Material Removal Characteristics of Single-Crystal 4H-SiC Based on Varied-Load Nanoscratch Tests

Single-crystal silicon carbide(SiC)has been widely applied in the military and civil fields because of its excellent physical and chemical properties.However,as is typical in hard-to-machine materials,the good mechanical properties result in surface defects and subsurface damage during precision or ultraprecision machining.In this study,single-and double-varied-load nanoscratch tests were systematically performed on single-crystal 4H-SiC using a nanoindenter system with a Berkovich indenter.The material removal characteristics and cracks under different planes,indenter directions,normal loading rates,and scratch intervals were analyzed using SEM,FIB,and a 3D profilometer,and the mechanisms of material removal and crack propagation were studied.The results showed that the Si-plane of the single-crystal 4H-SiC and edge forward indenter direction are most suitable for material removal and machining.The normal loading rate had little effect on the scratch depth,but a lower loading rate increased the ductile region and critical depth of transition.Additionally,the crack interaction and fluctuation of the depth-distance curves of the second scratch weakened with an increase in the scratch interval,the status of scratches and chips changed,and the comprehensive effects of the propagation and interaction of the three cracks resulted in material fractures and chip accumulation.The calculated and experimental values of the median crack depth also showed good consistency and relativity.Therefore,this study provides an important reference for the high-efficiency and precision machining of single-crystal SiC to ensure high accuracy and a long service life.

Growth Conditions of Φ100 mm n <111> FZ Silicon Single Crystal

For large diarneter silicon single crystal, the solid-liquid growth interface is necessary to be a coneaveshape with a certain radius range. If the change of the radius of growth interface is not in this limited range,the growth of DF (dislocation free) sinsle crystal is very difficult. The growth of FZ-Si single crystal was stud-ied. It is found that the growth speed ( 2. 5～2. 7 mm/min) as well as the rotation speed (3. 5 r/min) for theΦ100 mm crystal can be smaller . comparing with the Φ76. 2 mm crystal with the same coil. In order to satisfythe demand of large diameter crystal . the size of coil should be large enough, and the shape should satisfy theneed of the growth interface of crystal. With the increasing of diameter , the heating power , the anode voltageand the strength of electric field within the coil should be increased, and Ar pressure in surrounding circum-stance should also be higher , from 1. 96 × 1 0￣4 Pa to 4. 90 × 10￣4 Pa.According to the above growth factors, three rods of Φ100 mm FZ-Si single crystal were grown success-fully , the weights are 8～10 kg. When the diameter of crystal cone is increased to a limited size, “remeltingarca” will occur in the surface of the crystal , which cause a failure of growing DF crystal , this reason may bethat the recrystalliztion direction has been chansed , as it does.

MICRO/NANO-MACHINING ON SILICON SURFACE WITH A MODIFIED ATOMIC FORCE MICROSCOPE

To understand the deformation and removal mechanism of material on nano-scale at ultralow loads,a systemic study on AFM micro/nano-machining on single crystal ailicon is conducted. The results indicate that AFM nano- machining has a precisely dimensional controllability and a good surface quality on nanometer scale.A SEM is adopted to observe nano-machined region and chips,the results indicate that the material removal mechanisms change with the applied normal load. An XPS is used to analyze the changes of chemical composition inside and outside the nano-machined region respectively.The nano-indentation which is conducted with the same AFM diamond tip on the machined region shows a big discrepancy compared with that on the macro-scale. The calculated results show higher nano-hardness and elastic modulus than normal values .This phenomenon on be regarded as the indentation size effect(ISE).

Stress Relaxation Behaviors of Monocrystalline Silicon Coated with Amorphous SiO_(2) Film:A Molecular Dynamics Study

Long-lasting constant loading commonly exists in silicon-based microelectronic contact,as well as the chemical mechanical polishing area.In this work,the stress relaxation analysis of single crystal silicon coated with an amorphous SiO_(2) film is performed by varying the maximum indentation depth using molecular dynamics simulation.It is found that during holding,the applied indentation force declines sharply at the beginning and then steadily towards the end of the holding period.The stress relaxation amount of bilayer composites increases as the maximum indentation depth increases.It is also found that the deformation features of SiO_(2) film and silicon substrate during holding are inherited from the loading process.The SiO_(2) film during holding is further densified when the maximum indentation depth is equal to or less than a certain value(5.5 nm for the 0.8-nm film).The amount of generated phases and phase distributions of silicon substrate during holding are affected by the plastic deformation of silicon during loading.

Effect of Tool Geometry in Nanometric Cutting

With the development of science and technology, the ultra-precision manufacturing of the brittle and hard materials with superior quality have become a new attractive subject. Brittle materials (such as engineering ceramics, optical glass, semiconductor and so on) are widely used in electronics, optics, aeronautics and other high technology fields, so there are important theory significance and practical value to systematically study its machining mechanism and technology. Single crystal silicon is one of the typical brittle materials. Single crystal silicon wafer is a basic component of large and ultralarge integrated the circuit, its surface roughness and flatness are the key factor of improving its integration. With the successfully producing of the large diameter single crystal silicon wafer, its manufacturing technology became attractive subject again. This paper carries out computer simulation of nanometer cutting on single crystal silicon. Molecular Dynamics method which is different from continuous mechanics is employed to investigate the features of grinding energy dissipation, grinding force, stress state and grinding temperature, constructs the atom model of tool and work piece, and explains the microscale mechanism of material remove and surface generation of nanometer(subnanometer) manufacturing. This paper also investigates the variation of cutting force, thrust force, specific energy and surface deformation with different tool edge radius, different depth of cut.

Measurement Method of Compressibility and Thermal Expansion Coefficients for Density Standard Liquid at 2329 kg/m^3 based on Hydrostatic Suspension Principle

The accurate measurement on the compressibility and thermal expansion coefficients of density standard liquid at 2329kg/m3（DSL-2329） plays an important role in the quality control for silicon single crystal manufacturing. A new method is developed based on hydrostatic suspension principle in order to determine the two coefficients with high measurement accuracy. Two silicon single crystal samples with known density are immersed into a sealed vessel full of DSL-2329. The density of liquid is adjusted with varying liquid temperature and static pressure, so that the hydrostatic suspension of two silicon single crystal samples is achieved. The compression and thermal expansion coefficients are then calculated by using the data of temperature and static pressure at the suspension state. One silicon single crystal sample can be suspended at different state, as long as the liquid temperature and static pressure function linearly according to a certain mathematical relationship. A hydrostatic suspension experimental system is devised with the maximal temperature control error ±50 μK; Silicon single crystal samples can be suspended by adapting the pressure following the PID method. By using the method based on hydrostatic suspension principle, the two key coefficients can be measured at the same time, and measurement precision can be improved due to avoiding the influence of liquid surface tension. This method was further validated experimentally, where the mixture of 1, 2, 3-tribromopropane and 1,2-dibromoethane is used as DSL-2329. The compressibility and thermal expansion coefficients were measured, as 8.5′10–4 K–1 and 5.4′10–10 Pa–1, respectively.

Application of atmospheric pressure plasma polishing method in machining of silicon ultra-smooth surfaces

The modern optics industry demands rigorous surface quality with minimum defects,which presents challenges to optics machining technologies.There are always certain defects on the final surfaces of the compo-nents formed in conventional contacting machining proc-esses,such as micro-cracks,lattice disturbances,etc.It is especially serious for hard-brittle functional materials,such as crystals,glass and ceramics because of their special characteristics.To solve these problems,the atmospheric pressure plasma polishing(APPP)method is developed.It utilizes chemical reactions between reactive plasma and surface atoms to perform atom-scale material removal.Since the machining process is chemical in nature,APPP avoids the surface/subsurface defects mentioned above.As the key component,a capacitance coupled radio-fre-quency plasma torch is first introduced.In initial opera-tions,silicon wafers were machined as samples.Before applying operations,both the temperature distribution on the work-piece surface and the spatial gas diffusion in the machining process were studied qualitatively by finite element analysis.Then the following temperature measurement experiments demonstrate the formation of the temperature gradient on the wafer surface predicted by the theoretical analysis and indicated a peak temper-ature about 90uC in the center.By using commercialized form talysurf,the machined surface was detected and the result shows regular removal profile that corresponds well to the flow field model.Moreover,the removal profile also indicates a 32 mm^(3)/min removal rate.By using atomic force microscopy(AFM),the surface roughness was also measured and the result demonstrates an Ra 0.6 nm surface roughness.Then the element composition of the machined surface was detected and analyzed by X-ray photoelectron spectroscopy(XPS)technology.The results also demonstrate the occurrence of the anticipated main reactions.All the experiments have proved that this atmospheric pressure plasma polishing method has the potential to achieve the manufacture of high quality optical surfaces.

Surface bonding on silicon surfaces as probed by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been used to obtain the Raman signal of surface species on silicon single crystal surfaces without the necessity for surface enhancement by addition of Ag nanoparticles. By illuminating the hydrogen terminated silicon surface covered with a droplet of 4-vinylpyridine with UV light, a 4-ethylpyridine modified silicon surface can be easily obtained. By bringing a scanning tunneling microscope (STM) Au tip with a nanoscale tip apex to a distance of ca. 1 nm from the modified silicon surface, enhanced Raman signals of the silicon phonon vibrations and the surface-bonded 4-ethylpyridine were obtained. The Raman enhancement factor was estimated to be close to 107. By comparing the surface enhanced Raman scattering (SERS) signal obtained after surface enhancement with Ag nanoparticles and the TERS signal of the surface, the advantage of TERS over SERS for characterizing the surface species on substrates becomes apparent: TERS readily affords vibrational information about the system without disturbing it by surface enhancement. In this sense, TERS can be considered a truly non-invasive tool which is ideal for characterizing the actual surface species on substrates.

The chemisorption of Mg on the Si(100)-(2×1) surface

The adsorption of a half monolayer of Mg atoms on the Si（100）-（2×1） surface is studied by using the self-consistent tight binding linear muffin-tin orbital method.Energies of the adsorption systems of Mg atoms on the different sites are calculated.It has been found that the adsorbed Mg atoms are more favorable on the cave site above the surface than any other sites on the Si（100）-（2×1） surface and a metastable shallow site also exists above the surface.This is in agreement with the experimental results.The charge transfer and the layer projected density of states are also studied.