The influence of chemical etching on the surface quality of phase components

Large-aperture fused silica phase optical components such as continuous phase plate (CPP) are widely used in large-scale laser devices to achieve beam homogenization and improve beam quality. However, under the action of high-energy lasers, their lower damage threshold seriously restricts their service life and increases cost of using. Compared with other fused silica components, chemical processing technology with hydrofluoric acid solution (HF) is lacking in the processing of phase components because the residual root mean square value (RMS) of phase elements is very high, and it can not guarantee this. Therefore, it is necessary to carry out research on the influence of chemical treatment on phase components. In this paper, we improved the chemical treatment process to achieve the change of residual RMS value not more than 3 nm, and improved the ability of resisting laser damage that the damage threshold of 29J/cm2 was obtained under the test conditions 351nm@3ns. Finally, we successfully mastered the chemical control process of phase components and applied it to the CPPs as other fused quartz materials engineering production process.


Introduction
Chemical treatment is the most common and effective method in the process of improving the resistance of fused quartz elements to laser damage [1][2][3][4].Chemical treatment can greatly improve the surface service performance, but it also exposes sub-surface processing quality significantly and influences the surface accuracy of elements.By improving the processing technology, the sub-surface processing quality control has been achieved at this stage, and the surface accuracy changes of components in this process are also controlled within the required range.
The phase component is different from other fused silica samples in that its surface is covered with periodic structures with a depth of about 1-5 µm, and its surface shape accuracy is also required [5][6][7][8].The surface shape residual error is generally not more than tens of nanometers [8,9].Therefore, how to use chemical processing technology to achieve high damage threshold and ensure the surface accuracy of the element is the key to whether the chemical processing process of the phase element can be realized [10][11][12].Through the change of the parameters of the fused silica sample during the chemical treatment process, such as the control of the etching depth, and the process verification and strict control measures in the cleaning process [12][13][14], the surface accuracy of the component is guaranteed to be slightly changed while the damage threshold of the phase component is increased.
The continuous phase elements have higher requirements for surface accuracy, and its surface fluctuation also has a certain interference effect on uniform immersion chemical treatment.Therefore, it is necessary to study the effect of chemical treatment on the surface structure of phase elements.

Precision change of phase element during chemical treatment
In this paper, the 400mm fused sample whose multi-periodic phase superposition design is adopted is used, and the surface shape Peak-to-Veally value (PV) is 2.532 µm.After chemical treatment with 2.8% HF and 11% ammonium fluoride mixed solution, the surface accuracy changed more as the increase of the depth of chemical treatment.For example, the RMS value of the residual error of the 400 mm diameter phase element varied by tens of nanometers after etching 10 microns, as shown in Figure 1.Some changes changed from the initial residual error of 18 nm to 59 nm.This may be acceptable for other samples with a general surface shape RMS value of nearly 100 nm, but it is obviously unacceptable for the requirement that the phase element should not exceed 30 nm.

Chemical treatment experiment of 100mm diameter phase element
In order to determine whether the surface morphology of the phase element will affect the chemical treatment, this paper designed and processed two single-frequency phase experimental samples with different depths and different periods.The size of the phase elements is 100mmx100mm, the minimum design period is 10mm and 4mm respectively example as shown in Table 1, and the surface shape PV is 0.9um and 7um as shown in Table 1.The morphology is as shown in the Figure 2.  The processing shall be carried out according to the design, and the processing shall adopt the magnetorheological processing method (MRF) to ensure the processing surface shape control and the surface shape test after completion, and confirm that the test surface shape residual meets the requirements, and the residual RMS is as shown as Figure 3.   1, and the maximum change rate is about 4.4%, which is within the acceptable range.With the increasing of etching depth, the residual value tends to be stable, and the change is also within 5% after etching 20 µm depth.Sample 2 also has the same experimental conclusion as shown in Table 1.This variation range can be allowed, so the surface characteristics of the sample are not the cause of the remaining RMS changes during etching This experiment shows that the influence of etching on the surface residual RMS within the range of 100 mm aperture elements is within a controllable range, and the surface morphology of phase elements will not affect the chemical treatment basically.At the same time, it also proves that the uniformity of the aqueous solution of HF and ammonium fluoride itself can ensure that the etching of optical elements has a uniform effect.

Effect of chemical treatment on surface quality
The experiments were carried out with fused silica with a caliber of 400mm and a thickness of 3mm.The etching treatment was carried out under multiple parameters without the effect of ultrasonic field.Comparing the surface shape before and after etching, it was found that the surface accuracy changes still existed, and most of them were concentrated at the lower part of the element, 30-40mm from the edge of the element.Comparing the surface shape differences of four samples, it was found that they had similar difference distribution.The residual distribution diagram of the surface shape that was subtracted before and after element etching is as shown in the Figure 5.We found that the judgment was caused by etching parameters or solution environment.

Analysis and validation of chemical treatment process
By studying the effect of etching process on the residual rms of parts, we determine the improvement direction of etching process, and carry out the following work to optimize the etching.

Analysis of chemical etching parameters
In order to further clarify the influencing factors of this situation, we have analyzed the etching process of large-caliber CPP components.In the etching process, as shown in Figure 6, the ultrasonic auxiliary system and the upper and lower stirring system will be introduced.The ultrasonic cavitation can ensure the consistency of solution reaction in a small range, and help the diffusion of reaction products to prevent the formation of deposition at the same time.The upper and lower stirring system can ensure the uniformity of the solution in a large range, and the system can be observed to operate normally during the etching process.

Optimization of chemical etching parameters
In order to ensure that the temperature as another important parameter in the etching process is constant within a certain range, the equipment has also added a temperature-controlled circulating system, which can achieve stable and controllable temperature by directly heating and cooling the solution.However, in order to avoid solution sputtering, the circulating water outlet is placed at 1/3 below the water surface downward, and the heating device of the equipment is located at the bottom of the tank, resulting in stratification of the upper and lower temperature fields of the solution, and the bottom temperature is higher than the top temperature, This may be the main reason for uneven etching.In view of this, we closed the temperature control system and carried out a round of experiments.The experimental results show that the bottom part of the element has a significant easing trend, as shown in Figure 7, which shows that the control of the etching parameters is the key reason for uneven etching of phase elements.In order to achieve the uniformity of etching, the outlet position of the circulating water was changed to make it closer to the surface of the etching solution, and a horizontal outlet was added in the outlet direction.Secondly, the use method of the temperature control system was changed.The temperature control system was opened before the etching operation to reach the set value of the etching operation, and the constant temperature was maintained for a certain time.During the etching operation, the control mode of fast flow rate and small heating power was carried out.This solution is fully fused to ensure that the processed sample is in a constant temperature environment.At the same time, the temperature test at different points shall be added to detect the possible sudden uneven temperature in time.

Control results of chemical etching parameters
After the above optimization and upgrading of the etching device, two 400mm× 400mm fused elements has been experimentally verified.Compared with the surface profile distribution before and after etching 5µm, as shown in Figure 8, the surface profile PV and RMS are basically unchanged.Subsequently, the optimized etching parameters were verified with engineering CPP samples, and the verification results are shown in Figure 9

Summary
In this paper, the research on the surface shape control during the etching process of fused quartz phase samples is carried out to find out the influence rule of chemical post-processing on the of the residual RMS parameters of phase elements.Through the optimization of the relevant parameters such as temperature during the etching process, the application of chemical etching technology for largeaperture phase elements is realized.the control range of the residual RMS is not more than 3 nm.The laser damage threshold is increased that the damage threshold of 29J/cm 2 was obtained under the test conditions 351nm@3ns.Finally, we successfully mastered the chemical control process of phase components and applied it to the CPPs as other fused quartz materials engineering production process.

Figure 2 .
Figure 2. Surface shape design of the phase elements.

Figure 3 .
Figure 3. Residual RMS of surface shape.The components are etched in stages and the etching depth is controlled.Then we can compare the surface shape change and test the surface roughness.Figure 4 shows the change trend of sample 1 with the etching depth.It is found from the change trend in the figure that the sample has little change.The maximum change is at the initial stage of etching.The maximum change error is 1.2nm as shown in Table1, and the maximum change rate is about 4.4%, which is within the acceptable range.With the increasing of etching depth, the residual value tends to be stable, and the change is also within 5% after etching 20 µm depth.Sample 2 also has the same experimental conclusion as shown in Table1.This variation range can be allowed, so the surface characteristics of the sample are not the cause of the remaining RMS changes during etching Figure 4 shows the change trend of sample 1 with the etching depth.It is found from the change trend in the figure that the sample has little change.The maximum change is at the initial stage of etching.The maximum change error is 1.2nm as shown in Table

Figure 4 .
Figure 4. Variation trend of residual error with etching depth.

Figure 5 .
Figure 5. Distribution of surface shape subtraction before and after etching.

Figure 6 .
Figure 6.Etching process of phase element.

Figure 7 .
Figure 7. Etching influence without the effect of temperature control system.

Figure 8 .Figure 9 .
Figure 8. Surface shape of plane sample before and after etching.

Table 1 .
A table for characteristic parameters of two CPP samples. 3