- Email: [email protected]
Study on etching process of fused silica with concentrated HF
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Study on etching process of fused silica with concentrated HF Yong Shu
T
Air Force Engineering University, Aviation Maintenance NCO Academy, No. 23rd, Hangkong Road, Xinyang, 464000, PR China
A R T IC LE I N F O
ABS TRA CT
Keywords: Concentrated HF etching Fused silica Grinding Crack Roughness
Optical elements will go through grinding, lapping and polishing to reach the final accuracy, and large amount of time is consumed between grinding and polishing in order to remove cracks and defects left by grinding. This manuscript tries to employ HF etching process into the manufacturing of fused silica optical elements to improve the efficiency and quality, so the concentrated HF etching process of fused silica was studied systematically. Firstly, the principle of HF etching fused silica is introduced. Then, the process of concentrated HF is studied, which shows that the etching rate of concentrated HF is as high as 1.14 μm/min and the surface shape of workpiece can be basically kept unchanged after etching. Finally, the observation results of fused silica samples processed by concentrated HF show that the concentrated HF processing can eliminate the cracks on the surface left by grinding and improve the roughness of the surface. So the concentrated HF etching can be used to process the fused silica sample after grinding, it will eliminate cracks and defects quickly, improve the surface quality of the component, and promote the subsequent polishing process.
1. Introduction With the progress of society and technology, more and more high-tech equipment such as laser inertial confinement fusion system, telescope, lithography system are built, they all need a large number of fused silica optical elements. The processing of fused silica optical elements is generally composed of grinding, lapping, polishing and post-processing in order to achieve the final surface precision and surface quality. After grinding, the surface roughness of fused silica elements is poor, left a large number of cracks, scratches, defects and subsurface damage (SSD). So lapping is added to improve the surface roughness and reduce the depth of SSD with reducing size of abrasive particles. But Lapping cannot thoroughly remove SSD and will generate new SSD. Polishing is needed to improve surface quality, remove left SSD and further improve the surface precision [1]. Usually a lot of time is spent on lapping and rough polishing to remove SSD, so if we can quickly remove SSD left by grinding, the processing efficiency of optical elements will be greatly improved. HF can react with fused silica, and has been widely used in the processing of glass. Mikosza used HF to observe the Hertz type scratch on the glass to find the relationship between the load parameters and the geometric parameters of generated Hertz scratch [2]. Spierings deeply studied the glass etching process by HF, and pointed out that the interaction of HF and HF−2 could destroy silica bonds in fused silica and dissolve it, and he also described the surface and subsurface topography evolution in HF etching process [3]. Wong first studied the size and shape variation of a single crack during HF etching and found that a single crack will gradually expanded but will not disappear during etching; then he studied the morphological changes of the grinding surface during etching, and found that high concentration HF etching can help the extension and mutual interleaving of crack, eliminating the sub surface crack after grinding [4].
E-mail address: [email protected]. https://doi.org/10.1016/j.ijleo.2018.10.011 Received 9 August 2018; Accepted 2 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
As a chemical removal method, HF etching can achieve global removal and features high removal efficiency. Compared with traditional grinding and polishing methods, HF etching removes material by chemical reaction without production of surface fragmentation and scratches, so it will not introduce surface and subsurface damage. If HF etching can be applied in the process of fused silica components, the machining efficiency and the quality of processing will be improved. In order to achieve this goal, it is necessary to systematically study the processing principle, processing technology and processing effect of HF etching in order to determine the application position of HF etching in manufacture of optical elements. 2. The principle of HF etching The dissolution process of fused silica in liquid HF can be summed up in the following equation: (1)
SiO2 + 6HF → H2 SiF6 + 2H2 O
Eq. (1) is a simplification of a series complex chemical reaction processes occurring in the HF solution of fused silica. In fused silica materials, the four-dimensional SiO4 are combined with the adjacent four SiO4 by covalent ^SieOeSi^ (siloxane) bond to form interconnected three-dimension silicate reticular structures. The four siloxane keys must be broken to destroy the reticular structure to dissolve the fused silica. HF is a water soluble weak acid whose aqueous solution contains H+, F−, HF−2 as well as insoluble HF molecules. Their concentration can be calculated by following equations:
K1 = [H+]⋅[F −]/[HF ] K2 = [HF ]⋅[F −]/[HF2−] −4
(2) −1
−1
Hn F−n + 1
At 25 °C, K1 = 6.7 × 10 mol∙l , K2 = 0.26 mol∙l . In concentrated HF solutions, other higher-order polymers are appeared in addition to HF−2 [3]. The dissolution process of fused silica is mixed with many other reactions, so it is difficult to determine the leading factors of the dissolution process. It takes several steps to break the chemical bond to complete the reaction, and the reaction constant of the slowest step will determine the reaction rate of the whole dissolution process. The property that HF acid can dissolve fused silica is related to fluorine ions in solution, namely HF, F− and HF−2 . Studies have shown that NaF and NH4F solutions do not react with fused silica, so the effect of F− can be ignored [5,6]. Prokopowicz-Prigogine presented a detail model to describe the dissolving process of fused silica in HF [7], which believes that the etching rate is determined by the adsorption process of HF molecules, HF−2 and H+ ions. When HF corrodes fused silica, the HF−2 is adsorbed on the surface of silanol group, the HF molecule is adsorbed near the silanol group, and the H+ is adsorbed on the surface of the connecting oxygen atoms on the surface of siloxane element. When the surface of fused silica in gaseous HF was observed by an infrared spectroscopy, fluorine-containing compounds was found on the hydrolyzed surface of fused silica, which formed some atomic groups such as ^SieF and ^SieOeSiF3. HF and HF−2 can increase the electron density of the connecting oxygen atoms on the surface of the siloxane element, thereby absorbing more H+ ions, resulting in more siloxane bonds breaking in unit time, acting like a catalyzer. The breaking process of siloxane bonds by H+ also occurs when the glass dissolves in acid or alkaline solution. The above model holds that the etching rate is related to the concentration of HF, HF−2 and H+, and the relationship between them can be described as [7]:
VE = k1⋅Θ (H+)⋅{k2⋅Θ (HF2−) + k3⋅Θ (HF )} + k 4⋅Θ (H+)
(3)
here ki is a constant that contains the reaction rate, the adsorption equilibrium constant and the number of adsorption positions per unit region. Θ represents the coverage degree of adsorption position, which can be described with Langmuir isotherm [7]: (4)
Θ (HF ) = b⋅[HF ]/(1 + b⋅[HF ])
here b is the ratio between the adsorption rate and desorption rate. Although the model presents a specific formula for calculating the etching rate, the rate is still changeable in a real etching process, which can to be measured by experiments to obtain the final value. In practice, there are two commonly used HF etching methods, one is concentrated HF (49% HF in water) etching process, while another is BOE (Buffered oxide etch) etching. Due to the low etching efficiency of BOE, generally 2 nm/sec at 25 °C [8], we mainly discuss the characteristics of concentrated HF etching in this manuscript in order to quickly remove the material. 3. Processing technology of concentrated HF In order to apply the concentrated HF etching to optical manufacturing, it is necessary to study its technological characteristics first. There are mainly two concerns: one is the etching efficiency of HF, and the other is the effect of HF etching on the surface shape of optical elements. 3.1. Etching rate To measure the etching rate of HF, we need to measure the material removal before and after etching. The simplest way is to select a flat fused silica sample and divided it into two regions. Half of the sample is coated with photoresist, while the other remains 545
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
Fig. 1. HF etching rate test flow.
unchanged. Then the sample is put into concentrated HF acid for etching. The etched sample is cleaned and the surface photoresist is removed by acetone. As the photoresist won’t react with HF, the half sample covered by photoresist is protected while the other part is etched, so a step will form on the surface of the sample. A profiler is then employed to measure the surface profile, and the height of the step is the removal amount of etching. A fused silica sample was selected and divided into four different regions, and the etching time of 0 min, 20 min, 40 min, 60 min was obtained on four regions by alternate protection of each area. The etching depth of the concentrated HF (49% HF in water) was measured by the Talysurf PGI 1240 profiler, and then its etching rate could be calculated according to the etching time (Fig. 1). Fig. 2(a) gives the material removal with etching time of 20 min, 40 min, 60 min, and Fig. 2(b) presents the material removal rates at every stage. As shown by Fig. 2(b), the etching rate of concentrated HF is as high as 1.33 μm/min at initial stage, the etching rate decreases gradually as time goes on and the etching rate is only 0.88 μm/min at the last 20 min. The probable reason is that the HF was consumed continuously as the HF reacting with fused silica and the concentrated HF has strong volatility, so the content of HF in the solution decreases with the increase of time, which reduces the etching rate of the concentrated HF. Fig. 2(a) shows that in the 60 min etching time, HF removing 68 μm thick of materials, so the average etching rate is about 1.14 μm/min, which is quite considerable. According to experience of traditional processing, the removal rate of grinding is about 0.3–0.8 μm/min, the removal rate of lapping is about 0.03–0.17 μm/min, and the removal rate of polishing is about 0.002–0.008 μm/ min. The removal rate of concentrated HF etching is equal or much higher than that of traditional processing. At the same time, HF etching can simultaneously process the whole workpiece surface so the volume removal is only related to its etching rate. While other processing methods such as grinding, lapping and polishing, its volume removal is related to the size of the tool, so volume removal rate will be discounted. Therefore, concentrated HF etching of fused silica has the feature of high efficiency. 3.2. Etching uniformity The main purpose of the concentrated HF etching is to quickly remove large amount of material and eliminate the cracks after grinding. In order to facilitate subsequent processing, the surface shape accuracy of the workpiece should not be seriously damaged. Soaking optical elements in HF solution is a macroscopic controllable and microscopic uncontrollable material removal method. Macroscopically the removal amount can be determined by controlling the concentration, temperature and soaking time of HF solution. However due to the complexity of the micro-topography of the sample, this method cannot guarantee the uniform removal of whole surface, and will affect the roughness and shape of the workpiece [9]. Theoretically, in addition to the concentration of acid etching solution and temperature, contact area and gravity will also affect the reaction rate. Based on the theoretical knowledge of chemistry, when the concentration of chemical reaction and reaction temperature are the same, the greater the contact area between the reactants, the more the reaction in the unit time. For the etching process of optical elements, the contact area of the acid etching reaction will increase if the surface is rougher, and then more
Fig. 2. Etching rate of concentrated HF. 546
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
Fig. 3. Surface error variation during HF etching (a) Before HF etching (b) After HF etching.
material will be removed in unit time, that means the removal rate will be bigger. There are micro convex or concave structures on the surface of fused silica sample after grinding, and the reaction rate of the side wall of these parts is greater than that of the peak or bottom. This is because reactant on the side wall will quickly fall out of the side wall by gravity, and then new material will appear in the acid solution and continue to react. The reactants at the convex and concave part, especially the concave part, will stacked together and slow down normal reaction, which is more serious when the optical element statically resting in HF acid solution. In order to ensure the uniformity of etching, more attention should be paid to the placement of samples and etching technology. On the one hand, the workpiece can be inversely placed so as to reduce the accumulation of the reactant. In addition, ultrasonic vibration can be added to the etching process to facilitate the separation of the reaction-generated material from the surface of the optical sample. At the same time, it is necessary to keep the temperature constant during the etching process to ensure the stability of the etching process. By using these above methods, a grinding fused silica sample was etched by concentrated HF for about 1 h. In order to verify the uniformity of etching, a three-coordinate measuring machine was employed to measure the surface error of the fused silica sample before and after etching, and Fig. 3 gives the final measurement result. As can be seen from Fig. 3, after 1 h of etching, although approximately 60 μm material has been removed, the surface error of the sample is only increased from 0.876 μm (PV)/0.134 μm (RMS) to 2.107 μm (PV)/0.375 μm (RMS). Compared to the 60 μm amount of material removal, the surface shape before and after etching is basically consistent. Concentrated HF etching doesn’t do serious damage to the surface shape, and will not bring serious effect to subsequent processing.
4. Processing effect of concentrated HF The etching rate and etching uniformity of concentrated HF were studied in previous Section 3. The results show that the efficiency of HF etching is high, and the surface error is not deteriorated, so concentrated HF etching can be employed in optical manufacturing. At the beginning of the manuscript, we want to apply concentrated HF etching to grinding optical samples to remove surface cracks and subsurface defects. Then a set of experiments were done to verify if concentrated HF etching can effectively remove crack and improve the surface roughness. We selected the grinding fused silica samples and used concentrated HF to process them. Before and after etching, the sample was inspected by microscope and scanning electron microscope (SEM). At the same time, the surface roughness of the sample was
Fig. 4. Surface morphology after grinding (a) Microscope 500× (b) SEM 500× (c) SEM 5000×. 547
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
Fig. 5. Surface morphology after 30 min HF etching (a) Microscope 500× (b) SEM 500× (c) SEM 5000×.
measured by a profiler. In order to get more details of the sample, we slightly etched the grinding sample instead of direct observation. Fig. 4(a) is the result observed by the Keyence vhx-600e electron microscope, and the magnification in the figure is 500×. The shiny place in the picture is high part, and the darker position is the low point. The picture’s degree of light and shade is not uniform, that means the surface of sample is not flat. As the microscope could not see more details, a Phenom Desk top SEM (model G2 Pro) was employed to further observe the sample. The magnification of this SEM is from 80× to 45,000×. Fig. 4(b) is the result of SEM observation with the magnification of 500×. Similar to the microscope, the brighter part of the image is higher, and the darker part is lower. As can be seen from the picture, a large number of staggered ravines appeared on the sample, and the dark part represented the cracks left by grinding. The surface of the sample can be observed more clearly after further amplified to 5000× as Fig. 4(c). After grinding, the surface of the sample is uneven and there are many different cracks. These cracks need to be removed by subsequent processing to obtain smooth and non-damage surface. At the same time, the surface roughness of the sample is measured by the Talysurf PGI 1240 surface profiler, and the result is 1.21 μm (Ra). In order to obtain the intuitive impression, a grinding sample was etched by concentrated HF for 30 min first, and Fig. 5 presents the surface topography of the etched sample. It can be seen from Fig. 5(a) that after 30 min etching, the surface of the sample becomes uneven. Some parts become smoother, while others parts appear deep holes. As the depth of field of the microscope is relatively shallow, the SEM was employed to continue observing. The etching marks on the surface of the sample can be seen from the image of the SEM in Fig. 5(b). During etching the cracks were always expanding, and they will intersect in the process of expansion. The intersection will form deep grooves or high peaks, and the peaks always had sharp edges. The surface of the etched surface can be more clearly seen from the Fig. 5(c), and the sharp boundary line is the product of the crack expansion. Due to the existing deep trenches and pits, the sample went through another 30 min concentrated HF etching. After etching, it can be seen from Fig. 6(a) that the irregular structures on the surface were completely disappeared. The surface looks like a series of smooth convex overlapping together without sharp pits and protrusions, and the surface quality has been greatly improved. The observations results of SEM in Fig. 6(b) and (c) show a clearer picture of the smooth surface, and the cracks and flaws in the surface left by grinding have all disappeared. The surface was measured by a profiler to get the surface roughness and now the result is 0.19 μm (Ra). After concentrated HF etching, the surface quality increases significantly, and the surface integrity is also guaranteed. From the process of concentrated HF etching, it can be seen that as the reaction goes on, the surface quality of the sample is getting better and the surface roughness is getting smaller. Section 3.2 mentioned that etching rate will become bigger if surface roughness is larger, which also partly explains the decline in etching rates as the etching progresses. 5. Conclusion Through the study of the concentrated HF etching process, the results show that concentrated HF etching can be used to fabricate
Fig. 6. Surface morphology after 60 min HF etching (a) Microscope 500× (b) SEM 500× (c) SEM 5000×. 548
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
fused silica optical elements. The removal efficiency of concentrated HF etching is high and the sample can be processed globally, and concentrated HF etching can effectively remove the grinding cracks with an integrated surface, so concentrated HF etching can act as a connecting process between grinding and polishing. At the same time, concentrated HF etching will not seriously destroy the surface precision of the component, which can reduce the workload of the subsequent polishing. Application of concentrated HF etching will greatly improve the processing efficiency and processing quality of fused silica optical element. References [1] T. Suratwala, et al., Sub-surface mechanical damage distributions during grinding of fused silica, J. Non-Cryst. Solids 352 (52–54) (2006) 5601–5617, https://doi. org/10.1016/j.jnoncrysol.2006.09.012. [2] A.G. Mikosza, B.R. Lawn, Section and Etch study of hertzian fracture mechanics, J. Appl. Phys. 42 (13) (1971) 5540–5545, https://doi.org/10.1063/1.1659977. [3] G.A.C.M. Spierings, Wet chemical etching of silicate glasses in hydro- fluoric acid based solutions, J. Mater. Sci. 28 (23) (1993) 6261–6273, https://doi.org/10. 1007/bf01352182. [4] L. Wong, et al., The effect of HF-NH4F etching on the morphology of surface fractures on fused silica, J. Non-Cryst. Solids 355 (13) (2009) 797–810, https://doi. org/10.1016/j.jnoncrysol.2009.01.037. [5] J.S. Judge, A study of the dissolution of SiO2 in acidic fluoride solutions, J. Electrochem. Soc. 118 (11) (1971) 1772–1775, https://doi.org/10.1149/1.2407835. [6] S.T. Tso, J.A. Pask, Reaction of fused silica with hydrogen gas, J. Am. Ceram. Soc. 65 (9) (1982) 457–460, https://doi.org/10.1111/j.1151-2916.1982.tb10514.x. [7] M. Prokopowicz-Prigogine, Reactivity of a silica network of glass. Molecular mechanism of the dissolution of a silica network in aqueous HF-HCl solutions, Glastech. Ber. 62 (7) (1989) 249–255. [8] S. Wolf, R.N. Tauber, Silicon processing for the VLSI era, 2nd ed., Process Technology vol. 1, Lattice Press, California, 2001. [9] J.A. Menapace, et al., Combined advanced finishing and UV-laser conditioning for producing UV-damage-resistant fused silica optics, Proc. SPIE 4679 (2002) 56–68, https://doi.org/10.1117/12.461725. Yong Shu is a lecturer at the Air Force Engineering University. He received his BS degree in mechanical engineering from the Wuhan University in 2006, and his PhD degree in mechanical engineering from the National University of Defense technology (NUDT) in 2014. His current research interests include smoothing, computercontrolled optical manufacturing and fabricating of large mirrors.
549
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Study on etching process of fused silica with concentrated HF Yong Shu
T
Air Force Engineering University, Aviation Maintenance NCO Academy, No. 23rd, Hangkong Road, Xinyang, 464000, PR China
A R T IC LE I N F O
ABS TRA CT
Keywords: Concentrated HF etching Fused silica Grinding Crack Roughness
Optical elements will go through grinding, lapping and polishing to reach the final accuracy, and large amount of time is consumed between grinding and polishing in order to remove cracks and defects left by grinding. This manuscript tries to employ HF etching process into the manufacturing of fused silica optical elements to improve the efficiency and quality, so the concentrated HF etching process of fused silica was studied systematically. Firstly, the principle of HF etching fused silica is introduced. Then, the process of concentrated HF is studied, which shows that the etching rate of concentrated HF is as high as 1.14 μm/min and the surface shape of workpiece can be basically kept unchanged after etching. Finally, the observation results of fused silica samples processed by concentrated HF show that the concentrated HF processing can eliminate the cracks on the surface left by grinding and improve the roughness of the surface. So the concentrated HF etching can be used to process the fused silica sample after grinding, it will eliminate cracks and defects quickly, improve the surface quality of the component, and promote the subsequent polishing process.
1. Introduction With the progress of society and technology, more and more high-tech equipment such as laser inertial confinement fusion system, telescope, lithography system are built, they all need a large number of fused silica optical elements. The processing of fused silica optical elements is generally composed of grinding, lapping, polishing and post-processing in order to achieve the final surface precision and surface quality. After grinding, the surface roughness of fused silica elements is poor, left a large number of cracks, scratches, defects and subsurface damage (SSD). So lapping is added to improve the surface roughness and reduce the depth of SSD with reducing size of abrasive particles. But Lapping cannot thoroughly remove SSD and will generate new SSD. Polishing is needed to improve surface quality, remove left SSD and further improve the surface precision [1]. Usually a lot of time is spent on lapping and rough polishing to remove SSD, so if we can quickly remove SSD left by grinding, the processing efficiency of optical elements will be greatly improved. HF can react with fused silica, and has been widely used in the processing of glass. Mikosza used HF to observe the Hertz type scratch on the glass to find the relationship between the load parameters and the geometric parameters of generated Hertz scratch [2]. Spierings deeply studied the glass etching process by HF, and pointed out that the interaction of HF and HF−2 could destroy silica bonds in fused silica and dissolve it, and he also described the surface and subsurface topography evolution in HF etching process [3]. Wong first studied the size and shape variation of a single crack during HF etching and found that a single crack will gradually expanded but will not disappear during etching; then he studied the morphological changes of the grinding surface during etching, and found that high concentration HF etching can help the extension and mutual interleaving of crack, eliminating the sub surface crack after grinding [4].
E-mail address: [email protected]. https://doi.org/10.1016/j.ijleo.2018.10.011 Received 9 August 2018; Accepted 2 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
As a chemical removal method, HF etching can achieve global removal and features high removal efficiency. Compared with traditional grinding and polishing methods, HF etching removes material by chemical reaction without production of surface fragmentation and scratches, so it will not introduce surface and subsurface damage. If HF etching can be applied in the process of fused silica components, the machining efficiency and the quality of processing will be improved. In order to achieve this goal, it is necessary to systematically study the processing principle, processing technology and processing effect of HF etching in order to determine the application position of HF etching in manufacture of optical elements. 2. The principle of HF etching The dissolution process of fused silica in liquid HF can be summed up in the following equation: (1)
SiO2 + 6HF → H2 SiF6 + 2H2 O
Eq. (1) is a simplification of a series complex chemical reaction processes occurring in the HF solution of fused silica. In fused silica materials, the four-dimensional SiO4 are combined with the adjacent four SiO4 by covalent ^SieOeSi^ (siloxane) bond to form interconnected three-dimension silicate reticular structures. The four siloxane keys must be broken to destroy the reticular structure to dissolve the fused silica. HF is a water soluble weak acid whose aqueous solution contains H+, F−, HF−2 as well as insoluble HF molecules. Their concentration can be calculated by following equations:
K1 = [H+]⋅[F −]/[HF ] K2 = [HF ]⋅[F −]/[HF2−] −4
(2) −1
−1
Hn F−n + 1
At 25 °C, K1 = 6.7 × 10 mol∙l , K2 = 0.26 mol∙l . In concentrated HF solutions, other higher-order polymers are appeared in addition to HF−2 [3]. The dissolution process of fused silica is mixed with many other reactions, so it is difficult to determine the leading factors of the dissolution process. It takes several steps to break the chemical bond to complete the reaction, and the reaction constant of the slowest step will determine the reaction rate of the whole dissolution process. The property that HF acid can dissolve fused silica is related to fluorine ions in solution, namely HF, F− and HF−2 . Studies have shown that NaF and NH4F solutions do not react with fused silica, so the effect of F− can be ignored [5,6]. Prokopowicz-Prigogine presented a detail model to describe the dissolving process of fused silica in HF [7], which believes that the etching rate is determined by the adsorption process of HF molecules, HF−2 and H+ ions. When HF corrodes fused silica, the HF−2 is adsorbed on the surface of silanol group, the HF molecule is adsorbed near the silanol group, and the H+ is adsorbed on the surface of the connecting oxygen atoms on the surface of siloxane element. When the surface of fused silica in gaseous HF was observed by an infrared spectroscopy, fluorine-containing compounds was found on the hydrolyzed surface of fused silica, which formed some atomic groups such as ^SieF and ^SieOeSiF3. HF and HF−2 can increase the electron density of the connecting oxygen atoms on the surface of the siloxane element, thereby absorbing more H+ ions, resulting in more siloxane bonds breaking in unit time, acting like a catalyzer. The breaking process of siloxane bonds by H+ also occurs when the glass dissolves in acid or alkaline solution. The above model holds that the etching rate is related to the concentration of HF, HF−2 and H+, and the relationship between them can be described as [7]:
VE = k1⋅Θ (H+)⋅{k2⋅Θ (HF2−) + k3⋅Θ (HF )} + k 4⋅Θ (H+)
(3)
here ki is a constant that contains the reaction rate, the adsorption equilibrium constant and the number of adsorption positions per unit region. Θ represents the coverage degree of adsorption position, which can be described with Langmuir isotherm [7]: (4)
Θ (HF ) = b⋅[HF ]/(1 + b⋅[HF ])
here b is the ratio between the adsorption rate and desorption rate. Although the model presents a specific formula for calculating the etching rate, the rate is still changeable in a real etching process, which can to be measured by experiments to obtain the final value. In practice, there are two commonly used HF etching methods, one is concentrated HF (49% HF in water) etching process, while another is BOE (Buffered oxide etch) etching. Due to the low etching efficiency of BOE, generally 2 nm/sec at 25 °C [8], we mainly discuss the characteristics of concentrated HF etching in this manuscript in order to quickly remove the material. 3. Processing technology of concentrated HF In order to apply the concentrated HF etching to optical manufacturing, it is necessary to study its technological characteristics first. There are mainly two concerns: one is the etching efficiency of HF, and the other is the effect of HF etching on the surface shape of optical elements. 3.1. Etching rate To measure the etching rate of HF, we need to measure the material removal before and after etching. The simplest way is to select a flat fused silica sample and divided it into two regions. Half of the sample is coated with photoresist, while the other remains 545
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
Fig. 1. HF etching rate test flow.
unchanged. Then the sample is put into concentrated HF acid for etching. The etched sample is cleaned and the surface photoresist is removed by acetone. As the photoresist won’t react with HF, the half sample covered by photoresist is protected while the other part is etched, so a step will form on the surface of the sample. A profiler is then employed to measure the surface profile, and the height of the step is the removal amount of etching. A fused silica sample was selected and divided into four different regions, and the etching time of 0 min, 20 min, 40 min, 60 min was obtained on four regions by alternate protection of each area. The etching depth of the concentrated HF (49% HF in water) was measured by the Talysurf PGI 1240 profiler, and then its etching rate could be calculated according to the etching time (Fig. 1). Fig. 2(a) gives the material removal with etching time of 20 min, 40 min, 60 min, and Fig. 2(b) presents the material removal rates at every stage. As shown by Fig. 2(b), the etching rate of concentrated HF is as high as 1.33 μm/min at initial stage, the etching rate decreases gradually as time goes on and the etching rate is only 0.88 μm/min at the last 20 min. The probable reason is that the HF was consumed continuously as the HF reacting with fused silica and the concentrated HF has strong volatility, so the content of HF in the solution decreases with the increase of time, which reduces the etching rate of the concentrated HF. Fig. 2(a) shows that in the 60 min etching time, HF removing 68 μm thick of materials, so the average etching rate is about 1.14 μm/min, which is quite considerable. According to experience of traditional processing, the removal rate of grinding is about 0.3–0.8 μm/min, the removal rate of lapping is about 0.03–0.17 μm/min, and the removal rate of polishing is about 0.002–0.008 μm/ min. The removal rate of concentrated HF etching is equal or much higher than that of traditional processing. At the same time, HF etching can simultaneously process the whole workpiece surface so the volume removal is only related to its etching rate. While other processing methods such as grinding, lapping and polishing, its volume removal is related to the size of the tool, so volume removal rate will be discounted. Therefore, concentrated HF etching of fused silica has the feature of high efficiency. 3.2. Etching uniformity The main purpose of the concentrated HF etching is to quickly remove large amount of material and eliminate the cracks after grinding. In order to facilitate subsequent processing, the surface shape accuracy of the workpiece should not be seriously damaged. Soaking optical elements in HF solution is a macroscopic controllable and microscopic uncontrollable material removal method. Macroscopically the removal amount can be determined by controlling the concentration, temperature and soaking time of HF solution. However due to the complexity of the micro-topography of the sample, this method cannot guarantee the uniform removal of whole surface, and will affect the roughness and shape of the workpiece [9]. Theoretically, in addition to the concentration of acid etching solution and temperature, contact area and gravity will also affect the reaction rate. Based on the theoretical knowledge of chemistry, when the concentration of chemical reaction and reaction temperature are the same, the greater the contact area between the reactants, the more the reaction in the unit time. For the etching process of optical elements, the contact area of the acid etching reaction will increase if the surface is rougher, and then more
Fig. 2. Etching rate of concentrated HF. 546
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
Fig. 3. Surface error variation during HF etching (a) Before HF etching (b) After HF etching.
material will be removed in unit time, that means the removal rate will be bigger. There are micro convex or concave structures on the surface of fused silica sample after grinding, and the reaction rate of the side wall of these parts is greater than that of the peak or bottom. This is because reactant on the side wall will quickly fall out of the side wall by gravity, and then new material will appear in the acid solution and continue to react. The reactants at the convex and concave part, especially the concave part, will stacked together and slow down normal reaction, which is more serious when the optical element statically resting in HF acid solution. In order to ensure the uniformity of etching, more attention should be paid to the placement of samples and etching technology. On the one hand, the workpiece can be inversely placed so as to reduce the accumulation of the reactant. In addition, ultrasonic vibration can be added to the etching process to facilitate the separation of the reaction-generated material from the surface of the optical sample. At the same time, it is necessary to keep the temperature constant during the etching process to ensure the stability of the etching process. By using these above methods, a grinding fused silica sample was etched by concentrated HF for about 1 h. In order to verify the uniformity of etching, a three-coordinate measuring machine was employed to measure the surface error of the fused silica sample before and after etching, and Fig. 3 gives the final measurement result. As can be seen from Fig. 3, after 1 h of etching, although approximately 60 μm material has been removed, the surface error of the sample is only increased from 0.876 μm (PV)/0.134 μm (RMS) to 2.107 μm (PV)/0.375 μm (RMS). Compared to the 60 μm amount of material removal, the surface shape before and after etching is basically consistent. Concentrated HF etching doesn’t do serious damage to the surface shape, and will not bring serious effect to subsequent processing.
4. Processing effect of concentrated HF The etching rate and etching uniformity of concentrated HF were studied in previous Section 3. The results show that the efficiency of HF etching is high, and the surface error is not deteriorated, so concentrated HF etching can be employed in optical manufacturing. At the beginning of the manuscript, we want to apply concentrated HF etching to grinding optical samples to remove surface cracks and subsurface defects. Then a set of experiments were done to verify if concentrated HF etching can effectively remove crack and improve the surface roughness. We selected the grinding fused silica samples and used concentrated HF to process them. Before and after etching, the sample was inspected by microscope and scanning electron microscope (SEM). At the same time, the surface roughness of the sample was
Fig. 4. Surface morphology after grinding (a) Microscope 500× (b) SEM 500× (c) SEM 5000×. 547
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
Fig. 5. Surface morphology after 30 min HF etching (a) Microscope 500× (b) SEM 500× (c) SEM 5000×.
measured by a profiler. In order to get more details of the sample, we slightly etched the grinding sample instead of direct observation. Fig. 4(a) is the result observed by the Keyence vhx-600e electron microscope, and the magnification in the figure is 500×. The shiny place in the picture is high part, and the darker position is the low point. The picture’s degree of light and shade is not uniform, that means the surface of sample is not flat. As the microscope could not see more details, a Phenom Desk top SEM (model G2 Pro) was employed to further observe the sample. The magnification of this SEM is from 80× to 45,000×. Fig. 4(b) is the result of SEM observation with the magnification of 500×. Similar to the microscope, the brighter part of the image is higher, and the darker part is lower. As can be seen from the picture, a large number of staggered ravines appeared on the sample, and the dark part represented the cracks left by grinding. The surface of the sample can be observed more clearly after further amplified to 5000× as Fig. 4(c). After grinding, the surface of the sample is uneven and there are many different cracks. These cracks need to be removed by subsequent processing to obtain smooth and non-damage surface. At the same time, the surface roughness of the sample is measured by the Talysurf PGI 1240 surface profiler, and the result is 1.21 μm (Ra). In order to obtain the intuitive impression, a grinding sample was etched by concentrated HF for 30 min first, and Fig. 5 presents the surface topography of the etched sample. It can be seen from Fig. 5(a) that after 30 min etching, the surface of the sample becomes uneven. Some parts become smoother, while others parts appear deep holes. As the depth of field of the microscope is relatively shallow, the SEM was employed to continue observing. The etching marks on the surface of the sample can be seen from the image of the SEM in Fig. 5(b). During etching the cracks were always expanding, and they will intersect in the process of expansion. The intersection will form deep grooves or high peaks, and the peaks always had sharp edges. The surface of the etched surface can be more clearly seen from the Fig. 5(c), and the sharp boundary line is the product of the crack expansion. Due to the existing deep trenches and pits, the sample went through another 30 min concentrated HF etching. After etching, it can be seen from Fig. 6(a) that the irregular structures on the surface were completely disappeared. The surface looks like a series of smooth convex overlapping together without sharp pits and protrusions, and the surface quality has been greatly improved. The observations results of SEM in Fig. 6(b) and (c) show a clearer picture of the smooth surface, and the cracks and flaws in the surface left by grinding have all disappeared. The surface was measured by a profiler to get the surface roughness and now the result is 0.19 μm (Ra). After concentrated HF etching, the surface quality increases significantly, and the surface integrity is also guaranteed. From the process of concentrated HF etching, it can be seen that as the reaction goes on, the surface quality of the sample is getting better and the surface roughness is getting smaller. Section 3.2 mentioned that etching rate will become bigger if surface roughness is larger, which also partly explains the decline in etching rates as the etching progresses. 5. Conclusion Through the study of the concentrated HF etching process, the results show that concentrated HF etching can be used to fabricate
Fig. 6. Surface morphology after 60 min HF etching (a) Microscope 500× (b) SEM 500× (c) SEM 5000×. 548
Optik - International Journal for Light and Electron Optics 178 (2019) 544–549
Y. Shu
fused silica optical elements. The removal efficiency of concentrated HF etching is high and the sample can be processed globally, and concentrated HF etching can effectively remove the grinding cracks with an integrated surface, so concentrated HF etching can act as a connecting process between grinding and polishing. At the same time, concentrated HF etching will not seriously destroy the surface precision of the component, which can reduce the workload of the subsequent polishing. Application of concentrated HF etching will greatly improve the processing efficiency and processing quality of fused silica optical element. References [1] T. Suratwala, et al., Sub-surface mechanical damage distributions during grinding of fused silica, J. Non-Cryst. Solids 352 (52–54) (2006) 5601–5617, https://doi. org/10.1016/j.jnoncrysol.2006.09.012. [2] A.G. Mikosza, B.R. Lawn, Section and Etch study of hertzian fracture mechanics, J. Appl. Phys. 42 (13) (1971) 5540–5545, https://doi.org/10.1063/1.1659977. [3] G.A.C.M. Spierings, Wet chemical etching of silicate glasses in hydro- fluoric acid based solutions, J. Mater. Sci. 28 (23) (1993) 6261–6273, https://doi.org/10. 1007/bf01352182. [4] L. Wong, et al., The effect of HF-NH4F etching on the morphology of surface fractures on fused silica, J. Non-Cryst. Solids 355 (13) (2009) 797–810, https://doi. org/10.1016/j.jnoncrysol.2009.01.037. [5] J.S. Judge, A study of the dissolution of SiO2 in acidic fluoride solutions, J. Electrochem. Soc. 118 (11) (1971) 1772–1775, https://doi.org/10.1149/1.2407835. [6] S.T. Tso, J.A. Pask, Reaction of fused silica with hydrogen gas, J. Am. Ceram. Soc. 65 (9) (1982) 457–460, https://doi.org/10.1111/j.1151-2916.1982.tb10514.x. [7] M. Prokopowicz-Prigogine, Reactivity of a silica network of glass. Molecular mechanism of the dissolution of a silica network in aqueous HF-HCl solutions, Glastech. Ber. 62 (7) (1989) 249–255. [8] S. Wolf, R.N. Tauber, Silicon processing for the VLSI era, 2nd ed., Process Technology vol. 1, Lattice Press, California, 2001. [9] J.A. Menapace, et al., Combined advanced finishing and UV-laser conditioning for producing UV-damage-resistant fused silica optics, Proc. SPIE 4679 (2002) 56–68, https://doi.org/10.1117/12.461725. Yong Shu is a lecturer at the Air Force Engineering University. He received his BS degree in mechanical engineering from the Wuhan University in 2006, and his PhD degree in mechanical engineering from the National University of Defense technology (NUDT) in 2014. His current research interests include smoothing, computercontrolled optical manufacturing and fabricating of large mirrors.
549