PATENT ABSTRACT
A thermal diode for a photosensor of a thermal imaging camera includes a semiconductor substrate having a surface and two doped structures set apart from each other on the surface. Furthermore, a device is provided for influencing a current between the first and the second structure, in order to reduce a current density in an area near to the surface and to increase it in an area far from the surface. In addition, a topology having an even absorption layer is proposed. The measures proposed have the aim of realizing a low-noise diode for thermal applications.

PATENT DESCRIPTION
RELATED APPLICATION INFORMATION 
       [0001]    The present application claims priority to and the benefit of German patent application no. 10 2012 216 814.1, which was filed in Germany on Sep. 19, 2012, the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a photosensor for a thermal imaging camera. In particular, the invention relates to a thermally sensitive diode for the photosensor. 
       BACKGROUND INFORMATION 
       [0003]    A thermal imaging camera for spatially-resolved temperature measurement uses a thermally sensitive sensor array, in which a characteristic thermal radiation of an object is imaged onto an array of thermally sensitive sensor elements by a lens transmitting infrared radiation. For example, long-wave infrared radiation (LWIR) includes wavelengths in the range of approximately 8 to 14 μm. Due to the infrared radiation, the sensor elements warm up by amounts on the order of a few mK. An uncooled sensor array for characteristic thermal radiation may be realized cost-effectively on the basis of thermally sensitive diodes of silicon p-n junctions. The heating of the sensor element caused by the infrared radiation leads to a change in the current-voltage characteristic of a sensor element, which is able to be evaluated by electronics that are as low-noise as possible. 
         [0004]    For the practical applicability of such a sensor element, it is necessary to honor further boundary conditions. In order to minimize an unwanted heat transfer from the sensor element, for example, the sensor element may be exposed micromechanically. In this manner, heating of the sensor element may be increased, and therefore the signal may be increased, so that a high signal-to-noise ratio may be attained. The thermal resolution of the sensor may thereby be improved. Furthermore, the individual sensor elements must have a greater extension than the wavelength used of approximately 8 μm; surface areas of approximately 12 μm 2  are customary. In addition, each sensor element should have its own absorption layer in order to ensure the best possible absorption of the infrared radiation to be received. For example, silicon dioxide may be used for that purpose. In addition, the aim should be for sufficient mechanical stability of the sensor element with respect to vibrations. The observance of these boundary conditions is essential in constructing a low-noise sensor element for the photosensor of a thermal imaging camera, so that customary methods for improving a signal-to-noise ratio of the sensor element may not be usable. 
         [0005]    An object of the present invention is to indicate a thermal diode for a photosensor of a thermal imaging camera, the thermal diode exhibiting the lowest noise possible. A further object of the invention is to indicate a corresponding photosensor. The invention achieves these objectives by a thermal diode and a photosensor having the features set forth in the description herein. The further descriptions herein describe more specific embodiments. 
       SUMMARY OF THE INVENTION 
       [0006]    A thermal diode according to the present invention for a photosensor of a thermal imaging camera includes a semiconductor substrate having a surface and two doped structures set apart from each other on the surface. 
         [0007]    Furthermore, a device is provided for influencing a current between the first and the second structure, in order to reduce a current density in an area near to the surface and to increase it in an area far from the surface. 
         [0008]    In order to perform a measurement, the thermal diode may be traversed by an external current, or an external voltage may be applied. Arriving infrared radiation alters the voltage-current characteristic (U/I characteristic), so that the radiation may be inferred on the basis of the change. 
         [0009]    The noise of the thermal diode usually includes thermal noise, shot noise and 1/f noise. The 1/f noise of the thermal diode is produced in particular by structure defects within the semiconductor material or at an interface of the semiconductor material of the thermal diode. Such defects (traps) may trap charge carriers; the dwell time of trapped charge carriers may be highly variable. The entrance of a charge carrier into and its exit from such a defect alters the current flowing through the thermal diode and thus produces a current noise. Due to a great number of defects with different states, the spectra of the influences of all defects add up to a total spectrum that exhibits a 1/f characteristic. The amplitude of the 1/f noise decreases as the frequency rises, the noise-power density usually being cut in half upon doubling of the frequency. 
         [0010]    Since the defects described occur especially in the area of the surface of a semiconductor substrate, the 1/f noise in particular may be reduced by directing the current flowing through the thermal diode, away from the surface into deeper layers of the semiconductor substrate. The signal-to-noise ratio of the thermal diode may thereby be improved, which means an amplified thermal resolution of the thermal diode or of a photosensor having a plurality of such thermal diodes may be achieved. 
         [0011]    In a first variant, the device also includes a third doped structure disposed between the first and the second doped structure on the surface. The third doped structure need not be connected to a further electric potential, and in particular, need not lead to an electrical contacting which may be connected to an electrical or electronic element outside of the thermal diode. Such a structure is also referred to as potential-free or floating. The third doped structure forms a p-n junction with the surrounding semiconductor substrate, so that an electric field is formed in the area of the p-n junction. This field may cause a current flowing through the semiconductor substrate to be reduced in this area. The current flow between the first and the second doped structure may thereby be deviated into a deeper layer of the semiconductor substrate. 
         [0012]    In one specific embodiment, the third structure extends at least 100 nm below the surface of the semiconductor substrate. The current may thereby be diverted sufficiently into deeper layers of the semiconductor substrate. 
         [0013]    The thermal diode may be circular, the second structure surrounding the first structure at a constant distance. In this context, the third structure may be disposed concentrically between the first two structures, so that the second and third structures surround the first structure concentrically. Due to the circular formation of the thermal diode, the surface of the semiconductor substrate may be better utilized. At the same time, the circular third structure is able to assist efficiently in diverting the current between the first and second structures into deeper semiconductor layers. 
         [0014]    In another variant, which may be combined with the aforesaid variant, the device includes a doped base below the first or the second structure, the doping extending at least 1 μm below the surface of the semiconductor substrate. While customary doped structures on the surface of the semiconductor substrate have a thickness of several 10 nm to a maximum of 100 nm, by forming a depth structure of 1 μm and more, the current flow in deeper layers of the semiconductor substrate may be promoted. In so doing, the diode current may be distributed onto an enlarged cross-sectional area between the base and the semiconductor substrate. A portion of the current which flows in a layer of the semiconductor substrate near to the surface may thereby be reduced to the advantage of a current flowing in deeper layers. In one specific embodiment, the base may also extend to a greater depth, e.g., approximately 3 to 5 μm. 
         [0015]    In a first specific embodiment, the base is formed in one piece with the first or second structure. In other words, the first or second doped structure extends to the indicated depth in the semiconductor substrate. 
         [0016]    In another specific embodiment, the base may be doped differently from the first or second structure. This allows the use of a doped semiconductor substrate or a layer of a doped semiconductor substrate on the undoped semiconductor substrate. In addition, the base may be manufacturable separately from the first or second doped structure, which means a manufacturing process is able to be made easier. 
         [0017]    An absorption layer may be provided to cover the structures and the area situated between them on the surface of the semiconductor substrate, the absorption layer having a thickness modulated subject to the process, especially a uniform thickness. For example, the absorption layer may include silicon dioxide in order to improve absorption of the infrared radiation to be detected. By forming the absorption layer in a uniform thickness, the density of defects at the interface between the absorption layer and an adjacent layer may be reduced. In addition, different current paths are able to have more uniform lengths, that is, variance in the lengths of all current paths may be reduced. The 1/f noise is thereby able to be reduced overall. 
         [0018]    In one specific embodiment, one surface of the absorption layer may be even. Furthermore, the thickness of the absorption layer may not exceed 50 nm. In one specific embodiment, the absorption layer may reach a layer thickness of no more than 20 nm. Due to a reduced layer thickness, a reduced impurity concentration may be obtained, which means the 1/f noise may therefore be reduced overall. 
         [0019]    The thermal diode may be set up to provide a current-voltage characteristic, influenced by electromagnetic radiation in the infrared range, between the first and the second structure. In particular, the mid- or near-infrared range may be covered. In one specific embodiment, middle infrared radiation in a range of approximately 8 to 14 μm wavelength is utilized. 
         [0020]    A photosensor according to the present invention for a thermal imaging camera, especially in the infrared range, includes at least one thermal diode of the type described above. 
         [0021]    The invention will now be described in greater detail with reference to the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0022]      FIG. 1A  shows a top view of the thermal diode. 
           [0023]      FIG. 1B  shows a lateral view of the thermal diode. 
           [0024]      FIG. 2  shows a thermal diode with a field ring. 
           [0025]      FIG. 3  shows a thermal diode with base. 
           [0026]      FIG. 4  shows a thermal diode with base and a deep field ring. 
           [0027]      FIG. 5  shows a photosensor having thermal diodes according to one of  FIGS. 1A through 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 1A  shows a top view and  FIG. 1B  shows a lateral view of thermal diode  100 . In the specific embodiment shown, a concentric, especially a circular concentric formation is selected. In other specific embodiments, other concentric or linear formations may also be used. 
         [0029]    A substrate  105  of semiconductor material, e.g., silicon (Si), has a surface  110  on which a first doped structure  115  and a second doped structure  120  are disposed. In this context, structures  115  and  120  on surface  110  may be embedded into substrate  105 , as illustrated in  FIG. 1B . Structures  115  and  120  as well as the area of surface  110  of substrate  105  may be situated between them are covered uppermost with an absorption layer  125 . For example, absorption layer  125  may be formed of silicon dioxide (SiO 2 ). 
         [0030]    Structures  115  and  120  are doped with different polarity; for instance, first structure  115  may be N-doped, while second structure  120  is P-doped. A reversed polarity is likewise possible. A diode is thereby obtained between structures  115  and  120 . In the concentric type of construction shown, the diode is formed between outer structure  120  and inner structure  115 , which is why, for reasons of symmetry, two diodes are drawn in symbolically in  FIG. 1B . Functionally, however, there is only one diode. 
         [0031]    If electromagnetic radiation, especially in the infrared range, falls on the configuration, a change in the U/I characteristic between structures  115  and  120  is thereby brought about. At a given current, the change in voltage is a function of the intensity of the electromagnetic radiation, so that the radiation may be determined on the basis of the change in voltage. The change in voltage is usually amplified by a suitable measuring amplifier, which is connected to areas  115  and  120 , and evaluated. A photosensor for a thermal imaging camera may be formed by a plurality of thermal diodes  100  situated side-by-side. 
         [0032]    To reduce a 1/f noise of thermal diode  100 , absorption layer  125  is formed as thinly as possible. In one specific embodiment, the thickness of absorption layer  125  may be no more than approximately 50 nm, or, approximately 20 nm thick. Furthermore, it may be the case that the upper surface of absorption layer  125  be even. In this context, absorption layer  125  may have a uniform thickness. Owing to the formation of absorption layer  125  described, it is possible to reduce the defect density in the area of the interfaces between absorption layer  125  and adjacent layers  105 ,  115  and  120 . 
         [0033]      FIG. 2  shows a thermal diode  100  in a further specific embodiment. In accordance with the representation in  FIG. 1B , a thermal diode  100  is shown which additionally has a potential-free third structure  130  that is disposed between first structure  115  and second structure  120 . In the concentric specific embodiment depicted, third structure  130 , like second structure  120 , surrounds first structure  115 , so that third doped structure  130  may be referred to as a field ring. Like first structure  115  and second structure  120 , third structure  130  is formed by a doped semiconductor material, in one manner, the doping of outer second structure  120  may be adopted. However, a different doping may also be used. 
         [0034]    In contrast to structures  115  and  120 , third structure  130  is not set up to be connected to a further electrical or electronic element. In particular, no metal or other contacting surface is provided for the connection of third structure  130 . Between third doped structure  130  and the surrounding material, here substrate  105 , a p-n junction forms at which, upon the flow of current between structures  115  and  120 , an electric field may be established which impedes a flow of current close to third structure  130 . Density of the current flowing close to surface  110  of substrate  105  is thereby reduced, while the current density in deeper layers of substrate  105  is increased. A lower number of defect locations may be expected in the deeper layers, which is why the 1/f noise of thermal diode  100  may be reduced by diverting the current into deeper layers of substrate  105 . 
         [0035]      FIG. 3  shows a thermal diode  100  in a further specific embodiment. Corresponding to the representation in  FIGS. 1B and 2 , a variant is shown in which a base  140  is disposed below second doped structure  120 . In another specific embodiment, base  140  may also be disposed below first doped structure  115 . 
         [0036]    Base  140  continues second doped structure  120  downward in the direction of deeper layers of substrate  105 . In one specific embodiment, base  140  may be formed in one piece with second structure  120 . Expressed differently, second doped structure  120  may reach into greater depths of substrate  105 . In one specific embodiment, this depth is 100 to several 100 nm great and may reach 1 μm or more below surface  110 . 
         [0037]    In the specific embodiment shown, base  140  is produced from a semiconductor material which is doped differently than second structure  120 . In addition, base  140  is embedded into a doped substrate  145  which forms the upper section of substrate  105 . In the exemplary embodiment shown, first structure  115  is N+-doped, second structure  120  is P+, base  140  is P-well and doped substrate  145  is N-well. Other specific embodiments, particularly with polarity inverted at all elements, are likewise possible. Owing to base  140 , a surface is enlarged, at which contact exists to surrounding substrate  105  or doped substrate  145 . The diode current between first structure  115  and second structure  120  may thereby be distributed onto a larger cross-sectional area, whereby the current density may be reduced absolutely. All in all, due to the configuration shown, a greater portion of the diode current is able to flow in deeper layers of substrate  105  or of doped substrate  145 , where fewer defects are able to impede the current flow. 
         [0038]      FIG. 4  shows a thermal diode  100  in yet another specific embodiment based on the specific embodiment illustrated in  FIG. 2 , base  140  and doped substrate  145  of the specific embodiment shown in  FIG. 3  likewise being depicted without restricting the generality. The features of the various specific embodiments put forth within the scope of this invention may be combined with each other in any way desired in order to provide a thermal diode  100  which is as low-noise as possible. 
         [0039]    In the specific embodiment shown, third doped structure  130  is particularly deep, which may be 100 to several 100 nm. In one specific embodiment, the third structure may reach a thickness of 1 μm or even more. The effect of the diversion of the diode current into deeper areas of substrate  105  or of doped substrate  145  may be further promoted by the especially deep formation of third structure  130  shown. In combination with base  140  shown, this diversion is able to succeed particularly efficiently, so that thermal diode  100  may have an especially small 1/f noise component, and therefore a reduced noise. 
         [0040]      FIG. 5  shows a photosensor  150  having thermal diodes  100 . Thermal diodes  100  may be implemented to be concentric and round in the manner described above. In the specific embodiment shown, each thermal diode  100  includes a field ring in the form of third doped structure  130 ; however, another of the specific embodiments described or a combination of the specific embodiments described may also be used. Thermal diodes  100  are disposed on one common substrate  105 ; thermal diodes  100  may also be freed (exposed) micromechanically—individually or in groups—from substrate  105 . 
         [0041]    The edge length of thermal diodes  100 , which are assigned to a picture element (pixel) of photosensor  150 , may be at least as great as the wavelength of the infrared light to be detected. In the mid-infrared range (MWIR), the edge length may amount to approximately 3-5 μm, and in the long-wave infrared range (LWIR), to approximately 8-14 μm. In this context, one or more thermal diodes  100  may be assigned to one picture element. 
         [0042]    Photosensor  150  may be used for the imaging of electromagnetic radiation, particularly infrared radiation. With the aid of photosensor  150  and by adding only a few further components, a thermal imaging camera  155  may be made available in an easy manner.