A method has been developed for measuring the dependence of the electron probe diameter d in a scanning electron microscope SEM on the beam current J. The relationship for the CAMSCAN CS-44 SEM is d(J) ~ J 1/4 , whereas electron probe formation theory gives d(J) ~ J 3/8 ; the reasons for these differences are considered. Key words: electron probe, probe diameter, SEM. A major objective characteristic of a scanning electron microscope (SEM) is the minimum electron probe diameter d min ; it appears in the calculation of the resolution in microprobe analysis [1] and in algorithms for establishing the undis- torted video signal profile. Also, d min is important in measurements on the geometrical parameters of submicron relief in slow secondary electron collection [2, 3]. However, in most cases d min is not known during use. It is considered that the minimum probe size is attained with a working algorithm for the SEM amounting to the setting of the minimum possible stop for the objective lens, reducing the probe current to the noise limit, and reducing the working distance. In practice, other factors are often important, in particu- lar the signal-to-noise ratio for low-contrast objects, and the scope for parallel derivation of the image in back-scattered elec- trons, which requires currents larger by an order of magnitude and so on. The dependence of d min on the probe current J deter- mines the scope for adjusting the current in limiting magnification states. However, the literature shows that there are no methods of determining that dependence by experiment. Here we propose a method for the experimental determination of d min (J), which has been checked out on the CAMSCAN CS-44 SEM. Traditionally [1], d min (J) is derived theoretically. One assumes that the radius of the circle representing the cross sec- tion of the focused probe is determined by the joint action of the objective-lens aberrations [4]: r 2 min = r g 2 + r s 2 /16 + r c 2 + r d 2 , (1) where r g = (J / I) 1/2 (πγ) –1 is the radius of the gaussian image; r s = C s tan 3 γ is the spherical-aberration disk radius; r c = = C c tan γ (∆U /2U) is the chromatic-aberration disk radius; r d = 0.6λ/sinγ is the Airey disk radius; J the electron beam cur- rent; I electron source brightness; γ semivertex angle; C s and C c are the spherical and chromatic aberration coefficients; ∆U the electron energy spread; U electron energy; and λ electron wavelength. Formula (1) applies if the object is in the plane of minimal scattering and there are no other causes of increase in probe diameter. The position of that plane is dependent on various parameters such as the detailed values of C s and C c , the accelerating voltage, probe current, working distance, stop diameter, and so on. The procedure for adjusting the instrument does not usually provide the best set of parameters, but nevertheless, wide use is made of estimates of the minimum probe Measurement Techniques,Vol. 47, No. 5, 2004 BEAM CURRENT DEPENDENCE OF SEM ELECTRON PROBE DIAMETER LINEAR AND ANGULAR MEASUREMENTS Yu. A. Novikov, A. V. Rakov, and M. N. Filippov UDC 537.533 Translated from Izmeritel’naya Tekhnika, No. 5, pp. 13–15, May, 2004. Original article submitted February 9, 2004. 0543-1972/04/4705-0438 © 2004 Plenum Publishing Corporation 438