LOW TEMPERATURE STM/AFM

Transcription

1 * CreaTec STM of Au(111) using a CO-terminated tip, 20mV bias, 0.6nA* LOW TEMPERATURE STM/AFM High end atomic imaging, spectroscopy and manipulation Designed and manufactured in Germany by CreaTec Fischer & Co. GmbH

2 In adatoms on InAs (111)A which were assembled by vertical atom manipulation to form a linear chain. The In adatoms reside on In vacancy sites of the (2 2) surface reconstruction (Fölsch et al., PRL. 103 (2009), ) Spectroscopy of monatomic Cu chains on Cu (111) reveals that these Cu chains represent 1 D quantum wells as evident from a series of chain-localized bound states IMAGING, SPECTROSCOPY AND MANIPULATION Low Temperature Scanning Microscope STM/AFM FEATURES OF THE LOW TEMPERATURE STM/AFM CreaTec s LTSPM was developed with Dr. Gerhard Meyer at the FU Berlin in the group of Prof. Rieder. Following the demand of our customers we are continuously developing this instrument. It is very well known for its outstanding stability and performance in the scientific community. The LTSPM is specially designed for working at atomic scale at low temperature (i.e. to perform atomic/molecular manipulation or local spectroscopy). It has a modified Besocke beetle design which ensures its high stability (dz<1pm) and very low drift. The SPM is thermally isolated. Its temperature can be varied between 5 and 300K. The low temperature of the SPM freezes the motion of most adsorbates which can be evaporated onto the sample surface either inside or outside the SPM. The surface/adsorbates can be investigated for weeks. No degradation of the sample will be observed as long as the 4K radiation shield, that surrounds the SPM, remains cold. Due to the design of the cryostat, a very low helium consumption is achieved, which requires to refill only 4l of liquid helium every 72 hours when the SPM is at its lowest temperature. The whole scan unit is digitally controlled. Therefore a precise tip positioning on any spot of the sample with any set of tunneling parameters can be made. This can be used to do local spectroscopic measurements (i.e.i(v), I(Z)) to reveal electronic or vibrational properties of the sample or the interaction between tip and sample, and to perform different kinds of manipulation techniques. Any information (I(t), U(t), XYZ(t), di/du ) can be recorded during manipulation. The standard scan range of 1.2μm x 1.2μm x 0.2μm may be adapted according to the customer s requirements. The minimum tunneling current is 2pA (200fA using our internal preamplifier). The system provides optical access to the sample surface to control the exact position of the tip with the help of a telescope and to perform optical spectroscopy. The sample transfer can also be done at low temperature. A helium cooled manipulator allows you to prepare, analyze and transfer the samples at temperatures down to 15K. Superconducting coils providing in- and out-ofplane magnetic fields can be included according to the customer s needs.

3 STM: Si(111)# STM: Cu(111) TCNE/Ag(111)# STM of Au(111) using a CO-terminated tip, 20mV bias, 0.6nA #: D. Wegner, Westfälische Wilhelms-Universität Münster All other data: CreaTec LTSPM SYSTEM UHV SYSTEM Control system Load lock chamber In order to minimize internal vibrations, the size of the UHV system has been reduced to a minimum. Instead of having separate chambers for preparation and analysis the CreaTec system has only one chamber with an integrated gate valve. Manipulator This also minimizes the length of the He cooled manipulator, which can be used to prepare, analyze and transfer the samples and tips (T <15K). The preparation chamber can be equipped with a variety of customer specific preparation and analysis tools. Ion pump with integrated Ti sublimation pump Cryostat Integrated gate valve Ion pump with integrated Ti sublimation pump

4 STM/AFM Specifications BESOCKE BEETLE / PAN SLIDER - FEATURES - Combined LTSPM/AFM with qplus design - High stability Besocke beetle or Pan slider scanner - Scan range: 1.2μm x 1.2μm x 5K - Minimum tunneling current: <500fA (<200fA achieved) - Minimum AFM oscillation amplitude: 50pm (20pm achieved) - Very small drift at low temperature: <2Å/h in thermal equilibrium - Variable temperature: 5-300K (lower temperatures on request) - Outstanding hold time for both lhe and ln2: 72 hours guaranteed (up to 100h achieved) - Optical access to the tunneling junction - 6 electrical contacts to the sample in the STM - CreaTec or plate type sample holders - Sample transfer at low temperatures: <15K - Simple and user friendly tip transfer system - Clean in-situ dosing or evaporation - Optimized for spectroscopy and manipulation of atoms and molecules - Powerful software with spectroscopy and nanostructuring support Z slider with XY sample stage: Tip and sample are moving independently. Fixed tip position for optical experiments Besocke beetle scanning head for STM and AFM measurements XYZ slider: Tip moving in x, y and z. Available with superconducting coil for out-of-plane magnetic fields SUCCESSIVE STM IMAGES OF (60 x 60) NM 2 AU(111) First image, second image after ramping up 5mm and down again. Reproducibility below 10nm at 5K (CreaTec)

5 Electronics and Software The SPM control electronics are based on a digital design. A digital signal processor (DSP) controls the scanning parallel to the surface (x, y) as well as the tip sample separation. All scanning and feedback parameters are controlled through the SPM program. An additional high voltage amplifier and a current preamplifier complement the electronics. There is an option to read data from an external Agilent 34970A data logger into the program via the GPIB bus. The CreaTec LTSPM electronics controls Besocke beetle, Z slider and XYZ slider heads and implements a variety of different approach mechanisms. With a typical scan speed of about 100nm/s an image of (30nm) 2 with (256 pixel) 2 is acquired in less than 3 min (typical at 6K). The PC software allows to control the feedback as well as all scan parameters. It includes data acquisition and analysis as well as different spectroscopy modes: I(z), I(V), di/dv and IETS for single atom and molecule spectroscopy. The CreaTec control electronics allows access and control for direct IETS results with an integrated software lock-in amplifier. Optional external lock-in-amplifiers can be operated as well. SOFTWARE FEATURES In addition to standard features like 3D image rendering, histogram etc. It also includes a flexible atomic manipulation part with lateral manipulation in constant current and constant height mode, vertical manipulation and tip forming. The heart of the SPM electronics is a TSM320C6711 DSP by Texas Instruments. The DLPCA200 preamplifier allows to set the required gain through the digital I/O of the DSP board (103 to 1011 V/A, 500kHz to 1.2kHz). Other modes of operation or analysis may be implemented by the user. A special section within the software package allows user modifications which will be adopted into future software updates. This section is able to access the same range of DSP parameters as the rest of the program. The DSP program may be changed as well. Smaller software changes can be done by CreaTec and will be integrated into the regular updates. More complex adaptations are available on request. OTHER FEATURES OF THE SPM ELECTRONICS - The DSP is electronically decoupled from the analog I/O board - Analog I/O board consists of: - 6 D/A outputs (20 bit, > 200kHz, +/- 10 V) - 4 A/D inputs (18 bit, > 200kHz, +/- 10 V) - 32 digital I/O ports (16 input, 16 output) - 4 analog output channels are used to control the piezo voltages (X, Y, Z) and the sample bias voltage - tunneling current input - The feedback loop and all AD/DA channels operate with max. 200 khz, allowing a maximum speed of pixels /s. - 2 monitor outputs (ADC0 [I] and DAC0[Z]) - Commercial current preamplifier DLPCA 200 CREATEC HV AMPLIFIER - Range 200V, 60 khz (3dB), resolution 1mV - Controlled by the digital I/Os of the DSP board - Selectable gain (1,3,10, 30) - Modulation voltage (+/- 10 V input) and offset for all 3 channels INTERNAL PREAMPLIFIER - Internal Preamplifier for combined STM/AFM measurements located at the outer ln2 shield (77K) - Bandwidth approx. 500Hz at low temperatures - STM: Gain 1010 (Femto DLPCA 200:109) - AFM: Gain Integrated relay to switch between internal and external preamplifier

6 CREATEC qplus LT-AFM Dynamic STM and constant force AFM The qplus AFM is based on the well-known, high-quality Besocke-type STM. The head is equipped with an additional linear drive (main scanner, z main), with a tunnel current contact and two additional AFM contacts. The usual STM tip is replaced by a tuning fork AFM sensor with the following specifications: Resonance frequency: RT: ln2: lhe: Q factors: RT: > ln2: >10000 lhe: > ~20-28kHz, (depending on tip type and length) ~ 5-10 Hz ~ 1-2 Hz ~ Hz The AFM sensor consists of a tuning fork and a tungsten STM tip (typical length 1mm) glued on one of the legs of the tuning fork. The new z drive remains fully compatible with our standard STM tips. The diagram below shows a typical resonance spectrum at room temperature. The resonance spectrum of the tuning fork can be directly measured from within the AFM program. A separate software module is integrated into the SPM software to adjust the PLL control and the df signal. Our standard STM setup uses a Femto DLPCA200 preamplifier to measure the tunnel current outside the UHV chamber. For the AFM, we have integrated an additional in-situ preamplifier with the following specifications: Bandwith: ~500Hz Gain STM: Gain AFM: 10 8 Using an integrated relay, the operator is able to switch between the internal (gain ) and the external (Femto, gain 10 9 ) preamplifiers during the measurement. Tunneling currents down to 200 fa can be measured. AFM CONTROLLER The DSP controller is similar to the one used for the STM. It is extended by a second DSP card providing additional DAC and ADC channels. One DAC (driven at 200kHz) is used for the excitation of the AFM sensor, an additional ADC is used to measure the q-plus df signal. A power supply for the internal preamplifier is integrated. In addition, we deliver a special bandpass amplifier to reduce the noise and to amplify the q-plus signal. qplus TUNING FORK AFM - tuning fork sensor: f res =20-28kHz - FWHM (5K)=0.5-1Hz - large Q factor >20000 at 5K - available with W or PtIr tips (typical length 1mm) - Fully compatible to the CreaTec LTSPM - magnetically or mechanically fixed AFM tuning fork sensors - 3 separate point contacts: independent measurement of tunneling current and qplus signal Typical qplus resonance curve at RT (Q4300, FWHM 5Hz)

7 The CreaTec design of the tip movement and the sensor allows the direct and simultaneous measurement of tunneling current and df signal. Constant current measurements of the topography may be combined with df signal imaging (dynamic STM, CC+df). Other modes include constant height measuring the tunneling current combined with simultaneous df and constant df scans. High resolution AFM constant force scan of Au(111), slider, raw data (25Å) 2 Au (111), 10 Å/s, U=-80mV, df=-12hz Fast scan AFM constant force of Cu (111) at zero bias, slider, raw data (1600 Å) 2 Cu (111), 1200 Å/s, U=0V, df=-9.8 Hz Constant current df images of Si (111) at 5K: topography, current and df channel (from left to right), df=-22hz, 7 pa, tuning fork oscillation amplitude is 50pm Df feedback AFM images of Si (111) at 5K: topography, current and df channel (from left to right), f_sp =-22Hz, tuning fork oscillation amplitude is 50pm Constant force AFM of 100 Å of Au (111), topography, -1.3 Hz frequency shift, -1V bias Data: CreaTec

8 High resolution STM image of Si (111) at 5 K (courtesy: Dr. D. Wegner, Westfälische Wilhelms-Universität Münster) High resolution STM image of Cu(111) at 77 K (CreaTec) Low current image of Si(111) at 5K and 1pA, (CreaTec) High resolution low current STM images of Si(111): (a) 500 fa, (b) 200fA and (c) 100fA measured using the internal preamplifier (77K) (CreaTec) About us deposition of almost any material are our daily business. The combination with our characterization tools such as lowtemperature scanning probe microscopy, diffraction and other vacuum methods allow us to build you the tool that exactly fits your specific application. Our specialties are modifications and extensions of existing systems as well as the development of equipment for new emerging technologies where no standard solution exists. Since 1992, CreaTec manufactures custom-designed UHV systems and components for research and production. Many years of experience in the design and fabrication of UHV equipment allow us to meet even the most demanding specifications. High and ultrahigh temperatures, motion in vacuum, nanomanipulation, Manufacturing and testing of all our products takes place in an in-house clean room environment (class 1000 to class 10). Many of the innovations which today are an inherent part of our product range were generated from close cooperations with leading research institutions. CreaTec Fischer & Co. GmbH Industriestraße Erligheim, Germany General Managers: Dr. Wolfgang Braun, Albrecht Fischer Tel.: Fax: sales@ createc.de Internet: