Fabrication of ultra-shallow p-n junctions as piezoresistive sensors for AFM deflection sensing

Piezoresistive deflection sensing is a promising approach for massively parallel AFM cantilever arrays. It is well known, that p-type Si exhibits piezoresistivity [1]. We have fabricated boron-doped piezoresistors in n-type Si with pn-junction depths down to 10-20 nm. For p-type Si layers below 10 nm it is known, that the piezoresistive coefficient in <110> direction is much larger than for bulk p-type Si due to quantum confinement effects [2]. This was achieved by a sophisticated process technology called point defect engineering (PDE). Using 400 keV Si+ ion irradiation a vacancy-rich layer near the wafer surface was created [3]. A 10nm thick boron layer was deposited as a solid source for subsequent diffusion using Rapid Thermal Annealing (RTA) at temperatures between 900°C and 1000°C. Secondary ion mass spectrometry (SIMS) analyses were carried out to obtain and compare the boron atomic concentration profiles. These show, that the transient enhanced diffusion of boron can be retarded by this fabrication method, and pn-junction depths of about 10-20 nm can be achieved. In comparison, pn-junctions were fabricated using low energy ion implantation of 1 keV B+ ions and subsequent RTA. SIMS profile analyses show, that the pn-junction depth is larger, but still in the range of <100 nm.
The sheet resistance of the fabricated ultra-thin p-doped layers were measured using a dedicated layout, which allows extracting the sheet resistance using conventional two-point probe measurements of U-I-characteristics. These measurements show, that the PDE fabricated layers exhibit a lower sheet resistance than the B+ implanted layers, even though the conducting layer thickness is smaller. This shows, that the PDE process not only yields very shallow pn-junctions, but also that the boron activation during RTA is improved compared to the ion implanted samples.
Finally we present sensitivity measurements for the completed cantilever chip, which show excellent sub-nm sensitivity.

[1] C. S. Smith, Phys. Rev. 94 (1954) 42
[2] T. Ivanov, PhD Thesis (2004), University of Kassel
[3] K.-H. Heinig and H.-U. Jäger, Proc. of 1st workshop of ENDEASD (European Network on Defect Engin. of Advanced Semiconductor Devices), C. Claeys, (ed.), Santorini, Greece, April 1999, p. 294