Nanotechnology - Lithography Module
The Lithography Module
Typically lithography is performed as part of a well-characterized module, which includes the wafer surface preparation, photoresist deposition, alignment of the mask and wafer, exposure, develop and appropriate resist conditioning. The lithography process steps need to be characterized as a sequence in order to ensure that the remaining resist at the end of the modules is an optimal image of the mask, and has the desired sidewall profile.
The standard steps found in a lithography module are (in sequence): dehydration bake, HMDS prime, resist spin/spray, soft bake, alignment, exposure, post exposure bake, develop hard bake and descum. Not all lithography modules will contain all the process steps. A brief explanation of the process steps is included for completeness.
• Dehydration bake - dehydrate the wafer to aid resist adhesion.
• HMDS prime - coating of wafer surface with adhesion promoter. Not necessary for all
surfaces.
• Resist spin/spray - coating of the wafer with resist either by spinning or spraying. Typically
desire a uniform coat.
• Soft bake - drive off some of the solvent in the resist, may result in a significant loss of mass
of resist (and thickness). Makes resist more viscous.
• Alignment - align pattern on mask to features on wafers.
• Exposure - projection of mask image on resist to cause selective chemical property change.
• Post exposure bake - baking of resist to drive off further solvent content. Makes resist more
resistant to etchants (other than developer).
• Develop - selective removal of resist after exposure (exposed resist if resist is positive,
unexposed resist if resist is positive). Usually a wet process (although dry processes exist).
• Hard bake - drive off most of the remaining solvent from the resist.
• Descum - removal of thin layer of resist scum that may occlude open regions in pattern, helps
to open up corners.
We make a few assumptions about photolithography. Firstly, we assume that a well characterized module exists that: prepares the wafer surface, deposits the requisite resist thickness, aligns the mask perfectly, exposes the wafer with the optimal dosage, develops the resist under the optimal conditions, and bakes the resist for the appropriate times at the appropriate locations in the sequence. Unfortunately, even if the module is executed perfectly, the properties of lithography are very feature and topography dependent. It is therefore necessary for the designer to be aware of certain limitations of lithography, as well as the information they should provide to the technician performing the lithography.
The designer influences the lithographic process through their selections of materials, topography and geometry. The material(s) upon which the resist is to be deposited is important, as it affects the resist adhesion. The reflectivity and roughness of the layer beneath the photoresist determines the amount of reflected and dispersed light present during exposure. It is difficult to obtain a nice uniform resist coat across a surface with high topography, which complicates exposure and development as the resist has different thickness in different locations. If the surface of the wafer has many different height features, the limited depth of focus of most lithographic exposure tools will become an issue.
Typically lithography is performed as part of a well-characterized module, which includes the wafer surface preparation, photoresist deposition, alignment of the mask and wafer, exposure, develop and appropriate resist conditioning. The lithography process steps need to be characterized as a sequence in order to ensure that the remaining resist at the end of the modules is an optimal image of the mask, and has the desired sidewall profile.
The standard steps found in a lithography module are (in sequence): dehydration bake, HMDS prime, resist spin/spray, soft bake, alignment, exposure, post exposure bake, develop hard bake and descum. Not all lithography modules will contain all the process steps. A brief explanation of the process steps is included for completeness.
• Dehydration bake - dehydrate the wafer to aid resist adhesion.
• HMDS prime - coating of wafer surface with adhesion promoter. Not necessary for all
surfaces.
• Resist spin/spray - coating of the wafer with resist either by spinning or spraying. Typically
desire a uniform coat.
• Soft bake - drive off some of the solvent in the resist, may result in a significant loss of mass
of resist (and thickness). Makes resist more viscous.
• Alignment - align pattern on mask to features on wafers.
• Exposure - projection of mask image on resist to cause selective chemical property change.
• Post exposure bake - baking of resist to drive off further solvent content. Makes resist more
resistant to etchants (other than developer).
• Develop - selective removal of resist after exposure (exposed resist if resist is positive,
unexposed resist if resist is positive). Usually a wet process (although dry processes exist).
• Hard bake - drive off most of the remaining solvent from the resist.
• Descum - removal of thin layer of resist scum that may occlude open regions in pattern, helps
to open up corners.
We make a few assumptions about photolithography. Firstly, we assume that a well characterized module exists that: prepares the wafer surface, deposits the requisite resist thickness, aligns the mask perfectly, exposes the wafer with the optimal dosage, develops the resist under the optimal conditions, and bakes the resist for the appropriate times at the appropriate locations in the sequence. Unfortunately, even if the module is executed perfectly, the properties of lithography are very feature and topography dependent. It is therefore necessary for the designer to be aware of certain limitations of lithography, as well as the information they should provide to the technician performing the lithography.
The designer influences the lithographic process through their selections of materials, topography and geometry. The material(s) upon which the resist is to be deposited is important, as it affects the resist adhesion. The reflectivity and roughness of the layer beneath the photoresist determines the amount of reflected and dispersed light present during exposure. It is difficult to obtain a nice uniform resist coat across a surface with high topography, which complicates exposure and development as the resist has different thickness in different locations. If the surface of the wafer has many different height features, the limited depth of focus of most lithographic exposure tools will become an issue.
