Leyes conexas

Having demonstrated very promising results described in Section 4.4.1, a decision at M- Solv was made to promote the SMI technology at the Taiwan Printed Circuit-board Association trade show. To do so, the mask shown in Figure 4.17 was made to demonstrate various features enabled by the SMI system, including the ability to selectively scan small areas of the mask possible due to the small beam size, and then the ability to overlay these features at the substrate plane either by using the linear mask stage or the x-y substrate stage.

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Figure 4.17 CAD file and photograph of the chrome-on-quartz mask used to produce demonstrator samples for the TPCA trade show. The object field of the projection lens is large enough to capture all features in the y-axis; however the mask must be moved relative to the lens on a linear stage to image all features in the x-axis. The data in the CAD image represents transparent regions on the chrome on quartz mask.

Each of the squares shown at the top of Figure 4.17 were raster scanned individually, and the images of each overlaid on top of each other using the linear mask stage to produce pyramid structures. A map of the world, and the Taipai 101 tower, above and below the cross hair feature used to align the mask respectively, were ablated adjacent to the pyramids to demonstrate that there were no restrictions in feature shapes in contrast to a direct write system. The array of circuit tracks and pads were ablated and overlaid with their respective vias, with feature sizes down to 3 µm L/S in the smallest demo devices, and vias down to 10 µm diameter. The text ‘M-Solv’ and ‘TPCA 2014’ were then ablated in the centre of the circuitry array. Prior to the ablation, the mask was aligned to the cross hair feature labelled in Figure 4.17. The mask could be moved in the x-direction on a linear stage to centre all the features in x with the lens. By knowing the y offset of the features relative to the cross hair, it was possible to accurately scan each feature, and then arrange the features in the y-axis by using a y-offset on the x-y

Via s

Tracks and pads Cross

hair

Taipei 101

Squares for pyramids

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substrate stage to account for the offset with regard to the optical axis. A stitched microscope image of all the features arranged and ablated into polyimide on copper can be seen in Figure 4.18.

Figure 4.18 Stitched microscope image of the samples handed out at the TPCA trade show. The samples include demonstrator circuits with features down to 3 µm L/S and micromachined pyramids ablated into polyimide on copper. The sample has been cleaned to remove laser ablation debris using tape.

The pyramids shown in Figure 4.19 were both formed by overlaying the image of 4 square apertures. In the case of the inverse pyramid, the square apertures are simply getting smaller, such that only the central region is ablated with each scan of the different apertures. In the case of the other pyramid, there is an opaque chrome square of varying sizes centred on the transparent aperture, such that only the edge of the pyramid is ablated by all 4 scans of the apertures and is therefore the deepest. Although the pyramids are not of any obvious practical use, they demonstrate the accuracy with which one can overlay images using the linear stage in the mask plane, as well as the uniformity of the ablation even with several layers of ablation separated in time. More generally, it highlights the feasibility of overlaying any number of ablation layers, for

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complex, multilevel ablation of structures, with the primary constraint being space for the different layers of ablation on the mask.

Figure 4.19 SEM micrographs of an inverse pyramid and a pyramid ablated into BTX on copper and polyimide on copper respectively. The pyramids were formed by overlaying the image of different size square apertures in the substrate plane.

Figure 4.20 Laser ablation of the text ‘M-Solv’ and ‘TPCA 2014’ overlaid on an array of demonstrator devices. The devices surrounding the text have 3 µm L/S traces, as shown in the image on the right, and 10 µm diameter vias.

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Figure 4.21 Laser ablation of a map of the world and Taipai 101 tower in polyimide on copper. The map demonstrates the uniformity of ground plane ablation, given that the feature size is a factor 2 bigger than the beam size and no stitching effects are visible. Both features demonstrate high resolution ablation of irregular features of any shape.

Figure 4.20 has images of the device array, and the text aligned to it. The image on the right hand side is a close up of the smallest device size, with 3 µm L/S signal traces and 10 µm diameter vias accurately registered with their pads. Figure 4.21 shows a map of the world and the Taipei 101 tower ablated into polyimide. The map of the world demonstrates the uniform ablation of an area larger than the beam with no beam stitching effects visible. The Taipei 101 tower was added to demonstrate that there were no design restrictions in what could be ablated by the SMI system, unlike a direct write system with a focused laser beam. The halo surrounding the Taipei 101 tower is laser ablation debris deposited on the surface of the polyimide. The laser ablation debris is often not an issue for the chip packaging industry, since the samples go through a desmear acid etch process after laser ablation to roughen the surface of dielectrics which improves the adhesion of the copper in the subsequent electroless copper plating process. However the laser ablation debris can be easily removed by a number of cleaning processes. Figure 4.22 shows stitched microscope images of the demonstrator device array handed out at the TPCA show before and after cleaning with an adhesive tape. Alternatively, an ultrasonic bath with isopropyl alcohol (IPA) can be used to clean the samples. Simply wiping the samples with a cloth and IPA removes the laser ablation debris form the surface of the samples, but has a tendency to push and trap the laser ablation debris into the laser ablated features.

3.7

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Figure 4.22 Stitched microscope images of demonstrator device samples handed out at the TPCA show taken before and after tape cleaning respectively.