**2. Growth techniques of hydrogenated microcrystalline silicon**

The growth of µc-Si:H material uses silane (SiH4) and hydrogen as source-gas. It is currently admitted that free radical precursors (SiHx)-SiH3 is suspected to favor the µc-Si:H growthand H-enhances crystalline growth by etching of looser a-Si:H tissue-were needed to attain microcrystalline growth. In order to obtain such reactive species, decomposition of the source-gases is necessary. At first, this was obtained by using PE-CVD at high temperatures (600°C). The use of low deposition temperatures of 200-300°C with a plasma present in the

Novel Deposition Technique for Fast Growth

temperature plasma.

Microwave current, (b) Electric field.

(a) Microwave current

of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 361

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 low-

Fig. 2. The newly developed spoke antenna for introduction of microwave power (a)

*Power supply* 

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

E E

(b) Electric field

E

E

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