Mould insert fabrication of a single-mode fibre connector alignment structure optimized by justified partial metallization

Adhesive bonding is a standard joining technology in the field of micro-system technology and a reliable low-cost joining technology. A wide range of different adhesives designed to join a huge variety of different materials are commercially available. Dispensing of adhesive from dot sizes of some few 10 µm up to larger diameters is possible. Due to the gap filling behaviour of low viscosity adhesives also delicate joining tasks can be realized. The flow behaviour of low viscosity adhesives is used to realize repeatable joints with defined forms and surfaces using surface effects like adhesion. The fixation of the polymer master onto the copper substrate is realized by adhesive bonding. A second advantage in using adhesives, in addition to the fixation, is the sealing capability. Similar to results in standard welding processes a uniform circumferential pasted seam can be built at the edge between the polymer part and the copper plate (figure 3). This meniscus is built automatically depending on different boundary conditions like the contact angle between the adhesive and the components to be joined, the viscosity of the adhesive and of course the dispensing parameters to name a few. In an optimized process, undercuts and burrs in the mould insert can be avoided and the seam facilitates demoulding from the mould insert as well.

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For the mould insert fabrication using micro- and nanostructured components two kinds of adhesive techniques are currently applied at KIT, which differ in the way the adhesive is prepared. In general, the dispensing technique is used [12]. For special applications and for bonding microcomponents with through holes, the dip adhesive coating method is used.

2.1.1. Dip-in adhesive bonding.

The best results in adhesive bonding with fibre connector components (containing through holes) have been achieved with the dip-in adhesive bonding or adhesive coating method (figure 4). With this method, the adhesive is not directly dispensed on the component surface. Rather first an adhesive film or layer of a defined thickness is provided and in a second step the joining surface of the component is coated by shortly dipping it into that layer. In the first process step, a special dosing device is used. This device, made of steel, offers a cavity with a predefined and precisely manufactured deepness and flat surface. The cavity is overflowed with a certain amount of adhesive depending of the cavity size. With a precise blade the adhesive is manually distributed all over the area of the cavity. During this process the polished flat rim around the cavity serves as grinding structure for the blade. After distribution a homogeneous layer of adhesive with a defined thickness is prepared. The depth of the cavity is the essential factor of this method and is essential for the desired amount and distribution of adhesive on the bonding surface of the component. In addition, other influencing effects are the viscosity of the adhesive and the wettability or the layout of the joining surface. For adhesive transfer the component with its joining surface is shortly dipped in the surface of adhesive till the complete surface is wetted and pulled out again. After the dipping, the joining surface of the component is coated with the required amount of adhesive and is placed onto the copper substrate. For the dip-in adhesive bonding of the fibre connector components, a dosing device with a cavity depth of 10 µm was used. Advantages of this adhesive application process, especially in microcomponents containing through holes, are the adjustability of the defined adhesive layer thicknesses by using different notch depths and the reproducibility of the method. Also a continuous closed seam adhesive could be created at the edges of the contact surfaces. The seam is created not only around the entire component but also simultaneously into the through holes, which prevents an uncontrolled penetration of the electrolyte during the plating process.

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2.1.2. Finishing treatment of adhesive bonded components by laser ablation technology.

The nickel deposition in the electroforming process cannot be carried out if the copper substrate surface is covered by an adhesive layer. When too high a dosage of adhesive has been applied, especially in micro-holes and in long and narrow trenches, the presence of an adhesive layer can inhibit an electroforming process. In a reworking step, the adhesive layer can be treated by laser technology [13]. The finishing treatment is a laser ablation process in which an excimer laser is used to remove the adhesive layer in the narrow trenches to allow a proper initiation of the electroforming process (figures 5 and 17). The laser beam is positioned over the adhesive, then penetrates into the adhesive and destroys the layer down to the copper surface. Laser cleaning was performed using a short pulse excimer laser (ATLEX-500, ATL Lasertechnik GmbH) at a wavelength of 248 nm and maximum repetition rate up to 500 Hz. The laser pulse length was 5 ns (FWHM). A homogeneous beam with a 'flat top' profile (75 µm in diameter) and an intensity fluctuation of less than 5% was established.

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As an alternative to adhesive bonding, the master component can be joined with the copper substrate in a force-fit manner by means of clamping. In that case the copper substrate contains tap holes. The component is positioned onto the substrate and a pressure plate is placed on top (figure 6). Usually the pressure plate is made of PMMA by micro-milling and contains micro-channels to ensure that the electrolyte can flow into all structure cavities. The plate is tightened by a screw that presses the microcomponent onto the copper plate (figure 2). Clamping is used in electroforming for about 24 h. After a nickel layer of about 200 µm has formed on the copper plate, with the microcomponent being included, the clamping plate and screws can be removed and electroforming is continued. Clamping is used when e.g. the adhesive might damage optically relevant sidewall surfaces. The clamping technology cannot be used when the component does not possess any area that is large enough to introduce the necessary pressure forces and does not permit any pressure load without deformation taking place. Further requirements on the components are sharp edges and a plane base layer. Deburred or rounded edges and an uneven basis or contact surface will result in undercuts and burrs in the mould insert. When using the clamping technique, underplating occurs more often than when using the adhesive bonding technology. Underplating is defined as an uncontrolled penetration of the electrolyte under the structure or between the copper substrate and the master. Underplating is undesirable and forms burrs at the edge of the structure in the mould insert. Finally these burrs hinder the demoulding phase in the moulding techniques (e.g. hot embossing).

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Before starting electroforming, master microcomponents are usually coated at KIT with a conductive electroplating seed layer of chromium and gold using a physical vapour deposition (PVD) process. This seed layer enables a precise metal deposition along the microstructures. The copper substrate with the master microcomponent is fixed to a special holder that is immersed in the galvanic bath. The galvanic system with a nickel sulfamate electrolytes containing boric acid and a flourtensid (T = 52 °C; pH 3.4–3.6) was especially developed for the electroforming of microstructures at KIT. For electroforming nickel mould inserts, the standard parameters of manufacturing mould inserts based on LIGA structures are applied [2]. Electroforming starts with a low current density (0.5 A dm⁻²) at the seed layer and is continued with increased current density (<1.8 A dm⁻²) until the metal layer has reached a thickness of 6 mm (figure 2). This leads to a stiff and homogeneous metal block which can withstand the forces applied in the moulding process.

2.3.1. Electroforming of through holes.

If there are through holes in the master, as in case of the fibre connector components used in this work, a special PVD process must be carried out. In a standard electroforming process the complete surface of the component is coated with the electroplating seed layer. Hence a nickel layer covers the complete surface of the component, sidewalls and the upper side. In case of a through hole, the nickel deposition starts inside and outside the hole at the same time. The electroplating process shows that on the edge of the hole the nickel layer grows faster than inside the hole. In this situation, the nickel layer caps the hole and inside the electroplating process is stopping. The result is a nickel mould with defects such as electroplating growth seams or voids (figures 7 and 8).

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To optimize the nickel electroplating process for components with micro through holes a special PVD process for a justified partial metallization was developed at KIT [14]. For the electroforming process a new electroplating seed layer design was tested. This seed layer design allows guiding the formation of the electroplating nickel layer and reduces the velocity. The electroforming on the upper side does not start before the nickel has filled the complete through hole (figure 9). The new metallization design is composed of a sequence of circular spots with a diameter of 50 µm and a distance of 100 µm. The spots are created by a PVD mask and have to be adapted to the design of the master component and in particular to the position of the through holes.

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2.3.2. Fabrication of a PVD mask for the partial metallization.

First the layout of the circular spots alignment is designed using a computer-aided design (CAD) software tool. With the CAD layout a chromium mask can be fabricated (figure 10). Then in a UV-lithography process using this chromium mask, a structure with circular micro-pillars is created in a photoresist. In a nickel electroplating process the micro-pillars are surrounded with a nickel layer with a thickness of about 20 µm. Finally the resist pillars are dissolved and the PVD mask in nickel with circular holes, with a diameter of about 50 µm, is fabricated and lifted from the substrate (figure 11).

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To use the nickel mask in a PVD process, a special holder and positioning system is needed (figure 12). It is crucial that the mask is aligned with the master component. The circular holes must be placed exactly in the right position over the component to create the electroplating seed layer spots onto the component by PVD. To meet this requirement a special holder was designed and fabricated. When using this holder the mask is clamped over the component. Under a microscope the mask is aligned by means of screws with a metric fine thread. Finally the holder, together with the mask and the component, is fixed into the PVD equipment. The results after the PVD process are exactly positioned electroplating seed layer spots on the component surface (figure 13). The microcomponent with its partial metallization layer is then ready for the electroplating process.

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The electroplated nickel block will be separated from the substrate and processed by wire-cut EDM (figure 14).

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During post-process machining, the mould insert is cut to precise exterior dimensions such that it fits into hot embossing or injection moulding tools. The current standard mould insert dimensions at KIT are 66  ×  30  ×  5 mm³. Once the mould insert has its final dimensions, the original component is removed from the mould insert mechanically or chemically depending on the material it is composed of. Finally the mould insert can be used for the mass production of microcomponents.

At the beginning of this project, the clamping technique was used to perform electroplating tests with dummy masters with the fibre connector geometry and justified partial metallization. If the results were positive, the original master components, fabricated by VUB using DPW, could be used. For these tests, dummy fibre connectors were prepared in PMMA by means of micro-milling. The mechanically prepared component was fixed onto a substrate and the PVD mask was aligned and positioned on top of it. Component and mask were installed in the vapour deposition facility and coated with the justified partial metallization (figure 13). To carry out an electroforming process, the component has been clamped after the vapour deposition process on a copper substrate, and the plating is started (figure 6). The tests of the justified partial metallization and the electroforming delivered positive results. A nickel layer with a height of 6 mm was generated (figure 15). This nickel block was separated from the copper substrate and machined to the required outside dimensions by means of wire EDM (figure 16).

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After vapour deposition and electroplating tests by using the clamping technique, the gluing technique has been tested. The adhesive bonding technique tests have shown some advantages compared to the clamping technique. A particular advantage is the prevention of underplating which may occur frequently in the clamping technique. In the adhesive bonding technique, the adhesive prevents the underplating with its adhesive seam. The glue behaves like a gasket or sealing surface between the copper substrate and the component. Thus, the electrolyte cannot flow under the component and generate an uncontrolled nickel layer. A further advantage is the formation of the adhesive seam. The seam is distributed like a concave fillet weld, similar as in welding technology, both on the outer edges of the component and on the inside edges of a through hole (figure 5). This geometry generates nickel structures with rounded edges in the mould insert after electroforming (figures 17 and 18). These round edges facilitate the demoulding step in a moulding process and reduce the demoulding forces.

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A further effect was observed during the adhesive bonding of the component with long and narrow trench. The narrower the trench or the higher the amount of adhesive was, the higher the risk that adhesive seams from both sides of the trench flow together and form a closed concave adhesive surface at the bottom of the trench (as shown in image 1 of figure 5). The adhesion and capillary forces increase with the amount of adhesive applied. In this case the adhesive is distributed not only on the substrate but also on the sidewalls of the long trench. Especially at the ends of the trench, adhesive piles up upwards on the sidewalls. This behaviour has the consequence, that in this area a higher adhesive seam was formed and a stronger rounded edge was generated at the nickel structure after the electroforming (figure 18). Also after the adhesive bonding technique, a nickel layer with a height of 6 mm was electroplated. The nickel layer was separated from the copper substrate and machined to the required outside dimensions by means of wire EDM (figure 19).

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The justified partial metallization was carried out successfully. The nickel PVD mask was positioned and fixed with the adjusting unit on the top of the polymer master component. The complete holder with component and mask was installed in the PVD equipment. After the vapour deposition of chromium and gold, the component was coated with the electroplating seed layer spots in the necessary areas around the holes (figure 20). The results of the partial metallization also have demonstrated that the technique is applicable for different kinds and geometry of microstructures.

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After the justified partial metallization, the electroforming process was carried out on clamped as well as on adhesive bonded components. As required, the nickel deposition was started on the substrate surface, around the component and inside the through holes. First the nickel layer has completely filled the holes and reached the top of the component. Then the nickel layer was distributed evenly over the complete component thanks to the partial metallization (figure 21). During the electroforming process the outer nickel layer grew together with the inner layer. After an electroforming time of about 14 d, a nickel block was created with the required height of 6 mm (figure 22).

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During dip-in adhesive bonding the component is coated and placed manually onto the copper substrate. In future the process sequence will be realized with the micro-assembly system NAMOSE developed by KIT in cooperation with IEF-Werner GmbH (figure 23). The NAMOSE system is based on a high-precision four-axes system with XYZ-linear movements plus Z-rotation with a workspace of 400  ×  150  ×  35 mm³ and a resolution of around 0.5 µm. This system offers several possibilities for tool and module fixation. The main interface at the Z-axis is based on a Schunk MWS30-system. At this interface an alignment unit for manual angular adjustment of the horizontal axes can be fixed. This unit, developed at KIT (figure 23), is used in order to adjust the parallelism of the component and the copper plate to achieve a uniform distribution of adhesive during the dip-in adhesive bonding process.

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The resulting mould insert made from a DPW-fabricated PMMA master was installed in a hot embossing facility. The hot embossing system mainly consists of two separate heating plates. The mould insert is fixed onto one heating plate. Between the heating plates, a thermoplastic polymer foil is positioned, which is heated during the hot embossing step and pressed into the structures of the mould insert [1, 2]. A PMMA foil with a thickness of 1 mm was used. The mould inserts fabricated by the processes presented were tested at an embossing temperature of about 190 °C and an embossing pressure of 50 kN. In general, during hot embossing components are moulded with a residual layer (figure 24). This residual layer must be removed subsequently in one or two additional steps. In special cases and using a particular technique, hot embossing could be carried out with minimal or without residual layer [15]. After the residual-layer-free hot embossing of a fibre connector, a glass fibre was pushed into the spring-based through hole (figure 24). This paves the way towards fabrication of self-centring fibre alignment structures in large volumes.

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