去离子水中溶解二氧化碳对减少兆声清洗过程中晶片损伤的作用

Particle removal from wafer surfaces can be accomplished by  irradiation of cleaning fluid by sound waves in the MHz frequency  range. Unfortunately, unless proper cleaning conditions are chosen,  megasonic irradiation may also result in damage to fragile wafer  features. Here, we demonstrate a strong effect of dissolved CO₂ levels on the reduction of wafer damage during megasonic  cleaning. Test structures with L/S patterns were irradiated with  0.93 MHz sound waves at varying power densities and dissolved  CO₂ levels, in a single wafer spin cleaning tool, MegPie® .  Dissolution of increasing amounts of CO² in air saturated DI water  caused a significant decrease in the number of breakages to line  structures and also decreased the lengths of the line breakages, at  all power densities up to 2.94 W/cm² . This ability of dissolved CO₂ to protect against feature damage correlates well with its ability to  suppress sonoluminescence in sound irradiated DI water.

Megasonic cleaning is routinely employed in the semiconductor industry for removal of  contaminant particles and residues from wafer surface. With the progression of  technology nodes to smaller sizes, megasonic cleaning is faced with the challenge of  maintaining high cleaning efficiency without inducing damage to small features (~100  nm and less). Judicious choice of cleaning chemicals and control of dissolved gases has  been proposed to minimize wafer damage.

Sonoluminescence (SL), the phenomenon of release of light when liquid is irradiated  by sound waves of sufficient intensity, is a sensitive indicator of cavitation events (2). It  is widely believed that acoustic cavitation is responsible for both wafer cleaning and  damage. Intensity of SL correlates with intensity of cavitation, mostly transient cavitation  that is believed to cause damage to wafer features. Therefore, control of transient  cavitation has potential application in controlling damage.

The authors of this paper have previously reported the strong ability of dissolved CO₂ to quench SL generation in DI water . It was shown that increasing amounts of  dissolved CO₂ progressively decreased SL and ~150 ppm dissolved CO₂ was found to be sufficient to suppress SL generation in DI water under all conditions of power densityand duty cycle of acoustic energy at frequency 0.93 MHz It was also shown that CO isnot just incapable of supporting SL generation in DI water, but also a strong inhibitor ofSL generation in the presence of other gases that support SL generation.

Fig1

Damages resulting from the exposure to megasonic energy were examined at low  resolutions using an optical microscope and at high resolutions using a field emission  scanning electron microscope (Hitachi S-4800, FESEM) (Fig. 3 and Fig. 4). For  structures-I, width of the line features were small (~36 nm) and could not be resolved in  an optical microscope. At a magnification of 200X, a large number of damage sites  covering a representative area (0.3 mm x 0.4 mm) of the wafer, could be adequately  resolved. These damage sites appeared as small dark spots (Fig. 3). Images for ten nonoverlapping and contiguous regions of damages, covering an area of 1.2 mm²  and  occurring at a given power density, were collected and counted using the particle analysis  tool in ImageJ (version 1.43u). For test structures-II, width of the lines were thick enough  (~67 nm) to resolve in an optical microscope. A representative area of 0.08 mm² constituting ten pairs of thin and thick lines was examined and the damage sites were  counted manually. Total number of damages (line breakages) for both structures were  scaled to 1 mm² area of the wafer and plotted as a function of power density (Fig 5).