Process for production of field-emission cold cathode

Details Process for production of field-emission cold cathode Process for production of field-emission cold cathode

1998-10-01

2001-01-09

Ramsey, Kenneth J. (Department: 2879)

Electric lamp or space discharge component or device manufacturi

Process

With assembly or disassembly

C438S460000, C428S200000, C428S206000

Reexamination Certificate

active

06171163

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for production of a field-emission cold cathode having emitter electrodes each with an acute from end and particularly to an improved method for applying a protective sheet onto a wafer having a plurality of field-emitters formed at the surface, to later divide the cathodes into individual devices by dicing. The present invention also relates to a protective sheet suitably used in the method.
2. Description of the Related Art
“Field-emission cold cathode”, was developed as an electron emitting source replacing “thermionic cathode” which emits electrons upon application of heat. Field-emission cold cathode emits electrons into air by quantum-mechanical tunneling which takes place when a strong electric field (2 to 5E7 V/cm or more) is generated at the acute front end of each emitter electrode. Therefore, the property of field-emission cold cathode is dependent upon the acuteness of emitter front end of each field-emitter, and it is said that the emitter front end is required to have a radius of curvature of several hundreds of angstrom (A) or less. Further, to generate an electric field such as above, it is necessary to arrange the two electrodes (emitter electrode and gate electrode) of field-emitter in close vicinity to each other (the distance between them is about 1 &mgr;m or less) and applying a voltage of several tens to several hundreds volts (V). The surface cleanness of emitter electrode affects its work function as well and is an important factor for determining the emittability of the electrode.
In actual application of field-emission cold cathode, several thousands to several tens of thousands of field-emitters are formed on a wafer and used as an array where they are connected in parallel, in many cases. Therefore, utilizing the technique used for fine processing of semiconductor produces them.
As an actual production process of field-emission cold cathode, there is a process developed by Spindt et al. of SRI (Stanford Research Institute) of U.S.A., described in J. Appl. Phys. 39, p. 3504, 1968. In the process, a refractory metal (e.g. molybdenum) is deposited on a conductive substrate to form a structure having an acute front end. The process is shown in FIG.
26
.
First, on a silicon substrate
71
is formed an oxide film as an insulating layer
72
. Subsequently, molybdenum is vacuum-deposited as a gate layer
74
. Then, a resist
73
is applied, and photolithography and etching are conducted to form an aperture
77
having a diameter of about 1 &mgr;m. The insulating layer
72
is etched via the aperture
77
[FIG.
26
(
a
)]. Thereafter, oblique deposition with rotation is conducted to form a sacrifice layer
78
made of aluminum. Then, molybdenum is vacuum-deposited in a vertical direction to form an emitter electrode
75
and a molybdenum film
76
[FIG.
26
(
b
)]. Lastly, the sacrifice layer
78
is subjected to selective etching to lift off the molybdenum film
76
formed on the sacrifice layer
78
, whereby a device structure is obtained [FIG.
26
(
c
)].
In the device produced by the above process, when a voltage is applied so that the emitter electrode
75
becomes negative and the gate electrode
74
becomes positive, electrons are emitted from the front end of the emitter electrode
75
into a direction perpendicular to the substrate
71
. Such a structure is generally called a vertical type field-emitter.
The applications of such a field-emission cold cathode include use as electron sources such as electron tube or the like. In the above process, as in the process generally used for production of semiconductor device, several hundreds to several thousands of devices are produced simultaneously in one wafer. In order to mount these devices on an electron tube, the devices must be divided into individual devices by dicing.
Dicing is a step of cutting a wafer by the use of a grindstone (an abrasive such as diamond, C-BN or the like) rotating at a high speed. Dicing is generally conducted by injecting cutting water on the cutting area of wafer for the purpose of cooling and prevention of sludge scattering. However, the cutting water flows on the wafer surface having devices formed and carries sludge (of silicon wafer and conductive substance such as electrode material or the like) to the vicinity of each emitter electrode. Remaining of sludge in the vicinity of emitter invites deterioration of the insulating property of emitter and impairs the reliability of device.
For alleviation of the above problems, it has been conducted to cover, at the time of dicing, a wafer with a protective sheet comprising a resin base material and an adhesive coated thereon, whereby the insulating property of emitter has been maintained at a satisfactory level.
The general procedure of the dicing step is described below.
First, onto the wafer surface after emitter formation and lift-off is applied a protective sheet comprising a base material and an UV-curing adhesive coated thereon. An adhesive sheet is applied onto the backside of the wafer, and dicing is conducted at the protective sheet side. In dicing, the cutting water containing sludge is injected onto the wafer surface; however, the presence of the protective sheet prevents direct contact of the water with the emitter area, whereby no defects such as poor insulation caused by dust, stain and the like arise.
After dicing, the protective sheet is peeled from the wafer. This peeling can be conducted easily, for example, by using an UV-curing adhesive as the adhesive of the protective sheet and applying an ultraviolet light onto the protective sheet surface to cure the adhesive and lower its adhesivity.
However, the adhesive of the protective sheet flows on the wafer by the pressure applied to the protective sheet for its application onto the wafer. As a result, as shown in
FIG. 24
, at the area where an emitter is formed, the adhesive of a protective sheet
22
comprising a base material
23
and an adhesive layer
24
penetrates into the aperture
77
of a gate electrode
74
and contacts with the front end of an emitter electrode
75
. The depth of penetration of adhesive is about 0.25 &mgr;m when the gate aperture has a diameter of about 0.6 &mgr;m (an example of the measurement of penetration depth H (&mgr;m) by the present inventors was H=D×0.3+0.07 when the gate diameter D was 0.6 to 1.6 &mgr;m). By peeling the protective sheet after dicing, the adhesive is removed and remaining of adhesive is not detectable even by SEM observation or the like. However, a very small amount of the adhesive remains on the emitter surface actually, which has increased the effective work function of emitter surface and has deteriorated the property of device.
FIG. 25
shows the emission properties of (1) a device produced by the above standard process (Adhesive Free: defined as “◯”) and (2) a device also produced by the above standard process (in this case, application of protective sheet, and peeling of the sheet were conducted to examine the effect of remaining adhesive, Adhesive Remained: defined as “&Circlesolid;”). The emission property of the latter device formed using a protective sheet was apparently inferior to that of the former device.
Japanese Patent Application Kokai (Laid-Open) No. 356942/1992 discloses the followings. When an adhesive layer is formed selectively on the areas of protective sheet slightly larger than the areas to be cut, the resulting protective sheet is applied onto a wafer, and dicing is made at the protective sheet side using a dicing machine, or when an UV-curing type adhesive layer is formed on the whole surface of protective sheet, a light-shielding mask is placed only on the areas of protective sheet slightly larger than the areas to be cut, an ultraviolet light is applied onto the protective sheet to cure the adhesive layer at the areas other than the areas to be cut, the resulting protective sheet is applied onto a wafer, and dicing is made at th

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