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METHOD OF REDUCING ALKALI CONTAMINATION IN OXIDE FILMS FROM PLASMA PROCESSING STEPS

IP.com Disclosure Number: IPCOM000005575D
Original Publication Date: 2001-Oct-16
Included in the Prior Art Database: 2001-Oct-16
Document File: 4 page(s) / 62K

Publishing Venue

Motorola

Related People

Karl E. Mautz: AUTHOR

Abstract

METHOD OF REDUCING ALKALI CONTAMINATION IN OXIDE FILMS FROM PLASMA PROCESSING STEPS

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METHOD OF REDUCING ALKALI CONTAMINATION IN

OXIDE FILMS FROM PLASMA PROCESSING STEPS

by Karl E. Mautz

                 It is known in the industry that plasma processes can affect the concentration of alkali ions and other ionic contaminants in various oxide films, such as thermal oxides (or gate oxides), and dielectric oxides, such as BPSG, TEOS, BPTEOS, or PSG films.  The mechanism that causes the increase of alkali and mobile ions in the oxide is from 2 plasma processing effects.  The first is ion bombardment or energetic transfer of metallic ions in the plasma during etching to the wafer surface due to electrical attraction (from a voltage biased wafer chuck).  These ions are implanted at a depth determined by the wafer surface potential (dc bias), mass/charge ratio of the ion, and density of the oxide film exposed to the plasma (these are mobility factors).  The second effect is the tendency of the ions to migrate within the oxide film during high temperature plasma processes (such as photoresist removal - ashing), and this is a function of time the wafer is held at this higher temperature.

                 Typically, in the plasma etch processes the wafer temperature is held at a controlled value due to backside cooling of the wafer from the chuck.  The wafer surface temperature is typically higher than the chuck temperature due to surface chemistry reactions and physical processes such as ion bombardment.  Normal wafer surface temperatures are less than 125C for plasma metal etch processes and between 60-80C for other plasma etch processes.

                 However, some ash processes heat the wafer in excess of 250C, especially for post-metal etch ash processes in which higher temperatures are used to remove post-etch residual chloride species from the etched metal pattern wafers.  The process time is chosen to enhance this effect by using long wafer time-at-temperature effects to prevent corrosion defects (the total ash time of these processes greatly exceeds the normal photoresist removal and standard overash time).

                 It was found that longer ash time-at-temperature causes higher mobility of alkali and other contaminant ions, promoting migration from the wafer surface (or upper surface layer) deeper into the oxide film.  This effect makes it more difficult to remove or reduce the ions on the surface of the wafer (typically done using DI water rinse processes for surface removal).  This also increases the mobility of the ions implanted within the oxide (produced from the plasma ion bombardment effect) resulting in downward migration into the film.  This will reduce the efficiency of the surface-removing wet cleans.  These clean processes remove a thin layer of the surface oxide (skin) that is typically mobile ion rich.  Removing excessive film thicknesses causes subsequent processing problems, including loss of planarity and deterioration of the lithography quality.

                 The process optimization of the ash time and temperature is critical to prevent two major problems in integrated circuit manufacturing...