DEVELOPMENT OF A HIGH DENSITY FINITE SET OF UNIFORM FIELD EMITTERS ON A THIN FILM GLASS SUBSTRATE

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DEVELOPMENT OF A HIGH DENSITY FINITE SET OF UNIFORM FIELD EMITTERS ON A THIN FILM

GLASS SUBSTRATE

G. Kitzmann

To cite this version:

G. Kitzmann. DEVELOPMENT OF A HIGH DENSITY FINITE SET OF UNIFORM FIELD EMIT-

TERS ON A THIN FILM GLASS SUBSTRATE. Journal de Physique Colloques, 1986, 47 (C2),

pp.C2-79-C2-83. �10.1051/jphyscol:1986212�. �jpa-00225643�

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Colloque C2, suppl6ment au n03, Tome 47, mars 1986 page

DEVELOPMENT OF A HIGH DENSITY FINITE SET OF UNIFORM FIELD EMITTERS ON A THIN FILM GLASS SUBSTRATE

G.A. KITZMANN

D e p a r t m e n t of P h y s i c s , S t a t e U n i v e r s i t y of New Y o r k , New P a l t z , NY 12561, U.S.A.

Abstract - Copper, chrome tipped, field emitters have been made by directly depositing metals through a pair of magnetically held masks onto a thin film base of chrome/copper/chrome on a glass substrate. The emitter density is 1.OE6 emitters per square centimeter where the emitter base diameter is typically on the order of 1.5 microns and the emitters are on 10 micron centers. The individual emitters are typically 2 to 3 microns high with a radius of curvature of less than 0.2 microns.

Experiments have been conducted to determine the possibility of producing large area high density field emitter types of surfaces on glass substrates. From these experiments it has been tentatively concluded that the mean free path of the evapo- rant material can and may be changed in the vicinity of the substrate. These changes in the mean free path of the evaporant material can in turn lead to signif- icant changes in the final shapes of the emitter structure. In Figure 1 there are shown two emitter profile shapes which have resulted from altering the evaporant mean free path. In Figure la the mean free path of the evaporant is long while in lb the mean free path is short as compared to the diameter of the mesh hole.

In all of the experiments performed to date a nickel mesh (1) with a hole density of 1.OE6 square holes per square centimeter was used as the emitter mask. The mesh mask was held in place with a high gradient magnetic field which passed through the 0.16 cm thick prepared pyrex glass substrate (2.54 X 2.54 cm 2 ). The pyrex was ultrasonicly cleaned in alcohol and then dried in a Freon gas stream.

(1) Mesh was supplied by Dan Duheim of Buckbee-Mears, 245 E 6th St., St. Paul, &I.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986212

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. ¹ - Tne cusp form (a) i s caused by a long mean f r e e path while the cone form i s caused by a short mean f r e e path of the e v a p r a n t material r e l a t i v e t o the forming mesh hole.

Tne cleaned pyrex was placed i n the vacuum chamber and given an e v a p r a n t coating of chrome (1508) - copper (20008)- chrome (1508) which was nitrogen fixed.

'Ihese prepared substrates were then stored i n a desiccant chamber u n t i l ready f o r use. Upon use, the prepared substrates were placed on a magnetic holder assembly, the mesh mask was layed down and held with the magnetic f i e l d , a molybdenum d i s c with a precision hole was layed over the mesh mask, and a magnetic s t e e l keeper with a large hole which covered the molybdenum d i s c was layed down. B e e n t i r e assembly was then clamped mechanically 25 cm above the surface plane of the two Sloan E-beam evaporant cone sources on a rotating cage i n a Balzers 510 vacuum system. Evaporations were performed a f t e r the Balzers u n i t had pumped down to 2 E-7 Torr. Evaporations were done i n s e r i e s such that the substrate holder was f i r s t positioned above the copper source f o r the 5 micron copper evaporation and the 20013 chrome overcoat was then evaporated with the cage r o t a t i n g a t 0.5 rev/sec. Tne evaporation was monitored with an Inficon XTC. After the evapor- ation the system was allowed to cool f o r 1 hour and then brought up t o atmospheric pressure by allowing nitrogen to pass i n t o the system through the back to atmos- phere valve. l h e emitters and mesh masks were examined using a scanning electron microscope and on c e r t a i n occasions an analysis of the emitters was made using a K T 1000 x-ray analysis u n i t to inspect the material of the emitters. In a l l cases i t was found that the mesh mask was not closed o f f .

?he mean f r e e path of the copper used f o r these studies was calculatkd /1/ t o be

90.6

assuming the gas pressure was 5.3E-4 Torr above the copper melt pot

( 1 3 5 6 ~ ) . The calculated concentration was 3.8E12 atoms/cm and the Knudsen 3

numbers of L/a where L i s the mean f r e e path and a i s the characteristic dimension

=

5E-5 cm indicates that the flow i s completely molecular i n the chamber t o the masks and substrates.

In one s e t of experiments the mask-substrate assembly was t i l t e d 20' to the d i r e c t l i n e evaporation with the r e s u l t (see Figure 2) that the evaporant molecules a r e evidently being reflected from the overmasks and mesh walls. W e can conclude from t h i s that the sticking coefficient i s not equal t o I and that these reflected atoms have the e f f e c t of establishing different pressures i n the overmask hole and i n

the mesh hole (see Figure 2b). Also, since the atoms have been reflected i t i s most likely that they have l o s t energy t o the mesh and the overmask hole, thus creating thermal gradients near the edge of the overmask hole and the mesh holes.

In Figure 2b the f a i n t l i n e s can be associated with the reflected atoms off the overmask hole while the brighter lines a r e associated with the mesh hole. The mesh was hard clamped a t one edge and the l i n e s themselves a r e an indication of the

thermal expansion of the nickel mesh. Now, while i t i s possible t o thermally protect the nickel mesh by overcoating i t with high temperature oxides l i k e A1203, the problem of manipulating the gas pressure i n the mesh holes s t i l l remains.

Apparently the mesh hole gas pressure i s c r i t i c a l since one can develop emitters or not depending on the pressure and the subsequent reduction i n the mean f r e e path

Fig. 2 - The deposition from an edge clamped 0.50

overmask on a mesh hole mask

t i l t e d 20' to the copper e,aporant beam shows that the atoms a r e reflected both

from the edge of the overmask hole and the mesh holes when the eva orant mean f r e e

path i s long i n the chamber. Scanning electron micrograph of: (a7 the density

gradient depositions i n the 0.50

hole and (b) a close up i n the maximum

gradient area.

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of the evaporant atoms. In another set of experiments argon was admitted in pulses into the chamber up to a pressure of 0.1 Torr and the emitters so formed took on the cone shape as shown in Figure lb. Since the mesh masks were not closed off during the evaporation process the development of the emitter tips may be associated with the mesh cell gas pressures. It would appear that using side evaporation, as Spindt (2) does, /2,3/ to close off the mesh holes with a nonchemical interaction has the effect of changing the mesh hole cell pressure and thus the mean free path of the evaporant in the vicinity of the substrate.

In one set of direct line evaporation experiments the molybdenum overmask hole was varied from 0.5 m to 0.15 m with the result that the highest yield of emitters was obtained with the 0.15 nnn overmask. In Figure 3 there is shown a set of emitters developed by evaporating copper for 2 minutes at 300a/s after a 2 minute soak of the copper through an overmask of 0.15 m diameter. The bases of the emitters are shown with a tilt angle of 0 ' in Figure 3a where it should be noted that the bases are square like the holes in the mesh mask thus indicating a high cell pressure. In Figure 3b the resultant field of emitters is shown from this same evaporation at a tilt angle of 64.2'.

(a) (b)

Fig. 3 - Scanning electron micrographs of copper-chrome covered emitters developed from a fast evaporation through a 0.15 m overmask on a hole mesh.

(a) The square base image of the emitters from the hole mesh indicates that a short mean free path developed in the hole mesh (tilt angle 0 ' ) .

(b) The entire emitter set from the short mean free path development (tilt angle 64.2').

(2) C.A. Spindt, et al. J. de Ph si ue, Col. C9 Sup au n012, Tome 45, dec 84,

pp. C9-269 f f . ^{-- noFe E e} s:ru:tural shapes in figure 11.

S m r y

This study has attempted to understand the mechanism of the various structural shapes which arise when evaporations are made into small holes such as contained in the mesh masks. The studies conducted to date indicate that the shape of the deposition can be manipulated by designing equipment which will alter the mean free path of the evaporant near the substrate. With respect to broad area field emission it is not known yet whether the deposition cusps or wedge shapes will lead to the best long life uniform emission.

Acknowledgements

This work was supported in part by a grant from IBM-Kingston, N.Y. ?he author thanks Dr. R.J. Tofte, Chemistry Dept., SUNY-New Paltz, N.Y. for his helpful discussion and J.J. Sell for technical assistance.

References

/1/ S. Dushman, Scientific Foundation of Vacuum Techni ue, 2nd ed. (~ohn Wilev

Sons. New York.-pte-e

/2/ c.A.. Spindt, 'I. ~rodie; L. _and E.R. Westerberg, 1976: J . a. ^Phys.,

Vol. 47, No. 12, pp. 5248-5263.

/3/ C.A. Spindt, C.E. Holland and R.D. Stowell, Application of Surface Science 16

(North-~olland Pub. Co., 1983), pp. 268-276.