Aluminum-Silicon and Gold-Silicon Eutectics: New Opportunities for MEMS Technologies
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Aluminum-Silicon and Gold-Silicon Eutectics: New Opportunities for MEMS Technologies Ciprian Iliescu, Daniel P. Poenar and Jianmin Miao Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 ABSTRACT Our practical experiments highlight that aluminum-silicon (Al-Si) and gold-silicon (Au– Si) eutectics are fairly inert to the attack of both anisotropic and isotropic wet etchants of Si (e.g. KOH or HF/HNO3 solutions). Therefore, these interfacial eutectics can be used as etch-stop layers in wet etching-based bulk micromachining. This paper presents how Al-Si and Au–Si eutectic layers may be employed for applications whose production involves such operations (e.g. high-pressure sensors) and will discuss three major directions of interest: eutectic formation, diaphragm generation and application of the eutectic layer in a pressure sensor. INTRODUCTION It is well known that aluminum and gold form eutectics with silicon. For Au-Si the eutectic point is at 363OC with the concentration of 81 % Au and 19 % Si, whereas the Al–Si eutectic point is at 577OC with 87 % Al and 13 % Si (atomic percent). Initially, the Au-Si eutectic was first used in IC packaging process for die bonding. However, in time, with further development in packaging processes and materials employed in it, this technology was replaced by epoxidic bonding. Another important application of the eutectic materials is the wafer-to-wafer bonding for MEMS devices. Its main advantage lies in the low temperature required for the bonding: between 600 and 650OC for Al and from 385 to 450OC for Au eutectics, respectively, in contrast with the much higher range of 800-1100OC necessary for typical silicon fusion bonding. Furthermore, because the bonding mechanism relies on the formation of Al–Si or Au–Si eutectics at the interface of the wafers, other attractive advantages offered by the eutectic-based bonding are their very good hermeticity and reliability. EXPERIMENTS REGARDING Au-Si AND Al-Si EUTECTIC Experimental details Our experiments were carried out using 4-inch p type silicon wafers with a resistivity of 1-10 Ωcm. The deposition of the initial thin Au layer of gold and the influence of the subsequent anneal (temperature and time) are crucial in determining the quality and properties of the resultant wafer-to-wafer eutectic bonding. That is why, in the case of Au-Si eutectics, a Cr film around 50 nm thick was first sputtered unto the wafer in order to ensure a good adhesion of the subsequent Au layer, which was deposited to a thickness of 400 nm. The presence of the initial Cr layer does not prevent the eutectic formation, as it is well known that in the absence of a Ni layer (usually employed as a “barrier layer”), Au can easily diffuse into Si through the Cr layer. B5.5.1
Subsequently, annealing was performed at either 400OC or 450OC for periods of 30, 60, 90 and 120 minutes, respectively. The wafers were then cut after the annealing and the crosssection was inspected with a Hitachi SEM in order to determine the final thickness of the resulting
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