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

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THz CMOS FPA principle

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

Antenna

FET

3

Detector characterization

'

2

4

meas el

A

SNEP NEPG

1. Uniform NEP for different

elements of the array2. Minimal NEP

Goal

1. Maximal G and ηa

2. Uniform G and ηa

NEP – noise equivalent power. Minimal power which can be detected by detector

• NEPel electrical NEP of detector itself

• G the antenna gain• ηa matching between the

antenna and the detector

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System photograph (silicon FET array implementation)

Printed antennas on finite electrically thick substrate

Modelled system

5

System parameters

10

75.8

164

20

104

1a ,μm

2a ,μm

r,μm

d ,μm

φ,deg

Modeling using EMSS FEKO

10 mm

1 mm

1mm

The modeled system design: 8 antennas on a substrate of finite size. Antennas are positioned symmetrically relative to the substrate center

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Cut-off frequency of the first mode fc1 for infinite substrate

1 ,4 1c

r

cfh

h, µm εr=2 εr=7 εr =12

50 1.5THz 0.612 THz 0.452 THz

140 0.536 THz 0.219 THz 0.162 THz650 0.116 THz 0.047 THz 0.035 THz

Pozar, D.: Considerations for millimeter wave printed antennas. IEEE Trans. Antennas Propag. 31, 740–747 (1983)

1cf f 0.25critical dielh

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2r

7r

12r

1cf f

1cf f

1cf f

0.071diel

h

0.132diel

h

0.173diel

h

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

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

2r

7r

12r

1cf f

1cf f

1cf f

0.197diel

h

0.37diel

d

0.484diel

d

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

2r

7r

12r

1cf f

1cf f

1cf f

0.920diel

h

1.72diel

h

2.25diel

h

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Antenna pattern for different substrate relative permittivities

1cf f1cf f

Substrate thickness is h=140 μm

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Dependence of the calculated total antenna gain G in the normal direction on the substrate permittivity

2 4 6 8 10 12

-25

-20

-15

-10

-5

0

5

10

G, d

Bi

r

G1 G2 G3 G4

1 2 3 4 5 6 7 8

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Calculated gain for normal direction for 1st and 4th elements

280 290 300 310 320-30

-25

-20

-15

-10

-5

0

5

10

G, d

Bi

f, GHz

1, =2 4, =2 1, =12 4, =12

1 2 3 4 5 6 7 8

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Antenna – transistor matching

2max

Re Re4 ant tra

tr ant

P Z ZP Z Z

,int1

tr S G GSp

Z R R Zj C

Antenna FET

RG = 150 Ω, RS = 50 Ω, Cp= 4 fFZtr= (200 – j130) Ω at f = 300 GHz

1-μm Si MOSFETW/L = 20/2 (μm)

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

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Antenna-detector matching for different substrate thickness

1 2 3 4 5 6 7 8 • Optimal matching is not for electrically thinnest substrate

• Matching coefficient variation is less than gain variation

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System with the lens

lh la

123

4

1 2 3 40

5

10

15

20

25 1

2

3

Ang

le fo

r max

imum

gai

n

Element number

Modelled points Linear fit

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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

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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

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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.

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Thank You !