Effects of Diluted-NH4OH as Conductive Rinse Water in Single Wafer Cleaning Processes

During single wafer cleaning processes, silicon wafers are electrostatically charged by the high electrical resistance of de-ionized water (DI water). Recently this phenomenon has become more serious problems, because it causes particle attachment to the wafer surface by electrostatic attraction and electrostatic destruction of devices. So it is very important to prevent the electrostatic charge from the wafer surface. In order to prevent it, DI water is made conductive by adding gases. CO₂-dissolved water (CO₂ water) is widely used as conductive rinse water, but it causes corrosive dissolution of specific metals like copper due to its acidity. On the other hands, it is proposed to use diluted-NH₃ water (NH₃ water) as conductive rinse water for preventing electrostatic charge too. NH₃ water is typically generated by adding concentrated NH₃ water to DI water with an injection pump. However, it is difficult to generate NH₃ water of specific concentration constantly due to the extreme dilution ratio (hundreds of thousands times).

 Then we have developed a NH₃ water generator with a new dilution system and have validated the preventive effects against copper corrosion and electrostatic charge on wafer surface by using NH₃ water. The generator controls concentrated NH₃ water tank pressure with inert gas and supplies trace amount of concentrated NH₃ water to DI water through a tube by the tank pressure. The system can keep electric conductivity of NH₃ water stable easily. An electric conductivity is often used to determine the NH₃ concentration.

 In the experiment of copper corrosion, a Cu film (200 nm) was deposited on a Ti/Si wafer by the sputtering method as a test piece. Rinse of the Cu film was performed by use of a single wafer cleaner. The etching rate of the Cu film was measured by the changes of sheet resistance with the 4-point probe method. In the experiment of electrostatic charge, silicon wafer with thermal oxide film (100 nm) is used as a test piece. Rinse of the silicon wafer was performed by use of a single wafer cleaner. The surface voltage on wafer was measured with a surface electrometer.

 We have evaluated influence of copper corrosion with NH₃ water (10 μS/cm), CO₂ water (10 μS/cm) and DI water (18.2 MΩ·cm). The etching rate in each experiment was 0.2 Å/min (NH₃ water), 1.4 Å/min (CO₂ water) and 0.2 Å/min (DI water) respectively. It is obvious that NH₃ water inhibits copper corrosion compared with CO₂ water at the same electric conductivity. In the work with NH₃ water of varied electric conductivity, the each etching rate was 0.7 Å/min (20 μS/cm), 1.9 Å/min (30 μS/cm) and 65.5 Å/min (90 μS/cm). It is considered that copper corrosion is enhanced with increased amount of ammonia because complex ions like �(Cu(NH₃)₄�)^2� is generated from copper and ammonia. We have evaluated influence of electrostatic charge with NH₃ water (10 μS/cm), CO₂ water (10 μS/cm) and DI water (18.2 MΩ·cm). The most negative surface voltage in each experiment was -3.0 V (NH₃ water), -2.1 V (CO₂ water) and -44.6 V (DI water) respectively. It is obvious that electrostatic charge is occurred more negatively by DI water than by the others, and NH₃ water can prevent wafer surface from charging as effectively as CO₂ water at the same electric conductivity. In the work with NH₃ water of varied electric conductivity, the most negative surface voltage in each experiment was -6.1 V (5 μS/cm), -2.0 V (20 μS/cm) and -0.9 V (30 μS/cm). It is considered that even NH₃ water causes a bit of electrostatic charge in the case of the low electric conductivity level like 5 μS/cm.

 From these results, it is shown that NH₃ water is the most reasonable rinse water to wash wafers with copper exposure among these three kinds of rinse water, and it is very valuable to control the electric conductivity of NH₃ water precisely between 10 to 20 μS/cm in order not to charge wafer surface and etch copper out by rinse water.