Section 2 Lithography Jaeger Chapter 2 Litho Reader EE143 Ali Javey Slide 5 1 The lithographic process EE143 Ali Javey Slide 5 2 Photolithographic Process a b c d e f g EE143 Ali Javey Substrate covered with silicon dioxide barrier layer Positive photoresist applied to wafer surface Mask in close proximity to surface Substrate following resist exposure and development Substrate after etching of oxide layer Oxide barrier on surface after resist removal View of substrate with silicon dioxide pattern on the surface Slide 5 3 Photomasks CAD Layout Composite drawing of the masks for a simple integrated circuit using a four mask process Drawn with computer layout system Complex state of the art CMOS processes may use 25 masks or more EE143 Ali Javey Slide 5 4 Photo Masks Example of 10X reticle for the metal mask this particular mask is ten times final size 10 m minimum feature size huge Used in step and repeat operation One mask for each lithography level in process EE143 Ali Javey Slide 5 5 Lithographic Process EE143 Ali Javey Slide 5 6 Printing Techniques Contact printing Proximity printing Projection printing Contact printing damages the mask and the wafer and limits the number of times the mask can be used Proximity printing eliminates damage Projection printing can operate in reduction mode with direct step onwafer EE143 Ali Javey Slide 5 7 Contact Printing hv Photo Mask Plate photoresist wafer Resolution R 0 5 m mask plate is easily damaged or accumulates defects EE143 Ali Javey Slide 5 8 Proximity Printing hv g 20 m Photoresist wafer exposed R is proportional to g 1 2 1 m for visible photons much smaller for X ray lithography EE143 Ali Javey Slide 5 9 Projection Printing hv De Magnification nX lens 10X stepper 4X stepper 1X stepper focal plane P R wafer 0 2 m resolution deep UV photons tradeoff optics complicated and expensive EE143 Ali Javey Slide 5 10 Diffraction EE143 Ali Javey Slide 5 11 Aerial Images formed by Contact Printing Proximity Printing and Projection Printing EE143 Ali Javey Slide 5 12 Photon Sources Hg Arc lamps 436 G line 405 H line 365 I line nm Excimer lasers KrF 248nm and ArF 193nm Laser pulsed plasma 13nm EUV Source Monitoring Filters can be used to limit exposure wavelengths Intensity uniformity has to be better than several over the collection area Needs spectral exposure meter for routine calibration due to aging EE143 Ali Javey Slide 5 13 Optical Projection Printing Modules Optical System illumination and lens Resist exposure post exposure bake and dissolution Mask transmission and diffraction Wafer Topography scattering Alignment 14 Optical Stepper field size increases with future ICs scribe line 1 2 wafer Image field Translational motion EE143 Ali Javey Slide 5 15 Resolution in Projection Printing f focal distance d lens diameter Minimum separation of a star to be visible 16 Resolution limits in projection printing EE143 Ali Javey Slide 5 17 Depth of Focus DOF point EE143 Ali Javey Slide 5 18 EE143 Ali Javey Slide 5 19 Example of DOF problem Photo mask Field Oxide Different photo images EE143 Ali Javey Slide 5 20 Tradeoffs in projection lithography 1 lm 0 6 NA 2 DOF want small lm 2 NA 2 want large DOF 1 1 and and 2 2 require requireaacompromise compromisebetween between and andNA NA EE143 Ali Javey Slide 5 21 Sub resolution exposure Phase Shifting Masks Pattern transfer of two closely spaced lines a Conventional mask technology lines not resolved b Lines can be resolved with phase shift technology EE143 Ali Javey Slide 5 22 Immersion Lithography A liquid with index of refraction n 1 is introduced between the imaging optics and the wafer Advantages 1 Resolution is improved proportionately to n For water the index of refraction at 193 nm is 1 44 improving the resolution significantly from 90 to 64 nm 2 Increased depth of focus at larger features even those that are printable with dry lithography EE143 Ali Javey Slide 5 23 Image Quality Metric Contrast Contrast is also sometimes referred as the Modulation Transfer Function MTF EE143 Ali Javey Slide 5 24 Questions How does contrast change as a function of feature size How does contrast change for coherent vs partially coherent light EE143 Ali Javey Slide 5 25 Image Quality metric Slope of image simulated aerial image of an isolated line EE143 Ali Javey Slide 5 26 The need for high contrast Optical image Infinite contrast resist resist Finite contrast resist substrate resist substrate Position x EE143 Ali Javey Slide 5 27 Resists for Lithography Resists Positive Negative Exposure Sources Light Electron beams Xray sensitive EE143 Ali Javey Slide 5 28 Two Resist Types Negative Resist Composition Polymer Molecular Weight MW 65000 Light Sensitive Additive Promotes Crosslinking Volatile Solvents Light breaks N N in light sensitive additive Crosslink Chains Sensitive hard Swelling during Develop Positive Resist Composition Polymer MW 5000 Photoactive Dissolution Inhibitor 20 Volatile Solvents Inhibitor Looses N2 Alkali Soluble Acid Develops by etching No Swelling EE143 Ali Javey Slide 5 29 Positive P R Mechanism Photons deactivate sensitizer dissolve in developer solution polymer photosensitizer EE143 Ali Javey Slide 5 30 Positive Resist hv mask exposed part is removed 100 linear scale P R Resist contrast resist thickness remaining Q E 10 Q E fT exposure photon energy log scale 1 Qf log10 Q 0 EE143 Ali Javey Slide 5 31 Negative P R Mechanism hv remaining mask after development QET f Q E1 photon energy 0 hv cross linking insoluble in developer solution EE143 Ali Javey Slide 5 32 Positive vs Negative Photoresists Positive P R 9 higher resolution 9 aqueous based solvents 8 less sensitive Negative P R 9 more sensitive higher exposure throughput 9 relatively tolerant of developing conditions 9 better chemical resistance better mask material 9 less expensive 8 lower resolution 8 organic based solvents EE143 Ali Javey Slide 5 33 Overlay Errors alignment mask wafer photomask plate Alignment marks from previous masking level EE143 Ali Javey Slide 5 34 1 Thermal Run in Run out errors R r Tm m Tsi si run out wafer error radius Tm Tsi change of mask and wafer temp m si coefficient of thermal expansion of mask Si EE143 Ali Javey Slide 5 35 Rotational Translational Errors 2 Translational Error image Al n p 3 Rotational Error EE143 Ali Javey referrer Slide 5 36 Overlay implications Contacts Al SiO2 ideal SiO2 n p Si Al SiO2 Alignment error SiO2 n short ohmic contact p Si Solution Design n region larger than contact hole Al
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