1
Wet chemical textures for crystalline silicon solar cells
Dissertation submitted for the degree of Doctor of Natural Sciences Dr. rer. nat.
Presented by
Jose Nestor Ximello Quiebras at the
Faculty of Sciences Department of Physics
Konstanz, 2013
Tag der mündlichen Prüfung: 13.12.13 1. Referent: Prof. Dr. Giso Hahn
2. Referent: Prof Dr. Johannes Boneberg 3. Prof. Dr. Peter Nielaba
Abstract
In this work, two alternative solutions to the standard potassium hydroxide (KOH) – isopropyl alcohol (IPA) texturization process of as-cut mono-crystalline silicon (mono-Si) wafers used in photovoltaics are presented.
The standard KOH-IPA etch solution suffers from two drawbacks when the texturization process is carried out at 80oC, i.e. near the boiling point of IPA (82.4oC), as it is usually carried out in the photovoltaic community.
The first problem corresponds to the constant evaporation of IPA during the etching process. The quick solution here is the re-dosing of IPA, but unfortunately this solution has economic disadvantages, i.e., high costs.
The second problem corresponds to the high sensitivity of the KOH-IPA etch solution to the wafer characteristics of the as-cut mono-Si wafers. In other words, it means that different pyramidal results are obtained when different assortments of as- cut mono-Si wafers (e.g. from different manufacturers) are textured.
In this context, the introduction of new sawing methods for silicon also changes the surface morphology of the as-cut silicon (Si) wafers. This implies a modification of the etching process in order to obtain almost the same pyramidal texture on differently sawn wafers.
The solution proposed for the first problem corresponds to the usage of another alcohol. An alcohol with high boiling point in order to avoid constant evaporation of it during the etching process and to reduce etching time. The alcohol which fulfills these demands is polyvinyl alcohol (PVA) and the new solution is now called KOH- PVA solution. Due to the characteristics of the new alcohol an etching temperature of 100oC was used.
Although the pyramidal texture results produced on as-cut mono-Si wafers by using the new KOH-PVA etch solution are similar to those obtained by using the standard KOH-IPA etch solution, some well-defined differences were observed. For example, the pyramid size of the KOH-PVA texture (4 µm in average) is only about half of the pyramid size of the KOH-IPA texture (8 µm in average). Another important difference is that the KOH-PVA texture shows 1% lower weighted reflectance (between 400-1100 nm) compared to the reflectance of KOH-IPA textured wafers.
When trying to explain this difference by means of geometrical optics and the ray tracing software SUNRAYS, no difference was found. This is due to the fact, that in geometrical optical theory it is assumed that the size of the pyramids is bigger than the wavelength impinging on it, and because SUNRYAS does not take into consideration the random nature of the wet chemically produced pyramidal texture.
However, the difference in reflectance can be well explained by solving the wave equation, and by considering diffraction phenomena of pyramids (or structures) with sizes between 1-4 µm. With this method it is also possible to take into consideration the random nature of the wet chemically produced pyramidal texture.
Although some optical differences are observed between the KOH-IPA and KOH- PVA texture, at solar cell level both textures show similar results.
The second solution proposed in this work corresponds to the introduction of a closed etching bath in order to recover evaporated IPA (from the standard KOH-IPA etch solution) during the etching process. The closed etching bath was developed by the wet etching company LOTUS. A cooling system was adapted on top of the closed etching bath in order to liquefy evaporated IPA and conduct it again to the reservoir.
Furthermore, the new closed etching bath enables the periodic application of low pressure (below atmospheric pressure) inside the closed etching bath during the etching process. This accelerates the etching process, and therefore the etching time can be reduced to almost half of the time of a texturization process taking place at atmospheric pressure (by using the same standard KOH-IPA etch solution).
The periodic decrease of the pressure inside the bath, containing a standard KOH-IPA solution and the as-cut mono-Si wafers at constant temperature of 80oC, implies a decrease of the boiling point of IPA. Therefore IPA evaporates periodically when low pressure is applied, thereby removing the hydrogen bubbles and monosilicic acid particles from the surface of the silicon wafers. Thus the surface can be supplied with fresh etch solution and the etching process is accelerated.
Zusammenfassung
In dieser Arbeit werden zwei mögliche Alternativen zum Standard Kaliumhydroxid (KOH) – Isopropylalkohol (IPA) Texturierungsprozess von as-cut mono-kristallinen Silizium (mono-Si) Wafern vorgestellt. Der Ätzprozess entspricht dem Standardprozess in der Photovoltaik.
Die Standard KOH-IPA-Ätzlösung hat zwei Nachteile, wenn der Texturierungsprozess bei einer Temperatur von 80oC (nahe dem Siedepunkt von IPA) durchgeführt wird, wie es in der Photovoltaik üblich ist.
Das erste Problem ist das ständige Verdampfen von IPA während des Ätzprozesses. Eine schnelle Lösung für dieses Problem ist das Nachfüllen von IPA, was aber zu einem höheren IPA-Verbrauch und so zu höheren Kosten des Ätzprozesses führt.
Das zweite Problem ist die hohe Empfindlichkeit der Ätzlösung gegenüber den Oberflächeneigenschaften des gesägten mono-Si Wafers. Anders ausgedrückt bedeutet dies, dass je nachdem was für ein gesägtes Wafermaterial (zum Beispiel von verschiedenen Herstellern) prozessiert wird, unterschiedliche Texturergebnisse entstehen, obwohl die gleiche Ätzlösung und Ätzparameter verwendet werden.
Daher beeinflusst die Einführung neuer Sägemethoden für Silizium die Oberflächeneigenschaften der Wafer dermaßen, dass die Ätzlösung und der Ätzprozess angepasst werden müssen, um eine vergleichbare Pyramidentextur zu erzielen.
Zur Lösung des ersten Problems wird ein Alkohol mit hohem Siedepunkt verwendet, um somit Verdampfungsverluste zu vermeiden und die Ätzzeit zu verringern. Ein Alkohol, der solche Anforderungen erfüllt, ist Polyvinylalkohol (PVA).
Somit wird die neue Ätzlösung KOH-PVA-Lösung genannt. Die Eigenschaften des neuen Alkohols ermöglichen die Verwendung einer höheren Ätztemperatur von 100oC.
Obwohl die Texturergebnisse von KOH-PVA und KOH-IPA ähnlich sind, wurden auch deutliche Unterschiede beobachtet. Zum Beispiel ist die Pyramidengröße einer KOH-PVA-Textur (Mittelwert 4 µm) nur halb so groß wie die Pyramidengröße einer KOH-IPA-Textur (Mittelwert 8 µm). Ein weiterer wichtiger Unterschied ist die um 1%
niedrigere Reflexion (gewichtete Reflexion zwischen 400-1100 nm) einer KOH-PVA- Textur.
Wenn man versucht, den Reflexionsunterschied zwischen beiden Texturen mittels geometrischer Optik und unter Verwendung der Raytracing-Software SUNRAYS zu erklären, kommt man zu keinem befriedigenden Ergebnis. Dies liegt zum einen daran, dass in der Theorie für geometrische Optik folgende Annahme gemacht wurde: die Pyramidengröße sei viel größer, als die Wellenlänge des einfallenden Lichtes, und zum anderen daran, dass SUNRAYS die willkürliche Natur einer nasschemisch erzeugten Textur nicht berücksichtigt.
Jedoch lässt sich der Reflexionsunterschied durch Lösen der Wellengleichung und Berücksichtigung von Beugungseffekten an 1-4 μm großen Pyramiden (oder Strukturen) gut erklären. Mit dieser Methode ist es auch möglich die willkürliche Natur einer nasschemisch erzeugten Textur zu berücksichtigen.
Obwohl Reflexionsunterschiede zwischen der KOH-IPA und der KOH-PVA-Textur beobachtet werden können, sind die Ergebnisse auf Solarzellenebene nahezu identisch.
Als zweite Lösung wird in dieser Arbeit vorgestellt, wie sich der IPA-Dampf zurückzugewinnen lässt. Hierfür wird ein geschlossenes Becken verwendet. Das Becken wurde von der Firma LOTUS entwickelt. Ein Kühlsystem wurde über dem Ätzbecken installiert, wodurch die Zurückgewinnung von IPA-Dampf ermöglicht wird.
Außerdem ist es möglich, den Druck in dem geschlossenen Becken während des Ätzprozesses zu senken (unterhalb des Atmosphärendrucks). Dies beschleunigt den Ätzprozess dermaßen, dass sich die Ätzzeit auf etwa die Hälfte (im Vergleich zum Texturierungsprozess bei atmosphärischem Druck mit einer KOH-IPA Ätzlösung) verringern lässt.
Die periodische Absenkung des Drucks in dem geschlossenen Becken (mit einer Standard KOH-IPA-Ätzlösung und gesägten mono-Si Wafern bei einer konstanten Temperatur von 80oC) bedeutet auch, dass die Siedetemperatur des IPA gesenkt wird. Daher verdampft IPA periodisch, wodurch der IPA-Dampf Wassersoffblasen und Monokieselsäure-Partikel von der Oberfläche des Siliziumwafers entfernt.
Deshalb kann die Oberfläche des Wafers mit frischer Ätzlösung versorgt und so der Ätzprozess beschleunigt werden.
Contents
1. Introduction……….11 1.1 References……….14
2. Silicon (Si) solar cells……….17 2.1 Photovoltaic effect
2.2 History of silicon solar cells
2.3 Current-voltage characteristics of solar cells 2.4 Screen-printed silicon (Si) solar cells
2.4.1 Texturization: wet chemical texturization 2.4.2 POCl3 diffusion
2.4.3 PSG removal 2.4.4 SiNx:H deposition 2.4.5 Screen printing 2.4.6 Contact firing
2.4.7 Edge removal by sawing 2.5 References……….33
3. Theory of the chemical etching process and light trapping in textured silicon wafers……….35
3.1 Chemical etching
3.2 Influence of etching time on pyramidal texture using a potassium hydroxide (KOH)-isopropyl alcohol (IPA) solution
3.3 Determination of the heights of the pyramids (also referred to as pyramid size) 3.4 Theoretical calculation of light trapping in textured silicon wafers
3.5 References……….54
4. Sawing methods used to cut crystalline silicon and the influence of sawing and cleaning processes on texture……….57
4.1 New sawing methods
4.2 Texture on as-cut multi-crystalline silicon (multi-Si) Wafers
4.3 Texture on as-cut mono-crystalline silicon (mono-Si) Wafers 4.4 References……….64
5. New wet-chemical etching textures for mono-crystalline silicon ...65 5.1 Potassium hydroxide (KOH) – diethylene glycol monobutyl ether (DEG) etch solution
5.2 Potassium hydroxide (KOH) – glycerin etch solution
5.3 Potassium hydroxide (KOH) – polyvinyl alcohol (PVA) etch solution 5.3.1 Characteristics of PVA
5.3.2 Lifetime of the etch solution
5.3.3 Recovering PVA from the KOH-PVA etch solution
5.3.4 Independence of the KOH-PVA solution from the surface characteristics of as-cut Si-wafers
5.4 Comparison of pyramidal textures produced by the standard KOH-IPA and the new KOH-PVA etch solution
5.5 Shortening of etching time by first removing saw damage on as-cut Si-wafers 5.6 References……….92
6 Vacuum based wet chemical texturing……….95 6.1 Vacuum etching process
6.2 Vacuum etching process with KOH-IPA solution 6.3 Vacuum etching process with KOH-PVA solution 6.4 References……….104
7. Solar Cell Results……….105 7.1 KOH-IPA vs. KOH-PVA texture
7.2 KOH-PVA texture from Si-wafers with reduced saw damage 7.3 KOH-PVA texture for advanced solar cell processes
7.4 References……….111 8. Conclusions……….113
Chapter 1 Introduction
Currently, and in the foreseeable future, high worldwide energy demands represent (and will continue to represent) a serious problem for present and future generations. In the past and today, traditional fossil fuels such as oil, gas and coal have served as the standard energy sources. Unfortunately, these energy sources have the problems that: 1) they are limited in quantity (non-renewable) and 2) they release large volumes of carbon dioxide CO2, which presumably is at least partly responsible for international global warming.
With respect to the first problem, the limitations of standard energy sources must be overcome if the earth’s (already very large and still increasing) population is to enjoy rising standards of living. In my opinion, this can be achieved through the cooperation of countries with sufficient funding and actors (politicians or governments) that want to maintain a high standard of living, even after conventional energy resources have been largely exhausted. We already have available alternative energy sources such as water, nuclear, biomass, wind, and photovoltaic, among others.
Unfortunately, water power, for example, is only economical and feasible in countries with abundant rivers and waterfalls, such as, for example, Norway and Canada.
Nuclear energy has the problems of spent fuel waste storage and disposal and the danger of radioactive waste escaping into the environment in a nuclear reactor accident. Examples of such catastrophes are Chernobyl in 1986 (Russia, 26 April 1986) and more recently Fukushima in 2011 (Japan, 11 March 2011).
Energy sources such as biomass, wind and photovoltaic have the advantage that they do not generate much CO2. Therefore they can be used to reduce CO2 released into the atmosphere (problem 2). However, their main disadvantage are the relatively high production costs.
In this study, photovoltaic energy production technology is examined. Photovoltaic energy is electricity produced by solar radiation impinging on solar cells. Because with this form of energy production electricity is directly generated by solar cells exposed to solar radiation, this type of electricity is called solar electricity.
Solar cells are photovoltaic devices constructed meanly from semiconductor materials. The most common solar cells are, for example, made of: crystalline Gallium Arsenide (GaAs), crystalline Silicon (Si), crystalline Indium Phosphide (InP), Copper (Cu) - Indium (In) - Gallium (Ga) - Selenide (Se) Cu(In,Ga)Se2 (CIGS), amorphous Si, nano-crystalline dye, organic materials and cadmium telluride (CdTe).
In this study, solar cells based on crystalline silicon (Si) are the object of research.
Si-based solar cells could contribute to meeting increased world energy demands if they were not so expensive, and if they were adopted in most countries of the world.
Nowadays, China has greatly contributed to reduce the price of photovoltaic modules worldwide. Unfortunately it has also signified the decay of the German photovoltaic industry. In 2008, Germany was the largest producer of photovoltaic modules, now this role has been taken by China. Nevertheless, Germany is the country with the highest installation of photovoltaic modules, whose solar current production capacity reaches 34 GW [1: Kerler 2013] (which corresponds to approximately the electricity produced by 30 nuclear reactors), and such capacity will be increased in the near future (in 5 to 10 years) to 54 GW [1: Kerler 2013].
Reducing the cost of producing solar electricity plays a major role in spreading photovoltaic energy as a clean and renewable energy source. The costs of solar cells can be reduced in two ways. One is to increase their efficiency, and the other is to develop low-cost production technology. In order to increase solar cell efficiency and to develop new, economical technologies, research to increase our understanding of solar cell physics is of great importance.
The research literature often reports on high-efficiency solar cells, but the technological costs of producing them are still considerable. Therefore, not all of the reported processes are suitable for implementation in industrial production. Only those processes which permit low manufacturing costs have been introduced into mass production.
In this study, special attention is paid to the first processing step of manufacturing solar cells, namely the texturization process. The texturization of mono- and multi-Si- wafers is a process in which the surfaces of as-cut Si-wafers are modified to absorb more solar radiation, and also to remove saw damage from the wafers.
In the case of mono-Si-wafers, an alkaline solution is used to generate a pyramidal texture on the surface [2: Haynos 1974]. For multi-crystalline Si-wafers (multi-Si), an acidic etch solution is predominantly used to generate a “worm-like”
texture [3: Einhaus 1997]. This study focuses on the texturization of mono-Si-wafers by using wet chemical solutions. Exceptional cases will be noted in the text. Besides the chemical etching method used to texture mono-Si-wafers, texturization can also be carried out through mechanical grooving [4: Nunoi 1990], laser grooving [5:
Zolper 1989] or the plasma etching method [6: Dekkers 2000]. The chemical etching method is investigated because it is the cheapest method and because the
chemical etching method is the only one of these methods which allows a random pyramid texture due to the anisotropy of the chemical etching process. Here, anisotropy means that the etch rate depends on the crystal orientations of the mono- Si-wafer [7: Lee 1969].
The wet chemical texturization process of mono-Si-wafers is mainly carried out by using an aqueous solution of de-ionized water, isopropyl alcohol (IPA) and potassium hydroxide (KOH). The etch solution is heated to a temperature (T) of 80oC, and silicon wafers are immersed in the etch solution for a certain period of time. After approximately 30 min of etching, the silicon surface (both sides) is covered with small pyramids (pyramid size ≈ 10 µm, referred to as a ‘pyramidal texture’), and about 10 µm of silicon is removed on each side of the silicon wafers. This etching process is known in the photovoltaic industry as the standard KOH-IPA texturization process of mono-Si-wafers.
Although the texturing process using KOH-IPA is well established in the industrial production of screen-printed mono-Si solar cells, it still suffers from the steady evaporation of IPA, and the use of various different cleaners after the slicing process to remove slurry has considerable consequences for the further texturing process.
This last problem reduces the effectiveness of the standard KOH-IPA etch solution to achieve an optimal pyramidal texture on different assortments of as-cut silicon wafers [8: Chan 2009].
Furthermore, the introduction of new sawing methods changes the surface morphology of as-cut silicon wafers considerably [9: Aoyama 2010]. Thus, the texturing process has to be adjusted, or a new texturing solution has to be found.
In this work, two possible solutions to the KOH-IPA problem are presented (IPA evaporation and its high sensitivity to wafer characteristics). The first consists in the use of a different type of alcohol. We call this high boiling alcohol (HBA), because it has a boiling point above 200oC (higher than that of IPA and of water), and therefore we call the new solution KOH-HBA solution.
The second way to solve the problem consists in recovering the evaporated IPA.
Here, a new etching bath setup has been developed by the Lotus Systems wet etching company. In a cooling chamber located atop the new etching bath, IPA is cooled (liquefied) and then conducted to the etch solution. Apart from the cooling system of the new etching bath setup, a vacuum system has been adapted which allows the application of a vacuum in the etching chamber. The variation of pressure in the etching chamber by the application of a vacuum accelerates the etching process. Therefore, a considerable reduction in etching time is achieved.
In chapter 2 a short description of the photovoltaic effect and the history of silicon solar cell research is offered. The more important parameters describing the
efficiency of solar cells and the processing of screen-printed solar cells are also described.
In chapter 3, the etching process is described. A survey is offered of different theories that explain the etching process. The different theories describing light coupling to textured surfaces are also concisely presented.
In chapter 4, a survey of the actual sawing methods is given. This chapter illuminates the motivation for a great part this work. In this chapter, a comparison of wafers sawn using different methods and their influence on the optical characteristics after the same etching process is presented.
In chapter 5, different etch solutions are discussed that are used to texture mono- Si. Reflection measurements are made, and Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) pictures are made in order to characterize the texture on Si-wafers. Here the KOH-HBA etch solution is introduced. KOH-HBA texture is then simulated with the SUNRAYS program.
In chapter 6, the advantages of a new closed etching chamber are presented. The more important advantages of the new closed etching bath are the recovery of evaporated IPA and the employment of a vacuum that considerably reduces etching time. The vacuum etching process is also explained in this chapter.
In chapter 7, the results for different types of solar cells (screen-printed, selective- emitter, and photolithographic-based) cells processed with KOH-PVA texture are presented.
In Chapter 8, the overall conclusions of the study are summarized.
1.1 References
1. M. Kerler, Schatten über der Solarbranche, Südkurier Zeitung Nr. 228, 2, Dienstag, 1. Oktober (2013).
2. J. Haynos et al., The COMSAT non-reflective silicon solar cell: a second generation improved cell, Inter. Conf. on Photovoltaic Power Generation, 25 (1974).
3. R. Einhaus et al., Isotropic texturing of multicrystalline silicon wafers with acid texturing solutions, Proc. 26st IEEE PVSC, 167 (1997).
4. T. Nunoi et al., Cast polycrystalline silicon solar cells with grooved surface, Proc. 21st IEEE PVSC, 664 (1990).
5. J.C. Zolper et al., 16.7% efficient, laser textured, buried contact polycrystalline silicon solar cell, Appl. Phys. Lett. 55, 2363 (1989).
6. H.F.W. Dekkers et al., Silicon surface texturing by reactive ion etching, Opto-Electronics Review 8, 311 (2000).
7. D.B. Lee, Anisotropic etching of silicon, J. Appl. Phys. 40, 4569 (1969).
8. R. Chan et al., Wafer cleaning and its effects on subsequent texturing process, Proc. 24th EU PVSEC, 1199 (2009).
9. T. Aoyama et al., Fabrication of single crystalline silicon solar cell using wafer sliced by a diamond wire saw, Proc. 25th EU PVSEC, 2429 (2010).
Chapter 2 Silicon solar cells
The photovoltaic effect is the conversion of solar radiation into electrical energy by means of a photocell (nowadays a “photocell” is referred to as a “solar cell”).
2.1 Photovoltaic effect
The photovoltaic effect was first demonstrated by Becquerel in 1839. [1:
Becquerel 1839]. Figure 2.1 illustrates the experimental array used by Becquerel.
Becquerel illuminated a bath composed of an acidic solution containing silver chloride AgCl or silver bromide AgBr and two platinum Pt electrodes. Under illumination, AgCl decomposes into Ag+ and Cl- ions. Such ions then move to their respective electrode, thereby generating an electrical current.
Fig. 2.1: Experimental array used by Becquerel in 1839, Becquerel used this array to demonstrate the photovoltaic effect [1: Becquerel 1839].
The first silicon-based solar cell was produced by Ohl in 1946 [2: Ohl 1946], i.e., 107 years after the photovoltaic effect was first demonstrated. The significant
difference between the work carried out by Becquerel and that of Ohl is that Ohl replaced the acid solution (with AgCl) with a semiconductor (a silicon p-n junction).
A similar process took place with the transistor’s invention. The original idea of the transistor was based on a phenomenon occurring in an etch solution [3: Bardeen 1949]. In this case, the electrolyte was replaced by a metal.
These two inventions (solar cells and transistors) show the great importance of wet chemical processes. Wet chemical processes are also very important for further improvements in solar cells.
2.2 History of silicon solar cells
Ohl discovered the photovoltaic effect on Si during his attempts to produce high purity silicon crystals, because he was convinced that a high quality material could replace vacuum tubes in contemporary radio equipment (1939). Ohl’s experimental array for producing high quality silicon is shown in figure 2.2.
Fig. 2.2: Experimental array used by Ohl to produce high quality silicon crystals. The ingot is produced by fusing metallic silicon in powder form in a silica (SiO2) crucible in an electric furnace and slowly cooling the fused material until it solidifies [2: Ohl 1946].
In figure 2.2, two different regions of the Si crystal can be seen, one of them called the “p”-zone (top) and the other called the “n”-zone (bottom). Ohl cut a piece of silicon from the center of the Si crystal containing both zones (small rectangle in the center of figure 2.2), and in this way created the first Si p-n junction, see figure 2.3.
Fig. 2.3: Silicon piece on a Si crystal from figure 2.2 having two different zones; “p” and “n” [2:
Ohl 1946].
Ohl cleaned the Si grooves before he contacted both sides with rhodium plating, see below, figure 2.4.
Fig. 2.4: Silicon with “p” and “n” zones contacted with rhodium plating. This is the first silicon- based solar cell ever produced, with a solar cell efficiency of about 1% [2: Ohl 1946].
When the silicon grooves (p-n junction) of figure 2.4 were illuminated, the voltage was measured, so that the first silicon solar cell was produced. Ohl concluded that such voltage was produced by a barrier formed between the two different regions of the Si grooves. He decided that the difference between the zones was due to impurities. He defined the zones as the “n”-zone, for a region with an excess of electrons, and a “p”-zone, for a region with a low level of electrons.
A small version of Ohl’s solar cell was developed by Chapin et al. in 1954 [4:
Chapin 1954], see figure 2.5. Chapin’s silicon solar cell achieved a solar cell efficiency η of about 6%.
Fig. 2.5: Chapin’s silicon solar cell developed at Bell Laboratories in 1954 [4: Chapin 1954].
After experimental progress on the development of Si solar cells, some theoretical works appeared. One of them is the study presented by Schockley and Queisser in 1961 [5: Shockley 1961]. They found a maximal efficiency of Si solar cells of about 33%. The researchers also calculated the efficiency of solar cells in a way similar to that with Carnot’s machine. The two working temperatures are that of the Sun Ts and that of the solar cell Tc. Of course, some different assumptions were considered for solar cells. After the theory presented by Shockley and Queisser, other publications appeared that expanded on their work [6: Werner 1995, 7: Luque 1997, 8: Brown 2001].
Progress in solar cell research continued. To reduce reflections, the losses produced by a flat surface, the silicon surface was textured using a sodium hydroxide (NaOH) etch solution [9: Haynos1974]. The pyramidal texture clearly decreased reflection losses [10: Arndt 1975].
Further improvements in Si solar cells followed in the next few years. In 1975, the screen-printing method was used for the first time to produce front Silver (Ag) and back Aluminum (Al) contacts, replacing expensive vacuum metallization [11: Ralph 1975], see figure 2.6. Ralph et al. used titanium oxide and silicon dioxide as antireflective coatings. Subsequently, in the 1970s the cell shown in figure 2.6 became the standard Si solar cell. The efficiency of this solar cell was about 10%.
Another important step in the development of silicon solar cells was the introduction of silicon nitride (SiNx) as an antireflective coating in 1984 [12: Kimura 1984]. The present standard commercial cell is essentially that shown in figure 2.6, with SiNx as an antireflective coating and screen-printed contacts. An energy conversion efficiency of about 17-18% (for multi-Si) 19% (for mono-Si) has been achieved.
Fig. 2.6 Silicon solar cells with pyramidal texture, an antireflective coating and front and back screen-printed metal contacts [11: Ralph 1975].
In 1999, Zhao et al. [13: Zhao 1999] produced a Passivated Emitter, Rear Locally-diffused (PERL) silicon solar cell with the highest solar cell efficiency η = 24.7% ever achieved for silicon-based solar cells. The construction and structure of the solar cell are shown in figure 2.7.
Fig. 2.7: Silicon-based solar cell with the highest efficiency to date (η = 24.7%) [13: Zhao 1999].
For PERL Silicon solar cells, a high quality Float Zone (FZ) Si-wafer with low resistivity (1 Ω-cm) was used. Both sides of the PERL cell were passivated with silicon dioxide SiO2, thereby obtaining a high open-circuit voltage Voc = 706 mV.
In order to increase the short-circuit current Isc, inverted pyramids were produced on the front side of the cell. To further improve the light trapping, the flat back side of the cell was covered with evaporated aluminum, which works as a mirror on the back side. Furthermore, a double antireflective coating of ZnS/MgF2 was also deposited on the front side of the cell. Thus, a high short-circuit current density jsc was achieved (42.2 mA/cm2).
Using the new AM1.5 spectrum (IEC 60904: Ed. 2) to measure solar cells, the PERL solar cell achieved a calculated efficiency of 25.0% [14: Green 2009].
Although the PERL cell achieves the highest efficiency of silicon-based solar cells, such a solar cell has not been adopted for the industrial production of solar cells. The reason is, as mentioned in the introduction to this study, the high manufacturing costs. Solar cells with low production costs are clearly preferred by manufacturers.
Only those new solar cell concepts which fulfill this requirement have been adopted for the industrial production of solar cells [15: 2007 Neuhaus].
In 2006, photovoltaic industrial manufacturing was based mainly on mono- and multicrystalline silicon wafers. To be more exact, their percentage of the total cell production was of 89.9% [16: Hirshman 2006]. Another 7.4% of solar cells were produced from thin films (a-Si, CdTe, CIS), and 2.6% from silicon ribbons (EFG, string ribbons).
Of the mono- and multicrystalline silicon wafer solar cells produced in 2006 (see above), 86% were produced using screen printing to form the silver front and aluminum back contacts, and chemical vapor deposition was used to deposit SiNx as the antireflection coating on the front surface [16: Hirshman 2006].
The screen-printing process used to produce Si solar cells is the cheapest production method, and therefore it continues to be the one most commonly used in the photovoltaic industry [17: Green 2013]. Further improvements in screen-printing solar cells have been achieved, and to date efficiencies of about 19% have been reached in industrial production.
Two important factors have contributed to the dominance of silicon in the photovoltaic industry: Si is the second most abundant element in the earth’s crust, after oxygen, and it is preferred because of its electrical properties.
The most important electrical properties of silicon are: a) under solar radiation electron-hole pairs can be generated, and b) its ability to conduct electrons and/or holes, and the fact that such conductivity can be modified by introducing atoms of other elements.
In this study, the texturing process for mono-silicon wafers was investigated, and different kinds of solar cells were produced to test the textural effects. Before presenting the study, however, the principal parameters determining the efficiency of solar cells; open-circuit voltage Voc, short-circuit current density jsc, fill factor FF, and solar efficiency η will be explained.
2.3 Current-voltage characteristics of solar cells
As mentioned at the beginning of this work, solar cells are devices that use the sun’s radiation to produce an electrical current in an external circuit. The efficiency of solar cells is measured by applying voltage (V) to the cell and measuring the current (I) generated by it under solar illumination. A typical experimental voltage-current (I-
V) curve [18: McIntosh 2001] is shown in figure 2.8. The power P (I*V) produced by the cell as a function of the applied voltage is also shown in figure 2.8.
Fig. 2.8: Current-voltage curve of a solar cell obtained under illumination [18: McIntosh 2001].
The P-V curve shows a maximum value for the power P (Pmpp, Vmpp). This value corresponds to the maximum output power delivered by solar cells under the incident solar power Pin. From the Pmpp point, the maximal current point Impp and maximal voltage point Vmpp values can be determined.
The maximal conversion efficiency η of a solar cell is then defined as the maximal output power Pmpp produced by the cell under the incident power Pin of the sun.
To define the efficiency of solar cells η more exactly, other parameters are also defined.
The short-circuit current Isc is the I-axis intercept of the I-V curve. It is the current which flows when the solar cell is short-circuited, i.e., when the terminals of the solar cells have the same voltage, or when it is almost zero (V = 0). In other words, it means that the resistance (resistance in series with the solar cell) in the
circuit is almost zero. Thus, the current which flows in this case is only the current produced by illumination (IL).
From the equation which describes a solar cell under illumination [5: Shockley 1961]:
where: k = Boltzmann constant, IL = current generated under illumination, and by defining V = 0 (and therefore Isc) we obtain the result that the short-circuit current Isc
is equal to -IL; Isc = -IL.
The open-circuit voltage Voc is the V-axis intercept of the I-V curve. Voc is the voltage generated when the solar cell is open-circuited, i.e., when the circuit is open.
Therefore, the resistance in the circuit is infinite, which also means that no current is flowing. By setting the current at a value of zero, I = 0 (and therefore Voc) in the equation (2.2), we obtain:
From the last equation, it can be seen that Voc depends only on the current generated by illumination Isc and the saturation current density of the solar cell Io (in this case for the base and emitter). The current generated by illumination achieves saturation values immediately after illumination. Therefore, the unique possibility to increase Voc corresponds to the decrease in the saturation current density Io. From the equation for the saturation current density [19: Goetzberger 1997]
it can be seen that in order to decrease Io, high values of the diffusion length of the minority carriers (Ln and Lp) are necessary. Also, high values of the doping concentrations (NA and ND) are necessary. The employment of thin solar cells also decreases the Io.
The fill factor FF: Graphically the FF can be seen as the ratio between the area of the rectangle of the maximal point Impp*Vmpp and the area of the rectangle Isc*Voc, i.e., the FF reveals how much of the area Isc*Voc is filled with the area Impp*Vmpp. Mathematically it is expressed as:
Standard values of the FF for relatively efficient solar cells vary between 0.75 and 0.85.
The fill factor provides information about two important aspects. The first corresponds to the quality of the p-n junction. The recombination of charges in the space charge region of the p-n junction influences the FF very strongly. The second corresponds to the maximum electrical power which can possibly be generated by a solar cell utilizing optimal resistance (load controller). Here, the mean losses are due to the series and parallel resistance.
Therefore, solar cell efficiency can also be expressed in terms of Isc, Voc, and FF by combining equations (2.1) and (2.5):
In the next section, the state-of-the-art of screen-printed Si solar cells is described.
2.4 Screen-printed silicon solar cells
The method currently most often employed by the solar cell industry to produce silicon-based solar cells corresponds to the “screen printing” method. Therefore, below a more detailed view of the different processes involved in its elaboration is given.
It starts with a texturization process used to form small pyramids on a Si-wafer, then a POCl3 diffusion is carried out to form an emitter, and after that plasma edge isolation is carried out to isolate the front from the back side of the wafer. Then phosphorous glass (PSG) is removed in a hydrofluoric acid (HF) solution, after which a thin film of SiNx is deposited on the front side to passivate the emitter and as an antireflective coating. Then front and back contacts are deposited by the screen printing method, and finally a firing process is used to establish electrical contacts between the pastes and the Si-wafer. The firing process also allows the diffusion of hydrogen (H) into the wafer from the SiNx:H layer. Figure 2.9 gives a schematic diagram of this process, and figure 2.10 shows the structure of such a solar cell.
Fig. 2.9: Schematic process sequence for a Si solar cell using the screen-printing method.
Fig. 2.10: Silicon solar cell produced using the screen-printing method.
The increase in a solar cell’s efficiency in this study is due to further optimization of the different processes and material qualities used to make screen-printed silicon solar cells, rather than to the introduction of new solar cell concepts. An overview of the pros and cons of the different processing steps used in screen printing solar cells is given below.
2.4.1 Texturization: wet chemical texturization
The first process in fabricating a screen-printed silicon solar cell corresponds to the texturization of the as-cut silicon wafers. As-cut silicon wafers have saw damage on both surfaces, which has a depth of ca. 10 µm, see figure 2.11. The saw damage should be removed in order to avoid high charge carrier recombination at the surface.
Fig. 2.11: SEM pictures of an as-cut Si-wafer. On the left a cross sectional view is presented; on the right, the surface.
The texturization process for monocrystalline Si-wafers is usually carried out using an alkaline etch solution which consists of de-ionized (DI)-water, potassium hydroxide (KOH) and isopropyl alcohol (IPA). The KOH-IPA solution is heated to 80oC, and Si-wafers are immersed in it. In the photovoltaic industry the alkaline solution is widely known as the standard KOH-IPA solution. The experimental setup used in this work is shown in figure 2.12.
After an optimum etching time, the alkaline etch solution not only removes saw damage, it also forms small pyramids on the Si surface. An example of pyramidal texture generated with this etching process is shown in figure 2.13. We see in this picture that small pyramids cover the silicon surface (etching time = 25 min).
In figure 2.13 some regions of the surface are not covered with small pyramids.
The reason is the short etching time used to texture the wafer. After 30 min of etching, the entire surface is covered with small pyramids, see chapter 3 for more details.
Fig. 2.12: Experimental setup used in this study to texture mono-Si-wafers. Using a hotplate, the alkaline solution is heated in a glass beaker. As-cut Si-wafers are placed in the solution, and after a certain period of time a pyramidal texture is produced (see figure 2.13).
Fig. 2.13: Pyramid texture on mono-Si-wafer. The as-cut Si-wafer was etched for 25 min in a standard KOH-IPA solution at 80oC.
The formation of the pyramids is due to the anisotropy of the etch solution, i.e., different crystal orientations are etched at different rates, as explained in more detail in chapter 3.
The pyramidal texture reduces the total reflection of as-cut silicon wafers considerably; from approximately 25% to approximately 10% (in the wavelengths 850
– 1000 nm), see figure 2.14.
Fig. 2.14: Reflection measurements of an as-cut silicon wafer and an optimal KOH-IPA textured silicon wafer.
In the wavelength range between 850 nm and 1000 nm, a decrease in light reflection can be clearly seen, which corresponds to a reduction in the total light reflection of about 15% for wavelengths below 1000 nm.
How the light trapping process works with textured silicon wafers will be answered in chapter 3.
Although DI-water and IPA must be re-dosed after some period of time in order to maintain good textural characteristics, in the photovoltaic industry this method continues to represent a low-cost method to texture Si-wafers. Other etch solutions under investigation are ones which use sodium carbonate Na2CO3 [20: Melnyk 2004] or tetramethyl ammonium hydroxide TMAH [21: Papet 2006] as oxidants.
Unfortunately, the introduction of new sawing methods [22: Aoyama 2010], and different cleaning processes to remove slurry from as-cut mono-Si-wafers [23: Chan 2009] reduces the effectiveness of the standard KOH-IPA solution. Therefore, in the photovoltaic community, the investigation of other etch solutions to effectively texture Si-wafers is a matter of current research interest, as is the case in this study.
2.4.2 POCl3 diffusion
After the texturing process of Si-wafers has been carried out, a cleaning process is performed using DI-water, hydrochloric acid (HCl) and hydrofluoric acid (HF). After that, the wafers are dried in hot air. The clean textured silicon wafers are then introduced into a furnace for the POCl3 diffusion to form a p-n junction. For a standard screen-printed solar cell, the emitter has a sheet resistivity of about 50 Ω/□
[15: Neuhaus 2007].
The diffusion takes place at temperatures above 800oC in a tube-shaped oven.
The liquid phosphorous oxychloride POCl3 contained in a bubbler is gasified by nitrogen gas which flows through the liquid. Then, the gaseous POCl3 reacts with molecular oxygen O2 to form phosphorous oxide (P2O5), which is deposited on the Si surface. The deposited oxide represents the source of phosphorous (P) which diffuses into the wafer.
One disadvantage of this emitter is its poor blue response, i.e. high energetic photons are mainly absorbed on the surface. Due to the high doping on the n-region (front side of the cell), the Auger recombination mechanism is increased and poor surface passivation is observed. Therefore, emitters with low doping concentrations and thus high sheet resistivity should be suitable to solve this problem. However, the low doping concentration in the emitter makes it hard to form good electrical contacts with front-printed silver paste.
A practical solution to this problem is the formation of two different doping regions, one with a high doping concentration (under the contacts) and the other with a low doping concentration (between the contacts). This emitter is known as a selective emitter. This “simple” process raises solar cell efficiency up to 0.6% absolutely, see chapter 5 for more details.
2.4.3 PSG removal
After that, the phosphorous glass on Si-wafers is removed in a HF solution or, as mentioned above, by applying an acid solution. The solar cell is transported on the surface of an etching bath in such a way that only the back side of the wafer is wetted. Using a solution of H2O, HF, HNO3, and HsSO4, the emitter is completed removed from the back side of the wafer [24: Delahaye 2004].
2.4.4 SiNx:H deposition
The deposition of a SiN :H layer on the front side of the wafer is done for two
important reasons. The first is related to its passivation effect on the emitter. Also, because the SiNx:H layer is very rich in hydrogen H, hydrogen also diffuses into the Si-wafer and passsivates some impurities or crystal defects. The other reason is related to its antireflective characteristics. As the SiNx:H layer has a refraction index of about 2, i.e., between the refraction index of Si (3.5, for a wavelength of 600 nm) and that of air (approx. 1), it works very well as an antireflective coating. Thus the internal reflection of light in a Si (between Si and the SiNx:H layer) wafer is enhanced, and more light can be absorbed in the cell.
The SiNx:H layer is meanly deposited by using plasma-enhanced chemical vapor deposition (PECVD). The thickness of the silicon nitride (SiNx:H) layer is approximately 75 nm. With this thickness and a refraction index of about 2, the SiNx:H layer has been optimized to favorably absorb light with a wavelength of about 600 nm.
The application of two antireflective layers on the front side of the cell further increases light absorption. Such a double antireflective layer is mainly applied in the processing of more advanced lab-type solar cells. In chapter 5, a double antireflective layer of SiNx:H and magnesium fluoride (MgF2) is applied on a solar cell processed using a photolithographic-based method.
Of course, silicon dioxide (SiO2) could also be used as a passivation and antireflective layer (thickness of about 100 nm). However, the disadvantages of such a passivation layer are its high processing temperature (around 1000oC), the fact that it does not passivate the bulk of the cell, as the SiNx:H layer does, and because of its low refraction index of approximately 1.5.
2.4.5 Screen printing
Front silver and rear aluminum contacts are applied using the screen-printing method.
In the case of the front contacts (Ag), the paste has to establish contact with the emitter. To accomplish this, the paste contains lead borosilicate glass (PbO-B2O3- SiO2) and organic compounds. The lead borosilicate glass etches the SiNx
antireflection coating and stimulates the adhesion of Ag contacts. This process takes place only at high temperatures, i.e., during the firing process at temperatures of around 850oC.
One disadvantage of this method is the shadowing produced by the front silver contacts, which cover approximately 8% of the cell surface [15: Neuhaus 2007].
Therefore, the reduction of such shadowing is also a matter of investigation in the
photovoltaic community. In principle, the problem can be solved by using a photolithographic process; however, this method is more complicated.
To solve this problem, there are also solar cells that are in contact with metals on only one side of the cell. These cells are called interdigitated back contact solar cells.
In the case of the Al back contact, a similar process takes place. Here, the quantity of paste printed on the back side is very important for the formation of the Back Surface Field (BSF) and to overcompensate the phosphorous doping on the backside.
In chapter 5, a high efficiency solar cell produced by using the photolithographic based method is discussed. The current-voltage of the silicon solar cells shows the advantages of the method at the solar cell level.
2.4.6 Contact firing
After the screen printing of the front and back contacts, a firing process has to be carried out in order to establish electrical contact between the pastes and the silicon material. In the case of the front side, silver paste has to penetrate through the silicon nitride layer and contact the emitter.
The choice of the firing temperature is very important for good and optimal solar cell performance. Low firing temperatures (below 800oC) lead to very high series resistance, because at such temperatures the pastes do not establish good electrical contacts with Si. For high firing temperatures (higher than 900oC), the pastes penetrate too deeply into the cell, causing shunts (unwanted short circuits between the front and back contacts).
2.4.7 Edge removing by sawing edge isolation
In this work, the edges of the wafers were taken off by sawing. In the industrial production of solar cells the edges are removed by using plasma.
The diffusion of phosphorous to form the emitter results in a silicon wafer doped with phosphorous over its whole surface, i.e., “front” and “back” sides of the wafer, and also on the edges of the wafer. In order to separate the front from the back side, the edges of the wafers must be removed. One way to do this is by using the plasma etching method. The plasma used to do this is similar to that used to texture Si- wafers. To remove only the edges of the wafers, they are stacked, and at the bottom
and at the top of the stack, thick metals are put to protect the wafers. After that, one edge of the wafer is removed by the plasma. The process continues until the four edges of the wafers are taken off.
The edges of the wafers can also be removed by using a laser. Or the phosphor glass from one side can be etched back. The solar cell is transported on the surface of an etching bath in such a way that only the back side of the wafer is wetted. Using a solution of H2O, HF, HNO3, and HsSO4, the emitter is completed removed from the back side of the wafer [24: Delahaye 2004].
Finally, current-voltage measurements under AM1.5 spectrum illumination conditions are carried out in order to characterize the solar cells.
2.5 References
1. A.E. Becquerel, Les effets electriques produits sous l’influence des rayons solaires, C. R. Acad. Sci. 9, 561 (1839).
2. R.S. Ohl, Light-sensitive electric device, US Patent 2 402 662 (1946).
3. J. Bardeen et al., Physical principles involved in transistor action, Physical Review, 75, 8, 1208 (1949).
4. D.M. Chapin et al., A new silicon p-n junction photocell for converting solar radiation into electrical power, J. Appl. Phys. 25, 676 (1954).
5. W. Shockley et al., Detailed balance limit of efficiency of pn junction solar cells, J. Appl. Phys. 32, 510 (1961).
6. J.H. Werner et al., Radiative efficiency limit of terrestrial solar cells with internal carrier multiplication, Appl. Phys. Lett. 67, 1028 (1995).
7. A. Luque et al., Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels, Physical Rev. Lett. 78, 26, 5014 (1997).
8. A.S. Brown et al., Limiting efficiency of multiple band solar cells: an overview, Proc. 17th EU PVSEC, 246 (2001).
9. J. Haynos et al., The COMSAT non-reflective silicon solar cell: a second generation improved cell, Int. Conf. on Photovoltaic Power Generation, 25 (1974).
10. R.A. Arndt et al., Optical properties of the COMSAT non-reflective cell, Proc. IEEE PVSC, 40 (1975).
11. E.L. Ralph, Recent advancements in low cost solar cell processing, Proc.
IEEE PVSC, 315 (1975).
12. K. Kimura, Recent developments in polycrystalline silicon solar cell, Technical Digest, 1st Int. PV Science and Engineering Conf., 37 (1984).
13. J. Zhao et al., 24.5% efficiency silicon PERT cells on MCZ substrates and 24.7 efficiency PERL dells on FZ substrates, Prog. Photovolt.: Res. Appl. 7, 471 (1999).
14. M.A. Green, The path to 25% silicon solar cell efficiency: history of silicon cell evolution, Prog. Photovolt.: Res. Appl. 17, 183 (2009).
15. D.H. Neuhaus et al., Industrial silicon wafer solar cells, Advances in OptoElectronics, 2007, Article ID 24521 (2007).
16. W.P. Hirshman et al., Die neue Maßeinheit heißt Gigawatt, Photon 4, 52 (2006).
17. M.A. Green, Silicon solar cells: state of the art, Phil. Trans. R. Soc. A 371:
20110413 (2013).
18. K.R. McIntosh, Lumps, humps and bumps: three detrimental effects in the current-voltage curve of silicon solar cells, Dissertation, 8 (2001).
19. A. Goetzberger et al., Sonnenenergie: Photovoltaik, B. G. Teubner Stuttgart 78 (1997).
20. I. Melnyk et al., Na2CO3 as an alternative to NAOH/IPA for texturisation of monocrystalline silicon, Proc. 19th EU PVSEC, 1090 (2004).
21. P. Papet et al., Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching, Sol. Energy Mater. Sol. Cells 66, 1 (2006).
22. T. Aoyama et al., Fabrication of single-crystalline silicon solar cell using wafers sliced by a diamond wire, Proc. 25th EU PVSEC, 2429 (2010).
23. R. Chan et al., Wafer cleaning and its effects on subsequent texturing process, Proc. 24th EU PVSEC, 1199 (2009).
24. F. Delahaye et al., Edge isolation: innovative inline wet processing-ready industrial production, Proc. 19th EU PVSEC, 416 (2004).
Chapter 3
Theory of the chemical etching process and light trapping in textured silicon wafers
Although the etching process of silicon is “only” an oxidation-reduction process, there is as of yet no comprehensive theory able to explain the whole etching process.
This is because, among other things, such processes take place in very short periods of time (in nanoseconds – or even more quickly). Furthermore, the processes occur at the sub-atomic level. Both of these factors complicate the detailed study of the etching process.
What is known up until now about the etching process is: the end-products, i.e., the geometry formed on the etched mono-Si material and the by-products generated during the etching process. Such knowledge permits us to gain insight into the mechanisms involved in the etching process. However, a definitive theory which describes the etching process of semiconductors remains a “mystery.” At the beginning of the next section, a review of the etching process is presented.
3.1 Chemical etching
Before giving a review of the etching process, it is important to mention some of the main characteristics of the silicon semiconductor in order to gain a better understanding of the etching process.
Silicon is a semiconductor material with an electron conductivity σ between 104 >
σ > 10-8 (Ωcm)-1. This conductivity can be changed by doping. A silicon crystal is built up of many unit crystals arranged periodically. The unit crystal is a cubic structure, see figure 3.1.
Silicon atoms are covalently bonded with each other. Silicon has four valence electrons, and each silicon atom in the bulk is bonded with four neighboring atoms.
On the surface of a silicon wafer, for example, Si bonds are broken (these broken bonds are called “dangling bonds”), i.e., they are not bonded to other Si atoms. Some of these dangling bonds become saturated by forming unstable bonds with atoms in the atmosphere like oxygen, nitrogen, etc.
(0,0,0)
(1/4,1/4,1/4)
Fig. 3.1: The silicon crystal has a face centered cubic (fcc) structure with a base of two identical atoms, which are located at the (0,0,0) and (1/4,1/4,1/4) positions. This is a diamond structure. Every silicon atom is located in the center of a tetrahedron and covalently bonded with another four Si atoms.
Other important characteristics of silicon atoms correspond to the order (arrangement) of the atoms in the crystalline structure. The position of the atoms with respect to an x-y-z coordination system is well described by the Miller indices. The Miller indices are represented by the numbers in parentheses [1: Hummel 2004]. For example, the Miller index (100) describes the positions of atoms located on a plane perpendicular to the x-axes in a unit crystal, see figure 3.2.
Fig. 3.2: Miller indices in a cubic structure [1: Hummel 2004].
Also, from diagrams 3.2 and 3.1, it can be seen that the density of Si atoms on the (111) plane (direction) is higher than on the other planes (directions). This fact has very important consequences for the further explanation of the geometry produced by certain etching processes. Now a review of the etching process will be given.
After the microelectronics revolution and the construction of the first silicon solar cell (1946) and of the first germanium transistor (1949), interest in semiconductors was understandably great. Thus, the first attempts to etch silicon were carried out in the sixties [2: Finne 1967]. Also in 1967, an etch solution was used for the first time
to etch silicon. It consisted of potassium hydroxide (KOH), water (H2O) and isopropyl alcohol (IPA) [3: Price 1967].
In 1969, Lee [4: Lee 1969] used a ternary liquid etch solution for silicon, consisting of hydrazine, iso-propyl alcohol (IPA), and water. Hydrazine was employed as an oxidant, IPA as a complexing agent, and water appeared to function as a catalyst. He explained the etching process of silicon as follows: the oxidant oxidizes silicon to hydrated silica, and the complexing agent reacts with the silica and forms a soluble complex ion. Water provides excess (OH)- ions for the oxidation step.
The above-mentioned etch solutions used to etch silicon show two important characteristics: they act anisotropically and selectively. Anisotropic means that the etch rate depends on the crystal orientation, whereas selective means that the etch rate depends on the doping concentration of silicon.
Knowledge of the anisotropic and selectivity effects of alkaline etch solutions on mono-silicon has been used to produce microstructures in the microelectronic and micromechanical industries [5: Holmes 1974]. And it was also introduced for the first time in the photovoltaic industrial sector by Haynos et al. in the same year [6:
Haynos 1974].
The etch solution employed by Price [3: Price 1967] is now well known in the photovoltaic industry as the standard KOH-IPA solution used to texture mono-Si- wafers. The KOH-IPA solution worked very well for a long time after its introduction in 1974, but the development of new sawing methods and different chemicals to clean slurry from sawed wafers changed the surface characteristics of the as-cut silicon wafers, reducing the effectiveness of this etch solution (see chapter 4).
In 1983, Palik et al. [7: Palik 1983] measured the etching products by recording the Raman spectra in real time as the etching progressed in a 5M KOH solution. He found that the primary etching species were OH- ions, and the etching products were silicate SiO2(OH)=2. Isopropyl alcohol (IPA) does not appear to participate chemically in the etching process. Thus, the role of IPA in the etching process developed by Palik et al. conflicts with the observations of Lee [4: Lee 1969]. The results reported by Palik et al. with respect to the role of IPA are contrary to the findings of other authors, who clearly see the great influence of the complexing agents (IPA for example) on ternary etch solutions [8: Cho 2004]. Seidel also assumed that IPA does not play a major role in the silicon etching process, although he saw that the addition of IPA to the etch solution considerably decreases the etch rate of silicon [9:
Seidel 1986, 10: Seidel 1990]. From these results, it is clear that the role of the complexing agent (IPA for example) in ternary solutions used to etch silicon continues to be a mystery.
Nevertheless, the etching process of silicon (with a minimization of the role of IPA) was well described by Seidel in 1986 and 1990 [9: Seidel 1986, 10: Seidel 1990]. In his work, he also proposed an empirical equation that describes the etching rate of silicon, from which he also concluded that four electrons are needed to remove one silicon atom. Now we take a closer look at Seidel’s explanation of the silicon etching mechanism.
In a first oxidation step, two hydroxyl ions OH- from the alkaline solution are bonded to two Si dangling bonds, and thus Si-OH bonds are established. In this process, two electrons from the OH ions are injected into the conduction band of silicon.
[10: Seidel 1990]
The Si-OH bonds weaken the two silicon back bonds. In order to break the back bonds (which have already been weakened), two electrons from the Si-Si back bonds have to be excited to the silicon conduction band. When this happens, the Si-Si back bonds break, and a positive silicon hydroxide (Si-OH) complex is formed.
[10: Seidel 1990]
The silicon hydroxide complex further reacts with the other two hydroxyl ions from the solution to produce monosilicic acid.
[10: Seidel 1990]
The monosilicic acid can now diffuse into the solution.
The four electrons in the silicon conduction band, which are located near the surface, can be transferred to four water molecules, which are also located near the surface. Thus, a further four water molecules are decomposed into hydroxyl ions and atomic hydrogen. Atomic hydrogen further recombines to produce molecular hydrogen.
[10: Seidel 1990]
