Microscopic morphology evolution during ion beam smoothing of Zerodur® surfaces

Axisymmetric surface shape correction of optical elements by a wide-aperture ion beam

Methods for calculating the process of axisymmetric surface shape correction with a wide-aperture ion beam through a forming diaphragm for optical elements with a round and arbitrary border shape are presented. In the case of circular optical elements, an approach based on the separation of the axisymmetric component of the surface shape error from its decomposition by Zernike polynomials is used. In the case of optical elements with an arbitrary border shape (not round), an algorithm for searching the axisymmetric component of the shape error is proposed. The search criterion is to minimize the residual volume of the deviation of the surface shape from the calculated one. The proposed algorithm makes it possible to separate the axisymmetric component of the shape error relative to an arbitrary axis, including one located outside the workpiece. The results of surface treatment in accordance with the above methods are presented.

Curvature effect-based modeling and experimentation of the material removal in polishing optical surfaces using a flexible ball-end tool

Optical surfaces with high quality have been widely applied in high-tech industries for their excellent performances. To precision manufacture those surfaces efficiently and effectively, various machining technologies involved become extremely crucial. As one of the promising ultra-precision machining technologies, inflated or solid elastic tool polishing has attracted more attention for its own superiority. However, there is still lack of understanding on material removal mechanisms especially with the consideration of curvature effect, and it is of great importance to determine the surface quality and form control in ultra-precision polishing process. In this paper, originating from the famous macro-scale Preston equation, the curvature effect-based material removal model in polishing using a flexible ball-end tool has been developed successfully on the basis of two key sub-models, one is the generic model of effective relative velocity and the other refers to the semi-experimental contact pressure model. A series of spot polishing experiments subsequently are conducted on concave surfaces with a curvature radius range from 75 mm to 225 mm. The experimentally measured section profiles of polishing spots do match well with the predicted data, which verifies the effectiveness of the proposed material removal model. On the measured polishing spots, it is also observed that there have two nonuniform material removal phenomena, one is analyzed along the central axis and the other is discussed by two regions symmetrical about the central axis. Compared with the effective relative velocity, it is found that, the contact pressure is more sensitive to curvature effect by investigating the variation of maximum removal depth within a broader curvature radius range from 75 mm to 1000 mm. This study can provide a valuable foundation for polishing optical surfaces with deterministic removal.

Investigation of an Influence Function Model as a Self-Rotating Wheel Polishing Tool and Its Application in High-Precision Optical Fabrication

A new and patented polishing tool (ZL2020102387137) called a Self-rotating Wheel Polishing Tool (SWPT) was built, and its tool influence function (TIF) was investigated in this study. The polishing wheel is an innovative two-layer structure: a rigid hub inside and a flexible polishing pad outside. By using finite element analysis, the dynamic contact characteristics between the polishing wheel and the workpiece were studied, and the theoretical TIF was modeled. Due to the influence of friction resistance, the TIF is not circular, but oval. We then ran material removal experiment, and it was found that the experimental TIF and the theoretical TIF are very close and both are close to the Gaussian shape. Finally, optical fabrication was performed. After four times of about 3 h fabrication, the form error converged from PV-1.434λ (λ = 632 nm), RMS-0.308λ to PV-0.144λ, RMS-0.009λ, and PV and RMS converged by 90% and 97%, respectively. The form accuracy achieved the expected target of RMS-0.02λ, which proves that the SWPT has the characteristics of high convergence rate and high fabrication accuracy. The SWPT has a broad application prospect in the field of high-precision optical fabrication.

Recent advances in focused ion beam nanofabrication for nanostructures and devices: fundamentals and applications

The past few decades have witnessed growing research interest in developing powerful nanofabrication technologies for three-dimensional (3D) structures and devices to achieve nano-scale and nano-precision manufacturing. Among the various fabrication techniques, focused ion beam (FIB) nanofabrication has been established as a well-suited and promising technique in nearly all fields of nanotechnology for the fabrication of 3D nanostructures and devices because of increasing demands from industry and research. In this article, a series of FIB nanofabrication factors related to the fabrication of 3D nanostructures and devices, including mechanisms, instruments, processes, and typical applications of FIB nanofabrication, are systematically summarized and analyzed in detail. Additionally, current challenges and future development trends of FIB nanofabrication in this field are also given. This work intends to provide guidance for practitioners, researchers, or engineers who wish to learn more about the FIB nanofabrication technology that is driving the revolution in 3D nanostructures and devices.

Ultrasmooth beryllium substrates for solar astronomy in extreme ultraviolet wavelengths

The paper describes a multistage method of forming ultrasmooth substrates based on bulk beryllium. Such substrates are suggested to be used for multilayer extreme ultraviolet mirrors of spacecraft missions on solar corona investigations in the spectral range 17.1-58.4 nm. The technique for chemical nickel plating of the sample surface is described. The process parameters that provide the formation of an amorphous film with a thickness of about 100 microns are presented. The results of mechanical polishing are shown. The effective roughness of 1.3 nm is obtained, which is twice lower than one achievable for a nickel-free beryllium surface. The applicability of the ion beam figuring technique is demonstrated: the initial surface roughness of a nickel film after etching with Ar ions (E_ion=200-800  eV) to a depth of 250 nm does not deteriorate. The amorphous silicon film deposition followed by ion polishing made it possible to reduce the microroughness (atomic force microscope frame 2×2  μm) to σ_2×2=0.15  nm from the initial σ_2×2=0.46  nm. The reflectivity of multilayer mirrors deposited on these substrates turned out to be close to the values obtained on "witnesses" (supersmooth silicon substrates). Moreover, for the Mg/MoSi2 mirror optimized for the wavelength λ=58.4  nm the values of the reflection coefficients of structures on the beryllium substrate and on the silicon "witness" were identical (about 28%).

Research on the Surface Evolution of Single Crystal Silicon Mirror Contaminated by Metallic Elements during Elastic Jet Polishing Techniques

Metallic elements can contaminate single crystal silicon mirror during ion beam etching (IBE) and other postprocessing methods, which can affect the performance of components in an infrared laser system. In this work, scanning electron microscope (SEM) and atomic force microscope (AFM) were used to characterize the distribution of contaminant represented by aluminum (Al). After characterizing contaminated area, elastic jet polishing (EJP), EJP, and static alkaline etching (SAE) combined technique were used to process the mirror. The morphology and laser-induced absorption were measured. Results show that metallic elements can mix with silicon and generate bulges due to the sputtering effect. In addition, SAE and EJP combined technique can remove metallic contaminant and stabilize the surface quality. Research results can be a reference on conducting postprocessing technologies to improve laser damage resistance property of single crystal silicon mirror in infrared laser system.

X-ray scattering by the fused silica surface etched by low-energy Ar ions

Anomalously high x-ray scattering at a wavelength of 0.154 nm by super-polished substrates of fused silica, which were etched by the argon ions with the energy of 300 eV, is detected. The scattering intensity increases monotonically with increasing of the etching depth. The effect is explained by the scattering on the volume inhomogeneities with the lateral size greater than 0.5 μm of the subsurface "damaged" layer. The concentration of volume inhomogeneities increases with the increase of the fluence of argon ions, but the concentration of implanted argon atoms in the layer quickly reaches the maximum value and then begins a trend of going down. The thickness of the "damaged" layer is approximately equal to the penetration depth of the Ar atoms and can be directly determined from the x-ray specular reflection. It is shown that the presence of volume inhomogeneities of the subsurface "damaged" layer does not affect the geometric roughness of the surface. The observed effect imposes limitations on the usage of grazing incidence x-ray optics without reflective coatings and of the diffuse x-ray scattering (DXRS) method for studying the substrate roughness. A new method that potentially enables to evaluate the applicability of the DXRS method in practice is proposed.

Improved analysis model for material removal mechanisms of bonnet polishing incorporating the pad wear effect

Bonnet polishing technology has been widely applied in precision optical machining. Until now, most of the research concerning the modeling for material removal mechanisms of bonnet polishing have been presented based on the well-known Preston model. However, the various parameters involved in the bonnet polishing process are not formulated into that model, such as slurry characteristics, pad properties, bonnet sizes, processing conditions, etc. Recently, several analysis models capturing those various parameters have been developed and are even capable of interpreting non-Prestonian behaviors, but the pad wear effect has still not been taken into account. Hence, the purpose of this paper is to establish an improved analysis model by incorporating the pad wear effect with the cumulative polishing time. Compared with the previous analysis model and Preston model, the predicted results of the improved analysis model are much closer to the experimental data and become more acceptable. According to the analysis of key parameters, the understanding of material removal mechanisms in bonnet polishing is further completed, and the time-dependent pad wear effect should no longer be neglected.

Micro-analysis model for material removal mechanisms of bonnet polishing

There have been many researches concerning the modeling for material removal mechanisms of bonnet polishing (BP) based on the well-known Preston model. However, various parameters involved in the BP process are not formulated and considered in the classical model, such as slurry characteristics, pad properties, bonnet features, and processing conditions. In this paper, a micro-analysis model capturing those parameters is proposed based on the mutual interaction of the slurry, pad, and workpiece among the BP interfaces with the micro-contact theory and the tribology theory. The proposed model is validated by comparison with the experimental data, and good agreement can be obtained. According to the analysis of key parameters, the proposed model is capable of providing some insight into the material removal mechanisms of BP, and even those cannot be explained properly by the classical Preston model.

Fabrication of continuous phase plates with small structures based on recursive frequency filtered ion beam figuring

Large surface gradient and extensive mid-to-high spatial frequency in continuous phase plates (CPPs) with small structures make it difficult to achieve high-precision fabrication. An ion beam figuring (IBF) technology to fabricate CPPs with such characteristics is proposed in this paper. In order to imprint CPP microstructures with smaller spatial periods even down to 1mm in shorter time, we present a multi-pass IBF approach with different ion beam sizes based on the frequency filtering method. We discuss the selection principle and when to reduce ion beam sizes for different procedures to control dwell time and adequately exert the corrective capability in detail. This filtering method can obtains better surface quality in a faster way compared to the non filtering traditional IBF method. The experimental results verify this optimized method can effectively imprint complex microstructures with spatial period as small as 0.7 mm, surface peak-to-valleys (PV) smaller than 200nm and surface gradient as large as 1.8μm/cm to within 10 nm root-mean-square (RMS) of design specifications, which displays the advantages of our fabrication method.

Detailed subsurface damage measurement and efficient damage-free fabrication of fused silica optics assisted by ion beam sputtering

Formation of subsurface damage has an inseparable relationship with microscopic material behaviors. In this work, our research results indicate that the formation process of subsurface damage often accompanies with the local densification effect of fused silica material, which seriously influences microscopic material properties. Interestingly, we find ion beam sputtering (IBS) is very sensitive to the local densification, and this microscopic phenomenon makes IBS as a promising technique for the detection of nanoscale subsurface damages. Additionally, to control the densification effect and subsurface damage during the fabrication of high-performance optical components, a combined polishing technology integrating chemical-mechanical polishing (CMP) and ion beam figuring (IBF) is proposed. With this combined technology, fused silica without subsurface damage is obtained through the final experimental investigation, which demonstrates the feasibility of our proposed method.

Ion-beam polishing of fused silica substrates for imaging soft x-ray and extreme ultraviolet optics

We have studied the surface treatment of polished fused silica by neutralized Ar ions with energy of 500-1500 eV and incidence angles of 0-90°. We found the following regularities: for samples that passed the standard procedure of deep polishing (initial effective roughness σ(eff)∼0.5  nm), the effective roughness decreases to the ultrasmooth level (i.e., σ(eff)∼0.25  nm in the range of spatial frequencies q∈[4.9×10(-2)-63]  μm(-1)). The effect begins to be noticeable at the material removal of 150 nm and reaches saturation at depths of removal greater than 1 μm. For supersmooth samples (σ(eff)<0.3  nm), the effective roughness keeps the initial level at material removal down to tens of micrometers. The optimal ion energy range is 800-1300 eV (maximum smoothing effect); at higher energy some surface roughness degradation is observed. All the smoothing effects are observed at the incidence angle range θ(in)=0-35°. Increasing the ion energy above 1300 eV increases the etching rate by up to 4 μm per hour (J(ion)=0.8  mA/cm2), which allows for deep aspherization of sized substrates. The technique allows for manufacturing the optical elements for extreme ultraviolet and soft x-ray wavelength ranges with a numerical aperture of up to 0.6.