(First Best Student Paper Award) Polymer to Silicon Direct Bonding for Microelectronics
Authors: Dautriat Margaux , Montméat Pierre , Fournel Frank 
Published: February 2024

Direct bonding consists in spontaneously bringing into contact two solid surfaces without any intermediate liquid. A liquid is defined here as a being thick enough to have fluid characteristics. Silicon direct bonding is widely used in microelectronics, for instance for the production of silicon-on-insulator substrates (for transistor manufacturing). Direct bonding can be performed between identical surfaces, for example, two silicon substrates. However, direct bonding is also feasible with two different surfaces such as silicon and metal.

Direct bonding of two solid materials requires very strict surface conditions such as planarity, particle cleanliness and low surface roughness. The surface Root Mean Square (RMS) roughness should be lower than 0.5 nm for hydrophilic silicon bonding and 0.3 nm for hydrophobic silicon bonding [1].

We will focus here on direct bonding with polymer films. It is a quite innovative bonding as polymers are materials with very different properties compared to silicon. They are indeed not crystalline, much less rigid and most of the time hydrophobic.

Five different polymers thin films are used to evaluate direct bonding with polymers: LOR2A, BARC AR26N, SOC HM8102, TOK TDMR and LTC9310. Thicknesses are between 32 nm and 6 µm. Those polymers are deposited on 200 or 300 mm diameter Si substrates.

First of all, polymers are characterized by Fourier-Transform Infrared Spectroscopy (FTIR), spectroscopic ellipsometry and Thermogravimetric Analysis (TGA). Contact angles of the different polymers are in the 58° to 77° range, a feature which is a characteristic of hydrophobic surfaces. For comparison, the contact angle is lower than 5° for hydrophilic silicon (SiO2) and 78° for hydrophobic H-passivated silicon (Si). The surface roughness of the five different polymers is evaluated by Atomic Force Microscopy (AFM), with RMS roughness ranging from 0.29 to 0.40 nm found. Such values are in-between the 0.3 nm and 0.5 nm thresholds for hydrophobic and hydrophilic surface bonding, respectively. However, direct bonding is feasible between polymers and hydrophilic or hydrophobic silicon surfaces in all cases. Bonding waves are observed and characterized by infrared imaging. Adhesion energies for bonding with the five different polymers are between 40 and 70 mJ/m². Meanwhile, the bonding wave velocity is in the 10-19 mm/s range. Data for polymer to hydrophilic silicon bonding are provided in Table 1. They can be compared to Table 2 data for hydrophilic and hydrophobic silicon bonding (i.e. ceramic bonding) as they have equivalent water contact angles. Polymer to Si direct bonding has higher adhesion energy and bonding wave velocity than SiO2 to Si ceramic direct bonding. The bonding interface, characterized by Scanning Acoustic Microscopy (SAM), does not reveal any bonding defects. Bonded stacks can be annealed at different temperatures. Acoustic images of bonding with BARC are shown in Figure 1 for different annealing temperatures. It is possible to measure their adherence energy after annealing using the DCB (Double Cantilever Beam) technique [2]. BARC adherence energies are given in Figure 2. As with ceramic bonding, bonding strengthens as the temperature increases. The covalent bonds density should increase during the annealing. The bonding degradation at 250°C and especially 300°C evidenced by Scanning Acoustic Microscopy (SAM) is in line with TGA findings on polymer degradation, with a threshold at 280°C. X-ray Photoelectron Spectroscopy is being used to identify interactions between the silicon surface and the polymer. A mechanism will be proposed for these innovative bondings.

[1] H. Moriceau et al., « Overview of recent direct wafer bonding advances and applications », Adv. Nat. Sci. Nanosci. Nanotechnol., vol. 1, no 4, p. 043004, févr. 2011, doi: 10.1088/2043-6262/1/4/043004.

[2] W. P. Maszara, G. Goetz, A. Caviglia, et J. B. McKitterick, « Bonding of silicon wafers for silicon‐on‐insulator », J. Appl.
Phys., vol. 64, no 10, p. 4943-4950, nov. 1988, doi: 10.1063/1.342443.

Figure 1