Fabrication of thin-film transistors on polyimide films - Polyimides and Other High Temperature Polymers - page 458

tronics), negligible shrinkage during circuit fabrication, small coefficient of hu-
midity expansion, low solubility for water, low water and oxygen permeability,
and low surface roughness, become important. The standard process temperature
for the fabrication of amorphous silicon (a-Si:H) based electronics is ~ 250-
350ºC. If the process temperature is lowered somewhat, the high-temperature
polyimides meet well many of the requirements listed above.

2. SUBSTRATE SELECTION

A key aspect of using organic polymers as substrates for TFT electronics is the
initial surface passivation. The passivation layer “seals” the polymer foil and con-
verts the chemistry of the polymer to the chemistry of the passivating material. A
standardized substrate chemistry is important, because device fabrication requires
complex chemical sequences that cannot be altered to adapt their compatibility
with various substrate materials. Semiconductor devices are highly susceptible to
contamination, and the passivation layer substantially reduces the possibility of
contamination of the TFT layers during their growth, caused by the outgassing,
and oxygen and water release from the polymer foil. The passivation layer also
serves as an adhesion layer between the TFT layers and the substrate.
During TFT fabrication the polymer foil may be temporarily attached to a rigid
substrate, such as a silicon or glass wafer, or used as a free-standing substrate [13-
19]. If it is attached, the thermal expansion of the compliant foil substrate during
temperature cycling will be constrained by the coefficients of thermal expansion
of the stiff glass (

= 2.5 x 10 -6 /ºC).
These are much lower than those of organic polymer substrates and, therefore, are
more suitable for the TFT fabrication. While using temporary rigid substrates and
bonding agents is acceptable in the laboratory, less labor-intense techniques for
carriers will be needed in manufacture. After the fabrication the polymer foil that
now carries the TFTs is detached from the rigid substrate. If a free-standing
polymer foil is processed, its coefficient of thermal expansion becomes the most
important parameter of all. It has to be well matched to the coefficient of thermal
expansion of the TFT layers. Also the built-in stress in all TFT layers must be
very well controlled. Mismatch in the coefficients of thermal expansion between
any TFT layer and the polymer substrate and built-in stress may result in severe
substrate curving. Solving these aspects of circuit fabrication on free-standing
polymer foils will be important for future roll-to-roll manufacturing.
We selected polyimide Kapton ® E as a substrate for a-Si:H TFT fabrication. It is
stable to process chemicals, has a glass transition temperature > 350ºC, a coeffi-
cient of thermal expansion of 12x10 -6 /ºC [20], and RMS surface roughness
of ~ 30 nm [21]. A 75-

= 3.76 x 10 -6 /ºC for Corning 1737) or Si wafer (

α α

m thick foil shrinks ~ 0.04% after 2 hours at 200ºC, has a
relatively low humidity expansion coefficient of 9x10 -6 /%RH, a water permeabil-
ity of 4 g/m 2 /day, and oxygen permeability of 4 cm 3 /m 2 /day [20].

µ