JP6541739B2 - Plasma etching method - Google Patents

Description

  The present invention relates to a plasma etching method for plasma etching a silicon carbide substrate, and more particularly to a plasma etching method for forming a tapered etching structure on a silicon carbide substrate.

  In recent years, silicon carbide substrates, which are one of wide-gap semiconductor substrates, have attracted attention as semiconductor materials. This silicon carbide substrate is characterized in that the lattice constant of the crystal is small and the band gap is large as compared with the silicon (Si) substrate and the gallium arsenide (GaAs) substrate which are widely used conventionally, and the physical properties are excellent. It is expected to be applied to fields that can not be covered with a silicon substrate or a GaAs substrate.

  However, as described above, silicon carbide has a smaller lattice constant of crystal compared to silicon etc. In other words, since each atom is strongly bonded, it is difficult to break the bond between atoms, and a silicon substrate Compared to the above, it has the disadvantage that it is difficult to apply the etching process. Therefore, the present applicants have proposed a plasma etching method disclosed in JP 2011-96700 A as a method of plasma etching such a silicon carbide substrate.

This plasma etching method targets a substrate on which a silicon dioxide (SiO ₂ ) film is formed as a mask on the surface of a silicon carbide substrate as an etching target, supplies an inert gas such as He gas into the processing chamber and converts it into a plasma to deactivate it. C. or higher by applying bias power to a base on which a silicon carbide substrate is mounted and causing the generated ions to be incident on the silicon carbide substrate while generating ions derived from gas and the like. It heats to predetermined | prescribed etching process temperature. Then, an etching gas such as sulfur hexafluoride (SF ₆ ) gas is supplied into the processing chamber and plasmatized to generate ions, radicals, etc., and a bias power is applied to the base to set the temperature of the silicon carbide substrate The silicon carbide substrate is etched by sputtering with generated ions and a chemical reaction with radicals while maintaining the etching processing temperature.

  According to this plasma etching method, the bond between silicon (Si) and carbon (C) constituting the silicon carbide substrate is cut by heating the silicon carbide substrate placed on the base to a predetermined etching processing temperature Since a part of the energy necessary to do so can be given, and the bond between atoms can be easily broken, it becomes easy to carry out the etching process, and it becomes possible to carry out a highly accurate etching process.

JP 2011-96700 A

  By the way, when manufacturing a semiconductor from a silicon carbide substrate, the surface of the substrate is etched to form a groove or a hole (hereinafter referred to as a "recess"), and the recess is filled with metal to form a circuit. However, at this time, if the recess is not densely filled with metal, a circuit defect occurs, causing problems such as conduction failure. Therefore, it is preferable that the shape of the recess is a shape that facilitates the metal to be tightly filled, for example, a tapered shape in which the diameter decreases from the opening to the bottom.

  However, when the silicon carbide substrate is etched by the above-described conventional plasma etching method, the recess is not tapered but has a bowing shape in which the central portion of the side wall is curved in an arc. Thus, it is considered that the recess is formed into the bowing shape for the following reasons.

  That is, in the above-described conventional plasma etching method, as described above, the silicon carbide substrate is previously heated to the etching processing temperature, and etching is performed while maintaining the etching processing temperature. The bond is easy to break. Therefore, isotropic etching proceeds by the chemical reaction with the etching species such as radicals, and the shape of the recess formed in the silicon carbide substrate becomes the so-called bowing shape described above.

  As described above, when the shape of the recess is a bowing shape, not only the above-mentioned problem in the semiconductor manufacturing process occurs, but in addition, the film forming process is performed on the substrate by the chemical vapor deposition method (CVD method) At the same time, there also arises a problem that it becomes difficult to form a thin film on the side wall of the recess.

  Furthermore, in recent years, with the miniaturization of semiconductor elements, it is required not only to make the shape of the recess a tapered shape, but also to control the etching structure more precisely such as control of the taper angle. Also, in order to facilitate metal filling in the semiconductor manufacturing process, it is becoming more important to control the taper angle.

  Therefore, the inventors of the present invention conducted intensive studies on a plasma etching method capable of forming a tapered recess having a predetermined taper angle, and as a result, a raw material gas for forming a protective film containing a silicon-based gas and an oxygen gas. It is found that by simultaneously supplying the etching gas and the etching gas into the processing chamber at a predetermined flow ratio and etching the silicon carbide substrate, it is possible to form recesses having various taper angles determined according to the flow ratio. It came to

  The present invention was made as a result of intensive studies conducted by the present inventors, and provides a plasma etching method capable of forming a tapered recess having a desired taper angle in a silicon carbide substrate. To aim.

The way to achieve the above purpose is
A material gas for protective film formation containing at least a silicon-based gas and an oxygen gas and a reactive etching gas are simultaneously supplied into the processing chamber to be plasmatized, and carbonized placed on a base placed in the processing chamber A method of plasma etching a silicon substrate to form a tapered etched structure in the silicon carbide substrate, comprising:
A mask forming step of forming a mask having an opening on the surface of the silicon carbide substrate;
A ratio determined by the flow rate of the raw material gas for protective film formation and the flow rate of the reactive etching gas, which is set according to the angle between the side wall surface of the etching structure to be formed and the bottom surface Process,
The silicon carbide substrate is placed on a base in the processing chamber, the silicon carbide substrate is heated, and the reactive etching gas and the raw material gas for protective film formation have the set flow ratio. The carbon is supplied into the processing chamber at a predetermined flow rate to be plasmatized, and bias power is applied to a base on which the silicon carbide substrate is mounted to form a protective film on the silicon carbide substrate while the carbonization is performed. The present invention relates to a plasma etching method adapted to perform an etching step of etching a silicon substrate.

  According to this plasma etching method, first, when plasma etching a silicon carbide substrate, a mask having an opening on the surface of the silicon carbide substrate is formed.

  Next, a ratio determined by the flow rate of the raw material gas for protective film formation and the flow rate of the reactive etching gas is set according to the angle with respect to the bottom of the side wall surface in the etching structure to be formed on the silicon carbide substrate. The flow rate ratio is set to be a tapered shape having a taper angle of The value of the flow rate ratio is a value obtained by experimentally determining the correlation between the taper angle and the flow rate ratio, and for example, in determining the correlation between the taper angle and the flow rate ratio, reactive etching gas, silicon system Of the gas and oxygen gas, the flow rate of one of the gases may be fixed, the flow rate of the remaining gas may be changed to obtain correlation, or the flow rate of two gases may be fixed. It may be determined by changing the flow rate of the gas. Alternatively, with the flow rate of one gas fixed, the flow rate of each gas may be changed so as to equalize the remaining gas, and the correlation between the taper angle and the flow rate ratio may be obtained.

  Next, the silicon carbide substrate having a mask formed on the surface is placed on a base and heated. The heating temperature of the silicon carbide substrate is preferably 190 ° C. or higher, more preferably 200 ° C. to 1000 ° C., in order to effectively suppress the occurrence of the sub-trench and form the etching structure with high accuracy. .

  Thereafter, the reactive etching gas and the raw material gas for forming a protective film are simultaneously supplied into the processing chamber in a state in which the supply flow rate is adjusted so as to achieve the set flow ratio, and the reactive etching gas and the protective film The forming source gas is plasmatized. Thereafter, bias power is applied to the base on which the silicon carbide substrate is mounted, and plasma conversion is performed while forming a protection film (silicon dioxide) on the silicon carbide substrate by the plasma-formed material gas for forming a protection film. The silicon carbide substrate is etched by the etching gas.

  As described above, in the plasma etching method, the reactive etching gas and the protective film forming raw material gas are processed at a predetermined flow rate in the processing chamber such that a predetermined flow ratio is obtained according to the taper angle of the etching structure to be formed. At the same time, the concave portion (etching structure) having a predetermined taper angle can be formed by simultaneously supplying and plasmatizing the silicon carbide substrate and performing the etching process while protecting the silicon carbide substrate.

  It is considered that this is due to the correlation (eg, ratio) between the amount (the amount per time) of formation of the protective film on the side wall of the recess and the anisotropic etching rate by the ions and etching species. Hereinafter, a process of forming a tapered recess having a predetermined taper angle in a silicon carbide substrate will be described with reference to FIG. Incidentally, in FIG. 1, the symbols of K for the silicon carbide substrate, M for the mask, and H for the protective film are given.

  First, as shown in FIG. 1A, the portion of the silicon carbide substrate K located under the opening of the mask M has reactivity such as sputtering or radicals by ions generated by plasmatizing the reactive etching gas. While being etched by a chemical reaction with an etching species (hereinafter simply referred to as "etching species"), a reactive protective film-forming species (Si) such as Si derived from silicon-based gas (hereinafter simply referred to as "protective film formation" Chemical reaction between oxygen gas and protective film forming species (O) derived from oxygen gas, protective film forming species (Si) physically dissociated from silicon carbide substrate K by sputtering and protective film forming species derived from oxygen gas ( Protective film-forming species (S) derived from a chemical reaction with O) and a reaction product containing a silicon atom generated by etching a silicon carbide substrate ) And side wall protective film H of the recess by chemical reaction of the protective film forming species and (O) from the oxygen gas is formed. Thus, the silicon carbide substrate K is anisotropically etched. Incidentally, the protective film H is also etched not a little by sputtering by ions.

  Similarly, as shown in FIG. 1 (b), when the etching in the depth direction of the recess further proceeds while the side walls are protected by the protective film H, the bottom of the recess to be etched by ion sputtering is similarly performed thereafter. In the part where the distance between them is far, it is difficult to supply the protective film forming species (Si) physically dissociated from the silicon carbide substrate, but the material gas for forming the protective film contains a silicon-based gas, Since the protective film H is further formed by the chemical reaction between the protective film-forming species (Si) derived from the silicon-based gas and the protective film-forming species (O) derived from the oxygen gas, the protective film H is etched Its thickness will increase. Further, the bottom of the recess is physically dissociated from the silicon carbide substrate K, as in the above, with the chemical reaction between the protective film-forming species (Si) derived from the silicon gas and the protective film-forming species (O) derived from the oxygen gas. Chemical reaction between the protective film forming species (Si) and the protective film forming species (O) derived from oxygen gas, or the protective film forming species (Si) derived from a reaction product containing silicon atoms generated by etching and oxygen gas derived While the protective film H is formed on the side wall in the vicinity of the bottom by the chemical reaction with the protective film forming species (O), the sputtering by the ions and the etching by the chemical reaction with the etching species proceed. Under the present circumstances, as above-mentioned, the thickness of the protective film H formed in the side wall increases as etching progresses, and the opening part of the recessed part which is an entrance of the ion and etching species involved in etching gradually As the width of the recess bottom gradually narrows, the width of the recess to be formed gradually narrows from the opening to the bottom.

  Then, while the thickness of the protective film H formed on the side wall of the recess is increased, the etching in the depth direction of the silicon carbide substrate proceeds, and finally, as shown in FIG. A tapered recess is formed. Incidentally, the protective film H which becomes unnecessary in the post process is appropriately removed through the process.

  In the plasma etching method, as described above, the protective film is formed on the side wall of the recess also by the chemical reaction between the protective film forming species (Si) derived from the silicon-based gas and the protective film forming species (O) derived from the oxygen gas. H is to be formed. Therefore, a sufficient amount of protective film H is formed on the side wall of the recess without increasing the amount of protective film forming species (Si) which is dissociated by sputtering. It is possible to suppress the occurrence of sub-trenches caused by being physically etched.

In the present application, the “tapered shape” means that the width of the opening is wider than the width of the bottom in the recess, and the entire side wall is substantially straight. Further, in the present application, as shown in FIG. 1C, the angle (complementary angle θ) formed by the bottom surface and the side wall of the recess is defined as the “taper angle”. Assuming that the width of W is W1, the width of the bottom is W2, and the depth from the opening to the bottom is D, it can be calculated from the following equation.
(Equation 1)
θ (°) = tan ⁻¹ [D / {(W 1 −W 2) / 2}]

  In addition, as shown in FIG. 2, the state in which the sub-trench as referred to in the present application is generated refers to the difference Δd between the reference line SL passing through the lower end of the straight portion in the side wall and the etching structure bottom TB as the roundness When the roundness of the bottom of the etching structure TB below the SL (see the left figure in FIG. 2) is defined as “+”, and the roundness of the upper case (see the right in FIG. 2) is defined as “−” The case where the round degree is “−” is referred to, and the state where the sub-trench is not generated means the case where the round degree is 0 or more.

  In the plasma etching method, the taper angle of the recess to be formed depends on the ratio determined by the flow rate of the protective film forming raw material gas and the flow rate of the reactive etching gas. Therefore, in the flow rate ratio setting step, by setting the flow rate ratio according to the taper angle of the recess to be formed in the etching step, the taper angle of the recess formed in the etching step can be made a desired angle.

  As described above, the protective film H is also formed by the chemical reaction between the protective film forming species (Si) physically dissociated from the silicon carbide substrate K by sputtering and the protective film forming species (O) derived from oxygen gas. Therefore, it is possible to control the amount of these two protective film forming species and adjust the speed at which the protective film H is formed to make the taper angle of the recess a desired angle as described above. it is conceivable that. However, the amount of protective film-forming species (Si) physically dissociated from the silicon carbide substrate and the amount of protective film-forming species (Si) derived from the reaction product generated by etching are the magnitudes of the bias power applied to the substrate. It depends on several factors such as the pressure in the processing chamber, the flow rate of the gas, and the like. Therefore, the protective film H is formed by the chemical reaction between the protective film forming species (Si) dissociated from the silicon carbide substrate and the protective film forming species (Si) derived from the reaction product and the protective film forming species (O) derived from oxygen gas. If it is formed, in order to form a recess having a desired taper angle, a protective film forming species (see FIG. 2) which dissociates from the silicon carbide substrate by appropriately setting the flow rate of the etching gas, the magnitude of bias power, and the pressure in the processing chamber. The amount of Si) and the amount of protective film forming species (Si) derived from the reaction product must be strictly controlled, and the control is extremely complicated and not easy.

  On the other hand, in the plasma etching method, as described above, the protective film H is also formed by the chemical reaction between the protective film forming species (Si) derived from the silicon-based gas and the protective film forming species (O) derived from the oxygen gas. By setting the flow rate ratio, it is possible to easily and accurately form a recess having a desired taper angle.

  Also, conventionally, since the amount per unit area of the protective film forming species (Si) derived from the silicon carbide substrate is larger in the central portion than in the substrate peripheral portion, the central portion is formed rather than the substrate peripheral portion. The amount of the protective film is large, and as a result, the etching shape formed on the substrate is uneven. On the other hand, in the plasma etching method, as described above, by supplying the silicon-based gas, the protective film forming species (Si) derived from the silicon-based gas is uniformly spread over the entire substrate. Thereby, in addition to the protective film forming species (Si) derived from the silicon carbide substrate, the protective film forming species (Si) not depending on the etching amount also participates in the formation of the protective film, so the protective film derived from the silicon carbide substrate It becomes less susceptible to the wafer in-plane difference of the amount per unit area of the forming species (Si). Therefore, the angle of the etching structure can be precisely adjusted, and the uniformity of the shape can be maintained.

The raw material gas for forming a protective film is a mixed gas of a silicon-based gas containing halogen and an oxygen (O ₂ ) gas, and the silicon-based gas containing halogen is a silicon tetrafluoride (SiF ₄ ) gas or It is any one gas of silicon tetrachloride (SiCl ₄ ) gas.

  In this case, the silicon-based gas containing halogen is plasmatized to generate etching species such as halogen ions and halogen radicals, and the silicon carbide substrate is etched by sputtering with halogen ions and a chemical reaction with the etching species. As described above, since the silicon-based gas participates not only in the formation of the protective film but also in the etching of the silicon carbide substrate, the etching rate of the silicon carbide substrate is increased while protecting the side wall of the recess by the protective film. Can.

The reactive etching gas is preferably sulfur hexafluoride (SF ₆ ) gas, and the raw material gas for forming a protective film is a mixed gas of silicon tetrafluoride (SiF ₄ ) gas and oxygen gas. Is preferred.

  Further, in the case where the flow rate ratio determined in accordance with the angle is a ratio (first flow rate ratio) of the flow rate of the reactive etching gas to the flow rate of the raw material gas for protective film formation, the first flow rate ratio is It may be set to 0.140 or more and 0.35 or less. In this way, it is possible to form a recess having a desired taper angle while relatively suppressing the occurrence of so-called sub-trench.

  Furthermore, the first flow rate ratio may be set to 0.140 or more and 0.30 or less. In this way, it is possible to form a recess having a desired taper angle while suppressing generation of sub-trench more reliably.

The flow rate determined according to prior Symbol angle, to the sum of the flow rates between the reactive etching gas in the protective film forming material gas, the flow rate ratio of the silicon-based gas (the second flow ratio) In this case , setting the second flow ratio to greater than 0.40 and less than or equal to 0.632 can relatively suppress the occurrence of subtrenches, and may be greater than 0.4 and less than or equal to 0.6 Makes it possible to suppress the occurrence of sub-trench more effectively.

In addition, SF ₆ gas is used as a reactive etching gas, a mixed gas of SiF ₄ gas and oxygen gas is used as a source gas for forming a protective film, the flow rate of two of these gases is fixed, and the remaining gases are fixed. Changing the value of the first flow ratio by changing the flow rate of the first flow ratio, the value of the first flow ratio is set within the range of 0.140 to 0.186 according to the experimental results of the present inventors. Thus, it is possible to form a tapered recess having a desired taper angle in the range of 85.2 ° to 87.5 °.

In addition, the raw material gas for protective film formation may be a mixed gas of the same amount of SiF ₄ gas and O ₂ gas. In this case, according to the experimental results of the inventors of the present invention, by setting the value of the first flow ratio within the range of 0.250 to 0.333, the value within the range of 81.0 ° to 88.8 ° It is possible to form a tapered recess having a desired taper angle, and in this case, by increasing the value of the first flow ratio, the taper angle of the recess to be formed can be increased. The recess having a taper angle of can be easily formed.

  The reason for this is that the value of the first flow rate ratio increases, in other words, if the etching process is performed with the flow rate of the raw material gas for protective film formation reduced with respect to the flow rate of the reactive etching gas, the depth direction Since the anisotropic etching rate to the surface is relatively faster than the rate at which the protective film is formed on the side walls, the taper angle of the finally formed recess is increased. Also, conversely, if the etching process is performed in a state in which the value of the first flow ratio decreases, that is, the flow rate of the raw material gas for protective film formation is increased relative to the flow rate of the reactive etching gas, The anisotropic etching rate is relatively slow with respect to the rate at which the protective film is formed, which is considered to be because the taper angle of the formed recess is reduced.

  In this case, if the value of the first flow ratio is smaller than 0.250, the amount of the protective film to be formed will be large, and the etching in the depth direction of the recess will not progress easily. In addition, when the first flow ratio is more than 0.333, the amount of the protective film formed is too large, and the etching in the depth direction of the recess is difficult to progress.

  In the above-mentioned plasma etching method, in order to properly advance the etching of the silicon carbide substrate, the bias potential (Vpp) between the processing chamber and the silicon carbide substrate is applied to the base so as to be 430 V or more. It is preferable to set the magnitude of the bias power.

  As described above, according to the above-mentioned plasma etching method, a uniform tapered recess can be formed in a silicon carbide substrate, and further, the taper angle can be made a desired angle, and the sub-trench can be formed. It is possible to form a recess with a desired taper angle while suppressing the occurrence.

It is explanatory drawing for demonstrating the process in which a taper-shaped recessed part is formed in a silicon carbide board | substrate. It is explanatory drawing for demonstrating a round degree. It is sectional drawing which showed schematic structure of the plasma etching apparatus for enforcing the plasma etching method. It is a table summarizing the definitive etching conditions and the results in Example 1-3. It is an explanatory view for explaining the relationship between the taper angle of the concave portion for forming a first flow rate ratio in Example 1-3 and (SF ₆ gas / (SiF ₄ gas + _{O 2} gas)) in the silicon carbide substrate. It is the table | surface which put together the etching conditions and the result in Comparative Examples 1-6. It is explanatory drawing for demonstrating the loading effect, (a) shows the case where SiF ₄ gas is not used, (b) shows the case where SiF ₄ gas is used. It is the table | surface which put together the etching conditions and the result in Examples 4-10. It is a table summarizing the etching conditions and results in Example 11-18. Is a table summarizing the results of etching in Example 1 9-2 1. It is a graph showing the relationship between the substrate surface temperature and round degree in Example 1 9-2 1.

  Hereinafter, specific embodiments of the present invention will be described based on the attached drawings.

  First, an etching apparatus 1 for performing a plasma etching method will be described. The etching apparatus 1 includes a processing chamber 11 having a closed space, a base 15 disposed so as to be able to move up and down in the processing chamber 11, and a base 15 on which the silicon carbide substrate K is placed, and an elevator that raises and lowers the base 15. A cylinder 18, a gas supply device 20 for supplying an etching gas, a raw material gas for forming a protective film and an inert gas into the processing chamber 11, an etching gas supplied into the treatment chamber 11, a raw material gas for forming a protective film and It comprises a plasma generation device 30 for converting the active gas into plasma, a high frequency power supply 35 for supplying high frequency power to the base 15, and an exhaust device 40 for reducing the pressure in the processing chamber 11.

  The processing chamber 11 is composed of an upper chamber 12 and a lower chamber 13 having an internal space in communication with each other, and the upper chamber 12 is smaller than the lower chamber 13. The base 15 is composed of an upper member 16 on which the silicon carbide substrate K is placed and a lower member 17 to which the elevating cylinder 18 is connected, and is disposed in the lower chamber 13.

The gas supply apparatus 20, as the etching gas, SF ₆ and SF ₆ gas supply unit 21 for supplying a gas, the protective film as a raw material for forming a gas, SiF ₄ gas and _{O 2} gas supplied respectively SiF ₄ gas supply unit 22 And O ₂ gas supply unit 23, an inert gas supply unit 24 that supplies, for example, Ar gas as an inert gas, one end is connected to the upper surface of the upper chamber 12, and the other end is branched to be the SF ₆ A gas supply unit 21, a SiF ₄ gas supply unit 22, an O ₂ gas supply unit 23, and a supply pipe 25 connected to an inert gas supply unit 24 are provided, respectively, and an SF ₆ gas supply unit 21, a SiF ₄ gas supply unit 22, O ₂ gas supply unit 23 and the respective SF ₆ gas into the processing chamber 11 via the supply pipe 25 from the inert gas supply unit 24, SiF ₄ gas, supplies _{O 2} gas and an inert gas

The plasma generation device 30 is a device that generates so-called inductively coupled plasma (ICP), and includes an annular coil 31 disposed in the upper chamber 12, and a high frequency power supply 32 that supplies high frequency power to the respective coils 31. The high frequency power supply 32 supplies high frequency power to the coil 31 to plasmatize the SF ₆ gas, the SiF ₄ gas, the O ₂ gas and the inert gas supplied into the upper chamber 12.

Further, the high frequency power supply 35 supplies high frequency power to the base 15 to apply a bias potential between the base 15 and the plasma, thereby to supply SF ₆ gas, SiF ₄ gas, O ₂ gas and inert gas. The ions generated by the plasma formation of the above are made incident on the silicon carbide substrate K mounted on the base 15.

  The exhaust device 40 comprises a vacuum pump 41 for exhausting a gas, and an exhaust pipe 42 having one end connected to the vacuum pump 41 and the other end connected to the side surface of the lower chamber 13, via the exhaust pipe 42. A vacuum pump 41 evacuates the gas in the processing chamber 11 to maintain the inside of the processing chamber 11 at a predetermined pressure.

  Next, a method of plasma etching silicon carbide substrate K using etching apparatus 1 configured as described above will be described.

  First, in plasma etching the silicon carbide substrate K, the silicon carbide substrate K is subjected to a mask formation process.

  By this mask formation process, a mask is formed on the surface of silicon carbide substrate K using, for example, a vapor deposition method (chemical vapor deposition (CVD) or physical vapor deposition (PVD)) or the like, and then the mask Form a predetermined mask pattern with an opening. In the present embodiment, the mask is made of nickel (Ni), but is not limited to this. For example, the mask may be made of another metal mask or silicon dioxide.

Next, in order to make the concave portion (etching structure) formed in silicon carbide substrate K into a tapered shape having a desired taper angle, an angle formed by the sidewall surface and the bottom surface in the concave portion to be formed in silicon carbide substrate K A first flow rate ratio which is a ratio of the flow rate of SF ₆ gas to the total flow rate of SiF ₄ gas and O ₂ gas (SF ₆ gas / (SiF ₄ gas + O ₂ gas)) which is determined according to the angle) is set. The value of the first flow ratio, while a previously experimentally value obtained a correlation between the taper angle and the flow rate ratio, in the following description, with a fixed flow rate of the SF ₆ gas, SiF ₄ It is a value obtained by changing the flow rate of each gas so that the amount of gas and O ₂ gas become equal, and the numerical range is 0.250 to 0.333. Further, the setting of the first flow rate ratio may be made by adjusting the flow rate of gas for each of the gas supply units 21, 22, 23 so that the first flow rate ratio corresponds to the target taper angle, for example. Alternatively, the flow rate of the gas supplied from each of the gas supply units 21, 22, 23 may be appropriately set automatically by the control device by specifying the desired taper angle. When the flow rates of the respective gases are automatically set, the supply flow rates of the respective gases corresponding to the first flow rate ratio determined according to the predetermined taper angle are stored in advance in the control device.

  Next, a plasma etching process is performed on the silicon carbide substrate K on which the mask is formed.

  First, silicon carbide substrate K is carried into etching apparatus 1 and mounted on base 15, and the temperature of silicon carbide substrate K is heated to a predetermined etching processing temperature within the temperature range of 200 ° C. to 1000 ° C. Do. Specifically, the inert gas is supplied from the inert gas supply unit 24 into the processing chamber 11, and high frequency power is applied to the coil 31 and the base 15 by the high frequency power supplies 32 and 35. As a result, the inert gas supplied into the processing chamber 11 is plasmatized, and the ions generated by the plasmatization are mounted on the pedestal 15 by the bias potential generated by applying the high frequency power to the pedestal 15. The light impinges upon the placed silicon carbide substrate K. Then, the energy of the ions colliding with the silicon carbide substrate K is absorbed, the temperature rises, and eventually the equilibrium state is reached at the etching processing temperature. The pressure in the processing chamber 11 is maintained at a predetermined pressure by the exhaust device 40.

Then, the silicon carbide substrate K whose temperature has reached equilibrium at the etching temperature is etched. Specifically, SF ₆ gas, SiF ₄ gas and O ₂ gas are respectively supplied from the respective gas supply units 21, 22, 23 into the processing chamber 11 so as to have the first flow ratio set as described above, and a high frequency power supply High frequency power is applied to the coil 31 and the base 15 by means of 32 and 35. Thereby, the SF ₆ gas, the SiF ₄ gas, and the O ₂ gas supplied into the processing chamber 11 are plasmatized.

Then, the silicon carbide substrate K is etched by fluorine ions or etching species (for example, fluorine radicals) generated by plasmatizing SF ₆ gas and SiF ₄ gas, and a protective film forming species generated by plasmatizing O ₂ gas (O) is a protective film forming species (Si) generated by sputtering the silicon carbide substrate K by ions (Si), a protection derived from a reaction product (SiF ₄ ) containing silicon atoms generated by etching the silicon carbide substrate K A protective film is formed on the surface of the silicon carbide substrate K by reacting with the film forming species (Si) and the protective film forming species (Si) derived from the SiF ₄ gas. Thus, the etching of silicon carbide substrate K and the formation of the protective film are performed in parallel, and while being protected by the protective film, the anisotropic etching of silicon carbide substrate K proceeds to form silicon carbide substrate K. A tapered recess having the predetermined taper angle is formed.

  Further, in this plasma etching method, since the taper angle of the recess formed is determined by the first flow ratio, the recess having a desired taper angle can be formed by appropriately setting the first flow ratio.

  Thus, it is considered that the shape of the recess is tapered and the taper angle changes according to the first flow rate ratio because of the following reason.

That is, in this plasma etching method, SF ₆ gas, SiF ₄ gas and O ₂ gas are simultaneously supplied for plasmatization, whereby the protective film forming species physically dissociated from the silicon carbide substrate K ( A protective film-forming species (Si) derived from a reaction product (SiF ₄ ) containing a silicon atom generated by a chemical reaction between Si) and a protective film-forming species (O) derived from O ₂ gas and etching of a silicon carbide substrate K Not only by the chemical reaction with the protective film-forming species (O) derived from oxygen gas, but also by the chemical reaction between the protective film-forming species (Si) derived from SiF ₄ gas and the protective film-forming species (O) derived from O ₂ gas A protective film is formed. Therefore, as the etching progresses, the thickness of the protective film formed on the side wall increases, the opening of the recess gradually narrows, and the width of the portion to be etched, that is, the bottom of the recess gradually narrows. . Therefore, the width of the concave portion to be finally formed has a tapered shape in which the diameter is reduced from the opening side toward the bottom side. Then, when the first flow rate ratio to be set is changed, the anisotropic etching rate in the depth direction relative to the rate at which the protective film is formed changes relatively, so that recesses having finally different taper angles are formed. The

Incidentally, the inventors of this application apply this plasma etching method to set a predetermined flow rate within 30 to 40 sccm so that the flow rates of the SF ₆ gas are 20 sccm, the SiF ₄ gas and the O ₂ gas are equal to each other. At the same time, the silicon carbide substrate on which the Ni mask was formed was subjected to an etching process by supplying a coil applied power of 2000 W, a bias power of 700 W, and a pressure in the process chamber of 0.7 Pa. Incidentally, FIG. 4 is a table summarizing the etching conditions and the results in Example 1-3. Further, FIG. 5 is a view showing a shape of a recess formed in the etching conditions in Example 1 to 3, (a) Example 1, (b) Example 2, (c) is It shows the shape of the concave formed in example 3. In FIG. 5, reference symbol K indicates a silicon carbide substrate, reference symbol M indicates a mask, and reference symbol H indicates a protective film.

As shown in FIGS. 4 and 5, in the embodiment 1 in which the values of the first flow ratio 0.250 taper angle θ1 is formed a recess of 81.0 ° (see FIG. 5 (a)), the 1 the value of the flow ratio in example 2 was 0.286 taper angle θ2 is formed a recess of 84.4 ° (see FIG. 5 (b)), and the value of the first flow rate ratio is set to 0.333 in example 3 the taper angle θ3 formed recess of 88.8 ° (see FIG. 5 (c)).

As described above, in each case, the entire side wall has a tapered recess, and the taper angle increases as the value of the first flow rate ratio increases. Thus, as the value of the first flow ratio increases, the taper angle increases because the value of the first flow ratio increases, in other words, SF _{6 with} respect to the total flow of SiF ₄ gas and O ₂ gas. It is considered that the anisotropic etching rate in the depth direction becomes relatively faster than the rate at which the protective film H is formed on the side wall of the recess due to the increase of the gas flow rate.

Also, in order to examine the influence of the SiF ₄ gas as a raw material gas for forming a protective film on the etching shape to be formed, the inventors of the present invention have predetermined conditions of 40 sccm of SF ₆ gas and 0 to 100 sccm of O ₂ gas. A comparative experiment was conducted in which a silicon carbide substrate on which a Ni mask was formed was etched by supplying a coil power of 2500 W, a bias power of 700 W, and a pressure of 3 Pa in the processing chamber at the same time. . In addition, FIG. 6 is the table | surface which put together each etching conditions in the comparative examples 1-6, and its result. Further, as shown in FIG. 2, the “roundness” in the same figure is the difference Δd between the reference line SL passing through the lower end of the straight portion in the side wall of the etching structure and the etching structure bottom TB. Is defined as "+" when it is below the reference line SL and "-" when it is above, and when the round degree is less than 0, a sub-trench is generated When the round degree is 0 or more, it is assumed that no sub-trench has occurred.

As shown in FIG. 6, in Comparative Examples 1 to 3, although the sub-trench is not generated, the shape of the recess is a bowing shape, while in Comparative Examples 4 to 6, the shape of the recess is not a bowing shape. However, sub-trench has occurred. That is, when only O ₂ gas is used as the raw material gas for forming a protective film, it is impossible to form a recess having neither a sub-trench nor a bowing shape.

On the other hand, in this plasma etching method, since the SiF ₄ gas and the O ₂ gas are used as the source gases for forming the protective film, the tapered shape can be obtained while minimizing the generation of the sub-trench. Can be formed.

Furthermore, in this plasma etching method, as described above, since the SiF ₄ gas not only participates in the formation of the protective film but also participates in the etching of the silicon carbide substrate, the side wall of the recess is formed by the protective film. The etch rate of the silicon carbide substrate can be increased while protecting the As described above, the SiF ₄ gas contained in the protective film forming raw material gas also participates in the etching of the silicon carbide substrate. Therefore, since the flow ratio between the reactive etching gas and the raw material gas for forming a protective film can not be predicted simply, as described above, it is necessary to experimentally determine the correlation between the taper angle and the flow ratio.

Meanwhile, in the plasma etching method without using a silicon-based gas such as SiF ₄ gas by a chemical reaction between the etching species from the etching gas, such as silicon carbide and SF ₆ gas, such as contributing SiF ₄ to the formation of the protective film is produced (See Fig. 7 (a)). However, the amount of etching at the central portion is larger at the central portion and the peripheral portion of the substrate, so SiF ₄ or the like is larger at the central portion than at the peripheral portion of the substrate. Many are generated. Therefore, in the central portion of the substrate, the formation of the protective film is promoted more than the peripheral portion, and a difference occurs between the etching rate of the central portion of the substrate and the etching rate of the peripheral portion, and it is difficult to uniformly etch the entire substrate. Met.

In contrast, in the plasma etching process, by supplying the SiF ₄ gas, a sufficient amount of the protective film forming species derived from the SiF ₄ gas (Si) is distributed uniformly across to the peripheral portion of the substrate. Thereby, as shown in FIG. 7B, not only the protective film forming species (Si) derived from SiF ₄ generated by the chemical reaction of silicon carbide and the etching species, but the amount thereof is not dependent on the etching amount _Since the protective film-forming species (Si) derived from ₄ also participates in the formation of the protective film, the angle of the recess to be formed can be precisely adjusted as compared with the case where SiF ₄ gas is not used, and the entire substrate It is possible to make the shape of the recess in the uniform.

  As mentioned above, although the one form of the plasma etching method was demonstrated, the aspect which this plasma etching method can take is not limited to this at all.

In the above example, the value of the first flow rate ratio is obtained by changing the flow rates of the respective gases such that the SiF ₄ gas and the O ₂ gas have the same amount, with the flow rate of the SF ₆ gas fixed. However, the flow rate of the SiF ₄ gas and the flow rate of the O ₂ gas do not necessarily have to be the same. Further, the value of the first flow rate ratio to be set is not limited to the value obtained by changing the flow rate of the remaining gas while fixing the flow rate of one gas, and while the flow rates of two gases are fixed, It may be a value obtained by changing the flow rate of the remaining gas.

By the way, the inventors of the present invention simultaneously supply ₆ to 8 sccm of SF ₆ gas, 24 to 26 sccm of SiF ₄ gas, and O ₂ gas at a predetermined flow rate of 16 to 20 sccm, respectively, into the processing chamber to apply coil application power. The silicon carbide substrate on which the SiO ₂ mask was formed was etched using 1750 W, a bias power of 700 W, and a pressure in the processing chamber of 0.7 Pa. In all cases, the entire sidewall had a linear taper. Shaped recesses formed. FIG. 8 is a table summarizing the etching conditions and the results in Examples 4 to 10.

As can be seen from FIG. 8, even if the flow rates of SF ₆ gas, SiF ₄ gas and O ₂ gas are respectively changed to set a predetermined first flow rate ratio, the value of the first flow rate ratio is 0.140 to 0. Between .186, it is possible to form a tapered recess having a predetermined angle in the range of 85.2 ° to 87.5 °.

  In this plasma etching method, by setting the value of the first flow ratio to 0.140 or more and 0.35 or less, the generation of sub-trench can be suppressed as much as possible, and the value is 0.140 or more and 0.30 or less. The setting can further suppress the occurrence of sub-trench.

Incidentally, in order to examine the relationship between the value of the first flow ratio and the presence or absence of the occurrence of the sub-trench, the inventors of the present invention use SF ₆ gas, SiF ₄ gas and O ₂ gas as the first flow ratio. Is supplied to the processing chamber at a predetermined flow rate, the coil application power is 1750 W, the bias power is 200 to 400 W, and the pressure in the processing chamber is 0.1. An experiment was conducted to etch a silicon carbide substrate having a SiO ₂ mask formed at 8 Pa. Incidentally, FIG. 9 is a table summarizing the etching conditions and results in Example 11-18.

As shown in FIG. 9, in Example 11 in which the value of the first flow ratio was 0.26, the shape of the formed recess was neither a bowing shape nor a sub-trench. On the other hand, in Example 12 and Example 13 in which the first flow ratio was 0.31 and 0.35, although a small sub-trench was formed, a recess not having a bowing shape was formed, and a first flow ratio was also formed. in example 1 4 the value was 0.39, although larger subtrenches than examples 12 and 13 are formed, similarly, the recess not bowing shape is formed. Here, since it is preferable that the round degree is larger than −0.20, as described above, the value of the first flow ratio is preferably 0.35 or less, and in order to further increase the round degree, 0 It is preferable to set it as .30 or less. The bias potential (Vpp) between the processing chamber and the silicon carbide substrate in Examples 11 to 14 was 544 V, 542 V, 544 V, and 558 V, respectively.

  Here, focusing on the results of Examples 15 to 18, in Examples 15 and 16 in which the value of the first flow ratio is 0.23 which is 0.30 or less, it depends on the magnitude of the bias power. Also, the shape of the formed recess is neither a bowing shape nor occurrence of sub-trench. On the other hand, interesting results were obtained in Examples 17 and 18 in which the value of the first flow ratio was 0.33 which was larger than 0.30. In Example 18 in which the bias power was set to 250 W, a slight sub-trench was formed, whereas in Example 17 in which the bias power was set to 200 W, no generation of sub-trenches was observed. From this, it can be understood that the generation of the sub-trench can be suppressed by setting the bias power to an appropriate magnitude even in the range of the value of the first flow ratio at which the sub-trench can occur.

Moreover, the inventors have, to a bias potential between the processing chamber and the silicon carbide substrate (Vpp) to investigate the effect on the progress of etching, 11 sccm of SF ₆ gas, 30 sccm of SiF ₄ gas, _{O 2} A gas was supplied into the processing chamber at a flow rate of 18 sccm, and the silicon carbide substrate on which the SiO ₂ mask was formed was subjected to etching processing under different conditions of Vpp. did. On the other hand, when Vpp was 400 V, although etching progressed, the presence of residues was confirmed, and when it was 240 V, etching stop occurred. From this, it was found that the etching proceeds normally by etching the silicon carbide substrate under the condition that Vpp is 430 V or more.

Furthermore, in the above example, the flow rate ratio determined in accordance with the taper angle is the ratio of the flow rate of the reactive etching gas (SF ₆ gas) to the flow rate of the protective film forming material gas (SiF ₄ gas + O ₂ gas). Although the first flow rate ratio, the flow rate ratio of to the sum of the flow rates of the reactive etching gas and flow rate of the coercive Mamorumaku forming raw material gas, a silicon-based gas (SiF ₄ gas) (SiF ₄ gas / (SF ₆ gas The second flow rate ratio may be + SiF ₄ gas + O ₂ gas).

  In this case, as shown in FIG. 9, by setting the value of the second flow rate ratio to be larger than 0.40 and not larger than 0.632, the roundness becomes larger than −0.20 and generation of sub-trench is suppressed as much as possible. By setting the value of the second flow rate ratio to greater than 0.4 and less than or equal to 0.6, the occurrence of sub-trench can be further suppressed.

Also, in order to examine the influence of the temperature of the silicon carbide substrate at the start of etching on the occurrence of sub-trench, the present inventors changed three different types of silicon carbide substrate surface by changing the heating time of the silicon carbide substrate. After changing the temperature, an experiment was performed to etch the silicon carbide substrate. Incidentally, the preheating step supplies Ar gas into the processing chamber at a flow rate of 100 sccm, a coil applied power 2500W, the bias power 100W, the pressure in the processing chamber as 3 Pa, 10 seconds in Example 1 9, 25 seconds in example 2 0, in example 2 1 was carried out for 40 seconds processing. In the etching process, the flow rate of 9 sccm of SF ₆ gas, 12 sccm of SiF ₄ gas, 20 sccm of O ₂ gas, and Ar gas of 150 sccm is simultaneously supplied to the processing chamber, coil applied power is 2000 W, bias power is 330 W The pressure in the processing chamber was 0.8 Pa, and each reference example was processed for 3 seconds. Figure 10 is a table summarizing the results about the Example 1 9-2 1, 11 is a graph showing the relationship between the substrate surface temperature and round degree in Example 1 9-2 1 .

As shown in FIG. 10, as the processing time of the preheating step is longer, the amount of ions incident on and collided with the silicon carbide substrate is increased, so the surface temperature of the substrate is higher. Moreover, whereas in the embodiment 2 0 surface temperature of the substrate is Example 1 9 and 180 ° C. is 130 ° C., round degree is less than 0, the sub-trenches occurs in the etched structures, Oite example 2 1 surface temperature of the substrate is 220 ° C. is a round degree is 0 or more, the sub-trench is not generated. From this, it was found that when the etching conditions are the same, generation of sub-trench can be suppressed by raising the substrate surface temperature at the start of etching.

  Incidentally, as shown in FIG. 11, in order to make the roundness 0 or more, in other words, to effectively suppress the occurrence of the sub-trench, preheating is performed so that the surface temperature of the silicon carbide substrate is 190 ° C. or more. The processing time of the process may be adjusted.

In the above example, SF ₆ gas is used as the etching gas, but, for example, NF ₃ gas or F ₂ gas may be used. Furthermore, although SiF ₄ gas is used as the silicon-based gas, for example, silicon tetrachloride (SiCl ₄ ) gas which is a compound of silicon and chlorine which is a homologous element of fluorine may be used. Even in this case, as in the case of using the SiF ₄ gas, chloride ions and chlorine radicals generated by plasmatizing the SiCl ₄ gas are involved in the etching of the silicon carbide substrate, so that the corresponding effects can be obtained. Be

Furthermore, in the above example, the flow rates of SF ₆ gas, SiF ₄ gas and O ₂ gas are not particularly limited, but the peripheral portion of the silicon carbide substrate has less silicon carbide as compared to the central portion, ie, Since the amount of etching is small, the amount of protective film forming species (Si) derived from the reaction product generated by etching is small. Therefore, there is a difference in the amount of protective film forming species (Si) supplied between the peripheral portion and the central portion, so that at least this difference can be ignored so that the SiF ₄ gas can be supplied. Preferably, a sufficient amount of protective film forming species (Si) is distributed.

  In the above example, the plasma generator 30 has the coil 31 disposed in the upper chamber 12, but, for example, as an inductively coupled plasma (ICP) generator configured to be disposed above the top plate of the upper chamber 12. Also good.

DESCRIPTION OF SYMBOLS 1 etching apparatus 11 process chamber 15 base 20 gas supply apparatus 21 SF ₆ gas supply part 22 SiF ₄ gas supply part 23 O ₂ gas supply part 24 inert gas supply part 30 plasma generation apparatus 31 coil 32 high frequency power supply 35 high frequency power supply 40 Exhaust system K Silicon carbide substrate M Ni mask (mask)
H protective film

Claims (7)

The raw material gas for protective film formation containing at least a silicon-based gas and an oxygen gas and the reactive etching gas are simultaneously supplied into the processing chamber to be plasmatized, and are mounted on a base placed in the processing chamber. A method of plasma etching a silicon carbide substrate comprising carbon and silicon to form an etched structure in the silicon carbide substrate,
A mask forming step of forming a mask having an opening on the surface of the silicon carbide substrate;
Performing an etching step of etching the silicon carbide substrate while forming a sidewall protective film on the silicon carbide substrate;
The silicon-based gas is any one of silicon tetrafluoride gas and silicon tetrachloride gas,
The reactive etching gas is a gas selected from sulfur hexafluoride gas, nitrogen trifluoride gas and fluorine gas,
The supply flow rate of the silicon-based gas into the processing chamber is set to 12 to 40 sccm, and the protective film-forming species derived from the silicon-based gas is uniformly distributed from the central portion to the peripheral portion of the silicon carbide substrate. A plasma etching method characterized in that.   The plasma etching method according to claim 1, wherein the silicon carbide substrate placed on the base is heated to 190 ° C. or more in the etching step.   The plasma etching method according to claim 1, wherein the silicon carbide substrate placed on the base is heated to 200 ° C. or higher in the etching step.   4. The method according to any one of claims 1 to 3, wherein, in the etching step, a bias power having a magnitude such that a bias potential (Vpp) between the processing chamber and the silicon carbide substrate is 430 V or more is applied to the base. Plasma etching method.   The plasma etching method according to any one of claims 1 to 4, further comprising a removing step of removing the sidewall protective film. The reactive etching gas is sulfur hexafluoride gas,
The plasma etching method according to any one of claims 1 to 5, wherein the silicon-based gas is silicon tetrafluoride gas. The plasma etching method according to any one of claims 1 to 6, wherein the etching structure formed on the silicon carbide substrate has a tapered shape.