COMPARISON OF ADVANCED PLASMA SOURCES FOR ETCHING APPLICATIONS .4. PLASMA-INDUCED DAMAGE IN A HELICON AND A MULTIPOLE ELECTRON-CYCLOTRON-RESONANCE SOURCE

The effect of plasma induced damage on device yields becomes increasingly crucial as gate oxide thicknesses approach the 50-70 angstrom range for quarter micron design rules. To quantify plasma induced damage for new low pressure, high density plasma sources, inductively coupled carrier lifetime and photoluminescence are measured. Also, UV-visible spectroscopic ellipsometry is used to closely examine the wafer surface both during and after plasma exposure. This study is part of an ongoing research program that compares the etching performance of two commercially available, advanced plasma sources: a Lucas Labs helicon and a Wavemat multipole electron cyclotron resonance source. For n-type and p-type, nominally 100 mus lifetime wafers, plasma exposure in both sources under optimum polysilicon overetch conditions results in a degradation of carrier lifetimes of less than 5% for unprotected versus oxide protected crystalline silicon. For wafers with an initial carrier lifetime of 400 mus, exposure to plasmas in both sources leads to a decrease in carrier lifetimes to approximately 345 mus. The decrease in carrier lifetime, although measurable, is at least an order of magnitude smaller than measured in conventional reactive ion etching (RIE) processes. To simulate an RIE etching process, rf-bias power is applied without source power so that both sources behave like highly asymmetric, parallel plate RIE reactors with the wafer platen acting as a driven electrode and the source wall acting as a grounded electrode. Under these conditions carrier lifetimes below 45 mus were measured in both sources with wafers that had initial lifetimes of 400 mus. Photoluminescence measurements indicate incorporation of hydrogen into the etched material, but no formation of interstitial defects associated with plasma induced damage. For samples exposed to a plasma at 25 and 50 W rf-bias, hydrogen produced from HBr dissociation readily incorporates into the sample. Exposure to a plasma at 200 W applied rf-bias results in a decrease in carrier lifetime and an increase in hydrogen incorporation. Under high rf-bias conditions, hydrogen incorporation is probably aided by ion surface damage. Ultraviolet-visible ellipsometry does not detect any morphological damage induced in silicon wafers, but as with carrier lifetime and photoluminescence measurements, significant damage is detected after exposure to rf-bias power without applied source power. Ellipsometry also detects a difference between real-time and post process spectra after HBr and Cl2 plasma exposure corresponding to the desorption of a film approximately 12 angstrom thick. This is not observed after Ar plasma exposure. Instead, the roughened surface remains stable which shows that the differences observed during and after plasma exposure for HBr and Cl2 are indeed the result of a chemically reacting layer.