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aPhys. Status Solidi C 8, No. 3, 903–906 (2011) / DOI 10.1002/pssc.201000270

Dry texturing of mc-Si wafers

Garima Agarwal1,2, Simona De Iuliis*,1, Luca Serenelli1, Enrico Salza1, and Mario Tucci1 1 ENEA-Casaccia, Via Anguillarese 301, 00123 Rome, Italy 2 CNER, 14-Vigyan Bhawan, University of Rajasthan, 302004 Jaipur, India

Received 2 May 2010, revised 28 May 2010, accepted 31 May 2010 Published online 23 December 2010

Keywords Si, reactive ion etching, texturing, solar cell * Corresponding author: e-mail simona.deiuliis@enea.it, Phone: +39 063048 4515, Fax: +39 063048 6405

Texturing of mc-Si is a prevailing research topic to im-prove solar cell efficiency in production. Surface textur-ing for enhanced absorption in Si has been historically obtained by creating randomly distributed pyramids us-ing anisotropic etchants; but this preferential etching works only on single crystalline silicon because of its crystallographic orientations. A low-cost, large area, ran-dom, mask-less texturing scheme is expected to signifi-cantly impact terrestrial PV technology and reduce the amount of wet-chemical waste. We propose an approach based on randomly etched mc-Si by RIE system using NF3 instead of SF6 or CF4 to reduce the detrimental for-mation of carbonaceous or sulfurous contamination at the silicon surface, which results in a surface recombination. To obtain a fast process we have investigated the effect of the chemical etching due to the NF3 radicals and the

ion bombardment induced by Ar. We have found that Arions promote a helpful surface pre-conditioning, while fluorine radicals, produced by NF3 dissociation, are needed to increase the Si etching rate. Different combina-tions of flux ratios, gas pressures and RF power have been explored. Efforts have been devoted in obtaining a homogeneous texture on large area wafers, which is ines-capable for industrialization. After 10 minutes process ef-fective reflectance values have been measured within the range of 12-14%, and with a-Si/SiNx the value reduced to 7%. Post-processing minority carrier lifetime values in the range of 10 microseconds have been measured with-out applying any further chemical cleaning. Additionally, microscopic analysis has been performed to evaluate the surface microstructure morphology.

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1 Introduction Silicon surfaces usually reflect more than 30% of impinging solar light. To reduce these reflec-tivity losses surface texturing becomes important in addi-tion to standard anti-reflection coatings (ARC). Plasma in-duced geometrical texturing of multicrystalline silicon (mc-Si) wafers has proven its potential in the field of solar cell fabrication, by many researchers due to their keen in-terest in current developing photovoltaic technology [1-3]. Dry plasma texturing provides surface structures suitable for solar cell fabrication. Compared to wet chemical iso-tropic texturing, plasma processing offers some advantages such as less mechanical wafer breakage and use of de-ionized water consumption, which are beneficial for cost, environmental and safety perspectives. Moreover, it shows a high efficiency potential due to lower reachable reflec-tance values. Furthermore, the texturing is independent of silicon substrate material leading to high flexibility in case of varying wafer supply. It is more advantageous because of full control over all process parameters which allows for good reliability and reproducibility especially in industrial

production. Reactive Ion Etching (RIE) is a kind of dry plasma processing which not only use the chemical ability of plasma species, but also the kinetic energy of its ions [4]. This way of texturing is an idea for very fragile substrates and is not grain dependant under certain etching conditions. The uniformity over the substrate depends strongly on the used RIE parameters and reactor geometry. The aim of this work is to establish an isotropic plasma texturing process for mc-Si wafers using RIE technique for high wafer throughput. Main focus has been put on process homoge-neity and stability, high etch rate as well as low reflectivity and surface damage.

We propose an approach based on randomly etched Si using RIE by using NF3 instead of SF6 or CF4 to avoid un-wanted formation of sulfurous or carbonaceous compounds at the Si surface, which enhance the surface recombination [5]. It has been reported in literature that products of NF3 discharge can etch Si and SiO2, 1-2 orders of magnitude faster than can those of similar discharge in CF4. It has suggested that the discharge is much more effective at dis-

904 G. Agarwal et al.: Dry texturing of mc-Si wafers

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sociating NF3 than CF4, apparently due to the lower bond strength of NF3 molecule than CF4 molecular bonding and lower dissociation threshold energy too [6]. In this way NF3 has advantages over other fluorinated gases.

In the present study the etching has been developed us-ing NF3 mixed with an electropositive diluent gas, prefera-bly Ar, at a given range of concentration, pressure, flow rate and RF power, to obtain fine homogeneous textured surface along with good reflectance and fastest possible etch rates in a very short processing time.

2 Experimental details For the present study a plasma system produced by AXIC has been used, it is composed of two separate chambers to perform both Plasma Enhanced Chemical Vapour Deposition (PECVD) and RIE respectively, using 13.56 MHz RF generator. The RIE substrate holder is 23 cm of diameter, and 5 cm is the distance between the two electrodes. Texturing of mc-Si has been performed in RIE chamber and surface passiva-tion/ARC has been made in PECVD chamber. P-type doped mc-Si wafers, with thickness of 330 μm and resistiv-ity of 0.5-2 Ω cm have been used. Before dry etching proc-ess the wafers have been treated by alkaline wet etching followed by RCA2 solution for neutralization and cleaning. The reactant gases used in the investigation have been ni-trogen trifluoride (NF3) and argon (Ar); the flow of gases to the chamber has been controlled through a set of cali-brated mass flow controllers and the chamber pressure has been varied with the help of a throttle valve. The electrode area and the RF power have been suitably chosen to pro-duce a defect free texturized wafer surface. Etching has been performed with different gas ratios at different work-ing pressure, generally from 100 to 1000 mTorr, and at room temperature (RT), while the RF power has been ad-justed along with different flux ratios for a fixed interval time of 10 minutes. However, since the scope of this work is to discuss our best results, we present just the most effi-cient texturing processes.

For reliable texturing on mc-Si it has been found that the gas ratio NF3/Ar should be adjusted to 0.125 to 0.11. With an optimized gas ratio, the most interesting structures produced have been obtained at 200 W of RF power. Most of the processes have been performed for 10 min at a fixed pressure of 1000 mTorr. Before each texturing process, we have applied Ar plasma for 10 min to help the chemical etching by wakening of surface bonding. In this way the whole etching process has been done under a two step ap-proach.

Reflectance measurements have been performed using Perkin Elmer Lambda 9 UV/VIS/NIR spectrophotometer, for each kind of etching, to compare and individuate the best conditions that ensure the lowest reflectance. Also ef-fective reflectance Reff has been evaluated as the reflec-tance weighted with the sun spectrum. To get a deeper view of the surface after dry etching process scanning elec-tron microscopy (SEM) has been done to evaluate the mi-

crostructures through SEM micrographs. The equipment used for this task is Philips E-SEM XL30-LaB6.

To investigate the possibility to passivate the treated mc-Si surface we have covered both surfaces with a film composed by a double layer of amorphous material con-sisting of PECVD deposited amorphous silicon and silicon nitride (a-Si/SiNx), which has already been demonstrated to be effective [7]. The thicknesses of both layers have been dimensioned in such a way to get minimum reflection against solar spectrum. After passivation the effective mi-nority carrier lifetime τeff has been evaluated by using WT-85XA Sinton flash system.

3 Results and discussion RIE systems for silicon

surface texturing are based on a combination of chemical activity of reactive species generated in the plasma with physical effects caused by ion bombardment. For the pre-sent RIE etching we propose a two step approach including various combinations of physical and chemical process steps, to ensure which one is more dominant in particular dry etching conditions. It is observed that both steps are necessary for a particular time period in case of NF3/Ar plasma, for a fast and effective texturing. Following mechanism is proposed considering the dual steps dry etch processing:

1. Physical or sputter etching using plasma induced nonreactive Ar ion bombardment which strike vertically to the target surface, for non-selective removal of surface at-oms. In this process the plasma is helpful to energize a chemically inert projectile so that it moves at high velocity when it strikes to the surface. During this process momen-tum is transferred due to elastic collision. The surface at-oms of the surface are dislodged if projectile energy ex-ceeds bonding energy. Low-energy ions are used to avoid implantation damage. Using such step it is supposed that Ar ions are capable of making surface preconditioning which is helpful to fast etching in the following process.

2. Ion enhanced Chemical etching using plasma of NF3/Ar induced gaseous etchants along with energetic ions, for highly isotropic surface etching. The plasma has been used to produce chemically reactive species like free radi-cals, atoms and ions (NF2, F, F- etc) from inert molecular gas, which goes to diffuse the surface and some of them are adsorb on the surface. The chemical reaction starts be-tween free radicals and the Si, which may result to desorp-tion of by-products and their diffusion into bulk gases.

It is worth to note that the two steps have been per-formed in the same reactor without interrupting the vac-uum.

To obtain a fast process we have investigated the effect of the chemical etching due to the NF3 radicals and the ion bombardment induced by Ar. We have found that Ar ions promote a helpful surface pre-conditioning, while fluorine radicals, produced by NF3 dissociation, are needed to in-crease the Si etching rate. For the 1st step, induced Ar plasma treatment, a minimum condition has been opti-mized for a better surface preconditioning followed by

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chemical reactions in NF3/Ar states. The necessity of Ar ions in 1st step could be explained in a way that ion bom-bardment disrupting the Si network and thus enhancing its reactivity (for the following step) with fluorine by ap-proximately one hundred fold against of single use of chemistry phase. For a wide range of understanding of such kind of approach, mainly two regimes have been de-fined: low and high pressure regime. A graph is repre-sented in Fig. 1 for a low pressure regime.

Figure 1 mc-Si etching rate as a function of NF3 concentration in Ar (keeping total flux constant = 60 sccm) at a low pressure regime.

Figure 1 shows the mc-Si wafer etch rate as a function

of NF3 concentration in Ar varied from 10% to 100%. The plasma parameters as RF power at 200 W, pressure at 100 mTorr and an etching time of 10 min, have been main-tained. By visual assessment has been found that the grains are easily detectable using such conditions. From Figure 1 it is evident that increasing NF3 percentage in Ar (10%, 20%, 80%) or without Ar (NF3 100%), the etching rate of mc-Si linearly increases from 0.23 to 1.2 μm/min, which is attributed to the increase of fluorine radicals in the mixture of NF3/Ar gas plasma, while Reff is almost similar (30%) for all points. In pure NF3, the high fluorine atom concen-tration leads to a very fast chemical etching and substantial lateral attack on the surface [8], etching almost 12 μm of silicon. It means that, at low pressure, on one hand the high NF3concentration improves both the chemistry reaction and the etching process, on the other hand does not affect the reflectivity, which is also an important factor for our purpose. We can conclude that a low pressure regime does not produce a mutt and textured surface, due to the fact that the etching process is working in an anisotropic way, and does result in a very fast process.

Thus, to fabricate textured surfaces we have investi-gated the high pressure regime, and at the same time trying to maintain a high etching rate. We have performed several experiments in pressure range more above of 100 mTorr, at NF3/Ar ratio 0.25, keeping the same RF power of 200W and for the same etching time of 10 min.

Figure 2 NF3/Ar plasma working pressure vs mc-Si etching rate.

From the Fig. 2 it is clear that the best etching rate is

possible at the highest pressure, 1000 mTorr, while the Reff value, measured at this point, equals 16.8%. It is supposed that at high pressure there are less collisions between gase-ous plasma and hence less energy loss, which is useful to react most of the plasma instantly with the surface and re-new the chamber gases. Therefore, it is suggested that at high pressure, by keeping low the concentration of NF3 in Ar, is agreed number of reflectance and etching rate too.

Figure 3 Reflectance spectra of un-textured and textured mc-Si wafer at different total flux keeping NF3 percentage same as 10% along with SiNx passivation.

Figure 3 shows a comparison of reflectance data of

neighbour mc-Si wafers processed with an alkaline saw damage removal, dry NF3/Ar texturing with minimum NF3 concentration at different total fluxes, and SiNx passivated. It is observed from the reflectance spectra that by keeping the same percentage of NF3 at different total flux, resemble for similar results in terms of Reff (14% and 12%) and etch-ing rate (4.8 and 5.0 μm/min) for total flux 60 sccm and 80 sccm respectively. From a visual assessment the textured wafers are homogeneously mutt and look dark, leaving grain boundaries almost undetectable, signifying that the etching is working in an isotropic way. This is confirmed by the lowest reflectance value homogenously obtained on the whole processed area. It is worth to notice that ARC

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applied on the dry textured surface is able to reduce the ef-fective reflectance to a value of 7%. After mc-Si surface passivation a lifetime measurement in the range of 10 μs has been found comparable to that obtained on neighbour wafer after alkaline saw damage removal followed by sur-face passivation. This ensures the validity of the double step approach for the dry texturing of mc-Si surface. Also the uniformity of the etching treatment over the whole wa-fer surface has been successfully verified.

Figure 4 SEM images at different resolutions (50 μm upper side, 2 μm lower side) of mc-Si plasma dry etched surface in NF3/Ar ratio 0.1, total flux 60 sccm. Figure 5 SEM image (at resolution 10 μm) of mc-Si plasma dry etched surface in NF3/Ar ratio 0.1, total flux 80 sccm.

We have used SEM to monitor the surface morphology,

in order to explore the change in surface texturing depend-ing on a variety of plasma parameters. Figures 4 and 5 show the SEM micrographs at different resolution of RIE induced dry textured surfaces at two different total gas

fluxes by keeping the same percentage of NF3 in Ar. It is observable from micrographs that a porous sponge like mi-crostructure is created after RIE texturing which is similar in both cases of total gas fluxes. Because of such morphol-ogy a reduced effective reflectance value is obtained. The evaluated dimension of randomly distributed structures is less than 1 μm (varied from 400-700nm) in case of total flux 60 sccm, and equal to 1 μm in case of total flux 80 sccm.

4 Conclusions We have investigated a dry textured mc-Si surface in a direct parallel place RIE system using NF3 gas mixed with Ar in different chamber conditions. It is concluded that for fast reactivity between NF3 and Ar plasma, a preconditioning of surface is required through physical phenomenon of Ar ion bombardment, which is capable to promote the chemistry phase. When the surface is processed under a low pressure regime during chemistry phase, it is suggested to have high etching rate in contrast of poor reflectance, ensured the suitability of high pressure regime. Under such conditions we have observed an iso-tropic etching induced texturing which is found suitable for mc-Si in terms of reflectance and etching rate too. Hence, RIE induced NF3/Ar plasma processing has been devel-oped successfully as a method to texture mc-Si in a ho-mogenous way, independently of the grains orientation on the mc-Si surface.

Acknowledgements The authors gratefully would like to thank the International Centre for Theoretical Physics (ICTP, Tri-este Italy for providing TRIL fellowship to one of our authors (GA) to perform this work in collaboration with ENEA, Casaccia Research Centre, c-Si PV laboratory.

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