Dependence of Transport Rate on Area of Lithography and

Letter pubs.acs.org/Langmuir

Dependence of Transport Rate on Area of Lithography and Pretreatment of Tip in Dip-Pen Nanolithography Tzu-Heng Wu,† Hui-Hsin Lu,† and Chii-Wann Lin*,†,‡ †

Institute of Biomedical Engineering and ‡Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei 106, Taiwan S Supporting Information *

ABSTRACT: This study examines the lithographic capacity of tips in dip-pen nanolithography (DPN). The dependence of the transport rate (R) decay on the area of lithography (Alith), the dependence of Alith on the lithographic time (t), and the effect of piranha cleaning on the lithographic capacity are considered herein. The dependencies in the line-drawing lithography process are studied using 16-mercaptohexadecanoic acid (MHA) ink. On the basis of the linear decay dependence discovered in the R−Alith dependence, piranha treatment can increase the lithographic capacity by up to 35.5fold, an improvement that may originate from a change in the tip’s surface chemistry. Moreover, a theoretical model is derived to describe the Alith−t dependence accurately and to predict the tips’ lifetime. Furthermore, an experiment involving DPN-based nanostructure fabrication demonstrates the importance of monitoring the tips’ transport rate and lifetime. In addition to shedding light on the physical and chemical principles behind DPN, this study provides a comprehensive model for a quantitative analysis of the tips’ behavior.

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INTRODUCTION Ever since the development of dip-pen nanolithography (DPN),1,2 it has been applied in numerous applications3−5 including the fabrication of nanostructures, gas sensors, bioessay chips, and electronic devices. Owing to the recent development of parallel pen arrays,3,6 DPN has become a highthroughput process for rapid fabrication. Although pen arrays represent the potential use of DPN beyond laboratory-scale fabrication into industrial manufacturing, more challenges lie ahead. As noted by Saha et al.,7 the efficacy of applying DPN in large-scale fabrication is closely related to the lithography capacity of the tips. A tip’s lifetime is limited mainly, but not exclusively, by the fact that the tip is not an unlimited source of ink in practice. Some studies8,9 have noted that the transport rate of a tip (denoted hereinafter as R) decreases gradually in a manner determined by factors such as the coating method, writing conditions, and humidity. However, the interaction among the transport-rate decay, lithographic capacity, and lifetime of tips has seldom been studied thoroughly. Without monitoring, the R decay results in an inaccurate output pattern. Such errors in the lithography pattern can then hinder subsequent applications such as nanofabrication, thereby limiting the efficacy of DPN. Hence, thoroughly understanding the decay process is necessary to achieve the DPN’s full potential for large-scale synthesis. This study elucidates how R and Alith, Alith and the lithographic time (t) are related. These dependencies are examined by using MHA as an ink to carry out line-patterning © 2012 American Chemical Society

lithography on a gold substrate. Two types of tips are investigated: one treated with piranha solution before use and the other one used as purchased. Experimental results indicate that the transport rate decays linearly with an increasing area of lithography for both tip types. Closely examining the cutoff area reveals that piranha treatment increases the tip lithographic capacity by up to 35.5-fold. Exactly why this increase occurs is discussed in detail. Moreover, on the basis of a combination of the linear R−Alith decay equation with the nonlinear Alith−t dependence described herein, a theoretical model is derived to address issues involving the patterning precision and the tip’s lifetime. Furthermore, an experiment involving nanostructure fabrication demonstrates the effect of the R decay and the importance of monitoring the tips’ lifetime in DPN-based applications.

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MATERIALS AND METHODS

All gold substrates used in this study were prepared by the template stripping method.10 The tips were coated exclusively by the dipcoating method. Some tips were immersed in piranha solution for 30 min before use to study the effect of pretreatment on the lithographic capacity, and the others were used as purchased. DPN was then performed using the Nscriptor system purchased from Nanoink with type-A probes (side A-1, spring constant = 0.041 N/m). The transport rate of the tips was calibrated by using the built-in Ink-Cal function of Received: July 4, 2012 Revised: September 28, 2012 Published: September 28, 2012 14509

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Figure 1. Procedure for collecting tips’ (R, Alith, t) data: (a) transport rate calibration and (b) LFM image of the lithography following transport rate calibration. Steps a and b are performed in cycles until sufficient data points are collected to map the R−Alith and Alith−t dependences. (c) R−Alith for piranha-treated and as-purchased tips. The black dots/black circles denote piranha-treated tips 1 and 2, and the triangle/hollow triangles represent as-purchased tips 1 and 2. The fitted equations for as-purchased tips 1 and 2 (red/red dashed lines) and piranha-treated tips 1 and 2 (blue/blue dashed lines) are as follows: (as-purchased 1) 0.037−4.9 × 10−4(Alith); (as-purchased 2) 0.019−2.5 × 10−4(Alith); (piranha-treated 1) 0.090−4.7 × 10−5(Alith); and piranha-treated 2: 0.026−9.5 × 10−5(Alith). The R−Alith data are fitted with R2 = 0.95, 0.91, 0.70, and 0.74 for as-purchased tip 1, aspurchased tip 2, piranha-treated tip 1, and piranha-treated tip 2, respectively. (d) Alith−t for piranha-treated tip 1 (blue dots) and as-purchased tip 1 (red dots in the inset). Curves through circles in the plot correspond to the ideal constant-flux model (Alith = R0t). Curves through hollow triangles in the plot represent data modeled on the basis of eq 3 using R0 and D obtained from plot c.

where R0 represents the fitted initial transport rate and D denotes the decay constant of the tip. According to eq 1, the transport rates of all tips decay linearly at a rate that depends on D as Alith increases. The linear decay is attributed to the consumption of the ink molecules from the tip surface because Alith divided by the MHA molecular footprint (0.229 nm2) yields the number of ink molecules consumed from the tip during lithography. This finding closely resembles that of Giam et al.11 On the basis of that study, the transport rate scales linearly with the total amount of ink that is delivered to the cantilever. Figure 1c also illustrates the how piranha pretreatment affects the tips. First, both piranha-treated tips had a smaller D than that of the as-purchased tips, corresponding to their slower decay in R−Alith. Second, the cutoff area (R0/D) of the piranhatreated tips (i.e., the x-axis intercept of eq 1) markedly exceeds that of the as-purchased tips. The cutoff area is the maximum area that a tip can possibly write before R finally recedes to nearly zero. The R0/D values of piranha-treated tips are 1933 and 274 μm2, and those of the untreated tips are only 76 and 73 μm2. To incorporate additional data for analysis, Tables 1 and 2 are provided. Table 1 lists the cutoff area data for four piranhatreated tips, and Table 2 summarizes the characteristics of four as-purchased tips (Tables 1 and 2 include the data in Figure 1). In contrast to the cutoff area in Table 2, which ranges from 67.5 to 86.7 μm2, the treated tips presented in Table 1 show an exclusively larger cutoff area ranging from 274 to 2397 μm2.

the Nanoink software. The Supporting Information provides the calibration details. DPN is always performed at approximately 55% relative humidity by using a glovebox. Ferric nitrate and thiourea were purchased from Sigma-Aldrich. Additionally, a mixture of 1:1 v/v% 25 mM ferric nitrate/40 mM thiourea was used as the etchant in the nanostructure fabrication experiment. The Supporting Information introduces the procedure for evaluating instantaneous R.

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RESULTS AND DISCUSSION Figure 1a,b plots the procedure for collecting the R−Alith and Alith−t tip relationships. The procedure is described briefly as follows: (1) The tip transport rate is calibrated and collected along with the Alith that results from the process, as shown in Figure 1a. Here, the area of lithography (Alith) is defined as the area covered by MHA during lithography. In this study, Alith data are accumulated first by imaging the lithography pattern with lateral force microscopy (LFM) at a scanning rate of 7 Hz. Following imaging, Nanorule+ software from Pacific Technology is used to determine the area of the patterns, which result from lithography, to obtain Alith. (2) The tip is set to write a pattern, as shown in Figure 1b, and the Alith of lithography is measured. The lithographic time (t) is recorded in both steps 1 and 2. Steps 1 and 2 are repeated in cycles until sufficient data are gathered to map the dependencies. According to Figure 1c, the R−Alith dependence is optimally fitted with a linear function as R = R 0 − D × Alith

(1) 14510

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should be more aptly described by a nonlinear equation because R experiences a gradual decay over lithographic time. According to Figure 1d, the Alith of the piranha-treated tips varies only slightly from the constant-flux model for t = 3000 s (5% deviation). However, the Alith of an as-purchased tip (Figure 1d inset) decreased significantly compared to the constant-flux model for t = 3000 s (51% deviation). Although Figure 1d reveals that piranha-treated tips follow nearly ideal behavior within a given time frame, the as-purchased tip deviates strongly. Obviously, the constant-flux model fails to depict the tips’ behavior adequately in the case of the aspurchased tip, thereby necessitating the development of a more comprehensive model. According to Weeks et al.,14 for any given short period from t1 to t1 + δt, the following relationship holds

Table 1. Characteristics of Four Piranha-Treated Tips piranha-treated tip 1

tip 2

tip 3

tip 4

R0/D = 1997 (R0 = 0.090 μm2/s, D = 4.7 × 10−5 μm/s)

R0/D = 274 (R0 = 0.026 μm2/s, D = 9.5 × 10−5 μm/s)

R0/D = 2397 (R0 = 0.048 μm2/s, D = 2.0 × 10−5 μm/s)

R0/D = 561 (R0 = 0.050 μm2/s, D = 8.9 × 10−5 μm/s)

Table 2. Characteristics of Four As-Purchased Tips as-purchased tip 1

tip 2

tip 3

tip 4

R0/D = 75.5 (R0 = 0.037 μm2/s, D = 4.9 × 10−4 μm/s)

R0/D = 76 (R0 = 0.019 μm2/s, D = 2.5 × 10−4 μm/s)

R0/D = 86.7 (R0 = 0.026 μm2/s, D = 3.0 × 10−4 μm/s)

R0/D = 67.5 (R0 = 0.027 μm2/s, D = 4.0 × 10−4 μm/s)

v1w1 = R

(2)

where v1 denotes the tip writing speed that corresponds to a line width of w1. Combining eqs 1 and 2 yields eqs 3 and 4:

The above data demonstrate that piranha treatment increases the lithographic capacity by up to 35.5-fold. According to Lo et al.,12 AFM tips are commonly contaminated by small-molecule silane from the package used for shipping and storage purposes. That study further indicated that piranha pretreatment eliminates the organic contamination present on the tip. Therefore, we posit that the increased tip lithography capacity after piranha pretreatment is related to contaminant removal and the creation of additional silanol groups on the tip surface. Such a change in surface chemistry is also reported by Sirghi et al.13 The change in surface chemistry affects the interaction between ink molecules and the tip surface, leading to a higher lithographic capacity. Notably, the tip-to-tip variation, which is already found in other studies, is also observed in our experiment. For instance, the cutoff area of piranha−treated tips varies. However, the data of this study demonstrate that the piranha-treated tip and the as-purchased tips significantly differ from each other, reflecting the general nature of the tips. Besides the R−Alith dependence, this study also investigates the Alith−t dependence. Importantly, the Alith−t dependence reveals how a lithographic pattern deviates from the ideal size through lithographic time. Such a dependence can give an account of the lifetime of tips with respect to the precision of the output pattern. Tips are often assumed to exhibit an ideal constant-flux (R = R0) behavior in which the area of lithography increases linearly over the lithographic time following Alith = R0t. However, on the basis of the above discussion, Alith−t

Alith (t) =

R0 (1 − e−Dt ) D

R = R 0e−Dt

(3) (4)

The Supporting Information section provides a detailed derivation of eqs 3 and 4. According to Figure 1d, the piranha-treated tip follows the model from eq 3 within 2% error; in addition, the as-purchased tip (Figure 1d inset) follows the model within an error of 6%. This result indicates that eq 3 can accurately model the behavior of different types of tips. Beyond modeling the Alith−t dependence, eq 3 provides an effective mean of estimating the lifetime of the tips, which refers to the total amount of lithographic time that a specific tip could possibly function before the patterning precision is lower than a required level. This lifetime estimation is achieved by dividing eq 3 by R0t from the ideal model: precision of Alith at time t =

Alith (t )

Aconstant flux (t ) 1 (1 − e−Dt ) × 100% = Dt

(5)

Figure 2. Experiment involving DPN-based nanostructure fabrication. (a) Lithographic pattern: lithography is performed from L1 to L5 in each set (with line widths of 200, 300, 400, 500, and 600 nm) from sets 1 to 4. Notably, Alith of each set is 4 μm2. (b) AFM topographic image of nanowires (sets 1 and 2) following etching. The scale bar represents 2 μm. (c) Cross sections of four nanowires fabricated from L5-set1 to L5-set4 . (d) Rins for each set; Rinitial (circles−dotted line) and Rpost (hollow triangles−dashed line); Rins-set1 = 0.0248 μm2/s, Rins-set2 = 0.0227 μm2/s, Rins-set3 = 0.0207 μm2/s, Rins-set4 = 0.0171 μm2/s, Rinitial = 0.0236 μm2/s, and Rpost = 0.018 μm2/s. 14511

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Additionally, a theoretical model is derived thatis capable of describing the Alith−t dependences of tips accurately, which is important given the inability of the constant-flux model to explain the observed nonlinear dependence. The proposed model also provides an effective means of estimating the lifetime of a tip before lithography. Moreover, an experiment involving gold nanostructure fabrication demonstrates the importance of monitoring the tip lifetime and the role of R decay in DPN-based applications. Direct correlation between the instantaneous transport rate and the transport rate before and after lithography suggests that R decay directly transfers to a negative error in the fabricated nanostructure width, ultimately reducing the fabrication precision.

The lifetime of the tip (i.e., eq 5) is a measure of the tolerance in the deviation of Alith between the actual performance of the tip and that of the ideal tip at a specific lithographic time. Importantly, eq 5 helps DPN users to estimate the tips’ lifetime before patterning commences. By calibrating a few sets of (R, Alith) values, which can be easily obtained because the transport rate must be calibrated at the beginning of DPN, the user can glance briefly at the tip’s lifetime before lithography. Restated, by using eq 5, users can obtain time and precision information at the outset of the experiment by common experimental parameters. On the basis of this information, users can then determine the timing for either changing the tip or adopting other strategies to maintain a level of patterning precision. Interestingly, eq 4, as discovered in dots writing experiments, has been derived from eq 1. This derivation verifies Weeks’ recommendation14 that the exponential relaxation of R originates from a reduction of the transport driving force. The above data raises concerns about the importance of monitoring the lifetime of a tip in DPN. Restated, how does R decay affect DPN applications? To answer this question, this study uses an as-purchased tip to fabricate a gold nanostructure based on a method reported elsewhere.4,6 In this experiment, a mask pattern is drawn with lithographic sequence L1 to L5 in each set, from set 1 to 4. Figure 2a displays the dimensions and pattern of the mask. The sample is then etched with ferric nitrate/thiourea etchant to form the nanostructure. The MHA pattern functions as an etch-resistant mask during etching. After etching, the topography of the nanowire structures is scanned by atomic force microscopy (AFM, Figure 2b). To elucidate the effect of R decay on nanostructure fabrication, Figure 2c shows cross sections of four nanowires that are generated from the L5 mask (L5-set1 to L5-set4). According to Figure 2c, the nanowire width (w) gradually decreases from the designated L5 width (600 nm) as Alith is increased by 4 μm2 from set to set . The R decay, through writing 16 μm2 of Alith, has resulted in a 29% smaller nanowire width in w5-set4 than the designated 600 nm width. Additionally, the extent to which the R decay affects the fabricated structure is further quantified by comparing the instantaneous R (Rins) with R before (Rinitial) and after (Rpost) lithography, as shown in Figure 2d. The strong congruence between Rins‑set1 and Rinitial (0.0248 to 0.0236 μm2/s) and Rins‑set4 to Rpost (0.0171 to 0.018 μm2/s) reveals that the observed nanostructure width error correlates directly with the decay in the transport rate. To address the above-mentioned concern, these results confirm that the R decay poses a problem for DPN by yielding inadequate pattern precision, especially in tips without proper pretreatment. These findings, together with other results of this study, clearly demonstrate the importance of piranha pretreatment and monitoring of the tip’s lifetime.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the preparation of the substrate, piranha cleaning and coating of the tips, transport-rate calibration, calculation of instantaneous R, derivation of the theoretical model, and tip lifetime. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC-99-2221-E-002-117-MY3. The Center for Emerging Material and Advanced Devices of National Taiwan University is commended for its technical support. Ted Knoy is appreciated for his editorial assistance.

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REFERENCES

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SUMMARY This study examined the role of a tip -cleaning procedure and the area of lithography in influencing the tip lifetime in DPN processes. For both the piranha-treated and the as-purchased tips, R decays in a linear manner as Alith increases. The transport-rate decay is attributed mainly to the consumption of the ink molecule around the tip contact point upon lithography. Closely examining the obtained cutoff area reveals that the widely used piranha pretreatment increases the tip lithography capacity because the piranha solution removes surface contamination and renders a greater number of silanol groups on the tip surface. Restated, the change in tip surface chemistry after pretreatment improves the tips’ lithography capacity. 14512

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(10) Hegner, M.; Wagner, P.; Semenza, G. Ultralarge Atomically Flat Template-Stripped Au Surfaces for Scanning Probe Microscopy. Surf. Sci. 1993, 291, 39−46. (11) Giam, L. R.; Wang, Y.; Mirkin, C. A. Nanoscale Molecular Transport: The Case of Dip-Pen Nanolithography. J. Phys. Chem. A 2009, 113, 3779−3782. (12) Lo, Y.-S.; Huefner, N. D.; Chan, W. S.; Dryden, P.; Hagenhoff, B.; Beebe, T. P. Organic and Inorganic Contamination on Commercial AFM Cantilevers. Langmuir 1999, 15, 6522−6526. (13) Sirghi, L.; Kylián, O.; Gilliland, D.; Ceccone, G.; Rossi, F. Cleaning and Hydrophilization of Atomic Force Microscopy Silicon Probes. J. Phys. Chem. B 2006, 110, 25975−25981. (14) Weeks, B. L.; Noy, A.; Miller, A. E.; De Yoreo, J. J. Effect of Dissolution Kinetics on Feature Size in Dip-Pen Nanolithography. Phys. Rev. Lett. 2002, 88, 255505.

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