Wet and Dry Etching

1. Wet and Dry Etching
Unit 2 Wet and Dry Etching
By Dr. Ghanshyam Singh
Sharda University

2. 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

3. 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

4. Isotropic and Anisotropic Etch
UV expose Pattern development SiO2 Etching Resist Stripping
Isotropic Anisotropic
Completely
Anisotropic

5. 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 time
The presence of a surface layer that slows the etching process
A lowered temperature or weakened etch solution

6. 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 µm line when the patterned
resist line is 1 µm width. Thus, the undercut is 0. 1
µm 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….

7. • 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 µm / min
1 µm / min
target
Etch stop

8. Wet Etching
• Wet chemical etching is used for products with feature
sizes greater than 2 µm
• 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

9. • 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)

10. 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.

11. 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

12. 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

13. • 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)

14. – 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
DI
wafer
chuck

15. 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

16. Silicon Wet Etching
470 µm/min

17. 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

18. 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

19. (111)
[001]
[010]
[100]
(100)
[100]
[001]

20. 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

21. Deposited Oxide
• One of the final layers on a wafer is a SiO2film 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

22. 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

23. 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

24. Electrolytic etching and Chemical
mechanical polished
• Refer to notes………… : )

25. Dry Etching

26. 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.

27. – 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

28. – 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.

29. 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
E
E
E
E
E
E E E E E E
E
Damaged
E
E
E
EE
E
E
E
E
E
E
EB
B
B
B
B
BBBBBB
BE EEB
E

30. 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

31. Dry Etching Disadvantages
• Disadvantages:
– Some gases are quite toxic and corrosive
– Re-deposition of no volatile compounds
– Need for specialised expensive equipment

32. 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

33. 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

34. • 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

37. 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

39. 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(+)

40. Dry Etching Products