5, and (c) 10 kV. Figure 4d shows a BEI of the exceptional topographical and chemical contrast of this same region. This particular device had very low secondary emission characteristics. In figure 5a a BEI of a probe mark was used to reveal information not seen in (b), an SEI.

3. Accelerating Voltage

The accelerating voltage of the electron beam is a high potential difference applied between the filament (cathode) and anode of the SEM electron gun. This voltage accelerates the electrons emitted from the cathode towards the specimen surface. Commercial scanning electron microscopes are capable of a wide range of accelerating voltages from 1 to 50 kV. The use of either a high or low accelerating voltage is advantageous for different types of specimens. In general, the higher the accelerating voltage, the smaller the diameter of the primary beam, resulting in better point-to-point resolution. However, for semiconductors, the minimum spot size obtained by using a higher accelerating voltage does not always yield the optimum micrograph.

If the accelerating voltage is reduced, the electron beam penetration into the specimen is reduced. This may result in improved contrast due to both an increase in secondary electron emission at the surface of a device and a decrease in the background secondary emission generated by backscattered electrons at considerable distances from the beam impact point. The increased emission improves the SNR, thus the contrast.

To illustrate the effects of accelerating voltage on secondary electron emission, six different accelerating voltages were used to obtain SEM images of the same semiconductor device; the resulting micrographs are displayed in figure 6. The accelerating voltages chosen represent a typical range available on most commercial scanning electron microscopes: 2, 3, 5, 10, 20, and 30 kV. The same beam current was used in each case. The chemical and topographical contrast deteriorates as accelerating voltage is increased. There is also a progressive increase in the noise, as indicated by the grainy appearance of the micrographs made with higher voltages. The image with the most noise is at 30 kV where the beam penetration is at a maximum so that fewer secondary electrons are generated within an escape depth of the surface. It can be concluded that lower accelerating voltages are superior to higher accelerating voltages for obtaining better contrast and less noise in the imaging of this semiconductor device.

Figure 7 demonstrates the superiority of lower accelerating voltages for resolving surface detail at low magnification. Figure 7a shows an area of a microcircuit examined using a primary electron beam of 3 Note the resolution of surface structure on the metallization. Figure 7b shows this same area examined using a 30-kV primary electron beam, and it is obvious that most of the surface structure is obscured. The loss of visible surface structure is the result of excessive penetration and diminished surface electron emission.

Figure 5.

b.

Micrograph of a wire bond and probe mark in which the shadow effect produced by the backscattered electron image reveals some upturned metal not seen in the secondary image.

Figure 6.

a.

Micrograph recorded at 2 kV in which the chemical
and topographical contrast is outstanding.

b.

Micrograph recorded at 3 kV in which the chemical

and topographical contrast is still very good.

Series of micrographs which illustrates the change in con-
trast as the accelerating voltage is varied.

e.