Acceleration of Organic Contaminant Adsorption Onto Silicon Surfaces in the Presence of Residual Fluorine
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ABSTRACT The influence of fluorine atoms remaining after HF treatment on the adsorption of organic contaminants onto the surface of silicon wafers was investigated by analyzing the organic contaminants with gas chromatography-mass spectrometry following thermodesorption (TDGC/MS), and the surface composition with X-ray photoelectron spectroscopy (XPS). It has been found that residual fluorine on silicon surfaces after cleaning of the silicon wafers with either aqueous HF or anhydrous HF accelerates the adsorption of organic contamination onto the silicon surfaces. This would be due to the electrostatic force of attraction between the polar groups of organic compounds and the residual fluorine on the silicon surface.
INTRODUCTION As semiconductor devices continue to be highly integrated and their geometries also continue to shrink, not only particulate and metallic contaminants[l] but also trace organic contaminants adsorbed on the surface of silicon wafers[2-4]have been found to have an increasingly detrimental impact on the performance and yield of semiconductor products. When silicon wafers are exposed to the atmosphere in a conventional cleanroom, gaseous organic molecules in the air easily adsorb onto the surfaces of the silicon wafers in a short time[5]. In fact, polymeric materials are found almost everywhere in cleanrooms[4]. In order to avoid the adsorption of airborne organics on silicon surfaces, wafers should be placed in wafer storage boxes. However, it was recently shown that while wafers are stored in the boxes to protect them from airborne contaminants, organic additives, such as antioxidant, plasticizer, and crosslinking agent, can vaporize from plastic materials, such as polypropylene and polycarbonate of which the boxes are made and adsorb onto the wafer surfaces[6]. These organic contaminants can be easily removed by lifting off the native oxides using HF cleaning[6]. It is well known that aqueous HF treatment without subsequent DI water rinsing leaves fluorine atoms on a silicon surface terminated by atomic hydrogen[7]. It has been shown that for chemically reactive sites, such as the atomic steps of a silicon surface fluorinated after HF treatment, the fluorine coverage on a silicon surface is dependent upon the water rinse time[8], and that anhydrous HF treatment without subsequent water rinse results in much higher fluorine concentration than aqueous HF without subsequent water rinse[9]. It has also been shown that the existence of a critical amount of Si-F bonds (-- 12%) on the silicon surfaces can suppress the oxidation of silicon surfaces[8]. In this paper, the influence of surface fluorine on organic adsorption onto the silicon surface has been investigated by analyzing the organic contaminants and surface composition. Possible mechanisms for the organic adsorption on silicon surfaces with fluorine atoms are discussed.
387 Mat. Res. Soc. Symp. Proc. Vol. 477 @1997 Materials Research Society
EXPERIMENTAL Silicon wafers used for the present study were prepared from n-type Czochralski cr
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