SURFACE AND INTERFACE ANALYSIS, VOL. 25, 295È298 (1997)

SHORT COMMUNICATION

E†ects of Sample Cooling on Depth ProÐling ofNa in Thin FilmsSiO

2

John J. Vajo*Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265, USA

Depth proÐling using secondary ion mass spectrometry of a 100 keV Na implant in a 430 nm thick Ðlm hasSiO2been studied as a function of temperature from 295 to 93 K. At 295 K, and using a coincident electron Ñux to

reduce the extent of electric Ðeld-induced Na migration, accumulation of Na at the interface could be eliminated.However, proÐles at 295 K without Na accumulation were difficult to reproduce. In addition, the trailing edgesoften did not decrease exponentially and are broadened with decay lengths > 44 nm. At 93 K and using the electronÑux only to maximize the matrix secondary ion signals, accumulation of Na at the interface was eliminated, thetrailing edge decreased exponentially and the decay length was reduced to 27 nm. Sample cooling also improvedreproducibility and allowed a wider range of sputtering conditions to be used. 1997 by John Wiley & Sons, Ltd.(

Surf. Interface Anal. 25, 295È298 (1997)No. of Figures : 2 No. of Tables : 0 No. of Refs : 8

KEYWORDS: secondary ion mass spectrometry ; SIMS; depth proÐling ; Na ; ion implantation ; thin Ðlm; migration ;SiO2 ;Ðeld-induced migration ; charge compensation ; sampling cooling ; temperature ; decay length

INTRODUCTION

During ion sputtering, an electric Ðeld can form at thesputtered surface if the surface is electrically insulatingor if an electrically insulating layer is formed as a resultof chemical interaction between the ions and thesurface. Electric Ðelds can also form during electronirradiation of an insulating surface. An electric Ðeld atthe surface provides a driving force for migration ofcharged species, either towards or away from thesurface, depending on the direction of the Ðeld and thesign of the charge. The rate of migration depends on themagnitude of the Ðeld as well as on the charge, mobilityand di†usion coefficient of the migrating species and,therefore, also on the activation energy for di†usion andthe temperature. Occasionally, although particularly forsingly charged ions of groups 1 and 11, the rate ofmigration can be sufficiently large to severely distortcompositionÈdepth proÐle measurements obtainedusing secondary ion mass spectrometry (SIMS)1h3 orAuger electron spectroscopy (AES).4 An example is Nain thin Ðlms, where almost complete migration ofSiO2Na to the interface can occur duringSiO2/substrateSIMS analysis.2

This problem of electric Ðeld-induced migration hasbeen addressed by using a coincident electron Ñux toexactly compensate the charge produced by the sputter-ing ions, thereby eliminating the electric Ðeld and thedriving force for migration. Using this procedure,Magee and Harrington have reduced the fraction of Nathat migrated to the interface from 98% toSiO2/Si0.06% (for Na implanted at 150 keV to 1.5] 1014 cm~2

* Correspondence to : J. J. Vajo (e-mail :jjvajo=hrl.com).

into a 0.73 lm Ðlm on Si).3 When the substrate isSiO2conductive but forms an insulating layer as a result ofreaction with the primary ions, e.g. during near-normalincidence sputtering of Si, an alternate method toO2`eliminate the electric Ðeld is to adjust the sputteringconditions so that an insulating layer does not form.1This may be accomplished by increasing the angle ofincidence from near-normal to near 45¡, although thiscan lead to an unacceptably large loss of secondary ionsignal.

In contrast to the idea of eliminating the electric Ðeld,sample cooling can be used to reduce the di†usion coef-Ðcient and, thereby, the rate of migration. Using liquidnitrogen cooling, this technique has been used for AESanalysis of Na in soda-lime silicate glasses to achievestable surface concentrations and accurate depth pro-Ðles for glasses subject to di†erent surface treat-ments.4h6 Recently, sample cooling has been used forSIMS proÐling of Na implanted (at 30 keV to3.3] 1015 cm~2 Na) into glass.7 At room temperaturea distorted proÐle with several maxima was obtained,while cooling to 125 K produced an approximatelyGaussian shaped proÐle.

In this communication we compare depth proÐles ofNa implanted in thin Ðlms using electron ÑuxSiO2compensation and sample cooling. Sample cooling isfound to give more accurate and reproducible proÐlesfor a wider range of analysis conditions.

EXPERIMENTAL DETAILS

Depth proÐles were measured using a duoplasmatronion source and a PHI model 3500 SIMS II, with a 16mm quadruple mass spectrometer, attached to a PHI

CCC 0142È2421/97/040295È04 $17.50 Received 19 August 1996( 1997 by John Wiley & Sons, Ltd. Accepted 2 December 1996

296 J. J. VAJO

595/600 scanning Auger microprobe system. Sputteringwas performed using primary ions at various ener-O2`gies and angles of incidence that are described in thenext section. The current was (250^ 20) nA andO2`the rastered area was 0.5 mm] 0.5 mm. Secondary ionswere collected from the central 4% of the rastered areausing electronic gating.

Electron irradiation was accomplished with the elec-tron source housed coaxially in the cylindrical mirroranalyzer used for AES. The electron energy was 3 keV,the angle of incidence from the normal was 36È40¡ andthe current was 1È1.5 lA. The source was defocused tocover an area D0.6 mm in diameter and used withoutrastering. Heating from the electron Ñux is estimatedto be likely \10 ¡C and, at worst, \30 ¡C. Additionaldetails are given in the next section.

Samples were cooled using liquid nitrogen and amodiÐed PHI cooling stage. The copper cooling braidof the PHI stage was replaced by Ñexible tubing so thatliquid nitrogen could Ñow directly through the coppercooling block. This modiÐcation enabled temperatures,measured by a thermocouple attached to the samplestage not to the sample, as low as 93 K ([180 ¡C) to beobtained. During cooling, intermediate temperaturesstable to within ^3 K could be maintained by manu-ally restricting the Ñow of liquid nitrogen through thecooling system.

The samples were pieces cleaved from an Si(100)wafer with 430 nm of plasma-deposited implantedSiO27¡ o†-normal with Na at an energy of 100 keV and aÑuence of 9] 1013 cm~2. Although not measureddirectly, no heating was expected during Na implanta-tion. The samples were analyzed without coating.

RESULTS AND DISCUSSION

A representative matrix signal for 30Si`, the best NaproÐles obtained at room temperature and at 93 K anda calculated proÐle obtained using the TRIM code8 areshown in Fig. 1. (simulations were performed usingTRIM version 91.14 with an density of 2.32 gSiO2cm~3.) Figure 1(a) shows the interface using theSiO2/Sidecline in the 30Si` signal. This decline results from thematrix e†ect associated with sputtering fully oxidized Sifrom the layer and only partially oxidized Si fromSiO2the substrate.

ProÐles (a) and (b) in Fig. 1(b) show two of the bestproÐles obtained at room temperature using electronÑux charge compensation. Compared with the calcu-lated proÐle (proÐle (d) in Fig. 1(b)), proÐle (a) has abroad trailing edge extending into the Si substrate witha decay length of 67 nm and a small peak of Na accu-mulated at the interface. The amount of Na accumulat-ed at the interface is D0.3%. In addition, the depth ofmaximum Na concentration, is shifted from 210 nmRm ,for the calculated proÐle to 236 nm. For proÐle (b) thereis no accumulation of Na at the interface and Rm\ 220nm, which agrees better with the calculated value of 210nm. The trailing edge is overall steeper than for proÐle(a) but it is also distorted and does not simply decayexponentially. Averaged from 320 nm to 410 nm, the

Figure 1. Depth profiles of Na implanted (100 eV, 9 Ã1013

cmÉ2) in nm)/Si ; (a) representative matrix ion signal forSiO2(430

30Si½, which indicates the interface ; (b) quantified NaSiO2/Si

profiles together with a simulation. Profiles (a) and (b) in panel(b) were obtained at room temperature with an optimized electronflux charge compensation where the electron irradiated area onlypartially overlapped the sputtered area. Sputtering conditions were1.5 keV 50¡ angle of incidence from the surface normal andO

2½,

sputtering rate 19 nm minÉ1. For profile (a) the decay length is 67nm. Profile (c) in panel (b) was obtained at 93 K with the electronflux centered on the sputtered area, using 3.0 keV 47¡ angleO

2½,

of incidence and sputtering rate 25 nm minÉ1. The decay length is27 nm. Curve (d) in panel (b) was obtained from a TRIM simula-tion.8 Profiles (a)–(c) in panel (b) were quantified using theimplanted fluence without background subtraction.

decay length is 44 nm. ProÐle (b) is similar to proÐlesobtained commercially from Evans East (Plainsboro,NJ). The commercial proÐles (not shown) are sharperthan proÐle (b) but also have non-exponential trailingedges.

ProÐles (a) and (b) in Fig. 1(b) were difficult to obtain.The best proÐles, those with a minimum of Na migra-tion, resulted when the electron Ñux was defocused toapproximately the size of the sputtered area and cen-tered, without rastering, on the edge of the sputteredarea. Thus, the area of electron irradiation only par-tially overlapped the sputtered area. For reasons we donot understand, the amount of Na migration is verysensitive to the degree of overlap. If the electron Ñuxcompletely overlapped or overlapped and extendedbeyond the sputtered area, as is commonly prescribedfor insulating samples,2 more Na migration occurred.This sensitivity to the position of the electron-irradiatedand sputtered areas lead to a signiÐcant non-reproducibility of the room temperature proÐles. Fornominally identical conditions. less than half the proÐlesappeared as good as shown in Fig. 1. It is not known ifthe sensitivity to the electron compensation conditionsseen in this work is particular to our apparatus or

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 295È298 (1997)

EFFECTS OF SAMPLE COOLING ON DEPTH PROFILING OF Na In SiO2 THIN FILMS 297

Figure 2. Depth profiles for Na in nm)/Si at 295, 252,SiO2(430

202, 153 and 93 K using the electron flux and sputtering condi-tions for profile (c) in Fig. 1(b). The sputtering rate was 26–29nm minÉ1, independent of temperature. All profiles were quantifiedso that the integrated areas are 9 Ã1013 cmÉ2. This assumes that asingle sensitivity factor applies to Na both in the and at theSiO

2interface. This assumption is most questionable for theSiO

2/Si

profile at 295 K, where the large peak at the interface accounts for30% of the total Na.

applies more generally. Varying the electron energybetween 0.5 and 5 keV and the current up to 2 lA didnot improve the results. Interestingly, the electron Ñuxconditions necessary to only maximize and stabilize thematrix secondary ion signals were easy to obtain anddid not depend as sensitively on the overlap of theelectron-irradiated and sputtered areas, as long as sig-niÐcant (D50%) overlap occurred. In addition, theroom temperature proÐles were obtained with 1.5 keV

and an angle of incidence from the normal of 50¡.O2`Attempts to use higher energies, to better focus the ionsand to increase the sputtering rate, always resulted inmore Na migration, which could not be compensated.Similarly, angles of incidence \50¡ led to increased Namigration.

ProÐle (c) in Fig. 1(b) shows an Na proÐle obtainedwith the sample cooled to 93 K. Compared with thesimulation, this proÐle is more accurate than the proÐleat room temperature. At 93 K nm, whichRm\ 215nearly agrees with the calculated value of 210 nm, andthe decay length is reduced from [44 nm at room tem-perature to 27 nm. The proÐle is almost entirely con-tained within the layer, with an exponentiallySiO2decaying trailing edge from an Na concentration of2 ] 1018 cm~3 to \1 ] 1016 cm~3, and there is noaccumulation at the interface. In contrast to the elec-tron Ñux conditions used at room temperature, at 93 Kthe electron Ñux was (relatively easily) adjusted only tomaximize the matrix secondary ion signal. This wasaccomplished with the electron source defocused, notrastered, and (simply) centered on the sputtered area.Under these conditions, reproducible proÐles wereobtained.

ProÐling at 93 K also enabled a wider range of sput-tering conditions to be used. The proÐle at 93 K in Fig.1 was obtained with 3 keV and an angle of inci-O2`

dence of 47¡. Use of a higher energy increased theO2`sputtering rate and improved the focusing of the ions.The improved focusing may account for the lower back-ground of the 93 K proÐle. The lower angle of incidence(47¡ compared with 50¡), although slight, is important inour system because for angles of incidence greater thanD40¡ the secondary ion optics must be increasinglyretracted from its optimal position, to avoid hitting thesample holder, which reduces the signal intensities.Although the range of analysis parameters at 93 K isexpanded, Na accumulation at the interface and distor-tion of the proÐle occurred when, for example, the ionenergy was increased to 5 keV or the angle of incidencedecreased to 40¡. Thus, cooling the sample to 93 Kreduces but does not eliminate Na migration for allsputtering conditions.

Figure 2 shows Na depth proÐles as a function oftemperature using the sputtering and electron irradia-tion conditions determined at 93 K. At room tem-perature (295 K) the proÐle is severely distorted, with apeak Na concentration at 350 nm and a larger peak atthe interface. In this case, the fraction of Na inSiO2/Sithe peak at the interface is 30%. Cooling to 252 Kalmost eliminates the peak at the interface, although theproÐle is still distorted with a pronounced shoulder at110 nm and a peak at 310 nm. At 202 and 153 K theproÐles appear approximately Gaussian in shape, asexpected, but is shifted to 250 nm at 202 K and 235Rmnm at 153 K. As shown in Fig. 1(b), at 93 K the proÐlematches very closely the calculated proÐle. There is stilla slight shift by 5È10 nm, which is most clearly seen bycomparing the leading (rising) edge of proÐles (c) and (d)in Fig. 1(b). The di†erence in the trailing edge slopesmay result from ion mixing.

SUMMARY

Sample cooling to 93 K has been shown to reduce therate of Na migration away from the sputtered surfaceduring SIMS depth proÐling of Na implanted in an

thin Ðlm. Compared with using electron ÑuxSiO2charge compensation at room temperature, at 93 K amore accurate depth proÐle was obtained and a widerrange of sputtering conditions was possible withoutcausing distortion or Na accumulation at the SiO2/Siinterface.

Acknowledgements

We thank Doug Menke at Delco Electronics for providing the SiO2samples, Robert G. Wilson for the Na implantation and discussionsregarding SIMS, Leslie A. Momoda for support and discussionsregarding Na in passivation layers, a reviewer for raising theSiO2question of sample heating and John D. Williams for help with heatconduction calculations. The analysis at Evans East was performed bySteven W. Novak.

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 295È298 (1997)

298 J. J. VAJO

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( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 295È298 (1997)