Synthesis and Properties of UV Laser Debondable Temporary Bonding and Debonding Adhesives for 3D Multi-chip Packaging Process

Abstract

Recently, mobile devices with a focus on smartphone require both high performance and lightness at the same time, so TSV (Through Silicon Via) 3D multi-chip package technology was emerging. In order to realize this technique, temporary bonding and debonding adhesive is required to process silicon wafer and handling. However, using the existing adhesive handling a thin silicon wafer having a thickness of less than 50 μm is not easy. There are two main reasons for this. First, to maintain the high purity, it is required to have over 200 oC processing temperature during the process of bonding and debonding. Due to this thermal degradation, it generates low molecular weight substances and cause contamination to the thin silicon wafer. Second, while debonding, strong adhesive force can crack or cracking the thin silicon wafer and generate defects. In this study, the perspective on temporary bonding for TSV (Through Silicon Via) 3D multichip packaging and the perspective on debonding afterwards are divided into each technique elements, and these elements are to be used to newly synthesize the adhesive. Plus, the mechanisms of curing and debonding are to be analyzed through these property evaluations. In this study, the non-solvent type urethane acrylic adhesive was designed and manufactured to improve the disadvantages such as the phenomenon on flowing adhesive out from the silicon wafer after the spinning coating which the conventional solvent-based adhesive have during temporary bonding, the phenomenon of uneven coating thickness by contaminated wafer surface from the solvent evaporation and the phenomenon on the contamination of the work area. During the urethane synthesis, isophorone diisocyante was used as a hard segment and silicone-based diols (not conventional hydrocarbon-based diols) was used as a soft segment to improve the heat resistance of the polymer structure. Also, designing for dual curing adhesive that further introduce a photo-curing after thermal curing was made (rather than a single curable adhesive, such as the conventional light curing or thermal curing) to improve the heat resistance. A multi-functional acrylic monomer was end-capped to one end of the synthesized urethane oligomer and a monomer containing a fluorine was end-capped to the other end for to analyze the measurement of the number of functional groups of the multifunctional acrylic monomer, the density of the UV irradiation energy, hardening behavior, thermal stability and the peel strength depending on the amount of photoinitiator used. As a result, the number of functional acrylic monomer was in mono <di << tri, hexa order and increase on the indirect cure rate using the Gel-minute law was confirmed. This has initial reaction in the light-curing, which give an increase of the reaction sites and hence an increase in the crosslinking density. After tri-acrylate, it is confirmed that the gel fraction was not further increased by 60%, which can be interpreted that the reaction activity is falling by the radical trap of unreacted oligomers. The Gel fraction was confirmed to become constant at 60% when the UV irradiation energy density is at 400 mJ/cm2 or more. To decrease the peel strength due to the increase density of the UV energy, the pulsed irradiation method, which examine by dividing each into 100 mJ/cm2, was introduced to derive the molecular weight distribution of the polymer in a state in which heterogeneous. (The steady irradiation method, which is used primarily during irradiation, is not used.) In the steady irradiation method, the PDI value is 5.9, but the pulsed irradiation method showed 16.1 of high PDI value. This characteristic allowed showing lower intensity levels on the peel strength measurement of pulsed irradiation method than the steady irradiation method, concluding that the pulsed irradiation method is more suitable considering both temporary bonding and debonding. Type and the reaction mechanism of the photoinitiator was found through the existing studies, and the purpose of this study was to evaluate and analyze by varying the amount of light curing initiator. As a result, the gel fraction became constant without any further increase when adding the binder contrast of 2 phr or more photoinitiators. This is considered as an important research data on the optimal composition ratio of the initial dose, given a side reaction due to unreacted initiator, which can occur when an excess of added photoinitiator. Thermal decomposition behavior results, seen by varying the amount of UV energy density and photo initiator for the thermal stability and by using thermogravimetric analysis, was confirmed that the thermal stability is maintained at a high temperature condition over 250 oC. This can interpret that the heat resistance was improved by the introduction of dual-cure mechanism and the adhesive synthesis containing all hybrid type of silicone-based, epoxy-based, fluorine-based functional group. On the other hand, this study used the method of edge zone debonding, using the UV laser, to consider both temporary bonding and debonding. While the conventional method requiring a bonding temperature of 200 ~ 220 oC, this study was able to proceed the bonding at a low temperature of 80 ~ 150 oC. Furthermore, this study has done debonding within two minutes using a UV laser, rather than the conventional way in which this process takes time over 6 hours by a penetration of the solvent. For this purpose, BTHPEMA (2- [3- (2-Benzotriazol-2-yl) -4-hydroxyphenyl] ethyl methacrylate) was introduced when preparing the adhesive. When debonding, identification (BTHPEMA playing a role of being the LTHC (light to heat conversion)) for debonding progression of the polymer film formation by the heat cure adhesive was made through thermal curing mechanism of epoxy functionality by the FTIR-ATR analysis and the gel fraction measurement. Absorbency, for a UV laser of a synthesis adhesive, was determined according to wavelength using a UV-visible spectroscopy. As a result, 355 nm in wavelength bands indicate the absorption of up to 60% of the binder prepared in accordance with the increase amount of BTHPEMA comparison 0.4 phr input, and the UV laser absorbency of the adhesive synthesized was adjustable by varying the blending ratio. Moreover, it was confirmed that the joined debonding was composed in the effective energy density conditions of the UV laser irradiation with 5.65 to 6.72 J/cm2.