Oxygen Plasma and Warm Nitric Acid Surface Activation for Low-Temperature Wafer Bonding

Abstract

In this paper, the void formation and the surface energy for low-temperature Si-Si wafer bonding technique as a function of the annealing time and temperature as well as warm nitric acid and O 2 -plasma-assisted surface pretreatments are considered and compared. The analysis of the surface energy vs annealing time exhibits two main bonding mechanisms: ͑i͒ rapid reaction between silanol groups which leads to a quick enhancement of the bonding strength, and ͑ii͒ slow further increase of bonding strength and improvement of the bonding uniformity thanks to the out-diffusion of interface voids.Low-temperature wafer bonding technology is a very attractive technique which meets the complementary metal oxide semiconductor ͑CMOS͒ process requirements for the fabrication of threedimensional devices and packaging. A high bonding quality process should include high and uniform bonding strength over the whole bonded area, as well as a void-free bonding interface. Several published papers in the literature have demonstrated the possibility to reach sufficient bonding surface energy, without the use of an adhesive layer, at low temperature 1-8 or even at room temperature. [9][10][11][12] Recently, dry surface activation such as oxygen or argon plasma prior to bonding has been investigated. Higher surface energies were reported compared to the wet surface activation technique such as warm nitric acid; 5 however, many voids were raised by subsequent annealing steps for oxygen plasma activated Si-Si bonding. 5,9,13 In Ref. 14, we have shown the possibility to drastically reduce the number of interface voids by optimization of the plasma exposure time. In Ref. 15 we demonstrated that the annealing voids in the case of the plasma activation surface originate from excessive bonding reaction by-products and the distribution is guided by the presence of microcracks at the bonding interface. Our previous investigations 14-16 also showed that substantially long annealing time is needed to assure the completeness of the bonding reactions and therefore to reach saturated bonding surface energy.However, it is still unclear why the plasma exposure results in stronger bonding energy compared to wet surface activation techniques. In Ref. 5, the authors explained that higher surface energy could be related to the highly disordered surface structure caused by the plasma exposure, which enhances the diffusion of the water along the bonding interface. In Ref. 6 and 7 the self-bias voltage effects on the bonding quality were investigated and the correlation between bonding strength and self-bias voltage was proven. Unfortunately, the voids at the bonding interface were not considered together with the bonding strength in these studies. [5][6][7] In the present paper, we have investigated the void formation and the surface energy for low-temperature Si-Si wafer bonding technique as a function of the annealing time and temperature. Warm nitric acid and O 2 -plasma-assisted surface pretreatments have been considered. Based on the void distribution and...

A Review of Hydrophilic Silicon Wafer Bonding

“…There are many other papers concerning plasma activated wafer bonding, typically reporting on a new plasma generator source or an optimized set of parameters [18][19][20][21]41,50,52,64,65,67,[87][88][89][90][91][92][93][94][95][96][97][98][99][100] that allow for high strength bonding at low annealing temperatures and with minimal annealing voids. A significant number of these results are applicable only to the precise experimental setup of the reporting authors or are simply unrepeatable.…”

A Comparative Study: Void Formation in Silicon Wafer Direct Bonding by Oxygen Plasma Activation with and without Fluorine

“…11a). Many published articles [12][13][14][15] have reported that O 2 plasma activation alters the native oxide surfaces in the following ways: (1) removing organic contaminants, (2) increasing in the density of silanol groups (Si-OH) on oxide surfaces, and (3) creating subsurface reservoir (i.e., porous oxide layers, like a sponge) for water molecules, which may improve the low-temperature diffusivity of water molecules at the interface. After O 2 plasma treatment, the surface is very hydrophilic so that many water molecules are more easily adsorbed on the surface when it is exposed to moderately humid air (Figs.…”

Room Temperature Si/Si Wafer Direct Bonding in Air

“…Moreover, some small initial voids become to disappear at 8000C. It means no potential annealing voids would be generated continuously in longer heating time or higher temperature [11]. The remained voids at 8000C are mainly trapped voids, which is caused by initial particulates in our class 10 K clean room [ Fig.…”