Page 311 - Sami Franssila Introduction to Microfabrication

290 Introduction to Microfabrication

Wafer Mask

Chuck

BSA splitfield Focusing and storage of
microscope mask alignment marks
Mask
alignment
mark

Wafer
alignment
mark

Focusing of substrate alignment marks

Alignment
Figure 28.2 Double-side alignment. Figure courtesy of Suss Microtech GmbH

aligned to the bottom silicon wafer (photomasks are films on the backside. Single-wafer plasma etching is
glass plates with metal patterns). Bonding of two struc- an obvious choice and if wet etching is preferred (e.g.,
tured silicon wafers requires a tool similar to the double- because of surface quality considerations), the backside
side lithography system. Alignment marks on the first must be protected.
wafer are registered, the second wafer is aligned to those Protection by spin-coated polymers is a quick and
marks and the wafers are then brought to contact. The easy method. Photoresist is suitable for many applica-
critical step is to maintain the alignment while the wafers tions, such as mask oxide etching in BHF, but aggressive
are transferred to the bonding equipment. This is accom- etchants like KOH require either inorganic films (oxide,
plished by a special fixture that fits both the aligner nitride) or more stable polymers. CYTOP (cyclized per-
and the bonder, and therefore, wafers need not be han- fluoropolymer) can tolerate KOH and 49% HF. CYTOP
dled after alignment. Bonding is a process that can be can be removed by oxygen plasma. Blue tape common
repeated: wafer stacks with up to six wafers have been in wafer dicing can also be used as a protective layer, but
made, with ca. 1 µm alignment between the wafers. removal of the tape can be difficult if fragile freestanding
structures are present on the wafer.
A single-wafer holder that exposes only one side
28.1.5 Etching
of the wafer to the liquid is a universal solution. In
Wet etching (and wafer cleaning) in a tank takes place electrochemical etching or deposition, this holder also
on both sides simultaneously. It may be useful to etch provides the necessary electrical contacts to the wafer.
from both sides, either for symmetry reasons, or for However, some wafer front surface area is covered by
doubling the apparent etch rate. If all etching is on the holder, and single-wafer processing is more expen-
one side only, it is mandatory to preserve the protective sive than batch processing. With a holder, the topside