R. Pelzer

Growth behavior and physical response of Al-Cu intermetallic compounds

This review covers recent investigations and concludes our findings for the growth of Cu/Al intermetallic compounds (IMC). [1, 2] The corresponding copper-aluminum interfaces were either established by a physical vapor deposited (PVD) Cu layer on a PVD aluminum pad or a Cu thermosonic nailhead bond on a PVD aluminum-based pad metallization. The identification, growth kinetics and mechanical strength of the different Al-Cu intermetallic compounds have been investigated. The annealing matrix of these investigations covered the temperature range from 150-300 °C for 25-2000 h. The identification of the Al-Cu phases utilizes X-ray diffraction analysis (XRD), selected area diffraction pattern (SAD) and scanning electron microscopy (SEM) & energy dispersive X-ray spectroscopy (EDX). The main three IMC phases Al4Cu9, AlCu and Al2Cu were identified over the whole temperature range, whereas two additional phases (Al3Cu2, Al6Cu94) contribute to the total IMC growth at temperatures above 200 °C. Individual diffusion constants D0 and activation energies Ea of 1.0 eV for Al4Cu9 and AlCu, 1.2 eV for Al2Cu and 1.3 eV for the total IMC growth have been obtained. As the two slow growing phases Al3Cu2 and Al6Cu94 were not observed below 200 °C, lower activation energies for the total IMC stack were expected and have been measured to be in the range of 1.05-1.1 eV for thin film and bonded samples for temperatures below 200° C. Therefore it is recommended to use these lower activation energies for lifetime predictions in the typical regime of device application temperatures. The impact of IMC thickness and annealing conditions on bond strength was studied using ball shear test. The test results did not show any hints on interface strength degradation across the full experimental matrix even for the groups where Al was already fully consumed, in case of a tungsten barrier or adhesion layer between Al metallization and silicon-based dielectrics was used.

Application of quantitative modal analysis for investigation of thermal degradation of microelectronic packages

Abstract The influence of high temperature storage on the properties of epoxy based materials in microelectronic packages was investigated by application of a quantitative modal analysis technique. An electromagnetic shaker in combination with scanning laser Doppler vibrometry was used to extract the alteration of the modal response of the components in relationship with the thermal aging time. Data analysis was accomplished by the error ratio (ER) method, modal assurance criterion (MAC) and determination of an aspect ratio (AR) using an image correlation technique. Increasing the storage time resulted in a higher degree of degradation which was reflected by an increase in the values of ER, MAC and the AR. Microstructural investigations confirmed that alteration of the modal response is related to the temperature dependent gradual degradation and modification of the properties of the packaging material which was supported by finite element analysis. This present work shows the feasibility of modal analysis as an efficient and quantitative non-destructive testing method for evaluation of degradation of electronic packages.

Thickness dependency of adhesion properties of TiW thin films

The interfacial adhesion properties of Titanium-Tungsten (TiW) thin films with thickness in the range of 300 to 1300 nm to single crystal silicon have been studied by means of nano-indentation technique. The adhesion energy release rate of the coatings has been evaluated by using nanoindentation-induced blister technique. The radius of the blister around the indenter impression which was related to the delaminated film increased significantly with increasing the thickness of coatings, which indicates better adhesion of thinner TiW coatings on silicon substrate by a direct geometrical and qualitative comparison. The obtained steady-state strain energy (Gs) values decreased with increasing the thickness of TiW films. Furthermore at the same indentation load radial cracking at the corners of the indenter (de-cohesion) was observed in the thinner films, while thicker films did not show de-cohesion behavior but a more pronounced delaminated area around the indenter impression. It is suggested that the higher adhesion strength of the thinner films is related to the higher surface energy due to the smaller mean grain size and energy dissipation due to decohesion of the films.

High cycle fatigue testing of thermosonic ball bonds

Abstract During operation, miniaturized thermosonic Cu ball bond interconnects on Al pads occurring in microelectronic devices experience thermomechanical cyclic stresses, which lead to a degradation and subsequent fatigue fracture at the bond interface. Standard static tests, however, ignore the performance of such bonds under cyclic loads. Therefore, a new mechanical fatigue testing method tailored for such interconnects has been introduced, allowing to study their high cycle fatigue behavior in reasonable time. By means of a vibrating system and a special specimen setup cyclic stresses are mechanically induced at the bond interface causing fatigue lift off, where the bond is separated at its weakest site. For this purpose, two distinct specimen preparation methods basing on industrially applicable soldering techniques are suggested and can be used equally, depending on the focus of the investigation and the availability of the required testing structures. The first method - “single bond testing” - allows to test each bond individually regardless of the chip layout. In contrast, the second test method - “multiple bond testing” - allows to test several bonds simultaneously. To interpret and analyze the stresses occurring at the bond interface during these tests, finite element analyses were conducted. In the present study both methods are applied to Cu Al ball bonds of the same quality and chip layout. It is shown that the aluminum is responsible for the fatigue crack initiation and propagation processes as confirmed by fractographic analyses of the fatigued bond interfaces. It can be concluded that the proposed fatigue test method is a powerful alternative screening method for such miniaturized bond interfaces, which allows to reveal their mechanical fatigue behavior in reasonable time and to identify the weakest link of the tested bond interface.

Lifetime of thermosonic copper ball bonds on aluminum metallization pads

A novel accelerated mechanical fatigue testing system in combination with a special set-up was used to evaluate the reliability of Cu- ball bonds. Tailor made test structures were designed and prepared out of commercial wire bonded devices to assess bonding strength of the interconnects under various modes of cyclic loading. Fracture surface analysis of the interconnects showed wire bond lift-off as the dominant failure mode. Based on experimental results and FEM simulations, lifetime prediction curves for two different types of ball bonded devices could be established.