3 Oxide Layer Applications Name of the OxideThickness (Å)ApplicationTime in ApplicationNative15-20Undesirable-Screen~200ImplantationMid 70s to presentMasking~5000Diffusion1960s to mid 70sField & LOCOSIsolation1960s to 90sPad1960s to presentSacrificial<10001970s to presentGate30-120BarrierSTI1980s to presentNitride stress bufferDefect removalGate dielectric
4 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 substrateSilicon dioxide (oxide)
5 Oxide Applications: Gate Oxide Purpose: Serves as a dielectric between the gate and source-drain parts of MOS transistor.Gate oxideTransistor sitep+ Silicon substrateSourceDrainGateComments: Common gate oxide film thickness range from about 30 Å to 50 Å. Dry oxidation is the preferred method.
6 Oxide Applications: Field Oxide Purpose: Serves as an isolation barrier between individual transistors to isolate them from each other.Field oxideTransistor sitep+ Silicon substrateComments: Field oxide thickness ranges from 2,500 Å to 15,000 Å. Wet oxidation is the preferred method.
7 Oxide Applications: Barrier Oxide Purpose: Protect active devices and silicon from follow-on processing.Comments: Deposition to several hundred Angstroms thickness.Barrier oxideDiffused resistorsMetalp+ Silicon substrate
8 Oxide Applications: Pad Oxide Purpose: Provides stress reduction for Si3N4Comments: Very thin layer of oxide is deposited.Passivation LayerILD-4ILD-5M-3M-4Pad oxideBonding pad metalNitride
9 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 implantationScreen oxideHigh damage to upper Si surface + more channelingLow damage to upper Si surface + less channelingp+ Silicon substrateComments: Thermally grown
10 Oxide Applications: Insulating Layer between Metals Purpose: Serves as protective layer between metal lines.Comments: DepositionPassivation layerILD-4ILD-5M-3M-4Interlayer oxideBonding pad metal
11 LOCOS Process Cross section of LOCOS field oxide (Actual growth of oxide is omnidirectional)1. Nitride depositionPad oxide(initial oxide)Silicon2. Nitride mask & etchNitride3. Local oxidation of siliconSiO2 growth4. Nitride stripSiO2
13 STI Isolation Cross section of shallow trench isolation (STI) SiliconTrench filled with deposited oxideSidewall liner1. Nitride depositionPad oxide(initial oxide)2. Trench mask and etchNitride4. Oxide planarization (CMP)5. Nitride stripOxide3. Sidewall oxidation and trench fillOxide over nitride
14 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.
15 Pre-oxidation Cleaning Pre-oxidation cleaning is performed to remove particles, organic and inorganic contaminants, native oxide and surface defects.
16 RCA Cleaning RCA Standard Cleaning I (SC-1) NH4OH:H2O2:H2O 1:1:5 – 1:2:7 (70-80OC)DI waterRCA Standard Cleaning II (SC-2)HCl:H2O2:H2O 1:1:6 – 1:2:8 (70-80OC)RCA: Radio Corporation of America
17 RCA CleaningWhen 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.
18 HF etchingNative 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.
28 Si/SiO2 Interface Silicon Dangling Bond Si Si-SiO2 Interface Oxygen Interface State Charge (Positive)SiO2
29 Consumption of Silicon during Oxidation Before oxidationAfter oxidation
30 Wet OxidationAt 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.
34 Deal/Grove (Kinetic) Model Assumptions:Temperature: oCPressure: atmSiO2 thickness: μmx2 + A x = B(t + τ) ;τ = time for initial oxide thickness d0A = 2 D /κB = 2 D C0 / C1x = [B t A2 + d02 + A d0]0.5 – A / 2