Abstract:
For producing a layer having reduced mechanical stresses, the layer is composed of at least two sub-layers that are matched to one another such that stress gradients in the two layers substantially compensate. The method is particularly employable in the manufacture of structures in surface micromechanics.

Description:
BACKGROUND OF THE INVENTION 
     The development of mechanical stresses within layers often occurs in the manufacture of layers in semiconductor technology. These mechanical stresses are dependent on selected deposition conditions. These mechanical stresses are often referred to as stress. A distinction is made between absolute stress, which indicates the integral mechanical stressing of a layer, and stress gradient, which describes mechanical stresses within the layer and leads to a bending moment. 
     Such mechanical stresses are particularly observed in the production of layers of polysilicon, SiO 2 , Si 3  N 4  and metal. 
     Mechanical stresses influence diffusion processes within a layer. Given the employment of doped layers of, for example, polysilicon as a diffusion source, mechanical stresses of the polysilicon layer therefore have a disturbing effect. 
     Self-supporting structures are produced in micromechanics by structuring and partial under-etching of layers, these self-supporting structures being connected to a substrate via supporting points. Micromechanics on the basis of silicon process technology has increasing significance in sensor engineering since micromechanical systems therein can be manufactured by integration with an evaluation electronics on a substrate. A mechanical stress in the layer from which the structures are manufactured is likewise disturbing in the manufacture of self-supporting structures. The absolute stress in these systems can in fact be compensated by a suitable, stress-relaxing suspension of the structures. The stress gradient, however, leads to a bending of the self-supporting structure that makes it unusable. 
     For reducing mechanical stresses in a layer, it is known (see, for example, H. Guckel et al., Sensors and Actuators, 20 (1989), 117-122 &#34;Fabrication of Micromechanical Devices from Polysilicon Films with Smooth Surfaces&#34;) to dope the layer and to subsequently temper it. For effectively eliminating the stresses, however, temperatures are required that proceed substantially beyond the temperature stresses that can be accepted for modern electronic processes. This method is therefore unsuitable for utilization in microelectronics. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to specify a method for producing a layer with reduced mechanical stresses wherein unacceptable temperature stresses are avoided. 
     In the method of the invention, the layer is composed of at least one first sub-layer and one second sub-layer. The first sub-layer and the second sub-layer are matched to one another such that the stress gradients in the two layers substantially compensate. The bending moments of the sub-layers therefore cancel. A bending of a self-supporting structure that is formed from this layer later is thus avoided. This occurs, for example, in that the first sub-layer is deposited with a positive stress gradient and the second sub-layer is deposited with a corresponding negative stress gradient. It is critical for the method of the invention that the force moment in the middle plane of the manufactured layer resulting due to corresponding stress gradients in the two sub-layers becomes zero. 
     The first sub-layer is produced, for example, of Si 3  N 4  with a positive stress gradient, and the second sub-layer is produced of SiO 2  with a negative stress gradient. The first sub-layer can also be formed by deposition of, for example, tungsten, that is deposited with a tensile stress or, respectively, a compressive strain or suitable stress gradient, and the second sub-layer can be formed by deposition of tungsten that is deposited with a compressive strain or, respectively, a tensile stress or suitable stress gradient. 
     It is advantageous for producing a layer of polysilicon to form the first sublayer of polysilicon and the second sub-layer of amorphous silicon. It is advantageous in this case to produce an auxiliary layer of, for example, SiO 2  between the first sub-layer and the second sub-layer in order to avoid an epitaxial growth of the amorphous layer. Polycrystalline silicon arises from the second sub-layer of amorphous silicon by subsequent tempering of the two sub-layers. The temperature stresses required for this purpose are compatible with microelectronic manufacturing processes. 
     It particularly lies within the scope of the invention to compose the layer of more than two sub-layers. In this case, an averaging of the gradient of the mechanical stresses arises. According to the invention, the sub-layers are matched to one another such that the resultant force moment in the middle plane of the manufactured layer is zero. 
     A layer produced according to the method of the invention is particularly suitable as a diffusion source. Furthermore, it is suitable as a starting point for the manufacture of a micromechanical system. 
     The invention shall be set forth in greater detail below with reference to the figures and to the exemplary embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a substrate having a layer composed of two sub-layers; 
     FIGS. 2 and 3 show the manufacture of a micromechanical structure from the layer composed of two sub-layers; 
     FIG. 4 shows a layer composed of four sub-layers, whereby an auxiliary layer is arranged between two respective, neighboring sub-layers; and 
     FIGS. 5 and 6 show the manufacture of a micromechanical structure from the layer composed of four sub-layers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A sacrificial layer 12 is applied onto a substrate 11. The substrate 11, for example, is a silicon wafer or a SOI substrate. The sacrificial layer, for example, is composed of SiO 2  and is deposited in a thickness of, for example, 1 μm (see FIG. 1). 
     A first sub-layer 13 is applied onto the surface of the sacrificial layer 12. A second sub-layer 14 is applied onto the first sub-layer 13. The first sub-layer 13 and the second sub-layer 14 together form the layer 15. 
     The first sub-layer 13 is deposited, for example, as tungsten. The deposition thereof is managed such that the first sub-layer 13 is under a positive stress gradient. The second sub-layer 14, for example, is likewise formed of tungsten. The deposition thereof is managed such that the second sub-layer is under a negative stress gradient. The mechanical stress gradient in the first sub-layer 13 and in the second sub-layer 14 compensate, so that the force moment in the middle plane of the layer 15 is zero. 
     The layer 15 is suitable for manufacturing a self-supporting structure 151 for a micromechanical system. For that purpose, the layer 15 is first structured so that the shape of the self-supporting structure 151 is defined in the plane parallel to the surface of the substrate 11 (see FIG. 2). For that purpose, a mask and an etching process that is selective relative to the sacrificial layer 12 are employed. 
     Subsequently, the sacrificial layer 12 is etched selectively relative to the self-supporting structure 151 and relative to the substrate 11, for example with a buffered hydrofluoric acid. The sacrificial layer 12 is also partially removed under the self-supporting structure 151, so that the latter is firmly joined to the substrate 11 only via a supporting portion 121 (see FIG. 3). An sacrifical layer 22 is applied onto a substrate 21 (see FIG. 4). The substrate 21, for example, is a silicon wafer or a SOI substrate. The sacrificial layer 22 is, for example, formed of SiO 2  and is applied in a thickness of, for example, 1 μm. 
     A first sub-layer 23 is applied onto the sacrificial layer 22. An auxiliary layer 24 is applied onto the first sub-layer 23. A second sub-layer 25 is applied onto the auxiliary layer 24. An auxiliary layer 26 is applied onto the second sub-layer 25. A third sub-layer 27 is applied onto the auxiliary layer 26. An auxiliary layer 28 is applied onto the third sub-layer 27. A fourth sub-layer 29 is applied onto the auxiliary layer 28. The first sub-layer 23, the auxiliary layer 24, the second sub-layer 25, the auxiliary layer 26, the third sub-layer 27, the auxiliary layer 28 and the fourth sub-layer 29 together form a layer 30 having reduced mechanical stresses. 
     The first sub-layer 23 of, for example, polycrystalline silicon, is applied onto the sacrificial layer 22. Subsequently, the auxiliary layer 24 of SiO 2  is produced with a thickness of, for example, 0.1-3 nm. The auxiliary layer 24 is produced, for example, by a corresponding waiting time at atmosphere, an oxidizing cleaning, or by oxidation at an elevated temperature. The second sub-layer 25 is likewise produced from polycrystalline silicon. Since the auxiliary layer 24 offers conditions like the sacrificial layer 22 for the nucleation and the growth of the second sub-layer 25, the second sub-layer 25 grows analogous to the first sub-layer 23. The stress gradient in the second sub-layer 24 therefore corresponds to that in the first sub-layer 23. 
     The auxiliary layer 26 is likewise produced of SiO 2  with a thickness of approximately 0.1-3 nm. The third sub-layer 27, likewise formed of polycrystalline silicon, is deposited thereon. The growth of the third sub-layer 27 corresponds to that of the second sub-layer 25 as well as to that of the first sub-layer 23. The auxiliary layer 28 of SiO 2  is applied with a thickness between 0.1 and 3 nm onto the third sub-layer 27. The fourth sub-layer 29, likewise of polycrystaline silicon, is grown onto the auxiliary layer 28. 
     Since the first sub-layer 23, the second sub-layer 25, the third sub-layer 27 and the fourth sub-layer 29 have been grown under comparable conditions, the stress gradients are the same in all sub-layers. 
     When growing a layer of polysilicon, a growth of many small crystallites that compete for further growth during the course of the layer deposition arises in the lower part. In a polycrystalline layer that comprises a thickness of, for example, 200-2000 nm, this leads to a pronounced compressive strain in the approximately first 100 nm. The compressive strain decreases toward the top. 
     This stress gradient in each of the sub-layers 23, 25, 27, 29 is compensated in the method of the invention, since the sub-layers deposited over one another lead to an averaging of the stress gradient. The mechanical stress gradient of the individual sub-layers are compensated all the better in the layer 30 the more sub-layers of polysilicon are deposited under corresponding conditions. 
     In another exemplary embodiment of the invention, the first sub-layer 23 and the third sub-layer 27 are deposited of polycrystalline silicon. The second sub-layer 25 and the fourth sub-layer 29, by contrast, are deposited of amorphous silicon. The auxiliary layers 24, 26, 28 are again deposited of SiO 2  with a thickness between 0.1 and 3 nm. A compressive strain results in the layer, given the deposition of polycrystalline silicon. Given the deposition of amorphous silicon, by contrast, a tensile stress results in the arising layer. The auxiliary layers 24, 26, 28 assure that no epitaxial growth occurs when growing silicon. Due to a tempering of the layer 30, the second sub-layer 25 and the fourth sub-layer 29 of amorphous silicon are converted into polycrystalline silicon. 
     Dopant is introduced into the sub-layers 23, 25, 27, 29 for setting the electrical resistance of the layer 30. This can occur both after the deposition of the corresponding sub-layers as well as after the overall layer 30 has been finished. So that the layer 30 comprises a sheet resistivity corresponding to a single polysilicon layer having the same thickness, it is advantageous to break up the auxiliary layers 24, 26, 28 of SiO 2  with a tempering. For example, such a tempering is a ROA tempering at approximately 1000° C. Such a tempering is process-compatible. 
     The layer 30 is suitable for manufacturing a self-supporting structure in a micromechanical system. For that purpose, the layer 30 is structured so that a self-supporting structure 301 arises. This occurs with the assistance of a mask technique and with the assistance of an etching that is selective relative to the sacrificial layer 22 (see FIG. 5). subsequently, the sacrificial layer 22 is partially removed in an isotropic etching process, for example, with buffered hydrofluoric acid, this occurring selectively relative to the self-supporting structure 301 and relative to the substrate 21, so that only a supporting portion 221 of the sacrificial layer 22 remains via which the self-supporting structure 301 is connected to the substrate 21 (see FIG. 6). Components for a drive electronics of the micromechanical system can be realized in the substrate 21 before or after the manufacture of the self-supporting structure 301. 
     The first sub-layer 23, the second sub-layer 25, the third sub-layer 27 and the fourth sub-layer 29 are preferably produced with a thickness between 50 nm and 1 μm. 
     Although various minor changes and modifications might be proposed by those skilled in the art, it will be understood that I wish to include within the scope of the patent warranted hereon, all such changes and modifications as reasonably come within my contribution to the art.