Diffractive microlens integrated into Nb5N6 ... - OSA Publishing

Diffractive microlens integrated into Nb5N6 microbolometers for THz detection Xuecou Tu,1 Lin Kang,1,* Chao Wan,1 Lei Xu,1 Qingkai Mao,1 Peng Xiao,1 Xiaoqing Jia,1 Wenbin Dou,2 Jian Chen,1 and Peiheng Wu1,3 1

2

Research Institute of Superconductor Electronics, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China State Key Lab of Millimeter Waves, School of Information Science and Engineering, Southeast University, Nanjing 210096, China 3 [email protected] * [email protected]

Abstract: We fabricated square diffractive microlens array with five staircases in the THz wave band for Nb5N6 microbolometers. With each microlens intergrated with an Nb5N6 microbolometer on the same substrate, an array chip was fabricated in the 4 inches silicon wafer. The lens exhibits good focusing and improves the coupling efficiency. The voltage response of the microbolometer integrated with diffractive microlens is 16 times higher than that of the microbolometer fabricated on silicon substrate. The microbolometers used as room-temperature detectors yield a good responsivity of 71 V/W and a noise equivalent power of 1.0 × 10−10 W/Hz. The diffractive microlens array features light weight, low absorption loss, and high resolution and can be mass produced using standard microfabrication techniques. ©2015 Optical Society of America OCIS codes: (040.0040) Detectors; (040.2235) Far infrared or terahertz; (110.6795) Terahertz imaging; (050.1965) Diffractive lenses

References and links 1. 2.

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#236330 - $15.00 USD Received 18 Mar 2015; revised 8 May 2015; accepted 11 May 2015; published 18 May 2015 © 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.013794 | OPTICS EXPRESS 13794

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1. Introduction Terahertz (THz) wave band has received much attention for medical, communication, homeland security, and space technology applications [1,2], in which detectors are an important component. Nevertheless, the lack of low-cost and sensitive room-temperature array detectors in this spectral region hampers the development of detection and imaging systems. Thus, the development of sensitive detectors has attracted much interest [3– 8].Various types of THz detector arrays, such as uncooled microbolometers [9,10], miniature neon indicator lamps [11], InGaAs Schottky barrier diode arrays [12,13], and field-effect transistor arrays [14,15], have been developed. We recently reported the use of Nb5N6 microbolometer array for THz detection [16]. In this detector, the absorber unit is smaller than the wavelength of the signal to be detected, resulting in substantial decrease in signal-to-noise ratio. A critical aspect of this detector is the coupling efficiency of the antennae, which should be fabricated on substrates to form compact arrays with readouts. To improve the coupling efficiency of the incident signal to the detectors, research has been conducted to increase the gain of the receiving antenna in the THz wave ranges by using two methods. The first technique utilizes the combination of a thin-film antenna with a quasi-optical lens [17,18], and the other method uses a mechanical-made horn antenna [19,20]. However, these configurations are complex for large-scale array chips in real-time, compact, and costeffective THz imaging systems. To solve these limitations, we designed and fabricated a diffractive silicon microlens array with multiple staircases in the THz wave band and the focal position to obtain good focusing. The microlens utilizes the focusing effect to concentrate the electromagnetic wave into the dipole feeder antenna, thereby increasing the irradiated power density. Previous research on the silicon diffractive lens improved the efficiency of THz detectors well [21,22]. However,


due to the fact that the sizes are much larger than the wavelength and these lenses could not been integrated with THz detectors on the same silicon substrate for its focal position is not on the substrate. As the diameter of the microlens in this paper is comparable with the wavelength in free space, an imaging microlens array can be constructed, with each microlens functioning as a separate imaging element. Furthermore, the microlens array can be easily integrated using standard micro-fabrication techniques for mass production. 2. Design of diffractive microlen Figure 1 shows the principle of diffractive microlens with multilevel staircases. As described in Ref. 23, the number of levels is defined as N, and each step has the same height h. The multilevel surface of a diffractive small lens presents a continuous profile at sufficiently high N. Thus, an approximate extended hemispherical lens [24] can be obtained by increasing the number of multilevel staircases. At normal incidence without a binary diffractive small lens, a converged wave from the objective lens convenes at the focal point O1.

Fig. 1. Principle of diffractive lens with multilevel staircases.

After adding a diffractive small lens, the constructive interference criteria should be satisfied to maintain the converged wave from the objective lens to be focused at a new focal point O2, which is located on the center back surface of the small lens. Before and after adding the diffractive small lens, constructive interference can be achieved if the optical path of each light continuously changes the value of kλ0, where λ0 is the wave length in free space and k = 0, ± 1, ± 2... For a specific zone, the following equation represents constructive interference [23]: HO1 = n2 HO2 ± k λ0

where |HO1 | is the optical path from the reference point A (the intersection point of EHL and the specific step of the diffractive small lens) to the point O1, whereas n2|HO2| is the optical path from the reference point to the new focal point O2, where n2 is the refractive index of the lens. In Fig. 1, f = R + L, f0 = R + L + d, where L = R/n2 is the extended length of the diffractive small lens and d = n2 × R–L is the distance between O1 and O2. Equation (2) can be rewritten as

n2

[ f − (m − 1)h]

2

+ rm 2 =

[ f0 − (m − 1)h]

2

+ rm 2

where rm is the annular radius of any step with m ranging from 1 to N, here k = 0. The radius of any step can be obtained by solving Eq. (2). For a diffractive small lens used to approximate the EHL with radius R and extended length L, the extended length of the diffractive small lens may be selected over the other values by changing the value of k. This process simplifies the selection of dielectric substrates with different thicknesses. Therefore, the diffractive small lens exhibits more design freedom than that of the EHL.


As diffraction limited imaging [25] can be used to determine the lowest spatial resolution of THz systems, the radius of the silicon (n2 = 3.45) lens is set as R = 1.4 mm for 0.22 THz radiation detection. The other parameters of the lens determined by Eq. (2) are as follows: L = 0.407 mm, h = 0.14 mm, S = 2.8 mm, r1 = 0 mm, r2 = 0.610 mm, r3 = 0.840 mm, r4 = 1.00 mm, r5 = 1.12 mm, r6 = 1.21 mm, r7 = 1.28 mm, r8 = 1.34 mm, r9 = 1.37 mm, and r10 = 1.39 mm. Thus, the thickness of the silicon wafer should be chosen as T = L + 9 × h = 1.67 mm. Considering the complexity of micro-fabrication, we selected the first five staircases for demonstration. 3. FDTD simulation

In this paper, FDTD is implemented to obtain the diffraction field in the focal plane of the lens. For simplicity, only two-dimensional (2D) problems are considered. We assume that the incident wave is a polarized plane wave with the electric (E) field amplitude of 1 V/m, propagates along the x-axis, and contains electric component parallel to the y-direction. The cross-section of the simulation grid is 20 μm. With the 2D FDTD method, the transformation of the incident wave to the converging wave, which concentrates on the focal point of the lens, can be determined, as well as the other details of the diffraction mechanism. For comparison, the E field on a silicon substrate with the same thickness (T = 1.67 mm) of the diffractive lens is illustrated using the microlens in the simulation model. Figure 2(a) shows the simulated contour pattern of the E field on the microlens at 0.22 THz. The E field distribution in the x-y plane presents a focal spot at the end of the microlens (on the x = 0 plane). Diffraction fields are found in the region other than the focal region of the lens. At the front of the pattern, some reflected wave rays are observed in the first staircase, resulting in the low diffractive efficiency of the binary lens. The red dashed line in Fig. 2(b) shows the amplitude of the E field on the focal plane of the lens, which is also located at the end of the diffractive lens along the y-direction as indicated in (a). The blue dashed line in Fig. 2(b) shows the E field on the edge plane of the silicon substrate (on the x = 0 plane). At x = 0 and y = 0, the E field is 8.4 V/m, which is 3.5 times higher than that on the plane of the substrate. The voltage response of the Nb5N6 microbolometer is proportional to the square of the E field and can be improved by 12 times by integrating five silicon staircases.

Fig. 2. Simulated focusing characteristics at 0.22 THz. (a) E field distribution in the x-y plane. (b) The red dashed line shows the E field on the focal region of the lens (on the x = 0 plane) along the y-direction as indicated in (a). The blue dashed line shows the E field on the plane of the silicon substrate (on the x = 0 plane).

Figure 3(a) shows the frequency dependence of the E field at the focal region of the microlens (x = 0) as shown by the deep red dashed line in Fig. 2(a). As shown in Fig. 3(a),


from 0.16 THz to 0.26 THz, the incidence of the plane wave rays converges at the focal region and the area of the focal spot increases with the incident wavelength. This finding indicates that the microlens exhibits good focusing performance in the simulated THz range. Furthermore, the E field at the focal region displays multiple peaks and clear variations in different frequency ranges. The multiple peaks are fundamental modes (P1–P3) of the Fabry– Pérot cavity caused by the surface of the first staircase and the bottom surface of the microlens; these modes are well reproduced by simulation using the silicon substrate with the same thickness as the microlens.

Fig. 3. Frequency dependence and focusing performance of the microlens. (a) E field in the focal region of the silicon lens under different frequencies. (b) The red dashed line shows the E field extracted from (a) at y = 0 under different frequencies. The blue dashed line shows the E field at y = 0 on the silicon substrate plane under different frequencies. (c) shows the square of the E field enhancement at x = 0 and y = 0 extracted from (b).

The red dashed line in Fig. 3(b) shows the frequency distribution of the THz field at y = 0 extracted from Fig. 3(a). The blue dashed line shows the E field at y = 0 on the silicon substrate plane under different frequencies. The resonances P1–P3 correspond to the seventh, eight, and ninth cavity modes, respectively (n2T = 7λ/2, 4λ, and 9λ/2, with n2 = 3.45 as the refractive index of silicon and T = 1.67 mm as the thickness of the microlens and the substrate). Considering the focus of the microlens, we detected the enhanced THz field in all simulated frequencies from the two lines. At these resonances, the local THz field is further enhanced by the resonance effect caused by the surface of the first staircase and the bottom surface of the microlens. The enhancement factors extracted by E2microlens/E2substrate with Emicrolens and Esubstrate at x = 0 and y = 0 are denoted by the red and the blue dashed lines in Fig. 3(b), respectively. As shown in Fig. 3(c), the enhancement factor differs from the amplitude of the E field in Fig. 3(b). The peaks of the enhancement almost correspond to the troughs of the amplitude of the E field. This finding indicates that the enhancement is due to the diffractive effect of the staircases, rather than from the resonant effect. The maximum enhancement factor can reach 13 without a resonance frequency, but is only 3 at the resonant frequency demonstrated in Fig. 3 (c). Therefore, many staircases should be engineered to mitigate the refraction and achieve effective diffractive microlens. This requirement is verified by our further simulation and former theoretical analysis [23,26]. 4. Fabrication of Nb5N6 microbolometer integrated with diffractive microlens array

The microbolometer consists of a gold dipole planar antenna and a microbridge, which is the core element and is made of properly patterned Nb5N6 thin film. In terms of the antennacoupled microbolometer, the irradiation power is received by the antenna, then the induced current on the antenna passes through the microbolometer, and finally the resistance of the bolometer changes with the Joule heating. Therefore, in order to deliver the maximum power


from the antenna to the bolometer, the impedance matching between the antenna impedance and the resistance of the microbolometer becomes important. The resistance of the Nb5N6 microbolometer is about 1 kΩ, which is much larger than conventional metal microbolometers. Therefore,we need to desing an antenna with high impedance. A resonant dipole antenna with 0.6 kΩ input impedance was chosen for such a microbolometer. The fabricated wafer (4 inches) with diffractive microlens array and Nb5N6 microbolometers is shown in Fig. 4(a). Each microlens contains an Nb5N6 microbolometer at its center (Fig. 4). To fabricate the integrated chips, we used high-resistivity (ρ > 3000 Ω•cm) silicon wafers with a thickness of 1.67 mm. Standard micro-fabrication techniques were implemented as follows. First, a 100 nm-thick SiO2 was deposited on one side of the wafer through thermal oxidation. This combination was selected because of the low SiO2 loss under the millimeter wave frequency and the presence of easy-fabricating air bridges, which reduce the effective thermal conductance of the substrate and enhance the responsivity of microbolometers. To localize the Nb5N6 microbolometers at the center of the diffractive lenses, we made gold alignment marks for the following lift-off process. Second, fivestaircase square lens array was formed at the other side of the wafer through deep silicon etching (Fig. 4(b)). The parameters of the fabricated lens array are similar to that designed as mentioned above. Third, a 120 nm Nb5N6 film was deposited on the SiO2 film through radio frequency magnetron sputtering. Fourth, the Nb5N6 film was patterned into microbridges by using photolithography and the extra Nb5N6 film was etched by reactive ion etching (RIE). RIE was performed in an SF6 gas chamber for 55 s under a pressure of 4 Pa and a radio frequency power of 300 W. Fifth, we designed simple dipole antennae as contact pads to funnel the radiation into the absorber element, i.e., the Nb5N6 film microbridge. The dipole antennae enable the good response of the detector to the polarization of the incoming 0.2 THz and the surrounding radiation. The dipole antenna was formed with the apex connected to the Nb5N6 film microbridge by depositing a 300 nm gold film through the lift-off technology. Sixth, two square areas near the Nb5N6 microbridge were defined by photolithography, and the surficial SiO2 was etched in a buffered HF solution. The size of the microbridge was 3 μm × 3 μm, and the resistance was about 0.9 kΩ. The formed opening silicon was then etched by RIE to create air cavities in an SF6 gas chamber for 8 min at 8 Pa with a radio frequency power of 70 W. Finally, the air bridge under the Nb5N6 microbridge was formed by anisotropic etching (Fig. 4(c)). For comparison, an Nb5N6 microbolometer array chip without diffractive lens array was simultaneously fabricated on the silicon substrate with similar thickness by using similar steps.

Fig. 4. (a) Wafer (4 inches) with diffractive microlens array. (b) Optical microscope picture of fabricated five-staircase square silicon microlens array chip. (c) SEM micrograph of an Nb5N6 microbolometer fabricated at the back center of the microlens. The micrograph is acquired by tilting the sample at an angle of 60° with respect to the horizontal plane.


5.Experimental results and discussion

Nb5N6 thin film has been suggested as a bolometer candidate since thin-film Nb5N6 has a high value of the temperature coefficient of resistance (TCR), which is given by TCR = 1 / R × dR / dT . The value of the resistance is increased as the temperature drops, which is indicative of its semi-metallic character. The measured TCR of the Nb5N6 microbolometer is illustrated in blue in inset of Fig. 5. The Nb5N6 microbolometer has a negative TCR of −0.7% K−1 at 300 K. The current-voltage (I-V) curve of the fabricated Nb5N6 microbolometer is measured at room temperature using a bias source. The electric responsivity SE at each point on the I-V curve can be calculated by Jones’ expression [27]. Clearly shown in Fig. 5, the maximum electric responsivity is 560 V/W at a bias current of 0.24 mA, where is the best operation point for this microbolometer.

Fig. 5. The SE - Ib curves of the Nb5N6 microbolometer. The best electrical responsivity is 560 V/W at Ib = 0.24 mA. The inset shows the measured TCR of the Nb5N6 microbolometer.

To demonstrate that the microlens can improve the coupling efficiency of the detector, we measured the voltage response of the Nb5N6 microbolometers, which were integrated with the microlens and fabricated on the silicon with the same thickness as the microlens. The uniform fabrication process ensures that the electronic performances of the microbolometers are almost similar. Figure 6 shows the schematic illustration of the experimental setup for the optical responsivity measurements. THz source is provided by the multipliers from Virginia Diodes, Inc., Charlottesville [28] that multiplies a low-frequency signal to THz frequency range. The output power of THz source is about 0.3 mW, which is varied with signal frequencies. It was modulated using a 4 kHz TTL signal. The radiation is focused to yield the largest possible signal from the detector by two off-axis parabolic mirrors. For alignment procedure, a laser beam was used for rough adjustment, and then the microbolometer was moved until its response voltage reaches the maximum value. The voltage responses of the microbolometers are read out by a lock-in amplifier.


Fig. 6. Schematic of the experimental setup for optical responsivity measurements.

In Fig. 7(a), the red dashed line shows the measured optical responsivity (RO) of the Nb5N6 microbolometer integrated with diffractive lens as a function of the frequency from 0.165 THz to 0.255 THz. Peaks and troughs exists in the response of the detector as indicated by the interference between the first staircase and the bottom surface of the microlens of the diffractive lens. The optical responsivity was directly extracted from the measured voltage Δu by using the relationship RO = Δu/Pin, where Pin is the total radiation power focused by the off-axis parabolic lens. In the formula, the whole power incident on the lens effectively couples to the microbolometer. The maximum optical responsivity is 71 V/W at 0.21 THz, with a measured noise value of 7 nV/√Hz. The corresponding noise equivalent power (NEP) value is 1.0 × 10−10 W/√Hz, which significantly demonstrates the good performance of the Nb5N6 microbolometers integrated with diffractive lens. As the performance reflects the total signal power incident on the device, it is not corrected for the coupling efficiency of the radiation into the absorber element (Nb5N6 microbridge). Thus, the voltage responsivity values must be carefully compared with those derived from the effective signal power absorbed by the detector. The voltage response of the integrated microbolometer is proportional to the square of the incident E field. The square of the amplitude of the E field is presented as the blue line in Fig. 7(a); this amplitude was calculated by the FDTD simulation at x = 0 and y = 0, which is the center of the diffractive lens at which Nb5N6 microbridge was fabricated. For comparison, the measured optical responsivity (RO) of the Nb5N6 microbolometer fabricated on the silicon substrate is also presented as the dashed red line in Fig. 7(b). The square of the amplitude of the E field on the substrate is also presented as the blue line in Fig. 7(b); this amplitude was calculated by FDTD simulation at x = 0 and y = 0 at which the Nb5N6 microbridge was fabricated. The calculated value of E2x = 0, y = 0 fits the measured voltage response from 0.165 THz to 0.255 THz. The frequency shifts may be due to the thickness and reflection index deviation.


Fig. 7. (a) Red dashed line shows the measured response of the Nb5N6 microbolometer integrated with the five-step diffractive microlens. The blue line shows the calculated value of E2x = 0, y = 0 on the microlens. (b) The red dashed line shows the measured optical responsivity of the Nb5N6 microbolometer fabricated on the silicon substrate. The blue line shows the calculated value of E2x = 0, y = 0 on the substrate.

Figure 8 shows the enhancement of the optical response of Nb5N6 microbolometer integrated with the microlens compared with that of the microbolometer fabricated on the substrate. The red dashed line shows the measured optical responsivity enhancement of the Nb5N6 microbolometer integrated with the microlens by dividing the optical responsivity of the Nb5N6 microbolometer on the silicon substrate. The result demonstrates that the use of diffractive lens, which matches with the incoming beam from 0.165 THz to 0.255 THz, improves voltage responsivity from about three to 16 times in the designed frequency range; this increase corresponds to the calculated enhancement of the square E field. The electrical responsivity of the microbolometer calculated from the I–V curve is approximately 560 V/W, whereas the optical responsivity of the microbolometer integrated with diffractive lens is only approximately 13%. Hence, a portion of the incident power is lost through reflection and diffraction of the diffractive lens. With the fabrication of many staircases, the responsivity increases. In addition, the coupling efficiency can be improved using an anti-reflection coating on the surface of the lens.

Fig. 8. Enhancement of the optical response of Nb5N6 microbolometer integrated with the microlens compared with of the microbolometer fabricated on the substrate.


6. Conclusion

We demonstrated that five-staircase diffractive silicon lens array integrated into Nb5N6 microbolometers can be used in the 0.165 THz to 0.255 THz wave band for focusing in simulations and experiments. With each microlens intergrated with an Nb5N6 microbolometer on the same substrate, an array chip was fabricated in the 4 inches silicon wafer. The coupling efficiency of the microbolometer is improved by 16 times by etching five staircases. An optical voltage responsivity of 71 V/W and an NEP of 1.0 × 10−10 W/Hz are obtained when the detector operates at 0.21 THz. The resultant NEP is comparable with that of the commercial thermal THz uncooled detector, although further enhancement may be performed for future device optimization. These results show that staircase diffractive microlenses can be potentially integrated into Nb5N6 microbolometers for THz detection. A focal plane array can also be developed with these devices as detectors. In addition, the staircase diffractive lens can be easily integrated with other THz detectors for optical response enhancement. Acknowledgments

This work is supported by the National Basic Research Program of China (“973” Program) (Nos. 2014CB339800 and 2011CBA00100), the National High Technology Research Program of China (“863” Program) (No. 20l1AA010204) and the National Natural Science Foundation of China (Nos. 11227904). Also, it was partially supported by the Fundamental Research Funds for the Central Universities and Jiangsu Key Laboratory of Advanced Techniques for Manipulating Electromagnetic Waves.
