Impact of Rapid-Thermal-Annealing Temperature on the Polarization Characteristics of a PZT-Based Ferroelectric Capacitor

Abstract

A metal-ferroelectric-metal (MFM) capacitor was fabricated to investigate the effect of the rate-of-change of temperature in the rapid thermal annealing (RTA) process on the physical properties of the MFM capacitor’s ferroelectric layer [lead zirconate oxide (PZT)]. Remnant polarization (2 × P_r) is measured and monitored while performing the RTA process at 500 °C–700 °C. It turned out that, for a given target/final temperature in the RTA process, 2P_r of the ferroelectric layer decreases with a higher rate-of-change of temperature. This can provide a way to adjust the properties of the PZT layer, depending on the RTA process condition (i.e., using various rate-of-changes of temperature) for a given final/target temperature.

1. Introduction

Unlike conventional dielectric materials, ferroelectric materials have the property of maintaining electrical polarization without an external voltage. Based on this unique characteristic, they have been widely used in electronics. One of the representative materials being studied is lead zirconate titanate [Pb(Zr_1-xTi_x)O₃ (PZT)]. PZT has been mainly used in Ferroelectric Random Access Memory (FeRAM), Pyroelectric Infrared (IR) Sensors, Surface Acoustic Wave (SAW) devices, and so on [1,2,3,4,5]. Recently, ferroelectric material has been actively studied to take advantage of using the negative capacitance in itself, for low power applications [6,7].

As described above, ferroelectric films are widely used in electronics, and their properties vary greatly depending on synthesis conditions, the thickness of a ferroelectric layer, substrate/electrode materials on the top/bottom of the ferroelectric layer, and so on [8,9,10,11,12,13,14,15,16]. In the previous studies [17,18], it was investigated that the hysteresis of the polarization characteristic was increased at a higher annealing temperature. Afterwards, a follow-up study was done to investigate the dependence of the characteristics of a Pt/PZT/Pt capacitor on various rapid thermal annealing (RTA) temperatures. However, in terms of RTA temperature conditions, the details have been missed.

In this study, two different conditions on how to increase RTA temperature were suggested and set up; the target RTA temperatures were set/chosen in the range of 500 °C to 700 °C by 100 °C. (Case #1) a given target temperature was reached within 20 s, and then the temperature was maintained for 300 s. (Case #2) the rate-of-change of temperature was identical for all of the cases (i.e., the time duration for which the temperature was ramped up to a target temperature should not have been identical for all the cases, but the rate-of-change of temperature was identical). Once reaching the target temperature, the temperature was maintained for 300 s as in “Case #1”.

2. Fabrication and Measurement

To observe the polarization characteristics of a metal-ferroelectric-metal (MFM) capacitor, a series-connected resistor-capacitor (RC) network was built (see Figure 1). In order to fabricate the MFM capacitor in the RC circuit, a PZT solution with the ratio of lead:zirconium:titanium = 1:0.52:0.48 was spin-coated on a Pt layer at a rate of 3000 rpm for 30 s, to deposit an 80nm-thick PZT layer [8]. Afterwards, the RTA process was followed in two different ways.

(Case #1) as shown in Figure 2a, for 20 s, the temperature was ramped up to a target temperature. It is noteworthy that the rate-of-change of temperature depends on the target temperature (i.e., a faster rate-of-change is necessary to reach to a higher target temperature, for the given time duration of 20 s). The rate-of-change was 34 °C/s at 500 °C, 49 °C/s at 600 °C, and 62 °C/s at 700 °C. Once the temperature reached the target value, the temperature was maintained for 300 s.

(Case #2) as shown in Figure 2b, the RTA temperature was ramped up to the target value but with a given rate-of-change of temperature. Once the temperature reached the target value, the temperature was maintained for 300 s (as in Case #1). In both cases, the target temperature was set/chosen in the range of 500 °C to 700 °C by 100 °C. It is noteworthy that, in the case of the target temperature of 700 °C, the RTA process for both Case #1 and Case #2 was, in fact, identical.

Once the RTA process had been completed, a 40nm-thick gold layer with a diameter of 0.2 mm was deposited on the PZT layer. Afterwards, using a Radiant Precision LC II ferroelectric tester, the MFM capacitor’s polarization characteristic was measured by sweeping the voltage across the ferroelectric capacitor (i.e., 0 V → + 5V → −5 V → 0 V).

3. Results and Discussion

To experimentally analyze the impact of temperature (used in rapid-thermal-annealing, or RTA) on the polarization characteristics of the PZT-based MFM capacitor, various conditions for temperature have been developed. The final/target temperature, as well as the rate-of-change of temperature, would significantly affect the way that domains in the ferroelectric material of the MFM capacitor are formed and grown up with neighboring domains. In the previous study [19], depending on how the annealing temperature is developed in the RTA process, the grain size varies, and thereby, the hysteresis loop of the P-E curve is modified in proportion to the corresponding grain size. In this work, two ways of ramping up the RTA temperature from room temperature to the target temperature are proposed and developed (note that this was not discussed in the previous study). Each way has been applied to the RTA process for the MEM capacitor, and then the hysteresis characteristic was measured. In Figure 2, the measured temperature in the RTA process is drawn for two cases [i.e., “Case #1” shown in Figure 2a, and “Case #2” shown in Figure 2b].

The polarization-vs-voltage for “Case #1” and “Case #2” was measured and summarized in Figure 3a,b, respectively. The value of remnant polarization (i.e., 2 × P_r) was obtained from the experimental results and summarized in Figure 3c,d for Case #1 and Case #2, respectively. Comparing two cases at the same temperature, the decreasing rate of 2P_r with respect to the target temperature is higher in Case #2 than in Case #1. In reality, 2P_r in Case #2 (where the temperature is increased more rapidly than in Case #1) is decreased by 31% (vs. 2P_r at 700 °C). It turned out that 2P_r becomes larger when the RTA temperature slowly reaches the target temperature.

As known in previous studies, the conversion to perovskite is completed at temperatures above 450 °C [20]. The larger the rate-of-change is, the smaller the grain size is, and the greater the uniformity [21]. Further, it is confirmed that the value of 2P_r is proportional to grain size [22,23]. The previous study [24] confirmed that the larger the difference in rate-of-change of temperature for a given temperature, the greater the difference in grain size and the difference in 2P_r. However, in this study, the difference in rate-of-change of temperature at 500 °C is greater than that at 600 °C. It can be confirmed that the rate-of-change of 2P_r (which is originated from the rate-of-change of temperature) at 600 °C is greater than that at 500 °C. The reason for this difference is that 2P_r varies depending on the direction of grain formation, and (111) is known to show higher 2P_r than (100) [25]. At 500 °C, the Pt_xPb layer is dominant, so that the (111) orientation grain formation is dominant [26]. It can be seen that, when the rate-of-change is different, it is larger than at 500 °C by promoting the grain formation in the (100) orientation.

4. Conclusions

A metal-ferroelectric-metal (MFM) capacitor with lead zirconate oxide (PZT) was fabricated to study the impact of the rate-of-change of temperature in the rapid thermal annealing (RTA) process on the electrical properties of the MFM capacitor’s ferroelectric layer. It turned out that, for a given target temperature in the RTA process (500 °C–700 °C), 2P_r of the ferroelectric layer decreases with a higher rate-of-change of temperature. When the RTA process is carried out at a lower temperature, even though the difference in the rate-of-change in temperature becomes greater at a higher final/target temperature, the rate-of-change in polarization characteristics of PZT is lower due to the difference in the orientation of grains formed. This experimental study can provide a way to engineer the properties of a PZT layer, depending on the RTA process condition (i.e., using various rate-of-change of temperature) for a given final/target temperature.