Untersuchungen der Kontakteigenschaft beim Testen von integrierten Schaltungen (English shop)

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This work deals with the contact properties in the testing of integrated circuits. The focus lies on gaining a better understanding of the mechanisms and physical properties of the electrical and mechanical contact between the probing needles and the pads. Decisive here is the relationship between electrical contact resistance and the mechanical forces occurring. In order to measure the acting forces, in normal and lateral direction, and the electrical resistance in situ and under real-time conditions, a special measurement setup has been developed and constructed.

Due to the continuous increase in integration density, in current product technologies the probing pads are placed over active circuit areas in order to save further chip area. As a result, failures of the tested circuits occur more frequently, since the insulating layers between the metallization levels are mechanically destroyed by the force input from the probing needles. A reduction of the acting forces generally entails a deterioration of the electrical contact resistance. With the measurement setup developed in this work, the relationship between mechanical and electrical contact properties is investigated. It turned out that the native aluminium oxide on the probing pads is not the sole reason for a deterioration of the contact resistance, as frequently stated in publications. The measurements have shown that on aluminium an electrical contact resistance comparable to that on gold can be achieved. However, the forces occurring under identical conditions are higher on aluminium. With the aim of reducing the force input while keeping the resistance constant, various cleaning procedures for removing the aluminium oxide layer have been evaluated. It emerged that removal of the oxide is only barely possible with the methods investigated. Furthermore, measurements have shown that removal of the oxide and subsequent “clean” oxidation does not achieve any improvement of the contact properties.

Investigations of wafers from industrial IC production have made clear that, owing to the manufacturing process, additional contaminations on the pad surface are to be expected. Corresponding XPS analyses illustrate that a considerable proportion of fluorine contamination is to be found on the surface. In conjunction with the probing experiments carried out to determine the parameters contact resistance and force, a direct relationship between fluorine contamination and electrical contact resistance can be established. The contaminations form an insulating layer between probing needle and pad surface, which leads to a severe increase in resistance. Consideration of the theoretical description of the electrical contact resistance confirms the experimentally obtained results. The resistance value is mainly influenced by the presence of impurities on the surface. For this reason, the work goes on to address possible methods for removing the contaminations.

Based on the investigations carried out, both dry-chemical plasma processes and wet-chemical processes are examined more closely. One possibility is the addition of hydrogen directly to the fluorine plasma by which the impurities are caused. The wafers are effectively subjected to an in situ HF dip during processing, whereby the contaminations are reduced and thus an improvement of the contact resistance is achieved. The subsequent treatment of contaminated samples by an oxygen plasma leads to an improvement of the contact properties, since the fluorine contaminations are reduced. Additional experiments with a pure hydrogen plasma likewise show good results with regard to an improvement of the probing properties. Wet-chemical cleaning with tetrachloroethylene can likewise achieve an improvement of the contact resistance, though with increasing forces.

The results summarized in this work clearly show that a direct relationship exists between deterioration of the contact properties and fluorine contamination. With the cleaning procedures investigated, these can be removed and an improvement of the performance in probing can be achieved.