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軟性電子顯示器
Parylenes for flexible electronics and display
applications
A chemical vapor deposition (CVD) polymer technology has been
developed to use low cost monomers and provide highly crystalline
and thermally stable polymer films for the sub-65μm ICs and OLED
display applications. This thin film is expected to be a critical
enabler for making flexible touch screens, TFTs, solar cells and
displays, and very thin fuel cells. The applications include sheet
computers, e-books, e-paper, actuators, sensors, memories, RFID
tags, solar panel and batteries
.
New reactor chemistries have been developed ,
so low-cost monomers can be used to make various parylenes that
have a general composition of (I) (-CX2-Ar-CX2-)n, wherein, Ar is
an aromatic moiety, X can be H or F. CVD equipment that uses the
reactor chemistries for cracking monomers is commercially
available for deposition of various parylenes, including PPX-N and
PPX-F (Ar =C6H4, X=H & F). PPX-F has a higher thermal stability
and mechanical strength than any existing parylenes, including PPX-N.
It has been qualified for commercial applications as a low-k
intermetal dielectric (IMD) for sub-65nm ICs, and as encapsulation
of OLED displays. It is now undergoing evaluations for
manufacturing of flexible and transparent touch screen and
thin-film transistors (TFT), solar panels, and fuel cells.
Over the past 50 years, various parylenes or poly-para-xylylenes
have been made using the known Gorham method that involves thermal
cracking of dimers (-CX2-Ar-CX2-)2 in vacuum. Due to the presence
of the bucked benzene rings, dimers have high ring strain energy
(31 Kcal/Mol for Ar = C6H4, X=H in) , thus during synthesis of
dimers, formation of parylene is favored over that of dimers. To
gain high yield, low concentration of monomers (Y-CX2-Ar-CX2-Y,
Y=Br or I) in large equipment is employed to prevent
multi-molecular collisions of reactive diradical intermediate
(*CX2-Ar- X2C*, *= unpaired electron) and formation of parylenes.
The presence of bulkier F groups prevents the F-dimers (Ar= C6H4,
X=F in (II)) from becoming commercially available in large scale.
New monomer chemistry and implications
Before the invention of the Gorham method, many attempts were made
to thermally crack low cost monomers in vacuum. However, even the
temperature of the tubular reactor, or “cracker” was as high as
1000°C. Due to poor thermal conductance and the short residence
time (a few seconds) of the monomers in a vacuum of a few mTorrs,
these efforts resulted in a large amount of carbon deposit inside
the reactor and many C-Y bonds at polymer chain-ends. The
resulting parylenes have low molecular weight and poor thermal
stability (<150?200°C) and are useless for industrial
applications.
New reactor chemistry
Since 2002, new reactor chemistries have been
used to convert monomers into diradical (*CX2-Ar- X2C*) with more
than 99.9% yield. This was accomplished even though the reactor
temperature was only about 660°C, similar to processing conditions
for cracking of commercial dimers.
A newly designed reactor was constructed from a
chosen transition metal and was irradiation heated to the inside
space of the reactor, in contrast to the conductively heated
stainless tube used for cracking dimers. New regeneration
processes have been developed specifically for a given transition
metal to remove carbon deposits inside the reactor. The
regeneration chemistries comprise an oxidation step to remove the
carbon deposit by oxygen and a hydrogen reduction, and a second
step to recover the pure metallic inner surfaces from their metal
oxide state.
To maintain continuous deposition at a constant
rate, a dual-reactor set is used per CVD equipment, so when one
virgin reactor is under operation, the other deactivated reactor
will be under regeneration. To qualify the CVD equipment for
commercial operation, the regeneration chemistries have been fully
tested by continuous depositions of >500 wafers that resulted in a
+/- 2.5% wafer-to-wafer-film-thickness variation at 1σ.
Features of the CVD polymer process
In the CVD process flow, a reactive diradical
is formed in a reactor located outside the deposition chamber. The
diradical is transported from the reactor by diffusion to the
deposition chamber. Inside the chamber, the diradical is adsorbed
onto a low-temperature substrate and polymerizes very rapidly?even
at a substrate temperature as low as ?50°C?to form a polymer thin
film.
This process is capable of making very thin coatings, which is
important for coating applications relevant to nanotechnology. For
the deposition process using a monomer that has a high vapor
pressure at temperatures ranging from 60?80°C, a vapor flow
controller (VFC) kept at 120°C can be used to control not only the
monomers’ feed rate, but also the deposition end point. The
deposition rate of this CVD process depends only on the feed rate
of the monomers and the substrate temperature, or the adsorption
rate of the diradical on the substrate. When the feed rate of
monomers is kept at 3sccm by VFC, the deposition rate of PPX-F on
Si or glass substrate increases from 100 to 3000Å/min., as the
substrate temperature decreases from 10 to ?40°C. Due to good
end-point control using VFC, a consistent thickness of PPX-F film
at 20Å has been demonstrated. Therefore, this CVD process is now
available for making the organic buffer layer for ITO and as the
protective layer for the organic emitter compound.
In contrast, since all conventional dimers are solid and have low
vapor pressure, they need to be heated to temperatures above
120?140°C to obtain sufficient feed rates. But at 140°C,
condensation of dimers still occurred inside the VFC, thus
rendering the VFC useless for flow control. When VFC is above the
140?160°C range, the electronic components inside the VFC failed
quickly. In general, VFC is not used in conventional equipment for
controlling feed rate of dimers. Instead, dimer temperature in a
heated crucible regulates the dimers’ feed rate, while a
mechanical valve controls the deposition endpoint. Existing
commercial equipment is inherently incapable of accurately
controlling low feed rates from 0.1?1sccm due to temperature
fluctuation of dimers in vacuum, or for deposition of very thin
films that have only few Angstroms of thickness, due to the
absence of clear end-point deposition control.
Deposition can occur at a substrate temperature lower than 25°C,
in contrast to conventional CVD, which is important for coating
applications in the coming age of organic-electronics, and is
critical when the substrate is a heat-sensitive organic material.
Furthermore, because there is no plasma present in the deposition
chamber, plasma sensitive devices such as OLED displays can be
coated using the CVD deposition system.
Additionally, in contrast to all plasma CVD systems, the thermal
CVD chamber never needs to be cleaned because it is kept above the
maximum temperature for the adsorption of diradicals, or the
ceiling temperature, Tc. For a diradical derived from the F-dimer,
the Tc is ~10ºC, thus at room temperature, none of the diradical
will form a thin film on the inner wall of the deposition chamber.
Lastly, this CVD deposition is a molecular
layer deposition (MLD) process in that the thin film is formed one
molecular layer at a time on low-temperature surfaces. The MLD
deposition process can fill small gaps with high aspect ratios and
results in conformal coating for products with very complex
topology.
New parylenes
These new reactor chemistries enable the manufacture of newly
designed parylenes that would not be available from the Gorham
process, due to the presence of ring strain energy and unavailable
dimers.
PPX-F applications
Verified applications. The
PPX-F prepared from the patented monomer process resulted in thin
films consisting of more dimensionally stable β crystals with high
crystallinity and have a Young’s modulus up to 14GPa. In contrast,
the dimer process resulted in unstable
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crystal with low crystallinity and a low Young’s Modulus of
3.5GPa. Although the F-dimer process failed, the monomer process
rendered the success integration of PPX-F into sub-65nm ICs .
By coating only 0.1μm of PPX-F on a cathode,
one can avoid using liquid descant and expensive etched glass for
packaging of the resulting OLED displays and achieve a cost
savings >50% for OLED display packaging .
Potential applications. PPX-F exhibits
excellent chemical resistant, electrical insulation and mechanical
properties among all existing polymers. PPX-F also has better
light transparency and is less expensive than clear polyimides
(CPI) . The following summarizes the current studies being
undertaken for applications of PPX-F or PPX-N in flexible
electronics and displays.
PPX-F can replace both SiN, the topcoat for TFT,
and the polyimides (PIM), the alignment layer (AL) for LCD in
active matrix LCD (AMLCD) displays. Compared to SiN, PPX-F has a
lower dielectric constant and a lower leakage current . PPX-F also
has a lower (0%) water adsorption than PI (2-3%) and SiN (0.5?1%).
In addition, since both the topcoat and the alignment layer can be
deposited at one step in a CVD equipment to avoid the high cost
spin-on coating process for PIM , the cost of ownership for
manufacturing LCD can be lowered when PPX-F is used.
PPX-N can be used to bind nanotubes and replace the brittle ITO in
the commercial touch screen for a more flexible and transparent
touch screen (FTTS) . To make the FTTS, the carbon nanotubes (CNTs)
are first spray-coated over a plastic substrate, such as PET or
siloxane-hard-coated polycarbonate (HCPC) . After drying the
surfactant and solvents in coating, a thin layer of PPX-N is
deposited into pinholes in the CNT layer. The PPX-N coating will
bind all the CNTs together, and also bind the CNT layer to the
plastic substrate. For this application, the coating cost,
including the material and equipment is only ~5 cents/m2. Note
that a (PPX-F+CNT) film is a patternable organic conducting layer
that can also be used to fabricate a transparent TFT or make solar
cells .
Currently, to make flexible TFTs , SiO2 is deposited as a thermal
barrier layer to protect a lower cost plastic substrate, such as
PET. The SiO2 barrier layer is needed when PECVD is used to
deposit the ά
H-Si at 400°C and when an excimer laser is used to crystallize the
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H-Si into polycrystalline silicon, or LTPS . However, since the
SiO2 is brittle, an expensive clear polyimide (CPI) and a PES have
been studied for replacing the SiO2/PET substrate. To replace the
brittle SiO2 on PET, the CPI and PES substrate, PPX-F thin film is
used to overcoat the lower cost PET or a silicones-hard coated-PC
(HC PC) for the above applications . Potential advantages of PPX-F
over CPI and PES include:
PPX-F is a better dielectric material. It has lower water
adsorption (0% vs.2%) and the lowest leakage current due to the
absence of polarizable imide-groups in CPI.
PPX-F has lower CTE, so can provide smaller increases in threshold
voltage (Vth) that result from thermal annealing-induced interface
stress .
PPX-F has a higher Young’s modulus, thus it can result in lower
intrinsic stress during fabrication of LTPS.
A PPX-F thin film can be coated over the
cathode and anode as the backings. PPX-F anode backings, after
annealing at 400°C, would have a crystallinity of ~70% and their
polymer chain-to-chain distance would be at ~1.45Å. The annealed
PPX-F thin film is an ideal hydrogen/proton separator to prevent
diffusion of hydrogen through the proton exchange membrane (PEM)
and enter the cathode, and also to prevent diffusion of oxygen
through the PEM to the anode. In principle, the PPXX-F can also be
chemically modified by sulfonation so that the ?SiO3H groups will
be incorporated onto the aromatic moieties of the PPX-F. The
sulfonated PPX-F can be used to replace the Nafions that are
currently used as the PEM inside the membrane electrolyte assembly
(MEA).
Conclusions
The new reactor chemistries described here offer not only a lower
cost process to make existing parylenes, such as PPX-N from
monomers, but also provide parylenes such as PPX-F, that have more
attractive material attributes. Additionally, the new chemistries
offer an opportunity to make non-existing parylenes, such as FF-PPX,
that will have a lower dielectric constant, but higher mechanical
strength and higher thermal stability than PPX-F.
Flexible electronics that include organic TFTs and transparent and
flexible touch screens have become hot research subjects the last
few years. XBH have mentioned many advantages for ultra-light and
thin, flexible displays and electronics and their potential
applications for e-books, e-paper, actuators, sensors, memories,
RFID tags, and solar cells. Also, ultra-thin fuel cells having
much greater efficiencies are also needed for the coming age of
mobile electronics and displays. The CVD polymer technology
described in this article is expected to be a critical enabler for
making these flexible electronics and displays.
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