Etching In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films previously deposited and/or the substrate itself. In general, there are two classes of etching processes: Wet etching where the material is dissolved when immersed in a chemical solution Dry etching where the material is sputtered or dissolved using reactive ions or a vapour phase etchant

1. ETCHING
PRESENTED BY : SANGEET KHULE
ROLL NO : 60
COURSE : DIGITAL MANUFACTURING

2. CONTENTS
Etching
o Wet etching
o Dry etching

3. ETCHING
 After a thin film is deposited, it is usually etched to remove unwanted
materials and leave only the desired pattern on the wafer.
 The process is done many times.
 In addition to deposited films, sometimes we also need to etch the Si wafer
to create trenches.
 The masking layer may be photoresist, SiO2 or Si3N4.
 The etch is usually done until another layer of a different material is reached.
ETCHING

5.  DRY ETCHING
 WET ETCHING

6. INTRODUCTION
 Etching can be done ―wet‖ or ―dry‖
 Wet etching
o uses liquid etchants.
o Wafer is immersed in the liquid.
o Process is mostly chemical.
 Wet etching is not used much in VLSI wafer fabrication.

7. Dry etching
o Uses gas phase etchants in a plasma.
o The process is a combination of chemical and physical action.
o Process is often called ―plasma etching‖.
This is the normal process used in most VLSI fabrication.
The ideal etch produces vertical sidewalls.
In reality, the etch occurs both vertically and laterally .

8. There is undercutting, non vertical sidewalls, and some etching of the
Si.
The photoresist may have rounded tops and non-vertical sidewalls.
The etch rate of the photoresist is not zero and the mask is etched to
some extent.
This leads to more undercutting.
Etch selectivity is the ratio of the etch rates of different materials in the
process.

9. Etch selectivity is the ratio of the etch rates of different materials in the
process.
If the etch rate of the mask and of the underlying substrate is near zero, and
the etch rate of the film is high, we get high selectivity.
This is the normally desired situation .
If the etch rate of the mask or the substrate is high, the selectivity is poor.
Selectivity of 25 – 50 are reasonable.
Materials usually have differing etch rates due to chemical processes rather
than physical processes.

10. ETCH DIRECTIONALITY IS A MEASURE OF THE ETCH RATE IN
DIFFERENT DIRECTIONS (USUALLY VERTICAL VERSUS LATERAL).

11. TYPES OF ETCHING
We consider two processes :-
o ―wet‖ etching
o ―dry‖ etching
Wet process is well-established, simple, and inexpensive.
The need for smaller feature sizes could only be met with plasma
etching.
Plasma etching is used almost exclusively today.

12. WET ETCHING
In wet process by immersing the wafer in these chemicals, exposed
areas could be etched and washed away.
For SiO2, HF was used.
Wet etches work through chemical processes to produce a water
soluble byproduct.

13. In some cases, the etch works by first oxidizing the surface and then
dissolving the oxide.
An etch for Si involves a mixture of nitric acid and HF.
The nitric acid (HNO3) decomposes to form nitrogen dioxide (NO2).
The SiO2 is removed by the previous reaction.
The overall reaction is

14. SIMPLE IDEA OF WET ETCHING

15. Buffers are often added to keep the etchants at maximum strength
over use and time
Ammonium fluoride (NH4F) is often used with HF to help prevent
depletion of the F ions
This is called Basic Oxide Etch (BOE) or Buffered HF (BHF)
The ammonium fluoride reduces the etch rate of photoresist and helps
eliminate the lifting of the resist during oxide etching
Acetic acid (CH3COOH) is often added to the nitric acid/HF Si etch to
limit the dissociation of the nitric acid

16. Wet etches can be very selective because they depend on chemistry
The selectivity is given by
Material ―1‖ is the film being etched and material‖2‖ is either the mask
or the material below the film being etched
If S>>1, we say the etch has good selectivity for material 1 over
material 2

17. Most wet etches etch isotropically.
The exception is an etch that depends on the crystallographic
orientation.
Example—some etches etch <111> Si slower than <100> Si.
Etch bias is the amount of undercutting of the mask.
If we assume that the selectivity for the oxide over both the mask and
the substrate is infinite, we can define the etch depth as ―d‖ and the
bias as ―b‖.

18. We often deliberately build in some over etching into the process.
This is to account for the fact that
o The films are not perfectly uniform
o The etch is not perfectly uniform
The over etch time is usually calculated from the known uncertainties
in film thickness and etch rates
It is important to be sure that no area is under-etched; we can tolerate
some over- etching

19. DRY ETCHING (PLASMA ETCHING)
Plasma etching has (for the most part) replaced wet etching
There are two reasons:
o Very reactive ion species are created in the plasma that give rise to
very active etching
o Plasma etching can be very anisotropic (because the electric field
directs the ions)

20. Plasma systems can be designed so that either reactive chemical
components dominate or ionic components dominate.
Often, systems that mix the two are used
o The etch rate of the mixed system may be much faster than the sum
of the individual etch rates.
A basic plasma system is shown in the next slide

21. DRY ETCHING MECHANISMS
The main species involved in etching are
o Reactive neutral chemical species
o Ions
The reactive neutral species (free radicals in many cases) are
primarily responsible for the chemical component
The ions are responsible for the physical component
The two can work independently or synergistically

22. DRY ETCHING

23. If the area of the electrodes is the same (symmetric system) we get
the solid curve of 10-8.
The sheaths are the regions near each electrode where the voltage
drops occur (the dark regions of the plasma).
The sheaths form to slow down the electron loss so that it equals the
ion loss per RF cycle.
In this case, the average RF current is zero.

24. The heavy ions respond to the average voltage
The light electrons respond to the instantaneous voltage
The electrons cross the sheath only during a short period in the cycle
when the sheath thickness is minimum
During most of the cycle, most of the electrons are turned back at the
sheath edge
The sheaths are thus deficient in electrons
They are thus dark because of a lack of light- emitting electron-ion
collisions

25. DRY ETCHING
For etching photoresist, we use O2.
For other materials we use species containing halides such as Cl2, CF4, and
HBr.
Sometimes H2, O2, and Ar may be added.
The high-energy electrons cause a variety of reactions.
The plasma contains
o Free electrons
o Ionized molecules
o Neutral molecules
o Ionized fragments
o Free radicals

26. In CF4 plasmas, there are
o Free electrons
o CF4
o CF3
o CF3
+
o F
CF and F are free radicals and are very reactive
Typically, there will be 1015 /cc neutral species and 108-1012 /cc ions
and electrons

27. Features of this system
o Low gas pressure .
o High electric field ionizes some of the gas.
o Energy is supplied by 13.56 MHz RF generator.
o A bias develops between the plasma and the electrodes because the
electrons are much more mobile than the ions (the plasma is biased
positive with respect to the electrodes)

28. ADVANTAGES OF DRY ETCHING OVER WET
ETCHING
o Eliminates handling of dangerous acids and solvents.
o Uses small amounts of chemicals.
o Anisotropic etch profiles.
o High resolution and cleanliness.
o Less undercutting.
o Better process control.