Surface Treatment with UV-Excited Radicals for Highly-Reliable Gate Dielectrics
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In this paper, we demonstrate some applications of UV-excited chlorine and oxygen radicals for new surface treatments of ULSI production. Chlorine radicals are useful to remove trace metals from bare silicon surfaces and poly-buffered LOCOS structure, and to identify breakdown spots on a gate dielectric film as well as nonuniformities of native oxides. Oxygen radicals are effective to densify structural uniformity of a native oxide and to improve reliability of a gate dielectric film. UV-EXCITED CHLORINE RADICAL TREATMENTS REMOVAL OF TRACE METAL CONTAMINANTS We have proposed a removal technique of trace metals on Si surfaces by using UVUV (A =200-300 nm) Synthesized quartz
-Si wafer Gas inlet
Exhaust IR lamps
Fig.1 Schematic equipment for surface treatments with UV-irradiation 71 Mat. Res. Soc. Symp. Proc. Vol. 477 0 1997 Materials Research Society
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Fig.2 Fe contaminant removal from Si
20
30
40
50
etched depth (nm) l,:i
Si0 2
surfaces using UV-excited Cl radicals
excited Cl radicals" 3 ' . Figure 1 shows a schematic diagram for the cleaning experiment where the cleaning proceeds through a native oxide of Si leaving some amount of Si etched. Controlling the native oxide is important to get a uniform silicon surface. We have speculated that relatively large amount of Si " in native oxide disturbed chlorine radicals from diffusing uniformly, resulting in a rough surface. We initially thought a direct reaction between metals and CI radicals which produced volatile metal chlorides "' . Fe contaminants on a Si surface can easily be removed as shown in Fig.2. The problem is that it did not work on a thick Si0 2 surface. We investigated why Cl radicals hardly removed Fe contaminants on a SiO 2 surface as shown in Fig.2. We found that Cl radicals did not directly react with Fe contaminants which were in the form of oxides or hydroxides. One solution is to use silicon chlorides(SiClx) because it is feasible for them to thermodynamically react with Fe
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500
550
Substrate temperature (°C) Fig.3 Fe contaminant removal from Si and SiO 2 surfaces using SiCI, gas under UV irradiation 72
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Temperature: 500°C Cleaning time: 30s
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C12 :SICI4 flow rate (ml/min) Fig.4 Fe contaminant removal from Si surfaces using Cl. and SiCl, gas mixture I . The reaction formulas and standard free energies of oxides and hydroxides formations at BOOK( AGf, 800K))are shown as followings: 3C12 + 2Fe - 2FeCI2 , 3SiCl,
AGf,B00K = -232kJ
AGf,BaGK
+ 4Fe -- 4FeCI3 + 3
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