Advanced Workshop on Semiconductor Manufacturing Notes

Advanced Workshop on Semiconductor Manufacturing Overview

• Dates: May 21 to June 3.
• Hosting Institution: Center for Nanoscience Engineering (CeNSE) at the Indian Institute of Science (IISc), Bangalore.
• Participants: A diverse cohort of $16$ participants including students, researchers, faculty members, and industry professionals from:
  • Indian Institutes of Technology (IITs).
  • National Institutes of Technology (NITs).
  • Applied Materials.
  • McDermid Alpha.
  • University of Copenhagen (International participant).
  • ZIA Systems Private Limited.
  • Yashwantrao Chavan Institute of Science, Satara.
• Primary Points of Contact: Susan, Pavani, and Alit Seema (Head of the training program).

Participant Introductions and Goals

• Abhishek: PhD in Chemical Engineering from IISc; seeking career opportunities in semiconductors; attended the foundational workshop.
• Dr. Safraj Mujawa: Associate Professor in Maharashtra; Physics background; research in energy conversion/storage; interested in MOSFET fabrication and collaborative research with IISc.
• Shrikanta: Senior Manufacturing Engineer at Applied Materials; goal is to understand how devices are built and explore various manufacturing processes to innovate materials.
• Pratik Pitare: Pursuing MSc in Electronics; startup owner in robotics and agritech; goal is to start a venture involving semiconductor manufacturing equipment.
• Pragupeshwar: Second-year Electrical Engineering student at NIT Rourkela; interested in semiconductor manufacturing after attending a winter school.
• Kamaldeep Taral: PhD student at the University of Copenhagen; transitioning from academia to the semiconductor industry.
• Ramaphtha Sharma: Background in PhD from IIT and electrochemical fields; working with Alpha in advanced material packaging (PCB, copper pillars, metal plating); goal is to learn the full semiconductor fabrication ecosystem.
• Dr. Kupan: PhD from University of Delaware; postdoctoral work in Japan; expertise in thin films and magnetic materials; seeking exposure to the IISc ecosystem for career development.
• Aravind: EMS Manufacturing Engineer at ZIA Systems; over $10$ years of experience in electronics manufacturing; organization looks to implement semiconductor manufacturing.
• Deepa: Recent PhD graduate in Applied Physics; research in spectroscopic studies and nanoparticles; seeking hands-on fabrication experience.
• Ghanikilam: Second-year undergraduate; research interest in neuromorphic analog IC design and signal processing.
• Prajuk Patel: Product Manager at Applied Materials; $10$ years of experience (including automotive); interested in Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and Etch/Chemical Mechanical Planarization (CMP) processes.

National Nanofabrication Centre (NNFC) Safety and Protocols

• Facility Layout:
  • Total Size: $14,000$ square feet.
  • Corridor Area: $4,000$ square feet.
  • Clean Area: $10,000$ square feet.
  • Classification: Split between Class $100$ (Lithography) and Class $1,000$ (Etching, Deposition, Inline Characterization).
  • Equipment: Houses over $84$ pieces of equipment.
  • Operations: Multi-user facility open $24/7$.
• Clean Room Classifications:
  • ISO 14644 Standard: Defines classifications based on the number of particles per cubic meter.
  • Class 100: Maximum of $100$ particles of size 0.5 ̄\mu\text{m} and zero particles of size 5.0 ̄\mu\text{m}.
  • Class 1,000: Maximum of $1,000$ particles of size 0.5 ̄\mu\text{m}.
• Contamination Sources:
  • Personnel: Humans are the largest source of particles (millions generated just by standing or walking).
  • Molecular Contamination: VOCs, solvent vapors, outgassing from epoxies, body oils, perfumes, and cosmetics.
  • Particulate Contamination: Dust, clothing fibers (hoodies/jackets), equipment wear/tear, and cardboard.
  • Substrate Contamination: Fingerprints, chemical residues, and improper storage.

Clean Room Entry and Gowning Procedures

• Pre-Entry Hygiene: Face must be washed; no makeup, powder, perfumes, or deodorants. Mouth must be rinsed if the user is a smoker or has eaten aromatic food.
• Medical Restrictions: Nursing/expectant mothers, individuals with pacemakers or metallic implants must inform the NNFC office due to hazardous gases and high magnetic fields.
• Gowning Sequence:
  1. Face Mask.
  2. Hairnet (must cover all hair).
  3. Gown (Bunny suit).
  4. Booties (Clean room shoes).
  5. Gloves.
• Entry Air Shower: Requires a cycle of approximately $11$ seconds; the user must rotate to remove particles.
• Prohibited Items: Mobile phones, earplugs, gel pens, pencils, and rubbers. Only clean room paper and ballpoint pens are allowed.

Emergency Protocols and Chemical Safety

• Emergency Contact: Dial $115$ from internal fab intercoms to reach the Building Maintenance System (BMS).
• Chemical Spills:
  • Minor vs. Major: Major spills require calling BMS.
  • Personal Exposure: Use safety showers or eyewash stations for at least $15$ minutes.
  • Hydrofluoric Acid (HF/BHF): Extremely dangerous, colorless like water. $\text{H}^+$ ions are corrosive; $\text{F}^-$ ions penetrate skin and attack bones. Treatment involves thorough rinsing and application of Calcium Gluconate gel.
• Alarms:
  • Evacuation Alarm: Leave immediately via the nearest exit to the safe assembly point.
  • ETP Alarm: Effluent Treatment Plant alarm indicates high waste levels at the wet bench; not an immediate evacuation trigger for all.
• Facility Certifications: ISO $9001$ (Quality), ISO $14001$ (Environment), and ISO $45001$ (Health and Safety).

Wet Etch Bay Processes

• Core Functions: Substrate cleaning, isotropic/anisotropic etching, and MEMS structure release.
• Cleaning Mechanisms:
  • Piranha Clean: Mixture of Concentrated Sulfuric Acid ($\text{H}_2\text{SO}_4$) and Hydrogen Peroxide ($\text{H}_2\text{O}_2$) in a $3:1$ ratio. It is an exothermic reaction reaching 120 ̄\,^\circ\text{C}. It removes organic contaminants and photoresist, leaving the surface oxidized.
  • RCA Clean (Radio Corporation of America):
    • RCA1 (SC1): $5 : 1 : 1$ ratio of $\text{H}_2\text{O} : \text{NH}_4\text{OH} : \text{H}_2\text{O}_2$. Performed at 75 ̄\,^\circ\text{C}. Removes organic and particle contamination through electrostatic repulsion ($\text{OH}^-$ ions).
    • RCA2 (SC2): $6 : 1 : 1$ ratio of $\text{H}_2\text{O} : \text{HCl} : \text{H}_2\text{O}_2$. Performed at 75 ̄\,^\circ\text{C}. Removes metallic contamination by forming soluble chloride complexes.
  • HF Dip: Dilute Hydrofluoric acid used to remove native oxides. It makes the silicon surface hydrophobic.
• Crystal Orientations: Silicon wafers are typically (100), (110), or (111).
  • (111) planes have higher atomic density and etch more slowly than (100).
  • Anisotropic Etching: Angle of $54.7̄\,^\circ$ forms between (100) and (111) planes during wet etching.
• Etchant Comparison:
  • KOH (Potassium Hydroxide): Faster etch rate (≈ 1 ̄\mu\text{m/min}), cost-effective, but leaves rough surfaces and metallic ions.
  • TMAH (Tetramethylammonium Hydroxide): Slower (0.6̄\text{--}0.7 ̄\mu\text{m/min}), CMOS compatible (no metals), and creates smoother surfaces.
• MEMS Release:
  • HF Vaporizer: Uses gaseous HF to etch sacrificial oxide layers.
  • Critical Point Dryer (CPD): Uses supercritical $\text{CO}_2$ (where the fluid is neither gas nor liquid) to release delicate structures without surface tension-induced collapse.

Diffusion Bay and LPCVD

• Furnace Configuration: Horizontal stacks of four tubes each.
• LPCVD (Low Pressure Chemical Vapor Deposition): Operates at millibar pressures producing high-purity films through chemical reactions of gaseous precursors.
• Oxidation Types:
  • Dry Oxidation: $Si + O_2 → SiO_2$. Slower growth rate, higher quality/density, used for gate oxides. Thickness: 10̄\text{--}150 ̄\text{nm}.
  • Wet (Pyrogenic) Oxidation: $Si + 2H_2O → SiO_2 + 2H_2$. Faster, used for thick masking or isolation. Thickness: 150 ̄\text{nm} → 1 ̄̅\mu\text{m}.
• Doping: Introducing impurities like Boron (P-type) or Phosphorus (N-type).
  • Steps: Predeposition (atoms on surface) followed by Drive-in (annealing to penetrate and redistribute atoms).
• Materials Deposited via LPCVD:
  • Silicon Nitride ($\text{Si}_3\text{N}_4$): Precursors are Dichlorosilane (DCS) and Ammonia ($\text{NH}_3$).
  • Poly-Silicon/Germanium: Precursors are Silane ($\text{SiH}_4$) and Germane ($\text{GeH}_4$).
  • LTO (Low Temperature Oxide): Performed at 400̄\text{--}450 ̄\,^\circ\text{C} using Silane and Oxygen.
• Silicon Nanowires: Grown via Vapor-Liquid-Solid (VLS) method using Gold droplets as a catalyst.
• Contamination Levels (Cleanliness Hierarchy):
  • Level 1: Cleanest (Bare Silicon, RCA clean).
  • Level 2: Lithography/Dielectrics (Photoresist contamination).
  • Level 3: Metallization (Metal-bearing samples).
  • Level 4: Dirty (Specific materials like PZT).

Inline Characterization Tools

• Ellipsometer: Measures thin film thickness and optical constants ($n$ and $k$) by detecting changes in light polarization.
• Dektak Surface Profilometer: A contact stylus tool used to measure etch depth, step heights (16 ̄\text{nm to } 500 ̄̅\mu\text{m}), and surface roughness.
• K-MOS UltraScan: Uses a laser beam split into $12$ sub-beams to measure curvature and mechanical stress induced by thin films.
• Four-Point Probe: Measures sheet resistance ($\text{ohm/square}$) by passing current through outer probes and measuring voltage on inner probes.
• MDP (Microwave Detected Photoconductivity): Measures minority carrier lifetime (20 ̄\text{ns to ms}) and photoconductivity to assess purity and contamination impacts on electrical performance.

Questions & Discussion

• Q: How are particles measured in the clean room?
• A: specialized particle measuring equipment is used periodically, and data is documented to ensure compliance with standards. There are seasonal and occupancy-based trends in particle count.
• Q: Are water bottles allowed?
• A: Water bottles are not allowed in processing areas; they must be kept in the emergency exit room.
• Q: Can a single chemical reagent react preferentially with certain crystal planes?
• A: Yes, (100) planes etch much faster than (111) because (111) has higher atomic density and lower surface energy.
• Q: Is Dektak similar to AFM?
• A: Yes, both characterize surface topography, but AFM is considered superior as it provides higher resolution, whereas Dektak is a contact-mode surface profilometer.
• Q: How is the lifetime of charge carriers used to detect contamination?
• A: In pure silicon, lifetime is high. Contaminants act as traps for electrons and holes, significantly reducing recombination lifetime (e.g., dropping from 20 ̄̅\mu\text{s} to lower values after several process steps).