Novel liquid cobalt precursor for metal atomic layer deposition

Highly conformal atomic layer deposition (ALD) thin films are needed in a variety of technological applications. ^1–6) In particular, ALD cobalt (Co) thin films are desirable as barrier/liner films in Large Scale Integration (LSI) metallization, ^7–10) magnetic devices, ^11,12) and so on. There are many precursors for Co ALD. However, a large number of Co precursors are solid state at RT. ^13–18) There is no liquid Co precursor for the construction of the Co ALD process. Thus, this type of liquid Co precursor is desired for Co ALD processing, and a novel liquid Co precursor DIPRoBA-Co ¹⁹⁾ is proposed in this study. The DIPRoBA-Co is in a liquid state at RT and successfully achieves the construction of the Co ALD process with a liquid precursor instead of a solid one. Further, the fact that this precursor is in a liquid state at RT is important for constructing ALD systems due to it being free from solid adhering, particularly in device production factories. This study contains the transmission electron microscopy (TEM) observation that is added to the results presented at the ADMETA2022 conference. ²⁰⁾

In the following, we firstly describe the DIPRoBA-Co synthesis, followed by, secondly, the Co ALD examination with this as a source gas. Thirdly, we describe the DIPRoBA-Co characterization, and fourthly the Co metal ALD characterization with the DIPRoBA-Co.

First is the DIPRoBA-Co synthesis. All reaction was performed under a nitrogen gas atmosphere. Solvents were dried by 4 Å molecular sieves and deoxygenated. N-propyl lithium was prepared with small grains of lithium and n-propyl chloride in diethyl ether solvent. Cobalt chloride (CoCl₂) was dried at 150 °C in vacuo.

N, N'-diisopropylcarbodiimide (36 g, 0.285 mol) was slowly dropped into n-propyl lithium (0.284 mol) diethyl ether solution followed by stirring thereof at RT for 4 h. The reaction mixture was added dropwise to a suspension of CoCl₂ (18.4 g, 0.142 mol) in 100 ml of hexanes, followed by stirring thereof for 24 h. After evaporation of the solvents, 500 ml of hexanes was added thereto, followed by filtration of insoluble matters. After evaporation of the hexanes, distillation was performed under reduced pressure (13 Pa). Then, the DIPRoBA-Co could be obtained with a yield of 90%.

The second stage is the Co metal ALD examination with the DIPRoBA-Co. We selected an ammonia/hydrogen gas system as the reactant gas and argon gas as the purge gas for ALD processing with the DIPRoBA-Co. The ALD with DIPRoBA-Co as the source gas was carried out using a self-made apparatus. The apparatus consists of a reaction chamber with a heater heated up to 400 °C gas-valves controlled electrically, mass flow controllers for the ammonia/hydrogen gases and argon gas, a DIPRoBA-Co liquid pot with a heater heated up to 150 °C, and a rotary vacuum pump for exhausting introduced gases. In this experiment, the DIPRoBA-Co vaporization was carried out by the bubbling method. ^21–23) The DIPRoBA-Co, which was heated in the liquid pot to 90 °C and bubbled by 10 sccm argon gas, was injected into the reaction chamber under 120 Pa total pressure, where the amount of DIPRoBA-Co introduced was calculated ²³⁾ from the ratio of [vapor pressure]/[total pressure] to be about 1 sccm. A substrate, constructed of 50 nm thick HTO (high-temperature deposited silicon dioxide) film on a silicon wafer, was set in the middle of the reaction chamber.

The ALD cycle processes were: 1) exhaust for the reaction chamber; 2) source gas injection of the DIPRoBA-Co in argon gas; 3) 30 sccm flow rate argon gas purge; 4) exhaust for the reaction chamber; 5) injection of 60 sccm flow rate ammonia/hydrogen gases as reactant gas; and 6) 30 sccm flow rate argon gas purge. The argon gas purge is a very important process which eliminates excess sites of Co film bonds.

The Co ALD films with DIPRoBA-Co were fabricated, as follows, for characterization. The substrate, which was 50 nm thick HTO film on a silicon wafer, was set in the reaction chamber and was heated up to 400 °C in vacuo. Under this condition, the 100 sccm flow rate hydrogen gas was introduced for 30 min to obtain smooth surface ALD Co film. After this hydrogen gas treatment, the substrate was cooled down to ALD temperature, and the ALD Co films were fabricated with 300 cycles of the ALD cycle, at substrate temperatures of 180 °C, 200 °C and 220 °C, respectively.

The third stage is the DIPRoBA-Co characterization. The obtained DIPRoBA-Co is liquid at RT and its color is greenish-blue. The molecular structure is shown in Fig. 1. The four nitrogen atoms in an amidine skeleton coordinate to the Co atom, according to X-ray crystal structure determination by R. G. Gordon. ¹³⁾

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The melting point of this source is 15 °C and it is stable over 1 year in storage at RT. The melting point did not change after heating at 145 °C for 2 weeks.

The temperature dependence of the DIPRoBA-Co vapor pressure was determined by using the gas saturation method. ²⁰⁾ The relationship between vapor pressure p and absolute temperature T is shown in Fig. 2. The Clausius–Clapeyron equation was obtained from this result as follows

$$\begin{eqnarray}&&\mathrm{log}P\left(\mathrm{Pa}\right)=10.44-3380/T\left({\rm{K}}\right).\end{eqnarray} \tag{ 1 }$$

The DIPRoBA-Co has a vapor pressure of 11 Pa at 90 °C, which is enough for ALD processing.

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The fourth stage is the Co ALD characterization with the DIPRoBA-Co. The film thickness of the obtained samples was measured by a displacement sensor in a probe type step profiler. Thicknesses were about 30 nm for both films at substrate temperatures of 200 °C and 220 °C, respectively. Therefore, the growth rates are about 0.1 nm per cycle, meaning that the films grow by approximately one layer per cycle. This result indicates that both films at substrate temperatures of 200 °C and 220 °C, respectively, are formed by the ALD mode. The process of this phenomenon (growth of one layer per cycle) is considered here. In the first stage, the DIPRoBA-Co is introduced into the chamber and the DIPRoBA-Co goes onto the substrate Co surface. After chemisorption reaction, Co in the DIPRoBA-Co bonds to the surface Co. The DIPRoBA residue goes away. Then, the extra DIPRoBA-Co is exhausted. In the second stage, the reactant gases of ammonia and hydrogen are introduced, and these gases react with the DIPRoBA residue. Then, the residue is removed and one layer of Co grows.

On the other hand, the thickness was about 120 nm for the film at a substrate temperature of 180 °C. Therefore, the growth rate is about 0.4 nm per cycle, meaning that the film grows by approximately four layers per cycle, suggesting that the physical adsorption of the source gas on the substrate surface raised the growth per cycle rate.

The ALD film growth with the DIPRoBA-Co was examined using TEM. A sample for the TEM was obtained on a plasma silicon dioxide step pattern by ALD with the DIPRoBA-Co under conditions of 220 °C and 80 cycles. As shown in Fig. 3, 5–6 nm thick Co metal film grows on the step of the plasma silicon dioxide.

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This study is concluded as follows. Bis (diisopropylbutanamidinate)cobalt (DIPRoBA-Co) is proposed as the liquid Co precursor for metal ALD construction. This precursor is in a liquid state at RT and successfully achieves the construction of the Co ALD process with a liquid precursor instead of a solid one. Both films at substrate temperatures of 200 °C and 220 °C, respectively, are formed by the ALD mode, using the ammonia, hydrogen and DIPRoBA-Co gas system. TEM observation shows that 5–6 nm thick Co metal film grows on the step of the plasma silicon dioxide by this ALD with 80 cycles at 220 °C substrate temperature.