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Submitted on 1 Jan 1979
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SOME PROPERTIES OF THE LOW-PRESSURE DISCHARGE IN SILANE
P. Kocian, J. Mayor, S. Bourquard
To cite this version:
P. Kocian, J. Mayor, S. Bourquard. SOME PROPERTIES OF THE LOW-PRESSURE DISCHARGE IN SILANE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-169-C7-170.
�10.1051/jphyscol:1979783�. �jpa-00219489�
JOURNAL DE PHYSIQUE . CoZZoque 6'7, suppldment au n07, Tome 40, JuiZZet 1979, page C7- 169
SOME PROPERTIES OF THE LOW-PRESSURE DISCHARGE IN SILANE
P. Kocian, J.M. Mayor, S. Bourquard.
L&ora$ory o f Applied Physics, Federal I n s titut 1. INTRODUCTIOIT
During past several years, there has been a 7rowing interest in the silane (SiH ) an
4 a material For the production of silicon and its films /1,2/. There are severalme- thods to produce the films of Si. The depo- sition in a glow-discharge in silane seems to have some advantage regarding to other methods like evaporation in vacuum or ca- thodic sputtering /3/. Glow-discharge de- position is a complicated processus which is not fully known and understood. Numerous plasmachemical reactions occur in the elec- tric discharge in silane. To understand and to optimalize the reactions it will be ne- cessary to know the correlations between the plasmaparameters and the rates of the reactions in plasma. Unfortunately the sya- tematic investigation of the plasmaparame- ters in silane was not be made till now.
Only the dependences of the film qualities on the pressure, on the discharge power and on the flow rate were investigated.
The difficulty consist in the complexity of'the molecular plasma and its diagnostics The aim of this paper was the investigation of some basic parameters and phenomena in the low-pressure discharge in pure silane.
In order to obtain informations about these properties, we have performed our measure- ments in a DC glow discharge, in which the diagnostics are much easier than in the ge- nerally used R.F. discharge. Contamination of the electrodes is not immediate and lea- ves enough time to do the measurements.
2. EXPERIMENT
The electric discharge was produced in..a quartz tube 70 rnm in diameter and 800 mm long. It is well known that the impurities in'the $ischarge atmosphere influence con- siderably the plasmaparameters and therefo-
o f TechnoZogy, Lausanne, S w i t z e r l a n d .
re also the plasma reactions and their pro- ducts. Therefore a great precaution was ta- ken in the purity of the gas. A vacuum of
torr was obtained in the tube by means of a mechanical pump in series with a tur- bomolecular pump. The limit pressure was measured by means of the ionization gauge.
The evacuated tube was heated to outgas the discharge tube walls, the electrodes and the probes. Pure silane tapped from the cylinder passed through a flow meter to a needle valve and then entered the'dischar- ge tube. The discharge was produced between two stainless steel electrodes. Specially arranged cathode with adequate cooling mi- nimized the contamination of the discharge atmosphere (less than 0,01%). A highly sta- bilized DC power supply powered the dischar- ge.
3. MEASUREYENTS
Two basic barameters were investigated :the electric field E and the neutral gas tempe- rature T The electric field influences
g
-
the electron energy and density and deter- mines the electron energy distribution. The
electric field may be measured by means of I two probes placed in the positive column or by means of the mobile electrode.
The knowledge of the gas temperature is im- portant for the investigation of other plasma parameters like dissociation etc. It is necessary to note that T influences the
g
gas density and therefore also the produc- tion of charged particles and transport pro- cessus. There are several methods'to deter+
mine the gas temperature, most of them are available at the higher pressures and at the higher currents only. In our low-pre- ssure discharge where the temperature does
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979783
not exceed some tens of degrees the utili--- sation of the material thermometers seems to be the most convenient. It is necessary to note that the introducing this probe in a non isothermal plasma the measured value differs more or less from the true value.
Various reactions and processes occur at the probe surface. The most of these reac-- tions are exothermic, they relieve the heat and add a supplementary energy to the probe. The rates constants of these reac- tions depend on the probe surface. The con- venient choice of the surface may dimi- nish the influence of these reactions. Un- der suitable conditions the difference bet- ween the measured and true value does not exceed 1 % /4/.
Our measurements were realised between 0,05 and 1 torr and for plasma currents in the range 1
-
10' mA.At the low pressure standing striations oc- cur in the positive column. Their shape in- dicates that the radial electron density profile follows a Bessel function and that there are no negative ions. With increa- sing pressure moving striations occur and their shape is quite different, presenting an inflection point. This shape indicates a radial gaussian gradient of the electron density and a considerable concentration of negative ions. The deposited silicon at the anode and in their vicinity confirms
*his assumption. The discharge has quite similar properties as in organic vapours and gases /5/. The reduced electric field
~ / p is relatively low (less than 1 V/cmtor) and decreases with the increasing pressu- re and with increasing discharge current.
The detailed investigation of the electron density, electron energy and energy distri- bution as well as of the negative ions will be presented in other paper.
What about T we suppose that th9 gas is 9-
heated by ellastic collisions of molecules, atoms and radicals with the electrons. The influence of %he exothermic or endothermic reactions is neglected. Because of the lev.
electric field the gas temperature is lo?.
It depends on the pressure and on tLe dis- charge current. These dependences are illus trated in Figs 1. and 2.
23
Fig. l.:T in dependence on the psessure.
9
I
1 1
3 5
Fig. 2. : T in dependence on the current.
CJ
Ot-her inteesting phenomenon in the silane discharge is the deposition of silicon.
Ple observed that under certain conditions the deposition was in form of thin film while under other conditions in form of powder. This property,necessitates more detailed investigation.
As seen from this small paper the electric discharge in silane shows interesting pra- perties which will influence undoubtly the qualities of the deposited silicon films.
REFERENCES :
/1/ A.L. Armizotto,, Solid State Technolo- gy 1968, 43.
/2/ H.F. Sterling, C.G. Swann, Solid Sta- te electronics 8, 653 (1965).
/3/ R.C. Chittick, J.H. Alexander, H.F.
Sterling, J. Electrochem. Soc. 116, 77 (1969).
/4/ R. Seeliger, H. Strachler, Phys. Z.
28, 894 (1927).
/5/ P. Kocian, Czech. J. Phys. B 16, 47 (1966).
