Left panel: Graphite surface measured with the first prototype of the MultiScan electronics in 2002 using a Besocke-type STM scan head. Right panel: STM image with submolecular resolution of an α-sexithiophene monolayer on Cu(110)-(2x1)O, recorded with a homebuilt variable-temperature STM at 15 K.
Control Electronics for Scanning Probe Microscopes
Dr. Manfred Geretschläger, Ass.-Prof. Dr. Erich Steinbauer
We have developed a control electronics which can be used for almost any type of scanning probe microscopy, e.g., scanning tunneling microscopy (STM), atomic force microscopy (AFM) or near-field scanning optical microscopy (NSOM). It is characterized by the following features:
- various scan modes (constant height, constant current, constant external feedback)
- easy zooming and panning of the measured pictures
- electronic compensation of temperature induced mechanical drift of the scan head
- electronic plane subtraction for the compensation of sample inclination
- simultaneous measurement of up to 6 parameters, each selectable from a variety of 100 internal or external signals
- high speed surface scanning for measuring of surface adsorbates with a high mobility in real-time
- spectroscopy modes to measure local electronic or inelastic properties, force distance curves, etc.
- full software control of the microscope via a personal computer linked by high speed data communication
The scanning electronics is implemented by 6 types of electronic circuit boards which have been designed using multi-layer technology. Each board consists of a digital control unit which is built into a complex programmable logic device (CPLD) and analog or digital functional units. The electronic system is controlled by two self-designed CPLD microcontrollers (central processing unit and scanner control unit) with dedicated assembly commands and program memory. Control programs are generated using the connected personal computer and downloaded via a high speed data link which provides a high degree of flexible, distributed intelligence for complex measurement tasks. Output data are buffered in the memory banks of the central processing unit and can be efficiently retrieved by the personal computer.
The operating software of the scanning system consists of three parts:
- the main program MultiScan which runs on a PC under the operating system Windows NT / 2000 / XP
- programs for the CPLD microcontroller on the central processor board
- programs for the CPLD microcontrollers on the scanner boards.
The program MultiScan was written with the Microsoft Visual C++ development system. It implements functions for the operation of the scanning electronics and it provides a basic set of image processing and data handling tools. It is fully compatible with the Windows component object model (COM+ / ActiveX) and its functions can be controlled by
- direct user interaction using menus and dialogs
- script programs written in BASIC or JAVA
- programs written with the Microsoft Visual Basic development system
For the operation of the CPLD microcontrollers a special assembler program has been developed. It generates loadable modules of machine code which are downloaded to the controller main memory using the high speed parallel data link.
Two additional peripheral devices have been developed. First, a High Voltage Amplifier converts the low level output signals of the scanner electronics to a voltage range of ±200 V to operate the piezo electrodes of suitable scan heads. Its electronic noise is as low as 250 µV (RMS). The corresponding bandwidth is 18 kHz, the slew rate is 8 V/µs.
Second, a low noise current preamplifier converts the tip current of STM heads to a suitable voltage range of ±10 V which can be used as an input for the feedback loop. Its input stage can be switched either to standard mode (outside the vacuum chamber at room temperature) or to cryogenic mode (in vacuum at cryogenic temperature). In standard mode the electronic noise is 3.3 pA (RMS) at a bandwidth of 7.7 kHz.
The complete system was tested using a Besocke-type STM scan head operated at atmospheric pressure. As an example, the left panel in Figure 1 shows an atomically resolved image of a graphite surface recorded with the first prototype of the control electronics in 2002. The right panel displays an image with submolecular resolution of an α-sexithiophene monolayer on Cu(110)-(2x1)O, obtained more recently with our homebuilt variable-temperature STM (VT-STM).
We have also adapted and tested our control electronics for the use with a commercial AFM head. Continously, additional features which are available at the hardware level (drift compensation, plane subtraction, spectroscopy modes, …) are implemented into the software programs.