Rate Equation for TiN Etching in Dilute H2O2
Authors: Ueda Dai , Harumoto Akiko 
Published: December 2024

Introduction
Titanium nitride (TiN) has been widely used in semiconductor manufacturing as barrier metal, liner, work function metal, hard mask, and so on, and there are some wet processes for etch-back of TiN in confined narrow space [1]. Although, it has been reported that etching rate in the narrow space is much lower than that in bulk region [2], the mechanism is not well understood yet. S. Verhaverbeke et al. reported that TiN in APM is etched by mainly HO2
− ions generated from hydrogen peroxide (H2O2) [3], but quantitative relationship or rate equation was not discussed. Since hydrochloric acid peroxide mixture (HPM) is also proposed as an etching chemical [1], it is important for TiN etching to create rate equation over wide pH range to elucidate the mechanism. In this report, a rate equation for TiN etching in dilute H2O2 is proposed in various pH.
Experimental
TiN thin film with 10 nm of thickness were used as etching test sample. 30 vol% H2O2 was diluted with de-ionized water (DIW), and small amount of sulfuric acid (H2SO4) or ammonia ammonium hydroxide (NH4OH) were added to adjust pH. Mixing ratio of 30 vol% H2O2 : H2SO4 or NH4OH : DIW was 1 : x : (9 − x) in volume, and total volume was adjusted to constant indicative molar concentration of H2O2 (1.021 mol/L). Calculated pH was ranged from 0.004 to 9.274. Etching test was performed by immersing the sample in coupon to beaker. The temperature of the etching chemical was increased and kept at 50°C with heater. TiN film thicknesses before and after etching test was measured with ellipsometer and delta was calculated as etching amount. Etching rate was calculated by dividing the etching amount by immersion time.
As mentioned in the above, TiN is etched by HO2
− ions in dilute H2O2. In this report, H2O2 molecules was also considered because it is dominant when pH is low. Quantitative relationship between etching rate ER and [H2O2
] and [HO2
−
] was expressed in the following equation,
ER = k1
[H2O2
] + k2
[HO2
−
] [1]
where k1
and k2
are reaction rate constants for H2O2 molecules and HO2
− ions, respectively. Data sets were etching rates of [H2O2
], [HO2
−
], and TiN under each condition. [H2O2
] and [HO2
−
] were calculated from dissociation equation.
Results and Discussion
TiN etching rate increased with pH as shown in Figure 1. This result indicates the increase in HO2
− ion which reacts faster as pH increases [3]. On the other hands, TiN was etched in low pH even though the concentration of HO2
− ion is low. This result can be explained by applying rate equation. Table 1 shows obtained fitting parameters. Linear fitting of etching rate expressed by relation Eq. 1 with these constants, plotting experimental values and k1
[H2O2
] + k2
[HO2
−
], is shown in Figure 1. As shown in the table and the figure, standard error is small and p-values are reliable, indicating proposed model is relatively accurate, whereas fitting accuracy was not good if H2O2 was not taken into account. This result suggests that TiN is also etched by H2O2 molecule.
Conclusions
A rate equation of TiN etching in dilute H2O2 with various pH was proposed in the first-order equation of [H2O2
], [HO2
−
]. The proposed rate equation indicates that H2O2 molecules contributes to TiN etching.
References
Oniki, et al.,
Solid State Phenomena, 282, 132-138 (2018).
Oniki, et al., Solid State Phenomena, 314, (2021) 119-126.
Verhaverbeke, et al.,
Materials Research Society Symposium Proceedings, 477, 447–458 (1997).

Figure 1