| Summary: | Topic of adhesion has been well known in the scientific community due to its wide range of applications in microelectronics. Research on adhesion has been extensively carried out for improvement on the interfacial joining between materials. In microelectronic packaging, ceramic metallization has been the subject of interest due to its unique properties specifically under extreme environment. Although this topic has been extensively studied and reported using various bonding techniques, many open questions are still left to be addressed systematically to improve the device reliability.
In this thesis, a systematic study has been carried out for the methods to enhance the interfacial adhesion between Cu thin film and Al2O3 substrate. The Cu thin film was chosen for metallization onto the ceramic Al2O3 substrate by DC magnetron sputtering method as this method is simple, well-controlled, and compatible with industrial manufacturing process. Conventionally, an adhesion layer is required between Cu and Al2O3 due to poor wettability of Cu to the ceramic substrate. However such process inevitably requires a greater time and increases the production cost. A direct bonding of Cu thin films to Al2O3 substrate without adhesion layer was implemented in this study. The key study was focused on the impact of bonding parameters to the mechanical integrity of the interfacial joining. In general, the metal film properties are strongly affected by the deposition parameters. In addition, the residual film stress is the major challenge for good bonding. In this study, the effects of three deposition parameters on film stress have been investigated and identification on the film stress was analysed using XRD-sin2Ψ method. The effect of kinetics movement of Cu atoms to the residual film stress was studied and verified on the adhesion strength. The adhesion improved with an optimized working pressure due to the film stress minimization during the deposition step.
The surface condition of Al2O3 substrate prior to bonding has also been elucidated. Adhesive forces between materials are affected by the environmental factors, surrounding medium and composition. M.B. Ranage has mentioned that the particles in microelectronics are deposited from air, liquids or from human resources, in the size from 0.1 µm to several micrometres. Development of an effective cleaning method is mandatory for a robust bonding. The interaction between Cu and Al2O3 is believed to be affected by the surface cleanliness, as well as the modification of surface chemistry after treatments. Different surface cleaning techniques and treatment sequences were studied to understand their effect on Cu film adhesion with polycrystalline Al2O3 substrate. Excellent adhesion strength of more than 34 MPa has been achieved by argon plasma cleaning which is an abrupt increase compare to 6.1 MPa without plasma cleaning. The reason behind such drastic improvement is the effective removal of the surface contaminants as well as the creation of surface dangling bonds.
With further analysis, the study was extended on bonding mechanism was extended in terms of surface roughness, porosity density and surface morphology. The bonding mechanism and the area in contact between materials have been analysed using polycrystalline Al2O3 with three different range of surface roughness. A comparative study was also carried out using monocrystalline Al2O3 substrates with surface roughness smaller than 0.5 nm in order to provide additional evidence on the obtained results. The study of bonding mechanisms and their quantitative contribution to the Cu-Al2O3 bonding were carried out. It was revealed that surface adsorption provides majority of contribution to the observed adhesion strength. Mechanical interlocking is another contributing factor behind the observed adhesion strength increase in the latter especially for the substrate with higher surface porosity. For the substrate with surface roughness of 350 - 500 nm, it has contributed up to 18.6 % to the Cu film adhesion with Al2O3 polycrystalline. Diffusion bonding mechanism may be operative only with post deposition annealing treatment. From the XPS measurement, it was observed that Cu atoms have diffused deeper into the substrate without chemical reaction between Cu and Al2O3.
The current works conducted have achieved superb adhesion strength beyond 34 MPa, this value has far exceeded the MIL-STD-883E of 10 MPa. The guidelines for enhanced Cu-Al2O3 adhesion were proposed in this thesis with uncomplicated and trouble-free processing step. Extra insertion of glue layer such as titanium, tantalum or chromium is not required in this proposed bonding system. More importantly, types of bonding mechanisms have been justified and quantified to understand their contribution in Cu-Al2O3 bonding system.
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