In this chapter we will try to list all the information which is important for the simulation of electron-target interactions and the generation of events caused by these interactions in a Scanning Electron Microscope (SEM).
We will start this section with a description of a simple example. A more
general explanation of the features and options will be given at the end of
this chapter (see General Description below). For a quick information on
default values see Table 4.1.
Example:
By means of a simple example it will be demonstrated, step by step, how to perform an electron scattering simulation 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.
The geometry of this test example is shown in Figure 4.1. The geometry file describing this structure may be copied from
VMS SUBPAC::DEVPROSIM$DISK:[SLED.SESAME.EXAM]SEM_EXA1.GEOUnix
tcad/sesame/exam/sem_exa1.geo
To generate this geometry file (without using the GUI) you have to:
2 number of regions 6 1 number of vertices (1st region), segment 1 vertex number 2 '' 3 '' 4 '' 5 '' 6 '' 5 2 number of vertices (2nd region), segment 7 vertex number 8 '' 4 '' 3 '' 2 '' 8 overall number of vertices 0.0,8000.0 coordinates (x,z) of first vertex 40000.0,8000.0 coordinates (x,z) of second vertex 40000.0,15000.0 80000.0,15000.0 80000.0,60000.0 0.0,60000.0 40000.0,0.0 80000.0,0.0
The next step is to write an input file. Here is an example of an input file for this sample. This file might be generated either by means of the GUI or by using any text editor. You may also copy this file to your account from:
VMS: DISK$AXP_USER1:[SLED.NEW.SESAME.EXA]SEM_EXA1.INPUnix:
tcad/sesame/exam/sem_exa1.inp
&INIT CUSTXT='THIS IS A FIRST TEST EXAMPLE' JOB='TEXT_EXA1',TECHQU='SEM' SIGNAL='BE',UNITS='MICRON',&END &ETRNS E0=20.,EF=500.,NETRN=1000,&END &EBEAM ESTD=5,SCAN=T,SPSIZE=0.0035,DIVERG=1,&END &TARGET ATMAT='SI,O2','W' DENS=2.2,19.3 NREG=2,CGFILE='SEM_EXA1.GEO',&END &MODEL MODR(1)='HYB',&END &MSETUP TAKOFF=37.5,37.5,XDEROT=0,180 EBTILT=15,EBROT=180 MAGNI=1000,XESPOT=4,XRANGE=5,&END &OUTPUT SCREEN(1)='PET' SCREEN(2)='PEB,1' SCREEN(3)='ION;;W,M' SCREEN(4)='ION,3;;SI,K' SCREEN(5)='ION,3;;O,K' &END
For a detailed description of all namelists (input records) and their variables see 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 SEM_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 specified technique ( SEM is the default value). For the default settings for TECHQU=SEM see Table 4.1.
UNITS specifies the length units used in the current input file. This variable is relevant for all length dependent quantities (i.e. SPSIZE in the &EBEAM namelist (see Chapter 9.3), XESPOT and XRANGE in the &MSETUP namelist (see Chapter 9.6), and SCREEN, POST, PMSAV, and HIST in the &OUTPUT namelist (see Chapter 9.9) - the meaning of all these variables will be given in one of the next paragraphs). The default value is MICRON.
The next input record, &ETRNS (see Chapter 9.2), must be contained in each input file since the variables E0, the initial 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!).
The namelist &EBEAM (see Chapter 9.3) describes the features of the electron beam. SCAN=T means that the beam should be scanned, ESTD determines the standard deviation of electron energy in eV (Gaussian distributed), SPSIZE describes the spot size of the electron beam at the plane of beam focus (see Chapter 6.1 and 9.3), and DIVERG specifies the beam divergence in degrees.
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 atomic 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 out of one part Silicon and two parts Oxygen, which is equivalent to Silicondioxid, and the second region is pure tungsten. The relative fractions in each region will be automatically normalized to unity. This means 'Si,O2' and Si0.33,O0.67 will give (almost) the same result. For more information on specifying the target composition see Chapter 6.5.
DENS specifies the densities (in ) of the regions in the same
sequence in which the regions are defined in the geometry file, i.e. the first
real value in DENS corresponds to the first region, etc.. NREG
determines the number of regions and CGFILE specifies the file name for
the simulation geometry file ( Note: do not forget the single quotes!).
All these variables are obligatory and must be specified in the input file.
In &MODEL (see Chapter 9.5) the physical model may be specified using the MODR variable in dependence of the region (index is the region number). The default is the single scattering model SIN (see also Chapter A.3). If a model is specified only for the first region then the same model will be applied to all other regions too. The only restrictions one may face are according to the direct simulation model, which is not available for every element and compound (see Chapter A.5).
In the &MSETUP namelist (see Chapter 9.6) the measurement setup is defined. TAKOFF is the x-ray detector take-off angle, XDEROT the x-ray detector rotation angle, EBTILT the beam tilt angle, and EBROT the beam rotation angle (see Figure 6.2 and 6.3). All angles are measured in degrees. Actually, all detector angle variables are arrays which allow to specify more than one detector for one simulation. Up to five x-ray and electron detectors may be specified simultaneously. For more information on the specification of x-ray and electron detectors see Chapter 6.3 and 6.4, respectively.
MAGNI is the magnification and determines, together with the lateral scanning range XRANGE and the pixel distance on the used CRT, XPCRT (see Chapter 9.7), the number of scan positions. XESPOT is the lateral electron beam impact position (see Figure 6.4). Both, XESPOT and XRANGE, are in units specified by the UNITS variable in the &INIT namelist.
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.
If you never started SESAME before, then follow the instructions in Chapter 3 or 5, respectively. Otherwise simply start SESAME by executing the command "sesame" on your command line or invoke it from the VISTA TCAD framework (see Chapter 5).
For this particular input deck, one status window and two output windows will
appear on your screen. The status window will contain information like electron
energy, used CPU time, CPU time per electron, remaining CPU time, ionization
symbols legend, and some more. The first output window will contain the
electron beam ( PEB) and the trajectories of the primary electrons
( PET). In the second window the ionizations of the tungsten (W)
M-shells (from to
), of the silicon (Si) K-shell, and
of the oxygen (O) K-shell will be visualized. The "1" after
PEB in SCREEN(2) means that the information specified by this
variable should be directed to SCREEN(1). Similarily the output specified
in SCREEN(4) and in SCREEN(5) is directed to SCREEN(3).
All windows will show the whole simulation geometry and the sizes of the windows are the default sizes. For information on how to change the clipping visualized in the window and how to modify the window-size refer to Chapter 6.7 and 9.9. There you will also find more details about directing output to files using various formats.
So far we have seen only electron trajectories and the location of
ionizations. We shall now try, for instance, to visualize the
x-rays in a more quantitative way. For that reason, we put
the following two lines to the input deck ( &OUTPUT namlist) [all
extensions to the input file described in this chapter are contained in the
file SEM_EXA2.INP which also may be copied to your account]:
SCREEN(6)='XGD;;O,KA' SCREEN(7)='XDD;;O,KA;1'XGD stands for "generated x-ray distribution" and XDD for "detected x-ray distribution" (see Chapter 6.7 and 9.9). The integer after the third semicolon determines the x-ray detector to apply. Presently, up to 5 different detectors may be specified in the input deck. If no detector number is explicitly specified then detector number zero - the default detector - is to be used (see Chapter 6.3).
Furthermore, a grid is needed for this feature. If no grid is explicitly specified in the input deck a grid will be automatically generated by SESAME during runtime (see Chapter 6.8).
Running the extended input deck will bring two more windows to your screen. In
one window you will see the generated and in the other the detected x-ray
distribution of the O line. Intensities go from zero
(= white) to the maximum value (= dark red).
To demonstrate how the x-ray intensity changes with detector position we modify the input deck by adding the following line:
SCREEN(8)='XDD;;O,KA;2'For the second detector XDEROT is 180 degrees, this means, that this detector is rotated from the right hemisphere to the left - the take-off angle is the same. As can be seen on the output window, the location where the main part of the x-rays comes from is shifted.
Next we would like to simulate an x-ray scan of the same signal. To do that we put the following line into the input deck:
SCREEN(9)='XCY;;O,KA;1'An additional window opens up on the screen. This window shows a histogram of the x-ray scan over your target. Now you could change the position of the detector again (by specifying detector number 2 after the third semicolon) and investigate how the x-ray scan changes with detector position.
Another feature we would like to demonstrate here is a backscattered electron scan. To do this you have to put the following line into the input deck:
SCREEN(10)='BEY'In this case, all backscattered electrons will be collected and visualized in a histogram of the scan over your target, independent of their energies and directions, since no detector number is explicitly specified. To take into account an electron detector, a detector number must be spcified after a third semicolon in the string of SCREEN(10). For information on the specification of an electron detector, see Chapter 6.4 and 9.8.
Furthermore, we will briefly show how one can write generated output to files. Put the following lines to the &OUTPUT namelist of the input file:
PMSAV(1)='XGD;EXA1_XGD.SAV;O,KA' PMSAV(2)='XDD;EXA1_XDD1.SAV;O,KA;1' PMSAV(3)='XDD;EXA1_XDD2.SAV;O,KA;2' HIST(1)='BEY;EXA1_BEY.H1D' HIST(2)='XCY;EXA1_XCY1.H1D;W,MA;1' HIST(3)='XCY;EXA1_XCY2.H1D;W,MA;2' HIST(4)='IO2,60,4,8,45,0.0,1.5;EXA1_IO2.H2D;W,MA'PMSAV() produces a POSTMINI SAV file (PROMIS format), one named " EXA1_XGD.SAV", the second " EXA1_XDD1.SAV" and the third " EXA1_XDD2.SAV" containing the generated and detected (for detector 1 and 2) characteristic oxygen
In Figure 4.2 to 4.4 it can be seen that x-rays
from greater depths are almost completely absorbed in the sample. Furthermore,
it is clearly demonstrated that many more x-rays will be measured if the
detector is on the left side ( XDEROT=180) compared to the situation where
the detector is positioned on the right side ( XDEROT=0). This is because
of the strong absorption of the O x-rays in the tungsten
layer.
By means of the HIST() variable ASCII files are generated. Characters between the first and the second semicolon represent the file name. Numbers in front of the first semicolon in IO2 define the histogram intervals, in this particular case, the 2-dimensional distribution of ionisations ( IO2) shall be stored on a 2-dimensional histogram with 60 intervals in x-direction, expanding from 4 to 8 microns, and 45 intervals in z-direction, going from 0 to 1.5 microns. An integer after the third semicolon determines the detector to use. If no detector number is explicitly specified then the default detector (detector number 0) will be applied (see Chapter 6.3).
Such ASCII files can be visualized by using different tools, for instance
POSTMINI, GNUplot or MUFASplot (see Chapter
10.1, 10.3 and 10.4). For further postprocessing of
data VIDE may be applied
(see Chapter 10.2). Additional information on the use of HIST is
provided in Chapter 6.7 and 9.9. Figure 4.5
shows a plot of the W x-ray scan for two different detector
positions. The data has been smoothed prior to plotting. POSTMINI was
used to generate the encapsulated PostScript files plotted in this manual.
The last feature to be mentioned here is the possibility of splitting your input deck into up to 4 files. You have to provide a so called ''master input file" which is the actual input file read by SESAME. In this input file you may specify three further input files by means of the FINSTR, the FDETCT, and the FMYDEF variables of the &INIT namelist (see Chapter 9.1), which SESAME includes in the master input file.
FINSTR specifies the file name for a file containing instrument-specific parameters, FDETCT one for detector-specific parameters, and FMYDEF one for user-specific parameters. But principally, all of these three files may contain all of the namelist records, except the &INIT namelist ( Note: the &INIT namelist can only be specified in the master input file!). Important is to know that the specifications in the master input deck have the highest priority, followed by the FMYDEF file, the FDETCT file, and the FINSTR file. For instance, a take-off angle specified in the FDETCT file will be overridden if it is also specified in the FMYDEF or in the master input file.
For demonstration purposes we will split up our original input deck into different files. Then the master input deck may have the following form:
&INIT CUSTXT='THIS IS A FIRST TEST EXAMPLE' JOB='TEXT_EXA1',TECHQU='SEM' SIGNAL='BE',UNITS='MICRON' FMYDEF='SAMPLE_1.INP' FDETCT='DETECT_1.DAT' FINSTR='INSTR_1.DAT',&END &ETRNS E0=20.,EF=500.,NETRN=1000,&END &EBEAM SCAN=T,&END &MODEL MODR(1)='HYB',&END &OUTPUT SCREEN(1)='PET' SCREEN(2)='PEB,1' SCREEN(3)='ION;;W,M' SCREEN(4)='ION,3;;SI,K' SCREEN(5)='ION,3;;O,K' &ENDThe FMYDEF file may look like:
&TARGET ATMAT='SI,O2','W' DENS=2.2,19.3 NREG=2,CGFILE='EXA1.GEO',&END &MSETUP XESPOT=4,XRANGE=5,&ENDIn FDETCT some detector specific parameters should be specified
&MSETUP TAKOFF=37.5,37.5,XDEROT=0,180,&END &DETECT XFDEFF='DETECT_1.DAT','DETECT_1.DAT',&ENDand in FINSTR some parameters describing a typical measurement set-up for a specific instrument shall be defined:
&EBEAM ESTD=5,SPSIZE=0.0035,DIVERG=1,&END &MSETUP EBTILT=15,EBROT=180 MAGNI=1000,&END &INSTR XPCRT=0.25,&ENDIn that way it is very easy to combine the specifications for different instruments, detectors and other specific parameters. Thus, if a measurement is repeated with a different instrument then it might be sufficient to solely change the file names specified by the FDETCT and FINSTR variable in the master input file. If instrument or detector specific parameters are once provided for each individual instrument (or instrument setting), then simulations for different instruments/detectors can be performed while doing only a few modifications in the master input file. Furthermore, this feature also allows the users to specify their own set of default parameters. All in all, this option helps to keep the actual input file (master input file) short and clearly arranged.
Here, at the end of the example description it must be pointed out that only a very small proportion of available features has been presented so far. For more detailed information refer to Chapter 6, where SESAME features are described more comprehensively, and to Chapter 9, where all namelists and their variables are listed.
An example using a more sophisticated simulation geometry is described in
Chapter .
General Description:
In the input file (see Chapter 9) the measurement technique can be specified by means of the TECHQU variable (see Chapter 9.1). If this variable is set to SEM, which is the default value anyway, then many input variables are set to default values which are appropriate for this analysis technique. The most important parameters and their default settings are listed in Table 4.1. Thus, only those parameters with differing values must be explicitly specified in the input file.
Table 4.1: Default values for NAMELIST variables for TECHQU=SEM
As can be seen from the Table 4.1, the electron beam is,
by default, scanned over the whole lateral expansion of the specified
geometry. The default detector (detector number zero) for electrons is an ideal
2 detector. This means all backscattered electrons are fully collected
independent of their directions and their energies. In case of x-rays, the
detector is located by default at a take-off
angle of 30 degrees and a rotation angle of zero degrees (see Chapter
6.3 and 9.8). All x-rays leaving the sample in that direction
are counted, i.e. the default detector efficiency is one, independent of the
x-ray energy. More realistic detectors may be specified using the
&DETECT namelist (see Chapter 6.3 and 6.4).
The default signal in SEM mode is BE (backscattered electrons), the default model is the hybrid model (see Chapter A.4). This model is used because it is faster by approximately a factor of two compared to the single scattering model. But in cases where ionizations and characteristic x-rays are calculated, the single scattering model yields much better statistics for the same number of simulated electrons, i.e. with respect of reaching the same accuracy the single scattering model is faster.
The single scattering model uses a strict continuous slowing-down approximation. Therefore, ionizations are not explicitly calculated but the ionization probabilities along the electron trajectory are evaluated. This is why no ionizations can be exactly localized in the sample and ionization events are therefore randomly distributed along the electron trajectory.
But generally, other signals as well, like characteristic x-rays or secondary electrons, may be used and specified. But using the single scattering or the hybrid model it is not possible to create secondary electrons. The simulation of secondary electrons is invoked if either SIGNAL is set to SE or if LSECON in &MODEL (see Chapter 9.5) is set to T or if any output, specified in the input file in the &OUTPUT namelist (see Chapter 9.9), relies on the simulation of secondary electrons. Only DIR (direct simulation model) can be used in that case. For all other signals which may be chosen in SEM mode (see Table 9.3) all three models may be applied (see Table 9.2).
Since both, the single scattering model ( SIN) and the hybrid model
( HYB) calculate energy loss by means of a continuous slowing-down
aproach, the user must be aware that a simulated backscattered electron
spectrum cannot show the elastic peak (zero energy-loss electrons).
Due to this approach, each electron penetrating the sample suffers
a finite energy loss so that none of the electrons going back out of the sample
(backscattered) can retain their original energy. Therefore, if the elastic
peak of backscattered electrons is to appear in the simulations then
the direct simulation model must be applied. Presently, this is only
possible for the elements and compounds listed in Table A.1.