Lithography 1
Lithography Is the Designer s Brush Lithography is indispensible for defining locations and configurations of circuit elements/functions. 2
ITRS 2007 The major challenge in litho: CD, CD control, Overlay, Defect Control, Cost. 3
Critical Layers Passivation 2, nitride Passivation 1, USG Lead-tin alloy bump At least the following 4 Tantalum barrier layer Metal 4 Copper Active Gate Contact M1 Tungsten plug Tungsten local Interconnection FSG Metal 3 Copper FSG FSG Metal 2 Copper FSG M 1 Cu Cu FSG FSG PSG Tungsten n + n + STI USG p + p + P-well N-well P-epi P-wafer Nitride etch stop layer Nitride seal layer Tantalum barrier layer T/TiN barrier & adhesion layer PMD nitride barrier layer 4
Photo Lithography 5
Photo Lithography Photolithography is the patterning process that transfers the designed patterns from the mask to the resist on the wafer surface. It is the core of the manufacturing process flow Process Sequence: photoresist coating, alignment and exposure, photoresist developing Requirements High resolution High sensitive Precise alignment Low defect density High yield and good imaging 6
Photolithography Process Light Source Mask Positive Resist Photoresist Substrate Resist Coating Alignment & Exposure Developing 7
Positive and Negative Resist Processes Photoresist Mask/reticle Photoresist Positive Photoresist Substrate Substrate After Development UV light Exposure Positive PR: from insoluble to soluble Negative PR: from soluble to insoluble Negative Photoresist Substrate Substrate Dr. Ko, Institute of Nanotechnology, NCTU 8
Photoresist Photoresists are the photosensitive material coated temporarily on the wafer surface and used to transfer the image of designed circuit on the mask to the wafer surface The photosensitive chemical reaction (break bonds) mainly sensitive to the UV light and not sensitive to the visible light. Since resist is not sensitive to yellow light, all the fabs use yellow to illuminate lithography areas Ingredients: polymer, solvent, sensitizer, additives 9
Photoresist Material Parameter Primary two functions of resist Precise pattern formation Protection of the substrate during etch Parameters can be categorized as follows: Optical properties: resolution, photosensitivity, and index of refraction Mechanical/chemical properties: solid content, viscosity, adhesion, etch resistance, thermal stability, and sensitivity to ambient Processing and safety related properties: particle count, metals content, process latitude, shelf life, flash point, and threshold limit value 10
Ingredients in I-line Resist Polymer: Novolak (Etch mask) Photoactive compound (PAC, or called sensitizer): diazonaphthoquinone (DNQ) (Control photochemical reaction during exposure) Additive: phenolic materials (Modify photochemical reaction during exposure) Solvent: PGMEA, EL (Liquid suspension) O H O H O Source:Prof. Ko in NCTU H3C CH2 C O CH3 C CH3 H3C C OH C O C2H5 11
Chemical Reaction in I-line Resist 12
Photo-Resist Positive PR Becomes soluble after exposure When developed, the exposed parts dissolved Better resolution Negative PR Exposed PR becomes crosslinked polymer Cross-linked polymer has higher chemical etch resistance. Becomes insoluble after exposure When developed, the unexposed parts dissolved. Cheaper material Swelling lead to poor resolution Contrast ratio : γ ln E T E 1 1 13
Positive and Negative Resists Azide/isoprene negative resist Swells during develop Marginal step coverage Organic solvent developer Toxic strippers Sensitive to ambient oxygen Novolak resin resist No swelling during develop Good step coverage Aqueous developer Environmentally benign resist stripper Operate well in air 14
Wafer Exposure System Light Source 1:1 Exposure Systems Usually 4X or 5X Reduction Optical System Mask Photoresist Si Wafer Gap Contact Printing Proximity Printing Projection Printing 15
Projection Printing Project an image of the mask pattern onto a resist coated wafer several centimeters away from the mask. 1:1 projection optical system is easier to design. M:1 projection mask is easier to fabricate (5:1 most common or 10:1). Projection method Scan Step-and-repeat Step-and-scan Synchronized mask and wafer movement Light Source Light Source Scan Wafer Step and repeat Projection Lens Reticle Projection Lens Slit Lens Mask Lens Photoresist Wafer Wafer Stage 16
Diffraction Light in free space Light through a small aperture 17
Fresnel Diffraction g W Incident Plane Wave Mask Aperture Resist Wafer Light Intensity at Resist Surface Fresnel diffraction (near-field diffraction) applies when λ<g < W2 λ Within this range, the minimum resolvable feature size is Wmin λg Thus if g = 10 µm and an i-line (365nm) light source is used, W min?? 18
Shadowing Printing Contact printing Resolution ~ 1um Dust on mask will damage PR pattern. Mask pattern may be contaminated. Proximity printing A small gap of 10-50 um. Longer mask lifetime. Poorer resolution. R = λ gap 19
Fraunhofer Diffraction R Fraunhofer diffraction (far-field diffraction) is dominate in the projection system. According to the Rayleigh Criterion: 1.22λ f 1.22λf 0.61 λ R = = = d n(2 f sin α) n sinα 20
Fraunhofer Diffraction Light Source Condenser Lens Mask Objective or Projection Lens Photoresist on Wafer α x y Aperture z x 1 y 1 Plane x'y' Plane x y Plane 21
R = K NA λ 1 Resolution λ: wavelength, K 1 : system constant=0.61 (ideal) NA = n sinα= d/2f NA (numerical aperture): the capability of the lens to collect the diffraction light. NA is proportional to the lens diameter (camera with large lens) and reversely proportional to the distance between the wafer and the lens. Strayed refracted light D Mask Lens Diffracted light collected by the lens r o Less diffraction after focused by the lens Ideal light Intensity pattern 22
Resolution Comparison 23
Depth of Focus DOF = K λ 2 ( NA) 2 NA : numerical aperture, λ : wavelength, K 2 : system constant =0.5 (ideal) Ex: Point and shot cameras use small lens without focus but the resolution won t be great! Lens Very flat wafer surface is needed -> CMP DOF Center of focus Depth of focus Photoresist Substrate 24
Exposure Wavelength High pressure Hg or Hg/Se lamp Intensity (a.u) Deep UV (<260) I-line (365) H-line (405) G-line (436) Lamp intensity is too low at λ < 260 nm Rayleigh Equation R=k 1 λ/na 300 400 500 600 Wavelength (nm) 25
Exposure Wavelength 26
Photolithography Light Sources Photon beam: 365nm (I-line/UV), 248nm (KrF/DUV), 193nm (ArF/DUV), 157nm (F 2 /VUV), 13.5nm (EUV) Name Wavelength (nm) Application feature Size (nm) G-line 436 500 Mercury Lamp H-line 405 I-line 365 350 to 250 XeF 351 XeCl 308 Excimer Laser KrF 248 250 to 110 ArF 193 110 to 65 Fluorine Laser F 2 157 65 to 40 27
Photolithography Process Pre-baking 90-120 C for 60-120 sec. Adhesion promoter vapor deposition or spin coating PR spin coater 3000-6000 RPM for 1um with edge bead remove Soft baking 90-120 C for 60-120 sec. Exposure Post exposure baking (PEB) ~100 C for 10 min. PR Developing Hard baking 100-180 C for 10-30 min. 28
Gate Oxide Wafer Clean Wafer Clean STI Polysilicon USG P-Well Removing all contaminants from the surface Standard cleaning process can remove particle, organic and metal The native oxide can be removed by dipping in diluted HF Improved adhesion Dr. Ko, Institute of Nanotechnology, NCTU 29
Substrate Preparation Pre-bake and Primer Vapor Primer STI Polysilicon P-Well USG Dehydration bake Remove moisture from wafer surface Promote adhesion between PR and surface Usually around 100 C Integration with primer coating Promotes adhesion of PR to wafer surface Widely used: Hexamethyldisilazane (HMDS) HMDS vapor coating prior to PR spin coating Usually performed in-situ with pre-bake Chill plate to cool down wafer before PR coating Dr. Ko, Institute of Nanotechnology, NCTU 30
Substrate Preparation Prep Chamber Primer Layer Wafer HMDS Vapor Wafer Hot Plate Dehydration Bake Hot Plate Primer Vapor Coating H 3 C H CH 3 Si Si 晶圓表面 H 3 C Si N Si CH 3 H 3 C CH 3 OH Si Si OH 親水性表面 H 3 C H 3C CH 3 CH 3 Si Si H 3 C CH 3 O O + H 2 O + NH 3 Si Si 疏水性表面 Dr. Ko, Institute of Nanotechnology, NCTU 31
Resist Coating Primer STI Photoresist Polysilicon P-Well USG Wafer sits on a vacuum chuck Liquid photoresist applies at center of wafer Rotate at high speed Photoresist spreads by centrifugal force Evenly coat on wafer surface Dr. Ko, Institute of Nanotechnology, NCTU 32
Resist Spin Coater 33
Resist Thickness Viscosity Effect Dynamic Spin Rate Photoresist spread on spinning wafer surface Wafer held on a vacuum chuck Slow spin ~ 500 rpm Ramp up to ~ 3000-7000 rpm 34
Resist Spin Coater 35
Soft Baking Hot plate heating is widely used Evaporating most of solvents from PR film (5~20 % left) Solvents help to make a thin PR but absorb radiation and affect adhesion Soft baking time and temperature are determined by the matrix evaluations Over bake: polymerized, less photo-sensitivity Under bake: affect adhesion and exposure (solvent is not sensitive) Dr. Ko, Institute of Nanotechnology, NCTU 36
Alignment & Exposure Gate Mask Gate Mask Photoresist Alignment and exposure Photoresist Polysilicon Polysilicon STI USG STI USG P-Well P-Well Alignment mark Most critical process for IC fabrication Most expensive tool (stepper or scanner) in an IC fab Most challenging technology Determines the minimum feature size Currently 65 nm and pushing to 45 nm Dr. Ko, Institute of Nanotechnology, NCTU 37
Structure of Mask Cr 2 O 3, 30nm Cr, 70nm quartz, 2.3mm Electron beam lithography Cr 2 O 3 Cr quartz Which illumination mode is correct? Dr. Ko, Institute of Nanotechnology, NCTU 38
Masks Bright field Dark field 39
Post Exposure Baking λ/2n PR Photoresist Photoresist STI Polysilicon P-Well USG Overexposure Underexposure Substrate Photoresist has glass transition temperature (T g ) Baking temperature higher than T g Thermal movement of photoresist molecules Rearrangement of the overexposed and underexposed PR molecules Average out standing wave effect Smooth PR sidewall and improve resolution Dr. Ko, Institute of Nanotechnology, NCTU 40
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Development STI PR Polysilicon P-Well USG International standard of 2.38% TMAH (0.26 N) is used to reveal pattern Tetramethylammonium hydroxide, (CH 3 ) 4 NOH Developer solvent dissolves the softened part of photoresist Transferring the pattern from mask or reticle to photoresist Three basic steps: - Development -Rinse -Dry Dr. Ko, Institute of Nanotechnology, NCTU 42
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Development 44
Hard Baking STI PR Polysilicon P-Well USG Dr. Ko, Institute of Nanotechnology, NCTU Evaporating all solvents in PR Improving etch and implantation resistance Improve PR adhesion with surface Polymerize and stabilize photoresist PR flow to fill pinhole Under-bake Photoresist is not fully polymerized High photoresist etch rate Poor adhesion Over-baking PR flow and bad resolution 45
Hard Baking 46
Pattern Inspection STI PR Polysilicon P-Well USG Inspection: go next step or rework (stripping PR) Photoresist pattern is temporary Etch or ion implantation pattern is permanent Photolithography process can rework Can t rework after etch or implantation Scanning electron microscope (SEM) Optical microscope (OM) Dr. Ko, Institute of Nanotechnology, NCTU 47
Pattern Inspection Overlay or alignment run-out, run-in, reticle rotation, wafer rotation, misplacement in X-direction, and misplacement in Y-direction Critical dimension (CD) Surface irregularities such as scratches, pin holes, stains, contamination, etc. 48
Critical Dimension (CD) Measurement In-line SEM plane-view inspection Off-line SEM cross-sectional inspection CD Atomic Force Microscope 49
Critical Dimension (CD) 50
Misalignment 51