Abstract:
A method for manufacturing semiconductor components having micromechanical structures, micromechanical structures being patterned in a wafer for detecting a physical quantity acting on micromechanical structures, and semiconductor components for converting the physical quantity into an electrical signal proportional to the physical quantity being produced. The semiconductor components and the micromechanical structures are defined in a self-aligning manner by process steps acting on one side of the wafer to produce semiconductor components.

Description:
BACKGROUND INFORMATION 
     It is generally known to combine semiconductor components with micromechanical structures. The result is compact components, which are able to detect an acting physical quantity, such as a pressure, and simultaneously produce an electrical signal proportional to this physical quantity that is then fed to an evaluation circuit of the component. Components of this kind are manufactured, as is generally known, as monolithic components, the sensor part and the evaluation part being produced one after another in a wafer. The inherent drawback here is that because of the different fabrication techniques, significant interventions in each of the other fabrication steps have to be made. 
     Furthermore, it is known to manufacture the sensor part and the evaluation part separately and to subsequently join them to the component. The sensor part has the micromechanical structures and the semiconductor components for detecting an electrical signal that is proportional to the physical quantity. In the case of a pressure sensor, a membrane that deforms under applied pressure is produced in a silicon wafer. This deformation is taken up by piezoresistors (semiconductor components), which as a result undergo an analog change in resistance. This altered resistance is detected with a later applied evaluation circuit and used to obtain a pressure-proportional output signal. 
     It is known to fabricate the silicon membrane using an anisotropic etching process. The piezoresistors for detecting membrane deflection are then assigned to this silicon membrane through process steps in the semiconductor component production. The drawback here is that because of the separate processes, the distance between the piezoresistors and the membrane at maximum stress can only be realized with a relatively large deviation of about 50 μm. 
     SUMMARY OF THE INVENTION 
     The method of the present invention has the advantage that the piezoresistors are able to be placed with a much greater accuracy at the point of the membrane&#39;s maximum stress. This makes it possible to substantially reduce the size of the sensor part, so that given the same patterning on a wafer of the same size as those of the related art, the number of attainable components can be increased. Besides a higher yield, one derives the benefit here of reduced costs. 
     Due to the fact that the semiconductor components and the micromechanical structures are defined in a self-aligning manner by process steps acting on one side of the wafer to produce semiconductor components, both the micromechanical structure and the semiconductor components can be defined in the narrowest of spaces by simple process steps to be mastered with a high level of precision, since it is possible using the masking technique of semiconductor fabrication to achieve a very high accuracy, within the range of a few μm. The regions are defined relative to one another with self-alignment, producing by this means the mechanical structures and the semiconductor components. 
     The semiconductor components featuring the micromechanical structures are preferably able to be produced in successive process steps using a plurality of masking levels, the process steps being carried out from merely one side of the wafer. Besides the fact that this is easily feasible in terms of process engineering, a multiplicity of variants can be achieved with relatively little outlay for development. Moreover, it is advantageous that when the micromechanical structures are defined with the aid of the semiconductor component production method, the hollow space needed for membrane deflection is enclosed within the component. This simplifies the subsequent mounting of chips containing the evaluation circuit, since the connection between the evaluation circuit and the sensor element no longer needs to be designed compactly. Thus, for example, in the place of costly soldering, the chips containing the evaluation circuit can be bonded (adhesively applied) to the sensor element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a first process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 2 illustrates a second process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 3 illustrates a third process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 4 illustrates a fourth process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 5 illustrates a fifth process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 6 illustrates a sixth process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 7 illustrates a seventh process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 8 illustrates an eighth process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 9 illustrates a ninth process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 10 illustrates a tenth process step for producing the semiconductor component featuring the micromechanical structures of the present invention. 
     FIG. 11 shows a sectional view through a practical specific embodiment of a pressure sensor. 
     FIG. 12 shows a stress-path diagram of a pressure sensor. 
     FIG. 13 shows a layout of a pressure sensor. 
     FIG. 13 a  shows a detailed view of a first portion of the pressure sensor shown in FIG.  13 . 
     FIG. 13 b  shows a detailed view of a second portion of the pressure sensor shown in FIG.  13 . 
    
    
     DETAILED DESCRIPTION 
     First, the method for fabricating semiconductor components featuring micromechanical structures will be explained on the basis of FIGS. 1 through 10. A wafer  10  is shown schematically in a cutaway view in each of the Figures. 
     On the basis of the exemplary embodiment, the intention here is to disclose the manufacturing of a pressure sensor, which comprises a membrane that is movably arranged over a hollow space, and piezoresistors assigned to the membrane, for detecting the membrane&#39;s deflection. By way of an evaluation circuit, this pressure sensor is connected to a sensor element. 
     Wafer  10  is an SOI (silicon-on-insulator) wafer having a &lt;100&gt;crystal orientation. Above an introduced oxide layer  12 , wafer  10  has an n-doped zone  14 . Applied to n-doped zone  14  is a thermal oxide  16 , into which windows  18  are etched via a masking (not shown). Situated in the area of windows  18  is n-doped zone  14 , upwardly exposed in accordance with the illustration of FIG.  1 . 
     In a next process step elucidated on the basis of FIG. 2, a p + -diffusion  20  is produced through windows  18  in n-doped zone  14 . During formation of the p + -diffusion, a thermal oxide  22  grows into windows  18 , so that p + -zones  20  are delimited within n-doped zone  14  from oxide  12 , i.e., oxide  22 . P + -zone  20  constitutes the later connection region for the piezoresistors. 
     In accordance with the process step shown in FIG. 3, surface  24  is planarized, so that the various levels of thermal oxide  16 , i.e., of thermal oxide  22  are evened out. A CMP polish, for example, can be used for the planarization process. As FIG. 4 shows, a window  26  is opened into planarized oxide layer  16 ,  22 , e.g., by means of an etching process. Window  26  is situated in this case partially over a p + -doped zone and a region of n-doped zone  14 . The size of the later membrane is defined by window  26 . 
     As depicted by FIG. 5, a mask  28  used to introduce a p − -diffusion  30  is subsequently applied. By this means, a p − -zone  30  is created within the region of n-doped zone  14  surrounded by p + -zone  20 . Due to the fact that p − -zone  30  is delimited on both sides by p + -zones  20  (as becomes clear from the top view in FIG.  13 ), —given suitable patterning of p + -zones  20  —, the later piezoresistors formed by p − -zones  30  are automatically equal in size. 
     By means of masking  28 , p − -doped zone  30  is placed in the immediate vicinity of an edge  31  of oxide  22 , which forms an edge (still to be explained) of a membrane where a maximum stress occurs upon deflection of the membrane. Edge  31  is used at the same time as masking for introducing p − -doped zone  30 . 
     A thin layer of thermal oxide  32  is then grown within window  26 . This layer of thermal oxide  32  is thinner than the planarized layer of oxide  16 , i.e.,  22 , so that a depression  34  that opens toward surface  24  results. This depression  34  constitutes the later cavity for the pressure sensor, this cavity likewise being denoted by  34  and covered by the membrane. 
     As illustrated by FIG. 7, a substrate plate  36 , which, for example, can be a thin glass plate or a silicon wafer, is applied to surface  24 . Substrate plate  36  can be bonded anodically or directly to oxide  16 , i.e.,  22 . This results within the component shown in FIG. 7 in a cavity  34  formed by the depression, this cavity  34  being delimited —in accordance with the view shown in FIG.  7 —to the top by substrate plate  36  and to the bottom by a series of layers of oxide  32 , of oxide  12 , and regions of n-doped zone  14  situated between oxides  12  and  32 , of p + -doped zone  20 , and of p − -doped zone  30 —which form the later membrane. 
     As shown in FIG. 8, the silicon of wafer  10  is ablated to oxide layer  12 . This can be done, e.g., by means of abrasive trimming and/or through an overetching step. Vias (through-holes) ( 38 ), through which p + -zones  20  are accessible (FIG.  9 ), are produced, e.g., etched into oxide layer  12 , in conformance with the layout selected for the pressure sensor. A contacting point  40  (FIG. 10) for p + -zones  20  is formed above via  38 . 
     Thus, by means of the process steps elucidated on the basis of FIGS. 1 through 10, micromechanical structures, in this case the membrane covering cavity  34  and suitably doped regions are able to be patterned as semiconductor components to produce the piezoresistors. Depending on the masking and doping selected, semiconductor components featuring various micromechanical structures are able to be fabricated, a defined positioning of the semiconductor components relative to the micromechanical structures being possible in a self-alignment process. 
     The design of a practical pressure sensor  42  shall be elucidated in the following on the basis of FIGS. 12,  13 ,  13   a  and  13   b , the structures shown therein being produced by means of the process steps in accordance with FIGS. 1 through 10. 
     FIG. 11 shows a pressure sensor in a schematic sectional view, the illustration being rotated by 180° with respect to FIG.  10 . Parts equivalent to those in the preceding Figures are given the same reference numerals. 
     Pressure sensor  42  has a membrane  44  which extends over cavity  34 . Cavity  34  is delimited to the top and bottom by membrane  44  and substrate plate  36 , and laterally by edge  31  of oxide  22 . Cavity  34  is completely enclosed in pressure sensor  42 . Piezoresistors  46  formed by p − -doped zones  30 , which as generally known change their resistance value in response to a deflection of membrane  44 , are disposed within membrane  44 . By way of p + -doped zones  20 , piezoresistors  46  are electroconductively connected to contacting points  40 , so that when an electrical voltage is applied, a change in the resistance value of piezoresistors  46  can be detected. This change in the resistance value follows in proportion to a deflection of membrane  44 , to the inside given an externally applied pressure above atmosphere and upwards given an externally applied low pressure (partial vacuum), so that inferences can be made about the pressure effecting the membrane&#39;s deflection by evaluating the change in resistance. The evaluation circuit that completes the sensor element is not shown in detail here; however the evaluation circuit can be suitably completed with pressure sensor  10 . 
     In accordance with one practical exemplary embodiment, membrane size a=300±1 μm, membrane thickness d=7±0.5 μum, and the height h of cavity 34=1 μm. 
     This dimensioning can be easily achieved by adapting the process steps described on the basis of FIGS. 1 through 10. 
     In a diagram over membrane thickness d, FIG. 12 depicts stress o of membrane  46  (left axis of coordinate), and deflection y of membrane  46  (right axis of coordinate). Stress σ is expressed by the formula:        σ   =         p   ·     a   2         3.25                   d   2         .                            
     Deflection y is expressed by the formula:          y   =       p   ·     a   4         72.5                   E   ·     d   3             ,                          
     E being the modulus of elasticity of membrane  46 . 
     Different values are plotted in FIG. 12 for the case where pressure p equals 1 bar. Possible deflection values are characterized by ◯ and possible mechanical stress values by x. The plotted values reveal that, optimally, thickness d of membrane  46  should be 7±0.5 μm, since in this range a tolerance of responsivity (sensitivity) of ±20% is not exceeded. 
       
     FIG. 13 depicts a layout of a pressure sensor  42 . The top view reveals n-doped zone  14 , as well as p + -doped zones  20  embedded therein. Zones  20  are each provided with a contacting  40 , contacting  40 ′ being a ground connection, contacting  40 ″ a positive terminal, contacting  40 ′″ and  40 ″″ being terminals for tapping off a change in an electrical signal resulting through piezoresistors  46  from a deflection of membrane  44 . P + -doped zones  20  form the conductors leading to piezoresistors  46 , which are each delimited by two p + -doped zones  20 . All in all, p + -doped zones  20  being used as conductors for piezoresistors  46  have a diagonal design, so that they are not subject to any piezo effect. It is merely piezoresistors  46 , which are not diagonally aligned, that are subject to the piezo effect. N-doped zone  14  comprises an n-well terminal  48 . 
     An edge length k of pressure sensor  42  amounts, for example, to 1 mm. The piezoresistors have a resistance value of 2 kΩ, so that the interconnection configuration shown produces a total resistance of 2 kΩ of pressure sensor  42 . The bridge resistance is equal to the resistance of a single resistor. The circuit arrangement shown has pressure sensor  42  being connected in the manner of a Wheatstone bridge. 
     FIGS. 13 a  and  13   b  show details in accordance with FIG. 12 in a sectional representation. For the sake of clarity, the individual regions are designated by the process steps elucidated on the basis of FIGS. 1 through 10. 
     As can be easily inferred from FIGS. 13 a  and  13   b , it is possible for piezoresistors  46 , i.e., of p − -doped zones  30  to be placed with the techniques used (see FIGS. 1 through 10) with an accuracy of up to 2±1 μm to edge  31 , at the location of maximum stress upon deflection of membrane  46 . 
     By this means, an extremely small type of construction is rendered possible for pressure sensor  42 . For example, at least 10,000 pressure sensors  42  can be produced on a customary 6-inch wafer.