Thin Silicon Research Articles

22.8% full-area bifacial n-PERT solar cells with rear side sputtered poly-Si(n) passivating contact

High-temperature compatible passivating contacts based on phosphorus doped polysilicon (poly-Si(n)) on a thin silicon oxide, employed on the rear side of bifacial n-PERT like solar cells provide an efficiency increase while being compatible with current photovoltaic manufacturing processes, including the metallization process. However, a simple and cost-effective method for poly-Si(n) deposition is still elusive. Herein, we demonstrate DC-sputtering as a scalable and single side technique for poly-Si(n) manufacturing. First, it is demonstrated how DC-sputtered poly-Si(n) can provide high passivation (iV oc > 730 mV) and low contact resistivity (ρ c < 20 mΩ cm 2 ) when metallized by means of screen printing and firing through of Ag paste. Then, M2-sized bifacial n-PERT solar cells featuring the developed DC sputtered poly-Si(n) at the rear side and a homogenous front side B-emitter reaching a certified efficiency of 22.8% are demonstrated. Quokka simulations based on the developed DC sputtered poly-Si(n) indicate a potential up to 24.8% with an upward potential upon optimization of the front side emitter. This work highlights the feasibility of sputtered poly-Si(n) as an enabler for high-efficiency n-type solar cells. • Ex-situ P-doped PVD sputtered poly-Si passivating contacts integrated in M2 sized bifacial n-PERT solar cells. • M2 bifacial n-PERT devices with 22.8% certified efficiency demonstrated. • Integration of 100 nm of ex-situ doped sputtered poly-Si with screen printing and firing through of Ag paste. • Roadmap toward 25% efficiency presented.

Isotope doping-induced crossover shift in the thermal conductivity of thin silicon nanowires

Here, using homogeneous nonequilibrium molecular dynamics simulations, we report the thermal transport characteristics of thin Si nanowires (NWs) with varying size and isotope doping ratio. It is identified that crossover in the thermal conductivity (κ) of both isotope doping-free and isotope doped Si-NWs appears at critical sizes, below which κ is enlarged with decreasing size because the hydrodynamic phonon flow predominates, above which, due to the dominant phonon boundary scattering, opposite behavior is observed. With increasing isotope doping, however, the critical size in minimizing the κ is moved to small values because the phonon impurity scattering caused by isotope doping is critically involved. Moreover, there is a critical isotope doping (<50%) in the critical size motion, originating from that, above which, the critical size no longer moves due to the persistence of hydrodynamic phonon flow. This study provides new insights into the thermal transport behaviors of quasi-1D structures.

Investigation of the possibility of deposition of thin silicone films on the surface of quartz glasses by laser radiation from the gas phase

Investigation of the possibility of deposition of thin silicone films on the surface of quartz glasses by laser radiation from the gas phase

Ion Current Rectification and Long-Range Interference in Conical Silicon Micropores.

Fluidic devices exhibiting ion current rectification (ICR), or ionic diodes, are of broad interest for applications including desalination, energy harvesting, and sensing, among others. For such applications a large conductance is desirable, which can be achieved by simultaneously using thin membranes and wide pores. In this paper we demonstrate ICR in micrometer sized conical channels in a thin silicon membrane with pore diameters comparable to the membrane thickness but both much larger than the electrolyte screening length. We show that for these pores the entrance resistance is key not only to Ohmic conductance around 0 V but also for understanding ICR, both of which we measure experimentally and capture within a single analytic theoretical framework. The only fit parameter in this theory is the membrane surface potential, for which we find that it is voltage dependent and its value is excessively large compared to the literature. From this we infer that surface charge outside the pore strongly contributes to the observed Ohmic conductance and rectification by a different extent. We experimentally verify this hypothesis in a small array of pores and find that ICR vanishes due to pore-pore interactions mediated through the membrane surface, while Ohmic conductance around 0 V remains unaffected. We find that the pore-pore interaction for ICR is set by a long-ranged decay of the concentration which explains the surprising finding that the ICR vanishes for even a sparsely populated array with a pore-pore spacing as large as 7 μm.

Effect of sawing damage on flexibility of crystalline silicon wafers for thin flexible silicon solar cells

The cost of solar cell production can be reduced by wafer thinning. A thinner wafer provides flexibility, and crystalline silicon solar cells are promising as flexible solar cells due to their flexibility. However, as wafers become thinner, production yield decreases due to wafer breakage caused by sawing damage; thus, to further reduce wafer thickness, it is necessary to suppress sawing damage. We investigated the flexibility of wafers under various slice conditions by conducting biaxial bending tests and clarified the dominant factor causing sawing damage to further reduce the wafer thickness for crystalline silicon solar cells. The results of damage observation by scanning electron microscopy and evaluation of the crystal structure by Raman spectroscopy confirm that the damage structure changes significantly depending on wire specifications. The results from the biaxial bending tests indicate that the three-dimensional flexibility of a wafer is determined by wire specifications.

Development of multi-beam optics for time-resolved X-ray tomography: from π-polarization to σ-polarization

A multi-beam X-ray optics that can image a sample from different directions in a large angular range simultaneously without sample rotation is reported. It consists of 28 thin silicon crystals that are arranged in a σ-polarization diffraction geometry so that the diffracted X-rays from an incident white synchrotron radiation beam pass through the sample. A tomogram of a simple test sample was calculated from projection images recorded with the optics, showing that an alignment accuracy sufficient for X-ray tomography can be achieved.

Analysis of stresses and shape changes in thin substrates with stressed film patterning using femtosecond laser micromachining

The accurate, high-throughput fabrication of light weight optical systems for applications like next-generation space telescopes and semiconductor wafers is both crucial and challenging. A potential solution is to first fabricate thin substrates with traditional methods, then apply surface stress to bend them into desired shapes to correct residual height errors. We have developed a stress-based figure correction method for thin silicon mirrors using a femtosecond laser micromachining technique to generate patterned stress fields, through the removal of selective stressed film regions and adjacent substrate regions. In this paper, we present an in-depth analysis for the laser-induced stresses and resulting shape changes of thin mirrors due to the periodic patterning of a stressed film (silicon oxide) on silicon substrates using femtosecond laser surface ablation. Experimental results are presented, and a 3D finite element model (FEM) is developed to study the substrate curvature in directions parallel and perpendicular to the patterned troughs in the substrates. We simulate the stress relief process in the thin-film/substrate system, and the numerical predictions compare reasonably well with curvature measurements for several different geometrical combinations of depth, width and spacing of the patterned troughs, achieving >82% quantitative agreement with the experiments. It is also shown in the simulations that certain geometries of patterned troughs induce more dramatic shape changes and counter-intuitive reversals of curvature in the direction perpendicular to the troughs, and a qualitative explanation involving the Poisson effect is presented. The 3D finite element methods and findings of this paper can be used to determine the optimal parameters of the stressed film patterns for figure correction of thin telescope mirrors and other types of thin substrates used in the semiconductor industry.

Development of Heterojunction c-Si/a-Si1-xCx:H PIN Light-Emitting Diodes.

In this work, we explored the feasibility of the fabrication of PIN light-emitting diodes (LEDs) consisting of heterojunctions of amorphous silicon-carbide (a-Si1-xCx:H) thin films and crystalline silicon wafers (c-Si). The objective is the future development of electro-photonic systems in the same c-Si wafer, containing transistors, sensors, LEDs and waveguides. Two different heterojunction LEDs were fabricated consisting of PIN and PIN+N structures, where a-Si1-xCx:H thin films were used as P-type and I-type layers, while an N-type c-Si substrate was used as an active part of the device. The amorphous layers were deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique at a substrate temperature of 200 °C. The PIN device presented electroluminescence (EL) only in the forward bias, while the PIN+N device presented in both the forward and reverse biases. The EL in reverse bias was possible due to the addition of an N+-type a-Si:H layer between the c-Si substrate and the I-type a-Si1-xCx:H active layer. Likewise, the EL intensity of the PIN+N structure was higher than that of the PIN device in forward bias, indicating that the addition of the N-type a-Si:H layer makes electrons flow more efficiently to the I layer. In addition, both devices presented red EL in the full area, which is observed with the naked eye.

Sub-bandgap photoresponse and leakage current analysis in gold thin film-hyperdoped silicon photodiodes

We present a study of the sub-bandgap photoresponse and leakage current in gold-hyperdoped silicon photodiodes prepared using pulsed laser melting (PLM) of sub-nanometer gold films on n-type silicon substrates. Variable-temperature photo- and dark-current analysis provide insight into the role of PLM conditions on device performance. In general, we find photocurrent activation energies comparable to room temperature, suggesting a weak thermally-assisted optical photoresponse mechanism. Additionally, we establish a connection between repetitive PLM pulsing and increased device leakage current, which originates from electrically-active defects. Finally, we propose an explanation for the limited sub-bandgap external quantum efficiencies reported for hyperdoped silicon devices on the basis that the depletion layer largely does not encompass the hyperdoped layer where absorption occurs.

Breakage Ratio of Silicon Wafer during Fixed Diamond Wire Sawing.

Monocrystalline silicon is an important material for processing electronic and photovoltaic devices. The fixed diamond wire sawing technology is the first key technology for monocrystalline silicon wafer processing. A systematic study of the relationship between the fracture strength, stress and breakage rate is the basis for thinning silicon wafers. The external vibration excitation of sawing machine and diamond wire lead to the transverse vibration and longitudinal vibration for silicon wafers. The transverse vibration is the main reason of wafer breakage. In this paper, a mathematical model for calculating breakage ratio of silicon wafer is established. The maximum stress and breakage ratio for as-sawn silicon wafers are studied. It is found that the maximum amplitude of the silicon wafers with the size of 156 mm × 156 mm × 0.2 mm was 160 μm during the diamond wire sawing process. The amplitude, maximum stress and breakage rate of the wafers increased with the increase of the cutting depth. The smaller the silicon wafer thickness, the larger of silicon wafer breakage ratio. In the sawing stage, the breakage ratio of the 156 mm × 156 mm section with a thickness of 0.15 mm of silicon wafers is 6%.

A new approach to achieving high granularity for silicon diode detectors with impact ionization gain

Low Gain Avalanche Diodes (LGADs) are thin (20-50 μm) silicon diode sensors with modest internal gain (typically 5 to 50) and exceptional time resolution (17 ps to 50 ps). However, the granularity of such devices is limited to the millimeter scale due to the need to include protection structures at the boundaries of the readout pads to avoid premature breakdown due to large local electric fields. Here, we present a new approach – the Deep-Junction LGAD (DJ-LGAD) – that decouples the high-field gain region from the readout plane. This approach is expected to improve the achievable LGAD granularity to the tens-of-micron scale while maintaining direct charge collection on the segmented electrodes.

Femtosecond laser micromachining for stress-based figure correction of thin mirrors

The fabrication of a large number of high-resolution thin-shell mirrors for future space telescopes remains challenging, especially for revolutionary mission concepts such as NASA’s Lynx X-ray Surveyor. It is generally harder to fabricate thin mirrors to the exact shape than thicker ones, and the coatings deposited onto mirror surfaces to increase the reflectivity typically have high intrinsic stress that deforms the mirrors further. Since the rapid development of femtosecond laser technologies over the last few decades has triggered wide applications in materials processing, we have developed a mirror figure correction and stress compensation method using a femtosecond laser micromachining technique for stress-based surface shaping of thin-shell x-ray optics. We employ a femtosecond laser to selectively remove regions of a stressed film that is grown onto the back surface of the mirror, to modify the stress states of the mirror. In this paper, we present experimental results to create both isotropic and anisotropic stress states on thin flat silicon mirrors with thermal oxide ( S i O 2 ) films using femtosecond lasers. We show that equibiaxial stress can be generated through uniformly micromachined holes, while non-equibiaxial stress arises from the ablation of equally spaced troughs. We also present results from strength tests to show how this process minimally affects the strength of mirrors. These developments are beneficial to the high-throughput correction of thin-shell mirrors for future space-based x-ray telescopes.

Optical Polarization Sensor Based on a Metalens.

We investigated an optical microsensor of the polarization state of a laser light based on a metalens. In contrast to known polarization sensors based on metasurfaces that deflect different polarization types using various angles to the optical axis, the studied polarization sensor generated different patterns in the metalens focus to realize varied polarization states: left circular polarization generated a light ring in the focus, right circular polarization generated a circular focal spot, and linear polarization generated an elliptic spot with two sidelobes. Moreover, the tilt angle of the linear polarization matched the tilt angle of the elliptic focal spot. The simulation results were consistent with the theoretical predictions. A metalens with a diameter of several tens of microns was designed and fabricated in a thin amorphous silicon film with a thickness of 120 μm and a low aspect ratio, high numerical aperture, and short focal distance equal to a wavelength of 633 nm.

(Invited) Stretchable Sensing Devices Combining Ionic Liquids and Soft Electrodes

In recent years, a variety of ultra-flexible devices have been proposed. Their applications include wearable devices and soft robots. Among ultra-flexible devices, the devices with stretchablity is attracting attention as next-generation sensing devices. In such devices, ionic liquids can be used as a sensing material. Ionic liquids are polymers in a liquid state and their structure can be easily altered, For example, ionic liquids can be developed reactive to temperature, humidity, light, gases, and many other factors. Furthermore, since they are in a liquid state, they are durable against device stretching. In this study, we report on a stretchable sensing device using ionic liquids. In particular, by using liquid metal and CNT(Carbon nanotube) as electrode materials, we propose a super-stretchable device and a super-flexible device with transparency and high breathability.<Temperature/humidity/oxygen/optical sensors using liquid metal electrodes> liquid metal was used as electrode, and ionic liquids as a sensing material. Equivalent circuit of the sensors were established based on Nyquist plot The sensor showed stable sensitivity to temperature without hysteresis as shown with a 0.039/°C increase in conductivity, which is quite high compared to other reports. We also show proof of concept for humidity, oxygen gas, and optical sensings using four kinds of ionic liquids, 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][Otf]), 1-Butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPYR][NTf2]) and 1-butyl-3-(4-phenylazobenzyl)imidazolium bis(trifluoromethanesulfonyl)amide ([Azo][NTf2]) for optical sensing. The sensitivity of each ionic liquid to humidity and oxygen differed depending on the ionic liquid. For each type of stimuli, the sensing can be optimized by choosing the proper ionic liquid. In addition, using [Azo][NTf2], we demonstrate optical sensing and memory in this study.< Transparent and Breathable Ion Gel-based Sensors using CNT> highly transparent, ultra-flexible, and gas-permeable polymer thin-film sensors using ion gels as the sensing material, which demonstrated the capacity for selective detections, were proposed. Particularly, simultaneous and independent sensing of temperature and humidity was demonstrated in this study. The sensors were fabricated using a simple spray coating method on a thin silicone rubber film (around 25 µm thickness). Owing to their thin-film shape, they showed more than 80% visible light transmittance and a higher gas permeability of 58.7 g/m2 h than the human transepidermal water loss. Simultaneous and independent detection of temperature and humidity was achieved with a high sensitivity of 15.9%/°C and 2.5%/percentage of relative humidity, respectively, using two types of gels with ionic liquids.These results suggest that the easily modifiable nature of ionic liquids contribute to the development of stretchable electronics.

(Invited, Digital Presentation) Probing Triaxial Stress States Using Curvature-Based Methodology for Enabling Highly Robust and Reliable Integrated 3D Systems-on-Package in Advanced Microelectronics Industries

With the nascent of the systems-on-package in advanced 3D microelectronics, mechanical stress states in the system get much more complicated than in the past. Stress states are increasingly triaxial (as opposed to mostly biaxial stress states in the planar semiconductor devices), and with that, the roles of shear stresses in causing relatively new and unique mechanical damages and failures in the 3D stacks of chips and devices (memory, logic, sensors, actuators, data storage, etc.) cannot be undermined. The difficult stress management in a Through-Silicon Via (TSV) as the vertical interconnection between chips on stack has been an early indication of what more to come to the emerging, advanced microelectronics industries in the near, immediate future. Not only the mechanical damages that come into the picture, but also the delicate interrelation between the mechanical straining of the silicon and the corresponding electronic mobility affecting the very working of the semiconductor devices. The methods of stress determination used in the semiconductor industries so far (X-ray Diffraction, Raman Spectroscopy, etc.) have mostly worked under the assumptions of biaxial stress states which were mostly true in the planar semiconductor chip design. Such assumptions may no longer hold true for enabling highly robust and reliable integrated 3D Systems-on-Package in today’s and tomorrow’s microelectronics industries. In addition, once the 3D stacks of devices are fully integrated, it becomes next to impossible to manage the highly convoluted stress states originating from many silicon devices with mostly surface-based Raman techniques and low-penetration depth of conventional laboratory XRD. In this study, we propose to use the curvature-based approaches that would allow determination of the complete 3D stress states in every level of the stacking of silicon devices, and thus by following the stress state evolution during the stacking, the individual and overall resultant of the stress states in the highly integrated 3D systems may be analyzed. The approach originated from our previous work – on TSV for semiconductor chips as well as on thin silicon solar cells for the emerging photovoltaics industry – using synchrotron-based X-ray microdiffraction to determine the origin and evolution of the stress states in the devices (during thermal cycling or subsequent manufacturing steps). However, during those investigations, we realized that the diffraction science is not necessarily needed for highly accurate stress state determination, in fact, just the detection of the local curvature of the surface of the monocrystalline silicon in the devices would suffice to provide for equally highly accurate stress state determination. We have demonstrated the efficacy of this approach since in our work in silicon-based photovoltaics (Tippabhotla et al. Progress in Photovoltaics 2017). As long as we have the silicon surface from each level of devices (individually and after the stacking), a laser-based system could detect the change in local curvature in high spatial resolution and hence could provide with highly accurate stress state determination. No synchrotron radiation would be needed.

In situ TEM study of Ni-silicides formation up to 973K

Low-temperature solid-state reactions between Ni and Si were studied using in situ transmission electron microscopy (TEM). In the experiments thin amorphous silicon (a-Si) films were laid on Ni micro-grids and heated up to 973 K. In our approach the supporting Ni-grid serves as an unlimited source of nickel to successively form the whole range of Ni-silicide phases while diffusing into amorphous silicon. Unlike other thin film experiments where Ni and Si are layered on top of each other, our arrangement enables lateral diffusion of Ni along the Si layer and therefore enables the formation and study of successive Ni-Si phases side by side. That allowed us to observe in situ α-NiSi2 as the first reaction product, in contrast to most studies that had reported either δ-Ni2Si or θ-Ni2Si as the first phase to form. α-NiSi2 was continuously present at the reaction front propagating into the a-Si film. The phase sequence followed the increasing Ni concentration from a-Si towards the Ni-grid: α-NiSi2, NiSi, Ni3Si2, δ-Ni2Si, γ-Ni31Si12 and Ni3Si. Almost all known Ni-silicide phases were found to form at relatively low temperatures except the θ-Ni2Si, β-NiSi2 and β3-Ni3Si. The dominant phase was γ-Ni31Si12 which appeared in three structural modifications, differing in lattice periodicity along the c-axis. The periodicity of the basic γ-Ni31Si12 structure along the c-axis is ~12 Å (c0 = 12.288 Å) and that of the other two modifications were ~18 Å and ~36 Å, denoted by S12, S18 and S36 respectively. Of the three, only S12 has a structural model, S18 had been previously observed by Chen, but S36 had not been documented in previous works. During our in situ heating experiments, in addition to the Ni-silicide layer formation a new phenomenon was observed, namely the appearance, growth and transformation of Ni-silicide whiskers which was attributed to the accumulation of compressive stress in the thin layer.

Inter-pad dead regions of irradiated FBK Low Gain Avalanche detectors

Low Gain Avalanche Detectors (LGADs) are a type of thin silicon detector with a highly doped gain layer. LGADs manufactured by Fondazione Bruno Kessler (FBK) were tested before and after irradiation with neutrons. In this study, the inter-pad dead regions (IPDRs), defined as the width of the distances between pads, were measured with a TCT laser system. The response of the laser was tuned using β-particles from a 90Sr source. These insensitive “dead zones” are created by a protection structure to avoid breakdown, the Junction Termination Extension (JTE), which separates the pads. The effect of neutron radiation damage at 1.5 × 1015 neq/cm2, and 2.5 × 1015 neq/cm2 on IPDRs was studied. These distances are compared to the nominal distances given from the vendor, it was found that at the high fluence there is a better matching of the insensitive region to the designed inter-pad region.

Sparse multi-nanopore osmotic power generators

Ultrathin nanopore membrane is an emerging energy harvesting system capable of generating electricity from salinity gradients. Here, we report on the evaluation of its practical feasibility by exploring the energy conversion efficiency of single to densely packed multi-pores in a thin silicon nitride. The ionic current characteristics of single pores reveal a quasi-perfect cation selectivity when shrinking the diameter to 20 nm. The perm-selective nanopore is shown to yield osmotic power of 160 pW under a 1,000-fold transmembrane salt concentration difference. Meanwhile, whereas larger energy is gained by parallelly integrating multiple pores, excessive porosity also led to degraded energy conversion efficiency, thereby demonstrating an optimal power density of 100 W per square meter for 100 nm-sized multi-nanopores with a grid spacing of 1 μm. The present findings offer a guide to design highly efficient nanopore membrane osmotic power generators.

A 4.5-mW 22-nm CMOS Label-Free Frequency-Shift 3 × 3 × 2 3-D Biosensor Array Using Vertically Stacked 60-GHz LC Oscillators

In this brief, a label-free <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {3\times 3\times 2}$ </tex-math></inline-formula> three-dimensional frequency-shift biosensor array with vertically-stacked 60-GHz on-chip LC oscillators is presented. The 60-GHz LC oscillators function as sensor front-end since the frequency downward shifts due to dielectric derived from biomedical sensing targets. The proposed array element consists of the two vertically-stacked LC oscillators and 15-stage dividers necessary for extracting high-frequency output signals with a long distance. The proposed circuit was designed with Spectre RF simulation and the test chip was fabricated in 22-nm CMOS technology. The whole and element chip size is 0.04 mm2 and 0.0035 mm2 respectively. The measurement with thick and thin silicon resin samples successfully demonstrated their vertical and horizontal detection on the same chip while the prior works only achieved two-dimensional detection. The 22-nm CMOS prototype also showed high spatial resolution <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60 ~\mu {\mathrm{ m}} \times 60 ~\mu {\mathrm{ m}} \times 4 ~\mu {\mathrm{ m}}$ </tex-math></inline-formula> and high fill factor 73% while consuming 4.5 mW under 1.2 V per the single-element.

Acoustic wave amplification with thin film silicon bonded on lithium niobate

Signal processing with the use of acoustic waves is an important technology for various functions in radio frequency (RF) systems, including matched filtering in congested parts of the frequency spectrum. In order to generate long time delays on chip required for these applications, the acoustoelectric effect offers the ability to counter acoustic propagation losses while also generating inherent non-reciprocity. In this work, we demonstrate an approach to directly bond thin film silicon from 200 mm commercial silicon on insulator wafers on X-cut lithium niobate substrates with the use of plasma surface activation. The resulting delay line devices at 410 MHz demonstrate amplification of Rayleigh waves, with a peak non-reciprocal contrast between forward and reverse traveling waves of over 25 dB mm−1 under continuous direct current bias conditions. The demonstrated process can extend the functionality of traditionally passive piezoelectric RF microsystems.