1 basic principle
The basic Marangoni interface effect of the numerical control chemical polishing technology is shown in Figure 1. The application of this physical effect in the optical industry began in 1990, when Leenaars Huethorst proposed the principle of "Marangoni Drying". The basic idea is to adopt The volatile ethanol-like vapor-sprayed optical surface, due to the temperature gradient formed by the evaporation of ethanol (ie Marangoni surface tension gradient) will quickly shrink the surface of the liquid film to achieve the purpose of drying the surface, thus avoiding heat drying or centrifugal rotation The method of removing the liquid film easily forms a defect on the optical surface. Later, LLNL used this effect in the manufacture of diffractive optical elements, using the surface tension gradient formed by the evaporation of ethanol to control the etching area and etching speed of the HF etching solution on the surface of the fused silica, thereby completing the processing of high-precision diffractive optical elements. .
It can be seen from 1 that the numerical control chemical polishing technology draws on the basic process idea of ​​small-tool numerical control polishing, and adopts the mechanism of wet chemical etching, so that the process parameters are controllable. In addition, since the substrate is subjected to flexible processing using an etching solution, no additional mechanical contact stress and frictional thermal stress are generated during the processing, and subsurface defects are not formed.
The theoretical model of numerically controlled chemical polishing is also based on Preston's assumption that: h is the height of the material surface at a certain point, K is a proportional constant determined by factors such as material properties and temperature, and v is the chemical etching head at time t. The moving speed of a point on the surface, s is the concentration of the solution at the point where the point is at time t. Therefore, when the etching rate and the solution concentration of a certain region are known, the surface material removal amount can be calculated according to the action time t, and the surface height of the material after the elapse of time t.
2 manufacturing equipment and process
The overall structure of the numerical control chemical polishing equipment is shown in Fig. 2, and the inside of the broken line frame in 2(a) is the core of the processing, that is, the chemical etching portion. Due to the complex surface shape of CPP components, the small spatial period and the random variation, the phase depth is large, which makes the manufacturing process much more difficult than ordinary optical components. Effective machining begins with ensuring that there is a suitable set of etch heads, chemical grinding heads. The chemical grinding head first has to ensure an effective space period during the processing, and must have a good etching function shape.
Since the formation of chemical grinding heads is a complex dynamic process, it is difficult to establish a complete mathematical model. Therefore, it is necessary to make small-scale chemical grinding heads by changing the liquid outlet scale and the porous division of the liquid column. In view of the complexity of the surface shape of the CPP element, in order to be able to effectively obtain the detailed features of the surface of the element by etching, it is first required that the size of the etch head be as small as possible. In addition, in order to reduce the accumulation of errors that may occur during the etching process, the etched region is required to have a sharp edge. According to this design idea, the size of the chemical grinding head we produced is 5 mm, and the etching function has the form of a rectangular function, as shown in 3.
After the substrate is positioned during the processing, according to the design requirements of the CPP, the appropriate size of the grinding head is selected to etch the substrate.
The chemical etching solution effective for the optical glass type component is a hydrofluoric acid aqueous solution system. The hydrofluoric acid aqueous solution has strong volatility at a large concentration, which not only causes pollution to the working environment, but also causes uncontrolled etching pollution on the surface of the substrate to be processed. Therefore, we need to prepare an etching solution system with a lower concentration of hydrofluoric acid, a higher etching rate, and a relatively stable solution system.
The addition of NH4F to the HF solution to form a buffered HF solution (BHF) stabilizes the H+ concentration of the solution within a certain range and maintains the pH of the solution, thereby stabilizing the solution etch rate. A small amount of NH 4F can increase the concentration of HF2- ions in the solution, and the introduced NH4+ ions have a certain catalytic effect on the reaction, which can significantly increase the reaction rate. In order to find the right amount of addition, we carried out specific experiments on 5% hydrofluoric acid solution. 4 is the NH4F experimental curve. It can be seen from the figure that when the amount of NH 4F added is not large, the etching rate is proportional to the change of the addition amount, and when the addition amount reaches a maximum between 10% and 13%, the etching rate is followed. It tends to be saturated. The addition of 10% to 13% NH4F can increase the etching rate by about 3.5 times compared to the corresponding solution without NH4F. Therefore, selecting this interval as a working interval is advantageous for controlling the etching of the CPP substrate.
In addition, during the polishing process, the temperature of the etching solution also affects the processing process. The etching rate of the 5% HF solution at 20 ° C is 25.6 nm / min. When the temperature fluctuates by 1 ° C, the etching speed changes. Nm/min. If the constant temperature condition is ±1 °C, the controllable etch rate relative error is ±7.8%, so we must perform strict temperature control.
According to the CPP design requirements, combined with the existing equipment features and previous process experiments, the manufacturing process of large-diameter CPP components was determined, as shown in 6.
3 component manufacturing results
According to the designed process, we have processed CPP components with different calibers. Two typical results are given in this paper. Figure 7 shows the design and manufacturing results of a square CPP with a diameter of 300 mm × 300 mm. The interferometer detects the transmitted wavefront of the sample to obtain a reverse wavefront data. It is the result of the design and manufacture of a circular CPP of 320 caliber.
Comparing the two design results, it can be seen that the two CPPs not only have completely different shapes (square and round), but also the surface topography characteristics of the two components are different. The surface phase fluctuation of the square CPP has obvious directionality, and its periodic variation is different in different directions. The round-caliber CPP shape has no obvious directionality. However, from the results of the final interference detection, CPP components with different characteristics can be effectively processed by chemical polishing.
4 Conclusion
According to the needs of the ICF system beam smoothing, the chemical polishing method is used to face-process the large-diameter CPP components. Based on the principle of MARANGONI, the characteristics of this manufacturing method are analyzed, and the removal function of chemical etching is analyzed. Combined with the surface features of CPP components, a chemical grinding head was designed and its etching function was tested experimentally. In order to improve the stability and etching speed of the etching solution, NH4F was added to the HF solution, and the etching rates corresponding to different added concentrations were compared by experiments. The experimental results show that when the concentration of NH4F is 10%-13%, the etching rate is the highest. In addition, the other key parameters in the process, namely the influence of solution temperature, were analyzed experimentally. The results show that the temperature fluctuation of 1oC will bring the relative error of ±7.8% to the control of the etching rate, so it must be added in the specific equipment. A reliable temperature control system. According to the existing equipment, the machining process is designed, and CPP components with different shapes and surface features are manufactured. From the comparison of manufacturing results and design results, the use of chemical polishing process to manufacture CPP is a feasible solution.
(Finish)
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