Polished Sapphire Surface Research Articles

Study on material removal mechanism of sapphire wafer with CeO2 coated diamond composite abrasives via green polishing

Toxic and polluted ingredients are normally employed in the chemical mechanical polishing of sapphire wafers, which results in widespread environmental pollution. Thus, developing a novel green polishing technology is a significant challenge to achieve ultra-smooth and efficient polishing. In this study, novel CeO2 coated diamond composite abrasives were used in the semi-fixed abrasive polishing of sapphire wafers, which were successfully synthesized by a modified precipitation approach without using explosive hazardous chemicals. During the whole polishing process, the polishing slurry only contains deionized water, without any other chemical reagents, to avoid environmental pollution. Polishing results showed that the surface roughness of sapphire polished by the composite abrasives was reduced to 6.1 nm, which was 26.5 % lower than that of pure diamond. Meanwhile, the material removal rate was 1.46 nm/min, with an improvement of 10.6 %, as well as a reduction in residual stress by approximately 29.8 %. Combined with the theory of micro-contact mechanics, the polishing mechanism and interfacial contact behavior of composite abrasives were investigated in depth through the analysis of polished sapphire surface and wear debris. The improvement of polishing performance may be attributed to the enhancement of solid-state reaction and its mutual balance with the mechanical removal action of composite abrasives.

Novel model of material removal rate on ultrasonic-assisted chemical mechanical polishing for sapphire

AbstractUltrasonic-assisted chemical mechanical polishing (UA-CMP) can greatly improve the sapphire material removal and surface quality, but its polishing mechanism is still unclear. This paper proposed a novel model of material removal rate (MRR) to explore the mechanism of sapphire UA-CMP. It contains two modes, namely two-body wear and abrasive-impact. Furthermore, the atomic force microscopy (AFM) in-situ study, computational fluid dynamics (CFD) simulation, and polishing experiments were conducted to verify the model and reveal the polishing mechanism. In the AFM in-situ studies, the tip scratched the reaction layer on the sapphire surface. The pit with a 0.22 nm depth is the evidence of two-body wear. The CFD simulation showed that abrasives could be driven by the ultrasonic vibration to impact the sapphire surface at high frequencies. The maximum total velocity and the air volume fraction (AVF) in the central area increased from 0.26 to 0.55 m/s and 20% to 49%, respectively, with the rising amplitudes of 1–3 µm. However, the maximum total velocity rose slightly from 0.33 to 0.42 m/s, and the AVF was nearly unchanged under 40–80 r/min. It indicated that the ultrasonic energy has great effects on the abrasive-impact mode. The UA-CMP experimental results exhibited that there was 63.7% improvement in MRR when the polishing velocities rose from 40 to 80 r/min. The roughness of the polished sapphire surface was Ra = 0.07 nm. It identified that the higher speed achieved greater MRR mainly through the two-body wear mode. This study is beneficial to further understanding the UA-CMP mechanism and promoting the development of UA-CMP technology.

Polishing Performance and Removal Mechanism of Core-Shell Structured Diamond/SiO2 Abrasives on Sapphire Wafer.

Corrosive and toxic solutions are normally employed to polish sapphire wafers, which easily cause environmental pollution. Applying green polishing techniques to obtain an ultrasmooth sapphire surface that is scratch-free and has low damage at high polishing efficiency is a great challenge. In this paper, novel diamond/SiO2 composite abrasives were successfully synthesized by a simplified sol-gel strategy. The prepared composite abrasives were used in the semi-fixed polishing technology of sapphire wafers, where the polishing slurry contains only deionized water and no other chemicals during the whole polishing process, effectively avoiding environmental pollution. The experimental results showed that diamond/SiO2 composite abrasives exhibited excellent polishing performance, along with a 27.2% decrease in surface roughness, and the material removal rate was increased by more than 8.8% compared with pure diamond. Furthermore, through characterizations of polished sapphire surfaces and wear debris, the chemical action mechanism of composite abrasives was investigated, which confirmed the solid-state reaction between the SiO2 shell and the sapphire surface. Finally, applying the elastic-plastic contact model revealed that the reduction of indentation depth and the synergistic effect of chemical corrosion and mechanical removal are the keys to improving polishing performance.

Homogenization and reduction of the roughness of polished sapphire surfaces via inert gas plasma post-processing

In this paper, we report on the post-processing of classically precision polished sapphire surfaces by means of a dielectric barrier discharge plasma at atmospheric pressure. Here, the lateral distribution of roughness values over the surface was significantly reduced, leading to a higher lateral homogeneity. This effect is shown by a decrease in the standard deviation of the roughness values measured at different positions over the sample surface where an overall plasma-induced reduction by a factor of 8.7 was achieved. Moreover, the residual roughness was notably decreased by a mean factor of up to 2.9 to merely a few angstrom. The obtained results are of high interest for the enhancement of large-scale optical components made of sapphire, especially for UV or laser applications. This is finally visualized by the comparison of the total integrated scatter values calculated for untreated and post-processed surfaces.

Effects of quantities and pole-arrangements of magnets on the magneto-rheological polishing (MRP) performance of sapphire hemisphere

To improve the polishing effectiveness of sapphire hemisphere, the effects of quantities and pole-arrangements of magnets in magnetic field generator on the magneto-rheological polishing (MRP) performance of sapphire hemisphere were investigated through simulation and experiment. After analyzing the distribution of magnetic field on sapphire hemisphere surface and its change with the relative motion between sapphire surface and magnetic field generator during MRP, we developed a material removal rate (MRR) model based on the Preston equation, and the material removal distribution and average removal depth of polished sapphire surface could be predicted. Then according to the calculated material removal distribution, a surface roughness model of polished sapphire was established. Both simulation and experiment results showed that the MRR of polished sapphire increases with the quantity of magnets, while the surface roughness Ra of polished sapphire decreases with the quantity of magnets. As for the effect of pole-arrangement, the cross-stacking arrangement produced a surface of Ra 2.97 nm with MRR of 0.496 μm/h, and the cross-involute arrangement achieved a smoother surface of Ra 0.79 nm with MRR of 0.435 μm/h in MRP. These results showed that the cross-involute and cross-stacking pole-arrangements are favorable for the MRP of sapphire hemisphere. The good agreement between experimental and theoretical results verified the reliability of the established MRR and Ra models.

Preparation of a novel catalyst (SoFeIII) and its catalytic performance towards the removal rate of sapphire substrate during CMP process

Abstract Chemical mechanical polishing of sapphire with a novel catalyst (SoFeIII)-based colloidal silicon dioxide has been studied with high resolution transmission electron microscopy and X-ray diffraction. It has been found that a polytype of aluminun trihydroxide is formed on the polished crystal surface when SoFeIII-80 °C catalyst is used, while aluminum silicate hydrate layer is found on the polished sapphire surface when burnished with catalyst-free slurry. And the novel catalyst plays effective performance towards improving the removal rate of sapphire, and its removal rate is 7.21 μm/h, 1.66 times than the material removal rate obtained with catalyst-free slurry. Additionally, the optimum CMP removal by SoFeIII-80 °C yielded an ultra-smooth wafer surface with an average roughness of 0.0543 nm.

AFM and XPS studies on material removal mechanism of sapphire wafer during chemical mechanical polishing (CMP)

The material removal mechanism of sapphire wafer during chemical mechanical polishing has been studied through X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. XPS results indicate that alumina silicate (Al2Si2O7·2H2O) is generated on the polished sapphire surface by SiO2 slurry, otherwise alumina hydrate (AlO(OH)) on the polished surface by H2O solution. Meanwhile, ultra-smooth polished surface with extremely low Ra of below 0.1 nm and atomic step structure morphology via AFM is realized using SiO2 slurry. Through investigating the variations of the surface characteristics polished by different ingredients via the morphology and force curve measurements, it’s reveals that the product-aluminum silicate with stronger adhesion and lower hardness is more readily to generate and be removed than the product-alumina hydrate induced by H2O. Thus, except for atomic scale mechanical abrading, the abrasive SiO2 nanoparticle is used for anticipating in the chemical reaction, resulting in superior surface finish of sapphire wafer with high efficiency.

Surface Integrity Induced by Abrasive Machining Sapphire Wafer

Surface integrity of Sapphire wafer is the frontier technology to grow high quality AlN/GaN films for high brightness light-emitting diode. Surface damage measurement methods for single crystal sapphire were introduced firstly. Classical types of surface deformation induced by abrasive machining, such as lattice deformation, strain, scratch, crack versus abrasive grits and chemical reaction were compared. With the development of modern grinding technology, depth of the damaged surface layer decreases from 10μm to 2μm or much less. Both crack, lattice deformation, twinning and strain were detected along the ground surface, only dislocation and strain were appeared for polished sapphire surface. Usually, depth of the damaged layer was less than 300nm using mechanical polishing process. Chemical mechanical polishing achieved the lowest depth of the damaged surface and best surface roughness, scratch depth was used to descript the damaged depth, which is about 1 nm, and so called no damage to the layer remains during chemical mechanical polishing process.

High rate etching of sapphire wafer using Cl 2/BCl 3/Ar inductively coupled plasmas

Sapphire wafers which are generally used for the fabrication of GaN-based optoelectronic devices are found to be very difficult in lapping, polishing, and cutting for packaging due to the chemical stability and hardness of sapphire. To study possibilities of replacing some of these processes by dry etching, (0001) sapphire wafers were etched using inductively coupled plasmas as a function of gas combination of Cl 2/BCl 3 and Ar/Cl 2/BCl 3. The increase of BCl 3 in Cl 2/BCl 3 increased the etch rates. Also, the increase of BCl 3 in Cl 2 improved the etch selectivity over photoresist. With the mixture of 50% Cl 2/50% BCl 3, sapphire etch rates of 362.7 nm min −1 could be obtained and, by the addition of 20% Ar in this mixture, the etch rates increased further to 377.5 nm min −1. When the sapphire etching with 50% Cl 2/50% BCl 3 was applied to lapped wafers for polishing, the surface roughness was decreased from 12.95 to 1.43 nm and the smoothness was better than mechanically polished sapphire surface (5.3 8 nm).

Phonon boundary scattering at a silicon-sapphire interface

The authors have measured the low-temperature thermal conductivity of unusual geometry sapphire bars that have undergone various surface treatments. They show that after careful surface polishing a ten-fold enhancement over the correctly calculated Casimir limit is possible, and that the phonon specular reflection coefficient for such a surface is consistent with phonon scattering from surface asperities. Lightly abrading the surface reduces the specular reflection coefficient as expected; however, in this case the phonon scattering cannot be associated with surface roughness but rather with subsurface dislocations. Silicon deposited epitaxially onto the polished sapphire surface also reduces the ten-fold enhancement of the sapphire above. They show that this is in agreement with the quality of the silicon-sapphire interface and postulate that the thermal conductivity of the composite sample may provide a nondestructive technique for monitoring the average dislocation density at the interface.

Optical characterization of silicon and sapphire surfaces as related to SOS discrete device performance

Specular reflectance measurements were used in the quantitative characterization of sapphire and silicon film surfaces. Residual polishing damage in sapphire surfaces can be readily detected by infrared multiple reflectance measurements in the lattice-band region of sapphire at about 600 cm -1. Specular reflectance measurements in the ultraviolet, at a photon energy of about 4.3 eV (corresponding to the X 4− X 1 silicon transition), were used for the surface characterization of silicon films on sapphire. The reflectance methods were applied to the characterization of variously polished sapphire surfaces and to the characterization of heteroepitaxial silicon films grown on the substrates. The results of these measurements were correlated with various parameters of silicon-on-sapphire (SOS) devices fabricated in the silicon films. Measured device parameters include drain current, extrapolated threshold voltage, leakage current, and drain breakdown voltage. Most device data were automatically recorded on test patterns and statistical data were computed for the various device parameters.

Phonon scattering at crystal surfaces

Through measurements of the low-temperature (0.06-2.0 K) thermal conductivity of dielectric crystals in the boundary-scattering regime we are searching for causes of diffuse phonon scattering by crystal surfaces. Thin (\ensuremath{\le} 100 \AA{}) gold films on polished sapphire surfaces cause diffuse phonon scattering above 0.3 K. Below that temperature, specular reflection again begins to dominate. The same diffuse scattering can be caused by implantation of Xe ions which damage a layer $\ensuremath{\simeq}$500-\AA{} thick underneath the sapphire surface. For gold films, the effect diminishes with increasing thickness, i.e., greater perfection of the film. Because of the very small thickness of the layers in which the diffuse scattering occurs the phonon-scattering strength must be very much stronger than any phonon scattering previously observed in the bulk of dielectric crystals. Several potential explanations are discussed. However, none appear to be sufficiently general to account for both kinds of surface treatments.

Optical Reflectance Method for Determining the Surface uality of Sapphire ( Al2 O 3 )

Polished sapphire wafers are used as substrates for the growth of crystalline silicon. The quality of the substrate surface is an important factor in determining the crystalline quality of the silicon film and, in turn, relates to silicon device performance and reliability. To improve silicon‐on‐sapphire device manufacture, a sensitive and nondestructive optical reflectance technique has been developed to determine the quality of polished sapphire surfaces. The correlation between surface damage and single or multiple specular reflectance of sapphire in the lattice mode region, 900–300 cm−1, is reported. As a result of surface damage, the vibrational modes of sapphire are distorted and the optical constants associated with these modes change. To interpret the single and multiple reflectance spectra the optical constants of undamaged and damaged (11̅02) sapphire surfaces have been calculated from reflectance data using the Kramers‐Kronig method. From this analysis, the reflectance of sapphire at about 600 cm−1 is found to be very sensitive to surface quality. In this spectral region multiple reflectance can be used to measure the surface quality of sapphire with a high degree of sensitivity. The technique can, in principle, be used to measure the quality of any semiconductor or dielectric surface in spectral regions of high reflectance, such as regions of lattice band or bandgap absorption.