Non-uniformity Of Film Research Articles

A comprehensive study of nonuniformity properties of the LiCoO2 thin-film cathode fabricated by RF sputtering

The influence of nonuniformity properties of the LiCoO2 cathode film deposited by magnetron sputtering on the capacity of all-solid-state thin-film lithium-ion batteries (ASSLib) was studied. It was found that the film nonuniformity corresponds to the magnetron plasma density distribution and the angular distribution of sputtered particles. The capacity distribution of the ASSLib with LiCoO2 cathode depending on the distance to the substrate center was studied. The maximum capacity corresponded to the dense part of the toroidal region of the magnetron plasma. It was determined that the main causes of batteries capacity decline in the central part and on the edge of the substrate are the impurity phase of lithium cobaltate and the smaller thickness of the cathode layer, respectively.

In-situ optimization of lateral film uniformity in PVD system by using Fiber Bragg grating temperature sensors

Fiber Bragg grating (FBG) based temperature sensing method has been employed in this work for measuring the lateral temperature distribution on substrate plane during pulse DC magnetron sputtering deposition for optimization of lateral film uniformity. The evolution of temperature distribution with the variation of important process parameters like pulse width (496–1616 ns), deposition pressure (3.1 × 10−3–1.9 × 10–2 mbar) and sputtering power (25–250 W) have been measured over 40 mm radial distance on glass substrate horizon. To investigate the effect of substrate height on the temperature distribution, the later has been measured at two different substrate heights (60 mm and 90 mm) for varying sputtering power. Finally, the effect of variation in temperature distribution on uniformity of thickness and optical, morphological and structural properties have been investigated by separately depositing two HfO2 thin films at two representative extreme deposition powers (75 W and 200 W). The correlation of film non-uniformity of the above properties with the temperature distribution suggests that FBG based multipoint temperature sensing can be possibly used as an indicative tool for in situ optimization of the lateral film uniformity. In addition, the fast response and workability in plasma environment of FBG sensors enables precise in situ mapping of temperature distribution in sputtering process.

Magnetron sputtering system for depositing boron carbide film use as neutron detection.

Boron carbide (B4C) films used as neutron conversion layers were investigated in this paper to replace the traditional 3He detectors due to their shortage. A magnetron sputtering system was developed for depositing large-size B4C films with the 1500 × 400mm2 uniform-area. B4C films at the micron scale were deposited on aluminum (Al), float glass (SiO2), and silicon (Si) substrates with an inserting adhesion layer. The key characteristics, including surface morphology, thickness nonuniformity, purity, and neutron efficiency of B4C films, were characterized using atomic force microscopy, scanning electron microscopy, grazing incidence x-ray reflectivity, x-ray photoelectron spectroscopy, and neutron radiation metrology. The experimental results indicate that the deposition thickness nonuniformity across a 1500 × 400mm2 area was better than ±3%. The stoichiometric ratio of boron atoms and carbon atoms (B/C) is 5.18, with 6 at. % O and 0.79 at. % N concentrations. The measured neutron detection efficiency of a 3 µm 10B4C film for 25meV neutrons was 3.3 ± 0.3(sys)%, which is close to the simulated results (3.4%). The results show that the B4C neutron conversion layer is a promising substitute for 3He for neutron detection in the future.

High-quality c-axis oriented Al(Sc)N thin films prepared by magnetron sputtering

In order to adapt to the upgrading of wireless communication system, thin film bulk acoustic resonator (FBAR) has become one of the research hotspots in the communication field. The piezoelectric film, such as aluminum nitride (AlN), is the core functional layer of this device. However, previous reports have only focused on specific characteristics of the piezoelectric film, such as preferred orientation, piezoelectric coefficient, residual stress and so on. In this work, AlN and 6% scandium doped AlN films with (002) preferred orientation were deposited on (100) silicon wafers by direct-current magnetron reactive sputtering. We studied the crystallinity, roughness, element distribution, stress and additional characteristics of the films, and compared the quality changes of AlN thin films doped with Sc. Our results showed that all the deposited films had a uniform and dense structure with high crystallinity. But, the doping of Sc significantly reduced the nonuniformity, roughness, and stress of AlN films. Our research is of great significance for the preparation of high-quality piezoelectric thin films to improve the performance of FBAR filters, and ultimately improve the yield of devices.

Influence of rectangular substrate chamfer on edge bead effect of a spin-coated thin film

Spin coating is a common method for fabricating thin films. The edge bead effect is a major contributor to thin film non-uniformities. This study investigates the influence of chamfer angles and widths of a rectangular substrate on the edge bead formation mechanism in spin-coated films. Through the use of volume-of-fluid simulations and experiments, it was determined that the chamfer angle had a significant impact on the edge bead effect, while the chamfer width was not found to be a major factor. The use of a synchronous chamber in spin coating was found to negatively affect film planarization by restricting solvent evaporation and elevating its concentration, leading to a decreased film thickness. Additionally, the study concluded that the edge effect is not impacted by the Bernoulli effect or liquid accumulation along the edge if the average film thickness is below 1500 nm. The main reason for reducing the height of the edge bead was determined to be liquid fusion at the edge of the substrate, which only occurred when the chamfer width was close to the film thickness.

Heterogeneous integration of lithium tantalate thin film on quartz for high performance surface acoustic wave resonator

Surface acoustic wave (SAW) resonator based on the piezo film on a foreign substrate is promising to enhance the performance of radio frequency filters. In this work, the 4 inch wafer-scale lithium tantalate thin film on quartz (LTOQ) heterogenous substrate was fabricated by ion-cut process. The cut angle of quartz was optimized to achieve high-quality factor (Q) based on finite element analysis. The average film thickness and the film nonuniformity for the whole wafer are 602 nm and ±2.2%, respectively. The lithium tantalate film exhibits single-crystalline quality where the full width at half-maximum of high-resolution X-ray diffraction rocking curve is 47.4 arcsec. The shear horizonal surface acoustic wave resonator based on the LTOQ structure exhibits a maximum Bode-Q exceeding 3000 and the electromechanical coupling coefficient of 10.26%. The temperature coefficient of frequency at resonant frequency and anti-resonant frequency are −25.21 ppm °C−1 and −35.22 ppm °C−1, respectively.

Lead Selenide Thin Films and Uncooled Midinfrared Detectors by Vapor Phase Deposition.

The broad application of lead selenide (PbSe)-based uncooled midinfrared (MIR) detectors has been hindered by the nonuniformity of wafer-level films prepared by the conventional chemical bath deposition (CBD) method. Herein, using a vapor phase deposition (VPD) approach, we demonstrate the deposition of 3 in. wafer-scale uniform PbSe thin films with thicknesses of up to 1.5 μm. To trigger the MIR response, the as-grown films were sensitized at an elevated temperature in an oxygen-iodine atmosphere. We discovered that the key to spark off the MIR response of the PbSe detector originated from the self-assembled rodlike microstructures in the thin films, which can be controlled by the I2/PbSe flux ratio in the VPD process. At room temperature, the thin film detector exhibits an excellent optoelectronic performance, with detectivity up to 2.4 × 109 cm Hz1/2 W-1 achieved under optimized conditions. Our results show that the VPD method opens up a new avenue to the industrialization of uncooled lead-salt MIR detectors.

Simulation of the Nucleation and Crystal Growth Process in the Laser-Induced Deposition in Solution by a Lattice Boltzmann Method.

A Lattice Boltzmann model is proposed, combining the theories of nucleation and crystal growth for the study of the laser-induced deposition in solution (LIDS). The conjugate heat transfer and the natural convection of the liquid precursor were simulated with the evolving interface of crystal growth. In turn, the morphology of the deposited materials was affected by multiple process parameters, including conditions of chemical precursor and the laser-induced heat and mass transfer. Simulation results indicated that the morphology of deposited materials was mostly affected by the initial concentration of the precursor solution. Specifically, the nonuniformity of thin films was caused by the convection induced by the pulsed-laser, and the surface roughness was due to the competition of local structures for the precursor supply. A relationship of process-condition-material was established, providing guidance of choosing various parameters in LIDS for a desirable morphology of deposited material, facilitating the capabilities of pulsed lasers in precise control in nanomanufacturing.

(Invited) SiGe/SOI System: Mechanisms of Condensation and Strain Relaxation

Silicon-Germanium strain engineering has been used for more than two decades in silicon based devices and has contributed to the scaling down of a transistor’s size and to significant improvements in device performance. However, while conventional silicon-germanium based electronics has experienced rapid and steady growth, thanks to this continuous miniaturization of transistors, this trend cannot continue indefinitely. Industry has already moved to alternate methods such as FinFET devices, in which a thin silicon channel is placed vertically, and the FD-SOI (FD-SGOI) design consisting of a thin film Si(SiGe) channel placed horizontally. For nodes scaled down below 28 nm, low power operation will be inherently hindered by both the imperfect interface, non-uniformity of ultra-thin films and quantum confinement effects, which increase the effective bandgap. In these devices, despite the intense research activity on the strained SiGe ultra-thin body, which accounts for a large portion of such microelectronic devices (below the 45 nm node), we still fail to properly understand the mechanisms that limit hole and electron mobilities in SGOI layers. In addition, one of the main challenges for Si based devices remains the fabrication of efficient group-IV photon sources / photon detectors compatible with the microelectronic industry, which would usefully replace the integration of III-V heterostructures on Si. The major bottleneck is that group-IV semiconductor elements have indirect bandgaps, but with possibilities of being transformed to direct bandgaps using strain engineering strategies.In this presentation, we will review the formation mechanism of Ge-rich layers on SOI by condensation at different temperatures. TEM cross-section and GPA analysis of the heterostructures will be presented. We will also report the physical and optical properties of these heterostructures. Special attention is devoted to the influence of the SiGe thickness reduction (up to few MLs), where quantum confinement is prominent in the optical properties of the layers. Raman and PL results will be presented to better explain such confinement behavior. We show that novel SGOI substrates could represent a key strategy for the fabrication of future photonic devices.

Effect of K Doping on the Performance of Aqueous Solution‐Processed Cu(In,Ga)Se2 Solar Cell

Copper indium gallium selenide Cu(In,Ga)Se2 (CIGS) solar cells fabricated by vacuum thermal evaporation, magnetron sputtering, or organic solvent solution‐process either lead to high manufacturing cost and film nonuniformity or cause explosion, toxication and pollution. To address these issues, herein, water as a solvent to prepare the CIGS precursor solution is used and 7.04% efficient CIGS solar cells without MgF2 antireflection layer is obtained. Furthermore, for the first time, K doping on CIGS films via adding “K+” into the aqueous precursor solution is conducted. K–In–Se species is formed in the process of selenization and acts as a Se reservoir during CIGS grain growth, resulting in improved quality of the CIGS film with much larger grains. K doping can also inhibit the loss of Ga element. The high‐bandgap K–In–Se phase and higher Ga content combine to result in the increase in the CIGS bandgap. As a result, the K‐doped CIGS solar cell achieves a 33.1% enhancement in efficiency to 9.37%, with the significant increase in the open‐circuit voltage from 520.5 to 597.1 mV and fill factor from 50.77 to 55.36%. These results provide an important preliminary foundation for the development of aqueous precursor solution‐based CIGS solar cells.

An inductively coupled plasma metal organic chemical vapor deposition based on showerhead structure for low temperature growth

An inductively coupled plasma metal organic chemical vapor deposition (ICP-MOCVD) based on showerhead structure is proposed for the low temperature growth of thin solid films including GaN. The flow field of precursors in the chamber of ICP-MOCVD was analyzed and the structure of showerhead was optimized by changing the showerhead diameter to obtain uniform velocity field above the substrate. The thickness non-uniformity of GaN films grown at 600 °C was improved from 5.14% to 1.86% after the optimization of showerhead. On that basis, the influence of triethylgallium (TEG) and trimethylgallium (TMG) on low-temperature GaN growth were investigated and TEG was proved to be the more appropriate Ga source in this case. Finally, GaN film with high c-axis and in-plane orientations was obtained on sputtered AlN/sapphire template and the full width half maximums of (002) and (102) x-ray rocking curves are 0.45° and 0.57° respectively. Our results provide a practicable method for the optimization of low-temperature MOCVD, which has potential to obtain large-scale crystalline films at low temperature.

The impact of residual stress on resonating piezoelectric devices

Resonating piezoelectric devices, such as aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducers (PMUTs), display superior performance to previous generations of resonating microelectromechanical systems (MEMS). However, the quality of the piezoelectric thin film can greatly impact operating characteristics, such as the resonant frequency. Several AlN PMUT devices fabricated on the same silicon wafer exhibited a range of resonance frequencies (400-600 kHz), indicating that there is nonuniformity across the processed wafer. AlN film nonuniformity is likely introduced during the reactive sputtering process. Two key parameters identified as influencing the resonance frequency include: (i) the membrane diameter and (ii) residual stress. This work focuses on the residual stress, and uses X-ray diffraction technique (XRD) to determine the in-plane biaxial residual stress values, as a function of die position on the wafer. Results show that there is a compressive stress gradient along the wafer, ranging from -357 MPa to -56 MPa. A plot of in-plane biaxial residual stress as a function of resonance frequency shows a relation between the measured stress and frequency. As resonating piezoelectric devices require well defined operating frequencies and bandwidths, this work demonstrates the importance of studying not only global stresses, but also local residual stresses.

(Invited) New Strategies to Produce Light Emitters with Si-based Nanostructures

Silicon-Germanium strain engineering has been used for more than two decades in silicon based devices and has contributed to the scaling down of a transistor’s size and to significant improvements in device performance. However, while conventional silicon-germanium based electronics has experienced rapid and steady growth, thanks to this continuous miniaturization of transistors, this trend cannot continue indefinitely. Industry has already moved to alternate methods such as FinFET devices, in which a thin silicon channel is placed vertically, and the FD-SOI (FD-SGOI) design consisting of a thin film Si(SiGe) channel placed horizontally. For nodes scaled down below 28 nm, low power operation will be inherently hindered by both the imperfect interface, non-uniformity of ultra-thin films and quantum confinement effects, which increase the effective bandgap. In these devices, despite the intense research activity on the strained SiGe ultra-thin body, which accounts for a large portion of such microelectronic devices (below the 45 nm node), we still fail to properly understand the mechanisms that limit hole and electron mobilities in SGOI layers. In addition, one of the main challenges for Si based devices remains the fabrication of efficient group-IV photon sources / photon detectors compatible with the microelectronic industry, which would usefully replace the integration of III-V heterostructures on Si. The major bottleneck is that group-IV semiconductor elements have indirect bandgaps, but with possibilities of being transformed to direct bandgaps using strain engineering strategies.In this presentation, we will review the formation mechanism of Ge-rich layers on SOI by condensation at different temperatures. TEM cross-section and GPA analysis of the heterostructures will be presented. We will also report the physical and optical properties of these heterostructures. Special attention is devoted to the influence of the SiGe thickness reduction (up to few MLs), where quantum confinement is prominent in the optical properties of the layers. Raman and PL results will be presented to better explain such confinement behavior. We show that novel SGOI substrates could represent a key strategy for the fabrication of future photonic devices.

Effect of pH treatment on the structural and optical properties of Sn6Sb10S21 thin films facilely synthesized using a spin coating method

This study aimed to examine the effect of pH value for the Sn–Sb–S solution on the growth of tin antimony sulfide (TAS) thin films facilely synthesized and deposited on fluorine-doped tin oxide or borosilicate slide glass substrates using a spin coating method. Field-emission scanning electron microscopy (FE-SEM) micrographs confirmed that some aggregated NPs, the distribution of residuals and larger-sized clusters or non-uniform surfaces of thin films on the substrate surface were significantly affected by pH alteration. XRD analysis revealed that the most intense peak of TAS thin films exhibited a monoclinic Sn6Sb10S21 crystal structure with an orthorhombic Sn5Sb2S9 phase. However, the presence of a binary Sb2S3 phase was also observed; its peak intensity gradually reduced with increasing pH value. In addition, the investigation revealed that change in the structural and optical parameters under the alteration of the pH treatment in a solution had a significant impact on the formation, nucleation and growth of the clusters with increasing residuals and non-uniformity of thin film. It could be concluded from the overall results of our demonstration that TAS thin films with adjusted pH treatment can be attributed to the growth of TAS thin films for use as window layers in solar cell devices and may have potential for use in photodetector, optical communication, and optical dispersion devices in the future.

Effect of thickness non-uniformity of large-area grating metal film on grating diffraction wavefront

As the grating area increases, the non-uniformity of the metal film thickness becomes one of the main factors affecting both the grating surface error and the wavefront of the grating diffraction order. We study the theoretical model of the relationship between the metal film thickness non-uniformity and the grating diffraction wavefront error, analyze the effects of the relevant factors on the thickness of the metal films, and determine the values of these factors using theoretical and experimental methods. We also perform a ruling experiment to verify that the wavefront quality of the grating increases with improvement in the film thickness uniformity.

Highly efficient organic-inorganic hybrid perovskite quantum dot/nanocrystal light-emitting diodes using graphene electrode and modified PEDOT:PSS

Perovskite quantum dot (PQD)/nanocrystal (NC) light-emitting diodes (LEDs) have rapidly developed in last several years due to excellent optoelectronic properties of lead halide perovskites. However, the PDQ/NC LEDs using graphene electrodes haven't been reported despite of huge potential for applications in flexible displays and lighting sources. Herein, graphene is first used as electrodes of PQD and NC LEDs. To overcome film nonuniformity induced by graphene's hydrophobicity, a modification to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with dimethyl sulfoxide (DMSO) and Triton X-100 codoping is reported, which effectively improves the wettability of graphene surface and sequent film quality. More importantly, the synergistic effect of DMSO and Triton X-100 alters the PEDOT:PSS morphology from a coiled structure to a nanofibril conductive network, sufficiently enhancing the electrical conductivity of PEDOT:PSS. With this modification strategy, two kinds of green LEDs with CH3NH3PbBr3 PQD and FA0.8Cs0.2PbBr3 (FA = CH(NH2)2) NC emission layer are successfully fabricated on the graphene anodes, with 4.4 and 7.4 fold enhancements on current efficiency compared to the counterpart without PEDOT:PSS modification, respectively. The film modification strategy and graphene-based PQD/NC LEDs in this work are expected to shed light on further design and manufacture of flexible highly efficient PQDs/NCs display and lighting devices.

Simulations of film stress effects on mirror segments for the Lynx X-ray Observatory concept

The Lynx X-ray Observatory concept, under study for the 2020 NASA Decadal Survey, will require a telescope with ∼2 m2 of effective area and a point-spread function (PSF) with ∼0.5-arc sec half-power diameter (HPD) to meet its science goals. This requires extremely accurate thin grazing-incidence mirrors with a reflective x-ray coating. A mirror coating, such as 15-nm-thick iridium, can exhibit stress exceeding 1 GPa, significantly deforming segmented mirrors and blurring the PSF. The film stress and thickness are neither perfectly repeatable nor uniform. We use finite element analysis and ray tracing to quantify the effects of integrated stress inaccuracy, nonrepeatability, nonuniformity, and postmounting stress changes on segmented mirrors. We find that if Lynx uses segmented mirrors, it will likely require extremely small film stress (∼10 MPa) and nonuniformity (<1 % ). We show that realigning mirrors and matching complementary mirror pairs can reduce the HPD from uniform film stress by a factor of 2.3 × and 5 × , respectively. Doubling mirror thickness produces much less than the 4 × HPD reduction that would be expected from a flat mirror. The x-ray astronomy community has developed numerous methods of reducing the PSF blurring from film stress, and Lynx may require several of these in combination to achieve 0.5 arc sec HPD using segmented mirrors.

Technique for Calculating the Critical Current of Inhomogeneous Superconducting Films

A method for calculating the critical current of an inhomogeneous superconducting film (plate) in different external magnetic fields has been proposed. The superconducting properties are changed by varying the coherence length and London penetration depth over the thickness. The coherence length is maximum at the center of the plate and decreases upon approaching its boundaries and the London depth, on the contrary, is minimum at the center of the plate and increases toward its boundaries. Using the proposed approach, the magnetic field dependences of the critical current on the external magnetic field for the cases of nonuniform and uniform distributions of the superconducting properties over the film thickness have been calculated and compared. It has been established that at the nonuniform distribution of the superconducting properties the critical current of the plate is higher than at the uniform distribution at the same external magnetic fields. This is due to the fact that the order parameter is redistributed over the inhomogeneous plate thickness in such a way that the superconducting state becomes more stable against the current and magnetic field. It has been shown that the vortex-free Meissner state in inhomogeneous films is maintained at higher fields than in homogeneous films. The greater the film nonuniformity, the higher the stability of the Meissner state against an external magnetic field.

Organic-inorganic hybrid perovskite quantum dot light-emitting diodes using a graphene electrode and modified PEDOT:PSS.

Perovskite quantum dot (PQD) light-emitting diodes (LEDs) have rapidly developed in the past several years due to the excellent optoelectronic properties of lead halide perovskites. However, PQD LEDs using graphene electrodes have not been reported despite their huge potential for applications in flexible displays and lighting sources. Herein, graphene was first used as the electrode of PQD LEDs. To overcome graphene's limitations such as hydrophobicity and graphene-induced film nonuniformity, the modification of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with Triton X-100 and dimethyl sulfoxide (DMSO) codoping was reported, which not only improved the wettability of the graphene surface and the sequent film quality, but also reduced the dissolution of the PQD solvent to the bottom poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine] and PEDOT:PSS. More importantly, the synergistic effect of Triton X-100 and DMSO altered the PEDOT:PSS morphology from a coiled structure to a nanofibril conductive network, sufficiently enhancing the electrical conductivity of PEDOT:PSS. With this modification strategy, green PQD LEDs with CH3NH3PbBr3 emission layers were successfully fabricated on graphene anodes, with 3.7- and 4.4-fold enhancements in luminance and current efficiency, respectively, compared to those of their counterparts without PEDOT:PSS modification. The film modification strategy and graphene-based PQD LEDs in this work are expected to shed light on the further design and manufacture of flexible highly efficient PQD display and lighting devices.

Метод расчета критического тока неоднородных сверхпроводящих пленок

AbstractA method for calculating the critical current of an inhomogeneous superconducting film (plate) in different external magnetic fields has been proposed. The superconducting properties are changed by varying the coherence length and London penetration depth over the thickness. The coherence length is maximum at the center of the plate and decreases upon approaching its boundaries and the London depth, on the contrary, is minimum at the center of the plate and increases toward its boundaries. Using the proposed approach, the magnetic field dependences of the critical current on the external magnetic field for the cases of nonuniform and uniform distributions of the superconducting properties over the film thickness have been calculated and compared. It has been established that at the nonuniform distribution of the superconducting properties the critical current of the plate is higher than at the uniform distribution at the same external magnetic fields. This is due to the fact that the order parameter is redistributed over the inhomogeneous plate thickness in such a way that the superconducting state becomes more stable against the current and magnetic field. It has been shown that the vortex-free Meissner state in inhomogeneous films is maintained at higher fields than in homogeneous films. The greater the film nonuniformity, the higher the stability of the Meissner state against an external magnetic field.