Behavior of Nickel Deposition on Silicon Wafers from TMAH and Ammonia based SC1 Cleaning Process

Adverse affect of surface contamination, including metal, on silicon wafer resulted into continuous evolution of RCA cleaning process.¹ Deposition behavior of nickel on silicon wafer from SC1 solution, based on TMAH, is investigated here. A number of process parameters like chemical concentration, process time and temperature on deposition behavior of nickel from SC1 solution are considered. The mechanism of nickel deposition is discussed along with possible formation of some nickel silicide on the surface.

                Increased level of nickel deposition, as a function of TMAH concentration, from TMAH based SC1 solution is shown in Fig. 1. Time dependency of nickel deposition is carried out by exposing wafer

into a nickel-contaminated  SC1 solution with incremental period of time. The level of deposited nickel is found to increase with time as shown in Fig. 2. In other words, the non-equilibrium nickel deposition is observed, for the nickel contamination level used here and the period of investigation.

Additional nickel deposition study is performed by exposing wafer to TMAH based SC1 and ammonia based SC1 solution as a function of temperatures separately. Higher nickel deposition tendency, from SC1 bath, is observed in TMAH based SC1 solution compared to ammonia-based SC1 solution. Furthermore, the amount of deposited nickel from SC1 solution increases with temperature. The increased level of nickel deposition from TMAH based SC1 solution compared to ammonia- based SC1 solution is likely influenced by the increased stability of the nickel ion in ammonia with the formation of ammonia-nickel complex^{. 1}

Characterization of surface nickel from nickel-contaminated TMAH-based SC1 solution is carried out by dippingsilcon wafers in SC1 that contains 1ppb Nickel for 4.5 min., 9 min and 18 mins. respectively. This is shown in Figure 3. Amount of deposited nickel in chemical oxide was first analyzed with VPD-ICPMS. Subsequently, non-removable surface nickel on the surface was analyzed by ICP/MS following etching with surface with mixture of hydrofluoric and nitric acid.

The mechanism of nickel deposition can be considered to proceed by the reduction of Ni⁺² ion via electrochemical oxidation of silicon in the alkaline solution.² In other words the growth of chemical oxide on silicon is due to high redox potential of the SC1 solution containing peroxide. As indicated above the amount of metal oxide is expected to increase with treatment time observed in a similar study with copper.³

 Furthermore, the observation of silicon oxide between the deposited nickel and substrate in alkaline solution is observed in an EDX analysis of the interface.⁴ The oxide layer is proposed to be produced by reaction of oxidized silicon with OH- ion promoted by Ni⁺²ion. Part of Ni adsorbed on wafer from SC1 (TMAH+H2O2) possibly form nickel silicide, which may be difficult to remove in traditional low pH metal cleaning step.

Fig. 1 Amount of Nickel deposited on the wafer following SC1 traetment from  SC1 solution with increasing ratio of TMAH to H2O2 concentration.

Fig. 2. Nickel adsorption from SC1 solution contaminated with 1 ppb of nickel.

Figure 3. Nickel adsorption from TMAH-based SC1 solution and NH3-based SC1 solution as a function of temperature is shown below. Elevated nickel deposition is observed in TMAH-based solution.

Figure 4. Removal and non-removal nickel deposited on the wafer surface treated with dilute hydrochloric acid following depostion from from SC1 solution. Increased amount of non-removable nickel is observed with time.

References

1.W. Kern and D. A. Puotinen, RCA Rev., vol. 31, 187 (1970). 

2.P.M.M.C Bressers, S.A.S.P. Pagano, and J. J. Kelly, J. Electroanal. Chem., vol. 391, pg. 159 (1995).

3. J. S. Kim, H. Morita, J. D. Joo and T. Ohmi, J. Electrochem Soc. Vol. 144 (9), pg. 3275 (1997).

4. N. Takano, N. Hoseda, T. Yamada and T. Osaka, J. Electrochem Soc., vol 146 (4), pg. 1407 (1999).