Microelectronic Engineering 60 (2002) 365–379
`
`www.elsevier.com/locate/mee
`
`Microlens array produced using hot embossing process
`*
`N.S. Ong , Y.H. Koh, Y.Q. Fu
`School of Mechanical and Production Engineering,Nanyang Technological University,Nanyang Avenue,Singapore
`639798,Singapore
`Received 1 May 2001; accepted 30 August 2001
`
`Abstract
`
`In this paper, the fabrication of molds that are suitable for the production of microlens arrays using the
`replication technique is discussed. Variation of parameters in the replication process were investigated. A
`focused ion beam was used to fabricate the microlens cavities on three materials, with silicon showing the best
`result. Hot embossing was used to produce replicated polycarbonate lens array. The temperature of the mold and
`the embossing force were the two parameters varied. The microlens array produced using the embossing
`replication process demonstrates the possibility of nanometre fabrication. © 2002 Elsevier Science B.V. All
`rights reserved.
`
`Keywords: Microlens array; Focused ion beam; Hot embossing
`
`1. Introduction
`
`The increase in research into control-by-light systems has widened the market for the use of
`microlens array. Indeed, microlens array has a large field of application; for example high-speed
`photography, telecommunication industry that couple light in and out optical fiber waveguides and
`optical communication [1]. Also the use of optical control systems over their electrical counterparts
`offers a large number of advantages such as immunity to electromagnetic interference, safety in
`flammable areas, weight and cost savings, etc. Many methods of fabricating microlens were presented.
`Some examples are contactless embossing molding [2],
`the melted photoresist method [3] and
`microjet fabrication [4]. In this research, a focused ion beam is used to produce lens pattern on a mold
`material, which will then be used for embossing to produce microlens array.
`Focused ion beam (FIB) technology is well known and widely used in semiconductor manufactur-
`ing. The FIB system uses liquid metal gallium as the ion source. In the ionization process, gallium
`
`*Corresponding author. Tel.: 165-799-55-37; fax: 165-791-18-59.
`E-mail address: mnsong@ntu.edu.sg (N.S. Ong).
`
`0167-9317/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
`PII: S0167-9317( 01 )00695-5
`
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`Fig. 1. Steps involved in hot embossing process.
`
`atoms tend to lose one electron, thus becoming singly-charged positive ions. Being charged particles,
`ions can be accelerated, focused and controlled by electrostatic fields. Their relatively high mass
`(compared with that of subatomic particles) allows them to be used to induce the milling and
`deposition effects. Factors affecting FIB milling such as beam limiting aperture size, neighbouring
`space of beam spot, dwell time and milling sequence were reported [5–7].
`Fig. 1 shows the steps involved in hot embossing. A sheet of plastic foil/material is sandwiched
`between a mold (embossing tool) and an optically smooth backing plate was heated under pressure to
`a temperature (typically . 508C) above the softening temperature (T ) of the plastic. Higher
`g
`temperatures are favourable as the lower viscosity of the polymer facilitates the molding process.
`After molding, the polymer is cooled down to below the glass transition temperature. The molding
`force is maintained during cooling in order to preserve the polymer microstructures from distortion.
`Once the polymer is cooled to below T and the pressure is released, the plastic can be separated from
`g
`the mold to give a high quality copy of the planar microstructure. No material shrinkage was found
`during the hot embossing process [8]. A simple filling mechanism governing the flow of polymer in
`hot embossing was described [9]. The factors governing hot embossing were reported [10];
`temperature, embossing force and time were the three main factors. This technique works extremely
`well for shallow microstructures (relief depths less than 1 mm). Deeper structures, where aspect ratio
`was as high as seven, were also reported [11]. Such an embossing technique can be carried out in the
`laboratory using a relatively unsophisticated hot press.
`
`2. Fabrication of micro-molds
`
`The manufacturing of the microlens array involved 2 steps. Firstly, FIB milling is done on a mold
`material. Once the pattern obtained on the mold is deemed satisfactory, it is then used as a mold to
`emboss the plastic lens array.
`
`2.1. Focused ion beam milling
`
`The material used for the mold must be polished to surface finish of below 10 nm Ra. This is
`because when the ion hits the surface of the material, the depth of material being removed will be
`affected if there are large surface irregularities. The material was ground by abrasives paper of grit
`size 180, 400, 800, 1000 progressively. The surface was then polished with 3- and 1-mm sized
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`Fig. 2. Fabrication of microlens profile using FIB.
`
`diamond paste at a rotating speed of 150 rpm. The polished material was then cleaned in an acetone
`ultrasonic bath.
`Fig. 2 shows the lens profile generation. The inner circle path will be programmed with a higher ion
`dose while the ion dose in the outer circle path will be subsequently reduced. It is this varation in the
`ion dose that generates the depth of the lens profile. At a constant aperture, the depth increases as the
`ion dose increases. A higher ion dose indicates that there are a greater number of incident ions per
`unit area. Each of them removes particles from the material and thus generate a deeper feature. The
`sequence used in the fabrication of lens pattern was from periphery to the center of the profile. A
`250-mm aperture size was used for the FIB system. The dwell time was set at 5 ms. Fig. 3 shows the
`design lens profile. The diameter is 70 mm with a depth of 3.45 mm.
`Three different materials were used in the fabrication of the mold using FIB. As can be seen in
`Figs. 4 and 5, there are numerous small pitting holes on the mold surface. It is believed that one of the
`reasons for this problem is that the material is not homogenous. Another reason is because the grain
`
`Fig. 3. Microlens profile.
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`Fig. 4. SEM picture of a lens profile on pure nickel.
`
`structure for pure nickel and stainless steel materials were too big. This causes an uneven ‘tearing’
`effect due to the bombardment of the ions during the ion milling process. A lens profile was also
`milled on a silicon wafer using FIB (see Fig. 6). A surface finish of around 7 nm Ra was obtained.
`
`Fig. 5. SEM picture of a microlens profile on stainless steel.
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`Fig. 6. SEM picture of a microlens profile on silicon wafer.
`
`Silicon was therefore selected for the lens array mold fabrication. Furthermore, it was noted that
`silicon has several advantages [12]. The silicon must be thick enough to withstand the force applied
`during the embossing process. A 7 3 5 lens array was milled on a 9-mm thick silicon (see Fig. 7).
`
`2.2. Hot embossing
`
`In the embossing process, polycarbonate material was sandwiched between a flat nickel plate and
`the mold. Heat was applied to above the T temperature (1488C) of the polycarbonate material.
`g
`Embossing force was applied and held for 20 min. The mold was subsequently cooled to 268C with
`the force maintained to preserve the microstructures of the lens. Demolding was then performed.
`Demolding is the separation of the mold from the embossed polymer structure by a vertical movement
`of the mold.
`
`3. Results and discussion
`
`Fig. 8 shows a lens profile milled on the 9-mm thick silicon material. It can be seen that the surface
`profile was very well defined and the surface roughness was measured to be around 4 nm Ra.
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`3.1. Embossing with varying temperatures (temperature test)
`
`Fig. 7. Microlens array for mold 4.
`
`The silicon mold was used in this experiment. The temperature of the plastic (in this research, the
`plastic used was polycarbonate) was varied while the embossing force was kept constant at 2.22 kN. It
`was found that the surface finish of the mold deteriorates after each embossing. The following could
`be the reasons. Firstly, the polycarbonate (PC) was demolded at room temperature. There was friction
`
`Fig. 8. 2-D profile of mold 2 measured using WYKO NT 2000.
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`generated between the mold surface and the cooled polycarbonate material during demolding.
`Secondly, frictional force introduced during the flow of plastic into the lens cavity may have caused
`the Ra of the mold to deteriorate. However, the dimensions of the mold cavity were found to be
`unchanged. Therefore, new molds were milled and used in each embossing process so that a fair test
`is employed when comparing the surface finish of the mold cavity and the PC lens.
`In order to reduce thermally induced stresses in the material as well as replication errors due to the
`different coefficient of thermal expansion between the embossing tool and polymer, the change of
`temperature during the cooling should be as small as possible [13]. Therefore, a test was done to
`ascertain whether the demolding temperature could help reduce the surface roughness of the embossed
`lens and the mold cavity surface. A 4 3 4 lens pattern array (mold 3), of diameter 65.2 mm and depth
`4.6 mm, was fabricated using the FIB system. The demolding temperature was raised to 608C. The
`embossing temperature and force were 1988C and 2.22 kN, respectively. The replicated lens diameter
`was 63.5 mm with a depth of 4.9 mm.
`Fig. 9b shows the 3D profile of replicated lens molded by mold 3 with a demolding temperature of
`608C. It can be seen that a part of the replicated lens was damaged. It was discovered that at a higher
`demolding temperature, the PC lens tends to be sticky and was stuck inside the silicon mold. This
`behaviour makes demolding very difficult as it damages the replicated lens. The height of the
`replicated lens profile was large in comparison to the mold cavity. During demolding, the PC lens was
`stuck inside the cavity and became distorted when taken out. The PC lens, therefore, experience
`plastic deformation during the demolding process.
`Fig. 10 shows the comparison of the profiles of mold 3. The surface of the mold also deteriorates at
`a higher demolding temperature. But more importantly, plastic deformation and the damage
`introduced to the plastic lens at a higher demolding temperature renders the use of higher demolding
`temperature unfavourable in this work.
`Fig. 11(b) shows the lens produced by mold 3 using the same temperature and embossing force of
`1988C and 2.22 kN, respectively. The only difference was that the lens was demolded at room
`temperature. The profile was not damaged or distorted. The height measured was 4.6 mm. The
`mechanical behaviour of plastics is very dependent on the service temperature. At low temperatures,
`plastic behaves as a hard solid with a high modulus and a low extensibility. However, as the
`
`Fig. 9. Profile of replicated lens of mold 3 (demold at 608C).
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`Fig. 10. Profile of mold 3; (a) after molding, (b) before molding.
`
`temperature increases, the modulus decreases and extensibility increases slightly. Therefore, it can be
`said that distortion and extension of lens profile occurs more extensively at a higher demolding
`temperature.
`Fig. 12 shows SEM pictures of the replicated lens array molded using mold 4 where the embossing
`temperature and force were 1988C and 2.22 kN, respectively. Table 1 gives a summary of data for the
`various molds that were used in this test. The dimensions of the molds vary slightly as it is not
`possible to obtain the same dimensions using FIB. However, the purpose of this research work is to
`obtain the optimal temperature setting for hot embossing of PC material on a silicon mold.
`Four points on the replicated plastic lens were measured (see Fig. 13). The average Ra values were
`plotted (see Fig. 14). The results obtained from the second molding of the same mold were not used in
`this analysis as the mold surface deteriorated as mentioned earlier. From Fig. 14, it can be seen that
`the Ra value of the replicated plastic lens was higher than the mold. The surface roughness of the
`mold cavities before molding showed good consistency. Ra values of below 8 nm are obtainable.
`Therefore, the surface roughness (Ra) of the mold cavities before molding was considered to be
`constant. The Ra values of the replicated lens above 1768C shows an increasing trend that was similar
`to the Ra values of the mold cavities. But the result from the embossing done at 1688C shows a
`contradiction. It was thought that due to the low temperature, the viscosity of the polycarbonate
`
`Fig. 11. Profile of replicated lens of mold 3; (demold at room temperature, 268C).
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`Fig. 12. SEM picture of replicated 735 microlens pattern.
`
`material increases. This viscosity limits the smooth flow of the polycarbonate into the mold cavity as
`it seems unable to provide enough energy/work to form the curvature on the material surface [14].
`This could be the reason for the poor surface finish obtained at low embossing temperature. As
`reported [8], a higher temperature is favourable because of the lower viscosity that facilitates the
`molding process. However, an embossing temperature that is too high is also undesirable, as a large
`change in temperature during the cooling phase, will lead to poor surface finish of the replicated lens.
`In addition, a high temperature tends to ruin the surface profile of the microlens [13]. Therefore, to
`obtain a good surface finish, a balance between plastic viscosity and the change of temperature during
`the cooling phase had to be obtained. Judging from Fig. 14, the Ra value is lowest at around 1828C.
`
`Table 1
`Summary of mold data
`Mold no.
`2
`Ion dose (nC/ mm )
`per mill depth (mm)
`Extraction current
`(mA)
`Diameter
`Height
`
`1
`10
`
`5.5
`
`70.5
`6.3
`
`2
`5.3
`
`4
`
`67.8
`5.7
`
`4
`3.5
`
`3
`
`65.4
`5.5
`
`3
`3
`
`3.5
`
`65.2
`4.6
`
`5
`3
`
`2.2
`
`64.6
`3.4
`
`Number of
`embossing
`Embossing
`temperature (8C)
`Demolding
`temperature 8C
`
`1st trial
`
`2nd trial
`
`1st trial
`
`2nd trial
`
`1st trial
`
`1st trial
`
`2nd trial
`
`1st tri
`
`187
`
`26
`
`176
`
`26
`
`187
`
`26
`
`176
`
`26
`
`198
`
`26
`
`198
`
`60
`
`198
`
`26
`
`176
`
`26
`
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`Fig. 13. Four points were measured to obtain the average surface roughness of the replicated lens.
`
`3.2. Embossing with varying force (force test)
`
`Mold 5 (refer to Table 1 for mold dimensions) was used for this experiment with varying force. The
`temperature of the mold was set constant at 1828C and demolded at 268C. Fig. 15 shows the variation
`of the profiles of the lens with increasing force. In this work, zero applied force means the moving
`
`Fig. 14. Comparisons of the surface finish of the mold (before and after molding) and the surface finish of replication plastic
`lens.
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`Fig. 15. Embossing force applied, (a) Zero force (b) 4.44 kN (c) 6.66 kN (d) 11.1 kN.
`
`half of the hot press was closed, with the force sensor indicating a zero value. It was noted that with
`increasing force, the profile of the lens was more defined. Embossing force above 6.66 kN yields good
`results. However, too high a force may cause damage to the silicon mold. Crack was observed in the
`silicon mold when embossing was done at 11.1 kN. Fig. 16 shows the replicated lens array embossed
`using a force of 11.1 kN and a temperature of 1828C.
`Similar to the previous experiments, the surface roughness were measured at four locations on the
`replicated plastic lens. The difference in diameter and height were calculated as shown by:
`Mold dimension 2 replicated plastic lens dimension
`]]]]]]]]]]]]]]]]
`mold dimension
`Fig. 17 shows the difference in diameter and height versus embossing force. It can be seen that the
`difference in diameter decreases as the embossing force increases. The lens profile embossed with
`zero applied force was believed to be under-filled as it was significantly smaller than the original
`mold. The difference in diameter and height were 3.4 and 0.43%, respectively. It was thought that
`small fillet of the replicated plastic lens was formed at the edge of the mold (see Fig. 18) when a small
`embossing force was used. Thus, a smaller lens diameter was expected when a lower embossing force
`was used. However, when a higher embossing force was used, the polycarbonate was forced to form
`the shape of the mold cavity. Thus, the difference in diameter reduces.
`An interesting phenomenon was observed in that the height of the replicated lens was larger than
`the depth of the mold. Stretching of the total structure occurs during demolding due to two main
`reasons [9]: adhesion at the mold surface and friction due to surface roughness. It was therefore
`concluded that the polycarbonate undergoes plastic deformation during demolding. Stretching of the
`polymer should generally be avoided by ensuring good surface quality of the mold. This could be
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`Fig. 16. (a) Microscopic picture of replicated 735 lens array embossed using a force of 11.1 kN and temperature of 1828C;
`(b) zoom-in SEM picture of one of the replicated lens profile.
`
`done by adding demolding agent to the polymer or use of anti-adhesive layer on the mold cavity
`surface.
`Fig. 19 shows that there is an increase in mold surface roughness as the number of molding
`increases for the same mold. The Ra value of the mold cavity increases sharply for the first few
`moldings. After the fifth molding, the Ra values tend to stabilise at around 55 nm.
`Fig. 20 shows the average surface roughness of the replicated lens and as a comparison, the average
`surface roughness of the mold before molding was also plotted. It can be seen that at zero applied
`force, the surface finish is good (16.2 nm Ra) as some part of the plastic may not come in contact with
`the mold cavity surfaces. Thus, is the inherent advantage of contactless embossing molding, as the
`replicated plastic does not depend on the quality of the mold. But due to the low embossing force
`used, the profile and dimension of the lens are unpredictable and furthermore, the profile was not well
`
`Fig. 17. Molding force versus difference in replicated plastic lens dimensions.
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`Fig. 18. Small fillet of the replicated plastic lens formed with low embossing force.
`
`defined. The surface finish of the lens deteriorates when the embossing force was at 2.22 kN. At a low
`embossing force (2.22 kN), the polycarbonate material fills the cavity but did not stretch in the mold.
`As the embossing force increases, the polycarbonate was squeezed and stretched inside the mold.
`Therefore, this could be the reason for both the reduction of Ra values and better profiles (refer to Fig.
`15) for the replicated lens.
`Despite the poor surface finish of the mold as it deteriorates with the number of moldings, the
`replicated lens surface roughness remain constant at higher embossing force (greater than 4.4 kN).
`
`Fig. 19. Average mold roughness versus number of molding.
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`Fig. 20. Average surface roughness of replicated lens vs. embossing force. Average surface roughness of mold before
`molding included for comparative purpose.
`
`Therefore, using an embossing force of between 6.66 and 11.12 kN is favourable in this work. The
`best surface roughness of around 11 nm Ra was obtained.
`
`4. Conclusion
`
`The study was undertaken with the aim of using focused ion beam to mill microlens array on a
`mold material, which can be later used for replication using a hot embossing process. Conventional
`methods, such as photoresist-based techniques, cannot be used to produce patterns on the substrate
`material directly. Hot embossing was used primarily due to its low cost, short setup time and
`replication with close proximity.
`Materials such as pure nickel and stainless steel were milled using FIB, with very poor results.
`Silicon, which has several advantages [11], proved to be a suitable material for FIB. Embossing
`parameters investigated were temperature and embossing force. It was found that using high
`demolding temperature is not favourable, due to the increase in extensibility of plastic, which leads to
`distortion and extension of the lens profile. The optimal temperature for embossing polycarbonate into
`FIB machined silicon mold was found to be 1828C. It was thought that for a good replicated surface
`finish, a balance between the plastic viscosity and the change of temperature during the cooling phase
`had to be obtained.
`The diameter of the lens was found to be replicated with close proximity (0–3%) and the difference
`reduces when higher forces were used. It was thought that the formation of small fillet reduces the
`replicated diameter. Heights of the replicated lens were found to be larger than the mold. The
`adhesion at the surface and friction due to surface roughness of the mold could be a reason for this
`phenomenon.
`The embossing force test showed that the profile of replicated lens and the surface finish were
`generally better with a higher embossing force, despite the deterioration of the surface finish of the
`mold. A surface finish of around 11 nm Ra was obtainable for the lens.
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`Acknowledgements
`
`The authors would like to express their thanks to Cecila Chee from Inco Alloys Pte Ltd., and Daniel
`Loh and Gordon Brinser from Wacker Siltronic Pte Ltd. for sponsoring the mold materials for this
`research work.
`
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