1 Technion- Microelectronic Processing – Joseph Salzman – Fall 2006 Microelectronics Processing Microelectronics ProcessingEtching

1Technion- Microelectronic Processing – Joseph Salzman – Fall 2006

Microelectronics Processing Microelectronics Processing EtchingEtching


The Etch ProcessThe Etch Process

The properties of the etch process should be:

• Follow the photolithography step

• Remove selected materials from wafer surface

• Clean the wafer surface


Permanently transfer the mask pattern from photolithography into the surface layer of the wafer

Etching can be done in either “wet” or “dry” environment:

1. Wet etch – liquid etchants

2. Dry etch – gas phase etchants in a plasma

Goal of Etch ProcessGoal of Etch Process


Etching ProcessEtching Process


Basic concepts

• Etching process consists of three steps: 1. Mass transport of reactants (through a boundary layer) to the surface to be etched 2. Reaction between reactants and the film(s) to be etched at the surface 3. Mass transport of reaction products from the surface through the boundary layer

• Etching is usually done using liquid phase or gas phase reactants– liquid phase (wet) etching —reaction products soluble in solvent or gaseous– gas phase etching — reaction products gaseous / sublimation temperature


Etch Rate

• Rate at which material is removed from the wafer

Uniformity

• Etch rate constant across the wafer

Throughput

• Amount of wafers etched during one process cycle

Directional Control

• Controlling the horizontal and vertical etch rate: isotropic or anisotropic

Etch ParametersEtch Parameters


Selectivity

• Controlled by gas formula

• Controlled by etch rate

Selectivity = Surface Layer Material Etch Rate Underlying Layer Material Etch Rate


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Figures of merit


Directional Control – Isotropic Etch

• Etches equally in all directions

• Results in undercutting and sloping walls


Isotropic Etch


Directional Control – Anisotropic Etch

• The vertical etch rate is faster than the horizontal etch rate

• Produces straight sidewalls


Anisotropic Etch


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Wet Etch

• Chemical reaction between liquid etchants and surface

Process Techniques : Wet EtchProcess Techniques : Wet Etch


Wet Etch Process

• Performed in an immersion tank

• Wafers in wafer carrier are lowered into etchant solution

• Wafers remain in solution for specified amount of time .referred to as etch duration

• Wafer carrier is removed from solution

• Wafers are rinsed in deionized water

• Wafers are dried

Wet Etch ProcessWet Etch Process

Etch Duration

Etch Duration = Thickness of Layer (Å) Etch Rate of etchant


Cascade DI water rinse


Wet Etch Process - ExamplesWet Etch Process - Examples

End pointDetection!


Wet Etch Process – Examples - HFWet Etch Process – Examples - HF

End point detection

SiO2 wettsSi hydrofobic

Safety: HF inert in contact with skin Attacks bone!






Anisotropic Silicon Etching – Effect of Slow Anisotropic Silicon Etching – Effect of Slow {111} Etching{111} Etching

[110] Oriented Silicon


Anisotropic wet etching of SiAnisotropic wet etching of Si

{111} planes

(110) surface orientation

Heavily borondoped regionHole

Diaphragm V-groove{111} planes

54.74


Anisotropic Silicon Etching: ApplicationsAnisotropic Silicon Etching: Applications

(3) Field-Emission Tips


Anisotropic Silicon Etching: ApplicationsAnisotropic Silicon Etching: Applications

(4) MicroElectroMechanical Systems - MEMS


Etching selectively by doping


Characteristics of Wet Etch Process

• Highly selective

• Isotropic etch

• For use on features larger than 3 microns

• High throughput

• Low cost equipment

• Hazardous liquid chemicals

Wet Etch CharacteristicsWet Etch Characteristics


Wet Etch LimitationsWet Etch Limitations


Wet Etch Process Steps

• Etch

• Rinse

• Dry



Key Wet Etch Parameters

• Etchant concentration

• Temperature of etchant



Wet Etch in an Immersion Tank


Rinse in an Immersion Tank


Dry in Spin-Rinse-Dryer



Wet Etchants

• Acetic Acid HC2H3O2

• Hydrofluoric Acid HF

• Ammonium Fluoride NH4F

• Phosphoric Acid H3PO4

• Nitric Acid HNO3



Wet Chemical Etchants for VLSI Fab. Wet Chemical Etchants for VLSI Fab.


Chemical Mechanical Polishing (CMP)


Dry EtchDry Etch

Chemical reaction or physical etch between gas.etchants and surface material on wafer: Dry etching methods

Glow discharge methods

Dry physical etching (Sputter etching)

Plasma assisted etching

o Dry chemical etching (Plasma etching)

o Reactive ion etching (RIE)

Ion beam methods

Ion mlling

Reactive ion beam etching

Chemical assisted ion milling

Common materials to dry etch

Si, SiO2, Si3N4, Al, W, Ti, TiN, TiSi2, Photoresist

Difficult materials to dry etch

Fe, Ni, Co, Cu, Al2O3, LiNbO3, etc.


Characteristics of Dry Etch Process

• Highly selective

• Anisotropic etch

• For use of features smaller than 3 microns

• Expensive equipment

• Limited human exposure to hazardous chemicals

• RF Power safety risks

Dry Etch ProcessDry Etch Process


Dry Etch TechniquesDry Etch Techniques

Physical etch

Reactive Ion etch

Chemical (plasma) etch

Dry


Gas (plasma) etching advantages


Plasma: the 4th state of the matterPlasma: the 4th state of the matter

On earth we live upon an island of "ordinary" matter. The different states of matter generally found on earth are solid, liquid, and gas. We have learned to work, play, and rest using these familiar states of matter. Sir William Crookes, an English physicist, identified a fourth state of matter, now called plasma, in 1879. Plasma is by far the most common form of matter. Plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that which is not visible.

Natural plasma Man-made plasmaSources: http://www.plasmas.org/rot-plasmas.htm http://www.4thstate.com/publications/21stCentury.htm


Basic properties of plasmaBasic properties of plasma






Physical Etch

Also referred to as ion beam etching, sputtering, ion .milling

Ions bombard wafer surface causing molecules to .sputter off the surface - Argon introduced into RF Power Chamber

Advantages - Low level of undercutting - Anisotropic etch

Disadvantages - Low selectivity rate - Requires high level of RF Power



Sputter etchingSputter etching

In this process all of the electrical energy, usually RF, is applied to the substrate.Physically bombard the films to be etched with energized chemically inert ions or atoms.Material is removed by ion bombardment of the substrate. This process is most often used to pre-clean substrates prior to deposition.Glow discharge is used to energize chemically inert ions or atoms (e.g., Ar)

Highly anisotropic etching

– Damage to underlying material => may change device properties

– Rarely used in VLSI


Ion Beam Etcher



Dry chemical etching (Plasma etching)Dry chemical etching (Plasma etching)

Purely chemical etching.

RF energy is applied to a separate electrode with the substrates grounded. Chemical reaction between etchant gas and surface layer of wafer etches the wafer.

Glow discharge is used to produce chemically reactive species (atoms, radicals, or ions).

Advantage - High selectivity rate

Disadvantage - Isotropic etch


Plasma assisted etchingPlasma assisted etching

Plasma assisted etching sequence Take a molecular gas

CF4

Establish a glow dischargeCF4+e CF3 + F + e

Radicals react with solid films to form volatile product

Si + 4F SiF4

Pump away volatile product (SiF4 )


Etchants and etch products (different Etchants and etch products (different sources)sources)

Solid Etch gas Etch productSi, SiO2, Si3N4

PSG, and BPSGCF4, SF6, and NF3 SiF4

Si Cl2 and CCl2F2 SiCl2 and SiCl4Al BCl3, CCl4, Cl2 Al2Cl6 and AlCl3

Organic solidsPhotoresists, etc.

O2

O2 + CF4

CO, CO2, H2OCO, CO2, HF

Refractory metals(W, Ta, Ti, Mo, etc.)

CF4 WF6 …..

GaAs, InP Cl2 and CCl2F2 GaCl3, AsCl5, ….


RF-powered Plasma Etch SystemRF-powered Plasma Etch System

RF-powered plasma etch system

Steady-state voltage distribution inRF-powered plasma etch system


Plasma Etching MechanismsPlasma Etching Mechanisms

Chemical etching

Various reactions and speciespresent in a plasma

Due to their incomplete bonding, free radicals are highly reactive chemical species.


Combines both physical and chemical etching techniquesIf RF energy is applied to the substrates in a low pressure halogen-rich environment, material can be removed by both chemical means and ion bombardment of the substrate surface. Greater control over line widths and edge profiles is possible with oxides, nitrides, polysilicon and aluminum.Accomplished by replacing the neutral gas in a r.f. sputtering system by one or more chemical speciesGlow discharge is used to produce chemically reactive species (atoms, radicals, or ions) and chemically inert ionsWidely used in VLSI fabrication

Advantages - High selectivity rate - Anisotropic

Reactive Ion Etcing (RIE)Reactive Ion Etcing (RIE)


Dry Etch Process : RIEDry Etch Process : RIE


Ion Energy vs. Pressure for a Plasma Ion Energy vs. Pressure for a Plasma


Chemical vs. chemical/physical etchingChemical vs. chemical/physical etching

Purely chemical etching(using only reactive

neutral species)Isotropic etching

Chemical + physical etching(using reactive neutral species

and ionic species)Anisotropic etching


Effect of the inhibitorEffect of the inhibitor

w/o inhibitor=> Isotropic

w/ inhibitor=> Anisotropic

fast inhibitordeposition=> Bevelled walls


Importance of reactive ion etchingHighly anisotropic etchingHighly anisotropic etching

Precise pattern transferHigh resolution

Less consumption of chemicalsCost effectiveEnvironmentally benign

Clean processVacuum

Ease of automationEase of automation

Importance of RIEImportance of RIE


Barrel plasma systemBarrel plasma system

Quartz tube


High density plasma (HDP) systemHigh density plasma (HDP) system

ECR vs ICP

ECR was introduced at OPT in 1985.

ICP was introduced much later (1991- 1995) for plasma processing.

most important with both: independent control of ion energy and ion current density

lower (substrate) electrode grounded, RF driving opt.

Electron CyclotronResonance (ECR)

Inductively coupledplasma (ICP)


Sputter etch and ion milling systemSputter etch and ion milling system


RIE: ExamplesRIE: Examples


How to Control Anisotropy?How to Control Anisotropy?

1) Ionic bombardment damage exposed surfaces.2) Sidewall coating by inhibitor prevents sidewall etching.


How to Control Selectivity?How to Control Selectivity?

Example 1:


How to Control Selectivity?How to Control Selectivity?

Example 2: Si etching in CF4 + O2 mixture.


How to Control Smoothness?How to Control Smoothness?


Temperature Dependence of SelectivityTemperature Dependence of Selectivity


In General: Etching Process Includes In General: Etching Process Includes Several Sequential StepsSeveral Sequential Steps

Example 1: RIE of Al lines.


Sequential Steps in Etching processSequential Steps in Etching process

Example 2: Etching of deep trenches.


Sequential Steps in Etching deep trenchesSequential Steps in Etching deep trenches

Solution: Multiple step RIE sequence.


Summary of Plasma Systems & Summary of Plasma Systems & MechanismsMechanisms


The Four Classes of Plasma Etching The Four Classes of Plasma Etching MechanismsMechanisms


Factors Influencing Dry Etch Process

• Etch rate - RF Power level - Gas formula - Etch Temperature

• Pressure - Extremely high pressure results in an isotropic etch - Low pressure with high energy can damage wafer

Dry Etch Process SummaryDry Etch Process Summary


Factors Influencing Dry Etch Process

• Micro-loading - Different etch rates across wafer surface - Ashing can occur

• Post-etch corrosion - Due to residual etchant left on wafer after final rinse - Using a none Chlorine based etchant like Fluorine …..eliminates the problem.

Dry Etch Process SummaryDry Etch Process Summary


Plasma-Etching Gases Plasma-Etching Gases Used in VLSI Used in VLSI FabricationFabrication

PSGBPSG


Resist Stripping

• Removes residual resist after etch process

• Wet stripping - For use on wafers that have not been plasma …. …..etched - For non-metallic surfaces an acid solvent is used - For metallic surfaces an organic solvent is used

• Plasma Stripping - For use on wafers that have been etched by plasma - Uses oxygen as stripping plasma to remove …..photoresist

Wafer CleaningWafer Cleaning


Photoresist StrippingPhotoresist Stripping

Key components of a microwave plasma asher include a 2.45-GHz microwave generator and process chamber.


Photoresist StrippingPhotoresist Stripping


Etch check

Final check

Critical dimension measurement

Wafer Inspection and MetrologyWafer Inspection and Metrology

Documents

1 Technion- Microelectronic Processing – Joseph Salzman – Fall 2006 Microelectronics Processing Microelectronics ProcessingEtching