Effect of Process Temperature of Al2O3 Atomic Layer Deposition Using Accurate Process Gasses Supply System
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 . 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  , 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.
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 .
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)150oC, and (c)300oC. 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 300oC but also at 150oC. 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.
M. L. Green, et al., Microelectronic Eng., 48(1999) 25
S. Becu, et al., Microelectronic Eng., 83(2006) 2422
Xiongfei Yu, et al., IEEE electron device letters, 24(2003) 63
Joo-Hyung Kim, et al., Thin Solid Films, 516(2008) 8333
Min-Jung Choi, et al., Applied Surface Science, 301(2014) 451