Low D_k and D_f Photosensitive Insulation Material for Low Transmission Loss Packages Open Access

Today's packaging technologies need high speed interconnections among the different kinds of devices, specially (a) high frequency interposers and (b) mm-wave modules for next-generation mobile communication (5G). These applications require low electrical loss and fine circuits in order to realize high speed signal transmission and high density interconnections. The 3D vertical integration of DRAM on logic and side by side integration using silicon interposer (2.5 D) with TSV (Through Silicon Via) have been studied. However, TSV technology faces some issues such as poor high-frequency properties and high fabrication cost. Organic interposer (2.1D) and fan-out package with embedded high density circuitry is one of the key solutions for the trend of electronic package development ¹⁾ .

Current thermosetting materials with low D_k (dielectric constant) and D_f (dielectric tangent) are limited to via size larger than 20 μm by the laser compatibility. The cost of laser equipment is another bottleneck to be solved. On the other hand, photosensitive insulation material demonstrates fine via by photolithography process, however photosensitive insulation material is generally not applicable for high frequency applications because of the poor electrical properties.

To realize the organic interposer and fan-out package for both high frequency of transmission signals and high density interconnection, we have newly developed photosensitive insulation materials having low D_k and D_f as well as excellent lithographic and insulation properties. In this paper, we have newly developed three kinds of material having low D_k and D_f photosensitive insulation materials. The circuitry was assembled by semi-additive plating process (SAP) to check the process compatibility of the developed material. Then, the electrical performance was evaluated to fabricate micro-strip line using each material. Finally, the insulation reliability was checked by biased-HAST.

The material was designed using our accumulated resin solubility technology of core materials^2,3) . The low D_k and D_f photosensitive insulation material in this paper is negative tone using organic solvent developer to realize excellent electrical performance including excellent moisture absorption reliability. We have newly developed three kinds of material having different mechanical properties and electrical loss as summarized in Table 1. The D_f of Material A, B and C are 0.0018, 0.0035 and 0.0050 at 10 GHz, respectively. The D_k and D_f values of the materials were measured by split post dielectric resonator (SPDR) method. The glass transition temperature (T_g) of Material B and C are 190 and 200 °C, respectively. The T_g was evaluated by thermomechanical analysis. Fig. 1 shows the cross-sectional scanning electron microscope (SEM) and energy dispersive x-ray spectrometry (EDX) images after photolithography on copper. The photosensitive insulation material shows excellent resolution without any residue.

The circuitry was assembled by semi-additive plating process (SAP) using 300 mm wafer as shown in Fig. 2. The developed materials were coated on silicon wafer and the Ti/Cu seed layer was formed by sputtering. Then, the Line and Space(L/S) of 5/5 μm circuitry was fabricated by resist patterning, copper plating, resist stripping and seed etching. Fig. 3 and Fig. 4 show the overview and microscope observation results. The 5/5 μm circuitry was demonstrated without any material swelling, crack and delamination. Material C demonstrated chemical resistance and process compatibility for SAP fabrication.

The electrical properties of packaging become increasingly important. As a semiconducting material, standard silicon tends to have increased loss at higher frequencies. In this work, micro-strip lines with 2 vias were constructed on the developed materials, and impedance matched to 50 ohm. The structures where tested up to 20 GHz and transmission loss was characterized. The measurement results of Material C with D_f=0.0050 and the reference material with D_f=0.0350 are shown in Fig. 5. The transmission loss from transmission lines on Material C shows much lower than the conventional reference material at higher frequencies.

The insulation reliability is another key technology to realize the fine wiring using organic material. In the previous study, we evaluated insulation reliability below 2/2 μm using various kinds of materials and studied the required material properties to pass biased HAST for 200 h ^4)–6) . The newly developed photosensitive materials have quite low moisture absorption rate, low anion impurities, which satisfied the criteria to pass biased-HAST for 200 h on 2/2 μm. The test structure shows in Fig. 6. The 2/2 μm circuitry was covered with the cured material. The condition of biased HAST is 130 °C, 85 %RH and 3.3 V. Fig. 7 shows the electrical resistance changes of 2/2 μm circuitry during biased HAST. The developed material passed the criteria, which is the electrical over 10⁷ Ω over 300 h. The Cu migration was observed by STEM-EDX (Scanning transmission electron microscope energy dispersive X-ray spectrometry). Fig. 8 shows the overview of 2/2 μm wiring covered with Material C before (a) and after (b) biased HAST for 300 h. Fig. 9 shows the elemental mapping of the area around the 2/2 μm after biased HAST. The carbon and copper element mapping are reasonably assigned as Material C and the circuit line. The copper elemental mapping indicates no electrical migration under biased HAST condition. From these results, Material C realized excellent insulation reliability of 2/2 μm line/space.

We have newly developed low D_k and D_f photosensitive insulation materials. The dielectric tangents(D_f) of Material A, B and C were 0.0018, 0.0035 and 0.0050 at 10 GHz, respectively. The photosensitive insulation material demonstrated excellent resolution and process compatibility for SAP fabrication. The transmission loss from transmission lines on Material C shows much lower than the conventional reference material at higher frequencies and achieved excellent insulation property of 2/2 μm line/space.