Abstract:
A micromechanical sensor includes a support of silicon substrate having an epitaxial layer of silicon applied on the silicon substrate. A part of the epitaxial layer is laid bare to form at least one micromechanical deflection part by an etching process. The bared deflection part is made of polycrystalline silicon which has grown in polycrystalline form during the epitaxial process over a silicon-oxide layer which has been removed by etching. In the support region and/or at the connection to the silicon substrate, the exposed deflection part passes into single crystal silicon. By large layer thicknesses, a large working capacity of the sensor is possible. The sensor structure provides enhanced mechanical stability, processability, and possibilities of shaping, and it can be integrated, in particular, in a bipolar process or mixed process (bipolar-CMOS, bipolar-CMOS-DMOS).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 08/791,106 filed on Jan. 29, 1997 now U.S. Pat. No. 6,076,404, which is a continuation of application Ser. No. 08/470,373 filed on Jun. 6, 1995 (now U.S. Pat. No. 5,616,514), which is a continuation-in-part of application Ser. No. 08/253,883 filed on Jun. 3, 1994 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     German Patent Application No. 40 00 903 shows a micromechanical sensor configured as an acceleration sensor which is manufactured on the basis of silicon micromechanics. The sensor includes a support made of a silicon substrate with an epitaxial layer of silicon applied to the silicon substrate. A part of the epitaxial layer is laid bare by an etching process to provide micromechanical deflection parts in the form of tongues. One or more tongues are suspended for this purpose from one or more webs and are deflected from the rest of the sensor structure under the action of a force on the sensor. Means are furthermore provided for evaluating the deflection. 
     German Patent Application No. 40 03 473 discusses taking crystallographic angles of a single-crystal silicon wafer into consideration in connection with the development and arrangement, as well as for the etching process. To provide a means for evaluating the deflection of the tongues, individual electrodes, which are each electrically insulated from each other, are provided so that a capacitative change between tongue and electrode is measurable. 
     The laying bare of the tongues as a component of the epitaxial layer is effected by means of a rear etching. 
     This constitutes an additional process step as compared with an ordinary bipolar process. 
     From PCT Application No. WO 92/03740, it is known to apply, on a support made of a silicon substrate, a layer of polycrystalline silicon onto a silicon-oxide layer having contact windows in an LPCVD process (low pressure chemical vapor disposed). The silicon oxide layer is removed by an etching process, as a result of which the polycrystalline silicon layer is present at a distance from the silicon substrate as tongue or electrode on the supports formed in the contact windows. The deposition rate of low voltage LPCVD-poly is about 60 Å/min and is thus very slight as compared with the separation rate of about 1 μ/min of epitactic polysilicon. Due to this, for reasons of process economy, only relatively thin LPCVD layers can be produced. As the result, the working capacity of a lateral acceleration sensor, for example, is limited by the correspondingly small layer thicknesses of the tongues. Furthermore, additional silicon depositions are necessary for this procedure, as compared with a conventional bipolar process. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, production of the exposed deflection part of polycrystalline silicon, or the mechanically active layer, can be accomplished without additional expense as part of a bipolar or MOS process. Moreover, no additional silicon depositions are necessary. 
     Epitaxy is a known special process for the production of single-crystal layers of silicon. In accordance with the present invention, epitaxial layers deposited in polycrystalline form (over silicon-oxide), or other non-crystalline layers, are used which are applied in the course of a conventional bipolar process. 
     Since the epitaxial deposition rate is very high as compared with an LPCVD process, relatively thick layers of 10 to 30 μm can be produced in accordance with the present invention, thereby increasing the working capacity of the lateral sensor. 
     In accordance with the present invention, a silicon oxide layer is applied to a silicon substrate at a position where one or more micromechanical deflection parts are to be formed. Contact window openings are formed on the silicon substrate adjacent to the silicon-oxide layer. An epitaxial layer of silicon is then deposited on both the silicon oxide layer and the contact openings. The epitaxial layer grows in polycrystalline form on the silicon oxide layer. However, the epitaxial layer grows in single crystal form on the contact window openings to provide a direct connection to the silicon substrate. Lateral structural limitations of one or more micromechanical deflection parts are then formed by etching through the polycrystalline portion of the epitaxial layer in narrow etch pits to form trenches using, for example, an anisotropic plasma etching technique. Finally, the silicon-oxide layer is removed during an etching process to complete the formation of the one or more micromechanical deflection parts. 
     One particular advantage of the present invention is that the method according to the present invention can be universally employed for different designs. For example, arrangements of tongues which are supported on one side and plates which are supported on edge regions are both possible. Moreover, several layers can be produced one above the other. Another advantage is that, using the same method steps and without additional expense, integrated electronic circuits, particularly an evaluation circuit for the deflection, can be produced on the same support with the micromechanical sensor. Similarly, electrical insulation of the micromechanical sensor parts from other electronic components on the same support is possible with the customary manufacturing steps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  through  1   d  show cross sections through a sensor according to the present invention during different stages of manufacture. 
     FIGS. 2 a  through  2   e  show cross sections through a sensor according to the present invention, which includes a transistor, during different stages of manufacture by a bipolar process. 
     FIG. 3 a  is a top view of a sensor according to the present invention. 
     FIG. 3 b  is a cross section through the sensor of FIG. 3 a.    
     FIG. 4 is a top view of a second embodiment of a sensor according to the present invention. 
     FIG. 5 is a top view of a third embodiment of a sensor according to the present invention. 
     FIG. 6 is a cross-sectional view of a fourth embodiment of a sensor according to the present invention at a first stage of manufacture. 
     FIG. 7 is a cross-sectional view of the sensor shown in FIG. 8 along the line VII—VII. 
     FIG. 8 is a top view of the fourth embodiment of a sensor according to the present invention at an advanced stage of manufacture. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a support  1  of silicon substrate on which a silicon-oxide layer  2  is applied. Contact window openings  3 ,  4  to the silicon substrate are produced around this silicon-oxide layer  2 . 
     The silicon-oxide layer  2  can either be undoped or contain phosphorus, boron or As doping. Doping leads advantageously to a shorter etching process upon the subsequent removal of the silicon-oxide layer  2 , or can also serve for the doping of the mechanically movable silicon structure. 
     Still other layers such as silica nitride or polysilicon can, if desired, be applied on the oxide layer. 
     In accordance with FIG. 1 b , an epitaxial layer  5  of silicon is deposited on the support  1  and/or the silicon-oxide layer  2  and the contact-window openings  3 ,  4 . Epitaxy is a known special process for producing single-crystal layers of silicon. In accordance with the present invention, the epitaxial layer  5  grows, in single-crystal manner, only on support regions  6 ,  7  over the silicon substrate  1 . On the silicon oxide layer  2  within a region  8  corresponding approximately to the width of the arrow  9 , however, the epitaxial layer grows in polycrystalline form (indicated by the hatching). 
     The support as silicon wafer is preferably aligned in a crystal direction. Orientation in the ( 100 ) direction is technically important for an MOS and BICMOS process and the alignment ( 111 ) for a bipolar process. A ( 110 ) alignment is technically less relevant. 
     In order to improve the quality of the polycrystalline epitaxial layer (region  8 ), a poly-start layer  10  can be applied to the silicon-oxide layer  2  before the epitaxy, as indicated in dashed line in FIG. 1 a.    
     In accordance with an embodiment of the present invention, conductors or mating electrodes, which are limited spatially by pn-transitions, are provided below the etched-away sensor material on the substrate. For the electrical passivation, HF-resistant dielectric layers can be deposited on the substrate (for instance, nitride) before the deposition of the sacrificial oxide. They prevent high-leakage currents over the pn-transitions which are exposed after the etching of the sacrificial oxide layer  2 . 
     The micromechanical deflection parts are laid bare from the polycrystalline epitaxial layer in the region  8 . As shown in FIG. 1 c , deep narrow etching trenches are introduced for this purpose through the polycrystalline epitaxial layer  8  by a trenching process. For this purpose, a suitable mask is required, for instance as resist. The production of the trenches is effected by the technique of anisotropic plasma etching as dry etching process with high anisotropy. By the five trenches  11  shown, the lateral structural limitations of four tongue-shaped deflection parts  12 ,  13 ,  14 ,  15  are etched out. 
     In a further process step, the silicon-oxide layer  2  is removed as sacrificial layer. This removal is effected, with high selectivity with respect to the silicon, by hydrofluoric acid (HF). 
     As can be noted from FIG. 1 d , a micromechanical sensor  16  can thus be produced which has deflection parts  12 ,  13 ,  14 ,  15  of polycrystalline silicon which, in the support region at the connection to the silicon substrate  1 , pass into single-crystal silicon. Upon the action of a force on the sensor, these deflection parts  12 ,  13 ,  14 ,  15  are deflected with respect to the rest of the sensor structure, in particular the silicon substrate  1 . This deflection can be evaluated capacitively or piezoresistively for measurement purposes. 
     The method described above can be applied several times, one above the other, by alternate application of a silicon-oxide layer  2 , other layers  10 , and an epitaxial layer  5 , so that, after corresponding etching processes, several layers of deflection parts  12 ,  13 ,  14 ,  15  can be obtained one above the other. Such embodiments are suitable, in particular, for capacitive acceleration sensors. 
     The deposition rate for the epitaxial layer is relatively high, so that epitaxial layer thicknesses, and thus thicknesses of the deflection parts  12 ,  13 ,  14 , and  15 , of 10 to 30 μm can be obtained. 
     FIGS. 2 a  to  2   e  show the production and development of a concrete micromechanical sensor  16 , together with the integratability in a bipolar process for an adjacent transistor  17 . The transistor  17  represents, for instance, IC circuits. More particularly, the transistor  17  may be an evaluation circuit for the mechanical deflection of the deflection parts in the sensor  16 . 
     FIGS. 2 a  to  2   e  show an ordinary process stage in the bipolar technique after an n + -diffusion (buried layer diffusion) and a p-diffusion (lower insulation diffusion). FIG. 2 a  shows, as a starting part, a support  1  of p-doped silicon substrate. The layers  2  and  10  shown in the left-hand region of FIG. 2 b  correspond to the layers  2  and  10  of FIG. 1 a . The silicon-oxide layer  18  shown on the right side (the transistor is to be produced in the right-hand region) is removed for the further process steps, while the silicon layer  2  with the contact windows shown remains. As shown in FIG. 2 c , the n-epitaxial layer  5  is then applied over this structure, it growing over the remaining silicon-oxide layer  2  in polycrystalline form in the region  8  corresponding to the size of the arrow  9 . 
     In accordance with FIG. 2 d , an electric insulation is then produced by a p-insulation diffusion  19 , as well as a p-base diffusion  20 . Furthermore, an n + -collector connection diffusion  21  and an n + -emitter diffusion  22  are applied in a known manner in accordance with the bipolar process. An upper silicon-oxide layer  23  is furthermore applied. 
     In further process steps according to FIG. 2 e , a trench  11  is introduced for the lateral structural limitation of the tongue-shaped deflection part  12  and the silicon-oxide layer  2  is etched away as sacrificial layer with hydrofluoric acid in order to expose the underlying surface. Furthermore, contact openings and a metallization for connections are produced on the sensor  16 , as well as the transistor connections E, B, C on the transistor  17 . 
     As shown in FIG. 2 e , there is thus created a micromechanical sensor  16  having a tongue-shaped deflection part  12  which can be deflected under the action of force within the air gap  24 . Changes in capacitance can be tapped off and evaluated via the connections  25  and  26 . 
     FIGS. 3 a  and  3   b  show in detail a sensor  16  corresponding to a manufacturing process in accordance with FIGS. 2 a  to  2   e , left-hand side. FIG. 3 b  furthermore shows a corresponding cross section along a section line  27  from the top view of FIG. 3 a.    
     From FIG. 3 a  it can be seen that, by means of the trenching process, a trench  11  has been produced which delimits a plate-shaped structure to form deflection part  12 . Deflection part  13  is connected via two arms  28 ,  29  to the rest of the structure. As such, the sensor can be used as acceleration sensor, preferably for measuring movements vertical to the plane of the support. 
     In a further embodiment, shown in FIG. 4, a plate-shaped, approximately square deflection element  30  is held at its corners by four arms  31 ,  32 ,  33 ,  34 . Such an embodiment is particularly suitable as a capacitive acceleration sensor. 
     From a top view of a third embodiment in accordance with FIG. 5, it can be noted that, by the technique described above, sensors having a large number of electrodes  36 , possibly connected via conductive paths  35 , can be produced. Opposite these stationary electrodes  36 , there moves a free silicon mass produced in accordance with the method described above. It is provided, on its part, with electrodes which extend between the stationary electrodes  36 . The deflection of the mass  37 , as a result of acceleration in lateral direction, can thereby be sensed capacitively. 
     FIGS. 6,  7  and  8  show another embodiment of the present invention. FIG. 7 is a cross-section of FIG. 8 along the line VII—VII. FIGS. 6 and 7 show the fabrication process. 
     In FIG. 6, a single-crystalline silicon substrate  50  is covered by an etching-layer  51 . Silicon-dioxide (SiO 2 ) is the preferred material for the etching-layer  51 , although silicon nitride, glass or other materials can also be used as single layers or as multilayers on top of each other. On top of the etching-layer  51 , a polycrystalline layer  52  is formed by an epitaxial process. Normally the term epitaxial layer refers to a single-crystalline silicon layer. The layer  52  includes polycrystalline silicon material that is formed by a process that is normally used for the fabrication of a single-crystalline epitaxial layer on a single-crystalline silicon substrate. Epitaxial processes that are used for the fabrication of the layer  52  can be found, for instance, in the book VLSI Technology by Sze (McGraw-Hill, 1983, Chapter 2). As already described in FIG. 1 a , there might be a polysilicon start layer applied on top of the layer Si by a conventional polysilicon process (Sze, Chapter 3.3) before the polysilicon layer  52  is applied on top of the layer  51  by an epitaxial process. 
     FIG. 7 shows the further processing of the device shown in FIG.  6 . As already described, an etching process is applied to form the narrow-etched grooves  53  that extend from the upper side of layer  52  to the etching-layer  51 . As can be seen in the top view of FIG. 8, the trench  53  is used to form, out of the silicon layer  52 , a structure that can be used as a sensor. After that, an etching process that etches the etching-layer  51  is applied to etch the etching-layer  51  beneath part of the layer  52 . 
     FIG. 8 shows a top view of the structure that is formed out of the silicon layer  52 . The anchor area  54  remains connected to the silicon substrate  50  by the etching layer  51  that is not completely etched below the anchor area  52 . Below the movable tongue  55  the etching-layer  51  is completely removed, so that the tongue  55  may be moved by means of an acceleration. Means for detection of the deflection of the tongue  55  are not shown, but are well known in the art. Because of their different lateral dimension, the etching-layer  51  remains below the anchor area  52  while the etching-layer  51  is completely etched below the tongue area  52 . The etching process of the etching-layer  51  is stopped after the etching-layer is completely removed below the tongue area  54  and is still not removed below the anchor area  54 . A typical etching process for SiO 2  includes, for example, dipping the device in hydrofluoric acid. 
     FIG. 8 is only an example of a sensor that can be formed by the process illustrated in FIGS. 6 and 7. Since it is possible to fabricate anchor areas  54  that are firmly connected to the substrate  50  and movable areas  55  that can be moved with respect to the substrate  50 , this process can be used to fabricate all sorts of micromechanical sensor elements, especially, for example, sensor elements which are described in PCT Application No. WO 92/03740. Since the deposition rate of the epitaxial processes is very high, the sensor elements can be fabricated with much higher thickness than sensor elements described in PCT Application No. WO 92/03740.