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Current fluctuation of thermal field emission from tungsten emitter
Applications of Surface Science 5 (1980) 374—387 ©North-l-lolland Publishing Company
CURRENT FLUCTUATION OF THERMAL FIELD EMISSION FROM TUNGSTEN EMITTER Norio SAITOU and Shigehiko YAMAMOTO c~entralResearch Laboratory, Hitachi Ltd., Kokuhunji, Tokyo, 185 Japan
Received 15 August 1979 Revised manuscript received 14 January 1980
Field emission characteristics from heated tungsten emitter were experimentally investigated in several ambient gases. The emission current is stabilized by heat treatment at about 900°Cin CH 4. The relative fluctuation is about 2%. The observation of FE pattern shows that tungsten—carbon complex .structure is formed and kept for long time at 900’~C.The local probe currents from several planes of built-up W tip were measured. The current stability does not depend on the kind of ambient gases and pressure. The power spectrum density of built-up planes is usually 10 to 102 times larger than that of non-built-up plane throughout the frequency range below 500 Hz.
1. Introduction Field emission (FE) electron sources have attracted considerable attention to electron beam devices due to its high brightness and small source size. In 1970, Crewe [1] successfully observed an atom image in a scanning electron microscope with an FE electron gun. The important factors in practical application of FE gun are the current level and its stability. Many investigators [2,3] have been trying to find better tip materials and more stable operating conditions. Thermal field emission from tungsten emitters seems to be one of the most promissing electron sources for such devices as electron beam fabrication niachines and Auger electron microscope. There is, however, little published data on the current fluctuation of thermal field emission. Moreover, the mechanism of the fluctuation is not clear, either. In line with this, the field emission characteristics of single crystal tungsten (W) emitter have been experinientally studied at various temperatures in several ambient gases.
2. Experimental apparatus The experimental vacuum chamber was made of stainless steel (304). It was 30 cm in diameter and 25 cm in height. The chamber was evacuated by a 110 1/s
N. Saitou, S. Yamamoto
/ Thermal field emission
375
Tip Heater
1
Tip Tilting Mechanism
1Recorder I I kS2
I
Viewing
Fluor:sent
~I!__~.
I~ TotaL Current Emission 1 Local Probe Current
FE Tip
Pump
Vacuum Chamber
Anode
Micro-
Foradoy Cup
Ammeter
Recorder
Pure Gas Inlets Mass FiLter
Fig. 1. Schematic diagram of the experimental apparatus.
ion pump and the final pressure was maintained at 7 X 10_8 Pa (S X 10.10 Torr). Fig. 1 shows the schematic diagram of the experimental apparatus. A field emitter was heated by a stable dc current and the temperature was measured by an optical pyrometer. The temperature was corrected by taking the emissivity of 0.3 into consideration. The error is within ±30°C. Tungsten (W) single crystal, oriented to (310) direction, was used as a tip. Just immediately after observing the clean W FE pattern by tip flashing (2000°C, for 1—2 s), the tip was heated to the desired temperature and then all the measurements were started. A flat anode, which was coated with fluorescent phosphor, permitted the observation of FE pattern from the window and the measurement of total emission current, ‘E• The local probe current, I~,which is emitted from a special plane on the tip was measured by a Faraday cage under the anode. A tip tilting mechanism made it possible to change the measured crystal plane. The aperture angle 2a which is defined in fig. 1 was selected to be usually 18 mrad except for fig. 11 (2a = 75 mrad). This value is normally close to that used in scanning electron microscope. In this paper the angular confinement is defined by ‘p/’E for this aperture angle. The ‘E was recorded on a recorder chart (the frequency response is less than 10 Hz). The I~was either recorded on a recorder chart to measure the long term drift and relatively low frequency fluctuation or fed into a power spectrum analyzer (UA.
N. Saitou, S. Yamamoto / Thermal field emission
376
500, Federal Scientific Corporation) to measure the higher frequency fluctuation. The relative fluctuation of ‘F and I~are defined by ~Ji;/Ji. and ~ respectively, where ~ and ~ are the peak to peak fluctuation on the recorder chart over 1 mm recorded time. A cylindrical anode with a closed end was used for high current experiment, in which it is necessary to avoid the electron scattering. This type of anode can confine the electron inside the- cylinder. Several kinds of pure gases were introduced into the vacuum system through variable leak valves to change the ambient atmosphere. The gas components were always monitored by a quadrupole mass spectrometer.
3. Experimental results 3.]. Desorbedgas analysis In fig. 2, the dark portions are the residual background gas at pressure Pi
=
7X
10—8 Pa, when electrons are not emitted. Field emission electrons desorb some gases
from the anode or the wall of the vacuum system if the system is baked incompletely. The kind of desorbed gases were analyzed when 10 pA FE electrons were
5
Anode
Stao~less Steel
7 Pa FE on ( Pf= 3x10 FE off I Pi 7x 108 Pa
f 10
,
Some gases are desorbed I
,
Background gas I
Co 0
Co 2 5 H 20
0
10
20
30
40
50
60
Mass Number Fig. 2. Residual gas and desorbed gas analysis by a quadrupole mass spectrometer.
N. Saitou, S. Yamamoto / Thermal field emission
377
emitted. The pressure p~ became 3 X l0~ Pa. The white portions show the desorbed gases, when the anode is pure stainless steel. Fig. 2 shows that the main gas component is H 2 in UHV and CO, CO2 and H2 and some hydrocarbon with the mass number of about 40 and 56 are mainly desorbed from the vacuum system by electron bombardment. The components of the desorbed gases were almost the same when phosphor coated stainless anode was used. 3.2. Temperature dependence of field emission characteristics The temperature dependence of the FE pattern and the total emission current was investigated in an incompletely outgassed vacuum chamber. Fig. 3 shows the result taken at 1 h after tip flashing. The ‘E was kept to 10 pA. The pressure was 10 X i0~ Pa during the experiment. When the temperature was lower than 900°C,the ambient gases were adsorbed on a tip surface and the FE pattern became granular. Operation above 900°Cgave rise to the built-up of some planes on the W surface. At 1150°C{310} planes were built-up and at 1400°C the (100) plane was built.up. At higher than 1400°C, thermionic electrons were emitted. At 900°C a peculiar FE pattern appeared and the emission current was very stable. The temperature dependence of the relative
Clean Pattern Right after Ftasr’j~,q
[‘1
FEc~a~’er~
Temperature
floor,, Tema
~ ..-6OO~C
19 ~ —900C
~.
ll5O~C
~t4QO~C
Fig. 3. Temperature dependence of the field emission pattern and current stability for a (310) oriented tungsten tip. The initial pressure, p~= 1.3 X i0~ Pa, and final pressure Pf = 10 X i0~ Pa.
378
N. Saitou, S. Yainamoto
0.1
/ Thermal field emission
~
1005
0
0
500
1Q00
1500
Tip Temperature (°C) Fig. 4. Temperature dependence of relative fluctuation ~Iy/IF.
fluctuation ~Jl../IF is shown by the solid line in fig. 4. At room temperature and built-up temperature, the ‘~.IE/IEis larger than 4-—5%, but it is l—2% at 900°C. Under rigorous outgassing of the anode and the vacuum chamber, the ~.JI./Il.’ changed monotonously with temperature as shown by the dashed curve in fig. 4. In this experiment the cylindrical, type anode was used. The two curves indicate that some kind of gases previously analyzed, contributed to the stabilization of the emission current and the appearance of the peculiar FE pattern.
3.3. Field emission characteristics in specified ambient gas All the experiments in this section were done in a rigorously baked vacuum chamber. The anode was outgassed by electron bombardment. The amount of desorbed gas was reduced to 10% of that showed in fig. 2. The total emission current was usually kept under 1 pA to keep the desorbed gas as low as possible. To control the atmosphere, C0 2, H2, CH4 and CO gases were introduced into the vacuum chamber. The pressure of gas introdced was 8—10 X i0~ Pa. The local probe current was always kept at a dc level of lnA by changing the voltage applied to the tip. Field emission characteristics were observed at room temperature, 900°C and 1400°Cin the above ambient gases.
3.3.]. Field emission at room temperature and 1400°C (])FE pattern FE patterns were observed in several ambient gases at room temperature and
N. Saitou, S. Yamamoto
~
/ Thermal field emission
Clean Pattern
Leak Gas
14
(.~‘
°C
379
E~
cH4
Co
Fig. 5. Field emission pattern at room temperature and 1400°Cin several ambient gases.
1400°C. FE patterns shown in fig. S were taken at 1 h after flashing process. The photographs were taken from the slanting direction, about 45 degrees. At room temperature, all the FE patterns were granular. The ambient gases were adsorbed on the tip surface. At 1400°C,the patterns depended upon the gas present. In CO2, the (100) plane was built-up easily. But in H2, CH4 and CO, the {310} planes were built.up firstly and then (100) build-up followed. The (100) build-up occurring in CH4 may be due to a small amount of CO and CO2 residual gases.
(2) Relative fluctuation and angular confinement The long term drift of the (310) local probe current was recorded on a recorder chart. Each data in fig. 6 corresponds to the FE pattern in fig. 5. At room temperature in H2, and no introduction of gas (main component is H2), the M1,/I~is as small as 2% but the dclevel drift is observed. In CO2 or CO,I~,changes slowly with the period of 1—2 mm [4]. At 1400 C, the fluctuations in H2, CH4 and CO resemble each other. The is 15—20% and does not show an ambient gas dependence. The angular confinement was 1 X io~ for 2a = 18 mrad. This is almost the same with clean W(310) at room temperature. In case of no introduction of gas, and for CO2, the i~sI~/I~ is relatively small, 3 to 6%.
The relative fluctuation of the (100) local current is shown in fig. 7. Though the angular confinement is 1—2 X 10_2 the i~J~/I~ of the (100) plane of the (100) built-up tip is larger than 30%, but that of the (310) built-up tip is only 3%. it is seen that if one plane is built-up, the relative fluctuation ~ does not depend on the kind of ambient gas and its pressure.
380
/ Thermal field
N. Saitou, S. Yamamoto Introduced Gas
co2
non
__
Pa Pressure
H2 T
I— 2 6x10°
emission
CH4
9slO~
CO
9x)Q7
8a10°
B— iOxiO
atI4OOCIi~r~
-
~..—5 min-~
~11
at Room
L
___
I~
~LI
~
Fig. 6. Current drift of the (310) local probe current. Each data corresponds to the I F. patterns in fig. 5. 1100) Built - up Emitter
-6
I4O0~C~Pco
2~I3xlO
~ Pco~
26x ~
Pro,
cH4
=
P0
Pa
0’
1310) Built 1xi02
~
-
\=\
up Emitter )400C
H2,
I Oxl0~Pa
ixiO~
a
\\
E
\\ ~-i3i
~-I2nA
lxI~6
-
5
50
500
Hz
Frequency Fig. 7. The power spectrum density function and the current drift of the (100) local probe current.
/ Thermal field emission
N. Saitou, S. Yamamoto
Introduced Gas
CO
1*10-2
2
H2
381
Guilt—op Plane
1100) 1310)
‘I?
1*10
CH~
1310)
CO
13 I 0)
•~
~
I
lXiO~
isio
~
1at Room Temp.
\ \\\\ \~
1x106
Built up Temp
~~
~
Frequency
Fig. 8. The power spectrum density function of the (310) local probe current.
(3) Power spectrum density function w(f) To study the fluctuation more precisely, the power spectrum of the local current was measured. The frequency range was up to 500 Hz. The results are shown in fig. 7 (the (100) local current) and fig. 8 (the (310) local current) at 1400 C. The
solid line in fig. 7 is for the (100) built-up tip in CO2. No dependence is seen on the CO2 pressure. The chain line is for the (310) built-up tip in CO or H2. The w(J) was the same in both CO and H2. Fig. 8 shows the w(J) from the (310) plane at room temperature and 1400°Cin several ambient gases. The experimental conditions correspond to fig. S (and fig. 6). The following facts are apparent: (1) Three w(J) in H2, CH4 and CO at 1400°Cdo not have any differences and do
not show the gas dependence. (2) At room temperature the w(f) shows a gas dependence. (3) The w(f) of built-up planes is usually larger than that of non-built-planes. (4) The w(f) from the (310) local current of the (310) built-up tip is 10 times smaller than that from the (100) built-up tip.
382
N. Saitou, S. Ya,namoto
/ Thermal field
emission
NGULAR CONFI NEN[NT
PROBE
CURRE~4T
PoSITIoN
I
~(2c= 18
P
irii’ud)
Is nA (1) (310)
0.3x10~
~
—~—~~--—~—-—
-~4-0.9nA s- 1.2 n A -_
~) (116) ~0.9n4
r (3) (320)
-
~
-
I.O5nA
i.0x103
L-~~-~~°9 F°~ 2
(0) (311)
-
~
ni
O5nA
3 5x10
Fig. 9. Local probe current drift and angular confinement of a carbon adsorbed tip under optimum operating conditions.
N. Saitou, S. Yamamoto
/ Thermal field emission
383
3.3.2. Field emission at 900°C (1) FE pattern To know the conditions for making the pattern at 900°Cin fig. 3, a W tip was heated to 900°C in CO 2, 112, CH4 and CO. In case of C02, H2 and CO, the time
length T from flashing to the completion of the peculiar pattern was longer than 2 h. In CH4, T is about 30 mm at 900°Cand a few minutes at 1200°C. It was necessary to hold the tip temperature to 900°Cin order to keep the FE pattern, If the tip was operated below 900°C, the residual gases were adsorbed on the tip and the FE pattern disappeared. If the temperature was higher than 900°C, the tip was built-up and the pattern disappeared. Even if the tip with the peculiar pattern was operated at 900°C,the pattern was disappeared and bright spots twinkled in high
partial pressures of CO or CO2. (2) Relative fluctuation and angular confinement Field emission characteristics of the local probe current from four planes on the tip were measured at this optimum operating conditions. The pressure was 3—-7 X
l0~Pa during the measurement. Fig. 9 shows the relative fluctuation and angular confinement for 2cr = 18 mrad. The ~ was as small as 1—2% for (310), (116), and 3% for (320) planes. That for (311) plane was 6%. This is almost the same as that for clean W(3l0) at room temperature. At high partial pressure of CO or CO2, the ~ of the (310) plane became 6% in 2 h at 2 X 106 Pa and in I h at 4 X l06 Pa. On the contrary, the zV~/I~ was kept constant for longer than 10 h in 1 X i0~ Pa of CH4. These results show that CO or CO2 cause a deterioration of the current stability. The angular confinement of the (310) and (116) planes is about a half that of the clean W(310) at room temperature. That for the (320) is almost the same as W(3l0). That for the (311) is 3—4 times larger. From fig. 9 it is found that the angular confinement is almost proportional to the relative fluctuations in this tip. (3) Power spectrum density function w(f) Fig. 10 shows the w(J) measurement which corresponds to fig. 9. The w(J) for
lp*InA 1113)
l0~ ~‘..tO23)
c
~
~ 5
50
Frequency
500Hz
Fig. 10. The power spectrum density function. Each data corresponds to those in 1mg. 9.
384
N. Saitou, S. Yamamoto
/ Thermal field
Ihr
emission
2hr
3hr
T~850°C -60l~GttA~ -
~-~60O~rA’
....~
-
V
~
0~pA
--
1
~
5,36 KV
-
.
-
-
J4O0~LA
4~A
5.V KV
__ -
--.-~6OOj.ciS
..~
I
-
~
i
.—
0.
-
-
0,~
6.93 KV
..
-
-
___
Fig. 11. High
current characteristics of a carbon adsorbed tip operated at 850°C.The (310) local current was measured. The pressure was 5 X i0~ Pa.
the (311) plane around (211) plane has the largest amplitude throughout the frequency range below 500 Hz. But it is much smaller than that of built-up planes, seen in figs. 7 and 8.
(4) High current characteristics In order to know the stability of high current emission at 900°C,500 pA of total current was emitted at a pressure of 5 X i0~ Pa. The cylindrical type anode was used. The aperture angle was 2cr = 75 mrad. Before the emission, the anode was baked rigorously and the pressure increase was 2—4 X l0~ Pa for 100 pA of emission current. Fig. 11 shows that the ~I~/I~ was kept to less than 2%. For emission currents higher than 1 mA, the desorbed gas deteriorated the vacuum and the tip was ion bombarded,folluwed by a discharge in a few hours. The field emission current was so stable at a few hundreds pA of emission current that the tip was applied to the electron gun of an Auger microscope [51.
4. Discussion Two main causes of the current fluctuation in FE are (I) the surface migration of adsorbed atoms and (2) ion bombardment to the tip [61. Ion bombardment effects are important when the product of total emission current and pressure is larger than l0~~Pa A. The emission current in the present experiments except for
N. Saitou, S. Yamamoto
/ Thermal field
emission
385
the high current experiment of fig. 11 is so small that the effect can be neglected. The current fluctuation in the experiments is, undoubtedly, caused by the surface migration of the atom. The adsorption and the migration of the impurity atom will change the local work function. The current fluctuation is a function of (1) the diffusion constant of the adsorbed atom, D, (2) the work function change ~ (3) the average number of the adsorbed atom per unit area, ñ and (4) the area size A from which the local probe current is measured. By combining Kleint’s theory [7], Macfarlane’s [81 and Gomer’s theory [9], the power spectrum density function w(f) can be expressed in the equilibrium state as follows [10]:
w(~DmA~f~, (1) where c is the slope for frequency. The e is the complicated function of A, D and it takes between 0 and 2 [10]. In the present experiment, the w(f) of built-up plane did not depend on the ambient gas and the pressure. This means that some kind of strongly held gas or W atoms themselves are migrating on the built-up plane. It is not yet known what atom migrates. The w(f) of non-built-up plane was as small as lO~ to 10—2 of the built-up planes. The temperature of unbuilt-up planes should be the same as that of built-up planes because they are so close together. According to the eq. (1), this can be explained on the basis of a change in the effective emitting area. The peculiar FE pattern appeared at 900 C is due to carbon adsorption on W. The pattern is quite similar to MOller’s FE pattern which was obtained by evaporating carbon on a heated W [11]. Yates [12] and Shigeishi [13] reported that the thermal decomposition of hydrocarbon on heated W made tungsten—carbon structures on the surface. By the same reasoning, the hydrocarbon gas outgassed in fig. 3 was adsorbed, decomposed and then formed tungsten—carbon structures on the tip.
The detailed structure of C—W complex is not yet known. The FE current was very stable compared with that of clean W tip and/or built-up tip on which the flicker noise is mainly due to mobil W atoms [14]. Some explanations for the reduction of noise due to carbon adsorption can be given: (1) Carbon dramatically reduces the self diffusion of W atoms and then flicker
noise will be reduced at a specific temperature [15]. (2) The adsorbed gas like 112 is weakly coupled with W and the D is large. This kind of gas is easily desorbed at 900°C. (3) The sticking probability of the ambient gas to carbon is as small as l0’~
compared to W [16]. (4) Carbon itself is easily sputtered by ion bombardment but carburized tungsten will be resistant to ion sputtering. The w(f) for the (311) plane has the largest amplitude. According to FIM ob-
servations, Shigeishi [13] reported that the diffusion of carbon and tungsten atoms results in a pile-up into a thin wedge around the lattice step of the (110) and the edge of the (211) which corresponds to the (311). This pile-up may produce a high angular confinement and the large current fluctuations.
386
N. Saitou, S. Yamamoto
/ Thermal field emission
5. Summary The thermal field emission characteristics of tungsten (310) oriented single crystal were experimentally studied. The W tip was heated from room temperature to 1400°Cin CO 2,H2, CH4 and CO gases. On the built-up W emitter, it turned out that (1) The (100) plane is easy to build-up in CO2 and {310} planes are built-up in H2, CH4 and CO at 1150—1400°C. (2) The w(j) of the local probe current from built-up planes is not affected by the kind of ambient gases and the pressure. (3) Power spectrum theory suggests that some strong coupled atoms or W atoms themselves migrate on the built-up plane. (4) The relative fluctuation and the sv(J) of built-up planes are usually several times larger than that of unbuilt-up planes or that of room temperature field emission. This means that the effective emitting area of the built-up emitters is reduced. (5) The angular confinement of W(lOO) built-up plane is 10 times larger than that of room temperature W(3l0) plane. Therefore, the field emission current from the built-up plane can not be used in electron beam devices that require high stability. The field emission current is stabilized when the W tip is operated at 900°Cin
hydrocarbon gases. (1) A carbon adsorbed W tip is made when it is heated in hydrocarbon gases. (2) Field emission currents are much more stable than that of built-up tungsten when it is operated at 900°C. (3) This feature disappears in the presence of Co or CO2. (4) Total emission currents higher than 500 pA with 2% stability are obtained. (5) The angular confinement of this tip is 1/2 to 1 times of clean W(310) at room temperature. This tip can be applied to practical electron guns.
Acknowledgment The authors would like to thank Drs. U. Okano and T. Komoda for their valuable technical advice and encouragement.
References lii
A.V. Crewe, Quart. Rev. Biophys. 3(1970)137. [21 LW. Swanson, J. Vacuum. Sci. Technol. 12 (1975) 1228.
131 S. Ranc, M. Pitaval and G. Fontaine, Surface Sci. 57 (1976) 667. 141 S. Yamamoto, S. Fukuhara, N. Saitou and H. Okano, Surface Sci. 151 S. Goto, unpublished data.
61(1976) 535.
N. Saitou, S. Yama,’noto
/ Thermal field emission
[6] N. Saitou, Surface Sci. 66 (1977) 346. [71 C.H. Kleint, Surface Sci. 25 (1971) 394. [81 G.G. Macfarlane, Proc. Phys. Soc. B63 (1950) 807. [91 R. Gomer, Surface Sci. 38 (1973) 373. [10] N. Saitou, S. Yamamoto and H. Okano, Shinku 21(1978) 86. [In Japanese.] 1111 E.W. MUller, Feldem. Ergeb. exakt. Naturwissensch. Bd. XXVII, 290 (1953). [12] J.T. Yates, Jr. and T.E. Madey, Surface Sci. 28(1971)437. [131 R.A. Shigeishi, Surface Sci. 51(1975) 377. [14] LW. Swanson, Surface Sci. 70(1978)965. [15] M. Pichaud and M. Drechsler, Surface Sd. 32(1972) 341. [16] GA. Beitel, J. Vacuum. Sci. Technol. 9 (1971) 370.
387
CURRENT FLUCTUATION OF THERMAL FIELD EMISSION FROM TUNGSTEN EMITTER Norio SAITOU and Shigehiko YAMAMOTO c~entralResearch Laboratory, Hitachi Ltd., Kokuhunji, Tokyo, 185 Japan
Received 15 August 1979 Revised manuscript received 14 January 1980
Field emission characteristics from heated tungsten emitter were experimentally investigated in several ambient gases. The emission current is stabilized by heat treatment at about 900°Cin CH 4. The relative fluctuation is about 2%. The observation of FE pattern shows that tungsten—carbon complex .structure is formed and kept for long time at 900’~C.The local probe currents from several planes of built-up W tip were measured. The current stability does not depend on the kind of ambient gases and pressure. The power spectrum density of built-up planes is usually 10 to 102 times larger than that of non-built-up plane throughout the frequency range below 500 Hz.
1. Introduction Field emission (FE) electron sources have attracted considerable attention to electron beam devices due to its high brightness and small source size. In 1970, Crewe [1] successfully observed an atom image in a scanning electron microscope with an FE electron gun. The important factors in practical application of FE gun are the current level and its stability. Many investigators [2,3] have been trying to find better tip materials and more stable operating conditions. Thermal field emission from tungsten emitters seems to be one of the most promissing electron sources for such devices as electron beam fabrication niachines and Auger electron microscope. There is, however, little published data on the current fluctuation of thermal field emission. Moreover, the mechanism of the fluctuation is not clear, either. In line with this, the field emission characteristics of single crystal tungsten (W) emitter have been experinientally studied at various temperatures in several ambient gases.
2. Experimental apparatus The experimental vacuum chamber was made of stainless steel (304). It was 30 cm in diameter and 25 cm in height. The chamber was evacuated by a 110 1/s
N. Saitou, S. Yamamoto
/ Thermal field emission
375
Tip Heater
1
Tip Tilting Mechanism
1Recorder I I kS2
I
Viewing
Fluor:sent
~I!__~.
I~ TotaL Current Emission 1 Local Probe Current
FE Tip
Pump
Vacuum Chamber
Anode
Micro-
Foradoy Cup
Ammeter
Recorder
Pure Gas Inlets Mass FiLter
Fig. 1. Schematic diagram of the experimental apparatus.
ion pump and the final pressure was maintained at 7 X 10_8 Pa (S X 10.10 Torr). Fig. 1 shows the schematic diagram of the experimental apparatus. A field emitter was heated by a stable dc current and the temperature was measured by an optical pyrometer. The temperature was corrected by taking the emissivity of 0.3 into consideration. The error is within ±30°C. Tungsten (W) single crystal, oriented to (310) direction, was used as a tip. Just immediately after observing the clean W FE pattern by tip flashing (2000°C, for 1—2 s), the tip was heated to the desired temperature and then all the measurements were started. A flat anode, which was coated with fluorescent phosphor, permitted the observation of FE pattern from the window and the measurement of total emission current, ‘E• The local probe current, I~,which is emitted from a special plane on the tip was measured by a Faraday cage under the anode. A tip tilting mechanism made it possible to change the measured crystal plane. The aperture angle 2a which is defined in fig. 1 was selected to be usually 18 mrad except for fig. 11 (2a = 75 mrad). This value is normally close to that used in scanning electron microscope. In this paper the angular confinement is defined by ‘p/’E for this aperture angle. The ‘E was recorded on a recorder chart (the frequency response is less than 10 Hz). The I~was either recorded on a recorder chart to measure the long term drift and relatively low frequency fluctuation or fed into a power spectrum analyzer (UA.
N. Saitou, S. Yamamoto / Thermal field emission
376
500, Federal Scientific Corporation) to measure the higher frequency fluctuation. The relative fluctuation of ‘F and I~are defined by ~Ji;/Ji. and ~ respectively, where ~ and ~ are the peak to peak fluctuation on the recorder chart over 1 mm recorded time. A cylindrical anode with a closed end was used for high current experiment, in which it is necessary to avoid the electron scattering. This type of anode can confine the electron inside the- cylinder. Several kinds of pure gases were introduced into the vacuum system through variable leak valves to change the ambient atmosphere. The gas components were always monitored by a quadrupole mass spectrometer.
3. Experimental results 3.]. Desorbedgas analysis In fig. 2, the dark portions are the residual background gas at pressure Pi
=
7X
10—8 Pa, when electrons are not emitted. Field emission electrons desorb some gases
from the anode or the wall of the vacuum system if the system is baked incompletely. The kind of desorbed gases were analyzed when 10 pA FE electrons were
5
Anode
Stao~less Steel
7 Pa FE on ( Pf= 3x10 FE off I Pi 7x 108 Pa
f 10
,
Some gases are desorbed I
,
Background gas I
Co 0
Co 2 5 H 20
0
10
20
30
40
50
60
Mass Number Fig. 2. Residual gas and desorbed gas analysis by a quadrupole mass spectrometer.
N. Saitou, S. Yamamoto / Thermal field emission
377
emitted. The pressure p~ became 3 X l0~ Pa. The white portions show the desorbed gases, when the anode is pure stainless steel. Fig. 2 shows that the main gas component is H 2 in UHV and CO, CO2 and H2 and some hydrocarbon with the mass number of about 40 and 56 are mainly desorbed from the vacuum system by electron bombardment. The components of the desorbed gases were almost the same when phosphor coated stainless anode was used. 3.2. Temperature dependence of field emission characteristics The temperature dependence of the FE pattern and the total emission current was investigated in an incompletely outgassed vacuum chamber. Fig. 3 shows the result taken at 1 h after tip flashing. The ‘E was kept to 10 pA. The pressure was 10 X i0~ Pa during the experiment. When the temperature was lower than 900°C,the ambient gases were adsorbed on a tip surface and the FE pattern became granular. Operation above 900°Cgave rise to the built-up of some planes on the W surface. At 1150°C{310} planes were built-up and at 1400°C the (100) plane was built.up. At higher than 1400°C, thermionic electrons were emitted. At 900°C a peculiar FE pattern appeared and the emission current was very stable. The temperature dependence of the relative
Clean Pattern Right after Ftasr’j~,q
[‘1
FEc~a~’er~
Temperature
floor,, Tema
~ ..-6OO~C
19 ~ —900C
~.
ll5O~C
~t4QO~C
Fig. 3. Temperature dependence of the field emission pattern and current stability for a (310) oriented tungsten tip. The initial pressure, p~= 1.3 X i0~ Pa, and final pressure Pf = 10 X i0~ Pa.
378
N. Saitou, S. Yainamoto
0.1
/ Thermal field emission
~
1005
0
0
500
1Q00
1500
Tip Temperature (°C) Fig. 4. Temperature dependence of relative fluctuation ~Iy/IF.
fluctuation ~Jl../IF is shown by the solid line in fig. 4. At room temperature and built-up temperature, the ‘~.IE/IEis larger than 4-—5%, but it is l—2% at 900°C. Under rigorous outgassing of the anode and the vacuum chamber, the ~.JI./Il.’ changed monotonously with temperature as shown by the dashed curve in fig. 4. In this experiment the cylindrical, type anode was used. The two curves indicate that some kind of gases previously analyzed, contributed to the stabilization of the emission current and the appearance of the peculiar FE pattern.
3.3. Field emission characteristics in specified ambient gas All the experiments in this section were done in a rigorously baked vacuum chamber. The anode was outgassed by electron bombardment. The amount of desorbed gas was reduced to 10% of that showed in fig. 2. The total emission current was usually kept under 1 pA to keep the desorbed gas as low as possible. To control the atmosphere, C0 2, H2, CH4 and CO gases were introduced into the vacuum chamber. The pressure of gas introdced was 8—10 X i0~ Pa. The local probe current was always kept at a dc level of lnA by changing the voltage applied to the tip. Field emission characteristics were observed at room temperature, 900°C and 1400°Cin the above ambient gases.
3.3.]. Field emission at room temperature and 1400°C (])FE pattern FE patterns were observed in several ambient gases at room temperature and
N. Saitou, S. Yamamoto
~
/ Thermal field emission
Clean Pattern
Leak Gas
14
(.~‘
°C
379
E~
cH4
Co
Fig. 5. Field emission pattern at room temperature and 1400°Cin several ambient gases.
1400°C. FE patterns shown in fig. S were taken at 1 h after flashing process. The photographs were taken from the slanting direction, about 45 degrees. At room temperature, all the FE patterns were granular. The ambient gases were adsorbed on the tip surface. At 1400°C,the patterns depended upon the gas present. In CO2, the (100) plane was built-up easily. But in H2, CH4 and CO, the {310} planes were built.up firstly and then (100) build-up followed. The (100) build-up occurring in CH4 may be due to a small amount of CO and CO2 residual gases.
(2) Relative fluctuation and angular confinement The long term drift of the (310) local probe current was recorded on a recorder chart. Each data in fig. 6 corresponds to the FE pattern in fig. 5. At room temperature in H2, and no introduction of gas (main component is H2), the M1,/I~is as small as 2% but the dclevel drift is observed. In CO2 or CO,I~,changes slowly with the period of 1—2 mm [4]. At 1400 C, the fluctuations in H2, CH4 and CO resemble each other. The is 15—20% and does not show an ambient gas dependence. The angular confinement was 1 X io~ for 2a = 18 mrad. This is almost the same with clean W(310) at room temperature. In case of no introduction of gas, and for CO2, the i~sI~/I~ is relatively small, 3 to 6%.
The relative fluctuation of the (100) local current is shown in fig. 7. Though the angular confinement is 1—2 X 10_2 the i~J~/I~ of the (100) plane of the (100) built-up tip is larger than 30%, but that of the (310) built-up tip is only 3%. it is seen that if one plane is built-up, the relative fluctuation ~ does not depend on the kind of ambient gas and its pressure.
380
/ Thermal field
N. Saitou, S. Yamamoto Introduced Gas
co2
non
__
Pa Pressure
H2 T
I— 2 6x10°
emission
CH4
9slO~
CO
9x)Q7
8a10°
B— iOxiO
atI4OOCIi~r~
-
~..—5 min-~
~11
at Room
L
___
I~
~LI
~
Fig. 6. Current drift of the (310) local probe current. Each data corresponds to the I F. patterns in fig. 5. 1100) Built - up Emitter
-6
I4O0~C~Pco
2~I3xlO
~ Pco~
26x ~
Pro,
cH4
=
P0
Pa
0’
1310) Built 1xi02
~
-
\=\
up Emitter )400C
H2,
I Oxl0~Pa
ixiO~
a
\\
E
\\ ~-i3i
~-I2nA
lxI~6
-
5
50
500
Hz
Frequency Fig. 7. The power spectrum density function and the current drift of the (100) local probe current.
/ Thermal field emission
N. Saitou, S. Yamamoto
Introduced Gas
CO
1*10-2
2
H2
381
Guilt—op Plane
1100) 1310)
‘I?
1*10
CH~
1310)
CO
13 I 0)
•~
~
I
lXiO~
isio
~
1at Room Temp.
\ \\\\ \~
1x106
Built up Temp
~~
~
Frequency
Fig. 8. The power spectrum density function of the (310) local probe current.
(3) Power spectrum density function w(f) To study the fluctuation more precisely, the power spectrum of the local current was measured. The frequency range was up to 500 Hz. The results are shown in fig. 7 (the (100) local current) and fig. 8 (the (310) local current) at 1400 C. The
solid line in fig. 7 is for the (100) built-up tip in CO2. No dependence is seen on the CO2 pressure. The chain line is for the (310) built-up tip in CO or H2. The w(J) was the same in both CO and H2. Fig. 8 shows the w(J) from the (310) plane at room temperature and 1400°Cin several ambient gases. The experimental conditions correspond to fig. S (and fig. 6). The following facts are apparent: (1) Three w(J) in H2, CH4 and CO at 1400°Cdo not have any differences and do
not show the gas dependence. (2) At room temperature the w(f) shows a gas dependence. (3) The w(f) of built-up planes is usually larger than that of non-built-planes. (4) The w(f) from the (310) local current of the (310) built-up tip is 10 times smaller than that from the (100) built-up tip.
382
N. Saitou, S. Ya,namoto
/ Thermal field
emission
NGULAR CONFI NEN[NT
PROBE
CURRE~4T
PoSITIoN
I
~(2c= 18
P
irii’ud)
Is nA (1) (310)
0.3x10~
~
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-~4-0.9nA s- 1.2 n A -_
~) (116) ~0.9n4
r (3) (320)
-
~
-
I.O5nA
i.0x103
L-~~-~~°9 F°~ 2
(0) (311)
-
~
ni
O5nA
3 5x10
Fig. 9. Local probe current drift and angular confinement of a carbon adsorbed tip under optimum operating conditions.
N. Saitou, S. Yamamoto
/ Thermal field emission
383
3.3.2. Field emission at 900°C (1) FE pattern To know the conditions for making the pattern at 900°Cin fig. 3, a W tip was heated to 900°C in CO 2, 112, CH4 and CO. In case of C02, H2 and CO, the time
length T from flashing to the completion of the peculiar pattern was longer than 2 h. In CH4, T is about 30 mm at 900°Cand a few minutes at 1200°C. It was necessary to hold the tip temperature to 900°Cin order to keep the FE pattern, If the tip was operated below 900°C, the residual gases were adsorbed on the tip and the FE pattern disappeared. If the temperature was higher than 900°C, the tip was built-up and the pattern disappeared. Even if the tip with the peculiar pattern was operated at 900°C,the pattern was disappeared and bright spots twinkled in high
partial pressures of CO or CO2. (2) Relative fluctuation and angular confinement Field emission characteristics of the local probe current from four planes on the tip were measured at this optimum operating conditions. The pressure was 3—-7 X
l0~Pa during the measurement. Fig. 9 shows the relative fluctuation and angular confinement for 2cr = 18 mrad. The ~ was as small as 1—2% for (310), (116), and 3% for (320) planes. That for (311) plane was 6%. This is almost the same as that for clean W(3l0) at room temperature. At high partial pressure of CO or CO2, the ~ of the (310) plane became 6% in 2 h at 2 X 106 Pa and in I h at 4 X l06 Pa. On the contrary, the zV~/I~ was kept constant for longer than 10 h in 1 X i0~ Pa of CH4. These results show that CO or CO2 cause a deterioration of the current stability. The angular confinement of the (310) and (116) planes is about a half that of the clean W(310) at room temperature. That for the (320) is almost the same as W(3l0). That for the (311) is 3—4 times larger. From fig. 9 it is found that the angular confinement is almost proportional to the relative fluctuations in this tip. (3) Power spectrum density function w(f) Fig. 10 shows the w(J) measurement which corresponds to fig. 9. The w(J) for
lp*InA 1113)
l0~ ~‘..tO23)
c
~
~ 5
50
Frequency
500Hz
Fig. 10. The power spectrum density function. Each data corresponds to those in 1mg. 9.
384
N. Saitou, S. Yamamoto
/ Thermal field
Ihr
emission
2hr
3hr
T~850°C -60l~GttA~ -
~-~60O~rA’
....~
-
V
~
0~pA
--
1
~
5,36 KV
-
.
-
-
J4O0~LA
4~A
5.V KV
__ -
--.-~6OOj.ciS
..~
I
-
~
i
.—
0.
-
-
0,~
6.93 KV
..
-
-
___
Fig. 11. High
current characteristics of a carbon adsorbed tip operated at 850°C.The (310) local current was measured. The pressure was 5 X i0~ Pa.
the (311) plane around (211) plane has the largest amplitude throughout the frequency range below 500 Hz. But it is much smaller than that of built-up planes, seen in figs. 7 and 8.
(4) High current characteristics In order to know the stability of high current emission at 900°C,500 pA of total current was emitted at a pressure of 5 X i0~ Pa. The cylindrical type anode was used. The aperture angle was 2cr = 75 mrad. Before the emission, the anode was baked rigorously and the pressure increase was 2—4 X l0~ Pa for 100 pA of emission current. Fig. 11 shows that the ~I~/I~ was kept to less than 2%. For emission currents higher than 1 mA, the desorbed gas deteriorated the vacuum and the tip was ion bombarded,folluwed by a discharge in a few hours. The field emission current was so stable at a few hundreds pA of emission current that the tip was applied to the electron gun of an Auger microscope [51.
4. Discussion Two main causes of the current fluctuation in FE are (I) the surface migration of adsorbed atoms and (2) ion bombardment to the tip [61. Ion bombardment effects are important when the product of total emission current and pressure is larger than l0~~Pa A. The emission current in the present experiments except for
N. Saitou, S. Yamamoto
/ Thermal field
emission
385
the high current experiment of fig. 11 is so small that the effect can be neglected. The current fluctuation in the experiments is, undoubtedly, caused by the surface migration of the atom. The adsorption and the migration of the impurity atom will change the local work function. The current fluctuation is a function of (1) the diffusion constant of the adsorbed atom, D, (2) the work function change ~ (3) the average number of the adsorbed atom per unit area, ñ and (4) the area size A from which the local probe current is measured. By combining Kleint’s theory [7], Macfarlane’s [81 and Gomer’s theory [9], the power spectrum density function w(f) can be expressed in the equilibrium state as follows [10]:
w(~DmA~f~, (1) where c is the slope for frequency. The e is the complicated function of A, D and it takes between 0 and 2 [10]. In the present experiment, the w(f) of built-up plane did not depend on the ambient gas and the pressure. This means that some kind of strongly held gas or W atoms themselves are migrating on the built-up plane. It is not yet known what atom migrates. The w(f) of non-built-up plane was as small as lO~ to 10—2 of the built-up planes. The temperature of unbuilt-up planes should be the same as that of built-up planes because they are so close together. According to the eq. (1), this can be explained on the basis of a change in the effective emitting area. The peculiar FE pattern appeared at 900 C is due to carbon adsorption on W. The pattern is quite similar to MOller’s FE pattern which was obtained by evaporating carbon on a heated W [11]. Yates [12] and Shigeishi [13] reported that the thermal decomposition of hydrocarbon on heated W made tungsten—carbon structures on the surface. By the same reasoning, the hydrocarbon gas outgassed in fig. 3 was adsorbed, decomposed and then formed tungsten—carbon structures on the tip.
The detailed structure of C—W complex is not yet known. The FE current was very stable compared with that of clean W tip and/or built-up tip on which the flicker noise is mainly due to mobil W atoms [14]. Some explanations for the reduction of noise due to carbon adsorption can be given: (1) Carbon dramatically reduces the self diffusion of W atoms and then flicker
noise will be reduced at a specific temperature [15]. (2) The adsorbed gas like 112 is weakly coupled with W and the D is large. This kind of gas is easily desorbed at 900°C. (3) The sticking probability of the ambient gas to carbon is as small as l0’~
compared to W [16]. (4) Carbon itself is easily sputtered by ion bombardment but carburized tungsten will be resistant to ion sputtering. The w(f) for the (311) plane has the largest amplitude. According to FIM ob-
servations, Shigeishi [13] reported that the diffusion of carbon and tungsten atoms results in a pile-up into a thin wedge around the lattice step of the (110) and the edge of the (211) which corresponds to the (311). This pile-up may produce a high angular confinement and the large current fluctuations.
386
N. Saitou, S. Yamamoto
/ Thermal field emission
5. Summary The thermal field emission characteristics of tungsten (310) oriented single crystal were experimentally studied. The W tip was heated from room temperature to 1400°Cin CO 2,H2, CH4 and CO gases. On the built-up W emitter, it turned out that (1) The (100) plane is easy to build-up in CO2 and {310} planes are built-up in H2, CH4 and CO at 1150—1400°C. (2) The w(j) of the local probe current from built-up planes is not affected by the kind of ambient gases and the pressure. (3) Power spectrum theory suggests that some strong coupled atoms or W atoms themselves migrate on the built-up plane. (4) The relative fluctuation and the sv(J) of built-up planes are usually several times larger than that of unbuilt-up planes or that of room temperature field emission. This means that the effective emitting area of the built-up emitters is reduced. (5) The angular confinement of W(lOO) built-up plane is 10 times larger than that of room temperature W(3l0) plane. Therefore, the field emission current from the built-up plane can not be used in electron beam devices that require high stability. The field emission current is stabilized when the W tip is operated at 900°Cin
hydrocarbon gases. (1) A carbon adsorbed W tip is made when it is heated in hydrocarbon gases. (2) Field emission currents are much more stable than that of built-up tungsten when it is operated at 900°C. (3) This feature disappears in the presence of Co or CO2. (4) Total emission currents higher than 500 pA with 2% stability are obtained. (5) The angular confinement of this tip is 1/2 to 1 times of clean W(310) at room temperature. This tip can be applied to practical electron guns.
Acknowledgment The authors would like to thank Drs. U. Okano and T. Komoda for their valuable technical advice and encouragement.
References lii
A.V. Crewe, Quart. Rev. Biophys. 3(1970)137. [21 LW. Swanson, J. Vacuum. Sci. Technol. 12 (1975) 1228.
131 S. Ranc, M. Pitaval and G. Fontaine, Surface Sci. 57 (1976) 667. 141 S. Yamamoto, S. Fukuhara, N. Saitou and H. Okano, Surface Sci. 151 S. Goto, unpublished data.
61(1976) 535.
N. Saitou, S. Yama,’noto
/ Thermal field emission
[6] N. Saitou, Surface Sci. 66 (1977) 346. [71 C.H. Kleint, Surface Sci. 25 (1971) 394. [81 G.G. Macfarlane, Proc. Phys. Soc. B63 (1950) 807. [91 R. Gomer, Surface Sci. 38 (1973) 373. [10] N. Saitou, S. Yamamoto and H. Okano, Shinku 21(1978) 86. [In Japanese.] 1111 E.W. MUller, Feldem. Ergeb. exakt. Naturwissensch. Bd. XXVII, 290 (1953). [12] J.T. Yates, Jr. and T.E. Madey, Surface Sci. 28(1971)437. [131 R.A. Shigeishi, Surface Sci. 51(1975) 377. [14] LW. Swanson, Surface Sci. 70(1978)965. [15] M. Pichaud and M. Drechsler, Surface Sd. 32(1972) 341. [16] GA. Beitel, J. Vacuum. Sci. Technol. 9 (1971) 370.
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