Characterization of PEO-X Ionic Conductive Polymer for Anodic

Transcription

1 Characterization of PEO-X Ionic Conductive Polymer for Anodic Bonding Characterization of PEO-X Ionic Conductive Polymer for Anodic Bonding Xu Yin 1, Cuirong Liu 1*, Yue Nan 1, and Qingsen Meng 2 1 Taiyuan University of Science and Technology, 2 Taiyuan University of Technology Received: 14 April 2014, Accepted: 9 May 2014 Summary Anodic bonding is a common technology in MEMS packaging. However, only the bonding between glass and metal or semiconductor can be implemented at present. In this work, a new type of composite, solid polymer electrolyte (taking PEO as the matrix with a small amount of nano-sized inorganic filler), was prepared as a new anodic bonding material. We studied the interaction and conduction mechanism between PEO and the inorganic filler through DSC and XRD analyses, and also discussed the application feasibility of the solid polymer electrolyte being packaging material in anodic bonding. The results showed that adding inorganic filler can reduce the interface resistance of the macromolecule solid electrolyte, and control the diffusion of alkali metal ions in the solid lithium molecule electrolyte when under a strong electric field. The anodic bonding technique may promote the application of this new macromolecule material in MEMS. Keywords: MEMS, PEO, Anodic bonding, Solid electrolyte 1. Introduction During the manufacturing process of complicated MEMS, packaging is an importance step which directly affects the device s life cycle and application scope 1. MEMS packaging has always been one of the key techniques nagged by MEMS device development and practical application. Exploration of new packaging technique has become a research focus in the field 2. Glass is an ideal anodic bonding material often used in MEMS packaging 3, including Pyrex glass and silica glass, etc. However, bonding with glass requires high temperature and may cause large package-induced stress which would damage the structure in MEMS during the front-end process. Besides, cost reduction is another big issue that need to be addressed. Promotion of anodic bonding technique mainly depends on the application of new material. Smithers Information Ltd., 2014 Compared to glass, macromolecule with large molecular weight possesses low density as well as superior mechanical and heat-insulating properties. The study of replacing glass with material for packaging is of great importance to develop a new rapid anodic bonding technique with lowtemperature and high-quality. 2. Principle of Anodic Bonding with Solid Electrolyte Material Solid electrolyte is also called super ionic conductor. Its crystal structure is generally composed of two sets of crystal lattice, one as solid crystal with skeleton ions and the other as sub-lattice with migration ions. In the migration sub-lattice, the defect concentration is up to 10 2 /cm. That is, the number of migration lattice site *Corresponding Author: Cuirong Liu, Taiyuan University of Science and Technology, Tel: , yinxujia@163.com is more than the number of migration lattice itself. Therefore, all the lattices can move around and further increase the carrier concentration. Besides, the lattice synergic movement may reduce the electric conductance activation energy and thus increase the conductivity. Anodic bonding technique (also called as field diffusion bonding) is based on this property of solid electrolyte. Anodic bonding, in nature, is a solid electrochemical reaction. The bonding process is illustrated in Figure 1. When heated up to the bonding temperature, the alkali metal lattice in the solid electrolyte undergoes dissociation, With an electric field applied between the solid electrolyte and the anode metal, the alkali metal lattice in the solid electrolyte moves quickly from positive pole to negative pole, separates from the negative pole, and forms a polarized alkali metal lattice depletion layer. The depletion layer has a thickness of several microns, located near the interface between the solid electrolyte Polymers & Polymer Composites, Vol. 22, No. 8,

2 Xu Yin, Cuirong Liu, Yue Nan, and Qingsen Meng Figure 1. Diagram of bonding between metal and solid electrolyte found that the conductivity between PEO and alkali metal salt complex was mainly generated in the noncrystalline phase plateau zone To improve the conductivity of the solid macromolecule electrolyte represented by PEO-MX, restructuring and crystallization of the PEO molecular chain shall be prevented, and the relatively-large amorphous region need to be stabilized during the bonding process Preparation of Solid Macromolecule Electrolyte 3.1 Required Raw Material Required materials for preparation of are listed in Table 1. and positive pole where the negative charges accumulate. Meanwhile, image charges are generated in the positive anodized surface and a strong electric field occurs in the solid electrolyte deletion layer. The voltage drop in the solid electrolyte mainly lies in the depletion layer, while the voltage drop in non-depletion layer is much smaller. The electric field in the depletion layer can be as high as 10 6 V/ cm. This electric field produces strong electrostatic attraction in the interface between the anodized metal and the solid electrolyte. Therefore, the anions in the depletion layer move toward the anodized metal surface. As a result, the depletion layer in the solid electrolyte has elastic deformation and viscose flow, and finally impinges on the metal surface and enables irreversible oxidizing reaction between the surfaces of solid electrolyte and metal 4. latter two compositions are called as composite electrolyte 5. The connection performance of solid macromolecule electrolyte primarily depends on the conductivity, the migrated lattice quantity and the diffusion coefficient of metal salt 6. Among the three factors, high lattice conductivity is the key for anodic bonding. As a kind of polymer subject, PEO (short for polyethylene oxide) can dissolve a lot of inorganic lithium and salt, and thus possesses high conductivity at room temperature. Also, PEO has the best complexing effect and has been applied widely 7. With X-ray diffraction analysis, we found that the combination of PEO and alkali metal lattice had a stoicheiometric relationship as (PEO) 4-MX. That is, the ratio of oxygen atom to metal atom is 4. The study also 3.2 Preparation Method The was prepared as follows. Firstly, PEO and β-al 2 powder were mixed with the ratio of 20:1 in an agate jar for ball-milling. The agate balls 2, 4, 6 (6:3:1) with a ball to material ratio of 8:1 were applied with the rotating speed of 300 r/min for 10 h. Then the mixture was dried at 80 C under vacuum for 48 h. Secondly, the sintered material was compressed in a pressing machine to make a disc with the diameter of 25 mm and the thickness of 2 mm. On the other hand, anhydrous LiClO 4 was dissolved into a mixed solution of acetonitrile and acetone, with the concentration of LiClO 4 as 5%. Finally, the LiClO 4 solution was dropped on the PEO/β-Al 2 disc, which was naturally dried and used for future anodic boding. Solid macromolecule electrolyte material is composed of macromolecular polymer bulk with alkali metal salt and organic or inorganic filler. The macromolecular polymer bulk contains electro-donating groups for coordination, and the Table 1. Primary materials used in the experiment Name Molecular Formula Purity PEO H-(-OCH 2 CH 2 -) n -OH analytical pure lithium perchlorate LiClO 4 analytical pure acetonitrile CH 3 CN industrial pure inorganic filler β-al 2 analytical pure 724 Polymers & Polymer Composites, Vol. 22, No. 8, 2014

3 Characterization of PEO-X Ionic Conductive Polymer for Anodic Bonding 4. Results and Discussion 4.1 XRD Analysis Figures 2 and 3 show the XRD patterns of PEO-LiClO 4 and that of the composite solid macromolecule electrolyte prepared with β-al 2 as the additive. The scanning range (2θ) was 10 ~50. As can be seen, both patterns had two obvious diffraction peaks at 19 and 23. The addition of β-al 2 did not change the position of the diffraction peaks, but substantially reduced the peak intensity. It suggests that β-al 2 effectively hindered the crystallization of PEO-LiClO 4 and increased the PEO-LiClO 4 in amorphous region, which is beneficial to lithium ion migration in the solid macromolecule electrolyte during anodic bonding. nanoparticles 13. The large migration number makes it possible to obtain a high bonding density. 4.3 DSC Analysis Figures 4, 5 and 6 show the results of differential scanning calorimetry (DSC) for the composites with different contents of inorganic filler. It is known that the melting point of PEO is low. Therefore, the temperature scanning range was set from 0 C to 80 C, and the rise rate was 5 C/ min. The DSC curve in Figure 4 shows that there was an obvious Figure 2. XRD pattern of PEO-LiClO 4 peak at 66 C, while the peak in Figure 5 was around 77 C which is exactly the point of PEO transiting from crystalline state to amorphous state. Figure 6 shows that with the increase of the β-al 2 content, the crystallization temperature increased too. Therefore, the addition of inorganic filler β-al 2 promoted the glass transition temperature of the. This may explain why β-al 2 has an obvious inhibiting effect on PEO crystallization, and it is helpful to the ion diffusion in anodic bonding at low temperature. 4.2 Measurement of Lithium Ion Migration Number During anodic bonding, the migration number of lithium ion is an importance indicator for the bonding density. A large lithium ion migration number can promote the bonding density of the polymer electrolyte with aluminum. The migration number is usually measured by combining the steady-state current and the alternatingcurrent impedance 12. From Table 2, we can see that the addition of β-al 2 significantly increased the migration number of lithium ion, which made it easier for the lithium ion to transport positive charge in the composite solid macromolecule electrolyte. Under the exterior electric field during anodic bonding, Li + can not only migrate along the non-crystal chain segment in PEO, but also transfer electric charge through the vacancy on the surface of β-al 2 Figure 3. XRD pattern of PEO-LiClO 4 -β-al 2 Table 2. Number of Li + migration Content of β-al 2 Migration (W%) number of Li Polymers & Polymer Composites, Vol. 22, No. 8,

4 Xu Yin, Cuirong Liu, Yue Nan, and Qingsen Meng Figure 4. DSC curve of PEO-LiClO 4 Figure 5. DSC curve of PEO-LiClO 4 - β-al 2 (5% β-al 2 ) 4.4 Super-depth of Field Optical Microscopic Analysis The pure PEO and the composite were observed and compared with KEYENCE VHX-2000 superdepth of field microscope system, as shown in Figure 7 and Figure 8. On the PEO surface, there was sphere crystal structure composed of multiple inter-tangled molecular chains. The movement of polymer chains implements electric conduction through the repeated process of de-complexing, unify, and then complexing. Apparently, the masstransfer efficiency would be decreased if the molecular chain movement is under constraint. As comparison, the microstructure of the solid macromolecule electrolyte changed dramatically after adding the inorganic filler. As shown in Figure 8, the surface was much smoother, without the original ball structure. Therefore, the addition of inorganic filler affected the PEO crystallization. It may reduce the interface resistance of polymer electrolyte membrane and promote the conductivity at room temperature. As a result, it is easier for this composite material to transport charges during anodic bonding. 5. Conclusions Figure 6. DSC curve of PEO-LiClO 4 - β-al 2 (10% β-al 2 ) 1. According to the XRD and DSC results, the addition of inorganic filler restrained the crystallization of PEO, which is beneficial to the ion diffusion in the solid macromolecule electrolyte under strong electric field during anodic bonding; 2. By measuring the lithium-ion transference number, it is indicated that the inorganic filler can increase the migration of lithium ions in the polymer solid electrolyte; 3. As for the microstructure change, the addition of inorganic filler effectively reduced the resistance effect in the solid macromolecule 726 Polymers & Polymer Composites, Vol. 22, No. 8, 2014

5 Characterization of PEO-X Ionic Conductive Polymer for Anodic Bonding Figure 7. Micro-morphology of PE Figure 8. Micro-morphology of PEO-LiClO 4 -β-al 2 electrolyte interface and further promoted the conductivity at room temperature; 4. As a new packaging material, the may replace the Corning glass and be used in the anodic bonding technique. This study will promote the application of this new macromolecule material in MEMS. Acknowledgements Foundation item: supported by the National Natural Science Foundation of China ( ), Shanxi graduate outstanding innovative projects ( ). References 1. Rajarshi Saha, Nathan Fritz, Sue Ann Bidstrup-Allen, and Paul A. Kohl, Packaging-compatible wafer level capping of MEMS devices, Microelectronic Engineering, 104 (2013) Zhu Fulong, Some basic problems based on the mechanics of MEMS packaging process. Huazhong University of Science and Technology, Mrozek P., Glass-to-glass anodic bonding using TiNx interlayers for fully transparent device applications, Sensors and Actuators A: Physical, 174, (2012) Dehua Xiong, Jinshu Cheng, Hong Li, Wei Deng, and Kai Ye, Anodic bonding of glass ceramics to stainless steel coated with intermediate SiO2 layer, Microelectronic Engineering, 87(9) (2010) Masoud E.M., El-Bellihi A.-A., Bayoumy W.A., and Mousa M.A., Organic inorganic composite polymer electrolyte based on PEO LiClO4 and nano-al2o3 filler for lithium polymer batteries: Dielectric and transport properties, Journal of Alloys and Compounds, 575 (2013) Lishi Wang, Wensheng Yang, Jian Wang, and David G. Evans, New nanocomposite polymer electrolyte comprising nanosized ZnAl2O4 with a mesopore network and PEO- LiClO4. Solid State Ionics, 180(4-5) (2009) Masoud E.M., El-Bellihi A.-A., Bayoumy W.A., and Mousa M.A., Effect of LiAlO2 nanoparticle filler concentration on the electrical properties of PEO LiClO4 composite, Materials Research Bulletin, 48(3) (2013) Li Yi, Lu D., and Wong C.P., Electrical Conductive Adhesives with Nanotechnologies. NewYork:springer, Seok K. and Jin P.S., Preparation and Ion-conducting Behaviors of Poly(ethyleneoxide)-composite Electrolytes Containing Lithium Montmorillonite. Solid State Ionics, 178 (2009) Karmakar A. and Ghosh A., Dielectric permittivity and electric modulus of polyethylene oxide (PEO) LiClO4 composite electrolytes,current Applied Physics, Volume 12, Issue 2, March 2012, Pages Feng Qing, The preparation and electrochemical Properties of nano SiO2 composite polymer electrolytej]. rare metal materials and engineering, 2009,38 (1): Wang Lishi, Studies of Preparation and Properties of Nano-composite polymer lithium ion battery electrolyte D] 2009, Beijing University of Chemical Technology. 13. Jiang H.J., Chemical preparation of graphene-based nanomaterials and their applications in chemical and biological sensors. Small, 7 (2011) Polymers & Polymer Composites, Vol. 22, No. 8,

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