Thin Silicon Foils Research Articles

A setup for micro-structured silicon targets by femtosecond laser irradiation

We present a process to fabricate micro-structured thin silicon foils with highly light absorbing properties over a broad wavelength region based on short-pulse laser treatment. We employ this fascinating technique to the fabrication of targets for high-intensity laser-plasma experiments. A polished silicon wafer is processed with high-intensity femtosecond laser pulses resulting in conical spikes within the irradiated region. Height, distance, and shape of these spikes depend primarily on the number, energy, central wavelength and duration of the incident laser pulses, as well as the ambient medium. This method is of great value for the future development of target fabrication. The broad parameter base offers a huge selection regarding shape and dimensions of the resulting structure. It can be included in existing laser operations within the manufacturing chain while offering a high degree of customisability. Within the reach of higher repetition rates of high-power laser systems and an increased number of requested targets, this manufacturing method can be scaled while being cost efficient and easily adaptable.

X-ray absorption spectroscopy study of energy transport in foil targets heated by petawatt laser pulses

X-ray absorption spectroscopy is proposed as a method for studying the heating of solid-density matter excited by secondary X-ray radiation from a relativistic laser-produced plasma. The method was developed and applied to experiments involving thin silicon foils irradiated by 0.5–1.5 ps duration ultrahigh contrast laser pulses at intensities between 0.5×1020 and 2.5×1020 W/cm2. The electron temperature of the material at the rear side of the target is estimated to be in the range of 140–300 eV. The diagnostic approach enables the study of warm dense matter states with low self-emissivity.

Fabricating 40 µm-thin silicon solar cells with different orientations by using SLiM-cut method

Thin silicon foils with different crystal orientations were fabricated using the stress induced lift-off (SLiM-cut) method. The thickness of the silicon foils was approximately 40 µm. The foil had a smoother surface than the foil. With surface passivation, the minority carrier lifetimes of the and silicon foil were 1.0 µs and 1.6 µs, respectively. In this study, 4 cm2-thin silicon solar cells with heterojunction structures were fabricated. The energy conversion efficiencies were determined to be 10.74% and 14.74% for the and solar cells, respectively. The surface quality of the silicon foils was determined to affect the solar cell character. This study demonstrated that fabricating the solar cell by using silicon foil obtained from the SLiM-cut method is feasible.

Structural damage in thin SLIM-Cut c-Si foils fabricated for solar cell purposes: atomic assessment by electron spin resonance

Within the context of reducing production costs, thin (<90 μm) silicon foils intended for photovoltaic applications have been fabricated from standard (100)Si wafers using a low-temperature (<150 °C) stress-induced lift-off process. A multi-frequency electron spin resonance (ESR) study was performed in order to evaluate, at atomic scale, the quality of the material in terms of defects, including identification and quantification. Generally, a complex ESR spectrum is observed, disentangled as the superposition of three separate signals. This includes, most prominently (∼91% of total density) the D-line (Si3 ≡ Si· dangling bonds in a disordered Si environment), a set (∼6%) of highly anisotropic signals ascribed to dislocations (K1-like), and a triplet, identified as the Si-SL5 N-donor defect. Defect density depth profiling from the lift-off side shows all signals disappear in tandem after etching off a ∼33 μm thick Si layer, indicating a highly correlated−equal in relative terms−distribution of the three types of defects over the affected top part of the Si foil. The defect density is found to be highly non-uniform laterally, with the density peaking near the crack initiation point, from which defect generation spreads. It is thus found that the SLIM-Cut method for fabrication of thin Si foils results in the introduction of defects that would unacceptably impair the functionality of photovoltaic cells built on these substrates. Fortunately, this may be cured by etching off a thin top Si layer, resulting in a most useful thin Si foil of standard high quality.

Study of high-temperature Smart Cut™: Application to silicon-on-sapphire films and to thin foils of single crystal silicon

Two novel processes of elaboration of silicon thin films and silicon foils are proposed, based on the knowledge brought from the Smart Cut™. The first relies on the laser-beam annealing of an implanted silicon wafer in order to induce a separation layer within the implanted wafer and a transfer upon a transparent wafer. The second consists in depositing a layer of liquid silicon upon an implanted silicon wafer and to form a silicon foil by liquid phase epitaxial growth that can be separated from the substrate by Smart Cut™. The separation kinetics of an implanted silicon wafer is characterized for temperature between 450°C and 700°C, considering doses of implantation from 3.5∗1016Hcm−2 to 1.0∗1017Hcm−2. The out-diffusion of hydrogen is studied by Energy Recoil Detection Analysis and a model of diffusion of hydrogen in implanted silicon is proposed. Based on this analysis, a model for the kinetics of splitting at high temperature is established. Smart Cut™ separation is demonstrated for temperature up to 1250°C, considering an implanted silicon wafer bonded with a sapphire wafer, through which a laser beam anneals the structure. The kinetics of separation by laser beam annealing is characterized and compared to the kinetics established between 450°C and 700°C. The roughness of the silicon on sapphire film is characterized by Atomic Force Microscopy and a transfer is realized considering an implanted silicon bonded with a glass wafer of 200mm of diameter. Finally, this article presents results of liquid silicon deposition onto an implanted silicon substrate. These results demonstrate the possibility to detach the film grown by liquid phase epitaxy and the upper part of the implanted substrate by Smart Cut™. Electron Backscattering Diffraction Pattern analysis is considered in order to demonstrate the occurrence of epitaxy of the deposited liquid onto the implanted substrate.

Focused particle beam-induced processing.

Focused particle beam-induced processing.

Polycrystalline silicon foils by flash lamp annealing of spray-coated silicon nanoparticle dispersions

A novel technique for the preparation of thin silicon foils (d ≈ 10 µm) starting from silicon nanoparticle dispersions is reported. The basic steps are ultrasonic spray coating of mixtures of silicon nanoparticles and organosilicon compounds and subsequent flash lamp annealing (FLA). The organosilicon compounds are used to stabilize the silicon nanoparticles in dispersion. The FLA induces melting of the silicon nanoparticles which transform into polycrystalline silicon thin films. Parameter adjustment (e.g., pressure, flash duration, flash energy, and substrate temperature) allows for the preparation of thin films as well as free-standing and flexible silicon foils. If the sprayed film thickness reaches a critical value (d ≈ 20 µm) and the FLA is performed under vacuum conditions (5 × 10−3 mbar), then the film peels off from the molybdenum substrate. In a following step, the foil is flashed by FLA from the backside. This method targets thin silicon foils exhibiting thicknesses between 5 and 10 µm. The lateral size of silicon foils depends on the setup used. In this work, samples with a maximum area of approximately 5 × 5 cm2 were produced. The silicon foils were investigated by scanning electron microscopy and spectroscopic ellipsometry to determine thickness, surface structure, and the effective dielectric response. The compositions (Si, C, and O) and the bond characteristics of Si–O and Si–C were analyzed by means of energy dispersive X-ray spectroscopy and X-ray photoemission spectroscopy. Transmission electron microscopy and X-ray diffraction provided the average sizes of the silicon nanoparticles before and after FLA. A rough estimation of the free charge carrier concentration [n el ≈ (1013–1014) cm−3] was possible by electrical four-point probe measurements taking into account information on crystallite sizes.

Room temperature thin foil SLIM-cut using an epoxy paste: experimental versus theoretical results

The stress induced lift-off method (SLIM) -cut technique allows the detachment of thin silicon foils using a stress inducing layer. In this work, results of SLIM-cut foils obtained using an epoxy stress inducing layer at room temperature are presented. Numerical analyses were performed in order to study and ascertain the important experimental parameters. The experimental and simulation results are in good agreement. Indeed, large area (5 × 5 cm2) foils were successfully detached at room temperature using an epoxy thickness of 900 μm and a curing temperature of 150 °C. Moreover, three foils (5 × 3 cm2) with thickness 135, 121 and 110 μm were detached from the same monocrystalline substrate. Effective minority carrier lifetimes of 46, 25 and 20 μs were measured using quasi-steady-state photoconductance technique in these foils after iodine ethanol surface passivation.

Solar cell fabricated on welded thin flexible silicon

We present a thin-film crystalline silicon solar cell with an AM1.5 efficiency of 11.5% fabricated on welded 50 μ m thin silicon foils. The aperture area of the cell is 1.00 cm 2 . The cell has an open-circuit voltage of 570 mV, a short-circuit current density of 29.9 mA cm -2 and a fill factor of 67.6%. These are the first results ever presented for solar cells on welded silicon foils. The foils were welded together in order to create the first thin flexible monocrystalline band substrate. A flexible band substrate offers the possibility to overcome the area restriction of ingot-based monocrystalline silicon wafers and the feasibility of a roll-to-roll manufacturing. In combination with an epitaxial and layer transfer process a decrease in production costs can be achieved.

Directional Heating and Cooling for Controlled Spalling

The fabrication of thin solar cells by kerfless wafering techniques offers a high potential for the reduction of photovoltaic costs. We present an experimental setup for the exfoliation of thin crystalline silicon foils from a silicon substrate induced by the difference in thermal expansion coefficient of the silicon and an aluminum stressor layer at moderate temperatures. A moving temperature gradient across the substrate controls the crack propagation parallel to the silicon surface. We measure and simulate the spatial temperature distribution during thermal treatment and find that the direction of crack propagation is controlled by the temperature distribution. We detach foils with an area of 19.6 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with thickness values ranging from 50 to 80 μm within one layer. The foils have a smooth surface with some irregularities near the edge.

Kerfless layer-transfer of thin epitaxial silicon foils using novel multiple layer porous silicon stacks with near 100% detachment yield and large minority carrier diffusion lengths

Two important aspects for the success of the porous silicon-based layer transfer method in producing kerfless thin (<50µm) silicon foils for future silicon solar modules are addressed in this work: achieving high detachment yield and high minority carrier diffusion lengths. The detachment characteristics of the porous silicon-based lift-off process is studied using finite element modeling as well as experiments. It is shown that for easy detachment and high detachment yield, a low density of thin silicon pillars must be attained in the high porosity detachment layer (HP-DL) after high temperature sintering. This is elegantly achieved by increasing the thickness of the low porosity template layer (LP-TL) which acts as the vacancy supply to increase the post-anneal porosity of the HP-DL. In this way, near 100% detachment yield has been achieved. However, a thicker LP-TL results in a poorer quality epitaxial growth surface. To circumvent this trade-off, novel triple and quadruple layer porous silicon stacks are introduced which decouple the function of the LP-TL that acts as both the template for epitaxy and as the vacancy supply for the HP-DL. In these new stacks, a surface zone of very low void size and density (nearly void-free) is created which allows high quality epitaxy on easily-detachable porous silicon stacks. Minority carrier lifetime measurements on epitaxial foils grown on such a triple layer stack has resulted in an effective lifetime of ~350µs at the injection level of 1015cm−3 which corresponds to a minimum minority carrier diffusion length of ~670µm (> 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.

Thin silicon foils produced by epoxy-induced spalling of silicon for high efficiency solar cells

We report on the drastic improvement of the quality of thin silicon foils produced by epoxy-induced spalling. In the past, researchers have proposed to fabricate silicon foils by spalling silicon substrates with different stress-inducing materials to manufacture thin silicon solar cells. However, the reported values of effective minority carrier lifetime of the fabricated foils remained always limited to ∼100 μs or below. In this work, we investigate epoxy-induced exfoliated foils by electron spin resonance to analyze the limiting factors of the minority carrier lifetime. These measurements highlight the presence of disordered dangling bonds and dislocation-like defects generated by the exfoliation process. A solution to remove these defects compatible with the process flow to fabricate solar cells is proposed. After etching off less than 1 μm of material, the lifetime of the foil increases by more than a factor of 4.5, reaching a value of 461 μs. This corresponds to a lower limit of the diffusion length of more than 7 times the foil thickness. Regions with different lifetime correlate well with the roughness of the crack surface which suggests that the lifetime is now limited by the quality of the passivation of rough surfaces. The reported values of the minority carrier lifetime show a potential for high efficiency (&amp;gt;22%) thin silicon solar cells.

New Stress Activation Method for Kerfless Silicon Wafering Using Ag/Al and Epoxy Stress-Inducing Layers

The SLIM-cut technique provides a way to obtain thin silicon foils without a standard sawing step, thus avoiding kerf losses. This process consists of three steps: depositing a stress-inducing layer on top of the silicon surface; stress activation by heating and cooling, resulting in crack propagation in the silicon and detachment of a thin silicon layer; and a chemical cleaning to remove the stress-inducing layer. This paper describes a new stress activation method using Ag/Al and epoxy stress-inducing layers. The crack propagation is controlled along the sample length in order to avoid unwanted additional crack formation and interaction with other crack fronts. Silicon foils with thickness ranging between 50 and 130 μm were obtained with effective lifetimes between 1 and 81 μs.

Reorganization of Porous Silicon: Effect on Epitaxial Layer Quality and Detachment

Cost reduction is still a main goal in solar cell research and can be achieved by going towards thinner silicon bulk material. One way to avoid kerf loss and to reach thicknesses of less than 50 μm is a lift-off approach using porous silicon and epitaxial thickening for silicon foil fabrication. The porous layer structure requires a reorganization step that was varied in this work to optimize the detachment properties and the crystal quality of the epitaxial Si film. All processes were carried out in a quasi-inline Atmospheric Pressure Chemical Vapour Deposition (APCVD) reactor. Cross–sections were observed to see if the porous layer shows the desired structure. Stacking fault densities in epitaxial layers deposited on porous silicon layers significantly decrease with increasing reorganization time but are at least one order of magnitude higher than in epitaxial layers deposited on polished wafers. Microwave photoconductive decay (MWPCD) measurements and photo luminescence (PL) imaging were carried out to determine the effective carrier lifetimes of the detached foils and to correlate them with stacking faults and cracks. A detached 40 μm thin silicon foil with an averaged effective carrier lifetime of 22 μs is shown which corresponds to a diffusion length of over 200 μm. This investigation shows that silicon foils deposited in a quasi-inline APCVD reactor exhibit good detachment properties and a good crystal quality, which is both needed for high efficiency solar cell processing.

Solar Cell Processing of Foil Produced by Epoxy-induced Spalling of Silicon

To push silicon solar cells up to their theoretical limit and, in the meantime, minimize the amount of active material employed, many researchers have proposed to fabricate silicon solar cells starting from substrates of 100μm or below. A considerable number of methods have been proposed to overcome the limitations of the standard sawing technique but, all of them present some drawbacks. One of these methods, the Slim-cut method, exploits the stress-induced by the deposition of a layer on top of a thick silicon substrate to separate a thin silicon foil. This process however did not solve the issues linked to the processing of free-standing foils.In this work we present a flow to process the Slim-cut foils into solar cells by minimizing the number of process steps performed on free-standing foils to just one step. The possibility to tune the thickness of the obtained silicon foil is investigated both by analytical calculations and experimental results. Moreover, a solution to remove the stress-inducing layer compatible with the flow is reported and lifetime measurements of samples bonded on glass demonstrate that the quality of the samples is high enough for the fabrication of solar cells.

Room Temperature Spalling of Thin Silicon Foils Using a Kerfless Technique

An important factor for cost reduction of solar electricity is the reduction of silicon material quantity used for making a solar cell. It is well known in the PV industry that kerf losses associated with wafering are a limiting factor in the efforts to reduce material usage and some techniques have been developed to address this issue. The kerfless wafering technique described in this paper provides both things: a thin silicon layer foil while avoiding the sawing step. This technique consists of three steps: (i) dispensing a stress inducing layer on the silicon surface of a slab; (ii) thermal processing to activate the stress and detach a thin silicon layer of silicon; (iii) a chemical cleaning to obtain a flat thin foil of silicon. This technique was used with an epoxy stress inducing layer to obtain thin silicon foils, demonstrating for the first time the capability to obtain several foils out of the same substrate and using only room temperature cooling step, unlike other authors who used dipping in liquid nitrogen. The relationship between epoxy layer and foil thickness is presented, along with foils with bulk minority carrier lifetimes of 80μs.

Material Properties of Laser-Welded Thin Silicon Foils

An extended monocrystalline silicon base foil offers a great opportunity to combine low-cost production with high efficiency silicon solar cells on a large scale. By overcoming the area restriction of ingot-based monocrystalline silicon wafer production, costs could be decreased to thin film solar cell range. The extended monocrystalline silicon base foil consists of several individual thin silicon wafers which are welded together. A comparison of three different approaches to weld 50 μm thin silicon foils is investigated here: (1) laser spot welding with low constant feed speed, (2) laser line welding, and (3) keyhole welding. Cross-sections are prepared and analyzed by electron backscatter diffraction (EBSD) to reveal changes in the crystal structure at the welding side after laser irradiation. The treatment leads to the appearance of new grains and boundaries. The induced internal stress, using the three different laser welding processes, was investigated by micro-Raman analysis. We conclude that the keyhole welding process is the most favorable to produce thin silicon foils.

Improvement of seed layer smoothness for epitaxial growth on porous silicon

ABSTRACTIn the last decades many techniques have been proposed to manufacture thin (&lt;50µm) silicon solar cells. The main issues in manufacturing thin solar cells are the unavailability of a reliable method to produce thin silicon foils with contained material losses (kerf-losses) and the difficulties in handling and processing such fragile foils. A way to solve both issues is to grow an epitaxial foil on top of a weak sintered porous silicon layer. The porous silicon layer is formed by electrochemical etching on a thick silicon substrate and then annealed to close the top surface. This surface is employed as seed layer for the epitaxial growth of a silicon layer which can be partially processed while attached on the substrate that provides mechanical support. Afterward, the foil can be bonded on glass, detached and further processed at module level. The efficiency of the final solar cell will depend on the quality of the epitaxial layer which, in turn, depends on the seed layer smoothness.Several parameters can be adjusted to change the morphology and, hence, the properties of the porous layer, both in the porous silicon formation and the succeeding thermal treatment. This work focuses on the effect of the parameters that control the porous silicon formation on the structure of the porous silicon layer after annealing and, more specifically, on the roughness of the top surface. The reported analysis shows how the roughness of the seed layer can be reduced to improve the quality of the epitaxial growth.

Epoxy-Induced Spalling of Silicon

One of the main driving forces to lower the silicon solar cell modules overall cost is to decrease the thickness of the silicon substrate and, at same time, to reduce the material losses caused by the sawing of the silicon ingot. For this reason, a considerable number of methods have been proposed in the last decade to manufacture thin silicon foils which aim to replace the commonly used sawing technique. One of these methods is based on peeling off a thin layer from a silicon substrate by means of the residual stress induced either through the only deposition of a layer on a silicon substrate or through a succeeding thermal cycle of such a bilayer. Up to now, three different materials have been successfully employed in this technique: Nickel, Nickel/Chromium alloy and a Silver/Aluminium system. In this work we demonstrate the possibility to induce the lift-off of a thin silicon foil by means of curing an epoxy layer on top of a silicon wafer. This new method has the advantages of reducing metal contamination in the silicon and lowers the operating temperatures below 150°C.

Energy loss and angular dispersion of 2–200 keV protons in amorphous silicon

The energy loss of 2–200 keV protons in thin amorphous silicon foils has been measured for projectiles transmitted in the forward direction and as a function of the exit angle. At the lowest energies, differences of up to 30% with recently published values are observed. Angular effects in the energy loss, at low and high energies, have been investigated. The low-energy results are reproduced by model calculations and Monte Carlo simulations, which indicate that the inelastic energy loss does not show a dependence upon the impact parameter in the low energy region. A fitting formula for the present energy loss values is provided.