Fabrication of polymer microlens arrays using capillary forming with a soft mold of micro-holes array and UV-curable polymer

This paper reports a simple and effective method to fabricate microlens arrays with the ultraviolet-curable resins, and a soft mold of micro-holes array. During capillary forming operation, the surface of the soft mold of micro-holes array is being pressed against the ultravioletcurable resin layer coated on the plastic substrate. An array of convex lense can be formed in the circular holes of the soft mold due to the capillary filling and surface tension. The microlens arrays have smooth surface and uniform focusing function. The shape and height of micolens can be controlled with a proper combination of pressing pressure, pressing duration and UV curing dose. This technique shows great potential for fabricating polymer microlens arrays with high productivity and low cost. ©2006 Optical Society of America OCIS codes: (220.0220) Optical design and fabrication. (220.4000) Microstructure fabrication. (230.0230) Optical devices. (230.3990) Microstructure devices. References and links 1. D. Daly, R. F. Stevens, M. C. Hutley and N. Davles, “The manufacture of microlenses by melting photoresist ,” Meas. Sci. Technol. 1, 759-766 (1990), http://www.iop.org/EJ/toc/0957-0233/1/8 2. M. Fritze, M. B. Stern and P. W. Wyatt, “Laser-fabricated glass microlens arrays,” Opt. Lett. 23, 141-143 (1998). 3. W. X. Yu and X.-C. Yuan, “UV induced controllable volume growth in hybrid sol-gel glass for fabrication of a refractive microlens by use of a grayscale mask,” Opt. Express 11, 2253-2258 (2003). 4. D. L. MacFarlane, V. Narayan, J. A. Tatum, W. R. Cox, T. Chen and D. J. Hayes, “Microjet fabrication of microlens arrays ,” IEEE Photon. Technol. Lett. 6, 1112-1114 (1994). 5. S.-I. Chang and J.-B. Yoon, “Shape-controlled, high fill-factor microlens arrays fabricated by a 3D diffuser lithography and plastic replication method,” Opt. Express 12, 6366-6371 (2004). 6. W. Pan, X. Shen and L. Lin, “Micro-plastic lens array fabricated by a hot intrusion process, ” J. Micromech. Microeng. 13, 1063-1071 (2004). 7. C. Y. Chang, S. Y. Yang, L. S. Huang and C. H. Chang, “Fabrication of plastic microlens array using gasassisted micro hot –embossing with a silicon mold,” Infrared phys. Technol. 48, 163-173 (2006).


Introduction
In recent years, microlens arrays have been widely used in many fields such as flat panel display, micro-scanning system, fiber coupling and optical communication etc.Many methods of fabricating microlens array have been reported.Some examples are thermal reflow method [1], laser writing [2], gray scale mask photolithography [3] and microjet fabrication [4], 3D diffuser lithography [5], hot intrusion and gas-assisted hot embossing with nickel mold of micro-holes array [6,7].
Most of these techniques are complex, expensive and not easily accessible to most scientists and industrialists.Although the thermal reflow technique is regarded as a low cost mass-production process, it is time-consuming and difficult to control the precise shapes and heights.The hot intrusion and gas-assisted hot embossing with metal mold of micro-holes array are comparatively inexpensive to fabricate microlens array onto thermoplastic substrates.By adjusting the embossing load, temperature and time, the polymer melt is partially filled into the circular holes of mold, and forms a convex surface due to viscoelastic deformation and surface tension.However, the process involves high temperature and high pressure.It is still a time-consuming batch-wise process.
From this perspective, an effective capillary forming process for rapid fabricating microlens arrays has been proposed, developed and tested.The proposed technique uses a soft PDMS mold instead of the electroformed metal mold and uses UV-curable polymer to fabricate micro-optical structure.Compared with the conventional electroformed metal mold, the PDMS mold can be fabricated by casting process without expensive facilities.The usage of low-viscosity UV-curable resins allows rapidly fabrication at low pressure without heating cycles.It is a simple, inexpensive and reproducible method for rapid fabrication micro-and nanostructure.
In this study, the soft mold with micro-holes array is made by casting a pre-polymer of PDMS against a silicon master of micro-cylinders array.The silicon master of cylinders array is prepared using photolithography and deep reactive ion etching.During capillary forming operation, the surface of the soft mold of micro-holes array is pressing against the ultravioletcurable resin layer coated on the plastic substrate.Under proper pressing pressure and pressing duration, the resin will partially fill the circular holes, and due to surface tension form a convex surface.The resin is then cured by UV-irradiation at room temperature.After the soft mold is removed, the plastic substrate with microlens array patterns on the surface can be obtained.The total cycle time is less than 10 seconds.This method has many advantages over the conventional techniques.In addition to low cost and high productivity, the shape and height of microlens can be adjusted by changing the processing conditions.
In the experiment, a capillary forming system with UV exposure capacity has been designed, constructed and tested.The effects of processing conditions on the shape and quality of formed micolens are investigated.The optical property and surface roughness of fabricated microlens array are also measured and analyzed.

Fabrication of soft mold with micro-holes array
First of all, the silicon wafer with a SiO 2 layer is cleaned and coated with a 1.5μm thick positive photoresist (S1813) using a spin coater.The wafer is then exposed through a mask with 150×150 circle array features for 8 seconds using a UV mask aligner (EVG620).After dipping in the developer for 1 min, circular photoresist columns array are obtained.The second step is silicon dioxide layer etching by reactive ion etching in a CF 4 /O 2 plasma to make the etching mask.The deep reactive ion etching process then follows so as to define the shape of the micro-cylinders into the silicon wafer.The etching rate is about 1.5μm/min, and the total processing time is about 70 minutes.A silicon master with micro-cylinders array is obtained.The Polydimethylsiloxane (PDMS) pre-polymer solution (Dow Corning SYLGARD 184), a mixture of 9:1 silicon elastomer and the curing agent is then poured on the silicon master and cured at 80 o C for 2 hours.After the PDMS replica remove from the master, the soft mold with micro-holes array is obtained.Figure 1 shows a SEM image and crosssectional view of the soft PDMS mold with micro-holes array.The micro-holes array has a diameter of 100μm, a depth of 105μm and a pitch of 200μm.

The capillary forming facility and process
Figure 2 shows the capillary forming system with UV exposure capacity used in our experiments.The system consists of UV-transparent top and bottom plates, a charged coupled device (CCD) camera, an UV-lamp, a shutter and a micrometer scale resolution Z-stage with hydraulic unit.The Z-stage allows the control of the distance between the mold and substrate and thus the pressing pressure.The maximum pressing pressure is 500kPa.The wavelength of the UV-lamp is between 365-410nm.The UV intensity at 365nm is 100mJ/cm 2 .The UVcuring dose, which is the intensity of UV-light times the curing time, can be controlled by the shutter.An UV-curable epoxy resin UVA1321 (CHEM-MAT Technologies Inc., USA) is used.The refractive index is 1.50 at 633 nm wavelength.The viscosity is 2000cps at 25°C.
During the capillary forming operation, the UV curable epoxy resin is coated on the polycarbonate (PC) substrate of 180μm thickness.The stack of PDMS mold with array of micro-holes and the substrate coated with UV-curable resin is then placed in the capillary forming facility.The PDMS mold of micro-holes array is pressed against the ultravioletcurable epoxy layer on the PC substrate with proper pressing pressure for a certain period of time.An array of liquid convex lenses can be formed in the circular holes of the PDMS mold due to the capillary filling and surface tension.The UV curable epoxy resin is then cured by UV-irradiation at room temperature.After curing, the PDMS mold is removed from the substrate, and the plastic substrate with microlens array on its surface can be obtained.

Effect of process conditions on the capillary forming of microlens arrays
To study the effects of processing conditions on the capillary forming of microlens, three processing parameters, i.e., the pressing pressure, pressing duration and UV curing dose are investigated.The pressing pressure, pressing duration and UV curing dose used in the experiments are between 20kPa to 140kPa, 2 to 10 seconds and 250 to 1250mJ/cm 2 , respectively.The qualities of formed microlens arrays are visually inspected using a charged coupled device (CCD) camera.The microlens should be of spherical shape and free of defect.
The results show that the proper pressing pressure is between 50 and 80kPa.Too high pressure causes distortion of the pattern on the PDMS mold, resulting in sagging and collapse of microlens.The proper pressing duration is between 2 and 8 seconds.Array of microlens with spherical shape can be successfully formed.If pressing duration is too short, the liquid photopolymer will not fill the holes at all and fail to form any shape.On the other hand, if the pressing duration is above 8 seconds, the photopolymer completely fills the holes cavity, forming flat cylinders.The amount of UV curing dose has little effect on the forming of the microlens.The UV curing dose can be between 500 and 1000mJ/cm 2 .
Figure 3 shows a SEM image of the formed microlens array on polycarbonate (PC) substrate and the surface profile of a single microlens.The processing condition are 50kPa of pressing pressure, 4 seconds of pressing duration and 750mJ/cm 2 of UV dose for curing.The 150×150 array of microlens has been successfully fabricated over the epoxy coating on the plastic substrate.The microlens array has a diameter of 100μm, a sag height of 17.83μm and a pitch of 200μm.
Based on geometry and optical theory, the radius of curvature (R), focal length (f) and numerical aperture (NA) can be determined using the following equations: where D, h and n are the diameter, the sag height and the refractive index of epoxy material, respectively.Based on the calculation, the radius of curvature is 79μm, focal length is 158μm and numerical aperture of the microlens is 0.32, respectively.
Fig. 3. SEM image and the surface profile of the formed microlens array under the condition of 50kPa pressing pressure, 4 seconds pressing duration and 750mJ/cm 2 UV curing dose

The optical property of microlens array
The optical property of the fabricated microlens array is further measured using a beam profiler.The beam profiler is composed of expanding lenses, a filter, a micrometer scale resolution Z-stage, a microscope, a CCD system and a 633 nm laser light source.The average focal length is measured to be 160μm for the formed lens with a diameter of 100μm and a sag height of 17.83μm.The calculated and measured data agree well.durations increase from 6 to 8 seconds.However, there is little change in the shape of hemispherical profile and focal length of the cylindrical microlens.These results indicate that the shape, height and focal length of microlens are strongly dependent on the pressing duration in the capillary forming process.

Conclusion
This paper reports an effective method for rapid fabrication of microlens arrays using ultraviolet-curable resins and soft mold with micro-holes array.By pressing the PDMS mold of micro-holes array against the ultraviolet-curable epoxy on PC substrates with a proper combination of processing conditions, the polymer microlens array can be fabricated.An array of 150×150 microlens has been successfully produced with 50kPa of pressing pressure, 2～8 seconds of pressing duration and 750mJ/cm 2 of UV curing dose.The pitch of the focused light spots and spot intensity of the polymer microlens array are uniform.The average surface roughness of a typical microlens is measured to be 3.586nm.The shape, height and focal length of the formed microlens can be changed by adjusting the pressing duration in the process.With PDMS molds of micro-holes array and ultraviolet-curable polymer coated on the substrates, the great potential of the capillary forming process for rapid production of microlens arrays with versatile shape is demonstrated in this study.

Figure 4 (Fig. 4 .
Fig. 4. Light spot pattern and intensity profile of a microlens array, (a) light spot image of a microlens array, (b) intensity profiles at the focal plane of a microlens array

Fig. 5 .
Fig. 5. Surface roughness measured in a 5μm × 5μm area on the top surface of the fabricated microlens.(The average surface roughness is 3.586nm)

Figure 6
Figure6shows the effect of pressing duration on the shape of microlens.With the pressing pressure of 50kPa and UV curing dose of 750mJ/cm 2 , the pressing duration increases from 2 to 4 seconds.Perfect hemispherical microlens on PC substrate can be formed.The sag height of formed microlens increases from 4.75μm to 17.83μm.The measured focal length of the microlens decreases dramatically from 528μm to 158μm.When the pressing durations are above 4 seconds, cylinders with hemispherical profile are formed.The peak heights measured from the bottom of formed cylinders increases from 45.62μm to 79.24μm, as the pressing

Fig. 6 .
Fig. 6.Shape of microlens under various pressing duration in the UV-capillary forming process