Our experience in the design of an ultra-high speed image sensor targeting the theoretical maximum frame rate is summarized. The imager is the backside illuminated in situ storage image sensor (BSI ISIS). It is confirmed that the critical factor limiting the highest frame rate is the signal electron transit time from the generation layer at the back side of each pixel to the input gate to the in situ storage area on the front side. The theoretical maximum frame rate is estimated at 100 Mega-frames per second (Mfps) by transient simulation study. The sensor has a spatial resolution of 140,800 pixels with 126 linear storage elements installed in each pixel. The very high sensitivity is ensured by application of backside illumination technology and cooling. The ultra-high frame rate is achieved by the in situ storage image sensor (ISIS) structure on the front side. In this paper, we summarize technologies developed to achieve the theoretical maximum frame rate, including: (1) a special p-well design by triple injections to generate a smooth electric field backside towards the collection gate on the front side, resulting in much shorter electron transit time; (2) design technique to reduce RC delay by employing an extra metal layer exclusively to electrodes responsible for ultra-high speed image capturing; (3) a CCD specific complementary on-chip inductance minimization technique with a couple of stacked differential bus lines.
In CoCrTa "s, the relationship between the microstrnctnre of grain boundaries and degree of intergranular exchange coupling was investigated. The degree of intergranular exchange coupling decreases as Ta content increases. By TEM observation, it was revealed that the boundary phase, which appears to be amorphous, surrounds the grains. The width of this boundary phase tends to become thicker as Ta content increases From the result of an E M analysis, the Cr content at the grain boundaries is higher than that in the grains. The bonndary phase may be non-magnetic due to its high concentration of Cr. On the other hand, in this experiment, no boundary phase was observed in CoNiCr and CoCrPt filum which have the higher degree of intergranular exchange coupling than CoCrTa flhna Therefore, the change in the boundary phase may be effective in redncing the degree of intergranuhr exchange coupling.
Ultra clean sputtering process (UC-process) was introduced in the fabrication of Co,,Crl,,Ta&r thin f ¶lm media Magnetic properties, microstructure and their effect on media noise are discussed against the thickness of Cr underlayer. By applying the UGprocess, coerclve force Hc shows high values of about 2.3 kOe isotropically until 10 nm in Cr thickness. High He of about 1.5 kOe remains at even 2.5 nm. UC-process is found to enable the enhancement of the formation of Cr segregated grain boundary structure, which reduces intergranular exchange coupling even in the media with 2.5 nm in Cr thickness. The ratio of readback signal to media noise S/" gradually increases with decreasingCr thickness, and shows the highest value at 2.5-5 nm. In the media with sufficiently separated grains by segregated grain boundaries, the reduction of the grain size with decreasing Cr thickness is found to be most effective for the improvement of S/ Nm.
The media noise performance is discussed in connection with grain size and intergranular coupling for the Co85.5Cr10.5Ta4, Co78Cr17Ta5, and Co62.5Ni30Cr7.5 thin-film media fabricated under the ultraclean sputtering process (UC process). It is clarified that the value of S/Nm is quantitatively represented as functions of the value of Hc/Hkgrain and the grain size in every kind of media. The value of signal-to-media noise ratio (S/Nm) increases with the increment in Hc/Hkgrain and also with the decrement in grain size. In the media with grains adequately separated by segregated grain boundaries fabricated by the UC process, grain-size reduction, taking account of the decrease in intergranular magnetostatic coupling, is required to obtain even higher S/Nm values.
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