Controlled Spalling Technology Research Articles

Excellent HZO ferroelectric thin films on flexible PET substrate

The development of flexible memory devices has been attracting more and more interests to advance next-generation lightweight and wearable electronics. Though HfO2-based ferroelectric thin film has shown promising future for the mass-production of low-cost ferroelectric memory, the fabrication of HfO2-based ferroelectric devices on thermosensitive flexible substrates has been a practical challenge due to the necessity of a high temperature thermal process to reach its potential of ferroelectric high-performance. In this work, a flexible Pt/W/Hf0.5Zr0.5O2/W/Pt multilayer stack with excellent ferroelectric properties is fabricated on PET by a controlled spalling technology, which is simple, reliable and cost-efficient. AFM characterizations proved that multilayer film exhibited a smooth surface after spalling process, and the e-testing results showed that Hf0.5Zr0.5O2 film did not show obvious deterioration in ferroelectric properties during spalling and transfer. Furthermore, the flexible Hf0.5Zr0.5O2 ferroelectric films maintained superior polarization, retention, and endurance performance under a series of bending deformations and cycling tests, indicating its outstanding mechanical resilience. This work is of great significance for the practical application of new HfO2-based thin film flexible ferroelectric memory in wearable electronics.

Crystalline Silicon Spalling as a Direct Application of Temperature Effect on Semiconductors’ Indentation

Kerf-less removal of surface layers of photovoltaic materials including silicon is an emerging technology by controlled spalling technology. The method is extremely simple, versatile, and applicable to a wide range of substrates. Controlled spalling technology requires a stressor layer, such as Ni, to be deposited on the surface of a brittle material; then, the controlled removal of a continuous surface layer can be performed at a predetermined depth by manipulating the thickness and stress of the Ni layer, introducing a crack near the edge of the substrate, and mechanically guiding the crack as a single fracture front across the surface. However, spalling Si(100) at 300 K (room temperature RT) introduced many cracks and rough regions within the spalled layer. These mechanical issues make it difficult to process these layers of Si(100) for PV, and in other advanced applications, Si does not undergo phase transformations at 77 K (Liquid Nitrogen Temperature, LNT); based on this fact, spalling of Si(100) has been carried out. Spalling of Si(100) at LNT improved material quality for further designed applications. Mechanical flexibility is achieved by employing controlled spalling technology, enabling the large-area transfer of ultrathin body silicon devices to a plastic substrate at room temperature.

Flexible PLZT antiferroelectric film capacitor for energy storage in wide temperature range

Flexible energy-storage capacitor has attracted great attention due to deformable and lightweight, which could be applied to wearable electronics, bendable smartphones. It is urgent to manufacture a capacitor combining excellent electrical properties with mechanical bending capacity. Here, flexible Pb0.97La0.02Zr0.95Ti0.05O3 (PLZT 2/95/5) films are prepared by controlled spalling technology (CST) and exhibit outstanding electrical properties with polarization about 86.3 µC/cm2 at various tensile or compressive bending radii, which indicates excellent mechanical stability of films. It is worth mentioning that recoverable energy-storage density (Wre) of films has increased by 21% at 500 kV/cm compared to the original substrate, which is caused by change in stress. Meanwhile, flexible films were measured under wide temperature range from −70–200 °C with Wre fluctuating from 9.2 J/cm3 to 17.1 J/cm3, which is related to free energy barrier of phase transition. These studies provide reference for preparation of flexible capacitors which have great potential applications for future flexible micro-devices.

Flexible heterogeneous integration of PZT film by controlled spalling technology

Because of high temperature process, it has been a challenge to fabricate flexible piezoelectric or ferroelectric devices based on PbZr0.53Ti0.47O3 (PZT). In this work, a high-quality flexible PZT film is successfully fabricated by the controlled spalling technology (CST). The PZT film deposited on sapphire substrate using sol-gel method is transferred onto a flexible polyimide (PI) substrate through electroplating Ni stressor layer, realizing the flexible heterogeneous integration of PZT film and PI substrate. The obtained PZT film exhibits a good crystalline perovskite structure and a smooth surface before and after spalling process. The ferroelectric response of the spalled PZT film based on Au/PZT/Au structure does not show obvious deterioration under different mechanical deformation, which indicates good ferroelectric property and stable mechanical performance. Further, the film on flexible Ni layer has the better piezoelectric property compared with that on conventional rigid Pt/Ti/SiO2/Si substrate. This work has the important significance for the development of flexible ferroelectric and piezoelectric devices like sensors, memorizers and nanogenerators.

Controlled spalling and flexible integration of PZT film based on LaNiO3 buffer layer

As a functional ferroelectric material, PbZr0.53Ti0.47O3 (PZT) film is widely used in many electronic devices. However, with the rapid development of intelligent terminal, traditional PZT device cannot meet the demand for flexible wearable devices. In this work, a high-quality flexible PZT film is fabricated by the controlled spalling technology (CST) for the first time, which is novel, simple and cost-efficient. LaNiO3 (LNO) film by sol-gel process is used as the buffer layer for the PZT film growth due to good lattice match and low cost. The PZT film on silicon substrate is successfully transferred onto a flexible one by electroplating Ni stressor layer. LNO buffer layer thickness has an important influence on the spalled silicon depth and Ni stressor layer thickness during the spalling process. The exfoliated PZT film based on sandwich capacitor structure Au/PZT/Au with an area of 1 cm2 exhibits good ferroelectric properties under different mechanical deformation. Moreover, the ferroelectric response of flexible PZT film does not show obvious deterioration after repeated operation cycles. Therefore, the flexible PZT thin film has a good application prospect on wearable ferroelectric microelectronics.

Impact of process parameters on subsurface crack growth in brittle materials grinding

The main failure mechanism of brittle materials occurs through the initiation and propagation of cracks. Researches involved with various loading modes and material defects have been widely investigated to control the stability of subsurface crack. However, no detailed fracture mechanics analysis has been published to understand the direct effect of process parameters on crack growth. In this paper, taking the plastic deformation below the tool and the intrinsic line defect located at the plastic zone boundary into account, a mechanical and numerical study of the fracture mechanics is proposed from the perspective of process parameters in grinding of brittle materials. The stress intensity factors are computed in detail to analyze the various impacts of process parameters and tool geometry on the subsurface crack propagation. Results indicate that the main fracture mode for median crack induced in brittle material grinding is opening rather than shear. Although the residual stress caused by plastic zone plays an important role in fracture behavior, the effect of dislocations cannot be ignored as well. In addition, the starting point of opening fracture is also affected by grinding parameters and tool geometry. A small grinding speed, a sharp large tool, a large table speed and grinding depth will lead to strong anti-shielding effect on mode I crack propagation and strong shielding effect on mode II crack propagation. The results can be used to provide guidance for the development of controlled spalling technology which enables the reuse of cracking substrate.

61.3: High Performance Organic Light‐Emitting‐Diodes on Graphene Electrode and Thin c‐Si TFT for Flexible Display and Lighting

AbstractWe demonstrate high performance OLEDs with transparent graphene electrode and thin single crystal Si TFT. Green OLEDs on graphene exhibit power efficiency of >150lm/W at 10,000cd/m2 with external light extraction. OLEDs are also integrated with 60nm‐thick flexible single crystal Si TFT made by controlled spalling technology.

Layer transfer by controlled spalling

In this communication, we present what may be the simplest method yet devised for removing surface layers from brittle substrates. The process is called controlled spalling technology (CST) and works by depositing a tensile stressor layer on the surface of a substrate, introducing a crack near the edge of the substrate, and mechanically guiding the crack as a single fracture front across the surface. The entire process is performed at room-temperature using only common laboratory equipment. We present here, for the first time, the specific process conditions required for controlled spalling of Ge 〈0 0 1〉 substrates using Ni as the stressor layer. We also illustrate the versatility of CST by removing completed CMOS circuits from a Si wafer and demonstrate functionality of the flexible circuits. Raman spectroscopy of spalled circuits with the Ni stressor intact indicates a residual compressive Si strain of 0.0029, in good agreement with the calculated value of 0.0022. Therefore, CST also permits new opportunities for strain engineering of nanoscale devices.

(Invited) Cost-Effective Layer Transfer by Controlled Spalling Technology

Although historically studied as a failure mode, substrate spalling can be transformed into a versatile layer transfer method by carefully controlling the thickness and intrinsic stress of a surface layer, and mechanically guiding the fracture path. This Controlled Spalling process requires no specialized equipment and can be applied to essentially any brittle substrate. We have successfully demonstrated i) device fabrication in Si, Ge, and III-V based materials, ii) removal of fully-processed CMOS circuits, iii) kerf-free ingot slicing, iv) 300 mm diameter layer transfer and v) a wide range of other materials such as GaN grown on sapphire.

Extremely Flexible Nanoscale Ultrathin Body Silicon Integrated Circuits on Plastic

In recent years, flexible devices based on nanoscale materials and structures have begun to emerge, exploiting semiconductor nanowires, graphene, and carbon nanotubes. This is primarily to circumvent the existing shortcomings of the conventional flexible electronics based on organic and amorphous semiconductors. The aim of this new class of flexible nanoelectronics is to attain high-performance devices with increased packing density. However, highly integrated flexible circuits with nanoscale transistors have not yet been demonstrated. Here, we show nanoscale flexible circuits on 60 Å thick silicon, including functional ring oscillators and memory cells. The 100-stage ring oscillators exhibit the stage delay of ~16 ps at a power supply voltage of 0.9 V, the best reported for any flexible circuits to date. The mechanical flexibility is achieved by employing the controlled spalling technology, enabling the large-area transfer of the ultrathin body silicon devices to a plastic substrate at room temperature. These results provide a simple and cost-effective pathway to enable ultralight flexible nanoelectronics with unprecedented level of system complexity based on mainstream silicon technology.

Ultralight High‐Efficiency Flexible InGaP/(In)GaAs Tandem Solar Cells on Plastic

Ultralight flexible InGaP/(In)GaAs tandem solar cells with conversion efficiency of 28.1% are demonstrated using our novel layer transfer technique, called controlled spalling. Excellent stability of the flexible solar cell characteristics under different bending conditions in conjunction with its remarkably high specific power of 1995 W/kg indicates the prospects of the controlled spalling technology for enabling a viable high-efficiency flexible photovoltaic technology.

Cost-effective layer transfer by Controlled Spalling Technology

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Kerf-Less Removal of Si, Ge, and III–V Layers by Controlled Spalling to Enable Low-Cost PV Technologies

Kerf-less removal of surface layers of photovoltaic materials including silicon, germanium, and III-Vs is demonstrated by controlled spalling technology. The method is extremely simple, versatile, and applicable to a wide range of substrates. Controlled spalling technology requires a stressor layer, such as Ni, to be deposited on the surface of a brittle material, and the controlled removal of a continuous surface layer could be performed at a predetermined depth by manipulating the thickness and stress of the Ni layer. Because the entire process is at room temperature, this technique can be applied to kerf-free ingot dicing, removal of preformed p-n junctions or epitaxial layers, or even completed devices. We successfully demonstrate kerf-free ingot dicing, as well as the removal of III-V single-junction epitaxial layers from a Ge substrate. Solar cells formed on the spalled and transferred single-junction layers showed similar characteristics to nonspalled (bulk) cells, indicating that the quality of the epitaxial layers is not compromised as a result of spalling.

High-efficiency thin-film InGaP/InGaAs/Ge tandem solar cells enabled by controlled spalling technology

In this letter, we demonstrate the effectiveness of the controlled spalling technology for producing high-efficiency (28.7%) thin-film InGaP/(In)GaAs/Ge tandem solar cells. The controlled spalling technique was employed to separate the as-grown solar cell structure from the host Ge wafer followed by its transfer to an arbitrary Si support substrate. The structural and electrical properties of the thin-film tandem cells were examined and compared against those on the original bulk Ge substrate. The comparison of the electrical data suggests the equivalency in cell parameters for both the thin-film (spalled) and bulk (non-spalled) cells, confirming that the controlled spalling technology does maintain the integrity of all layers in such an elaborate solar cell structure.