The present disclosure relates to thermoelectric thermal detectors.
Optical detectors are light-sensitive sensors, which determine presence and/or intensity of incident electromagnetic radiation and output a measured value in terms of a suitable electrical signal. Optical detectors are split into quantum-type and thermal-type detectors. Quantum detectors, or photovoltaic and photoconductive detectors, are typically faster, and have often higher sensitivity than thermal detectors but are relatively complex. Quantum detectors operating in the infrared range are often made of expensive and/or toxic materials, and need to be operated at low temperatures to achieve high sensitivity (due to suppression of noise by reduced temperature). The thermal-type sensors are devices configured to determine a power of electromagnetic radiation by converting it into heat, and determining the generated temperature in terms of a suitable electrical signal.
Thermal sensors utilize various technologies, but the most relevant commercial technologies are resistive and thermoelectric thermal detectors. A thermal detector consists of an absorber of incident radiation and a transducer, which converts the change of the temperature of the absorber into an electric signal. A resistive thermal detector, sometimes referred to as a bolometer, uses temperature dependent resistors as transducers. A thermoelectric thermal detector, often a thermopile or a thermocouple, uses thermoelectric transduction based on the thermoelectric effect.
According to some aspects, there is provided the subject-matter of the independent claims. Some embodiments are defined in the dependent claims.
According to a first aspect of the present disclosure, there is provided a detector comprising an optically absorbing membrane suspended over a cavity between the membrane and a substrate, the substrate comprised in the detector, and a thermoelectric transducer attaching the optically absorbing membrane over the cavity, wherein the optically absorbing membrane forms a contacting element between n-type and p-type thermoelectric elements of the thermoelectric transducer.
According to a second aspect of the present disclosure, there is provided a method of manufacturing a detector, comprising obtaining a substrate wafer and depositing an oxide layer on the substrate wafer, depositing a thermoelectric transducer layer on the oxide layer, depositing an optically absorbing layer on the oxide layer or on the thermoelectric transducer layer, and etching a recess into the oxide layer to form a cavity, leaving an optically absorbing membrane comprising the optically absorbing layer and part of the thermoelectric transducer layer suspended over the cavity by the thermoelectric transducer layer.
A detector constructed as disclosed herein comprises an optically absorbing membrane suspended by a thermoelectric transducer, for example over a cavity. The cavity may have a reflector at its bottom to reflect back the fraction of incident light the membrane did not absorb, to enhance sensitivity of the detector. The cavity may have a resonant function. The membrane may be a nanomembrane, its thickness being in the nanometre scale. The nanoscale membrane is light-weight, enabling it to warm up faster as a response to incident radiation, which enhances a response speed of the detector. The disclosed detector further does not have a separate support structure, as the membrane is directly suspended and attached over the cavity by the thermoelectric transducer itself. The fact the detector has no separate support structure also enhances response time, as in systems with support structures the support structure slows down the response time by increasing the heat capacity of the detector. Beneficially, the herein disclosed detector may be manufactured using safer (e.g. less toxic) materials. Such materials may also be cheaper. Further, the herein disclosed detector may be manufactured using microelectromechanical, MEMS, methods.
Quantum detectors, or photovoltaic and photoconductive detectors, require cooling solutions for high sensitivity, and often have high cost and need exotic and/or toxic materials, such as HgCdTe which is needed in long-wave infrared detection. Cooling systems incur complexity, power consumption and cost. The main limitation of uncooled optical detectors of both thermal and quantum types is poor performance in terms of sensitivity as described by specific detectivity. State-of-the-art thermal detectors are typically slower than quantum detectors. Thermoelectric transduction provides the benefits of increased sensitivity and low power consumption, compared to resistive detectors requiring active power. Thermoelectric transduction doesn't require external power, since it inherently generates a voltage. Furthermore, since electric current is not needed in signal transduction in the case of thermoelectric elements, there are less noise sources, which results in higher signal-to-noise ratio (that is, increased sensitivity).
A thermoelectric transducer consists of two dissimilar thermoelectric materials joined together by a contacting element. The dissimilar thermoelectric materials comprise an n-type semiconductor with negative charge carriers, and a p-type semiconductor with positive charge carriers. In
Stubs 122, 132 may provide electrical connections between the thermoelectric transducer 120, 130 and readout electronics configured to process a signal from the detector. For example, these electrical connections may be built using wire bonding using metallic bonding pads, flip-chip bonding or wafer bonding techniques. As a further alternative, substrate 140 may comprise a CMOS circuit. Further, the detector may be interfaced with other optical devices, such as microspectrometer films (e.g. Fabry-Pérot interferometers). Readout electronics are not illustrated in
The optically absorbing membrane 110 is illustrated in
In yet further embodiments, there may be two optically absorbing layers, one on either side of thermoelectric transducer layer 112a, 112b. In other words, the optically absorbing membrane may comprise two optically absorbing layers and the thermoelectric transducer layer, the optically absorbing layers being disposed on either side of the thermoelectric transducer layer. On the other hand, in some embodiments the optically absorbing membrane 110 comprises one and only one optically absorbing layer 111 and exactly one thermoelectric transducer layer 112, the optically absorbing layer 111 being disposed on one and only one side of the thermoelectric transducer layer 112. Specific examples of various membrane embodiments will be discussed in more detail in connection with
Where two optically absorbing layers are present in optically absorbing membrane 110, they may be of the same material , or of different materials. The optically absorbing membrane 110 may have a thickness of less than 800 nanometres, less than 200 nanometres, less than 180 nanometres, less than 160 nanometres, less than 100 nanometres, less than 60 nanometres or less than 20 nanometres, for example. As disclosed above, a thin membrane has low heat capacity, Further, membrane phonon thermal conductivity of in-membrane materials decreases when the thickness is reduced to the nanoscale.
Optically absorbing layers, such as optically absorbing layer 111, may be comprised of metals, semimetals or highly doped semiconductors. Examples include TiW (titanium-tungsten), Ti (titanium), W (tungsten), TiN (titanium nitride), NbN (niobium nitride), MoN (molybdenum nitride), Mo (molybdenum), thin Al , a-Si (amorphous silicon), Al:ZnO (aluminium-doped zinc oxide), highly-doped single and poly crystalline silicon and doped SrTiO3 (strontium titanate). A further example of the absorber material is infrared absorbing insulators, such as silicon nitride or aluminium oxide. These materials absorb well in a band of infra-red. In the absorbing layers, the conductivity of the material may be selected such that it enables impedance matching to the vacuum impedance with a low thermal mass, that is, the resistance should not optimally be too high, but high enough for good absorptance. For plasmonic absorbers, the permittivity of the selected material , and pattern feature sizes, may beneficially be matched to the desired wavelength. Concerning electrical requirements, the selected absorbing layer 111 material beneficially has low contact resistance with the materials of semiconductor elements 120 and 130 (and thus with thermoelectric transducer layer 112a, 112b). This contact resistance should be much lower than the total resistance of the thermoelectric legs 120, 130, as otherwise performance of the detector is reduced by the contact resistance.
The thermoelectric materials used for thermoelectric elements 120, 130 and thermoelectric transducer layer 112a, 112bmay have a thickness, when applied in the detector, of less than 200 nm. The one is an N-type thermoelectric material and the other is a P-type thermoelectric material. Suitable materials include highly doped N(P)-type silicon, polysilicon and other semiconductors. Doping may be performed with ion implantation, diffusion or other suitable methods. Beneficially, the thermoelectric materials have a high thermoelectric figure of merit, ZT (see e.g. A. Varpula et al., Appl. Phys. Lett. 110, 262101 (2017) for a definition of ZT). For maximal sensitivity of the optical detector the effective thermoelectric figure of merit, the effective ZT, of the device should be maximized. As to mechanical requirements of the thermoelectric materials of elements 120, 130 and thermoelectric transducer layer 112a, 112b, they should have low or moderate tensile stress for suitable suspension of the absorber. Less suitable stress conditions can be handled by benefiting from a frame 160 to tune the stresses in the thermoelectric material.
Examples of suitable thermoelectric materials include Bi2Te3 (bismuth telluride), Bi2Se3 (bismuth selenide), HgCdTe (mercury cadmium telluride), ZnO2 (zinc peroxide), SrTiO3 (strontium titanate), silicon nanowires, thin single-crystalline silicon, thin polysilicon, Bi2Te3 (bismuth telluride) and Sb2Te3 (antimony telluride).
Optionally, a passivation layer, which is not illustrated in
When the wavelength the detector is intended to detect is known, the cavity may also be dimensioned accordingly, such for resistive absorbers that the height of the cavity is a quarter of a center wavelength the thermal detector is arranged to detect. For plasmonic absorbers, the cavity may be different from the quarter of the center wavelength.
In general , there may be provided a detector comprising an optically absorbing membrane 110 suspended over a cavity between the membrane 110 and a substrate 140, and a thermoelectric transducer 120, 130 attaching the optically absorbing membrane 110 over the cavity, wherein the optically absorbing membrane 110 forms a contacting element between n-type 120 and p-type 130 thermoelectric materials of the thermoelectric transducer 120, 130. Membrane 110 may be patterned, for example by perforating it with a plurality of holes. When membrane 110 is patterned, both thermoelectric transducer layer 112 and optically absorbing layer 111 may have the same pattern, such that holes of the pattern, for example, extend through the entire membrane 110.
By being attached over the cavity by the thermoelectric transducer it may be meant, that the legs 120, 130 connecting membrane 110 with the rest of the detector (e.g. stubs 122, 132) do not comprise non-thermoelectric materials. The legs may be connected with or between further structures, such as stubs 122, 132 and frame 160, but the legs themselves may be comprised solely of the thermoelectric materials.
The detector may comprise a back reflector attached in an inside edge of the cavity, arranged to reflect an optical signal not absorbed by the membrane back toward the membrane 110. Thus membrane 110 may have two chances to absorb energy from the optical signal.
The detector may be only passively cooled, by which it is meant the detector does not have an active cooling mechanism. In other words, the detector may be uncooled. Where the detector is actively cooled, it may be cooled using a Peltier chip, for example. An uncooled detector provides, in general , the benefit of slightly better sensitivity.
The detector may comprise a frame 160 either on top of the thermoelectric transducer 120, 130 or between the thermoelectric transducer 120, 130 and stubs 122, 132 defining a height of the cavity. As discussed above, presence of the frame 160 enables using a broader range of thermoelectric materials to build the thermoelectric transducer 120, 130 and the thermoelectric transducer layer 112.
The optically absorbing membrane 110 may be a resistive impedance matched absorber or a plasmonic absorber. Where the membrane is a plasmonic absorber, it may be a broad-band absorber, for example. For plasmonic absorbers the absorbing material permittivity and feature sizes of the pattern may be matched to the wavelength that it is desired to detect with the detector. Where the optically absorbing membrane is a resistive impedance matched absorber, the height of the cavity may be a quarter of a wavelength the detector is arranged to detect.
Processing then advances to the phase illustrated in
Processing then advances to the phase illustrated in
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The arrangement of
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The arrangement of
The following combinations of materials may be employed in construction of the detector. A single combination of materials is disclosed on a single row:
It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.
Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.
As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.
Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.
The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, that is, a singular form, throughout this document does not exclude a plurality.
At least some embodiments of the present invention find industrial application in using and manufacturing detectors. Examples of potential detector applications include infrared imaging, infrared chemical analysis based on absorption spectroscopy, for example, and temperature measurements. These devices can also be used as calorimetric sensors.
ALD atomic layer deposition
CVD chemical vapour deposition
LPCVD low-pressure CVD
LTO low temperature oxide
PECVD plasma-enhanced CVD
Number | Date | Country | Kind |
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20195772 | Sep 2019 | FI | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FI2020/050582 | 9/11/2020 | WO |