The instability of polycrystalline thin films: Experiment and theory
- PDF / 1,061,992 Bytes
- 10 Pages / 593.28 x 841.68 pts Page_size
- 26 Downloads / 210 Views
D. B. Marshall Structural Ceramics Group, Rockwell International Science Center, Thousand Oaks, California 91360 (Received 29 August 1988; accepted 6 September 1989)
Dense polycrystalline thin films of ZrO2 (3 and 8 mol % Y2O3) were produced by the pyrolysis of zirconium acetate precursor films, which were deposited on single crystal A12O3 substrates by spin-coating aqueous solutions of zirconium acetate and yttrium nitrate. Dense films were heat treated to encourage grain growth. With grain growth, these films broke into islands of ZrO2 grains. Identical areas were examined after each heat treatment to determine the mechanism that causes the polycrystalline film to uncover the substrate. Two mechanisms were detailed: (a) for a composition which inhibited grain growth and produced a polycrystalline film with very small grains, the smallest grains would disappear to uncover the substrate, and (b) for a composition which did not inhibit grain boundary motion, larger grains grew by enveloping a smaller grain and then developed more spherical surface morphologies, uncovering the substrate at three grain junctions. In both cases, the breakup phenomenon occurred when the average grain size was larger than the film thickness. Thermodynamic calculations show that this breakup lowers the free energy of the system when the grain-size-to-filmthickness ratio exceeds a critical value. These calculations also predict the conditions needed for polycrystalline thin film stability.
I. INTRODUCTION
Liquid precursors (for example, solutions of arkoxides, acetates, and citrates) can be used to form ceramic thin films. In this process, the liquid precursor is deposited on a chemically compatible substrate by either spin-coating or dip-coating. A ceramic thin film is then formed by the pyrolysis of the precursor film. To better understand this process, a program of study has been initiated, emphasizing the relations between processing and microstructural development during heat treatment required to convert the precursor into a dense ceramic thin film. Water soluble zirconium acetate was the precursor used in this study. A water soluble yttrium salt was added to form ZrO2-rich compositions within the ZrO2-Y2O3 binary system. This system includes the transformation toughening tetragonal structure (3 mol % Y2O3) and a cubic structure (8 mol % Y2O3). These two compositions were chosen because grain growth is extremely sluggish for the tetragonal composition and very rapid for the cubic composition.1 Single crystal wafers of A12O3, which is chemically compatible with ZrO2, were used as substrates. Initial experiments showed that dense, polycrystalline thin films of ZrO2 could be formed. When subjected to further heat treatment, the films would uncover
the substrate by breaking up into an interconnected network of zirconia islands. Such behavior has been observed for a variety of metal thin films, including nickel,2 gold,2"6 silver,7"9 tin,1011 copper,12 and platinum.13 In these studies, thin films were observed to develop holes which wo
Data Loading...
