Nihon Micro Coating Co., Ltd. (Tokyo, JP) earned U.S. Patent 7,857,876 for diamond clusters are used as a polishing material of free abrading particles. Each is a combination of artificial diamond particles having primary particle diameters of 20 nm or less and impurities that are attached around these diamond particles.
According to inventors Noriyuki Kumasaka, Yuji Horie, Mitsuru Saito and Kazuei Yamaguchi, the density of non-diamond carbon contained in the impurities is in the range of 95% or more and 99% or less, and the density of chlorine contained in other than non-diamond carbon in the impurities is 0.5% or more and preferably 3.5% or less.
The diameters of these diamond clusters are in the range of 30 nm or more and 500 nm or less, and their average diameter is in the range of 30 nm or more and 200 nm or less.
Such polishing material is produced first by an explosion shock method to obtain diamond clusters and then removing the impurities such that density of non-diamond carbon contained in the impurities and density of chlorine contained in other than non-diamond carbon in the impurities become adjusted.
The object of this invention to provide a polishing material capable of producing with good reproducibility products of a specified level of accuracy without spots, as well as a method of producing such polishing material.
Spots come about for the following reasons. Diamond clusters are pressed against the surface of a target object to be polished only while they are being held in the surface portion of a polishing cloth which is pressed against the surface of the target object. Since this polishing cloth and the surface of the target object are moving relative to each other, the diamond clusters on the surface of the polishing cloth are caused to act on and polish the target object surface.
Particles of artificial diamond obtained by an explosion shock method have rounded corners, and there are no sharp edges. Diamond clusters are clusters of such diamond particles, having an indefinite shape without sharp edges, but they have protruding parts. During a texturing process, these protruding parts engage with the surface portion of the polishing cloth so as to support the clusters in the surface part of the polishing cloth. In other words, it may be thought that the diamond clusters are then temporarily attached to the surface portion of the polishing cloth.
Since diamond clusters have indefinite shapes as a whole, the force for attaching them temporarily to the polishing cloth is individually different. Moreover, the force for the attachment due only to the protruding parts of the diamond clusters is not sufficiently strong such that the density of diamond clusters supported in the surface part of the polishing cloth also varies locally, and this is believed to be the cause of spots that are generated. It is also believed that the diamond clusters in the slurry are in a condition of easily being taken into the internal part of the polishing cloth.