Damage to shallow n+/p and p+/n junctions by CHF3+CO2 reactive ion etching

Abstract

The damage incurred during contact etch is studied, with emphasis on determining those defects responsible for leakage current of shallow junctions. Shallow p+/n and n+/p junctions have been prepared with depths of 160–180 nm. Junction leakage and contact resistance measurements have been made for various amounts of silicon loss up to within 20 nm of the junctions during contact etch through a 300 nm of SiO2 film by using a CHF3+CO2 plasma. For p+/n junctions, the leakage current density was found to depend strongly on contact area and increase rapidly with etch depth after the etched surface has extended to within 80 nm of the junction boundary. On the other hand, the leakage current stays constant even when the etched surface approaches within 40 nm of the junction boundary for n+/p junctions. Further etching of the n+/p junction only induces punch-through. Contact resistance was found to increase with etch depth for the p+ junctions after 50 nm of silicon was removed from the surface, and stays constant for all etched n+ junctions. Etching induces a prominent feature at 970–975 meV in photoluminescence (PL) spectra. The occurrence and peak height of this luminescence, which is associated with H and independent of C concentrations, are found to correlate directly with leakage current of as-etched junctions. The annealing behavior of reactive ion etching (RIE) damage is somewhat more complicated, with a change in defect PL before there is a noticeable change in the leakage current. A transmission electron microscopic study showed no evidence of defects.