This document discusses etching techniques used in semiconductor manufacturing. It describes wet etching where wafers are immersed in chemical etchants, and dry etching techniques like plasma etching and reactive ion etching that use gas phase etchants. Wet etching allows isotropic removal of material but has issues with uniformity and hazardous chemicals. Dry etching allows anisotropic removal and avoids chemical handling, though some gases used are also hazardous. Selectivity and etch rates are important properties that determine process control.

1. ETCHING
NEHA SHARMA
ASSISTANT PROFESSOR
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
ENGINEERING COLLEGE JHALAWAR

2. CONTENTS
■ WET ETCHING
■ DRY ETCHING
■ PROPERTIES OF ETCHING
■ Refer:
• S.M Sze, VLSI technology. McGraw-hill.
• S. K. Ghandhi, VLSI fabrication principles: silicon and
gallium arsenide. John Wiley & Sons.

3. Etching
■ Etching refers to the removal of material from the wafer surface.
■ The process is usually combined with lithography in order to select
specific areas on the wafer from which material is to be removed.
■ Etching represents one way of permanently transferring the mask
pattern from the photoresist to the wafer surface.

4. Types of etching
■ Dry etching
o Occurs in gas phase
o Reactive Ion etching
o Plasma etching
■ Wet etching
o Etching by chemicals in liquid state (etchant)
o Oxidation od reduction reactions for removal

5. WET ETCHING
(liquid state)

6. • In wet etching, the wafers are immersed in
a tank of the etchant (mix of chemicals), as
shown in figure 1.
• There is a chemical reaction between the
wafer surface and the etchants that helps in
material removal.
• Either a photoresist layer or a hard mask
like oxide or nitride layer is used to protect
the rest of the wafer.
• The time for etching depends on the amount
and type of material that needs to be
removed. KOH (potassium hydroxide) is a
common etchant used to remove Si.
• After etching, the wafers are rinsed, usually
in DI water, for removal of etchant and then
finally dried.

7. Etchants
■ For Si etching, KOH is used or a mixture of nitric acid and
hydrofluoric acid (HF).
■ For silicon oxide etching, usually a mixture of HF and
ammonium fluoride (NH4F) is used, called as BOE (buffered
oxide etchants).
■ For silicon nitride, usually a strong acid like hot phosphoric
acid is used at high temperatures (180 ◦C) since it is a very
good passivating layer and hard to remove under normal
conditions.

8. PROPERTIES OF ETCHING
AND PROBLEMS
(Directivity, Selectivity, Etch rate)

9. DIRECTIVITY
OF ETCHING
• ISOTROPIC: IN ALL
DIRECTIONS
• ANISOTROPIC: IN ONE
DIRECTIONS (IDEALLY)

10. 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
• Too short an etch time
• The presence of a surface
layer that slows the etching
process
• A lowered temperature or
weakened etch solution

11. SELECTIVITY

12. PROBLEMS ■ OVER ETCHING AND
UNDERCUTTING
■ Severe undercutting takes place
when
1. Excessive etch time
2. High temperature
3. Strong etchant solution
4. Adhesion between resist and wafer
is weak.

13. PROBLEMS WITH WET ECTHING
1. Wet etching is used for large pattern sizes, usually larger
than 2 µm.
2. It is an isotropic process - sloped sidewalls rather than
straight walls.
3. Wet etch has to be combined with subsequent rinse and dry
steps. This increases chances of defects or contamination.
4. Hazardous chemicals and conditions are used, so safety is
an issue. Safe disposal of chemicals is essential.
5. Undercutting and resist peel off can happen if time is not
controlled or etch conditions change during process.

14. DRY ETCHING
(REACTIVE ION ETCHING, PLASMA ETCHING, ION BEAM MILLING)
etchant gases are the primary medium for the removal of material.

15. WHAT IS A PLASMA ?
■ FORTH STATE OF MATTER
■ WHEN GAS IS EXPOSED TO VERY
HIGH TEMPERATURE OR VERY HIGH
VOLTAGE, ITS MOLECULES BREAKS
INTO IONS AND THEN PLASMA IS
CREATED
■ IT IS AN IONISED GAS.
■ CONDUCTS ELECTRICITY AND CAN
PRODUCE MAGNECTIC FIELD.

16. PLASMA
ETCHING
• The chemical etchant is
introduced in the gas phase.
• For etching silicon oxide, CF4
(tetrafluoromethane) is used.
• The chamber is first evacuated
before introducing the gas.
• Radio frequency (RF) electrodes
are then used to generate the
plasma that ionizes the gas.
• This ionized gas attacks the
oxide layer, removing the layer

18. Ion beam
etching
• Ion beam etching is similar to the
ion beam milling process that is
used for transmission electron
microscopy sample preparation.
• This is a physical process where
ionized inert gas ions (usually Ar)
are used to remove material from
the wafer.
• The process is not selective but it
is highly directional

19. Reactive Ion
Etching (RIE)
• It combines the plasma and ion beam
etching process
• good directionality but low selectivity
• process in which chemical etching is
accompanied by ionic bombardment
• It is accomplished by replacing neutral
gas with one or more chemical species in
RF sputtering environment.
• Plasma is formed by these species.
• Substrate/wafer are normal to the
plasma flow and rf field, thus highly
directional and rapid movement of
ionized particles.

20. How anisotropic nature
/directionality can be increased?
• Using chemicals whose reaction
products have large ionized
components.
• increasing voltage drop across
cathode, as increasing
impingement velocity.
• Reduced pressure reduced
collision with cathode
Why low selectivity?
• Ion bombardment
• Energetic neutral
species isotropic in
nature also patriciates.

21. 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
Disadvantages
■ Some gases are quite toxic
and corrosive
■ Re-deposition of no volatile
compounds
■ Need for specialised
expensive equipment