Synthesis and dynamic random access memory behavior of a functional polyimide

Article abstract of DOI:10.1021/ja062489n

The device under testing was a plastic dynamic random access memory based on a donor-functionalized polyimide (TP6F-PI), which exhibited the ability to write, read, erase, and refresh the electrical states. The device had an ON/OFF current ratio up to 10⁵, promising minimal misreading error. Both the on and off states were stable under a constant voltage stress of 1 V and survived up to 10⁸ read cycles at 1 V. Copyright

Full text of DOI:10.1021/ja062489n

Published on Web 06/15/2006
Synthesis and Dynamic Random Access Memory Behavior of a Functional
Polyimide
†
‡
§
§
‡
Qi-Dan Ling, Feng-Chyuan Chang, Yan Song, Chun-Xiang Zhu, Der-Jang Liaw,
§
,†
†
Daniel Siu-Hhung Chan, En-Tang Kang,* and Koon-Gee Neoh
Department of Chemical and Biomolecular Engineering and Department of Electrical and Computer Engineering,
National UniVersity of Singapore, Kent Ridge, Singapore 119260, and Department of Chemical Engineering,
National Taiwan UniVersity of Science and Technology, Taipei, Taiwan
Received April 10, 2006; E-mail: cheket@nus.edu.sg
Polyimides (PIs) have been widely used in microelectronics,
owing to their excellent mechanical, dielectric, thermal, and
1
chemical properties. In recent years, PIs have also been explored
2
for applications in organic electronics, such as light-emitting diodes,
3
4
photovoltaics, and xerography. Polymer memories exhibit simplic-
ity in structure, good scalability, low-cost potential, 3D stacking
capability, and large capacity for data storage. In the pioneering
5
works on polymer memories, polymers were used as polyelectro-
lytes, matrixes of dyes or nanoparticles, or components of charge
transfer (CT) complexes in doped or mixed systems.6 The design
and synthesis of polymers that can provide the required memory
properties within a single molecule is an alternative approach.11
Taking into account the demand for thermal stability in polymeric
materials,12 the present work explores a thermally stable PI for
memory applications. The present memory device is based on a
functional PI containing both electron-donor (D) and electron-
acceptor (A) moieties within a single macromolecule (TP6F-PI,
Figure 1). Differing from that of the widely reported nonvolatile
flash and write-once read-many-times polymer memories, the device
based on TP6F-PI exhibits dynamic random access memory
-10
Figure 1. Molecular structure (top) of the functional PI (TP6F-PI) and
schematic diagram (bottom) of the single-layer memory devices.
(DRAM) behavior.
The functional PI was synthesized by the reaction of 4,4′-
diaminotriphenylamine with hexafluoroisopropyl bis(phthalic di-
anhydride), to form the corresponding poly(amic acid).13 Imidization
gave rise to TP6F-PI with a number-average molecular weight of
4
4
.16 × 10 and a polydispersity index of 3.70 (see Supporting
Information). TP6F-PI exhibited excellent thermal stability, with
a 10% weight-loss temperature of 524 °C and a glass transition
temperature of 316 °C. The memory device consisted of a NMP
solution spin-coated TP6F-PI film (∼50 nm in thickness) sand-
wiched between an indium-tin oxide (ITO) bottom electrode and
a 0.3 µm-thick Al top electrode (Figure 1).
Figure 2. Current density-voltage (J-V) characteristics of a 0.16 mm2
Al/TP6F-PI/ITO device. The ON state was maintained by refreshing at 1
V every 5 s. Sweeps 1, 2, 5, 6, 7, and 8: 0 to +4 V (with power off for 1
min for sweeps 6 and 7). Sweeps 3 and 4: 0 to -4 V.
device transition from the ON state back to the OFF state. This
electrical transition served as the “erasing” process for the memory
device. The device remained in the OFF state after this erasing
process, as indicated by the subsequent negative sweep (the fourth
sweep). The erased (“0”) state could be further written to the stored
The memory effect of TP6F-PI is shown in the current density-
voltage (J-V) characteristics of Figure 2. In the 1st sweep from 0
to 4 V, an abrupt increase in J was observed at a switching threshold
voltage of about 3.2 V, indicating the device transition from a low-
conductivity (OFF) state to a high-conductivity (ON) state (the
(“1”) state when the switching threshold voltage was reapplied,
“
writing” process). The device remained in this high-conductivity
indicating that the memory device was rewritable (the fifth and
sixth sweeps). The seventh sweep was conducted after turning off
the power for about 1 min. It was found that the ON state had
relaxed to the steady OFF state without an erasing process. The
short retention time of the ON state indicated that the memory
device was volatile. However, the device could be reprogrammed
to the ON state (the seventh and eighth sweeps). The unstable ON
state could be electrically sustained by a refreshing voltage pulse
of 1 V in every 5 s (the ninth trace).
state during the subsequent positive scan (the second sweep). The
distinct bielectrical states in the voltage range of 0 to 3.2 V allowed
a voltage (e.g., 1.0 V) to read the “0” or “OFF” signal (before
writing) and “1” or “ON” signal (after writing) of the memory. In
the third sweep from 0 to -4 V, an abrupt decrease in J was
observed at a threshold voltage of about -2.1 V, indicating the
†
Department of Chemical and Biomolecular Engineering, National University
of Singapore.
‡
National Taiwan University of Science and Technology.
Department of Electrical and Computer Engineering, National University of
The J-V characteristics were repeatable with good accuracy,
and device degradation was not observed. The current magnitude
§
Singapore.
8732
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J. AM. CHEM. SOC. 2006, 128, 8732-8733
10.1021/ja062489n CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 4. (a) ON/OFF current ratio and effect of operation time and (b)
read pulses on the device in the OFF state and ON state. The inset in (b)
shows the pulses used for the measurements.
Figure 3. Molecular orbitals (left) of the basic unit of TP6F-PI and the
transitions (right) from the ground state to the CT state induced by the
electric field.
4
for the first 0.5 h, an ON/OFF current ratio of ∼10 was maintained
during the long-term testing under ambient conditions. No obvious
degradation in J was observed for the ON and OFF states after
more than 10 read cycles at a read voltage of 1 V (Figure 4(b)).
Thus, both states were stable under the voltage stress and were
insensitive to read pulses.
was proportional to the device area, that is, J was independent of
this parameter. This ability to write, read, erase, and refresh the
electrical states fulfils the functionality of a DRAM.
The mechanism of field-induced conductivity is probably similar
to that of the photoinduced CT in photoconductive PIs.14 The
incorporation of Ds has been known to enhance the photocurrent
in PI by several orders of magnitude, arising from the improved
CT complex formation in the PI backbone.15 In the present TP6F-
PI, triphenylamine acts as a D, while phthalimide acts as an A to
promote the CT complex formation. Molecular simulation of the
basic unit of TP6F-PI was carried out at the DFT B3LYP/6-31G(d)
level with the Gaussian 03 program package.16 Figure 3 shows the
resulting HOMO and LUMOs, and the plausible electronic pro-
cesses. The calculated energy levels are comparable to the HOMO
8
In summary, a plastic DRAM, based on a donor-functionalized
5
PI (TP6F-PI), exhibited an ON/OFF current ratio up to 10 . Both
the ON and OFF states were stable under a constant voltage stress
8
of 1 V and survived up to 10 read cycles at 1 V. A PI memory
device distinguishes itself from other polymer devices by its superior
thermal and chemical properties, while still possessing good
processability and scalability.
Supporting Information Available: Details on polymer synthesis
and characterization, molecular simulation, device fabrication, and ref
1
6. This material is available free of charge via the Internet at http://
(-5.13 eV) and LUMO2 (-2.03 eV) energy levels of TP6F-PI,
pubs.acs.org.
measured by cyclic voltammetry (see Supporting Information). The
HOMO is located on D, while the first and second LUMOs are
located on A. At the threshold voltage, one of the electrons transits
from the HOMO to the LUMO3 within D to form an excited state.
Excitation of D leads to a decrease in ionization potential and
consequently promotes intra- or intermolecular CT at the excited
state. CT can occur indirectly from the LUMO3 of D to the
LUMO2, then to the LUMO of A, or directly from the HOMO to
the LUMO2 and LUMO at the excited state, to form a conductive
complex. These processes are supported by the electronic absorption
spectrum of TP6F-PI. The respective absorption maximum and
absorption edge at ∼290 nm (4.28 eV) and ∼400 nm (3.10 eV)
correspond to the HOMO f LUMO3 (4.60 eV, highest probability)
and HOMO f LUMO2 (2.96 eV, low probability) transitions (see
Supporting Information). The charges can be further segregated
under an electric field and delocalized to the conjugated triphen-
ylamine, thus stabilizing the CT state to some extent. However,
the ON state of TP6F-PI could not be sustained due to limited
delocalization in the triphenylamine moieties. A reverse bias of
about -2.1 V, or removal of the electric field, can dissociate the
CT complex and return the device to the initial OFF state.
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