Method for manufacturing a silicon substrate having a silicon oxide film

Abstract

Provided is a manufacturing method for manufacturing a silicon substrate having a silicon oxide film with a film thickness of 2 μm or more, which is less likely to warp or deform with a small stress in an extremely short time and has excellent film quality (hardness, refractive index, light transmittance). 【Solution means】A first step of patterning an etching mask for deep trenches on the surface of a silicon substrate, a second step of forming deep trenches in the silicon substrate using deep reactive ion etching (D-RIE), and oxidizing the surface of the silicon substrate in which the deep trenches are formed by a thermal oxidation method to oxidize all of the silicon of the ridges defined by the deep trenches and filling the deep trenches with silicon oxide that expands in volume, thereby forming a thick silicon oxide film.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a silicon substrate having a thick silicon oxide film.

Background Art

[0002] In applications such as power devices, optical devices, and MEMS, the demand for silicon substrates with high-quality silicon oxide films having a thickness of 2 μm or more formed on silicon substrates is increasing. As methods for forming a silicon oxide film on a silicon substrate, a flame deposition method, a thermal oxidation method, a CVD method, etc. are known.

[0003] The flame deposition method is a method in which gaseous raw materials are reacted in a flame to generate high-temperature fine particles (SiO2) and sprayed and deposited on a silicon substrate. Since the oxide is in a particulate state during deposition, the light transmittance is small both inside and between the particles, and it cannot be used as it is. In order to obtain a high-quality oxide film, a remelting process at a high temperature (1250 to 1500 °C) is essential. Since it is formed at a high temperature, a large stress is generated due to the difference in the thermal expansion coefficients of the substrate and the oxide film. For this reason, the silicon substrate may be warped, deformed, or the oxide film may crack or peel off.

[0004] The thermal oxidation method is a method of obtaining a high-quality oxide film by leaving the silicon substrate itself in an oxidation atmosphere at a high temperature (900 - 1200 °C). According to Deal-Grove's law, in the case of a thin film, the film thickness is proportional to the oxidation time (reaction rate-limiting at the interface), and when it becomes a certain thickness, it is proportional to the square root of the oxidation time (diffusion rate-limiting in the oxide film). To obtain a thicker silicon oxide film, an exponential long time is required. For example, it is said that at 1100 °C, it takes more than 5000 minutes to obtain a thickness of 5 μm. Although it can be fabricated at a lower temperature (900 - 1200 °C) than the flame deposition method (1250 - 1500 °C), a thick silicon oxide film is formed not only on the front surface (used surface) but also on the back surface (unused surface). Since the silicon oxide film is formed on both the front and back surfaces, warping is not visually observed. However, in reality, a large strain (stress) occurs at the interface between the substrate and the silicon oxide film, and defects may occur in the substrate or the silicon oxide film. Since the silicon oxide film is also formed on the back surface (unused surface), problems may occur in handling and processing in subsequent steps.

[0005] The CVD method is roughly classified into the thermal CVD method and the plasma CVD method. The thermal CVD method includes atmospheric pressure CVD and reduced pressure CVD. Under atmospheric pressure or reduced pressure, a material gas (such as SiH4, SiH2Cl2, etc.) and oxygen (O2) are reacted to deposit an oxide film on a substrate. This is a method that can form a silicon oxide film at a relatively low temperature (600 - 900 °C). However, a large stress is generated at the interface, which may cause cracks or peeling of the silicon oxide film. The silicon oxide film formed by the thermal CVD method has poor film quality and requires an annealing process at a high temperature. The plasma CVD method includes capacitively coupled plasma CVD, inductively coupled plasma CVD, microwave ECR (Electron Cyclotron Resonance) plasma CVD, etc. By plasmaizing a material gas (such as SiH4, TEOS (Tetraethyl orthosilicate), etc.) and an oxygen gas (O2) and reacting them in a high-energy state (excited state), a method can deposit a film at a lower temperature (300 - 600 °C) and at a higher speed than other methods. Different from other methods, the film quality (hardness, refractive index, transmittance, etc.) can be changed according to the film formation conditions (pressure, temperature, power, gas flow rate). However, the film quality is not as good as that of the thermal oxidation method. Also, depending on the film formation conditions and gas species, impurities (organic components) may be included. Since it is formed on only one side, a certain amount of stress is generated even during low-temperature treatment. When the silicon oxide film becomes thick, the stress increases, causing the substrate to warp and making the silicon oxide film prone to cracking and peeling.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] Each of the above-described methods has its advantages and disadvantages, and it has been difficult to simultaneously satisfy the requirements of preventing warping of the silicon substrate, improving the quality of the film properties (hardness, refractive index, light transmittance), and shortening the film formation time. Also, Patent Document 1 and Patent Document 2 disclose techniques for forming a thick silicon oxide film on a silicon substrate, but they are insufficient to meet the currently required conditions.

[0008] The present invention has been made in view of the above problems, and an object thereof is to provide a method for manufacturing a silicon substrate having a high-quality silicon oxide film with a thickness of 2 μm or more and without warping in a shorter time.

Means for Solving the Problems

[0009] The present invention is a method for manufacturing a silicon substrate having a thick silicon oxide film, a step of patterning an etching mask for deep trenches having a regular pattern on the surface of the silicon substrate, a step of forming deep trenches in the silicon substrate using deep reactive ion etching (D-RIE), a step of oxidizing the surface of the silicon substrate on which the deep trenches are formed by a thermal oxidation method to oxidize all of the silicon of the ridges defined by the deep trenches and filling the deep trenches with silicon oxide that expands in volume, thereby forming a thick silicon oxide film. It is a method for manufacturing a silicon substrate having a silicon oxide film.

[0010] The thick silicon oxide film has a film thickness of at least 2 μm or more, and has the silicon oxide film according to claim 1.

[0011] After forming the thick silicon oxide film, it may further have a step of annealing.

[0012] After forming the thick silicon oxide film, it is also possible to further have a step of planarizing the surface of the thick silicon oxide film.

[0013] Preferably, in the first step, a silicon oxide film of a thin film formed by oxidizing the surface of the silicon substrate by a thermal oxidation method is patterned.

[0014] Preferably, the ridge is formed to be a hexagonal prism and arranged in a honeycomb shape.

[0015] In the second step, a configuration can be adopted in which etching is performed so that the side wall of the deep trench slopes in a flaring shape from the bottom surface of the deep trench toward the opening.

[0016] Preferably, the etching mask can be formed such that the ratio of the width of the deep trench to the width of the ridge is 0.5 to 1.25:1.00.

[0017] The present invention forms a first thick silicon oxide film in a first region of the silicon substrate using the first step to the third step, and using the first step to the third step, a second thick silicon oxide film having a film thickness different from that of the first thick silicon oxide film can be formed in a second region of the silicon substrate.

[0018] Using the first to third steps in first and second regions of the silicon substrate that are separated from each other, first and second thick silicon oxide films having the same film thickness can be formed.

Effect of the Invention

[0019] According to the present invention, by patterning deep trenches in a silicon substrate, the amount of silicon to be oxidized when forming a thick silicon oxide film by a thermal oxidation method can be significantly reduced. Therefore, compared with the case of simply forming a thick silicon oxide film on a silicon substrate by a thermal oxidation method, a silicon substrate having a thick silicon oxide film with excellent film quality (hardness, refractive index, light transmittance), a small stress that is less likely to cause warping or deformation, and a film thickness of 2 μm or more can be manufactured in an extremely short time. According to the present invention, it is possible to form a thick silicon oxide film partially rather than over the entire surface of a silicon substrate. Therefore, semiconductor devices can be formed in regions of the silicon substrate other than the region of the thick silicon oxide film. According to the present invention, by varying the depth of the deep trench, a silicon substrate having a plurality of silicon oxide films with different thicknesses can be manufactured.

Brief Description of the Drawings

[0020]

Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 1E

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Figure 8A

Figure 8B

Figure 9A

Figure 9B

Figure 9C

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Figure 9E

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Figure 10

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description, the same reference numerals are given to the same elements, and redundant descriptions will be omitted as appropriate. First Embodiment First, as shown in FIG. 1A, a thin film silicon oxide film 12 is formed by dry oxidation on the surface of the silicon substrate 10 as an etching mask for forming deep trenches. The silicon substrate 10 is obtained by single-crystallizing high-purity silicon while precisely controlling the crystal structure and slicing it into wafers. The silicon oxide film 12 is a thermal oxide film formed by thermally oxidizing the surface of the silicon substrate 10. The thermal oxidation furnace can be either vertical or horizontal. From the viewpoint of etching resistance, a thermal oxide film is preferred as the etching mask. However, as long as it is a material that can withstand deep etching, an insulating film such as a CVD oxide film or a nitride film, a photoresist, a resin, etc., or a combination thereof may be used. Depending on the material of the etching mask, some of the steps described below will change, but the details will be omitted.

[0022] Next, a photoresist is applied to the silicon oxide film 12 of the thin film. The photoresist can be either positive or negative. The photoresist is exposed and developed using a photomask for deep trenches. The details of the pattern of the photomask for deep trenches (hereinafter referred to as the trench pattern) will be described later, but it is designed to have a regular pattern. Next, as shown in FIG. 1B, using the developed photoresist 14 as a mask, the silicon oxide film 12 is etched to pattern the etching mask 12A. The etching method used at this time can be either wet etching or dry etching, but dry etching is preferred when dimensional accuracy is emphasized. After forming the etching mask 12A, the photoresist 14 is removed. The above is the first step of the present invention.

[0023] If the width between adjacent etching masks 12A shown in FIG. 1B is defined as the trench width W1 and the width of the etching mask 12A is defined as the ridge width W2, the etching mask 12A (photomask) is designed so that the deep trench 20T described later is just filled with the silicon oxide film. Theoretically, the etching mask 12A is designed so that the trench width W1:ridge width W2 = 0.5 to 1.25:1.00.

[0024] Next, as shown in FIG. 1C, a deep trench 20T with a trench depth dp is formed in the silicon substrate 10 by deep reactive ion etching (D-RIE) (the second step of the present invention). Due to the formed deep trench 20T, regularly arranged ridges 10R are formed on the silicon substrate 10. Depending on the trench width W1 and the trench depth dp, for D-RIE, considering the protection and smoothing of the sidewalls of the deep trench 20T and the adjustment of the trench angle, etc., it is preferable to use the high-speed switching Bosch process that alternately performs etching and sidewall protection. With this process, the scallop on the sidewall of the deep trench 20T is smooth and a large aspect ratio can be obtained.

[0025] In FIG. 1C, the sidewalls of the deep trench 20T are formed perpendicular to the bottom surface. However, depending on the conditions, as shown in FIG. 2, the angle θ of the sidewall with respect to the bottom surface can be adjusted. When the aspect ratio is large, in order to prevent a shortage of oxidizing gas at the bottom of the deep trench 20T, the angle θ may be made larger than 90 degrees. That is, the sidewalls of the deep trench 20T are etched so as to be tapered from the bottom portion of the deep trench 20T toward the opening. By adjusting the balance between the isotropic etching of the Bosch process and the formation of the sidewall protective film, the sidewalls can be inclined. By making the angle θ larger than 90 degrees and inclining the sidewalls, in the process of forming the thick silicon oxide film described later, the oxidizing gas can easily reach the bottom of the deep trench 20T.

[0026] Next, the etching mask 12A made of a silicon oxide film existing on the upper surface of the ridge 10R is removed. For the removal of the etching mask 12A, wet etching or dry etching may be used. Also, on the back side of the silicon substrate 10, there is a thin silicon oxide film, which can be easily removed if not necessary. Depending on the wet etching method, it can be removed simultaneously from the front and back.

[0027] Next, the surface of the silicon substrate 10 on which the deep trench 20T and the ridge 10R are formed is oxidized by a thermal oxidation method (the third step of the present invention). As the thermal oxidation method, pyrogenic oxidation (wet oxidation) is used. When the surface of the silicon substrate 10 is thermally oxidized, theoretically, when 1.0 of silicon is consumed, silicon oxide with a volume about 2.25 times that amount is formed. Therefore, as the thermal oxidation progresses and the silicon constituting the ridge 10R and the silicon at the bottom of the deep trench 20T are oxidized, as shown in FIG. 1D, the silicon oxide film 30 that expands in volume fills the deep trench 20T, and the volume of the deep trench 20T decreases, and the volume of the ridge 10R decreases. The thermal oxidation of the silicon substrate 10 is further advanced to completely oxidize the silicon of the ridge 10R and completely fill the deep trench 20T with the silicon oxide film 30. As a result, as shown in FIG. 1E, an integrated thick silicon oxide film 40 can be formed. It can be seen that the film thickness Th of the silicon oxide film 40 is larger than the trench depth dp shown in FIG. 1C.

[0028] When the interface of the silicon oxide film 30 grown from the side walls on both sides of the deep trench 20T becomes a problem, an annealing process is performed to make the interface more uniform. For example, as shown in FIG. 3, there may be a non-uniform interface NU or extremely small voids VD in the thick silicon oxide film 40. The annealing process is a continuous process from thermal oxidation in a thermal oxidation furnace. Alternatively, another diffusion furnace (oxidation furnace) or a rapid thermal annealing (RTA) apparatus may be used. The annealing temperature is the same as or higher than the thermal oxidation temperature, and gases such as N2, O2, H2O, or a mixed gas thereof are used. If there are slight irregularities on the surface 40f of the thick silicon oxide film 40, it is also possible to planarize them by the annealing process.

[0029] When irregularities due to the trench pattern are observed on the surface 40f of the thick silicon oxide film 40 as shown in FIG. 1E, the surface 40f of the thick silicon oxide film 40 is planarized to obtain a flat surface 40f1 as shown in FIG. 4. For planarization, CMP (Chemical Mechanical Polish) can be used, but planarization is also possible by an etch-back method, a remelting method, or the like.

[0030] By the manufacturing process as described above, a silicon substrate 10 having a thick silicon oxide film 40 with a film thickness of 2 μm or more, which is difficult to warp or deform with a small stress in an extremely short time and has excellent film quality (hardness, refractive index, light transmittance), can be manufactured.

Example

[0031] Example 1 is an example of the conditions when forming a thick silicon oxide film 40 with a film thickness of 20 μm on the silicon substrate 10. (1) The silicon oxide film 12 of the thin film formed on the silicon substrate 10 is formed to have a film thickness of 0.1 μm by dry oxidation (oxidation temperature: 1100 °C, gas used: O2, oxidation time: 110 minutes). (2) A negative photoresist is used as the photoresist. (3) Using a photomask for trenches designed with a pattern of trench width W1: ridge width W2 = 1.25 μm: 1.00 μm, expose and develop using an i-line reduction projection exposure machine (stepper). (4) Etch the silicon oxide film 12 with 3% BHF (buffered hydrofluoric acid) until the surface becomes hydrophobic to pattern the etching mask 12A. (5) Using the Bosch process (high-speed switching), etch the silicon substrate 10 by D-RIE to make the trench depth Th of the deep trench 20T 19 μm. The sidewall angle θ of the deep trench 20T is 90° (vertical). (6) In the process of forming the thick silicon oxide film 40, pyrogenic oxidation with an oxidation temperature: 1100 °C, gas used: H2O, and oxidation time: 300 minutes is used.

[0032] Under the above conditions, when the silicon of the ridge 10R is thermally oxidized, it becomes approximately 2.25 times the volume, so the deep trench 20T can be filled, and a thick silicon oxide film 40 with a film thickness of 20 μm can be fabricated.

Example

[0033] Example 2 is an example of the conditions for the annealing process to be performed when the interface between the thermal oxide films becomes a problem in the process of Example 1. The silicon substrate 10 obtained by the process of Example 1 is continuously processed in a common thermal oxidation furnace. The annealing temperature is annealed at the same temperature as the thermal oxidation temperature or a higher temperature. If the thermal oxidation temperature is 1100 °C, the annealing temperature is annealed at 1100 - 1150 °C.

Example

[0034] Example 3 is an example of the conditions when the annealing process of Example 2 is performed using a high-temperature short-time annealing apparatus. The annealing process is carried out with a heating rate of 100 °C / sec, an annealing temperature of 1150 °C, a gas used: O2, an annealing time of 30 minutes, and a cooling rate: rapid cooling by N2 injection.

[0035] Next, the trench pattern will be described. FIG. 5 shows a slit pattern in which the trench portion and the ridge portion are arranged to extend in a slit shape. When the thick-film silicon oxide film 40 is formed with the slit pattern, stress biased in the slit direction A1 in which the trench portion and the ridge portion extend is generated, and thereby the silicon substrate 10 may be warped.

[0036] FIG. 6 shows a concentric pattern in which the trench portion and the ridge portion are arranged concentrically. When the thick-film silicon oxide film 40 is formed with the concentric pattern, stress biased in the circumferential direction A2 is generated.

[0037] FIG. 7 shows a square pattern. In this case, the ridges 10R are arranged at equal intervals as regular square prisms. When the thick-film silicon oxide film 40 is formed with the square pattern, no problematic biased stress is generated. However, as shown in FIG. 7, voids Vd may be formed between the silicon oxide films formed in the trench portion.

[0038] FIG. 8A shows a honeycomb pattern in which the ridge portions are arranged in a honeycomb shape. When the trench portion and the ridge portion are formed with the honeycomb pattern, the ridges 10R are formed on the silicon substrate 10 as hexagonal prisms. As shown in FIG. 8B, regarding the arrangement relationship of the ridge portions, when the distance between the opposing hexagonal sides of the hexagonal ridge portion is d1 and the distance between adjacent ridge portions is d2, for example, when the distance d1 is 1.0, the distance d2 is set to 0.5. When the thick-film silicon oxide film 40 is formed using such a honeycomb pattern, no problematic biased stress occurs, no voids are generated in the trench portion, and a high-quality thick-film silicon oxide film 40 can be obtained.

[0039] Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIGS. 9A to 9F. In the present invention, it is possible to form the thick silicon oxide film 40 not over the entire surface of the silicon substrate 10 but partially. Further, by changing the trench depth dp of the deep trench 20T, it becomes possible to manufacture the silicon substrate 10 having a plurality of silicon oxide films 40 with different thicknesses. Hereinafter, an example of the manufacturing process of the silicon substrate 10 having silicon oxide films with three different film thicknesses will be described.

[0040] As shown in FIG. 9A, a first deep trench 20T1 is formed in a partial region of the silicon substrate 10 by the same process as the first and second steps of the first embodiment. Next, the surface of the silicon substrate 10 on which the first deep trench 20T1 and the first ridge 10R1 are formed is oxidized by a thermal oxidation method in the same manner as the third step of the first embodiment, and as shown in FIG. 9B, a first thick silicon oxide film 40A is formed. Next, as shown in FIG. 9C, after removing the silicon oxide film on the surface of the silicon substrate 10, a second deep trench 20T2 shallower than the first deep trench is formed in a partial region of the silicon substrate 10 by the same process as the first and second steps of the first embodiment. Note that the silicon oxide film on the surface of the silicon substrate 10 can also be used as an etching mask when forming the second deep trench 20T2. Next, as shown in FIG. 1D, the surface of the silicon substrate 10 on which the second deep trench 20T2 and the second ridge 10R2 are formed is oxidized by a thermal oxidation method in the same manner as the third step of the first embodiment, and a second thick silicon oxide film 40B is formed. Next, as shown in FIG. 1E, after removing the silicon oxide film on the surface of the silicon substrate 10, a third deep trench 20T3 shallower than the second deep trench is formed in a part of the region of the silicon substrate 10 by the same process as the first and second steps of the first embodiment. Note that the silicon oxide film on the surface of the silicon substrate 10 when forming the second thick silicon oxide film 40B can also be used as an etching mask when forming the third deep trench 20T3. Next, as shown in FIG. 1F, the surface of the silicon substrate 10 on which the third deep trench 20T3 and the second ridge 10R3 are formed is oxidized by the thermal oxidation method in the same manner as the third step of the first embodiment to form a third thick silicon oxide film 40C. Through the above process, a plurality of thick silicon oxide films 40A to 40C with different film thicknesses can be formed on the silicon substrate 10.

[0041] Third Embodiment Next, FIG. 10 shows an example of the structure of a semiconductor device fabricated using a silicon substrate on which a thick silicon oxide film is formed in the manufacturing process of this embodiment. In FIG. 10, silicon oxide films 40D are formed in regions separated from each other on the silicon substrate 10. The thick silicon oxide film 40D may be formed at once as in the manufacturing process of the first embodiment, or may be formed separately as in the manufacturing process of the second embodiment. For example, a high-voltage vertical MOS transistor is formed between the two separated silicon oxide films 40D. In this way, if the silicon substrate on which the thick silicon oxide film obtained according to this embodiment is formed is used for a semiconductor device, the elements can be separated from each other.

[0042] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific examples, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

Description of Reference Numerals

[0043] 10 Silicon substrate 10R Ridge 12 Etching mask 14 Photoresist 20R Trench 30 Silicon oxide 40 Thick silicon oxide film W1 Width of trench W2 Width of ridge Th Thickness of thick silicon oxide film dp Depth of deep trench

Claims

1. A method for manufacturing a silicon substrate having a thick silicon oxide film, comprising: a first step of patterning an etching mask for deep trenches having a regular pattern on the surface of the silicon substrate; a second step of forming deep trenches in the silicon substrate using deep reactive ion etching (D-RIE); a third step of oxidizing the surface of the silicon substrate in which the deep trenches are formed by a thermal oxidation method to oxidize all of the silicon of the ridges defined by the deep trenches and filling the deep trenches with silicon oxide that expands in volume to form a thick silicon oxide film. The method for manufacturing a silicon substrate having a silicon oxide film according to claim 1, wherein in the second step, the ridges are formed in a hexagonal prism shape and arranged in a honeycomb pattern, and the deep trenches are arranged between adjacent ridges.

2. The method for manufacturing a silicon substrate having a silicon oxide film according to claim 1, wherein when the distance between opposite hexagonal sides of the ridges formed as hexagonal prisms is d1 and the distance between adjacent ridges is d2, a relationship of d1 > d2 is set.

3. A method for manufacturing a silicon substrate having a thick silicon oxide film, comprising: a first step of patterning an etching mask for deep trenches having a regular pattern on the surface of the silicon substrate; a second step of forming deep trenches in the silicon substrate using deep reactive ion etching (D-RIE); a third step of oxidizing the surface of the silicon substrate in which the deep trenches are formed by a thermal oxidation method to oxidize all of the silicon of the ridges defined by the deep trenches and filling the deep trenches with silicon oxide that expands in volume to form a thick silicon oxide film. The thick silicon oxide film has a film thickness of at least 2 μm or more. The method for manufacturing a silicon substrate having a silicon oxide film further comprises a step of annealing to flatten the surface of the thick silicon oxide film after forming the thick silicon oxide film.

4. The method for manufacturing a silicon substrate having a silicon oxide film according to claim 1, wherein the thick silicon oxide film has a film thickness of at least 2 μm or more.

5. The method for manufacturing a silicon substrate having a silicon oxide film according to claim 1, further comprising a step of planarizing the surface of the thick silicon oxide film after forming the thick silicon oxide film.

6. The method for manufacturing a silicon substrate having a silicon oxide film according to claim 1, wherein the first step is to pattern a thin silicon oxide film formed by oxidizing the surface of the silicon substrate by a thermal oxidation method.

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