Alkaline silicon etching solution and application thereof

Abstract

The invention discloses an alkaline silicon etching solution and application thereof. The composition comprises an inorganic metal hydroxide, hydroxylamine, and further comprises catechol and / or resorcinol. The alkaline silicon etching liquid comprises the following components in percentage by weight: 0.1 to 2.5 percent of catechol and / or resorcinol. By adding 0.1%-2.5% of catechol and / or resorcinol, the etching rate stability of the etching liquid can be remarkably improved. Because catechol and / or resorcinol has reducibility, catechol and / or resorcinol can preferentially react with an oxidizing agent such as oxygen in the solution, the possibility that hydroxylamine is oxidized is reduced, and decomposition of hydroxylamine is inhibited. Meanwhile, by controlling the specific dosage of the added catechol and / or resorcinol, the situation that excessive catechol and / or resorcinol are / is adsorbed to the silicon surface, and the effect is affected is avoided. The etching solution provided by the invention has high initial etching rate and high etching rate stability.

Description

Technical Field

[0001] The present invention particularly relates to an alkaline silicon etching solution and application thereof. Background Art

[0002] Anisotropic etching of single-crystal silicon is widely used to fabricate various microstructures in the field of microelectromechanical systems (MEMS). It is also widely used to texturize single-crystal silicon surfaces, thereby reducing light reflection and improving the efficiency of crystalline silicon solar cells. In single-crystal silicon wet processing, the etch rate is a major factor affecting productivity. Slow silicon etch rates significantly increase device fabrication time, thereby reducing production efficiency. Therefore, in the MEMS industry, which uses wet anisotropic etching for silicon bulk micromachining, the silicon etch rate is of great concern, especially when removing underlying material through undercutting to create deep cavities and independent microstructures.

[0003] Commonly used wet anisotropic silicon etching solutions are alkaline etching solutions, such as potassium hydroxide (KOH) solution and tetramethylammonium hydroxide (TMAH) solution. Among them, the etching rate of tetramethylammonium hydroxide solution is greatly affected by temperature and concentration, and its safety is not good. Potassium hydroxide (KOH) solution has anisotropic etching properties for silicon. <100> The etching rate of the crystal direction is much faster than <111> Crystal direction.

[0004] Although potassium hydroxide has a higher silicon etching rate than tetramethylammonium hydroxide solution at the same concentration, it is still relatively low, for example, only 1µm / min at 80°C, which is difficult to meet the high silicon etching rate requirements in the MEMS field. Furthermore, in silicon-based micro-nanofabrication (such as MEMS and semiconductor manufacturing), the performance of the etching solution directly affects the precision, yield, and reliability of the device. Currently, silicon etching solutions still face difficulties and challenges in terms of etching rate control, surface smoothness, and stability.

[0005] For potassium hydroxide solution etching solution, there are usually the following problems: 1. Bubble problem: Silicon reacts with hydroxide in alkaline etching solution to generate hydrogen bubbles. These bubbles will adhere to the surface of silicon wafer, acting as a pseudo mask, increasing the roughness of the etched surface, and may also damage the microstructure. In addition, bubbles attached to the silicon surface will hinder the contact between the etching solution and the silicon wafer, reducing the etching rate; 2. Crystal orientation effect: KOH etching is anisotropic, and the etching rate is seriously affected by the orientation of the silicon crystal. The etching rate of different crystal planes varies greatly. On the wafer plane on (100) silicon, the etching rate is different from that on the wafer plane on (100) silicon. <110> Directionally aligned rectangular openings result in rapid etching of the exposed {100} planes and slow etching of the {111} planes, which can lead to issues related to mask alignment, where small rotational errors can lead to feature weakening and enlargement.

[0006] Adding hydroxylamine to a KOH solution can significantly increase the initial etching rate of silicon. For example, adding 15% hydroxylamine to a 20wt% KOH solution can achieve a Si{110} etching rate four times that of a pure 20wt% KOH solution. This is because hydroxylamine participates in the redox reaction of silicon, promoting chemical reactions on the silicon surface to form silicon dioxide. Hydroxylamine also has the effect of quickly removing hydrogen bubbles produced during etching from the silicon surface. However, hydroxylamine is very unstable in strongly alkaline KOH solutions and tends to decompose rapidly, resulting in a decrease in the effective hydroxylamine content in the etchant, affecting its effectiveness.

[0007] Although some prior art methods add other auxiliary components to alkaline silicon etching solutions containing hydroxylamine to inhibit the decomposition of hydroxylamine, these auxiliary components may inhibit the silicon etching rate and reduce the silicon etching rate, such as the initial etching rate, making it difficult for alkaline etching solutions to achieve a high initial etching rate. Achieving a high initial etching rate while ensuring that the etching solution is stable and the hydroxylamine content is substantially non-decomposed (thus ensuring that the etching solution can maintain a high etching rate level after long-term storage) is a challenge. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to address the shortcomings and deficiencies of the existing technology and provide an improved alkaline silicon etching solution. The etching solution has a high initial etching rate and is highly stable. The etching solution can maintain a high etching rate after being stored for a long time, such as several days. In other words, the etching rate is highly stable and safe. To solve the above technical problems, the present invention adopts the following technical solutions: An alkaline silicon etching solution, comprising an inorganic metal hydroxide, hydroxylamine, and catechol and / or resorcinol; the alkaline silicon etching solution comprises 0.1%-2.5% of the catechol and / or resorcinol in weight percentage. Although adding hydroxylamine to a potassium hydroxide solution etching solution can increase the initial etching rate of the etching solution, the inventors of the present application have found that the hydroxylamine component in the etching solution is unstable and tends to decompose rapidly after the etching solution is prepared, resulting in a reduction in the effective hydroxylamine component content, resulting in a significant decrease in the etching rate after the etching solution is left for a long time, that is, the etching rate stability of the etching solution is low. Typically, etching solutions containing potassium hydroxide and hydroxylamine are difficult to use after being left for one day after preparation because the etching rate has already significantly decreased by then.

[0009] The inventors of the present application have found through research that by adding 0.1%-2.5% of catechol and / or resorcinol to an etching solution containing potassium hydroxide and hydroxylamine, the etching rate stability of the etching solution can be significantly improved. As mentioned above, hydroxylamine is very unstable in potassium hydroxide solution. In the absence of a stabilizer, after the etching solution is prepared, hydroxylamine will quickly decompose, resulting in a decrease in the etching rate of the potassium hydroxide solution, that is, poor etching rate stability. The catechol and / or resorcinol added in the present application have reducing properties and can preferentially react with the oxidant (such as oxygen, hydrogen peroxide, etc.) in the etching solution, and themselves are oxidized, thereby consuming the oxidant in the system, reducing the possibility of hydroxylamine being oxidized, indirectly protecting hydroxylamine, avoiding its decomposition, making it stable for a long time, and thus making the content of effective hydroxylamine stable, so that the etching solution not only has a high initial etching rate, but also has a long-term stable etching rate, that is, after the etching solution is placed for a long time, the etching rate still maintains a high level, that is, the stability of the etching rate is improved. At the same time, the present application controls the added catechol and / or resorcinol to a specific amount, that is, in terms of weight percentage, it accounts for 0.1%-2.5% of the etching solution, which can further improve the etching rate stability of the etching solution. This is because although the added catechol and / or resorcinol can play a role in avoiding the decomposition of hydroxylamine, if the amount is too much, the excessive catechol and / or resorcinol will be adsorbed to the silicon surface to be etched or the surface of the silicon etching product that has been etched, which will affect its reaction with the oxidant and avoid the effect of hydroxylamine decomposition.

[0010] The hydroxylamine component in the etching solution of the present application is highly stable and does not decompose even after prolonged storage, such as several days. This ensures that the etching solution can still achieve a high etching rate even after prolonged storage, significantly extending the service life of the etching solution while maintaining a high etching rate. Other additives, such as alcohols and alcoholamines, struggle to achieve both a high initial etching rate and a stable, high etching rate even after prolonged storage.

[0011] In some embodiments, the alkaline silicon etching solution comprises 0.3%-2.5% of the catechol and / or resorcinol by weight. The alkaline silicon etching solution can comprise, for example, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.2%, 2.3%, 2.4%, 2.5% of catechol and / or resorcinol.

[0012] In some embodiments, the alkaline silicon etching solution contains 0.7%-2.5% of the catechol and / or resorcinol by weight.

[0013] In some embodiments, the alkaline silicon etching solution comprises 0.8% to 1.2% of catechol and / or resorcinol by weight. Within this dosage range, optimal etching rate stability and high etching rate can be achieved.

[0014] In some embodiments, the inorganic metal hydroxide is selected from a combination of one or more of potassium hydroxide, cesium hydroxide, and sodium hydroxide. The inorganic metal hydroxide is the main component of etching.

[0015] In some embodiments, the alkaline silicon etching solution comprises 10%-30% of the inorganic metal hydroxide by weight. The alkaline silicon etching solution may comprise, for example, 10%, 15%, 20%, 25%, or 30% of the inorganic metal hydroxide.

[0016] In some embodiments, the alkaline silicon etching solution contains 20%-30% of inorganic metal hydroxide by weight.

[0017] In some embodiments, the alkaline silicon etching solution comprises 1% to 15% hydroxylamine by weight. The alkaline silicon etching solution may comprise, for example, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, 10%, or 15% hydroxylamine. Hydroxylamine can increase the initial etching rate.

[0018] In some embodiments, the alkaline silicon etching solution contains 5%-10% hydroxylamine by weight.

[0019] In some embodiments, the alkaline silicon etching solution further comprises a chelating agent.

[0020] In some embodiments, the chelating agent is selected from ethylenediaminetetraacetic acid (EDTA), aminotriacetic acid (NTA), ethylenediaminedisuccinic acid (EDDS), diethylenetriaminepentaacetic acid (DTPA), ethylene glycol tetraacetic acid (EGTA), nitrilotriacetic acid (NTA), glucoheptonic acid, hydroxyethylethylenediaminetriacetic acid (HEDTA), glutamic acid-N,N-diacetic acid (GLDA), iminodisuccinic acid (IDS), methylglycinediacetic acid (MGDA), and salts thereof. Adding a chelating agent alone cannot effectively inhibit the decomposition of hydroxylamine.

[0021] In some embodiments, the alkaline silicon etching solution comprises 0.001%-3% chelating agent by weight. The alkaline silicon etching solution can comprise, for example, 0.001%, 0.01%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 1.0%, 1.5%, 2.0%, 2.5%, or 3.0% of a chelating agent.

[0022] In some embodiments, the alkaline silicon etching solution further comprises water, which may be, for example, ultrapure water.

[0023] In some embodiments, the alkaline silicon etching solution consists of, by weight percentage, 10%-30% of an inorganic metal hydroxide, 1%-15% of hydroxylamine, 0.1%-2.5% of catechol and / or resorcinol, 0.001%-3% of a chelating agent, and the balance of water.

[0024] The present invention also provides the use of the aforementioned alkaline silicon etching solution for silicon etching in the fields of microelectromechanical systems (MEMS) or photovoltaics (solar cells). Both fields have very high requirements for silicon etching.

[0025] Due to the implementation of the above technical solution, the present invention has the following advantages compared with the prior art: 1) The present invention, by adding 0.1%-2.5% of catechol and / or resorcinol to an etching solution containing an inorganic hydroxide and hydroxylamine, can inhibit the decomposition of hydroxylamine, thereby maintaining the etching rate level after the etching solution is placed for a long time, while also significantly improving the initial etching rate of the etching solution. Specifically, the etching solution of the present invention has a high initial etching rate and good stability. Even after a long period of placement, the hydroxylamine is essentially not decomposed, and the etching rate remains high even after placement.

[0026] 2) The alkaline etching solution of the present invention has a maximum etching rate of 5.14µm / min on the silicon {100} plane, which is higher than the existing technical level and has a longer storage time.

[0027] 3) The alkaline etching solution of the present invention has a high etching rate on different planes of silicon.

[0028] 4) The alkaline etching solution of the present invention can maintain a high etching rate even after being placed for 8 days or even 40 days after preparation, and the service life of the etching solution is far higher than the existing technical level.

[0029] 5) The alkaline etching solution of the present invention is safer and has a higher etching rate than the TMAH system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a metallographic microscope image of the polished silicon 100 surface after etching in the etching solution for 0 days in Example 1; Figure 2 This is a metallographic microscope image of the polished silicon 100 surface after etching in the etching solution for 8 days in Example 1; Figure 3 This is a metallographic microscope image of the polished silicon 100 surface after etching in the etching solution for 0 days in Example 5; Figure 4 This is a metallographic microscope image of the polished silicon 100 surface after etching in the etching solution for 0 days in Comparative Example 4; Figure 5 This is a metallographic microscope picture of the polished silicon 100 surface after etching in the etching solution for 0 days in comparative example 5. DETAILED DESCRIPTION

[0031] The technical solutions of the present invention are described in detail below in conjunction with specific embodiments so that those skilled in the art can better understand and implement the technical solutions of the present invention, but the present invention is not limited to the scope of the examples.

[0032] Preparation of silicon etching solution and visual observation of its stability (this observation is performed before etching, and stability observation is one of the means to characterize the stability of the etching solution): After the components of the silicon etching solution are fully mixed, observe the generation of bubbles in the solution under static conditions. The various results and their definitions are as follows: Basically no bubbles: no obvious bubbles are observed with the naked eye, and there are very few bubbles in the solution and they disappear quickly; Obvious bubbles: bubbles are visible to the naked eye, and the number of bubbles continues to increase.

[0033] The silicon single crystal etching process is as follows: 2cm×2cm silicon {100} and {110}, single crystal sample weighed m before the experiment 前 , soaked in 0.2% ammonium fluoride solution with a pH of about 3 for 5 minutes to remove the surface silicon oxide layer and then rinsed with plenty of water; then etched the silicon single crystal sample in the embodiment solution at 80°C for 20 minutes, took out the sample and rinsed it with plenty of water, dried it with nitrogen and weighed it m 后 ; According to the weight loss Δm(=m 前 -m 后 ) Calculate the corrosion rate of silicon single crystal according to the following formula: Silicon single crystal corrosion rate = Δm / (ρ*a*t) Where ρ is the density of silicon 2.33 g / cm 3 , a is the corrosion area of ​​silicon single crystal, and t is the corrosion time.

[0034] Example 1: This example provides an alkaline etching solution, and uses it for etching silicon: The alkaline etching solution is composed of the following components by weight: 24% KOH, 10% hydroxylamine, 0.1% EDTA, 64.9% ultrapure water, and 1% catechol. Visual observation of the etching solution stability and the initial silicon etching rate (etching was performed immediately after the etching solution was prepared) are shown in Table 1.

[0035] Examples 2-5 and Comparative Examples 1-3: Examples 2-5 and Comparative Examples 1-3 are essentially the same as Example 1, differing only in the composition of the alkaline etching solution, as shown in Table 1 below. Comparative Examples 1-3 used excessive amounts of catechol or resorcinol. The results are shown in Table 1 below ( / indicates no addition).

[0036]

[0037] As shown in Table 1, the present invention solves the problem of instability (e.g., unstable etching rate) of conventional potassium hydroxide and hydroxylamine solution etching solutions by adding a specific amount of catechol and / or resorcinol to an alkaline etching solution containing potassium hydroxide and hydroxylamine (after the solution is prepared and before etching, bubbles are generated due to the decomposition of hydroxylamine, which is a sign of instability of the etching solution). By adding catechol and / or resorcinol, they can react with oxidants such as oxygen in the solution first, thereby avoiding the decomposition of hydroxylamine caused by the reaction of the oxidant with hydroxylamine, so that the effective amount of hydroxylamine in the etching solution remains high. At the same time, the amount of catechol and / or resorcinol is controlled not to be excessive, otherwise it will adsorb on the silicon surface and affect the aforementioned effects. As a result, the alkaline etching solution of the present invention has a high initial etching rate and high etching rate stability. A comparison of Example 1 and Example 4 (both of which keep the same amount of catechol) shows that, within a certain dosage range, increasing the amount of hydroxylamine can increase the etching rate. From the comparison of Examples 1-4 and Comparative Example 1, it can be seen that when the amount of hydroxylamine is kept constant at 10%, the etching rate is the highest when the amount of catechol is 1%, and the etching rate is greater than that of Example 2 at 0.7%. When the amount of catechol increases from 1% to 2.5%, although the initial etching rate decreases by 0.24, the degree of decrease is not as much as the degree of decrease of 0.70 when the amount of catechol increases from 2.5% to 5%. That is, when the amount of catechol exceeds 2.5%, the initial etching rate decreases faster. The present invention controls the amount of catechol and / or resorcinol to below 2.5%, which can achieve better results.

[0038] Comparative Example 4: Similar to Example 1, except that the alkaline etching solution was composed of 24% KOH and 76% ultrapure water by weight. The initial etching rate on the {100} plane was 1.36 µm / min, significantly lower than that in Example 1.

[0039] In addition, the etching solutions of Example 1 and Comparative Example 4 were respectively tested on different silicons to obtain the corrosion rates on different silicon planes. The results are shown in Table 2 below: It can be seen that the etching solution of Example 1 has a high initial etching rate for various silicons.

[0040]

[0041] Comparative Example 5: This was essentially the same as Example 1, except that catechol was omitted from the alkaline etching solution, and the ultrapure water content was adjusted to 65.9%. Visual observation of the etching solution stability revealed a significant number of bubbles, and the etching rate for the {100} plane was 3.98 µm / min, significantly lower than that of Example 1. This indicates that the addition of catechol improves the stability of the etching solution and increases the initial etching rate.

[0042] After the etching solutions of Example 1, Example 5 and Comparative Example 5 were prepared, they were allowed to stand for a period of time before etching. The corresponding corrosion rates on the {100} plane were tested. The results are shown in Table 3 below, where "-" indicates not tested:

[0043] In Comparative Example 5, the etching rate of the etching solution decreased significantly after one day of storage, so there was no need to test the etching rate after three days of storage. As shown in Table 3, the present invention improves the hydroxylamine decomposition stability of the alkaline etching solution containing potassium hydroxide and hydroxylamine by adding a specific amount of catechol or resorcinol. The etching solution can remain stable for a long time, and the etching rate remains high even after etching is performed after a long period of storage. In contrast, the etching rate of existing etching solutions decreases significantly after long-term storage due to the instability and decomposition of hydroxylamine.

[0044] The metallographic microscope picture of the polished silicon 100 after etching in the etching solution for 0 days in Example 1 is as follows: Figure 1 As shown, the picture after 8 days is as follows Figure 2 As shown; the metallographic microscope picture of Example 5 when the etching solution is placed for 0 days is as shown Figure 3 As shown; Comparative Example 4, Comparative Example 5 in the etching solution when placed for 0 days when the metallographic microscope pictures are as follows Figure 4-5 As shown. It can be seen that after 8 days of storage, the surface state of the silicon 100 after etching by the etching solution of Example 1 did not change significantly. The surface state of the silicon 100 after etching by the etching solution containing resorcinol in Example 5 was rougher than that of Example 1, but better than that of Comparative Example 5. Although the corrosion rate of the potassium hydroxide solution in Comparative Example 4 was relatively low, the surface state was worse than that of Example 1. The surface state of the silicon 100 after etching by the silicon etching solution containing hydroxylamine without adding catechol in Comparative Example 5 was significantly rougher than that of Example 1.

[0045] Comparative Examples 6-8 are essentially the same as Example 1, except that catechol is replaced with glycerol, xylitol, and sorbitol, respectively. Visual observation of the etching solution stability and the initial corrosion rate on the {100} plane are shown in Table 4 below:

[0046] As can be seen from Table 4, when glycerol, xylitol or sorbitol is used instead of the catechol or resorcinol of the present invention, more bubbles are generated when the etching solution is left to stand, the etching solution is unstable, and the etching rate is reduced.

[0047] Comparative Example 9: This is essentially the same as Example 1, except that 1% catechol was replaced with 10% isopropyl alcohol, and the amount of ultrapure water was adjusted to 55.9%. It was found that the etching solution stratified, and etching could not be performed. This was because isopropyl alcohol was insoluble in the etching solution system of this example. When the etching solution was composed by weight as follows: 24% KOH, 10% hydroxylamine, 0.1% EDTA, 60.9% ultrapure water, and 5% isopropyl alcohol, isopropyl alcohol was soluble in the etching solution system, but the initial etching rate for the silicon 100 surface dropped to 1.10µm / min.

[0048] Comparative Example 10: Similar to Example 1, except that hydroxylamine was replaced with hydroxylamine sulfate. The etching solution still contained crystals, preventing etching. This is because hydroxylamine sulfate has low solubility in the potassium hydroxide aqueous solution used in this example, and most of it remains insoluble, forming crystals.

[0049] Comparative Example 11: Similar to Example 1, except that catechol was replaced with monoethanolamine. The etching rate on the {100} plane was reduced to 2.16 μm / min, significantly lower than that in Example 1.

[0050] Comparative Examples 12-14 are essentially the same as Example 1, except that the potassium hydroxide system is replaced with a TMAH system and the dosages of some components are changed (TMAH is a toxic substance, and the mass concentration of TMAH aqueous solution in industry is generally not too high). The results of visual observation of stability and initial corrosion rate are shown in Table 5 below:

[0051] As can be seen from Table 5, when the TMAH system is used, even with the addition of catechol, the initial corrosion rate is significantly lower than that of KOH, and is much lower than the initial corrosion rate of the present invention.

[0052] The above embodiments are only intended to help understand the method and core concept of the present invention. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from the principles of the present invention, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. An alkaline silicon etching solution, characterized in that: The alkaline silicon etching solution contains inorganic metal hydroxide, hydroxylamine, and also contains catechol and / or resorcinol; in terms of weight percentage, the alkaline silicon etching solution contains 0.1%-2.5% of the catechol and / or resorcinol.

2. The alkaline silicon etching solution according to claim 1, wherein: In terms of weight percentage, the alkaline silicon etching solution contains 0.3%-2.5% of the catechol and / or resorcinol.

3. The alkaline silicon etching solution according to claim 1, wherein: In terms of weight percentage, the alkaline silicon etching solution contains 0.7%-2.5% of the catechol and / or resorcinol.

4. The alkaline silicon etching solution according to claim 1, wherein: In terms of weight percentage, the alkaline silicon etching solution contains 0.8%-1.2% of the catechol and / or resorcinol.

5. The alkaline silicon etching solution according to claim 1, wherein: The inorganic metal hydroxide is selected from a combination of one or more of potassium hydroxide, cesium hydroxide and sodium hydroxide.

6. The alkaline silicon etching solution according to claim 1, wherein: In terms of weight percentage, the alkaline silicon etching solution contains 10%-30% of inorganic metal hydroxide.

7. The alkaline silicon etching solution according to claim 1, wherein: In terms of weight percentage, the alkaline silicon etching solution contains 1%-15% hydroxylamine.

8. The alkaline silicon etching solution according to claim 1, wherein: The alkaline silicon etching solution further comprises a chelating agent.

9. The alkaline silicon etching solution according to claim 8, wherein: The chelating agent is selected from the group consisting of ethylenediaminetetraacetic acid, aminotriacetic acid, ethylenediaminedisuccinic acid, diethylenetriaminepentaacetic acid, ethylene glycoltetraacetic acid, nitrilotriacetic acid, glucoheptonic acid, hydroxyethylethylenediaminetriacetic acid, glutamic acid-N,N-diacetic acid, iminodisuccinic acid, methylglycinediacetic acid and their salts, and / or the alkaline silicon etching solution contains 0.001%-3% of the chelating agent by weight.

10. The alkaline silicon etching solution according to claim 1, wherein: The alkaline silicon etching solution further contains water.

11. The alkaline silicon etching solution according to claim 1, wherein: In terms of weight percentage, the alkaline silicon etching solution consists of 10%-30% of an inorganic metal hydroxide, 1%-15% of hydroxylamine, 0.1%-2.5% of catechol and / or resorcinol, 0.001%-3% of a chelating agent, and the balance of water.

12. Use of the alkaline silicon etching solution according to any one of claims 1 to 11 for silicon etching in micro-electromechanical systems or photovoltaic fields.

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