Developments in Ceramic Materials Research

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Li Chen
technology moved to the Laboratoire d'Electronique de Technologie et de l'Informatique
(LETI), a research arm of the French Atomic Energy Commission, in Grenoble. Earlier field
emission studies showed that electron emission current from the Spindt micro tip field emitter
was unstable, and might trigger irreversible catastrophic break down of the device [6, 9]. This
was a key technological hurdle that slowed down the FED development. To tackle this
problem, Robert Meyer of LETI patented a method for fabricating an array of molybdenum
micro tips with a lateral ballast resistive layer [15]. The ballast resistive layer can effectively
reduce the potential difference between the cathode tip and the gate electrode as more
electron current flows out of the emitter [16]. Therefore, it evens out the electron emissions
from the individual arrays across the display. The ballast resistive layer also helps to reduce
the undesirable electric arcing between the field emitter cathode and phosphor anode, and
thus improve the performance and prolong the life of FED [17, 18]. The LETI group was able
to demonstrate publicly that FEDs constructed with lateral resistive layers could be
manufactured with desired uniformities and had expected lifespans needed for commercial
displays [19]. This stimulated the development of FEDs by many researchers in America,
Europe and Asia in the 1990’s.
However, the difficulty associated with scale-up of the Spindt type micro field emitters is
the large size of electron beam evaporator required to deposit molybdenum micro tips. To
ensure the evaporant reaches the cathode faceplate to form sharp cone tips, the distance
required between the molybdenum source and the sample panel holder must be large. A study
performed at Samsung showed that for an evaporator with throw of 72 cm, an angle variation
of 0.8° resulted in a tip radius of curvature change of 2 nm over a 6 cm distance along the
substrate [20]. This tip radius variation resulted in a 75% decrease in emission current for tips
at the edge of the substrate versus those tips in the centre. The Spindt type micro tips based
FED belongs to the first generation FEDs, which relay on thin film technology and
semiconductor processing methods. Despite its success in achieving performance, it is still a
high cost technology. In addition, it suffers from scalability problem.
In the development of second generation FEDs, researchers have directed efforts towards
reducing the overall display cost by replacing the refractory metal emitter material with
alternative cold field emitters. If the switching voltage for the alternative field emitters is
reduced below the Spindt type micro tip cathode required gate voltage value, which is
typically between 20~60 V, this allows the use of smaller and cheaper driving circuits.
Research has been investing materials which possess either a lower work function or material
structures with larger intrinsic electric field enhancement.
Among the investigated material candidates of field emission cathode for FED
application, carbon based materials have experimentally demonstrated the most encouraging
field emission properties [21, 22]. Early investigation into diamond showed excellent field
emission property, which is believed benefit from the property called negative electron
affinity [23], which is related to the material’s work function. However, the challenge here is
how to pump electrons into the conduction band for these materials. Diamond like carbon
materials also have negative electron affinity, and electrons can be emitted into the vacuum at
a very low electric field from the materials [24]. Subsequent investigation into pure graphitic
materials including nanostructure graphite was also proofed to be good field emission
materials [25].
Since the multi-walled carbon nanotubes was discovered by Sumio Iijima in 1991 [26]
and the single walled nanotubes was synthesized giving carbon structures of 1.4 nm in