Low-Frequency Scanning Capacitance Microscopy
- PDF / 1,977,664 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 34 Downloads / 230 Views
AFM).
In the past three basically different approaches were used to capacitance imaging. The microscopes based on the RCA videodisk pickup [1, 3 - 5] use the change of the resonant frequency of a microstrip resonator, caused by changes of the probe/sample capacitance, connected to it. They operate at about 1 GHz. A similar heterodyne solution, based on a lumped element circuit, was followed in reference [6]. It had a working frequency of 90 MHz. The second possibility was the application of a scanning force microscope, with a charged tip attracted to the surface by Coulomb interaction [7]. The representative of the third concept is the low-frequency capacitance microscope [8], using phase-sensitive demodulation of the ac current flowing through the probe/sample capacitor. Until now it is the only concept that makes the imaging of the Mat. Res. Soc. Symp. Proc. Vol. 500 © 1998 Materials Research Society
components of the complex capacitance possible. It operates in the MHz region. By means of active shielding of both probe and input stage - a typical low frequency technique - the parasitic stray capacitance of the probe could be significantly reduced. The measurement principle itself helped to separate it from the capacitance of the input of the electronics, also suppressed by bootstrapping. By further modifications the probe/sample capacitance was reduced to a few hundred aF [9]. Recently SCMs became commercially available [10, 11]. They combine the SCM with AFM and use a metal-coated AFM cantilever as SCM probe. This approach makes the separation of contributions of surface topography, detected by AFM, and of inhomogeneity of dielectric film, possible [12]. Most of the important properties, like the lateral resolution, signal-to-noise ratio, etc., strongly depend on the shape of the probe. Its stray capacitance and its dependence on the distance from the probe axis play a crucial role [13]. In this paper we shall present an analysis of a shielded, conical probe with spherical apex. The assumed imaged surface was a conducting plane or a conducting plane covered by an insulating film. The lateral sensitivity to capacitance and to dielectric losses has been derived. The obtained data have been used for a discussion of optimal input stage of the capacitance microscope, designed to work at still lower frequencies. MAPPING THE ELECTROSTATIC FIELD The exact calculation of the capacitance between a surface with arbitrary topography and a needle perpendicular to it, is a rather complex problem. It would require the solution of the Laplace equation AV = 0 pertinent to the space between electrodes and the electrostatic field at the electrodes, coupled to the charge density through the Poisson equation, with complicated boundary conditions. The problem is further complicated by the local presence of dielectric, properly described by Maxwell's 1st equation. Therefore numerical computation, using the Finite Element Method (FEM), has been used [9, 14]. The employed program [15] is able to calculate the flux and capacitance bet
Data Loading...
