EE 5621

PROBLEM SET 3

S. G. Burns

Due: Friday, 15 October

1. You are to ion implant B (boron) through a 0.5 mm SiO₂ masking oxide on a Si wafer such that the peak concentration will be at the SiO₂-Si interface. Use graphs to estimate the acceleration potential in keV. Repeat for As. For both cases estimate the resultant straggle.

2. How many ions are required to obtain a uniform doping profile of N_D = 2 x 10¹⁷ cm^-3 to a depth of 1μm? How long will it take to ion implant a 12 inch (300 mm) diameter wafer assuming a beam current of 200 μA? Assume singly ionized ions.

3. Similar to Problem 2, use ion implantation with a variable acceleration and design your ion implant to obtain a uniform doping profile with N_D = 1x10¹⁹ atoms/cm³ and form an abrupt junction at 1μm. This comprises the predeposition phase.

(i) What is the dose?

(ii)

n(x)

How long will it take to ion implant an 8 inch (200 mm= 20cm) diameter wafer assuming a beam current of 700 μA? Assume doubly ionized ions.

This predeposition ion implantation is now capped and we proceed with a finite (limited) source diffusion at a temperature T₁ for a time t₁.

(a) Sketch the resultant new doping profile and answer the following:

(b) During this time used for this finite source diffusion, the surface concentration will (INCREASE, DECREASE, REMAIN THE SAME); the junction depth, x_j, will (INCREASE, DECREASE, REMAIN THE SAME); and the dose, Q_o, will (INCREASE, DECREASE, REMAIN THE SAME). Circle your choices.