Contents Photolithography Process - Wafer Preparation - Photoresist Coating - Align & Expose - Photoresist Development Process Control CD Measurement Equipment Expose System & Wafer Track Consumables Chemicals & Gases Process Characterization Exposure System & Photoresist Design Rule & Key Target Analysis
Photolithography Process Technology
Photolithography Process Technology Introduction Purpose Specification Variable Process Description Process Step Parameter Process Control Consumables Chemicals Gases
Equipments Exposure Tool Wafer Track Oven Microscope Mask / Reticle Lithography Characterization Resist Characterization Tool Characterization
LITHOGRAPHY THE AIM IS TO REPRODUCE THE FEATURES ON THE MASK WITH THE HIGHEST POSSIBLE RESOLUTION IN THE PHOTORESIST ON THE SUBSTRATE
LITHOGRAPHY SYSTEMS NEEDS SMALL DIMENSIONS (LINEWIDTH) SMALL VARIATIONS IN DIMENSION (LINEWIDTH CONTROL) LARGE DEPTH OF FOCUS (TOLERANCE OF NON FLAT WAFERS AND THICK RESISTS) ACCURATE ALIGNMENT OF ONE PATTERN TO ANOTHER LOW DISTORTION OF IMAGE AND SAMPLE LOW COST (FAST EXPOSURE) HIGH RELIABILITY TOLERANCE OF CONTAMINATING PARTICLES ON MASK AND SAMPLE
Goal : To transfer patterns from a mask onto a wafer surface using a photoresist layer. Transfer Process : 1. Transfer pattern from the mask to the photoresist layer. 2. Transfer pattern from the photoresist layer to the wafer surface.
Photolithography Specification : 1. Class : 10 ( Better class will produce better yield ) 2. Humidity : 40 + / - 5% 3. Temperature : 20 + / - 1 Degree Celcius 4. Pressure : The highest in the cleanroom
Photolithography Philosophy Philosophy of the photolithography process is to hold all other process parameters constant and to vary exposure energy.
Photolithography Wafer Preparation Photoresist Coating Align and Expose Photoresist Development
Photolithography Process Steps : 1. Dehydration Bake 2. HMDS Priming and Cool Down Wafer 3. Photoresist Coating 4. Softbake 5. Align and Expose 6. Post Exposure Bake and Cool Down Wafer 7. Photoresist Development 8. Hardbake
DEHYDRATION BAKE To remove moisture from the wafer surface Moisture on wafer surface reduces resist odhesion Wafer on an oxide surface will allow wet etchants to penetrate easily between the resist and the wafer surface Parameters : - Temperature
HMDS PRIME To promote adhesion of the photoresist to the wafer surface Hexamethyldisilazane Parameter : - Temperature - Time
COOL PLATE To cooldown a wafer to the ambient temperature after HMDS baking process and before resist coating process Parameters : - Cooldown rate - Temperature uniformity - Substrate temperature
PHOTORESIST COATING To coat a resist onto the Si wafer for patterning with a required resist thickness and uniformity Parameters : Resist viscocity Resist dispense method Resist dispense velocity Resist dispense volume Resist dispense time Nozzle position Spin speed Spread time
Acceleration / deceleration Final spin speed Final spin time Chuck diameter Chuck vacuum Controlled exhaust Ambient atmosphere (RH & Temperature) Cup ambient (RH & Temperature) Wafer centring Machine and chuck laveling Resist temperature control
EDGE BEAD REMOVAL To remove resist that formed at the edge of the wafer after resist coating operation Parameters : Dispense type Dispense volume Dispense speed Spin speed Nozzle position
SOFTBAKE To remove a solvent from photoresist and activates photoactive compound To remove stress in the photoresist To improve photoresist adhesion Parameters Type of heat transfer (Hot plate or convection) Temperature Temperature uniformity Time Exhaust Cooldown rate
POST EXPOSURE BAKE To reduct the effect of standing waves which occur during exposure To increase the resistance of the resist prior to the etching process Parameters : Type of plate (hot plate or convection oven) Temperature Temperature uniformity Time Exhaust Cooldown rate
COOL PLATE To cooldown a wafer to the ambient temperature before development process Parameters : Cooldown rate Temperature uniformity Substrate temperature
DEVELOPMENT To remove area of resists that have been exposed/unexposed to form a pattern The development rate for positive resist much less than negative resist Parameters : Development technique (Immersion/Spray/Puddle) Developer type (Metal Ion/metal ion free) Developer concentration Developer temperature Development time Carbon dioxide absorption
Agitation Time between exposure and develop Time between develop and rinse Resist thickness Post exposure bake temperature Exposure energy
HARDBAKE To increase the resistance of the resist To remove any residual solvent Parameters : Type of plate (conduction or convection) Temperature Temperature uniformity Time Exhaust Cooldown rate
Photolithography Equipment 1. Exposure Tool 2. Wafer Track 3. Vacuum Oven 4. Optical Microscope with CD Measurement
Advantages of Projection Printing Method a. No mask or reticle damage b. No photoresist damage c. Mask or reticle will last more longer d. No mask change during operation
WAFER TRACK Photoresist coating application Photoresist development PHOTORESIST COATING HMDS Prime Cool down wafer Photoresist coating Edge bead removal Softbake
PHOTORESIST DEVELOPMENT Post exposure bake Cool down wafer Development Hardbake
SELECTION GUIDE OF WAFER TRACK SYSTEM I. Sender and Receiver Station II. Centring Module (Accuracy) III. Hotplate and Coolplate - Temperature range - Temperature uniformity - Cooldown rate - Plate type - Exhaust uniformity
IV. Handling System V. Process Modules for Coating and Development - Dispense type - Dispense accuracy - Coating uniformity across wafer and Between wafer - Development type - Development uniformity VI. Environmental Control - Humidity uniformity - Temperature uniformity
MASK A glass plate with a thin film of chromium Contain complete pattern for the whole wafer Same feature size on the wafer The pattern is transferred by using mask aligner
PHOTOMASK REQUIREMENT Consistent in quality Provides high resolution Low defect level Optical compatible with exposure equipment, resist and cleaning Cost effective
RETICLE A glass plate with a thin film of chromium Contains multiple image fields (two or three) and each field contains numerous die patterned Images on the reticle are protected by a pellicle
RETICLE CLEANING Not so critical Reduction reticle smaller particles could be ignored Covered by the pellicle Not contacted with a resist on wafer
What limit the resolution of optical lithography?
DIFFRACTION KILLER MASK ALIGNER STEPPER Reduce Gap Improve Lenses ( N.A ) Matching Exposure Light Exposure Source Improve Registration Between Mask Exploit Phase-Shift Mask
LITHOGRAPHY THROUGHPUT
EXTENDING USABLE OF OPTICAL LITHOGRAPHY OR TO IMPROVE RESOLUTION INCREASE THE NUMERICAL APERTURE REDUCE THE WAVELENGTH BOTH INCREASE THE N.A & REDUCE THE WAVELENGTH EXPLOIT PHASE SHIFT MASK EXPLOIT MULTI LAYER RESIST SCHEME
NEXT? 1. Multilevel Resist Schemes 2. Phase Shift Mask
Selection Guide Of Mask Aligners And Steppers Below are several factors that can be used for selecting particular mask aligners and Steppers. i.aligner type Scanning Projection, Projection Step And Repeat and Contact Proximity. ii.contact / Proximity - Contact Pressure ( kpa) - Proximity Separation ( um ) iii.scanning / Projection & Steppers - Lens Numerical aperture - Depth Of Focus - Field Size ( mm x mm ) - Reduction Ratio - Magnification Distortion ( nm ) iv.resolution ( um ) v.usable linewidth ( um ) vi.exposure Source - Type - Power ( w )
- Wavelength ( nm ) - Illumination Uniformity ( % ) - Luminance ( mw / cm2 ) vii.wafer Stage - Max wafer diameter ( mm ) - X Y range ( mm x mm ) - 0 range ( degrees ) - 0 accuracy ( urad ) viii.alignment - Global / site by site - Manual / Automatic - Pre - alignment accuracy ( um ) - Overlay accuracy ( nm ) - Machine to machine overlay repeatability - Machine alignment stability ( um ) ix.reticle Size ( Max ) - Width ( mm ) - Length ( mm ) - Thickness ( mm ) - Accommodates masks with pellicles. x.throughput rate ( wph )
Polymer Photoresist Components - Phenol- formaldehyde - React when exposed to energy (Photosolublization) -Dark Solvent - Vehide for polymers and sensitizers - Ethoxyethyl acetate & methoxyethyl acetate - Determine the thickness of resist Sensitizers - Control / modify chemical reaction
Ability of the Photoresist Adhesion Factors Moisture content of the wafer surfaces Wetting characteristics of the resist on surfaces Type of primer used and method of application Delays in the wafer imaging process Resist chemistry (solvent) Wafer surface smoothness Stress force in the resist coating Contamination or surface defects
Photoresist Performance Factors Resolution - Smallest opening can be resolved in photoresist layer - Positive resist negative resist Adhesion - Ability to adhere to the variety of surface - Negative resist positive resist Exposure Speed - The speed with which resist react to exposure - Negative resist positive resist Pinhole count - Pinhole increase layer thickness decrease - Negative resist has fewer pinholes than negative photoresist
Photoresist Thickness Measurement Ellipsometer -He Ne - Refractive Index is required - Thickness and uniformity Nanospec - UV Light - Refractive Index is not required - Thickness
Comparison Coating Thickness Step Coverage Exposure Developing Chemical stability Oxygen Exposure Time Material Cost Chemical resistance Ion Implantation Negative Photoresist Coating thickness must be 1/3 the minimum image size but this increase pinhole density Marginal due to thin coating limitation Depend upon cross linking for image formation Solvent developing resulting in image swelling.solvent disposal is more difficult Marginal chemical stability requires filtration prior to use. Oxygen sensitivity causes exposure problems. Faster 2 3 seconds 1/ 4 to 1 / 3 lower cost per wafer Excellent chemical resistance Stable at high dose Positive Photoresist Coating thickness can be equal to or greater than minimum image size. Extremely accurate image. Excellent as thick coating (2-3 micron) can be used. Chemical change takes place in resist that is removed. Aqueous developing and the image is only affected by the developer.disposal is relatively simlpe. Reasonable chemical stability but high viscosity makes filtration difficult. No oxygen sensitivity Slower 10 15 seconds High: more than material cost Chemical resistance is less Melts under high dose.
Storage and Handling of Photoresist Photoresist should be contained in the dark bottle Photoresist should be operated in the yellow or gold lighting area Photoresist must be stored under constant temperature condition Photoresist bottles should be tightly closed Phoresist should be filtered before use
Photoresist selection guide o Type of resist positive & negative o Exposure wavelength g/h/i o Energy density for 1 um film thickness (mj/cm2) o Resolution limit (um) o Contrast o Film thickness range at 4000 rpm (um) o Thickness uniformity across wafer o Viscosity range (centipoise) o Solid content range (weight percent)
o Recommended temperature for softbake (c) o Recommended temperature for hardbake (c) o Reflective index solution o Reflective index dry film o Maximum content of particles 0.5 um (particles / ml) o Maximum Sodium, Boron, Metal and Iron content (ppm) o Water content (%) o Flash point temperature (c) o Absolute filtration to (um) o Shelf life (month)
Primer Application Spin Coating Heated Vapor Coating The most popular - Wafer more - uniformity good - HMDS used reduced - Contamination less Spray Coating Dip Priming
Advantages of HMDS Not containing chlorine Not required hardbaking Adhesion promoter remains effective on a wafer more longer Pinhole and undercutting problem reduced Profile of resist increase
Positive Photoresist Developer Metal Ion Content Advantages - Higher development rate at exposure area - Better resist profile - Attack unexposed positive resist are less Disadvantages - Left residue - Cause contamination - Change device performance Metal Ion Free Advantages - Avoid from residue formation on wafer - Good for small geometry size Disadvantages - Attack unexposed positive photoresist - Low development rate - Poor resist profile
Why photoresist profile extremely important? - The structure of the layer underneath is depend on the structure of the resist. - Will change actual geometry size
Line And Space Definition i. Line a. Line is a bright field or the remain photoresist. b. To increase linewidth, decrease the exposure time or develop time. c. To decrease linewidth, increase the exposure time or develop time. ii. Space a. Space is a dark field or the area where the photoresist is removed after developed. b. To increase space width, increase the exposure time or develop time. c. To decrease space width, decrease the exposure time or develop time.
Lithography Process Control To control the lithography process is by measuring the critical dimension (CD) of the image Critical dimension the smallest dimension in the integrated circuit CD measurement consist of 3 steps - CD on mask - Cd on developed photoresist - Cd on patterned layer after etch
CONCLUSION Equipment - Exposure Tool - I line Stepper - Mask aligner - Wafer Track System - Vacuum Oven - Optical Microscope with CD Measurement - I line Positive Photoresist Lithography Process Specification Dehydration Bake Temp: 145 +/- 5 o C Vacuum: 10 mtorr Time: 15 minutes
Photoresist Coating Vapor Prime: HMDS Resist: I line +ve resist Velocity: 5000 rpm Time: 30 seconds Thickness: 1.2 um EBR: 1 mm Softbaking Temp: 90 +/- 1 o C Time: 60 seconds Align & Expose Tools: 1 line stepper & Mask aligner. Exp Energy: 200 mj / mm Exp Latitude: 20 MJ Focus Latitude: 4 um PE Baking Temp; 110 +/- 1 o C Time: 95 seconds Development Technique: Spray & Puddle Velocity: 4500 rpm Time; 60 seconds Hardbake Temp: 120 +/- 1 o C Time: 80 seconds CD Measurement Tolerance: 0.3 um
Photolithography Process Characterization
PHOTORESIST THICKNESS DETERMINATION Starting Material : Bare Si Wafer FSI Clean Dehydration Bake HMDS & Cooldown Wafer Resist Coating - Resist Type : PRH - Variable : Spin Speed - Constant : Others Softbake Resist Thickness Measurement - Ellipsometer or Nanospec Resist Thickness Determined - At Tentative Spin Speed
PHOTORESIST THICKNESS UNIFORMITY CHARACTERIZATION Starting Material Base Silicon Dehydration Bake HMDS & Cool Down Wafer Resist Coating Resist Type: PRH Constant : Spin Speed Variables : Dispense Volume Dispense Speed Dispense time Dispense spin speed Exhaust Flows Softbake Evaluation
Phase 1 Resist Thickness Uniformity Across wafer - Ellipsometer Wafer To Wafer - Ellipsometer PHOTORESIST THICKNESS UNIFORMITY DETERMINED
EXPERIMENTAL RESULT FORMAT FOR PHOTORESIST THICKNESS UNIFORMITY CHARACTERIZATION
PHOTORESIST THICKNESS UNIFORMITY & REPEATABILITY CHARACTERIZATION Starting Material Base Silicon Dehydration Bake HMDS & Cool Down Wafer Resist Coating Resist Type : RPH Constant : Spin Speed Variables : Dispense Volume Dispense Speed Dispense time Dispense spin speed Exhaust Flows Softbake Evaluation.
Phase 1 Resist Thickness Uniformity Across wafer - Ellipsometer Wafer To Wafer - Ellipsometer Phase 2 Resist Thickness Uniformity & Repeatability Across Wafer - Ellipsometer Wafer To Wafer - Ellipsometer Characterize Variables For Resist Thickness Uniformity And Repeatability Are Determined
Aim - PHOTORESIST CONTRAST ASSESSMENT To identify the best photoresist profile with a vertical sidewall Image Contrast - Different in density or optical gradient at image sidewall Parameters - Exposure energy & development time HIGH CONTRAST LOW CONTRAST
PHOTORESIST CONTRAST ASSESSMENT FLOW CHART Starting Material - Si Bare wafer with specific Resist thickness Softbake Align & Expose - Energy (E1) - Reticle : Test reticle (1.0 um resolution) Development - Time : Increase or decrease Repeat the whole process Hardbake - Adjust develop time PHOTORESIST PROFILE EVALUATION Cleave Wafer SEM Examination Photoresist profile _ Slope sidewall Photoresist Profile - Vertical Sidewall Exposure energy with vertical photoresist profile determined E1 + develop time, DS
EXPOSURE LEVEL DETERMINATION Starting Material Si wafer with photoresist Coated - Specific resist thickness - Equipment : stepper - Reticle : test reticle ( 1.0 um resolution ) Align & Expose wafer - Exposure matrix ( 30 Mj 300 mj in steps of 10 mj ) Development - Standard Parameters Energy level Evaluation Light Microscope ( Observe at 1.0 um resolution ) The tentative energy level at 1.0 um resolution is determined ( Eo )
Align & Expose Wafer - 5 different energy Align & Expose with Eo Align & Expose with Eo - 10 mj Align & Expose with Eo - 20 Mj Align & Expose with Eo + 10 mj Align & Expose with Eo + 20 mj Development Energy Level Evaluation Linewidth measurement ( 1.0 um resolution ) SEM The Best Energy Level Determined ( E1 )
EXPOSURE LATITUDE MEASUREMENT Starting Material - Bare SiWafer - photoresist thickness : Ao Softbake Align & Expose -Exposure Matrix - Energy : ( E1 n10mj ) & ( E1 + n10mj ) Development Hardbake EXPOSURE LATITUDE MEASUREMENT Wafer Cleaving SEM Examination EXPOSURE LATITUDE DETERMINED EMAX & Emin
EXAMPLE RESULT OF EXPOSURE LATITUDE DETERMINED Example typical result Let say E1 = 170 mj / cm 2 Exposure Energy ( mj / cm 2 ) Measurement ( um ) 140 1.10 150 1.05 160 1.00 170 1.00 180 1.00 190 0.95 200 0.90 The exposure latitude for 1.00 um geometry size is 160 mj / cm 2 to 180 mj / cm 2 or 30 mj / cm 2.
FOCUS LATITUDE MEASUREMENT Starting Material - Bare Si Wafer Photoresist Thickness : Ao Softbake Align & Expose 1 - Reticle : Test reticle ( 1.0 um resolution ) - Focus level : The best ( Fo ) Align & Expose 2 - Reticle : Test reticle ( 1.0 um resolution ) - Focus level : Increase 4 steps focus in the steps of 1 um from Fo Align & Expose 3 - Reticle : Test reticle ( 1.0 um resolution ) - Focus level : Decrease 4 steps focus in the steps of 1 um from Fo Development
Hardbake FOCUS LATITUDE MEASUREMENT Wafer Cleaving SEM Examination - Linewidth Measurement ( 1.0 um resolution ) FOCUS LATITUDE DETERMINED + / - Fx um
Example result of focus latitude determination Focus Level ( um ) Linewidth Measurement ( um ) +4 1.08 +3 1.03 +2 1.00 +1 1.00 0 1.00-1 1.00-2 1.00-3 0.95-4 0.90 The focus latitude for 1.0 um geometry size is +/- 2 um or 4 um range. Focus latitude should be greater than photoresist thickness.