1. Field of the Invention
This invention relates to a signal detector to be used with a scanning probe in various observation and/or recording apparatuses utilizing the atomic force that is generated when a probe and a medium are brought into close proximity to each other. More particularly, the present invention relates to a technology of controlling the offset current in such apparatuses.
2. Related Background Art
An atomic force microscope (to be referred to as an AFM hereinafter) is a scanning probe microscope adapted to detect the atomic force generated when the surface of a specimen and the front end of the probe are brought into close proximity to each other by way of the deflection of the probe. While the atomic force may be repulsive or attractive, an AFM generally utilizes the effective region of the repulsive atomic force.
The optical lever system is the most well known system for detecting the amount of deflection of the probe. With this system, a laser beam irradiates the front end of the probe and the displacement of the reflected beam caused by the deflection of the probe is detected by means of a photoelectric conversion device such as a photodiode and the profile of the surface of the specimen is observed by way of the obtained displacement signal. The optical lever system is advantageous in that the amount of displacement can be boosted to improve the resolution by providing a long optical path for the reflected beam. However, use of a long optical path requires a large optical system and a large detection system. Another drawback of the optical lever system is that it involves a cumbersome operation of making a laser beam accurately irradiate the minute end of the probe.
Recently, a technique of detecting the deflection of the probe as a change in the electric resistance of the probe has been attracting attention. A piezoelectric element (piezo resistance element) is typically used as detection means of the probe. A piezo resistance element is affected by external force in two ways; distortion of external profile and its energy band structure so that the electric conductivity and hence the electric resistance of the element is changed by external force. Unlike the optical lever system, a system using such a piezo resistance element does not require an optical system and hence an AFM utilizing such a system can be made very compact and easy to handle.
In recent years, research efforts have been made in the technological field of multi-probe AFM adapted to scan the surface of a specimen by means of a plurality of probes arranged in parallel. This technology is particularly advantageous when the specimen has a large surface area to be scanned because the classical AFM is developed to observe the surface of a specimen in terms of atomic dimension. Since the AFM deals with signals representing the atomic force generated between the probe and the surface of the specimen, which is extremely small, the scan rate of the microscope is inevitably limited and hence it is practically not possible to provide an AFM that can accurately scan a large area. Thus, the multi-probe AFM is designed to scan an extended area by using a relatively large number of probes for scanning without trying to raise the scan rate of each probe. The technique of detecting the amount of deflection of each probe by way of the change in the piezo resistance is more suited than the optical lever system when observing the surface of a specimen by means of multi-probe AFM because, if the optical lever system is used, it is highly difficult to accurately irradiate each of the plurality of probes with a light beam unless a very large optical system is employed.
For detecting the amount of deflection of a probe by way of the change in the piezo resistance, a constant bias has to be applied to the probe. The bias may be a constant current bias or a constant voltage bias, with which the change in the piezo resistance will be detected as a change in the voltage or current. A piezo resistance element that can be used with this system shows a specific resistance (piezo resistance) and the piezo resistance element comes to constantly generate a so-called DC type signal when a bias is applied to it. Additionally, the change in the piezo resistance of a piezo resistance element due to deflection of the probe is as small as 10xe2x88x928 times of the initial resistance of the piezo resistance element and hence the signal representing the amount of deflection generated by the change is accordingly very small. Therefore, a highly sensitive detection system is required to detect such signals and additionally the above pointed out DC type signal has to be removed if such signals are to be detected.
Still further, the piezo resistance of a piezo resistance element is significantly affected by temperature. Therefore, the above described DC type signal is also affected by temperature and hence it does not show a constant value but changes with time as the temperature of the element normally changes with time. The influence of this temperature change is particularly remarkable when the probe is brought close to the surface of the specimen. In such a case, the temperature of the piezo resistance element changes due to heat exchange with the probe. This causes signal drift. Finally, when a plurality of probes are used for an AFM, the values of the piezo resistance specific to the respective probes may differ from each other. Thus, there arises the problem that, in addition to, the DC type signals, the signals generated by each of the plurality of probes drift differently from each other.
In view of the above circumstances, it is therefore an object of the present invention to provide a signal detector adapted to be used with a scanning probe as it can remove the offset signal contained in the output signal representing the change in the piezo resistance of the probe by means of a simple arrangement. Such a signal detector can reliably detect the signal and be used in multiple as it is made free from variances of piezo resistance. Another object of the present invention is to provide an atomic force microscope comprising such a signal detector.
According to the invention, there is provided a signal detector to be used with a scanning probe having a piezo resistance cantilever comprising:
a signal detection circuit for detecting the change in the piezo resistance; and
a circuit for removing an offset component from the signal.
According to the invention, there is also provided an atomic force microscope comprising such a signal detector.