Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 121

In an anisotropic wet-etching process, hydroxides react with silicon in the following steps [7] :

2

2OH e / Si þ Si ð OH Þ þ 4e e

4e e / 4OH e þ

(4.5)

4H 2 O þ 2H 2

2

4OH e / SiO 2 OH 2 2

Si ð OH Þ þ þ H 2 O :

The overall reaction is

2 2

2OH e þ

Si þ 2H 2 O / Si ð OH Þ þ 2H 2 : (4.6)

In the steps of (4.5), four electrons are transferred from each silicon atom to the conduction band.
The presence of electrons is important for the etching process. Manipulating the availability of
electrons makes a controllable etch stop possible. Silicon etchants, which can provide hydroxide
groups, are categorized as [7] :

Alkali hydroxide etchants e KOH, NaOH, CsOH, RbOH, or LiOH;

Ammonium hydroxide etchants e ammonium hydroxide NH 4 OH, tetramethyl ammonium

hydroxide (TMAH) (CH 3 ) 4 NOH;

Ethylene diamine pyrochatechol (EDP, which is hazardous and causes cancer, and should be
accompanied by safety measures) a mixture of ethylenediamine NH 2 (CH 2 ) 2 NH 2 ,

e

pyrochatechol C 6 H 4 (OH) 2 , and water; and

Other etchants hydrazine/water and amine gallate etchants.

e

Silicon atoms in {111}-planes have stronger binding forces, which make it more difficult to release
electrons from this plane. This fact leads to the low etch rates of {111}-planes. Anisotropy or
orientation dependence is caused by the different etch rates in different crystal planes. Table 4.3 [8,17]
compares the most important parameters of common anisotropic etchant solutions. KOH offers the
best selectivity between the {100}-plane and the {111}-plane. However, KOH attacks aluminum
structures on the wafer. TMAH etches faster in the {111}-plane but does not attack aluminum.

Table 4.3 Characteristics of Different Anisotropic Wet Etchants

Characteristics KOH NH 4 OH TMAH EDP Hydrazine

References [8,10] [11,12] [13,14] [9] [15,16]

See a See b

Concentration (weight%) 40 50 1 18 10 40

e e e

Temperature ( C)

80 75 90 90 70 97 100

e e

{111} etch rate (nm/min) 2.5 5 — 20 60 5.7 17 2

e e e

{100} etch rate (mm/min) 1 2 0.1 0.5 0.5 1.5 0.2 0.6 2

e e e e

{110} etch rate (mm/min) 1.5 3 — 0.1 — —

e

Si 3 N 4 etch rate (nm/min) 0.23 — 1 e 10 0.1 —

SiO 2 etch rate (nm/min) 1 e 10 — 0.05 e 0.25 0.2 0.17

Al attack Yes No No Yes —

a 1 L ethylene diamine NH 2 e CH 2 e CH 2 e NH 2 , 160 g pyrocatechol C 6 H 4 (OH) 2 , 6 g pyrazine C 4 H 4 N 2 , 133 mL H 2 O.
b 100 mL N 2 H 4 , 100 mL H 2 O (explosive, very dangerous!).