The present invention is directed to the manufacture of an electrostatic element for steering a charged particle beam and, in particular, to facilitating the manufacturing process and providing an improved product.
Various instruments are known which rely on interaction of charged particles emitted from a sample to derive characteristics of the sample. Examples of such instruments are an electron microscope and a focused ion beam microscope.
For facilitating the description of the present invention, it will be explained in connection with a scanning electron microscope (xe2x80x9cSEMxe2x80x9d). However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to other instruments and machines that require a focused beam of charged particles.
An SEM operates by generating a primary scanning electron beam that impacts a sample, a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and collected by a detector which is arranged near the surface of the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is used to display an image of the surface on a video screen.
On its way between the electron source and the sample, the beam is deflected by various electromagnetic and electrostatic elements that are used to, for example, align, focus and scan the beam, as well as correct its shape. All the elements that deflect the beam from one path to another, for whatever purpose, are referred to herein as steering elements. The present invention is particularly directed to a steering element of the electrostatic type.
Prior art electrostatic steering elements have a cylindrical body made of a material which must be an electrical insulator and non-magnetic. The body is formed with a center bore. For an octopole element in common use, eight small, metallic pole pieces are individually inserted into the bore and fixed in place. This approach has several drawbacks. The entire process is labor intensive and time consuming because all eight pole pieces are manufactured as separate parts, and these must be individually affixed to the body. Also, it is very difficult with this approach to achieve precise alignment of the small pole pieces with respect to each other and with respect to the body. A positional error for any pole piece will cause aberrations in the electrostatic field created by the element and resultant inaccuracies in steering the electron beam. Since the pole pieces must be equidistantly spaced from each other, a slight positional error in placing any one of the pole pieces relative to the body will be carried through when aligning subsequent pole pieces with the misaligned pole piece. Thus, because of the tight available space within the bore, it may not be possible to insert the final pole piece in accurate alignment relative to the other pole pieces. In fact, it may not be possible to insert the final pole piece at all. In addition, because the metal used for the pole pieces, such as copper, is subject to oxidation, the pole pieces have to be covered with a non-magnetic coating, such as rhodium. However, due to the required non-linear shape of the slots between adjacent pole pieces, it is difficult to completely coat the pole pieces at the deep end portion of the slots.
One object of the invention is to provide an improved technique for manufacturing an electrostatic steering element for a charged particle beam.
Another object of the present invention is to manufacture an electrostatic steering element for a charged particle beam with less labor and high accuracy.
A further object of the present invention is to manufacture an electrostatic steering element for a charged particle beam without having to coat the pole elements to prevent oxidation.
Yet another object of the present invention is to manufacture an electrostatic steering element for a charged particle beam which is rugged and robust so that it is not subject to misalignment during handling.
These and other objects are attained in accordance with one aspect of the invention directed to a process for manufacturing an electrostatic element for steering a charged particle beam which provides a cylindrical, non-conductive body having a bore therethrough, and also a cylindrical, conductive core having a bore therethrough and sized to fit within the bore of the body. The core is secured within the bore of the body, and longitudinally extending slots are cut completely through the core to create pole pieces that are electrically isolated from each other.