Angle-resolved X-ray Photoelectron Spectroscopy Analysis Research Articles

Backside Analysis of Ultra-Thin Film Stacks in Microelectronics Technology Using X-ray Photoelectron Spectroscopy

AbstractX-ray photoelectron spectroscopy (XPS) has become increasingly important over the past few years for supporting the development of ultra-thin layers for high-k metal gates. As the analysis depth of XPS is however limited to about 5-7 nm, it would be extremely useful if the analysis could be carried out from the backside using standard silicon wafers. This approach puts extreme requirements on the sample preparation as hundreds of micrometers of bulk silicon have to be removed and one has to stop with nanometer precision when reaching the interface to the ultra-thin layer stack. Therefore, we have developed dedicated procedures for preparing and analyzing samples for backside XPS analysis. This paper presents the developed approach with a focus on sample preparation using plan-parallel polishing, endpoint detection by interference fringes, and selective wet etching. First angle-resolved XPS (ARXPS) analysis results of metal gate stacks demonstrate the power of such backside analysis.

Film continuity and interface bonding of thin boron carbonitride films on Ge(100) and Si(100)

Motivated by the need for a passivation layer for Ge, thin (<10 nm) films of amorphous boron carbonitride (BCN) were deposited on Ge(100) surfaces to study film continuity and interface bonding. Chemical vapor deposition on Ge(100) and Si(100) (studied for comparison) produced BC0.75N0.07 and BCN0.07 stoichiometry films, respectively. When BCN is nominally 2.5–5 nm thick, the film’s coverage of Ge(100) is continuous. This range was determined by (i) 0.5 kV He+ ion scattering spectroscopy scans, in which the underlying Ge(100) substrate signal disappears upon continuous BCN coverage and (ii) the convergence to unity of the ratio of BCN thicknesses estimated by two angle resolved x-ray photoelectron spectroscopy (ARXPS) techniques. Only 1.5–2 nm BCN was required for continuous coverage of Si(100), possibly due to better nucleation on this surface. ARXPS analysis indicates that the BCN-Ge(100) interface is clean and abrupt, with no evidence of an interfacial layer due to elemental intermixing. Constituent bonding in the BCN bulk is maintained at the Ge(100) interface. Preservation of the Ge 3d and Ge 2p full width at half maximum values after BCN deposition suggests a lack of distortion of Ge surface bonds, a criterion necessary for passivation if low interface state densities are to be achieved. BCN-Si(100) interface characteristics were similar to BCN-Ge(100).

Assembly of titanosilicate ETS-10 crystals on organosilane-functionalized gallium nitride surfaces

Integration of titanosilicate ETS-10 crystals with optoelectronic semiconductors is hypothesized to allow activation of these crystals for photocatalytic applications without direct exposure to light, and to more efficiently use the photons available. ETS-10 crystals were assembled on 3-aminopropyltrimethoxysilane (APTMS)-functionalized n-type GaN (0 0 0 2) substrates. Strong attachment of crystals to the functionalized substrates was shown by minimal to no loss of crystals on the GaN surfaces as tested by 30 min substrate sonication in water. Successful substrate functionalization was confirmed by the appearance of the Si 2p peak at 102.2 eV in the XPS spectra. The observation of the N 1s photoelectron peak at 401.19 eV suggested the presence of amino groups protonated by the surface hydroxyl groups of the substrate. Angle resolved XPS analysis of the APTMS-functionalized GaN surfaces showed a decrease in the relative N:Si ratio upon decreasing the take-off angle from 90° to 30°. These results suggest Si–O terminal groups are preferentially located at the surface of the APTMS linker, while the protonated amino groups are likely located at the substrate-linker interface. Therefore, the strong ETS-10 crystal attachment to the functionalized GaN (0 0 0 2) substrates can be hypothesized to be due to covalent bonding between the external ETS-10 surface hydroxyl groups and the Si–O terminal groups of the APTMS-functionalized GaN surfaces.

Electroless deposited Ni–P alloys: corrosion resistance mechanism

Electroless Ni-P alloys are produced as coatings on a broad variety of substrates. They exhibit a corrosion resistance that is superior to pure nickel but do not form a NiO oxide film (passive film) as pure nickel does. Despite the fact that many mechanisms have been proposed to explain this superior corrosion behaviour, no consensus has yet been reached. In this work electrochemical and XPS surface analytical methods have been combined in order to gain a deeper insight into the mechanisms underlying the corrosion resistance of electroless deposited Ni-P alloys with phos- phorus content between 18 and 22 at.%. The anodic polarization curves in acidic and neutral solutions confirm a broad current plateau followed by a region with increasing current density. During potentiostatic polarization in the plateau region the current decays according to a power law with exponent ca. -0.5 indicating diffusion-limited disso- lution of nickel. XPS/XAES measurements performed after potentiostatic polarization show that phosphorus is present in three different chemical environments. Based on the Auger parameter concept and on the chemical state plot, the three phosphorus states were assigned to phosphorus in the bulk alloy, phosphates and an intermediate phosphorus com- pound attributed to elemental phosphorus. Angle-resolved XPS analysis has shown that the elemental phosphorus is enriched at the interface between the alloy and the outermost surface in contact with the corrosive solution. These results suggest the following conclusions: the high corrosion resis- tance of electroless deposited Ni-P alloys can be explained by a strong enrichment of elemental phosphorus at the interface which limits the dissolution of nickel via a diffusion mechanism. A complementary explanation--not yet advanced--for the high corrosion resistance may lie in the electronic state of nickel in the Ni-P alloys.

Oligo(ethylene glycol) monolayers by silanization of silicon wafers: Real nature and stability

Grafting silicon wafers with CH 3O (CH 2 CH 2 O) n C 3H 6-trimethoxysilane and -trichlorosilane ( n = 6 to 9) was performed in different conditions (solvent, reaction time, washing) in order to select procedures compatible with the design of nanostructured surfaces for biomaterial applications, using electron-beam lithography. After a first screening by principal component analysis (PCA), the X-ray photoelectron spectroscopy (XPS) data were analyzed by plotting the carbon to oxygen molar ratio vs the molar ratio of carbon singly bound to oxygen [ C O] over carbon bound only to carbon and hydrogen [ C (C,H)]. This was found to be a convenient method for discarding samples containing free polymerized silane. Such excess occurred as a result of insufficient washing or unsuitable solvent for the reaction (ether), as confirmed by AFM and thickness measured by X-ray reflectometry. Angle resolved XPS analysis indicated that the grafted silane layer had a 1–2 nm thickness and was covered by a thin layer of adventitious contaminant. As a result, the surface chemical composition obtained covered a broad range (O/C of 0.4 to 1.1; C O/ C (C,H) of 2.5 to 6.5); variations could not be related to the nature of the silane reagent and no significant difference was found between hexane and toluene as solvent for the reaction. The grafted silane layer was not stable upon incubation during 24 h in phosphate buffered saline (PBS) at 37 °C, which mimics biological environments. As a consequence, the grafted wafers did not show protein repellent properties. This alteration was not observed at room temperature. XPS analysis demonstrated that silane layer detachment was due to a hydrolysis within the SiO 2 layer initially present at the wafer surface.

Electrochemical and Surface Characterization of 4-Aminothiophenol Adsorption at Polycrystalline Platinum Electrodes

The formation of a self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) on polycrystalline platinum electrodes has been characterized by surface analysis and electrochemistry techniques. The 4-ATP monolayer was characterized by cyclic voltammetry (CV), linear sweep voltammetry, Raman spectroscopy, reflection-absorption infrared (RAIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). CV was used to study the dependence of the adsorption time and 4-ATP solution concentration on the relative degree of coverage of 4-ATP monolayers on polycrystalline Pt electrodes. The adsorption time range probed was 24-72 h. The optimal concentration of 4-ATP needed to obtain the highest surface at the lowest adsorption time was 10 mM. RAIR and Raman spectroscopy for 4-ATP-modified platinum electrodes showed the characteristic adsorption bands for 4-ATP, such as nuNH, nuCH(arom), and nuCS(arom), indicating the adsorption on the platinum surface. The XPS spectra for the modified Pt surface presented the binding energy peaks of sulfur and nitrogen. High energy resolution XPS studies, RAIR, and Raman spectrum for platinum electrodes modified with 4-ATP indicate that the molecules are sulfur-bonded to the platinum surface. The formation of a S-Pt bond suggests that ATP adsorption leads to an amino-terminated electrode surface. The thickness of the monolayer was evaluated via angle-resolved XPS (AR-XPS) analyses, giving a value of 8 A. As evidence of the terminal amino group on the electrode surface, the chemical derivatization of the 4-ATP SAM was done with 16-Br hexadecanoic acid. This surface reaction was followed by RAIR spectroscopy.

Surface characterization of ion-enhanced implanted photoresist removal

We characterize the chemical constitutents of high dose implanted deep ultraviolet photoresist before and after dual-mode oxygen plasma processing, where a remote rf-plasma source is operated simultaneously with rf bias. Raman spectroscopy indicates that the organic composition of the crust comprises a mixture of sp2 graphite and sp3 diamondlike carbon structures. High dose ion implantation reduces the hydrogen content by about 50 at. % as measured by hydrogen forward scattering and explains the reduced optical emission signal intensity observed during crust removal. The crust thicknesses extracted from the secondary-ion-mass spectroscopy profile correspond well to prior scanning electron microscopy characterization [Kawaguchi et al., J. Vac. Sci. Technol. B (submitted)] and support the existence of a transitional layer between the hardened crust and the underlying photoresist. Angle-resolved x-ray photoelectron spectroscopy analysis of arsenic implanted photoresist shows that dual-mode plasma processing causes substantial oxidation deep into the bulk. This result contrasts with downstream plasma processing, which proceeds by a near-surface mechanism. In addition, surface arsenic levels increase by an order of magnitude, which suggests that ion bombardment does not significantly sputter the dopant.

Quantitative XPS analysis of hydrosilated 1-alkene and 1-alkyne at terraced, dihydrogen-terminated, 1 × 1 (100) silicon

A quantitative angle-resolved XPS analysis was carried out of the carbonaceous films resulting from the derivatization under mild thermal activation of nearly flat, terraced, dihydrogen-terminated, 1 × 1 (100) Si with 1-octene or 1-octyne. The analysis of the C 1s signal gave evidence for the presence of carbon in carbide configuration (SiC bonds) at the substrate–film interface, in addition to the alkanic carbon and adventitious oxidized carbon (CO bonds) produced by the oxidizing impurities flawing the reaction. Assuming the surface as uniformly covered, the analysis showed that for both reactants the films were closely packed. Copyright © 2006 John Wiley & Sons, Ltd.

Si 0.85Ge 0.15 oxynitridation in wet-nitric oxide ambient

Nitric oxide (NO) aided Si 0.85Ge 0.15 wet-oxynitridation has been performed at 400–700 °C, while the wet-NO feed gas was preheated to higher temperatures before entering the reaction zone. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy data suggests that both nitrogen and oxygen incorporation increases within the dielectric bulk with increasing wet-oxynitridation temperature, while there is no apparent germanium segregation towards the dielectric/substrate interface at all temperatures studied. Moreover, angle-resolved XPS analysis suggests that wet-oxynitridation at temperatures higher than 600 °C volatilizes some germanium oxide from the surface region, while silicon monoxide is outgassed from the dielectric bulk and accumulates near the surface. Nitrogen incorporation is found to hinder germanium segregation. Z-contrast imaging with scanning transmission electron microscopy shows that oxynitrides grown in wet-NO have sharp interfaces with bulk Si 0.85Ge 0.15, while the roughness of the dielectric/Si 0.85Ge 0.15 substrate interface is less than 2 Å. These results are discussed in the context of an overall mechanism of SiGe wet-oxynitridation.

Etching of porous SiOCH materials in fluorocarbon-based plasmas

This work focuses on the etching of different porous methylsilsesquioxane materials (spin on SiOCH, k=2.2) with different porosity (30%, 40% and 50%) in fluorocarbon-based plasmas (CF4∕Ar). The etching of these materials is performed on blanket wafers in a magnetically enhanced reactive ion etcher. The surface and bulk modification after partial etching are studied using different surface analysis techniques such as quasi-in-situ x-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), and attenuated total reflection spectroscopy (FTIR-ATR). Similar to nonporous SiOCH materials, a decrease in etch rate of porous SiOCH films is observed with either increasing Ar dilution or polymerizing gas addition (CH2F2), which can lead in this last case to an etch stop phenomenon. The etch rate increases with higher porosity in the SiOCH film, since less material per unit thickness needs to be removed as the porosity increases. The XPS results show that a fluorocarbon layer is formed at the surface of the porous material and complementary angle resolved XPS analyses reveal that fluorocarbon species diffuse through the pores into the material. After partial etching, FTIR and ATR analyses reveal a carbon depletion in the remaining film, which indicates that the porous material is altered during plasma exposure. The film degradation is more important as the porosity increases. The etch rate evolution and film degradation are discussed and interpreted in terms of etching mechanisms and plasma surface interaction.

Correlation between hydroxyl fraction and O/Al atomic ratio as determined from XPS spectra of aluminium oxide layers

AbstractA set of five different aluminium oxide layers has been investigated using XPS. The oxide layers were made by oxidizing aluminium in a vacuum, with an alkaline and acidic pretreatment and in boiling water. Hydroxyl fractions of the aluminium oxide layers ranging from 0.0 to 0.5 were determined by constrained curve‐fitting of the O 1s peak. The O/Al atomic ratios of the aluminium oxide layers, ranging from 1.5 to 2.0, were determined from the O 1s and Al 2p photoelectron intensities. A method is presented to account for the attenuation of the photoelectron intensities by the contamination overlayer. For the studied oxide layers, a linear relation is observed between the hydroxyl fraction and the O/Al atomic ratio of the aluminium oxide layers. It is concluded that the results obtained by the curve‐fitting procedure are reliable. Furthermore, a linear relation is observed between the hydroxyl fraction and the O 1s peak width. The O 1s binding energies of the O2− and OH− components of the oxide layers correspond to 531.0 ± 0.1 eV and 532.4 ± 0.1 eV, respectively. Only pseudoboehmite showed 0.5 eV lower binding energies for these components. Angle‐resolved XPS analysis showed that most of the studied oxides are enriched in hydroxyl groups at their outermost surface. Copyright © 2004 John Wiley & Sons, Ltd.

Atomic layer deposition of ZrO2 on W for metal–insulator–metal capacitor application

A metal–insulator–metal (MIM) capacitor using ZrO2 on tungsten (W) metal bottom electrode was demonstrated and characterized in this letter. Both ZrO2 and W metal were synthesized by an atomic layer deposition (ALD) method. High-quality 110∼115 Å ZrO2 films were grown uniformly on ALD W using ZrCl4 and H2O precursors at 300 °C, and polycrystalline ZrO2 in the ALD regime could be obtained. A 13∼14-Å-thick interfacial layer between ZrO2 and W was observed after fabrication, and it was identified as WOx through angle-resolved x-ray photoelectron spectroscopy analysis with wet chemical etching. The apparent equivalent oxide thickness was 20∼21 Å. An effective dielectric constant of 22∼25 including an interfacial WOx layer was obtained by measuring capacitance and thickness of MIM capacitors with Pt top electrodes. High capacitance per area (16∼17 fF/μm2) and low leakage current (10−7 A/cm2 at ±1 V) were achieved.

Low temperature Si0.85 Ge0.15 oxynitridation in wet-nitric oxide ambient

AbstractNitric oxide (NO) aided Si0.85Ge0.15 wet-oxynitridation has been performed at 400 700°C, while the wet-NO feed gas was preheated to higher temperatures before entering the reaction zone. X-Ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) data suggests that both nitrogen and oxygen incorporation increases within the dielectric bulk with increasing wet-oxynitridation temperature, while there is no apparent germanium segregation towards the dielectric/substrate interface at all temperatures studied. Angle-resolved XPS analysis shows that increase in wetoxynitridation temperature above 600°C is likely to volatilize some germanium oxide from the surface region. Nitrogen incorporation is found to hinder germanium segregation. These results are discussed in the context of an overall mechanism of SiGe wet-oxynitridation.

XPS Study of Corrosion Behavior of Ti-18Nb-4Sn Shape Memory Alloy in a 0.05 mass% HCl Solution

The Ti-18Nb-4Sn alloy has been confirmed to possess much higher corrosion resistance compared to the NiTi alloy throughout immersion time up to 604.8 ks in a 0.05 mass% HCl at 310 K open to air. The Ti-18Nb-4Sn alloy shows higher open circuit potential, compared to titanium metal and NiTi alloy. The higher open circuit potential results in an increase in the thickness of the passive film formed on the alloy. The passive film on the Ti-18Nb-4Sn alloy is rich in niobium as well as tin due to preferential dissolution of titanium. The degree of enrichment of niobium and tin in the film increases with an increase in immersion time. On the other hand, the passive film on the NiTi alloy consists exclusively of titanium. The angle-resolved XPS analysis reveals that the outer part of the passive film on the Ti-18Nb-4Sn alloy is rich in niobium and deficient in titanium. Furthermore, the passive film on the Ti-18Nb-4Sn alloy is composed of a double oxyhydroxide containing titanium, niobium and tin cations. It can, therefore, be said that the formation of the thick and homogeneous double oxyhydroxide film whose outer part is rich in niobium seems responsible for the excellent corrosion resistance of the Ti-18Nb-4Sn alloy.

Wetting of heterogeneous surfaces of block copolymers containing fluorinated segments

The wetting of well-characterized heterogeneous surfaces of block copolymers has been studied by low-rate dynamic contact angle measurements using axisymmetric drop-shape analysis. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the roughness, the heterogeneity and the chemical composition of the surfaces. By changing the block length of polysulfone and semifluorinated polyester segments in the block copolymers, the surface heterogeneity of thin films prepared on silicon wafers could be controlled. Tapping-mode AFM measurements showed that soft, hydrophobic domains of varying size on the submicrometer length scale were obtained on these surfaces (60–250 nm). The mean roughness was of the order of several nanometers. The results of the contact angle measurements showed that neither roughness nor heterogeneity had a significant effect on the advancing contact angle of water, at the scale of the features present; however, the contact angle hysteresis increased with increasing percentage of the soft domains. We assume that liquid retention by the solid upon retraction of the three-phase line is the main cause for the observed increase in contact angle hysteresis. Concerning the molecular composition of these block copolymer surfaces, angle-resolved XPS analysis showed a surface segregation of fluorine within the surface region. A direct correlation was found between the fluorine content of the block copolymer surfaces and the advancing contact angle of water.

Formation of Alkanethiol Monolayer on Ge(111)

Alkanethiols, CH3(CH2)(n-1)SH, are shown to react readily with HF-treated Ge(111) surface at room temperature to form a high-quality monolayer. The resulting films are characterized by using contact angle analysis (CAA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and high-resolution electron energy loss spectroscopy (HREELS). The octanethiol and octadecanethiol films on Ge(111) both exhibit 101 degrees and 40 degrees water and hexadecane contact angles, respectively. These values indicate that the thiol surface coverage is relatively high, and that the films possess a high degree of orientational ordering. The angle-resolved XPS analysis supports that thiols are bound to the Ge surface by Ge-S bonds at the monolayer/Ge interface. The film thickness values obtained by XPS and SE agree well with the earlier reported values on alkyl monolayers on Ge(111) prepared by Grignard reaction. On the basis of HREEL spectra taken after thermal annealing steps, the monolayers are found to be thermally stable up to 450 K. The thermal stability provides further evidence that thiols are covalently bonded to Ge(111).

Synthesis, characterization and anti-fouling properties of poly(ethylene glycol) grafted poly(vinylidene fluoride) copolymer membranes

Methoxypoly(ethylene glycol) monomethacrylate (PEGMA) graft-copolymerized poly(vinylidene fluoride) (PVDF) (the P(PEGMA)-graft-PVDF copolymer) was synthesized. The PVDF homopolymer in N-methyl-2-pyrrolidone (NMP) solution was first pretreated with ozone and the peroxide content of the ozone-treated PVDF was determined by assaying with 2,2-diphenyl-1-picrylhydrazyl (DPPH). The activated PVDF was then subjected to graft copolymerization with the PEGMA macromonomer in NMP. The microstructures and compositions of the P(PEGMA)-graft-PVDF copolymers were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS), elemental analysis, differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. Ultra-filtration (UF) membranes were prepared from the P(PEGMA)-graft-PVDF copolymer by the phase-inversion method. The bulk and the surface compositions of the membranes were determined by elemental analysis and XPS, respectively. In general, the graft concentration increases with the PEGMA macromonomer concentration. Angle-resolved XPS analyses of the UF membranes prepared from the copolymer revealed a substantial surface enrichment of the hydrophilic PEGMA component. The morphology of the membranes was studied using scanning electron microscopy (SEM). The UF membrane prepared from the P(PEGMA)-graft-PVDF copolymer with a graft concentration ([PEGMA]/[CH2CF2] ratio) above 0.06 was effective in preventing bovine serum albumin (BSA) adsorption.

Development of a lifting wavelet representation for surface characterization

This paper reviews the existing numerical analysis methods and their problems in surface metrology. Based on the requirements of functional analysis of surfaces, this paper proposes a lifting wavelet representation for extraction of different components of a surface. The theory of the lifting wavelet is introduced and a fast algorithm is developed. Different frequency components of the surface can be separated, extracted and then reconstructed according the intended requirements of functional analysis. The surface textures can be highlighted and multi-scalar topographical features can be identified and clearly recovered. In order to verify the behaviour of the new model, a computer simulation based on sinusoidal and triangular waveforms is used. Case studies are conducted using a series of typical surfaces of engineering and bioengineering, such as planes, cylinders and curves, measured by contact (stylus) and non-contact (phase-shifting interferometry) instruments, to demonstrate the feasibility and applicability of using the lifting wavelet model in the analysis of these surfaces.

X-ray photoelectron spectroscopy characterization of AlGaN surfaces exposed to air and treated in NH4OH solution

Chemical properties of Al0.17Ga0.83N surfaces exposed to air and treated in an NH4OH solution were systematically investigated by x-ray photoelectron spectroscopy (XPS). An air-exposed sample showed highly nonstoichiometric surface which included a large amount of Ga and Al oxides. The angle-resolved XPS analysis revealed that the natural oxide layer possessed a complicated composition distribution in depth and that the Al-oxide component was dominant on the topmost layer. A drastic reduction of such Al-oxide component as well as Ga-related oxide was achieved after the treatment in an NH4OH solution at 50 °C for 10 min, resulting in a constant in-depth composition distribution. The NH4OH-based treatment was found to enhance the intensity of the E2 (high) Raman peak and to reduce the root-mean-square value of surface roughness.

Molecular structure of interfaces between plasma-polymerized acetylene films and steel substrates

Plasma-polymerized films of acetylene were deposited onto steel substrates in an inductively coupled reactor by exciting the plasma in an argon carrier gas and then injecting the monomer into the afterglow region. The molecular structure of the film/substrate interface was determined using reflection-absorption infrared spectroscopy (RAIR) and X-ray photoelectron spectroscopy (XPS) to characterize the films as a function of thickness. RAIR showed that thick (∼ 900 A) as-deposited plasma-polymerized acetylene films had a complicated molecular structure and contained methyl and methylene, mono- and disubstituted acetylene, vinyl, and cis- and trans-disubstituted olefin groups. Evidence of oxidation resulting from the reaction of trapped radicals with atmospheric oxygen and moisture to form O-H and C=O groups was also obtained. The molecular structure of thin films (∼ 60 A) was similar although evidence was obtained to indicate that acetylide groups (H-C≡C - were present at the film/substrate interface. Results obtained using angle-resolved XPS analysis showed that carbonaceous contamination was removed from the substrate and that oxides and hydroxides on the substrate surface, especially FeOOH, were chemically reduced during deposition of the films. XPS also confirmed that plasma-polymerized acetylene films deposited on steel substrates contained C-O- and C=O groups. Preliminary results also showed that films deposited in an inductively coupled reactor were good primers for rubber-to-metal bonding, whereas films deposited in a capacitively coupled reactor were not. The differences may be due to the wide variety of functional groups found in the former type of films but not in the latter.