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Passive Die Alignment in Glass Embedded Fan-Out Packaging
Keywords: Glass Embedding, Fan-Out Packaging, Heterogeneous Integration
Fundamentally glass is arguably one of the most interesting materials for heterogeneous integration. Whether for its RF properties, its high surface quality, its hermeticity or tunable coefficient of thermal expansion (CTE) and low cost, there are several reasons to favor glass for advanced packaging applications. Consequently there are researchs going on to improve current Fan-Out-Packaging schemes by making use of solid thin glass [1]. Unfortunately and up to now, processability of glass has not been among those above reasons. This is because current glass processing technologies typically induce micro-cracks and stresses which contribute to glass reputation of being prone to brittle fracture. In contrast, unoffended glass has excellent mechanical properties as it can be demonstrated when it is processed by Laser Induced Deep Etching (LIDE). The LIDE technology is a two-step process using first of all a fast laser beam modifying the glass through its entire thickness followed by a chemical etching of the glass in a second step. As shown by the Authors before [2], LIDE technology is capable of generating high aspect and high quality microfeatures in various alumino-boro-silicate glass at high speed and can be applied to economically manufacture open glass cavity substrates. These glass mounting substrates enable Glass Embedded Fan-Out Packaging with die thicknesses of over 500 �m. The technology supports all wafer sizes and also panel formats of up to 20x20� and can readily combined with through glass vias (TGV) and integrated passive devices (IPD) [2]. The objective of this paper is to demonstrate the feasibility of glass mounting substrates made by LIDE which include newly developed passive die alignment structures. The aim of these strucutes is to compensate for potential die misalignements and die shift issues which become severe when moving to panel level fan out packaging. The passive alignment structures are created with the same LIDE process step as the open cavities and are located at two adjacent edges of the rectangular cavity. The filigree spring-like alignment structures benefit in particular from being processed in a crack and stress free manner. Although the spring elements have a minimal dimension of less than 100 �m, these structures show an outstanding break strength when they are deformed while active dies are placed in the mounting cavity. Depending on the design, the spring elements can have a stroke of sevelraltenths of micrometer which enable the compensation of rather large die displacements. Authors will present examples for LIDE based mounting glass substrates with the described features. The performance of the proposed design and method is evaluated with a die accuracy study. Test dies with alignment marks are placed in the cavities and measured relatively to alignments marks on the mounting glass substrate. The Fan-Out packaging concept based on the research shown here combines several advantages. Due to the relatively high Youngs Modulus of the glass the reconstituted wafer shows less warpage than in the state of the art. The passive alignment structures reduce the die shift to a minimum (depending on dicing accuracies and through package vias for package-on-package or antenna on package application) and can be readily integrated.
Roman Ostholt, Vice President LIDE
LPKF Laser & Electronics AG
Garbsen, Lower Saxony

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