(SEM-V) THEORY EXAMINATION 2021-22 VLSI TECHNOLOGY

SECTION A (Any 2 Questions Explained Briefly)
 

What is electronic grade silicon?
Electronic grade silicon is a highly purified form of silicon used for semiconductor device fabrication. It has extremely low impurity concentration, usually one impurity atom in a billion silicon atoms. Such high purity is necessary to ensure predictable electrical properties and reliable performance of integrated circuits.
 

What are positive and negative photoresists?
Photoresists are light-sensitive materials used in lithography. In positive photoresist, the exposed portion becomes soluble and is removed during development, whereas in negative photoresist, the exposed portion becomes hard and remains on the wafer while the unexposed part is removed. Positive photoresists provide better resolution and are more commonly used in VLSI fabrication.
 

SECTION B (Any 2 Questions Explained)
 

Explain Chemical Vapor Deposition (CVD) process.
Chemical Vapor Deposition is a process used to deposit thin solid films on a wafer surface through chemical reactions of gaseous precursors. The wafer is heated, and reactive gases are introduced into the chamber. These gases react or decompose on the wafer surface to form a solid film such as silicon dioxide, silicon nitride, or polysilicon. CVD provides good uniformity, step coverage, and is widely used in VLSI manufacturing.
 

Explain Czochralski method of single crystal growth.
The Czochralski method is used to grow large single crystals of silicon. In this method, high-purity polycrystalline silicon is melted in a quartz crucible. A small seed crystal is dipped into the molten silicon and slowly pulled upward while rotating. As the seed is withdrawn, silicon solidifies on it, forming a single crystal ingot. This method is widely used because it allows precise control over crystal diameter and doping concentration.
 

SECTION C (Any 2 Questions Explained)
 

Explain Float-Zone method of single crystal generation.
The Float-Zone method is a crystal growth technique in which a molten zone is moved along a silicon rod using localized heating. Impurities are carried along with the molten zone, resulting in a highly pure crystal. This method produces silicon with very low impurity levels and is mainly used for high-power and high-frequency devices, although it is more expensive than the Czochralski method.
 

Explain RCA cleaning process.
RCA cleaning is a standard wafer cleaning process used to remove organic, ionic, and metallic contaminants from the silicon surface. It involves multiple steps using chemical solutions such as ammonium hydroxide, hydrogen peroxide, and hydrochloric acid. Proper cleaning is essential to ensure good oxide growth, reliable device fabrication, and high yield in VLSI processes.
 

Most Questions in This PDF Are Related To
 

Most questions in the VLSI Technology (KEC-053) paper are related to silicon crystal growth, oxidation techniques, lithography, diffusion and ion implantation, thin film deposition, and CMOS fabrication steps. The paper mainly focuses on fabrication processes and manufacturing technologies used in IC design, rather than circuit analysis.