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.
As the name implies, CMP acts through both chemical and mechanical action. The chemicals of the CMP slurry, typically water and ammonia, oxidize the surface of the wafer, causing the surface material of the wafer to corrode. After parts of the surface are corroded, the mechanical action of the rotating polishing pad and the abrasive particles within the slurry act mechanically to flatten the wafer surface and remove corroded residue.
In practice, several polishing steps are involved in polishing the surface of a wafer. Each successive polishing step uses a slurry having smaller abrasive particles than the previous slurry. The use of the different sizes of particles allows the polishing of the wafer to begin with a rough polishing and to finish with fine polishing. The wafer is rinsed with water or some other neutralizing substance between each step in order to neutralize the action of the etchant. The flow of water or neutralizing substance across the surface of the wafer also acts to rinse away the abrasive particles.
The process of polishing a wafer is thought to have a great influence on the degree of pitting that occurs within the wafer during subsequent processing. Pitting is a deformity in the crystalline structure of a wafer surface which impairs the ability of the wafer to be properly photolithographically processed and potentially interferes with the functioning of a semiconductor device formed upon the wafer. Although pitting forms within the surface of a semiconductor wafer during the cleaning phase of wafer production, the pitting is due to defects which originate during the polishing phase of production. The adherence of abrasive particles to the surface of the wafer during polishing leads to uneven etching by the polishing slurry, causing small defects within the wafer. During the wafer cleaning process, the defects from the polishing phase become pits in the surface of the wafer. Though the occurrence of pitting has been lessened by the formulation of advanced polishing slurries and the implementation of new polishing techniques, pitting is still a very real problem which causes the significant rejection of a portion of semiconductor wafers produced.
What is needed is a method of polishing the surface of semiconductor wafers which allows for the global planarization characteristic of chemical mechanical polishing, but prevents surface pitting and other surface defects. Further needed is a method of polishing which prevents the subsequent pitting of a wafer surface which is compatible with standard chemical mechanical polishing procedures so as not to require a significant capital outlay for new polishing equipment.