3 Materials and Processing of TSV
55
of the hole, so that the diameter of the hole does not increase as the depth of the
hole is increased. With the help of schematic shown in Fig. 3.6, we describe different
steps of this process below:
1. The top layer of the Si wafer is patterned using standard photolithography (see
Fig. 3.6a). The plasma assisted etching process, as described in (b), does not etch
the polymeric photo-resist coated over the Si wafer. A SiO 2 layer may or may not
be grown on top of Si wafer.
2. The Si wafer is etched by highly energetic plasma of a suitable gas, e.g., SF 6 ,
CF 4 , NF 3 , Cl 2 , etc. (see Fig. 3.6b) [21]. The gas creates a plasma consisting of
electrons and neutral species as well as positively charged species. The positively
charged species, which accelerate under an applied electric field, impinge on the
Si surface normal to its motion path at high speed, removes any passivation layer
or debris from there, and then reacts with Si [21]. Interestingly, due to the high
momentum of the plasma, it also removes the formed chemical compound from
the end of the hole in Si. The most common etching gas (and also the gas originally
suggested in the Bosch process) is sulfur hexafluoride, SF 6 [32]. The following
chemical reaction occurs between SF 6 and Si:
Si + 2SF 6 (g) −→ SiF 4 (g) + 2SF 4 (g)
T > 100
◦ C
(3.1)
2Si + 2SF 6 (g) −→ 2SiF 4 (g) + S 2 F 4 (g)
T < −30
◦ C
(3.2)
Since all reaction products in above equations are gaseous, i.e., volatile, Si atoms
are gradually removed from the wafer as the above process continues. The above
reactions are highly exothermic and hence Bosch process is often performed at
low temperatures (<5
◦ C). In addition, a good and uniform etch rate is achieved
if the temperature is uniformly maintained.
3. Following the above etching process, another gas, for example C 4 F 8 , C 4 F 6 , etc., is
passed over Si for depositing a thin layer of passivation polymer layer on the walls
Fig. 3.5 A high aspect ratio
blind hole etched in Si wafer
by cryogenic plasma etching
process [31]
55
of the hole, so that the diameter of the hole does not increase as the depth of the
hole is increased. With the help of schematic shown in Fig. 3.6, we describe different
steps of this process below:
1. The top layer of the Si wafer is patterned using standard photolithography (see
Fig. 3.6a). The plasma assisted etching process, as described in (b), does not etch
the polymeric photo-resist coated over the Si wafer. A SiO 2 layer may or may not
be grown on top of Si wafer.
2. The Si wafer is etched by highly energetic plasma of a suitable gas, e.g., SF 6 ,
CF 4 , NF 3 , Cl 2 , etc. (see Fig. 3.6b) [21]. The gas creates a plasma consisting of
electrons and neutral species as well as positively charged species. The positively
charged species, which accelerate under an applied electric field, impinge on the
Si surface normal to its motion path at high speed, removes any passivation layer
or debris from there, and then reacts with Si [21]. Interestingly, due to the high
momentum of the plasma, it also removes the formed chemical compound from
the end of the hole in Si. The most common etching gas (and also the gas originally
suggested in the Bosch process) is sulfur hexafluoride, SF 6 [32]. The following
chemical reaction occurs between SF 6 and Si:
Si + 2SF 6 (g) −→ SiF 4 (g) + 2SF 4 (g)
T > 100
◦ C
(3.1)
2Si + 2SF 6 (g) −→ 2SiF 4 (g) + S 2 F 4 (g)
T < −30
◦ C
(3.2)
Since all reaction products in above equations are gaseous, i.e., volatile, Si atoms
are gradually removed from the wafer as the above process continues. The above
reactions are highly exothermic and hence Bosch process is often performed at
low temperatures (<5
◦ C). In addition, a good and uniform etch rate is achieved
if the temperature is uniformly maintained.
3. Following the above etching process, another gas, for example C 4 F 8 , C 4 F 6 , etc., is
passed over Si for depositing a thin layer of passivation polymer layer on the walls
Fig. 3.5 A high aspect ratio
blind hole etched in Si wafer
by cryogenic plasma etching
process [31]