Selective Functionalization of Silicon Micro/Nanowire Sensors via Localized Joule Heating

Inkyu Park 1,*, Student Member IEEE, Zhiyong Li 2, and Albert P. Pisano1, Member IEEE

1 Berkeley Sensor and Actuator Center (BSAC), University of California at Berkeley, Berkeley, CA, USA 94720

2 Quantum Science Research, Hewlett-Packard Laboratory, Palo Alto, CA, USA 94303

Abstract - A novel approach to achieve localized surface functionalization of silicon-based micro and nanoscale linear structures (eg., silicon nanowire sensors) is proposed in this paper. This method is based upon the protection of silicon surface by hydrophobic polymer layers such as polytetrafluoroethylene (PTFE). These layers are used as a protective, patterning barrier against surface functionalization of silicon or silicon oxide surface. Subsequently, these polymer layers undergo selective thermal ablation along the silicon micro / nanowire sensors by localized Joule heating. This local ablation is then followed by a brief low power O2 plasma for activating the surface of silicon oxide with more hydroxyl (-OH) groups for further surface functionalization. Next, organosilane groups including amino-propyltriethoxysilane (APTES) and 3-mercaptopropyltri-methoxysilane (3-MPTMS) are used as a chemical linker between silicon oxide surface and protein or DNA molecules. We first present a finite element analysis (FEA) of localized Joule heating of silicon nanowires and experimental results of localized ablation of the protective polymer by Joule heating. We also verify localized surface modification of ablated surface of silicon with 3-MPTMS by selective binding of gold nanoparticles on the thiolized silicon surface. This localized functionalization is expected to have great advantages such as increasing the sensitivity and lowering the detection limit of silicon micro/nanowire-based sensors.

Keywords – Silicon Nanowire Sensor, Selective Surface Functionalization, Micro/nano-scale Joule Heating, Thermal Ablation of Polymer

I. INTRODUCTION

Recently, there has been a great interest and active

development in the biochemical sensors based upon silicon nanowires. A number of silicon nanowire sensors have been developed for the chemical detection of pH level, streptavidin [1], NH3 or O2 gas [2], DNA [3], and prostate specific antigen (PSA) [4]. Its sensing mechanism is generally believed to be the change of its electrical conductance upon surface protonation / deprotonation or exposure to charged biological molecules at its surface. The electrical charges on the surface of silicon nanowire sensor cause either depletion or accumulation of mobile charge carriers (holes for p-type silicon nanowire and electrons for n-type silicon nanowire). The depletion of charge carriers causes decrease of electrical

conductance while their accumulation gives the opposite effect [5]. Due to the large surface-to-volume ratio and quasi-1D characteristics, silicon nanowire sensor provides a high sensitivity in the chemical detection. Its detection limit has been pushed down to a few fM level for DNA [6] and protein detection [4]. Its potential applications are expanding from detection of pre-concentrated and purified solution of target molecules to the real-time detection from neural cells [7]. For the detection of various biomolecules such as protein or DNA, the surface of silicon nanowires should be functionalized with probe molecules that are complementary to the target molecules to be detected. Currently, the surface modification of silicon nanowire sensors is done by flowing probe molecules in microchannels or spotting a small volume of solutions of probe molecules onto the sensing region [4] by using micropipette or syringes. Unfortunately, during this process, probe molecules get bound not only to the active sensors but also to the surrounding regions due to nonspecific binding of probe molecules to the silicon-based substrates. In silicon nanowire sensor device, the active sensing component (Si nanowire) has a thin native SiO2 surface. Unfortunately, the surrounding regions are also silicon-based materials such as silicon oxide or silicon nitride. Therefore probe molecules get bound not only to the silicon nanowire but also to the surrounding areas. If we have low concentration of target molecules, only a small portion of target molecules will be actually detected by active sensing region while the majority will be wasted by binding to the surrounding region as shown in Fig.1. This will not only reduce the sensitivity and raise the detection limit of the sensor, but also introduce sensing noise, which will be very critical in the in-vitro detection from individual cells with extremely small number of target molecules for detection. To the authors’ best knowledge, there have been no developments in the localized surface functionalization of silicon micro/nanowire sensor. There have been previous efforts on the localized surface functionalization by electrostatic attraction of probe molecules on the electrodes [8], photolithographic definition of hydrophobic layer [9], or microcontact printing lithographty [10]. However, electrostatic attraction scheme does not provide a great selectivity of surface modification against surrounding regions and also cannot be used for the attachment of neutral or weakly charged probe molecules. Photolithographic and microcontact printing method cannot provide ultrafine localization in nanoscale dimensions. It also requires intensive alignment to the prefabricated silicon nanowires.

This project has been supported by Center for Nanoscale Mechatronics and Manufacturing (Grant No. 019997), one of the 21st Century Frontier Research Programs, which are supported by Ministry of Science and Technology, Republic of Korea *Contact author: Mr. Inkyu Park can be reached at Berkeley Sensor and Actuator Center (BSAC), UC Berkeley (Phone : +1-510-643-1099; E-mail : inkyu@eecs.berkeley.edu)

(a) (b)

Fig1. Conceptual image for the comparison between global (a) and local (b) functionalization. In the case of (a), only a small fraction of target molecules are bound to the active sensing area. In contrast, majority of target molecules will get bound to the active sensing area in the case of (b). The circled molecules indicate those attached to the silicon nanowire and used for the detection.

In this paper, we introduce a novel approach to achieve a

selective surface functionalization of silicon micro/nanowire. This method is based upon a highly localized ablation of hydrophobic background polymer by Joule heating along the silicon nanowire and additional post-processing. In Fig.2 is shown the overall procedure of the selective surface functionalization by localized Joule heating. First, a hydrophobic protection layer is deposited on prefabricated silicon nanowires either by spin-coating or plasma-based deposition. Second, electrical current is passed through silicon nanowire for the localized Joule heating of silicon nanowire and localized ablation of protection layer. After a low power O2 plasma treatment of the surface, the surface is modified with organosilane linkers for further biochemical funtionalization. After additional rinsing step, probe molecules are finally immobilized on the organosilane linkers that have been locally bound to the silicon nanowire surfaces. This novel approach enables selective surface functionalization only along the silicon nanowire while keeping the surrounding regions with protection layer from being chemically modified via self-aligned Joule heating of silicon nanowire.

1. Uniform coating of hydrophobic protection layer

2. Joule heating of silicon micro/nanowire for localized ablation of protection layer

4. Localized functionalization with probe molecules

3. Low power O2 plasma treatment Surface modification with organosilane linker

SiO2Si Organosilane linker

Hydrophobic protection layer

Fig.2. Operational principle of thermally assisted selective surface funtionalization of silicon nanowire. Uniform coating of hydrophobic protection layer is locally ablated at desired silicon nanowire. Since the underlying silicon oxide layer is thermally insulative, very localized heating is ensured. After local removal of protection layer, exposed silicon nanowire is functionalizaed with organosilane linker and probe molecule.

II. METHODS AND MATERIALS

In order to prove a localized Joule heating in silicon micro/nanowires, we have employed the multiphysics module of FEMLAB®, a finite element analysis (FEA) software. For the simplicity of simulation, we have excluded the theory of nanoscale heat transport phenomena such as quantum effect or phonon boundary scattering. The physics of including these effects would be to reduce thermal conductivity and increase thermal gradients. Thus, the simulations made for this paper are conservative estimates of the spatial resolution of the dimensions of the ablated portion of the protective, patterning polymer. Silicon nanowires have height and width of 100nm, separated from each other by 100nm, and situated on 380nm thick SiO2 film on Si substrate. The surface of structure is coated with 50nm thick Polytetrafluoroethylene (PTFE, Teflon®). Joule heating of nanowire #1 (left-most nanowire) or multiple nanowires (#1, 4, 7, and 10) by application of electrical bias (50V) and energy generation density of

316 /102.8 mW× is a heat source. Common heat transfer modes (conduction between each layers, and convection and radiation to surroundings) have been modeled in the simulation.

For experimental verification of selective ablation of the protective polymer by localized Joule heating, we have fabricated polysilicon microwires with 100nm thickness, 2µm width, and 16 µm length by microfabrication process. First, 1µm thick thermal SiO2 was grown by wet oxidation at 1000ºC on (100) p-type lightly doped Si substrate. Then a 100nm thick polysilicon layer was grown by LPCVD process with SiH4 as a precursor of Si and PH3 as a source for the phosphorous dopant. By controlling a partial pressure of PH3 gas, we could obtain a moderately doped polysilicon film with a resistivity of m⋅Ω×= −41013.1ρ , which is equivalent to the doping level of 318 /100.2 cmC ×= for n-doped (phosphorous) single crystalline silicon. The patterns for microwires are made by photolithography of G-line photoresist, followed by reactive ion etching (RIE) in CHF3 + O2 environment. Then Al metal layer was patterned for the electrical interconnect and a 50nm thick PTFE layer was deposited by vapor-based deposition system (STS plasma etching system). Electrical bias was applied across silicon microwires by HP4145B semiconductor parameter analyzer for the localized Joule heating of silicon microwire and polymer layer deposited on it. The locally ablated microwires were observed by optical microscope and atomic force microscope (AFM) for topological analysis. After the localized ablation, the sample was treated with low power (10W) O2 plasma for short time period (~10sec) for further activation of silicon surface. Afterwards, vapor phase deposition of 3-mercaptopropyltrimethoxysilane (MPTMS) was done in a reaction chamber with N2 gas as a carrier gas for the 3-MPTMS at room temperature for 30 minutes. After the deposition of 3-MPTMS, the sample was thoroughly rinsed with deionized water and isopropyl alcohol (IPA). Finally, the sample was immersed in a solution of gold nanoparticles for 2 hours in order to verify the localized surface modification of MPTMS only on the surface of silicon, not on the surrounding

polymer layers by visualizing selective binding of gold nanoparticles.

In order to study the surface chemistry of polymer by heating at elevated temperatures, we have used pieces of

211 cm× (100) p-type silicon substrates. After piranha cleaning (H2SO4 + H2O2, 120ºC) and low power (10W) O2 plasma treatment, the samples were coated with PTFE by vapor-based deposition. Then they were heated at different elevated temperatures (300 ºC, 400 ºC, and 540 ºC) on hotplates for various time periods. Their contact angle and removal rate was studied by surface profiler and contact angle analyzer. The surface chemistry of thermally ablated samples was investigated by x-ray photoelectron spectroscopy (XPS).

III. RESULTS AND DISCUSSION

A steady state heat transfer analysis was done by finite element analysis (FEA) with FEMLAB® as shown in Fig. 3 (a). When thermal energy is applied to nanowire #1 (left-most nanowire) by Joule heating, the temperature increase is highly localized. The temperature of the mid-point of nanowire #1 is 815.3K while those of nanowire #2 and 3 are 511.7K and 405.9K. From our thermal ablation test with hot plate, 815.3K is a temperature high enough to ablate PTFE layers in a short time period (~1 minute). In contrast, the temperatures of neighboring nanowires are not sufficiently high for the ablation of PTFE. Therefore, we can expect a very localized ablation of PTFE only on the nanowire #1 and its vicinity while other nanowires and their surrounding polymers are still protected by PTFE. When the electrical bias is applied on the nanowire #1, 4, 7, and 10, the temperature distribution is also localized on these nanowires as shown Fig.3 (b). The temperatures of these nanowires are increased up to 813.6K. Although the temperatures of the neighboring nanowires increase, there temperature is maintained below 600K, which is a low temperature for the ablation of the PTFE. Therefore, PTFE is thermally ablated only along nanowire #1, 4, 7, and 10, while the rest of nanowires are still protected by PTFE layers. Since this simulation has excluded the possibilities of nanoscale heat transfer phenomena [11-14], there could possibly be a considerable discrepancy from the real

Temperature distribution by localized heating

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Fig. 3. Temperature distribution in silicon nanowire arrays by Joule heating through silicon nanowire (a) heat applied only on nanowire #1 (left-most nanowire) (b) heat applied on nanowire #1, #4, #7, and #10.

experimental results. If the thickness of the silicon thin film gets smaller than 100 nm level, the effect of phonon scattering at the film boundaries cannot be ignored since the mean free path of phonon is in the order of ~100nm [11]. Due to the boundary phonon scattering effect, the effective thermal conductivity of silicon can be reduced down to 30~50% for a nanowire of 100nm thickness and width [11,12,14]. When we consider this thermal conductivity reduction in the numerical simulation, the temperatures of nanowires by Joule heating actually shows a slight increase of temperature (820K for 30% of conductivity reduction) for nanowire #1. For more accurate estimation of temperature distribution, further detailed heat transfer modeling that includes nanoscale heat transfer phenomena in the thermal conduction, convection, and radiation should be used. Also, multiphase heat transfer including the thermal decomposition of PTFE should be included for really accurate calculation. Due to the lack of established and accurate analysis method in the literature for this kind of multiphase / nanoscale heat transfer problems, we are not currently able to do more precise estimation of temperature distribution by nanoscale Joule heating. However, the numerical analysis conducted in this paper will provide preliminary information for the first order approximation.

The selective ablation of protective polymer layer by localized Joule heating has been verified by polysilicon microwire. Fig. 4 shows a local removal of PTFE by applying Joule heating across the silicon microwire of 100nm thickness, 2µm width, and 16 µm length. The removal of PTFE is visually observed instantly (<1 sec) after applying a bias of 25V for 5 seconds across the silicon microwire. As shown in Fig.4 (a), the removal of PTFE occurs only on the silicon microwire and its close proximity. In the electrical circuit, most of the electrical potential drop occurs across the silicon microwire since it contributes to most of the electrical resistance. Therefore, most of the electrical energy is dissipated on the silicon microwire by Joule heating. AFM scanning image in Fig.4 (b) also shows the local removal of PTFE film only on the silicon microwire and its close proximity. There is also a formation of slight bump in a semicircular shape by ~80nm at both ends of the silicon microwire. The removal depth of the PTFE is measured to be ~35nm, which is 15 nm smaller than the thickness of deposited PTFE thin film. Further Joule heating for 2 minutes allowed the complete removal of deposited PTFE thin film along the microwire. With this protocol it is possible to achieve sufficient removal of the protective, patterning polymer (i.e., PTFE), so that a subsequent O2 plasma cleaning step may result in a “clean” nanowire surface and therefore makes possible the subsequent functionalization. If we apply a bias of 30V, excessive Joule heating causes the destruction of silicon microwire. Our speculation is that this fracture of microwire is due to the melting of polysilicon by excessive Joule heating.

We have verified the localized ablation of protective polymer by numerical analysis and experiment above. However, the surface chemistry of the ablated location is more critical than the physical removal of the polymer film. If the

Temperature distribution by localized heating of nanowire

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Fig. 4. Experimental verification of localized thermal ablation of hydrophobic protection layer (PTFE) in silicon microwire (2µm width and 16 µm length). Optical micrograph (a) and AFM scanning (b) shows that uniformly deposited PTFE was locally ablated along the silicon microwire after heating with a bias of 25V bias across the microwire (c) polysilicon microwire was broken by applying bias of 30V across the wire. We speculate that this is due to melting of polysilicon by excessive Joule heating. surface of silicon is left with highly carbonized chemistry or severely contaminated with impurities after ablation of polymer films at elevated temperatures, it will no more be available for further surface functionalization. In order to find the change of surface chemistry by thermal ablation of PTFE on silicon nanowire, we prepared 211 cm× die size samples coated with PTFE thin films and tested their thermal ablation at different elevated temperatures for different time periods on a hotplate. We measured the removal rate of PTFE, contact angle, and XPS spectra of the surface. In Fig. 5 is shown the change of thickness and contact angle of polytetra-fluoroethylene (PTFE) film by thermal ablation at different temperatures (300, 400, and 540ºC). All the samples started with PTFE film with thicknesses of 51-54nm. As shown in Fig.5 (a), the removal rate of PTFE is a strong function of temperature. The film is completely removed after 60 seconds of heating at 540 ºC while it takes 240 seconds to completely remove the film at 400 ºC. Furthermore, the film thickness reduces but eventually is saturated approximately at 26nm by heating at 300 ºC. Fig.5 (b) shows that the contact angle change is also a strong function of ablation temperature. The sample surface remains hydrophobic (contact angle > 100º) by ablation at 300ºC. This is because PTFE layer is still remained by heating at this temperature as shown in Fig. 5 (a). The sample surface turns from a hydrophobic surface to a semi-hydrophilic (contact angle ~ 40º) surface by ablation at 540 ºC. This surface can be made further hydrophilic (40º → <3º) by low power (10W) O2 plasma for short time period (10 sec). In contrast, the sample heated at 300 ºC shows a drop of contact angle only down to 97º by O2 plasma treatment. These results indicate that the best strategy to selectively functionalize the silicon micro/nanowire is to apply Joule heating for an optimal time period for the maximization of film thickness contrast between higher and lower temperature locations, and then to apply a low power O2 plasma for further activation of surface.

Thickness change of PTFE film by thermal ablation at 300 - 540 degC

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Fig.5. Thickness and contact angle change of polytetrafluoroethylene (PTFE) film by thermal ablation at different temperatures (300, 400, and 540ºC). This result shows a strong dependence of removal rate and contact angle of PTFE coated Si substrate on the ablation temperature.

In Fig. 6 is shown the XPS spectra of the PTFE-coated Si samples that were heated at 400ºC for 4 minutes and 540ºC for 2 minutes and its comparison with control sample (piranha cleaned and O2 plasma treated bare Si substrate). The survey scan in Fig. 7 (a) shows that the peak of fluorine (F 1s) from PTFE is remained after heating at 400ºC for 4 minutes while the F 1s peak is greatly reduced by heating at 540ºC for 2 minutes. Detailed XPS spectra in Fig. 7 (c) also verifies a fluorinated surface of Si after thermal ablation at 400ºC for 4 minutes. The peaks of carbon (C 1s) of both 400 ºC and 540ºC ablated samples resemble that of clean Si substrate due to the contaminants on the sample surface as shown in Fig. 7 (b). In all the samples, the peak of oxygen (O 1s) is strong, indicating the native oxide on the Si substrate. The results of XPS spectra, contact angle, and thickness plot of ablated PTFE assure that the temperature gradient in silicon micro/nanowire will enable a very localized surface functionalization.

After selective removal of PTFE film by localized Joule heating and low power O2 plasma treatment, we have functionalized the surface silicon microwire with 3-mercaptopropyltrimethoxysilane (3-MPTMS) by vapor phase deposition. This particular chemical group is a common linker for the attachment of protein or DNA molecules on the silicon-based surface. The thiol (-SH) group also has a strong affinity to the surface of gold. Because of this reason, we have used gold nanoparticles to verify that 3-MPTMS was selectively bound to silicon microwire, not on the surrounding regions covered with PTFE. After through rinsing with DI water and IPA for the removal of nonspecifically bound 3-MPTMS groups on the PTFE film, we have immersed the sample in a solution of gold nanoparticles with a diameter of 40nm and 100nm. After storing at room temperature for 2

XPS spectra of thermally ablated surface of Polytetrafluoroethylene (PTFE) on Si

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Fig. 6. XPS spectra of thermally ablated surface of PTFE on Si at 400 ºC and 500ºC. Survey scan (a) and detail scan of F1s (c) shows that the thermal ablation at 400ºC for 4 minutes leaves fluorinated surface on the silicon substrate while 540 ºC for 2 minutes removes both carbon (C) and fluorine (F) from the substrate. hours and additional gentle rinsing with DI water, we could observe that gold nanopartciles were selectively bound to the surface of silicon microwire only as shown in Fig. 7. Although there are some physically adsorbed gold nanoparticles on the surface of PTFE, the contrast of surface affinity to gold nanoparticles is huge. This result proves that we could successfully achieve a selective surface functionalization of 3-MPTMS functional group on silicon by the scheme of localized Joule heating of silicon microwire.

Fig. 7. Selective attachment of Au nanoparticles on 3-MPTMS group attached selectively on the surface of silicon. This verifies that selective surface functionalization can be done by thermal ablation based on local Joule heating

IV. CONCLUSION

In this paper, we have developed a novel method for

selective surface functionalization of silicon micro/nanowire by thermal ablation of hydrophobic background polymer layers with self-aligned local Joule heating of silicon micro/nanowire. This thermal ablation is followed by an O2 plasma cleaning step before subsequent functionalization by an organosilane polymer. The simulation and experimental results show that application of a moderate electrical bias (<50V) across silicon micro/nanowire can enable highly localized thermal ablation of PTFE thin film from silicon substrate. Contact angle, thickness removal, and XPS spectra results indicate that Joule heating at 540 ºC for a short time period (<2 min), with additional low power O2 plasma treatment, can readily make the surface of silicon for surface functionalization. This is done without O2 plasma damage to the unablated protective, patterning polymer. We have used gold nanoparticles to visualize selective surface modification of silicon surface with 3-MPTMS.

ACKNOWLEDGEMENTS

This project has been supported by Center for Nanoscale Mechatronics and Manufacturing (Grant No. 019997), one of the 21st Century Frontier Research Programs, which are supported by Ministry of Science and Technology, Republic of Korea. We also thank Dr. William Flounders at Berkeley Sensor and Actuator Center (BSAC) and Dr. Steven Ruzin at Biological Imaging Facility (BIF) of UC Berkeley for valuable discussions.

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