Reactive sputtering control for industrial CIGS deposition

Introduction

Reactive feedback control in magnetron sputtering deposition technology has increasingly been accepted in the field of high speed PVD. The introduction of suitable feedback control in certain industrial processes has made it possible to increase the deposition rates keeping an excellent degree of uniformity. This has rendered in some of the fastest return of investment seen in the deposition industry.

Feedback control in processes requiring reactive gases such as oxygen or nitrogen are relatively easy to implement via fast mass flow controllers [1]. However, certain industrial processes require the injection and control of species that are not so easy to be delivered in a vapour phase, e.g. chalcogens used for CIGS production.
In recent years, the need for high quality and productivity CIGS for photovoltaic applications to a low cost has increased dramatically. While laboratory results have achieved efficiencies as high as 20.4% [2], the mass production of panels using CIGS solar cell technology has proven to be extremely difficult. The implementation of a large-scale, all-vacuum sputtering deposition process of entire CIGS cell stacks has to this day not been successfully achieved.

Regarding the development of a single-step deposition of chalcogen-based compounds, different ways to inject S or Se into the vacuum system has been presented. Some of them would involve evaporation methods where the flows of the elements are very difficult to control. In some other cases the delivery form could have severe health & safety issues, (e.g. processes involving the manipulation of H2S or H2Se).

Experimental setup

A novel reactive sputtering process control for the deposition of chalcogen compounds is being developed. As shown in Figure 1, two rectangular magnetrons operated in AC mode at 40kHz are used for sputtering of the metal part [(Cu-based alloys, Zn and In)], while the injection and control of the evaporated chalcogen species (Selenium and Sulphur) is performed by a pulsed valve cracker linear effusion cell [3].

The linear effusion cell is composed of an evaporation reservoir for the generation of the vapour stream, a fast actuating valve, which is heated to avoid material re-condensation, and a cracking zone for dissociation of the evaporated molecules. The Se, S flux is controlled by adjusting the valve opening time (in the 10 ms range) and repetition frequency (up to 10Hz) at a stable reservoir temperature, thus achieving a reproducible flux control for long deposition times The chalcogen evaporation rate was shown to be linearly proportional to the valve aperture time.

A Speedflo control unit developed by Gencoa Ltd [3], is used to monitor the plasma emission lines, actuate the pulsed valve cracker linear effusion cell and by means of its feedback dynamic control, stabilize the sputtering process. As changes occur in the plasma, the optical sensor monitors the variations in the intensity of the emission lines. This provides an input to the fast process control that automatically actuates the effusion cell valve to adjust the chalcogen flux [4]. The balance of metal and chalcogen atoms is maintained at the optimum level for obtaining high deposition rates and accurate control of the film stoichiometry, which is crucial for obtaining high efficiency devices combined with efficient Se, S material consumption.

An optical plasma emission spectrum taken with the Speedflo control unit spectrometer for Cu, In and Zn sputtered in the presence of Ar gas and S vapour is shown in Figure 3.

The hysteresis behaviour of the chalcogen reactive sputtering process is studied in order to understand the poisoning and de-poisoning process. To perform this study, the response of the sensor signal is plotted as a function of the time, while the flow of evaporated chalcogen is ramped up and down. As shown in Figure 4, a clear transition between the ‘metal’ to ‘fully poisoned’ states is observed. As the deposition rate is strongly reduced in the poisoned state, the plasma conditions must be maintained in the transition regime to perform a fast industrial deposition of chalcogen compounds .

In reactive sputtering, it is well known that the plasma is unstable even when the involved parameters remain constant. For that reason, and once the dynamics of the target contamination has been characterized, a feedback control is implemented in order to stabilize the sputtering process. Moreover, in some cases it is desirable to move a process from different working set-points to obtain for example, composition gradients. This should be performed with high speed and accuracy. In Figure 5, it is shown a feedback control response of the sensor signal (Cu emission line at =514nm) as a function of the time for different working set-points. It can be seen that the developed fast process control can stabilized the sensor signal at a given set-point value with high accuracy and move the process between different working set-points within seconds.

References

[1]  V. Bellido-González, B. Daniel, J. Counsell, D. Monaghan. Thin Solid Films 502 (2006) 34 – 39.
[2]  M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovolt: Res. Appl., 21 (2013) 1-11.
[3]  http://www.gencoa.com/speedflo/
[4]  V. Bellido-González and I. Fernández-Martínez: GB Patent 1307097.4 (2013)