SMD-enabled infrared thermopile sensor

Abstract

An SMD-enabled infrared thermopile sensor has at least one miniaturized thermopile pixel on a monolithically integrated sensor chip accommodated in a hermetically sealed housing which consists of an at least partially non-metallic housing substrate and a housing cover. A gas or a gas mixture is contained in the housing. The sensor has a particularly low overall height, in particular in the z direction. This is achieved by virtue of an aperture opening being introduced in the housing cover opposite the thermopile pixel(s), which aperture opening is closed with a focusing lens which focuses the radiation from objects onto the thermopile pixel(s) on the housing substrate, and by virtue of a signal processing unit being integrated on the same sensor chip next to the thermopile pixels, wherein the total housing height and the housing cover are at most 3 mm or less than 2.5 mm.

Claims

1. An SMD-enabled thermopile infrared sensor for contactless temperature measurement, as a hotspot, or for gesture detection, having at least one thermopile pixel on a monolithic integrated sensor chip, which is arranged in a hermetically sealed housing, the housing comprising an at least partially nonmetallic housing substrate and a housing cover, wherein a gas or gas mixture is located in the housing, wherein an aperture opening (26) is introduced into the housing cover (3) opposite to the at least one thermopile pixel (29), which opening is closed using a focusing lens (4), which focuses radiation of objects onto the at least one thermopile pixel (29) on the housing substrate (1), wherein the housing substrate (1) is an insulator, and is provided with a recess (30) and side walls (31) for accommodating the sensor chip (2), wherein the housing cover (3) has angled lateral edges (32), which are supported and hermetically sealed on the side walls (31) of the housing substrate (1), wherein the housing substrate (1) has metallic connections (9) extending from a front side of the housing substrate to a rear side of the housing substrate, the metallic connections being provided on the rear side of the housing substrate (1) with contact surfaces (10) for a surface mounting, wherein a signal processing unit (12) is integrated on the sensor chip (2) adjacent to the at least one thermopile pixel (29), and wherein an overall height of the housing is at most 3 mm.

2. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the focusing lens (4) images object radiation on the at least one thermopile pixel (29) from a viewing angle of at most 40°.

3. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the housing cover (3) consists of a metal or nonmetal having high thermal conductivity and is a deep-drawn, injection-molded, or diecast part.

4. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the housing cover (3) and the housing substrate (1) are bonded to one another by an adhesive, are soldered to one another, or are welded to one another, and wherein an insulation layer (19) is located above the metallic connections (9) and wherein a metal layer (18) is located thereon.

5. The SMD-enabled thermopile infrared sensor as in claim 1, wherein a reference pixel (15), which receives infrared radiation from the housing cover (3) located above it, is arranged in the sensor chip (2) adjacent to the at least one thermopile pixel (29) arranged below the focusing lens (4), and wherein the housing cover (3) has an absorbing layer (18) or another type of cover on an inner side in a region above the reference pixel (15).

6. The SMD-enabled thermopile infrared sensor as claimed in claim 5, wherein a distance of the housing cover (3) in a region of the focusing lens (4) from the at least one thermopile pixel (29) is significantly greater than a distance in a region of the absorbing layer (18) from the reference pixel (15).

7. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the housing cover (3) has, in a region of the focusing lens (4) on an inner side, a bulge (33) formed according to an outer shape of the focusing lens (4) or an indentation (34) adjoining an elevation (28) of the housing cover (3) for fixing the focusing lens (4) in its position above the at least one thermopile pixel (29).

8. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the least one thermopile pixel comprises multiple sensor pixels (17) that are arranged in matrix form under the focusing lens (4).

9. The SMD-enabled thermopile infrared sensor as in claim 8, wherein a size of the at least one thermopile pixel (29) is at most 500 μm.

10. The SMD-enabled thermopile infrared sensor as in claim 8, wherein the signal processing unit (12) comprises a storage of calibration data for the sensor pixels (17) is configured to compute an object temperature, and is configured to store measured values.

11. The SMD-enabled thermopile infrared sensor as in claim 1, wherein a length or a width of the housing substrate (1) is at most 5 mm.

12. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the focusing lens (4) is diffractive or refractive and has either aspheric or spheric surfaces.

13. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the focusing lens has a focal length between 0.5 mm and 1.9 mm.

14. The SMD-enabled thermopile infrared sensor as in claim 1, wherein a thickness of a bottom surface of the housing substrate (1) and a thickness of the housing cover (3) are at most 0.5 mm.

15. The SMD-enabled thermopile infrared sensor as in claim 1, wherein the signal processing unit (12) comprises measuring units for temperature and voltage references and also an operating voltage and furthermore contains signal amplifiers, low-pass filters, analog-to-digital converters, a microprocessor unit having data memory, and an interface for communication with external electronics of a mobile device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: shows the basic structure of a single-element thermopile infrared sensor according to the invention as a sectional illustration in an SMD housing having single-chip, installed lens optical unit, and monolithic integrated signal processing;

(2) FIG. 1a: shows a detail A from FIG. 1 with highlighting of several details;

(3) FIG. 1b: shows a modification of the miniaturized SMD sensor with particularly low structural height;

(4) FIG. 2: shows the top view of a single-element thermopile infrared sensor according to the invention in an SMD housing;

(5) FIG. 3: shows a design of the SMD sensor according to the invention having an additional reference pixel for compensation purposes;

(6) FIG. 3a: shows a design of the miniaturized SMD sensor having a reference pixel having different cover height above the signal pixel and the reference pixel;

(7) FIG. 3b: shows a further design of the miniaturized SMD sensor having a lens mounted from the outside on the housing cover;

(8) FIG. 4: shows the top view of the thermopile infrared sensor in the SMD housing having reference pixel; and

(9) FIG. 5: shows a further embodiment according to the invention of the SMD thermopile infrared sensor having multiple sensor pixels and monolithic integrated signal processing.

DETAILED DESCRIPTION

(10) FIGS. 1 and 1a show the schematic structure of an SMD-enabled miniaturized thermopile infrared sensor according to the invention, in which a monolithic sensor chip 2 is arranged in a sensor housing, which consists of a bottom plate/housing substrate 1 provided with a recess 30 and side walls 31, and a housing cover 3. The housing substrate 1 consists of an insulator, such as ceramic. The sensor chip 2 is provided with a membrane 6 (sensor pixel) and an absorber layer for infrared radiation located thereon, under which a cavity 5 having vertical walls is located, i.e., the sensor chip encloses the cavity like a frame, which is also used as a heat sink for the thermopile, as will be described. The membrane 6 is suspended with the aid of beams 23 above the cavity 5, wherein the beams 23 connect the membrane 6 to the sensor chip 2. Slots are located between the beams 23 and the membrane 6 and/or the sensor chip 2 for thermal insulation. The cavity 5 simultaneously delimits the area of the pixel 29 (FIG. 1b). In addition to the membrane 6, a signal processing unit 12 is located on the sensor chip 2.

(11) On each of the sides of the bottom plate 1, metallic connections 9 from the front side to the rear side of the bottom plate 1 (FIG. 1) are located laterally to the sensor chip 2. The metallic connection 9 ends on the inner side of the bottom substrate 1 in a contact surface 8 on the inner side of the side wall 31 in the form of a metallization as a connecting surface for a bond wire 7 for the electrical connection of a contact pad 25 on the sensor chip 2 and the contact surface 8 on the bottom substrate 1 (FIG. 1a). The metallic connections 9 each end in a contact surface 10 for the surface mounting, for example, on a circuit board or a substrate, on the rear side of the housing substrate 1, i.e., on the bottom in the drawing. In FIG. 1, a metallic connection 9 having a bond wire 7 is illustrated on each of the right and the left sides as representative of further adjacent metallic connections 9, which extend into the depth of the image.

(12) The housing substrate 1 is covered by a housing cover 3, the lateral edge 32 of which, which is angled outward, is supported hermetically sealed on the side wall 31 of the housing substrate 1 (FIG. 1). A gas or gas medium 11 having special properties which are still to be described is located in the interior, i.e., between the housing cover 3 and the housing substrate 1.

(13) Furthermore, an aperture opening 26, in front of which a focusing lens 4 is arranged on the inner side of the housing cover 3 (FIG. 1a), is located in the housing cover 3. Alternatively, the lens 4 can also be mounted from the outside on the housing cover 3, as shown in FIG. 3b. It is essential to the function that the center axis of the aperture opening 26 and/or the focusing lens 3 corresponds to the axis of symmetry of the membrane 6. The lens 4 focuses the infrared radiation of a measured object located in front of the aperture opening 26 from a viewing angle (“field of view”) FOV 24 on the sensor pixel having the absorber surface on the membrane 6 and/or the thin carrier membrane 22 of the pixel.

(14) The housing cover 3 is a metallic deep-drawn part to ensure good thermal conductivity, which is adhesively bonded, soldered, or welded onto a contact point 18 of the housing substrate 1. Alternatively, the housing cover 3 can also be a metallic or nonmetallic injection-molded or diecast part, which preferably has good thermal conductivity and has prepared coatings for a hermetic adhesive, soldered, or welded closure in relation to the lateral edge 31 of the housing substrate 1 at the contact points to the housing substrate 1.

(15) In the case of soldering or welding, the metallic contact point 18, which can be welded or soldered, has to be applied to the housing substrate 1 above an insulation layer 19 on the metallic connection 9 (FIG. 1a) to prevent a short circuit between the metal conductor tracks 9 on the side wall 31 of the housing substrate 1. The required hermetic housing closure is ensured with the aid of a peripheral soldered or adhesive joint 20.

(16) The housing substrate 1, which comprises a central planar bottom plate having side walls 31 enclosing it, predominantly consists of a nonmetallic material (for example, ceramic), onto which metallic conductor tracks 9 are applied. These conductor tracks 9 end on the upper side with inner contact pads 8 and on the lower side with outer contact surfaces 10 for surface mounting. The contact surfaces 10 contain a coating, which can be soldered or welded, for the mechanical and electrical mounting on circuit boards or other circuitry carriers (not shown) located underneath.

(17) The housing substrate 1 can be provided (FIG. 1a) on the upper side below the sensor pixel, i.e., in the recess 30, with a reflective metal layer 21 (for example, gold, silver, aluminum), to enhance the signal sensitivity of the sensor pixel by reflection of transmitted radiation components.

(18) The absorber surface on the membrane 6 covers the so-called “hot” contacts of the thermopile sensor, which are arranged on the thin membrane 22, which consists, for example, of silicon oxide or silicon nitride, or of other insulating materials. The membrane having the absorber layer 6 is suspended above the recess 5 in the monolithic sensor chip 2 (silicon substrate). In the solution according to FIG. 1, the recess 5 has vertical or nearly vertical walls and is driven into the sensor chip 2 from the wafer rear side, for example, by reactive ion etching (a so-called RIE process). Alternatively, the cavity below the sensor pixel can also (as shown in FIG. 3) be driven from the front side into the sensor chip 2 (silicon wafer). In this case, instead of the reactive ion etching, a wet-chemical etching method known per se is used, in which a sacrificial layer (for example, porous silicon) or parts of the silicon substrate are dissolved and etched through slots known per se between the membrane 6 and the beams 23, and/or the beams 23 and the side wall of the recess 5. The inclined walls 13, 14 of the recesses 5 in FIG. 3 are an example of this.

(19) The thermocouples of the thermopile structure are produced from thermoelectric materials known per se of differing thermoelectric polarity. These can be semiconductor materials applied in a CMOS process, for example, n-conductive and p-conductive polysilicon, (doped) amorphous silicon, germanium, or a mixed form of silicon and germanium, or also applied thin thermoelectric metal layers (for example, bismuth, antimony, inter alia), and have a thickness of less than 1 μm.

(20) While the “hot” contacts of the thermocouples are located on the thin membrane 22 below the absorber layer, the “cold” contacts are arranged on the edge of the silicon substrate 2, so that the greatest possible temperature difference is achieved between both contacts. The small pixels particularly preferably have long beams 23 known per se, which are insulated from the silicon substrate of the sensor chip 2 and from the absorber region by slots etched from the front side into the membrane. Since such high-sensitivity thermopile pixels are sufficiently known from the prior art, further details (such as the embodiment of the beams and the insulating slots) are not shown in the figures of the drawings.

(21) The signal processing unit 12 integrated in the sensor chip 2 contains, for example, temperature and voltage references, signal amplifiers, possibly low-pass filters, analog-to-digital converters, a microprocessor unit having data memory (for example, EEPROM), which are known per se, and an interface for communication with the external electronics of the mobile device (for example, smart phone). The temperature reference circuit integrated on the sensor chip 2 (for example, a so-called PTAT-(Proportional To Ambient Temperature)) measures the temperature of the sensor chip and thus of the “cold” contacts.

(22) The preamplifiers contained in the signal processing unit 12 amplify the very low signal voltages of the thermopile sensor pixel or pixels and compare them to the voltage and ambient temperature references. All of these analog signals are converted in the analog-to-digital converter into digital values, which are in turn used in the microprocessor unit together with the correction values stored in the data memory (for example, EEPROM) for the object temperature computation.

(23) The lens optical unit/focusing lens 4 mounted in the housing cover 3 has focusing lens surfaces on the lower side, the upper side, or on both sides. The mounting typically takes place in such a way that the distance to the sensor pixel corresponds precisely to the focal length (and/or the so-called “back focal length”) of the lens 4. The lens is produced in a mass production method known per se (for example, pressing, molding) or particularly preferably in a wafer-level method, for example, on silicon wafers, in which many thousand lenses are produced simultaneously on a wafer and are provided with infrared filter layers. Such wafer lenses have the additional advantage that they may be “picked off” from the silicon wafer and mounted using the same automated handling system (so-called pick & place or die bonder) as normal semiconductor chips during the mounting of the lenses in the housing cover 3. In this case, both refractive lenses (having typical spheric or aspheric curvature of the active surface) and also diffractive lenses (for example, Fresnel lenses) can be used.

(24) The mounting of the lens 4 in the housing cover 3 can be performed, for example, by adhesive bonding, soldering, or welding, wherein the edge of the lens receives a metallization layer (not shown) for the soldering or welding. The mounting of lens 4 and housing substrate 1 to the cover 3 is usually carried out by adhesive bonding if the gas or gas mixture to be enclosed is to have atmospheric normal pressure. If the pressure of the gas medium 11 in the housing is to be significantly lower than normal pressure, a soldering or welding method is preferably used for the mounting.

(25) The viewing angle of a sensor pixel is ideally to be <40°, preferably <15° for the application. A formula which sets the pixel size in a ratio to the focal length may be used most simply to estimate the viewing angle FOV 24. The ARCTAN of the ratio of half the pixel size to focal length thus supplies half the viewing angle (FOV/2) as a result. It results therefrom, for example, that at a focal length of the lens of 1 mm and a pixel size of 0.5 mm, a viewing angle of 30° results. A pixel size of 0.26 mm supplies a viewing angle of approximately 12°. At greater focal length of the lens, the viewing angle is reduced, but at a lens focal length of 1.5 mm and the typical thicknesses of lens (for example, 0.26 mm), sensor chip (0.4 mm), cover and bottom plate (each 0.2 mm), housing heights of 2.6-3 mm already result depending on the construction of the lens—too much for most smart phones.

(26) FIG. 1b shows a modification of the miniaturized SMD sensor having particularly low structural height. In this case, the housing cover 3 receives a bulge 33 upward, which approximately corresponds to the lens curvature. The curved (convex) side of the focusing lens 4 faces upward, so that the vertex of the lens 4 approximately corresponds to the maximum height of the cover 3 and thus defines the total housing height. In this manner, the total height of the housing can be reduced by approximately the thickness of the cover in comparison to the arrangement according to FIG. 1a.

(27) Alternatively, the housing cover 3 can also have an additional elevation 28, which merges into an indentation 34 to accommodate the lens 4 and encloses its outer circumference. The lens 4 is inserted with the curved side downward into the indentation 34 in this case (FIG. 3b). The aperture opening 26 forms the inner edge of the indentation 34 here, which also supports the focusing lens 4.

(28) The medium 11 enclosed in the housing is a gas or gas mixture, the thermal conductivity of which is substantially lower than air or nitrogen at normal pressure. The gas medium 11 is preferably a gas having high molar mass (for example, xenon, krypton, or argon) or a gas having an internal pressure significantly reduced in relation to normal pressure. The housing has to be sealed in such a way that no gas exchange takes place with the surroundings.

(29) FIG. 2 shows a top view of a surface-mountable IR single element sensor according to the invention as shown in FIG. 1 with further details.

(30) The lens 4 is located here precisely symmetrically above the sensor pixel 6, wherein multiple terminal contacts 9 establish an electrical connection from the sensor chip 2 in the interior of the housing to the terminal contacts 10 for the SMD mounting on the housing lower side.

(31) FIG. 3 shows a further design of the SMD thermopile sensor according to the invention, which can be miniaturized, and in which, adjacent to the active sensor pixel 6, a further pixel is provided as a reference pixel 15, which is arranged at the remote end of the sensor chip 2. This reference pixel 15 does not receive infrared radiation from the measured object itself, but rather “looks” toward an emitting surface 16, which is applied or mounted on the inner side of the cover. The compensation of measurement errors due to temperature gradients in the sensor housing, induced, for example, by the so-called “heat shock” effect, is thus possible. This emitting layer 16 is preferably produced by printing or spraying and/or vapor deposition via changing masks. Alternatively, an additional part having high emission can be adhesively bonded on the cover inner surface or mounted in another manner having good thermal contact in relation to the housing cover 3.

(32) In the embodiment in FIG. 3, the thermopile pixels are produced in surface micromechanics, i.e., by etching free the cavities 5 below the membranes 6 and 15 from the front side through slots in the freestanding membranes. Both pixels 6, 15 can also, however, like the sensor pixels according to FIG. 1 or FIG. 3a, have a cavity having vertical walls 5, which have been driven from the rear side by DRIE into the sensor chip 2.

(33) FIG. 3a shows a further design according to the invention of the miniaturized SMD sensor having a reference pixel 15. In this arrangement, the housing cover for the lens has an additional elevation 28, to achieve the distance (so-called “back focal length”) necessary for sharp imaging between the lens 4 and the absorber 6 on the membrane, while the cover 3 having the absorbing layer 16 above the reference pixel is embodied particularly flat. The reference pixel having the absorber 15 may thus be moved closer to the sensor pixel having the absorber 6 in an effective manner, and nonetheless the reference pixel does not receive radiation of the object via the lens. The special elevation 28 can also be designed in a formfitting manner so that the lens 4 is mounted precisely symmetrically above the absorber 6 of the sensor pixel during the mounting to avoid optical imaging errors, such as “squinting”, if the optical axis of the lens is located precisely vertically above the absorber center point.

(34) FIG. 4 shows the top view of a design according to the invention of the miniaturized SMD sensor according to FIGS. 3 and 3a. It can be seen therefrom that the active sensor pixel 6 looks outward through the lens optical unit 4 of the aperture opening, while the reference pixel 15, in contrast, looks at the absorbing layer 16 located behind the cover and measures the housing temperature at the same time. In principle, depending on the space requirement in the terminal, square or approximately square housing shapes may also be achieved. It is possible more easily to construct the entire housing approximately square using the design in FIG. 3a, because the distance between the absorbers 6 and 16 may be significantly reduced.

(35) Using the additional reference pixel 15, for compensation purposes of slow and fast ambient temperature changes, both the chip temperature and thus the temperature of the “cold” thermopile contacts are available by means of integrated PTAT reference, and also the housing temperature of the cover 3 having the absorber layer 16. This enables an effective compensation of the short-term temperature changes occurring in mobile devices (for example, smart phones), for example, after the switching on, when “taking out of the trouser pocket”, when holding to the face or ear, or when changing from the temperature-controlled interior to the outside, where significantly higher or lower temperature changes can suddenly occur.

(36) The design of the active pixel as a multiple pixel represents an expansion of the invention which is of great interest for mobile devices. A corresponding solution is apparent from FIG. 5. In this case, instead of a single pixel, multiple pixels 17 (for example, 2×2, 4×4, 8×8, 16×16) are arranged below the lens, to capture a spatial distribution of the temperature distribution of the object or also the easy recognition of gestures. The cavities under the pixels can again be driven into the sensor chip 2 from the upper side or the lower side using the slotted membranes.

(37) The sensor chip 2 according to the invention as shown in FIG. 5 contains multiple individual cells 17 having slotted membrane and beam structure known per se, on which the thermocouples (not shown in detail) are housed.

(38) The signal processing of the individual pixels takes place in the signal processing unit 12, at the output (interface) of which items of digital signal information of the individual pixels are output together with the correction values for measuring the temperature distribution or, for example, a gesture controller. The lens optical unit is designed so that the remote measured objects are sharply imaged on the sensor pixels.

LIST OF REFERENCE NUMERALS

(39) 1 housing substrate (bottom plate) 2 monolithic sensor chip 3 housing cover 4 focusing lens 5 cavity under pixel 6 membrane having absorber layer 7 bond wire 8 contact surface (metallization) on housing substrate 9 metallic connection 10 contact surface on housing rear side 11 gas medium in hermetically sealed housing 12 signal processing unit 13 cavity under active pixel 14 cavity under reference pixel 15 membrane having absorber layer of the reference pixel 16 absorbing layer 17 sensor pixels in matrix form 18 metallized bonding surface, which can be soldered, on housing substrate 19 insulation layer 20 soldered or adhesive joint 21 reflective metal layer 22 thin carrier membrane of the pixel 23 beam 24 outer beam path 25 terminal contact 26 aperture opening 27 viewing angle of the reference pixel 28 elevation of the housing cover above sensor pixel 29 thermopile pixel 30 recess 31 side wall 32 lateral edge 33 bulge 34 indentation