Device for the Detecting of Charged Secondary Particles

1. Field of the Invention

The invention relates to a device for the detection of charged secondary particles which are generated by a primary particle beam incident on a specimen.

2. Description of the Prior Art

When a particle beam is projected on a specimen, secondary particles are emitted due to an interaction with the primary particles of the solid body. The energy and angular distribution of these secondary particles are dependent, interalia, on the chemical and physical composition present in the interaction region which is on the surface of the specimen, and on the distribution of the potential present in the specimen. The secondary particles which are triggered are usually sensed by a detector arranged laterally above the specimen. These secondary particles are of particular significance for imaging and measuring potentials in scanning electron microscopes. Prior art devices, such as German OS 35 90 146, produce an undesirably high asymmetrical extraction field, which will permit the quantitative measurements of potential to be made only within an extremely small scan field.

It is known to use a plurality of detectors arranged symmetrically relative to the primary beam axis to guarantee that the detection of the secondary electrons is independent of the trigger point and emission angle. Such a multi-detector system is disclosed in JP-A-58 35 854. These systems have the disadvantage of a noise signal which will be generated by backscattered electrons. This noise signal will be superimposed on the secondary electron signal causing the detector to receive a signal that is multiplied in comparison to individual detectors. In these detector systems the scan field that is available for quantitative measurements is limited to a few mm.sup.2.

An object of the invention is to provide a secondary particle detector in which secondary particles triggered on a specimen will be successfully detected regardless of the location of the measuring point within a large-area scan field. In particular, the device should be suitable for electron beam testing of printed circuit boards.

An advantage of the invention is that there will be no location-dependent disturbances caused by the detector geometry which might be superimposed on the measured signal. Moreover, the signal contributions of backscattered primary electrons are considerably reduced.

FIG. 1 is a schematic side view of a device for the detection of secondary electrons constructed in accordance with the principles of the present invention.

FIG. 2 is a schematic plan view of an electrostatic octupole for the device of FIG. 1.

FIG. 3 is a graph showing the voltage distribution within the device of FIG. 1.

FIG. 4 is a graph showing the calculated secondary particle paths for the case wherein the secondary electrons start in the center of the printed circuit board.

FIG. 5 is a graph showing the calculated secondary particle path for the case wherein secondary electrons start on a diagonal about 10 cm outside the center of the printed circuit board.

FIG. 6 is a plan view of a magnetic octopole for the deflection of secondary electrons having inner and outer pole shoes for pole elements, which can be used in the device of FIG. 1.

The device for the detection of secondary electrons schematically shown in FIG. 1 can be used in electron beam measuring equipment for testing printed circuit boards and wiring modules. It replaces the conventional detector system and is arranged above the printed circuit board LP to be tested. The printed circuit board is placed in an evacuated chamber K on a table T that can be aligned relative to the primary beam axis OA. The particle beam for charging the line networks and for reading the distribution of potential is formed according to the network geometry and is usually generated in a modified scanning electron microscope whose column contains a beam generator, one or more condenser lenses, an objective lens and deflection system PA for positioning the primary electron beam within the scanned field that can be as large as 20.times.20cm.sup.2 in size. The secondary electrons that are triggered at the respective measuring points are accelerated in the direction of the deflection unit SA which is arranged above a tube electrode RE. The particles are accelerated into an electrical field that is rotationally symmetrical with respect to the primary beam axis OA. The secondary particles are decelerated in an opposing electrical field generated between two hemispherical-like electrodes K1 and K2 and are extracted in the direction of the detector DT which is composed of a scintillator held at a high, positive potential of 10 kV, and which includes a light conductor and a photomultiplier.

The tube electrode RE, which is arranged directly above the printed circuit board LP, is symmetric relative to the primary beam axis OA and preferably has a circular cross-section, whereby the tube diameter is selected somewhat larger than the diagonal of the printed circuit board. The cross-sectional area of tube electrode RE can exhibit an arbitrary shape. The tube diameter, however, must then be selected such that the electrical field within the scan field continues to maintain rotational symmetry with respect to the primary beam axis OA. This tube electrode is placed at ground or at a low negative potential U.sub.RE which may be between -5V and -50V. It is composed of two aluminum carrier rings R1 and R2 connected by a plurality of threaded rods and of a stainless steel wire net DN (wire diameter d.apprxeq.0.25 mm, mesh width w.apprxeq.1 mm) which is held in by guide rails. Threaded rods G1 through G4 are used to fasten the tube electrode RE and the deflection unit SA to the aluminum plate TP which is screwed into the chamber wall W. Each of these threaded rods G1 through G4 are connected by bushings and washers of polymerized polystyrol to the carrier rings R1 and R2 in an insulated fashion.

An electrical octupole element is arranged symmetrically relative to the carrier beam axis and serves as a deflection unit SA. The frame of the deflection unit, which is composed of two aluminum rings R3 and R4, as well as the aluminum plate TP are placed at a potential U.sub.A of, for example, U.sub.A =1 through 3 kV. Strips IS of polymerized polystyrol are attached to the narrow side of each of the deflection plates AP of the octopole. These strips IS are fitted into a guide channel present in the carrier rings R3 and R4. In order to increase the mechanical stability, the carrier rings R3 and R4 are interconnected to one another via threaded rods.

In order to deflect the secondary electron particles generated in the rotationally symmetrical electrical extraction field in the direction of the detector DT, which is arranged laterally above the octupole SA, the deflection plates AP are preferably composed of a wire network and are charged with the potentials recited in FIG. 2. The positive extraction potential U.sub.A is also superimposed on the respective deflection potentials U.sub.x or U.sub.y.

After the deflection in the field of electrostatic octupole SA (the deflection direction is indicated by an arrow in FIG. 2), the secondary electrons pass through an opposing electrical field generated between two approximately hemispherical electrodes K1 and K2. These electrodes K1 and K2 arranged on the upper carrier ring R4 are each composed of a hard-soldered wire ring and of a wire weave having a mesh width of about 1.6 mm and a wire thickness of about 0.22 mm. While the electrode K1 connected to the carrier ring R4 in conductive fashion is held at the extraction potential U.sub.A, the net electrode K2 is placed at a potential U.sub.G between -15 volts and +15 volts. The K1 and K2 electrode arrangement will allow only secondary particles having energies above a threshold, prescribable with the potential U.sub.G, to reach detector DT (resulting in an improvement of the voltage contrast signal).

FIG. 3 shows the calculated field distribution inside the device of the invention. It was assumed that the tube electrode RE and the deflection unit SA lie at the potential of U.sub.RE =-18 volts and U.sub.A =3 kV respectively (U.sub.x =U.sub.y =U.sub.G =0 volts). As a result, the extraction field immediately above the specimen under test will be uniform.

The focusing affect of the extraction field that is rotationally symmetrical with respect to the primary beam axis OA is shown in FIGS. 4 and 5. These respectively show the calculated trajechories for that case wherein the secondary electrons SE start in the center of the printed circuit board (FIG. 4) or, on the diagonal about 10 cm outside the center with an energy of 20 eV and an angle between 40.degree. and 180.degree. with respect to the plane of the printed circuit board (FIG. 5). Independently of the trigger location, the secondary electrons SE enter nearly perpendicularly into the opposing field generated between electrodes K1 and K2. It may also be seen that the secondary electrons SE emitted approximately perpendicularly relative to the primary axis OA are also accelerated in the direction of the deflection unit SA.

The invention is not limited to the exemplary embodiments that have been set forth above. Thus, it is possible to use a magnetic octupole SAM for the deflection of secondary electrons. As FIG. 6 shows, this octopole SAM is composed of eight inner pole pieces which PI which are placed at the potential U.sub.A and which are separated from their respective outer pole pieces by insulated ring IR. These outer pole pieces are placed at ground potential and are each provided with an excitation winding S. For the deflection of the secondary electrons in the direction indicated by the arrow, the currents that are respectively indicated must flow in the excitation windings S.

The described device can also be used for the detection of ions or photoelectrons when the specimen under test is scanned with an ion beam or laser beam.