System schematic and design considerations

To allow flexibility in beam homogeniser choice and to simplify changing and testing different homogenisers after the initial setup, it was decided to image the plane of uniformity formed by an arbitrary homogeniser down to the mask plane. An infinite conjugates imaging system was used, formed of a plano-convex singlet and a telecentric f-theta scan lens. Using a telecentric scan lens means that a double telecentric projection lens can be used to form the image of the mask, and it also limits the size of the first element in the projection lens to approximately the size of the object since the beam deviated across the mask by the scanner is approximately normal to the mask at all points in the scan field. The scan lens also has a flat field in its focal plane which keeps the image of the homogenous beam in focus across the scan field. The use of a double telecentric imaging system gives a more precise definition of magnification which is less sensitive to object and image position. This is key for the application, given that the magnification must be defined to 3 decimal places to give substrates the dimensional accuracy required to accurately register with the microbumps on the chips I/Os. The divergence of the beam after the mask is then defined by the homogeniser system and the lenses in the infinity imaging system, as well as the inherent properties of the multimode beam. Care must be taken to design the system such that the aperture of the scanner and the entrance pupil of the f-theta lens are not overfilled, but that the

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divergence at the mask is high enough to obtain the desired numerical aperture on the object side of the projection lens with the optimum partial coherence factor. There are a number of ways to homogenise a multimode laser beam, the most common being microlens arrays and diffractive optic element (DOE) homogenisers. The DOE type homogenisers are less sensitive to alignment in the system, and can generate a more homogenous beam with multimode lasers. The beam homogeneity is improved with multimode lasers by reducing the high intensity speckle occurring due to diffraction effects in beams with long coherence lengths. For these reasons it was decided to use a DOE type homogeniser to form a flat top, square beam profile. The homogeniser used forms a square, flat top plane at the beam waist of a plano-convex singlet. The DOE manufacturer provided the following equation to calculate the square beam size:

𝑡 = 𝑓 × tan 𝛼 (4.1)

where the width of the square formed, t, is dependent on the diffusion angle of the homogenizer, α, and the focal length of the singlet, f. This introduces another degree of control and flexibility into the system, since both the beam size and the divergence of the beam at the mask can be adjusted by changing the DOE for one with a different diffusion angle, or by changing the focal length of the plano-convex singlet after the DOE. A schematic of the system can be seen in Figure 4.1. The focal length of the second plano-convex singlet can also be adjusted to change the magnification of the infinite conjugates imaging system, and hence the beam divergence at the mask and the filling of the entrance pupil of the f-theta lens and projection lens.

Having planned the illumination system, the lens specification was discussed with Sill Optics, a commercial lens supplier. A proposal was made for a 0.11 NA double telecentric imaging lens with 2-3 µm resolution across its image field. The maximum object size, image size and magnification of the lens required careful consideration with regard to the proposed application. The maximum object size is limited by the availability of metal-on-quartz photomasks. A single photomask is required to have a minimum of two equally sized patterned areas, defining the information for the redistribution of the signal I/Os from the chip and the vias to connect signals in different planes of the package. It is feasible to have this information on two separate masks, with a mechanical solution to align the masks to each other to allow the accurate overlay and registration of the layers. However, in order to reduce the mechanical complexity of the system, and to improve the accuracy of the image overlay, the

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information for the redistribution layer and its complimentary via layer was put on the same photomask. Photomasks are standard consumables in the semiconductor industry and chrome and aluminium-on-quartz photomasks are readily available on quartz substrates up to 7×7” in size. The defect free area as defined by the manufacturer of a 7×7” photomask is up to ~164×164mm. Additional space is required between the two layers on the photomask to define features for mask alignment, resolution charts for focusing and any other assembly or tooling marks which may be required on the panel for subsequent process steps. Allowing for some space between features and layers on the mask, to allow for overscan, dictated that the maximum object size at the mask for two equally sized layers on a 7×7” photomask is ~70×70mm.

Figure 4.1 Schematic of the SMI system. The beam passed through a DOE and forms a homogenous, square flat top beam at the beam waste of a singlet lens. This plane of homogeneity is then imaged by a singlet and telecentric f-theta lens in an infinite conjugate imaging system down to the plane of the photomask. The beam is rastered across the photomask by the scanner and the image of the photomask is projected onto the substrate by a double telecentric projection lens.

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Having calculated the maximum object size based on readily available, low cost photomask sizes, the magnification and image size of the lens was considered with the following constraints. The image size should be as large as possible to capture the largest possible portion of the chip packaging industries. Chip-scale-packages (CSPs) are by definition approximately the same size as the chips they house, and are commonly used in consumer electronics. Thus most of the CSP market can be captured by an image field of ~20×20 mm. There are high end package types such as ball-grid- arrays (BGA) that are commonly around 30×30 mm or larger, and can be as large as 50×50 mm, which are used in high end applications such as graphics processing units (GPUs) and servers. However, because of the limit on the object size, targeting the larger substrate BGA market would require a low magnification imaging system with a demagnification of less than 2. This restricts the fluence at the substrate to below 0.8 J cm-2 because of the damage threshold of chrome- and aluminium-on-quartz masks. Photomasks with dielectric coatings can have damage thresholds up to 2 J cm-2, but are significantly more expensive. An optical system with a demagnification less than 2 also increases the challenge of the optical system design to achieve high NA optics with a low demagnification. It was therefore decided to target package sizes below 20×20 mm, with a demagnification of 3.5x in the imaging system, allowing fluences of up to 3 J cm-2 at the substrate.

Other constraints on the lens design were total track length and the front and back working distances of the lens. For practical reasons the track length from substrate to scanner was limited to ~1 m. Given that a long focal length, telecentric f-theta was required with a scan field of 70 mm or greater, it was decided to restrict the track length from mask to substrate to ~600 mm to allow room for the f-theta lens and scanner assembly. The projection lens also required a front working distance of greater than 50 mm to allow room for the mechanical mounting of the photomask on a linear stage, and a back working distance of greater than 50 mm to allow room for a debris extraction nozzle to extract fumes and debris from the laser ablation process. Given the small spectral bandwidth of frequency tripled DPSS lasers, it was possible to use an all silica design for the lens without the need to compensate for chromatic aberration with CaF2

elements, as is the case with some high resolution excimer laser projection lenses.

Based on the above considerations, a custom projection lens was acquired with 0.11 NA across a 20x20 mm image field and a magnification of 3.5x. All other components in

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the system were off-the-shelf components, including an f-163 mm f-theta lens with an NA of 0.03 and a scan field of 70x70 mm, a DOE beam homogeniser with a diffusion angle of 0.17° and several singlets to interchange in order to form different spot sizes at the mask.