Room Temperate Bonding of Al2O3 Layers by Atomic Layer Deposition on Polyimide Substrates

Atomic layer deposition (ALD) gas barrier layer coatings on polymer substrates have been studied to address the problem of short-lifetime of polymer devices. ALD Al₂O₃ layers deposited with trimethylaluminium (TMA) and H₂O vapor precursors are relatively dense and conformal, so ALD Al₂O₃ layers are good candidates for effective gas barriers against water vapor and oxygen permeation.  In order to seal such ALD Al₂O₃ coated polymer substrates without organic adhesives, the bonding technique of choice has to satisfy low temperature bonding requirements.  Utilizing a high temperature bonding process would damage thermally sensitive polymer substrates during thermal compression bonding. Furthermore the gas barrier property of organic adhesives would be too low for effective sealing.

The polymer substrates used in this study are polyimide (PI) sheets from XENOMAX® from Toyobo Co., Ltd that are widely used as substrates for flexible electronics.  These polyimide sheets have extremely smooth surfaces (RMS ~1 nm), which are conducive for bonding, because good large contact areas can be obtained. These polyimide substrate sheets of 35 μm thickness were coated with Al₂O₃ films of 250 ALD cycles using different process temperatures ranging from 80 ºC, 150 ºC and 250 ºC, while 100 ALD cycles were used at 250 ºC. The warpage of the polyimide sheets, the surface morphology, and the results of bonding for these ALD Al₂O₃ coated polyimide samples were investigated as a function of different ALD deposition temperatures.

 The polyimide sample processed at the higher ALD Al₂O₃ temperature of 250 ºC exhibited much larger warpage, which is detrimental for bonding. We prepared PI sheets whose size was 10 cm x 10 cm. The ALD Al₂O₃ on polyimide samples deposited at 80, 150, and 250 ºC display warpage, and these edges lifted up to 1 mm, 3 mm, and 5 mm from a horizontal plane, respectively. This warpage is attributed to the difference of thermal expansion coefficients between the polyimide sheet and the deposited ALD Al₂O₃ layer.  The surface morphology of the ALD layer was studied using atomic force microscopy (AFM) surface profiling. The sample deposited at a lower ALD temperature has smoother surface morphologies. Our ALD Al₂O₃ films synthesized at 80 ºC exhibited a very smooth surface morphology indicative of amorphous material, while the Al₂O₃ film synthesized at 250 ºC was observed to possess a granular structure demonstrating recrystallization into a polycrystalline film. The rough grain structure of ALD Al₂O₃ films deposited at 250 ºC exceeds the critical surface roughness and as a consequence prevents surfaces from bonding.

For the bonding experiment, all PI samples coated by ALD Al₂O₃ were bonded using a modified method of Surface Activated Bonding (SAB) employing a nano-adhesion layer. First, ALD Al₂O₃ layers of about 25 nm are deposited on the surface of the polyimide substrates using trimethylaluminium (TMA) and water as precursors. Next, the Al₂O₃-coated surfaces are moved inside a high vacuum bonding machine and cleaned using Ar-ion beam bombardment. After the surface cleaning process step, Si nano-adhesive layers are deposited via ion-beam sputtering. Subsequently, the surface of the nano-adhesion layers are activated using the Ar-ion beam SAB method. Finally, surfaces were brought into close contact and bonded under compression in ultra-high vacuum. The structure of the bond interface was PI/Al₂O₃/Si/Si/Al₂O₃/PI. Here the thickness of each ALD Al₂O₃ layer and Si layer are 25 nm and 10 nm, respectively. We compared the results of the bonding of PI films coated at different ALD process temperatures. All bonding processes were performed at room temperature in ultra high vacuum. While the ALD Al₂O₃ samples deposited at 150 ºC suffered various non-bonded areas due to warpage and surface roughness, the ALD films deposited at 80 ºC revealed near perfect bonding without any voids, which is desirable for tight sealing of gas barrier layers.

Room temperature bonding of ALD Al₂O₃ films on Si and polyimide substrates was successfully performed. Due to high temperature gradient induced warpage of the polymer substrate and elevated surface roughness of the ALD layer, a lower process temperature of 80 ºC for ALD synthesis of Al₂O₃ provided optimum gas barrier conditions. The results of our bonding experiments demonstrate that larger bonded areas were achieved when the ALD synthesis of Al₂O₃ gas barrier layers was performed at 80 ºC. This bonding method is expected to be used for future microelectronics packaging and gas barrier sealing for polymer substrate.