In Situ EC-STM Studies of Silicon and Copper Single Crystal Surfaces
in Aqueous Solutions
Sang-Eun Bae
Electrical & Computer Engineering, University of Houston, Houston, TX
Date: May 4, 2007, Time: 3:30 pm, Location: W122-D3 Engineering Building 1, The University of Houston
Abstract:
In situ electrochemical scanning tunneling microscopy (EC-STM) is employed to examine the surface reactions of Si(111) wafer and Cu(100).
Preparation of atomically flat silicon wafer surface has become one of key issues in semiconductor industry as the length scales of Si IC chip is going down to nano size. In 1990, Higashi et al. discovered that atomically flat Si(111) surface can be prepared in a 40 % NH 4F solution at pH 8. In order to explain the etching reaction of the Si(111) surface in a 40 % NH 4F solution at pH 8, a step flow reaction (SFR) mechanism was introduced, in which unstable atoms on the silicon surface, such as dihydride, trihydride, and kink silicon sites, are preferentially dissolved by anisotropic etching. According to the in situ EC-STM results in the literatures, the etching rate ratio of dihydride silicon steps to monohydride steps ranges from 2 to 4. Thus, the SFR mechanism has been considered as a reasonable explanation for the Si(111)-H surface reaction. However this model does not give a flawless explanation for the etching reaction of Si(111)-H surface. For example, the SFR mechanism cannot explain the constant terrace width of Si(111)-H surface. Firstly, the in situ EC-STM images obtained from Si(111)-H surface will be presented to explain the etching reaction.
Cu is one of the most useful metals. In particular, Cu is used as an interconnect of Si IC chip as well as an electrocatalyst for nitrate reduction. The Cu interconnect is typically deposited using a process known as ‘superfilling’, or ‘bottom up plating’ which is the differential acceleration between Cu electrodeposition in trenches and planar surfaces. To produce the superfilling condition, small amounts of organic and inorganic reagents are employed in commercial electroplating baths as additives in order to obtain desirable deposit morphology and texture. Some of these additives act as accelerators which work to decrease the overpotential for deposition relative to additive free solutions. In order to investigate the mechanisms for the accelerators, we served Cu(100) as a model surface and observed interactions between the Cu(100) surface and the accelerators.
Additionally, the investigation for nitrate reduction will be able to be presented. The adsorption and reduction of nitrate at metal surfaces has long attracted considerable interest because of its environmental importance and its putative role in the production of useful nitrato compounds. Although it is well known that Cu shows the best catalytic effect on the nitrate reduction, there is no literature directly examining nitrate adsorption and reduction on copper electrode surfaces. Adlayer structures according to applied potential during nitrate reduction on Cu(100) surface are observed and discussed.
