Radical Formation in Hydroxylamine-Copper Chemical Mechanical Planarization Processes

Abstract

Hydroxylamine is commonly employed in laboratory chemistry as a reducing agent, such as for reduction of Cu͑II͒ to Cu͑I͒, yet a sulfate salt oxidizes copper during chemical mechanical planarization ͑CMP͒ processing employed in the manufacture of semiconductors. Electrochemical, Fourier transform infrared, and electron paramagnetic resonance measurements were conducted to verify formation of a nitrosyl radical complex, similar to Fremy's salt, that is believed to be the molecular species responsible for catalytic copper oxidation during CMP processing.Semiconductor memory devices fabricated with submicrometer copper interconnect traces typically employ chemical mechanical planarization ͑CMP͒ of the silicon wafers following deposition of the copper interconnect layer. The CMP process removes excess metal, resolves individual conductor traces, and produces a planarized base for fabrication of the succeeding layers. Metal CMP may be considered a process of chemical oxidation of the surface metal layer with concomitant removal of the oxidized surface using a microslurry polish. Several classical chemical oxidizing agents, including hydrogen peroxide, peracetic acid, ammonium persulfate, and ferric nitrate, have been used to oxidize copper. The rate of copper metal removal during CMP processing of copper-coated wafers using hydrogen peroxide is believed to be related to the concentration of hydroxyl radical. 1 Soluble complexes of iron and copper have been reported 1 to promote production of the hydroxyl radical, speeding copper removal by CMP. Application of copper complexing agents under CMP conditions, in the absence of hydrogen peroxide, may also function as oxidizing agents to affect the removal rate of copper as well.Hydroxylamine, a strong reducing agent, is known to reduce soluble copper as Cu(II) → Cu(I) and soluble manganese as Mn(III) → Mn(II), 2,3 yet copper metal becomes oxidized in the presence of hydroxylamine sulfate or nitrate during CMP processing. Nitrogen atoms found in common nitrogen-containing compounds reside in oxidation states between Ϫ3 or N Ϫ3 and ϩ5 or N ϩ5 . Hydroxylamine is an intermediate nitrogen, and others that have been well documented. Nitrogen compounds from nitrous oxide through nitrate are clearly oxidizing agents, while those including ammonia through hydroxylamine are considered to be reducing agents with nitrogen gas being a relatively unreactive neutral compound. Previous efforts have been conducted employing hydroxylamine-based slurries for CMP of copper films for which the chemical oxidation of copper metal was proposed to be a free radical process. 4,5 This implies that normally reducing hydroxylamine becomes converted to an oxidizing-type compound during the reactive chemical process. Voltammetric and other electrochemical measurement techniques 6-10 have been employed to study the reactive nature of hydroxylamine toward transition metals. Electroreduction of NO on a single crystal of gold revealed a direct transition to hydroxylamine. 9 Voltammetric determination of hydr...

Electrochemical Measurements of Passivation Bilayers on Copper in a CMP System

“…The data of Table X indicated that Cu 2 O was abundant at pH 3.5 while CuO/Cu͑OH) 2 was abundant at pH 7.5. Previous measurements 25 demonstrated that the redox potential for HAS at pH 3.5 was 0.65 V so that the 0.52 V reduction of Cu͑I͒ to Cu͑0͒ was indicated. At pH 7.5, the redox potential for HAS ͑hydroxylamine͒ was only 0.26 V, insufficient to reduce copper oxide.…”

“…͑See the discussion on the XPS data later in this section.͒ Hydroxylamine is a strong reducing agent capable of removing the oxide layer from the copper wafer surface, but in the presence of unoxidized copper metal, and certain other metals, HAS is catalytic for radical oxidation of the metal. 25 The chemical composition of a copper surface film is affected by air oxidation, chemical oxidation, and parasitic passivation reactions represented by the following equations as …”

Electrochemical characterization of Cu dissolution and chemical mechanical polishing in ammonium hydroxide–hydrogen peroxide based slurries

“…Sodium based solutions can not be used for actual semiconductor processing due to contamination issues. However, in order to understand the electrochemical aspects of metal dissolution in slurry chemicals, salts such as Na 2 SO 4 , NaNO 3 and NaClO 3 have been used as supporting electrolytes to reduce the solution resistivity [10,14,[21][22][23][24]. A kink was observed in the anodic branch of the plot for solutions containing hydrogen peroxide concentration of less than 0.5%.…”