OTS (July 2007) -- At the April FSI Knowledge Services Seminar™ Series in Singapore and Shanghai, China, Ms. Ling Tze Tan of Chartered Semiconductor Manufacturing described applications of the ANTARES® cryokinetic aerosol cleaning tool in 90nm BEOL production. Her presentation focused on cleaning after via etch and after dielectric film deposition reflecting a growing widespread concern in the industry about the potential for water and chemical damage to copper and porous dielectrics.
She reported good control of particle excursions and high removal rates of surface defects on patterned wafers, which translated to improved yield, and concluded that cryogenic aerosol cleaning with the ANTARES® system could be used for 90 nm technology and beyond with no adverse impact to device performance.
Both etching and ashing steps create particulate contaminants that must be removed before further processing, usually through chemical- or water-based cleaning processes. In full-via-first integration schemes, high aspect ratio openings create deep wells through the dielectric layer that make removal of chemical and water and subsequent drying a challenge. Combinations of hydrophobic and hydrophilic surfaces in a via opening can result in drying residues or watermarks which interfere with subsequent patterning steps and conductive layer formation.
Figure 1: SEM images of wafer maps and SEM images showing dramatic defect reduction after using the ANTARES® System
Additionally, materials used in advanced processes, particularly copper and porous dielectrics, are adversely affected by exposure to water and chemical. Vias in copper interconnect structures are typically formed by reactive ion etching through the deposited low-k dielectric layer. A thin etch stop layer below the dielectric terminates the etch process and protects the underlying copper line. Etch stop layers are deposited using chemical vapor deposition processes that may leave small pinhole defects. The etch process, and the subsequent ash process used to remove photoresist, can stress and enlarge any pinhole that occurs at the location of a via, exposing the underlying copper to any subsequently applied chemical agent. Water or chemicals can attack the underlying copper through the pinhole defects in the etch stop layer causing pits and corrosion that interfere with good electrical continuity between the via and the copper line.
Porous dielectrics are also prone to water damage. In addition to surface defects, such as watermarks or particulate contaminants, the porous materials may absorb water, changing their electrical characteristics. The ANTARES® system cryokinetic aerosol cleaning process is a dry process operating at cryogenic temperatures and therefore not subject to these limitations. The cleaning media is a high velocity aerosol mixture of solid, liquid, and gaseous nitrogen or nitrogen-argon gases. The aerosol clusters collide with and detach contaminant particles from the wafer surface. The particles are subsequently entrained in a laminar flow of gas, which carries them out of the cleaning chamber. No rinsing or drying is required.
Cryokinetic aerosol cleaning provides a desirable alternative to water and chemical based cleaning processes. The nitrogen and argon gases used are completely inert and will not attack copper or adversely affect the performance porous dielectric materials. Its importance will increase as the use of these water sensitive materials continues to grow in BEOL processes. There are also a number of FEOL applications, such as cleaning after gate formation, silicidation levels and post ion implant where customers are benefiting from the defect removal / yield enhancement effects of the ANTARES® systems.
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