NANO-LEVEL CHARACTERIZATION OF THE SURFACE PROPERTIES OF ENGINEERED SILICON WAFER

Open Access
- Author:
- Xi, Xiaoning
- Graduate Program:
- Engineering Mechanics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Bernhard R Tittmann, Thesis Advisor/Co-Advisor
Bernhard R Tittmann, Thesis Advisor/Co-Advisor - Keywords:
- direct bonding
wetting properties
surface roughness
atomic force microscopy - Abstract:
- Surface properties of silicon are the key parameter to the success of silicon direct bonding technique in the semiconductor industry. The hydrophilicity and the roughness of silicon surfaces processed with three typical surface treatments in the direct bonding technique were systematically evaluated by atomic force microscopy (AFM). HF-treated silicon surfaces used for hydrophobic bonding showed negligible dependence of adhesion forces on humidity. However, HF treatment resulted in a considerable increase of the roughness, suggesting that additional smoothing processes are desired. RCA 1 treated and thermal oxidized silicon surfaces, without roughness increase, exhibited substantial increases in adhesion forces when environmental humidity increased from 10% to around 60%. Further rise of humidity resulted in a drop of the adhesion force. Molecular dynamics simulations of the water layer absorbed on an oxidized silicon surface suggested that there is a densification in the water structure at the vicinity of the liquid-solid interface. Such a change in the atomic structure of the water layer, depending on its thickness, gives rise to the non-monotonic adhesion force – humidity relationship on hydrophilic silicon surfaces. Based on this, a quantitative model to describe the relationship was introduced. Since hydrophilic treated surfaces exhibit strong adhesion forces over a wide range of humidity, the direct bonding should be done at a low humidity to reduce water bubble forming at the bonding interface during annealing process.