http://www.intel.com/research/silicon/mooreslaw.htm 1
Moore s law only holds due to photolithography advancements in reducing linewidths 2
All processing to create electric components and circuits rely on photolithography 3
Typical MOS transistor NMOS = n-type carrier across gate 4
Transistor fabrication N-MOS P-MOS 5
Interconnect 6
Chapter 1 sections 1-7 : 7
8
Basic process flow 9
10
Wafer clean: removal of Organics and metalics 11
HMDS Hexamethyldisilazane Prime: Replaces surface adsorbed H 2 O and gives off Ammonia. This material produces a bond with the wafer surface creating a polar surface ( electrostatic). No surface wetting by Photoresist occurs on an un-treated SiO2 surface with these bonded hydroxyl groups. Basically the Photoresist is hydrophobic and will not adhere to a hydrophilic surface. The HMDS is a hydroxyl getter and creates a hydrophobic surface, which the Photoresist had good adhesion. BOTTOMLINE: Priming adjusts the surface energy of the wafer so that it is comparable to the surface energy of the Photoresist. 12
Photoresist 13
Spin Coat 14
Spin Coat: RPMs: Spread or cast, Ramp,and terminal 15
Spin Coat 16
Spin Coat 17
Softbake Removes solvent from film and stablizing coating: typical: 90C to 120C I-line DNQ 100C to 130C: DUV CAR 18
Alignment 19
Photoresist Exposure DNQ photoresist actinic radiation 20
DUV: Photoresist Exposure wavelengths below 200nm All use excimer lasers Note 248nm = KrF laser 21
Photoresist Exposure 22
Exposure and feature type 23
Positive and Negative Tone Photoresists 24
Contact/Proximity/Projection printing 25
Projection printing: Typical stepper 26
Projection printing: Numerical Aperture 27
Projection printing: High NA lens http://www.research.ibm.com/journal/rd/411/singh.html 28
Projection printing: Resolution http://www.research.ibm.com/journal/rd/411/singh.html 29
Projection printing: Depth of Focus http://www.research.ibm.com/journal/rd/411/singh.html 30
Projection printing: Depth of Focus http://www.research.ibm.com/journal/rd/411/singh.html 31
Optical lithography Performance: Resolution NA and wavelength coherent systems 32
Photoresist Standing waves: reflection/interference 33
Photoresist Post Exposure Bake Purposes: key idea DNQ/Novolak positive tone: diffusion bake : Diffusion of PAC to improve CD contact by removing standing waves. PAG/Novolak negative tone: (acid hardened resist: AHR) Diffusion of H+ ion to react with polymer causing polymer to become insoluble.( PAG: Triazine) DUV PAG/Blocking group/phs: Diffusion of H+ ion to react with blocking group causing PHS to become soluble 34
Photoresist exposure and dissolution Key ideas: DNQ and Novolak Resin I-line 365nm system 1. DNQ or Photo Active Compound PAC is an Inhibitor: It inhibits Development rate when present! There is very little dissolution in an OH solution. 2. Photolytic conversion of DNQ to ICA in by exposure to Near UV radiation ( Hg lamp) increases development rate PAG and Poly Hydroxystyrene PHS DUV CAR 248nm system 1. PAG Photo-Acid-Generator creates an Acid (H+) upon exposure to 248nm radiation. There is very little dissolution in an OH solution. 2. The addition of thermal energy using a PEB bake causes the H+ to diffuse and react with the blocking group, causing the exposed area to become soluble. 35
Photoresist Development Threshold dose Develop exposed photoresist in TMAH (2.38%) basic solution 36
Photoresist CDs Dose Vs linewidth Polysilicon Linewidth DUV photoresist 0 Focus offset. 0.7 Photoresist linewidth CD um 0.68 0.66 0.64 0.62 0.6 y = -0.0267x + 1.0649 R 2 = 0.9727 ACEN ALL ALR AUL AUR ZCEN ZLL ZLR ZUL ZUR Linear (AUL) 0.58 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19 Exposure Dose 37
Photoresist CDs Dose Vs Spacewidth Contact CD Vs Exposure Dose Mj/cm2 IX405 i-line Photoresist Nominal 0.80u 0.95 Contact Photoresist CD microns 0.90 0.85 0.80 0.75 0.70 0.65 201CEN DI Linear (201CEN DI) y = 0.0013x + 0.5853 R 2 = 0.927 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 ASML 5500/100C Exposure dose mj/cm2 38
I-line Positive tone Photoresist 365nm http://www.jsrusa.com/resists.htm Line dense isolated line isolated space contact 39
I-line negative tone Photoresist 365nm http://www.jsrusa.com/resists.htm 40
DUV Photoresist KrF 248nm http://www.jsrusa.com/krfr11je.htm 41
DUV Photoresist OH contamination issue http://www.iemw.tuwien.ac.at/publication/workshop0600/hudek.html 42
Photoresist Post-develop bake Hardbake Improve adhesion of photoresist for subsequent wet processing: Wet etches: BHF, Acetic acid, H 2 O 2 Plateup: Au or Cu Increasing hardbake temperature will cause photoresist patterns to flow. 43
Photoresist Etch 44
Photoresist Etch: RIE http://www.iemw.tuwien.ac.at/publication/workshop0600/hudek.html 45
Photoresist removal: Strip Post etch 46