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In early 2017, the world record efficiency for single-junction crystalline silicon (c-Si) solar cells was achieved by merging amorphous silicon (a-Si:H)/c-Si heterojunction technology and back-contact architecture. However, to fabricate such silicon heterojunction interdigitated back-contact (SHJ-IBC) solar cells, complex a-Si:H patterning steps are required to form the interdigitated a-Si:H strips at the back side of the devices. This fabrication complexity raises concerns about the commercial potential of such devices. In this work, a novel process scheme for a-Si:H patterning using damage-free laser ablation is presented, leading to a fast, simple and photolithography-free emitter patterning approach for SHJ-IBC solar cells. To prevent laser-induced damage to the a-Si:H/c-Si heterocontact, an a-Si:H laser-absorbing layer and a dielectric mask are deposited on top of the a-Si:H/c-Si. Laser ablation only removes the top a-Si:H layer, reducing laser damage to the bottom a-Si:H/c-Si heterocontact under the dielectric mask. This dielectric mask is a distributed Bragg reflector (DBR), resulting in a high reflectance of 80% at the laser wavelength and thus providing additional protection to the a-Si:H/c-Si heterocontact. Using such simple a-Si:H patterning method, a proof-of concept 4-cm(2) SHJ-IBC solar cell with an efficiency of up to 22.5% is achieved. The authors gratefully acknowledge the financial support of IMEC's Industrial Affiliation Program for Si-PV. This project has also received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no. 727523 (NextBase).

A fully dry and hydrofluoric-free low-temperature process has been developed to passivate n-type crystalline silicon (c-Si) surfaces. Particularly, the use of a hydrogen (H2) plasma treatment followed by in situ intrinsic hydrogenated amorphous silicon (a-Si:H) deposition has been investigated. The impact of H2 gas flow rate and H2 plasma processing time on the a-Si:H/c-Si interface passivation quality is studied. Optimal H2 plasma processing conditions result in the best effective minority carrier lifetime of up to 2.5 ms at an injection level of 1 × 1015 cm−3, equivalent to the best effective surface recombination velocity of 4 cm/s. The reasons that enable such superior passivation quality are discussed in this paper based on the characterization of the a-Si:H/c-Si interface and c-Si substrate using transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, and deep-level transient spectroscopy.

One of the advantages of an epitaxially grown emitter is that its doping profile is not limited by the diffusion process and that almost any arbitrary profile shape can be realized. In this simulation study we make use of the epitaxial doping profile’s flexibility to optimize the emitter of an nPERT solar cell. Initially an optical and electrical model is set up in Sentaurus TCAD and calibrated based on measurements of an actual device with an efficiency of 22.5%. Next, simulations for a box shaped profile with varying thicknesses and doping concentrations are carried out. Additionally, a two-step emitter consisting of a combination of two box profiles – one located at the surface and another deep profile underneath the former one – is also simulated. The results clearly identify the optimal doping concentrations and thicknesses which result in the highest efficiencies. Outside of these optimal regions the most significant loss mechanisms are pointed out. To be highlighted is that the switch from a the single box profile emitter to an optimal two-step profile emitter realizes a gain of 0.4% absolute resulting in an efficiency of 22.9%. This demonstrates that the freedom in the shape of the epitaxial doping profile can be employed to further increase the solar cell efficiencies.

Previously, IMEC proposed the i(2)-module concept which allows to process silicon heterojunction interdigitated back-contact (SHJ-IBC) cells on thin ( < 50 mu m) Si wafers at module level. This concept includes the bonding of the thin wafer early on to the module cover glass, which delivers mechanical support to the wafer and thus significantly improves the production yield. In this work, we test silicone and ethylene vinyl acetate bonding agents and prove them to be resistant to all rear side processes, including wet and plasma processes. Moreover, a lift-off process using a sacrificial SiOx layer has been developed for emitter patterning to replace conventional lithography. The optimized process steps are demonstrated by the fabrication of SHJ-IBC cells on 6-inch 190 mu m-thick wafers. Efficiencies up to 22.6% have been achieved on reference freestanding wafers. Excellent V-oc of 734 my and J(sc) of 40.8 mA/cm(2) lead to an efficiency of 21.7% on silicone-bonded cells, where the high V-oc indicates the process compatibility of the bonding agent. The developments that enabled such achievements and the key factors that limit the device performance are discussed in this paper. The authors would like to thank the financial support of imec's industrial affiliation program for Si-PV. This work was also partially funded by Kuwait Foundation for the Advancement of Sciences under project code: P115-15EE-01. The research has received funding from the European Union's Seventh Programme for research, technological development and demonstration under grant agreements no. 609788 (Cheetah), and also funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270. The authors gratefully acknowledge Joachim John for the spectral response measurements and Yiming Yang for the SEM measurements.

Both Kelvin Probe Force Microscopy and Scanning Electron Microscopy enable assessment of the effect of electrical bias on the surface potential of the layers of a solar cell. We report on a comprehensive comparison of surface potential measurements on an interdigitated back contact solar cell using these two techniques. Measurements under different values of electrical biases are performed on and between the metallic contacts. They show a good agreement between the surface potential obtained with Kelvin Probe Force Microscopy and the Scanning Electron Microscopy signal. In order to provide an accurate comparison, the scanned areas are adjacent to each other and accurate repositioning is achieved thanks to a nano-indentation between the contacts. We show that measurements under reverse bias are of interest to locate nano-defects and measurements under forward bias are relevant to identify local series resistance issues. We suggest that a setup combining Scanning Electron Microscopy and Kelvin Probe Force Microscopy under different values of the electrical bias should be valuable since the former is a high throughput technique enabling measurements on large scan areas, while the latter is a quantitative, low noise, and unintrusive local technique.

Utilisation of expensive silicon (Si) material in crystalline Si modules has come down to 4 g Si per watt-peak in 2018, mainly as a result of reduction in wafer thickness and kerf losses as well as increase in module efficiencies. With continued progress in conventional multi-wire sawing of ingots, wafers as thin as 100 mu m could eventually be produced. Beyond this, kerfless lift-off technologies are being investigated which enable wafer thicknesses well below 100 mu m with negligible Si kerf waste. Such thin Si wafers and foils would be much lighter in weight than today's standard 165-180 mu m-thick wafers and would exhibit considerable flexibility and fragility. This necessitates a rethink about how to handle and process thin Si into solar devices in a manufacturing line with high mechanical yield and high throughput. This paper gives a broad overview of the different approaches for fabricating solar cells on thin Si foils. In particular, three routes are discussed in detail, namely (1) freestanding processing of thin Si, (2) processing of thin Si supported mechanically on a conductive low-cost Si substrate ("wafer-equivalent" approach) and (3) processing of thin Si bonded to a transparent glass superstrate. In each case, the main challenges are explained and the recent progress in addressing them are summarised. Kerfless 50 mu m-thick epitaxial Si foils lifted-off using porous Si and thinned-down Si wafers (below 70 mu m) are used as model substrates for this work. The authors gratefully acknowledge the efforts of several people who were involved in this work over the last few years, namely Jonathan Govaerts, Stefano Granata, Menglei Xu, Ergi Donercark, Ivan Sharlandshiev, Shashi Kiran Jonnak, Shruti Jambaldinni, Robert Roozeman, Jarkko Heildcinen, Thomas Kaden, Zuzana Kovacova, Erich Neubauer, Michael Grimm and Kaori Nagaoka. The authors also acknowledge the funding received for this work from the European Commission for the H2020 project CABRISS under grant agreement No. 641972. Imec is a partner in EnergyVille (www.energyville.be), a collaboration between the Flemish research partners KU Leuven, VITO, imec, and UHasselt in the field of sustainable energy and intelligent energy systems. Radhakrishnan, HS (reprint author), Imec Vzw Partner EnergyVille, Kapeldreef 75, B-3001 Leuven, Belgium. sivarama@imec.be

The influence of the plasma ignition condition during PECVD deposition from a silane/hydrogen mixture on the amorphous silicon passivation of crystalline silicon surface is investigated. The changes in this process step mainly consist in varying the power density for very brief durations in between 1 s and 3 s. We find that the ignition phase contributes significantly in the film growth, especially in the a-Si:H/c-Si interface formation. In particular, the deposition rate increases with ignition power density. TEM cross-section inspection presents a rougher a-Si:H/c-Si interface with higher plasma power and thus, a tendency for nano-clusters formation caused by the crystalline nature of the substrate. In-situ plasma diagnostics reveal the gradual raise up of I Ha*/ I SiH* with the power density leading to worse SiH* abstraction to the surface. Whereas, time-resolved optical emission spectroscopy explains the possible recombination mechanism in the plasma due to higher-silane related reactive species (HSRS) formation via polymerization reactions. Our results point out that the ignition conditions with a rather low power for longer time give the best passivation, resulting an effective lifetime up to 9 ms.

In order to reduce the material cost for silicon solar cells, several research groups are investigating methods to minimize the silicon consumption for making monocrystalline silicon wafers. One promising approach is deposition of an epitaxial layer on porous silicon, followed by detachment of the layer. This contribution discusses improvements in the epitaxial wafer fabrication by optimization of the porosification process. The introduction of a layered porous silicon structure allows to independently improve both epitaxial layer quality and detachment yield. In this way, we have managed to obtain 100µm thick silicon wafers with effective lifetimes up to 1.3ms, and 40µm thick wafers with effective lifetimes up to 700µs. We will also review the current status of the process development for solar cells made on thin wafers. Two approaches are presented. In the first approach, heterojunction solar cells are fabricated on freestanding epitaxial wafers of 40µm thickness. In the second approach, high efficiency (21%) heterojunction back-contacted cells are fabricated on wafers that are bonded to a glass superstrate. Challenges for device processing and limitations in cell performance are discussed.

Copyright © 2018 John Wiley & Sons, Ltd. In this work, the ATOM (intrinsic a-Si:H/TiO x /low work function metal) structure is investigated to realize high-performance passivating electron-selective contacts for crystalline silicon solar cells. The absence of a highly doped Si region in this contact structure is meant to reduce the optoelectrical losses. We show that a low contact resistivity (ρ c ) can be obtained by the combined effect of a low work function metal, such as calcium (Φ 2.9 eV), and Fermi-level depinning in the metal-insulator-semiconductor contact structure (where in our case TiO x acts as the insulator on the intrinsic a-Si:H passivating layer). TiO x grown by ALD is effective to achieve not only a low ρ c but also good passivation properties. As an electron contact in silicon heterojunction solar cells, inserting interfacial TiO x at the i-a-Si:H/Ca interface significantly enhances the solar cell conversion efficiency. Consequently, the champion solar cell with the ATOM contact achieves a V OC of 711 mV, FF of 72.9%, J SC of 35.1 mA/cm 2 , and an efficiency of 18.2%. The achievement of a high V OC and reasonable FF without the need for a highly doped Si layer serves as a valuable proof of concept for future developments on passivating electron-selective contacts using this structure. ispartof: PROGRESS IN PHOTOVOLTAICS vol:26 issue:10 pages:835-845 status: published

We present a novel approach to simplify the rearside a-Si:H patterning of silicon heterojunction interdigitated back-contact solar cells. In our current process, laser ablation and lift-off are used. Since lift-off is not industrially-viable, we propose to replace it with a selective deposition process which ensures a-Si: H is realised only on c-Si surface and not on the SiOx mask, at the end of the process, based on cycles of a-Si: H deposition and etching. While neither the deposition nor the etching is truly selective, this method relies on the difference of a-Si: H etch rates on c-Si and SiOx surfaces to achieve selectivity as the net end-result. The main challenge is addressing the trade-off between selectivity and c-Si surface passivation. The authors gratefully acknowledge the financial support of imec’s industrial affiliation program for Si-PV. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 727523 (NextBase).

One of the promising ways to increase the efficiency of silicon heterojunction (SHJ) cells is to replace the doped hydrogenated amorphous silicon (a-Si:H) contact layer by doped hydrogenated microcrystalline silicon (mu c-Si:H) which has better suited opto-electrical properties. In this work, we report on the development of a plasma-enhanced chemical vapor deposition process for mu c-Si:H. We demonstrate an n-type mu c-Si:H layer with high crystalline fraction and low parasitic absorption. The developed layer is implemented as the front electron contact of a SHJ cell. A significant improvement in J(sc) of 0.9 mA/cm(2) is achieved on device level while maintaining high V-OC values (> 725 mV), leading to an efficiency of 20.6% for the best cell. Cell efficiency is limited by a decreased FF which is attributed to the increased sensitivity of mu c-Si:H layers to ITO sputtering damage. The authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. Part of this project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270.

This chapter provides an overview of contamination that can be encountered during semiconductor device manufacturing. Contamination can have a detrimental impact on yield and reliability, but also on process control. In the conventional classification scheme, a distinction is made between particle contamination, metallic contamination, and airborne molecular contamination. For each of these contaminants, a brief overview is given of the sources of contamination and the mechanisms by which the contamination is deposited on the substrate surface. Also general properties and possible effects of the contamination are discussed. In addition, possible detrimental effects of contaminant removal and cleaning are reviewed.

Recently, large area hybrid silicon heterojunction cells (SHJ) with nickel/copper (Ni/Cu) plated front contacts were shown to benefit from a simpler processing sequence than n-PERT cells featuring a boron diffused rear emitter. Hybrid refers here to the combination of a diffused front surface field (FSF) and a SHJ rear emitter. In this work, further process simplifications for such hybrid SHJ cells are evaluated. Based on reflectance and adhesion considerations, stacks of tin-doped indium oxide and copper (ITO/Cu) and chromium/silver (Cr/Ag) are tested as alternative to the previously chosen ITO/Ag rear contact stack. Using ITO/Cu/Al as rear contact stack, average efficiencies of 20.8±0.2% are presented on 6-inch wafers. We also demonstrate that the excimer laser annealing step previously employed to form nickel silicides at the front side can be skipped without comprising solder tab adhesion quality.

Argon (Ar) and oxygen (O2) plasmas are performed on silicone adhesives to eliminate the negative influence of silicone on amorphous silicon (a-Si:H) surface passivation of wafers bonded to glass. Both the Ar and O2 plasmas lead to oxidation of the silicone surface, consisting in an increase of oxygen/carbon ratio, of degree of crosslinking, and of material density. The oxidized silicone is more resilient than pristine and does not interact with the a-Si:H passivation process, allowing for state-of-the-art surface passivation of wafers bonded to glass. Similarities between the modifications induced by the Ar and O2 plasmas on the silicone indicate the secondary role of the O2 radicals in the oxidation process. Moreover, amorphous/crystalline heterojunction interdigitated back contact solar cells (a-Si:H/c-Si HJ i-BC) are fabricated on freestanding and bonded wafers treated with Ar plasma. The devices show comparable open-circuit voltages of up to 675 mV, confirming at device level the efficacy of the treatment.

Two important aspects for the success of the porous silicon-based layer transfer method in producing kerfless thin ( 15 cm −3 which corresponds to a minimum minority carrier diffusion length of ~670 µm (> 16 times the silicon thickness). With such high quality epitaxial foils combined with high detachment yield, very high efficiency solar devices on thin silicon substrates would be a reality in the near future.

In the future solar devices will be manufactured onto thin mono-crystalline silicon wafers (foils) potentially bonded to substrate carriers. In this study, the influence of the substrate carrier potential (grounded or floating) on the amorphous silicon a-Si:H surface passivation of the bonded foil is discussed for the cases of conductive and insulating substrate carriers. A conductive carrier leads to an increase in deposition rate and ion bombardment energy, which is detrimental for sample lifetime. An insulating carrier, e.g., a glass substrate, inverts the trend and improves the a-Si:H surface passivation.

The influence of the cleaning process on the amorphous silicon passivation of homojunction emitters is investigated. A significant variation in the passivation quality following different cleaning sequences is not observed, even though differences in cleaning performance are evident. These results point out the effectiveness of our cleaning treatment and provide a hydrogen termination for intrinsic amorphous silicon passivation. A post-deposition treatment improves the passivation level yielding emitter saturation currents determined by Auger recombination in the order of 70 fA/cm2 and below.

A detailed investigation of the laser damage to amorphous silicon (a-Si:H) layers patterned by laser ablation (LA) and wet chemical etching is presented. This approach can be applied to pattern the rear side of silicon heterojunction interdigitated back-contact solar cells. Only the top sacrificial a-Si:H laser-absorbing layer of an a-Si:H/SiOx/a-Si:H/c-Si stack is ablated. Laser damage in the bottom a-Si:H layer and a-Si:H/c-Si interface is analyzed by both scanning electron microscopy and transmission electron microscopy. We show that the a-Si:H/c-Si passivation is degraded by laser damage and that this degradation can be diminished by increasing laser processing speed. This is attributed to a decrease of laser-irradiated area, and particularly smaller overlapping zones of adjacent laser pulses. The re-passivation quality after LA and wet etching is similar to that of as-passivated samples. This indicates that laser damage is not present in the bulk c-Si substrate but only in the a-Si:H passivation layer, which is removed during subsequent wet etching, thus allowing high quality repassivation., Marie Sklodowska-Curie Action; Epitaxial silicon foil solar cells with interdigitated back contacts 657270 — EpiSil-IBC published in physica status solidi (RRL) - Rapid Research Letters doi:10.1002/pssr.201700125

Texturing of crystalline silicon at the light wavelength scale is a promising approach for light management in ever thinner cells. However, this approach requires adapting the device design and process, which has so far revealed to be a challenging task. Indeed, successful integration requires compromising the excellent optical properties for preserving the electrical ones. We briefly summarize what has been learned so far in the community and illustrate some of these learnings with the fabrication and improvement of 1-μ m-thin monocrystalline nanotextured silicon solar cells.

Eliminating a doping process could be an effective way to reduce the production cost of c-Si cells. However, in absence of highly doped Si, the formation of a high quality contact is not straightforward. The lack of field-effect passivation from a lowly doped region can lead to a high recombination current density at the contacts (J(0,metal)) and moreover, contact resistivity (rho(c)) typically increases when doping level is decreasing. In this work we focus on reducing the contact resistivity of an electron-selective contact for doping-free cells. Although the effect of low work function metals (LWMs) in combination with an i-a-Si:H layer has already been reported, the synergy effect of a LWM and a MIS (Metal-Insulator-Semiconductor) contact structure on top of the ia-Si:H has not been reported yet. Here, we demonstrate a new ATOM (i-a-Si:H / TiOX / low workfunction metal) contact structure as an electron-selective contact using an i-a-Si:H layer, a TiOX interfacial layer and Ca (Phi 2.9eV) without requiring an additional n(+) doping process. The addition of TiOX in between the i-a-Si:H layer and the Ca decreases the pc by about 2 orders of magnitude. Despite of increased J(0,metal) due to e-beam processing of TiOX, the Ca based ATOM contact increases the potential max efficiency up to 25 %. To the best of our knowledge, this is the first demonstration of an electron-selective contact comprising a low work function metal, an interfacial TiOX and an i-a-Si:H passivation layer. This type of contact could be a promising route for the optimization of doping-free cells (C) 2017 The Authors. Published by Elsevier Ltd. The authors gratefully acknowledge the financial support of imec's industrial affiliation program for Si-PV. And part of this project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270.

We present a novel process scheme for a-Si:H patterning using damage-free laser ablation, resulting in a simple, fast, and photolithography-free emitter patterning of silicon heterojunction interdigitated back-contact (SHJ-IBC) solar cells. An a-Si:H laser-absorbing layer and a stack of sacrificial dielectric layers are deposited on top of a-Si:H/c-Si heterocontact to prevent laser damage. Laser ablation only removes the top aSi: H layer, which limits laser damage to the surface of dielectric layers. These dielectric layers form a distributed Bragg reflector with a high reflectance of 80% at the laser wavelength which results in additional protection of the bottom a-Si:H/c-Si heterocontact. The significant reduction of laser damage is confirmed by atomic-force microscopy and photo-luminescence measurements. Such damage-free laser ablation process was successfully incorporated in a SHJ-IBC process flow and a best efficiency of 21.8% was achieved. The authors gratefully acknowledge the financial support of imec’s industrial affiliation program for Si-PV. The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 727523.

A parametric study of post-deposition hydrogen plasma treatment of intrinsic a:Si:H films is performed. We demonstrate a significant improvement in passivation of c-Si(100) promoting epitaxy after an in-situ hydrogen plasma treatment depending mainly on the pressure and slightly on the power. Plasma diagnostic indicates an increase of Hα* signal with high power and low pressure. However, our analysis reveals a better hydrogen incorporation with high pressure and a slight increase in monohydride with high power. Longer H2 plasma duration up to 50 s shows no detrimental effect on the passivation quality. Optimizing the in-situ H2 plasma treatment, high minority carrier lifetime over 15 ms was achieved after short thermal annealing. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Interdigitated back contacted (IBC) silicon solar cells have long been shown as high conversion efficiency solar cells, with values as high as 25.0% reported by Sunpower [1]. Such high efficiencies are enabled by the cell structure, with the absence of front side metal shading, which can then be optimised from optical and passivation perspectives. High bulk lifetime silicon and a low series resistance metallisation scheme can also enhance the efficiencies. Other advantages include the easier module incorporation given both metal contacts are located on the rear of the cell.

The passivation characteristics of thermally grown silicon dioxide (SiO 2 ) and hydrogenated amorphous silicon (a-Si:H) layers are investigated, using a combination of photoluminescence and capacitance-voltage analysis techniques. Key findings are the significant passivation degradation of SiO 2 and a-Si:H layers induced by metallization through electron beam evaporation. The degradation correlates with an increase in silicon dangling bond defect density at the interface with silicon (for both SiO 2 and a-Si:H) or in the passivation layer (a-Si:H). Performing the metallization by thermal evaporation is an effective method to avoid such process-induced damage, as is forming gas annealing at 450°C, which effectively recovers the interface characteristics of SiO 2 layers. Deposition of amorphous silicon on a thermal SiO 2 layer induces bulk and interface defects in the SiO 2 layer-but in this case, a 450°C forming gas anneal is not possible due to the thermal budget limitations of a-Si:H, thereby posing problems for solar cell structures which rely on a combination of PECVD a-Si:H and thermal SiO 2 passivation layers.

A scenario for future silicon photovoltaics is to use wafer-based solar cells with a thickness below 100 μm. A way to realize this scenario is the merging of cell and module processing, with thin wafers processed while attached to the superstrate glass. One of the challenges for this type of processing is the achievement of high performing surface passivation, i.e., with surface recombination velocities below 10 cm/s. In this paper, a detailed explanation for lifetime degradation on wafers bonded to glass by means of silicone is proposed. The degradation is due to cyclic silicone molecules outgassing during a-Si:H deposition from the adhesive used to bond the wafers. The cyclic molecules are incorporated in the a-Si:H layer and modify the amorphous silicon network. By the application of specific outgassing conditions before a-Si:H deposition, a significant amount of cyclic molecules is removed from the adhesive. Non-degraded a-Si:H layers and surface passivation comparable to standalone wafers are obtained, as shown with measures of lifetime and solar cell open circuit voltage.

Future wafer-based silicon solar cells will be fabricated on thin (

A possible scenario for wafer-based silicon photovoltaics is the processing of solar modules starting from thin silicon wafers bonded to glass. However, interactions between the adhesive used for bonding and the solar cell processing can affect the surface passivation of the bonded wafer and decrease cell performances. A method that suppresses these interactions and leads to state-of-the-art a-Si:H surface passivation is presented in this Letter. The method is based on an increase of the surface cross-linking of a silicone adhesive by means of an O2 plasma and it is successfully tested on three different silicones. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogenated amorphous silicon (a-Si:H) layers deposited by chemical vapour deposition provide an attractive route to achieve high performance crystalline silicon (c-Si) solar cells due to their deposition at low temperatures and their superior passivation quality. A post-deposition annealing of such layers typically enables a further strong reduction in defect states at the a-Si:H/c-Si interface, due to the saturation of dangling bonds by atomic hydrogen. In this work, we present the evolution of the effective lifetime during annealing at different temperatures. We find that at lower deposition temperatures (150 °C) the high density of Si-H 2 bonds in as deposited layers results in higher defect density and worse passivation quality. In contrast, deposition at higher temperatures (200 °C) leads to a structural improvement of the short range order in the a-Si:H layer and a decrease of the disorder in the film network. The presence of a dense material mixed with a fraction of SiH 2 at the interface favors further improvement of the passivation during a post-deposition anneal, resulting in the best case in an effective lifetime of 10ms.

In order to relax the mechanical constraints of processing thin crystalline Si wafers into highly efficient solar cells, we propose a process sequence, where a significant part of the process is done on module level. The device structure is an interdigitated-back-contact cell with an amorphous silicon back surface field. The record cell reaches an independently confirmed efficiency of 18.4%. Although the device deserves further optimization, the result shows the compatibility of processing on glass with efficiencies exceeding 18%, which opens the door to a high-efficiency solar cell process where the potentially thin wafer is attached to a foreign carrier during the full processing sequence.

In this study, A H2-plasma is studied as a dry method to etch thin layers of amorphous silicon aSi:H(i) deposited on a crystalline wafer. It is found that H2-plasma etches aSi:H(i) selectively toward silicon nitrides hard masks with an etch rate below 3nm/min. Depending on power density and temperature of the substrate during the H2-plasma, the energy bandgap, the hydrides distribution and the void concentration of the aSi:H(i) layers are modified and the amorphous-to-crystalline transition is approached. At high temperature (>250C) and low plasma power (2), the dihydride (SiH2) content increases and the bandgap widens. The etch rates stays below 0.5 nm/min. At low temperature (70mW/cm2), the void concentration increases significantly and etch rates up to 3nm/min are recorded.These findings are supported by a theoretical model that indicates formation of Si-H-Si precursors in the layer during exposure to H2-plasma. According to the experimental conditions, these precursors either diffuses and forms Si-Si strong bonds or are removed from the film, causing layer etching.

Optical antennas have the prospect to redirect light rays into engineered directions at the subwavelength scale. They offer new options for photodetection, color routing, fluorescence emission and sensing applications. Previously, metallic nanoantennas based on localized surface plasmon resonance (LSPR) have commonly been exploited to fulfill this purpose, e.g. the gold Yagi-Uda array and split-ring resonator. However, the intrinsic ohmic losses of metals are large especially in the visible range, hindering further efficiency improvement for practical applications. In addition, the interaction of the metallic nanoantennas with the magnetic component of light is relatively weak, adding to their lack of highly tunable scattering directionality. In this presentation, we will demonstrate our recent experimental progress on all-dielectric nanoantennas made of amorphous silicon with tunable scattering directionality. Our nanoantennas are designed as V-shaped single element and fabricated using electron-beam lithography followed by dry reactive ion etching. It is illustrated that the scattering cross section of the silicon nanoantenna can be considerably higher than that of comparable Au antennas. In addition, the extinction coefficient of amorphous silicon is adequately low in the considered wavelength range, resulting in minimal absorption losses and an enhanced scattering efficiency. More interestingly, compared to Au nanoantennas that exhibit light scattering in a single particular direction, by carefully engineering the geometry of the silicon nano-antennas, their scattering can be effectively tuned into two opposite directions within the visible range (Supporting figure). Over a spectral range of less than 100 nm, the scattering directionality gradually shifts from the leftward to the rightward. More simulation results based on the finite difference time domain (FDTD) methods are available to perfectly match and corroborate our experimental measurement. Initial analysis of the underlying physics for the tunable scattering directionality will also be discussed. Such unique optical properties make the silicon nanoantenna promising candidates for novel low-loss optical devices that can enable unprecedented control over the scattering directionality.

An optical antenna forms the subwavelength bridge between free space optical radiation and localized electromagnetic energy. Its localized electromagnetic modes strongly depend on its geometry and material composition. Here, we present the design and experimental realization of a novel V-shaped all-dielectric antenna based on high-index amorphous silicon with a strong magnetic dipole resonance in the visible range. As a result, it exhibits extraordinary bidirectional scattering into diametrically opposite directions. The scattering direction is effectively controlled by the incident wavelength, rendering the antenna a passive bidirectional wavelength router. A detailed multipole decomposition analysis reveals that the excitation and abrupt phase change of an out-of-plane polarized magnetic dipole and an in-plane electric quadrupole are essential for the directivity switching. Previously, noble metals have been extensively exploited for plasmonic directional nanoantenna design. However, these inevitably suffer from high intrinsic ohmic losses and a relatively weak magnetic response to the incident light. Compared to a similar gold plasmonic nanoantenna design, we show that the silicon-based antennas demonstrate stronger magnetic scattering with minimal absorption losses. Our results indicate that all-dielectric antennas will open exciting possibilities for efficient manipulation of light-matter interactions.

The use of Kelvin Probe Force Microscopy to investigate PN junctions under illumination and electrical bias has been shown to be an effective way to analyze the electrical properties of solar cells at the nanoscale. We use it here, for the first time, on interdigitated back contact solar cells. Measurements are performed between the metallic fingers under an electrical bias in the dark and under illumination in open circuit conditions. Our results show that that KPFM could be used to identify and localize diodes resistance losses at the nanoscale.

Module-level processing of silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells while bonded to glass, in the so-called i2-module concept, is discussed. In this approach, a key challenge is the interdigitated patterning of a-Si:H without compromising on the rear-surface passivation. Process adaptations involving more resistant bonding agents and a milder wet etchant enabled bonded cells with the best efficiency of 21.7% and Voc of 734 mV, which are the highest reported for bonded cells processed partially at module level, and which undoubtedly proves the potential of this i2-module concept to reach high Voc and efficiency. Yet another challenge is to make the process flow cost-effective and industrially-relevant, i.e. a litho-free, all-dry process flow, analogous to thin-film PV module fabrication. As a first step, dry etching of a-Si:H was developed to replace wet etching, and was successfully incorporated in a SHJ-IBC process flow to fabricate freestanding cells with the best efficiency of 22.9% and above 20% on thick (190 μm) and thin (56 μm) EVA-bonded silicon.

Thermal atomic layer deposition (ALD) of Al2O3 provides an adequate level of surface passivation for both p-type and n-type Si solar cells. To obtain the most qualitative and uniform surface passivation advanced cleaning development is required. The studied pre-deposition treatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation - after cleaning and Al2O3 deposition - carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivation level is found. Si-H starting surfaces lead to equivalent passivation quality but worse passivation uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and consequently exhibits better surface passivation uniformity.

Crystalline Si (c-Si) technology is dominating the photovoltaics market. These modules are nonetheless still relatively expensive, in particular because of the costly silicon wafers, which require large thickness mostly to ease handling. Thin-film technologies, on the other hand, use much less active material, exhibit a much lower production cost per unit area, but achieve an efficiency still limited on module level, which increases the total system costs. A meet-in-the-middle is possible and is the object of this paper. The development of c-Si thin-foil modules is presented: first, the fabrication of the active material on a glass module and then the processing of the Si foils into solar cells, directly on module level. The activity of IMEC in this area is put into perspective with regard to worldwide research results. It appears that great opportunities are offered to this cell concept, although some challenges still need to be tackled before cost-effective and reliable industrial production can be launched. Copyright © 2012 John Wiley & Sons, Ltd.

The potential of contact passivation for increasing cell performance is indicated by several results reported in the literature. However, scant characterization of the tunneling layers used for that purpose has been reported. In this paper, contact passivation is investigated by insertion of an ultra-thin AlOx layer between an n-type emitter and a Ti/Pd/Ag contact. By using a 1.5nm thick layer, an increase of the minority carrier lifetime by a factor of 2.7 is achieved. Since current-voltage measurements indicate that an ohmic behavior is conserved for AlOx layers as thick as 1.5nm, a 1.5nm AlOx layer is found to be a candidate of choice for contact passivation.

In this work, an extensive characterisation of intrinsic amorphous silicon (a-Si) passivation layers deposited on n- and p-type silicon is reported. Low temperature capacitance-voltage measurements are utilised to enable parameter extraction from the c-Si/a-Si interface and a-Si bulk. Electron spin resonance enables atomic identification of defects present. Results reveal the presence of electrically active defects at the c-Si/intrinsic a-Si interface (∼1x1012 cm-2), and throughout the amorphous silicon layer bulk (∼8x1016 cm-3), which are atomically identified as Pb0 centres and D centres silicon dangling bond defects, respectively. The value of this work is the atomic identification of these defects in this stack, coupled with their electrical activity. That they can be detected by these techniques demonstrates the power of the methodology used to assess and quantify these defects. Therein lies the significance of this work: a methodology capable of fundamentally optimising amorphous silicon processing from an atomic perspective.

Silicon photovoltaic (PV) roadmaps indicate the reduction of wafer thicknesses and the need for innovation in wafering method and cell processing. Within this framework, Imec proposes the i2-module device [1], i.e. an heterojunction interdigitated back-contact (HJ i-BC) solar module [2] processed on 40-μm thick epitaxial wafers bonded to carriers by means of silicone. In the i2-module concept, the Rear Side (RS) of the solar cell is passivated while the wafer is bonded to the module glass and the influence of the silicone on the passivation process is reduced by an O2 plasma realized in an Reactive Ion Etching (RIE) chamber [3]. In this contribution, the effect of different post-bonding cleaning sequences on the passivation of wafers/silicone/glass stacks treated with an O2 plasma is investigated and a simplified post-bonding cleaning sequence leading to state-of-the-art passivation is proposed.

Single-wafer cleaning based on megasonic nozzle systems can be used for removing nano-particles. An optimized cleaning process must deliver a high degree of cleaning uniformity with a minimal amount of structural damage. However, the cleaning liquid is confined to a sonicated liquid jet, which is ejected by the nozzle. As a result, a high degree of complexity is introduced to the cleaning process due to the strong interdependencies between acoustic, electric and hydrodynamic characteristics of such systems. In the present work, the influence of the gas content of the cleaning liquid on the performance of a 1 MHz nozzle system is investigated and related to the intrinsic properties of the water jet. Cleaning tests are performed and the cleaning results are correlated to the electrical responses of the driving transducer. The electrical response depends both on the gasification of the liquid and the occurrence of acoustic reflections. Furthermore, Schlieren- and Sonoluminescence-imaging of the cleaning system are performed. The imaging techniques can identify the impact of bubbles on the propagation of acoustic waves and the resonant excitation of cleaning cavitation throughout the liquid column.

In this paper, we report on the design, fabrication, packaging, and testing of very reliable CMOS-integrated 10-cm2 11-megapixel SiGe-based micromirror arrays on top of planarized six-level metal 0.18-?m CMOS wafers. The array, which is to be used as a spatial light modulator (SLM) for optical maskless lithography, consists of 8 ?m × 8 ?m pixels, which can be individually addressed by an analog voltage to enable accurate tilt angle modulation. Due to very stringent requirements on mounted-die flatness (< 0.01 mrad), the first level packaging of SLM die is done using specially designed SiC holders. To avoid trapped particles between the die and holder, which would jeopardize the flatness spec, special backside cleaning of the dies (less than or equal to one 0.8-?m particle/cm2) is needed before mounting the SLM die on the holder. To enable this backside cleaning and to avoid front-side particles during dicing, handling, and wire bonding, a temporary waferor zero-level packaging cap, which can be placed and removed at room temperature, was developed. The dynamic white light interferometer measurements of packaged dies showed that 99.5% of the 123 648 mirrors tested are within the spec. In addition, a stable average cupping of below 7 nm, an rms roughness of below 1 nm, and a stable actuation of over 2.5 teracycles are demonstrated.

A new method for the fast evaluation of photoresist residue removal efficiency is discussed in this paper. In this method ldquohazerdquo which is the low-frequency component of the background signal of a light scattering instrument is mapped over the entire wafer. Since the background signal is sensitive to any kind of surface anomaly, it can be used as a metrology for any kind of surface roughness or residues. The goal of this paper is to devise a fast and cheap screening method for photoresist residue removal efficiency. Using this method we show that cleaning solutions can be easily screened for their residue removal efficiencies based on the haze signal of the light scattering instrument.

Particle removal without damage addition remains a grand challenge. Particle removal and damage addition are macroscopic effects of the cleaning technique. In order to improve the understanding of cleaning techniques like high-velocity-aerosol cleaning, these macroscopic properties need to be linked to the fundamental properties of the technique like its spray characteristics. Here we show that using the particle removal or damage addition frequency, we can estimate the fraction of droplets contributing either to particle removal or damage addition. We find that only a very small fraction of the droplets leads to damage formation. We also show that for particle removal only a very small fraction of droplets is needed, or the droplets only need to clean a very small surface area. The fraction of droplets contributing to particle removal depends on the effective area used to analyze the particle removal frequency.

In many process steps of integrated circuits (IC’s) fabrication, silicon wafers are coming in contact with process liquids such as ultra pure water (UPW) and aqueous and non-aqueous chemical mixtures. During these process steps, liquid-borne particle contamination can deposit on the wafer surface. Particle contamination from UPW is an important factor influencing random yield loss of IC’s [ ]. A number of yield models are used to predict yields including Poisson, Murphy, Seeds, and negative binomial models [ , ]. However, these models are based on the assumption that particles are randomly deposited on the wafer surface [ ].

The increasing complexity of semiconductor devices imposes challenging requirements on particle contamination and surface damage. To meet these requirements novel surface-cleaning processes are evaluated, which combine physical energy with organic solvents. In this work, the performance of megasonic cleaning with deionized water (DIW) and N-methylpyrrolidone (NMP) was evaluated in terms of particle removal efficiency (PRE) and damage analysis. The goal was to define an optimum process window where the PRE was maximum and the damage was minimum. Particle removal and damage analysis were performed on unpatterned silicon wafers and with patterned polysilicon lines, respectively, under identical sonic power and process parameters. A comparison between these two solvents reveals that at low sonic power the particle-cleaning performances in DIW and NMP are similar. At high sonic power, in both solvents a detailed analysis of the PRE and damage indicates a non-homogeneous trend over the surface of the wafer. Mor...

High velocity aerosol cleaning using ultrapure water or dilute aqueous solutions (e.g. dilute ammonia) is common in semiconductor IC fabrication [1]. This process combines droplet impact forces with continuous liquid flow for improved cleaning efficiency of sub-100nm particles. As with any physically enhanced cleaning process, improved particle removal can be accompanied by increased substrate damage, especially to smaller (