Measuring the critical thickness of thin metalorganic precursor films
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Measuring the critical thickness of thin metalorganic precursor films Ryan K. Roeder and Elliott B. Slamovich School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907-1289 (Received 5 August 1998; accepted 8 March 1999)
Successful application of sol-gel, metalorganic decomposition, or hydrothermal routes to ceramic thin films depends on the mechanical integrity of the precursor film. Above a critical thickness, a precursor film will crack or decohere from the substrate during drying. The cracking and thickness of thin metalorganic precursor films were simultaneously observed during drying using a standard optical microscope. Isochromatic color fringes produced by interference of reflected white light were used to monitor film thickness. The critical film thickness was determined by the color fringe corresponding to the thickness at which propagating cracks terminated. As a demonstration of the technique, the critical thickness of titanium di(isopropoxide) bis(ethyl acetoacetate) films was measured, showing increased critical thickness with the addition of small amounts of an elastomeric polymer.
Amorphous precursor thin films are converted into crystalline ceramic thin films by a variety of techniques, including sol-gel processing, metalorganic decomposition (MOD), and hydrothermal processing. In each of these methods, a substrate is first coated with a precursor film, usually by spin or dip coating. The precursor film is then dried and converted into a ceramic thin film by a chemical and/or thermal processing step. During each step, the film volume may decrease due to loss of volatile organics and increased film density. If the film adheres to the substrate, volume changes constrained in the plane of the substrate induce internal stresses which may cause film cracking.1–7 Moreover, cracking of the precursor film consequently affects the mechanical integrity of the final ceramic film. The mechanical integrity of thin films has long been observed to be dependent on film thickness. Above a critical thickness, brittle films under a tensile stress are observed to crack spontaneously or propagate cracks from preexisting flaws.2–5 Cracking occurs when the strain energy release rate exceeds the film fracture toughness, or (in concept) when internal stresses exceed the cohesive strength of the film.6–10 Films above a critical thickness can also decohere from the substrate when interfacial stresses exceed the adhesive strength.8–11 The critical film thickness for cracking has typically been observed in the range 0.4–1.0 mm for a variety of materials and conditions.2–6 These thicknesses are suitable for measurement by optical techniques. Isochromatic color fringes produced by interference of reflected white light can be used to measure the thickness of transparent thin films in the micrometersubmicrometer range.12,13 The major disadvantage of the technique is that the refractive index of the film must be known.12,14 However, when the refractive index 236
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