Room Temperature Bonding with Lift-Off Resist Using the Surface Activated Bonding Method for a Layer Transfer Platform
Authors: Matsumae Takashi , Suga Tadatomo 
Published: February 2020

This study demonstrates the practicability of surface activated bonding for polymethylglutarimide (PMGI), as a temporary wafer bonding method at room temperature for future material integration. A bonded area covering over 92% of the wafer surface, with a room temperature bond strength of ~2 J/m2 is achieved using the bonding method. To overcome the fundamental transistor-scaling limit, novel material integration methods such as the 3D stacking are being researched. There is a growing interest in temporary bonding as one of the key technologies for 3D stacking. Recently, a wafer bonding method for polymethylglutarimide (PMGI) has been proposed for layer transfer. It has been demonstrated that the PMGI layer can be effectively bonded using thermal compression bonding. Moreover, debonding using N-Methyl-2-pyrrolidone (NMP) results in negligible amounts of the PMGI sacrificial layer as a residue on the debonded wafer surface and is therefore suitable for a transfer process.
However, the high temperature annealing process in the bonding process requires a temperature of 250ºC to bond most of the loaded area; this high temperature process may damage the transferred material. Moreover, the high temperature process limits usage in applications such as integration of black phosphorous. For low temperature bonding, a surface activated bonding (SAB) process has been proposed as a room temperature bonding method. It has been proposed that polymer materials could be bonded using this process with thinly deposited layers called nano-adhesion layers at room temperature.
In this study, PMGI layer is bonded to a Si substrate using the SAB method at room temperature (around 25 ºC); this method is intended to be used as a layer transfer process. One Si substrate is coated with PMGI layer, and the substrate is bonded to other Si support substrate via a proposed modified method of SAB. The proposed room temperature bonding process consists of five steps: spin-coat and soft-bake, bake in a vacuum, ion beam supper-deposition, surface activation, and bonding.
Using the proposed method, a bonded area covering over 92% of the wafer surface, with a room temperature bond strength of ~2 J/m2, is achieved. It is concluded that soft-baking at 120ºC is most suitable for obtaining the strongest bond as well as the largest bonded area. Moreover, even the samples that aren’t baked after spin-coating achieved a bonded area of 81 %. This is because the solvent cyclopentanone is evaporated in the bake-in-vacuum step.
The surface profile viewed using an atomic force microscope revealed that the PMGI surface was sufficiently smoothened for bonding at room temperature, after the vacuum baking process. It has been reported that it is difficult to bond a rough surface using room temperature bonding with SAB. The root means square (RMS) values of PMGI surfaces before and after vacuum annealing are 1.5 nm and 0.5 nm, respectively. The results of the AFM indicate that the surface becomes smoother after baking in a vacuum. This smooth surface is one the reasons for a considerable bonded area as well as a strong bonding strength using the proposed bonding technique.
To investigate the chemical reaction between the PMGI and the deposited Si layer caused by the bonding process, attenuated total reflectance/Fourier-transform infrared spectroscopy (ATR/FT-IR) was performed on the PMGI layers before and after the deposition of the Si nano-adhesion layer. On comparing these spectra, the following features were observed: The first feature is that the peak height derived from the N-H bond that was at around 1550 cm-1, decreased after the Si deposition procedure. The second is that the peak from the C=O stretch band at around 1700 cm-1also decreased. From these results, it can be assumed that during the Si deposition process, a considerable modification of the N–H and C=O groups of the PMGI occurred after the Si deposition. This chemical modification is the key for the strong bonding of the PMGI at room temperature.
In summary, we demonstrated that PMGI bonding at room temperature is capable of providing large and strong bonding interfaces. This process can be applied for 3D integration and material integration, without thermal damage. Thus, the proposed method can provide a platform for a layer transfer process suitable for 3D material integration of thin-layered systems like black phosphorous.

Figure 1