Wet Cleaning and k-restore of low-k and ultra low-k Dielectrics
Low-k and ultralow-k dielectrics are and will be used within the interconnect system to lower the RC delay due to shrinking feature dimensions. The substitution of easily polarizable Si-O bonds by Si-C bonds, the formation of a loose network structure and the incorporation of porosity lead to the low dielectric constants of the materials. Low mechanical strength and damage of the dielectric material by usage of plasma processes for patterning, photoresist and etch residue removal are the main challenges integrating those materials in copper damascene technology.
Removal of plasma etch residues by wet cleaning – Surface energies and wetting behaviour of solids and liquids
The low-k dielectrics are patterned using CF-based gases in a plasma process. To achieve an anisotropic etching process, polymers are deposited during etching to protect the feature sidewalls. To avoid adhesion issues of the following layers, high contact resistance and to keep critical dimensions these residues have to be removed. Common residue removal processes include plasma cleaning in combination with a wet cleaning or a wet step alone. Besides the compatibility of a wet cleaning solution to Cu/low-k technology the wetting of the solid surface by the liquid is essential for an efficient cleaning process. The surface energies / surface tensions of liquid and solid are the key to wetting. The measurement of these parameters can help to find or develop a suitable solution to clean a specific surface.
Recovery of the k-value of plasma damaged low-k dielectrics
Plasma damage effects like carbon depletion due to the break of Si-C bonds, hydrophilization of the low-k surface and densification of the surface near areas of the material lead to increasing k-values and leakage currents. Besides the development of less damaging plasma processes for patterning, photoresist stripping and cleaning of low-k materials the recovery of the k-value by a repair process is one major research topic. On the surface of the damaged low-k dielectric silanols can be formed which lead to moisture adsorption. Those silanols can be removed and substituted by trimethylsiloxane groups by a silylation reaction using chemicals like HMDS or OMCTS in gas or liquid phase. This repair process can be combined with a curing process, e.g. thermal or UV-assisted which advances the k-recovery treatment.
Compatibility of cleaning solutions to Cu/low-k:
- Spectroscopic ellipsometry: refractive index, film thickness, homogeneity
- Hg-Probe (CV measurement): k-value, leakage currents, break through voltage
- FTIR: structural / chemical analysis
- Nanoindentation: mechanical properties (hardness, Young's modulus, yield stress)
Evaluation of the wetting behaviour of cleaning liquids and solid surfaces:
- Contact angle measurement and pendant drop method: determination of surface energies of solid and liquid (dispersive, polar and total values)
- Testing the wetting behaviour of liquids (cleaners and other solutions of interest) on solid surfaces