How to achieve a homogeneous sensitivity in THz detector arrays

1. How to achieve a homogeneous sensitivity in THz detector arrays M. Sakhno, J. Gumenjuk-Sichevska, F. Sizov Institute of Semiconductor Physics NASU, Kiev, Ukraine, e-mail: sakhno.m@gmail.com

2. THz CMOS FPA principle Antenna • Advantages of Si FET THz Detectors • Based on standard silicon technology with high level of integration • Un-cooled • Can be assembled into arrays for real time THz/mm wave imaging; • Mechanically robust; • Low costs at high volumes FET

3. Detector characterization NEP – noise equivalent power. Minimal power which can be detected by detector Goal Uniform NEP for different elements of the array Minimal NEP • NEPelelectrical NEP of detector itself • G the antenna gain • ηa matching between the antenna and the detector 1. Maximal G and ηa 2. Uniform G and ηa

4. System photograph (silicon FET array implementation) Printed antennas on finite electrically thick substrate Modelled system

5. System parameters 1 mm 1mm 10mm The modeled system design: 8 antennas on a substrate of finite size. Antennas are positioned symmetrically relative to the substrate center Modeling using EMSS FEKO

6. Cut-off frequency of the first mode fc1 for infinite substrate Pozar, D.: Considerations for millimeter wave printed antennas. IEEE Trans. Antennas Propag. 31, 740–747 (1983)

7. Linear gain diagram for substrate thickness h=50 μm, f=300GHz Each antenna was simulated and the results were combined on one picture to facilitate the comparison of different elements

8. Linear gain diagram for substrate thickness h=140 μm, f=300GHz

9. Linear gain diagram for substrate thickness h=650 μm, f=300GHz

10. Antenna pattern for different substrate relative permittivities Substrate thickness is h=140 μm

11. Dependence of the calculated total antenna gain G in the normal direction on the substrate permittivity 5 7 6 8 4 2 3 1

12. Calculated gain for normal direction for 1st and 4th elements 5 7 6 8 4 2 3 1

13. Antenna – transistor matching 1-μm Si MOSFET W/L = 20/2 (μm) Antenna FET RG = 150 Ω, RS = 50 Ω, Cp= 4 fF Ztr= (200 – j130) Ω at f = 300 GHz Sakhno, M., Golenkov, A., & Sizov, F. (2013). Uncooled detector challenges: Millimeter-wave and terahertz long channel field effect transistor and Schottky barrier diode detectors. Journal of Applied Physics, 114(16), 164503. doi:10.1063/1.4826364

14. Antenna-detector matching for different substrate thickness 5 7 6 8 4 2 3 1 • Optimal matching is not for electrically thinnest substrate • Matching coefficient variation is less than gain variation

15. System with the lens The angle of maximum gain versus the element position for the system with the lens (only the first four elements are shown because of the mirror symmetry). The substrate parameters are h=50 μm, r=2, the incident radiation frequency is 300 GHz

16. Conclusions • The substrate electric thickness in THz FPAs plays a crucial role in the frequency characteristics of the system • Electrically thick substrate makes NEP of elements non-uniform • Degradation of antenna pattern can be explained by excitation of substrate modes. • Critical substrate thickness is approximately 0.25 wavelength in dielectric • Simulation shows that Si CMOS system (substrate thickness h = 50μm and εr = 2) with the lens can operate as FPA

17. Acknowledgements • This work is partly supported by the SPS:NUKR.SFP 984544 Project and a joint grant 01-02-2012 from the National Academy of Sciences of Ukraine and Russian Academy of Sciences.

18. Thank You !