Biomedical Engineering Reference
In-Depth Information
Table 4.5
Recipes of Dry Etchant Gases for Thin Films of Functional Materials (After
[3]
)
Material
Etchant Gases
Selective To
Si
BCl
3
/Cl
2
, BCl
3
/CF
4
, BCl
3
/CHF
3
,Cl
2
/CF
4
,Cl
2
/He, Cl
2
/CHF
3
, HBr, HBr/Cl
2
/
He/O
2
, HBr/NFl
3
/He/O
2
, HBr/SiF
4
/NF
3
, HCl, CF
4
SiO
2
SiO
2
CF
4
/H
2
,C
2
F
6
,C
3
F
8
, CHF
3
, CHF
3
/O
2
, CHF
3
/CF
4
, (CF
4
/O
2
)
Si (Al)
Si
3
N
4
CF
4
/H
2
, (CF
4
/CHF
3
/He, CHF
3
,C
2
F
6
)
Si (SiO
2
)
Al
BCl
3
, BCl
3
/Cl
2
, BCl
3
/Cl
2
/He, BCl
3
/Cl
2
/CHF
3
/O
2
, HBr, HBr/Cl
2
, HJ, SiCl
4
,
SiCl/Cl
2
,Cl
2
/He
SiO
2
Organics
O
2
,O
2
/CF
4
,O
2
,/SF
6
—
Chemical dry etching
uses a chemical reaction between etchant gases to attack the substrate
material. Gaseous reaction products are conditions for this etching concept because deposition of solid
products will protect the surface and stop the etching process. Chemical dry etching is isotropic and
exhibits relatively high selectivity. Etchant gases either can be excited in an RF field to become plasma
or react directly with the etched material. Chemical dry etching is often used for cleaning wafers. For
instance, photoresist and other organic layers can be removed with oxygen plasma.
Table 4.5
lists some
typical recipes of dry etchant gases.
Physical
chemical etching
is further categorized as reactive ion etching (RIE), anodic plasma
etching (APE), magnetically enhanced reactive ion etching (MERIE), triode reactive ion etching
(TRIE), and transmission-coupled plasma etching (TCPE)
[3]
. RIE is the most important technique for
micromachining. Reactant gases are excited to ions. Under low pressures and a strong electrical field,
ions are directed to the substrate surface almost perpendicularly. Therefore, this method can achieve
relatively high aspect ratios. The etch rates lie between the ranges of physical etching and chemical
etching.
Dry etching using plasma is a better process for achieving precisely defined features. However,
most conventional plasma-assisted dry-etching processes are isotropic, which limits their applications
to etching of thin films. The common problem of physical
e
chemical dry etching (or RIE) used in
microelectronics is the trench effect where etch trenches are not vertical. The trench is wider on the top
because the top section of a trench is exposed longer to etching plasma and ions. The wall should be
protected during the dry-etching process to keep trench walls parallel and to achieve a high aspect
ratio. For microchannels, a special technique called deep reactive ion etching (DRIE) is needed for the
fabrication of high-aspect-ratio structure. The DRIE process does not depend on crystal orientation of
the wafers. Two major approaches of DRIE are:
e
Etching assisted by cryogenic cooling;
Alternate etching and chemical vapor deposition.
In the first approach, the substrate is cooled with liquid nitrogen. The cryogenic temperatures allow
reactant gas, such as SF
6
or O
2
, to condense on the trench surface. While the condensation film protects
the sidewall from etching, it is removed at the bottom by ion bombardment. Because the trench bottom
is not protected, it is etched further into the substrate (
Fig. 4.4
(a)).
The second approach uses chemical vapor deposition to protect the sidewalls
[19]
. This technique
was invented and patented by Robert Bosch GmbH in Reutlingen, Germany. Therefore, the technique
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