Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

Summary

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with H₂O₂/H₂O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

Abstract

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H₂O₂/H₂O solution did not show measurable change in their initial contact angle (CA) ~75°. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H₂O₂/H₂O solution at 250 and 65 mJ/cm², respectively, has decreased the CA to near 15°, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer’s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH₃OH) using ArF laser operating at pulse fluence of 65 mJ/cm² and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103°. The XPS results indicate ArF laser induced formation of Si–(OCH₃)_x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH₃ radicals.

Introduction

The remarkable electronic and chemical properties as well as its high mechanical strength have made silicon (Si) an ideal choice for microelectronic devices and biomedical chips¹. Selective area control of the Si surface has received significant attention for applications involving microfluidic and lab-on-chip devices^2,3.This is often obtained either by nano-scale modification of the surface roughness or by chemical treatment of the surface⁴. The surface roughening or patterning to produce disordered or ordered surface structures on the Si surface include photolithography⁵, ion beam lithography⁶ and laser techniques⁷. Compared with these methods, laser surface texturing process is reported to be less complicated with the potential to produce microstructures with high spatial resolution⁸. However, as Si has an elevated texturing threshold, requiring irradiation with pulse fluence to induce surface texturing in excess of its ablation threshold (~500 mJ/cm²)⁹, texturing of Si surface has frequently been assisted by employing reactive gas atmospheres, such as that of a high pressure SF₆ environment^4,7,8. Consequently, to modify wettability of the Si surface, numerous works have focused on chemical treatment by depositing organic¹⁰ and inorganic films², or using plasma or electron beam surface treatment^11,12. It is recognized that hydrophilicity of Si originating from the existence of singular and associated OH groups on its surface could be achieved by boiling it in a H₂O₂ solution at 100 °C for several minutes¹³. However, the hydrophobic Si surface states, most of which are due to the presence of Si-H or Si-O-CH₃ groups, could be achieved by wet chemical handling involving etching with HF acid solution or coating with photoresist^13-15. To achieve selective area control of wettability of Si, complex patterning steps are usually required, including treatment in chemical solutions¹⁶. The high chemical reactivity of UV laser radiation has also been used to selective area process organic film coated solid substrates and modify their wettability¹⁷. However, a limited amount of data is available on laser-assisted modification of Si wettability by irradiation of samples immersed in different chemical solutions.

In our previous research, UV laser irradiation of III-V semiconductors in air^18-20 and NH₃²¹ was successfully used to alter the surface chemical composition of GaAs, InGaAs and InP. We established that UV laser irradiation of III-V semiconductors in deionized (DI) water decreases surface oxides and carbides, while the water adsorbed on semiconductor surface increases²². A strongly hydrophobic Si surface (CA~103°) was obtained by ArF laser irradiation of Si samples in methanol in our recent work ²³. As indicated by X-ray photoelectron spectroscopy (XPS), this is primarily due to the ability of the ArF laser to photodissociate CH₃OH. We have also used KrF and ArF lasers to irradiate Si (001) in a 0.01% of H₂O₂ in DI water. This allowed us to achieve selective area formation of superhydrophilic surface of Si (001) characterized by the CA of near 15°. The XPS results suggest that this is due to generation of Si-OH bonds on the irradiated surface²⁴.

A detailed description of this new technique using KrF and ArF lasers for selective area in situ modification of the hydrophilic/hydrophobic surface of Si surface in low concentration of H₂O₂/H₂O and methanol solutions is demonstrated in this article. The details provided here should be sufficient to allow similar experiments to be performed by interested researchers.

Protocol

1. Sample Preparation

1. Use a diamode scribe to cleave an n-type (P-doped) one-side polished Si wafer (resistivity 3.1~4.8 Ω.m) which is 3 inch in diameter, 380 µm thick, into samples of 12 mm x 6 mm; clean the samples in OptiClear, acetone and isopropyl alcohol (5 min for every step).
2. Etch samples in a ~0.9% HF solution for 1  min to etch away initial oxide; rinse in DI water and dry in high-purity (99.999%) nitrogen (N₂).
3. Store prepared samples in N₂ bag to curb their  oxidation in air.

2. Irradiate Samples by ArF (λ=193 nm) and KrF (λ=248 nm) Lasers.

1. Place samples in a 0.74 mm tall chamber and then seal the chamber with a fused silica window that has high transmission in UV (≥90%). Fill the chamber with H₂O₂/H₂O solution in the range of 0.01-0.2 % or with degassed methanol using a microfluidic channel.
2. Irradiate samples with homogenized ArF or KrF lasers at demagnification of 2.6 and 1.8, respectively. Irradiate only  2 sites on each sample by increasing laser pulses from 100 to 600 in step of 100 pulses  through a circular mask (4 mm in diameter). Irradiate the samples in the same way with a “maple leaf” (9 mm x 7.2 mm) mask.
3. Rinse samples in DI water,  dry with N₂ flush; place the samples in a sealed container, then quickly fill this container with N₂, in order to avoid exposure to air prior to further experiments.

3. Immobilization of Bio-conjugated Nanospheres

1. Dilute biotin-conjugated and fluorescein stained 40-nm-diameter nanospheres in a pH 7.4 phosphate buffered saline (PBS, 1X) solution to 10¹² particles/ml at RT (~25 °C). Immerse ArF or KrF laser irradiated samples for 2  hr in this solution at RT.
2. Wash samples with PBS to eliminate physically bound fluorescein stained nanospheres on the surface.

4. Surface Characterization

1. Contact angle (CA) measurement
   1. Carry out static CA measurements with a goniometer in an environment of RT and ambient humidity.
   2. Employ high purity DI water (resistivity 17.95 MΩ·cm) in a micro-syringe; generate similar volume (~5 µl) drops on the sample surface by lowering the micro-syringe to a similar height for every measurement.
   3. Capture and save the water drops profile images by CCD camera with software. Measure independently 4 different sites with same irradiation conditions.
   4. Estimate and average the CA values in drop analysis module from ImageJ software; load the image and change it into grayscale; launch the plugin Dropsnake; place roughly a few knots on the drop contour (~10 knots) from left to right to initialize snake; accept the curve connecting these knots and evolve the curve by pressing snake button. Note: contact angles are displayed in the image and the table.
2. XPS measurement
   1. Investigate surface chemical modification with a XPS spectrometer(1x10^-9 Torr base pressure) outfitted with an Al Kα source working at 150 W:
      1. Load the samples into the vacuum chamber.
      2. Acquire the surface survey data in constant energy modes of 50 eV pass energy from an area of 220 µm x 220 µm.
      3. Acquire high resolution scans data from the same analyzed area at 20 eV pass energy.
   2. Process XPS spectra data with XPS spectra quantification software, as referenced^25,26.
3. Fluorescence microscope imaging
   1. Excite samples, which were irradiated through “maple leaf” mask and exposed to fluorescein stained nanospheres, using a blue light source (λ=450~490 nm).
   2. Observe fluorescent images, emitting at 515 nm, with a fluorescence inverted microscope in magnification of 4X.
   3. Characterize the surface morphology of these samples with AFM, as referenced²⁷.

Results

These representative results have been presented in our previous published work^23,24. Figure 1 shows the CA vs. N (number of pulses) on sites irradiated by KrF laser at 250 mJ/cm² in DI H₂O for different concentrations of H₂O₂/H₂O solutions (e.g., 0.01, 0.02, 0.05 and 0.2%). The CA decreases with increasing pulse number for all the H₂O₂ solutions. The minimum CA (~15°) for the 0.02 and 0.01% H₂O

Discussion

We have proposed a protocol of UV laser irradiation of Si wafer in a microfluidic chamber filled with low concentration of H₂O₂ solution to induce a superhydrophilic Si surface, which is mainly due to the generation of Si-OH. UV laser photolysis of H₂O₂ was supposed to form negatively charged OH^- radicals. Also, UV laser photoelectric effect leads to the formation of a positively charged surface³⁷. Therefore, the interaction of these negative OH^-

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
fluorescein stained nanospheres	Invitrogen	F8795
OptiClear	National Diagnostics	OE-101
ArF laser (λ=193 nm)	Lumonics	pulse master 800
KrF laser (λ=248 nm)	Lumonics	pulse master 800
XPS	Kratos Analytical	AXIS Ultra DLD
Fluorescence microscope	Olympus	IX71
XPS quantitification software	CasaXPS	2.3.15