Fabrication of plastic microlens array using gas-assisted micro-hot-embossing with a silicon mold

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Fabrication of plastic microlens array using gas-assisted

micro-hot-embossing with a silicon mold

C.-Y. Chang

a

, S.-Y. Yang

a,*

, L.-S. Huang

b

, J.-H. Chang

a a

Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan

b

Institute of Applied Mechanics, National Taiwan University, Taipei 106, Taiwan Received 3 May 2005

Available online 29 November 2005

Abstract

This paper reports an innovative method for fabrication of plastic microlens arrays. By using gas pressure to press the plastic ﬁlm onto silicon mold of holes array, microlens array can be directly fabricated. A machine with closed chamber for gas-assisted micro-hot-embossing was constructed and tested. The 300· 300 plastic microlens array with a diameter of 150 lm and a pitch of 200 lm were successfully produced. Under the condition of 150C, 10–40 kgf/cm2

gas pressure and 30–90 s duration, the microlens with uniform and strong focusing function were formed on the polycarbonate ﬁlm. The shape and height of microlens can be changed by adjusting the processing temperature, pressure and duration. This technique shows great potential for fabricating microlens array on large plastic ﬁlms with high productivity and low cost. 2005 Elsevier B.V. All rights reserved.

Keywords: Gas-assisted micro-hot-embossing; Hot embossing; Deep reactive ion etching; Replication; Silicon mold; Microlens array

1. Introduction

In recent years interest has grown in fabrication of microlens arrays due to their wide applications in optical computing, optical signal processing, opti-cal interconnection, optiopti-cal data storage, display, etc. Many methods for fabricating microlens array have been proposed and demonstrated, such as ther-mal reﬂow [1–3], excimer laser ablation [4], gray scale photolithography [5], microjet fabrication[6], hot embossing of plastic material on a lens array

mold made by focused ion beam milling [7] and

hot intrusion process [8]. Among them, Most are expensive and not easily accessible to scientists and industrialists. Although the thermal reflow technique is regarded as a low cost mass-production process, the reflow of photoresist is difficult to con-trol to yield precise shape.

The conventional hot embossing [7] and hot

intrusion process[8]are comparatively inexpensive, but there are inherent problems due to the pressing mechanism using hot plates of press. The pressure between the mold and plastic substrate is higher in the center and lower in the edge. The pressure distri-bution is not uniform. The embossing area is thus limited. Besides, the mold material is limited to metal. Glass or silicon molds are often too brittle to be pressed by hot plates.

* _{Corresponding author. Fax: +886 2 8369 5574.}

E-mail address:[email protected](S.-Y. Yang).

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In order to overcome the problem, we developed an innovative method using gas to exert isotropic pressure for micro-hot-embossing. Perfectly uni-form embossing pressure throughout the whole area can be achieved. In addition, silicon molds can be used.

In this study, gas-assisted micro-hot-embossing is used to fabricate plastic microlens array. A silicon mold with holes array microstructures is ﬁrst

fabri-cated by conventional photolithography and deep reactive ion etching process. Plastic film is then placed on top of the mold, and the stack is placed in the closed chamber. Upon heating above the glass transition temperature (Tg) of the plastic film, nitro-gen gas is introduced into the chamber. Under gas pressure, the polymer material is partially filled into the circular holes, and a convex surface is formed due to viscoelastic deformation and surface tension.

Fig. 1. Procedures for fabricating silicon holes array mold. (a) Photolithography, (b) mask etching, (c) silicon etching and (d) SEM image and surface proﬁle of silicon mold.

Nitrogen tank Pressure regulator Valve Silicon mold Heating/cooling plate Chamber Gas in Gas out Plastic film Pressure gauge

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Finally, the stack is cooled down, the gas is vented, the chamber is opened, and the plastic ﬁlm of micro-lens array is removed.

To verify the quality of microlens, the shape and height of embossed microlens are measured using surface proﬁler (Alpha-Step 500, TENCOR, USA) and inspected by scanning electron microscopy (JSM-5600, JEOL, Japan). Also, the surface rough-ness and optical property of plastic microlens array are measured and analyzed. This study further investigates the eﬀects of heating temperature, gas pressure and pressing duration on the shape of formed microlens.

2. Silicon mold of holes array

The procedures for fabricating a silicon mold of holes array are shown in Fig. 1. The silicon mold with 300· 300 holes array of 150 lm in diameter,

200 lm in pitch and 61.44 lm in depth is fabricated by photolithography and deep reactive ion etching process described as follows.

The ﬁrst step is conventional photolithography. The patterns on the mask are transferred onto the photoresist (PR) on top of the (1 0 0)-oriented sili-con wafer (Fig. 1a). SiO2 layer was ﬁrst thermally grown on top of the (1 0 0)-oriented silicon wafer. Then a 1.5 lm thick AZ 5214 positive resist was

Gas pressure (kgf/cm2) Temperature (οC) T1 T_g Troom P1 Temperature profile Pressure profile t1 Time (seconds) t2

Heating Pressing Cooling and Demolding

stage stage packing stage stage Pre-loading

Fig. 3. Temperature, pressure and time proﬁles during gas-assisted micro-hot-embossing.

Table 1

The processing conditions used in the experiments Processing parameters Run Processing temperature (C) Processing pressure (kgf/cm2₎ Processing time (s) 1 140 10 30 2 150 20 60 3 160 30 90 4 170 40 120 5 180 50 150

Note. Reference parameters are underlined.

130 140 150 160 170 180 190 Processing temperature (°C) 0 20 40 60 Peak height ( μ m) P=30kg/cm2 t=120s Lens profile

Fig. 4. Peak height of embossed microstructure for various processing temperature in the gas assisted micro-hot-embossing process.

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spun over the wafers at 4000 rpm followed by a

100C softbake for 2 min. The wafer was then

exposed through a mask with circular holes feature for 10 s. For this exposure, a UV Karl–Suss double side mask aligner was used. The aligner is equipped with ultra-violet wavelength 365–405 nm. The UV intensity at 365 nm is 150 mJ/cm2. The resists pat-terns were then developed using AZ 400 k devel-oper, diluted 1–4 with de-ionized water (DI), followed by a thorough rinse in DI. Following resist pattern deﬁnition, the wafers were baked at a

tem-perature of 120C in a oven for an additional

15 min in order to harden the resist structures. By hardening the resist, the feature patterns become less susceptible to degradation by ion bombardment during the reactive ion etching.

The second step is mask etching. Etching of sili-con dioxide layer is done by selective reactive ion etching (RIE) to make the etching mask (Fig. 1b). The ﬁnal step is anisotropic silicon etching and removal of the masking silicon dioxide layer (Fig. 1c). The silicon mold with holes array pattern

Fig. 5. SEM of the typical cylindrical microlens array. (a) The SEM image of a cylindrical microlens array and (b) the zoomed cross-section view of a single cylindrical microlens.

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light spot through the microlens array is uniform and intensive. This study demonstrates the great poten-tial of the gas-assisted micro-hot-embossing process with a silicon holes array mold for eﬃcient produc-tion of microlens arrays.

Acknowledgements

This work was partially supported by the Na-tional Science Council (Series No. NSC93-2218-E-002–019) of Taiwan. The experimental work was carried out at the MEMS Laboratory in the Nano-Electro-Mechanical-Systems Research Center at NTU is acknowledged.

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