1 Photolithography I ( Part 2 ) Chapter 13 : Semiconductor Manufacturing Technology by M. Quirk & J. Serda Bjørn-Ove Fimland, Department of Electronics and Telecommunication, Norwegian University of Science and Technology ( NTNU )
2 Objectives After studying the material in this chapter, you will be able to: 1. Explain the basic concepts for photolithography, including process overview, critical dimension generations, light spectrum, resolution and process latitude. 2. Discuss the difference between negative and positive lithography. 3. State and describe the ten basic steps to photolithography. 4. Explain how the wafer surface is prepared for photolithography. 5. Describe photoresist and discuss photoresist physical properties. 6. Discuss the chemistry and applications of conventional i-line photoresist. 7. Describe the chemistry and benefits of deep UV (DUV) resists, including chemically amplified resists. 8. Explain how photoresist is applied in wafer manufacturing. 9. Discuss the purpose of soft bake and how it is accomplished in production.
3 Ten steps of Photolithography UV Light HMDS Resist Mask λ λ 1-3) Vapor prime 4) Spin coat 5) Soft bake 6) Alignment and Exposure 7) Post-exposure bake (PEB) 8) Develop 9) Hard bake 10) Develop inspect
4 Vapor Prime Wafer Cleaning Dehydration Bake Wafer Priming Priming Techniques Puddle Dispense and Spin Spray Dispense and Spin Vapor Prime and Dehydration Bake
5 Wafer Cleaning Resist liftoff Remove particles, grease and so on Particles/grease reduces adhesion Photoresist can loosen from the wafer Can etch when below photoresist Affects the properties of the semiconductor Metals can give impurities (doping) on the surface Particles can give an uneven thickness of the photoresist and also make holes. Important to do this step fast (i.e. go to next step quickly) Prevent further contamination before next layer
6 Dehydration Bake Remove moisture (Water) from the wafer surface Water reduces adhesion between photoresist and surface Use a dehydration bake between cleaning and wafer priming 200-250 C for about one minute (Si wafer) To remove all water, 750 C is needed for silicon No point in using this temperature as water dampens the surface when temperature is lowered
7 Wafer Priming Put a material between the wafer and photoresist which improves adhesion Better than wafer <--> resist hexamethyldisilazane (HMDS) mostly used Applied by either Puddle Spray Vapor (most common)
8 HMDS Puddle Dispense and Spin Puddle formation Spin wafer to remove excess liquid Uses a lot of HMDS Figure 13.14 Quirk & Serda
9 Wafer Priming Spray and spin Helps remove residue particles Takes a long time Uses a lot of HMDS
10 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. Minimizes HMDS consumption No contact of liquid HMDS with wafer less contamination Chamber cover Wafer Hot plate Exhaust Figure 13.15 Quirk & Serda
11 Ten steps of Photolithography UV Light HMDS Resist Mask λ λ 1-3) Vapor prime 4) Spin coat 5) Soft bake 6) Alignment and Exposure 7) Post-exposure bake (PEB) 8) Develop 9) Hard bake 10) Develop inspect
12 Spin Coat Photoresist Types of Photoresist Negative Versus Positive Photoresists Photoresist Physical Properties Conventional I-Line Photoresists (365 nm light) Negative I-Line Photoresists Positive I-Line Photoresists Deep UV (DUV) Photoresists (248 or 193 nm) Photoresist Dispensing Methods
13 Types of Photoresists Two Types of Photoresist Positive Resist Negative Resist CD Capability Conventional Resist (CD 0.35 μm) Deep UV Resist (CD 0.25 μm) Process Applications Non-critical Layers Critical Layers
14 Photoresist Physical Characteristics Resolution Contrast High contrast => Straight resist wall Sensitivity Minimum exposure dose (mj/cm 2 ) for given λ (λ-dependent) Viscosity How thick liquid ( affecting resist thickness ); Step coverage pinholes pattern resolution Adhesion Adhesion on various materials ( Semiconductors, Dielectrics and Metals ) Etch resistance e.g. thermal (shape) stability at ~ 150 C during dry etch Surface tension High resist strength vs. good resist flow Storage and handling Storage-lifetime and -temperature, Contamination and evaporation after opening. Contaminants and particles Mobile ionic impurities are damaging
15 Resist Contrast Poor Resist Contrast Sloped walls Swelling Poor contrast Good Resist Contrast Sharp walls No swelling Good contrast Resist Resist Film Film Figure 13.16 Quirk & Serda
16 Photoresist Physical Characteristics Resolution Contrast High contrast => Straight resist wall Sensitivity Minimum exposure dose (mj/cm 2 ) for given λ (λ-dependent) Viscosity How thick liquid ( affecting resist thickness ); Step coverage pinholes pattern resolution Adhesion Adhesion on various materials ( Semiconductors, Dielectrics and Metals ) Etch resistance e.g. thermal (shape) stability at ~ 150 C during dry etch Surface tension High resist strength vs. good resist flow Storage and handling Storage-lifetime and -temperature, Contamination and evaporation after opening. Contaminants and particles Mobile ionic impurities are damaging
17 Surface Tension Low surface tension from low molecular forces High surface tension from high molecular forces Figure 13.17 Quirk & Serda
18 Components of Conventional Photoresist Solvent: gives resist its flow characteristics Resin: mix of polymers used as binder; gives resist mechanical and chemical properties, inert Sensitizers: photosensitive component of the resist material Additives: chemicals that control specific aspects of resist material
19 Negative Resist Crosslinking Unexposed areas remain soluble to developer chemical. UV Areas exposed to light become crosslinked and resist the developer chemical. Photoresist Oxide Substrate Unexposed Exposed Pre-exposure - photoresist Soluble Post-exposure - photoresist Crosslinks Post-develop - photoresist Figure 13.19 Quirk & Serda
20 PAC as Dissolution Inhibitor in Positive I-Line Resist Unexposed resist, containing PACs, remain crosslinked and insoluble to developer chemical. Photoresist UV Resist exposed to light dissolves in the developer chemical. Oxide Substrate Exposed Unexposed PAC Pre-exposure + photoresist Soluble resist Post-exposure + photoresist Post-develop + photoresist Figure 13.20 Quirk & Serda
21 Good Contrast Characteristics of Positive I- Line Photoresist Positive Photoresist: Sharp walls No swelling Good contrast Resist Film Figure 13.21 Quirk & Serda
22 DUV Emission Spectrum Relative Intensity (%) KrF laser emission spectrum 100 80 60 40 20 0 248 nm Relative Intensity (%) 120 100 80 60 40 20 Emission spectrum of high-intensity mercury lamp DUV* 248 nm i-line 365 nm h-line 405 nm g-line 436 nm 0 200 300 400 500 600 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. Mercury lamp spectrum used with permission from USHIO Specialty Lighting Products Figure 13.22 Quirk and Serda
23 Chemically Amplified (CA) DUV Resist Unexposed resist remains crosslinked and PAGs are inactive. UV Resist exposed to light dissolves in the developer chemical. Photoresist Oxide Substrate Exposed Unexposed PAG H + PAG PAG H + PAG PAG PAG PAG H + PAG PAG Pre-exposure + CA photoresist Acid-catalyzed reaction (during PEB) Post-exposure + CA photoresist Unchanged Post-develop + CA photoresist Figure 13.23 Quirk and Serda
24 Exposure Steps for Chemically Amplified DUV Resist 1. Resin is phenolic copolymer with protecting group that makes it insoluble in developer. 2. Photoacid generator (PAG) generates acid during exposure. 3. Acid generated in exposed resist areas serves as catalyst to remove resin-protecting group during post exposure thermal bake. 4. Exposed areas of resist without protecting group are soluble in aqueous developer. Table 13.5 Quirk & Serda
25 Ten steps of Photolithography UV Light HMDS Resist Mask λ λ 1-3) Vapor prime 4) Spin coat 5) Soft bake 6) Alignment and Exposure 7) Post-exposure bake (PEB) 8) Develop 9) Hard bake 10) Develop inspect
26 Spin Coat Process Summary: Wafer is held onto vacuum chuck Dispense ~5ml 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 Figure 13.10 Quirk & Serda
27 Steps of Photoresist Spin Coating 1) Resist dispense 2) Spin-up 3) Spin-off 4) Solvent evaporation Figure 13.24 Quirk & Serda
28 Spin Coat Apply photoresist Applied with dispenser Applied continuously, not as droplets (important) Radial application Dispenser moves radially while applying photoresist Reduces the amount of photoresist used Static application Apply the photoresist before starting the spinner Spin the wafer slowly to distribute the resist (Typically 500 rpm) fast to get a homogenous film (typically around 4000 rpm) Dynamic application Apply at 100-200 rpm Then spin up to correct speed (again, ~ 4000 rpm) Removes solvents Photoresist thickens (solvent content from ~ 75 % to ~ 15 %)
29 Resist Spin Speed Curve 80000 Spin Speed Curve of IX300 70000 Resist Thickness (Å) 60000 50000 40000 30000 110 cp 70 cp 1 thickness RPM 1 2 20000 21 cp 10000 01000 2000 3000 4000 5000 6000 7000 Spin Speed (RPM) Used with permission from JSR Microelectronics, Inc. Figure 13.27 Quirk and Serda
30 Photoresist Dispense Nozzle Bottom side EBR Resist dispenser nozzle Air flow X θ Z Y Nozzle position can be adjusted in four directions. Resist flow Wafer Vacuum chuck Air flow Stainless steel bowl Exhaust Spin motor Drain Vacuum Figure 13.26 Quirk and Serda
31 Ten steps of Photolithography UV Light HMDS Resist Mask λ λ 1-3) Vapor prime 4) Spin coat 5) Soft bake 6) Alignment and Exposure 7) Post-exposure bake (PEB) 8) Develop 9) Hard bake 10) Develop inspect
32 Soft Bake Characteristics of Soft Bake: Improves Photoresist-to-Wafer Adhesion Promotes Resist Uniformity on Wafer Drives Off Most of Solvent in Photoresist (reduced to ~ 5 %) Typical Bake Temperatures are 90 to 100 C For about 60 Seconds On a Hot Plate Followed by Cooling Step on Cold Plate
33 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 Solvent exhaust Hot plate Figure 13.28 Quirk and Serda
34 Ten steps of Photolithography UV Light HMDS Resist Mask λ λ 1-3) Vapor prime 4) Spin coat 5) Soft bake 6) Alignment and Exposure 7) Post-exposure bake (PEB) 8) Develop 9) Hard bake 10) Develop inspect
35 Automated Wafer Track for Photolithography Load station Vapor prime Resist coat Develop and rinse Edge-bead removal Transfer station Wafer stepper (Alignment/Exposure system) Wafer Transfer System Soft bake Cool plate Cool plate Hard bake
36 Photolithography Track System Photo courtesy of Advanced Micro Devices, TEL Track Mark VIII Photo 13.2 Quirk and Serda
37 Objectives After studying the material in this chapter, you will be able to: 1. Explain the basic concepts for photolithography, including process overview, critical dimension generations, light spectrum, resolution and process latitude. 2. Discuss the difference between negative and positive lithography. 3. State and describe the ten basic steps to photolithography. 4. Explain how the wafer surface is prepared for photolithography. 5. Describe photoresist and discuss photoresist physical properties. 6. Discuss the chemistry and applications of conventional i-line photoresist. 7. Describe the chemistry and benefits of deep UV (DUV) resists, including chemically amplified resists. 8. Explain how photoresist is applied in wafer manufacturing. 9. Discuss the purpose of soft bake and how it is accomplished in production.
38 Thank You