3. Influence of the deposition parameters

#### 3.1. Materials and Methods

This section describes the influence of the main deposition parameters on the film properties, being a practical guide of parameter selection for a desired characteristic of the deposited a-SiC thin film. The experiments were performed on 4 inch diameter, p type, 500um-thick silicon wafers with a (100) crystallographic orientation. The wafers were initially cleaned in piranha (H₂SQ;H₂O₂ in the ratio of 2:1) at 120°C for 20 minutes and rinsed in DI water. The native silicon oxide layer was then removed by immersing the wafers in a classical BOE solution for 30 seconds. Stress measurement was performed using a KLA Tencor FLX-2320 system while the thickness uniformity of the thin filmswere measured with a refractometer (Filmetrics F50, USA). The deposition of the tested a-SiC layers was performed using a STS Multiplex Pro-CVD PECVD system described in detail elsewhere [35, 36]. This system enables two RF deposition modes: a low frequency (LF) mode at 380 kHz with a tuning power between 0 to 1 kW, and/or a high frequency (HF) mode at 13.56 MHz with a selected power in the range between 0 to 600 W. The depositions of the a-SiC layers were performed using pure SiH (pure) and CHz as precursors, with Ar as an overall dilution gas.

#### 3.2. Pressure

The a-SiC deposition's uniformity is strongly dependent on the pressure in the reactor chamber (Fig. 1a).

Figure 1. Variation of: (a)uniformity, and (b)deposition rate, with the pressure for α-SiC PECVD deposition in a STS Multiplex Pro-CVD PECVD system.

Below 800mTorr, large non-uniformity values were observed, but the deposition uniformity linearly improved as the pressure was varied between 500 to 800mTorr, finally settling to a constant valueof the uniformities under 2% for all the pressures in the range 900 to 1400mTorr. The thickness uniformity's map presented a "donut" shape, which means that the gas molecules present a high velocity, increasing the deposition rate at the edge of the wafer.

Another important aspect of the pressure is its influence on the deposition rate (Fig. 1b): low pressures reduce the concentration of reactive species thus resulting in a low deposition rate, which increases quasi-linearly with pressure.

#### 3.3. RF Power

Both the RF power and the power deposition mode are key parameters for tuning the optical and mechanical properties of the deposited PECVD a-SiC layer. A low value of the residual stress is required in MEMS applications where free standing structure is fabricated. Meanwhile, a high deposition rate is desired mainly in industrial applications. Fig. 2 illustrates the variation of the deposition rate and residual stress versus the RF power for the HF mode. The linear dependence of the deposition rate on the HF power noticed for power levels below 300W can be explained by the proportional increase in the dissociation of reactant gases with increasing the power. After a 'threshold' value (300W) the dissociation into reactive species is no longer a crucial factor as probably all reactive species are easily and fully dissociated, hence further increasing the power has little effect on the deposition rate.

Figure 2. Variation of the deposition rate and residual stress with the HF power in the STS Multiplex Pro-CVD PECVD reactor. The deposition temperature was 300°C, the pressure 1100mTorr, and the gas flow rates of SiH, CH, and Ar were 45, 300 and 700sccm, respectively.

An interesting characteristic of PECVD α-SiC thin films deposited using the dual mode technique, particularly when compared to other PECVD deposition methods, is the very low value of the internal average stress, which can range between 50MPa and -70MPa. Fig. 2 shows the variation of this residual average with the RF power. Our results indicate that the average stress variation of a -SiC film deposited in the HF deposition regime is not due to )+ %"%0%+\*/z+"z0\$!z"%()j/z\$!)%(z+),+/%0%+\*\_z10z.0\$!.z 1!z0+z%\*0!.\*(z/0.101.(z.!w.¥ .\*#!)!\*0z!1/!z0\$!z%\*.!/%\*#(5z\$%#\$!.z2(1!/z+"zz,+3!.z-1%'(5z,.+2% !z!\*+1#\$z!\*¥ ergy for the adsorbed species to migrate and find the more energetically favorable sites for the film growth to take place.

better dissociation of the gasses that is reflected in a higher deposition rate.The deposition in LF mode is more suitable when PECVD -SiC is used as masking layer or in applications for

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http://dx.doi.org/10.5772/51224

137

z2!.5z%\*0!.!/0%\*#z\$.0!.%/0%z+"z0\$!zzw%z !,+/%0%+\*z%\*z0\$!zz)+ !z\* z3\$%\$z\* be useful in many applications is that the deposition rate as well as the residual stress do not ,.!/!\*0z.!(!2\*0z2.%0%+\*/z/zz"1\*0%+\*z+"z0!),!.01.!zeFJf^z \*z+\*0./0\_z%\*z0\$!zz)+ !\_z0!)¥ perature has an important effect on the stress value, although the deposition rate is not much ""!0! z5z0!),!.01.!z%\*zz)+ !z/z3!((zeFJf^z/z%/z !,%0! z%\*z%#1.!zG\_z0\$!z%\*0!.\*(z2!.¥ #!z/0.!//z%/z+),.!//%2!zc.+1\* zPDDC
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Figure 4. Variation of the average residual stress of PECVD i-SiC films with the deposition temperature. The films were deposited in the HF mode at a power of 150W, at a pressure of 1100mTorr, and with gas flow rates of SiH4, CH4

The influence of the SiH4/CH4z#/z"(+3z.0%+\_z%((1/0.0! z%\*z%#^zH\_z/\$+3/zz(%\*!.z !,!\* ¥ ence of the (compressive) residual stress on the SiH4/CH4 ratio for depositions in the HF mode. As expected, increasing the SiH4 z+\*0!\*0z ".+)zD`DCz 0+zE`DCz%\*z 0\$!z#/z "(+3z)'!/ 0\$!z !,+/%0! zw%z"%()z)+.!z]%w.%\$kz\* z0z0\$!z/)!z0%)!z !.!/!/z0\$!z)#\*%01 !z+"

harsh environments (due to the densification of the layer).

6!.+dz3\$!\*z0\$!z !,+/%0%+\*z0!),!.01.!z%/z/%010! z!03!!\*zFCCz\* zFHC[^

rangement, not from modifications of its chemical composition [37].

and Ar of 45, 300 and 700sccm, respectively.

3.5. SiH4/CH4 ratio

3.4. Temperature

\$!z .!".0%2!z%\* !4z3/z ()+/0z +\*/0\*0z%\*z 0\$!z .\*#!z !03!!\*z E^Hz \* z E^Iz3\$%(!z 0\$!z 1\*%¥ formity of the deposition and uniformity of the refractive index was below 1.5%.

Figure 3. Variation of the deposition rate and residual stress with the LF power in the same PECVD reactor under the same conditions as in Fig. 2.

The variation of the deposition rate and residual stress of PECVD-SiC films deposited in LF mode are presented in Fig. 3. The differences between HF mode and LF mode are quite evident if one compares the results shown in Fig. 3 with those shown in Fig. 2^z\$!z !,+/%¥ tion rate depends linearly with the LF power. However, for low values of LF power, the stress is highly compressive (even below P1200MPa) but quickly reduces to about PHCC and remains almost constant at this value for any LF power above 300W. This variation can be explained by the densification of the layer determined by the increasingly more energetic ion bombardment with increasing the LF power, which characterizes the LF deposition mode. At high frequencies, only the electrons are able to follow the RF field while the ions cannot follow the instantaneous variations of the electric field due to their heavier mass. The cross over frequency at which the ions start following the electric field is between 1 and H
6z !,!\* %\*#z1,+\*z0\$!z)//z+"z0\$!z%+\*/^z+\*/!-1!\*0(5\_z!(+3zD
6\_z0\$!z%+\*z+). ¥ ment is significantly higher, which not only enhances chemical reactions but also densifies the film. However, the refractive index of -SiC film decreases with the increasing the LF power (from 2.9 to 2.5). It can be concluded that, for a low value of the residual stress in the -SiC layers, the HF deposition mode is more suitable. Meanwhile, the HF mode assures a better dissociation of the gasses that is reflected in a higher deposition rate.The deposition in LF mode is more suitable when PECVD -SiC is used as masking layer or in applications for harsh environments (due to the densification of the layer).

#### 3.4. Temperature

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Figure 3. Variation of the deposition rate and residual stress with the LF power in the same PECVD reactor under the

The variation of the deposition rate and residual stress of PECVD-SiC films deposited in LF mode are presented in Fig. 3. The differences between HF mode and LF mode are quite evident if one compares the results shown in Fig. 3 with those shown in Fig. 2^z\$!z !,+/%¥ tion rate depends linearly with the LF power. However, for low values of LF power, the stress is highly compressive (even below P1200MPa) but quickly reduces to about PHCC and remains almost constant at this value for any LF power above 300W. This variation can be explained by the densification of the layer determined by the increasingly more energetic ion bombardment with increasing the LF power, which characterizes the LF deposition mode. At high frequencies, only the electrons are able to follow the RF field while the ions cannot follow the instantaneous variations of the electric field due to their heavier mass. The cross over frequency at which the ions start following the electric field is between 1 and H
6z !,!\* %\*#z1,+\*z0\$!z)//z+"z0\$!z%+\*/^z+\*/!-1!\*0(5\_z!(+3zD
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formity of the deposition and uniformity of the refractive index was below 1.5%.

the film growth to take place.

136 Physics and Technology of Silicon Carbide Devices

same conditions as in Fig. 2.

z2!.5z%\*0!.!/0%\*#z\$.0!.%/0%z+"z0\$!zzw%z !,+/%0%+\*z%\*z0\$!zz)+ !z\* z3\$%\$z\* be useful in many applications is that the deposition rate as well as the residual stress do not ,.!/!\*0z.!(!2\*0z2.%0%+\*/z/zz"1\*0%+\*z+"z0!),!.01.!zeFJf^z \*z+\*0./0\_z%\*z0\$!zz)+ !\_z0!)¥ perature has an important effect on the stress value, although the deposition rate is not much ""!0! z5z0!),!.01.!z%\*zz)+ !z/z3!((zeFJf^z/z%/z !,%0! z%\*z%#1.!zG\_z0\$!z%\*0!.\*(z2!.¥ #!z/0.!//z%/z+),.!//%2!zc.+1\* zPDDC
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Figure 4. Variation of the average residual stress of PECVD i-SiC films with the deposition temperature. The films were deposited in the HF mode at a power of 150W, at a pressure of 1100mTorr, and with gas flow rates of SiH4, CH4 and Ar of 45, 300 and 700sccm, respectively.

### 3.5. SiH4/CH4 ratio

The influence of the SiH4/CH4z#/z"(+3z.0%+\_z%((1/0.0! z%\*z%#^zH\_z/\$+3/zz(%\*!.z !,!\* ¥ ence of the (compressive) residual stress on the SiH4/CH4 ratio for depositions in the HF mode. As expected, increasing the SiH4 z+\*0!\*0z ".+)zD`DCz 0+zE`DCz%\*z 0\$!z#/z "(+3z)'!/ 0\$!z !,+/%0! zw%z"%()z)+.!z]%w.%\$kz\* z0z0\$!z/)!z0%)!z !.!/!/z0\$!z)#\*%01 !z+" the compressive stress from about -70MPa to -20MPa, while at the same time the refractive index increases from 2.6 to 2.8.

Figure 5. Variation of the average residual stress with the SiHz/CH ratio for PECVD a-SiC films deposited at 300°C. The SiHz flow rate was kept constant at 45sccm, while all the other depositions were the same as those indicated in Fig. 4.

To conclude, an optimized process recipe for the deposition of a low stress a-SiC film using a STSP PECVD reactor has the following parameters:


Table 1.
