introduction after a thin film is deposited, it is usually etched to remove unwanted materials and...

IntroductionIntroduction After a thin film is deposited, it is usually After a thin film is deposited, it is usually

etched to remove unwanted materials and etched to remove unwanted materials and leave the desired pattern on the waferleave the desired pattern on the wafer

Need to etch the Si wafer to create trenches Need to etch the Si wafer to create trenches for trench capacitors, recessed gate for trench capacitors, recessed gate transistors, and a number of MEMS devicestransistors, and a number of MEMS devices– The masking layer may be photoresist, SiOThe masking layer may be photoresist, SiO22 or or

SiSi33NN44

– The etch is usually done until another layer of a The etch is usually done until another layer of a different material is reacheddifferent material is reached Monitoring systems to detect change in material type or Monitoring systems to detect change in material type or

a selective etch may be used.a selective etch may be used.

IntroductionIntroduction

IntroductionIntroduction Etching can be done “wet” or “dry”Etching can be done “wet” or “dry” Wet etching Wet etching

– uses liquid etchantsuses liquid etchants– Wafer is immersed in the liquidWafer is immersed in the liquid– Process is mostly chemicalProcess is mostly chemical

Rarely used in VLSI wafer fab todayRarely used in VLSI wafer fab today– ParticulatesParticulates– Isotropic nature of most wet etchesIsotropic nature of most wet etches

Equal etch rate in all directionsEqual etch rate in all directions– Hard to control etch rate and selectivityHard to control etch rate and selectivity– Difficult to integrate monitoring systemsDifficult to integrate monitoring systems


Dry etchingDry etching– Uses gas phase etchants in a plasmaUses gas phase etchants in a plasma

Combination of chemical and physical actionCombination of chemical and physical action

– Process is often called “plasma etching”Process is often called “plasma etching” The ideal etch produces vertical sidewallsThe ideal etch produces vertical sidewalls

– Anisotropic etching is usually desired, but Anisotropic etching is usually desired, but extremely difficult to achieveextremely difficult to achieve Etch rate depends significantly on directionEtch rate depends significantly on direction Undercut changes linewidth of etch region Undercut changes linewidth of etch region

compared to originial linewidth of masking materialcompared to originial linewidth of masking material


There is undercutting, non-vertical There is undercutting, non-vertical sidewalls, and some etching of the Sisidewalls, and some etching of the Si– The photoresist may have rounded tops The photoresist may have rounded tops

and non-vertical sidewallsand non-vertical sidewalls The etch rate of the photoresist is The etch rate of the photoresist is

not zero and the mask is etched to not zero and the mask is etched to some extentsome extent– This leads to more undercuttingThis leads to more undercutting


Etch selectivity is the ratio of the etch Etch selectivity is the ratio of the etch rates of different materials in the processrates of different materials in the process– High selectivity if the etch rate of the mask High selectivity if the etch rate of the mask

and of the underlying substrate is near zero, and of the underlying substrate is near zero, and the etch rate of the film is highand the etch rate of the film is high

– Poor selectivity if the etch rate of the mask Poor selectivity if the etch rate of the mask or the substrate is highor the substrate is high

Selectivities of 25 – 50 are desiredSelectivities of 25 – 50 are desired– Materials usually have differing etch rates Materials usually have differing etch rates

due to chemical processes rather than due to chemical processes rather than physical processesphysical processes

IntroductionIntroduction Etch directionality is a measure of the etch Etch directionality is a measure of the etch

rate in different directions rate in different directions – Usually vertical versus lateralUsually vertical versus lateral– Can be along particular crystalline planesCan be along particular crystalline planes


Anisotropic etching or etch directionality Anisotropic etching or etch directionality is often related to physical processesis often related to physical processes– ion bombardment and sputteringion bombardment and sputtering– Directionality is often desired in order to Directionality is often desired in order to

maintain the lithographically defined maintain the lithographically defined featuresfeatures

However, perfect anisotropic etch can However, perfect anisotropic etch can lead to step coverage issues and other lead to step coverage issues and other problems during subsequent processing problems during subsequent processing stepssteps


Selectivity is very desirableSelectivity is very desirable– The etch rate of the material to be The etch rate of the material to be

removed should be fast compared to removed should be fast compared to that of the mask and of the substrate that of the mask and of the substrate layerlayer

It is hard to get good directionality It is hard to get good directionality and good selectivity at the same and good selectivity at the same timetime


Other system requirements includeOther system requirements include– Ease of transporting gases/liquids to the Ease of transporting gases/liquids to the

wafer surfacewafer surface– Ease of transporting reaction products Ease of transporting reaction products

away from wafer surfaceaway from wafer surface– Process must be reproducible, uniform, Process must be reproducible, uniform,

safe, clean, cost effective, and have low safe, clean, cost effective, and have low particulate productionparticulate production

Basic ConceptsBasic Concepts

We consider two processesWe consider two processes– ““wet” etchingwet” etching– ““dry” etchingdry” etching

In the early days, wet etching was used In the early days, wet etching was used exclusivelyexclusively– It is well-established, simple, and inexpensiveIt is well-established, simple, and inexpensive

The need for smaller feature sizes could The need for smaller feature sizes could only be met with plasma etchingonly be met with plasma etching– Plasma etching is used almost exclusively Plasma etching is used almost exclusively

todaytoday


The first wet etchants were simple The first wet etchants were simple chemicalschemicals

By immersing the wafer in these chemicals, By immersing the wafer in these chemicals, exposed areas could be etched and washed exposed areas could be etched and washed awayaway

Wet etches were developed for all stepWet etches were developed for all step For SiOFor SiO22, HF was used., HF was used. Wet etches work through chemical Wet etches work through chemical

processes to produce a water soluble processes to produce a water soluble byproductbyproduct

O2HSiFH6HFSiO 2622


In some cases, the etch works by first oxidizing In some cases, the etch works by first oxidizing the surface and then dissolving the oxidethe surface and then dissolving the oxide

An etch for Si involves a mixture of nitric acid An etch for Si involves a mixture of nitric acid and HFand HF

The nitric acid (HNOThe nitric acid (HNO33) decomposes to form ) decomposes to form nitrogen dioxide (NOnitrogen dioxide (NO22))

The SiO2 is removed by the previous reactionThe SiO2 is removed by the previous reaction The overall reaction isThe overall reaction is

22222 2HNOHSiOO2H2NOSi

222623 HOHHNOSiFH6HFHNOSi

Basic ConceptsBasic Concepts Buffers are often added to keep the etchants at Buffers are often added to keep the etchants at

maximum strength over use and timemaximum strength over use and time Ammonium fluoride (NHAmmonium fluoride (NH44F) is often used with HF F) is often used with HF

to help prevent depletion of the F ionsto help prevent depletion of the F ions This is called Basic Oxide Etch (BOE) or Buffered This is called Basic Oxide Etch (BOE) or Buffered

HF (BHF)HF (BHF) The ammonium fluoride reduces the etch rate The ammonium fluoride reduces the etch rate

of photoresist and helps eliminate the lifting of of photoresist and helps eliminate the lifting of the resist during oxide etchingthe resist during oxide etching

Acetic acid (CHAcetic acid (CH33COOH) is often added to the COOH) is often added to the nitric acid/HF Si etch to limit the dissociation of nitric acid/HF Si etch to limit the dissociation of the nitric acid the nitric acid


Wet etches can be very selective because they Wet etches can be very selective because they depend on chemistrydepend on chemistry

The selectivity is given byThe selectivity is given by

Material “1” is the film being etched and Material “1” is the film being etched and material”2” is either the mask or the material material”2” is either the mask or the material below the film being etchedbelow the film being etched

If S>>1, we say the etch has good selectivity If S>>1, we say the etch has good selectivity for material 1 over material 2for material 1 over material 2

2

1

r

rS


Most wet etches etch isotropicallyMost wet etches etch isotropically The exception is an etch that depends on the The exception is an etch that depends on the

crystallographic orientationcrystallographic orientation Example—some etches etch <111> Si slower Example—some etches etch <111> Si slower

than <100> Sithan <100> Si Etch bias is the amount of undercutting of the Etch bias is the amount of undercutting of the

maskmask If we assume that the selectivity for the oxide If we assume that the selectivity for the oxide

over both the mask and the substrate is infinite, over both the mask and the substrate is infinite, we can define the etch depth as “d” and the we can define the etch depth as “d” and the bias as “b”bias as “b”


We often deliberately build in some We often deliberately build in some overetching into the processoveretching into the process

This is to account for the fact that This is to account for the fact that – the films are not perfectly uniformthe films are not perfectly uniform– the etch is not perfectly uniformthe etch is not perfectly uniform

The overetch time is usually calculated from The overetch time is usually calculated from the known uncertainties in film thickness and the known uncertainties in film thickness and etch ratesetch rates

It is important to be sure that no area is under-It is important to be sure that no area is under-etched; we can tolerate some over-etchingetched; we can tolerate some over-etching


This means that it is important to have as This means that it is important to have as high a selectivity as possible to eliminate high a selectivity as possible to eliminate etching of the substrateetching of the substrate

However, if the selectivity is too high, However, if the selectivity is too high, over-etching may produce unwanted over-etching may produce unwanted undercuttingundercutting

If the etch rate of the mask is not zero, If the etch rate of the mask is not zero, what happens?what happens?

If If m is the amount of mask removed, we m is the amount of mask removed, we get unexpected lateral etchingget unexpected lateral etching


m is called “mask erosion”m is called “mask erosion” For anisotropic etching, mask erosion For anisotropic etching, mask erosion

should not cause much of a problem should not cause much of a problem if the mask is perfectly verticalif the mask is perfectly vertical

Etching is usually neither perfectly Etching is usually neither perfectly anisotropic nor perfectly isotropicanisotropic nor perfectly isotropic

We can define the degree of We can define the degree of anisotropy byanisotropy by

vert

latf r

rA 1


Isotropic etching has an AIsotropic etching has an Aff = 0 while = 0 while anisotropic etching has Aanisotropic etching has Aff = 1 = 1

There are several excellent examples There are several excellent examples in the text that do simple in the text that do simple calculations of these effectscalculations of these effects

These examples should be studied These examples should be studied carefullycarefully

ExampleExample Consider the structure belowConsider the structure below

The oxide layer is 0.5 The oxide layer is 0.5 m. Equal structure widths and m. Equal structure widths and spacings, Sspacings, Sff, are desired. The etch anisotropy is 0.8., are desired. The etch anisotropy is 0.8.

If the distance between the mask edges, x, is 0.35 If the distance between the mask edges, x, is 0.35 m, m, what structure spacings and widths are obtained?what structure spacings and widths are obtained?

ExampleExample

To obtain equal widths and spacings, STo obtain equal widths and spacings, Sff, the mask , the mask width, Swidth, Smm, must be larger to take into account the , must be larger to take into account the anisotropic etchinganisotropic etching

SinceSince

where b is the bias on each side, andwhere b is the bias on each side, and SinceSince

ThusThus

bSS fm

d

bAf 1

fffm AxSS 12

ExampleExample This result makes senseThis result makes sense

– For isotropic etching, AFor isotropic etching, Aff=0 and S=0 and Smm is a maximum is a maximum

– For perfectly anisotropic etching, AFor perfectly anisotropic etching, Aff=1 and S=1 and Smm=S=Sff and is and is a minimuma minimum

The distance between the mask edges (x) is the The distance between the mask edges (x) is the minimum feature size that can be resolvedminimum feature size that can be resolved

ButBut

Substitution and rearranging yields (note typo in Substitution and rearranging yields (note typo in text)text)

mF SSx 2

fff

fff

AxxS

AxSx

12

12

ExampleExample Substituting numbers for the problemSubstituting numbers for the problem

This result shows that the structure size can This result shows that the structure size can approach the minimum lithographic dimension only approach the minimum lithographic dimension only when the film thickness gets very small OR as the when the film thickness gets very small OR as the anisotropy gets near 1.0anisotropy gets near 1.0

Very thin films are not always practicalVery thin films are not always practical This means we need almost vertical etchingThis means we need almost vertical etching Wet etching cannot achieve the desired resultsWet etching cannot achieve the desired results

m 55.0

8.01m 0.52m 35.0

fS

Plasma EtchingPlasma Etching

Plasma etching has (for the most part) Plasma etching has (for the most part) replaced wet etchingreplaced wet etching

There are two reasons:There are two reasons:– Very reactive ion species are created in the plasma Very reactive ion species are created in the plasma

that give rise to very active etchingthat give rise to very active etching– Plasma etching can be very anisotropic (because Plasma etching can be very anisotropic (because

the electric field directs the ions)the electric field directs the ions) An early application of plasma etching (1970s) An early application of plasma etching (1970s)

was to etch Siwas to etch Si33NN4 4 (it etches very slowly in HF (it etches very slowly in HF and HF is not very selective between the and HF is not very selective between the nitride and oxide)nitride and oxide)


Plasma systems can be designed so that Plasma systems can be designed so that either reactive chemical components either reactive chemical components dominate or ionic components dominatedominate or ionic components dominate

Often, systems that mix the two are usedOften, systems that mix the two are used– The etch rate of the mixed system may be The etch rate of the mixed system may be

much faster than the sum of the individual much faster than the sum of the individual etch ratesetch rates

A basic plasma system is shown in the A basic plasma system is shown in the next slidenext slide


Features of this systemFeatures of this system– Low gas pressure (1mtorr – 1 torr)Low gas pressure (1mtorr – 1 torr)– High electric field ionizes some of the gas High electric field ionizes some of the gas

(produces positive ions and free electrons)(produces positive ions and free electrons)– Energy is supplied by 13.56 MHz RF generatorEnergy is supplied by 13.56 MHz RF generator– A bias develops between the plasma and the A bias develops between the plasma and the

electrodes because the electrons are much electrodes because the electrons are much more mobile than the ions (the plasma is more mobile than the ions (the plasma is biased positive with respect to the electrodes)biased positive with respect to the electrodes)


If the area of the electrodes is the same If the area of the electrodes is the same (symmetric system) we get the solid curve (symmetric system) we get the solid curve of 10-8of 10-8

The sheaths are the regions near each The sheaths are the regions near each electrode where the voltage drops occur electrode where the voltage drops occur (the dark regions of the plasma)(the dark regions of the plasma)

The sheaths form to slow down the electron The sheaths form to slow down the electron loss so that it equals the ion loss per RF loss so that it equals the ion loss per RF cyclecycle

In this case, the average RF current is zeroIn this case, the average RF current is zero


The heavy ions respond to the average voltageThe heavy ions respond to the average voltage The light electrons respond to the instantaneous The light electrons respond to the instantaneous

voltagevoltage The electrons cross the sheath only during a short The electrons cross the sheath only during a short

period in the cycle when the sheath thickness is period in the cycle when the sheath thickness is minimumminimum

During most of the cycle, most of the electrons During most of the cycle, most of the electrons are turned back at the sheath edgeare turned back at the sheath edge

The sheaths are thus deficient in electronsThe sheaths are thus deficient in electrons They are thus dark because of a lack of light-They are thus dark because of a lack of light-

emitting electron-ion collisionsemitting electron-ion collisions


For etching photoresist, we use OFor etching photoresist, we use O22

For other materials we use species containing halides For other materials we use species containing halides such as Clsuch as Cl22, CF, CF44, and HBr, and HBr

Sometimes HSometimes H22, O, O22, and Ar may be added, and Ar may be added The high-energy electrons cause a variety of reactionsThe high-energy electrons cause a variety of reactions The plasma contains The plasma contains

– free electronsfree electrons– ionized moleculesionized molecules– neutral moleculesneutral molecules– ionized fragments ionized fragments – Free radicalsFree radicals


In CFIn CF44 plasmas, there are plasmas, there are– Free electronsFree electrons

– CFCF44

– CFCF33

– CFCF33++

– FF CFCF and F are free radicals and are very and F are free radicals and are very

reactivereactive Typically, there will be 10Typically, there will be 101515 /cc neutral /cc neutral

species and 10species and 1088-10-101212 /cc ions and electrons /cc ions and electrons

Plasma Etching MechanismsPlasma Etching Mechanisms

The main species involved in etching areThe main species involved in etching are– Reactive neutral chemical speciesReactive neutral chemical species– IonsIons

The reactive neutral species (free radicals in The reactive neutral species (free radicals in many cases) are primarily responsible for many cases) are primarily responsible for the chemical component the chemical component

The ions are responsible for the physical The ions are responsible for the physical componentcomponent

The two can work independently or The two can work independently or synergisticallysynergistically

Plasma Etching MechanismsPlasma Etching Mechanisms

When the reactive neutral species When the reactive neutral species act alone, we have chemical etchingact alone, we have chemical etching

Ions acting by themselves give Ions acting by themselves give physical etchingphysical etching

When they work together, we have When they work together, we have ion-enhanced etchingion-enhanced etching

Chemical EtchingChemical Etching

Chemical etching is done by free radicalsChemical etching is done by free radicals Free radicals are neutral molecules that have Free radicals are neutral molecules that have

incomplete bonding (unpaired electrons)incomplete bonding (unpaired electrons) For exampleFor example

Both F and CFBoth F and CF33 are free radicals are free radicals Both are highly reactiveBoth are highly reactive F wants 8 electrons rather than 7 and reacts F wants 8 electrons rather than 7 and reacts

quickly to find a shared electronquickly to find a shared electron

eFCFCFe 34


The idea is to get the free radical to react The idea is to get the free radical to react with the material to be etched (Si, SiOwith the material to be etched (Si, SiO22))

The byproduct should be gaseous so that it The byproduct should be gaseous so that it can be transported away (next slide)can be transported away (next slide)

The reaction below is such a reactionThe reaction below is such a reaction

Thus, we can etch Si with CFThus, we can etch Si with CF44

There are often several more complex There are often several more complex intermediate statesintermediate states

4SiFSi4F


Gas additives can be used to increase the Gas additives can be used to increase the production of the reactive species (Oproduction of the reactive species (O22 in CF in CF44))

The chemical component of plasma etching The chemical component of plasma etching occurs isotropicallyoccurs isotropically

This is becauseThis is because– The arrival angles of the species is isotropicThe arrival angles of the species is isotropic– There is a low sticking coefficient (which is more There is a low sticking coefficient (which is more

important)important) The arrival angle follows what we did in The arrival angle follows what we did in

deposition and there is a cosdeposition and there is a cosnn dependence dependence where n=1 is isotropic where n=1 is isotropic


The sticking coefficient isThe sticking coefficient is

A high sticking coefficient means that the A high sticking coefficient means that the reaction takes place the first time the ion reaction takes place the first time the ion strikes the surfacestrikes the surface

For lower sticking coefficients, the ion can leave For lower sticking coefficients, the ion can leave the surface (usually at random angles) and the surface (usually at random angles) and strikes the surface somewhere elsestrikes the surface somewhere else

incident

reactedc F

FS


One would guess that the sticking One would guess that the sticking coefficient for reactive ions is highcoefficient for reactive ions is high

However, there are often complex However, there are often complex reactions chained together. This reactions chained together. This complexity often means low sticking complexity often means low sticking coefficientscoefficients

SScc for O for O22/CF/CF44 on Si is about 0.01 on Si is about 0.01 This additional “bouncing around” of This additional “bouncing around” of

the ions leads to isotropic etchingthe ions leads to isotropic etching


Since free radicals etch by Since free radicals etch by chemically reacting with the material chemically reacting with the material to be etched, the process can be to be etched, the process can be highly selectivehighly selective

Physical EtchingPhysical Etching

Due to the voltage drop between the plasma Due to the voltage drop between the plasma and the electrodes and the resulting electric and the electrodes and the resulting electric field across the sheaths, positive ions are field across the sheaths, positive ions are accelerated towards each electrodeaccelerated towards each electrode

The wafers are on one electrodeThe wafers are on one electrode Therefore, ionic species (ClTherefore, ionic species (Cl++ or Ar or Ar++) will be ) will be

accelerated towards the wafer surfaceaccelerated towards the wafer surface These ions striking the surface result in the These ions striking the surface result in the

physical processphysical process The process is much more directional because The process is much more directional because

the ions follow the field linesthe ions follow the field lines


This means n is very large in the cosThis means n is very large in the cosnn distributiondistribution

But, because the process is more physical But, because the process is more physical than chemical, the selectivity will not be as than chemical, the selectivity will not be as good as in the more chemical processesgood as in the more chemical processes

We also assume that the ion only strikes the We also assume that the ion only strikes the surface once (which implies that the surface once (which implies that the sticking coefficient is near 1)sticking coefficient is near 1)

Ions can also etch by physical sputtering Ions can also etch by physical sputtering (Chapter 9)(Chapter 9)

Ion-Enhanced EtchingIon-Enhanced Etching

The ions and the reactive neutral The ions and the reactive neutral species do not always act species do not always act independently (the observed etch independently (the observed etch rate is not the sum of the two rate is not the sum of the two independent etch rates)independent etch rates)

The classic example is etching of Si The classic example is etching of Si with XeFwith XeF22 and Ar and Ar++ ions are introduced ions are introduced


The shape of the etch profiles are The shape of the etch profiles are interestinginteresting

The profiles are not the linear sum of The profiles are not the linear sum of the profiles from the two processesthe profiles from the two processes

The profile is much more like the The profile is much more like the physical etch alone (c)physical etch alone (c)


If the chemical component is increased, the If the chemical component is increased, the vertical etching is increased, but not the lateral vertical etching is increased, but not the lateral etchingetching

The etch rate is also increasedThe etch rate is also increased The mechanisms for these effects are poorly The mechanisms for these effects are poorly

understoodunderstood Whatever the mechanism, the enhancement Whatever the mechanism, the enhancement

only occurs where the ions hit the surfaceonly occurs where the ions hit the surface Since the ions strike normal to the surface, the Since the ions strike normal to the surface, the

enhancement is in this directionenhancement is in this direction This increases the directionalityThis increases the directionality


Possible models include Possible models include – Enhancement of the etch reactionEnhancement of the etch reaction– Inhibitor removalInhibitor removal

The reaction takes place only where the ions The reaction takes place only where the ions strike the surface strike the surface

Since the ions strike normal to the surface, Since the ions strike normal to the surface, removal is thus only at the bottom of the wellremoval is thus only at the bottom of the well

It is assumed that etching by radicals (chemical It is assumed that etching by radicals (chemical etching) is negligible etching) is negligible

Note that even under these assumptions, the Note that even under these assumptions, the side walls may not be perfectly verticalside walls may not be perfectly vertical


Note that an inhibitor can be removed Note that an inhibitor can be removed on the bottom, but not on the on the bottom, but not on the sidewallssidewalls

If inhibitors are deliberately If inhibitors are deliberately deposited, we can make very deposited, we can make very anisotropic etchesanisotropic etches

If the inhibitor formation rate is large If the inhibitor formation rate is large compared to the etch rate, one can compared to the etch rate, one can get non-vertical walls (next slide)get non-vertical walls (next slide)

Types of Plasma SystemsTypes of Plasma Systems

Several different types of plasma Several different types of plasma systems and modes of operation systems and modes of operation have been developedhave been developed– Barrel etchersBarrel etchers– Parallel plate systems (plasma mode)Parallel plate systems (plasma mode)– Parallel plate systems (reactive ion Parallel plate systems (reactive ion

mode)mode)– High density plasma systemsHigh density plasma systems– Sputter etching and ion millingSputter etching and ion milling

Barrel EtchersBarrel Etchers Barrel etchers were one of the earliest types of systemsBarrel etchers were one of the earliest types of systems VT has a small oneVT has a small one Here, the electrodes are curved and wrap around the Here, the electrodes are curved and wrap around the

quartz tubequartz tube The system is evacuated and then back-filled with the The system is evacuated and then back-filled with the

etch gasetch gas The plasma is kept away from the wafers by a perforated The plasma is kept away from the wafers by a perforated

metal shieldmetal shield Reactant species (F) diffuse through the shield to the Reactant species (F) diffuse through the shield to the

waferswafers Because the ions and plasma are kept away from the Because the ions and plasma are kept away from the

wafers, and the wafers do not sit on either electrode, wafers, and the wafers do not sit on either electrode, there is NO ion bombardment and the etching is purely there is NO ion bombardment and the etching is purely chemical chemical

Barrel EtchersBarrel Etchers

Barrel EtchersBarrel Etchers

Because the etches are purely chemical, Because the etches are purely chemical, they can be very selective (but is almost they can be very selective (but is almost isotropic)isotropic)

The etching uniformity is not very goodThe etching uniformity is not very good The systems are very simple and The systems are very simple and

throughput can be highthroughput can be high They are used only for non-critical steps They are used only for non-critical steps

due to the non-uniformitydue to the non-uniformity They are great for photoresist strippingThey are great for photoresist stripping

Parallel Plate SystemsParallel Plate Systems

Parallel plate systems are commonly Parallel plate systems are commonly used for etching thin filmsused for etching thin films

Parallel Plate SystemsParallel Plate Systems

This system is very similar to a PECVD system This system is very similar to a PECVD system (Chapter 9) except that we use etch gases (Chapter 9) except that we use etch gases instead of deposition gasesinstead of deposition gases

These systems are much more uniform across These systems are much more uniform across the wafer than the barrel etcherthe wafer than the barrel etcher

The wafers are bombarded with ions due to the The wafers are bombarded with ions due to the voltage drop (Figure 10-8)voltage drop (Figure 10-8)

If the plates are symmetric (same size) the If the plates are symmetric (same size) the physical component of the etch is found to be physical component of the etch is found to be rather small and one gets primarily chemical rather small and one gets primarily chemical etching etching

Parallel Plate SystemsParallel Plate Systems By increasing the energy of the ions (increasing By increasing the energy of the ions (increasing

the voltage) the physical component can be the voltage) the physical component can be increasedincreased

This can be done by decreasing the size of the This can be done by decreasing the size of the electrode on which the wafers sit and changing electrode on which the wafers sit and changing which electrode is groundedwhich electrode is grounded

In this configuration, we get the reactive ion In this configuration, we get the reactive ion etching (RIE) mode of operationetching (RIE) mode of operation

Here, we get both chemical and physical Here, we get both chemical and physical etchingetching

By lowering the gas pressure, the system can By lowering the gas pressure, the system can become even more directionalbecome even more directional

High-Density Plasma High-Density Plasma EtchingEtching

This system is becoming more popularThis system is becoming more popular These systems separate the plasma density and These systems separate the plasma density and

the ion energy by using a second excitation the ion energy by using a second excitation source to control the bias voltage of the wafer source to control the bias voltage of the wafer electrodeelectrode

A different type of source for the plasma is used A different type of source for the plasma is used instead of the usual capacitively coupled RF instead of the usual capacitively coupled RF sourcesource

It is non-capacitively coupled and generates a It is non-capacitively coupled and generates a very high plasma density without generating a very high plasma density without generating a large sheath biaslarge sheath bias


These systems still generate high ion These systems still generate high ion fluxes and etch rates even though they fluxes and etch rates even though they operate at much lower pressures (1—10 operate at much lower pressures (1—10 mtorr) because of the higher plasma mtorr) because of the higher plasma densitydensity

Etching in these systems is like RIE Etching in these systems is like RIE etching with a very large physical etching with a very large physical component combined with a chemical component combined with a chemical component involving reactive neutralscomponent involving reactive neutrals

They thus give reasonable selectivityThey thus give reasonable selectivity

SummarySummary

introduction after a thin film is deposited, it is usually etched to remove unwanted materials and...

Documents

achieveetch rate

drywet etching

good selectivity

plasma etchingplasma

good directionality

wafer surfaceprocess

wafer surfaceease

processhigh selectivity