Stress Reduced Embedded Die Substrate Structure Fabricated for Heterogeneous Integration Using Selective Laser Ablation
Authors: Matsuura Masamitsu , Asano Tanemasa , Kanaya Haruichi 
Published: January 2025

The demand for high-speed, multi-functionality, and miniaturization of devices increases with a general acceptance of mobile communication devices such as smartphones and smartwatches in the next-generation 5G and IoT era. Undoubtedly, the semiconductor package of the mobile communication devices needs to be downsized with keeping function. One of the solutions is embedded die substrate technology, which is able to integrate passive components and active components with the minimum volume in the same body to implement heterogeneous integration. However, in the conventional embedded die substrate, the embedded die is fully covered by the substrate materials. This fact results in the generation of significant residual stress in the embedded components. Accordingly, the stress-sensitive components such as micro electro mechanical systems (MEMS) sensor, bulk acoustic wave (BAW) resonator, and surface acoustic wave (SAW) filter are regarded unsuitable for embedding.

In our previous study, we newly developed an embedded die substrate structure having a hollow chamber at both sides of the die to mitigate a severe limitation for the stress-sensitive devices [1]. Measurements of stress in a sensor chip embedded in substrate have indicated a significant reduction in stress achieved by the formation of the hollow chamber. The results have also suggested technology options for further reduction of stress. This paper will report the formation of slots in the resin material at the periphery of the die, which is newly developed for stress-free embedded die.

The manufacturing flow of embedded die substrate with a hollow chamber starts from the formation of the cavity core frame using a conventional PCB process. The through-hole cavity is formed on an FR4 core frame by a router. The single-sided adhesive tacky tape, which enables to hold the die, is laminated by a roll laminator at the bottom side of the frame. The die is placed into the cavity with a face-down using a flip-chip bonder. After that, the cavity filling material is laminated by a vacuum laminator from the opposite side of the tacky tape. The die into the cavity is completely covered by filling material and proceed the cure at 190 °C for 60 min. After die embedding, the thin layer of the filling material on the die is removed by CO2 laser ablation. After that, the tacky tape is peeled off at room temperature. Accordingly, both side of the die is exposed into the air, and the die is only held at the peripheral by the filling material. After embedding the die, a 6-um-thick copper redistribution (RDL) layer is formed by a semi-additive process (SAP) to connect the IO pads on the die. Then, the hollow chamber at both sides was formed by using cavity prepreg and FR4 core as a cap layer. Finally, laser via, 2nd copper RDL, solder mask, and singulation proceed, then the embedded die substrate is finalized. As a result, cavities are formed at the topside and bottom side of the die, and the die is mechanically held by the filling material only at the periphery.

The above process was applied to embedding a piezo-resistance gauge chip, which is able to measure the stress and its distribution generated at the chip surface and evolution of the stress with the progress of the fabrication steps. After completing the packaging, the measurement was extended to the temperature dependency from -60°C to 100°C.

The results clearly showed a significant reduction of residual die stress. They also revealed the effects of the material properties of the filling resin. However, even with low modulus filling material, the residual stress remained more than 22 MPa at 50°C environments.

To further reduce stress on the die for stress-free packaging, we newly introduce slits in the filling material at the periphery of the die. CO2 laser pulses were applied around the die to make a through-hole via or a through-hole slit in the filling material. We report a change in stress with the ratio of the slit from zero up to 80% as measured with the piezo-resistance gauge chip. Moreover, the effects of the position of the slit are discussed by taking stress concentration at the corner of the die into consideration.

To summarize, this paper reports a newly developed fabrication process of embedded die substrate technology for mechanical stress-free packaging. The characteristic of the laser ablation is fully utilized to form a stress-releasing structure around the embedded die. This new technology will enable heterogeneous integration of stress-sensitive components in an embedded die substrate.