ADVANCED PECVD REACTOR FOR THIN FILM SOLAR APPLICATIONS
A. Taha, D. Chaudhary, M. Klindworth, F. Leu, J. Martin, A. Salabas,
W. Wieland, D. Zorzi, Ch. Ellert
OERLIKON Solar Ltd., Trübbach
Hauptstrasse 1a, CH – 9477 Trübbach, Switzerland
e-mail: abed.taha@oerlikon
ABSTRACT: Oerlikon Solar introduced in 2007 the micromorph technology [1] into the photovoltaic market.
Oerlikon Solar’s strategy adapted the PECVD KAI deposition systems, approved in mass production and developed originally for TFT display applications, to thin film silicon solar cells. Hereby the base of Oerlikon’s thin film silicon solar device technology origins from the long-term research carried out at the Institute of Microtechnique of the University of Neuchatel (IMT-NE) over the last 25 years on amorphous and microcrystalline silicon. This is based on the well-known very high frequency glow discharge (VHF-G
D) deposition technique [2]. Oerlikon Solar is developing and providing mass fabrication equipment including processes for entire thin film silicon manufacturing lines for module areas of 1.3m x 1.1m. This has led to successful production lines on various customer sites in Europe and Asia. In order to successfully implement the solar cell and module processes developed within R&D Oerlikon Solar into a production line, the PECVD reactor is continuously adapted and improved such that the process parameters can be transferred straightforward into the mass production equipment. The reactor development plays a key role to enable this transfer. Relevant parameters for optimum performance of the PECVD equipment are the uniformity of the deposition over the entire surface of the substrate as well as the deposition rate.
In addition to optimum deposition quality the reactors need to be cleaned after each deposition step in order to provide for any deposition run identical starting conditions. This in turn is prerequisite for stable and reproducible run to run deposition results.
1 INTRODUCTION
Year after year the interest of using Plasma Enhanced
Chemical Vapour Deposition (PECVD) method for
silicon based thin films solar cells is growing rapidly.
This was triggered by the introduction of the micromorph
cell design in 1996 by Meier et al. [1]. The combination
of a microcrystalline with an amorphous cell allows the
improvement of the cell efficiency to far beyond 11 % in
the so called micromorph tandem cell. It was also proved
that using (PECVD) methods is high productiv and
economically viable to produce micromorph tandem cell.
For this purpose high plasma excitation frequency was
required. In addition to keep the productive time of a potential deposition tool as high as possible the self cleaning method developed for PECVD tools in LCD industry was transferred to the solar applications. In this paper we present the necessary steps which led to the presently available KAI syst
em of Oerlikon Solar and the improvements that was done in the 2nd generation of Oerlikon advanced (PECVD) reactor.
2 MOTIVATION
In order to achieve high deposition rate in the PECVD steps of the thin film silicon solar very high RF-frequency was found very advantageous [2-5], see Figure 1 as taken from [6]. Figure 1: Increase of the deposition rate under equivalent experimental conditions in a PECVD reactor.
Higher frequencies do not only allow more efficient dissociation of the precursor molecules like SiH4but as well yield higher rates while keeping the ion bombardment on the deposited layers low [7,8].
The most challenging point in using high frequencies RF plasma is the non uniform voltage distribution across the electrodes due to what is know as standing wave effect which becomes significant when the dimensions of the electrodes become comparable to a quarter of the vacuum wave length of the electromagnetic wave at the selected RF-frequency [9].
In order to overcome this effect Schmitt [10] suggested already for the application in LCD technology t
he introduction of a compensating dielectric layer into one of the two flat electrodes facing the large area plasma slab.
3 TECHNICAL DESCRIPTION
The predominant limiting factor to a flat distribution of the deposition rate is the above mentioned standing wave effect. At Oerlikon Solar a frequency of 40 MHz was selected since it represents the best combination
of
layer quality, deposition rate increase and availability of RF-equipment. As can be seen by the example
shown in Figure 2 the thickness of a deposited amorphous single layer is more than a factor of 2 higher in the center than on the edge, which is due to the peak of the electric field in the center of the reactor, i.e. the standing wave.
Figure 2: Distribution of the thickness of an amorphous silicon layer in a commercial large area PECVD reactor deposited on a glass substrate of 1100mm x 1300mm at 40MHz.
By introducing a dielectric layer into one of the electrodes a local voltage distribution
according to the respective capacitances of this layer and the plasma gap is achieved. The key point is the proper selection of the distribution of the width of that dielectric layer. A very schematic sketch is shown in Figure 3. It is important to cover the vacuum gap which represents the dielectric compensatin
g layer by a non-conducting material which prevents the plasma to fill that gap, which would eliminate the necessary voltage distribution over the two gaps.
Figure 3: Schematics of the design of the RF-electrodes, which allows overcoming the standing wave effect.
4  DEPOSITION RESULTS
Using the mentioned concept Oerlikon solar designed the first PECVD reactor generation in 2007 providing good layer quality for thin film solar applications using two different reactor types (one for amorphous layers and one for microcrystalline) - since the plasma conditions for the deposition of these two materials are quite different both require a different shape of the electrodes – After over 2 years of industrial use of this two reactor types, the collected experience leads to optimization of the hardware to get higher performance efficiency and process flexibility.
The main goal was to improve the gas usage to get more gas in the plasma zone (over the glass), then to reduce the powder formation and powder trapping as good as possible. Then the last step was to redesign the electrical field distribution (using the electrode shape) to decrease the RF losses and to optimize the one reactor concept to provide both layers (amorphous and microcrystalline)
with suitable deposition uniformity and material quality for solar cell applications. Thus, doors are opened for further process optimization and to get higher productivity.
In Figure 4 and Table 1 the thickness distribution of an amorphous layer is shown on 15x13 matrix of measured points. The uniformity is described by U = (Max – Min)/ (Max + Min), which means that all thickness value lie within the interval ±U.
Power  DR  DR
U%  U%  Reactor [W] (15X13)
(13X11)
(15X13) (13X11)
St.d aSi 370    3.33    3.34 13.16    6.95 One Box 370    3.19    3.23 14.35    6.24 One Box 400    3.42    3.47 13.83    5.81 One Box
440    3.63
3.66
14.64    6.03
Table 1: Comparison between amorphous layer uniformity in standard a-Si reactor box and in the new advanced plasma box with 20 mm edge exclusion.
Figure 4: Distribution of the thickness of an amorphous silicon layer in 1100mm x 1300mm glass using the advanced plasma Box
The new plasma box was also optimized to get the best microcrystalline layer quality and to increase the gas usage efficiency for mc-Si layers as what is shown in Table 2 and Figure 6.
Table 2: Comparison in performance capitalist between amorphous layers uniformity in standard aSi box and in the new advanced plasma box with 20 mm edge exclusion
Figure 5: Thickness distribution of microcrystalline layer deposited at about 5.5 A/s, with a uniformity U(15X13) = 8.9%.
However, in order to judge about the uniformity of a deposited microcrystalline layer not only the thickness but as well the Raman crystallinity (RC) is relevant, since the quality of the absorbing microcrystalline layer is critically determined by RC. This parameter is defined by
the ratio of the crystalline fraction (peaks at 520cm -1
and
505cm -1
) with respect to the total response including the
amorphous peak at 480cm -1
. These spectra are measured in a common Raman spectrometer. The distribution of the RC value over the glass substrate with an edge exclusion of 20 mm is shown in Figure 5.
Figure 5: Distribution of the Raman crystallinity of a microcrystalline layer. This distribution can be characterized by an interval of ±11% at low crystallinity level (0.56).
Figure 6 is a direct comparison between mc-Si layers done in the 1st  and 2nd  mc-Si reactor generations with the same Raman crystallinity. (around 0.6). It shows that less gas is needed to get the same deposition rate. In the same time less RF power needed to be delivered for the same mc-Si layer crystallinity. These two advantages allow the user either to use the same deposition rate with less production costs or to increase the deposition rate to the new limits.
Figure 6: mc-Si deposition rate in PECVD reactor (V1.0 & V2.0 generation) with same Raman Crystalli
nity and SiH4 gas concentration
5  SUMMARY
In summary, the OC Oerlikon Solar PECVD reactor was described in its basic principles. The standing wave effect present at 40MHz in conventional reactors was overcome, thus    a flat thickness and crystallinity distribution was achieved for amorphous and microcrystalline layers.
The upcoming 2nd  generation of Oerlikon’s microcrystalline PECVD reactor will offer improved layer uniformity in thickness and crystallinity distribution as well as significantly better resource usage efficiency in terms of gases and RF power in the near future. As an addition feature it may as well replace the 1st  generation of a-Si deposition reactors with its comparable growth rates and layer homogeneity.
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