Adhesion of critical interfaces in microelectronics
Research output: Thesis › Doctoral Thesis
In modern microelectronic devices a wide variety of thin film materials are utilized such as metals, ceramics and glasses in specially tailored structures to fulfill and enhance the device performance. The adhesion of the individual thin film layers is crucial for the longevity and reliability of the integrated circuits. Especially the interfaces between the isolating layers, e.g. doped silica glass, and the metallization can be a critical due to their different bonding types. As adhesion is affected by many factors such as thermal treatments or film stresses it is important to evaluate the critical interfaces in these devices quantitatively. In the course of this thesis, residual compressive stresses, nanoindentation and scratch testing were utilized to produce defined areas of delamination, called buckles. The materials’ response to the used techniques was investigated via focused ion beam (FIB) cross-sectioning to determine the cause for the delamination, thereby furthering the understanding and applicability of the methods. Three different interfaces exemplary for today’s microelectronic devices have been tested ranging from metal/polymer to ceramic/glass. In a first step, the effect of residual stress and a tantalum (Ta) interlayer on the adhesion in a gold-polyimide system was investigated. It was found that spontaneous buckles formed in the Au film without the interlayer after deposition, while external loading is required to delaminate the Au film with the Ta adhesion layer indicating a higher adhesion energy. Secondly, the three techniques, residual stress, nanoindentation and scratch test were utilized to induce interface delamination of a tungsten-titanium (WTi) film on two different borophosphosilicate glass (BPSG) films. Although buckles induced by compressive stress and nanoindentation have been extensively investigated over the last decades, the scratch test is usually regarded as a semi-quantitative method but is, under the right conditions, capable to induce buckles suited for adhesion evaluation. The comparison of the results calculated from the buckles induced by compressive stress and scratch testing were in good agreement for the two types of BPSG. However, FIB cross-sectioning of the nanoindentation buckles revealed that additional fracture events in the BPSG led to incorrect adhesion energies in both cases. The fracture events following indentation are a serious concern when utilizing nanoindentation for adhesion measurement. To further analyze these fracture events, cross-sectional investigation of indentation buckles on a silicon nitride (Si3N4) film on a BPSG film with a compressively stressed WTi overlayer was performed to illustrated that the fracture paths are very sensitive to the applied loads. In a narrow load range two types of buckles were produced by indentation. While type one of the buckles were the result of through-thickness cracks, type two were representative of the Si3N4/BPSG interface. The scratch test on the other hand, does not result in BPSG fracture under the buckled region because the scratch induced stresses lead to initial delaminations parallel to the scratch trace which acted as points of origin for spontaneous buckles. Therefore, the scratch test was chosen to investigate the effects of annealing duration, from 30 minutes to 2 hours, at 400°C on the adhesion of the WTi film on BPSG. As the annealing duration increased, the adhesion energy increased to about double the value of the unannealed WTi film. The study illustrates the applicability of nanoindentation and scratch testing in compressively stressed film systems for quantitative adhesion measurement. Special emphasis was put on the investigation of the failure modes responsible for buckle development in nanoindentation and scratch testing, thereby furthering the understanding of buckle formation.
|Translated title of the contribution||Haftung von kritischen Grenzflächen in der Mikroelektronik|
|Publication status||Published - 2018|