Some properties of a room temperature THz detection array
Irmantas Kašalynas
*a,b,Aurèle J. L. Adam
†a, Tjeerd O. Klaassen
a, Niels J. Hovenier
a,
Grégory Pandraud
c, Ventzeslav P. Iordanov
‡cand Pasqualina M. Sarro
da
Kavli Institute of NanoScience Delft, Delft University of Technology, P.O. Box 5046, 2600 GA
Delft, The Netherlands
b
Semiconductor Physics Institute, A.Goštauto 11, 01108 Vilnius, Lithuania
c
Electronic Instrumentation Laboratory, Faculty of Electrical Engineering, Mathematics and
Computer Science, Delft University of Technology, P.O. Box 5031, 2600 GA Delft, The
Netherlands
d
Delft Institute of Microelectronics and Submicron technology (DIMES), Technology and Materials
Lab, Delft University of Technology, P.O. Box 5053, 2600 GB Delft, The Netherlands
ABSTRACTDetection peculiarities of an un-cooled (room temperature) 8×8 pixel array designed to image broadband THz radiation were investigated. Each pixel consists of a thin conductive film absorber on a dielectric membrane with thermopile temperature readout. It was designed and tested for four combinations of two different types of absorber and thermopile materials. The photo-response profile, determined by scanning the pixels through the focus of a THz laser beam, was wider than expected from a 2-D convolution of the Gaussian beam and the absorber surface. Also the time response did depend on the position of the beam relative to the pixel. Simulations show that those properties are due to the fact that also the thermopiles absorb THz radiation. For the best composition of absorber and thermopile, the responsivity, the noise equivalent power, and the bandwidth were estimated to be of 28 V/W, 5×10-9 W/Hz1/2 and 50 Hz, respectively. Keywords: THz frequency detectors, THz detection array
1. INTRODUCTION
Nowadays a rapidly growing interest in the terahertz frequency range (0.3 – 5 THz) has been triggered by potential applications in fields such as imaging, medical diagnostics, communications and security.1-4 Astronomers and
spectroscopists as a pioneer users of this frequency range have mastered highly sensitive, bolometer-like, THz detection systems but operating at (sub-) liquid He temperatures.5 For many modern applications however, simple broadband room
temperature detection systems are necessary. At present, single pixel pyro-electric detectors are often used as simple broadband, tabletop, detectors with a fair speed and sensitivity.6 However, 2-D pyro-electric imagers are rare, bulky and
quite expensive due to employed hybrid technology. Recently, a micro-bolometer camera designed for wavelengths of 7.5-14 µm have been used successfully for detection of images at 2.52 THz.7 But, the sensitivity of such
micro-bolometers, with a pixel size of about 46 µm, is unknown and probably far away from optimum.
One of the goals of the (past) European HP-RTN project “Terahertz Electronics: Components and Systems (INTERACTION)” 8] was the design and fabrication of a 2-D array for the detection of broad band THz radiation at
room temperature, fast enough to enable real time imaging (30 frames/s or more), and cheap to produce. In order to ensure easy mass production, the design should be realized within standard Complementary Metal–Oxide– Semiconductor (CMOS) technology.
In this paper we report on the detection peculiarities of a room temperature THz detection array designed to image radiation in the wavelengths range of 50-500 µm. A pixel essentially consists of a thin film absorber upon a silicon
* e-mail: irmantak@ktl.mii.lt; phone (+370-5) 2312418, fax: (+370-5) 2627123
1nA/ /
7/f
/
//
//
///
/1/////
1
'Vi/ /
E E Readout pads Thermopile constant T reservoirnitride air bridge, with thermopile temperature readout. We investigated samples from the first production run of 8×8 pixel arrays, which are divided into four quadrants, one for each of the 4 combinations of absorber (TiN, highly doped polySi) and the thermopile material (p-polySi/n-polySi, p-polySi/Al). We used a sharply focused Gaussian laser beam (mainly at λ=118.8 µm) to investigate the pixel photo-response, such as the responsivity, time response and noise equivalent power. It was observed that the single pixel photo-response profile was wider than expected, and that the time response depended on the position of the beam with respect to the pixel. Simulations of a two-dimensional convolution of the Gaussian laser beam and the detector functions indicated that the thermocouples absorb THz radiation also. The THz detection arrays with TiN as absorber and p-polySi/n-polySi as thermocouples demonstrated to have the highest responsivity of about 28 V/W and a bandwidth of about 50 Hz.
2. ARRAY DESIGN AND EXPERIMENTAL SET UP 2.1 Design and fabrication
The design of the array is largely based on that of a mid-infrared detection array reported earlier.9 Briefly, the basic pixel
has a size of 1×1 mm2 and consists of an approximately 500 nm thick, 720×720 µm2, SiN membrane supported at the 4
edges by 280 µm wide silicon separation beams. At the centre a THz absorbing layer of 500×500 µm2 is deposited. The
temperature increase due to absorption of THz radiation is monitored using a thermopile, placed between the absorber film and the silicon beams that act as the “constant temperature” reservoir. The thermopile consists of 4×10 thermo-couples connected in series and placed at each side of the absorber. Two types of thermo-thermo-couples are considered for the first try out: (i) n- polySi /p-polySi, and (ii) p-polySi /Al.
Figure 1. Schematic view of the design of a single pixel. Figure 2. Schematic view of the cross section of the pixel.
The sensitivity of such a bolometric system depends on the thermal characteristics of the absorber. For a simple “ ideal” bolometer, the temperature increase as a result of optical absorption can equals ∆T = Pabs/G. Here Pabs is the power
absorption and G the heat conductivity towards the constant temperature reservoir. The time constant τ equals C/G, with C the heat capacity of the absorber. Clearly, in order to have a system with a short time constant, the heat capacity of the absorber should be small, and the heat conductivity should be chosen as small as possible to obtain a large temperature increase and at the same time reach the set value for τ. Clearly for the absorber a thin layer has to be used to minimize its heat capacity. As we aim at the detection of 50-500 µm radiation, it is not possible to find a “bulk” absorber with a large, wavelength independent absorptivity for this region. Therefore we have chosen for a thin metal film absorber with a 188 Ω sheet resistance as demonstrated in Ref. 10. Because the film is deposited on a dielectric membrane with an optical thickness much less then the THz wavelength, it can be considered to be “free standing” so it will act as a broadband absorber with a wavelength independent absorption coefficient of 50%. Two options were considered for the absorber sheet: one using a thin metal film, the other one using a highly doped polySi layer, both with a 188 Ω sheet resistance. Based on test of the THz absorption of various metal films available, we have chosen to use TiN with a thickness of about 30 nm. For the first try-outs it was decided to design the arrays without on chip readout of the thermopile voltages, but rely on an external readout system.
process, sheet resistances of deposited materials were checked. At the first run three wafers with 54 arrays per wafer were processed. After cleaving of the wafers, arrays were placed in a 144-pins chip carrier and bonded. Fig. 3 shows a
Table 1 The materials used for the absorber and the
thermopile in the production of THz detection array.
Quadrant Absorber Thermocouple
I TiN p-polySi/n- polySi
II polySi p-polySi/n-polySi
III polySi p-polySi /Al
IV TiN p-polySi/Al
Figure 3. Photo of an array split into four quadrants
with one for each combination of the 2 types of the absorber film and the thermopile materials which are summarized in Table 1.
photo of a THz detection array: the 4 different quadrants are visible due to different visible light reflection properties of the two absorber materials.
2.2 Experimental setup
Our experimental setup is shown in Fig. 4. The THz radiation source in the experiment was an optically-pumped molecular laser delivering a pulse of 60 ms duration with a repetition rate of 30 Hz. A small part of the laser beam was directed to the pyro-electric detector to monitor the illumination intensity simultaneously. The remaining beam was focused with a 25 mm focal length lens and positioned on the THz array or on a second pyro-electric detector calibrated by a home made absolute power meter. The positioning of the detector or array was performed using a three-axis translation stage. The pyro-electric detector has a circular sensitive element with a diameter of 5 mm, and could also be used with an additional pinhole with a diameter d of 0.15 mm, 0.3 mm, or 0.5 mm, to investigate the beam profile. The intensity of the laser beam could be changed by placing white paper sheets in the beam in front the Mylar beam splitter. Most experiments were performed at 2.52 THz (λ = 118.8 µm).
An electronic system that consists of four separate but identical channels designed for thermopile signal readout from each quadrant of the THz array has been designed (see Fig.4). Each of the four multiplexers connects one of 16 pixels of a quadrant to a low drift, low noise preamplifier. A personal computer with a National Instruments® data acquisition board controls the four multiplexers in parallel via opto-couplers, and acquires the thermopile voltage of four pixels at a time (one pixel per quadrant). By scanning the 16 possibilities, the system enables us to have a “live” image at a speed of 50 frames per second. We used an oscilloscope for the investigation of the time resolved photo-response of the pixels and the system noise.
We used a sharply focused beam to be able to study the photo response of a single pixel in detail. The beam shape at the focal point of the lens was determined by scanning the pyro-electric detector with pinhole. The scan result was normalized to the peak value and was fit to a two-dimensional (2D) convolution of the Gaussian beam and the detector functions. The Gaussian beam intensity in the x-y plane perpendicular to the beam is given by:
(
)
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
+
=
exp
2
2 2 2)
,
(
ω
y
x
y
x
f
. (2.1)Here the beam waist ω is the radius by which the intensity decreases by a factor of 1/e2.
The convolution of the two functions is described by:
( ) (
t
p
x
y
t
)
d
dt
f
p
f
∫ ∫
∞ ∞ − ∞ ∞ −−
−
=
∗
τ
,
τ
,
τ
. (2.2)Pyr S 3 axis translation stage Lens Beam 25 mm Splitter
Mirror THz detection array
placed in 144-pin socket
/ Pyroelectricwith
\
s
Pinhotewhichd: 0 II
• 0.15mm 0.. I I C/)i•
•
0.50mm•
/
2DModeling i I Gaussianu 0.5mm Pinhole diameterd:\
" /I
—0.15mm.
II / ——— 0.50mm \ 1E-5 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 Positionof detector (mm)1r.
Pimentv
I eraCam1St Quad:• 5th Pixel • 12th Pixel
2D Modeling
Gaussian u 0.5 mm
Square Pixel width:
---;;
\\•.•. .
06 04 02 00 02 04 06 08
Positionof pixel (mm)
important to determine the value of the pixel responsivity E (V/W), given by the ratio of the thermopile output voltage and the terahertz-laser power impinging on the absorbing surface.
Figure 4. Schematic diagram of the experimental setup. THz
detection array (THz camera) is on a 3 axis translation stage. Replacing the THz array by the calibrated pyro-electric detector is needed for the determination of the laser beam profile and the beam power.
Figure 5. Photo of part of the electronic board with the THz
detection array placed in a 144-pin socket, and the thermopile readout system.
The Noise Equivalent Power (NEP) is described by:
f
E
VND
NEP
∆
=
, (2.3)Here VND is the voltage noise density of the pixel measured using an oscilloscope at a frequency interval of ∆f under ambient conditions.
The time response of the pixel photo-response, that is both the rise and the fall time τRISE and τFALL, was estimated fitting
the time resolved measurements to an exponential function. The THz array bandwidth was estimated using the expression: BW = 0.35 / τFALL.
3. RESULTS AND DISCUSSION
Figure 6. Points: Beam profile of the FIR laser beam in
the focal plane measured using a 5mm pyro-electric detector with 0.15 mm and 0.5 mm diameter pinhole in front. Lines: The fit of experimental data with 2D convolution of the Gaussian beam and the pinhole functions. Clearly the beam waist is about 0.5 mm.
Figure 7. Points: Photo-response of pixels scanned in
Lxperiment TeraCam 4th Quad A 1st Pixel • ttthPixet 2D Modeling Gaussian u 0.5mm ---0.6mm
•
/ —0.7mm 0.01 .11 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 Position of pixel (mm)J/H, H
0.5 CS C) CS CSo.o
-0.5 L. oectricdetectj\.
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Time (s)The result of a FIR laser beam profile scan at the focal distance of the lens using the pyro-electric detector with pinhole is shown in Fig. 6. To enable the investigation of the wings of the laser beam, also measurements with lower spatial resolution (larger pinhole) were performed. Equation (2.2) is used to fit the data with a Gaussian function using ω as an adjustable parameter; the fits are shown in Fig. 6. The simulation data assuming a beam waist of about 0.5 mm fit the experiments very well for both pinhole diameters.
A large variation was observed in the responsivity of the pixels. The pixels of quadrant I appeared to perform quite well, that is according to expectations, whereas those of quadrants II-IV exhibited in general a much too low responsivity. Typical results of photo-response measurements of two pixels from the quadrant I with TiN absorber film and p-polySi/n-polySi thermocouples are shown in Fig.7. The experimental data cannot be described accurately using the 2D convolution of a Gaussian beam with ω = 0.5 mm and a 0.5 mm square absorber surface. However, assuming a width of about 0.6 mm for the square absorbing surface, a much better fit is obtained. This result seems to indicate that the thermocouples also absorb THz radiation. And this leads to an effective increase of the size of the absorber surface. A typical photo-response profile of a pixel with a low responsivity (quadrant IV) is shown in Fig. 8. In this case the response with the beam in the center of the pixel is smaller than the response with the beam near the edges of the (TiN) absorber. Fitting the experimental data in the wings of the photo-response curve is possible only assuming a width of about 0.7 mm for the square absorbing surface. This maximum responsivity near the edge of the thin film absorber clearly proves the contribution to the photo-response from THz absorption of those parts of the thermocouples that are nearest to the thin film absorber. It should be noted that the response attributed to the thermopile absorption is at least a factor of 5 smaller than the response in the center of a high responsivity pixel. This fact does not show up in Fig.’s 7-9 because all those data are normalized.
In Fig. 9 a typical results of time resolved photo-response experiments are shown. With the beam focus at the center of the pixel the rise and fall times of time response are
considerably longer (τRISE≈ 4 ms and τFALL≈ 6 ms) than with
the focus at the thermocouples (τRISE≈ 2 ms and
τFALL≈5 ms). The smaller time constants for the
contribution due to the THz absorption of the thermo-elements are expected because of the good heat contact between the thermo-elements and the Si supporting beams and possibly the small heat capacity.
We have estimated the responsivity and transient constants of single pixels with the focused laser beam in the center of the pixel. The voltage noise amplitude measured using an oscilloscope at a frequency interval ∆f = 1 kHz at room temperature and normal pressure was of the order of 2-4 µV.
Figure 8. The same as in Fig.7 but for 4th quadrant
consisting of TiN absorber film and p-polySi/Al thermocouples. Note the dip in the response near the 0.0 position
Figure 9. Oscilloscope trace of the transient response of a
pixel placed at two different positions in the focal plane. Also the response of the fast pyro-electric detector used for reference is shown. The “overshoot” in the rising slope reflects the actual laser pulse shape; the overshoot in the falling slope is an electronic feature.
Table 2. Typical values for the response time, responsivity
E and the NEP of a pixel of the 4 different quadrants of the
All properties are summarized in Table 2. The values observed for the pixels in quadrant I are pretty close to the expected from simulations values of the responsivity, time response and NEP of about 25 V/W, 10 ms, and 4×10-9
W/Hz1/2, respectively. It should be noted that the responsivity here is given as voltage per incoming power; only 50% of
that power is absorbed by the thin film.
4. CONCLUSIONS
The detection properties such as the responsivity, E, the noise equivalent power, NEP, and the bandwidth, BW, of this novel room temperature THz detection array consisting of a free standing thin film absorber with thermopile temperature readout have been investigated. We studied four detector configurations, one for each combination of the two types of thin film absorber (p-polySi or TiN) and two types of thermocouple (p-polySi/n-polySi or p-polySi/Al) materials. It appears that only the combination of TiN absorber and p-polySi/n-polySi thermo-elements leads to well behaving detectors; all other combinations resulted in poorly performing systems. Possibly the highly doped p-polySi does not function as a macroscopic thin film absorber and/or the thermal connection between absorber surface and thermo-elements is not inferior. Together with the observed THz absorption of the thermopiles, that leads to a low overall responsivity, which is dominated by the relatively fast thermopile contribution. As a result these pixels exhibit smaller time constants than observed for well behaving pixels from quadrant I, with a dominant contribution to the responsivity from the absorber surface. The best combination of absorber and thermopile leads to values of E ≈ 28 V/W, NEP ≈ 5×10 -9 W/Hz1/2 and BW ≈ 50 Hz. Such an array certainly enables real time imaging with 30 frames/second.
ACKNOWLEDGMENT
This work has been performed within the framework of the European Union HP-RTN project “Terahertz Electronics:
Components and Systems (INTERACTION)”under contract No HPRN-CT-2002-00206.
REFERENCES
1. R.M. Woodward, B.E. Cole, V.P. Wallace, R. J. Pye, D.D. Arnone2, E.H. Linfield, and M. Pepper, “Terahertz pulse imaging in reflection geometry of human skin cancer and skin tissue”, Phys. Med. Biol. 47, pp. 3853–3863, 2002. 2. M. Nagel, M. Först and H. Kurz, “THz biosensing devices: fundamentals and technology”, J. Phys.: Condens.
Matter. 18, pp. S601–S618, 2006.
3. J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovannini, J. Faist, C. Kremser, and P. Debbage, “Imaging with a terahertz quantum cascade laser”, Opt. Express 12, 9, pp. 1879-1884, 2004.
4. P.H. Siegel, “Terahertz Technology,” IEEE Transactions on Microwave Theory and Techniques, pp. 910-928, 2002. 5. S. Verghese, P.L. Richards, S.A. Sachtjen, K. Char, “Sensitive bolometers using high-Tc superconducting
thermometers for wavelengths 20-300 µm”, J. Appl. Phys. 74, pp. 4251-4253, 1993.
6. R.W. Whatmore and R. Watton, “Pyroelectric materials and devices”, in: P.Chaper, C.T. Elliot (Eds.), Infrared Detectors and Emiters: Materials and Devices, Kluwer Academic Publisher, Boston, 2000.
7. A.W.M. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging using a microbolometer focal-plane array,” Opt. Lett. 30, 19, pp. 2563–2565, 2005.
8. http://www.ee.leeds.ac.uk/interaction/
9. A.W. van Herwaarden, F.G. van Herwaarden, S.A. Molenaar, E.J.G. Goudena, M. Laros, P.M. Sarro, C.A. Schot, W. van der Vlist, L. Blarre, and J.P. Krebs, “Design and fabrication of infrared detector arrays for satellite attitude control”, Sens. Actuators A 83, pp. 101-108, 2000.
