Elsevier

Microelectronic Engineering

Volume 115, 1 March 2014, Pages 46-50
Microelectronic Engineering

Rapid fabrication of semiellipsoid microlens using thermal reflow with two different photoresists

https://doi.org/10.1016/j.mee.2013.10.025Get rights and content

Highlights

  • A simple and rapid fabrication method for a semiellipsoid microlens array has been developed in this study.

  • This technique can be applied in arbitrary shape microlens.

  • A coupling efficiency of up to 85% can be obtained using a semiellipsoid microlens fabricated by our proposed method.

Abstract

A simple and rapid fabrication method for a semiellipsoid microlens array has been proposed in this study. Low-loss coupling can be achieved between a laser diode and a single-mode fiber. The single-mode fiber is tipped with a newly developed semiellipsoid microlens in this technique. Average array coupling loss is as low as 15%. Two different photoresist layers are firstly coated on a silicon wafer in sequence. After the lithography process, the elliptical photoresist column with two different photoresists can be obtained. During the thermal reflow processing, upper photoresist layer (AZ-4620) reaches the glass transition temperature (Tg), which is transformed from a glassy state into a rubbery state. Due to their high surface tension, liquid photoresist tend to minimize the structural energy and reduce their surface trying to achieve a lens shape. Since the glass transition temperature of lower photoresist layer (AZ-5214E) is higher than the temperature of thermal reflow, the lower photoresist layer still maintained solid state. The elliptical base can precisely define the bottom of the liquid photoresist as an elliptical shape. Electroforming technology is then used to convert the photoresist patterns into a metallic mold for PDMS semiellipsoid microlens. A coupling efficiency of up to 85% can be obtained using a semiellipsoid microlens fabricated by our proposed method.

Introduction

The coupling efficiency between a laser diode (LD) and a single-mode optical fiber (SMF) is a big concern in many applications such as pumping erbium-doped optical fiber amplifiers and optical communication systems [1], [2], [3], [4]. The laser light of edge emitting laser is launched from the side edge, thus the shape of side edge decides the shape of diffraction pattern. If the shape of diffraction pattern is rectangular, the shape of diffraction pattern will become oval gradually because the light intensity at four corners of the rectangle is relatively weak after the laser light travels for a distance. In addition, the divergence angle of laser will form a curved wavefront, but the single mode fiber produces circular diffraction pattern and plane wavefront. The mode mismatch will cause a great loss in coupling [5], [6]. Many schemes have been proposed to match the modal profiles of laser and fiber in order to achieve higher coupling efficiency. There are three main methods to couple and reshape the diode laser beams. The first and most straightforward method is the use of lenses to focus the light, in which the laser beams are collimated and focused separately in the parallel and perpendicular directions. The second technique uses optical fibers in which the fiber-ends are positioned close to the laser strips. The light is coupled into and guided along the fibers. The resulting combination of the fibers in a bundle results in the compression of the beam. Another method to reshape the beam makes use of a tapered duct, which has a bigger entrance than exit plane, with gradually decreasing diameter. The light is reflected at the sides of the duct and as it is tapered, the duct compresses the light into the smaller area at the end-face [7].

Optical coupling devices have been widely applied in fiber-optic communication systems for efficient coupling performance. A micro-ball lens can be used to obtain more efficient light coupling from a laser diode into a single-mode fiber. A ball lens can achieve low-cost coupling, but the coupling efficiency of the general ball lens is limited to around 10%. This poor performance is caused by the large spherical aberration size. Increasing ball lens mode converter performance through a more reliable design has been discussed by many researchers using theoretical or experimental methods. Reith et al. [8] showed that coupling efficiencies as high as 34–40% can be maintained using the relaxed alignment tolerance technique during the package assembly process. Shen et al. [9] showed that batch fabrication of an eyeball-like spherical microlens array (ESMA) can reduce micro assembly cost without sacrificing performance, and can also replace the conventional ball lenses or costly gradient refractive index lenses. Yang et al. [10] proposed a mathematical model to design and fabricate micro-ball lens arrays using thermal reflow in two polymer layers.

Based on the light gathering property of microlens, the curved wavefront is converted to plane wavefront through the ellipsoid microlens and the oval diffraction pattern of laser is converted to a circular diffraction pattern, which increases the light intensity of laser and optical coupling efficiency of fiber so as to reduce the loss of misalignment between laser diodes and fiber. Miyamoto et al. [11] fabricated ellipsoidal liquid lenses by depositing silicone oil into elliptical hydrophilic domains on a glass wafer. They use a laser beam to measure the two focal lengths corresponding to major and minor diameters by observing the image formed by a laser beam. Chao et al. [6] achieved a high optical coupling efficiency between a single-mode fiber and an edge emitting laser diode using a semiellipsoid microlens on the single-mode fiber endface. The semiellipsoid microlens was fabricated using photolithography, incomplete thermal reflow, mold electroforming and polydimethylsiloxane (PDMS) injection. It was shown that the microlens array can provide high coupling efficiency of a laser diode to a single mode fiber, and relax the lateral and axial misalignment tolerance.

In order to satisfy the needs of photoelectric systems, many researchers have been exploring various ways to fabricate refractive microlens arrays on different lens materials. Various lens materials and corresponding technologies have been presented, like polycarbonate [12], silica (glass) [13], diamond and sol–gel [14]. For the modern science and technology, most of fabrication technologies for these materials must involve the LIGA process, leading to the high-priced charges. PDMS is one polymeric material that attracted the attention of researchers because of its favorable properties, such as optically transparent, ease of manufacturing, and low cost, which suits different micro-optics applications. However, PDMS is not suitable to fabricate very high aspect-ratio microstructure because of its low Young’s modulus [15]. Also, PDMS undergoes a small degree of shrinkage during curing. Curing can shrink PDMS by 0.5–2.5%, depending on curing temperature, PDMS component ratios, and layer thickness [16]. To solve this problem, alignment methods for wafer level [16] and multilayer [17] have been reported. Wu et al. [18] found that PDMS shrinks about 3% during cooling from high temperature to room temperature. To ensure the dimension accuracy to be as designed after peeling from masters, the PDMS prepolymer was cured at room temperature overnight to prevent the shrinkage of PDMS. Jeong and Konishi [19] controlled the size of replicable PDMS molds/stamps using a stepwise thermal shrinkage process. Thin PDMS films can be formed either by spin-coating onto a master or by “sandwich mold fabrication” [20], in which liquid PDMS is squeezed between a polyethylene sheet and a patterned mold master, clamped, and cured to produce a patterned PDMS thin film. This film preferentially sticks to the polyethylene sheet. The PDMS film can then be transferred and permanently bonded to glass or another PDMS layer, before peeling away the polyethylene. Although PDMS is not suitable for all micro-optics applications, it has remained as the most popular material for easy and effective micro-optics device fabrication.

Any replication technique requires the fabrication of a master mold that has the negative features of the final desired patterns. Hot embossing and injection molding are two major tools that can be used to mass produce plastic microlens. Dutta et al. [21] reported a method for fast prototyping of microlens array with a combination of proton beam writing (PBW) and nanoimprint lithography (NIL) technique. PBW technique was used to fabricate a high quality mold on which Ni electroplating was done and a high quality metallic stamp was made. This stamp was successfully used to replicate good quality microlens array. Hung [22] used the Ni–Co/nano-Al2O3 composite electroforming to make the metallic micro-mold for a microlens array. The Ni–Co/nano-Al2O3 composite deposit was obtained by air-pressurized electroforming technology. His experimental results proved that pressurized electroforming provides an effective way to improve the surface morphology and increase the hardness of the micromold. The SNAOA optimum algorithm was applied to reduce the deformation (by internal stress) and increase the service life of the micro-mold during the hot embossing process. A novel fabrication scheme to develop high-throughput plastic microlenses using injection-molding techniques was proposed by Appasamy et al. [23]. The master mold insert for the injection-molding process was fabricated using metal electroforming. The electroplating process was optimized for obtaining a low stress electroform. This technique enables high-volume production of plastic microlenses with cycle times for a single chip being of the order of 60 s.

Due to the surface tension, only the microlens in spherical cap shape can be produced by the traditional thermal reflow method. Ashraf et al. [24] explored techniques to characterize important geometrical parameters of microlens and used them for process development (to optimize and control the fabrication of microlens by thermal reflow technique), optical characteristics prediction (to predict important optical parameters like focal length and spherical aberration), and model verification (to compare profiles based on theoretical model and experimental data). We proposed a simple and rapid fabrication method for a semiellipsoid microlens array. Two different photoresist layers were firstly coated on a silicon wafer in sequence. After the lithography process, elliptical columns with two different photoresists on a wafer can be obtained. Since the glass transition temperature (Tg) of photoresist AZ-5214E is higher than that of photoresist AZ-4620, the upper photoresist column (AZ-4620) had converted into liquid state, the lower photoresist column (AZ-5214E) still maintained solid state during the thermal reflow process at a temperature of 160 °C. The lower layer can precisely define the bottom of the upper liquid photoresist as an elliptical shape (as shown in Fig. 1). Electroforming technology is then used to convert the photoresist patterns into a metallic mold for PDMS ellipsoidal microlens. Due to the rapid fabrication method developed, it is suitable for the mass production of semiellipsoid microlenses.

Section snippets

Fabrication methods

In this experiment, silicon wafers were used as the substrate. The wafers were firstly cleaned and dehumidified in an oven at a temperature of 150 °C for 30 min. The wafers were then coated with a 5 μm thick photoresist layer (AZ-5214E) using a spin coater. The spin condition was 5500 rpm for 30 s. After that, this photoresist film was prebaked on a hot plate at 110 °C for 10 min. Before thermal reflow process, prolate spheroid approximation method [25] was applied to estimate the required thickness

Results and discussion

The optical coupling efficiency C.E. from laser diode to the fiber can be defined asC.E.=PfiberPLDwhere PLD, Pfiber are output powers of the laser diode and optical fiber, respectively. We used a wavelength of 980-nm LD and the power was 60 mW. According to our previously proposed optical simulation method [28] and the SNAOA optimum algorithm [29], the semiellipsoid microlens with the height of 20 μm has an optimum coupling efficiency in the major axis length of 110 μm and the minor axis length of

Conclusion

We presented a new semiellipsoid microlens fabrication method using the thermal reflow with two different photoresists. Since both photoresists were positive-type, the exposure was only required once. We choosed the AZ-4620 and AZ-5214E as photoresists in the lithography process. During the thermal reflow processing, upper photoresist column (AZ-4620) reached the glass transition temperature, which was transformed from a glassy state into a rubbery state. Since the glass transition temperature

Acknowledgment

This work is supported by the National Science Council of Taiwan, through Grant No. NSC-100-2221-E-252-009-MY3.

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