Abstract:
A sensor, in particular for the spatially resolved detection, includes a substrate, at least one micropatterned sensor element having an electric characteristic whose value varies as a function of the temperature, and at least one diaphragm above a cavity, the sensor element being disposed on the underside of the at least one diaphragm, and the sensor element being contacted via connecting lines, which extend within, on top of or underneath the diaphragm. In particular, a plurality of sensor elements may be formed as diode pixels within a monocrystalline layer formed by epitaxy. Suspension springs, which accommodate the individual sensor elements in elastic and insulating fashion, may be formed within the diaphragm.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a sensor, in particular for the spatially resolved detection, and to a method for its production. 
     BACKGROUND INFORMATION 
     DE 101 14 036 describes a method for producing micropatterned sensors, in which openings are introduced into a semiconductor substrate, which transform themselves into cavities underneath a sealed diaphragm cover in the depth of the substrate in a subsequent thermal treatment. This makes it possible to produce a capacitive pressure sensor, the cavity in the substrate being developed between two doping zones, which form a plate-type capacitor having a capacitance as a function of the spacing of the doping zones. The doping zones are connected to a corresponding evaluation circuit by deep contacting. 
     DE 10 2004 043 357 describes a method for producing a cavity in a semiconductor substrate, in which a lattice-type structure on the surface of the substrate is first produced from substrate material not rendered porous, between which or underneath which a porous region is subsequently formed into the depth of the semiconductor substrate. The porosified region is relocated into a cavity by a subsequent thermal treatment, the lattice-like structure being developed into a diaphragm or into part of a diaphragm above the cavity, if appropriate. 
     However, such production methods often do not allow the development of more complex sensors having high resolution and low noise. 
     SUMMARY 
     In contrast, the micropatterned sensor according to example embodiments of the present invention and the method for its production have a number of advantages. At least one, preferably several sensor elements that are laterally set apart are formed within a substrate, each being suspended underneath a diaphragm made of dielectric material. The sensor elements may be diodes, in particular, but basically also transistors, for example. Important is that the individual sensor elements have a temperature-dependent electric characteristic whose values are able to be read out via lead wires. 
     The individual sensor elements are suspended in one or several cavities formed underneath the diaphragm. In this context, a separate cavity may be provided for each sensor element, or several or all of the sensor elements may be disposed within one shared cavity. 
     The individual sensor elements are contacted via lead wires, which run within, on top of or underneath the diaphragm. The diaphragm may be patterned such that it forms individual suspension springs, which link each sensor element to the surrounding mainland or to surrounding webs of an epitaxy layer formed on top of or above the substrate. 
     According to an example embodiment, reinforcements, specifically LOCOS (local oxidation of silicon) reinforcements produced by local oxidation, are formed in the dielectric layer constituting the diaphragm, which increase the mechanical stability considerably. The reinforcements may be formed especially at the lateral edge of the diaphragm, so that they surround the particular sensor element; furthermore, they may extend at the lateral edge of the mainland or the remaining webs supporting the sensor elements and thereby accommodate the suspension springs with high stability. The ultimate tensile strength of the suspension springs at the sensor elements and the mainland or the remaining webs is able to be increased in this manner. 
     Because of the diaphragm, in particular because of the suspension springs in the diaphragm, excellent thermal decoupling of the sensor elements with respect to each other and the mainland is achieved. Developing the sensor elements in an epitaxial and thus monocrystalline layer makes it possible to keep the signal noise very low. This is advantageous in particular when forming diodes or transistors. 
     Thus, a component array having high resolution or a high number of sensor elements and low noise is formed, which may have a mechanically very sturdy design. The individual lead wires to the sensor elements can be connected to shared lead wires, so that the individual components may be read out via successive addressing. Due to the high integration, the power requirement is low. 
     In particular, this makes it possible to produce a diode array for the spatially resolved temperature measurement and/or for the spectroscopic measurement of a gas concentration. Another field of application is a fingerprint sensor. 
     According to an example embodiment, the sensor not only includes the detector region having the sensor elements but, laterally adjacent and advantageously isolated therefrom, a circuit region including additional components to evaluate the signals output by the sensor elements. At least a few of the process steps of forming the sensor elements of the detector region may also be utilized to produce the circuit region, so that a rapid and cost-effective production is possible. Thus, a MEMS (micro electro mechanical system) component having a combined sensor system and electronic evaluation circuit is able to be formed on one chip. 
     The production may be implemented entirely by surface-micromechanical process steps, so that only one surface needs to be processed. The production may be implemented at the level of the wafer with subsequent sectioning. 
     To begin with, a first region of the doped substrate (or a doped layer formed on the substrate) is rendered porous for the production, a lattice-like structure and a second region surrounding the first region first being protected from the subsequent etching process by suitable doping. Thus, the first region underneath the lattice-type structure may subsequently be rendered selectively porous in electrolytic manner; if appropriate, complete removal of the material in this region is also possible already. An epitaxial layer may then be grown on the lattice-like structure and the surrounding mainland, annealing of the porous region being implemented during the growing process (or possibly also in an additional step) while forming a cavity. 
     Thus, an epitaxial monocrystalline layer in which the sensor elements are subsequently developed by additional process steps, e.g., by doping corresponding diode regions, may be formed above the cavity. Since the sensor elements are developed in the monolithic epitaxial layer, they exhibit low signal noise. The cavity already thermally insulates them from the substrate. 
     Further insulation is achieved by developing a diaphragm underneath which the sensor elements are suspended. To this end, one (or several) dielectric layer(s) is/are applied on the epitaxy layer and then patterned. In particular, the dielectric layer may be formed by oxidation or deposition of an oxide layer, formation of etching accesses through the dielectric layer and the epitaxy layer, as well as subsequent sacrificial layer etching of the epitaxy layer. The at least one dielectric layer thus forms a diaphragm, which is self-supporting above the cavity and accommodates the particular sensor element in thermally and mechanically decoupled manner. Further thermal decoupling may be achieved by patterning suspension springs in the diaphragm, thereby making it possible to route the electrical lead wires to the sensor elements via the suspension springs. 
     Example embodiments of the present invention are explained in greater in the following text with the aid of the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  through  1   f  illustrate the process steps of the production of a micromechanical sensor according to an example embodiment, exemplarily for one diode pixel of the sensor; 
         FIG. 1   a  illustrates a preliminary circuit process and a process step for developing a lattice structure in the detector region; 
         FIG. 1   b  illustrates the process step of an n-layer epitaxy; 
         FIG. 1   c  illustrates the process steps of the implantation or diffusion in the circuit region, production of dielectric layers, and patterning of the contact holes in the dielectric layers; 
         FIG. 1   d  illustrates the back end circuit process with the development of a full-area metal cover of the diaphragm, and of passivation layers in the circuit region; 
         FIG. 1   e  illustrates the process steps of removing the layer stack above the first metallization layer, patterning the metallization in the region of the diode pixels, and opening sacrificial-layer etching accesses, or suspension springs in the remaining dielectric layer; 
         FIG. 1   f  illustrates the isotropic sacrifical layer etching to expose the diode pixel; 
         FIG. 2  illustrates an example embodiment as an alternative to  FIG. 1   f  with retroactive formation of a cavity from the rear side; 
         FIG. 3  illustrates a plan view of a finished diode pixel according to  FIG. 1   f  or  FIG. 2 ; 
         FIG. 4  illustrates a cross section through a sensor having a plurality of diode pixels and continuous, shared free space; 
         FIG. 5  illustrates a plan view of the sensor of  FIG. 4  having a plurality of diode pixels. 
     
    
    
     DETAILED DESCRIPTION 
     In the production process, a detector region  2  and, laterally spaced apart or abutting, a circuit region  3  are formed on a p-semiconductor substrate  1 , e.g., p-doped (100) silicon; the development of the two regions  2 ,  3  is able to be fully or partially combined in the subsequent process steps. 
     According to  FIGS. 1   a  through  1   f , both the circuit region  3  and detector region  2  are implemented in surface-micromechanical manner from the top surface of p-substrate  1 . To this end, preliminary circuit processes to form circuit region  3  may be implemented to begin with. Process steps for forming circuit region  3  may also be added between the subsequent process steps  1   a  through  1   f . In an advantageous manner, one or several of the following process steps for developing detector region  3  is/are simultaneously utilized to produce circuit region  3 . 
     Using p +  doping, for example, a lower iso-layer  6 , which has the shape of a trough in cross section, may be formed in p-substrate  1  between detector region  2  and circuit region  3 , lower iso-layer  6  being supplemented toward the top in a later process step and utilized to insulate detector region  2  from circuit region  3 . 
     For each sensor element to be produced, a first region  12  is rendered porous in detector region  2 , a lattice-like structure  14  having lattice webs  16  remaining on the surface of first region  12 . In the lateral region, first region  12  is advantageously delimited by an annular second region  18 . First region  12  and second region  18  are doped to different extents, especially by a different type of charge carrier. First region  12  is p-doped, for example, that is to say, it may be formed directly out of p-substrate  1  in particular, and second region  18  is n + -(or also n-)doped. In principle, first region  12  may also be completely removed already so that a free space remains as “100% porosity” underneath lattice-type structure  14 . 
     The production of this array of a porosified first region  12 , a surrounding second region  18 , and a spared lattice-type structure  14  is described in DE 10 2004 036 035 as well as DE 100 32 579, for example, to which reference is made here for individual details. Second region  18  is produced at the lateral edge of first region  12 , for instance by redoping, such as with the aid of implantation and/or diffusion methods. Furthermore, lattice-type structure  14  having lattice webs  16  is formed by n-doping, and lower iso-layer  6  is formed by p + -doping. These designs of second region  18 , lattice-like structure  14 , and lower iso-layer  6  is realizable with the aid of, for example, resist masks prior to the further process steps, i.e., also prior to the etching. 
     Subsequently, an etching mask  20  of SiO2 and/or Si3N4, for example, is deposited on detector region  2  and circuit region  3  and patterned such that first region  12  having lattice-like structure  14  is spared. Only then will first region  12  be rendered porous by electrochemical etching in an electrolyte containing hydrofluoric acid. A spreading agent such as isopropanol, ethanol, or a tenside may be added in order to reduce the surface tension. Depending on the substrate doping and the desired micropattern, the concentration of hydrofluoric acid may range from 10 to 50%. The porosity of first region  12  is adjustable by the selected current density. 
     Lattice webs  16  and annular, n + -doped second region  18  are not attacked by the electrochemical etching process since holes (defect electrons) are required for the dissolution process of silicon, of which a sufficient number is available in the p-silicon but not in the n-Si. Second region  18  therefore delimits first region  12  in the lateral direction, and the depth of first region  12  is defined by the etching duration and current intensity. 
     According to  FIG. 1   b , an n-epi layer  24  is subsequently deposited or grown on p-substrate  1  epitaxially, such layer extending across detector region  2  and circuit region  3 . During this epitaxial growth process, annealing of porous first region  12  also takes place, which leads to thermal relocation of the porous material and thus to the formation of a cavity  26  underneath n-epi layer  24 . In the process, a monocrystalline layer precipitates from the porous material and deposits on the walls of cavity  26 . Lattice webs  16  relocate to form, for example, a monocrystalline layer  28  between cavity  26  and n-epi layer  24 . This annealing step may be implemented at approximately 900 to 1200° C., for example. The formation of a cavity  26  out of a porous region is described in DE 10 2004 036 035, for instance. 
     According to example embodiments of the present invention, larger-area regions may optionally remain in lattice-type structure  14 , so that only a weak porosification takes place underneath them, i.e., merely by lateral etching. These more weakly porosified regions may form temporary support points  30  inside cavity  26  during annealing, which thus support layer  28  and n-epi layer  24  above cavity  26 . 
     According to  FIG. 1   c , suitable structures are subsequently developed within and/or on top of n-epi layer  24 , which may be implemented both in detector region  2  and also in circuit region  3 . Different implantation- or diffusion-process steps for developing the circuits may be utilized in circuit region  3  in a manner known per se. An n + -region  32  and a p + -region  34  are formed in detector region  2  in n-epi layer  24  for each future pixel via implantation and/or diffusion. N-epi layer  24  together with p + -region  34  forms a diode in the process. Furthermore, one or a plurality of dielectric layer(s)  36  is/are developed, e.g., by oxidation to SiO2, locally thicker LOCOS reinforcement regions  38  being developed at least in detector region  2  by a LOCOS method. To this end, stronger oxidation accompanied by a corresponding increase in volume and thus thickening in the vertical direction are obtained in SiO2 layer  36  by suitable masking. LOCOS reinforcement regions  38  are formed at the edge of the future pixels in particular, i.e., above the edges of cavity  26 . Corresponding LOCOS reinforcement regions  38  may also be formed in circuit region  3 . 
     Furthermore, the one or the several dielectric layer(s)  36  is/are patterned in circuit region  3  and in detector region  2 . In so doing, access holes  40 ,  42  for the subsequent contacting are patterned above n + -region  32  and p + -region  34 . Different components  44 , for example, are patterned in circuit region  3 . LOCOS reinforcements  38  may be formed here as well. 
     According to  FIG. 1   d , metallizations and passivations, e.g., a metallization layer  48  including contact pad  50 , and one or a plurality of passivation layer(s)  54  are applied in a backend circuit process. Metallization layer  48  of Al, for example, contacts n + -region  32  and p + -region  34  in cut-out access holes  40 ,  42  of dielectric layer  36 . Metallization layer  48  is utilized accordingly also in circuit region  3  for contacting the components  44  formed there, for supply lines and possibly also for components. Metallization layer  48  advantageously also forms connecting lines  56  between detector region  2  and circuit region  3  so that an integrated component is produced, which has a detector region  2  and a circuit region  3 . 
     One or a plurality of metallization layer(s)  50  made of, e.g., Al may be developed in the process. N + -region  32  is provided merely for contact with metallization layer  50  so that no Schottky contact occurs between the metal and the heavily doped region. Actual diode  35  is formed between n-epi layer  24  and p + -region  34 , which because of its heavy doping likewise does not cause any Schottky contact with metallization layer  50 . As can be gathered from  FIG. 1   d , n-epi layer  24  is able to be insulated from circuit region  3  in the lateral direction by an upper iso-layer  31  and, above this, by a LOCOS reinforcement  38 . 
     The one or the plurality of metallization layer(s)  48  is/are also used to prevent the deposition of the one or the plurality of passivation layer(s)  54  above diode  35 . 
     Passivation layer  54  is subsequently removed above diode  35 , metallization layer  48  serving as etching stop. Metallization layer  48  is then suitably patterned above diode  35 , so that only n+-region  32  and p + -region  34  are contacted by connecting lines  60 ,  62 , as can be gathered from the plan view of  FIG. 3  (the additional patterning of  FIG. 3  takes place only subsequently). 
     Furthermore, according to  FIG. 1   e , sacrificial-layer etching accesses  66  are opened in n-epi layer  24 , preferably by reactive ion etching through n-epi layer  24 . A BRIE method may be used for this purpose or, since the n-epi layer has a thickness of only a few μm, for example, a conventional reactive ion etching method, such as a Bosch etching method, as well. In the process, sacrificial-layer etching accesses  66  are produced in n-epi layer  24 , which forms the preliminary diaphragm. Sacrificial-layer etching accesses  66  are already visible in the plan view of  FIG. 3 , although connecting lines  60 ,  62  are not yet undercut by etching. According to  FIG. 1   f , this takes place in a subsequent isotropic sacrificial-layer etching step using ClF3, XeF2, for example, or some other etching gas that selectively etches silicon, until the monocrystalline region of diode pixel  52  has been exposed by etching. A portion of the one or the plurality of dielectric layer(s)  36  is undercut by etching with the aid of the sacrificial-layer etching and exposed as diaphragm  36 . 1  in this way. Patterned diaphragm  36 . 1  forms elastic suspension springs  70  on which connecting lines  60 ,  62  to n + -doped region  32  and to p + -doped region  34  extend as well. 
     If temporary supports  30  are formed according to  FIG. 1   b , then they will be removed as well during the underetching according to  FIG. 1   f.    
     Diode pixel  52  is therefore supported by the, e.g., four suspension springs  70 , which hang freely now, LOCOS reinforcements  38  being formed in suspension springs  70  or at the transition of suspension springs  70  to the mainland. As a result, individual diode pixels  52  are thermally well insulated from one another and from the remaining mainland via suspension springs  70  made of the insulating SiO2. 
     Diode pixel  52  shown in  FIG. 3  may be used for direct temperature sensing, in particular. In addition, an absorption material for absorbing IR radiation may be applied on diode pixel  52 . 
     The precise design of LOCOS reinforcement  38  may be selected according to the particular mechanical requirements; according to the plan view of  FIG. 3 , it is possible, in particular, to provide an annular reinforcement at the inner end of suspension springs  70 , i.e., at the outer end of diode pixel  52 , and at the outer edge of suspension springs  70 , i.e., in the connection to the mainland. In this manner, two concentric, annular or rectangular LOCOS reinforcements  38  and  38  are formed. 
       FIG. 1   f  and, in a plan view,  FIG. 3  therefore show finished sensor  72 , which as a rule includes a plurality of diode pixels  52  and circuit region  3  having a suitable evaluation circuit. 
     In the example embodiment of  FIG. 2  as an alternative to that in  FIG. 1   f , a cavity  74  is formed from rear side  76  of p-substrate  1  or the entire wafer in addition. To this end, a bulk etching process may be implemented from rear side  76  of p-substrate  1 . To protect the structures of diode pixel  52 , proceeding from  FIG. 1   f , an oxide layer  78  may first be formed at the boundary surfaces of all structures as first process step, i.e., at p-substrate  1 , n-epi layer  24 , both in the mainland region and at diode pixel  52 , and furthermore at second region  18  having n + -doping. This oxidation of the silicon to SiO2 may therefore first be implemented from the direction of the front side, whereupon a deep-trenching etching process is then carried out from rear side  76  of p-substrate  1 , and cavity  74  is formed, which thus is situated underneath individual diode pixel  52 . 
     Cavity  74  may thereupon be sealed using a suitable material, e.g., a material having low thermal conductivity. With the exception of additional cavity  74  underneath diode pixel  52 , sensor  82  of  FIG. 2  therefore corresponds to sensor  72  shown in  FIG. 1   f.    
       FIG. 4  shows an additional example embodiment of a sensor  92 , which basically corresponds to sensor  72  of the first specific embodiment according to  FIGS. 1   f ,  3 ; however, instead of a plurality of separate cavities  26  being developed underneath the plurality of diode pixels  52 , only one continuous cavity  94  is formed, which therefore surrounds all of the diode pixels  52  or a number of diode pixels  52 . In contrast to the first example embodiment, the support of diaphragm  36 . 1  in p-substrate  1  is therefore omitted. However, webs  96  from n-epi layer  24  remain between individual diode pixels  52  and are not etched off, these webs  96  or the lattice-type structure formed thereby being utilized for heat dissipation. During operation, the plurality of diode pixels  52  initially heat up slightly, and the heat they generate is output in lateral direction to webs  96  via diaphragm  36 . 1  formed from dielectric layer  36 , the silicon material of webs  96  having high thermal conductivity. As a result, it is possible to dissipate the heat generated in individual diode pixels  52  to the outside in the lateral direction. In the specific embodiment of  FIG. 4 , a single continuous cavity  94  is therefore produced, at whose underside individual diode pixels  52 , which were formed out of n-epi layer  24 , are suspended. 
       FIG. 5  shows a plan view of a diode array made up of four diode pixels  52 . Connecting lines  60 ,  62  of each diode pixel  52  may be connected to shared connecting lines  98 ,  100 ; as a result, (cathode) connecting lines  60  contacting the particular n + -region  32 , are connected to a shared cathode connecting line  100 - 1 ,  100 - 2 , . . . , and (anode) connecting lines  62  contacting the particular p + -region  34  are connected to one or a plurality of shared anode connecting line(s)  98 - 1 ,  98 - 2 , . . . . The individual diode pixels  52  are therefore able to be read out via corresponding addressing of shared connecting lines  98 - 1 ,  98 - 2 , . . . , as well as  100 - 1 ,  100 - 2 . 
     Given such an array, it is therefore possible to form a complex diode array  110  having relatively few connecting lines. When forming a larger cavity  94  according to  FIG. 4 , shared connecting lines  98 - 1 ,  98 - 2 , . . . ,  100 - 1 ,  100 - 2 , . . . , may be applied on diaphragm  36 . 1  above webs  96 ; contacting of connecting lines  98 - 1 ,  98 - 2 , . . . ,  100 - 1 ,  100 - 2 , . . . at the points of intersection is prevented by a corresponding insulation layer.