JP7218226B2 - Plasma etching method - Google Patents

Description

The present invention relates to a plasma etching method.

An etching method including a first step and a second step is used to form a recess in a silicon substrate. _In the first step, recesses are formed in the silicon substrate by using a mixed gas of HBr gas and CHF3 gas and supplying bias power to the stage of the silicon substrate. _In the _second step, a mixed gas of SF6 gas and CHF3 gas is used, and bias power is not supplied to the silicon substrate stage. In the second step, the vicinity of the bottom of the recess formed in the first step is isotropically etched to form a concave curved surface at the bottom of the recess (see, for example, Patent Document 1).

JP 2004-259927 A

By the way, the CHF ₃ gas used in the second step forms a protective film containing carbon on the inner surface of the recess. At this time, in the above-described second step, the etchant, which is fluorine-containing ions, is not actively drawn into the recesses, and the etching proceeds by allowing the etchants to enter the recesses on an air current or the like. As a result, the flow rate of the etchant entering the recesses varies from silicon substrate to silicon substrate. remains on the inner surface of the recess as a residue. Conversely, when the flow rate of the etchant entering the recesses was high, the isotropic etching in the recesses proceeded excessively and had undercuts that spread outward from the openings in the mask. A concave portion is formed.

As described above, in the etching method including the first step and the second step described above, the optimum range of etching conditions for forming recesses having a desired shape is very narrow. It is difficult to reproducibly form such concave portions.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a plasma etching method capable of improving the reproducibility of the shape of recesses.

A plasma etching method for solving the above problems is a method of etching a silicon substrate having a mask formed thereon using a fluorine-containing gas and an oxygen-containing gas, which are gases capable of forming a protective film on a silicon substrate, comprising: forming a recess opening in the surface of the silicon substrate by supplying high-frequency power to a bias electrode at a first duty ratio; and forming a concave curved surface at the bottom of the recess that tapers toward the back surface of the silicon substrate by supplying the bias electrode with a ratio of the bias electrode.

According to the above configuration, the fluorine-containing gas and the oxygen-containing gas form a recess having a concave curved surface at the bottom while forming a protective film on the inner surface of the recess. At this time, since the duty ratio for forming the concave surface is smaller than the duty ratio for forming the recess, it is possible to form the concave surface while actively drawing fluorine-containing ions into the recess. can. In other words, since the active attraction of fluorine-containing ions can be performed both in the formation of the concave portion and in the formation of the concave curved surface, it is possible to improve the reproducibility of the shape of the concave portion.

In the above plasma etching method, the first duty ratio may be greater than 50% and 100% or less, and the second duty ratio may be 50% or less.

According to the above configuration, since the first duty ratio is included in the range of more than 50% and 100% or less, side etching in the silicon substrate is suppressed. In addition, since the second duty ratio is 50% or less, the amount of ions drawn into the silicon substrate can be suppressed to an amount that can form a concave curved surface.

In the above plasma etching method, forming the concave surface on the bottom of the recess may include supplying the high-frequency power at a frequency of 1 Hz or more and 100 Hz or less.

According to the above configuration, when forming the concave curved surface, by repeating the supply and stop of the high-frequency power, the period during which the etching of the silicon substrate can mainly be performed and the period during which the formation of the protective film can be mainly performed. , can be sufficiently secured separately. Thereby, it is possible to form a concave curved surface at the bottom of the concave portion.

In the above plasma etching method, forming the concave curved surface on the bottom of the concave portion causes the high-frequency power to be lower than the second duty ratio after the high-frequency power is supplied to the bias electrode at the second duty ratio. The method may include supplying the bias electrode with a third duty ratio.

According to the above configuration, since the concave curved surface is formed using the third duty ratio that is even smaller than the second duty ratio, the concave curved surface has a more rounded shape than when only the second duty ratio is used. It can be curved.

In the above plasma etching method, in the recess including the recessed curved surface formed by forming the recessed portion in the silicon substrate and forming the recessed curved surface on the bottom of the recessed portion, the opening of the recessed portion is A ratio of the depth of the recess to the diameter may be 1 or more and 100 or less. According to the above configuration, it is possible to reproducibly form recesses having a high aspect ratio, such as a depth of 1 or more and 100 or less with respect to the diameter of the opening.

In the above plasma etching method, the frequency of the high-frequency power may be 100 kHz or more and 40 MHz or less. According to the above configuration, since the frequency of the high-frequency power is 100 kHz or more and 40 MHz or less, it becomes easy to draw the etchant, which is ions containing fluorine, into the recess.

FIG. 1 is an apparatus configuration diagram schematically showing an example of the configuration of a plasma etching apparatus; 4 is a timing chart for explaining the high frequency power supplied to the bias electrode in the first step; 4A to 4C are cross-sectional views schematically showing an example of recesses formed in the silicon substrate by the first step; FIG. (a) A timing chart for explaining a first example of the high-frequency power supplied to the bias electrode in the second step, (b) A first example of the concave curved surface formed on the silicon substrate in the second step. 3 is a cross-sectional view of FIG. (a) A timing chart for explaining a second example of the high-frequency power supplied to the bias electrode in the second step, (b) a schematic representation of a second example of the concave curved surface formed on the silicon substrate in the second step. 3 is a cross-sectional view of FIG. (a) A timing chart for explaining a third example of the high-frequency power supplied to the bias electrode in the second step, (b) A schematic representation of a third example of the concave curved surface formed on the silicon substrate in the second step. 3 is a cross-sectional view of FIG. (a) SEM image of the cross-sectional structure of the recess formed in Test Example 1; (b) SEM image of the cross-sectional structure of the recess formed in Test Example 2;

One embodiment of the plasma etching method is described with reference to FIGS. The configuration of a plasma etching apparatus for performing the dry etching method, the plasma etching method, and test examples will be described in order below.

[Configuration of plasma etching apparatus]
The configuration of the plasma etching apparatus will be described with reference to FIG.
As shown in FIG. 1, the plasma etching apparatus 10 includes a chamber body 11 having a cylindrical shape with a bottom. An upper opening of the chamber body 11 is sealed with a quartz plate 12 . A space defined by the chamber main body 11 and the quartz plate 12 is a chamber space 11S. A stage 13 for holding a silicon substrate S to be etched is arranged in the chamber space 11S.

A bias electrode 13P incorporated in the stage 13 is connected to a bias power source 15 via a bias matching device 14. As shown in FIG. The bias power supply 15 outputs high frequency power of 100 kHz or more and 40 MHz or less. In this embodiment, the bias power supply 15 outputs high frequency power of 400 kHz, for example. The bias matching device 14 matches the output impedance of the bias power supply 15 and the input impedance of the load to which high frequency power is input.

An ICP antenna 21 is arranged on the opposite side of the quartz plate 12 to the chamber space 11S. The ICP antenna 21 is composed of, for example, a two-stage spiral coil that is wound two and a half times in the circumferential direction of the silicon substrate S. As shown in FIG. The ICP antenna 21 has an input terminal 21I, which is the central end of the spiral shape, and an output terminal 21O, which is the outer end of the spiral shape.

An input terminal 21I of the ICP antenna 21 is connected to an antenna power supply 23 via an antenna matching box 22 . The antenna power supply 23 outputs high frequency power of 13.56 MHz, for example. An output terminal 21O of the ICP antenna 21 is connected to the ground potential through a blocking capacitor 24. FIG. The antenna matching box 22 matches the output impedance of the antenna power supply 23 and the input impedance of a load to which low-frequency power is input. The blocking capacitor 24 has the function of increasing the amplitude of the potential at the output terminal 21O compared to when the output terminal 21O of the ICP antenna 21 is connected to the ground potential. The blocking capacitor 24 adjusts the current flowing through the ICP antenna 21 so as to obtain a desired plasma density in the chamber space 11S. The range of the capacitance value of the blocking capacitor 24 is, for example, 10 pF or more and 100 pF or less.

A magnetic field coil 25 that forms a magnetic neutral line in the chamber space 11S is arranged on the outer circumference of the quartz plate 12 . The magnetic field coil 25 is composed of an upper coil 25A, a middle coil 25B, and a lower coil 25C. Each of the three coils of the magnetic field coil 25 is separately connected to a current source 26 that supplies a current for forming a magnetic neutral line.

An upper current source 26A is connected to the upper coil 25A, a middle current source 26B is connected to the middle coil 25B, and a lower current source 26C is connected to the lower coil 25C. The upper stage current source 26A and the lower stage current source 26C output currents in the same direction to the upper stage coil 25A and the lower stage coil 25C, which are the supply destinations of the currents. Middle-stage current source 26B outputs a current to middle-stage coil 25B in a direction opposite to that of the current sources other than middle-stage current source 26B. In each of the current sources 26A, 26B, and 26C, the direction of flow of each current and the magnitude of each current are set so that a magnetic neutral line is formed in the chamber space 11S.

An exhaust port 11P1 formed in the chamber main body 11 is connected to an exhaust portion 31 for exhausting the fluid in the chamber space 11S. The exhaust unit 31 includes, for example, a pressure regulating valve that regulates the pressure of the chamber space 11S and various pumps.

A gas supply port 11P2 formed in the chamber main body 11 is connected to a gas supply section 32 for supplying an etching gas to the chamber space 11S. The gas supply unit 32 includes a mass flow controller that supplies the fluorine-containing gas and a mass flow controller that supplies the oxygen-containing gas. In this embodiment, the fluorine-containing gas is sulfur hexafluoride gas (SF ₆ ) gas, and the oxygen-containing gas is oxygen (O ₂ ) gas.

The plasma etching apparatus 10 controls the driving of each of the bias matching box 14, the bias power supply 15, the antenna matching box 22, the antenna power supply 23, the current source 26, the exhaust section 31, and the gas supply section 32. A device 33 is provided.

The control device 33 stores a target value of high frequency power to be supplied to the bias electrode 13P and a target value of high frequency power to be supplied to the ICP antenna 21 as one of process conditions. The control device 33 controls the output operations of the power supplies 15 and 23 so that the output power of the bias power supply 15 and the antenna power supply 23 reach their target values. .

The controller 33 stores the target value of the current supplied to the magnetic field coil 25 and the target value of the etching gas flow rate as one of the process conditions. The controller 33 controls the output operation of the current source 26 so that the output current of the current source 26 reaches its target value and the flow rate of the etching gas supplied by the gas supply unit 32 reaches its target value. It controls the output operation of the gas supply unit 32 . Furthermore, the control device 33 controls the output operations of the matching devices 14 and 22 so that the reflected power toward the antenna power supply 23 and the bias power supply 15 become smaller. do.

For example, the control device 33 controls the output operation of each of the bias power source 15, the antenna power source 23, the current source 26, and the gas supply section 32 based on the process conditions stored in the control device 33. , a plasma P using a fluorine-containing gas and a plasma P using an oxygen-containing gas are supplied to the silicon substrate S at the same time. Plasma P using a fluorine-containing gas is densified by the magnetic neutral lines formed by the magnetic field coil 25, and promotes the formation of recesses extending in the thickness direction of the silicon substrate S. FIG. In addition, the plasma P using the oxygen-containing gas is densified by the magnetic neutral line formed by the magnetic field coil 25, and a protective film is formed to protect the side wall of the recess formed in the silicon substrate S from being etched. It is formed on the sidewall of the recess.

[Plasma etching method]
A plasma etching method will be described with reference to FIGS.
The plasma etching method is a method of etching the silicon substrate S on which the mask is formed using a fluorine-containing gas and an oxygen-containing gas. A protective film can be formed on the silicon substrate S by using a mixed gas of a fluorine-containing gas and an oxygen-containing gas. The plasma etching method includes forming a recess in the silicon substrate S and forming a concave curved surface at the bottom of the recess. In forming the concave portion in the silicon substrate S, the concave portion opened in the surface of the silicon substrate S is formed in the silicon substrate S by supplying high-frequency power to the bias electrode 13P at the first duty ratio. By forming the concave curved surface on the bottom of the concave portion, by supplying high-frequency power to the bias electrode 13P at a second duty ratio smaller than the first duty ratio, the concave curved surface tapering toward the back surface of the silicon substrate is formed on the bottom of the concave portion. Form from the bottom. The plasma etching method will be described in more detail below with reference to the drawings.

FIG. 2 schematically shows the high-frequency power supplied to the bias electrode 13P in forming the concave portion in the silicon substrate S. As shown in FIG. Forming the concave portion in the silicon substrate S is hereinafter also referred to as the first step.

As shown in FIG. 2, in the first step, high frequency power is supplied to the bias electrode 13P with a first duty ratio. The duty ratio is the ratio of the period during which the high-frequency power is supplied to the period during which the high-frequency power is repeatedly supplied. In this embodiment, the first duty ratio is 100%. Note that the first duty ratio is not limited to 100%, and the first duty ratio may be any other value within the range of greater than 50% and less than or equal to 100%. Since the first duty ratio is included in the range of more than 50% and 100% or less, the period during which ions are not drawn into the silicon substrate S can be reduced to less than half the period of the first step. As a result, isotropic etching of the silicon substrate S is suppressed from causing side etching in the silicon substrate S. FIG.

The frequency of the high frequency power supplied to the bias electrode 13P is 100 kHz or more and 40 MHz or less. If the frequency of the high-frequency power is 100 kHz or more and 40 MHz or less, it becomes easy to draw the etchant, which is fluorine-containing ions, into the concave portions by the bias voltage.

FIG. 3 schematically shows the state of the silicon substrate S in the first step.
As shown in FIG. 3A, a mask M is formed on the surface of the silicon substrate S etched in the first step. The mask M has an opening Ma corresponding to the shape and position of the recess to be formed in the silicon substrate S. As shown in FIG. The mask M has a plurality of openings Ma.

As shown in FIG. 3B, recesses SR are formed in the silicon substrate S by etching using _SF6 gas and _O2 gas. An etchant contained in plasma generated from _SF6 gas is supplied to the portion of the silicon substrate S exposed from the opening Ma. Etchants are, for example, fluorine-containing ions and fluorine-containing radicals. Thereby, the silicon substrate S is etched. At this time, oxygen-containing ions and oxygen-containing radicals contained in the plasma generated from the oxygen gas are also supplied to the silicon substrate S. As a result, the protective film SP is formed on the portion of the silicon substrate S exposed from the opening Ma, which is more difficult to etch by the plasma generated from the _SF6 gas than silicon. The protective film SP contains silicon and may contain at least one of oxygen and sulfur.

Since high-frequency power is supplied to the bias electrode 13P, fluorine-containing ions in the etchant contained in the plasma are drawn into the silicon substrate S along the direction perpendicular to the surface of the silicon substrate S. Fluorine-containing radicals are also supplied to the silicon substrate S along the direction perpendicular to the surface of the silicon substrate S. As a result, the protective film SP formed on the bottom SRb of the recess SR is removed, and etching along the thickness direction of the silicon substrate S proceeds. On the other hand, although fluorine radicals are supplied to the side surfaces of the recess SR, almost no fluorine ions are supplied. Therefore, the protective film SP formed on the side surface of the recess SR remains on the side surface of the recess SR.

As shown in FIG. 3C, by continuing the etching of the silicon substrate S, a recess SR having a shape extending along the thickness direction of the silicon substrate S and having a side surface on which the protective film SP is located. is formed.

4 to 6 each show an example of the second step. 4(a), 5(a), and 6(a) are timing charts of the high-frequency power supplied to the bias electrode 13P in the second step. On the other hand, FIGS. 4(b), 5(b), and 6(b) schematically show the state of the silicon substrate S at the end of the second step. 4(b), 5(b), and 6(b) show cross-sectional structures along a plane orthogonal to the surface of the silicon substrate S. As shown in FIG.

As shown in FIG. 4A, in the second step, the period during which the bias electrode 13P is supplied with the high frequency power is the first period T1, and the period during which the bias electrode 13P is not supplied with the high frequency power is the second period. It is period T2. A first period T1 and a second period T2 following the first period T1 form one cycle C. As shown in FIG. The percentage of the first period T1 to the period C (T1/C×100) is the second duty ratio. The second duty ratio is 50% or less. Note that in the example shown in FIG. 4, the second duty ratio is 50%.

In the second step, the first period T1 and the second period T2 are alternately repeated according to the period C and the second duty ratio. In the first period T1, high-frequency power is supplied to the bias electrode 13P, so that the formation of the protective film SP on the side surface and the etching on the bottom of the recess SR proceed in the same manner as in the first step described above.

On the other hand, since high-frequency power is not supplied to the bias electrode 13P during the second period T2, ions are less likely to be supplied to the silicon substrate S than during the first period T1. Thereby, in the silicon substrate S, the reaction between the silicon substrate S and the radicals supplied to the silicon substrate S mainly proceeds. Here, the fluorine-containing radicals functioning as an etchant for the silicon substrate S and the oxygen-containing radicals for forming the protective film SP are supplied to the entire surface defining the recesses SR.

However, on the surface that partitions the recess SR, a distribution is formed in the probability of each radical being supplied due to the shape of the recess SR. Specifically, in the depth direction of the recess SR, the deeper the portion of the surface defining the recess SR, the more difficult it is for each radical to enter. In addition, when viewed from the direction facing the surface of the silicon substrate S, the radicals are less likely to enter the edge of the bottom SRb of the recess SR and the vicinity of the edge. Therefore, among the surfaces that partition the recess SR, each radical is least likely to enter the corner formed by the bottom SRb and the side surface. Accordingly, once the protective film SP is formed on the corner, there is a high probability that the protective film SP remains in the recess SR without being etched. As a result, in the protective film SP, the thickness of the portions located at the corners tends to be thicker than the thickness of the portions located at other sites.

Then, in the first step after the second step, the fluorine-containing ions are drawn into the silicon substrate S again along the direction perpendicular to the surface of the silicon substrate S. At this time, since the portion of the protective film SP located at the corner is thicker, the etching in the vicinity of the corner in the silicon substrate S progresses more slowly than the etching in the central portion of the bottom SRb.

As shown in FIG. 4B, by repeating the first step and the second step in accordance with the period C, in the second step, silicon is removed from the bottom SRb of the recess SR formed in the first step. It is possible to form a concave curved surface SC tapering toward the back surface of the substrate S. The center of curvature of concave curved surface SC is located within the region defined by concave portion SR.

In the second step, high-frequency power can be supplied at a frequency of 1 Hz or more and 100 Hz or less. That is, in the second step, the length of the period C can be set to any length within the range of 10 milliseconds or more and 1 second or less. In this way, when forming the concave curved surface SC, by repeating supply and stop of the high-frequency power, a period mainly for etching the silicon substrate S and a period mainly for forming the protective film SP are repeated. be able to. That is, the period for etching the silicon substrate S and the period for growing the protective film SP can be secured by the first period T1 and the second period T2.

FIG. 5(a) is a timing chart when the second duty ratio is 30%, and FIG. 6(a) is a timing chart when the second duty ratio is 10%. FIG. 5B shows the shape of the recess SR formed in the silicon substrate S when the second duty ratio is 30%, and FIG. 6B shows the shape when the second duty ratio is 10%. The shape of the recess SR formed in the silicon substrate S in this case is shown.

4(b), 5(b), and 6(b), the smaller the value of the second duty ratio, the more the silicon substrate S is formed. , the radius of the circle containing the concave surface SC tends to increase. Further, when the number of periods C is the same, the smaller the value of the second duty ratio, the larger the proportion of the concave curved surface SC in the surface formed by etching the bottom SRb formed in the first step. have

The smaller the value of the second duty ratio, the easier it is for the protective film SP to be formed at the corner formed by the side surface and the bottom SRb. Therefore, the smaller the value of the second duty ratio, the thicker the portion of the protective film SP located at the corner. As a result, in the first step after the second step, the area of the bottom SRb etched along the depth direction of the recess SR becomes smaller. As a result, the smaller the value of the second duty ratio, the larger the radius of the circle including the concave surface SC, and the larger the proportion of the concave surface SC in the surface formed by etching the bottom SRb. have a tendency That is, the smaller the value of the second duty ratio, the more rounded the concave curved surface SC becomes.

As shown in FIGS. 4(b), 5(b), and 6(b), in the recess SR formed through the first step and the second step, the depth relative to the diameter φ of the opening of the recess SR is The ratio of the height D (D/φ) is the aspect ratio of the recess SR. It is preferable that the recessed portion SR formed through the first step and the second step has an aspect ratio of 1 or more and 100 or less. As the aspect ratio increases, the amount of etchant that reaches the bottom of the recess SR decreases, making it difficult to control the shape of the recess SR. Therefore, in the concave portion SR having a large aspect ratio of 10 or more, the technique of controlling the shape of the concave curved surface SC by supplying high-frequency power at the second duty ratio is a more useful technique.

In this embodiment, the frequency of the high frequency power supplied to the bias electrode 13P in the second step is equal to the frequency of the high frequency power supplied to the bias electrode 13P in the first step. Also, the peak-to-peak value of the high-frequency power supplied to the bias electrode 13P in the second step is equal to the peak-to-peak value of the high-frequency power supplied to the bias electrode 13P in the first step. In this way, when proceeding from the first step to the second step, by not changing the frequency and magnitude of the high-frequency power, it is possible to suppress unstable plasma generation in the chamber space 11S. Furthermore, when proceeding from the first step to the second step, by not changing the magnitude of the high-frequency power, compared to the case where the magnitude of the high-frequency power is reduced, the depth direction of the recess SR in the second step It is possible to prevent the etching along the edge from becoming difficult to proceed.

The high-frequency power supplied to the bias electrode 13P in the second step may be different from the high-frequency power supplied to the bias electrode 13P in the first step in at least one of frequency and magnitude.

[Test example]
A test example will be described with reference to FIG.
[Test Example 1]
A silicon substrate having a mask formed thereon was prepared. A plurality of recesses were formed on the silicon substrate under the following conditions. Thus, a silicon substrate of Test Example 1 was obtained.

・Fluorine-containing gas SF ₆ gas ・Flow rate of fluorine-containing gas 300 sccm
・Oxygen-containing gas O ₂ gas ・Flow rate of oxygen-containing gas 150 sccm
・Pressure in chamber space 10 Pa
・Frequency of high frequency power 400kHz
・High frequency power 20W
・Period of the first process: 2 minutes ・First duty ratio: 100%
・Period of second process 1 minute ・Second duty ratio 10%
・Frequency of 2nd process 10Hz

[Test Example 2]
A concave portion was formed in the silicon substrate under the same conditions as in Test Example 1, except that the period of the first step was set to 3 minutes and the concave portion was formed without performing the second step. Thus, a silicon substrate of Test Example 2 was obtained.

[Evaluation results]
The silicon substrate of Test Example 1 and the silicon substrate of Test Example 2 were cut along a plane perpendicular to the surface of each silicon substrate. Thereby, cross sections for observation were formed for each of the silicon substrate of Test Example 1 and the silicon substrate of Test Example 2. FIG. Using a scanning electron microscope, a cross section for observation in each silicon substrate was photographed. A SEM image shown in FIG. 7A was obtained by photographing a cross section for observation in the silicon substrate of Test Example 1. FIG. Further, by photographing a section for observation in the silicon substrate of Test Example 2, an SEM image shown in FIG. 7B was obtained.

As shown in FIG. 7A, in the silicon substrate S of Test Example 1, it was found that the bottom of the recess SR was formed by a concave curved surface tapering toward the back surface of the silicon substrate S. As shown in FIG. It was confirmed that, as in Test Example 1, it was possible to form the recess SR having a concave curved surface by forming the recess SR through the first step and the second step.

On the other hand, as shown in FIG. 7B, in the silicon substrate S of Test Example 2, the bottom of the recess SR was found to be formed by a plane substantially parallel to the surface of the silicon substrate S. was taken. As in Test Example 2, when the recessed portion SR was formed through only the first step, it was confirmed that the bottom of the recessed portion SR was formed by a flat surface.

As described above, according to one embodiment of the dry etching method, the following effects can be obtained.
(1) Since the second duty ratio when forming the concave curved surface SC is smaller than the first duty ratio when forming the recess SR, the concave curved surface SC is formed while actively drawing the etchant into the recess SR. can be formed.

(2) Since the first duty ratio is in the range of more than 50% and less than or equal to 100%, side etching in the silicon substrate S is suppressed. Further, since the second duty ratio is 50% or less, the amount of etchant drawn into the silicon substrate S can be suppressed to an amount that allows the formation of the concave curved surface SC.

(3) When the high-frequency power is supplied at a cycle of 10 milliseconds or more and 1 second or less, the period during which etching of the silicon substrate S can be mainly performed and the period during which the protective film SP can be formed are respectively set. can be sufficiently secured separately.

(4) In the concave portion SR having a large aspect ratio of 10 or more, the technique of controlling the shape of the concave curved surface SC by supplying high-frequency power at the second duty ratio is a more useful technique.

(5) When the frequency of the high-frequency power is 100 kHz or more and 40 MHz or less, it becomes easy to draw the etchant into the recesses SR by the bias voltage.

In addition, the embodiment described above can be implemented with the following changes.
[Plasma etching method]
- Forming the concave curved surface SC at the bottom of the recess SR is to supply high-frequency power to the bias electrode at a second duty ratio and then supply high-frequency power to the bias electrode at a third duty ratio smaller than the second duty ratio. may include

According to this modification, since the concave curved surface SC is formed using the third duty ratio that is even smaller than the second duty ratio, the shape of the concave curved surface SC is made more rounded than when only the second duty ratio is used. can be a curved surface.

[Fluorine-containing gas]
- The fluorine _- containing gas is not limited to _SF6 gas, and may be, for example, CF4 gas, NF3 gas _, or the like.

[Oxygen-containing gas]
- The oxygen-containing gas is not limited to _O2 gas, and may be, for example, carbon dioxide gas, nitrogen dioxide gas, or the like.

DESCRIPTION OF SYMBOLS 10... Plasma etching apparatus, 11... Chamber main body, 11P1... Exhaust port, 11P2... Gas supply port, 11S... Chamber space, 12... Quartz plate, 13... Stage, 13P... Bias electrode, 14... Bias matching device, 15... Bias power supply 21...ICP antenna 21I...Input terminal 21O...Output terminal 22...Antenna matching box 23...Antenna power supply 24...Blocking capacitor 25...Magnetic field coil 25A...Upper stage coil 25B...Middle stage Coil 25C Lower coil 26 Current source 26A Upper current source 26B Middle current source 26C Lower current source 31 Exhaust unit 32 Gas supply unit 33 Control device C Cycle D... depth, M... mask, Ma... opening, P... plasma, S... silicon substrate, SC... concave curved surface, SP... protective film, SR... concave part, SRb... bottom, T1... first period, T2... second 2 periods.

Claims (6)

A method of etching a silicon substrate on which a mask is formed using a fluorine-containing gas and an oxygen-containing gas, which are gases capable of forming a protective film on a silicon substrate, comprising:
forming, in the silicon substrate, a recess opening in the surface of the silicon substrate by supplying high-frequency power to a bias electrode at a first duty ratio;
By supplying the high-frequency power to the bias electrode at a second duty ratio smaller than the first duty ratio , the formation of the protective film on the side surface of the recess and the etching on the bottom of the recess are advanced. forming a concave curved surface tapering toward the back surface of the silicon substrate at the bottom of the recess located in the silicon substrate so as to have a diameter smaller than that of the opening by plasma etching method. . the first duty ratio is greater than 50% and equal to or less than 100%;
The plasma etching method according to claim 1, wherein the second duty ratio is 50% or less. 3. The plasma etching method according to claim 1, wherein forming the concave surface on the bottom of the recess includes supplying the high-frequency power at a frequency of 1 Hz or more and 100 Hz or less. Forming the concave curved surface on the bottom of the recess means supplying the high-frequency power to the bias electrode at the second duty ratio, and then supplying the high-frequency power to the bias electrode at the third duty ratio smaller than the second duty ratio. 4. The plasma etching method according to any one of claims 1 to 3, comprising supplying a bias electrode. In a concave portion including the concave curved surface formed by forming the concave portion in the silicon substrate and forming the concave curved surface in the bottom of the concave portion, the depth of the concave portion with respect to the diameter of the opening of the concave portion. The plasma etching method according to any one of claims 1 to 4, wherein the height ratio is 1 or more and 100 or less. The plasma etching method according to any one of claims 1 to 5, wherein the frequency of the high-frequency power is 100 kHz or more and 40 MHz or less.

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