Lecture 16 – Introduction to Optical Lithography EECS 598-002 Winter 2006 Nanophotonics and Nano-scale Fabrication P.C.Ku Optical Lithography An optical system that transfers the image from the mask to the resist layer + the process of forming an etching mask (i.e. the resist development and etc.) EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 2 Resolution limits for imaging Small features correspond to large (kx, ky) components. In traditional optical microscopes, the detector sees the light in the far fieldk region. 2 2 222 k==++ωµε0 kkkxyz 22 ⇒+<⇒kkxyω nck/2/&,max =πλ n 17.5 15 Resolving power 12.5 k-space real-space 10 = λλ/2( n) ≡ eff /2 7.5 5 = diffraction limit 2.5 −2/π n λ 2/π n λ k& -2 -1 1 2 λ / n EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 3 Finite-size lens In a real system, the cutoff spatial frequency is often limited by the size of the lens which is quantitatively described by a numerical aperture (NA). NA≡ n sinθ k 2π ⇒=⇒=&,max sinθ k NA k &,max λ θ Resolving power Æ λλ/2NA( ) ≡ eff /2 where λeff = λ /NA EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 4 Patterning process Dissolution rate resist x I aerial image x Dissolution + rate I EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 5 Some clarifications The minimum feature size: The fundamental limit of optical lithography is not determined by the optical system alone but rather is an overall contributions from the optics, resist, develop and etching processes. Process window: Capability of printing small features does not always guarantee a good quality and a repeatable and controllable patterning. Alignment: Alignment to the underlying layer is equally as important as the optics. EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 6 How was our prediction in the past? 1.0 µm 0.7 µm 0.5 µm 0.35 µm 0.25 µm 0.18 µm 0.13 µm 0.10 µm ? EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 7 ITRS prediction in 1998 ITRS 1998: 193 DUV litho cannot produce 65 nm process. EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 8 ITRS 1999 157 nm appears on the map. EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 9 ITRS 2005 report Note: 157 nm off the chart now. EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 10 Major challenges (at this moment…) Data from ENIAC. EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 11 Evolution of optical lithography Contact and proximity printing Defects, gap control 1:1 projection printing Overlay, focus, mask cost Step-and-repeat projection Reduction possible printing Easier focus; Step-and-scan projection better usage of lens printing area EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 12 A step-and-scan system (stepper or scanner) Mask Wafer EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 13 Step-and-repeat vs step-and-scan Step-and-repeat Step-and-scan scan EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 14 Evolution of optics From Introduction to Microlithography EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 15 An example of the optics (NA=0.6, 4X reduction) US Patent 5969803 EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 16 Challenges in lens design Larger lens (required by better resolution) Æ aberration Suitably rotating the lens in the step-and-scan system can minimize the aberration Finite linewidth of laser source Æ dispersion Aspheric lens Æ more expensive Tighter spec on surface quality of lens Shortening the wavelength Æ more expensive raw materials EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 17 Resolution vs minimum linewidth Resolution often refers to the smallest pitch of a dense line/space pattern. It is limited by the diffraction limit. Important for DRAM/flash. Minimum linewidth is the minimum line or space that we can resolve. It has no fundamental limit. Important for logic chips (e.g. the gate length of a transistor) EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 18 There’s no fundamental limit to optical lithography! EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 19 Fundamentals of lithographic optics Diffraction Partial coherence Depth of focus Reflection and interference Polarization dependence EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 20 Fraunhofer diffraction (scalar; far-field) η y ξ x z Mask plane Image plane k ixy()22+ eeikz 2z Uxy(, )= FU[] (,)ξη fx =xz/λ izλ f y =xz/λ EM field EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 21 Diffraction from an aperture ⎛⎞ax FU[]()ξ = a sinc⎜⎟ fxzx = /λ ⎝⎠λz 22⎛⎞ax a Intensity ∝ a sin c ⎜⎟ ⎝⎠λz 1 0.9 0.8 0.7 Before the lens 0.6 0.5 0.4 0.3 0.2 0.1 0 -4 -3 -2 -1 0 1 2 3 4 λz/a EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 22 Diffraction of a line/space (N spaces) pattern 22 ⎛⎞⎛⎞Npxππ sx ⎜⎟⎜⎟sin sin Ix()∝ λλ s p ⎜⎟⎜⎟ππpx sx ⎜⎟⎜⎟sin ⎝⎠⎝⎠λλ 1 1 1 0.9 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.7 0.6 0.6 0.6 0.5 0.5 0.5 0.4 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0 0 0 -5 0 5 -5 0 5 -5 0 5 N=5 N=10 N=100 EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 23 Basic lithographic optics configuration illumination mask projection lens photoresist (image plane) EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 24 Image formation st Need to have at least the 0-th and the 1 diffraction orders being collected to recover the pitch information. +1 0 0 -1 -1 Oblique incidence can improve the minimum pitch but result in a less image contrast. EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 25.