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. 3 3 3 3 3 3 4 4 4 3 3

Photolithography is at the Center of the Wafer Fabrication Process Photolithography -- Definitions * Thin Films Implant Diffusion Etch Test/Sort Polish Photo Patterned wafer Photolithography is at the Center of the Wafer Fabrication Process 4 4 4 4 4 4 5 5 5 4 4

Resist Tone Negative: Prints a pattern that is opposite of the pattern that is on the mask. Positive: Prints a pattern that is the same as the pattern on the mask. 10 10 10 10 10 10 11 11 11 10 10

Resist Tone Negative Lithography photoresist oxide silicon substrate Ultraviolet Light Exposed area of photoresist Shadow on photoresist Chrome island on glass mask Areas exposed to light become polymerized and resist the develop chemical. Island silicon substrate oxide photoresist Window Resulting pattern after the resist is developed. Negative Lithography 11 11 11 11 12 12 12 11 11

Resist Tone Positive Lithography photoresist silicon substrate oxide Ultraviolet Light Areas exposed to light become photosoluble. Chrome island on glass mask Island Shadow on photoresist Window photoresist Exposed area of photoresist oxide silicon substrate Resulting pattern after the resist is developed. Positive Lithography 12 12 11 12 12 12 13 13 13 12 12

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

Ten Basic Steps of Photolithography Introduction to the Lithography Process 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 Ten Basic Steps of Photolithography This following slides addresses Engineering Technology Content Standard 3 and 4 on Manufacturing and Materials. * Some processes may include a Post-exposure Bake 13 13 14 14 14 13

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. 14 13 14 14 15 15 15 14

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: time speed thickness uniformity particles & defects vacuum chuck spindle to vacuum pump photoresist dispenser 15 14 15 15 16 16 16 15

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 photo-resist solvents Improves adhesion Improves uniformity Improves etch resistance Improves linewidth control Optimizes light absorbance characteristics of photoresist 16 15 16 16 17 17 17 16

4. Alignment and Exposure UV Light Source Mask Resist l Transfers the mask image to the resist-coated wafer Activates photo-sensitive components of photoresist Quality measures: linewidth resolution overlay accuracy particles & defects 17 16 17 17 18 18 18 17

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/NA ~ ~

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 windows islands Quality measures: line resolution uniformity particles & defects vacuum chuck spindle developer dispenser to vacuum pump 18 18 18 17 19 19 19 18

6. Hard Bake Evaporate remaining photoresist Improve adhesion Higher temperature than soft bake 19 18 19 19 20 20 20 19

7. Development Inspection Optical or SEM metrology Quality issues: particles defects critical dimensions linewidth resolution overlay accuracy 20 19 20 20 21 21 21 20

8. Plasma Etch-Or Add Layer Selective removal of upper layer of wafer through windows in photoresist: subtractive Two basic methods: 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 CF4 21 20 21 21 22 22 22 21

8. Plasma Etch-Or Add Layer

9. Photoresist Removal (strip) No need for photoresist following etch process Two common methods: wet acid strip dry plasma strip Followed by wet clean to remove remaining resist and strip byproducts O2 Plasma 22 21 22 22 23 23 23 22

10. Final Inspection Photoresist has been completely removed Pattern on wafer matches mask pattern (positive resist) Quality issues: defects particles step height critical dimensions 23 22 23 23 24 24 24 23

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 clean-rooms-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?