Wafer Samples Research Articles

An Alternate Approach to Backside Thinning: A Doping Selective Silicon Wet Etch

Backside illuminated technologies have been used in complementary metal–oxide–semiconductor image sensors for years and are finding a renewed interest and are being investigated for imagers and detectors in light detection and ranging applications. Coupled with the improved performance facilitated through 3D integration, the thinning of bonded wafers has become a crucial processing step. In this study, an alternate and potentially more cost-effective method has been explored using an HNA (hydrofluoric, nitric, acetic)wet etch for backside thinning of bonded epitaxial P+/P- silicon wafers. By utilizing a dopant concentration gradient, P+ silicon is selectively etched and P- silicon can be effectively used as an etch stop. Several HNA concentrations were explored to determine ratios that yield high P+ silicon etch rates while maintaining high P+/P- selectivity. Furthermore, bath life, spiking scheme, and repeatability were examined using a large wafer sample size with the goal of developing a full process flow.

Variation of Surface Nanostructures on (100) PbS Single Crystals during Argon Plasma Treatment

The nanostructuring of the (100) PbS single crystal surface was studied under varying argon plasma treatment conditions. The initial PbS single crystals were grown by high-pressure vertical zone melting, cut into wafer samples, and polished. Subsequently, the PbS single crystals were treated with inductively coupled argon plasma under varying treatment parameters such as ion energy and sputtering time. Plasma treatment with ions at a minimum energy of 25 eV resulted in the formation of nanotips with heights of 30–50 nm. When the ion energy was increased to 75–200 eV, two types of structures formed on the surface: high submicron cones and arrays of nanostructures with various shapes. In particular, the 120 s plasma treatment formed specific cruciform nanostructures with lateral orthogonal elements oriented in four <100> directions. In contrast, plasma treatment with an ion energy of 75 eV for 180 s led to the formation of submicron quasi-spherical lead structures with diameters of 250–600 nm. The nanostructuring mechanisms included a surface micromasking mechanism with lead formation and the vapor–liquid–solid mechanism, with liquid lead droplets acting as self-forming micromasks and growth catalysts depending on the plasma treatment conditions (sputtering time and rate).

Measurements of spectral emissivity, reflectance and transmittance at high temperatures using laser heating and auxiliary light source

A multi-wavelength measurement method for the high-temperature radiation properties of semi-transparent materials heated by high power laser and irradiated by the infrared source with alternating spectral distributions is introduced. The method avoids the strong dependence of existing methods on accurate temperature measurements or emissivity model of semi-transparent samples. The spectral emissivity, reflectance, transmittance and temperature of semi-transparent samples can be determined simultaneously. Furthermore, we established the experimental device, including a FTIR spectrometer (spectral range of 2.0∼16 μm), a 500 W 915 nm fiber laser, and an infrared SiC emitter irradiation source. The radiation properties of the oxidized stainless-steel sample (opaque materials) and n-type, phosphorous-doped silicon wafer sample (semi-transparent materials) were investigated experimentally at different temperatures (400∼800 °C). With the increase of laser power, the upper limit of sample temperature can be extended to higher temperatures. The measurement results show that the infrared spectral emissivity of stainless-steel increases with the increase of temperature. The spectral emissivity of semi-transparent n-type, phosphorous-doped silicon wafer increases with the increase of temperature, while the spectral directional-hemispherical reflectance decreases with the increase of temperature. The results of temperature and wavelength dependent emissivity of silicon wafer sample are consistent with the literatures, which verifies the applicability of the device. The research work provides a useful reference for measuring the high-temperature radiation properties of semi-transparent materials.

Current tuned slotted Y‐branch laser for wafer thickness measurements with THz radiation

A slotted Y-branch laser diode that emits two spectral modes at a difference frequency of about 1 THz is investigated in the context of photonic based cw-THz measurements. The beating frequency of the emitted laser light can be tuned by ±10.5 GHz around 1 THz by changing the applied laser current, which allows for potentially fast measurements. A second spectral window of ±6.5 GHz was found at 850 GHz. Pointwise scanning of the difference frequency is demonstrated with thickness determination of HRFZ-Si wafer samples as a possible application scenario.

Measurement of Nanometre-Scale Gate Oxide Thicknesses by Energy-Dispersive X-ray Spectroscopy in a Scanning Electron Microscope Combined with Monte Carlo Simulations.

A procedure based on energy-dispersive X-ray spectroscopy in a scanning electron microscope (SEM-EDXS) is proposed to measure ultra-thin oxide layer thicknesses to atomic scale precision in top-down instead of cross-sectional geometry. The approach is based on modelling the variation of the electron beam penetration depth and hence the depth of X-ray generation in the sample as a function of the acceleration voltage. This has been tested for the simple case of silica on silicon (SiO2/Si) which can serve as a model system to study gate oxides in metal-on-semiconductor field-effect transistors (MOS-FETs). Two possible implementations exist both of which rely on pairs of measurements to be made: in method A, the wafer piece of interest and a reference sample (here: ultra-clean fused quartz glass for calibration of the effective k-factors of X-ray lines from elements O and Si) are analysed at the same acceleration voltage. In method B, two measurements of the apparent O/Si ratio of the same wafer sample need to be made at different acceleration voltages and from their comparison to simulations the SiO2 layer thickness of the sample can be inferred. The precision attainable is ultimately shown to be limited by surface contamination during the experiments, as very thin carbonaceous surface layers can alter the results at very low acceleration voltages, while the sensitivity to ultra-thin surface oxides is much reduced at higher acceleration voltages. The optimal operation voltage is estimated to lie in the range of 3–15 kV. Method A has been experimentally verified to work well for test structures of thin oxides on Si-Ge/Si.

Improvement in the Output Power of Near-Ultraviolet LEDs of p-GaN Nanorods through SiO2 Nanosphere Mask Lithography with the Dip-Coating Method.

In this paper, the conditions of the dip-coating method of SiO2 nanospheres are optimized, and a neatly arranged single-layer SiO2 array is obtained. On this basis, a “top-down” inductively coupled plasma (ICP) technique is used to etch the p-GaN layer to prepare a periodic triangular nanopore array. After the etching is completed, the compressive stress in the epitaxial wafer sample is released to a certain extent. Then, die processing is performed on the etched LED epitaxial wafer samples. The LED chip with an etching depth of 150 nm has the highest overall luminous efficiency. Under a 100 mA injection current, the light output power (LOP) of the etched 150 nm sample is 23.61% higher than that of the original unetched sample.

Reflectivity of VUV-sensitive silicon photomultipliers in liquid Xenon

Silicon photomultipliers are regarded as a very promising technology for next-generation, cutting-edge detectors for low-background experiments in particle physics. This work presents systematic reflectivity studies of Silicon Photomultipliers (SiPM) and other samples in liquid xenon at vacuum ultraviolet (VUV) wavelengths. A dedicated setup at the University of Münster has been used that allows to acquire angle-resolved reflection measurements of various samples immersed in liquid xenon with 0.45° angular resolution. Four samples are investigated in this work: one Hamamatsu VUV4 SiPM, one FBK VUV-HD SiPM, one FBK wafer sample and one Large-Area Avalanche Photodiode (LA-APD) from EXO-200. The reflectivity is determined to be 25–36 % at an angle of incidence of 20° for the four samples and increases to up to 65 % at 70° for the LA-APD and the FBK samples. The Hamamatsu VUV4 SiPM shows a decline with increasing angle of incidence. The reflectivity results will be incorporated in upcoming light response simulations of the nEXO detector.

A minimally invasive electrostatic particle extractor for nanodusty plasmas and its application for the verification of in situ Mie polarimetry

We present an electrostatic particle extractor system (EPEX) which extracts particles from a reactive, particle-growing plasma by using the electrostatic force. The system is able to extract eight samples during a single growth cycle without the need to interrupt the plasma process. Using video microscopy, imaging-Mie, and Mie polarimetry we demonstrate the minimal invasiveness of EPEX. The extraction voltage and its duration are optimized to tune the particle density on the sample wafer. Compared to other invasive extraction methods, EPEX is fast and reliable. Using atomic force microscopy the size of the particles can be determined from samples extracted from the nanodusty plasma during one growth cycle and can be compared to the results of simultaneously performed in situ Mie polarimetry for the first time. We show that the extraction process can be divided in three distinct phases and that its physics can be understood in the simple framework of the force balance for a single dust particle. Using this single particle model, we present a new method for determining the particle charge of dust particles in dense dusty plasmas. The determined particle charge of (−9.2 ± 2.8) elementary charges reveals a strong Havnes effect.

Effect of rotational speed ratio between platen and work piece on lapping processes

Process parameters that are addressed in literature to affect lapping process includes, abrasive grain size, types of abrasive, concentration of abrasives, viscosity of the compound, lapping pressure, lapping speed, lapping time, and lap material/disc material. However, a trajectory analysis of a single fixed abrasive particle shows various paths for different rotational speed ratio between lapping plate and work piece. Therefore, the objective of this paper is to investigate the effect of rotational speed ratio on conventional loose abrasive lapping process. In the study, kinematic trajectories of single fixed abrasive particle were presented for different rotational speed ratios between lapping plate and work piece. Then, experimental study was conducted on sliced silicon wafer samples under different rotational speed ratios and the MRR and surface roughness of each sample was measured. The result reveals that rotational speed ratio between the lapping plate and work piece has an effect on favoring slurry transport between plate and work piece which affects the performance of material removal rate. The future focus of study would be to study optimal rotational speed ratio as one process parameter.

Cryogenic cleaning of tin-drop contamination on surfaces relevant for extreme ultraviolet light collection

Improvement of tool reliability and uptime is a current focus in the development of extreme ultraviolet lithography. The lifetime of collection mirrors for extreme ultraviolet light in tin-based plasma light sources is limited considerably by contamination with thick tin deposits that cannot be removed sufficiently fast by plasma etching. For tin droplet splats sticking to large substrates, we have developed and compared several efficient cleaning techniques based on cryogenic cooling. A silicon carbide substrate and different silicon wafer samples with up to 6 in. diameter with the surface uncoated, multilayer-coated, unstructured, and grating-structured were tested. After tin dripping onto heated samples, embrittlement of droplet contamination is induced in situ by stresses during phase transformation, following the initiation of tin pest with seed crystals of gray tin. Conversion of initially adhesive deposits to loose gray tin has been reached in less than 24 h on all tested surfaces by continuous cooling with cold nitrogen vapor to temperatures in the range of −30 to −50 °C. Alternatively, stress-initiated tin removal by delamination of β-Sn droplet splats has been attained via contraction strain induced by strong cooling to temperatures of around −120 °C. Profilometry has been used to analyze the bottom side of tin droplet splats removed from a grating-structured wafer. The in situ tin cleaning techniques give results comparable to fast ex situ cleaning that has been achieved either by sample immersion in liquid nitrogen or by splat removal after CO2 snowflake aerosol impact using a hand-held jet-nozzle. The implementation of the in situ phase conversion concept for the cleaning of collector mirrors in commercial light sources for lithography is discussed.

Characterization and Reduction of Defects in 4H-SiC Substrate and Homo-Epitaxial Wafer

4H-SiC substrates and homo-epitaxial layers were obtained using the traditional methods of physical vapor transport and chemical vapor deposition. Defect morphology has been studied using both Synchrotron White Beam X-ray Topography and Monochromatic Beam X-ray Topography. Molten KOH etching method was adopted to further investigate the dislocation behavior mechanisms. Deflected dislocations were observed at the periphery regions in both substrate and epitaxial wafers. 3C polytypes and half loop arrays were observed in the 4H-SiC epitaxial wafer. It is also found that the majority of basal plane dislocations are converted to threading edge dislocations in the epitaxial wafer samples. The proportion of BPD to TED conversion depends on the surface step morphology and growth mode in epitaxial growth which in turn depends on the C/Si ratio. By the optimization of etching time prior to epitaxy and C/Si ratio, high-quality epitaxial wafers with extremely low basal plane dislocations densities (＜0.1 cm-2) was obtained.

Two-dimensional detection of subsurface damage in silicon wafers with polarized laser scattering

Abstract This study proposes a polarized laser scattering (PLS) method for detection of the two-dimensional distribution of the grinding-induced subsurface damage (SSD). This study also uses two destructive methods to observe the distribution of SSD in the ground silicon wafers by cross-sectional microscopy and by layer-by-layer microscopy. An experimental investigation is arranged for detecting the two-dimensional damage from diamond wheel grinding of silicon wafers through wafer sample preparation and SSD observations with the cross-sectional and layer-by-layer microcopy. It also includes PLS detection of the ground silicon wafers for SSD information. The results show that both the depth of SSD and the crack density contribute to the PLS detection signals. The study reveals the characteristics of the PLS detection, which illustrates the potential application of the PLS method to in-process detection.

Texturization of diamond wire sawn multi-crystalline silicon wafers by micro-droplet etching

Diamond wire sawn multicrystalline silicon (DWS mc-Si) solar cell wafers have been known to be difficult to texturize by conventional acidic wet etching texturization process. A novel texturization method based on localized micro-droplet etching (MDE) has been developed, in which the micro-droplet is generated by controlled condensation of thermal vapor from a HF–HNO3–H2O bath. Uniformly distributed micron scale etch-pit type surface textures on DWS mc-Si wafers are made by the MDE process. Their light reflectivity is reduced to 18–21%, and their sawmarks are removed. Full size DWS mc-Si wafer samples were MDE textured, and send to a solar cell fabrication line along with the conventionally wet textured DWS mc-Si wafers as reference, without any adjustment of processing parameters for the MDE textured wafers. The average short circuit current of the MDE textured cells has been found to increase as compared to the reference cells, in a relative ratio approximately matching the final light reflection reduction ratio induced by the MDE textures. The average open circuit voltage and fill factor of the MDE textured cells are found to increase slightly, indicating that the MDE texture has not induced any significant increase in surface recombination of carriers, neither poorer contact of printed grid contact on cell surfaces. In average, 0.52% absolute increase of energy conversion efficiency is obtained.

Ti+ and Mg+ Ion Beam Extraction from the Modified Bernas Ion Source

The korea multi-purpose accelerator complex (KOMAC) has a metal-ion-beam irradiation facility (150 keV/1 mA) equipped with a Bernas ion source. Metal-ion beams such as Cr+, Fe+, Co+, and Cu+ have been successfully extracted from our facility. Recently, some research groups have requested us to irradiate Ti+ and Mg+ ions into materials. These two kinds of ions have never been provided before. In response to these requests, we carried out the extraction of Ti+ and Mg+ beams at our facility. The vaporization temperatures of TiCl(III) and MgCl(II) under a pressure of 10−4 Torr were less than 300 °C and more than 600 °C, respectively. In this study, optimal extraction conditions for Ti+ and Mg+ ion beams were acquired successfully. To confirm the fluences of the Ti+ and the Mg+ beams, we prepared a silicon wafer sample and a gallium nitride (GaN) sample. Ti+ ions were irradiated with a current density of ∼6 μA/cm2 onto a silicon wafer sample for 90 min, and the dose was 1.68 × 1016 ions/cm2, we measured by using Rutherford backscattering spectrometry (RBS) analysis. Mg+ ions implanted in GaN were confirmed by using secondary ion mass spectroscopy (SIMS). The depth and the concentration of Mg+ ions at the peak were found to be 130 nm and 7.4 × 1019 ions/cm3, respectively. We would expect the newly extracted Ti+ and Mg+ ions to be helpful for research in the fields of material science and condensed matter physics.

Modelling of Phosphorus and Boron Doping Concentration on SOI Wafer Based Diffusion Process

High concentration of Boron and Phosphorus elements are required in diffusion process during the fabrication of semiconductor devices such as diode and transistor based on Silicon On Insulator (SOI) wafer. Achieving high level of these elements’ concentration is the entry point for further research in the field of electronics. For this reason, the concentration of the both elements was tested by flowing Boron and Phosphorus gas with flow rate of 1.5 litre per minute into the Nitrogen furnace for 5 minutes towards the surface of the SOI wafer samples at temperatures of 880, 900 and 950 degrees Celsius. This test was carried out at Michiharu Tabe Laboratory, Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan. Furthermore, the resistivity measurements of  samples with Boron and Phosphorus doping were carried out. The results of resistivity were then converted to obtain the concentrations of Boron and Phosphorus on the surface of SOI wafer sample. From the concentration and temperature data, it is obtained the modelling of concentration to temperature function for Boron and Phosphorus. The modelling results show that there is a linear correlation between high concentrations of Boron and Phosphorus to temperature.

Mechanism study of SiO2 layer formation and separation at the Si die sidewall during nanosecond laser dicing of ultrathin Si wafers with Cu backside layer

Laser dicing of ultrathin dies is promising and is gaining importance because of its cost and quality advantages over mechanical and plasma dicing. However, the effects of laser dicing on the mechanical strength and microstructure of ultrathin Si dies need to be further understood, especially when dicing through Si wafers with backside Cu layer. A critical phenomenon effecting the Si die sidewall strength after nanosecond laser dicing of Si wafers with backside Cu is the formation and separation of a SiO2 layer at the sidewall. The mechanisms behind the SiO2 layer formation and separation were studied in this work. Si wafer samples without and with backside Cu layer were prepared by dicing with nanosecond laser using standard production parameters. The microstructure and phases formed were investigated by energy dispersive spectroscopy and nanobeam diffraction in a transmission electron microscope. In die samples without backside Cu, the sidewall consists of a thin surface layer of amorphous Si, followed by a polycrystalline Si layer, and finally an epitaxial Si layer. In die samples with backside Cu, the sidewall microstructure was observed to be vastly different. At the upper region of the sidewall, a surface layer of polycrystalline Cu was found, followed by a polycrystalline Cu3Si layer, a SiO2 layer mixed with Cu3Si, and finally a thick SiO2 layer. The Cu3Si catalyzes the growth of the SiO2 through an oxidation step of the Cu3Si on the sidewall surface as well as at the SiO2/Si interface. In the lower region of the sidewall, the microstructure is similar to the upper region, but there is a separation of the SiO2 layer from the crystalline Si. The SiO2 undergoes a decomposition reaction at the SiO2/Si interface, releasing volatile SiO which causes microvoids to form and grow laterally at the interface. The growth and coalescence of the microvoids eventually lead to the separation of the SiO2 layer from the crystalline Si, leaving behind a clean and rough crystalline Si surface with a peak-to-peak roughness of 100–200 nm. In the areas where the SiO2 layer has separated from the Si die sidewall, the fracture strength of the sidewall is dependent on the material property and surface roughness of the crystalline Si, and not on the SiO2 layer. In the sidewall region near the die frontside, the SiO2 thickness is more than regions near the die backside, and no microvoiding and separation at the SiO2/Si interface were detected. This is hypothesized to be due to a higher O2 pressure at the upper region of the narrow dicing trench which is open to the atmosphere compared to the lower regions where there could be O2 deprivation and lower O2 pressure.

Quantification of heavy metals in oils with µL volume by laser induced breakdown spectroscopy and minimazing of the matrix effect

In this work we describe a method for preparation and analysis of oils by Laser Induced Breakdown Spectroscopy (LIBS), aimed to minimize the necessary sample volume and the matrix effect while maximizing the detection sensitivity and measurement's repeatability. The preparation procedure consists in stabilizing the oil sample and silica wafer substrate at a fixed temperature, here of 40 °C, and in delivering an oil droplet on the wafer rotated by a spin coater. In this way, a uniform oil film is obtained, which thickness is controlled through the rotation speed. So prepared target is then scanned by using the LIBS instrument. From comparative measurements on the pure oil and oil containing 2100 ppm of various elements, we studied different potentials sources of the matrix effect. During the sample preparation, above a certain rotation speed the thickness of the liquid film is the same for the two oils although their kinematic viscosities are very different, meaning that the volume sampled by LIBS is the same. The measured oil transmissivity at the laser wavelength of 1064 nm significantly decreases with concentration of impurities, but this effect could be neglected when dealing with very thin films. The plasma formation threshold measured on the bulk oil samples decreases with the impurity content. In case of pure oil, also for the maximum laser energy here used (165 mJ), the plasma is mainly initiated on the wafer while the presence of impurities increases screening of the substrate by the plasma formed directly on the oil. The matrix effect disappears on a very thin film, here of 0.74 μm, where the C I line intensity in plasma does not vary with the total concentration of impurities between zero and 2100 ppm; simultaneously, the plasma emission becomes stable from one laser pulse to another, contrary to the case of a thick liquid layer. At 0.5 μs from the laser pulse the plasma electron density is much higher in presence of oil than on the bare substrate because of the initial plume confinement. In the optimized experimental conditions the plasma emission from oil was very intense although the sample volume probed by each laser pulse was of 0.3 nL only. By choosing properly the signal acquisition delay and the calibration procedure, the latter is dependent on the excitation energy of the analytical lines, we obtained the detection limits of 3.9 ppm, 0.49 ppm, 0.16 ppm and 0.082 ppm for Zn, Cd, Cu and Cr in oil, respectively.

Surface grafted agents with various molecular lengths and photochemically active benzophenone moieties.

Homologues of benzophenone silane, a covalently graftable, photochemically active surface functionalizing agent, are investigated as surface functionalization agents for both small particles and planar substrates. In these homologues, a chlorosilane functional group and a photochemically active benzophenone oxo moiety are separated with an aliphatic spacer of varying length. The species obtained are first investigated by surface grafting on substrates (Si wafers, glass plates, and indium tin oxide coated glass plates). Si wafer samples are investigated with ellipsometry clearly indicating monolayer formation. The monolayer thickness can be controlled by the size of the aliphatic spacer group and also by the doping concentration of the solution used in the spin-casting step. The functionalized surfaces are further investigated by measuring the contact angle of a suitable organic fluid, a nematic liquid crystal. Photo exposure of these samples results in a drastically varied contact angle: The surface grafted species are still photochemically active and photo exposure leads to the addition of a nearby organic molecule (from the liquid crystalline phase) to each activated surface agent molecule. The synthesized species are then investigated as (covalently binding) surfactants in the wet planetary ball milling process aimed to fabricate solid-liquid dispersions (of Fe doped lithium niobate particles). It was found that the use of species with higher molecular length results in dispersions of small particles, functionalized with photochemically active surface agents. Indeed, they show better performance than conventional surfactants.

Development of standard samples with programmed defects for evaluation of pattern inspection tools for 7-nm and smaller nodes

Background: Continued shrinkage of pattern size has caused difficulties in detecting small defects. Multibeam scanning electron microscopy (SEM) is a potential method for pattern inspection below 7-nm node. Performance of the tool depends on charge control, resolution, and defect detection capability. Aim: The goal of this study is to develop a method for evaluating the performance of multibeam SEM for 7-nm nodes. Approach: By developing various standard samples with programmed defects (PDs) on 12 in. Si wafer, we evaluate the performance of multibeam SEM. Results: The first wafer had line and space (LS) patterns and PDs with varying contrast. A second wafer had various shaped small PDs, ∼5 nm in size in 16- to 12-nm half-pitch LS patterns. A third wafer with extremely small PDs of around 1 nm was fabricated in LS patterns with ultralow line-edge roughness (LER) of less than 1 nm. The first wafer was effective for charge control, whereas second and third wafer confirms resolution and defect detection capability. Conclusions: A set of minimum three standard wafer samples is effective to confirm the performance of multibeam SEM for below 7-nm nodes. Besides, we proposed a method to verify the LER values measured by a critical-dimension SEM.

P‐1.6: Highly Stable LTPS TFT on Si Wafer under High Current Stress

In this work, we report low temperature polysilicon (LTPS) thin film transistor (TFT) on the glass substrate and silicon wafer substrate by using blue laser annealing (BLA). The TFTs evaluated the high current stress (HCS) at the gate voltage (VGS) ‐20 V and the drain voltage (VDS) ‐6 V for 1 h. As a result, the performance of TFT on the silicon wafer shows excellent stability of HCS compared to that of TFT on the glass substrate. Also, viewed in its entirety, the break down current (IBK) of glass sample smaller than silicon wafer sample at breakdown voltage.