Safe, sustainable solvent alternatives for wafer cleaning and drying
Smaller chip features, increasingly complex structures and concerns about safety and materials compatibility have made semiconductor cleaning and drying more challenging than ever before.
To help meet these challenges, wafer fabs around the world are recognizing the outstanding safety and performance advantages of 3M™ Novec™ Engineered Fluids in a growing number of critical cleaning and drying processes.
Semiconductor wafer cleaning
Novec fluids are a family of nonflammable, non ozone-depleting, low global warming solvents that are both highly effective cleaners and are compatible with delicate films, photo resists and most materials of construction - providing a safe, sustainable, high performance alternative to IPA, NMP and other conventional solvents in many applications.
For example, the excellent solubility of Novec fluids for fluoropolymer residues facilitates efficient post-etch contaminant removal during MEMS and Through-Silicon-Via (TSV) fabrication. And when blended with co-solvents, Novec fluids can effectively remove photoresist from water-sensitive substrates common to Cu/low-k devices.
Process flow schematic of the Bosch etch process
The Bosch etch process (Figure 1) is commonly used to micromachine MEMs, optical waveguides and DRAM deep trench capacitors. Formation of through-silicon via interconnects using the Bosch etch process results in the deposition of a fluoropolymer residue on the sidewall of the vias that is difficult to remove with conventional cleaning solvents.
Figure 2: NMP Comparison
Figure 1: Bosch Etch Process Flow Schematic
In this study (Figure 2), NMP and Novec 7200 fluid were compared for their ability to remove fluoropolymer sidewall residues left as a result of a Bosch etching process. As seen here, Secondary Ion Mass Spectometry showed a strong signal for mass number 343, representing a fluoropolymer fragment on the wafer. Sample cleaned with Novec 7200 show the lowest signal.
Novec Fluids can be used neat, as azeotropes or in co-solvent processes to effectively remove sub-micron particulate and ionic soils. Increased solvency and low surface tension allow these fluids to penetrate into tight spaces and complex geometries – ensuring higher wafer cleaning efficiency, particularly in 3D wafer cleaning.
Figure 3: Surface tension gradient draws water away from surface to Marangoni Dry Mechanism
Semiconductor Wafer Drying
The low surface tension, low viscosity, and fast evaporation rate of Novec fluids make them particularly effective in drying hydrophobic and high aspect ratio structures. Novec fluids evaporate completely, leaving no residue behind, and will not etch delicate substrates.
This is particularly critical in certain applications, such as surface tension gradient drying, which typically employs IPA in nitrogen. As semiconductor features become smaller, more complex and densely packed, even the slightest amount of unwanted etching from aggressive solvents such as IPA can cause serious defects that increase the chance of device failures and rejects. In addition, IPA can remain on a wafer surface after drying, leaving a residue that could form Si-C bonds during a subsequent high temperature process, thereby degrading thin oxide reliability.
The unique physical characteristics of certain Novec fluids make them ideal for use in surface tension gradient drying. And because Novec fluids are nonflammable, they can be used under a wider range of process conditions compared to IPA, which has flammability limits of 2% in air.