Surface patterning nanoparticle-based arrays
- 232 Downloads
In this study, focused ion beam lithography is used to pattern different size and shape island arrays on silicon wafers. Cavity arrays of inverse shapes are then made on silicone mold surfaces by polymerization. After that, Al2O3 nanoparticle-based island arrays are created by a surface feature transfer and freeze casting process using an Al2O3 colloidal suspension. The effects of silicone mold surface wettability and freezing rate on the Al2O3 nanoparticle pattern quality are investigated. The results show that coating the silicone mold surface with a 10 nm thick Au–Pt layer makes the Al2O3 nanoparticle suspension more wetting on the mold surface and also likely reduces the dry Al2O3 nanoparticle adhesion to the mold surface. Freezing rate should be lower than 1 °C/min to avoid cracks or loose Al2O3 nanoparticle packing in the freeze cast features. When these factors are properly controlled, the reported patterning process allows reproduction of micron-size feature arrays from Al2O3 nanoparticle suspensions. The studied approach should be applicable to most nanoparticle-based materials and open numerous opportunities for direct-device fabrication.
KeywordsMold Mold Cavity Mold Surface Freezing Rate Al2O3 Nanoparticle
Patterning has attracted considerable attention because of its potential applications in microelectronics [1, 2], optoelectronics, sensing , and biomedical devices . The methods used for patterning can be classified into two categories: top–down and bottom–up. The top–down approach uses a patterned template as a mask/mold to create patterns. The process includes imprinting and lithographic patterning [5, 6, 7]. The top–down method has the advantages of precise control of feature size, easy preparation of patterns on masks/molds, and ability to fabricate large area patterns . The bottom–up approach involves self-assembly of ions/molecules/nanoparticles on substrates via electrostatic attraction , van der Waals force, covalent bond , or molecular recognition ; it is usually used to obtain parallel or single-line patterns for device applications.
The top–down methods are preferred for creation of ordered patterns. The bottom–up methods can offer finer resolution but the process is time-consuming . If the top–down and the bottom–up approaches can be combined, new pattern creation capability might be possible. For example, the combination of contact charge transfer and polydimethylsiloxane mold enabled deposition of patterned nanoparticles . Chemically directed nanoparticle assembly in combination with nanoimprinting allowed patterning of nanoparticles on different substrates .
Nanoparticles are widely available and have been the subject of intensive research for many years because of their promising applications [14, 15, 16], such as photonics, electronics, catalysis , and sensing . Nanoparticles can be used as simple and versatile building blocks to form new devices and three-dimensional structures when they are properly arranged and the inter-particle or particle/substrate interactions are properly invoked [19, 20].
One exciting area to explore is to use templated molds to regulate and confine nanoparticles into a pre-determined pattern arrays. The combination of templating and nanoparticle processing provides a simple and effective technique to pattern different types of nanoparticles and can be used for fabrication of novel micro- and even nano-devices . The process consists of three consecutive transfer steps: patterning of mold core, mold making, and nanoparticle-based feature creation . However, formation of patterns with good feature fidelity and integrity is still a challenge. The interaction between a specific nanoparticle suspension and the corresponding mold surface needs to be better understood in order to avoid feature sticking and breakage. Also, packing of deposited nanoparticles needs to be reasonably dense in order to retain the formed feature arrays.
In this work, a nanoparticle-based surface templating process is studied. First, silicone molds are made by focused ion beam (FIB) lithography and templated feature array transfer. Second, a 35 vol% solids loading Al2O3 nanoparticle suspension is produced. Finally, different Al2O3 nanoparticle-based feature arrays are obtained by freeze casting. Wettability of the Al2O3 nanoparticle suspension on the silicone mold surface and freezing rate are studied in order to obtain well-defined feature arrays. This templating and freeze casting approach allows creation of micron-size feature arrays from the Al2O3 nanoparticle suspension.
In this study, Si wafer (p type, (100) orientation) was used as the mold core and was first patterned using a dual beam FIB microscope (FEI Helios 600 NanoLab, Hillsboro, OR). The instrument was composed of a sub-nanometer resolution field emission scanning electron microscope (SEM) and a field emission scanning Ga+ beam column. Samples can be moved by 150 mm distance along X and Y axes and tilted from −5° to 60° by a high precision specimen goniometer. The Ga+ source had a continuously adjustable energy range from 0.5 to 30 kV and an ion current between 1.5 pA and 21 nA. The patterning beam spot size and resolution were a function of the ion beam current and voltage and can be tuned to as small as 5 nm. The FIB microscope can read 24 bit digital patterns and reproduce them on a substrate using the ion beam.
Silicone molds were produced using the templated silicon wafer as the mold core [23, 24, 25, 26, 27]. First, the patterned silicon wafer was glued onto a 25 mm diameter and 2.5 mm thick aluminum disk. Silicone base compounds and curing agent (RTV 664, General Electric Company, Waterford, NY) were homogeneously mixed at 10:1 ratio before being poured over the silicon wafer and the aluminum disk assembly. A 30 mm diameter and 10 mm high plastic tube with a 1.0 mm thick wall was used to obtain a short silicone cylinder. The silicon wafer/aluminum disk assembly and the silicone mixture were placed under 40 Pa pressure for the removal of the air bubbles in the silicone compounds. Since the silicone mixture had high viscosity, the vacuuming process was repeated three to five times before being kept at 40 Pa for 15 minutes. Then the silicone mixture was cured for 24 hrs at ambient conditions before being separated from the silicon wafer. One end of the silicone cylinder receded inwards with a 1.5 mm thick wall and the receded surface was defined by the assembled aluminum disk and the silicon wafer. The silicone sample was termed ‘mold’ in this study.
Al2O3 suspension preparation
Al2O3 nanoparticles with a specific surface area of 45 m2/g were used in this study (Nanophase Technologies, Romeoville, IL). Average Al2O3 particle size was around 38 nm with a normal size distribution from 10 to 50 nm . For the Al2O3 nanoparticle suspension preparation, poly(acrylic acid) (PAA, M W 1,800, Aldrich, St Louis, MO) was used as a dispersant; glycerol (C3H8O3, Fisher Chemicals, Fairlawn, NJ) was used as an anti-freezing agent. A water-glycerol mixture at a ratio of 9:1 (water:glycerol) was used as the dispersing medium. The mixture was homogenized for 5 min using a ball mill before use. Al2O3 nanoparticles were added into the dispersing medium in 10 g increments along with 2.0 wt% of PAA dispersant (on Al2O3 basis). Since low pH promotes PAA dispersant adsorption onto Al2O3 nanoparticles , HCl solution was added to lower the pH to 1.5. The suspension was ball milled for 12 hrs with periodic adjustment of pH to 1.5. Suspensions of approximately 20 vol% Al2O3 solids loading were made by this procedure. After this step, Al2O3 nanoparticles were again added in 10 g increments, along with 2.0 wt% of PAA dispersant (on Al2O3 basis) to make 35 vol% solids loading suspension. NH4OH was used to adjust the suspension pH to 9.5. The suspension was then mixed for 24 hrs for complete homogenization.
Contact angle measurement
The contact angle between the Al2O3 nanoparticle suspension and the silicone mold was measured using a 3D Digital Video Microscope (KH-7700, Hirox-USA, Inc., River Edge, NJ). First, the Al2O3 nanoparticle suspension was placed on the surface of the silicone mold by a syringe (1 cc 25G 5/8 Tuberculin, Becton–Dickinson & Company, Franklin Lakes, NJ). Then the droplet and the silicone mold images were taken using the microscope. The contact angle was measured from the images. An average value from ten different measurements was taken as the final contact angle result.
Templated feature array making and characterization
The experimental parameters varied in this study
Mold surface coating
Freezing rate (°C/min)
With Au–Pt coating
Without Au–Pt coating
The templated patterns and microstructural differences of the freeze cast features were analyzed using a LEO550 field emission SEM (Carl Zeiss MicroImaging, Inc, Thornwood, NY). To illustrate the three-dimensional nature of the templated features, the SEM images of the patterned arrays were taken at 45° tilt angle.
Template and mold making
Under 30 kV voltage and 21 nA beam current patterning condition, the arrays generated have well-defined size, shape, and feature arrangement. The surfaces of different features are smooth. The feature height is controlled by the patterning time and can be easily varied. The height of the homogenous circle and square island arrays is 4.8–5.0 μm. The height of the heterogeneous circle array is ~2.5 μm. During the FIB patterning, it is important to obtain well-focused ion beam. Good beam focus produces smoother feature surfaces in a faster rate . However, it should be pointed out that the patterned feature shapes in Fig. 1 are slightly tapered in the height direction. This is a result of the Gaussian distribution shape of the ion beam. The diameter of the ion beam increases as its upper portion interacts with the silicon feature side surface. Scanning of the ion beam removes materials from the sides of the features while the features grow taller.
Au–Pt nanolayer effect
Coating the molds with a Au–Pt nanolayer shows to be an effective approach to reducing the patterned feature distortion and damage. The fundamental cause for the distortion of the features seen in Fig. 3 can be understood by examining the freeze casting process in two stages: the wet stage of the Al2O3 nanoparticle suspension and the dry stage of the Al2O3 nanoparticles in the molds.
After the Al2O3 nanoparticle samples are dried, the removal process from the silicone molds is affected by the mold surface smoothness. With the Au–Pt coating, the mold surface should be smoother since the Au–Pt layer is only 10 nm thick. The Au–Pt nanolayer decreases the adhesion between the silicone mold and the Al2O3 nanoparticle sample. This facilitates the Al2O3 sample removal process without causing undesirable feature damage.
It should be pointed out that the above explanations need to be experimentally verified to be sure. Also, the Au–Pt nanolayer is very thin and should not be scratched in order to avoid damage. The Au–Pt nanolayer can stick to the Al2O3 nanoparticle sample and get removed from the mold surface during the mold release. So the Au–Pt nanolayer is mostly only effective for one time use.
Freezing rate effect
Al2O3 nanoparticle-based feature arrays are patterned by a surface templating and freeze casting process. Mold surface wettability and freezing rate greatly affect the pattern quality and the feature microstructures. Coating the silicone mold surface with a 10-nm thick Au–Pt layer makes the surface more wetting for the Al2O3 nanoparticle suspension and also likely reduces the dry Al2O3 nanoparticle adhesion to the mold surface. Freezing rate should be lower than 1 °C/min in order to avoid microcracks and loose nanoparticle packing for the freeze cast features. When these factors are properly controlled, the patterning and freeze casting process allows reproduction of different micron-size feature arrays. This approach should be applicable to most nanoparticle suspensions and open numerous opportunities for direct-device fabrication.
The authors acknowledge the financial support from National Science Foundation under Grant No. CMMI-0824741.
- 22.Lu K, Hammond C, Int J Appl Ceram Technol (submitted)Google Scholar
- 25.Lu K (2007) J Am Ceram Soc 90(12):3753Google Scholar
- 27.Lu K, Kessler CS (2006) In: Mullins WN, Wereszczak A, Lara-Curzio E (eds) Ceram engineering and science proceedings, vol 27(8), pp 1–10Google Scholar