VAMAS interlaboratory study on organic depth profiling. Part I: Preliminary report

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Research articleReceived: 2 July 2010 Revised: 24 September 2010 Accepted: 24 September 2010 Published online in Wiley Online Library: 9 November 2010

(wileyonlinelibrary.com) DOI 10.1002/sia.3705

VAMAS interlaboratory study on organic depthprofiling†

A. G. Shard,∗ S. Ray, M. P. Seah and L. Yang

We present the results of a VAMAS (Versailles project on Advanced Materials and Standards) interlaboratory study on organicdepth profiling, in which twenty laboratories submitted data from a multilayer organic reference material. Individual layerswere identified using a range of different sputtering species (C60

n+, Cs+, SF5+ and Xe+), but in this study only the C60

n+ ionswere able to provide truly ‘molecular’ depth profiles from the reference samples. The repeatability of profiles carried out onthree separate days by participants was shown to be excellent, with a number of laboratories obtaining better than 5% RSD(relative standard deviation) in depth resolution and sputtering yield, and better than 10% RSD in relative secondary ionintensities. Comparability between laboratories was also good in terms of depth resolution and sputtering yield, allowing usefulrelationships to be found between ion energy, sputtering yield and depth resolution. The study has shown that organic depthprofiling results can, with care, be compared on a day-to-day basis and between laboratories. The study has also validatedthree approaches that significantly improve the quality of organic depth profiling: sample cooling, sample rotation and grazingangles of ion incidence. c© Crown copyright 2010.

Keywords: sputtering; molecular depth profiling; SIMS; XPS

Introduction

The technological need for structured organic films and coatingshas been driven by the requirements of innovative industries, suchas organic electronics, tissue engineering and drug delivery. Whilstthere are numerous methods to produce such films, there are fewmethods that are capable of determining the three-dimensionalmicro- and nanostructures of individual organic components. Suchinformation is important to ensure that the envisaged design iscorrectly implemented and for determining the cause of failuredue to, for example, segregation and migration of componentsor localized contamination. The ability to perform ion sputterdepth profiles of organic materials enables the development ofuseful analytical techniques for this problem. Attempts to developsuitable ion sources for organic depth profiling have included thefollowing sputtering ions: SF5

+,[1] C60n+,[2] C24H12

+,[3] Ar>500+,[4]

Cs+,[5] and O2+.[6] The main characteristic that is sought is the

ability to induce negligible retained damage in the organic materialduring sputtering. This is a criterion that is hard to define anddepends upon the sensitivity of the technique used for chemicalanalysis of the sputtered surface and the level of informationrequired. For example, in SIMS analysis, the amount of damageobserved is a function of the size (and, therefore, mass) of thesecondary ion which is monitored. Several other key characteristicshave also received attention and relate to the practical applicationof organic depth profiling. The measurement of sputtering yieldis a key parameter in determining the sputtered depth, and themeasurement of depth resolution is important in the interpretationof data acquired from a depth profile. These characteristics aremuch easier to define and measure than the extent of chemicaldamage and provide a greater fundamental insight into themechanisms that operate during organic depth profiling.

It has become clear in recent years, that some means of com-parison was required to establish the important parameters thatinfluence the quality of organic depth profiles. Comparisons were

generally made on an ad hoc basis with a range of different mate-rials of varying thicknesses using different experimental protocols.In most cases, single layer materials on inorganic substrates havebeen used. Despite some commonality in the materials employed(for example, spin-cast poly (methyl methacrylate) on silicon waferis often used), different thicknesses led to a poor comparabilityin a dose-dependent sputtering rate. Furthermore, the large (ap-proaching two orders of magnitude) sputtering rate change atthe interface between organics and inorganics led to problemsin determining a depth resolution.[7,8] In light of this issue, theUK National Physical Laboratory (NPL) undertook an interlabo-ratory comparison under the auspices of VAMAS TWA2: SurfaceChemical Analysis, to assess the level of reliability, reproducibilityand comparability in organic depth profiling. VAMAS (Versaillesproject on Advanced Materials and Standards, www.vamas.org)conducts collaborative projects to harmonize measurements, test-ing, specifications and standards. Previous VAMAS interlaboratorystudies in SIMS have covered the repeatability of static SIMS,[9,10]

mass-scale calibration,[11] relative quantification,[12] and linearityof the intensity scale consecutively with this study.[13] The presentcomparison used a reference material capable of providing de-tailed information on sputtering yield behavior, depth resolution,molecular signal behavior and damage effects.

The requirement in organic depth profiling for a material withsharply defined interfaces between different organic materials

∗ Correspondence to: A. G. Shard, National Physical Laboratory, Hampton Road,Teddington, Middlesex, TW11 0LW, UK. E-mail: [email protected]

† This article VAMAS interlaboratory study on organic depth profiling was writtenby A. G. Shard, S. Ray, M. P. Seah and L. Yang of National Physical Laboratory.It is published with the permission of the Controller of HMSO and the Queen’sPrinter for Scotland.

National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW110LW, UK

Surf. Interface Anal. 2011, 43, 1240–1250 c© Crown copyright 2010.

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VAMAS interlaboratory study on organic depth profiling

was initially addressed by Sostarecz,[14] who employed Langmuir-Blodgett layers, and Wagner,[1] who spin-coated thin films of differ-ent polymers to investigate the characteristics of SF5

+ sputtering.However, these materials are not suitable for purposes of com-parison because of the problems associated with obtaining repro-ducible, homogenous and planar films with independently deter-mined thicknesses. The considerable effort of preparing large num-bers of samples is also a limiting factor. An alternative approach us-ing vacuum sublimation was reported by NPL[15] and subsequentlydeveloped to create organic ‘delta-layers’[16] using two types ofIrganox which were suitable for creating thin, homogenous filmsand had very similar sputtering yields under C60

n+ ion bombard-ment. Because this method allowed for the batch production ofsamples and generated uniform, high-quality films, it was selectedas the reference material for the VAMAS interlaboratory study.

A preliminary report on the results of the study was presented atthe SIMS XVII conference in Toronto, 2009.[17] This article presentsthe complete results of the study. The samples have proven tobe extremely useful to participants and have led to a number ofpublications in which new advances and approaches in organicdepth profiling have been demonstrated.[18 – 21] The referencematerials have already provided insight into fundamental aspectsof organic depth profiling, such as the dose and energydependence of the sputtering yield, the depth resolution, sputter-induced roughening and molecular secondary ion behavior.This study has enabled new approaches to organic depthprofiling to be quantitatively tested, including sample rotation,[19]

cooling,[19,22,23] low incidence impact angles[18,24] and bevels,[20,25]

and also shown that the repeatability and reproducibility of mostaspects of organic depth profiling is excellent.

Experimental

Silicon wafers with (100) surfaces were cleaved to ∼10 mm× 10 mm squares. Lint-free tissue was used to gently removeparticulates on the surface and the wafers were soaked overnightin isopropanol and dried using a flow of nitrogen. Nine wafers wereplaced face down in a holder positioned above two crucibles in anEdwards AUTO306 vacuum coater. A quartz crystal microbalance(QCM) was positioned at the side of the holder and used tomonitor the thickness of each Irganox layer. The depositiongeometry is outlined in Fig. 1a. The holder could accommodateup to 25 wafers, but only the central 9 were used due to theconstraints of obtaining a constant (±5%) Irganox 1010 thicknessacross the wafer. Tantalum boats were filled with either Irganox1010 or Irganox 3114 (CIBA, Macclesfield, UK); the boat filledwith Irganox 1010 was placed directly under the central siliconwafer, and the boat filled with Irganox 3114 to the side. Thisresulted in some heterogeneity in the Irganox 3114 thicknesswithin each batch, but much smaller heterogeneity in the Irganox1010 thickness. Since the latter thickness defines the depth ofthe Irganox 3114 marker layers and the marker layer thickness isnot critical for the purpose of comparability, this is the correctcompromise.

The thicknesses of films deposited from each boat wereestablished using spectroscopic ellipsometry (M2000, Woollam,NE, USA) in calibration runs. These calibration runs were performedbefore and after all samples were made for both Irganox 1010 andIrganox 3114 crucibles and after every second batch for Irganox1010. The calibrations were used to provide a sensitivity factor forthe QCM that could be used during sample construction to stop

deposition when the correct thickness had been achieved on thecentral sample. The geometry of the reactor was held fixed, andgood repeatibility was obtained in the QCM calibration factor forIrganox 1010, with a relative standard deviation (RSD) of 3.2%.We also established that the QCM and ellipsometric thicknesseswere proportional over a range of thicknesses from 50 to 200 nm,and that the relationship between thickness and position on theholder is well described (∼0.5% RSD) by the relationship given inEqn (1)

t (ri)

t (ri = 0)= z4

(z2 + ri

2)2 (1)

in which t is the film thickness, z is the vertical height of theholder above the boat and ri is the horizontal distance betweenthe vertical projection of the centre of boat i on the holder andthe position of the sample on the holder (Fig. 1a). This equationis derived from simple geometric considerations, assuming thethickness is proportional to: (i) the inverse square of the distancefrom the boat to the sample; (ii) the cosine of the angle between thesubstrate normal and the direction to the boat; and (iii) the cosineof the angle between the boat normal and the direction to thesubstrate. The first term assumes that evaporation is a smoothlyvarying function of evaporation direction and arises from massconservation. The second term accounts for the substrate notnecessarily facing the evaporation source. And the last term treatsthe evaporation source as an area source, so that the depositionrate depends upon the area of the source seen by the substrate.In our setup, both angles can be assumed to be the same. Fromthis relationship, the maximum difference in thickness within eachbatch is calculated to be 4%, and across any single sample themaximum variation in total thickness is less than 2%.

The construction of samples was achieved by the alternateevaporation of Irganox 1010 and Irganox 3114, the first and lastbeing Irganox 1010. The intended Irganox 1010 thicknesses were100 nm for all layers except the top two which were 50 nm, andthe intended Irganox 3114 thicknesses were 3 nm. The idealizedstructure for the reference materials is shown in Fig. 1b. Thethickness of each layer in each sample was calculated from theQCM readings, the QCM calibration factor and Eqn (1). Afterconstruction, samples were immediately placed in wafer traysto prevent contamination, wrapped in aluminium foil to excludelight, and sealed in bags ready for despatch to participants. Wehave previously found that exposure to light causes the lossof molecular secondary ion intensity from the surface of thesematerials over the course of six months. The thickness of onesample from each batch was checked by ellipsometry and themean error between the calculated and ellipsometric thicknesswas 1.7%, with a maximum error of 4.0%.

Analytical protocol

The protocol for analysis[26] was sent to all participants. Thisprovided guidance on measuring the sputter beam diameter andsetting the sputter and analysis areas to ensure a constant ion doseacross the analysis area. Specifically, the width of the sputteringbeam was set to be larger than twice the spacing between theraster lines. The analysis dimensions were restricted to less thanthe difference between the sputter dimensions and six timesthe sputtering beam width, as shown in Fig. 2. This was a veryconservative restriction, which should ensure that the analysisarea received a constant sputtering ion dose. Participants were

Surf. Interface Anal. 2011, 43, 1240–1250 c© Crown copyright 2010. wileyonlinelibrary.com/journal/sia

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A. G. Shard et al.

(b)

50 nm

50 nm

100 nm

100 nm

100 nm

13 mm

Holder

silicon wafer

QCM

TO

P V

IEW

SID

E V

IEW

Irganox crucibles

z = 140.5 mm

r1010

10103114

(a)

Pitchlength

Irgano x 1010

Irgano x 1010

Irgano x 1010

Irgano x 1010

Irgano x 1010

silicon

Figure 1. Production of multilayer reference materials. (a) Schematic, top and side views of the arrangement of samples inside the vacuum coater withrelevant dimensions. (b) Ideal structure, Irganox 3114 layers are 3 nm thick and interspersed between each Irganox 1010 layer.

Analysis width

Dose variation ~0.01%

Raster width

AnalysisBorder width

Dose variationBorder width

Sputter beam FWHM

Figure 2. Diagrammatic representation of the recommended analysiswidth for depth profiling in relation to the beam diameter and constantdose regions.

asked to establish whether they were able to detect any of theIrganox 3114 marker layers using their standard conditions and, ifsuccessful, they were to repeat the analysis twice more on differentdays. Finally, they were asked to alter the analysis conditions toinvestigate the parameters that may affect the quality of the depthprofile. The participants were asked to measure the sputter beamcurrent before and after each analysis to provide an estimate ofthe stability of the current during the experiment.

Upon return of the data to NPL, the areic dose (number ofparticles per unit area of sample surface) at each time point wascalculated from the average beam current, the rastered area ofthe sputter beam on the sample and the sputter time. A numberof signals are unique to the Irganox 3114 marker layer, in SIMSanalysis,[16] the most intense of these are: CN−, 26.003 u; CNO−,41.998 u; C18H24N3O4

−, 346.177 u; C33H46N3O5−, 564.344 u, and

in XPS analysis the N1s intensity was used. The intensities of theseunique signals were fitted with response functions comprisinga convolution of an exponential decay length and a Gaussianbroadening (a simplification of the Dowsett response function[27]

with the exponential growth length, λg, set to zero), which alsoprovided the position of the layer (taken as the maximum of theresponse function curve) and the intensity integrated over dose.Since the depth of each layer was known, the dose positions wereused to calculate the sputtering yield as a function of dose usingthe exponential function described previously[15] although, in afew cases, poor beam current stability rendered this inappropriate.A depth scale could then be applied to the data and the full widthat half maximum (FWHM) of each layer signal determined.

Participant details

In all, 8 batches of 9 samples were made, and 55 sampleswere despatched to 29 separate institutions for analysis on 35instruments, including both SIMS and XPS analysis. 20 sets of data(participants A to T) were returned to NPL. Of the respondents,15 employed TOF-SIMS IV or TOF-SIMS V instruments (ION-TOF,Germany), 4 used other SIMS instrumentation, some of which werein the earliest stages of development (C, K, S, T) and 1 used anXPS instrument (Q). 14 used C60

n+ ions as the sputtering source, 3

wileyonlinelibrary.com/journal/sia c© Crown copyright 2010. Surf. Interface Anal. 2011, 43, 1240–1250

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Table 1. Summary of the sputtering and analysis beams and energies used by participants in the VAMAS interlaboratory study on organic depthprofiling

Sputter beam (S) Analysis beam (A)

Code Species Energy (keV) Angle(◦) Species Energy (keV) Notes and additional data

A C60+ 10 45 Bi3+ 25 S: C60

n+ : 20; 30 keV

B C60++ 20 45 Bi3+ 25

C C60+ 15 42 C60

+ 15 A: Ga+: 18 keV

D Cs+ 1 45 Bi3++ 20 S: Cs+: 3 keV

E C60++ 20 45 Bi3+ 25 Sample rotation

F C60+ 10 45 Bi3+ 25 A: C60

+: 10 keV

G C60+ 10 45 Bi3++ 50 S: C60

++ : 20 keV

S: Cs+: 1 keV

H Cs+ 0.15 45 Ga+ 15 Two datasets

I SF5+ 5; 8 45 Bi3+ 25 Sample cooling

J SF5+ 5 45 Bi3+ 25 S: SF5

+: 3 keV

K C60+ 40 40 C60

+ 40 Angle: 30◦ ; 60◦

L C60+ 10 45 Bi3+ 25 Sample cooling

S: C60++ : 20 keV

M Cs+ 3 45 Bi3+ 25 S: Ar+ : 3 keV

N Xe+ 1.2 45 Ga+ 15

O C60+ 10 45 Bi3+ 25

P C60++ 20 45 Bi3+ 25 Two datasets

S: C60+++ : 30 keV

Q C60+ 10 67 XPS AlKα

R C60n+ 10; 20; 30 45 Bi3+ 25

S C60+ 20 48 C60

+ 20 One dataset

A: Bi3++ 50 keV

Angle: 76◦

T C60+ 40 40 C60

+ 40 S: C60++ : 80 keV

used Cs+ (D, H, M), 2 usec SF5+ (I, J), and 1 respondent used Xe+

(N). None of the participants used coronene or argon cluster ionbeams. A summary of the experimental setups employed by theparticipants is given in Table 1.

Results and discussion

Ability of ions to profile the reference material

Within the study, a successful depth profile was classified as onein which at least one of the Irganox 3114 layers could be clearlyidentified in at least one of the detection channels. Accordingto this criterion, the data provided for Xe+ sputtering at roomtemperature were not successful, and for SF5

+ sputtering, the firstlayer was clearly identified in some, but not all, of the data. Theclearest evidence of success for this ion beam was from participantI, who employed sample cooling in a number of experiments anddemonstrated a clear signal for the first layer at ∼50 nm depthfrom both CN− and CNO− secondary ions at both 5 and 8 keVimpact energies, as shown by the peak at a dose of ∼4 × 1018 ionsm−2 (note that as dose, this is per m2 of sample surface) in Fig. 3a.The second layer at ∼100 nm depth is possibly indicated by thevery broad peak at a dose of ∼25 × 1018 ions m−2, if this is so,it indicates a rather catastrophic reduction in sputtering yieldbetween the first and second layer. In this profile, the analyticalbeam was 25 keV Bi3+, as indicated in Table 1.

Participant M provided a nominally successful depth profileusing 3 keV Ar+ ion sputtering, although the only detectionchannel which clearly identified the marker layers was the CN−

secondary ion at 26 u (Fig. 3b). We can infer that this arisesfrom highly damaged and degraded remnants of the Irganox 3114molecule. Notwithstanding this, all four layers are clearly identifiedand have similar shapes and intensities, implying that a steadystate has been achieved before the first layer has been exposed.The dose, D, vs depth, x, positions of the four layers are fitted wellby a linear function, implying a constant sputtering yield volumeof 0.313 nm3 per ion, however, the function intercepts the depthaxis at ∼18 nm for zero dose, implying a significantly larger initialsputtering yield, S(0).

In this work, the sputtering yield volume at depth x, S(x), isdeduced from the change in depth sputtered with dose using therelation

S(x) = dx

dD(2)

On the same sample, 3 keV Cs+ ion sputtering was able toidentify all four layers using both CN− (26 u) and CNO− (42 u)secondary ions, as shown in Fig. 3c. Once again, the sputteringyield was close to constant at 0.146 nm3 per ion, with an initialoffset of ∼15 nm at zero dose. However, in this case, there is anobvious change in the sputtered surface until the third markerlayer at ∼200 nm is reached. This is evident because of the distinctbroadening in the last two peaks and a changing ratio in theintensities of CNO− to CN− secondary ions between the first threepeaks. For the final peak, this ratio is identical to the third peak.Participant D found similar effects using 3 keV Cs+ ion sputteringin a single experiment, although the derived sputtering yield wasslightly lower at 0.125 nm3 per ion and all four peaks were differentin width and relative secondary ions intensities. These differences


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A. G. Shard et al.

0 10 20 30 40 50In

ten

sity

(42

u)

Participant I: 8 keV SF5+ sample at -100 °C

0 200 400 600 800 1000 1200

Inte

nsi

ty (

26 u

)

0 500 1000 1500 2000 2500

Inte

nsi

ty (

26 u

)

0 5000 10000 15000 20000

Dose (1018 ionsm–2)

Inte

nsi

ty (

42 u

)

Participant M: 3 keV Ar+

Participant M: 3 keV Cs+

Participant H: 150 eV Cs+

(b)

(d)

(a)

(c)

Figure 3. Examples of SIMS depth profiles that were able to detect Irganox 3114 layers using the CN− (26 u) and/or CNO− (42 u) secondary ions, but notusing higher mass, molecular secondary ions. Data are shown as crosses, fits (in 3b and 3c) using the Dowsett function are shown as solid lines. (a) SF5

+sputtering at 8 keV with sample cooling; (b) Ar+ sputtering at 3 keV; (c) Cs+ sputtering at 3 keV and (d) Cs+ sputtering at 150 eV.

are probably a consequence of the failure of the analytical beamdose to be fully static in these dual beam experiments, since thedifferences in marker layer depths are hardly significant (∼3 nm).With a very low sputtering yield for the sputtering species, the dosefrom the analysis beam ions becomes significant and this effecthas recently been the subject of some investigation.[28,29] Using1 keV Cs+ ion sputtering, both participants D and G were able toclearly distinguish the first two layers using low-mass secondaryions, but the depth resolution was worse than at 3 keV, and thelast two layers were very broad and overlapped. With 150 eV Cs+,participant H was able to clearly resolve all four layers using bothCN− and CNO− secondary ions, as shown in Fig. 3d. The onset ofeach layer is very sharp, implying good depth resolution, but thedecay of signal from each layer is extended and has significantstructure. If the sputtering can be attributed only to the low-energycaesium ions and not the 15 keV gallium ions used for analysis,then the mean sputtering rate was 0.0195 nm3 per ion.

The vast majority of participants used C60n+ ions as the

sputtering species. All participants were able to detect more thanone of the layers using their standard experimental procedures.

Moreover, participants using SIMS were able to detect theselayers using the high-mass molecular secondary ions at 346u (C18H24N3O4

−) and 564 u (C33H46N3O5−), as expected from

our previous work on these systems.[16] Within this VAMASinvestigations, C60

n+ ions were the only species capable ofdetecting these characteristic ions during a depth profile, althoughsubsequently[21] it has been shown that argon cluster ions havethe potential to perform as well, if not better than C60

n+. In Fig. 4, aselection of SIMS profiles is shown, with a range of impact energiesand an impact angle of 45◦. It is clear that the deeper layers cannotbe adequately identified using the low-energy sputtering ions,and that the depth resolution for the shallower layers becomesworse as the energy of the incident C60

n+ ions is increased.

Repeatability of organic depth profiling

Most participants carried out repeat experiments on the referencesamples using, nominally, the same conditions on separatedays. This permitted the repeatability of organic depth profilingusing the same instrument to be established. Four measuresof repeatability were employed in this study using the mean


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0 1 2 3 4 5 6 7In

ten

sity

(56

4 u

)

0 0.5 1 1.5 2 2.5

Inte

nsi

ty (

564

u)

0 0.1 0.2 0.3 0.4 0.5

Dose (1018 ions m–2)

Inte

nsi

ty (

564

u)

Participant A: 10 keV, 45°

Participant R: 30 keV, 45°

Participant T: 80 keV, 45°

(a)

(b)

(c)

Figure 4. Examples of SIMS depth profiles using C60n+ as a sputtering source, the intensity of the secondary ion at 564 u is shown as a function of C60

n+dose. The kinetic energy of C60

n+ ions are (a) 10 keV, (b) 30 keV and (c) 80 keV. Data are shown as crosses and solid lines connect points from the fit usingthe Dowsett function.

of the RSD for each clearly distinguishable (FWHM <30 nm)Irganox 3114 layer signal in the three repeats: (i) the depthresolution (FWHM) determined from the Dowsett function fitto the layer; (ii) the sputtering yield at each layer determined fromthe exponential fit to the peak positions of the layers (an examplefit is shown in Fig. 5); (iii) the absolute (raw) intensity of eachlayer integrated over dose and; (iv) the relative intensity of thelower-mass secondary ions (either 26.003 u, 41.992 u or 346.177u) to the intensity of the characteristic ion from Irganox 3114with the highest observed mass (either 41.992 u or 564.344 u).Using a wider mass range is expected to degrade the repeatabilityunless significant care is taken in the instrumental procedures,[12]

however, many participants demonstrated excellent repeatabilityin relative intensity across the full mass range, as shown later.For participant Q who employed XPS, the final measure ofrepeatability, (iv), is not appropriate. The average RSD for (i) isshown in Fig. 6a; for (ii) in Fig. 6b; and for both (iii) and (iv) inFig. 6c.

The repeatability for depth resolution by participant is shownin Fig. 6a. It is clear that this repeatability can be excellent,with most participants achieving better than 10% RSD. Excellentrepeatability in this measure is not very surprising given that thedepth scale is independently determined from the spacing of thelayers, providing an internal calibration for the FWHM. The value,therefore, does not depend upon the ability of participants tomeasure beam currents or raster areas consistently. ParticipantG was unable to achieve a steady C60

+ beam current, and the

0

50

100

150

200

250

300

350

400

0 1 2 3 4


Dep

th (

nm

)

Participant B

20 keV C60++

Figure 5. Example data and fit for the determination of sputtering yield,S(x) from Eqn (2). The depth of each layer is plotted against the peakposition in terms of dose, markers show data and the solid line is the fitusing a previously determined relationship.[16]

functional form of the smooth line used to translate sputter timeinto depth was not able to adequately correct for this, leadingto a higher RSD than other participants. This variation in beamcurrent is the cause of the poor performance shown by thisparticipant in Figs 6b and c. Participant Q used XPS analysis, and


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A. G. Shard et al.

0

2

4

6

8

10

12

14

16

18

20

A B C D E F G K L M O Q R T

RS

D (

%)

(a) FWHM

94.5%

0

2

4

6

8

10

12

14

16

18

20


RS

D (

%)

(b) Sputtering yield

0

20

40

60

80

100


Participant

RS

D (

%)

Raw

Normalized

(c) Intensities

Figure 6. Repeatability of organic depth profiling shown as the averagerelative standard deviation (RSD) in four parameters over all clearlydistinguishable (FWHM <30 nm) layers and plotted for each participant.The four parameters are: (a) depth resolution, expressed as FWHM of theDowsett function; (b) sputtering yield determined from the exponentialfit to peak positions; (c) absolute, integrated intensities (filled bars) andrelative integrated intensities (open bars).

both the sparsity of the data (one point every ∼5.5 nm) and noisein the low-intensity N1s peak led to considerable uncertaintyin determining the FWHM of the narrow peaks for the first andsecond layers. The standard deviation of the FWHM values (∼3 nm)is actually similar to the spacing of the experimental data points,and this result simply arises from the low data density.

The repeatability in sputtering yield determination is given inFig. 6b for each participant. Once again, the consistency shown inthe results from the participants is generally excellent, with onlyparticipants C, G and T providing repeat data with larger than10% RSD. This is a testament to the consistency and care thatthe participants have shown in measuring their sputtering beamcurrents both before and after each experiment, and the stabilityof the sputtering sources. The poor results from participant G have

already been explained with participant C experiencing similarbeam stability problems, but not as severe as those of participantG. However, participant T reported steady and consistent beamcurrents and, therefore, the variation shown here is difficult toexplain.

The repeatability in integrated signal intensities is shown inFig. 6c as the filled bars. Only participants F and L were able toachieve better than 10% RSD in this measure although B, D, Eand Q achieved less than 20% RSD. This measure of repeatabilitydepends upon being able to obtain stable and repeatable fluxesfrom both the sputtering and analysis sources and having a stableanalyzer, it is therefore a very stringent measure. Because SIMSwas able to detect the Irganox 3114 layers using more thanone secondary ion, the repeatability of the relative intensitiesfor each layer could be tested which provides an indication ofthe constancy of the analyzer efficiency. Some contribution maybe expected from variability in the analysis beam current if thedetector does not have a linear response. The repeatability ofrelative secondary ion intensities is provided in Fig. 6c as whitebars. Note that participant Q used XPS and, therefore, this measureis not relevant. For most participants, the repeatability of relativesecondary ion intensity is much better than the repeatability ofraw ion intensities as may be expected. Only participants C, K, Oand T had RSDs larger than 20% in this measure and it is notablethat three of these participants (C, K and T) used the same C60

n+

source for both sputtering and analysis on instruments with adifferent design to those used by the majority of participants.Participant F provided two depth profiles using the same C60

n+source for analysis and sputtering with an ION-TOF instrument,which provided consistent secondary ion intensity ratios, verysimilar to the secondary ion intensity ratios using Bi3+ as theprimary ion source for analysis. This indicates that use of the samesource for analysis and sputtering should not be a limiting factorin obtaining repeatable relative secondary ion intensities, butthe level of instrumental development is implicated. ParticipantO provided one set of data with secondary ion intensities afactor of ∼4 times lower than the other two sets of repeat data.For high-intensity secondary ions this factor was much lower,∼2 for the CNO− secondary ion, and it appears likely that thedetector experienced saturation for these ions, leading to a poorerperformance than other instruments of the same type used inthe study. It is noteworthy that participant G who experiencedsputtering current instability, performed very poorly in the rawintegrated intensity measure of repeatability as expected, but theirrelative secondary ion intensities were very consistent.

For participants employing C60n+ as the sputtering species,

there is an interesting correlation between the repeatability ofrelative secondary ion intensities and the relative intensity of thesecondary ions. This is demonstrated in Fig. 7a, where the averageRSD of the normalized secondary ion intensities for all layersand ions is plotted against the average normalized secondaryion intensities for the low-mass 42 u secondary ion. A similarrelationship can be found for the 26 u secondary ion. It is clearthat as the intensity for the high-mass secondary ion decreaseswith respect to the low-mass secondary ions, the repeatability ofthe intensity ratios deteriorates. Figure 7b shows a subset of theintensity ratio data, which compares the standard deviation andmean secondary ion intensity ratios of the two high-mass ionsat 364 u and 564 u for each participant. There are two distinctsets of results, a cluster with good repeatability where the 346u secondary ion intensity is approximately half that of the 564u secondary ion, and a second set where the two intensities are


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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.5 1.5

I(346)/I(564)

SD

I(34

6)/I(

564)

05

1015202530354045

0 50 100 150

I(42)/I(564)

RS

D (

%)

1

(a) (b)

Figure 7. Correlation between relative secondary ion intensities and the repeatability of relative secondary ion intensities. (a) The RSD of normalizedintegrated intensities is plotted against the average value of the intensity ratio of the 42 u and 564 u secondary ions. (b) Comparison of the standarddeviation to the mean value of the intensity ratio of the two high mass secondary ions at 346 u and 564 u.

similar but the standard deviation is significantly larger. For thelower-mass ions, there is general agreement from all but twoparticipants that the CNO− ion at 42 u is ∼12 times more intensethan the CN− ion at 26 u, the major cause of poor repeatability forthe CNO− ion intensity being detector saturation and therefore,nonlinear detection efficiency. The cause of poor repeatability inthe intensity ratio for higher-mass ions cannot be due to detectorsaturation and may or may not be due to low and variabledetection efficiencies for high-mass ions, or dark noise resultingfrom incorrect analyzer and detector settings.

Comparability of C60n+ organic depth profiling

To compare results across the different ion energies used byparticipants, the most appropriate measures are the sputteringyields and depth resolutions obtained for the first Irganox 3114layer at ∼50 nm depth into the sample. Secondary ion intensitiesare poorly comparable, as indicated in the previous section and,therefore, are not considered further here. The first layer waschosen because at this relatively shallow depth the sputteringyield decay and catastrophic roughening effects are small even for5 keV C60

+ sputtering.[16] Data are only shown for profiles taken atroom temperature and with incidence angles close to 45◦.

Figure 8a demonstrates that there is good comparabilitybetween the sputtering yields derived from the data provided byparticipants, which indicates that the participants were diligent indetermining both sputtering ion beam currents and the sputteredareas. Up to 40 keV, the sputtering yield follows an apparentlylinear dependence on ion impact energy. The higher value forparticipant E, marked ‘14 Hz rotation’, may be due to the constantsputtering yield in these experiments. This is described in detailelsewhere.[19] Participant T provided data using 80 keV C60

2+ ionsfor sputtering, which indicated a sputtering yield in excess of1000 nm3 per incident ion at the first Irganox 3114 layer. This isnot plotted on the graph for reasons of clarity, but would indicatethat the sputtering yield increases in a nonlinear fashion at higherimpact energies. A little caution should be applied to this resultas participant T demonstrated one of the largest uncertainties insputtering yield determination. From the results we can provide anapproximate relationship between sputtering yield, Y , and C60

n+impact energy, E, for these materials as follows:

Y ≈ 8.4(nm3ion−1keV−1) × [

E − 1.5(keV)]

(3)

This relationship is entirely in accord with previous work usingenergies from 5 to 30 keV on the same material,[16] the results of a

linear fit to the energy-dependent sputtering yields determined at55 nm depth in that work are shown in Fig. 8a and closely overlaysthe relationship found in this work.

It was indicated in the preliminary report[17] that the depthresolution (FWHM) had good comparability between participantswho used 10 keV C60

+ ions on similar instruments. Figure 8bextends this comparison to all the energies used and plots theFWHM at the first layer against the cube root of the sputteringyield volume at that layer. The cube root of this volume providesthe characteristic length scale of the sputter crater from anindividual ion impact. A linear correlation exists between thesetwo parameters. The relationship should be expected if the depthresolution is limited by the topography induced by individualsputter craters, and the shapes of those craters do not changesignificantly with their volumes. A practical estimation of depthresolution at moderate (∼50 nm) depths in these materials usingC60

n+ at ∼45◦ is given by

FWHM ≈ 33√

Y (4)

If the craters simply were a constant shape of volume Y and wererandomly disposed, the FWHM would rise with the square rootof the number of sputtering events and, hence, with the squareroot of the depth. However, in the continued sputtering process,it is likely that the exposed rims are sputtered more strongly thanthe crater floors so that, in analogue to the model of Seah et al.[30]

the increase with depth soon ceases and Eqn (4) is obtained inwhich the value of the coefficient, 3, depends on the differencein the above sputtering rates. From the limited data available, itseems that the precision of this estimate is not affected by the useof sample rotation or cooling but the use of different angles ofincidence will change the relationship.

Improved methods of organic depth profiling

Participants in this study have demonstrated three practicalmethods of improving the quality of organic depth profiling.The use of sample cooling and rotation is described in detailelsewhere,[19] and the use of more grazing angles of incidence onthese samples has been briefly described.[18] Figure 9 displaysprofiles from participants who used incidence angles of 60◦

or greater. Notwithstanding other issues, some of which arisefrom the nonoptimal geometry of instruments for this type ofexperiment, it is clear that as the incidence angle increases, thewidth of the marker layer signals decreases. Less obviously, the


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0

5

10

15

20

25

30

35

0 5 10

Cube root of sputtering yield (nm)

FW

HM

of

firs

t la

yer

(nm

)

0

50

100

150

200

250

300

350

400

450

500(a) (b)

0 20 40

C60n+ kinetic energy (keV)

Sp

utt

erin

g y

ield

vo

lum

e (n

m3

per

ion

)

14 Hz rotation

Participant T

Figure 8. Comparison of sputtering yields and depth resolutions at the first (50 nm deep) layer for C60n+ profiles from participants. (�) Plots repeat data

shown with error bars of one standard deviation, (×) plots an unrepeated datum. (a) Sputtering yield, expressed as a volume per incident C60n+ ion,

plotted against kinetic energy, the thin solid line shows an unconstrained linear regression, the dashed bold line shows a similar fit to data from the firstinvestigation on this material.[16] (b) FWHM plotted against the cube root of the sputtering yield volume, a linear regression constrained to intercept theorigin is shown.

0 0.2 0.4 0.6 0.8 1 1.2

Inte

nsi

ty (

564

u)

0 1 2 3 4 5

Inte

nsi

ty (

N 1

s)

0 0.5 1.5 2 2.5 3.5


Inte

nsi

ty (

564

u)

Participant K: 40 keV, 60°

Participant Q: 10 keV, 67°

Participant S: 20 keV, 76°

(a)

(b)

(c)

31

Figure 9. Examples of depth profiles using C60n+ as a sputtering source, the intensity of either the secondary ion at 564 u or the N1s signal is shown as

a function of C60n+ dose. The kinetic energy and incident angles of C60

n+ ions are: a, 40 keV, 60◦ ; b, 10 keV, 67◦, using XPS, and c, 20 keV, 76◦ . Data areshown as crosses, and straight solid lines connect points from the fit using the Dowsett function, these points have the same dose positions as the dataand overly the data in (c) because the number of points in each peak is similar to the degrees of freedom in the fit.


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sputtering yield also becomes less dependent upon ion dose.There are insufficient data returned within this study to determinethe key relationships between the angle of incidence of the primaryions and both the sputtering yield and depth resolution. However,it is clear that this is a topic worthy of further detailed investigation.

Conclusion

The first interlaboratory study on organic depth profiling has suc-cessfully been completed and has provided a wealth of informationon the repeatability of the technique and comparability betweenlaboratories. Of the ion species employed for organic sputteringin this study, C60

n+ is the only one that provides molecular in-formation during the depth profile. We find that, given sufficientcare in obtaining instrumental stability, the repeatability of or-ganic depth profiling in terms of depth resolution and sputteringyields is excellent (better than 5% RSD) and can be close to thelimits set by the samples themselves. For SIMS analysis, obtainingrepeatability in raw secondary ion intensities requires extremecare in reproducing instrumental conditions, however, in mostcases, the relative secondary ion intensities are highly repeatable.The good comparability in depth resolution and sputtering yieldbetween laboratories in this study have enabled us to confirm therelationship between impact energy and sputtering yield for thesematerials and show that there is an apparently linear dependenceof initial depth resolution to the characteristic length scale of theindividual ion sputter crater. Furthermore, the study has shownthat values obtained for depth resolution and sputtering yieldcan be compared, with some confidence, between laboratories.The comparability of secondary ion intensities, however, must betreated with caution. The best comparability seems to be possibleonly when similar instruments and identical ion sources are used,and the intensity ratio between two ions of a similar mass arecompared.

Three methods for improving the quality of C60n+ organic depth

profiles have been validated during the study: sample cooling,sample rotation and increasing the ion impact angle. The first twoare dealt with in detail elsewhere,[19] and in this paper we show theresults obtained using high-impact angles, which indicate that thehigher (more grazing) the ion impact angle, the better the depthresolution and maintenance of sputtering yield. This study hasprovided an important base for the development of standards andhas enabled further progress in understanding the fundamentalsof organic depth profiling.

Acknowledgements

This work forms part of the Chemical and Biological Programmeof the National Measurement System of the UK Department ofBusiness, Innovation and Skills (BIS). We are greatly indebted tothe participants in the interlaboratory study for the time and effortthey have taken in analyzing the samples. They are listed below, inalphabetical order, with the names of their institutions at the timeof the study.

J. Brison, S. Muramoto, D. Castner: University of Washington,WA, USA.

R. Canteri: FBK, Trento, Italy.I. W. Fletcher, S. F. Davies: Intertek MSG, Wilton, Redcar, UK.J. Fletcher, J. Vickerman: University of Manchester, UK.S. Hinder, J. F. Watts: The Surface Analysis Laboratory, University

of Surrey, UK.

S. Iida : ULVAC-PHI Inc., Chigasaki, Japan.T. G. Lee: Nanobio Fusion Research Centre, Korea Research

Institute of Standards and Science, Korea.C. Lu, N. Winograd: Penn State University, PA, USA.C. Mahoney: National Institute of Standards and Technology,

Gaithersburg, MD, USA.N. Mine, L. Houssiau: FUNDP, Namur, Belgium.T. Mouhib, C. Poleunis, P. Bertrand : Universite Catholique de

Louvain, Belgium.D. Rading: ION-TOF GmbH, Munster, Germany.A. Rafati, D. Scurr, M. R. Alexander, M. C. Davies: Laboratory of

Biophysics and Surface Analysis, University of Nottingham, UK.P. Sjovall: SP Technical Research Institution of Sweden, Borås,

Sweden.P. M. Thompson: Eastman Kodak Co., Rochester, NY, USA.M. Wagner: Procter and Gamble, Cincinnati, OH, USA.D. Wells, B. Yatzor, S. Burns, R. Goacher, J. A. Gardella: University

at Buffalo, NY, USA.Y. Yamamoto: Asahi Glass Company Ltd., Kanagawa, Japan.Z. Zhu: Environmental Molecular Sciences Laboratory (EMSL),

Pacific Northwest National Laboratory, Richland, WA, USA. EMSLis a User Facility operated for the Office of Biological andEnvironmental Research of the US Department of Energy.

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VAMAS interlaboratory study on organic depth profiling. Part I: Preliminary report