AGEING TESTS ON GOLD LAYERS AND BONDED CONTACTS

Studies have been made of the adhesion of gold thin films on NiCr and Mo layers deposited on ceramic substrates. It is shown that the ageing behaviour and the related long-term adhesion losses are complex processes and the various contributing mechanisms are identified.


INTRODUCTION
Searching for physical reasons for the degradation of integrated circuits, we find that the weak links are often the thin-film metallizations, the bonded or soldered contacts, and the hybrid metallizations. The metal layers mainly limit the lifetime of the microelectronic devices during their application.
Several attempts have been made to undertake an analysis of the degradation kinetics of gold thin and thick films which is closely related to the material structure.1,2 This is in contrast to the analysis of statistical reliability, where usually a black box analysis of the electronic behaviour of a device leads to probability data. a resistors electromigration 0 ....m rnicrowelding and adhesive spots / pull-off structures FIGURE 1 The test pattern, used for ageing measurements by DC resistivity and adhesion tests 89 All material failure is dynamic. It advances by rate processes which have threshold conditions and characteristic growth kinetics. 4  such as the grain size, the lateral homogeneity of the grain structure, the chemical composition, the defect accumulation at the free surface and the interfaces, and finally the mechanical stresses. Knowledge about the processes concerning the ageing rnakes it possible to establish a thermodynamical calculation for simple cases e.g. like the recovery of defects or the diffusion of humidity through encapsulations the latter of which is thought to govern layer degradation by corrosion. In Section 2 of this paper we shall describe the structural defect dependent ageing processes in pure and sandwich layers of Au.
The term "ageing" is used to describe variation with time, temperature and electrical stress which produces irreversible change in physical and chemical properties during the treatment. In Section 3 the problem of the adhesion between the layers and the substrate will be discussed in terms of the ageing kinetics. === resistivity decrease due to recovery ' resistivity increase due to Cr diffusion into the Au

THE AGEING KINETICS OF GOLD LAYERS
Pure, fine-grained fcc. metal films with a thickness of the order of 0.5/am to 3/am have, in contrast to the respective bulk metals, a high density of atomistic defects. This is expressed in terms of a high internal stress 6 and of a high dislocation density. 7 Both show high rates of recovery, due to diffusion and transformation in the crystalline structure, at temperatures below the bulk recrystallization temperature of 300-360C for pure gold. 8 For a precise determination of the ageing kinetics using the MEECHAN-BRINKMAN (M-B) method, 9 the DC resistivity is a useful physical property: it is sensitive to the defect concentration and it is measured in a simple way with high accuracy.' The applied test pattern, Figure 1, contains strips with different cross sections. The two configurations with wide strips are used for investigating the electromigration kinetics. The narrow strips are used for the DC measurements. The other sections of the test pattern contain square and round spots used for adhesion tests on bonded contacts, and areas for diffusion tests using electron microprobe analysis.
The DC measurements were carried out at 78 K (liquid nitrogen) after each isochronal (increasing temperature and constant time interval) and isothermal (increasing time, constant temperature) heat treatment. The classification of the measured The diffusion sensitive systems, e.g. Au thin films with NiCr adhesion layers show a remarkable increase in the DC resistivity during the isochronal heat treatment, Figure 2. This is explained by the diffusion of adhesion layer components into the pure gold layer. It should be noted that an additional DC current during the.heat treatments increases the residual resistivity. In this case, additional defects near grain boundaries are introduced, the diffusion of the Ni and Cr into the Au is not markedly disturbed.
The non-diffusion sensitive systems show a continuous decrease of the DC resistivity within the limits of a few percent. Figure 3 shows the behaviour of a 0.5/am Au thin film on a 50 nm Mo adhesion layer. In the bulk we do not find any solid state miscibility. The recent observations of the diffusion of Mo through Au layers o must be interpreted by grain boundary diffusion.
Thus we find two basic kinds of behaviour which govern, in a more or less complex manner, the ageing characteristics of the measured systems, Au layer/ adhesion layer. If the diffusion and the recovery both contribute to the change in the DC resistivity, the resulting plots can be separated to show the contributions. Figure 4

ADHESION LOSSES OF BONDED CONTACTS
If the adhesion layers react by diffusion with the Au layers, the residual gold-rich interface to the oxide substrate does not have sufficient adhesion. Consequently the "adhesion" must be related to the time and temperature dependent longterm ageing kinetics.12 The term "adhesion" is to be understood as the mechanical resistance which a multilayer structure consisting of a thin film and a substrate has to "pull-off," blistering and other irreversible kinds of destruction.
In this section we do not consider the zero-hour adhesion which can be related closely to fracture mechanics. The long term adhesion is based on the creep fracture of composite structures involving the influence of corrosion at the crack tip. The adhesion layer also reacts chemically with the oxide substrate. Figure 6 shows the corrosion in the flaw between the substrate and the Au layer. Figures 7a, b show the formation of crystals but of the plane of the adhesion layer. These chemical reaction products are growing through the Au layer (Figure 7c). It should be noted that these reactions of crystalline transformation are not stress dependent as in the case of the whisker growth through a hard cover layer. 13 (The driving force originates from the loss in the free enthalpy by means of the large crystal formation.) The effect of an external, tensile force e.g. attached to a microwelded beam lead is shown diagrammatically in Figure 8.12 The different geometrical relationships of the arrangement consisting of the substrate/thin film/beam lead system lead to different kinds of tensile stress distributions.
The sharper the crack tip the higher will be the local maximum of the normal stress in the thin film. Table shows the lifetimes of diffusion sensitive adhesion layer/Au layer systems which were contacted by thermocompressive beam lead bonds and subjected to steady-state creep tests.
The diffusion governed process of the adhesion loss by sharp crack propagation is explained by the model shown in Figure 9. The creep stability of the fine grained thin film is governed by the grain boundary creep rate. This is accompanied by creep induced diffusion of the adhesion layer elements through the grain boundaries in the gold. The kinetics is determined by the vacancy migration shown in Figure 9. This leads to a steady-state creep rate b Using the simplification that no chemical bonds are ruptured in the cracking interface, we get, for the example of Au grains (with R 100 nm, d 0.3 nm, 2 0.007 nm a o 10 MPa, T 300-400 K,) and Cr as the tracer diffusor, (the grain boundary self diffusivity of Au D1 as in Gupta reference, 2 the Cr diffusivity in Au D2 as in Holloway reference: 14 3 x 10 -10 s-1 3 x 10 -7 S -1 if we assume grain boundary occupation densities of between 0 and 10%. The layer thicknesses, the grain and the atomistic defect structure, and the geometry of the introduced external force determine the kinetics of the thin film degradation and of the adhesion losses. a curvature radius of 0.4 mm and a thickness of 0.1 mm 12 will provide crack velocities between 10 -9 mm s -1 and 10 -6 mm s -* The total crack lengths of 0.2 to 0.3 5 mm yield lifetimes between 2 h at T 400 K and several days at T 300 K. This is in a good agreement with the experimental results.

CONCLUSIONS
It is evident that the ageing kinetics of an Au thin fdm/adhesion layer composite and the related long term adhesion losses are complicated effects which result from the occurrence of several reactions, namely:the recovery of atomistic defects