Hybrid Bonding Based on Polyimide for Low Temperature Process

As the input/output (I/O) density of semiconductor has increased to meet More Moores and More than Moores law, the pitch of electrodes is becoming narrower and narrower in recent year. Hybrid bonding isemerging as a technology to realize narrow-pitch electrodes with a wiring pitch of 15m or less, replacing solder-die wiring for higher I/O density integration. Hybrid bonding is divided into two main types, which are wafer-to-wafer (W2W) and chip-to-wafer (C2W) bonding. W2W hybrid bonding technology is already mature process for CMOS image sensors and NAND Flash memory,but it is less flexible than C2W since it requires the chips to be the same size and it is not always possible to bond good chips together (cumulative yields should be low). C2W hybrid bonding technology is being developed to implement multiple semiconductor chips to the substrate for heterogeneous integration packages. The advantage of C2W is possible to; 1) select known good die (KGD), 2) bond different size chips, and 3) stack multi chips. In the case of multi-chip stacks (e.g., High Band Width Memory (HBM) packages), the thickness of a chip must be reduced to keep the overall package thickness down. However, chip warpage of thin thickness chips is big issue. Hybrid bonding based on SiO2 dielectric which is conventional technology is assumed to be disadvantage of warpage due to its high modulus and Coefficient of Thermal Expansion (CTE) mismatch to Cu electrode. On the other hand, hybrid bonding technology using organic polymers is expected to suppress the warpage because of the lower modulus than SiO2 and of low temperature bonding and annealing for thermal budget reduction. Another advantage of polymer is adjustable mechanical properties easily. Knowing the robust mechanical and thermal properties for polyimides (PIs), these polyimide materials must be the best fit ones for hybrid bonding.In this work, low temperature processable PIs as dielectric materials have been developed for polymer C2W hybrid bonding.In order to apply to the low-temperature 3D stacking process, a low-temperature curable polyimide was developed at less than 200C for curing and less than 250C for bonding. In designing materials, low temperature processable feature was achieved by low temperature imidization technology. During the low temperature PI-PI bonding test, higher bond strength compared to SiO2-SiO2 was confirmed by shear test. Plasma activation is important process for hybrid bonding to enhance bonding strength and reliability of packages. We investigated physicochemical state of PI surface after plasma activation treatment process before bonding by Surface Free Energy (SFE) and X-ray Photoelectron Spectroscopy (XPS) analysis process so as to try to understand the mechanism of PI-PI bonding. From the results of the analysis, it became evident that plasma treatment could increase the hydrophilicity and hydrogen-bonding energy of the polyimide surface. Moreover, amount of reactive functional group such as carboxyl group and amino group increased after plasma treatment. From this evidence, we have proposed bonding mechanism via covalent bonding formation between two polyimide interface. According to process optimization of bonding process, we investigated relationship between bonding pressure and temperature to find sweet spot at prebonding process. As a result, bonding at pressures from 1.6 MPa to 8 MPa and temperatures from 40C to 250C offered in good yields in case of low modulus PI (PI-A), while high modulus PI (PI-B) showed good bonding yield at pressures from 1.6 MPa to 8 MPa and temperatures from 80C to 250C. In addition, CTE of PI larger than Cu is also important in order to complete Cu/PI hybrid bonding in annealing process. Cu/PI wafers with copper protrusion geometry whose surface is controlled by a chemical mechanical polish (CMP) process have been fabricated. We prepared different height of Cu protrusion after CMP and demonstrated C2W Cu/PI hybrid bonding at low temperatures in case of 5 to 15nm Cu protrusion height without voids at the bonding interface. Hence, it is expected that these materials could be applied to future packaging technology at low temperature and contribute to reduce warpage of substrates as well as to decrease the thermal damage for the devices.Hitoshi Araki received Master of degree and joined Toray Industries in 2005. He moved to University of Illinois for research of flexible electronics from 2014 to 2016 by study abroad program of Toray. Returned to Toray in 2016, and from there to the present he has been involved in research on polyimides for semiconductor packaging.