Semiconductor Manufacturing Technology - …jupiter.math.nctu.edu.tw/~weng/courses/IC_2007/... ·...

© 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian Serda

Semiconductor Manufacturing Technology

Michael Quirk & Julian Serda© October 2001 by Prentice Hall

Chapter 16

Etch

Semiconductor Manufacturing Technology

Michael Quirk & Julian Serda© October 2001 by Prentice Hall

Chapter 16

Etch



Applications for Wafer Etch in CMOS Technology

Photoresistmask Film

to be etched

(a) Photoresist-patterned substrate (b) Substrate after etch

Photoresistmask Protected

film

Figure 16.1


Process Flow in a Wafer Fab

Implant

Diffusion

Test/Sort

Etch

Polish

PhotoCompleted wafer

Unpatternedwafer

Wafer startThin Films

Wafer fabrication (front-end)

Used with permission from Advanced Micro Devices

Figure 16.2


Etch Process

Categories of Etch Processes• Wet Etch• Dry Etch• Three Major Materials to be Etched

– Silicon– Dielectric– Metal

• Patterned Etch Versus Unpatterned Etch


Etch Parameters

• Etch rate• Etch profile• Etch bias• Selectivity• Uniformity• Residues• Polymer formation• Plasma-induced damage• Particle contamination and defects


Etch Rate

∆T

Start of etch End of etch

t = elapsed time during etch

∆T = change in thickness

Figure 16.3


Wet Chemical Isotropic Etch

Isotropic etch - etches in all directions at the same rate

Substrate

Film

Resist

Figure 16.4


Anisotropic Etch with Vertical Etch Profile

Anisotropic etch - etches in only one direction

Resist

Substrate

Film

Figure 16.5


Sidewall Profiles for Wet Etch Versus Dry Etch

Table 16.1

Type of Etch Sidewall Profile Diagram

Wet Etch Isotropic

Isotropic (depending on equipment & parameters)

Anisotropic (depending on equipment & parameters)

Anisotropic – Taper

Dry Etch

Silicon Trench


Etch Bias

(b)

Bias

Substrate

Resist

Film

(a)

Bias

Resist

FilmFilm

Substrate

Wb

Wa

Figure 16.6


Etching Undercut and Slope

Undercut

Substrate

Resist

FilmOveretch

Figure 16.7


Etch Selectivity

S = Ef

Er

Ef Nitride

Oxide

Er

Figure 16.8


Etch Uniformity

Measure etch rate at 5 to 9 locations on each wafer, then calculate etch uniformity for each wafer and compare wafer-to-wafer.

Randomly select 3 to 5 wafers in a lot

Figure 16.9


Polymer Sidewall Passivation for Increased Anisotropy

Plasma ions

Resist

Oxide

Polymer formationSilicon

Figure 16.10


Dry Etch

• Advantages of Dry Etch over Wet Etch• Etching Action• Potential Distribution


Advantages of Dry Etch over Wet Etch

1. Etch profile is anisotropic with excellent control ofsidewall profiles.

2. Good CD control.

3. Minimal resist lifting or adhesion problems.

4. Good etch uniformity within wafer, wafer-to-waferand lot-to-lot.

5. Lower chemical costs for usage and disposal.

Table 16.2


Plasma Etch Process of a Silicon Wafer

8) By-product removal

1) Etchant gases enter chamber

Substrate

Etch process chamber

2) Dissociation of reactants by electric fields

5) Adsorption of reactive ions on surface

4) Reactive +ions bombard surface 6) Surface reactions of

radicals and surface film

Exhaust

Gas delivery

RF generator

By-products

3) Recombination of electrons with atoms creates plasma

7) Desorption of by-products

Cathode

AnodeElectric field

λ

λ

Anisotropicetch Isotropic etch

Figure 16.11


Chemical and Physical Dry Etch Mechanisms

Reactive +ions bombard surface Surface reactions of

radicals + surface film

Desorption of by-products

Anisotropic etch Isotropic etch

Sputtered surface material

Chemical EtchingPhysical Etching

Figure 16.12


Chemical Versus Physical Dry Plasma Etching

Etch Parameter

Physical Etch(RF field

perpendicularto wafersurface)

Physical Etch(RF field

parallel towafer surface)

Chemical EtchCombined

Physical andChemical

Etch Mechanism

Physical ionsputtering Radicals in

plasma reactingwith wafersurface*

Radicals inliquid reactingwith wafersurface

In dry etch,etching includesion sputteringand radicalsreacting withwafer surface

Sidewall Profile Anisotropic Isotropic Isotropic Isotropic toAnisotropic

SelectivityPoor/difficultto increase(1:1)

Fair/good (5:1to 100:1)

Good/excellent(up to 500:1)

Fair/good(5:1 to 100:1)

Etch Rate High Moderate Low Moderate

CD Control Fair/good Poor Poor to non-existent Good/excellent

* Used primarily for stripping and etchback operations.Table 16.3


Schematic View of Reactor Glow Discharge with Potential Distribution

Plasma (+Vp)Ion

sheath

RF

Powered electrode (Vt)

Grounded electrode

-V 0 +V

Vp

Vt

Figure 16.13


Effects of Changing Plasma Etch Parameters

Table 16.4

Ion Energy DC Bias Etch Rate SelectivityPhysical

Etch

↑ ↓ ↓ ↓ ↑ ↓

↓ ↑ ↑ ↑ ↓ ↑

↑ ↑ ↑ ↑ ↓ ↑

↓ ↓ ↓ ↓ ↑ ↓

↑ ↑ ↑ ↑ ↓ ↑

↓ ↓ ↓ ↓ ↑ ↓

↑ ↓ ↓ ↓ ↑ ↓

↓ ↑ ↑ ↑ ↓ ↓

DC Bias

Electrical Size

Increase (↑) or Decrease (↓) in Etch Control Parameters

RF Frequency

RF Power


Plasma Etch Reactors

• Parallel plate (planar) reactor• Downstream etch systems• Triode planar reactor• Reactive ion etch (RIE)• High-density plasma etchers• Etch System Review• Endpoint Detection• Vacuum for Etch Chambers


Parallel Plate Plasma Etching

Roots pump

Process gases

Exhaust

Gas- flow controller

Pressure controller

Gas panel

RF generatorMatching network

MicrocontrollerOperator Interface

Gas dispersion screen

Electrodes

Endpoint signal

Pressure signal

Roughingpump

Wafer

Figure 16.15


Schematic of a Downstream Reactor

Plasma chamber

Diffuser

Wafer chuck

Heat lamp

To vacuum system

Microwave energy Microwave source 2.45 MHz

Figure 16.16


Triode Planar Reactor

Inductively-coupledRF generator (3.56 MHz)

Capacitively-coupled RF generator (100 kHz)

Induction coil

Capacitor

Figure 16.17


Parallel Plate RIE Reactor

RF generator

Wafer

Powered electrode(cathode)

Grounded electrode

(anode)

Ar+

(physical etch component)

F(chemical etch

component)

Figure 16.19


Schematic of Electron Cyclotron ReactorMicrowave source 2.45 MHz

Wave guide

Diffuser

Quartz window

Electrostatic chuck

Cyclotron magnet

Plasma chamber

Wafer

Additional magnet

13.56 MHz

Vacuum systemRedrawn from Y. Lii, “Etching,” ULSI Technology, ed. by C. Chang & S. Sze, (New York: McGraw-Hill, 1996), p. 349.

Figure 16.20


Inductively Coupled Plasma Etch

Electromagnet

Dielectric window

Inductive coil

Biased wafer chuck

RF generator

Bias RF generator

Plasma chamberPlasma

chamber

Redrawn from Y. Lii, “Etching,” ULSI Technology, ed. By C. Chang and S. Sze (New York: McGraw-Hill, 1996), p. 351.

Figure 16.21


Magnetically Enhanced Reactive Ion Etch (MERIE)

Electromagnet (1 of 4)

13.56 MHz

Biased wafer chuck

WaferWafer

Redrawn from Wet/Dry Etch (College Station, TX: Texas Engineering Extension Service, 1996), p. 165.

Figure 16.23


Dry Etcher ConfigurationsConfigurations Activity Pressure

(Torr) ArrangementHigh

DensityPlasma

Biasing BiasSource Profile

Barrel Reactive 10-1 to 1 Coil or electrodes outsidevessel No In cassette (bulk) RF Isotropic

Parallel Plate (Plasma) Reactive 10-1 to 1 Planar diode (two electrodes) No On poweredelectrode (anode) RF Anisotropic

Downstream Plasma Reactive 10-1 to 1 Coil or electrodes outsidevessel No

In cassette (bulk)downstream ofplasma

RF orMicrowave Isotropic

Triode Planar Reactive 10-3 Triode (three electrodes) No On platformelectrode Anisotropic

Ion Beam Milling Inert 10-4 Planar triode No On poweredelectrode (anode) Anisotropic

Reactive Ion Etch(RIE) Reactive < 0.1 Planar or cylindrical diode No On cathode Anisotropic

Electron CyclotronResonance (ECR) Reactive

10-4 to 10-

3

(low)

Magnetic field in parallel withplasma flow Yes On cathode RF or DC Anisotropic

Distributed ECR Reactive (low) Magnets distributed aroundcentral plasma Yes On cathode RF or DC Anisotropic

Inductively CoupledPlasma (ICP) Reactive (low) Spiral coil separated from

plasma by dielectric plate Yes On cathode RF or DC Anisotropic

Helicon Wave Reactive (low)

Plasma generated byelectromagnets and plasmadensity maintained at wafer bymagnetic field

Yes On cathode RF or DC Anisotropic

Dual Plasma Source Reactive (low) Independent plasma and waferbiasing Yes On cathode RF or DC Anisotropic

MagneticallyEnhanced RIE(MERIE)

Reactive (low) Planar diode with magneticfield confining plasma Yes On cathode RF or DC Anisotropic

Table 16.5


Endpoint Detection for Plasma Etching

Endpoint detection

Normal etch Change in etch rate - detection occurs here.

Endpoint signal stops the etch.

Time

Etch

Par

amet

er

Figure 16.24


Characteristic Wavelengths of Excited Species in Plasma Etch

Material Etchant Gas Emitting Species ofsome Products Wavelength (nm)

SiliconCF4/O2

Cl2

SiFSiCl

440; 777287

SiO2 CHF3 CO 484

AluminumCl2

BCl3

AlAlCl

391; 394; 396261

PhotoresistO2 CO

OHH

484309656

Nitrogen(indicatingchamber vacuumleak)

N2

NO337248

Table 16.6


Dry Etch Applications

• Dielectric Dry Etch– Oxide– Silicon Nitride

• Silicon Dry Etch– Polysilicon– Single-Crystal Silicon

• Metal Dry Etch– Aluminum and Metal

Stacks– Tungsten Etchback– Contact Metal Etch


Requirements for Successful Dry Etch

1. High selectivity to avoid etching materials that are not to be etched (primarily photoresist and underlying materials).

2. Fast etch rate to achieve an acceptable throughput of wafers.

3. Good sidewall profile control.4. Good etch uniformity across the wafer.5. Low device damage.6. Wide process latitude for manufacturing.


Dry Etch Critical Parameters

Equipment Parameters:• Equipment design• Source power• Source frequency• Pressure• Temperature• Gas-flow rate• Vacuum conditions• Process recipe

Other Contributing Factors:• Cleanroom protocol• Operating procedures• Maintenance procedures• Preventive maintenance schedule

Process Parameters:• Plasma-surface

interaction: - Surface material- Material stack of

different layers- Surface temperature- Surface charge- Surface topography

• Chemical and physical requirements

• Time

Quality Measures:• Etch rate• Selectivity• Uniformity• Feature profile• Critical dimensions• Residue

Plasma-etchinga wafer

Figure 16.25


Oxide Etch Reactor

CF4

C3F8

C4F8

CHF3

NF3

SiF4

ArWafer

Electrostatic chuck

Plasma

Selection of fluorocarbon and hydrocarbon chemicals

HF CF2

F

CHF

CH4

Figure 16.26


Etch Stop Hard Mask Layer

n-well p-well

LI Oxide

p+ Silicon Substrate

p- Epitaxial Layer

2 Doped oxide CVD

Nitride etch

5

Oxide CMP 3 4 Oxide etch1 Nitride CVD

Example: Silicon nitride, Si3N4, serves as etch-stop during LI oxide etch. Note: The numbers show the order of the five operations.

Figure 16.27


Contact Etching to Varying Depths

Contact holes

S DG

Figure 16.28


Polysilicon Conductor Length

Polysilicon gate Gate oxide

The gate length determines channel length and defines boundaries for source and drain electrodes.

DrainSource

Gate

Figure 16.29


Polysilicon Gate Etch Process Steps

1. Breakthrough step to remove native oxide and surface contaminants

2. Main-etch step to remove most polysiliconwithout damage to gate oxide

3. Overetch step to remove remaining residues and poly stringers while maintaining high selectivity to gate oxide


Undesirable Microtrenching duringPolysilicon Gate Etching

Substrate

Poly

Resist

Gate oxide

Ions

Trench in gate oxide

Figure 16.30


Silicon Trench Etching

Figure 16.31


Major Requirements for Metal Etching

1. High etch rates (>1000 nm/min).2. High selectivity to the masking layer (>4:1),

interlayer dielectric (>20:1) and to underlying layers.3. High uniformity with excellent CD control and no

microloading (<8% at any location on the wafer).4. No device damage from plasma-induced electrical

charging.5. Low residue contamination (e.g., copper residue,

developer attack and surface defects).6. Fast resist strip, often in a dedicated cluster tool

chamber, with no residual contamination.7. No corrosion.


Metal Stack for VLSI/ULSI Integration

TiN Al + Cu (1%)Ti

p+ Silicon substrate

p- Epitaxial layer

n-well p-well

LI Oxide

ILD-1

Metal etchPhotoresist mask

Figure 16.32


Typical Steps for Etching Metal Stacks

1. Breakthrough step to remove native oxide.2. ARC layer etch (may be combined with above step).3. Main etch step of aluminum.4. Overetch step to remove residue. It may be a

continuation of the main etch step.5. Barrier layer etch.6. Optional residue removal process to prevent

corrosion.7. Resist removal.


Tungsten Etchback

Metal-2 stack

(d) Metal-2 deposition

Tungstenplug

(a) Via etch through ILD-2 (SiO2)

Metal-1 stackILD-2

ILD-1

Via SiO2

(c) Tungsten etchback

SiO2Tungstenplug

(b) Tungsten CVD via fill

Tungsten

Figure 16.33


Wet Etch

• Wet Etch Parameters• Types of Wet Etch

– Wet Oxide Etch– Wet Chemical Strips


Wet Etch Parameters

Parameter Explanation Difficulty to Control

ConcentrationSolution concentration(e.g., ratio of NH4F:HFfor etch an oxide).

Most difficult parameter tocontrol because the bath

concentration is continuallychanging.

TimeTime of waferimmersion in the wetchemical bath.

Relatively easy to control.

Temperature Temperature of wetchemical bath. Relatively easy to control.

Agitation Agitation of the solutionbath.

Moderate difficulty toproperly control.

Table 16.7


Approximate Oxide Etch Rates in BHF Solution at 25° C

Table 16.81 Approximate Oxide Etch Rates in BHF Solution at 25°Ca

Type of Oxide Density (g/cm3) Etch Rate (nm/s)Dry grown 2.24 – 2.27 1Wet grown 2.18 – 2.21 1.5

CVD deposited < 2.00 1.5b – 5c

Sputtered < 2.00 10 – 20a) 10 parts of 454 g NH4F in 680 ml H2O and one part 48% HFb) Annealed at approximately 1000°C for 10 minutesc) Not annealed

1 B. El-Kareh, ibid, p. 277.

Table 16.8


Historical Perspective -Polysilicon Etch Technology Evolution

GeometryRequirements

Time Frameand Reactor

DesignChemistries Strengths Limitations and

Problems Controls

4 to 5 µm,isotropic etch

Pre-1977: wetetch

HF/HNO3buffered withacetic acid or H2O

Batch Process Resist lift; bath aging;temperature sensitive

Operator judgementfor endpoint

3 µm1977: barreletcher

CF4/O2 Batch Process Non-uniformity,isotropic etch, largeundercut

Manometer andtimer

2 µm

1981: singlewafer etch

CF4/O2 Single wafer; individualetch endpoint,improvement inrepeatability

Low oxide selectivity;isotropic process

Endpoint detection

1.5 µm

1982: singlewafer RIE

SF6/Freon 11,SF6/He

MFCs; independentpressure and gas flowcontrol, improvement inrepeatability

Low oxide selectivity;profile control

MFCs; separate gasflow and pressurecontrol

To 0.5 µm1983: variablegap; load-locked

CCl4/He, Cl2/He,Cl2/HBr

Load-locked chamber;variable gap, improvementin repeatability

Microloading in highaspect ratios; profilecontrol

Control ofelectrode gap;computer controls

To 0.25 µmand below

1991:inductivelycoupledplasma (ICP)

Cl2, HBr High-density plasma; lowpressure; simple gasmixtures, improvement inrepeatability

Complex tool; manyvariables

Independent RFcontrol for plasmageneration andwafer bias

Table 16.9


Photoresist Removal

Plasma Ashing• Asher Overview• Plasma Damage• Residue Removal


Atomic Oxygen Reaction with Resist in Asher

SubstrateResist

Asher reaction chamber

2) O2 dissociates into atomic oxygen 3) Plasma energy

turns oxygen into + ions

4) Neutral O and O+ react with C and H atoms in resist

Neutral oxygen radicals

5) By-productdesorption

6) By-product removal

Exhaust

Gas delivery

Downstream Plasma

1) O2 molecules enter chamber

+

++

++

++

λλ

+

Figure 16.34


Post Etch Via Veil Residue

Via veils

Polymer residue

Figure 16.35

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