Extraction of inhomogeneous chemical species from TOF-SIMS images withtopographic artifacts by using correlation coefficient mapping

Shin-ichi Iida a,*, Noriaki Sanada a, Scott R. Bryan b, Mineharu Suzuki a

a ULVAC-PHI, Inc., 370 Enzo, Chigasaki, Japanb Physical Electronics, 18725 Lake Drive East, Chanhassen, MN 53317, USA

Applied Surface Science 255 (2008) 1603–1605

A R T I C L E I N F O

Article history:

Available online 14 May 2008

Keywords:

TOF-SIMS imaging

Image processing

Correlation coefficient mapping (CCM)

Surface topography

A B S T R A C T

It is often difficult to extract chemical distributions from TOF-SIMS (time-of-flight secondary ion mass

spectrometry) images because intensity contrast can originate from several sources, including; (1)

sample chemistry; (2) surface topography; (3) matrix effects; and (4) differential charging. Several

methods, to extract the chemical distribution from TOF-SIMS images that are affected by surface

topography, such as normalization by total ion intensity and PCA (principal component analysis), have

been carried out. However, these methods have not been successful in all cases. In this study, we have

tried to remove topographic artifacts by using correlation coefficient mapping (CCM) method. In order to

investigate the usefulness of CCM, it was applied to three types of specimens that had topographical

artifacts and/or chemical distributions. These results suggest that topographic artifacts can be removed in

TOF-SIMS images using CCM, revealing more accurate inhomogeneous chemical distributions.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

Applications of TOF-SIMS (time-of-flight secondary ion massspectrometry) analysis have expanded into a wide variety ofscientific fields because of its very high sensitivity to both elementsand molecular species. Imaging applications require an accurateevaluation of chemical distributions on a surface without artifactscaused by surface topography. This is especially true whenanalyzing rough specimens such as particles, tubes, and thebottom of trenches. Several methods have been proposed forremoving topographic contrast. For instance, Rangarajan and Tylerapplied principal component analysis (PCA) in the study of TOF-SIMS chemical images with topographic effects [1]. In the presentstudy, we show successful results extracting inhomogeneouschemical distributions from TOF-SIMS images with topographicartifacts by using correlation coefficient mapping (CCM).

2. Experimental

We used three kinds of specimens; (1) a cross-sectional surfacecleaved from p-type Si (5 1 1) wafer in the atmosphere; (2) asurface of Si wafer coated by hydroxyquinolinato aluminum (Alq3)

* Corresponding author.

E-mail address: shinichi_iida@ulvac.com (S.-i. Iida).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.05.144

(DOJINDO Laboratories, Japan), and; (3) a polystyrene (PS)spherical particle (DYNOSPHERES SS-950-P, JSR Corp., Japan) ona PS sheet.

Positive ion images were collected using a TRIFT IV (ULVAC-PHI,Inc., Japan) with Au+ projectiles from a 30 kV Au ion source. Typicalbeam size and mass resolution (m/Dm) in the present study wereapproximately 0.2 mm and 1000 at m/z 28, respectively. A pulsedlow energy electron gun for charge neutralization was used duringmeasurement of the polystyrene sample. TOF-SIMS images con-sisted of 256 � 256 pixels with a full mass spectrum at every pixel.

3. Results and discussion

The image was divided into a 32 � 32 (= 1024) grid array andspectra were extracted from 1024 sectioned areas and binned to1 amu. Correlation coefficients (CCs) were calculated by usingdigitized mass peak intensities at two specified sectioned areas. Ingeneral, ion intensities in the mass spectra decreased exponen-tially with increasing mass [2]. In this study, ion intensities weremultiplied by 1/exp(�bx) in order to add weight for the intensitiesof high mass ions, where x is in mass. Though b is an arbitraryconstant, 0.017 of b was adopted in this study. It is noted that thisweighting was especially useful for the specimen of Alq3 on Sisubstrate and there was no contribution for the specimens ofcleaved Si wafer and PS spherical particle on PS sheet since the CCswere mainly determined in the low mass region because there wasno significant differences in high mass region.

Fig. 1. (a) Total ion image (50 mm � 50 mm) of cross-section of a cleaved Si wafer, (b) intensity distribution in 5 level gradations, and (c) CCM.

Fig. 2. (a) Total ion and (b) AlC16H12N2O2+ ion image (150 mm � 150 mm) of a Si wafer covered with Alq3, and (c) CCM.

S.-i. Iida et al. / Applied Surface Science 255 (2008) 1603–16051604

First we chose the specimen which has topographic artifactswithout differences in chemical distribution to investigate theusefulness of CCM for removing topographic contrast. Fig. 1(a)shows the total ion image of the cross-section surface of a cleavedSi wafer. The analysis area was 50 mm � 50 mm. For sake ofsimplicity, the intensity scale was reduced to 5 levels of intensityas shown in Fig. 2(b). In order to examine the cause of intensitydistribution in the image, CCs between the spectra at the specifiedsectioned area (indicated by the arrow in Fig. 1(b)) and all othersectioned areas was investigated. All of the CCs were in the range0.95–1.0. Fig. 1(c) shows the planar distribution of the CC’s thatwere described in the 5 intensity levels. In the present study, allCCs were positive and less than 1, as shown in Fig. 1(c). Theseresults indicate that surface chemistry is the same at eachsectioned area, indicating that contrast in the image was due totopographic effects. By using CCM, topographic effects in TOF-SIMSimages can be clearly removed.

Figs. 2(a and b) show the total ion and AlC16H12N2O2+ (m/

z = 315 amu) ion image, respectively, from the surface at the

Fig. 3. TOF-SIMS spectra from the area that have CCs in the range of (a) 0.2–0.4 and

(b) 0.8–1.0 in Fig. 2(c).

boundary of Alq3 coated area and Si wafer. The analysis area was150 mm � 150 mm. No contrast was observed in the total ionimage of Fig. 2(a), since the majority of the total ions were Si+,which had no lateral variation. Half of the analysis area wascovered with Alq3 as shown in the 315 amu image of Fig. 2(b). Thisrepresents a case where the total ion image shows no contrast eventhough there is chemical variation in the image. In order todetermine if there was any chemical variation, CCM was alsoapplied to this sample. Fig. 2(c) shows the CCM which wasobtained by calculation of CC’s between the spectra at thesectioned area (lowest left corner of the image) and all othersectioned areas. Roughly two specified areas were observed,corresponding to the intensity distribution of AlC16H12N2O2

+ ions.Especially, CC’s were in the range of 0.2–0.4 in the sectioned areanear the boundary of the Alq3 side, and those were almost 1 in theuncovered Alq3 area. Fig. 3 shows the summed spectra extractedfrom the area that has CCs in the range of (a) 0.2–0.4 and (b) 0.8–1.0. A difference in 315 amu is observed in Fig. 3. These resultsindicate that CCM can be used to determine the number ofchemically different regions within an image when the total ionimage gives no indication and there is no topographic contrast.

Fig. 4(a) shows the total ion image of a PS spherical particle on aPS sheet. Analysis area was 150 mm � 150 mm. The dark ovalfeature on the right side in the spherical shape shown in Fig. 4(a) isnot distinguished from surrounding regions in the five levelgradations (Fig. 4b). CCs between the spectra at the specifiedsectioned area (indicated by the arrow in Fig. 4b) on thepolystyrene particle and all other sectioned areas were examined.The result of the CCM is shown in Fig. 4(c). We can roughly dividethe image into two specified areas, one with CCs more than 0.8 andthe other with CCs less than 0.2. Fig. 5 shows the summed spectraextracted from the two areas. The spectrum from the pixels withCCs more than 0.8 corresponds to PS contaminated by Na and K.The spectrum from pixels with CCs less than 0.2 corresponds to PScontaminated by polydimethylsiloxane. These results suggest thata heterogeneous chemical distribution in TOF-SIMS images thatalso contains topographic contrast can be distinguished by usingthe CCM method.

Fig. 5. TOF-SIMS spectra extracted from the area that has CCs in the range of (a) 0–

0.2 and (b) 0.8–1.0 in Fig. 4(c).

Fig. 4. (a) Total ion image (150 mm � 150 mm) of PS spherical particle on a PS sheet, (b) intensity distribution in five level gradations, and (c) CCM.

S.-i. Iida et al. / Applied Surface Science 255 (2008) 1603–1605 1605

CCM and score mapping of first PC (principal component) werecompared. PCA was performed with multivariate analysis softwareafter normalizing total ion counts, which is the common normal-

ization method to minimize topographic effects [3]. It is noted thatmapping of the principal components in the PCA analysis could notremove the topographic effects. Details of this comparison will bedescribed elsewhere.

4. Conclusions

We have investigated the use of correlation coefficient mapping(CCM) to extract the distribution of chemical species from TOF-SIMS images with topographic contrast. The CCM method wasapplied to three types of specimens: (1) a rough surface with asingle chemical phase; (2) a flat surface with two chemical phases;and (3) a spherical particle with heterogeneous contamination onsubstrate of the same material. For each of the three specimens, itwas possible to extract the heterogeneous chemical distributionsusing the CCM method.

References

[1] S. Rangarajan, B. Tyler, J. Vac. Sci. Technol. A 24 (2006) 1730.[2] I. Gilmore, M.P. Seah, Surf. Sci. 161 (2000) 465.[3] M.S. Wagner, D.J. Graham, B.D. Ratner, D.G. Castner, Surf. Sci. 570 (2004) 78.