有机污染物在晶圆表面的吸附行为

ABSTRACT

Silicon wafers can adsorb volatile organic compounds present in the air of the enclosed environmentsin the cleanrooms where they are manufactured following their reversible outgassing. Hence, organiccontamination even at ppt levels is recognized as being the vector facilitating unexpected detrimentalimpact on silicon wafers during the fabrication processes in the cleanroom environment.Therefore, in order to better characterize these processes, it is of paramount importance to study theadsorption phenomena of organic compounds on silicon wafers.The silicon wafer surface was previously characterized by determining the concentrations and bondingenvironments of the most abundant surface atoms: Si, O and C by the XPS technique.Two typical organic solvents largely used in semiconductor fabrication processes were selected, i.e.propylene glycol methyl ether acetate (PGMEA) and ethyl acetate (EA) in order to characterize theadsorption phenomena and evaluate kinetic parameters.The adsorption enthalpies were discussed in terms of physico-chemical properties of both moleculesunder study and to simulate more realistic conditions, the adsorption of a mixture of them was studied at different mixing ratios.The results outlined in this study give an improved understanding of the adsorption properties oftypical organic contaminants in clean rooms which in turn may help to optimize the performance of thesemiconductor devices.

1. Introduction

The fast development ofhigh performance devices in the micro-electronic industries requires a very good maintenance of a cleanenvironment. It has been shown that cleanrooms are indispensableto provide a suitable environment for processing semiconductordevices.However,even in such environments,the organic submonolayercontamination of wafer surfaces may occur.In cleanrooms of class10 and higher, the concentration oforganic compounds mayexceedby a few orders of magnitude the concentrations of particles (1].In general, the organic contamination emerges either fromthe process environment such as the cleanroom itself or from theplastic storage boxes used at different stages of the fabricationprocesses (2]. The adsorbed organic compounds on wafer surfaceshave been found to induce defects such as increased surface roughness, formation of haze, streaking, damage to epitaxialgrowth, and degradation of gate oxide integrity [3-6]. Kitajima andShiramizu (7] for instance, have reported that device character-istics may be damaged if the organic molecular density on a baresilicon surface exceeds approximately 1 x 1013 (Catoms cm-2).

The organiccontaminants are either physisorbed orchemisorbed depending on their physico-chemical properties(7]. In general, the adsorption properties of monolayer organiccompounds on solid surfaces are governed by their vapor pres-sures, dipole moments and molecular weights (8]. The controlof such contamination and especially the knowledge of organicdeposition kinetics on silicon wafers are of crucial interest for theindustry that deals with the fabrication of semiconductor devices.Although the adsorption kinetics of certain compounds such asphthalates have been reported to be of up to several days (1,9,10in actual semiconductor fabrication processes, the wafer exposuretime is usually limited to only a few hours (111. However, evena few hours of exposure can lead to contamination of the wafersurface with low molecular weight organic compounds. Suchcontamination, in turn may induce deterioration of the electricalcharacteristics ofthe semiconductors.

Thermodesorption-gas chromatography/mass spectrom-etry (TD-GC/MS) is used in most studies for both air and wafersurface analysis (1,11,12,16]. The major disadvantage of this typeof experimental technique is the setting of the heating tempera-ture because excessive heating can not only decompose the parentcontaminants but also induce unintended materialoutgassing fromchamber components that may interfere with those from the waferoutgassing[11].

Other wafer studies based on X-ray photoelectron spectroscopy(XPS) and time of flight secondary ion mass spectrometry (TOFSIMS) have been used for the characterization of the silicon wafers17,18]. Both methods require high vacuum conditions underwhich the loss of volatile components ofthe samples could becomesignificant. Furthermore, these methods cannot give informationabout the kinetic behavior of the contaminants.In this study, XPS technique was exclusively used for the char-acterization of the wafer surface. The adsorption of the organicmolecules was monitored using a flow tube reactor coupled tothe state-of-the-art proton transfer reaction mass spectrometry(PTR-MS). We studied the adsorption properties of two organiccompounds: propylene glycol methyl ether acetate (PGMEA) andethyl acetate (EA) that are widely used as solvents in cleanroomsettings [19].

To study the impact ofsurface concentration oforganic contaminants on silicon wafers, first, the silicon wafer surface was characterized by XPS technique, then, the adsorption behaviors ofPGMEAand EA at different gas phase concentrations were determined. Theeffect oftemperature on the adsorption kinetics ofboth compoundswas evaluated.In order to simulate more realistic conditions exist-ing in the cleanrooms, we investigated the effect of the mixtureon the adsorption kinetics. Finally, based on the restrictions suggested by the International Technology Roadmap for Semiconductors(20] for the maximum allowed organic contaminationlevels, an estimation of the maximum exposure time for differentgas phase concentrations was established for both compounds.