Low-Temperature Direct CVD Oxides to Thermal Oxide Wafer Bonding in Silicon Layer Transfer

Abstract

The bonding strength of low-pressure chemical vapor deposition and plasma-enhanced chemical vapor deposition ͑PECVD͒ oxides to thermal oxide is studied. Prior to bonding, all CVD oxide wafers are subjected to careful surface preparation including densification, chemical-mechanical polishing, activation, and post-bond annealing to ensure high-quality bonding. All wafers show surface roughness and wafer bow suitable for bonding after the surface preparations. It is found that bonding strength increases upon annealing and saturates beyond 2 h of annealing for the temperature range of 200-300°C. Tetraethyl orthosilicate source PECVD oxide is found to exhibit suitable bonding properties and can be used in applications such as silicon layer transfer.Three-dimensional ͑3D͒ integration, in the form of a vertical stack of several interconnected device layers, has many performance, integration, and cost advantages. 1 Direct Cu-to-Cu wafer bonding 2,3 has been proposed as a method to fabricate 3D structures. In this method, a thinned silicon-on-insulator ͑SOI͒ device layer is bonded to a substrate device wafer in a back-to-face fashion. This requires bonding of the SOI wafer to a top wafer, known as a handle wafer, to assist in SOI wafer handling in subsequent processes followed by SOI wafer etchback from the bottom. The thinned SOI layer is then permanently bonded to the substrate device wafer using Cu as the bonding medium. The bonding between the SOI wafer and the handle wafer is a temporary sacrificial bond, as the handle wafer will be released from the final 3D stack. Hence, this bond has to be strong enough to hold the SOI wafer during subsequent processes. However, the same bond should release readily during SOI thin-film transfer onto the substrate wafer. Epoxy 4 and other adhesives 5 have been investigated, but they are not compatible with the 400°C process temperature during Cu wafer bonding and the chemical attack during SOI wafer etchback. In this paper, we investigate lowtemperature direct oxide wafer bonding for this application.Oxide wafer bonding has been studied and applied extensively for application in SOI wafers 6,7 and microelectromechanical system ͑MEMS͒ 8 fabrication. Most research has concentrated on bonding thermal oxide wafers and silicon wafers that have suitable properties for bonding. In the silicon layer transfer flow described above, the SOI wafer has completed the front-end-of-line ͑FEOL͒ processes and any subsequent high-temperature oxidation is prohibited. In this case, one might have to resort to oxides prepared at low temperature ͑400°C and below͒ by means of chemical vapor deposition ͑CVD͒. 9 However, CVD oxides prepared at low temperature do not possess properties favorable for wafer bonding. In this work, we investigate the necessary surface preparations needed for successful bonding of CVD oxide wafers deposited at low temperature to thermal oxide wafer grown at high temperature for the above application. Bonding strength of the bonded wafer pairs with regards to CVD oxide types, anne...

Low Temperature Bonding of PECVD Silicon Dioxide Layers

“…Low temperature PECVD oxide layers offer a solution to this problem. Studies have confirmed that with proper layer preparation PECVD oxide bonding can be achieved (6,7).…”

Direct Bonding of Ge-Ge Using Epitaxially Grown Ge-on-Si Wafers

“…Extremely low surface roughness is essential for high quality bonding due to higher contact area between the pairing wafers. 3,17 Oxygen (O 2 ) plasma activation is widely used in wafer bonding for bond strength enhancement at low temperature. 18 In order to determine if O 2 plasma activation can enhance the Ge-Ge bond strength, the Ge/Si wafers were split into two groups.…”

Fabrication of Strained-Si/Strained-Ge Heterostructures on Insulator

“…An oxide-to-oxide bond is utilized to fuse the two wafers together. 6 The bond process begins by cleaning the wafer pairs and then exposing them to an oxygen plasma. The wafers are then brought into forced contact and annealed to strengthen the bond.…”