etching 2014

1SNT5039 Nano Processing (Etching Technology)

1. PLASMA ETCHING FUNDAMENTALS- PLASMA ETCHING MECHANISMS- ETCHING PROCESS VARIABLES- ETCHING PROCESS REQUIREMENTS

2. ISSUES OF ETCHING FOR NANO-PROCESSING- PLASMA ETCHING MODELS- CHAGE UP DAMAGE- MICROLOADING

PART III: ETCHING TECHNOLOGY


Difference between Plasma Etching and Wet Etching

PLASMA ETCHING MECHANISMS



Four Different Plasma Etching Mechanisms


Sputtering:- Positive ions are accelerated across

the sheath with high kinetic energy.- Directional but has low selectivity.

Chemical etching: - Active species from the gas phase

encounter the surface and react to form volatile product.

- Non-volatile reaction product would remain on the surface and impede further etching.

- Non-directional but can have high selectivity.



Ion-enhanced directional etching:- Where neutral species cause little etching, ion bombardment can make the substrate more reactive, the effects of radicals and ions can be synergistic to enhance etching rate.

- Ions accelerated across the plasma sheath edge strike the surface vertically with kinetic energy, causing directional etching.

Inhibitor-driven ion-assisted etching:- Etching by neutral radicals is spontaneous so ion bombardment does not cause etching reaction.

- Ions sputter off substrate materials to form inhibitor-films on the sidewall, resulting in anisotropic etching.



Plasma parameters

Plasma properties(Collision)

- RF power (source power, bias power)- Frequency of power- Pressure- Gas combination- Gas flow rate- Temperature

- Energy of particles (Te, Ti)- Particle density (n, n*, np)- Ionization and dissociation ratios- Ion energy (Ei)- Electron energy distribution

Etching properties

-Etching rate-Uniformity- Selectivity-Anisotropy-Loading (microloading)- Plasma induced damage

ETCHING PROCESS VARIABLES


Electron-Atom CollisionsElastic scattering: e + Ar e + ArIonization: e + Ar 2e + Ar+Excitation: e + Ar e + Ar*

Ion-Atom CollisionsElastic collision: Ar+ + Ar Ar+ + ArResonant charge exchange: Ar+ + Ar Ar + Ar+Non-resonant charge exchange: B+ + C B + C+

Radiative relaxation: Ar* Ar + photon (Used for OES)

Recombination: e + Ar+ Ar*** e + Ar+ + Ar Ar + Ar*** e + Ar+ + wall Ar + wall

Q) Why are the last two reactions are dominant in the plasma, rather than direct recombination ?



Pressure affects the following properties

(1) Sheath potential and ion energy bombarding the surface(2) Electron temperature and electron energy distribution(3) Ionization ratio and collision frequency (4) Flux of ions and radical species to the surface(5) Surface coverage and chemical reaction rate(6) Mass transport rate

(* Pressure range in processing plasma: 1mTorr to 10Torr)

Physical sputtering

Chemical etching

Ion-assisted etching

Pressure

Ion energy

ETCHING PROCESS VARIABLES (PRESSURE EFFECTS)


- Electron energy and ionization rate tend to decrease with increasing pressure, and this results in less efficiency to regenerate the charge lost by diffusion and recombination.- However, electron density (plasma density) does not change very much due to the compensating effect between the larger number of neutral reactants and lower ionization rate at higher pressure.- That is, plasma density (1010 1011 cm-3) tends to be insensitive to the pressure change in the low pressure processing plasmas in the range of 1mTorr 1 Torr.- In general, the neutral gas temperature tends to increase with pressure whereas the electron temperature deceases. This is due to the thermalization of electrons (energy transfer from electrons to neutrals).

ETCHING PROCESS VARIABLES (PRESSURE EFFECT)


- The ratio of neutrals to ions is higher at high pressure (surface flux of neutrals is much larger). In high pressures, etching is mainly performed chemically. Isotropic etching profile and undercutting are common phenomena observed in the chemical etching.

- The rate of chemical etching tends to be proportional to the concentration of neutral etchant species (first order reaction rate), whereas the rate of ion-enhanced etching tends to be independent of the concentration of neutral etchant species (zero order reaction rate).

- Lower pressure brings about the increase of sheath voltage and ion bombardment energy.

ETCHING PROCESS VARIABLES (PRESSURE EFFECT)


In the case of elementary reactions: A + B = products.

In this case, reaction rate can be expressed as:(product formation)/(time) = k(T) nAnB (ni =xip)

Effects of wall on pressure dependenceMany cases, heterogeneous reaction on the wall is

more efficient, since it can frequently be first orderreaction:

A + B + wall = AB + wall.In this case, reaction rate can be expressed as:

(product formation)/(time) = k(T) nAAw

PRESSURE EFFECT IN CHEMICAL KINETICS


Overall reactionEffective Pressure

Dependence

F + Sisurf = SiF p

CF3 + F = CF4 p2

CF2 + F2 = CF3 + F p2

CF2 + CF2 = C2F4 p2

3CF2 = C3F6 p3

ETCHING PROCESS VARIABLES (PRESSURE)


QAR

xQxARx

andQAR

xQxARx

seseSiO

seseSi

619

14

519

14

1021.81068.2

102.2

1087.11068.2

105

2

moleculesCFoffluxInputatomsSioffluxEtch

4

Utilization factor

Flow rate effect (low flow rate regime)

: For Si

: For SiO2

where Re = etch rate (/min), As = substrate area, Q = flow rate (sccm).

Q) In the CF4/O2 plasma where the single 8-inch silicon wafer has an opening ratio of 50% and its etching rate is 3000A/min with CF4gas flow of 25sccm,1) Estimate utilization factor of CF4 molecules for Si etching (0.36)2) Estimate a maximum possible etching rate (8300A/min)

ETCHING PROCESS VARIABLES (FLOW RATE)


Etch rate Limited by Residence time

Overall etching rate

100% utilization line

Gas flow rate

Overall flow rate effect- When pumping speed or flow rate exceeds to a certain point, residence

time of reactive gases becomes too short to etch the substrate. - Short residence time can lower the utilization factor. This can be

minimized by increasing RF power to increase the generation rate of reactive species

- Overall etching rate will be determined by lack of reactant gases at the low flow rate regime and by insufficient residence time of reactant gases at the high flow rate regime.



If utilization factor is high, reaction products can affect gas composition of the etching plasma. This is often observed in the low flow regime. The following figures show consumption of the etching gas under the condition with high utilization factor.

Q) Explain the effect of increasing the power in both graphs?



Areas that RF frequency can influence to plasma and etching properties

(1) Spatial distribution of species and electrical fields across the discharge(2) Energy distribution of species (whether it is constant or oscillating withtime)(3) Minimum voltage required to start and operate a plasma and theenergy with which ions bombard the surface(4) Electron energy distribution function (EEDF)

- If wt > 1, the process is too slow to respond and it reachesa static state in equilibrium with the time average conditions.(w is input power frequency and t is relaxation time)

ETCHING PROCESS VARIABLES (FREQUENCY)


ETCHING PROCESS VARIABLES (FREQUENCY)

V = 109V (V N)160MHzMomentum CollisionFrequency

experimentalVu(Vu N)10MHzElectron EnergyModulation

0.3Torr/v+, aveLitf ~ 200KHzUitf ~ 6MHz

Ion-SheathTransit

Kex = 10-10 10-9N = 1016cm-3

Kex ni N160KHz 1.6MHzCharge Exchange

Ka = 1.510-9cm3sec-1N = 1016cm-3Ka = 310-10cm3sec-1N = 1016cm-3

Ka ne N2.5MHz (0.08eV)

500KHz (3eV)

Attachment

L = 2cm(2/L2)(v/3)ne50KHz (1eV)2MHz (3ev)

FreeDiffusion

D = 250cm2sec-1L = 2cm[Cl] ~ 51015cm-3

2D[Cl] / L2100HzAtom-AtomRecombination(Heterogeneous)

Kri = 510-8cm3sec-1

n+ n- = 31010cm-3Kri n+ n-250HzIon-Ion

Recombination

Kei 10-7 cm3sec-1Kei ni ne500HzElectron-IonRecombination

ne = 31010Wp = (neoe2/moeo)1/21.55GHzPlasma Frequency

L = 2cmDa ~ 2103cm2sec-1

(2Da / L2)ne800HzAmbipolarDiffusion

Kra = 3.4710-32cm-6sec-1[Cl] ~ 51015cm-3

Kra[Cl]2N0.3HzAtom-AtomRecombination(Homogeneous)

ParametersAssumed

Rate Expression(dn/dt)

Approx. Frequency[d(2logn)/dt]-1

Process

V = 109V (V N)160MHzMomentum CollisionFrequency

experimentalVu(Vu N)10MHzElectron EnergyModulation

0.3Torr/v+, aveLitf ~ 200KHzUitf ~ 6MHz

Ion-SheathTransit

Kex = 10-10 10-9N = 1016cm-3

Kex ni N160KHz 1.6MHzCharge Exchange

Ka = 1.510-9cm3sec-1N = 1016cm-3Ka = 310-10cm3sec-1N = 1016cm-3

Ka ne N2.5MHz (0.08eV)

500KHz (3eV)

Attachment

L = 2cm(2/L2)(v/3)ne50KHz (1eV)2MHz (3ev)

FreeDiffusion

D = 250cm2sec-1L = 2cm[Cl] ~ 51015cm-3

2D[Cl] / L2100HzAtom-AtomRecombination(Heterogeneous)

Kri = 510-8cm3sec-1

n+ n- = 31010cm-3Kri n+ n-250HzIon-Ion

Recombination

Kei 10-7 cm3sec-1Kei ni ne500HzElectron-IonRecombination

ne = 31010Wp = (neoe2/moeo)1/21.55GHzPlasma Frequency

L = 2cmDa ~ 2103cm2sec-1

(2Da / L2)ne800HzAmbipolarDiffusion

Kra = 3.4710-32cm-6sec-1[Cl] ~ 51015cm-3

Kra[Cl]2N0.3HzAtom-AtomRecombination(Homogeneous)

ParametersAssumed

Rate Expression(dn/dt)

Approx. Frequency[d(2logn)/dt]-1

Process


1) When excitation frequency is significantly higher than the ion transit frequency,ions are accelerated toward the electrode during negative half cycle, and the fastest ions with transit times lasting over many cycles are accelerated by the sheath electric field, E(x,t), to an energy,

Since the excitation voltage is, Vo sin wt, is applied to the sheath,

During half cycle,

2) When excitation frequency is significantly lower than the ion transit frequency,

n THF s dxdttxeE

nTeV

10

2/

0max),(1

tVdxtxEs

sin),(0 0

00

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2sin2

VtdtVeeV HF

0max VVLF

FREQUENCY EFFECT: ION TRANSIT FREQUENCY (ITF)


FREQUENCY EFFECT: ELECTRON ENERGY OSCILLATION

- While electrons in a dischargelose only a small energy duringcollisions with neutrals, gasmolecules translate energy tosurroundings. Therefore electrontemperatures are much higherthan gas temperature.

- The diagram shows electronenergy as a function of excitationtime for different ratios of electronenergy loss frequency (nu) vsexcitation frequency (w).

1) When nu > w


We should be able to differentiate the gas temperature from the surfacetemperature. The gas temperature is not easily controllable because itdepends on power input and heat transfer.

Instead, the surface temperature is more practically used, because in theprocessing plasmas the thermal boundary layer (the distance in which thegas temperature is maintained close to the wall temperature due to heattransfer) is much thicker than the mean free path.

The etching properties affected by the temperature can be chemicalreaction rates, selectivity, surface morphology, and degradation of photo-resist.

When the etching rates are controlled by the reaction which is a functionof the temperature, it can be represented as Arrhenius Equation in the formof )/exp( kTEAR ae

ETCHING PROCESS VARIABLES (TEMPERATURE)


Q) From the below Arrhenius plotof Si and SiO2 etching reactionsin a fluorine containing plasma,

(a) Estimate activation energyfor overall etching reactions of Siand SiO2

(b) Find complete Arrheniusequations, and

(c) Plot the etching selectivitybetween Si and SiO2 as afunction of the temperature )/exp( kTEAR ae

ETCHING PROCESS VARIABLES (TEMPERATURE)


Q) Based on these results, explain why fluorine-based gases are preferred to chlorine-based gases for Si etching?

ETCHING PROCESS VARIABLES (TEMPERATURE AND VOLATILITY)


ETCHING PROCESS VARIABLES (WALL EFFECT)


Plasma parameters

Plasma properties (Collision)

- RF power (source power, bias power)- Frequency of power- Pressure- Gas combination- Gas flow rate- Temperature

- Energy of particles (Te, Ti)- Particle density (n, n*, np)- Ionization and dissociation ratios- Ion energy (Ei)- Electron energy distribution

Etching properties

-Etching rate-Uniformity- Selectivity-Anisotropy-Loading (microloading)- Plasma induced damage



Etching process is normally characterized by etching rate, selectivity,uniformity, and surface quality.

- Etch rate: High etch rate results in high throughput.

- Anisotropy: Most cases of IC manufacturing, vertical andanisotropic profiles are required. Some cases, sloped profile isnecessary to guarantee adequate step coverage during subsequentdeposition.

- Surface quality: Undesirable combination of substrates and gasescan result in pitting or unwanted surface residue that can degradesmoothness of surface. This problem becomes more severe asdevice size becomes smaller and new materials are introduced foradvanced devices.

ETCHING PROCESS REQUIREMENTS


Etch rate (Re)

- Overetch ratio =(Additional etch time / target etch time) x 100%

- Uniformity = [(Rmax Rmin) / (2Rave)] x 100%

- Anisotropy = 1- Rh/Rv

- Selectivity = Rf/Rs

ETCHING PROCESS REQUIREMENTS


SELECTIVITY REQUIREMENT

Selectivity Analysis:Selectivity is the etching rate ratio between two neighboring materials such as a) film versus mask and b) film versus underlying layer.

Si-substrate

SiO2 thin oxide

Poly-Si gate

MaskD/2


SELECTIVITY REQUIREMENT

-The vertical resist profile withthe angle of 90 degrees iscommonly obtained in the ICprocesses, from the advancedlithography machines using193nm-365nm wavelengths.

-However, it can be lowered bybaking processes beforeetching to 70-80 degrees.

- Substrate-to-mask selectivityrequirements become lessstringent with more anisotropicetching and a vertical mask-edge profile.


The figure shows process uniformityas a function of the selectivity foretching poly-silicon on the underlying400A gate oxide. The cross-hatcheddamage region indicates theunacceptable region in which morethan 100A oxide is removed. Withimproved uniformity, processselectivity requirement can berelaxed. The numbers in the mapshows the amounts of oxide removed.The results show that, the higher thepoly-silicon to oxide selectivity, theless the gate oxide removed. Also, ifthe process uniformity is poor, higherselectivity is necessary to ensure theremaining oxide thicker than theminimum requirement.

SELECTIVITY REQUIREMENT (TRADE-OFF WITH UNIFORMITY)

Si-substrate

SiO2 thin oxide

Poly-Si gate

Si-substrate

SiO2 thin oxide

Poly-Si gate


(a) In practical etching applications, there always exist non-uniformities in the filmthickness and in the etching rate across the wafer. We can define the uniformity offilm thickness , the uniformity of film thickness , and etch rate of the film Rf. Whenthe etching is carried out, we can define the time to take for all the films to be cleared,tc. Considering other factors to affect device properties, the total etching time, ttot,usually include some over-etching,

(b) Since the etching selectivity is required to prevent the under-layer from beingattacked (For example, think of the conventional gate structure where the thin gateoxide is between the poly-silicon gate conductor and the Si substrate.), the time totake to expose the substrate first, tmin, needs to be considered

(c) If the maximum allowable consumption of thin underlayer, ys max, was alreadydetermined, it can be expressed as a function of its etching rate Rs,ave. In this way,minimum selectivity requirement of film to substrate, Sf/s,min, is determined

SELECTIVITY ANALYSIS DURING ETCHING

Selectivity is the etching rate ratio between two neighboring materials such as a) substrate versus mask and b) film versus underlying layer


- Due to non-uniformity in thicknesses and etch rates across the wafer

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avefavesavefsf y

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ETCHING PROCESS REQUIREMENTS (SELECTIVITY)

Si-substrate

SiO2 thin oxide

Poly-Si gate

Si-substrate

SiO2 thin oxide

Poly-Si gate


Q1) Using the given equation, estimate minimum etching selectivity for poly-silicon to gate oxide in the following gate structure and processing condition, and also estimate the etch rate of gate oxide.

a) average thickness of poly-silicon gate is 2000Ab) thickness uniformity of poly-silicon gate is 5%c) average etch rate of poly-silicon is 4000A/mind) etch rate uniformity of poly-silicon gate is 7%e) over-etching toward the gate oxide is 100%, and f) allowable oxide consumption is 50A

(Ans: Selectivity is 55 and etch rate is 73A/min)Q2) Compare the results of Q1 with the results of in the following cases.

a) average thickness of poly-silicon gate changes to 1000Ab) average etch rate of poly-silicon changes to 3000A/minc) over-etching toward the gate oxide changes to 50%d) allowable oxide consumption changes to 30A

)1()1(

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ETCHING PROCESS REQUIREMENTS (SELECTIVITY)


Metalelectrode

Si wafer

Selectivity(Channel/SiO2) SiO2

Channel

Selectivity (Metal/Channel)

Photoresist

Selectivity(PR/Metal)

SELECTIVITY REQUIREMENTS IN GATE STACK ETCHING


Where etching rate is controlled by the transport of etchant gases. ie, the amount of available gases on the wafer surface, the etching rate decreases as the open area on the wafer surface increases. This is known as Loading Effect.

ETCHING PROCESS REQUIREMENTS (LOADING)


1. PLASMA ETCHING FUNDAMENTALS- PLASMA ETCHING MECHANISMS- ETCHING PROCESS VARIABLES- ETCHING PROCESS REQUIREMENTS

2. ISSUES OF ETCHING FOR NANO-PROCESSING- PLASMA ETCHING MODELS- CHAGE UP DAMAGE- MICROLOADING

ETCHING TECHNOLOGY


A. Ionization: e + Cl, Cl2 Cl+, Cl2+ + 2e

B. Dissociation: e + Cl2 2Cl + 2e

C. Adsorption: Cl, Cl2 Sisurf-nCl

D. Product formation: Si-nCl SiClx (adsorbed)

E. Product desorption: SiClx (adsorbed) SiClx (gas)

PLASMA ETCHING MODELS

Various steps of ion-assisted etching of silicon in chlorine plasmas are illustrated as follows:


Si + CF4 = SiF4 + C

SiO2 + CF4 = SiF4 + CO2

H + F = HF

C + O2 = CO or CO2

Q) Is CF4 reactive ?

PLASMA ETCHING MODELS (F/C)

- F for etching and C for polymerization

F/C ratio = ?


(1) Increasing the F/C ratio increases Si etching rates and decreasing the F/C ratio lowers them. The etching rates of SiO2, Si3N4, Ti, and W are affected similarly by the F/C ratio.

(2) F/C ratio of CF4 gas is four. As the etching of Si is carried out and the etch products SiF4 are generated, and F/C ratio decreases.

(3) Addition of H2 to CF4 forms HF, thereby the F/C ratio and the etching rate are reduced. This effect is also observed for CHF3 and C3F8 where the F/C ratio is lower than 4.

(4) Addition of O2 can increase the F/C ratio, because oxygen tends to consume more carbon than fluorine by forming CO or CO2.




The high selectivity of etching ratesof SiO2 versus Si in CF4+H2 plasmascan be explained by the F/C model

- Although the F/C ratio is less than 4due to the formation of HF, SiO2 cancompensate for the decreased F/Cratio since it generates oxygen

- Etching rates of SiO2 do notdecrease significantly with theaddition of H2

- Etching rate of Si is decreasedsignificantly as the F/C ratio islowered with the addition of H2

Si-substrate

SiO2

Mask



- The F/C ratio can also be used to differentiate the process regimes between etching and polymerization.

- In reactive ion etching, the self-bias voltage enhances ion bombardment at the surface and thereby promotes the removal of nonvolatile etch products.

- This enlarges the possible etching process regime towards lower F/C ratios.


PLASMA ETCHING MODELS (ETCHANT-UNSATURATE MODEL)

(1) Halocarbons and their mixtures with oxidants are widely used for plasma etching applications. Unsaturated halocarbon radicals generated from the gas usually become sidewall inhibitors for anisotropic etching.

(2) In the halocarbon plasmas, balance between unsaturated species and etchant/oxidant atoms is required. Etchant-unsaturate model can be a guide in predicting the effects of the plasma composition.

(3) Unsaturated fluoro- and chloro-carbon polymers that are generated in the plasmas can be saturated during reactions with atoms and reactive molecules. The most reactive species are preferentially removed by the saturation reactions.

- Continued


(4) Relative reactivity of atoms and molecules in the saturation reactions are F~O > Cl > Br and F2 > Cl2 > Br2. This order is used to predict the predominant etching species and reaction products for given gas compositions.

O, O2 + CxF2x COF2, CO, CO2 + F, F2O, O2 + CxF2x-yCly COF2, COFCl, CO, CO2 + F, F2, Cl, Cl2

(5) When atoms can react with the substrate to form volatile products, etching can occur. When excessive unsaturates adsorb on the surface, films are formed via polymerization. The unsaturates also form sidewall films, resulting in anisotropic etching with the help of ion bombardment.

(6) Addition of oxidants to the plasmas changes the concentration of halogen atoms and unsaturates. As the oxidants consume unsaturates, the relative concentration of less reactive halogen atoms increases.



O, O2 + CxF2x COF2, CO, CO2 + F, F2

O, O2 + CxF2x-yCly COF2, COFCl, CO, CO2 + F, F2, Cl, Cl2



GAS ADDITIVES IN ETCHING


Charge-up Model of Poly-Si GateNOTCH FORMATION: PROFILE CONTROL OF POLY-SI ETCHING

-Sidewall facing the open area is irradiated by electrons, and therefore the potential of the edge line is lower than that of the other inner lines in the normal array structure. However, the recombination between ions and electrons occurs at the inner sidewall of the edge line pattern. This induces the notching at the edge lines.

-Extent of notching is dependant on the electron flux to the side wall facing the open area, electron temperature and ion current density towards the wafer surface.


Devices can be damaged by plasma-generated charges trapped in thin gate oxides, and can show leaky behavior of thin gate oxides and notching in the poly-silicon gates. Also this causes non-uniform breakdown of gate oxides and non-uniform conductivity of gates.

Electron ShadingPlasma (or topography) non-uniformity

PLASMA INDUCED DAMAGE


Positive ions, accelerated by the electric field of the plasma sheath region,bombard the wafer surface perpendicularly. Electrons, repelled by theelectric field of the plasma sheath region, reach the surface with largeangular distribution.

Electrons are easily trapped at the sidewall of small patterns (usually< 0.2mm) causing excess positive ions at the bottom of high aspect ratiopatterns. Hence, notching occurs at the bottom of poly-Si gate structures.

PLASMA INDUCED DAMAGE (CHARGE-UP)


A. Oxygen radicals generated in oxygen plasmas can attack organic photo-resist materials to break polymer chains and form volatile products CO, CO2, and H2O.

B. Stripping rates of photo-resist increase with the increase of the oxygen concentration and temperature. Processing temperature is above the glass transition temperature, Tg.

C. Exposure of device structures to the plasma during photo-resist stripping can cause electrical damage and charging to the devices. Downstream plasma systems are widely used to avoid these problems.

PHOTO-RESIST STRIPPING (ASHING)


MICROSCOPIC UNIFORMITY IN ETCHING

MICROLOADING


MICROSCOPIC UNIFORMITY IN ETCHING

- It is differently named asRIE-lag,micro-loading,aspect-ratio dependent etching (ARDE), andpattern dependent etching.

- Defined as a phenomenon where the etching rates in thesmall patterns are either lower or higher than the etchingrates in the open area, it can be understood as aphenomenon where the etching rates change as etching iscarried out and thereby the aspect ratio of structuresbecomes higher.

MICROLOADING


According to the equation, the relative deposition and etching rates are alsodependent on the flux ratio of ions over neutrals.

For device structures of the high aspect ratio, the relative flux ratio betweenions and neutrals is high due to the directionality of the ions. Thus, theetching dominates the polymer deposition but the selectivity is low.

In the open area, relative flux ratio of ions over neutrals is lower comparedto that of the high aspect ratio structures. Here the deposition dominates theetching, but the selectivity can be high.

nniii

iiie JSVJEV

JEVR0/1

MICROSCOPIC NON-UNIFORMITY IN ETCHING (MICROLOADING)

Consider a situation where significantpolymeric species are generated in the plasmaand utilized for anisotropic, selective etching.For example, consider the etching of SiO2 withSi as an underlayer in the CHF3 plasmas wherethe etching selectivity is determined by polymerdeposition and etching of these materials.


Ion-Neutral Synergy Effects In Etching

Etching rate from the point of ions

Etching rate from the point of neutrals

Combining both,

nne JSVR )1(0

iiie JEVR

nniii

iiie JSVJEV

JEVR

0/1

where Vi is the volume removed per unit bombardment energy (cm3/eV)for the saturated surface, is the surface coverage of the chemicallyassisting neutral species, Ei is the average ion energy (eV), and Ji is theion flux (cm-2 sec-1) to the surface. Vn is the volume removed per reactingneutral (cm3), S0 is the sticking coefficient on the bare surface, and Jn isthe neutral flux (cm-2 sec-1) to the surface.



The etch rate becomes larger when there are appreciable contributions fromboth ions and neutrals than when there is only one major contribution eitherfrom neutrals or ions. When the surface is saturated with neutrals and the etching rate becomescontrolled by ions: . Also, when the surface is depleted with neutralsand the etching rate becomes controlled by neutrals: . At constant ion energy flux, the etching rate increases in proportion to theneutral flux and then saturates with the further increase. This is also thesame for the increase of the ion energy flux.

nniii

iiie JSVJEV

JEVR0/1

Etch

rate

s

Ion energy flux or neutral flux

Ion-neutral synergistic effects can beunderstood by the equation. That is, theetching rate becomes very small when theneutral flux is negligible. Also, the etchingrate becomes very small when the ion energyflux is negligible.

iiie JEVR nne JSVR 0



Mask

Large contact

Small contact

Inverse RIE-lag

SiO2

Mask

Large contact

Small contact

RIE-lag

SiO2

Flux ratio for ions over neutrals

Etch rates of SiO2

Higher A/R structuresLower pressure



For a given pressure and gas composition, the optimum flux ratio of ions over neutrals can be obtained for a certain aspect ratio.

(a) At low pressures, the flux ratio of ions over neutrals is so large that the etching process becomes dominated by neutral flux at the high aspect ratio structures but not at open areas where appreciable neutral flux is available. Thus the etching rates at the open areas can be higher than those at the high aspect ratio structures.

(b) At high pressures, the flux ratio of ions over neutrals becomes low and the etching process becomes faster at the high aspect ratio structures because enough neutral flux is available. But at open structures, the etching rates become slower due to polymer deposition that is resulted from the increased neutral flux. Thus the etching rates at the open structures become lower than those at the high aspect ratio structures.

Q) Considering (a) and (b), plot qualitatively etch rates as a function of aspect ratio for cases of low pressure and high pressure.


etching 2014

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