Sapphire Surface Research Articles

Compact speckle spectrometer based on CNN-LSTM denoising

We propose a compact speckle spectrometer that utilizes micro-nanostructures processed by femtosecond lasers on sapphire surfaces as scattering media. The spectral resolution is 0.5 nm, and the entire system is compact and stable. At the same time, the convolutional long short-term memory network (CNN-LSTM) was introduced into the denoising algorithm. Compared with traditional reconstruction algorithms, this method not only ensures rapid spectral reconstruction but also offers better reconstruction accuracy. It can effectively reduce the reconstruction error caused by the reduction of speckle autocorrelation caused by environmental noise and prolong the stability time of the system.

Orientation-dependent structural properties during growth and growth mechanism of CoO films

The orientation-dependent local structural properties of CoO films on sapphire substrates during growth were investigated through linearly-polarized extended X-ray absorption fine structure (EXAFS) measurements. Specifically, CoO(111) and (100) crystals with a rock-salt structure (Fm3m) were epitaxially grown on α-Al2O3(0001) and (101¯2) substrates, respectively, at 700℃ using a radio-frequency sputtering system. The local structural properties of CoO films in the in-plane and out-of-plane orientations were quantitatively determined using linearly-polarized EXAFS at the Co K-edge during growth. The EXAFS analysis revealed that during the initial stages of growth, the local structural properties exhibit significant differences compared to thick films, with short atomic distances and large (small) Debye-Waller factors observed in the out-of-plane (in-plane) orientations. The local structural strain mostly diminished when approximately 20 CoO layers accumulated on the substrate. Density functional theory (DFT) calculations further supported these findings by confirming that cobalt atoms initially form stable bonds with the sapphire surface, leading to the simultaneous growth of oxygen and cobalt layers in a coordinated manner through layer-by-layer growth.

Spiral feed polishing uniform removal and compensation strategy of sapphire components

A single crystal sapphire component has been widely used in various high-tech fields because of its significant advantages such as high hardness, high stability, and excellent optical and mechanical properties, and has put forward high requirements for surface accuracy and quality. The existing sapphire polishing technology has problems such as low polishing efficiency, difficult control of polishing accuracy, and difficulty in removing surface defects and subsurface damage introduced by the front grinding process. Therefore, for the polishing and damage removal stage of sapphire optical components, the surface shape accuracy should be strictly controlled, especially for the surface shape accuracy after ultra-precision grinding. It is of great significance to study the uniform removal technology of grinding damage based on not destroying the surface shape. This study focuses on the mechanical polishing of sapphire. Firstly, the characteristics of the sapphire removal function of the elastic polishing tool are analyzed and the stability of the polishing tool is verified. Secondly, by establishing the relationship between the amount of workpiece material removal and the radius of rotation, the feed rate is planned to achieve the effect of uniform spiral removal. By optimizing the center feed speed of the tilting axis small-size polishing tool, the center feature of the polishing surface is controlled to be sharp. Finally, based on the sub-aperture polishing figuring theory, the influence of the center feature on the surface profile is reduced by using the spiral grating processing method. This study provides an efficient and stable strategy for the uniform removal and polishing of rotationally symmetric optical elements (such as aspheric surfaces) of high-hardness material sapphire and is expected to play a role in scenes that are particularly difficult to process, difficult to detect full aperture, difficult to calculate removal function and dwell time, such as higher steep aspheric surfaces and Gaussian aspheric surfaces.

Influences of ultrasonic vibration directions, amplitudes, and frequencies on sapphire polishing studied by molecular dynamics

The sapphire surface morphology, atom removal rate, temperature, polishing force, subsurface damage, dislocation, and stress were explored under different ultrasonic directions, frequencies and amplitudes through molecular dynamics (MD). For both vertical and horizontal vibration, the rising ultrasonic frequency and amplitude will reduce the tangential and normal force, and increase the subsurface temperature and the material removal rate (MRR). Higher frequencies promote the basal dislocation, thus reducing the subsurface damage. Higher amplitudes cause thinner subsurface damage layer under horizontal vibration. However, it is opposite at vertical vibration. The horizontal vibration can obtain a flatter polished surface and a thinner subsurface damage layer due to the longer trajectory and less impact on sapphire surface. This study can provide reference for sapphire high-quality polishing.

Imaging of micro-steps on as-grown surface of sapphire with X-ray phase contrast technique

Basal-faceted sapphire ribbons grown using the Stepanov–LaBelle technology have a low density of surface steps. We present our results on a study of steps on the surface of a ribbon, misoriented relative to the singular face (0001) by several arc minutes. The ribbon has been characterized by means of in-line phase contrast imaging technique at Pohang Light Source, South Korea. It was shown for the first time that a step height of about 1 μm can be determined directly from an image. The step height obtained using the phase contrast method was confirmed by atomic force microscopy measurements. We have found that the experimental contrast matches the theoretical simulations only if the calculated intensity profile has been convolved with a Gaussian function. The full width at half maximum of the Gaussian was independently got from previous measurements. We have obtained an analytical solution in the case of theoretical fully coherent phase contrast image. The inverse problem is easy to solve, since there is a direct proportionality between the contrast and the step height.

Study on the polishing performance and mechanism of sapphire wafers by different types of degradable surfactants

Traditional chemical-mechanical polishing (CMP) slurry suffers from significant environmental pollution, and easy reaggregation of ultra-fine abrasives, all of which hinder its further application. In this study, the processing performance and mechanism of three different degradable surfactants to sapphire wafers were investigated, utilizing degradable aminomethylpropanol and xylitol as pH regulator and complexing agent of polishing slurry, respectively, which is aimed at solving the problem of the easy agglomeration of ultra-fine abrasives while achieving the super-precision and pollution-free process of sapphire wafers. After polishing, the surface roughness (Sa) of sapphire wafers decreases from 7.240 nm to 0.396 nm, which is 44.8 % lower than that without surfactant. Only <1 nm is the PV value, and there is a 12.9 % increase in the material removal rate. The action mechanism of biodegradable surfactants during CMP was revealed through TEM, XPS, electrochemical testing, contact angle measurement, and particle size analysis. The results showed that surfactants can effectively improve the wettability of the polishing slurry on the wafer and the dispersity of the ultra-fine abrasives in the slurry, thus facilitating the interfacial reaction between the sapphire surface and polishing slurry and the homogeneity of the material removal. This work truly realizes a green and pollution-free process that meets the requirements of ultra-smooth and efficient polishing of sapphire wafers.

Modulating growth of graphene on sapphire by chemical vapor deposition

The direct growth of graphene on insulating substrates presents significant promise for the advancement of next-generation semiconductor technologies. Here, the synthesis of graphene directly on sapphire substrates via chemical vapor deposition (CVD) is reported. The CVD growth parameters are systematically manipulated to optimize the crystal quality of graphene. Furthermore, the dependency of graphene growth on the crystallographic orientation of the sapphire substrate is explored to elucidate the underlying mechanisms. It is found that robust C-O chemical bonds are established between the oxygen atoms on the sapphire surface and the carbon atoms in the graphene, predominantly influencing the nucleation process. The oxygen-rich nature of the c-plane sapphire surface contributes to a higher graphene nucleation density and the formation of smaller grains comparing to r-plane sapphire. Accordingly, high quality and uniform monolayer graphene is successfully obtained on both c-plane and r-plane sapphire.

Study on wetting mechanism of Sn3Sc alloy on silica and sapphire surfaces

To further investigate the wetting mechanism of filler alloys containing active elements on oxide ceramics surfaces, we prepared a new filler alloy (Sn-3at.%Sc filler alloy) containing trace rare earth elements by vacuum melting furnace. Under high vacuum, its wetting behavior on silica and sapphire substrates was studied using a modified sessile drop method. The final contact angles of liquid Sn-3at.%Sc filler alloy on silica and sapphire surfaces are 6.0° and 9.8°, respectively, showing good wetting properties, and exhibiting similar wetting behavior at the same temperature. The same interfacial product Sc2O3 was detected at the interface of Sn-3at.%Sc/silica and Sn-3at.%Sc/sapphire systems. However, thermodynamic and spreading analysis suggests that Sc2O3 is precipitated in different ways at the interface of the two systems. The former is generated by an interfacial reaction, while the latter precipitates during the cooling process. The wettability of the Sn-3at.%Sc/silica system may depend on the interaction between the interface reaction products and residual scandium adsorption at the solid/liquid interface, while the wettability of Sn-3at.%Sc/sapphire system may depend on the adsorption of Sc-O clusters at the interface. These findings provide a theoretical basis and design ideas for further development of tin-based filler alloys and design of new type filler alloys.

Fabrication of sapphire/MgAl2O4/sapphire laminated transparent composite by brazing with BaO–B2O3–SiO2 glass filler

For improving the mechanical properties of MgAl2O4 ceramic and maintaining its wide band transparency, the sapphire/MgAl2O4/sapphire laminated transparent composites with thin sapphire layers were fabricated by brazing with BaO–B2O3–SiO2 (BBS) glass filler. The BBS glass filler had an appropriate CTE (coefficient of thermal expansion) value and good wettability on the surface of sapphire and MgAl2O4 ceramics. Joining experiments were first carried out to optimize the preparation process parameters of laminated composites. The results indicated that high bonding temperature promoted mutual dissolution and diffusion of BBS glass and MgAl2O4 ceramic, but led to the intergranular infiltration of glass filler and the formation of pores due to volatilization of B2O3. When the bonding temperature was 1000 °C, the sapphire/MgAl2O4 joints with the best transparency and acceptable shear strength could be obtained. On this basis, the laminated composites with 0.15 mm thick sapphire layers were fabricated by brazing using BBS glass at 1000 °C. The flexural strength of the laminated composite reached 320 ± 10 MPa, which was higher than that of MgAl2O4 (195 ± 15 MPa). The in-line transmittance of laminated composite was 80.9 % at 1000 nm, which was slightly lower than that of MgAl2O4 (82.2 %).

Step-edge-guided nucleation and growth mode transition of α-Ga2O3 heteroepitaxy on vicinal sapphire

Abstract Controlling the epitaxial growth mode of semiconductor layers is crucial for optimizing material properties and device performance. In this work, the growth mode of α-Ga2O3 heteroepitaxial layers was modulated by tuning miscut angles (θ) from 0° to 7° off the (10-10) direction of sapphire (0002) substrate. On flat sapphire surfaces, the growth undergoes a typical three-dimensional (3D) growth mode due to the random nucleation on wide substrate terraces, as evidenced by the hillock morphology and high dislocation densities. As the miscut angle increases to θ=5°, the terrace width of sapphire substrate is comparable to the distance between neighboring nuclei, and consequently, the nucleation is guided by terrace edges, which energetically facilitates the growth mode transition into the desirable two-dimensional coherent growth. Consequently, the mean surface roughness decreases to only 0.62 nm, accompanied by a significant reduction in screw and edge dislocations to 0.16×107 cm-2 and 3.58×109 cm-2, respectively. However, the further increment of miscut angles to θ=7° shrink the terrace width less than nucleation distance, and the step-bunching growth mode is dominant. In this circumstance, the misfit strain is released in the initial growth stage, resulting in surface morphology degradation and increased dislocation densities.

Molecular dynamics simulation to investigate titanium metallization on sapphire for fiber optic-based pressure sensing application

Brazing of sapphire or ceramic with metal is an essential manufacturing process for sensor and ultra-high vacuum applications. For an efficient brazing of such materials with metals, a prerequisite of metallization of sapphire with titanium (Ti) layers with good adhesion is required. In this direction, the present work is carried out to investigate the phenomena occurring at the interface of titanium–sapphire during metallization process and observe the strength of joint of sapphire with respect to Ti layer. To carry out such study, a model was constructed using molecular dynamics simulation (MDS) for both sapphire and Ti. The MDS model showed that the chemical reaction between Ti and sapphire takes place to form new chemical bonds of Ti–O, Ti–O–Al–O–Ti, and Al–O–Ti. These new bonds are much stronger than the chemical bonds of parent materials and they are the key factor for brazing of sapphire surface using Ti layer. This study shows that chemical reaction in the metallization is a spontaneous and exothermic process which increases the temperature of Ti layer by 450 K. Moreover, it is also observed that the metallization quality mainly depends on the extent of proximity between sapphire and Ti, and it is possible by surface heating up to 1,200 K. The present work is for the understanding and optimization of metallization process, which could be useful toward an efficient execution of brazing between sapphire and metal.

Observation and analysis of the ultra-rapid cooling effect of cryoprotectant droplets on a sapphire surface

Cryopreservation has emerged as a promising technology to realize semipermanent storage of biomaterials widely used in assisted reproduction and cell therapy. A large portion of successful cryopreservation requires the combination of ultra-rapid cooling and appropriate low-toxicity cryoprotectant agents, which inhibits the formation of ice crystals that damages the cells lethally. Conventional approaches are not competent in accurately capturing high enough cooling rate and more importantly, not able to simultaneously track the instant appearance of the sample at a microscale level during the momentary cooling process. A rapid freezing device for the droplet sample study purpose was designed and developed, which is capable of visualization of the sample on a sapphire surface at a vitrification cooling rate over 104 K/min. The instantaneous temperature changes and the appearance of the cooling samples were recorded at the speed of 10 kHz and 2000 frames. Typical vitrification and crystallization processes were observed and analysed. Low concentrations of the CPA solution inhibited the formation and growth of the crystals, and resulted in the formation of layered ice, punctate ice and scattered small ice. The experimental results could contribute to a better understanding of the ultra-rapid cooling mechanism on cryoprotectant droplets.

Femtosecond laser welding of sapphire-copper using a thin film titanium interlayer

The ability to weld metals and sapphire is widely required in aerospace, micro-electro-mechanical systems and microwave energy delivery devices. The experiments of femtosecond laser welding between sapphire and copper (Cu) were conducted under non-optical contact conditions. And the effect of pre-fabricating a Ti film on the sapphire surface using femtosecond laser on the bonding strength of ssapphire/Cu joint was investigated. The interface of the sapphire/Cu directly welded by femtosecond laser contains a mixture of sapphire and copper, and the shear strength of the sapphire/Cu interface was approximately 46.4 MPa. The results indicated that the laser-induced molten state of sapphire and the laser-ablated copper mixed together at the interface, filling the gaps and forming an effective metallurgical bond. And then, in the case that the Ti film was prepared on the sapphire surface through femtosecond laser-induced backward transfer (LIBT), a Ti transition layer between sapphire and cooper was observed on the bonding interface of the joint, and the shear strength of the joint increased to approximately 115.5 MPa. A mixture of sapphire, copper, titanium, and titanium oxide was observed at the interface of the joint. The Ti layer between sapphire and copper may effectively alleviated residual stresses at the interface and promoted bonding between copper and sapphire, thereby improving the strength of the joint. The methods and results of this study have reference value for the ultrafast laser bonding between other transparent brittle materials and metals.

A Method for Preparing Surface Sub-Microstructures on Sapphire Surfaces Using Femtosecond Laser Processing Technology

Femtosecond laser processing technology is an advanced sub-micro-processing technique that enables the non-contact processing of various materials. This technology can be used to apply sub-micro structures for purposes such as hydrophilicity enhancement, optical transmittance improvement, and photonics detection. However, when it comes to processing micro/nanostructures on highly brittle materials using femtosecond lasers, there are challenges such as low processing efficiency, generation of debris, and microcracking. In this paper, we propose a method called the out-of-focus femtosecond laser direct writing technique combined with wet etching. This method offers simplicity, speed, and flexibility in preparing dense, large-area sub-microstructured surfaces on the brittle material sapphire. Our detailed investigation focuses on the impact of laser processing parameters (direct writing period, distance of focusing, direct writing speed, etc.) on the sub-microstructures of Al2O3 surfaces. The results demonstrate that this method successfully creates embedded sub-microstructures on the sapphire surface. The microholes, with a diameter of approximately 2.0 μm, contain sub-micro structures with a minimum width of 250 ± 20 nm. Additionally, we conducted experiments to assess the optical transmittance of sapphire nanostructures in the range of 350–1200 nm, which exhibited an average transmittance of approximately 77.0%. The water contact angle (CA) test yielded a result of 52 ± 2°, indicating an enhancement in the hydrophilicity of the sapphire nanostructures with only a slight reduction in optical transmittance. Our efficient fabrication of sub-microstructures on the sapphire surface of highly brittle materials offers a promising method for the production and application of brittle materials in the field of micro-optics.

Wetting of Al2O3 by Cu-xTi alloys at 1373 K

Wetting of Al2O3 by molten Cu-xTi alloys with wide composition range was deeply studied. The results show that the precipitation of TiO at the interface is depended on the Ti concentration in the alloys as well as the oxygen partial pressure in the atmosphere. When Ti concentration is lower than the critical Ti concentration, the wettability of the system is mainly determined by the precipitated TiO and the adsorption of Ti-O clusters at the interface; conversely, it is mainly determined by the adsorption of Ti-O clusters on the surface of alumina. For the spreading dynamics of: Cu-Ti alloys on the sapphire surface, the existence of two stages were observed: firstly, an initial fast spreading stage, which is mainly limited by the reaction rate of TiO precipitation; followed by a later slow spreading stage, which is mainly limited by the precipitation reaction of Al2O, and exhibits a stepwise spreading characteristic.

Large-area single-crystal TMD growth modulated by sapphire substrates.

Transition metal dichalcogenides (TMDs) have recently attracted extensive attention due to their unique physical and chemical properties; however, the preparation of large-area TMD single crystals is still a great challenge. Chemical vapor deposition (CVD) is an effective method to synthesize large-area and high-quality TMD films, in which sapphires as suitable substrates play a crucial role in anchoring the source material, promoting nucleation and modulating epitaxial growth. In this review, we provide an insightful overview of different epitaxial mechanisms and growth behaviors associated with the atomic structure of sapphire surfaces and the growth parameters. First, we summarize three epitaxial growth mechanisms of TMDs on sapphire substrates, namely, van der Waals epitaxy, step-guided epitaxy, and dual-coupling-guided epitaxy. Second, we introduce the effects of polishing, cutting, and annealing processing of the sapphire surface on the TMD growth. Finally, we discuss the influence of other growth parameters, such as temperature, pressure, carrier gas, and substrate position, on the growth kinetics of TMDs. This review might provide deep insights into the controllable growth of large-area single-crystal TMDs on sapphires, which will propel their practical applications in high-performance nanoelectronics and optoelectronics.

Orthogonal experimental study on the influence of machining parameters on flat lapping of sapphire substrate

Sapphire is extensively utilized in the optical, aerospace, and civil electronic industries due to its favorable optical, physical, and chemical characteristics. To enhance the efficiency and quality of sapphire lapping, an orthogonal experiment was conducted on a single-side sapphire substrate using a ceramic lapping plate. The study examined the impact of lapping plate material, lapping pressure, lapping speed, and abrasive particle size on the surface roughness, profile, and removal rate. By analyzing the signal-to-noise ratio and variance of machining parameters, the influence rule and influence of the weight of machining parameters on machining results were obtained. The results of the experiment demonstrate that the material removal rate of sapphire was positively affected by an increase in lapping pressure, speed, and abrasive particle size. Moreover, the removal rate of the SiC lapping plate was the highest among the experimental materials. The roughness of the sapphire surface decreased with increasing lapping pressure, speed, and abrasive particle size, while the SiC plate had the lowest surface roughness. The profile tolerance of sapphire diminished as the lapping pressure, lapping speed, and the abrasive particle size increased. Additionally, the ZrO2 lapping plate exhibited the most minor profile tolerance. The size of the abrasive particle significantly impacted the material removal rate, with a specific gravity exceeding 70%. Similarly, the lapping pressure had a significant effect on both the surface roughness and the profile tolerance. The ideal machining parameter combination comprised an abrasive particle size of 10 µm, a lapping pressure of 22 785.0 Pa, a lapping speed of 60 rpm, and a lapping plate of SiC. Under optimal machining conditions, sapphire exhibited a material removal rate of 0.65 µm/h, a surface roughness of 0.0920 µm, and a profile tolerance of 2.0915 µm after 20 min of lapping. This demonstrated that the lapping process enables highly efficient and high-quality machining of sapphire substrates.

Influence of surface damage on the optical properties of sapphire and its etching repair method

Sapphire optical components are prone to cracks and scratches during grinding, installation, and service, which affects the laser damage resistance. In order to improve the laser damage resistance of sapphire damaged surface, the influence of surface damage on the laser damage resistance was analyzed. The repair mechanism of wet etching on the damaged surface was expounded. The results show that the reduction of damage frequency and depth can reduce the LIMF, and the LIMF caused by ellipsoidal damage is less than prism damage. The texture direction relationship between the two surfaces influences the internal LIMF. Compared with the vertical surface damage, the parallel surface damage has a larger LIMF value. The LIMF value of the etching surface is minimal. The characteristics of the damaged surface before and after etching were compared. The results show that the surface damages are passivated and merged after etching, and the surface transforms into ellipsoidal features. These changes are conducive to improving the laser damage resistance. Finally, the effect of the etching process on surface quality is analyzed. The formation mechanism and etching repair mechanism of sapphire damaged surface at the atomic scale were revealed by molecular dynamics simulation. Finally, the laser damage resistance of sapphire surface was tested, and the feasibility of wet etching was verified.

Nanomaterials and Equipment for Chemical–Mechanical Polishing of Single-Crystal Sapphire Wafers

Single-crystal sapphire (α-Al2O3) has been widely used in semiconductor, optics, communication, national defense, and other fields. Before application, an ultra-smooth surface which is scratch free and subsurface damage free is essential. Furthermore, the sapphire has unique qualities such as significant rigidity and chemical stability, which make it extremely arduous to process. Chemical mechanical polishing (CMP) is recognized as the final process to reduce the roughness and eliminate surface defects of a sapphire surface. In this review, the materials and equipment used for the chemical polishing of a sapphire wafer are summarized, and the surface nanoscale changes of sapphire wafer are reviewed from the angles of regulating polishing-process parameters, composition of polishing slurry including that which is nano-abrasive, a pH regulator, a complexing agent, and other additives, as well as hybrid CMP technologies. The outlook and future applications are also summarized.

Single Shot Generation of High‐Aspect‐Ratio Nano‐Rods from Sapphire by Ultrafast First Order Bessel Beam

AbstractEngineering the polarization and spatial phase of ultrafast laser pulses represents a compelling strategy for enhancing control over laser–matter interaction and enabling rapid and innovative nano‐fabrication processes. Here, the single‐shot, ultrafast laser fabrication of high‐aspect‐ratio, vertically standing nano‐pillars with a diameter of nm and height up to 15 µm on the surface of sapphire, is reported. To achieve this, the distinctive properties of diffraction‐free, first‐order Bessel beams endowed with either radial or azimuthal polarization distributions, are harnessed under tight focusing conditions. The highly intense laser–matter interaction in this configuration generates a tubular‐shaped, high‐pressure field beneath the material surface, leading to the rapid expulsion of material across the surface. Three distinct regimes for the pillar generation are identified in addition to a mechanism based on the Rayleigh‐Plateau theory that explains the distinct morphological regimes observed. The findings not only shed light on the underlying physical mechanisms of intense excitation of transparent dielectrics but also offer exciting prospects for the rapid fabrication of positive nano‐structures and material compression across various fields of application.