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  • Aniket Adak

Semiconductor Lithography: Explained!

What is semiconductor Lithography?

Semiconductor lithography or photolithography is a fabrication process that transitions from highly complex circuit patterns drawn on a photomask made of a large glass plate to a reduced version of ultra-high-performance lenses and exposed onto a silicon substrate which is usually known as a wafer. In lucid words or lament terms, fabrication of an integrated circuit (IC) requires an amalgamation of physical and chemical processes performed on a semiconductor (e.g., silicon) substrate. In process classification, the various processes are segregated into: film deposition, patterning, and semiconductor doping. Films of both conductors, which are polysilicon, aluminum and with the help of more recently copper and insulators which are bifurcated as various forms of silicon dioxide, silicon nitride, and others are used to connect and beset transistors and their components. Selective doping of various regions of silicon permits the conductivity of the silicon wafer to be changed with the application of voltage. With the creation of structures or models of these varied components, millions of transistors can be constructed and wired together to form the complex circuitry of a modern microelectronic device. Fundamental to all of these processes is called lithography or photolithography i.e., the construction of three-dimensional relief images on the substrate for subsequent pattern transition to the substrate.

Process of semiconductor lithography:

1. Lithography

Lithography is used to go under transformational patterns from a photomask to the surface of the wafer. The patterned information is recorded on a layer of photoresist which is applied on the top of the wafer. The photoresist is forced to change its physical properties when it is subjected to light (often ultraviolet) or another source of illumination (e.g., X-ray). The photoresist, which is the main aspect of lithography is either developed by (wet or dry) etching or by converting to volatile compounds through exposure. The pattern defined by the mask is either eliminated or remains after development, depending on if the type of resistance is positive or negative.

2. Etching

Etching is useful for the removal of unwanted material selectively in order to create patterns. This pattern of eliminating materials is known as the etching mask. The parts of the material which will be subjected to further processes should remain and are protected by the mask. The unmasked material can be removed by two methods, 1) wet (chemical) and 2) dry (physical) etching. Wet etching has a very quality strength to become isotropic (it measures the same value when it measures in the different directions) which results in limiting its application, and there will be difficulty in catching the time. On the other side, dry etching, as compared to wet etching, is highly anisotropic but less selective.

3. Deposition

A horde of layers of different materials, having different properties, is made to be deposited during the Integrated Circuit fabrication process. The two most crucial aspects of the deposition phase of lithography are the deposition methods 1) physical vapour deposition (PVD) and 2) chemical vapour deposition (CVD). During physical vapour deposition, gas ions possessing high kinetic energy sputter particles from a sputter target to create a low-pressure plasma chamber. The methodology of CVD is that a chemical reaction involving both, a gas mixture on the substrate surface at high temperatures. The requirement of staggering rate of temperatures (high temperatures) is the most curtailment factor for applying CVD. This problem can be avoided with the help of plasma enhanced chemical vapour deposition, also known as PECVD, where the chemical reaction is enhanced with radio frequencies instead of high temperatures. A critical aspect and deciding factor for the deposition technique is the non-variability of the deposited material, especially the layer thickness.

4. Chemical Mechanical Planarization