High efficiency slicing of low resistance silicon ingot by wire electrolytic-spark hybrid machining

As the semiconductor industry requires the cutting of silicon ingots into wafers, the slicing of large, ultra thin wafers is one of the main technologies to prevent wastage. Recently, apart from conventional inner diameter (ID) blade and multi-wire saw methods, wire electrical discharge machining (WEDM), which has no cutting force, has been introduced to this area and low resistance silicon may be sliced by WEDM. In this paper, a novel approach, based on wire electrolytic-spark slicing strategy using hybrid oil/aqueous electrolyte, combining electric discharge and anodic etching into a single process, is investigated experimentally. Some improvements, such as a new wire winding system, hybrid electrolyte and high efficiency pulse generator, have been adopted in a kind of high speed (HS)-WEDM machine. Experiments have been conducted to evaluate the machining rate, surface quality and wafer thickness of low resistance (0.5–3 Ω cm) mono-crystalline silicon. It has been demonstrated that with properly selected electrical parameters and electrolyte, a maximum machining rate of 600 mm²/min can be obtained and with a wafer thickness less than 120 μm. Furthermore, in comparison with WEDM, heat affected zone and harmful metal residues are considerably diminished, which provides significant theoretical and experimental support for future applications.     

Effect of metal coating on machinability of high purity germanium using wire electrical discharge machining

This paper reports on the research of wire electrical discharge machining (WEDM) as a cutting process for n-type high purity germanium (HP Ge). WEDM requires sufficient electrical conductivity of the work piece for discharges to occur. Owing to the very high material resistivity of HP Ge (32.8 Ω cm), the electrical conduction is too low for WEDM to be efficient. To temporarily enhance the conduction, metals (aluminum and nickel) were deposited on the HP Ge on 1 or 2 sides with various thicknesses (1.0, 2.0, and 3.0 μm) using sputter deposition. This shortens the path of conduction between the HP Ge and the WEDM ground and also serves to trigger the discharges. Machining experiments were performed to determine the correlation between the slicing rate and locally enhanced HP Ge through various discharge energies (potential voltage: 150, 200, 250 and 300 V and capacitance: 1, 3.3, 5.5, 9.9 and 21.4 nF). From the results, the obtained maximum slicing rate is 7.7 mm²/min for Al coating (2 sides, 1.0 μm thickness) at high energy (300 V, 21.4 nF), which is improved as much as 27 times over uncoated HP Ge. The fastest cutting without creating subsurface microcracks was measured as 1.12 mm²/min performed at 150 V and 9.9 nF. Additional slicing experiments at reverse polarity (positive wire and negative work piece, uncommon polarity for WEDM) were performed at 150 V and various capacitances. The experiment proved that there were rectifying contacts at the metal coating surface. It was found that under identical EDM settings, a faster slicing rate also showed a reduction in kerf size due to less lateral discharge energy. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were used to investigate microcracks and to analyze surface impurities.     

Electrodeposited Ni microcones with a thin Au film bonded with Au wire

In wire bonding, the bonding quality between the substrate Au film and metal wire has an effect on productivity and reliability. Au film thickness is important for substrate bondability. It is required to reduce the Au film thickness as thin as possible without deteriorating the level of bondability to cut down extremely high cost of Au consumption. In this study, electrodeposited Ni/Au microcones were fabricated and thermosonic bonded with Au wire. The thickness of Au film was only 0.05 μm. Bonded with Au wire 17.5 μm in diameter, the 0.4 μm-height microcones showed excellent and stable bondability with average pull strength 6.29 gf and small standard deviation. Microscopic observation showed that Ni/Au microcones inserted into Au wire effectively, thus insertion weld between microcones and Au wire was formed. Pull fracture scanning electron microscopy (SEM) images showed an improvement of stitch bonding quality when using Ni/Au microcones. Mechanism of bonding process between the Ni/Au microcones and Au wire was put forward by three stages.     

Grinding characteristics of micro-abrasive pellet tools fabricated by a LIGA-like process

The purpose of this paper was to investigate the wearing and grinding characteristics of the micro-abrasive pellet tools with 4–6 μm diamond particles fabricated by a LIGA-like process that has micro-lithography with photoresist mold and nickel/diamond composite electroforming. The results showed that when the micro-pellet tool containing partial resist joint with a root on the substrate was designed and fabricated, the tool against alumina sandpaper in wear test showed lesser amount of micro-pulled-out pellets than the tools with flat joint type, which displayed the tool to have better adhesion strength. In addition, when the micro-diamond tools were used to grind silicon wafers, the surface appearance of wafers showed ductile behavior. The surface roughness of wafers ground with increased pellet tool rotation speed became better and R_a=0.05 μm was achieved.     

Residual stress and subsurface damage in machined alumina and alumina/silicon carbide nanocomposite ceramics

We have used TEM and Hertzian indentation to study the interrelation between subsurface damage and residual stress introduced by grinding and diamond polishing surfaces of polycrystalline alumina and 5%SiC/alumina nanocomposites. In all cases a layer of high dislocation density was found near the surface. This varied in thickness from about 300 nm for alumina polished with 1 μm diamond grit to greater than 6 μm for a nanocomposite surface wheel-ground with 150 μm diamond grit. For a given finishing process the nanocomposites showed a greater depth of dislocation activity than alumina. In alumina, extensive basal twinning was found beneath the ground surfaces. Hertzian indentation data indicates a residual compressive stress of about 1500 MPa confined to the dislocation-containing region. Mechanisms for the enhanced dislocation activity in the nanocomposites are discussed.     

An evaluation of the evolution of workpiece surface integrity in hole making operations for a nickel-based superalloy

White layers and extensive material drag introduced during rough machining are regarded as detrimental to surface integrity. As such a sensible method for determining the amount of material to be removed in a roughing process would be to understand the relationship and interaction between roughing (i.e. drilling) and finishing (i.e. plunge milling) operations. Within this work non-standard cutting parameters were employed during the roughing process to generate a white layer and material drag up to a depth of 20 μm. Various plunge milling cutting strategies followed, with radius removal ranging from 25 μm to 250 μm in order to identify the amount of material removal necessary to eliminate the anomalies previously generated from mistreated surface history. The results show that finishing with a depth of cut between 50 μm and 125 μm removes all anomalies from the roughing process, leaving behind a negligible amount of material drag (3–4 μm). X-ray diffraction demonstrates significant tensile residual stresses (1000–2000 MPa) were generated in the axial and hoop direction by abusive hole drilling while subsequent plunge milling operation leaves compressive surface stresses in the region of ?500 MPa in both the axial and hoop directions; in both cases the depth of the surface stresses extended to around 125 μm from the drilled surface. It was also found that a depth of cut of 25 μm was not sufficient to recover the abused surface; this was due to intense material drag accompanied by surface cracking (i.e. 2 μm depth). The research shows that understanding the interaction between successive cutting operations can provide a suitable machining route to fulfil the industrial quality requirements in terms of the machined surface mechanical/metallurgical properties.     

Effect of oxide scale on temperature-dependent interfacial heat transfer in hot stamping process

An optimization-based numerical procedure was developed to determine the temperature-dependent interfacial heat transfer coefficient (IHTC). The effects of temperature, pressure and oxide scale thickness were analyzed, for oxide thickness between 9 μm and 156 μm and pressure from 8 MPa to 42 MPa. Oxide scales and contact pressure both show distinctive effects on IHTC in the cooling process. The average IHTC decreases about 2461 W/(m² °C) with the increase of oxide scale thickness and increases 2620 W/(m² °C) with the increase of pressure. Based on the two-way ANOVA, the effect of contact pressure influences the IHTC most. Their mutual interaction is negligible. The IHTC decreases when the average temperature between the blank and die surface is above 250 °C and increases when the latent heat release.     

Microstructure and mechanical properties of biomedical Co–29Cr–8Mo alloy wire fabricated by a modified melt-spinning process

A modified in-rotating-water spinning process has been applied for producing the alloy wire of Ni-free Co–29Cr–8Mo suitable for biomedical use. The microstructure and tensile properties of the as-spun and heat-treated wires were investigated using backscattered electron microscopy, X-ray diffraction analyses and tensile tests. The microstructure of the as-spun wire exhibits a cellular structure and evolves into an equiaxed and fine-grained structure with an average grain size of several micrometers, containing σ-phase precipitates after heat treatment at 1373 K. Grains increase in size and reach an average diameter ranging from 10 to 20 μm at 1473 K. The crystal structure of the as-spun wire changes from face-centered cubic to strain-induced hexagonal close-packed martensite through wiredrawing. The wiredrawing, combined with heat treatments, improves the mechanical properties of the as-spun wire. The present modified melt-spinning process is an effective method to produce Ni-free Co–Cr–Mo alloy wire for biomedical applications.     

Experimentation on MR fluid using a 2-axis wheel tool

This paper is focused on magnetorheological (MR) fluid assistive polishing of optical aspheric components. MR fluid is a functional mixture of non-colloidal magnetic particle of micrometer size suspended in a host fluid, with the special property that its viscosity can be varied by the application of a magnetic field. This paper introduces the basic principles of the methodology and presents experiment results on MR fluids using a 2-axis wheel-shaped tool supporting dual magnetic fields. Mathematical models taking into account the pressure and the tool velocity are derived. The experiments serve to evaluate the effects of process parameters on material removal and performance using a K9 glass parabolic lens of 60 mm diameter as work-piece. It is shown that surface roughness can be reduced from an initial value of 3.8–1.2 nm after 10 min of polishing. The form errors can also be improved from an initial 2.27 μm rms and 7.89 μm peak-to-valley to become 0.36 μm rms and 2.01 μm peak-to-valley after 60 min of polishing.     

Development of a reversible machining method for fabrication of microstructures by using micro-EDM

A new micromachining method for the fabrication of micro-metal structures by using micro-reversible electrical discharge machining (EDM) was investigated. The reversible machining combines the micro-EDM deposition process with the selective removal process, which provides the ability of depositing or removing metal material using the same micro-EDM machining system. From the discharge mechanism of micro-EDM, the process conditions of micro-EDM deposition were analyzed firstly. Using the brass and steel materials as a tool electrode, the micro-cylinders with 200 μm in diameter and height-to-diameter ratio of more than 5 were deposited on a high-speed steel surface. Then the machining procedure was transformed easily from deposition to selective removal process by switching the process conditions. Different removal strategies including micro-EDM drilling and micro-EDM milling were used in the machining. Micro-holes with 80 μm in diameter are drilled successfully in the radial direction of the deposited micro-steel cylinder. Also, a brass square column with 70 μm in side length and 750 μm in height, and a micro-cylinder with 135 μm in diameter and 1445 μm in height are obtained by using micro-EDM milling. Finally, the characteristics of the deposited material were analyzed. The results show that the material components of a deposited micro-cylinder are almost the same as those of the tool electrode, and the metallurgical bonding has been formed on the interface. In addition, the Vickers-hardness of 454Hv of the steel deposited material is higher when compared to the hardness of 200Hv of the raw steel electrode.     

Femtosecond laser machining of cooling holes in thermal barrier coated CMSX4 superalloy

Machining of cooling holes on thermal barrier coated superalloy components using a nanosecond (ns) laser generates considerable collateral damage such as recast layer, spatter and delamination of the ceramic coating. However, recent studies have suggested that these damages can be virtually eliminated by machining with femtosecond (fs) lasers. A detailed study on the microstructural characteristics of fs laser machined holes with diameters of 300 μm and 600 μm, generated on thermal barrier coated superalloy CMSX4 under various processing conditions has been conducted. Features examined include the shape, size and the surface finish of the hole wall. Femtosecond laser machined holes with a surface roughness of less than 2 μm and no major collateral damage could be generated in coated samples up to a thickness of 1.5 mm. The machining was found to cause minor ablative material removal from the top ceramic layer within 100 μm of the outer edge of the hole. The presence of machined holes did not affect the thermal cycling life at 1100 °C of the coated samples.     

Multi-wire sawing of sapphire crystals with reciprocating motion of electroplated diamond wires

This study examined the multi-wire sawing of C-plane sapphire ingots using diamond wires. Feeding new wire during the reciprocating motion of the wire was found to vary the cutting force, wafer shape, and roughness as a result of the break-in effect. The break-in and wire wear seemed to cause a gradual change in the cutting performance along the ingot position. The cutting force results indicated that an inappropriate supply of wire yielded an unbalanced force between the front and back sides of the ingot, which was caused by a difference in the cutting depth along the ingot. The results showed that controlling the wire consumption resulted in an average flatness of 16 μm, with a maximum value of 26 μm.     

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

Laser beam dispersion affects the resolution of Raman and photo-stimulated luminescence piezo-spectroscopy measurements of transparent materials. In this paper, we investigate the lateral spreading of the laser beam and the axial sampling depth of Raman spectroscopy measurements within thermal sprayed yttria-stabilized zirconia (YSZ) thin coatings. The lateral diameters of the laser beams (λ = 632.8 nm and 514 nm) reach approximately ～160 μm after travelling through a thickness of 200 μm of air plasma sprayed (APS) YSZ and ～80 μm after travelling through 120 μm of electron beam physical vapour deposited YSZ. The Raman spectroscopy sampling depth was found to be between 30 and 40 μm in APS YSZ. The beam dispersions within these two coatings were simulated using the ray-tracing software ZEMAX to understand the observed scattering patterns. The results are discussed with respect to the application of these two spectroscopic techniques in multi-layered thermal barrier coating systems.     

Effect of initial particulate and sintering temperature on mechanical properties and microstructure of WC–ZrO2–VC ceramic composites

Owing to improving the mechanical properties of cemented carbides in high speed machining fields, a new composite tool material WC–ZrO₂–VC (WZV) is prepared from a mixture of yttria stabilized zirconia (YSZ) and micrometer VC particles by hot-press-sintering in nitrogenous atmosphere. Commercial WC, of which the initial particle sizes are 0.2 μm, 0.4 μm, 0.6 μm and 0.8 μm, is mixed with zirconia and VC powder in aqueous medium by following a ball mill process. The sintering behavior is investigated by isostatic pressing under different sintering temperature. The relative density and bending strength are measured by Archimedes methods and three-point bending mode, respectively. Hardness and fracture toughness are performed by Vickers indentation method. Microstructure of the composite is characterized by scanning electron microscopy (SEM). The correlations between initial particles, densification mechanism, sintering temperature, microstructure and mechanical properties are studied. Experimental results show that maximum densification 99.5% is achieved at 1650 °C and the initial particle size is 0.8 μm. When temperature is 1550 °C and particle size is 0.4 μm, the optimized bending strength (943 MPa) is obtained. The best hardness record is 19.2 GPa when sintering temperature is 1650 and particle size is 0.8 μm. The indention cracks propagate around the grain boundaries and the WC particles fracture, which is associated with particle and microcrack toughening mechanism.     

Influence of casting surface on creep behaviour of thin-wall Ni-base superalloy Inconel100

The conventionally cast nickel-base superalloy Inconel100 was investigated with the focus on the influence of the surface condition and the thin-section size on the creep properties. Flat samples with 0.9 mm and 1.3 mm thickness were wire eroded from precision cast ingots. Thin cast specimens with a remaining casting surface were compared with machined specimens which flat surfaces were subsequently ground to the final thicknesses. Microstructures were examined by optical and scanning electron microscopy. Creep tests in air at 980 °C under a constant load of 150 MPa revealed decreasing creep strength with decreasing sample thickness. However, thin cast samples and samples with a machined surface having the same thickness showed similar creep properties.     

Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining

Processing of highly abrasive materials via powder injection molding or extrusion requires mold materials with high wear resistance to increase the durability of the tools and to sustain a high quality of the manufactured products. High performance ceramics which exhibit high hardness, bending strength and toughness show the perfect combination of properties for these applications. However they also have the usual drawback that they cannot be economically customized in complex shapes and low quantities, as they are required for tool and mold design. Recent material development enabled EDM of electrically conductive oxide ceramics, the most widespread machining process for machining of hard materials, as an alternative to conventional ceramic manufacturing and hard machining technologies.This study focuses on the influence of TiC particle sizes on material properties and EDM machinability of ZTA–TiC ceramics with 24 vol.% TiC, 17 vol.% ZrO₂ and 59 vol.% Al₂O₃. Fracture toughness, bending strength and electrical conductivity were analyzed for samples produced from TiC powders with particle sizes varying from 0.43 μm to 2.54 μm. Surface integrity of wire cut samples and feed rate during machining were investigated. It was shown that reducing the size of electrical conductive grains strongly increases the electrical conductivity and slightly decreases mechanical properties. Therefore also the machining characteristics are influenced by TiC grain size. The feed rate increases with decreasing particle size to a maximum at d₅₀ = 1–1.3 μm. Reduction of TiC particle size also leads to significantly decreasing surface roughness after the main cut. Additionally the necessary number of trimming steps to achieve a distinct surface roughness is also minimized for low particle sizes.     

Analysis of kerf width in micro-WEDM

In micro-wire electrical discharge machining (micro-WEDM), the machined kerf width varies with different machining parameters, which greatly influences the machining precision. In order to study the kerf variations in micro-WEDM, the mathematical model of wire lateral vibration in machining process is established and its analytical solution is obtained in this paper. The model is practically verified on a self-developed micro-WEDM machine. Under this model, a 30.8 μm width slot is achieved on a stainless steel workpiece with ?30 μm wire-tool.     

Effects of diamond size of CMP conditioner on wafer removal rates and defects for solid (non-porous) CMP pad with micro-holes

Wafer removal rates and defects were investigated for 200 mm tetraethyl orthosilicate (TEOS) oxide chemical mechanical planarization (CMP) processes using two types of CMP pads: a porous pad and a solid pad with micro-holes. An initial CMP test conducted with fumed silica based-slurry and a conditioner with 180 μm diamond revealed that the wafer removal rates by the solid pad with micro-holes were approximately 10% lower than those by the porous pad, but scratch type defects were reduced. In order to increase the removal rate of a solid pad with micro-holes to the comparable level of a regular porous pad without changing process parameters, it was decided to modify conditioner design by using different diamond size from 70 to 130 μm. It was found that wafer removal rates increased from 2973 to 2587 Å/min and defect counts reduced from 5.3 to 1.7 by decreasing the diamond size from 180 to 70 μm in the case of the solid pad with micro-holes. Various pad surface analysis results, including contact area estimation and microscopic observations, also revealed that a smaller diamond conditioner generated the pad texture with finer and more regular pad asperities.     

Optical properties of films of polycarbazolyldiacetylene PDCHD-HS for photonic applications

Spun films of polycarbazolyldiacetylene polyDCHD-HS were prepared from toluene solutions with a thickness ranging from 9 nm up to 3.6 μm. Thicker films were characterized as waveguides at 849 and 1321 nm; 5 dB/cm propagation losses were measured at 1321 nm. The off-resonance third-order nonlinearity of ultra-thin samples (9–14 nm) was measured at 1064 nm with picosecond pulses and using surface plasmon spectroscopy. The very large value χ⁽³⁾=4.4×10⁻¹⁷ m²/V² was detected for the real part of the third-order susceptibility.     

Mechanical properties of as-fabricated and 2300 °C annealed tungsten wire tested up to 600 °C

Recent efforts dedicated to the mitigation of tungsten brittleness have demonstrated that tungsten fiber-reinforced composites acquire pseudo ductility even at room temperature. Crack extension and fracture process is basically defined by the strength of tungsten fibers. Here, we move forward and report the results of mechanical and microstructural investigation of different grades of W wire with a diameter of 150 μm at elevated temperature up to 600 °C. The results demonstrated that potassium doping to the wire in the as-fabricated state does not principally change the mechanical response, and the fracture occurs by grain elongation and delamination. Both fracture stress and fracture strain decrease with increasing test temperature. Contrary to the as-fabricated wire, the potassium-doped wire annealed at 2300 °C exhibits much lower fracture stress. The fracture mechanism also differs, namely: cleavage below 300 °C and ductile necking above. The change in the fracture mechanism is accompanied with a significant increase of the elongation to fracture being ~ 5% around 300 °C.