Process News Spring 2015 | OI Plasma Technology

Role of plasma enhanced chemical vapour deposition (PECVD) in thin film silicon photovoltaics

Dr Sekhar Bhattacharya, Dr Balai Bhargav, Mr Nafis Ahmed, Mr Balaji Chandra and Mr Arrokiya Doss, SSN Research Centre, Chennai-603110, INDIA

• 0.5-2.0 Torr operating pressure and 0.02-2.0 Wcm-2 power density The Oxford Instruments Plasma Pro ® 100 system was utilised for the following applications at the SSN Research Centre: • To deposit amorphous silicon films (doped and intrinsic) for fabrication of thin film solar cell on glass • To deposit silicon nanowires In PECVD amorphous silicon (a-Si) is deposited by plasma from a mixture of SiH 4 and H 2 gases at substrate temperature of 200-3000 o C. A typical recipe for deposition of amorphous silicon is given below:

Plasma Enhanced Chemical Vapor Deposition (PECVD) is an excellent process for depositing a variety of thin films at lower temperatures. It is a process used to deposit thin films from a gaseous state to a solid state on a substrate. Chemical reactions occur after creation of plasma of the reacting gases. Since the formation

of the reactive and energetic species in the gas phase occurs by collision in the gas phase, the substrate can be maintained at a low temperature. Hence, film formation can occur on substrates at a lower temperature than is possible in the conventional CVD process. Some of the desirable properties of PECVD films are good adhesion, good step coverage and uniformity of deposition. Some of the popular applications of PECVD include deposition of passive and active waveguide layers, dielectric films such as silicon dioxide, low stress and low temperature silicon nitride, amorphous silicon and silicon carbide.

SiH

4 flow = 20 sccm , H 2

flow = 60 sccm, Process pressure =

1000 mTorr,

Substrate temperature = 200 C, Plasma power = 31 mW/cm 2 .

If doped amorphous silicon is to be deposited then B 2 H 6 flow of 1 - 2 sccm or PH 3 flow of 1 - 2 sccm is introduced into the process chamber for P and N doping respectively. The structure of an amorphous silicon thin film solar cell is given below. Substrate is a transparent conducting oxide (TCO) coated glass on which P-I-N amorphous silicon layers are deposited by PECVD followed by metallisation in a vacuum thermal evaporator.

P a-Si layer

I a-Si layer

N a-Si layer

SiH

4 : 20 sccm

SiH

4 : 20 sccm

SiH

4 : 20 sccm

B

H

6 :1.2 sccm

PH

3 : 1.5 sccm

2

PECVD Reactor: Oxford Instruments Plasma Pro 100 system

H

2 : 100 sccm

H

2 : 100 sccm

H

2 : 100 sccm

The key features of the system are: • Top electrode RF driven (13.56 MHz) • Substrate is transferred from loadlock and sits directly on heated electrode • Gas injected into process chamber via top “Showerhead”

20-30 nm

300-350 nm

30-40 nm

Silicon nanowires can be made by PECVD by VLS Mechanism. The vapor-liquid-solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowire, from chemical vapor deposition.

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