1. Field of the Invention
The invention concerns apparatus and methods for scanning probe microscopy (SPM).
Scanning force microscopy (SFM) (AFM—atomic force microscopy) is one form of scanning probe microscopy. An important field of application of scanning probe microscopy is the determination of the topography of a specimen surface with high lateral and vertical resolution. The term “lateral resolution” in this context refers to the resolution in a plane of the surface under examination. The direction perpendicular to this plane is called vertical direction. In vertical direction, the topography of the surface is determined by vertical resolution. In addition to the topology, other characteristics of a specimen to be examined can be measured, such as the elasticity or forces of adhesion. Also optical near field microscopes belong to the class of scanning probe microscopes (SNOM—scanning near field optical microscope).
To be able to undertake scanning probe microscopy, the spacing between a probe and the specimen to be examined must be adjustable and measurable very precisely. Probes used in connection with scanning probe microscopes, for instance, are measuring beams which are called cantilevers. A force between the cantilever and the specimen under examination is evaluated as a measurement parameter, especially in scanning force microscopes, a force which may be described, in the simplest case, by a Lenard-Jones potential.
There are several ways of detecting the force. In the simplest case, the excursion of the probe is measured. With scanning force microscopy, when using a cantilever, the probe typically is designed as a thin spring pole. Likewise known are measuring methods with which the cantilever is excited so as to oscillate. Then the damping of the amplitude of the resulting oscillation is controlled. What the known measuring methods have in common is that the interaction between the cantilever and the specimen under examination is measured. As used in the present context, the term “scanning probe microscopy” comprises all these methods and the respective microscopes which are made use of with them.
With one known measuring method, the force acting on the cantilever is detected by applying a light spot principle (light pointer). According to this principle, a measuring ray of light, especially a laser beam is directed at the cantilever, with focusing being provided, if desired. In response to bending of the cantilever, the light beam is reflected at a certain angle with respect to the direction of the incident light, either from the cantilever or from a structural member connected to the cantilever. The reflected light beam is directed at a photodiode which comprises a detector surface having at least two segments. A difference in the light signals received at the two segments is an indication that the measuring light beam is remote from a midposition between the two segments. The midposition is defined as being located where equal portions of the reflected light beam impinge on both segments. Bending of the cantilever provokes a change in the equal distribution of the reflected light beam across both segments. If it is desired, in addition, to detect torsion of the cantilever a photodiode having four segments may be used since that permits the position of the reflected light beam to be determined in two directions on the photodiode. Knowing a cantilever spring rate, the force between the cantilever and the specimen under examination can be determined based on the measurement of the bending of the cantilever.
With a scanning probe microscope, the cantilever may be made, for instance, of silicon. Materials, such as SiN3 or diamond likewise may be used. Basically, the measuring method according to the light spot approach is independent of the material of which the cantilever or measuring tip is made.
When scanning the specimen with the help of the cantilever, usually the distance in vertical direction between the specimen and the cantilever must be adjusted accurately by means of relative movement between the specimen and the cantilever. In this manner, for example, a constant force ratio may be adjusted. Piezoelectric elements may be used to adjust the spacing. During a measurement, the cantilever at the same time carries out scanning-type motion in lateral direction with respect to the specimen. In principle, either the specimen or the cantilever may be moved. If it is the cantilever that moves this is referred to as a “stand-alone scanning probe microscope”. However, the cantilever also might be moved laterally and the specimen to be examined might be moved vertically, or vice versa.
2. Discussion of the Related Art
Two approaches are known, at the present time, in connection with stand-alone scanning probe microscopes to implement the light spot principle. With one approach, all the components of the light spot are moved along in all three directions in space. In this case the light spot is independent of the cantilever movement and simply indicates bending of the cantilever. This kind of implementation is disadvantageous in that it requires various setting means for adjusting the light source which generates the measuring light rays. The complete mass of the resulting mechanical structure must be moved along, and a mechanical resonant frequency of the measuring system is greatly reduced, especially also in vertical direction. The mechanical structure altogether must be implemented in but little space.
With a second type of implementation, the cantilever alone is moved in all three directions in space. In this case, however, measures must be taken to make sure that the measuring light rays still impinge on the cantilever as it moves so as to be reflected from the same. Imaging of the reflected measuring light on the photodiode is not possible unless lateral tracking of the measuring light rays is provided, especially when measurements are made which require scanning of large areas by means of the cantilever. Various methods have been proposed for tracking the measuring light. These methods are successful in that the intensity of the reflected measuring light rays remains unchanged or is varied only a little as the cantilever is moved. If the cantilever does not bend the photodiode signal obtained due to the reflected measuring light rays is completely or almost completely constant.
However, this method has the disadvantage that the lateral tracking of the measuring light rays does not permit simultaneous correction of a vertical measuring error which also exists. A vertical measuring error occurs when the cantilever, rather than being oriented at right angles to the direction of incidence of the measuring light rays, is slightly inclined.
When making measurements with scanning probe microscopes it is frequently desired to also examine the specimen by transillumination using an optical microscope for a broader analysis. To accomplish that, the specimen must be illuminated by condenser light in order to obtain optimized results. But the implementations described above of the light spot principle with stand-alone scanning probe microscopes do not allow the probe to be examined by means of an optical microscope disposing of condenser illumination when the specimen is positioned on a specimen support (slide) so as to be measured by means of the scanning probe microscope.