Chip scale packaging continues to draw attention for applications that require high performance or small form factor solutions. The term chip scale package (CSP) has become synonymous with "fine pitch BGA" as the distinction between a ball grid array (BGA) and some chip scale packages becomes nearly indistinguishable. The cost of chip scale packages also continues to draw attention as one of the barriers to wide scale industry adoption. Sometimes lost in the chip scale debate is the discussion about wafer level chip scale packages, which offer the fastest path to small form factor, high performance and cost effective solutions. In this paper, we describe an approach to wafer level chip scale packaging that is an extension of integrated passive device processing, which results in low cost.Index Terms-Chip-scale packaging, integrated passive devices, wafer-level processing.
Interfacial strength of Cr/polyimide thin film structures and its durability to thermal exposure as a function of process variables was determined by 90° peel testing. Results were correlated with surface and interfacial characterization of thin film structures and peel failure surfaces using Auger, x-ray photoelectron spectroscopy, and secondary ion mass spectroscopy techniques. Initial adhesion in as-deposited Cr/polyimide structures exhibits a mild dependence on radio-frequency Ar plasma surface modifications and deposition process conditions. However, the durability of these structures under thermal process cycles depends strongly on these process parameters, greater doses of energetic ion bombardment were seen to yield greater durability. Characterization of peel failure surfaces revealed that fracture in the as-deposited and peeled samples occurs cohesively in the near surface region within the polyimide. Cohesive fracture was also seen in some samples after thermal process exposures, with peel strengths that were acceptable but somewhat less than the corresponding unannealed samples. Failure at an oxidized Cr to polyimide interface was observed in other thermally annealed samples whose peel strengths were very low. It is proposed that both types of degradation are caused by the water absorbed in the polyimide. The slight adhesion degradation appears to be due to water reaction and hydrothermal weakening of the modified near surface polymer region during thermal processing. The more catastrophic degradation is due to the oxidation of the Cr by the absorbed water during the anneal cycles. Results of critical experiments that verify these hypotheses are presented.
Adhesion durability of dc magnetron sputtered tantalum to in situ radio frequency (rf) plasma treated biphenyl tetracarboxylic acid dianhydride-para phenylene diamine (BPDA-PDA) polyimide films was determined as a function of repeated 400°C thermal exposure cycles and 85 °C/80% temperature!humidity (T/H) exposures. rf plasma treatments in Ar, oxygen and a sequential combination of oxygen followed by Ar were evaluated. Using 90° peel testing, it was found that all the plasma treatments resulted in high peel strengths (greater than 60 g/mm) initially and after high temperature thermal exposures, but structures fabricated with rf oxygen plasma treatment rapidly degraded in peel strength during T / H exposure. It is proposed that this hydrothermal degradation is a result of the instability of the nonstoichiometric tantalum oxide present at the metal/polyimide interface which transforms to stoichiometric Ta205 during the T / H exposure. Supporting evidence for this model is presented based on failure analysis of peeled strips by Auger electron spectroscopy as well as in situ x-ray photoelectron spectroscopy observations of the Ta bonding chemistry to the rf plasma treated polyimide.
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