^{1,a)}, M. Yildirim

^{1}and H. R. Stapert

^{1}

### Abstract

X-ray photoelectron spectroscopy(XPS) is widely applied for the chemical characterization of surfaces and multilayers of thin films. In order to obtain quantitative results, XPS peak areas generally are divided by sensitivity factors and normalized to to obtain so-called raw concentrations. For homogeneous materials, materials with randomly distributed atoms within the analyzed surface layer, these concentrations may be a useful quantity. Yet, for a material consisting of a substrate on top of which a number of chemically different layers are present, the raw concentrations depend on measuring details like the takeoff angle during the XPSanalyses and clearly are not a satisfactory way to describe the sample. The main purpose of this article is to present a calculation method that converts raw concentrations into more meaningful quantities. The method is applicable to a restricted but technologically relevant class of samples: substrates on top of which one or more homogeneous layers are present. Examples are: gate dielectrics on Si or GaAs, self-assemblingmonolayers on a metallic substrate, thin oxide films on metals with an organic contamination on top. The method is based upon standard exponential attenuation of the photoelectron intensity as a function of traveled distance. For each element or chemical state in the system it has to be known to which layer(s) it belongs. Sensitivity factors are corrected for matrix effects and for intrinsic excitations. Starting from the raw concentrations, the method calculates in a self-consistent way the composition of all layers in the system and the thickness of each layer. Only one measurement at one measuring angle is required to obtain these results. To obtain insight into the accuracy of the calculation method, XPS results obtained on ultrathin layers on Si that were slightly contaminated with hydrocarbons have been analyzed with the method. The obtained thicknesses were in good agreement with values for the thickness of the layer and the organic surface contamination as obtained by other methods. Consistent values were also obtained for the concentration ratio in the layers. The calculation method has also been verified for three types of self-assembledmonolayers(SAM layers) on gold. Layers of C18 (octadecane-thiol) and of EG4 (a mercaptoalkyloligo-ethyleneglycol) deposited from solutions with different concentrations were examined. Also, SAM layers deposited from mixtures with molecules with different chain lengths, mercapto-undecanol (MUO), and a biotinylated oligo-ethyleneglycol-alkyl thiol (BAT), were investigated. The model analysis provided the thickness of the organic layers, the concentrations of the components in the layers, and the coverage of the gold with sulphur (in ). Rutherford backscattering spectrometry (RBS) was applied to determine (in an independent way) the amount of sulphur at the gold surface. The RBS results correlated well with the XPS data. The obtained values for the concentration ratios of the SAM layers were in agreement with the theoretically expected values. It is shown in the article that it is essential to model the mixtures of MUO and BAT as a three-layer system (gold substrate, aliphatic interlayer, and top layer containing the ethylene oxide groups) in order to obtain agreement.

I. INTRODUCTION

II. DESCRIPTION OF THE MODEL CALCULATION

A. Principle

B. Calculation

C. Implementation

III. APPLICATIONS

A. Measurements on ultrathin on Si

B. Measurements of SAM layers on gold

C. Measurements on SAM layers of C18 (octodecane thiol) on gold

D. Measurements of SAM layers of EG4 on gold

E. Measurements of mixed SAM layers of MUO and BAT on Au

F. Final remarks on the measurements on SAM layers on gold

IV. CONCLUSIONS

### Key Topics

- Gold
- 130.0
- Self assembly
- 39.0
- X-ray photoelectron spectroscopy
- 38.0
- Chemical bonds
- 13.0
- Elasticity
- 11.0

## Figures

Results of a three-layer model analysis of the apparent concentrations obtained for the samples. The results are plotted as a function of the thickness according to the standard equation, [Eq. (1) in Ref. 12]. (a) Optical thickness , thickness organic contamination , thickness layer according to model calculation , and total thickness according to the model calculation . (b) Concentration ratio /“Si as ” plotted as a function of . In this figure, the theoretical ratios for and also are shown.

Results of a three-layer model analysis of the apparent concentrations obtained for the samples. The results are plotted as a function of the thickness according to the standard equation, [Eq. (1) in Ref. 12]. (a) Optical thickness , thickness organic contamination , thickness layer according to model calculation , and total thickness according to the model calculation . (b) Concentration ratio /“Si as ” plotted as a function of . In this figure, the theoretical ratios for and also are shown.

Bond-line structure of the thiols tetraoxatridecane-thiol (EG4), mercapto-undecanol (MUO), and biotinylated alkyl thiol (BAT) (see Ref. 6).

Bond-line structure of the thiols tetraoxatridecane-thiol (EG4), mercapto-undecanol (MUO), and biotinylated alkyl thiol (BAT) (see Ref. 6).

spectrum for a sample consisting of C18 on gold [Sample 1(F) in Table V]. low denotes the doublet at corresponding to Au–thiolate. high denotes the doublet at corresponding to unbound thiol groups.

spectrum for a sample consisting of C18 on gold [Sample 1(F) in Table V]. low denotes the doublet at corresponding to Au–thiolate. high denotes the doublet at corresponding to unbound thiol groups.

(a) Thickness of the EG4 layers vs concentration in solution. The theoretical thicknesses in the figure below were obtained using a value for the length of the EG4 molecule of (based on a planar configuration (see Ref. 25)) and making use of the experimentally determined values of the coverage ṈS [see Table VIII(B)]. (b) Concentration ratio in the deposited SAM layers vs concentration in solution.

(a) Thickness of the EG4 layers vs concentration in solution. The theoretical thicknesses in the figure below were obtained using a value for the length of the EG4 molecule of (based on a planar configuration (see Ref. 25)) and making use of the experimentally determined values of the coverage ṈS [see Table VIII(B)]. (b) Concentration ratio in the deposited SAM layers vs concentration in solution.

(a) Thickness of the SAM layers consisting of a mixture of MUO and BAT according to the model calculations , theoretical thickness , and thickness as determined by ellipsometry. (b) denotes the fraction of BAT in the deposited layers according to Eq. (36) and denotes the fraction BAT in the deposited layers assuming a relative sensitivity factor for the transition that is 10% less than .

(a) Thickness of the SAM layers consisting of a mixture of MUO and BAT according to the model calculations , theoretical thickness , and thickness as determined by ellipsometry. (b) denotes the fraction of BAT in the deposited layers according to Eq. (36) and denotes the fraction BAT in the deposited layers assuming a relative sensitivity factor for the transition that is 10% less than .

Coverage with sulphur (in ) of a series of MAOEG layers on gold. Values obtained with RBS are plotted as a function of values obtained with XPS (model analysis). The composition of the SAM layers is the implicit variable.

Coverage with sulphur (in ) of a series of MAOEG layers on gold. Values obtained with RBS are plotted as a function of values obtained with XPS (model analysis). The composition of the SAM layers is the implicit variable.

## Tables

Investigated samples and thickness layer according to ellipsometry measurements (nine-point Woollam). The variation in the thickness across the nine measurement positions is also given.

Investigated samples and thickness layer according to ellipsometry measurements (nine-point Woollam). The variation in the thickness across the nine measurement positions is also given.

Raw concentrations (at. %) obtained for the sample series in Table I. Sample D has been measured in duplicate (sample number ). The result for sample L is the average of ten separate measurements at different positions. The contribution of suboxides to the concentration of “ as ” was taken into account by adding a weighed average [similar to Eq. (3) in Ref. 12].

Raw concentrations (at. %) obtained for the sample series in Table I. Sample D has been measured in duplicate (sample number ). The result for sample L is the average of ten separate measurements at different positions. The contribution of suboxides to the concentration of “ as ” was taken into account by adding a weighed average [similar to Eq. (3) in Ref. 12].

Results of a three-layer model analysis of the apparent concentrations in the samples. In the right-hand column, the concentration ratio is given.

Results of a three-layer model analysis of the apparent concentrations in the samples. In the right-hand column, the concentration ratio is given.

Bulk composition (at. %) and surface composition according to XPS analyses of two organic substances (the latter calculated from PHI sensitivity factors). The thick polyimide layer was based on preimidized polyimide type AL3046 (JSR). PEDOT–PSS was spin-coated starting from a solution of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(styrene sulfonic acid) (PSS), and NaOH.

Bulk composition (at. %) and surface composition according to XPS analyses of two organic substances (the latter calculated from PHI sensitivity factors). The thick polyimide layer was based on preimidized polyimide type AL3046 (JSR). PEDOT–PSS was spin-coated starting from a solution of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(styrene sulfonic acid) (PSS), and NaOH.

Raw concentrations of the two series of C18 samples on gold, obtained by dividing the peak areas by standard PHI sensitivity factors and normalizing to .

Raw concentrations of the two series of C18 samples on gold, obtained by dividing the peak areas by standard PHI sensitivity factors and normalizing to .

Estimated values for the IMFP in polymers, based on values for of Cumpson (see Table 2 in Ref. 24) and the energy dependence given in Eq. (35). The values in the second row are valid for hydrocarbon polymers. The values in the third row are valid for CHO polymers. In the fourth row, values are given for the practical effective attenuation length (PEAL) according to Petrovykh *et al.* ^{a}

Estimated values for the IMFP in polymers, based on values for of Cumpson (see Table 2 in Ref. 24) and the energy dependence given in Eq. (35). The values in the second row are valid for hydrocarbon polymers. The values in the third row are valid for CHO polymers. In the fourth row, values are given for the practical effective attenuation length (PEAL) according to Petrovykh *et al.* ^{a}

Results of the model calculation for the SAM layers consisting of C18 deposited on gold. The concentrations in the composition column denote the weighted average of the concentrations in the interlayer and in the top layer. The concentration sulphur denotes the total concentration of both S in Au–thiolate bonds and S in unbound thiol groups. The parameter ṈS is based only upon the concentration of sulphur in Au–thiolate bonds.

Results of the model calculation for the SAM layers consisting of C18 deposited on gold. The concentrations in the composition column denote the weighted average of the concentrations in the interlayer and in the top layer. The concentration sulphur denotes the total concentration of both S in Au–thiolate bonds and S in unbound thiol groups. The parameter ṈS is based only upon the concentration of sulphur in Au–thiolate bonds.

(A) Raw concentrations obtained for layers of pure EG4 on gold and (B) Results of the model calculation for the EG4-samples.

(A) Raw concentrations obtained for layers of pure EG4 on gold and (B) Results of the model calculation for the EG4-samples.

“Raw concentrations” of the samples containing a mixture of MUO and BAT, obtained by dividing the peak areas by standard PHI sensitivity factors and normalizing to .

“Raw concentrations” of the samples containing a mixture of MUO and BAT, obtained by dividing the peak areas by standard PHI sensitivity factors and normalizing to .

Results of the model calculations for the MUO–BAT mixtures. The concentrations in the composition column denote the weighted average of the concentrations in the interlayer and in the top layer. The parameter ṈS is based only upon the concentration of sulphur in Au–thiolate bonds. ṈS denotes the coverage with sulphur in gold–thiolate bonds, the thickness of the SAM layers according to the model calculations, and the theoretical thickness.

Results of the model calculations for the MUO–BAT mixtures. The concentrations in the composition column denote the weighted average of the concentrations in the interlayer and in the top layer. The parameter ṈS is based only upon the concentration of sulphur in Au–thiolate bonds. ṈS denotes the coverage with sulphur in gold–thiolate bonds, the thickness of the SAM layers according to the model calculations, and the theoretical thickness.

Results of model calculations for the MUO–BAT mixtures for a two-layer model (one single organic SAM layer on top of a gold substrate) and for a three-layer model (an aliphatic and an ethylene oxide part on top of the gold substrate). The theoretical concentration ratios and are also given.

Results of model calculations for the MUO–BAT mixtures for a two-layer model (one single organic SAM layer on top of a gold substrate) and for a three-layer model (an aliphatic and an ethylene oxide part on top of the gold substrate). The theoretical concentration ratios and are also given.

Thickness of some of the MUO–BAT mixtures on gold according to the model calculations, , and as determined using the ion-etch method, [Eq. (37)].

Thickness of some of the MUO–BAT mixtures on gold according to the model calculations, , and as determined using the ion-etch method, [Eq. (37)].

Article metrics loading...

Full text loading...

Commenting has been disabled for this content