J. Phys. Chem. B 2006, 110, 10365-10373
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Kinetics of Atomic Force Microscope-Based Scanned Probe Oxidation on an Octadecylated Silicon(111) Surface Menglong Yang, Zhikun Zheng, Yaqing Liu, and Bailin Zhang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry and Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: September 19, 2005; In Final Form: March 1, 2006
Atomic force microscope (AFM)-based scanned probe oxidation (SPO) nanolithography has been carried out on an octadecyl-terminated Si(111) surface to create dot-array patterns under ambient conditions in contact mode. The kinetics investigations indicate that this SPO process involves three stages. Within the steadily growing stage, the height of oxide dots increases logarithmically with pulse duration and linearly with pulse voltage. The lateral size of oxide dots tends to vary in a similar way. Our experiments show that a direct-log kinetic model is more applicable than a power-of-time law model for the SPO process on an alkylated silicon in demonstrating the dependence of oxide thickness on voltage exposure time within a relatively wide range. In contrast with the SPO on the octodecysilated SiO2/silicon surface, this process can be realized by a lower voltage with a shorter exposure time, which will be of great benefit to the fabrication of integrated nanometersized electronic devices on silicon-based substrates. This study demonstrates that the alkylated silicon is a new promising substrate material for silicon-based nanolithography.
Introduction Scanned probe oxidation (or tip-induced local anodic oxidation, abbreviated as SPO or LAO, respectively) using a conductive atomic force microscope (C-AFM) has been developing into a routine technique for nanopatterning and nanofabrication for its availability, reliability, reproducibility, and easy operation.1-5 In the past decade, the AFM-based local oxidation on the silicon-based substrates has attracted much interest for its potential to fabricate nanometer-sized integrated systems, such as functionalized nanotemplates, chemical or biological nanosensors, and electronic and optoelectronic nanodevices.1,6-19 Several kinds of silicon-based substrates, such as hydrogen-terminated silicon,5,20,21 silica-coated silicon,22,23 alkylsilated SiO2/silicon,9-19,24-33 and SiO2/silicon modified with another organic film,34-36 have been popularly used in this process. However, the H-terminated silicon surface can be spontaneously oxidized in an ordinary atmosphere; the preoxidation layer of silica-coated silicon greatly degrades the mask effectiveness of the SPO oxide; as for the alkylsilated SiO2/ silicon, the charge-trapped oxide layer existing between the silicon bulk and alkylsilyl monolayer possesses a high density of electrical active defects, degrading the electrical characteristics of the semiconductor. It is possible to avoid these inherent disadvantages of the above substrates with alkyl-terminated silicon. Since Linford and Chidsey’s pioneering work of attaching 1-alkenes on H-terminated Si(111) surfaces through a hydrosilylation reaction to form the Si-C linked robust alkyl monolayers,37 such alkylated silicon surfaces have been consistently focused on for their several outstanding advantages over either H-terminated silicon or alkylsilated SiO2/silicon:38-43 (1) A large atomically * Corresponding author. Phone/Fax: +86 431 5262430. E-mail: blzhang@ ciac.jl.cn.
flat surface of densely packed alkyl monolayers can be routinely prepared with greater reproducibility, homogeneousness, and terminal variability than the alkylsilyl monolayers;42 (2) a true silicon/organic monolayer interface possesses excellent electrical characteristics;44-47 (3) high thermal stability43,48 and excellent chemical stability49-52 ensure the compatibility with pattern transfer processes. These features indicate that such alkylated silicon is a promising substrate for scanning probe microscope (SPM)-based nanofabrication. Recently, AFM-based SPO nanolithography has been applied on the alkylated silicon to fabricate the protrudent silicon oxide patterns as a mask for selective etching53-56 or to functionalize the alkyl derivative monolayers to fabricate templates for protein arrays.57,58 Both potential and reliability for pattern transfer have been demonstrated. To get a better control and more detailed insight of this SPO process, a kinetics investigation is essentially necessary. Although the kinetics of SPO on H-terminated silicon surfaces has been extensively investigated,5,59-66 only a couple of kinetic investigations have been made on alkylsilated SiO2/silicon so far.24,29 As for SPO on alkylated silicon, to our knowledge, few investigations on the kinetics have been reported to date. Several investigations indicate that the SPO process on organic-filmmodified SiO2/silicon is more complex than that on H-terminated silicon.29,33-36,67 Just recently, we have conducted a SPO process on a 1-octadecene passivated Si(111) surface in the tip-moving oxidation fashion to produce the continuous oxide lines.68 However, the line-writing experiments of that could not directly clarify the variation of SPO domains with the voltage exposure time. In this study, we carried out the SPO process in the stationary oxidation fashion to produce oxide dot arrays under ambient conditions in order to detail the different stages of the SPO process and focused on the kinetic dependences of oxide growth on the applied voltage and exposure time.
10.1021/jp0553030 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/11/2006
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Figure 1. Contact mode AFM topography and friction images of dot arrays produced by SPO under an ambient humidity of ∼60%. The oxidation parameters of pulse voltages and durations for each oxide dot are listed in the scheme in the bottom right corner. For each voltage, 16 dots were oxidized for different durations in every two rows from top to bottom. Topography images were synchronously obtained at a -90° scan angle in the forward scanning (a) and backward scanning (b) or at a 0° scan angle in the forward scanning (d) and backward scanning (e). Friction image c was obtained in the forward scanning at a -90° scan angle. The arrows in this figure indicate the fast scanning direction. Topographic images were displayed under a gray z scale of 0-3 nm in order to highlight the low oxide dots. The high features over 3 nm were all shown as saturated white color.
Experimental Section Materials. Single-side polished p-type Si(111) wafers (borondoped, resistivity of 10-35 Ω cm) were purchased from Institute of Microelectronics of Chinese Academy of Sciences (Beijing, China). 1-Octadecene (C18H36, tech. 90% grade from Acros Organics) was vacuum-distilled, and the intermediate fraction (∼60 vol %) was collected and then stored at 4 °C in the dark. It was melted at room temperature and deoxygenated by bubbling with pure Ar gas for at least 1 h just before use. The reagents for silicon wafer cleaning and preparation, including ammonium fluoride (40% solution), hydrofluoric acid (40%), hydrochloric acid (36%), hydrogen peroxide (30%), and sulfuric acid (98%), were all MOS grade from Beijing Institute of Chemical Reagents (Beijing, China). Other reagents, including ammonium sulfite ((NH4)2SO3‚H2O), are all of analytical reagent (AR) grade from Beijing Chemical Reagents Company (Beijing, China). Ultrapure water (18 MΩ cm) was obtained from a Millipore system. Preparation of Octadecyl-Terminated Si(111). The octadecylated Si(111) was prepared through the same process that we previously reported.68 In brief, a flat H-terminated Si(111) surface was first prepared by controlled etching in the deoxygenated NH4F solution, and then, the octadecylated Si(111) was formed by the UV-promoted reaction between the H-terminated Si(111) surface and 1-octadecene. The octadecyl monolayer was checked by water contact angle measurement. The contact angles of 109 ( 3° for octadecene-modified silicon samples and 78 (
2° for H-terminated silicon samples indicated that an octadecyl monolayer had been formed on the silicon surface with wellorganized film structures.37 The monolayer’s surface roughness was evaluated in terms of the root-mean-square (rms) roughness and the average roughness (Ra) from AFM images. The roughness values of rms < 0.15 nm and Ra < 0.1 nm in optional 5 µm × 5 µm (some times 2 µm × 2 µm) square areas indicated that highly uniform flat surfaces were formed. SPM Oxidation. Experiments were carried out using an AFM (SPA-400 SPM Unit + SPI-3800N Probe Station, Seiko Instruments Inc., Japan) with a Pt-coated silicon cantilever (nominal tip radius of about 25 nm, rectangular cantilever with a length of 450 µm, a width of 50 µm, and a force constant of 0.2 N/m, Innovative Solutions Bulgaria Ltd.). The oxidation and imaging were both performed in contact mode under ambient conditions at a room temperature of 22 ( 2 °C and relative humidity (RH) of ∼60 or ∼70%. SPO processes were performed in vector scan mode, and a dc positive pulse was applied to the silicon substrate (the voltage values mentioned below were the voltages of the silicon substrate relative to the probe). The probe approached the sample surface at a preset load force of ∼1 nN in the SPO process and 0.1 nN in the imaging mode. Unless mentioned elsewhere, topographic images were obtained in the forward scanning with the same probe just after patterning. Results and Discussion Writing dots is a basic fashion in SPM-based nanolithography. In dot-array writing, the AFM tip moves to the predetermined
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Figure 2. Oxide dot height as a function of pulse duration for different voltages. (a) Data were measured from parts a (solid signs) and b (hollow signs) of Figure 1. (b) Data were measured from parts d (solid signs) and e (hollow signs) of Figure 1. The data in parts c and d were measured from parts a and d of Figure 1, respectively. These data were fitted by the logarithmic curves (h ) k ln(t) + A) within the regime of stage iii (R is the correlation coefficient), simply linked by thin lines within the regime of stage ii and linked by dotted lines within the regime of stage i. There were no data plotted within the regime of “stage i” in part c because no oxide bumps over the threshold of the 0.3 nm height can be observed in these duration ranges in Figure 1a.
sites and then stays as pulse voltage is applied for the desired duration. We performed AFM-based SPO on an octadecylated Si(111) surface in this fashion to investigate the kinetic dependences of oxide height and width on applied voltage and exposure time (i.e., pulse duration). 1. The division of the Kinetic Stages in the SPO on Alkylated Silicon. Since high resolution and precise locating are much more important than oxide thickness in most applications of SPO lithography, we first select pulse durations no more than 5 s in view of the fact that this time scale is long enough to produce several nanometers of oxide height without obvious shape distortion resulting from instrument drift. A dot array produced by the SPO process under ambient relative humidity (RH) of ∼60% is presented in Figure 1. The oxidation parameters of applied voltages (10, 9, 8, and 7 V) and pulse durations (from 5 s to 0.1 ms) are herein marked in the scheme at the bottom right corner of Figure 1. To clarify the cross-talk effect of the friction signal on the topographic imaging, the topographic imaging was performed in different fast scan directions either perpendicular or parallel to the cantilever’s longitudinal axis. In detail, when the scan angle was set as -90°, the scanning was performed in the so-called lateral force microscopy (LFM) mode where the fast scan direction is perpendicular to the cantilever axis. As a contrast, when the SPM unit is performed at a 0° scan angle, the fast scan direction is along the cantilever axis. The oxide dot heights (h) versus the pulse durations (t) and voltages (U) obtained from Figure 1 are plotted, as shown in Figures 2 and 3, respectively. The dependences of oxide dot diameters (D, full width at half-maximum (fwhm)) on pulse voltages and durations will be discussed later. When the topographic images were obtained in LFM mode, no remarkable differences in the observed dot heights between the opposite
Figure 3. Oxide dot height as a function of applied voltage for different pulse durations. Data were obtained from the regime of stage iii in Figure 2c and fitted by the straight lines.
scanning directions were seen, as shown in Figure 2a. However, when the topographic images were obtained at a 0° scan angle, Figure 2b indicates that the observed heights of the modified domains in the forward image are remarkably greater than those in the backward image. In this case, the significant differences in the observed topography of the modified domains between the opposite scanning directions are caused by the cross-talk effect of the friction signal on the topographic imaging. It is well-known that the SPO process on the octadecylsilated (e.g., octadecyltrichlorosilane (OTS)-modified) SiO2/silicon changes the local surface properties of the oxidized domains from hydrophobic to hydrophilic. The same change takes place in the SPO process on the octadecylated silicon surface. As a result, the local friction of the oxidized domains becomes greater than that of the surrounding alkyl monolayer in the contact AFM scanning. Just as the greater friction can result in greater torsion of the cantilever in LFM mode, the greater friction can also cause greater up-down bend of the cantilever when the AFM
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SCHEME 1: Schematic Illustrations of the Three Stages of the SPO Process on an Alkylated Silicon Surfacea
a (a) The initial stage (stage i). The methyl end groups of the monolayer are locally oxidized into the carboxylic acid end groups, without the oxidation of the underlying silicon. (b) The transitional stage (stage ii). The alkyl monolayer is degraded, accompanied by the oxidation of the underlying silicon. (c) The steadily growing stage (stage iii). The organic film has been farthest degraded, and the increasing of the oxide height fully depends on the growth of the silicon oxide until its final stop.
is used at a 0° scan angle. This cantilever’s bend can make the friction signal coupled with the height signal by means of the four-quadrant photodetector. Subsequently, through the feedback systems, the cross-talk between friction and height signals can introduce the deviations into the observed heights of oxidized domains. At a 0° scan angle, since the high friction of oxidized domains can cause the cantilever’s bend to be upward in the forward scanning and downward in the backward scanning, the observed heights of oxidized domains can be artificially increased in the forward image and decreased in the backward image, as shown in parts d and e of Figure 1, respectively. The similar topography contrast between the opposite scanning directions has also been observed by Schubert,33 who used it as evidence to identify the different stages of the SPO process on the OTS-modified SiO2/silicon. As the pulse duration is shortened, the fade-out of oxide dots in Figure 1a indicates that a time threshold (tth) for a given voltage exists, or in view of voltage, an apparent voltage threshold (Uth) for a given pulse duration, which is necessary to produce a minimum-detectable oxide bump protruding from the alkylated silicon surface (a 0.3 nm bump height is generally regarded as the threshold20). Usually, only applied voltage obviously over Uth could produce oxide dots. The tth value decreases with applied voltage. For example, a pulse duration of 20 ms in Figure 1a is just long enough to produce an AFMdetectable oxide bump when a 7 V voltage is applied, while this time threshold can be reduced to 1 ms when the applied voltage is increased to 10 V. A critical experiment shows that a 5 V pulse longer than 0.2 s can produce an oxide bump. SPM and TOF-SIMS (time-of-flight secondary ion mass spectrometry) investigations conducted by Pignataro’s group67 have revealed that the SPO process on the octadecylated silicon surfaces proceeds probably with a similar mechanism to that on the OTS-modified SiO2/Si surfaces where organic film will first suffer from the local oxidation of the end groups from methyl to carboxyl, followed by the partial degradation before the underlying silicon bulk starts to be oxidized.29,33-36 Therefore, different from the SPO on H-terminated silicon, the apparent voltage threshold in this process is first dependent on the resistance of alkyl film against tip-induced oxidation. Furthermore, as shown in Scheme 1c, the AFM-measured oxide height (h) is just the apparent oxide thickness relative to the surrounding alkyl film surface, which will be essentially smaller than the total oxide thickness (H). The time consumption for the previous oxidation of alkyl film and the growth of “submerged” oxide thickness (i.e., the sum of thickness of x and d in Scheme 1c) will be reduced with elevated voltage, which leads to a shorter time threshold for a higher voltage, or in terms of voltage, a higher voltage threshold for a shorter pulse
duration. The time thresholds of 1 ms for a 10 V pulse and 20 ms for a 7 V pulse in this process are much shorter than those in the SPO on OTS-modified SiO2/silicon, for example, 20 ms for a 10 V pulse and 100 ms for a 7 V pulse.24 This benefits from the absence of a native SiO2 layer which may block the drift of reactive oxyanions. Considering the dependence of the oxide height on pulse duration in Figure 2 together with the topographic contrast and the frictional information in Figure 1, we recommend that a whole process of tip-induced anodic oxidation on octadecylated silicon could be divided into three stages (Scheme 1). The First Stage. When the pulse duration is short enough (e.g., 0.1-0.5 ms for a 10 V pulse, 0.1-2 ms for a 9 V pulse, 0.5-7 ms for an 8 V pulse, and 10 ms for a 7 V pulse in this example), the significant bumps (of the order of 0.3 nm in height) cannot be observed at the modified domains in the topographic images by LFM mode (Figure 1a and b), whereas the friction image (Figure 1c) depicts that the short-pulsemodified domains obviously exhibit greater frictional force than the surrounding alkyl monolayer. However, comparing the forward and backward topographic images obtained at a 0° scan angle, it can be found that these short-pulse-modified domains can display faint positive “heights” in the forward topographic image (Figure 1d) but negative ones in the backward topographic image (Figure 1e). In other words, the plus-minus of observed heights of the short-pulse-modified domains depends on the scanning directions at the 0° scan angle. As demonstrated by Schubert,33 the apparent height signal of the short-pulse regime shown in Figure 1d and e should be regarded as an AFMimaging artifact caused by the cross-coupling between the friction and height signals. The variations of the short-pulsemodified domains can be ascribed to the local oxidation of the end groups of the alkyl monolayer from methyl (-CH3) into the oxygenic end groups, typically carboxylic acid groups (i.e., -COOH).14-19,31-33 In contrast to the oxidation of the underlying silicon causing the actual increase of height due to the volume expansion of silicon oxide, the conversion of the end groups of monolayer from methyl to carboxyl cannot materially vary the height of the modified domains. However, the great change of local surface properties from hydrophobic to hydrophilic can strongly increase the local friction of the end-groupmodified surface. Just like the surface terminal oxidation in the SPO on the OTS-modified SiO2/silicon, as demonstrated by Sagiv17 and Schubert,33 the higher friction of such end-groupmodified surfaces may lead to an artificial height deviation in the topographic imaging by the contact AFM. The tip-induced oxidative conversion of the monolayer end groups indicates the beginning of the SPO process, and thereby, as schematically shown in Scheme 1a, this short-pulse regime can be defined as
AFM-Based SPO on an Octadecylated Si(111) Surface the initial stage in the SPO process on alkylated silicon. The stage is characterized by the local oxidative conversion of the end groups of monolayer from methyl to carboxyl, without significant variation of height in the modified domain. On the bare silicon substrates, the rate-limiting factors of the SPO process are usually attributed to the buildup of space charge63,66,69-71 as well as the oxide stress59,60,72 in the oxide layer. However, on the alkylated silicon surfaces, the water meniscus between the tip and the sample will strongly limit the SPO process, especially in the initial stage. It has been demonstrated that the meniscus status and the rate-limiting factors are inherently coupled and both affect the total current passing through the tip-sample junction at a given exposure stage.73 SPO on silicon substrates is a well-known intricate electrochemical process69,74-77 in which the water meniscus plays a key role in producing and transmitting the active oxyanions. However, the formation of a meniscus is the collective result of electric-field-induced condensation and capillary condensation when a voltage is applied between the tip and the sample.73,74,78-80 On the highly hydrophobic surfaces under moderate humidity, the field effect tends to dominate the development of a meniscus.65,73,79,80 During most of the initial stage, the duration of voltage may be too short to develop a so-called field-induced meniscus between the tip and the hydrophobic octadecyl surface. As shown in Scheme 1a, the SPM oxidation process will be restricted by the supply of oxyanions produced by electrolyzing the water vapor in the ambient humidity rather than by electrolyzing the meniscus.65 Under these conditions, the produced oxyanions are deficient and mainly consumed to oxidize the end groups of the alkyl monolayer. Only a few oxyanions may penetrate into the alkyl monolayer to degrade the carbon chains. With SPO proceeding, the local surface of the oxidized domain becomes more hydrophilic and favorable to the formation of a meniscus. The Second Stage. As the pulse duration is increased into stage ii (e.g., 1-10 ms for a 10 V pulse, 4-40 ms for a 9 V pulse, 10-70 ms for an 8 V pulse, and 20-100 ms for a 7 V pulse in this example), Figure 1a and b shows that the oxide bump starts to grow over the modified domain with a height no lower than 0.3 nm, which indicates that the underlying silicon starts to be oxidized into silicon oxide. A 0.3 nm high bump is usually considered to be a criterion for the formation of the first silicon oxide monolayer.20 Figure 1e shows those oxide bumps of about 0.3 nm in height in Figure 1a, whose oxidation parameters are 1 ms for a 10 V pulse, 10 ms for an 8 V pulse, 20-70 ms for a 7 V pulse, and so forth, are still displayed as negative height domains. It may be because a 0.3 nm height is not high enough to off-set the friction-caused negative deviation in height. However, they are the transition points in nature, where the center part of the modified domain grows from negative height to positive height as the pulse duration is prolonged, while the edge of the modified domain remains negative in height. As noted by Schubert,33 the increased height contrast at the center part of the modified domain can also clarify the oxidative growth of the underlying silicon, competing with the degradation of the carbon chains in the monolayer. Figure 2c and d illustrates that the variation of the dot height with the pulse duration within the regime of stage ii is complicated, which is dependent on the SPO conditions and different from the logarithmical growth mode in the long-pulse regime of stage iii. As schematically shown in Scheme 1b, this duration range can be defined as the transitional stage in the SPO process on
J. Phys. Chem. B, Vol. 110, No. 21, 2006 10369 alkylated silicon. In this stage, the alkyl monolayer is degraded, accompanied by the oxidation of underlying silicon. In the transitional stage, a vulnerable field-induced meniscus has been developed on the oxidized surface, whose electrolyzing ensures a relatively sufficient supply of oxyanions. Therefore, competing with the degradation of the alkyl monolayer, plenty of oxyanions drift through the remnant organic monolayer and the produced silicon oxide layer to oxidize the deeper silicon. The growth of the underlying silicon oxide increases the height of the oxidized domain, while the degradation of the alkyl monolayer decreases the height. As a result, the variation of oxide height with exposure time is determined by the competition of the alkyl monolayer, thinning with the silicon oxide layer thickening. Both the degrading rate of carbon chains in the monolayer and the growing rate of the underlying silicon oxide depend on the SPO conditions. Besides the oxidation parameters of bias voltage and pulse duration, many external factors, such as humidity, water meniscus, the doping of the silicon substrate, the local surface properties of the sample, the state of the tip apex, and the tip-sample contact status, can strongly influence these two rates in different ways because the biased tip can directly contact the monolayer surface but cannot touch the interface of the silicon bulk. The oxyanions can directly attack the carbon chains to cause the degradation of the monolayer. As a contrast, the oxyanions must break through the block effects of both the remnant organic monolayer and the produced silicon oxide layer to oxidize the underlying silicon. The degradation of carbon chains makes the organic monolayer thinner and looser, which reduces the block effect on the oxyanions. However, the growth of silicon oxide causes the accumulation of space charge63,66,69-71 and structure stress59,60,72 in the oxide layer, which enhances the block effect on the oxyanions. All of these factors together determine the growing rate of silicon oxide. Furthermore, the rapid growth of silicon oxide and decomposition of organic monolayer in the transitional stage will consume large amounts of electrolyzed water, which may make the water meniscus unstable and inversely the state of the water meniscus can influence the SPO process. Consequently, the competition of the degradation of the alkyl monolayer and the growth of the silicon oxide complicate the height variation of stage ii as well as transition points between the different stages. An intrinsical difference between stage ii and stage iii should be whether the monolayer has been farthest degraded so that the last remnant organic film (or probably an organic/inorganic hybrid layer) does not continue to affect the growth of the underlying silicon oxide in the succeeding stage iii. A transition point between stage ii and stage iii can usually be identified in the function of oxide height versus pulse duration, for example, the plots of “9 V” and “8 V” in Figure 2c. However, sometimes, it may be difficult to be exactly identified, for example, the plots of “10 V” and “7 V” in Figure 2c. In this case, comparing parts c and d of Figure 2 can facilitate the identification of the transition point between stage ii and stage iii, which will be discussed below. The Third Stage. When the pulse duration is long enough (e.g., >0.01 s for a 10 V pulse, >0.04 s for a 9 V pulse, >0.07 s for an 8 V pulse, and >0.1s for a 7 V pulse in this example), the high correlativity of R2 (R is the correlation coefficient) of the logarithmic curve fitting in Figure 2c illustrates that the oxide height increases logarithmically with the pulse duration within stage iii. Furthermore, the straight-line fitting with high correlativity (R2 > 0.95) in Figure 3 indicates that the oxide height
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Figure 4. Oxide dot height as a function of pulse duration for different voltages. Curve fittings with the direct-logarithm model (h ) k1 ln(t) + A) (a) and with the power-of-time law model (h ) k2tγ) (b) within the regime of stage iii were illustrated by the bold lines. For the three data series of “10V”, all the fitting correlativity of R2 was 0.99 in part a and 0.97 in part b. The data points outside stage iii were simply linked by the thin lines (a).
increases linearly with the applied voltage within stage iii. It can be found that the kinetic laws of the SPO process on the alkylated silicon in the regime of stage iii are very similar to those in the SPO process on the bare silicon substrate. This indicates that, in this stage, the organic film should have been degraded to the maximum and the increase of the oxide height fully depends on the growth of the silicon oxide. Therefore, stage iii is regarded as the steadily growing stage of oxide in the SPO process on alkylated silicon. As schematically shown in Scheme 1c, within the regime of stage iii, since the H/d ratio, known as the volume expansion ratio of SPM oxide, can approximately be taken as a constant for the p-type silicon substrate71 and the thickness (x) of the surrounding alkyl film can also be regarded as a constant, the AFM-measured oxide height (h) reliably depends linearly on the total oxide thickness (H). Therefore, it can be inferred that the total oxide thickness will also increase logarithmically with the pulse duration and linearly with the applied voltage within stage iii. In the steadily growing stage, a stable field-induced meniscus has been developed and the supply of oxyanions is very ample. The buildup of space charge63,66,69-71 and the block effect of the stressful oxide layer59,60,72 gradually increase with oxide layer thickening and dominate the oxide growth.65 The nonuniformity of some oxide dots is also discernible, as shown in Figure 1c due to the nonuniformity of the tip apex and wearing of the conductive metal coating layer. Clearly, the above-mentioned division of the kinetic stages of the SPO on octadecylated silicon emphasizes that the different reactions of the oxidative conversion of the monolayer end groups, the degradation of carbon chains in the monolayer, and the silicon oxide growth in the silicon bulk influence the variation of the topographic height of modified domains in different ways. It is different from the division of the oxide growth kinetic regions of SPO on H-terminated silicon previously reported by Dagata et al.66 Compared with Figure 2c, three features of Figure 2d can be found. First, although the “observed heights” in the regime of stage i in Figure 2d are a friction-caused artifact, the relative magnitude of these observed heights agrees with the relative intensity of the frictional signals in Figure 1c, which suggests that the observed height in stage i in Figure 2d is probably proportional to the local friction of the end-group-modified surface. It is a reasonable explanation that a higher or longer pulse usually produces a greater conversion ratio of top methyl to carboxylic groups, which can result in a greater friction contrast to increase the cross-talk effects of friction on height signal, bringing about a higher observed height. Therefore, the observed heights in stage i in Figure 2d may provide an approach
to identify the surface variation of the end-group-modified domains. Second, Figure 2d enhances the difference in the variation of height with duration between stage ii and stage iii to some degree, which can be ascribed to the different surface frictions of oxidized domains in different stages. The oxidized domain in stage ii is coated by a remnant organic film with some thickness and loose structure, and consequently, its surface is relatively soft. The surface of the oxidized domain in stage iii is relatively hard because the organic film has been farthest degraded. Therefore, the local friction can be greatly different between these two kinds of surfaces, bringing about different degrees of cross-talk effects between friction and height signals. Consequently, the enhanced contrast in the height’s variation with duration between stage ii and stage iii in Figure 2d can be regarded as an indication to identify the different states of the degraded monolayer. Clearly, a greater difference in the variation trend of height with duration before and after the transition point is favorable to identify the transition point between these two stages. Third, although Figure 2d displays the modified domain with an increased apparent height due to the cross-talk effects of friction signals on the forward topographic image at a 0° scan angle, the plots within stage iii in Figure 2d illustrate the kinetic law of the same direct-log form (h ) k ln(t) + A) just with the differences in the fitting parameters of k and A. This indicates that, within the steadily growing stage, the cross-talk effects between the friction and height signals may just introduce a systematic error into the AFM-measured heights, which does not affect the reliability of the kinetic laws in principle. In general, it is of great significance for the kinetic investigation on the SPO process to combine the different topographic images and frictional image, as shown in Figure 1. 2. Comparison between Kinetic Models in the Steadily Growing Stage. It is a reasonable inference that the tip-induced oxide growth on the alkylated silicon in the steadily growing stage is similar to that on the bare silicon substrate. Considering the fact that, besides the direct-log kinetic model (h ) k1 ln(t) + A),5,59,60 a different model of the power-of-time law form (h ) k2tγ)61-63 is also generally adopted to explain the relation of the SPO oxide height to the exposure time in the SPO process on the hydrogen-terminated silicon, it is significant to compare the applicability of these different kinetic models for the SPO on alkylated silicon in the steadily growing stage. The SPO experiments were conducted within a wide range of exposure time. The AFM-measured heights and diameters of the oxide dots are plotted versus the pulse durations in Figures 4 and 6c, respectively. Data series “10V(1)” and “10V(2)” in Figure 4 were obtained from the dot arrays shown in Figure 5.
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Figure 5. Three-dimensional AFM images of two series of dot arrays produced by a 10 V voltage with varying pulse duration from 100 to 0.001 s (a) and 200 to 0.002 s (b) under an ambient humidity of ∼70% (RH). A topography image of the rectangular area in part b is displayed at the top of part c and the cross-section profile at the bottom.
Figure 6. Oxide dot diameters as a function of the pulse voltages (a) or durations (b and c). These functions were fitted by straight lines (a) or logarithmic curves (b and c), respectively. The data in parts a and b were obtained from the dot array in Figure 1a. Data series “10V(1)” and “10V(2)” in part c were obtained from the dot arrays in Figure 5.
Both dot-array series were produced by a 10 V pulse under a relative humidity (RH) of about 70% on the same substrate sample, while the series “10V(3)” was produced on another substrate sample. There are not remarkable differences between the two series “10V(1)” and “10V(2)” in the oxide dot heights (Figure 4) or diameters (Figure 6c). However, they are obviously different from the series “10V(3)” in the oxide heights (Figure 4) as well as the transition points between stage ii and stage iii. It indicates that, besides the oxidation parameters of pulse voltage and duration, several external factors, such as the local surface properties of the sample or the state of the tip apex, can strongly influence the reproducibility of the SPO process.
Contrasting parts a and b of Figure 4, it can be concluded that, within a relatively wide duration range in the steadily growing stage, the direct-log model is more applicable than the power-of-time model for this SPO process in demonstrating the kinetic dependence of oxide dot height on pulse duration. The conclusion is basically drawn from the following two facts: first, the logarithmic fitting curves match experimental data with a higher correlativity within the regime of stage iii in Figure 4a than the power-law fitting curves do in Figure 4b; second, the distribution of data points is relatively random on either side of the logarithmic fitting curve within the regime of stage iii in Figure 4a, which conforms to the statistical law of random errors. As a contrast, if the same experimental data are fitted with the power-law curves, as shown in Figure 4b, the negative deviation relative to experimental data appears in the midsection of power-law fitting curves, while the positive deviation remarkably appears in both of the end-sections. The logarithmic dependence of oxide height on exposure duration is in conformity with the result of our previous line-writing experiments where the continuous oxide lines were created by the same SPO process and the height of the oxide line increases logarithmically with the reciprocal of the tip-moving speed.68 In addition, the exponential model81 with the expression h ) A - Be-t/τ does not satisfactorily fit our experimental data. These results are in agreement with the Avouris kinetic investigations on SPO of H-terminated silicon in which the direct-log model was demonstrated to work well in the dot oxidation process within a wide kinetic range (pulse duration of 0.01-1000 s).59,60 This feature supports the previous inference of the kinetic similarity between the SPO process on the alkylated silicon and that on the bare silicon within the steadily growing stage. The similar kinetic behavior reveals that these two processes essentially possess the same oxidation mechanism in the steadily growing stage except that an additional oxidative degradation of alkyl film delays the oxidation of the silicon bulk when SPO is performed on the alkylated silicon surface. The logarithmical increase of oxide dot height with pulse duration means that the growing rate of oxide thickness decreases exponentially with the thickness of oxide.59,60 It follows that the SPO oxidation of the underlying silicon is initiated at a great initial rate, rapidly slowing down until finally being terminated with the thickened oxide layer. That is to say, within the steadily growing stage, the so-called self-limiting behavior59,60,73 controls the SPO process on alkylated silicon. Two phenomena appear when the pulse duration is prolonged out of stage iii in Figure 4. One is that the oxide practically stops growing when the dot height has reached the limit, for example, h ≈ 3 nm for a 5 V pulse, as shown in Figure 4a. This is an unavoidable result of the self-limiting effect. Taking the submerged oxide thickness of x + d > 2.1 nm (i.e., thickness
10372 J. Phys. Chem. B, Vol. 110, No. 21, 2006 of the octadecyl monolayer of x ≈ 2.1 nm47,67) into account, the total oxide thickness of H ) h + x + d > 5.1 nm has reduced the electric-field intensity (i.e., E ) Vbias/rH) below 1 × 107 V/cm for Vbias ) 5 V. (Here, r is the relative dielectric constant of the silicon oxide layer, r > 1.) The intensity of 1 × 107 V/cm has been regarded as the critical electric field in the SPO process, where the electric field has been too weak to drive the oxyanions through the oxide layer to oxidize the deeper silicon bulk.5,59,60 The other phenomenon is that a narrow center domain grows out of the broad oxide base so as to form the so-called “twostoried shape”75,82 which was usually produced by a 1ong and high pulse under great ambient humidity. In the SPO process on the octadecylated silicon under an ambient humidity of ∼70%, typically shown in Figure 5c, a 10 V pulse shorter than 50 s almost always produces a simple domelike oxide dot, while a long pulse longer than 200 s usually results in a two-storied shape. The pulse duration of 100 s seems to be a critical point under our experimental conditions. The occurrence of a twostoried shape also results from the self-limiting. The radial diffusion of oxyanions results in radial growth of the oxide, which extends the oxide region. Consequently, it facilitates the widening of the meniscus size and the defocusing of the electric field. Since the intensity of the electric field close to the sample surface decays from the center to the edge, when the outer domain of the oxide region has been oxidized thickly enough to stop growing, the center domain can continue growing by virtue of the strongest intensity in the center of the electrical field. An investigation for SPO on the native p-type Si(001) surface reported that the critical duration for the transition of dot shape from the simple dome to the two-storied feature was 10 s for a 10 V pulse under a relative humidity of 70%.82 The critical duration of this experiment is much longer than that. This is probably due to the highly hydrophobic alkylated surface outside of the oxide region suppressing the formation of the water layer, which is greatly advantageous to limit the size of the water meniscus, weaken the radial diffusion of oxyanions, and focus the electric field. The lateral size of the oxide feature (i.e., the diameter of the oxide dot) is also an important target concerned in the nanofabrication. The AFM-measured oxide dot diameters are plotted versus pulse voltages in Figure 6a and durations in Figure 6b and c, respectively. Figure 6 illustrates that the oxide dot diameter increases linearly with applied voltage (straight-line fitting with R2 > 0.94 in Figure 6a) and logarithmically with duration (logarithmic curve fitting with R2 > 0.94 in Figure 6b and >0.95 in Figure 6c) within the steadily growing stage. These figures indicate that the kinetic characters of radial growth of oxide dots are similar to those of axial growth under our experimental conditions. Sugimura’s group has also observed that both dot height and lateral dot size increase logarithmically with duration during SPO on an octadecylsilated SiO2/Si surface.24 Conclusions We have performed the AFM-based SPO process on an octadecyl-terminated Si(111) surface to create protuberant oxidedot-array patterns and investigated the kinetics of this nanolithography process. Our experiments indicate that this SPO process involves three stages. The different reactions of the oxidative conversion of the end groups of the monolayer, the degradation of carbon chains in the monolayer, and the oxide growth of the underlying silicon appear in different stages and dominate the variation of the modified domains in different
Yang et al. ways. The oxide dot height increases logarithmically with exposure time and linearly with applied voltage within the steadily growing stage. From the comparison of the direct-log form and the power-of-time law form kinetic models for this process, it is found that the direct-log kinetic model is more applicable for SPO on alkylated silicon in demonstrating the dependence of oxide thickness on exposure time of applied voltage within a relatively wide range. Furthermore, it is found that the threshold of exposure time in this process is obviously shorter than that in the SPO on an octadecylsilated SiO2/silicon surface. The lateral size of the oxide dot also tends to increase linearly with applied voltage and logarithmically with exposure time. A distinct advantage of SPO on octadecylated silicon over SPO on octadecylsilated SiO2/silicon is that a shorter exposure time and/or a lower voltage can create remarkably protuberant oxide patterns. A short pulse of several milliseconds with a 10 V voltage, which is just able to result in the surface oxidization of alkyl film on octadecylsilated silicon,15 can produce a remarkable oxide bump on octadecylated silicon. It could be predictable that shorter exposure time as well as lower voltage in SPO lithography on a silicon-based substrate would reduce the detrimental effects on the electrical characteristics of the adjacent region, which is of great benefit to the fabrication of integrated nanometer-sized electronic devices. This study shows that alkylated silicon is a promising substrate material for tipinduced anodization lithography. Acknowledgment. The authors are grateful to financial support from the National Natural Science Foundation of China (no. 20375038) and Nano Project of Changchun Institute of Applied Chemistry. References and Notes (1) Wouters, D.; Schubert, U. S. Angew. Chem., Int. Ed. 2004, 43, 2480. (2) Kra¨mer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367. (3) Nyffenegger, R. M.; Penner, R. M. Chem. ReV. 1997, 97, 1195. (4) Baski, A. A. AdVanced Semiconductor and Organic Nanotechniques: Fabrication of Nanoscale Structures using STM and AFM; Academic Press: London, 2003. (5) Fontaine, P. A.; Dubois, E.; Stie´venard, D. J. Appl. Phys. 1998, 84, 1776. (6) Snow, E. S.; Juan, W. H.; Pang, S. W.; Campbell, P. M. Appl. Phys. Lett. 1995, 66, 1729. (7) Chien, F. S.; Wu, C. L.; Chen, T. T.; Gwo, S.; Hsieh, W. F. Appl. Phys. Lett. 1999, 75, 2429. (8) Held, R.; Heinzel, T.; Studerus, P.; Ensslin, K.; Holland, M. Appl. Phys. Lett. 1997, 71, 2689. (9) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. Langmuir 2000, 16, 4409. (10) Zheng, J.; Chen, Z.; Liu, Z. Langmuir 2000, 16, 9673. (11) Li, Q.; Zheng, J.; Liu, Z. Langmuir 2003, 19, 166. (12) Ling, X.; Zhu, X.; Zhang, J.; Zhu, T.; Liu, M.; Tong, L.; Liu, Z. J. Phys. Chem. B 2005, 109, 2657. (13) He, M.; Ling, X.; Zhang, J.; Liu, Z. J. Phys. Chem. B 2005, 109, 10946. (14) Maoz, R.; Cohen, S. R.; Sagiv, J. AdV. Mater. 1999, 11, 55. (15) Liu, S.; Maoz, R.; Schmid, G.; Sagiv, J. Nano Lett. 2002, 2, 1055. (16) Hoeppener, S.; Maoz, R.; Cohen, S. R.; Chi, L. F.; Fuchs, H.; Sagiv, J. AdV. Mater. 2002, 14, 1036. (17) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 725. (18) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 424. (19) Liu, S.; Maoz, R.; Sagiv, J. Nano Lett. 2004, 4, 845. (20) Snow, E. S.; Campbell, P. M. Appl. Phys. Lett. 1994, 64, 1932. (21) Kinser, C. R.; Schmitz, M. J.; Hersam, M. C. Nano Lett. 2005, 5, 91. (22) Day, H. C.; Allee, D. R. Appl. Phys. Lett. 1993, 62, 2691. (23) Kim, Y.; Kang, S. K.; Choi, I.; Lee, J.; Yi, J. J. Am. Chem. Soc. 2005, 127, 9380. (24) Sugimura, H.; Hanji, T.; Hayashi, K.; Takai, O. Ultramicroscopy 2002, 91, 221.
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