Study on La2O3 Wet Clean by pH Controlled Functional Water
Authors: Ogawa Yuichi , Hideaki Iino , Takeo Fukui , Oniki Yusuke , Akanishi Yuya , Altamirano-Sánchez Efraín , Holsteyns Frank 
Published: February 2020

Introduction

For the dimensional scaling and performance enhancement of upcoming semiconductor devices, some novel materials have been implemented. In the high-k metal gate (HKMG) of logic and memory periphery transistors, some lanthanides and alkaline earth metal or their oxides, e.g. lanthanum oxide (La2O3) and magnesium oxide (MgO), have been proposed as a zero-thickness work function shifter [1-7]. These dielectric films are typically patterned by wet processing to enable a multiple threshold voltage [6,7]. However, the wet processes on the new materials are challenging because they dissolve easily in de-ionized wafer (DIW). In this paper, the impact of functional water rinsing on the dissolution and passivation of La2O3 films have been investigated.

Experimental detail

Coupon testFirst, the effect of etching due to pH was tested. The pH of the solution was adjusted by ammonia or hydrochloric acid. Moreover, O3, H2, H2O2 was added to the ammonia water which dissolution suppression has confirmed, and the effects of their solutions were tested.(2) Wafer test

We tested using 10 mm × 10 mm coupon which was cut from 300 mm La2O3 wafers. And the La2O3 coupon was immersed in various dilute solutions under open air conditions. After immersion, the La concentration in the solution was analyzed by ICP-MS. And we calculated the La dissolution amount from these results.

The DIW and some functional waters, e.g. CO2 dissolved water (CO2Water), ultra-diluted NH4OH (NH4OH water) , ultra-diluted NH4OH and ultra-diluted H2O2 (NH4OH/H2O2 water), were prepared by using a Kurita functional water unit (HS-13, Kurita Water Industries), which has the capability of injection and mixing CO2 (1 – 40 ppm) and NH4OH (1 – 100 ppm). The 300 mm blanket La2O­3 wafers were processed on a single wafer wet cleaning tool (SU-3200, SCREEN Semiconductor Solutions) connected with the functional water unit.
First, We had cleaning tests using CO2 water, DIW, NH4OH water, NH4OH/H2O2 water. Moreover, in order to clarify the dissolution suppression mechanism, the wafer surfaces after cleaning with each solution were analyzed by XRR.

Results and discussion

Coupon test La2O3 + 6H+ → 2La3
+ +3H2O (1).La2O3 + 3H2O → 2La(OH)3
(2).In addition, when H2, O3 and H2O2 were added to ammonia water of pH 10.4, Suppression effects were confirmed by H2O2 added, but we did not have the effect by H2 and O3 gases added(2) Wafer test Several functional waters have been evaluated for the introduction of novel materials and applications for leading edge semiconductor device manufacturing. In case HKMG patterning is required to remove La2O3, CO2W rinsing is preferred. Meantime, NH4OH water and NH4OH/H2O2 water rinsing could preserve the La2O3 layer.

Conclusions

Cleaning test was performed on a 300 mm La2O3 wafer and the dissolution rate was calculated by performing wafer weight change. Figure 3 shows the etching behavior of La2O3 films treated by DIW, CO2water and NH4OH water and NH4OH/H2O2 water. The results show that the La2O3 film dissolves in CO2water and DIW, of which the pH is acidic or neutral. And XRR measurement ressults are Figure 4, It was suggested that the La2O3 surface after various rinse reatments was formed of La (OH)3. The La (OH) 3 film is thicker by the NH4OH water and NH4OH/H2O2 water cleaning than in the DIW only cleaning. Therefore, it is considered that passivation with a hydroxide film is generated and dissolution is suppressed by setting it as an alkaline region.

Regarding the RMG La2O3 patterning for the PMOS devices, it would be required to remove La2O3. Meantime, the La2O3 layer should be preserved for the NMOS devices [6]. Relevant wet etching and rinsing solutions should be used for each patterning step.

On the other hand, the dissolution of La2O3 is drastically suppressed in NH4OH water, which is an alkaline solution. Therefore, the La2O3 surface would be passivated by its hydroxide:

The results show that La2O3 have following reaction in acidic or neutral solution:

Figure 1 is shown that the influence pH on the etching rate of La2O3. By the coupon test, we clarified that the dissolution of La2O3 can be suppressed by raising pH. From this test, it was found that the etching amount was sharply reduced from about pH 1.2 to about 8, and was hardly dissolved over pH 10.

Figure 1