Ion Beam Etching of Silicon: Implantation and Diffusion of Noble Gas Atoms, and Gettering of Copper
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ION BEAM ETCHING OF SILICON: IMPLANTATION AND DIFFUSION OF
NOBLE GAS ATOMS, AND GETTERING OF COPPER WILLIAM D, SAWYER ,OI SCJ-RdLIN" and JARG WEBER Max-Planck-Institut fOr Festk6rperforschung, Heisenbergstr. 1, 7000 Stuttgart 80, Federal Republic of Germany + Fakult~t fOr Physik, Albert Ludwigs Universit~t, Hermann Herder Str. 3, 7800 Freiburg, Federal Republic of Germany * Now at Mobil Solar Energy Corporation, 4 Suburban Park Drive, Billerica, Mass. 01821-3980
ABSTRACT Defects introduced into silicon by ion beam etching are investigated by low-temperature photoluminescence (PL) and Rutherford backscattering (RBS) measurements. The RBS results show that during the ion beam etch a highly damaged surface layer is formed which contains a large concentration of Ar atoms. The Ar atoms then diffuse out of the surface and into the crystalline bulk by some form of radiation enhanced diffusion. Annealing of the etched samples at 350*C results in the formation of noble gas defects known from previous PL studies of ion implanted silicon. When the samples are annealed at 650*C PL lines due to new defects are formed. Although little is known about their structure, we show that the new Ar defects getter small copper contaminations very effectively. INTRODUCTION Dry etching has become an important process necessary for the production of semiconductor devices. Ion beam etching (IBE) exposes the surface of the material being etched to bombardment by noble gas ions with energies from 100 eV
to
several
keV.
Significant
semiconductor
surface damage
is
reported
[i]. Previous research also gave some indications that the damage created during etching was penetrating the bulk of the material [2). In this paper we investigate the damage caused by IBE of silicon. Using RBS we show that a large concentration of noble gas atoms is implanted in the surface layer of the material etched. Low temperature PL measurements enable us to characterize the small concentration of defects in a thin region below the surface. EXPERIMENTAL Samples cut from polished silicon wafers were cleaned in acetone, propanol and deionized water, and the native oxide was stripped by a short HF dip. They were then immediately mounted in a Veeco 3" Microetch system with good thermal contact to a water cooled stage. The sample temperature was monitored during etching by a thermocouple. A maximum temperature of 40*C was not exceeded. The samples were etched for 15 min in a noble gas ion 2 beam with a constant current density of 0.2 to 0.4 mA/cm , depending on the gas used. The ion beam energy was 1 keV. The samples were annealed in 50'C steps for 30 min under nitrogen gas flow. Rutherford backscattering measurements were performed in the channeling mode with a 2 MeV He ion beam. Backscattered ions were detected with an Ortec Si detector. Photoluminescence spectra of the samples were recorded at 4.2 K using an unfocused Ar laser, a cooled Ge detector, and conventional lock-in techniques. The surface of some of the samples was anodically oxidized and removed in ord
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