Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

Acoustic emission during laser annealing of silicon single crystals

The acoustic response of silicon single crystals to the action of a millisecond laser pulse, with an excitation wavelength 1.06 μm, has been investigated. It has been discovered for the first time that additional acoustic emission, delayed in time with respect to the photoacoustic response, is observed for laser energy flux density above the threshold corresponding to surface melting. The delay time depends linearly on the laser radiation power and varies from one to tens of milliseconds. It is shown, by comparing the parameters of the acoustic emission with the dynamical development of thermoelastic stresses in the laser action zone as well as with the kinetics of melting of the irradiated surface, that crack formation under the action of the thermoelastic stresses is the source of the additional acoustic emission. Fiz. Tverd. Tela (St. Petersburg) 39, 505–509 (March 1997)     

Acoustic emission monitoring of surface hardening by laser

The acoustic emission signals from the laser surface heating of En8 steel, a stainless steel, 0.25% C mild steel, an /β brass, and Al-4% Cu were analysed. The stainless steel was acoustically inactive but it was found that transformation processes in En8 and mild steel emitted a signal which could be used as part of a feedback loop to control the extent of the process. Similarly the process could be controlled in the case of the non-ferrous alloys by the monitoring of the emissions associated with one or more of the following: surface melting, cracking and an order-disorder transformation.

The acoustic emission signals measured during laser cladding, in which powder is blown into the laser generated weld pool, were totally dominated by the noise from the powder striking the surface. These signals (in particular the RMS) appeared capable of being used to monitor the powder feed rate and the behaviour of any auxillary equipment such as an ultrasonic vibrator.

The acoustic signals from a cooling clad trace of Stellite 6 on En3 steel plate varied with the size of the deposit and the intensity of the residual stresses allowing a method of comparing the effect of vibration on residual stress in vibrated or non-vibrated specimens.     

Absorption bleaching by stimulated emission in erbium-doped silicon-rich silicon nitride waveguides

Stimulated emission from sensitized erbium ions in silicon-rich silicon nitride is demonstrated by pump-probe measurements carried out in waveguides. A decrease in the photoinduced absorption of the probe at the wavelength of erbium emission is observed and is attributed to stimulated emission from erbium excited indirectly via localized states in the silicon nitride matrix.     

The influence of high pressures on structure and properties of silicon nitride

Abstract

Silicon nitride as a material with both small coefficient of thermal expansion and high oxidation resistance has long attracted the attention of specialists in various fields of science and technology'. Covalent bonding in this compound is the reason for the use of high temperature sintering.     

Acoustic emission and deformation processes in aluminum at high temperatures

Processes of high-temperature deformation of polycrystalline aluminum are investigated. It is found out that at high temperatures monotonous deformation transforms into macroscopic deformation jumps accompanied by single high-amplitude acoustic emission pulses correlated with the strain rate. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 33–38, November, 2005.     

Acoustic emission technique for monitoring the pyrolysis of composites for process control

Carbonization is the first step in the heat and pressure treatment (pyrolysis) of composites in preparing carbon-carbon parts. These find many uses, including aircraft brakes, rocket nozzles and medical implants. This paper describes the acoustic emissions (AE) from various stages of the manufacturing process of carbon-carbon composites. This process involves carbonization at a high temperature and this results in both thermal expansion and volume change (due to pyrolysis in which a sacrificial polymer matrix is converted to carbon). Importantly the resultant matrix is porous and has a network of small intra-lamina cracks. The formation of these microcracks produces AE and this paper describes how this observation can be used to monitor (and eventually control) the manufacturing process. The aim is to speed up manufacture, which is currently time-consuming. The first section of the paper describes the design of unimodal waveguides to enable the AE to propagate to a cool environment where a transducer can be located. The second part of the paper describes various experimental observations of AE under a range of process conditions. In particular, this paper presents a technique based on detecting acoustic emissions and (1) uses wire waveguides to monitor parts within the autoclave to 800 degrees C, (2) monitors microcracking during pyrolysis, (3) uses a four-level threshold to distinguish between low- and high-amplitude cracking events, (4) recognizes the occurrence of harmful delaminations, and (5) guides the control of the heating rate for optimum efficiency of the pyrolysis process. In addition, supporting data are presented of in situ measurements of porosity, weight loss, cross-ply shrinkage, and mass spectroscopy of gases emitted. The process evolution is illustrated by the use of interrupted manufacturing cycle micrographs obtained by optical, scanning acoustic (SAM) and scanning electron (SEM) microscopy. The technique promotes in-process monitoring and control but also contributes to current understanding of pyrolysis.     

Acoustic waves of GaN nitride nanowires

Acoustic waves of GaN nanowires are studied. The materials have a wurtzite structure and thus the elastic anisotropy is included. We use the xyz-algorithm originally employed in the analysis of resonant ultrasound spectroscopy results. We have considered hexagonal cross-section nanowires as those experimentally grown. We have studied also circular cross-section nanowires (nanotubes) for comparison. It was found that the lower frequency modes have a very similar dispersion relation both in shape and frequency value, but higher frequency modes exhibit more important differences. The nanowires are solid or hollow and the influence of the thickness on the dispersion relation and the elastic displacement pattern is considered. Nanowires with a zinc-blende structure have also been considered because of theoretical studies allowing for their existence. We have studied also the squared displacement vectors to see the spatial distribution of the different modes in the nanowires. It was found that there are many modes whose elastic displacement components concentrate around the borders of the nanowire and they do not penetrate almost in the inner part of the nanowire.     

Surface plasmon enhanced light emission of silicon-rich silicon nitride: Dependence on metal island size

Surface plasmon coupled light emission of silicon-rich silicon nitride (SRSN) was investigated as a function of metal island size. It was found that the emission intensity was enhanced by surface plasmon (SP) and the enhancement factors increase greatly with the increase of metal island size. Moreover, SP coupled emission spectral shape was found also correlating with Ag island size. By calculating the extinction characteristics of the Ag islands, it was believed that SP scattering and absorption efficiency of the metal islands decide the photoluminescence (PL) changes including emission intensity enhancement and band position shift.     

Imaging microanalysis of silicon nitride ceramics with a high resolution scanning ion microprobe

Samples of silicon nitride sintered with Y₂O₃ and MgO are examined by scanning ion microscopy and analyzed by secondary ion mass spectrometry. High lateral resolution elemental images of both fractured and polished ceramic surfaces are interpreted within the context of crystal morphology and intergranular composition. Advantages as well as limitations of the SIMS method are discussed.     

Airborne sound emission as a process monitoring tool in the cut-off grinding of concrete

A detailed analysis of the airborne sound emitted in the cut-off grinding of concrete with a diamond grinding disk is presented. It is shown that the frequency spectra of airborne sound emitted in the cutting process contain detailed information about the process conditions. As long as the machining parameters are appropriate for the work piece, the airborne sound spectra show statistically excited natural frequencies and turning frequencies. In this case the total signal level gives sufficient evidence of the work piece composition and machinability. The effects of inappropriate machining parameters, like tool deflection or very high friction forces acting on the tool, can be identified by means of distinct frequencies in the airborne sound spectra. In addition, the emitted airborne sound can be used to image the structure of the flat joint surface, which cannot be determined by another method. With regard to everyday applications, the results obtained by this procedure can be used to apply airborne sound analysis systems to machines and detect process parameters which are overstressing the cutting tool.     

Analysis of thin thermal silicon nitride films on silicon

We have investigated the chemical and electrical properties of very thin (<32 Å thick) silicon nitride films grown by rapid thermal nitridation of silicon. These films were of interest as a possible means of tailoring the barrier heights of silicon Schottky barrier diodes. Auger and XPS analysis showed that the level of oxygen contamination in the films was very low ([N]/[N]+[O]) =0.85 to 0.95). The oxygen is located primarily at the surface and interface of the films. Metal-nitride-silicon devices were characterized by I-V and C-V techniques. These measurements indicated an increase in barrier heights to p-type substrates and a decrease in barrier heights to n-type substrates compared to values measured in the absence of the nitride layers. The magnitude of the change in barrier height increases with increasing nitride thickness. The barrier height can be varied reproducibly over a wide range. For molybdenum on p-type, this range is greater than half the bandgap. For titanium and molybdenum on p-type diodes, barrier heights higher than 1.0 V can be achieved. These measurements could be explained by a reduction in the density of silicon interface states with increasing nitride thickness or by the presence of positive fixed charge in the nitride layer.     

Bandwidth-tunable silicon nitride microring resonators

We designed a reconfigurable dual-interferometer coupled silicon nitride microring resonator.By tuning the integrated heater on interferometer's arms,the"critical coupling"bandwidth of resonant mode is continuously adjustable whose quality factor varies from 7.9×10⁴ to 1.9×10⁵ with the extinction ratio keeping higher than 25 dB.Also a variety of coupling spanning from"under-coupling"to"over-coupling"were achieved,showing the ability to tune the quality factor from 6.0×10³ to 2.3×10⁵.Our design can provide an adjustable filtering method on silicon nitride photonic chip and contribute to optimize the nonlinear process for quantum photonics and all-optical signal processing.     

Elasticity and inelasticity of silicon nitride/boron nitride fibrous monoliths

A study is reported on the effect of temperature and elastic vibration amplitude on Young’s modulus E and internal friction in Si₃N₄ and BN ceramic samples and Si₃N₄/BN monoliths obtained by hot pressing of BN-coated Si₃N₄ fibers. The fibers were arranged along, across, or both along and across the specimen axis. The E measurements were carried out under thermal cycling within the 20–600°C range. It was found that high-modulus silicon-nitride specimens possess a high thermal stability; the E(T) dependences obtained under heating and cooling coincide well with one another. The low-modulus BN ceramic exhibits a considerable hysteresis, thus indicating evolution of the defect structure under the action of thermoelastic (internal) stresses. Monoliths demonstrate a qualitatively similar behavior (with hysteresis). This behavior of the elastic modulus is possible under microplastic deformation initiated by internal stresses. The presence of microplastic shear in all the materials studied is supported by the character of the amplitude dependences of internal friction and the Young’s modulus. The experimental data obtained are discussed in terms of a model in which the temperature dependences of the elastic modulus and their features are accounted for by both microplastic deformation and nonlinear lattice-atom vibrations, which depend on internal stresses.     

Si_3N_4波导上的费涅尔透镜

本文提出一种新型的费涅尔波导透镜.它不仅具有衍射受限的聚焦特性,而且有与理想费涅尔波导透镜几乎一样的效率,并使工艺简化.在氮化硅波导上进行的工艺实验表明,实验结果与理论分析具有较好的一致性.     

High efficiency silicon nitride grating coupler

In this paper, we have designed, fabricated and characterized silicon nitride grating couplers with high efficiency at 1490 nm. The devices are fabricated using deep UV photolithography with resolution requirement of ～500 nm. The grating coupler fabricated yields a peak coupling efficiency of ?5.1 dB. The 1-dB bandwidth of the grating coupler is 60 nm.     

Luminescent-wavelength tailoring silicon-rich silicon nitride LED

Wavelength tunable photoluminescence （PL） of Si-rich silicon nitride （SRSN） film with buried Si nanocrystals （Si-ncs） grown by plasma enhanced chemical vapor deposition （PECVD） under Sill4 and NH3 environment is investigated. Intense broadband visible emissions tunable from blue to red can be obtained from the as-deposited SiNs thin films with increasing NH3 flow rate from 150 to 250 sccm and detuning the SiH4/NH3 flow ratio during deposition. To date, the normalized PL wavelength of SiNx films after anneal- ing could be detuned over the range of 385-675 nm by decreasing the NH3 flow rate, corresponding to an enlargement on Si-nc size from 1.5-2 to 4-5 nm. The PL linewidth is decreased with increasing ammonia flow rate due to the improved uniformity of Si-ncs under high NH3 flow rate condition. In addition, the PL intensity is monotonically increasing with the blue shift of PL wavelength due to the increasing density of small-size Si-ncs. The ITO/SiNx/p-Si/Al diode reveals highly resistive property with the turn-on voltage and power-voltage slope of only 20 V and 0.18 nW/V, respectively. The turn-on voltage can further reduce from 20 to 3.8 V by improving the carrier injection efficiency with p-type Si nano-rods.