In-situ study of the stiffness of alumina thin films during vapor deposition
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In-situ study of the stiffness of alumina thin films during vapor deposition Joris Proost and Frans Spaepen Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA ABSTRACT The evolution of the biaxial modulus of amorphous alumina thin films has been studied insitu during electron-beam evaporation at 400°C on Si and sapphire substrates. A constant value of 197 ± 30 GPa was measured up to a thickness of 1.17 µm when depositing at 3 Å/s. This value was identical to that of films thinner than 0.30 µm deposited at an average rate of 6 Å/s. Above 0.30 µm, the modulus in these films decreased continuously to a value of 50 ± 8 GPa at a final thickness of 2.05 µm. Based on their refractive indices, the films deposited at 6 Å/s are 30% less dense than those deposited at 3 Å/s. This density deficit is insufficient to account for the modulus decrease ; the remainder is probably the result of slit-like microcracks.
INTRODUCTION Bulk ceramics are well-known to exhibit a large variation in elastic constants due to porosity, inherent in their processing. For example, in α-alumina Coble and Kingery [1] measured a decrease in Young’s modulus of 80% for a 50% volume fraction of isolated pores in a continuous solid phase. In ceramic thin films, an increased compliance is a well-known effect, often studied in relation to the film deposition conditions, as in the case of thermal barrier coatings consisting of columnar grains [2]. In this work, we report on an in-situ study of the evolution of the biaxial modulus with thickness during deposition of alumina thin films.
EXPERIMENTAL PROCEDURE Alumina thin films were electron-beam evaporated from powdered Al2O3 source material onto silicon (100) and sapphire (0001) substrates at 400°C. The chamber pressure during deposition was 1-5·10-6 Torr. No excess O2 was added. An O/Al atomic ratio of 1.49 ± 0.01 was measured by Rutherford backscattering spectroscopy for 300-400 nm thick films deposited on glassy carbon substrates. Electron diffraction in cross-sectional transmission electron microscopy (TEM) of 750 nm thick films showed an amorphous structure, as expected for a deposition temperature below 500°C [3]. The evolution of the biaxial modulus during film growth was monitored with an in-situ measurement of the substrate curvature. The technique is based on monitoring the spacings D between multiple laser reflections off a cantilevered substrate on a CCD, as described in Ref. [4]. From the mean differential spacing ∆D/Do, the average growth stress in the film can be determined as :
O9.9.1
σ f ⋅t f =
Ys ⋅ ts2 ⋅ cos α ∆ D F ⋅ ≡ 12 ⋅ L Do w
(1)
Ys and ts are, respectively, the biaxial modulus and thickness of the substrate ; α is the angle of incidence of the laser beam relative to the sample normal and L is the distance from the sample to the CCD. The ratio cosα/L was calibrated from mirrors with known curvature. Note that the fundamental parameter being measured is not the stress, but the stress-thickness product, commonly denoted as the force per uni
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