CHAPTER 3. DEVICE FABRICATION
3.6 Process Flow For PDMS Integration
Figure 3.4: A figure illustrating the FLICE technique which was optimized in collaboration with the FEMTOprint<
focal volume to the laser was around 523 m3, obtained working with a 50x objective with 0.65 NA. Than the glass slide was kept in a high concentrated KOH etching bath for 8h. The SMRs were designed considering different geometries, in particular four different type of SMRs were fabricated as shown in Figure 3.5b,Figure 3.5d,Figure 3.5fandFigure 3.5h. Each of them shows a constriction of the channel path in the middle of the suspended beam which are also shown in Figure 3.5a,Figure 3.5c,Figure 3.5e andFigure 3.5g. This constrictions have been taken into account to evaluate the possibility to slow down the velocity of the fluid in the central part of the resonator, which is characterized by the maximum amplitude of displacement at the resonance frequency of the SMR, increasing the permanence time of the fluid in that region. The first micro resonator that was fabricated is illustrated inFigure 3.5a. The Z constriction is just an insertion of two solid trapezium within the channel. The second resonator that was analyzed is shown in Figure 3.5c. The constriction is very similar to the previous resonator in the sense that it also having the two trapezium solid that have the presence of the 10x10x35 rectangular solid located within them as shown in Figure 3.5d. The third micro resonator under consideration which is illustrated in Figure 3.5e, shows the constriction which is X shaped and can be represented as two solid trapezium adjacent to one each other as shown inFigure 3.5f. The main reason why the constriction was introduced into the design of the micro channel resonator was to aid in slowing down the nanoparticles motion through the channel. In our quest to simulate and characterize the micro-channel resonator it was important to understand the dimensions of the fabricated channel Figure 3.5 and to evaluate the theoretical volume occupied by the constriction as shown in Table 3.1.
500-55-Z 500-55-Z0 750-30-XX 500-55-X
Length [µm] 500 500 750 500
Outer Width [µm] 75 75 75 75
Inner Width [µm] 55 55 30 55
Volume Of Constriction[pm3] 0.105 0.108 0.036 0.0405 Volume Outer Plate[pm3] 1.6875 1.6875 2.53125 1.6875 Volume Of Hallow Plate [pm3 ] 0.825 0.825 0.675 0.825 Volume Accessible To Fluid[pm3] 0.72 0.717 0.639 0.7845 Mass Of The Resonator[µg] 2.1314 2.1380 4.1687 1.9871
Table 3.1: A table illustrating the summary of the geometrical characteristics associated with the different micro resonator.
3.6 Process Flow For PDMS Integration
In this section the idea is to explain the process flow necessary in order to integrate Polydimethylsiloxane with the suspended micro channel resonator. Plasma oxygen bonding was used to integrate the suspended micro channel resonator with the Polydimethylsiloxane. The motive for this
CHAPTER 3. DEVICE FABRICATION
(a) The 500-55-Z resonator (b) The Z constriction In 3D
(c) The 500-55-Z0 resonator
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(d) The Z0 constriction In 3D
(e) The 500-55-X resonator
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(f) The X constriction In 3D
(g) The 750-30-XX resonator
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(h) The XX constriction In 3D
Figure 3.5: A figure illustrating the images and the schematics associated with different constriction of the suspended micro channel resonators.
integration was to aid the flow the liquids through the micro fluidic channel.
3.6.1 Silicon Mold Process Flow
The PDMS microfluidic channel was realized by following these different steps as illustrated inFigure 3.7.
The first step was coating the silicon wafer with a negative photoresist. This negative photoresist is the kind of resist in which the portion of the resist that becomes exposed to light remains after developments . The second step deals with the transferring of the fluidic patterns by means of the laser direct writing. The photoresist development only results in the removal of the unexposed photoresist.
In the third step there is the dry etching of the unprotected silicon surface. This dry etching process is extremely important because it allows for the correct transfer of the microfluidic patterns onto the silicon substrate. The final step deals with the stripping the photoresist and the passivation of the silicon surface
3.6. PROCESS FLOW FOR PDMS INTEGRATION
Figure 3.6: A figure showing the SEM image for A. 55µm B.30µm channel micro-channel resonator
Figure 3.7: A figure showing the silicon mold process A. Deposition of the negative photoresist B.
Exposure C. Development D. Dry Etching E. Stripping F. Passivation with PFOT G. PDMS deposition H. PDMS Mold
with Perfluorodecyltrichlorosilane (PFOTS). The Perfluorodecyltrichlorosilane(PFOTS) deposition was performed overnight at room temperature. The advantage of this passivation is to ensure that the PDMS demold after polymerization as shown inFigure 3.7.
3.6.2 PDMS Glass Bonding Process Flow
The PDMS(Sylgard 184) was prepared by mixing the polymer and the cross-linking agent in a ratio of 10:1. The mixture was then poured unto the above silicon mold for a duration of 2 hours at a temperature of 600C. The PDMS mold was formed and cutting the single PDMS blocks which is known to contain the microfluidic patterns. The oxygen plasma bonding was undertaking to enhance the PDMS bonding with that of the glass. The bonding did occur at a power of 20 Watt, a duration of 40 seconds and a flow rate of 30 standard cubic centimeters per minutes of the oxygen gas. Careful attention was given to the alignment of the microfluidic patterns within the PDMS and the SMR within the glass. Contact and heating treatment was performed at 800C for a duration of 10 minutes was performed. TheFigure 3.8is showing the final product after the bonding process which is linked to the connectors which will finally allow the flow of the liquids through the micro channel resonator.
CHAPTER 3. DEVICE FABRICATION
Figure 3.8: A figure showing the final product after the bonding
3.6. PROCESS FLOW FOR PDMS INTEGRATION