In integrated circuits, Metal-Insulator-Metal (MIM) capacitor is the key passive element for filtering, decoupling and oscillating, and so on. Followings are especially important requirements; high capacitance density, low leakage current density [1]. To meet these requirements, various high-k materials have been investigated for dielectric materials of MIM capacitors [2,3].
Atomic layer deposition (ALD) is one of the most promising methods. It is considered that the temperature of ALD is one of important parameter in relation to the supply of Trimethylaluminum (TMA) for Al2O3. A recent study reported that an excessive high temperature may cause decomposition of TMA and the substrate material must not be oxidized during the oxidation process. H2O oxidation at stage temperature of 75 °C is shown to be effective because the oxidation ability of H2O becomes low, so it is effective to form the high quality interface of Al2O3/metal. Then, it is expected to be effective to improve Al2O3 film quality by post deposition annealing (PDA).
In this paper, using the developed ALD process equipment, Al2O3 film was deposited by H2O oxidation ALD at stage temperature of 75C°. The impact of the annealing on the electrical characteristics of MIM and metal-insulator-silicon (MIS) capacitors was investigated.
Experiment
Figures 1 and 2 show the process flow and the cross sectional schematic image of the fabricated MIM capacitors, respectively. After n-type (100) Si wafers Cleaning, SiO2 films were formed by thermal wet oxidation at 1000°C. Next, bottom TiN electrodes were formed by DC sputtering at 1kW(2.5W/cm2) with Ar/N2 flow at 300°C. To form active region, SiO2 films were deposited by atmospheric pressure chemical vapor deposition with SiH4/O2/N2 mixed gases at 400°C and the wet etching was carried out. Al2O3 films were deposited by ALD. TMA was supplied at 50°C to the chamber with the high temperature flow control system (HT-FCS) [5]. After the four times cyclic purge, H2O gas was supplied to the chamber and then TMA was oxidized to Al2O3 film. At this time, the pressure of the chamber was 133Pa and the wafer stage was 75°C. At this process condition, Al2O3 growth rate per cycle was 0.35nm/cycle nearly equaled to the thickness of mono-layer Al2O3 and high uniformity of Al2O3 film thickness in the Si-wafer was obtained [4]. In this work, 40 cycles were carried out. Next, the three types of PDA conditions were applied to investigate the effect of PDA to improve Al2O3 films: (1) without annealing; (2) O2 annealing at 400°C for 30min; (3) N2 annealing at 400°C for 30min. Finally, the top Al electrode was formed by high vacuum vaporization. As reference samples, MIS with three types of Al2O3 dielectric were prepared on n-type (100) Si wafers.
Result and Discussion
Figure 3 shows the leakage current density of MIM capacitors with the area of 1.0×10-4cm2 as a function of applied voltage. The thickness of Al2O3 films for without annealing, O2 annealing and N annealing were 13.7, 11.0 and 11.2nm, respectively. Here these films thickness were measured with the reference Al2O3 samples deposited on Si wafers by spectroscopic ellipsometry. Regarding the measurement conditions of leakage current, the delay time at each applied bias was set to 40sec to eliminate the transition and displacement current to accurately measure the quality of Al2O3 films. Figure 4 shows the capacitance density of MIM capacitors with the area of 1.0×10-4cm2 as a function of applied voltage at a frequency of 100kHz. As a result, the leakage current density and the capacitance density at 1V for without annealing, O2 annealing and N2 annealing were 7.2×10-10 and ,1.2×10-10 and 1.5×10-10 A/cm-2 and 5.7, 6.8 and 6.3 fF/um-2, respectively. Figure 6 shows the comparison result on capacitance density and leakage current between this work and reported MIM capacitors with various materials. From this result, the leakage current density of both of MIM and MIS using Al2O3 without the annealing were about the same level as previous works. The reduction of the leakage current and increase of capacitance density were confirmed especially for the O2 annealing and the superior characteristics in comparison to the previous works were successfully obtained. Furthermore, by measuring C-V characteristic of MIS as reference samples, the absolute value of the fixed charge density of Al2O3 film was reduced by 2.04×1012cm2 by both O2 and N2 annealing processes. Therefore, O2 annealing at 400°C is considered to be an effective PDA for the formed Al2O3 film.
In conclusion, by O2 annealing at 400°C for Al2O3 film deposited by H2O oxidation ALD at relatively low temperature of 75°C is effective process sequence to achieve low leakage current MIM capacitors.
Reference
[1]Chit Hwei Ng, et al., IEEE Trans, vol.52(2005) 1399-1408
[2]S. Becu, et al., Microelectronic Eng., vol.83(2006) 2422
[3]Sang-UK Park, et al., Microelectronic Eng., vol.88(2011) 3389-3392
[4]Hisaya Sugita, et al., ECS Trans, vol.66 (2015) 305-314
[5]Michio Yamaji, et al., ECS Trans, vol.45 (2012) 429
