Wafer Backside Surface Modification

Abstract

Microchips are made from circular silicon discs called wafers and are manufactured with photolithography. A critical step of a photolithography process is when a wafer with a photoresist is placed on a wafer table (WT) and exposed to light to form patterns that build up components of an integrated circuit (IC). The dimensions of the IC are in nanometer scale and are becoming increasingly smaller. Any changes to the wafer table can contribute to alignment issues or increase overlay between successive exposures and is dependent on interaction with the wafer backside.
Common wafer backside materials are silicon, silicon oxide and silicon nitride, which are hydrophilic, so they readily interact with water and adsorb it from the atmosphere. Furthermore, their surface conductivity is highly dependent on the relative humidity (RH) of the air. Additionally, silicon oxide and silicon nitride generate surface charge from single-wafer wet spin tools that are a part of the critical toolset for advanced IC fabrication.
In the presence of water, a charged wafer backside can conduct unwanted current through the surface. The occurrence of this phenomena can affect both the performance and yield of the components that the wafer backside comes in contact with during photolithography.
This thesis studies reducing the charge generation and surface conductivity of a hydrophilic silicon and silicon nitride wafer backside by depositing hydrophobic self-assembled monolayers (SAM) with silane coupling agents. They are less likely to form surface charge and have lower surface conductivity. The hydrophobic monolayers that are studied are deposited from hexamethyldisilazane (HMDS) and a non-disclosed fluorinated molecule, called F-SAM. HMDS is already used in photolithography on the wafer frontside to promote adhesion to the photoresist and fluorinated molecules are known to be hydrophobic, inert and with high electrical resistance.
This surface modification tackles both the water content on the surface and the charging via single wafer wet-spin tools. The monolayers are characterized in terms of surface chemistry, surface free energy, wear resistance, zeta potential, surface charge and conductivity. Lastly, the charge diffusion is measured when a wafer is put in contact with a wafer table in an experimental setup, where the discharge current is registered.
The outcome of this study shows that modified surfaces are found to be homogenous to a sub nanometer level and uniform. Compared to typical silicon nitride wafers, the surface conductivity is lowered by a factor of 1000 and is not affected by the RH. This results in the total measured charge diffused to a wafer table to be reduced by at least a factor of 17. As a conclusion, modified surface can be considered a promising option in reducing any unwanted current leakage from a wafer backside to surrounding components.