CN102575961A

CN102575961A - Terahertz detector comprising a capacitively coupled antenna

Info

Publication number: CN102575961A
Application number: CN2010800473984A
Authority: CN
Inventors: D·高恩; T·莫尔弗; I·博格; D·伊拉德
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2009-10-23
Filing date: 2010-10-21
Publication date: 2012-07-11
Also published as: WO2011048170A1

Abstract

A THz radiation detector (10) comprises a plurality of antenna arms (12) separated from a suspended platform (16) by an isolating thermal air gap (18). The detector functions to concentrate THz radiation energy into the smaller suspended MEMS platform (e.g., membrane) upon which a thermal sensor element (24) is located. The THz photon energy is converted into electrical energy by means of a pixilated antenna using capacitive coupling in order to couple this focused energy across the thermally isolated air gap and onto the suspended membrane on which the thermal sensor is located.

Description

Terahertz detector comprising a capacitively coupled antenna

Technical Field

The present invention relates to the field of semiconductor imaging devices, and in particular to monolithic passive THz (terahertz) detectors.

Background

THz radiation imaging is a research area that is currently evolving at exponential speed, with inherent applications like THz security imaging, which can reveal weapons hidden under clothing from distances of ten meters away, or medical THz imaging, which can reveal skin cancer tumors hidden under skin, for example, and perform fully safe dental imaging. Constructing prior art THz detectors is often a challenging task because both the radiation source and the radiation detector are complex, difficult and expensive to manufacture.

THz radiation is non-ionizing and therefore, unlike X-ray radiation, it is completely safe for humans. THz imaging for security applications uses, for example, passive imaging techniques, i.e., the ability to perform remote THz imaging without the use of any THz radiation source, and thus relies solely on the very low power of natural THz radiation that would normally be emitted from any room temperature volume, in accordance with the well-known physics of black body radiation. Passive THz imaging requires extremely sensitive sensors to remotely image this very low power radiation. Prior art passive THz imaging utilizes a hybrid technique of superconducting single detectors cooled to a temperature of about 4 degrees Kelvin, which makes the system extremely complex (e.g., it takes more than 12 hours to merely adjust the temperature before any imaging can be performed) and expensive (e.g., 10 ten thousand dollars or more). A detector that can be used to detect THz radiation and that has a much lower potential cost than existing superconducting solutions is desirable. However, passive THz imaging requires three orders of magnitude higher sensitivity than passive Infrared (IR) imaging, which is a challenging gap.

Disclosure of Invention

In one embodiment of the invention, the THz radiation detector comprises a plurality of antenna arms separated from a suspended platform by an insulating air gap. The detector is used to concentrate THz radiation energy into the smaller suspended MEMS platform (e.g., membrane) where the thermal sensor elements are located. THz photon energy is converted to electrical energy by means of pixilated antennas (pixilated antenna), capacitive coupling being used to couple this concentrated energy across the insulating air gap onto the suspended membrane on which the thermal sensor is located.

This detector mechanism achieves a much stronger focused THz-induced heating of the suspended membrane, so that even if the detector is operated at room temperature, this thermal signal becomes much stronger than the detector temperature noise. This much higher thermal signal than thermal noise is then converted by the thermal sensor element into an electrical signal much higher than electrical noise.

Thus, in accordance with the present invention, there is provided a terahertz (THz) detector comprising a dielectric substrate, an antenna fabricated on the substrate, and a suspended platform comprising a thermal sensor for receiving THz radiation concentrated by the antenna via capacitive coupling and for converting the received THz radiation into an electrical signal, wherein the capacitive coupling provides thermal isolation between the antenna and the thermal sensor.

In accordance with the present invention, there is also provided a terahertz (THz) detector comprising a dielectric substrate, an antenna fabricated on the substrate and configured to receive THz radiation, a load resistor capacitively coupled to the antenna, and a suspended platform comprising a thermal sensor thermally isolated from the antenna and thermally coupled to the resistor and configured to convert THz radiation collected by the antenna into an electrical signal.

In accordance with the present invention, there is further provided a method of detecting terahertz (THz) radiation, the method comprising: providing an antenna fabricated on a dielectric substrate and configured to receive THz radiant energy, coupling the THz radiant energy received by the antenna to a resistor, thermally coupling heat generated by the resistor to a suspended platform comprising a thermal sensor thermally isolated from the antenna and thermally coupled to the resistor, and converting THz radiation incident on the thermal sensor into an electrical signal.

Drawings

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

fig. 1 is a diagram illustrating one exemplary embodiment of a passive THz radiation detector;

fig. 2 is a circuit diagram illustrating an equivalent circuit of the THz radiation detector of fig. 1;

FIG. 3 is a diagram illustrating a metal pattern layer of a wide bandwidth reflector structure of a detector;

figure 4 is a diagram illustrating multiple layers of a wide bandwidth reflector structure of a detector;

FIG. 5 is a diagram illustrating a side view of the THz radiation detector of FIG. 1;

FIG. 6 is a circuit diagram of a first exemplary embodiment of a thermal sensor of the THz detector;

FIG. 7 is a circuit diagram of a second exemplary embodiment of a thermal sensor of the THz detector;

FIG. 8 is a circuit diagram of a third exemplary embodiment of a thermal sensor of the THz detector;

FIG. 9 is a circuit diagram of a fourth exemplary embodiment of a thermal sensor of the THz detector;

fig. 10 is a diagram illustrating a second exemplary embodiment of a passive THz detector;

fig. 11 is a diagram illustrating an exemplary 2 × 2 pixel matrix using the THz detector of fig. 10;

FIG. 12 is a diagram illustrating an enlarged view of an exemplary THz detector including a support arm structure and a suspended thermal sensor;

fig. 13 is a diagram illustrating a side view of the THz radiation detector of fig. 10; and

fig. 14 is a diagram illustrating a side view of a supporting arm portion of the THz radiation detector.

Detailed Description

A diagram illustrating one exemplary embodiment of a passive THz radiation detector is shown in fig. 1. The THz radiation detector embodiment, generally designated 10, includes an antenna bar 12, a capacitive coupling gap 18, a suspended platform 16, a resistor 22, a thermal sensing element 24, a support arm 14, and a substrate 28. The etched area, indicated by 29, provides isolation from other detectors in the surroundings.

The THz detector 10 utilizes electromagnetic coupling technology to first absorb optical energy (i.e., THz energy) through an antenna 12 (which in this particular example comprises a cross-dipole bow-tie antenna), wherein the antenna 12 is used to convert the optical energy to electrical energy, which is then capacitively coupled to a thermally isolated, released thermal sensor element (e.g., diode, transistor, etc.). Capacitively coupling the antenna to the thermal sensor element provides thermal isolation of the sensor from the antenna.

In one exemplary embodiment, a plurality of detectors are arranged to receive THz radiation energy in a 2D array configuration. In this case, the radiation energy received in each pixel of a 2D imaging array, which in one exemplary embodiment is on the order of hundreds of microns in size (e.g., 300 microns square), and concentrated at each pixel at a frequency on the order of 1THz, enters into a much smaller suspended MEMS platform (on the order of tens of microns) on which the THz detector resides (thus, a fully suspended and thermally isolated MEMS structure has minimal thermal mass and thermal conductivity). As described above, this is accomplished by converting THz photon energy into electrical energy using a pixilated antenna, and by coupling this focused antenna energy across an insulating air gap into a suspended platform in which the thermal sensor is located using capacitive coupling. This approach achieves focused THz-induced heating of the suspended platform, so that even if the detector is operated at room temperature, this THz-induced thermal signal becomes much stronger than the detector temperature noise. This thermal signal, which is higher than the thermal noise, is then converted into a signal with a larger electrical signal-to-noise ratio by the sensing active device (e.g., a transistor). In other words, a thermal conduction discontinuity (e.g., capacitive coupling gap 18) is formed between the antenna and the relatively small suspended platform by the MEMS process.

The technique of using capacitive coupling to concentrate antenna energy onto the isolated sub-pixel floating platform 16 can be used with a variety of on-chip pixilated antennas, such as the antenna with the higher bandwidth shown in fig. 10. Concentrating the THz energy via the antenna 12 helps to filter out competing received infrared radiation because infrared radiation not received by the antenna is absorbed by a small suspended platform 16 of a size significantly smaller than the pixel size. Note that the capacitor can be formed by combining several back end of line (BEOL) metal layers of silicon technology and the capacitive coupling can be enhanced by increasing the capacitor area using an interdigitated (i.e., comb-like) structure.

In addition, the detector provides impedance matching across the coupling capacitance between the pixilated antenna 12 and the thermal sensor. It is preferred to use an antenna having a reactive impedance over the bandwidth of interest that cancels out the coupling capacitance. This can be achieved, for example, by using bowtie dipole wires 12 having a length greater than a half wavelength, or by appropriate design of the antenna shown in fig. 10. A more than half wavelength antenna also provides a high impedance of several hundred ohms, which helps match the antenna with the thermal sensor element across a given impedance of the coupling capacitor.

Impedance matching between the antenna and the thermal sensor element is achieved by capacitively coupling the signal to a matching resistor, preferably made of polysilicon. Note that it is preferable to use a cross-shaped resistor made of polysilicon paired with a standard NMOS transistor located near the resistor. Note that in an alternative embodiment, this approach is modified to allow separation of two different polarizations of the received THz radiation. This can be used to identify polarized radiation such as that obtained from reflection from a flat surface.

The cross bow-tie antenna shown in fig. 1 comprises two orthogonal bow-tie antennas 12 shaped and long to create a small amount of reactive impedance over the desired THz imaging spectrum. The energy from the antenna is then capacitively coupled to the sub-pixel floating platform 16 that remains after the etching process. Alternatively, stronger capacitive coupling can be achieved by using several silicon process BEOL metal layers connected by dense vias (not shown) at the edges of the antenna and the platform.

Note that platform 16 comprises a suspended thermally isolated platform having thermal sensing elements (e.g., transistors, diodes, etc.) thereon. The platform is designed to have dimensions typical of existing infrared detectors. The platform is connected to a silicon substrate 28 by a support arm 14. The support arm defines a thermal resistance that, in conjunction with the platform thermal capacitance, adjusts the platform to have a desired thermal time constant suitable for video imaging (i.e., about 70 milliseconds or less).

It should be appreciated that the detector shown in FIG. 1 is one exemplary embodiment for achieving a heat flow discontinuity between the antenna and the micro-platform through a MEMS process. For example, the detector of fig. 1 illustrates the option of having the support arm extend within one of the bowtie trapezoids. It will be appreciated that other arrangements are possible, such as having the support arm extend diagonally beyond the antenna. Alternatively, the support arm may extend in a large circle around the antenna until reaching the silicon substrate, which results in a much lower thermal conductivity of the support arm.

Other possible antenna types include helical antennas, tooth antennas, and slot antennas. Although it is not critical which type of antenna is used, it is preferred that the energy from the antenna is not directly coupled but capacitively coupled to the detector so that the thermal sensor element is thermally isolated.

The capacitive coupling between the suspended thermally isolating platform and the antenna arm can be significantly enhanced if the length of them parallel to each other is increased. To achieve this, the coupling capacitor at the edge of the platform may be configured with a saw-tooth (i.e., comb-like) edge structure, and the antenna strip may be shaped to have a complementary saw-tooth edge structure, so that the structure becomes an interdigitated capacitor structure. This allows the coupling surface between the coupling capacitor and the antenna to be significantly increased without increasing the capacitor area much (i.e., without increasing the thermal capacitance of the coupling capacitor). This same technique can be used if several BEOL metal layers are used to form the coupling capacitor. The above-described coupling capacitance can be increased by making the parallel spacing between the antenna and the stage smaller. However, this depends on the quality of the MEMS process used. Better MEMS processing can reduce this spacing without risking electrical shorting between the antenna and the coupling capacitance metal in the platform.

A circuit diagram illustrating an equivalent circuit of the THz radiation detector of figure 1 is shown in figure 2. The detector, generally designated 50, includes a plurality of antenna bars 52 (e.g., a cross-dipole bow-tie antenna), a coupling capacitor 54, a heating element (e.g., a resistor) 56, and a thermal sensor 58 thermally coupled to the resistor 56.

In one embodiment, the resistors comprise polysilicon resistors and the thermal sensor elements comprise SOI transistors located on the floating platform 16 (fig. 1). In one embodiment, the region labeled 24 comprises the diffusion region of this transistor. In another embodiment, the transistors are made relatively small in size so as to be located at one corner between cross-shaped polysilicon lines.

The portion labeled 28 includes a silicon wafer substrate for support. In one embodiment, the bow-tie antenna is located directly on the silicon wafer, such as where the MEMS post-processing is only used to release the small suspended platform from below and from above. However, some unwanted reflection of THz energy may occur near the antenna due to the high dielectric constant of silicon and associated losses. To prevent this, in an alternative embodiment, the silicon wafer is etched back from under the antenna using Deep Reactive Ion Etching (DRIE) in which the buried oxide layer (BOX) of the SOI silicon process is used as an etch stop to form a large opening. It is therefore preferred to have the bow-tie antenna strip configured to be located within a region of pure silicon oxide and without any silicon underneath. The antenna is then still located within the oxidized and buried oxide layer, which is not separated from the surrounding silicon wafer. Note that in one embodiment, the entire area under the two-dimensional active pixel array is etched using well-known Deep Reactive Ion Etching (DRIE) techniques.

Reactive Ion Etching (RIE) is then applied from the front side to completely release only the suspended platform 16 and its support arms 14. The existing silicon process BEOL metal layer is used as an RIE etch mask.

When forming a 2D pixel array such as that shown in fig. 1 (e.g., an antenna capacitively coupled to a thermally isolated bolometer), it is desirable to reduce electrical crosstalk between pixels. In one embodiment, this may be accomplished by rotating each pixel antenna by 45 degrees relative to the neighboring antennas of adjacent pixels, thereby forming a checkerboard structure in which the cross-shaped antennas are straight or tilted by 45 degrees. This checkerboard structure has the further benefit that it provides more space to build the antenna arms a bit longer, which helps to become more flexible and provide the desired large reactive impedance over the desired THz frequency bandwidth. Note that the above technique can be used with the wide bandwidth reflectors described above, i.e., the metal cross of the reflector can be rotated to form the checkerboard 2D array structure described above where the cross is straight or tilted by 45 °.

To further improve both the sensitivity and spectral selectivity of the THz detector, a high bandwidth back reflector face is added to the pixilated antenna wafer. This back reflector is located on a second plate parallel to the THz detector chip but spaced a specific distance from the back of the THz detector chip using a dedicated spacer. If the metallic back surface is placed with a given separation distance from the antenna, it acts as an effective reflector at a corresponding frequency at which the separation distance is equal to a quarter of the wavelength (calculated in the dielectric between the antenna and the reflective metallic surface). This is sufficient if one wishes to have a good narrow-band reflector at a given frequency. However, for high bandwidth applications (i.e., frequency ratios of about 1: 1.5 or even greater than 1: 2 or 1: 3), an effective reflector across the entire frequency band is desired.

To achieve such a high bandwidth, a back reflector comprising several metal layers is constructed, each patterned as an array of metal crosses as shown in fig. 3, where all layers are constructed within the same background dielectric constant of the encapsulation material. Fig. 3 illustrates a top view of a single metal patterned layer of a high bandwidth reflector structure.

Such a cross-shaped 2D ordered array acts as a filter reflecting back radiation having a half wavelength less than the length of the cross-bar, while allowing radiation having a longer wavelength to pass through. This is similar to the shape used in Yagi-Uda antennas. Depending on its length relative to half the wavelength of the dipole antenna element itself, either rod element of the yagi antenna may operate as a reflector or director. Thus, the back reflector of the yagi antenna is slightly longer than a half wavelength, while the front director is shorter than a half wavelength. In one embodiment of the detector of the invention, a plurality of parallel layers having the pattern type shown in fig. 3 are combined to form a stack as shown in fig. 4, wherein the further away from the pixilated antenna wafer plane the layers have longer cross arm lengths. Each of these patterned layers is spaced apart from the pixilated antenna plane by a distance equal to a quarter of a wavelength, wherein the length of the cross-arm in the patterned layer is adjusted to be slightly larger than half the wavelength. This rule of thumb is used as a guide to tuning and optimizing such structures in Electromagnetic (EM) solution analysis applications, and then slight modifications are made to the patterned layer design to account for the interactions that exist between elements of different lengths.

It is recommended to implement such a layered structure within any high quality dielectric material, such as multilayer alumina packaging technology. Although care should be taken with the quality of the dielectric material used at THz frequencies, some losses can be tolerated.

Alternatively, the readout circuits of several pixels are combined electrically, which results in higher sensitivity (i.e., higher signal-to-noise ratio) at lower THz frequencies, but at the expense of reduced resolution at higher THz frequencies. This technique can be implemented dynamically by switching quickly between the two options in a timely manner, or used with a slower switch between the two image options. Since the video rate used has a relatively low frame rate of about 15Hz (in order to have the largest possible integration time in each sensing device to achieve the largest possible signal-to-noise ratio), the available "dead" time between frames can be used to alternately display both images with small pixel size (i.e., higher resolution) and images with large pixel size (i.e., higher sensitivity and higher clothing penetration). The two images combine in the eye of the user viewing the images to form a higher quality image with both higher resolution and higher penetration/sensitivity. This can be achieved without increasing the actual detector sampling rate and therefore without adding any noise. By displaying many image frames during the sensor integration time, some with small pixels and others with larger pixels combined, this technique can be used to form a continuous function between the two extremes of the large and small pixel size. By changing the ratio of image frames displayed in larger pixels, scanning can be performed by moving or rotating a knob in a camera or other image detection device to alternate between resolution and sensitivity/penetration.

A diagram illustrating a side view of the THz radiation detector of figure 1 is shown in figure 5. The detector, generally designated 100, includes a bow-tie antenna strip 126, a suspended platform 128, a silicon substrate 102, and a reflector 120. Bow-tie (or any other type) antenna strips 126 are constructed on a silicon substrate 102 and include metal 104 (e.g., aluminum or copper) on a silicon oxide dielectric (BOX) 112. The suspended platform 128 structure includes a silicon oxide dielectric layer 114, a silicon thermal sensor element (e.g., a sense transistor body or bulk wafer) layer 114, a polysilicon layer 118, a silicon oxide dielectric (BOX)108, and a metal (e.g., aluminum or copper) 106. The high bandwidth reflector 120 includes a dielectric layer 122 and a plurality of layers 124 having a metal mesh pattern. In one embodiment, the antenna and the opening under the suspension platform are constructed using well-known Deep Reactive Ion Etching (DRIE) techniques. In accordance with the present invention, the suspended platform and antenna strip are separated by a capacitive coupling gap 110.

A circuit diagram of a first exemplary embodiment of a thermal sensor of a THz detector is shown in fig. 6. In this first exemplary embodiment, the thermal sensor element 60 includes a MOS transistor 62 operating in a sub-threshold region. Thermal energy thermally coupled from resistor 56 (fig. 2) causes an electrical signal to be generated in the transistor, which is then amplified and processed by the readout circuitry.

A circuit diagram of a second exemplary embodiment of a thermal sensor of a THz detector is shown in fig. 7. In this second exemplary embodiment, the thermal sensor element 70 includes a forward biased diode 72. Thermal energy thermally coupled from resistor 56 (fig. 2) causes an electrical signal to be generated in the diode, which is then amplified and processed by the readout circuitry.

A circuit diagram of a third exemplary embodiment of a thermal sensor of a THz detector is shown in fig. 8. In this third exemplary embodiment, the thermal sensor element 80 includes a forward biased bipolar transistor junction 82. Thermal energy thermally coupled from resistor 56 (fig. 2) causes an electrical signal to be generated in the bipolar transistor, which is then amplified and processed by the readout circuitry.

A circuit diagram of a fourth exemplary embodiment of a thermal sensor of a THz detector is shown in fig. 9. In this fourth exemplary embodiment, the thermal sensor element 90 comprises a resistive bolometer 92. Thermal energy thermally coupled from resistor 56 (fig. 2) causes an electrical signal to be generated in the bolometer, which is then amplified and processed by a readout circuit.

A diagram illustrating a second exemplary embodiment of a passive THz radiation detector is shown in figure 10. The detector shown in fig. 10 is similar to that of fig. 1, the main difference being the type of on-chip antenna used. In fig. 10, the antenna has a square tooth shape. The detector, generally designated 130, includes a plurality of antenna arms 132 (four in this example), a silicon substrate 144 surrounding the antenna 130, a suspended platform 136, support arms 138, sensor signals 142, and readout circuitry 140. In operation, the antenna tabs 134, which are approximately equal to the length of the suspended platform, capacitively couple the thermal energy gathered by the antenna across the isolation gap around the suspended platform to corresponding coupling capacitors (not shown) on the suspended platform. The energy coupled across the gap heats the resistor, which is sensed by a thermal sensor (not shown) built into the suspended platform. The output of the sensor is processed by readout circuitry 140 for display or further post-processing.

A diagram illustrating an illustrative example of a small 2 x 2 pixel imaging matrix using the THz detector of fig. 10 is shown in fig. 11. Note that using the techniques described herein, one skilled in the art can construct a much larger matrix with hundreds of pixels in order to form the desired high resolution image. An imaging matrix, generally designated 150, includes a plurality of detectors 152 (four in this exemplary embodiment), sensor signal lines 154, and readout circuitry 156. Each detector includes an antenna arm 157, a suspended platform 160, and a support arm 158. The output of the sensors located on the platform is input into readout circuitry 156 for display or further post-processing. Note that in one embodiment, the pixel array is surrounded by dummy (dummy) pixel rows and columns. The rows and columns of dummy pixels are used to maintain the same MEMS and VLSI fabrication conditions for the pixels located at the periphery of the two-dimensional array.

A diagram illustrating an enlarged view of an exemplary THz detector including a support arm structure and a suspended thermal sensor is shown in fig. 12. The exemplary detector, generally designated 170, includes a plurality of antenna bars 172 (four in this example), a suspended platform 174 and a support arm 186. The suspended platform 174 includes coupling capacitors 178 along the four edges of the platform, where these capacitors are formed by a thermally insulating air gap 176 around the platform, cross-shaped polysilicon resistors 180 that heat the entire platform, and thermal sensors 182. Support arms 186 are attached to a silicon substrate 188 for supporting the suspended platform. Sensor output signals 184 are transmitted from the thermal sensor along the support arm to readout circuitry (not shown).

A diagram illustrating a side view of the THz radiation detector of figure 10 is shown in figure 13. The detector, generally designated 190, includes an antenna arm 191 and a floating platform 193. The antenna arm includes a BOX layer 204, silicon dioxide layers 200 and 196, a polysilicon portion 208, a metal portion 212, a metal layer 194, and an oxide layer 192. The floating platform includes BOX layer 206, silicon dioxide layer 202, polysilicon layer 218, silicon dioxide layer 198, and metal portion 212.

A diagram illustrating a side view of a supporting arm portion of the THz radiation detector is shown in fig. 14. The support arm, generally designated 220, includes a BOX layer 224, a silicon dioxide layer 225, a silicon nitride layer 227, a polysilicon sensor signal line 228, a silicon nitride layer 226, and a silicon dioxide layer 222.

To aid in understanding the operation of the THz detector of the present invention, the following provides a quantitative illustration of exemplary calculations that can be performed with the detector for room temperature passive THz imaging at video rates.

In this example, assume that the predetermined bandwidth is 0.5 to 1.5THz, which includes I ═ 2.857 × 10 at a temperature of 300 degrees kelvin^-5W/cm²The black body power of (a). When from 0.5 to 1.5THz integral, at T-300 degrees kelvin, the corresponding blackbody power temperature sensitivity per degree kelvin is given by:

dI/dT＝1.043×10^-7 Watt/cm²/°K (1)

let us assume that the total radiation reception efficiency is given by the following equation, taking into account atmospheric losses, lens losses, target emissivity less than 1, pixel fill factor, antenna efficiency, and impedance matching losses, etc.:

η_total＝η_env×η＝0.3 (2)

wherein,

η is the efficiency of the detector;

η_envis the efficiency of the environment (i.e., anything other than the detector).

Note that it is reasonable to assume a value of 0.3 at this stage. Let us assume that the pixel size is A_D＝200×200μm²This is the wavelength at the higher end frequency of 1.5THz and is therefore the best resolution limit we can achieve at this frequency. Note that in lower frequencies, several pixels are used together to obtain the higher sensitivity of the larger pixel in one piece.

Let us assume a frame time of 70ms, which corresponds to a video frame rate of 14Hz, which is sufficient for the human eye. Higher frame rates are also possible, but they can reduce the signal-to-noise ratio. Let us further assume that we are using F_#1 (i.e., the ratio of the focal length F to the lens diameter D) plastic THz lens. Alternatively, better optics may be used at the expense of a large lens diameter or mirror combination, etc. However, for this exemplary calculationWe assume a simple viable optical device. Let us now assume that the Noise Equivalent Temperature Difference (NETD) in the target is NETD 0.5 ° K, which is sufficient for high quality thermography.

Assuming the above values, when the target temperature change is Δ T — NETD — 0.5 degrees kelvin, the THz signal received by each pixel is Ps — NEP — 1.56 picowatts. Note that this is the received power change per 0.5 degree change of the target, where the total target power received per pixel, i.e., the background power, is approximately 8.57 × 10^-10W is added. This is calculated using the following well-known equation:

<math> <mrow> <mi>Ps</mi> <mo>=</mo> <mi>dI</mi> <mo>/</mo> <mi>dT</mi> <mo>×</mo> <mi>ΔT</mi> <mo>×</mo> <msub> <mi>A</mi> <mi>D</mi> </msub> <mo>×</mo> <mfrac> <mn>1</mn> <msup> <msub> <mrow> <mn>4</mn> <mi>F</mi> </mrow> <mo>#</mo> </msub> <mn>2</mn> </msup> </mfrac> <msub> <mi>η</mi> <mi>total</mi> </msub> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>3</mn> <mo>)</mo> </mrow> </mrow> </math>

where we replace dI/dT Δ T with power density I for the calculation of background power.

Let us now make an ideal assumption, assuming that the dominant detector noise is the fundamental thermal fluctuation noise caused by the finite thermal capacity of the platform. This is appropriate for the initial start, since thermal fluctuation noise will always remain even after we minimize additional electrical noise from the thermal sensor element itself. When so assumed, we can use the known relationship for the detector Noise Equivalent Power (NEP) due to thermal fluctuation noise only and make it equal to the above calculated signal to achieve unity signal-to-noise ratio, as follows:

assuming that capacitive coupling can be used to concentrate the above-mentioned received power from the receive antenna into a thermally isolated suspended platform, we obtain a desired threshold unit signal-to-noise ratio when the thermal conductivity of the platform is given by:

this equation yields 1.243 x 10 when assuming a detector temperature of 315 degrees Kelvin (40 degrees Celsius)^-7The required support arm thermal conductivity of W/° K. Note that better results are obtained when the detector is cooled and maintained slightly using a closed circulation system at a fixed temperature slightly below room temperature.

Material properties using an exemplary 0.18 μm SOI semiconductor process result in a 50 micron by 50 micron suspensionPlateau corresponding thermal conductivity having Cth of 8.70 × 10^-9The heat capacity of the coke/on, in the sense that they both provide the desired time constant of 70 milliseconds. After the same calculations, we obtain better performance than this (i.e., lower NEP) when all the energy is concentrated on a platform smaller than 50 microns x 50 microns, which is limited by the ability to design and release small platforms, and by the physically required size of the thermal sensor elements (e.g., sense transistors) located on this platform.

Let us now consider the electrical noise handling of the sensor element (e.g. the sense transistor). When additional electrical noise is also considered, the expression for the NEP of the detector is:

assuming the transistor is sub-threshold, these electrical noises include two additional terms in the NEP expression, which are the fundamental shot noise (middle term) and the technology-dependent 1/f noise (right term). To reduce the relative effect of shot noise, we need a sufficiently large detector threshold current I_DFor example, about several hundred nA. To reduce 1/f noise, a higher TCC for the same bias current and also a lower K is used_fLarger transistors. It can also be derived from equation (6) that if we further reduce the support arm thermal conductivity, we can have the relative effect of these additional electrical noises significantly reduced. We can also address platforms much smaller than 50 microns x 50 microns-in principle as small as 10 microns x 10 microns-doing so reduces the electrical noise 625 times and the thermal fluctuation noise 25 times.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, "a," "an," "the," and "the" are also intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the limited number of embodiments disclosed herein. It is therefore to be understood that all suitable variations, modifications and equivalents may be resorted to, falling within the spirit and scope of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

1. A terahertz (THz) detector, comprising:

a dielectric substrate;

an antenna fabricated on the substrate;

a suspended platform comprising a thermal sensor, wherein the thermal sensor is to receive THz radiation focused by the antenna via capacitive coupling and to convert the received THz radiation into an electrical signal; and

wherein the capacitive coupling provides thermal isolation between the antenna and the thermal sensor.

2. A terahertz (THz) detector, comprising:

a dielectric substrate;

an antenna fabricated on the substrate and for receiving THz radiation;

a load resistor capacitively coupled with the antenna; and

a suspended platform comprising a thermal sensor, wherein the thermal sensor is thermally isolated from the antenna and thermally coupled to the resistor, and is configured to convert THz radiation collected by the antenna into an electrical signal.

3. A detector according to claim 1 or claim 2, further comprising a reflector parallel to and spaced a predetermined distance from the substrate, the reflector being arranged to reflect radiation having a half wavelength less than the length of the antenna, thereby improving the sensitivity and selectivity of the detector.

4. The detector of claim 3, wherein the reflector comprises a plurality of metal layers, each metal layer patterned as an array of metal crosses.

5. A detector according to claim 1 or claim 2, wherein the antenna is adapted to focus THz induced heating of the suspended thermal sensor such that the resulting thermal signal is substantially greater than thermal sensor temperature noise.

6. A detector according to claim 1 or claim 2, wherein the antenna comprises a cross dipole bow tie antenna.

7. A detector according to claim 1 or claim 2, wherein the antenna comprises a square-toothed antenna.

8. The detector of claim 6, wherein the thermal sensor comprises a pair of independent thermal detectors for independently sensing two orthogonal polarizations of THz radiation received by the cross-dipole bow-tie antenna, and the thermal detectors are selected from the group consisting of: MOS transistors, forward biased diodes, bipolar transistors, multiple active devices that together function as a thermal detector.

9. A detector according to claim 1 or claim 2, wherein the antenna presents a reactive impedance for cancelling out the coupling capacitance over a desired bandwidth.

10. A detector according to claim 1 or claim 2, wherein the antenna concentrates THz radiation to attenuate and filter out competing received infrared radiation.

11. A detector according to claim 1 or claim 2 wherein the suspended platform is attached and secured to the substrate via a support arm.

12. The detector of claim 11, wherein said support arm is configured to determine a thermal resistance of said suspended platform, wherein said thermal resistance is configured to tune said suspended platform to have a desired thermal time constant.

13. The detector of claim 2, wherein the resistance of the resistor is configured to match the impedance of the antenna.

14. A method of detecting terahertz (THz) radiation, the method comprising:

providing an antenna fabricated on a dielectric substrate and for receiving THz radiation energy;

capacitively coupling THz radiation energy received by the antenna to a resistor;

thermally coupling heat generated by the resistor to a suspended platform comprising a thermal sensor thermally isolated from the antenna and thermally coupled to the resistor; and

converting THz radiation incident on the thermal sensor into an electrical signal.

15. The method of claim 14, wherein the suspended platform is attached and secured to the base plate via a support arm that determines a thermal resistance of the suspended platform, wherein the thermal resistance is used to tune the suspended platform to have a desired thermal time constant.

16. The method of claim 14, wherein the thermal sensor comprises a pair of independent thermal detectors for independently sensing two orthogonal polarizations of THz radiation received by the cross-dipole bow-tie antenna, and the thermal detectors are selected from the group consisting of: MOS transistors, forward biased diodes, bipolar transistors, multiple active devices that together function as a thermal detector.