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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is formed by deep reactive ion etching of silicon wafer. The etching rate was about 1.5 lm/min,

and the total processing time was about 41 min. Each hole in the holes array is of diameter

Fig. 6. The perfect hemispherical microlens array and its surface proﬁle. (a) SEM image of the hemispherical microlens array and (b) surface proﬁle of a single hemispherical microlens.

0 25 50 75 100 125 150 30 20 10 Width (µm) Height (µm)

Theoretical spherical curve Actual data

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150 lm, a pitch of 200 lm and a depth of 61.44 lm.

Fig. 1d shows a SEM image of the silicon mold and its surface proﬁle scanned by the Alpha-step 500. 3. The gas assisted micro-hot-embossing process

After the silicon mold with holes array is made, gas-assisted micro-hot-embossing is used to fabri-cate the plastic microlens arrays. A ﬁlm made of optical grade polycarbonate (PC, glass transition temperature Tg130C, refractive index 1.59) is used as the substrate. The thickness of the PC ﬁlm is 180 lm.

Fig. 2shows the gas-assisted micro-hot-emboss-ing system. The system is composed of a nitrogen tank (120 kgf/cm2 max.), a pressure regulator, valves, a stainless steel chamber and a heating/cool-ing plate. The gas pressure can be regulated with the pressure regulator. Electrical heating elements in the heating/cooling plate are used to heat the mold and plastic substrate, while water is used to cool the sys-tem. The mold can be a silicon wafer, glass, nickel mold, or other stamper with micro-features.

The four stages of the gas-assisted micro-hot-embossing process as illustrated in Fig. 3 are explained as following:

(1) Heating stage: The plastic ﬁlm/silicon mold stack is placed in a closed chamber and hot plate is heated to processing temperature

(T1) which is above the Tgof the plastic mate-rial. During the heating process, low gas pres-sure is applied to the ﬁlm to prevent the ﬁlm from creasing.

(2) Pressing stage: When the processing tempera-ture is reached, the gas is introduced into the chamber to exert gas pressure (P1) over the ﬁlm, forcing the ﬁlm in close contact with the mold. Microlens will be formed in the holes. 0 10 20 30 40 50 60 Processing pressure (kg/cm2₎ 0 20 40 60 Peak height ( μ m) T = 150°C t=120s Lens profile

Fig. 8. Peak height of embossed microstructure increase with processing pressures in the gas-assisted micro-hot-embossing process. 0 10 20 30 40 50 Processing pressure (kg/cm2₎ 80 120 160 200

Radius of curvature of microlens (

μ m) 0 10 20 30 40 50 Processing pressure (kg/cm2₎ 120 160 200 240 280 320

Focal length of microlens (

μ

m)

(a)

(b)

Fig. 9. Radius of curvature and focal length of microlens under various processing pressure in the process. The eﬀect of process-ing pressure on (a) the radius of curvature of a microlens and (b) the focal length of a microlens.

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(3) Cooling and packing: After the processing time period (t1–t2), the polymer is cooled down to below the glass transition tempera-ture, while maintaining the pressure (P1) to

prevent uncontrolled shrinkage and

distortion.

(4) Demolding: At the de-molding temperature, the gas is vented, the chamber is opened, and the ﬁlm with microlens array is removed.

4. Processing conditions for microlens forming To study the eﬀects of processing conditions on the surface proﬁle of microlens, three processing parameters, i.e., the processing temperature, pres-sure, and time, were chosen. The values used in the experiments are listed inTable 1.

By changing one parameter, with other parame-ters fixed at reference states (underlined in Table 1), the effect of each parameter on the surface profile of microlens using gas-assisted micro-hot-emboss-ing can be determined.

4.1. Eﬀects of temperature on the surface proﬁle of microlens

Fig. 4shows the eﬀect of processing temperature on the surface proﬁle of microlens. With the pro-cessing pressure of 30 kgf/cm2 and time of 120 s,

when processing temperature increases from

140C to 180 C, the peak height of the embossed microstructure is increases dramatically. The shape of the embossed microstructure changes from sim-ple hemisphere to cylindrical hemisphere, and ﬁnally cylinder. When processing temperature is below or at 140C, the plastic is too rigid and no lens shape is formed. On the other hand, when the temperature is above 150C, the plastic is intruded into the hole and forms cylinder with or

0 30 60 90 120 150 Processing time (s) 0 20 40 60

Sag height of microlens (

μ

m)

T=150°C P=30kg/cm2

Lens profile

Fig. 10. Sag height of microlens increase with processing duration in the gas-assisted micro-hot-embossing process.

0 30 60 90 120 150 Processing time (s) 80 120 160 200 240

Radius of curvature of microlens (

μ m) 0 30 60 90 120 150 Processing time (s) 160 200 240 280 320 360

Focal length of microlens (

μ

m)

(a)

(b)

Fig. 11. Radius of curvature and focal length of microlens under various processing time in the process. The eﬀect of processing time on (a) the radius of curvature of a microlens and (b) the focal length of a microlens.

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even without hemispherical proﬁle on the top sur-face. Typical cylinders with hemispherical proﬁle and its cross-sectional view are shown inFig. 5.

Perfect hemispherical microlens on PC films can be formed at the processing temperature of 150C, with pressure of 30 kgf/cm2 and duration of 120 s. Fig. 6 shows a SEM image of the hemi-spherical microlens array and surface profile of a single microlens. A microlens in the lens array is of diameter 150 lm, a pitch of 200 lm and a sag height of 25.15 lm.Fig. 7shows its measured pro-file as compared to the theoretical spherical curve (solid line).

The radius of curvature (R) and focal length (f) of the microlens can be determined using equations based on the basic geometric and optical theory[9]

as follows: R¼D 2_{þ 4h}2 8h ; f ¼ R n 1

where D, h and n are diameter, sag height of micro-lens and the refractive index of the PC material, respectively. The calculated radius of curvature and focal length of the present plastic microlens are 124.4 lm and 210.8 lm, respectively.

4.2. Eﬀects of pressure on the surface proﬁle of microlens

Fig. 8 shows the eﬀects on the surface proﬁle of microlens when the processing pressure is changed from 10 kgf/cm2 to 50 kgf/cm2, with the tempera-ture and duration maintained at 150C and 120 s. When the gas pressure increases from 10 kgf/cm2 to 40 kgf/cm2, the sag height of microlens increases, and the radius of curvature and focal length decreases with the processing pressure as shown in

Fig. 9. However, if the processing pressure is too high (beyond 50 kgf/cm2), cylinders with hemispher-ical proﬁle on the top are resulted.

4.3. Eﬀects of duration on the surface proﬁle of microlens

Fig. 10shows the eﬀects of the pressing duration on the surface proﬁle of microlens, with the pressure remained at 30 kgf/cm2and temperature at 150C. The sag height of the microlens increases with press-ing duration, and the radius of curvature and focal length of the microlens decreased with the duration as shown inFig. 11. Further increase of processing

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duration beyond 90 s results in very small increase in the sag height of microlens.

Based on the above study, the processing temper-ature and pressure are the two most critical process-ing parameters in gas-assisted micro-hot-embossprocess-ing

process. At the temperature of 150C (for PC mate-rial), when the gas pressure is between 10 and 40 kgf/cm2, microlens of increased sag heights can be formed. The increase in pressing duration also results in increase in sag height.

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5. The surface quality and optical property of plastic microlens

Fig. 12shows a plastic ﬁlm with 300· 300 micro-lens array fabricated using gas-assisted micro-hot-embossing process, under the condition of 150C, 30 kgf/cm2 and 120 s. It is observed that array of microlens were successfully fabricated over the whole plastic ﬁlm.

In order to characterize the surface morphology of the plastic microlens array, the surface roughness

was measured by atomic force microscope

(DIMENTION 3100, Digital Instrument, USA).

Fig. 13shows the AFM image and roughness anal-ysis of a randomly picked microlens. The averaged surface roughness (Ra) of microlens is 4.014 nm over an area of 5 lm by 5 lm on the top surface of microlens.

The focused light spot was measured by a beam proﬁler, consisting of expanding lenses, ﬁlter and CCD system using a 665 nm laser light source.

Fig. 14 shows the setup and a portion of the spot pattern produced by a plastic microlens array. The

image shows the uniform and intensive focusing function of the microlens array.

6. Conclusions

In this paper, we proposed a novel fabrication method of plastic microlens array using gas-assisted micro-hot-embossing process with a silicon holes array mold. A large array of 300· 300 plastic micro-lens with diameter of 150 lm and pitch of 200 lm has been successfully produced. The experimental results show that the shape and height of embossed micro-lens depend on processing temperature, pressure and duration. The peak height of embossed micro-lens increases signiﬁcantly with the increase in processing temperature and pressure. For PC micro-lens fabrication, the optimal processing temperature is 150C, the optimal pressure is 10–40 kgf/cm2and the optimal processing duration is 30–90 s. Micro-lens of diﬀerent curvatures and focal lengths can be obtained with a proper combination of pressure and duration. The measured surface roughness of a plastic microlens formed is 4.014 nm and the focused

Fig. 14. Optical experimental setup and a light spot pattern produced by a plastic microlens array. (a) Schematic of the optical experimental setup and (b) light spot image.

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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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