2.1. Reactor and processes using chlorine trifluoride gas

In order to etch silicon carbide by chlorine trifluoride gas, the horizontal cold-wall reactor shown in Figure 1. is used. This reactor consists of a gas supply system, a quartz chamber \* z%\*"..! z(),/^zzFCz))z3% !z4zGCz))z(+\*#z4zC^EwDz))z0\$%'zFw/%(%+\*z.% !z/1¥ strate manufactured using chemical vapor deposition (CVD) (Admap Inc., Tokyo) is held horizontally on the bottom wall of the quartz chamber.

Figure 1. Horizontal cold-wall reactor used for etching polycrystalline 3C-silicon carbide substrate.

The gas supply system introduces chlorine trifluoride gas, nitrogen gas and hydrogen gas. 5 .+#!\*z#/z%/z1/! z0+z.!)+2!z0\$!z/%(%+\*z+4% !z"%()z+\*z0\$!z/%(%+\*z.% !z/1/0.0!z/1.¥ face, the same as those on the silicon surface [22f^z\$!z\$!%#\$0z\* z3% 0\$z+"z0\$!z-1.06z\$)¥ ber are compactly designed to be 10 mm and 40 mm, respectively, similar to the chamber in our various studies [22, 30].

The etching using chlorine trifluoride gas is carried out following the process shown in %#¥ ure 2. There are mainly three steps.

(a) cleaning the silicon carbide substrate surface by baking in ambient hydrogen at 1370 K for 10 min,

(b) changing the gas from hydrogen to nitrogen, and

(c) etching the silicon carbide substrate surface using chlorine trifluoride gas.

Figure 2. Process for cleaning and etching polycrystalline 3C-silicon carbide surface.

During step (a), hydrogen gas is introduced at atmospheric pressure into the reactor at a flow rate of 2 slm. Next, in step (b), the quartz chamber and the silicon carbide substrate are cooled to room temperature. The hydrogen gas present in the quartz chamber must be sufficiently purged with nitrogen gas to avoid an explosive reaction between hydrogen and chlorine 0.%"(1+.% !^z1.%\*#z/0!,zcd\_z0\$!z/%(%+\*z.% !z/1/0.0!z%/z\$!0! z\* z &1/0! z0+z0!),!.¥ 01.!/z!03!!\*zIJCzz\* zLJCz^z\$!z/%(%+\*z.% !z/1/0.0!z%/z!0\$! z5z\$(+.%\*!z0.%"(1+.¥ ide (>99.9 %, Kanto Denka Kogyo Co., Ltd., Tokyo) at a flow rate of 0.1-0.25 slm without further purification and without dilution. In order to evaluate the gaseous products, part of the exhaust gas from the reactor is fed into a quadrupole mass spectra (QMS) analyzer, as shown in Figure D^z\$!z/%(%+\*z.% !z!0\$%\*#z.0!z1/%\*#z\$(+.%\*!z0.%"(1+.% !z#/z%/z!2(10! z".+)z0\$!z !¥ .!/!z%\*z0\$!z3!%#\$0z+"z0\$!z/%(%+\*z.% !z/1/0.0!^z\$!z/1."!z)+.,\$+(+#5z+"z0\$!z,+(5.5/¥ talline 3C-silicon carbide substrate before and after the etching is observed using an optical microscope. The root-mean-square (RMS) surface roughness and the average roughness, *R* a, are measured. In order to evaluate the condition of the chemical bonds of the silicon carbide surface before and after the etching, X-ray photoelectron spectra (XPS) are obtained.

#### 2.2. Etching rate

In Section 2, details of polycrystalline 3C-silicon carbide etching using chlorine trifluoride gas [23, 24fz .!z .!2%!3! \_z,.0%1(.(5z "+1/%\*#z+\*z 0\$!z!0\$%\*#z .0!\_z#/!+1/z,.+ 10/\_z/1.¥ face chemical bonds and the surface morphology of the silicon carbide. In Section 3, the dry etching of single-crystalline 4H-silicon carbide using chlorine trifluoride gas [25-29] over the 3% !z0!),!.01.!z.\*#!z+"zHJCwDHJCzz%/z.!2%!3! \_z,.0%1(.(5z+10z0\$!z!0\$%\*#z.0!\_z/1.¥

In order to etch silicon carbide by chlorine trifluoride gas, the horizontal cold-wall reactor shown in Figure 1. is used. This reactor consists of a gas supply system, a quartz chamber \* z%\*"..! z(),/^zzFCz))z3% !z4zGCz))z(+\*#z4zC^EwDz))z0\$%'zFw/%(%+\*z.% !z/1¥ strate manufactured using chemical vapor deposition (CVD) (Admap Inc., Tokyo) is held

face chemical reaction rate constant, surface morphology and etch pits.

2.1. Reactor and processes using chlorine trifluoride gas

horizontally on the bottom wall of the quartz chamber.

Trifluoride Gas

100 Physics and Technology of Silicon Carbide Devices

our various studies [22, 30].

for 10 min,

ure 2. There are mainly three steps.

(b) changing the gas from hydrogen to nitrogen, and

2. Polycrystalline 3C-Silicon Carbide Etching Using Chlorine

Figure 1. Horizontal cold-wall reactor used for etching polycrystalline 3C-silicon carbide substrate.

The gas supply system introduces chlorine trifluoride gas, nitrogen gas and hydrogen gas. 5 .+#!\*z#/z%/z1/! z0+z.!)+2!z0\$!z/%(%+\*z+4% !z"%()z+\*z0\$!z/%(%+\*z.% !z/1/0.0!z/1.¥ face, the same as those on the silicon surface [22f^z\$!z\$!%#\$0z\* z3% 0\$z+"z0\$!z-1.06z\$)¥ ber are compactly designed to be 10 mm and 40 mm, respectively, similar to the chamber in

The etching using chlorine trifluoride gas is carried out following the process shown in %#¥

(a) cleaning the silicon carbide substrate surface by baking in ambient hydrogen at 1370 K

The etching rate of the polycrystalline 3C-silicon carbide substrate surface is shown in %#¥ ure 3, which was obtained using 100% chlorine trifluoride gas at various gas flow rates in 0\$!z0!),!.01.!z.\*#!z+"zIJCz0+zLJCzz0z0)+/,\$!.%z,.!//1.!^z\$!z/-1.!/\_z%.(!/z\* z0.%¥ angles show the etching rate at the chlorine trifluoride gas flow rate of 0.2, 0.1 and 0.05 slm, respectively. As shown in Figure 3, the etching rate at the substrate temperature less than 670K is quite low; its value is less than 1 µm/min. However, with the increasing substrate temperature, the etching rate significantly increases particularly near 720 K. At the substrate temperature of 770 K, the etching rate at the flow rate of 0.2 slm becomes 20 µ)u)%\*az%0z.!¥ mains constant at substrate temperatures greater than 770 K.

As shown in Figure 3, the etching rate changes with the flow rates. The etching rates are 25, 10 and 5 µm/min at the flow rates of 0.2, 0.1 and 0.05 slm, respectively. For each chlorine trifluoride gas flow rate, the trend in the flat etching rate at temperature greater than 770K is maintained.

In order to evaluate the influence of chlorine trifluoride gas concentration, the etching rate of the polycrystalline 3C-silicon carbide substrate surface using 10-100% chlorine trifluoride gas in ambient nitrogen was measured at the flow rate of 0.2 slm, atmospheric pressure and 670-970 K, as shown in Figure 4. In this figure, the square, reverse triangle, circle, diamond, and triangle show the substrate temperatures of 670, 730, 770, 870 and 970 K, respectively. Being consistent with Figure 3, the etching rate at the substrate temperature of 670K is very low. The etching rates at 730 K are significantly higher that those at 670 K.

0z 0\$!z /1/0.0!z 0!),!.01.!z +"z JJCz \_z 0\$!z !0\$%\*#z .0!z%/z ,.+,+.0%+\*(z 0+z 0\$!z \$(+.%\*!z 0.%¥ fluoride gas concentration. At 870 and 970 K, the etching rate at each chlorine trifluoride gas concentration is the same as that at 770 K. Therefore, when the substrate temperature is higher than 770 K, the etching rate over a very wide chlorine trifluoride gas concentration range is not affected by the substrate temperature.

Figure 4. Etching rate of the polycrystalline 3C-silicon carbide substrate surface using chlorine trifluoride gas at 10-100%, 0.2 slm, atmospheric pressure, and 670-970K. Square: 670 K, reverse triangle: 730K, circle: 770K, diamond:

Etching of Silicon Carbide Using Chlorine Trifluoride Gas

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

103

The change in the surface morphology of the 3C-silicon carbide is explained. Figure 5 shows photographs of the silicon carbide surface etched using chlorine trifluoride gas at the flow rate of 0.1 slm and atmospheric pressure at 670, 720 and 770 K for 15 min. The values in ,.!\*0\$!/!/z.!z0\$!z!0\$z !,0\$^z%#1.!zHzcdz/\$+3/z0\$!z%\*%0%(z/1."!z3\$%\$z\$/z0\$!z,!.%+ ¥ ically line-shaped hills and valleys. As shown in Figure 5 (b), although the surface still has the line-shaped morphology at 670 K after 15 minutes, its pattern remains but is unclear. 0z 0\$!z \$%#\$!.z 0!),!.01.!/z +"z JECz \* z JJCz\_z 0\$!z(%\*!w/\$,! z ,,!.\*!z +!/z \*+0z .!¥ main, as shown in Figures 5 (c) and (d). In these figures, there are very small and very shallow pits having a round edge. This shows that the etching using chlorine trifluoride can

 \*z+. !.z0+z/\$+3z0\$!z !0%(z+"z/1."!z/)++0\$%\*#z!""!0\_z0\$!z\$\*#!z%\*z0\$!z/1."!z,,!.¥ ance is shown, in Figure 6, along etch period at a substrate temperature of 770 K and a flow rate of 0.1 slm of chlorine trifluoride. This figure shows photographs of the etched silicon carbide surface at (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, and (e) 30 min. The values in parenthes are the etch depth. Figures 6 (a) and (d) are the same as Figures 5 (a) and (d), .!/,!0%2!(5^z\$!z.!(0%+\*/\$%,z!03!!\*z0\$!z!0\$z,!.%+ z\* z0\$!z!0\$z !,0\$z%/z/\$+3\*z%\*z%#¥ ure 7 (a). The line-shaped pattern in Figure 6 (a) is slightly rounded after 5 minutes. At 10 minutes, there is only a trace of the line-shaped appearance, as shown in Figure 6 (c). The 3C-silicon carbide surface has a round-shaped morphology after 15 minutes as shown in Figure 6 (d), since the line-shaped pattern is removed during the etch period between 10

smooth the large hills and valleys which existed on the silicon carbide surface.

870 K, and triangle:970K.

2.3. Surface morphology and roughness

Figure 3. Etching rate of the polycrystalline 3C-silicon carbide substrate surface by chlorine trifluoride gas (100%) at atmospheric pressure in the temperature range between 670 and 970 K. Square: 0.2 slm, circle: 0.1 slm, and triangle: 0.05 slm.

Figure 4. Etching rate of the polycrystalline 3C-silicon carbide substrate surface using chlorine trifluoride gas at 10-100%, 0.2 slm, atmospheric pressure, and 670-970K. Square: 670 K, reverse triangle: 730K, circle: 770K, diamond: 870 K, and triangle:970K.

#### 2.3. Surface morphology and roughness

temperature of 770 K, the etching rate at the flow rate of 0.2 slm becomes 20 µ)u)%\*az%0z.!¥

As shown in Figure 3, the etching rate changes with the flow rates. The etching rates are 25, 10 and 5 µm/min at the flow rates of 0.2, 0.1 and 0.05 slm, respectively. For each chlorine trifluoride gas flow rate, the trend in the flat etching rate at temperature greater than 770K

In order to evaluate the influence of chlorine trifluoride gas concentration, the etching rate of the polycrystalline 3C-silicon carbide substrate surface using 10-100% chlorine trifluoride gas in ambient nitrogen was measured at the flow rate of 0.2 slm, atmospheric pressure and 670-970 K, as shown in Figure 4. In this figure, the square, reverse triangle, circle, diamond, and triangle show the substrate temperatures of 670, 730, 770, 870 and 970 K, respectively. Being consistent with Figure 3, the etching rate at the substrate temperature of 670K is very

0z 0\$!z /1/0.0!z 0!),!.01.!z +"z JJCz \_z 0\$!z !0\$%\*#z .0!z%/z ,.+,+.0%+\*(z 0+z 0\$!z \$(+.%\*!z 0.%¥ fluoride gas concentration. At 870 and 970 K, the etching rate at each chlorine trifluoride gas concentration is the same as that at 770 K. Therefore, when the substrate temperature is higher than 770 K, the etching rate over a very wide chlorine trifluoride gas concentration

Figure 3. Etching rate of the polycrystalline 3C-silicon carbide substrate surface by chlorine trifluoride gas (100%) at atmospheric pressure in the temperature range between 670 and 970 K. Square: 0.2 slm, circle: 0.1 slm, and triangle:

low. The etching rates at 730 K are significantly higher that those at 670 K.

range is not affected by the substrate temperature.

mains constant at substrate temperatures greater than 770 K.

is maintained.

102 Physics and Technology of Silicon Carbide Devices

0.05 slm.

The change in the surface morphology of the 3C-silicon carbide is explained. Figure 5 shows photographs of the silicon carbide surface etched using chlorine trifluoride gas at the flow rate of 0.1 slm and atmospheric pressure at 670, 720 and 770 K for 15 min. The values in ,.!\*0\$!/!/z.!z0\$!z!0\$z !,0\$^z%#1.!zHzcdz/\$+3/z0\$!z%\*%0%(z/1."!z3\$%\$z\$/z0\$!z,!.%+ ¥ ically line-shaped hills and valleys. As shown in Figure 5 (b), although the surface still has the line-shaped morphology at 670 K after 15 minutes, its pattern remains but is unclear. 0z 0\$!z \$%#\$!.z 0!),!.01.!/z +"z JECz \* z JJCz\_z 0\$!z(%\*!w/\$,! z ,,!.\*!z +!/z \*+0z .!¥ main, as shown in Figures 5 (c) and (d). In these figures, there are very small and very shallow pits having a round edge. This shows that the etching using chlorine trifluoride can smooth the large hills and valleys which existed on the silicon carbide surface.

 \*z+. !.z0+z/\$+3z0\$!z !0%(z+"z/1."!z/)++0\$%\*#z!""!0\_z0\$!z\$\*#!z%\*z0\$!z/1."!z,,!.¥ ance is shown, in Figure 6, along etch period at a substrate temperature of 770 K and a flow rate of 0.1 slm of chlorine trifluoride. This figure shows photographs of the etched silicon carbide surface at (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, and (e) 30 min. The values in parenthes are the etch depth. Figures 6 (a) and (d) are the same as Figures 5 (a) and (d), .!/,!0%2!(5^z\$!z.!(0%+\*/\$%,z!03!!\*z0\$!z!0\$z,!.%+ z\* z0\$!z!0\$z !,0\$z%/z/\$+3\*z%\*z%#¥ ure 7 (a). The line-shaped pattern in Figure 6 (a) is slightly rounded after 5 minutes. At 10 minutes, there is only a trace of the line-shaped appearance, as shown in Figure 6 (c). The 3C-silicon carbide surface has a round-shaped morphology after 15 minutes as shown in Figure 6 (d), since the line-shaped pattern is removed during the etch period between 10 and 15 minutes. The surface morphology in Figure 6 (d) is maintained at 30 minutes in Figure 6 (e), and the rounded edges of the very shallow pits do not become sharp during the last DHz)%\*10!/^z\$1/\_z 0\$!z.+1\* w/\$,! z)+.,\$+(+#5z+"z/%(%+\*z.% !z/1."!z3%((z!z)%\*¥ tained during etching for longer than 30 minutes.

Figure 5. Photograph of the silicon carbide substrate surface etched using chlorine trifluoride at atmospheric pressure after 15 min at (b) 670 K, (c) 720 K, and (d) 770 K. (a) is the silicon carbide substrate surface before etching. The values in parentheses are the etch depth.

Figure 7. Etch depth and roughness of the silicon carbide surface etched by chlorine trifluoride gas at atmospheric

Etching of Silicon Carbide Using Chlorine Trifluoride Gas

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

105

Figure 8 shows photographs of the polycrystalline 3C-silicon carbide surface etched using a gas mixture of chlorine trifluoride and nitrogen at 670-870K for 15 min. The concentration and the flow rate of the gas mixture are 10-100% and 0.2 slm, respectively. Figure 9 shows the RMS roughness of the polycrystalline 3C-silicon carbide surface etched using chlorine trifluoride gas at atmospheric pressure and 670-870K for 15 min. The triangle, diamond and circle show the RMS roughness at the chlorine trifluoride gas concentrations of 10, 50 and

The photograph indicated by 'Before etch' in Figure 8 shows the initial surface, which has 2!.5z\*..+3\_z2#1!z\* z/\$((+3z0.!\*\$!/z"+.)! z5z)!\$\*%(z,+(%/\$%\*#^z/%\*#z0\$!z\$(+.¥ ine trifluoride gas concentration of 10%, the change in the surface morphology is explained. The surface etched at 730K and 10% is recognized to have circular-shaped pits. Although the etching rate under this condition is very low, its surface shown in Figure 8 has an etched depth of 15 µm. This surface shows many circle-like pits, the edge of which is clearly shown. \$!/!z)5z!z0\$!z#.%\*z+1\* .5z+.z/+)!z %/+. !.! z.!#%+\*z3\$%\$z\*z!z!0\$! z0zz/(%#\$0¥

At 770 K and 10 %, the shape of the circular-shaped pits still clearly exists, similar to that at 730 K and 10 %. The photograph at 870 K and 10% shows pits smaller than those at 770 K and 10%. Simultaneously, the conical shape of the pits still exists. This shows that chlorine 0.%"(1+.% !z#/z0z0\$!z(+3z+\*!\*0.0%+\*z+"zDCMz\$/zz-1%0!z/)((z.+(!z+"z/)++0\$%\*#z0\$!z/1.¥

pressure and 770 K within 30 min: (a) etch depth, and (b) root mean square (RMS) roughness.

100 %, respectively.

ly higher etching rate.

Figure 6. Photograph of the silicon carbide surface etched using chlorine trifluoride at atmospheric pressure and 770 K at (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, and (e) 30 min. The values in parenthes are the etch depth.

In order to evaluate the surface smoothing effect of silicon carbide by chlorine trifluoride gas, the surface roughness is measured using the root-mean-square (RMS) roughness as shown in Figure 7 (b). Figure 7 (a) also shows the etch depth. The initial surface has an RMS roughness of 5 µ)^z\$!z/1."!z.+1#\$\*!//z !.!/!/z3%0\$z%\*.!/%\*#z!0\$z,!.%+ ^z0zDCz)%¥ nutes, when 180 µm has been etched, the RMS roughness becomes a low value of 1 µm. Consistent with Figures 6 (d) and (e), the RMS roughness is maintained at nearly 1 µm at 30 minutes when the etch depth becomes greater than 500 µm. Solid line in Figure 7 (b) %\* %¥ cates the possibility that the chlorine trifluoride gas has a smoothing effect on the 3C-silicon carbide surface.

and 15 minutes. The surface morphology in Figure 6 (d) is maintained at 30 minutes in Figure 6 (e), and the rounded edges of the very shallow pits do not become sharp during the last DHz)%\*10!/^z\$1/\_z 0\$!z.+1\* w/\$,! z)+.,\$+(+#5z+"z/%(%+\*z.% !z/1."!z3%((z!z)%\*¥

Figure 5. Photograph of the silicon carbide substrate surface etched using chlorine trifluoride at atmospheric pressure after 15 min at (b) 670 K, (c) 720 K, and (d) 770 K. (a) is the silicon carbide substrate surface before etching. The values

Figure 6. Photograph of the silicon carbide surface etched using chlorine trifluoride at atmospheric pressure and 770

In order to evaluate the surface smoothing effect of silicon carbide by chlorine trifluoride gas, the surface roughness is measured using the root-mean-square (RMS) roughness as shown in Figure 7 (b). Figure 7 (a) also shows the etch depth. The initial surface has an RMS roughness of 5 µ)^z\$!z/1."!z.+1#\$\*!//z !.!/!/z3%0\$z%\*.!/%\*#z!0\$z,!.%+ ^z0zDCz)%¥ nutes, when 180 µm has been etched, the RMS roughness becomes a low value of 1 µm. Consistent with Figures 6 (d) and (e), the RMS roughness is maintained at nearly 1 µm at 30 minutes when the etch depth becomes greater than 500 µm. Solid line in Figure 7 (b) %\* %¥ cates the possibility that the chlorine trifluoride gas has a smoothing effect on the 3C-silicon

K at (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, and (e) 30 min. The values in parenthes are the etch depth.

tained during etching for longer than 30 minutes.

104 Physics and Technology of Silicon Carbide Devices

in parentheses are the etch depth.

carbide surface.

Figure 7. Etch depth and roughness of the silicon carbide surface etched by chlorine trifluoride gas at atmospheric pressure and 770 K within 30 min: (a) etch depth, and (b) root mean square (RMS) roughness.

Figure 8 shows photographs of the polycrystalline 3C-silicon carbide surface etched using a gas mixture of chlorine trifluoride and nitrogen at 670-870K for 15 min. The concentration and the flow rate of the gas mixture are 10-100% and 0.2 slm, respectively. Figure 9 shows the RMS roughness of the polycrystalline 3C-silicon carbide surface etched using chlorine trifluoride gas at atmospheric pressure and 670-870K for 15 min. The triangle, diamond and circle show the RMS roughness at the chlorine trifluoride gas concentrations of 10, 50 and 100 %, respectively.

The photograph indicated by 'Before etch' in Figure 8 shows the initial surface, which has 2!.5z\*..+3\_z2#1!z\* z/\$((+3z0.!\*\$!/z"+.)! z5z)!\$\*%(z,+(%/\$%\*#^z/%\*#z0\$!z\$(+.¥ ine trifluoride gas concentration of 10%, the change in the surface morphology is explained. The surface etched at 730K and 10% is recognized to have circular-shaped pits. Although the etching rate under this condition is very low, its surface shown in Figure 8 has an etched depth of 15 µm. This surface shows many circle-like pits, the edge of which is clearly shown. \$!/!z)5z!z0\$!z#.%\*z+1\* .5z+.z/+)!z %/+. !.! z.!#%+\*z3\$%\$z\*z!z!0\$! z0zz/(%#\$0¥ ly higher etching rate.

At 770 K and 10 %, the shape of the circular-shaped pits still clearly exists, similar to that at 730 K and 10 %. The photograph at 870 K and 10% shows pits smaller than those at 770 K and 10%. Simultaneously, the conical shape of the pits still exists. This shows that chlorine 0.%"(1+.% !z#/z0z0\$!z(+3z+\*!\*0.0%+\*z+"zDCMz\$/zz-1%0!z/)((z.+(!z+"z/)++0\$%\*#z0\$!z/1.¥ face, but rather tends to roughen it. This trend is measured using the RMS roughness, shown by the triangles in Figure 9. The RMS roughness before etching is nearly 0.5 µm; it %\*.!/!/z3%0\$z0\$!z%\*.!/%\*#z/1/0.0!z0!),!.01.!z0z0\$!z\$(+.%\*!z0.%"(1+.% !z#/z+\*!\*0.¥ tion of 10%.

Figure 9. RMS roughness of the polycrystalline 3C-silicon carbide substrate surface etched using chlorine trifluoride

Etching of Silicon Carbide Using Chlorine Trifluoride Gas

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

107

From the view point of the effect of the etchant gas concentration at each temperature, the /\$,!z+"z0\$!z,%0/z0!\* /z0+z!+)!z1\*(!.z3%0\$z0\$!z%\*.!/%\*#z\$(+.%\*!z0.%"(1+.% !z#/z+\*¥ !\*0.0%+\*^z\$!.!"+.!\_z/z 0\$!z+2!.((z 0.!\* \_z 0\$!z\$%#\$!.z 0!),!.01.!z\* z 0\$!z\$%#\$!.z\$(+.¥ ine trifluoride gas concentration produces a smoother surface of the polycrystalline 3C-

The polycrystalline 3C-silicon carbide etching rate can be adjusted using the combination of gas flow rate, gas concentration and the substrate temperature, in order to obtain surfaces suitable for various purposes. This technique is expected to be used for various applications, such as the dry cleaning of the silicon carbide substrate surface instead of wet method, and the removal of the damaged layer formed during the chemical mechanical polishing using

The fraction of silicon and carbon on the silicon carbide surface remaining after the etching %/z1/!"1(z%\*"+.)0%+\*z "+.z !2!(+,%\*#z2.%+1/z,.+!//!/z3\$%\$z.!z,!."+.)! z"0!.z 0\$!z!0\$¥ %\*#^z\$1/\_z0\$!z\$!)%(z+\* /z0z0\$!z/%(%+\*z.% !z/1."!z.!z)!/1.! z1/%\*#zw.5z,\$+0+¥ electron spectroscopy (XPS) before and after etching by chlorine trifluoride gas. Figure 10 shows the fraction of carbon, silicon, oxygen, chlorine and fluorine on the silicon carbide substrate before and after etching the depth greater than 150 µ)z1/%\*#z0\$!z\$(+.%\*!z0.%"(1+.¥

The silicon carbide surface before the etching has a carbon fraction nearly equal to that of silicon. However, the fraction of carbon significantly increases to 75 % after the etching. This %\* %0!/z0\$0z0\$!z,.+ 10%+\*z+"z2+(0%(!z.+\*z+),+1\* z%/z/(+3!.z0\$\*z0\$0z+"z/%(%+\*z+)¥

gas at atmospheric pressure and 670-870 K for 15 min. Triangle: 10 %, diamond: 50 %, and circle: 100%.

silicon carbide.

diamond slurry.

pound at this temperature.

2.4. Surface chemical condition and etching rate

ide gas for 15 min at atmospheric pressure and at 720 K.

Figure 8. ,@GLG?J9H@G>L@=HGDQ;JQKL9DDAF=KADA;GF;9J:A<=KMJ>9;==L;@=<MKAF?;@DGJAF=LJA>DMGJA<=?9K9L9LEGKs pheric pressure for 15 min at 670-870 K, 10-100% and 0.2 slm.

!40\_z0z0\$!z"%4! z\$(+.%\*!z0.%"(1+.% !z#/z+\*!\*0.0%+\*z+"zHCM\_z0\$!z\$\*#!z%\*z0\$!z!0\$! z/1.¥ face morphology is explained using Figure 8. The surface etched at 730 K and 50% shows the clear edge shape of the pits. The surface etched at 770 K and 50% still has a clear edge of the conical-shaped pits. Although some pits still have such the clearly observed edge shape, the rest of the surface has no clear edges. The surface morphology at 870 K and 50% shows both clear and vague edges. Since semi-smoothed and clear pits coexist there, the RMS roughness, indicated by the diamonds in Figure 9, still slightly increases with the increasing substrate temperature.

The change in the surface morphology etched at 100% is also shown in Figure 8. The surface etched at 670 K and 100% shows the clear edge of the pits. In contrast to this, the surface etched at 730 K and 100% shows the slightly vague edge of the pits. For the surface etched at 770 K and 100 %, the conical-shaped pits still remain, but are few. The edge of the conicalshaped pits disappears, when the surface is etched at 870 K. Since this trend in smoothing the surface appears in the RMS roughness behavior, the RMS roughness at 100%, indicated using circles in Figure 9, slightly decreases with the increasing substrate temperature.

Figure 9. RMS roughness of the polycrystalline 3C-silicon carbide substrate surface etched using chlorine trifluoride gas at atmospheric pressure and 670-870 K for 15 min. Triangle: 10 %, diamond: 50 %, and circle: 100%.

From the view point of the effect of the etchant gas concentration at each temperature, the /\$,!z+"z0\$!z,%0/z0!\* /z0+z!+)!z1\*(!.z3%0\$z0\$!z%\*.!/%\*#z\$(+.%\*!z0.%"(1+.% !z#/z+\*¥ !\*0.0%+\*^z\$!.!"+.!\_z/z 0\$!z+2!.((z 0.!\* \_z 0\$!z\$%#\$!.z 0!),!.01.!z\* z 0\$!z\$%#\$!.z\$(+.¥ ine trifluoride gas concentration produces a smoother surface of the polycrystalline 3Csilicon carbide.

The polycrystalline 3C-silicon carbide etching rate can be adjusted using the combination of gas flow rate, gas concentration and the substrate temperature, in order to obtain surfaces suitable for various purposes. This technique is expected to be used for various applications, such as the dry cleaning of the silicon carbide substrate surface instead of wet method, and the removal of the damaged layer formed during the chemical mechanical polishing using diamond slurry.

#### 2.4. Surface chemical condition and etching rate

face, but rather tends to roughen it. This trend is measured using the RMS roughness, shown by the triangles in Figure 9. The RMS roughness before etching is nearly 0.5 µm; it %\*.!/!/z3%0\$z0\$!z%\*.!/%\*#z/1/0.0!z0!),!.01.!z0z0\$!z\$(+.%\*!z0.%"(1+.% !z#/z+\*!\*0.¥

Figure 8. ,@GLG?J9H@G>L@=HGDQ;JQKL9DDAF=KADA;GF;9J:A<=KMJ>9;==L;@=<MKAF?;@DGJAF=LJA>DMGJA<=?9K9L9LEGKs

!40\_z0z0\$!z"%4! z\$(+.%\*!z0.%"(1+.% !z#/z+\*!\*0.0%+\*z+"zHCM\_z0\$!z\$\*#!z%\*z0\$!z!0\$! z/1.¥ face morphology is explained using Figure 8. The surface etched at 730 K and 50% shows the clear edge shape of the pits. The surface etched at 770 K and 50% still has a clear edge of the conical-shaped pits. Although some pits still have such the clearly observed edge shape, the rest of the surface has no clear edges. The surface morphology at 870 K and 50% shows both clear and vague edges. Since semi-smoothed and clear pits coexist there, the RMS roughness, indicated by the diamonds in Figure 9, still slightly increases with the increasing

The change in the surface morphology etched at 100% is also shown in Figure 8. The surface etched at 670 K and 100% shows the clear edge of the pits. In contrast to this, the surface etched at 730 K and 100% shows the slightly vague edge of the pits. For the surface etched at 770 K and 100 %, the conical-shaped pits still remain, but are few. The edge of the conicalshaped pits disappears, when the surface is etched at 870 K. Since this trend in smoothing the surface appears in the RMS roughness behavior, the RMS roughness at 100%, indicated

using circles in Figure 9, slightly decreases with the increasing substrate temperature.

pheric pressure for 15 min at 670-870 K, 10-100% and 0.2 slm.

substrate temperature.

tion of 10%.

106 Physics and Technology of Silicon Carbide Devices

The fraction of silicon and carbon on the silicon carbide surface remaining after the etching %/z1/!"1(z%\*"+.)0%+\*z "+.z !2!(+,%\*#z2.%+1/z,.+!//!/z3\$%\$z.!z,!."+.)! z"0!.z 0\$!z!0\$¥ %\*#^z\$1/\_z0\$!z\$!)%(z+\* /z0z0\$!z/%(%+\*z.% !z/1."!z.!z)!/1.! z1/%\*#zw.5z,\$+0+¥ electron spectroscopy (XPS) before and after etching by chlorine trifluoride gas. Figure 10 shows the fraction of carbon, silicon, oxygen, chlorine and fluorine on the silicon carbide substrate before and after etching the depth greater than 150 µ)z1/%\*#z0\$!z\$(+.%\*!z0.%"(1+.¥ ide gas for 15 min at atmospheric pressure and at 720 K.

The silicon carbide surface before the etching has a carbon fraction nearly equal to that of silicon. However, the fraction of carbon significantly increases to 75 % after the etching. This %\* %0!/z0\$0z0\$!z,.+ 10%+\*z+"z2+(0%(!z.+\*z+),+1\* z%/z/(+3!.z0\$\*z0\$0z+"z/%(%+\*z+)¥ pound at this temperature.

12 (a) shows that chemical bonds of carbon with silicon dominate at the substrate surface before etching. After etching, an almost amount of carbon has chemical bonds with carbon as shown in Figure 12 (b). The surface after etching is covered with large amount of carbon having carbon-carbon bonds. This result is consistent with the dark appearance of the silicon

Etching of Silicon Carbide Using Chlorine Trifluoride Gas

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

109

Figure 12. XPS spectra of C 1s measured (a) before and (b) after the etching using chlorine trifluoride gas for 15 min

The gaseous products and the chemical reactions associated with silicon carbide etching are explained. Figure 13 shows the mass spectra of the gaseous species existing in the exhaust #/z%))! %0!(5z"0!.z!#%\*\*%\*#z/%(%+\*z.% !z/1/0.0!z!0\$%\*#z0zJJCzz1/%\*#z\$(+.%\*!z0.%¥ "(1+.% !z#/z 0z 0)+/,\$!.%z,.!//1.!^z\$!z,.0%(z,.!//1.!/z .!z\*+.)(%6! z1/%\*#z 0\$!z,.!/¥ sure at the mass of 28 a.m.u., which is the largest partial pressure in this measurement and which can be assigned to silicon from the silicon tetrafluoride and nitrogen remaining in the

5,%(z)//z /,!0.\_z /\$+3\*z%\*z %#1.!z DF\_z .!z%\*0!.,.!0! z 5z 0'%\*#z%\*0+z +1\*0z 0\$!z ".#¥ mentation in the QMS analyzer and the isotopic abundance of chlorine [31-33]. In this figure,

gas. The low partial pressures corresponding to chlorine trifluoride and its fragment, ClF3 <sup>+</sup>

chlorine gas. Chlorine gas is produced due to the chemical reaction during silicon carbide

, which is the fragment of nitrogen

, which is assigned to

z0zHGz\* zHIz^)^1^\_z.!z !0!0! ^z\$!z,.0%(z,.!/¥

,

at atmospheric pressure. The temperature of the silicon carbide substrate is 720 K.

the ion species at a mass of 14 a.m.u. is assigned to N+

sures observed at masses of 70, 72 and 74 a.m.u. correspond to Cl2 <sup>+</sup>

at masses of 92 and 94 a.m.u., and ClF+

carbide surface after the etching.

2.5. Chemical reactions

QMS system.

Figure 10. Fraction of carbon, silicon, oxygen, chlorine and fluorine on the silicon carbide substrate surface before and 9>L=J=L;@AF?MKAF?;@DGJAF=LJA>DMGJA<=>GJEAF9L9LEGKH@=JA;HJ=KKMJ=AFL@=J=9;LGJ0@=L=EH=J9LMJ=G>L@=KADAs con carbide substrate is 720 K.

Figure 11. XPS spectra of Si 2p measured (a) before and (b) after etching using chlorine trifluoride gas for 15 min at atmospheric pressure. The temperature of the silicon carbide substrate is 720 K.

In order to evaluate the state of silicon and carbon, the XPS spectra of Si 2p and C 1s are shown in Figures 11 and 12, respectively. The conditions of etching are the same as in Figure 10.

An almost amount of silicon at the silicon carbide surface has chemical bond with carbon before etching as shown in Figure 11 (a). However, after the etching, a significant amount of silicon oxides and oxidized or halogenated silicon carbide are present as shown in Figure 11 (b). The chemical bonds of carbon simultaneously change, as same as those of silicon. Figure 12 (a) shows that chemical bonds of carbon with silicon dominate at the substrate surface before etching. After etching, an almost amount of carbon has chemical bonds with carbon as shown in Figure 12 (b). The surface after etching is covered with large amount of carbon having carbon-carbon bonds. This result is consistent with the dark appearance of the silicon carbide surface after the etching.

Figure 12. XPS spectra of C 1s measured (a) before and (b) after the etching using chlorine trifluoride gas for 15 min at atmospheric pressure. The temperature of the silicon carbide substrate is 720 K.

#### 2.5. Chemical reactions

Figure 10. Fraction of carbon, silicon, oxygen, chlorine and fluorine on the silicon carbide substrate surface before and 9>L=J=L;@AF?MKAF?;@DGJAF=LJA>DMGJA<=>GJEAF9L9LEGKH@=JA;HJ=KKMJ=AFL@=J=9;LGJ0@=L=EH=J9LMJ=G>L@=KADAs

Figure 11. XPS spectra of Si 2p measured (a) before and (b) after etching using chlorine trifluoride gas for 15 min at

In order to evaluate the state of silicon and carbon, the XPS spectra of Si 2p and C 1s are shown in Figures 11 and 12, respectively. The conditions of etching are the same as in

An almost amount of silicon at the silicon carbide surface has chemical bond with carbon before etching as shown in Figure 11 (a). However, after the etching, a significant amount of silicon oxides and oxidized or halogenated silicon carbide are present as shown in Figure 11 (b). The chemical bonds of carbon simultaneously change, as same as those of silicon. Figure

atmospheric pressure. The temperature of the silicon carbide substrate is 720 K.

con carbide substrate is 720 K.

108 Physics and Technology of Silicon Carbide Devices

Figure 10.

The gaseous products and the chemical reactions associated with silicon carbide etching are explained. Figure 13 shows the mass spectra of the gaseous species existing in the exhaust #/z%))! %0!(5z"0!.z!#%\*\*%\*#z/%(%+\*z.% !z/1/0.0!z!0\$%\*#z0zJJCzz1/%\*#z\$(+.%\*!z0.%¥ "(1+.% !z#/z 0z 0)+/,\$!.%z,.!//1.!^z\$!z,.0%(z,.!//1.!/z .!z\*+.)(%6! z1/%\*#z 0\$!z,.!/¥ sure at the mass of 28 a.m.u., which is the largest partial pressure in this measurement and which can be assigned to silicon from the silicon tetrafluoride and nitrogen remaining in the QMS system.

5,%(z)//z /,!0.\_z /\$+3\*z%\*z %#1.!z DF\_z .!z%\*0!.,.!0! z 5z 0'%\*#z%\*0+z +1\*0z 0\$!z ".#¥ mentation in the QMS analyzer and the isotopic abundance of chlorine [31-33]. In this figure, the ion species at a mass of 14 a.m.u. is assigned to N+ , which is the fragment of nitrogen gas. The low partial pressures corresponding to chlorine trifluoride and its fragment, ClF3 <sup>+</sup> , at masses of 92 and 94 a.m.u., and ClF+ z0zHGz\* zHIz^)^1^\_z.!z !0!0! ^z\$!z,.0%(z,.!/¥ sures observed at masses of 70, 72 and 74 a.m.u. correspond to Cl2 <sup>+</sup> , which is assigned to chlorine gas. Chlorine gas is produced due to the chemical reaction during silicon carbide etching, similar to that for the silicon etching [22, 34]. The partial pressures at masses of 35 and 37 a.m.u. can be assigned to Cl+ , which is a fragment from both chlorine trifluoride and chlorine (Cl2). The partial pressures observed at masses of 19 and 20 a.m.u. correspond to F+ and HF+ , respectively. F+ and HF+ .!z,.+ 1! z 1!z0+z0\$!z".#)!\*00%+\*z+"z\$(+.%\*!z0.%"(1+.¥ 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 of the cold wall reactor used in this study.

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

thick produced by the chemical vapor deposition method (Admap Inc., Tokyo).

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

Etching of Silicon Carbide Using Chlorine Trifluoride Gas

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

111

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 X-ray topograph of Si-face and C-face of 4H-silicon carbide was taken at the beam-line

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,'¥

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

3SiC · + ·8ClF3 · ¨ ·3SiF4 · + ·3CF4 · + ·4Cl2 (2)

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.¥

Trifluoride Gas



3.1. Substrate, reactor and process

the wide 4H-silicon carbide substrate.

ious temperatures were evaluated.

3.2. Numerical calculation of etching rate

age, Fluent version 6 (Fluent, Inc., Lebanon, NH, USA).

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

The partial pressures at masses of 66, 85 and 104 a.m.u. can be assigned to SiF2 <sup>+</sup> , SiF3 <sup>+</sup> and SiF4 <sup>+</sup> , 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!¥ cause the partial pressures at masses of 50 and 69 a.m.u. correspond to CF2 <sup>+</sup> and CF3 <sup>+</sup> , respectively, which can be assigned as fragments of carbon tetrafluoride.

\$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 trifluoride is as follows:

$$\text{3SiC} \cdot + \cdot \text{8Cl} \\ \text{F}\_3 \cdot \rightarrow \cdot \text{3SiF}\_4 \\ \cdot + \cdot \text{3CF}\_4 \\ \cdot + \cdot \text{4Cl}\_2 \tag{1}$$
