ES2435198T3 - A pixel structure that has an umbrella-type absorbent with one or more recesses or channels calibrated to increase radiation absorption - Google Patents

Abstract

A pixel structure (100) for use in an infrared imager, comprising: - a substrate (102), and - a bolometer (104) comprising: a transducer (108) having an offset relationship with respect to said substrate, the transducer having an electrical resistance that varies in response to changes in transducer temperature, and an absorber (110) having a thermal connection to the transducer that allows radiation absorbed by the absorber to heat the transducer; wherein the absorber has an upper side (130) defining a recess (112a, 112b) in the absorber, the recess being adapted to affect the propagation path of a portion of radiation received by the absorber such that the portion of radiation is absorbed by the absorber instead of leaving the absorber; characterized in that the absorber has a spaced apart relationship to the transducer and includes an absorption layer (124) having a thickness that is 10 nanometers or less.

Description

A pixel structure having an umbrella-type absorbent with one or more recesses or channels calibrated to increase radiation absorption 5

Field of the Invention

The present invention relates to a pixel structure of a focal plane matrix based on bolometers.

Background of the invention

Infrared detectors are used in a variety of applications to provide an electrical output that is a useful measure of incident infrared radiation. For example, quantum detectors are a type of infrared detector that is often used for night vision purposes in a variety of military, industrial applications.

15 and commercial. Quantum detectors generally work at cryogenic temperatures and therefore need a cryogenic cooling device. As a result, quantum detectors that operate at cryogenic temperatures can have a relatively complex design and generally consume significant amounts of energy.

Another type of infrared detector is a thermal detector. Thermal detectors are typically not refrigerated and therefore generally operate at room temperature. One type of thermal detector that has developed and is becoming increasingly popular is an uncooled focal plane matrix based on microbolometers. A focal plane matrix generally includes a plurality of pixel structures, each of which includes a bolometer disposed on a common substrate. Each bolometer includes a transducer element that has

25 an electrical resistance that varies as a result of the temperature changes produced by the incident infrared radiation. A measure of the incident infrared radiation can be obtained by detecting changes in electrical resistance. Since the design of a non-refrigerated focal plane matrix based on bolometers is generally less complex than cryogenically cooled quantum detectors, and since these uncooled focal plane arrangements generally require significantly less energy than cryogenically cooled quantum detectors, Non-cooled focal plane matrices based on bolometers are increasingly being used.

Each pixel structure of a conventional uncooled focal plane matrix has a bolometer that includes an absorbent element to absorb infrared radiation and a transducer element associated with a resistor.

35 that varies as its temperature varies accordingly. Although the absorbent and transducer elements can be separate layers of a multilayer structure, the absorbent element and the transducer element can sometimes be the same physical element.

In operation, the infrared radiation incident on the absorbent element will heat the absorbent element. Since the absorbent element and the transducer element are in thermal contact, heating of the absorbent element will correspondingly heat the transducer element, thereby causing the electrical resistance of the transducer element to change in a predetermined manner. A measure of the incident radiation can be obtained by measuring the change in the electrical resistance of the transducer element, such as passing a known current through the transducer element.

To allow the bolometer to be sensitive to changes in incident infrared radiation, the bolometer is generally designed to minimize thermal loss to the substrate. Thus, conventional focal plane matrix bolometers have separate absorbent and transducer elements relative to the substrate to substantially thermally decouple the relatively massive substrate from the pixel. In this sense, each bolometer generally includes two or more legs that support the transducer and absorbent elements above the substrate. The legs can extend between the absorbent and transducer elements and the substrate, or the legs can connect the absorbent and transducer elements to pillars or the like that support the absorbent and transducer elements above the substrate.

In order to provide thermal contact between the absorbent and transducer elements while electrically isolating the transducer element from the absorbent element, the bolometer also generally includes an electrically conductive and thermally conductive insulating layer disposed between the absorbent element and the transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate that serves to structurally support the other layers and to protect the other layers during the manufacturing process. See for example US patents. No. 5,286,976, No. 5,288,649, No. 5,367,167 and No.

6,307,194 describing the pixel structures of conventional focal plane matrices based on bolometers, the contents of which are incorporated herein by reference.

In order to further improve the performance of conventional pixel structures, each bolometer can include

65 a reflector disposed on the surface of the underlying substrate under the absorbent and transducer elements. As such, the infrared radiation that is incident on the bolometer but passes through it and is not absorbed by the absorbent element, will be reflected by the reflector back to the absorbent element. At least a portion of the reflected radiation will therefore be absorbed by the absorbent element during the second passage through the absorbent element, thereby increasing the percentage of the incident radiation that is absorbed by the absorbent element.

5 However, the pixel structure of conventional focal plane matrices based on bolometers, as described in US Pat. No. 6,307,194, still allows a substantial percentage of the reflected radiation to pass through and exit the absorbent without being absorbed. On the other hand, in conventional focal plane matrices based on bolometers, the absorbent has a resistive absorption layer having a typical thickness of 5.0 nm or more using current manufacturing and deposition techniques. Consequently, the absorber in conventional focal plane matrices based on bolometers has a resistivity of 200-350 ohms per square, which limits the absorption capabilities of the conventional pixel structure.

Therefore, an improved absorbent is needed that overcomes the problems mentioned above and others

15 previously experienced for bolometers. In particular, a bolometer is needed that has an absorbent with high resistivity and increased absorption capabilities to effectively absorb substantially all of the incident radiation.

US 5602393 shows a microbolometer detector element, which includes an optically absorbent material structure that absorbs a portion of incident radiation, and has a plurality of openings that diffract other portions of the incident radiation. The absorbent structure has an electrical resistivity that varies depending on its temperature. An optical radiation director redirects the diffracted optical radiation back to the absorbent structure. The distance between the radiation director and the absorbent structure is tuned so that, for a predetermined design wavelength, the optical radiation that is redirected back to the

The absorbent structure constructively interferes with incident optical radiation that is not reflected by the absorbent structure. Constructive interference causes the absorbent structure to absorb substantially more optical radiation at the design wavelength than at other wavelengths. Redirected radiation also destructively interferes with the incident optical radiation that is reflected by the absorbent structure, thereby suppressing the radiation reflection. A circuit is connected to the absorbent structure to measure its electrical resistivity.

EP 0859413 A2 shows a two-dimensional infrared focal plane matrix of temperature sensing units. The temperature detection units are arranged for each pixel in a two-dimensional arrangement in a semiconductor substrate. Each temperature detection unit is integrally formed

35 with reading circuits. Each temperature sensing unit has a temperature sensing portion that is supported by support legs made of a high thermal resistance material to reduce heat flow to the semiconductor substrate. Each temperature sensing unit also has a temperature sensing element with an infrared ray absorbing portion that is spliced by at least one splice pillar with the temperature sensing element.

US 2004053435 A1 shows a method for manufacturing an electronic device that includes the steps of: preparing a cavity defining a sacrificial layer, of which at least the upper surface is covered with a chemical attack stop layer; forming at least a first opening in the chemical attack stop layer, thereby partially exposing the surface of the cavity defining the sacrificial layer; attack

Chemically the cavity defining the sacrificial layer through the first opening, thereby defining a provisional cavity under the chemical attack stop layer and a support portion that supports the chemical attack stop layer thereon; and removing a portion of the chemical attack stop layer by chemical attack, thereby defining at least a second opening that reaches the provisional cavity through the chemical attack stop layer and expanding the provisional cavity to a final cavity.

EP 0867701 A1 shows a method of manufacturing an infrared detector, a method for controlling the voltage in a layer of polycrystalline SiGe and an infrared detector device. The manufacturing method includes the steps of forming a surface layer on a substrate; model the sacrificial layer; establish a layer consisting essentially of polycrystalline SiGe in the sacrificial layer; deposit an absorbent of

Infrared in said polycrystalline SiGe layer; and then remove the sacrificial layer. The method of controlling the tension in a layer of polycrystalline SiGe deposited on a substrate is based on varying the deposition pressure. The infrared detector device comprises an active area and an infrared absorber, in which the active area comprises a layer of polycrystalline SiGe and is suspended above a substrate.

Summary of the invention

A pixel structure according to the present invention is described in claim 1. More advantageous embodiments are described in dependent claims 2-15.

65 Brief description of the drawings

The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate an implementation of the present invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings:

Figure 1 is a perspective view of an exemplary pixel structure of a focal plane matrix consistent with the present invention;

Figure 2 is a cross-sectional view in side elevation of the pixel structure along the line A-A indicated in Figure 1;

Figure 3 is an exemplary expanded exploded view of a section of the pixel structure with reference C in Figure 2, where the pixel structure section represents a recess or channel formed in an absorption layer of the pixel structure according to with the present invention;

Figure 4 depicts a top level view of the pixel structure plot with a bolometer with an absorbent formed in accordance with an example that is not part of the present invention, in which the absorbent is formed without a recess or channel ;

Figure 5 represents a top level view of the layout of a pixel structure with a bolometer with an absorbent formed in accordance with the present invention, in which the absorbent is formed to have a first predetermined number of channels formed through it. ;

Figure 6 represents a top level view of the layout of a pixel structure with a bolometer with an absorbent formed in accordance with the present invention, in which the absorbent is formed to have a second

The predetermined number of channels formed through it and the second number of channels is greater than the first number of channels; Y

Figure 7 is a graph representing the percentage of radiation reflected (or not absorbed) versus the radiation wavelength for a prior art bolometer, the bolometer of Figure 4, the bolometer of Figure 5, and the bolometer of figure 6.

Detailed description of the invention

Reference will now be made in detail to an implementation according to methods, systems and products consistent with the present invention as illustrated in the accompanying drawings.

Figure 1 is a perspective view of an embodiment of a pixel structure 100 consistent with the present invention. Figure 2 illustrates a cross-sectional view in side elevation of the pixel structure 100. As shown, the pixel structure 100 includes a substrate 102 and a bolometer 104 disposed on the substrate. As those skilled in the art know, a focal plane array typically includes an array of pixel structures for use in an imaging device, such as an uncooled infrared detector. Although the matrix may have different sizes, such as 320 rows by 240 columns, the focal plane matrix of a common example includes an array of pixel structures arranged in 640 rows by 240 columns. For the purposes of the illustration, however, a single pixel structure is represented with the understanding that the

Other pixel structures of the focal plane matrix can be constructed in a similar manner.

Although not illustrated, the focal plane matrix for an infrared detector is typically disposed within a housing that is sealed to establish a vacuum chamber. The housing typically includes a window formed of a material transparent to infrared radiation so that the incident of infrared radiation on the housing can be felt by one or more pixel structures of the focal plane matrix. See, for example, US 625229B1 which describes housings and techniques for sealing the housings in which the focal plane matrices are arranged, the contents of which are incorporated herein by reference.

The substrate 102 is a microelectronic substrate and, as such, is typically formed of silicone although they can be

55 use other materials. The circuit is typically formed above or below the substrate surface in a manner known to those of ordinary skill in the art in order to adequately provide process signals and signals that are received from the respective pixel structures, thus allowing each structure of pixel is interrogated to determine the radiation incident. As shown in Figure 1, in conjunction with an embodiment in which the focal plane matrix includes a pixel structure matrix 100, the circuit preferably includes row and column selection elements 106 to allow other individual pixel structures To be addressed. As is known to those skilled in the art, the circuit may also include another circuit arranged on the substrate in electrical communication with the row and / or column selection elements to process the signals transmitted or received from the respective pixel structures.

The bolometer 106 includes a transducer 108 with a separation ratio with respect to the substrate 102 and an absorbent 110 supported in a separation relationship with respect to both the substrate and the transducer. He

The absorbent has a thermal connection (for example, through a position 111 in Figures 1-2) with the transducer allowing the radiation absorbed by the absorbent to heat the transducer. Since the absorbent 110 is separated from the transducer 108, the characteristics of the transducer and the absorbent can be optimized individually. For example, the absorption characteristics of the absorbent can be maximized to increase the sensitivity of the respective pixel structure. As described herein further, absorbent 110 has one

or more recesses or channels (for example, 112a and 112b) in the absorbent 110 which effectively increases the resistivity, sheet strength, and absorbability of the absorbent 110.

The transducer 108 is formed of a material that has an electrical resistance that varies in response to changes in its temperature. For example, transducer 108 may be formed of vanadium oxide VOx, titanium oxide TiOx, or other material having an electrical resistance that varies predictably in a significant way in response to changes in its temperature. The transducer 108 may be disposed on an insulating layer 114 that serves to protect and support the transducer 108. While the material forming the insulating layer is electrically insulating, the material is also preferably selected and the insulating layer is preferably molded to help

15 in thermally insulating the transducer from the substrate 102, thereby reducing the thermal loss to the substrate. The insulating layer 114 may be formed of SiO2 silicone dioxide, or other molded insulating material to define a pair of legs 116 with a meandering design to increase the thermal insulation of transducer 108. For example, US 6,307,194 describes the formation of a pair of legs 116 with a meandering design to support a transducer on a substrate, the contents of which are incorporated herein by reference. Although not illustrated, transducer 108 may also be covered with a protective layer, just like another insulating layer formed of a silicon dioxide layer, a nitride layer, or a polyamide passivation layer. For the purposes of the description, the transducer element 108 as well as the surrounding insulating layers will be collectively referred to as the transducer layer 109.

25 As shown in Figure 2, the bolometer 104 may also include a reflector 118 disposed on or in the substrate 102 and below the transducer 106 such that the reflector 118 also underlies at least a portion of the absorbent 110. For example, then that the circuit on the substrate 10 has been formed, as on receiving the substrate from a foundry IC, a layer of material is modeled on the surface to be the reflector. This layer defines the lower side of the optical cavity for absorption. The upper side of the cavity is defined by the absorbent 110 formed later in the manufacturing process as described herein in more detail. The reflector 118 may be formed from a variety of materials, such as aluminum or titanium.

As shown in Figure 2, the transducer layer 109 is spaced from the underlying substrate 102 by a gap G1. Although the G1 gap can have different sizes without leaving the spirit and scope of the present

In the invention, the gap is preferably within a range of 0.8 to 1.2 microns. The absorbent 110, in turn, is spaced at a predetermined distance from the transducer layer 109. In one implementation, the absorbent 110 is spaced by a gap G2, which is within a range of 0.8 to 2.4 microns from the transducer layer 109. The combined thickness G3 of the two gaps G1 + G2 has a combined separation of about a quarter of the wavelength of the radiation that the absorbent 110 is designed to preferentially absorb, such as about 2 to 3 , 5 microns for an absorbent designed to preferably absorb infrared radiation having a wavelength of 8 to 14 microns.

The transducer layer 109 may be spaced from the substrate 102 in a variety of ways. For example, the insulating layer 114 may include a pair of legs 116 that are angled or inclined downwardly to the substrate 102.

Alternatively, the legs 116 of the bolometer can be extended as shown in Figure 1 to respective abutments 120 that extend outwardly orthogonally from the substrate 102 to support the transducer layer 109 above the substrate 102. As the Transducer layer, the abutments 120 are typically a composite structure formed of an electrically conductive material, such as chromium or nickel-chromium (commonly known as Nichrome or NiCr), which extends through an electrically insulating material such as SiO2, with in order to provide electrical contact between the transducer element 108 and the circuit disposed above the substrate 102. Thermally isolating the transducer element 108 from the substrate, the thermal loss from the bolometer to the substrate is reduced.

The bolometer further includes conductive fingerprints 122 extending from the transducer element 14 to about

55 respective pillars 22 and, more particularly, to the electrically conductive portion of each pillar, thereby electrically connecting the transducer element to the circuit disposed above the substrate 102. As described above in conjunction with the transducer element, the conductive fingerprints are deposited generally above the insulating layer 112 and, although not illustrated, the conductive fingerprints may also be covered with another insulating layer in order to protect the conductive fingerprints and the signals that propagate along it. As described below, the conductive fingerprints are more preferably deposited on the winding legs 116 of the insulating layer 114 in order to extend from the transducer element 108 to the respective pillars 120. As explained below, the circuit is therefore It can be controlled to pass the current through the transducer element 108 so that the resistance of the transducer element can be monitored accordingly, thereby providing a measure of the radiation incident on the bolometer.

65 As noted above, absorbent 110 is spaced from transducer layer 109 by means of

one or more posts 111, which function as a thermal connection between the absorbent 110 and the transducer 108. The single or more posts 111 also allow the absorbent to cover the transducer in an umbrella-like configuration. In the implementation shown in Figure 2, the bolometer includes a single post 111 extending outwardly from the transducer element 108 in order to support a central portion of the absorbent 110 in a spacing relationship with respect to the transducer 108. However, in an alternative implementation, the bolometer 104 may employ one or more posts 111 located in positions other than a central portion of the absorbent 110. Each post 111 is formed of a conductive material. In one implementation, each post 111 is formed of SiO2 silicone dioxide, which can also be used to form the insulating layer 114 covering the transducer. However, in this implementation, each post 111 has a size and shape that allows heat to be effectively transferred from the absorbent 110 to the transducer element 108. Conversely, the legs 116 of the insulating layer 114 has a size and shape that effectively limits the heat transmission between the transducer element and the substrate 102. In this sense, the length of the respective post 111 is generally much less than the width of the post so that the post is quite thermally conductive. For example, while each post 111 may have a variety of sizes, in one implementation the post has a length that is equal to or less than 1 micron and a width that is equal to or greater than 4 microns. In contrast, the length of each leg of the insulating layer is many times greater than the respective width. While multiple posts can be used to support the absorbent in a separation relation with respect to the transducer layer, the entire thermal mass of the post (s) is preferable and relatively small with respect to the absorbent and the transducer in order to allow The time constant of the bolometer is also relatively small. The thermal mass of the absorbent 110 and the transducer layer 109 must also be small to produce a faster thermal time constant. The thermal time constant is defined as the thermal mass of the pixel divided by the thermal conduction of the pixel. Thus, a reduction in thermal mass leads directly to a smaller / faster thermal time constant. As described in more detail below, a recess or channel 112a or 112b in the absorbent 110 advantageously contributes to a reduction in thermal mass and therefore to a smaller / faster time constant

25 for bolometer 104.

As shown in Figures 1-2, the absorber 110 comprises an absorption layer 124 formed of a material that strongly absorbs radiation of a predetermined wavelength or wavelength range of interest, such as infrared radiation with a length wave from 8 to 14 microns. For example, the absorption layer124 may be formed of nichrome (NiCf) or other highly resistant metal or alloy. In addition, to increase the sheet strength of the absorbent and improve the absorption capacities of the absorption layer 124, the absorption layer 124 is formed to have a thickness (eg, reference "d1" in Figure 3) which is smaller 10.0 nm and preferably within a range of 1.5 to 5.0 nm. The effect of the thickness (d1) of the absorption layer 124 on the sheet strength (Rs) of the absorption layer124 is described below. The resistance

35 (R) of the absorption layer 124 is reflected in equation 1:

R = Rho * L / A (1)

where Rho is the resistivity of the absorption layer 124 and L and A are its length and cross-sectional area, respectively. If W is the width of the absorption layer 124 and d1 its thickness (i.e., -A = Wt), then the resistance can be represented as shown in equation 2:

R = (Rho / d1) (L / W) = Rs (L / W) (2)

where Rs = Rho / d1 is the sheet resistance of the absorption layer 124. Since L / W has no units and the resistance R is expressed in ohms, to avoid confusion between the resistance R and the sheet resistance Rs of a

45 layer, the sheet resistance Rs is commonly specified in unit of "ohms per square" and the ratio L / W has references as the number of square units (of any size) of material in the resistor. Accordingly, the sheet strength (Rs) of the absorption layer 124 can be increased independently of the cross-sectional area or the size of the absorption layer 124 by forming the absorption layer 124 to be as thin as possible using techniques of known deposition. In an implementation in which the absorption layer 124 is formed of NiCr to be 10.0 nm thick, the absorption layer 124 has a sheet resistance of 100 ohms per square. In another implementation in which the absorption layer124 is formed of NiCr to have a thickness of 5.0 nm, the absorption layer 124 has a sheet resistance of approximately 400 ohms per square.

However, when the absorption layer 124 is formed of highly resistant metal or alloy material (such as NiCr) to have a thickness of 10.0 nm or less, the absorption layer 124 becomes less rigid and may require a base layer of support for maintaining the gap G2 between the absorbent 110 and the transducer 110. Accordingly, the absorbent 110 may include a base layer 126 on which the absorption layer 124. is formed. In this implementation, the base layer 126 is formed in the post 111. The base layer 126 is formed of a rigid and light material, (such as SiOx silicone oxide, or silicone nitride, SixNy) that maintain its shape in a relatively thin thickness (for example, "d2" in the figure 3) so that the base layer 126 maintains the spatial relationship (for example, the gap G2) between the absorbent 110 and the transducer layer 109. In one implementation, when the absorption layer124 is formed to have a thickness d1 which is equal to or less than 10.0 nm the c apa base 126 or the

Absorbent 110 is formed of SiOx, to have a thickness that is equal to or greater than 100.0 nm.

As shown in Figures 1-3, the absorbent 110 may also comprise a protective layer 128 formed on the absorption layer 124 so that the absorption layer124 is sandwiched between the base layer 5 126 and the protective layer 128. The layer Protective 128 may be formed of SiOx, SixNy, or other passivation type material. In one implementation, when the protective layer 124 is formed to have a thickness d1 that is equal to or less than 10.0 nm, the base layer 126 of the absorbent 110 is formed of SiOx, to have a thickness d3 that is equal

or greater than 40.0nm.

It is noted that in Figures 1-3 the base layer thickness d2, the absorption layer thickness d1 and the protective layer thickness d3, are not drawn to scale. While the absorption layer has a substantially smaller thickness than both the base layer and the protective layer, for purposes of the illustration, the thickness of the absorption layer is visibly discernible from the base layer and the protective layer.

As previously discussed, a significant portion (for example, 20% or more) of infrared radiation that is incident on a conventional bolometer passes through and is not absorbed by the absorbent element. The reflector in the bolometer reflects the portion of non-absorbed radiation back towards the absorbent element. However, a significant portion (for example, 20% or more) of the reflected radiation is not absorbed by the absorbent element of the conventional bolometer during its second pass through the absorbent element, but passes through and exits the absorbent element.

In accordance with the present invention, the absorption layer 124 of the bolometer 104 is formed of highly resistive metal or alloy material (such as NiCr) to have a thickness d1 of 10.0 nm or less, which significantly improves the absorption capacity of the bolometer 104 on the conventional bolometer so that the

25 bolometer 104 absorbs more radiation incident on an upper side 130 of bolometer 104 and more radiation reflected back to bolometer 104 through reflector 118. However, the absorption layer 124 of bolometer 104 may still allow a portion (e.g., less than 20%) of reflected radiation to pass through and out of the absorption layer124 without being absorbed.

Accordingly, to improve the sheet strength and absorption capacity of the absorbent 110, the absorbent 110 is formed to have one or more recesses or channels 112a and 112b defined by the upper side of the absorbent 110 as shown in Figures 1- 3. Each recess or channel 112a and 112b is adapted to affect the propagation path of a portion of radiation received by the absorbent so that the radiation portion is absorbed by the absorbent 110 rather than leaving the absorbent 110. Each recess or channel 112a and 112b

35 has a characteristic width or length (w) that is less than a predetermined wavelength or a predetermined wavelength range (eg, the infrared wavelength band of 8 to 14 microns) associated with the portion of radiation to be absorbed by the absorption layer 124. In one implementation, each recess or channel 112a and 112b is formed to have a characteristic width or length (w) that is within a range of 1.4 microns to 3 microns in shape. that the absorber 110 is adapted to effect the radiation propagation path received by the absorber 110 that has a wavelength equal to or greater than 3 microns and, thus, those wavelengths that are in the infrared band of 8 to 14 microns .

As shown in Figure 3, a portion of the radiation 302 that is incident on the upper side 130 of the absorption layer 124 and that has a wavelength greater than the width (w) of the recess or channel 112a can

45 propagate in a path to the recess or channel 112a. Since the recess or channel 112a has a characteristic width or length (w) that is less than the wavelength of the incident radiation 302, the propagation path of the incident radiation 302 is altered or redirected to allow the absorption layer 124 to absorb the radiation portion 302 instead of exiting the absorbent 110. Furthermore, as shown in Figure 3, each recess or channel 112a and 112b may be arranged in the absorbent 110 relative to the reflector 118 such that the respective recesses 112a and 112b effect the radiation propagation path 304 reflected by the reflector 118 towards the absorbent 110 so that the reflected radiation 110 is absorbed by the absorption layer 124 instead of leaving the absorbent 110.

The shape of the recess or channel 112a or 112b may have a square, trapezoidal, rectangular, circular or any other shape.

Another with a characteristic width or length (w) that can be calibrated to be less than a predetermined wavelength or a range of predetermined wavelengths of interest. The characteristic length for a circular recess or channel 112a or 112b, for example, is its diameter. Although the absorption layer 124 is represented with channels 112a and 112b formed as a respective passage through the upper side 130 and a lower side 132 of the absorbent 110, the channels 112a and 112b can be formed as recesses on both the upper side 130 or lower side 132 of the absorbent 110 without forming a passage to the other side 132 or 130. The recesses or channels 112a and 112b can be formed in the absorbent 110 using a photolithographic pattern and chemical attack techniques.

An additional advantage of increased sheet strength (Rs) of the absorption layer124 is achieved by selectively removing the base layer material (for example, SiOx) and / or the protective layer material (for example, SiOx) 65 same as the material of absorption layer 124 (for example NiCr) to form the recesses or channels 112a and 112b. When the absorption layer 124 is deposited or forms a predetermined thickness d2, the sheet strength (Rs =

Rho / d1) is increased by reducing the percentage of material density of absorbent 110 as reflected in equation 3:

Rs with channels = (Rs without channels) / (% density of material remaining) (3)

5 For example, in an implementation of the bolometer 104, as discussed below, the absorption layer 124 was formed by a thickness d1 corresponding to 350 ohms per square before forming channels 112a and 112b. In this implementation, a plurality of channels 112a and 112b were formed around a center of the absorbent 110 so that the percentage of material density remaining or volume of the absorbent 110 was a corresponding half (1/2) or 50 percent. The sheet resistance Rs with channels 112a and 112b was increased to 700

10 ohms per square according to equation 3 as shown below:

Rs with channels (700 ohms per square) = Rs without channels (300 ohms per square) / 50% density of remaining material

To measure the increased absorption capacity of a bolometer formed in accordance with the present invention against a prior art bolometer, four were manufactured and tested under the same radiation conditions four

15 different pixel structures with four different bolometers. The first pixel structure (not shown in the drawings) was manufactured to have a conventional bolometer structure as described in Figures 1-2 of US 6,307,194. The conventional bolometer was formed to have a NiCr absorption layer with a thickness of 10.0 nm corresponding to a sheet resistance (Rs) of 200 ohms per square.

Figure 4 represents a top level view of the arrangement of the second pixel structure 400 manufactured and tested. The second pixel structure 400 was manufactured according to an example that is not part of the present invention to have a bolometer 402 consistent with the bolometer 104 of the pixel structure 100, except the absorber 404 (upper side in view of the Figure 4) of bolometer 402 was formed without a recess or channel 112a and 112b. Post 406 (consistent with post 111) of bolometer 402 is shown through absorbent 404

25 in Figure 4, but it is not a recess or channel of the absorbent 404. The absorbent 404 was formed to have a NiCr absorption layer with a thickness of approximately 5.7 nm, which corresponds to a sheet resistance (Rs) 350 ohms per square.

Figure 5 represents a top level view of the third pixel structure 500 manufactured and tested. The

Third pixel structure 500 was manufactured in accordance with the present invention to have a bolometer 502 consistent with bolometer 104 of pixel structure 100, where absorber 504 of bolometer 502 was formed to have seventy eight (78) channels 508 (consistent with channels 112a and 112b) formed in or through it. Channels 508 formed around a center of absorbent 502. Each of channels 508 was formed to have a characteristic width or length (w) of approximately 1.4 microns, which is less than

35 predetermined wavelength of 8 microns associated with the beginning of the infrared radiation band (for example, 8-14 microns). The post 506 (consistent with the post 111) of the bolometer 502 reflects the central position of the absorbent 502. The post 506 of the bolometer 502 is shown through the absorbent 504 in Figure 5 but is not a recess or channel of the absorbent 504. The Absorber 504 was formed to have a NiCr absorption layer with a thickness of approximately 5.7 nm, which corresponds to a sheet resistance (Rs) of 350 ohms per square

40 before the 78 channels were formed in the absorbent 504. As shown in Figure 5 the channels 508 collectively displace approximately one third of a volume of an absorbent 504 so that approximately two thirds of the absorbent material 504 remains, resulting in an increase in sheet resistance from 350 ohms per square to approximately 525 ohms per total square, or 700 ohms per square in proximity to the channels according to the present invention.

Figure 5 represents a top level view of the layout of the fourth pixel structure 600 manufactured and tested. The fourth pixel structure 600 was manufactured in accordance with the present invention to have a bolometer 602 consistent with the bolometer 104 of the pixel structure 100, where the absorber 604 of the bolometer 602 was formed to have one hundred three (103) channels 608 ( consistent with channels 112a and 112b) formed in or through the

50 same. Channels 608 formed around a center of absorbent 602. Each of channels 608 was formed to have a characteristic width or length (w) of approximately 1.4 microns, which, as noted above, is less than a length of Default 8 micron wave associated with the beginning of the infrared radiation band (for example, 8-14 microns). The post 606 (consistent with the post 111) of the bolometer 602 reflects the central position of the absorbent 602. Although the post 606 of the bolometer 602 is visible through the

Absorber 604 in Figure 6, post 606 is not a recess or channel of absorbent 604. Absorber 604 was formed to have a NiCr absorption layer with a thickness of approximately 5.7 nm, corresponding to a resistance of sheet (Rs) of 350 ohms per square before 103 channels were formed in absorbent 604. As shown in Figure 6, channels 608 collectively displace approximately half of the volume of absorbent 604 so that approximately half of the material absorbent 604 remains, resulting in

An increase in sheet resistance from 350 ohms per square to approximately 700 ohms per square in accordance with the present invention. Although not shown in the drawings, the umbrella structure of an absorbent formed in accordance with the present invention may have a sheet strength of approximately 1000 ohms per square when the absorption layer has a thickness (d1) of approximately 100 nm or less, the base layer has a thickness (d2) of about 90.0 nm or greater, the protective layer has a thickness (d3) of about 40.0 nm or greater and the absorption layer is formed to have a channel density four fifths or less (that is, the density of the remaining absorption layer material is one fifth or more) based on the thickness of the absorption layer.

Each absorber (for example, the prior art absorbent and the absorbents 404, 504 and 604) of the four different pixel structures was radiated with the same level of radiation for wavelengths of 8 to 16 microns. The amount of radiation reflected by the respective absorber was measured by each radiation wavelength and separately plotted on the graph depicted in Figure 7. It should be noted that the reflected radiation measured

10 represents the radiation that was not absorbed by the respective absorber or when the radiation initially passes through the absorbent or after being reflected by the reflector implemented in the respective bolometer. A reflectance of "0" in Figure 7 represents no radiation exiting the absorbent or, in other words, 100% of the radiation was absorbed by the absorbent. Conversely, a reflectance of "1" (not shown in Figure 7) indicates that all radiation was reflected by the absorber or, in other words, 0% of the radiation

15 was absorbed by the absorbent.

As shown in Figure 7, graph 702 of the portion of radiation reflected (instead of absorbed) by the absorbent of the conventional bolometer discussed above is approximately 20% or more than graphs 704, 706 and 708 of the respective portion of radiation reflected (instead of absorbed) by bolometers 402, 502 and

20 602 manufactured in accordance with the present invention. The difference in absorption capacity between the absorbent of the conventional bolometer and the absorbent 402 without recesses or channels in or through it is attributed to the difference in structure of the respective absorbents and the difference in thickness of the respective absorbent layers.

25 Graph 704 of the portion of radiation reflected (instead of absorbed) by the absorber of bolometer 402 without recesses or channels in or through it is significantly larger than graphs 706 and 708 of the respective portion of reflected radiation (instead absorbed) by bolometers 502 and 602 manufactured in accordance with the present invention to have channels 508 and 608 especially within the wavelength range of 9 to 10 microns. The difference in absorption capacity between absorber 404 of bolometer 402 and each of the

302502 or 602 absorbents are attributed to the recesses or 508 or 608 channels formed in the respective 502 absorbents and

602. The recesses or channels 508 or 608 each have a characteristic width or length (w) that is less than 8 microns or less than any of the radiation wavelengths in the 8-16 micron wavelength band used to test the absorbers as shown in Figure 7. Thus, the recesses or channels 508 or 608 were able to refract or effect the propagation path of a substantial portion (for example, approximately 95% on average) of radiation received by the absorbent 504 and 604 so that the radiation portion is absorbed by the absorbent instead of allowing it to exit the absorbent. In the range of 9 to 10 microns, the effect of recesses or channels 508 and 608 is even more substantial. The oxide material used to form the base layer and protective layer for the absorber 404 without recesses or channels has an impact on the ability of a portion of the radiation within the range of 9 to 10 microns incident on the absorber 404 to reach and, thus , 40 being absorbed by the absorber layer of absorbent 404. As one third or more of the base layer and protective layer is removed or moved through channels 508 or 608 of absorbent 504 or 604, the remaining base layer material and material The protective layer of the absorbent 504 or 604 affects less the ability for wavelengths of 9 to 10 microns to reach the absorption layer due to channels 508 and 608. Channels 508 and 608 also significantly decrease the thermal mass of the absorbent, allowing the respective 502 or 602 bolometer to have a

45 faster thermal time constant.

In addition, graph 706 of the portion of radiation reflected (instead of absorbed) by absorber 504 of bolometer 502 with a predetermined number of channels 508 (i.e., 78 channels) is also significantly higher than graph 708 of the respective portion of radiation reflected (instead of absorbed) by the absorbent

50 604 of bolometer 602 with a second predetermined number of channels 608 (ie 103 channels). Thus, an increase in the density of the channel and reduction in the density of the remaining absorbent material (for example, further reduction in the base layer and protective layer) results in a further increase in the absorption capacity as well as sheet strength of an absorbent (for example, absorbent 604) manufactured in accordance with the present invention.

Claims (11)

1. A pixel structure (100) for use in an infrared imager, comprising: 5 -a substrate (102), and

-

a bolometer (104) comprising:

a transducer (108) having a separation relation with respect to said substrate, the transducer having an electrical resistance that varies in response to changes in the temperature of the transducer, and an absorbent (110) having a thermal connection with the transducer that allows the radiation absorbed by the absorbent to heat the transducer; 15 in which the absorbent has an upper side (130) defining a recess (112a, 112b) in the absorbent, the recess being adapted to affect the propagation path of a portion of radiation received by the absorbent such that the radiation portion is absorbed by the absorbent instead of leaving the absorbent; characterized in that the absorbent has a separation relation with respect to the transducer and includes an absorption layer 20 (124) having a thickness that is 10 nanometers or less. 2. A pixel structure according to claim 1, wherein the recess is a channel through the absorbent. A pixel structure according to claim 1, wherein the radiation portion corresponds to a range of and the recess has a width less than the range of wavelengths. 4. A pixel structure according to claim 1, wherein the bolometer further comprises a reflector (118) arranged on the substrate and under the transducer, in which the recess is arranged relative to the reflector of 30 such that the recess effects the radiation propagation path reflected by the reflector towards the absorbent so that the reflected radiation is absorbed by the absorbent instead of leaving the absorbent. 5. A pixel structure according to claim 1, wherein the absorbent covers the transducer in a umbrella type configuration. 35 6. A pixel structure according to claim 1, wherein the recess is one of a plurality of channels (112a, 112b) through the absorbent. 7. A pixel structure according to claim 6, wherein the channels are spaced about 40 an absorbent center. 8. A pixel structure according to claim 1, wherein the thickness of the absorption layer is in a range of 5 nanometers to 10 nanometers. A pixel structure according to claim 1, wherein the recess is one of a plurality of channels through the absorbent and spaced around the absorbent such that the absorption layer has a sheet resistance within the range of 350 ohms per square to 1000 ohms per square. 10. A pixel structure according to claim 1, wherein the recess is one of a plurality of 50 channels through the absorbent and the channels collectively displace a third or more of a volume of the absorption layer. 11. A pixel structure according to claim 1, wherein the recess is one of a plurality of recesses in the absorbent and the collective density of the recesses is selected such that the absorbent 55 has a sheet resistance that is equal to or greater than 400 ohms per square. 12. A pixel structure according to claim 1, wherein the absorbent includes a base layer (126) disposed below the absorption layer and with a thickness equal to or greater than 90 nanometers. 13. A pixel structure according to claim 12, wherein the bolometer further comprises a pole (111) extending between the transducer and the absorbent base layer to support the absorbent in the separation relationship with the transducer. 14. A pixel structure according to claim 1, wherein the absorption layer comprises NiCr.