Lithography: The Complex Process of Creating Microelectronics - Prof. Aurangzeb Khan, Study notes of Electrical and Electronics Engineering

Download Lithography: The Complex Process of Creating Microelectronics - Prof. Aurangzeb Khan and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! 1 1 Lithography • A light sensitive photoresist is spun onto the wafer forming a thin layer on the surface. The resist is then selectively exposed by shining light through a mask which contains the pattern information for the particular being fabricated. The resist is then developed which completes the pattern transfer from the mask to the wafer. Lithography comes from two Greek words, “lithos” which means stone and graphein which means write. “ writing a pattern on stone” 2 Lithography • Lithography is the most complicated, expensive, and critical process of modern IC manufacturing. • Lithography transforms complex circuit diagrams into pattern which are define on the wafer in a succession of exposure and processing steps to form a number of superimposed layers of insulator, conductor, and semiconductors materials. • Typically 8-25 lithography steps and several hundred processing steps between exposure are required to fabricate a packed IC. • The minimum feature size i. e., the minimum line width or line to line separation that can be printed on the surface, control the number of circuits that can be placed on the chip and has a direct impact on circuit speed. The evolution of IC is therefore closely linked to the evolution of lithographic tools. 3 Lithography Overview • While the lithography concept is very simple, the actual implementation is very complex, because of the following demands placed on this process: • Resolution - demand for smaller device structures • Exposure field - chip size and need to expose at least one full chip (8” wafer) • Placement accuracy - alignment with respect to the existing pattern • Throughput - manufacturing cost • Reduction of defects density - yield loss 0.7X in linear dimension every 3 years. • Placement accuracy ≈ 1/3 of feature size. • ≈ 35% of total wafer manufacturing costs for lithography. • Note the ???. This represents the single biggest uncertainty about the future of the roadmap. 4 Light Sources • Decreasing feature sizes require the use of shorter λ. • Traditionally Hg vapor lamps have been used which generate many spectral lines from a high intensity • plasma inside a glass lamp. • (Electrons are excited to higher energy levels by collisions in the plasma. Photons are emitted when the energy is released.) • g line - λ = 436 nm • i line -λ = 365 nm (used for 0.5 µm, 0.35 µm) 2 5 Historical Development and Basic Concepts • Patterning process consists of mask design, mask • fabrication and wafer printing. • It is convenient to divide the wafer printing process into three parts A. Light source - Shorter wavelength photons B. Wafer exposure system. C. Resist. 6 Light Sources: Laser Sources • The most powerful and commonly used laser sources for deep UV photolithography are the excimer lasers. • Kr +NF3 → KrF→ photon emission Energy KrF - λ = 248 nm (used for 0.25 mm) ArF - λ = 193 nm • Issues include finding suitable resists and transparent optical components at these wavelengths. The combination of the high power and deep UV lines makes ArF and KrF attractive sources for advance optical lithography 7 Wafer Exposure Systems Contact printing is capable of high resolution but has unacceptable defect densities. Inexpensive, diffraction effects are minimize. • Proximity printing cannot easily print features below a few µm (except for x-ray systems). Poor resolution due to diffraction effects, required 1 X mask. • Projection printing provides high resolution and low defect densities and \ dominates today. • Typical projection systems use reduction optics (2X - 5X), step and repeat or step and scan mechanical systems, print ∼ 50 wafers/hour and cost $5 - 10M. 8 Photomask • A mask for optical lithography consists of a transparent plate called blank, covered with a patterned film of opaque material. • The blank is made of soda lime, borosilicate glass, or fused quartz. The advantage of the quartz is that it is transparent to deep UV (≤365nm) and has a very low thermal expansion coefficient.