Photolithography is one of the main parts of the microprocessor manufacturing steps.
There are currently three types of photolithography processes: contact alignment photolithography, proximity alignment photolithography, projection or stepper lithography.
Contact alignment lithography is the simplest and involves a light source, a lens, a mask and a substrate positioned on a stand and secured by a chuck.
Projection alignment was first introduced in the early 70s by semiconductor giant Perkin Elmer and involves a complete set of optics between the mask and the substrate. Until then, the market was dominated by contact printing mask aligners, simple machines for optical lithography limited to features of few µm.
As the microprocessors world was slowly heading towards thinner features, new solutions had to be explored.
In projection printing, the wafer contact is completely avoided, a lens projects the image of the photomask over the substrate, previously coated with photoresist.
There are basically three different types of printing machines: projection scanners, single step-and-repeat projection machines and multiple step-and-repeat projection machines.
Scanners can expose a wafer using by multiple scans but without the capacity of realigning the masks between two consecutive scans.
Steppers instead are capable of exposing small parts of the wafer multiple times until the whole wafer is covered with the ability of aligning each field and making adjustments in all directions.
Modern steppers make use of reduction lenses (5:1 to 10:1) therefore being able to reduce the pattern on the mask and rendering the potential mask defects irrelevant
Reductions are however limited by diffraction considerations, without the help of tricks like immersion and multiple patterning, stepper lithography can achieve critical dimensions down to tens of nanometers.
Lens resolution in projection printing is given by the following formula
where K1 is a parameter which depends on the resists and process conditions, λ is the wavelength of the light used to transfer the pattern and NA is the numerical aperture of the lens system
Similarly, depth of focus law is given by:
Depth-of-focus =±k2 λ/NA2
Both laws are not good predictors of achievable resolution, as factors the kind of resist used, influence coefficient k1 and k2
In the early 80s, a state-of-the-art lens had a resolution of about 1µm and ±0.75µm as depth of focus, which meant k1 and k2 values of 0.8 and 0.12 respectively.
It was therefore assumed that optical lithography would never reach sub-micron capabilities, but this proved wrong for the following factors:
- Lens capabilities improved immensely. For example i-line lenses for steppers today reach easily a resolution of under 0.7µm with depth-of-focus of ±2.5µm, while a few years ago such values were limited to 1.3µm of resolution and depth-of-focus of ±0.75µm
- Smaller wavelengths have been used. In the last decades the industry has moved from g-line mercury lamps (436nm) to 365nm i-line lamps, to KrF excimer steppers (248nm) to ArF steppers (193nm)
- Resists have improved considerably
- Depth of focus requirements have decreased due to utilization of chemical mechanical polishing. Depth of focus of ±0.2µm are now considered adequate
Recently, new tricks like multi-patterning and immersion lithography have extended the life of optical steppers well below 45nm of CD (critical dimension)
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