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Highly flexible coating system for the PV industryArticle PV Production Annual 2014

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Introduction

With the launch of the multiple application inline MAiA 2.1 platform, Roth&Rau has brought together all inline processes into a single-machine platform. The common element of the platform is the wafer tray size. The use of trays with the same external dimensions in all processes enables the number of different non-process-specific machine components to be restricted to a sensible minimum. For example, it allows all coating machines in a production line to be loaded and unloaded using identical automation solutions. The advantage for the system operator lies in the substantially reduced costs for the training of maintenance personnel and for the spare parts inventory. System reliability is increased by focusing

the development capacities on fewer individual components as well as by the use of field-proven components. It is also conceivable that a number of processes which previously required several individual machines could be combined into a single machine, subject to the process requirements. The major advantage of this lies in the ability to utilize consecutive processes without breaking the vacuum, thus affording the highest possible process quality in volume production.

MAiA PECVD systems – a modular product family for PECVD in the PV industry

MAiA is the name given by Roth&Rau to an equipment family for a wide-reaching spectrum of plasma applications.

The MAiA 2.1 platform from Roth&Rau – the latest generation of a highly flexible coating system for the PV industryDr. Detlef Sontag, Dr. Hermann Schlemm, Gunnar Köhler, Matthias Uhlig, Hans-Peter Sperlich, Dr. Mirko Kehr & Dr. Egbert Vetter, Roth&Rau AG , Hohenstein-Ernstthal, Germany

ABSTRACT For many years plasma-enhanced chemical vapour deposition (PECVD) technology has been an indispensable part of PV development. In addition to the deposition of a PECVD silicon nitride layer as an anti-reflective coating (ARC) on the front of a solar cell, the passivation of the rear surface by PECVD for the passivated emitter and rear cell (PERC) concept is growing in popularity. Roth&Rau’s multiple application inline MAiA platform was conceived for industrial-scale applications of this type as well as for R&D purposes, and is demonstrating outstanding results in these areas. Thanks to the modular design and process flexibility of the new MAiA 2.x machine generation, additional application areas are being opened up, such as plasma-based texturing of wafer surfaces and coating deposition for the manufacture of heterojunction (HJT) solar cells. As well as for the amorphous silicon layers needed for this purpose, just a few nanometres thick, the new MAiA platform can also be used for transparent conductive oxide (TCO) coatings by means of a conversion to a physical vapour deposition (PVD) sputtering unit.

PECVD process MAiA configuration

Front side SiN 1 × PECVD front side

Back side AlOx/SiN 2 × PECVD back side

Front side SiN + Back side AlOx/SiN 1 × PECVD front side + 2 × PECVD back side

Back side SiO2/SiN 2 × PECVD back side

Plasma texturing 1 × Plasma etch front side

Table 1. Overview of MAiA applications for silicon solar cell technology.

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Table 1 shows the potential applications for silicon solar cell technology, extending from plasma-enhanced chemical vapour deposition (PECVD) for silicon nitride on the cell front side (for which Roth&Rau has set up more than 400 systems all over the world since 2000), to current pilot applications, such as plasma texturing.

MAiA systems with two PECVD processes for one layer stack are proving highly successful in the early phase of their industrial introduction. Fig. 1 shows the basic structure of

an inline plasma-coating system along with the associated system components: 24 solar wafers travel from left to right through the system on a high-temperature-resistant carrier measuring approximately 1.0m × 0.8m. During this phase they are subjected to alternating vacuum cycles and also have to be heated up to the typical plasma process temperatures of 200–400°C. Cycle times of 30–60 seconds allow around 2000–4000 wafers to be processed per hour. In the PECVD process chamber the wafers travel through

Figure 1. Basic arrangement of a MAiA system (with a plasma process).

Figure 2. MAiA system for two PECVD processes (back-side AlOx/SiN): schematic arrangement of the processes (top); view of the system (bottom).


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a continuously operating process under linear plasma sources; as they do so, they are coated in 30–60 seconds, which places considerable demands on the PECVD process deposition rates.

Once the carriers have been placed under vacuum, they can be subjected to a second plasma process with which multiple coating systems can be realized by a single plasma unit. Fig. 2 depicts a system of this type for coating the rear surface of passivated emitter and rear cells (PERCs) with greater than 20% efficiency using a coating system consisting of aluminium oxide and silicon nitride (see MB-PERC section).

Because both PECVD processes work with precursor gases that must not be allowed to mix, there is an additional process gas lock between the two PECVD processes (Fig. 2 top). Whereas the length of the first generation of MAiA systems could still be as much as 30m, for a second generation MAiA it has been reduced to 10–15m for two processes as a result of optimization of the carrier size and locks (Fig. 2 bottom).

Thanks to the modular design consisting of individual vacuum chambers (see Fig. 1), even large MAiA systems can be transported and installed with ease. Moreover, a wide range of customer wafer throughput rates and process requirements can be accommodated by means of customized module configurations.

The systems are equipped with plasma or sputter sources that can be arranged above or below the carriers, enabling the solar cell wafers to be coated from above, from below or even from both sides.

Linear microwave plasma sources as core components of MAiA PECVD systemsThe first linear MW plasma sources were employed industrially by Roth&Rau in 2000 [1], and more than 2000 of these first-

generation plasma sources have been delivered to date. Since 2009, an enhanced version of the MW plasma source has been used (with 50–100% greater coating rates [2]).

Fig. 3 shows a cross-sectional view of the MW plasma source in operation, with ammonia and silane for PECVD of silicon nitride. Precursor gas 1 (in this case NH3), entering from above, surrounds this high-density plasma, resulting in a high degree of dissociation for the precursor, before the resulting radicals react in close proximity to the substrate, with the silane being introduced approximately 20mm above the level of the wafer.

Fig. 4 illustrates the linear plasma source and the side-mounted MW generators. The generators produce a surface wave discharge plasma of high charge-carrier density along

Figure 3. Linear MW plasma source in cross-sectional view.Figure 4. Linear MW plasma source and MW generators (from Mehlich et al. [2]).

Figure 5. Dynamic coating rate and refractive index for PECVD of silicon nitride as a function of the mean MW power.



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the quartz tube by means of introducing microwaves into an air-cooled coaxial system consisting of a quartz tube and inner conductor.

Fig. 5 shows the dynamic coating rate and the refractive index of the MW plasma source as a function of the MW power used. Typical industrial processes operate at around 1750W.

MAiA PECVD applications with optional linear RF plasma sourcesSince 2005 linear MW plasma sources have been joined by linear RF plasma sources [3] excited by a frequency of 13.56MHz, which can be used interchangeably as MW plasma sources. This system of modular linear plasma sources will open up further customer-specific MAiA PECVD solutions. Because RF-excited plasma generates different plasma characteristics, it will be possible to tackle new applications with the RF plasma source.

Fig. 6 shows the RF plasma source in schematic form. An electrode, measuring approximately 100mm × 1000mm, embedded in a housing and shielded with insulators, is located ~40mm above the carrier that passes through with the solar wafers. This can be used to carry out PECVD as well as plasma-etching processes at process pressures of 0.1–1.0 mbar.

Fig. 7 shows, with the aid of the ion current density distribution measured at the substrate level by means of plasma probes, that the plasma source produces an approximately 100mm-wide plasma, with maximum current densities in excess of 1mA/cm2.

A key difference between the linear RF plasma source and the MW plasma source is that, in the former, substantially higher ion energies are produced on the substrate (Fig. 8). Whereas the MW source exhibits typical ion energies of 5–10eV, the RF plasma generation occurs primarily at higher RF power outputs of up to 60eV.

Figure 7. Ion current density distribution in the direction of travel of the carrier beneath the linear RF plasma source, as a function of RF power, during PECVD of silicon nitride.

Figure 8. Ion energy distribution beneath the linear RF plasma source, as a function of RF power, during PECVD of silicon nitride.

Figure 6. Schematic of the linear RF plasma source (view in the direction of travel of the carrier) [4].


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Double layer anti-reflective coating

For the production of a conventional crystalline silicon solar cell, it is common to use a single-layer anti-reflective coating (SLARC). The state-of-the-art coating is a hydrogen-containing amorphous silicon nitride layer (a-SiN:H), which is created from silane (SiH4) and ammonia (NH3) by PECVD. This layer provides a unique combination of outstanding front-side surface passivation and excellent optical properties for light trapping. Typically, the layer thickness is around 75nm with a refractive index (RI) around 2.08, depending on the type of surface texture and wafer material.

The PV industry is driving towards higher cell efficiency because this is the key to significant reductions in production cost. One approach to achieving this is to further improve light trapping by introducing a double layer (DLARC) for the front side of the cell. The new MAiA 2.1 platform from Roth&Rau is specially designed for such combinations of dielectric layers. The tool is equipped with linear MW plasma sources which create a soft plasma without ion bombardment and with a much higher deposition rate than that attainable with conventional RF-induced plasmas. Significantly improved anti-reflection conditions can be achieved by using different dielectric layers with varying RIs and thicknesses. In Fig. a comparison of reflectivity spectra is presented for mono and multi-crystalline solar cells with SLARC and DLARC. It can be calculated that the weighted reflectivity (Rw; weighted with an air mass of 1.5, solar spectrum in the range 350–1200nm) improves from 4% down to 2.5% for cz wafers and from 10% to 7% for mc wafers. The DLARC is particularly superior to the SLARC in the short-wavelength range 350–500nm (important for selective-emitter cells) and in the long-wavelength range 800–1100nm (important for PERC cells).

In the pilot production line at the Roth&Rau Technology Centre, marked improvements in cell efficiency have been

observed solely as a result of the implementation of a DLARC in the cell process. With the MB-PERC cell design (see next section), an average cell efficiency boost from 19.60% to 19.99% has been obtained, with a cell efficiency of 20.04% for the best cell. A detailed comparison of cell parameters is given in Fig. 10. The first key driver is the open-circuit voltage (Voc), which increases by 0.9% from 646mV to 652mV because of an improved passivation quality of a higher RI of the first layer. Even more significant is the gain in short-circuit current

density (Jsc), which improves by 1.1% from 38.32mA/cm2 to 38.75mA/cm2.

Designed as a modular platform, the new MAiA 2.1 platform is fully capable of depositing a wide range of DLARCs. In addition to pure a:SiN:H or a:SiSiO:H layers, an amorphous silicon oxinitride (a:SiON:H) layer, commonly used as the second layer, is also possible. Such a mixed layer increases the range of possible RIs from 1.47 to 1.90 without changing the thickness. Taking into account the glass on top of a DLARC, simulations show that an a:SiON:H-layer is more suitable for integrated cell and module manufacturers for achieving optimized light trapping. A throughput of up to 3400 wafers per hour is possible, independently of the layer properties. The new MAiA 2.1 platform is therefore perfectly suited to all kinds of cell design (MWT, PERC, IBC, etc.), for p-type or n-type silicon, and applicable in the R&D lab or in high-volume production.

MB-PERC

The MB-PERC solar cell concept is based on the MAiA 2.1 platform developed by Roth&Rau as the centrepiece of the cell process. It is a logical development of conventional PERC technology, with a clear industry-oriented focus aimed at high-volume cell production. Fig. 11 shows the MB-PERC

Figure 9. Typical reflectivity spectra for a front-side-textured solar cell, showing a comparison of SLARC and DLARC for cz and mc wafers.

Figure 10. Implementation of DLARC into PERC cell design: cell data for SLARC and DLARC of a p-type cz-silicon wafer.



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process flow for a standard Al-BSF (back-surface field) solar cell (in grey) and the additional steps required for MB-PERC technology (in blue).

It is apparent at a glance that MB-PERC is not a totally new cell concept, but rather an evolutionary progression of the standard cell processes adopted all over the world. The entire standard cell production-line equipment is still used, complemented solely by an MAiA 2.1 for the deposition of the rear dielectric passivation coatings and a laser system for the local opening of these coatings. Fine-tuning of the upstream and downstream processes is of course required in order to exploit the full potential of MB-PERC technology. These adaptations are purely of a technological nature – no hardware interventions are required.

As indicated in Fig. 11, the two process steps of AlOx and SiNx deposition in the MAiA 2.1 take place in two process chambers. Should the need arise, for example to deposit the front ARC, a third process chamber can also be added. The DLARC mentioned in the previous section may also be realized by means of an additional process chamber.

In contrast to the conventional PERC process, MB-PERC technology employs aluminium oxide (AlOx) as the dielectric passivation coating on the rear surface. In addition to very good chemical passivation properties, AlOx possesses negative fixed charges, which are responsible for its outstanding passivation effect. This enables very low surface recombination velocities to be achieved, as can be seen in Fig. 12.

Fig. 13 depicts the homogeneity of the AlOx passivation coatings produced using the MAiA 2.1 platform, within one wafer and across the entire width of a carrier. Results are shown for three different AlOx-coating thicknesses, each with seven measurement points per wafer at five positions all across the carrier, from the far left (LL), through the centre (C), to the far right (RR).

After the deposition, the dielectric coating on the rear is

opened up locally using a laser: the opened part must be selected in such a way as to achieve the optimum ratio of contacted and passivated surfaces. The subsequent screen printing takes place in the same way as in standard cell processing, except for the use of aluminium pastes with modified properties. During the contact sintering process, a local BSF forms in the laser-ablated areas, as a result of which the recombination at the contact openings is reduced. Here, the formation of a BSF and the filling of the contacts are dependent on the interaction between the laser employed, the opening geometry and the aluminium paste, as well as the temperature/time allowed during the sintering.

Fig. 14 shows a scanning electron microscope (SEM) image of a local contact following appropriate sample preparation.

Figure 11. Process flow diagram for MB-PERC cell technology.

Figure 12. Comparison of surface recombination velocities for different passivation coatings [5].


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The dielectric passivation coating is not visible owing to its small thickness at the selected magnification. The BSF can be clearly seen as a strip, with a width of ~6µm around the eutectic system.

Very good results have been achieved at the Roth&Rau Technology Centre using the MB-PERC process. Table 2 presents a summary of the cell parameters of the best cells. The transition to high-volume production by Roth&Rau’s

customers has also proved very successful. With cell efficiencies of up to 20.7% and average efficiencies of 20.3%, these cell manufacturers are exceptionally well equipped to meet ever-higher customer expectations.

Plasma-activated dry etching using the Roth&Rau MAiA®Tex 2.1 inline platform

The modular design concept used in the Roth&Rau MAiA system family not only supports a wide range of coating processes used in PV technology (front- and/or rear-surface coating with different individual coatings and multiple coating systems such as SiN, SiO, AlOx, ITO or different doping coatings), but also provides for a wide variety of dry etching processes [6]. An example of this is the MAiA®Tex system (see Fig. 15), designed for the production of textured substrate surfaces.

The system enables single-sided texturing (essential for various high-efficiency solar cell technologies) of all silicon-based substrate materials (SiC or diamond-sliced mono or

Figure 13. Homogeneity of the AlOx coating within a single wafer and from wafer to wafer, across the entire width of a carrier from the far left (LL) to the far right (RR).

Figure 14. SEM image of a local contact in cross section. The BSF area is clearly discernible as a strip around the Si/Al eutectic system.

Material/size η [%] FF [%] Jsc

[mA/cm2] Voc

[mV]

cz 6” / pseudo square 20.49 79.18 39.1 662.3

mc 6” / full square 18.57 78.5 36.6 646.3

Table 2. Cell parameters for MB-PERC technologies for mono- and multicrystalline cells produced at the Roth&Rau Technology Centre.



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multi, UMG, string ribbon, cast mono, kerf-less material, etc.); it also allows especially thin substrates (≥ 50µm material thickness). Fig. 16 shows a cast mono Si wafer, consisting of both large monocrystalline and multicrystalline areas, before and after plasma texturing with the MAiA®Tex. The two different crystalline areas exhibit equally good reflective properties, even when viewed at an oblique angle. All these substrates can be handled with just one fluorine and oxygen-based process, resulting in very good homogeneity and very low reflective properties (necessary in order that the maximum light possible enters the solar cell to generate photocurrent) [7].

Fig. 17 shows the results relating to the reflective properties of different Si materials after processing with the MAiA®Tex compared with wafers textured by means of the wet chemical process. The absolute efficiency of the solar cells

can be increased thanks to the improved reflective properties of the wafer surfaces (by as much as 0.5% in the case of multicrystalline solar cells).

Highly passivating amorphous silicon coatings for HJT cells

An MAiA process chamber, as shown in Fig. 18, was set up to carry out investigations to compare the performance of linear MW and RF plasma sources in the deposition of intrinsic a-Si:H for HJT cells. Amorphous intrinsic silicon coatings of silane and hydrogen at substrate temperatures of 200°C were deposited [4] using just one plasma source type at a time as an alternative.

Fig. 19 clearly shows that far thicker coatings, with thicknesses of 7–10nm, can be deposited using the linear RF plasma source. Furthermore, only the coatings deposited with RF plasma achieved the energy gap of 1.7–1.8eV that is typical of amorphous silicon. This shows that only the RF

Figure 15. Inline MAiA®Tex system for the production of plasma-textured Si surfaces.

Figure 17. Wavelength dependence of the reflective properties of different Si surfaces.

Figure 18. Schematic representation of the process chamber of an MAiA system for the comparison of an MW plasma source with an RF plasma source for PECVD of amorphous silicon ([4]).

Figure 16. Reflective properties of cast mono Si surfaces: (a) untextured; (b) plasma-textured; (c) plasma-textured with ARC.

(a) (b) (c)


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plasma source is a promising candidate for the deposition of a-Si coatings for HJT cells using inline systems of type MAiA.

Fig. 20 shows that if these intrinsic a-Si coatings are deposited on float-zone Si wafers, charge carrier lifetimes in excess of 10ms can be achieved with passivation coatings of the order of 10nm. With a coating thickness of 7nm, required for HJT cells, excellent lifetimes of 5–8ms are also achieved.

Initial test results for the purpose of demonstrating the performance of the inline MAiA process with linear RF plasma sources for the production of HJT solar cells, in accordance with the sequence described in [8], yielded efficiencies of greater than 20%. In this case the standard PECVD process for HJT cells (S-Cube static parallel plate reactor from Roth&Rau AG with 13.56MHz) was replaced by the above-described process for the deposition of the intrinsic amorphous silicon layer. Since the deposition of the doped amorphous silicon coatings places quality requirements on the PECVD process that are less demanding, the performance of inline MAiA processes is therefore also demonstrated in the area of PECVD of pure semiconductor layers for PV applications.

PVD applications on the MAiA 2.1 platform

The physical vapour deposition (PVD) process module rounds off the coating capabilities of the MAiA 2.1 platform. The first PVD produced by Roth&Rau was developed primarily with the requirements of the solar industry in mind. The use of rotating tubular targets results in an optimum target utilization of up to 85% and therefore a reduction in operating costs. The relatively short target length (of just 1400mm) in comparison to glass-coating systems enables the target to be switched quickly and easily. This means that, in addition to production use, the machine is also exceptionally well suited to R&D operation, in which a wide variety of processes have to be reproduced. Manufacturers of tubular targets offer a wide choice of target materials in different qualities for this purpose. A maximum of four tubular targets can be fitted per process chamber, allowing the user to determine, on an individual basis, whether the substrate is coated from above or below for each target. Using open trays, the double-sided coating of substrates can thus be accomplished in a single pass. A subsequent change of coating direction is also possible by fitting or removing the corresponding spacers. Depending on the process sequence required and the deposition period, any desired number of process modules can be directly connected in series. Should a gas separation be required between different processes, this can be fully guaranteed by means of an additional transfer chamber.

During the production of HJT solar cells at the Roth&Rau Technology Centre, the PVD process module is used for depositing the transparent conductive oxide (TCO) coating on the front of the cell and for depositing the TCO coating

Figure 19. Comparison of coating thicknesses, coating roughness and energy gaps of intrinsic amorphous silicon coatings using (a) RF plasma source, and (b) MW plasma source [4].

Figure 20. Effective minority-carrier lifetime of intrinsic a-Si:H on polished float-zone silicon wafers deposited using the linear RF plasma source in the MAiA [4].

(a)

(b)



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and/or the metal coating for the rear contact. With other cell concepts, the PVD process module is notably well suited to the production of flat metal contacts on the rear of the cells. The homogeneities achieved in the deposition lie in the range of ±5% about the mean value. The coating thickness distribution of an ITO coating with a target coating thickness of 110nm is shown in Fig. 21.

References

[1] Schlemm, H. et al. 2003, “Industrial large scale silicon nitride deposition on photovoltaic cells with linear microwave plasma sources”, Surf. Coat. Technol., Vol. 174–175, pp. 208–211.

[2] Mehlich, H. et al. 2010, “PECVD Processes with increased deposition speed with linear microwave plasma sources”, Proc. 25th EU PVSEC, Valencia, Spain, pp. 2659–2662,

[3] Schlemm, H., Fritzsche, M. & Roth, D. 2005, “Linear radio frequency plasma sources for large scale industrial applications in photovoltaics”, Surf. Coat. Technol., Vol. 200, No. 1–4, pp. 958–961.

[4] Decker, D. et al. 2012, “Inline a-Si:H PECVD using linear microwave, and new linear RF plasma sources”, Proc. 27th EU PVSEC, Frankfurt, Germany, pp. 1624–1629.

[5] Schmidt, J. et al. 2008, “Progress in the passivation of silicon solar cells”, Proc. 23rd EU PVSEC, Valencia, Spain, pp. 974–981.

[6] Grimm, M. et al. 2008, “MAiA – A new versatile in-line plasma etching/PECVD tool concept for processing crystalline silicon solar cells”, Proc. 23rd EU PVSEC, Valencia, Spain, pp. 1801–1804.

[7] Uhlig, M., Köhler, G. & Schlemm, H. 2013, “Single side microwave plasma texturization with inline MAiATex tool”, Poster presentation 28th EU PVSEC, Paris, France.

[8] Descoeudres, A. et al. 2012, “Silicon heterojunction solar cells on n- and p-type wafers with efficiencies above 20%”, Proc. 27th EU PVSEC, Frankfurt, Germany, pp. 647–651.

Figure 21. Homogeneity of the ITO coating, sputtered using the Roth&Rau MAiA 2.1 platform.


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