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Farshid Karbassian Oxidation. Oxide Layer Applications Types of Oxidation Dry Oxidation Wet Oxidation Modeling C-V Measurement Outline 2.

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Presentation on theme: "Farshid Karbassian Oxidation. Oxide Layer Applications Types of Oxidation Dry Oxidation Wet Oxidation Modeling C-V Measurement Outline 2."— Presentation transcript:

1 Farshid Karbassian Oxidation

2 Oxide Layer Applications Types of Oxidation Dry Oxidation Wet Oxidation Modeling C-V Measurement Outline 2

3 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 1960s to present Barrier 100-200 STI 1980s to present Nitride stress buffer Defect removal Gate 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 substrate Silicon dioxide (oxide) 4

5 Oxide Applications: Gate Oxide Purpose: Serves as a dielectric between the gate and source-drain parts of MOS transistor. Comments: Common gate oxide film thickness range from about 30 Å to 50 Å. Dry oxidation is the preferred method. Gate oxide Transistor site p + Silicon substrate SourceDrain Gate 5

6 Oxide Applications: Field Oxide Purpose: Serves as an isolation barrier between individual transistors to isolate them from each other. Comments: Field oxide thickness ranges from 2,500 Å to 15,000 Å. Wet oxidation is the preferred method. Field oxide Transistor site p + Silicon substrate 6

7 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 7

8 Oxide Applications: Pad Oxide Purpose: Provides stress reduction for Si 3 N 4 Comments: Very thin layer of oxide is deposited. Passivation Layer ILD-4 ILD-5 M-3 M-4 Pad oxide Bonding pad metal Nitride 8

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. Comments: Thermally grown Ion implantation Screen oxide High damage to upper Si surface + more channeling Low damage to upper Si surface + less channeling p + Silicon substrate 9

10 Passivation layer ILD-4 ILD-5 M-3 M-4 Interlayer oxide Bonding pad metal Oxide Applications: Insulating Layer between Metals Purpose: Serves as protective layer between metal lines. Comments: Deposition 10

11 Cross section of LOCOS field oxide (Actual growth of oxide is omnidirectional) 1. Nitride deposition Pad oxide (initial oxide) Silicon 2. Nitride mask & etch Silicon Silicon Nitride 3. Local oxidation of silicon SiO 2 growth Silicon 4. Nitride strip Silicon SiO 2 Nitride Silicon LOCOS Process 11

12 Silicon oxynitride Nitride oxidation mask Birds beak region Selective oxidation Pad oxide Silicon substrate Silicon dioxide Selective Oxidation and Birds Beak Effect 12

13 Cross section of shallow trench isolation (STI) Silicon Trench filled with deposited oxide Sidewall liner 1. Nitride deposition Pad oxide (initial oxide) Silicon 2. Trench mask and etch Silicon Silicon Nitride 4. Oxide planarization (CMP) Silicon 5. Nitride strip Oxide 3. Sidewall oxidation and trench fill Oxide over nitride Silicon STI Isolation 13

14 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 14

15 Pre-oxidation cleaning is performed to remove particles, organic and inorganic contaminants, native oxide and surface defects. Pre-oxidation Cleaning 15

16 RCA Standard Cleaning I (SC-1) NH 4 OH:H 2 O 2 :H 2 O 1:1:5 – 1:2:7 (70-80 O C) DI water RCA Standard Cleaning II (SC-2) HCl:H 2 O 2 :H 2 O 1:1:6 – 1:2:8 (70-80 O C) DI water RCA Cleaning RCA: Radio Corporation of America

17 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. H 2 O). In RCA II, H 2 O 2 oxidizes the inorganic contaminants and HCl reacts with the oxides to form soluble chlorides, which allows desorption of contaminants from the wafer surface. RCA Cleaning 17

18 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:H 2 O 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. HF etching 18

19 Wet Clean Chemicals % solution Temperature Time Oxidation Furnace O 2, H 2, N 2, Cl Flow rate Exhaust Temperature Temperature profile Time Inspection Film thickness Uniformity Particles Defects Thermal Oxidation Process Flow Chart 19

20 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. Thermal Oxidation 20

21 Dry Oxidation In dry oxidation, pure oxygen gas (5s at least) is used. At high temp. O 2 molecules diffuse across an existing oxide layer to reach the Si/SiO 2 interface. 21

22 Dry Oxidation 22

23 Diffusion of Oxygen Through Oxide Layer 23 TEM image of Si/SiO 2

24 Horizontal Diffusion Furnace 24

25 Vertical Diffusion Furnace 25

26 Horizontal and Vertical Furnace 26

27 Vertical Furnace Process Tube Heater 1 Heater 2 Heater 3 Thermocouple measurements Temperature controller Profile TCs Control TCs Overtemperature TCs System controller TC 27

28 Si/SiO 2 Interface 28

29 tt 0.55t0.55t 0.45t0.45t Before oxidation After oxidation Consumption of Silicon during Oxidation 29

30 At high temp. H 2 O dissociates and form hydroxide, HO, which can diffuses in the SiO 2 layer faster than O 2. 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 30

31 Wet Oxidation System Pyrogenic Steam System 31

32 Bubbler System Wet Oxidation System 32

33 Dry Oxidation Vs. Wet Oxidation Dry oxidation Wet oxidation 33

34 B = 2 D C 0 / C 1 A = 2 D / κ x = [B t + 0.25 A 2 + d 0 2 + A d 0 ] 0.5 – A / 2 x 2 + A x = B(t + τ) ; τ = time for initial oxide thickness d 0 Deal/Grove (Kinetic) Model Assumptions: Temperature: 700 - 1300 o C Pressure: 0.2 - 1.0 atm Pressure: 0.2 - 1.0 atm SiO 2 thickness: 0.03 - 2 μm SiO 2 thickness: 0.03 - 2 μm 34

35 Color chart Oxide Measurement 35

36 C-V Measurement Oxide Measurement 36

37 Any questions?

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