Novel Low Temperature Curable Photo-Patternable Polyamide with High Planarity for Wafer Level Fan-Out Packaging (WLFO)

WLFO has expanded its design applicability due to increased demands on package functionality, advantages of cost reduction and enhanced capabilities for 2D and 3D technologies WLFO packages are designed to offer increased I/O densities with a reduced die footprint, through higher resolution capabilities from wafer-level processes. [1] Development of 5G packaging technologies is pushing WLFO heterogeneous integration for RFIC, data communication, processing and storage. [2,3]

The most recent packaging designs have put a greater reliance on polymeric systems. The core of the WLFO package design technology is the use of reconstructed epoxy molded wafers with Redistribution layers. WLFO extends the interconnects beyond the chip surface allowing for a less restrictive design space at a lower cost. There remain many challenges, including yield loss and manufacturability as this technology extends to larger substrates and more complicated package designs that incorporate multi-chip, SiP, CoC or PoP. [4]

There are two main areas of the RDL polymer dielectric requiring further enhancement to meet future design, yield, and reliability targets. First; current and future polymer dielectrics must be able to overcoat high and severe topography. Different sized devices embedded in the same WLFO package causes issues of non-planarity which inhibit SiP integration. Second; non-uniform stress created during in-line wafer processing impacts the reconstructed wafer warpage and die stress. Further, warpage can be exacerbated by the mold compound if post processing temperatures are too high.

MicroChem Corp and Nippon Kayaku have jointly developed a novel photosensitive polyamide formulation called KMRD-015A for use as a redistribution polymer dielectric to address these issues (Figure 1.1). KMRD is a low temperature curable photo-patternable polymer layer capable of post cure thicknesses between 5μm to 15μm.

The negative-tone photosensitive Polyamide (PA) is patterned and exposed with a high-pressure mercury lamp at ~150 mJ/cm² that forms a latent image in the dielectric layer. This latent image is developed using an aqueous developer tetramethylammonium hydroxide (TMAH). The PA system uses a single stage cure with a peak temperature of 185 °C for 1 hour in a standard convection oven (Figure 1.2). During the curing process, the PA undergoes branched cross-linking followed by self-polymerization of the resin. The polymer structure and cross-linkage system results in low post cure shrinkage, ~3–5% when cured at 185 °C, and 8–10% when cured at 200 °C.

KMRD is capable of via patterning at aspect ratios close to 1.5. Minimum via pattern resolution is dependent on film thickness. 4um vias are capable with 5um thick films. 7 um wide vias are capable in 10um films. 9um wide vias are capable in 15um films and 15 um vias are capable in 20um thick films. KMRD offers a wide process latitude and ease of use. (Table 1.1)

The cured properties of various dielectrics are listed and compared in Table 1.2. Cure temperatures <200 °C are qualified as low temperature. The nominal cure temperature for KMRD is 185 °C, significantly lower compared to polyimide material. In-order to control inherent stresses within the dielectric layer, lower modulus and higher elongation to break are required. The maximum elongation percentage and the Young's modulus are 70% max, and 2.0 Gpa respectively. In the case of KMRD, the percent elongation is over twice the % elongation for some polyimides and equal to the literature elongation value for PBO based products. The Young's modulus for KMRD is approximately 60% lower compared with some Polyimides and equivalent to PBO based products. Mechanical properties are maintained for cure temperature 185 °C through 200 °C.

The glass transition temperature (Tg) for KMRD is ~240 °C using a 185 °C cure, or ~250 °C using a 200 °C cure (Figure 1.5). KMRD is also a good dielectric insulator with 3.2 dielectric constant and a 0.01 dissipation factor. The combination of low curing temperatures (more compatible with WLFO EMC Overmold process), the higher percent elongation (better toughness) and lower modulus (lower residual stresses) makes KMRD a good candidate for WLFO applications.

The chemical stability of the RDL is critical for manufacturability of the WLFO device. The RDL must be able to survive multiple photoresist stripping and etching steps in the initial device build. Further it must be able to survive various flux chemistry is the solder flux processing. Chemical stability was tested with no degradation in film loss or appearance for a variety of Acids and Alkaline's such as NMP, DMSO, Acetone, Sulfuric Acid, Potassium Hydroxide, Ammonium Fluoride (BOE) and Peroxide silicon etching. (Table 1.2).

The polyamide structure of KMRD most likely allows for enhanced adhesion to copper. Shear adhesion testing demonstrated excellent adhesion of KMRD to inorganic substrates such as SiN and Cu as well as very good adhesion to polymer layers such as polymides, epoxy and itself (Figure 1.6). The adhesion values indicate that KMRD is well suited for use in multi layers as needed for the RDLs in WLFO package designs.

The cured storage modulus values are virtually identical for KMRD cured at 185 °C and 200 °C. The storage modulus represents the rigidity of the material. Change in the modulus of the material with temperature indicates how the material will perform with regards to changes in temperature. Ideally there is minimal change in modulus with temperature. There is a relatively low change in storage modulus of KMRD when compared at low, mid and high temperature. The modulus or rigidity of the KMRD layer increased slightly at very low temperatures (−55 °C) with minimal change from ambient to elevated temperature. (Figure 1.8) This data supports the excellent temperature cycling performance demonstrated with minimal stress induced failure in the aggressive bow tie test structure evaluation even at 1000 and 2000 temperature cycles. (Table 1.3)

The temperature cycling test (TCT) results are highly dependent on the toughness of the film. Specifically, the TCT evaluates the polymers ability to not distort at high temperatures and not crack at low temperature. Typically during heating, wafer stresses are released as the temperature increases. After thermal processing, the wafer inherently yields a high stress value at ambient temperature. Further, during cooling the wafer can be stressed beyond its intrinsic property and can no longer support the film itself. The film cracks or delaminates until such stress is relieved. Reliability testing was completed using short-loop test vehicles for HAST (Highly Accelerated Temperature/Humidity Stress Testing) and TCT (Temperature Cycle Testing). In all tests performed no failure was found with KMRD, validating its performance for WLFO. Figure 1.7 is KMRD patterned with 15um wide vias on Cu and undergone TCT for 2000 cycles from −55°C to +125°C. There is no appearance of KMRD breakdown, no cracking or deformation. The protected Cu areas remain free of oxidative attack.

Adhesion by shear test was also performed post TCT and HAST. No degradation in the shear adhesion strength was measured. (Table 1.3.)

Figures 1.9 examines the interface between patterned KMRD and the Cu. The KMRD successfully protected the underlying Cu from attack during the HAST test conditions. No loss of adhesion/delamination is noted for KMRD at the interface.

KMRD demonstrates robust ability to planarize over large topography. Figure 2.0 shows 10um of KMRD planarizing over 40um wide Cu vias. KMRD films measure <10% shrinkage post-cure and have a residual stress value of ~20 Mpa. These values make KMRD well suited for applications requiring an overcoat over high and severe topography.

KMRD demonstrated excellent compatibility with solder flux processing and solder reflow temperatures in device build simulations. Good Cu adhesion was maintained post solder flux and reflow at 260 °C ×3. Figure 2.1a &b is a top down image of pattern Cu via interconnects along with a SEM image via post addition of a typical SAC (Tin-Silver-Copper) solder ball. No deformations or cracks were observed. The successful functional test correlates very well with the measured thermal and material analysis.

5G technology integration is driving aggressive packaging designs. New designs are creating challenges for packaging materials. Specifically, redistribution layers are inducing defects from excessive shrinkage due to high curing temperatures, low degrees of planarity and limited resolution at both thick and thin layer thicknesses. KMRD overcomes such issues and provides a high level of planarity with shrinkage values <10%. It is capable of via patterning at aspect ratios close to 1.5. KMRD provides low temperature, 185 °C, curing with robust performance for current and future WLFO designs.

MCC/NKC would like to acknowledge the UMASS Amherst for their use of facilities for measuring electrical performance as well as UMASS Lowell CRF for use of their FIB tool for generating our reliability data.