Sn particles produced by plasma-induced dewetting and its applications

Abstract

Metal film deposited on glass substrate by physical vapor deposition is energetically in a metastable state. When sufficient energy is applied, the film dewets into a set of particles by means of diffusion to minimize the total energy in the system. Metal film dewetting is initiated by hole formation, which is activated by either spinodal dewetting or hole nucleation and growth mechanism. Depending on the type of hole formation, the resulting particles vary in size and size distribution. Such observations have generally been used to distinguish the dominant mechanisms of the dewetting process. In this work, the dewetting of Sn films was investigated. Behaviors of both spinodal dewetting and hole nucleation and growth were observed when 50 to 2000 nm thick Sn films were dewetted under ICP-generated H2 plasma. At low film thicknesses, the dewetted particle diameter exhibited a spinodal-like behavior by varying exponentially to the film thickness with powers close to 5/3. At higher film thicknesses, a linear growth was observed in particle diameter with respect to film thickness, which implied that film dewetting was activated by hole nucleation and growth. Several attempts were made to control the size and size distribution of the dewetted Sn particles. Input RF power was a dominant factor controlling the electron temperature and the density of plasma. When this parameter was increased, varying particle size and size distributions were observed at same film thickness. The capability of film dewetting was expanded when introducing repeated dewetting and templated substrates. Implementation of repeated dewetting resulted in higher particle density and bimodal particle distribution at appropriate first and second film conditions. Also, templated substrates enabled the production of particles of similar size at desired locations. The preliminary studies of Sn film dewetting were applied in two industrial applications. The first application was in the fabrication of solder bumps, which are interconnections built to transfer signals between a semiconductor chip and its external circuitry. The current solder bump process requires a lengthy procedure, which includes two sets of photolithography. The number of photolithography was reduced to one by utilizing the self-assembling nature of metal film dewetting. Sn film dewetting was also applied in producing metal-based mesh-type transparent conductive film. The rapid and non-thermal characteristics of plasma-induced dewetting allowed a uniform distribution of Sn particles on polymer substrates. These particles were used as a mask layer to outline the mesh areas in the subsequent conductive layer. By controlling the size of the particle mask, conductive films with varying transmittance and resistance values were obtained. The deposition of Sn films and other metal films were conducted by magnetron DC sputters. Plasma treatment of the Sn films was carried out in a separate vacuum chamber with an external ICP source installed. The surface morphologies of the dewetted particles and transparent conductive film samples were monitored by an ex-situ field emission scanning electron microscopy. The SEM images were analyzed using a computer software to perform a statistical analysis of dewetted particles.