Embedded Si Fan Out: A Low Cost Wafer Level Packaging Technology Without Molding and De-bonding Processes
Authors: Xie Hong , Yu Daquan , Huang Zhenrui , Xiao Zhiyi , Yang Li , Xiang Min 
Published: September 2017

The growing and diversifying system requirements have continued to drive the development of a variety of new package technologies and configurations: small form factor, low weight, low profile, high pin count and high speed and low cost. Embedded chip in EMC, also called fan-out wafer-level packaging (FOWLP), has been used in various products such as baseband, RF (radio frequency) transceiver, and PMICs (power management ICs). Currently, INFO technology developed by TSMCÂ®, NANIUMÂ® were in mass production for 3D integration for processor and memory, which inspires other packaging foundries to develop their own embedded FOWLP for the forecasted explosive growth of this market in the next few years.

There are a number of challenges for FOWLP. For process point of view, temporary bonding and de-bonding are required. EMC wafers are difficult to handle due to its large warpage driven by the big CTE difference between the Si and molding material. In addition, the manufacturing of fine pitch RDL on EMC surface is also difficult.

In this paper, the concept of wafer level embedded Si Fan-Out (eSiFO) technology was introduced and the development progress was reported. For eSiFO, cavities with certain depth were formed by Si dry etch. Then device dies were thinned to designed thickness. The dice were then placed into the cavities and bonded by the attached film on the bottom of the dice. A reconstructed wafer was formed. The micro gap between the chip and sidewall of the cavity as well as the surface of the reconstructed wafer were filled by dry film using vacuum process. Next, the pads were opened, followed RDL fabrication, repassivation, BGA, wafer thinning and dicing. Finally, an eSiFO package was fabricated.

There are a number of advantages for eSiFO technology. There is nearly no warpage since the Si was used as reconstruct substrate. The process is relatively simple since no molding, temporary bonding and de-bonding are required. RDL manufacturing is easier on Si wafer vs with molding compounds and can achieve high density routing. Furthermore, it can provide small form factor since the thinning of wafer is the last step.

To prove the concept of eSiFO, a 3.3 x 3.3mm package with 50 BGA bumps at 400Î¼m pitch was fabricated. The device wafer was thinned to 100Î¼m. The die size is 1.96 Ã— 2.36mm with pad pitch at about 90Î¼m. The depth of the cavities on 8 in. wafer formed by Bosch process on bare Si wafer was 107Î¼m with 8Î¼m variation. The length and width of Si cavities is 20Î¼m larger than die size. In the package, there is one layer Cu RDL with thickness of 3Î¼m, minimum line width of 13.72Î¼m. The BGA ball diameter is 280Î¼m. All the processes were evaluated and the results showed such packages can be produced. Reliability tests including THS, T/C, HTS and HAST were carried out and no failure issue was observed. Mechanical simulation was used to analyze the stress distribution during TC test and the results showed the maximum stress was located at the RDL near the UBM.

In summary, a low cost wafer level fan out technology using reconstructed Si wafer was developed. The process is simple without molding, temporary bonding and de-bonding. The reliability tests of test vehicles proved that such package is reliable. The newly developed eSiFO technology can be widely used for chips requiring fan-Out, small form factor and high density interconnects.