Kelvin probe force and scanning capacitance microscopy on MOS structures

As the size of semiconductor devices is decreasing permanently, new techniques are required to probe their dopant profile reliably on a nanometer scale. Kelvin probe force microscopy (KPFM) and scanning capacitance microscopy (SCM) are the most promising techniques for this demand. KPFM (LevelAFM from Anfatec) enables the detection of the contact potential difference (CPD) between tip and sample and SCM (DI3100 from Veeco Instruments) probes the capacitance of the metal oxide structures formed in contact. In order to demonstrate the complementary information obtained by KPFM and SCM measurements, we used the pn junction in a static random access memory integrated circuit device where the n-well with a donor concentration of 2E17 cm⁻³ has been fabricated by implanting the p-epi with a nominal acceptor concentration of 2E16 cm⁻³ with P+ ions of energy 900 keV and a fluence of 1.7E13 cm⁻² [1]. As expected, KPFM yields a smooth variation of the CPD between the p-epi and the n-well amounting to 230 meV. SCM clearly shows the boxlike doping profile of this pn junction. The CPD variation can be modelled by assuming a partially compensated donor concentration in the n-well.
[1] M.W. Nelson et al., Electrochemical and Solid-State Letters 2 (1999) 475.