Today, scanning probe microscopy as other micromechanical cantilever applications (generally termed “SPM applications” in the following) have become standards in fields extending from laboratory tests to industrial applications in batch production lines and medical applications in hospitals. Thus, samples to be investigated may be tiny, isolated particles or living cells, typically in the μm range, as well as relatively large samples like structures and surfaces of industrial products or tissue probes in medical applications, often measured in cm in size.
Considering the extremely small range of operation of SPMs, both in the z-axis, i.e. the “altitude” dimension, as well as in the x/y-dimension, i.e. the scannable area, it is obvious that multiple readjustments are required when scanning large, uneven, or three-dimensional objects.
To give examples of actual sizes, an area scannable by a typical SPM may have a size of 10 μm×10 μm to 150 μm×150 μm at most and the operational range along the z-axis is often no more than a few μm, e.g. 5 μm. Completely scanning an object of e.g. 1 mm diameter width and 10 mm length is thus obviously a lengthy task, taking altogether between 1'000 and 100'000 scans, each scan requiring a readjustment of the sample. Considering that it is often sufficient to scan only certain significant or critical parts or areas of an object, it becomes apparent that a possibly automated method for selecting those significant areas would be very advantageous.
The present invention provides this solution. It is a multi-step process. In a first step, the complete surface of an object or a large part of it is mapped in its entirety, not by SPM scanning, but by a “coarse” method, e.g. by optical means, developing an integral map of the object, preferably in digital form. In a second step, the thus produced integral map is subjected to a, preferably mathematical, analysis to identify and localize areas or spots of interest or significance, thus providing a map or listing of test areas or locations. These areas are e.g. for SPM flat areas which are accessible with a SPM probe, e.g. a cantilever with integrated tip. In a third step, these test areas or locations are “fine” scanned by the SPM. The results of this SPM scanning can be displayed, stored, printed, or subjected to a further analysis. The latter could be a fourth step of the method according to the invention.
All above steps are automated so that human interaction with the system is limited to the insertion of the probe or sample and the reading of the results, or the use of a printout, or the use of the results of the further processing, resp. human errors or misinterpretations are thus practically excluded.
It is usual practice in scanning probe microscopy, as in other micromechanical cantilever applications, to bring the sensing probe close to the surface of the sample by a multiple step process, e.g. with a coarse approach mechanism and a fine adjustment by the z-actuator used for scanning the probe. The sensing probe consists in case of SPM usually of a cantilever with integrated sharp tip at its end. This approach step is finished when a criterion such as defined cantilever deflection or, in case of an oscillating cantilever, a predefined oscillation amplitude or frequency change is detected. To prevent, during the approach step, the sharp end or tip of the probe from damage by touching the sample surface, elaborate detection mechanisms have been developed, described e.g. in lyoki et al. U.S. Pat. No. 8,024,816 B2. However, all these procedures are time consuming.
In order to reduce the time consumption of the approach step(s), various rather complex procedures to automatize a scanning probe microscope have been pursued, e.g. described in Struckmeier et al. U.S. Pat. No. 7,810,166 B2. Therein, the main problems addressed are the optimizing of scan parameters and the subsequent processing of measurements using predefined parameter sets.
All efforts so far do not provide an automated method or device for a pre-evaluation of any measurements and automated search for optimized measurement conditions or preferable measurement positions, i.e. positions which promise high yield of good quality results.
On the other hand, and in a totally different technological field, i.e. not related to raster scanning microscopy, there are relatively coarse methods—as compared to SPM methods—for the optical mapping of much larger samples than usually scanned with SPMs.
One example of a—in relation to scanning probe microscopy—“coarse” optical scanning methods is disclosed in Schwertner EP 2 437 027. The disclosed three-dimensional scanning method uses structured illumination of a sample and has the advantage that large samples can be scanned rather quickly, even samples that are larger than the optical field of view. This is done by relative movement between the sample plane and sample. It involves providing rectilinear relative movement between a sample plane and a sample which plane is inclined with respect to direction of the relative movement. Then the surface configuration of the sample is reconstructed from a set of calculated optical section images.
Another example for such coarse optical scanning methods discloses Wang et al. U.S. Pat. No. 8,454,512, showing a confocal photoacoustic microscopy system. It includes a laser which focuses a light pulse onto an area inside of an object and an ultrasonic transducer receiving acoustic waves emitted by the object in response to the light pulse. An electronic system processes the acoustic waves and generates an image of the area. The focal point of the laser preferably coincides with the focal point of the ultrasonic transducer.
However, none of these coarse optical scanning methods provides a resolution of the obtained data that is comparable to the resolution obtainable with an SPM. Further, none of them is in any way suited for or adapted to an SPM. First, the method of raster scanning differs fundamentally from optical surface investigation methods. Second, the obtained data of the two methods are incompatible. Third, optical scanning apparatuses are so different from raster scanning devices that it requires inventive effort to adapt one to the other.