Cl 2 Plasma Etching of Si(100): Surface Chemistry and Damage
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ABSTRACT The interaction of a C12 plasma with a Si(100) surface has been investigated by angle resolved x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. From XPS, it was found that the amount of chlorine incorporated at the Si surface increases with ion energy. Chlorine is present as SiClx (x = 1-3) with average relative coverages (integrated over depth) of [SiCl]:[SiC12]:[SiCI 3] 1:0.33:0.1. These relative coverages don't depend strongly on ion energy between 40 and 280 eV. Real-time spectroscopic ellipsometry measurements showed that the layer present during etching is stable when the plasma is extinguished and the gas pumped away. In addition, the equivalent thickness of damaged silicon and silicon-chloride within the surface layer increases with ion energy.
INTRODUCTION Low pressure, high density C12 plasmas are widely used to etch Si and other materials for Si integrated circuits [1]. The overall mechanism for Si etching in a C12 plasma is well established: C12 and Cl react with the Si surface to form a chlorinated layer that is removed by ion bombardment. The nature of that chlorinated surface layer (thickness, stoichiometry, and structure) are, however, largely unknown.
Both the chemical nature
(chlorine content, surface stoichiome try) and physical nature (depth and degree of damage) of the chlorinated silicon surface layer must be clarified to better understand the chemical and physical etching mechanisms in high density plasmas and suggest new directions for etching processes and plasma reactor designs. In this paper, angle-resolved XPS and spectroscopic ellipsometry are used to study respectively the surface chemistry and damage of the Si surface during and after C12 plasma etching over a range of ion energies and densities. More detailed accounts of these studies are published elsewhere [2,3].
EXPERIMENTAL PROCEDURE The chemical nature of the Si surface layer etched with C12 plasma was investigated in a helical resonator reactor coupled to an x-ray photoelectron spectrometer with a vacuum transfer chamber [2,4]. The physical nature was studied in situ by spectroscopic ellipsometry mounted on a helicon reactor [3]. Monocrystalline Si (100) wafers (p-type, 15-25 02-cm) were stripped of their native oxide before etching. The reactors were evacuated to a pressure of 10-6 Torr before the plasma was ignited. Two etching modes were evaluated: 1) reactive ion etching (RIE) with radiofrequency (rf) power applied only
39 Mat. Res. Soc. Symp. Proc. Vol. 448 0 1997 Materials Research Society
to the wafer stage and 2) a high density (HD) plasma with and without rf power on the stage.
RESULTS AND DISCUSSION After etching under steady state conditions, low resolution (1.3 eV) XPS spectra were recorded as a function of take-off angle 0 (with respect to the wafer plane), and the integrated intensities of the core-levels peaks were used to determine the total chlorine coverage (Fig. 1). The highest ion energy (RIE, -240 V DC bias) leads to the highest chlorine coverage (37% at 0 = 200). On t
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