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Optical quality study of refractive lenses made out of oxide glass using hot embossing
Infrared Physics & Technology 73 (2015) 212–218
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Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Optical quality study of refractive lenses made out of oxide glass using hot embossing R. Kasztelanic a,b,⇑, I. Kujawa b, H. Ottevaere c, D. Pysz b, R. Stepien b, H. Thienpont c, R. Buczynski a,b a
University of Warsaw, Department of Physics, ul. Pasteura 7, 02-093 Warsaw, Poland Institute of Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warsaw, Poland c Vrije Universiteit Brussel, Department of Applied Physics and Photonics, Brussels Photonics Team, Pleinlaan 2, B-1050 Brussel, Belgium b
h i g h l i g h t s We discuss hot embossing process to low-cost, low-volume lenses replication. We discuss the various process parameters that influences the lens characteristics. We analyze the geometrical and optical characteristics of the lenses.
a r t i c l e
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Article history: Received 5 July 2015 Available online 1 October 2015 Keywords: Hot embossing Mid-infrared optical elements Micro-optics fabrication Refractive lenses Interferometry Optical quality
a b s t r a c t In this paper we discuss the fabrication of lenses made out of multi-component oxide glasses by using double-sided hot embossing to allow low-cost, low-volume replication of these lenses. We focus on the choice of the glass for both the stamps as well as for the molded component and we discuss the various process parameters of the hot embossing process that influences the lens characteristics. We also analyze the geometrical and optical characteristics of the lenses using state-of-the-art optical characterization tools. We find that lenses with radii of curvatures up to 8 mm for a diameter of 4.5 mm can be obtained under standard operation conditions (without vacuum) and that the stamp material should be further optimized in case lenses with higher radii of curvature need to be achieved. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The current dynamic development of technology results from working on new materials. Such materials allow for fabricating components characterized by better performance in particular circumstances and extending the scope of their applications. New technologies, or those adapted from other domains, allow for a faster, more efficient and cost-effective production, they allow to reduce the size and improve the quality of the fabricated components. This type of development can also be noticed in the domain of optics. On the one hand, new materials with improved optical properties are being developed, which allows for smaller losses and operating in a wider wavelength range. On the other hand, there are materials whose properties are similar to those already available on the market, but cheaper to produce and easier to handle or manufacture. Progress in the methods of fabrication leads to producing smaller and lighter optical systems. It also allows for
mass fabrication of optical elements, which reduces costs and ensures their widespread use. The key example of the latter will be, for instance, miniaturized camera lenses installed in mobile phones. The primary aim of the research presented here was to develop oxide glasses that are easy to fabricate and cheap in production. Second, we aimed to produce diffractive and refractive optical elements working in the range from visible to midIR light, with the use of the simplest possible fabrication method, which is the hot embossing (HE) process. This paper briefly presents the results of our work on the selection of the oxide glasses and the modification of the HE technology. The main focus of the paper is on presenting the quality analysis of the fabricated lenses and how their parameters and quality changes in time during the fabrication for a large amount of elements.
2. Material selection and fabrication technology ⇑ Corresponding author at: Institute of Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warsaw, Poland. http://dx.doi.org/10.1016/j.infrared.2015.09.024 1350-4495/Ó 2015 Elsevier B.V. All rights reserved.
There are a number of different technologies for the fabrication of optical components [1,2], among others: mechanical processing,
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lithography and ion technologies, injection molding and hot embossing. Mechanical processing is mainly used in the production of elements in the centimeter scale. Its main disadvantage is the time devoted to the grinding and polishing steps and high cost material processing. Lithography [3,4] and ion technologies [5] are best suited for manufacturing diffractive elements in the micro scale. They offer a very good precision of the micro-optical elements but these methods are expensive and they require specialized equipment. Injection molding [6,7] and hot embossing [2] are relatively cheap and both allow for high-volume manufacturing of elements in the micro and centimeter scale. These technologies are primarily used for the fabrication of polymer components [1,8], but HE and similar precision glass molding techniques can also be used for the fabrication of glass elements [1,9–15]. Compared to polymer HE, the HE in glass molding requires higher operating temperatures, larger pressure forces and longer production times. It is also critical to avoid glass crystallization, which may occur during heating, embossing or cooling of the element, and which renders the element optically useless. Despite these problems the HE method is used for manufacturing both refractive [16] and diffractive [17] optical elements, as well as for planar waveguide elements [10] made from various types of glasses: borosilicate glasses [18] low-meltingtemperature glasses [14], metallic glasses [9,15], chalcogenide glasses [10] and inorganic glasses [19]. In this paper we are testing if it is possible to simplify the hot embossing method for fabricating glass-based optical components, and lenses in particular. Thus, we use the HE without the protective atmosphere and at normal pressure. The process of producing a single lens in the HE proceeds according to the following scheme (Fig. 1). In the first phase an appropriate glass sample is prepared. For this purpose, a glass block is formed, first using mechanical methods such as grinding and polishing. A cylinder is formed, whose diameter is about 110% of the targeted lens diameter. Then the cylinder is cut and polished into disks. Their thickness is selected in such a way that their volume is similar to the volume of the final element. Finally, a piece of glass prepared in this way is placed on the bottom half of the stamp. During the second HE phase, both parts of the stamp and the glass disk are heated to the forming temperature. At this time, the glass softens and, thanks to its viscosity, it starts to assume a more spherical shape. In the next step, the upper stamp is lowered and pressed with a fixed pressure force onto the softened glass that is positioned onto the bottom stamp. The lens is formed. After a predetermined time the pressure is reduced, the stamps are separated and the temperature is reduced. Glass is separated from the
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stamp and is slowly cooled down to the room temperature in order not to introduce unnecessary stress. Obtained in this way, the lens requires no additional processing, such as polishing. Its quality can be examined in the third and last phase of the process. Apart from the technical details like the speed of heating and cooling, the temperature of the embossing process and the force and time of pressure, there is also an important issue of selecting the glass material for the stamp and for the lens. The molded glass should neither wet the stamp, nor adhere to the stamp surface. This parameter of glass is important because the embossed lens must be easy to remove from the stamp, but also the glass should protect the surface of the stamp against erosion. On the other hand, we know from experimental practice that for every glass the right material for the stamp must be selected. The choice of the stamp materials includes [1] stainless steel, tungsten carbide, silicon carbide, aluminum nitride [20], often coated with additional layers like NiP, TiAlN [21] and ceramics [22]. In our study aiming to obtain lenses working from visible to MidIR range [10,15], we first employed an initial selection procedure with regard to the above-mentioned criteria and optical properties of glasses. Next, we chose for further tests a relatively cheap tungsten–tellurium–niobate glass labeled TWPN/I/6 (oxide composition synthesized in-house: WO3 – 34.8%, Na2O – 1.8%, TeO2 – 60.3%, Nb2O5 – 3.1%). Its spectral transmission and viscosity characteristics are presented in Fig. 2 [11,16,23]. All experimental results presented later will relate to this particular glass. However, we obtained similar results for other composite metal-oxideglasses, which is why we believe that the conclusions of the present are of a more general nature, and will apply irrespective of the selection of specific materials. The desired shape of the stamp, i.e. the negative of each lens surface, was selected from commercial available concave lenses. The tests on glass adhesion of TWPN/I/6 glass to different metal, non-metal and ceramic materials allowed us to choose fused quartz glass as the best material for the stamp [17]. We tested other stamp materials [17], but the multicomponent oxide glasses adhered to the stamp surface and it was not possible to separate the lenses from a stamp. Sometimes, the surface of the stamp eroded, which did not allow for fabricating a larger number of lenses. 3. Lens fabrication Having chosen the glass for the lens and the material for the stamp we optimized the hot embossing process. The most important factor was the temperature of forming the glass element. When the temperature is too low, the glass showed not ductile
Fig. 1. The scheme for lens fabrication using the hot embossing process.
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Fig. 2. The basic data for TWPN/I/6 glass: (a) spectral transmission and (b) viscosity characteristics with characteristic temperatures (Tg – transition temperature, Tz – ovalization point, Tk – sphere point, Tpk – hemisphere point, Tr – spreading point).
enough, which required the use of larger pressure forces and, as a result, it leads to cracking the stamps. Running the process at higher temperatures resulted in difficulties to separate the lens from the stamp [24]. Finally, we found that the optimum molding temperature is at 468 °C. At this temperature the dynamic viscosity of glass is near 104 Poise (Fig. 2b), which means that the glass during the molding process behaves as a viscous liquid. However, this value is not exceeded, because the given temperature of the molding process refers to the temperature close to the stamp, while the temperature of the glass is a couple of degrees lower. The further parameters affecting the quality of the obtained lens were the glass’s heating rate and the force and duration of the pressure. As the optimal process parameters we specified the heating rate at 5 °C/min, the force at 1.22 N, and the duration of the molding at 80 min [11], further labeled as Process 1. Secondly, we attempted to reduce the formation time to 5 min by applying pressure equal to 200 N, further labeled as Process 2. The parameters of the two hot embossing processes (Process 1 and 2) are shown in Table 1 [11,16]. Next, we examined the quality of the lenses made with the hot embossing Process 1 and Process 2 and we investigated the durability of the stamps for the two processes, when fabricating a large Table 1 The parameters of the two hot embossing processes. Parameter
Process 1
Molding temperature (°C) Demolding temperature (°C) Heating rate (°C/min) Cooling rate (°C/min) Molding time (min) Molding force (N) Total process time (min)
468 468 5 2 (to 450), 15 (to room temp.) 80 5 1.22 200 210 135
Process 2
number of consecutive optical elements. As a test element, we chose a biconvex lens with two different radii of curvature R1 = 3.9 mm and R2 = 8.2 mm, a diameter / of 4.5 mm and a lens height h of 2.15 mm. For Process 1, 30 lenses were made using the optimal process parameters shown in Table 1. After the first visual inspection, we did not note a significant change in the quality of the lenses, nor in the appearance of the stamps. An example of a lens fabricated with Process 1 parameters is shown in Fig. 3. In contrast, in case of the attempt to shorten the time of the embossing (Process 2) we observed a gradual deterioration in the quality of the lens and one half of the stamp for a larger number of embossing steps. Moreover, applying higher pressures led to the gradual degradation of the stamp, which broke after applying 15 embossing steps.
4. Measuring the lens parameters and the optical quality of the fabricated elements First, the optical properties of the embossed lenses were measured using a Mach–Zehnder interferometer [25] working at a wavelength k = 632.8 nm. In particular, the following parameters were determined: the focal length (Fig. 4a), the numerical aperture (NA) (Fig. 4b), the spot size (Fig. 4c) and the cut-off frequency (Fig. 4d). The presented data show that the first lenses obtained in Process 1 and Process 2 have similar characteristics, and are suitable to be used in optical setups [11,16]. However, they differ from those predicted at the design stage (Table 2). Moreover, in Process 2, after producing approximately 4 lenses, the parameters begin to change. Starting from the latter lens the focal length has increased from 3.85 to 6.47 mm for the next 10 fabricated lenses. Assuming the parameters of the glass have not changed, it means
Fig. 3. Example of a fabricated lens: (a) top view, and (b) size comparison.
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Fig. 4. Optical properties of the embossed lenses (measured with a Mach–Zehnder interferometer at k = 632.8 nm): (a) focal length (tolerance ±.01 mm), (b) numerical aperture (±.5), (c) spot size (±0.1 lm), and (d) cut-off frequency (±1 lp/mm).
Table 2 The theoretical designed values and the mean value of the measured parameters of the lenses fabricated with both processes (all values are measured at k = 632.8 nm). Parameter
f (mm) NA Spot size (lm) Cut-off frequency (lp/mm) R1 Radius of curvature (mm) FWHM (mm) R2 Radius of curvature (mm) FWHM (mm)
Theoretical designed values
Measurements Process 1
Process 2
2.57 0.10 7.45 260.00
3.66 0.08 10.35 245.58
– – – –
3.90 –
4.78 1.023
– –
8.20 –
8.41 0.076
8.41 0.125
that the radii of curvature of the lens must have decreased. This is correlated with the reduction of the NA value from 0.07 to 0.04, as well as with the increase in the spot size from 10.9 to 17.2 lm, and with the reduction of the cut-off frequency from 224 to 140 lp/mm. In the case of Process 1 we have not noticed this type of systematic changes, although the repeatability of the measured parameters is worse than for Process 2 (histograms on Fig. 5, Table 2). The systematic difference between the measured and the theoretical values, as well as the correlated shape of the various curves in Fig. 4 suggest that the errors are due to incorrect copying of the lens curvature from the mold. Further measurements were taken using a white light interferometer (Bruker Contour GT-I) to check whether the change of the radius of curvature of the lens surface is indeed responsible for the change of its optical properties. With the given curves for Pro-
cess 1 (Fig. 5a) and for Process 2 (Fig. 5c) and the histograms (Fig. 5b and d), it appears that for both processes the flatter surface (R2) is copied with higher repeatability (FWHM = 0.076 mm and FWHM = 0.125 mm, respectively) compared to the more convex surface (R1), which is copied with the more fluctuations on the curvature (FWHM = 1.023 mm). Moreover, in Process 2 the curvature of the more convex surface (R1) decreases systematically, which corresponds to the increase in the focal length of the lens. The white light interferometer measurements also allow us to evaluate the roughness parameters. The results obtained show that the quality of both surfaces does not change over time. But the distribution of the individual parameters, presented in Fig. 6, shows that especially for Process 2 the surface quality of the flatter surface (R2) is slightly worse than the one of the more convex surface (R1). Because the interferometric measurements were only taken in the central part of the lens (diameter of 0.6 mm) a multisensor coordinate measuring machine with chromatic focus probe (Werth UA 400) was used to determine the spherical surface of the full lens diameter (diameter of 3.5 mm). The results obtained allowed us to identify the cause of the almost random values of the curvature radius for the more convex surface (R1). Fig. 7a shows the typical change in the curvature of the surface when moving away from the center of the lens. Thus, while the surface curvature of R2 = 8.2 mm is maintained for the entire measured surface, the radius of curvature R1 = 3.9 mm, has a value about 6 mm near the center and decreases when moving away from the center of the lens (Fig. 7, red1 curves). In addition, the radius of curvature of the whole lens is larger than the corresponding curvature of the stamp 1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
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Fig. 5. The surface quality of the fabricated lenses measured with a white light interferometer: (a) and (b) for Process 1, (c) and (d) for Process 2 (Radius tolerance ±.01 mm).
Fig. 6. Roughness parameters of the fabricated lenses obtained with the white light interferometer: Ra – arithmetic average of absolute values, Rq – root mean squared of absolute values, Rt – maximum height of the profile, Rv – maximum valley depth and Rp – maximum peak height. The numbers in the upper right corner of each of the histograms indicate the expected value and standard deviation, respectively (all roughness values are given in lm and were taken in an area of 1.236 0.941 lm2).
(red curves on Fig. 7). This is caused by the fact that the embossing process was carried out in air at normal pressure, which led to trapping a cushion of air between the glass and the stamp (Fig. 8b). Because the amount of the trapped air was different every time, the radius of curvature changed in a random way. It is possible to
conclude that the smaller is the curvature of the lens and the less air is accumulated between the lens and the stamp, the better the copying of the stamp’s shape occurs. What is also important, when the process was conducted in optimal conditions (Process 1), the randomness of the surface R1 was also smaller than that in Process 2.
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Fig. 7. The change of the curvature radius as a function of the distance from the center of the lens: (a) typical shape, and (b) other measured shapes.
Concluding, in order to ensure a reliable reproduction of the shape and especially for high-curvature surfaces it is necessary to carry out the hot embossing process under pressure reduced to the level of 1 mbar. What is more, for several R1 surfaces we obtained cross sections shown in Fig. 7b (red curve). According to them, the lens assumes the curvature corresponding to the stamp only close the edge where glass is touching directly to the surface of the stamp (Fig. 8b). Closer to the center of the lens, the stamp has no direct contact with the glass due to an air cushion. Therefore, the roughness of the lens surface in the central area does not result from the process of hot embossing but from the manner in which the sample of glass was prepared before the embossing. This is also the reason why the roughness of the more curved surface (R1) is smaller than that of the flattened surface (R2) (Fig. 6). A detailed analysis of all the lenses made with Process 1 shows that for three out of 30 lenses the desired shape of the surface (R2) was only achieved at a certain distance from the center. This effect occurred sporadically and had a much smaller range compared to the lenses made with Process 2. We also observed lenses where the curvature of the lens for both surfaces changed in the opposite direction with the distance from the center of the lens (Fig. 7b, black curve). This effect might have been caused by a partial sticking of the molded glass to the stamp due to the vacuum formed between the molded element and the stamp. As determined in preliminary studies, the fused silica and TWPN/I/6 glass adhere to each other to a small extent at the temperature at which the hot embossing was performed.
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The curvature of the fabricated lenses can also be affected by the way the stamps were positioned during embossing, in other words which of the stamps is at the bottom and which one at the top. In the experiments conducted, the bottom stamp had a smaller curvature (R2) than the top stamp (R1). The glass sample lay on the bottom stamp during heating from room temperature to the temperature at which the hot embossing process took place. The stamp with the larger curvature (R1) was pressed into the sample only during the molding of the lens. The sample lying on the bottom stamp softens when the temperature is increased and takes the form of a flattened sphere tightly filling the cavity in the stamp (Fig. 8a). When the upper stamp is pressed, air is trapped in the space between the glass and the stamp (Fig. 8), which changes the curvature of the molded lens. Probably, placing the stamp with the larger curvature at the bottom could lead to a better filling of the melted glass in the cavity of the stamp. The flatter curve at the top part of the stamp would push out the air cushion leading to a proper formation of the lens surface. Due to technical reasons related to the hot embossing machine used, we could not reverse the position of the stamps and carry out more measurements. In any case, the results of our measurements indicate that the optical quality of the lenses obtained in Process 1 allow for usage in optical setups [11,16]. 5. Conclusions The results of the experiments presented here show that the hot embossing method is suitable for producing lenses made of oxide glasses. We found that the optical quality of the obtained lenses significantly depends on the configuration of the setup and the choice of parameters during the hot embossing process. Selecting less optimal parameters results in changes in the shape of the fabricated elements and ultimately leads to the destruction of the stamp. In addition, the presented data indicate that there is a limit on the curvature of the produced spherical surface when the process in performed in standard atmospheric pressure conditions (no vacuum applied). For a higher curvature, air may get trapped between the stamp and the molded lens, causing accidental changes of the curvature radius. However, increasing the curvature radius to approximately 8 mm eliminates this problem. Another important aspect is the long molding time of 80 min for the embossing process in optimal conditions. An attempt to shorten this molding time to 5 min led to a destruction of the stamp. It seems that the main problem in this case is the material for fabricating the stamp. Fused silica was chosen as the stamp material mainly due to its minimal adhesion to TWPN/I/6 glass. However, the processes of heating, pressing and rapid cooling resulted in a degradation of the stamp. Hence, a way to increase
Fig. 8. Schema of the hot embossing process leading to the creation of the air cushion: (a) before molding step, and (b) during molding step.
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the performance is to find a material which will not undergo thermal degradation, which will not be wetted by the glass from which the lenses are made of and which will be highly resistant to the oxidized atmosphere coming from the oxide glasses. Unfortunately, classic materials studied so far such as ceramics, or metals coated with anticorrosion and antiadhesive layers turned out unsuitable. We can conclude that fabricating lenses from metaloxide-glass using a hot embossing process is possible for producing short series of lenses of limited curvature ranges. For smaller radii of curvature an optimization of the stamp material is needed. Acknowledgements This research was carried out with the financial support from MPNS COST Action MP1205 Advances in Optofluidics: Integration of Optical Control and Photonics with Microfluidics, the European Access Centre for Photonics Innovation Solutions and Technology Support (ACTPHAST), and program PhoQuS – Fostering Excellence in Photonics and Quantum Science on University of Warsaw, Poland. This work was supported in part by FWO (G008413N, Belgium), IWT (Belgium), BELSPO IAP Photonics@be, the Methusalem and Hercules foundations, Flanders Make, and the OZR of the Vrije Universiteit Brussel, Belgium. References [1] M. Worgull, Hot Embossing, Theory and Technology of Microreplication (Micro and Nano Technologies), William Andrew, 2009. [2] H.P. Herzig, Micro-Optics: Elements, Systems and Applications, CRC Press, 1997. ISBN 9780748404810. [3] D.C. O’Shea, Diffractive Optics: Design, Fabrication, and Test, SPIE Press, 2004. ISBN 9780819451712. [4] E. Di Fabrizio, L. Grella, M. Baciocchi, M. Gentili, Fabrication of Diffractive Optical Elements by Electron Beam Lithography, Diffractive Opt. Opt. Microsyst. 149–160 (1997). [5] A. Vijayakumar, U. Eigenthaler, K. Keskinbora, G.M. Sridharan, V. Pramitha, M. Hirscher, J.P. Spatz, S. Bhattacharya, Optimizing the fabrication of diffractive optical elements using a focused ion beam system, Proc. SPIE 9130 (2014), http://dx.doi.org/10.1117/12.2051925. [6] T. Hoang, Injection Molding Technology of Optical Elements in LED Illumination, LAP LAMBERT Academic Publishing, 2013. [7] E.J. Carvalho, E.S. Braga, L.H.D. Cescato, Replication of diffractive optical elements by injection molding, Proc. SPIE 5622 (2014), http://dx.doi.org/ 10.1117/12.592206. [8] M. Heckele, W.K. Schomburg, Review on micro molding of thermoplastic polymers, J. Micromech. Microeng. 14 (2004) R1–R14.
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Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Optical quality study of refractive lenses made out of oxide glass using hot embossing R. Kasztelanic a,b,⇑, I. Kujawa b, H. Ottevaere c, D. Pysz b, R. Stepien b, H. Thienpont c, R. Buczynski a,b a
University of Warsaw, Department of Physics, ul. Pasteura 7, 02-093 Warsaw, Poland Institute of Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warsaw, Poland c Vrije Universiteit Brussel, Department of Applied Physics and Photonics, Brussels Photonics Team, Pleinlaan 2, B-1050 Brussel, Belgium b
h i g h l i g h t s We discuss hot embossing process to low-cost, low-volume lenses replication. We discuss the various process parameters that influences the lens characteristics. We analyze the geometrical and optical characteristics of the lenses.
a r t i c l e
i n f o
Article history: Received 5 July 2015 Available online 1 October 2015 Keywords: Hot embossing Mid-infrared optical elements Micro-optics fabrication Refractive lenses Interferometry Optical quality
a b s t r a c t In this paper we discuss the fabrication of lenses made out of multi-component oxide glasses by using double-sided hot embossing to allow low-cost, low-volume replication of these lenses. We focus on the choice of the glass for both the stamps as well as for the molded component and we discuss the various process parameters of the hot embossing process that influences the lens characteristics. We also analyze the geometrical and optical characteristics of the lenses using state-of-the-art optical characterization tools. We find that lenses with radii of curvatures up to 8 mm for a diameter of 4.5 mm can be obtained under standard operation conditions (without vacuum) and that the stamp material should be further optimized in case lenses with higher radii of curvature need to be achieved. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The current dynamic development of technology results from working on new materials. Such materials allow for fabricating components characterized by better performance in particular circumstances and extending the scope of their applications. New technologies, or those adapted from other domains, allow for a faster, more efficient and cost-effective production, they allow to reduce the size and improve the quality of the fabricated components. This type of development can also be noticed in the domain of optics. On the one hand, new materials with improved optical properties are being developed, which allows for smaller losses and operating in a wider wavelength range. On the other hand, there are materials whose properties are similar to those already available on the market, but cheaper to produce and easier to handle or manufacture. Progress in the methods of fabrication leads to producing smaller and lighter optical systems. It also allows for
mass fabrication of optical elements, which reduces costs and ensures their widespread use. The key example of the latter will be, for instance, miniaturized camera lenses installed in mobile phones. The primary aim of the research presented here was to develop oxide glasses that are easy to fabricate and cheap in production. Second, we aimed to produce diffractive and refractive optical elements working in the range from visible to midIR light, with the use of the simplest possible fabrication method, which is the hot embossing (HE) process. This paper briefly presents the results of our work on the selection of the oxide glasses and the modification of the HE technology. The main focus of the paper is on presenting the quality analysis of the fabricated lenses and how their parameters and quality changes in time during the fabrication for a large amount of elements.
2. Material selection and fabrication technology ⇑ Corresponding author at: Institute of Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warsaw, Poland. http://dx.doi.org/10.1016/j.infrared.2015.09.024 1350-4495/Ó 2015 Elsevier B.V. All rights reserved.
There are a number of different technologies for the fabrication of optical components [1,2], among others: mechanical processing,
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lithography and ion technologies, injection molding and hot embossing. Mechanical processing is mainly used in the production of elements in the centimeter scale. Its main disadvantage is the time devoted to the grinding and polishing steps and high cost material processing. Lithography [3,4] and ion technologies [5] are best suited for manufacturing diffractive elements in the micro scale. They offer a very good precision of the micro-optical elements but these methods are expensive and they require specialized equipment. Injection molding [6,7] and hot embossing [2] are relatively cheap and both allow for high-volume manufacturing of elements in the micro and centimeter scale. These technologies are primarily used for the fabrication of polymer components [1,8], but HE and similar precision glass molding techniques can also be used for the fabrication of glass elements [1,9–15]. Compared to polymer HE, the HE in glass molding requires higher operating temperatures, larger pressure forces and longer production times. It is also critical to avoid glass crystallization, which may occur during heating, embossing or cooling of the element, and which renders the element optically useless. Despite these problems the HE method is used for manufacturing both refractive [16] and diffractive [17] optical elements, as well as for planar waveguide elements [10] made from various types of glasses: borosilicate glasses [18] low-meltingtemperature glasses [14], metallic glasses [9,15], chalcogenide glasses [10] and inorganic glasses [19]. In this paper we are testing if it is possible to simplify the hot embossing method for fabricating glass-based optical components, and lenses in particular. Thus, we use the HE without the protective atmosphere and at normal pressure. The process of producing a single lens in the HE proceeds according to the following scheme (Fig. 1). In the first phase an appropriate glass sample is prepared. For this purpose, a glass block is formed, first using mechanical methods such as grinding and polishing. A cylinder is formed, whose diameter is about 110% of the targeted lens diameter. Then the cylinder is cut and polished into disks. Their thickness is selected in such a way that their volume is similar to the volume of the final element. Finally, a piece of glass prepared in this way is placed on the bottom half of the stamp. During the second HE phase, both parts of the stamp and the glass disk are heated to the forming temperature. At this time, the glass softens and, thanks to its viscosity, it starts to assume a more spherical shape. In the next step, the upper stamp is lowered and pressed with a fixed pressure force onto the softened glass that is positioned onto the bottom stamp. The lens is formed. After a predetermined time the pressure is reduced, the stamps are separated and the temperature is reduced. Glass is separated from the
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stamp and is slowly cooled down to the room temperature in order not to introduce unnecessary stress. Obtained in this way, the lens requires no additional processing, such as polishing. Its quality can be examined in the third and last phase of the process. Apart from the technical details like the speed of heating and cooling, the temperature of the embossing process and the force and time of pressure, there is also an important issue of selecting the glass material for the stamp and for the lens. The molded glass should neither wet the stamp, nor adhere to the stamp surface. This parameter of glass is important because the embossed lens must be easy to remove from the stamp, but also the glass should protect the surface of the stamp against erosion. On the other hand, we know from experimental practice that for every glass the right material for the stamp must be selected. The choice of the stamp materials includes [1] stainless steel, tungsten carbide, silicon carbide, aluminum nitride [20], often coated with additional layers like NiP, TiAlN [21] and ceramics [22]. In our study aiming to obtain lenses working from visible to MidIR range [10,15], we first employed an initial selection procedure with regard to the above-mentioned criteria and optical properties of glasses. Next, we chose for further tests a relatively cheap tungsten–tellurium–niobate glass labeled TWPN/I/6 (oxide composition synthesized in-house: WO3 – 34.8%, Na2O – 1.8%, TeO2 – 60.3%, Nb2O5 – 3.1%). Its spectral transmission and viscosity characteristics are presented in Fig. 2 [11,16,23]. All experimental results presented later will relate to this particular glass. However, we obtained similar results for other composite metal-oxideglasses, which is why we believe that the conclusions of the present are of a more general nature, and will apply irrespective of the selection of specific materials. The desired shape of the stamp, i.e. the negative of each lens surface, was selected from commercial available concave lenses. The tests on glass adhesion of TWPN/I/6 glass to different metal, non-metal and ceramic materials allowed us to choose fused quartz glass as the best material for the stamp [17]. We tested other stamp materials [17], but the multicomponent oxide glasses adhered to the stamp surface and it was not possible to separate the lenses from a stamp. Sometimes, the surface of the stamp eroded, which did not allow for fabricating a larger number of lenses. 3. Lens fabrication Having chosen the glass for the lens and the material for the stamp we optimized the hot embossing process. The most important factor was the temperature of forming the glass element. When the temperature is too low, the glass showed not ductile
Fig. 1. The scheme for lens fabrication using the hot embossing process.
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Fig. 2. The basic data for TWPN/I/6 glass: (a) spectral transmission and (b) viscosity characteristics with characteristic temperatures (Tg – transition temperature, Tz – ovalization point, Tk – sphere point, Tpk – hemisphere point, Tr – spreading point).
enough, which required the use of larger pressure forces and, as a result, it leads to cracking the stamps. Running the process at higher temperatures resulted in difficulties to separate the lens from the stamp [24]. Finally, we found that the optimum molding temperature is at 468 °C. At this temperature the dynamic viscosity of glass is near 104 Poise (Fig. 2b), which means that the glass during the molding process behaves as a viscous liquid. However, this value is not exceeded, because the given temperature of the molding process refers to the temperature close to the stamp, while the temperature of the glass is a couple of degrees lower. The further parameters affecting the quality of the obtained lens were the glass’s heating rate and the force and duration of the pressure. As the optimal process parameters we specified the heating rate at 5 °C/min, the force at 1.22 N, and the duration of the molding at 80 min [11], further labeled as Process 1. Secondly, we attempted to reduce the formation time to 5 min by applying pressure equal to 200 N, further labeled as Process 2. The parameters of the two hot embossing processes (Process 1 and 2) are shown in Table 1 [11,16]. Next, we examined the quality of the lenses made with the hot embossing Process 1 and Process 2 and we investigated the durability of the stamps for the two processes, when fabricating a large Table 1 The parameters of the two hot embossing processes. Parameter
Process 1
Molding temperature (°C) Demolding temperature (°C) Heating rate (°C/min) Cooling rate (°C/min) Molding time (min) Molding force (N) Total process time (min)
468 468 5 2 (to 450), 15 (to room temp.) 80 5 1.22 200 210 135
Process 2
number of consecutive optical elements. As a test element, we chose a biconvex lens with two different radii of curvature R1 = 3.9 mm and R2 = 8.2 mm, a diameter / of 4.5 mm and a lens height h of 2.15 mm. For Process 1, 30 lenses were made using the optimal process parameters shown in Table 1. After the first visual inspection, we did not note a significant change in the quality of the lenses, nor in the appearance of the stamps. An example of a lens fabricated with Process 1 parameters is shown in Fig. 3. In contrast, in case of the attempt to shorten the time of the embossing (Process 2) we observed a gradual deterioration in the quality of the lens and one half of the stamp for a larger number of embossing steps. Moreover, applying higher pressures led to the gradual degradation of the stamp, which broke after applying 15 embossing steps.
4. Measuring the lens parameters and the optical quality of the fabricated elements First, the optical properties of the embossed lenses were measured using a Mach–Zehnder interferometer [25] working at a wavelength k = 632.8 nm. In particular, the following parameters were determined: the focal length (Fig. 4a), the numerical aperture (NA) (Fig. 4b), the spot size (Fig. 4c) and the cut-off frequency (Fig. 4d). The presented data show that the first lenses obtained in Process 1 and Process 2 have similar characteristics, and are suitable to be used in optical setups [11,16]. However, they differ from those predicted at the design stage (Table 2). Moreover, in Process 2, after producing approximately 4 lenses, the parameters begin to change. Starting from the latter lens the focal length has increased from 3.85 to 6.47 mm for the next 10 fabricated lenses. Assuming the parameters of the glass have not changed, it means
Fig. 3. Example of a fabricated lens: (a) top view, and (b) size comparison.
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Fig. 4. Optical properties of the embossed lenses (measured with a Mach–Zehnder interferometer at k = 632.8 nm): (a) focal length (tolerance ±.01 mm), (b) numerical aperture (±.5), (c) spot size (±0.1 lm), and (d) cut-off frequency (±1 lp/mm).
Table 2 The theoretical designed values and the mean value of the measured parameters of the lenses fabricated with both processes (all values are measured at k = 632.8 nm). Parameter
f (mm) NA Spot size (lm) Cut-off frequency (lp/mm) R1 Radius of curvature (mm) FWHM (mm) R2 Radius of curvature (mm) FWHM (mm)
Theoretical designed values
Measurements Process 1
Process 2
2.57 0.10 7.45 260.00
3.66 0.08 10.35 245.58
– – – –
3.90 –
4.78 1.023
– –
8.20 –
8.41 0.076
8.41 0.125
that the radii of curvature of the lens must have decreased. This is correlated with the reduction of the NA value from 0.07 to 0.04, as well as with the increase in the spot size from 10.9 to 17.2 lm, and with the reduction of the cut-off frequency from 224 to 140 lp/mm. In the case of Process 1 we have not noticed this type of systematic changes, although the repeatability of the measured parameters is worse than for Process 2 (histograms on Fig. 5, Table 2). The systematic difference between the measured and the theoretical values, as well as the correlated shape of the various curves in Fig. 4 suggest that the errors are due to incorrect copying of the lens curvature from the mold. Further measurements were taken using a white light interferometer (Bruker Contour GT-I) to check whether the change of the radius of curvature of the lens surface is indeed responsible for the change of its optical properties. With the given curves for Pro-
cess 1 (Fig. 5a) and for Process 2 (Fig. 5c) and the histograms (Fig. 5b and d), it appears that for both processes the flatter surface (R2) is copied with higher repeatability (FWHM = 0.076 mm and FWHM = 0.125 mm, respectively) compared to the more convex surface (R1), which is copied with the more fluctuations on the curvature (FWHM = 1.023 mm). Moreover, in Process 2 the curvature of the more convex surface (R1) decreases systematically, which corresponds to the increase in the focal length of the lens. The white light interferometer measurements also allow us to evaluate the roughness parameters. The results obtained show that the quality of both surfaces does not change over time. But the distribution of the individual parameters, presented in Fig. 6, shows that especially for Process 2 the surface quality of the flatter surface (R2) is slightly worse than the one of the more convex surface (R1). Because the interferometric measurements were only taken in the central part of the lens (diameter of 0.6 mm) a multisensor coordinate measuring machine with chromatic focus probe (Werth UA 400) was used to determine the spherical surface of the full lens diameter (diameter of 3.5 mm). The results obtained allowed us to identify the cause of the almost random values of the curvature radius for the more convex surface (R1). Fig. 7a shows the typical change in the curvature of the surface when moving away from the center of the lens. Thus, while the surface curvature of R2 = 8.2 mm is maintained for the entire measured surface, the radius of curvature R1 = 3.9 mm, has a value about 6 mm near the center and decreases when moving away from the center of the lens (Fig. 7, red1 curves). In addition, the radius of curvature of the whole lens is larger than the corresponding curvature of the stamp 1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
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Fig. 5. The surface quality of the fabricated lenses measured with a white light interferometer: (a) and (b) for Process 1, (c) and (d) for Process 2 (Radius tolerance ±.01 mm).
Fig. 6. Roughness parameters of the fabricated lenses obtained with the white light interferometer: Ra – arithmetic average of absolute values, Rq – root mean squared of absolute values, Rt – maximum height of the profile, Rv – maximum valley depth and Rp – maximum peak height. The numbers in the upper right corner of each of the histograms indicate the expected value and standard deviation, respectively (all roughness values are given in lm and were taken in an area of 1.236 0.941 lm2).
(red curves on Fig. 7). This is caused by the fact that the embossing process was carried out in air at normal pressure, which led to trapping a cushion of air between the glass and the stamp (Fig. 8b). Because the amount of the trapped air was different every time, the radius of curvature changed in a random way. It is possible to
conclude that the smaller is the curvature of the lens and the less air is accumulated between the lens and the stamp, the better the copying of the stamp’s shape occurs. What is also important, when the process was conducted in optimal conditions (Process 1), the randomness of the surface R1 was also smaller than that in Process 2.
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Fig. 7. The change of the curvature radius as a function of the distance from the center of the lens: (a) typical shape, and (b) other measured shapes.
Concluding, in order to ensure a reliable reproduction of the shape and especially for high-curvature surfaces it is necessary to carry out the hot embossing process under pressure reduced to the level of 1 mbar. What is more, for several R1 surfaces we obtained cross sections shown in Fig. 7b (red curve). According to them, the lens assumes the curvature corresponding to the stamp only close the edge where glass is touching directly to the surface of the stamp (Fig. 8b). Closer to the center of the lens, the stamp has no direct contact with the glass due to an air cushion. Therefore, the roughness of the lens surface in the central area does not result from the process of hot embossing but from the manner in which the sample of glass was prepared before the embossing. This is also the reason why the roughness of the more curved surface (R1) is smaller than that of the flattened surface (R2) (Fig. 6). A detailed analysis of all the lenses made with Process 1 shows that for three out of 30 lenses the desired shape of the surface (R2) was only achieved at a certain distance from the center. This effect occurred sporadically and had a much smaller range compared to the lenses made with Process 2. We also observed lenses where the curvature of the lens for both surfaces changed in the opposite direction with the distance from the center of the lens (Fig. 7b, black curve). This effect might have been caused by a partial sticking of the molded glass to the stamp due to the vacuum formed between the molded element and the stamp. As determined in preliminary studies, the fused silica and TWPN/I/6 glass adhere to each other to a small extent at the temperature at which the hot embossing was performed.
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The curvature of the fabricated lenses can also be affected by the way the stamps were positioned during embossing, in other words which of the stamps is at the bottom and which one at the top. In the experiments conducted, the bottom stamp had a smaller curvature (R2) than the top stamp (R1). The glass sample lay on the bottom stamp during heating from room temperature to the temperature at which the hot embossing process took place. The stamp with the larger curvature (R1) was pressed into the sample only during the molding of the lens. The sample lying on the bottom stamp softens when the temperature is increased and takes the form of a flattened sphere tightly filling the cavity in the stamp (Fig. 8a). When the upper stamp is pressed, air is trapped in the space between the glass and the stamp (Fig. 8), which changes the curvature of the molded lens. Probably, placing the stamp with the larger curvature at the bottom could lead to a better filling of the melted glass in the cavity of the stamp. The flatter curve at the top part of the stamp would push out the air cushion leading to a proper formation of the lens surface. Due to technical reasons related to the hot embossing machine used, we could not reverse the position of the stamps and carry out more measurements. In any case, the results of our measurements indicate that the optical quality of the lenses obtained in Process 1 allow for usage in optical setups [11,16]. 5. Conclusions The results of the experiments presented here show that the hot embossing method is suitable for producing lenses made of oxide glasses. We found that the optical quality of the obtained lenses significantly depends on the configuration of the setup and the choice of parameters during the hot embossing process. Selecting less optimal parameters results in changes in the shape of the fabricated elements and ultimately leads to the destruction of the stamp. In addition, the presented data indicate that there is a limit on the curvature of the produced spherical surface when the process in performed in standard atmospheric pressure conditions (no vacuum applied). For a higher curvature, air may get trapped between the stamp and the molded lens, causing accidental changes of the curvature radius. However, increasing the curvature radius to approximately 8 mm eliminates this problem. Another important aspect is the long molding time of 80 min for the embossing process in optimal conditions. An attempt to shorten this molding time to 5 min led to a destruction of the stamp. It seems that the main problem in this case is the material for fabricating the stamp. Fused silica was chosen as the stamp material mainly due to its minimal adhesion to TWPN/I/6 glass. However, the processes of heating, pressing and rapid cooling resulted in a degradation of the stamp. Hence, a way to increase
Fig. 8. Schema of the hot embossing process leading to the creation of the air cushion: (a) before molding step, and (b) during molding step.
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the performance is to find a material which will not undergo thermal degradation, which will not be wetted by the glass from which the lenses are made of and which will be highly resistant to the oxidized atmosphere coming from the oxide glasses. Unfortunately, classic materials studied so far such as ceramics, or metals coated with anticorrosion and antiadhesive layers turned out unsuitable. We can conclude that fabricating lenses from metaloxide-glass using a hot embossing process is possible for producing short series of lenses of limited curvature ranges. For smaller radii of curvature an optimization of the stamp material is needed. Acknowledgements This research was carried out with the financial support from MPNS COST Action MP1205 Advances in Optofluidics: Integration of Optical Control and Photonics with Microfluidics, the European Access Centre for Photonics Innovation Solutions and Technology Support (ACTPHAST), and program PhoQuS – Fostering Excellence in Photonics and Quantum Science on University of Warsaw, Poland. This work was supported in part by FWO (G008413N, Belgium), IWT (Belgium), BELSPO IAP Photonics@be, the Methusalem and Hercules foundations, Flanders Make, and the OZR of the Vrije Universiteit Brussel, Belgium. References [1] M. Worgull, Hot Embossing, Theory and Technology of Microreplication (Micro and Nano Technologies), William Andrew, 2009. [2] H.P. Herzig, Micro-Optics: Elements, Systems and Applications, CRC Press, 1997. ISBN 9780748404810. [3] D.C. O’Shea, Diffractive Optics: Design, Fabrication, and Test, SPIE Press, 2004. ISBN 9780819451712. [4] E. Di Fabrizio, L. Grella, M. Baciocchi, M. Gentili, Fabrication of Diffractive Optical Elements by Electron Beam Lithography, Diffractive Opt. Opt. Microsyst. 149–160 (1997). [5] A. Vijayakumar, U. Eigenthaler, K. Keskinbora, G.M. Sridharan, V. Pramitha, M. Hirscher, J.P. Spatz, S. Bhattacharya, Optimizing the fabrication of diffractive optical elements using a focused ion beam system, Proc. SPIE 9130 (2014), http://dx.doi.org/10.1117/12.2051925. [6] T. Hoang, Injection Molding Technology of Optical Elements in LED Illumination, LAP LAMBERT Academic Publishing, 2013. [7] E.J. Carvalho, E.S. Braga, L.H.D. Cescato, Replication of diffractive optical elements by injection molding, Proc. SPIE 5622 (2014), http://dx.doi.org/ 10.1117/12.592206. [8] M. Heckele, W.K. Schomburg, Review on micro molding of thermoplastic polymers, J. Micromech. Microeng. 14 (2004) R1–R14.
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