Unit
- Ancient origins: Lithography began in 1796 with Alois Senefelder's invention of chemical printing on limestone, enabling high-quality reproductions without engraving. This technique used oil-based inks and water-repellent surfaces.
- Photolithography birth: In 1826, Joseph Nicephore Niepce created the first photoresist using bitumen of Judea, which hardened upon exposure to light. This laid the groundwork for modern light-based patterning.
- Semiconductor revolution: The 1959 invention of the planar transistor by Jean Hoerni required precise patterning of silicon dioxide. Photolithography became essential for mass-producing integrated circuits.
- Shrinking features: The 1970s saw commercial steppers using UV light to print features below 5 microns. The industry adopted the micrometer as a standard unit for critical dimensions.
- Deep UV adoption: In the 1980s, mercury-xenon lamps provided 365 nm i-line radiation, enabling 0.5 micron resolution. This wavelength dominated for over a decade.
- Chemical amplification: The 1990s introduced chemically amplified resists using photoacid generators, boosting sensitivity and allowing higher throughput for deep UV systems.
- 193 nm era: ArF excimer lasers at 193 nm replaced KrF as the workhorse for 130 nm nodes and below. Immersion lithography later extended this wavelength to print 7 nm features.
- Extreme ultraviolet: EUV lithography at 13.5 nm began high-volume manufacturing around 2019, enabling single-patterning for 7 nm and smaller nodes.
- Multiple patterning: When single exposures could not reach smaller nodes, techniques like double patterning and quadruple patterning emerged. These split dense patterns across two or more masks, effectively multiplying resolution but increasing cost and cycle time.
- Computational lithography: Optical proximity correction and source mask optimization use algorithms to pre-distort mask patterns, compensating for diffraction and interference. This allows printing features smaller than the exposure wavelength by shaping illumination and mask design.
- Directed self assembly: Block copolymers can spontaneously form nanoscale patterns, such as cylinders or lamellae, when guided by prepatterned substrates. This method achieves sub 10 nm pitches without expensive optics, though defect control remains challenging.
- Maskless lithography: Electron beam direct write and multibeam systems pattern wafers without physical masks, offering flexibility for prototyping and low volume production. The main drawback is reduced throughput compared to optical projection.
Design Principle
Principle: Resolution vs. Throughput Trade-off
Definition: The balance between achieving fine feature sizes and maintaining high wafer output. Higher resolution often requires slower exposure tools, longer process times, or more complex masks.
Application: In early steppers, exposure time per field was limited by lamp intensity and resist sensitivity. Engineers optimized numerical aperture and wavelength to push resolution without sacrificing throughput. Modern scanners use high-speed stages and advanced illumination to maintain/200 wafers per hour at sub-10 nm resolution.
Quick Revision
- Lithography originated in 1796 as a printing method and evolved into semiconductor patterning.
- Key milestones include the photoresist (1826), planar transistor (1959), and stepper (1970s).
- Resolution improved from microns to single nanometers through shorter wavelengths and immersion.
- Chemical amplification boosted resist sensitivity for deep UV and EUV.
- The resolution-throughput trade-off remains a central design challenge in lithography tools.