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Influence of the phase transformation of a metal on hollow cathode discharge characteristics
Materials Science attd Engineering, A 139 ( 1991 ) 29-32
29
Influence of the phase transformation of a metal on hollow cathode discharge characteristics C. Simon and S. H e u r a u x Laboratoire de Physique des Milieux lonis(s ( Unit~ de Recherche associate au CNRS 835), Faculty; des Sciences, UniversitO de Namy L B.P. 239, 54506 Vandoeuvre-I~s-Nancy C(dex (France)
H. Michel Laboratoire de Science et G(nie des Surfaces (Unit6 de Recherche associ6e au CNRS 1402), Ecole des Mines, 54042 Nancy (France)
M. Belmahi, B. Petat and M. Remy Laboratoire de Physique des Milieux lonisOs (Unit6 de Recherche associOe au CNRS 835), FacultO des Sciences, Universit{ de Nancy L B.P. 239, 54506 Vandoeuvre-l(s-Nancy (~dex (France)
Abstract The influence of properties of metals (copper, tantalum, titanium, Ti-A1 and nickel) constituting the cathode on the characteristics of a hollow cathode discharge has been investigated for different pressures and currents. We point out that the phase transformations of the cathode material induce a modification in the current-voltage characteristics of the discharge. This phase transformation effect of the cathode material combines with the geometrical effect induced by the typical hollow cathode discharge which seems to be independent of the material properties. The evolution of the electron density and the evolution of the intensities of emission lines in the plasma are also affected by the phase transition. This effect is related to a work function change of the metal at the phase transition. It can be used as a test of work function variation of materials or for optimization of plasma processes.
1. Introduction Hollow cathode discharges (HCDs) have been studied for a long time [1-3] and are used as light sources and for surface treatment processes and laser technology [4-7]. Nevertheless, some basic processes have been incompletely described up to now. In particular, the investigation of the voltage-current (V-I~) characteristics is fragmentary [8, 9], and the effect of some properties of the cathode material, such as the work function, has never been studied. Abnormal H C D s with different cathode materials have been investigated with a particular study of V-1 d characteristics associated with electron density and emission line intensity measurements. For the first time a modification of the V-1 d curves is observed when materials undergoing a phase transformation are used as cathodes. We propose and discuss an interpretation of the H C D voltage evolution with the discharge 0921-5093/91/$3.50
current, in terms of work function changes at the phase transformations of the cathode material.
2. Experimental set-up and methodology T h e hollow cathode is a metallic tube of 21 mm length, of 7 mm inner diameter and open at both ends. Titanium, T i - A l alloy, nickel, tantalum and copper have been used as cathode materials. Adequate electrical isolation of the outer cathode surface is used to avoid cathode pollution and to maintain the cathode processes of the self-sustained discharge inside the cathode cavity [10]. T h e ultimate pressure in the vessel is about 10 -s Torr. T h e gas in the vessel is high purity (99.9999%) research grade argon. As the cathode is not cooled, a calibrated thermocouple gives the temperature Tc of one point at the centre of its inner surface. Particular attention was paid to this measurement during the © Elsevier Sequoia/Printed in The Netherlands
30
discharge, an accuracy of better than 5 K was reached; this is necessary for the interpretation of the experimental results. The d.c. discharge current I d is adjusted between 15 mA and 1 A, and the working pressure p varies in the range 0.1-10 Torr (measured by means of a capacitance manometer) with a constant gas flow of 8 cm 3 min -1. The electron density n e is obtained by electrostatic probe measurements inside the hollow cathode cavity. The emission line intensity I r is measured on the discharge axis with an optical fibre and is analysed with a Jobin-Yvon THR monochromator. A full description of the apparatus has been given in ref. 11. For the best reproducibility of the experimental results, some rules must be followed for each experiment. (1) A very low background pressure between experiments must be maintained. (2) There must be a constant vessel temperature of 380 K between and during each experiment by means of an external heating cell to limit vessel-exterior thermal exchanges. (3) The cathode should be prepared at the beginning of each experiment by a discharge with I d = 50 mA and p = 1.35 Torr for 1½h to test the reproducibility. (4) Thermal equilibrium of the cathode-vessel system must be obtained before any measurements are made. This state is reached in 1 h or longer after any modification of a discharge parameter and depends on the nature of the cathode, the gas pressure and the cathode temperature. (5) First, with a new cathode, a discharge must be made for at least 1 day before any measurements are taken. After this time, the surface state of the cathode is considered to be nearly stabilized.
For this study the following materials were chosen for the cathodes: copper and tantalum which do not have phase transformations, titanium and Ti-A1 alloy which have a crystallographic phase transformation, and nickel which has a magnetic phase transformation (at the Curie point). Experimental results on copper cathodes (not presented here) are similar to those for tantalum because they have no phase transformation. The interpretation of the shape of the voltage-current characteristics presented in Fig. 1 is as follows: as the length of the negative glow increases with increasing current at constant pressure [7], the decreasing part of V - 1 d (before point A) is connected to the self-penetration of the negative glow, while the increasing part (portion AC of the curves) corresponds to the obstructed regime inside the hollow cathode. The glow-to-arc discharge transition appears beyond point C for higher currents at constant pressure with a high cathode temperature (1720K for tantalum, 1620 K for titanium, and 1170 K for copper). This discharge mode is characterized by a considerable drop in the voltage when the discharge current increases, the thermal electron emission being the dominant process in the physics of the discharge. During the abnormal HCD, the photoelectric effect [3, 12] and the positive ion bombardment [13] are mainly responsible for the
500
....
i ....
,50
i ....
C,,j
....
i-'Ci
....
i ....
I ....
i ....
/-x,
i ....
i""
oT,
0+ Ni
I ~ 1
Ta
4o0
~
350
i/
B
l
,,
°
C
3. Experimental results and discussion A preliminary study of the HCD properties has led to the voltage-pressure and voltage-current characteristics which have already been discussed [7, 11]. Each curve presents a minimal voltage which can be associated with the geometrical effect induced by the typical HCD, for an optimal working pressure or an optimal current. This optimal current seems to be independent of the cathode material (see Fig. 1, point A).
25O
200
,
, 50
~ 100
~ 150
. 200
. 250
, 300
.... 350
400
,_.., 450
.... 500
,.1. 550
600
discharge current (mA) Fig. 1. Voltage-current characteristics at a given pressure for different (uncooled) hollow cathode materials (tantalum, titanium and nickel): point A, separation of two cathode geometry effects of the abnormal HCD; point B, cathode material effect (phase transformation); after point C, transition to arc.
31
secondary emission of electrons from the cathode. Voltage-current curves for cathode materials with a phase transformation are different. The observed voltage variations beyond point B (Fig. 1, titanium and nickel) in the voltage-current characteristics of the abnormal discharge cannot be explained solely by the H C D geometry effect. In the case of the titanium cathode this event always occurs, for different pressures, at the same cathode temperature To= 1155 _+ 10 K (Fig. 2), which exactly corresponds to the temperature of the a-to-fl phase transition of titanium. To confirm this material effect induced by the titanium phase transition, a Ti-6at.%A1 alloy was used as the cathode material. This binary alloy has the transition from a to ct+fl phase at 1210 K and the transition from a +fl to fl to 1250 K [14]. Figure 3 shows that the perturbation follows exactly the evolution of the different phase transformations of the Ti-Al alloy with a flat part during the a + fl phase. In the case of nickel the magnetic phase transformation and the voltage variation occur at 623 K. We have verified that this variation occurs always at the Curie point and is independent of the pressure as in the case of titanium. In the obstructed regime of the abnormal HCD, a larger current increases the discharge voltage but, if more primary (fast) electrons are emitted into the discharge at constant current, the voltage decreases. More electrons can be
obtained from the cathode if the secondary emission coefficient ~, increases. This modification of can be induced by a change in the work function [15] of the cathode material. In the case of titanium, we associate the decreasing voltage in the obstructed regime with a diminution in the work function at the phase transition [ 11 ] and we show this diminution for the first time. During the self-penetration of the negative glow, a larger current decreases the discharge voltage. In the case of nickel, the magnetic phase transformation occurring in this part of the discharge induces a known increase (5.05-5.1 eV) in the work function [16] and the voltage must increase to compensate for the loss of primary electrons. This known variation in the nickel work function confirms the findings for titanium. As the temperature of the inner surface of the cathode is not uniform, the voltage variation is not sudden. The voltage variations due to the phase transformation begin when the hotter surface of the cathode reaches the transformation temperature (at the centre of the inner surface) and finish as the transformation is complete, e.g. when the transition point reaches both edges of the cathode. The variation in the work function modifies the evolution of the plasma parameters. For example, it induces an increase in the slope of the n¢- I d curve in the fl phase at constant pressure and an increase in the intensity of emission lines of titanium atoms as shown in Figs. 4 and 5. 400
400 .............
~
......... ~
"7
' u" "0:6'T:~"
/
,
i ....
"
i ....
p-
i ....
i ....
J ....
1.35 T o r r
O.SWorr
.,e"
350
....
350
.
~-
g
I ~ ,
\\
°
i ....
n
Ti-6%AI
o
Ti
ct+[3 Ti
,
"..al~.~"
~l Ti a l l o y :
\
~ 3oo
~ 3oo
ct T i a l l o y
i ....
Qa
TC = 1210 K
i
~,
T c = 1250 K
t~ t~ 250 "~
250 ', T c - 1155 K tl Ti 200 200
....
i .... 50
i .... 100
i .... 150
i .... 200
i .... 250
i .... 300
i .... 350
....
' .... 50
400
discharge current (mA) Fig. 2. Effect of crystallographic phase transformation occurring at 1155 K on voltage-current characteristics of (uncooled) titanium HCD at different pressures.
J .... 1(30
~ Ti , .... 150
,..',.~ 200
.... 250
, .... 300
, .... 350
400
discharge current (mA) Fig. 3. Dependence of voltage-current characteristics upon the temperature of crystallographic phase transformations, at a given pressure, for two different (uncooled) hollow cathode materials (titanium and Ti-6at.%Al).
32 4
'7
....
i ....
, ....
, ....
, ....
, ....
i ....
[]
ne
0
Vd
, ....
400
;t 3
ct Ti
L
B
/
e~
o~ 0
. . . .
~'ri
{
' . . . .
50
i . . . i ,
100
/
~
350
P = 1.35 T o r r . . . .
150
i . . i . f
200
. . . .
250
i
. . . .
300
i
....
200
350
400
discharge current (mA) Fig. 4. Evolution of electron density with discharge current, at a given pressure, according to a and/3 phases of titanium used as (uncooled) hollow cathode material and related to the corresponding voltage-current characteristic evolution. The significance of points A and B is the same as in Fig. 1.
1000
....
i ....
i ....
, ....
i ....
800
f ....
Til 3 6 5 . 3 5 n m
4-
Ti I 364.27 nm
• •
ct
Ti
,
i ....
o
B
i ....
References
f
Ti I 363.55 nm./j a 4
[~ T i
-
-
~600
0~
6
400
0~ e~
20O
0
.
.
.
.
50
. 100
.
. 150
i,, 200
.i
....
250
300
voltage, electron density and light emission. Hence, in numerical simulations of discharges which mainly depend on the secondary emission coefficient, it is necessary to include in the code the material properties of the cathode. To our knowledge, no theory has been developed in any cathode geometry which takes into account this secondary emission modification. On the other hand, a possible application of this effect is its use as an in situ diagnostic tool for the existence of a work function drop or step in the cathode surface during a process. In the present case, for some technological applications of titanium, it could be interesting to use this modification because the plasma electron density no increases with the discharge current Id more quickly in the fl than in the a phase, as well as the spectroscopic emission intensity of titanium. This effect has been used in the physical vapour deposition of TiN [17] without knowledge of this phenomenon.
350
400
discharge current (mA) Fig. 5. Evolution of titanium emission lines with current discharge at a given pressure, according to a and fl phases of the titanium cathode. The significance of points A and B is the same as in Fig. 1. (a.u. = arbitrary units.)
4. Conclusion The first point of these experiments is to show that the properties of the cathode material must be taken into account to understand the evolution of the discharge parameters such as current,
1 F. Paschen, Ann. Phys., (Leipzig), 4 (1916) 901. 2 A. Giintherschulze, Z. Teeh. Phys., 2 (1930) 49. 3 P. F. Little and A. yon Engel, Proc. R. Soe. London, Ser. A, 224 (1954) 209. 4 S. Caroli, Prog. Anal At. Spectrosc., 6 (1983) 253, and references cited therein. 5 S. Caroli, J. Anal At. Spectrom., 2 (1987) 661, and references cited therein. 6 R. Mavrodineanu, J. Res. NatL Bur. Stand., 89 (1984) 143, and references cited therein. 7 K. Rozsa, Z. Naturforseh., 35a (1980) 649. 8 T. Musha, J. Phys. Soc. Jpn., 17(1962) 1440, 1447. 9 M.E. Pillow, Spectrochim. Acta B, 36 ( 1981 ) 821. 10 C. Simon, Envelope Soleau 16895, 1989. 11 C. Simon, Contribution ~ l'&ude d'une d6charge en cathode creuse de titane, Th~se d'Universit~, Nancy I, 1989. 12 H. Falk, Ann. Phys., (Leipzig), 16 (1965) 160. 13 A. V. Bondarenko, Sov. Phys.--Tech. Phys., 21 (1976) 1497. 14 R. E Elliot, Constitution of Binary Alloys, First Supplement, McGraw-Hill, New York, 1965, p. 59. 15 R. A. Baragiola, E. V. Alonso, J. Ferron and A. OlivaFlorio, Surf. Sci., 90(1979) 240. 16 A.B. Cardwell, Phys. Rev., 76(1949) 125. 17 H. Michel, M. Gantois and C. H. Luiten, Proc. Conf. on Heat Treatment, London, 1984, The Metals Society, 1984, p. 1.
29
Influence of the phase transformation of a metal on hollow cathode discharge characteristics C. Simon and S. H e u r a u x Laboratoire de Physique des Milieux lonis(s ( Unit~ de Recherche associate au CNRS 835), Faculty; des Sciences, UniversitO de Namy L B.P. 239, 54506 Vandoeuvre-I~s-Nancy C(dex (France)
H. Michel Laboratoire de Science et G(nie des Surfaces (Unit6 de Recherche associ6e au CNRS 1402), Ecole des Mines, 54042 Nancy (France)
M. Belmahi, B. Petat and M. Remy Laboratoire de Physique des Milieux lonisOs (Unit6 de Recherche associOe au CNRS 835), FacultO des Sciences, Universit{ de Nancy L B.P. 239, 54506 Vandoeuvre-l(s-Nancy (~dex (France)
Abstract The influence of properties of metals (copper, tantalum, titanium, Ti-A1 and nickel) constituting the cathode on the characteristics of a hollow cathode discharge has been investigated for different pressures and currents. We point out that the phase transformations of the cathode material induce a modification in the current-voltage characteristics of the discharge. This phase transformation effect of the cathode material combines with the geometrical effect induced by the typical hollow cathode discharge which seems to be independent of the material properties. The evolution of the electron density and the evolution of the intensities of emission lines in the plasma are also affected by the phase transition. This effect is related to a work function change of the metal at the phase transition. It can be used as a test of work function variation of materials or for optimization of plasma processes.
1. Introduction Hollow cathode discharges (HCDs) have been studied for a long time [1-3] and are used as light sources and for surface treatment processes and laser technology [4-7]. Nevertheless, some basic processes have been incompletely described up to now. In particular, the investigation of the voltage-current (V-I~) characteristics is fragmentary [8, 9], and the effect of some properties of the cathode material, such as the work function, has never been studied. Abnormal H C D s with different cathode materials have been investigated with a particular study of V-1 d characteristics associated with electron density and emission line intensity measurements. For the first time a modification of the V-1 d curves is observed when materials undergoing a phase transformation are used as cathodes. We propose and discuss an interpretation of the H C D voltage evolution with the discharge 0921-5093/91/$3.50
current, in terms of work function changes at the phase transformations of the cathode material.
2. Experimental set-up and methodology T h e hollow cathode is a metallic tube of 21 mm length, of 7 mm inner diameter and open at both ends. Titanium, T i - A l alloy, nickel, tantalum and copper have been used as cathode materials. Adequate electrical isolation of the outer cathode surface is used to avoid cathode pollution and to maintain the cathode processes of the self-sustained discharge inside the cathode cavity [10]. T h e ultimate pressure in the vessel is about 10 -s Torr. T h e gas in the vessel is high purity (99.9999%) research grade argon. As the cathode is not cooled, a calibrated thermocouple gives the temperature Tc of one point at the centre of its inner surface. Particular attention was paid to this measurement during the © Elsevier Sequoia/Printed in The Netherlands
30
discharge, an accuracy of better than 5 K was reached; this is necessary for the interpretation of the experimental results. The d.c. discharge current I d is adjusted between 15 mA and 1 A, and the working pressure p varies in the range 0.1-10 Torr (measured by means of a capacitance manometer) with a constant gas flow of 8 cm 3 min -1. The electron density n e is obtained by electrostatic probe measurements inside the hollow cathode cavity. The emission line intensity I r is measured on the discharge axis with an optical fibre and is analysed with a Jobin-Yvon THR monochromator. A full description of the apparatus has been given in ref. 11. For the best reproducibility of the experimental results, some rules must be followed for each experiment. (1) A very low background pressure between experiments must be maintained. (2) There must be a constant vessel temperature of 380 K between and during each experiment by means of an external heating cell to limit vessel-exterior thermal exchanges. (3) The cathode should be prepared at the beginning of each experiment by a discharge with I d = 50 mA and p = 1.35 Torr for 1½h to test the reproducibility. (4) Thermal equilibrium of the cathode-vessel system must be obtained before any measurements are made. This state is reached in 1 h or longer after any modification of a discharge parameter and depends on the nature of the cathode, the gas pressure and the cathode temperature. (5) First, with a new cathode, a discharge must be made for at least 1 day before any measurements are taken. After this time, the surface state of the cathode is considered to be nearly stabilized.
For this study the following materials were chosen for the cathodes: copper and tantalum which do not have phase transformations, titanium and Ti-A1 alloy which have a crystallographic phase transformation, and nickel which has a magnetic phase transformation (at the Curie point). Experimental results on copper cathodes (not presented here) are similar to those for tantalum because they have no phase transformation. The interpretation of the shape of the voltage-current characteristics presented in Fig. 1 is as follows: as the length of the negative glow increases with increasing current at constant pressure [7], the decreasing part of V - 1 d (before point A) is connected to the self-penetration of the negative glow, while the increasing part (portion AC of the curves) corresponds to the obstructed regime inside the hollow cathode. The glow-to-arc discharge transition appears beyond point C for higher currents at constant pressure with a high cathode temperature (1720K for tantalum, 1620 K for titanium, and 1170 K for copper). This discharge mode is characterized by a considerable drop in the voltage when the discharge current increases, the thermal electron emission being the dominant process in the physics of the discharge. During the abnormal HCD, the photoelectric effect [3, 12] and the positive ion bombardment [13] are mainly responsible for the
500
....
i ....
,50
i ....
C,,j
....
i-'Ci
....
i ....
I ....
i ....
/-x,
i ....
i""
oT,
0+ Ni
I ~ 1
Ta
4o0
~
350
i/
B
l
,,
°
C
3. Experimental results and discussion A preliminary study of the HCD properties has led to the voltage-pressure and voltage-current characteristics which have already been discussed [7, 11]. Each curve presents a minimal voltage which can be associated with the geometrical effect induced by the typical HCD, for an optimal working pressure or an optimal current. This optimal current seems to be independent of the cathode material (see Fig. 1, point A).
25O
200
,
, 50
~ 100
~ 150
. 200
. 250
, 300
.... 350
400
,_.., 450
.... 500
,.1. 550
600
discharge current (mA) Fig. 1. Voltage-current characteristics at a given pressure for different (uncooled) hollow cathode materials (tantalum, titanium and nickel): point A, separation of two cathode geometry effects of the abnormal HCD; point B, cathode material effect (phase transformation); after point C, transition to arc.
31
secondary emission of electrons from the cathode. Voltage-current curves for cathode materials with a phase transformation are different. The observed voltage variations beyond point B (Fig. 1, titanium and nickel) in the voltage-current characteristics of the abnormal discharge cannot be explained solely by the H C D geometry effect. In the case of the titanium cathode this event always occurs, for different pressures, at the same cathode temperature To= 1155 _+ 10 K (Fig. 2), which exactly corresponds to the temperature of the a-to-fl phase transition of titanium. To confirm this material effect induced by the titanium phase transition, a Ti-6at.%A1 alloy was used as the cathode material. This binary alloy has the transition from a to ct+fl phase at 1210 K and the transition from a +fl to fl to 1250 K [14]. Figure 3 shows that the perturbation follows exactly the evolution of the different phase transformations of the Ti-Al alloy with a flat part during the a + fl phase. In the case of nickel the magnetic phase transformation and the voltage variation occur at 623 K. We have verified that this variation occurs always at the Curie point and is independent of the pressure as in the case of titanium. In the obstructed regime of the abnormal HCD, a larger current increases the discharge voltage but, if more primary (fast) electrons are emitted into the discharge at constant current, the voltage decreases. More electrons can be
obtained from the cathode if the secondary emission coefficient ~, increases. This modification of can be induced by a change in the work function [15] of the cathode material. In the case of titanium, we associate the decreasing voltage in the obstructed regime with a diminution in the work function at the phase transition [ 11 ] and we show this diminution for the first time. During the self-penetration of the negative glow, a larger current decreases the discharge voltage. In the case of nickel, the magnetic phase transformation occurring in this part of the discharge induces a known increase (5.05-5.1 eV) in the work function [16] and the voltage must increase to compensate for the loss of primary electrons. This known variation in the nickel work function confirms the findings for titanium. As the temperature of the inner surface of the cathode is not uniform, the voltage variation is not sudden. The voltage variations due to the phase transformation begin when the hotter surface of the cathode reaches the transformation temperature (at the centre of the inner surface) and finish as the transformation is complete, e.g. when the transition point reaches both edges of the cathode. The variation in the work function modifies the evolution of the plasma parameters. For example, it induces an increase in the slope of the n¢- I d curve in the fl phase at constant pressure and an increase in the intensity of emission lines of titanium atoms as shown in Figs. 4 and 5. 400
400 .............
~
......... ~
"7
' u" "0:6'T:~"
/
,
i ....
"
i ....
p-
i ....
i ....
J ....
1.35 T o r r
O.SWorr
.,e"
350
....
350
.
~-
g
I ~ ,
\\
°
i ....
n
Ti-6%AI
o
Ti
ct+[3 Ti
,
"..al~.~"
~l Ti a l l o y :
\
~ 3oo
~ 3oo
ct T i a l l o y
i ....
Qa
TC = 1210 K
i
~,
T c = 1250 K
t~ t~ 250 "~
250 ', T c - 1155 K tl Ti 200 200
....
i .... 50
i .... 100
i .... 150
i .... 200
i .... 250
i .... 300
i .... 350
....
' .... 50
400
discharge current (mA) Fig. 2. Effect of crystallographic phase transformation occurring at 1155 K on voltage-current characteristics of (uncooled) titanium HCD at different pressures.
J .... 1(30
~ Ti , .... 150
,..',.~ 200
.... 250
, .... 300
, .... 350
400
discharge current (mA) Fig. 3. Dependence of voltage-current characteristics upon the temperature of crystallographic phase transformations, at a given pressure, for two different (uncooled) hollow cathode materials (titanium and Ti-6at.%Al).
32 4
'7
....
i ....
, ....
, ....
, ....
, ....
i ....
[]
ne
0
Vd
, ....
400
;t 3
ct Ti
L
B
/
e~
o~ 0
. . . .
~'ri
{
' . . . .
50
i . . . i ,
100
/
~
350
P = 1.35 T o r r . . . .
150
i . . i . f
200
. . . .
250
i
. . . .
300
i
....
200
350
400
discharge current (mA) Fig. 4. Evolution of electron density with discharge current, at a given pressure, according to a and/3 phases of titanium used as (uncooled) hollow cathode material and related to the corresponding voltage-current characteristic evolution. The significance of points A and B is the same as in Fig. 1.
1000
....
i ....
i ....
, ....
i ....
800
f ....
Til 3 6 5 . 3 5 n m
4-
Ti I 364.27 nm
• •
ct
Ti
,
i ....
o
B
i ....
References
f
Ti I 363.55 nm./j a 4
[~ T i
-
-
~600
0~
6
400
0~ e~
20O
0
.
.
.
.
50
. 100
.
. 150
i,, 200
.i
....
250
300
voltage, electron density and light emission. Hence, in numerical simulations of discharges which mainly depend on the secondary emission coefficient, it is necessary to include in the code the material properties of the cathode. To our knowledge, no theory has been developed in any cathode geometry which takes into account this secondary emission modification. On the other hand, a possible application of this effect is its use as an in situ diagnostic tool for the existence of a work function drop or step in the cathode surface during a process. In the present case, for some technological applications of titanium, it could be interesting to use this modification because the plasma electron density no increases with the discharge current Id more quickly in the fl than in the a phase, as well as the spectroscopic emission intensity of titanium. This effect has been used in the physical vapour deposition of TiN [17] without knowledge of this phenomenon.
350
400
discharge current (mA) Fig. 5. Evolution of titanium emission lines with current discharge at a given pressure, according to a and fl phases of the titanium cathode. The significance of points A and B is the same as in Fig. 1. (a.u. = arbitrary units.)
4. Conclusion The first point of these experiments is to show that the properties of the cathode material must be taken into account to understand the evolution of the discharge parameters such as current,
1 F. Paschen, Ann. Phys., (Leipzig), 4 (1916) 901. 2 A. Giintherschulze, Z. Teeh. Phys., 2 (1930) 49. 3 P. F. Little and A. yon Engel, Proc. R. Soe. London, Ser. A, 224 (1954) 209. 4 S. Caroli, Prog. Anal At. Spectrosc., 6 (1983) 253, and references cited therein. 5 S. Caroli, J. Anal At. Spectrom., 2 (1987) 661, and references cited therein. 6 R. Mavrodineanu, J. Res. NatL Bur. Stand., 89 (1984) 143, and references cited therein. 7 K. Rozsa, Z. Naturforseh., 35a (1980) 649. 8 T. Musha, J. Phys. Soc. Jpn., 17(1962) 1440, 1447. 9 M.E. Pillow, Spectrochim. Acta B, 36 ( 1981 ) 821. 10 C. Simon, Envelope Soleau 16895, 1989. 11 C. Simon, Contribution ~ l'&ude d'une d6charge en cathode creuse de titane, Th~se d'Universit~, Nancy I, 1989. 12 H. Falk, Ann. Phys., (Leipzig), 16 (1965) 160. 13 A. V. Bondarenko, Sov. Phys.--Tech. Phys., 21 (1976) 1497. 14 R. E Elliot, Constitution of Binary Alloys, First Supplement, McGraw-Hill, New York, 1965, p. 59. 15 R. A. Baragiola, E. V. Alonso, J. Ferron and A. OlivaFlorio, Surf. Sci., 90(1979) 240. 16 A.B. Cardwell, Phys. Rev., 76(1949) 125. 17 H. Michel, M. Gantois and C. H. Luiten, Proc. Conf. on Heat Treatment, London, 1984, The Metals Society, 1984, p. 1.