Page 2 - Molecules for Charge-Based Information Storage

Molecules for Charge-Based Information Storage Lindsey and Bocian

and one capacitor (Figure 1). Charge stored on the capacitor
indicates the bit level (either 0 or 1). To store sufficient charge
(∼200000 electrons) for accurate readout, a trench (or stack)
2
type capacitor is utilized. These designs lead to extremely
high aspect ratios, which are not easily scalable to the smallest
feature sizes now achievable via lithography (∼20 nm).
Another disadvantage arising from the small dimensions
of the transistor gate in the DRAM cell is that the charge on
the capacitor decays rapidly (tens of milliseconds in current
constructs) via a leakage current through the transistor.
Thus, the data stored in each memory location must be
FIGURE 1. Trench capacitor. Conceptual illustration showing crossbars periodically refreshed by reading it out and then writing it
(red, blue), transistor, and trench capacitor (left). Image of a 78-nm trench back again. The frequency at which this refresh must be
DRAM cell (with 70:1 aspect ratio) showing numerous trenches (right).
repeated is determined by the magnitude of the capacitance
and the leakage current; thus, a key design feature in the
electronic properties over a broad range through molecular miniaturization of DRAM cells is the available capacitance as
1
design and chemical synthesis. However, the molecular the feature size shrinks. If DRAM cells could reach the same
components employed must remain robust under daunting feature size now possible via lithography, a memory density
2
conditions including high-temperature processing steps dur- in excess of 1 terabit/cm in a planar format could be
ing manufacture (∼400 °C), relatively high-temperature achieved (or 10 000 terabit/cm 3 in a three-dimensional
operating conditions (up to 140 °C), and very large numbers format). A DRAM based on molecular properties would be
12 2
of operational cycles over a lifetime (g10 ). compatible with this memory density. 2
One of the long-term objectives of molecular electronics The use of molecules as the charge-storage element in a
is to learn to construct circuitry using molecular components DRAM device has the general attributes of (1) electrical
in a bottom-up approach. A more near-term strategy entails writing/reading, (2) operation under ambient conditions,
the development of hybrid semiconductor/molecular de- (3) low power consumption, (4) no moving parts, (5) reliable
vices wherein molecules are used to augment the features of operation under multiple cycles, (6) scalability to small
traditional, semiconductor-based, photolithographically dimensions, and (7) fault tolerance because a number of
constructed circuitry, thereby taking advantage of the vast discrete molecules constitute a single memory storage
infrastructure of the semiconductor industry. Toward this element. Our program to develop porphyrin-based
latter goal, our laboratories have been engaged over the charge-storage molecules for DRAM focused both on de-
past decade in the development of a molecular-based signing the requisite features for facile information storage
information-storage medium that can be readily incorpo- and on developing strategies for the incorporation of
rated into existing semiconductor processing technologies. 352 the molecules into hybrid semiconductor/molecular mem-
Our general approach uses a collection of redox-active ory chips. A cycle of molecular design, chemical synthesis,
porphyrin molecules covalently attached to an electroactive and physical characterization was iterated over the course
surface wherein information is stored in the discrete redox of which >350 compounds were prepared and tested
states of the molecules. 3 during the past decade. The program grew out of our long-
Our porphyrin-based charge-storage medium forms the standing research in molecular photonics 53 and necessi-
basis for constructing a hybrid semiconductor/molecular tated substantial advances in porphyrin synthetic
2
dynamic random access memory (DRAM). DRAM serves chemistry. 10,40,52,54 The chief results of this program are
as the central memory element in numerous devices, most the focus of this Account.
notably in the standard desktop and laptop computers. A
key advantage of DRAM over other types of memory Porphyrin-Based Charge-Storage Memory
technologies, such as static random access memory Elements
(SRAM), is its small feature size, hence the capability of The basic concept of the porphyrin-based charge-storage
achieving high-memory density. The small feature size of memory element is illustrated in Figure 2. Porphyrins were
DRAM arises because the basic cell consists of one transistor chosen owing to certain specific characteristics of their redox

Vol. 44, No. 8 ’ 2011 ’ 638–650 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 639