Observation of anomalous high resistance to implanted areas caused by reactive ion etching (RIE)

Abstract

Oxide etching for contact hole definition and sidewall spacer formation is normally performed using fluorocarbon gas mixtures in a plasma reactor. A significant amount of over-etching must be allowed to compensate for uneven oxide thickness and poor process uniformity. During this time underlying silicon material is subjected to energetic particle bombardment which under severe circumstances can induce polymer deposition and radiation damage. Recent results indicate that contacts made to n + and/or p + areas, defined by reactive ion etching, suffer from high resistance which can lower circuit yield. In view of the foregoing, experiments were performed to investigate this phenomena as well as to find a solution whereby the problem of high contact resistance could be circumvented. Reactive ion etching (RIE) was performed in an hexode etcher using CHF 3, O 2 gas mixture at 50 mtorr. Plasma etching was performed in a single wafer etcher using a gas mixture of CHF 3, C 2F 6 and He at 1.5 torr. The as-etched samples were analysed using spreading resistance profiling (SRP), RBS-channeling measurements, ESCA and 4-point probe techniques. These measurements were used to investigate both the presence and the electrical activity of the implanted dopants. The nuclear reaction 1H( 15N, αγ) 12C at 6.38 MeV was employed to profile the hydrogen depth distribution. Cross sectional TEM was performed in order to characterize the extent of the damage. Contact chains defined by dry processing were used for electrical evaluation. It is shown that the silicon surface can be restored after RIE via an in situ low power etch with NF 3. The effectiveness of such a procedure was verified by contact resistance and thermal wave measurements.