A Low-Temperature Curable Conformal Adhesive Layer for Monolithic Lamination of Thin Film Encapsulation

Abstract

Lamination of a thin film encapsulation (TFE) layer is regarded as one of the most promising methods that enable the reliable operation of organic electronic devices by attaching the TFE layers thereon directly using an adhesive layer. In this study, a low-temperature curable adhesive thin film with low glass transition temperature (T _g) is newly designed and synthesized. Low T _g allows conformal contact at the interface of the adhesive layer and the substrate subsequently leads to the enhancement of adhesion, and thus the barrier performance of the lamination of barrier film increases. In order to fabricate a low-T _g adhesive layer, glycidyl methacrylate (GMA) was copolymerized with a 2-hydroxyethyl acrylate (HEA) monomer in the vapor phase via initiated chemical vapor deposition. With a 5 µm thick p(GMA-co-HEA) adhesive layer, a strong adhesion was readily achieved by curing it at 60 °C for 1 h, with the peel strength of 16.6 N/25 mm, and the water vapor transmission rate of the glass-laminated encapsulation was as low as 3.4 × 10⁻³ g/m² · day under accelerating conditions (38 °C, 90% relative humidity). We believe the low-temperature curable thin adhesive layer will serve as a powerful material for the lamination of organic electronic devices in a damage-free way.

Key words

initiated chemical vapor deposition (iCVD) - thin film encapsulation (TFE) - low-T _g adhesive

• References and Notes

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• 26 Peel strength was measured by T-peel test using universal testing machine (UTM, Zwick) with 5 kN load cell after deposition of p(GMA-co-HEA) on a polyethylene terephthalate (PET) film and cured at 60 °C. For the T-peel test, the sample number of n = 3 was prepared and the average value with standard deviation of the measured peel strength was calculated.
• 27 A Ca test was conducted to calculate the water vapor transmission rate (WVTR). The 50 nm thick Ca (Junsei Chemical, 99.5‍%) was deposited with the deposition rate of 0.7 Å/s via a thermal evaporator (Daeki Hi-Tech Co., Ltd.) installed in a N₂-filled glove box on the glass substrate. The Ca test samples were stored in a thermohygrostat (JSRH-R70CPL, JS RESEARCH INC.) under the accelerating conditions of 38 °C and 90‍% RH. After the Ca thin film is oxidized after contact with the water vapor or oxygen, it becomes transparent. Therefore, the barrier property could be monitored in the oxidized area visually. The change of Ca area was analyzed by a digital camera (Nikon P300) and ImageJ v.1.46 (National Institutes of Health). The WVTR could be calculated as: WVTR (g/m²·day) = n · δ_Ca (M(H₂O))/(M(Ca)) · h · dA/dt, where n represents the molar equivalent of the Ca oxidation reaction with a water molecule (n = 2) and δ_Ca is the Ca density (1.55 g/cm³). Here, M(H₂O) and M(Ca) are the molecular weights of the water molecule and Ca, respectively. h is the height of the Ca film, and dA/dt is the slope of the graph plotting the Ca-oxidized area versus time.
• 28 The transmittance of 5 µm thick p(GMA-co-HEA) deposited on 1 mm thick glass slide was also obtained by using a spectroscopic ellipsometer (J.A. Woollam Co., Inc.).
• 29 For the organic layer and inorganic layer of TFE, poly(1,3,5-trimethyl-1,3,5-trivinylcyclosiloxane) (pV3D3) and Al₂O₃ were deposited, respectively. pV3D3 was deposited via iCVD by using V3D3 (Gelest, 95‍%) as a monomer and tert-butyl peroxide (TBPO, Aldrich, 98‍%) as an initiator. V3D3 and TBPO were heated to 40 °C and 30 °C, respectively, and fed into a custom-built iCVD system. During the polymer deposition, the process pressure, substrate temperature, and the filament temperature were maintained at 600 mTorr, 50 °C, and 140 °C, respectively. To deposit the Al₂O₃ film via atomic layer deposition (ALD), trimethyl aluminium (TMA, 99.999‍%, EG Chemical Co. Ltd.) and deionized H₂O were injected sequentially into the ALD chamber. To deposit Al₂O₃ at 90 °C, a single ALD cycle consisted of TMA pulse for 0.5 s, N₂ purging for 15 s, H₂O pulse for 0.5 s, and N₂ purging for 15 s. To accelerate the nucleation step, TMA was injected for 2 s and purged for 30 s in the initial three cycles. The total chamber volume of the reactor was 9210 cm³, and base pressure was maintained lower than 0.1 Torr.