monolithic multifunctional integration in fused silica

Monolithic Multifunctional Integration in Fused Silica Yves Bellouard*

Mechanical Engineering Dept, Technische Universiteit Eindhoven, Eindhoven, the Netherlands.

ABSTRACT

At energy level below the ablation threshold, femtosecond-laser irradiation of Fused Silica (a-SiO2) can induce significant changes in the material properties: the refractive index and the chemical etching susceptibility can both be significantly increased. Using these effects one can scan a femtosecond laser beam through the substrate volume in order to create three-dimensional patterns with tailored material properties. Based on this approach, optical elements such as waveguides can be embedded in glass substrates. The elements can be provided with various functionalities such as for instance fluidic channels or micro-mechanical elements.

In this paper, we show that femtosecond laser irradiation applied to Fused Silica (a-SiO2) defines a novel technology platform for highly integrated all-optical microsystems. In contrast to the common approaches that rely on combining materials to achieve particular functions, our technology utilizes a single piece of material, whose properties are locally functionalized through femtosecond laser irradiation.

This leads to a new micro-systems design paradigm, whose potential is illustrated with a few examples in this paper.

INTRODUCTION

For the last decades, micro-technologies have nurtured many innovations that have significantly impacted our daily life and contributed to the advancement of science by providing new investigation and analytical tools. In biotechnologies for instance, the concept of miniaturized laboratories – the so-called “Lab-on-a-Chip” – that can be implemented on a single chip has emerged as a major paradigm toward faster and portable analytical tools. Further progress in microtechnologies depends on our ability to increase the level of integration of functionalities in miniaturized platform but also, on appropriate micro-manufacturing technologies, in particular high-aspect ratio machining that are particularly important for devices that integrate micro-mechanical functions.

This paper presents a micromachining process that combines femtosecond exposure and chemical etching to fabricate three-dimensional structures in synthetic Fused Silica (a-SiO2). This machining method not only allow the fabrication of high-aspect ratio structures but also, offers the ability to integrate, at the same time and using a single process flow, optical waveguides. This process particularity opens new opportunities for integrated biochips and micromechanical sensors design.

This publication is intended to provide the reader with an overview of our laser-assisted micro-manufacturing process and its applicability to the fabrication of integrated devices in glass substrates.

1. Femtosecond lasers pulses and their effects on synthetic Fused Silica

1.1. Femtosecond lasers The fabrication process is based on femtosecond laser irradiation and chemical etching. Femtosecond lasers [1] are characterized by ultra-short pulse (typically 10-13s) that leads to ultra high peak power (intensities in the Terawatt/cm2 or even Petawatt/cm2 can be reached with relatively simple commercial systems).

The femtosecond pulse-matter interaction is unusual in several ways. Due to these high power intensities the fast energy deposition, any material can be turned into plasma. The diffusion of heat away from the interaction region proper can be

* [email protected], Phone: ++31-40-2473715.

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almost completely prevented. Furthermore the extreme optical intensities can lead to strong non-linearity through multi-photons absorption process. As a consequence, the modification induced in the material occurs only at the laser focal point. The latter observation is of particular technological importance for the machining of “transparent material” (i.e. transparent for the laser-wavelength): the material can be modified beneath its surface and throughout its volume. We use femtosecond laser to pattern glass substrate and more specifically synthetic fused silica.

1.2. Synthetic Fused Silica (a-SiO2)Synthetic Fused Silica, later referred in this paper as a-SiO2, is a glass material transparent over a wide optical spectrum (t>0.90% typically from 200 nm to 2.5 um). Key properties of Fused Silica are a near zero thermal expansion, an exceptionally good thermal shock resistance, very good chemical inertness, low dielectric constant and low dielectric loss. Typical of glasses, it lacks long-range order in its atomic structure. The geometry is described as a three-dimensional network of rings of various sizes anchored around local SiO4 tetrahedrons.

Fused silica resists to high temperature environment (melting point is around 2000°C). It has a very low coefficient of thermal expansion and exhibits excellent chemical stability to a broad range of chemicals.

Table I summarizes typical physical properties of silica glass. These properties make silica glass an interesting substrate material for bio-MEMS in general as well as for harsh-environment sensors.

In our experiments, we used only high purity synthetic fused silica substrates (Dynasil® 1100). This glass is characterized by OH content in the range of 600-1000 ppm, a Cl content of 90 ppm and total metallic impurities content 1-2 ppm.

1.3. Effects on Silica glass Femtosecond laser illumination affects two properties of fused silica. First it increases locally its refractive index [2] (increase of refractive index as high as 0.007 has been reported [8]). This property makes the fabrication of optical waveguides possible by simply connected laser-affected zone (LAZ) together to form a light path (Davis et al. [2]). Second, it also increases the Hydrofluoric acid (HF) chemical etching selectivity [3] - i.e. laser-exposed regions are etched several order of magnitude faster than unexposed regions. Etching anisotropy induced by laser exposure of 100:1 was reported [4]. Although, the exact origins of these phenomena are still subject to discussion (see [6] for instance about the origin of index of refraction changes), systematic SEM investigations [4] and recently nanoindentation [10] and thermal conductivity measurements [18], strongly support the idea that both phenomena - increase of index of refraction and higher chemical etching selectivity - originate from a material densification effect and more specifically, a change of silanol rings-order sizes in the a-SiO2. This view is supported by earlier work made in non-laser irradiated bulk material [5].

Symbol Quantity Typical values

E Young’s modulus

70 GPa a

Poisson ratio 0.17Density 2200 kg/mm3

CTE Expansion coefficient

0.55 (106/°C)

t Opticaltransparency (spectral range 0.2 to 2.5 µm)

> 0.9

aGPa : Giga-Pascal = 103 N/mm2

Table I – Fused Silica main properties




MICRO-FABRICATION PROCESS

2. Process overview

To form three-dimensional structures the following two-steps procedure is applied (see Fig. 1):

1) The material is selectively exposed in defined volume: This is done by rasterizing a pattern according a volume sampling technique described in [4] and briefly outlined in the next paragraph. The laser used in our experiment is a Ti:Sapphire laser (RegA from Coherent) operating at 800 nm. This laser setup is installed in Translume Inc facilities. The pulse width is typically 100-fs, and the repetition rate is set at 250 kHz. The average power ranges from 20 to 400 mW, which corresponds to pulse energies ranging from 55-nJ to 1 micro-Joule. The linear spot size is approximately 1 micron. In our experiments, we used 20X and 50X long-focal objectives from Mittutoyo. With these objectives one can work well below the surface of thick glass substrates. Typical writing speeds are 0.5 mm/s to 2 mm/s. Considering these writing speed, affected regions are hit multiple times by the laser (typically 500 to 2500 times).

2) After laser-exposure, the part is etched in a low-concentration HF-bath. Concentrations between 2.5% and 5% are used. Etching time depends on pattern sizes and typically varies from an hour to several hours for the thickest substrates. Following etching the part is rinsed in de-ionized water (DI) and dried.

3. Volume sampling [4]

The laser affected zone (LAZ) has the shape of an ellipsoid stretched along the optical axis. The LAZ shape and size was determined using either a refractive index map technique [8] or more recently a novel technique based on Scanning Thermal Imaging [18]. The LAZ shape can be correlated to laser beam parameters such as beam-size, waist and energy. The stretching along the vertical direction depends on the chosen focusing optics. Noteworthy, fs-laser matter interaction involved non-linear processes, the ellipsoid short axis can be smaller than the spot-size itself. These observations are supported by near-field optical profilometry measurements which provide a refractive index map of the region of interest (details on this characterization technique can be found in [7]). A refractive map of a single line written in the glass using fs-laser is shown on Fig. 2.

The volume of matter to be etched away is defined by stacking and arranging spatially laser-exposed regions so that a continuous contour around the volume to remove is defined. Further details about the volume sampling can also be found in [4].

Fig. 1. Process steps. 1- The material is exposed to femtosecond laser irradiation. Exposed regions are etched faster than unexposed regions. 2 -Chemical etching. The part is etched with Hydro-Fluoric acid (typically 2.5 %) for a time varying typically from an hour to several hours. The etchingrate is polarization dependant.




4. Etched Microstructures

4.1. Experimental conditions The laser-exposure is made by moving the specimen under the laser beam using a setup somewhat similar to Computerized-Numerically-Controlled (CNC) machine tools use for conventional macro-scale machining. The specimen is moved under the laser beam along X-and Y-axis. The laser focal point is moved independently along the Z-axis. It is therefore possible to describe any arbitrary three-dimensional shapes by defining appropriate laser-paths. Due to the non-linear absorption mechanisms involved in the femtosecond laser matter/interaction with transparent materials, the material is only modified at the focal point. As a consequence, the glass can be modified below its surfaces. We used that property to form tunnels below the glass surface. Although the written pattern was 5 mm long, we noticed a maximum etching penetration depth of about 1.2 mm for a width, measured at the entrance, varying from 70 microns to a millimeter. This depth limit was also reported recently by other authors [19, 20]. Depletion of the chemical reagent as the etching front progresses may explain this limitation. An alternative hypothesis is that byproducts of the etching reaction redeposit and form a protective layer preventing further etching.

The average etching rate in the longitudinal direction, measured for a period of three hours, is in the order of 5 microns/min. This etching rate is measured for a transverse polarization, i.e. perpendicular to the laser writing direction. In comparison, the etching rate for non-laser exposed fused silica, for the same material and under the same etching conditions, is typically 3 microns/hour [4]. Therefore, LAZ are etched about 100 times faster than non-affected regions.

Fig. 3. Micro-tunnels [4]- The tunnel lies at about 20 microns from the glass surface. The length is about 1.2 mm and the entrance diameter is about70 microns. The tunnel is located 50 microns below the surface. Tunnel located deeper (420 um) were also produced using the same method.

Fig. 2. Refractive index map of a single line written in a fused silica glass [8]. The center of the line has the highest index of refraction which, asAgrawal et al. [5] emphasized, can be correlated with a local increase of glass density which in turn, can be related with the local increase of etchingrate.




Figure 3 shows a micro-tunnel manufactured through selective etching. The writing speed of 100 microns/s was used and pulsed energies of 135 nJ. The volume is described by a set of equally spaced parallel lines spaced by 3 microns. The left picture is an optical microscope view of the tunnel tails. Dark lines correspond to the etching pattern. The right picture is an SEM view of the tunnel entrance. The tunnel has a slight conical structure with an angle of 2.7 deg for the first portion (0 - 0.5 mm) and 1.8 deg for the deeper section (0.5 to 1 mm). Additional observations on partially etched tunnels (see [4]) suggest that the etching profile is propagating faster along the illuminated track.

Figure 4 shows a scanning electron microscope (SEM) picture of two branching micro-fluidic channels. The picture below shows an example of three-dimensional micro-machined V-groove. The V-contour was scanned with the fs laser beam. Worth noticing, optical waveguides can be written at the same time as the pattern that defines the microstructure (see Figure 5). Therefore, the relative positioning accuracy of waveguides relatively to microstructures only depends on the performance of the stages used to move the glass specimen under the laser beam. This is a unique characteristic of this process: to the best of our knowledge, it is the only micro-manufacturing process that allows the simultaneous integration of optical and 3D structural functionalities.

Fig. 4. Example of 3D-micro-machined structures. Top – V-groove shape / Bottom – Branching micro-channels.

Fig. 5. Optical microscope of a micro-channel with two waveguides going accross. (left) The channel is 30 microns wide and 60 microns deep. The waveguides are 8 microns in diameter and are positioned 50 microns below the glass surface. Waveguide array used for sensing micro-displacement (right). Waveguides are spaced every 30 microns.

Trench (through) – about 20um)

Waveguides array (movable part)

Stationary waveguides




PROCESS CHARACTERIZATION

5. Etching rate To measure the etching rate, a 30-microns wide and 675-microns deep channel-pattern was positioned in the substrate such that the HF acid can only penetrate inside the glass through the top (Fig. 6). In this case, the pattern is made of 15 x 90 lines with a line density of 2 microns. The channel was placed enough close to the glass edge such that it is possible to measure the etched depth through direct optical observations. The measured etching speed is 1.2 microns/min for transverse polarization and about three times less for parallel polarization. The influence of the polarization on the etching rate was reported [21]. Our observation confirms their findings. In addition, the polarization also influences the wall roughness quality as it can be seen on Fig. 11.

Laser Writing direction

Parallel polarization

Transverse polarization

Fig. 6. Etching rate as a function of the polarization. (The convention used to define parallel and transverse polarization is shown on the left.)

Best results are obtained with transverse polarization. The difference of etching rate between perpendicular and parallel polarization is also clearly visible in the left picture in Fig. 7. Both channels have the same depth and were made using the same laser pattern but were made under different last polarization conditions. Polarization also leads to different machining quality (Fig. 7 right).

Fig. 7. Effect of polarization: the top photograph shows two trenches etched with the same energy and writing speed parameters but with twodifferent polarizations.

Parallel polarization Transverse polarization

Transverse polarization

Parallel polarization




5.1. Surface roughness measurement

Atomic Force Measurements were made to investigate the surface roughness on etched profile. For this measurement, a 2 mm-thick slab was used on which a line was written all the way through to split the slab in two parts. The polarization used was transverse. Results are shown in Figure 8.

The cut surface was investigated using an AFM at various locations across the cut edge. The higher roughness was found in the middle of the substrate which corresponds to the last part etched. Etching time was 23 hours in a 2.5 % concentration (time required for the two part to split). The average roughness measured on the edge is 200 nm and 400 nm in the center.

5.2. Edge profiles Beam with rectangular cross-sections where manufactures and observed in an SEM. The beam thickness was 500 microns. The cutting trench is made of pattern lines stacked in the Z-direction. Since the pattern extends from one surface to the opposite one, we assume that the etching profile propagates from both surfaces at the same speed. The etching pattern was written all-the way so that the part defined by the contour is removed from its substrate (see Fig. 9).

We first locate the edges (point A and B in Fig. 9) under the SEM at high magnification (x10000) and record the SEM stages coordinates. Using this method, the point’s coordinates are known with accuracy better than 100 nm (SEM stages resolution). The location of point M is then calculated. Moving the SEM to the calculated location, we then look actual edge position (point C). The distance defines the relative error between the desired profile and the etched profile. With this information, we estimate the angle . We found an average angle alpha to be of 0.23 deg with a standard deviation of 0.22 deg.

Fig. 8. AFM based surface roughness measurement. Average roughnesses are rms values.




5.3. Nanoindentation This study was reported in [10]. Here we just recall the main results. The specimen preparation procedure is outlined in Fig.10. The patterns are written 400 microns below the surface. To generate the lines, the laser beam is scanned at a speed of 500 microns/s. Once the patterns are written, the specimen is cut in its middle using a diamond saw. The cut surfaces are polished down to optical transparency.

Fig. 10. Nanoindentation experiments: (left) Sample preparation procedure and experimental results. Category I corresponds to successful measurement. Cat. II and III are failed test. Reference tests are shown for comparison [10].

Femtosecond laser patterns are localized using the nanoindenter (MTS XP) embedded microscope and indents are made approximately in the middle of the patterns. We used the so-called “Continuous Stiffness Measurement” technique that has the advantage of a continuous measurement of the stiffness during the indent loading phase as opposed to just one performed during unloading.

Having in mind all the usual limitations and imprecision inherent to nanoindentation experiments, a consistent trend is observed. A slight increase of 2 to 3 GPa is observed. The laser pattern has a composite structure being made of parallel lines alternating irradiated and non-irradiated zones. It is therefore difficult to extrapolate from these measurements the effective increase in the laser irradiated zone. Nevertheless, these observations point to structural changes.

5.4. Scanning Thermal Imaging of fs-laser affected zone The specimen preparation is similar to the one described for nanoindentation except that the cut specimen is analyzed using a Scanning Thermal Microscope (SThM) (PSIA). This microscope is an Atomic Force Microscope (AFM) equipped with a cantilever incorporating an embedded thermal probe. As in conventional contact-probe AFM, the SThM cantilever is first brought in contact with the sample. When scanning the surface, irregularities in its morphology

Fig. 9. Etching profile measurement. Schematic of measurement point localization procedure.




deflect the cantilever. A Z-scanner is used to counter this deflection by adjusting the height of the probe, thus maintaining a constant force to the surface. From the Z-scanner information one creates the topography profile of the scanned surface. The temperature of the sample is constantly monitored during the scanning. From this data a thermal map is obtained. We use the probe in conductivity contrast imaging (CCM) to measure local changes in the thermal conductivity. A resistive element located on the thermal probe locally heats the substrate to a temperature much higher than room temperature. The energy required to maintain the set temperature is representative of the local thermal conductivity. When the heated probe makes contact with the substrate, heat flows from the probe to the sample, resulting in the cooling of the probe. The closed-loop feedback senses this shift, balances the bridge voltage, and restores the probe's resistance (or temperature) to its preset value. The more current needed to maintain the probe set temperature, the higher the thermal conductivity of the specimen.

Laser Affected Zones are clearly identifiable in the SThM. Fig 11 shows a part of the patterned line as seen through their thermal conductivity image. The conductivity is decreased in the laser-affected zone which also points to structural changes. Although it sounds counter-intuitive, these observations support further the hypothesis that fs-laser exposed regions are densified. The lower conductivity for denser silica glass is related to phonons vibration anomalies whose origins are still subject to discussion and not fully understood [14,15]. However, Zhu [16] observed experimentally that at room temperature the thermal conductivity of densified fused silica is slightly lower than that of normal fused silica.

Fig 11. Scanning Thermal Imaging of a laser-exposed area. The left picture shows two groups of two adjacent lines. The right shows the corresponding topography which emphasizes the fact the observed SThM patterns can only be attributed to material structural changes.

5.5. Phenomenological Model

A phenomenological model was proposed in [4]. Here we just summarize its main features. Based on our observations [4, 8, 10, 18] as well as additional data reported in the literature, we formulate the following hypothesis:

1. For doses below a certain threshold, there is a clearly well defined isotropic etched zone at the laser focus volume that seems to correspond to a different material phase than the surrounding volume.

2. For this regime, the compaction mechanism is driven by a long-order structural change where the laser is focused.

3. Beyond a certain threshold of deposited energy, ablation occurs due to the fact that no more compaction is possible and that Si2+ and Si3+ are formed that tends to lower the ablation threshold.




MONOLITHIC MULTIFUNCTIONAL INTEGRATION IN FUSED SILICA

6. General concept

In the previous sections, it was shown that femtosecond laser patterning of fused silica substrate is an efficient way to fabricate three-dimensional structures in which optical features like waveguides can be embedded using a single process-flow.

This process is very attractive for monolithic integration as it significantly limits the number of fabrication steps to only two and offers new opportunities for miniaturized devices. Waveguides may be combined to form optical functions as diverse as inter-leavers, interferometers (Mach-Zehnder) or encoders. Many papers have reported on combination of waveguides. These optical functions can be very accurately positioned and embedded in complex fluidic channels network or micro-mechanical devices.

The positioning accuracy is intrinsically high as the process of patterning channels and waveguides does not require the need for repositioning elements as it is a single-step process-flow. All this makes the process described in this paper attractive for all-optical sensors where only optical signals are processed.

On the other hand, fused silica is a quite robust material rather well-adapted to harsh environments that are characterized either by high temperatures and large pressures, or unusual environmental conditions like vacuum, nuclear radiation or high electromagnetic fields. These environments are hostile for humans, and necessitate specific technologies that can survive these tough conditions. For some applications (insertion inside Magnetic Resonance Instruments for instance), it is also desired to have technologies that do not interfere with their environment.

Fig. 13. Illustration of the concept of all-optical sensor with remote processing unit. An optical link is used to carry the information from the sensing unit to the processing unit.

Compaction Stress-field

Fig. 12. Proposed model to describe the etching phenomenon. The central portion of the beam is densified and a phase change is found. This issupported by the higher contrast found in central portion of partially etched structure [4].

Laser track




Sensors are usually made of two parts: the sensing head that interacts physically with the element to measure and the processing unit. The basic idea of “all-optical sensor” is to decouple the sensing head from the processing unit. The transfer of information between the sensing-head and the processing unit is made using optical waveguides and fibers. The sensing information is exclusively carried by photons in the visible or near-infrared spectrum. Considering optical losses in fibers (typically less than a fraction of one dB/km), the processing unit that is potentially sensitive to surrounding environmental conditions can be placed far away from the hazardous zone.

7. Illustration: micro-force sensor

We have used the process described throughout this paper to machine a micro-force sensor (shown in Figure 14 and Figure 15) out of a 500 microns thick wafer of fused silica.

This force sensor demonstrates the simultaneous integration of optical and mechanical functionalities in a single substrate. Furthermore, as it makes use of integrated optics, the optical signal can be brought through optical fibers giving the opportunity to decouple the sensing head from the processing and signal conditioning unit. The ability to incorporate integrated optics in three-dimensional shapes using a single machining process opens tremendous opportunities for high accuracy miniaturized sensors that are suitable for harsh environments. In particular such sensors are immune to electromagnetic radiation, can withstand high temperatures, are chemically inert and biocompatible.

7.1. Design and simulation

The device consists of a flexure made of two 40 microns-thin bars that holds a central portion on which a V-groove is machined. The flexure has a low bending stiffness along the z direction. The mechanical structure was calculated using an analytical model and optimized using FEM cad software (Ansys). The flexure is used in its linear regime (small deflection).

A waveguide divided in three portions is “written” below the surface and goes all the way across the structure along the X-axis (Figure 2 shows a close-view of the central portion of the waveguide). Once a force is applied on the V-groove, the flexure deflects along the Z direction and doing so, moves the central portion of the waveguide down.

Fig. 14. Left: Schematic of a micro-force sensor entirely fabricated using fs-laser and chemical etching. Right: SEM picture of the micro force sensor. As it can be seen high aspect ratio are obtained: the wafer is 500 microns thick and the trenches are less than 20 microns.

This misalignment between waveguides creates optical coupling losses at the interfaces. Through measurement of the optical loss one can extract the flexure position. Further as the stiffness of the flexure is known one can calculate the force applied to the flexure.




Fig. 15. Left: Working principle of the force sensing element. Right: Close view of the embedded waveguides for the displacementsensing.

7.2. Experimental results

Figure 16 shows the variation of light intensity as a function of the flexure deflection as well as the measured force/displacement characteristic.

This result demonstrates the principle of this micro-displacement sensor. Repeatable sub-micron displacements were measured. Our current measurements were limited by the resolution of the sensor use to characterize the structure. We suspect that the resolution of the glass sensor is below 100 nm. Further experimental work will be done to fully characterize the structure.

CONCLUSION

This paper has proposed a detailed review of a micro-manufacturing process based on femtosecond laser patterning of fused silica glass. It was shown that both waveguides and microstructures as diverse as fluidic channels or flexures can be machined in a single process-flow and integrated in a single substrate. This process is particularly interesting for all-optical devices.

Fig.16. Photo-detector signal as a function of the position. The signal rapidly decreased when the waveguides are no longer aligned. The signal shown on the upper-right corner is not filtered.

Mobilepart




ACKNOWLEDGEMENT

This research is made in collaboration and is partially supported by Translume Inc. The specimens presented in this review have been machined at Translume facility in Ann Arbor, MI, USA. The author thanks Mark Dugan, Ali Said and Philippe Bado for their contribution to this work.

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monolithic multifunctional integration in fused silica

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