The invention relates to a method for the production of a vacuum measuring cell with a diaphragm.
It is known to measure pressures or pressure differences thereby that a thin diaphragm is pressurized and its deflection measured. A known and suitable method for measuring the deflection of such diaphragms comprises implementing the diaphragm arrangement as a variable electric capacitor, wherein, via measuring electronic circuitry, the capacitance change, which correlates with the pressure change, is evaluated in known manner. The capacitor is implemented by disposing the thin flexible diaphragm surface at a short distance opposite a further surface and coating both opposing surfaces with an electrically conducting coating or implementing them of electrically conductive material. Due to the deflection, the distance between the two electrodes changes upon pressurization of the diaphragm leading to a capacitance change of the arrangement which can be evaluated. Sensors of this type are produced of silicon in large production numbers. The areal base body as well as also the diaphragm are herein often entirely comprised of silicon material. There are also designs with combined material composition, for example silicon with glass base. The sensors can thereby be produced cost-effectively. As a rule, pressure sensors of this type are only applicable for relatively high pressure ranges of approximately 10−1 mbar to a few bars. High resolution at lower pressures starting at approximately 10−1 mbar are no longer realizable using the material silicon. Sensors of this type are not suitable for typical vacuum applications. The reason is inter alia that the silicon on the surface reacts with the environment and the sensitive sensor characteristic is thus disturbed. Water vapor contained in normal atmospheric air already leads to corresponding reactions on the surfaces. The problem is additionally exacerbated if the sensor is employed in chemically aggressive atmospheres, which is increasingly common in current reactive vacuum plasma processes. Attempts have therefore been made to protect such silicon sensors by passivating the surfaces against aggressive external actions. Attempts have also been made to provide the surface with a protective coating in order to increase the durability and resistance against the chemically aggressive environment, as has been described in DE 41 36 987 A1. Such measures are difficult to realize and, in the case of mechanically deformable parts, such as diaphragms, only yield limited success, in particular in the presence of especially aggressive media, such as fluorine, bromic acid and their compounds, as are utilized in the semiconductor industry, for example in vacuum etching processes.
It has therefore been proposed to produce measuring cells for vacuum pressure measurements of corrosion-resistant materials such as Al2O3. EP 1 070 239 B1 describes a capacitive vacuum measuring cell which is substantially completely built of ceramic and, consequently, is to a high degree corrosion-resistant. To be able to measure very low pressures up to 10−6 mbar with high accuracy, a very thin ceramic diaphragm of <250 μm thickness is utilized, which is disposed free of tension and substantially symmetrically in a ceramic housing. Although this diaphragm-based vacuum measuring cell is very successful in operation and represents a significant advance with respect to corrosion resistance, it was found that the joining regions between diaphragm and housing as well as the joining region for the connection fitting and, if applicable, the connection fitting itself, during operation used in aggressive process environments which contain, for example acids, halogens, such as chlorine and fluorine, etc., represent a weak point regarding the service life of the cell even if the entire cell is substantially comprised of a corrosion-resistant ceramic. In the assembled state the measuring cell includes extremely small voids exposed to the process gases, which voids are offset in the form of labyrinths and accessibility to the regions where the joining sites of the parts are located entails significant difficulty. Coverage of such regions in such small and difficult to access voids through a suitable coating also appears scarcely possible, especially since during the coating the particles would have to be guided around edges and corners of this labyrinth.