Jaime Gilberto Duduch

Ductile and brittle modes in single-point-diamond-turning of silicon probed by Raman scattering

In recent years, considerable progress has been made on the study of the machinability of fragile materials as crystalline semiconductors because of the demand for faster fabrication processes of complex surface shapes for optoelectronic applications. The most-studied machined semiconductors are silicon and germanium [1– 3]. Some studies indicate that ductility could be related to the high-pressure metallization (brittle-to-ductile) transformation that occurs when these materials are subjected to high hydrostatic pressures [4–7]. Because of this, most of the studies reported previously are devoted mainly to microindented or microcut Si and Ge. Among the experimental techniques used to probe their effects, Raman scattering has been successfully employed, mainly to exploit the presence of residual stresses around the indentations and grooves made by indenters [8]. It is well known that, due to the machining (or to the polishing or lapping) process, semiconductor surfaces can undergo structural damage [9]. Raman scattering is a powerful characterization technique in these cases because the vibrational spectrum of the material is greatly influenced by disorder and residual strains: These lead to changes in phonon frequencies, broadening of Raman peaks and breakdown of Raman selection rules [10]. For bulk crystalline Si (c-Si), the triple degenerate optical phonons display in the first-order Raman spectrum one sharp peak at 521 cm−1. Due to the positive phonon deformation potentials of Si, compressive (tensile) strains lead to positive (or negative) frequency shifts. On the other hand, due to the loss of phonon correlation length and the consequent breakdown of the q = 0 Raman selection rules (q is the phonon wave vector), disorder effects can lead to an asymmetric broadening and shifting of Raman peaks compatible to the dispersion relation of the material [11]. In the silicon case, the frequency and asymmetry point to lower values because the dispersion relationship presents decreasing optical frequencies, increasing phonon wave vectors. At maximum disorder (amorphous material, aSi), the first-order Raman spectrum reflects the phonon density of states: It presents two broad bands centered at about 100 cm−1 (acoustic band) and 470 cm−1 (optical band) [12]. In this letter, an original (macro-) Raman investigation of single-point-diamond-turned silicon samples machined in ductile and brittle modes is presented. To probe the depth profile of disorder effects, the 457.9, 488.0 and 514.5 nm lines of an argon ion laser were used as exciting light. For these lines, the penetration depth of the light is about 140, 270 and 340 nm for c-Si, respectively. For a-Si, the optical absorption coefficient can reach one order of magnitude higher, leading to penetration depths of about tenths of nanometers, depending on the degree of amorphization [13, 14]. All measurements were performed at room temperature, with special care taken taken to avoid overheating the samples. Cutting tests were performed on Si samples on the (1 0 0) surface. The samples were single-pointdiamond-turned using a facing operation on a RankPneumo ASG 2500 diamond-turning machine. Alkalisol 900 coolant cutting fluid was continuously sprayed onto the workpiece during machining to avoid overheating the sample. A 0 (−25) degree rake angle diamond tool with nose radius R= 1.52 mm and a clearance angle of 12◦ was used in all tests. The feed rate used was 12.5 (1.25) μm rev−1 and the nominal depth of cut was 10 (1)μm. These conditions provide brittle (ductile) mode during machining, with an opaque (mirrorlike) surface. The finished surfaces were observed using scanning electron microscopy (SEM). Fig. 1 shows a

Raman characterization of structural disorder and residual strains in micromachined GaAs

Structural disorder and strain effects in ductile-regime single-point-diamond-turned gallium arsenide monocrystalline samples were probed by Raman scattering. The positive frequency shift of the longitudinal and transverse optical phonons observed in the machined samples indicate a residual compressive stress of about 1.5 GPa. This residual strain was attributed to the hysteresis of phase transformation generated by the high pressure imposed by the cutting tool tip during the machining process. The broadening of the Raman peaks indicate a high degree of structural disorder in the GaAs lattice. Moreover, the Raman spectrum of annealed samples, after machining, shows a less disordered but still misoriented matrix. In addition, it was found that crystalline arsenic formed into the surface vicinity.

Dependence of brittle-to-ductile transition on crystallographic direction in diamond turning of single-crystal silicon

The objective of this paper is to show the dependence relationship between the crystallographic orientations upon brittle-to-ductile transition during diamond turning of monocrystalline silicon. Cutting tests were performed using a −5° rake angle round nose diamond tool at different machining scales. At the micrometre level, the feedrate was kept constant at 2.5micrometres per revolution (µm/r), and the depth of cut was varied from 1 to 5 µm. At the submicrometre level, the depth of cut was kept constant at 500 nm and the feedrate varied from 5 to 10 µm/r. At the micrometre level, the uncut shoulder generated with an interrupted cutting test procedure provided a quantitative measurement of the ductile-to-brittle transition. Results show that the critical chip thickness in silicon for ductile material removal reaches a maximum of 285 nm in the [100] direction and a minimum of 115 nm in the [110] direction, when the depth of cut was 5 µm. It was found that when a submicrometre depth of cut was applied, microcracks were revealed in the [110] direction, which is the softer direction in silicon. Micro Raman spectroscopy was used to estimate surface residual stress after machining. Compressive residual stress in the range 142 MPa and smooth damage free surface finish was probed in the [100] direction for a depth of cut of 5 µm, whereas residual stresses in the range 350 MPa and brittle damage was probed in the [110] direction for a depth of cut of 500 nm.

Influence of the mechanical and metallurgical state of an Al-Mg alloy on the surface integrity in ultraprecision machining

This work presents the experimental results of the facing turning of an Al-Mg alloy. This aluminium alloy was mechanically and metallurgically modified, by means of cold rolling reduction and refining grain size previous to machining. The samples were cut and compared with samples in the as-received form, machined under the same cutting conditions. Surface finishing and work hardening were measured. Results show that theoretical surface roughness values are always smaller than the measured ones for all samples. Also, the surface roughness of the as-received samples is larger than that of mechanically modified samples. This difference of surface roughness was attributed to the swelling effect of the material. Microhardness values of the machined samples showed a decreasing trend with increasing loads. The surface of the cold rolled sample did not present a detectable microhardness alteration. Optical microscopy was used to observe qualitative aspects of the machined surface.

Structure evaluation of submicrometre silicon chips removed by diamond turning

Continuous chips removed by single-point diamond turning of single crystal silicon have been investigated by means of scanning electron microscopy/transmission electron microscopy and micro-Raman spectroscopy. Three different chip structures were probed with the use of electron diffraction pattern: (i) totally amorphous lamellar structure, (ii) amorphous structure with remnant crystalline material and (iii) partially amorphous together with amorphous with remnant crystalline material. Furthermore, micro-Raman spectroscopy from the chips remained in the cutting tool rake face depicted different silicon phases. We have found, from a detailed analysis of the debris, five different structural phases of silicon in the same debris. It is proposed that material removal mechanisms may change along the cutting edge from shearing (yielding lamellar structures) to extrusion. Shearing results from structural changes related to phase transformation induced by pressure and shear deformation. Extrusion, yielding crystalline structures in the chips, may be attributed to a pressure drop (due to an increase in the contact area) from the tool tip towards the region of the cutting edge where the brittle-to-ductile transition occurs. From this region upwards, pressure (stress) would be insufficient to trigger phase transformation and therefore the amorphous phase would not form integrally along the chip width.

Multiple phase silicon in submicrometer chips removed by diamond turning

Continuous chips removed by single point diamond turning of single crystal silicon have been investigated by means of Scanning Electron Microscopy/Transmission Electron Microscopy and micro-Raman Spectroscopy. Three different chip structures were probed with the use of electron diffraction pattern: (i) totally amorphous lamellar structure, (ii) amorphous structure with remnant crystalline material and, (iii) partially amorphous together with amorphous with remnant crystalline material. Furthermore, micro-Raman spectroscopy from the chips left in the cutting tool rake face showed different silicon phases. We have found, from a detailed analysis of the debris, five different structural phases of silicon in the same debris. It is proposed that material removal mechanisms may change along the cutting edge from shearing (yielding lamellar structures) to extrusion. Shearing results from structural changes related to phase transformation induced by pressure and shear deformation. Extrusion, yielding crystalline structures in the chips, may be attributed to a pressure drop (due to an increase in the contact area) from the tool tip towards the region of the cutting edge where brittle-to-ductile transition occurs. From this region upwards, pressure(stress) would be insufficient to trigger phase transformation and therefore amorphous phase would not form integrally along the chip width.

Micropositioning device using solid state actuators for diamond turning machines: a preliminary experiment

Ultra-precision machining to sub-micrometer depths is shown to be a threshold area of work requiring a combination of the best of materials, sensors, positioning devices and control strategies. In ultra-precision design, it is extremely likely that there will be few design options for selection. In the case of ultra fine tool feed, the piezoelectric actuator is a potential choice. Currently, European and Japanese tool manufacturers are investigating magnetostrictive actuators for this purpose. Both piezoelectric and magnetostrictive materials suffer from hysteresis type non-linearity, so that the output of such systems depends upon the previous input, and absolute positioning is only achievable with the aid of feedback control. This work compares several modern control techniques for the positioning of a tool post for ultra- precision machining of brittle materials (in the nanometric range), e.g., lead-lag filter, PI+D, PID+feedforward, and fuzzy logic/neural network. The performance of the micropositioning device using both piezoelectric and magnetostrictive (solid-state) actuators are assessed by means of simulation techniques. The performance results are compared with results obtained by other authors.

Investigation on diamond turning of silicon crystal - generation mechanism of surface cut with worn tool

This paper discusses the effect of tool wear on surface finish in single-point diamond turning of single crystal silicon. The morphology and topography of the machined surface clearly show the type of cutting edge wear reproduced onto the cutting grooves. Scanning electron microscopy is used in order to correlate the cutting edge damage and microtopography features observed through atomic force microscopy. The possible wear mechanisms affecting tool performance and surface generation during cutting are also discussed. The zero degree rake angle single point diamond tool presented small nicks on the cutting edge. The negative rake angle tools presented more a type of crater wear on the rake face. No wear was detected on flank face of the diamond tools.

Critical aspects on the behavior of material from the mechanical tool-workpiece interaction in single point diamond turning

Some material aspects such as grain size, purity and anisotropy exert an important influence on surface quality, especially in single point diamond turning. The aim of this paper is to present and discuss some critical factors that can limit the accuracy of ultraprecision machining of non-ferrous metals and to identify the effects of them on the cutting mechanism with single point diamond tools. This will be carried out through observations of machined surfaces and chips produced using optical and scanning electron microscopy. Solutions to reduce the influence of some of these limiting factors related with the mechanism of generation of mirror-like surfaces will be discussed.

In-situ raman spectroscopy analysis of re-crystallization annealing of diamond turned silicon crystal

Mechanical material removal during ultraprecision machining of semiconductors crystals normally induces surface damage. In this article, Raman micro-spectroscopy has been used to probe structural alteration as well as residual stresses in the machined surface generated by single point diamond turning. The damage found is characterized by an amorphous phase in the outmost surface layer. In addition, it is reported, for the first time, the results of in-situ re-crystallization annealing of micromachined silicon monitored by micro-Raman spectroscopy. It is also shown that the annealing heat treatment influenced surface roughness: results were Rmax equal to 24.2 nm and 47.3 nm for the non treated and for the annealed surfaces, respectively.