Advances in nanotechnology make it possible to produce components with increasingly smaller structural elements. To show and process the nanostructures, tools that can scan the structures in multiple dimensions are required, so that images can be generated from the measurement data of these tools. Furthermore, to produce microstructured components, photomasks are required, the pattern elements of which can project the tiny structural elements of the components or nanostructures into the photoresist applied to a wafer.
A powerful tool for locally analyzing a specimen is a scanning electron microscope (SEM), the electron beam of which can be focused very finely, so that the beam diameter at the focal point is in the single-digit nanometer range. This measuring instrument scans the electron beam over the surface of a specimen. Among the effects of the interaction of the electrons with the specimen are that secondary electrons (SE) and back-scattered electrons (BSE) are generated.
However, the resolution of a scanning electron microscope and the quality of the images generated falls sharply if the specimen to be examined has a significant surface charge. FIG. 1 illustrates this problem. If a specimen has a positive electrical charge, an electron beam produces too large an image of a structural element arranged on the specimen surface. If, on the other hand, the surface of the specimen is negatively charged, the SEM forms too small an image of the structural element. For example when examining the critical dimension (CD) and/or positioning errors of structural elements of a photomask, of a developed photoresist or of a component on a wafer, this can lead to great uncertainty in determining the process yield.
Scanning probe microscopes (SPM) are likewise powerful analytical tools in nanotechnology. SPMs scan a specimen or its surface with a probe tip and thus produce a realistic topography of the specimen surface. Depending on the kind of interaction between the measuring tip and the specimen surface, a distinction is made between different types of SPM, for example scanning tunnelling microscopes (STMs) or scanning force microscopes (SFMs), which are also known as atomic force microscopes (AFMs). For example, scanning force microscopes scan a specimen surface by their probe tip or measuring tip being passed over the surface of the probe at a very small distance (i.e. in the range of a few nanometers) (non-contact mode) or else while touching the surface (contact mode).
So if there is an electrical charge distribution on a specimen or its surface, i.e. the specimen surface and the measuring tip of the AFM are at different electrical potentials, compensating currents may flow when there is contact between the measuring tip and the specimen surface or when there is a small distance from the specimen, or electrical flashovers may occur, and these may lead to damage or destruction of a fine measuring tip and/or a sensitive specimen. FIG. 2 illustrates this matter.
Electrical charging of an insulating and/or semiconducting specimen may be caused by irradiation of the specimen with an electron beam or generally with a charged particle beam. Furthermore, even handling of a specimen may lead to an electrostatic charge of its surface. If the specimen is a wafer to be processed, coating processes and/or etching processes may also result in an electrical charging of the specimen.
Consequently, electrical charging of the specimen surface is extremely undesirable for examinations that are performed using a scanning electron microscope or a scanning probe microscope.
In the case of specimens that have an electrically conductive surface, electrical charging can be avoided by earthing the specimen. In the case of electrically insulating or semiconducting specimens, surface charges can be prevented by vapor-depositing a thin conductive layer onto the surface of a specimen to be examined. However, the latter is not possible for many applications, in particular whenever the aforementioned analytical tools are used for example in the production of microstructured semiconductor components or in the production of photomasks.
The authors K. M. Satyalakshmi et al. report in the article “Charge induced pattern distortion in low energy electron beam lithography”, J. Vac. Sci. Technol. vol. 18(6), November/December 2000, pp. 3122-3125 on investigations into determining the deflection of an electron beam as a function of charge distributions on a specimen surface that are produced in the vicinity of the point of incidence.
To prevent an electrical charge from impairing the measurement of structural elements of semiconductor components during their production, U.S. Pat. No. 5,736,863 proposes the application of earthed test structures in the intermediate spaces between individual chips.
US 2002/0070340 A1 describes the use of two electron beams of differing energy, the lower-energy beam, which has an electron generation rate of <1, compensating for the electron generation rate of the higher-energy beam, which is >1, so that no surface charges occur.
To compensate for surface charges, U.S. Pat. No. 6,507,474 B1 proposes measuring the charging of the surface with a charge sensor and compensating for it with the aid of an ionizer.
US 2004/0051040 A1 discloses a scanning electron microscope that controls the irradiating intensity of the electron beam in such a way that a change in the volume of a photoresist as a result of the electron irradiation remains as small as possible.
The aforementioned documents are primarily concerned with the avoidance or reduction of electrical charges.
US 2004/0211899 A1 describes the use of one or more electrometers to determine the electrical charging of a wafer surface.
In some examples, electrometers may have the disadvantage that they cannot measure an electrical charge distribution locally but only over a large area, i.e. in the range of square millimeters to square centimeters.