Impact of an Amino-Alcohol Organic Molecule on SiO2 and Si Bonding Energy
Authors: Fournel Frank , Calvez Aziliz , Larrey Vincent , Eleouet Gaëlle , Morales Christophe , Rieutord François 
Published: December 2020

Direct bonding energy relates to the well-known “strength” or “toughness” of direct bonding interfaces. It is the energy needed to separate two bonded surfaces. Its evolution with the post bonding annealing temperature is an important parameter closely linked to the fundamental direct bonding mechanism [1]. There are different ways of modifying the bonding energy and especially to increase it at low annealing temperatures. Plasma activation, chemical mechanical polishing (CMP) are for instance well known surface preparations that enhance the bonding energy at temperatures below 500°C [2].
Usually, for hydrophilic direct bonding, surfaces have to meet various specifications. A small high frequency roughness, a low nanotopology amplitude, a small wafer deformation, no particle contamination and a good hydrophilicity are mandatory to obtain spontaneous bonding at room temperature without any pressure [3, 4]. Moreover, the surface cleanliness in terms of hydrocarbon contamination is also very important to avoid bonding interface defectivity during the post bonding annealing. However, in this study, an intentional organic surface contamination will be shown to have a positive impact on direct bonding energy. An amino-alcohol molecule (N,N-diéthyléthanolamine also called DEAE) is sent vapor deposited on the surface of 200 mm silicon oxide or silicon. In an airtight reactor, 100 ml of water with different DEAE concentrations yield different DEAE saturated atmospheres in equilibrium with the liquid phase. As fast as possible, two wafers are loaded in the reactor and left there for different exposure times. Then, wafers covered with 145 nm of thermal silicon oxide surfaces are bonded just out of the reactor and annealed at different temperatures, for direct bonding energy measurements in an anhydrous atmosphere. As shown in figure 1a, different exposure times from 15 s up to 5 min to pure DEAE saturating vapor give almost the same bonding energy values. To have surfaces in good equilibrium with the reactor atmosphere, 5 min. of exposure time is selected to test various liquid DEAE concentrations. As shown on figure 1b, with the smallest tested concentration (2%) in the liquid phase, the direct bonding energy increases by more than 70% at 300°C compared to a reference bonding. Moreover, no bonding defect could be detected by acoustic microscopy. A positive impact of DEAE on silicon to silicon oxide bonding will also be shown.
As silicon oxide direct bonding involves water oxide hydrolysis [1], a mechanism could be proposed to explain the beneficial impact of DEAE. Indeed, it is known that a high pH solution can enhance the silicon oxide hydrolysis [5]. As this molecule is very hydrophilic due to its alcoholic group, this molecule can be chemically adsorbed on a hydrophilic surface and can then be trapped at the bonding interface. After bonding, this molecule can increase the trapped water pH, enhancing the silica hydrolysis at the direct bonding interface and yielding higher bonding energies at low temperatures. As organic contamination is known to be detrimental for direct bonding, high concentration on the surface might be an issue. This should be the reason why a low concentration is more interesting.

REFERENCES

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[4] K.T. Turner, Wafer Bonding: Mechanics-Based Models and Experiments, Massachusetts Institute of Technology, 2004.
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Figure 1