1. RIE - Reactive Ion Etching
Yannik Junk
FYS Project

2. (chemical and physical)
Why RIE?
Desired properties of etching processes:
Anisotropy
Selectivity
Wet etching: Selective, but not anisotropic
Dry etching:
Plasma etching (chemical)
Selective
Not anisotropic
Ion beam etching (physical)
Not selective
Anisotropic    
 Reactive ion etching
(chemical and physical)
Selective
Anisotropic  

3. Reactive ion etching - RIE
Plasmas are utilized for etching
Two different types of reactors: Capacitively coupled (CPP) and inductively coupled (ICP) plasma reactor
Capacitively coupled plasma reactor:
Gas injected between two capacitor plates
RF voltage ionizes the gas and generates the plasma
High RF frequencies  Electrons are strongly accelerated and hit the electrodes
Depletion of the electrons at the upper electrode causes an effective dc bias voltage between the electrodes
Ions are accelerated towards the lower electrode

4. Reactive ion etching - RIE
Inductively coupled plasma reactor:
Plasma generated by a magnetic field induced by a coil
Sample holder coupled to a second RF generator
Possible to control the plasma density independently of the dc bias
Advantages:
Anisotropic etching at higher rates
Less damage

5. Requirements for successful etching
Volatile etching products
Appropriate masking material
Choice of pressure and temperature
Choice of plasma power

6. Usual chemistries: Silicon
Chlorine:
Advantage of Cl-based RIE: Highly anisotropic due assistance of ion bombardment
Undoped Si is etched very slowly in Cl-based chemistries without ion bombardment
Highly n-doped Si etches very quickly in Cl2-atmospheres
Flourine:
Possible etch gases are SF6, CF4, SiF4, NF3, XeF2 and F2
Problem: Isotropic etching
 Bosch process

7. Etching until the passivation layer is gone
The Bosch process
Etching until the passivation layer is gone
Passivating the walls
e. g. SF6
e. g. C4F8

8. Etching of III-V semiconductors
Flourides of elements III are not volatile
F-based chemistries are not suitable for etching of III-V semiconductors
Cl-based chemistries are used
Mixtures of Cl- and F-based etchants might be used

9. Annoying effects in RIE
Loading effect:
Change of the etch rate with the sample size
Caused by a faster depletion of etchants for large samples
Strongly dependent on the respective chemistry and the material of the lower electrode
If the electrode can be etched, it is contributing to this effect
For similar etch rates for different sample sizes, the sample to be etched can be positioned on a large sacrificial sample of the same material

10. Annoying effects in RIE
Lag effect:
When etching with a mask, etch rates in large openings are higher than in small openings
Caused by differences in diffusion depending on the size

11. Annoying effects in RIE
Micro-masking:
Formation of nano-pillars on etched surfaces
Can be caused by etching products with low volatility (e. g. InCl3 when etching InP in Cl2-based chemistry
Solution: Decreasing the cooling of the sample

12. Annoying effects in RIE
Micro-masking:
Different origin: Induced by interplay of substrate and etching chemistry
Example: semi-insulating InP etched in Cl2:CH4:H2
Formation of “whiskers” on top of the sample
Caused by heating  Polymerization of CH4
Can be remedied by thermal coupling between the sample and the (cooled) electrode

13. Sources
F. Karouta: A practical approach to reactive ion etching. J. Phys. D: Appl. Phys. 47, (2014)
H. Jansen et al.: A survey on the reactive ion etching of silicon in microtechnology. J. Micromech. Microeng. 6, (1996)
Image sources:
kiel.de/matwis/amat/semitech_en/kap_7/backbone/r7_2_2.html
J. Karttunen, J. Kiihamäki, S. Franssila: Loading effects in deep etch silicon. Proc. SPIE 4174, Micromachining and Microfabrication Process Technology VI
S. A. Campbell: Fabrication engineering at the micro- and nanoscale. Fourth edition, Oxford University Press, 2013