Wafer-level sensor structure for the measurement of ion energy and ion angle distribution functions in low pressure plasmas - plasma sensor
Low-pressure plasmas are among the most important tools in microtechnology and can be found today in a wide range of applications. Special importance is given to the processes of microstructure technology, which form the basis for the entire semiconductor technology. In most of these processes, ions generated in the plasma are accelerated by means of electric fields onto a workpiece to be treated, e.g. in the form of a silicon wafer, where they are used, among other things, to produce layers or to generate defined geometries. The individual ions striking the wafer have different kinetic properties, which can be described within certain limits by functional relationships. The two most important models are the ion energy distribution function (IEDF), which describes the frequencies of occurrence of ions within a certain energy range, and the ion angle distribution function (IADF), which is formed by the summed fractions of ions within different incident angle ranges. The characteristics of these two distribution functions are decisive for most of the results of the plasma processes performed.
The goal of the DFG-funded project "plasma sensor" is to investigate a sensor element that can measure the ion distribution functions, IEDF and IADF, in low-pressure plasmas. The sensor represents a combination of the well-known principle of the retarding-field analyzer RFA and a novel MEMS-based sensor principle. The RFA slows down the impacting ions by means of an electric field, which allows only ions with sufficient kinetic energy to enter the sensor and be measured. The angle analyzer consists of a micro-engineered, piezoelectrically tiltable perforated grid, which, depending on the tilt angle, is only permeable to ions that strike the wafer surface within the acceptance angle. The sensor can be constructed in the form of a silicon wafer compatible with CMOS and is intended for the analysis of industrial plasma processes.
The dimensioning of the components of the RFA is supported by the Ruhr University Bochum. Based on given reactor geometries, the expected distribution functions are calculated by means of corresponding simulations. Based on these results, the structural elements of the sensor, the detector and the static and moving gratings, are dimensioned and built.The technology established at the Center for Microtechnologies with piezoelectric aluminum nitride will be used. The piezoelectric actuation principle was chosen because the MEMS ultimately has to be operated directly in a plasma array and capacitive technologies are ruled out due to the strong electromagnetic fields within this setup. The sensor element is tested in a test reactor available at ZfM. The measurement results will finally be compared with corresponding simulations, which are performed at the Ruhr University Bochum and reproduce the trajectories of the ions through the sensor element. The aim of the project is to investigate and describe the functional principle of such a novel sensor element. If successful, further envisaged projects will focus on the introduction of such sensors into industrial production.
Perforated MEMS element for measuring the ion angle and ion energy distribution function. In transmitted light, the holes for ion angle selection are visible (hole diameter 1µm, hole depth 30µm).
Resonance function of piezoelectrically actuated MEMS element in an ion beam.
