P. A. Rosenthal and E. T. Yu
Department of Electrical and Computer Engineering and Materials Science and Engineering Program
University of California San Diego, La Jolla, CA. 92093-0407

P. J. Zampardi
IBM Microelectronics Division
1000 River Road M/S 861A, Essex Junction, VT 05452

Accurate determination at or near the nanometer scale of layer thickness, chemical composition, and dopant distributions in semiconductor heterostructure materials is crucial for analysis and optimization of advanced device structures. Scanning probe techniques offer unique and powerful capabilities in this regard.  In this work, we have used cross-sectional scanning force microscopy performed on cleaved samples in an ambient environment to measure quantitatively nanometer-scale differences in base-layer thickness and to assess dopant distributions in AlGaAs/GaAs heterojunction bipolar transistor structures.  Based on local variations in electronic structure, differences in base-layer thickness between samples can be measured with accuracy better than 10 nm, and variations in dopant concentration of a factor of two are readily detected.

The technique employed entails the application of a bias voltage with components at dc and frequency w between an electrically conductive probe tip and grounded sample surface. Specifically, we have used a variation of the standard EFM technique, referred to as capacitive force imaging, in which the amplitude of the scanning probe tip is detected as it oscillates in response to the electrostatic force component at frequency 2w. We have performed a detailed analysis of the contrast mechanisms in this mode of operation and find that the amplitude contrast is dependent on the bias voltage and the local dopant concentration through the dependence of the depletion layer depth below the sample surface on these quantities. Our studies also provide an approximate measure of the charge concentration on the cleaved cross-sectional surface, which we find to be of order ~ 4x10^-8 coulombs/cm², corresponding to a charged surface state density of ~ 3x10¹¹cm^-2.

We have used capacitive force imaging to characterize the structural and electronic properties of cleaved cross-sections of AlxGa1-xAs/GaAs epitaxial layer heterojunction bipolar transistor (HBT) structures, one with 50 nm base width and the other with 120 nm base width. The contrast observed allows us to clearly delineate the emitter, base, collector, and subcollector regions within the device structure, and to distinguish regions within the collector differing in dopant concentration by a factor of two (N_D varies between 3x10¹⁶ cm^-3 and 6x10¹⁶ cm^-3). We have also distinguished clearly between the base widths in these samples and have precisely measured the difference to be 63±3 nm, in excellent agreement with the nominal difference of 70±7 nm. These results demonstrate clearly the ability, using the capacitive force imaging technique, to obtain a reliable and precise measure of differences in device dimensions between samples with accuracy better than 10 nm, and to detect relatively small differences in dopant concentration, based upon direct measurement of electronic properties.  Furthermore, the demonstration of these capabilities in ambient air in a cross-sectional geometry implies the applicability of these techniques to realistic device structures with modest sample preparation requirements.