# Fabrication and characterization of a streamlined micro hot wire probe to reduce aerodynamic disturbances

Baptiste Baradel, Olivier Léon, Léo Chamard, Philippe Combette, Fabien

Méry, Alain Giani

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Baptiste Baradel, Olivier Léon, Léo Chamard, Philippe Combette, Fabien Méry, et al., Fabrication and characterization of a streamlined micro hot wire probe to reduce aerodynamic disturbances. Micro and Nano Engineering, 2025, 29, pp.100326. 10.1016/j.mne.2025.100326. hal-05344624

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# Fabrication and characterization of a streamlined micro hot wire probe to reduce aerodynamic disturbances☆

Baptiste Baradel<sup>a,∗</sup>, Olivier Léon<sup>b</sup>, Léo Chamard<sup>a</sup>, Philippe Combette<sup>a</sup>, Fabien Méry<sup>b</sup>, Alain Giani<sup>a</sup>

## ARTICLE INFO

### Keywords:

MEMS

Thermal sensor

HWA

Turbulence measurements

## ABSTRACT

The miniaturization of sensing elements in hot-wire anemometry through micro-fabrication techniques enables high temporal and spatial resolution, essential for investigating complex and high-speed flows. However, hot-wire probes remain inherently intrusive, and minimizing disturbances caused by the probe is critical for accurate measurements at small scales. This study focuses on the development of a micro hot-wire (xµHW) sensor with reduced disturbances through the implementation of a streamlined geometry. By developing a micro-fabrication process, based among others on silicon's aspect ratio-dependent etching properties, a new sensor prototype was produced. Experimental characterization in a wall-bounded flow highlights the potential and limitations of this streamlined design for aerodynamic applications.

## 1. Introduction

Among the technologies used to study velocity fluctuations in turbulent flows, Hot Wire Anemometry (HWA) remains one of the most commonly used measurement techniques [1,2]. This method provides continuous measurements, offering access to a significant portion of the velocity spectra in turbulent flows, and is relatively easy to set up. However, HWA has some limitations, notably due to the sensor's dimensions relative to the spatial scales of turbulence, which can lead to spatial filtering issues [3]. Poor probe design can also result in internal heat losses within the sensor, adversely affecting measurements [4], particularly if the wire's aspect ratio (length to diameter) is too low. Additionally, HWA is an intrusive measurement technique, and these aerodynamic disturbances caused by the probe can lead to measurement inaccuracies [5].

Over the years, the aerodynamics community has made significant efforts to improve HWA probes, beginning with the use of Wollaston wires [6,7]. These advanced hot-wire probes have demonstrated improved performance, particularly in addressing the spatial filtering limitations of classical HWA and in achieving high aspect ratio geometries. However, they present several challenges: they require a high level of technical expertise, are mechanically fragile, and further miniaturization beyond the current wire diameter limit (d = 0.5 µm) remains difficult. As a result, probing even finer turbulent

scales remains challenging. Following these initial improvements — and the demonstrated need to reduce wire dimensions — the most significant advancements emerged with the development of recent micro-fabrication techniques. The most notable micro-sensor is the NSTAP (Nanoscale Thermal Anemometry Probe), created by Princeton University [8], with dimensions of $ℓ × w × t = 60 × 1 × 0.1$ µm³. This sensor also features an aerodynamically optimized geometry, the result of iterative design enhancements [9]. It has been tested under various flow conditions and has demonstrated significantly improved spatial resolution, particularly in high Reynolds number flows. For more details on the NSTAP's applications, readers may refer to the review made by Fan et al. [10] and the recent work of Gunady et al. [11]. Following a similar path, a micro-hot-wire (µHW) probe was developed at ONERA and University of Montpellier. This probe was designed with a similarly small sensing element ($ℓ × w × t = 60 × 1.5 × 0.4$ µm³) but a different design that has proven effective in capturing small turbulent scales [12].

Micro-fabrication processes have helped mitigate spatial filtering issues by achieving small dimensions while maintaining a good wire aspect ratio, thereby minimizing detrimental heat losses by conduction in the sensor prongs. Despite these advances, current micro-wire sensors still face challenges related to flow-induced disturbances, which stem from geometric features intrinsic to their design [11–13]. These disturbances can compromise measurement accuracy. The present study

<footnote><sup>a</sup> IES, Univ Montpellier, CNRS, Montpellier, 34000, France</footnote>

<footnote><sup>b</sup> DMPE, ONERA, Université de Toulouse, Toulouse, 31000, France</footnote>

<footnote>☆ This article is part of a Special issue entitled: 'NanoSystems 2024' published in Micro and Nano Engineering.</footnote>

<footnote>* Corresponding author.
E-mail address: baptiste.baradel@umontpellier.fr (B. Baradel).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1. Schematic view of the µHW probe.</figcaption></figure>

aims to revisit the µHW probe design by developing and fabricat-
ing a three-dimensional streamlined geometry, inspired by the NSTAP
concept. The objective is to evaluate the potential improvements this
approach could bring to the existing µHW design for wall-bounded
turbulence measurements. Achieving this requires the development
of specialized micro-fabrication processes capable of producing the
desired aerodynamic shape.

The present article is structured as follows: first, a reminder of the µHW probe concept is provided in Section 2, followed in Section 3 by the development of the new streamlined µHW probes, the experiments performed with them, a critical analysis, and finally, concluding remarks in Section 4.

## 2. Initial prototype

This section provides an overview of the design, fabrication, and
preliminary results obtained with the µHW probe, which have already
been detailed in Baradel et al. [12]. Readers are encouraged to refer to
this work for a comprehensive discussion on this micro-hot-wire probe.

2.1. *Design and fabrication*

The geometry of the µHW probe is illustrated in the schematic
shown in Fig. 1. The sensor has a relatively simple design based on a
silicon (Si) structure with two prongs. A 900 µm long bridge connects
these prongs. The bridge is 10 µm wide, with a narrowed section in
the center that reduces to a width of 1.5 µm over a length of 60 µm.
This narrowed section serves as the sensing element, while the wider
parts are referred to as the stubs. Previous research [12] has examined
different sensing lengths, concluding that the length does not influence
measurements in the studied flow. Therefore, this study focuses solely
on µHW probe with a 60 µm sensing length. This long bridge design
is motivated by two key considerations. Firstly, an analytical study of
thermal conduction within the sensor showed that this geometry helps
reduce end-conduction heat losses for the sensing element, thereby
enabling a satisfactory frequency response with an order of several
tens of kilohertz. Secondly, the elongated bridge increases the distance
between the sensing element and the supporting prongs, which likely
reduces aerodynamic interference from the prongs in the measurement
region. This configuration helps to minimize disturbances to the flow
streamlines crossing the micro-wire, thereby improving the accuracy of
the resulting measurements.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Schematic of the manufacturing steps of the µHW probes (cross-sectional view containing the micro-wire).</figcaption></figure>

Regarding the materials used, the sensor is fabricated from a
200 µm-thick, double-side polished, (100) oriented Si wafer. A Sili-
con Nitride (SiNₓ) layer is deposited on both sides of the wafer to
serve as an electrical insulator as well as a masking layer for the
subsequent silicon wet etching process. This SiNₓ layer is deposited
via low-pressure chemical vapor deposition (LPCVD), a method that
ensures both mechanical robustness and excellent flatness of the bridge
structure, owing to its low residual tensile stress [14]. The final layer
is the sensitive and conductive material, platinum (Pt), chosen for its
durability and stability. A 200 nm thick layer of platinum is deposited
using electron beam physical vapor deposition (EBPVD). Crucially, a
very thin (20 nm) adhesive layer of tantalum oxide (TaₓOᵧ) is applied
before the platinum deposition [15] using the same deposition system
without breaking the vacuum. This adhesive layer is essential for
ensuring the long-term stability of the thermo-resistive properties and
maintaining parameters close to those of bulk platinum.

For the fabrication of the µHW probe, the final substrate is a stack
consisting of the following layers: SiNₓ/Si/SiNₓ/TaₓOᵧ/Pt.

Once the substrate is obtained, the manufacturing process can be-
gin, utilizing standard MEMS fabrication techniques. The different steps
are summarized in Fig. 2. Before each step, the samples are cleaned
using ultrasound baths with solvent, rinsed with de-ionized water, and
finally dehydrated. Regarding the thin film material layers, specifically
the Pt and SiNₓ layers, they are sequentially patterned using pho-
tolithography and etched by Reactive Ion Etching (RIE). The Pt layer is
etched in an argon sputtering plasma, whereas the SiNₓ layer is etched
using a fluorine-based chemistry (CHF₃/SF₆) with an O₂ admixture.
The next step involves shaping the sensor by etching the silicon sub-
strate. For this initial µHW probe design, it was decided not to profile
the sensor. Consequently, a potassium hydroxide (KOH) solution is used
for etching. This step releases the bridge housing the sensing element. It
is important to note that KOH is an anisotropic etchant, etching silicon
depending on its crystal planes. Using a (100) oriented silicon wafer
results in a 54.7° angle on the flanks of the sensor, as illustrated in
Fig. 2(d). This anisotropy was taken into account in the design of the
photolithography masks to prevent any under-etching of the silicon
beneath the SiNₓ. It should be noted that the sensors are fabricated
in a matrix, with each sensor separated by a SiNₓ/Si/SiNₓ framework
structure and connected by a strip. To enable singulation, the strip
is opened on the front side of the SiNₓ layer. After KOH etching, a
thin layer of silicon remains, which can then be individually cleaved
to release each sensor.

The µHW probe obtained after these fabrication steps is presented
in Fig. 3. As shown in the image, the sensor is mounted on a custom
printed circuit board (PCB) connector, specifically designed to fit into
<figure><img src="image_5.png" /><figcaption>Fig. 3. Scanning electron microscopy (SEM) image of the µHW probe.</figcaption></figure>

standard probe supports. The µHW probe is connected to the PCB using micro-bonding aluminum wires to ensure minimal contact resistance. This type of bonding has proven to be highly resistant to high-speed flows.

## 2.2. Experimental characterization

Two aspects are of interest in characterizing the sensor: its thermo-electrical properties and its performance in turbulent flows. These will be discussed in the following paragraphs.

The principle of HWA is based on the Joule effect and the use of thermo-electrical materials. The resistance ($R$) of the sensing element depends on its temperature ($T$) with the first-order linear approximation: $R(T) = R_0(1 + \alpha T)$, where $R_0$ and $\alpha$ are parameters that need to be determined for each sensor. To determine these parameters, the sensors are placed in an oven with varying temperatures. By performing low-current (0.1 mA) measurements of the sensor resistance, a linear regression for $R(T)$ can be obtained. The resistance at 0 °C, $R_0$, depends on the sensor geometry, such as the lengths of the stubs and the sensing element, and is typically around 100 Ω for µHW with a sensing length of 60 µm. The temperature coefficient of resistance, $\alpha$, depends on the µHW material. The values obtained are around 0.0036 K⁻¹, which is close to that of bulk platinum (approximately 0.0039 K⁻¹ for bulk Pt [16]).

Additionally, the stability of this parameter has been tested, which is crucial for performing repeatable measurements. By setting a µHW probe at an operating temperature of around 200 °C for 17 days and monitoring its resistance, only a ±1.1 ‰ variation was observed. This indicates excellent stability of the micro-sensors over time, avoiding additionally any annealing of the sensor.

The main purpose of the µHW probe is to perform velocity fluctuation measurements in turbulent flows. Consequently, the sensors have been characterized in wind tunnel facilities. Measurements were performed in a turbulent boundary layer (TBL) and compared with three references: (i) a classic commercial hot-wire probe (Dantec 55P15, with a sensing length of 1.25 mm, hereafter referred to as the “long HW”), (ii) a NSTAP provided by Princeton University, and (iii) numerical data extracted from the Direct Numerical Simulation (DNS) of Schlatter and Örlü [17] for a similar Reynolds number $Re_τ = 1100$. All sensors were operated using a standard Constant Temperature Anemometer (CTA) system with a 1:1 bridge configuration and an operating temperature of approximately 200 °C. Each sensor was calibrated in-situ relying on King's law [1] before performing TBL measurements.

The results are presented in Fig. 4, reproduced from [12]. The graphs provide profiles of mean axial velocity $U^+$ and axial velocity

<figure><img src="image_6.png" /><figcaption>Fig. 4. Velocity statistics as a function of wall distance ($z^+$) in a TBL at $Re_τ = 1100$ for different sensors: (a) Mean velocity profile $U^+$, (b) Velocity variance profile $\overline{u^2}$. Experimental data reproduced from Baradel et al. [12]. DNS dataset extracted from Schlatter and Örlü [17].</figcaption></figure>

$\overline{u^2}$ in the wall-normal direction $z^+$, as the probe traverses the boundary layer starting from the wall. These results are normalized using wall units as indicated using the superscript +. The most important point to note is that the µHW probe provided accurate measurements of flow velocity fluctuations (Fig. 4(b)), similar to the NSTAP, which is a significant improvement compared to the results obtained with the long HW commercial probe. This is the result of a better resolution of small scale turbulent structures in the near-wall region of the TBL. However, an examination of the mean velocity profile (Fig. 4(a)) reveals a bias in the measurements obtained with the µHW probe in the near-wall region ($5 < z^+ < 15$), where the estimated mean velocities are lower than those reported in DNS data [17]. A similar trend is observed with the NSTAP sensor, although the discrepancy appears to be less pronounced than for the µHW probe. In terms of fluctuating velocity (Fig. 4(b)), another noteworthy point is that the µHW probe slightly overestimates the velocity fluctuations in the logarithmic region ($50 < z^+ < 500$) compared to both the NSTAP and DNS data. Even if the deviations are minor they raise concerns about the µHW probe's reliability in both the near-wall and logarithmic regions. The underlying causes of these discrepancies remain under investigation, but one hypothesis is that they stem from the relatively large prongs of the µHW sensor, which may introduce a blockage effect and interfere with both the flow and the wall. This suggests the need for further exploration of the probe's geometric design parameters.

# 3. Streamlined µHW probe

## 3.1. Design and fabrication

Building on the previous results, a modification of the µHW probe design is proposed to mitigate the assumed aerodynamic disturbance effects. While a complete redesign of the sensor geometry could be considered, a simpler and more pragmatic approach was chosen. The overall shape – including the silicon prongs and the suspended bridge (Fig. 1) – is retained. Instead, the back face of the sensor will be profiled, in a manner similar to the NSTAP design [8]. This modification
preserves key advantages of the original geometry, such as reduced end-conduction losses due to the stubs, while shifting the measurement zone further away from the prongs. Furthermore, the NSTAP, which features a profiled backside, showed results suggesting reduced aerodynamic disturbances. This approach thus intends to combine the strengths of both sensor types. The modified sensor is referred to as the streamlined µHW, and the target profile slope is approximately 7°, a standard angle for aerodynamically shaped bodies. Finally, to isolate the effect of probe thickness, the prong width remains unchanged in this study.

To shape the back face of the µHW probe, Aspect Ratio Dependent Etching (ARDE) of silicon is employed. This technique, well-documented in the literature [18–20], leverages the dependence of etch depth on the size of mask openings, arising from complex physical and chemical interactions during the etching process. ARDE has been widely used to fabricate 3D MEMS structures [8,21]. Among the various available methods, Deep Reactive Ion Etching (DRIE) was selected for its ability to achieve deep etching with vertical sidewalls, as highlighted in a previous study [22]. That study also established a correlation between etch depth and mask opening size, which was used to guide the photolithography mask design. In this work, the mask openings range from 1.5 µm to 80 µm.

The use of DRIE on the back face of the substrate necessitates changing the masking layer, currently SiNₓ. Silicon Nitride is etched by the same gases as silicon, and no photoresist was available at our facility to achieve the required precision for the opening sizes while being thick enough to resist to the DRIE step. Consequently, the nitride layer on the back face was not added and was replaced by a stack of two layers. First, a 3 µm thick Silicon Dioxide (SiO₂) layer serves as a hard mask for the Si etching through DRIE. This layer is deposited via Plasma-Enhanced Chemical Vapor Deposition (PECVD) and was chosen for its high selectivity (65:1) compared to silicon regarding DRIE. Secondly, a 100 nm thick Chromium (Cr) layer is added by EBPVD on the SiO₂ to act as a mask for the latter layer.

Finally, the substrate for this process consists of the following stack: Cr/SiO₂/Si/SiNₓ/Pt.

The manufacturing process to fabricate the streamlined µHW probe is summarized in Fig. 5. Regarding the top layers (SiNₓ and Pt), the process remains the same as before. For the back face, starting with the Cr layer, photolithography is performed, and the layer is then etched using a standard Chromium etchant solution (Sigma-Aldrich). It then acts as a mask for the SiO₂ etching by RIE. The etching recipe used here was customized to achieve highly straight directional etching, guaranteeing vertical sidewalls along the 3 µm depth in the layer. Once the chromium layer is removed, the next step is to perform silicon etching via DRIE. The goal is to etch 190 µm of silicon with vertical sidewalls. The process used is a standard Bosch-based commonly employed for traversing silicon wafers. It consists of three cycles: an etching phase, a passivation phase and a removal phase of the passivation layer. Typical results obtained after this etching are presented in Fig. 6. Finally, to remove excess Si and smooth the backside surface, the samples undergo an isotropic etching process. Xenon difluoride (XeF₂) gas was selected for this step due to its high selectivity and efficiency in isotropic silicon etching [23]. The samples were placed in a XeF₂ etching system and subjected to 20 cycles of etching, each lasting 30 s, at a pressure of 3 mTorr.

The final sensors are visible in Fig. 7, showing the profiled back face. Streamlined µHW probes have a more slender appearance than the initial µHW probe. A point of improvement regarding these new micro-sensors is the presence of a step at the end of the profiled zone. This could be detrimental and be a source of velocity field perturbation. However, given the distance between the micro-wire and this step (around 1 mm), it can be reasonably assumed that no significant interference should appear. Similar to the previous µHW probe, the newly developed sensors are mounted onto PCB connectors using micro aluminum wire bonding.

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Schematic of the manufacturing steps of the streamlined µHW probes (cross-sectional view perpendicular to the micro-wire).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** SEM picture of the back face of a streamlined µHW probe after DRIE and before XeF₂ etching.</figcaption></figure>

## 3.2. Experimental characterization

To assess the performance of the streamlined µHW probe, the same procedure as for the µHW was followed. First, regarding the thermoelectrical parameters, similar values were found after the characterization steps. No issues with the new sensors' resistance were observed during days of measurement.

Secondly, for the aerodynamic measurements, the same experimental procedure described in paragraph 2.2 was followed: a calibration followed by velocity profile measurements in a TBL. The resulting measurements were compared to the same reference dataset used in Fig. 4. It should be noted that the slight discrepancies observed in the outer part of the boundary layer ($z^+ > 1000$) are attributed to
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** SEM picture of the streamlined µHW probe: (a) Front view, (b) Back view.</figcaption></figure>

minor differences in the Reynolds number between the experiments. The results obtained with the new streamlined sensor are shown in Fig. 8. This figure again underscores the advantage of micro-sensors in capturing velocity fluctuations–reflected in the velocity variance profile (Fig. 8(b)), particularly in the near-wall region to capture the peak of velocity variance.

A closer look at Fig. 8(a) shows that the mean velocity measured by the streamlined µHW probe in the near-wall region (for $z^+ < 50$) is very similar to that recorded by the original µHW probe. However, an improvement is observed in the logarithmic region of the velocity variance profile inset shown in Fig. 8(b), where the data from the streamlined µHW align more closely with the DNS results. These findings suggest that profiling the µHW probe to reduce the associated aerodynamic disturbances is beneficial for measurements in the outer region of the boundary layer. This improvement does not, however, appear to extend to the near-wall region, where signs of intrusive effects remain visible. This persistence suggests that the primary source of interference in that region may not be driven by the thickness of the prongs, but rather their width. This hypothesis points to a need for further probe design refinements, particularly focused on reducing prong dimensions, to enhance measurement accuracy in the near-wall. This motivates future investigations into optimized sensor geometries.

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Velocity statistics as a function of wall distance ($z^+$) in a TBL at $Re_t \approx 1100$ for the µHW probe and its streamlined version: (a) Mean velocity profile $U^+$, (b) Velocity variance profile $\overline{u'^2}$. Identically to Fig. 4, the µHW probe experimental data are reproduced from [12] and the DNS dataset is extracted from [17].</figcaption></figure>

## 4. Conclusion

Reducing the aerodynamic disturbances of hot-wire sensors is essential for improving measurement accuracy in fluid flow studies. This work presents the development of a streamlined µHW probe, fabricated through a combination of established micro-fabrication techniques. To shape the back face of the probe, the silicon ARDE behavior was exploited using DRIE, in conjunction with $XeF_2$ isotropic etching to smooth the surface. This process resulted in the streamlined µHW sensor presented in this study.

Characterization of the redesigned sensor demonstrated that the streamlined geometry can offer some advantages for free-stream turbulent flow measurements. However, it showed no significant improvement in performance for measurements in the near-wall region of a turbulent wall-bounded flow. Given the increased complexity introduced by the developed manufacturing process and its limited benefit for measurements in the present wall-bounded flow configuration, the current approach requires reconsideration. Nevertheless, these results provide valuable insight and help guide future efforts toward more effective strategies for improving the current µHW probe design.

Rather than modifying the overall profile, future efforts will focus on reducing the width of the prongs, which may be a more critical factor influencing near-wall disturbance effects. To support this new design direction, Computational Fluid Dynamics (CFD) simulations can offer a way to evaluate and optimize sensor geometry prior to fabrication. This will guide the development of new manufacturing processes for next-generation micro-sensors.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Baptiste Baradel reports financial support was provided by Occitanie Region. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
## Acknowledgments

The authors acknowledge the support of the staff at the Centrale de Technologie en Micro et Nanoélectronique (CTM), Université de Montpellier, France, for their assistance with the fabrication process. They also thank V. Lechien for valuable help with sensor integration, and express special gratitude to Prof. M. Hultmark for providing the NSTAPs used in this study.

This research was partly funded by Région Occitanie, France with the funding program ALTDOC. For the purpose of Open Access, a CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the author-accepted manuscript arising from this submission.

## Data availability

Data will be made available on request.

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