ELECTRON-BEAM QUALITY CONTROL OF THE CONDUCTIVE LAYER OF RESISTORS

V. I. GORBUNOV, D. I. SVIRYAKIN, N. Ya. FILISHOV, A. N. KOLUPAEV

A scanning electron beam and, in particular, electron-beam microscopy are finding ever broader application in the electronics industry in the inspection of various radio components. Scanning electron microscopy is used in the development and manufacture of semiconductor devices, materials research, determination of crystallographic orientation, measurement of the concentration of an alloying impurity, structural analysis, control of electrical characteristics, etc. Owing to the diversity of the fields of application of the scanning electron beam, newly developed methods of investigation must meet the specific requirements of each individual problem.

Thus, for example, the method described in [1] for checking diffusion processes in transistor multicomponent integrated circuits (ICs) by means of a scanning electron beam serves to reveal electronic junctions. The method is based on the principle of separation, by the internal field of the diffusion junctions of multijunction devices, of the charges of current carriers induced by the incident electron beam. But in order to reveal the boundaries of junctions, it is necessary to make a polished section on the specimens, so that the diffusion regions become accessible to the high-energy scanning beam, which, at the points where it strikes the semiconductor, generates electron–hole pairs as a result of the transition of electrons from the valence band to the conduction band. Moreover, the number of generated carriers is proportional to the energy of the primary beam. Current carriers created within the diffusion length of the junction are affected by the internal field of the junction. Directly at the junction all induced carriers are separated and, thus, a maximum current flows in the external circuit (the load circuit). After the beam has passed the junction, the number of separated carriers decreases and the current amplitude also decreases. In regions outside the influence of the junction field, the carriers recombine and do not affect the total current flowing in the external circuit. These current changes are the useful signal, which, after amplification, is reproduced on the screen of a cathode-ray tube by means of amplitude or brightness modulation.

For its practical implementation, this method requires a high-performance vacuum unit, a powerful electron gun with a refined electron-optical system for focusing and deflecting the beam, allowing the beam aperture to be reduced to fractions of a micron. The procedure for checking transistor structures cannot be classified as a nondestructive testing method, since preparation of the polished section leads

lead to destruction of the product being inspected and to its loss of operability. Therefore the electron scanning method can be used only in developing the technology for manufacturing multicomponent ICs and is unsuitable for mass production.

To carry out mass quality control of the current-conducting coating of resistors, it is necessary to have an electron beam with relatively low penetration capability, an aperture measured in units and tens of microns, and a low degree of evacuation of the vacuum system \((10^{-5} \div 10^{-6}\ \text{mm Hg})\). Proceeding from these requirements, an installation for electron-beam quality testing of the current-conducting coatings of resistors was developed, consisting of an electron-beam vacuum chamber, a focusing-deflecting system, a video amplifier, and a video monitoring device.

Fig. 1. Block diagram of the installation.

The block diagram of the installation is shown in Fig. 1. For rapid replacement of the items being inspected, the vacuum chamber is made demountable.

It consists of a four-electrode electron projector, a transition section (8), in which there are a branch pipe (12) for pumping air out of the instrument and pressure-gauge sensors (9) in the vacuum system, and a unit for fastening the product (10). The electron projector, consisting of a modulator (2), the first (3) and second (4) anodes, is taken from a standard LI-421 vidicon and makes it possible to obtain, in the plane of the product being inspected (11), a focused electron beam of the order of 20 μm. As the electron source, the cathode unit (1) from a demountable electron-optical system of VNIIЭLP design was used, which withstands repeated admission of air into the system. The electron gun assembly is inserted into a glass tube (6) with an internal conductive coating, which, in turn, is fastened to the transition section (8) of the instrument by means of a union nut (7). On the other side of the transition section a bulb (10) with a vacuum-tight electrical lead-through is installed for placing the products to be inspected and connecting the load resistance, from which the video-signal voltage is taken. The video signal is amplified by means of a UV-49 preliminary video amplifier (13) and fed to the video monitoring device (14) of the PTU-26M industrial television installation. Focusing and control of the electron beam are carried out by a standard FOS-34 focusing-deflecting system (5), placed on the neck of the electron-beam vacuum chamber. On the screen of the video monitoring device the operator observes the image of the surface layer of the product being inspected.

During operation of the electron projector, the focused beam

electrons scans over the surface of the item being inspected and charges it to the cathode potential. Then, during the interval between commutations, equal to the frame time of a television image, the potential of portions of the item surface rises to values proportional to their secondary-emission properties. As a result, a potential relief is formed on the surface, by which the current of the incident beam is modulated in the subsequent commutation periods. Secondary-electron emission from the surface creates a current in the opposite direction with respect to the current of the primary beam. The resulting value of the signal current is approximately equal to the difference between the number of primary electrons reaching the surface of the item and the number of secondary electrons leaving it.

The primary current is equal to the beam current if the potential of the item surface is not very close to the cathode potential, while the magnitude of the secondary current is determined by the secondary-emission coefficient \(\sigma\) and the primary beam current \(I_n\). Consequently, the signal current in the slow-electron mode will be equal to [2]

\[ I_c = I_n - \sigma I_n = I_n(1-\sigma). \]

The current of the primary beam reaching the surface of the item depends primarily on the distribution of potential over its surface, i.e., on the conductivity of its elementary regions and on the applied external voltage. If the potential of surface regions with low conductivity is close to the cathode potential, then not all electrons can reach it. A portion of the electrons, having a normal velocity component insufficient to reach the surface, is reflected back to the anode without reaching the item. The dependence of the secondary-emission coefficient \(\sigma\) on the properties of the scanned surface is even more complex. The value of the coefficient \(\sigma\) is defined as the ratio of the number \(N_2\) of all electrons emitted by the surface during time \(t\) to the number \(N_1\) of primary electrons reaching the target during this time [2].

\[ \sigma = \frac{N_2}{N_1}. \]

The value of \(\sigma\) depends on many factors of the material of the irradiated item, the nature of its treatment, especially the condition of the surface, temperature, etc. Having studied the dependence of \(\sigma\) on these factors, it is possible to monitor the surface layers of various items with fairly high accuracy.

As an example, the photographs presented in Fig. 2 and Fig. 3 show images of MUN-1-type resistors without a protective varnish coating and MLT-2-type resistors with the varnish coating removed, photographed from the screen of a VKU industrial television unit. In the photographs, individual points and bands that are defects of the resistive coating are clearly visible. In addi-

Fig. 2. Photograph of the image of a MUN-1-type resistor without a protective varnish coating.

Fig. 3. Photograph of the image of an MLT-2 type resistor with the lacquer coating removed.

As a result, on the VCU screen an image of the inspected article is obtained, enlarged by tens of times, which helps to improve the detectability of defects in conducting and dielectric coatings.

The images presented were obtained at a beam energy of about 600 eV and a voltage on the surface of the resistor of about 70 V. When the accelerating voltage on the anodes of the electron projector is increased, the depth of penetration of electrons into the substance increases, which makes it possible to perform inspection to a greater depth (on the order of 10 µm); in addition, the contrast and sharpness of the image increase, and by reducing the aperture of the beam it is possible to increase the magnification factor of the image on the VCU screen. Fig. 4 shows a photograph of the image of an MLT-2 type resistor at a voltage on the second anode of the electron projector of about 2 kV and a potential on the resistor surface equal to 150 V.

Fig. 4. Photograph of the image of an MLT-2 type resistor obtained at an increased voltage on the electron projector.

Work on a demountable vacuum electron-beam television installation has shown the fundamental possibility of effective television nondestructive quality control of current-conducting and lacquer coatings of radio components by means of a well-focused scanning beam of slow electrons. The described inspection method can provide both selective and 100% inspection of articles during their production.

REFERENCES

1. G. V. Lukianoff. Electrical Junction Delineation by scanning Electron Bean Technique, “Solid State Technol.”, vol. 14, No. 13, 1971, pp. 39–43.
2. A. E. Gershberg. Transmitting Television Tubes Using the Internal Photoeffect. Moscow–Leningrad, “Energiya,” 1964.
3. I. M. Bronshtein, B. S. Fraiman. Secondary Electron Emission. Moscow, “Nauka,” 1969.