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We present a simple, fast, and repeatable method for fabricating nano-fluidic channels based on two-photon absorption (TPA) polymerization. Since our method does not require any mask, it is relatively cheaper and faster than other conventional lithography techniques. We illustrate that SU-8 has pronounced photoinitiation threshold behavior, which linearly decreases as the repetition rate increases. If the pulse energy and the repetition rate are controlled, channel width can be easily controlled. We report fluidic channels up to 110 nm in width, between polymerized parallel ribs, utilizing TPA and the photoinitiation threshold properties of SU-8. Finally, we also show that high repetition rate laser presents greater controllability in size of the polymerized region by varying fluence.
©2009 Optical Society of America
1. Introduction
In last two decades, microfluidics has rapidly expanded its interest into many multidisciplinary fields of study, such as chemistry, fluid dynamics, biology, and micro-fabrication technology. Applications of fluidic channels include: protein separation and DNA sequencing, drug delivery, biomolecular sorting, and single molecule detection [1]. Since the emergence of the reliable, reconfigurable, and scalable systems, such as, lab-on-a-chip (LOC) and micro-total-analysis-systems (μTAS), the needs of innovative procedure for creating nano-fluidic channels has accelerated significantly.
There have been many techniques developed to create nano-fluidic channels by lithography. Lithography is pervasive in micro-fabrication because of its capability of transferring complex nano-patterns to the substrate with high aspect ratio. Conventional techniques for high resolution nano- and micro-fabrication of fluidic channels are Soft X-ray lithography, extreme UV (EUV) lithography, electron beam lithography (EBL), focused ion beam (FIB) and Nano-imprinting lithography (NIL) [2]. Since these lithography techniques greatly rely on a mask or mold fabrication, which is very expensive and time consuming, alternative patterning techniques are required. In addition, fabrication processes are complicated, require various special equipments, and limit to certain materials. Finally, conventional polymerization based on single-photon absorption (SPA), has limited spatial resolution because of diffraction, and limited flexibly in direct lithographic patterning of 3D parts [3].
Two-photon absorption (TPA) has attracted great research interest because of its potential in creating 3D micro and nano features. In TPA polymerization, laser is focused inside the polymer which induces chemical crosslinking and solidification. The polymerization rate is quadratically dependent on the photon-flux density [4]. Hence, polymerization occurs near the vicinity of the focal point of the laser light, which enables solidification of photo-resist in the sub-diffraction limit [5]. In this research work, we explore the feasibility of using TPA over SPA for the fabrication of nano-fluidic channels.
Proposed method involving TPA
Fig. 1. TPA process for fabricating fluidic channels. (a) During exposure, femtosecond laser is focused inside SU-8 resist. (b) After development, exposed resist turns into parallel ribs. (c) SEM image of two parallel ribs. (d) Magnification of channel between two ribs
SPA processes are widely adopted in photo-resist mask fabrication, but it has limited applicability when attempting to fabricate 3D features. In SPA, the polymerization starts from the surface and penetrates into the resist, which limits 3D structuring resolution. In contrast, TPA polymerization occurs near the vicinity of the focal point, which allows for layer-by-layer construction of 3D profiles. Thus, 3D nano-fluidic channels with complex cross section profiles can be fabricated. Also, TPA does not require a mask therefore it is relatively inexpensive and particularly suitable for rapid prototyping. Finally, TPA is fast, reproducible, and configurable to the design requirements. To create nano-channels, a femtosecond laser beam is tightly focused onto the SU-8 resist and scans in the pattern of parallel lines with fixed pitch, as shown in Fig. 1(a). After development, the exposed pattern of lines converts into parallel ribs. Two consecutive ribs that are separated by spacing form a channel, as shown in Fig. 1(b). By carefully controlling the spacing, nanofluidic channels can be obtained.
Unlike SPA, TPA has threshold and nonlinear behavior, because the absorbed energy is proportional to the square of the intensity of light [6]. Generally, every photo-resist has a photoinitiation threshold. The cross linking occurs only if the energy absorbed by the resist reaches a certain value that is called photoinitiation threshold fluence, expressed by laser energy per unit area [7]. The photoinitiation threshold is precisely defined for SU-8 resist. Since the energy distribution of machining spot follows a Gaussian profile, desired thickness of the ribs can be achieved by controlling laser pulse energy and the number of applied pulses. In addition to the rib thickness, the nano-scaled spacing can also be achieved by positioning two consecutive ribs with high precision nano positioning stages or laser beam steering mechanisms. Therefore, the accuracy and the resolution of the spacing are solely determined by the mechanisms.
2. Experiment setup and fabrication process
The experiment was carried out using a diode-pumped Yb-doped fiber oscillator/amplifier system capable of producing pulses of 214 fs duration and variable repetition rates from 200 kHz to 26 MHz, with a central wavelength of 1030 nm. To obtain better feature resolution, we used second harmonic (515 nm) central wavelength. Previous works have demonstrated that second harmonic increases the efficiency and the ease with which the micromachining of the features were carried out due to the reduction in the order of multiphoton absorption [8].
Figure 2 is a schematic representation of the machine setup used to expose resist with a controlled laser beam. A plano-convex lens of 500 mm focal length and a plano-concave lens of 150 mm focal length were used to reduce the laser beam diameter and a λ/2 wave-plate is used in between these lenses to control the polarization of the beam in order to increase the efficiency of second harmonic. A harmonic generator converts the laser beam to the second harmonic (515 nm) central wavelength. The 515 nm mirrors were used to dump the 1030 nm wavelength out of the beam. Afterwards, a plano-concave lens of 75 mm focal length and a plano-convex lens of 300 mm focal length were utilized to expand the beam diameter by 4 times to 10 mm. A quarter waveplate was used to rotate the polarization state of the laser beam to circular. In addition, a diaphragm is used to perfect the beam profile. Finally, the laser beam was scanned onto the substrate surface with SU-8 coating using a piezo tip/tilt mirror. A telecentric lens with a 12.478 mm focal length was utilized to focus the laser beam. The theoretical laser minimum spot diameter (Do) was calculated using D0≈1.27λ 0 f/D [9], where f is the effective focal length of the telecentric lens, λo is the wavelength of the laser and D denotes the laser beam diameter. Values for these parameters are 12.478 mm, 515 nm and 8 mm, respectively. Hence, the theoretical spot size diameter was calculated to be 1.02 μm. The scan field for this setup is 1mm × 1mm, hence the maximum channel length is 1mm. Nonetheless, it can be increased by increasing spot size or by translating positioning stage.
The very important distinction between SPA and TPA is that in the case of TPA, the photo resists are usually transparent instead of highly absorptive behavior in SPA. Hence, TPA is capable of polymerizing the resist by adjusting the laser intensity at the focal point to be above the threshold so that the diffraction limit simply becomes a measure of the focal spot size. In our experiment we observed TPA polymerization at the focal point inside the volume of resist. The overall process of TPA is very similar to UV lithography except femtosecond laser is used for exposure. SU-8 resist of 0.5μm thickness was deposited on a borosilicate glass substrate by spin coating. The substrate then soft baked on a hot-plate at the constant temperature of 95°C for 1 min with coated surface on top. Glass substrate with SU-8 coating then exposed under controlled laser beam for specific geometry created in AutoCAD. When SU-8 resist is exposed, it goes through thermal expansion and swelling depending on the pulse energy, number of pulses, and the absorption time. As a result, one can achieve nano-fluidic channel between two ribs, if the process parameters are controlled.
After exposure, the resist was hard-baked on a hot-plate at 95°C for 2 min. SU-8 crosslinking occurs during this step in the regions that contain the acid catalysts generated during exposure [3]. Once substrate was cooled, it was immersed in SU-8 developer solution for 2 min. Finally, the parallel fluidic channels formed by TPA polymerization were observed under Scanning Electron Microscope (SEM).
3. Discussion and results
The fabrication of nano-channels on glass substrate using TPA technique is fairly simple and repeatable. We created fluidic-channels arrays with various laser parameters with the attempt to achieve the smallest channel width with highest controllability. Fig. 3 is an SEM image of parallel fluidic-channels formed between two successive ribs. Channels of around 1 μm width were formed in between ribs measured 5 μm in width. Our goal was to bring two ribs as close as possible, by increasing their width precisely, to create nano-scale channels between them.
Photoinitiation threshold can be determined by extrapolating linear plot of squared diameter D 2 of polymerized circular pit versus the laser fluence to D 2=0 [10]. However from the experiment, we determined the photoinitiation threshold of SU-8 resist are 0.989 J/cm2, 0.819 J/cm2, 0.734 J/cm2, and 0.400 J/cm2, for repetition rates of 4.33 MHz, 8.67 MHz, 13 MHz, and 26 MHz, respectively. In addition, the threshold energies are determined to be 8.08 nJ/pulse, 6.69 nJ/pulse, 6.00 nJ/pulse and 3.07 nJ/pulse for 4.33 MHz, 8.67 MHz, 13 MHz and 26 MHz, respectively. In other words, each pulse carries less energy for high repetition rate laser.
The threshold fluence and threshold power were plotted in Fig. 4 for their respective repetition rates. From Fig. 4, it is evident that photoinitiation threshold fluence for SU-8 decreases linearly, and the average threshold power increases, as the repetition rate increases. This agrees with the results obtained in similar process of thin film ablation by femtosecond laser [10]. The threshold pulse energy required to initiate the polymerization reduces with the decrease of pulse repetition rate, thus the threshold fluence reduces with repetition rate. By increasing the repetition rate, the effective number of pulses is increased. The effective number of pulses, Neff is a convenient measure of the feed rate in order to compare the results to stationary process of polymerization by multiple pulses. Neff can be calculated from: Neff = (π/2)0.5 ω0f/v [11]. Here, ω0 is the machining spot radius which is 0.51 μm, f is the laser repetition rate and v is the piezo scanning speed, which kept constant at 300 mm/s. The expression relates accumulated fluence of multiple pulses with a Gaussian intensity profile, and subsequent spots separated by v/f. As shown in Fig. 4, Neff increases linearly with the increase of repetition rate, f. The insert of Fig.4 shows that, opposite to threshold fluency, the average threshold power increases with the increase of repetition rate and reaches to saturation after 13 MHz. This can be explained by the combined effect of effective number of pulses and threshold fluence.
Fig. 4. Threshold fluence and number of effective pulse for a for various repetition rates; inset figure represents average threshold power for given repetition rates
To study the effect of fluence and repetition rate on the feature size, particularly on channel width, the fluence and channel width were plotted in Fig. 5 for various repetition rates. It is observed that high fluence and high repetition rate are preferred to minimize the channel width. At high repetition rate, the increment of fluence is small, which provides better control on the line width. This phenomenon can be explained by the Gaussian energy distribution of a laser pulse. Fig. 6 shows the Gaussian intensity profile of an effective machining spot [12]. The lower intensity outer edge of the beam does not polymerize at effective pulse number of 10, but does polymerize at effective pulse number of 56, due to the effect of heat accumulation. Thus, at a given laser fluence, the machine spot diameter for 56 pulses, Φ(56), is greater than the effective machining spot for 10 pulse, Φ(10), resulting in a wider area of exposure of SU-8. As shown in Fig. 4, the threshold fluence reduces with the increase of Neff. Since the spacing between two laser paths is constant, the ribs width increases as the energy increases, thus channel width decreases.
Scanning resolution, R, is given by the distance between two consecutive scan points, as shown in Fig. 7. Similar to Neff, scanning resolution also has an effect on size of polymerized area, which can be determined from R=v/f, where the scanning speed,v, is constant at 300 mm/s. Thus, R is inversely proportional to the repetition rate f. Higher the repetition rate, smaller the value of R. If the value of R is smaller than the spot diameter, there will be an overlap. The percent overlap can be given by %Overlap = (1-R/D0)×100. The percent overlap for 26 MHz, 13 MHz, 8.67 MHz, and 4.33 MHz are calculated to be 99.43%, 98.87%, 98.30% and 96.60%, respectively. High repetition rate with tighter resolution and greater overlap results in higher degree of polymerization. Also, the linearity of the scan line significantly improves with high percent overlap, which is necessary to have good sidewalls of fluidic channel.
Fig. 8. Fluidic channels obtained for 26 MHz. (a) Change in Channels width due to change of pulse energy of 26 MHz from left to right: 1.23 μm, 0.62 μm, 0.42 μm, 0.69 μm and 0.93 μm. (b) fluidic channels left to right: 110 nm, 150 nm.
Low pulse energy in high repetition rate laser provides the flexibility and controllability in fluence increments required to gradually increase the width of ribs. Hence, for both 26 MHz and 13 MHz the channel width decrease gradually as the fluence increases. In contrast, a minor increment of fluence for low repetition rate laser results in a sudden decrease in channel width, because of high pulse energy in low repetition rate. We can also see that it is very difficult to control the channel width at low fluence, just above the threshold fluence; a minor increment in fluence above threshold fluence results in significant reduction in channel width. Nevertheless, it is evident from the results that the width of the polymerized ribs can be well controlled at high repetition rates and fluence well above the threshold.
Figure 8(a) shows the fluidic channels created by 26 MHz repetition rate. However, this time the pulse energy is not same for each line. From left to right the pulse energy for each line were 4.86 nJ/pulse, 7.35 nJ/pulse, 8.90 nJ/pulse, 9.30 nJ/pulse, 7.70 nJ/pulse, and 5.63 nJ/pulse, respectively and the channel created between them are 1.23 μm, 0.62 μm, 0.42 μm, 0.69 μm and 0.93 μm, respectively. We can see that the smallest channel is between two lines with highest the pulse energy. Fig. 8(b) shows the smallest channel width of 110 nm achieved with the pulse energy of 9.30 nJ/pulse and the repetition rate of 26 MHz.
4. Conclusion
We have demonstrated a simple, fast and inexpensive process using TPA polymerization in SU-8 by femtosecond laser. Unlike other lithography processes, our process does not require a mask and it is configurable to design requirements. By polymerizing multiple parallel ribs with nano-scale pitch, we were able to achieve fluidic channel as small as 110 nm in width. We also demonstrate that SU-8 has a threshold pronounced behavior, and it does not polymerize until laser fluence overcomes the photoinitiation threshold. The photoinitiation threshold fluence for SU-8 decreases linearly as the repetition rate increases. Experimental results showed that high repetition rate is preferred to obtain minimum channel width due to the construability. Other than the mechanical positioning, the channel width can be controlled by selecting optimum effective number of pulses and scanning resolution. Future work will be focused on creating 3D channels with desired cross section profile using the property of within-volume-polymerization of TPA.
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