Semiconductor fabrication involves deposition, patterning and removal of various layers of materials on semiconductor wafers. Frequently, after deposition of a layer, it is necessary to planarize the deposited layer using chemical mechanical polishing (CMP). CMP involves abrasion of the layer with a polishing pad, in the presence of a slurry. Following the completion of the CMP, the slurry debris and contaminants must be removed to avoid contamination of integrated circuit layers. The scribe lines between integrated circuit dies are susceptible to such infiltration. U.S. Pat. No. 6,214,441 (the '441 patent) is incorporated by reference herein in its entirety. The '441 patent describes a method for sealing the circumferential edge of the wafer using a wafer edge exposure unit. A photosensitive material is applied to the wafer, and the edge of the wafer is exposed in a wafer edge exposure (WEE) unit. An edge sealing ring is thus formed, preventing infiltration of contaminants in the scribe lines.
U.S. Pat. No. 5,996,628 (the '628 patent) also discusses application of a WEE unit in a photo process for removing dielectric material from the circumferential edge of the wafer. By removing the edge material, damage to the dielectric layer and propagation of cracks to the wafer interior during handling is prevented.
In a typical configuration, a WEE has an irradiation system including a first light source, a lens for focusing the light, and a mask to limit the light to a small region. The WEE includes a movable rotating chuck for passing the circumference of the wafer under the first light source. The WEE further includes a notch searching unit that includes a second light source and an optical sensor, such as a charge coupled device (CCD) sensor or a CMOS imaging sensor. When a single notch at the wafer's perimeter passes between the second light source and sensor of the notch searching unit, the beginning of a full wafer rotation is noted. When the single notch is again detected by the optical sensor, the wafer has rotated through a complete rotation, and the entire circumferential edge of the wafer has been exposed to the irradiating system light source.
For a given photosensitive material, each part of the wafer edge is given a minimum dose of irradiating light to develop the photosensitive material. For a given light source of fixed intensity, the amount of time needed to expose the entire wafer edge is proportional to the circumference of the wafer. A 50% increase in wafer diameter (e.g., from 300 mm to 450 mm) results in a 50% increase in exposure time. Additionally, the notch searching unit may rotate the wafer by nearly a full rotation to locate the notch. So a 50% increase in wafer diameter also results in about a 50% increase in the notch location time. The increased delay in the WEE can reduce the duty cycle of downstream tools, reducing fabrication facility (“fab”) yield and profitability.