Layout Of A Inverter Topic 3 - Imperial College London

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Layout of a InverterTopic 3VDDCMOS Fabrication ProcessQpPeter CheungDepartment of Electrical & Electronic EngineeringImperial College LondonvoviQnGNDURL: www.ee.ic.ac.uk/pcheung/E-mail: p.cheung@ic.ac.ukPYKC Nov-27-09Lecture 3 - 1E4.20 Digital IC DesignThe CMOS Process - photolithography (1)PYKC Nov-27-09Lecture 3 - 2E4.20 Digital IC DesignThe CMOS Process - photolithography (2)(d) Expose resist to UV lightthrough a MASK(a) Bare silicon wafer(f) Etch away oxideSilicon WaferSilicon Wafer(b) Grow Oxide layerSiO2 1μmSilicon Wafer(g) Remove remaining resistSilicon Wafer(e) Remove unexposed resist(c) Spin on photoresistSilicon WaferphotoresistSilicon WaferPYKC Nov-27-09E4.20 Digital IC DesignSilicon WaferLecture 3 - 3PYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 4

Mask 1: N-well Diffusion SiO2 is etched usingMask 1.Mask 2: Define Active Regions Mask 2 creates theactive regions wherethe MOSFETs will beplacedPhosphorous stratep-substrateA thick field oxide is grown using acontruction technique called Local OxidationOf Silicon (LOCOS). Phosphorous isdiffused into theunmasked regions ofsilicon creating an n-well for the fabricationof p-channel devicesPYKC Nov-27-09SiO2n-well The thick oxide regionsprovides isolationbetween the MOSFETsp-substrateLecture 3 - 5E4.20 Digital IC DesignPYKC Nov-27-09Mask 3: Polysilicon Gate A high quality thin oxideis grown in the activearea ( 100A- 300A) Mask 3 is used todeposit the polysilicongate (most critical step)Thin Oxiden-wellp-substrateLecture 3 - 6E4.20 Digital IC DesignMask 4: n DiffusionSiO2n-wellp-substrateThe polysilicon layer is usually arsenic doped(n-type). The photolithography in this step isthe most demanding since it requires the finestresolution to create the narrow MOS channels. Mask 4 is used tocontrol a heavy arsenicimplant and create thesource and drain of then-channel devices. This is a self-alignedstructure.Arsenic ImplantPhotoresistSiO2n-wellp-substrateThe polysilicon gate acts like a barrier for thisimplant to protect the channel region.SiO2n n SiO2n-welln-wellp-substratePYKC Nov-27-09E4.20 Digital IC Designn p-substrateLecture 3 - 7PYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 8

Mask 5: p Diffusion Mask 5 is used tocontrol a heavy Boronimplant and create thesource and drain of then-channel devices. This is a self-alignedstructure.Mask 6: Contact Holes A thin layer of oxide isdeposited over the entirewafer Mask 6 is used to patternthe contact holes Etching opens the holes.Boron ImplantPhotoresistSiO2n-welloxidep n n p-substraten SiO2p Etched contact holesp n p n n-welln Lecture 3 - 9E4.20 Digital IC DesignPYKC Nov-27-09p n n SiO2p p n n-wellLecture 3 - 10E4.20 Digital IC DesignCross section of a CMOS InverterMask 7: Metalization A thin layer of aluminumis evaporated orsputtered onto thewafer. Mask 7 is used topattern theinterconnection.SiO2p-substratep-substratePYKC Nov-27-09p n n-wellp p-substrateThe polysilicon gate acts like a barrier for thisimplant to protect the channel region.p n SiO2p p n n-wellvivoVDDp-substratep Aluminum Interconnectionp n n SiO2p p n n p Qnp-substraten-welln Qpn-wellSource-BodyConnectionn p Source-BodyConnectionp-substratePYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 11PYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 12

Physical Layout of an InverterDimension of transistorsn-wellPMOS active regionVDDLQpNMOS active regionn Wn diffusionp diffusionLvoviPolyWGatep n-wellp PolyDrainSourceDrainSourcePoly 1 (poly-Si gate)n GateQnContact Holen-channel MOSFETGNDp-channel MOSFETMetal 1PYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 13Photo cross-section of a transistorPYKC Nov-27-09E4.20 Digital IC DesignPYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 14Advanced metalization with polishingLecture 3 - 15PYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 16

Latch-up problem (1) Latch-up (con’t)As shown above, the p region of the p-transistor, the n-well and the p- substrate form aparasitic pnp transistor T1.The n- well, the p- substrate and the p source of the n-transistor forms another parasiticnpn transistor T2.There exists two resistors Rw and Rs due to the resistive drop in the well area and thesubstrate area. PYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 17T1 and T2 form a thyristor circuit.If Rw and/or Rs are not 0, and for somereason (power-up, current spike etc), T1or T2 are forced to conduct, Vdd will beshorted to Gnd through the smallresistances and the transistors.Once the circuit is 'fired', both transistorswill remain conducting due to the voltagedrop across Rw and Rs. The only way toget out of this mode is to turn the poweroff.This condition is known as latch-up.To avoid latch-up, substrate-taps (tied toGnd) and well-taps (tied to Vdd) areinserted as frequently as possible. Thishas the effect of shorting out Rw and Rs.PYKC Nov-27-09E4.20 Digital IC DesignLecture 3 - 18

The CMOS Process - photolithography (1) Silicon Wafer Silicon Wafer SiO 2 1μm Silicon Wafer photoresist (a) Bare silicon wafer (b) Grow Oxide layer (c) Spin on photoresist Lecture 3 - 4 The CMOS Process - photolithography (2) Silicon Wafer (d) Expose resist to UV light through a MASK Silicon Wafer (e) Remove unexposed resist Silicon Wafer

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