In this investigation, the influence of a thin gold (Au) layer on the growth behavior of the intermetallic compound (IMC) in a Nickel-Tin-Solder (NiSn-Solder) was studied. The reaction kinetics was studied in the temperature range of 232 ℃ to 330 ℃ using cross-sectional scanning electron microscope (SEM) images. The kinetics of the reaction was determined using the empirical power law and the research showed that the introduction of an Au layer changes the reaction kinetics of the solder significantly. Furthermore, the change in reaction kinetics was accompanied by a change in morphology of the developing grains. The grain morphology of the IMC was studied for samples annealed at 290 ℃using cross-sectional and top-view samples and compared to Au free NiSn-Solder.
The study presented previously focused on the influence of temperature on the morphology and growth kinetic of Ni3Sn4 in a NiSn-Solder system [
The metal stack used in this investigation was deposited onto a 150 mm, p-doped (boron), single flat silicon wafer with a (100) surface normal, and an initial thickness of 625 µm, using sputtering in high vacuum. All materials used for the investigation are grade 5N (99.999% pure) materials. The deposition was done using a commercially available cluster tool for the physical vapor deposition of thin films. The material stack consists of a Titanium (Ti) adhesion layer, deposited on the native oxide of the silicon wafer. After the Ti adhesion layer a nickel layer, with a thickness of 500 nm, was deposited followed by a 120 nm layer of Au, and a 1000 nm thick layer of Tin (Sn) which was covered with 100 nm Ti to prevent the oxidation of the Sn. The process gas used during the sputtering processes was Argon. During deposition the wafers remained in a high vacuum, ensuring that the materials remain oxygen and contaminant free. After the deposition of the material stack, the wafer was thinned to a thickness of 150 µm using a standard wafer thinning process. The samples used to determine the growth kinetic of the IMC are square pieces with a side length of 1.4 mm, the samples used to determine the morphology of the grains at 290˚C and 10 s of annealing are square pieces with a side length of approximately 1 cm, the preparation procedures for the samples will be explained in the respective sections.
In order to generate samples for the determination of the growth kinetics of the IMC, the 150 µm thick wafer is separated into square pieces with a side length of 1.4 mm using a low power, nano-second, laser dicing process. During the processing of the wafer great care was taken not to expose it to temperatures above room temperature, in order to eliminate the possibility of the materials reacting before the experiments. Additionally, during the SEM investigation, only regions outside of the heat effected zone of the laser dicing process were used for analysis, in order not to alter the results of the annealing experiments. The samples were annealed at the temperatures 232˚C, 260˚C, 290˚C, 310˚C, and 330˚C for various annealing durations (1 s, 3 s, 5 s, 10 s, 30 s, 60 s) using a heated bond head tool. The process used for annealing is described in detail by Wendt et al. [
To determine the growth of the IMC during annealing the cross-section of the samples was investigated using scanning electron microscope (SEM) images. In order to generate a clear SEM image of the cross-section, the samples were embedded into an epoxy resin, ground, polished, ion milled, and a thin Gold-Palladium film was deposited on the surface of the sample to ensure conductivity and avoid charging of the sample during SEM analysis. The cross-section of an as-deposited sample embedded into an epoxy resin is displayed in
The samples used for determining the morphology of the IMC are square pieces with a side length of approximately 1 cm. They were generated by initiating cracks with a diamond tipped stylus on the surface of the 150 µm thick wafer in the desired grid and then separating the wafer into pieces. The pieces were annealed using a hot plate (Präzitherm, PZ 28-2T) at a temperature of 290˚C for 10 s. After annealing the remaining elementary
Sn and parts of the AuSn4 were selectively etched away using an aqueous solution of HCl (38%) and H3PO4 (85%), an etch duration of 60 s was selected in order not to over etch the samples, the solution was kept at room temperature during etching.
The morphology, and chemical composition of the IMC formed during heat treatment was investigated using a Zeiss Leo Gemini 1530 SEM, and an AMETEK EDAX APOLLO X silicon drift detector.
The growth kinetic of the IMC was studied using cross-sectional samples. Investigating samples after deposition showed that the Au and the Sn layer already reacted to form a mixture of AuSn4 and Sn, as seen in
The IMC composition is closest to the compound AuNi2Sn4 which was first reported by Neumann et al. [
The cross-section images of samples annealed at 290˚C and above show that the AuNi2Sn4 grows in large rounded grains. In the NiSn-Solder system a needle-like growth of Ni3Sn4 is observed at these particular temperatures, which is discussed by Wendt et al. [
As seen in
where X represents the thickness of the IMC at time t, X0 represents the thickness at t = 0, k is the rate constant of the reaction, and n is the time exponent. The empirical power law is frequently used to describe the growth of intermetallic compounds and the value of n gives important insight into the growth rate controlling steps in the reaction. Solid state diffusion controlled reactions exhibit a parabolic dependence of IMC layer thickness on time, while grain-boundary diffusion controlled reactions often show a t1/3 dependence [
1) The decrease in Ni thickness is directly proportional to the increase in IMC thickness
2) The solubility of Ni in the liquid AuSn4/Sn is negligible
3) The growth of the IMC during heating and cooling is negligible
Assumption 1 is valid if the IMC in question is constituted of only one phase, AuNi2Sn4, with a constant Ni concentration, which is in accordance with our EDX results, and if assumption 2 is valid. Since the amount of Ni detected using EDX analysis in the AuSn4/Sn section of the cross-sectional samples is 2 at% and lower, and since the solubility of Ni in liquid Sn is less than 1 at%, assumption 2 is considered to be valid [
The amount of Ni consumed was plotted versus annealing time in a logarithmic plot to determine the time exponent n, as seen in
The time exponent n was determined for all annealing temperatures, the results are displayed in
The resulting value of n was expected to be similar to the result of Ni3Sn4, which was determined to be in the range of 0.26 to 0.33 by Wendt et al. but the current results show that it is significantly lower, which could not be matched to a currently known growth model [
The IMC morphology of the growing AuNi2Sn4 was investigated using etched samples and compared the results presented in [
The top-view and cross-section SEM images of the NiSn-Solder sample displayed in
T [˚C] | n |
---|---|
232˚C | 0.12 |
260˚C | 0.13 |
290˚C | 0.18 |
310˚C | 0.16 |
330˚C | 0.12 |
displays the AuNi2Sn4 grains with some remaining AuSn4 on top. The AuSn4 grains also display a needle-like morphology which is attributed to the solidification morphology of the compound when cooled from temperatures above the melting point of the solder. This effect has been previously observed and is reported in the literature [
Since the time exponent differed significantly form the results expected, the investigation focused to find the reason responsible for the deviation from the expected results. A close look at the interface between the IMC and the liquid component of the solder as well as the solid component of the solder can give insight into the mechanism controlling the IMC growth. When investigating the cross-sectional SEM images of the embedded samples, individual AuNi2Sn4 grains were observed spalling from the IMC layer.
The spalling of grains is often attributed to the crumbling mechanism, proposed by Görlich et al. [
solder moves along grain-boundaries towards the Ni during the growth of the IMC.
The reaction of an AuNiSn-Solder was studied in the temperature range of 232˚C to 330˚C. The growth kinetic was analyzed using the empirical power law and the time exponent n was determined to be between 0.12 and 0.18. Additionally, the morphology of the grains was studied using cross-sectional and top-view SEM images of etched samples and a comparison between a NiSn-Solder and the AuNiSn-Solder was made. It became clear that the Au layer had a pronounced effect on the time exponent n of the IMC growth as well as on the morphology of the developing grains. The crumbling of IMC grains into the solder was observed and could be linked to the crumbling mechanism proposed by Görlich et al. Additionally, the cross-sectional SEM images showed that the grain-boundaries seemed the be a preferred transport path of the liquid component of the solder towards the Ni interface, causing new grains to develop at the intersections between the grain-boundaries and the Ni layer.
Mathias Wendt,Andreas Weimar,Marcus Zenger,Klaus Dilger, (2016) Changing the Growth Behavior of a NiSn-Solder Using Gold. Journal of Materials Science and Chemical Engineering,04,31-38. doi: 10.4236/msce.2016.44004