Thin film solar cells are one of the most promising and cost effective means of generating electricity from sunlight. However, their manufacture requires the use of precision film thickness process control sensors in order to produce high efficiency and economically viable cells. Current film thickness sensors are not reliable enough to meet these demands, failing after continuous use in a matter of hours as a result of high heat and excessive layer build-up. We will develop a state of the art sensor that addresses these shortcomings. We propose replacing the current water-cooled quartz crystal microbalance (QCM) based process control sensors with a new class of high temperature microbalances made from doubly rotated quartz, gallium orthophosphate or langasite. These sensors can be constructed to clean themselves after repeated use by heating to temperatures in the range of 500- 1300C. In addition they offer more stable performance under normal process conditions due to their improved resistance to film stress and radiation induced noise. In our Phase I investigation we demonstrated a 500C sensor that is stable and that can regenerate itself after a thin film deposition. For a CIGS thin film process, we successfully removed selenium films completely off the crystal. With higher temperature capability, indium, copper and gallium films can be as well. In Phase II we will improve this capability to 1000 C, leading to the removal of nearly all commercially deposited thin films in solar cell manufacture. Essentially, an unlimited lifetime sensor is the end result. Commercial Applications and Other
Benefits: The successful construction of a self cleaning and heated microbalance sensor will have applications far beyond thin film solar cell production. This includes furnace processing, chemical vapor deposition (CVD), atomic layer deposition (ALD) and even low temperature thin film processing (PVD techniques such as thermal evaporation and sputtering) where the failure of the sensor during long processing runs can be a major problem. Many optical devices are made under conditions that fall into this latter category. A new class of organic thin film devices, including OLEDs (Organic Light Emitting Diodes) and organic thin film photovoltaic devices are well known to be difficult to monitor in vacuum processing lines. This new class of microbalance eliminates many of the failure modes. This sensor will be a revolutionary development in continuous vacuum coating technology. Roll coating systems have seen limited use of film thickness monitoring systems because of the short life of the sensors. Our innovation will remove that limitation completely.
