Immersion Lithography ECE487 Spring 09 Raj Chakraborty Outline • Motivation for Immersion Lithography (need this first to understand what immersion litho is) • What are we immersing in? • Implementation Issues • Implementation Solutions • Beyond 45nm node – The march towards 1.65 NA • Key Players in Immersion Litho • Q&A Motivation for Immersion Lithography Rayleigh equation: K1: resolution/process factor (currently ~0.25-0.4). Function of RET (Resolution Enhancement Techniques) and resist quality Lambda: wavelength NA: Numerical Aperture Lambda: – 1.2μm and larger linewidths: G-line output of mercury lamps (436nm) was used – 0.8μm linewidths: I-line output of mercury lamps (365nm) was introduced for critical layers – 0.35um linewidths: I-line use continued to the 350nm linewidth – 130nm linewidths: Krypton Fluoride (KrF) Excimer Lasers (248nm) – 90nm linewidths: KrF being replaced by Argon Fluoride (ArF) Excimer lasers (193nm) Motivation NA: - Mid eighties ~ NA value of approximately 0.4  - Today ~ NA greater than 0.8.  KEY POINT: The physical theoretical limit to NA for exposure systems using air as a medium between the lens and the wafer is 1! Practical limit ~ 0.93 The resolution limit for 193nm exposure systems may be calculated using the Rayleigh equation Lambda = 193nm, NA = 0.93 and k1 = 0.25 A highly optimized ArF exposure system has an absolute maximum resolution of 52nm, sufficient for 65nm linewidths, but not capable of meeting 45nm linewidths. Motivation NA is actually determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens and is given by: where, n is the index of refraction of the medium surrounding the lens and α is the acceptance angle of the lens KEY POINT: What if a medium with a higher index of refraction is substituted for air? Microscopy does this currently.. What are we immersing in? The medium between the lens and the wafer being exposed needs to have: - index of refraction >1 - low optical absorption at 193nm - be compatible with photoresist and the lens material - be uniform and non-contaminating. Surprisingly, UltraPure Water (UPW) may meet all of these requirements. Water has an index of refraction n ~1.47, absorption of <5%, is compatible with photoresist and lens, and in it’s ultrapure form is non-contaminating. Plugging in n = 1.47 and assuming sinα can reach 0.93 (NA = 1.36) the resolution limits for 193nm immersion lithography are: Implementation Issues The stage on a 193nm exposure tool steps from location to location across the wafer scanning the reticle image for each field ~ Maintaining a consistent bubble free liquid between the lens and the wafer is very difficult. Complete removal of liquid after exposure is completed Liquid Temperature control ~ variations in n. Maintaining temperature uniformity with a rapidly moving stage and a pulsed laser passing through the fluid is a challenge. Implementation Issues • Consistent Water purity (both particles and gasses)~ affects n • Leaching (and swelling) from resist ~ need water compatible resists  • Outgassing from resist • Conventional Lens material degradation  Implementation Solutions Water puddle technique: dispense water between the lens and the wafer with a specialized nozzle and rely on surface tension to maintain a bubble-free puddle Specialized vacuum nozzle assembly to remove liquid Transparent top-coat layer on resist and low outgassing resists Lutetium Aluminum Garnet (LuAG) lens material is more compatible with immersion liquids and also has higher n.  Point of Use (POU) temperature control unit for liquid achieves temp control to within 0.01C Point of Use (POU) water microfiltration (down to 3nm) and degassing of water. Beyond 45nm node – The march towards 1.65 NA Immersion applied to 157nm (F2 laser) exposure could carry optical lithography even further (<30nm), although water is not a usable medium at 157nm (because it absorbs too much) and suitable mediums are still being researched. Exposure source stability is also an issue at 157nm. Due to the issues with 157nm exposure, carrying immersion into the 32nm node will require higher NA scanners ~1.65. Current immersion systems are at about 1.35.  Refractive index of medium, resist and lens need to be increased to achieve 1.65 NA Doped water (mixing water with Sulfates, phosphates) is a possibility (n~1.65) but slightly higher absorbance due to dopants. Other exotic liquids. Resist polymer engineering to increase resist n. Challenges will include development of a suitable medium, high index resist, and high index lens material with the required inter- compatibility Beyond 45nm node Strong contender with EUV for 32/22nm node. Matter of fact, being implemented by Intel at 32nm (1268 process). Key concern will be defects. It has been estimated that wet lithography (immersion) is statistically ~ 20% more vulnerable to defect related issues compared to dry, based on End of Line (EOL) yield data.  Defect sources will primarily be due to microbubbles and particle carrying properties of liquids. Will require many vendors to work closely together to resolve the impending issues (resist vendors, scanner vendors, filtration and purification vendors, lens optics vendors, laser vendors) Key Players in Immersion Litho • Scanner OEM’s: ASML (Twinscan), Nikon, Canon • Track OEM’s: TEL (Lithius), DNS • Resist vendors: JSR Micro, TOK, Dupont, Fujifilm. • Laser OEM’s: Cymer • Filtration and Purification product vendors: Pall, Mykrolis • Lens and optics vendors: Carl Zeiss • Finally, IDM’s using immersion litho: Matsushita (at 45nm node), Intel (at 32nm), TSMC (at 90nm). References • www.solidstate.com/articles/article_display.html?id=20502 • www.nikon.com/about/technology/core/optical_u/immersio n_e/index.htm • www.almaden.ibm.com/st/chemistry/lithography/immersio n/ • spie.org/x8368.xml • www.asml.com/immersion/ • www.eetimes.com/news/latest/showArticle.jhtml?articleID= 177103820 • www.future-fab.com/documents.asp?d_ID=1896 • www.nec.co.jp/techrep/en/journal/g09/n01/090114.html Q&A Thank You!
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