Figure 5.1 Relative size of one micron. • Major contaminants: 1. Particles 2. Metallic ions 3. Chemicals 4. Bacteria • Bacteria: Need const. running water to prevent bacterias Figure 5.2 Relative size of contamination. (Hybrid Microcircuit Technology Handbook) Figure 5.3 Relative size of airborne particles and wafer dimensions. • As lines get smaller, particle control becomes more important. • Killer defects: particles in critical location Contamination caused problems: Device processing yield Device performance Device reliability Contamination sources 1. Air 2. The production facility 3. Clean room personnel 4. Process water 5. Process chemicals 6. Process gases 7. Static charge Figure 5.4 Example resist stripper trace metal contents (EKC Technology—830 Photoresist stripper) Process chemicals and process water can be contaminated with trace chemicals from wafer process. • MOS Grade = Low sodium grade (sodium is the most prevalent mobile ionic contaminant) • MICs: Mobile ion contaminants • Metals in an ionic form can cause electrical failure even after final test since they are movable. • Exist in most chemical • Need to be < 1010 atoms/㎝2 Figure 5.5 Relative size of airborne particulates (microns). Figure 5.6 Air cleanliness classes standard 209E. •regular air in city: 5M /ft3 Figure 5.7 Typical class numbers for various environments. • Clean room design strategies: 1. Clear air station 2. Tunnel design 3. Total clean room 4. Mini environments Figure 5.8 Hepa filter. • Clean room starts with space program: NASA, a single speck can cause satellite to fail. • • • Fragile fiber with small holes in accordion design (手風琴) Air pass with large volume and low velocity (not to cause air currents) air flow → 90 - 100 ft/min Figure 5.9 Cross section of VLF hood. • HEPA (High Efficiency Particulate Attenuation) ~ 99.99+ efficiency • (work station) vertical laminar flow • *important: (1) HEPA (2) positive pressure Figure 5.10 Cross section of a VLF-fume-exhaust hood. Safety + No contamination • Wet Chemical Process Hood wafer storage Figure 5.11 Cross section of clean-room tunnel. Divide fabrication area into separate tunnels or bays • To prevent contamination from too many people working in the same room: use Tunnel/Bay concept Fewer people work in one bay Figure 5.12 Cross section of laminar flow clean room. (Courtesy of Semiconductor International.) • Recovery: the time required for the filters to return the area to acceptable condition after a shift start, personnel break or other disturbance. Class 1:6 seconds Figure 5.12 air return open work station with perforations Figure 5.13 Wafer transfer microenvironment. • Cost billion USD to build clean room. Use micro / mini environments to reduce cost → isolate the wafer in as small an environment as possible. Figure 5.13 pressure air / N2 Figure 5.14 Minienvironment system elements. • (WIT): wafer isolation technology • low construction and operating cost • with > 8’wafer → too heavy to carrier, too expensive to drop Figure 5.14 • Mechanical interface Figure 5.15 Fab area with growing area, air showers, and service aisles. • Temperature: 72℉±2℉ (stable chemical reaction) • Humidity: important for polymer (too wet, polymer is not sticky) (too dry,static charge) • relative humidity: 15 ~ 50% • smog control: ozone filtered by carbon Figure 5.15 Service bay Positive air pressure static control Double door Shoe glove cleansers Adhesive flow mats Figure 5.16 Triboelectric series. (Hybrid Circuit Technology Handbook, Noyes Publications) • Static charge formed by triboelectric charge (formed when two materials initially in contact are separated) • One surface loses e• One surface gains e- Figure 5.16 • loses e- Figure 5.16 • gains e- Figure 5.16 • (1) High density circuits with submicron feature size vulnerable to smaller particles attracted by static charge. Static charges build up on wafer storage boxes, work surface, equipments. Can be as high as 50,000 volt to attract aerosols from air or personnel garments. Very difficult to remove. • (2)ESD (Electric Static Charge) can destroy devices. Need to package devices with antistatic materials Figure 5.17 Static-charge reduction techniques. Preventions: Use antistatic materials in garments and storage boxes. (2) Use antistatic solution (apply to the wall),but not in critical area to prevent contamination from the solution (3) Grounded static discharge straps. (4) Ionizer (underneath the Hepa or close to the Nitrogen blow gun) to neutralize the charge built up in the filtered air. (1) Other static change examples: (1) Photomask and rectile damage. ESD discharge can vaporize and destroy the chrome pattern. (2)ESD discharge between package material (PFA) for wafer and equipment produce EM interference with machine operation. Static Charge Prevention (1) Use antistatic materials in garments and storage boxes. (2) Use antistatic solution (apply to the wall) (3) Grounded static discharge straps. (4) Ionizer (underneath the Hepa) to neutralize the charge built up in the filtered air. Figure 5.18 Activity-caused increase in particles. over background=1 (Hybrid Microcircuit Technology Handbook,Noyes Publications) • Human: biggest source of contamination • After showering and sitting, gives off 100,000 ~ 1,000,000 particles/ min • Hair spray cosmetics, smoking, pencil must be prevented. • Gowning from top to bottom. Undress from bottom to top. Figure 5.19 Resistivity of wafer versus concentration of dissolved solids. A Fab uses millions of gallons of water per day. Water contaminants if nor processed: 1.Dissolved minerals 2.Particulates 3.Bacteria 4.Organics 5.Dissolved oxygen 6.Silica D.I water 18,000,000 Ohms-cm at 25 degree C Process chemicals • Contaminants: metallic; particulates; chemicals • Grades: commercial ~ too dirty for IC reagent ~ too dirty for IC electronic ~ cleanliness depends on manufacturer semiconductor ~ cleanliness depends on manufacturer • usually MIC level 1 ppm, some supplier provide 1 ppb • particle filtering level 0 - 2μm or lower. • Usually purchase bulk quantities (prevent container contamination) Process for Cleaner Chemicals • BCDS (Bulk Chemical Distribution Systems) →cleaner chemical / lower cost • Point of use (POU), mix chemical at process vessel. • Point of use chemical generation (POUCG) Chemical made at process station, to reduce contamination and cost. (such as NH4OH,HF, H2O2) Gas Quality 1. Purity 2. Water vapor contents 3. Particulates 4. Metallic ions Process with gas reactions Oxidation Reactive ion etch Sputtering Plasma etch CVD Contamination Control • • • • • Contamination may change the chemical reaction Gas Purity 99.99 ~ 99.999999%(Highest purity, with six 9’s) Water vapor is limited to 3 - 5 ppm, or it can oxide the Si surface Gas filtered (Particulates ~ 0.2μm) MIC < ppm Water Requirements • 1. 2. 3. 4. 5. 6. • • Regular wafer contains: Dissolved minerals ~ removed by ion exchange system. Particulates ~ removed by sand, earth, membrane filtration. Bacteria ~ removed by sterilizer Organics ~ removed by carbon bed filtration. Dissolved oxygen ~ removed by decarbonator & vacuum degasifier Silica Monitor water resistivity in several points in fabrication area. Standard 18,000,000Ω-㎝ at 25℃ (18 megohm 18mΩ) water Clean Room Materials and Supplies: Notebook Tools Pencils Storage boxes Cartwheels Need special materials, which don’t generate particles Clean Room Maintenance: Cleaner Applicator Wiper Vacuum cleaner with Hepa filter all need special materials Wafer Surface Cleaning • Wafer surface contamination: 1. Particulates 2. Organic residues 3. Inorganic residues 4. Unwanted oxide layers • Wafer surface roughness requires 0.15 nm (nmRMS) root mean square of vertical surface roughness. • In 2010 (nmRMS)<0.1nm • Excess surface roughness effect device performance and layer unifromaity Surface Contamination • Surface contaminant type 1.Particulates 2.Organic residues 3.Inorganic residues 4.Unwanted oxides • Gate oxide need <0.02 defects/㎝2 when tested at 5MV/㎝ 30 secs • Zn, Na, Fe, Ni, Ca<2.5*109 atoms/㎝2 • Al,Ca < 5*109 atoms/㎝2 • FEOL: Front End of the Line (from active layer) • BEOL: Back End of the Line Figure 5.20 DRAM water specs. (Semiconductor International, July 1994, p. 178) Figure 5.21 Typical deionized water system. Water stored is blanked with nitrogen to prevent the Absorption of carbon dioxide.Carbon dioxide interfere with resistivity may cause wrong reading Figure 5.22 Sources of particulate contamination. This analysis, shown at SEMI Forecast by Dr. C. Rinn Cleavelin, Texas Instruments, revealed equipment-generated particles as the top enemy in 1985. • PWP: Particles per Wafer Pass • Equipment need material and design selection and assembled in a clean room environment. Figure 5.23 Typical FEOL cleaning process steps. Standard clean Particulate removal Small particulate held to surface by : 1.van der Waals force ( strong inter-atomic attraction between the electrons of one atom and nucleus of another) 2. Capillary force (Occurred when there’s liquid bridge between particle and the Surface) Zeta Potential: arises from a charge zone around particles that is Balanced by opposite charge zone in the cleaning liquid Van der Waals force can be minimized by Zeta potential The charge in the liquid varies with cleaning liquid speed, PH of the solution, concentration of the electrolytes in the solution, additives in the solution, such as surfactant. These conditions create a large number of charge that has same polarity of the wafer and create repulsive force to keep the particle in the solution and off the wafer surface Surfactant and mechanical assist tool (such as megasonics) are used to dislodge the particle attached to the surface by capillary force Figure 5.24 Capillary force from film. Occurred when there’s liquid bridge between particle and the surface Surfactant and mechanical assist are used to dislodge the particle Most commonly used cleaning process Nitrogen blow off: 1. Spray of filtered high pressure nitrogen 2. Ionizer strip static charges from the nitrogen stream and neutralize the wafer surface Figure 5.25 Mechanical scrubber. Wafer hold by a rotating vacuum Chuck Brush and wafer rotation create high energy cleaning action Liquid forced between the space with high velocity adds to cleaning Dilute NH4OH is added to the cleaning solution to control zeta potential RCA clean SC1H2O,H2O2,NH4OH ratio from 5:1:1-7:2:1 Used at 75-85 degree C Oxide keeps forming and dissolving Removes organic residues and sets up a condition for desorption of trace metal from the surface SC2H2O,H2O2, HCl ratio from 6:1:1 to 8:2:1 Used at 75-85 degree C Removes alkali ions, hydroxides and complex residue metal If oxide free surface is needed, HF etch is added before or after RCA clean Metal ions are not dissolved in most cleaning solution, need to add chelating agent (ethylenediamine-tetra-acetic acid) Dilute solutions, SC1 (1:1:50), SC2 (1:1:60) are usually used with same effectiveness and with less roughness on the surface. Photomask Cleaning High pressure water spray (2000-4000 psi) Add small amount of surfactant as destatic agent Organic residue removal Use TCE, Aceton, Alcohol Problems: Solvent cleaning is difficult to dry and contains contaminants Chemical Cleaning H2SO4 + Oxidant { (H2O2, [(NH4)2S2O3],HNO3, OZONE)} H2SO4 is effective cleaner for inorganic residues and particles from 90o C to 120oC Oxidants are added to remove carbon residues by converting C to CO2 which leaves as vapor C+O2=CO2 (gas) Chemical Cleaning (CONTINUE) H2SO4 + 30% H2O2 (by volume) (Carro’s acid or Piranha acid) Used for all stages of processing and photoresist stripper Exothermic reaction, T=110-130 degree C Need to add H2O2 to maintain the cleaning rate (As time proceeds, temperature falls and reaction rate falls) Ozone addition Ozone can be used in the sulfuric acid instead of oxidant additive Ozone can also be added to D.I. Water (1-2ppm) to provide a cleaning solution for light organic contaminants (for 10 minutes at room temperature) Oxide layer removal Thin oxide (100-200Å) formed in air or in the heated chemical bath with the presence of oxygen Thin oxide is an insulating layer which prevents Electrical contact between Si and metal, also prevents silicon surface from chemical processes Hydrascopic-silicon surfaces with oxide Hydrophobic-silicon surfaces that are oxide free Oxide layer removal (continue) Before Oxidation process Si surface is cleaned in 49% HF, which etches oxide but not Si In later process, oxides in patterned holes are etched in water and HF solution (strength from 100:1 to 10:1) Strength is chosen that solution will etch oxide in the Hole, but not the silicon (typical dilutions from 1:50-1:100) Pregate cleaning uses HF as the last chemical step (HF-last)- surface is hydrophobic and low metal contamination Figure 5.26 RCA clean formulas. Developed by RCA engineer Werner Kern in mid 1960s to remove Organic and inorganic residues from silicon surface Figure 5.27 Experimental room temperature cleaning process. Combine Water, HF + Ozone in Megasonic for cleaning Spray cleaning Standard cleaning process using immersion in chemical baths performed in wet bench Immersion process is expensive (needs a lot of solution), causes redeposition of contaminants on the surface, and smaller and deeper patterns are difficult to clean Spray cleaning advantages: Chemical costs are down, cleaning efficiency is low, less recontamination due to spray Pressured spray assists in cleaning small patterns Rinsing after cleaning in the same machine without separate station Figure 5.28 CO2 “SNOW” cleaning (Courtesy of Walter Kern). High pressure CO2 is directed at the surface from the nozzle Pressure drop causing rapid cooling and forms CO2 particles and snow Impinging particles dislodge the surface particles and flow carry them away Can also use argon: Argon aerosol is large and heavy can dislodge the particle when directed to the wafer under pressure Nitrogen/argon can also be used for this technique: (cryokinetic) Water Rinse Wet cleaning is followed by rinse in D.I. Water Rinsing functions: Removing cleaning chemicals Stop oxide etch Future direction: Higher rinse efficiency, from 30 gal/sq.in. of Si to 2 gal/sq.in. of Si in 2012 Dry cleaning Vapor or gas phase cleaning (E.g. HF/water mixtures) Plasma etch UV ozone cleaning: oxidize and photo-dissociate contaminants from the wafer surface Figure 5.29-(1) Rinse systems: (a) single overflow D.I.water from bottom flow through around the wafer, exitiy into over a dam into drain system Nitrogen bubbles up through the water, aids th mixing of the chemical with the water on wafer surface (bubbler) Figure 5.29-(2) Rinse systems: (b) three-stage overflow Figure 5.30 Parallel down flow rinsing (Courtesy of Walter Kern). Water brought into the system from outside the rinser and flow down through the wafer Rinse usually take 5 minutes with water flow rate equivalent to 5 times the volume of the rinser per minutes (5V/min) Rinse time can be determined by measuring the resistivity of the water (water resistivity meter is used, usually exiting water is 15-18 megohm) Figure 5.31 Spray-dump rinser. Overflow rinser with spray capability Spray rinsing Flow water removes water soluble chemicals and carries the chemicals away Faster flow rate will speed up the rinsing process Spray rinsing removes the chemical with a physical force from momentum and has a faster rinse rate Advantages: Faster rinse rate, more efficient rinsing Use less water Disadvantage: Carbon dioxide from air get trapped in the spray and form charged particles and resistivity meter reads them as contaminants Figure 5.32 Ultrasonic/megasonic wafer cleaning/etching bath. Ultrasonic:20,000-50,000 Hz,waves passes liqyid causing microscopic bubbles to form and collapse rapidly creating scrubbing action that dislodge the particles (cavitaion) Megasonic:850KHz, small particles held on the wafer surface due to slow moving boundary layer on the wafer surface, leaving the particle unexposed to the cleaning chemicals,megasonic energy reduces the stagnant layer on the wafer surface, exposing particles to the cleaning solutions,also,acoustic streaming fosters an increase in the velocity of the rinse and cleaning solutions past the wafer surface, increasing cleaning efficiency Figure 5.33-(1) Spin rinse dryer styles. (a) Multiboat Rinse the wafer with Water from central pipe, than rotate with high speed with hot nitrogen from the center pipe.The rotation throw water off the wafer And the hot nitrogen remove the water droplet Drying Technique • Spin Rinse dryers • Isopropyl Alcohol (IPA) Vapor dry • Surface Tension/ Marangoni Drying Figure 5.33-(2) Spin rinse dryer styles. (b) single boat axial Axial dryer:water and nitrogen come through the side, rinse and drying take place while spinning Figure 5.34 Vapor dry (Courtesy of Walter Kern). Alcohol drying: Heated reserve of Liquid IPA with vapor Cloud above it. When Wafer with residual Water on surface is Suspended in the Vapor zone, the IPA Replaces the water, Chilled coils around the vapor zone condense the water vapor Out of the IPA, leaving The surface water free Figure 5.35 SIA Roadmap Projections (Micro October 1998 p. 54). Surface Tension/Marangoni Drying • Surface tension draws the water away from the surface, leaving it dry. • IPA and nitrogen are directed at wafer water level interface. • IPA/Nitrogen flow created a surface tension gradient causing a water flow from surface into the water. This internal flow further enhances the removal of water from the wafer.