Sawing methods used to cut crystalline silicon and the influence of sawing and cleaning processes on texture
Chapter 6 Vacuum based wet chemical texturing
A texturization process in a closed etching bath with cycling vacuum pulses appears to have been introduced for the first time by Ximello et al. [1: Ximello 2011, 2: Ximello 2012, 3: Ximello 2012, 4: Ximello 2012, 5: Ximello 2012, 6: Ximello 2012], as no earlier reports of using this technique to texture mono-Si-wafers have been found. However, it should be mentioned that this principle was already suggested for cleaning processes [7: Gray 2002].
The cleaning process proposed by Gray et al. is also known as cavitation and it is a well-accepted means for cleaning surfaces. Typically, ultrasonic sound waves are used to produce tiny collapsing bubbles at the solid surface. The energy of the ultrasonic waves is released into the fluid and the heat created by this energy evaporates small volumes of the fluid at the surface of the object, forming vapor bubbles. The vapor bubbles are cooled by the surrounding fluid and collapse, releasing the energy on implosion.
Gray et al. explained the cycling vacuum cleaning process in a closed bath as follows (taken literally from the patent of Gray [7: Gray 2002]):
Decompression processing is the production of vacuum bubbles at a solid surface, to produce an energy release at the solid surface. The process is accomplished by alternating vacuum and pressure to produce a pulsing action within a fluid. The release of the pressure produces vapor bubbles at the solid surface, which are collapsed when pressure is re-applied. The level of the vacuum, and/or pressure, the temperature, rate of introducing vacuum and/or pressure can control the rate of growth and size of the bubbles, and the total energy release [7: Gray 2002].
The explanation of Gray et al. (above mentioned) suggests that the decrease of the pressure in the closed bath automatically implies the formation of bubbles at the solid surface. But he did not mention which is the reason of that.
Six year later (2008), he published an extension of his patent [8: Gray 2008].
Here, he mentioned three “new” processes occurring during the cleaning process.
1. - The growing edge of the bubble actually acts as a forced convection removal process for particles.
2. - Bubble formation facilitates the diffusion of particles into the etch solution.
3. - The leading edge of the growing bubbles is the main latent heat transfer area since the film thickness in this area is very small. In other words, the third particle removal mechanism is the fluid evaporation within this region.
From the points mentioned above, it can be concluded that bubbles encapsulate particles at the surface of the solid and then bubbles are detached, removing the particles inside the bubbles.
It was clearly mentioned that in this process a fluid evaporates, but it is not directly explained why.
Considering that the cleaning process is taking place at a constant temperature near the boiling point temperature of the fluid used inside the closed bath, a decrease of the pressure also means a decrease of the boiling point temperature of the fluid. And because the etching process is taking place at higher temperature (initial constant temperature, near the boiling point temperature at atmospheric pressure), the fluid starts to boil.
To explain this more clearly, the texturing process of mono-Si wafers with vacuum by using a KOH-IPA solution is now considered. The KOH-IPA etch solution consists of water, KOH and IPA. The etching temperature of the standard texturization process of mono-Si is dictated by the boiling point of IPA (82.4o) and it corresponds to 80oC in order to avoid evaporation of IPA. Because water has a boiling point of 100oC, it is considered that water does not boil during this etching vacuum process.
Therefore, only the vapor pressure vs. temperature of IPA is used to explain this vacuum etching process of mono-Si wafers.
6.1 Vacuum etching process
The vacuum etching process in a closed etching bath (see figure 6.1) could also be considered as a “thermodynamic method” to texture mono-Si-wafers, because it can be explained by means of thermodynamic variables like pressure P and temperature T.
Fig. 6.1: A closed etching bath developed by the LOTUS wet etching company. Such a prototype enables the periodic introduction of pressures below atmospheric pressure (ATM) in the etching chamber.
To understand the vacuum etching process, the vapor pressure P – temperature T curve for isopropyl alcohol (IPA) will be considered, see figure 6.2.
0.6
A B
80 70
Fig. 6.2: Vapor pressure – Temperature curve for IPA. For the vacuum-assisted etching process, the atmospheric pressure (1 atm. ≈ 1 bar, point A) in the closed etching chamber is decreased to values of about 0.6 atm. (≈ 0.6 bar, point B) for short periods. After the vacuum is turned off, the pressure in the etching chamber returns to its initial value of one atmosphere (point A) [9: Wikipedia 2013].
The vacuum process of mono-Si in a KOH-IPA solution (at 80oC) works as follows:
1. – At the start, the etching process takes place at atmospheric pressure and at a temperature of about 80oC (point A in figure 6.2), i.e., near the boiling point of IPA (82.4oC). At this point, the surfaces of silicon wafers are covered by hydrogen bubbles developed by the chemical etching process (not by applying vacuum) see figure 6.3.
The etching process is carried out at 80oC in order to avoid the massive evaporation of IPA from the etch solution, although this new prototype recovers IPA.
In order to remove hydrogen bubbles from the surfaces of silicon wafers, IPA will be evaporated in the etch solution for short periods of time, which can be achieved by decreasing the pressure in the etching chamber (as it was suggested above).
Fig. 6.3: At the beginning of the etching process, the pressure in the etching chamber corresponds to that of the atmosphere (1 atm. ≈ 1 bar, point A in figure 6.2). At this point, hydrogen bubbles (not created by vacuum, if not by the etching process itself) cover the surfaces of the silicon wafers.
2. – The pressure in the etching chamber is decreased to values lower than atmospheric pressure (atm), for example 0.6 atm, for very short periods of time, for example 1 second. When the pressure in the closed etching chamber is decreased (for one second), the boiling point of IPA is also theoretically decreased to approximately 70oC (point B in figure 6.2) for very short periods of time. At point B of figure 6.2, IPA starts to boil for only one second, because the bath temperature (almost the water temperature in the etch solution) is maintained at constant temperature of 80oC. The sudden change of IPA´s boiling point of 10oC produces its evaporation, which can be indirectly observed by means of the detachment of the hydrogen bubbles. During the formation of IPA vapor some particles on the surface of
Si-wafers are encapsulated. These particles consist mostly on monosilicic acid particles produced as end products during the anisotropic etching of the mono-Si wafers (see chapter 3.1).
Fig. 6.4: Pressure below atmospheric pressure is applied to the closed etching bath. In this way, the IPA is brought to a boil. The IPA boiling process removes hydrogen bubbles and monosilicic acid particles from the silicon surface. Thus the etching process is accelerated.
In the figure above, the hydrogen bubbles and monosilicic acid particles covering the Si surfaces are detached from the surfaces as a result of the process of boiling the IPA for short periods of time.
3. – After turning off the vacuum, normal atmospheric pressure is restored in the closed etching chamber. The surfaces of the silicon wafers are now almost free from hydrogen bubbles and monosilicic acid particles, and so the anisotropic etching process can continue, see figure 6.5.
Fig. 6.5: The vacuum in the etching bath is turned off, and atmospheric pressure is restored.
The silicon wafer surfaces are now free of hydrogen bubbles, monosilicic acid particles, and fresh etch solution is applied to these surfaces. The removal of hydrogen bubbles, monosilicic acid particles, and the application of fresh etch solution to the surfaces of silicon wafers explains why the etching process is accelerated.
Thus, the etching process continues at normal atmospheric pressure and with fresh etch solution on the Si surfaces. The periodic removal of hydrogen bubbles monosilicic acid particles, and the application of fresh etch solution to Si wafer surfaces (see figure 6.6) throughout the whole etching procedure is the reason for the acceleration of the etching process.
During the etching process under atmospheric pressure conditions (periods of about 15 seconds), the classic anisotropic etching process takes place, i.e., atoms on the (100) crystal orientation are etched faster than atoms on the (111) crystal orientation. The reduction of the period of time in which the etching process takes place under atmospheric pressure, while maintaining the low-pressure period of one second, further accelerates the etching process, however, at the cost of anisotropy (pyramid formation). Thus it was important to find a compromise solution between the acceleration process and the quality of the pyramidal texture.
Fig. 6.6: Schematic representation of vacuum pulses applied during the etching process in a closed etching chamber.
6.2 Vacuum etching process with KOH-IPA solution
The solution consists of 60 liters of DI-water, KOH and IPA. A temperature of 80oC, and etching times of respectively 30 min and 16 min are used. Here, new etching equipment is used (no glass beaker). In the closed etching chamber (see figure 6.1), below-atmospheric pressures are applied. The magnitude and duration of the vacuum pulses can be set by the operator.
The application of vacuum pulses during the texturization procedure accelerates the etching process, and therefore it was possible to reduce the etching time to 16 min. If no vacuum pulses are employed during the etching process, an etching time of 30 min is required. Furthermore, with the new etching equipment it was possible to recover IPA from the etching chamber. Here, only 12.5x12.5 cm2 Cz-Si-wafers were used.
In order to characterize the pyramidal texture, reflection measurements were carried out (see figure 6.8), and scanning electron microscope (SEM) pictures were taken (see figure 6.7).
Fig. 6.7: SEM pictures of textured Si-wafers. Wafers were textured in a KOH-IPA solution. a) shows the surface of a silicon wafer textured at atmospheric pressure, and b) shows a textured silicon wafer produced using an extra vacuum step process.
Comparing Fig. 6.7a) and b) we observe a decrease in pyramid size in b). The decrease in pyramid size is due to the vacuum process used during the texturization.
Vacuum steps in the etching chamber allow a very fast detachment of hydrogen bubbles from the silicon surfaces. Hydrogen bubbles do not have enough time to increase, and so the chemical etching process can continue. The small pyramid size is comparable to that observed with KOH-PVA textured Si-wafers, and the reflection values of such small pyramidal textures are also lower than for larger pyramids.
Figure 6.8 shows reflection measurements of the textured silicon wafers shown in Figure 6.7.
400 600 800 1000 0
10 20 30
KOH-IPA texture
KOH-IPA texture
with vacuum steping process
Reflection (%)
Wavelength (nm)
Fig. 6.8: Reflection measurements of Cz-Si-wafers textured with a KOH-IPA solution. Vacuum pulses were used during the texturization procedure to accelerate the etching process.
In fig. 6.8 we see that Cz-Si-wafers textured with the KOH-IPA solution and with the vacuum process show slightly lower reflection values for wavelengths lower than 850 nm, which is consistent with the results observed for the KOH-PVA texture.
For the vacuum-assisted etching process an etching time of 16 min was used.
Compared with the etching time used in the standard KOH-IPA etching process, which lasts between 30 and 40 min, a decrease in etching time of about 50% was achieved.
6.3 Vacuum etching process with KOH-PVA solution
In this case, the results were not satisfactory. The etching time was only very slightly reduced – it fell from 30 min to 28 min. Here, technical problems with the vacuum pump system did not allow the application of lower pressure values in the closed etching bath. The pressure in the closed etching bath (at approx. 100oC) was simply too high to produce values under atmospheric pressure and thereby bring the water to a boil. The employment of more powerful vacuum pumps should overcome this problem. The vacuum pump could not be changed during this research work, and thus pressure remains a problem to be dealt with in the future.
In the next section, the performance of the pyramidal texture produced by either the KOH-IPA or the KOH-PVA solution is tested at the solar cell level.
6.4 References
1. N. Ximello et al., UP to 20% efficient solar cell on monocrystalline silicon wafers by using a KOH-high boiling alcohol (HBA) texturing solution, Proc. 26th EUPVSEC, 849 (2011).
2. J.N. Ximello-Quiebras et al., Vacuum enhanced process for texturisation of monocrystalline silicon wafers, Galvanotechnik, (2012).
3. J.N. Ximello-Quiebras et al., Solutions used in the texturization of monocrystalline silicon, Photovoltaics International, 73 (2012).
4. J.N. Ximello-Quiebras et al., Method for texturing a surface of a semiconductor substrate and device for carrying out the method, Patent, WO 2010/136387 A1, Dec. 2, (2010).
5. J.N. Ximello-Quiebras et al., Verfahren zum Texturierung einer Oberfläche eines Halbleitersubstrates sowie Vorrichtung zum Durchführen des Verfahrens, Patent, DE 10 2009 022 477 A1, Dec. 16 (2010).
6. J.N. Ximello-Quiebras et al., Method for texturing a surface of a semiconductor substrate and device for carrying out the method, Patent Application Publication, Pub. No. US 2012/0129355 A1, May 24, (2012).
7. D. Gray et al., Solvent and aqueous decompression processing system, Patent No.: US 6,418,942 B1, Jul. 16, (2002).
8. D. Gray et al., Cyclic nucleation process, Patent No.: PCT/US2008/081397, Oct. 27, (2008).
9. Wikipedia, 2-Propanol, de.wikipedia.org/wiki/2-Propanol, downloaded:
October 20, (2013).