Electron beam test probe for integrated-circuit testing

Abstract: An electron beam test probe apparatus and a method for use of said apparatus enable measurement to be less sensitive than prior art systems to local electric fields in the vicinity of the point at which the potential of the specimen is being measured. The apparatus consists of an electron beam for bombarding the specimen at the point at which the potential of the specimen is to be measured, a magnetic lens for collimating the secondary electrons emitted from the specimen in response to this bombardment, and a detector system for measuring the energy distribution of the secondary electrons so collimated. The apparatus may be used to produce an image of the specimen in the vicinity of the point under bombardment while measuring the potential at said point. The methods taught that were disclosed allow the measurement of the potential on buried conductors located beneath an insulating layer. These methods also prevent drift in the electron beam resulting from varying surface electric fields on the specimen.

Description: The present invention relates to an electron beam test probe system having the features set forth in the preamble of patent claim 1. Such a system of the prior art is disclosed in SCANNING vol. 5, 2 1983 p. 71-83. In this known system, the electron beam column is stationary. The probe card means are displaceable relative to the column in three orthogonal directions so as to make contact to the unit under test. The latter in turn is displaceable relative to the electron beam column together with the probe card means in order to permit proper alignment with the electron beam. The probe card means is connected by flexible leads to the exterior via vacuum feedthroughs so that it can follow the displacements of the unit under test. The leads transmit forces to the probe card means tending to move it relative to the unit under test, and this is particularly pertinent where stiff coaxial leads and/or a great number of leads are necessary to test e.g. a complex circuit. The document JP-A-51-87955 discloses an electron beam tester wherein the electron beam column can be moved relative to a device under test. The present invention as defined by patent claim 1 provides for displacement of the electron beam generating means relative to the specimen positioning means which in turn are used to bring the specimen into proper position relative to the probe card means. The dependent claims define particularities of embodiments of the invention. The invention will be described in detail with reference to the accompanying drawings. Fig. 1 shows a test probe according to the present invention in a somewhat schematic manner, and Fig. 2 illustrates the mechanical adjustments provided in accordance with the present invention. A test probe constructed according to the present invention is shown schematically at 100 in Figure 1. It consists of an electron beam source consisting of a tungsten filament 102 which emits electrons when heated by passing an electrical current through it. The factors which influence the design of the electron beam source will be discussed below. These electrons are accelerated by a potential difference maintained between the filament 102 and an acceleration electrode 104. The electrons accelerated by electrode 104 are then formed into an electron beam by the orifice 106 in electrode 104. The electron beam is focused onto a point in a specimen 108 by two magnetic lenses, magnetic lens 110 and collimating magnetic lens 112. Deflection coils 114 are used to correct for any slight misalignment between the filament 102 and the axis 116 of the electron beam column. The secondary electrons produced in response to the bombardment of the specimen 108 by this electron beam are formed into a second collimated beam by the collimating magnetic lens 112. This second collimated beam travels in a direction parallel to that of the electron beam used to bombard the specimen 108 and in a direction opposite to that of said bombarding beam. Those secondary electrons with sufficient energy to pass through a filter electrode 118 are deflected into a secondary electron detector 120 by the combination of a negative voltage applied to a suppressor electrode 122 and a positive potential applied to the surface 124 of the secondary electron detector 120. In the preferred embodiment, the collimating magnetic lens 112 consists of a coil wound in an annular duct formed in a pole piece having a slot slanted downward toward the point at which the specimen is bombarded by the electron beam. The pole piece is toroidal in shape having a passageway which is cylindrical in shape with an axis coincident with the axis 116 of the electron beam column. The pole piece is constructed from a ferromagnetic material having a high magnetic permeability. When a current is passed through the coil, a magnetic field is generated. The magnetic field so produced forms a magnetic circuit in which the field lines lie in a plane which is perpendicular to the direction of the wires in the coil. Since the pole piece is constructed of a high magnetic permeability material, most of this magnetic circuit is constrained to flow through the pole piece. However, since the slot does not contain such highly permeable material, the flux lines "bulge" out at the position of the slot forming a region of intense magnetic field at the point. The small fraction of the magnetic field which is not constrained to the pole piece forms a region of uniform weaker magnetic field in the bore of the magnetic lens as illustrated by flux line. The collimating magnetic lens 112 operates on the secondary electrons in a manner analogous to the action of an optical lens on the photons emitted from a point light source located at the focal point of the lens. Referring to Figure 1, the potential barrier used to sense the energy distribution of the secondary electrons is created by applying a potential to a filter electrode 118. In the preferred embodiment, the filter electrode 118 is a tubular electrode located in the bore of the collimating magnetic lens 112. It will be apparent to those skilled in the art that a uniform potential barrier will be created at the entrance 157 of said tubular electrode when a potential is applied to said tubular electrode, provided the diameter of said tubular electrode is small compared to its length. In the present invention, the length of the tubular electrode is preferably at least two times its diameter. The optimum placement of the filter electrode 118 is in the region of uniform magnetic field created by the collimating magnetic lens 112, since the secondary electrons are collimated in this region. If the filter electrode 118 were placed in the region in which the magnetic flux was diverging, the secondary electrons would also have diverging velocity vectors and hence the improvements in energy resolution obtained from collimation would be reduced. Those secondary electrons which have sufficient energy to overcome the potential barrier created by the filter electrode 118 are "swept" into the secondary electron detector 120 by an electrostatic sweeping field created by a combination of a negative potential applied to suppressor electrode 122 and a positive potential applied to the surface 124 of the secondary electron detector 120. It will be apparent to those skilled in the art, that the positive potential applied to the surface 124 could also be applied to an electrode located between the secondary electron detector 120 and the suppressor electrode 122. In the preferred embodiment, the suppressor electrode is a tubular electrode having an axis that is parallel to that of the electron beam column. A section 158 is removed from the side closest to the secondary electron detector 120 to provide a passageway for the secondary electrons. The potential applied to the suppressor electrode 122 is chosen to discriminate between secondary electrons from the specimen and backscattered electrons. The backscattered electrons have energies which are much larger than those of the secondary electrons. Hence, a much larger electrostatic field is required to sweep the backscattered electrons into the secondary electron detector 120. The potential applied to the suppressor electrode 122 is chosen to be high enough to sweep the secondary electrons into the secondary electron detector 120 and low enough to allow any backscattered electrons exiting from the filter electrode to continue into the bore 160 of the suppressor electrode 122. Most of these backscattered electrons eventually strike some structure above the suppressor electrode 122. Any low energy electrons produced by these collisions will not affect the potential measurement of the specimen, since such electrons will not be able to enter the secondary electron detector 120. The precise value for the suppressor electrode potential depends on the physical dimensions of the suppressor electrode 122 and its positioning relative to the other components of the electron beam column. In the preferred embodiment, the energy of the electron beam is considerably lower than that used in prior art systems. Most prior art systems are essentially adaptations of scanning electron microscopes. As such, they are optimized for electron beam energies in the 10 to 20 keV range. The test probe of the present invention employs an electron beam energy in the 500 to 1500 volt range. Prior art test probe systems had insufficient "fields of view" to probe an entire VLSI circuit. In the present invention, this problem is overcome by providing both coarse and fine adjustment means for positioning the electron beam relative to the specimen. The fine adjustment means consists of steering magnets 186, shown in Figure 1, which deflect the electron beam relative to the specimen. The steering magnets 186 can produce electron beam deflections in either of two orthogonal directions perpendicular to the direction of the electron beam. Thus, they may be used to scan a region on the specimen as well as to provide fine positioning of the electron beam on the specimen. This scanning motion can be used to construct an image of the surface of the specimen which is accentuated to show the different electrostatic potentials present on said specimen. In the preferred embodiment, said steering magnets consist of steering coils wound on slightly conducting plastic supports to improve their frequency performance. It will be apparent to those skilled in the art that the steering magnets 186 could be replaced with electrostatic deflection plates which cause the electron beam to be similarly deflected when an appropriate potential is applied thereto. The coarse adjustment means is illustrated in Figure 2. The electron beam column 187 is mounted within a vacuum vessel having a boundary shown at 188, 190 and 192. This boundary also encloses a stage 194 having a platform 196 for holding the specimen 108 being probed. The electron beam column includes means for positioning the column, shown at 198 and 200, by translation in two orthogonal directions, x and y, perpendicular to the electron beam direction. The electron beam column may be moved using micrometers 202 and 204 which control the positioning means 198 and 200, respectively. In addition, the stage 194 provides a means for positioning the specimen by translation in any of three directions, x, y, and z and by rotation through an angle A about an axis parallel to the electron beam direction. The various translational movements carried out by the stage 194 are controlled by micrometers 206, 208, and 210. The rotational motion is controlled by a micrometer 212. In the preferred embodiment, these micrometers are controlled by stepping motors under computer control. A flexible bellows 190 provides a means for maintaining the vacuum in the vessel when the electron beam column is moved with respect to the vacuum chamber. The bellows 190 allows the column positioning means to remain outside the vacuum vessel. This positioning means preferably utilizes conventional bearings or other lubricated surfaces to support the column load. Outgassing from such lubricants during the pump down process increases the length of time needed to pump down the vacuum chamber when a new specimen is introduced into the test probe. In addition, the volume of the vacuum vessel which must be pumped down each time a specimen is introduced into the apparatus is also minimized in this design. A probe card 214 Provides a means for making electrical connections to the specimen 108 mounted on the platform or table 196. This probe card 214 contains a hole through which the electron beam passes. One of the intended uses of the apparatus of the present invention is for making potential measurements on integrated circuits. As is well known to those skilled in the integrated circuit arts, integrated circuits are normally constructed on wafers containing hundreds of such circuits. The apparatus of the present invention may be used to examine an individual integrated circuit prior to its separation from other integrated circuits on the wafer. Such measurements are made using the present invention by positioning the desired integrated circuit, referred to as a die, in the field of view of the electron beam and then moving the electron beam to the various points on the die at which potential measurements are desired. The wafer is mounted on table 196 at 108. The positional controls 206, 208, 210, and 212 are used to position the desired die relative to the probe card 214. The probe card includes mechanical probes 216 for connecting power and test signals to the die under examination. Once the desired die is correctly positioned relative to the probe card 214, the region of interest on said die is selected by translating the electron beam column 187 relative to the die using the coarse x-y positional controls 198 and 200. The region of interest is then examined using the steering magnets 186. The present invention includes a means for pulsing the electron beam in short pulses. This is accomplished by applying a suitable potential across the blanking electrodes 220 shown in Figure 1. In the absence of a potential across said blanking electrodes, the electron beam is imaged by a magnetic lens 110 into the aperture 160. The collimating magnetic lens 112 then images the electron beam to a small spot on the specimen 108. When a potential is applied across said blanking electrodes, the electron beam is deflected sufficiently to prevent it entering the aperture 160.

Claims: An electron beam test probe system for measuring the potential at one or more points on a specimen (108), said system comprising: an electron beam column (187), a vacuum vessel (188, 190, 192), a probe card (214) for providing electrical connections to said specimen (108), specimen positioning means (194, 196) for positioning said specimen (108) relative to said probe card (214) by translation in two orthogonal directions (X, Y) perpendicular to a predetermined axis (116) and for rotation by an angle (A) about an axis parallel to the predetermined axis (116), said electron beam column (187), said probe card (214) and said specimen positioning means (194, 196) all being contained in said vacuum vessel (188, 190, 192), characterized by column positioning means (198, 200) for positioning said electron beam column (187) relative to said probe card (214) by translation in two orthogonal directions (X, Y) perpendicular to the predetermined axis (116), said column positioning means (198, 200) being outside the vacuum vessel (188, 190, 192). The test probe system of claim 1 wherein said electron beam column is characterized by: electron beam source means (102) for producing an electron beam having an energy between 500 and 1500 eV, alignment means (114) for aligning said electron beam relative to the predetermined axis (116), first aperture means (106) for defining the angular divergence of said electron beam, means for generating a blanking signal, blanking electrode means (220) for deflecting said electron beam when said blanking signal is generated, second aperture means (160) for intercepting said electron beam when said electron beam is deflected by said blanking electrode means (220), first magnetic lens means (110) for imaging the electrons not removed by said first aperture means (106) into said second aperture means (160) when no blanking signal is generated, electron beam steering means (186) for deflecting said electron beam in either of two orthogonal directions, each of which being perpendicular to said predetermined axis (116) such that said electron beam bombards said specimen (108) a selected point thereon, collimating magnetic lens means (112) for focussing said electron beam onto said specimen (108) and for collimating the secondary electrons produced in response to said electron beam bombardment of said specimen (108) wherein said first magnetic lens means (110) focuses said electron beam into said collimating magnetic lens means (112), said electron detecting means (120) for sensing secondary electrons collimated by said collimating magnetic lens means (112) which have energies greater than a first specified energy and less than a second specified energy, wherein said second specified energy is less than the energy of the electrons from said electron beam which are backscattered by said specimen (108). The test probe system of claim 2 characterized in that said collimating magnetic lens means (112) comprises a magnetic lens having a region of intense magnetic flux substantially coincident with the point at which said electron beam bombards said specimen (108) and a region of uniform magnetic flux having field lines substantially parallel with said predetermined axis (116), and wherein said electron detecting means (120) comprises: electron counting means for counting electrons, potential barrier means (118) for preventing electrons having energies less than said first specified energy from reaching said electron counting means; and electron deflecting means (122) for deflecting electrons having energies less than said second specified energy into said electron counting means. The test probe system of any of claims 1 to 3 characterized in that said specimen positioning means comprises means (210) for translating said specimen (108) in a direction parallel to said predetermined axis (116).

Inventors: RICHARDSON NEIL, RICHARDSON, NEIL
Applicants: SCHLUMBERGER TECHNOLOGIES INC, SCHLUMBERGER TECHNOLOGIES, INC.
Application year: 1986
Main classification: G01R31
IPC codes: G01R31, H01J37, G01Q60