School of Microelectronic Engineering EMT362: Microelectronic Fabrication CMOS ISOLATION TECHNOLOGY Part 2 Ramzan Mat Ayub School of Microelectronic Engineering

Lecture Objectives Understand the basic operation of MOS Capacitor Able to calculate the Threshold Voltage for MOS Capacitor and Transistor Understand why isolation is needed in CMOS process Understand the isolation requirements and related design rules Able to describe in terms of wafer cross section, the process steps for Semirecessed LOCOS, Fully Recessed LOCOS, STI and several advanced isolation structures formation.

School of Microelectronic Engineering PWELL NWELL POLY p+ n+ L W p+ A A’ BB’

School of Microelectronic Engineering p-well n+ NMOS CROSS SECTION ALONG A TO A’ LINE n+ Device with the same polarity - simpler

School of Microelectronic Engineering CROSS SECTION ALONG B TO B’ LINE n or p-substrate p-well p+ n+ PMOS NMOS n-well Device with the different polarity – more complicated

School of Microelectronic Engineering NMOS#2 DRAIN SOURCE NMOS#1 Field transistor MOS Device Isolation Requirements MOS Transistors are isolated as long as; source-substrate and drain-substrate pn junctions are held at reverse bias unwanted channels are prevented from forming among adjacent devices

School of Microelectronic Engineering Electric circuit in VLSI technology is implemented by connecting isolated devices through specific conducting path. To fabricate monolithic ICs, electrically isolated devices must be created in the silicon substrate. Only later they are connected. Improper isolated device will result; total circuit failure high leakage (large dc power dissipation) noise margin degradation voltage shift, cross talk between transistors and etc. The challenge is VLSI device only allows single transistor leakage < 10 pA/um). On the other hand, process integration imposed a stringent requirement on the isolation technology; spacing between actives should be as small as possible to produce the surface topography as planar as possible isolation process module must be simple to implement and easy to control

School of Microelectronic Engineering

V TF is the threshold (minimum) voltage to turn on the parasitic MOS (field transistor) V TF is normally at least 8 V above supply voltage to ensure less than 1 pA/um between isolated MOS device 2 methods of increasing the V TF ; making a thicker field oxide Increase the doping beneath field oxide (channel stop implant) NMOS#2 DRAIN SOURCE NMOS#1 M-1 Field transistor

School of Microelectronic Engineering MOS Device Isolation Characterization Test Structures for NMOS Isolation poly n+ Aluminum

School of Microelectronic Engineering The purpose; To find the V TFiso : The gate voltage at which the maximum allowable leakage current arise To find the optimum n+ to n+ spacing Gate voltage (V TFiso ) at drain 1nA or 1pA, at V D = Vcc, is measured VTFiso is plotted against n+ to n+ spacing to find the optimum n+ to n+ at certain V DS values

School of Microelectronic Engineering V TFiso Volts 0.1 u A 1 n A 10 p A n+ to n+ spacing in micron Channel current

School of Microelectronic Engineering Overview on CMOS Isolation Techniques Grow and etch thick oxide (1970) Semi recessed LOCOS (1980) Basic LOCOS Poly buffered SILO and etc Fully recessed LOCOS (1980) Side Wall Mask Isolation (SWAMI) Self Aligned Planar Oxidation (SPOT) FUROX (Fully Recessed Oxide) Shallow Trench (STI) (1990) SOI + STI (2000)

School of Microelectronic Engineering Grow and Etch Technique substrate a) Grow thick oxide b) Pattern and etch c) S/D diffusion

School of Microelectronic Engineering A)Grow and etch (used until late 70s) Thick oxide is grown thermally in the furnace Wafer is patterned and etch Disadvantages Sharp corners, difficult to cover in the latter process steps Channel stop must be implanted before oxide is grown (active to be aligned with channel stop region – low packing density)

School of Microelectronic Engineering LOCOS Isolation Technology oxidation nitride removal oxidation nitride removal a) Semi recessed LOCOSb) Fully recessed LOCOS

School of Microelectronic Engineering Basic Semi-recessed LOCOS Process Step-1: Pad Oxide Layer Wafer is cleaned using RCA cleaning technique A SiO2 (called pad or buffer oxide) is thermally grown The function of this oxide is to cushion the transistion of stress between the silicon substrate and the subsequently deposited nitride. Silicon substrate

School of Microelectronic Engineering Step-2: Silicon Nitride Layer A thick layer of CVD silicon nitride is deposited. The function of this nitride is as mask to the oxidation process. Silicon nitride is very effective as oxidation mask because oxygen and water vapor diffuse very slowly through it, preventing oxidizing species from reaching the silicon surface under the nitride. Silicon nitride however exhibiting a very high tensile stress (10 10 dynes/cm 2 ), hence used with minimal thickness. Silicon substrate

School of Microelectronic Engineering Step-3: Photolithography-Active Area Definition To define the active area (where the transistors to be put) Silicon substrate

School of Microelectronic Engineering Step-4: Nitride Etch To cover the active regions, expose areas to form LOCOS Silicon substrate

School of Microelectronic Engineering Step-5: Channel stop implant To create a channel stop doping layer under Field Oxide. In NMOS circuit, a p implant (boron, keV) is used, while in PMOS, arsenic is used. PR is removed after the implant Silicon substrate

School of Microelectronic Engineering Step-6: Grow Field Oxide Field oxide is thermally grown by wet oxidation at temperatures around 1000C to the thickness ,000A. Oxide will grows where there is no masking nitride, but at the nitride’s edges, some oxidation occurred. This caused the nitride’s edges to lift. Because of the shape, this structure is called bird’s beak. Silicon substrate

School of Microelectronic Engineering The bird’s beak is a lateral extension of the field oxide into the active area of the devices. For a typical 8000A LOCOS, bird’s beak ~ 5000A. Limiting factor for the usage of LOCOS. ORIGINAL MASK BIRD’S BEAK FINAL ACTIVE AREA 8000 A 5000 A

School of Microelectronic Engineering SEM picture of Semi-Recessed LOCOS

School of Microelectronic Engineering Step-7: Strip Masking Nitride Layer Oxynitride etch ( A top layer of nitride) – deglaze process Wet hot phosphoric process to remove nitride (good selectivity to oxide) Tricky process, deglaze process must be carefully characterized. Silicon substrate

School of Microelectronic Engineering Step-8: Regrow and strip sacrificial oxide Kooi et al discovered that a thin layer of silicon nitride can form on the silicon surface (pad oxide – silicon interface). This nitride spot is called white ribbon or Kooi Effect and must be removed to prevent defect from occuring when growing gate oxide. This can be done by regrowing a pad oxide and subsequently removed.

School of Microelectronic Engineering

Factors Affecting Bird’s Beak Length and Shape 1)Pad oxide thickness Lateral oxidation can be reduced by using a thinner pad oxide, leading to a shorter bird’s beak. 2)Pad layer composition – CVD oxynitride 3)Silicon crystal orientation – shorter bird’s beak in compared to 4)Field oxide process temperature – Shorter with higher oxidation temperature. 5)Thickness and mechanical properties of nitride layer – the thicker the nitride, the shorter the bird’s beak 6)Mask stack geometry – depends on the shape and size of the structures

School of Microelectronic Engineering Advanced Semi-Recessed LOCOS Process Based on the fact that a thinner pad oxide will produce a shorter bird’s beak. Usual pad oxide is replaced with a polybuffered layer; poly 500A:oxide 100A Thicker nitride is used to suppress the bird’s beak more, 1000 – 2500A A) Poly Buffered LOCOS B) Sealed Interface LOCOS Reduce the bird’s beak by depositing nitride layer directly onto the silicon. Lateral diffusion of oxidants is suppressed better, resulting a shorter bird’s beak. Q7, Tutorial 1 Q8, Tutorial 1

School of Microelectronic Engineering Basic Fully-recessed LOCOS Process Step-1: Pad Oxide Layer Wafer is cleaned using RCA cleaning technique A SiO2 (called pad or buffer oxide) is thermally grown The function of this oxide is to cushion the transistion of stress between the silicon substrate and the subsequently deposited nitride. Silicon substrate

School of Microelectronic Engineering Step-2: Silicon Nitride Layer A thick layer of CVD silicon nitride is deposited. The function of this nitride is as mask to the oxidation process. Silicon nitride is very effective as oxidation mask because oxygen and water vapor diffuse very slowly through it, preventing oxidizing species from reaching the silicon surface under the nitride. Silicon nitride however exhibiting a very high tensile stress (10 10 dynes/cm 2 ), hence used with minimal thickness. Silicon substrate

School of Microelectronic Engineering Step-3: Photolithography-Active Area Definition To define the active area (where the transistors to be put) Silicon substrate

School of Microelectronic Engineering Step-4: Nitride Etch, Oxide Etch, Silicon Etch To cover the active regions, expose areas to form LOCOS Silicon substrate

School of Microelectronic Engineering Step-5: Channel stop implant To create a channel stop doping layer under Field Oxide. In NMOS circuit, a p implant (boron, keV) is used, while in PMOS, arsenic is used. PR is removed after the implant Silicon substrate

School of Microelectronic Engineering BIRD’S BEAK Step-6: Grow Field Oxide Field oxide is thermally grown by wet oxidation at temperatures around 1000C to the thickness ,000A. Oxide will grows where there is no masking nitride, but at the nitride’s edges, some oxidation occurred. This caused the nitride’s edges to lift. Because of the shape, this structure is called bird’s beak. BIRD’S HEAD

School of Microelectronic Engineering SEM picture of Fully-Recessed LOCOS

School of Microelectronic Engineering Step-7: Strip Masking Nitride Layer Oxynitride etch ( A top layer of nitride) – deglaze process Wet hot phosphoric process to remove nitride (good selectivity to oxide) Tricky process, deglaze process must be carefully characterized. Step-8: Regrow and strip sacrificial oxide Kooi et al discovered that a thin layer of silicon nitride can form on the silicon surface (pad oxide – silicon interface). This nitride spot is called white ribbon or Kooi Effect and must be removed to prevent defect from occuring when growing gate oxide. This can be done by regrowing a pad oxide and subsequently removed.

School of Microelectronic Engineering Advanced Fully-Recessed LOCOS Process A)Side Wall Masked Isolation (SWAMI) Bird’s beak free structure, very planar process Silicon substrate 1.Pad Oxidation 2.CVD Nitride Deposition

School of Microelectronic Engineering 3.Oxide / Nitride Etch 4.Silicon Etch Sloping sidewall, help to reduce the stress during oxidation

School of Microelectronic Engineering 5.Second layer of pad oxide and nitride 6.CVD Oxide

School of Microelectronic Engineering 7.Etch Oxide, Etch Nitride 8.Field Oxidation 9.Nitride / Oxide strip Active LOCOS

School of Microelectronic Engineering B) Self Aligned Planar Oxidation Technology (SPOT) Another modified fully-recessed LOCOS to eliminate the bird’s beak and head. Conventional semi-recessed LOCOS is grown using high pressure oxidation. The LOCOS then removed using BOE SiO2

School of Microelectronic Engineering Second pad oxide is grown, followed by deposition of a second CVD nitride Nitride and oxide then anisotropically etched. Second LOCOS is then grown using High Pressure Oxidation LOCOS

School of Microelectronic Engineering C) Fully Recessed Oxide (FUROX) D) OSELO E) ETC

School of Microelectronic Engineering 4 major applications Locos replacement for isolation within the well (STI) Isolation in bipolar (Moderate Trench) Latch prevention in CMOS (Moderate Trench) Trench capacitor in DRAM (Deep Trench) 3 categories Shallow trench <1 um Moderate 1-3 um Deep >3um deep Advantages Increase the packing density tremendously Disadvantages Complex to fabricate, very expensive machines Poor uniformity, Low throughput Trench Isolation Technology

School of Microelectronic Engineering Trench etched CVD oxide deposited Oxide polished to surface by CMP

School of Microelectronic Engineering Shallow Trench Isolation (STI)

School of Microelectronic Engineering

Silicon On Insulator (SOI) with STI Isolation Technology Completely isolate the transistor on silicon surface from the bulk silicon substrate. Tremendously increase the packing density of IC chip Mainstream isolation technology for high performance ICs for feature size below 0.13um process technology Normally coupled with Copper Interconnect Technology and Low-k Interlevel Dielectric.

School of Microelectronic Engineering