Gj. Norga et al., DETECTION OF METALLIC CONTAMINANTS ON SILICON BY SURFACE SENSITIVE MINORITY-CARRIER LIFETIME MEASUREMENTS, Journal of the Electrochemical Society, 145(7), 1998, pp. 2602-2607
Citations number
26
Categorie Soggetti
Electrochemistry,"Materials Science, Coatings & Films
We present the use of in situ carrier lifetime measurement by radio-fr
equency photoconductance decay (RFPCD) to detect metallic contaminatio
n during aqueous HF cleaning of silicon wafers. The effect of metal io
n deposition from dilute HF solutions on carrier recombination at the
Si/HF junction is examined. Ions of the more electropositive metals, s
uch as Fe2+ and V2+, do not undergo spontaneous reduction from dilute
HF. When present in HF, they do not measurably increase surface recomb
ination and hence cannot be detected by RFPCD. Reduction of Cu2+ ions
at the Si/HF interface occurs by a slow (conduction band) process whic
h involves nucleation of nanometer sized precipitates, whose areal den
sity can be measured by RFPCD. Using RFPCD in conjunction with atomic
force microscopy, we have found that the presence of 2 ppm of chloride
ions in dilute HF increases considerably both nucleation and growth o
f the-copper clusters. Air exposure prior to RFPCD measurement in HF i
ncreases the recombination activity of deposited copper on the surface
drastically. The reason for this effect remains poorly understood at
present. In contrast with copper, gold ions deposit from HF on Si by a
much faster valence band process. The initial deposition rate of gold
ions is diffusion limited, as evidenced by the t(-1/2) dependence of
the surface minority carrier lifetime, tau(surf) (t is immersion time
in gold-dosed solution).