Plasma Etching

1. Plasma Etching
By Student : EE576
(VLSI Processing)
Minh Anh Thi Nguyen
11/8/01

2. Outline
•What is plasma Etching
•Parameters in plasma
•Isotropic and Anisotropic
•Types of plasma etching system
•Reactive plasma etching and equipment
•Processes of plasma etching
•Advantages and disadvantages of plasma etching
•How plasma etching works
•Why use plasma etching

3. When Plasma Etching applies in
Microelectronic
•By the early 1970s, plasma etching was first
widely adapted to device manufacturing, which
was mainly oxygen plasma. At this time, it was
primarily used for photo resist ashing.
•By the late 1970s, it was widely recognized that
plasma etching can offer the possibility of
anisotropy etching, then rapidly increasing
microelectronic industry drives the transition from
wet etching to plasma etching and continuously
promotes its development.

4. What is Plasma Etching
•Also known as dry etching
•One of the most important processes in IC
manufacturing
•A key process for removing material from surface
•has a number of advantage over chemical etching
•uses a gas that is subjected to an intense electric
field
• the only commercially usable technology for
anisotropy removal of material from surface

5. Types of plasma etching system
1.None plasma based
uses spontaneous reaction to appropriate
reactive gas mixture
2. Plasma based
uses radio frequency (RF) power to drive
chemical reaction

6. None plasma based
•Isotropic etching of silicon
•Typically fluorine-containing gases
(interhalogens) that is readily to etch si
•Highly selectivity making layers
•No need processing plasma equipment

7. Plasma based
•RF power is used to drive chemical
reactive
•Plasma takes place of elevated temperature
or very reactive chemicals

8. Isotropic etching
•A process that etches at the same rate in all
direction
•Very good pattern transfer
•Poor selectivity
•Can etch anything
•High etch rate

9. Anisotropic etching
•A process that etches only in one direction.
•Preferable for smaller feature dimension

10. Isotropic Vs. Anisotropic
•Isotropic: vertical and horizontal
•Anisotropy: much higher vertical rate than
horizontal

Isotropic Anisotropic

11. Isotropic vs. Anisotropic
•hf = thickness of the thin film
•l= lateral distance etched underneath
•Af = degree of anisotropic
•Af = 1- l/hf
Anisotropic
•Af = degree of isotropic
•hf = l
•Af = 0
isotropic

12. Plasma etching
•There are five classes of plasma etching
mechanisms
–Sputter etching
–Chemical etching
–Accelerated ion-assisted etching
–Sidewall-protected ion-assisted etching
–Reactive ion etching (RIE)

13. Sputter etching
•Highly anisotropic
•A purely physical process
•Very similar to ion implantation but low energy
ions are used to avoid implantation damage

14. Chemical etching
•isotropic
•Plasma is used to produce chemically reactive
species (atoms radical and ions) from inert
molecular gas.
•Production of gas by products is extremely
important

15. Accelerated ion –assisted etching
•Like the sputter etching ions are accelerated
by the sheath potential

16. Sidewall-protected ion-assisted
etching
•Can be anisotropic
•Involves additional species to create a
protective sidewall barrier.

17. Reactive ion etching
•Process in which chemical etching is accompanied by no undercutting
since side-wall are not exposed (ie. Ion assisted etching)
•Bombardment opens areas for reactions
•Low selectivity

18. Parameters in plasma
•Temperatures: etching rate and directivity
•Pressure: ion density and ion directivity
•Densities :charged and neutral particles
•Power: ion density and ion kinetic energy
•Other variable: gas flow rate, loading,
reactor material and masking material

19. Plasma etch process temperature
control
•Process temperature is the single most
important parameter in the plasma process.
•Process temperature has primary control
over etch rate and has a secondary effect on
etch uniformity
•the temperature at which the process
operates has a major influence on
processing rates.

20. Plasma temperature control
(cont.)
•The higher the process temperature, the
faster the processing rate.
•Process temperature control is mandatory
when processing temperature sensitive
devices.
•Uncontrolled process temperature can cause
distortion.

21. Low pressure
-To help etching by product diffuse out the
via
- The mean free path lengths of gas molecules
and ion are longer and this reduces
scattering collision that can cause a loss in
profile control

22. How Plasma etching works
•Chamber is evacuated
•Chamber is filled
with gases
•RF energy is
applied to pair
of electrodes

23. How Plasma etching works
•Applied energy accelerates electron increasing kinetic
energy
•Electron collide with neutral gas molecules forming ions
and more electrons
•Plasma discharge is bulk region and dark or sheath region
near electrodes
•Bulk region is semi-neutral or nearly equal to number of
electrons and ions
•Sheath region is nearly all of the potential drop;
accelerates “+” ions from bulk region which bombard the
substrate
•Maintained at 1 Pa(75mtorr) to 750 Pa (56torr) with gas
density of 27x10ˆ14 to 2x 10^17 molecules / cm^3

24. Reactive plasma etching and
equipment
•Barrel reactor
•Reactive Ion Etcher (RIE)
•Magnetic-Enhanced RIE (MRIE)
•electron cyclotron resonance plasma etcher
(ECR)
•inductively coupled plasma (ICP)
•Clustered plasma processing

25. Barrel Reactor
•The main components of the barrel reactor
is the cylindrical vacuum chamber with a
pair of RF electrodes concentrically inside

26. RIE system
•Asymmetrical parallel plate system
•Plasma, sheath and boundary layer
•Combination physical and chemical etching
•Advantage: Anisotropy etching
•Disadvantage:
low etch rate
low selectivity
Surface damage

27. MRIE system
•Advantage:
large anisotropic
high etch rate
reduced surface damage
etching relative independent of loading

28. ERC system
•Higher plasma density at low pressure
•Control the density of
reactive ion and their
kinetic energy separated

29. ICP system
•Simple system
•Almost same process
result as that from
ERC system
•Two RF power
generators to control
ion energy and ion
density separately

30. Clustered plasma system
•Use a wafer handle to pass wafers from one
process chamber to another in a vacuum
environment
•can also increase throughput, and provide an
advantage of high chip yield, since wafer is
exposed to less contamination.
• minimize particulate contamination

31. Processes of plasma etching

32. Advantages of Plasma etching
•Easy to control and reproduce
•Operated in low pressure chamber
•No liquid waste
•Uses small amounts of chemicals
•Eliminates handling of dangerous acids
•can be highly selective to underlying layers
•causes little damage to the photo resist

33. Disadvantages of plasma etching
•the wafer can be damaged from the RF
radiation.
•Tendency for isotropic etching
•poor pattern transfer
•Some gases are quite toxic
•Expensive equipment

34. When do we want to use plasma
etching
•The first thing you should note about this
technology is that it is expensive to run
compared to wet etching.
•Need vertical side walls for deep etching in
the substrate
•Adopted plasma etching to achieve small
features

35. Why use plasma etching?
•Fast
•Selective
•Low damage
•Anisotropic

36. References
•Glow Discharge Processes
•http://personal.cityu.edu.hk/~appkchu/AP4120/6.PDF
•An introduction to plasma etching for VLSI
circuit technology
•Plasma etching processes for ULSI
semiconductor circuits

37. Thank You