Patent Publication Number: US-2006016995-A1

Title: Microstructured infrared sensor and method for its manufacture

Description:
FIELD OF THE INVENTION  
      The present invention relates to a microstructured infrared sensor and a method for its manufacture.  
     BACKGROUND INFORMATION  
      Microstructured infrared sensors may be used, e.g., in gas detectors, in which IR (infrared) radiation emitted by a radiation source, an incandescent bulb operated in the low-current range, or an IR LED, for example, is transmitted over a measuring path and subsequently received by the infrared sensor, and the concentration of the gases to be detected in the measuring path is estimated from the absorption of the infrared radiation in specific wavelength ranges. Gas sensors of this type may be used, e.g., in automobiles, for example, for detecting a leak in an air conditioning unit operated using CO 2 , or for checking the air quality of the ambient air.  
      In general, microstructured infrared sensors have a sensor chip as a substrate in which a diaphragm, underetched by a cavity, is formed. At least one thermopile structure, having two bonded printed conductors made of different conductive materials, e.g., polycrystalline silicon and a metal, and an absorber layer for absorbing the incident IR radiation is deposited on the diaphragm. The incident IR radiation is absorbed by the absorber layer, whereupon the latter is warmed according to the intensity of the absorbed radiation. The thermal voltage across the bonded printed conductors resulting from the temperature increase is read as a measuring signal. In general, a cap chip is attached in a vacuum-tight manner to the sensor chip, whereby a sensor space shielded from the exterior is formed for the thermopile structure. The sensor may be placed into a package provided with a cover having a screen for the passage of the IR radiation. The IR radiation to be detected thus strikes the absorber layer essentially vertically after passing through the screen of the cover and the silicon cap chip which is transparent to IR radiation. The screen has approximately the same diameter as the absorber layer beneath it.  
      To achieve sufficient sensitivity for detecting the gas concentration, a relatively large thermopile detector having a large number of thermopiles, i.e., printed conductors,,is generally formed. These may be run from the diaphragm to the surrounding substrate material in a cruciform shape.  
      Due to the large surface area needed and the complex design of the large thermopile structures, high costs are incurred in manufacturing the infrared sensor and the sensor module made up of the sensor, the package, and the cover.  
      An object of the present invention is to provide a method for manufacturing an infrared sensor such that a high sensitivity level is achieved for the sensor at a relatively low manufacturing cost.  
     SUMMARY  
      In accordance with the present invention, the incident IR radiation is focused onto the absorber layer through a convergent, i.e., convex, lens. The convergent lens is formed on top of the sensor, i.e., on top of the cap chip or a lens chip additionally attached to the cap chip, so that no additional optical aids need to be mounted and adjusted.  
      Due to the increased sensitivity, the number of thermopiles, i.e., printed conductors, may be reduced. According to the present invention, the lateral dimensions of the diaphragm and of the absorber layer may also be reduced.  
      The present invention utilizes the fact that when the radiation is focused onto the absorber layer by a convergent lens, a measuring signal which is proportional to the radiation may be obtained. According to the present invention, the surface of the screen may be selected to be several times larger than the screens normally used. The convergent lens is formed by the convex lens area on top of the cap chip or of the additional lens chip and the bottom of the cap chip, which may be flat, i.e., as a convex-planar convergent lens in particular. Optical focusing may be achieved here due to the difference between the refractive indices of the air inside the package and of the semiconductor material of the cap chip or of the additional lens chip, and the difference between the refractive indices of the semiconductor material and of the vacuum of the sensor space.  
      According to the present invention, the number of thermopiles may be reduced to the point that they run only to one side of the diaphragm.  
      According to an example embodiment of the present invention, the convex lens area on the sensor surface may be formed as a dried lacquer layer. In this case, a liquid spherical cap of an optically transparent lacquer is formed on the surface; this lacquer forms a convex shape having the desired radiation-focusing effect due to the surface tension of the liquid and the wetting of the surface. A solid spherical cap may thus be formed as a convex lens area by subsequent drying.  
      The drop of lacquer may be formed by first applying a lacquer layer having a larger surface area and structuring a cylindrical area, which is then liquefied by inspissating a solvent.  
      Alternatively, a liquid lacquer droplet may be directly dispensed for this purpose, e.g., via a piston dispenser having a precision needle. Time and material are saved here compared to forming and structuring the lacquer layer and inspissating solvents. The advantages of using a piston dispenser are, e.g., that changes in pressure and viscosity have no effect on the dispensed volume. Furthermore, very small volumes may be metered, volumetric reproducibility is high (e.g., ±2%), low-viscosity materials do not reflow, and the material is not modified by shearing.  
      Compared to photolithography or special lithography, spin-on deposition and a prebake step of the first layer, spin-on deposition and prebake step of the second layer, edge lacquer removal, exposure, subsequent developing, and the required lacquer height control are no longer needed in the case of direct dispensing. The 10-minute dispensing step, for example, is also considerably shorter than the 45-minute swelling process required in special lithography, and the 2-hour drying, for example, according to the present invention is somewhat shorter than the 3-hour drying, for example, required for special lithography. The time for the overall process may thus be reduced by 60%, for example, and handling time by workers may be reduced by as much as over 80%.  
      Furthermore, smaller amounts of material are used in direct dispensing, because no excess material remains at the end of the process, in contrast to a process in which layers are applied and subsequently structured. Also, no developer, no solvent for swelling, and no photoresist are required, so that a considerable additional savings in materials may also be achieved.  
      Furthermore, in another example embodiment of the present invention, the convex lens area may also be formed in the substrate itself, i.e., in the cap chip or the additional lens chip. In this case, as in the above embodiments, a spherical cap of dried lacquer is first formed, and the spherical lacquer cap and the surrounding substrate material are then etched, e.g., dry etched. The shape of the lens formed in the substrate corresponds to the shape of the original spherical lacquer cap if the etching selectivity of the substrate material and the lacquer is selected to be 1:1; by varying the etching selectivity during the etching process, a non-spherical shape may also be achieved in the substrate, so that in principle complex geometries may also be formed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a cross-sectional view of an infrared sensor according to an example embodiment of the present invention.  
       FIG. 2  shows a top view of a sensor chip in the diaphragm area.  
       FIGS. 3   a  through  3   c  show the various steps of an example method for the manufacture of the cap chip of the sensor shown in  FIG. 1 .  
       FIGS. 4   a  through  4   d  show the various steps of another example method for the manufacture of a lens on the cap chip.  
       FIG. 5  shows a piston dispenser for carrying out the method shown in  FIG. 4 .  
       FIG. 6  shows a cross-sectional view of an infrared sensor according to another example embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      As shown in  FIG. 1 , infrared sensor module  1  has a package  2  made of a molded compound or ceramic, for example, and a cover  3  attached to package  2  having a screen aperture  4 . An infrared sensor  6  is placed in package inner space  5  formed between package  2  and cover  3 . The infrared sensor  6  has a sensor chip  9  glued onto the bottom of package  2  and a cap chip  11  attached to sensor chip  9  by seal glass bond  10 . Situated on the sensor chip  9 , above a cavity  14  of the sensor chip  9 , is a diaphragm  12 . Diaphragm  12  and cavity  14  may be formed, for example, by forming or depositing an SiO 2  or Si 3 N 4  layer on the substrate of sensor chip  9 , structuring etched openings, etching cavity  14  underneath the layer, and subsequently sealing the etched openings.  
      Alternative to the embodiment shown in  FIG. 1 , a cavity  14  may be formed from the bottom of sensor chip  9  via KOH etching, for example, and the etching process may be stopped when a sufficiently thin diaphragm  12  has formed on the top or front of substrate  9 . In this alternative embodiment, unlike that of  FIG. 1 , cavity  14  extends to the bottom of sensor chip  9 .  
      Continuing with  FIG. 1 , at least one thermopile structure  17  having printed conductors  19  and  20 , in contact with one another and made of different electrically conductive materials, e.g., polycrystalline silicon and aluminum or another metal, is deposited on diaphragm  12 . The at least one thermopile structure  17  is formed such that the “warm contact area” of printed conductors  19  and  20  is located on diaphragm  12  and the “cold contact area” is located outside of diaphragm  12  on silicon substrate  9 . An infrared absorber layer  21  is applied to the contact area of printed conductors  19 ,  20  on diaphragm  12  and is heated by the incident IR radiation, the temperature increase generating a thermal voltage across printed conductors  19 ,  20  which is measurable as an electrical signal.  
      A sensor space  23 , in which a vacuum is insulated from the package inner space  5  by a seal glass bond areas  10 , is formed between cap chip  11  and sensor chip  9 . For this purpose, a cavity may be formed on the bottom of cap chip  11  via KOH etching, for example, this cavity forming sensor space  23  after cap chip  11  has been attached to sensor chip  9  in seal glass bond areas  10 . An advantageously spherical convex lens area  24 , e.g., made of silicon, is formed on top  22  of cap chip  11  in an area above thermopile structure  17 . Convex silicon lens area  24  is formed in this embodiment in a depression  27  on top  22  and adjoins package inner space  5  which is filled with air, a protective gas, or vacuum, for example. Below the convex lens area  24 , a flat boundary surface  25  adjoins sensor space  23  which is under vacuum. Thus, the combination of the convex lens area  24  and the flat boundary surface  25  acts as a convex-planar convergent lens  26 , which focuses incident IR radiation from the outside through screen aperture  4  into package inner space  5  onto absorber layer  21 . The focal point of the IR radiation is advantageously located in absorber layer  21  as a wide spot.  
      As an alternative example embodiment to the embodiment having a convex-planar convergent lens  24 , a biconvex convergent lens or a convergent lens as a structure made up of a plurality of adjoining convex areas may also be formed. Furthermore, instead of the convergent lens, a prism-type structure having a tip pointing upward and obliquely descending planar surfaces may be formed as a beam-focusing device. In this case, it is relevant that the incident IR radiation is focused by the beam-focusing device onto absorber layer  21 . The focal point or spot is advantageously located in absorber layer  12 .  
      The surface area of screen aperture  4  is significantly larger than the surface area of absorber layer  21 , e.g., 2 to 10 times larger, in the example embodiment shown in  FIG. 1 . Several times more IR radiation strikes convergent lens  26  in this way than without the use of such a beam-focusing device, the IR radiation being focused onto absorber layer  21 . The heat introduced into absorber layer  21 , which is increased proportionally to the incident light, results in a proportional increase in sensitivity, while the number of thermopile structures  17  remains the same.  
      If the same sensitivity of IR sensor  6  compared to an IR sensor designed without the use of a convergent lens  26  is desired, the number of thermopile structures  17  may be proportionally reduced, which reduces the dimensions of thermopile structures  17  and of sensor chip  9  accordingly.  
       FIG. 2  shows a top view of diaphragm  12  having a plurality of thermopile structures  17 , each having bonded printed conductors  19 ,  20 . According to the present invention, they may be conducted away in a single direction, in  FIG. 2  downward, instead of to all sides as in the currently customary cruciform embodiments.  
      In all embodiments shown, IR sensor  6  may be formed on the wafer level. For this purpose, a plurality of diaphragms  12 , cavities  14 , and thermopile structures  17  are formed in a sensor wafer, a plurality of convex lens areas  24  are formed on the top of a cap wafer, and cavities for sensor spaces  23  are formed on the bottom. Furthermore, seal glass, i.e., a low-melting lead glass, is applied to the sensor wafer around thermopile structures  17 , and the cap wafer is placed in a bonding position onto the sensor wafer. By heating or baking the resulting wafer stack and subsequent singulation, individual IR sensors  6  may then be manufactured in a cost-effective manner.  
       FIGS. 3   a  through  3   c  show the various steps of such a manufacturing process according to the present invention on the wafer level, i.e., prior to singulation. For this purpose, a minimally sensitive lacquer layer  29  is applied to the cap substrate, i.e., cap wafer  27 , and structured photolithographically to form a cylinder  30 , as shown in  FIG. 3   a . Subsequently, the lacquer of cylinder  30  is liquefied at a suitable temperature of 60° C. to 80° C., e.g., 75° C., while adding solvent vapor, e.g., acetone vapor, for 25 minutes. The liquefied lacquer forms, as shown in  FIG. 3   b , a liquid spherical cap  34  due to its wetting properties and the effect of gravity and surface tensions. The liquid spherical cap  34  is then rehardened, as shown in  FIG. 3   c , at a high temperature of 100° C. to 120° C., for example, to form a solid spherical cap  24 . It is also possible to melt cylinder  30  by increasing the temperature to 150° C. to 160° C. without adding solvent vapor, and to then let the melted area harden. However, as a result of treatment with solvent vapor and subsequent hardening, changes in the lacquer during melting, in which solvent diffuses out and thus the lacquer changes its chemical consistency, are avoided. In particular, possible deviations from the desired target structure and resulting imaging errors due to the evaporation of the solvent, which may affect functioning of the optical system, are prevented or at least largely prevented.  
      In a dry etching system, the dried, solid spherical lacquer caps  34  and the surrounding silicon of cap wafer  27  are etched in such a way that the shape of the lacquer is transferred to the silicon of cap wafer  27  and convex lens area  24  is formed in cap wafer  27  as shown in  FIG. 3   c . If the silicon to lacquer etching selectivity is selected to be 1:1, the shape of convex lens area  24  in cap wafer  27  corresponds to the shape of the original spherical lacquer cap  34  as shown in  FIG. 3   b . However, by varying the etching selectivity during the etching process, a non-spherical shape may also be produced in the silicon of cap wafer  27 .  
      Alternative to the process shown in  FIGS. 3   a  through  3   c , spherical caps  34  of liquid lacquer may also be applied directly to cap wafer  27 , as shown in  FIGS. 4   a  through  4   d . In this case, small droplets  42  of a lacquer liquid  45  or a liquid lacquer from a precision needle  43  are applied to cap wafer  27  using a piston dispenser  40 , an example of which is shown in  FIG. 5 , and the droplets  42  subsequently form convex spherical caps  34  due to their surface tension. The relatively extensive, more time-consuming and more material-intensive photolithographic process of  FIGS. 3   a  through  3   c  is replaced by this dispensing, i.e., metering procedure. The. above-mentioned changes in the lacquer during a melting process, e.g., possible deviations from the desired target structure and the resulting imaging errors, are largely or completely avoided in the method illustrated in  FIGS. 4   a  through  4   d.    
       FIGS. 4   a  through  4   d  schematically show a bottom portion of piston dispenser  40  in various steps of forming the spherical cap  34 . As shown in  FIG. 4   a , cylinder  46  of the dispenser filled with lacquer liquid  45  is displaced toward cap wafer  27  until precision needle  43  is sufficiently close above the wafer. Subsequently a droplet  42  of lacquer liquid  45  is deposited on cap wafer  27  by a descending piston  49 , as shown in  FIG. 4   b . The surface of cap wafer  27  may be wetted as soon as droplet  42  is formed on precision needle  43 , as shown in  FIG. 4   c , so that even very small droplets may be formed. As shown in  FIG. 4   d , cylinder  46  is removed again vertically, so that initially liquid spherical cap  34  of liquid lacquer remains on cap wafer  27  and then hardens in this shape.  
      As shown in  FIG. 5 , piston dispenser  40  may have the following components. A cartridge, for example, may be used as container  50  for lacquer liquid  45 , lacquer liquid  45  being conducted under a low pressure of 0.3 bar to 0.8 bar, for example, through a channel  52  to a pump chamber  53 . When piston  49  moves upward, it produces a partial vacuum, causing lacquer liquid  45  to flow into the pump chamber  53 . When the piston moves downward, the material supply is interrupted and piston  49  presses the desired amount of lacquer liquid  45  through the precision needle  43 .  
       FIG. 6  shows another example embodiment of the sensor according to the present invention, having a package  2  and a cover  3  which are substantially identical to the first example embodiment of  FIG. 1 . Positioned within package  2  is IR sensor  106  having a sensor chip  9  with membrane  12 . However, in the embodiment shown in  FIG. 6 , cap chip  111  has a flat top on which a silicon lens chip  114  is attached over an adhesive layer  112  made of an optically transparent adhesive. Lens chip  114  has convex lens area  24  on its top. Convex lens area  24  may be formed using any of the above-described processes, e.g., the example method shown in  FIGS. 3   a  through  3   c , or the example method shown in  FIGS. 4   a  through  4   d.    
      As an alternative example embodiment, sensor  106  may also be manufactured on the wafer level by manufacturing a sensor wafer, a cap wafer, and a lens wafer separately. In this embodiment, the cap wafer is to be structured only from one side to form sensor space  23 , and the lens wafer is designed as cap wafer  27  shown in the first embodiment of  FIG. 1 . A wafer stack, in which the cap wafer is attached to the sensor wafer in seal glass bonding areas and the lens wafer is attached to the cap wafer by an adhesive layer, is subsequently produced from these three wafers. Alternative to the embodiment shown in  FIG. 6 , lens chip  114  may extend laterally to the width of cap chip  111  and sensor chip  6 , so that the manufacture as a wafer stack and the subsequent singulation are easily facilitated.