Abstract:
A micromechanical component having a substrate made from a substrate material having a first doping type, a micromechanical functional structure provided in the substrate and a cover layer to at least partially cover the micromechanical functional structure. The micromechanical functional structure has zones made from the substrate material having a second doping type, the zones being at least partially surrounded by a cavity, and the cover layer has a porous layer made from the substrate material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a micromechanical component having a substrate made from a substrate material having a first doping, a micromechanical functional structure provided in the substrate and a cover layer to at least partially cover the micromechanical functional structure. The present invention also relates to a method for manufacturing a micromechanical component. 
     BACKGROUND INFORMATION 
     Micromechanical function is understood to include active function, e.g., a sensor function, or passive function, e.g., a printed conductor function. 
     Although it may be applied to any micromechanical component and structure, such as, for example, sensors and actuators, an exemplary embodiment according to the present invention and the underlying problem are elucidated with reference to a micromechanical component, e.g., an acceleration sensor, which may be manufactured, for example, using a silicon surface micromachining technology. 
     Monolithically integrated inertial sensors produced by surface micromachining technology, in which the sensitive movable structures are situated on the chip without protection (analog devices), may result in increased expenses for handling and packaging. 
     This problem may be circumvented by a sensor having an evaluation circuit on a separate chip, e.g., covering the structures produced by surface micromachining with a second cap wafer. This type of packaging may constitute a large share of the cost of manufacturing an acceleration sensor by surface micromachining. These costs may arise, for example, as a result of the high surface area required between the cap wafer and the sensor wafer and due to structuring (2-3 masks, bulk micromechanics) of the cap wafer. 
     The structure of a functional layer system and a method for the hermetic capping of sensors using surface micromachining is referred to in German Published Patent Application No. 195 37 814, in which the production of a sensor structure is explained. The cited hermetic capping is performed using a separate cap wafer of silicon, which may be structured using expensive structuring processes, such as KOH etching. The cap wafer is applied to the substrate with the sensor (sensor wafer) using a seal glass. This requires a wide bonding frame around each sensor chip to ensure an adequate adhesion and seal integrity of the cap. This may limit the number of sensor chips per sensor wafer. Due to the large amount of space required and the expensive production of the cap wafer, sensor capping may incur considerable costs. 
     FIG. 10 is a schematic cross-sectional view of a micromechanical component. 
     As shown in FIG. 10, a semiconductor substrate is identified as  10 , a sacrificial layer as SL, a functional level having a micromechanical functional structure (e.g., an acceleration sensor) as FS, a seal glass as SG, a cavity as CA and a cap wafer as CW. As described above, the corresponding manufacturing process may be expensive since it requires two wafers, for example, a substrate wafer  10  and a cap wafer CW, which may be adjusted to each other. 
     The production of a cavity under a porous silicon layer is referred to in G. Lammel, P. Renaud, “Free-standing mobile 3D microstructures of porous silicon,” Proceedings of the 13 th  European Conference on Solid-State Transducers, Eurosensors XIII, The Hague, 1999, pages 535-536. 
     SUMMARY OF THE INVENTION 
     It is believed that an exemplary micromechanical component and manufacturing method according to the present invention allow a simple and cost-effective manufacturing of a micromechanical component, e.g., an acceleration sensor, a micropump, a flow channel, a check valve, a flow regulator, etc., using porous substrate material. 
     The use of such porous substrate material, e.g., porous silicon, may permit simple production of a cavity having a superimposed diaphragm in one process step. The micromechanical structures may be produced in the same process step. Thus, it is believed that advantages of an exemplary micromechanical component according to the present invention and an exemplary method for manufacturing the same include: 
     the production of micromechanical structures in a cavity having a superimposed diaphragm in one process step; 
     the exclusion of the cap wafer with wafer-to-wafer adjustment; 
     the inclusion of a vacuum in the cavity; and 
     the production of structures having complex depth profiles. 
     An exemplary embodiment according to the present invention is based on the micromechanical functional structure having zones made from the substrate material having a second doping, the zones being at least partially surrounded by a cavity, and the cover layer having a porous layer made from the substrate material. During manufacturing, the p-doped zones may be readily etched, when the substrate is anodized. However, the n-doped zones may not be etched or only their surfaces may be insignificantly etched. 
     According to an exemplary embodiment of the present invention, a sealing layer seals the pores of the porous layer. In this manner, a predetermined atmosphere under the diaphragm may be set. 
     According to another exemplary embodiment of the present invention, the sealing layer has an oxide layer formed on the porous zone. 
     According to still another exemplary embodiment of the present invention, at least one of the zones made from the substrate material having the second doping type has a supporting zone to support the porous zone. 
     According to yet another exemplary embodiment of the present invention, at least one of the zones made from the substrate material having the second doping type is completely detached from its surroundings. 
     According to still another exemplary embodiment of the present invention, the cavity includes a flow channel, which may be connected by at least two back openings. 
     According to yet another exemplary embodiment of the present invention, the back openings are each connected by one transfer opening, which is formed in the zone. 
     According to still another exemplary embodiment of the present invention, a sealing layer seals the pores of the porous layer and a detection device situated on the sealing layer piezoresistively detects the flow rate. 
     According to yet another exemplary embodiment of the present invention, a check valve device is provided above a corresponding transfer opening within the flow channel, the check valve device having at least one of the zones made from the substrate material having the second doping type, which is detached from its surroundings or is resiliently connected to the substrate material. 
     According to still another exemplary embodiment of the present invention, two check valve devices of different dimensions are provided above a corresponding transfer opening, a sealing layer sealing the pores of the porous layer and the porous zone, the sealing layer being operable as a pump diaphragm. 
     According to yet another exemplary embodiment of the present invention, the cavity includes a circular inner flow channel and a concentric outer flow channel, which are connected by radial ports in a separation zone made from the substrate material having the second doping type, the inner flow channel being interrupted by a bar and a back inlet opening being provided on one side of the bar and a first back outlet opening being provided on the other side of the bar and a second back outlet opening being provided in the outer flow channel, so that a medium, flowing through the back inlet opening, may be separated, specific to mass, by centrifugal force, through the first and second back outlet opening. 
     According to still another exemplary embodiment of the present invention, the substrate has at least one trench, which is partially filled with a doping material of the second doping type and partially filled with a filler. 
     According to yet another exemplary embodiment of the present invention, the substrate material is silicon. 
     According to still another exemplary embodiment of the present invention, the zones made from the substrate material having the second doping type are provided in the substrate at different depths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a - 1   c  are cross-sectional views of a micromechanical component manufactured using an exemplary manufacturing method for manufacturing the micromechanical component according to the present invention. 
     FIG. 2 is a micromechanical structure manufactured using another exemplary manufacturing method according to the present invention. 
     FIGS. 3 a  and  3   b  are cross-sectional views of a micromechanical component manufactured using another exemplary manufacturing method according to the present invention. 
     FIGS. 4 a - 4   d  are cross-sectional views of a micromechanical component manufactured using yet another exemplary manufacturing method according to the present invention. 
     FIGS. 5 a  and  5   b  are cross-sectional views of an exemplary micromechanical component according to the present invention. 
     FIGS. 6 a  and  6   b  are cross-sectional views of a second exemplary micromechanical component according to the present invention. 
     FIGS. 7 a  and  7   b  are cross-sectional views of a third exemplary micromechanical component according to the present invention. 
     FIGS. 8 a  and  8   b  are cross-sectional views of a fourth exemplary micromechanical component according to the present invention. 
     FIGS. 9 a  and  9   b  are cross-sectional views of a fifth exemplary micromechanical component according to the present invention. 
     FIG. 10 is a cross-sectional view of a micromechanical component. 
    
    
     DETAILED DESCRIPTION 
     In the figures, identical symbols denote identical or functionally equivalent components. 
     FIGS. 1 a - 1   c  are schematic views in cross-section of an exemplary micromechanical component manufactured using manufacturing method for manufacturing the micromechanical component according to the present invention. 
     As shown in FIG. 1 a , the micromechanical component includes a p-doped wafer substrate  10  made of silicon, n-doped zones  15  in substrate  10 , a metal mask  20  and metal mask openings  21 . 
     First, the n-doped zones  15  are produced in p-doped substrate  10 , for example, using standard semiconductor processes, such as an implantation method, in which the penetration depth with a corresponding distribution may be determined by adjusting the energy. The n-doped zones  15  are situated at a specific depth below the substrate surface and may, for example, also be situated on the substrate surface (not shown). 
     Then, parts of the substrate surface are masked using metal mask  20 . A nitride mask, an oxinitride mask, etc., may be used instead of the metal mask  20 . 
     As shown in FIG. 1 b , the n-doped zones  15  of substrate  10  defined by mask  20  are etched electrochemically by hydrofluoric acid (HF) to make them porous. The porosity is controlled by the current density. Initially, a low current density is applied, resulting in the production of a layer of low porosity. The current density is then raised above a critical value. In addition, the hydrofluoric acid concentration may be reduced, or other solutions that inhibit H 2  formation may be used. As a result, the pores in the lower zone of a porous layer  30  become sized, so that the substrate material is essentially or entirely etched away and a cavity  50  is formed under the remaining porous layer  30 . In this case, the term electropolishing is used. The material is removed through porous layer  40 . 
     The structure formed in the functional level by n-doped zone  15  includes exposed structures  60 , permanent structures  70  and structural elements, which are connected to porous layer  30  by a supporting zone  40 , thus forming a diaphragm support. Depending on the width of the n-doped structures, the structures may also be undercut and exposed as exemplified by exposed structures  60  of FIG. 1 b.    
     An exemplary manufacturing method according to the present invention may consider the different dopings, n and p, for example, reacting differently to the electrochemical etching attack in semiconductor substrate  10 . For example, the p-doped zones in semiconductor substrate  10  may be anodized well. However, the n-doped zones  15  may resist the etching attack. Consequently, the buried n-doped zones  15  may not be attacked during the anodizing. A porous film, which may superficially form on n-doped zones  15 , may be eliminated by tempering in H 2  or by a short dip in silicon-etching solutions, such as, e.g., TMAH or solutions containing KOH. In this case, the etch front passes around n-doped zones  15 . 
     As shown in FIG. 1 c , the pores of porous silicon zone  30 , which form an upper limit of cavity  50 , are sealed by different processes. The deposition of a layer with oxide, nitride, metal, epitaxy or the oxidation of porous layer  30  to form sealing layer  75  are exemplary arrangements. Tempering in H 2 , for example, at temperatures above 1000° C., may also result in a vacuum-tight seal. The pressure ratios during the sealing process determine the internal pressure arising in cavity  50 , and H 2  may diffuse out by tempering. 
     The structure exemplified in FIG. 1 c  may be used as an acceleration sensor. Exposed structures  60  may be capable of vibrating in transverse accelerations, as a result of which the distance between exposed structures  60  and permanent structures  70  may periodically change. The change in distance may be analyzed capacitively by an interdigital capacitor. If a vacuum is to be enclosed under the sealing diaphragm made up of porous zone  30  and sealing layer  70 , the sealing diaphragm may be stabilized by cited supporting zones  40 . 
     Alternatively, all micromechanical structures manufactured using an exemplary method according to the present invention may be produced together with a corresponding integrated circuit, e.g., an evaluation circuit. For this purpose, an epitaxy layer may be deposited on the porous zone. The corresponding circuit components may be produced, for example, using CMOS, bipolar or mixed processes. 
     FIG. 2 is a micromechanical structure manufactured using another exemplary manufacturing method according to the present invention. 
     As shown in FIG. 2, reference symbol  200  denotes a doping mask and  201  denotes a doping mask opening. In contrast to metal mask  70  of FIGS. 1 a - 1   c , an n-doping is used as mask  200  in this exemplary embodiment. The combination of an n-doping as a mask and an additional mask layer on the doped substrate surface, e.g., nitride, may also be used. 
     FIGS. 3 a  and  3   b  are cross-sectional views of a micromechanical component manufactured using another exemplary manufacturing method according to the present invention. 
     As shown in FIG. 3, n-doped zones  15   a ,  15   b  are provided at different depths, made possible by the selection of different implantation energies. As a result, structures having very complex depth profiles may be produced. As shown in FIGS. 3 a  and  3   b , two different implantations are performed to produce the upper functional level having n-doped zones  15   a  and to produce the functional level having n-doped zones  15   b . In other respects, the method steps occur in a similar manner, described above with reference to FIGS. 1 a - 1   c.    
     The second functional level may be incorporated by depositing an epitaxy layer, into which the second functional level is implanted after the first functional level has been implanted. 
     FIGS. 4 a - 4   d  are cross-sectional views of a micromechanical component manufactured using yet another exemplary manufacturing method according to the present invention. 
     In addition to the reference symbols previously introduced, FIG. 4 a  shows trenches  80  in p-doped semiconductor substrate  10 . The trenches  80  may be introduced in semiconductor substrate  10 , for example, using an etching method in combination with a hard mask. 
     As shown in FIG. 4 b , after the trenches  80  have been created, a chemical vapor deposition occurs with an n-doped deposition layer  90 , e.g., epitaxial silicon, to form n-doped zones  15   c . Subsequently, as shown in FIG. 4 c , the trenches  80  are filled with a filler, e.g., polysilicon, and the resulting structure is planarized. Finally, an epitaxial polysilicon layer  150  is deposited as shown in FIG. 4 d.    
     This procedure of trench formation, doping, filling and epitaxial deposition may be repeated cyclically to produce complex depth profiles. For example, this exemplary method according to the present invention may permit a very high-definition doping profile to be produced with a high aspect ratio. In addition to n-doped polysilicon, for example, oxide, BPSG, etc., may be used for filling. For example, filler  100  may be n-doped or p-doped, depending on the intended appearance of the resulting structure. 
     Following the exemplary procedure shown in FIG. 4 d , the further process steps described above with reference to FIGS. 1 b  and  1   c  occur. 
     FIGS. 5 a  and  5   b  are cross-sectional views of an exemplary micromechanical component according to the present invention. 
     FIGS. 5 a  and  5   b  illustrate a branched flow channel having defined transfer openings. In this exemplary embodiment according to the present invention, the transfer openings are provided as back openings  510 , while porous zone  30  is hermetically sealed by a sealing layer  75 . N-doped zones  15  define the lower limit of cavity  50   a  and thus the bottom of the flow channel. The Y-shaped structure of the flow channel may be attained by suitable masking. 
     For example, transfer openings  520  may be provided in the structure shown in FIGS. 5 a  and  5   b , which are provided in n-doped zone  15 , so that, when back openings  510  are etched from the back, the passages do not become too large, which is indicated by the corresponding bell-shaped back etching profile. In this respect, n-doped zone  15  also acts as an etching stop for the etching from the back. 
     In the exemplary embodiment according to the present invention described with reference to FIGS. 5 a  and  5   b , an additional epitaxy layer (not shown) may be deposited and power components (not shown), e.g., power transistors, may be implemented on the additional epitaxy layer. The flow channel may then carry a coolant liquid or a coolant gas or another coolant, so that the power components may be cooled from the back by thermal contact. Compared with cooling from the front, cooling from the back may not require the surface to be protected from the coolant. The flow channel may have a meandering shape or may be entwined in another direction for this application (not shown). 
     FIGS. 6 a  and  6   b  are cross-sectional views of a second exemplary micromechanical component according to the present invention. 
     In this exemplary embodiment according to the present invention, piezoresistive resistors  630 ,  630 ′ are provided on the sealing layer above porous zone  30 . Varying flow rates in flow direction FD result in a varying pressure, which subjects the diaphragm and thus piezoresistive resistors  630 ,  630 ′ to a voltage of varying strength. The resulting change in resistance may be analyzed. A heating structure having temperature sensors analogous to the previous thermal mass flow sensors may also be used. 
     It is believed to be advantageous in that, due to the supply of the mass flow from the back, it may not be necessary to protect resistance elements  630 ,  630 ′ against media. 
     FIGS. 7 a  and  7   b  are cross-sectional views of a third exemplary micromechanical component according to the present invention. 
     An exemplary embodiment according to the present invention shown in FIGS. 7 a  and  7   b , relates to a check valve, and includes a micro-sealing ball  730  and a micro-sealing plate  740  which, together with transfer opening  720  in n-doped zone  15   b , form a check valve. Micro-sealing ball  730  and/or micro-sealing plate  740  are formed simultaneously with the flow channel and/or transfer opening  720  during the anodization process and seal off transfer opening  720  in the event of a return flow. 
     FIGS. 8 a  and  8   b  are cross-sectional views of a fourth exemplary micromechanical component according to the present invention. 
     The exemplary embodiment described with reference to FIGS. 8 a  and  8   b  is a micropump. The diaphragm contains porous zone  30 , and sealing layer  70  is thinner and may be deflected in direction DD. 
     A deflection may be implemented, for example, by using a magnetic layer as sealing layer  75 , which may be deflected by an electromagnet. The diaphragm may be thermally or electrostatically deflected. In doing so, cavity  50   d  is enlarged or reduced in volume, and the use of two different check valves  830 ,  830 ′ permit a flow direction FD to be imposed. Check valve  830  has the shape of a ball, and check valve  830 ′, in the form of an ellipsoid, which interacts with an elliptical, elongated opening. 
     When the diaphragm is deflected upwards, check valve  830 ′ closes the right inlet, and liquid may flow past the check valve. Thus, liquid is drawn into the left transfer opening. With a downward deflection, left check valve  830  closes the round transfer opening, while liquid may flow past right check valve  830 ′. Thus, the liquid drawn in is pressed out through the right transfer opening. 
     FIGS. 9 a  and  9   b  are cross-sectional views of a fifth exemplary micromechanical component according to the present invention. 
     The exemplary structure according to the present invention and described with reference to FIGS. 9 a  and  9   b  is a gas centrifuge. The gas centrifuge includes a circular inner flow channel  50   e  and a concentric outer flow channel  50   f , which are connected by radial ports  905  in a separation zone  15  made of the substrate material. The inner flow channel is interrupted by a bar  910 . A back inlet opening I is located on one side of the bar and a first back outlet opening O 1  is provided on the other side of bar  910 . A second back opening O 2  is provided at the end of outer flow channel  50   f . Thus, a medium flowing through back inlet opening I may be routed to first or second back outlet opening O 1 , O 2  specific to mass by centrifugal force. That is, heavier gas components are pressed into outer flow channel  50   f  due to the centrifugal force, while the lighter gas components remain in inner flow channel  50   e . To intensify the affected separation effect, a plurality of such gas centrifuges may, for example, be connected in series, one after the other. 
     It should be noted that the present invention is not limited to the various exemplary embodiments described above, but rather may be modified in a variety of ways. 
     For example, micromechanical base materials such as, e.g., germanium, may be used instead of the silicon substrate. Also, sensor structures may be formed.