Room-temperature thermal detection at a wavelength of 2 µm in the short-wave infrared range (1.7–3 µm) was demonstrated for the first time using a Nb5N6 microbolometer. The photothermal responses of two types of Nb5N6 microbolometers were evaluated. By suspending Nb5N6 microwires in the air above the substrate, a reduction in thermal conductance of the device by a factor of 39 was achieved. The measured optical voltage responsivity RO of the Nb5N6 microbolometer reached the value of 61.5 V/W. A noise equivalent power of 8.5 × 10−11 W/√Hz (at 1 kHz) and a detectivity D* = 2.0 × 107 cm√Hz /W with a typical response time as small as 0.17 ms was obtained at a wavelength of 2 µm for a 10 × 30-µm2 device. The performance could be improved further by optimizing the design and operating parameters. This study revealed a simple low-cost technique to develop a large-scale focal plane array in silicon for infrared detection.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The detection of short-wave infrared (SWIR) radiation near 2 μm, which typically relies on thermal and photo responses , attracts significant research interest owing to the applications in the fields of medical treatment , remote atmospheric sensing , hyperspectral imaging , meat production monitoring , gas sensing , and telecommunications . In general, infrared (IR) photon detectors, including photoconductors , quantum well photodetectors , and superconductor detectors , exhibit rapid response and high sensitivity; however, they usually require complex fabrication processes and low-temperature operation with a cooling equipment. Thermal detectors, such as thermocouple detectors and resistance thermal detectors, can operate at room temperature with a broadband response; however, they usually suffer from a slow response owing to the relatively large thermal inertia of the sensitive elements. Microbolometer [11–14] is a type of thermal detector, constructed using materials with a high temperature coefficient of resistance (TCR = ), so that the absorbed IR radiation changes the resistance. Microbolometers based on different materials are integrated in various technologies for IR detection and thermal imaging. They operate at room temperature; however, they exhibit a low overall performance; their slow response (in the range of milliseconds) is a typical disadvantage. In order to improve the response of a thermal detector, the thermal inertia and size of the sensitive element were reduced ; the responsivity was also improved by employing an air-bridge microstructure . Recently, micro- and nanobolometers based on novel materials have attracted significant attention for IR detection [17–22]. However, these materials cannot be easily obtained in large quantities. In addition, the fabrication process cannot be easily integrated with standard silicon technologies; therefore, it is challenging to prepare large-scale arrays for IR imaging.
A typical microbolometer detector structurally consists of several stacked layers constructed in a Fabry Perot cavity configuration to achieve maximum absorption of the incident IR radiation and to deliver the maximum energy to the heat sensitive layer. The layer stack typically consists of (top-to-bottom): An absorber layer, a heat sensitive layer, a mechanical support layer, a thermal isolation layer and a reflector layer (mirror) . The most widely used heat sensitive layer material in microbolometers is vanadium-oxide (VOx). By using Micro-Electro-Mechanical System (MEMS) techniques, VOx thin-films have shown high temperature coefficients of resistance (TCR), and suitable pixel resistances for CMOS readout circuits [24–27]. Despite those advantages, VOx thin-films have a low infrared optical absorption coefficient in the IR band. Thus an absorber material layer is required inside the detector stack to perform absorption and then transfer heat to microbolometer by virtue of thermal conduction. Silicon nitride (Si3N4) , Titanium (Ti) , Nichrome (NiCr) , and black gold  have been used as absorber materials in the VOx-based microbolometers. This increases the difficulty of the preparation process. In addition, semiconductor films of vanadium oxide have a large resistance at 300 K, so the thermal noise will be relatively large. Our earlier studies  demonstrated the possibility of using Nb5N6 thin-films as the absorber layer and also the heat sensitive layer in microbolometer fabrication. In the last years, we have proposed the use of sputtered Nb5N6 thin film as sensing material for terahertz detection [32–34] for it exhibited moderate values of the electrical resistivity to obtain low-noise resistors. In our recent experiments, we revealed that these films have a strong absorption in the IR regime. Moreover, the material is processed at temperatures below 50°C and patterned using standard reactive ion etching (RIE), being therefore fully compatible with conventional technologies in silicon integrated circuits (ICs).
In this study, the photothermal responses of Nb5N6 microbolometers with different structures are investigated for a 2-µm detection. Moreover, a sensitive room-temperature thermal detector with a D* = 2.0 × 107 cm√Hz /W and typical response time as small as 0.17 ms is achieved by suspending Nb5N6 microwires in the air above the substrate. Nb5N6 microbolometer was proposed for a 2-µm detection, which exhibited relatively high sensitivity, low cost, and wideband detection. The approach could be applied for infrared focal-plane imaging array.
2. Device design and fabrication
The performance of a microbolometer can be improved by increasing its thermal impedance . For example, the performance of an air-bridge microbolometer is optimized by suspending the device in air above the substrate. The only conduction path is from the ends of the detector to the metal pads. In this study, two Nb5N6 microbolometer samples are fabricated. The microwire sizes of the two microbolometers are equal to 10 μm × 30 μm. Sample A is fabricated on a thermally oxidized silicon substrate with a 200-nm-thick layer of SiO2. The SiO2 layer acts as a thermal insulator between the Nb5N6 microwire and Si substrate. Sample B is fabricated on the same thermally oxidized silicon substrate; the middle part of the wire is suspended above the air to obtain isolation from the substrate. The SiO2 layer in sample B supports the suspending Nb5N6 microwire in case of fractures of the Nb5N6 thin film. Radio-frequency (RF) magnetron sputtering is used to deposit a 120-nm-thick Nb5N6 film on the substrate. The square resistance of the Nb5N6 thin film is approximately 0.5 kΩ with a TCR of up to −0.7% K−1 at 300 K. Subsequently, the Nb5N6 thin film is patterned into microbridges using photolithography and reactive ion etching (RIE). The resistance of the Nb5N6 microbridge depends on the dimensions of the Nb5N6 film. Test leads are integrated with the Nb5N6 thin-film microbridge by depositing a 5-nm-thick Ti film and 220-nm-thick gold layer, and using the lift-off technology. The Nb5N6 microwire is placed across two Ti/Au electrodes sputtered on the plate, as illustrated in Fig. 1(a). For sample B, two etching areas at both sides of the Nb5N6 microbridge are defined by patterning the substrate with a photoresist and etching the surficial SiO2 in a buffered-HF solution. The opening formed on silicon is then etched using RIE to create an air cavity [Fig. 1(b)]. RIE is performed in SF6 gas at a pressure of 8 Pa and RF power of 70 W. The air-bridge [Fig. 1(c)] under the Nb5N6 microbridge is formed by anisotropic etching at 3 μm of the Si part of the substrate. The fabricated sample B is shown in Fig. 1.
The electrical properties of the bolometers were investigated by measuring current–voltage (I–V) curves using a source meter (Keithley 2400) in a DC current bias mode from −0.8 to 0.8 mA, as shown in Fig. 2. The linear shape of the curve for sample A in the range of −0.8 to 0.8 mA reveals good ohmic contacts between the Nb5N6 microbridge and electrodes, while sample B exhibits semiconducting properties, which is desirable for IR detection. Sample B exhibited a nonlinear behavior with the increase of the bias current. This is related to the bolometric effect, as the bias current power increases the temperature. It is attributed to the air-bridge structure, which yields a low thermal conductance [16, 17]. The voltage responsivity of the microbolometer can be expressed as :35, 36]:33]. The measured resistances at room temperature of samples A and B are 1.48 kΩ and 1.42 kΩ, respectively. By substituting the measured resistance in Eq. (1) and extracting the slope of the fitted curves (α/G), shown in the inset of Fig. 2, G values are calculated to be 7.1 × 10−5 W/K and 1.8 × 10−6 W/K for samples A and B, respectively. The thermal conductance of the air-bridge microbolometer is approximately 39 times smaller than that of the microbolometer without an air-bridge. The thermal conductance is an important factor that directly affects the performance of microbolometers. A low thermal conductance is desirable; therefore, the thermal isolation and air-bridge technique employed in this study can improve the sensitivity of the detectors.
3. Nb5N6 microbolometer for a 2-µm detection
Figure 3 illustrates the setup used to characterize the Nb5N6 microbolometer for a 2-μm detection. The optical responses of the microbolometers were measured using a single-mode (SM) fiber-coupled mini laser diode source (Thorlabs FPL2000S); the modulation frequency could be set by an arbitrary waveform generator (AWG, Agilent 8357D) in the range of 0.01–100 kHz. The laser light was focused approximately 1.2 mm away from the microbridge of the device through the SM fiber (Thorlabs SM2000) centered at a wavelength of 2.0 µm, used to irradiate the microbolometers. The temperature of the Nb5N6 microwire increased with the light absorption, leading to a change in the resistance, which can be used to obtain the intensity of the incident light. The microbolometer connected in series with a bias resistance of 50 kΩ was DC-biased using a low-noise battery-power current source; the response voltage was measured by a lock-in amplifier (Stanford SR830).
Figure 4 shows the optical voltage responses under a 2-μm irradiation as a function of the bias current for samples A and B. A pulse with a wavelength of 2 µm, period of 1 ms, and width of 0.5 ms was irradiated on the microbridge. At low bias currents, the optical response voltage increased linearly with the bias current. At bias currents of 1.8 mA and 0.57 mA for samples A and B, respectively, the responsivity voltages of the microbolometers reached their maximum values, as shown in Fig. 4. The bias current could be increased up to 2.0 mA without burning out sample B, which demonstrates the robustness of this detector. The optical responsivity is defined as:Eq. (1)). The comparison of the above thermal conductances of the devices A and B reveals that the optical voltage response of the device B should be larger than 61.5 V/W. This difference could be attributed to two factors: misalignment between the light from the fiber and Nb5N6 microbolometer and interference attenuation caused by the formation of an optical cavity at the air bridge with a depth of 3 µm. The optical voltage response of the detector could be improved by a resonant optical cavity .
The response time of the Nb5N6 microbolometer detection, as a type of thermal detection, can be estimated as :
As depicted in Fig. 3, by changing the modulation frequency of the laser source and measuring the corresponding peak-to-peak value of the response voltage by the lock-in amplifier, we can obtain the response time of the detector . Figure 5 shows the modulation-frequency dependence of the resistance change of the Nb5N6 microbolometers caused by the incident pulses. The amplitude of the response voltage decreases with the increase of the modulation frequency. The measured 3-dB roll-off times of the Nb5N6 microbolometer samples A and B for a 2-μm detection were approximately 0.1 ms and 0.17 ms, respectively, smaller than those of microbolometers based on other materials reported in [14, 21], and one order of magnitude smaller than those of ZnO IR detectors .
A standard figure of merit to evaluate the performance of bolometers is the specific detectivity D*, expressed as :Eq. (5) that noise can severely limit the performance of the device. A is the active area of 10 μm by 30 μm. A dynamic signal analyzer (Agilent 35670A) and low-noise amplifier (LNA, Stanford SR560) with a voltage gain of 1,000 were used to measure the noise spectrum of the microbolometer in a shielded room. In experiments, the voltage noise spectral density of the microbolometer is obtained by subtracting the noise value of the cascaded LNA . The noise spectra of the Nb5N6 microbolometers tended to be constant when the modulation frequency was larger than 4 kHz . This result can be attributed only to the thermal noise (, kB is the Boltzmann constant and T is the absolute temperature ). Noise intensities of 4.5 nV/√Hz and 5.2 nV/√Hz (at 1 kHz), corresponding to NEPs of 1.1 × 10−10 W/√Hz and 8.5 × 10−11 W/√Hz, and were obtained for samples A and B, respectively. The experimental results for the two Nb5N6 microbolometers are summarized in Table 1. According to Eq. (5), a maximum detectivity of about 2.0 × 107 cm√Hz /W was reached for sample A.
A rapid thermal detection of a 2-μm irradiation using a Nb5N6 microbolometer was demonstrated for the first time. An NEP of 8.5 × 10−11 W/√Hz (at 1 kHz) and response time of 0.17 ms were obtained at a wavelength of 2 µm. Although a monochromatic 2-µm-wavelength laser was employed, the rapid sensitive response of the Nb5N6 microbolometer was extended to a wider IR spectrum owing to the broadband absorption of the Nb5N6 thin film in the IR spectral range and its bolometric detection nature. We aim to design an optical cavity with an increased optical absorption in a future study to enhance the sensitivity of the microbolometer. It is worth noting that a combination of the air-bridge Nb5N6 microbolometer array with a proper readout integrated circuit could be an efficient approach to develop sensitive low-cost focal-plane arrays for SWIR imaging applications.
National Basic Research Program of China (“973”) (2014CB339800), National Natural Science Foundation of China (11227904, 61521001, 61571217), Natural Science Foundation of Jiangsu Province (BK20160635), Fundamental Research Funds for the Central Universities, and Jiangsu Key Laboratory of Advanced Techniques for Manipulation of Electromagnetic Waves.
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