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THIẾT KẾ VI MẠCH TƯƠNG TỰCHƯƠNG 2: CMOS Technology
Hoàng Trang-bộ môn Kỹ Thuật Điện Tử
TP.Hồ Chí Minh 12/2011
Hoàng Trang bộ môn Kỹ Thuật Điện Tử[email protected]
11
1. Overview
- IC technology
CMOS BJT- CMOS vs BJT
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What is Integrated Circuit Technology?
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How Does IC Technology Influence Analog IC Design?
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Classification of Silicon Technology
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Why CMOS Technology?
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Components of a Modern CMOS Technology
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CMOS Components – Transistors
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2.BASIC IC PROCESS TECHNOLOGYFUNDAMENTAL IC PROCESSING STEPS
100‐200‐300‐450mm
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Oxidation
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Diffusion
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Ion implantation
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Deposition
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Etching
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Shallow Trench Isolation
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Epitaxial
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Photolithography
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Illustration of Photolithography ‐ Exposure
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Illustration of Photolithography –Positive Photoresist
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Major Fabrication Steps for a DSM CMOS Process1 ) p and n wells1.) p and n wells2.) Shallow trench isolation3.) Threshold shift4.) Thin oxide and gate polysilicon5.) Lightly doped drains and sources6.) Sidewall spacer) p7.) Heavily doped drains and sources8.) Siliciding (Salicide and Polycide)9 ) Bottom metal tungsten plugs and oxide9.) Bottom metal, tungsten plugs, and oxide10.) Higher level metals, tungsten plugs/vias, and oxide11.) Top level metal, vias and protective oxide
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 1 – Starting MaterialThe substrate should be highly doped to act like a good conductor
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 2 - n and p wellsThese are the areas where the transistors will be fabricated -NMOS in the p well and PMOS in the n wellNMOS in the p-well and PMOS in the n-well.Done by implantation followed by a deep diffusion.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 3 – Shallow Trench IsolationThe shallow trench isolation (STI) electrically isolates one region/transistor from anotherregion/transistor from another.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 4 – Threshold Shift and Anti-Punch Through Implants- The natural thresholds of the NMOS is about 0V and of the PMOS is about –1.2V. An p-implant is used to make the NMOS harder to invert and the PMOS p peasier resulting in threshold voltages balanced around zero volts.- Also an implant can be applied to create a higher-doped region beneath the channels to prevent punch-through from the drain depletion region extending to source depletion region.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 5 – Thin Oxide and Polysilicon GatesA thin oxide is deposited followed by polysilicon. These layers are removed where they are not wantedremoved where they are not wanted.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 6 – Lightly Doped Drains and SourcesA lightly-doped implant is used to create a lightly-doped source and drain next to the channel of the MOSFETsand drain next to the channel of the MOSFETs.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 7 – Sidewall Spacers A layer of dielectric is deposited on the surface and removed in such a way as to leave “sidewall spacers” next to the thin oxidesuch a way as to leave sidewall spacers next to the thin-oxide-polysilicon-polycide sandwich. These sidewall spacers will prevent the part of the source and drain next to the channel from becoming heavily doped.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 8–Implantation of the Heavily Doped Sources and DrainsNote that not only does this step provide the completed sources and drains but allows for ohmic contact into the wells andand drains but allows for ohmic contact into the wells and substrate.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 9 – SilicidingSiliciding and polyciding is used to reduce interconnect resistivity by placing a lowresistance silicide such as TiSi2 WSi2 TaSi2 etcby placing a lowresistance silicide such as TiSi2, WSi2, TaSi2, etc. on top of the diffusions.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 10 – Intermediate Oxide LayerAn oxide layer is used to cover the transistors and to planarize (or polish >CMP) the surfacepolish->CMP) the surface.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 11- First-Level MetalTungsten plugs are built through the lower intermediate oxide layer to provide contact between the devices wells and substratelayer to provide contact between the devices, wells and substrate to the first-level metal.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Step 12 – Second-Level MetalThe previous step is repeated to from the second-level metal.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Completed Fabrication After multiple levels of metal are applied, the fabrication is completed with a thicker top-level metal and a protective layer to hermetically seal the circuit from the environment. Note that metal is used for the upper level metal vias The chip is electrically connected byNote that metal is used for the upper level metal vias. The chip is electrically connected byremoving the protective layer over large bonding pads.
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
Scanning Electron Microscope (SEM) of a MOSFET cross-section
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
SEM: Showing Metal Levels and Interconnect
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3. TYPICAL DEEP SUBMICRON (DSM) CMOS FABRICATION PROCESS
DSM CMOS Technology Summary
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4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
What is UDSM CMOS Technology?Vindication of Moore’s Law“The minimum feature size decreases by approximately 0.7 every two years.”y pp y y y
Minimum feature size ~25 nanometers (2012)2006 state of the art:- 65 nm drawn length - 35 nm transistor gate length- 1.2 nm transistor gate oxide - 8 layers of copper interconnect
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4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
Example: about 65 Nanometer CMOS Technology
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4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
UDSM Metal and Interconnects
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4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
Advantages of UDSM CMOS Technology:
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4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
Disadvantages of UDSM CMOS Technology (for Analog)?• Reduction in power supply resulting in reduced headroom• Gate leakage currents
R d d ll i l i t i i i• Reduced small-signal intrinsic gains• Increased nonlinearity (IIP3)• Noise and matching?
Intrinsic gain and IP3 as a function of the gate overdrive for decreasing VDS:
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Anne-Johan Annema, et. Al., “Analog Circuits in Ultra-Deep-Submicron CMOS,” IEEE J. of Solid-State Circuits, 2005
4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
the Gate Leakage Problem?
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4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
Gate Leakage and fgate
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4. ULTRA DEEP SUBMICRON (UDSM) CMOS FABRICATION PROCESS
UDSM CMOS Technology Summary
• Increased transconductance and frequency capabilityL l lt• Low power supply voltages
• Reduced parasitics• Gate leakage causes challenges for analog applications of g g gUDSM technology• Other . . . ?
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5. PN JUNCTIONS in CMOS
How are PN Junctions used in CMOS?• PN junctions are used to electrically isolate one semiconductor region from anotheranother• PN diodes• Creation of the thermal voltage for bandgap purposes• Depletion capacitors – voltage variable capacitors (varactors)
Components of a PN junction:1.) p-doped semiconductor – a semiconductor having atoms containing a lack of electrons (acceptors). The concentration of acceptors is NA in atoms per cubic centimeter.2.) n-doped semiconductor – a semiconductor having atoms containing an excess of electrons (donors) The concentration of these atoms is ND in atomsexcess of electrons (donors). The concentration of these atoms is ND in atoms per cubic centimeter.
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6. MOS TRANSISTORPHYSICAL ASPECTS OF MOS TRANSISTORS
Physical Structure of MOS Transistors in an n-well Technology
Width (W) of the MOSFET = Width of the source/drain diffusionLength (L) of the MOSFET = Width of the polysilicon gate between the S/D diffusionsNote: the MOSFET is isolated from the well/substrate by reverse biasing the resulting
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PN junction
6. MOS TRANSISTORPHYSICAL ASPECTS OF MOS TRANSISTORS
Enhancement MOSFETsThe channel between the source and drain of an enhancement MOSFET is formed when the proper potential is applied to the gate of the MOSFET. This potential inverts th t i l i di t l b l th t t th t f i it th dthe material immediately below the gate to the same type of impurity as the source and drain forming the channel.
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6. MOS TRANSISTORPHYSICAL ASPECTS OF MOS TRANSISTORS
Depletion Mode MOSFETThe channel is diffused into the substrate so that a channel exists between the source and drain with no external gate potential.
The threshold voltage for a depletion mode NMOS transistor will be negative (a negative gate potential is necessary to attract enough holes underneath the gate to
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negative gate potential is necessary to attract enough holes underneath the gate to cause this region to invert to p-type material).
6. MOS TRANSISTORPHYSICAL ASPECTS OF MOS TRANSISTORS
W k I i O iWeak Inversion Operation
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6. MOS TRANSISTORLAYOUT OF MOS TRANSISTORS
Layout of a Single MOS transistor
Comments:• Make sure to contact the source and drain with multiple contacts to evenly distribute the current flow under the gate
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the current flow under the gate.• Minimize the area of the source and drain to reduce bulk-source/drain capacitance.
6. MOS TRANSISTORLAYOUT OF MOS TRANSISTORS
Geometric Effects
Orientation:Devices oriented in the same direction match more precisely than those oriented in other directions
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6. MOS TRANSISTORLAYOUT OF MOS TRANSISTORS
Diffusion and Etch Effects
• Poly etch rate variation – use dummy elements to prevent etch rate differences.• Do not put contacts on top of the gate for matched transistors.• Be careful of diffusion interactions for diffusions near the channel of the MOSFET
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6. MOS TRANSISTORLAYOUT OF MOS TRANSISTORS
Thermal and Stress Effects• Oxide gradients – use common centroid geometry layout• Stress gradients – use proper location and common centroid geometry layout• Thermal gradients – keep transistors well away from power devices and use common centroid geometry layout with interdigitated transistors
Examples of Common Centroid Interdigitated transistor layout:Examples of Common Centroid Interdigitated transistor layout:
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6. MOS TRANSISTORLAYOUT OF MOS TRANSISTORS
MOS Transistor LayoutPhotolithographic invariance (PLI) are transistors that exhibit identical orientation. PLI comes from optical interactions between pthe UV light and the masks.
Examples of the layout of matched MOS transistors:1 E l f i t d h t lith hi i i1. Examples of mirror symmetry and photolithographic invariance.
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6. MOS TRANSISTORLAYOUT OF MOS TRANSISTORS
Examples of the layout of matched MOS transistors (cont)2. Two transistors sharing a common source and laid out to
hi b th h t lith hi i i d t idachieve both photolithographic invariance and common centroid.
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6. MOS TRANSISTORLAYOUT OF MOS TRANSISTORS
Examples of the layout of matched MOS transistors (cont)3. Compact layout of the previous example.
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7. CAPACITORS in CMOS technology
Types of Capacitors for CMOS Technology
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7. CAPACITORS in CMOS technology
Characterization of Capacitors
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7. CAPACITORS in CMOS technologyPN JUNCTION CAPACITORS
PN Junction Capacitors in a Well
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7. CAPACITORS in CMOS technologyPN JUNCTION CAPACITORS
PN Junction Capacitors in a Well
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7. CAPACITORS in CMOS technologyMOSFET GATE CAPACITORS
MOSFET Gate Capacitor StructureThe MOSFET gate capacitors have the gate as one terminal of the capacitor and some combination of the source, drain, and bulkthe capacitor and some combination of the source, drain, and bulk as the other terminal.In the model of the MOSFET gate capacitor shown below, the gate capacitance is really two capacitors in series depending ongate capacitance is really two capacitors in series depending on the condition of the channel.
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7. CAPACITORS in CMOS technologyMOSFET GATE CAPACITORS
MOSFET Gate Capacitor StructureThe MOSFET gate capacitors have the gate as one terminal of the capacitor and some combination of the source, drain, and bulkthe capacitor and some combination of the source, drain, and bulk as the other terminal.In the model of the MOSFET gate capacitor shown below, the gate capacitance is really two capacitors in series depending ongate capacitance is really two capacitors in series depending on the condition of the channel.
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Reference
Phillip E.Allen, Douglas R.Holberg, “CMOS Analog Circuit Design”, 2nd Edition, Oxford Univeristy Press, 2002.g , , y ,
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