**3. Novel fast deposition techniques of microcrystalline silicon**

Now-a- days, for the high throughput of high-efficiency µc-Si solar cells in PV industry, one of the most crucial requirements is fast deposition of µc-Si without deteriorating the optical, structural and electronic properties of the film. To overcome the difficulty, several highdensity plasma sources have been developed, such as very high frequency (VHF) plasma, inductive coupling plasma (ICP) and surface wave plasma (SWP). As it has been reported, the excitation frequency of a plasma source has an important effect on the electron acceleration in the plasma, and a high excitation frequency is expected to result in a high electron density and a low electron temperature. Therefore two new microwave plasma sources have been developed i.e. Low-pressure high-density microwave plasma source utilizing the spoke antenna and the remote-type high-pressure microwave plasma using a quartz tube having an inner diameter of 10 mm and applied those for the fast deposition of µc-Si films for Si thin-film solar cells. The remote-type high-pressure microwave plasma will be discussed in elsewhere.

#### **3.1 Low-pressure high-density microwave plasma source utilizing the spoke antenna**

The microwave plasma source is shown schematically in Fig. 1, which is composed of the combination of a conventional microwave discharge and a spoke antenna. Its chamber size is 22 cm in diameter, which enables large-scale film processing. The spoke antenna is located on a 15 mm-thick quartz plate, which is not inside of the vacuum chamber. The antenna system is shown in Fig. 2 more in detail. The length of each spoke is 4 cm, which is about 1/4 of the wavelength of a 2.45 GHz wave. The design of the spoke antenna assembly

Fig. 1. A schematic illustration of the microwave plasma source

deposition chamber, the so called Plasma-Enhanced Chemical Vapor Deposition technique (PE-CVD) was developed later on and allowed the low-temperature deposition of µc-Si:H films, and rapid progresses have been achieved. Unfortunately, "state-of-the-art" microcrystalline silicon solar cells consist of intrinsic µc-Si:H layers that are deposited by rf and VHF PE-CVD at deposition rates of only 1-5 Å/s. On the other hand, a µc-Si:H film with a 2-µm- thickness intrinsic absorption layer is required for application to Si thin-film solar cells because of the low optical absorption in the visible region. The µc-Si:H i-layer deposition step is the most time consuming step in the deposition sequence of the solar cell.

Now-a- days, for the high throughput of high-efficiency µc-Si solar cells in PV industry, one of the most crucial requirements is fast deposition of µc-Si without deteriorating the optical, structural and electronic properties of the film. To overcome the difficulty, several highdensity plasma sources have been developed, such as very high frequency (VHF) plasma, inductive coupling plasma (ICP) and surface wave plasma (SWP). As it has been reported, the excitation frequency of a plasma source has an important effect on the electron acceleration in the plasma, and a high excitation frequency is expected to result in a high electron density and a low electron temperature. Therefore two new microwave plasma sources have been developed i.e. Low-pressure high-density microwave plasma source utilizing the spoke antenna and the remote-type high-pressure microwave plasma using a quartz tube having an inner diameter of 10 mm and applied those for the fast deposition of µc-Si films for Si thin-film solar cells. The remote-type high-pressure microwave plasma will

**3.1 Low-pressure high-density microwave plasma source utilizing the spoke antenna**  The microwave plasma source is shown schematically in Fig. 1, which is composed of the combination of a conventional microwave discharge and a spoke antenna. Its chamber size is 22 cm in diameter, which enables large-scale film processing. The spoke antenna is located on a 15 mm-thick quartz plate, which is not inside of the vacuum chamber. The antenna system is shown in Fig. 2 more in detail. The length of each spoke is 4 cm, which is about 1/4 of the wavelength of a 2.45 GHz wave. The design of the spoke antenna assembly

Spoke antenna

Chamber

4cm

**Spoke antenna** 

Therefore, a novel fast deposition technique of µc-Si:H is required.

Fig. 1. A schematic illustration of the microwave plasma source

Power Supply

22 cm

be discussed in elsewhere.

Quartz Plate

**3. Novel fast deposition techniques of microcrystalline silicon** 

is based on an inter-digital filter composed of parallel cylindrical rods (spokes) arranged between parallel-grounded plates. The spokes are resonantly coupled by the stray capacitance between adjacent spokes and the inductance of the spokes themselves. The resonance condition of an introduced angular frequency is given by ω=2π*f*=1/(C×L)1/2, where *f* is the introduced frequency, C is the array capacitance, and L is the antenna inductance. Thus, the antenna operates as a band-pass filter. The spokes are arranged like those in a wheel, and the plasma serves as one of the grounded plates. The electromagnetic wave propagates through the spokes consecutively with a phase difference of 90°, and microwave current flows in every spoke. The current in the spokes couples inductively and capacitively to the plasma ("CM coupling"), and the induction current in the plasma accelerates the electrons to sustain the plasma, as shown in Fig. 2 & 3. The power is supplied from the center of the antenna, and the plasma under the spoke antenna is radially discharged because induction current flows near every spoke. As a result, uniform microwave plasma over an area of diameter greater than 20 cm can be generated efficiently. As well, since no magnetic field is required to generate the high-density microwave plasma, it is possible to design a simple source yielding high-density and lowtemperature plasma.

(a) Microwave current

Fig. 2. The newly developed spoke antenna for introduction of microwave power (a) Microwave current, (b) Electric field.

From a material processing standpoint, large-area microwave plasmas (MWPs) have several advantages in comparison with other types of high-density sources. First, MWPs, being no magnetized sources, are free from such magnetic field induced problems as inhomogeneous density profile and charge-up damage, which is often, experienced in electron cyclotron resonance (ECR) or helicon plasma sources. Second, MWPs can be enlarged to diameters

Novel Deposition Technique for Fast Growth

the spoke antenna

microwave power.

**6 1010**

**3**

**)**

**1.2 1011**

**Electron density ne (cm-**

**1.8 1011**

**2.4 1011**

10 mTorr

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 363

Fig. 5. The schematic diagram of the low-pressure high-density microwave plasma utilizing

Z=6 cm A

Fig. 6. Electron density, ne, and electron temperature, Te, measured as a function of input

**200 400 600 800 1000 1200**

**Power (W)** 

**1**

**1.5**

**2**

**2.5**

**Electron temperature Te**

Z=6 cm

SiO2

**3**

**3.5**

**4**

**( V)**

Fig. 3. The coupling of the spoke antenna with microwave plasma [x]

Fig. 4. Images of Ar plasma at a) 80 mTorr and b) 20 Torr. The plasma maintains uniform state under a wide pressure regime.

longer than 1 m more easily than inductively coupled plasmas (ICPs). Thus, the application of MWPs to giant electronic devices such as solar cells is promising. Third, MWPs have lower bulk-electron temperature. Fourth, MWPs can be operated stably from atomic pressure down to below 10 mTorr. Fig. 4. demonstrates that Ar plasma maintains a uniform state over 22 cm in diameter up to 20 Torr. The schematic diagram of the low-pressure highdensity microwave plasma utilizing the spoke antenna is shown in Fig. 5.

**Power supply** 

Fig. 3. The coupling of the spoke antenna with microwave plasma [x]

(a) (b)

density microwave plasma utilizing the spoke antenna is shown in Fig. 5.

state under a wide pressure regime.

Fig. 4. Images of Ar plasma at a) 80 mTorr and b) 20 Torr. The plasma maintains uniform

longer than 1 m more easily than inductively coupled plasmas (ICPs). Thus, the application of MWPs to giant electronic devices such as solar cells is promising. Third, MWPs have lower bulk-electron temperature. Fourth, MWPs can be operated stably from atomic pressure down to below 10 mTorr. Fig. 4. demonstrates that Ar plasma maintains a uniform state over 22 cm in diameter up to 20 Torr. The schematic diagram of the low-pressure high-

80 mTorr ~20 Torr

Fig. 5. The schematic diagram of the low-pressure high-density microwave plasma utilizing the spoke antenna

Fig. 6. Electron density, ne, and electron temperature, Te, measured as a function of input microwave power.

Novel Deposition Technique for Fast Growth

microwave power was fixed at 700W.

**4 cm** 

**SiO2**

**SiH4 using low-pressure high-density microwave plasma** 

**0**

**SiH4 H2** 

**Ts**

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 365

In this study, a new source gas supply method was introduced, i.e., the SiH4 was introduced using a shower head placed 2 cm above the substrate holder under a steady flow of the H2 plasma supplied by the ring. The results from these gas supply method were compared with the results from the another gas supply method, i.e. a SiH4-H2 mixture was fed into the chamber using a ring just beneath the quartz plate. Figure 9 shows the schematic of the two different gas supply methods. The film deposition parameters were included the SiH4 concentration R=Fr(SiH4)/[Fr(SiH4)+Fr(H2)] (Fr is the flow rate). The SiH4 concentration was varied in a range from 5% to 67% by increasing Fr(SiH4) from 3 to 30 sccm with a constant H2 flow rate of 15 sccm. The film depositions were performed at the distance (Z) between the quartz plate and the substrate holder of 6 cm and the working pressure of 80 mTorr. The

**ne &Te**

**SiH4+H2** 

**SiH4+H2**

**SiH4 H2**

**3.2 Fast deposition of highly crystallized μc-Si:H films with low defect density from** 

Fig. 9. Schematic of the two different gas supply methods used in this study. The distance

**Vsub**

*ne*

*Te*

Fig. 10 shows the deposition rate dependence of ESR spin density, Ns for the corresponding μc-Si films fabricated using two different SiH4 gas supply methods at Ts of 150 and 250°C. Here, the film deposition rate was controlled by varying Fr(SiH4) from 3 to 30 sccm under constant Fr(H2) of 15 sccm and working pressure of 80 mTorr. For all samples, the film thickness was ~ 1.5 μm and the ESR measurements were performed directly on these films. It is to be noted that Ns was decreased by about one order of magnitude when the shower head was used for both Ts conditions despite the other deposition conditions being the same. However, Ns was almost independent of Fr(SiH4) on the order of (3-4)×1016 cm-3, which was still one order of magnitude larger than that of high quality μc-Si films reported

(Z) between the quartz plate and substrate holder was 6 cm.

**Z (cm)** 

elsewhere.

A uniform, high-density (electron density, ne: >1011 cm-3) and low-temperature (electron temperature, Te:1~2 eV) plasma can be generated by the microwave plasma source utilizing a spoke antenna without using complex components such as magnetic coil as shown in Figures 6 & 7 . The Te is almost independent of working pressure up to ~150 mTorr as shown Figure 8, which is suitable for the large area thin film processing.

Fig. 7. Electron density and electron temperature plotted against working pressure.

Fig. 8. The radial and axial distributions of ne and Te in microwave Ar plasma under microwave power of 700 W at 80 mTorr with Ar flow rate of 20 sccm.

A uniform, high-density (electron density, ne: >1011 cm-3) and low-temperature (electron temperature, Te:1~2 eV) plasma can be generated by the microwave plasma source utilizing a spoke antenna without using complex components such as magnetic coil as shown in Figures 6 & 7 . The Te is almost independent of working pressure up to ~150 mTorr as

shown Figure 8, which is suitable for the large area thin film processing.

Fig. 7. Electron density and electron temperature plotted against working pressure.

**20 40 60 80 100 120 Pressure (mTorr)** 

**0**

**1**

**2**

**3**

**4**

**5**

**Electron tem**

Z=6 cm

SiO2

**1**

**0 5 10 15 Z (cm)** 

**1.5**

**2**

**2.5**

**Electron temperature Te**

**3**

Z

**3.5**

**perature Te**

**6 1010**

**8 1010**

**1 1011**

**1.2 1011**

**Electron density ne (cm-3)** 

(a) (b)

**1.4 1011**

**1.6 1011**

**1.8 1011**

**6**

Power: 700 A

**7**

**10<sup>10</sup>**

**3)** 

**10<sup>11</sup>**

**Electron density ne (cm-**

**10<sup>10</sup>**

**10<sup>11</sup>**

**Electron density ne (cm-3)** 

**10<sup>12</sup>**

**10<sup>12</sup>**

Fig. 8. The radial and axial distributions of ne and Te in microwave Ar plasma under

**0.5**

**1**

**1.5**

**Electron temperature Te**

**2**

**2.5**

Z=6cm r

**3**

microwave power of 700 W at 80 mTorr with Ar flow rate of 20 sccm.

**0 2 4 6 8 10 12 r (cm)** 

#### **3.2 Fast deposition of highly crystallized μc-Si:H films with low defect density from SiH4 using low-pressure high-density microwave plasma**

In this study, a new source gas supply method was introduced, i.e., the SiH4 was introduced using a shower head placed 2 cm above the substrate holder under a steady flow of the H2 plasma supplied by the ring. The results from these gas supply method were compared with the results from the another gas supply method, i.e. a SiH4-H2 mixture was fed into the chamber using a ring just beneath the quartz plate. Figure 9 shows the schematic of the two different gas supply methods. The film deposition parameters were included the SiH4 concentration R=Fr(SiH4)/[Fr(SiH4)+Fr(H2)] (Fr is the flow rate). The SiH4 concentration was varied in a range from 5% to 67% by increasing Fr(SiH4) from 3 to 30 sccm with a constant H2 flow rate of 15 sccm. The film depositions were performed at the distance (Z) between the quartz plate and the substrate holder of 6 cm and the working pressure of 80 mTorr. The microwave power was fixed at 700W.

Fig. 10 shows the deposition rate dependence of ESR spin density, Ns for the corresponding μc-Si films fabricated using two different SiH4 gas supply methods at Ts of 150 and 250°C. Here, the film deposition rate was controlled by varying Fr(SiH4) from 3 to 30 sccm under constant Fr(H2) of 15 sccm and working pressure of 80 mTorr. For all samples, the film thickness was ~ 1.5 μm and the ESR measurements were performed directly on these films. It is to be noted that Ns was decreased by about one order of magnitude when the shower head was used for both Ts conditions despite the other deposition conditions being the same. However, Ns was almost independent of Fr(SiH4) on the order of (3-4)×1016 cm-3, which was still one order of magnitude larger than that of high quality μc-Si films reported elsewhere.

Novel Deposition Technique for Fast Growth

corresponding μc-Si films is shown in Fig. 14.

concentration R: Fr(SiH4)/Fr(SiH4)+Fr(H2).

2.5 3 3.5 4 4.5 5

MW plasma

**Photon Energy (eV)** 

layers optical model.

0

5

10

15

**2** 

20

25

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 367

A very fast deposition rate of 65Å/s has been realized for µc-Si:H films with a Raman crystallinity ratio of Ic/Ia of about 3.5 under very low H2 dilution (i.e. with high SiH4 concentration of 67%) as shown in Fig. 12 and low defect density of (1-2) ×1016 cm-3 using high-density and low-temperature microwave plasma. The imaginary part of the dielectric function <2> spectra of µc-Si:H films fabricated from SiH4 using high-density and lowtemperature microwave plasma is shown in Fig. 13 along with that using rf PE-CVD methods. Using the optical model the best fitted volume fraction of c-Si and void i.e. *f*c-Si and *f*void in the bulk layer and void in surface layer, *f*void, with SiH4 concentration R for the

Fig. 12. Film deposition rate and Raman crystallinity, Ic/Ia as a function of SiH4

**SiH4 concentration (%)** 

rf plasma

Fig. 13. Imaginary part of the pseudo dielectric function<2> spectra for the µc-Si films fabricated from SiH4 using MW Plasma along with that using rf PECVD methods and five

**d5 d3 d1 d2** 

**d4 (f**α**-Si, fSiO2, fvoid)**

**(fpoly-Si, fa-Si, fvoid) (f**α**-Si, fvoid) (fpoly-Si, fa-Si, fvoid)**

**(fpoly-Si, fa-Si, fvoid)**

Fig. 10. ESR spin density, Ns for corresponding μc-Si films fabricated using different gas supply method as well as that for samples prepared at Ts=150C are plotted as a function of film deposition rate Rd.

Fig. 11. μc-Si:H film microstructure

**Ts=250** 

**5 30sccm** 

**Fr(SiH4)** 

**Ts=150** 

℃

℃

**Ring** 

**Ts=250** 

**Shower head** 

℃

**SiH** 

**@80mTorr** 

Fig. 10. ESR spin density, Ns for corresponding μc-Si films fabricated using different gas supply method as well as that for samples prepared at Ts=150C are plotted as a function of

**a-Si matrix c-Si grain** 

**: Si : H**

**0 10 20 30 40 50 60 70**

**Ts=150** ℃

**1-2**×**1016 cm-3**

**Deposition rate (Å/s)** 

film deposition rate Rd.

**SiH2**

**10<sup>16</sup>**

**10<sup>17</sup>**

**ESR spin density (cm-3)** 

Fig. 11. μc-Si:H film microstructure

A very fast deposition rate of 65Å/s has been realized for µc-Si:H films with a Raman crystallinity ratio of Ic/Ia of about 3.5 under very low H2 dilution (i.e. with high SiH4 concentration of 67%) as shown in Fig. 12 and low defect density of (1-2) ×1016 cm-3 using high-density and low-temperature microwave plasma. The imaginary part of the dielectric function <2> spectra of µc-Si:H films fabricated from SiH4 using high-density and lowtemperature microwave plasma is shown in Fig. 13 along with that using rf PE-CVD methods. Using the optical model the best fitted volume fraction of c-Si and void i.e. *f*c-Si and *f*void in the bulk layer and void in surface layer, *f*void, with SiH4 concentration R for the corresponding μc-Si films is shown in Fig. 14.

Fig. 12. Film deposition rate and Raman crystallinity, Ic/Ia as a function of SiH4 concentration R: Fr(SiH4)/Fr(SiH4)+Fr(H2).

Fig. 13. Imaginary part of the pseudo dielectric function<2> spectra for the µc-Si films fabricated from SiH4 using MW Plasma along with that using rf PECVD methods and five layers optical model.

Novel Deposition Technique for Fast Growth

selection of deposition precursor.

Fig. 16. µc-Si:H films fabricated from SiH4 using MW plasma

**SiH (µc-Si phase)**

**SiH2Cl2 using low-pressure high-density microwave plasma** 

for the dissociation instead of SiH4.

**SiH2Cl2 and SiH4**

Table 1.

Highly crystallized μc-Si:H films with a preferred (220) crystal orientation at a high deposition rate of 65 Å/s were fabricated from SiH4 with a negligibly small volume fraction of amorphous Si but μc-Si network included high volume fraction of voids as shown in Fig. 15. which was hardly compatible with a device quality material. To overcome this problems, the fast deposition of highly photoconductive hydrogenated chlorinated microcrystalline Si (μc-Si:H:Cl) films with amorphous Si phase and with less volume fraction of void have been fabricated from SiH2Cl2 with higher threshold energy

**3.3 Fast deposition of highly crystallized μc-Si:H:Cl films with low defect density from** 

**3.3.1 Fine structure of Si network of microcrystalline silicon thin-film fabricated from** 

The typical FTIR spectra of 1-μm-thick μc-Si:H:Cl films fabricated from a SiH2Cl2-H2 mixture, compared with those of μc-Si:H films from SiH4 as shown in Fig. 17. Here, the peak assignments of SiH (bulk and surface stretching) and SiH2 bulk stretching are also shown in

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 369

IR absorption peak at 2090 cm-1 corresponding to the surface SiH mode in the µc-Si phase appeared as a shoulder in the high-density film. These results suggest that the c-Si phase is isolated in a-Si network as shown in Fig. 11 & 16, which is not preferable for the Si thin-film solar cells. Therefore, the suppression of the excess film crystallization is required by the

**µc-Si phase** 

Fig. 14. Changes in *f*c-Si and *f*void in the bulk layer and surface layer, with SiH4 concentration R for the corresponding μc-Si films shown in Fig. 11.

Fig. 15. The FTIR spectra for the corresponding µc-Si:H films fabricated from SiH4 using MW plasma.

Highly crystallized µc-Si:H film was synthesized despite low H2 dilution ratio rather than the conventional rf and VHF plasmas, because of high generation efficiency of atomic hydrogen. FTIR spectra and microstructure of µc-Si:H film and of SiHn absorption region are shown in Fig. 11 & 15 for the corresponding µc-Si film. Generally, two IR absorption peaks are observed at 2000 and 2100 cm-1, which are attributed to the bulk SiH in a-Si and SiH2 in µc-Si phase, respectively, in the film fabricated by the rf plasma CVD. However, no SiH absorption peak at 2000 cm-1 is observed in the film fabricated by high-density microwave plasma. These imply that the film crystallization is promoted extremely in the high-density plasma with negligibly small fraction of amorphous Si phase. In addition, the

Fig. 14. Changes in *f*c-Si and *f*void in the bulk layer and surface layer, with SiH4 concentration

Fig. 15. The FTIR spectra for the corresponding µc-Si:H films fabricated from SiH4 using

Highly crystallized µc-Si:H film was synthesized despite low H2 dilution ratio rather than the conventional rf and VHF plasmas, because of high generation efficiency of atomic hydrogen. FTIR spectra and microstructure of µc-Si:H film and of SiHn absorption region are shown in Fig. 11 & 15 for the corresponding µc-Si film. Generally, two IR absorption peaks are observed at 2000 and 2100 cm-1, which are attributed to the bulk SiH in a-Si and SiH2 in µc-Si phase, respectively, in the film fabricated by the rf plasma CVD. However, no SiH absorption peak at 2000 cm-1 is observed in the film fabricated by high-density microwave plasma. These imply that the film crystallization is promoted extremely in the high-density plasma with negligibly small fraction of amorphous Si phase. In addition, the

**2300 Wave number (cm 1800 -1)** 

R for the corresponding μc-Si films shown in Fig. 11.

MW plasma.

**Transmittance (%T)** 

IR absorption peak at 2090 cm-1 corresponding to the surface SiH mode in the µc-Si phase appeared as a shoulder in the high-density film. These results suggest that the c-Si phase is isolated in a-Si network as shown in Fig. 11 & 16, which is not preferable for the Si thin-film solar cells. Therefore, the suppression of the excess film crystallization is required by the selection of deposition precursor.

Fig. 16. µc-Si:H films fabricated from SiH4 using MW plasma

Highly crystallized μc-Si:H films with a preferred (220) crystal orientation at a high deposition rate of 65 Å/s were fabricated from SiH4 with a negligibly small volume fraction of amorphous Si but μc-Si network included high volume fraction of voids as shown in Fig. 15. which was hardly compatible with a device quality material. To overcome this problems, the fast deposition of highly photoconductive hydrogenated chlorinated microcrystalline Si (μc-Si:H:Cl) films with amorphous Si phase and with less volume fraction of void have been fabricated from SiH2Cl2 with higher threshold energy for the dissociation instead of SiH4.
