Difference between revisions of "Parameter"

Latest revision as of 11:18, 21 December 2016

Scan

The position of the tip (x,y,z) is controlled by three 20bit D/A converters. Range ( -524288 .. +524287 corresponding to –10V to +10V).The orientation is chosen in the following way: negative X Values are located on the left side of the image, positive Y values are located at the bottom of the image. The origin of the image is without additional offset values at point (0,0). This corresponds to the point in the Y top and X-center of the image. (The origin of the image can be changed using the setxyoffset – command, which changes the OffsetX,Y values). The size of the image is defined by the distance between the image points in DAC units (Delta X,Y) and the number of image points. The corresponding size of the image is in Angström: DeltaX*NumX/(524287)*10V* GainX*XPIEZOCONST[Å/V] (similar to Y, Z). T Size, Speed, Pixelnumber(with certain limitations) can be changed during scanning. Top-down and Bottom-Up scanning is supported.

Scan operation is controlled by the following parameters:

Image Size(Pixel): Input the number of pixels of the image to be acquired. X is the value of the horizontal scan direction.

Image Size(Å): Input the size of the image [Å]. It is important that the Piezo-Sensitivity factors have been correctly chosen. The deflection sensitivity is temperature dependent!

Scanning Speed[Å/sec]: Value of the scanning speed [Å/s]. Input values will be adjusted regarding to hardware restrictions. For higher speeds larger internal step sizes will be chosen. Also the time it takes for a single line (only forward direction time) and the time to take a complete image are displayed.

Biasvoltage[V]: Input of tunneling bias voltage [V]. The bias voltage value can be also changed with the Voltage - Slider. The Bias-Polarity can be switched by pressing the + or - Keys or by mouse clicking on the '+V' or '-V' - Label at the voltage - slider.

Rotation[Deg]: Rotation of the scanning direction in degree. Rotation is around the center point in the top image scan line (0,0 point)

Rot_center: If checked the rotation is done around the image center.

Channels: Select the channels which will be measured during a scan operation.

I-Preamp Gain: Select the gain of the gain switchable current preamplifier. The value is the base 10 logarithm of the gain in [Å/V].

Constant Current - Constant Height: Select either scanning in normal constant current or in constant height mode. In constant height mode the topography signal is automatically substituted by the current signal.

Forward - Backward Scandirection: Normally data is taken only when scanning from left to right. If forward+backward is selected then data is acquired in both directions. As an example if only one channel is active the uneven lines are taken from left to right and the even lines are taken from right to left.

Multivoltage: Another option is the multiple voltages mode. In this mode a series of images are taken in parallel line by line, while for each image a different bias voltage is applied. The different bias voltages are taken from the neighboring MultiVolt table. Only the voltages up to the first voltage which is equal to zero are used up to a maximum of 6 voltages. The individual images are saved in the same file and appear in one single image window

PlanDX, PlanDY: An average tilt of the sample can be compensated by using these parameters. The user can input these parameters manually or use the automatic feature. This is done the following way. Take an image. Use the <Pln> Button and input 3 points to subtract a plane. Then select the Set_Plan_DX_DY feature under the DSP item in the main menu. It is important to realize that thermal drift or piezo creep will significantly disturb the Plan_DY value. To improve that you could take an image rotated by 90deg and follow the above procedure. This will give you a better Plan_DY value, but less precise Plan_DX value. Combination of both allow you to both values with higher precision. Important: Check the new values, especially before you take a constant height image.

Offset_X,Offset_Y: The X,Y offset voltages of the origin of the image. Double Click the Offset_X Label to switch between [V] and [Å] units

Repeat Control: Use these parameters to acquire image series with precise timing.

Count: The number of images to be taken.

Interval: Select the time interval [s] when a new image scan is started. (must be larger then the time to acquire a single image)

The files are automatically saved with the file extension R.DAT

Const. Height Z-Offset, Preamp gain, Bias Voltage: In constant height mode different values for the Preamp Gain and the Biasvoltage can be applied. In constant height mode scanning Zoffset determines an additional perpendicular shift of the tip after the feedback loop has been switched off.

Piezoconstants: These are calibration constants to determine the piezo sensitivity [Å/V]. Be aware that these parameters are temperature dependant.

HV_Gain: Displays the gain of the x,y,z channels of the Createc HV Amplifier

DSP

The digital feedback controller is operated by the following parameters.

FB-Mode: Select whether the current signal from the FB Channel is applied directly to the digital feedback controller (Lin(I)), or the logarithm of the signal (Log(I)) or the absolute value (Abs(I)) is calculated first.

FB-Channel: Here the value of the channel can be selected which is used as input signal of the Feedback Loop controller.

SetPoint[A or Hz]: Setpoint value of the current or df signal.

Integrator-RC: This value sets the response time of the integrator part of the PI controller. Only the integrator part is usually installed.

P-Gain: This value sets the proportional part of the PI controller.

Current Low Pass[Hz]: The current signal from ADC0 can be recorded during scanning. Before the signal is stored on the PC the signal is low pass filtered. The edge frequency of this low pass filter can be set by this parameter.

Integrator and P-Gain Slider: The Integration and P-Gain values can be continuously modified using the two sliders. The original values input manually in the text boxes can be changed by 0.1 - 10.0 by slider movement. Both values can be changed during scan operation to optimize feedback operation.

Tip Forming

Tip forming (controlled crashing of the tip) is similar to the vertical manipulation procedure, but uses independent parameters to simplify the intermittent use of tip forming and vertical manipulation/spectroscopy. Tip forming is performed by first pressing on the tip forming button <TF> and then marking a particular point in the image by clicking with the left mouse button. Similar to vertical manipulation the data taken during the tip formation are displayed in the vertical manipulation form. The parameters used are TipForm_Vol, which is the voltage applied to the sample during the tip forming process. Then TipForm_Z [mV] , which means that the tip is moved towards the sample by Z x Zpiezoconst for the tip forming. (The gain of the z - HV amplifier is automatically included). The preamplifier gain is always set to gain 106.

Voltage[V] Voltage applied during the tip forming manipulation process.

Z-Approach Defines how far the tip moves towards the sample during a tip forming process

Pulslength[sec] Defines how long a complete tip forming approach takes.

Lateral Speed[A/sec] Defines how fast the tip moves to the selected tip contact position

Lockin

The new version of STMAFM has an internal lockin integrated in the DSP software. The internal lockin modulates the bias voltage directly and measures the modulated current signal on ADC0. No extra cabling has to be installed. It is still possible to switch between the internal lockin and an external one whose output has to be connected to ADC1(ADC2).

The internal lockin measures both the signal at the modulation frequency and the second harmonic.

The parameters:

Frequency: Modulation frequency

Amplitude: Peak to Peak modulation amplitude

Ref. Amplitude: Peak to Peak amplitude of the external reference signal supplied by DAC4. (Discontinued under AFM Operation)

RC: Time constant of the lock-in low pass filter

Phase1, Phase2: Phase value for first and second harmonic

Lockin-Mode:

Internal Off (External): The internal lockin is switched off. The modulation has to be taken from an external lockin. Connect the dI/dV signal to channel ADC1 and the d2I/dV2 to ADC2.

Internal: Modulation and Signal detection done by the internal lockin. The signal is extracted from the current signal (ADC0).

Internal + Scan off: Internal Lockin Mode but the modulation is switched off during scanning

Internal + Spectrum only: The modulation is applied only during spectroscopy

External + Internal Ref.on DAC 5: The modulation is done by the internal lockin, but lockin detection is performed by the external lockin. A reference signal for the external lockin is supplied by DAC 5. Connect the dI/dV signal to channel ADC1 and the d2I/dV2 to ADC2.

Output Channel: Determines the output channel of the Lockin. (BiasVoltage, PLL df and Ampl, Z)

Lockin-Channel: Determines the input channel of the Lockin (ADC0-3, df Z(DAC0))

2F: Enables 2nd harmonic mode

Panel

Sensors On Panel additional information is available, regarding the temperature sensors etc.

Memo The memo field is used to input extra information about the current image. The memo info is stored with the image.

Info

Lists all paramter sets.

Load,Save Press to load or save the selected parameter set.

Info Use the info field to input information which should be later available, but not contained in any other datafile etc. The contents of the info field is saved in info.cfg. Double clicking in the info field, will insert an actual time stamp.