GOLD-ALUMINIUM INTERMETALLIC COMPOUND FORMATION

During investigations of failure mechanisms and failure avoidance we have studied the kinetics and the mechanisms responsible for the growth of the Au-Al compounds. Au4Al, Au5Al2, Au2Al, AuAl, AuAl2 have been observed and the growth kinetics of the Au2Al, AuAl, AuAl2 follow a (time)1/2 dependence. The activation energy for the growth of Au2Al is 1 eV and 1.2 eV was found for AuAl and AuAl2. The kind of compound formed depends on the samples preparation and the results indicate that the kinetics of the mixing is the main mechanism responsible for the sequence of formation of the various compounds.

1. INTRODUCTION Au-A1 thermocompression bonds are generally used for monolithic and hybrid integrated circuits as well as for discrete devices.Their break-down is caused in most cases by the failure of the Au-AI bonds.Degradation 0 f the bonds appears in their increased resistance or total rupture.It has been observed that in certain cases loose, voided phases appear at the failed Au-A1 bond, which are described in the literature as Au m A1 n intermetallic phases. 1,2,3These phases do not cause failure by themselves but certain types of the failures, i.e. broken metallization around the bond, lifted bonds, cracks are related to the formation of these phases. 4 ,s , 6 The purpose of the present paper is to show, in the form of an investigation into the failure mechanisms and the possibility of avoidance of them, a) the kinetics of the compound formation and b) the mechanisms responsible for the.growth.

EXPERIMENTAL METHOD
On thermally oxided Si single crystal, Au and A1 films were deposited in a high vacuum oil-free system by e-gun.To minimize interface effects, mainly due to oxygen contaminations, gold was deposited first followed by A1, without breaking the vacuum.The thicknesses of Au and A1 range between 1000 A and 3000 &.Thermal annealing was carried out in a 10 -7 torr furnace in 50C to 500C temperature range.
The different compounds were identified by X-ray diffraction and MeV 4 He + Rutherford backscattering was used to measure the thicknesses of the compound layers with a resolution of about 200 A. Both the techniques are widely described in the literature.AuA12, AuA1, Au2 A1, AusAI:, Au4AI.The sequence of formation for the various compounds is resumed in the same figure.The first phase formed at low temperature is Aus AI: in competition with AuA1.
The question mark near Aus A12 indicates that we are not sure if Aus A12 is formed before Auz A1 or not.By pushing the heat treatment Au: A1 is formed.Now depending upon the relative quantity of Au and A1 three different sequences can be constructed.The end phases are AuAI: in the case Au A1, AuA1 in the case Au AI and Au4 AI for Au >> A1.These sequences follow a precise scheme: AuAI: starts to form only when all Au is consumed, Au4A1 only when all AI is consumed and AuA1 starts to develop only when both A1 and Au are consumed.Au: A1 and AusA12 are end phases only when the thicknesses of Au and A1 films were chosen in such a way that the Au/A1 atomic ratios were made 2/1 and 5/2 respectively.The end phases have been found to be stable up to 500C.The list of the samples made is given in Table I, together with thicknesses of Au, A1 and resulting compound formed after heat treatment.There data have been obtained by a backscattering technique.
The growth rate proportional to /t and controlled by a single activation energy E T for different times and temperatures has been determined for Au2 A1, 9 AuAI and AuA1.
Figure 2 shows the typical backscattering spectra of the sample as evaporated and after heat treatment at 247C for different times.The Au and AI thicknesses are 1500 A and 1400 A respectively.The formation of the compound occurs at room temperature and the high ratio between Au unreacted and Au in the compound indicates the presence of Aus A12 in agreement with X-ray diffraction analysis.
After heat treatment at 247C for 5 min only the compounds Au: A1 and AuAI: have been observed as confirmed by X-ray diffraction data.These compounds constitute two layers, not mixed, Au: A1 ENERGY (MeV) He* MeV backscattering spectra of the samples as evaporated and after annealing at 247C for 5 min, 15 min, 120 min.. near the substrate and AuA12 on top of it.By further heating at the same temperature for 15 min a step at energies around 1.55 MeV starts to develop.This step corresponds to the formation of a layer of AuA1 between the two previous ones.The AuA1 layer grows utilizing material either from Au2 A1 or from AuA12.
By increasing the annealing time, AuA1 grows and after 120 min Au2 A1 and AuAI2 are almost completely converted into about 3000 A of AuAI.
Figure 3 reports at two different temperatures, 217C and 247C, the dependence of the square of the thicknesses o f AuA1 upon the annealing time.The experimental data are fitted by a straight line indicating that diffusion through AuA1 is the limiting process for the growth of the compound.The data show the presence of a delay in AuA1 formation, a delay which is attributed to the time necessary to consume all the A1 to form AuA12. Also, in the same figure, the kinetic growth of AuA12at 217C obtained by backscattering data, is reported.This phase has been identified by X-ray diffraction data.
The growth rate of AuAl and AuA12 as a function of 1/T is shown in Figure 4 compared with the data obtained for AuA12 and Au2 AI compounds by Campisano et al. 9 AuA1 has a growth rate controlled by a single activation energy ET 1.2 + 0.1 eV, the same has been obtained from AuA12.The growth rate of AuAI2 measured here is in agreement with the data of Campisano et al. 9 The growth rates of Au4 AI and Aus A12 are not measured yet.Moreover, although quantitative data are not available yet, the impurities play a fundamental role in the kinetics and/or nucleation of intermetallic compound formation.In the Au-Al system, for instance, the hydrogen inhibits the phases formation, the presence of Si accelerates the formation of some phases and the oxygen decomposes the AuAlz

DISCUSSION AND CONCLUSION
All the phases predicted by the phase diagram of Au-A1 system were obtained with a low temperature process by thin film interactions.The sequence and the kind of compounds formed strongly depend upon the thickness of deposited material: samples made differently give different compounds, even if the same heat treatment is performed.This indicates that the kinetics of compound formation and, above all, the end phases, are not controlled by the stability of the compound itself but by the quality and kind of the material available for the reaction.In other words the nucleation of a compound seems to be controlled by the boundary conditions.Also the delay in AuA1 formation shown in Figure 3 is due to a nucleation phenomenon inhibited by the presence of Au or A1 more than to a different growth rate of AuA1 compared to AuAI or Au: A1.The growth kinetic of Au2 AI, 9 AuAI., AuAI= phases is proportional to x/t indicating that the transport across the compound layer is the limiting step in the process.In the temperature range considered one single activation energy ET is found.Marker experiments are necessary in order to identify the moving species.
In conclusion we have shown that the sample preparation plays a fundamental role in the Au-A1 phases which are mainly responsible for the failure observed when Al and Au are used.

FIGURE 3
FIGURE 3 Square of AuA1 and AuA12 thickness as a function of time.D corresponds to time necessary for the formation of about 1500 A of AuA12.

TABLE
End phases in the Au-A1 thin films interactions is shown.Five different compounds are predicted: