Mar. 20, 2008  9  US 2008/0067077 Al  [0108]  The electric conductivity of the electrolytic liquid  is preferably 5 to 200 mS/cm. When the electric conductivity  of the electrolytic liquid is low, a higher voltage needs to be  applied to carry out electrolytic polishing at an intended  processing rate. The application of higher voltage will  adversely affect electrolytic polishing, such as lowering of  current efficiency in polishing due to the generation of  oxygen, the formation of pits in a processed surface, lower  flattening effect due to breakage of a protective film, etc.  Thus, electrolytic polishing is desirably carried out at a low  voltage. To this end, the electric conductivity of the elec› trolytic liquid is preferably 5 to 200 mS/cm.  [0109]  In order to increase the polishing rate in electrolytic  polishing and to prevent surface roughening of a polished  surface, the electrolytic liquid preferably contains the same  conductive material as a polishing object, e.g., copper, as a  starting component. The concentration of such a starting  conductive material, e.g., copper, in the electrolytic liquid is  generally not less than 0.001 % by weight, preferably not less  than 0.005% by weight, more preferably not less than 0.01 %  by weight. The inclusion of a conductive material as a  starting component in the electrolytic liquid can promote  diffusion of the same conductive material, e.g., copper, or  polishing products including the conductive material from a  surface of a polishing object into the electrolytic liquid  during polishing. The concentration of a starting conductive  material, e.g., copper, in the electrolytic liquid is generally  not more than 10% by weight, preferably not more than 1 %  by weight. This is because when the concentration of a  starting conductive material is more than 1 % by weight, the  conductive material is likely to consume other component(s)  of the electrolytic liquid. When the conductive material of a  polishing object is a compound, the respective components  of the conductive material may be contained in the electro› lytic liquid as starting components in a proportion corre› sponding to the compositional ratio of the compound.  [0110]  An example of the electrolytic liquid comprises (1)  2 to 80% by weight of an organic acid, (2) 2 to 20% by  weight of a strong acid having a sulfonic acid group, (3) 0.01  to 1 % by weight of a corrosion inhibitor, (4) 0.01 to 1 % by  weight of a water-soluble polymeric compound, (5) 0.01 to  2% by weight of abrasive particles, (6) 0.01 to 1 % by weight  of a surfactant, and an aqueous solvent. The aqueous solvent  may be deionized water, preferably ultrapure water.  [0111]  Electrolytic polishing using the electrolytic liquid  of the present invention preferentially processes raised por› tions of irregularities present in a surface of a conductive  material, such as copper, formed over a surface of a work› piece, such as a substrate, while protecting recessed portions  of the irregularities with a corrosion inhibitor, thereby  processing and flattening the surface of the conductive  material. The electrolytic liquid is especially effective for  use in electrochemical mechanical polishing which com› prises elctrolytically polishing a surface of a conductive  material while rubbing the surface with a polishing pad. In  particular, a protective film is first formed by a corrosion  inhibitor on a surface a conductive material to prevent  excessive etching, followed by rubbing of the surface of the  conductive material with a polishing pad having appropriate  hardness and flatness, thereby selectively removing the  protective film formed on the surfaces of raised portions of  the conductive material while leaving the protective film  formed on the surfaces of recessed portions of the conduc- tive material. By subsequent electrolytic polishing of the  conductive material, the raised portions of the conductive  material can be preferentially processed, whereby the sur› face irregularities of the conductive material can be  smoothed out. A polishing pad for CMP, for example, can be  used as the polishing pad.  [0112]  A description will now be made of experiments on  processing (polishing) of copper which were carried out  with various electrolytic liquids, using a chip-testing  machine capable of processing a 40 mm-diameter area. The  chip-testing machine is capable of controlling the electrode  potential of copper and performs processing of copper by  polishing an exposed copper surface with a polishing pad  attached to a processing table while applying a voltage  between the copper and an opposing electrode. Polishing  (processing) of copper was carried out by rotating the  polishing table at 250 rpm (linear velocity 0.78 mis at a  position of 30 mm distance from the center) while pressing  the polishing pad against a substrate sample having a copper  surface at a pressure of 0.5 psi (35 g/cm 2 ).  During process› ing, current density or the electrode potential of copper was  kept constant. Current density or the electrode potential can  be measured even when the polishing pad is not in contact  with a copper surface (pressure  O  psi) as in the below› described test for determining the effect of a corrosion  inhibitor.  [0113]  The substrate sample, which was used in the pro› cessing experiments, is a silicon substrate with a copper film  (copper blanket wafer), having surface layers of: a oxide  film (SiO$_{2}$ )  having a thickness of about 250 nm; a barrier  metal film ofTaN having a thickness of30 nm; a copper seed  film having a thickness of 100 nm; and a copper plated film  having a thickness of 1500 nm, formed in this order.  A. Relationship of Various Organic Acids with Electrode  Potential  [0114]  Table 1 shows electrode potentials in polishing of  the copper surface of the above-described sample wafer,  carried out at a constant current density ( 40 mA/cm 2 )  in an  electrolytic liquid, using the above-described chip-testing  machine and various types of organic acids in the electro› lytic liquid. A lower electrode potential in the table indicates  that a higher electric current will flow and thus the process› ing rate will be higher, as compared to a higher electrode  potential, if processing is carried out to the same level of  electrode potential. A lower electrode potential is therefore  preferred.  TABLE 1  Concentration  Electrode potential at  No. Electrolyte  [mol/1]  pH 40 mA/cm $^{2 }$[VvsAg/AgCl]  Malanie acid  4  4.03  2  Maleic acid  4  4.33  3  Citric acid  4  5.36  4  Glycolic acid  4  5.47  5  Malic acid  4  5.73  6  Glyoxylic acid  4  4.99  7  Ascorbic acid  4  7.55 