Although any given micromechanical components are also usable, the present invention and its underlying problems will be described with reference to components including gas sensors based on silicon, including a heating device (hotplate).
Micro hotplates are an important component for micromechanical sensors. Micro hotplates are utilized in sensor principles which require an elevated temperature for the functional principle. Gas sensors based on the chemical transducer principle are worth mentioning, in particular: The desired chemical reaction does not yet take place at room temperature, but rather requires a certain activation energy and, therefore, an elevated operating temperature. Classical sensors of this type are, for example, metal oxide gas sensors which typically must be operated between 250° C. and 400° C.
Hot plates are utilized not only for chemical sensors, but also for sensors based on the physical transducer principle, such as, for example, thermal conductivity sensors, Pirani elements (vacuum sensors), or mass flow sensors.
Micro hotplates are manufactured, according to the related art, either as closed diaphragms or via suspended diaphragms, such as described, for example, in “Micromachined metal oxide gas sensors: opportunities to improve sensor performance,” Isolde Simon et al., Sensors and Actuators B 73 (2001), pp. 1-26.
Such sensor elements including micro hotplates conventionally have typical lateral dimensions of greater than 1 mm×1 mm. In order to meet the requirements of consumer electronics, such as those present, for example, in smartphones, a miniaturization of the lateral dimensions of less than ˜1 mm×1 mm is presently sought and, simultaneously, a reduction of the power consumption is required. In addition to the challenges of special heater designs, the surface area available for chip bonding is therefore becoming smaller and smaller and, therefore, the challenges placed on a production-suitable assembly and joining technology are also elevated.
Suspended diaphragms, such as those manufactured, for example, with the aid of the SMM technology, have advantages with regard to “chip handling” and bonding, since, in this case, chips may be bonded on the entire back side surface and, therefore, the possible bonding area is much larger than is the case for a diaphragm which was exposed from the back side via a wet chemical exposure (using, for example, KOH) or dry etching with the aid, for example, of DRIE. Closed diaphragms, which are typically under tensile stress, have advantages, however, with regard to robustness and compatibility with various coating methods, and therefore these closed diaphragms retain their right to exist, despite the smaller bonding area, even in highly miniaturized systems.
A combined pressure, humidity, temperature, and gas sensor was recently offered on the market. The gas sensor must be operated at elevated temperatures, for example, of approximately 200° C. to 400° C., in order to achieve a good gas reaction (catalytic conversion) and is therefore implemented in a diaphragm on a miniaturized hotplate including a heater. Typical dimensions of the hotplate-silicon substrate are 0.9×0.5 mm2 in the case of a diaphragm size of 300×300 μm2. The heat output of the hotplate should be optionally dimensioned in such a way that the other sensors, which are integrated together in a very small housing (for example, 3×3×0.9 mm3), are not excessively affected.
The actual sensitive material of the gas sensor, whose resistance is measured, is mounted on the surface of the diaphragm, in this case, using methods known today, i.e., for example, via a dispensing method, wet-chemically manufactured materials, namely “sensor pastes,” being processed herewith. This method presently imparts a limitation on a further miniaturization, since not only is the minimum size of an applied drop predefined in this case, but also usually its shape, since the material is often undesirably self-leveling.
Other methods for manufacturing gas sensors utilize ink jet methods for the deposition of the gas-sensitive material or a thin-film technology including a shadow mask for the deposition. The minimum achievable size is substantially limited in all the methods. Sizes, for example, below approximately 100×100 μm2, for the dimensions of the area coated with the gas-sensitive material are not attainable.
With respect to future sensors, an application of several sensitive materials is additionally desirable. In this case, the overall size of the heated area and the diaphragm is to be further reduced. In any case, the areas on which the materials are applied must be substantially reduced in size. This is not possible by way of a drip method or even by way of an ink-jet dispensing method.
It is therefore desirable to utilize thin films, i.e., layers which are applied via physical or chemical deposition methods, for example, via chemical vapor deposition, atomic layer deposition, sputter deposition, ion beam-assisted deposition, vacuum evaporation methods, etc. (see, for example, German Patent No. DE 3 322 481 A1).
When thin-film methods are utilized for a gas-sensitive layer, it is problematic that this layer is not compatible, in many cases, with normal structuring processes of the semiconductor industry. The gas-sensitive layers are often porous and sensitive to contamination or soiling, and often even to components of lithographic varnishes.
In standard structuring methods, a layer is generally applied in a planar manner over an entire wafer, then a mask is applied with the aid of photolithography and then a restructuring of the layer is carried out in the unprotected areas with the aid of wet etching or dry etching or sputtering methods. The mask is usually made of photoresist; a multilayer mask, a so-called “hard mask,” is also utilized, if necessary. These masks are removed after this structuring step. Etching methods are used again in this case, which means there is an undesirable interaction in the case of gas-sensitive layers lying under the mask.
With the aid of this standard structuring method in the semiconductor industry, structure widths in the range of a few dozen nanometers may be achieved.
The exposure of a gas-sensitive layer to the chemicals used (for example, photoresist for a resist mask, solvents for the removal of the resist mask, etchants for removing a hard mask, argon- or oxygen-ion bombardment, or the like) usually substantially changes the layer, primarily when this process must be carried out multiple times and the first deposited and structured layers are then subjected to this process multiple times. In this case, etching media or etching methods would have to be found, each of which selectively etches only the desired layer and the attack on the other gas-sensitive layers is minimal. The fact that the aforementioned metal oxides have, in part, very similar etching properties is a great challenge. In addition, the gas-sensitive layers are mostly porous. Residue of mask materials and process chemicals may therefore remain in the gas-sensitive material throughout the entire process sequence, in a barely controllable manner, and influence the subsequent gas reaction.