Research On Technologies And Applications Of Low Temperature Wafer Bonding

As one of the newly-emerged enabling technologies, wafer direct bonding has been playing key roles in more and more fields. Without any kind of glue, wafer bonding technology is able to integrate two polished semiconductor wafers and thus be used widely in fields like micro-electronics micro-mechanics and opto-electronics. Researching deeply into detailed aspects of wafer bonding technologies and applications will help to promote its maturity in semiconductor industry. This research work covers some of the aspects in wafer bonding technologies and applications. As the foundation of device manufacturing, the bonding technologies introduced in this dissertation covers the aspects of theories, processes and reliability tests. For the applications, wafer bonding are applied to multi-layer wafer bonding, patterned wafer bonding and micro fluidic devices.In the research of bonding theories, starting from the basic mechanism of plate contacting theory, a set of bonding model is established according the properties of the silicon wafers. In the model, the surface topography is described to obey the Gaussian distribution. By calculating and analyzing the model in detail, the relations, in the silicon wafer direct bonding, among the elastic force due to surface deformation, the adhesive force due to surface activation and the distance due to surface separation are unveiled. The impact of separation distance on bonding force is explained in detail. By comparing the results of models and the conditions of the real bonding, a practical surface roughness criterion for spontaneous bonding, which includes the impact of surface activation quality, is proposed and proved to be in agreement with the experimental data.In the process exploration, based on the idea of combining both wet chemical activation and dry plasma activation, UV exposure is employed in wafer bonding process. Results of comparative trials show that proper UV exposure increases the bonding quality dramatically. From the view of the chemical thermodynamics, by comparing the energy between UV and chemical bonds, the mechanisms of UV cleaning and UV activation are explained in detail, as well as different ways by which hydroxyl groups are accumulated on the surfaces.In the quality tests and reliability evaluations, different aspects of bonded samples are examined by carrying out experiments of constant temperature and humidity treatment, thermal cycling treatment, as well as the air tightness evaluation. Besides, It is found that prolonging the exposure time may cause decrease in bond quality and the reason for which is explained based on the results of surface roughnesses and bear ratios from AFM measurement.Wafer bonding applications: By applying what we have explored into application of wafer bonding, triple-layer bonding is carried out and satisfying results are achieved to lay a foundation for 3D interconnected structures. Besides, the ability of this bonding process in cleaning and activating the silicon surface is confirmed again by observing uniform interfaces achieved by bonding bare silicon wafers with patterned wafers. Finally, by combining wafer bonging technology with other processes, a micro fluidic chip for hemocytometer is prepared. After analyzing the function, components and process in detail, wafer bonding is served as a packaging method to integrate the structural layer and sealing layer.