AU2004203904B2 - Thermal sensor and method of making same - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: Actual Inventors: Address for Service: Invention Title: Honeywell Inc.

Higashi, Robert E.

DAVIES COLLISON CAVE, Patent Attorneys, 1 Nicholson Street, Melbourne, 3000 Thermal sensor and method of making same The following statement is a full description of this invention, including the best method of performing it known to us.

Q:\OPERJPN\2004 JULDEC\12487850 DIV FILINGDOC 17/8/04 P.%OPERUPM124878SO Ispl 4o17/O/2007 -1- THERMAL SENSOR AND METHOD OF MAKING SAME FIELD OF THE INVENTION The invention relates to an improvement in bi-level infrared detectors with respect to which U.S. patent Re. 36,136 is an leading example and a method of making bi-level infrared detectors.

BACKGROUND OF THE INVENTION There are a number of prior art bi-level infrared thermal detectors. Examples include U.S. Patents Re. 36,136; 5,286,976 and 5,450,053.

The technology disclosed in U.S. Re. 36,136; 5,286,976 and 5,540,053 has proven to be a commercial success (a cross section ofa pixel using this prior art technology is shown in Figure however, this technology is relatively complex due in part to the utilization of a so-called wet etch process used in the fabrication. A relatively large number of masks must be used in order to produce an array of pixels. This produces more mass and hence requires more energy to change temperature.

The technical disclosures of U.S. Re. 36,136; 5,286,976 and 5,450,053 are incorporated herein for reference.

SUMMARY OF THE INVENTION In accordance with one aspect of the present invention there is provided a two-level microbridge infrared thermal detector apparatus comprising: a pixel on a semiconductor substrate, said pixel having a lower section on the surface of said substrate and an upper detector planar section spaced from and immediately above said lower section; said lower section including integrated circuit means; said upper detector planar section comprising a temperature responsive detector of an oxide of vanadium having a high TCR and a resistivity in the range of 5K ohms to 300K ohms per square sheet resistance, and being supported via posts above said lower section by P:\OPERUPI I2487850 Ilspdoll-1701/2007 -1Aleg portions of an oxide of vanadium having a resistivity in the range of 250 to 1,000 ohms per square sheet resistance; and said leg portions being electrically connected to said temperature responsive detector and said integrated circuit means, and providing thermal isolation for said detector.

In accordance with another aspect of the present invention there is provided method of fabricating an array of two-level microbridge infrared thermal detector pixels on a substrate wherein each of said pixels has a lower section on a surface of said substrate, a microbridge upper detector section spaced from and immediately above said lower section, and each of said lower sections includes integrated circuit means, the method comprising, for each pixel, the steps of: depositing on said microbridge upper detector section a thin film of an oxide of vanadium having a high TCR and a resistivity of 5K ohms to 300K ohms per square sheet resistance to thus form a temperature responsive detector; depositing at least two legs connecting said temperature responsive detector electrically via posts to its associated integrated circuit, said legs as deposited being a thin film of an oxide of vanadium having a resistivity of 5K to 300K ohms per square sheet resistance; passivation of said thin film legs; exposing of selected portions of said passivated thin film legs wherein low resistance is desired; and performing an argon gas backsputter on the exposed selected portions of said thin film legs to reduce the resistivity thereof to a range of 250 to 1000 ohms per square sheet resistance.

In one example, there is provided a two-level microbridge infrared thermal detector comprising a pixel on a semiconductor substrate. The pixel has a lower section on the surface of the substrate which lower section includes integrated circuit means. The pixel also has an upper detector planar section spaced from and immediately above the lower section.

The upper detector planar section comprises a temperature responsive detector of an oxide of vanadium characterized by having a high temperature coefficient of resistance (TCR) and a resistivity in the range of 5K ohm to 300K ohms per square sheet resistance. Further, the upper detector planar section is mechanically supported above the lower section by means including leg portions of an oxide of vanadium characterized by having a resisivity in the range of approximately 250 ohms to 1,000 ohms per square sheet resistance. The leg portions are electrically connected both to the detector and to the integrated circuit means. Further and 0o importantly, the leg portions provide thermal isolation for the temperaure responsive detector.

In a typical application the invention takes the form of a large number of pixels arranged in an array and being positioned on a common semiconductor substrate. The thennal detector apparatus is further typically charactrized by having the upper detector planar section for each of the pixels including absorber means. Also the thermal detector apparatus may be chmcteried by including a reflective layer on the substrate below the upper detector section so as to increase the overall sensitivity and efficiency of the apparatus.

There are provided support legs and contacts which use only a minimum area to thus nmamize the thimal detectance absorption area; this is a signifiat attribute Additionally, the thermal conductance from the detector is minimized Further, the process for fabricating the sensor uses a much smaller number of photomasks to surpass the performance of the sensors ofthe prior art "wet etch" process which use up to 13 photomask. Thus, the unique process of this invention minimizes the thermal mass, thermal conductance, and pixel size limits, and (ii) provides a smaller, lower cost detector.

BRIEF DESCRIPTION OF THE DRAWINGS Figure I is a cross section view of a prior art pixel using, in part, the technology of U.S. Re. 36,136; 5,286,976 and 5,450,053; Figure 2 is a cross section view of a pixel fabricated from the technology of the present invention; Figure 3 is a top or plan view of the temperature responsive detector 40; and Figure 4 is an annotated isometric view of the device of Figure 2 as viewed from the top left thereof It should be noted that both Figures 1, 2 and 4 have the horizontal scale compressed so as to permit satisfactory depiction of the vertical scale of the disclosed pixels.

DETAILED DESCRIPTION OF THE INVENTION The prior art bi-level microbridge pixel 10 shown in Figure 1 comprises in part a substrate 12 which includes integrated circuit means 12' and thus comprises a lower section of the pixel. The integrated circuit means may follow the teaching of U.S. Re 36,136. The upper section 11 is a complex multilayer structure made from the use of the technology, with a plurality of steps, as disclosed in U.S. Re. 36,136; 5,286,976 and 5,450,053 the result being an upper detector section 11 spaced from and supported immediately above the lower section 12, a spacing of 1.8 microns being a desirable spacing for many applications and utilizations of the pixel as is well understood by those skilled in the art More specifically the prior art detector 10 comprises a series of successive layers 13-19: a first layer 13 of Si 3

N

4 on which are deposited a thin layer 14 of VOx (an oxide of vanadium); a layer 15 ofNiCr, a layer 16 of Si 3

N

4 a layer 17 ofCr, a plug 18 of Cu; and a layer 19 of NiCr.

Thus the prior art pixel of Figure 1 has a large number of layers which causes the pixel to hang a relatively heavy mass which then necessitates a larger amount of energy to change the temperature of the pixel.

A pixel 30 fabricated in accordance with the technology of the present invention is depicted in Figure 2. As in the prior art device, the pixel includes a lower section 33 having a semiconductor substrate 31 which includes integrated circuit means 32; the integrated circuit means may be achieved utilizing the teaching of U.S. Re 36,136.

A reflective layer 34 may be applied to the top surface of substrate 31; a thin layer of indium tin oxide may be selected for this layer. The reflective layer functions to increase the sensitivity of the detectoriz accordance wth the teaching of U.S. 5,286,976. A vertical post 36 is provided to connect both mechanically and electrically the lower section 33 and the upper detector plana section 38 of the pixel. More specifically, post 36 is made of a conductor such as alunintun tha has its lower and as shown in Figure 2 electrically coginectMd to the initegrate circuit means 32 and mechanically supported on substrate 3 1. Refering to Figure 4, it is Seem that ther are two diagonally positioned posts 36 and 36AA. The upper ends of posts 36 and 36AA are adapted to be connected to the ends of the leg portions; 41 and 42 of the detector discussed below.

in figure 2, it is seen that the upper detector plana section 38 is spaced from and is positioned immediately above the lower section 33; again a spacing of 1.8 microns is used in this example.

The upper deetr planar section 38 comprises in part a-temperature respoinsive detesqcr 40 of VOx (an oxide of vanadium) and shown in Figures 2 ad 3; this material is characterized by having a high temperature coefficient of resisumc (UtC) and a resistivity in the range of SK ohms to 300K ohms per square sheet resistance. For. example, one successful example comprised, in part, a detector 40 having a VOx film thickness of 700 and a resistivity of 50K ohm per square sheet ressame. The top plan view of the temperature responsive detectoir 40 is shown in Figure 3. It is shown to have a generally sua shape except for notches in two opposite comers to thus provide a high form factor, area available for sensing jinfrre ehergy. Rerenice numerals 40' vid 40" identify spcdapart rcgions o~f iho sensor 40; more specifically region 40V is most proximat to the top 36' of post 36 ad region is motproimute to the top 36AA of post 36AA. Integrally connected (both mechanically and electrically) with detecto 40 are a pair of legs 41 and 42 also of an oxide of vanadium. As shown in Figure 3, leg 41 is connected to detector 40 at region 40V with a short downwardly extending section 4 thence a horizont a xending section 41 thence a long vertical extending portion 4 1 -which, terminates with a generally hollow square shaped section or conector 4 1" adapted to be electrically connected to the top 36AA' of aluminum post 36AA.

In the samne manner, the leg 42 is Connected to sensor layer 40 at 40" and comprises a short upwardly, (as shown in Figure 3) extending vertical section 42', thence a horiz~nWa extending section 42", thence a vertical downwardly extending section 42' ternmn in a generally hollow shaped section or connector 42"" adapted to be electricaly ctonnected to the top 36' of aluminum post 36.

A cliejetc layer 49 may be provided on the underside of the sensor Layer 40 including legs 41 anid 42; it can contribute to the mechanical support of the sensor by the legs 41 and 42.

An absorber Layer 54 may be provided on the underside of the dielectric layer as shown in Figure 2 to increase the sensitivity of the sMIsor.

A topdtic ec 0o icon aneidddotpofila w40asisshon ot i Figures 2 and 4. Post caps 52 and 52AA wre added on top of 4 r" I r 4V' and 41 is respectively as shown in Figure 4, an intennediat layer 52 being of a suitable materia sudh a Cr.

A very unique aspect is the utilization of an oxide of vanadium for the legs for supporting the upper deetrplowa section 38 which incude the deetr40 of an oxide of vaniadium. Fist of A the legs 41 and 42 provide excellent thermal isolaio Of the upper detector planar section from the aluminum posts 36 and 36AA, and by extenson, fromi the ipe surhce of the underlying substrate 31 with its associated integratd circuit means 32.

However, the utilimaio of untreated oxide of vaniadium for the leg 41 and 42 would hav, an unaccptaly high iupdsefor providing an electrical connection between dhe sea" 40 and the integratedcircuitmiin the substrate~ This apparentiavnu iaberaoead in hxct is converwed to an advantag through the use of a unique processing step following the deposition and patterning of a filmn of high TCR oxide of vanadium with its attndan properties of a very high shoe resistance. The steps required for the solution of the problem is to first passivate the oxide of vanadiwrn and then expose the areas where it is desired to have low electrical resistace, the legs 41 and 42. The exposed legs then are subjected to an argon gas back sputtering process which then dramatically drops the resisumne of the exposed film to a sheet resistance in the range of 250 ohms to 1,000 ohms per square sheet resistance.

One example is to have the legs 41 and 42 with a film thickness of 700 Angstroms and a sheet resistivity of 500 ohms.

An advantage can be measured in part by a knowledge that the prior art micro lmeter "wet etch" process as set forth, in part, in U.S. Reissue 36,136 required 12 photo lithography steps and fourteen depositions following the fabrication ofthe read out electronics or semiconductor means in the substrate. The relatively large number of steps and depositions translates directly to device manuactoring costs for fabrication as well as minimum size constraints through density of featurewhen avoiding coincident edge featres In contrast, examples of the present invention using "dry etch" (oxygen plasma) processing, have a reduced number of independent process steps. Further the prior art device/process used a very narrow NiCr layer 14 to form an interconnect between the high TCR VOx material 14 and the readout electronics 12'; examples of the present invention avoid that expensive complexity by the provisions of legs 41 and 42 which support sensor 40, and provide thermal isolation for sensor 40, and provide an electrical connection means.

Thus since the invention discosed herein may be embodied in other specifc forms without departing fom the spirit or general ch istics hee some of which forms have been indicated, the examples described herein are to be considered in all respects illustrative and not restrctive. The scope of the nvemon is to be ind--- by the appended daim rather than be the foregoing description, and all changes which come within the meaning md range of equivalency of the claims are intented to be embraced therein.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

P \OPERUPI12487850 lspa dl1oc17A1/2007 -7- The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims (9)

1. A two-level microbridge infrared thermal detector apparatus comprising: a pixel on a semiconductor substrate, said pixel having a lower section on the surface of said substrate and an upper detector planar section spaced from and immediately above said lower section; said lower section including integrated circuit means; said upper detector planar section comprising a temperature responsive detector of an oxide of vanadium having a high TCR and a resistivity in the range of 5K ohms to 300K ohms per square sheet resistance, and being supported via posts above said lower section by leg portions of an oxide of vanadium having a resistivity in the range of 250 to 1,000 ohms per square sheet resistance; and said leg portions being electrically connected to said temperature responsive detector and said integrated circuit means, and providing thermal isolation for said detector.

2. The thermal detector apparatus of claim 1, wherein an array of said pixels is positioned on a common semiconductor substrate.

3. The thermal detector apparatus of claim 2, wherein the upper detector planar section of each of said detectors includes an absorber layer.

4. The thermal detector apparatus of claim 3, including a reflective layer on said substrate below said upper detector section.

5. The thermal detector apparatus of claim 1, wherein the ratio of the resistivity of said upper detector planar section to the resistivity of said leg portions is in the range of 10:1 to 600:1.

6. A method of fabricating an array of two-level microbridge infrared thermal detector pixels on a substrate wherein each of said pixels has a lower section on a surface of said substrate, a microbridge upper detector section spaced from and immediately above said lower section, and each of said lower sections includes integrated circuit means, the method comprising, for each pixel, the steps of: depositing on said microbridge upper detector section a thin film of an oxide of P:\OPERUPN\I248780 Isp.doc-170I/2W07 -9- vanadium having a high TCR and a resistivity of 5K ohms to 300K ohms per square sheet resistance to thus form a temperature responsive detector; depositing at least two legs connecting said temperature responsive detector electrically via posts to its associated integrated circuit, said legs as deposited being a thin film of an oxide of vanadium having a resistivity of 5K to 300K ohms per square sheet resistance; passivation of said thin film legs; exposing of selected portions of said passivated thin film legs wherein low resistance is desired; and performing an argon gas backsputter on the exposed selected portions of said thin film legs to reduce the resistivity thereof to a range of 250 to 1000 ohms per square sheet resistance.

7. The method of claim 6, wherein the ratio of the resistivity of said temperature responsive detectors to the resistivity of said thin film legs is in the range of 10:1 to 600:1.

8. A two-level microbridge infrared thermal detector apparatus substantially as hereinbefore described with reference to the drawings.

9. A method of fabricating an array of two-level microbridge infrared thermal detector pixels on a substrate, substantially as hereinbefore described with reference to the drawings.