Novel Method for Metal-Insulator-Metal (MIM) Plasma Etching Residue Removal
Authors: Chen Hsin-An , Tou Pei-Ting , Huang Hui-Chin , Shih Kai-Yao , Lu Ming-Chen 
Published: February 2020

After top plate etching process by metal-insulator-metal (MIM) manufacturing, a hard metallic polymer is formed and it is not easily to remove by ashing process or traditional polymer wet clean. The carbon fluoride gas with too much etching power such as CF4 is used as the etchant in current techniques, such that the second metal layer and the insulation layer may be damaged. In this paper, lower cost chemical was used for etch polymer removed. The chemical contain metal oxide etchant to oxidize the metal and metal oxidized polymer will be etched by fluorine function group which is mixed with this chemical. This method also can avoid metal or high K film damaged by CF4 gas in plasma etching process or O2 ashing process.

The manufacturing method of the metal-insulator-metal device was shown in Figure1. However, this process has some issues. There have some useless polymer residue around the pattern sidewall when etching the top plate tantalum, tantalum nitride step. Figure 2 showed the polymer is hard to remove after subsequent wet cleaning process.

In a subsequent process, these polymers make the film surface uneven and become the cause for short circuits during electrical connection, such that the performance of the metal-insulator-metal device is affected, and the yield and reliability are worsened as a result. Based on these issues, this paper provides a polymer cleaning method of a metal-insulator-metal device. Use carbon less etching gas for Ta/ TaN etching to avoid carbon rich polymer formation and post clean chemical composed by oxidant, such as H2O2, and metal oxide etchant, such as fluorine to remove excess polymer remaining on the metal-insulator-metal structure.

The DSP is chemical used to remove the polymer. Two ratio of the DSP mixed solution with the hydrofluoric acidare 10000:1 and 1000:1.The baseline etching gases containing carbonand wet polymer remove is DSP mixed HF 10000:1. After etch, the CD-SEM result was showed in Figure 3(a). It showed the corner of MIM device top plate had heavy polymer residue. In order to remove the heavy polymer residue after etch, increasing the concentration of HF and the result was showed in Figure 3(b). Unfortunately, the polymer still remain and no any improvement.

In order to reduce C source and TaClX can more efficient to oxidize, replace the etch gases from C-rich to C-less . Figure 4 showed the etch gases changed to C-less gas and the polymer seems become thinner than C-rich gas. And after DSP with HF 1000:1 the polymer seems almost removed but DSP with HF 10000:1 and ST-250 poly still have polymer remain. It seems the DSP with HF 10000:1 the HF concentration too low to corrosion the metal oxide. ST-250 of the fluorine function group is NR3HF and the solution pH=8, the fluorine function group didn’t react with metal oxide in the alkaline environment. Figure 4(b) shows higher concentration of HF is benefit for polymer remove because increase the metal oxide etchant and the TaClX almost oxidized to Ta2O5 then be react to H2[TaF7] which can be dissolved.

The cross section SEM image shows the polymer around the pattern sidewall after the carbon rich etching gas, O2 asher and post-etched clean step, it seems hard and thicker which is shown as Figure 5(a). At the Figure 5(b), there seems no polymer remain by replacing C-less etch gases.

In order to totally remove the polymer, prolong the process time of DSP with HF 10000:1 and 1000:1 and the CD-SEM data is shown in Figure 5. After wet process with the higher concentration of HF the polymer is totally remove but the lower HF concentration the polymer still remain. To verify the remove mechanism, double the wet process time with lower concentration of HF. And the result be shown in Figure 6(c), the polymer still remain. It proves that the concentration of HF is the key point to corrosion the metal oxide.

The polymer types and polymer removed methods after etching process were studied. Carbon rich etch gases will restrain the metal polymer be oxidize at the O2 asher step. By change etch gas to lower carbon content and following post-etched clean step is easily to corrosion the metal oxide which be oxidized by O2 asher and wet oxidize chemical. The concentration of HF in DSP chemical is key point to remove the polymer. DSP to HF concentration between 1000:1 to 100:1 is suitable for post clean. The method is low cost and prevent use too much etching power etching gas to damage the device pattern.

Figure 1