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Femtosecond laser ablation of dielectric layers for high-efficiency silicon wafer solar cells
Solar Energy 164 (2018) 287–291
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Femtosecond laser ablation of dielectric layers for high-efficiency silicon wafer solar cells
T
Jaffar Moideen Yacob Alia,b, Vinodh Shanmugama, Bianca Limc, Armin G. Aberlea,b, ⁎ Thomas Muellera, a
Solar Energy Research Institute of Singapore, National University of Singapore, Singapore 117574, Singapore Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117583, Singapore c Institute for Solar Energy Research, D-31860 Emmerthal, Germany1 b
A R T I C L E I N F O
A B S T R A C T
Keywords: Femtosecond laser Silicon wafer solar cells Dielectric ablation Photoluminescence imaging
Laser ablation of dielectric layers to form local contacts while inducing negligible electronic damage to the underlying substrate is crucial for high-efficiency silicon solar cell fabrication. In this work, ablation of dielectric layers such as SiNX, AlOX/SiNX stacks and thermal SiO2/PECVD SiNX stacks is performed using laser pulses with a pulse duration of 480 fs and a wavelength of 515 nm. At this wavelength the femtosecond (fs) laser pulses are typically absorbed in the underlying silicon (indirect ablation), causing defects. However, by precisely determining the single pulse ablation properties such as threshold fluence, spot radius (r) and energy penetration depth (z0) and by optimising the line ablation process, it is possible to ablate the dielectrics with little to no damage to the substrate. Upon analysing r and z0, two distinct ablation regimes, ‘gentle’ and ‘strong’, are identified. Etch-back processing and photoluminescence analysis shows that the strong ablation regime needs more than 15 min of damage etching, corresponding to an etch depth of about 18 µm and thus indicating bulk damage. However, in the gentle regime the defects can be etched off in less than 5 s, indicating that the damage is confined to less than ∼100 nm from the wafer surface, which agrees with the z0 determined for the gentle regime. Hence, gentle fs laser ablation is highly suitable for industrial mass production of ablation intensive and high-efficiency silicon wafer solar cells.
1. Introduction Laser processing in the photovoltaics (PV) industry has gained significant interest due to its inherent advantages such as high-throughput processing, versatility and precision that are vital in the fabrication of cost effective and highly efficient solar cells (Niyibizi et al., 2012). For example, in the fabrication of interdigitated back contact (IBC) solar cells, laser processing is used for selective ablation of dielectrics and micro-structuring of underlying silicon for local contact formation (Franklin et al., 2016). Dielectric layers used in solar cell fabrication are typically for surface passivation, masking, or anti-reflection coating (Blakers et al., 1989). Various high-efficiency solar cell architectures require the dielectric layer to be locally opened in order to form an ohmic contact to the underlying silicon (Franklin et al., 2016). Semiconductor industry methods such as photolithography (Zhao et al., 1999) are slow and expensive, and thus not suitable for low-cost mass production of high-efficiency solar cells. Laser systems are steadily improving with excellent translational control and very high ⁎
1
throughput. Different laser sources with pulse lengths ranging from nanosecond to femtosecond and wavelengths ranging from UV to IR have been used for dielectric ablation (Kautek et al., 1996; Heinrich et al., 2011; Abbott and Cotter, 2006; Iyengar et al., 2011; Binetti et al., 2016). Recently, ultrashort pulse laser technology using femtosecond laser sources are being adapted into solar cell processing (Hermann et al., 2006). A fs laser pulse is shorter than the timescale of photonelectron-lattice interactions and thus the laser pulse ends before the excited electrons can transfer energy to the lattice, leading to “cold” ablation (Shirk and Molian, 1998). It is possible to obtain highly localized energy deposition with a negligible heat affected zone (HAZ). Though fs ablation is known to produce cleaner structures, the laser absorption inside silicon is known to introduce defects in its bulk (Hayafuji et al., 1981). The laser induced defects act as recombination centres for minority carriers and thus reduce the open-circuit voltage and efficiency of the solar cell. A precise and damage-free ablation of dielectric layers is useful for high-efficiency solar cell architectures such as IBC and heterojunction
Corresponding author. E-mail address: [email protected] (T. Mueller). Current address.
https://doi.org/10.1016/j.solener.2018.02.046 Received 27 October 2017; Received in revised form 6 February 2018; Accepted 17 February 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.
Solar Energy 164 (2018) 287–291
J.M. Yacob Ali et al.
solar cells. Achieving this requires a comprehensive understanding of the basic ablation properties such as the ablation threshold fluence (FTH), spot radius (r0) at the interaction plane and the energy penetration depth (z0) into the substrate. In this paper, the ablation of three widely used dielectric layers in silicon solar cell fabrication – SiNX, AlOX/SiNX stacks and thermal SiO2/PECVD SiNX stacks – that are deposited onto planar n-type silicon wafers is investigated. Initially, the single pulse ablation behaviour was analysed to determine the basic ablation properties. The laser ablated spots were measured using a 3D optical microscope. Most of the laser processing techniques in solar cell manufacturing involve continuous laser scribing by overlapping pulses. As a result, the equivalent fluence at any point along the line is much higher than the incoming single pulse fluence, causing more damage to the underlying silicon. Therefore, the required fluence (FTH_LINE) with respect to pulse spacing will be estimated for line ablation. In order to determine the laser induced defects, the dielectric layer is opened using various pulse fluences and the samples are subsequently etched in KOH solution for various time periods and then re-passivated using SiNX on both sides. The resulting surface quality is analysed using a photoluminescence (PL) imaging tool (BTI Imaging, Australia) (Trupke et al., 2006).
Fig. 1. Schematic of a sample illustrating the process of indirect ablation.
Table 2 Dielectric layers and their thicknesses.
2. Experimental details
Dielectric layer
Thickness (nm)
SiNX AlOX/SiNX stack SiO2/SiNX stack
70 10/70 30/160
2.1. Laser system The laser source used in this work produces 480 fs laser pulses at 1030 nm wavelength and has a pulse repetition rate frep ranging from 200 kHz to 2000 kHz. In this study, the laser source is used in the Second Harmonic Generation (SHG) mode at 515 nm. The maximum average laser power Pavg irradiating the sample is 15.8 W, measured using an in-built power meter. The laser beam scanner has a maximum scan speed of 20 m/s. A summary of the laser parameters used in this study is given in Table 1. The incoming pulse has a Gaussian intensity profile where the beam radius r is defined as the distance from the beam axis where the intensity drops to 1/e2 of its peak. The beam spot area is calculated as πr2. From the measured Pavg value, the pulse energy EP and pulse fluence FP are calculated as follows:
EP =
FP =
Pavg frep
(1)
2 EP πr 2
(2)
Fig. 2. Process sequence for characterizing laser induced damage.
2.3. Characterization of laser induced damage 2.2. Sample specifications In order to characterize the laser induced damage, the samples were processed as shown in Fig. 2. The dielectric layer was completely ablated in square regions measuring 3 cm × 3 cm, using selected pulse fluence values from both the gentle and the strong ablation regimes. The samples were then dipped in a 10% HF solution for 5 min at room temperature to remove any remaining dielectric layers in the non-laser processed regions and on the opposite side of the wafer. Following that, the samples were etched in KOH solution for a duration in the range of 5 s to 15 min in order to determine the etching time required for laser damage removal. This corresponds to etching depths of about 100 nm to about 18 µm, respectively (note that the etching rate is ∼1.2 µm/min per side). Finally, the samples were re-passivated with 70 nm SiNX on both sides and then analysed using the PL imaging technique.
The investigated samples consist of planar n-type silicon wafers (3.5 Ωcm resistivity) passivated with dielectric layers on both sides. A schematic of the samples is shown in Fig. 1. The dielectric layers include SiNX, AlOX/SiNX stacks and SiO2/SiNX stacks. SiNX and AlOX were deposited using a PECVD reactor, while SiO2 was grown via dry thermal oxidation at 950° C in a tube furnace. The thickness of the dielectric layers was determined using an ellipsometer from SEMILAB. The results are listed in Table 2. Table 1 Laser parameters used in this study. Wavelength λ (nm) Pulse duration τL (s) Repetition rate frep (kHz) Beam propagation factor M2 Theoretical focus spot radius r (µm) Max. average laser power Pavg (W) Max. pulse energy EP (µJ) Max. peak fluence FP (J/cm2)
515 480 × 10−15 200–2000 1.3 24.6 15.8 at 200 kHz 79 at 200 kHz 8.3 at 200 kHz
3. Results and discussion 3.1. Single pulse ablation properties In this section, the single pulse ablation properties such as FTH, r and 288
Solar Energy 164 (2018) 287–291
J.M. Yacob Ali et al.
0.6
5000 SiNX
nm,
AlOX / SiNX
4000
0.5
SiO2 / SiNX
z ( m)
d2 ( m2)
0.4
3000
fs - gentle
Linear Fit
fs
fs - strong
0.3 fs - gentle
0.2
1000 0
nm,
AlOX / SiNX SiO2 / SiNX
fs - strong
Linear Fit
2000
SiNX
fs
0.1 5
6
7
8
0.0
9
5
6
7
8
9
Ln(Peak Fluence in mJ/cm2)
2
Ln(Peak Fluence in mJ/cm )
Fig. 3. d2 (left) and z (right) versus Ln(Peak Fluence), for different dielectric layers.
electrons. However, in the strong ablation regime, rS and z0_S increase at a faster rate with respect to pulse fluence due to the enhanced electronic heat conduction discussed above. Though z0_S is confined to < 300 nm for the ablation of 70 nm SiNX, the impact of strong ablation is seen to affect the bulk of the underlying silicon. This will be discussed in more detail in Section 3.3.
z0 are experimentally determined for all the dielectric layers. For a Gaussian laser beam profile, the diameter d and depth z of an ablated crater are related to the peak fluence as
F d2 = 2r 2ln ⎛ P ⎞ F ⎝ TH ⎠
(3)
F z= z0ln ⎛ P ⎞ ⎝ FTH ⎠
(4)
⎜
⎜
⎟
3.2. Threshold fluence vs pulse spacing in line ablation
⎟
While performing laser scribing with pulse overlap, the equivalent fluence received by the substrate increases and ablation is observed at a fluence much lower than that of FTH. The pulse spacing Δ is defined as the distance between the centres of two adjacent pulses along the direction of ablation. As the pulse spacing is reduced, more pulses overlap with each other and this can shift the ablation regime from gentle to strong. In this section, the line threshold fluence (FTH_LINE) versus Δ is determined for all three dielectric layers. Δ can be controlled by adjusting the pulse repetition rate (frep) and laser beam scan speed (v) as shown in Eq. (5).
where FTH is the ablation threshold fluence which is defined as the minimum peak fluence required to completely remove the dielectric layer. In Eq. (3), if FP ≤ FTH, we get d2 = 0. Thus, plotting d2 against the Ln(peak fluence) and extrapolating the linear fit will give us FTH (Liu, 1982). The spot radius r on the sample surface and the energy penetration depth z0 can be estimated from the slope of the linear fit of Eqs. (3) and (4) respectively. For statistical purposes, the value of each individual parameter set is taken as the mean of five ablated spots. Fig. 3 shows a plot of d2 (left) and z (right) versus the logarithm of the peak fluence, for different dielectric layers. The slopes in Fig. 3 reveal the presence of o different ablation regimes, termed as ‘gentle’ and ‘strong’ as indicated in the graphs, and are distinguished using the subscripts G and S, respectively. In ultrashort pulse ablation, the excited hot electrons and the lattice are not in thermal equilibrium resulting in electronic heat conduction (Gattass and Mazur, 2008). In the gentle ablation regime (at lower fluence), the density and temperature of hot electrons are lower resulting in a lower electronic heat conduction rate. Therefore, in the gentle regime, the ablation depth is mainly governed by optical penetration length of the laser beam (Mannion et al., 2003). In the strong ablation regime (at higher fluence), the temperature and density of hot electrons are very high causing an excessive electronic heat conduction, so it takes longer time to achieve thermal equilibrium due to the large temperature difference. Such rapid electron-lattice collisions result in bulk heating and damage. As a result, in the strong regime, the ablation depth is mainly governed by the heat transport of thermally excited electrons (Schille et al., 2015). FTH, r and z0 for the two regimes are detailed in Table 3. The required FTH values increase with layer thickness. The ablation spots (rG) in the gentle regime are much smaller than the focus spot radius r (24.6 µm) indicating negligible thermal diffusion of excited
Δ = v/frep
(5)
FTH_LINE is extracted as described in Section 3.1 by obtaining the plot of square of line width (WLINE2) versus Ln(Peak Fluence) at a fixed Δ and extrapolating the linear fit. Fig. 4 shows the plot of FTH_LINE vs Δ, along with the single pulse threshold fluence values (FTH_G) from Table 3. It is seen that the required threshold fluence decreases as we move from single pulse ablation to line ablation. In line ablation, the FTH_LINE decreases with decreasing pulse spacing and remains fairly constant below ΔX = 8 µm. As the pulse spacing decreases, the surface roughness increases, leading to better absorption of the subsequent laser pulses. However, beyond a certain point the increase in surface roughness is not significant enough to cause enhanced absorption. This is in accordance with earlier studies (Häfner et al., 2016; Di Niso et al., 2013; Oosterbeek et al., 2016) conducted on multi pulse laser ablation behaviour where a single spot was irradiated with multiple laser pulses and the corresponding threshold fluence was extracted. The threshold fluence decreased with increasing pulses but remained fairly constant for higher number of pulses due to saturation in either accumulation of defects or surface roughness caused by the individual overlapping
Table 3 FTH, r and z0 for different dielectric layers. Dielectric layer
SiNx AlOx/SiNx SiO2/SiNx
Gentle ablation
Strong ablation
FTH_G (J/cm2)
rG (µm)
z0_G (nm)
FTH_S (J/cm2)
rS (µm)
z0_S (nm)
0.137 ± 0.007 0.299 ± 0.014 0.463 ± 0.03
18.0 ± 0.9 16.5 ± 0.7 19.5 ± 1.1
81.5 ± 4.1 61.2 ± 2.9 54.6 ± 3.3
1.44 ± 0.06 2.33 ± 0.12 2.97 ± 0.12
35.7 ± 1.4 35.6 ± 1.9 41.8 ± 1.7
264.9 ± 10.6 252.7 ± 13.9 421.5 ± 17.3
289
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J.M. Yacob Ali et al.
0.6
pulses. nm,
fs
SiNX
FTH_LINE ( J/cm2)
AlOX / SiNX
3.3. Laser induced damage
SiO2 / SiNX
0.4
As discussed in Section 2.3, the laser ablation of SiNX was carried out at selected pulse fluence values from both ablation regimes and the samples were then etched in KOH solution for varying durations. Fig. 5 details the test structure along with the pulse fluence values and the corresponding ablation regime. The pulse spacing in both horizontal and vertical directions is maintained at 10 µm. The area in the centre of the wafers acts as a reference. The values shown in Fig. 5 in each box represent the single pulse fluence applied, however, the equivalent fluence received by the substrate will be higher due to pulse overlap. As a result, the strong ablation behaviour starts at a pulse fluence less than the FTH_S = 1.44 J/ cm2 determined for the ablation of 70 nm SiNX.
0.2
0.0
F20 TH_G
16
1 pulse
12
8
4
( m)
Fig. 4. Line ablation threshold (FTH_LINE) versus pulse spacing (Δ).
3.3.1. Photoluminescence imaging and analysis The effective carrier lifetime of five different samples after etching and re-passivation are presented in Fig. 6. The values inside each box denote the effective minority carrier lifetime (in µs) averaged over its area. A bright area after re-passivation represents a higher effective lifetime, indicating that the laser induced damage was completely etched off. Similarly, a dark image represents a lower lifetime due to the presence of laser damage. The ablated areas appear dark after HF treatment since the laser pulse is absorbed by silicon, causing defects. In the gentle regime, the laser induced damage could be etched off in less than 1 min in 20% KOH/Di-water solution. However, in the strong ablation regime the ablated areas appear dark even after 15 min of etching, which corresponds to an etch depth of ∼18 µm (etching rate ∼1.2 µm/min). This indicates the presence of laser induced bulk damage although the energy penetration depth in the strong ablation regime (z0_S) is only 264.9 nm for the ablation of SiNX (see Table 3). Moreover, it is
Fig. 5. Test structure with pulse fluence values and the corresponding ablation regime.
10% HF for 5 min
1 min KOH Etching
10 min KOH Etching
5 min KOH Etching
15 min KOH Etching
Fig. 6. PL imaging analysis after re-passivation at various stages. The boxes highlighted by dashed lines indicate the effect of pulse overlap. The value in each box is the effective minority carrier lifetime in µs. The images are scaled identically.
290
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J.M. Yacob Ali et al.
Test structure
10% HF for 5 min
5 s KOH etching
Fig. 7. Gentle ablation followed by 5 s of KOH etching.
interesting to see that at FP = 0.89 J/cm2 (gentle regime), the effect of pulse overlap resulted in a strong ablation that could not be completely etched off (boxes highlighted in Fig. 6). Since the energy penetration depth in the gentle regime is 81.5 nm (z0_G for SiNX ablation), it should be possible to remove the laser damage within as little as 5 s of KOH etching, corresponding to an etch depth of ∼100 nm. In order to verify this hypothesis, a test structure for gentle ablation is carried out at a pulse repetition rate of 2 MHz which enables an area of 3 cm × 3 cm to be hatched in a few seconds and the entire test structure (∼30% of the wafer area) could be ablated in less than 1 min. The pulse fluence values at this repetition rate fall into the gentle regime for the entire range of laser power settings as per Eqs. (1) and (2). Fig. 7 shows the test structure for gentle ablation and the corresponding PL image after 5 s of KOH etching and re-passivation. From Fig. 7 it can be seen that operating in the gentle regime induces negligible electronic damage to the underlying silicon which can be etched off in a few seconds, thereby saving a considerable amount of processing time. The required etching time is also in agreement with the determined z0_G for SiNX ablation. Hence, fs gentle ablation will be highly beneficial in the fabrication of high-efficiency IBC and heterojunction silicon solar cells that require selective ablation and patterning of dielectric layers.
Acknowledgements The authors thank their colleagues from the Silicon Materials and Cells cluster of the Solar Energy Research Institute of Singapore (SERIS) for their assistance in sample processing and characterization. SERIS is sponsored by National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through Singapore Economic Development Board (EDB). This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Clean Energy Research Programme project grants (NRF2012EWTEIRP001-023). References Abbott, M., Cotter, J., 2006. Optical and electrical properties of laser texturing for high-efficiency solar cells. Prog. Photovoltaics Res. Appl. 14, 225–235. Binetti, S., Le Donne, A., Rolfi, A., Jäggi, B., Neuenschwander, B., Busto, C., et al., 2016. Picosecond laser texturization of mc-silicon for photovoltaics: a comparison between 1064 nm, 532 nm and 355 nm radiation wavelengths. Appl. Surf. Sci. 371, 196–202. Blakers, A.W., Wang, A., Milne, A.M., Zhao, J., Green, M.A., 1989. 22.8% efficient silicon solar cell. Appl. Phys. Lett. 55, 1363–1365. Di Niso, F., Gaudiuso, C., Sibillano, T., Mezzapesa, F., Ancona, A., Lugarà, P., 2013. Influence of the repetition rate and pulse duration on the incubation effect in multiple-shots ultrafast laser ablation of steel. Phys. Procedia 41, 698–707. Franklin, E., Fong, K., McIntosh, K., Fell, A., Blakers, A., Kho, T., et al., 2016. Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell. Prog. Photovoltaics Res. Appl. 24, 411–427. Gattass, R.R., Mazur, E., 2008. Femtosecond laser micromachining in transparent materials. Nat. Photonics 2, 219–225. Häfner, T., Heberle, J., Dobler, M., Schmidt, M., 2016. Influences on incubation in ps laser micromachining of steel alloys. J. Laser Appl. 28, 022605. Hayafuji, Y., Yanada, T., Aoki, Y., 1981. Laser damage gettering and its application to lifetime improvement in silicon. J. Electrochem. Soc. 128, 1975–1980. Heinrich, G., Bähr, M., Stolberg, K., Wütherich, T., Leonhardt, M., Lawerenz, A., 2011. Investigation of ablation mechanisms for selective laser ablation of silicon nitride layers. Energy Procedia 8, 592–597. Hermann, J., Benfarah, M., Bruneau, S., Axente, E., Coustillier, G., Itina, T., et al., 2006. Comparative investigation of solar cell thin film processing using nanosecond and femtosecond lasers. J. Phys. D Appl. Phys. 39, 453. Iyengar, V.V., Nayak, B.K., More, K.L., Meyer Iii, H.M., Biegalski, M.D., Li, J.V., et al., 2011. Properties of ultrafast laser textured silicon for photovoltaics. Sol. Energy Mater. Sol. Cells 95, 2745–2751. Kautek, W., Krüger, J., Lenzner, M., Sartania, S., Spielmann, C., Krausz, F., 1996. Laser ablation of dielectrics with pulse durations between 20 fs and 3 ps. Appl. Phys. Lett. 69, 3146–3148. Liu, J.M., 1982. Simple technique for measurements of pulsed Gaussian-beam spot sizes. Opt. Lett. 7, 196–198 (1982/05/01 1982). Mannion, P., Magee, J., Coyne, E., O'Connor, G.M., 2003. Ablation thresholds in ultrafast laser micromachining of common metals in air. In: Opto Ireland, 2003, pp. 470–478. Niyibizi, A., Ikua, B.W., Kioni, P.N., Kihato, P., 2012. Laser material processing in crystalline silicon photovoltaics. In: Proceedings of Sustainable Research and Innovation Conference, 2012, pp. 69–74. Oosterbeek, R.N., Corazza, C., Ashforth, S., Simpson, M.C., 2016. Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon. Appl. Phys. A, 122, 449. Schille, J., Schneider, L., Loeschner, U., 2015. Process optimization in high-average-power ultrashort pulse laser microfabrication: how laser process parameters influence efficiency, throughput and quality. Appl. Phys. A 120, 847–855. Shirk, M., Molian, P., 1998. A review of ultrashort pulsed laser ablation of materials. J. Laser Appl. 10, 18–28. Trupke, T., Bardos, R.A., Schubert, M.C., Warta, W., 2006. Photoluminescence imaging of silicon wafers. Appl. Phys. Lett. 89. Zhao, J., Wang, A., Green, M.A., 1999. 24.5% efficiency silicon PERT cells on MCZ substrates and 24.7% efficiency PERL cells on FZ substrates. Prog. Photovoltaics Res. Appl. 7, 471–474.
4. Conclusions Gentle ablation of dielectric layers with minimal damage to the underlying silicon was successfully performed using a femtosecond laser source at a wavelength of 515 nm. The single pulse ablation behaviour revealed the presence of two different ablation regimes, gentle and strong, and their corresponding ablation properties such as FTH, r and z0 were precisely determined. It was found that the required threshold fluence increases with layer thickness, irrespective of the operating regime. From a micro-structuring point of view, moving from the gentle to the strong regime causes a twofold increase in the interaction spot radius and an order of magnitude increase in the energy penetration depth. This has to be taken into account when carrying out complex structuring of dielectrics. Etch-back processing and photoluminescence analysis revealed that ablation in the gentle regime induces negligible damage to the underlying silicon that can be etched off in a short time (5 s). However, in the strong ablation regime, the laser damage cannot be completely removed even during 15 min of KOH etching, indicating the presence of bulk damage. It was also shown that the ablation regime can shift from gentle to strong with sufficient pulse overlap, and hence the incoming single pulse fluence has to be modified with respect to pulse spacing. Thus, gentle fs laser ablation at very high repetition rate (2 MHz) and with short etch-back duration (5 s) will result in considerable savings in processing time and fabrication cost. Hence, this process seems highly suitable for industrial mass production of IBC and heterojunction silicon wafer solar cells. 291
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Femtosecond laser ablation of dielectric layers for high-efficiency silicon wafer solar cells
T
Jaffar Moideen Yacob Alia,b, Vinodh Shanmugama, Bianca Limc, Armin G. Aberlea,b, ⁎ Thomas Muellera, a
Solar Energy Research Institute of Singapore, National University of Singapore, Singapore 117574, Singapore Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117583, Singapore c Institute for Solar Energy Research, D-31860 Emmerthal, Germany1 b
A R T I C L E I N F O
A B S T R A C T
Keywords: Femtosecond laser Silicon wafer solar cells Dielectric ablation Photoluminescence imaging
Laser ablation of dielectric layers to form local contacts while inducing negligible electronic damage to the underlying substrate is crucial for high-efficiency silicon solar cell fabrication. In this work, ablation of dielectric layers such as SiNX, AlOX/SiNX stacks and thermal SiO2/PECVD SiNX stacks is performed using laser pulses with a pulse duration of 480 fs and a wavelength of 515 nm. At this wavelength the femtosecond (fs) laser pulses are typically absorbed in the underlying silicon (indirect ablation), causing defects. However, by precisely determining the single pulse ablation properties such as threshold fluence, spot radius (r) and energy penetration depth (z0) and by optimising the line ablation process, it is possible to ablate the dielectrics with little to no damage to the substrate. Upon analysing r and z0, two distinct ablation regimes, ‘gentle’ and ‘strong’, are identified. Etch-back processing and photoluminescence analysis shows that the strong ablation regime needs more than 15 min of damage etching, corresponding to an etch depth of about 18 µm and thus indicating bulk damage. However, in the gentle regime the defects can be etched off in less than 5 s, indicating that the damage is confined to less than ∼100 nm from the wafer surface, which agrees with the z0 determined for the gentle regime. Hence, gentle fs laser ablation is highly suitable for industrial mass production of ablation intensive and high-efficiency silicon wafer solar cells.
1. Introduction Laser processing in the photovoltaics (PV) industry has gained significant interest due to its inherent advantages such as high-throughput processing, versatility and precision that are vital in the fabrication of cost effective and highly efficient solar cells (Niyibizi et al., 2012). For example, in the fabrication of interdigitated back contact (IBC) solar cells, laser processing is used for selective ablation of dielectrics and micro-structuring of underlying silicon for local contact formation (Franklin et al., 2016). Dielectric layers used in solar cell fabrication are typically for surface passivation, masking, or anti-reflection coating (Blakers et al., 1989). Various high-efficiency solar cell architectures require the dielectric layer to be locally opened in order to form an ohmic contact to the underlying silicon (Franklin et al., 2016). Semiconductor industry methods such as photolithography (Zhao et al., 1999) are slow and expensive, and thus not suitable for low-cost mass production of high-efficiency solar cells. Laser systems are steadily improving with excellent translational control and very high ⁎
1
throughput. Different laser sources with pulse lengths ranging from nanosecond to femtosecond and wavelengths ranging from UV to IR have been used for dielectric ablation (Kautek et al., 1996; Heinrich et al., 2011; Abbott and Cotter, 2006; Iyengar et al., 2011; Binetti et al., 2016). Recently, ultrashort pulse laser technology using femtosecond laser sources are being adapted into solar cell processing (Hermann et al., 2006). A fs laser pulse is shorter than the timescale of photonelectron-lattice interactions and thus the laser pulse ends before the excited electrons can transfer energy to the lattice, leading to “cold” ablation (Shirk and Molian, 1998). It is possible to obtain highly localized energy deposition with a negligible heat affected zone (HAZ). Though fs ablation is known to produce cleaner structures, the laser absorption inside silicon is known to introduce defects in its bulk (Hayafuji et al., 1981). The laser induced defects act as recombination centres for minority carriers and thus reduce the open-circuit voltage and efficiency of the solar cell. A precise and damage-free ablation of dielectric layers is useful for high-efficiency solar cell architectures such as IBC and heterojunction
Corresponding author. E-mail address: [email protected] (T. Mueller). Current address.
https://doi.org/10.1016/j.solener.2018.02.046 Received 27 October 2017; Received in revised form 6 February 2018; Accepted 17 February 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.
Solar Energy 164 (2018) 287–291
J.M. Yacob Ali et al.
solar cells. Achieving this requires a comprehensive understanding of the basic ablation properties such as the ablation threshold fluence (FTH), spot radius (r0) at the interaction plane and the energy penetration depth (z0) into the substrate. In this paper, the ablation of three widely used dielectric layers in silicon solar cell fabrication – SiNX, AlOX/SiNX stacks and thermal SiO2/PECVD SiNX stacks – that are deposited onto planar n-type silicon wafers is investigated. Initially, the single pulse ablation behaviour was analysed to determine the basic ablation properties. The laser ablated spots were measured using a 3D optical microscope. Most of the laser processing techniques in solar cell manufacturing involve continuous laser scribing by overlapping pulses. As a result, the equivalent fluence at any point along the line is much higher than the incoming single pulse fluence, causing more damage to the underlying silicon. Therefore, the required fluence (FTH_LINE) with respect to pulse spacing will be estimated for line ablation. In order to determine the laser induced defects, the dielectric layer is opened using various pulse fluences and the samples are subsequently etched in KOH solution for various time periods and then re-passivated using SiNX on both sides. The resulting surface quality is analysed using a photoluminescence (PL) imaging tool (BTI Imaging, Australia) (Trupke et al., 2006).
Fig. 1. Schematic of a sample illustrating the process of indirect ablation.
Table 2 Dielectric layers and their thicknesses.
2. Experimental details
Dielectric layer
Thickness (nm)
SiNX AlOX/SiNX stack SiO2/SiNX stack
70 10/70 30/160
2.1. Laser system The laser source used in this work produces 480 fs laser pulses at 1030 nm wavelength and has a pulse repetition rate frep ranging from 200 kHz to 2000 kHz. In this study, the laser source is used in the Second Harmonic Generation (SHG) mode at 515 nm. The maximum average laser power Pavg irradiating the sample is 15.8 W, measured using an in-built power meter. The laser beam scanner has a maximum scan speed of 20 m/s. A summary of the laser parameters used in this study is given in Table 1. The incoming pulse has a Gaussian intensity profile where the beam radius r is defined as the distance from the beam axis where the intensity drops to 1/e2 of its peak. The beam spot area is calculated as πr2. From the measured Pavg value, the pulse energy EP and pulse fluence FP are calculated as follows:
EP =
FP =
Pavg frep
(1)
2 EP πr 2
(2)
Fig. 2. Process sequence for characterizing laser induced damage.
2.3. Characterization of laser induced damage 2.2. Sample specifications In order to characterize the laser induced damage, the samples were processed as shown in Fig. 2. The dielectric layer was completely ablated in square regions measuring 3 cm × 3 cm, using selected pulse fluence values from both the gentle and the strong ablation regimes. The samples were then dipped in a 10% HF solution for 5 min at room temperature to remove any remaining dielectric layers in the non-laser processed regions and on the opposite side of the wafer. Following that, the samples were etched in KOH solution for a duration in the range of 5 s to 15 min in order to determine the etching time required for laser damage removal. This corresponds to etching depths of about 100 nm to about 18 µm, respectively (note that the etching rate is ∼1.2 µm/min per side). Finally, the samples were re-passivated with 70 nm SiNX on both sides and then analysed using the PL imaging technique.
The investigated samples consist of planar n-type silicon wafers (3.5 Ωcm resistivity) passivated with dielectric layers on both sides. A schematic of the samples is shown in Fig. 1. The dielectric layers include SiNX, AlOX/SiNX stacks and SiO2/SiNX stacks. SiNX and AlOX were deposited using a PECVD reactor, while SiO2 was grown via dry thermal oxidation at 950° C in a tube furnace. The thickness of the dielectric layers was determined using an ellipsometer from SEMILAB. The results are listed in Table 2. Table 1 Laser parameters used in this study. Wavelength λ (nm) Pulse duration τL (s) Repetition rate frep (kHz) Beam propagation factor M2 Theoretical focus spot radius r (µm) Max. average laser power Pavg (W) Max. pulse energy EP (µJ) Max. peak fluence FP (J/cm2)
515 480 × 10−15 200–2000 1.3 24.6 15.8 at 200 kHz 79 at 200 kHz 8.3 at 200 kHz
3. Results and discussion 3.1. Single pulse ablation properties In this section, the single pulse ablation properties such as FTH, r and 288
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0.6
5000 SiNX
nm,
AlOX / SiNX
4000
0.5
SiO2 / SiNX
z ( m)
d2 ( m2)
0.4
3000
fs - gentle
Linear Fit
fs
fs - strong
0.3 fs - gentle
0.2
1000 0
nm,
AlOX / SiNX SiO2 / SiNX
fs - strong
Linear Fit
2000
SiNX
fs
0.1 5
6
7
8
0.0
9
5
6
7
8
9
Ln(Peak Fluence in mJ/cm2)
2
Ln(Peak Fluence in mJ/cm )
Fig. 3. d2 (left) and z (right) versus Ln(Peak Fluence), for different dielectric layers.
electrons. However, in the strong ablation regime, rS and z0_S increase at a faster rate with respect to pulse fluence due to the enhanced electronic heat conduction discussed above. Though z0_S is confined to < 300 nm for the ablation of 70 nm SiNX, the impact of strong ablation is seen to affect the bulk of the underlying silicon. This will be discussed in more detail in Section 3.3.
z0 are experimentally determined for all the dielectric layers. For a Gaussian laser beam profile, the diameter d and depth z of an ablated crater are related to the peak fluence as
F d2 = 2r 2ln ⎛ P ⎞ F ⎝ TH ⎠
(3)
F z= z0ln ⎛ P ⎞ ⎝ FTH ⎠
(4)
⎜
⎜
⎟
3.2. Threshold fluence vs pulse spacing in line ablation
⎟
While performing laser scribing with pulse overlap, the equivalent fluence received by the substrate increases and ablation is observed at a fluence much lower than that of FTH. The pulse spacing Δ is defined as the distance between the centres of two adjacent pulses along the direction of ablation. As the pulse spacing is reduced, more pulses overlap with each other and this can shift the ablation regime from gentle to strong. In this section, the line threshold fluence (FTH_LINE) versus Δ is determined for all three dielectric layers. Δ can be controlled by adjusting the pulse repetition rate (frep) and laser beam scan speed (v) as shown in Eq. (5).
where FTH is the ablation threshold fluence which is defined as the minimum peak fluence required to completely remove the dielectric layer. In Eq. (3), if FP ≤ FTH, we get d2 = 0. Thus, plotting d2 against the Ln(peak fluence) and extrapolating the linear fit will give us FTH (Liu, 1982). The spot radius r on the sample surface and the energy penetration depth z0 can be estimated from the slope of the linear fit of Eqs. (3) and (4) respectively. For statistical purposes, the value of each individual parameter set is taken as the mean of five ablated spots. Fig. 3 shows a plot of d2 (left) and z (right) versus the logarithm of the peak fluence, for different dielectric layers. The slopes in Fig. 3 reveal the presence of o different ablation regimes, termed as ‘gentle’ and ‘strong’ as indicated in the graphs, and are distinguished using the subscripts G and S, respectively. In ultrashort pulse ablation, the excited hot electrons and the lattice are not in thermal equilibrium resulting in electronic heat conduction (Gattass and Mazur, 2008). In the gentle ablation regime (at lower fluence), the density and temperature of hot electrons are lower resulting in a lower electronic heat conduction rate. Therefore, in the gentle regime, the ablation depth is mainly governed by optical penetration length of the laser beam (Mannion et al., 2003). In the strong ablation regime (at higher fluence), the temperature and density of hot electrons are very high causing an excessive electronic heat conduction, so it takes longer time to achieve thermal equilibrium due to the large temperature difference. Such rapid electron-lattice collisions result in bulk heating and damage. As a result, in the strong regime, the ablation depth is mainly governed by the heat transport of thermally excited electrons (Schille et al., 2015). FTH, r and z0 for the two regimes are detailed in Table 3. The required FTH values increase with layer thickness. The ablation spots (rG) in the gentle regime are much smaller than the focus spot radius r (24.6 µm) indicating negligible thermal diffusion of excited
Δ = v/frep
(5)
FTH_LINE is extracted as described in Section 3.1 by obtaining the plot of square of line width (WLINE2) versus Ln(Peak Fluence) at a fixed Δ and extrapolating the linear fit. Fig. 4 shows the plot of FTH_LINE vs Δ, along with the single pulse threshold fluence values (FTH_G) from Table 3. It is seen that the required threshold fluence decreases as we move from single pulse ablation to line ablation. In line ablation, the FTH_LINE decreases with decreasing pulse spacing and remains fairly constant below ΔX = 8 µm. As the pulse spacing decreases, the surface roughness increases, leading to better absorption of the subsequent laser pulses. However, beyond a certain point the increase in surface roughness is not significant enough to cause enhanced absorption. This is in accordance with earlier studies (Häfner et al., 2016; Di Niso et al., 2013; Oosterbeek et al., 2016) conducted on multi pulse laser ablation behaviour where a single spot was irradiated with multiple laser pulses and the corresponding threshold fluence was extracted. The threshold fluence decreased with increasing pulses but remained fairly constant for higher number of pulses due to saturation in either accumulation of defects or surface roughness caused by the individual overlapping
Table 3 FTH, r and z0 for different dielectric layers. Dielectric layer
SiNx AlOx/SiNx SiO2/SiNx
Gentle ablation
Strong ablation
FTH_G (J/cm2)
rG (µm)
z0_G (nm)
FTH_S (J/cm2)
rS (µm)
z0_S (nm)
0.137 ± 0.007 0.299 ± 0.014 0.463 ± 0.03
18.0 ± 0.9 16.5 ± 0.7 19.5 ± 1.1
81.5 ± 4.1 61.2 ± 2.9 54.6 ± 3.3
1.44 ± 0.06 2.33 ± 0.12 2.97 ± 0.12
35.7 ± 1.4 35.6 ± 1.9 41.8 ± 1.7
264.9 ± 10.6 252.7 ± 13.9 421.5 ± 17.3
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0.6
pulses. nm,
fs
SiNX
FTH_LINE ( J/cm2)
AlOX / SiNX
3.3. Laser induced damage
SiO2 / SiNX
0.4
As discussed in Section 2.3, the laser ablation of SiNX was carried out at selected pulse fluence values from both ablation regimes and the samples were then etched in KOH solution for varying durations. Fig. 5 details the test structure along with the pulse fluence values and the corresponding ablation regime. The pulse spacing in both horizontal and vertical directions is maintained at 10 µm. The area in the centre of the wafers acts as a reference. The values shown in Fig. 5 in each box represent the single pulse fluence applied, however, the equivalent fluence received by the substrate will be higher due to pulse overlap. As a result, the strong ablation behaviour starts at a pulse fluence less than the FTH_S = 1.44 J/ cm2 determined for the ablation of 70 nm SiNX.
0.2
0.0
F20 TH_G
16
1 pulse
12
8
4
( m)
Fig. 4. Line ablation threshold (FTH_LINE) versus pulse spacing (Δ).
3.3.1. Photoluminescence imaging and analysis The effective carrier lifetime of five different samples after etching and re-passivation are presented in Fig. 6. The values inside each box denote the effective minority carrier lifetime (in µs) averaged over its area. A bright area after re-passivation represents a higher effective lifetime, indicating that the laser induced damage was completely etched off. Similarly, a dark image represents a lower lifetime due to the presence of laser damage. The ablated areas appear dark after HF treatment since the laser pulse is absorbed by silicon, causing defects. In the gentle regime, the laser induced damage could be etched off in less than 1 min in 20% KOH/Di-water solution. However, in the strong ablation regime the ablated areas appear dark even after 15 min of etching, which corresponds to an etch depth of ∼18 µm (etching rate ∼1.2 µm/min). This indicates the presence of laser induced bulk damage although the energy penetration depth in the strong ablation regime (z0_S) is only 264.9 nm for the ablation of SiNX (see Table 3). Moreover, it is
Fig. 5. Test structure with pulse fluence values and the corresponding ablation regime.
10% HF for 5 min
1 min KOH Etching
10 min KOH Etching
5 min KOH Etching
15 min KOH Etching
Fig. 6. PL imaging analysis after re-passivation at various stages. The boxes highlighted by dashed lines indicate the effect of pulse overlap. The value in each box is the effective minority carrier lifetime in µs. The images are scaled identically.
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Test structure
10% HF for 5 min
5 s KOH etching
Fig. 7. Gentle ablation followed by 5 s of KOH etching.
interesting to see that at FP = 0.89 J/cm2 (gentle regime), the effect of pulse overlap resulted in a strong ablation that could not be completely etched off (boxes highlighted in Fig. 6). Since the energy penetration depth in the gentle regime is 81.5 nm (z0_G for SiNX ablation), it should be possible to remove the laser damage within as little as 5 s of KOH etching, corresponding to an etch depth of ∼100 nm. In order to verify this hypothesis, a test structure for gentle ablation is carried out at a pulse repetition rate of 2 MHz which enables an area of 3 cm × 3 cm to be hatched in a few seconds and the entire test structure (∼30% of the wafer area) could be ablated in less than 1 min. The pulse fluence values at this repetition rate fall into the gentle regime for the entire range of laser power settings as per Eqs. (1) and (2). Fig. 7 shows the test structure for gentle ablation and the corresponding PL image after 5 s of KOH etching and re-passivation. From Fig. 7 it can be seen that operating in the gentle regime induces negligible electronic damage to the underlying silicon which can be etched off in a few seconds, thereby saving a considerable amount of processing time. The required etching time is also in agreement with the determined z0_G for SiNX ablation. Hence, fs gentle ablation will be highly beneficial in the fabrication of high-efficiency IBC and heterojunction silicon solar cells that require selective ablation and patterning of dielectric layers.
Acknowledgements The authors thank their colleagues from the Silicon Materials and Cells cluster of the Solar Energy Research Institute of Singapore (SERIS) for their assistance in sample processing and characterization. SERIS is sponsored by National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through Singapore Economic Development Board (EDB). This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Clean Energy Research Programme project grants (NRF2012EWTEIRP001-023). References Abbott, M., Cotter, J., 2006. Optical and electrical properties of laser texturing for high-efficiency solar cells. Prog. Photovoltaics Res. Appl. 14, 225–235. Binetti, S., Le Donne, A., Rolfi, A., Jäggi, B., Neuenschwander, B., Busto, C., et al., 2016. Picosecond laser texturization of mc-silicon for photovoltaics: a comparison between 1064 nm, 532 nm and 355 nm radiation wavelengths. Appl. Surf. Sci. 371, 196–202. Blakers, A.W., Wang, A., Milne, A.M., Zhao, J., Green, M.A., 1989. 22.8% efficient silicon solar cell. Appl. Phys. Lett. 55, 1363–1365. Di Niso, F., Gaudiuso, C., Sibillano, T., Mezzapesa, F., Ancona, A., Lugarà, P., 2013. Influence of the repetition rate and pulse duration on the incubation effect in multiple-shots ultrafast laser ablation of steel. Phys. Procedia 41, 698–707. Franklin, E., Fong, K., McIntosh, K., Fell, A., Blakers, A., Kho, T., et al., 2016. Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell. Prog. Photovoltaics Res. Appl. 24, 411–427. Gattass, R.R., Mazur, E., 2008. Femtosecond laser micromachining in transparent materials. Nat. Photonics 2, 219–225. Häfner, T., Heberle, J., Dobler, M., Schmidt, M., 2016. Influences on incubation in ps laser micromachining of steel alloys. J. Laser Appl. 28, 022605. Hayafuji, Y., Yanada, T., Aoki, Y., 1981. Laser damage gettering and its application to lifetime improvement in silicon. J. Electrochem. Soc. 128, 1975–1980. Heinrich, G., Bähr, M., Stolberg, K., Wütherich, T., Leonhardt, M., Lawerenz, A., 2011. Investigation of ablation mechanisms for selective laser ablation of silicon nitride layers. Energy Procedia 8, 592–597. Hermann, J., Benfarah, M., Bruneau, S., Axente, E., Coustillier, G., Itina, T., et al., 2006. Comparative investigation of solar cell thin film processing using nanosecond and femtosecond lasers. J. Phys. D Appl. Phys. 39, 453. Iyengar, V.V., Nayak, B.K., More, K.L., Meyer Iii, H.M., Biegalski, M.D., Li, J.V., et al., 2011. Properties of ultrafast laser textured silicon for photovoltaics. Sol. Energy Mater. Sol. Cells 95, 2745–2751. Kautek, W., Krüger, J., Lenzner, M., Sartania, S., Spielmann, C., Krausz, F., 1996. Laser ablation of dielectrics with pulse durations between 20 fs and 3 ps. Appl. Phys. Lett. 69, 3146–3148. Liu, J.M., 1982. Simple technique for measurements of pulsed Gaussian-beam spot sizes. Opt. Lett. 7, 196–198 (1982/05/01 1982). Mannion, P., Magee, J., Coyne, E., O'Connor, G.M., 2003. Ablation thresholds in ultrafast laser micromachining of common metals in air. In: Opto Ireland, 2003, pp. 470–478. Niyibizi, A., Ikua, B.W., Kioni, P.N., Kihato, P., 2012. Laser material processing in crystalline silicon photovoltaics. In: Proceedings of Sustainable Research and Innovation Conference, 2012, pp. 69–74. Oosterbeek, R.N., Corazza, C., Ashforth, S., Simpson, M.C., 2016. Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon. Appl. Phys. A, 122, 449. Schille, J., Schneider, L., Loeschner, U., 2015. Process optimization in high-average-power ultrashort pulse laser microfabrication: how laser process parameters influence efficiency, throughput and quality. Appl. Phys. A 120, 847–855. Shirk, M., Molian, P., 1998. A review of ultrashort pulsed laser ablation of materials. J. Laser Appl. 10, 18–28. Trupke, T., Bardos, R.A., Schubert, M.C., Warta, W., 2006. Photoluminescence imaging of silicon wafers. Appl. Phys. Lett. 89. Zhao, J., Wang, A., Green, M.A., 1999. 24.5% efficiency silicon PERT cells on MCZ substrates and 24.7% efficiency PERL cells on FZ substrates. Prog. Photovoltaics Res. Appl. 7, 471–474.
4. Conclusions Gentle ablation of dielectric layers with minimal damage to the underlying silicon was successfully performed using a femtosecond laser source at a wavelength of 515 nm. The single pulse ablation behaviour revealed the presence of two different ablation regimes, gentle and strong, and their corresponding ablation properties such as FTH, r and z0 were precisely determined. It was found that the required threshold fluence increases with layer thickness, irrespective of the operating regime. From a micro-structuring point of view, moving from the gentle to the strong regime causes a twofold increase in the interaction spot radius and an order of magnitude increase in the energy penetration depth. This has to be taken into account when carrying out complex structuring of dielectrics. Etch-back processing and photoluminescence analysis revealed that ablation in the gentle regime induces negligible damage to the underlying silicon that can be etched off in a short time (5 s). However, in the strong ablation regime, the laser damage cannot be completely removed even during 15 min of KOH etching, indicating the presence of bulk damage. It was also shown that the ablation regime can shift from gentle to strong with sufficient pulse overlap, and hence the incoming single pulse fluence has to be modified with respect to pulse spacing. Thus, gentle fs laser ablation at very high repetition rate (2 MHz) and with short etch-back duration (5 s) will result in considerable savings in processing time and fabrication cost. Hence, this process seems highly suitable for industrial mass production of IBC and heterojunction silicon wafer solar cells. 291