As for dry chemical or plasma texturing processes, there are several ways to excite plasma. Those are described in detail, for example, in [37]. The most common plasma sources for texturing purposes are microwave (MW) and radio frequency (RF) excitations or the combination of both (RF-MW). RF plasma in the use of reactive ion etching (RIE) reveal an anisotropic etch character, whereas MW plasma tend to etch more isotropically. In conjunction with both one can, within certain limits of course, control the degree of anisotropic or isotropic etching character. The latter provides a powerful tool for designing textures with defined feature sizes.
The most prominent etch reactants used for plasma texturing purposes are chlorine (Cl) and fluorine (F). Both halogens etch silicon, when provided in atomic form by breaking off Si-Si bonds. Due to the toxic, explosive, and very
corroding nature of chlorine- based chemistry, the investigations undertaken in this thesis are restricted to fluorine, using sulphur-hexafluoride (SF6) as precursor.
For plasma texturing, also two possible processing routes can be pursued: pre-structured and self-masking processes.
4.3.1 Pre-structured plasma texture processes
Pre-structured texturing uses some kind of patterned etch stop mask that defines surface areas where the etch process is allowed and areas where it is suppressed.
Etch stop masks used for this purpose consist, for example, of photo resists, SiOx, SiNx or colloidal spin-on layers. The photo resists can be structured by photolithography or a lately presented nano-imprint process [38]. SiOx and SiNx
have to be patterned by an additional photolithography step. The colloidal spin-on layers define by the mere existence of the colloids nspin-on-etchable and etching areas. Only after the desired pattern is defined can the actual abrasion process start. By adjusting the etch characteristics of the plasma process, a conform reproduction of the etch pattern or an under-etching of the mask can be obtained. This implies the possibility of producing nearly any desired structure shapes.
Pre-structured texturing processes have the potential for low reflectance, very good light trapping, and due to the designable and ordered structuring, excellent electrical properties for solar cell applications. But they also imply increased and sophisticated labour and costs. Pre-structured texturing processes are therefore mostly used in laboratory and high efficiency approaches, such as shown by [39, 40]. Up to now only the nano-imprint technology [38] seems to offer a cost effective way of industrializing micro patterning satisfying the needs of the PV market.
4.3.2 Self masking plasma texture processes
More likely to be integrated in standard solar cell production lines are plasma texturing processes exploiting the so-called “self masking” process. Inomata et al [41] first introduced a mask-less texturing process. This process was RIE based, using Cl2 and resulting in pyramid-like structures. Several fluorine incorporating gasses are known and have been tested as well for plasma etching in general, and as for plasma texturing in particular in the past. Most of these are either expensive, like xenon difluoride (XeF2), toxic like nitrogen trifluoride (NF3) or leave unwanted polymer residuals like fluorocarbon gasses.
The most promising precursor is SF6. It is non-toxic, comparatively cheap and also reveals the possibility of maskless texture formation. The latter can be observed by adding other gasses such as oxygen (O2) and was described, for example, by [42]. Here, atomic fluorine is provided by dissociation within the plasma which, when in contact with a wafer, penetrates the silicon surface at random. By breaking the Si-Si bonds, the fluorine starts local etching and thereby roughens the surface at an atomic level. An etch prohibiting layer consisting of polymers of Si, O and F (SixOyFz) is deposited simultaneously and randomly on the surface. A constant process of deposition-, etching- and re-deposition starts. The statistical nature of the process leads to more or less defined structures on the substrate surface. As the etch stop layer mostly suppresses chemical etching, the depths of the obtainable structures tend to increase with a higher directional acceleration of ions onto the substrate surface.
Three different configurations for plasma texturing processes shall be compared in this thesis:
• MW induced plasma using SF6 and O2
• MW induced plasma using SF6, O2 and NH3
• Combined RF and MW induced plasma using SF6 and O2.
Some of the findings presented in the following were discovered in the frame of diploma thesis projects [43, 44].
All texturing results shown in the following were obtained using a SiNA® setup built by Roth & Rau and described in [45]. The SiNA is a well established tool for low cost, high throughput application for the PECVD deposition of SiNx layers. A detailed description and characterisation of the SiNA® at ISE can be found in [46].
Figure 7: Sketch of plasma source, by [47].
Our SiNA® is equipped with several plasma sources, of which one is dedicated for etching and texturing purposes only. It allows the combined use of MW and RF plasma generation and thereby either plasma- or reactive ion etching in an in-line setup. More details describing the plasma source are given in [48]. A principle sketch of the plasma source design is shown in Figure 7.