# Quantifying the performances of SU-8 microfluidic devices: high liquid water tightness, long-term stability, and vacuum compatibility

Said Pashayev, Romain Lhermerout, Christophe Roblin, Éric Alibert, Jerome Barbat, Rudy Desgarceaux, Remi Jelinek, Edouard Chauveau, Said Tahir, Vincent Jourdain, et al.

► To cite this version:

Said Pashayev, Romain Lhermerout, Christophe Roblin, Éric Alibert, Jerome Barbat, et al. "Quantifying the performances of SU-8 microfluidic devices: high liquid water tightness, long-term stability, and vacuum compatibility." Microfluidics and Nanofluidics, 2024, 28 (5), pp.25. 10.1007/s10404-024-02720-4. [hal-04754131](https://hal.science/hal-04754131v1)

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Quantifying the performances of SU-8 microfluidic
devices: high liquid water tightness, long-term
stability, and vacuum compatibility

Said Pashayev¹,², Romain Lhermerout¹,³, Christophe Roblin¹,
Eric Alibert¹, Jerome Barbat¹, Rudy Desgarceaux¹, Remi Jelinek¹,
Edouard Chauveau¹, Saïd Tahir¹, Vincent Jourdain¹,
Rasim Jabbarov², Francois Henn¹, Adrien Noury¹*

¹Laboratoire Charles Coulomb (L2C), Univ. Montpellier, CNRS,
Montpellier, France.

²Institute of Physics of Azerbaijan National Academy of Sciences, Baku, Azerbaijan.

³Laboratoire Interdisciplinaire de Physique, Univ. Grenoble Alpes & CNRS, Grenoble, France.

*Corresponding author(s). E-mail(s): adrien.noury@umontpellier.fr;

Abstract

Despite several decades of development, microfluidics lacks a sealing material that can be readily fabricated, leak-tight under high liquid water pressure, stable over a long time, and vacuum compatible. In this paper, we report the performances of a micro-scale processable sealing material for nanofluidic/microfluidics chip fabrication, which enables us to achieve all these requirements. We observed that micrometric walls made of SU-8 photoresist, whose thickness range from 35 µm to 135 µm, are at least leak-tight to 1.5 bars and up to 5.5 bars, exhibit no water porosity even after 2 months of aging, and are able to sustain under 10<sup>-5</sup> mbar vacuum. This sealing material is therefore reliable and versatile for building microchips, part of which must be isolated from liquid water under pressure or vacuum. Moreover, the fabrication process we propose does not require the use of either aggressive chemicals or high-temperature or high-energy plasma treatment. It thus opens a new perspective to seal microchips with sensitive surfaces containing nanomaterials.
# 1 Introduction

Sealing is one of the inevitable processes in micro and nanofluidic chip fabrication. Ideally, low-temperature sealing methods are desirable to use with sensitive materials and avoid the thermal expansion of substrates. Also important is to achieve elevated burst pressure tightness, both for the reliability of the device and to operate small channels with large fluidic resistance. Additionally, microfluidics is gaining popularity for highly sensitive mass measurements in liquid environments to weigh biomolecules, single cells, etc. In these measurements, experiments are carried out in a vacuum with microfluidic nanomechanical resonators Martín-Pérez et al (2019); Burg et al (2007). Therefore, low-temperature bonding, high liquid water pressure leak-tight, vacuum compatible and microscale processable material is requested.

Various methods are available in microfluidics depending on the constraints of the materials to bond: plasma bonding Xu et al (2013) is suitable for bonding poly-dimethylsiloxane (PDMS) on silicon or glass, anodic bonding can be employed to bond hard materials together (glass and/or silicon) Gray et al (1999), adhesives are more versatile and suitable for both plastic and silicon Borók et al (2021), and finally ultrasonic or laser welding can be used to bond thermally plastics together Tsao and DeVoe (2009). These materials and methods typically require at least one of the following conditions: activation of the extremely clean surfaces, and surface chemistry Temiz et al (2015); Liu et al (2021). So far, PDMS has been the most common material in chip sealing. However, this material has several drawbacks such as the adsorption of hydrophobic molecules, poor stability after surface treatment, swelling by organic solvents, vapor water permeability, and breaking under high-pressure operations Mukhopadhyay (2007); Sollier et al (2011). Moreover, sealing with PDMS requires activating at least one of the surfaces, typically using O₂ plasma. This can limit its application because surface activation can damage any delicate material used on the surfaces, such as nanomaterial Mathur et al (2012).

Photoresists such as SU-8, when used as an adhesive material to bond hard surfaces, might present better performances than PDMS while allowing bonding substrates without surface activation or high temperature. Indeed, SU-8 is an epoxy-based negative tone photoresist, developed by IBM in the mid-1990s, and initially used as an inexpensive mold maker. Soon after it was used for the fabrication of high aspect ratio micro-electromechanical system (MEMS) Lorenz et al (1997) and later became a widely used material in microfluidics, particularly for replica molding PDMS or other polymers Kamande et al (2015). Interestingly, SU-8 being photosensitive allows forming of patterns by photolithography with micrometric dimensions, very well suited for microfluidics. SU-8 is also used as both a structural material and an intermediate layer for adhesive bonding Salvo et al (2012), exhibiting excellent physical and optical properties, good chemical stability, and low porosity Narayan et al (2018). However, its leak-tightness while it is submitted to a liquid under pressure, stability, and vacuum compatibility is still to be quantified. Furthermore, SU-8 can be subject to outgassing, as reported from mass spectrometry and gas chromatography techniques Melai et al (2009). This clearly indicates that a hard bake is needed to boost the cross-linking degree of the resin, to remove the remaining initial constituents, and
hence to provide the best performance when it has to be used under vacuum.
Bonding strength measurements are mainly based on three methods: tensile strength
Chow et al (2004), burst pressure Borók et al (2021), and liquid leakage Abidin
et al (2019). The bonding devices where SU-8 was used as an adhesive material
had been mainly tested with the tensile strength method. It was reported that the
bonding strength was in the range of from 10 bar Admassu et al (2021) up to 450
bar Steigert et al (2008). However, the tensile strength method does not provide
practical information on device performance in use conditions. The most practical
method to quantify the strength of the bonding and assess the functionality of a
microfluidic chip is the liquid burst pressure test. The liquid burst pressure tightness
of the SU-8-based devices when it is used as an adhesive material has been reported
as ranging from 1 bar Sip and Folch (2010) when bonding PDMS layers up to the
highest 38 bar Lima et al (2013) when bonding glass substrates. The durability of the
so far reported SU-8-based device was 1-2 days Narayan et al (2018). In comparison,
the performances of the PDMS-based devices were reported with the tensile strength
from 3.2 bar Bhattacharya et al (2005) up to 20 bar Hammami et al (2022), with
the liquid burst pressure method from ~2 bar Gonzalez-Gallardo et al (2021) up to
the highest 8.5 bar Hammami et al (2022), when adhesion is enhanced by plasma
pre-treatment of the surfaces and the durability from 3 days Song et al (2018) or the
longest of 1 month Baraket et al (2013).

To the best of our knowledge, the quantitative analysis of SU-8 microfluidic device durability, ionic leakage, and water vapor leakage under vacuum has not been reported.

In this work, we propose an easy and permanent sealing process to quantify the bond-
ing performance of the microfluid device when SU-8 is used as an adhesive material
to bond glass with silicon/silicon dioxide or quartz. The sample fabrication and bond-
ing require basic clean room facilities and low temperatures. Since our goal is to
develop the sealing process of microchips, our investigation focuses on the search for
the thinnest possible wall of SU-8 that can exhibit high-pressure tightness, high vac-
uum compatibility, and no liquid water porosity over a long time. The latter criterion
is checked using an original and very sensitive method based on ionic conduction mea-
surements. The sealing method investigated here is non-destructive, reliable, versatile,
and does not require the use of aggressive chemicals, high temperatures, or high-energy
plasma treatment. It provides up to 5.5 bar liquid water burst pressure leak-tightness
for wall thickness of 135 µm, vacuum compatibility, and extremely low liquid porosity
over long times.

2 Materials and Methods

2.1 SU-8 patterning.

SU-8 preparation was performed in the cleanroom. Because of its improved adhesion
to glass (as specified by the manufacturer i.e., Kayaku - MicroChem®), we employed
SU-8-3025 instead of the more widely used SU-8-20xx series. Optimal adhesion was
obtained using Omicoat (Kayaku - MicroChem®) as a primer. A glass substrate
of 20x40 mm, 4 mm thick, was used to fabricate SU-8 patterns. First, it was milled
to allow the subsequent placement of capillaries of 1.6 mm diameter for the fluidic
connection (Figure 1). In the case of ionic conductivity measurements, two holes were
additionally drilled to insert Ag/AgCl electrodes (Figure 3). Surface cleanliness of the
substrate was ensured by Piranha cleaning (3:1 ratio H₂SO₄:H₂O₂). Then, Omnicoat
primer was spin-coated in 2 steps: first at 500 rpm for 5 s and then at 3000 rpm for
30 s and baked at 200 °C for 2.5 minutes. The substrate was cooled down to 70 °C
and a few milliliters of SU-8 were poured all over the substrate. This temperature was
recommended in order to remove bubbles MEMScyclopedia (1999). Once the reaction
between the Omnicoat primer and the SU-8 was complete, the substrate was cooled
down to room temperature and spin-coated in 2 steps: first at 500 rpm for 5 s and
then at 2000 rpm for 30 s. The SU-8 coated substrate was then baked at 100 °C for 35
minutes and slowly cooled down to room temperature. The coating was exposed for
45 s to 365 nm UV photolithography, with a power density of 5.8 mW cm⁻² through
a photolithography mask. A second baking treatment at two different temperatures,
i.e., 70° C during 2.5 min and 100 °C during 6.5 min, was applied, followed by slowly
cooling down to room temperature. Finally, the pattern of SU-8 was developed for 15
minutes in SU-8 Developer (Kayaku - MicroChem®) and rinsed with isopropanol. The
height of the so-obtained SU-8 pattern was measured with mechanical profilometry to
35 µm.

## 2.2 Device assembling, Bonding & Fluidic connections

The SU-8 pattern on the glass substrate was bonded on a Si/SiO₂ wafer to form the reservoirs to be tested. We used Si wafer covered with a 90 nm thick SiO₂ dry oxide layer. The Si wafer was not treated with Omnicoat or any adhesion promotor. 2 mm thick brass rectangular frames with a dimension of 30 x 50 mm were placed on either side and maintained with 4 screws (Figure 1c). The influence of pressure applied on the SU-8 pattern was assessed by observing optically the device (Figure S1). This crucial step was strictly controlled to obtain homogeneity by observing interferences/reflectivity with an optical microscope in order to ensure the adhesion of the SU-8 wall on both the glass substrate and the wafer and to avoid any deformation of the pattern. The last step of the process consisted of a thermal treatment at 200 °C for 2 hours. It allowed us to obtain surfaces that were completely in contact with the SU-8 pattern (Figure S1c). Noteworthy, during this baking step the thickness of the SU-8 layer extended compare to its fabrication thickness from 20 µm to 35 µm, 30 µm to 50 µm, 60 µm to 85 µm, and 100 µm to 135 µm. Note that the frames were kept after bonding and during all experiments reported here. After assembly, a capillary was inserted into the milling area of the glass substrate and sealed with Stycast 2850FT epoxy glue. The device was left at 65 °C for 2 hours in order to let the Stycast ensure efficient sealing.
# 3 Results and discussion

## 3.1 Liquid water pressure tightness

Water pressure leak-tightness is visually assessed with an optical microscope, change in the contrast/reflectivity indicates the presence of water outside of the SU-8 reservoir. A schematic of the setup is shown in Figure 1a. Our process allows us to build a SU-8 reservoir whose wall thickness is as small as 35 µm. The pattern is designed on the glass substrate in order to form a microfluidic reservoir. This reservoir is filled with water at a pressure piloted by an Elveflow® OB1 MK3+ pressure controller. We tested SU-8 wall thicknesses ranging from 135 µm down to 35 µm. The given two boundary dimensions are set by our method of chip assembly. 135 µm is the maximum compressible structure (given the large area of our device) that can be assembled, while 35 µm is the minimum structure below which the SU-8 rolls during assembly.

<figure><img src="image_2.png" /><figcaption>**Fig. 1:** Liquid water pressure leak-tightness measurement setup. (a) Schematic of the setup. A pressure controller allows applying water pressure to the SU-8 reservoir. A binocular microscope is used for visual observation of the material's performance. (b) 3D artistic view of the device. (c) Photography of the device with frames added to improve bonding of the material, and capillary for fluidic connection to the pressure controller.</figcaption></figure>

The burst pressure distribution measured for ten devices with 85 µm thick SU-8 walls is plotted in Figure 2a. In Figure 2b, we report the burst pressure distribution obtained for various values of SU-8 wall thickness. The symbols represent the mean, maximum, and minimum values of the burst pressure for each corresponding SU-8 wall thickness. The device's performance was measured for each applied pressure over 1 hour. One of the main outcomes is that the water pressure tightness does not depend on the SU-8 wall thickness. The absence of a trend of leak-tightness versus width (Figure 2b) is an indication that the resistance to pressure is more governed by random imperfections of the surface and dust rather than porosities or cracks. Another interesting feature of our SU-8 devices is that they can sustain pressures ranging from minimum of 1.5 bar
<figure><img src="image_3.png" /><figcaption>**Fig. 2:** Liquid water burst pressure leak-tightness. (a) Liquid water burst pressure distribution for 10 devices with 85 µm thick SU-8 wall. (b) Water burst pressure distribution for various SU-8 wall thicknesses.</figcaption></figure>

measured with 35 µm thick SU-8 wall up to 5.5 bar measured with 135 µm. The defects during sample assembling might also explain the scattering observed in Figure 2a.

## 3.2 Liquid water Porosity, Ageing, and Vacuum compatibility

To test the liquid porosity of the SU-8 wall, we propose an original and extremely sensitive method based on the measurement of ionic conduction between two reservoirs filled with an electrolyte and separated by a SU-8 wall. Indeed, it can be assumed that if the wall exhibits percolating paths through the entire thickness then an ionic current will be observed between both reservoirs. Knowing that ionic currents as low as 1 pA were reported from extremely small porosities such as carbon nanotubes (~1 nm) Yazda et al (2017), it can be assumed that in our case nanoporosity can be detected if a similar ionic current measurement method is used.

For that purpose, patterns of SU-8 walls were designed with 2 reservoirs on the glass substrate and assembled with an electrically insulating piece of quartz in order to lower the ionic noise Bsawmaii et al (2024). The ionic current is measured via 2 Ag/AgCl electrodes placed in each micro reservoir and tightly sealed with Stycast glue in order to avoid electrolyte leak (Figure 3).

Finally, the electrodes are connected to a high-impedance source-amplifier (Keithley 2636B), which enables to the measurement of currents in the pA range.

First, blank measurement is carried out when both reservoirs are filled with Milli-Q water© whose resistivity is very high, about 18 $MΩ·cm^{-1}$. Then, reservoirs are filled with KCl electrolytes at various concentrations. The resistance R of the SU8 wall is readily determined from the current/voltage plot which follows Ohm's law, as can be seen in Figure 4a. Then the conductivity $\sigma$ of the wall is calculated using the following
<figure><img src="image_4.png" /><figcaption>Fig. 3: Schematic of the setup designed to measure the ionic conductivity between 2 micro-reservoirs filled with Milli-Q water or KCl electrolyte and separated by a SU-8 wall.</figcaption></figure>

equation:

$$ \sigma = \frac{w}{Rlh} \qquad (1) $$

where w, l, and h are the thickness, length, and height of the SU-8 wall after baking respectively. The ionic conductivity of our set-up filled with Milli-Q water is found to be about 1 µS cm⁻¹ and seems independent of the SU-8 wall thickness (Figure 4b). The dependence of the ionic conductivity with the ionic concentration in the log-log scale is reported in Figure 4b. We observe that the conductance does not follow the Kohlrausch’s law for bulk conductivity of diluted electrolytes, i.e.,

$$ \sigma = \sum_{i} \lambda_{i} C_{i} \qquad (2) $$

where λᵢ and Cᵢ are the molar conductivity and the concentration of the solvated ion i respectively. Indeed, the log-log dependence would match with a straight line of slope 1 if Kohlrausch’s law was obeyed. The evolution observed here for device S#1 (w=85 µm) is similar to the usual behavior reported for ionic conductivity through nanopores, i.e., a power law dependence of the ionic conductivity with the ionic concentration at low concentration, with an exponent much lower than 1 or even as low as 0.14. We also observe a flat slope for the conductivity of S#2 (w=50 µm) as a function of the concentration. The fact that reducing dimension leads to the absence of transport is an indication that transport occurs through fabrication defects rather than porosities.

In addition, the comparison of the ionic conductivity obtained here with the one reported for similar walls made of PDMS shows that the SU-8 resin is similar or even less permeable to electrolytes and hence to water (Table S1).

The measurement of the ionic conductivity as a function of time (Figure 4c) shows that there is no significant aging effect over up to 16 weeks, while we may expect a constant decrease or increase of the ionic conductivity if the nanoporosity plugged or opened respectively when it is filled with the electrolyte. This shows that our
<figure><img src="image_5.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_7.png" /></figure>

**Fig. 4:** Ionic conductivity measurement between two micro-reservoirs filled with either 300 mbar Milli-Q water or KCl electrolyte and separated by a SU-8 wall. (a) A typical example of a current/voltage plot obtained with a KCl 10⁻³ M electrolyte and a SU-8 wall of w, l, h equal to 85 µm, 10800 µm, 29 µm. (b) Log-log plot of the ionic conductivity versus KCl concentration for two SU-8 walls with different thicknesses. The dashed line with a slope equal to 1 represents the behavior expected for bulk ionic conductivity, i.e., Kolhrausch's law (Equation 2). (c) Conductivity normalized by its initial value as a function of time for SU-8 walls of 3 different thicknesses (KCl concentration of 0.1 M).

SU-8 wall remains watertight for a longer period of time than usually observed on micrometric walls made of other materials Klammer et al. (2006).

Finally, we assessed the ability of SU-8 to operate fluidics under a vacuum. Noteworthy, it must be recalled that curing SU-8 at 200° C for 2 hours significantly increases the reticulation rate of the resin, hence it is essential to limit outgassing as much as possible Melai et al. (2009). The proposed set-up used for this test is schematically
drawn in Figure 5. It works on the principle of the so-called pressure rising method in dynamic vacuum Silverio et al (2022). In brief, the aim is to see an increase in the cell pressure while introducing water inside the test reservoir. The pressure rise would indicate a leak. The device as shown in Figure 1 is mounted inside a vacuum chamber under a secondary vacuum, i.e., 5 × 10⁻⁵ mbar. First, we verify that the presence of the SU-8 device does not significantly degrade the vacuum in the cell. Then, the reservoir is filled with liquid water under pressure and the change in the gauge level is recorded. It is observed that applying a pressure of 1.5 bar on water for one hour does not induce any decrease of the vacuum which remains constant at 5 × 10⁻⁵ mbar under pumping. This means that the outgassing from SU-8 and the SU-8 porosity to water must be extremely small.

<figure><img src="image_8.png" /><figcaption>Fig. 5: Schematic of the vacuum setup. The device is mounted in the chamber and SU-8 performance is tested with applying liquid water pressure under a secondary vacuum</figcaption></figure>

We now estimate the leak rate $Q_{leak}$ of particles, passing through our SU-8 device into the cell: $Q_{leak}=P_{cell}×D_{leak}$, with $P_{cell}$ the pressure at the level of the cell (as measured with our Gauge) and $D_{leak}$ the debit in volume at a given pressure, expressed in L·s⁻¹. Since the pressure in the cell is not changing over the leak test, one can write that the effective pumping speed at the level of the cell compensates for the leak rate of our device and the outgassing from the cell: $D_{leak} + D_{outgassing} = D_{cell}$. For the sake of simplicity, we will estimate an upper bond of $D_{leak}$ by writing:

$$D_{leak} \leq D_{cell} \qquad (3)$$

To compute $D_{cell}$, we recall that the flux of particles $Q$ is equal everywhere (from the cell to the pump) and can be written as:

$$Q = P_{cell} \times D_{cell} = P_{pump} \times D_{pump} = C(P_{cell} - P_{pump}) \qquad (4)$$
Where $P_{cell}/P_{pump}$ and $D_{cell}/D_{pump}$ are respectively the pressure and the debit in volume at the level of the cell/pump, C is the conductance of the circuit connecting the cell to the pump (pipes and valve). From Equation 4, we can write: $\frac{1}{C} + \frac{1}{D_{pump}} = \frac{1}{D_{cell}}$ it comes

$$D_{cell} = C \frac{D_{pump}}{C + D_{pump}} \qquad (5)$$

$D_{pump} \sim 85 Ls^{-1}$ is obtained from the datasheet of our pump. Because $D_{pump}$ is constant for pressures below $10^{-4}$ mbar, we approximate that $D_{pump}(P_{pump}) \sim D_{pump}(P_{cell})$. We will see in what follows that $D_{cell}$ is anyway dominated by C. We now need to compute C, which is composed of 2 terms: $C_{pipes}$ and $C_{valve}$. From the datasheet of our valve, $C_{valve} = 160 L \cdot s^{-1}$. $C_{pipes}$ is given in the molecular regime by Agilent Technologies (2022)

$$C_{pipes} = \frac{d^3}{L} \sqrt{\frac{2\pi k_B T}{m_{N_2}}}, \text{ thus:}$$

$C_{pipes} = 16.2 L \cdot s^{-1}$ where $L = 2.4 m$ and $d = 6 cm$ are length and diameter of the pipes respectively.

Finally, $\frac{1}{C} = \frac{1}{C_{pipes}} + \frac{1}{C_{valve}}$ and $C \sim 14.7 L \cdot s^{-1}$.

Injecting in Equation 5 we get $D_{cell} \sim 12.5 Ls^{-1}$.

And finally, Equation 4 gives us $Q = P_{cell} \times D_{cell} \sim 6.3 \times 10^{-4} \pm 1.9 \times 10^{-4} mbar \cdot L \cdot s^{-1}$, with the error bar being set by resolution of the Gauge.

The calculated upper bond of the leak rate is better than the leak rate corresponding to the water vapor leak tightness which is in the range of $10^{-3} mbar \cdot L \cdot s^{-1}$ Rottländer et al (2016).

# 4 Conclusion

We reported that micrometric walls of SU-8 photoresist can be leak-tight from 1.5 up to 5.5 bars, for thicknesses ranging from 135 µm down to 35 µm. We also demonstrated the absence of porosity even after 2 months of aging. We also find that SU-8 can be operated under a secondary vacuum as a microfluidic seal with no degradation of the vacuum (neither outgassing of the SU-8 nor liquid water or gas permeability). Moreover, the fabrication process we propose requires neither aggressive chemical nor high temperature nor high energy plasma treatment. It thus opens a new perspective to seal microchips when sensitive surfaces or materials have to be used.

## Author Contributions

A.N., S.P., F.H., and R.L. conceived the experiment. S.P. fabricated the samples with the help of J.B., E.C., S.T., Re.J., R.D., R.L., and C.R.. S.P. carried out the experiment with inputs from V.J., E.A., and A.N.. A.N., F.H., and Ra.J. supervised the work. All authors have read and agreed to the manuscript.

## Conflicts of interest

There are no conflicts of interest to declare.
Acknowledgments

We acknowledge financial support from CNRS-MITI (France) through a 'Momentum' grant. SP acknowledges support from ANAS (Azerbaijan) for the Ph.D. scholarship. The work was partly supported by the French Renatech network. 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, in accordance with the grant's open access conditions.

**Supplementary information.**

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