygen. A return to the 100% N2 atmosphere results in the reestablishment of high nitrogen content in the films. One can conclude that the concentration of this element in the coatings varies in a periodic manner, following periodic alterations of the N2/O2 proportion in the working atmosphere. Similar is the behavior of oxygen. In the case of the best samples deposited under pure nitrogen conditions, its amount in the coating totals 2.8 at.%, only to increase up to 54 at.% for samples obtained in pure oxygen. Just as is the case with nitrogen, the concentration of oxygen in the coatings varies in a periodic manner, following periodic alterations of the O2/N2 ratio in the working atmosphere.

The concentration of carbon in the films follows the nitrogen-to-oxygen proportions in the working gas. However, these changes are not so evident, and they result from the periodicity of chemical reactions taking place in the glow discharge. As discussed earlier, the coatings deposited under oxygen-rich conditions are stripped of carbon because of a swift removal of volatile carbon oxides from the reaction mixture.

#### *3.3. "Cold Mirror" Interference Optical Filter with a Gradient of Refractive Index*

The effect of optical interference taking place in the coating deposited with the four-fold exchange of the working atmosphere led us to believe that it was possible to design and to construct the entire interference filter with a gradient change of refractive index. The design of a "cold mirror" type filter was accomplished with the use of a TFCalc.TM 3.5 (*transmision filter calculation*) specialized software. An initial stage of that design comprised the construction of a database containing all the magnitudes of refractive index and extinction coefficient of the respective materials obtained at different compositions of the working atmosphere. It was then assumed that the filter of interest will be of a band type with the absorption threshold set at 300 nm. The calculations showed that it takes a ten-fold exchange of the working gas atmosphere along the nitrogen–oxygen–nitrogen sequence to obtain of desired filter structure. The designed gradient change of refractive index for such a filter is presented below, in Figure 12.

**Figure 12.** A model gradient change of refractive index of the "cold mirror" interference filter.

Reflection spectra of the filter model as well as of the real device are presented in Figure 13. It appears from the figure that the optical characteristics of the real filter are not much different from those of the model. According to the design, the 50% reflection level should be observed at the wavelength of 290 nm, while in reality it is recorded at 313 nm. This difference is likely to be a result of deposition rate fluctuations taking place in the course of the changes of the working atmosphere. It may also be a consequence of a moderate temperature increase in the reactor during deposition. In any case, a manufacture of the presented filter constituted a test of both the deposition equipment and the technology developed. This proves the high stability and high repeatability of the process, thus opening a path to depositing much more complex optical coatings.

**Figure 13.** Reflection spectra of the "cold mirror" type of interference filter: theoretical model, and real device.

The coatings with a continuous change of refractive index resulting from gradients of their chemical composition, constitute a subject of studies carried out in numerous laboratories. Some publications deal with the methods using two different sources of material. There are reports of gradient coating deposition from solid SiO2, Ta2O5, or Nb2O5 targets using sputtering techniques. Optical properties of the coatings acquired are good [9,46,47]. However, the deposition technique itself is somewhat problematic, because of different sputtering rates of different targets. Retaining appropriate proportions between particular materials require a continuous control of the magnetron sputtering power. It is also possible to sputter materials with a use of either electron or ion beams, but these techniques are very costly both in the capital investment phase and in the course of operation. Use of two different precursor compounds is also practiced in the PE CVD technologies. In the work cited, HSDSN and TaCl5 were used as sources of low and high refractive index materials, respectively. The gradient coatings were deposited with the help of plasma enhanced technology using oxygen as one of the reactive mixture components [48]. Aggressive chlorine-containing chemicals are obtained as by-products in this technique, which enforces the application of highly (chemically) resistant materials in the construction of deposition equipment.

In the literature, there are reports concerning coatings with a gradient of physical properties based on a single precursor compound. These publications usually deal with silane SiH4 playing the role of that precursor [22,49]. As working gases in these works, mixtures of oxygen and nitrogen of different proportions are applied, with the resulting structure of the PE CVD coatings varying from silicon oxide to silicon nitride through intermediate materials of a SiOxNy composition. Silane, however, is a very volatile, flammable and explosive chemical, and its use requires extra safety precautions. The method of deposition of gradient coatings described in the present work removes the above-mentioned inconveniences. Instead of a hazardous gas, easy to handle liquid organosilicon compounds are used as precursors. As far as literature reports are concerned, thus far there has been none whatsoever that refers to gradient coatings deposited from these precursors.

The optical "cold mirror" filter, whose model and real transmission spectra are presented in Figure 13, has also been investigated with the help of scanning electron microscopy. The SEM image of the manufactured filter cross-section is presented in Figure 14.

In the micrograph presented above, one can identify subsequent "sublayers" present in the optical coating. The thickness of the coating equals ca. 1045 nm. From the design assumptions, that thickness should amount to 1050 nm. That agreemen<sup>t</sup> is quite satisfactory, and it proves the quality of the technology developed. That technology, and the method of deposition of single-layer optical coatings with the gradient of refractive index using organosilicon precursor and oxygen-nitrogen mixture as working atmosphere in particular, constitutes a subject of patent application number PL423097 (A1), PL233603 (B1).

**Figure 14.** SEM image of the cross-section of the optical interference filter manufactured.
