Oxidation Farshid Karbassian

Outline Oxide Layer Applications Types of Oxidation Modeling Dry Oxidation Wet Oxidation Modeling C-V Measurement

Oxide Layer Applications Name of the Oxide Thickness (Å) Application Time in Application Native 15-20 Undesirable - Screen ~200 Implantation Mid 70s to present Masking ~5000 Diffusion 1960s to mid 70s Field & LOCOS 3000-5000 Isolation 1960s to 90s Pad 100-200 1960s to present Sacrificial <1000 1970s to present Gate 30-120 Barrier STI 1980s to present Nitride stress buffer Defect removal Gate dielectric

Oxide Applications: Native Oxide Purpose: This oxide is a contaminant and generally undesirable. Sometimes used in memory storage or film passivation. Comments: Growth of native oxide layer at room temperature takes 3-4 hours up to about 12 Å. p+ Silicon substrate Silicon dioxide (oxide)

Oxide Applications: Gate Oxide Purpose: Serves as a dielectric between the gate and source-drain parts of MOS transistor. Gate oxide Transistor site p+ Silicon substrate Source Drain Gate Comments: Common gate oxide film thickness range from about 30 Å to 50 Å. Dry oxidation is the preferred method.

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.

Oxide Applications: Barrier Oxide Purpose: Protect active devices and silicon from follow-on processing. Comments: Deposition to several hundred Angstroms thickness. Barrier oxide Diffused resistors Metal p+ Silicon substrate

Oxide Applications: Pad Oxide Purpose: Provides stress reduction for Si3N4 Comments: Very thin layer of oxide is deposited. Passivation Layer ILD-4 ILD-5 M-3 M-4 Pad oxide Bonding pad metal Nitride

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 Screen oxide High damage to upper Si surface + more channeling Low damage to upper Si surface + less channeling p+ Silicon substrate Comments: Thermally grown

Oxide Applications: Insulating Layer between Metals Purpose: Serves as protective layer between metal lines. Comments: Deposition Passivation layer ILD-4 ILD-5 M-3 M-4 Interlayer oxide Bonding pad metal

LOCOS Process Cross section of LOCOS field oxide (Actual growth of oxide is omnidirectional) 1. Nitride deposition Pad oxide (initial oxide) Silicon 2. Nitride mask & etch Nitride 3. Local oxidation of silicon SiO2 growth 4. Nitride strip SiO2

Selective Oxidation and Bird’s Beak Effect Silicon oxynitride Nitride oxidation mask Bird’s beak region Selective oxidation Pad oxide Silicon substrate Silicon dioxide

STI Isolation Cross section of shallow trench isolation (STI) Silicon Trench filled with deposited oxide Sidewall liner 1. Nitride deposition Pad oxide (initial oxide) 2. Trench mask and etch Nitride 4. Oxide planarization (CMP) 5. Nitride strip Oxide 3. Sidewall oxidation and trench fill Oxide over nitride

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.

Pre-oxidation Cleaning Pre-oxidation cleaning is performed to remove particles, organic and inorganic contaminants, native oxide and surface defects.

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) RCA: Radio Corporation of America

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.

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 the silicon surface.

Thermal Oxidation Process Flow Chart Wet Clean Chemicals % solution Temperature Time Oxidation Furnace O2, H2 , N2 , Cl Flow rate Exhaust Temperature profile 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.

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.

Dry Oxidation

Diffusion of Oxygen Through Oxide Layer TEM image of Si/SiO2 Si SiO2 O, O2 Oxide-silicon interface Oxygen-oxide interface Oxygen supplied to reaction surface

Horizontal Diffusion Furnace

Vertical Diffusion Furnace

Horizontal and Vertical Furnace

Vertical Furnace Process Tube Heater 1 Heater 2 Heater 3 Thermocouple measurements Temperature controller Profile TCs Control TCs Overtemperature TCs System TC

Si/SiO2 Interface Silicon Dangling Bond Si Si-SiO2 Interface Oxygen Interface State Charge (Positive) SiO2

Consumption of Silicon during Oxidation Before oxidation 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.

Wet Oxidation System Pyrogenic Steam System

Wet Oxidation System Bubbler System

Dry Oxidation Vs. Wet Oxidation

Deal/Grove (Kinetic) Model Assumptions: Temperature: 700 - 1300 oC Pressure: 0.2 - 1.0 atm SiO2 thickness: 0.03 - 2 μm x2 + A x = B(t + τ) ; τ = time for initial oxide thickness d0 A = 2 D /κ B = 2 D C0 / C1 x = [B t + 0.25 A2 + d02 + A d0]0.5 – A / 2

Oxide Measurement Color chart

Oxide Measurement C-V Measurement

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