Increase in silicon charge coupled devices speed with focused ion beam implanted channels

Abstract

The clocking frequency of long channel CCDs is limited by the time taken to transfer charge from one well to the next. The bulk of the charge transfers rapidly by Coulomb repulsion but residual charge has to transfer by thermal diffusion, which is a slow process. If the n-type dopant density has a gradient in the direction of current flow, a built-in electric field is created, speeding up the charge transfer process. The focused ion beam system is uniquely suited to implant such a gradient of doping and has been used to implant buried-channel CCDs with 26-μm-long storage gates. The devices built to demonstrate this concept were shallow-buried-channel CCDs driven by 2-phase 5-V clocks. Arsenic ions were implanted at an energy of 220 keV and a gradient of doping increasing from 0 to 1.5×1011 ions/cm2 was superimposed on a uniform phosphorus implant. The challenge here was to get a low, monotonically increasing implant dose. Defocusing of the 25-pA As+ + beam to a diameter of ∼1 μm was found to be necessary. The maximum operating (clocking) frequency of the graded-implant devices was 41 MHz compared to a maximum frequency of 2.5 MHz for the same devices with uniformly doped channels. Such long-channel CCDs are of interest in 2-dimensional imaging applications and signal processing applications where high well charge capacity is needed without compromising speed or charge transfer efficiency.