High value implanted resistors for microcircuits

Boron ion implantation has been used to fabricate high sheet resistance p-type junction resistors in silicon substrates. Thermally grown SiO
2
and conventional photolithography were employed to define the resistor geometries. Ion doses in the range 0.5 × 10
¹³
to 10 × 10
¹³
ions/cm
²
with energies ranging from 30 to 55 keV followed by anneal at 950°C were used. The temperature coefficient of resistance (TCR), voltage coefficient of resistance (VCR), junction Characteristics, and noise level of these resistors have been studied for sheet resistances ρ
x
, from 0.8 to 11 kΩ/square. Over this range of sheet resistances the TCR increases smoothly from approximately 800 to 4000 PPM/°C with the lower TCR corresponding to the lower sheet resistance. For 3 kΩ square implanted resistors, the variation of resistance with temperature closely matches that found for a standard boron base and resistor (B&R) diffusion having a sheet resistance of 140 Ω/square. The junction leakage and the noise level of the implanted resistors can be made comparable to that obtained for diffused resistors. The implanted resistor exhibits a positive VCR, which increases with increasing sheet resistance as a result of depletion-layer pinch-off action from the substrate. Details of the implant conditions and process control are discussed. Experimental results demonstrating the compatibility of the resistor implantation process with microcircuits using low current, high β diffused bipolar transistors are presented.