213 Original scientific paper  MIDEM Society Temporary Bonding Using Paper Inserted PPC Layer Zhiyuan Zhu 1 , Kequan Xia 1 , Zhiwei Xu 1 , Hongze Zhang 3 , Haijun Lou 2 1 Ocean College, Zhejiang University, Zhejiang, China 2 Institute of Advanced Technology, Zhejiang University, Zhejiang, China 3 Nanjing Electronic Devices Institute, Nanjing, China Abstract: Temporary bonding using paper inserted polypropylene carbonate (PPC) layer is demonstrated. The inserted paper layer can absorb photo acid generator (PAG)-induced acid and protect the substrate. Large improvements of bonding strength are achieved using paper inserted PPC layer. Especially, the bonding strength is much higher than that of PPC/PAG-PPC bonding for tissue paper. The results show that the paper fibers can absorb decomposed PPC and PAG-induced acid, thus protecting the substrate. Keywords: temporary bonding; debonding; polypropylene carbonate Začasno bondiranje z uporabo s papirjem vnesene PPC plasti Izvleček: Prikazano je začasno bondiranje z vnosom plasti polipropilen karbonata (PPC) s pomočjo papirja. Vložena papirna plast lahko absorbira kislino generatorja fotokisline (PAG), s čimer zaščiti substrat. Uporaba PPC plasti omogoča velike izboljšave vezne trdnosti bonda. Zlasti je vezna trdnost precej višja kot pri uporabi PPC/PAG-PPC tkanine. Rezultati kažejo, da lahko papirna vlakna absorbirajo razgrajeno kislino, ki jo povzroča PPC in PAG, s čimer se zaščiti substrat. Ključne besede: začasno bondiranje; debondiranje; polipropilen karbonat * Corresponding Author’s e-mail: xuzw@zju.edu.cn, Journal of Microelectronics, Electronic Components and Materials Vol. 48, No. 4(2018), 213 – 216 1 Introduction Silicon wafer is the fundamentals of semiconductor industry [1-3]. Particular, the application of thin silicon wafers is an enabling technology for modern electron- ics as reduced wafer thickness integration enables shorter processing times and less cost [4-6]. All of this is advantageous for a low package height for chip cards, the requirement for higher power, and the search for System-in-a-Package (SiP) using chip stack methods. Temporary bonding and debonding are essential for fabricating thin wafers [7, 8]. On the one hand, the bonding strength should be high in order to endure the mechanical polishing/thinning. On the other hand, debonding process is required to be simple, low cost and of high reliability. To achieve the above require- ments, we propose photo acid generator (PAG) loaded polypropylene carbonate (PPC) debonding at room temperature without any solvent based or ultraviolet (UV) treatment [9]. Subsequently, PPC/PAG-PPC bond- ing structure is proposed [10]. The inserted PPC layer serve as the passivation layer, which can absorb the PAG-induced acid. The bonding layer loses adhesion to the substrate and automatic debonding is achieved when center region of the PPC layer is decomposed. The PAG-induced acid is separated from the bonding substrate, which improve the bonding strength and protect the substrate. Application of PPC as temporary adhesive for fabrication of sensor chip has also been demonstrated [11]. The main aim of this work is to develop a method to further increase the PPC temporary bonding strength. Herein, we propose temporary bonding using paper inserted PPC layer. Further improvement of bonding strength is achieved. https://doi.org/10.33180/InfMIDEM2018.403 214 2 Experimental Temporary bonding and debonding using paper in- serted PPC layer is proposed to protect the substrate and improve the bonding strength. The bonding struc- ture is shown in Fig. 1a. The PPC polymers (QPAC 40) used in this work were obtained from Empower Materi- als and have a molecular weight of 196 kg ∕ mol. The PPC solution was prepared by dissolving pure PPC in acetone under constant stirring to form a 20 wt% so- lution. Tetrakis(penta-fluorophenyl) borate-4-methyl- phenyl [4-(1-methylethyl)-phenyl] iodonium tetrakis (pentafluorophenyl) borate is used as the PAG and ob- tained from tokyo chemical industry Co. The PAG-PPC solution was prepared by adding PAG (about 1 wt% of the PPC) to the PPC solution. Printer paper, tissue paper and silicone paper are used as the paper insertion layer. The main chemical compositions of the paper insertion are cellulose, hemicellulose, lignin, calcium carbonate, etc. The paper fiber can interlace with PPC polymer and absorb PAG-induced acid. Silicon paper is fabricated by treating printer paper with silicone, so it is nonstick and heatproof. The glass substrates (10mm×10mm) are obtained from Dongsheng Corporation with density of 2.2 g/cm 3 , tensile strength of ~ 50 MPa and inflection resistance of 60-70 MPa. The PPC solution was poured onto the glass substrate facade. The acetone was then removed by baking the specimen for 10 min at 60 °C. Subsequently, paper layer is attached on PPC surface, followed by dropping of PAG-PPC solution and sol- vent evaporation. Bonding of the fabricated samples is achieved using a hot press (Carver, Inc.) at bonding pressure of 0.2MPa. The bonding temperatures for ther- mal bonding 1 and 2 are 100 ℃ and 200 ℃, respectively. The morphology of the substrate surface is examined using Hitachi TM3000 scanning electron microscope (SEM). 3 Results and Discussions The decomposition products of PPC include cyclic propylene carbonate (cPC), propylene glycol, and CO 2 , which indicates that decomposed PPC is liquid at room temperature. The (SEM image of typical paper fiber is shown in Fig. 2. It is obvious that the cellulose fiber of paper can absorb decomposed PPC and PAG- induced acid. Thus, PPC is less consumed and there is PPC remained beneath the paper layer, the substrate is protected and the bonding strength is improved. Un- der high bonding temperature ( ＞170 ℃, represent as thermal bonding 2 in Fig. 1a1), sufficient PAG-induced acid is produced, consuming the entire PPC layer be- tween paper layer and achieving automatic debonding at room temperature without any solvent based or UV treatment. Under low bonding temperature ( ＜130 ℃, represent as thermal bonding 1 in Fig. 1a2), a portion of PAG is activated and less PAG-induced acid is produced, and a considerable portion of acid is also absorbed by paper fiber. Thus, a subsequent UV irradiation or heat- ing is required to achieve complete debonding. Figure 1: Schematic of temporary bonding and debonding process a PPC/Paper/PAG-PPC bonding structure a1 Bonding at high temperature( ＞170 ℃) and subse- quent automatic debonding for PPC/Paper/PAG-PPC structure a2 Bonding at low temperature( ＜130 ℃) and subse- quent debonding for PPC/Paper/PAG-PPC structure Figure 2: SEM image of typical paper fiber Glass substrates are used to demonstrate and compare the bonding results. The results are shown in table 1. It a a1 a2 120µm Z. Zhu et al; Informacije Midem, Vol. 48, No. 4(2018), 213 – 216 215 can be seen from table 1 that large improvements of bonding strength are achieved using paper inserted PPC layer. This effect is especially obvious for tissue pa- per, which absorb more PAG-induced acid. For printer paper and tissue paper, the bonding strength decrease significantly when bonding temperature increases to 200 ℃, which is probably caused by damage of fiber structure. On the contrary, there is no significant de- crease of bonding strength of silicone paper inserted PPC layer, which can be attributed to the heat resist- ance property of silicone paper. In order to adjust the debonding time, heating or UV irradiation processes after bonding is used. All the bonded samples fabricated in this work are success- fully debonded. Table 1: Comparison of the bonding strength of differ- ent methods demonstrated using glass substrates Ref. Bonding strength (MPa) Thermal bonding 1 Thermal bonding 2 Printer paper in this work 3.9±0.45 MPa 1.7±0.33 MPa Tissue paper in this work 4.1±0.29 MPa 2.3±0.12 MPa Silicone paper in this work 3.5±0.19 MPa 3.2±0.43 MPa PAG-PPC bonding in [9] 3.0±0.34 MPa 2.1±0.18 MPa PPC/PAG-PPC bonding in [10] 3.3±0.26 MPa 2.7±0.25 MPa The surface morphologies of debonded samples were observed by SEM. Fig. 3 shows the SEM image of typi- cal surface morphology of debonded samples for PAG- PPC bonding under thermal bonding 2. PPC residue was observed. The results correspond well with the bond mechanism which state that under high bonding tem- perature, excessive PAG-induced acid is produced, con- suming the PPC protection layer at multiple locations and and leaving PPC residue. The PAG-induced acid can cause damage to the substrate and decrease the reliabil- ity of circuit where PPC protection layer is consumed. Fig. 4 shows the SEM image of typical surface morphol- ogy of debonded samples for PPC/silicone paper/PAG- PPC under thermal bonding 2. The results show that the upper surface of paper is the debonding interface, which indicate that the PPC layer beneath paper inser- tion is hardly decomposed. This corresponds well with the bond mechanism which state that paper fibers can absorb decomposed PPC and PAG-induced acid. Thus, PPC is less consumed and there is PPC remained be- neath the paper layer, the substrate is protected and the bonding strength is improved. The improvements of bonding strength have also been demonstrated in Table 1. Figure 3: SEM image of typical surface morphology of debonded samples for PAG-PPC bonding under ther- mal bonding 2 Figure 4: SEM image of typical surface morphology of debonded samples for PPC/silicone paper/PAG-PPC under thermal bonding 2. 4 Conclusion This letter proposes temporary bonding and debond- ing using paper inserted PPC layer. The paper insertion can absorb PAG-induced acid and increase bonding strength. In order to adjust the debonding time, heat- ing or UV irradiation processes after bonding is used. 5 Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 61804132). The author would like to thank the Animal Science Experimental Teaching Center of Zhejiang University for SEM charac- terization. The author would like to thank Prof. Yufeng Jin and Prof. Min Yu from Peking University for their help in previous work. The author would also like to Z. 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