Phase transformations in metallization systems under conditions of nonstationary thermal action

Abstract

In assessing the reliability of metallization systems, most researchers focus their attention on long term experiments under conditions of subcritical current densities and study degradation processes that are not related to the formation of fused zones [1,2]. How ever, increased electric powers and other severe work ing conditions (in particular, in high current electron ics) favor the degradation of metallization layers up to their fusion. Special investigations have been devoted to the development of methods for diagnostics of met allization systems [3][4][5], including their operation under high thermal loads leading to local phase transi tions [6,7].The present work was aimed at studying phase tran sitions in metallization systems under conditions of thermal impacts caused by rectangular current pulses with amplitude up to j = 8 × 10 10 A/m 2 and durations within τ i = 100-1000 μs.The experimental setup was analogous to that used in [8] and included a source of rectangular current pulses with amplitude of up to 60 A and durations up to 1 ms, a master oscillator, a digital storage oscillo scope, and a microscope (Metam R1) with "numeri cal" eye glass for determining the length of a fused zone. The measurements were performed on test sam ples with a metallization stripe (length, 3 mm; width, 75 μm; thickness, up to 5 μm) on a semiconductor substrate (Fig. 1, inset A).The main current carrying layer in test structures was made of aluminum, which is the main metal used for the metallization of semiconductors. The substrate was made of phosphorus doped (111) oriented silicon plate with a resistivity of 0.01 Ω cm covered with a 60 μm thick n type epilayer (15 Ω cm), which pre vents shunting of the metallization. Some substrates were covered by additional dielectric layers of silicon oxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) (Fig. 1, inset B). The nitride layer was deposited via reaction of dichlo rosilane with ammonia at reduced pressure (~50 Pa) in a temperature interval of 700-900°C. The thermal oxide layer was grown in a temperature interval of 1150-1250°C by standard technology in a diffusion furnace filled with dry oxygen. Some samples were Abstract-First order phase transitions induced in aluminum metallization layers by the passage of single rectangular current pulses with amplitude up to 8 × 10 10 A/m 2 and durations within 100-1000 μs are consid ered. The formation of local fused zones and their subsequent migration during current passage have been experimentally studied. The main mechanisms of interphase boundary propagation due to heat evolution at the solid/liquid interface under conditions of nonstationary heating of the metal film are established. The velocity of liquid phase propagation (~25 m/s) along the metallization stripe has been determined in exper iment and a method of calculating the length of a fused zone upon the current pulse passage is proposed.

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