VI - Miscellaneous
Standard operating procedures
While many settings within WinSPA can be accessed independetly via non-modal forms, it is important to understand how they influence the acquisition of scan data, conversion of units, correction of instrumental errors etc. . In fact, it is helpful to consider the combination of different calculation tools and settings as "standard operation procedures", i.e., applying tools and changing settings always after certain changes of the LEED system, sample, etc.
These standard operation procedures are described in the following. It is strongly recommended to print out this section and keep it available at the SPA-LEED control PC for everyday use.
For a new SPA-LEED system (or after the first installation of WinSPA)
- Calculate the k space sensitivity for internal and external geometry.
The k space sensitivity is used for the conversion from deflection voltages to all other k-space units. Its correctness is therefore mandatory for a correct k-space scaling.
- Use a well known sample type with a well-ordered reconstruction (such as Si(111)(7x7)).
- Open a 2D scan form. Choose Volts as scan units.
- Do a large overview 2D scan over about two Brillouin zones.
- Select a known distance with the line tool, for example a full BZ or the distance between two superstructure spots.
- Copy the line parameters to clipboard.
- Open the k sensitivity calibration form.
- Paste the line parameters. Enter also the corresponding length in %BZ.
- Click on Calculate, and then on Apply. The sensitivity value is then copied to the software settings form.
- Apply new k sensitvity value in the "software settings > LEED system" tab.
Note: It is helpful - as a zeroth order sensitivity analysis - to repeat the above described procedure 3-4 times with different selected lines to estimate the accuracy.
Note: In case you use an "external" electron gun in a RHEED-like geomtry you will notice a very pronounced nonlinearity of the reciprocal space metrics. Using other units than Volts in the external geometry does therefore usually not make sense.
Either derive these values from drawings of the manufacturer or measure them physically in your system: Distance d1 between gun and sample, distance d2 between gun and channeltron aperture, the needed angle is then simply arctan (d1/d2). Some default values can be found in the hardware section.
Refer to the manufacturer's manuals, enter the documented gain values in the "software settings > LEED system" tab and apply them. Finally check, whether the voltages displayed in the voltage meter form are the same as either displayed on your HV amplifier or as measured (test probe).
- Use a sample with well-defined known (or previously measured) geometrical dimensions (use a caliper rule).
- Do an SEM scan of your sample with Volts as scan units. Focus the image with the gun lens(es).
- Draw a line which corresponds to a known dimension of your sample with the line tool.
- Copy the line parameters to the clipboard.
- Open the R sensitivity calibration form and paste the line parameters.
- Enter the dimension you marked with the line.
- Click on Calculate, and then on Apply.
- Apply the new R sensitvity value in the "software settings > LEED system" tab.
After changing between internal and external geometry and vice versa
- Change the geometry settings in the "Software Settings > LEED" tab.
After changing the type of sample
- Choose the correct sample type in the "Software Settings > Sample" tab. If the sample is not yet in the list, enter its parameters manually. It will automatically added to the samples list.
The sample constants are used for the calculation of the k space units %BZ and S and their correctness is therefore mandatory for a correct k-space scaling.
After changing the position of the sample (sample transfer, maybe even heating)
- Calculate Image Shift Correction
It is assumed that you have already centered the (00) spot at very high energies (450eV) solely mechanically by using sample rotation and sample tilt (if possible in your system). The following steps make only sense, if you have already roughly aligned your system by doing so. If you can not reproducibly align the sample rotational angles, you should do the (00) centering at high energies as the 0th step before the image shift correction described in the following.
Note: For most UHV sample manipulation systems with well-defined caliper scales you can probably simply use the "once found" optimum mechanical alignment settings, and just leave the rest to the electrical Image Shift Correction.
- Open the Image Shift Calculation form.
- Open two 2D scan forms. Choose Volts as scan units.
- Repeat the following steps (approx. 4 to 12 times) with different energies, for example 40eV, 80eV, 140eV, 180eV, 280eV, 400eV
- Do a large 2D scan (~50%..100%BZ). Identify the (00) spot. Avoid out-of-phase conditions.
- Select a region (rectangle tool) around the (00) spot.
- Copy rectangle parameters to clipboard and paste into the second 2D scan.
- Set resolution of second scan to about 150x150 pixels.
- Measure the center of intensity of the (00) spot with the ellipse tool.
- Copy the coordinates of the center of intensity to the clipboard and paste them into a new line of the table in the Image Shift Calculation form.
Note: At low energies you could be fooled by charging effects in the SPA-LEED or at your sample holder.
After each change of scan energy
- Verify that the beam park position is located in a low intensity region
Whether in industrial or academic user environments, it has become normality for multiple users to access one scientific instrument independently. Due to different personal preferences as well as knowledge and experience, however, the settings for the daily work may significantly differ between individual users.
Therefore, starting from release V2.1 on, WinSPA offers a user management in order to enable the clash-free sharing of a SPA-LEED instrument between multiple users.
This user management has two different aspects: All users have their own individual config.txt file thus allowing to keep all of their software settings separated; starting from mere GUI selections such as color tables, but also data path preferences and - last but certainly not least - vital settings of the instrument such as sensitivities, gains, angles, counter settings, timing etc. The second aspect of the user management involves dedicated access rights to different setup levels based on user experience. The purpose is to allow users to operate the SPA-LEED instrument safely according to their experience level. For example, a beginner will and should not tinker with settings of the counter timing, amplifier gain etc., because incorrect settings can lead to unusable scan data, slow operation and other undesirable problems.
Simply select the relevant user at the program start. You may change the name and rights of the user in the software settings form.
The graphic tools mainly support you in the selection of parts of the reciprocal space for further investigation. For example, you can draw a line within a 2D scan to define a 1D scan, you can select an elliptical region in 2D scans to get its center of mass for a spot tracking measurement, or you can define a rectangular region in a 2D scan for a detailed (sub-)scan.
Secondly, some graphic tools provide rough data analysis functionality. They should not replace a thorough offline data analysis, but in many cases you may want wo get a quick answer to questions like
- What are the two angles within a reconstruction's unit cell?
- Is the width of the (00) spot small enough to judge that the surface is clean and well-ordered?
- What is the angle of facet rods and what are the corresponding facet orientations?
The line tool for 2D scans
The line tool allows you to draw a line between two crosshairs selected with the mouse. A section through the 2D data along the drawn line is displayed, as well as the parameters of the drawn line. For easier and more exact placement of the two limiting crosshairs the relative intensities at the two end points of the line are also displayed - this way you can place the two crosshairs exactly on diffraction spots.
The line parameters can be copied to the clipboard in order to use them for a more detailed 1D scan.
The rectangle tool for 2D scans
The rectangle tool allows you to select a rectangular region of interest within a 2D scan by selecting two marker positions, which define two diagonal points of the rectangle. You will get some basic information about the selected region.
The parameters of the rectangle can then be copied to the clipboard in order to use them for a detail 2D scan.
The ellipse tool for 2D scans
The ellipse tool allows you to select an elliptical region of intereset within a 2D scan by selecting two marker positions, which define two diagonal points of the outer rectangle of the ellipse. The center of mass of all data points within the ellipse is calculated. This gives you a more representative value for the center of a diffraction spot than the crosshair tool.
The center of mass within the ellipse can be copied to the clipboard in order to be used in 0D scans (spot tracker) as well as in the image shift calculation form.
The angle tool for 2D scans
The angle tool allows you to select three points within a 2D scan via positioning of three crosshair markers. These three markers define an angle. For an exact placement of the markers on diffraction spots the relative intensity of each marker in comparison to the maximum intensity within the 2D scan is shown. You can use this tool for estimating the symmetry of a diffraction image.
No copy to clipboard option is given for this tool.
The crosshair tool for 2D scans
The crosshair tool allows you select a point within a 2D scan with a marker. The coordinates of this point are displayed as well as the relative intensity in comparison to the maximum intensity of the scan. For easier and more precise selection of the correct point the local gradient (arrow) and curvature (circle) are also displayed.
The k-space calculator provides four types of data and/or units conversion for convenient planning of measurements:
- Conversion from 2D rectangle definition by center and rotation to edge coordinates.
- Conversion between edge and center definition of 1D scans.
- Conversion between k|| units (1/A, %BZ and Volts).
- Conversion between electron energy and scattering phase(S).
For example, if you want to define a 1D scan exactly through the centers of two diffraction spots, just measure the two spots in an 2D overview (ellipse tool) and put the two points' coordinates into the k calculator and calculate the 1D scan definition. Enter the values in a 1D scan form and then just increase the scan length appropriately in order to have both spots fully scanned.
The first type of conversions is between 2D scan center and border coordinates.
|GUI element||Meaning and Usage|
|2D Center Coordinates (x0,y0,dx,dy,angle)||Center coordinates (x,y), sizes (dx,dy) and angle of rotation of a rectangular 2D scan.|
|2D Border Coordinates (x1..x4,y1..y4)||Coordinates of the four border points of the 2D scan.|
The second type of k space coordinate conversions is between 1D center and border coordinates.
|GUI element||Meaning and Usage|
|1D Center Coordinates (x0,y0,length,angle)||Center coordinates (x0,y0), length and angle of rotation of a linear 1D scan.|
|1D Border Coordinates (x1,y1,x2,y2)||Coordinates of the two border points of the 1D scan.|
The third type of k space coordinate conversions allows to convert values between different kpar coordinates.
|GUI element s||Meaning and Usage|
|k space value 1||Value of one of the two k space coordinates for conversion|
|k space unit 1||Units of one of the two k space coordinates for conversion|
|k space value 2||Value of the other of the two k space coordinates for conversion|
|k space unit 2||Units of the other of the two k space coordinates for conversion|
|Surface Row Distance||Required for conversion|
|k Sensitivity||Required for conversion|
|Electron Energy||Required for conversion|
The fourth type of k space coordinate conversions allows to convert between energy and scattering phase.
|GUI element||Meaning and Usage|
|Energy||Can be either input to or result of the conversion.|
|Scattering phase||Can be either input to or result of the conversion.|
|Step Height||Required for conversion|
|Incidence Angle||Required for conversion|
The Stop watch
The stop watch is a small but very helpful tool for efficient measurements. It basically defines a local time in in-situ experiments for the start of the measurement, the beginning of the deposition etc..
|GUI Element||Meaning and Usage|
|Time display||Display of the stopwatch timer's local time.|
|Start||Start the stopwatch timer.|
|Stop||Stop the stopwatch timer.|
|Pause||Pause the stopwatch timer. This may be used if you interrupted a deposition in between, but you should normally not need this function.|
|Reset||Reset the stopwatch timer.|
|log this:||Click here to write a comment text line into the log file, along with a precise time information.
The topmost input line is automatically cleared upon click and is thus meant for unique events during your experiment ("Start of deposition"), the lower ones are not and can be used for repetetively used comments, such as "Focus changed", "Emission reduced", "sample flashed" and so forth.
|Log Text||Text line to be written into log file.|
Note: Not being meant as a replacement for a well-documented lab book, of course,
the "log this:" buttons and edits allow you to store short text lines along
with a precise time stamp into the text log file of WinSPA.
This text information can be valuable for the documentation and analysis of your experiments.
The PWM form
The more modern versions of the SPA-LEED DAQ hardware (Meilhaus ME-1400B) offer a simple approach to pulsed heating and gated detection. The idea is to detect electrons only when the sample is not heated (field-free), for further details see the pulse heating entry in the refrences section.
Essentially, the heating of the sample thus needs to be done by heating pulses followed by voltage- and current-free pauses. The time scale of those heating pulses is about 1ms..20ms, depending on temperature, emissivity and heat capacity of your sample.
Depending on these parameters and the speed of your power supply used for sample heating, changing the heat and idle times (or, their ratio, the duty cycle) is usually required in order to find a compromise between stable operation (long heating) and high fraction of available time for measurement (short heating).
A digital (on/off) signal with variable duty cycle is usually referred to as pulse width modulation (PWM), hence the name of the respective form in WinSPA.
|GUI Element||Meaning and Usage|
|Use PWM signal for counter gating||Check this, if you want to use chopped/pulsed sample heating|
Note that this slows down all scans by approximately (1-duty cycle).
|PWM duty cycle slider||Select the duty cycle in percent.|
|Base clock frequency||In Intel 8254 PWM mode, base clock frequency and pre-divider determine the resulting PWM frequency. The recommended range is between 1ms and 50ms.|
|PWM frequency pre-divider||see above.|
|PWM signal display||This display shows the resulting PWM signal according to your settings.|
The ramp generator form
Depending on your DAQ hardware, you have a least one D/A channel available for spare use: In the case of the BMCM PCI-Base II for example, a 4 channel D/A board is used for energy and x/y control, leaving the fourth channel available.
Many users want to remotely control other devices such as evaporators or heating devices (e.g., sample heating)
with this analog control voltages of -10..10V.
The ramp generator form is a simple tool to create voltage ramps of different kind on the fourth analog output channel of your SPA-LEED DAQ system.
You can select the boundary values, step sizes/numbers, symmetry etc. of the voltage ramps to
be output. A schematic representation of the waveform according to your settings is shown on top
of the form.
Please not that this ramp generator is not meant for fast waveform generation, but rather for ramps with output voltage updates each 100ms or slower.
|GUI Element||Meaning and Usage|
|Ramp schematic display||Shows the resulting shape of the waveform, not to scale, though.|
|Ramp progress bar||Shows the progress of the output relative to the display above.|
|Ramp start value||Start value for the signal ramp.|
|to/over||Select whether stop value is absolute or relative to start value.|
|Ramp stop value||Stop/Range value for the signal ramp.|
|step size||Step size after which the output signal is updated. Too fine step sizes create high CPU load, too coarse ones lead to steps in the signal.|
|duration||Overall duration of the ramp.|
|stay there/jump back/ramp back||Select, how the ramp should be finished.|
You can use this feature for well-defined and repeatable temperature cycles for your sample, even involving very slow annealing curves.
The log file
WinSPA saves all scan events (start/stop of scans), stopwatch actions and text comments ("log this:") into the log file. This file can therefore be a valuable help for your data analysis, since even complex measurements can be sorted and analyzed afterwards.
The start times of the scans are also written into each scan file, as well as the local stopwatch time. This information is therefore redundantly saved. Your comments and all start/stop information of scans you do not save to disc are only written into the logfile.
The about window
The "about" window gives you the information about the precise release version of your copy of WinSPA.
All "real" releases are numbered with an X.YY scheme (X=main release number, YY=sub-release number), beta versions are numbered with an X.YY.ZZ scheme where ZZ counts all compilation steps (=sub-sub-releases).
The sub-sub-releases are preceding the sub-releases. For example, a version 2.00.12 is a beta of V2.00.