Apparatus and method for reducing topographical effects in an auger image

A system for producing an Auger image which is substantially independent of the topographical contour of the sample surface includes at least two counting means. Preferably, the two counting means are adapted to be dedicated counters, one for background incidents and one for signal incidents. The system also includes counter control means for regulating the counting time and the immersion incident energy threshold for each counter.

BACKGROUND OF THE INVENTION 
The present invention generally relates to an apparatus for producing an 
Auger image and, in particular, relates to such an apparatus which is 
substantially independent of the topographical contour of the sample under 
test. 
Auger analysis in a sophisticated analytical technique whereby the surface 
of a sample is examined for its elemental composition. When discussing 
Auger systems, the term "surface" is generally considered to be that 
portion of the sample which is on the order of a few atomic layers deep. 
During an Auger analysis, a primary electron source bombards a segment of 
the sample surface to release secondary electrons (i.e. Auger electrons) 
therefrom, which secondary electrons are collected and analyzed. The 
liberated particles are usually analyzed as a function of their energy. 
As well known in the art, one particularly convenient means for evaluating 
the data generated by an Auger system is to create an Auger image, or map. 
To create such a map, the technologist generally measures the intensity of 
an Auger peak for a two-dimensional array of points on the sample being 
tested. In such an analysis, the primary electron beam is digitally 
controlled by a computer and stepped through a rectangular area of the 
sample surface. The points on the map are usually scanned in a raster 
pattern simlilar to that used for producing a television image. To form 
such an Auger image of a surface, the magnitude of the Auger peaks are 
first measured at each point in the raster matrix. This information is 
generally stored in a memory device and later displayed on, for example, 
an oscilloscope or other form of recorder where the intensity at each 
point of the raster is proportional to the magnitude of the Auger peak. 
The intensity of a particular Auger peak is generally obtained by measuring 
the magnitude, N(e.sub.p) of the peak at the energy, e.sub.p, giving 
maximum intensity and subtracting the background magnitude N(e.sub.b) at 
an energy e.sub.b sufficiently removed from the peak that Auger electrons 
forming the peak do not contribute. In conventional computer-controlled 
Auger systems, Auger maps are determined by the following steps: First, an 
electron-pass energy, e.sub.p, is selected. After the pass energy is 
determined and the mechanism set, the number of incidences N(e.sub.p) is 
measured and stored for each point in a selected line of the raster 
matrix. Thereafter, the pass energy for the detector is set at another 
base line, e.sub.b, representative of the electron energy of the 
background which is present. Thereafter, the incidences of background 
N(e.sub.b) is measured for each point in the same matrix. These 
measurements N(e.sub.p) and N(e.sub.b), taken at e.sub.p and e.sub.b, 
respectively, are repeated for each line in the matrix. From the 
accumulated data an Auger image is constructed by conventional arithmetic 
processing equipment by subtracting the number of counts at e.sub.b from 
the number of counts e.sub.p at each point in the matrix. 
A first order topographical correction can be achieved by dividing the peak 
height determined from above by the background incidences, i.e., 
[N(e.sub.p)-N(e.sub.b)]/N(e.sub.b). This is a fairly accurate correction 
to the topographical variations which modulate the background and peak 
height uniformly. This particular method is advantageous in that it is 
independent of the incident beam current since both the background and the 
peak are proportional to the excitation beam. Therefore, beam current 
variations having a period longer than the time required to scan each line 
do not affect the normalized Auger intensities. 
Unfortunately, the above normalization scheme only removes beam current 
noise of rather low frequency. For example, if a particular line of the 
matrix contains 200 measurement points with a typical measurement time at 
each point of ten milliseconds, the elapsed time between the peak and the 
background measurements is therefore two seconds. Hence the technique is 
only effective in removing noise components having a frequency less than 
0.5 Hz. 
Another disadvantage is that by the use of equal measurement intervals at 
each spatial point, non-uniform statistical noise levels are created when 
topographical effects, i.e., surface depth variations, vary the signal 
magnitude either through scattering, absorption or miscellaneous 
reflections. Hence, even using a first order topographical correction, the 
variation of the Auger peak due to actual compositional changes of the 
surface is difficult to distinguish from changes due to noise variations 
along each particular line, as well as between different lines. As a 
result, topological variations can result in a complete 
mischaracterization of the elemental composition of a surface. 
SUMMARY OF THE INVENTION 
Accordingly, it is one object of the present invention to provide an 
apparatus for yielding a uniform background noise level in an Auger image 
and to prevent low frequency noise in the beam current on other 
instrumental factors from affecting the Auger image. 
This object is achieved, at least in part, by measuring the background and 
the signal of interest in rapid succession at each point and by varying 
the measurement time per point to achieve a uniform noise level due to 
statistical noise in the background. 
Other objects and advantages will become apparent to those skilled in the 
art from the following detailed specification, the accompanying drawing 
and the claims appended hereto.

DETAILED DESCRIPTION OF THE INVENTION 
A system, generally indicated at 10 in the drawing and embodying the 
principles of the present invention, for reducing topographical effects in 
forming an Auger image includes a conventional Auger analyzer 12 for 
detecting secondary, or Auger, electrons. The system 10 also includes 
means 14 for amplifying the detected signal, the means 14 is preferably a 
conventional electron multiplier well known in the art. The signal from 
the multiplier 14 is digitized by a conventional V/F digitizer 16 which 
transforms the signals from the electron multiplier 14 into digital 
pulses. The signal from the digitizer 16 is then provided via a switch 
means 18 to first and second counters 20 and 22, respectively. The 
counters, 20 and 22, can be of any type known in the art; for example, 
they can be semiconductor chips such as TI 74LS191 manufactured and 
marketed by Texas Instruments Corp. The switch means 18 is preferably a 
logical AND gate, such as an 74LS08-TI also manufactured and marketed by 
Texas Instruments Corp. The outputs from the counters, 20 and 22, are 
directed into a statistical processing control unit 24 the output of which 
is provided to a recorder and/or display mechanism 26 which can be, for 
example, a video display unit. 
In one mode of operation, the primary electron source bombards one point of 
the surface under test to liberate secondary electrons therefrom. The pass 
energy for the analyzer 12 is set at e.sub.b by the control 24, one output 
of which is coupled to the analyzer 12 via an analyzer control unit 27. 
The digitized signals are counted, via the electron multiplier 14 and 
signal digitizer 16, into the first counter 20. A timer 28 is 
simultaneously activated and used to measure the time, .DELTA.T, required 
to bring the signal level in the first counter 20 to a preset value. The 
pass energy of the analyzer 12 is then set at e.sub.p and the second 
counter 22 is used to measure the signal level during the same time 
interval, .DELTA.T. Thus, the number of incidences of signals at energy 
level e.sub.b is initially accumulated by the first counter 20 until a 
particular preselected value has been reached. Then, the number of 
incidences of particles at energy e.sub.p is counted for the same point on 
the surface and for the same period of time. This value is recorded in 
second counter 22. As a result, the normalized Auger signal is the direct 
difference, which can be determined by, for example, a computer means 30, 
between this measured N(e.sub.p) signal and the preselected level 
N(e.sub.b). This signal is then provided to the display mechanism 26. 
Thus, the difference is provided on the screen of an oscilloscope as, for 
example, a function of the intensity. After both of these measurements 
have taken place, the primary electron source, not shown, is focused on 
the next adjacent point in the particular line of a raster pattern and the 
process repeated. This particular embodiment greatly reduces instrument or 
beam current errors by reducing the time interval between the e.sub.p and 
the e.sub.b measurement. For example, for a .DELTA.T measurement time of 
10 milliseconds per point, noise components having frequencies less than 
100 Hz are rejected. In addition, by varying the .DELTA.T per point along 
each line to establish a constant background signal, the statistical 
uncertainty, i.e., the shot noise level, is identical for each point in 
the raster scan. 
Utilizing the same components, an Auger image can be produced which may, in 
fact, provide a larger signal-to-noise ratio capability. In this 
alternative embodiment, the first counter 20 is arranged, via the computer 
30 and the control unit 24, to count the number of incidences of the 
background signal whereas the second counter 22 is arranged to count only 
those instances at the peak Auger energy, e.sub.p. During a preselected 
particular time interval, the signal from the signal digitizer 16 is 
switched, via switch means 18, at a high frequency between the first and 
second counters 20 and 22, respectively. Simultaneously, the pass energy 
of the analyzer 12 is switched, via the control unit 24, between e.sub.b 
and e.sub.p. In this manner, a signal proportional to the two signal 
levels, e.sub.b and e.sub.p, is simultaneously accumulated in the counters 
20 and 22. In this embodiment, the measurement at each point of the matrix 
is terminated when the background count, for example, in counter 20 
reaches a predetermined level. In this fashion, the normalized Auger 
signal is then the difference between the value in the second counter 22 
and the predetermined background level. 
One important aspect in implementing this alternative embodiment is to 
ensure that the switching time between the counters, 20 and 22, is small 
compared with the time required for the number of background counts to 
achieve its predetermined level. As a result, by reducing the switching 
time, the frequency of the noise rejection is extended upwardly. 
The embodiments described herein are particularly advantageous for use with 
primary electron guns which are conventionally characterized as having low 
frequency flicker noise in addition to inherent white or shot noise. 
While the above embodiments are specifically described, they are intended 
to be exemplary of the present invention and other variations may be 
recognized by those skilled in the art, without departing from the true 
scope and spirit of the present invention. As such, the scope and spirit 
of this invention is deemed limited only by the claims appended hereto and 
the reasonable interpretation thereof.