Development of Photosensitve Polyimide B-stage Sheet Having High Cu Migration Resistance

Transcription

1 Journal of Photopolymer Science and Technology Volume 30, umber 2 (2017) C 2017SPST Development of Photosensitve Polyimide B-stage Sheet Having High Cu Migration Resistance Masao Tomikawa *, Kazuyuki Matsumura, Yu Shoji, Yoshiko Tatsuta, and Ryoji Okuda Electronic & Imaging Materials Research Laboratories, Toray Industries Sonoyama, Otsu-City, Shiga-Ken , Japan Phone: *Masao_Tomikawa@nts.toray.co.jp Fan-out type wafer level packaging (FO-WLP) technology is one of promising next generation semiconductor package. The FO-WLP technology requires fine pattern pitch re-distribution layer (RDL) with good electrical insulation. In order to meet those requirements, we examined Cu migration resistance of two types of polyimides and photosensitive system under high temperature, high humidity, and high electronic field in order to meet the requirement electrical insulation of fine pattern pitch. We found that there is no large difference between negative photosensitivity and positive photosensitivity. Polyimide structure significantly affects Cu migration under bias HAST condition. From these results, we will describe highly reliable PSPI dry film and coatings. From those results, we developed a reliable positive tone photosensitive polyimide B-stage dry film. Keywords: Polyimide, Insulation, Reliability, Migration 1. Introduction Recently a Fan Out Wafer Level Packaging (FO-WLP) technology applys for various types of semiconductor devices, such as power controller, RF device, and application processor of smart phone [1]. Because FO-WLP offers low profile of package with high reliability, so it is desirable for apply for the package in the smart phone. FO-WLP was composed of Si chip was placed on re-distribution layer (RDL) and molded by resin. In general process flow of FO-WLP is placing a Si chip in a mold resin, then RDL was formed on the Si chip and mold resin. So due to thermal stability limitation of molding resin, RDL process should be below 200 o C. Photosensitive polyimide or PBO were good candidates for RDL insulator due to its good mechanical properties. However most of polyimides and PBOs were required rather higher curing temperature. So low temperature curable photosensitive polyimides and PBOs with mechanical toughness were desired and reported [2]. In addition it is desired to show good insulation properties. Especially the FO-WLP for an application processor required good insulation property at fine Cu pattern [3]. In this paper we are examined the effect of polyimide structure and photosensitive system (negative and positive) on Cu migration at high temperature humid condition under high electronic field. 2. Experimental 2.1. Polyimide preparation The polyimide resin was obtained by polycondensation of tetracarboxylic dianhydrides and diamines. A fixed amount of diamines were placed in a 4 neck flask with a mechanical stirrer, thermometer and nitrogen inlet, and dissolved into -methyl-2-pyrridone (MP, Mitsubishi Chem.) and heated to 60 o C under nitrogen flow. A fixed amount of tetracarboxylic dianhydrides was added to the diamines solution with MP at 30 wt% concentration. The mixture was stirred for 1 hour at 60 o C, then heated to 180 o C. The polycondensation reaction was carried out at 180 o C for 4 hour. After cooling the poiyimide solution to room temperature, the solution was poured into the water to precipitate the polyimide. The polyimide precipitate was collected by filtration and washed Received April 2, 2017 Accepted May 12,

2 J. Photopolym. Sci. Technol., Vol. 30, o. 2, 2017 by water 3 times. The obtained polyimide was dried at 80 o C for 72 hour in a convection oven Photosensitive polyimide solution preparation The photosensitive polyimide solution was obtained by the following procedure. Polyimide was measured 10 g then dissolved in a -butylolactone (GBL, Mitsubishi Chem,) at 35 wt% concentration. (a) (b) (c) (d) RO H 3 COH 2 C R= H 3 COH 2 C SO 2 O 2 CH 2 OCH 3 CH 2 OCH 3 CH 2 OCH 3 OR OR : H = 2.8:0.2 CH 2 OCH 3 Fig. 1. Chemical structures of (a) TKF-280, (b) MW-100LM, (c) OXE-02, and (d) BPE-6. A diazonaphthoquinone compound (TKF-280, Sanbo Chem. 1g) and thermal cross-linker (MW-100LM, Sanwa-Chem, 0.4 g) were added to the solution. The obtained solution was filtered by 0.47 m PTFE filter prior to use. A negative tone photosensitive polyimide solution was obtained by following procedure. Polyimide was weighed about 10 g and add GBL to obtain 35 wt% solution. A photo-iniator (OXE-02, BASF), 0.3 g and acrylic monomer (BPE-6E, Shin-nakamura Chemical), 2 g were added to the solution. The solution was filtered prior to use by 0.47 m PTFE filter. Chemical structures of those additives are shown in Fig. 1. UV-vis spectra were taken on a Shimadzu UV-2400 PC. The patterns of the resists were observed using a Keyence digital microscope VHX-200. Screen printing was carried out using a screen-printing machine (LZ-1232, ewlong Seimitsu Kogyo Co., Ltd) Photosensitive dry film preparation Photosensitive dry film was prepared by the following procedure. The photosensitive polyimide solution with various types of additive (0.3 g) was mixed and coated on surface treated PET film at 50 m wet thickness by bar coater. The coated film was dried in a convection oven at 80 o C for 15 min. The dried film was used as a photosensitive dry film. The photosensitive dry film was laminated on a Si wafer by pressing the dry film at 120 o C by hand-roller. The laminated photosensitive polyimide was exposed at 300 mj/cm 2 (@ 365 nm) by a contact aligner through a photo mask. The exposed photosensitive dry film was developed by 2.38% tetramethylammonium solution (TMAH) at 23 o C for a fixed time. It was then cured at 120 o C for 10 min and heated to 200 o C and stored 200 o C for 1 hr under nitrogen flow Cu migration measurement The photosensitive polyimide solutions were coated on a TEG with a comb-type electrode (Philtech) to cover the pad area by scotch tape to coat completely by spin coating method then heated to 180 to 250 o C for 1 hour under nitrogen flow. The electric resistance change (Bias HAST test) was monitored by SIR-13 system (ESPEC corporation) under 130 o C 85% HAST condition at 2 MV electric field strength. Test piece structure was shown in Fig

3 J. Photopolym. Sci. Technol., Vol. 30, o. 2, 2017 (a) Fig. 2. Cu migration test structure. 3. Results and discussion 3.1. Polyimide preparation The obtained polyimides molecular weights were measured by GPC to check the reaction. In this work, the molecular weights of polyimides were distributed from 20K to 40K at Mw. This molecular weight range is suitable for photo-lithographic performance. We changed the diamine structure and dianhydride structure from flexible/soft to rigid. The rigid structure was created with an aromatic ring or aliphatic ring. The soft structure was created with alkyene groups. 3.2 Bias HAST test Results of bias HAST test examples are shown in Fig. 3 and summarized in Table 1. As shown the figure, the polyimide having the soft segment exhibits better reliability. We examined the tested TEG and found cracks in the TEG using the rigid polyimide after HAST test. We expected that the rigid polyimide would show better resistivity because of the rigid structure. During the HAST treatment, the rigid polyimide cracked due to poor Cu adhesion or low elongation to break. We expected the polyimide having the soft segment to show lower reliability due to its low Tg. Because the molecular motion of a low Tg polyimide is not restricted, the polymer motion allowed for low break down voltage in general [4]. However, the soft polyimide shows good insulation reliability. We think following facts are considered. One is that the 114 o C Tg of the soft polyimide is still too high for complete molecular relaxation at HAST condition. The other fact is the soft polyimide shows intrinsic high resistivity. To understand further, reliability investigations of polyimides having a wide range of Tg are required Preparation of photosensitive sheet In order to obtain a positive tone photosensitive dry film, we examined various types of additives to (b) (c) (d) Fig. 3. Resistivity change of (a) Positive-tone, Rigid Polyimide, (b) egative-tone, Rigid Polyimide, (c) Positive-tone, Soft Polyimide, (d) egative-tone, Soft Polyimide. 183

4 J. Photopolym. Sci. Technol., Vol. 30, o. 2, 2017 Table 1. Summary of this work decrease melt viscosity at B-stage. The examined compounds were epoxy resin, oxetane compound, methyrol compound. The positive tone photosensitive polyimide was obtained by mixing with diazonaphthoquinone photo active compound and polyimide. In case of positive tone photosensitive chemistry, unexposed area remains due to dissolution inhibition by the diazonaphthoquinone compound. The diazonaphthoquinone compound is decomposed to indene carboxylic acid by UV exposure [5]. So the remaining polyimide pattern is composed of heat-decomposed diazonaphthoquinone compound and polyimide. Hayase et. al. reported that negative tone photosensitive polyimide pattern was obtained by mixing with polyimide precursor (Poly(amic acid)) and diazonaphtoquinone compound with relatively high post bake temperature (140 o C) by diluted TMAH solution [6]. We expect the decomposition of diazonaphthoquinone compound at heat curing process was initiated to release nitrogen and form a carbene. The carbene was reactive intermediate to react with polyimide to form inert resin. But there is other possibility to form sulfonic acid to decompose the diazonaphthoquinone compound. We checked the ph of the cured polyimide resin to immerse in water. The ph is neutral (about 7), so we conclude the sulfonic acid was not formed in this experiment. In case of negative tone photosensitive polyimide, radical type photo initiator,, acrylic monomer and polyimide were used. Muramatsu et. al. examined the photo reaction of oxime ester type photo-initiator [7]. In their work, photo initiating product was secondary decomposed methyl radical. Then, acrylic monomer was converted to cross-linked network with polyimide by initiating radical. After forming the cross linking network, molecular motion of the polyimide was limited due to network formation. So, the networked polyimide shows good electrical insulation. From these results, we successfully developed a positive high reliable photosensitive polyimide B-stage dry film. The B-stage dry film covers the topological surface completely and forms a 10 m fine pattern resolution in a 22 m thick film (Fig. 4) and shows fine pattern resolution (Fig. 5). Fig. 4. Cu migration test structure. Fig. 5. Positive photosensitive polyimide pattern after Cure (10 m bia with 22 m thick). 4. Conclusion We investigated the effect of polyimide structure having soft segment or hard segment on the reliability of electronic resistance under HAST condition. A polyimide having soft segments shows better reliability than polyimides having hard segments. This is due to the Cu adhesion or polyimide elongation. From these results, we developed reliable photosensitive polyimide B-stage dry film with positive photosensitive. References 1. C. Lu, th International Microsystems, Packaging, Assembly and Circuits Technology Conference (IMPACT), 184

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