Corrosion Behaviour of Aluminum Metallization During Aqueous Exposure
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bayerite (AI(OH) 3) crystals [2] were observed on the aluminum metallization of Al bond pads that exhibited poor wire bondability. The objective of the study was to investigate the aluminum metallization surface chemistry as a function of aqueous exposures used to clean the circuit board after the direct chip attach (DCA) stage in the second level packaging process. It is known that aluminum readily forms aluminum hydroxide in water of moderate temperature (temperature greater than 50'C) [2,3]. The hydrogen concentration in the hydroxide film can be as high as 3.7 - 4.1 x 1020 atoms/cm 3 [4]. The surface layer usually consists of a hydrogen-enriched deposit, AI(OH) 3 [4]. The growth mechanism has been described as:
+ 3H20 + 3H20 A120 3 + 5H20 A120 3 A1203
-
2AI(OH)+ 2 + 40H2AI(OH) 2+ + 20H2AI(OH) 4- + 2H+
(1) (2) (3)
With an increase in temperature and pH, the most likely surface hydrolysis reaction shifts from reaction 1 to 2 to 3. The kinetics of aluminum hydroxide growth study was done by Alwitt et al. 12 - 4]. Figure 1 shows hydroxide film growth on an aluminum substrate in DI water in the temperature range of 50'C to 100°C. It was found that induction periods of 123 Mat. Res. Soc. Symp. Proc. Vol. 390 © 1995 Materials Research Society
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Figure 1 - Film growth on aluminum in distilled water, a logarithmic plot of weight gain versus reaction time following the induction period 12]. hydroxide formation are temperature dependent; for example, at 40'C, the induction period for hydroxide formation is 4 hours, whereas the induction time at 70'C is reduced to 2 min. It is believed that an induction period is required for the initial hydrolysis reaction to take place at surface sites with a critical size of nucleus to sustain the reaction [4]. The reaction rate showed two distinct regions after the induction period: (a) an initial period of high growth rate, followed by (b) a much slower rate of growth after a certain thickness of the oxide film was established. Figure 2 shows the results obtained by Alwitt [21 of total weight gain of the aluminum specimen at 40'C DI water exposure. It is believed that the initial formation of hydroxide is in the amorphous form. The subsequent transformation by the action of water at neutral or mildly alkaline pH can be described as follows: Amorphous hydroxide -- pseudoboehmite -- trihydroxide (bayerite, gibbsite, or nordstandite) 121 The final reaction leads to crystalline phases (trihydroxide), which are less soluble in water. Based on the data of Alwitt [2,3], the rate limiting step for pseudoboehemite produced between 50'C and 100°C is diffusion of water into the oxide surface. The pseudoboehmite is formed by a condensation-polymerization mechanism. The condensation involves the boehmitic group formation and ligand displacement to form an hydroxide link. Formation of pseudoboehmite is accelerated by mildly elevated pH. However, at 40'C, a thick porous hydrous oxide layer only showed limited g
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