Passive Alignment of Optical Elements in a Printed Circuit Board T. Lamprecht1, F. Horst1, R. Dangel1, R. Beyeler1, N. Meier1, L. Dellmann1, M. Gmür2, C. Berger1 and B.J. Offrein1 1 IBM Research GmbH, Zurich Research Laboratory, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland E-mail:
[email protected], tel. +41-1-724 84 90 2 Varioprint AG, CH-9410 Heiden, Switzerland Abstract A successful implementation of optics into PCBs (printed circuit boards) requires a precise passive alignment of optical elements relative to the optical waveguides in the board. We tackled this challenge with a novel concept that allows the passive alignment onto a PCB of any optical or optoelectronic building block with a precision of a few micrometers. Markers, structured into a copper layer during manufacturing, are used as a position reference for the polymer waveguide fabrication and for the formation of mechanical alignment features. To form the latter, laser drilling, a standard process for via formation in PCBs, is used. Thereby, we exploit the fact that the laser light is reflected at the copper surface, such that the copper marker can act as a mask for the laser beam. An opening in the copper marker can then be used to define an accurately positioned alignment slot, independently of the low positioning accuracy of the drilling laser. We were able to demonstrate repeated insertions of adapter elements into these alignment slots with a standard deviation of 3 µm for in-plane displacements. Afterwards, optical modules were mounted onto the adapters, using a standard MT interface provided by the adapter. We measured a standard deviation of the order of 5 µm for the in-plane and out-of-plane misalignments of the module with respect to the optical waveguides. The passive alignment concept demonstrated enables accurate and simple plug-in of any kind of element, in particular of optical and opto-electronic elements, into a PCB. The concept is based on established PCB manufacturing processes, which is crucial for the development towards a low-cost optical interconnect technology. Optical interconnects The increasing performance of microprocessors leads to higher bandwidth requirements for the data flow to and from the processor. Today, all signaling on a PCB is performed electrically, using copper lines that are integrated in the board. However, issues such as propagation loss and inter-channel crosstalk, limit the scalability of electrical interconnects to ever higher bandwidth densities. Optical interconnects feature a higher bandwidth × length product, are more powerefficient and allow a higher bandwidth density than electrical interconnects. It is especially the higher bandwidth density of optics that drives the research on optical PCB technology for inter-system interconnects [1] [2]. Several subsystems have to be developed to integrate optical communication links in a PCB [3]. Transceiver elements perform the conversion of the signals from the
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electrical to the optical domain and vice versa, multimode polymer waveguides route the optical signals in the board, and optical connectors are required to couple the signals to other boards or to a fiber network. One major obstacle for the integration of optical elements, such as optical interconnects, into standard electronic systems, is the large gap in positioning tolerances between the optics and the electronics world. In standard PCB processing, manufacturing tolerances of as much as 100 µm, or even 200 µm for large backplanes, are acceptable. In the world of optics, however, the tolerances for mutual alignment between building blocks are only 5-10 µm. This implies the use of the less cost-extensive and more tolerant multimode optics. The tolerances for optics are more than one order of magnitude smaller than those of standard PCB technology. In this paper we will present our solution for this alignment problem with the requirement that costs be kept as low as possible. To achieve this, the resulting alignment should be passive, whereby the optical elements are simply plugged or ‘clicked’ into the board without the need for optical monitoring and/or further fine adjustment. Also, the production of the PCB should, wherever possible, use the existing standard equipment at the PCB manufacturer. Combining optical waveguides and mechanical alignment structures on PCBs Because of the high requirements regarding the smoothness of the waveguide sidewalls, the definition of the actual waveguides is one process step that cannot be performed using standard PCB processing equipment. For this step, a new process method and corresponding machinery must be introduced. Common methods to define the waveguide structure are: (hot) embossing, molding, optical definition through a photolithographic mask, and optical definition using a laser writer [4] [5] [6] [7]. The first three methods have in common that the dimensions of the waveguide pattern are fixed and defined by the preform, mold or mask used. Waveguide definition using a laser writer is more flexible and allows the writing pattern to be adjusted for individual PCBs, e.g. to compensate for PCB deformations. The passive mechanical alignment of modules in the PCB needs robust alignment structures, which are accurately aligned relative to the waveguides. An elegant method to obtain such structures is by making them in the same step and material as the waveguide core layer. However, the resulting structures are quite small and brittle, and it is difficult to preserve the structures throughout, the complete board fabrication cycle and retrieve them afterwards. As will be shown in the following sections, more robust structures can be obtained by adding a second step in which
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the alignment structures are formed by photolithography in a copper layer on the board. In the case of embossed, molded or mask-exposed waveguides, the definition of the copper layer needs a high-accuracy (< 5 µm) optical mask and alignment as well as precise control of the board temperature to minimize board deformations between waveguide and alignment structure production. When using laser-writing for the waveguide definition, the copper alignment structures can be produced first using simple and inexpensive standard PCB processes. Then, the waveguides are laser-written and the pattern is adjusted to compensate for the long-distance inaccuracy of the copper structures. In our work we are focused on the latter combination, laser-written waveguides with photolithographically formed copper alignment markers. The proposed alignment concept is in principle also applicable to other waveguide definition methods, e.g definition through a photolithographic mask. The copper markers are produced using — and with the same accuracy as — standard PCB layer processes. A drawing of a marker pattern is shown in Figure 1. These markers contain both the mechanical alignment structures and the optical reference patterns. The optical references are used to determine the adjustments needed during the subsequent laser-writing of the waveguide pattern.
On top of the substrate with the markers, the optical waveguide layers are deposited by doctor-blading or spraycoating. The cladding layers are solidified by flood-exposure with UV light. The positions of the optical reference markers are measured using a camera system and image-recognition software. Based on the marker positions, the proper position and adjustment of the waveguide core pattern are determined after which the pattern is exposed into the waveguide core layer using the laser-writing tool (see Figure 2b).
Figure 2b: Waveguides built on top of substrate using optical reference structures in markers for positioning. On top of the waveguide layer, the top FR4 multilayer board is laminated, thereby completely burying the waveguide layers and marker patterns. The board buildup after this step is shown in Figure 2c.
Figure 1: Layout of copper marker, showing optical and mechanical alignment structures. Marker size is typically 1.5 mm × 1.5 mm. Optical PCB fabrication procedure The fabrication of the optical PCB starts with a substrate, which can be just a thin foil as well as a complete multilayer FR4 board with various electrical traces inside. On the top of this substrate, there is a continuous layer of copper, onto which a thin RCC (resin-coated copper) foil is laminated. In the top copper layer of the RCC foil, the copper alignment markers, as shown above, are defined. This structure is shown in Figure 2a.
Figure 2a: Substrate with marker structure defined in top layer of RCC foil.
Figure 2c: Waveguides buried into board by adding top FR4 multilayer. To retrieve the markers, a first milling step removes the board material below the markers. The milling depth is controlled by noting the moment of electrical contact between the milling tool and the continuous copper layer below the RCC foil. The milling then continues for a few more micrometers to remove the copper and stops in the resin of the RCC foil (see Figure 2d).
Figure 2d: Removal of substrate below markers by milling until electrical contact on continuous copper below RCC.
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The remainder of the resin at the bottom of the markers is ablated using a drilling laser. This laser stops on copper, but continues to drill through the mechanical XY reference opening in the marker so that at this position we obtain a reference slot. However, the position of this reference slot is not defined by the inaccurate laser drill, but rather by the opening in the marker, which is accurately positioned relatively to the optical reference structures. This laserdrilling step is shown in Figure 2e.
Figure 3: Photograph of a set of four mechanical alignment markers after processing.
Figure 2e: Cleaning away of resin from marker using a drilling laser. The laser stops on copper, but drilling continues through the mechanical XY reference opening. After the laser-drilling step, we obtain a structure (shown at the left in Figures 2e and 2f) with a clean copper area on the bottom surface of the marker, which can be used for accurate Z alignment of a mechanical element. At the right, there is now an accurately-positioned alignment slot that can be used for XY alignment of the mechanical element, as shown in Figure 2f.
Passive alignment adapters We typically use the passive alignment system with an intermediate adapter that provides a link between the mechanical alignment structures in the PCB and the optical elements. Currently we use two different types of adapters. The first type, called a ‘silicon adapter’, provides alignment rails for in-house-fabricated optical transmitter and receiver modules [8]. The second type is an ‘MT adapter’, which provides an industry-standard MT-pin interface that accepts any element conforming to this standard, be it transmitters, receivers, ribbon cables, or connectors. Figure 4 shows, at the left, a top-view drawing of the silicon adapter. The alignment studs that mate with the alignment structures on the PCB are placed at the four corners of the silicon adapter. In the center are the alignment rails that accept a silicon-based optical module. At the right, a front view of the adapter is shown, including an optical module that is accurately seated on the alignment rails in the adapter. The position of the optical module in the adapter is defined by the side and bottom facets on the module. The adapter and the module are made out of silicon using MEMS technologies such as photolithography and DRIE (deep reactive ion etching). The rails and the XY-alignment studs on the adapter, as well as the side and bottom facets on the module are each structured using a single photolithography step, providing sub-micrometer position precision.
Figure 2f: Bottom of marker structure serves as mechanical Z reference, the edge of marker-defined slot as mechanical XY reference. A photograph of the complete laser-drilled marker structures, consisting of a group of four markers surrounding an optical access slot in the PCB, is shown in Figure 3. The end facets of the optical waveguides in the board (not visible in this photograph) can be accessed through the side-walls of this slot.
Figure 4: Layout of the silicon adapter. Left: top view. Right: front view with optical element seated on the alignment rails.
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1.
Figure 5: Silicon Adapter Figure 5 shows such a silicon adapter, and one can clearly see the Z- and XY-alignment studs, which mate with the alignment slots in the copper markers. The second type of adapter, the MT adapter is shown in Figure 6. At the corners of this adapter there are again alignment studs that mate with the mechanical alignment slots in the copper markers on the PCB. However, here, the two MT pins to the left and right of the waveguide array provide the reference structure for the optical element or cable.
Figure 6: MT adapter and its mating site in the optical PCB, waveguides illuminated from the back with red light. Passive alignment accuracy For the passive alignment system as described in the preceding paragraph, the total alignment error between waveguide and optical element results from the accumulation of a number of alignment errors in all intermediate alignment steps. The following table lists all steps, together with (in parentheses) the process steps that determine the accuracy of each step. Starting from the optical waveguide, the steps are:
Position accuracy in optical PCB between waveguides and optical reference on marker (laser-writer accuracy, optical layer thickness control) 2. Position accuracy on copper marker between optical and mechanical reference (marker lithography) 3. Mechanical fit of adapter to the marker (marker etching, marker laser drill, adapter fabrication) 4. Position accuracy on adapter between marker reference and optical module reference (adapter fabrication) 5. Mechanical fit of optical module to adapter (adapter fabrication, optical module fabrication) 6. Position accuracy on optical module between adapter reference and optical element (optical module fabrication). The main focus of this report is on the steps 2 and 3, which specifically determine the performance that can be achieved using the approach employing a laser-drilled copper marker. To measure this performance, we followed two paths. On the one hand, we directly measured the alignment accuracy between an optical module and embedded waveguides. In this measurement, we tried to minimize the misalignment contributions in steps 1, 4, 5 and 6, to emphasize the effects of steps 2 and 3. On the other hand, we also performed individual measurements on various aspects of steps 2 and 3 to obtain an exact determination of the contributions of various possible causes of inaccuracy. These measurements as well as the direct measurement of the passive alignment of a full optical module and the detailed measurements for individual components will be described in the following sections. Measurement system Most measurements were performed using an automated three-axis positioning system, in which a measurement platform is supported by high-precision air bearings. A video camera and a height sensor are mounted on the measurement platform. The video camera and image recognition software are used to detect and measure alignment features. The height sensor is based on the chromatic-coded confocal imaging principle and used to measure vertical dimensions. All measurements provide a statistical distribution over a multiple of passive alignment marker sites or over a multiple of passive alignment insertions on one site. The measured standard deviation is a sum of the alignment accuracy and the measurement accuracy. Therefore, the actual standard deviation for the passive alignment will be smaller, and the value measured gives an upper limit. For the measurement system, we expect a measurement accuracy of better than 0.5 µm for the measurements performed. The following axis convention is used: the Z-direction is the vertical, out-ofplane dimension and the X- and Y-directions are the horizontal, in-plane dimensions, with the X-direction along the optical axis of the waveguides. PCB test cards We designed test cards with 10 passive alignment sites on each card, each site comprising a group of four markers, as
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shown in Figure 3. The test cards contain arrays of 12 waveguides, which pass through and are aligned with each marker site. Test-card manufacturing, including all milling and laser drilling steps, was done at Varioprint using standard PCB manufacturing tools, except for the waveguide layer, which was fabricated at IBM Research. Measurement of position accuracy on the copper markers At PCB manufacturing, the marker pattern is defined using a photolithography foil mask with a resolution on the order of 5 µm. The projection optics and the wet etching of the copper provide a smoothing of the marker edge. Also, as reference pattern we used a circle with a size that is much larger than the resolution of both the mask and the measuring camera. This means that the circle edge, which is used for determining the circle center position, spans many mask and camera pixels. The resulting averaging over many pixels would, in principle, allow the position of the center of the circle to be determined with an accuracy of approximately 0.5 µm, far below the resolution of mask and camera. A photograph of an etched copper marker is shown in Figure 7.
alignment slot. Assuming a uniform distribution over the resulting possible position error range from -5.8 µm to +5.8 µm, the theoretical standard deviation of the position error of perfect adapters in these alignment slots is 3.4 µm. For the variation of the alignment slot radius over the boards we measured a standard deviation of 3.3 µm. As indicated by this relatively large standard deviation, the amount of over-etching varies significantly, which makes it difficult to apply an etch compensation. Measurement of the alignment of silicon adapters to the alignment slots In this measurement we determined the position accuracy of a silicon adapter (see Figures 4 and 5) inserted into the test card, with respect to the copper markers. The measurement starts by determining the exact position of the alignment slots on all 10 sites of the test card using two perpendicular comb scans with the height sensor to determine the edge of a alignment slot and calculate the center of the alignment slot. Afterwards, silicon adapters, one for each of the 10 sites on the card, were inserted and slightly clamped. Insertion into the boards was done manually, without any alignment effort. Then, the video camera with image recognition was used to detect the edge of the rails and calculate the position of each silicon adapter. With the two resulting position matrices, the misalignment of the adapters with respect to their corresponding marker sites could be calculated. Table 1 shows the in-plane and out-of-plane misalignments of the inserted adapters with respect to the copper marker sets, measured for 10 sites on two different boards. Table 1: Adapter to copper marker set misalignment for two test cards with 10 alignment sites each.
Figure 7: Copper marker Board We measured the position error between the optical and the mechanical reference structures on 160 markers spread over four different test cards from two different production batches. The mean position error averaged over all markers was found to be a negligible 0.2 µm. The more important number is the standard deviation which, at 1.2 µm, confirms that the local accuracy in a copper structure produced with standard PCB manufacturing technology can be sufficiently high for passive alignment purposes. Measurement of mechanical alignment slot size The size of the mechanical alignment slot after laser drilling is crucial for accurate passive alignment because a too large slot can introduce play between alignment stud and slot. We measured the diameter of the alignment slots using the height sensor on the positioning system. With the sensor, 12 points on the edge of the alignment slot are found, and the radius of the circle fitting through these points is determined. This procedure was applied to 53 alignment slots on two different boards. The average slot radius was found 255.8 µm, with a standard deviation of 3.3 µm. Assuming that the adapters that must fit in these slots have alignment studs with a perfect diameter of 500 µm, there would be 5.8 µm of play of the reference stud in the
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