On a possibility of condensed matter surface profiling by a high brightness electron probes

Abstract

As is known, laser ablation is widely used in time-of-flight mass spectrometry as well as in modem micro-and nanoelectronics, microelectromechanics for structural surface modification and profiling of condensed substances [1,2].In the present work the phenomenon of ablation is treated as a fast process of phase bound-free transition in excited or ionized structural elements (atoms, molecules) of condensed substance exposed to intense corpuscular radiation, in particular, intense electron fluxes. It is known that interaction of a substance with intense either laser or electron beams exhibits some common regular features, which have not yet been interpreted unambiguously. These are the following. 1. The phenomena of laser and electron ablation are of threshold nature, i.e., they appear only when the beam intensity or the electron flux power density (P) exceeds some threshold value (PI) characteristic of a given substance. For a wide range of condensed media, PI � 50 MW/cm 2 [3].2. There is a finite time delay (M ) between the onset of the substance exposure to a concentrated energy flux and the moment of phase transition of the substance to plasma. The time I1t strongly depends on the P level and can widely vary from � 10-8 s (P � PI) to � 10-13 s (at P » PI)The purpose of the present work is to study the physical mechanism of the above features.To calculate the rate of incident radiation energy absorption by a condensed substance (in units of W/m 2 ), we consider the condensed substance as a set of two interrelated subsystems:those of electrons and phonons. Based on the above scheme, we calculate estimates for the thermal conduction flux power density (e) existing in this case. In a one-dimensional approximation, the thermal flux propagating from the surface into the bulk of the substance e , can be expressed as 1where n f is the phonon concentration, V s is the sound velocity, & is the average energy transferred by a phonon at its transition with a local temperature T + I1T to the region with the temperature T. In Debye's approximation, the average phonon energy is defined by the known expreSSIOnwhere N is the concentration of atoms in the substance, k is the Boltzmann constant, Debye temperature, � == % . Integrating (2) in view of (1) yields e � r . N . k(} . � .(2) () is theWhere r = r(�) is a dimensionless parameter whose numerical value is close to r � 0.5 at 978-1-4244-6644-3/10/$26.00 ©2010 IEEE 178