# On-Chip Characterization of High-Loss Liquids between 750 GHz and 1100 GHz

Juan Cabello-Sánchez, Vladimir Drakinskiy, Jan Stake, Helena Rodilla
Manuscript received July 17, 2020; revised September 5, 2020; accepted October
1, 2020. This work was supported by the Swedish Research Council
(Vetenskapsrådet) under grant 2015-03981 , and Knut and Alice Wallenberg
Foundation under grant 2014.0275.Juan Cabello-Sánchez, Vladimir Drakinskiy,
Jan Stake and Helena Rodilla are with the Terahertz and Millimetre Wave
Laboratory, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
(e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS>rodilla@chalmers.se)Color versions of one or more of
the figures in this article are available online at
http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TTHZ.2020.3029503

###### Abstract

Terahertz spectroscopy is a promising tool for analyzing the picosecond
dynamics of biomolecules, which is influenced by surrounding water molecules.
However, water causes extreme losses to terahertz signals, preventing
sensitive measurements at this frequency range. Here, we present sensitive on-
chip terahertz spectroscopy of highly lossy aqueous solutions using a vector
network analyzer, contact probes, and a coplanar waveguide with a 0.1 mm wide
microfluidic channel. The complex permittivities of various deionized
water/isopropyl alcohol concentration are extracted from a known reference
measurement across the frequency range 750–1100 GHz and agrees well with
literature data. The results prove the presented method as a high-sensitive
approach for on-chip terahertz spectroscopy of high-loss liquids, capable of
resolving the permittivity of water.

###### Index Terms:

Coplanar waveguides (CPW), isopropyl alcohol (IPA), material properties,
microfluidic channels, on-wafer measurements, permittivity, scattering
parameters, terahertz spectroscopy, vector network analyzers (VNA), water

##  I Introduction

Terahertz (THz) spectroscopy is an indispensable tool to analyze light-weight
molecules with applications in astronomy and chemistry. With new technological
developments, the application of THz technology has extended to fields as
diverse as security [1], communications [2], pharmaceutical control [3],
medicine and biology [4]. In biology, THz waves have shown to be a relevant
method for studying picosecond dynamics of biomolecules [5, 6], predicted to
be key for their biological function [7] in which water plays an important
role [8]. Aqueous samples have been measured with time-domain spectroscopy
(TDS), one of the most common THz spectroscopy methods, with free-space
transmission [9], free-space reflection [10], and on-chip setups [11, 12].
However, despite having a relatively high dynamic range, necessary for
measuring high-loss samples, the low time-averaged power and wide bandwidth
typically yield measurements with a low signal-to-noise ratio (SNR), limiting
the smallest detectable signal change [13]. The low SNR further plummets when
measuring high-loss aqueous samples (around 100 dB/mm [9]) in higher-loss chip
setups, hindering pure water measurements at frequencies above
$0.5\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ [12], or having to avoid liquid
sample in the region with the most intense electric field to minimize losses
[14, 11], but sacrificing sensitivity.

A promising method for obtaining high SNR for on-chip applications is
measuring S-parameters using vector network analyzers (VNA), a common
measuring method at microwave and millimeter-wave frequencies. This method is
based on an electronic heterodyne technique, and benefits from having first-
class frequency resolution ($\sim$1\text{\,}\mathrm{H}\mathrm{z}$$), about
$20\text{\,}\mathrm{d}\mathrm{B}$ higher dynamic range than TDS systems at
$1\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ [15], traceability to the
International System of Units [16], and can use calibration techniques to move
the reference plane to the region of interest [17]; whereas the downside is
that the bandwidth is limited to a waveguide band. The frequency range of VNA
measurements applied to biology has been typically restricted to microwave
[18] and millimeter-wave frequencies [19, 20]. However, recent development in
heterodyne technology has increased the maximum frequency of VNA analysis up
to 1.5 THz [21] in rectangular waveguides, and up to 1.1 THz for on-chip
measurements using contact probes [22]. Contact probes offer an efficient way
to guide the generated power directly into the sensing chip (compared to free-
space coupling), thus increasing the sensibility of the method.

In this letter, we demonstrate the use of a vector network analyzer and
contact probes for THz spectroscopy of high-loss aqueous samples contained in
a chip. We describe the design of the low-loss sensing waveguide and its
fabrication, the measurement setup, and how the complex refractive index was
extracted from the measured complex transmission coefficient. This is a first
step towards a miniaturized chip sensor for high-loss liquid samples at THz
frequencies.

##  II Method

For sensing liquid samples on a chip, we used a coplanar waveguide (CPW) [23],
which provides easy interfacing with ground-signal-ground probes (Fig. 1.a).
The CPW was designed to suit both the measurement probe’s pitch
($25\text{\,}\mathrm{\SIUnitSymbolMicro m}$) and characteristic impedance
($50\text{\,}\mathrm{\SIUnitSymbolOhm}$) [22], yielding a central strip width
of $23.5\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and a ground separation of
$1.5\text{\,}\mathrm{\SIUnitSymbolMicro m}$. The CPW was designed on a ultra-
thin $23\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thick polyethylene
terephthalate (PET) film substrate ($\epsilon^{\prime}=3.15$ and
$\tan(\delta)=0.017$ at 1 THz [24]), to avoid power leakage [25] to
undesirable substrate modes [26]. The CPWs and their dedicated calibration
standards were fabricated using e-beam lithography and evaporation of
$20\text{\,}\mathrm{nm}$ Ti and $350\text{\,}\mathrm{nm}$ Au on top of the PET
substrate.

Figure 1: (a) Micrograph of the CPW being excited by the ground-signal-ground
probe. (b) Photograph of measurement setup during measurements.

The PET substrate containing the CPWs is held on top of a polyethylene
supporting substrate to avoid coupling with the probe station’s metal chuck
during the measurements (Fig. 1.b). The polyethylene supporting substrate had
a $1\text{\,}\mathrm{mm}$ deep and $1\text{\,}\mathrm{mm}$ long air cavity
under the measured CPW, suspending it on air (Fig. 2). On top of the PET
substrate, an interchangeable polydimethylsiloxane (PDMS) microfluidic channel
was clamped to the polyethylene supporting substrate. The microfluidic channel
was designed to have a $100\text{\,}\mathrm{\SIUnitSymbolMicro m}$ wide square
cross-section, whereas the cross-section of the PDMS containing the channel is
$1\text{\,}\mathrm{m}\mathrm{m}$ wide by $10\text{\,}\mathrm{m}\mathrm{m}$
tall (Fig. 2). To deliver the sample into the microfluidic channel, input and
output tubings were connected to the microfluidic channel.

We measured the complex transmission coefficient, $\hat{T}_{s}$, between
$0.75\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ and
$1.1\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ using a VNA Keysight N5242A
connected to two VDI WR1.0SAX frequency extenders [27], having a
typical/minimum dynamic range of 65/45 dB, respectively, and a continuous wave
signal power higher than $-40\text{\,}\mathrm{d}\mathrm{B}\mathrm{m}$. To
couple the signal to the sensing chip, we used DMPI T-Wave ground-signal-
ground probes [22]. We used dedicated multi-line TRL [28] calibration
structures to set the calibration plane at the air-PDMS interface (Fig. 2).

Figure 2: Illustration of the cross-section of the microfluidic channel filled
with sample intersecting the CPW.

The aqueous samples consist of propan-2-ol (IPA) in deionized-water
(DI-$\ce{H_{2}O}$) with different concentrations—0, 10, 20, 30, 40, and 50% of
IPA in volume. The samples were pumped into the microfluidic channel from
higher to lower concentration of IPA through the input tube using a syringe
and flushing air between samples. The pressure was adjusted to atmospheric
pressure by removing the syringe momentarily after introducing each sample.
The probes were kept in contact with the chip throughout all measurements to
minimize probing position uncertainty. Each sample was measured five times in
consecutive VNA sweeps with an intermediate frequency bandwidth of
$50\text{\,}\mathrm{H}\mathrm{z}$.

The effective refractive index of the sample-loaded CPW
($\hat{n}_{e}=n_{e}-j\kappa_{e}$) was calculated by comparing the measured
transmission with a reference measurement, $\hat{T}_{r}$ (average of five
measurements with DI-water as sample), following the equation:

$\begin{split}\hat{n}_{e}&=\hat{n}_{r}-\frac{\ln{\left[\hat{T_{s}}/\hat{T_{r}}\right]}}{jk_{0}l_{s}}\end{split}$
(1)

where $k_{0}$ is the vacuum wavenumber, $l_{s}$ is the effective sample
length, and $\hat{n}_{r}$ is the effective refractive index of the reference.
$\hat{n}_{r}$ was obtained from analytical expressions for multilayered
substrates on CPWs [29] and using a double Debye model for water [9] in order
to include the frequency-dependent permittivity.

Finally, the sample’s refractive index $\hat{n}_{s}$ was found from measured
CPW’s effective refractive index, $\hat{n}_{e}$, by using the same analytical
expressions for multilayered substrates on CPWs [29]. According to it, the
equation relating the sample permittivity with the CPW’s effective
permittivity is:

$\begin{split}\hat{n}_{e}^{2}=\hat{\epsilon}_{e}=1+\frac{1}{2}(\hat{n}_{s}^{2}-1)\frac{K(k)K(k^{\prime}_{s})}{K(k^{\prime})K(k_{s})}+\\\
+\frac{1}{2}(\hat{n}_{sub}^{2}-1)\frac{K(k)K(k^{\prime}_{sub})}{K(k^{\prime})K(k_{sub})}\end{split}$
(2)

where $K$ is the complete elliptical integral of the first kind, and $k$,
$k^{\prime}$, $k_{s}$, $k^{\prime}_{s}$, $k_{sub}$, $k^{\prime}_{sub}$, are
terms depending on the geometry of the CPW cross-section, superstrate (sample)
and substrate, respectively [29]. The analytical expression (eq. 2) agrees
well with more detailed 3D electromagnetic simulations of the multilayered
CPW.

##  III Results

We measured the transmission of a 1 mm long CPW with the PDMS microfluidic
channel containing IPA/DI water solutions from
$0.75\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ to
$1.1\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$. For DI-water, the typical
insertion loss was in the order of $25\text{\,}\mathrm{d}\mathrm{B}$. Fig.
3a-b shows the phase and magnitude, respectively, of the transmission for each
sample normalized to the reference measurement (DI-water). Solid lines
represent the average of five successive measurements, whereas the shadows
represent their standard deviation. Both the normalized magnitude and phase
consistently decrease for increasing IPA concentration, as expected from
literature values [30, 9]. The artifacts observed around
$0.95\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ and
$1\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$, which appear as resonances in both
magnitude and phase of the transmission, were also observed without any
microfluidic channel. A possible explanation is parasitic probe-to-probe
coupling effects or calibration artifacts [31]. The measurements’ noise
increases noticeably after $1.05\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ due
to a drop in the dynamic range at the end of the frequency extender band. The
relative error shows to be smaller for $\kappa_{e}$ than for $n_{e}$, implying
that in this case, smaller sample changes can be detected with attenuation
measurements than with phase measurements. For water measurements the SNR was
typically higher than $25\text{\,}\mathrm{d}\mathrm{B}$.

Figure 3: Transmission measurements can resolve magnitude and phase
differences between high-loss aqueous samples. Transmission (a) phase
difference, and (b) power ratio with respect to reference measurement for the
IPA/DI-$\ce{H_{2}O}$ samples (from 50% to 100% DI-$\ce{H_{2}O}$ in steps of
10%). (c) Real and (d) imaginary part of effective refractive indices of the
CPW with samples. Lines indicate the mean of five measurements, whereas
shadows indicate the standard deviation.

The real and imaginary effective refractive indices are shown in Fig. 3.c-d,
where all high-loss samples have been successfully resolved. A relatively
large effective length $l_{s}=$250\text{\,}\mathrm{\SIUnitSymbolMicro m}$$ was
used since part of the liquid was found to extend between the PDMS and CPW
substrate. Fig. 4 shows the samples’ extracted real and imaginary
permittivities at $0.8\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ versus sample
DI-water concentration, showing error bars with 95% confidence interval. Both
the real and imaginary parts of the permittivities change consistently with
the changes of IPA concentration. The analytically extracted sample
permittivities are also plotted with the permittivity of the IPA/water
mixtures using literature values, using a modified Bruggeman’s approximation
for binary mixtures of polar liquids [32], with equation:

$\left[\frac{\epsilon_{m}-\epsilon_{{}_{\text{H}_{2}\text{O}}}}{\epsilon_{{}_{\text{IPA}}}-\epsilon_{{}_{\text{H}_{2}\text{O}}}}\right]\left[\frac{\epsilon_{{}_{\text{IPA}}}}{\epsilon_{m}}\right]^{1/3}=1-\left[a-(a-1)\alpha_{{}_{\text{H}_{2}\text{O}}}\right]\alpha_{{}_{\text{H}_{2}\text{O}}}$
(3)

where $\epsilon_{{}_{\text{H}_{2}\text{O}}}$ and $\epsilon_{{}_{\text{IPA}}}$
are literature values of the permittivities of water [9] and IPA [30]
respectively, $\alpha_{{}_{\text{H}_{2}\text{O}}}$ is the volume concentration
of water, $\epsilon_{m}$ the permittivity of the mixture, and $a=1.33$ is the
fitting factor for propanol-water samples [32].

Figure 4: Obtained sample permittivities show consistent change with varying
water concentration. Permittivity of IPA/DI-$\ce{H_{2}O}$ samples vs. water
concentration at $0.8\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ for this work
(blue and red errorbars for 95% confidence interval) and literature values.
Literature values of ($+$) IPA [30, 33, 34], ($\vartriangle$) water[9, 35, 36,
37], and ($\medwhitestar$) IPA/$\ce{H_{2}O}$ mixtures measured with a similar
on-chip method [12] are plotted. The dashed lines show Bruggeman’s model
approximation for the change in water concentration, based in eq. (3).

##  IV Conclusion

This letter presents an on-chip dielectric spectroscopy method capable of
measuring high-loss aqueous samples enabled by VNA and ground-signal-ground
probes. Similar attempts of broadband measurements of aqueous solutions with
TDS had problems resolving the complex refractive index for high water content
solutions at frequencies higher than
$0.5\text{\,}\mathrm{T}\mathrm{H}\mathrm{z}$ [12] or lost sensitivity from not
placing sample where the sensor’s field is strongest [11]. The main reasons
why the presented method could measure aqueous samples sensitively is due to
(1) a higher coupling efficiency when exciting the CPWs using the contact
probes compared to other methods which couple a free-space beam into the
substrate via an antenna; (2) the higher average power of continuous-wave THz
signals produced by the VNA compared to other pulsed methods, like TDS; (3) a
low-loss design of the CPW. The high dynamic range (typically
$50\text{\,}\mathrm{d}\mathrm{B}$ at calibration plane) allows to measure
water with a sample length up to approximately
$0.6\text{\,}\mathrm{m}\mathrm{m}$. Some drawbacks of this method are that (1)
is limited to frequencies up to 1.1 THz, due to lack of contact probes at
higher frequencies; (2) being limited to rectangular waveguide bands for a
given setup, and (3) a higher cost. In perspective, this method is a first
step towards a miniaturized system for sensing high-loss aqueous samples at
THz frequencies, which could provide an integrated, accurate, and controlled
platform to study fundamental biological phenomena in their native
environment.

## Acknowledgment

The authors would like to thank Mr. Mats Myremark for machining parts for the
measurement setup and Dr. Kiryl Kustanovich for helping to fabricate the
microfluidic channel. The devices were fabricated and measured in the
Nanofabrication Laboratory and Kollberg Laboratory, respectively, at Chalmers
University of Technology, Gothenburg, Sweden.

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|  Juan Cabello-Sánchez (S’ 17) was born in Madrid in 1992. He received the
Bachelor diploma in Electrical Engineering and Computer Science (ABET-
accredited) from the Technical University of Madrid, Spain, in 2015, and MSc
in Wireless, Photonics and Space Engineering from Chalmers University of
Technology, in Gothenburg, Sweden, in 2017. From July 2017 he is pursuing his
PhD in the THz and Millimetre-wave Laboratory at Chalmers University of
Technology.
---|---
|  Vladimir Drakinskiy Vladimir Drakinskiy was born in Kurganinsk, Russia, in
1977. He received the Diploma degree in physics and informatics (with honors)
from the Armavir State Pedagogical Institute, Armavir, Russia, in 2000, and
the Postgraduate degree from Moscow State Pedagogical University, Moscow,
Russia, in 2003. From 2000 to 2003, he was a Junior Research Assistant in the
Physics Department, Moscow State Pedagogical University. Since 2003, he has
been in the Department of Microtechnology and Nanoscience, Chalmers University
of Technology, Gothenburg, Sweden. During 2003–2005, he was responsible for
mixer chips fabrication for the Herschel Space Observatory. Since 2008, he has
been a Research Engineer with the Department of Microtechnology and
Nanoscience, Chalmers University of Technology. He is currently responsible
for terahertz Schottky diodes process line at MC2, Chalmers University of
Technology. His research interests include microfabrication and
nanofabrication techniques, detectors for submillimeter and terahertz ranges,
and superconducting thin films.
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|  Jan Stake (S’ 95–M’ 00–SM’ 06) was born in Uddevalla, Sweden, in 1971. He
received the M.Sc. degree in electrical engineering and the Ph.D. degree in
microwave electronics from the Chalmers University of Technology, Goteborg,
Sweden, in 1994 and 1999, respectively. In 1997, he was a Research Assistant
with the University of Virginia, Charlottesville, VA, USA. From 1999 to 2001,
he was a Research Fellow with the Millimetre Wave Group, Rutherford Appleton
Laboratory, Didcot, U.K. He was then a Senior RF/Microwave Engineer with Saab
Combitech Systems AB, until 2003. From 2000 to 2006, he held different
academic positions with the Chalmers University of Technology and, from 2003
to 2006, was also the Head of the Nanofabrication Laboratory, Department of
Microtechnology and Nanoscience, MC2. During Summer 2007, he was a Visiting
Professor with the Submillimeter Wave Advanced Technology Group, Caltech/JPL,
Pasadena, CA, USA. He is currently a Professor and the Head of the Terahertz
and Millimetre Wave Laboratory, Chalmers University of Technology. He is also
a Cofounder of Wasa Millimeter Wave AB, Goteborg, Sweden. His research
interests include graphene electronics, high frequency semiconductor devices,
THz electronics, submillimeter wave measurement techniques (“THz metrology”),
and THz in biology and medicine. Prof. Stake served as Editor-in-Chief for the
IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY from 2016 to 2018.
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|  Helena Rodilla (M’ 16–SM’ 20) was born in Salamanca, Spain, in 1982. She
received the B.S. and Ph.D. degrees in Physics from the University of
Salamanca, Salamanca, Spain, in 2006 and 2010, respectively. From 206 to 2010,
she was with the Electronics Group, Department of Applied Physics, University
of Salamanca, Spain, where her research interest was semiconductor physics.
From 2011 to 2013 she was Postdoctoral Researcher with the Microwave
Electronics Laboratory, Department of Microtechnology and Nanoscience (MC2),
Chalmers University of Technology, Gothenburg, Sweden, where she worked on
very low-noise InP HEMTs for cryogenic low noise amplifiers. Since 2013 she
has been with the Terahertz and Millimetre Wave Laboratory, MC2, Chalmers
University of Technology, Gothenburg, Sweden, where she became Associate
Professor in 2020. Her current research interests include the use of
millimeter wave and terahertz technology in life science applications, sensing
and on-wafer terahertz measurements.
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