Method for forming trench in silicon carbide substrate

Abstract

To provide a plasma processing method that can form a trench structure having an improved steepness around the bottom in a simple manner on a silicon carbide substrate.SOLUTION: A plasma processing method for forming a trench in a silicon carbide substrate according to the present invention performs a reactive ion etching process using plasma while maintaining the silicon carbide substrate, which is an etching target, at a temperature of -20°C or less, to form a trench in the silicon carbide substrate. The temperature of the silicon carbide substrate during the plasma processing is preferably -100°C or less and -196°C or more.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for forming trenches in a silicon carbide substrate. [Background technology]

[0002] In trench MOSFETs used as power transistors, the gate is composed of an electrode placed inside a trench dug in the substrate via an insulating film. If the trench has a steep edge, when a voltage is applied to the gate electrode, an electric field will concentrate at the steep edge, destroying the insulating film on the inner surface of the trench. Therefore, it is desirable to form the bottom of the trench in a U-shape in order to increase the breakdown voltage of the device.

[0003] As one such method, Patent Document 1 discloses a method having a first step of forming a trench on a semiconductor substrate by performing anisotropic etching, a second step of processing the corners of the bottom of the trench into a forward tapered shape by performing forward taper etching after the first step, and a third step of rounding the corners of the bottom of the trench by performing isotropic etching after the second step. Specifically, as the second step of processing the corners of the bottom of the trench into a forward tapered shape, a method of etching while depositing a protective film (silicon oxide film) on the sidewall of the trench by increasing the oxygen concentration in an etching tank, and a method of decreasing the vertical component of the incident direction of ions by increasing the pressure in the etching tank and decreasing the input high frequency output (power) are disclosed.

[0004] Patent Document 1 targets a silicon substrate as a semiconductor substrate, but for higher voltage applications, silicon carbide trench MOSFETs using silicon carbide (SiC) as a substrate, which has a higher voltage resistance, are used. Silicon carbide has a smaller crystal lattice constant than silicon, and the atoms are strongly bonded, so silicon carbide substrates are more difficult to etch than silicon substrates. Therefore, it is not easy to etch silicon carbide substrates using the same method as silicon substrates.

[0005] Patent Document 2 discloses a method for rounding the bottom of a trench in a silicon carbide trench MOSFET. In the method disclosed in Patent Document 2, first, a mixed gas of a fluorine-containing etching gas and a protective film forming gas is turned into plasma, and then, while flowing an inert gas from the surface of a base on which the substrate is placed toward the back surface of the silicon carbide substrate, the silicon carbide substrate is etched to form a trench. After forming the trench, at least one of the following (A) to (C) is performed to etch the bottom of the trench. (A) The flow rate of the etching gas is reduced more than in the step of forming the trench. (B) The flow rate of the protective film forming gas is increased more than in the step of forming the trench. (C) The pressure of the inert gas is reduced more than in the step of forming the trench. In (A), the etching ability of the etching gas is reduced, so that the peripheral edge of the trench bottom is less likely to be scraped off, and the central part of the trench bottom is more easily etched than the peripheral part, making it easier to round the trench bottom. In (B), a thick protective film is formed on the sidewall of the trench, making it difficult for plasma ions of the etching gas to hit the peripheral portion of the bottom of the trench, which makes it easier to round the bottom of the trench.In (C), the temperature of the silicon carbide substrate is difficult to decrease, making it easier for the bottom of the trench to be isotropically etched, which makes it easier for the bottom of the trench to be rounded.

[0006] In either of the methods described in Patent Document 2, the etching gas is concentrated at the center of the trench bottom, so while the center of the trench bottom is dug, the peripheral part of the trench bottom is likely to remain in the state formed in the trench formation process, and the steep shape of the peripheral part of the trench bottom may not be sufficiently improved. Therefore, the applicant proposed the technology described in Patent Document 3.

[0007] The method described in Patent Document 3 is characterized by comprising a first step of forming a trench in the silicon carbide substrate by performing a reactive ion etching process using plasma, a second step of forming a protective film on the inner surface of the trench by using isotropic plasma, and a third step of digging the bottom of the trench by performing an anisotropic etching process that strongly etches the trench in the depth direction. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2001-044216 A [Patent Document 2] JP 2015-162630 A [Patent Document 3] Patent No. 7220455 Summary of the Invention [Problem to be solved by the invention]

[0009] The method described in Patent Document 3 requires multiple steps, which is time-consuming and labor-intensive.

[0010] An object of the present invention is to provide a plasma processing method capable of forming, in a silicon carbide substrate, a trench structure with an improved steepness around its bottom by a simple method. [Means for solving the problem]

[0011] The present invention, which has been made to solve the above problems, is a plasma processing method in which a silicon carbide substrate to be etched is subjected to a reactive ion etching process using plasma while maintaining the substrate at a temperature of -20°C or lower, thereby forming a trench in the silicon carbide substrate.

[0012] The low-temperature plasma processing apparatus according to the present invention may be of either an inductively coupled type or a capacitively coupled type.

[0013] In the method according to the present invention, the temperature of the silicon carbide substrate during the reactive ion etching process using plasma is preferably −100° C. or lower, and more preferably −150° C. or lower.

[0014] In the method according to the present invention, the temperature of the silicon carbide substrate when carrying out the plasma reactive ion etching process is preferably set to -196°C or higher.

[0015] The etching gas used in the reactive ion etching process of the plasma processing method according to the present invention can be any gas that contains carbon (C), sulfur (S), fluorine (F) or chlorine (Cl) and is in a gaseous state at room temperature.

[0016] Specifically, fluorocarbon (CF) gas or fluorosulfur (SF) gas is preferable. As the fluorocarbon (CF) gas, for example, C4F8 (boiling point -5.8°C), CHF3 (boiling point -82°C), C2F6 (boiling point -78°C) or CF4 (boiling point -128°C) can be used. As the fluorosulfur (SF) gas, for example, SF6 (boiling point -63.9°C) can be used.

[0017] When these etching gases are plasmatized in the processing chamber, the silicon carbide substrate is cooled to the above temperature, so that a part of them liquefies (condenses) on the sidewall of the trench (groove), or the reaction products (e.g., SiF4 (boiling point -94.8°C), CF4 (boiling point -128°C), CHF3 (boiling point -82°C), C2F6 (boiling point -78°C), C3F6 (boiling point -31°C), C3F8 (boiling point -36°C), C4F8 (boiling point -5.8°C)) generated by reacting with the silicon carbide substrate, which is the material to be etched, or the mask material on the surface liquefies (condenses) on the sidewall of the trench (groove) and functions as a mask (protective film) that blocks incident ions. Then, etching of the outer periphery of the bottom of the trench is suppressed, etching of the center of the bottom of the trench is facilitated, and the bottom of the trench becomes rounded. The amount of reaction product (thickness of the protective film) that functions as a mask (protective film) depends on the temperature of the trench sidewall, so the lower the temperature of the silicon carbide substrate, the more incident ions are blocked, and a round shape with a large curvature tends to be obtained. Therefore, the temperature of the silicon carbide substrate in the present invention must be a temperature below the boiling point of the reaction product, which is -20°C or lower, preferably -100°C or lower, and more preferably -150°C or lower. The boiling point of the reaction product is a value under normal pressure, and when the reactive ion etching process is performed under reduced pressure, the reaction product will liquefy at a temperature slightly lower than that.

[0018] In the plasma processing method of the present invention, other conditions for the reactive ion etching process are not particularly limited so long as a trench can be formed at a temperature equal to or lower than the temperature at which a reaction product generated in the reactive ion etching process liquefies on the trench sidewall of the silicon carbide substrate. For example, if the low-temperature plasma processing apparatus is an inductively coupled type, the ICP high-frequency power may be 1000 W or less, the bias output may be 1000 W or less, and the pressure inside the processing chamber may be 0.1 to 10 Pa. Effect of the Invention

[0019] When a trench is formed in a silicon carbide substrate by the plasma processing method according to the present invention, the steep shape around the bottom is improved. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram of a low-temperature plasma processing apparatus used to confirm the effects of the present invention. [Diagram 2] FIG. 2 is a cross-sectional view of a lower electrode of the low-temperature plasma processing apparatus. [Diagram 3] FIG. 4 shows a heating channel pattern on the top surface of a separator member in the lower electrode. [Figure 4] FIG. 4 is a diagram showing a cooling flow path pattern on the lower surface of a separator member in the lower electrode. [Diagram 5] Graph showing etching rate measurement results for silicon carbide substrates at room temperature (20°C), -100°C, -150°C, and -170°C. [Figure 6] Cross-sectional photographs of a silicon carbide substrate after trenches were formed in the substrate by plasma treatment at temperatures of 20°C, -100°C, -150°C, and -170°C. [Figure 7] 1 is an enlarged cross-sectional photograph of the bottom of the trench in the substrate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In order to confirm the effect of the present invention, a test was carried out to form a trench using a commercially available low-temperature plasma processing apparatus, and the results will be described.

[0022] First, an inductively coupled low-temperature reactive ion etching apparatus (product name: RIE-800iPLN, manufactured by Samco Corporation), which is the low-temperature plasma processing apparatus used in the test, will be described. FIG. 1 is a schematic diagram of the low-temperature plasma processing apparatus 1. The low-temperature plasma processing apparatus 1 has a processing chamber 10 for performing plasma etching, and a lower electrode 11 is provided at the bottom thereof on which a substrate (hereinafter, referred to as a workpiece) W, which is an object to be processed, is placed. The lower electrode 11 is provided with an electrostatic adsorption mechanism or mechanical chuck mechanism (not shown) for fixing the workpiece W during plasma processing, and a temperature control mechanism for cooling the workpiece W to a temperature below room temperature and controlling it to a predetermined temperature below room temperature. The lower electrode 11 is also provided with a high-frequency power source 101 for drawing ions in the plasma into the workpiece W.

[0023] A gas inlet 12 for introducing an etching gas and a gas exhaust port 13 for exhausting the inside of the processing chamber 10 are provided on a side wall of the processing chamber 10. A gas supply unit 121 for supplying a plasma gas and a flow rate control unit (MFC) 122 for adjusting the flow rate of the gas are connected to the gas inlet 12, and a vacuum pump 131 is connected to the gas exhaust port 13.

[0024] A spiral high-frequency coil 141 serving as an upper electrode is provided at the top of the processing chamber 10 via a dielectric window 14, and high-frequency power is supplied to the high-frequency coil 141 from a high-frequency power source 142 for converting the plasma gas into plasma.

[0025] The low-temperature plasma processing apparatus 1 is provided with a control unit 15 configured by a computer, and an operator performs various processes by the low-temperature plasma processing apparatus 1 through the control unit 15. In order to perform a predetermined plasma process on the workpiece W, the control unit 15 appropriately operates the vacuum pump 131, the flow rate control unit 122, the high-frequency power supply 142, etc. based on conditions specified by the operator and preset conditions, etc., and also controls each part of the temperature adjustment mechanism provided in the lower electrode 11 in order to control the temperature of the workpiece W during the process.

[0026] The detailed structure of the lower electrode 11 is shown in Fig. 2. The lower electrode 11 is composed of a base part 111 having a cavity below the upper surface on which the workpiece W is placed, a separator member 112 housed in the cavity, an internal partition wall 113 for sealing the separator member 112 within the cavity, and a lower partition wall 115 for forming a vacuum space 114 below the internal partition wall 113. A shallow recess 116 is provided on the upper surface of the base part 111, i.e., the surface that contacts the workpiece W, and helium gas is introduced and sealed in this recess during processing to improve heat transfer between the base part 111, i.e., the lower electrode 11, and the workpiece W. The vacuum space 114 is provided to prevent condensation of the lower electrode 11 due to liquid nitrogen.

[0027] The separator member 112 separates the cavity formed by the base portion 111 and the internal partition wall 113 into upper and lower portions in an airtight and liquidtight manner, and the upper surface of the separator member 112 is in close contact with the lower surface of the base portion 111, and the lower surface of the separator member 112 is in close contact with the upper surface of the internal partition wall 113. A heating flow path 117 having a pattern as shown in FIG. 3 is formed on the upper surface of the separator member 112, and a cooling flow path 118 having a pattern as shown in FIG. 4 is formed on the lower surface of the separator member 112. Nitrogen gas (G-N2) is supplied from a heating control unit 151 to the heating flow path 117 on the upper surface, and liquid nitrogen (L-N2, -196°C) is supplied from a liquid nitrogen supply unit 152 to the cooling flow path 118 on the lower surface. In the heating control unit 151, the nitrogen gas (G-N2) is heated by a heater 153, and the temperature is controlled.

[0028] The lower partition 115 is provided with a suction port 119 for drawing a vacuum into the vacuum space 114 below the internal partition 113. A turbo molecular pump is preferably used as a pump for drawing a vacuum into the vacuum space 114. In addition, a seal material that can be used down to -60°C is preferably used as a seal for airtightly maintaining the pressure difference between the vacuum space 114 and other parts such as the processing chamber 10. Examples of such a seal material include ethylene propylene diene rubber (EPDM) and indium.

[0029] When low-temperature etching is performed on the workpiece W using the lower electrode 11 having such a structure, the control unit 15 operates as follows. First, liquid nitrogen is supplied from the liquid nitrogen supply unit 152 to the cooling flow passage 118 through a pipe equipped with a valve 162, and heated nitrogen gas is supplied from the heating control unit 151 to the heating flow passage 117. Then, the amount and / or timing of liquid nitrogen supplied to the cooling flow passage 118 is adjusted by pulse-controlling the opening and closing and opening degree of the valve 162 to control the temperature of the workpiece W during plasma processing, and the temperature of the nitrogen gas supplied to the heating flow passage 117 is controlled by the heater 153 to improve the temperature controllability of the workpiece W. The heated nitrogen gas is also used to quickly raise the temperature when raising the temperature from a low temperature. According to the present invention, the temperature of the workpiece W can be uniformly controlled to a low temperature during plasma processing, and the control can also be performed quickly.

[0030] Next, a mask having a predetermined opening was formed on a silicon carbide substrate (workpiece W), and the workpiece W was maintained at room temperature (20°C), -100°C, -150°C, and -170°C. A CHF3 / Ar / O2 mixed gas was introduced as an etching gas into the processing chamber 10 of the low-temperature plasma processing apparatus 1 to perform reactive ion etching, thereby forming a trench in the silicon carbide substrate.

[0031] The results of measuring the etching rate of silicon carbide at each temperature are shown in Figure 5. It can be seen that the etching rate increases as the temperature decreases.

[0032] Furthermore, a silicon carbide substrate was used as the workpiece W, and reactive ion etching was performed using a low-temperature plasma processing apparatus 1 to form a trench having a width of 1 μm and a depth of 3 μm. That is, the workpiece W having a mask having the trench pattern formed on its surface was placed on the lower electrode 11 as shown in FIG. 1, and the workpiece W was held at room temperature (20°C), -100°C, -150°C, and -170°C. CHF3 / Ar / O2 mixed gas was introduced as an etching gas into the processing chamber 10 to perform reactive ion etching, thereby forming a trench in the silicon carbide substrate. The processing time was set to be until the trench depth reached approximately 3 μm.

[0033] Figure 6 shows photographs of the cross sections of the substrate after trenches were formed at each temperature. Figure 7 shows enlarged photographs of the bottom of the trench. At room temperature (20°C), the corners of the trench bottom are not rounded, but the corners are slightly rounded in the trench treated at -100°C, and the bottoms of the trenches treated at -150°C and -170°C are entirely rounded. Due to this trench bottom shape, silicon carbide substrates in which trenches are formed by low-temperature plasma treatment are expected to have a high breakdown voltage. [Explanation of symbols]

[0034] 10…Processing chamber 11...Lower electrode 111…Base 112...Separator member 113…Internal bulkhead 114...Vacuum space 115…Lower bulkhead 116…Helium gas recess 117...heating channel 118...Cooling channel 119...Vacuum suction port 12...Gas inlet 121...Gas supply section 122...Flow rate control section 13...Gas exhaust port 131…Vacuum pump 14...Dielectric window 141...High frequency coil 142...High frequency power supply 15...Control section 151... Heating control unit 152...Liquid nitrogen supply unit 153…Heater

Claims

1. A plasma processing method in which a silicon carbide substrate to be etched is subjected to a reactive ion etching process using plasma while being maintained at a temperature of -20°C or lower, thereby forming a trench in the silicon carbide substrate.

2. 2. The plasma processing method according to claim 1, wherein an etching gas in the plasma reactive ion etching process contains a fluorocarbon gas.

3. The fluorocarbon gas is CHF 3 , C.F. 4 , C 2 F 6 or C 4 F 8 The plasma processing method according to claim 2 , wherein the gas is a gas.

4. 2. The plasma processing method according to claim 1, wherein an etching gas in the plasma reactive ion etching process contains a sulfur fluoride-based gas.

5. The sulfur fluoride gas is SF 6 The plasma processing method according to claim 4, wherein the gas is a gas.

6. 2. The plasma processing method according to claim 1, wherein the temperature of the silicon carbide substrate is set to a temperature not higher than a temperature at which an etching gas or a reaction product produced in a reactive ion etching process liquefies on a side wall of the trench.

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