Why dry etching?

Dry etching advantages

• Eliminates handling of dangerous acids and solvents
• Uses small amounts of chemicals (gas)
• Isotropic or anisotropic/vertical etch profiles
• Directional etching without using the crystal orientation of Si
• High resolution and cleanliness
• Less undercutting
• Better process control -> reproducibility (??)

Dry etching disadvantages:

• Some gases are quite toxic and corrosive.
• Re-deposition of non-volatile compound on wafers.
• Expensive equipment ($200-500K for R&D, few million for industrial tools ).

1
 Types of dry etching
Non-plasma based - uses spontaneous Plasma based - uses radio frequency (RF)
reaction of appropriate reactive gas power to drive chemical reaction.
mixture.

Xenon di-fluoride (XeF2) etching of Si:

2XeF2 + Si  2Xe (g) + SiF4 (g)
• XeF2 is a white powder, with vapor pressure 3.8 Torr at 25oC.
• Isotropic etching, non-polish etching (rough)
• High selectivity for Al, SiO2, Si3N4, photoresist.
• Typical etch rate 1μm/min
• Heat is generated during exothermic reaction
• XeF2 reacts with water (or vapor) to form HF

4Si(s) + 2Cl2 (g) ---> 4SiCl4 (g) + 130 kcal/mole

Although there is a large gain in free energy, the large activation energy does
not allow low temperature processes - reaction is only effective above  800°C.
In order to succeed with “gas” etching, one has to go out of equilibrium.
2
The solution is plasma etching.
 Plasma-based etching
• Directional etching due to presence of ionic species in plasma and (self-) biased
electric field. (The self-bias electric field is not applied externally, but is created
spontaneously in RF plasma)
• Two components exist in plasma
o Ionic species result in directional etching.
o Chemical reactive species result in high etch selectivity.
• Control of the ratio of ionic/reactive components in plasma can modulate the dry
etching rate and etching profile.

Plasma
Neutrals (etchant gas)
Gaseous products
Ions
Free radicals

react
adsorb

surface

Si ( s )  4 F ( g )  SiF4 ( g ) 3
 RF plasma chemistry

CF4 plasma

In equilibrium, degree of ionization typically 10-3 - 10-6, very low, meaning majority
gas not ionized.
(plasma density = number of ions/cm3  typically 109 – 1013/cm3.) 4
 Chemical etch: highly selective, but isotropic
• Due to their incomplete bonding (incomplete outer shells), free radicals (neutral,
e.g. CF3 and F from CF4 plasma) are highly reactive chemical species.

• Free radicals react with film to be etched and form volatile by-products.

• Pure chemical etch is isotropic or nearly isotropic,

and the etching profile depends on arrival angle
and sticking coefficients of free radicals.
• Free radicals (un-charged) in plasma systems have
isotropic arrival angles.
• The sticking coefficient S is very low, typically only
S0.01 (i.e. most free radicals adsorb then just
bounce back without reaction).
• This leads to isotropic character of etch, as free
radicals can etch area beneath the mask due to
bouncing, as seen in the figure. The resulted
profile has large undercut.
5
 Etch byproducts should have low boiling point
Low boiling point means very volatile, so it can be pumped away.
This is not necessary for physical etching/sputtering, where etch product is
sputtered off that ideally doesn’t fall on the other part of the wafer (re-deposition).
Boiling points of typical etch products

6
 Physical etch component in a plasma etch system
(much less important than chemical etch)
1. Ionic species are accelerated toward each electrode by built-in self-bias.
2. The ionic species strike wafer surface and remove the material to be
etched.
3. Directional, non-selective - similar sputter yield for different materials.
It may result in significant re-deposition.

Pure physical etch: sputter etching system

• Self-bias few 100V, but low ion energy
(order 10V) due to collision energy loss.
• Thus very low milling rate in a sputter
system, often for surface cleaning only.
• For a dedicated ion milling system (no Ar plasma
plasma, see later slides), the pressure is
10-4Torr or even lower (cannot sustain a
plasma), leading to large mean free path,
high ion energy and high milling rate.

7
 Sputter etching and ion milling

Problems associated with sputter etching (or any etching that has a
high degree of physical/ionic etching):
a) trenching at bottom of sidewalls;
b) redeposition of photoresist and other materials;
c) charging and ion path distortion.

8
 Chemically assisted ion beam etching system

• RIBE: reactive IBE, reactive gases are • CAIBE: chemically assisted ion beam etching,
introduced into plasma region together with inert Ar ion, neutral reactive gas is introduced
Ar gas, so they are ionized. RIBE is virtually into lower chamber, so not ionized, though
the only example where the same ion has some may be ionized due to backflow into
both a physical (ion impact) and chemical plasma region or bombardment by Ar ion.
(reactive etching) component.
9
 High inhibitor Low inhibitor Example:
deposition rate deposition rate
etching profile of Si or SiO2

Teflon

• Fluoropolymer (like Teflon) in CHF3 or CF4+H2 RIE of Si

or SiO2 is the inhibitor.
• If Ar gas is added, inhibitor is mainly removed by ion
bombardment. So less attack of inhibitor on sidewall.
• If O2 gas is added, inhibitor on sidewall is removed at
faster rate than Ar ion, but the etch of inhibitor at
horizontal surface is even faster.
• Yet at very low temperature, inhibitor SiOxFy (not act
as inhibitor at higher temperature when it is volatile)
forms when O2 is added, which is the mechanism for
fast anisotropic etching of Si using cryo-etcher. (deep
Si etcher, popular for MEMS – micro electro
mechanical systems)

10
Figure 10-14
 Anisotropy due to ion bombardment: summary
• Due to its extremely low density, ions don’t contribute much to etching; neutral radicals do.
• So even with directional ion bombardment, the overall etching can still be pretty isotropic.
• For instance, SF6 etch of Si is very isotropic with large undercut like wet etch.
• To achieve anisotropy, there are two mechanisms:
o Energy-driven anisotropy: bombardment by ion disrupts an un-reactive substrate and
causes damages such as dangling bonds and dislocations, resulting in a substrate more
reactive towards etchant species (electron or photon can also induce surface activation).
o Inhibitor-driven anisotropy: ion bombardment removes the inhibitor layer from horizontal
surface (sidewall remain passivated), and reaction with neutrals proceed on these un-
passivated surfaces only.
One may think that ions won’t help much due to its much lower density than radicals. But ion
has sticking coefficient S1 (every ion bombardment counts), whereas radicals S0.01 (most
radicals hit the surface and left without doing anything).

Inhibitor-driven
Energy-driven anisotropy
anisotropy
11
 Downstream etchers

• Plasma is formed in a cavity which is

separated from the etching chamber.
• Wafers are shielded from bombardment.
• Only neutral free radicals reach wafers.
• Etching is completely chemical and
isotropic.
• High selectivity achievable - Si:SiO2 = 50:1
• Plasma may be generated by RF
(13.56MHz) or by microwave (2.45GHz).

12
 Plasma etching in parallel plate systems – plasma mode

Parallel plate = capacitively coupled plasma (CCP)

• Both chemical and physical etch occur (wafer “in contact” with plasma), though the later is
weak, particularly at higher pressure when DC voltage drop near wafer is smaller.
• Etching is fairly isotropic and selective due to the strong chemical component.

13
 Parallel plate etchers (regular RIE, low density plasma)

• Absolutely the most important form of dry

etching, though recently ICP (see later slides) is
becoming more and more popular.
• Compared to plasma mode: smaller wafer
electrode (counter electrode grounded to
chamber wall), lower pressure (<100mTorr), more
physical bombardment (voltage drop many 100V).
• Ion enhanced etching mechanism, (usually)
directional/anisotropic and selective.

RIE using parallel plate setup is low

density plasma system (ions 108 –
1010/cm3), thus low etch rate.
Here low (ion) density plasma also
implies low density of free radicals.
Thus low etching rate. VERY roughly, one can say that plasma consists of order
14
1% radicals (reactive neutral species) and 0.01% ions.
 Reactive ion etch (RIE)
Schematic RIE process

• Due to its simultaneous anisotropy and

selectivity, RIE is intensively used.
• Works for most semiconductors and
dielectrics.
• OK for few metals that form volatile etch a) Ion sputtering, b) reactive ion etching, c)
products: Al (form AlCl3), Cu (CuCl2) (not radical formation (?), d) radical etching
really), Ti (TiF4, TiCl4), W (WF6), Cr (CrO2Cl2).
(most important)

In RIE, ion energy is low (several 10s eV, << voltage drop near wafer surface, due to collision
energy loss), and its number density is very low, thus negligible etching by ion bombardment.
The name reactive “ion” etching is very misleading since ions don’t contribute directly to
etching – it just “helps” chemical etching.
15
 Ion energy vs. pressure for a plasma
• Lower pressure (<10mTorr) increases mean free
path as well as voltage drop near wafer electrode,
both of which leads to more energetic and
directional ion bombardment, thus more
anisotropic, but less selective and slower etching
rate due to low ion/free radicals density.
• High pressure (>100mTorr), short mean free path,
low voltage drop, isotropic chemical etching.
• Thus it is desirable to have a low pressure plasma
with high ion density.

Plasma mode: >100mTorr

RIE mode: 10-100mTorr
Sputter etching: pressure as low as possible, as long as
plasma can be sustained, but still very slow etching rate.

16
 Electron cyclotron resonance (ECR)
and inductively coupled plasma (ICP)
ECR was introduced in 1985.
ICP was introduced much later (1991- 1995).
Dual plasma source:
Top one (ECR or ICP RF power) generates HDP,
determines ion density/current.
Bottom one (CCP RF power) generates bias voltage like
regular RIE, determines ion energy.

Typical parameters for HDP and conventional plasma etcher

ions/cm3
should be lower

17
CCP: capacitively coupled plasma, parallel plate, used for conventional regular RIE.
 ECR and ICP
Electron cyclotron resonance plasma Inductively coupled plasma (ICP)
(less common nowadays)
ICP RF power
(for dense plasma)

plasma

RF bias power
(similar to RIE, parallel plate)
• High magnetic field in the coil, so electrons move in circles with long path, leading to
higher collision and ionization probability, and much less electron loss to chamber wall
and the bottom plate where sit the wafer. Moreover,
• For ICP, AC magnetic field induces circular electrical field, which accelerates electrons.
• For ECR, DC magnetic field, electron cyclotron =qB/m; electrons accelerated if this
frequency matches the microwave frequency. 18
 Schematic of ECR etcher
Microwave source 2.45 MHz Quartz window
Wave guide
Plasma chamber

Diffuser

Cyclotron magnet
Wafer

Additional magnet

13.56 MHz

Electrostatic chuck

Vacuum system

19
 Schematic of ICP etcher
RF generator Inductive coil

Dielectric
window

Plasma Electromagnet
chamber

Biased wafer chuck Bias RF generator

As you see, there is practically no top plate as in parallel plate regular RIE.
The wafer sees the ICP power – the two power sources are not physically separated.
Otherwise, even though the plasma density in the upper part is high, it will get lost due
to re-combination and de-excitation when it travels through the bottom part. 20
 Magnetically enhanced reactive ion etch (MERIE)
Like regular parallel plate RIE, but magnetic field forces electron to go circles, increasing
collision with gas molecules and decreasing loss to chamber walls or top/bottom plates.
However, now that electrons don’t loss to bottom plate, no or little bias voltage – need to
apply an external bias to accelerate ions.

I haven’t seen any MERIE, so

it is not popular.
On the contrary, magnetron
Electromagnet sputtering is very popular.
(1 of 4) This is probably because
there are many ways to
increase etching rate; but
Wafer sputter without magnetron
is always very slow:
few nm/min, vs. 10s to 100s
Biased wafer chuck nm/min RIE etching rate.

13.56 MHz 21
 Why deep RIE (DRIE)?
• Plasma etching can produce deeper trenches than wet
etching, but with tapered angles.
• Tapered trenches are not desirable in many
applications such as resonators that involve pairs of
“centipedes-like” micro-devices with overlapped
“fingers”.
• DRIE process may produce deep high aspect ratio
structures with vertical sidewall (θ ≈ 0o).
• It is the most important breakthrough in drying etching
Working principle:
in recent years, popular for MEMS (micro electro
• The DRIE process provides thin film
mechanical systems) fabrication.
of a few microns protective coating
on the sidewalls during the etching
process.
• It involves the use of a high-density
plasma source.
• The process allows alternating
process of plasma (ion) etching of
the substrate material and the
deposition of etching-protective
material on the sidewalls.
22
Deep Si etch: ICP -
“Bosch” process
ICP: inductively coupled plasma
ECR: electron cyclotron resonance

Besides Bosch process, the other

very popular deep Si etch is cryo-
etch (i.e. at very low temperature,
order -100oC, SiOxFy as inhibitor).
Often a deep RIE tool can do both
processes.

• Uses high density plasma (ICP is used, but ECR also works) to alternatively etch silicon
and deposit an etch-resistant polymer on sidewalls.
• SF6 etch 5-13 sec; followed by C4F8 fluorocarbon polymer deposition 5-10 sec.
• Etch rate several m/min, capable of etching several hundred m with vertical walls.
• Sidewall is rough, depending on cycle times (longer cycle, more zigzag).
• Process recipe depends on geometry (aspect ratio…).
• More popular for MEMS, less common for nano-fabrication due to sidewall zigzag. 23
 Deep Si etch - Bosch process
Non è possibile v isualizzare l'immagine.

DRIE uses lower energy ions  less

damage and higher selectivity.
Plasma maintained at 0.5 to 3mTorr.

Rough sidewall due to scalloping effect.

20 m deep pores

1 cycle

24
 More examples of deep Si etch

High aspect ratio

Micro-gripper

Zigzag sidewall profile

25
 DRIE issues: “footing”

Due to 200:1 selectivity, the (vertical) etch practically just

stops when it reaches SiO2 stop layer.
Problem: lateral undercut at Si/SiO2 interface  “footing”
caused by charge accumulation at the insulator.

Charging-induced
potential perturbs the
Poor charge electric field, distorts the
relaxation and lack ion trajectory.
of neutralization by
electrons at Result: strong and
insulator leads to localized damage
ion flux into (“footing”) to the
substrate builds up
structure at Si-SiO2
positive potential.
interface. 26
 Summary: plasma etching mechanism
• Chemical etching: free radicals react with material to be removed. E.g. plasma etching at high
pressure close to 1Torr.
• Physical etching or sputtering: ionic species, accelerated by the built-in electric field (self-bias),
bombard the materials to be removed. E.g. sputter cleaning using Ar gas in sputter deposition
system.
• Ion enhanced etching: combined chemical and physical process, higher material removal rate
than each process alone. E.g. reactive ion etching (RIE), which is the most widely used dry
etching technique.

Sputter etching
Plasma etching

Physical
Reactive Ion

Process
Ion milling &
Wet etching
Chemical
Process

etching
Pressure
Energy (power)

Selectivity

Anisotropicity 27
Figure 10-19
Modes of plasma etching

28
 Main issues in plasma etching

PR: photoresist
CD: critical dimension

Etch profiles 29
 Etched profile control

Lateral etch of resist widens the

opening gradually. Ion trajectory problem causes bowing profile
30
 Aspect ratio and micro-loading effect
• Micro-loading: etch rate depends on local pattern density.
• Aspect-ratio dependent etching:
o Etchants are more difficult to pass through the smaller hole.
o products are harder to diffuse out.
• Lower pressure can minimize the effect: more directional and longer mean free path,
easier for etchants to reach the trench/hole bottom and for etch byproducts to get out.

mask
silicon

31
Plasma etch methods for various films: overview

• Most reactant gasses contain halogens: F, Cl, Br, or I

• Exact choice of reactant gasses to etch each specific film
depends on
o Ability to form volatile by-products that can be
removed by pumping
o Selectivity and anisotropy.
• Boiling points are good indicators of volatility of species
o Lower boiling point, higher tendency to evaporate.
o High boiling point may need etching at elevated
temperatures.

32
RIE/plasma etch gases: Typical gases for films used in IC fabrication

Most lab systems have only fluorine-based gases (SF6, CF4, CHF3) since they are relatively
safe; chlorine-based gases are corrosive. Most RIE has Ar and O2 gas, some has H2 and He.
33