When designing a beam scanning microscope, there is a trade-off between various operational parameters. A particularly important relationship is that between the size of the beam spot used to scan the surface, and the resolution of the microscope: the smaller the beam spot size, the higher is the resolution. Since the resolution is one of the most important features of a microscope, all systems are designed to have smallest possible beam spot size. Usually a small beam spot size is maintained even with large field of view.
When a two dimensional image of secondary electrons (SE) or back-scattered electrons (BSE) is produced, the pixel dimensions should be smaller than the spot size in order to reconstruct the information properly (Nyquist sampling theorem). If an improper ratio between the pixel size and the spot size is employed, and the information contains high frequency components, the information will be undersampled and aliasing effects will be generated. Aliasing errors, which are well known in the art, are exemplified in FIG. 1A, which is a scanning electron microscope (SEM) image of a photoresist on a silicon wafer. The aliasing errors are especially obvious at the fringes of the dark squares in the image, which are unequal in their intensity (Moire fringes). As a result, the squares appear contorted. In contrast, FIG. 1B, which is an image of the same object taken according to the invention, as more fully described below, is essentially free from aliasing effects. This is apparent from the equal light distribution along the fringes of the dark squares in the image. As a result, the squares appear undistorted.
In all beam scanning devices, where the beam is scanned either in a raster manner or in a bi-directional manner, the sampling is carried out in a different way in the different axes. Along the horizontal (fast) scanning axis, the pixel size is defined by determining a specific sampling time. Thus the pixel size in this axis is defined by the sampling time interval, multiplied by the beam linear speed. In the horizontal axis the actual spot size is smeared during the sampling time. A proper match of the sampling bandwidth, as known and easily carried out by the skilled person, usually compensates for proper sampling of the data. In the vertical axis, on the other hand, the pixel dimension is defined by the vertical distance that the beam travels from a given scanning line to the next one. When the pixel dimension in the vertical direction exceeds a proper ratio of the spot size in this direction, undersampling phenomena may occur (for object information that has high frequency components in the vertical direction).
There are two common ways to overcome aliasing effect. The first way involves generating an image with sufficient number of pixels at all fields of view, i.e., increasing the number of pixels in the vertical and horizontal direction. This method is discussed in detail in "New Generation Scanning Electron Microscope Technology Based On The Concept Of Active Image Processing", [Esikahu Oho et al, Scanning Vol. 19 483-488, 1997]. This method, also called "active image processing", for removing the effects of aliasing errors, employs a procedure by which a large amount of data is acquired, and is then reduced by averaging into new pixel data. This procedure is efficient to remove aliasing errors, but presents other substantial drawbacks, inasmuch as when using this method one is required to generate larger and larger images the bigger the field of view is, which is a major disadvantage when further image processing is required.
The second method involves increasing the spot size to match it to the sampling pixel size. This can be done symmetrically or asymmetrically, by increasing the spot size in one direction more than in the other. This can be done very simply by defocusing the beam or by more complicated electron optics means. The major drawback of this method is that it either reduces the actual depth of focus of the system (in the first case), or involves a complicated control over the beam shape electron optics (in the second case). Moreover, it requires increasing spot size in a non-symmetrical way in the two axes.
It is therefore clear that the art has not yet provided a simple and efficient way to overcome the aforesaid drawbacks and to eliminate aliasing effects in scanning beam microscopy.
It has now been found that by increasing the number of pixels in the vertical direction, by generating a plurality of successive images of the same object, wherein each image is shifted in the vertical direction by a distance smaller than the distance between two lines, and then averaging all the images together, a substantial reduction of the aliasing effect can be obtained. This method, which is explained in more detail below, further has the advantage of providing an enhanced signal-to-noise ratio, which would not be obtained by using the above prior art methods of modifying the electron beam shape or defocusing. Furthermore, the method of the invention avoids enlarging the pixel number in both vertical and horizontal directions, which is necessary for the above-described method of Oho et al.
Therefore, the method of the present invention avoids expenses in terms of computer power and scanning ability and expenses in terms of beam shaping means associated with the various prior art methods, while at the same time, maintaining the focus and even reducing the signal-to noise ratio.
It is therefore an object of the present invention to provide a method for eliminating the undersampling effect in scanning beam microscopes, without the need to defocus the beam, or to generate large format images.
It is another object of the invention to provide apparatus for eliminating such undersampling effects.
Other objects of the invention will become apparent as the description proceeds.