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LITHOGRAPHY Presentation Transcript
1.LITHOGRAPHY
2.Why this course is included in B. Tech.? To let you improve your % of marks Just be attentive in the class and attend it regularly
3.Step No. 6: Pattern Development Purpose: to develop the desired pattern in the Photo-resist Equipment: Baths for developing and cleaning chemicals and spin dryer Method: Dip the wafer in the developing and rinsing chemicals for desired times and then spin dry.
4.Step No. 7: Hard Bake Purpose: to remove water and any other liquid and to harden the photoresist present on the wafer. Equipment: Oven Method: Heat at ~ 200 C for about 20 minutes
5.Step No. 8: Develop Inspect Optical Inspection under microscope (100X) is done to check for Line Resolution Line Width Resolution Particles and Defects .
6.What is Photoresist? The photosensitive compound used in microelectronics is called Photoresist. Certain properties of these compounds change when they are exposed to light of a particular wave length. Photoresists are used to transfer the pattern on the substrate.
7.What is a pattern? The arrangement of black and white areas on the mask (glass plate) is called pattern. The pattern indicates the areas through which light will expose the photoresist
8.Requirements of a photoresist PR should be sensitive to the desired frequency and insensitive to yellow or red light. It should have fine line definition that should be retained during subsequent processing while it is still present on the wafer. The exposed resist should undergo chemical changes
9.Requirements of a photoresist (contd.) The ‘HARD RESIST’ (chemically inert part of the resist) should bind strongly to the substrate or the layer below PR. The ‘SOFT RESIST’ (chemically active part of PR) should be easily removable from the wafer surface. The Hard PR should be able to sustain further processing (Etching) without losing fine line definition. The PR must not contribute impurities, introduce defects or in any other way degrade the performance of the device being fabricated.
10.Requirements of a photoresist (contd.) The Hard PR should be easily removable when it is no longer required, without adversely affecting the other layers present. This process of Hard PR removal is called STRIPPING Chemical used for stripping the hard PR is called ‘STRIPPER’ ‘Plasma Aching’ is the technique used for removing hard PR by plasma technique
11.Types of PR Photoresists are of 2 types Positive – it creates a + ve image of the pattern on the mask Negative – it creates a – ve image of the pattern on the mask
12.Two Types of Photoresists Negative PR And Positive PR Negative PR The exposed parts become crosslinked and polymerized due to the photochemical reaction, which hardens and remains on the wafer surface after development, whereas the unexposed parts are dissolved by the developer.
13.Positive Photoresist The main component is novolac resin, which is a crosslinked polymer before the exposure. After the exposure process, the exposed part’s cross-links break down and become “softened” due to the photochemical reaction called photosolubilization, and will be dissolved by the developer, while the unexposed parts remain on the wafer surface.
14.Use of Photo Active Compound
15.DQN Positive Photoresist Diazo-Quinone Novalac is a +ve PR Its Photo-active Compound is Diazo-quinone Its matrix material is novalac
16.Photoresist Parameters Dose – it is the total quantity per unit area of photons falling on PR. Dose ? = Photon intensity x Exposure time Units: Energy (calories or joules) per unit area Sensitivity – amount of light energy necessary to create the chemical change Resolution – Smallest feature size that can be reproduced in a photoresist. Contrast – is the difference in appearance of two or more parts of a field seen simultaneously or successively.
17.What is a Mask? A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.
18.Dark Field Mask and Clear Field Mask Active Region - region of interest Field Region – rest of the glass plate region If the field region is dark it is a dark field mask If the field region is clear , it is a clear field mask
19.Preference between Dark Field Mask and Clear Field Mask For aligning mask with the pattern on the wafer we must see the wafer pattern through the mask. Therefore a clear field mask is preferred.
20.Imaging errors due to dust particles: error is critical - if the particle is in active region - if the mask is dark field .
21.Flow diagram for mask generation process
22.Electron-Beam Lithography .
23.Photoresist Parameters (contd. ..) Contrast –is the difference in appearance of two or more parts of a field seen simultaneously or successively. Contrast: ? = 1 / [log10(D100/D0)] Where D100 = lowest energy density for which all the resist is removed, D0 = lowest energy density needed to begin photo chemistry .
24.Critical Modulation Transfer Function (CMTF) CMTF is the minimum optical modulation transfer function necessary to obtain a pattern. It is defined by: CMTF = (D100 – D0)/(D100 + D0) 25.The alignment systems and misalignments 26.Photolithography .
27.Photolithography (continued) .
28.What is PEB ? How it works ? Why is it useful? .
29.Questions Explain the working of + & - PR Why + PR gets higher resolution What is resolution? 2 basic techniques for transferring resist features into a layer with proper schematic representation Working of electron lithography .
30.Resolution and Focusing
31.Can we continue to reduce wave length to improve resolution?
32.Reducing wave length.
33.Intensity of DUV sources << mercury lamps DUV requires different PR Chemically amplified PR for DUV Catalysis effect is used to increase the effective sensitivity of the PR A photo acid is created in the PR when it is exposed to the DUV light. In the post exposure bake (PEB) process, the wafer is heated, and the heat drives acid diffusion and amplification in a catalytic reaction.
34.X-ray Lithography Wavelength < 5 nm; Higher resolution No materials that can reflect or refract x-rays Must be accomplished by the direct printing process – similar to proximity printing
35.Thin layers of chromium can block UV light Thick layer of gold is required to block X – rays. Therefore thickness to gap ratio changes.
36.Extreme Ultra Violet (EUV) radiation (11 – 14 nm) for sub-0.1 micron features No known material can be used to make lenses for EUV – strong absorption at short wave lengths Therefore EUV systems must be mirror-based Light sources still under development Material of masks – multilayer coatings such as Pd/C , Mo/Si
37.Ion-beam Lithography Similar to e-beam lithography – higher resolution Can be both – direct writing and projection resist exposing Advantage – direct ion implantation and ion-beam sputtering patterned etch Disadvantage – throughput is very low Application – mask/reticle repair
38.Calculate NA, R and DOF
39.Differentiate between good and bad developments
40.Explain with the help of diagrams Overlay Budget, Misalignments, Resolution and Depth of Focus
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