A METAL IMPURITY EFFECTS ON DEVICES

It is well established that contamination of silicon wafers with transition metals can reduce device yield even if the concentration of metals is below 10 ¹² cm ^-3. When dissolved in silicon, transition metals can form deep levels [1] which increase p-n junction leakage currents by generation [2 4] of carriers in reverse biased depletion regions (or by recombination in small forward biases) [5 7]. Furthermore, metal impurities in p-n junctions can lower the reverse bias breakdown voltage [8]. Metal precipitates can also form bands of deep levels [9,10], which greatly increase leakage currents and can even directly short p-n junctions [11 13]. In bipolar junction transistors, dissolved metals generally increase the base currents, power consumption and heat production, and degrade the emitter efficiency and base transport factors [2]. MOS devices are sensitive to metal contamination [14] since many transition metals precipitate at the Si/SiO ₂ interface or become trapped in the oxide layer [15,16]. The resulting reduction of the dielectric strength of the oxide layer leads to oxide breakdown device failures [5,17 27] and/or an increase in generation rates [28], often referred to as a decrease in gate oxide integrity (GOI). Furthermore, transition metals have been linked to poor retention times (high refresh rates) in DRAMs [29] and pixel failures [3,4,18] in CCD devices. CCD devices can...