Scanning Electron Beam Apparatus For Viewing Potential Distribution On Specimen Surfaces

Abstract

In electron beam apparatus, in which a finely focused probe is caused to impinge on the surface of a specimen and the resulting secondary electrons are collected and used to show potential distribution on the specimen surface the detector includes, between the collector and the specimen, a positive grid and a further grid, this further grid being of variable low or negative potential to form effectively a potential barrier of adjustable height to discriminate between electrons coming from regions of different potential.

Description

This invention relates to electron beam apparatus of the kind in which a beam of electrons, known as an electron probe, is caused to impinge on a specimen under examination and information about the voltage distribution at the specimen surface is obtained by using a detector to pick up electrons emanating from the specimen. Normally the beam will be focused to a fine spot and will be caused to scan the surface of the specimen and the signal is used to control the brightness of a cathode-ray tube display synchronized with the scanning of the beam, so that an image of the specimen surface is formed.

Voltage contrast in the image may itself be of significance, for example where the specimen is a solid-state electronic component or a portion of an integrated circuit, or the voltage contrast may be indicative of some other physical phenomenon which it is desired to examine.

The detector is normally placed on the same side of the specimen as the beam but to one side of the beam and is intended to pick up the secondary electrons emerging from the specimen. The detector normally subtends a relatively small solid angle at the specimen surface. As the beam scans over an area of uniform potential the mean number of secondary electrons picked up by the detector remains the same, but when a region of different potential is reached the absolute energy distribution of secondaries is upset and there is a change in the number of secondaries picked up, resulting in image contrast.

However, such an arrangement is not predictable. For a given voltage, difference dark may correspond to positive and light to negative or vice versa.

Further, it is not normally possible to arrange that, for a given electron probe current, the collected current changes by a certain amount for a given change in specimen surface voltage. Hence, the magnitude and sign of the specimen surface voltage difference cannot be deduced from the change in collected current.

A further difficulty is that the secondary electrons, as they move away from the point of impact and are attracted in curved paths and at relatively low velocities towards the detector may also be influenced by fields produced by potential gradients or potential differences in other parts of the specimen surface. Moreover the detector picks up not only true secondary electrons of low energy but also high-energy reflected primaries which are substantially unaffected by potential contrast. Finally it also picks up other low-energy secondary electrons generated in the surrounding casing and specimen mount by impingement of the reflected primaries, and these likewise are not affected by the potential contrast in the specimen, and so swamp still further the wanted signal. They may form as much as 30 percent of the low-energy electrons reaching the detector.

The main aim of the present invention is to provide a configuration of electrodes, including an electron detector, such that the sense of the change in collected current for a given change in specimen surface voltage is always the same, irrespective of the absolute value of either voltage, that is to say the collected current is a monotonic function of surface voltage. A further aim is to attempt to ensure that this configuration of electrodes can be made to give the same change in collected current for the same change in specimen surface voltage, whether the specimen surface voltage change be in space or in time, and wherever the electron probe might be situated or might move upon the specimen surface. A further aim is to provide that the collected electron current has as far as possible a linear relationship with the potential at the specimen surface. A further aim is to provide that the collected electron current is so far as possible independent of all influence other than the number of electrons emitted from the small area of specimen surface where the electron probe is instantaneously incident, and the voltage of that same point.

According to the invention we propose to provide a detector subtending a substantial solid angle at the point of impact of the probe and to provide grids between the detector and the point of impact, the potential on at least one of the grids being such as to allow the passage only of the electrons of above a certain level of energy, the "threshold energy," which may be set to any chosen value, and the potential on at least one other grid being at a substantial positive value to attract a high proportion of the available secondary electrons.

Preferably it is the first grid encountered by the electrons that is at a substantial positive potential. The presence of this grid also produces a field that masks any disturbances of the field above the specimen induced by potential differences on the specimen surface in the neighborhood of the point of impact. The grids and the detector may be of part-spherical form, with their common center of curvature at the point of impact, or they could be planar (this is necessitated in many cases by the nature of the detector) or cylindrical, frustoconical, or of any other convenient form.

The invention will now be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of the apparatus according to the invention;

FIG. 2 is a graph showing the energy distribution of secondary electrons; and

FIG. 3 is a graph showing the relationship between collector current and potential difference.

Referring first to FIG. 1, the probe-forming system is orthodox, comprising an electron gun G, an anode A, a first electron-optical lens L1 and a final electron lens L2. The beam or probe of electrons formed by this system impinges on a specimen S and is caused to scan a small area of the specimen, for example a square measuring a fraction of a millimeter each way, by scanning coils C fed from a time base T which also controls the deflection plates of a cathode-ray tube CR. The brightness of the spot on the screen of the tube is controlled by the signal from a detector, indicated generally at D, so that there is reproduced on the screen an image of the scanned area of the specimen, the contrast in the image being determined by the change in signal at the detector D as the probe moves over the surface of the specimen.

Where the detector is simply picking up the secondary electrons from the specimen and the contrast to be displayed is potential contrast at the specimen surface, the known detectors suffer from the defects mentioned above. The changes that are to be detected are masked by backscattered high-energy primary electrons, by secondary electrons generated by impact of backscattered primary electrons on parts of the instrument around the specimen, and the by fact that the very potential contrast which one is seeking to detect produce fields above the surface of the specimen which themselves influence the paths of the low-energy secondary electrons and which thus mask unpredictably the contrast one is seeking to observe.

Normally the electron gun will be at a large negative potential, (for example -20 kv.), and the specimen at earth potential, so that the electrons of the probe will have an energy (in the example quoted) of 20 kev. Any of these which are backscattered from the specimen will have energies of a similar order, i.e., of various values up to 20 kev. Consequently they will be fast and not easily deflected by small electric or magnetic fields. The true secondary electrons, on the other hand have low energies of varying values, mostly under 5 electron volts, with a peak around about 21/2 electron volts. This is shown diagrammatically in FIG. 2.

Those secondary electrons leaving a point in the specimen surface which is at earth potential have, on arriving at given grid or other surface of a certain potential an amount of energy represented by their intrinsic energy around 21/2 to 4 electron volts, to which is added (or from which is subtracted) that energy gained or lost as a result of the potential difference between the earthed specimen and the grid or other surface in question. Where the specimen surface is not at earth potential the energy gained or lost will be different.

In the detector illustrated we provide a grid G1 at a relatively high-positive potential, a control grid G2, of adjustable low potential, and a third grid G3 for a purpose to be explained below, between the specimen surface and an electron collector in the form of a scintillator SC, the light from which is taken by a light pipe to a photomultiplier P to produce the electrical signal that is amplified and used to modulate the beam in the cathode-ray tube CR.

The grid G1 serves simply to attract the secondary electrons from the point of impact of the beam on the specimen surface towards the detector and has the additional function of producing an electrical field above the specimen surface that masks any field distortions caused in this region by the presence of potential differences in the specimen. In a typical example the grid G1 may be at a positive potential of 500 volts but the actual voltage will depend on the distance from the specimen; it is desirable that the potential gradient should be not less than 5,000 volts per meter. This would give a value of 25 volts where the grid is 5 millimeters from the specimen surface. Preferably the gradient is at least 10.sup.5 volts per meter. However, regardless of distance the potential on G1 should be at least 25 volts to have a worthwhile effect.

The heart of the invention lies in the control grid G2, which forms a potential barrier of variable height to discriminate between secondary electrons emanating from regions of the specimen surface of different potential. The grid G2, like the grids G1 and G3 can be in the form of a mesh of wires and its effective potential is not that on the wires themselves but that of the field in the gaps between the wires, which is largely determined by the potential on the wires but is also influenced to some extent by potentials of surrounding electrodes. If this effective potential is zero, secondary electrons coming from a point on the specimen surface which is at zero potential will get through the grid G2 as they will have a potential energy of about 21/2 electron volts or more. Electrons coming from a region of the specimen surface which is at, say, -5 volts will get through it easily as they will have the added energy of 5 electron volts derived from their acceleration in the field from -5 volts up to zero. Electrons coming from a region at +5 volts, on the other hand, are retarded and only those which started with an intrinsic energy of greater then 5 electron volts will get through the grid. As will be seen from FIG. 2, this will only be a small proportion. Thus the signal picked up by the detector will vary as the probe scans over a region of the specimen surface that varies in potential, those portions which are more negative producing a larger signal in the detector than those which are more positive.

In practice the absolute potentials are influenced by the work function potential of the elements of which the specimen is made but as this is the same all over the specimen it will not affect the reasoning above.

The detector is thus able to discriminate between regions of the specimen surface of different potential and a potential contrast image is formed on the screen of the cathode-ray tube CR. By adjusting the potential of the grid G2 it is possible to vary the mean potential level at which the scanned region just produces no signal in the detector, and so one can discriminate at will between regions of positive and negative potential or between regions having different levels of potential of the same sign. Effectively the grid G2 forms a barrier B as indicated in FIG. 2, all those electrons which lie to the right of the barrier being able to pass through it. For maximum discrimination the barrier will normally be set approximately to the peak of the curve, or to the point of the curve of steepest slope, or the point of inflexion, to the right of the peak, or somewhere between these two points. Linearity of response is also desirable, i.e., a uniform change in signal at the detector for a uniform change in potential at the specimen surface, and this is best achieved by working at the point of inflexion. FIG. 3 shows approximately how the collector current Ic varies with the value of the difference between the effective potential Vg on the grid G2 and the potential Vs on the specimen surface. Effectively this represents the integral of the curve of FIG. 2, but with the zero of the horizontal axis shifted by approximately 5 volts to allow for the fact that the potential difference in question has to be up to 5 volts negative to overcome the intrinsic energy of the secondary electrons. It is desirable to work over the nearly linear part of the curve around -21/2 volts.

A scintillator is preferably used to pick up the electrons, as it has a very low-noise factor. Its potential is unimportant to the above reasoning and is of any typical value used for scintillators for example 10 kv. The grid G3 has the function of a suppressor grid to avoid the signal being upset by secondary emission from the collector itself, resulting from impact of high-energy backscattered primary electrons. For convenience we connect this third grid to the first grid G1, so that it has the same potential of 500 volts in the present example.

In order to improve linearity of response we may vary the potential on the control grid G2 cyclically in a sinusoidal or other manner about its mean value. As the absolute value of sensitivity varies to some extent with the potential on the control grid, i.e., away the straight part of the curve of FIG. 3, the cyclic variation does itself introduce some distortion and so its amplitude should be not more than necessary.

In the example illustrated the grids and the scintillator are arranged in line on an axis which is normal to the surface of the specimen, and the electron probe is brought in at an angle to clear them but it would be possible for the probe to be normal to the surface, the detector then being to one side. The grids need not be flat but could, for example be part-spherical or even cylindrical, surrounding the normal to the specimen, and in that case the collector may be other than a scintillator, since a scintillator is generally necessarily flat.

In the example illustrated we provide a window W defining the area of scintillator exposed. The purpose of this window is to increase the proportion, in the collected electron current, of electrons emitted with small energies (and thus contributing to the voltage contrast) to the proportion of electrons emitted with high energies. The positive first grid G1 will tend to pull in towards the normal to the specimen the preferred low-energy secondaries, more than it does high-energy electrons. Again, the high-kinetic energy of these preferred electrons at the first grid G1 means they are less bent by the retarding field between the grids G1 and G2 than they would be without the additional kinetic energy given by the first grid. Finally, the positive third grid G3 will again pull in towards the specimen normal those electrons penetrating the grid G2 with a low energy. At the other extreme, an electron emitted with high energy will be much less deflected from its course by the various grid potentials. The window W allows through to the collector only those electrons which are close to the normal to the specimen. Hence the net effect is that electrons from a larger solid angle of emission are collected for low-emission energies than for high-emission energies.

The detector arrangement in accordance with the invention may be used in conjunction with AC sorting methods, or in conjunction with the stroboscopic scanning methods disclosed in our copending U.S. Pat. application Ser. No. 813,176 filed on Apr. 3, 1969.

Claims

We claim:

1. Electron beam apparatus comprising electron gun means for forming a high-energy probe of electrons having an energy of the order of thousands of electron volts, lens means focusing said probe, a specimen having a surface thereof in the path of said probe for impact thereon of said probe, a detector adapted to receive secondary electrons emerging from said specimen surface as a consequence of the impact thereon of said probe said detector subtending a restricted solid angle at said specimen surface, image display means connected to said detector to receive signals therefrom generated by said secondary electrons, deflection means acting on said probe to deflect said probe laterally, said image display means being synchronized with said deflection means such as to produce an image of a region of said specimen surface in terms of said secondary electrons, a first grid between said specimen surface and said detector, said first grid being at a substantial positive electric potential with respect to said specimen, such as to create a voltage gradient between said first grid and specimen surface which is not less than 5,000 volts per meter, and said potential being not less than 25 volts, a second grid between said first grid and said detector, said second grid being at a potential which is negative and low but variable with respect to the specimen, the potential of said second grid being of the order of between zero and -5 volts.

2. Electron beam apparatus as set forth in claim 1 including a third grid between said second grid and said collector.

3. Electron beam apparatus as set forth in claim 2 wherein said third grid is electrically connected to said first grid.

4. Electron beam apparatus as set forth in claim 1 wherein each of said grid and said collector are planar and are mutually parallel.

5. Electron beam apparatus as set forth in claim 4 wherein said grids and collector lie perpendicular to a normal to the specimen surface.

6. Electron beam apparatus as set forth in claim 1 wherein said voltage gradient is not less than 10.sup.5 volts per meter.