Metal Lift-Off
Using Solstice® Systems
Metal-Lift Off (MLO) remains a productive and cost-effective metallization scheme for a broad array of applications including probe card, microfluidics and any of a vast number of MEMs devices. Effective metallization through MLO demands complete removal of photoresist and all organic residual materials, including along feature sides where chemical action can be impaired as well as within trench features. This seemingly simple objective drives certain constraints and requirements, including human safety considerations, into the process.
Successful metal line formation by MLO requires a reliable and robust process window, including for the narrowest of features for which the lithography step will allow. The narrower the feature, the more sensitive and prone to delamination. A properly designed MLO process will produce kinetic activity aggressive enough to fully remove photoresist from feature corners and narrow areas, but not so aggressive as to delaminate narrow features. Batch systems have long proven highly effective in many MLO applications, especially vertical-wafer spray systems where deluge-level chemistry application replicates immersion but adds kinetic scrubbing action to clear tougher areas. For the finest features, or for integration schemes where photoresist requires additional mechanical action, single wafer systems tend to dominate, due to widening of the applicable process window. Single wafer systems provide direct, perpendicular chem impingement to the wafer surface that can be more readily optimized to a given need. For instance, high pressure spray can be employed for stronger kinetic removal of resist.
Redeposition of released sacrificial metal constitutes a yield impacting issue that must be avoided. Most single-wafer systems have a face-up architecture due to the simplicity of construction. However, dislodged metal has a genuine risk of re-adhesion to the wafer surface as it is carried directly across the wafer toward the edge. Higher wafer rotation speeds can help to prevent genuine fall-out of metal fragments through entrainment, but the resulting turbulence can active ‘stir’ fragments onto the surface. On the other end, lower wafer rotation rates avoid turbulent mixing but risk settling out of debris. Face-down single wafer systems open a wider process window with regard to redeposition because metal flakes and fragments are much more prone to fall away from the wafer as they are lifted off the surface.
Solvation of photoresist still routinely involves the use of strong organic solvents such as NMP, acetone, etc., which poses important considerations around human safety and potential exposure. Open, benchtop style systems pose the biggest challenge to limiting personnel exposure. Batch spray systems offer excellent personnel protection because they fully seal the process environment prior to introduction of solvent and they fully remove and rinse all solvent prior to releasing the chamber door. Likewise in single wafer systems, considerations of tool architecture are important. Face up single wafer chambers are inherently ‘open’ systems though a box or enclosure can be added so that the surrounding area of the chamber can be exhausted. This of course consumes tool space. Face down chambers operate much like batch spray systems in that the chamber is always free of any solvent any time the chamber is opened.
The Solstice from ClassOne Technology offers the broadest process flexibility with flood-flow and high-pressure flow options to accommodate multiple device types within a given fab while maintaining a wide process window. And its face-down architecture ensures personnel safety and avoids redeposition of lifted metal debris.
Typical Metal Lift-Off (MLO) Performance
- Complete lift-off of sacrificial metal
- No damage to features
- Broad flexibility with options for high-flow, high-pressure, soak
Metal-Lift Off (MLO) remains a productive and cost-effective metallization scheme for a broad array of applications including probe card, microfluidics and any of a vast number of MEMs devices. Effective metallization through MLO demands complete removal of photoresist and all organic residual materials, including along feature sides where chemical action can be impaired as well as within trench features. This seemingly simple objective drives certain constraints and requirements, including human safety considerations, into the process.
Successful metal line formation by MLO requires a reliable and robust process window, including for the narrowest of features for which the lithography step will allow. The narrower the feature, the more sensitive and prone to delamination. A properly designed MLO process will produce kinetic activity aggressive enough to fully remove photoresist from feature corners and narrow areas, but not so aggressive as to delaminate narrow features. Batch systems have long proven highly effective in many MLO applications, especially vertical-wafer spray systems where deluge-level chemistry application replicates immersion but adds kinetic scrubbing action to clear tougher areas. For the finest features, or for integration schemes where photoresist requires additional mechanical action, single wafer systems tend to dominate, due to widening of the applicable process window. Single wafer systems provide direct, perpendicular chem impingement to the wafer surface that can be more readily optimized to a given need. For instance, high pressure spray can be employed for stronger kinetic removal of resist.
Redeposition of released sacrificial metal constitutes a yield impacting issue that must be avoided. Most single-wafer systems have a face-up architecture due to the simplicity of construction. However, dislodged metal has a genuine risk of re-adhesion to the wafer surface as it is carried directly across the wafer toward the edge. Higher wafer rotation speeds can help to prevent genuine fall-out of metal fragments through entrainment, but the resulting turbulence can active ‘stir’ fragments onto the surface. On the other end, lower wafer rotation rates avoid turbulent mixing but risk settling out of debris. Face-down single wafer systems open a wider process window with regard to redeposition because metal flakes and fragments are much more prone to fall away from the wafer as they are lifted off the surface.
Solvation of photoresist still routinely involves the use of strong organic solvents such as NMP, acetone, etc., which poses important considerations around human safety and potential exposure. Open, benchtop style systems pose the biggest challenge to limiting personnel exposure. Batch spray systems offer excellent personnel protection because they fully seal the process environment prior to introduction of solvent and they fully remove and rinse all solvent prior to releasing the chamber door. Likewise in single wafer systems, considerations of tool architecture are important. Face up single wafer chambers are inherently ‘open’ systems though a box or enclosure can be added so that the surrounding area of the chamber can be exhausted. This of course consumes tool space. Face down chambers operate much like batch spray systems in that the chamber is always free of any solvent any time the chamber is opened.
The Solstice from ClassOne Technology offers the broadest process flexibility with flood-flow and high-pressure flow options to accommodate multiple device types within a given fab while maintaining a wide process window. And its face-down architecture ensures personnel safety and avoids redeposition of lifted metal debris.
Typical Metal Lift-Off (MLO) Performance
- Complete lift-off of sacrificial metal
- No damage to features
- Broad flexibility with options for high-flow, high-pressure, soak
