In phase-change memory (PCM) low-field electrical resistance is typically used to quantify the programmed cell state. However, this metric has several disadvantages. First, it exhibits temporal drift, which is a significant challenge for realizing multilevel PCM. Moreover, because of cell-geometry effects, this metric saturates after a certain point and thus masks the fact that the amorphous size increases with increasing input power. Finally, the resistance is typically measured as the current for a fixed bias voltage, which adversely affects the signal-to-noise ratio at high resistance values. A new metric for the programmed state in a PCM cell is proposed that has significant advantages over the resistance metric in all these aspects and is more representative of the fundamental programmed entity, which is the amorphous/crystalline phase configuration in the PCM cell. Analytical and experimental results are presented that demonstrate the efficacy of the proposed metric.
Phase-change memory (PCM) is one of the most promising candidates for next-generation non-volatile memory technology [1,2]. The cross-sectional tunneling electron microscopy (TEM) image of a mushroom-type PCM cell is shown in Figure 1 (a) . The cell consists of a layer of phase-change material, such as germanium antimony telluride (GST), sandwiched between a bottom and a top electrode. In this architecture, the bottom electrode has a radius (denoted as rE ) of approx. 15 nm and is fabricated by sub-lithographic means. The top electrode has a radius in excess of 100 nm and the thickness of the phase change layer is approx. 100 nm. A transistor or a diode is typically employed as the access device.
FIG. 1. (a) Cross-sectional TEM image of a PCM mushroom cell. (b) Schematic of a PCM mushroom cell. (c) Approximation of the a-GST geometry. (d) Cylindrical approximation of the a-GST geometry.
The GST is in the crystalline state prior to operation of the device. By application of programming pulses (voltage or current) and the subsequent Joule-heating-induced melting and quenching process, amorphous GST (a-GST) regions are created within the crystalline GST (c-GST) volume as shown in Figure 1(a) and 1(b). By varying the amplitude of the programming pulse, the size of the amorphous region and its thickness (uA) can be increased or decreased in a continuous manner, and this is the basis for multi-level storage in PCM .
FIG. 2. A typical experimental I-V characteristic corresponding to a PCM cell showing the threshold switching behavior and sub- threshold conduction.
In PCM, the electrical resistance of the cell is used to measure uA. As c-GST has much lower resistivity than a-GST, the electrical transport is dominated by the amorphous region. A typical I-V characteristic corresponding to a PCM cell programmed to a certain amorphous thickness is shown in Figure 2. The resistance of the a-GST region varies as a function of the electric field and hence the bias voltage.
Moreover, beyond a critical electric field, the resistivity drops to a very low value, so that a large current flows through the cell, dissipating significant power and thus potentially changing the amor-phous/crystalline phase configuration. This drop in resistance is known as the threshold switching phenomenon [5,6], and has to be avoided when measuring the resistance of a PCM cell as it may disturb the cell state. To measure the resistance, the PCM cell is typically biased with a constant read voltage (VR) that corresponds to the sub-threshold regime.
Because of the field-dependent nature of threshold switching, the threshold switching voltage is different for different programmed states and hence it is not possible to apply a high bias voltage to measure the electrical resistance for every state. Therefore, a low read voltage is typically used, and hence the most common cell-state metric is the low-field electrical resistance. This metric however has several undesirable traits that pose significant challenge to the realization of multilevel PCM.
This is a condensed version of the original article, which is posted on the Journal of Applied Physics website. The rest of the paper covers low-field resistance metric, alternate cell-state metric, experimental results, conclusions, and references.