JP2003151960A - Trench etching method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new technique for etching a trench with a flat trench wall face on a silicon substrate. SOLUTION: In the etching method, a silicon substrate, in which its surface is covered with a mask having a trench-shaped opening, is exposed to a reactive gas (for example, NF3 , O2 , and HBr) in the trench etching step, the concentration of the gas (for example, O2 ) included in the reactive gas for forming a sidewall protective layer is varied in time in the trench etching step. In the trench etching step, a planarizing state of the trench wall face can be obtained and a trench having a flat wall face can be obtained during etching.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for forming a trench in which a surface of a wall forming the trench (hereinafter referred to as a trench wall surface) is flat by etching a silicon substrate.

[0002]

2. Description of the Related Art A technique for manufacturing a semiconductor device by etching a silicon substrate to form a deep trench has become widespread. Usually, the wall surface of the trench is covered with an insulating film, and the electrode is filled inside the insulating film. In this type of semiconductor device, in order to increase the breakdown voltage of the insulating film formed on the wall surface of the trench, it is necessary to avoid the formation of stress concentration parts and electric field concentration parts in the insulating film. Since stress concentration and electric field concentration are likely to occur at the transition part (corner part) of the trench bottom wall and side wall, a technology has been developed to relax the stress and electric field concentration by increasing the radius of curvature at the trench corner part. Has been done. In Japanese Patent Laid-Open No. 2001-44216, first, anisotropic etching is performed to form a trench, then the bottom wall of the trench is forward-taper etched, and finally isotropic etching is performed to make a transition portion between the bottom wall and the side wall of the trench. The technique for increasing the radius of curvature in (1) is described.

[0003]

However, the breakdown voltage of the insulating film formed on the wall surface of the trench cannot be sufficiently increased only by increasing the radius of curvature at the corner portion of the trench. This is because if the surface of the trench wall surface is rough, stress concentration and electric field concentration are likely to occur in the insulating film formed on the wall surface due to the unevenness of the trench wall surface. A fundamental object of the present invention is to provide a novel technique capable of etching a trench having a flat trench wall surface in a silicon substrate. A further object of the present invention is to provide an improved technique capable of etching a trench having a flat trench wall surface and having a large radius of curvature at a corner portion in a silicon substrate. Still another object of the present invention is to provide an improved technique capable of etching a trench having a flat trench wall surface and a large radius of curvature at a corner portion in one etching step.

[0004]

Means for Solving the Problems In order to form a basic problem of the present invention, that is, to form a trench having a flat trench wall surface by an etching technique, the present invention devises a reactive gas used for etching. The reactive gases used to etch the silicon substrate to form trenches include:
A gas mainly for separating silicon from the silicon substrate to promote etching, and a layer for protecting the sidewall from the reactive gas on the sidewall of the trench formed by etching are deposited to realize anisotropic etching. Gas and are used. In the present invention, the concentration of the gas for forming the sidewall protective layer is utilized by utilizing the phenomenon that the concentration of the gas for forming the sidewall protective layer is closely related to the progress of the trench etching and the roughness of the wall surface of the trench formed by etching. Trench etching is performed while changing the time. According to the invention,
The trench having a flat wall surface can be etched, and the withstand voltage of the insulating layer formed on the wall surface of the trench can be increased.

In one invention of the present application, when a silicon substrate whose surface is protected by a mask having an opening having a shape corresponding to a trench shape is exposed to a reactive gas containing NF ₃ and O ₂ to perform trench etching, _The trench etching is performed by changing the ratio of ₃ and O ₂ with time. According to this method, a state in which the etching progresses efficiently and a state in which the trench wall surface is flattened can be obtained during the trench etching, and the trench having a flat wall surface can be etched. In Japanese Patent Publication No. 2874233, a deep trench is anisotropically etched in a silicon substrate by alternately using a mixed gas of halogen and nitrogen and an oxygen gas for a high frequency bias application type electron cyclotron resonance plasma etching apparatus. The technology is described. In the present invention, only the ratio is changed without changing the type of mixed gas, whereas the technique described in Japanese Patent No. 2874233 is completely different in that the gas type is exchanged.

[0006] In the above method, and O ₂ is low concentration state NF ₃ is a high concentration, it is preferable that the NF ₃ is O ₂ at low concentrations to trench etching by switching a high concentration state. According to this method, a state where the trench etching is efficiently performed due to the high concentration of NF ₃ and a state where the trench wall surface is flattened due to the high concentration of O ₂ are obtained, and the trench having the flat wall surface is etched. can do. Moreover, according to this method, the trench having a large radius of curvature at the corner portion is etched in a state where the NF ₃ concentration is high. According to this method, it is possible to etch not only a flat wall surface but also a trench having a large radius of curvature at the corner portion.

According to the above-mentioned method, NF ₃ is contained at a low concentration in O ₂
It is preferable to finish the trench etching in a high concentration state. The wall surface of the trench formed in this case is extremely flat.

The reactive gas is HB in addition to NF ₃ and O _2.
It is more preferred to include r. In this case, NF ₃ and O
It is preferable to keep the total flow rate of ₂ constant to continue the trench etching. According to this method, a trench having a corner portion having a large radius of curvature and a flat wall surface is formed.

NF ₃ and O ₂ in the same etching chamber
If the ratio of is changed over time, the cost and time required for loading and unloading the silicon substrate can be reduced.

Generally speaking, the method of the present invention is a method in which a silicon substrate whose surface is protected by a mask having an opening having a shape corresponding to a trench shape is exposed to reactive gas ions and trench etching is performed. It can be said that trench etching is performed by varying the concentration of the type of gas that forms the included sidewall protection layer with time. According to this method, during the trench etching, a state in which the etching progresses efficiently and a state in which the trench wall surface is flattened are obtained, and the trench having a flat wall surface can be etched.

From another viewpoint of the present invention, in a method of trench etching by exposing a silicon substrate whose surface is protected by a mask having an opening having a shape corresponding to a trench shape to a reactive gas containing oxygen, to perform trench etching. Can be said to be a method of performing trench etching by changing the value with time. In this case, oxygen becomes a gas that forms the sidewall protective layer. The sidewall protective layer forming gas is not limited to oxygen and may be nitrogen or the like. The sidewall protective layer forming gas of claim 7 is not limited to oxygen.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION First, the main features of the embodiments described below will be summarized. (Embodiment 1) A silicon substrate is NF ₃ is O ₂ at a high concentration repeated several cycles a cycle in NF ₃ is low concentration O ₂ is exposed to a high concentration reactive gas after exposure to a low concentration reactive gases trench How to etch. (Mode 2) A trench is etched by repeating a plurality of cycles of exposing a silicon substrate to a reactive gas having a low concentration of a sidewall protective layer forming gas (eg, oxygen) and then exposing the silicon substrate to a highly reactive gas having a sidewall protective layer forming gas. how to. (Feature 3) The method for etching a trench according to each of the claims or the features, wherein a trench is formed by using a reactive gas in a reactive ion etching apparatus.

[0013]

EXAMPLE FIG. 1 shows a silicon substrate 2 whose surface is protected by a mask 4 having an opening 6 having a shape corresponding to a trench shape. The mask 4 is formed of a silicon oxide film (or a silicon nitride film). The silicon substrate 2 whose surface is protected by the mask 4 is called a wafer 8. Figure 2 shows wafer 8
1 shows a device for reactive ion etching. A pair of parallel plate electrodes (upper electrode 18 and lower electrode 20) are provided in the etching chamber 19. A high frequency voltage 22 is applied between the pair of parallel plate electrodes. Lower electrode 2
The wafer 8 is set on the wafer 0. A coil 24 is provided outside the etching chamber 19 and applies a magnetic field inside the etching chamber 19. The etching chamber 19 is evacuated (26) by a vacuum pump. On the other hand, the reactive gas is introduced. Reactive gas is H
It is a mixed gas of Br, NF ₃ , and O ₂ , and the flow rate of each gas can be independently controlled by the solenoid valves 10, 12, and 14. The solenoid valves 10, 12, 14 are controlled by the controller 16, and the opening degree of each solenoid valve 10, 12, 14 can be programmed with respect to the elapsed time from the etching start time. By applying a high frequency voltage to the reactive gas introduced into the etching chamber, the reactive gas is turned into plasma and the wafer 8 is exposed to the reactive gas ions and the neutral active species.

The following reactions proceed on the surface of the wafer 8.
It Reaction 1: Si + Br → SiBr_Four By this reaction, Si is etched from the Si substrate 2. Reaction 2: Si + F → SiF_Four By this reaction, Si is etched from the Si substrate 2. Reaction 3: SiBr_X  ＋ O → SiBr_YO_W The product of this reaction, SiBr_YO_WOn the trench wall
Pile up. Reaction 4: SiF_X  + O → SiF_YO_W The product of this reaction, SiF_YO_WOn the trench wall
Pile up. Reaction 5: SiBr_YO_W＋ F → SiBr_Four＋ SiF_Four The deposit is etched by this reaction.

In this embodiment, trench etching is performed under the conditions shown in FIG. Immediately after the start of trench etching, N
The ratio of the O ₂ flow rate to the total flow rate of F ₃ and O ₂ is 33%. This condition is called condition A, and NF ₃ has a high concentration. After that, the NF ₃ flow rate is reduced and the O ₂ flow rate is increased, and the ratio of the O ₂ flow rate to the total flow rate of NF ₃ and O ₂ is increased to 52%. This condition is called condition B. Under the condition B, O ₂ has a high concentration. While etching under condition A in step 1 and condition B in step 2,
The total flow rate of NF ₃ and O ₂ is kept constant. Also, NF
_{In addition to 3} and O ₂ gas, HBr gas is kept flowing at a constant flow rate.

FIG. 4 shows the cross-sectional shape of the trench 30 obtained by trench etching in which a nearly vertical forward taper shape is obtained under the condition of low oxygen concentration A, and the curvature at the transition between the bottom and side surfaces of the trench. The radius is large. On the other hand, the surface of the wall 32 having a substantially vertical forward taper forming the trench is rough. FIG. 5 shows a cross-sectional shape of the trench 34 obtained when the trench etching is performed under the condition that the oxygen concentration is high to obtain a substantially vertical forward taper shape. The surface of the wall of the almost vertical forward taper forming the trench is shown. Is flat. On the other hand, the radius of curvature at the transition portion 36 between the bottom surface and the side surface of the trench is small. As long as condition A and condition B are used independently, it is impossible to etch a trench having a large radius of curvature at the transition between the bottom surface and the side surface of the trench and having a flat trench wall surface.

FIG. 6 shows a cross-sectional shape of the trench 40 obtained by first etching under the condition A and then etching under the condition B according to FIG. The etching time under condition A and the etching time under condition B are equal. In this case, the trench 4 has a large radius of curvature at the transition portion 44 between the bottom surface and the side surface of the trench and has a flat trench wall surface 42.
0 is etched. The radius of curvature at the transition portion 44 between the bottom and the side of the trench is greatly etched during the etching under the low oxygen concentration A condition, and the wall surface of the trench is flat during the high oxygen concentration B condition etching. It is speculated that it will be etched.

Under the condition of low oxygen concentration, the concentration of F is high and the reaction for etching the deposit, namely: SiBr _Y O _W +
The reaction of F → SiBr ₄ + SiF ₄ occurs actively, and the deposition on the trench surface becomes slow. Normally, the forward tapered trench is not processed so that it is etched, but the bottom of the trench is etched so that it remains parallel to the surface of the silicon substrate. The radius of curvature at the part becomes large. When the insulating film is formed along the wall surface of the trench, it is possible to prevent the stress and the electric field from being concentrated on the insulating film at the transition portion (that is, the corner portion) between the bottom surface and the side surface of the trench. Experiments have confirmed that the larger the radius of curvature of the corner portion, the higher the withstand voltage of the insulating film can be maintained.

Under the condition of high oxygen concentration B, SiBr _X + O
→ SiBr _Y O reaction of _W takes place in lively, product SiBr _Y
O _W are deposited on the wall of the trench, to protect the trench wall from etching. When the product SiBr _Y O _W is deposited on the uneven Si surface forming the wall surface of the trench, it is preferentially deposited on the recess having a low equilibrium vapor pressure to fill the recess. The SiBr _Y O _W filling the recess is difficult to etch. Si on the protrusion
Since Br _{Y OW} is not deposited, Si is exposed, and the exposed Si is etched to remove the convex portion. Since the convex portions are removed and the concave portions are filled, the trench wall surface is flattened. When the insulating film is formed along the wall surface of the trench, if the wall surface of the trench is rough, stress or electric field may be concentrated at a specific portion of the insulating film, and current is likely to leak. According to this example, since the trench wall surface is flattened, stress concentration and electric field concentration are less likely to appear in the insulating film formed along the trench wall surface, and it has been confirmed that the dielectric strength of the insulating film can be maintained high. .

FIG. 7 shows the withstand voltage of the oxide insulating film formed on the wall surface of the trench, and shows the case where the trench etching is performed only under the condition A where the oxygen concentration is low and the case where the trench etching is performed only under the condition B where the oxygen concentration is high. In contrast, it has been confirmed that the breakdown voltage of the oxide insulating film formed on the wall surface of the trench etched by switching the etching condition from the A condition to the B condition with time is high. Even if the etching was first performed under the condition A and then under the condition B, or if the etching was first performed under the condition B and then under the condition A, the same performance was obtained.

FIG. 8 shows a case where the condition B of the condition A is repeatedly switched for two cycles for etching. When a plurality of cycles (two cycles in the case of FIG. 8) are repeatedly etched, a deep and narrow trench wall surface is smoothed. Therefore, a large radius of curvature at the corner can be secured.

FIG. 9 shows another etching condition. Under the A condition where the oxygen concentration is low, O occupies the total flow rate of NF ₃ and O _2.
The ratio of ₂ flow rates is 20%. On the other hand, the ratio of the O ₂ flow rate to the total flow rate of NF ₃ and O ₂ is set to 70% under the B condition where the oxygen concentration is high. Through experiments, it was confirmed that when the O ₂ flow rate in the total flow rate of NF ₃ and O ₂ was 20 to 35%, the radius of curvature at the corner portion of the trench could be increased. In addition, the ratio of the O ₂ flow rate to the total flow rate of NF ₃ and O ₂ is
It was confirmed that the flatness of the trench wall surface was remarkably improved at 50 to 70%. In the present invention, oxygen is used as the gas that is deposited on the trench wall surface to form the layer that protects the trench wall surface, but a product other than oxygen can be deposited on the trench wall surface to protect the wall surface. By continuing the etching while varying the concentration of the gas forming the trench wall surface protective layer between the high concentration state and the low concentration state, it is possible to form a trench having a large radius of curvature at the corner and a smooth wall surface.

[0023]

According to the present invention, the concentration of the gas forming the sidewall protective layer, eg, O ₂ gas, contained in the reactive gas, eg, the reactive gas containing NF ₃ and O ₂ , is varied with time. Since the trenches are etched by forming the trenches with flat walls, the trenches can be formed, and the breakdown voltage of the insulating layer formed on the trench wall faces can be increased.

[Brief description of drawings]

FIG. 1 shows a cross section of a silicon substrate whose surface is protected by a mask.

FIG. 2 shows an apparatus for etching a silicon substrate.

FIG. 3 shows etching conditions for a silicon substrate.

FIG. 4 shows a trench etched under a low oxygen concentration condition.

FIG. 5 shows a trench etched under a high oxygen concentration condition.

FIG. 6 shows a trench etched by switching between a low oxygen concentration condition and a high oxygen concentration condition.

FIG. 7 shows a withstand voltage of an insulating film formed on a wall surface of a trench. A indicates etching when the oxygen concentration is low, B indicates etching when the oxygen concentration is high, and A + B indicates etching when the oxygen concentration is changed to the high concentration condition.

FIG. 8 shows another example of etching conditions for a silicon substrate.

FIG. 9 shows still another example of etching conditions for a silicon substrate.

[Explanation of symbols]

2: Silicon substrate 4: Mask 6: Open 8: Wafer

Claims (8)

[Claims] 1. A silicon substrate whose surface is protected by a mask having an opening having a shape corresponding to a trench shape is formed of NF ₃ and O.
A method for trench etching by exposure to a reactive gas containing _2, a method for trench etching over time varying the ratio of NF ₃ and O _2. 2. The trench etching method according to claim 1, wherein the trench etching is performed by switching between a state where NF ₃ is high concentration and O ₂ is low concentration and a state where NF ₃ is low concentration and O ₂ is high concentration. 3. The trench etching method according to claim 1, wherein the trench etching is finished in a state where the concentration of NF ₃ is low and the concentration of O ₂ is high. 4. The trench etching method according to claim 1, wherein the reactive gas further contains HBr. 5. The trench etching method according to claim 1, wherein the trench etching is performed while maintaining the total flow rate of NF ₃ and O ₂ constant. 6. NF ₃ and O in the same etching chamber
The trench etching method according to claim 1, wherein the ratio of ₂ is changed with time. 7. A method of trench etching by exposing a silicon substrate, the surface of which is protected by a mask having an opening having a shape corresponding to a trench shape, to reactive gas ions,
A method of performing trench etching by varying the concentration of a type of gas that forms a sidewall protective layer contained in a reactive gas with time. 8. A method of performing trench etching by exposing a silicon substrate, the surface of which is protected by a mask having an opening having a shape corresponding to a trench shape, to a reactive gas containing oxygen, wherein the oxygen concentration is varied with time to form a trench. How to etch.