Page 322 - Sami Franssila Introduction to Microfabrication

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Processing on Non-silicon Substrates

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29.1 SUBSTRATES loses its stiffness above ca. 500 C (very much depen-
dent on exact composition). Secondly, sodium diffusion
We are already familiar with devices made on non- at elevated temperatures can be detrimental to elec-
silicon substrates: the acoustic resonator of Figure 7.9 tronic devices.
and the passive integrated chip of Figure 24.13 were
Quartz is pure silicon oxide, just like fused silica,
fabricated on glass/fused silica because substrate capac-
so there is no alkali metal contamination risk. While
itances had to be eliminated. The photomask is also
fused silica is glass in the sense of being amorphous,
a microstructure on glass, even though it is not usu- quartz is crystalline, but the word quartz is often used as
ally considered one. It shows many of the issues that shorthand for fused silica. Etching of crystalline quartz.
make non-silicon substrates different: it is square, thick The etching of quartz in HF-based solutions leads to
and made of glass, which is not a well-defined material
like silicon. The coefficient of thermal expansion (CTE) crystal plane-dependent etching, just like silicon etching
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for soda lime glass is 10 ppm/ C (2.6 ppm/ C for Si), in alkaline solutions. This crystallinity has important
and as a photomask material soda lime glass is limited implications for piezoelectric devices, which must be
to applications above 3 µm linewidths in which dimen- oriented along proper crystal axes.
sional control requirements are lax (remember exercise Flat panel displays (FPDs) are the most important
9.3). The big difference in CTE relative to silicon makes devices fabricated on glass, by sales volume. Radiation
soda lime glass unsuitable for anodic bonding. detectors and photodetectors of various designs have
Glasses contain, by definition, alkali metals, usually been made on glass substrates, using a-Si, SiC and
sodium. These alkali ions are essential for some diamond as active materials. Glass substrates have
several advantages from a manufacturing point of view:
applications, such as anodic bonding even though they
they are available in large sizes; 50 × 60 cm is fairly
are detrimental to electronic devices. Pyrex glass has
typical, and 140 × 185 cm is available. Secondly, glass
composition SiO 2 :B 2 O 3 :Al 2 O 3 :Na 2 O in the approximate
ratio 80:10:5:5. Pyrex glass is available in round formats is cheap. Thirdly, it is fairly smooth and can be cleaned
and is extensively used in anodic bonding, because its with RCA-cleans just like silicon wafers; in fact, the
CTE matches that of silicon. In photoactive glasses there RCA-clean was invented for glass cleaning in TV-
are also lithium and other exotic metals, which are major picture tube manufacturing.
contamination risks. Photoactive glasses have CTEs four Some problems of non-silicon substrates are related to
times that of silicon, which excludes anodic bonding. processing them in a silicon-oriented lab. Even though
Fused silica is 100% SiO 2 and is quite compatible with fused silica wafers are round like silicon, have flats
silicon processes. It is mechanically strong enough to like silicon and are available in the same thicknesses
withstand standard high-temperature process steps and as silicon, complications can still arise, especially in
it is available up to the 300 mm wafer size, which has automated tools. The detection of the presence and the
made it the material of choice for some silicon-based movements of wafers are based on either optical or
optical devices. However, because of the lack of mobile capacitive sensors, and these are fooled by transparent
ions, it is not amenable to anodic bonding. dielectric wafers. Amorphous silicon or polysilicon
The limited temperature range available for process- deposition on the wafer backside can be used as a
ing is a hindrance for processing on glass. This comes preventive measure, but the role of this extra film needs
from two main factors: glass is mechanically soft and it to be considered for all process steps and tools.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)