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The paper reports an effective method to fabricate micro-lens arrays with the ultraviolet-curable polymer, using an original pneumatically diaphragm-driven drop-on-demand inkjet system. An array of plano convex micro-lenses can be formed on the glass substrate due to surface tension and hydrophobic effect. The micro-lens arrays have uniform focusing function, smooth and real planar surface. The fabrication process showed good repeatability as well, fifty micro-lenses randomly selected form 9 × 9 miro-lens array with an average diameter of 333.28μm showed 1.1% variations. Also, the focal length, the surface roughness and optical property of the fabricated micro-lenses are measured, analyzed and proved satisfactory. The technique shows great potential for fabricating polymer micro-lens arrays with high flexibility, simple technological process and low production cost.
©2012 Optical Society of America
1. Introduction
Micro-lenses and micro-lens array are finding wide applications mainly in the domain of micro optical system, e.g. flat panel display, fiber coupling, optical trapping, optical interconnects, biomedical instruments, optical data storage and optical communications [1–5]. Several methods for fabricating micro-lens and micro-lens array using glass, silicon and polymer have been reported [6–8]. Among them, polymeric micro-lenses have recently attracted considerable interest due to its flexibility, light-weight, mechanical property and simple fabrication process.
Some typical reported fabrication methods of polymer micro-lens are thermal reflow method [9], UV curing of liquid drop [10,11], hot embossing [12], ink-jet printing [13] and so on. In order to have a large scale production of such micro-structures, the following criteria needs to be fulfilled: (1) high through-put, (2) low cost, (3) high efficiency, (4) simple process.
The fabrication of micro-lens using droplet-on-demand inkjet method has attracted considerable research because of the potential applicability to micro-optical system. The ink-jet printing technology, firstly developed by MicroFab [14], provides an additive manufacture technique of non-contact and data-driven type. The distinct advantage of the ink-jet printing technology is its one step forming without etching. To our knowledge, ink jet printing technology includes piezoelectric actuating mode and pneumatically actuating mode. The former is a commercially available generator with a piezoelectric tube forcing out a droplet when a voltage pulse is applied on. However, the system is costly because of the expensive driving equipment. Also, the poor power supply makes drop squeeze out very difficult. The latter is simple structure and easy operation. Yet, the constant change of driving force location along with the liquid surface dropping leads to lack of reliability and stability. Combining with the above two structures, an accurate and low-cost drop-on-demand(DOD) droplet generator for the ink-jet process with an in situ image measurement system has been developed by the authors [15,16].
In this paper, we demonstrate the application of a novel pneumatically diaphragm-driven drop-on-demand inkjet system to fabricate micro-lens array using optical UV-curable polymer. Due to surface tension and hydrophobic effect, a plano convex micro-lens array can be formed on the glass substrate. Detailed optical measurements of micro-lenses fabricated by the technique will be made. The focal lengths, surface roughness and optical aberrations of the lenses are optically and mathematically characterized.
2. Inkjet printing method
2.1 Experimental set-up
Figure 1 and Fig. 2 show the micro-droplet drop-on-demand inkjet system with UV exposure capacity used in our experiments. The system consists of a pneumatically diaphragm-driven inkjet generator, a micrometer scale resolution motion stage, an in situ vision system based on delay trigger and a UV-lamp. As shown in Fig. 3 , diaphragm and compressed gas pulse are used as driving element and driving source respectively. The pneumatically diaphragm-driven drop-on-demand inkjet generator produces micro-droplets by volumetric change of liquid chamber through deformation of diaphragm. Figure 4 shows the picture of the glass nozzles fabricated with the homemade equipment, the minimum diameter is 30μm or even smaller. The wavelength of the UV-lamp is between 365~410nm and is not shown in Fig. 2. After the ejection, the substrate with liquid micro-lenses is in the original position to avoid damaged, and the UV-lamp will be installed to replace the droplet generator and complete the curing.
2.2 Ejection and deposition process of a single micro-droplet
Figure 5 describes the ejection process of a single micro-droplet with the diameter about 300μm. The liquid is glycerin/water (60/40, mass ratio), and the viscosity and surface tension are 10.0mPa•s and 69mN/m respectively. The used liquid is glycerin/water mixture instead of optical epoxy which will be used to fabricate micro-lens. This is to avoid the solidification of the optical epoxy inside the glass nozzle under illumination, which is indispensable for the in situ vision system. Figure 6 illustrates the deposition process of the micro-droplet on three substrates having different wettability. Contact angles are 29.9°, 57.9° and 114.2° respectively. Due to surface tension and hydrophobic effect, the convex liquid droplet having stable shape will form. As shown in Fig. 6, through adjusting control parameters, glass nozzle with different outlet sizes and substrate of different wettability, micro-lenses have variable diameters and curvature radiuses.
Fig. 6 Deposition process of micro-droplets on substrates with different contact angle (diameter: about 300μm), (a) contact angle = 29.9°, (b) contact angle = 57.9°, and (c) contact angle = 114.2°.
2.3 UV-curable polymer
UV curing optical epoxies are a preferred class of material for micro-lens printing, because of their good characters such as thermal and chemical durability, excellent optical performance, high adhesion, photo curing rapidly, low volume shrinkage, and so on. The parameters of our selected optical epoxy are shown in Table 1 . Its viscosity is 21.5cps at room temperature to enable DOD printing. The index of refraction is 1.50. The transmittance of an epoxy sheet of 300μm thickness as function of the wavelength for a range of 400~900nm was detected by Ultraviolet and visible spectrophotometer (PerkinElmer, Lambda 35). As Fig. 7 shown, the transmittance is very high (up to 90%) for visible and near IR wavelength. It is confirmed that the UV-curable adhesive is very suitable for propagating micro-lens array. The fabricated micro-lens can be used in application requiring integration of optoelectronic devices and optical communication.
Table 1. Material Performance of UV-curable Adhesive
During the inkjet operation, the UV curable epoxy resin is deposited on the 1mm thick glass substrate coated with hydrophobic optical film of 114.2° contact angle. An array of liquid convex lenses can be formed. The UV curable epoxy resin is then cured by UV-irradiation at room temperature. After curing, the glass substrate with micro-lens array on its surface can be obtained.
3. Results and discussion
3.1 Micro-lens morphology
Figure 8 shows optical micrographs of the formed micro-lens array on glass substrate. The resultant refractive micro-lens array has good structural and dimensional uniformity. To quantitatively analyze the uniformity of the micro-lens, 50 micro-lenses of the sample shown in Fig. 8 were randomly chosen to be measured the diameter, sags and pitch by three-dimensional optical microscopy of ultra depth of field (Keyence, VHX-600E). The micro-lens array has an average diameter of 333.28μm, a sag height of 94.13μm and a pitch of 354μm. The optical epoxy contact angle for the fabricated samples is 59.1°. Furthermore, the variation of the diameters is about 1.1%, as shown in Fig. 9 . The results indicate high uniformity of the micro-lens array and good controllability of the system.
3.2 Surface quality of formed micro-lens
In order to quantitatively analyze the surface roughness of the micro-lenses, a sampling area of 5 × 5μm was stochastically selected for the measurement using a scanning probe microscope (SPM, Veeco NanoScope MultiMode). The specimen is randomly selected from a single micro-lens array. Figure 10 shows the surface roughness of the fabricated micro-lens. It was observed that the arithmetical mean deviation of the profile Ra is 0.243nm, maximum height of the profile Ry is 9.495nm, root mean square roughness Rq is 0.454nm. The resultant surface shows an excellent surface smoothness.
3.3 Focal length
The focal length of the micro-lenses was obtained by the two independent methods described below: 1) Calculation of focal length based on geometry and optical theory, 2) Measurement of focal length using collimator.
3.3.1 Calculated focal length
The resultant polymeric micro-lens is plano convex. Based on geometry and optical theory, the radius of curvature can express by
where dL, HL and K represent the diameter of micro-lens, the sags height and asphericity coefficient, respectively. Here, K = 0, where the section of the micro-lens is spherical. Then, the focal length can express byTen micro-lenses randomly chosen from the fabricated micro-lens array, shown in Fig. 3, are measured by three-dimensional optical microscopy of ultra depth of field (Keyence, VHX-600E). From the data measured as shown in Table 2 , we could obtain the focal length of the micro-lenses is 389μm and non-uniformity of the micro-lens array is:
Table 2. Diameter, Sags and Focal Length of 10 Micro-lenses Randomly Chosen from Micro-lens Array
3.3.2 Measured focal length
A method of accurately measuring the focal length of the micro-lens using a collimator is introduced. As Fig. 11 shown, the measurement system is equipped with collimator, Porro reticle, CCD and image grabbing card. A highly corrected collimator set has a Porro reticle with three pairs of spaced lines located in its focal plane, the image of the Porro reticle is projected over the micro-lens under test and focused in its focal plane. By means of CCD and image grabbing card, the image of spaced lines appears on the computer screen and the size is determined. Finally, the focal length of the micro-lens can be calculated by the equation: f = f′ × (y′/y), where, f′, y and y′ represent the focus of the parallel light tube, the pitch of the spaced line and the pitch of the image.
The focus of the adapted Ultra short focal parallel collimator (SSFC-IV) is 51.48mm. The pitches of the three pairs of scribed lines are 10mm, 4mm and 2mm respectively. Figure 12 shows image of Porro reticle with spaced lines. And Table 3 shows the measured focal lengths. The average focal length of the micro-lenses is 482μm and non-uniformity of the micro-lens array is 1%.
Table 3. Measured Focal Length
As can be seen from Table 2 and 3, there are distinct deviation of the above two results. The first approach uses mathematical equation to calculate the focus length based on the micro-lens profile and refraction index of the UV-curable adhesive. Therefore, the precision and accuracy of the measurement will affect the results. However, the second approach uses the optical measurement system to achieve the focus length by Porro reticle. The method has more power to reflect the actual situation taking many factors into account, such as the role of glass substrate, clamping situation and so on. As a result, the focal lengths obtained by the two ways will be different.
3.4 Optical quality
Star testing method can be used to test the imaging defects caused by spherical aberration, chromatic aberration, coma aberration or other drawbacks. The smaller the diffraction pattern, the more concentrated of the energy and the better the imaging quality. The optical property of the fabricated micro-lens array is measured using the above system (as Fig. 11 shown) just replacing Porro reticle with star orifice plate. Figure 13 shows a portion of the spot patterns produced by the formed micro-lens array. The images reveal that the pitch and the intensity of the focused light spots are uniform. And the view of the focal plane of the micro-lens array shows uniform arrays of focal points.
3.5 Defect analysis
As shown in Fig. 14 , there are some defects observed by SEM and optical microscope. Figure 14(a) shows edge defect with the contact diameter of 89.16μm. The possible reason for edge defects is that the substrate surface is rough. Figure 14(b) and Fig. 14(c) show particle contamination and cracked surface with contact diameters of 79.16μm and 94.50μm respectively. One possible reason for particle contamination is the non-clean surface or low cleanliness of work environment .Yet, the reason for cracked surface is likely due to over-exposure. Figure 14(d) shows non-circular profile with the contact diameter about 528.06μm. The actually reason is likely that jetting direction deviates from vertical direction of the surface.
Fig. 14 Defects of micro-lens, (a) edge defect ( × 532), (b) particle contamination and cracked surface ( × 1059), (c) cracked surface ( × 850), and (d) non-circular profile ( × 300).
4. Conclusion
In this paper, an innovative method for fabricating micro-lens array using pneumatically diaphragm-driven drop-on-demand inkjet technology is reported. Under the proper processing conditions, an array of 9 × 9 polymeric micro-lenses with a diameter of 333.28μm, a pitch of 354.04µm and a sag height of 94.13µm has been successfully fabricated. The measured surface roughness of the micro-lens Ra is 0.243nm. The pitch and intensity distribution of the micro-lens array are uniform. The resulting micro-lens array displays very high optical quality. The technique shows great potential for fabricating polymer convex or hemispherical refractive micro-lens arrays with the diameter around 80μm~1mm due to its high flexibility, simple technological process and low production cost. Furthermore, the micro-lens and micro-lens array can be written directly onto optical substrates (quartz glass or polymer plate) or optical fiber end, waveguide, and diode lasers cover.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 51105321 and Grant No. 50775087) and the Xiamen Municipal Science and Technology Plan Project (Grant No. 3502Z20113036).The experimental work is mainly carried out at the Institute of Micro-Systems in School of Mechanical Science and Engineering in Huazhong University of Science and Technology. The financial and technique supports are gratefully acknowledged.
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