Processes at the cathode surface
![[Vdc vs Gamma]](images/second11.gif)
Secondary electron Emission
The secondary electron yield, g, 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]](Images/EQ_impeda3.gif){kind=small}
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) |
| Ag | 4.26 | 0.120 | 748 |
| Al | 4.28 | 0.119 | 544 |
| Au | 5.1 | 0.067 | 792 |
| Bi | 4.4 | 0.122 | |
| C | 4.8 | 0.086 | |
| Ca | 2.9 | 0.208 | |
| Co | 5.0 | 0.073 | 752 |
| Cr | 4.5 | 0.105 | 700 |
| Cu | 4.65 | 0.096 | 740 |
| Fe | 4.5 | 0.105 | 720 |
| Ga | 3.9 | 0.144 | |
| Ge | 4.8 | 0.086 | |
| Mg | 3.66 | 0.159 | 476 |
| Mn | 3.8 | 0.150 | |
| Mo | 4.6 | 0.099 | 724 |
| Ni | 4.9 | 0.080 | 740 |
| Pb | 4.25 | 0.121 | 760 |
| Pt | 5.3 | 0.054 | |
| Sb | 4.1 | 0.131 | |
| Si | 4.85 | 0.083 | 608 |
| Sn | 4.42 | 0.110 | 692 |
| Sr | 2.7 | 0.221 | |
| Ti | 4.33 | 0.116 | 628 |
| V | 4.3 | 0.118 | 648 |
| W | 4.55 | 0.102 | |
| Zn | 4.33 | 0.116 | 640 |
| Zr | 4.05 | 0.134 | 664 |
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. Sci. 90 (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, Anal. Bioanl. Chem, 2006,
First published on the web: 14 June 2000.
Authors: Richard Payling and Thomas Nelis