Copper-copper Direct Bonding At Low Temperature Based On Formic Acid Pretreatment

At present,the application of two-dimensional integrated packaging technology in the semiconductor field is more mature than the current three-dimensional integrated packaging technology.However,in the past few decades,the development of integrated circuits has almost completely followed the development of Moore's Law.The packaging technology develop on the basis of the original two-dimensional packaging process to the direction of three-dimensional packaging,due to increasing packaging density,shortening the interconnection wire length and reducing power consumption.With the continuous development of packaging technology,copper has gradually become the main interconnect material.Copper has better conductivity compared with the gold used in the previous packaging process.Under higher elongation,it can guarantee higher strength,and the bonding process can reduce the formation of intermetallic compounds.In the three-dimensional system packaging technology,metal thermal bonding is a key technology for realizing multi-layer chip stacking and vertical interconnection.However,the high temperature will cause the performance of the chip to change.In order to solve the adverse effects of high temperature generated by high temperature bonding,we have developed a low-temperature bonding technology.In this paper,we mainly study the Cu-Cu low temperature direct bonding technology based on formic acid pretreatment.The technology is mainly divided into two parts:the first is the surface treatment of the copper film before bonding,and the second is the bonding process after the surface treatment.At the same time,in order to realize the in-situ pretreatment of formic acid on copper surface and copper-copper direct bonding,we designed and developed a copper-copper low temperature direct bonding machine based on formic acid pretreatment according to the reaction principle of formic acid on the surface and oxide of copper.In the surface treatment part,first of all,the surfaces of the copper thin film were treated with formic acid gas and a solution respectively,and then the copper thin films before and after treatment were tested by X ray photoelectron spectroscopy?XPS?and atomic force microscopy?AFM?.The reduction of the surface of the copper film before and after treatment and the change of the surface morphology were studied and analyzed.The treatment time is changed when the surface of the copper film is treated with formic acid gas.XPS analysis showed that the reduction of copper surface was more complete with the increase of reaction time within a certain time range?0min³0min?.AFM analysis showed that the surface roughness of Cu film increases gradually with the increase of formic acid gas treatment time within a certain time range.When a formic acid solution was used,the treatment time and the formic acid solution concentration were changed.XPS analysis shows that within a certain time range?0min³0min?,the surface of the copper film is gradully reduced with the increase of the treatment time,and the reduction of copper film surface is the best when the solution concentration is 50%.AFM analysis shows that the surface roughness of the copper film is significantly increased after treatment with formic acid solution.Because the formic acid solution has a certain corrosion effect on the surface of the copper film,the surface roughness of copper film treated by formic acid gas is smaller than that of the formic acid solution.In the direct bonding part,first of all,the copper thin film samples after 30min treatment by formic acid gas were heated to 200 degrees in the atmosphere of N₂,and then the copper thin film samples under aload of 1200N,so that the upper and lower copper films were contacted and directly bonded.Then the bonding strength is tested with tensile testing,and the tensile strength is about 10MPa.Finally,Cu-Cu directly bonded interface after formic acid pretreatment was observed by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.Copper atoms at the bonding interface were tightly bound.Therefore,the copper-copper bonding technology by formic acid pretreatment can obtain high quality copper-copper bonding,providing an effective bonding technology for high-performance electronic device packaging.