Currently available flow sensors are often embodied in the form of thin-film membrane sensors, using the anemometric method to determine the particular flow.
In addition, PCT Publication No. 98/50763 has already described an integrated gas flow sensor in which a recess is produced in a silicon wafer and then is covered with an unsupported membrane having a two-ply layer of polysilicon/silicon dioxide or polysilicon/silicon nitride. In addition, integrated thermocouples and a resistance heating conductor are also provided on this unsupported membrane. The recess beneath the unsupported membrane provides thermal insulation of the thermocouples from the silicon substrate. To produce this integrated flow sensor, a surface micromechanical structuring method for silicon is used in which a layer of porous silicon functions as a sacrificial layer. The thermocouples are made of polycrystalline p-type silicon/aluminum, for example, or polycrystalline n-type silicon/polycrystalline p-type silicon. The resistance heating conductor is a strip of polycrystalline silicon.
German Published Patent Application No. 100 30 352 describes a sensor element situated on an unsupported membrane which is stabilized with webs, a recess being created beneath the unsupported membrane by converting silicon to porous silicon and/or porous silicon dioxide. In addition, that patent application describes the use of a such a sensor structure in a flow sensor.
In exposing membranes, a fundamental distinction is made between two methods, namely surface micromechanics, which generally uses a sacrificial layer produced on the front side of the substrate, e.g., a wafer, before deposition of the membrane. This layer is later removed from the front side through detachment openings in the membrane or in the substrate at the edge of the membrane. In addition, there is also the bulk micromechanical method in which the membrane which has already been produced is exposed from the rear side of the substrate by various etching methods, e.g., wet chemical methods or by a plasma etching method through an opening etched there.
The formation of porous silicon, which is known as a possible sacrificial layer for a surface micromechanical process, involves an electrochemical reaction between hydrofluoric acid and silicon in which a spongy structure is formed in the silicon. For this method, the wafer must be anodically polarized with respect to a hydrofluoric acid electrolyte. Due to the resulting porous structure, the silicon has a large internal surface area and therefore has different chemical and physical properties than the surrounding bulk silicon. In particular, the reactivity of porous silicon is greatly increased, thus permitting selective dissolution of porous silicon with respect to bulk silicon. To produce porous silicon, various doped silicon substrates are suitable, but p-doped wafers are generally used. The doping determines the size of the structure within the porous silicon.
Various masking methods and/or masking layers and an electrochemical etching stop are used in locally defined production of porous silicon. A thin layer at the surface of the p-doped silicon substrate is frequently redoped into n-doped silicon to function as the masking layer, e.g., by implantation or diffusion of a dopant into it, so that porous silicon is formed only in the p-doped regions in the subsequent electrochemical etching. Furthermore, the formation of porous silicon in this electrochemical etching process is isotropic, so the masking layer, which is applied first, is undercut completely, thus forming unsupported structures.
Another possibility of masking in addition to the use of redoped silicon is to use silicon oxide layers and silicon nitride layers as the masking layer, which may also be removed again in a subsequent process step. In this case the masking layer is also undercut isotropically.
For dissolving out porous silicon thus produced within a defined area, diluted potassium hydroxide solution and hydrofluoric acid may be used, but in the latter case it is necessary to convert the porous silicon which is produced first into porous silicon oxide in an additional oxidation step.
An object of the present invention was to implement a surface micromechanical flow sensor so that it will have an improved stability and an improved thermal insulation of the actual sensor elements with respect to the substrate. In addition, another object was to develop a flow sensor which would also allow angle-dependent detection of a flow, in particular a gas flow, and would permit an inexpensive and at the same time very flexible production method with regard to the layout of the flow sensor.