3. Single-Crystalline 4H-Silicon Carbide Etching Using Chlorine Trifluoride Gas

### 3.1. Substrate, reactor and process

etching, similar to that for the silicon etching [22, 34]. The partial pressures at masses of 35

chlorine (Cl2). The partial pressures observed at masses of 19 and 20 a.m.u. correspond to F+

ide. Since the partial pressure of fluorine (F2) at mass 38 a.m.u. did not appear in the mass spectra, the thermal dissociation of chlorine trifluoride gas [35] is negligible in the gas phase

Figure 13. )9KKKH=;LJ9G>?9K=GMKKH=;A=K=PAKLAF?AFL@==P@9MKL?9K>JGEL@=J=9;LGJ<MJAF?L@==L;@AF?G>L@=KADAs ;GF;9J:A<=KMJ>9;=MKAF?;@DGJAF=LJA>DMGJA<=?9K9L9LEGKH@=JA;HJ=KKMJ=0@=L=EH=J9LMJ=G>L@=KADA;GF;9J:A<=KM:s

, respectively, whose parent is silicon tetrafluoride, like silicon etching [34f^z \$!z #/¥ eous carbon compound produced by etching is identified as carbon tetrafluoride (CF4d\_z!¥

\$1/\_z/%(%+\*z\* z.+\*z!+)!z0\$!z#/!+1/z/,!%!/z+"z/%(%+\*z0!0."(1+.% !z\* z.+\*z0!0¥ rafluoride, respectively. The overall chemical reaction between silicon carbide and chlorine

3SiC· + · 8ClF3 · ¨ ·3SiF4 ·+ · 3CF4 · + ·4Cl2 (1)

The partial pressures at masses of 66, 85 and 104 a.m.u. can be assigned to SiF2 <sup>+</sup>

cause the partial pressures at masses of 50 and 69 a.m.u. correspond to CF2 <sup>+</sup>

respectively, which can be assigned as fragments of carbon tetrafluoride.

, which is a fragment from both chlorine trifluoride and

, SiF3 <sup>+</sup>

and CF3 <sup>+</sup>

and

,

and HF+ .!z,.+ 1! z 1!z0+z0\$!z".#)!\*00%+\*z+"z\$(+.%\*!z0.%"(1+.¥

and 37 a.m.u. can be assigned to Cl+

110 Physics and Technology of Silicon Carbide Devices

of the cold wall reactor used in this study.

strate is 770 K. The ionization conditions are 70 eV and 1.73 mA.

, respectively. F+

and HF+

SiF4 <sup>+</sup>

trifluoride is as follows:

The substrate used is the n-type single-crystalline 4H-silicon carbide wafer having a (0001) surface, 8-degrees off-oriented to <11-20>. This substrate has nitrogen as the n-type dopant at the concentration of 3 - 5 x 1018 cm-3. The 4H-silicon carbide substrate, having 5 mm wide x 5 mm long x 400 µm thick dimensions, is placed on the center of the polycrystalline 3Csilicon carbide susceptor, which has the dimension of 30 mm wide × 40 mm long × 0.2 mm thick produced by the chemical vapor deposition method (Admap Inc., Tokyo).

The reactor shown in Figure 1 is used following the process shown in Figure 2 except of hydrogen baking. The Si-face (0001) and C-face (000-1) of the 4H-silicon carbide substrates are etched using chlorine trifluoride gas. The etching is performed at the temperatures between 570 – 1570 K at the chlorine trifluoride gas flow rate of 0.1 – 0.3 slm. The average etching rate of 4H-silicon carbide is determined by the decrease in the substrate weight. -!1/!z0\$!zFw/%(%+\*z.% !z/1/!,0+.z\* z0\$!zGw/%(%+\*z.% !z/1/0.0!z.!z/%)1(0\*!¥ ously etched in the reactor, the etching rate obtained is comparable to the average value for the wide 4H-silicon carbide substrate.

The X-ray topograph of Si-face and C-face of 4H-silicon carbide was taken at the beam-line -DHz +"z 0\$!z\$+0+\*z 0+.5z +"z 0\$!z%#\$z\*!.#5z!(!.0+.z!/!.\$z.#\*%60%+\*z c.+¥ posal No. 2006G286), in order to evaluate the crystalline defects. The density and behavior +"z!0\$z,%0z,.+ 1! z+\*z0\$!zGw/%(%+\*z.% !z/1/0.0!z1/%\*#z\$(+.%\*!z0.%"(1+.% !z#/z0z2.¥ ious temperatures were evaluated.

#### 3.2. Numerical calculation of etching rate

The etching rate of single-crystalline 4H-silicon carbide is numerically calculated [29]. The geometry of horizontal cold-wall reactor, shown in the previous section (Figure 1), is taken into account for a series of calculations. In order to evaluate the silicon carbide etching rate and the overall rate constant in steady state in non-uniformly distributed temperature and #/z "(+3z "%!( /\_z 0\$!z 03+w %)!\*/%+\*(z!-10%+\*/z+"z)//\_z)+)!\*01)\_z!\*!.#5\_z/,!%!/z 0.\*/¥ port and surface chemical reaction, linked with the ideal gas law, are solved. The discretized equations are coupled and solved using the SIMPLE algorithm [36fz+\*zzz/+"03.!z,'¥ age, Fluent version 6 (Fluent, Inc., Lebanon, NH, USA).

The silicon carbide etching is assumed to follow the overall reaction in Eq. (2) [23, 24],

$$\text{\textbullet SiC} \cdot + \text{\textbullet CIF}\_3 \cdot + \text{\textbullet SiF}\_4 \cdot + \text{\textbullet CF}\_4 \cdot + \text{\textbullet CI}\_2 \tag{2}$$

Mass changes due to the chemical reaction of Eq. (2) are taken into account in the boundary conditions at the surface of silicon carbide. The overall reaction shown in Eq. (2) is assumed to be a first-order reaction.

$$\text{SiC etching rate} = 6 \times 10^7 M\_{\text{SiC}} \text{[CIF}\_3\text{]} \cdot / \cdot \rho\_{\text{SiC}} \quad \text{(\text{\$\mu\$mm min}^{-1}\$)}\tag{3}$$

Figure 14. Etching rate of 4H-silicon carbide using chlorine trifluoride gas at 100 %, 0.1 slm and various temperatures.

Etching of Silicon Carbide Using Chlorine Trifluoride Gas

http://dx.doi.org/10.5772/50387

113

Figure 15. !L;@AF?J9L=G>/A>9;= O@AL=;AJ;D=9F<>9;= <9JC;AJ;D=\$KADA;GF;9J:A<=;@9F?AF?OAL@;@DGJAF=LJAs

The relationship between the etching rate and the chlorine trifluoride gas flow rate is shown in Figures 16. Figures 16 (a) and 16 (b) show the etching rate of the Si-face and the C-face, .!/,!0%2!(5\_z+"z0\$!zGw/%(%+\*z.% !z/1/0.0!z5z\$(+.%\*!z0.%"(1+.% !z#/zcDCCMdz0z0)+/¥ pheric pressure, when changing with the chlorine trifluoride gas flow rate. The circle, square \* z0.%\*#(!z !\*+0!z0\$!z!0\$%\*#z.0!z0z0\$!z/1/0.0!z0!),!.01.!/z+"zJJC\_zKJCz\* zLJCz\_z.!¥ spectively. As shown in these figures, the Si-face and C-face etching rates increase with the gas flow rate of the chlorine trifluoride gas. Additionally, the etching rate at 770, 870 and 970 K overlap each other for both the Si-face and the C-face, consistent with Figures 14. The etching rate of the Si-face is about 60 % of that of the C-face. The relationship between the Siface etching rate and the C-face etching rate is similar to that of another empirically known

fluoride gas concentration, at 1370 K and at the total gas flow rate of 0.2 slm for 5 min.

etching technique, such as the potassium hydroxide method [41].

Dark circle: C-face, and white circle: Si-face.

where k SiC is the density of solid silicon carbide (kg m-3). *M* SiC is the molecular weight of silicon carbide (kg mol-1), respectively. The factor 6 x 107 is used for the unit conversion of m s-1 to µm min-1. *k* SiC is the overall rate constant for the reaction of Eq. (3). [ClF3fz%/z0\$!z+\*!\*¥ tration of chlorine trifluoride gas at the silicon carbide surface (mol m-3). The concentration of each species at the surface is governed by a balance between the consumption due to the chemical reaction and the diffusion fluxes driven by the concentration.

The gas velocity and pressure at the inlet are 0.08 m s-1 and 1.0133 x 105 Pa, respectively. The heat capacities of chlorine trifluoride, nitrogen, tetrafluorosilane, tetrafluorocarbon and chlorine are taken from the literature [37]. The gas properties, such as the viscosity and the 0\$!.)(z+\* 10%2%05z+"z\$(+.%\*!z0.%"(1+.% !\_z0!0."(1+.+/%(\*!\_z\$(+.%\*!z\* z\*%0.+#!\*z.!z!/0%¥ mated with the method described in the literature [38]. The Lennard-Jones parameters of l and c/*k*z "+.z \$(+.%\*!z 0.%"(1+.% !z .!z G^IFz\*#/0.+)/z \* z FHHz \_z .!/,!0%2!(5\_z3\$%\$z .!z +¥ tained using a theoretical equation [38] taking the value of viscosity [39] into account. Each physical constant is expressed as a function of temperature. The properties of the mixed gas are estimated theoretically [40f^z\$!z%\*.5z %""1/%+\*z+!""%%!\*0/z+"z\$(+.%\*!z0.%"(1+.% !\_z0!0¥ rafluorosilane and chlorine are estimated using the method described in the literature [38].

The overall rate constant, *k* SiC in Eq. (3) is obtained so that the calculated etching rates agree with those measured at various conditions.

#### 3.3. Etching rate

%#1.!zDGz/\$+3/z 0\$!z!0\$%\*#z.0!z+"z 0\$!z%w"!z\* zw"!z+"zGw/%(%+\*z.% !z0z 0\$!z/1¥ strate temperatures between 570 K and 1570 K. The etching rate of the C-face of 4H-silicon carbide is slightly higher than that of the Si-face of 4H-silicon carbide. The etching rate of the Si-face and C-face of 4H-silicon carbide is near 5 µm min-1z\* z%0z%/z/0%((z"(0z0z0\$!z0!),!.¥ 01.!/z!03!!\*zJJCzz\* zDHJCz^z\$%/z"(0z!0\$%\*#z.0!z!\$2%+.z%/z/%)%(.z0+z0\$0z+"z,+(5.5/¥ talline 3C-silicon carbide, shown in Figure 3 in the previous section. The various surface morphologies at higher temperatures shown in the latter part of this section are obtained at nearly the same etching rate.

%#1.!zDHz/\$+3/z0\$!z05,%(z!\$2%+.z+"z0\$!z!0\$%\*#z.0!z\$\*#%\*#z3%0\$z0\$!z\$(+.%\*!z0.%"(1+.¥ ide gas concentration, obtained at 1370 K. The etching rate is proportional to the chlorine trifluoride gas concentration, similar to that of polycrystalline 3C-silicon carbide, shown in Figure 4. The fluctuation of the etching rate, shown in Figures 14 and 15, is entirely 20 %, 3\$%\$z%/z 1!z 0+z 0\$!z+\*/% !.(!z %/0.%10%+\*z+"z 0\$!z!0\$%\*#z.0!z+2!.z 0\$!zFw/%(%+\*z.¥ bide susceptor.

Mass changes due to the chemical reaction of Eq. (2) are taken into account in the boundary conditions at the surface of silicon carbide. The overall reaction shown in Eq. (2) is assumed

where k SiC is the density of solid silicon carbide (kg m-3). *M* SiC is the molecular weight of silicon carbide (kg mol-1), respectively. The factor 6 x 107 is used for the unit conversion of m s-1 to µm min-1. *k* SiC is the overall rate constant for the reaction of Eq. (3). [ClF3fz%/z0\$!z+\*!\*¥ tration of chlorine trifluoride gas at the silicon carbide surface (mol m-3). The concentration of each species at the surface is governed by a balance between the consumption due to the

The gas velocity and pressure at the inlet are 0.08 m s-1 and 1.0133 x 105 Pa, respectively. The heat capacities of chlorine trifluoride, nitrogen, tetrafluorosilane, tetrafluorocarbon and chlorine are taken from the literature [37]. The gas properties, such as the viscosity and the 0\$!.)(z+\* 10%2%05z+"z\$(+.%\*!z0.%"(1+.% !\_z0!0."(1+.+/%(\*!\_z\$(+.%\*!z\* z\*%0.+#!\*z.!z!/0%¥ mated with the method described in the literature [38]. The Lennard-Jones parameters of l and c/*k*z "+.z \$(+.%\*!z 0.%"(1+.% !z .!z G^IFz\*#/0.+)/z \* z FHHz \_z .!/,!0%2!(5\_z3\$%\$z .!z +¥ tained using a theoretical equation [38] taking the value of viscosity [39] into account. Each physical constant is expressed as a function of temperature. The properties of the mixed gas are estimated theoretically [40f^z\$!z%\*.5z %""1/%+\*z+!""%%!\*0/z+"z\$(+.%\*!z0.%"(1+.% !\_z0!0¥ rafluorosilane and chlorine are estimated using the method described in the literature [38]. The overall rate constant, *k* SiC in Eq. (3) is obtained so that the calculated etching rates agree

%#1.!zDGz/\$+3/z 0\$!z!0\$%\*#z.0!z+"z 0\$!z%w"!z\* zw"!z+"zGw/%(%+\*z.% !z0z 0\$!z/1¥ strate temperatures between 570 K and 1570 K. The etching rate of the C-face of 4H-silicon carbide is slightly higher than that of the Si-face of 4H-silicon carbide. The etching rate of the Si-face and C-face of 4H-silicon carbide is near 5 µm min-1z\* z%0z%/z/0%((z"(0z0z0\$!z0!),!.¥ 01.!/z!03!!\*zJJCzz\* zDHJCz^z\$%/z"(0z!0\$%\*#z.0!z!\$2%+.z%/z/%)%(.z0+z0\$0z+"z,+(5.5/¥ talline 3C-silicon carbide, shown in Figure 3 in the previous section. The various surface morphologies at higher temperatures shown in the latter part of this section are obtained at

%#1.!zDHz/\$+3/z0\$!z05,%(z!\$2%+.z+"z0\$!z!0\$%\*#z.0!z\$\*#%\*#z3%0\$z0\$!z\$(+.%\*!z0.%"(1+.¥ ide gas concentration, obtained at 1370 K. The etching rate is proportional to the chlorine trifluoride gas concentration, similar to that of polycrystalline 3C-silicon carbide, shown in Figure 4. The fluctuation of the etching rate, shown in Figures 14 and 15, is entirely 20 %, 3\$%\$z%/z 1!z 0+z 0\$!z+\*/% !.(!z %/0.%10%+\*z+"z 0\$!z!0\$%\*#z.0!z+2!.z 0\$!zFw/%(%+\*z.¥

chemical reaction and the diffusion fluxes driven by the concentration.

*<sup>M</sup>*SiC*k*SiC ClF3 · / ·kSiC (*µ*mm min<sup>1</sup>

), (3)

to be a first-order reaction.

112 Physics and Technology of Silicon Carbide Devices

SiC etching rate = 6x10<sup>7</sup>

with those measured at various conditions.

3.3. Etching rate

bide susceptor.

nearly the same etching rate.

Figure 14. Etching rate of 4H-silicon carbide using chlorine trifluoride gas at 100 %, 0.1 slm and various temperatures. Dark circle: C-face, and white circle: Si-face.

Figure 15. !L;@AF?J9L=G>/A>9;=O@AL=;AJ;D=9F<>9;= <9JC;AJ;D=\$KADA;GF;9J:A<=;@9F?AF?OAL@;@DGJAF=LJAs fluoride gas concentration, at 1370 K and at the total gas flow rate of 0.2 slm for 5 min.

The relationship between the etching rate and the chlorine trifluoride gas flow rate is shown in Figures 16. Figures 16 (a) and 16 (b) show the etching rate of the Si-face and the C-face, .!/,!0%2!(5\_z+"z0\$!zGw/%(%+\*z.% !z/1/0.0!z5z\$(+.%\*!z0.%"(1+.% !z#/zcDCCMdz0z0)+/¥ pheric pressure, when changing with the chlorine trifluoride gas flow rate. The circle, square \* z0.%\*#(!z !\*+0!z0\$!z!0\$%\*#z.0!z0z0\$!z/1/0.0!z0!),!.01.!/z+"zJJC\_zKJCz\* zLJCz\_z.!¥ spectively. As shown in these figures, the Si-face and C-face etching rates increase with the gas flow rate of the chlorine trifluoride gas. Additionally, the etching rate at 770, 870 and 970 K overlap each other for both the Si-face and the C-face, consistent with Figures 14. The etching rate of the Si-face is about 60 % of that of the C-face. The relationship between the Siface etching rate and the C-face etching rate is similar to that of another empirically known etching technique, such as the potassium hydroxide method [41].

rate over the 4H-silicon carbide substrate and the 3C-silicon carbide susceptor can increase 3%0\$z0\$!z%\*.!/%\*#z"(+3z.0!z+"z\$(+.%\*!z0.%"(1+.% !z#/^z\$!z!0\$%\*#z.0!z"+.zw"!z(1(0¥ ed using Eq. (5) also showed the typical behavior of the measurement. Therefore, Eqs. (4) and (5) are applicable to reproduce the behavior of the 4H-silicon carbide (Si-face and C-

Etching of Silicon Carbide Using Chlorine Trifluoride Gas

http://dx.doi.org/10.5772/50387

115

Figure 17. Rate constants for etching of Si-face (broken line) and C-face (solid line) of 4H-silicon carbide using chlorine

Figure 18. Arrhenius plot of 4H-silicon carbide Si-face etching rate at chlorine trifluoride flow rate of 0.1 and 0.2 slm.

face) etching rate.

trifluoride gas, obtained by numerical calculation.

Square: measurement, solid line: calculation.

Figure 16. Etching rate of (a) Si-face and (b) C-face of 4H-silicon carbide substrate by chlorine trifluoride gas (100%) at atmospheric pressure in the flow rate range between 0.1 and 0.3 slm. Circle, square and triangle show the etching rates at the substrate temperatures of 770, 870 and 970 K, respectively.

#### 3.4. Surface reaction rate constant

Figure 17 is the Arrhenius plot of the rate constants for etching of Si-face and C-face of 4Hsilicon carbide. The rate constants is expressed in Eqs. (4) and (5).

$$k\_{\rm SiC} = 4.1 \exp\{-6.6 \times 10^4/RT\} \qquad \text{(m/s)} \qquad \text{for Si} - \text{face} \tag{4}$$

and,

$$k\_{\rm SiC} = 98 \times \exp\left\{-8.3 \times 10^4 / RT\right\} \quad \text{(m/s)} \qquad \text{for C-face} \tag{5}$$

where *R* is the gas constant (J mol-1 K-1).

In order to show that the rate constant of Eq. (4) can reproduce the measured etching rate behavior, the measured and the calculated etching rate values of Si-face are shown in Figure 18, as the Arrhenius plot. The calculation shows that the etching rate at the temperatures near 670K is near 1 µm/min, and it becomes near 10 µ)u)%\*z0zDCCCz^z\$!z(1(0! z!0\$¥ ing rate tends to become flat at the higher temperatures at the chlorine trifluoride gas flow rate of 0.1 and 0.2 slm. Additionally, the etching rate obtained by the calculation increases with increasing the chlorine trifluoride gas flow rate. Because the great etchant flow rate can moderate the etchant depletion occurring in the downstream region, the average etching rate over the 4H-silicon carbide substrate and the 3C-silicon carbide susceptor can increase 3%0\$z0\$!z%\*.!/%\*#z"(+3z.0!z+"z\$(+.%\*!z0.%"(1+.% !z#/^z\$!z!0\$%\*#z.0!z"+.zw"!z(1(0¥ ed using Eq. (5) also showed the typical behavior of the measurement. Therefore, Eqs. (4) and (5) are applicable to reproduce the behavior of the 4H-silicon carbide (Si-face and Cface) etching rate.

Figure 17. Rate constants for etching of Si-face (broken line) and C-face (solid line) of 4H-silicon carbide using chlorine trifluoride gas, obtained by numerical calculation.

Figure 16. Etching rate of (a) Si-face and (b) C-face of 4H-silicon carbide substrate by chlorine trifluoride gas (100%) at atmospheric pressure in the flow rate range between 0.1 and 0.3 slm. Circle, square and triangle show the etching

Figure 17 is the Arrhenius plot of the rate constants for etching of Si-face and C-face of 4H-

In order to show that the rate constant of Eq. (4) can reproduce the measured etching rate behavior, the measured and the calculated etching rate values of Si-face are shown in Figure 18, as the Arrhenius plot. The calculation shows that the etching rate at the temperatures near 670K is near 1 µm/min, and it becomes near 10 µ)u)%\*z0zDCCCz^z\$!z(1(0! z!0\$¥ ing rate tends to become flat at the higher temperatures at the chlorine trifluoride gas flow rate of 0.1 and 0.2 slm. Additionally, the etching rate obtained by the calculation increases with increasing the chlorine trifluoride gas flow rate. Because the great etchant flow rate can moderate the etchant depletion occurring in the downstream region, the average etching

*<sup>k</sup>*SiC =4.1 exp( 6.6x10<sup>4</sup> / *RT* ) (m/s) for Siface (4)

*<sup>k</sup>*SiC =98 x exp( 8.3x10<sup>4</sup> / *RT* ) (m/s) for Cface (5)

rates at the substrate temperatures of 770, 870 and 970 K, respectively.

silicon carbide. The rate constants is expressed in Eqs. (4) and (5).

3.4. Surface reaction rate constant

114 Physics and Technology of Silicon Carbide Devices

where *R* is the gas constant (J mol-1 K-1).

and,

Figure 18. Arrhenius plot of 4H-silicon carbide Si-face etching rate at chlorine trifluoride flow rate of 0.1 and 0.2 slm. Square: measurement, solid line: calculation.
