Wafer Surface Charge Reversal as a Method of Simplifying Nanosphere Lithography for Reactive Ion Etch Texturing of Solar Cells

Hindawi Publishing Corporation Advances in OptoElectronics Volume 2007, Article ID 32707, 4 pages doi:10.1155/2007/32707 Research Article Wafer Surface Charge Reversal as a Method of Simplifying Nanosphere Lithography for Reactive Ion Etch Texturing of Solar Cells Daniel Inns, Patrick Campbell, and Kylie Catchpole Centre of Excellence for Advanced Silicon Photovoltaics and Photonics, University of New South Wales, Sydney NSW 2052, Australia Received 23 April 2007; Accepted 18 July 2007 Recommended by Armin G. Aberle A simplified nanosphere lithography process has been developed which allows fast and low-waste maskings of Si surfaces for subsequent reactive ion etching (RIE) texturing. Initially, a positive surface charge is applied to a wafer surface by dipping in a solution of aluminum nitrate. Dipping the positive-coated wafer into a solution of negatively charged silica beads (nanospheres) results in the spheres becoming electrostatically attracted to the wafer surface. These nanospheres form an etch mask for RIE. After RIE texturing, the reflection of the surface is reduced as effectively as any other nanosphere lithography method, while this batch process used for masking is much faster, making it more industrially relevant. Copyright © 2007 Daniel Inns et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION Multicrystalline Si wafers make up the majority of the photovoltaic market. Due to the variety of grain orientations, they cannot be textured cheaply by the anisotropic wet chemical etches used for monocrystalline silicon. One possibility for texturing is reactive ion etching (RIE). RIE can randomly texture a Si surface, with the minimum reflectance of below 5% achieved vcommentWe changed “masking” to “maskings” for the sake of clarity. Please check. [1, 2]. This maskless RIE requires the etching of several microns of Si and has the resulting long process times. The use of a mask during RIE allows greater control over the surface topography than maskless RIE and results in shallower textures with reduced etch times. The best reflection achieved to date using an RIE mask applied a photolithography process to form pyramids [3], with minimum reflectances around 5% achieved. As an alternate to a photolithography mask, RIE masks can be formed by nanospheres. Nanospheres are highly uniform spheres (typically of silica or polystyrene) and are commercially available and very cheap. In previous work with nanosphere masks, the mask has been formed by spincoating, resulting in a uniform monolayer of spheres across the surface. These particles are then the etch mask for subsequent RIE [4]. Without an antireflection coating, the mini- mum reflectance is reduced around 13% by etching less than 500 nm of Si—making the process fast and high-throughput compared to other RIE textures, with or without masks. Spinning-on of the nanosphere mask requires very careful control of the nanoparticle solution used, with many critical factors including spin speed, substrate surface treatments, and particulate concentration [5]. The typically rough wafer surface complicates spinning as some large regions are left unmasked [4]. Additionally, the throughput would be low and the masking solution wastes high for a factory application. A solution with a high nanosphere solids content is required for this spinning process and accentuates the waste problem. Dipping a wafer into a masking solution would be attractive in a manufacturing environment, as the batch process could have very high throughput. Dipping could also coat large area wafers, 1 m2 glass sheets, or other objects which are not suitable for spinning due to size, asymmetry, or weight. For some applications, it would also be an advantage to mask both sides of a wafer at once, to achieve a double-sided texture. Silicon dioxide, which comprises the surface of the standard wafer and the silica beads commonly used for the masking, has a negative surface charge when immersed in a solution with a pH above its isoelectric point of ∼2.5 [6]. To electrostatically attract the negative spheres to the negative wafer 2 Advances in OptoElectronics (a) Figure 1: Dark field optical microscope image of the nanosphere mask applied to a wafer by reversing its surface charge. The image is 180 × 160 µm. surface, the surface charge of the wafer must be reversed by the application of a positive coating. This work shows that a simple dipping process can reverse the surface charge of a wafer, and another dip can mask the surface. Masking based on this idea can lead to high-nanosphere surface coverage, and etching through the nanosphere masks is well controlled by RIE. A variety of textures can be formed and very good reflectance reduction achieved. The geometry of the texture features can be altered by changing RIE processing parameters (primarily pressure and gas mixtures), which also controls the etch rate of the nanospheres that act as the mask. 2. (b) EXPERIMENTS AND DISCUSSION (c) Below is outlined the process used to optimize the two solutions used for dip coating wafers, first for wafer surface charge reversal, followed by electrostatic attraction of a nanosphere lithography mask. Afterwards, some RIE textured surfaces are examined by SEM and the reflectance of the best texture assessed. 2.1. Reversing the surface charge of silicon wafers Layer-by-layer coating can be used to build up a variety of types of films, by alternate deposition of positive followed by negative ions, molecules, or colloids [7]. The use of colloidal particles such as nanospheres in the layer-by-layer method was pioneered by Iler [8]. In this work, Iler’s technique was used to deposit a single bilayer of hydrolyzed Al+ ions followed by negatively charged SiO2 nanospheres. The composition of the Al+ solution and the pH of the nanosphere solution were adjusted to optimize the mask coatings. Before any surface treatments, a polished and oxide-free Cz Si wafer surface was oxidized by a 5-minute piranha clean (H2 SO4 : H2 O2 solution, 2 : 1 mixture) and then rinsed for 10 minutes. A polished surface was chosen as its predictably high reflection is best for quantifying the benefits of the resulting texture. In order to attach a positive Al coating to the wafer, a set of AlNO3 solutions (concentration varying from 400–800 µM) was prepared, and small amounts of 50 mM NaOH were added to adjust the pH and concentration of the solutions. The cleaned wafer was dipped into the solution Figure 2: SEM images of the nanosphere masks attached to a silicon wafer surface using 3 different pH masking solutions: (a) pH = 4, (b) pH = 3.5, (c) pH = 3.2. The images are 80 µm × 80 µm. for 30 seconds to allow adsorption of a positive layer. Immediately afterwards, a 10 seconds dip-in-deionized water was followed (...truncated)