INFICON Guardian EIES Controller - Manual (Page 128)

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Guardian Co-Deposition Controller Operating Manual
However, for Cu the line near 3250 Å is not only attenuated, but is also near the
secondary Al line around 3100 Å. To accurately measure the Cu emission, we need
a discriminating device with the conflicting goals of a narrow bandwidth and low
attenuation of the desired signal.
Thin film optical filters are both efficient and selective. In addition they are readily
available, small, and relatively inexpensive. The main disadvantages are that a
specific filter is required for each material, and the filters degrade over time.
Periodic spectrometer verification of filter central wavelength and pass band is
required.
An alternative to optical filters are monochromators. Both wavelength and
bandwidth are tunable, but the efficiency does vary with the square of the
bandwidth. For applications requiring deposition of many different materials,
monochromator size and expense may be an acceptable trade-off to stocking and
changing filters.
6.2.3 Detector
Once the characteristic wavelength is discriminated by a filter or monochromator,
it must be accurately measured by a detector. The EIES-IV Guardian uses a
photomultiplier tube (PMT) to convert the optical signal to electric current. See
Figure 6-3 .
Figure 6-3 PMT
In addition to the PMT, a programmable high voltage power supply and
microprocessor are housed within the detector module. This allows each detector
to be calibrated to a PMT voltage specific to the material and deposition rate being
measured. A single DB-9 cable carries the RS-232 communications and low
voltage power between the detector and Guardian controller.
0 - 1000V
20 Bit
D to A
32 Bit
C
Switching
Power
Supply
0 - 2500 nA
PMT
I/V
F
I
L
T
E
R
Δ-∑
μ