CHARACTERIZATION OF ENHANCED FIELD EMISSION SITES ON NIOBIUM SURFACES DUE TO HEAT TREATMENT

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CHARACTERIZATION OF ENHANCED FIELD EMISSION SITES ON NIOBIUM SURFACES DUE TO

HEAT TREATMENT

N. Sankarraman, Ph. Niedermann, R. Noer, O. Fischer

To cite this version:

N. Sankarraman, Ph. Niedermann, R. Noer, O. Fischer. CHARACTERIZATION OF ENHANCED FIELD EMISSION SITES ON NIOBIUM SURFACES DUE TO HEAT TREATMENT. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-133-C7-138. �10.1051/jphyscol:1986724�. �jpa-00225917�

JOURNAL DE PHYSIQUE

Colloque C7, suppl&ment a u n o 11, T o m e 4 7 , Novembre 1 9 8 6

CHARACTERIZATION O F ENHRNCED FIELD EMISSION SITES O N NIOBIUM SURFACES D U E T O H E A T TREATMENT

N. SANKARRAMAN, Ph. NIEDERMANN, R.J. NOER* and 0. FISCHER Departement d e Physique d e la Matiere Condensee, Universite de Geneve, 24, Quai Ernest Ansennet, CH-1211 Geneve 4, St~itzerland

ABSTRACT

Nous presentons une etude de l'emission de champ augmentde sur des surfaces de niobium. Notre experience a ete effectuee dans un "Vacuum Generators Escalabn, equip8 d'un microscope Auger B balayage, d'un microscope B emission de champ et dfun systeme de chauffage in situ de la cathode. Nous observons que la densiti de sites dr6mission augmente syst6matiquement lors de traitements thermiques entre 650°C et 900°C. Par contre il est possible d'obtenir des surfaces sans emission (I

<

The study of enhanced field emission from broad area niobium cathodes, and the characterization of individual sites are presented. This experiment was built[l] in a UHV surface analysis system which included a SEM, a scanning Auger microscopy facility and also an in situ heat treatment unit where the samples could be annealed up to 2000°C. Upon heat treatment of the niobium samples, it is observed that the density of field emitting sites increases with the heat treatment tempera- ture up to 900°C whereafter the density decreases sharply. After heat treatments at 1400°C and higher we have been able to obtain, consistently, emission free surfaces at 100 MV/m. Further heat treatments of the emission free sample show a maxima in the density of field emitting sites at 800°C. These sites were either activated or created during the heat treatment. Typical examples of the activated and created sites are presented.

I. INTRODUCTION

Enhanced field emission from broad area cathodes is a subject of considerable interest at presentI2.31 because of its technological importance in high voltage vacuum devices and high field rf superconducting accelerator cavities141. This emission originates at certain sites, however its mechanism is still basically unknown. It has thus become essential to characterise and study the individual field emitting sites, in order to understand the underlying mechanisms of EPE and also to control this field emission. In the early years of investigations, EFE was explained on the basis of geometric field enhancement from sharp metallic protru- sions on the surface of the sample. At present, with the use of high resolution Secondary Electron Microscopy(SEM), it has been observed that these metallic pro- trusions are very rarely the cause of EFE[5] and often the localised sites are found to be particles or metallic inclusions on the surface. A strong shift in the energy spectrum of .the emitted electrons(61 has also been observed indicating a non-metallic nature of the emitter.

In our previous work(l1, we have studied the occurrence of such sites on chemically polished niobium surfaces and their behaviour as introduced and also after in situ heat treatments up to 2000°C. After a 800°C heat treatment a marked increase in the density of the field emitting sites was observed whereas subsequent higher tempera- ture heat treatments reduced the number of sites on the surface. Consistently ve were able to obtain surfaces without field emission up to 100 MV/m, with a current limit of 40 nA, by heat treatments to 1600°C or higher. After further heat treat-

* Permanent address : Carleton College. Northfield. MN-55057. U.S.h

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986724

JOURNAL D E PHYSIQUE

ment at 800°C of such emission free surfaces, a high density of field emitting sites were observed again. A vast majority of the sites localised on these surfaces were micron size particles made up of foreign elements. The most common sites after the 800°C heating were carbon particles or particles composed of molybdenum and sulfur. The emitting particles are only a small fraction of the total number of particles seen on the surface, most of which do not emit up to 100 MV/m. From these observations, it is evident that very special conditions on the particle or the interface between the particle and the surface are essential to produce enhanced field emission.

Our measurements at that time were done only after heat treatments of 800°C and above. It vas not clear vhether lower temperature heat treatments would show a reduced or enhanced density of field emitting sites. In this paper, we present the results of heat treatment of the niobium samples over a range of temperatures from 350°C to 1800°C. These heat treatments are repeated over three cycles. We also pre- sent a study of the elemental characterisation of the sites, with special emphasis on the activation and creation of the field emitting sites with heat treatment.

11. EXPEIUHENTAL SETUP ANDPROCEDURE

This study was carried out in a commercial UHV surface analysis system (Vacuum Gen- erators Escalab) which was equipped with a Scanning Electron Microscope (SEM), a facility for microfocus Auger and also Scanning Auger Microscopy. For the field emission study this system had been modified to include a Field Emission Scanning Hicroscope[l], consisting of a micro-manipulator for precision cathode motions along the x-, y- and z-axes, a rotatable anode holder to accommodate anodes of dif- ferent tip geometries, and a fast high voltage regulator for constant current measurement of the distribution of weak and strong field emitting sites. The system also contains a separately pumped preparation chamber where the samples could be transported without removal from the UHV and heat treated, up to a temperature of 2500°C by electron bombardnient heating. During the heat treatment the pressure in the preparation chamber remains below mbar

.

High purity(99.93Z) accelerator grade niobium samples were machined into discs of 12mm diameter from 2mm thick sheet niobium. In order to avoid electrostatic edge effects these discs had their top edge rounded. These flat discs were then etched chemically for 15 min with a standard recipe[7], which is used for niobium RF cavi- ties in accelerators. They were then rinsed in distilled water and cleaned ultra- sonically in alcohol before being fan dried and introduced in to the system for measurements. Care vas taken to keep the time between the ultrasonic cleaning in alcohol and the introduction in the system very short.

The field emission scans of the entire surface was done with a 1 mm diameter tung- sten anode at a constant gap of 200 urn. The sample was scanned in a raster pattern with the anode held in a fixed position and at a constant high voltage. During the scan when a field emission site passes under the anode, and as soon as the current reaches the set value (usually 4 0 nA) the voltage is regulated to maintain a con- stant current. With the help of these scans ve are able to obtain a distribution of both weak and strong sites on the sarface of the sample. Each of the sites was individually localised with a fine tip and analysed for its surface chemical compo- sition by Auger analysis. After all the sites are analysed the sample is trans- ferred to the preparation chamber and heat treated, typically for 30 minutes, after which it is retransferred to the analysis chamber for further investigations.

The study of the evolution of the sites and microstructures due to heat treatment, was done by recording the SEH image of the entire surface, at a suitably high mag- nification, on a commercial video recorder, before and after heat treatments. This enabled us to determine whether the particle associated with a new field emitting site existed before the heat treatment or not. The sites were characterised as a) Activated Sites - those which had an existing particle or structure before heat treatment but vith no field emission and b) Created Sites - field emission sites vith the appearance of a new particle or structure after the heat treatment.

111. RESULTS

In this paper we present the results obtained on two identically prepared niobium samples. In one of the samples major emphasis was given to the measurement of the density of the field emission sites, and the characterization of the individual sites, with different heat treatments. In the second sample the principal objective was to study of activation and creation of the sites with heat treatment. These results are consistent with all the samples measured in our laboratory.

(A) Heat treatrent results

A selection of the field emission scans on the niobium surface, taken at a gap of 200pm between the anode and the sample surface, with a 1 mm broad area tungsten anode is presented in figure 1. These scans are for an as introduced sample and after subsequent heat treatments from 350°C to 1200°C. The heating time was main- tained constant, at 30 minutes, for all the heat treatments. Samples were systemat- ically scanned, after each heat treatment, starting at 25 UV/m, and then at 40, 50, 60, 75, 90 and 100 HV/m. From the series of scans it is evident that the density of the sites increases up to heat treatments of 90O0C where after it decreases very rapidly and there is no emission observed to 100 HV/m, at a current of 40 nA. In order to study the evolution of the sites with temperature the electrically clean

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Fig. 1. A selection of field emission scans for the first thermal cycle.

Fig. 2. A plot of the number of field emit- ting sites as a function of the heat treat- ment temperature at 90 HV/m for the three

0 200 603 800 1m ~ 3 3 GOO cycle, (a)Second thermal cycle and (0)Third

T ('C) thermal cycle.

C7-136 JOURNAL DE PHYSIQUE

surface was again heat treated from 350°C, in order to identify the temperature at which the sites reappear. The scan fields and the heat treatment temperatures were maintained the same as in the first cycle. This procedure of studying the emission from the niobium surface after heat treatments on the electrically clean surfaces, was repeated over three thermal cycles. We reproducibly observed an increase in the density of field emitting sites after heat treatments in the range 650°C - 900°C, and a decrease above 900°C. In certain cases, a time gap of over 25 days between subsequent heat treatments did not change the nature of the surface nor the emis- sion. This result confirms the long term stability of the sites, which was also observed in an earlier work on intentionally deposited field emitting sites on a niobium surface[8].

Figure 2 gives the number of sites taken from the field emission scans (fig. 1) as a function of the heat treatment temperature for the three cycles at 90 HV/m. From the series of scans (fig. 1) and fig. 2, i t is evident that the density of sites on the surface increases gradually up to a heat treatment of 650°C whereafter a sharp increase in the density of sites is observed up to 900°C. On heating at higher tem- peratures the density of sites falls abruptly, and after 1200°C and above, surfaces with no field emission up to 100 MV/m are obtained consistently. The appearance of new field emission sites after heat treatments between 700°C and 900°C, on an elec- trically clean surface, was again observed consistently and our results indicated that this happens both by activation and creation of the sites.

In this sample, a total of 115 different sites were localised and analysed during the three thermal cycles. The distribution of the different types of sites is pre- sented in Table I. Of these, the sulfur containing sites constitute 33%, all of which started emitting after heat treatments between 650°C and 900°C. Host of these sites show a strong concentration of molybdenum. On heating to higher temperatures the sulfur concentration disappears, leaving behind a particle of molybdenum with an increased concentration of niobium. The carbon sites constitute about 21% of the sites which are predominant in the as introduced samples and also up to heat treat- ments of 650°C. About 10% of the sites TABLE I: Percentage distribution observed showed no visible features to a of the different sites observed resolution of 0.5pm and in 18% of the during the heat treatments. sites, the Auger analysis of the emitting Total number of sites = 115. particle did not show any variation in the composition from the background surface.

While trying to localise weak field emit- ting sites, silver flakes are released from the anode tip holder during sparks and these sites constitute 6% of the total.

Finally 12% of the sites, which we char- acterise as "random sites" contained par- ticles with other elements like Calcium, Chromium or Aluminium. The distribution of the 115 sites on the surface of the sample indicates that there is no preferential region with an enhanced density of field emitting sites.

(B) Activation and Creation of sites

In order to study the activation and creation of the field emitting sites, a niob- ium sample was initially heat treated at 1600°C for 30 minutes and a field emission scan, with a fine tungsten tip anode, at 100 HV/m in a 3x3 mm2 area of the surface did not show any field emitting sites. The SEM image of the entire area of the field emission scan was video recorded at a high magnification. After this record- ing process, the sample was heat treated at 800°C for 30 minutes and a subsequent field emission scan of the same 3x3 mm2 area of the sample at a field of 100 W / m showed a high density of field emitting sites. Each of the individual field emit- ting sites was localised and the SEM image of the corresponding region of a site was relocated on the recorded image, taken before the heat treatment.

Site characteristics Carbon

Sulfur containing particles

No feature to 0.5pm No change in Auger

analysis comp.

Silver Others

Typical examples of the activation and creation of field emitting sites after the Percentage

21 33 10 l8 6 12

Fig. 3. SEM images of the sites before and after heat treatments.

(a)Field emitting carbon sites localised after a 800°C heating.

(b)Video recorder image of the same area before the heat treatment indicating the presence of the par- ticle. (c)Field emitting particle containing sulfur, molybdenum and niobium. (d)Video recorded image of the same region before the heating indicating that the particle has been created by the heating.

Fig. 4. Auger analysis of the emit- ting particles shown in fig. 3 along with the background refer- ence. (a)On the particle of fig. 3(a) which has an increased carbon concentration. (b)Background reference. (c)The created particle of fig: 3(c).

800°C heat 'treatment are shown in figure 3. ( a )

In figure 3(a) the emitting site was loca-

lised to be a particle of 6pm in size and it ⁰

was found, on the video recorded image, to

have already existed before the heat treat- - dN

ment (fig. 3b), indicating that the site had dE

been zctivated during the heat treatment. The Auger analysis of the particle, presented in fig. 4(a) along with the Auger analysis of

the background niobium (fig. 4b), shows that IN^

it has a strong concentration of carbon. Fig- s

ure 3(c) is an example of a created site ₁₀₀ ^{I .} ₂₀₀ ^{I .} ₃₀₀ ^{, .} ₄₀₀ ^{I .} ₅₀₀ ^, after it appeared following the 800°C heat E(eV)

treatment. The video recorded image of the same region (fig. 3d) does not show the pres-

ence of the particle which indicates that the particle had been created during the heat treatment of the sample. The Auger analysis of the particle, shown in fig. 4(c), indicates that the particle has a strong concentration of sulfur along with molybdenum as compared to the background Auger analysis(fig. 4b) of the niob- ium surface. The size of the particle was 2um, but the sulfur concentration though being maximum on the particle was spread over a 50pm diameter around the particle.

In contrast, the molybdenum concentration was localised only to the particle.

In the 3x3 mm2 area of the surface, in all, 11 field emitting sites were localised and of these 7 sites were activated and 4 sites were created during the heat treat- ment. A field emission scan of the same 3x3 mm2 area, after a final heat treatment of the sample at 1600°C, did not have any emission sites up to 100 MV/m indicating that the sites have been destroyed by the heat treatment. Similar experiments on other niobium samples have produced identical results. In general, the particles which were activated were composed of carbon and those which were created contained a strong concentration of sulfur along with molybdenum.

(IV) DISCUSSION

The increase in the density of the field emission sites after heat treatments up to 800°C and a subsequent decrease in the emission has been observed consistently on the niobium surfaces studied. The most common sites observed in the as introduced state are sites containing a strong concentration of carbon and also sites associ-

C7-138 JOURNAL DE PHYSIQUE

ated with etch pits and mechanical defects. On heat treatment in the temperature range of 600" - 900°C the most predominant new sites are those containing a strong concentration of sulfur along with molybdenum. On further heat treatment above 1100°C there is a drastic decrease in the density of the sites with the diffusion of the particles into the bulk of the sample.

An analysis of the accelerator grade niobium used for the study was found to have 25 ppm level molybdenum impurities in the bulk. At temperatures of 650° - 800°C sulfur impurities are known to segregate to the surface of the sample[9,10], allow- ing the formation of molybdenum sulfide(MoS2)[ll] with the molybdenum inclusions close to the surface. On further heat treatment of the sample we have observed that the sulfur concentration in the MoS particle disappears at 1000°C leaving behind a particle of molybdenum. On heatlng the sample above 1200°C very small particles ?

diffuse into the bulk of the sample, whereas larger particles remain with an increased concentration of niobium which can be attributed to the diffusion from the bulk.

The activation of the carbon sites after a 800°C heating could be a consequence of the segregation of impurities to the surface of the sample which create the neces- sary conditions, at the interface, in order to have enhanced field emission. It is interesting to note that most of the strong field emitting sites located on the sample are associated with materials which have a two-dimensional layered struc- tures. Graphitic carbon is a strong emitter and has a layered structure like MoSZ.

In order to understand the independent behaviour of these layered structure materl- als on the niobium surfaces, we have artificially deposited micron sized particles of carbon and MoS on niobium surfaces and found them to emit strongly[8]. The question as to wiy layered structure materials, like MoS2 particles, emit strongly is still not answered.

(V) CONCLUSIONS

We have characterized the field emitting sites on the niobium surface produced dur- ing heat treatments. It is observed that there is a increase in the density of sites up to 800°C, whereas above 900°C heating the density reduces and surfaces with no field emission to 100 MV/m are obtained consistently. On a repeated heating of the emission free surface around 800°C new sites are activated or created. The activated sites were composed of carbon and the created sites contained a strong concentration of sulfur and molybdenum. Though a large percentage of the emitters observed are associated with materials having a two dimensional layered structure, their emission mechanism is still unknown.

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Ph.Niedermann, N.Sankarraman, R.J.Noer, 0.Fischer; J. Appl. Phys., 2 , 892, 1986.

R.J.Noer; Appl. Phys.,

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R.V.Latham; High Voltage Insulation: The Physical Basis, Academic Press; Lon- don and New York, 1981.

H.Piel IEEE Trans. Nucl. Sci. NS-32, 3565, 1985.

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D.Bloess; Proc. 2nd Workshop on RF ~ G e r c o n d u c tivi ty, Gengve 1984, editor H.Lengeler, CERN, p. 409.

R.J.Noer, Ph.Niedermann, N.Sankarraman, 0.Fischer; J. Appl. Phys., 2 , 3851, 1986.

L.A.Harris; J. Appl. Phys., 2 , 1428, 1968.

T.W.Hass, J.T.Grant, G.J.Dooley 111; J. Vac. Sci. Techn., 1, 43, 1970.

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