Investigation of Surface Pre-Treatment Methods for Wafer-Level Cu-Cu Thermo-Compression Bonding

Abstract

To increase the yield of the wafer-level Cu-Cu thermo-compression bonding method, certain surface pre-treatment methods for Cu are studied which can be exposed to the atmosphere before bonding. To inhibit re-oxidation under atmospheric conditions, the reduced pure Cu surface is treated by H2/Ar plasma, NH3 plasma and thiol solution, respectively, and is covered by Cu hydride, Cu nitride and a self-assembled monolayer (SAM) accordingly. A pair of the treated wafers is then bonded by the thermo-compression bonding method, and evaluated by the tensile test. Results show that the bond strengths of the wafers treated by NH3 plasma and SAM are not sufficient due to the remaining surface protection layers such as Cu nitride and SAMs resulting from the pre-treatment. In contrast, the H2/Ar plasma–treated wafer showed the same strength as the one with formic acid vapor treatment, even when exposed to the atmosphere for 30 min. In the thermal desorption spectroscopy (TDS) measurement of the H2/Ar plasma–treated Cu sample, the total number of the detected H2 was 3.1 times more than the citric acid–treated one. Results of the TDS measurement indicate that the modified Cu surface is terminated by chemisorbed hydrogen atoms, which leads to high bonding strength.

Bonding-Based Wafer-Level Vacuum Packaging Using Atomic Hydrogen Pre-Treated Cu Bonding Frames

“…The AES and TDS results indicate that the atomic hydrogen pre-treatment removes Cu native oxide and then forms hydride-like layers such as surface hydrogen or hydroxyl group, which suppress re-oxidation on the Cu surfaces during the wafer storage in atmospheric condition. The hydride-like layer growth on the Cu surface by hydrogen plasma treatment has been reported [ 21 , 22 , 38 ], and such surface hydrogen species can also suppress the re-oxidation of the Cu surfaces [ 38 ]. Such re-oxidation suppression by the chemisorbed hydrogen may enable vacuum sealing at far lower temperature by the atomic hydrogen pre-treatment in comparison with the conventional wet oxide reduction.…”

“…It should be noted that the Cu hydride-like layer does not need to be removed by an additional process before bonding, while the surface protection layer such as Cu 3 N or SAM must be removed by thermal treatment before bonding [ 22 , 39 ]. It is considered that desorbed hydrogen atoms can diffuse into the Cu crystals due to their small atomic radius.…”

“…Atomic hydrogen generated by H 2 plasma cannot only reduce existing Cu oxides, but also form a Cu hydride-like layer, which prevents re-oxidation at the same time. Even though the Cu bonding frames were exposed to the air after H 2 /Ar plasma pre-treatment, sufficient bond strength for vacuum sealing at 300 °C was demonstrated [ 22 ]. Metal corrosion can be avoided by H 2 plasma pretreatment.…”

Time-Dependent Evolution Study of Ar/N2 Plasma-Activated Cu Surface for Enabling Two-Step Cu-Cu Direct Bonding in a Non-Vacuum Environment

“…To mitigate this issue, surface treatments such as chemical modifications, surface assembled monolayer (SAM), specialized chemical mechanical polishing (CMP), and plasma treatment, have been studied. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Among them, plasma treatment is the most favorable process as it does not leave behind any residue. Plasma-treated Cu-Cu direct bonding is generally evaluated at specific conditions such as at high temperature (>300°C) and/or under ultra high vacuum (UHV) for an extended period of time.…”

Optimization of Copper Thermocompression Diffusion Bonding under Vacuum: Microstructural and Mechanical Characteristics

“…Bonding (tensile) strength (BS) values of up to 70 MPa have been reported in the case of thermocompression between 200 and 400 • C [8][9][10], while a BS value around 10 MPa is achieved at room temperature compression [11]. Exceptionally high BS values (>200 MPa) have been referred to in the case of annealed and pre-treated copper surfaces using H 2 /Ar plasma or formic acid vapor [12].…”