II-Lithography Fall 2013 Prof. Marc Madou MSTB 120 Content Lithography definitions Resist tone Introduction to the lithography process Surface Preparation Photoresist Application Soft Bake Align & Expose Develop Hard Bake Inspection Etch Layer or Add.
Download ReportTranscript II-Lithography Fall 2013 Prof. Marc Madou MSTB 120 Content Lithography definitions Resist tone Introduction to the lithography process Surface Preparation Photoresist Application Soft Bake Align & Expose Develop Hard Bake Inspection Etch Layer or Add.
II-Lithography Fall 2013 Prof. Marc Madou MSTB 120 Content Lithography definitions Resist tone Introduction to the lithography process Surface Preparation Photoresist Application Soft Bake Align & Expose Develop Hard Bake Inspection Etch Layer or Add Layer Resist Strip Final Inspection Clean- Room, Wafer Cleaning CD and Tg Making a Mask Moore’s ‘Law’ Photolithography -- Definitions Photolithography is used to produce 2 1/2-D images using light sensitive photoresist and controlled exposure to light. Microlithography is the technique used to print ultra-miniature patterns -- used primarily in the semiconductor industry. Photolithography -- Definitions Patterned wafer Test/Sort Diffusion Thin Films Polish Photo Etch Implant Photolithography is at the Center of the Wafer Fabrication * Process Resist Tone Negative: Prints a pattern that is opposite the pattern that is on the mask. of Positive: Prints a pattern that is the same as the pattern on the mask. Resist Tone Ultraviolet Light Chrome island on glass mask Areas exposed to light become polymerized and resist the develop chemical. Island Exposed area of photoresist Window photoresist Shadow on photoresist photoresist oxide oxide silicon substrate silicon substrate Negative Lithography Resulting pattern after the resist is developed. Resist Tone Areas exposed to light become photosoluble. Ultraviolet Light Chrome island on glass mask Island Shadow on photoresist Window photoresist Exposed area of photoresist photoresist oxide oxide silicon substrate silicon substrate Positive Lithography Resulting pattern after the resist is developed. Resist Tone Resist Tone Photoresist profiles – Overcut (LIFT-OFF) – Vertical – Undercut Resist Tone Photoresist profiles – Overcut (LIFT-OFF) – Vertical – Undercut Dose : High Developer: Low Dose : Medium Developer: Moderate Dose : Low Developer: Dominant Introduction to the Lithography Process Ten Basic Steps of Photolithography 1. Surface Preparation 2. Photoresist Application 3. Soft Bake 4. Align & Expose* 5. Develop 6. Hard Bake 7. Inspection 8. Etch 9. Resist Strip 10. Final Inspection * Some processes may include a Post-exposure Bake 1. Surface Preparation (HMDS vapor prime) Dehydration bake in enclosed chamber with exhaust Clean and dry wafer surface (hydrophobic) Hexamethyldisilazane (HMDS) Temp ~ 200 - 250°C Time ~ 60 sec. HMDS 1. Surface Preparation (HMDS vapor prime) 1. Surface Preparation (HMDS vapor prime) 2. Photoresist Application Wafer held onto vacuum chuck Dispense ~5ml of photoresist Slow spin ~ 500 rpm Ramp up to ~ 3000 - 5000 rpm Quality measures: – – – – – photoresist dispenser time speed thickness uniformity particles & defects vacuum chuck to vacuum pump spindle 2. Photoresist Application Resist spinning thickness T depends on: – Spin speed – Solution concentration – Molecular weight (measured by intrinsic viscosity) In the equation for T, K is a calibration constant, C the polymer concentration in grams per 100 ml solution, h the intrinsic viscosity, and w the number of rotations per minute (rpm) Once the various exponential factors (a,b and g) have been determined the equation can be used to predict the thickness of the film that can be spun for various molecular weights and solution concentrations of a given polymer and solvent system 2. Photoresist Application 2. Photoresist Application 3. Soft Bake Partial evaporation of photoresist solvents Improves adhesion Improves uniformity Improves etch resistance Improves linewidth control Optimizes light absorbance characteristics of photoresist 4. Alignment and Exposure Transfers the mask image to the resistcoated wafer Activates photosensitive components of photoresist Quality measures: UV Light Source Mask l – linewidth resolution – overlay accuracy – particles & defects Resist 4. Alignment and Exposure Alignment errors (many different types) Mask aligner equipment Double sided alignment especially important in micromachines 4. Alignment and Exposure 4. Alignment and Exposure 4. Alignment and Exposure Contact printing Proximity printing Self-aligned (see next) Projection printing : R = 2bmin = 0.6 l/NA z R = 2b min ~ 3 l (s ) 2 z R = 2b min ~ 3 l ) 2 4. Alignment and Exposure 4. Alignment and Exposure The defocus tolerance (DOF) Much bigger issue in miniaturization science than in ICs http://www.newport.com/tutornew/optics/ Optics_Reference_Guide.html 4. Alignment and Exposure 5. Develop Soluble areas of photoresist are dissolved by developer chemical Visible patterns appear on wafer developer dispenser – windows – islands Quality measures: – line resolution – uniformity – particles & defects vacuum chuck to vacuum pump spindle 6. Hard Bake Evaporate remaining photoresist Improve adhesion Higher temperature than soft bake 7. Development Inspection Optical or SEM metrology Quality issues: – – – – – particles defects critical dimensions linewidth resolution overlay accuracy 8. Plasma Etch-Or Add Layer Selective removal of upper layer of wafer through windows in photoresist: subtractive Two basic methods: CF4 – wet acid etch – dry plasma etch Quality measures: – – – – defects and particles step height selectivity critical dimensions Adding materials (additive) Two main techniques: – Sputtering – evaporation Plasma 8. Plasma Etch-Or Add Layer 9. Photoresist Removal (strip) No need for photoresist following etch process Two common methods: O2 – wet acid strip – dry plasma strip Followed by wet clean to remove remaining resist and strip byproducts Plasma 10. Final Inspection Photoresist has been completely removed Pattern on wafer matches mask pattern (positive resist) Quality issues: – – – – defects particles step height critical dimensions Clean-rooms, Wafer Cleaning Yellow light and low particle size/density curves Cleaning steps – RCA1-peroxides and NH3-removes organics – RCA2-peroxide and HCl-removes metals Dry vs. wet cleaning Supercritical cleaning-no liquid phase Clean-rooms, Wafer Cleaning Clean-rooms, Wafer Cleaning Clean-rooms, Wafer Cleaning Clean-rooms, Wafer cleaning Yield is the reason for the cleanrooms-the smaller the features the more important the cleanroom In the future people will work outside the cleanroom and only wafers will be inside the clean environment At universities, modularity (many different materials and processes) is more important than yield CD and Tg CD (e.g. 90 nm) i.e. critical dimension (the smallest feature made in a certain process) Glass transition temperature, above Tg the resist picks up dirt quite readily and the profile might get degraded Making a Mask Software Mask Moore’s ‘Law’ Observation and self fulfilling prophecy --not a physical law Is it running out of steam?