WO2005091380A1

WO2005091380A1 - Infra red sensing devices

Info

Publication number: WO2005091380A1
Application number: PCT/IB2005/000593
Authority: WO
Inventors: Appolonius Jacobus Van Der Wiel; Guy Cools
Original assignee: Melexis Nv
Priority date: 2004-03-09
Filing date: 2005-03-09
Publication date: 2005-09-29
Also published as: GB0405201D0

Abstract

A method of manufacturing a vacuum packaged Infra Red (IR) sensing devices. The sensing elements of the sending devices are contained within a silicon chamber. The silicon chamber protects the sensing element from damage but allows good detection by the elements due to the transparency of silicon to IR radiation.

Description

INFRA RED SENSING DEVICES

The present invention relates to infra red (IR) sensing devices and in particular to IR sensing devices comprising an integrated circuit incorporating IR sensing elements wherein said IR sensing elements are encapsulated within a vacuum. Infra red (IR) sensing devices incorporating integrated circuits typically comprise one or more IR sensing elements each having an area of IR absorbing material provided on a wafer, the IR absorbing material arranged to be exposed to incoming IR radiation, and temperature measurement means. The temperature of said area of IR absorbing material is thus dependent upon IR radiation incident thereon. By measuring the temperature difference between the IR absorbing material and surrounding areas of wafer, the intensity of the incident IR radiation can be determined.

The temperature difference is measured by one or more thermocouples having their hot junctions in proximity to and responsive to the temperature of the said IR absorbing material and their cold junctions at the temperature of the surrounding areas of wafer. The temperature of said surrounding areas of wafer may be determined as a reference by separate temperature sensing means such as a thermistor.

The accuracy and responsiveness of such an IR sensor is determined in part by the thermal mass of the IR absorbing material and the thermal mass of the area of wafer upon which it is provided and by the thermal isolation between the said IR

absorbing material and the surrounding areas of wafer. Thermal isolation of the IR

absorbing material may be enhanced by reducing the amount of wafer directly in contact with the IR absorbing material and by encapsulating the IR absorbing material in a vacuum.

Vacuum encapsulation is conventionally achieved by packaging individual integrated circuits in a conventional manner in a vacuum environment or depositing a cover or thin film over the sensing elements in a vacuum environment or soldering a cap onto the surface of the integrated circuit in a vacuum environment.

The first method requires the packaging operation to be performed in a vacuum environment wherein a fresh the vacuum is provided for each integrated circuit and is thus expensive. The second method is less costly but such covers typically only provide a small separation between the IR sensing material and the external environment, often of the order of 2 microns, which is too small to provide significant thermal isolation from the external environment. The third method requires metallization of part of the surface of the wafer in order to accept the solder, typically achieved by using noble metals. This is however a non CMOS standard plating process which increases the cost of the procedure. Additionally, the metallized area of wafer can normally not be used for circuitry which means that a larger wafer must be provided for a given level of operability.

A further disadvantage of the above methods is that they do not easily enable the implementation of filtering layers or lens structures on the wafer. It is therefore the object of this invention to provide a new method of vacuum encapsulating integrated circuit sensing elements. According to a first aspect of the present invention there is provided an infra red (IR) sensing device comprising: a first wafer upon an upper surface of which is provided an integrated circuit, said integrated circuit incorporating one or more IR sensing elements; a second wafer bonded to the upper surface of the first wafer via glass frit, said glass frit being arranged to surround said IR sensing element or elements such that the first wafer, the second wafer and the glass frit providing the bonding define a sealed chamber encapsulating said IR sensing element and wherein a vacuum is provided in said chamber.

According to a second aspect of the present invention there is provided a method of manufacturing an infra red (IR) sensing device comprising the steps of: providing an integrated circuit on a surface of a first wafer, said integrated circuit incorporating one or more IR sensing elements; providing a second wafer, said second wafer having provided thereon a glass frit pattern; aligning said wafers such that said glass frit pattern provided on said second wafer surrounds said IR sensing element or elements provided on said first wafer; and bonding said wafers together via said glass frit to form a sealed chamber defined by said wafers and said glass frit encapsulating said IR sensing element or elements, wherein said aligning and bonding steps are carried out in a vacuum thus encapsulating said IR sensing element or elements in a vacuum. In this manner a low cost CMOS process compatible method of vacuum packaging IR sensing devices is provided. By encapsulating the sensing elements within a silicon chamber a robust device is provided which by virtue of the transparency of silicon to IR radiation provides good detection properties. Preferably both said first and said second wafers are silicon wafers and most preferably said first and said second wafers have the same coefficient of thermal expansion. Preferably, said first wafer is a standard CMOS wafer.

Preferably, the or each IR sensing element comprises an area of IR absorbing material, means for measuring the temperature of the IR absorbing material relative to the temperature of the first wafer, means for measuring the absolute temperature of the first wafer means for interfacing signals generated by the temperature sensing means with external circuitry. Most preferably, a cavity in the surface of the first wafer is provided for the sensing element or individual cavities are provided for each sensing element and the area of IR absorbing material for each sensing element is provided on a beam extending over said cavity. Said cavities may be formed by etching or by any other suitable process.

Preferably the said second wafer may have a cavity provided on a surface opposite said first wafer to increase the dimensions of said chamber. Said cavity may be formed by etching or any similar method. Preferably, said cavity is adapted to be positioned substantially opposite said IR absorbing material when the wafers are bonded together.

Preferably, said sensing element or elements on said first wafer are electrically connected to a number of bond pads to facilitate connection to external circuitry. Preferably, said bond pads are provided in one or more groups, wherein within each group the bond pads are adjacent to one another and apertures are provided in said second wafer, said apertures being aligned with said groups or groups of bond pads on said first wafer when said wafers are bonded together. Said apertures are preferably formed by etching the second wafer on one or preferably two opposite surfaces or by any other similar process.

In an alternative preferred embodiment, said second wafer may be provided with one or more grooves or channels facing said bond pads, said grooves or channels preferably being formed by etching or similar. In such embodiments, said second wafer may be back lapped, etched or similar to provide apertures aligned with said bond pads.

Preferably the said glass frit material is provided on said second wafer, by screen printing or any other suitable method. Most preferably said glass frit material is processed to remove any organic compounds. In order to bond said wafers together using said glass frit said wafers are preferably heated such that said glass frit melts and thereby bond said wafers together.

Preferably one or more of the following are provided on the second wafer: filter means, lens means or masking means.

Preferably the said filter means comprises one or more layers of material allowing the passage of IR radiation provided on one or more surfaces of said second wafer. Preferably, the said masking means comprises one or more layers of material opaque to IR radiation provided on one ore more surfaces of said second wafer, said layers of defining apertures through which radiation may pass through the second wafer to said sensing elements. Said filter material or said masking material may be provided on one or more surfaces of the wafer by deposition or any other suitable method.

Preferably the lens means comprises a series or plurality of notches provided on one or more surfaces of the said second wafer, said notches adapted to reduce reflection or improve the transmission of IR radiation through said second wafer. The lens means may also be adapted to focus IR radiation on the sensing element or elements. The notches may be formed by etching or any other suitable method.

Preferably if a plurality of IR sensing elements are incorporated into a single integrated circuit on said first wafer, all the IR sensing elements sealed within a single chamber and are thus maintained at the same pressure.

Preferably the sensing device may additionally incorporate absolute pressure sensing means arranged to monitor the pressure local to the IR sensing elements and electronic circuitry means to interface said absolute pressure sensing means to external processing means to electronically compensate for variations in IR sensitivity of the sensing elements caused by pressure variations in the chamber.

Preferably, a plurality of such IR sensing devices are manufactured in parallel. To facilitate this, an array of integrated circuits incorporating one or more sensing elements is provided on said first wafer and corresponding glass frit and aperture or groove/channel patterns are provided on said second wafer. The second wafer is then aligned with said first wafer and once in contact, both wafers are heated to melt said glass frit and thus fuse the wafers together. Most preferably, said individual IR - 1 - sensing devices are subsequently separated by making a series of cuts along scribe lines running between adjacent integrated circuits in the array.

In order that the invention is more clearly understood, it will now be described further herein, by way of example only and with reference to the following drawings in which:

Figure 1 is a cross-section of a vacuum encapsulated infra red (IR) sensing integrated circuit according to the present invention;

Figures 2a-d show cross-sections of alternative embodiments of the vacuum encapsulated IR sensing integrated circuit of figure 1. Referring to Figure 1, an infra red (IR) sensing device is shown in the process of manufacture. A first silicon wafer 101 has one or more IR sensing elements 100 provided thereupon. Each IR sensing element 100 comprises IR absorbing material 104 mounted on a beam 103, each said beam 103 extending over a cavity 102 provided in an upper surface of wafer 101. By providing the IR absorbing material 104 upon the beam 103 extending over the cavity 102, the IR absorbing material 104 is effectively thermally isolated from the wafer 101 such that the temperature of the IR absorbing material is indicative of the intensity of the incoming IR radiation. Suitable cavities 102 can be provided by etching the wafer 101.

Temperature sensing means and electronic circuitry means, not shown, are implemented within the first wafer to: measure the temperature of the IR absorbing material 104 relative to the temperature of the first wafer 101; to measure the absolute temperature of the first wafer 101; and to interface the signals generated by the temperature sensing means to external processing means (not shown) via electrical connections pads, 109.

A second silicon wafer 106 is mounted over the surface of the fist silicon wafer 101, to provide the roof 107 of a sealed chamber 111 encapsulating said IR sensing elements 100. The walls of the chamber 111 are provided by glass frit 105 used to bond the second wafer 106 to the first wafer 101. The sealed chamber 111 allows the IR sensing elements 100 to be enclosed within a vacuum, which thereby provides additional thermal isolation from the silicon wafers 101, 106 and the external environment.

The roof portion 107 of the second wafer 106 does not extend over the connection pads 109, which are provided to one side of the wafer 101. This allows connections to be made between the IR sensing elements 100 and external circuitry without compromising the vacuum within which the IR sensing elements 100 are encapsulated.

The IR sensing device is one of a plurality of such devices manufactured in parallel. This is achieved by forming an array of said integrated circuits of the type described above on a single wafer 101.

A glass frit pattern 105 is provided on a second wafer 106, the glass frit pattern 105 being such that when said first and said second wafers 101, 106 are brought together in a desired alignment, the glass frit pattern 105 surrounds the IR sensing element or elements 100 of each integrated circuit in the array. The glass frit pattern 105 is typically formed on the second wafer 106 by screen printing.

The second wafer 106 is also etched to provide apertures or openings 108 in the wafer which may be aligned with the connection pads 109 of the integrated circuits on the first wafer 101, said aperture also define a boundary between roof portions 107 of adjacent devices. The apertures 108 allow access to the pads 109 for connection to external circuitry. By providing apertures 108 in the wafer, rather than providing individual roof portions 107, the structural integrity of the second wafer 106 is maintained. The apertures 108 may be formed either before or after the formation of the glass frit pattern 105, by etching from one or both sides of the wafer 106 or by any other suitable means.

The second wafer, 106, is aligned with first wafer, 101, and the two wafers brought together under vacuum conditions. The wafers 101, 106 are raised in temperature to reflow the glass frit and thus bond the wafers 101, 106 together in the desired alignment. In this manner the glass frit 105 and the two wafers 101, 106 define a chamber 111 within which the IR sensing elements 100 are encapsulated. The volume of the chamber 111 can be varied by adjusting the thickness of the glass frit 105. By performing the steps of aligning and bonding the wafers 101, 106 in a vacuum, the chamber 111 consequently contains a vacuum. After bonding, individual IR sensing devices are separated by cutting the bonded wafers along scribe lines such as scribe line 110 shown in figure 1. The connecting pads 109 are thus exposed and accessible for subsequent connection to external circuitry.

Referring now to figures 2a-d, a number of alternative embodiments of roof portions 201 are shown being modifications of roof portion 107 shown in figure 1. hi the embodiment of figure 2a an additional cavity 204 is etched in a lower surface of roof portion 201. This increases the separation between the IR sensing elements 100 and the roof portion 201 to improve the thermal insulation effect of the vacuum chamber 111. hi figure 2b a further embodiment is shown wherein the roof portion 201 is provided with an etched cavity 204. The cavity and the rest of the lower surface of the roof portion 201 are coated by an IR filtering material 205. This reduces or prevents non-IR radiation from being incident upon the IR sensing elements 100. An

IR opaque coating 202 is provided on the top surface of the wafer 201, to define an aperture through which IR radiation must pass to be incident on the sensing elements 100. This masks tliose areas of first wafer 101 which are not IR sensing elements 100 from any heating effect that might adversely affect the accuracy of the IR sensor.

This also has the effect of restricting the IR sensors field of view to a desired direction. Additionally, the top surface of roof portion 201 is provided with a lens pattern 203 to focus or concentrate incident IR radiation onto the IR sensing elements 100 and thus improve the accuracy and efficiency of the device. The lens pattern may typically be formed by etching or other similar process. In the embodiment of figure 2c a lens pattern 212 is provided on the lower surface of roof portion 201 and a layer of IR filtering material 211 is applied to the upper surface of roof portion 201. In the embodiment shown, there is no cavity 204 on the lower surface of roof portion 201 nor is there an IR opaque coating 202 on either surface of roof portion 201. It is of course possible that alternative embodiments of roof portion 201 optimised for particular application may have any desired combination of one or more of a cavity 204, a lens pattern 203, 212, an opaque coating 202 or an IR filtering layer 205, 211 any or all of which can be provided on either surface of roof portion 201. In a further embodiment shown in figure 2d, a lens pattern 222 is provided on the lower surface of roof portion 201 and additionally a channel 223 is also provided. The channel 223 is positioned such that it is aligned with the connection pads on wafer 101 when the wafers 101, 106 are bonded together. After bonding the upper part of wafer 201 is back lapped or etched to line 224, removing the portion labelled 221. The channel 223 thus provides an aperture over connection pads 109 allowing access for connection to external circuitry. The wafers 101, 106 can then be cut as before to separate them into a plurality of individual IR sensing devices.

It is of course to be understood that the invention is not to be limited to the details of the above embodiment which is described by way of example only.

Claims

1. An infra red (IR) sensing device comprising: a first wafer upon an upper surface of which is provided an integrated circuit, said integrated circuit incorporating one or more IR sensing elements; a second wafer bonded to the upper surface of the first wafer via glass frit, said glass frit being arranged to surround said IR sensing element or elements such that the first wafer, the second wafer and the glass frit providing the bonding define a sealed chamber encapsulating said IR sensing element and wherein a vacuum is provided in said chamber.

2. A method of manufacturing an infra red (IR) sensing device comprising the steps of: providing an integrated circuit on a surface of a first wafer, said integrated circuit incorporating one or more IR sensing elements; providing a second wafer, said second wafer having provided thereon a glass frit pattern; aligning said wafers such that said glass frit pattern provided on said second wafer surrounds said IR sensing element or elements provided on said first wafer; and bonding said wafers together via said glass frit to form a sealed chamber defined by said wafers and said glass frit encapsulating said IR sensing element or elements, wherein said aligning and bonding steps are carried out in a vacuum thus encapsulating said IR sensing element or elements in a vacuum.

3. A sensing device according to claim 1 and claim 2 wherein said first and said second wafers are silicon wafers.

4. A sensing device accordingly to claim 1 or claim 2 wherein the first and second wafers have the same coefficient of thermal expansion.

5. A sensing device according to any one of claims 1 to 4 wherein the first wafer is a standard CMOS wafer.

6. A sensing device according to claim 1 or claim 2 wherein the or each IR sensing element comprises an area of IR absorbing material, means for measuring the temperature of the IR absorbing material relative to the temperature of the first wafer, means for measuring the absolute temperature of the first wafer means for interfacing signals generated by the temperature sensing means with external circuitry.

7. A sensing device according to claim 6 wherein a cavity is provided to the surface of the first wafer for sensing element or individual cavities are provided for each sensing element and the area of IR absorbing material for each sensing element is provided on a beam extending over said cavity.

8. A sensing device according to claim 7 wherein said cavities are formed by etching.

9. A sensing device according to any one of claims 1 to 8 wherein said second wafer has a cavity provided on a surface opposite said first wafer including the dimensions of said chamber.

10. A sensing device according to claim 9 wherein the cavity is adapted to be positioned substantially opposite said IR absorbing material when the wafers are bounded together.

11. A sensing device according to any one of claims 1 to 10 wherein said sensing element or elements on said first wafer are electrically connected to a number of bond pads to facilitate connection to external circuitry.

12. A sensing device according to claim 11 wherein said bond pads are provided in one ore more groups wherein, within each group or groups, the bond pads are adjacent to one another and apertures are provided in said second wafer, said apertures being aligned with said group or groups of bond pads on said first wafer when the wafers are bonded together.

13. A sensing device according to claim 12 wherein said apertures are formed by etching the second wafer on at least one, preferably two, opposite surfaces.

14. A sensing device according to any one f claims 1 to 10 wherein said second wafer is provided with one ore more grooves or elements facing said bond pads.

15. A sensing device according to claim 14 wherein the grooves or channels are formed by etching.

16. A sensing device according to claim 14 wherein said second wafer is back lapped or etched to provide apertures adjacent with bond pads.

17. A sensing device according to any one of claims 1 to 16 wherein the glass frit material is provided on said second wafer by screen printing.

18. A sensing device according to claim 17 wherein the glass first material is processed to remove any organic compounds.

19. A sensing device according to claim 17 or claim 18 wherein the said wafers are bonded together by heating said wafers until said glass fit melts and thereby bonds the wafers together.

20. A sensing device according to any one of claims 1 to 19 wherein said second wafer comprises one or more of a filter means, lens means or a masking means.

21. A sensing device according to claim 20 wherein said filter means comprises one or more layers of material allowing passage of IR radiation, said material being provided on one or more surfaces of said second wafer.

22. A sensing device according to claim 20 or claim 21 wherein said masking means comprises one or more layers of material opaque to IR radiation, said material being provided on one ore more surfaces of said second wafer, said layers defining apertures through which radiation may pass through the second wafer to said sensing elements.

23. A sensing device according to any one of claims 20 to 22 wherein said filter material or masking material is provided on said one or more surfaces of the wafer by deposition.

24. A sensing device according to any one of claims 20 to 22 wherein the lens means comprise a series or plurality of notches provided on one or more surfaces of said second wafer, said notches being adapted to reduce reflection or improve the transmission of IR radiation through said second wafer.

25. A sensing device according to claim 23 wherein said lens means is also adapted to form IR radiation on the sensing element or elements.

26. A sensing device according to claim 24 or claim 25 wherein said notches are formed by etching.

27. A sensing device according to any one of claims 1 to 26 wherein a plurality of IR sensing elements are incorporated into a single integrated circuit on said first wafer, all IR sensing elements are sealed within a single chamber and can thus be maintained at the same pressure.

28. A sensing device according to any one of claims 1 to 27 wherein said sensing device includes absolute pressure sensing means arranged to monitor pressure local to the IR sensing elements and electronic circuitry means to interface said absolute pressure sensing means to external processing means to electronically compensate for variations in IR of the sensing element caused by pressure variations in the chamber.

29. A method according to claim 2 wherein one or more sensing elements is provided on said first wafer and a corresponding glass frit and aperture or groove/channel position as provided on said second wafer, said second wafer is aligned with said first water and once in contact, both wafers are heated to melt the glass frit and thus fuse the wafers together.

30. A method according to claim 29 wherein included sensors can be separated by making a series of cuts along scribe lines running adjacent integrated circuits in the array.