EE143 F05 Lecture 11 Photolithography Key Topics Photo s through light Litho s stone Graphy writing Resolution Depth of Focus Overlay Errors Photoresist Response E beam and X ray lithography Professor N Cheung U C Berkeley 1 EE143 F05 Professor N Cheung U C Berkeley Lecture 11 2 EE143 F05 Professor N Cheung U C Berkeley Lecture 11 3 EE143 F05 Lecture 11 Contact Printing hv Photo Mask Plate photoresist wafer Resolution R 0 5 m mask plate is easily damaged or accumulates defects Professor N Cheung U C Berkeley 4 EE143 F05 Lecture 11 Proximity Printing hv g 20 m Photoresist wafer exposed R k g 1 2 1 m for visible photons much smaller for X ray lithography Professor N Cheung U C Berkeley x 5 EE143 F05 Lecture 11 Projection Printing hv De Magnification nX 10X stepper 4X stepper 1X stepper lens focal plane P R wafer 0 2 m resolution deep UV photons tradeoff optics complicated and expensive Professor N Cheung U C Berkeley 6 EE143 F05 Lecture 11 Aerial Images formed by Contact Printing Proximity Printing and Projection Printing Professor N Cheung U C Berkeley 7 EE143 F05 Lecture 11 Optical Stepper field size increases with future ICs scribe line 1 2 wafer Image field Professor N Cheung U C Berkeley Translational motion 8 EE143 F05 Lecture 11 Excimer Laser Stepper Light is in pulses of 20 ns duration at a repetition rate of a few kHz About 50 pulses are used See Plummer et al Ch 5 Professor N Cheung U C Berkeley 9 EE143 F05 Lecture 11 Excimer Laser Stepper Professor N Cheung U C Berkeley 10 EE143 F05 Lecture 11 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 Professor N Cheung U C Berkeley 11 EE143 F05 Professor N Cheung U C Berkeley Lecture 11 12 EE143 F05 Professor N Cheung U C Berkeley Lecture 11 13 EE143 F05 Professor N Cheung U C Berkeley Lecture 11 14 EE143 F05 Lecture 11 Qualitative Explanation of image degradation by lens Mask 2 lens wafer plane 1 0 1 parallel optical beam 2 grating with spatial frequency 1 P L L P Professor N Cheung U C Berkeley P sin n n 0 1 2 sin NA of lens P 2L 15 EE143 F05 Why Mask Intensity Imax lm NA Lecture 11 The lens has to collect at least the n 1 diffracted beams to show any spatially varying intensity on wafer Therefore Pm 2 lm NA O Image on wafer x optical system x For advanced reading see Microlithography Science and Technology by Sheats and Smith Professor N Cheung U C Berkeley 16 EE143 F05 Lecture 11 Effect of Fourier Components on aerial image ofn 0 a rectangular waveform n 0 n 1 n 0 n 1 n 3 n 0 n 1 n 3 n 5 Professor N Cheung U C Berkeley 17 EE143 F05 Lecture 11 Depth of Focus DOF off point best Professor N Cheung U C Berkeley 18 EE143 F05 Lecture 11 Simulated aerial images with various degree of defocus No defocus Note degradation of image contrast and image slope Defocus increases Professor N Cheung U C Berkeley 19 EE143 F05 Lecture 11 Example of DOF problem Photo mask Field Oxide Different photo images Professor N Cheung U C Berkeley 20 EE143 F05 Lecture 11 Focus versus Extreme Defocus an illustration Large P features Small P features For ForReference Referenceonly only Best focus Extreme Defocus Professor N Cheung U C Berkeley 21 EE143 F05 Lecture 11 1 lm 0 6 NA 2 DOF want small lm 2 NA 2 want large DOF lm 1 1 and and 2 2 require requireaacompromise compromisebetween between and andNA NA Professor N Cheung U C Berkeley 22 EE143 F05 Lecture 11 Image Quality Metric Contrast Professor N Cheung U C Berkeley 23 EE143 F05 Lecture 11 Image Quality metric Slope of image simulated aerial image of an isolated line Professor N Cheung U C Berkeley 24 EE143 F05 Lecture 11 Two Resist Types Negative Resist Polymer Molecular Weight MW 65000 Light Sensitive Additive Promotes Crosslinking Volatile Solvents Light breaks N N Crosslink Chains Sensitive hard Swelling during Develop Positive Resist Polymer MW 5000 Photoactive Inhibitor 20 Volatile Solvents Inhibitor Looses N2 Alkali Soluble Acid Develops by etching No Swelling Professor N Cheung U C Berkeley 25 EE143 F05 Lecture 11 Positive Resist hv mask exposed part is removed 100 linear scale P R ET resist sensitivity 1 Resist contrast log10 1 LOG TO BASE 10 Professor N Cheung U C Berkeley resist thickness remaining E1 ET exposure photon energy log scale 5 to 10 Note Note In Inthe the143 143Reader Reader isisdefined definedas asnatural naturallog log 26 EE143 F05 Lecture 11 Positive P R Mechanism Photons deactivate sensitizer less cross linking dissolve in developer solution polymer photosensitizer Professor N Cheung U C Berkeley 27 EE143 F05 Lecture 11 diazide Positive Resist Exposure Reaction ketene a C 0 group The ketene is shortlived intermediate PAC carboxylic acid moisture The carboxylic acid can react with the alkaline solution the soluble ester developer to form a soluble ester Professor N Cheung U C Berkeley 28 EE143 F05 Lecture 11 Negative P R Mechanism hv remaining mask after development ET 1 E1 log ET E1 photon energy hv cross linking insoluble in developer solution Log to base 10 Professor N Cheung U C Berkeley 29 EE143 F05 Infinite InfiniteContrast ContrastResist Resist DD00 D D critical D100 100 Dcritical Lecture 11 Finite FiniteContrast ContrastResist Resist DD00 D D100 100 energy Professor N Cheung U C Berkeley 30 EE143 F05 Professor N Cheung U C Berkeley Lecture 11 31 EE143 F05 Lecture 11 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 Professor N Cheung U C Berkeley 32 EE143 F05 Lecture 11 Overlay Errors alignment mask wafer photomask plate Professor N Cheung U C Berkeley Alignment marks from previous masking level 33 EE143 F05 Lecture 11 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 Professor N Cheung U C Berkeley 34 EE143 F05 Lecture 11 run out 2 Translational Error Al image n p Professor N Cheung U C Berkeley referrer 35 EE143 F05 Lecture 11 3 Rotational Error Professor N Cheung U C Berkeley 36 EE143 F05 Professor N
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