Patent Application: US-201113230653-A

Abstract:
a superconducting metallic glass transition - edge sensor and a method for fabricating the mgtes are provided . a single - layer superconducting amorphous metal alloy is deposited on a substrate . the single - layer superconducting amorphous metal alloy is an absorber for the mgtes and is electrically connected to a circuit configured for readout and biasing to sense electromagnetic radiation .

Description:
in the following description , reference is made to the accompanying drawings which form a part hereof , and which is shown , by way of illustration , several embodiments of the present invention . it is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention . superconducting metallic glasses exhibit a number of desirable properties for applications of embodiments of the invention , and will exceed the performance of the soa for each tes figure of merit : improved energy resolution ; lower excess noise ; amorphous alloy may be a self - absorber of radiation ( with 4d and 5d transition metal additions to alloy ( s )); precisely controlled superconducting transition temperature , t c ; simplified detector architecture ; and mechanically & amp ; chemically robust design . to illustrate the design advantages of mgtes detectors in accordance with embodiments of the invention , the application of the mgtes detectors as x - ray radiation and infrared radiation ( ir ) detectors is described herein . for ir sensing , the device design would be very similar , except without the addition of an x - ray absorbing layer on the tes thermistor . in either case , each mgtes device may be run in the deep electro - thermal - feedback ( etf ) mode ( see [ 1 ]). the mgtes detector arrays operate as micro - calorimeters , and are scalable to large arrays . the mgtes pixels are read out using superconducting quantum interference ( squid ) amplifiers . based on mgtes design details , squid multiplexing techniques based on transformer ratio multiplexing can be explored ( see [ 9 ]). as a starting point , for a discussion of mgtes alloys , the device architecture shown in fig3 is presented , which is similar to that employed in soa tes arrays . consequently , fig3 illustrates a schematic of a mgtes device architecture in accordance with one or more embodiments of the invention . an mgtes element 302 can be synthesized as a 140 μm × 140 μm sized device , on a low - stress - silicon - nitride ( lsn ) substrate 304 of varying thickness ( 0 . 25 μm & lt ; t lsn & lt ; 1 μm ). to tune the thermal conductance g , membrane - isolated tes sensor designs are used , wherein the superconducting film 302 is deposited onto a thin silicon - nitride ( si — n ) membrane 304 , which is thermally isolated by thin si — n support beams 305 of low thermal conductance . the mgtes design has an advantage in that its superconducting leads 306 can be deposited using higher transition temperature compositions from the same alloy system as the tes element itself . shown in fig3 is the squid 308 current readout , whose output is used in feedback to the bias voltage 310 to maintain the operating point ( 10 %- 20 % bias point ) on the mgtes transition curve . superconducting metallic glass transition - edge sensors ( mgtes ) 302 can be fabricated using a single - layer glassy ( amorphous ) superconductive thin film as the active device element , which is overall a much simpler device architecture compared to the soa multilayer tes designs . the simplified architecture greatly increases the ability to fabricate large arrays of mgtes detectors 302 , with minimal variation in t c , from one element to another . mgtes films may be deposited onto substrates held at or near room temperature , which greatly improves device yield compared to the soa proximity effect tes devices , where the substrate temperatures are raised to 600 - 700 ° c . during the deposition process . mgtes films deposited onto substrates held between 77 - 300 k would exhibit very uniform amorphous structures for good glass forming alloys . the superconducting metallic glass transition - edge sensors ( mgtes ) introduce the novel concept of using film chemistry modulations alone to control the transition temperature t c . this well - studied technique offers the possibility of smoothly tuning the superconducting transition temperature of the mgtes device , while also adjusting its composition for high physical and chemical stability . the energy resolution limit of the tes device ( including johnson noise in the film and the thermodynamic fluctuations between the detector and the heat bath ) is given by reference [ 1 ]: δ ⁢ ⁢ e fwhm = k b ⁢ t 2 ⁢ c ⁡ ( 1 / α ) ⁢ 8 ⁢ n eqn . ⁢ - 1 where c is the total heat capacity of the superconducting thin film and absorber . the factor n = d log p / d log t is the logarithmic derivative of the bias power with respect to temperature , typically , n = 3 - 5 . in eqn .- 1 , α is the logarithmic sensitivity factor the α value in soa tes devices based on polycrystalline films is typically 20 & lt ; α & lt ; 200 . the α value is a measure of the sharpness of the superconducting transition , and it is strongly dependent on the properties and the state of the film . the α values in soa tes devices are reduced , manifest by the complexity of the elemental metallic bilayer design , which results in a broadened transition widths of the order δt c ˜ 2 - 30 mk . the mgtes may exhibit a significantly improved energy resolution , because amorphous superconducting films can be fabricated with very narrow transition widths , as low as δt c ≈ 100 μk . further , such transition widths may only be limited by intrinsic superconducting fluctuations . due to the findings presented in numerous studies , it is accepted that for amorphous superconductors , the contribution to the transition width δt c from superconducting fluctuations is well described using the aslamazov - larkin ( al ) theory . as a means of estimating the performance of mgtes devices , the contribution to the transition width δt may be calculated using the aslamazov - larkin ( al ) theory for superconducting fluctuations ( see [ 10 ]). al fluctuations contribute to broadening of the transition width , and are dominant near the lower end of the transition , i . e ., near the zero in resistance . the resistance per square r □ ( ω ) was calculated from data presented in ref . [ 11 ] by using its definition r • ⁡ ( ω ) = ρ t eqn . ⁢ - 4 where ρ is the film resistivity in ω - m , and t is the film thickness in meters . narrow superconductive transition widths δt c predicted by eqn . 3 & amp ; 4 above are somewhat universal to superconducting amorphous thin films . fig4 shows the superconducting transition observed in an amorphous zr 75 rh 25 ( at . %) splat - quenched foil ( e . g ., of ref . [ 7 ]) in accordance with one or more embodiments of the invention . the zr — rh and zr — cu alloys are model systems , and therefore are relevant for a discussion of the magnitude of superconductive fluctuations near t c . to estimate the performance of an mgtes device fabricated from amorphous zr 75 rh 25 and zr 40 cu 60 films , the al theory is applied to calculate the superconductive transition width δt c . the results of these estimates are provided in table 2 . in table 2 , the aslamazov - larkin theory is applied to calculate the transition width δt c and the calculated logarithmic sensitivity factor α in zr 75 rh 25 and zr 40 cu 60 films . as described above , narrow transition widths δt c are somewhat universal to superconducting amorphous thin films . the electron transport and superconductive properties of amorphous metals are reported in reviews by mizutani , howson & amp ; gallagher , and naugle ( see [ 20 ]-[ 22 ]). reports of superconductive behavior at low temperatures , t c & lt ; 1 k , in two alloy systems : cu 35 ti 65 ( t c = 58 mk ) and cu 60 zr 40 ( t c = 320 mk ) are useful . based upon such alloys , a number of alloy compositions , with ternary additions , may exhibit t c &# 39 ; s in the range t c ≈ 65 - 75 mk . the alloys in both of these systems are strongly scattering metallic glasses ( i . e ., amorphous metals ), where the conduction is due to the d - band conduction . such alloys exhibit a negative temperature coefficient of resistivity α r & lt ; 0 , with resistivity values that are well above the mooij correlation limit ρ ≈ 150 μω - cm ( see [ 12 ]). for alloys with resistivities above this limit , the temperature dependence , ρ ( t ), is small and can be tuned via composition . using the calculated superconductive transition width , the logarithmic sensitivity factor α can also be estimated ( and is included in table 2 above ): the predicted superconductive transition widths are δt c = 1 . 53 mk and δt c = 0 . 24 mk , for zr 75 rh 25 and zr 40 cu 60 , respectively . δt c &# 39 ; s of this magnitude provide large logarithmic sensitivity factors , and the calculated results are promising . the estimated al fluctuation spectrum contribution to δt c = 1 . 53 mk for zr 75 rh 25 , is especially promising : 1 ) for the qualitative agreement with the experimentally determined δt c = 7 mk value for a 45 μm thick foil ; and 2 ) the large t c , which suggests the fabrication of mgtes arrays operating at higher temperatures . temperatures in this range , ˜ 4 k , are commercially viable for medical imaging applications . the calculated logarithmic sensitivity factors are much greater than the known values for the soa proximity effect bilayer tes devices . in calculating a for the amorphous superconductors , it may be assumed that δr / r ≈ 1 . this is reasonable for low bias points on the zr 75 rh 25 and zr 40 cu 60 transition curves . these alloys are strongly scattering metallic glass , where the conduction is due to the d - band conduction ( see [ 11 ]). this alloy exhibits a negative temperature coefficient of resistivity α r & lt ; 0 , with resistivity values well above the mooij correlation limit ≈ 150 μω - cm ( see [ 12 ]). for alloys with resistivities above this limit , the temperature dependence , ρ ( t ), is small and can be tuned via composition . the temperature dependence of resistivity curves for : a ) crystalline metals , b ) weakly - scattering metallic glasses , and c ) strongly - scattering metallic glasses , are shown in fig5 in accordance with one or more embodiments of the invention . in other words , fig5 illustrates the schematic temperature dependence of the electrical resistivity , ρ e , for : a ) crystalline metals , b ) weakly - scattering metallic glasses , and c ) strongly - scattering metallic glasses . in the strongly scattering alloys of the type c , at low temperatures quantum corrections to the temperature dependence of resistivity become important in the weak - localization regime ( wlr ). in the wlr , the electron propagation between scattering events is no longer classical as there is significant interference between scattered partial waves . there are two sources of correction ; ( a ) the ‘ localization ’ effect , which considers the quantum interference effects and ( b ) the ‘ interaction ’ effect , which considers the modification of the e − - e − interaction in the wlr . these ideas have been very successful in describing the behavior in various examples of disordered metals and they represent a fundamental difference between the transport properties of highly resistive metals and metals for which ρ & lt ; 100 μω - cm . in amorphous metals , the disordered structure is the primary electron scattering mechanism , so impurities and defects have little effect on the properties . therefore , the residual - resistance - ratio ( rrr )= r ( 300 k )/ r ( 4 . 2 k ) for an mgtes device ( in accordance with embodiments of the invention ) may be of the order rrr ≈ 1 . such an embodiment reduces the effects of film non - uniformities , providing narrow transitions . amorphous superconductors have no long - range order , and so they show no anisotropy effects , brillouin - zone effects or precipitation effects . obviously , they also lack crystal interfaces ( grain boundaries ) and crystalline imperfections ( dislocations , twins , stacking faults , and interstitial impurities ). these properties have made superconducting metallic glasses popular for testing theories of transition widths , e . g ., naugle et al &# 39 ; s results that agree well with the al theory ( see [ 13 ]-[ 14 ]) thus , the use of the al theory to predict δt c &# 39 ; s is reasonable . further , there is evidence of 2d effects in thin cu — ti films that may be examined . in high - quality mgtes thin films held under low - bias and low - t , smaller δt c could be realized ; with logarithmic sensitivity scaling as α ≈[ δt c ] − 1 and values approaching α ≈ 10000 possible . the impact of a larger logarithmic sensitivity α is immediate . the resolution of tes devices is given by eqn . 1 , shown again below ( see ref . [ 3 ]). compared to the soa tes with 50 & lt ; α & lt ; 200 , the resolution of embodiments of the invention may be improved by factors proportional to 1 /√{ square root over ( α ′)}, which could yield reductions of up to 250 % in δe fwhm . similarly , mgtes device embodiments may exhibit near - theoretical noise - equivalent - power ( nep ) values . this performance improvement may arise from the intrinsic factors described above , and also extrinsic factors . for example , thin metallic glass films tend to grow with very little residual stress compared to polycrystalline metal films ( where the stresses can be quite large ) ( see [ 15 ]). the work by r . r . hake further summarizes the universal trend of narrow δt c &# 39 ; s for metallic glasses , in almost bulk form , i . e ., in melt - spun ribbon form ( see [ 16 ] which reports work for binary transition - metal alloys based on zirconium ( zr )). of particular note is a zr 62 co 38 ( at . %) specimen with t c = 2 . 10 k , and δt c = 4 mk ; which yields δt c / t c = 1 . 91 × 10 − 3 . in addition to the above , embodiments of the invention may utilize two early - transition - metal / late - transition - metal ( etm - ltm ) alloy systems known to exhibit superconductive behavior below 1 k : 1 ) amorphous cu 35 ti 65 ( at . %), which has t c ˜ 58 mk ; and 2 ) amorphous cu 60 zr 40 may be used . embodiments of the invention may use these alloy and near - neighbor compositions in the respective alloy systems ( see refs . [ 18 ] and [ 19 ]). performance metrics for the base amorphous metal systems that may also be examined , i . e ., cu 1 - x ti x and cu 1 - x zr x ( at . %), are shown in table 3 ( see refs . [ 18 ] and [ 19 ]): to estimate the performance of an mgtes device fabricated from an amorphous cu 60 zr 40 film , the al theory can be applied to calculate the superconductive transition width δt c . this information is provided in table 4 : the predicted superconductive transition width is δt c = 90 . 7 μk . in mgtes device embodiments , δt c &# 39 ; s of this magnitude may provide large logarithmic sensitivity factors . this δt c value yields the following conservative estimate for the logarithmic sensitivity factor α , this number is much greater than the value for the soa proximity effect tes device . in calculating α , δr / r ≈ 1 is assumed . it may be recognized that an a of this magnitude is not realized when biasing a tes device in the etf mode but , one can expect that the large value intrinsic to superconducting amorphous films may enable lower biasing points or a reduced noise equivalent power value . accordingly , embodiments of the invention may synthesize single - layer superconducting amorphous alloy films from the new class of multicomponent bulk metallic glass forming alloys , which exhibit a greater resistance to crystallization , and a higher glass - forming - ability ( gfa ). there are a number of reports of superconductive behavior for these alloys at temperatures below 1 k . therefore , ternary , quaternary , and higher order alloying additions may be added to the binary alloys to tune the t c value and examine the effect of alloying on the mgtes device performance . in view of the above , the single - layer superconducting amorphous metal alloy used in embodiments of the invention may consist of an early - transition - metal / late - transition - metal ( etm / ltm ) alloy system that exhibits superconductive behavior . the early transition metal ( etm ) elements may be drawn from elements in groups ivb , vb , and viib of the periodic table , and the late - transition ( ltm ) metal elements may be drawn from elements in groups viiib , ib , and iib of the periodic table . such alloys may be deposited onto a substrate using sputtering , atomic - layer - deposition , electron - beam evaporation , or thermal evaporation . as an efficient x - ray absorber , a modified laminated high fill - fraction array of bi absorber structures may be employed , similar to that employed by chervenak et al ( e . g ., the laminated bi / cu absorbers ) ( see ref [ 17 ]). this absorber consists of 8 total alternating individual bi / cu films , with layer thicknesses of 8 . 5 and 0 . 8 μm , respectively . this mushroom - type absorber structure is shown in fig6 in accordance with one or more embodiments of the invention . however , to improve the x - ray efficiency , a more efficient design may be adopted ( e . g ., a reduced bi - thickness to increase intra - layer x - ray photon thermalization , coupled with an au - interlayer instead of cu ). the electronic heat constant γ e for cu and au are similar , 0 . 695 and 0 . 729 mj / mol - k 2 , respectively . the higher z of au will improve the x - ray quantum efficiency . measurements will be conducted to establish the heat capacity c . depending on the superconducting metallic glass alloy used , mgtes device embodiments may utilize a self - absorbing mgtes superconducting thermistor element , using alloys selected from the mo — re , mo — ru , zr — rh , cu — sn , pt — zr , pt — ti , hf — ni , hf — cu , hf — pt , w — cu , ta — cu , and similar superconducting alloys drawn from combination of 4d - 3d transition metal alloys and 5d - 3d transition metal alloys , and 5d - 4d transition metal alloys , greatly simplifying the device design . further , superconducting leads that connect the single - layer superconducting amorphous metal alloy to the circuit may be comprised of a different composition within the same alloy system as the superconducting amorphous metal alloys used as the thermistor element . for example , the superconducting leads may be made using zr 76 cu 24 ( at . %) with t c = 3 . 5 k ; and the mgtes element ( thermistor ) may be made using zr 40 cu 60 ( at . %) with t c = 0 . 32 k . mgtes detectors may be designed for measurements conducted on a platform that operates at low temperatures (≈ 50 mk ). each mgtes device may be run in the deep electro - thermal feedback ( etf ) mode , and the noise characteristics of our sensors may be tested as described by irwin et al . [ 1 ]-[ 3 ]. depending on the alloy system under investigation and characteristics of the amorphous alloys prepared , the noise performance on thermally isolated island structures or small volume isolation architectures may be investigated . examples of both structure types are shown in fig7 and fig8 . in this regard , fig7 illustrates four tes sensors on a sin thermally isolated island structure in accordance with one or more embodiments of the invention . fig8 illustrates distributed antenna - coupled small - volume e arrays in accordance with one or more embodiments of the invention . the left side of fig8 illustrates a meandering aluminum structure ( g ) that forms an array of slot antennas . the two sides can be connected periodically with an array of mgtes microbolometers ( b ), each with a normal state resistance of tens of ohms ( see ref [ 10 ]). this concludes the description of the preferred embodiment of the invention . the following describes some alternative embodiments for accomplishing the present invention . in view of the above , alloys used in mgtes device embodiments have significantly higher superconducting transition temperatures than the soa tes devices based on thin films of ti ( t c ˜ 0 . 5 k ), or multi - layer moau designs that exploit the superconducting proximity effect ( t c ˜ 0 . 1 k ). the complicated soa tes device design requires a complex fabrication process ; making it difficult to produce large arrays with uniform performance . the unexplained excess noise equivalent power ( nep ) spectrum values observed in soa tes systems ( noise excess high - frequency noise , 4 × over theoretical expectations ) is introduced in part by the complicated multi - layer design and other factors intrinsic to the soa tes design . soa tes operating temperatures in the range t ˜ 100 mk require complex cryogenic cooling systems ; e . g ., a continuous adiabatic demagnetization refrigerator ( adr with 4 - stages ) or a he 3 / he 4 dilution refrigerator . adr systems for operation for t & lt ; 1 k are very expensive , and in the early stages of development . the superconducting transition temperature for some mgtes alloy films are t c & gt ; 4 k . temperatures in this range , enable the use of cheaper , cryogen free cooling systems ; e . g ., pulse - tube - refrigerator ( ptr ) systems that are commercially available for the t & lt ; 70k and t ˜ 4k temperature ranges . these systems do not vibrate like traditional stirling or gifford - mcmahon cryocoolers . another advantage of the mgtes concept is that the superconducting transition temperature is controlled by alloy composition alone , and not film thickness like in soa tes systems . this affords a greater control over the t c and a more robust , reliable detector . the width of the superconducting transition region in superconducting metallic glasses , δt c , is significantly less than in crystalline alloys . the unexplained excess noise equivalent power ( nep ) spectrum values observed in soa tes systems ( noise excess ˜ 5 - 10 % over theoretical expectations ), will be reduced in mgtes systems due to : 1 ) the simpler device design ( single thin film ); 2 ) the intrinsically high electrical resistivity ( yields rrr ˜ 1 ); and 3 ) the intrinsically narrow superconducting transition region in superconducting metallic glasses , δt c , which provides an increased energy resolution , provided by correspondingly larger logarithmic sensitivity factors α = d log r ( t )/ d log t ≈[ t c / δt c ]= 3000 are possible . compared to α ≦ 100 in soa tes designs . as described herein , embodiments of the invention provide the ability to use the mgtes detector in cryogenically cooled x - ray microcalorimeter detector arrays , for terrestrial applications . in this regard , a mgtes microcalorimeter array may establish a new paradigm as x - ray detectors for energy dispersive spectroscopy ( eds ) systems in scanning electron microscopes . the mgtes device embodiments offer order of magnitude or better increase in energy resolution , i . e ., decreased δe , compared to soa tes or si - drift detectors , δe ˜ 10 ev vs . δe ˜ 200 ev . other applications include : 1 ) x - ray detectors for medical systems ; and 2 ) mgtes microcalorimeters as radioactive α - particle or γ - ray detectors for nuclear forensics applications . in view of the above , the mgtes sensors of embodiments of the invention are a break - though superconducting transition - edge sensor with unprecedented performance . in particular , mgtes sensors may achieve the following improvements over that of the prior art : develop superconducting amorphous metal alloys , with tunable t c for application to mgtes sensors ; mgtes sensors that take full advantage of the intrinsic fluctuation limits in amorphous superconductors ; and mgtes array resulting in enhanced energy resolution and reduced noise floor . the foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . many modifications and variations are possible in light of the above teaching . it is intended that the scope of the invention be limited not by this detailed description , but rather by the claims appended hereto . k . d . irwin , “ an application of electrothermal feedback for high resolution cryogenic particle detection ” appl . phys . lett . 66 . 1998 ( 1995 ). s .- f . lee , j . m . gildemeister , w . h . holmes , p . l . richards , “ voltage biased superconducting transition - 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