The WFPC2 CCDs are thick, front-side illuminated devices, with a format of 800x800, 15x15 micron multi-pinned phase (MPP). MPP allows CCD exposure with the total inversion of all phases. The Si-SiO_2 interface, at the surface of the CCD, is pinned at the substrate potential, directing signal charge away from the Si-SiO2 interface states towards the buried n-channel. Figure 4.1 shows a schematic which illustrates the principle of MPP (modified from Janesick et al. 1989). The front-side Si-SiO_2 interface significantly affects the performance of CCDs, so MPP operation yields many practical benefits including reduced dark noise, better charge transfer efficiency (CTE), rapid removal of residual images, excellent pixel-to-pixel uniformity, and improved radiation hardness. MPP technology has been demonstrated and characterized in both Loral (Janesick, et al., 1989) and Tektronix devices (Woodgate, et al., 1989). The CCD sensors for WFPC2 were made by Loral in 1991 and processed and packaged for flight at JPL.
Figure 4.1: MPP Operating Principle.
The Loral CCDs are illuminated from the 'front' surface, i.e., the light passes through the polysilicon gate structure overlying the 10 micron thick active silicon layer. Because the WFPC2 devices are front-side illuminated and supported by a bulk silicon substrate, the CCD surface is flat, which has reduced the uncertainties in the astrometric calibration to about the 1/10 pixel level.
In this section the performance of the WFPC2 CCDs is reviewed, and compared to the WF/PC-1 devices. A summary of device characteristics is given in Table 4.1.
Table 4.1: Comparison of WF/PC-1 and WFPC2 CCDs.