**1. Introduction**

Due to their wide-ranging applications in optics, interference filters constitute a group of broadly developed and manufactured optical devices [1]. Among others, they are applied as coatings on lenses, camera and projector optics [2], telescope components [3], solar panels [4], and car and aircraft windows [5].

Initially, interference filters were composed of stacks of alternated homogeneous films characterized by the low and high magnitude of refractive index. These systems exhibited numerous shortcomings, one example being an occurrence of secondary harmonic bands in their spectra, substantially lowering optical quality of a filter [6]. Another often encountered problem was poor adhesion between the subsequent layers with the resulting interfacial penetration of water frequently leading to complete filter destruction [7]. This is a reason for which a new generation of interference filters was designed, assuming a continuous periodical (sinusoidal) change of their index of refraction [8]. A gradient inhomogeneous structure of a coating allows one to find a photonic forbidden band similar to that of Bragg's mirror, but one that is narrower (by a coe fficient of π/4) than the width of the quarter-wave band. This is one reason these structures are characterized by better optical properties, with perfectly suppressed secondary and higher harmonic bands [7]. However, one needs to remember that the more complex the coating structure, the higher the di fficulty of the technological problems connected with its deposition.

Despite numerous advantages of inhomogeneous interference filters, there is a rather limited number of scientific publications dealing with these devices present in the literature. One of the principal reasons for that is the fact that the manufacture of these filters requires very precise control of their optical parameters, which is extremely di fficult (if not impossible) in the case of such conventional deposition techniques as the sol–gel method [9]. The realization of this type of project requires the availability of optical materials of variable magnitude of refractive index, controlled by the continuous (and high-precision) change of their elemental composition. The deposition techniques most often used for the manufacture of optical coatings include co-sputtering [10], co-evaporation [11], ion-assisted deposition [12], and glancing angle deposition [13]. In all these cases, both a precise control of the coating composition and safeguarding of its high optical quality require excessive financial investment.

Among the di fferent materials used for the purpose of manufacturing inhomogeneous optical filters, perhaps silicon and its derivatives are the most popular. In recent years, there have been several articles published dealing with the technology of porous silicon for these applications. The first paper on that subject was published in 1997 by Berger [14]. Later, in 2002, Cunin et al. showed an application potential for the filter in screening biomolecular studies [15]. This type of filter, produced by Chhasatia et al. with the help of a thermal hydrosilylation method, was used in the construction of an insulin detection biosensor [16]. Regrettably, the above-mentioned filters are not transparent to visible light, which substantially limits their applications. It was Zhang et al. who produced antireflective narrow line-width filters with a refractive index gradient using silicon oxide (SiO2) as a low (1.50)-index material and niobium oxide (Nb2O5) as a high (2.14)-index material, applying the glancing angle deposition technique for that purpose [17]. The magnetron sputtering technique is also used to manufacture rugate type optical filters. With the help of the reactive magnetron sputtering method, Bartzsch et al. produced silicon oxynitride (SiON) antireflection filters, wherein index of refraction varied within a relatively narrow range of 1.46 to 1.99 [18].

The application of the RF PECVD technique for that purpose appears to be an interesting and fruitful solution [19,20]. The technique allows one to selectively control the chemical vapor deposition processes by means of appropriate adjustment of energy and intensity of substrate bombarding ions. By using this method, Larouche et al. obtained stable filters with very good optical characteristics, stripped of harmonic reflexes. These filters were made of titanium dioxide/silicon oxide (TiO2/SiO2) coatings with their refractive index varying between 1.5 and 2.35, depending on the TiO2/SiO2 ratio [20]. Rats et al., on the other hand, produced inhomogeneous silicon oxide/silicon nitride (SiO2/Si3N4) types of filters using RF/MW PE CVD technique with silane as a source of silicon and N2, NH3 and N2O gas mixture as a source of nitrogen and oxygen [21]. Lin et al. obtained inhomogeneous optical SiON filters by using a PE CVD method and a helium diluted mixture of nitrogen and nitrogen suboxide. With silane as a source of silicon, the resulting magnitude of refractive index varied between 1.5 and 1.85 [22]. In the majority of works dealing with the deposition of SiON types of coatings with the help of the RF PE CVD method, silane is used as a source of silicon. It is a flammable and explosive gas and working with it requires special safety precautions. Therefore, a precious alternative for this precursor is presented by organosilicon connections. There are several literature positions dealing with a use of this group of compounds in the deposition processes of silicon-containing films [23–25]. It should be stressed, however, that these works usually concern either single homogeneous coatings [23] or stacks of a few homogeneous layers [25]. As far as an application of these precursors in the production of "rugate" filters is concerned, there is practically no information available.

The present work reports a process of deposition of silicon carbon-oxynitride (SiONC) coatings of variable composition for the purpose of the construction of a "rugate" optical filter. To do that, the RF

PE CVD technique was applied with either hexamethyldisilazane (HMDSN) or tetramethyldisilazane (TMDSN) used as a source of silicon. A mixture of gaseous N2 and O2 of continuously changing composition was used as a working gas. The coatings were characterized in detail with respect to their optical properties as well as their composition, chemical structure and surface morphology. Since better optical properties were characteristic of the films deposited from the HMDSN, this precursor was finally selected as a starting material for the production of a inhomogeneous SiOC/SiONC/SiNC films with the gradient of refraction index. The solution constitutes a subject of national patent procedure with several PL423097 (A1) PL233603 (B1). An application of that system as a "cold mirror" type of rugate interference filter is also described in this work.

#### **2. Materials and Methods**
