Optical lithography

Optical lithography

Definition: Optical Lithography

Optical lithography, also known as photolithography, is a key process used in semiconductor manufacturing to transfer intricate patterns onto a silicon wafer. It is a technique that utilizes light to project a pattern onto a photosensitive material, known as a photoresist, which is coated on the wafer surface.

History 

The earliest optical lithography tools used in the manufacturing of semiconductor devices were of a type classified at contact printers. In these systems, a mask is placed in direct contact with the photoresist-coated wafer and light is shined through the mask. Patterned areas on the mask served to block the light causing the negative of the pattern to be transferred to the wafer. The problem with the contact approach, however, was the rapid generation of defects on the mask, which are subsequently replicated in all exposures. The industry addressed this problem with the introduction of proximity lithography which is essentially the same as contact lithography but with a small air gap maintained between the surface of the mask and the wafer. This mitigated the defect problem but at the cost of resolution limitations arising from diffraction, or spreading of the light, upon propagation of the light through the free-space gap between the mask and wafer. 

The free-space diffraction problem was eventually solved by introducing an imaging system between the mask and the wafer. The gap can now effectively be eliminated since the function of the imaging system is to replicate the electric field present in its object plane to its image plane. Any focus error in this optical system can be thought of simply as equivalent to the gap present in the proximity tool with the further benefit of allowing the gap to effectively become negative thereby expanding the acceptable gap or focus operating range. In addition to solving the proximity diffraction problem, using an imaging system enables demagnification from the mask to the wafer. This is beneficial since it greatly relaxes mask requirements both in terms of feature quality and defects. The demagnification cannot be made too large, however, since mask size would become an issue. Modern projection optical lithography tools use a demagnification of 4.

Figure: Schematic of pattern transfer:(a) Negative Resist and (b) Positive Resist 

Process Steps

  1. Mask Design: The first step in optical lithography is the creation of a mask, also called a photomask or reticle. The mask contains the desired pattern that needs to be transferred onto the wafer. It is typically made of a glass substrate with a thin layer of chrome or other light-blocking material.
  2. Mask Alignment: The mask is aligned with the wafer using precision alignment systems. This ensures that the pattern on the mask is accurately positioned on the wafer surface.
  3. Photoresist Coating: A thin layer of photoresist is applied to the wafer surface using spin coating or other deposition techniques. The photoresist is a light-sensitive material that undergoes chemical changes when exposed to light.
  4. Exposure: The mask is placed in a lithography tool, which projects light through the mask onto the wafer. The light passes through the transparent regions of the mask, exposing the underlying photoresist. The exposed areas of the photoresist undergo a chemical reaction, either becoming more soluble (positive resist) or less soluble (negative resist).
  5. Development: After exposure, the wafer is immersed in a developer solution that selectively removes either the exposed or unexposed regions of the photoresist, depending on the resist type. This step reveals the desired pattern on the wafer.
  6. Etching: The patterned photoresist acts as a mask for subsequent etching processes. The exposed areas of the wafer, not covered by the photoresist, are etched away using plasma or chemical etchants. This step transfers the pattern into the underlying layers of the wafer.
  7. Resist Stripping: Once the pattern transfer is complete, the remaining photoresist is removed from the wafer surface using solvents or plasma-based processes. This step prepares the wafer for further processing or inspection.

Figure: Flow Chart for the pattern transfer process


Figure: Steps in opening a window in making a film, using positive photoresist.
Figure: Optical lithography replicates the mask pattern through an imaging lens.



Figure: Optical lithography

Figure: Exposed resist image transferred to the underlying thin film by isotropic etching, anisotropic etching, electroplating, lift-off, and ion implant.
Figure: Schematic depiction of the reflective lithography configuration required at EUV wavelengths

Advantages and Limitations

Optical lithography has been the workhorse of the semiconductor industry for several decades due to its cost-effectiveness and scalability. It has enabled the miniaturization of electronic devices by allowing the fabrication of smaller and more complex features on silicon wafers.

However, as the demand for even smaller feature sizes increases, optical lithography faces limitations due to the diffraction of light. The wavelength of light used in optical lithography limits the achievable resolution. To overcome this limitation, advanced techniques such as multiple patterning, immersion lithography, and extreme ultraviolet (EUV) lithography have been developed.

Overall, optical lithography remains a critical process in semiconductor manufacturing, playing a vital role in the production of integrated circuits and other microelectronic devices.





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