Transcript Oxidation
Oxidation Farshid Karbassian Outline • Oxide Layer Applications • Types of Oxidation • Dry Oxidation • Wet Oxidation • Modeling • C-V Measurement 2 Oxide Layer Applications Name of the Oxide Thickness (Å) Application Native 15-20 Undesirable Time in Application - Screen ~200 Implantation Mid 70s to present Masking ~5000 Diffusion 1960s to mid 70s 3000-5000 Isolation 1960s to 90s Field & LOCOS Pad 100-200 Nitride stress buffer 1960s to present Sacrificial <1000 Defect removal 1970s to present Gate 30-120 Gate dielectric 1960s to present Barrier 100-200 STI 1980s to present Oxide Applications: Native Oxide Purpose: This oxide is a contaminant and generally undesirable. Sometimes used in memory storage or film passivation. Silicon dioxide (oxide) p+ Silicon substrate Comments: Growth of native oxide layer at room temperature takes 3-4 hours up to about 12 Å. 4 Oxide Applications: Gate Oxide Purpose: Serves as a dielectric between the gate and source-drain parts of MOS transistor. Gate oxide Gate Source Drain Transistor site p+ Silicon substrate Comments: Common gate oxide film thickness range from about 30 Å to 50 Å. Dry oxidation is the preferred method. 5 Oxide Applications: Field Oxide Purpose: Serves as an isolation barrier between individual transistors to isolate them from each other. Field oxide Transistor site p+ Silicon substrate Comments: Field oxide thickness ranges from 2,500 Å to 15,000 Å. Wet oxidation is the preferred method. 6 Oxide Applications: Barrier Oxide Purpose: Protect active devices and silicon from followon processing. Barrier oxide Metal Diffused resistors p+ Silicon substrate Comments: Deposition to several hundred Angstroms thickness. 7 Oxide Applications: Pad Oxide Purpose: Provides stress reduction for Si3N4 Nitride Pad oxide Passivation Layer Bonding pad metal ILD-5 M-4 ILD-4 M-3 Comments: 8 Very thin layer of oxide is deposited. Oxide Applications: Implant Screen Oxide Purpose: Sometimes referred to as “sacrificial oxide”, screen oxide, is used to reduce implant channeling and damage. Assists creation of shallow junctions. Ion implantation p+ Silicon substrate High damage to upper Si surface + more channeling Comments: 9 Low damage to upper Si surface + less channeling Thermally grown Screen oxide Oxide Applications: Insulating Layer between Metals Purpose: Serves as protective layer between metal lines. Interlayer oxide Passivation layer Bonding pad metal ILD-5 M-4 ILD-4 M-3 Comments: 10 Deposition LOCOS Process 2. Nitride mask & etch 1. Nitride deposition Nitride Pad oxide (initial oxide) 3. Local oxidation of silicon Silicon SiO2 growth SiO2 SiO2 Nitride 4. Nitride strip Silicon Cross section of LOCOS field oxide (Actual growth of oxide is omnidirectional) 11 Selective Oxidation and Bird’s Beak Effect Silicon oxynitride Nitride oxidation mask Bird’s beak region Selective oxidation Silicon dioxide Pad oxide Silicon substrate 12 STI Isolation 1. Nitride deposition 2. Trench mask and etch Nitride Silicon 3. Sidewall oxidation and trench fill Oxide over nitride Pad oxide (initial oxide) 4. Oxide planarization (CMP) 5. Nitride strip Trench filled with deposited oxide Oxide Sidewall liner Silicon 13 Cross section of shallow trench isolation (STI) Pre-oxidation Cleaning • Crystallization of silicon dioxide is very undesirable, since it is not uniform and crystal boundaries provide easy paths for impurities and moisture. Therefore, pre-oxidation wafer cleaning is performed to eliminate crystallization. 14 Pre-oxidation Cleaning • Pre-oxidation cleaning is performed to remove particles, organic and inorganic contaminants, native oxide and surface defects. 15 RCA Cleaning • RCA Standard Cleaning I (SC-1) NH4OH:H2O2:H2O 1:1:5 – 1:2:7 (70-80OC) • DI water • RCA Standard Cleaning II (SC-2) HCl:H2O2:H2O 1:1:6 – 1:2:8 (70-80OC) • DI water Wafer RCA: Radio Corporation of America Spindle Chuck to vacuum pump RCA Cleaning • When wafers are submerged in RCA I solution, particles and organic contaminants oxidize, and their byproducts are either gaseous (e.g. CO), or soluble in the solution (e.g. H2O). • In RCA II, H2O2 oxidizes the inorganic contaminants and HCl reacts with the oxides to form soluble chlorides, which allows desorption of contaminants from the wafer surface. 17 HF etching • Native oxide on Si is of poor quality and needs to be stripped, especially for the gate oxide which requires the highest quality. • This is performed either in HF:H2O solution or in HF vapor etcher. • After native oxide stripping, some F atoms bind with Si atoms and form Si-F bonds on 18 the silicon surface. Thermal Oxidation Process Flow Chart Wet Clean • Chemicals • % solution • Temperature • Time 19 Oxidation Furnace • O2, H2 , N2 , Cl • Flow rate • Exhaust • Temperature • Temperature profile • Time Inspection • Film thickness • Uniformity • Particles • Defects Thermal Oxidation • Depending on the quality and thickness which is required for the oxide layer, wet or dry oxidation may be used. • Former is faster, but latter is cleaner and makes a better interface. 20 Dry Oxidation • In dry oxidation, pure oxygen gas (5s at least) is used. At high temp. O2 molecules diffuse across an existing oxide layer to reach the Si/SiO2 interface. 21 Dry Oxidation Exhaust System MFCs Control Valves coil Regulator Purge N2 O2 Process N2 HCl Process Tube Temp. Flat Zone distance Gas Cylinders 22 Diffusion of Oxygen Through Oxide Layer Oxygen supplied to reaction surface O, O2 Oxygen-oxide interface SiO2 Oxide-silicon interface Si TEM image of Si/SiO2 23 Horizontal Diffusion Furnace 24 Vertical Diffusion Furnace 25 Horizontal and Vertical Furnace Performance Factor Typical wafer loading size Clean room footprint Horizontal Furnace Vertical Furnace Small, for process flexibility Small, to use less space Ideal for process flexibility 200 wafers/batch 100 wafers/batch Larger, but has 4 process tubes Not capable Gas flow dynamics (GFD) Optimize for uniformity Boat rotation for improved film uniformity Temperature gradient across wafer Particle control during loading/unloading Ideal condition Worse due to paddle and boat hardware. Bouyancy and gravity effects cause non-uniform radial gas distribution. Impossible to design Smaller (single process tube) Capable of loading/unloading wafers during process, which increases throughput Superior GFD and symmetric/uniform gas distribution Parallel processing Quartz change 26 Performance Objective Wafer loading technique Pre-and postprocess control of furnace ambient Easy to include Ideally small Large, due to radiant shadow of paddle Small Minimum particles Relatively poor Easily done in short time Ideally automated More involved and slow Improved particle control from top-down loading scheme Easier and quicker, leading to reduced downtime Easily automated with robotics Excellent control, with options of either vacuum or neutral ambient Control is desirable Difficult to automate in a successful fashion Relatively difficult to control Vertical Furnace Process Tube Thermocouple measurements Temperature controller Control TCs Profile TCs Heater 1 Heater 2 Heater 3 TC 27 Overtemperature TCs System controller Si/SiO2 Interface Silicon Oxygen Interface State Charge (Positive) Dangling Bond SiO2 Si-SiO2 Interface Si 28 Consumption of Silicon during Oxidation t 0.55t 0.45t Before oxidation 29 After oxidation Wet Oxidation • At high temp. H2O dissociates and form hydroxide, HO, which can diffuses in the SiO2 layer faster than O2 . • A wet oxidation system may have a boiler or a bubbler or maybe it is a pyrogenic steam system, which is more common. 30 Wet Oxidation System Control Valves coil Process Tube Exhaust Gas Cylinders 31 Purge N2 O2 Process N2 H2 Burn box MFCs Regulator Pyrogenic Steam System Wet Oxidation System N2 MFC N2 bubbles N2 + H2O Process tube Exhaust Heated gas line Water Heater 32 Bubbler System Dry Oxidation Vs. Wet Oxidation Dry oxidation 33 Wet oxidation Deal/Grove (Kinetic) Model 2D 2 DC0 x x ( t ) C1 2 Assumptions: Temperature: 700 - 1300 oC Pressure: 0.2 - 1.0 atm SiO2 thickness: 0.03 - 2 μm ( d 02 2 Dd 0 / )C1 / 2 DC0 x2 + A x = B(t + τ) ; τ = time for initial oxide thickness d0 A = 2 D /κ 34 B = 2 D C0 / C1 x = [B t + 0.25 A2 + d02 + A d0]0.5 – A / 2 Oxide Measurement Color chart • 35 Oxide Measurement • C-V Measurement Large Resistor Capacitor Meter Oxide Aluminum Silicon Metal Platform Heater 36 Heater Any questions?