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“Alternative” auger analysis reveals important properties of M-type and scandate cathodes
Applied Surface Science 24 (1985) 330-339 North-Holland, Amsterdam
330
“ALTERNATIVE” AUGER ANALYSIS REVEALS IMPORTANT PROPERTIES OF M-TYPE AND SCANDATE CATHODES J. HASKER
and H.J.H.
Philips Research Laboratories, Received
STOFFELEN 5600 JA Eindhoven,
5 May 1984; accepted for publication
The Netherlands
12 March 1985
It is shown that in the evaluation of Auger spectra, as measured on cathode surfaces, the elemental sensitivities have to be corrected for differences in elemental number density. The substrate and the surface covering material have to be treated differently in the analysis. The result for normal M-type cathodes is that-besides the Ba-0 cover - there is an excess oxygen concentration on the surface of about two times the Ba-0 concentration. This in spite of the fact that the oxygen to barium peak-to-peak height ratio in the measured spectra is about equal to 2. For a degraded M-cathode the excess oxygen level is found to be much higher than for the normal cathodes. A new type of scandate cathode is described and discussed. Its analysis shows more Ba and, relatively, much less excess oxygen than for the M-type cathodes. This may account for the substantially higher emission.
1. Introduction In the mutual comparison of cathodes, and for better understanding of emission properties, both the composition and the concentration of the surface covering material are important. For this reason there have already been many valuable contributions on the application of surface science to cathodes, some of which appeared in the Proceedings of the previous “T&Service Cathode Workshops” [l-3]. Among the various techniques Auger Electron Spectroscopy plays an important role. It is the purpose of the present paper to contribute to improvement of the evaluation of measured Auger spectra. The results will be applied to M-cathodes and to a new type of scandate cathode. According to ref. [3] (pages 21-23), a B-type cathode has about 0.7 monolayer of Ba-0 with some areas of the tungsten matrix covered by oxygen alone, while an M-type has a few percent more Ba. According to ref. [3] (page 55) the M-type may have about 20% more Ba. Thus an M-type cathode is currently thought to have a Ba-0 cover of about a monolayer. Even more certain is the conclusion that the O/Ba atomic ratio on its surface is close to unity. In the evaluation referred to above use is made of elemental Auger sensitivity factors [4-61. In doing so, differences in back scattering and elemental 0169-4332/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J. Hasker, H. J. H. Stoffelen / M-type
and scandate cathodes
331
number densitieswhich, according to analysis from first principles (see e.g. ref. [7]) have to be taken into account - are left out of consideration. Considering the uncertainties with respect to, e.g., the sensitivity factors, the differences in back scattering are of minor importance. However, it will be shown in the next section that the number density correction is very important, in particular when Ba is involved. This gives rise to a substantial increase in the O/Ba ratio as calculated from measured spectra.
2.
Evaluation of relative atomic concentrations from the Auger spectrum
In this section a homogeneous mixture of m componentsas may be in the substrate of a dispenser cathodewill be considered (the next section deals with the covering material which has to be treated differently). It is rather common practice to calculate the relative concentration ci of component i in the mixture from
0) where Hi is the peak-to-peak height of any selected f‘line” of element i occurring in the d N/d E versus E spectrum measured on the mixture and S, is the corresponding elemental sensitivity. The latter quantity is given by [4] S, =
Heli/HelAg
(2)
9
height for element i as measured on a solid where He,, is the peak-to-peak quantity for the 351 eV silver piece of element i and HelAg the corresponding “ line”. Obviously, the peak-to-peak height can be written as H, = a h,ni
/0
aexp(
-x/h)
dx = CCh,n,X,
(3)
where a is a proportionality constant, hi the signal per atom of element i, n, the number density of element i in the mixture, x the coordinate perpendicular to the surface and h the escape depth. Similarly eq. (2) can be written as (4)
S, = (h,nel,heli)/(hAgnApXe,Ap). Elimination
of h, from eqs. (3) and (4) yields (5)
n, = (YH,)/S,(~.,/~,,,)~ where Y = (L/WWL&lAg) is a proportionality
constant
because
A,,, and A apply
to the same energy
so
J_ Hmker, H.J. H. Stofferen / M-type
332
and scandate cathodes
that h,,,/X = 1 (in first order the escape depth only depends on energy). According to eq. (5) the relative atomic concentration of element i in our homogeneous mixture is given by eq(1) if Si is replaced by !$. where
The correction factor density n is given by
n.,/n,,i
can easily be calculated
n = pN/M,
because
the number
(7)
where p is the specific weight, N Avogadro’s number and A4 the atomic weight. The question now is, of course, whether this correction is important. Considering the values of n.,/neu for Ni, W, SC, Ba and Cs - which are 0.64, 0.92, 1.46, 3.80 and 6.87, respectivelythe answer is clear. In particular in the examination of thermionic cathodes (Ba) and photocathodes (Cs), application of the number density correction will make sense.
3. Sub-monolayer cover, cathode models and data to be used in the evaluation The following simplified model will be used. For a substrate covered wit1 one monolayer the peak-to-peak heights for the cover and for the substrate art H, = ah,n,i2R’exp( H, = ah,n,
-x/X,)
O”exp( -x/X,) / 2R,
dx, dx,
(9)
where 2R, is the thickness of the layer. Thus, when Hcb is the peak-to-peak signal of a bulk consisting of the same material as the cover and Hsb is the peak-to-peak value for the uncovered substrate, we obtain
Ij m f4=Hsb 11
K = Kb
2R’exp( -x/X,)
dx
0
exp( -x/h,)
ZR,
dx
1
(10)
/A,.
01)
/A,,
1
It goes without saying how this procedure works in cases of partial cover, or of cover by two layers - or, e.g., for a combination of partial cover by one layer, partial cover by two layers and partial lack of cover. Both M-type and scandate cathodes contain tungsten in their substrate. Because of the evidence that at least on W the Ba will be situated on top of 0 if 0 is present [8], the same will be assumed for the cathodes to be examined. No preference for any of the substrate components is assumed. Hence, the cover will consist of Ba-0, Ba-0 + excess 0 or Ba-0 + excess Ba.
J. Hasker, H. J. H. Stoffelen
/ A4 type and scandate cathodes
333
Table 1 Sensitivity factors for 5 keV primary energy Element
Energy
s
n &net
s
0.93 0.82 0.80 3.81 1.09 0.56
0.074 0.048 0.011 0.457 0.437 0.082
(eV) W OS RU Ba 0 C
1736 1853 2260 584 510 272
0.080 0.058 0.014 0.120 0.400 0.140
[4] [4] [4] [4] [4] [4]
SC(in Sc,O,)
333
$ x 0.14 [6]
1.74
0.097
0 (in Sc,O,)
510
4 x 0.14 [6]
1.16
0.097
0.063 [4] 0.215 [4]
1.25 0.83
Al (in Al 203) 0 (in Al@,)
1385 510
??
0.079 0.179
For both Ba and 0 the value of 2 R, is taken equal0 to 2.8 A. The energy of the various “lines” is shown in table 1. Hence, X,. = 9 A (0 and Ba) and X, = 7 A (SC) or 17 A (W, OS and Ru) in the evaluation [9]. The spectra were measured with the aid of a single pass CMA (Physical Electronic Industries 10-150) with 5 keV primary beam energy, 3 PA beam current and - 0.6 mm spot diameter. To allow comparison with other work, all sensitivity factors with their references and number density corrections are summarized in table 1. Concerning this table a few remarks have to be made. The spectrum for Al,O, in ref. [4] applies to 3 keV primary energy. First, the sensitivity (S) for both Al and 0 was calculated from this spectrum. Next, the values found were multiplied by the same factors as the elemental values when going from 3 to 5 keV (these can be taken from ref. [4]). For Ba and 0 the elemental sensitivities are used (in contradistinction with the values which can be obtained from the bulk BaO spectrum). This is justified by the predominance of the metallic character exhibited by the low-energy Ba spectrum (5 80 eV) obtained in our experiments. The oxygen sensitivities of the oxides are only used to subtract the fractions belonging to S%O, and to the small amount of Al,O, from the oxygen signal measured on a scandate cathode. Before showing and discussing the results, some more information with respect to the examined cathodes will be provided.
4. Cathodes under examination The M-cathodes are conventional: after impregnation with the well-known 4-l-l composition, a tungsten plug has been provided with a sputtered OS-Ru layer with a thickness of about 5000 A. These plugs are welded to MO sleeves containing the heater.
J. Hasker, H.J. H. Stoffelen / M-type
334
and scandate cathodes
With respect to the scandate cathodes a few more remarks have to be made. Together with J. van Esdonk of the tube- and cathode-technological department of their laboratory, the first author of the present paper was confronted with the following scandate-cathode history a couple of years ago. The first scandate cathodes, as described in the patent of Figner (US No. 3,358,178), were pressed from a mixture of W powder and about 7% by weight Ba,Sc,O, powder and subsequently sintered. The first objective was to obtain a cathode being insensitive to moisture but, in addition, the emission proved to be surprisingly high. However, the plug remains porous, the more so because the sinter temperature has to be kept relatively low (- 1500°C) to avoid decomposition of the scandate. This results in a relatively short cathode life. The next step was thus to add Ba,Sc,O, or Sc,O, to the impregnant of a B-type cathode. This procedure’ has been described in patents by Koppius (US No. 3,719,856) and by Zalm and van Stratum (US No. 4,007,393). Though in general somewhat lower than for the pressed scandate cathode, the emission from this type of cathode is:much higher than from B- and M-type cathodes. However, after damage by ion sputtering there is little or no recovery. In view of these facts, we concluded that a cathode as shown schematically in fig. 1 should have the advantages of pressed and impregnated scandate cathodes with fewer if any of their disadvantages. The plug consists of about 0.4 mm W covered with a top layer of about 0.1 mm of W and about 5% by weight of Sc,O,. Pressing and sintering are performed in such a way that the porosity is the same as for usual B- and M-type cathodes. The plug can be impregnated without problems with, e.g., a 4-l-l or 5-3-2 impregnant without additives. Another advantage of the top layer structure is that it provides no problems with respect to the impregnant-tungsten chemistry. The surface roughness is approximately the same as for a B-matrix. Fig. 2 shows the current density (logarithmic scale) versus the square root of the anode voltage for both a top-layer scandate cathode and an M-cathode in diodes with about 0.3 mm distance between cathode and anode. The temperature is brightness temperature on MO and the measurement was performed in single-pulse operation. The emission capability of the scandate cathode can be seen clearly. With respect to the emission from the scandate cathode, it may not be concluded that-similar to Ba on SrO for the oxide-coated cathode [lo]-it
impregnant
Fig. 1. Schematic
representation
of the top-layer
scandate
cathode.
J. Hasker, H.J. H. Stoffelen
/ M-type
and scandate cathodes
335
t j(A/cm’)
10
30
20 V”2. V in volts
-
Fig. 2. Current density versus square root of anode voltage for both a top-layer scandate cathode and an OS-Ru coated cathode in diode configuration with a distance of about 0.3 mm between cathode and anode.
originates from (sub)monolayer cover of Ba on bulk Sc,O,. This has been shown by spraying usual cathode-Ni with a mixture of BaCO, and Sc,O, in a binder and by processing and treating this system as a normal oxide-coated cathode. Moreover, it seems essential for high emission to have W in the surface: the emission of a B-cathode sputtered with a SC layer (which will at least be partly oxidized by the impregnant) is much lower than that of the top-layer cathodes. It is noted that Auger analysis of an activated SC-sputtered cathode did not show W in the SC layer. These experiments indicate that the emission must be due to Ba-0 and/or Ba on W. Then the question concerning the role of Sc,O, remains. In our opinion the scandia regulates the oxygen concentration on the surface. This view is supported by the fact that in a mixture of W, WO, and Sc,O, the scandia can reduce WO, with the formation of Sc,WO,, [ll] and by the results to be discussed in the next section of the present paper. Finally, it may be useful to remark that a batch of 10 toplayer cathodes has been operated at 950°C MO temperature in diodes with 0.3 mm cathode to anode distance under a continuous load of 1 A/cm2 for more than four years (i.e., about 39,000 h) without significant degradation of the emission as measured from time to time with a 1000 V pulse.
336
J. Hasker, H.J. H. Stojjelen / M-type
and scandate cathodes
5. Results and discussion To start with, three M-cathodes operated in diodes at 1 A/cm2 and a cathode temperature of 1050°C have been examined. Immediately after activation the current-voltage characteristics of these cathodes were similar. Cathodes M, and M, have been operated for 100 and 1250 h, respectively, both without degradation as measured with a 1000 V pulse. Cathode M, has been operated for 650 h with a degradation in current at V= 1000 V by a factor of about 4. It must be noted that the latter is very unusual. The cathodes were taken from the tubes, transported through air and reactivated in the Auger system. At that time the emission could not be measured in the system. Apart from the Ba-0 concentration, the cathodes showed a substantial concentration of excess 0. In all cases the Ba-0 cover was the same (16%) while for M, and M, the ratio [excess O]/[Ba-0] was equal to 2 and for M, equal to 4. The latter high value may be related to the low emission from M,. It must be remarked, however, that - for some unknown reason - the substrate of this cathode showed a lack of Ru. Moreover, it is clearly possible that (some of the) excess oxygen is so situated that it does not reduce the emission. After the above measurements a fresh, i.e. not activated, cathode M, was put into the Auger system and its Auger spectrum was measured after activation and emission monitoring at 1050°C. This spectrum is shown in fig. 3. The ratio [excess OJ/[Ba-0] was again equal to 2. However, the level of Ba-0 cover was about 50% higher than for M,, M, and M,. After a relative short exposure to air and subsequent reactivation and emission control the Ba-0 cover was found to be 19%. Obviously, the results presented above deviate markedly from accepted views of the M-type cathode (as discussed in section 1): the Ba-0 cover has found to be much less than - 1008, i.e. 2058, and there is a substantial concentration of excess oxygen. This is due to the number density correction discussed in section 2. As an interesting exercise, we calculated that without the number density correction the result for M, before its exposure to air is: 87% Ba-0 cover and no excess oxygen on the surface. This result agrees fairly well with the current picture of an M-type cathode. It must be noted that our conclusion concerning a substantial excess oxygen besides the Ba-0 concentration applies within the framework of sensitivity values used in the calculations (table 1). Though these are fairly generally accepted and used, the value for oxygen in particular is subject to criticism. Taking the sensitivity values in table 1 for granted, our conclusion can also be stated and discussed as follows. The oxygen to barium peak-to-peak height ratio of nearly 2 : 1 in the spectrum of fig. 3 does not mean that the oxygen to barium particle ratio is 1 : 1 (the ratio calculated from this spectrum for cathode M, is 3 : 1). This is so in spite of the fact that an oxygen to barium peak-to-peak height ratio of 2 : 1 is also found for bulk BaO [5]. It is noted that
J. Hasker, H.J. H. Stoffeien
/ M-type
and scandate cathodes
331
t1
i!
eV)
l(
Fig. 3. Auger spectrum brightness).
1500 Electron of a normal
/ 2000 energy [eVI OS-Ru
coated
cathode,
after activation,
at T = 1320 K (MO
a Ba-0 cover of about 20%, as found above, does not necessarily imply that Ba is not distributed homogeneously over the cathode surface: the distance between adjacent Ba atoms may be greater than in bulk Ba. Moreover, a Ba atom needs not be conventionally married to one specific oxygen atom (as our notation might suggest). The scandate top-layer cathode was mounted directly, i.e. before activation, into the Auger system. After activation and emission control at 950°C the Auger measurements were performed. The spectrum is shown in fig. 4. First, the fractions belonging to Sc,O, and Al,O, were subtracted from the oxygen signal. It is noted that, because the Sc,O, contribution is substantial, the accuracy in the remaining oxygen will be relatively low. The result of further analysis is a Ba-0 cover of about 15% and about the same amount of excess
J. Hasker, H.J. H. Stojjelen / M -
338
t dN dE
C
0 (5JOeVl 500
1000
W (1736eVl 1500 Electron
2000 energy [eVl +
Fig. 4. Auger spectrum of a top-layer scandate cathode, after activation, at T=1220 brightness).
K (MO
Ba. Thus - compared with the M-cathode - there is certainly no excess oxygen, and the total amount of Ba is about 50% higher. This may account for the higher emission of the scandate cathode. However, in this comparison it should be borne in mind that the substrates are also different. The above results and discussion for M-type and scandate cathodes were presented in summary form at the 1984 Tri-Service Cathode Workshop. Obviously, the role of excess oxygen in the emission behavior has to be examined in more detail. For this and other reasons a scanning-Auger program on scandate cathodes has recently been started in cooperation with J.E. Crombeen of our laboratory. Pressed, pressed + sintered and pressed + sintered + impregnated plugs will be examined before considering complete cathodes. On a pressed W + Sc,O, plug the Sc,O, spectrum has already been measured on a small grain without spurious charging. According to this measurement the oxygen to scandium peak-to-peak height ratio is 1.1. This is
J. Hasker, H.J.H. Stoffelen / M-type and scandate cathodes
339
very different from the 1.5 ratio taken from ref. [6] which we used in the above evaluation. In addition, the SCsensitivity found via a measurement on tungsten was much higher than in ref. [6]. The following may provide some confidence with respect to these new values: when the BaO spectrum [5] is substracted from the Ba,Sc,O, spectrum in ref. [6], the remaining oxygen to scandium peak-to-peak height ratio is close to 1. Use of the new values in the evaluation for the scandate cathode gives results which are of course different from those presented at the Workshop. Compared with the latter, the new total Ba concentration is about 50% greater and now- again apart from the Ba-0 concentration- there is excess oxygen. However, the latter is smaller than for M-type cathodes: according to the calculations the ratio [excess O]/[Ba-0] = 1. So, in the comparison with the M-type cathode the trend has remained the same. The scandate cathode shows a higher Ba concentration and, relatively, much less excess oxygen. Nevertheless, improvement of the standards and further examination of reproducibility will be necessary. Finally, it may be useful to remark that - qualitatively - our main results, i.e. substantial excess oxygen on M-cathodes and much less excess oxygen on scandate cathodes, would remain the same when a coplanar [12] rather than a stacked surface structure would be used in the calculations.
References [l] [2] [3] [4]
1978 Tri-Service Cathode Workshop, Appl. Surface Sci. 2, No. 2 (1979).
1980 T&Service Cathode Workshop, Appl. Surface Sci. 8, Nos. l/2 (1981). 1982 Tri-Service Cathode Workshop, Appl. Surface Sci. 16, Nos. l/2 (1983). L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy, 2nd ed. (Physical Electronics Industries, Eden Prairie, MN, 1976). [5] A. Shih, C. Hor and G.A. Haas, Appl. Surface Sci. 2 (1979) 112. (61 A. van Oostrom and L. Augustus, Appl. Surface Sci. 2 (1979) 173. [7] R. Shimizu, Japan. J. Appl. Phys. 22 (1983) 1631. [8] R. Forman, Appl. Surface Sci. 2 (1979) 258. [9] A.W. Czanderna, Ed., Methods of Surface Analysis (Elsevier, Amsterdam 1975). [IO] P. Zalm, in: Advances in Electronics and Electron Physics, Vol. 25 (Academic Press, New York, 1968). [11] V.A. Levitakii, Inorg. Mater. 16 (1981) 1489. [12] G.A. Haas, A. Shih and C.R.K. Marrian, Appl. Surface Sci. 16 (1983) 139.
330
“ALTERNATIVE” AUGER ANALYSIS REVEALS IMPORTANT PROPERTIES OF M-TYPE AND SCANDATE CATHODES J. HASKER
and H.J.H.
Philips Research Laboratories, Received
STOFFELEN 5600 JA Eindhoven,
5 May 1984; accepted for publication
The Netherlands
12 March 1985
It is shown that in the evaluation of Auger spectra, as measured on cathode surfaces, the elemental sensitivities have to be corrected for differences in elemental number density. The substrate and the surface covering material have to be treated differently in the analysis. The result for normal M-type cathodes is that-besides the Ba-0 cover - there is an excess oxygen concentration on the surface of about two times the Ba-0 concentration. This in spite of the fact that the oxygen to barium peak-to-peak height ratio in the measured spectra is about equal to 2. For a degraded M-cathode the excess oxygen level is found to be much higher than for the normal cathodes. A new type of scandate cathode is described and discussed. Its analysis shows more Ba and, relatively, much less excess oxygen than for the M-type cathodes. This may account for the substantially higher emission.
1. Introduction In the mutual comparison of cathodes, and for better understanding of emission properties, both the composition and the concentration of the surface covering material are important. For this reason there have already been many valuable contributions on the application of surface science to cathodes, some of which appeared in the Proceedings of the previous “T&Service Cathode Workshops” [l-3]. Among the various techniques Auger Electron Spectroscopy plays an important role. It is the purpose of the present paper to contribute to improvement of the evaluation of measured Auger spectra. The results will be applied to M-cathodes and to a new type of scandate cathode. According to ref. [3] (pages 21-23), a B-type cathode has about 0.7 monolayer of Ba-0 with some areas of the tungsten matrix covered by oxygen alone, while an M-type has a few percent more Ba. According to ref. [3] (page 55) the M-type may have about 20% more Ba. Thus an M-type cathode is currently thought to have a Ba-0 cover of about a monolayer. Even more certain is the conclusion that the O/Ba atomic ratio on its surface is close to unity. In the evaluation referred to above use is made of elemental Auger sensitivity factors [4-61. In doing so, differences in back scattering and elemental 0169-4332/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J. Hasker, H. J. H. Stoffelen / M-type
and scandate cathodes
331
number densitieswhich, according to analysis from first principles (see e.g. ref. [7]) have to be taken into account - are left out of consideration. Considering the uncertainties with respect to, e.g., the sensitivity factors, the differences in back scattering are of minor importance. However, it will be shown in the next section that the number density correction is very important, in particular when Ba is involved. This gives rise to a substantial increase in the O/Ba ratio as calculated from measured spectra.
2.
Evaluation of relative atomic concentrations from the Auger spectrum
In this section a homogeneous mixture of m componentsas may be in the substrate of a dispenser cathodewill be considered (the next section deals with the covering material which has to be treated differently). It is rather common practice to calculate the relative concentration ci of component i in the mixture from
0) where Hi is the peak-to-peak height of any selected f‘line” of element i occurring in the d N/d E versus E spectrum measured on the mixture and S, is the corresponding elemental sensitivity. The latter quantity is given by [4] S, =
Heli/HelAg
(2)
9
height for element i as measured on a solid where He,, is the peak-to-peak quantity for the 351 eV silver piece of element i and HelAg the corresponding “ line”. Obviously, the peak-to-peak height can be written as H, = a h,ni
/0
aexp(
-x/h)
dx = CCh,n,X,
(3)
where a is a proportionality constant, hi the signal per atom of element i, n, the number density of element i in the mixture, x the coordinate perpendicular to the surface and h the escape depth. Similarly eq. (2) can be written as (4)
S, = (h,nel,heli)/(hAgnApXe,Ap). Elimination
of h, from eqs. (3) and (4) yields (5)
n, = (YH,)/S,(~.,/~,,,)~ where Y = (L/WWL&lAg) is a proportionality
constant
because
A,,, and A apply
to the same energy
so
J_ Hmker, H.J. H. Stofferen / M-type
332
and scandate cathodes
that h,,,/X = 1 (in first order the escape depth only depends on energy). According to eq. (5) the relative atomic concentration of element i in our homogeneous mixture is given by eq(1) if Si is replaced by !$. where
The correction factor density n is given by
n.,/n,,i
can easily be calculated
n = pN/M,
because
the number
(7)
where p is the specific weight, N Avogadro’s number and A4 the atomic weight. The question now is, of course, whether this correction is important. Considering the values of n.,/neu for Ni, W, SC, Ba and Cs - which are 0.64, 0.92, 1.46, 3.80 and 6.87, respectivelythe answer is clear. In particular in the examination of thermionic cathodes (Ba) and photocathodes (Cs), application of the number density correction will make sense.
3. Sub-monolayer cover, cathode models and data to be used in the evaluation The following simplified model will be used. For a substrate covered wit1 one monolayer the peak-to-peak heights for the cover and for the substrate art H, = ah,n,i2R’exp( H, = ah,n,
-x/X,)
O”exp( -x/X,) / 2R,
dx, dx,
(9)
where 2R, is the thickness of the layer. Thus, when Hcb is the peak-to-peak signal of a bulk consisting of the same material as the cover and Hsb is the peak-to-peak value for the uncovered substrate, we obtain
Ij m f4=Hsb 11
K = Kb
2R’exp( -x/X,)
dx
0
exp( -x/h,)
ZR,
dx
1
(10)
/A,.
01)
/A,,
1
It goes without saying how this procedure works in cases of partial cover, or of cover by two layers - or, e.g., for a combination of partial cover by one layer, partial cover by two layers and partial lack of cover. Both M-type and scandate cathodes contain tungsten in their substrate. Because of the evidence that at least on W the Ba will be situated on top of 0 if 0 is present [8], the same will be assumed for the cathodes to be examined. No preference for any of the substrate components is assumed. Hence, the cover will consist of Ba-0, Ba-0 + excess 0 or Ba-0 + excess Ba.
J. Hasker, H. J. H. Stoffelen
/ A4 type and scandate cathodes
333
Table 1 Sensitivity factors for 5 keV primary energy Element
Energy
s
n &net
s
0.93 0.82 0.80 3.81 1.09 0.56
0.074 0.048 0.011 0.457 0.437 0.082
(eV) W OS RU Ba 0 C
1736 1853 2260 584 510 272
0.080 0.058 0.014 0.120 0.400 0.140
[4] [4] [4] [4] [4] [4]
SC(in Sc,O,)
333
$ x 0.14 [6]
1.74
0.097
0 (in Sc,O,)
510
4 x 0.14 [6]
1.16
0.097
0.063 [4] 0.215 [4]
1.25 0.83
Al (in Al 203) 0 (in Al@,)
1385 510
??
0.079 0.179
For both Ba and 0 the value of 2 R, is taken equal0 to 2.8 A. The energy of the various “lines” is shown in table 1. Hence, X,. = 9 A (0 and Ba) and X, = 7 A (SC) or 17 A (W, OS and Ru) in the evaluation [9]. The spectra were measured with the aid of a single pass CMA (Physical Electronic Industries 10-150) with 5 keV primary beam energy, 3 PA beam current and - 0.6 mm spot diameter. To allow comparison with other work, all sensitivity factors with their references and number density corrections are summarized in table 1. Concerning this table a few remarks have to be made. The spectrum for Al,O, in ref. [4] applies to 3 keV primary energy. First, the sensitivity (S) for both Al and 0 was calculated from this spectrum. Next, the values found were multiplied by the same factors as the elemental values when going from 3 to 5 keV (these can be taken from ref. [4]). For Ba and 0 the elemental sensitivities are used (in contradistinction with the values which can be obtained from the bulk BaO spectrum). This is justified by the predominance of the metallic character exhibited by the low-energy Ba spectrum (5 80 eV) obtained in our experiments. The oxygen sensitivities of the oxides are only used to subtract the fractions belonging to S%O, and to the small amount of Al,O, from the oxygen signal measured on a scandate cathode. Before showing and discussing the results, some more information with respect to the examined cathodes will be provided.
4. Cathodes under examination The M-cathodes are conventional: after impregnation with the well-known 4-l-l composition, a tungsten plug has been provided with a sputtered OS-Ru layer with a thickness of about 5000 A. These plugs are welded to MO sleeves containing the heater.
J. Hasker, H.J. H. Stoffelen / M-type
334
and scandate cathodes
With respect to the scandate cathodes a few more remarks have to be made. Together with J. van Esdonk of the tube- and cathode-technological department of their laboratory, the first author of the present paper was confronted with the following scandate-cathode history a couple of years ago. The first scandate cathodes, as described in the patent of Figner (US No. 3,358,178), were pressed from a mixture of W powder and about 7% by weight Ba,Sc,O, powder and subsequently sintered. The first objective was to obtain a cathode being insensitive to moisture but, in addition, the emission proved to be surprisingly high. However, the plug remains porous, the more so because the sinter temperature has to be kept relatively low (- 1500°C) to avoid decomposition of the scandate. This results in a relatively short cathode life. The next step was thus to add Ba,Sc,O, or Sc,O, to the impregnant of a B-type cathode. This procedure’ has been described in patents by Koppius (US No. 3,719,856) and by Zalm and van Stratum (US No. 4,007,393). Though in general somewhat lower than for the pressed scandate cathode, the emission from this type of cathode is:much higher than from B- and M-type cathodes. However, after damage by ion sputtering there is little or no recovery. In view of these facts, we concluded that a cathode as shown schematically in fig. 1 should have the advantages of pressed and impregnated scandate cathodes with fewer if any of their disadvantages. The plug consists of about 0.4 mm W covered with a top layer of about 0.1 mm of W and about 5% by weight of Sc,O,. Pressing and sintering are performed in such a way that the porosity is the same as for usual B- and M-type cathodes. The plug can be impregnated without problems with, e.g., a 4-l-l or 5-3-2 impregnant without additives. Another advantage of the top layer structure is that it provides no problems with respect to the impregnant-tungsten chemistry. The surface roughness is approximately the same as for a B-matrix. Fig. 2 shows the current density (logarithmic scale) versus the square root of the anode voltage for both a top-layer scandate cathode and an M-cathode in diodes with about 0.3 mm distance between cathode and anode. The temperature is brightness temperature on MO and the measurement was performed in single-pulse operation. The emission capability of the scandate cathode can be seen clearly. With respect to the emission from the scandate cathode, it may not be concluded that-similar to Ba on SrO for the oxide-coated cathode [lo]-it
impregnant
Fig. 1. Schematic
representation
of the top-layer
scandate
cathode.
J. Hasker, H.J. H. Stoffelen
/ M-type
and scandate cathodes
335
t j(A/cm’)
10
30
20 V”2. V in volts
-
Fig. 2. Current density versus square root of anode voltage for both a top-layer scandate cathode and an OS-Ru coated cathode in diode configuration with a distance of about 0.3 mm between cathode and anode.
originates from (sub)monolayer cover of Ba on bulk Sc,O,. This has been shown by spraying usual cathode-Ni with a mixture of BaCO, and Sc,O, in a binder and by processing and treating this system as a normal oxide-coated cathode. Moreover, it seems essential for high emission to have W in the surface: the emission of a B-cathode sputtered with a SC layer (which will at least be partly oxidized by the impregnant) is much lower than that of the top-layer cathodes. It is noted that Auger analysis of an activated SC-sputtered cathode did not show W in the SC layer. These experiments indicate that the emission must be due to Ba-0 and/or Ba on W. Then the question concerning the role of Sc,O, remains. In our opinion the scandia regulates the oxygen concentration on the surface. This view is supported by the fact that in a mixture of W, WO, and Sc,O, the scandia can reduce WO, with the formation of Sc,WO,, [ll] and by the results to be discussed in the next section of the present paper. Finally, it may be useful to remark that a batch of 10 toplayer cathodes has been operated at 950°C MO temperature in diodes with 0.3 mm cathode to anode distance under a continuous load of 1 A/cm2 for more than four years (i.e., about 39,000 h) without significant degradation of the emission as measured from time to time with a 1000 V pulse.
336
J. Hasker, H.J. H. Stojjelen / M-type
and scandate cathodes
5. Results and discussion To start with, three M-cathodes operated in diodes at 1 A/cm2 and a cathode temperature of 1050°C have been examined. Immediately after activation the current-voltage characteristics of these cathodes were similar. Cathodes M, and M, have been operated for 100 and 1250 h, respectively, both without degradation as measured with a 1000 V pulse. Cathode M, has been operated for 650 h with a degradation in current at V= 1000 V by a factor of about 4. It must be noted that the latter is very unusual. The cathodes were taken from the tubes, transported through air and reactivated in the Auger system. At that time the emission could not be measured in the system. Apart from the Ba-0 concentration, the cathodes showed a substantial concentration of excess 0. In all cases the Ba-0 cover was the same (16%) while for M, and M, the ratio [excess O]/[Ba-0] was equal to 2 and for M, equal to 4. The latter high value may be related to the low emission from M,. It must be remarked, however, that - for some unknown reason - the substrate of this cathode showed a lack of Ru. Moreover, it is clearly possible that (some of the) excess oxygen is so situated that it does not reduce the emission. After the above measurements a fresh, i.e. not activated, cathode M, was put into the Auger system and its Auger spectrum was measured after activation and emission monitoring at 1050°C. This spectrum is shown in fig. 3. The ratio [excess OJ/[Ba-0] was again equal to 2. However, the level of Ba-0 cover was about 50% higher than for M,, M, and M,. After a relative short exposure to air and subsequent reactivation and emission control the Ba-0 cover was found to be 19%. Obviously, the results presented above deviate markedly from accepted views of the M-type cathode (as discussed in section 1): the Ba-0 cover has found to be much less than - 1008, i.e. 2058, and there is a substantial concentration of excess oxygen. This is due to the number density correction discussed in section 2. As an interesting exercise, we calculated that without the number density correction the result for M, before its exposure to air is: 87% Ba-0 cover and no excess oxygen on the surface. This result agrees fairly well with the current picture of an M-type cathode. It must be noted that our conclusion concerning a substantial excess oxygen besides the Ba-0 concentration applies within the framework of sensitivity values used in the calculations (table 1). Though these are fairly generally accepted and used, the value for oxygen in particular is subject to criticism. Taking the sensitivity values in table 1 for granted, our conclusion can also be stated and discussed as follows. The oxygen to barium peak-to-peak height ratio of nearly 2 : 1 in the spectrum of fig. 3 does not mean that the oxygen to barium particle ratio is 1 : 1 (the ratio calculated from this spectrum for cathode M, is 3 : 1). This is so in spite of the fact that an oxygen to barium peak-to-peak height ratio of 2 : 1 is also found for bulk BaO [5]. It is noted that
J. Hasker, H.J. H. Stoffeien
/ M-type
and scandate cathodes
331
t1
i!
eV)
l(
Fig. 3. Auger spectrum brightness).
1500 Electron of a normal
/ 2000 energy [eVI OS-Ru
coated
cathode,
after activation,
at T = 1320 K (MO
a Ba-0 cover of about 20%, as found above, does not necessarily imply that Ba is not distributed homogeneously over the cathode surface: the distance between adjacent Ba atoms may be greater than in bulk Ba. Moreover, a Ba atom needs not be conventionally married to one specific oxygen atom (as our notation might suggest). The scandate top-layer cathode was mounted directly, i.e. before activation, into the Auger system. After activation and emission control at 950°C the Auger measurements were performed. The spectrum is shown in fig. 4. First, the fractions belonging to Sc,O, and Al,O, were subtracted from the oxygen signal. It is noted that, because the Sc,O, contribution is substantial, the accuracy in the remaining oxygen will be relatively low. The result of further analysis is a Ba-0 cover of about 15% and about the same amount of excess
J. Hasker, H.J. H. Stojjelen / M -
338
t dN dE
C
0 (5JOeVl 500
1000
W (1736eVl 1500 Electron
2000 energy [eVl +
Fig. 4. Auger spectrum of a top-layer scandate cathode, after activation, at T=1220 brightness).
K (MO
Ba. Thus - compared with the M-cathode - there is certainly no excess oxygen, and the total amount of Ba is about 50% higher. This may account for the higher emission of the scandate cathode. However, in this comparison it should be borne in mind that the substrates are also different. The above results and discussion for M-type and scandate cathodes were presented in summary form at the 1984 Tri-Service Cathode Workshop. Obviously, the role of excess oxygen in the emission behavior has to be examined in more detail. For this and other reasons a scanning-Auger program on scandate cathodes has recently been started in cooperation with J.E. Crombeen of our laboratory. Pressed, pressed + sintered and pressed + sintered + impregnated plugs will be examined before considering complete cathodes. On a pressed W + Sc,O, plug the Sc,O, spectrum has already been measured on a small grain without spurious charging. According to this measurement the oxygen to scandium peak-to-peak height ratio is 1.1. This is
J. Hasker, H.J.H. Stoffelen / M-type and scandate cathodes
339
very different from the 1.5 ratio taken from ref. [6] which we used in the above evaluation. In addition, the SCsensitivity found via a measurement on tungsten was much higher than in ref. [6]. The following may provide some confidence with respect to these new values: when the BaO spectrum [5] is substracted from the Ba,Sc,O, spectrum in ref. [6], the remaining oxygen to scandium peak-to-peak height ratio is close to 1. Use of the new values in the evaluation for the scandate cathode gives results which are of course different from those presented at the Workshop. Compared with the latter, the new total Ba concentration is about 50% greater and now- again apart from the Ba-0 concentration- there is excess oxygen. However, the latter is smaller than for M-type cathodes: according to the calculations the ratio [excess O]/[Ba-0] = 1. So, in the comparison with the M-type cathode the trend has remained the same. The scandate cathode shows a higher Ba concentration and, relatively, much less excess oxygen. Nevertheless, improvement of the standards and further examination of reproducibility will be necessary. Finally, it may be useful to remark that - qualitatively - our main results, i.e. substantial excess oxygen on M-cathodes and much less excess oxygen on scandate cathodes, would remain the same when a coplanar [12] rather than a stacked surface structure would be used in the calculations.
References [l] [2] [3] [4]
1978 Tri-Service Cathode Workshop, Appl. Surface Sci. 2, No. 2 (1979).
1980 T&Service Cathode Workshop, Appl. Surface Sci. 8, Nos. l/2 (1981). 1982 Tri-Service Cathode Workshop, Appl. Surface Sci. 16, Nos. l/2 (1983). L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy, 2nd ed. (Physical Electronics Industries, Eden Prairie, MN, 1976). [5] A. Shih, C. Hor and G.A. Haas, Appl. Surface Sci. 2 (1979) 112. (61 A. van Oostrom and L. Augustus, Appl. Surface Sci. 2 (1979) 173. [7] R. Shimizu, Japan. J. Appl. Phys. 22 (1983) 1631. [8] R. Forman, Appl. Surface Sci. 2 (1979) 258. [9] A.W. Czanderna, Ed., Methods of Surface Analysis (Elsevier, Amsterdam 1975). [IO] P. Zalm, in: Advances in Electronics and Electron Physics, Vol. 25 (Academic Press, New York, 1968). [11] V.A. Levitakii, Inorg. Mater. 16 (1981) 1489. [12] G.A. Haas, A. Shih and C.R.K. Marrian, Appl. Surface Sci. 16 (1983) 139.