Silicon Alloyed with Gold Forms a Eutectic Alloy Which Can Be Used As a Low Melting Point

In Situ Observation of Stress-Induced Au-Si Phase Transformation

D. Huitink1, L. Peng2, R. Ribeiro2, and H. Liang1,2[*]

Department of Mechanical Engineering1 and Materials Science and Engineering,2TexasA&MUniversity, College Station, TX77843-3123

Materials

Gold and silicon were chosen in this study for their relatively low energy of mixing, as indicated by the Au-Si low eutectic temperature. To coat the tip, a vacuum enabled thermal evaporator was employed to deposit an intermediate ~5 nm thick layer of germanium (99.9999%, Alfa Aesar) on the probe, followed by a 250 nm layer of gold (99.99%, Alfa Aesar). This enhances the adhesion of the gold to the probe.

According to the equilibrium phase diagram (not shown), the gold (Au) and silicon (Si) system forms a eutectic with small quantity of about 5%wt Si at 3630C and exists as a perfectly mixed liquid over a fairly broad temperature and composition range.[1] Previously, equilibrium eutectic systems have been used to fabricate nanorods and nanowires at a relatively low-temperature through vapor-liquid-solid synthesis methods.[2],[3],[4],[5],[6],[7] Among those, metastable phases of AuSi and Au7Si appeared with different structural and electronic properties.

Silicon alloyed with gold forms a eutectic alloy which can be used as a low melting point solder for silicon devices.[8],[9] During soldering, it is desirable for silicon to have a high diffusion rate inside the gold.[10],[11]

AFM Experiments

An AFM facilitated the control of sliding the probe on the surface of the silicon substrate in a method analogous to traditional friction tests in the mild wear regime. After initially contacting the surface of the Si, the AFM controller increased the loading on the gold coated probe tip/surface contact point by increasing the deflection of the probe cantilever in 5 µm increments. The deflection of the cantilever is directly proportional to the force at the probe-surface interface, where the proportionality constant is a function of the cantilever composition and dimensions. The AFM then controlled the reciprocating linear motion of the probe tip along the surface of the substrate with a velocity of 480 µm/s. After this procedure, an alternate, uncoated AFM probe was used in "close-contact" mode (i.e. no physical contact) to observe any morphological changes to the surface of the silicon wafer. Unlike in traditional wear tests, where a trough or wear track is observed where sliding occurred, a deposition of new material was observed on the surface of the Si. A line of 55 nm width and 1.3 nm height formed on the surface, as shown in Figure 1. Furthermore, it was previously demonstrated that the formation of this structure is not directionally dependent (or related to the crystal direction of the substrate) as indicated by non-linear structures created using a patterning software.1

In Situ TEM Experiments

Inside this system, the nanoindentation tip can be coarsely positioned in three dimensions with screws, and further finely positioned and actuated with a 3D piezoceramic actuator which can control the movement and applied force according to the applied voltage. A digital video recorder equipped with the TEM captured the events of the indentation in real time for bright-field or dark-field imaging conditions.

A worn diamond nanoindenter tip of Berkovich-type geometry was sputtered with ~1 µm gold (Mohs Hardness of 2.5-3,[12] although likely softer due to the porosity resulting from sputtering) and pressed into Si (Mohs Hardness of ~7[13]) while monitoring by TEM. Prior to indentation, the Si was ion milled by focused ion beam (FIB) to reduce the thickness of the Si to less than approximately 250 nm in order to facilitate TEM observation. To achieve this electron transparent thin feature, two opposing trenches are milled with the ion beam on a Si wafer. This process was optimized to reduce the amount of damage by the 30 kV Ga+ ion beam in the FIB.

References

1 C.D. Thumoond and M. Kowalchik, “Germanium and Silicon Liquidus Curves,” Bell Syst. Tech. J. 39, (1) p.169-204, (1960).

2 M.K. Sunkara, S. Sharma, and R. Miranda, G. Lian, and E.C. Dickey, “Bulk Synthesis of Silicon Nanowires Using a Low-temperature Vapor-Liquid-Solid Method,” App. Phys. Lett., Vol. 79, No. 10, p.1546-1548, (2001).

3 R.S. Wagner and W.C. Ellis, Appl. Phys. Lett. 4, 89 (1964).

4. J. Westwater, D.P. Gosain,S. Tomiya, S. Usui, and H. Ruda,J. Vac. Sci. Technol. B15,554 (1997).

5 A.M. Morales and C.M. Lieber, Science, 279, 208(1998).

6 X. Duan and C. M. Lieber, Adv. Mater.12, 298 (2000).

7 S.Q. Feng, D.P. Yu, H.Z. Zhang, Z.G. Bai, and Y. Ding, J. Cryst. Growth209, 513 (2000).

8F. G. Yost, J. Electr. Mdlt.3, 353 (1974).

9H. Kato, J. Electrochem. Soc.134,1750(1987).

10 A. Hiraki, Jap. J. Appl. Phys.22, 549 (1983).

12A. Cros, P. Muret, Mater. Sci. Reports, 8, p.271, (1992).

13Marcus Materials Co. catalog, 2006. Newark, Delaware, (2006).

14 Goodfellow catalog, Cambridge, England, (1996/1997).

[*] Corresponding Author:

[1]. C.D. Thumoond and M. Kowalchik, “Germanium and Silicon Liquidus Curves,” Bell Syst. Tech. J. 39, (1) p.169-204, (1960).

[2]. M.K. Sunkara, S. Sharma, and R. Miranda, G. Lian, and E.C. Dickey, “Bulk Synthesis of Silicon Nanowires Using a Low-temperature Vapor-Liquid-Solid Method,” App. Phys. Lett., Vol. 79, No. 10, p.1546-1548, (2001).

[3]. R.S. Wagner and W.C. Ellis, “Vapor-Liquid-Solid Mechanism of Single Crystal Growth,” Appl. Phys. Lett., 4, p.89, (1964).

[4]. J. Westwater, D.P. Gosain,S. Tomiya, S. Usui, and H. Ruda, “Growth of Silicon Nanowires via Gold/Silane Vapor-Liquid-Solid Reaction,” J. Vac. Sci. Technol. B, 15, p.554, (1997).

[5]. A.M. Morales and C.M. Lieber, “A Laser Ablation Method for the Synthesis of Crystalline Semiconductor Nanowires,” Science, 279, p.208, (1998).

[6]. X. Duan and C. M. Lieber, Adv. Mater. “General Synthesis of Compound Semiconductor Nanowires,” 12, p.298, 2000.

[7]. S.Q. Feng, D.P. Yu, H.Z. Zhang, Z.G. Bai, and Y. Ding, “Metal-induced Nickel Silicide Nanowire Growth Mechanism in the Solid State Reaction,” J. Cryst. Growth, 209, p.513, (2000).

[8]. F. G. Yost, “Ultimate strength and morphological structure of eutectic bonds,” J. Electr. Mdlt., vol. 3, pp. 353-369, May (1974).

[9]. H. Kato, “Effects of Silver Dissolved in Eutectic Gold-Silicon Solder on Silicon Devices,” J. Electrochem. Soc., 134, p.1750-1753, (1987).

[10]. A. Hiraki, “Low Temperature Reactions at Si-Metal Contacts - From SiO2 Growth due to Si–Au Reaction to the Mechanism of Silicide Formation,” Jap. J. Appl. Phys., 22, p.549, (1983).

[11]. A. Cros, P. Muret, “Properties of noble-metal/silicon junctions ,” Mater. Sci. Reports, 8, p.271, (1992).

[12]. Marcus Materials Co. catalog, 2006. Newark, Delaware, (2006).

[13]. Goodfellow catalog, Cambridge, England, (1996/1997).