Lecture 14 Taken in part from Chapters 13

Lecture 14 Taken in part from Chapters 13 -15 Semiconductor Manufacturing Technology 14 - 1

Objectives • Basic concepts for photolithography, including process overview, critical dimension generations, light spectrum, resolution and process latitude. • Difference between negative and positive lithography. • Eight basic steps to photolithography. • Wafer surface preparation for photolithography. • Photoresist physical properties. • Applications of conventional i-line photoresist. • Deep UV resists • Photoresist application techniques • Soft bake processing 14 - 2

Wafer Fabrication Process Flow Wafer fabrication (front-end) Wafer start Thin Films Polish Photo Etch Unpatterned wafer Completed wafer Test/Sort Diffusion Implant 14 - 3

Photolithography Concepts • Patterning process – Photomask – Reticle • • • Critical dimension generations Light spectrum and wavelengths Resolution Overlay accuracy Process latitude 14 - 4

Three Basic Exposure Methods 1: 1 Exposure ~5: 1 Exposure 14 - 5

• Contact printing capable of high resolution but has unacceptable defect densities. May be used in Development but not manufacturing. • Proximity printing cannot easily print features below a few mm in line width. Used in nano-technolgy. • Projection printing provides high resolution and low defect densities and dominates today. Typical projection systems use reduction optics (2 X - 5 X), step and repeat or step and scan. They print » 50 wafers/hour and cost $5 - 10 M. 14 - 6

Steps in Lithography Process Lithography has three parts: (1) Light source, (2) Wafer exposure (3) Resist 14 - 7

14 - 8

Photomask and Reticle for Microlithography 1: 1 Mask 4: 1 Reticle 14 - 9

Three Dimensional Pattern in Photoresist Linewidth Space Photoresist Thickness Substrate 14 - 10

Section of the Electromagnetic Spectrum Visible Gamma rays f (Hz) (m) 10 10 22 -14 (nm) 10 20 -12 10 157 X-rays 10 10 193 -10 Infrared UV 18 10 10 248 VUV DUV -8 16 Microwaves 10 10 14 10 -6 10 365 i 12 -4 10 10 Radio waves 10 -2 10 10 0 8 10 10 2 6 10 4 405 436 h g Common UV wavelengths used in optical lithography. 14 - 11

Light Sources Decreasing feature sizes requires shorter λ. • Hg vapor lamps: Hg plasma inside glass lamp – – Produces multiple wavelengths Limited in intensity “g” line: λ = 436 nm (used to mid 1980 s) “I” line: λ = 365 nm (early 1990 s, >0. 3 μm) • Deep UV by excimer lasers – Kr + NF 3 + (energy) → Kr. F + (photon emission) • Kr. F: λ = 248 nm (used for 0. 25 μm) • Ar. F: λ = 193 nm (used for 0. 12 μm) 14 - 12

Important Wavelengths for Photolithography Exposure Table 13. 1 14 - 13

Importance of Mask Overlay Accuracy Top view of CMOS inverter The masking layers determine the accuracy by which subsequent processes can be performed. The photoresist mask pattern prepares individual layers for proper placement, orientation, and size of structures to be etched or implanted. PMOSFET NMOSFET Small sizes and low tolerances do not provide much room for error. Cross section of CMOS inverter Figure 13. 4 14 - 14

Photolithography Processes • Negative Resist – Wafer image is opposite of mask image – Exposed resist hardens and is insoluble – Developer removes unexposed resist • Positive Resist – Mask image is same as wafer image – Exposed resist softens and is soluble – Developer removes exposed resist 14 - 15

Negative Lithography Ultraviolet light Chrome island on glass mask Areas exposed to light become crosslinked and resist the developer chemical. Island Exposed area of photoresist Window Photoresist Shadow on photoresist Photoresist Oxide Silicon substrate Resulting pattern after the resist is developed. 14 - 16

Positive Lithography Ultraviolet light Areas exposed to light are dissolved. Chrome island on glass mask Shadow on photoresist Island Window photoresist Photoresist Exposed area of photoresist Photoresist oxide Oxide silicon substrate Silicon substrate Resulting pattern after the resist is developed. 14 - 17

Relationship Between Mask and Resist Desired photoresist structure to be printed on wafer Island of photoresist Substrate Chrome Window Mask pattern required when using negative photoresist (opposite of intended structure) Quartz Island Mask pattern required when using positive photoresist (same as intended structure) 14 - 18

Clear Field and Dark Field Masks Clear Field Mask Simulation of metal interconnect lines (positive resist lithography) Dark Field Mask Simulation of contact holes (positive resist lithography) 14 - 19

Eight Steps of Photolithography 14 - 20

Eight Steps of Photolithography UV Light HMDS Resist Mask 1) Vapor prime 2) Spin coat 3) Soft bake 4) Alignment and Exposure 5) Post-exposure bake 6) Develop 7) Hard bake 8) Develop inspect 14 - 21

Photolithography Track System 14 - 22

Vapor Prime The First Step of Photolithography: • Promotes Good Photoresist-to-Wafer Adhesion • Primes Wafer with Hexamethyldisilazane, HMDS • Followed by Dehydration Bake • Ensures Wafer Surface is Clean and Dry 14 - 23

Spin Coat Process Summary: • Wafer is held onto vacuum chuck • Dispense ~5 ml of photoresist • Slow spin ~ 500 rpm • Ramp up to ~ 3000 to 5000 rpm • Quality measures: – time – speed – thickness – uniformity – particles and defects Photoresist dispenser Vacuum chuck To vacuum pump Spindle connected to spin motor 14 - 24

Soft bake Characteristics of Soft Bake: • • • Improves Photoresist-to-Wafer Adhesion Promotes Resist Uniformity on Wafer Improves Linewidth Control During Etch Drives Off Most of Solvent in Photoresist Typical Bake Temperatures are 90 to 100 C – For About 30 Seconds – On a Hot Plate – Followed by Cooling Step on Cold Plate 14 - 25

Alignment and Exposure UV light source Process Summary: • Transfers the mask image to the resistcoated wafer • Activates photo-sensitive components of photoresist • Quality measures: – linewidth resolution – overlay accuracy – particles and defects Mask Resist 14 - 26

Post-Exposure Bake • Required for Deep UV Resists • Typical Temperatures 100 to 110 C on a hot plate • Immediately after Exposure • Has Become a Virtual Standard for DUV and Standard Resists 14 - 27

Photoresist Development Process Summary: • Soluble areas of photoresist are dissolved by developer chemical • Visible patterns appear on wafer - windows - islands • Quality measures: - line resolution - uniformity - particles and defects To vacuum pump Develop dispenser Vacuum chuck Spindle connected to spin motor 14 - 28

Hard Bake • • A Post-Development Thermal Bake Evaporate Remaining Solvent Improve Resist-to-Wafer Adhesion Higher Temperature (120 to 140°C) than Soft Bake 14 - 29

Develop / Inspect • Inspect to Verify a Quality Pattern – Identify Quality Problems (Defects) – Characterize the Performance of the Photolithography Process – Prevents Passing Defects to Other Areas • Etch • Implant – Rework Mis-processed or Defective Resist-coated Wafers • Typically an Automated Operation 14 - 30

Vapor Prime • Wafer Cleaning • Dehydration Bake • Wafer Priming – Priming Techniques • Puddle Dispense and Spin • Spray Dispense and Spin • Vapor Prime and Dehydration Bake 14 - 31

Effect of Poor Resist Adhesion Due to Surface Contamination Resist liftoff 14 - 32

HMDS Puddle Dispense and Spin Puddle formation Spin wafer to remove excess liquid 14 - 33

HMDS Hot Plate Dehydration Bake and Vapor Prime Process Summary: • Dehydration bake in enclosed chamber with exhaust • Hexamethyldisilazane (HMDS) • Clean and dry wafer surface (hydrophobic) • Temp ~ 200 to 250 C • Time ~ 60 sec. Chamber cover Wafer Hot plate Exhaust 14 - 34

Purpose of Photoresist in Wafer Fab • To transfer the mask pattern to the photoresist on the top layer of the wafer surface • To protect the underlying material during subsequent processing e. g. etch or ion implantation. 14 - 35

Progressive Improvements in Photoresist • Better image definition (resolution). • Better adhesion to semiconductor wafer surfaces. • Better uniformity characteristics. • Increased process latitude (less sensitivity to process variations). 14 - 36

Spin Coat • Photoresist – Types of Photoresist – Negative Versus Positive Photoresists • Photoresist Physical Properties • Conventional I-Line Photoresists – Negative I-Line Photoresists – Positive I-Line Photoresists • Deep UV (DUV) Photoresists • Photoresist Dispensing Methods 14 - 37

Types of Photoresists • Two Types of Photoresist – Positive Resist – Negative Resist • CD Capability – Conventional Resist – Deep UV Resist • Process Applications – Non-critical Layers – Critical Layers 14 - 38

Negative Versus Positive Resists • Negative Resist – Wafer image is opposite of mask image – Exposed resist hardens and is insoluble – Developer removes unexposed resist • Positive Resist – Mask image is same as wafer image – Exposed resist softens and is soluble – Developer removes exposed resist • Resolution Issues • Clear Field Versus Dark Field Masks 14 - 39

Photoresist Physical Characteristics ¥ ¥ ¥ ¥ ¥ Resolution Contrast Sensitivity Viscosity Adhesion Etch resistance Surface tension Storage and handling Contaminants and particles 14 - 40

Resist Contrast Poor Resist Contrast • Sloped walls • Swelling • Poor contrast Good Resist Contrast • Sharp walls • No swelling • Good contrast Resist Film 14 - 41

Surface Tension Low surface tension from low molecular forces High surface tension from high molecular forces 14 - 42

Components of Conventional Photoresist Solvent: gives resist its flow characteristics Resin: mix of polymers used as binder; gives resist mechanical and chemical properties Sensitizers: photosensitive component of the resist material Figure 13. 18 Additives: chemicals that control specific aspects of resist material 14 - 43

Negative Resist Cross-Linking Unexposed areas remain soluble to developer chemical. Areas exposed to light become crosslinked and resist the developer chemical. UV Photoresist Oxide Substrate Unexposed Exposed Soluble Pre-exposure - photoresist Crosslinks Post-exposure - photoresist Post-develop - photoresist 14 - 44

PAC as Dissolution Inhibitor in Positive I-Line Resist Unexposed resist, containing PACs, remain crosslinked and insoluble to developer chemical. Resist exposed to light dissolves in the developer chemical. UV Photoresist Oxide Substrate Exposed Unexposed PAC Pre-exposure + photoresist Soluble resist Post-exposure + photoresist Post-develop + photoresist 14 - 45

Good Contrast Characteristics of Positive I-line Photoresist Positive Photoresist: • Sharp walls • No swelling • Good contrast Resist Film 14 - 46

DUV Emission Spectrum Kr. F laser emission spectrum Emission spectrum of high-intensity mercury lamp 100 120 80 100 80 60 h-line 405 nm 60 40 g-line 436 nm 40 20 Relative Intensity (%) i-line 365 nm 0 248 nm 20 0 DUV* 248 nm 200 300 400 500 600 Wavelength (nm) * Intensity of mercury lamp is too low at 248 nm to be usable in DUV photolithography applications. Excimer lasers, such as shown on the left provide more energy for a given DUV wavelength. 14 - 47

Chemically Amplified (CA) DUV Resist Unexposed resist remains crosslinked and PAGs are inactive. Resist exposed to light dissolves in the developer chemical. UV Photoresist Oxide Substrate Exposed Unexposed PAG PAG H+ H+ PAG Acid-catalyzed reaction (during PEB) Pre-exposure + CA photoresist PAG PAG Unchanged Post-exposure + CA photoresist Post-develop + CA photoresist 14 - 48

Exposure Steps for Chemically. Amplified DUV Resist Table 13. 5 14 - 49

Steps of Photoresist Spin Coating 1) Resist dispense 3) Spin-off 2) Spin-up 4) Solvent evaporation 14 - 50

Automated Wafer Track for Photolithography Load station Vapor prime Resist coat Develop Edge-bead removal and Rinse Wafer stepper (Alignment/Exposure system) Transfer station Wafer Transfer System Soft bake Cool plate Hard bake 14 - 51

Photoresist Dispense Nozzle Z Y X q Nozzle position can be adjusted in four directions. Resist flow Resist dispenser nozzle Wafer Stainless steel bowl Bottom side EBR Air flow Vacuum chuck Air flow Exhaust Spin motor Drain Vacuum 14 - 52

Resist Spin Speed Curve of IX 300 80000 70000 Resist Thickness (Å) 60000 50000 40000 110 c. P 30000 70 c. P 20000 10000 21 c. P 0 1000 2000 3000 4000 5000 6000 Spin Speed (RPM) 7000 14 - 53

Soft Bake on Vacuum Hot Plate Purpose of Soft Bake: • Partial evaporation of photoresist solvents • Improves adhesion • Improves uniformity • Improves etch resistance • Improves linewidth control • Optimizes light absorbance characteristics of photoresist Chamber cover Wafer Hot plate Solvent exhaust Figure 13. 28 14 - 54

Residual Solvent (% w/w) Solvent Content of Resist Versus Temperature During Soft Bake DNQ/Novolak resist Bake Temperature (°C) Figure 13. 29 14 - 55