CN100383932C - A silicon wet etching process - Google Patents

Abstract

The invention discloses a silicon wet etching process. The steps are: ① Sputtering a Cr mask on a clean silicon wafer, the thickness of the Cr mask is 50nm-600nm; ② On the Cr mask, use a photolithography process to prepare patterns; ③ On (NH ₄ ) ₂ Ce(NO ₃ ) The Cr mask is etched in _{the 5} solution, and the composition ratio of the (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution is: per 100ml of the etching solution, the cerium ammonium nitrate is: 5-20 grams, the highest Chloric acid: 1.5-8 ml, the balance is water; ④ use KOH solution to wet-etch the etched silicon wafer; ⑤ use (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution to remove silicon On-chip Cr mask. Since Cr is closely combined with the silicon material, it can ensure sufficient firmness during wet etching, and the selectivity of the Cr mask in the KOH etching solution is very good. The invention has the characteristics of low toxicity and simple processing.

Description

A silicon wet etching process

technical field

The invention belongs to etching mask technology, in particular to a silicon wet etching process.

Background technique

The core process of wet etching is selective etching, that is, to obtain the required pattern on the substrate by using the characteristics that the etching rate of the mask and the substrate is very different in the etching solution. Therefore, the fabrication of the mask is an important process link to realize the wet method.

Currently, SiO ₂ (silicon dioxide) and Si ₃ N ₄ (silicon nitride) are commonly used as mask materials in wet etching of silicon, and concentrated alkali such as KOH is generally used as an etching solution. Using SiO ₂ (Silicon Dioxide) and Si ₃ N ₄ (Silicon Nitride) as etch masks has some problems, making the etch effect not very good. Therefore, it is necessary to find a new mask material and work out a corresponding scheme.

The selectivity of the SiO ₂ film mask to the KOH etching solution is not very good. Generally, the ratio of the etching rate of thermally grown SiO ₂ to silicon in a KOH solution at 50°C is about 1:100, while the thermally grown SiO 2 ₂ film has the best corrosion resistance to KOH among various SiO ₂ films. Therefore, the SiO ₂ mask can only be used in shallow silicon wet etching; and the processing of SiO ₂ usually uses highly toxic HF (hydrofluoric acid), which is dangerous to the environment and the human body.

The etch rate of the Si ₃ N ₄ mask in KOH is extremely small, so its selectivity is very good. However, the thermal expansion coefficient of Si ₃ N ₄ is quite different from that of silicon, and its film will crack or even fall off on the silicon wafer, so it cannot play the role of a mask at all. Therefore, it is necessary to use the SiO ₂ layer as the intermediary layer, first grow a layer of SiO ₂ on the silicon wafer, and then deposit Si ₃ N ₄ at low temperature, so as to ensure that the Si ₃ N ₄ mask is firm and reliable in the etching solution. Moreover, the wet etching method of Si ₃ N ₄ is very complicated, generally only RIE (Reactive Ion Etching) can be used, the process is relatively complicated, and the equipment requirements are high.

Contents of the invention

The purpose of the present invention is to overcome the disadvantages of the prior art and provide a silicon wet etching process, which has the characteristics of low toxicity and simple processing.

A kind of silicon wet etching process provided by the invention, its step comprises:

(1) Sputtering a Cr mask on a clean silicon wafer, the thickness of the Cr mask is 50nm-600nm;

(2), on the Cr mask, utilize a photolithography process to prepare a photoresist pattern;

(3) Etching the Cr mask in (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution, the composition ratio of (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution is: per 100ml of etching solution Among them, ammonium cerium nitrate: 5 grams to 20 grams, perchloric acid: 1.5 milliliters to 8 milliliters, and the balance is water;

(4), use KOH solution to carry out wet etching to the silicon wafer after the Cr mask is etched;

(5) Using (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution to remove the Cr mask on the silicon wafer.

The present invention uses metal Cr as the mask material, specifically, has the following technical effects:

(1) Cr is closely combined with silicon materials, and is often used to deposit intermediate layers of other metal materials on silicon substrates, so it can ensure sufficient firmness during wet etching.

(2) Cr has stable chemical properties, good resistance to strong alkali, and hardly reacts in KOH etching solution, so the selectivity of Cr mask in KOH etching solution is very good.

(3) Cr can be rapidly dissolved in (NH ₄ ) ₂ Ce(NO ₃ ) ₅ (cerium ammonium nitrate) solution, while (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution cannot etch silicon, and it is basically non-toxic , harmless to the environment. Therefore, (NH ₄ ) ₂ Ce(NO ₃ ) ₅ can be used to pattern the Cr mask.

(4) The template pattern can be transferred to the Cr mask by using standard photolithography technology, the process is simple, and there is no special requirement for equipment.

The comparison result of the present invention and prior art is as follows:

Table 1 Comparison of properties of various mask materials

mask material SiO₂ Si₃N₄ Cr Typical preparation method thermal oxidation CVD sputtering Typical processing method HF wet etching   RIE dry etching Cerium ammonium nitrate wet etching Processing characteristics The etchant is toxic   The equipment is expensive and the process is complicated The etchant is non-toxic and the process is simple.

In summary, the present invention has the characteristics of low toxicity and simple processing technology, which will greatly reduce the production cost of wet etching and significantly improve production efficiency.

Description of drawings

Fig. 1 is a process flow diagram of the present invention;

Figure 2 is a 30-minute etching pit SEM image (side length 4 μm);

Figure 3 is a 30-minute etching pit SEM image (side length 40 μm);

Figure 4 is a SEM image of the pits etched for 60 minutes.

Fig. 5 is a topography diagram of peeling off in the center of the Cr mask.

Detailed ways

Process flow of the present invention is as shown in Figure 1. The pattern processing of the Cr mask uses a standard photolithography process: firstly, the pattern to be etched on the silicon wafer is made into a template, and the photoresist is coated and then exposed and developed. Then the Cr mask is etched in an etchant. The (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution is prepared by mixing cerium ammonium nitrate, 70% perchloric acid and water. The content of cerium ammonium nitrate in 100ml of etching solution is 5 grams to 20 grams. The content of chloric acid is: 1.5 milliliters ~ 8 milliliters, and the rest is water. During etching, the photoresist can protect the graphics that need to be preserved, transfer the graphics on the stencil to the Cr mask, and finally perform KOH wet etching on the masked silicon wafer.

In terms of process flow, the pattern processing of the Cr mask uses a standard photolithography process: first, the pattern to be etched on the silicon wafer is made into a template, and the photoresist is coated and then exposed and developed. Then the Cr mask is etched in the solution, the pattern to be preserved is protected by photoresist, the pattern on the template is transferred to the Cr mask, and finally, the masked silicon wafer is subjected to KOH wet etching.

The essence of the present invention is to use Cr as the mask material in the silicon wet etching process, and use the mixed solution of (NH ₄ ) ₂ Ce(NO ₃ ) ₅ , perchloric acid and water as the etching solution.

Example 1:

aExperimental process

1. Clean silicon chip;

2. Prepare a sputtering Cr mask with a thickness of 200nm, and the sputtering device is a high-frequency magnetron sputtering system of ANELVA Company;

3. On the Cr mask, use the photolithography process to prepare the pattern, that is, the pattern that needs to be etched on the silicon wafer is made into a template, and the photoresist is applied and then exposed and developed;

4. Etching the Cr mask in (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution, using photoresist to protect the pattern to be preserved, and transferring the pattern on the template to the Cr mask. The (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution only reacts with Cr which is not protected by photoresist, but does not react with photoresist and Si. In every 100ml of (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution, the content of ammonium cerium nitrate is 15 grams, the content of perchloric acid is: 5 milliliters, and the rest is water.

5. The silicon wafer of the mold layer is wet-etched with KOH, and a concentrated KOH solution (KOH:H ₂ O=50g:100ml) is used. The wet etching system is heated at a constant temperature of 50°C in a HHS-1 water bath, and the temperature control accuracy is ±1°C. The silicon wafer used is φ100mm, N-type, thickness ≥510μm, resistivity 3Ω·cm, crystal orientation <100>.

6. Using (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution to remove the Cr mask.

bTest results

As shown in the SEM image of Figure 2, after 30 minutes of etching, the square pit with a side length of 4 μm has etched an inverted pyramid structure, and the spire has appeared. According to the characteristics of monocrystalline silicon wet etching, the four walls of the inverted pyramid should be the <111> crystal orientation plane, which has an inclination angle of 53.7° with the <100> crystal orientation plane. The side length of the pit in Fig. 3 is 40 μm, and the projection of the <111> plane on the <100> plane is about 15 μm, so it can be deduced that the depth at this time is about 15×tan 53.7°=20.42 μm. The etching depth at this time is 21.74 μm after the measurement of the step instrument; while Figure 4 is the SEM image of the pit after etching for 60 minutes, the projection of the <111> plane on the <100> plane is about 28 μm, similarly, it can be deduced that this The etching depth was 38.12 μm, while the depth measured by the step meter was 41.01 μm. As mentioned above, the etching solution used in the test is KOH at 50°C (KOH:H ₂ O = 50g:100ml), the theoretical etching speed is 1 μm per minute, and the etching for 60 minutes should be able to etch a depth of 60 μm. left and right graphics. There is a relatively large gap between the test etching rate and the theory, and whether the etching rate is slowed down due to the use of a Cr mask remains to be further studied.

The relatively large white particles in the pit may be unwashed KOH crystals, and the flocs should be reaction products such as K ₂ SiO ₃ (potassium silicate), which are insoluble in organic solvents and easily adhered to The surface of the silicon wafer, which was not washed off during the subsequent cleaning, remained on the surface of the pit after drying. At the same time, IPA (isopropanol) is not added to the etching solution, so the etching surface is not very smooth.

Example 2

aExperimental process

1. Clean silicon chip;

2. Prepare a sputtering Cr mask with a thickness of 50nm, and the sputtering device is a high-frequency magnetron sputtering system of ANELVA Company;

3. On the Cr mask, use the photolithography process to prepare the pattern, that is, the pattern that needs to be etched on the silicon wafer is made into a template, and the photoresist is applied and then exposed and developed;

4. Etching the Cr mask in (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution, using photoresist to protect the pattern to be preserved, and transferring the pattern on the template to the Cr mask. The (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution only reacts with Cr which is not protected by photoresist, but does not react with photoresist and Si. In every 100ml of (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution, the content of cerium ammonium nitrate is 5 grams, the content of perchloric acid is: 1.5 ml, and the rest is water.

5. The silicon wafer of the mold layer is wet-etched with KOH, and a concentrated KOH solution (KOH:H ₂ O=50g:100ml) is used. The wet etching system is heated at a constant temperature of 50°C in a HHS-1 water bath, and the temperature control accuracy is ±1°C. The silicon wafer used is φ100mm, N-type, thickness ≥510μm, resistivity 3Ω·cm, crystal orientation <100>.

6. Using (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution to remove the Cr mask.

bExperimental results

The experimental results of the 50nm Cr mask are very similar to Example 1, but because the film thickness is relatively thin, the etched pattern is not as clear as that of the 100nm Cr mask. In addition, no obvious cracks were found in the center after 60 minutes of etching in the etchant.

In this example, since the concentration of the (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etchant is not high, the etching speed is relatively slow. That is, it takes a long time to transfer the pattern from the photoresist to the Cr mask.

Example 3

aExperimental process

1. Clean silicon chip;

2. Prepare a sputtering Cr mask with a thickness of 600nm, and the sputtering device is a high-frequency magnetron sputtering system of ANELVA Company;

3. On the Cr mask, use the photolithography process to prepare the pattern, that is, the pattern that needs to be etched on the silicon wafer is made into a template, and the photoresist is applied and then exposed and developed;

4. Etching the Cr mask in (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution, using photoresist to protect the pattern to be preserved, and transferring the pattern on the template to the Cr mask. The (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution only reacts with Cr which is not protected by photoresist, but does not react with photoresist and Si. In every 100ml of (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution, the content of cerium ammonium nitrate is 20 grams, the content of perchloric acid is: 8 ml, and the rest is water.

5. The silicon wafer with the Cr mask is wet-etched using KOH, using a concentrated KOH solution (KOH:H ₂ O=50g:100ml). The wet etching system is heated at a constant temperature of 50°C in a HHS-1 water bath, and the temperature control accuracy is ±1°C. The silicon wafer used is φ100mm, N-type, thickness ≥510μm, resistivity 3Ω·cm, crystal orientation <100>.

6. Using (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution to remove the Cr mask.

bExperimental results

Using a 600nm sputtering film can achieve relatively good results in an etching process that does not require a long time, similar to Example 1, and the pattern is relatively clear. However, since the 600nm sputtered film has a dense structure and strong bonding force, its internal stress is also relatively large. This makes the silicon wafer relatively fragile in the violently reacting KOH solution, and the center film often cracks. Once the central film cracks, the Si wafer will lose its protection and the graphics will be damaged. As shown in Figure 5, after 30 minutes of wet etching, the center of the silicon wafer lost its protection due to cracking and peeling of the Cr mask, and the etched pattern could no longer be recognized.

In this example, since the concentration of the (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etchant is relatively high, the etching speed of the Cr mask is fast.

Claims (1)

1. A silicon wet etching process, the steps comprising: (1) Sputtering a Cr mask on a clean silicon wafer, the thickness of the Cr mask is 50nm-600nm; (2), on the Cr mask, utilize a photolithography process to prepare a photoresist pattern; (3) Etching the Cr mask in (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution, the composition ratio of (NH ₄ ) ₂ Ce(NO ₃ ) ₅ solution is: per 100ml of etching solution Among them, ammonium cerium nitrate: 5 grams to 20 grams, perchloric acid: 1.5 milliliters to 8 milliliters, and the balance is water; (4), use KOH solution to carry out wet etching to the silicon wafer after the Cr mask is etched; (5) Using (NH ₄ ) ₂ Ce(NO ₃ ) ₅ etching solution to remove the Cr mask on the silicon wafer.

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