Surface Oxide Evolution on Al-Si Bond Wires
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B7.11.1
Surface Oxide Evolution on Al-Si Bond Wires 1
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Wentao Qin, Ray Doyle, Tom Scharr , Mahesh Shah , Mike Kottke, Dennis Werho, N. David Theodore DigitalDNATM Labs, Motorola Inc., Tempe, AZ 85284, USA. 1 Radio Frequency & Digital Signal Processing, Motorola Inc., Tempe, AZ 85284, USA. Contact Author: [email protected] ABSTRACT Al-Si wires are often used to make contact to bond-pads of semiconductor chips and devices. During operation in certain types of devices the wires typically reach relatively high temperatures. Under such circumstances, the oxidation could lead to a decrease of wire conductivity. It is of interest therefore to understand the oxidation behavior of the bond wires, particularly at elevated temperatures. In the current study, Al-Si wires were characterized as received from the supplier, and after thermal annealing at 240°C and 300°C. The surface oxides were found to evolve from a single-layer oxide to a double-layer oxide with varying chemistry. Oxide thicknesses were substantially lower than the wire diameter, even after 3000 hours of annealing. Therefore oxidation of the Al-Si wires has an insignificant impact on their electrical conductivity. INTRODUCTION Wire-bonding has been used for many years to make contact between semiconductor chips and the outside world. The Al-Si wire is a candidate material for such applications, such as power transistors for cellular base stations. Approximately 1 at. % Si is added to the Al for the purpose of dispersion strengthening of the wires. An optical micrograph showing such bond wires is shown in Fig. 1. The wire diameter is about 40 microns. During operation of power transistors, the device temperatures can reach up to 200 °C. Since the bond-wires are exposed to the air, it is necessary to understand the extent to which the wires are likely to be oxidized. The nature of the resulting oxide is also of interest. If the oxides grow significantly thick and in turn reduce the conducting cross-section, the electrical resistance of the wire will increase. This will lead to a degradation of the device performance.
Fig. 1 Optical image showing Al-Si wires bonded to semiconductor chip.
B7.11.2
Upon exposure to atmospheric oxygen, Al develops an oxide layer which protects the metal from further weathering or oxidation. The surface oxide has been reported to grow to a thickness of 4.5 nm after exposure to air for a month [1]. In the presence of 100% humidity, oxide growth was slower than in dry air, but the final oxide thickness was reported to be about twice the value that resulted in dry air [2]. At elevated temperatures of 350 °C and 520 °C, surface oxides developing on Al-0.9%Si sheets were estimated to be 20-100 nm thick [3]. For temperatures up to 300 °C, the surface oxide was found to be uniform and to remain below 20 nm in thickness [4]. Oxide growth follows an inverse logarithm law, for temperatures up to 300-350 °C [4]. At these temperatures, oxide film growth appears to be controlled by migration of Al ions through the growing oxide film [5]
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