JP2006278836A - Etching method, etching apparatus, computer program, and computer memory medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for suppressing the damage of a silicon nitride film when etching the silicon nitride film whose underlayer is a silicon oxide film by using a hard mask having silicon oxide as its main component. <P>SOLUTION: In the method for suppressing the damage of a silicon nitride film; etching is so performed as to include a process for forming a mask pattern in a hard mask by using a resist film as a mask, a process for ashing the resist film, a process for oxidizing the hard mask, a process for performing the over-etching of a silicon nitride film in a high selecting ratio of the silicon nitride film to a silicon oxide film, and a process for performing the main etching of the silicon nitride film before its over-etching and after the process for forming a mask pattern in a hard mask in a smaller selecting ratio than its selecting ratio used when performing its over-etching. In the case of using such a method, the oxide film formed in the hard mask so plays the role of a protective layer when performing the over-etching of the silicon nitride film as to suppress the damage of the silicon nitride film by preventing the generation of its pitting. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an etching method, and more particularly, to an etching method and an etching apparatus for etching a silicon nitride film with a silicon oxide film as a base using a hard mask containing silicon oxide as a main component as a mask, and to execute the method. The present invention relates to a computer program and a computer storage medium.

  Various studies have been made on film types and structures in order to promote high integration and thinning of semiconductor devices. For example, a new structure has been proposed for a gate structure of a CMOS. In this background, in a conventional CMOS gate structure, a gate insulating film is formed on a silicon film, and thinning of the gate insulating film has been promoted. However, as this thinning proceeds, leakage current increases. Therefore, there is a situation that the conventional gate structure has reached its limit.

  As a new CMOS gate structure, there is a so-called three-dimensional gate structure, for example. According to this structure, it is necessary to form a fine and three-dimensional gate electrode. In terms of materials, metal gate electrodes are being studied in place of conventional polycrystalline silicon electrodes, and the next-generation semiconductor element fabrication process requires a rather complicated process.

As one of these processes, a process of etching a silicon nitride (SiN) film whose base is a silicon oxide (SiO ₂ ) film has been studied, and the pattern of the silicon nitride film formed by the etching is a gate after It will be used for electrode formation.
By the way, in general, when etching a certain type of film, the etching rate of the substrate surface is not completely uniform, and it is difficult to make the etching rate uniform between the central part and the peripheral part. Etching is continued after the underlying film is exposed, and this etching is called over-etching. In the case of normal over-etching, it is necessary to ensure a high selection ratio with respect to the base film, but this selection ratio is about 7 to 10 at most with respect to the base film.

However, in the semiconductor element structure studied here, the silicon oxide film as the base film is extremely thin, for example, 5 nm. In such a case, a high selection ratio of, for example, about 20 to 40 is required to perform overetching. Is done. Patent Document 1 includes CH ₃ F gas and O ₂ gas as etching gases in order to increase the selectivity of the silicon nitride film to the silicon oxide film when the silicon nitride film is etched using the silicon oxide film as a mask. A technique is disclosed in which etching is performed using a mixed gas in which the mixing ratio of O ₂ gas to CH ₃ F gas (O ₂ / CH ₃ F) is 4 to 9. Therefore, in the process of etching a silicon nitride film whose base is a silicon oxide film, if this mixed gas is used during over-etching, the silicon nitride film can be formed while suppressing the reduction of the base film even if the base silicon oxide film is thin. It can be etched.

However, when the mixed gas is used, the following problems occur. FIG. 10A is a view showing a stacked body before etching the above-described silicon nitride film, in which 11 is a silicon (Si) film, 12 is a silicon oxide film, 13 is a silicon nitride film, and 14 is A nitrogen-containing silicon oxide (SiON) film as a hard mask, 15 is a resist film, and 16 is a resist pattern formed on the resist film 15. If general etching is performed on this laminate, the SiON film 14 is etched using the resist film 15 as a mask, and the resist film 15 is removed by ashing, and then the mixed gas described in Patent Document 1 is used. Etching is performed using the SiON film 14 as a mask.
However, as described above, etching with a gas having an extremely high selectivity with respect to the silicon oxide film 12 means that the etching action on the silicon nitride film 13 is large, and this silicon nitride. Since the material of the film 13 and the SiON film 14 constituting the hard mask are similar, the mixed gas locally damages the SiON film 14. The SiON film 14 is as thin as 50 nm, for example, and even if the in-plane uniformity corresponding to the specifications is ensured, the SiON film 14 is not completely uniform, so that holes 18 are drilled in the SiON film 14. . This point has been confirmed by experiments. If the phenomenon in which the hole 18 is perforated is called a punching phenomenon (pitting), the surface of the silicon nitride film 13 is damaged through the hole 18 when the pitting occurs, which affects the subsequent process.

Therefore, it is conceivable to protect the SiON film 14 by leaving the resist film 15 without ashing when the silicon nitride film 13 is etched. However, since the mixed gas contains a large amount of oxygen, the silicon nitride film Since the ashing of the resist film 15 also proceeds during etching, the thickness of the resist film 15 must be increased if the SiON film 14 is to be protected. That is, in order to improve the etching shape, there is a demand for reducing the thickness of the resist film 15. Under such circumstances, an etching method that does not damage the silicon nitride film is required in performing the above-described etching.
In Patent Document 2, as a method of forming a hard mask of silicon nitride, an antireflection film on the silicon nitride layer is oxidized by oxygen plasma at the time of ashing the resist mask, and this is used as a protective layer. Thus, although a technique capable of thinning the silicon nitride layer is described, it is not a technique that can solve the problems of the present invention.
JP 2003-229418 A JP 2000-269220 A

  The present invention has been made to solve such a problem, and its purpose is to etch a silicon nitride film based on a silicon oxide film using a hard mask mainly composed of silicon oxide as a mask. It is to provide a technique that does not cause damage to the silicon nitride film.

According to the etching method of the present invention, a silicon nitride film having a silicon oxide film as a base is covered with a hard mask mainly composed of silicon oxide, and a resist film having a pattern is formed on the hard mask. In the etching method for etching a silicon nitride film for a stacked body, a step of etching a hard mask using a resist film as a mask, forming a mask pattern on the hard mask, a step of ashing the resist film, and a surface portion of the hard mask With a high selection ratio of the silicon nitride film to the silicon oxide film, the oxidation step for oxidizing under conditions different from the ashing and the etching of the underlying silicon oxide film exposed at the bottom of the silicon nitride film pattern are sufficiently suppressed, Overetching process for overetching silicon nitride film and Main etching is performed after the step of forming a mask pattern on the hard mask and before the over-etching step, and etching the silicon nitride film with a selectivity smaller than the selectivity when over-etching the silicon nitride film. And a process. In the present invention, oxidizing the surface portion of the hard mask means oxidizing a portion in the thickness direction without oxidizing the back surface of the hard mask, and only oxidizing the surface in general. It does not stay.

  The main etching step is performed, for example, after the step of ashing the resist film and before the oxidation step. The main etching step is performed after the step of forming the mask pattern on the hard mask and before the step of ashing the resist film. The oxidation step may be performed by plasma obtained by converting oxygen gas into plasma. Further, the over-etching step may be performed by plasma obtained by converting a mixed gas containing, for example, a gas containing carbon, fluorine and hydrogen, and an oxygen gas into plasma.

  In this laminated body, the thickness of the hard mask is, for example, 50 nm or less, the thickness of the silicon oxide film is, for example, 5 nm or less, and the thickness of the silicon nitride film, for example, is 50 nm or more.

  An etching apparatus according to the present invention includes an airtight processing container provided with a mounting table on which a substrate is mounted, means for supplying a processing gas into the processing container, means for adjusting the pressure in the processing container, And a control unit for controlling each means so as to carry out the above-described etching method in an apparatus for etching a substrate with plasma obtained by converting the processing gas into plasma. It is characterized by.

The computer program of the present invention is a computer program that operates on a computer and is used in an apparatus that introduces a processing gas into a processing container and performs etching on a substrate, and performs the above-described etching method. The steps are organized as follows. Further, the computer storage medium of the present invention stores the above-described computer program.

In the present invention, in etching a silicon nitride film having a silicon oxide film as a base and covered with a hard mask containing silicon oxide as a main component, the ashing is performed on the surface portion of the hard mask in advance after ashing the resist film. Oxidation is performed under different conditions, and the silicon nitride film is over-etched with a high selectivity with respect to the underlying silicon oxide film. Therefore, since the oxide film formed on the surface of the hard mask serves as a protective layer for the hard mask during overetching, damage to the hard mask is suppressed and the occurrence of pitting is prevented. As a result, good etching can be realized without damaging the surface of the silicon nitride film.
If the silicon oxide film as the base film is very thin, for example, 5 nm or less, and the silicon nitride film to be etched is sufficiently thicker than the base film, the silicon oxide as the base during overetching The selection ratio of silicon nitride to the film needs to be quite high, and the etching action on the hard mask increases accordingly, so that it can be said that this method is particularly effective when the hard mask is thin.

  Hereinafter, an etching method according to the present invention will be described. First, an example of an etching apparatus used for carrying out the method will be described with reference to FIG. The etching apparatus 2 shown in FIG. 1 has a processing surface 21 whose surface is anodized, for example, a sealed space inside, a mounting table 3 disposed in the center of the bottom surface in the processing chamber 21, and a mounting table 3. And an upper electrode 4 provided to face the mounting table 3.

  The processing chamber 21 is electrically grounded, and an exhaust device 23 is connected to an exhaust port 22 on the bottom surface of the processing chamber 21 via a pipe 24. The exhaust device 23 includes a pressure adjusting unit (not shown). When the pressure adjusting unit receives a control signal from the control unit 2A described later, the exhaust device 23 moves inside the processing chamber 21 in accordance with the signal. The processing chamber 21 is maintained at a desired degree of vacuum by evacuation. In FIG. 1, reference numeral 25 denotes a transfer port for an object to be processed formed on the side wall of the processing chamber 21, and the transfer port 25 is configured to be opened and closed by a gate valve 26.

  The mounting table 3 includes a lower electrode 31 and a support 32 that supports the lower electrode 31 from below. The mounting table 3 is disposed on the bottom surface of the processing chamber 21 via an insulating member 33. An electrostatic chuck 34 is provided on the mounting table 3, and the wafer W is mounted on the mounting table 3 via the electrostatic chuck 34. The electrostatic chuck 34 is made of an insulating material, and an electrode plate 36 connected to a high-voltage DC power source 35 is provided inside the electrostatic chuck 34. When a voltage is applied to the electrode plate 36 from the high-voltage DC power source 35, static electricity is generated on the surface of the electrostatic chuck 34. As a result, the electrostatic chuck 34 is configured to electrostatically attract the wafer W placed thereon. ing. The electrostatic chuck 34 is provided with a through hole 34 a for releasing a backside gas described later to the upper portion of the electrostatic chuck 34.

A refrigerant flow path 37 through which a predetermined refrigerant (for example, a conventionally known fluorine-based fluid, water, etc.) passes is formed in the mounting table 3, and the mounting table 3 is cooled by the refrigerant flowing through the refrigerant flow path 37. The object to be processed placed on the mounting table 3 is cooled to a desired temperature via the mounting table 3. In addition, a temperature sensor (not shown) is attached to the lower electrode 31, and the temperature of the object to be processed on the lower electrode 31 is constantly monitored via the temperature sensor.
Further, a gas flow path 38 for supplying a heat conductive gas such as He (helium) gas as a backside gas is formed inside the mounting table 3, and the gas flow path 38 is provided at a plurality of locations on the upper surface of the mounting table 3. It is open at. These openings communicate with the through hole 34a provided in the electrostatic chuck 34. When a backside gas is supplied to the gas flow path 38, the backside gas passes through the through hole 34a. To the top of The backside gas is evenly diffused throughout the gap between the electrostatic chuck 34 and the object to be processed placed on the electrostatic chuck 34, so that the thermal conductivity in the gap is increased.

  The lower electrode 31 is grounded via a high pass filter (HPF) 3a, and a high frequency power source 31a of 13.56 MHz, for example, is connected to the lower electrode 31 via a matching unit 31b. A focus ring 39 is disposed on the outer peripheral edge of the lower electrode 31 so as to surround the electrostatic chuck 34, so that the plasma is focused on the object to be processed on the mounting table 3 through the focus ring 39 when plasma is generated. It is configured.

The upper electrode 4 is formed in a hollow shape, and a plurality of holes 41 for dispersing and supplying the processing gas into the processing chamber 21 are formed on the lower surface thereof, for example, so as to be evenly distributed to constitute a gas shower head. Yes. A gas introduction pipe 42 is formed at the center of the upper surface of the upper electrode 4, and the gas introduction pipe 42 passes through the center of the upper surface of the processing chamber 21 through the insulating member 27. The gas introduction pipe 42 is branched into five when going upstream to constitute branch pipes 42A to 42E, and the processing gas supplied into the processing chamber 21 is stored at the ends of the branch pipes 42A to 42E. CF ₄ (carbon tetrafluoride) gas supply source 45A, CH ₃ F (methyl fluoride) gas supply source 45B, O ₂ (oxygen) gas supply source 45C, Ar (argon) gas supply source 45D and CHF ₃ (three fluorine Methane gas) gas supply source 45E. In the branch pipes 42A to 42E, valves 43A to 43E and flow rate control units 44A to 44E are sequentially provided upstream. The valves 43A to 43E and the flow rate control units 44A to 44E constitute a gas supply system 46. The gas supply system 46 supplies each processing gas from each gas supply source 45A to 45E by a control signal from the control unit 2A described later. Control disconnection and flow rate.

  The upper electrode 4 is grounded through a low pass filter (LPF) 47, and a high frequency power source 4a having a frequency higher than that of the lower electrode 31, for example, 60 MHz, is connected to the upper electrode 4 through a matching unit 4b. ing. Although not shown, the high frequency power sources 4a and 31a are connected to the control unit 2A, and the power supplied from each high frequency power source to each electrode is controlled in accordance with the control signal.

  Such an etching apparatus 2 evacuates the inside of the processing chamber 21 by the exhaust device 23, and in the state where a predetermined processing gas is supplied from the processing gas supply sources 45A to 45E into the processing chamber 21 at a predetermined flow rate. When high frequency power is applied to the electrode 31 and the upper electrode 4, the processing gas is turned into plasma (activated) in the processing chamber 21 by the high frequency power applied to the upper electrode 4, and the high frequency power applied to the lower electrode 31. As a result, a bias potential is generated in the wafer W, and a predetermined etching process or oxidation is performed on the object to be processed mounted on the mounting table 3 so as to enhance the perpendicularity of the etching shape by drawing ion species to the object to be processed. It is configured to be processed.

  The etching apparatus 2 is provided with a control unit 2A composed of, for example, a computer. The control unit 2A includes a data processing unit including a program, a memory, and a CPU. The control unit 2A sends a control signal to each unit of the etching apparatus 2 in the program, and proceeds with each step described later to be processed. Instructions are set up so that a pattern can be formed on the body. In addition, for example, the memory has an area in which processing parameter values such as processing pressure, processing time, gas flow rate, and power value are written, and these processing parameters are read when the CPU executes each instruction of the program. A control signal corresponding to the parameter value is sent to each part of the etching apparatus 2. This program (including a program related to a process parameter input screen) is stored in a computer storage medium such as a flexible disk, a compact disk, or an MO (magneto-optical disk) and installed in the control unit 2A.

Next, an embodiment of the etching method of the present invention using the etching apparatus 2 will be described with reference to FIGS. First, the gate valve 26 is opened, and a wafer W as a substrate is loaded into the processing chamber 21 by a transfer mechanism (not shown). After the wafer W is placed horizontally on the placement table 3, the transfer mechanism moves away from the processing chamber 21 and the gate valve 26 is closed. Subsequently, the backside gas is supplied from the gas flow path 38 and the thermal conductivity between the wafer W and the electrostatic chuck 34 is increased, whereby the wafer W is cooled to a predetermined temperature. Thereafter, the following steps are performed. Here, the wafer W will be described first. In addition, about the material of a film | membrane, in order to make correspondence with drawing easy, it describes with a chemical symbol. The wafer W is a laminated body as shown in FIG. 2A. To explain, the SiO ₂ film 52, the SiN film 53, and the SiON film 54 are laminated on the Si layer 51, and an organic substance is formed on the SiON film 54. A resist film 55 is formed which is an organic film containing as a main component. A resist pattern 56 is formed on the resist film 55.

As will be described later, the SiON film 54 not only functions as a hard mask when the SiN film 53 is etched, but also functions as an antireflection film when the resist film 55 is exposed in the resist pattern 56 forming step. By the way, the thickness of the SiON film 54 is, for example, 50 nm or less, the thickness of the SiN film 53 is, for example, 50 nm or more, and the thickness of the SiO ₂ film 52 is, for example, 5 nm or less.

[First Embodiment of Etching Method of the Present Invention]
(Step 1: Etching of SiON film 54)
Taking place the exhaust of the exhaust pipe 24 within the processing chamber 21 through the exhaust system 23, the processing chamber 21 is maintained at a predetermined pressure, CHF ₃ gas underwent flow rate control into the process chamber 21, CF ₄ gas And a mixed gas composed of Ar gas is supplied. Subsequently, high frequency power is applied to the upper electrode 4 and the lower electrode 31, respectively, and each processing gas is turned into plasma. In this way, as shown in FIG. 2B, the SiON film 54 is etched along the resist pattern 56 using the resist film 55 as a mask, and a mask pattern 57 is formed.

(Step 2: Ashing of resist film 55)
The high frequency power supplies 4a and 31a are turned off to stop the generation of plasma, and the supply of CHF ₃ gas, CF ₄ gas and Ar gas into the processing chamber 21 is stopped. The gas remaining in the processing chamber 21 is exhausted by the exhaust device 23, and thereafter, O ₂ gas is supplied into the processing chamber 21. When the gas in the processing chamber 21 is replaced with the O ₂ gas from the mixed gas used in step 1, the O ₂ gas is turned into plasma by applying predetermined high-frequency power to the upper electrode 4 and the lower electrode 31, respectively. By this plasma formation, the resist film 55 remaining on the SiON film 54 is ashed and removed (FIG. 2C). The flow rate of O ₂ gas in this ashing process is, for example, 300 sccm, the supply power of the upper electrode 4 is, for example, 300 W, and the supply power of the lower electrode 31 is, for example, 100 W.

(Step 3: Formation of precursor pattern 58)
The high frequency power sources 4a and 31a are turned off to stop the generation of plasma and the supply of O 2 gas into the processing chamber 21. O ₂ gas remaining in the processing chamber 21 is exhausted, and then CF ₄ gas, CHF ₃ gas, and Ar gas whose flow rates are controlled are supplied into the processing chamber 21. When the gas in the processing chamber 21 is replaced with a mixed gas composed of these processing gases, a predetermined high-frequency voltage is applied to the upper electrode 4 and the lower electrode 31, respectively, and the mixed gas is turned into plasma, whereby FIG. As shown in FIG. 6, the main etching of the SiN film 53 is performed using the SiON film 54 as a mask, and a precursor pattern 58 is formed in the SiN film 53. For etching to reduce damage to the securing and the SiON film 54 of vertical shape in this process, the selection ratio of the SiN film 53 for the SiO ₂ film 52 (etching rate of / SiO ₂ film 52 of SiN film 53) is, for example, It is performed under the conditions of 1 to 3.

In Step 3, CF ₄ gas, which is a CF-based gas, is used as a part of the processing gas, so that a polymer component is attached to the sidewall of the precursor pattern 58 as a protective film due to the active species of the CF ₄ gas. Etching progresses. In this way, the sidewall of the precursor pattern 58 is formed with high verticality, and the shape of the pattern 59 to be finally formed is controlled to form a concave portion with high verticality.

The main etching process in step 3 prevents the SiO ₂ film 52 from being etched, so that the bottom of the precursor pattern 58 remains on the SiN film 53 as shown in FIG. The process is performed so as to stop at a stage just before the SiO ₂ film 52 that is the base film of the SiN film 53 is exposed.
Over-etching performed in step 5 described later is performed under a condition in which the selection ratio of the SiN film 53 to the SiO ₂ film 52 is considerably higher than that in step 3. It cannot be expected and the damage to the SiON film 54 increases as the over-etching time performed in step 5 becomes longer. In consideration of this point, it is preferable that the precursor pattern 58 is formed as deep as possible in the SiN film 53. Considering in-plane variation of the etching rate, for example, the thickness of the SiN film 53 is 85 on average. It is preferable to be formed to a depth of about%.

(Step 4: oxidation of SiON film 54)
The high frequency power supplies 4a and 31a are turned off to stop the generation of plasma, and the supply of CF ₄ gas, CHF ₃ gas and Ar gas into the processing chamber 21 is stopped. The gas remaining in the processing chamber 21 is removed by the exhaust device 23, and then O ₂ gas is supplied into the processing chamber 21. When the gas in the processing chamber 21 is replaced with O ₂ gas, a predetermined electric power is applied to the upper electrode 4 and the lower electrode 31 respectively to make the O ₂ gas into plasma and oxidize the SiON film 54. In the oxidation process of Step 4, the flow rate of O ₂ gas is, for example, 1200 sccm, the supply power of the upper electrode 4 is, for example, 300 W to 1500 W, and the supply power of the lower electrode 31 is, for example, 50 W to 200 W. Is performed under conditions different from those in the ashing process in Step 3.
FIG. 3B shows the wafer W after the completion of the oxidation, and the surface of the SiON film 54 is oxidized by plasma processing using O ₂ gas. An oxide layer 54a is obtained by oxidizing the SiON film 54. This step 4 is performed in order to improve the selectivity of the SiN film 53 with respect to the SiON film 54 during the overetching in the step 5, that is, to enhance the resistance of the SiON film 54 during the overetching and to protect the SiON film 54. However, it is only necessary to oxidize, for example, one to several tens of atomic layers on the surface of the SiON film 54. If this oxidation treatment is excessively performed, the surface of the precursor pattern 58 is oxidized, and in step 5, the etching is normally performed. There is a concern that it may be difficult to perform, and there is a possibility that the formation of the pattern 59 may be hindered.

(Step 5: Formation of pattern 59)
The high frequency power supplies 4a and 31a are turned off, and the generation of plasma is stopped. Further, the gas remaining in the processing chamber 21 is exhausted, and O ₂ gas, CH ₃ F gas, and Ar gas whose flow rates are controlled are supplied into the processing chamber 21. When the gas in the processing chamber 21 is replaced with a mixed gas composed of these processing gases, predetermined electric power is applied to the upper electrode 4 and the lower electrode 31 to turn the mixed gas into plasma. In this way, the SiN film 53 remaining in the main etching process in Step 3 is etched, and the exposed SiO ₂ film 52 is etched (over-etched) at a timing at which the SiN film 53 can be surely removed in the entire surface. Thus, a pattern 59 is formed (FIG. 3C).

Thus to suppress the etching of the SiO ₂ film 52 in step 5, the reaction is carried out under conditions high selectivity of the SiN film 53 for the SiO ₂ film 52 than the step 3. Specifically, for example, the processing chamber 21 is supplied so that the ratio of CH ₃ F gas to O ₂ gas is 4 to 9, for example, and the etching is performed under the condition that the selection ratio is 20 or more, for example.

As described above, in the etching method according to the first embodiment, the SiON film 54 is used as a hard mask to perform main etching until the underlying SiO ₂ film 52 is exposed to the SiN film 53 to form a precursor pattern 58. Then, after the surface portion of the SiON film 54 is oxidized, over etching is performed to etch the remaining part of the SiN film 53 so that the SiO ₂ film 52 is exposed. Since the underlying SiO ₂ film 52 is extremely thin, the over-etching process is performed in a state in which the selection ratio of the SiN film 53 to the SiO ₂ film 52 is considerably high. For this reason, a hard mask similar to the component of the SiN film 53 is used. Although a large etching action also acts on a certain SiON film 54, this SiON film 54 is oxidized and a protective film is formed, and most of the SiN film 53 has already been removed by the main etching and is overetched. Since the processing time is short, damage to the SiON film 54 during over-etching is suppressed, and generation of pudding is prevented. As a result, damage to the surface of the SiN film 53 can be suppressed. Therefore, a good pattern 59 can be formed even for the above-described laminate in which the resist film 55 is thinly formed.

  In the present invention, a silicon oxide-based film such as a SiON film is used as a hard mask for etching a SiN film. However, the present invention is not limited to SiON, and other materials such as SiOC or SiCOH may be used as a hard mask. Can do.

[Second Embodiment of the Etching Method of the Present Invention]
Hereinafter, another embodiment of the present invention will be described with reference to FIGS. First, a wafer W having a stacked structure similar to that of the previous embodiment as shown in FIG. 4A is carried into the processing chamber 21, and a mixed gas composed of CHF ₃ , CF ₄ and Ar is put into the processing chamber 21. 4B, using the resist film 55 as a mask, etching of the SiON film 54 and main etching of the SiN film 53 are performed to form a mask pattern 57 and a precursor pattern 58, as shown in FIG. The main etching is stopped just before the SiO ₂ film 52 is exposed in the same manner as in Step 3 of the embodiment described above, and therefore the bottom of the precursor pattern 58 remains in the SiN film 53.

Subsequently, the remaining resist film 55 is removed by ashing as in step 2 of the first embodiment (FIG. 4C), and the surface of the SiON film 54 is oxidized and oxidized layer as in step 4 above. After forming 54a (FIG. 5A), the pattern 59 is formed by over-etching the remaining SiN film 53 in the precursor pattern 58 until reaching the SiO ₂ film 52 in the same manner as in Step 5 (FIG. 5 (FIG. 5A). b)).

Also in this embodiment, since the surface portion of the SiON film 54 that is a hard mask is oxidized before over-etching, damage to the SiON film 54 during over-etching is suppressed, and pitting in the SiON film 54 can be prevented. Therefore, the same effect as the previous embodiment can be obtained.
As still another embodiment, step 4 and step 3 in the first embodiment may be interchanged. That is, as shown in FIG. 2C, after removing the resist film 55 on the SiON film 54 by ashing, the surface portion of the SiON film 54 may be oxidized as described above, and then main etching may be performed. .

Example 1
In Example 1, a pattern 59 is formed on the wafer W according to the steps described in the first embodiment using the etching apparatus 2 described above on the wafer W having the laminated structure described in the above-described embodiment. went. The conditions in each step of Example 1 were set as follows.

(Step 1: Etching process of SiON film 54)
Pressure of mixed gas: 20 to 50 mTorr (2.67 to 6.67 Pa)
High-frequency power supply (U / L): 300 to 600 W / 0 to 400 W (where U is the upper power supply and L is the lower electrode)
Mixed gas flow ratio: CHF ₃ / CF ₄ / Ar = 0-200 / 200-400 / 600 sccm
(Step 2: Ashing process of resist film 55)
O ₂ gas pressure: 200 mTorr (26.7 Pa)
High frequency power supply (U / L): 300W / 100W
O ₂ gas flow rate: 300 sccm
(Step 3: Forming process of precursor pattern 58)
Pressure of mixed gas: 20 to 50 mTorr (2.67 to 6.67 Pa)
High frequency power supply (U / L): 300-600W / 0-400W
Flow rate ratio of mixed gas: CHF ₃ / CF ₄ / Ar = 0 to 200/200 to 400/600 sccm
(Step 4: Oxidation process of SiON film 54)
O ₂ gas pressure: 200 mTorr (26.7 Pa)
High frequency power supply (U / L): 300-1500W / 50-200W
O ₂ gas flow rate: 1200 sccm
(Step 5: Pattern 59 forming step)
Pressure of mixed gas: 120 mTorr (16.0 Pa)
High frequency power (U / L): 500 W / 100 to 300 W
CH ₃ F / O ₂ / Ar = 3/13/90 sccm

(Example 2)
In Example 2, the pattern 59 was formed on the wafer W having the same configuration as the wafer W used in Example 1 according to the procedure described in the second embodiment. In the second embodiment, the step of etching the SiON film 54 and the SiN film 53 using the resist film 55 first as a mask was performed under the following conditions.
Pressure of mixed gas: 20 to 50 mTorr (2.67 to 6.67 Pa)
High frequency power supply (U / L): 300-600W / 0-400W
CHF ₃ / CF ₄ / Ar = 0-200 / 200-400 / 600 sccm
The process of ashing the resist film 55, the process of oxidizing the SiON film 54, and the process of over-etching the SiN film 53, which are performed following this process, are performed under the same reaction conditions as in the first embodiment, that is, the steps in the first embodiment. The treatment was sequentially performed under the reaction conditions described in Step 2, Step 4, and Step 5, respectively.

(Comparative example)
In the comparative example, the etching apparatus 2 was used to etch a wafer W having the same configuration as the wafer used in each example as shown in FIG. That is, the SiON film 54 is etched in the same manner as shown in FIG. 2 according to the first embodiment, and then the resist film 55 is ashed (FIG. 6A). Thereafter, the SiON film 54 is oxidized in the same manner as in the step shown in FIG. 3B (FIG. 6B), and then the SiN film 53 is etched to a high selectivity under the condition of overetching. The SiN film 53 is removed to form a pattern 59 (FIG. 6C). The reaction conditions for each step of this comparative example were the same as the conditions for each step described in Example 1.

7 and 8 schematically show patterns 59 formed by the respective examples and comparative examples. This schematic diagram is drawn based on the result of observing the surface of the processed wafer W with a scanning electron microscope. FIG. 7A corresponds to the first embodiment, and the side wall of the formed pattern 59 is formed substantially vertically. FIG. 7B corresponds to Example 2, and a slight amount of resist residue 61 scattered on the bottom of the formed pattern 59 was observed, but this residue 61 was not a particularly problematic amount. In the second embodiment, as in the first embodiment, the side wall of the pattern 59 is formed substantially vertically. FIG. 6C corresponds to the comparative example, and the pattern 59 has a tapered shape, and a large residue 62 like a sword mountain is generated at the bottom of the pattern 59. The taper shape was formed because the pattern 59 was formed under the condition that a high selection ratio was obtained with respect to the SiO ₂ film 52 without forming the precursor pattern 58. It is thought that this is because the side wall protecting action of the concave portion due to adhesion was weak. The residue 62 is the surface portion of the SiN film 53 was exposed with the SiON film 54 when also to oxidize the SiON film 54 was appeared oxidation, the etching with high selectivity conditions to the SiO ₂ film 52, the oxide It can be considered that the etched portion became a thin mask and the etching of the SiN film 53 was hindered to become a residue. Therefore, each example and comparative example showed that by applying the etching method of the present invention, a pattern 59 having a good shape in which the side wall is substantially vertical and the generation of residue is suppressed can be obtained.

  Further, in FIG. 8, a side view of the patterns in the respective examples and comparative examples is shown in the upper stage, and a top view of these patterns is shown in the lower stage. In Example 1 and Example 2, no pitting was observed in the SiON film 54, but in the comparative example, pitting occurred, and the bottom portion penetrated through the SiON film 54 and reached the SiN film 53 as shown in the figure. 71 was perforated. In the comparative example, since the precursor pattern 58 is not formed, the etching time performed on the SiN film 53 under the high selection condition is longer than that in each example, and the damage received on the SiON film 54 is increased. Is considered to be the cause. Therefore, it was shown from the respective examples and comparative examples that the etching method of the present invention is effective for suppressing damage to the SiON film 54 and preventing the occurrence of pitting.

  Subsequently, in the step of oxidizing the SiON film 54 in the process of forming the pattern 59, the SiON film 54 is oxidized by changing the power applied to the upper electrode 4 and the lower electrode 31, respectively. A test was conducted to verify the relationship with the shape. The procedure for forming the pattern 59 in this verification test is based on the steps substantially the same as those of the first embodiment described above, but the process of forming the mask pattern 57 by etching the SiON film 54 is divided into two steps. went. More specifically, first, etching is performed using the resist film 55 as a mask under conditions with low selectivity with respect to the SiN film 53, and then the resist film 55 is masked under conditions with high selectivity with respect to the SiN film 53. Overetching was performed. Thereafter, the reaction proceeded through steps 2 to 5 similar to those of the above-described embodiment, and ashing was performed again after step 5 to remove a trace amount of polymer attached by etching.

In order to etch the SiON film 54 under high selection conditions, an etching apparatus having substantially the same structure as the etching apparatus 2 described above was used as the etching apparatus, but the etching apparatus was provided in the etching apparatus 2 as a processing gas supply source. In addition to the gas supply sources 45A to 45E, a C ₄ F ₈ (octafluorocyclobutane) gas supply source, which is a CF-based gas, is provided. Like other processing gases, A C ₄ F ₈ gas having a predetermined flow rate is supplied into the processing chamber 21.

The reaction conditions in each step are shown below.
First stage (stage in which the SiON film 54 is etched under a condition with low selectivity to the SiN film 53)
Pressure of mixed gas: 20 to 50 mTorr (2.67 to 6.67 Pa)
High frequency power supply (U / L): 300-600W / 0-400W
Flow rate ratio of mixed gas: CHF ₃ / CF ₄ / Ar = 0 to 200/200 to 400/600 sccm

Second stage (stage in which the SiON film 54 is over-etched under conditions with high selectivity to the SiN film 53)
Pressure of mixed gas: 50 to 100 mTorr (6.67 to 13.3 Pa)
High frequency power (U / L): 100W / 500W
Flow rate ratio of mixed gas: C ₄ F ₈ / Ar / O ₂ = 0 to 50/800/0 to 50 sccm

Third stage (corresponding to step 2 of the first embodiment)
O ₂ gas pressure: 200 mTorr (26.7 Pa)
High frequency power supply (U / L): 300W / 100W
Oxygen gas flow rate: 300 sccm

Fourth stage (corresponding to step 3 of the first embodiment)
Pressure of mixed gas: 20 to 50 mTorr (2.67 to 6.67 Pa)
High frequency power supply (U / L): 300-600W / 0-400W
Flow rate ratio of mixed gas: CHF ₃ / CF ₄ / Ar = 0 to 200/200 to 400/600 sccm

5th stage (equivalent to Step 4 of the first embodiment)
O ₂ gas pressure: 200 mTorr (26.7 Pa)
High frequency power supply (U / L): 300-1500W / 50-200W
O ₂ gas flow rate: 1200 sccm

6th stage (corresponding to step 5 of the first embodiment)
Pressure of mixed gas: 120 mTorr (16.0 Pa)
High frequency power (U / L): 500 W / 100 to 300 W
CH ₃ F / O ₂ / Ar = 3/13/90 sccm

7th stage (process to remove a small amount of polymer attached by etching)
O ₂ gas pressure: 200 mTorr (26.7 Pa)
High frequency power supply (U / L): 300W / 100W
O ₂ = 1200 sccm

The result formed through the above steps is shown in FIG. The vertical axis in the left table in FIG. 9 indicates the power applied to the upper power supply 4 in the fifth stage, and the horizontal axis indicates the power applied to the lower power supply 31, respectively. The numbers in the table are the θ shown on the right in the figure. That is, the angle of the side wall of the formed pattern 59 with respect to the horizontal plane is shown. From this figure, when the electric power applied to the lower electrode 31 is reduced with respect to the electric power applied to the upper electrode 4 and the drawing of O ₂ plasma into the precursor pattern 58 is reduced, a good pattern shape with θ close to 90 ° can be obtained. Became clear.

It is a vertical side view which shows an example of the etching apparatus used with the etching method of this invention. It is explanatory drawing which shows an example of the process in the 1st Embodiment of this invention. It is explanatory drawing which shows an example of the process in the 1st Embodiment of this invention. It is explanatory drawing which shows another example of the process in the 2nd Embodiment of this invention. It is explanatory drawing which shows another example of the process in the 2nd Embodiment of this invention. It is explanatory drawing which shows the process in a comparative example. It is explanatory drawing which shows the state of the pattern formed in the Example and the comparative example. It is explanatory drawing which shows the state of the pattern formed in the Example and the comparative example. It is the table | surface which showed the test result which verified the electric power and pattern shape which are given to the electrode in the said etching apparatus. It is a vertical side view of the pattern formed by the conventional etching method.

Explanation of symbols

21 processing chamber 3 mounting table 31 lower electrode 4 upper electrode 52 SiO ₂ (silicon oxide) film 53 SiN (silicon nitride) film 54 SiON (nitrogen-containing silicon oxide) film 55 resist film 58 precursor pattern 59 pattern

Claims (11)

A silicon nitride film having a silicon oxide film as a base is covered with a hard mask mainly composed of silicon oxide, and a patterned film is formed on the hard mask. In an etching method for etching
Etching the hard mask using the resist film as a mask and forming a mask pattern on the hard mask; and
Ashing the resist film;
An oxidation step of oxidizing the surface portion of the hard mask under conditions different from the ashing;
An overetching step of overetching the silicon nitride film with a high selection ratio of the silicon nitride film to the silicon oxide film to such an extent that etching of the underlying silicon oxide film exposed at the bottom of the silicon nitride film pattern is sufficiently suppressed;
After the step of forming a mask pattern on the hard mask and before the overetching step, the silicon nitride film is etched with a selectivity smaller than the selectivity when overetching the silicon nitride film. An etching method comprising: an etching step.   2. The etching method according to claim 1, wherein the main etching step is performed after the step of ashing the resist film and before the oxidation step.     2. The etching method according to claim 1, wherein the main etching step is performed after the step of forming a mask pattern on the hard mask and before the step of ashing the resist film.   The etching method according to claim 1, wherein the oxidation step is performed by plasma obtained by converting oxygen gas into plasma.   5. The over-etching process is performed by plasma obtained by converting a mixed gas containing a gas containing carbon, fluorine, and hydrogen and an oxygen gas into plasma. 6. Etching method.   6. The etching method according to claim 1, wherein the hard mask has a thickness of 50 nm or less.   The etching method according to claim 1, wherein the silicon oxide film has a thickness of 5 nm or less. 8. The etching method according to claim 1, wherein the thickness of the silicon nitride film is 50 nm or more. An airtight processing container having a mounting table on which a substrate is placed, means for supplying a processing gas into the processing container, means for adjusting the pressure in the processing container, and means for converting the gas in the processing container into plasma In an apparatus for etching a substrate with plasma obtained by converting a processing gas into plasma,
9. An etching apparatus comprising a control unit for controlling each means so as to carry out the etching method according to claim 1. A computer program that is used in an apparatus that introduces a processing gas into a processing container and performs etching on a substrate and operates on a computer,
9. A computer program comprising steps for performing the etching method according to claim 1. A computer storage medium in which the computer program according to claim 10 is stored.

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