Modified Surface Activated Bonding Using Si Intermediate Layer for Bonding and Debonding of Glass Substrates
Authors: Takeuchi Kai , Fujino Masahisa , Suga Tadatomo 
Published: February 2020

The Liquid Crystal (LC) displays using ultra-thin glass substrates of under 100 µm thick are necessary to realize the next generation display devices including the touch screen of the mobile phone, the tablet electronics and the flexible display. However, the ultra-thin glass substrates are easily deformed and damaged during the display process such as the color filter, LC and Thin Film Transistors (TFTs) at 350-550°C. Therefore the handling of the ultra-thin glass substrates in the process is a key issue for the limitation of the thickness of the display devices.
In these days, in order to realize the thin display devices, the glass thinning process by chemical etchant such as HF is widely employed for the fabrication process. In this method, thick and rigid glass substrates are used during the LC display process not to deteriorate the handling quality. After assembling and sealing process, the fabricated displays are immersed in HF and thinned to the target thickness. Although this method has been successfully developed, HF is very toxic and a big environmental load. Therefore, an alternative method to handle the ultra-thin glass substrates without HF process is desirable for the display fabrication.
As an alternative method to the glass thinning method, the handling by the carrier glass is has been proposed. The carrier glass is popular way to handle thin and fragile substrates in manufacture and it will enable the handling of the ultra-thin glass substrates in the process and realize the thinner display devices than by the current process without HF. In order to use the carrier glass, the ultra-thin glass should be bonded to the carrier glass and the bonding needs to be endurable for the high temperature process of TFT. Additionally, the ultra-thin glass must be released from the carrier glass after the fabrication process.
However, there is no suitable method to bond the glasses and debond after high temperature process. For example, the indirect bonding method such as the adhesive glue cannot keep the bonding during high temperature process and the residue of the adhesive after debonding becomes a problem for the process and the quality of the display. The direct bonding methods like thermal compression bonding and hydrophilic bonding are successfully employed in the manufacturing process of electronics, however, the bond strength is so high that the debonding without mechanical damage to the glass substrates is not possible especially after high temperature process.
In this study, we proposed applying Surface Activated Bonding (SAB) method to the bonding and debonding of the glasses for the fabrication of LC display. In the SAB method, the bonding surfaces are activated by Ar ion beam and bonded by atomic force in high vacuum at room temperature. Recently, the SAB method succeeded in bonding of glass using Si intermediate layers of under 20 nm thick which are deposited on the bonding surfaces by ion beam sputtering, nevertheless, the bond strength is too high to debond. In this case, we changed the conventional SAB method using Si layers for glass and controlled the bond strength before after heating. The Si layer was formed on one side of the glass surface in the contrary to the conventional SAB form the Si layers on the both side of the bonding surfaces. After deposition of Si, the bonding surfaces were exposed to nitrogen gas and bonded in vacuum. The bond strength was evaluated by blade insert test and was around 0.9 J/m2. The bonding endured heating at 350-550°C for 90 min and the bond strength after heating was around 0.5 J/m2. The bond strength by this method decreased after heating, whereas generally the glass bonding gets stronger after heating by the other direct bonding method. The decrease of the bond strength enables an easy debonding after the high temperature process in the display fabrication. The small voids can be seen in the bonding interface and this is the reason why the bond strength is controlled. It is indicated that the small voids are derived from water and OH groups on the bonding surfaces that were formed during the exposure to nitrogen gas, and the decomposed OH groups formed the small hydrogen bubbles after heating as a similar process to hydrophilic bonding.
In this study, we succeeded in bonding the glass substrates and controlling the bond strength to achieve an easy debonding for the display fabrication. This technique will realize the thinner display devices than by current manufacturing method and reduce the environmental load.