More Accurate Treatment of Two-Dimensional Ion Profiles

The following three factors must be taken into account to give more accurate two-dimensional ion profiles [2]:

• Masking layers are usually tapered at the edge and not perfectly abrupt, so that ions are gradually prevented from entering the silicon. Fig 4(a) shows the effect of implanting through a 60°C sloping mask edge with a stopping power similar to that of silicon.

• Secondly, lateral standard deviation or transverse straggle, DR_^ varies with the depth. This effect is most important in case of high-energy, light-ion profile such as 200 keV boron in silicon where the profiles have large negative skewness. In this case the peak of the lateral distribution moves closer to the surface than the projected range by 1500°A.

• Lastly we must consider the shadowing effect of the 7° beam tilt used to minimize channeling. For example if the gate stack is 0.5 microns high and has vertical walls, then the shadow can be 600°A long. This introduces an unexpected series resistance into the device. Also the length and side of the shadow for a particular case depends on the rotation angle of the wafer with respect to the beam tilt.[2]

Fig. 4 Modified masked ion profiles for 50keV arsenic. Contours show concentration of arsenic after implanting a dose of 10¹⁵ cm^-2. (a) Ions penetrate through the thinner regions of a sloping mask edge, increasing the doping near the corner. (b) a thick mask casts a shadow at the base which is 12% of the mask height for a 7^o beam tilt. Note also the variation in lateral standard deviation with depth.[2]