Atomically Flat Silicon Oxide Monolayer Generated by Remote Plasma

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Atomically Flat Silicon Oxide Monolayer Generated by Remote Plasma Dickson Thian, Yonas T. Yemane, Manca Logar, Shicheng Xu, Peter Schindler, Martin M. Winterkorn, J Provine, and Fritz B. Prinz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00768 • Publication Date (Web): 30 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

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The Journal of Physical Chemistry

Atomically Flat Silicon Oxide Monolayer Generated by Remote Plasma Dickson Thian1#, Yonas T. Yemane1#, Manca Logar2, Shicheng Xu3, Peter Schindler3, Martin M. Winterkorn3, J Provine3, Fritz B. Prinz3,4* 1

Department of Applied Physics, Stanford University, Stanford, CA 94305, USA

2

Laboratory for Materials Chemistry, National institute of Chemistry, Ljubljana, Slovenia

3

Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA

4

Department of Material Science and Engineering, Stanford University, Stanford, CA 94305, USA

#

These authors contributed equally to this work.

*Corresponding author: [email protected], (650) 725-2018

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Abstract We demonstrate stable, atomically-smooth monolayer oxidation of Si(111) using a remote plasma. Scanning tunneling microscopy (STM) confirms the atomically flat nature of the oxidized surface while cross sectional transmission electron microscopy (TEM) proves the monolayer to bilayer oxide thickness. Fourier transform infrared spectroscopy (FTIR) and atomic layer deposition (ALD) indicate oxygen is incorporated onto the silicon surface in the form of Si-O-Si and Si-OH bonds. The incorporation of Si-OH bonds is inferred by using TiCl4, a highly specific ALD precursor, for TiO2 ALD. This plasma technique provides precise control of the surface chemistry and yields abrupt yet stable SiO/Si interfaces. It enables production of atomically flat, ALD-active silicon surfaces that could serve as a well-defined platform for investigation of various surface chemistries via STM. Using this substrate, we present the first ever STM observations of ALD TiO2 on silicon oxide.


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Introduction Continued device scaling in the microelectronics industry requires further development of atomic layer deposition (ALD) to enable deposition of more conformal and stable ultrathin films1. In the model ALD system, each precursor cycle results in one complete monolayer of material deposition. In practice, however, most precursor-substrate combinations involve an initial nucleation phase2,3. This nucleation phase is not well understood and often overlooked when growing films over 10-20 nm in thickness. Surface termination has been shown to significantly influence the initial growth of ALD4. This indicates nucleation is strongly controlled by interfacial and surface energies between the precursor and the substrate. The nucleation phase limits the minimum thickness of complete coverage films in many ALD chemistries. To better understand the nucleation phase, novel characterization methods capable of resolving individual nucleation sites must be developed. Commonly used characterization techniques for ALD include Fourier transform infrared spectroscopy (FTIR)5, X-ray photoelectron spectroscopy (XPS)6– 8

and more recently, scanning tunneling microscopy (STM)2,9. Each method provides different but

complementary information for characterization. A scanning probe method such as STM is valuable because it enables direct spatial observation of nucleation on the substrate surface. For conductive substrates, STM offers numerous advantages such as atomic resolution10, feature tracking, and local density of state (LDOS) measurements. STM observation of ALD requires the underlying substrate to be atomically smooth prior to deposition. Atomically smooth substrates such as Au(111) and SrTiO3(001) have been used to observe the nucleation phase of ZnS ALD2 and Pt ALD11, respectively. However, substrates typically used for ALD in research and industry, such as HF etched silicon or RCA cleaned SiO2, have root mean square roughness > 1 nm and, hence, do not fulfil this smoothness criterion12. The surface roughness of these substrates may exceed the size of individual ALD nucleation sites, preventing direct observation via STM. The inability to make atomically smooth silicon-based ALD substrates poses difficulties for STM surface observation of ALD growth. 3 ACS Paragon Plus Environment


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Surface roughness also has a drastic effect on silicon-based nanoscale device performance. In these devices, it is best that surface and semiconductor-insulator interfaces be flat at the atomic scale. Surface roughness increases tunneling currents and the number of traps and charges at the interfaces13. Furthermore, a flat surface minimizes carrier scattering and electric field fluctuations in electron transport14. To minimize surface roughness on silicon, an ex-situ wet etching process can be used to produce an atomically smooth hydrogen terminated Si(111) (Si(111):H or Si:H) surface15,16. However, Si:H is not an ideal substrate for ALD as the hydrogen termination can render the substrate unreactive to many ALD processes. In contrast, hydroxyl surface termination has been shown to initiate the reaction in many ALD chemistries17,18. Michalak et. al.15 showed it is possible to convert ~30% of the –H termination of a smooth Si(111):H surface to an –OH termination (Si(111):OH) without greatly affecting the surface roughness. However, the method requires arduous care as it involves multiple wet chemical steps, each of which can contaminate the surface with particulates and chemical residues. The chemical treatment process also may etch the surface, affecting atomic flatness19. Another method of obtaining –OH termination is to create an ultrathin oxide layer. This has been attempted using nitric acid20,21, hydrogen peroxide22 and rapid thermal oxidation12. In each case, however, surface smoothness is compromised. An alternative method for surface modification— treating etched bare silicon with oxygen plasma—has been shown successfully to oxidize the topmost surface to varying extents23–26. Depending on the plasma treatment method, the thickness of the oxide layer grown ranges from approximately one nanometer26 to hundreds of nanometers24. Most recent studies23,27 show that the oxide layers produced using oxygen plasma have similar properties to those grown using conventional thermal methods. Plasma processing also allows relatively good control of the layer thickness and reduces trap state densities. But the effect of specific plasma parameters on surface roughness has not been well investigated. Previous studies indicate surface roughness increases with increasing oxygen incorporation and SiO2 thickness, especially for the first few nanometers of surface oxide28,29.


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To utilize STM to observe ALD nucleation growth on a silicon surface, the substrate should fulfil three criteria: it should be (1) atomically flat to clearly observe ALD growth, (2) -OH terminated, and (3) sufficiently conductive for STM. These three criteria point to a silicon sample with an ultrathin oxide layer at the surface that is flat and maintains conductivity. However, current techniques for generating oxide terminated silicon cannot satisfy these criteria simultaneously. Atomically flat oxide substrates show only partial –OH termination and are prone to surface contamination that hinders STM15. Fully –OH terminated oxide substrates are not atomically flat and have insulating oxide layers too thick for the STM tunneling current to penetrate. In this paper, we present a method using low power remotely generated inductively coupled plasma (ICP) to modify an atomically smooth Si(111):H surface to incorporate a monolayer of oxygen. The optimized plasma treatment terminates the surface with –OH bonds while preserving atomic scale flatness. This process can be performed below 150 °C, which is ideal for further semiconductor processing. We will show that this process is robust and repeatable, and provide evidence of a resulting atomically flat surface that is both suitable for STM observation and active for ALD. Specifically, we used STM, TEM and FTIR to provide evidence of the incorporation of oxygen onto the resultant flat surface, and used a highly specific ALD chemistry, the deposition of TiO2 via TiCl4 and H2O, to indirectly probe the –OH termination of the surface. With the aforementioned properties, this process yields a practical platform to study ALD nucleation. The –OH surface termination provides chemical similarity to thicker silicon oxide layers more typically used as ALD substrates, while the atomically smooth surface enables STM observation capable of resolving individual nucleation sites. As a proof of concept, we examine the nucleation phase of TiO2 ALD to show the advantages of this substrate. This represents the first STM observations of ALD on an oxide terminated silicon surface and opens up the possibility of studying the nucleation phase of many other ALD chemistries.



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Experimental Section Four-inch phosphorus doped Si(111) wafers (ρ = 320-480 Ω-cm) were used for all substrates mentioned in this paper. For STM work, 2 cm x 1 cm samples were cleaved from the wafers. Ammonium fluoride (NH4F) solution (Aldrich, 40%, semiconductor grade) was first sparged with ultra-high purity (>99.999%) argon in a Teflon beaker for at least 30 minutes to remove traces of dissolved oxygen before being used for etching16. The STM used in this paper is a home-built station capable of high vacuum pressures and atomic resolution9. The STM scanner is modified from an Agilent 5500 SPM system and mechanically cut Pt/Ir tips were used as the scanning probe. STM images were processed using plane subtraction and line leveling, which are described briefly in the Supporting Information. The remote plasma tool used for plasma treatment is a Fiji F202 ALD system (Cambridge Nanotech / Ultratech) which uses an inductively coupled plasma (ICP) method for plasma generation. Subsequent ALD of TiO2 was performed in another home-built hot-wall reactor. Fourier transform infrared spectroscopy (FTIR) was conducted on a nitrogen purged Thermo-Fisher IS50 FTIR system with a Ge attenuated total reflectance (ATR) accessory (Harrick’s Scientific) and a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. Five hundred and twelve spectra were averaged for each sample and background subtraction was performed using the OMNIC software. The ATR method was favored over transmission method because of its increased sensitivity to surface bonds. The transmission electron microscope (TEM) used in this experiment is an aberration corrected JEOL ARM 200 CF capable of both atomic resolution images in Scanning TEM (STEM) mode, as well as electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX) with high spatial resolution. X-ray photoelectron spectroscopy (XPS) was performed for elemental analysis of samples on a SSI S-Probe XPS Spectrometer and a PHI VersaProbe Scanning XPS Microprobe. Si(111) samples were etched in argon-sparged ammonium fluoride (NH4F) sequentially for 10 min, 10 min and 3 min. NH4F anisotropically etches Si(111) by preferentially attacking step edges16, leaving behind atomically flat, hydrogen terminated silicon terraces. The NH4F solution was replaced between 6 ACS Paragon Plus Environment

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each etching cycle. The first etching step removes most of the silicon dioxide layer, and any surface impurities and contaminants. The second etching step removes surface contamination that lies immediately below the surface of the silicon dioxide layer. The last etching step was time-optimized for flattening the surface while minimizing pit formation. For piranha treatment, samples were placed in 90 °C piranha solution (40% H2O2, 98% H2SO4; 1:3 by volume) for 30 minutes before rinsing with 18 MΩcm MilliQ H2O for 30 s. [Caution: This process is hazardous. The reaction between H2O2 and H2SO4 is highly exothermic and possibly explosive. Proper training is necessary before handling these chemicals.] Samples were placed in a nitrogen glovebox (< 0.5 ppm H2O, < 0.04 ppm O2) immediately after treatment to minimize silicon oxidation or excessive carbon contamination. Si(111):H remains stable for weeks in this nitrogen environment. For plasma treatment, samples were placed in a Fiji ALD system and treated with an Ar and O2 low power plasma mixture with varying power conditions. Including argon in the gas mixture helps strike the plasma, enabling the use of low power plasma processing. Argon is a spectator species in the plasma treatment. No argon was detected in any subsequent XPS sample analysis. Unless otherwise stated, 200 sccm of Ar and 30 sccm of O2 were used in the plasma gas mixture. During plasma treatment, a separately plumbed 60 sccm Ar supply acts as a purge gas to assist in the removal of plasma species from the sample. The sample temperature for all plasma treatments was 150 °C. As will be presented in the results, this plasma treatment re-oxidizes the sample to create an ultrathin oxide layer which can be as thin as only a single monolayer. Samples were then removed and placed either in the STM’s vacuum chamber, the nitrogen purged FTIR chamber, the vacuum ALD chamber or prepared for conventional cross sectional TEM immediately after plasma treatment. For TiO2 deposition, the precursors used were TiCl4 and distilled H2O. With the sample at 140 °C, 45 alternating cycles of TiCl4 and H2O were performed, with Ar gas flowing between each precursor pulse to purge any remnant precursor. After deposition, all samples were analyzed via XPS for Ti content. For STM analysis, the TiO2 ALD process was repeated for 15 cycles of deposition. 7 ACS Paragon Plus Environment


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TEM simulation was performed using QSTEM code for the quantitative simulation of STEM images30. Results and Discussion STM requires atomically flat substrates due to the sensitivity of the tunneling process. Therefore, minimizing surface corrugation is essential for optimal STM substrates. Figure 1a shows an STM topograph of Si(111):H produced using the NH4F procedure, and Figure 1b shows the corresponding cross sectional TEM image. The STM topograph of Si(111):H shows several atomically flat terraces, each 20-50 nm wide with minimal surface corrugation as indicated by rms roughness. The measured surface corrugation on each terrace is 0.20 ± 0.03 Å. The height differential between adjacent terraces is approximately 3 Å in height which corresponds to a single atomic bilayer of silicon31. The inset shows an atomically resolved image of Si(111):H illustrating the 1x1 atomic arrangement of a hydrogen terminated silicon surface. This provides additional evidence that the surface is completely hydrogen terminated. The inset also shows two typical defects on the silicon surface, which are most likely missing silicon atoms (which appear as darker spots) and missing hydrogen atoms (which appear as brighter spots)32. In the TEM image, the silicon surface is very well-defined with a sharp Si-H interface. The characteristic dumbbell structure for crystalline silicon atoms can be resolved. Hydrogen atoms cannot be resolved by the TEM and do not appear in the image. Figure 1c and d show a sample after treatment with 300 W remote plasma for 300 s in the Fiji ALD system. In the STM image, the silicon surface is clearly more corrugated with increased surface roughness within the terraces. Atomic step heights remain the same between terraces. Although corrugation of the terrace surface increased slightly, the surface remains atomically smooth with a measured corrugation of 0.32 ± 0.07 Å. Note that the corrugation is an order of magnitude less than a single atomic layer. This is important for subsequent STM observations of ALD growth because the height of a potential nucleation site (≥ 1 atomic layer) still greatly exceeds the surface corrugation. Note that STM does not measure actual topography of the surface, but it is an indirect probe of the electronic


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density of the surface. Therefore, topography as measured by STM can be described loosely as a convolution of topography and local conductivity at the tip position. As such, the corrugation seen can be caused by either actual topography (physical roughness), or by local variations in electronic density due to implantation of oxygen atoms in the silicon crystal lattice. Most likely, the corrugation is a combination of both. The darker areas of the corrugation correspond to areas of lower conductivity and indicate either O atoms on the surface or in the underlying Si lattice33. Based on our STM images and observations from Watanabe et. al.34, the plasma treated sample yields a surface less than two monolayers thick that is smoother than oxides formed by other methods34. The TEM image in Figure 1d provides additional insight into how the plasma treatment modified the surface. The interface between silicon and glue is no longer a sharp interface as in Figure 1b. The first terminal layer of dumbbells that can be resolved is slightly distorted in and out of the plane and is no longer in focus. This distortion represents a disruption in the silicon lattice, likely due to oxygen atoms. The STM images and complementary TEM images suggest the remote plasma treatment did not sputter the surface, and the slight increase in surface corrugation is due to charge density variations when oxygen is incorporated. Figure 2a shows a zoomed in view of the cross sectional bright field STEM (BF-STEM) image. The single row of atoms lying on top of the terminal silicon plane is marked by white arrows. EELS was performed across the interface as marked in Figure 2b. Near the edge of the silicon sample, a loss peak is observed at 532 eV, corresponding to the oxygen K-edge loss peak. No oxygen is detected in the area with the carbon glue. This suggests oxygen is localized around a narrow band < 1 nm above the silicon surface. In fact, Figure 2a shows only a single monolayer of oxygen is present. To further validate the cross sectional TEM results, a simulation model was constructed (Figure 2c) where one atomic layer of oxygen was placed at the terminal plane of a monocrystalline Si(111) surface. Above the oxygen atoms, a layer of amorphous carbon was added to best approximate the carbon glue used in TEM sample preparation. Parameters used for the simulation were in accordance with the experimental parameters.



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The simulation shows an approximation of the expected TEM image and supports the experimental TEM observations in Figure 2a. FTIR analysis gives deeper insight into the chemical bonds formed after plasma processing. Figure 3 shows representative FTIR spectra taken of six different samples: (1) a freshly etched Si:H sample, (2) an etched sample treated with low power 40 W + 10 W plasma for 30 s (40 W power for the initial 10 s to strike the plasma, 10 W power for subsequent 20 s), (3) an etched sample treated with low power 25 W plasma for 60 s, (4) an etched sample treated with high power 300 W plasma for 300 s (previously used for STM and TEM analysis in Figures 1c and 1d), (5) an etched sample treated with high power 300 W plasma for 1 hr, and (6) a silicon sample with its native oxide, cleaned for 30 min in 90 °C piranha. Since the etched Si:H sample has no surface oxide, it shall be treated as the reference spectrum for comparison. The Si-H stretching mode (ߥSi-H) can be seen as a very sharp peak at 2085 cm-1, suggesting a high degree of uniformity of the Si-H bonds perpendicular from the surface15. But after plasma treatment, the Si-H stretching mode completely disappears, indicating that no Si-H bonds can be detected on the surface. There are multiple signatures for the Si-O bond depending on the vibrational mode, the bond environment, and the technique used for IR measurements. Typically the longitudinal optical (LO) and transverse optical (TO) vibrational modes for Si-O-Si bonds lie in the 1100 cm-1 to 1250 cm-1 region35, while the Si-OH modes lie around 800 cm-1.15 The Ge ATR characterization method is typically more sensitive to the LO mode at 1250 cm-1. Unfortunately, it is not sensitive to the weak Si-OH bond at 800 cm-1 because the IR transmission characteristics of the Ge crystal greatly deteriorate below 1000 cm-1. Compared to the reference Si:H spectrum, the low power plasma treated samples show an additional broad absorbance peak around 1200 cm-1. This absorbance increases and shifts to higher energies (1217 cm-1) with increasing plasma power, corresponding to the LO mode for Si-O-Si bonds. With the high power plasma treated samples, a small shoulder develops around 1150 cm-1 which corresponds to the TO mode for Si-O-Si. The piranha cleaned native SiO2 sample shows both the LO and TO Si-O-Si modes at 1234 cm-1 and 1150 cm-1, respectively. With increasing intensity of the plasma treatment there is a


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gradual increase in the LO peak intensity, as well as a gradual shift in the vibrational energy. (Figure S1 of the Supporting Information displays the same data normalized by subtracting the Si:H spectrum, which more clearly shows the gradual shift in energy and the TO mode.) Note that the 300 W, 1 hr treated sample has a LO peak position higher than the piranha cleaned native SiO2. Via EELS and XPS, Schaefer et al.36 showed that the asymmetric stretching mode vibrational frequency of the oxide increases linearly with the surface coverage for low coverage of oxygen atoms on a silicon surface. We hypothesize that the similar shift that we see in the LO mode can be explained by the same phenomenon, that there is a slight change in the average Si-O-Si bond angle in the ultrathin oxygen layer as plasma-induced oxygen incorporation increases. It has also been shown that the LO mode occurs at a slightly different frequency in oxides grown by different methods37 and of different thickness38. This is likely related to both the bond angle as well as the local bond environment (including next nearest neighbor influence) and consequently the amount of strain placed on the Si-O-Si bond38. Our results suggest that, even at very low power, the remote plasma completely strips all hydrogen atoms from the surface of Si:H. Oxygen is then incorporated into the silicon surface. The amount of oxygen incorporation is a function of both plasma power and duration. Further incorporation of oxygen into the silicon to allow for thicker oxide formation is likely diffusion limited and thus temperature dependent. This has been elucidated by studies in thermal oxidation39, although for oxide thicknesses much greater than in the current study. The maximum oxide thickness grown using this remote plasma technique has been suggested to be limited by a space charge effect40 where the plasma imparts a residual charge onto the silicon surface and causes a “plasma sheath” where further attack by plasma is prevented by electrostatic repulsion. This allows for as thin an oxide layer as possible which minimizes surface corrugation. Using a simple model, increasing surface oxidation can be shown to create inherent roughness of the surface41. In fact, the TEM results suggest that for the plasma treatments attempted in this study, only one to two monolayers of oxygen were incorporated onto the silicon surface, in contrast with 1-2 nm minimum thickness shown in other studies25,26,42. This leads to a very sharp Si/SiOx interface. 11 ACS Paragon Plus Environment


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All TEM/STM/FTIR measurements were performed on samples after a brief atmospheric exposure during instrument transfer. This suggests the Si/SiOx interface is at least temporarily stable in atmosphere. The continued stability of these samples with respect to lifetime and temperature requires further study. FTIR suggests the plasma treatment likely incorporated oxygen atoms onto the surface of silicon in a random fashion and probably created a mix of Si-O- dangling bonds and Si-O-Si bonds. Once in atmosphere, Si-O- dangling bonds will form Si-OH bonds, which are believed to be the most active species during an ALD reaction and of most interest in this study. Although the number of Si-O-Si bonds is directly correlated to the number of surface Si-OH bonds formed, the Ge FTIR-ATR method was unable to directly measure the presence of Si-OH bonds on the Si surface. In order to gauge the ALD reactivity of the surface, the same number of TiO2 ALD cycles was performed on all samples using TiCl4 and H2O. TiCl4 is highly selective toward –OH bonds while its deposition on hydrogen terminated Si surfaces is strongly inhibited17,43,44. In this regard, the presence of Ti within the first few ALD cycles can be an indirect measure of the –OH bonds present on the Si surface. The results of the XPS analysis of the various treated samples after TiO2 ALD are shown in Figure 4. For an objective comparison, ALD on the various samples was performed concurrently. We have also included deposition on an untreated native silicon oxide sample for comparison. From the Ti 2p XPS peak height, the –OH bond saturation on the surface can be inferred to decrease in the following order: piranha cleaned native SiO2; an Si:H sample after 25 W, 60 s plasma treatment; an Si:H sample after 300 W, 300 s plasma treatment; an Si:H sample after 300 W, 1 hr plasma treatment; an Si:H sample after 30 s plasma treatment consisting of 40 W, 10 s and 10 W, 20 s power (the higher initial power helps to strike the plasma); and untreated native SiO2 sample. Ti is not detected on the Si:H sample. To explain these results, we note that piranha treatment has been shown to be particularly effective in removing carbon contamination from the surface and hydroxylating the silicon oxide surface, saturating the surface with –OH bonds. Accordingly, the highest TiO2 growth was detected on the piranha cleaned


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native SiO2 sample. In comparison with the untreated native SiO2 sample, all plasma treated samples show higher ALD activity. To explain the difference in activity between the different plasma treated samples, we hypothesize that the intensity of plasma treatment has a dramatic effect on the active surface Si-OH coverage. For the 30 s treated sample, the plasma likely did not oxidize the surface completely, resulting in reduced TiO2 deposition. As hypothesized, Si-O- dangling bonds are formed in this plasma process. As the length of treatment increased, Si-Si bonds adjacent to these dangling bonds are attacked by the plasma and broken, facilitating the formation of Si-O-Si bonds. The ratio of surface Si-O- dangling bonds to Si-O-Si bonds directly relates to the ALD activity of the sample. The 25 W, 60 s treatment and 300 W, 300 s treatment appear to be the best plasma parameters showing the highest level of activity for ALD. A wide range of plasma power and duration may provide suitable surface activation for ALD. Increasing plasma duration to 1 hr, more dangling bonds form Si-O-Si bridge bonds which reduces the activity of the surface, leading to lower TiO2 deposition. This corroborates the FTIR data in Figure 3, which shows increasing incorporation of Si-O-Si bonds on the surface with increasing plasma intensity. The only outlier in the FTIR data is the piranha cleaned native SiO2, which shows a lower Si-O-Si absorption energy but a greater amount of TiO2 deposition in the XPS. This discrepancy can be explained by the fact that the Si-O-Si was formed as native oxide (~2 nm thick) at the manufacturer by slow oxidative processes such as thermal oxide growth through diffusion, allowing for greater lattice relaxation. In contrast, the Si-O-Si formed by the plasma process is likely to be strained and confined to a few monolayers on the surface. In line with the goal of obtaining an atomically flat substrate suitable for STM observations of the nucleation phase of ALD, Figure 5 presents STM observations of the nucleation phase of TiO2 ALD using TiCl4 and water as precursors. This represents, to our best knowledge, the first ever observations of the nucleation phase of ALD on an oxide substrate using STM. Figure 5a and b show STM surface topographs of 15 complete cycles of ALD TiO2 deposited on a Si:H substrate and a substrate after 300 W, 300 s plasma treatment (as described above), respectively. The 13 ACS Paragon Plus Environment


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white dashed lines indicate line profiles for each topograph. Comparing with the STM topographs prior to ALD (Figure 1a and c), there is clearly little to no deposition on the Si:H sample after 15 cycles of TiO2 ALD, reiterating previous studies that hydrogen terminated silicon is passive for ALD via TiCl417,45. On the plasma treated sample, deposition of TiO2 is apparent. The atomic steps of silicon are mostly obscured due to the uniform deposition. Each circular feature seen in the topograph is likely an individual nucleation seed of TiO2, ranging from 2 to 6 Å in height. All STM observations were corroborated by XPS analysis shown in Figure 5c. No Ti signal was detected on the SiH substrate while the plasma treated sample displayed a complete Ti 2p spectrum. This drastic difference in deposition behavior has been explained in literature17. The hydrogen passivation of Si:H produces a high kinetic barrier for the adsorption of Ti; once adsorbed, the release of Cl atoms is also energetically unfavorable. In contrast, Ti adsorption and the release of Cl atoms are much more favorable on the OH terminated silicon surface. STM observation of ALD on an oxide substrate proves the advantages of an atomically flat, silicon oxide monolayer, and demonstrates the power of using a high resolution technique such as STM in observing the nucleation stage of ALD. These STM observations can complement traditional techniques such as XPS and ellipsometry, both of which are insufficient for probing incomplete nucleating films.

Summary and Conclusions We have demonstrated the surface monolayer oxidation of atomically flat, hydrogen terminated silicon using a remote plasma. The oxide generated remains atomically flat which is crucial for subsequent scanning probe analysis (e.g. observation of ALD nucleation). High resolution TEM images have revealed monolayer oxygen incorporation onto the silicon surface, corroborating the limited surface corrugation seen via STM. Both FTIR spectroscopy and XPS have proven oxygen is incorporated in SiO-Si and Si-OH bonds, with the surface bonding dependent on plasma power and duration. The surface 14 ACS Paragon Plus Environment

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saturation of Si-OH bonds was indirectly measured with TiO2 ALD by using the chemical specificity of TiCl4 to react only with –OH groups. The level of TiO2 ALD on the plasma treated samples was comparable to that of fully –OH saturated piranha cleaned SiO2. These atomically flat oxide substrates were used as a platform to observe TiO2 ALD via STM. TiO2 nucleation sites were observed directly on the atomically flat oxide substrates as compared to no growth on the Si:H substrates, corroborating previous literature on TiCl4 selectivity. This represents the first observation of ALD on an oxidized silicon substrate using STM, previously not possible because of conductivity and flatness requirements. The reported plasma-based process enabled the production of samples that are atomically flat, active for ALD and have an abrupt atomically smooth SiOx/Si interface. It can be used to obtain ALD active, smooth samples suitable for scanning probe observation to further elucidate ultrathin film deposition on atomically defined surfaces. This platform, combined with high resolution STM could reveal interesting ALD physics not apparent with techniques such as XPS, especially in the nucleation phase. The method may also be used for functionalization of etched silicon surfaces for subsequent low temperature thermal ALD of dielectrics on the atomically smooth surface. The stable and sharp oxide interface might help reduce silicon surface trap densities and consequently facilitate further optimization in microelectronics. 1. Acknowledgements The authors thank Chris Jezewski, Harsono Simka, Scott Clendenning and Don Gardner of Intel Corp. and Jan Torgersen for useful discussions and suggestions. This research was funded by the Semiconductor Research Corporation under Task ID 2454.001. Plasma treatment was performed in part at the Stanford Nanofabrication Facility (SNF), which is supported by National Science Foundation through the NNIN under Grant ECS-9731293, and the authors would like to thank Michelle Rincon and the SNF staff for assistance with the SNF tools. D.T. gratefully acknowledges fellowship support from the Agency



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for Science, Technology and Research Singapore. Y.Y. gratefully acknowledges fellowship support from the Ford Foundation and the Stanford DARE program. 2. Associated Content Supporting Information Expanded discussion of FTIR spectra, STM characterization of various plasma conditions, XPS analysis of plasma treated silicon surfaces, cross-sectional TEM of various plasma conditions, and explanation of STM image processing. This material is available free of charge via the Internet at http://pubs.acs.org.

3. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

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Atomically Flat Silicon Oxide Monolayer Generated by Remote Plasma

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