Intrinsic stress in dielectric thin films for micromechanical components
Introduction
Modern information technologies make use of micro-optical components for display and data transmission purposes. To obtain high reflection of light on surfaces, special thin-film systems are usually utilized. For that purpose dielectric multilayers are used, which are mostly deposited by electron-beam evaporation [1] or sputtering methods [2], [3], [4].
In this work, the mechanical stress in highly reflecting thin-film systems on micromechanical silicon mirror plates for laser-beam deflection has been investigated. The mirror plates have a typical size of about 4 mm×4 mm and a thickness of approximately 30 μm [5]. The thin-film system investigated consists of stacks of alternating quarterwave films of Nb2O5 and SiO2 deposited by magnetron sputtering. Without special measures, sputtered oxide films exhibit a compressive film stress of several hundred MPa, which would bend a 30 μm thick silicon plate to a radius of curvature of a few tenths of a metre. This would lead to an unacceptable divergence of the reflected laser beam.
In order to influence the film stress we have varied the substrate bias voltage, substrate temperature and operating gas pressure.
Since low compressive stress or even tensile stress is likely to occur for low ion impact, a special electrode was applied to reduce the plasma density in front of the substrate. Finally, stress compensation by means of a metal underlayer having tensile stress was investigated as well.
Section snippets
Experimental
The oxide deposition was performed in a turbomolecular-pumped vacuum system having a residual gas pressure below 5×10−5 Pa.
Two circular magnetron sources (4 in. diameter) were fixed side-by-side in front of a rotable substrate holder which could be biased by a separate radio-frequency (RF) power supply. By using movable shutters having a special shape, the inhomogeneity of the film thickness on a 4 in. silicon wafer could be improved to ≤5%. The process gas flow was controlled by mass-flow
Variation of the substrate temperature: niobium oxide
At increasing substrate temperature a distinct increase of the compressive film stress in Nb2O5 was found (Fig. 2). Moreover, between 250°C and 400°C, the slope of the curve was significantly larger than in the low-temperature range. This is an indication of a modification of the film structure resulting in varied thermal expansion and/or a change in the intrinsic film stress. However, this assumption has to be proved by further investigations of the film structure and by measurements of the
Conclusions
Film stress in magnetron-sputtered Nb2O5 and SiO2 films has been investigated as a function of deposition parameters. The final goal of this work was to grow low-stress films of these materials as part of dielectric multilayers suitable for coating very thin silicon mirror plates. In Nb2O5 films, stress is usually compressive and grows with increasing substrate bias voltage. Low compressive stress and even slight tensile stress (+20 MPa) could be achieved by using an additional ring-shaped
Acknowledgments
Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The authors are indebted to the Zentrum für Mikrotechnologien of the Technische Universität Chemnitz for preparation of laser mirrors and S. Collard for X-ray investigations.
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In-situ high-temperature X-ray diffraction investigations of magnetron sputtered niobium oxide layers up to 900 °C
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Effect of sputtering anisotropic ejection on the optical properties and residual stress of Nb <inf>2</inf> O <inf>5</inf> thin films
2010, Applied Surface ScienceCitation Excerpt :The precise knowledge of the optical properties and the stresses are important in the design of optical thin-film systems. Since various deposition techniques showed the different properties and stress behaviours [24–29]. Comparisons of the film properties have shown significant variations in the refractive index, absorption, residual stress and surface roughness.
Optical properties and mechanical stress in SiO<inf>2</inf>/Nb<inf>2</inf>O<inf>5</inf> multilayers
2001, Thin Solid FilmsCitation Excerpt :We found that chromium, nickel silver, stainless steel, copper and aluminium exhibited tensile stress in the films while niobium, titanium and zirconium exhibited compressive stress. Details of this work have been published elsewhere [12]. Chromium had the best values and was chosen as material for the additional stress-compensating film.