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Cathode Processes

Secondary electron Emission

[Vdc vs Gamma]

The secondary electron yieldg, is the average number of electrons emitted per incident ion.

Empirically it has been found that g increases as the work function f  decreases, i.e.(1-4)

[gamma]

where Ei is the effective potential energy (assumed to be the argon ionization energy, 15.76 eV).(5)

A theoretical interpretation of this empirical equation involves several assumptions about ion bombardment. There are also significant variations in measured work functions, especially for elements with low ionization potential, and care is required when the term in brackets is very small.(6)Hence the estimates for g given are indicative only of general trends.

Note that the values for g differ from ref (3) because of the more recent values for f  used here.


Element

Work Function (eV)(7,8)

Secondary Electron Yield, g 

DC Bias Voltage (V)

Ag4.260.120748
Al4.280.119544
Au5.1  0.067792
Bi4.4   0.122 
C4.8   0.086 
Ca2.9   0.208 
Co5.0  0.073752
Cr4.5  0.105700
Cu4.650.096740
Fe4.5  0.105720
Ga3.9   0.144 
Ge4.8   0.086 
Mg3.660.159476
Mn3.8   0.150 
Mo4.6  0.099724
Ni4.9   0.080740
Pb4.250.121760
Pt5.3   0.054 
Sb4.1   0.131 
Si4.850.083608
Sn4.420.110692
Sr2.7   0.221 
Ta4.10.13
Ti4.330.116628
V4.3  0.118648
W4.550.102 
Zn4.330.116640
Zr4.050.134664

The DC bias voltage was measured using the purest materials available, at constant power (40 W) and pressure (700 Pa) using a 4 mm anode on a JY 5000 RF instrument. The actual plasma power may be a little lower then the 40 W delivered by the rf power genertor.

The figure displayed above shows the  DC bias voltage measured on near pure metals as a function of their calculated secondary electron yield, g

The results show the expected trend: DC bias voltage decreases with increasing secondary electron yield. The large scatter of the exerimental data is due to several factars. The secondary electron emission yield is not the only factor influencing the dc-bias voltage. Source cleanliness, in particular the presences of traces gases in the plasma carrier gas, do influence the dc-bias voltage. The secondary electron emission yiels also varies strongly with the state of the surface of the sputtered species.


References

  1. R A Baragiola, E V Alonso, J Ferron and A Oliva Florio, Surf. Sci90 (1979) 915.
  2. L M Kishinevsky, Radiation Effects 19  (1973) 23.
  3. L Ohannessian, PhD Thesis, Universite Claude Bernard, Lyon, France (1986).
  4. H Hocquaux, in R K Marcus (Ed), Glow Discharge Spectroscopies, Plenum, New York (1993), p 351.
  5. A Bogaerts, private communication (2000).
  6. R A Baragiola, private communication (2000).
  7. CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton (1990), pp 12-84 to 12-87.
  8. E A Brandes and G B Brook (Eds), Smithells Metals Reference Book, Butterworth-Heinemann (1998), p 18-4.
  9. A. Bengtson, Th. Nelis, The concept of constant emission yield in GDOES, Anal. Bioanal. Chem, 2006, English, DOI: 10.1007/S00216-006-0412-7

First published on the web: 14 June 2000.

Authors: Richard Payling and Thomas Nelis