Photosensitive Polyimide for Packaging Applications

Transcription

1 Journal of Photopolymer Science and Technology Volume 28, Number 1 (2015) SPST Photosensitive Polyimide for Packaging Applications Masao Tomikawa, Ryoji kuda, and Hiroyuki hnishi Electronic & Imaging Materials Research Laboratories, Toray Industries, Inc Sonoyama, tsu-shi, Shiga-Ken , Japan Masao_Tomikawa@nts.toray.co.jp Recently packaging technologies progress so much for high density, low profile and small foot print. For those applications, Photo sensitive polyimide (PSPI) applied as a re-distribution layer for the package. The PSPI was required to have good adhesion to wiring metal with low temperature curable nature. We developed the PSPIs with low temperature curability for emerging packaging applications. Keywords: Positive-tone photosensitive polyimide, low residual stress, low temperature curable 1. Introduction Polyimide is widely used as a surface protective layer, interlayer dielectrics of semi-conductor devices for its excellent heat stability, electrical and mechanical properties [1]. For those applications, polyimide was required to obtain suitable pattern. So photo sensitive polyimide (PSPI) which was able to obtain the polyimide pattern by using common photo lithographic technique was required. There are two types of PSPI. ne is negative tone PSPI of which exposed area is remained. The other is positive tone type PSPI of which unexposed area is remained. Negative-tone photosensitive polyimide was applied due to obtaining highly reliable films. Positive tone photosensitive polyimide is used widely because of its fine pattern forming property and environmental friendly alkaline development. Recently, progress of semiconductor packaging is drastic for developing multi-layer stacking and paper like thinning, and fan out type wafer level packaging [2]. For those packages chip bending due to stress miss-match was severe issue. To overcome this issue, low stress buffer coating and re-distribution layer materials were desired. Low temperature curable positive tone photosensitive polyimide is one of good candidates for those applications. In this article we studied for characteristics of positive-tone photosensitive polyimide that satisfies the needs mentioned above by molecular designing of polyimide polymer and cross-linking agent. 2.Experimental 2.1. Polyimide polymer preparation Polyimide can be obtained by reacting tetracarboxylic dianhydride and diamine in N-metyl-2-pyrolidone ( NMP, Mitsubishi Chemical)at 30wt% solid contents. Those mixture was completely imidized at 160 for 4 hours under N 2 gas flow, to obtain polyimide polymer. The chemical structure of the polymer is shown in Figure 1. The molecular weight of the polyimide was measured by gel permeation chromatography (GPC). The GPC measurement was carried out by Waters Model510 with TSK Gel a2500+ Received April 10, 2015 Accepted May 11,

2 TSK-Gel-a4000 (Toso) at 40. NMP contains 50 mmol/l of LiCl; 50 mmol/l of H 3 P 4 was used as an eluent. The flow rate of eluent was 0.4mL/min. Standard mono-disperse polystyrene (Toso) was used for the molecular weight calibration. The GPC chart of the obtained polymer is shown in Figure 2. N X Figure 1 Polyimide structure used in this work. X or/and Y : rigid and flexible structure. Mw :15700 Mn :7430 Mw/Mn :2.11 N Y hydroxide solution (TMAH) at Curing method Soft-baked 7um PSPI coated on a wafer was cured at clean oven(koyo Thermosystem), min under N2 atmosphere. In comparison, PW-1500 (positive-tone photosensitive polyimide precursor, Toray) was evaluated as a reference. We obtained PW-1500 film on an 8 in. Si wafer at the same conditions, the film was cured at min Evaluation of Adhesion PSPI was coated on wafer, then soft-baked at 120C for 3min. Soft-baked wafer was exposed by i-line stepper with test pattern reticle. The exposed wafer was developed by TMAH and cured at 200C for 1hr as shown in pattern formation.. Adhesion strength was measured as shown in Fig. 3. Equipment: DAGE Arctek Shear tester series 4000 Load cell: BS250 Test Speed; 100um/sec Height of rod from substrate: 1um Figure 2 GPC chart of Polyimide 2.2. Low temperature curable Positive-tone photosensitive polyimide preparation PSPI was obtained by following procedure. The dried polyimide polymers (3.51g), naphthoquinone diazide sulfonate (DNQ, 0.62g), cross-linker (0.12g), were mixed into 5.75g of gamma butyrolactone (Mitsubishi Chemical). The solution was filtered through a 1.0um pore poly (tetrafluoro-ethylene) filter to remove the gel particles Pattern formation The PSPI was coated on an 8-in Si wafer by a spin-coater (ACT-8, Tokyo-Electron). Then, the coated wafer was soft-baked at sec on a hot plate (Act-8). The baked film thickness was measured by STM-802(Dainippon-screen). The soft-baked wafer was exposed by an i-line stepper (Nikon,NSR-2005i9C) from 200mJ/cm2 to 800mJ/cm2. The exposed wafer was developed by 2.38% tetra-methyl ammonium Figure 3 Adhesion test procedure 2.6. Evaluation for chemical stability In case of using PSPI as re-distribution layer, it requires to have not only high heat stability but also stability of various chemicals treatments for solder bump formation. We evaluated pattern PSPI cure film immersed in various types of chemicals for defined conditions. Then, the treated PSPI film was observed by micro-scope to check crack and measured film thickness by STM Packaging test The PSPI was coated on Daisy chain TEG as shown in Fig. 5. Then thermal cycle test was carried out from -40C to 125C for 650cycles. Resistivity was measured to check open failure. 74

3 Table1. Physical properties of cured PSPI Figure 4 Cross section structure of Packaging test Figure 5 Packaging test procedure 3. Results and Discussion 3.1. PSPI preparation Conventional positive-tone photosensitive polyimide is composed of alkali soluble polyimide precursor and napthoquinonediazide photosensitive compound (DNQ) [3, 4]. To complete imidization, it requires to be heated more than 300, and it has high residual stress. To improve these characteristics, we designed the pre-imidized polyimide polymer which has low residual stress but soluble to alkaline developer, and flexible structured cross-linking agent. We developed various types of low temperature curable PSPIs for customer requirements. LT-6100 is focused on low residual stress. LT-6300 shows high photo speed. LT-6500 is focused on High Tg. LT-6600 is balanced type Evaluation of hardened layer Table 1 summarizes the properties of low temperature curable PSPIs. Particularly, PSPI has lower residual stress and lower temperature curable than those of conventional PSPI[5] with almost same mechanical and thermal properties. 3.3 Chemical resistance Furthermore, chemical resistance test of PSPI was investigated. Investigated chemicals were as follows; N-methylpyrolidone (NMP), propyleneglycol monomethyl ether (PGME), ethyl lactate (EL), isopropyl alcohol (IPA)were used. As Resist stripper, we chose monoethanol amine and dimethyl sulfoxide mixture (TK106), 25 % NaH, 2.38 % TMAH for alkaline, aqueous mixture of sulfuric acid and Hydrogen peroxide(4:1 respectively), 1%Hydrofluoric acid aqueous solution as Etchant. The chemical resistance test was performed just dipping the cured Si wafer in the solution as a fixed condition as shown in Table 2. PSPI shows good chemical resistance. Table 2.Chemical resistance of LT-PSPI layer Chemical resistance NMP rt/15min no change Solvent PGME rt/15min no change EL rt/15min no change IPA rt/15min no change Resist stripper TK106 rt/15min no change Alkakine 25%NaH rt/15min no change 2.38%TMAH rt/5min no change Etchant H2S4/H22 rt/5min no change 1%HF rt/5min no change 3.4 Evaluation for pattern shape PSPI was coated on 8inch Si wafer at the same condition as 2.3. Pattern after cure from PSPI without cross-linker shows round shape due to flow during cure process (Figure 6). In order to suppress the flow, we investigated addition of heat cross-linkers. The patterned wafer was cured under N 2 atmosphere at 170 ~ min. to compare pattern shapes, in a clean oven (Koyo Thermosystem). The SEM photos of the obtained PSPI patterns (5 m thickness after curing, 3 m and 5 m mask size) are shown at Figure 7. As shown in the Figure, 75

4 they show good pattern shapes and those taper angles are about 60. Table 5 Adhesion of LT-6600 Figure 6 Pattern profile of PSPI Left without heat cross-linker Right with heat cross-linker Figure 7 Pattern profile of obtained PSPI 3.5. Evaluation for Adhesion Adhesion of PSPI to various substrates such as Si, Si, SiN and Cu is quite important for semiconductor applications. In general, siloxane couplers were used for improvement of adhesion to Si, Si and SiN [6]. Aluminum chelate was added for metal and Si substrates [7]. In order to obtain good adhesion to various substrates, we investigated various types of couplers and other chemicals. We measured the adhesive strength of PSPI by following the Figure 3. The results were summarized at Table 3, 4, 5. As shown in the Tables, PSPIs exhibit good adhesion to various types of substrates. Table 3 Adhesion of LT-6100 Table 4 Adhesion of LT Evaluation for Packaging reliability We evaluated the Packaging reliability by using daisy chain TEG. Cross section structure of the TEG was shown in Figure 4. Results were summarized at Table 4. As shown in the Table, we did not observe any failure after 650cycles. The PSPIs show good reliability after thermal cycle test. Unfortunately we did not observe the effect of residual stress on reliability. This result means that reliability of low temperature curable PSPI shows as same as that of conventional PSPI which was used as packaging re-wiring layer [5].. Table 6 Result of thermal cycle test 4. Conclusion We successfully developed a positive-type photosensitive material that is curable under 200, by compounding flexible structured polyimide resin, photosensitive DNQ, and cross-linking agent reacting below 200. The obtained PSPIs have drastically small residual stress compared to conventional PSPI. It also has good chemical resistance to each common chemicals, and heat stability up to 300. These properties exhibit that PSPIs are suitable for low stress buffer coating and re-distribution layer materials for new packaging applications. References 1. J. H. Kim, J. Photopolym. Sci. & Technol., 22 (2009) K. Yoshinaga, M.Nomura Science & Trends, 23 April T. Yuba, R. kuda, M. Tomikawa and J-H. Kim, 76

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