effect of rca cleaning on the surface chemistry of glass and polysilicon films as studied by...

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 270È277 (1998)

Effect of RCA Cleaning on the Surface Chemistryof Glass and Polysilicon Films as Studied by ToF-

andSIMS XPS

E. C. Onyiriuka,1* C. B. Moore,1 F. P. Fehlner,1¤ N. J. Binkowski,1 D. Salamida,2 T-J. King3 andJ. G. Couillard41 Corning Incorporated, Science and Technology Division, SP-FR-3, Corning, NY 14831, USA2 Xerox Corporation, Webster, NY 14580, USA3 University of California at Berkeley, Berkeley, CA 947204 Bard Hall, Cornell University, Ithaca, NY 14853, USA

The surface chemistry of an alkaline-earth boroaluminosilicate glass is changed by contact with chemical solutions.The present study shows that RCA cleaning creates a silica-rich surface on the glass. This altered surface can beremoved by hydroÑuoric acid etching. During the RCA cleaning process, glass components can be transferred to apolysilicon Ðlm placed in the same alkaline solution. The acidic solution in turn removes most of the contaminationfrom the polysilicon. 1998 John Wiley & Sons, Ltd.(

Surf. Interface Anal. 26, 270È277 (1998)

KEYWORDS: RCA clean ; glass ; surface chemistry ; polysilicon ; thin Ðlm transistor ; ToF-SIMS; XPS; x-ray photoelectronspectroscopy

INTRODUCTION

The development of liquid-crystal displays, especiallythe active matrix versionÈactive-matrix liquid-crystaldisplay (AMLCD)Èinitiated the era of large-area elec-tronics based on glass substrates. A representative sub-strate used for this purpose is Corning Code 1737glass.1 The technology follows the lead of silicon-integrated circuits, which demand strict attention tocleanliness during their fabrication. A hydrogenperoxide-based cleaning method called RCA cleaning,developed by RCA laboratories,2 has been adopted asthe standard for silicon semiconductor technology andcan also be used to clean glass substrates for AMLCDs.

This method, which evolved from the cleaning of elec-tron tube components, uses a sequence of alkaline andacidic peroxides to remove both organic and inorganiccontamination and/or residues from the silicon surfaces.A hydroÑuoric acid etch can also be included to removethe thin oxide layer formed during the oxidizing clean-ing of silicon.3 Variations of the method have beentried,4 but the original procedure continues to be usedwith some modiÐcation.5 The following reagents wererecommended in the paper authored by Kern and Puo-tinen.2 The Ðrst solution (SC-1) consists of a mixture of

in the proportions 1 :1 : 5 toNH4OH/H2O2/H2O1 : 2 : 7. This is designed to remove organic moleculesand complex some Group I and II metals. It is followedby a deionized water rinse. The second solution (SC-2)is composed of in the ratio 1 :1 : 6 toHCl/H2O2/H2O

* Correspondence to : E. C. Onyiriuka, Corning Incorporated,Science and Technology Division SP-FR-03, Corning, NY 14831,USA.

E-mail : onyiriukaec=coming.com

1 : 2 : 8. This solution mixture is designed to remove andcomplex heavy metal ions to prevent displacementreplating from solution. Formic acid can be substitutedfor the hydrochloric acid in the solution. The hydrogenperoxide should be used in a vented container to avoidgas pressure build-up. The other reagents are deionizedwater, 37% hydrochloric acid and 27% ammoniumhydroxide. All reagents should be electronic grade.Cleaning should be carried out at 75È85 ¡C for timesranging from 10 to 20 min. The substrate wafers mustbe kept wet at all times. The Ðnal steps in the processare a thorough rinse in deionized water, a spin dry andtransfer to a closed container Ðlled with inert gas. Kernand Puotinen2 also have shown that the process is ase†ective with fused silica as with silicon, and capable ofremoving more than 90% of the heavy metal ions fromthe surface of the substrate. Recently, Ojima et al.6developed a new megasonic-excited ozonized watercleaning method that excels in the removal of hydrocar-bon contamination from Si surfaces but is unprovenwith respect to inorganics.

RCA cleaning has been one of the choices adoptedfor glass cleaning in the display industry. However,there is a concern that the cleaning process may changethe chemistry of the glass surface. It is known that soda-lime glass surfaces do change when exposed to dampatmospheres7,8 and that a borosilicate glass surface canbe altered by chemical reagents.9 The work of Hair etal.10 shows that extensively leached borosilicate,aluminosilicate and soda-lime glass surfaces di†ergreatly in composition from the bulk glass. The popu-lation of Lewis acid sites is increased in many cases.Leaching a phase-separated glass can create increasedsurface area. Hydration of the glass surface also occurs.Some of the treated surfaces were found to be cata-lytically active or useful as supports for gasÈliquid chro-matography.11

CCC 0142È2421/98/040270È08 $17.50 Received 2 May 1997( 1998 John Wiley & Sons, Ltd. Accepted 1 December 1997

RCA CLEANING OF GLASS AND POLYSILICON FILMS 271

The present experiment was designed to evaluate twoaspects of RCA cleaning of glass. The Ðrst was to deter-mine whether a polysilicon (poly-Si) Ðlm deposited onone side of the glass would be contaminated with glasscomponents from the other side during RCA cleaning.The second was to measure changes in the surfacechemistry of Code 1737 glass caused by cleaning. Time-of-Ñight secondary ion mass spectrometry (ToF-SIMS),x-ray photoelectron spectroscopy (XPS) and atomicforce microscopy (AFM) were used to analyze the glasssurfaces.

EXPERIMENTAL

Materials

The samples examined in the present work were asfollows :(1) Corning Code 1737 glass, fusion drawn and protect-

ed with a polymer coating. The glass is an alkalineearth boroaluminosilicate.

(2) Code 1737 glass, cleaned and coated on both sideswith 120 or 500 nm of silica deposited by atmo-spheric pressure chemical vapor deposition(APCVD).

(3) Code 1737 glass, cleaned and coated on both sideswith 120 nm of poly-Si deposited by low-pressurechemical vapor deposition (LPCVD).

(4) Single-crystal silicon covered with 800 nm of ther-mally oxidized silicon.

(5) Single-crystal silicon coated with 120 nm of LPCVDpoly-Si.

The initial cleaning of the glass consisted of removal ofthe protective polymer coating, followed by oxygenplasma exposure for at least 5 min. A detergent washwas used to remove any residual inorganic dirt. TheÐnal deionized water rinse was followed by an acetoneand isopropyl alcohol rinse with a subsequent nitrogenblow dry. Both the “controlÏ and “testÏ glass sampleswere subjected to the same initial cleaning process. Thiswas done to eliminate any inÑuence by the oxygenplasma and detergent on the Ðnal surface composition.The “cleanÏ glass samples were then subjected to furtherRCA cleaning prior to the coating of the glass withpoly-Si. The base clean consisted of four parts of waterto one part of ammonia to one part of hydrogen peroxi-de. The acid clean comprised of 12 parts of water to 2.5parts of hydrochloric acid to 2.5 parts of hydrogen per-oxide. Both the base and acid cleaning protocols werefollowed by a deionized water rinse. To determine theinitial contamination of the RCA cleaning baths, apolysilicon-on-silicon control wafer was RCA cleanedfor 10 min in each bath, along with three thermally oxi-dized silicon wafers and three precleaned fused-silicawafers.

The e†ectiveness of the silica barrier layers to blockcross-contamination of poly-Si layers by glass com-ponents was tested in the following experiment. Each oftwo polysilicon-on-silicon wafers was sandwichedbetween two Code 1737 glass substrates coated withsilica. One polysilicon sample was pulled out after thebase clean and the other was run through both the baseand acid cleaning procedures. The base and acid clean-

ing times were 15 min each. The study of glass com-ponent transfer from bare Code 1737 glass ofpolysilicon-on-silicon wafers was carried out using twopoly-Si samples sandwiched between three pieces ofglass. The spacing was D2 mm. Again, one of the poly-silicon samples was pulled out after the base clean andthe other was run through the entire cleaning pro-cedure. Exposure times to the base or acid were 15 mineach. Immersing the polysilicon-on-silicon samples wasdone to check the purity level of the original base solu-tion : one sample was exposed to the base as initiallyformulated ; a second sample was exposed to the basesolution after 21 bare glass samples had been processedthrough the same solution. The possibility of glass com-ponent transfer implies that the surface of the glass ischanging in composition. Two checks were made to testthis hypothesis by preparing the following samples ofCode 1737 glass :(1) Rinsed in deionized water.(2) Exposed to the base portion of the RCA clean for 60

min.(3) Exposed to the acid portion of the RCA clean for 60

min.(4) RCA cleaned for the normal 10 min in base and 10

min in acid.(5) RCA cleaned and dipped for 3 s in 10% HF solu-

tion.The used solutions from the Ðrst three samples wereanalyzed using inductively coupled plasma (ICP) atomicemission spectroscopy. The glass surface chemistry wasdetermined using both ToF-SIMS and XPS. Surfacemorphology was evaluated with AFM.

Characterization

Time-of-Ñight secondary ion mass spectrometry (ToF-SIMS). The ToF-SIMS analysis was performed using aCharles Evans & Associates TFS surface analyzerequipped with a liquid-metal Ga positive ion sourceand pulse electron Ñooding. The instrument was oper-ated in an ion microprobe mode in which the primaryion beam (25 kV, nanosecond-pulsed, 600 pA) was ras-tered over a 140] 140 lm area. A 0.2 lm beam wasused for the analyses and positive secondary ions weredetected. The counting times were typically 22 min foreach element. Charging problems were subdued byplacing a metal grid on top of the samples and raisingthe bias voltage to 3030 V. This allowed the ion images/maps to be obtained. Typical primary ion doses were ofthe order of 2 ] 1012 ions cm~2. Under these condi-tions the ToF-SIMS has a probing depth of one mono-layer with detection limits \1 ppm or 1010 atomscm~2. The pressure in the analytical chamber was in the10~8È10~9 Torr range during analysis. The ion imageswere displayed as integrated intensities of mass-selectedions of the glass components within the Ðeld of view asa function of picture element (or pixel) position.

The elements Si, Al, B, Mg, Ba, Ca, Sr and Na weremonitored by ToF-SIMS using masses 28, 27, 11, 24,138, 40, 88 and 23, respectively. In this particular study,only changes of at least 50% are considered in theanalysis of the ToF-SIMS data. The relative intensitiesshown were not corrected for individual atomic sensiti-vities. Calibration with known standards would have

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 270È277 (1998)

272 E. C. ONYIRIUKA ET AL .

allowed for more quantitative determinations to bemade. Unfortunately, it is impractical to obtain stan-dards for a multicomponent glass of the type studied inthis work. Therefore, “controlÏ samples were deliberatelyintroduced to establish cross-contamination during theRCA cleaning process.

X-ray photoelectron spectroscopy (XPS). The XPS analyseswere carried out using a Physical Electronics (PHI)Model 5500 Multitechnique XPS/SAM spectrometer.The instrument is equipped with a monochromatic AlKa x-ray source (1486.6 eV) and a dual anode Mg/Alstandard source. Small-area (800 lm) XPS spectra wereobtained using either the Mg Ka (1253.6 eV) or Al Ka(1486.6 eV) standard x-ray source, operated at 400 W(15 kV, 28 mA). The XPS surface elemental compositiondata were obtained using 23.5 or 58 eV pass energies inthe high and medium energy resolutions, respectively.The sputterÈdepth proÐles were obtained by inertargon-ion sputtering (4 kV, 25 mA, 20 mPa) rasteredover a 3] 3 mm area and the data collected from an800 lm area in the bottom of the crater using a 117.4eV pass energy. Under the sputtering conditions used,the sputter rate for thermally grown was deter-SiO2mined to be D6 nm min~1. To minimize reaction of theexposed glass surface with atmospheric moisture, theglasses were fractured in a nitrogen-Ðlled glove bag. Thefractured glass was then introduced directly to the XPSspectrometer for analyses without exposure to theambient laboratory conditions.

Atomic force microscopy (AFM). Surface topographic datawere obtained in the TappingModeTM AFM fromDigital Instruments Dimension 3000 Multimode Nano-Scope IIIa Scanning Probe Microscopy (SPM) system.Three-dimensional surface plots, section and roughnessanalysis were obtained from a 4 lm section. Theaverage roughness value was taken as being the(Ra)most representative of the mean surface roughness in allsamples examined.

RESULTS

The combined used of SIMS with another complemen-tary technique, XPS, allowed us to obtain a quantitative

analytical comparison of the surfaces, including the dis-tribution of the species and trace element (such assodium) concentrations. Contamination of poly-Sicaused by the RCA cleaning process itself and the ToF-SIMS results for both bare and barrier-coated Code1737 glass surfaces are compiled in Table 1. Compari-son of the uncleaned polysilicon-on-silicon sample (no.1) with the ones that were pulled from the base portionof the RCA clean (nos 2 and 8) shows that the base cleanled to an increase in the concentration of Al and Bdopants on the poly-Si Ðlm surface only if glass waspresent. The used base caused an especially largeincrease in signal intensities for these elements. Sodium,Mg and Ca levels also increased but there was less dif-ference between the fresh and used base solutions. Com-plete RCA cleaning of sample no. 3 removed theresidual Al contamination deposited during the baseclean while leaving behind three times higher B levelthan was found in the control sample. Sodium, Mg andCa concentrations were also higher than the levelsdetected for the non-RCA-exposed sample.

Comparing samples 4 and 5 with samples 6 and 7 canassess the role of a silica barrier layer in preventing thetransfer of glass components. The samples pulled outafter the base portion of the RCA clean both had com-parable concentrations of Al, B, Mg and Ca except forNa, which was higher for the bare glass when comparedto the coated glass. Similar results were found for bothsamples that went through the complete RCA clean. Asbefore, the acid clean removed most of the contami-nants transferred to the glass during the base clean.Both samples ended up with approximately the samelevel of contamination, which was close to that of thecleaned control sample 3 specimen.

The ToF-SIMS analysis results of Code 1737 glasssurfaces after the RCA cleaning process are collected inTable 2. All the ToF-SIMS ion images obtained showedthat the distribution of residual cations was uniformexcept for relative intensity di†erences, which wereobserved for some elements. In the tabulated data, thedeionized rinsed samples (nos 1 and 4) represent thecontrols for two di†erent experiments. Note that theinitial cleaning of the controls followed by a deionizedwater gave rise to di†erent ratios of the elementsdetected on the glass surface. Changes in surface chem-istry induced by RCA cleaning are clearly evident in

Table 1. The ToF-SIMS element peak intensities for polysilicon Ðlms on siliconafter 15 min (except sample 3, which was 10 min) of RCA cleaning

Relative intensitiesa

Sample description Al B Mg Ca Na

1. Control—no RCA clean 0 405 60 370 115

2. Pulled from fresh base 7070 315 165 1210 2070

3. Full clean with oxidized Si and

fused silica (fresh solutions) 0 1100 125 560 815

4. Cleaned with silica-coated glass

—pulled after base clean 52 910 840 390 1175 120

5. Full clean with silica-coated glass 0 1090 45 560 1030

6. Cleaned with bare glass—

pulled after base clean 60 500 665 630 1645 1200

7. Full clean with bare glass 0 1460 37 480 580

8. Pulled from used base (after 21

bare glass substrates) 23 725 4255 245 1365 3785

a A 10 min count time (zero indicates a background of a few counts per second).

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 270È277 (1998) ( 1998 John Wiley & Sons, Ltd.


Table 2. The ToF-SIMS element peak intensities for Code 1737 glass surfaces aftervarious cleaning processes

Relative intensitiesa

Sample description Si Al B Mg Ca Na

1. Deionized water rinse 261 700 49 200 2705 2715 16 700 8250

2. After a 60 min base clean 209 000 192 000 5785 6825 25 700 4730

3. After a 60 min acid clean 138 000 17 500 585 965 6930 3560

4. Deionized water rinse 24 000 15 500 1200 800 3600 2100

5. RCA clean, 10 min each

bath 85 000 14 000 1000 0 1000 1500

6. RCA clean ½HF dip

(3 s in 10% HF) 51 700 20 800 1700 800 2500 3300

a A 10 min count time for samples 1–3 cleaned) ; 1 min for samples 4–6 (no clean).(CO2

CO2

samples 2, 3 and 5, respectively. Sixty minutes in thebase clean alone resulted in enhancement of the surfaceconcentrations of all the cations monitored except forsilicon and sodium. Exposure of a deionized water-

rinsed sample for 60 min to the acid clean alonedecreased the concentrations measured for all cations,leaving Si and Al as the major components remaining.Although, Al is still a major component, it should be

Table 3. The XPS atomic concentrations and AFM surface roughness data for Code 1737 glass samples

Atomic concentration (%)a

Si Al B Mg Ca Sr Ba Na

Roughness

Sample description Ra

(nm)

Fractured surface — 18.2 4.9 6.5 0.3 1.4 0.4 1.0 —

Deionized water rinse (nos 2–1) 0.18 22.2 4.4 3.2 0.3 1.0 0.3 0.8 0.1

RCA clean (nos 2–7) 0.20 26.6 2.0 1.5 0.2 0.4 0.1 0.2 0.1

RCA clean/HF dip

(nos 2–9) 0.49 24.9 4.2 4.1 0.4 0.8 0.3 0.7 0.2

Pellicle glass

(fractured surface) 18.9 4.8 6.1 0.3 1.5 0.4 1.0 —

a Corrected by eliminating C and N concentrations ; the remainder in the atomic concentration table rep-resents the oxygen concentration for each sample.

Figure 1. An XPS sputter depth–composition profile of Code 1737 alkaline-earth boroaluminosilicate glass with pellicle to protect theas-drawn glass surface.



Figure 2. An XPS sputter depth–composition profile of Code 1737 alkaline-earth boroaluminosilicate glass after a deionized water rinse.

noted that its concentration is reduced from the levelfound in the deionized rinsed sample (no. 1). CompleteRCA cleaning (10 min in each bath) resulted in a dra-matic increase in Si, unchanged Al and B levels and adecrease in Mg, Ca and Na concentrations when com-pared to the control sample (no. 4). RCA cleaning fol-lowed by an HF dip (10% solution of 3 s) resulted in anincrease in concentration for all elements except Siwhen compared with samples (see RCA results).However, the original chemical composition was not

attained, as indicated in the results obtained for sample4.

The ToF-SIMS results were supported by XPS dataobtained from samples treated the same way. The XPSresults are summarized in Table 3, along with AFMsurface roughness data. In the tabulated data, actualatomic concentrations are given, corrected for the pres-ence of carbon and nitrogen, which were also detectedon the glass surface. Even though oxygen is deliberatelyomitted in the table, its concentration can be obtained

Figure 3. An XPS sputter depth–composition profile of Code 1737 alkaline-earth boroaluminosilicate glass after RCA cleaning.



Figure 4. An XPS sputter depth–composition profile of Code 1737 alkaline-earth boroaluminosilicate glass after RCA cleaning followed byHF dipping.

by di†erence from the percentages indicated for the ele-ments in the table. The fractured surfaces of two Code1737 glass samples (an unprotected and a pellicle-protected glass surface), which are both representativeof the bulk glass composition in each case, were alsoexamined by XPS. The quantitative XPS analyses of theglass fracture surfaces are in good agreement with theknown “bulkÏ composition of Code 1737 glass deter-mined by ICP wet chemical analysis. It is apparent fromthe XPS data that even a deionized water rinse changesthe ratios of the cations on the glass surface. Evidently,RCA cleaning results in a silica-rich glass surface, whilethe HF dip strips o† some of this layer. X-ray photo-electron spectroscopy sputter depth proÐling comparedthe surface and bulk glass compositions. Figure 1 is anXPS depth proÐle of a Code 1737 glass with pellicle toprotect the as-drawn glass surface for comparison pur-poses. Similarly, the XPS depth proÐles of the DI rinsedcode 1737 glass control (no pellicle), an RCA clean andan RCA clean followed by HF dip, showing the degreeof alteration of the glass surfaces, are collectively dis-played in Figs 2È4, respectively.

DISCUSSION

Cross-contamination of poly-Si Ðlms on Code 1737 glassduring RCA cleaning

Based on the ToF-SIMS data compiled in Table 1, thefollowing conclusions can be drawn. First, the baseclean itself leads to an increase in the concentration ofglass components on the poly-Si Ðlm. The fresh solution(no. 2) compared to the control (no. 1) showed that theAl contamination level increased. A plausible explana-

tion for the increased Al level may be due to contami-nation from residual impurities present in the basicsolution from previous runs. The concentrations of allimpurities were found to be dramatically increased ifthe solution has been previously. However, completeRCA cleaning (no. 3) reduced the Al level to valuesbelow the SIMS detection limit, while leaving the othercomponents at two to three times the concentrationfound in the control sample. The presence of a silicabarrier layer on the glass did not prevent the transfer ofglass components to the surface of the poly-Si Ðlm.Comparison of the coated sample (no. 4) with the bare(no. 6) sample shows that comparable levels of contami-nation were transferred during the base clean except forthe Na level, which was higher for the bare glass. Aplausible explanation is that the edges of the glass werenot completely coated with the silica barrier layer andhence were susceptible to attack by the base solution.However, the acid clean (nos 5 and 7) corrected the situ-ation, which regenerated approximately the same levelof contamination as was detected in sample 3. The sig-niÐcance of these contamination levels is unclear, espe-cially as the ToF-SIMS data are reported as relative(uncorrected for individual atomic sensitivities) ratherthan absolute intensities.

In any event, the relative intensities reported here aresufficient and would have no inÑuence on a qualitativecomparison, as was done in this study.

Altered surfaces on Code 1737 glass caused by RCAcleaning

Both ToF-SIMS and XPS were used to characterize thesurface compositions of the glass samples after RCAcleaning and HF etching. Both sets of data showed thesame trends. It is apparent from the results in Table 2



that the starting composition of the glass is greatlya†ected by prior cleaning. Samples 1 and 4 representtwo di†erent runs and were considered nominally thesame at the time of the analysis. However, the di†erencein surface compositions, when corrected for countingtimes, suggests that some variation in precleaning musthave occurred. One known di†erence is that sample 1was cleaned using a snow jet just prior to analysis,CO2but sample 4 did not receive this treatment.

The base clean (no. 2) altered the glass surface com-position, as seen from the decrease in the Si levels, whilethe rest of the cations increased in concentration. Thisobservation is consistent and typical of glass substratesexposed to basic solutions (Ref. 12, p. 122). On the otherhand, the acid clean (no. 3) essentially leached out allthe cations except Si, leaving a silica-rich glass surface.A similar result was obtained for the complete RCA-cleaned (no. 5) sample. The results in the present studyare in agreement with the acid attack on a silicate glassobserved and documented by other workers (Ref. 12, p.122). However, when the RCA clean was followed by anHF dip, the surface reverted back towards the originalone. This is not surprising given the di†erence in reacti-vity between HCl and HF toward silicate glasses. It iswell known that hydroÑuoric acid solution attacks thesilica glass network, releasing all the components of theglass into solution or resulting in the precipitation ofthe components as Ñuoride salts (Ref. 12, p. 115). Incontrast, hydrochloric acid solutions leach out alkaliand alkaline-earth cations without attacking the silicaglass network.

A more quantitative analysis of the glass surfaces wascarried out with XPS. Table 3 contains the results forsamples similar to those used for ToF-SIMS experi-ments. Even though the total oxygen content can beinferred, albeit by di†erence in each case in the tabulat-ed data, it should be noted that some of the oxygens aredue to contributions from the hydroxyl groups on theglass surface. Data from fractured surfaces of two di†er-ent glass samples (a control glass used for the treat-ments, and a pellicle-protected glass) are included asbeing representative of the bulk glass compositions. Theelemental compositions of both fractured glass surfaces

are in good agreement, showing no detectable levels ofsodium. The surface composition of the deionizedwater-rinsed sample di†ers from the fractured surface,indicating that the initial state of the surface is di†erentfrom that of the bulk due to the precleaning and/orsurface contamination. The RCA-cleaned glass produc-ed a silica-rich surface, which was in turn partiallyremoved by the HF etch process. This observation is ingood agreement with the ToF-SIMS results shown inTable 2.

The roughness of the treated surfaces as determinedusing AFM are also compiled in Table 3. The datashow that there is essentially no di†erence in surfaceroughness between the deionized water-rinsed surfaceand the HCl-based RCA-cleaned surface. However, withthe HF-exposed glass, an increase in surface roughnessby a factor of 2.5 times the value for a deionized water-rinsed surface was observed. This is again consistentwith HF etching of the glass resulting in rougher topog-raphy. The surface roughness of the fractured surfacewas not measured because a cleaved glass surface isseldom Ñat and smooth.

X-ray photoelectron spectroscopy depthÈcomposition proÐles of the glass were also determined.Figures 1, 2 and 3 present the depth proÐles for Si, Al, Band Ba. A total estimated depth of 30 nm was probed.It is apparent that the surface composition of the glassis di†erent from the bulk, in agreement with the x-ray-scattering study of Umbach and Blakely.13 It is sur-prising that the depth of the altered surface is approx-imately the same in all three cases, because both theRCA clean and the HF etch removed components fromthe glass surface. In the former case, the treatment mustpenetrate below the glass surface, while in the latter theadded surface roughness masks the true surface com-position. The same roughness argument will render anydepth proÐle of the fractured glass meaningless, henceno such proÐles were obtained. Results from the Ðguresare collected in Table 4. The proÐled samples are com-pared with the surface results of Table 3. The oxygenconcentrations were used to correct for the Si, Al, B andBa levels reported in the table. The surface concentra-tions are in good agreement for both sets of data.

Table 4. XPS elemental composition data comparing the surface to the depth proÐle data at30 nm (“bulkÏ) into the Code 1737 glass as a function of surface treatment

Atomic concentration (%)a

Surface ‘Bulk’

Treatment Sample Si Al B Ba Si Al B Ba

Deionized water rinse 2–1 22.6 4.6 3.2 0.8

2–3 22.7 4.1 3.0 1.0 18.8 5.8 5.3 2.0

RCA clean 2–7 26.8 2.1 1.5 0.2

2–5 26.5 2.1 1.5 0.1 19.5 6.0 5.2 2.0

RCA/HF 2–9 25.3 4.3 4.3 0.7

2–11 24.1 3.1 3.1 0.5 18.4 5.8 6.5 2.1

Pellicle glass 22.0 4.4 2.4 0.8 18.5 5.7 4.2 2.0

Code 1737 ‘Control’

(fractured surface) 18.2 4.9 6.5 1.0

Pellicle glass

(fractured surface) 18.9 4.8 6.1 1.0

a Corrected to include only Si, Al, B, Ba and O; oxygen concentration was calculated by differ-ence for samples 2–3, 2–5 and 2–11; the remainder, which represents the percentage of oxygen,is deliberately omitted in the table for clarity.



However, the “bulkÏ values determined at 30 nm depthvia sputter depth proÐling do not agree with the resultsfor the fractured surfaces with respect to the Ba levels,even though depth proÐles to greater depths showedthat the equilibrium concentration values had beenachieved. It is known that the composition of multi-component oxides can be altered during sputtering.14 Ithas also been shown that ion-beam-induced di†usionand surface segregation, which occur during sputtering,can lead to distortions in the depth distribution of theelements.15 The various sputtering artefacts and theire†ect on the original depth distribution proÐlesobtained by surface techniques were recently discussedin a review article by Bach.16 The extent of the distor-tion is sensitive to the charge and size of the cations. Inthis context, at 30 nm depths, even though the proÐleindicates that the equilibrium concentrations have beenattained, the depths should be thought of as essentiallynew or ion-beam-modiÐed surfaces and, as such, theconcentrations may indeed be di†erent from the valuesobtained from a static fractured glass. We have pre-viously observed ion-beam sputter-induced surfacemigration of alkali and alkaline-earth cations, such asNa and Ba, during SIMS depth proÐling of tantalum-Ðlmed barium boroaluminosilicate glass.17 The sputter-induced migration phenomenon would explain why theBa levels determined at 30 nm depth in the presentstudy are higher than the values determined for the frac-

tured glass surfaces. It is unlikely that subnanometersurface roughness will a†ect the depth proÐle resultsbecause both the “controlÏ and the “testÏ samplesreceived the same treatment protocols and wereanalyzed under the same conditions. Any such e†ectsshould cancel out and will not change the conclusions.

CONCLUSION

In summary, it has been shown that cross-contamination of surfaces can occur during RCA clean-ing of glass. The cleaning to produce a silica-richsurface changes the surface of the glass itself. The unin-tentional thin silica-rich layer resulting from the alter-ation of the glass by RCA cleaning has recently beenfound to be beneÐcial in eliminating the thin-Ðlm tran-sistor instabilities usually encountered when no formalbarrier layer was used to separate the glass substratefrom the thin-Ðlm transistor.18

Acknowledgements

We thank Bryan Wheaton for the AFM roughness determination andRon Burdo for inductively coupled plasma spectroscopic elementalanalysis. The contributions of Lyle Kinney are gratefully acknow-ledged.

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effect of rca cleaning on the surface chemistry of glass and polysilicon films as studied by...

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