Intrinsic stress in dielectric thin films for micromechanical components

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Abstract

The film stress in coated micromechanical elements may cause bending of such elements and thus impair their performance. In these cases, stress reduction within a single layer by proper choice of deposition parameters or stress compensation within multilayer systems is necessary. In this paper, possibilities for stress reduction in high-reflection (Nb2O5/SiO2)n quarterwave multilayers for thin silicon laser mirrors have been investigated.

Film deposition was performed by reactive direct-current (Nb2O5) and non-reactive radio-frequency magnetron sputtering (SiO2), respectively. The film stress was investigated as a function of process gas pressure, substrate temperature and ion bombardment of the growing film. At zero bias voltage, a total stress of about −30 MPa was obtained in the Nb2O5 films. Utilization of an additional electrode to reduce the plasma density in front of the substrate did change the stress to a small tensile value. SiO2 films show a compressive stress that could not be reduced below 100 MPa within the parameter range investigated.

Complete stress compensation in the multilayer film systems was only possible by application of an additional tensile-stressed metal interlayer. Chromium films deposited prior to the growth of a (4×2) stack of Nb2O5 and SiO2 did compensate — within the error of measurement of ±25 MPa — the average stress in the multilayer system to zero.

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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