Journal of the Korean Physical Society, Vol. 33, No. , November 1998, pp. S108∼S111

Dry Cleaning of Fluorocarbon Residues by Low-PowerElectron Cyclotron Resonance Hydrogen Plasma

Seung-Hyun Lim, Jin-Won Park, Hwan-Kuk Yuh and Euijoon Yoon

School of Materials Science & Engineering and Inter-university Semiconductor Research Center (ISRC),Seoul National University, Seoul 151-742

Sang In Lee

Semiconductor R&D Center, Samsung Electronics Co. Ltd., Yongin 449-900

A low-power (50 W) electron cyclotron resonance hydrogen plasma cleaning process was demon-strated for the removal of fluorocarbon residue layers formed by reactive ion etching of silicon diox-ide. The absence of residue layers was confirmed by in-situ reflection high energy electron diffractionand cross-sectional high resolution transmission electron microscopy. The ECR hydrogen plasmacleaning was applied to contact cleaning of a contact string structure, resulting in comparable con-tact resistance arising during by a conventional contact cleaning procedure. Ion-assisted chemicalreaction involving reactive atomic hydrogen species generated in the plasma is attributed for theremoval of fluorocarbon residue layers.

I. INTRODUCTION

Reactive ion etching (RIE) using fluorocarbon chem-istry is widely used to open contact holes in dielectriclayers for very large scale integration (VLSI) since it pro-vides high anisotropy and selectivity over Si. However,it is known that the RIE leaves fluorocarbon residueson the exposed surface after etching of silicon dioxide[1,2]. These residue layers were reported to be non-volatile, chemically and thermally stable, and that theyresulted in high contact resistance and degradation ofmetal-silicon interfacial quality [3]. Therefore, fluorocar-bon residues need be removed prior to the metal deposi-tion processes by appropriate means.

Conventionally, fluorocarbon residues can be removedby a two step cleaning procedure. First, wafers are ex-posed to an oxygen plasma. During this process theresidue layers are oxidized, and at the same time thesilicon dioxide film is formed. Second, the oxide layer issubsequently removed by an HF solution. Although theoxidizing process is one of the most effective methodsto remove the fluorocarbon residues, silicon consump-tion leads to changes in critical dimensions of the devicestructure [4]. Furthermore, wet cleaning processes suchas HF dip may have limitations in cleaning high aspectratio contact holes [5]. It is also difficult to integratesuch wet processes with other dry processes in a clustertool environment for the fabrication of VLSI [6]. Sputtercleaning techniques based on high density argon plasmaswere reported for the removal of RIE residues [7,8], how-ever, it is likely that the high power levels may deterio-

rate the electrical properties of the contacts by energeticion bom bardment. Thus, an alternative plasma cleaningmethod at a much reduced power level is advantageousfor this purpose.

In this paper, we report the successful removal of fluo-rocarbon residues on silicon substrate using low power(50 W) electron cyclotron resonance (ECR) hydrogenplasma. The ECR plasma is advantageous over radiofrequency plasma since it provides higher plasma den-sity with lower incident ion energy.

II. EXPERIMENT

Silicon dioxide films (7000 A) were deposited by chem-ical vapor deposition on a chemically clean boron-doped(100) silicon wafers (resistivity, 10∼20 Ωcm). Then, thefilms were removed by RIE using a CHF3/CF4/Ar gasmixture. They were overetched for 110 sec after reachingthe SiO2/Si interface. These RIE-contaminated waferswere cleaned in an ultrahigh vacuum electron cyclotronresonance (UHV-ECR) hydrogen plasma system at vari-ous conditions. The schematic diagram of the UHV-ECRplasma system used in this study is shown in Fig. 1. Adetailed description of the system is reported elsewhere[9]. To briefly summarize, the base pressure of the sys-tem was around 1×10−9 Torr. Pd-purified H2 was intro-duced in the main chamber through an ECR cavity. H2

flow rate, total pressure and magnet current were fixedat 74 sccm, 2 mTorr, and 50 A, respectively [10]. Sub-strate DC bias was held at floating condition throughout

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Dry Cleaning of Fluorocarbon Residues by Low-Power Electron Cyclotron ... - Seung-Hyun Lim et al. -S109-

Fig. 1. A schematic diagram of the UHV-ECR plasmasystem employed in this study.

the experiment. Substrate temperature was increased toabout 100 C during hydrogen plasma cleaning due to ionbombardment though the substrate heater was turnedoff. Microwave power and cleaning time varied.

The wafer surface was monitored by in-situ reflectionhigh energy electron diffraction (RHEED) installed inthe main chamber. Transmission electron microscopy(TEM) and high resolution TEM (HRTEM) were usedto evaluate either the presence of fluorocarbon residuesor defects induced by ECR hydrogen plasma cleaningof RIE-contaminated silicon surface. To evaluate theelectrical properties of the contact prepared by hydro-gen plasma cleaning, contact strings (1064 contacts) werefabricated by a standard 1.5 µm CMOS silicon integrated

Fig. 2. (a) RHEED pattern and (b) cross-sectionalHRTEM image of a reactive ion etched silicon surface.

Fig. 3. RHEED patterns of silicon surface after ECR hy-drogen plasma cleaning (a) 50 W, 3 min, (b) 50 W, 5 min,and (c) 100 W, 5 min.

circuit process. N+ and P+ contact resistances were mea-sured using a HP-4155A semiconductor analyzer.

III. RESULT AND DISCUSSION

The surface after RIE was examined by in situRHEED. As shown in Fig. 2(a), the hallo and haze pat-tern was observed, indicating that an amorphous phasewas present on the surface. To investigate the amorphouslayer in detail by cross sectional HRTEM, the samplewas in situ capped with amorphous silicon in the cham-ber. An amorphous residue layer, 70 A in thickness, wasclearly observed between the silicon substrate and theamorphous silicon capping layer. The residue layer wasuniform in thickness. It was also observed that defectiveregions were formed by RIE to the depth of as much as500 A from the silicon surface but crystallinity of siliconsubstrate was maintained.

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Fig. 4. Cross-sectional TEM images of silicon surface after(a) hydrogen plasma cleaning: 50 W, 5 min and (b) argonplasma cleaning: 50 W, 20 min

The RIE-contaminated silicon wafers were cleaned byECR hydrogen plasma and the cleaned surface was ex-amined by RHEED. As shown in Fig. 3(a), the RHEEDpattern was still hazy, but smeared spots appeared after3 mins of hydrogen plasma exposure at microwave power50 W. When the cleaning time was increased from 3 minto 5 min under the same condition, the spots becamebrighter. When the microwave power was increased to100 W, a similar RHEED pattern was obtained after 5mins of hydrogen plasma exposure, as shown in Fig. 3(c).

The result of Figs. 3(b) and (c) implies that theamorphous residue layers were almost completely re-moved from the silicon surface after 5 mins of hydrogenplasma exposure. The fact that the RHEED patternswere spotty implies that the surfaces were rough. It islikely that the fluorocarbon residues were not uniformlyremoved and some part of the silicon surface was exposedearlier than others. Then, the exposed silicon surface wasetched preferentially by hydrogen plasma [11], renderingsilicon surface microscopically rough after the completeremoval of fluorocarbon residues. Since the RHEED pat-terns of Fig. 3(b) and (c) were similar and, furthermore,excessive hydrogen plasma exposure may have resultedin silicon loss and additional defect formation, a cleaningcondition of microwave power of 50 W and cleaning timeof 5 mins was chosen for further studies.

The cross-sectional TEM image of the sample in Fig.3(b) clearly shows that residue layer was removed, How-ever, silicon surface was microscopically made to becomemore rough after 5 min ECR hydrogen plasma cleaning,

Fig. 5. Comparison of P+ and N+ contact resistances aftervarious cleaning methods (H: ECR hydrogen cleaning, O: O2

ashing+conventional wet cleaning).

as shown in Fig. 4(a). It was also observed that thetop surface was damaged. However, when the hydrogengas was substituted by argon gas at the same microwavepower (50 W) and when exposed four times longer (20min), a continuous residue layer still remained, as shownin Fig. 4(b). It was reported by other researchers thatfluorocarbon residues could be removed by ECR argonplasmas [7,8]. However, their cleaning conditions weremore severe than ours. The microwave power rangedfrom 1.1 to 1.5 kW, and the substrates were biased at−100 V. From these results, it is confirmed that thedominant cleaning mechanism in our study is not thesputtering due to physical ion bombardment, but ion-assisted chemical reaction involving reactive atomic hy-drogen speices generated in the ECR hydrogen plasma.

The hydrogen plasma cleaning at this condition wasapplied for the removal of fluorocarbon residues formedat the contact regions, and contact resistances were com-pared with the ones prepared by combining conventionaloxygen plasma ashing with wet etching. Contact sizeswere 2×2, 2.5×2.5, and 3×3 µm2. Fig. 5 shows thatlow-power hydrogen plasma contact cleaning gave com-parable contact resistances for both N+ and P+ contacts.

IV. CONCLUSIONS

Low-power ECR hydrogen plasma cleaning of fluo-rocarbon residues formed by oxide RIE were success-fully demonstrated. RHEED and HRTEM results con-firmed that the residues were completely removed. Ion-assisted chemical reaction involving reactive atomic hy-drogen species generated in the plasma are attributed forthe removal of fluorocarbon residues. The ECR hydro-gen plasma cleaning was applied for the contact cleaning,resulting in comparable contact resistance arising duringconventional contact cleaning procedure.

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Dry Cleaning of Fluorocarbon Residues by Low-Power Electron Cyclotron ... - Seung-Hyun Lim et al. -S111-

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