Abstract:
An optoelectronic gas sensor based on optodes, where multiple separate photosensitive elements and an opto-transmitter located centrally between them are integrated into or onto a semiconductor substrate is characterized in that the photosensitive elements lie in one plane in the substrate, and together with a lateral emission area of the opto-transmitter emitting light laterally, they are covered by sections of the optode material whose thickness and refractive index are selected so that light emitted laterally from the emission area is guided to the photosensitive elements by total reflection in the optode material in each transmission branch. Such a chip-shaped gas sensor based on optodes can be implemented in a very small design, e.g., in an area of 2×2 mm 2  and a thickness of 250 μm.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optoelectronic gas sensor based on optodes as well as an electronic component for producing such an optoelectronic gas sensor. 
     BACKGROUND INFORMATION 
     An optoelectronic gas sensor is described in a technical article “A field hardened optical waveguide hybrid integrated-circuit, multi-sensor chemical probe and its chemistry” by Richard J. Polina et al. in SPIE, vol. 3105, pages 71-78. This known gas sensor based on optodes is diagramed schematically in FIG. 6, and its properties are described briefly below. 
     From an LED  33 , a light bundle is divided vertically by two mirrors  35 ,  36  into two parts L 1 , L 2  and reflected laterally, so it goes to a measuring segment  38  made of optode material and a reference segment  39 . Light beam L 2  passing through optode segment  38  is in turn reflected down vertically on a mirror surface  37 , so it reaches the photosensitive surface of a first photodiode  32 , while light beam L 1  passing through reference segment  39  is reflected down vertically onto the photosensitive surface of another photodiode  31  at another mirror surface  34 . Optode segment  38  and reference segment  39 , mirror surfaces  34 - 37  and photodiodes  31  and  32  are arranged symmetrically about LED  33  which is arranged at the center. The optode material of measuring segment  38  is exposed opposite the gas to be measured (arrow), so this gas has access through an opening provided in the chip casing (not shown). 
     FIG. 7 shows schematically a gas measuring sensor  30  equipped with three successive sensor units  301 ,  302  and  303  according to FIG.  6 . 
     Due to the method of coupling and reflection of light bundles L 1  and L 2  and mirror surfaces  34 - 37 , first from the vertical into the lateral direction and then from the lateral back into the vertical direction, the known gas sensor chip shown in FIG.  6  and described in the technical article cited above is relatively long and broad (e.g., 3 cm long and 0.35 cm broad), so a gas measuring sensor  30  according to FIG. 7 constructed using multiple gas sensor chips  301 - 303  arranged in successive rows turns out to be rather long. In addition, such a known gas sensor chip and gas measuring sensor  30  implemented with it is relatively expensive. Furthermore, various aging phenomena on separate chips  301 - 303  can lead to unwanted measurement errors. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to make possible an improved optoelectronic gas sensor based on optodes, so that it will be less expensive and will have much smaller dimensions, and so that an electronic component according to the present invention can be made available for manufacturing an optoelectronic gas sensor without requiring additional optical components such as mirrors and prisms. 
     According to a first embodiment of the present invention, the object is achieved by providing an optoelectronic gas sensor based on optodes, where multiple separate photosensitive elements and an opto-transmitter located centrally between them are integrated into or onto a semiconductor substrate; this is characterized in that the photosensitive elements lie in one plane in the substrate, and together with a lateral emission area of the opto-transmitter emitting light laterally they are covered by sections of the optode material whose thickness and refractive index are selected so that light emitted laterally from the emission area is guided to the photosensitive elements by total reflection in the optode material in each transmission branch. 
     According to a second embodiment of the present invention, an optoelectronic gas sensor based on optodes achieving the above object is made available, where multiple separate photosensitive elements and an opto-transmitter located centrally between them are integrated into or onto a semiconductor substrate; this is characterized in that the photosensitive elements lie in one plane in the substrate and are each covered by a section of the optode material, the opto-transmitter is spaced a distance away from the sections of the optode material through an annular gap, and the thickness of the optode material is much smaller than the height of the opto-transmitter, so the light emitted by the opto-transmitter is emitted into air and then reaches the photosensitive elements through the optode material either directly or after being reflected on the inside walls of a casing surrounding the gas sensor chip. 
     One of the photosensitive elements and the layer covering it may form a reference segment. The optode material of the measuring segments is made of a gas-sensitive polymer carrier material to which is added at least one indicator substance from the group of compounds including, for example, azobenzenes, acetophenones, corrins, porphyrins, phthalocyanines, macrolides, porphyrinogens, nonactin, valinomycin and/or complexes thereof with transition metals of secondary groups I-II and V-VIII. However, the layer covering the reference segment may be made of a polymer carrier material without any added indicator substance. 
     In an embodiment of a layout according to the present invention, the photosensitive elements of the optoelectronic gas sensor with the sections of the optode material covering them or with the polymer carrier layer covering the reference segment may be arranged in sectors with central symmetry around the opto-transmitter. For example, in this way four symmetrical transmission branches may be formed, including three sensor segments and one reference segment. 
     The chip forming the optoelectronic gas sensor may be designed to be square, pentagonal, hexagonal, heptagonal or octagonal or even circular, for example. Of course, such an optoelectronic gas sensor may also include less than or more than four transmission branches. 
     With an optoelectronic gas sensor implemented according to the first embodiment, the individual transmission branches are separated by barriers, so that the individual transmission branches are not influenced optically by the stray light coming from the optode material. The height of these barriers may be selected to be approximately the same as the height of the central photosensor. In addition, all locations on the chip that are not photosensitive may be mirrorized if necessary, likewise the side walls of the barriers. 
     The substrate of the chip may be made of n-type silicon, and the photosensitive elements may be made of p-type regions of silicon integrated into the n-type silicon substrate. In this way, the photosensitive elements form photodiodes. The opto-transmitter is preferably an LED, but multiple LEDs may also be used to define the wavelength. 
     The thickness of the optode material over the photosensitive elements may be 200 μm to 300 μm and preferably in the range of 220 μm to 260 μm. 
     With an optoelectronic gas sensor constructed according to the second embodiment of the present invention, the thickness of the optode material is much less than the height of the LED and amounts to approx. 5 μm to 20 μm, preferably 5 μm to 10 μm, while the height of the opto-transmitter (LED) is much greater, and may amount to approx. 300 μm. 
     To produce an optoelectronic gas sensor, an electronic component provided for this purpose may be designed so that multiple separate photosensitive elements are integrated into or onto a semiconductor substrate in sectors with central symmetry while maintaining a certain mutual spacing; a thin dielectric insulation layer covers all the photosensitive semiconductor areas; contact openings are provided with contacts to the photosensitive semiconductor elements at defined peripheral locations on the photosensitive semiconductor elements; and metallized strips are provided in the spaces between the photosensitive elements leading to a central contact pad for connecting the LED functioning as the opto-transmitter. 
     In this way, four equally large photodiodes having a common cathode formed by the substrate and an area of the individual photosensitive areas of 0.8 to 1 mm 2  may be integrated into in a chip having an edge length in the range of approx. 200 μm to 300 μm, with the total chip height being approx. 400 μm to 500 μm, in an electronic component suitable for production of an optoelectronic gas sensor. 
     For practical use, the chip of the optoelectronic gas sensor is mounted in a casing (preferably SMD) and protected by a cover. This cover has openings over the locations coated with gas-sensitive material so that gas can penetrate. 
     With respect to good processability, it is possible to mount the casing cover on a circuit board before applying the gas-sensitive materials and to solder the electronic component which later forms the optoelectronic gas sensor into a corresponding electronic circuit without coating it. Therefore, the gas-sensitive materials cannot be destroyed or damaged in any way by the heat of the soldering operation. The openings in the casing cover through which the coating is subsequently applied or through which the gas enters in subsequent use can be sealed with an adhesive film, for example, in the soldering operation to prevent flux from entering. 
     The present invention offers at least the following advantages, in particular, in comparison with conventional optoelectronic gas sensor based on optodes: 
     This sensor has extremely small geometric dimensions due to the integration of all function units of the sensor (i.e., the electronic components and the optode paths on a silicon chip). 
     According to the first embodiment of the present invention, the optode segments, i.e., the gas-sensitive polymer and the reference segment, can assume the function of the passive optical system otherwise necessary, such as prisms or mirrors in guiding the light from the opto-transmitter to the optode and from the optode to the photodiode, because of the small distance between the opto-transmitter (LED) and the photosensitive elements (photodiodes) This guarantees that the atmosphere surrounding the gas can act on a large surface area of the optode. 
     The small distance between the LEDs and the photodiode causes a high efficacy in coupling light between these two components. This means a low power consumption. 
     The adjusted emission characteristics of the LEDs laterally, achieved by mirrorizing the top and bottom sides of them, enhances this effect. 
     coupling losses are extremely low, i.e., no additional passive optical system is necessary, due to direct coupling of light from the LEDs to the optode segments and the reference segment and from there to the photodiodes. 
     The barriers guarantee low crosstalk. 
     The reference and measuring branches have a high symmetry because the photodiodes are monolithically integrated. 
     The optoelectronic gas sensor implemented according to the second embodiment of the present invention has an advantage in comparison with that described above in that the optode layer and the layer of the reference segment can be much thinner. These layers have a thickness of approx. 5-10 μm. They do not function primarily as light guides because the light is first emitted into air by the LED and then passes through the optode layers and/or the reference layer to the photodiodes either directly or after being reflected by the surrounding casing. The barriers have only a subordinate importance here. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a layout of a first embodiment of an optoelectronic gas sensor based on optodes according to the present invention; 
     FIG. 2 shows schematically a central cross section through the gas sensor chip illustrated in FIG. 1; 
     FIG. 3 shows schematically a layout of an electronic component suitable for producing an optoelectronic gas sensor based on optodes according to the present invention; 
     FIG. 4 shows schematically a cross section through the chip illustrated in the layout in FIG. 3; 
     FIG. 5 shows schematically an optoelectronic gas sensor chip according to the present invention produced according to FIGS. 3 and 4 using the electronic component, having a casing surrounding it; 
     FIG. 6 shows an optoelectronic gas sensor chip. 
     FIG. 7 shows a gas measuring sensor produced with the optoelectronic gas sensor chip of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     The layout of a first embodiment of an optoelectronic gas sensor based on optodes shown schematically in FIG. 1 contains four equally large transmission branches a, b, c and d. One of the transmission branches, namely transmission branch c in this example, is designed as a reference segment, as indicated by diagonal shading. The reference segment, three optode segments, each made of optode material  5   a - 5   d , and a photosensitive area  2   a - 2   d  of a photodiode (shown with dotted lines) below that are arranged in a star pattern around a central LED  3  on a common substrate  1 . Individual transmission branches a-d are separated by barriers  6   a - 6   d  located in the interspaces between the optode segments. 
     The sectional view of the optoelectronic gas sensor based on optodes shown in FIG. 1, as seen along line II—II of intersection in FIG. 2, illustrates the integration of the photosensitive areas (only areas  2   a  and  2   c  are shown here) of p-type Si in substrate  1  made of n-type Si. Central LED  3  is mirrorized on its top and bottom sides with a mirror layer  7 ,  8  made of gold, for example, so the beams of light emitted by it are emitted mainly laterally, i.e., into the optode material of optode segments  5   a  and  5   c , where they are totally reflected at the interface of the optode material with air, as illustrated by two beam paths L 1  and L 2  shown as examples, so the beams of light are thus directed at photosensitive areas  2   a  and  2   c  of the photodiodes. 
     As shown in FIG. 2, the optode material is applied approximately to the height of central LED  3 , the areas of the optode material adjacent to both sides of the LED and/or layer  5   c  covering the reference segment are designed with a slight descending curve, and the outer sections of the optode segments and the reference segment are rounded to improve the total reflection of light beams L 1 , L 2  emitted into the optode segments and the reference segment. In this way, a large portion of the light input by the LED into the optode segments and the reference segment reaches photosensitive areas  2   a - 2   d  of the photodiodes. 
     It should also be pointed out that barriers  6   a - 6   d  which are not shown in FIG. 2 have a height approximately up to the height of LED  3 , so the individual transmission branches do not have any mutual optical influence on one another. Gas to be measured can flow over optode segments a, b, d (see arrow in FIG. 2) through windows open to the ambient atmosphere in a casing not shown in FIGS. 1 and 3. 
     A prototype of the optoelectronic gas sensor based on optodes according to FIGS. 1 and 2 was produced using a silicon chip with, for example, the basic structure of photodiode BPW 34 having an area of 2×2 mm 2  and a thickness of 250 μm as the carrier chip, but it was not designed to be photosensitive over the entire area, but instead having photosensitive areas of p-type silicon only selectively at defined locations  2   a - 2   d  (cf. FIG.  1 ). 
     An LED chip  3  as an opto-transmitter was glued to the center of the top side of the carrier chip. The LED had mirror layers  7 ,  8  produced by gold layers on its top and bottom sides (cf. FIG.  2 ). In this way, light which would otherwise be emitted by the LED with a spherical characteristic could be emitted primarily laterally. This gave a higher intensity accordingly. Optode layers  5   a ,  5   b ,  5   d , designed as measuring segments, were coated with a gas-sensitive polymer, i.e., a polymer material to which an indicator substance had been added. The reference segment was preferably coated with an optode carrier material, i.e., a polymer material without any added indicator substance. The design was largely symmetrical, so that in addition to influences of the electronics, changes such as aging and soiling involving the carrier material of the optodes could also be compensated. Light (cf. L 1 , L 2  in FIG. 2) reached the photodiode from the LED by total reflection in the optode layers and/or the reference layer. Locations of the Si chip that were coated but were not photosensitive were mirrorized by gold plating, for example, before being coated. Finally, barriers  6   a - 6   d  were preferably applied by a screen printing method which guarantees that the barrier height required for effective separation can be achieved. 
     The thickness of the Si chip was 250 μm, as mentioned above. The LED was 300 μm wide and 300 μm high. The barrier height was also 300 μm. 
     For practical use, the chip was mounted in a casing, preferably SMD, and protected by a cover having openings over the locations coated with gas-sensitive optode materials, i.e., over optode segments  5   a ,  5   b  and  5   d  so that gas could penetrate. 
     The prototype gas sensor chip had extremely small geometric dimensions due to the integration of all function units of the optoelectronic gas sensor such as the electronic components and the optodes on one silicon chip. 
     Because of the small distance between the LED and the optical receivers, i.e., the photosensitive areas of the photodiodes, the optode segments with the gas-sensitive polymer were able to assume the function of the passive optical system that would otherwise be necessary. This concerns the guidance of light from LED  3  functioning as the opto-transmitter to the optode and from the optode to the photodiode. This guarantees that the gas of the ambient atmosphere can act on a large surface area of the optode. 
     Due to the small distance between the LED and the photosensitive areas of the photodiode, a high efficacy was achieved in coupling light between these components. This resulted in a low power consumption. 
     The adjusted emission characteristic of the LED laterally achieved due to the mirrorized top and bottom sides of the LED enhanced this effect. Extremely low coupling losses occurred due to the direct coupling of light from the LED into the optode and from there into the photodiode. Thus, additional passive optical components could be eliminated. 
     The barriers created between the optode segments and the reference segment guaranteed minimal crosstalk, which would otherwise be caused by stray light that could come from the optodes and enter the adjacent optode segments. The similar and centrally symmetrical embodiment of the measuring segments and the reference segment yielded an optoelectronic gas sensor based on optodes where changes such as aging and soiling involving the carrier material of the optodes could be compensated in addition to compensating for influences of the electronics. 
     Of course, the square embodiment illustrated in FIGS. 1 and 2 having three optode measuring segments and one reference segment should be regarded only as an example. With similar process steps and features, it is also possible to implement chip shapes that are pentagonal, hexagonal, heptagonal, octagonal or even round, optionally having fewer than or more than three optode measuring segments. 
     FIGS. 3 through 5 show one embodiment of an electronic component which is also suitable for a customer-specific design of an optoelectronic gas sensor based on optodes according to the present invention. 
     The layout illustrated in FIG. 3 shows that, as in the first embodiment described above and illustrated in FIGS. 1 and 2, four photosensitive areas  12   a - 12   d  are integrated on a substrate  10 , e.g., made of n-type silicon, and together with an optode material not shown in FIGS. 3 and 4 and/or a reference layer to be applied later by the customer, they form four transmission branches a, b, c and d. 
     Photosensitive areas  12   a - 12   d  are integrated on square silicon substrate  10  in the form of square photosensitive elements made of p-type Si so that they maintain a certain spacing. 
     Metallizing strips  13  and  14  are arranged in the interspaces between photosensitive elements  12   a - 12   d , providing the electric connection to an LED to be glued in place later and functioning as the opto-transmitter, Contact pads  11   a - 11   d  are provided for contacting photosensitive elements  12   a - 12   d.    
     As shown by the sectional view in FIG. 4, contact pads  11  and the central end of metallizing strip  14  are contacted through contact openings through an insulation layer  18  made of SiO 2 , for example, covering substrate  10  and photosensitive elements  12 , in each case to photosensitive elements  12  of p-type Si or to substrate  10  of n-type Si. 
     The electronic component illustrated in FIGS. 3 and 4 has a chip edge length of approx. 250 μm, and the area of the individual photosensitive areas amounted to approx. 0.8-1 mm 2 . The metallization was gold or aluminum. 
     Use of the electronic component illustrated in FIGS. 3 and 4 is especially suitable for producing an optoelectronic gas sensor based on optodes. 
     This will now be explained on the basis of FIG.  5 . 
     The chip is mounted in a casing  20  (preferably SMD) and protected by a cover  21 . Cover  21  has openings  22  over the locations to be coated with gas-sensitive materials, so that gas can penetrate (see arrow). For good processability, casing cover  21  can be soldered into a suitable electronic circuit on a circuit board before applying the gas-sensitive materials, and the component shown in FIGS. 3 and 4 can be soldered there without being coated. Since the chip had not yet been coated with the gas-sensitive materials, they are protected from the soldering operation. In the case of a customer-specific implementation, the coating is applied by the customer on site through openings  22  in casing cover  21  through which the gas enters in later use. These can be sealed with an adhesive film during the soldering operation, for example, to prevent the penetration of flux. 
     FIG. 5 shows the finished optoelectronic gas sensor chip based on optodes. To simplify the drawing, all the metallized segments have been omitted. 
     Photosensitive elements  12  arranged in one plane are covered with a much thinner layer  15  of the optode material or reference segment material than was the case in the embodiment illustrated in FIGS. 1 and 2. Due to an annular gap  16 , a central opto-transmitter  23  is arranged a distance away from the sections of the optode material and projects far above the top surface of optode material  15 . The inside wall of casing cover  21  has a mirrorized surface  24 . In this way, light emitted by LED  23  is first emitted into air, i.e., to the gas atmosphere in chamber  25  above optode layers  15 , and then reaches photosensitive elements  12  through optode material  15  either directly or after being reflected on the inside wall or on mirrorized inside wall  24  of casing cover  21 . 
     To illustrate this, FIG. 5 shows a few light beams reflected on mirror layer  24 . It is noticeable here that the top side of LED  23  is not mirrorized in the embodiment according to FIG.  5 . The height of central LED  23  shown in FIG. 5 is about 300 μm, as in the embodiment illustrated in FIGS. 1 and 2, while the layer thickness of optode layers  15  and the reference layer in FIG. 5 is 5-10 μm, i.e., is much thinner than the optode layer of the embodiment shown in FIGS. 1 and 2, because the optodes and the reference layer do not function primarily as light guides. 
     The barriers are not shown in FIG. 5 for clarity. 
     The photodiode array shown in FIGS. 3 and 4 and used in FIG. 5 can be produced in a size of 2×2 mm 2  smaller with traditional thin-film processes with standard process dimensions. Additional designs such as a pentagon, a hexagon, a heptagon, an octagon or even a circular shape can also be implemented with the embodiment illustrated in FIGS. 1 and 2. 
     LED  23  can be glued in place with a conductive silver adhesive applied by screen printing. The barriers not shown in FIG. 5 can also be applied by a similar screen printing operation using a conductive silver adhesive at a height of approx. 50 μm. 
     At least the following advantageous properties may apply to the embodiment of an optoelectronic gas sensor based on optodes illustrated in FIG.  5 : the sensor has extremely small geometric dimensions due to the integration of all of its function units on one Si chip; due to the small distance between LED  23  and photodiodes  12 , the optode made of the gas-sensitive polymer makes a passive optical system unnecessary (apart from possible mirrorizing of the inside wall of casing cover  21 ). 
     This guarantees that the gas flowing through windows  22  into interior space  25  can act on a large optode surface. Here again, a high efficacy in coupling light between the components is achieved due to the small distance between LED  23  and photodiodes  12 , which causes a low power consumption.