Method of reducing particulate contamination during polishing of a wafer

#The invention relates to the production of wafers for use in semiconductor devices. More specifically, the invention relates to a method of polishing wafers with emphasis on the reduction of particulate contamination remaining upon the wafer surface after the polishing process.

#In the production of semiconductor wafers, it is important to produce a wafer having a very smooth and planar surface. Irregularities in the wafer surface introduced during wafer processing may be amplified during further processing and may result in an unusable wafer. Irregularities or defects within the wafer surface may also lead to ineffective lithographic processing, resulting in a defective semiconductor device. Therefore, the minimization of defects on the wafer surface is of the utmost importance.

#Chemical mechanical polishing, or CMP, is the most common method of polishing wafer surfaces in order to obtain a planar surface. In CMP, a slurry of etching chemicals and abrasive particles is applied to the surface of a wafer through a porous polishing pad. The action of the polishing pad contacts the slurry with the wafer surface.

#Having thus described the invention in general terms, reference will now be made to the accompanying drawing, wherein:

#FIG. 1 is a schematic flow diagram which illustrates an embodiment of the present invention.

#The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

#During the production of a wafer, such as a silicon wafer, the surface of the wafer is polished with a polishing slurry containing an alkali etchant chemical, such as ammonia, and abrasive particulates, usually silica. It is theorized that problems with pitting of the surface, observed during the cleaning phase of wafer production, first develop during the polishing of the wafer. During polishing, the silica particles in the polishing slurry develop an ionic attraction to the pure silicon surface of the wafer. The attraction is due to slight charge differentials between the silicon and silica and is heightened by rapid pH changes in the polishing slurry caused by the pH of the rinse water used between polishing steps. Because of the charge differential, the silica strongly adheres to the surface of the silicon wafer. After polishing, the silica particles which are adhered to the silicon prevent the chemical etchant within the polishing slurry from acting on the silicon material underlying the silica particles. The differential etching between those areas of the wafer that are and are not protected by a silica particle causes defects within the wafer surface. During subsequent cleaning steps, such as SC 1 cleaning, the defects develop into pitting within the wafer surface. The pits typically cause crystal defects which are highly undesirable in wafer production. Thus, pitting which is first evidenced during the cleaning stage of wafer production, is thought to be rooted in the adherence of abrasive particles to the wafer surface during the polishing stages of wafer production. The ability to remove particles from the surface of the wafer prior to the cleaning stage, therefore, lessens the occurrence of pitting within the wafer.

#Referring now to FIG. 1, as shown in block 1, semiconductor wafers to be polished are supplied, such as from a rough polishing operation. Prior to beginning the main polishing operation, the sizes and types of abrasive particles and the types of etchants to be used during polishing are determined, as shown in block 2. Polishing with the abrasive slurry may theoretically be accomplished in one step, but is generally accomplished in multiple steps, with each polishing step using smaller abrasive particles than previous polishing steps. By utilizing multiple polishing slurries having abrasive particles of decreasing particle size, large surface imperfections are polished out by the large abrasive particles and smaller defects are polished out of the surface by the smaller sized abrasive particles.