Biomedical Engineering Reference
In-Depth Information
micromachining. See Further Reading at the end of this chapter for references on etching
and microfabrication.
1.4.2 Deposition and “Lift-Off”
he photoresist pattern is almost never the last step of a microfabrication process. For example,
it can be used to mask the deposition or growth of another layer (such as a uniform gold thin
ilm), as shown in
Figure 1.10
; the photoresist is removed with a solvent, a procedure known as
“
lit-of
.”
As an example, let us consider the creation of a suspended membrane of silicon nitride across
a silicon cavity. It would require the following steps, approximately:
Deposition of a silicon nitride layer on a silicon wafer using a high-temperature
furnace.
Photolithography to create a photoresist pattern atop the silicon nitride layer.
Dry etch of the silicon nitride (that is not protected by photoresist) until the silicon
substrate is reached; typically, a reactive gas SF
6
would be introduced into the plasma
etcher chamber.
Removal of the photoresist with acetone or with an oxygen plasma; at this point, the
silicon nitride layer would contain the same pattern as the original photoresist layer.
Wet etch of the silicon (that is not protected by the silicon nitride) with an aqueous
solution containing a strong base such as KOH, which etches the Si(100) crystalline
plane around 100 times faster than the Si(111) plane, resulting in a pit with slanted
walls; if the pit is wide enough, the etch can reach the other side of the wafer, creating
a suspended silicon nitride membrane.
his process has been the starting process of many micromachined devices. As a relatively
simple example, consider the fabrication of a silicon tip on a silicon nitride cantilever (
Figure
1.11
). Still, it involves three photolithography (P) steps, two deposition steps (D), three dry-
etching steps (E
d
), and three wet-etching steps (E
w
, not counting the three dissolutions of the
photoresist), in this order: D-P-E
d
-E
w
-E
w
-D-P-E
d
-P-E
d
-E
d
. All the etches used diferent chemi-
cals or gases.
Figure 1.11
only shows the last etching step (a dry etch that forms the Si tip by
Photoresist pattern
Blanket deposition
Selective growth
Lift-off
Metal pattern
FIGURE 1.10
Metal.deposition.and.lift-off.
