Partha Mishra_Lithography - Seminar & Technical Writing Topic.pptx

The presentation slides were submitted as a part of the Seminar and Technical Writing course at NIT Rourkela. The topic of the presentation is Lithography.

1. Seminar and Technical Writing (CR4900) Topic: Lithography Course Instructor: Prof. Debasish Sarkar Slides Prepared By: Partha Mishra (B.Tech. Ceramic Engineering Dept. 8 th semester)

2. OUTLINE Nanotechnology – Top Down vs. Bottom Up Approaches Lithography Technique & its Importance Lithography Process Sequence Wafer Surface Preparation Photoresist for Lithography Photoresist Chemistry Requirements & Properties of Photoresist Chemically Amplified Photoresists Masks for Lithography Main Exposure Modes in Lithography Performance Optimization Advanced Lithography Lithography: Future & Challenges References

3. Illustration of the top-down (a) and bottom-up (b) lithographic principles. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Nanotechnology – Top Down vs. Bottom Up Approaches Kailasa Temple: Notable vertical excavation. Carved out of a Megalith structure. Konark Sun Temple: Constructed by carving out big granite stones places one on top of other. Top Down Approach Taking material away to make structures Principle: Adding Layer material followed by Patterning it E.g. Lithography Techniques, Liquid Phase Exfoliation, Ball Milling, Mechanical Attrition Bottom Up Approach Selectively adding atoms to create structure Principle: Growth & Assembly E.g. Sol-gel, CVD, MBE Analogies in Real life

4. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Lithography Technique & its Importance Lithography is a top-down manufacturing process, with temporary patterns added or removed from a given area. Focused radiant energy + Chemical films Temporary patterns on Si or other wafers Applications: IC patterning process, printed electronic boar, printed plate, etcetera. This technique helps determine the min. feature size that limits throughput achieved in IC manufacturing. To decrease decrease (visible>Extreme UV (EUV)>Deep UV (DUV)). increase NA (or μ ) using Immersion Lithography technique.     NA : Numerical aperture of lens λ : Wavelength of radiant energy k 1 : Rayleigh const. (0.6 typically) μ : Refractive Index of medium in b/w Lens & Substate ϴ : Semi-angle of Lens   Feature Size ( )  

5. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Lithography Process Sequence Surface Cleaning  Barrier Layer Formation (Oxidation)  Spin coating with photoresist (PR)  Soft baking  Mask alignment  Exposure  Development  Hard baking  Post process cleaning

6. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Wafer Surface Preparation 1) Wafer Cleaning 2) Wafer Priming Courtesy: Dr. R. B. Darling (UW) lecture notes on photolithography

7. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Photoresist for Lithography Photoresist (PR) Photo sensitive material Temporarily coated on wafer surface Transfer design image on it through exposure Very similar to the photo sensitive coating on the film for camera Photoresist (PR) types Positive PR Negative PR Transfer pattern image same as that on the mask Transfer –ve of pattern image on the mask Becomes soluble post exposure Becomes insoluble post exposure Energy from light dissociates the +ve PR sensitizer breaking the cross-links Exposed PR becomes cross-linked polymer When developed, the exposed parts are dissolved When developed, the unexposed parts are dissolved Better resolution Poor resolution due to PR swelling as polymer absorbs development solvent More expensive Cheaper DQN (PAC – diazoquinone & Resin – novolac), PMMA (Polymethyl methacrylate) Polyisoprene

8. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Photoresist Chemistry PR generally consists of 3 parts: Resin: “plastic like” or “glue-like” compound that acts as a binder providing mechanical properties. Changes solubility due to photochemical reaction when exposed. Solvent: Chemicals used to dissolve the resin & control its mechanical properties like viscosity. Allows resin to be applied in a liquid state by spinning. Photoactive Compound (PAC): Act to inhibit or promote the dissolution of the resin in the developer. PAC inhibits dissolution in +ve PR before light exposure. After exposure the PAC promotes dissolution of the resin. Fig. Chemical Reaction of +ve PR based on DQN where diazoquinone (DQ) – PAC & novolac (N) – Resin Fig. Chemical Reaction of -ve PR based on photopolymerization of polystyrene

9. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Requirements & Properties of Photoresist High resolution Thinner PR film  Higher resolution Thinner PR film  Lower etching & Ion implantation resistance High etch resistance Good adhesion Wider process latitude Higher tolerance to process condition change PR must be able to withstand process conditions Coating, spinning, baking developing Etch resistance Ion implantation blocking Today DUV (deep UV) 193nm is used for IC industry, the +ve or –ve PR are no longer in use. For R&D many fancy resists are being used (e.g. AZ-5214 resist), which can be utilized as both +ve and –ve PR! Requirements of PR PR Physical Properties PR Performance Factors Resolution Adhesion Expose rate, Sensitivity and Exposure source Process latitude Pinholes Particle and Contamination Levels Step Coverage Thermal Flow

10. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Chemically Amplified Photoresists Chemical amplification can improve sensitivity significantly, with effective quantum efficiency>>100% DUV resists are all chemically amplified resist Photo-acid generator (PAG) is converted to an acid by photon exposure. Later, in a post exposure bake (PEV), the acid molecule reacts with a “blocking” molecule (protection groups) on a polymer chain, making it soluble in developer and regenerating the acid molecule. It is basically a catalytic chain reaction. In principle, only one photon is needed to generate one “seed” (acid catalyst), and all the rest reaction takes place during PEB* (thus PEB temperature needs to be tightly controlled for reproducible result). Fig. Basic operation of a chemically amplified resist. PAG is photo-acid generator, INSOL and SOL are the insoluble and soluble portions of the polymer base. *PEB – Post Exposure Baking

11. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Masks for Lithography Masks are used to produce a pattern on a substrate/ a wafer as in the case of chip manufacturing. Several masks are used in turn, each one reproducing a layer of the completed design, and together they are known as a mask set. Mask making is a fabrication process where a computer-aided design (CAD) is transferred to a thin (80-100 μ m) layer of metal in a glass or fused silica substrate, known as mask or photomask. The metal works as an absorption layer for light at different wavelengths. E-beam writers or Laser/LED writers are used to write onto the mask substrate. Binary mask Phase-shifting mask Consists of a transparent plate called blank, covered with a patterned film of opaque material. Adding a phase-shifting function to the on-off property of binary masks yields a higher resolution at the same or even larger amount of depth of focus. The transmission characteristic is a binary one, i.e., “1” for transparent and “0” for opaque. Both amplitude and phase are used to store the information about the image on the mask. The blank Is made of soda lime, borosilicate glass, or fused quartz. Recently enormous efforts for industrial application have been made, and pilot production has already started. Types of masking

12. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Main Exposure Modes in Lithography Contact Printing Proximity Printing Projection Printing The mask is directly in contact with the wafer surface. The mask is above the wafer surface. An optical system focusses the light source & reduces the mask image for exposure on the surface. Simple Low Cost Mask damage is minimal Good registration possible Low-cost processes Higher resolution Less system reduces diffraction error Poor for small features Mask damage may occur from contact Defects from contaminants on mask or wafer due to contacting Poorer resolution due to distance from the surface Diffraction errors Errors due to focus of lens system may occur Limiting factor in resolution can be due to optical system

13. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Performance Optimization Ways to improve resolution and decrease feature sizes Obvious ways: Thinner photoresist & larger NA Shorter wavelength (DUV, and even X-rays) Phase-shifting masks Diffraction-compensating pattern design Immersion photolithography Smart ways:

14. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Advanced Lithography E-beam Lithography X-ray Lithography Ion-beam Lithography Electron beam is used for direct writing. Primarily used to produce photomasks X-ray source is used High energy ion beam is used for writing Advantages Extremely small wavelength (<0.01 A or shorter). Sub-micro resolution (even 20nm resolution can be achieved). Direct patterning without a mask. No mirrors required. Greater depth of focus. Highly automated and precise control. Proximity effect due to electron scattering. Advantages Very small wavelength (1.5 Angstroms or shorter). Can be performed in air, but particles are a problem. Advantages Higher resolution than optical, x-ray or e-beam lithography because ions have a higher mass and therefore scatter less than electrons. Disadvantages Very low throughput (scanning of 10 wafers per hr.). Even though small features can be directly written, it often has large pitch due to intense sidelobes from diffraction. Bright sources of do not exist resulting in very limited throughput. Vacuum based technology (very expensive). Disadvantages Bright sources of X-rays exist, but are still not bright enough for high throughput. Polished mirrors are very difficult and expensive to make and maintain. Shares all of EUVL problems. Disadvantages Ion bean lithography may suffer from random space-charge effects, causing broadening of ion beam.

15. Since the minimum feature size ( ) is , the long term solution to obtaining smaller device sizes is to lower . Changes that must be made in current systems: Efficient small wavelength sources must be developed Reflective optics and masks must be developed Resists responsive in the extreme UV range must be developed Cost issues must be addressed   PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR Lithography: Future & Challenges Courtesy : Hong Xiao, www2.austin.cc.tx.us/HongXiao/Book.htm In 2021 Samsung used Extreme UV Lithography in production of 7 nm DRAM. Advanced EUV Stepper Courtesy : Dr. Alan Doolittle, Georgia Tech, ECE 6450 course lecture slides

16. PARTHA MISHRA CERAMIC ENGINEERING DEPT, NITR References Ceramic Coating Class Lecture (CR4204), 4 th year Spring Semester, Dr. Sudip Dasgupta, Ceramic Engineering Dept. NIT Rourkela. Photolithography. Hong Xiao, Ph. D. www2.austin.cc.tx.us/HongXiao/Book.htm. Lithography course ECE 6450 slides, Dr. Alan Doolittle, Georgia Tech. https://alan.ece.gatech.edu/ECE6450/Lectures/ECE6450L8-Photoresists%20and%20Nonoptical%20Lithography.pdf Photolithography lecture notes by Dr. R. B. Darling (UW). https://users.wfu.edu//ucerkb/Nan242/L15-Photolithography.pdf Coursera | Online Courses & Credentials From Top Educators. Join for Free | Coursera. (n.d.). Coursera. https://www.coursera.org/learn/nanotechnology/home/ Photolithography Applications – Membrane Solutions. (n.d.). https://www.membranesolutions.com/application_photolithography.htm Electron Beam Lithography (EBL), Pala, N., & Karabiyik, M. (2016), In Springer eBooks (pp. 1033–1057). https://doi.org/10.1007/978-94-017-9780-1_344 Photolithography, E-beam lithography, Ion beam lithography - For the “Nanochemistry” course / June 2023, B.S. Forough Zahedpour.