STM Electronics

AG Moeller – University of EssenM. Matena

25th May 2004

Contents

1 The different modules 11.1 The Signal-Ranger-Adapter . . . . . . . . . . . . . . . . . . . . . 11.2 The Input Filter Module . . . . . . . . . . . . . . . . . . . . . . . 21.3 The X/Y-Module . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.4 The App. Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.5 The STM-Interface . . . . . . . . . . . . . . . . . . . . . . . . . . 51.6 The HV-Amp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.7 The RTM VMOD . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.8 The InMod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2 Block diagrams 10

1

1 The different modules

This chapter describes the function of the different components used to connecta scanning tunneling microscope to a signal ranger board (Signal Ranger SP2SPM, SOFT DB). This description starts with the components that are directlyconnected to the DSP board and that for dealing with the processing of the X-,Y - and Z-signals to control the piezo.

1.1 The Signal-Ranger-Adapter

Figure 1: Signal Ranger Adapter

The Signal-Ranger-Adapter (Picture 1 on page 1) takes the signals from theDSP board and adds the center of the scan Xoffs and the scan position Xin.The offset signal is sent through a low-pass filter of sixth order with a cut-offfrequency of 20 Hz and after that it is amplified with a factor of five. The factorof five provides the right amplification of the working voltage of the DSP (±2 V )to the working voltage of the electronic (±10 V ). The amplification factors forthe scan signals (Xin, Yin, Zin) can be adjusted with the respective switches.

The modulation input Zmod is amplified with a factor 1100

. This damping allowsthe modulation of the Z-signal with an external lock-in providing a sinusoidaloutput voltage with an amplitude in the order of several volts.

1

The separation of input and offset variables can be understood with the helpof the picture 2 on page 2.

Figure 2: Relative Coordinates

Introducing relative coordinates makes a high resolution scan possible. Thecoordinates (Xoffs/Yoffs) determine the center of the scanned section while thesection itself is described by (Xin/Yin). So all bits being available by the DSP aresolely assigned to the scanned section so that an optimal resolution is granted.In this case an optimal resolution means: 1 Bit is assigned to the least possibledeviation of voltage.

1.2 The Input Filter Module

The Input Filter Module (Picture 3 on page 3) provides low-pass filters for thethree Cartesian coordinates and the tunneling voltage. They smooth the signalcoming from the D/A-Converter. The cut-off frequencies are 100 Hz for the X-and Y -coordinates and can be switched between 10 kHz and 20 kHz for theZ-component as well as between 500 Hz and 5 kHz for the V -component. Allthese filters are of sixth order.A cut-off-frequency of 100 Hz is chosen for the X- and Y -components as a com-promise: The given signal for these coordinates is a triangle signal which has ofcourse components of high frequencies in its Fourier series. On the other handthe signal from the DSP has to be smoothed so that 100 Hz are chosen as asensible value which might only flatten the triangle tip a little bit.For the other signals (V and Z) higher cut-off frequencies have to be used because

2

Figure 3: Input Filter Module

a fast response is needed for the Z-component and it might also be interesting tomodulate both components.

1.3 The X/Y-Module

After the last module the X- and Y -components are seperated from the Z-component.On the one hand the X/Y-Module (Picture 4 on page 4) deals with the X- andY -coordinates whereas the STM-Interface which is described in one of the nextsections handles the Z-coordinate.

The switch Gain X/Y provides different gains for the X- and Y -components.In position “0” the tip remains on the same place during the whole scan. Thismight be important to detect mechanical and electrocinal variations of the STM:If the tip of the STM is not moved in X- or Y -direction and neither the Z-component nor the tunneling voltage is modulated the Z-component won´t change.On the other hand: If the Z-component changes under these conditions theremust be some kind of disturbance.In contrast to the STM-Interface which is described in one of the following sec-tions the amplification factors of this module only refer to the X/Y-Module. Thetrimmer Gadj Y is used to adjust the proportion between the voltages in X-and Y -direction so that unsymmetries of the piezo can be corrected. A short

3

Figure 4: X/Y-Module

comment to this unsymmetry: In general the piezo constants for X and Y aredifferent, i.e. the same voltage applied to the X- and Y -components will result indifferent movements of the piezo in both directions. Of course, GXSM can handlethis, but the program only displays data points. So a scan which is displayedas a square is in real coordinates a rectangle beacuse the distance between twodatapoints is different for X and Y . If the same piezo constants for the X- andY -components are realized by the Gadj Y GXSM will draw pictures withoutdistortion.

The limiter prevents the tip from jumping if the X- or Y -voltages are changedtoo fast. This is useful to prevent a crash of the tip in case of a software or hard-ware problem. The limiter is turned on by the switch Limit and its sensitivitycan be adjusted by the trimmers Lim. X and Lim. Y. Typical values for thevelocity of the Limiter are around 1 V

s.

The main potentiometers are used to add an offset to the X- and Y -componentse.g. to correct the offset of the DSP or the scan position.

The outputs of this module are directly connected to the HV-Amp so that thehandling of the coordinates ends here and the next modules are merely importantfor the processing of the Z-component.

4

1.4 The App. Control

Figure 5: App Control

The sparely equipped front panel of this module (Picture 5 on page 5) showsthat it is in an early state of development. It is planned in the near future toprovide a logarithmic calculus. Compared to the digital logarithmic calculus aanalog solution has one advantage: The digital version first discretises the signaland then takes the logarithm so that errors in ranges of small values are highcompared to those ones in ranges of high values. So the noise of the signalinevitably increases. To prevent this it is useful to discretise the signal after thelogarithmic calculus. This is provided by the analog solution.

Until now the App. Control is used to control an important feature of theSTM-Interface. The input is connected to the InMod (which provides the elec-tronics with the tunneling current). The potentiometer is used to set a thresholdand the output is set to 5 V if the input i.e. the tunneling current reaches thisthreshold. The output is connected to the STM-Interface (input Speed Con-trol) to retard the speed of the approaching tip.

1.5 The STM-Interface

The STM-Interface (Picture 6 on page 6) is responsable for the control of the ap-proach of the tip. The necessary procedure to guarantee a safe approach is similarto the description of the relative coordinates used by the Signal Ranger Adapter

5

Figure 6: STM-Interface

(Picture 7): To achieve the optimum resolution in Z-dircetion the DSP only con-trols a small part of the total Z-range. Depending on the polarity of the DSPvoltage the electronics will move the zero point (the whole range controlled bythe DSP is moved too) to the corresponding direction which is realized by sim-ply integrating the DSP signal and adding this “offset” to the DSP voltage. Thespeed of this movement is always porportional to the amplitude of the DSP and itis adjusted by the potentiometer called Speed. As already mentioned the inputSpeed Control is connected to the App. Control and if this input recognisesthe 5 V the speed is lowered to prevent the tip from crashing into the surface.The purpose of this feature:As shown in figure 7 the DSP voltage shall be negative. So the STM-Interfacemoves the zero point of the DSP to the surface. If a tunneling current is detectedthe voltage of the DSP turns to zero. But the electronics moves the zero pointof the DSP further to the surface because it integrates the signal of the DSP.So the tunneling current exceeds the threshold and the DSP voltage turns – ac-cording to figure 7 – to positive values. Because of that the electronics will thencorrect the zero point of the DSP upwards until the tunneling current falls belowthe threshold and the process is repeated. The consequence: The DSP and theSTM-Interface work in opposite directions. To solve this problem the speed ofthe electronics is lowered if a tunneling current is detected.

6

Figure 7: Z-control of the STM-Interface

If the piezo moves to its limits without reaching a sufficient tunneling currentthe output MSCU will provide a signal for the piezo engine. The approach canbe initiated either by pressing the so called button or by setting the switch toApp. If this switch is set to Reset the tip is retracted from the probe.

Another interesting feature called tip guard – which is activated by a jumper(default: inactive) on the board – uses the approach control during the scanningprocess to prevent the tip from crashing. Example: The automatic control isturned off. In this case the tip can only be moved in the small range the DSPcontrols. A tilted sample might cause the DSP operating at its limits so that thetip cannot be retracted anymore. With tip guard enabled the electronic retractsthe whole range controlled by the DSP – i.e. it adds an offset to the DSP voltage.

If tip guard is disabled and the switch is set to Hold the STM-Interface hasonly on task: It takes the Z-coordinate from the input connected to Z-PC andforwards it to the output called HVAmp after having added the signal from Z-Mod which is used to modulate the Z-component. The input Z-Mod is dampedby a factor of 100.The output Z-Buffer provides the same signal as the output HVAmp. So itcan be used to monitor the Z-position with an oscilloscope.

The switch Z-Factor has three different positions. As an example: Position1:10 provides a total signal amplification of 10. In this case total amplifica-tion means: The amplification of this module multiplied by the amplificationof the HV-Amp. Let the amplification factor of the HV-Amp be 15. So theSTM-Interface itself only provides an amplification of 10

15.

7

1.6 The HV-Amp

Figure 8: HV-Amp

The HV-Amp (Picture 8 on page 8) receives the signals for the three Carte-sian coordinates and ampflies them to be able to manage the piezo. The finalamplification factor is 15.

Now the modules which are necessary to process the X-, Y - and Z-componentsare specified and the last two modules describe the way back from the STM tothe DSP board.

1.7 The RTM VMOD

The RTM VMOD (Picture 9 on page 9) has two voltage sources. On the onehand it provides a voltage which can be adjusted by the potentiometer Is between−10 V and +10 V . The output for this voltage is called Is,out. Originally itprovided a setpoint for an analog control loop but at the moment the control isdone by GXSM so that this output is used as a variable voltage source.

On the other hand this module controls the tunneling voltage. It adds thesignals of the inputs Udiff ,in and Umod,in. To this sum a voltage can be addedwhich is adjusted by the three remainig controllers. The output Vout providesthe sum of all these voltages.The potentiometer allows to apply this voltage between 0 V and the value

8

Figure 9: RTM VMOD

which is defined by the switch Range. The other switch changes the polarityof the voltage. Another thing should be mentioned here: The different settingsof the switch Range causes different attenuations for the input Umod,in: Thesetting 0.1 causes the attenuation factor 10000, 10 the factor 100 and 1 the factor1000.The Udiff ,in inputs can be used to connect the DSP board. The input Umod,in isused to connect a LockIn amplifier in order to measure dI

dU.

Another interesting feature is Vcomp,out. The wire providing the tunnelingvoltage – which is not perfectly smoothed – for the STM induces an electric fieldon the surface which can affect the tunneling current and thus the measurement.Because of that the Vcomp,out is connected to another wire being positionedparallel to the primarily mentioned one. Vcomp,out then provides a potentialbeing out of phase to the first one which has to compensate the electric field onthe surface caused by the tunneling voltage. Vcomp,out can be adjusted by thetrimmer −α V. The phase shift is fixed to 180◦.

1.8 The InMod

The InMod (Picture 10 on page 10) gets the tunneling current from the IV-converterof the STM input IVC-In. The special connector on the front panel also providesthe power supply for the IVC whereas V-in is solely an input which can be usedfor another IVC. The signal of the IVC is then put through different low-pass

9

filters and forwarded to the different connectors on the lower side of the frontpanel. For STM a cut-off frequency of 10 kHz is generally recommended.

Figure 10: InMod

2 Block diagrams

The first picture (figure 11) shows how the signal is processed on its way towardsthe STM. To prevent a rather confusing picture the different inputs and outputs(e.g. Xin,Xoffs, ...) are symbolized by one input or output. As mentioned inthe previous chapter the processing of the X- and Y -components is made by theX/Y-Module whereas the Z-component is connected to the STM-Interface.The STM-Interface also needs – in order to realize the approach of the tip –informations about the tunneling current provided by the App. Control whichis connected to the InMod. The output of 5 kHz is usually appropiate foran adequate control of the App. Control. Picture 12 shows this part of theelectronic.

This picture also shows the connection of the RTM VMOD providing thetunneling voltage for the STM. The output of 10 kHz of the InMod is directlyconnected to the DSP.

10

Figure 11: Block diagram: direction from the DSP to the STM

Figure 12: Block diagram: direction from the STM to the DSP

11