Mediated patterning of sol-gel thin layers: Shrinkage, decohesion, and lift-off

This thesis explores the unique de-cohesion and lift-off processes that occur during mediated patterning of integrated sol-gel thin films of functional materials. Mediated deposition, an additive patterning approach, was introduced by Jeon et al. (J. Mater. Res., 10[12]: 2996–9 (1995)), where microcontact printing (μ-CP) facilitated the selective transfer of a molecular ‘ink’ (e.g., octadecyltrichlorosilane, ODTS) to the substrate, and de-cohesion of sol-gel films occurred exclusively above the printed ink regions after heat treatment (<150°C). Patterned sol-gel films of uniform thickness were revealed after facile lift-off of cracked fragments, without the use of etchants. In this thesis, the origin and mechanics of de-cohesion and lift-off were determined through direct observations and measurements. The disparate mechanical behaviors of sol-gel films on functionalized and unmodified surfaces were determined, with emphasis on interfacial strength, in-plane biaxial stress, thin film mechanics, and crack propagation for sol-gel layers on silicon, glass and fused silica substrates. In addition, direct patterning of sol-gel layers was demonstrated for the first time on curved glassy surfaces using μ-CP and mediated patterning methods.; The shrinkage and densification of sol-gel layers were determined by in-situ and ex-situ measurements, where a ∼30% decrease in thickness (t) was typical during drying of sol-gel layers (T < 300°C). Constrained shrinkage, resulting from adhesion to the supporting silicon substrate, led to a coherent (crack-free) film with biaxial stresses of ∼200 MPa (t ∼ 50 nm). For ODTS-mediated sol-gel films, however, precise in-situ wafer curvature measurements determined that film stresses never exceeded 75 MPa. Additionally, interfacial adhesive strength between the substrate and the sol-gel film were evaluated for the first time using a unique pulsed-laser stress-wave technique. An adhesive strength threshold of 15 MPa was determined for sol-gel films on ODTS-functionalized silicon, whereas, the threshold strength of films deposited on unmodified silicon was determined as >25 MPa and greater than the fracture strength of silicon. In this work, hydrophobic surface regions, facilitated by directed μ-CP of a molecular film, affected adhesion so as to reduce the substrate constraint and promote de-cohesion of the sol-gel film on the sub-micron scale. That is, selective de-cohesion and lift-off were enabled by the control of local mechanical interfacial constraints.