Effect of Process Temperature of Al2O3 Atomic Layer Deposition Using Accurate Process Gasses Supply System

Introduction

 On-chip capacitors are used for various purposes in electronic devices, for example, AC coupling capacitors, bypass capacitors, sample and hold circuits for analog signals, and so on. For these purposes, a high capacitance density is required for on- chip capacitors in order to shrink the chip size or to reduce the thermal noise. However, it has been difficult to minimize the capacitor size with a same capacitance owing to stagnation of capacitance density improvement [1]. To maintain the capacitance with a small capacitor size and a same dielectric material used, the dielectric film thickness must be thinner. However, an excess shrinking of the thickness of dielectric film causes the large leakage current and degrades the reliability. Therefore, an introduction of high-k material is a very efficient way to increase the capacitance density. In particular, Al₂O₃, HfO₂, and ZrO₂have been extensively studied as candidates for dielectric materials of metal-insulator-metal capacitors [2-4].

 Atomic layer deposition (ALD) is one of the most promising methods, because a good step coverage is obtained and the process temperature is relatively low. Most ALD is carried out at around 300 ^oC [4][5] , it is considered that the temperature of ALD is one of important parameter in relation to supply the metal organic (MO) gasses, such as Trimethylaluminum (TMA).

 In this paper, using the developed ALD process equipment with high accurate gas flow system adapted to high temperature usage, the impact of the process temperature on the electrical characteristics was investigated.

Experiment

 Cz-n type 33mm Si wafers (8-12Wcm) were used. MOS capacitors were fabricated as follows. Si wafers were cleaned to remove native oxide film by diluted HF (DHF 5%), and after that Al₂O₃films were deposited on Si surface directly by ALD, followed by forming gate and bottom electrodes of Al by the evaporation, respectively. Electrical characteristics (I-V, C-V) were measured at five points on each wafer.

 Figure 1 shows the ALD sequence. At first H₂O was injected 1sec to make Si surface OH-terminated, after that the following ALD cycle was started. TMA was injected 1 sec, and H₂O was injected 1 sec for oxidation. This cycle was repeated until the Al₂O₃ film thickness is about 20 nm. The following gas purge was repeated 4 times every time before TMA or H₂O was injected. Ar was purged, by changing pressure in the reactor chamber from 133 to 27 Pa, and getting back to 133 Pa.

 Here, TMA gas was accurately supplied as follows. TMA was supplied as vaporized gases, with heated to 60 ^oC (TMA vapor pressure: 9 kPa). In addition, the tube supplying the TMA gas was also heated to 60 ^oC to keep TMA gaseous. The TMA gas flow rate of 3 sccm was controlled by flow control system (FCS) adapted to high temperature usage [6].

 We prepared three kind of Al₂O₃ films formed at (a)room temperature (RT), (b)150 ^oC, and (c)300 ^oC.

Result and Discussion

 Figure 2 shows J-E characteristics of Al₂O₃ films formed at (a)RT, (b)150^oC, and (c)300^oC. The variation of J-E characteristic increases, with the temperature increasing. As the result, The RT sample has small variation and high breakdown field intensity.

 Figure 3 shows growth rate per cycle as a function of temperature. The average, maximum, and minimum values are also plotted on respectively. It is found that the growth rate and its variability increase, with the temperature increasing. Only RT case, the uniformity of the growth rate is good and the value is about 0.3 nm/cycle , which is almost as same as the monolayer Al₂O₃. However, the growth rates of the others are larger than the thickness of monolayer, and theirs variability is very large.

 These means that the complete ALD was performed at only RT, while the others’ deposition mechanisms were occurred at relatively high temperature. It is suggested that the decomposition of TMA occurred not only at over 300^oC but also at 150^oC. And almost no decomposition occurs at RT, so that the high quality Al₂O₃films are formed.

 These data indicated it is important that maintaining the process temperature in order to go on the ALD reaction on the surface, and not to decompose MO gasses.

Reference

[1]M. L. Green, et al., Microelectronic Eng., 48(1999) 25

[2]S. Becu, et al., Microelectronic Eng., 83(2006) 2422

[3]Xiongfei Yu, et al., IEEE electron device letters, 24(2003) 63

[4]Joo-Hyung Kim, et al., Thin Solid Films, 516(2008) 8333

[5]Min-Jung Choi, et al., Applied Surface Science, 301(2014) 451

[6]Michio Yamaji, et al., ECS Trans, 45 (2012) 429