Method for manufacturing a component, in particular a thermal sensor, and thermal sensor

A method of manufacturing a component, in particular a thermal sensor, and a thermal sensor. The component has at least two regions having different heat conductivities, a surface region being created in a substrate and the heat conductivity of the surface region being lower than that of the surrounding substrate. For producing a flat topography on the component a layer is created which covers the surface region. The layer and the surface region have at least approximately similar physical properties.

BACKGROUND INFORMATION

In practice, numerous micromechanical thermal sensors having a thermally decoupled region are known which are used in many fields of everyday applications, such as temperature sensors or flow sensors. Such sensors have the predominant common feature that sensitive measuring elements are thermally decoupled from the surroundings to allow very rapid and/or highly sensitive measurements to be carried out. In practice, such thermal decoupling is achieved for example by etching on the backside of a substrate such as silicon, for example, a free-standing membrane being created as a result of the etching.

Furthermore, a “sacrificial layer” technology is known by which a cavern is created in a substrate to achieve the thermal decoupling. A layer to be sacrificed may optionally remain. If this layer has a sufficiently low heat conductivity with respect to the surrounding material, thermal decoupling of the sensitive measuring elements in the sensors results without this layer being removed.

However, it is disadvantageous that such methods for manufacturing sensors having a thermally decoupled region are difficult to manage from a process engineering standpoint, complicated to carry out, and costly. An additional disadvantage is that in layers which are created in a substrate such as silicon, for example, undesired arching occurs on the surface during subsequent processes on account of structural differences, volumetric expansion, and the resulting mechanical stress.

SUMMARY OF THE INVENTION

The method according to the present invention for manufacturing a component, in particular a thermal sensor, having a thermally decoupled region in which for producing a flat topography on the sensor a layer covering is created which covers the surface region, the layer and the surface region having at least approximately similar physical properties, has the advantage that a component is provided which has good thermal decoupling in partial areas while at the same time having a flat topography or surface.

Using the method according to the present invention for manufacturing a component, in particular a thermal sensor, having a thermally decoupled region, in which a thermally decoupled surface region of a substrate is created by spin-on deposition or dispensing of sol gel into a recess in the substrate and a subsequent solidification process for the sol gel, during which a porous solid is produced which has a firm connection to the substrate as well as thermal and mechanical resistance, a region or a layer which has a lower heat conductivity than the material which surrounds this region is advantageously provided in the substrate in a simple manner. At the same time, a sensor having a flat topography is provided by the method according to the present invention, since undesired arching of the surface region is eliminated or greatly reduced.

The component according to the present invention, whose surface region is formed, at least in partial areas, from a solidified sol gel embedded in a substrate which is preferably silicon, advantageously has good thermal decoupling and at the same time has a flat topography. This is accomplished by using a sol gel which for the definition of a thermally decoupled region is introduced in a locally delimited manner into a recess in the substrate and in a subsequent solidification process is converted to a porous solid, and which advantageously forms a flat topography on the surface of the substrate.

The term “sol gel” is used here to describe the subject matter of the present invention, and its meaning is described in greater detail in the introduction to the reference book “Sol Gel Science: the Physics and Chemistry of Sol Gel Processing,” C. Jeffrey Brinker, George W. Scherer, 1990, Academic Press Inc., San Diego. In the cited work, the definition of the term “sol gel” is further discussed as the manufacture of ceramic materials by producing a sol, gelling this sol, and removing a solvent. The sol may be produced using an inorganic or organic precursor, for example metal oxides or nitrates, and may preferably be composed of compact oxidic particles or polymer chains. In addition, the term “ceramic” is also intended to include organically modified materials such as ORMOSILs, CERAMERs, or ORMOCERs. In particular, materials based on silicates (SiOx, for example) are semiconductor-compatible, and compared to silicon have a much lower heat conductivity which preferably is in the range of two orders of magnitude smaller.

Alternatives to a solidification process in which a fluid sol gel is converted to a porous solid are provided by complete condensation, drying, or pyrolysis of the sol gel, the porous solid as the end product possibly being SiOx, for example. In particular, a sol gel may be dried by simple evaporation of the solvent contained in a sol gel at a drying temperature of room temperature or higher, the resulting porous solid also being known as xerogel or “dry” gel. If the drying process is carried out in a supercritical region of the phase diagram of a sol gel, the porous solid ultimately produced on the surface region in the substrate is known as aerogel.

In both cases, depending on the type of drying selected, a more or less porous solid structure, composed of SiOx, for example, is produced which has a correspondingly lower heat conductivity. The above-mentioned aerogels have a very high degree of porosity characterized by a solids fraction of down to 1% or even less, relative to the total volume of the aerogel. The aerogels therefore have very low heat conductivities. Such high degrees of porosity are achievable in particular on account of the greatly reduced surface tension when sol gels are dried in the supercritical region.

The starting materials for the substrate of the component according to the present invention as well as for the surface region created therein are economical, and may be semiconductor-compatible components. A sol gel is introduced into a recess in the substrate, using simple processes such as dispensing or spin-on deposition, and then, using the solidification process, a thermally and mechanically stable, porous solid region having low heat conductivity is produced in the substrate.

DETAILED DESCRIPTION

FIG. 1illustrates in highly schematic form a substrate1for a component designed as a thermal sensor2having a thermally decoupled region3. Substrate1and thermally decoupled region3have different heat conductivities, thermally decoupled region3forming a surface together with substrate1and having a heat conductivity that is lower—preferably by two to three orders of magnitude—than that of substrate1.

Surface region3is composed of oxidized, porous silicon3A which is formed directly from substrate1by an appropriate surface treatment of the substrate. To this end, a region of substrate1, which is composed of silicon, present as the result of an electrochemical etching process known as such, is converted to a porous state and is subsequently oxidized to produce a stable material phase. The silicon in substrate1is converted in partial areas, using the etching process, into a porous sponge-like structure having altered physical properties. The sponge-like structure in porous silicon region3A is characterized by the fact that a large part of the silicon is absent and that only small crystallites are present, with the result that this porous silicon region3A has a lower heat conductivity than substrate1surrounding it.

As the result of volumetric changes and stresses as porous silicon3A in surface region3is oxidized, undesired arching occurs on the surface of surface region3, as illustrated in FIG.1.

The various manufacturing phases for porous silicon region3A in substrate1are illustrated in a highly schematic form inFIGS. 2 and 3,FIG. 2showing initially untreated substrate1with a masking4applied in partial areas, andFIG. 3showing substrate1including silicon region3A which has been formed having a porous structure up to a defined layer depth.

FIG. 4illustrates substrate1which has an additional porous silicon layer5extending over the entire cross section, the silicon layer having a smaller layer thickness or lesser layer depth than porous silicon region3A and being additionally created on substrate1before silicon region3A is oxidized and after masking4is removed. Additional porous silicon layer5and porous silicon layer3A have essentially the same porous structure.

As the result of additional porous silicon layer5, arching of surface region3during the oxidation of porous silicon regions3A and5of substrate1following production of same is reduced or eliminated altogether, since the stresses and expansions in volume of porous silicon region3A which cause arching are reduced by the presence of additional porous silicon layer5. The degree of arching of surface region3is significantly influenced by the thickness of additional porous silicon layer5.

To obtain a region of distinct thermal decoupling on the side of substrate1on which surface region3is provided, after the oxidation process additional porous silicon layer5which is applied to form a flat surface on sensor2or for planarization of surface region3is removed again from substrate1and surface region3by back-etching, resulting in two regions precisely delimited with respect to one another having markedly different heat conductivities and a sensor2having a flat topography.

The flat topography of sensor2is achieved primarily by the fact that surface region3and additional porous silicon layer5which covers it have mutually corresponding physical properties, and that arching of surface region3during oxidation is avoided. If arching nevertheless occurs during oxidation, or if additional porous silicon layer5is not intended after oxidation, the two porous and oxidized silicon regions3A and5are uniformly etched at the same etch rate, which results from the mutually corresponding physical properties of surface region3and additional silicon layer5, using the back-etching process. After back-etching, previously arched surface region3together with the surface of surrounding substrate1of sensor2form a flat topography, the surface of substrate1which surrounds surface region3again being formed from unoxidized, nonporous silicon having a higher heat conductivity.

FIGS. 5 through 7illustrate the stepwise manufacture of a thermal sensor2having a thermally decoupled region.FIG. 5shows substrate1, formed from silicon, in which an oxidized, porous silicon region, i.e., surface region3, produced from substrate1in the above-mentioned manner is situated. On its side facing surrounding substrate1, surface region3has an arch which rises above the surface of surrounding substrate1, resulting in an uneven topography of sensor2.

To manufacture a blank for sensor2having a flat topography, as shown inFIG. 6, a layer5A of sol gel covering surface region3is applied to arched surface region3and surrounding substrate1, the sol gel layer after a solidification process having at least approximately similar physical properties and a similar degree of porosity with regard to surface region3. Layer5A, composed of solidified sol gel or a porous solid which in the meantime has become thermally and mechanically stable, and surface region3are then etched at the same etch rate, so that layer5A and the arching on surface region3are equally removed up to the point of the original surface of substrate1, and the blank for sensor2has a flat topography. This design of the blank for sensor2is illustrated in FIG.7.

Metal alkoxide precursors such as tetraethylorthosilicates (TEOS), for example, are used among other materials to manufacture a sol gel. By admixture of an acidic or basic catalyst such as hydrochloric acid or NH4OH, for example, hydrolysis is initiated during which the alkoxide (OCxHy) groups are replaced by hydroxyl (OH) groups. The subsequent condensation reaction then creates the intended siloxane bonds (Si—O—Si), with alcohols or water as by-product, which constitutes the gelling process. Depending on the type and quantity of catalyst used, such a gelling process may last from several minutes to days. It is possible to establish an intended viscosity of a sol gel by modifying this procedure.

To prevent too rapid gelling and to maintain the sol gel in a workable state, after a certain burn-in period or a certain degree of gelling the hydroxyl groups are in turn replaced by nonreactive alkoxide groups. This preferably is achievable by adding chemical substances such as hexamethyldisilazane (HDMS) and/or hexane. If the gelling has already advanced too far in partial areas, a sol gel may be mechanically reliquefied by dispersion, for example.

The sol gel is applied at the intended or preferred viscosity to substrate1and surface region3by spin-on deposition or dispensing. After application on a substrate1, the sol gel is then solidified by pyrolysis or further condensation, preferably at temperatures around 400° C., and forms a porous solid.

FIGS. 8 and 9show substrate1having a recess6, grooved from substrate1with steep etched edges, using an anisotropic physical-chemical etching method. Substrate1is formed from silicon, as before. To produce surface region3, a sol gel is introduced into recess6, andFIG. 9illustrates an intended or ideal flat surface condition after sol gel is filled into recess6and solidified. Surface region3, composed of solidified sol gel or solid which in the meantime has become porous, together with substrate1form a flat topography.

As shown inFIGS. 10 and 11, recess6is provided in substrate1composed of silicon, using an anisotropic, wet chemical etching method which produces a recess having sloped etched edges. According to the illustration inFIG. 11, recess6is likewise filled with a sol gel to form surface region3, which, after solidification of the sol gel, together with substrate1likewise form a flat topography. The sol gel is introduced into recess6of substrate1according toFIG. 9, and also according toFIG. 11is introduced by spin-on deposition, it naturally being within the discretion of one skilled in the art to introduce sol gel into recess6by dispensing or another suitable filling method.

FIG. 12illustrates the case in which arching in surface region3results when sol gel is introduced into recess6of substrate1and is subsequently solidified or converted into a porous solid phase. To achieve a flat topography of sensor2by planarization, as illustrated inFIG. 13a thin layer5B of sol gel is applied to substrate1and surface region3in a second spin-on deposition procedure. The sol gel in layer5B is then solidified, resulting in a flat topography made of porous solid. If necessary, additional layer5B together with the arching in surface region3are uniformly etched, so that the blank for sensor2has a surface, formed after etching of substrate1and the porous area of surface region3, of varying heat conductivity.

To protect surface region3from environmental influences, the surface region is preferably provided with a passivation layer7, illustrated inFIGS. 14 and 15, which is applied to surface region3before sensor structures8are applied.

The starting materials used here for manufacturing sensor2are economical and semiconductor-compatible. In addition, very simple processes such as dispensing or spin-on deposition are used in the manufacture to create a thermally decoupled region in a substrate. Only surface micromechanical processes are used in the manufacturing method, which in particular improves the mechanical stability as a result of filling the recess produced in the substrate, and thus simplifies the packaging of such sensors as well.

It is understood that the thermal sensors may be sensors which are used in micromechanical fields, the subject matter of the present invention not being limited to thermal sensors as such. Of course, within the discretion of one skilled in the art, components may be manufactured according to the present invention which are used for other applications. Thus, it is conceivable to design actuators according to the present invention and to integrate these into ignition devices for airbag systems, for example.