(Invited) Al-Al Waferbonding Process Development for Heterogeneous Integration
Authors: Schulze Sebastian , Wietstruck Matthias , Voß Thomas , Krüger Patrick 
Published: February 2024

3D heterogeneous integration provides the possibility to combine dissimilar semiconductor technologies and add new components to obtain a higher performance and increase the functionality. Aligned Wafer level bonding, where the chips are vertically interconnected is known as one of the key technologies needed for 3D integration. Here, Aluminum to aluminum bonding is very attractive due to its easy compatibility to complementary metal-oxide-semiconductor (CMOS) processes. Furthermore Al-Al bonding provides not only the mechanical attachment between different wafers but also enables a good electrical and thermal conductivity. One of the main challenges for Al-Al thermocompression bonding is the formation of aluminum oxide which inhibits uniform and reliable Al-Al bonding at low temperatures with minimum bond forces. To overcome this limitation, this work presents a surface activated bonding process carried out in an EVG® ComBond® 200 mm automated high vacuum wafer bonding system. Here, the surface oxide is first removed with an argon plasma treatment. Without breaking vacuum to avoid a reoxidation, the wafers are optically aligned to each other and then bonded by applying force and temperature.

This paper shows an integration concept for patterned 200 mm Al-Al bonded wafers and focusses on different factors influencing the quality of the Al-Al bonding. The bond quality is evaluated with various characterization methods. For electrical characterization single contacts with sizes down to 10 µm x 10 µm are analyzed. To characterize the bond interface C-mode scanning acoustic microscopy (C-SAM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used. To calculate the bond strength, shear force measurements are carried out. The alignment accuracy is evaluated with an infrared microscope measuring the overlay of specific alignment marks at different dice across the wafer.

A great factor influencing the bonding quality are the process parameters during the actual bonding process. Here, we vary the duration of the argon plasma activation from 1.5 to 5 min. Additionally, the impact of the bonding temperature in the range of 200 to 300 °C and the influence of the bonding force between 20 and 60 kN is investigated. Furthermore the bonding time is varied from 15 to 60 min. The best results with contact resistances in the mΩ-Range and a bonding yield >95% are achieved for wafers bonded for 1 h at 300 °C with 60 kN and an activation time of 5 min.

Besides the bonding parameters, also the wafer fabrication prior to the bonding process has a significant influence on the Al-Al bond quality. In this context, the local and global topography of the wafers play an important role. An improved wafer flatness increases the contact area during bonding and can thus contribute to a better bonding result. In order to reduce the surface roughness of the Al bonding pads, we vary the parameters for the aluminum deposition and analyze the impact of different adhesion/barrier layers below the aluminum bonding film. The paper shows that also the tungsten plugs under the Al bonding pads affect their topography and thus the bond quality. In addition, we modify an etch-mask influencing the condition of the Al bonding pads and directly correlate it to the bonding yield and electrical resistance. Moreover, the impact of the wafer bow on the bond strength is studied.

Even the processes that follow the Al-Al bonding can still negatively affect the final result. After bonding a backside grinding process is used, which causes silicon dust on the wafer surface and requires an additional cleaning step. This study shows the effect of an ultrasonic clean on the measured bond strength.

In summary we present a surface activated, low temperature Al-Al bonding process of patterned 200 mm wafers with a high bonding yield, low contact resistances and an alignment accuracy close to 1 µm. Our work presents an integration concept for Al-Al bonded wafers and highlights different factors that influence the bonding result.