A simple and cost-effective approach for fabricating pyramids on crystalline
silicon wafers
A.K. Chu, J.S. Wang, Z.Y. Tsai
Ã
, C.K. Lee
Institute of Electro-optical Engineering and Semiconductor Technology Research and Development Center, National Sun Yat-sen University,70 Lien-hai Road, Kaohsiung 804, Taiwan
article info
Article history:
Received 9 September 2008
Received in revised form
19 January 2009
Accepted 28 January 2009
Available online 9 March 2009
Keywords:
Silicon
Texture
Pyramid
Pseudo-mask
abstract
A simple and cost-effective approach for texturing crystalline silicon wafers is proposed. The advantage
over conventional texturization processes is the fact that no surfactant is added in alkaline etchants.
Hydrogen bubbles, regarded as residuals in conventional texturization processes, are utilized in this
approach as etch masks on a silicon surface. This is accomplished by placing a metal grid with suitable
openings on silicon wafers to confine the hydrogen bubbles created during etching. Pyramids with a size
ranging from 6 to 9
m
m are uniformly fabricated using this method. Without antireflection coating, an
average weighted reflectance of 15.1% is achieved.
Crown Copyright & 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction
Surface texturization of (10 0)-oriented crystalline silicon
wafers is a frequently used technique in modern solar cell
processing to reduce optical reflections. Many successful ap-
proaches based on texturing silicon surface utilizing alkaline
solutions [1,2] have been reported. Alkaline etchants such as
sodium hydroxide (NaOH) [3] or potassium hydroxide (KOH) [4]
at low concentration in water expose Si {111} faces resulting in
square-based pyramids randomly distributed over the cell surface.
This texture will enhance light trapping capability by increasing
both coupling of lights into the cell and reflectivity [5,6] of lights
trying to escape from the cell [7,8]. In general, isopropyl alcohol
(IPA) is added to the alkaline etchants to improve the uniformity
of the random pyramid texture. Though IPA is expensive, it is
essential for the process because it removes hydrogen bubbles
sticking on the silicon wafer by improving the wettability of the
wafer surface. In a typical texturing condition, high concentration
of IPA is required [9] to obtain uniform pyramid textures.
However, the etching rate of silicon decreases drastically with
IPA concentration [10]. In addition, IPA is volatile in a heated
etching bath. To maintain a sufficient wettability of the silicon
surface, IPA must be constantly added to the solution to
compensate the amount lost due to evaporation. In current
commercial texturing technique based on alkaline anisotropic
etching, the cost of IPA can be the key factor of overall
texturization cost reduction approach.
Recent research on low-cost silicon texturization centers on
investigating alternative solutions [11,12] for surface texturing
and reducing amount of IPA in the solutions [13]. Several novel
processing techniques were reported using different chemicals,
such as tetramethyl ammonium hydroxide (TMAH) and sodium
phosphate (Na
3
PO
4
). Moreover, Nishimoto and Namba [14]
successfully textured the silicon surface with sodium carbonate
(Na
2
CO
3
) solution and claimed that no IPA was needed for
the process. In this paper, we propose a different approach
based on KOH texturization without adding any surface-active
additive in the solution. In a conventional alkaline etching
process, KOH reacts with silicon in water producing hydrogen
bubbles. These hydrogen bubbles with 2–3 mm diameter can
stick on the silicon surface and suppress chemical reaction
between the silicon surface and the KOH solution. This will lead
to insufficient pyramid nucleation and result in poor texture
uniformity and reproducibility. However, these bubbles can also
be used as a ‘‘pseudo-mask’’ [15] as long as they can be constantly
trapped on the silicon surface during the etching process. This is
accomplished by placing a metal grid with periodic opening at
close proximity to the silicon wafers. The hydrogen bubbles
created due to KOH-Si reaction can be confined by the openings
of the metal grid because of surface tension of the bubbles. Thus
the hydrogen bubbles, which are regarded as residuals in the
conventional texturization processes, are utilized in this approach
as the etch-mask on the silicon surface. Pyramids with a size
ranging from 6 to 9
m
m are uniformly fabricated using this
method.
ARTICLE IN PRESS
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/solmat
Solar Energy Materials & Solar Cells
0927-0248/$ - see front matter Crown Copyright & 2009 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2009.01.018
Ã
Corresponding author. Tel.: +886 7 5252000 Â 4466; fax: +886 7 5254456.
E-mail addresses: chu5066@faculty.nsysu.edu.tw (A.K. Chu),
intergrand@hotmail.com (Z.Y. Tsai).
Solar Energy Materials & Solar Cells 93 (2009) 1276–1280