Study on a High Performance MEMS Infrared Thermopile Detector

micromachines

Article Study on a High Performance MEMS Infrared Thermopile Detector

Aida Bao 1,2, Cheng Lei 2,*, Haiyang Mao 3, Ruirui Li 1,3 and Yihao Guan 2

1 School of Information technology and electronics, Beijing Institute of Technology, Beijing100081, China; [email protected] (A.B.); [email protected] (R.L.) 2 School of Instrument and Electronics, North University of China, Taiyuan 030051, China; [email protected] 3 Institute of Microelectronics of Chinese Academy of Sciences, Beijing 100029, China; [email protected] * Correspondence: [email protected]

Received: 28 September 2019; Accepted: 12 December 2019; Published: 13 December 2019

Abstract: This paper presents a high-performance micro-electromechanical systems (MEMS) thermopile infrared detector. It consists of a double-end beam and a dual-layer thermocouple structure, which improves the responsivity of the detector. The etch-stop structure is integrated into the detector to prevent isotropic etching-caused damage on the device. The responsivity of the detector achieved 1151.14 V/W, and the measured response time was 14.46 ms. The detector had the potential to work as a high-precision temperature sensor and as a vacuum sensor.

Keywords: MEMS; infrared detector; thermopile; etch-stop

1. Introduction Infrared detector technology has developed rapidly due to the increasing demand for wider applications, such as thermal imaging and resource exploration in industry and civilian fields [1–3]. Modern infrared detectors are developed based on the infrared heat effect and the photoelectric effect. The thermal infrared detector has become a hot topic because of its advantages of uncooling at room temperature, wide spectrum response, no chopping requirements, and low cost. The thermopile infrared detector has several advantages compared to other detectors. It consists of a series of thermocouples connected in a series with each other. Therefore, compared to a thermocouple element, the thermopile device can obtain a higher output signal. It has been reported that some thermoelectric materials can improve the performance of the Micro-electromechanical Systems (MEMS)thermal reactor infrared detector. The complementary metal oxide semiconductor (CMOS) process-compatible cantilever thermopile infrared detector was developed, which utilizes Al/n-poly Si as a thermocouple material and silicon oxide/ Silicon nitride (SiO2/Si3N4) as a dielectric support film material [4]. Thermo-electric materials such as Bi2Te3 and Sb2Te3 were integrated in the infrared detector to obtain high-performance [5]. Single-layer thermocouple strips (SLTS) are also adopted in the infrared detector [6]. However, the number of thermocouple strips is limited due to its size limitation and relatively low performance. Meanwhile, thermopile-based devices require a thermal isolation layer between the hot junctions and the infrared (IR) absorber area, which causes the decrease in performance. In this paper, a MEMS thermopile infrared detector is proposed. In the detector introduced in the article, a double-end beam structure is adopted. In addition, the detector uses a double-layer thermocouple structure (DLTS), that is, the N-type thermocouple strips and the P-type thermocouple strips in the detectors are located in different planes, respectively, and the size of the detector device, based on this structure, can be further reduced while maintaining high-performance. A thermal

Micromachines 2019, 10, 877; doi:10.3390/mi10120877 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 877 2 of 10 Micromachines 2019, 10, x 2 of 10 insulation structure is applied to cold and hot junctions in the device, keeping the temperature of the device’s hot junction equal to the infrared absorption regions while keeping the temperature of the device’s hot junction equal to the infrared absorption regions while keeping the temperature of the cold junction equal to the heat sink. Moreover, a novel etch stop structure is integrated into the cold junction equal to the heat sink. Moreover, a novel etch stop structure is integrated into the detector detector to prevent over-etching during isotropic dry etching from the front, which prevents cold to prevent over-etching during isotropic dry etching from the front, which prevents cold junctions and junctions and output electrodes from floating and causing damage in the fabrication process. output electrodes from ﬂoating and causing damage in the fabrication process. 2. Theoretical Analysis of Infrared Detector 2. Theoretical Analysis of Infrared Detector The thermopile based infrared detector is shown in Figure 1. In the device, there is a layer of The thermopile based infrared detector is shown in Figure1. In the device, there is a layer suspended dielectric film under the thermocouples, and the thermal junction of the thermopile of suspended dielectric ﬁlm under the thermocouples, and the thermal junction of the thermopile contact with the infrared absorption zone. The cold junction of the thermocouple is located on the contact with the infrared absorption zone. The cold junction of the thermocouple is located on the heat heat sink, which is made of silicon with good thermal conductivity. The heat sink is consistent with sink, which is made of silicon with good thermal conductivity. The heat sink is consistent with the the ambient temperature. ambient temperature.

Figure 1. The schematic diagram of the thermopile based infrared micro-electromechanical systems (MEMS)Figure 1. detector. The schematic diagram of the thermopile based infrared micro-electromechanical systems (MEMS) detector. When infrared radiation is applied to the thermopile device, a temperature difference (Tdiff) is createdWhen between infrared the “hotradiation junction” is applied and the to “cold the thermopile junction” of device, the device, a temperature and according difference to the Seebeck (Tdiff) is ecreatedffect, the between thermal the response “hot junction” voltage and (∆U the) of “cold the thermopile junction” of device the device, is generated. and according The response to the Seebeck output voltageeffect, the of thethermal thermopile response device voltage can ( beΔU expressed) of the thermopile as [7]: device is generated. The response output voltage of the thermopile device can be expressed as [7]: ∆U = NT (α αB) = NT α (1) Δ=di f f ααA − − =di f f αAB UNTdiff() A B NT diff AB (1) Among them, N is the total number of the thermocouples [7]. αA and αB are Seebeck coefficients Among them, N is the total number of the thermocouples [7]. α and α are Seebeck of materials A and B, respectively. αAB is the difference in the Seebeck coefficientA of materialsB A and B. The response rate and time are important parametersα in evaluating the performance of the thermopile coefficients of materials A and B, respectively. AB is the difference in the Seebeck coefficient of infraredmaterials detector. A and TheB. responseThe response rate of rate the deviceand time can beare expressed important as [parameters8]: in evaluating the performance of the thermopile infrared detector.∆U The response∆U rate of the device can be expressed as [8]: Rv = = (2) P0 ϕ0Ad ΔΔUU where P0 is the infrared radiation power,Rϕ==0 is the radiation power density, and Ad is the device v ϕ (2) absorption area. According to Stefan–BoltzmannPA00 law, the powerd density of infrared radiation on the device surface can be expressed as [9] (assuming Tdiff << T0): where P0 is the infrared radiation power, φ0 is the radiation power density, and Ad is the device absorption area. According to Stefan–Boltzmann law, the4 power4 density of infrared radiation on the Cr σ ε1 (T 1 T 0) ϕ = · · · − (3) device surface can be expressed as [9]0 (assuming Tdiff << T20): As π d · · 0 ⋅⋅⋅σε 44 − CTTr 11() 0 where Cr is the root mean square conversionϕ = factor of the chopper, σ is the Stefan–Boltzmann constant,(3) 0 ⋅⋅π 2 ε1 is the blackbody emissivity rate, T1 is the temperatureAds 0 of the infrared source, T0 is the ambient temperature, As is the radiation area of the infrared source, and d0 is the distance between the infrared where Cr is the root mean square conversion factor of the chopper, σ is the Stefan–Boltzmann constant, ε1 is the blackbody emissivity rate, T1 is the temperature of the infrared source, T0 is the ambient temperature, As is the radiation area of the infrared source, and d0 is the distance between the infrared source and the surface of the thermopile device. In case the effect of the Joule heat and

Micromachines 2019, 10, 877 3 of 10

Micromachinessource and the2019 surface, 10, x of the thermopile device. In case the effect of the Joule heat and Peltier eff3 ectof 10 is neglected, the temperature difference between cold and hot junctions, Tdiff, can be expressed as [10]: Peltier effect is neglected, the temperature difference between cold and hot junctions, Tdiff, can be expressed as [10]: η P0 Tdi f f = · (4) ηG⋅ th P0 T = (4) where η is the infrared absorption rate of the materialdiff in the infrared absorption region and Gth is the Gth total thermal conductivity of the thermopile. As shown in Figure2, according to energy conservation wherelaw, the η infraredis the infrared absorption absorption region rate of theof the device material converts in the the infrared absorbed absorption infrared region radiation and intoGth is heat the Qtotalabsorb thermaland transmits conductivity it through of the the thermopile. three mechanisms: As shown Fromin Figure the hot2, according end to the to coldenergy end conservation through the law,heat sink,the infrared that is, theabsorption thermal conductanceregion of theof device the structure, convertsG thes, the absorbed radiation infrared thermal radiation conductance into Gheatr in Qtheabsorb absorption and transmits region, it through and the thermalthe three conductance mechanisms: of From the gas the in hot the end microcavity to the cold structure, end throughGg. the heat sink, that is, the thermal conductance of the structure, Gs, the radiation thermal conductance Gr in the absorption region, and the thermalG conductanceth = Gs + Gg of+ Gther gas in the microcavity structure, Gg. (5)

Figure 2. ThermalThermal conductance distribution of the packaged thermoelectric device.

After substituting Equations (1) and (4) into Equation (2), the responsivity of the device can be =++ further simpliﬁed to: GGGGth s g r (5) N η αAB After substituting Equation (1) and (4)Rv into= Equation· · (2), the responsivity of the device can (6)be Gth further simplified to: Thermal conductance of the atmospheric gas and radiation are usually negligible when the device operates in a vacuum and at a low temperature.N ⋅⋅ Therefore,η α the entire thermal conductance of the R = AB thermopile can also be written as: v (6) Gth Gth = Gs (7) Thermal conductance of the atmospheric gas and radiation are usually negligible when the where G can be expressed as: device operatess in a vacuum and at a low temperature. Therefore, the entire thermal conductance of X4 the thermopile can also be written as: λi di wi Gs = N · · (8) li i=1 = GGth s (7) Herein, λi, wi, di, and li, respectively, are the thermal conductivity, width, thickness, and length of where Gs can be expressed as: each thermocouple strip (i = 1 for the P-type thermocouple strips; i = 2 for the N-type thermocouple strips; i = 3 for the isolation layer; and i = 4 for the4 supportingλ ⋅⋅dw membrane). Further, the responsivity of GN= ii i the device can be calculated by using the expression:s  (8) i=1 li

Herein, λ , w , d , and l , respectively, are ηtheα thermal conductivity, width, thickness, and i i i i Rv = · (9) 4 length of each thermocouple strip (i = 1 for theP P-typeλi di wi thermocouple strips; i = 2 for the N-type · l · thermocouple strips; i = 3 for the isolation layer; andi=1 i =i 4 for the supporting membrane). Further, the responsivity of the device can be calculated by using the expression: ηα⋅ R = v 4 λ ⋅⋅dw  ii i (9) i=1 li

Micromachines 2019, 10, 877 4 of 10

Another important parameter is detectivity, which can be determined by: p Rv Ad∆ f D∗ = (10) Un where ∆ f is the measurement frequency bandwidth and Un is the noise voltage of the thermopile, which can be written as: p Un = 4kR0T0∆ f (11) The noise equivalent power (NEP) can be expressed as:

U NEP = n (12) Rv where k is the Boltzmann constant and R0 is the electrical resistance of the thermopile strips. Then, the detectivity can be calculated as: r Ad D∗ = Rv (13) 4kT0R0

3. Design of the Detector As expressed in Section2, the performance of the thermopile detector lies in the high-di fference in the Seebeck coefficient and relatively low thermal conductance of the thermocouple strips. The traditional single-layer thermocouple structure (SLTS) detector has drawbacks in regards to these two points. Herein, the thermocouple strips of the SLTS were constructed from the N-type or P-type Poly-Si and aluminum. To overcome the issues mentioned, we propose a thermopile infrared detector based on a double-layer thermocouple structure. Compared to a single-layer thermocouple structure detector, the double-layer thermocouple structure detector maintains high-performance while scaling down the size, in comparison to the single layer device. In the meantime, to avoid the over-release of the cold end of the probe and the output electrode, a novel etch stop structure is also designed in the detector. In Figure3, the detector based on the double-layer thermocouple structure adopts N-type and P-type polysilicon as the thermocouple materials, in which the N-type thermocouple and P-type thermocouple strips are located on the bottom layer and are connected in a series with each other by aluminum wires. In the hot junction of the detectors, N-type and P-type thermocouples are connected by a “climbing” aluminum structure, and the N-type and P-type thermocouple strips are interconnected by a “diagonal” shaped aluminum structure in the cold junction. This arrangement of thermocouples actually increases the detector duty cycle and reduces the lateral dimensions of the device compared to conventional four-end-beam-based thermopile devices [6]. From Table1, it can be seen that the Seebeck coefficients of the doped polysilicon material are much larger than that of aluminum, but the thermal conductivity is lower than that of aluminum. The difference in the Seebeck coefficient of the double-layer thermocouple structure of the thermopile detector is calculated at about twice as much as the thermopile detector with aluminum and the N-type or P-type polysilicon. Meanwhile, the thermal conductance of Al is much higher than that of the poly-Si. As shown in Equation (9), the double-layer thermocouple structure (DLTS) device has more than twice the sensitivity of the SLTS device. Besides this, according to Equation (9), the structural size of the DLTS device may be further scaled down by reducing the length and maintaining the relatively higher sensitivity. The resistivity of the aluminum is also much greater than that of the polysilicon. Therefore, the resistance of a thermopile detector, based on a double-layer thermocouple arrangement, is approximately double that of the single-layer thermocouple structure detector. Meanwhile, the noise voltage of the detector based on the double-layer thermocouple structure is about the square root of two of the single-layer structure detectors, according to Equation (11). The size of the double-layer thermocouple structure can be further reduced while maintaining relatively high-performance. Micromachines 2019, 10, x 5 of 10

Micromachines 2019, 10, x 5 of 10 size of the double-layer thermocouple structure can be further reduced while maintaining relatively high-performance. Micromachinessize of the double-layer2019, 10, 877 thermocouple structure can be further reduced while maintaining relatively5 of 10 high-performance.

Figure 3. Schematic of a thermopile infrared detector based on a double-layer thermocouple structure. Figure 3. Schematic of a thermopile infrared detector based on a double-layer thermocouple structure. Figure 3. Schematic of a thermopile infrared detector based on a double-layer thermocouple structure. Table I. Theoretical thermoelectric properties of materials [11,12]. Table 1. Theoretical thermoelectric properties of materials [11,12]. Table I. Theoretical thermoelectricN-type prop Poly-Sierties of materials [11,12].P-type Poly-Si Material Type Al N-Type Poly-Si P-Type Poly-Si Material Type Al (doped @ 3.64 × 102020 cm−3) 3 (doped @ 1.82 × 102020 cm−3) (DopedN-type @ 3.64 Poly-Si10 cm− ) (DopedP-type @ 1.82 Poly-Si10 cm − ) Material Type −1 Al × × Seebeck coefficient (μVK1 ) −1.66 −124.17 20 −3 105.76 20 −3 Seebeck coeﬃcient (µVK− ) 1.66 (doped @ 3.64124.17 × 10 cm ) (doped @ 105.761.82 × 10 cm ) Thermal Conductivity − − SeebeckThermal coefficient Conductivity (μVK−1) −2371.66 −124.1735 105.76 30 1−1 −11 237 35 30 Thermal(Wm(Wm Conductivity− KK− ) Resistivity (µΩ m) 0.03237 35 2.7 6.55 30 Resistivity(Wm−1 (KμΩ−1) m) 0.03 2.7 6.55 Resistivity (μΩ m) 0.03 2.7 6.55 The detector was modeled and simulated with Ansys workbench 14.0. The temperature of the cold junction was 22 °C, radiant power density (φ0) was 66.732 W/m2, and the output electrode cold junctionThe detector was 22 was◦C, modeled radiant power and simulated density (ϕ with0) was Ansys 66.73 workbench W/m , and the14.0. output The temperature electrode potential of the potential of the N-type polysilicon was set as 0 V. It can be seen that the simulated temperature of the ofcold the junction N-type polysilicon was 22 °C, was radiant set as 0power V. It can density be seen ( thatφ0) was the simulated 66.73 W/m temperature2, and the ofoutput the hot electrode junction washotpotential junction approximately of thewas N-type approximately the same,polysilicon and the the was same, average set andas 0 temperaturetheV. It average can be seen temperature diﬀerence that the between simulated difference the temperature coldbetween junction the of cold and the junction and the hot junction was about 0.067 °C, as shown in Figure 4. thehot hotjunction junction was was approximately about 0.067 ◦theC, assame, shown and in the Figure average4. temperature difference between the cold junction and the hot junction was about 0.067 °C, as shown in Figure 4.

Figure 4. Simulation results of the temperature distribution of the detector. Figure 4. Simulation results of the temperature distribution of the detector.

4. Micro-FabricationFigure of 4. the Simulation Detector results of the temperature distribution of the detector. 4. Micro-Fabrication of the Detector The designed thermopile infrared detector with a double-layer thermocouple structure was microfabricated4. Micro-FabricationThe designed with thermopile the of CMOSthe Detector infrared compatible detector process. with However, a double-layer isotropic drythermocouple etching of the structure device oftenwas microfabricatedleadsThe to a suspensiondesigned with thermopile ofthe the CMOS cold junctioninfrared compatible anddetector outputprocess. with electrode Howe a double-layerver, during isotropic the thermocouple release dry etching process, structureof which the device causes was oftendamagemicrofabricated leads to theto a device. suspension with Tothe avoid CMOSof the this, cold compatible the junction etch stop process.and structure output Howe electrode is alsover, designedisotropic during and thedry release applied etching process, in of the the detector. whichdevice oftenThe leads micro-fabrication to a suspension of process the cold of junction the thermopile and output infrared electrode detector during is shownthe release in Figure process,5. which First, in order to build a release barrier structure, a deep etching process is used to form a closed-ring deep trench structure. Then, the thermal oxide and low pressure chemical vapor deposition (LPCVD) Micromachines 2019, 10, x 6 of 10 causes damage to the device. To avoid this, the etch stop structure is also designed and applied in the detector. MicromachinesThe micro-fabrication2019, 10, 877 process of the thermopile infrared detector is shown in Figure 5. First,6 of in 10 order to build a release barrier structure, a deep etching process is used to form a closed-ring deep trench structure. Then, the thermal oxide and low pressure chemical vapor deposition (LPCVD) polysilicon areare usedused toto fillfill thethe deepdeep trench, trench, and and then then an an 8000 8000 Å Å silicon silicon oxide oxide dielectric dielectric support support layer layer is grownis grown on on the the wafer wafer surface. surface. Then, Then, a a thin thin film film of of silicon silicon nitride nitridewith with highhigh thermalthermal conductivityconductivity and electrically insulation is filled filled at the cold end. The cold end of the thermal strip is in good thermal contact with the heat sink (Figure 5 5a).a). The LPCVD process isis used to deposit polysilicon, silicon oxide, and polysilicon thin filmsfilms sequentially;sequentially; the thickness of the polysilcon filmsfilms are 5500 Å and are P-type and N-typeN-type doped,doped, respectively. respectively. The The silicon silicon oxide oxid film,e film, which which is located is located between between two polysilicontwo polysilicon films, isfilms, utilized is utilized as a thermal as a thermal insulation insulation layer withlayer a with thickness a thickness of 4000 of Å 4000 (Figure Å (Figure5b). The 5b). three-layer The three-layer films arefilms patterned are patterned to form to aform double-layer a double-layer polysilicon polysilic thermocoupleon thermocouple structure. structure. Then, theThen, film the is film re-etched, is re- suchetched, that such the topthat polysilicon the top polysilicon (N-type) is(N-type) shorter thanis shorter the bottom than the polysilicon bottom (P-type)polysilicon to facilitate (P-type) the to connectionfacilitate the of connection multiple pairs of multiple of thermocouples pairs of thermocouples (Figure5c). (Figure 5c).

Figure 5. Fabrication process of the detector with a double-layer thermocouple structure and etch Figure 5. Fabrication process of the detector with a double-layer thermocouple structure and etch stop stop structure. (a) formation of etching barrier structures, (b) Poly-Si-SiO2-Poly-Si deposition and structure. (a) formation of etching barrier structures, (b) Poly-Si-SiO2-Poly-Si deposition and implantation, (c) Photo-patterning of the Poly-Si-SiO2-Poly-Si layers, (d) Passivation layer deposition implantation, (c) Photo-patterning of the Poly-Si-SiO2-Poly-Si layers, (d) Passivation layer deposition afer Al patterning, (e) Deposition and patterning of a SiNx layer, (f) XeF2 release of Si substrate. afer Al patterning, (e) Deposition and patterning of a SiNx layer, (f) XeF2 release of Si substrate. After that, a layer of silicon oxide on the surface of the double-layer polysilicon thermocouples is depositedAfter that, as aa protectivelayer of silicon film foroxide the on structure. the surfac Aftere of the filmdouble-layer is patterned, polysilicon an ohmic thermocouples contact hole inis deposited the structure as a is protective formed, and film then for the a patterned structure. aluminum After the wirefilm is layer patterned, is formed. an ohmic The cold contact area hole and hotin the junction structure areas is formed, are, respectively, and then a connected patterned by aluminum the “diagonal” wire layer shape is formed. and the The “climbing” cold area shape and aluminumhot junction line, areas which are, respectively, realizes the electricalconnected connection by the “diagonal” between theshape thermocouples, and the “climbing” as shown shape in Figurealuminum3. Then, line, thewhich plasma realizes enhanced the electrical chemical co vapornnection deposition between (PECVD) the thermocouples, process is used as shown to grow in aFigure silicon 3. oxideThen, filmthe plasma as a passivation enhanced layer chemical with vapo a thicknessr deposition of 4000 (PECVD) Å on the process wafer surface.is used to After grow the a passivationsilicon oxide layer film isas patterned, a passivation the siliconlayer with oxide a th layerickness on theof 4000 surface Å on of the hotwafer end surface. is removed After andthe passivation layer is patterned, the silicon oxide layer on the surface of the hot end is removed and the hot end of the P-type thermocouple is exposed (Figure5d). Then, a SiN x film, with a thickness of the hot end of the P-type thermocouple is exposed (Figure 5d). Then, a SiNx film, with a thickness of ~6000 Å, is deposited on the SiO2 layer and patterned into the absorber (Figure5e). Later on, a SiO 2 dielectric~6000 Å, is layer deposited is further on depositedthe SiO2 layer by PECVD, and patterned and then, into releasing the absorber windows (Figure are 5e). opened Later in on, this a layer.SiO2 dielectric layer is further deposited by PECVD, and then, releasing windows are opened in this layer. Finally, the thermopile device is released by isotropic etching of XeF2 gas (Figure5f). Finally,The the thermopile thermopile infrared device detector is released has by been isotropic successfully etching prepared, of XeF2 asgas shown (Figure in 5f). Figure 6a. The size of the detector is only 1.5 1.5 mm2; Figure6b is the photo of the detector after vacuum encapsulation. × Micromachines 2019, 10, x 7 of 10

MicromachinesThe thermopile 2019, 10, x infrared detector has been successfully prepared, as shown in Figure 6a. The7 ofsize 10 of the detector is only 1.5 × 1.5 mm2; Figure 6b is the photo of the detector after vacuum encapsulation. MicromachinesThe thermopile2019, 10, 877 infrared detector has been successfully prepared, as shown in Figure 6a. The7 ofsize 10 of the detector is only 1.5 × 1.5 mm2; Figure 6b is the photo of the detector after vacuum encapsulation.

Figure 6. The fabricated thermopile infrared detector (a) and the packaged device (b ). Figure 6. The fabricated thermopile infrared detector (a) and the packaged device (b). 5. CharacterizationFigure 6. ofThe the fabricated Detector thermopile infrared detector (a) and the packaged device (b). 5. Characterization of the Detector 5. CharacterizationAn infrared testing of the system, Detector based on the water cooling system, was established. The water coolingAn system infrared ensures testing that system, the basedtemperature on the waterof the coolingdetector system, is consistent was established. with room Thetemperature. water cooling The An infrared testing system, based on the water cooling system, was established. The water systemtesting system ensures consists that the of temperature a black body of infrared the detector radiation is consistent source (Eletrip with room BR 500, temperature. Optris GmbH, The testingBerlin, cooling system ensures that the temperature of the detector is consistent with room temperature. The systemGermany), consists a chopper of a black(Stanford body Research infrared Systems radiation SR540, source Standford (Eletrip BRResearch 500, Optris Systems, GmbH, Sunnyvale, Berlin, testing system consists of a black body infrared radiation source (Eletrip BR 500, Optris GmbH, Berlin, Germany),CA, USA), a choppercustom water (Stanford cooling Research system, Systems an infrared SR540, detector, Standford a low-pass Research filter Systems, circuit Sunnyvale, module, and CA, Germany), a chopper (Stanford Research Systems SR540, Standford Research Systems, Sunnyvale, USA),a B1500A a custom semiconductor water cooling characteristics system, ananalyzer infrared (Keysight detector, Technologies, a low-pass filter Santa circuit Rosa, module,CA, USA). and a CA, USA), a custom water cooling system, an infrared detector, a low-pass filter circuit module, and B1500AThe semiconductor vacuum encapsulated characteristics thermopi analyzerle infrared (Keysight detector Technologies, is fixed in the Santa water Rosa, cooling CA, USA).system, and a B1500A semiconductor characteristics analyzer (Keysight Technologies, Santa Rosa, CA, USA). the temperatureThe vacuum of encapsulated the heat sink thermopileof the device infrared is controlled detector at 22 is °C. fixed The in chopper the water is coolingplaced between system, The vacuum encapsulated thermopile infrared detector is fixed in the water cooling system, and andthe detector the temperature and the ofblack the heatbody sink to ofcontrol the device the chopper is controlled frequency, at 22 ◦ C.and The the chopper low-pass is placed filter circuit between is the temperature of the heat sink of the device is controlled at 22 °C. The chopper is placed between theused detector to suppress and high-frequency the black body noise to control in the the test. chopper The detector’s frequency, infrared and theradiation low-pass response filter circuitsignal is the detector and the black body to control the chopper frequency, and the low-pass filter circuit is usedprocessed to suppress by a low-pass high-frequency filter circuit noise and in therecorded test. The with detector’s the semiconductor infrared radiation characteristics response analyzer. signal is used to suppress high-frequency noise in the test. The detector’s infrared radiation response signal is processedDuring by the a low-pass test, the filter temperature circuit and of recorded the blackbody with the is semiconductorset to 500 K. At characteristics this time, the analyzer. infrared processed by a low-pass filter circuit and recorded with the semiconductor characteristics analyzer. radiationDuring power the test, density thetemperature on the surface of theof the blackbody detector is is set 66.73 to 500 W/m K.2 At. In this addition, time, the the infrared frequency radiation of the During the test, the temperature of the blackbody2 is set to 500 K. At this time, the infrared powerchopper density is set onto 4 the Hz. surface The tested of the detectorresult of isthe 66.73 ther Wmopile/m . In infrared addition, detector the frequency is shown of thein Figure chopper 7,8. is radiation power density on the surface of the detector is 66.73 W/m2. In addition, the frequency of the setFigure to 4 7 Hz. shows The the tested I-V resultcharacteristic of the thermopile curves of infraredthe device. detector The tested is shown results in Figuresshow that7 and the8. resistance Figure7 chopper is set to 4 Hz. The tested result of the thermopile infrared detector is shown in Figure 7,8. shows(R0) of the I-Vdevice characteristic is 458.5 kΩ curves. of the device. The tested results show that the resistance (R0) of the deviceFigure is7 shows 458.5 k theΩ. I-V characteristic curves of the device. The tested results show that the resistance (R0) of the device is 458.5 kΩ.

Figure 7. I-V characteristic curves of the thermopile infrared detector. Figure 7. I-V characteristic curves of the thermopile infrared detector. Figure8a shows the output voltage waveform of the detector in three periods. The tested results Figure 7. I-V characteristic curves of the thermopile infrared detector. showFigure that the 8a responseshows the voltage output of voltage the detector waveform is 7.47 of mV the underdetector a black in three body periods. radiation The of tested 500 K results and a choppershow that frequency the response of 4 voltage Hz. Figure of the8b detector shows the is 7.47 enlarged mV under rising a edge black of body the waveformradiation of in 500 Figure K and8, Figure 8a shows the output voltage waveform of the detector in three periods. The tested results R whichshow that shows the thatresponse the detector voltage responseof the detector time isis 14.467.47 mV ms. under The responsivity a black body radiationv of the device of 500 canK and be calculated as 1151.14 V/W. Similarly, NEPcan be calculated as 7.51 10 2 nW/Hz1/2, and the detectivity × − D * can be calculated as 4.15 108 cm Hz1/2/W. × Micromachines 2019, 10, x 8 of 10 Micromachines 2019, 10, x 8 of 10 a chopper frequency of 4 Hz. Figure 8b shows the enlarged rising edge of the waveform in Figure 8, a chopper frequency of 4 Hz. Figure 8b shows the enlarged rising edge of the waveform in Figure 8, which shows that the detector response time is 14.46 ms. The responsivity Rv of the device can be which shows that the detector response time is 14.46 ms. The responsivity Rv of the device can be calculated as 1151.14 V/W. Similarly, NEPcan be calculated as 7.51 × 10−2 nW/Hz1/2, and the detectivity calculatedMicromachines as2019 1151.14, 10, 877 V/W. Similarly, NEPcan be calculated as 7.51 × 10−2 nW/Hz1/2, and the detectivity8 of 10 D* can be calculated as 4.15 × 108 cm Hz1/2/W. D* can be calculated as 4.15 × 108 cm Hz1/2/W.

Figure 8. Response voltage of the detector under a black body radiation of 500 K and a chopper Figure 8. ResponseResponse voltage voltage of of the the detector detector under under a a black black body body radiation of of 500 K and a chopper frequency of 4 Hz (a) and the enlarged rising edge (b). frequency of 4 Hz (a) and the enlarged risingrising edgeedge ((b).).

This detector can also function as a vacuum sensor and a temperature sensor with high This detectordetector can can also also function function as a vacuumas a vacuum sensor sensor and a temperature and a temperature sensor with sensor high sensitivities.with high sensitivities. The vacuum sensing capability of the device was tested with a temperature and pressure Thesensitivities. vacuum The sensing vacuum capability sensing of capability the device of was the testeddevice withwas tested a temperature with a temperature and pressure and controlled pressure controlled cavity. The MEMS vacuum sensor is placed in the cavity, and the black body at a specific controlledcavity. The cavity. MEMS The vacuum MEMS sensor vacuum is placed sensor in is the placed cavity, in and the thecavity, black and body the at black a speciﬁc body temperatureat a specific temperature is used to provide a constant heating power to the chip, so that the heat absorption area temperatureis used to provide is used a constantto provide heating a constant power heating to the po chip,wer so to thatthe chip, the heat so that absorption the heat area absorption maintains area a maintains a constant surface radiation power density. The vacuum pump is used to control the maintainsconstant surface a constant radiation surface power radiation density. power The vacuum density. pump The is vacuum used to controlpump theis used vacuum to control conditions the vacuum conditions in the chamber and the output voltage is recorded by B1500A. vacuumin the chamber conditions and in the the output chamber voltage and is the recorded output byvoltage B1500A. is recorded by B1500A. When the vacuum conditions in the chamber are changed, the response signals are measured. When the vacuum conditions in the chamber ar aree changed, the the response response signals signals are are measured. measured. As shown in Figure 9, the output voltage of the device gradually decreases as the cavity pressure As shown in Figure9 9,, thethe outputoutput voltagevoltage ofof thethe devicedevice graduallygradually decreasesdecreases asas thethe cavitycavity pressurepressure increases. We obtained the response sensitivities of the devices at the surface radiant power densities increases. We We obtained the response sensitivities of the devices at the surface radiant power densities of 51, 47, 40, 34, and 28 W/m2, which are 0.83, 0.73, 0.53, 0.36, and 0.30 μV/Pa, respectively. It can be of 51, 47, 40, 34, and 28 W/m2, W/m2, which are 0.83, 0.73, 0.53, 0.36, and 0.30 μµV/Pa,V/Pa, respectively. It can be seen that, with the increase of surface radiant power density, the sensitivity of the device increases, seen that, with the increase of su surfacerface radiant power density, density, the sensitivity of the device increases, and the upper limit of the sensitivity of the detector reaches approximately 1055 Pa. and the upper limit of the sensitivity of the detector reaches approximatelyapproximately 10105 Pa.

Figure 9. Curves for cavity pressure versus output voltage of the thermopile detector exposed to Figure 9. Curves for cavity pressure versus output voltage of the thermopile detector exposed to Figurediﬀerent 9. powerCurves densities. for cavity pressure versus output voltage of the thermopile detector exposed to different power densities. different power densities. The temperature responses of the thermopile infrared detectors under diﬀerent vacuum conditions The temperature responses of the thermopile infrared detectors under different vacuum are tested.The temperature The vacuum pressureresponses in theof chamberthe thermopile is kept atinfrared a constant detectors value of 5under mTorr, different 5 Torr, and vacuum 50 Torr. conditions are tested. The vacuum pressure in the chamber is kept at a constant value of 5 mTorr, 5 Whenconditions the temperature are tested. The of the vacuum blackbody pressure changes in the from chamber 0 to 125 is keptC, the at outputa constant signal value ofthe of 5 detector mTorr, is5 Torr, and 50 Torr. When the temperature of the blackbody changes◦ from 0 to 125 °C, the output signal Torr,shown and in Figure50 Torr. 10 When. The sensitivitythe temperature of the detectorof the blackbody with a cavity changes pressure from of0 to 5 mTorr,125 °C, 5 the Torr, output and 50 signal Torr µ µ µ reaches 10.50 V/◦C, 7.80 V/◦C, and 5.00 V /◦C, respectively. The output signal of the detector increases gradually with the temperature increase. Its behavior is caused by the direct interdependence of the black body temperature and the detector surface radiation power density [6]. Micromachines 2019, 10, x 9 of 10 of the detector is shown in Figure 10. The sensitivity of the detector with a cavity pressure of 5 mTorr, 5 Torr, and 50 Torr reaches 10.50 μV/°C, 7.80 μV/°C, and 5.00 μV /°C, respectively. The output signal of the detector increases gradually with the temperature increase. Its behavior is caused by the direct Micromachines 2019, 10, 877 9 of 10 interdependence of the black body temperature and the detector surface radiation power density [6].

Figure 10. Temperature response of the thermopile infrared detector in different vacuum conditions. Figure 10. Temperature response of the thermopile infrared detector in different vacuum conditions. 6. Conclusions 6. Conclusion Due to the novel DLTS structure, the performance of the device has been greatly improved. A novel etch-stopDue structureto the novel is utilized DLTS instructure, the detector the toperformance prevent etching of the damage device to has the been structures greatly in improved. the releasing A process.novel etch-stop The responsivity structure is ofutilized the sensor in the achievesdetector to 1151.14 prevent V/ W,etching NEP damage reaches to7.51 the structures10 2 nW/ Hzin the1/2, × − thereleasing detectivity process.D* reachesThe responsivity 4.15 108 ofcm the Hz 1sensor/2/W, and achieves the response 1151.14 timeV/W, is NEP 14.46 reaches ms. The 7.51 response × 10−2 × sensitivitiesnW/Hz1/2, the in detectivity different temperature D* reaches 4.15 and × vacuum 108 cm Hz conditions1/2/W, and were the measured,response time and is can 14.46 be ms. applied The inresponse high-precision sensitivities infrared in different detection temperature applications. and The vacuum MEMS conditions infrared detector, were measured, with a double-layer and can be thermocoupleapplied in high-precision structure, was infrared designed detection and micro-fabricated applications. and The can MEMS be integrated infrared with detector, CMOS circuitswith a indouble-layer the future. thermocouple structure, was designed and micro-fabricated and can be integrated with CMOS circuits in the future. Author Contributions: Conceptualization, A.B. and C.L.; methodology, H.M.; validation, C.L.; formal analysis, R.L.;Author data Contributions: curation, Y.G.; Conceptualization, writing—original draft A.B. preparation,and C.L.; methodolo A.B. andgy, C.L.; H.M.; project validation, administration, C.L.; formal H.M. analysis, Funding:R.L.; data curation,This work Y.G.; was writing—original supported by National draft prep Naturalaration, Science A.B. and Foundation C.L.; project of China administration, (Grant No. H.M. 61401458, 61335008, 61136006 & 51205373), Jiangsu Natural Science Foundation (Grant No. BK20131098), Shanxi Natural ScienceFunding: Foundation This work (Grant was supported No. 201801D121157 by National and Natural 201801D221203), Science Foundation Scientific of and China Technological (Grant No. Innovation 61401458, Programs61335008, of61136006 Higher & Education 51205373), Institutions Jiangsu Natural in Shanxi Science (1810600108MZ). Foundation (Grant No. BK20131098), Shanxi Natural ConflictsScience Foundation of Interest: (GrantThe authors No. 201801D121157 declare no conflict and of201801D interest.221203), Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (1810600108MZ).

ReferencesConflicts of Interest: The authors declare no conflict of interest.

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