In order to optimize the properties of each layer and therefore the characteristics of the electronic devices, it is desirable to deposit each layer, and in particular the active semiconductor layer, in high purity conditions; it is therefore necessary to minimize all external sources of contamination as well as cross contaminations among the different layers of the device. Use of a UHV system which can achieve a good ultimate vacuum and therefore a low outgassing rate minimizes external sources of contamination, and therefore the incorporation in the deposited films of residual impurities such as oxygen, nitrogen and carbon, which can have a detrimental effect on the properties of silicon thin films.
Deposition Process Chambers (DPC’s) are constructed according to ultra high vacuum (UHV) standards (stainless steel construction and all metal seals) which allow ultimate pressures in the 10-9 mbar range at room temperature and in the 10-7 mbar range at process temperature to be readily reached with turbomolecular pumping For a UHV system to maintain a good base vacuum with as little contaminants as possible it is desirable to keep the process chamber always under ultra high vacuum (UHV) conditions, using a Load Lock Chamber (LLC) isolated from the process chamber by a gate valve: only the Load Lock chamber is vented to atmosphere for substrate loading.
Cross contamination among different layers is avoided using a system configuration with multiple separate deposition process chambers (DPC’s) for the deposition of each layer with a specific gas mixture. The cluster multichamber configuration consists of multiple reaction chambers configured in a circular geometry; the contamination at the interface between layers is avoided using a central vacuum chamber connected to all process chambers to transfer the substrate from one process chamber to another without venting it, named transfer chamber (TC). Multichamber cluster systems with a vacuum transfer chamber are commonly used for research and production of thin film devices. The vacuum transfer chamber is kept under vacuum during normal operation and a Load Lock chamber is used for substrate loading.
A five chamber cluster system using RF-PECVD in three process chambers (DPC1 for intrinsic silicon, DPC2 for doped silicon, DPC4 for silicon alloys), ECR-PECVD in DPC3 and plasma etching in DPC5 has been manufactured and installed at the ENEA laboratories in Portici The system features complete control by PC with capability of simultaneous indipendent operation of each DPC and is used for R&D on various types of thin film electronic devices.