INTEGRATED INFRARED SENSORS WITH OPTICAL ELEMENTS AND METHODS
An infrared (IR) radiation sensor device (27) includes an integrated circuit radiation sensor chip (1A) including first (7) and second (8) temperature-sensitive elements connected within a dielectric stack (3) of the chip, the first temperature-sensitive element (7) being more thermally insulated from a substrate (2) than the second temperature-sensitive element (8). Bonding pads (28A) on the chip (1) are coupled to the first and second temperature-sensitive elements. Bump conductors (28) are bonded to the bonding pads (28A), respectively, for physically and electrically connecting the radiation sensor chip (1) to corresponding mounting conductors (23A). A diffractive optical element (21,22,23,31,32 or 34) is integrated with a back surface (25) of the radiation sensor chip (1) to direct IR radiation toward the first temperature-sensitive element (7).
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This application is a continuation of U.S. patent application Ser. No. 12/645,267, entitled “INTEGRATED INFRARED SENSORS WITH OPTICAL ELEMENTS, AND METHODS,” filed on Dec. 22, 2009, which is related to U.S. patent application Ser. No. 12/380,316, entitled “INFRARED SENSOR STRUCTURE AND METHOD,” filed Feb. 26, 2009. Each of these applications is incorporated herein by reference for all purposes.
BACKGROUND OF THE INVENTIONThis invention is related to the assignee's pending application “INFRARED SENSOR STRUCTURE AND METHOD” by Walter B. Meinel and Kalin V. Lazarov, Ser. No. 12/380,316, filed Feb. 26, 2009, and incorporated herein by reference.
The present invention relates generally to various semiconductor-processing-compatible infrared (IR) sensor structures and fabrication methods, and more particularly to improved IR radiation sensing structures and processes which reduce size and cost of IR sensors and which provide smaller, more economical, more sensitive IR radiation intensity measurements. More particularly, the invention also relates to improved IR radiation sensing structures and processes which also avoid the costs and difficulties associated with discrete lenses as used in the closest prior art to collimate or shift the angle of incoming IR radiation, by integrating diffractive optical elements, such as Fresnel lenses or diffraction gratings, into an integrated circuit chip including an IR sensor.
The closest prior art is believed to include the article “Investigation Of Thermopile Using CMOS Compatible Process and Front-Side Si Bulk Etching” by Chen-Hsun-Du and Chengkuo Lee, Proceedings of SPIE Vol. 4176 (2000), pp. 168-178, incorporated herein by reference. Infrared thermopile sensor physics and measurement of IR radiation using thermopiles are described in detail in this reference. “Prior Art”
Referring to Prior Art
Prior Art
A second thermocouple group (not shown) essentially similar to the one shown in
Incoming IR radiation indicated by arrows 5 in Prior Art
Unfortunately, the floating membrane supporting one of the group of thermocouple junctions over the cavity as shown in
The IR radiation sensor in Prior Art
Above mentioned Prior Art
The above described prior art IR sensors require large, expensive packages. The foregoing prior art IR radiation sensors need to block visible light while transmitting IR radiation to the thermopiles in order to prevent false IR radiation intensity measurements due to ambient visible light. To accomplish this, the packages typically have a silicon (or other material transmitting infrared radiation but blocking visible light) window or a window with baffles. Furthermore, the “floating” portion of dielectric membrane 3 over cavity 4 in Prior Art
The prior art also is believed to include use of a discrete Fresnel lens in conjunction with an infrared sensor of the kind typically used in motion sensing alarms. Such devices include a relatively large plastic Fresnel lens, and the infrared sensor typically is a pyroelectric sensor which is substantially different than a thermopile sensor. The discreet Fresnel lens is used to collimate ambient IR radiation from a relatively large, distant source and focus it on to a relatively small IR sensor to improve its signal to noise ratio. Somewhat similarly to the Melexis prior art shown in
Alignment of a small thermopile sensor of the kind shown in the above mentioned Du and Lee article or included in the Melexis IR thermometer with a discrete collimating lens is difficult and expensive because of the relatively small size of the thermopile membrane. The thermopile might have a dimension of only approximately 200 microns across, so misalignment of the discrete collimating lens by more than approximately 50 microns may result in substantial errors in focusing the incoming IR radiation onto the thermopile. Furthermore, if the collimating lens is located a few centimeters from the thermopile, tilt of more than a few degrees of the plane of the collimating lens with respect to the plane of the thermopile will cause the focal point to be substantially misaligned with the thermopile. The degree of precision alignment typically needed is achievable but quite costly using conventional alignment methods.
It would be highly desirable to provide smaller, more economical, more sensitive, and more robust IR sensors than are known in the prior art for various applications such as non-contact measurement of temperature and remote measurement of gas concentrations.
Thus, there is an unmet need for an IR radiation sensor which is substantially smaller, more sensitive, and less expensive than the IR radiation sensors of the prior art.
There also is an unmet need for an IR radiation sensor which avoids the costly problems associated with the packaging of the prior art sensors and the alignment of discrete diffractive optical elements with thermopiles, thermocouple groups, or even individual thermocouples of the sensors.
There also is an unmet need for a low cost IR radiation sensor having a response which is sensitive to the wavelength of incoming IR radiation.
There also is an unmet need for a low-cost IR radiation sensor which is substantially smaller and more sensitive than the IR radiation sensors of the prior art, wherein the IR-radiation-sensitive elements are composed of thermocouples.
There also is an unmet need for a low-cost IR radiation sensor which is substantially smaller and more sensitive than the IR radiation sensors of the prior art, wherein the IR-radiation-sensitive elements are composed of temperature-sensitive resistive elements.
There also is an unmet need for a more accurate IR radiation sensor than has been found in the prior art.
There also is an unmet need for an IR radiation sensor which provides more sensitive, more accurate measurement of IR radiation than the IR radiation sensors of the prior art.
There also is an unmet need for a CMOS-processing-compatible IR radiation sensor chip which does not need to be packaged in a relatively large, expensive package having a discrete window.
There also is an unmet need for a CMOS-processing-compatible IR radiation sensor chip which is substantially more robust than those of the prior art.
There also is an unmet need for an improved method of fabricating an infrared radiation sensor.
There also is an unmet need for an improved method of fabricating a CMOS-processing-compatible IR sensor device which does not require bonding the CMOS-processing-compatible IR sensor chip in a relatively large, expensive package having an infrared window therein.
SUMMARY OF THE INVENTIONIt is an object of the invention to provide an IR radiation sensor which is substantially smaller and less expensive than the IR radiation sensors of the prior art.
It is another object of the invention to provide a more accurate IR radiation sensor than has been found in the prior art.
It is another object of the invention to provide a low cost IR radiation sensor having a response which is sensitive to the wavelength of incoming IR radiation.
It is another object of the invention to provide a low-cost IR radiation sensor which is substantially smaller and more sensitive than the IR radiation sensors of the prior art, wherein the IR-radiation-sensitive elements are composed of thermocouples.
It is another object of the invention to provide a low-cost IR radiation sensor which is substantially smaller and more sensitive than the IR radiation sensors of the prior art, wherein the IR-radiation-sensitive elements are composed of temperature-sensitive resistive elements.
It is another object of the invention to provide an IR radiation sensor which avoids the costly problems associated with the packaging of the prior art sensors and the alignment of discrete diffractive optical elements with thermopiles, thermocouple groups, or even individual thermocouples of the sensors.
It is another object of the invention to provide an IR radiation sensor which provides more sensitive, more accurate measurement of IR radiation than the IR radiation sensors of the prior art.
It is another object of the invention to provide an IR radiation sensor chip which does not need to be packaged in a relatively large, expensive package having an infrared window.
It is another object of the invention to provide an IR radiation sensor chip which is substantially more robust than those of the prior art.
It is another object of the invention to provide an improved method of fabricating an infrared radiation sensor.
It is another object of the invention to provide an improved method of fabricating a CMOS-processing-compatible IR sensor device which does not require packaging the CMOS-processing-compatible IR sensor chip in a relatively large, expensive package having a window therein.
It is another object of the invention to provide an improved method of fabricating an IR sensor device which is more robust than those of the prior art.
Briefly described, and in accordance with one embodiment, the present invention provides an infrared (IR) radiation sensor device (27) which includes an integrated circuit radiation sensor chip (1A) including first (7) and second (8) temperature-sensitive elements connected within a dielectric stack (3) of the chip, the first temperature-sensitive element (7) being more thermally insulated from a substrate (2) than the second temperature-sensitive element (8). Bonding pads (28A) on the chip (1) are coupled indirectly or directly to one or both the first and second temperature-sensitive elements. Bump conductors (28) are bonded to the bonding pads (28A), respectively, for physically and electrically connecting the radiation sensor chip (1) to corresponding mounting conductors (23A). A diffractive optical element (21,22,23,31,32 or 33) is integrated in or on a back surface (25) of the radiation sensor chip (1) to direct IR radiation toward the first temperature-sensitive element (7). In the described embodiments, concentric regions of the Fresnel lens (21,22 or 23) are circular.
In one embodiment, the invention provides an infrared (IR) radiation sensor device (27) including an integrated circuit radiation sensor chip (1 or 1A) including first (7 in
In one embodiment, the diffractive optical element includes a Fresnel lens (21,22, or 23) focused on a portion of the dielectric layer (3) bounding the cavity (4). In one embodiment, the Fresnel lens is a binary Fresnel lens (21) formed of concentric regions etched into the back surface (25) of the radiation sensor chip (1A). In another embodiment, the Fresnel lens is a binary Fresnel lens (22) formed of concentric rings of infrared-opaque material deposited on the back surface (25) of the radiation sensor chip (1A).
In yet another embodiment, the diffractive optical element includes a diffraction grating (33) formed of a plurality of elongated parallel rectangular regions (33A) etched into the back surface (25) of the radiation sensor chip (1A).
In one embodiment, the first (7) and second (8) temperature-sensitive elements include first (7) and second (8) thermocouple groups, respectively, connected in series to form a thermopile (7,8), and wherein the dielectric stack (3) is a semiconductor process dielectric stack including a plurality of SiO2 sublayers (3-1,2 . . . 6) and various polysilicon traces, titanium nitride traces, tungsten contacts, and aluminum metallization traces between the various sublayers patterned to provide the first (7) and second (8) thermocouple groups connected in series to form the thermopile (7,8). CMOS circuitry (45) is coupled between first (+) and second (−) terminals of the thermopile (7,8) to receive and operate upon a thermoelectric voltage (Vout) generated by the thermopile (7,8) in response to infrared (IR) radiation received by the radiation sensor chip (1 or 1A), the CMOS circuitry also being coupled to the bonding pads (28A).
In the described embodiments, the substrate (2) is composed of silicon to pass infrared radiation to the thermopile (7,8) and block visible radiation. A passivation layer (12) is disposed on the dielectric stack (3), a plurality of generally circular etchant openings (24) being located between the various traces and extending through the passivation layer (12) and the dielectric layer (3) to the cavity (4) for introducing silicon etchant to produce the cavity (4) by etching the silicon substrate (2).
In one embodiment, the first (R2) and second (R4) temperature-sensitive elements include first (R2) and second (R4) resistive devices, respectively.
In one embodiment, the invention provides a method for making a radiation sensor device (27), including providing first (7 in
In one embodiment, a Fresnel lens (21,22, or 23) is formed on a back surface (25) of the radiation sensor chip (1) so that the Fresnel lens is focused on a portion of the dielectric layer (3) bounding the cavity (4). In another embodiment, a diffraction grating (33) is formed on a back surface (25) of the radiation sensor chip (1) so that the diffraction grating directs infrared radiation to the first temperature-sensitive element (7 or R2). In one embodiment, a Fresnel lens is formed as a binary Fresnel lens (21) by etching a plurality of concentric regions into the back surface (25) of the radiation sensor chip (1A).
In one embodiment, the invention provides an infrared radiation sensor device (27), including a radiation sensor chip (1 or 1A) including first (7 in
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.
The various layers shown in dielectric stack 3, including polysilicon layer 13, titanium nitride layer 15, aluminum first metallization layer M1, aluminum second metallization layer M2, and aluminum third metallization layer M3 each are formed on a corresponding oxide sub-layer of dielectric stack 3. Thermopile 7,8 thus is formed within SiO2 stack 3. Cavity 4 in silicon substrate 2 is located directly beneath thermocouple group 7, and therefore thermally insulates thermocouple group 7 from silicon substrate 2. However thermocouple group 8 is located directly adjacent to silicon substrate 2 and therefore is at essentially the same temperature as silicon substrate 2. A relatively long, narrow polysilicon trace 13 is disposed on a SiO2 sub-layer 3-1 of dielectric stack 3 and extends between tungsten contact 14-2 (in thermocouple group 7) and tungsten contact 14-1 (in thermocouple group 8). Titanium nitride trace 15 extends between tungsten contact 15-1 (in thermocouple group 8) and tungsten contact 15-2 (in thermocouple group 7). Thus, polysilicon trace 13 and titanium nitride trace 15 both function as parts of thermopile 7,8. Thermopile 7,8 is referred to as a poly/titanium-nitride thermopile, since the Seebeck coefficients of the various aluminum traces cancel and the Seebeck coefficients of the various tungsten contacts 14-1, 14-2, 15-2, and 17 also cancel because the temperature difference across the various connections is essentially equal to zero.
The right end of polysilicon layer 13 is connected to the right end of titanium nitride trace 15 by means of tungsten contact 14-2, aluminum trace 16-3, and tungsten contact 15-2 so as to form “hot” thermocouple group 7. Similarly, the left end of polysilicon layer 13 is connected by tungsten contact 14-1 to aluminum trace 11B and the left end of titanium nitride trace 15 is coupled by tungsten contact 15-1, aluminum trace 16-2, and tungsten contact 17 to aluminum trace 11A, so as to thereby form “cold” thermocouple group 8. The series-connected combination of the two thermocouple groups 7 and 8 forms thermopile 7,8.
Aluminum metallization interconnect layers M1, M2, and M3 are formed on the SiO2 sub-layers 3-3, 3-4, and 3-5, respectively, of dielectric stack 3. A conventional silicon nitride passivation layer 12 is formed on another oxide sub-layer 3-6 of dielectric layer 3. A number of relatively small-diameter etchant holes 24 extend from the top of passivation layer 12 through dielectric stack 3 into cavity 4, between the various patterned metallization (M1, M2 and M3), titanium nitride, and polysilicon traces which form thermocouple groups 7 and 8. As subsequently explained, silicon etchant is introduced through etchant holes 24 to etch cavity 4 into the upper surface of silicon substrate 2. Note that providing the etchant openings 24 is not conventional in standard CMOS processing or bipolar integrated circuit processing, nor is the foregoing silicon etching used in this manner in standard CMOS processing or bipolar integrated circuit processing.
The small diameters of etchant holes 24 are selected in order to provide a more robust floating thermopile membrane, and hence a more robust IR radiation sensor. The diameters of the etchant hole openings 24 can vary from 10 microns to 30 microns with a spacing ratio of 3:1 maximum to 1:1. The spacings between the various etchant openings 24 can be in a range from approximately 10 to 60 microns. The smaller spacing ratio (i.e., the distance between the edges of the holes divided by the diameter of the holes) has the disadvantage that it results in lower total thermopile responsivity, due to the packing factor (the number of thermocouple groups per square millimeter of surface area) of the many thermocouple groups (see
Referring back to
The back surface 25 of silicon substrate 2 receives IR radiation 5 and passes it through chip 1A to SiO2 stack 3 while filtering out any ambient visible light. Thermocouple group 7 is thermally insulated, by cavity 4, from silicon substrate 2. Thermocouple group 8 is formed by dissimilar materials within dielectric stack 3 directly adjacent to silicon substrate 2. The portion of SiO2 stack 3 spanning the opening of cavity 4 forms a thin “floating” membrane which supports thermocouple group 7. IR radiation impinges uniformly on the back surface 25 of silicon substrate 2, differentially heating thermocouple groups 7 and 8. This results in the temperature T1 of thermocouple group 7 and the temperature T2 of thermocouple group 8 being different because of the insulative or thermal resistance properties of cavity 4. The thermoelectric output voltage difference Vout between thermopile terminals 11A and 11B in
Vout=(T1−T2)(S1−S2),
where T1-T2 is the temperature difference between the two thermocouple groups 7 and 8, and where S1 and S2 are the Seebeck coefficients of thermopile 7,8.
The differential voltage Vout generated between (−) conductor 11B and (+) conductor 11A can be applied to the input of the CMOS circuitry in block 45. Block 45 can include gain/filter amplifier and various “mixed signal” circuitry 45B as shown in
Still referring to
Fresnel lens 21 in
Typically, each feature of Fresnel lens 21 is approximately half of the IR radiation wavelength. Imagine a Fresnel lens having a diameter of about 500 microns. Half of a wavelength is about 4 microns. The cavities 4 of the prior art IR sensor chips are etched from the back surface of the integrated circuit chip, and that eliminates the possibility of forming a Fresnel lens on the back of the chip.
Use of “binary” diffractive optical elements may be beneficial if it is desired to perform a differential infrared measurement and to differentiate wavelength.
The use of bridge circuitry as described above increases the sensitivity of the IR sensor chip of
The basic structure of the embodiment of the invention shown in
Thus, the invention provides an infrared sensor chip integrated into and forming a part of a package, thereby eliminating the need for a package having a window. The sensitivity of the sensor chip is greatly improved by providing an optical element, such as a Fresnel lens, formed directly on/in a back surface of the chip. This avoids the need for using expensive external infrared optics. In a described embodiment, the back side of the silicon sensor die utilized as part of a wafer scale chip package (WCSP) forms a window for IR radiation and functions as a visible light filter. The integration and precise self-alignment achievable by conventional photolithography allows the creation of the Fresnel lens in/on the silicon chip surface to collimate the beam and focus it on to the thermocouple group and thereby increase the sensor sensitivity, thereby integrating the lens, silicon sensor chip, and package at very low cost. The increase in sensitivity and the shaping of the field of view of the sensor makes it more suitable for measurement of the temperature of remote objects. The invention achieves very low cost by providing previously un-achieved very precise alignment of features on the front surface of the wafer with optical element features on the back surface of the wafer.
Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.
Claims
1. An infrared (IR) radiation sensor device comprising:
- an integrated circuit radiation sensor chip including first and second temperature-sensitive elements in a dielectric stack of the radiation sensor chip, the first temperature-sensitive element being more thermally insulated from a substrate of the radiation sensor chip than the second temperature-sensitive element;
- a plurality of bonding pads on the radiation sensor chip coupled to the first and second temperature-sensitive elements;
- a plurality of bump conductors bonded to the bonding pads, respectively, for physically and electrically connecting the radiation sensor chip to corresponding mounting conductors, respectively; and
- an diffractive optical element integrated with a back surface of the radiation sensor chip.
2. The radiation sensor device of claim 1 wherein the optical element is a diffractive optical element, and wherein the first temperature-sensitive element is insulated from the substrate by means of a cavity between the substrate and the dielectric stack.
3. The radiation sensor device of claim 2 wherein the diffractive optical element includes a Fresnel lens focused on a portion of the dielectric layer bounding the cavity.
4. The radiation sensor device of claim 3 wherein the Fresnel lens is a binary Fresnel lens formed of concentric regions etched into the back surface of the radiation sensor chip.
5. The radiation sensor device of claim 3 wherein the Fresnel lens is a binary Fresnel lens formed of concentric rings of infrared-opaque material deposited on the back surface of the radiation sensor chip.
6. The radiation sensor device of claim 2 wherein the diffractive optical element includes a diffraction grating.
7. The radiation sensor device of claim 6 wherein the diffraction grating is formed of a plurality of elongated parallel rectangular regions etched into the back surface of the radiation sensor chip.
8. The radiation sensor device of claim 3 wherein concentric regions of the Fresnel lens are circular.
9. The radiation sensor device of claim 3 wherein the Fresnel lens includes at least approximately 100 concentric regions.
10. The radiation sensor device of claim 5 wherein the infrared-opaque material is composed of metal.
11. The radiation sensor device of claim 2 wherein the first and second temperature-sensitive elements include first and second thermocouple groups, respectively, connected in series to form a thermopile, and wherein the dielectric stack is a semiconductor process dielectric stack including a plurality of SiO2 sublayers and various polysilicon traces, titanium nitride traces, tungsten contacts, and aluminum metallization traces between the various sublayers patterned to provide the first and second thermocouple groups connected in series to form the thermopile.
12. The radiation sensor device of claim 11 including CMOS circuitry coupled between first and second terminals of the thermopile to receive and operate on a thermoelectric voltage generated by the thermopile in response to infrared (IR) radiation received by the radiation sensor chip, the CMOS circuitry also being coupled to the bonding pads.
13. The radiation sensor device of claim 12 wherein the substrate is composed of silicon to pass infrared radiation to the thermopile and block visible radiation, and further including a passivation layer disposed on the dielectric stack and a plurality of generally circular etchant openings located between the various traces and extending through the passivation layer and the dielectric layer to the cavity for introducing silicon etchant to produce the cavity by etching the silicon substrate.
14. The radiation sensor device of claim 1 wherein the first and second temperature-sensitive elements include first and second resistive devices, respectively.
15. A method for making a radiation sensor device, comprising:
- providing first and second temperature-sensitive elements connected in a dielectric stack of a radiation sensor chip, and thermally insulating the first temperature-sensitive element from a substrate of the radiation sensor chip;
- forming a plurality of bonding pads on the radiation sensor chip, the bonding pads being coupled to the first and second temperature-sensitive elements;
- bonding the bonding pads to a plurality of corresponding mounting conductors, respectively; and
- integrating an optical element with a back surface of the radiation sensor chip.
16. The method of claim 14 wherein the optical element is a refractive optical element, the method including insulating the first temperature-sensitive element from the substrate by etching a cavity in the substrate between the first temperature-sensitive element and the substrate.
17. The method of claim 16 wherein step (d) includes forming a Fresnel lens on a back surface of the radiation sensor chip so that the Fresnel lens is focused on a portion of the dielectric layer bounding the cavity.
18. The method of claim 16 wherein step (d) includes forming a diffraction grating on a back surface of the radiation sensor chip so that the diffraction grating directs infrared radiation to the first temperature-sensitive element.
19. The method of claim 17 including forming the Fresnel lens as a binary Fresnel lens by etching a plurality of concentric regions into the back surface of the radiation sensor chip.
20. A infrared radiation sensor device, comprising:
- a radiation sensor chip including first and second temperature-sensitive elements connected in a dielectric stack of a radiation sensor chip, and thermally insulating the first temperature-sensitive element from a substrate of the radiation sensor chip;
- means for thermally insulating the first temperature-sensitive element from a substrate of the radiation sensor chip;
- bump conductor means bonded to a plurality of bonding pads coupled to the thermopile, respectively, for physically and electrically connecting the radiation sensor chip to corresponding mounting conductors; and
- optical means integrated with a back surface of the radiation sensor chip for directing incoming infrared radiation to a portion of the dielectric stack bounding the thermally insulating means.
Type: Application
Filed: Oct 19, 2012
Publication Date: Feb 21, 2013
Applicant: Texas Instruments Incorporated (Dallas, TX)
Inventor: Texas Instruments Incorporated (Dallas, TX)
Application Number: 13/656,352
International Classification: H01L 31/0232 (20060101); H01L 31/18 (20060101);