XPS depth analysis of CuO by electrospray
droplet impact
†
Yuji Sakai,a *Satoshi Ninomiya b and Kenzo Hiraoka a
The electrospray droplet impact (EDI)is prented as a new etching technique that induces almost no surface damage left after irradiation.We have previously reported that lective etching was not obrved after EDI irradiation for organic materials.The prent work made a comparative XPS study on the CuO etched by Ar +and EDI.Although marked reduction of CuO was obrved with Ar +etching,no chemical modi fication was occurred by EDI.The etching rate of a few nm/min with EDI was estimated for copper oxide under the prent experimental conditions.Copyright ©2012John Wiley &Sons,Ltd.Keywords:XPS depth analysis;CuO;Electrospray Droplet Impact;EDI
Introduction
Transition metal oxides are uful in numerous practical applica-tions such as catalysis,electric or magnetic devices,and anticor-rosion coatings.For the analysis of metal oxides,it has been reported t
hat ion beam irradiation induces reduction for transi-tion metal oxides.[1–7]For example,almost all of the transition metal oxides suffer from reduction under Ar +etching.In a review article,[8]Hashimoto classi fied the XPS spectral changes in transi-tion metal oxides into three types.In type 1oxides,additional peaks appear becau of reduction;under Ar +etching,the less oxidative components appear at binding energies lower than the oxide binding energy,and the alterations hinder the deter-mination of oxidation states in XPS depth analysis.In attempts to suppress reduction occurring under Ar +beam irradiation,ion beams with lower-energy and different glancing angles were examined,but the deoxidative etching could not be avoided.[9]In type 2oxides,the spectra are broadened,and Ar +etching is accompanied by broadening of the full width at half maximum for the XPS peaks,caud by morphology change,for example,crystalline to amorphous.In type 3oxides,the oxidized state is almost unchanged with Ar +etching.
We propod a new etching method that us the electrospray droplet impact (EDI)in particular for cluster SIMS.[10,11]The pro-jectiles ud in EDI are extremely large multiple-charge water cluster ions with mass in the range 106–107u.EDI is capable of performing atomic or molecular-level etching without leaving damage on the etched surface.In related works,miconductors (SiO 2and InP)[12,13]and organic materials (polyethylene tere-phthalate,polystyrene,polymethylmethacrylate,p
olyvinyl chlo-ride and polyimide)[14–17]were etched by EDI and examined by XPS.It was found that surfaces etched by EDI did not exhibit preferential sputtering of particular elements.Moreover,we have reported [18,19]that deoxygenation was not obrved for transition metal oxides,TiO 2,Ta 2O 5,etched by EDI.
In this work,we examined CuO,which is a type 1oxide accord-ing to Hashimoto ’s classi fication,[8]that is,CuO is reduced by Ar +irradiation.By measuring the depth pro file of copper oxide formed on the Cu film deposited on Si,the formation of a double-layer of oxide and metallic Cu was con firmed.
Experimental
The standard bulk CuO sample (99.998%,powder)was purchad from Newmet Koch (UK).The oxidation of copper to form thin CuO film was performed from a Cu film evaporated on a Si (001)sub-strate as follows.The Cu was deposited on Si substrate using a vac-uum evaporator.The thickness of the deposited Cu was measured using a stylus pro filer (DEKTAK150,Veeco,USA)and a spectro-scopic ellipsometer (SE800:SENTECH Instrument,Germany).The film thickness of evaporated Cu was prepared to be approximately 10nm.After the deposition of Cu on Si,it was heated in ambient air at 180 C for 6h in an electric oven to oxidize Cu to CuO.
The experimental apparatus was consisted of an XPS instrument (JPS-9010MC with a MgK a X-ray source,JEOL,Japan)equipped with the Ar +and EDI guns.For the comparative study of Ar +and EDI etching,both ion guns were mounted in the same vacuum chamber.The analyzing area was lected to be approximately 1mm in diame-ter by using a lected area aperture.An electron flood gun was ud to neutralize sample charging by photoelectron emission.
In the EDI experiment,multiply-charged water droplets were formed by electrospraying 0.01M of tri fluoroacetic acid aqueous solution at atmospheric pressure.They were introduced into the vacuum chamber,and lected by the ion guide in the m/z range of 1Â104À5Â104.Typical charged water clusters may be repre-nted [10,11]as [(H 2O)90000+100H]100+.The droplets were accel-erated by 10kV,focud on the target with a diameter of 0.2mm,and scanned on a 4Â4mm 2area.The beam current was 1nA at 8kV accelerating voltage.The details of the EDI gun were described elwhere.[20]
*Correspondence to:Yuji Sakai,Clean Energy Rearch Center,University of Yamanashi,Takeda 4-3-11,Kofu,Yamanashi,400-8511,Japan.E-mail:ysakai@yamanashi.ac.jp
†
Paper published as part of the ECASIA 2011special issue.
a Clean Energy Rearch Center,University of Yamanashi,Takeda 4-3-11,Kofu,Yamanashi,400-8511,Japan
b Interdisciplinary Graduate School of Medical and Engineering,University of Yamanashi,Takeda 4-3-11,Kofu,Yamanashi,400-8511,Japan
Surf.Interface Anal.(2012)Copyright ©2012John Wiley &Sons,Ltd.
ECASIA special issue paper
复合句例句Received:23August 2011应傍战场开的上一句
Revid:8November 2011
Accepted:20December 2011
Published online in Wiley Online Library
()DOI 10.1002/sia.4843
Results and discussion
Low-damage etching
The characteristic binding energies for Cu2p 3/2are shown in Table 1(bad on Poulston et al .[22]).CuO is characterized by a shake-up satellite at a binding energy approximately 9eV higher than that of Cu2p 1/2and Cu2p 3/2.The full-width at half-maximum values of the Cu2p peaks of CuO were larger than tho of metal-lic Cu,and this broadening has been attributed to a shake-up process.[21]In the ca of Cu 2O,distinguishing it from metallic Cu is not easy,and sometimes the X-ray excit
ed Cu-LMM Auger spectrum is ud for this purpo.The Cu 2O Auger peak appears approximately 1.9eV lower in kinetic energy compared with Cu.To examine the capabilities of EDI etching,a comparative depth analysis was carried out on bulk CuO irradiated with Ar +and EDI.Figure 1shows the XPS spectra of bulk CuO etched with 400eV Ar +as a function of etching time.Before etching,the peak energy and shape of Cu2p 3/2were identi fied as CuO (e Table 1).After Ar +etching for 5s,the peak position and shape of Cu2p 3/2changed to tho for metallic Cu,and the shake-up peak of Cu2p disappeared.The Cu-LMM Auger peaks (not shown)were mea-sured simultaneously during depth analysis.It was found that CuO has changed to metallic Cu,and no intermediate Cu 2O was obrved.The results clearly show that CuO was deoxi-dized to Cu by 400eV Ar +irradiation.
Figure 2shows the XPS spectra of bulk CuO etched with 8keV EDI as a function of etching time.All Cu2p peaks were obrved as tho for CuO.The shake-up peaks of Cu2p appear unchanged under EDI.The XPS spectra for bulk CuO did not show any recog-nizable change after 60min EDI etching;that is,the EDI process caud no recognizable chemical modi fication for CuO.The depth
etched in 60min EDI was estimated to be approximately 18nm as described in the following ction.Depth analysis of CuO
Figure 3shows the changes in the Cu2p spectra for the 10-nm copper oxide film on Si substrate as a function of Ar +etching time.The obrved trend is similar to that shown in Fig.1,the occur-rence of reduction for CuO.The result means that Ar +etching of the CuO thin film was accompanied by reduction,similar to the effect on bulk CuO.Figure 4shows the depth pro file of the copper oxide prepared on Si substrate etched by with Ar +.The CuO was only detected at the beginning of irradiation time becau of the rapid reduction of CuO by Ar +irradiation.Here we assumed that the peak of Cu2p 3/2is consisted of two components,CuO and Cu.Figure 4shows the deconvoluted pro files (dotted line)of CuO and Cu obtained by curve-fitting calculation for the Cu2p 3/2pro file.The CuO intensity decreas rapidly with Ar +irradiation time.The film thickness of CuO formed on the Cu film deposited on the Si substrate could not be estimated becau of rapid reduction of CuO to Cu by Ar +irradiation.
Figure 5shows the depth analysis of the EDI-etched copper oxide film on Si.The intensities of the main and shake-up peaks
Table 1.Binding energies of Cu2p3/2for metallic Cu,Cu 2O,and CuO (from Poulston et al .[22])
Cu2p 3/2Binding energy
天灵盖是什么意思
Metallic Cu 932.6Cu 2O 932.4CuO
933.6
Figure 1.Cu2p XPS spectra of CuO (powder)etched by 400eV Ar +as a function of etching time.Figure 2.Cu2p XPS spectra of the CuO (powder)etched by 8kV EDI as a function of etching time.
Figure 3.XPS depth analysis of CuO/Si etched by 400eV Ar +as a function of time.
Y.Sakai,S.Ninomiya and K.Hiraoka
Copyright ©2012John Wiley &Sons,Ltd.Surf.Interface Anal.(2012)
of Cu2p decread monotonically during 15min etching.It ems likely that the copper oxide sample prepared in ambient air is consisted of a double layer of CuO and Cu.Figure 6shows the Cu2p 3/2peaks with the deconvoluted spectra for EDI etching for 0,20,and 40min.The Cu2p 3/2peaks at 0,20,and 40min may be assigned to CuO,[CuO +Cu],and Cu,respectively.Cu 2O was not obrved in
the Cu-LMM Auger spectra (not shown).Figure 7shows the depth pro file of the 10-nm oxidized copper film on Si obtained by EDI irradiation.CuO is obrved initially,and its decrea after 30min is accounted for by the increa of Cu.The dotted pro files in Fig.7show the double layer structure of CuO and Cu.It is likely that the copper film is partially oxidized on the surface of original Cu deposited on Si by heating it in the ambient air.
The 10-nm thin film compod of CuO and Cu was completely etched off with 60min EDI irradiation in Fig.7.The interface be-tween Cu2p3/2and Si2p3/2appeared at approximately 30min in Fig.7.We estimated that the EDI etching rate of [CuO +Cu]was approximately 0.3nm/min under the prent experimental conditions.In sum,the results show that EDI is applicable to the depth pro filing of copper oxide film formed on the Cu surface.The ability of EDI with atomic or molecular-level etching and with little surface modi fication made it possible to obrve the double layer structure of CuO and Cu components.This unique
characteristic of EDI might be originated from the water droplets affecting the surface with supersonic velocity.The H atoms in H 2O molecules in the droplet projectile have the lightest mass among all elements,and the momentum of the impinging dro-plets would be prerved mainly by backscattered H atoms at the instant of collision.The ef ficient dissipation of the collision energy as s
hockwaves propagating through the water droplets and the substrate may explain the atomic or molecular-level desorption with high ionization ef ficiency.[15–17]In a n,the kinetic energy of water droplets is ef ficiently converted to internal energies in supersonic collisions in EDI.
Conclusion
We performed the XPS depth analysis with EDI for CuO.The experimental results con firmed that the deoxidization of CuO was negligible with EDI etching.The test sample of copper oxide was found to be consisted of a double layer structure compod of CuO and Cu on Si.Apparently,EDI is capable of shallow surface etching causing almost no damage on the etched surface,and thus it may be applicable to the characterization of organic and inorganic multilayer systems with high depth
resolution.
Figure 4.XPS depth pro file of CuO/Si by Ar +etching.The Cu2p 3/2was parated into the deconvoluted pro files (dotted line)of CuO and
Cu.
Figure 5.XPS depth analysis of CuO/Si by EDI etching as a function of etching
time.
Figure 6.Cu2p 3/2peaks (a)before etching,(b)20min of EDI etching,and (c)40min of EDI etching.The deconvoluted spectra in panels a,b,and c correspond to CuO,[CuO +Cu],and Cu,
respectively.
Figure 7.XPS depth pro filing of CuO/Si by EDI etching.The Cu2p 3/2was deconvoluted into the pro files (dotted line)of CuO and Cu.
XPS depth analysis of CuO by EDI
Surf.Interface Anal.(2012)Copyright ©2012John Wiley &Sons,Ltd.
Acknowledgements
This work was supported by the Rearch and Development of Nanodevices for the Practical Utilization of Nanotechnology Pro-gram of the New Energy and Industrial Technology Development Organization of the Japane Government.
References
[1]H.M.Nagiub,R.Kelly,Rad.Eff.1975,25,1.
[2]K.S.Kim,W.E.Battinger,J.W.Amy,N.Winograd,J.Electron Spectrsc.
Relat.Phenom.1974,5,351.
[3]S.Hashimoto,K.Hirokawa,Y.Fukuda,K.Suzuki,T.Suzzuki,N.Usuki,
N.Gennai,S.Yoshida,M.Koda,H.Sezaki,H.Horie,A.Tanaka,T.Ohtsubo, Surf.Interface Anal.1992,18,799.
[4]U.Bardy,K.Tamura,M.Owari,Y.Nihei,Appl.Surf.Sci.1988,32,352.
[5]R.Kelly,Surf.Sci.1980,100,85
[6]S.Hofmann,J.M.Sanz,J.Trace Microprobe Technol.1982–1983,
1,213
[7]G.Panzner,B.Egart,H.P.Schmidt,Surf.Sci.1985,151,400
[8]S.Hashimoto,J.Surf.Anal.2003,10,230
自寻短见[9]S.Hashimoto,A.Tanaka,J.Surf.Anal.2001,8,192
[10]K.Hiraoka,D.Asakawa,S.Fujimaki,A.Takamizawa,K.Mori,Eur.Phys.
J.D.2006,38,225
[11]K.Hiraoka,K.Mori,D.Asakawa,J.Mass Spectrom.2006,41,894
[12]Y.Sakai,Y.Iijima,K.Mori,K.Hiraoka,Surf.Interface Anal.2008,我属羊老婆属狗
40,1716
[13]Y.Sakai,Y.Iijima,K.Mori,K.Hiraoka,J.Surf.Anal.2008,14,466
[14]Y.Sakai,Y.Iijima,R.Takaishi,D.Asakawa,K.Hiraoka,J.Vac.Sci.
赞赏反义词
Technol.A,2009,27,743
[15]K.Hiraoka,R.Takaishi,D.Asakawa,Y.Sakai,Y.Iijima,J.Vac.Sci.
Technol.A2009,27,748
[16]K.Hiraoka,Y.Sakai,Y.Iijima,D.Asakawa,K.Mori,Appl.Surf.Sci.2009,
255,8947.破五是什么意思
[17]K.Hiraoka,Y.Sakai,Y.Iijima,J.Vac.Sci.Technol.A2010,28,510
[18]Y.Sakai,Y.Iijima,K.Hiraoka,J.Surf.Anal.2008,15,172
引力红移[19]Y.Sakai,S.Ninomiya,K.Hiraoka,Surf.Interface Anal.,2011,43,1605.
[20]Y.Sakai,Y.Iijima,D.Asakawa,K.Hiraoka,Surf.Interface Anal.2010,
42,658
[21]K.S.Kim,J.Electron Spectrosc.1974,3,217
[22]S.Poulston,P.M.Marlet,P.Stone,M.Bowker,Surf.Interface Anal.
1996,24,811
Y.Sakai,S.Ninomiya and K.Hiraoka
