Modelo de cálculo de los costos de importación

Having set up the infinity imaging system, it was decided to investigate why beam stitching effects had not been more apparent in ground plane ablation. It had been expected when setting up the system that some artefact from the edge of the beam might be visible in the ablation and depth profile of areas much larger than the beam. It is certainly possible to observe periodic variation in the ablation depth of large features, with the period being consistent with the pitch and mark speed of the raster pattern used to scan the mask, however this was easy to tune out. To investigate whether the absence of beam stitching effects was a result of the sloped edge profile of the top hat beam formed by the DOE, a square aperture was placed at the plane of uniformity and imaged down onto the mask to give the beam sharp edges. Prior to installing the aperture, the imaging performance of the system was tested with the infinite conjugate imaging of the uniform beam to confirm that any change brought about in the beam divergence after the mask had not detrimentally affected the performance of the imaging system by changing the filling of the object side NA of the projection lens. Figure 4.23 shows that the performance of the imaging system is at least as good as with the finite conjugate imaging system by demonstrating 2 µm L/S ablation in an ABF-type build up substrate.

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Figure 4.23 Microscope micrograph of resolution chart patterned by SMI after installing the infinite conjugate imaging system of the plane of uniformity. The substrate is an ABF-type substrate of epoxy with glass filler particles. The image demonstrates 2 µm L/S laser ablation of the substrate and good performance of the imaging system. The measurement error is ±0.14 µm.

Having confirmed the imaging system was still performing as expected, the aperture was installed in the plane of uniformity on a micrometer stage, and focused by moving the aperture along the optical axis until the image of the aperture at the mask plane, relayed down to the substrate by the projection lens, was in focus at the substrate plane. A mask was then scanned at less than 1 shot per area both with and without the aperture and images of the laser ablation can be seen in Figure 4.24. On the left hand side of the image, the blue areas between the laser pulses demonstrate the sloped edge of the beam, where there is partial or lower fluence ablation of substrate. In the image on the right hand side, these areas of lower fluence ablation are clearly not present, due to the sharp beam edge brought about by imaging the aperture. The angle of the lines in the image on the right is due to the misalignment of the aperture orientation with the axes of the scanner.

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Figure 4.24 Image of a mask scanned at less than 1 shot per area both without (left) and with (right) an aperture imaged onto the mask from the plane of uniformity formed by the DOE. The orientation of the aperture in the image on the right hand side has not been aligned to the axes of the scanner.

The aperture alignment was then corrected and the dimensions of the image of the square aperture calculated so that the mask could be scanned with precisely one shot per area. Figure 4.25 shows the corrected aperture alignment, and the imaged aperture size was measured to be (200±2) µm by (205±2) µm. Thus the mask was scanned with a pitch of 718 µm and a mark speed of 3500 mm s-1 such that it was scanned with 1 shot per area as can be seen in Figure 4.26. The beam stitching effects are very apparent, with unacceptable ridges and lines in the laser ablation.

Figure 4.25 Optical micrographs of ablation when the mask is scanned with a square aperture imaged onto the mask. The image on the left shows the orientation of the square aperture has been adjusted such that it is aligned with the axes of the scanner. The image on the right shows the dimensions of square aperture, used to calculate the parameters required to scan the mask with the correct shot overlap.

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Figure 4.26 Mask scanned with square aperture in beam with 1 shot per area. The beam stitching effects are clearly visible.

The mask was then scanned at 4 shots per area with 2 shots in the horizontal direction and 2 shots in the vertical direction. This was done by both scanning the mask 4 times at 1 shot per area, with each scan of the mask in a 2x2 array with a 0.35 mm pitch, and by halving the pitch and mark speed of the original vector file to put down the 4 shots in a single scan of the mask. The shots were distributed in this fashion in an attempt to homogenise the illumination of the mask and alleviate the stitching effects observed in Figure 4.26. The results can be seen in Figure 4.27, and clearly the pitch in x and y is half that of the 1 shot per area scan shown in Figure 4.26 as expected. The key difference between the 2 images is the amount of laser ablation debris left on the surface of the substrate. In the image on the left, in which the mask was scanned 4 times at 1 shot per area, the debris from the previous scans is removed in each subsequent scan of that mask, such that only the debris generated in the final scan is deposited on the surface. In the image on the right, most of the laser ablation debris from the single pass is deposited on the surface of the ablation. However, the additional shots in both cases have not helped reduce the visibility of the beam stitching effects. Therefore one can conclude that the soft beam edge shown in the beam profile in Figure 4.5 obtained as a result of using the DOE in conjunction with a multimode beam is essential to masking the beam stitching effects to obtain uniform ground plane ablation as shown in Figure 4.21.

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Figure 4.27 Mask scanned with a square aperture in the beam at 4 shots per area. In the image on the left the mask was scanned at 1 shot per area 4 times, with each scan offset in a 2x2 array with a pitch equal to half the spot size. In the image on the right the mask was scanned with 4 shots per area in a single pass of the mask.

4.5 Summary

A custom imaging projection lens and illumination system was specified, and the performance simulated and evaluated using a Zemax ray tracing model. The illumination system was installed and an unexpected beam waist was observed close to the plane of the first element of the projection lens. As a temporary solution, the f-theta lens below the scanner was used in a finite conjugates imaging system to image the plane of beam uniformity formed by the DOE onto the mask. This enabled the production of demonstrator samples to promote the SMI technology, however resulted in non-optimal performance of the f-theta lens since it was being used outside of its focal plane. After the samples had been produced, the illumination system was reverted back to the infinite conjugates imaging system of the plane of uniformity, however with a shorter focal length lens following the DOE. This led to a smaller beam reduction prior to the scanner, and a higher fluence at the mask plane which necessitated the use of Aluminium-on-quartz masks. The imaging system was found to be performing well and an aperture was introduced to the plane of uniformity and imaged onto the mask to investigate the impact of a steep edge to the beam profile. It was found that beam stitching effects become very noticeable when the beam profile has a sharp edge, and that the sloped beam edge formed by the DOE when used in conjunction with a multimode laser is ideal for masking beam stitching effects and obtaining uniform large area ablation. Thus the optimum configuration for the custom imaging lens and illumination system was determined, enabling further investigation of the performance of the imaging system and some of the unique aspects of the SMI approach.

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