Wet and Dry Etching

Unit 2 Wet and Dry Etching By Dr. Ghanshyam SinghSharda University

Etching: Outline

• Important factors:– Uniformity

– Etch Control

– Etch Selectivity

• Wet Etching– Physical Mechanism of Wet Etching

– Etching Methods

» Immersion wet etching

» Spray wet etching

– Silicon wet etching: Isotropic and Anisotropic

– Etchant

– Selective Etch Stop

– Silicon Dioxide wet etching

Etching: Outline

– Silicon Nitride wet etching

– Metal Wet Etching

• Dry Etching• Features of a dry etch process

– Etch rate

– Plasma/radiation damage

– Anisotropy

– Uniformity

– Selectivity

– Cleanliness: Contamination, residues, corrosion

• Plasma

• Dry Etching Mechanism

• RIE

• Products

Isotropic and Anisotropic Etch

UV expose Pattern development SiO2 Etching Resist Stripping

Isotropic Anisotropic Completely Anisotropic

Goals (Uniformity, Control and Selective)

• Uniformity:• is defined as the percentage change in etch rate across the entire

etched region

• Etch Control:• Etch rate is defined as the amount of the film etched in a given time.

• Possible problems

Uniform Non-Uniform

Complete Incomplete

Too short an etch timeThe presence of a surface layer that slows the etching processA lowered temperature or weakened etch solution

Severe undercutting• Problem: Over-etch and undercutting

Undercut

Reverse undercut

Tapered undercut

Re-entrant undercut

In any etch process there is always some degree of overetch planned into the process

One way to quantify the undercut is the undercut distance per side. For e.g. a particular etching process may produce 0.8 m line when the patterned resist line is 1 m width. Thus, the undercut is 0. 1 m per side.

Severe undercutting takes place when

1. Excessive etch time

2. High temperature

3. Strong etchant solution

4. Adhesion between resist and wafer is weak….

• Etch Selectivity• One goal in etching step is the preservation of the surface underlying

the etched layer.

• The protected layer is too thin and important. Etch rate is too fast

• Etch Selectivity (S) is expressed as the ratio of the etch rate of the layer material (rm) to the etch rate of the underlying surface (rs)

– S = rm/rs

500 m / min

1 m / min

target

Etch stop

Wet Etching• Wet chemical etching is used for products with feature

sizes greater than 2 m

• Advantages:• Damage-free finish to wafer surface where surface morphology is typically

smooth and shiny

• fast etch rate especially for blanket etch

• simple and direct etching process since simple resist can be used as etch mask

• process occur at atmospheric environment

• cheaper cost

• high etch selectivity easily available for etchants, resist and etched materials

• good etch uniformity across wafer

• Disadvantages:• Isotropic etching

• No control for precision etching

• Excessive particle contamination is possible

• Bubbles can grow during etching that act as localised mask

• The resist cumming when the resist is incompletely exposed or insufficiently developed and this residual resist act as etching mask. (Not a problem in Dry Etching)

Physical mechanism of Wet Etching

• Wet chemical etching of a solid in a solution is a heterogeneous process.

• Contain mixture of oxidising agent and reducing agent whereby etching reactions involve oxidation-reduction mechanisms.

• Oxidising agent will oxidise the wafer material and the reducing agent will dissolve the oxide product.

Physical mechanism of Wet Etching• Three major process stages:

• The diffusion of the reacting ions or molecules from etchant solution towards the exposed film on the wafer surface through the boundary layer.

• The formation of a soluble or/and gaseous by-products through the chemical reaction between the etchant and the exposed film.

• The diffusion of the reaction by-product from the surface of the wafer through boundary layer into the bulk of the etchant solution.

Diffusion Reaction Diffusion

Reactants ProductsBoundary layer

Etching Methods• Immersion wet etching:

– For Silicon, Silicon dioxide, Silicon nitride and Aluminium

– Steps:• Wafers are immersed in a tank of an etchant solution for a specific

time for the wet chemical reaction between the etchant and etched material to occur

• Transferred to a rinse station for acid removal

• Transferred to a station for final rinse and a spin dry step

– Etching uniformity and process control can be enhanced by the addition of heaters and agitation devices.

• Advantages: • The simplest and most economical techniques

• Good selectivity

• Particle contamination can be flittered

• Spray wet etching– For Silicon, Silicon dioxide, Silicon nitride and Aluminium

– Steps:• Etchants are sprayed onto the surface of the etched film in surplus,

covering all surface area to be etched for a specific time

• DI water is then sprayed onto the surface of the etched film for etchant removal

• Final rinse and spin dry

• Advantages over immersion etching– Added definition gained from the mechanical pressure of the spray

– Spray etching also minimises contamination form the etchants

– More controllable (Can be removed instantly)

– Better etch uniformity (Fresh etchant is constantly supplied)

– Uses less amount of chemicals

– Faster etching process than immersion technique.

• Disadvantages– System cost

– Safety considerations associated with caustic etchants in a pressurised system

– Requirement of etch-resistant materials used for the systems to prevent the deterioration of the machine.

Etchants

vacuum

DIwafer

chuck

Silicon Wet Etching• The Well Known isotropic Etchant for Silicon is HNO3

(70% as Oxidising agent) and HF (49% as reduction agent). Water or acetic acid can be used to reduce etching rate.

• Oxidisation : Si+2HNO3-->SiO2+HNO2

• Reduction: SiO2+6HF-->H2SiF6+2H2O--->H2+SiF6+2H2O

• Etch stop material: Boron doped silicon p-type

• Method to make a Si membrane

Silicon Wet Etching470 m/min

Why acetic acid?• Role of acetic acid (CH3COOH):

– Acetic acid is frequently substituted for water as the dilutent.

– Acetic acid has a lower dielectric constant than water• 6.15 VS 81

• This produces less dissociation of the HNO3 and yields a higher oxidation power for the etch

– Acetic acid is less polar than water and can help in achieving proper wetting of slightly hydrophobic Si wafers

Anisotropic Silicon Wet Chemical Etching• The anisotropy is obtained through the different etch rates

that selected chemicals exhibit against different crystalline planes. >>>Silicon directional etching

• Atoms lying on (111) planes appear more densely packed than those on the (110) and (100) plane.

• As a consequence, certain etching formulations are favoured in removing atoms from (110) and (100) planes but not from the (111) planes.

• V-groove, square-ribs

(111)

[001]

[010]

[100]

(100)

[100]

[001]

SiO2 wet etching• Thermal oxide:

• The most common etched layer is a thermally grown silicon dioxide. The basic etchant is hydrofluoric acid (HF) which gives isotropic etch of silicon dioxide.

• HF is able to dissolve silicon dioxide without attacking silicon because the etch selectivity of silicon oxide to silicon is 100:1. For silicon nitride at 1 nm/min

• Etch stop material Silicon Nitride

• The full strength HF (49%) has an etch rate of about 30 nm/s or 1800 nm/min at room temperature>>>>Too fast

• In pratice, the HF is diluted with ammonium fluoride and water, which give a typical etch rate of 100 nm/min at room temperature

• Bubble problem

Deposited Oxide• One of the final layers on a wafer is a SiO2 film deposited

over the Al metallisation pattern. These films are known as vapox or silox film

• The HF content attacks the underlying Al pads, causing bonding problems in the packaging process.

• Etchant: 1 NH4F: 2 CH3COOH, 100 nm/min

• Bubble problem

• NH4F<--->NH3+HF

• SiO2+6HF--->H2SiF6+2H2O--->H2+SiF6+2H2O

Silicon Nitride Wet Etching• Etchant is a hot (140-200ºC) phosphoric acid at 10

nm/min.

• Etching process must be done in a closed reflux container equipped with a cooled lid to condense the vapours.

• Bubble problem

• 10:1 Silicon dioxide; 30:1 Silicon

Metal Wet Etching• Metal wet etching is also used to pattern metal lines.

• Polycrystalline metal such as aluminium is commonly used and the etched Al metal always has ragged edges.

• Etchant:– 4H3PO4: 1H2O:1 HNO3:4CH3COOH at 35 nm/min.

• Selectivity: Do not attack the Si, SiO2, Silicon Nitride

Electrolytic etching and Chemical mechanical polished

• Refer to notes………… : )

Dry Etching

Dry Etching• Dry Etching : Material removal reactions occur in the gas

phase• Types of Dry Etching: Non-plasma/Plasma based• Dry etching :

– Etch rate (Factors):– The system design

– The chemistry and physics of etch mechanisms

– The ion density: Increasing the power supply to electrodes

– System pressure

– Plasma radiation damage– Preferred system: high density sources with low pressure

– Within the plasma field are energetic atoms, ions, electrons and photons. These species, depending on their concentration and energy levels, cause various kinds of damage in semiconductors.

– Anisotropy:– Ions are travelling mainly in a direction perpendicular to the wafer

surface.

– Ion-neutral particle collisions from the plasma sheath result in a fraction of the ions hitting the sidewall and some lateral etching may occurs.

– Lowering the pressure reduces ion-neutral collisions and enhances anisotropic etching

– Uniformity– Achieving microscopic uniformity in etching across wafer and between

wafers can be a problem because of

» Variation etch rate

» Dependence of profile on pattern density and feature size.

– Selectivity– Selectivity is defined as the rate of etching one film with respect to the

film under the same conditions.

– Chemical wet etches are generally more selective than plasma etches

– Cleanliness: contamination, residues, corrosion– Cleaner than wet etching

– Use an electrostatic wafer holder rather than mechanical holders. Mechanical holders generate particles and cause wafer breakage and clamps cast a shadow on part of the wafer surface.

– Plasma etch environment is highly reactive and many chemical reactions take place. These reactions create contamination and can interfere with the deposition process.

– Post-etch corrosion is caused by etch residues left on metal patterns after the etch process. For example, the addition of Cu to Al metal and the use of Ti or W metallisation increases the corrosion problem from residual Cl after plasma etch. Minimising this problem includes substituting fluorine-based etchants for chlorine etchants, passivating the sidewalls and surfaces.

Proposed mechanisms for anisotropic plasma etching

• (1) Damage mechanism.• Perpendicular ion bombardment creates a damaged surface that is

then more reactive toward neutral etchants.

• Ion transport must be perpendicular to the surface so that only the etch rate of the bottom surface is enhanced.

• (2) Blocking mechanism• Ions help to desorb etch-inhibitiong species, such as etch products

from the surface

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Damaged

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Dry Etching Advantages• Eliminates handling of dangerous acids and solvents

• Uses small amounts of chemicals

• Isotropic or anisotropic etch profiles

• Directional etching without using the crystal orientation of Si

• High resolution and cleanliness

• Less undercutting

• No unintentional prolongation of etching

• Better process control

• Ease of automation

Dry Etching Disadvantages• Disadvantages:

– Some gases are quite toxic and corrosive

– Re-deposition of no volatile compounds

– Need for specialised expensive equipment

Plasma• Plasma=partially ionised gas consisting of equal numbers

of “+”(ions) and “-” (electrons) charges and a different number of neutral (un-ionised) molecules

• An ion-electron pair is continuously created by ionisation and destroyed by recombination

• Typical kinetic energy of an electron in plasma is 2-8eV

• KE=1/2 mv2 = 3/2 kT

• 2eV electron has T~15000 K, V=6*107 cm/s

Plasma Formation• Chamber is evacuated

• Chamber is filled with gas

• RF energy is applied to a pair of electrodes

• Applied energy accelerates electrons increasing kinetic energy

• Electrons collide with neutral gas molecules, forming ions and more electrons

• Steady state is reached (plasma); ionisation=recombination

• Plasma discharge is characterised by central glow or bulk region and dark or sheath regions near electrodes

• Bulk region=semi-neutral (nearly equal number of electrons and ions)

• Sheath regions=nearly all of the potential drop; accelerates “+” ions from bulk region which bombard the substrate

• Maintained at 1 Pa to 750 Pa with gas density of 27*1014 to 2*1017 molecules/cm3

Reactive Ion Etching (RIE)• RIE=process in which chemical etching is accompanied by

ionic bombardment (ie ion assisted etching)

• Bombardment opens areas for reactions

• Ionic Bombardment:– No undercutting since sidewalls are not exposed

– Greatly increased etch rate

– Structural degradation

– low selectivity

Disadvantages of RIE• Conflict between etching rate and anisotropic profile

– Etching rate (+) ---> Reactive species--->concentration (+)----> Gas pressure (+)---> Collision (+)--->Anisotropic (decreases)

• Conflict between damage of high etching rate and anisotropic profile– KE(+)--->Etching rate (+)--->damage(+)

Dry Etching Products