INTERDIFFUSION OF Au/Ni/Cr ON SILICON SUBSTRATE

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INTERDIFFUSION OF Au/Ni/Cr ON SILICON SUBSTRATE

B. Yan, D. Lin, D. Mao

To cite this version:

B. Yan, D. Lin, D. Mao. INTERDIFFUSION OF Au/Ni/Cr ON SILICON SUBSTRATE. Journal de

Physique Colloques, 1988, 49 (C5), pp.C5-631-C5-634. �10.1051/jphyscol:1988580�. �jpa-00228077�

JOURNAL DE PHYSIQUE

Colloque C5, supplbment au n010, Tome 49, octobre 1988

INTERDIFFUSION OF Au/Ni/Cr ON SILICON SUBSTRATE

B. YAN, D. LIN and D. MA0

Shanghai Jiao Tong University, Department of Materials Science and Engineering, Shanghai 200030, China

ABSTRACT

By using surface resistance measurement and Auger Electron Spec- troscopy, the interdiffusion behavior of Au/Ni/Cr metallization on si- licon substrate was examined. Cr was proved to be an excellent diffu- sion barrier between Au and S i up to 450°C for a Cr layer of 500A thick which would avoid serious degradation of solderability and resistivity of the metallization. Ni was found much less effective a s a diffusion barrier and therefore was mainly to improve the solderability. It was shown that silicon outdiffusion was dominant in the interdiffusion be- tween gold overlayer and silicon substrate. It was attributed to the large grain boundary area and high defect density in the polycrystal- line gold film.

I. Introduction

Au/Ni/Cr multilayer metallization i s widely used in microelectro- nics for interconnections, such a s bonding and soldering. Especially, it is used in silicon power devices in order to improve the thermal fatigue life. Interdiffusion of the Au/Ni/Cr metallization with silicon substrate is of great practical importance for it h a s both beneficial and harmful effects to the service properties of the metallization.

Proper interdiffusion can improve the cohesivity among the thin films, whereas excessive interdiffusion will deteriorate the conductivity and solderability of the metallization. Cr is an ideal element to improve the adhesion to silicon and silicon dioxide, moreover, i t h a s the thermal expansion coefficient very close to that of silicon, which is an important factor in thermal fatigue. Au h a s good solderability and i s an extremely inert element to protect the metallization from being oxidized during storage. Ni is expected to suppress the interdiffusion between Au and Cr (1) and has good solderability a s well. However the effectiveness of Ni a s diffusion barrier h a s never been proved experi- mentally.

There have been extensive work on the metallizations. In order to simplify the problem, most of the work were carried out on the multi- layer thin films deposited on sapphire substrate or Si02 to avoid the interdiffusion between the metallization and the substrates, and the focus h a s mainly been on the interdiffusion within the metallizations.

However, silicon outdiffusion through the metallization which i s of great importance i n the service properties i s frequently ignored.

T h e primary objective of this paper w a s to study the interdiffu- sion between the gold base metallization and the silicon suhstrate under various annealing temperatures by using the sheet resistivity measurement and Auger spectroscopy profiling technique. Therefore the effectiveness of Cr and Ni a s diffusion barriers could be verified and the proper annealing temperature could be determined.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988580

JOURNAL DE PHYSIQUE

11. Experimental

Thin films used in this study were deposited on polished silicon substrates. Cr and Ni were deposited in a three-hearth EB evaporator without breaking the vacuum. T h e vacuum before evaporation was loe6 torr. Because of its limited availability, gold was evaporated sepe- rately in the resistance-heated evaporator. T h e exposure time to the air between the two evaporations was kept a s short a s possible. The thicknesses of the films a r e listed in Table 1 measured by the stylus profilometer.

After evaporation, the specimens were annealed in N2 a t 250°C, 380°C, 450°C and 530°C respectively for 1 hr.

Table 1 Film thicknesses of the specimens Thickness

( A )

c

I

I

Cr to 0 at the surface i s 0.71, very Fig.1 Sheet resistances close to that of Cr203 (0.66). The out- vs. annealing temperature diffusion of C r observed in this study

1 3 5 0

Sheet resistance measurement was carried out at the STZ-1 digital four-point probe. Detailed information of interdiffusion w a s obtained by using the Auger spectroscopy profiling technique on the P H I 6 1 0 Scanning Auger Spectroscope.

111. Results 1. Sheet resistances

The measurements of the sheet resistance vs. annealing t.empera- ture for the three specimens are plotted in Fig.1. T h e sheet resis- tances of Au/Cr and Au/Ni in the as-deposited state are similar. If w e assume that the gold layer h a s the main contribution to the electric conduction, the resistiviti-es calculated from the sheet resistances and film thicknesses of Au are 3.06 and 3.08 pn-cm respectively, which is slightly higher than the bulk resistivity (2.35 PO-cm) due to higher defect densities. T h e higher value of Au/Ni/Cr was probably the result of measurement scatter, it fell down to a lower value after annealing at 250°C for 1 hr.

T h e sheet resistances increased with increasing temperature. At lower temperature, they increased rather slowly and then increased drasticall-y with the annealing temperature. For Au/Cr t h e turning point was at about 380°C, but for Au/Ni/Cr the drastic increase of the sheet resistance occurred a t a l o w e r temperature, 250°C, and the resis- tances were much higher than that of the bilayer metallizations.

Notably, Au/Ni showed lower resistances when annealed at 530°C for 1 hr.

is i n agreement with the previous re-

2200

2. Auger spectroscopy S o

The depth profiles were obtained by Auger spectroscopy. It was shown that no significant interdiffusion was observed after annealing at 250°C for

I,..

1 hr which was in good agreement with the minor changes of the sheet resis-

tances. : , , -

The composition distribution of

as-deposited Au/Cr specimens and after #,,-

annealing at 450°C and 530°C a r e given in Figure 2. Oxygen was found t o exist in the Cr layer which was proved to be incorporated into t.he film during depo-

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,,--.- *urn, ^I

A~,,',,S, Ii ./

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.9,G+"

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sition process. On annealing a t 450°C ^{,a,, .."a 3 0 0 .no 5 0 0}

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for 1 hr, Cr and O diffused to the outer surface of the specimen. T h e ratio of

ports (2-4). It should be noted that although the concentration of the C r layer became rather l o w , ~ 2 0 a t % , very little interdiffusion took

place between Au and S i , showing excellent behavior a s diffusion barrier. Once the chromium-rich layer disappeared, e,g. at 530°C, interdiffusion between silicon and gold took place significantly.

Figure 3 shows the depth profiles of BulNi. Notice that no oxygen existed in the Ni layer which was attributed to its lower tendency to oxidation. After annealing a t 380°C, Ni layer diffused to both sides of the film a s shown in Fig.3b. Contrary to Au/Cr, significant inter- diffusion between Au and Si took place in spite of the existance of the Ni-rich layer with Ni concentration of -30at%. When the annealing temperature w a s raised t o 450°C, Ni peak no longer existed.

The composition profiles of Au/Ni/Cr specimens are given in Fig.4.

Again, i t confirms the existance of oxygen i n the Cr layer. T h e inter- diffusion behavior i s almost the combination of Au/Cr and Au/Ni. It i s interesting t o note that Cr outdiffusion occurred a t 250°C which was lower than that in the Au/Cr specimen, moreover, Cr layer also disap- peared a t a lower temperature, 450°C, instead of 530°C a s in the Au/Cr specimen.

IV. Discussion

T h e present results clearly show the excellent property of Cr in suppressing the interdiffusion between Au and Si. As can be seen from Fig.Zb, S i had much less concentration in the Cr-rich layer than Au.

T h i s fact i s also confirmed in the Au/Ni/Cr specimen shown i n Fig.4b.

It implies that S i h a s much lower diffusivity in the Cr layer than Au.

However, after the diffusion barrier disappeared and the interdiffusion between Au and Si took place, Siconcentration in the Au layer was much higher than that of Au in silicon substrate a s shown in F i g . 4 ~ . Since silicon and gold have negligible solubility between each other, the higher concentration of Si in the gold layer might be due to the higher defect density and large grain boundary area of the polycrystalline gold f i l m , i.e. S i can be segregated a t the crystal defects and grain boundaries, whereas no grain boundary exists in the silicon substrate resulting in the very low Au concentration. By the same reason, the diffuspvity of Si i n the Au layer is higher than that of Au in S i sub- strate. In other words, silicon outdiffusion is dominant in the inter- diffusion between S i and Au.

N i showed rather poor barrier behavior to the interdiffusion be- tween S i and Au. As a matter of f a c t , Au has higher grain boundary diffusivity i n , C r ( E=0.68eV(5)) than i n Ni (E=2.6eV, estimated from its bulk value(6)). Since Au diffusion is not the controlling process, the poor barrier behavior shows a higher diffusivity of Si in Ni. More quantitative work is underway and will be reported elsewhere.

It is observed that interdiffusion between the Au and S i i s the dominating factor in t.he increase of the sheet resistivity. Surpri- singly, the sheet resistance of Au/Ni specimens after annealing at 530°C decreased although significant interdiffusion between Au and S i occurred a s shown in F i g . 3 ~ . T h e depth profile of F i g . 3 ~ shows that even after high temperature annealing, the Au concentration at the surface I.ayer is still relatively high (-80at%). Our SEM observations of the surface morpholog,y, which will be published seperately, showed that many nickel silicide crystals surrounded by Au lumps were observed on the surface of the Au/Ni specimens. As the annealing temperature increased, the silicide grew and the surrounging Au lumps were more likely to be connected together so a s to decrease the sheet resistance when four probe measurement was made. Contrary, in the Au/Cr and

~ u / N i / C r specimens, due to the outdiffusion of Cr and the formation of chromium oxide at the surface, gold concentration at the surface layer was low (-50-60at%). T h e existance of the rather thick layer of chro- mium oxide (-3001) and the high S i concentration in the underneath layer contri.bute to, the- higher sheet resistance.

It was.observ&d that the Au/Ni/Cr specimens showed higher inter- diffusion than in the bilayer specimens. T h e chromium outdiffusion seemed t o be enhanced in the three layer metallization. There are two possibilities First, the thickness of Cr layer in the triple layer specimen (2501) was less than that in the bilayer specimen (350ij.

C5-634 JOURNAL DE PHYSIQUE

Second, the outdiffusion of Cr through the Au layer might be enhanced by the existance of Ni a s solute. The enhancing effect of Ni on the self diffusivity of Cr was reported by Askill(7). We think the solute enhancement in the present c a s e i s very likely. Further study i s needed.

V. Conclusions

1. Interdiffusion between Au and S i substrate causes significanz increase of sheet resistivi-ty and deteriorate the solderability.

2. Cr is not o n l y . a n adhesive layer to the silicon substrate, but also an ideal diffusion barrier for the interdiffusion between S i and Au. A Cr layer of 5001 thick can maintain its barrier property up to 450°C.

3. Silicon outdiff usion i s dominant in the interdif fusion between gold overlayer and siiicon substrate. It is attributed to the large grain boundary area and high defect density in the polycrystall.in~

thin fiim. Therefore, a good diffusion barrier must have low Si dif- fusivity.

4. Ni can neither suppress the interdiffusion between Au and Si nor the outdiffusion of Cr a s expected.

References

1. J.R.Rairden, C.A.Neugebauer, and R.A.Sigsbee

,

2 ,

197 1

2. P.H.Holloway, Solid State Technology, Feb. 1980, p.109 3. A.Munitz and Y.Komen, Thin Solid Films,

2,

4. L.S.Weinman, T.W.Orent, and T.S.Lin, Thin Solid Films,

72,

1976

6. T.G.M.Van den Belt and J.H.W.de W i t , Thin Solid Films,

109,

2 ,

Fig.2 Depth profiles of Fig.3. Depth profiles of Fj.g.4. Depth profiles o f Au/Cr spec:imens Au/Ni specimens Au/Ni/Cr specimens