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Auger Electron Spectroscope (AES)

 

In this chapter we will try to list all of the information which is important for the simulation of electron-target interactions and the generation of events caused by these interactions in an Auger Electron Spectroscope (AES).

The section will start off with a description of an example showing the basic features of SESAME. It is followed by a summary of features and options concerning Auger electron simulation.

Example:

By means of this simple example it will be demonstrated, step by step, how to perform a simulation of an Auger scan starting from the generation of a geometry and an input file to the visualization of the results and their interpretation.

The actual geometry and input files as read by SESAME during program execution are described here. These files might be also generated using the graphical user interface (GUI). For more information on the GUI see Chapter 5.

We will simulate Auger line scans of the following sample: An Al line of 1.5 microns width and 1 micron height containing 1% Cu on a substrate. On the surface of the line a precipitate is embedded of the size 1000x1000 Å. It is assumed to consist of Cu. The appropriate geometry of the example is shown in Figure 4.18.

  
Figure 4.18: Geometry of the example AES EXA1

First of all a geometry file which describes the sample has to be generated.

To generate this geometry file (without using the GUI) you have to:

  1. Determine how many different regions your sample consists of. In this case there are three different regions. You have to write this number in the first line of your geometry file.
  2. Give each vertex (point) of your geometry a consecutive number starting with one.
  3. Starting from region 1 you have to define each region by going from vertex to vertex around the regions in clockwise direction. For each region you have to write the number of vertices of this region on a separate line followed by all the numbers of the vertices, again each on a separate line (see copy of input file below).
  4. Enter the number of different vertices of the whole structure in a separate line.
  5. Write coordinate pairs (x,z) for each vertex in a separate line of the input file. Note: The unit of the coordinates is always Å!
  6. Save file (for instance: AES_EXA1.GEO).

For a more accurate description of the geometry specification see Chapter 6.6.

The geometry file should look like:

3                               number of regions
6                               number of vertices for first region
 1                              vertex number
 2                                   ''
 3                                   ''
 4                                   ''	
 5                                   ''
 6                                   ''
8                               number of vertices for second region
 7                              vertex number
 8                                   ''
 12                                  ''
 11                                  ''
 9                                   ''
 10                                  ''
 3                                   ''
 2                                   ''
4                               number of vertices for third region
 8                              vertex number
 9                                   ''
 11                                  ''
 12                                  ''
12                              overall number of vertices
     0 10000                    coordinates (x,z) of first vertex
 10000 10000                    coordinates (x,z) of second vertex
 20000 10000                    etc.
 30000 10000
 30000 30000
     0 30000
 10000     0
 17000     0
 18000     0
 20000     0
 18000  1000
 17000  1000
The next step is to write an input file. Following, you will find an example of an input file for this sample. You may also copy this file to your account from:

 
VMS 		  DISK$AXP_USER1:[SLED.NEW.SESAME.EXA]AES_EXA1.INP

Unix tcad/sesame/exam/aes_exa1.inp

 &INIT
   CUSTXT = 'THIS IS A AUGER TEST EXAMPLE'
   JOB    = 'AES_EXA1'
   TECHQU = 'AES'
   SIGNAL = 'AE'
   UNITS  = 'ANGSTROM'

 &END

 &ETRNS
   E0     = 6.000000
   EF     = 700.000000
   NETRN  = 6400
 &END

 &EBEAM
   SCAN   = T
 &END

 &TARGET
   CGFILE = 'AES_EXA2.GEO'
   NREG   = 3
   ATMAT  = 'Si,O2','Al','Cu'
   DENS   =  2.200, 2.700, 8.960
 &END

 &MODEL
   FMAT   = 'SIO2.DIEL','AL.DIEL','CU.DIEL'
 &END

 &MSETUP
   XRANGE = 13000.000000
   XESPOT = 18500.000000
 &END

 &INSTR
   NLSCP = 64
 &END

 &DETECT
   EDTHMN=36
   EDTHMX=48
 &END

 &OUTPUT
   NSCRUP = 500
   LGRAY=.T.
   SCREEN(1)='PEB,1,,'
   SCREEN(2)='AEY;;Al,KLL;1'
   SCREEN(3)='AEY;;Cu,LMM;1'
 &END

A detailed reference of all namelists (input records) and their variables is given in Chapter 9. Here only short explanations of the variables used in this specific input deck will be given.

The input file should start with the &INIT namelist (see Chapter 9.1) although no obligatory variables exist for this namelist, i.e. all variables of this input record have a default value.

CUSTXT is a user-defined text which should contain a short description of the example and which will appear in all generated output files. With the variable JOB a job name may be specified ( Note: No blanks are allowed!). The job name will appear in the names of generated output files if no file names for such files are explicitly specified in the input deck. The default value for the job name is the name of the input file without the extension, this would be AES_EXA1 in this case.

TECHQU defines the measurement technique used. This variable guarantees the presetting of default values for many different variables appropriate for the used technique. SEM is the default setting for TECHQU, we will set TECHQU to AES. The default settings for TECHQU=AES can be seen in table 4.4.

  
Table 4.4: Default values for NAMELIST variables for TECHQU=AES

UNITS specifies the length units used in the current input file. This length unit applies to all of the variables in the input file which have the dimension of a length but not to the geometry file ! The default value is MICRON, but we will set it to ANGSTROM

The next input record, &ETRNS (see Chapter 9.2), must be contained in each input file since the variables E0 for the primary electron energy, and NETRN, the total number of electrons, are obligatory. For EF, the final electron energy, the default value is 100 eV ( Note: unit for E0 is keV, for EF it is eV!), we will take 500eV as we are not interested in electrons with lower energy and we will save CPU time with a higher cut-off energy.

The namelist &EBEAM (see Chapter 9.3) describes the features of the electron beam. SCAN=T means that the beam should be scanned. Here it is also possible to specify a beam divergence, a spot size, etc.. More detailed information is given in Chapter 6.1 and 9.3, here we will use default values.

The specimen is described by the &TARGET namelist (see Chapter 6.5, 6.6 and 9.4). ATMAT is a string array containing the chemical abbreviations of all elements of the target and their relative fractions according to the different regions. The chemical abbreviations in one region as well as the strings describing different regions must be separated by commas ( Note: strings must be enclosed by single quotes!). Relative fractions of elements in one region must, if necessary at all, be specified right after the corresponding chemical abbreviation without a blank, comma or any other character inbetween. In this particular example, for instance, the first region is composed of one part Silicon and two parts Oxygen, which is equivalent to Silicondioxide, the second region consists of 99 parts Aluminum and one part Copper, and the third region is pure Copper. The relative fractions in each region will be automatically normalized to unity. This means 'Si,O2' and Si0.33,O0.77 will give the same result. For more information on specifying the target composition see Chapter 6.5.

DENS specifies the densities of the regions in the same sequence in which the regions are defined in the geometry file, i.e. the first value in DENS corresponds to the first region, etc.. Densities are to be specified in units of or equivalently

CGFILE specifies the file name for the simulation geometry file ( Note: do not forget the single quotes!).

In MODEL (see Chapter 9.5) the physical simulation model may be specified using the MODR variable in dependence of the region (index is the region number). The default setting is SIN for single scattering, Auger simulation is only possible in the direct simulation mode ( DIR). If a model is specified only for the first region then the same model will continue to be applied to all of the other regions. The direct simulation model requires an appropriate dielectric data file describing the dielectric properties in each region. These are specified for each region in the variable FMAT.

As the dielectric function is a solid state property, it cannot be calculated from atomic data. Thus a dielectric data file is needed for each compound used. In the second region we use the dielectric data for pure Al assuming the influence of 1 percent Cu to be negligible.

In &DETECT the electron detector can be specified. In this example it is assumed that an cylindrical mirror analyzer (CMA) with a polar acceptance angle of is used. The polar acceptance angle is specified in EDTHSZ (see Chapters 6.4 and 9.8).

The variable XESPOT = 17000 and XRANGE = 8000 in the record &MSETUP sets the scan range to 17000Å 4000Å. The setting of NLSCP in &INSTR sets the number of scan points for the scan (see Chapters6.2).

The variables of the &OUTPUT namelist (see Chapter 9.9) control the output generated by SESAME. For a more detailed description it shall be also referred to Chapter 6.7.

In this example one status window and four output windows will appear on the screen. The status window will contain information like electron energy, used CPU time, CPU time per electron, remaining CPU time, ionization sympols legend, and some more. The first output window shows a detail of the geometry and the primary electron trajectories (PET). The section of the geometry to be shown is defined by the values following the second comma after the keyword PET. The values are in the order . As we previously defined in the variable UNITS, these values are all to be given in Å. In the remaining three screens can be seen the results of the Auger scan (AEY) respectively for the lines Si-KLL, Al-KLL and Cu-LMM. NSCRUP = 100 tells SESAME to update the status window and histogram screens after each 100 primary electrons.

It is also possible to obtain histogram results (in our case the Auger scan data) in form of ASCII files. To do this we add following lines to the &OUTPUT record :

   HIST(1)='AEY,,12000.000000,25000.000000;AL.KLL;Al,KLL;0'
   HIST(2)='AEY,,12000.000000,25000.000000;CU.LMM;Cu,LMM;0'

This will create two files namely

 AL.KLL and CU.LMM
containing the Auger yields for each scan. The contents of these two histogram files are visualized by means of GNUPLOT in Figure 4.19.

Furthermore by setting the

 &OUTPUT
variable LGRAY to .T. you obtain more realistic looking like scan images of the Auger scans (see figure 4.20).

A more complex example is decribed in Chapter gif.

 

 



next up previous contents
Next: The Graphical User Up: Analysis Methods Previous: Transmission Electron Microscope



Horst Wagner
Tue Mar 19 10:24:55 MET 1996