Etching method

Abstract

This invention provides an etching method for removing the native oxide film formed on the surface of a silicon nitride film. [Solution] The method involves supplying hydrogen fluoride gas to the etching target, followed by supplying radicals to etch the surface oxide layer of the silicon nitride film (steps S13 and S14), and repeating the cycle of supplying hydrogen fluoride gas to the etching target from which the surface oxide layer has been etched, followed by supplying radicals, multiple times (steps S16 to S18), thereby selectively etching the silicon nitride film relative to the silicon oxide film. The ratio of the amount of silicon nitride film etched to the silicon oxide film is the selection ratio, and the first selection ratio of the processing conditions when etching the surface oxide layer is smaller than the second selection ratio of the processing conditions when selectively etching the silicon nitride film.

Description

Technical Field

[0001] The present disclosure relates to an etching method.

Background Art

[0002] Among wafers in which a silicon nitride film and a silicon oxide film are adjacent to each other, an example of a method for selectively etching the silicon nitride film includes a step of supplying hydrogen fluoride gas to a processing space in which the wafer is accommodated, and a step of supplying radicals of an inert gas to the processing space. In the step of supplying fluorine gas and the step of supplying radicals of an inert gas, the temperature of the wafer is maintained at a low temperature. In the etching method, first, by supplying hydrogen fluoride gas to the wafer, hydrogen fluoride is adsorbed on the surface of the silicon nitride film. Subsequently, radicals of an inert gas are supplied to the wafer, whereby energy equal to or higher than the activation energy of the etching reaction between hydrogen fluoride and silicon nitride is applied to the wafer. As a result, etching of the silicon nitride film proceeds (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, a natural oxide film may be formed on the surface of the silicon nitride film that is exposed to the outside. In this case, in the etching conditions for selectively etching the silicon nitride film, etching of the natural oxide film hardly proceeds. Therefore, it is required to remove the natural oxide film formed on the surface of the silicon nitride film.

Means for Solving the Problems

[0005] An etching method for solving the above problems is a method for selectively etching the silicon nitride film among an etching target including a silicon nitride film and a silicon oxide film. This etching method includes supplying hydrogen fluoride gas to the etching target, then supplying radicals to etch the surface oxide layer of the silicon nitride film, and repeating the cycle of supplying hydrogen fluoride gas to the etching target from which the surface oxide layer has been etched, and then supplying radicals, multiple times, thereby selectively etching the silicon nitride film with respect to the silicon oxide film. The ratio of the amount of etching of the silicon nitride film to the amount of etching of the silicon oxide film is the selectivity ratio, and the first selectivity ratio of the processing conditions when etching the surface oxide layer is smaller than the second selectivity ratio of the processing conditions when selectively etching the silicon nitride film.

[0006] According to the etching method described above, when etching the surface oxide layer formed on the surface of the silicon nitride film, the surface oxide layer is etched with a first selectivity ratio smaller than the second selectivity ratio. Therefore, it is possible to remove the surface oxide layer without excessively etching the silicon oxide film before selective etching of the silicon nitride film.

[0007] In the etching method described above, etching the surface oxide layer may involve repeating a first cycle multiple times, which includes supplying the hydrogen fluoride gas to the etching target and supplying the radicals to the etching target after the hydrogen fluoride gas has been supplied.

[0008] According to the etching method described above, the surface oxide layer is etched by repeating the first cycle multiple times. Therefore, the likelihood of etching the surface oxide layer across the entire silicon nitride film is increased without excessively etching the silicon oxide film.

[0009] In the etching method described above, at least one of the radicals used for etching the surface oxide layer and the radicals used for selective etching the silicon nitride film may be oxygen atom-containing radicals. According to this etching method, hydrogen fluoride molecules adsorbed on the object to be etched are activated by oxygen atom-containing radicals, which have a relatively long radical lifetime.

[0010] In the etching method described above, the cycle in which the hydrogen fluoride gas is supplied to the etching target after the surface oxide layer has been etched, and then radicals are supplied, is the second cycle, the amount of hydrogen fluoride molecules supplied to the etching target at one time when etching the surface oxide layer is the first supply amount, and the amount of hydrogen fluoride molecules supplied to the etching target in one second cycle when selectively etching the silicon nitride film is the second supply amount, and the first supply amount may be greater than the second supply amount.

[0011] According to the etching method described above, since the first supply amount is greater than the second supply amount, the reaction between activated hydrogen fluoride molecules and the surface oxide layer is more likely to occur during etching of the surface oxide layer. As a result, the first selectivity ratio can be made smaller than the second selectivity ratio.

[0012] In the etching method described above, the etching target is heated to a temperature of 80°C to 400°C before etching the surface oxide layer, and the temperature of the etching target is maintained within the range of 80°C to 400°C when etching the surface oxide layer and selectively etching the silicon nitride film.

[0013] According to the etching method described above, heating the etching target to a temperature of 80°C or higher reduces the likelihood of variations in the amount of hydrogen fluoride molecules adsorbed at each part of the etching target. Furthermore, heating the etching target to a temperature of 400°C or lower prevents the etching target from becoming excessively hot, thus suppressing the difficulty in adsorbing hydrogen fluoride molecules onto the etching target.

[0014] In the etching method described above, the cycle in which the hydrogen fluoride gas is supplied to the etching target from which the surface oxide layer has been etched, and then radicals are supplied, is the second cycle, and selective etching of the silicon nitride film may include etching the silicon nitride film to a thickness greater than 0 nm and less than 5 nm in one second cycle.

[0015] According to the etching method described above, the thickness of the silicon nitride film etched in one cycle is less than 5 nm, so the amount of etching is less likely to vary at each location in the silicon nitride film. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a diagram showing the configuration of an etching apparatus. [Figure 2] Figure 2 is a diagram showing the configuration of the etching chamber included in the etching apparatus shown in Figure 1. [Figure 3] Figure 3 is a flowchart illustrating the etching method. [Figure 4] Figure 4 is a timing chart illustrating the operation of each supply unit in the etching apparatus. [Figure 5] Figure 5 is a process diagram illustrating one of the steps included in the etching method. [Figure 6] Figure 6 is a process diagram illustrating one of the steps included in the etching method. [Figure 7] Figure 7 is a process diagram illustrating one of the steps included in the etching method. [Figure 8] FIG. 8 is a process diagram for explaining one step included in the etching method. [Figure 9] FIG. 9 is a process diagram for explaining one step included in the etching method. [Figure 10] FIG. 10 is a process diagram for explaining one step included in the etching method. [Figure 11] FIG. 11 is a process diagram for explaining one step included in the etching method. [Figure 12] FIG. 12 is a process diagram for explaining one step included in the etching method. [Figure 13] FIG. 13 is a process diagram for explaining one step included in the etching method.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] Referring to FIGS. 1 to 13, one embodiment of the etching method will be described. [Configuration of Etching Apparatus 10] Referring to FIG. 1, the etching apparatus 10 will be described.

[0018] As shown in FIG. 1, the etching apparatus 10 includes an etching chamber 11, a load lock chamber 12, and a gate valve 13. The etching apparatus 10 includes a radical generation gas supply unit 21, a hydrogen fluoride gas supply unit 22 (HF gas supply unit), a plasma supply unit 23, and an inert gas supply unit 24. The etching apparatus 10 includes a control unit 10C.

[0019] The etching chamber 11 defines a processing space 11S (see Figure 2) that accommodates a substrate S (see Figure 2), which is an example of a substrate to be etched. The substrate S includes a silicon nitride film S1 and a silicon oxide film S2 (see Figure 5). The etching chamber 11 etches the silicon nitride film S1 within the processing space 11S. The load lock chamber 12 loads the substrate S into the etching chamber 11 from outside the etching apparatus 10 before etching. The load lock chamber 12 then loads the substrate S out of the etching chamber 11 to outside the etching apparatus 10 after etching.

[0020] The gate valve 13 is positioned between the etching chamber 11 and the load lock chamber 12. When the gate valve 13 opens, the etching chamber 11 communicates with the load lock chamber 12. When the gate valve 13 closes, the etching chamber 11 is isolated from the load lock chamber 12.

[0021] The load lock chamber 12 is connected to the cooling gas supply unit 12A. The cooling gas supply unit 12A supplies cooling gas to the load lock chamber 12. The cooling gas is an inert gas used to cool the substrate S after etching.

[0022] The etching chamber 11 includes a heating section 11A and an exhaust section 11B. The heating section 11A heats the etching chamber 11, thereby heating the substrate S in the processing space 11S. The exhaust section 11B reduces the pressure of the etching chamber 11 to a predetermined pressure.

[0023] The etching chamber 11 is connected to an HF gas supply unit 22 and a plasma supply unit 23. The HF gas supply unit 22 supplies HF gas to the processing space 11S. The HF gas supply unit 22 is configured to supply hydrogen fluoride gas (HF gas) to the processing space 11S at a predetermined flow rate. The HF gas supply unit 22 is, for example, a mass flow controller.

[0024] The plasma supply unit 23 supplies plasma to the processing space 11S, thereby supplying radicals 33 (see Figure 8) contained in the plasma into the processing space 11S. The plasma supply unit 23 comprises a discharge tube 23A, a waveguide 23B, and a microwave irradiation unit 23C. The microwave irradiation unit 23C irradiates the discharge tube 23A with microwaves through the waveguide 23B. The discharge tube 23A is connected to the radical generation gas supply unit 21. The inner surface of the discharge tube 23A is made of an inorganic oxide. The inorganic oxide constituting the inner surface of the discharge tube 23A may be silicon oxide or aluminum oxide. The discharge tube 23A may be, for example, a quartz tube.

[0025] The radical-generating gas supply unit 21 supplies radical-generating gas 32 to the discharge tube 23A to generate radicals 33. The radical-generating gas supply unit 21 supplies the radical-generating gas 32 to the discharge tube 23A at a predetermined flow rate. The radical-generating gas supply unit 21 is, for example, a mass flow controller.

[0026] The radical-generating gas 32 may be a gas containing oxygen atoms or a noble gas. The noble gas may be argon (Ar) gas or helium (He) gas. The oxygen-containing gas may be oxygen gas, a mixture of oxygen gas and hydrogen gas, or nitrogen oxides (N x O y The nitrogen oxide gas may be at least one selected from the group consisting of ) gases. The nitrogen oxide gas may be at least one selected from the group consisting of nitric oxide (NO) gas, nitrogen dioxide (NO2) gas, nitrous oxide (N2O) gas, dinitrogen trioxide (N2O3) gas, and dinitrogen pentoxide (N2O5) gas.

[0027] The plasma supply unit 23 generates plasma in the discharge tube 23A by irradiating the radical-generating gas 32 with microwaves inside the discharge tube 23A. The plasma contains radicals 33 that activate hydrogen fluoride molecules (HF molecules 31, see Figure 6). The radicals 33 that activate the HF molecules 31 may be oxygen atom-containing radicals, oxygen radicals, or noble gas radicals.

[0028] The inert gas supply unit 24 supplies inert gas 35 to the processing space 11S. The inert gas supply unit 24 is configured to supply inert gas 35 to the processing space 11S at a predetermined flow rate. The inert gas supply unit 24 is, for example, a mass flow controller. The inert gas 35 may be nitrogen (N2) gas or argon (Ar) gas. The inert gas supply unit 24 may supply the inert gas 35 into the processing space 11S using the same piping as the HF gas supply unit 22, or it may supply the inert gas 35 into the processing space 11S using separate piping.

[0029] The control unit 10C includes a memory unit 10CM. The memory unit 10CM stores process conditions for etching the silicon nitride film S1. The process conditions include the pressure in the etching chamber 11, the temperature of the substrate S, the flow rates of various gases, and the output of the microwave irradiation unit 23C. The control unit 10C controls the operation of the heating unit 11A, the exhaust unit 11B, the radical generation gas supply unit 21, the HF gas supply unit 22, the plasma supply unit 23, and the inert gas supply unit 24 so that the etching conditions match the process conditions.

[0030] The control unit 10C instructs the heating unit 11A to heat the substrate S to a predetermined temperature between the first and second temperatures, while simultaneously supplying HF gas to the HF gas supply unit 22, and then supplying radicals 33 to the plasma supply unit 23.

[0031] The control unit 10C includes electronic circuits such as a CPU and an MPU. The control unit 10C includes storage such as an SSD or an HDD. The control unit 10C includes memory such as ROM, RAM, or registered memory. The control unit 10C may also include integrated circuits such as an ASIC or an FPGA. All processing to be performed by the control unit 10C may be performed by the software provided by the control unit 10C, or by a combination of the integrated circuits and software provided by the control unit 10C.

[0032] [Configuration of etching chamber 11] As shown in Figure 2, the etching chamber 11 houses the support section 10A. The support section 10A is capable of supporting multiple substrates S. The multiple substrates S supported by the support section 10A are stacked with gaps between adjacent substrates S. Each substrate S comprises a silicon nitride film S1 and a silicon oxide film S2, as described above. An example of a substrate S is a disc shape.

[0033] An example of a substrate S comprises a plurality of silicon nitride films S1 and a plurality of silicon oxide films S2 (see Figure 5). In the substrate S, the silicon nitride films S1 and silicon oxide films S2 are alternately stacked. The substrate S has pores SA that penetrate the plurality of silicon nitride films S1 and the plurality of silicon oxide films S2. The sides defining the pores SA include the end faces of each silicon nitride film S1 and the end faces of each silicon oxide film S2. The end faces of each silicon nitride film S1 are oxidized. That is, a surface oxide layer SA1 of the silicon nitride film S1 is located at the end face of each silicon nitride film S1. The surface oxide layer SA1 of the silicon nitride film S1 is composed of at least one of silicon oxide and silicon oxynitride. The surface oxide layer SA1 of the silicon nitride film S1 may be composed of silicon oxide only, or of silicon oxynitride only, or of silicon oxide and silicon oxynitride.

[0034] The etching chamber 11 is equipped with a shower head 11D. The shower head 11D is connected to a discharge tube 23A. There may be one discharge tube 23A connected to the shower head 11D, or two or more. Figure 2 shows an example in which two discharge tubes 23A are connected to the shower head 11D. The shower head 11D is equipped with multiple supply ports. The supply ports of the shower head 11D are aligned along the direction in which the substrates S are stacked. The supply ports of the shower head 11D supply plasma supplied from the discharge tubes 23A toward the substrates S.

[0035] The etching chamber 11 is equipped with a rotating part 11E. The rotating part 11E rotates the support part 10A in the circumferential direction of the substrate S. The rotating part 11E disperses the plasma supplied from the shower head 11D toward the substrate S and the HF gas supplied from the HF gas supply unit 22 toward the substrate S in the circumferential direction of the substrate S.

[0036] The etching chamber 11 is equipped with a temperature measuring unit 11F. The temperature measuring unit 11F measures the temperature inside the etching chamber 11 as the temperature of the substrate S. The temperature measuring unit 11F is connected to the control unit 10C. The temperature measurement result from the temperature measuring unit 11F is input to the control unit 10C. The control unit 10C controls the driving of the heating unit 11A based on the measurement result from the temperature measuring unit 11F.

[0037] [Etching method] The etching method will be explained with reference to Figures 3 and 4. The etching method is a method for selectively etching the silicon nitride film S1 from among an etching target that includes a silicon nitride film S1 and a silicon oxide film S2. The etching method includes etching the surface oxide layer SA1 formed on the silicon nitride film S1. The etching method includes selectively etching the silicon nitride film S1 with respect to the silicon oxide film S2 in an etching target in which the surface oxide layer SA1 formed on the silicon nitride film S1 has been etched. The selectivity ratio is the ratio of the etching amount of the silicon nitride film S1 to the etching amount of the silicon oxide film S2, that is, the ratio of the etching amount of the silicon nitride film S1 with respect to the etching amount of the silicon oxide film S2. The selectivity ratio of the processing conditions when etching the surface oxide layer SA1 is the first selectivity ratio. The selectivity ratio of the processing conditions when selectively etching the silicon nitride film S1 is the second selectivity ratio. The first selectivity ratio is smaller than the second selectivity ratio.

[0038] Etching of the surface oxide layer SA1 includes supplying HF gas to the etched object, and supplying radicals 33 generated from radical generating gas 32 to the etched object after the HF gas has been supplied.

[0039] Selective etching of the silicon nitride film S1 involves repeating a second cycle multiple times, which includes supplying HF gas to the etching target and supplying radicals 33 generated from radical generating gas 32 to the etching target after the HF gas has been supplied. The radical generating gas 32 used for selective etching of the silicon nitride film S1 may be the same as or different from the radical generating gas 32 used for etching the surface oxide layer SA1.

[0040] According to the above etching method, when etching the surface oxide layer SA1 formed on the surface of the silicon nitride film S1, the surface oxide layer SA1 is etched with a first selectivity ratio smaller than the second selectivity ratio. Therefore, it is possible to remove the surface oxide layer SA1 before selective etching of the silicon nitride film S1.

[0041] Figure 3 is a flowchart illustrating the etching method. The process described below is performed by the control unit 10C executing an etching program that includes the process conditions stored in the control unit 10C.

[0042] As shown in Figure 3, the etching method includes a substrate placement step (step S11) and a heating step (step S12). In the substrate placement step, multiple substrates S are placed on the support section 10A. At this time, each substrate S is placed on the support section 10A located inside the load lock chamber 12. Subsequently, the support section 10A with the multiple substrates S placed on it moves from the load lock chamber 12 to the etching chamber 11. After that, the gate valve 13 is closed, and the exhaust section 11B reduces the pressure inside the etching chamber 11 to a predetermined level.

[0043] In the heating step, the heating unit 11A heats the substrate S. The heating unit 11A may heat the substrate S to a temperature of 80°C to 400°C before etching the surface oxide layer SA1 of the silicon nitride film S1. The temperature of the substrate S may be maintained within the range of 80°C to 400°C when etching the surface oxide layer SA1 and when selectively etching the silicon nitride film S1.

[0044] By heating the substrate S to a temperature of 80°C or higher, variations in the amount of HF molecules 31 adsorbed at different parts of the substrate S are less likely to occur. In particular, when the substrate S has pores SA that penetrate multiple silicon oxide films S2 and multiple silicon nitride films S1, preferential adsorption of HF molecules 31 at the openings of the pores SA is suppressed. As a result, variations in the amount of HF molecules 31 adsorbed on the side surfaces defining the pores SA are less likely to occur in the depth direction of the pores SA. Furthermore, by heating the substrate S to a temperature of 400°C or lower, the temperature of the substrate S does not become excessively high, thus suppressing the difficulty in adsorbing HF molecules 31 onto the substrate S.

[0045] Next, the surface oxide layer SA1 formed on the surface of the silicon nitride film S1 is etched. The etching method includes an adsorption step of HF molecules 31 (step S13), a supply step of radicals 33 (step S14), and a step of determining whether the first cycle including the adsorption step and the supply step has been repeated a predetermined M times (2 ≤ M) (step S15).

[0046] In the HF molecule 31 adsorption step, the HF gas supply unit 22 supplies HF gas to the etching chamber 11. In the radical 33 supply step, the radical generation gas supply unit 21 supplies radical generation gas 32 to the discharge tube 23A. Next, the microwave irradiation unit 23C irradiates the discharge tube 23A with microwaves, generating a plasma containing radicals 33 within the discharge tube 23A. The plasma supply unit 23 supplies the plasma containing radicals 33 to the etching chamber 11. The radicals 33 may be, for example, oxygen radicals (O radicals).

[0047] The first cycle, which includes the adsorption step of HF molecules 31 and the supply step of radicals 33, is repeated M times. The number of times the first cycle is repeated may be predetermined. Thus, in the etching method described above, etching the surface oxide layer SA1 involves repeating a first cycle multiple times, which includes supplying HF gas to the substrate S and supplying radicals 33 to the substrate S after the HF gas has been supplied. Since the surface oxide layer SA1 is etched by repeating the first cycle multiple times, the accuracy of etching the surface oxide layer SA1 is increased.

[0048] Next, the silicon nitride film S1 is etched onto the substrate S from which the surface oxide layer SA1 has been etched. The etching method includes an adsorption step of HF molecules 31 (step S16), a radical supply step of radicals 33 (step S17), and a step of determining whether the second cycle, including the adsorption step and the supply step, has been repeated a predetermined number of N times (2 ≤ N) (step S18).

[0049] In the HF molecule 31 adsorption step, the HF gas supply unit 22 supplies HF gas to the etching chamber 11. In the radical 33 supply step, the radical generation gas supply unit 21 supplies radical generation gas 32 to the discharge tube 23A. Next, the microwave irradiation unit 23C irradiates the discharge tube 23A with microwaves, generating a plasma containing radicals 33 within the discharge tube 23A. The plasma supply unit 23 supplies the plasma containing radicals 33 to the etching chamber 11. The radicals 33 may be, for example, O radicals.

[0050] The second cycle, which includes the adsorption step of HF molecules 31 and the supply step of radicals 33, is repeated N times. The number of times the second cycle is repeated may be predetermined. By selectively etching the silicon nitride film S1, it is possible to etch the silicon nitride film S1 to a thickness greater than 0 nm but less than 5 nm in a single second cycle. Since the thickness of the silicon nitride film S1 etched in a single second cycle is less than 5 nm, the amount of etching is less likely to vary at each location on the silicon nitride film S1. In particular, when the substrate S has holes SA that penetrate multiple silicon oxide films S2 and multiple silicon nitride films S1, the amount of etching of the silicon nitride film S1 becomes less likely to vary in the depth direction of the holes SA.

[0051] The etching method further includes a substrate recovery step (step S19). In the substrate recovery step, the pressure inside the etching chamber 11 is increased to the same level as the pressure inside the load lock chamber 12. Next, after the gate valve 13 opens, the support part 10A moves from the etching chamber 11 to the load lock chamber 12. Subsequently, after the gate valve 13 closes, the cooling gas supply unit 12A supplies cooling gas into the load lock chamber 12. Then, after the temperature of the substrate S drops to below a predetermined temperature, the cooling gas supply unit 12A stops supplying cooling gas. After that, the etched substrate S is recovered from the load lock chamber 12.

[0052] In the etching method, the amount of HF molecules 31 supplied to the substrate S at one time when etching the surface oxide layer SA1 is the first supply amount. That is, the amount of HF molecules 31 supplied to the substrate S in one first cycle is the first supply amount. When selectively etching the silicon nitride film S1, the amount of HF molecules 31 supplied to the etching target in one second cycle is the second supply amount. The first supply amount is greater than the second supply amount.

[0053] By making the first supply amount greater than the second supply amount, the reaction between the activated HF molecules 31 and the silicon oxide film S2 becomes more likely to occur. As a result, the first selectivity ratio can be made smaller than the second selectivity ratio.

[0054] Figure 4 shows an example of how the control unit 10C drives the heating unit 11A, the radical generation gas supply unit 21, the HF gas supply unit 22, the microwave irradiation unit 23C, and the inert gas supply unit 24. Figure 4 shows how each unit is driven during the heating process (step S12) and one first cycle.

[0055] Furthermore, in Figure 4, the state where heating by the heating unit 11A is stopped is indicated as "OFF," and the state where heating by the heating unit 11A is being performed is indicated as "ON." Also, the state where microwave irradiation by the microwave irradiation unit 23C is stopped is indicated as "OFF," and the state where microwave irradiation by the microwave irradiation unit 23C is being performed is indicated as "ON." In addition, the state where gas supply from each gas supply unit 21, 22, 24 is stopped is indicated as "OFF," and the state where gas supply from each gas supply unit 21, 22, 24 is being performed is indicated as "ON."

[0056] As shown in Figure 4, when etching the substrate S in the etching chamber 11, first, at timing t0, the substrate S is placed inside the etching chamber 11. Next, at timing t1, the control unit 10C starts heating the heating unit 11A. As a result, the temperature T of the substrate S begins to rise, and at timing t2, the temperature T of the substrate S reaches a predetermined temperature within the range of 80°C to 400°C.

[0057] Subsequently, at timing t3, the control unit 10C starts supplying HF gas to the HF gas supply unit 22. Next, at timing t4, the control unit 10C stops supplying HF gas to the HF gas supply unit 22 and starts supplying radical generating gas 32 to the radical generating gas supply unit 21. Then, at timing t5, the control unit 10C starts irradiating the microwave irradiation unit 23C with microwaves.

[0058] Next, at timing t6, the control unit 10C stops supplying radical-generating gas 32 to the radical-generating gas supply unit 21, stops microwave irradiation to the microwave irradiation unit 23C, and starts supplying inert gas 35 to the inert gas supply unit 24. Then, at timing t7, the control unit 10C stops supplying inert gas 35 to the inert gas supply unit 24.

[0059] Thus, in the process that the control unit 10C has the etching chamber 11 perform, the process of heating the substrate S is started at timing t1, and the heating of the substrate S continues until the etching of the substrate S is completed. Furthermore, of the processes that the control unit 10C has the etching chamber 11 perform, the process from timing t3 to timing t4 is the process of supplying HF gas and adsorbing it onto the substrate S. Furthermore, the process from timing t4 to timing t5 is the process of supplying radical generating gas 32. Furthermore, of the processes that the control unit 10C has the etching chamber 11 perform, the process from timing t5 to timing t6 is the process of supplying radicals 33. Furthermore, the process from timing t6 to timing t7 is the process of supplying inert gas 35.

[0060] In other words, the processing from timing t3 to timing t7 constitutes one first cycle. The control unit 10C causes the etching chamber 11 to execute multiple first cycles until the amount of etching of the surface oxide layer SA1 on the substrate S reaches a predetermined amount. When the first cycle is repeated, the timing t7 of the (m-1)th first cycle and the timing t3 of the mth first cycle may occur simultaneously. Alternatively, the timing t3 of the mth first cycle may start after a predetermined period has elapsed since the timing t7 of the (m-1)th first cycle.

[0061] In a single second cycle, the same processing as in the first cycle, from timing t3 to timing t7, is performed. However, the length from timing t3 to timing t4 in the first cycle is longer than the length from timing t3 to timing t4 in the second cycle. That is, the period during which HF gas is supplied in the first cycle is longer than the period during which HF gas is supplied in the second cycle. As a result, the first supply amount in the first cycle is greater than the second supply amount in the second cycle. Note that the timing t7 in the final first cycle and the timing t3 in the first second cycle may occur simultaneously. Alternatively, the timing t3 in the first second cycle may start after a predetermined period has elapsed since the timing t7 in the final first cycle.

[0062] [Effect] The operation of the etching method will be explained with reference to Figures 5 to 13. As shown in Figure 5, the substrate S has pores SA extending along the thickness direction. The pores SA penetrate two or more silicon nitride films S1 and two or more silicon oxide films S2. The substrate S is provided with a support substrate S3. The multilayer film, including the silicon nitride films S1 and silicon oxide films S2, is formed on the support substrate S3. Although only one pore SA is shown in Figures 5 to 13 for illustrative purposes, the substrate S has multiple pores SA. The substrate S may be, for example, a substrate for 3D-NAND.

[0063] As shown in Figure 6, in the etching method, first, the etching apparatus 10 supplies HF gas to the substrate S. As a result, the HF molecules 31 supplied to the substrate S are also supplied to the inner surface of the pores SA containing the surface oxide layer SA1. HF molecules 31 have high adsorption properties to the substrate S. Therefore, the amount of HF molecules 31 adsorbed near the opening of the pores SA tends to be greater than the amount of HF molecules 31 adsorbed at the bottom of the pores SA. In this regard, heating the substrate S makes it less likely for the HF molecules 31 supplied to the substrate S to be consumed near the opening of the pores SA, and as a result, it becomes easier for HF molecules 31 to be introduced to the bottom of the pores SA.

[0064] In the step of supplying HF gas to the substrate S, the pressure in the processing space 11S to which the HF gas is supplied may be 500 Pa or higher. When the pressure in the processing space 11S is 500 Pa or higher, HF molecules 31 are more easily adsorbed onto the substrate S.

[0065] As shown in Figure 7, after the supply of HF gas, the etching apparatus 10 switches the type of gas supplied to the substrate S from HF gas to radical-generating gas 32. In this case, if the radical-generating gas 32 is an oxygen atom-containing gas, the affinity of the radical-generating gas 32 to the silicon oxide film S2 is higher than the affinity of the HF molecules 31. Therefore, the HF molecules 31 adsorbed on the silicon oxide film S2 that defines the pores SA are replaced by the radical-generating gas 32 with a higher probability than the surface oxide layer SA1, which is a thin oxide formed on the surface of the silicon nitride film S1. Furthermore, since the radical-generating gas 32 has low adsorption capacity, it is unlikely that the radical-generating gas 32 will remain on the silicon oxide film S2.

[0066] If radical-generating gas 32 is supplied to the substrate S between supplying HF gas to the substrate S and supplying radicals 33 to the substrate S, at least some of the unwanted HF molecules 31 located on the substrate S will be removed from the substrate S by the flow of radical-generating gas 32.

[0067] As shown in Figure 8, after the supply of radical generating gas 32, the etching apparatus 10 supplies radicals 33 generated from the radical generating gas 32 to the substrate S. This generates etchant 34 that etches the surface oxide layer SA1 from HF molecules 31 adsorbed on the silicon nitride film S1 defining the pores SA and radicals 33. In this way, the surface oxide layer SA1 is etched by the surface reaction between HF molecules 31 and radicals 33 on the substrate S. The etching of the surface oxide layer SA1 proceeds along a direction perpendicular to the depth direction of the pores SA.

[0068] When the surface oxide layer SA1 is etched, it is etched with a first selectivity ratio smaller than the second selectivity ratio. However, as described above, the HF molecules 31 adsorbed on the silicon oxide film S2 are replaced by radical-generating gas 32 with a higher probability than the surface oxide layer SA1, which is a thin oxide formed on the surface of the silicon nitride film S1. Therefore, it is possible to preferentially remove the surface oxide layer SA1 compared to the silicon oxide film S2 before the selective etching of the silicon nitride film S1.

[0069] When the radical-generating gas 32 is an oxygen atom-containing gas, the oxygen atom-containing radical 33 is particularly difficult to deactivate, making it easier for radicals 33 to be supplied to the interior of the pores SA. This makes it possible to proceed with etching of the surface oxide layer SA1 even inside the pores SA. Furthermore, when radicals 33 are generated from a gas containing oxygen atoms, the surface of the silicon oxide film S2 that does not have adsorbed HF molecules 31 is repaired by the radicals 33. This allows for even more preferential etching of the surface oxide layer SA1 relative to the silicon oxide film S2.

[0070] As shown in Figure 9, after supplying radicals 33 to the substrate S, the etching apparatus 10 switches the type of gas supplied to the substrate S from radicals 33 to inert gas 35. The inert gas 35 may be nitrogen gas, as described above. When inert gas 35 is supplied to the substrate S, it is supplied into the pores SA, and the HF molecules 31, radicals 33, and etchant 34 remaining in the pores SA are replaced by the inert gas 35. When etching of the surface oxide layer SA1 is repeated over multiple cycles, the temperature of the substrate S is maintained at a predetermined value from the first cycle of the first round until the end of the first cycle of the final round.

[0071] As described above, in the etching method, one first cycle includes supplying HF gas to the substrate S and supplying radicals 33 to the substrate S. The etching method may also include repeating multiple first cycles. The first cycle includes supplying inert gas 35 to the substrate S after supplying radicals 33 to the substrate S. This makes it less likely that radicals 33 supplied to the etching target in the mth first cycle will be present near the substrate S at the start of the m+1th first cycle. Therefore, etching is suppressed in areas other than the surface oxide layer SA1 in the m+1th first cycle.

[0072] As shown in Figure 10, after etching of the surface oxide layer SA1, the etching apparatus 10 supplies HF gas to the substrate S again. As a result, the HF molecules 31 supplied to the substrate S are supplied to the inner surface of the pore SA, which is formed from the end face of the silicon nitride film S1 and the end face of the silicon oxide film S2 after etching the surface oxide layer SA1. At this time, the heating of the substrate S makes it less likely for the HF molecules 31 supplied to the substrate S to be consumed near the opening of the pore SA, and as a result, it becomes easier for HF molecules 31 to be introduced to the bottom of the pore SA. In the step of supplying HF gas to the substrate S, the pressure in the processing space 11S to which the HF gas is supplied may be greater than or equal to the pressure in the processing space 11S to which the HF gas was supplied during etching of the surface oxide layer SA1.

[0073] As shown in Figure 11, after the supply of HF gas, the etching apparatus 10 switches the type of gas supplied to the substrate S from HF gas to radical-generating gas 32. In this case, if the radical-generating gas 32 is an oxygen atom-containing gas, the affinity of the radical-generating gas 32 to the silicon oxide film S2 is higher than the affinity of the HF molecules 31. Therefore, the HF molecules 31 adsorbed on the silicon oxide film S2 that defines the pores SA are replaced by the radical-generating gas 32 with a higher probability than the silicon nitride film S1.

[0074] As shown in Figure 12, after the supply of radical generating gas 32, the etching apparatus 10 supplies radicals 33 generated from the radical generating gas 32 to the substrate S. This generates etchant 34 that etches the silicon nitride film S1 from the HF molecules 31 adsorbed on the silicon nitride film S1 and the radicals 33. In this way, the silicon nitride film S1 is etched by the surface reaction between the HF molecules 31 and the radicals 33 on the substrate S. The etching of the surface oxide layer SA1 proceeds along a direction perpendicular to the depth direction of the pores SA.

[0075] When the silicon nitride film S1 is etched, it is etched with a second selectivity ratio greater than the first selectivity ratio. At this time, the HF molecules 31 adsorbed on the silicon oxide film S2 are replaced by radical-generating gas 32 with a higher probability than the silicon nitride film S1. Therefore, selective etching of the silicon nitride film S1 is promoted. Furthermore, if radicals 33 are generated from a gas containing oxygen atoms, the surface of the silicon oxide film S2 that does not have adsorbed HF molecules 31 is repaired by the radicals 33. This allows for even more preferential etching of the silicon nitride film S1 compared to the silicon oxide film S2.

[0076] As shown in Figure 13, after supplying radicals 33 to the substrate S, the etching apparatus 10 switches the type of gas supplied to the substrate S from radicals 33 to inert gas 35. The inert gas 35 may be nitrogen gas, as described above. By supplying inert gas 35 to the substrate S, the inert gas 35 is supplied into the pores SA, thereby replacing the HF molecules 31, radicals 33, and etchant 34 remaining in the pores SA with the inert gas 35. The temperature of the substrate S is maintained at a predetermined value from the first second cycle until the end of the final second cycle. One second cycle includes supplying HF gas to the substrate S and supplying radicals 33 to the substrate S. The second cycle includes supplying inert gas 35 to the substrate S after supplying radicals 33 to the substrate S. As a result, radicals 33 supplied to the etching target in the nth second cycle are less likely to be present near the substrate S at the start of the (n+1)th second cycle. Therefore, in the n+1th second cycle, etching is suppressed in areas other than the silicon nitride film S1.

[0077] As described above, according to one embodiment of the etching method, the following effects can be obtained. (1) When etching the surface oxide layer SA1 of the silicon nitride film S1, the surface oxide layer SA1 is etched with a first selectivity ratio smaller than the second selectivity ratio. Therefore, it is possible to remove the surface oxide layer SA1 without excessively etching the silicon oxide film S2 before selective etching of the silicon nitride film S1.

[0078] (2) The surface oxide layer SA1 is etched by repeating the first cycle multiple times. This increases the likelihood of etching the surface oxide layer SA1 over the entire silicon nitride film S1 without excessively etching the silicon oxide film S2.

[0079] (3) When radical 33 is an oxygen atom-containing radical, the HF molecule 31 adsorbed on the etching target is activated by the oxygen atom-containing radical, which has a relatively long radical lifetime.

[0080] (4) Because the first supply amount is greater than the second supply amount, the etching of the surface oxide layer SA1 is likely to cause a reaction between the activated HF molecules 31 and the surface oxide layer SA1. (5) By heating the substrate S to a temperature of 80°C or higher, variations in the amount of HF molecules 31 adsorbed in each part of the substrate S are less likely to occur. Also, by heating the substrate S to a temperature of 400°C or lower, the temperature of the substrate S does not become excessively high, so the difficulty in adsorbing HF molecules 31 onto the substrate S is suppressed.

[0081] (6) When the thickness of the silicon nitride film S1 etched in one first cycle is less than 5 nm, the amount of etching is less likely to vary at each position of the surface oxide layer SA1.

[0082] The above-described embodiment can be implemented with the following modifications. The etching method may involve lowering the temperature of the substrate S during the etching of the surface oxide layer SA1 compared to the selective etching of the silicon nitride film S1, thereby making the first selectivity ratio smaller than the second selectivity ratio.

[0083] The etching method may involve increasing the pressure in the etching chamber 11 during the etching of the surface oxide layer SA1 compared to the selective etching of the silicon nitride film S1, thereby making the first selectivity ratio smaller than the second selectivity ratio.

[0084] The etching method may involve increasing the flow rate of HF molecules 31 during the etching of the surface oxide layer SA1 compared to the selective etching of the silicon nitride film S1, thereby making the first selectivity ratio smaller than the second selectivity ratio.

[0085] The etching method may be modified so that the first selectivity ratio is smaller than the second selectivity ratio, by using different radical generating gases 32 for etching the surface oxide layer SA1 and for selective etching the silicon nitride film S1.

[0086] The etching method may be modified to make the first selectivity ratio smaller than the second selectivity ratio by making at least one difference in the supply period of the HF molecules 31, the temperature of the substrate S, the pressure of the etching chamber 11, the flow rate of the HF molecules 31, and the type of radical generating gas 32. [Explanation of Symbols]

[0087] S... Circuit board S1...Silicon nitride film S2...Silicon oxide film SA1...Surface oxide layer T…Temperature 10…Etching equipment 10C…Control Unit 11…Etching Chamber 11S… Processing space 21…Radical generation gas supply unit 22…HF Gas Supply Unit 23…Plasma supply unit 24...Inert gas supply unit 31...HF molecule 32…Radical-generating gases 33… Radical

Claims

1. A method for selectively etching a silicon nitride film among an etching target including a silicon nitride film and a silicon oxide film, After supplying hydrogen fluoride gas to the etching target, radicals are supplied, thereby etching the surface oxide layer of the silicon nitride film, and The process includes repeatedly supplying the hydrogen fluoride gas to the etching target from which the surface oxide layer has been etched, followed by supplying radicals, thereby selectively etching the silicon nitride film with respect to the silicon oxide film. The ratio of the etching amount of the silicon nitride film to the etching amount of the silicon oxide film is the selectivity ratio. The first selectivity ratio of the processing conditions for etching the surface oxide layer is smaller than the second selectivity ratio of the processing conditions for selectively etching the silicon nitride film. An etching method characterized by the following features.

2. Etching the aforementioned surface oxide layer is Supplying the hydrogen fluoride gas to the etching target, and The first cycle, which includes supplying the radicals to the etching target after the hydrogen fluoride gas has been supplied, is repeated multiple times. The etching method according to claim 1.

3. At least one of the radicals used for etching the surface oxide layer and the radicals used for selective etching the silicon nitride film is an oxygen atom-containing radical. The etching method according to claim 1 or 2.

4. The second cycle is the cycle in which the hydrogen fluoride gas is supplied to the etching target from which the surface oxide layer has been etched, and then the radicals are supplied. In etching the surface oxide layer, the amount of hydrogen fluoride molecules supplied to the etching target at one time is the first supply amount. In selective etching of the silicon nitride film, the amount of hydrogen fluoride molecules supplied to the etching target in one second cycle is the second supply amount. The first supply quantity is greater than the second supply quantity. The etching method according to claim 1 or 2.

5. The process further includes heating the object to be etched to a temperature of 80°C to 400°C before etching the surface oxide layer. In etching the surface oxide layer and selectively etching the silicon nitride film, the temperature of the object to be etched is maintained within a range of 80°C to 400°C. The etching method according to claim 1 or 2.

6. The second cycle is the cycle in which the hydrogen fluoride gas is supplied to the etching target from which the surface oxide layer has been etched, and then the radicals are supplied. Selective etching of the silicon nitride film is The second cycle includes etching the silicon nitride film to a thickness greater than 0 nm and less than 5 nm. The etching method according to claim 1 or 2.

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