Process for production of field-emission cold cathode

Description Submit all comments and votes

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 .mu.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 .mu.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 .mu.m when the gate aperture has a diameter of about 0.6 .mu.m (an example of the measurement of penetration depth H (.mu.m) by the present inventors was H=D.times.0.3+0.07 when the gate diameter D was 0.6 to 1.6 .mu.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 ".largecircle.") 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 ".circle-solid."). 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 the protective sheet side using a dicing machine, there is no staining of the device surface by the adhesive and, because the protective sheet is bonded to the wafer only at the areas to be cut and the vicinities, there is no staining of the device surface by sludge, etc.

When the above practice is applied in production of field-emission cold cathode, it is presumed that no adhesive contacts with the front end of emitter electrode and that the reduction in emitter property caused by remaining of adhesive is prevented. However, formation of adhesive layer only on the areas of protective sheet slightly larger than the areas to be cut may cause partial peeling of applied protective sheet owing to the pressure applied thereto during dicing, the cutting water may penetrate into the device from the peeled area during dicing, and the intended purpose may not be achieved.

In order to prevent the above partial peeling of applied protective sheet, it is considered to use an adhesive of high adhesivity in formation of a protective film. The protective sheet used for dicing generally comprises a synthetic resin film having a thickness of about 50 to 100 .mu.m and an adhesive layer formed thereon, and its rigidity is not so high. Usually, such a protective sheet is delivered from the supplier in the form of a roll and is used by drawing from the roll. When the protective sheet is drawn from its roll or subjected to looseness removal or aligning, the thin protective sheet, which shows considerable expansion and contraction, tends to expand towards a direction in which the protective sheet receives a force and contracts in a direction perpendicular thereto. When the adhesive layer is formed only on the areas of protective sheet slightly larger the areas to be cut, for example, in a width of the cutting width plus 60 .mu.m (30 .mu.m at one side of the cutting width) as described in Example of the above-mentioned literature, this 60 .mu.m corresponds to 0.06% of wafer diameter when a wafer of 4 in. (10 cm) in diameter is used; when such a protective sheet is applied onto such a wafer, aligning at an accuracy of 0.06% is possible for one particular point of the wafer but is absolutely impossible for the whole wafer surface in view of the expansion and contraction of the protective sheet. Further, the wafer size is increasingly becoming larger, which makes the accuracy of aligning even more difficult. Misalignment and consequent adhesive absence on the areas to be cut or consequent presence of cured adhesive on the areas to be cut invites the penetration of cutting water into device and resultant reduction in emitter property.

Therefore, the width of adhesive application onto protective sheet must be large. However, when the width is large, the adhesive layer may cover the emitter area owing to the misalignment caused by the expansion and contraction of protective sheet. Thus, alignment conducted in application of protective sheet onto wafer is very difficult.

SUMMARY OF THE INVENTION

The present invention alleviates the above-mentioned problems of the prior art and provides a process for producing a field-emission cold cathode, in which process no adhesive is allowed to remain on the front end of emitter electrode by a simple method and thereby reduction in emitter property is suppressed.

The present invention lies firstly in a process for producing a field-emission cold cathode, which comprises steps of applying a protective sheet comprising a base material and an adhesive layer formed thereon, onto a wafer having a plurality of field-emitters formed at the surface and then dicing the resulting material to obtain individual field-emitters, wherein the protective sheet is fitted to a frame for protective sheet and, in that state, is provided with preventive means for flow of adhesive, at the areas of the adhesive layer corresponding to the emitter areas consisting of the plurality of field-emitters. In this process, it is desirable that the frame for protective sheet is provided with aligning means and that the formation of the preventive means for flow of adhesive and the application of the protective sheet onto the wafer are conducted by utilizing the aligning means.

The present invention lies secondly in a process for producing a field-emission cold cathode, which comprises steps of applying a protective sheet comprising a base material and an adhesive layer formed thereon, onto a wafer having a plurality of field-emitters formed at the surface and then dicing the resulting material to obtain individual field-emitters, wherein the application of the protective sheet onto the wafer is conducted by the use of a pressing tool having concave parts at the positions corresponding to each emitter area of cathode.

The present invention lies thirdly in a process for producing a field-emission cold cathode, which comprises steps of applying a protective sheet comprising a base material and an adhesive layer formed thereon, onto a wafer having a plurality of field-emitters formed at the surface and then dicing the resulting material to obtain individual field-emitters, wherein the protective sheet contains, in the adhesive layer, particles having diameters of 10 to 90% of the thickness of the adhesive layer.

In the present process, a protective sheet having protective areas for each emitter area can be applied onto a wafer having field-emitters at the surface, at a high aligning accuracy; or there is used a method for application of protective sheet onto wafer, which requires no alignment of protective sheet, or a protective sheet used in the method. Therefore, in the present process, it is possible to prevent the reduction in emitter property caused by the contact of adhesive with emitter (the reduction has been a problem), and a field-emission cold cathode of high reliability can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an example of the field-emitter obtainable by the present process.

FIG. 2 is a perspective view showing an embodiment of the present process.

FIG. 3 is sectional views showing steps of the present process.

FIG. 4 is a schematic sectional view showing a method for forming protective areas (for each emitter area) in a protective sheet by UV application.

FIG. 5 is a perspective view showing an embodiment of the present process.

FIG. 6 is a schematic sectional view showing an example of the step of applying a dicing sheet on the back side of a wafer.

FIG. 7 is a schematic sectional view showing a protective sheet having protective areas (for each emitter area) formed by UV application and curing and a wafer portion of single cathode.

FIG. 8 is a plan view showing positions of dicing.

FIG. 9 is a schematic sectional view showing dicing from the backside of wafer.

FIG. 10 is perspective views showing other examples of the field-emitters obtainable by the present process.

FIG. 11 is a perspective view showing an embodiment of the present process, wherein alignment is conducted by an improved method.

FIG. 12 is a perspective view showing an embodiment of the present process, and explains the step of the application of protective sheet onto wafer conducted after the operation of FIG. 11.

FIG. 13 is a schematic sectional view showing a case of forming, on a protective sheet, protective areas (for each emitter area) in the form of thin films, by deposition.

FIG. 14 is a schematic sectional view showing a protective sheet having protective areas (for each emitter area) formed in the form of thin films and a wafer portion of single cathode.

FIG. 15 is a schematic sectional view showing a protective sheet having protective areas (for each emitter area) formed in the form of adhesive-free areas and a wafer portion of single cathode.

FIG. 16 is schematic sectional views showing steps of forming concave areas in the adhesive layer of the protective sheet by pressing a convex tool against the adhesive layer.

FIG. 17 is a schematic sectional view showing a protective sheet having protective areas (for each emitter area) formed in the form of concave areas by pressing a convex tool against the adhesive layer of the protective sheet and a wafer portion of single cathode.

FIG. 18 is a sectional view showing an embodiment of the second process of the present invention.

FIG. 19 is a schematic sectional view showing a protective sheet having protective areas (for each emitter area) formed by the method of FIG. 18 and a wafer portion of single cathode.

FIG. 20 is a sectional view showing other embodiment of the second process of the present invention.

FIG. 21 is a schematic sectional view showing a protective sheet having protective areas (for each emitter area) formed by the method of FIG. 20 and a wafer portion of single cathode.

FIG. 22 is a sectional view showing an embodiment of the third process of the present invention and shows a protective sheet and a wafer portion of single cathode.

FIG. 23 is a sectional view showing the structure of the emitter area of FIG. 22.

FIG. 24 is a schematic sectional view showing a problem of the prior art.

FIG. 25 is a graph showing the effect of adhesive on emitter property.

FIG. 26 is schematic sectional views showing conventional steps of producing a Spindt type cold cathode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first process of the present invention, a protective sheet comprising a base material and an adhesive layer formed thereon is fitted to a frame, after which the protective sheet is provided with preventive means for flow of adhesive, at the adhesive layer; therefore, the misalignment between protective sheet and wafer, caused by the expansion or contraction of protective sheet can be prevented. The preventive means for flow of adhesive, formed at the adhesive layer of the protective sheet for safe application of protective sheet onto wafer are specifically either one of the following preventive means. In the following description, the areas of the protective sheet having such preventive means are called protective areas for each emitter area (hereinafter referred to simply as "protective areas").

1. The cured areas of the adhesive layer of protective sheet, obtained by using a photo-curing adhesive as the adhesive of the adhesive layer and, before the application of the protective sheet onto a wafer, applying a light to the areas of the adhesive layer corresponding to each emitter area of cathode, to cure the areas.

2. The thin films made of a material having a rigidity higher than that of the adhesive of the adhesive layer, formed on each area of the adhesive layer of protective sheet, correspond to each emitter area of cathode; or foils applied onto said area of the adhesive layer.

3. The adhesive-free areas of the adhesive layer of protective sheet, formed by coating an adhesive only on the areas of the base material of protective sheet, other than those corresponding to each emitter area of cathode, or by coating an adhesive on the whole surface of the base material and, before the application of the resulting protective sheet onto a wafer, removing the adhesive of only the areas corresponding to each emitter area of cathode.

4. The concave areas of the adhesive layer of preventive sheet, formed on the areas of the adhesive layer corresponding to each emitter area of cathode by pressing a convex tool.

The size of each protective area formed as above can be appropriately determined by taking into account the size of each emitter area of field-emission cold cathode and the adhesivity of adhesive. However, for example, when an emitter area of 0.1 to 0.8 mm in diameter is formed in a chip of 2 mm.times.2 mm, the protective area for this emitter area can be formed in a size about equal to that of the emitter area or in a size having a diameter larger by about 0.2 mm than that of the emitter area, i.e. a diameter of 0.3 to 1.0 mm.

In the second process of the present invention, a pressing tool having concave parts is used and the concave parts are allowed to correspond to each emitter area of cathode, independently of whether or not the protective sheet shows expansion or contraction; thereby, the pressure applied to each emitter area is reduced, the flow of adhesive is suppressed, and the contact of adhesive with front end of emitter electrode can be prevented.

In the third process of the present invention, the adhesive layer of the protective sheet contains fine spheres of given diameters; thereby, the flow of adhesive is suppressed and the contact of adhesive with front end of emitter electrode can be prevented.

By using any of the present processes, a field-emission cold cathode showing no reduction in emitter property can be provided.

EXAMPLE 1

In this Example is described a case wherein there is used a protective sheet comprising a synthetic resin film and an UV-curing type adhesive layer formed on one side of the film and, before the application of the protective sheet onto a wafer having a plurality of field-emitters formed at the surface, only the areas of the adhesive layer of the protective sheet corresponding to the emitter areas of the cathodes of the wafer are irradiated with an ultraviolet light to cure said areas of the adhesive layer.

In a field-emission cold cathode, several hundreds to several tens of thousands of very small field-emitters each having a gate aperture of about 0.6 .mu.m in diameter, such as shown in FIG. 26(c) are arranged in parallel connection and forms an emitter array.

FIG. 1 is a perspective view showing the appearance of a field-emitter device. In FIG. 1, a gate electrode 12 is formed on a substrate 11 via an insulating layer (not shown in FIG. 1); and the above-mentioned emitter array 10 is formed in the center of the gate electrode 12. To the gate electrode 12 is connected a bonding pad 13 for application of gate voltage. The size of the device is ordinarily a square having a side of several millimeters (about 2 to 10 mm) when the device is used as electron sources such as electron tube or the like. Field-emitters are not produced each independently, but are produced by a wafer process such as used in the semiconductor industry and then divided into individual devices.

In this Example is described the ordinary operational process employed after wafer process, with reference to some of the accompanying drawings. FIG. 3(a) is a sectional view of a wafer 21 which has completed the wafer process. On the surface of the wafer 21 [the upper side of FIG. 3(a)] are arranged emitter areas 20 each containing an emitter array, in a matrix form. Since each emitter area is actually too small to identify, it is written in a much larger size in FIG. 3(a) to show its position.

A protective sheet 22 is applied on the surface of the wafer 21 in order to protect each emitter area 20 and each device surface from cutting water, sludge, etc. during dicing. The protective sheet 22 comprises (1) a base material made of a polyolefin, a polyethylene or the like and having a thickness of about 50 to 100 .mu.m and (2) an adhesive layer made of an acrylic, a urethane adhesive or the like, formed on one side of the base material in the thickness of about 5 to 20 .mu.m. In this Example, there is used an adhesive layer made of an UV-curing acrylic adhesive. For both the base material and the adhesive layer, it is desirable to use transparent materials for alignment conducted for dicing.

The protective sheet is fixed to a frame for protective sheet before its application onto a wafer. For example, the protective sheet 22 is stuck to a frame 25 for protective sheet, at the adhesive layer 24 side, as shown in FIG. 2. The position relationship between the protective sheet 22 and the frame 25 may be opposite to the above. In that case, an adhesive is beforehand coated on the area of the frame to contact with the base material 23 of the protective sheet 22, or the protective sheet 22 is fixed to the frame 25 by a mechanical means. In a state that the protective sheet 22 is fixed to the frame 25, only the areas of the protective sheet 22 corresponding to each emitter area are irradiated with an ultraviolet light. Specifically, as shown in FIG. 4, a mask 29 having apertures 30 of about the same size as each emitter area at the areas corresponding to each emitter area, is placed on or in close vicinity to the protective sheet 22; and an ultraviolet light is applied to the protective sheet 22 via the mask 29. As the mask 29, there can be used, for example, a mask obtained by forming, by etching, apertures 30 in a stainless steel sheet (a rolled foil) having a thickness of about 0.1 mm. The UV irradiation is conducted via the mask 29 by placing the mask 29 at the adhesive layer 24 side or base material 23 side of the protective sheet 22. As a result of the UV irradiation, the irradiated areas of the adhesive layer 24 are cured and become protective areas (cured areas) 19. Since it is difficult to distinguish the cured areas from the uncured areas, it is desirable to form aligning marks before or after or simultaneously with the UV application, as mentioned later.

As shown in FIG. 2, the application of the protective sheet onto a wafer is conducted by moving the frame 25 for protective sheet in X, Y and .theta. directions so that the emitter areas on the wafer 21 and the protective areas 19 formed in the protective sheet 22 can match with each other. In that case, any of the wafer and the frame 25 may be moved.

By forming the protective areas after the protective sheet has been fixed to the frame, there occurs no deformation of the protective sheet of low rigidity, the relative positions of the formed protective areas can be maintained, and the positioning of the protective sheet relative to the wafer can be conducted at a high accuracy. Further fixation of the protective sheet to the frame makes easy the handling of the protective sheet.

The material for the frame 25 can be any as long as it has a sufficient rigidity, and a metal, a plastic of high rigidity, etc. can be used. The shape of the frame 25 can be any as long as the above requirement of sufficient rigidity is satisfied, and any of rectangular shape (as shown in FIG. 2), circular shape, polygonal shape, elliptical shape and the like can be used. With respect to the thickness of the frame 25, a frame having a thickness larger than that of a wafer is used in FIG. 2; however, the thickness of frame may be about the same as the thickness of wafer, or may be smaller than the thickness of wafer as long as the frame has a sufficient rigidity.

As mentioned above, the positions of the protective areas (cured areas) 19 can be allowed to agree with the positions of the gate apertures of the devices of the wafer 21; therefore, there is no contact of the adhesive with the front end of each emitter electrode in the gate aperture. As a result of the alignment, the state of FIG. 3(b) is obtained.

Successively, onto the backside of the wafer 21 is applied a dicing sheet 26 which is necessary for holding of the wafer 21 during dicing and also for stretching of the wafer 21 after dicing. As the dicing sheet 26, there can be used, for example, a sheet obtained by forming an acrylic adhesive layer on a polyolefin base material, as in the case of the protective sheet 22. The dicing sheet 26, however, need not be an UV-curing type. When a ring-shaped frame 27 for dicing is fitted to the periphery of the wafer 21, the handling of the wafer 21 after the application of the dicing sheet 26 onto the wafer 21 becomes easier. By cutting and removing the unnecessary portion of the dicing sheet 26 outside the frame 27, a state of FIG. 3(c) is obtained. The position relationship between the frame 25 for protective sheet and the frame 27 for dicing is not restricted to that shown in FIG. 3(c). The frame 25 for protective sheet can function also as a frame for dicing and, in this case, no frame for dicing is necessary.

Application of the dicing sheet 26 onto the wafer 21 may be conducted, for example, by using a base 35 having a clearance 34 at least at the area contacting with the emitter areas 20, such as shown in FIG. 6 and applying a pressure by the use of a roller 33. This method of application is desirable because the protective sheet 22 receives no further pressure.

Next, dicing is conducted. Dicing is conducted in full cutting; that is, the protective sheet 22, the wafer 21 and the upper portion (about 20 .mu.m) of the dicing sheet 26 are cut and the cut wafers 21 are separated from each other completely. Thereby, a state of FIG. 3(d) is obtained. The relation between each device and dicing positions is shown in FIG. 7 and FIG. 8. In the protective sheet 22, only the protective areas 19 corresponding to the emitter areas 20 are cured by irradiating UV light, are low in fluidity (deformability), and make no contact with the emitters; meanwhile, the protective areas 19 have low adhesivity to the wafer 21. However, the areas of the protective sheet 22 other than the protective areas 19 are adhered to the wafer 21 at an ordinary strength. In FIG. 8, the dicing positions 28 are shown by each two parallel broken lines between adjacent devices, and dicing is conducted at the areas where the protective sheet 22 and the wafer 21 are bonded at a sufficient strength. Therefore, the emitter areas 20 are protected from cutting water and sludge during dicing.

After dicing, the cutting water and sludge adhering on the protective sheet surface are removed by washing, followed by drying; an ultraviolet light is applied from the base material 23 side of the protective sheet 22 to lower the adhesivity of the adhesive layer 24 of the protective sheet 22; then, a release sheet obtained by coating an adhesive on a base material (a synthetic resin film) is applied onto the protective sheet, and the protective sheet 22 is peeled together with the release sheet by utilizing the adhesivity of the release sheet [FIG. 3(e)].

In this Example, there was explained a case of applying a protective sheet 22 on the surface of a wafer 21 and a dicing sheet 26 on the backside of the wafer 21 and then conducting dicing from the surface side of the wafer. However, dicing from the backside of a wafer, such as shown in FIG. 9, is possible. In this case, it is possible to set standard points beforehand at the periphery of the wafer 21 and decide dicing positions based on the standard points, or, in the case of a silicon wafer, to conduct positioning by the use of a transmitted light (an infrared light).

In this Example was explained a case of producing a device as shown in FIG. 1, i.e. a device containing one emitter array. However, this Example is also applicable to a case of producing a device such as shown in FIG. 10(a), i.e. a device having a focussing electrode 14 for converging electron beams, at the periphery of a gate electrode; in this case, the protective areas 19 of protective sheet can be formed so as to correspond to each emitter array 10. This Example is further applicable to a case of producing a device such as shown in FIG. 10(b), i.e. a device containing a plurality of emitter arrays 10; in this case, one protective area 19 may be formed so as to correspond to one emitter array 10, or one protective area may be formed so as to cover a plurality of emitter arrays 10.

In this Example, the areas of the adhesive layer of protective sheet corresponding to the emitter areas of wafer are cured by UV irradiation, prior to the application of the protective sheet onto the wafer; thereby, the deformation (flow) of adhesive during the application of the protective sheet onto the wafer is suppressed, the amount of the adhesive penetrating into gate aperture is minimized, and there is no contact of adhesive with emitter. As a result, there is no remaining of adhesive on emitter surface and an excellent emission property (a curve containing o marks, in FIG. 25) is obtained. In the above was described a case where aligning marks are formed on both the protective sheet and the wafer, and application of the protective sheet onto the wafer is conducted utilizing the aligning marks. Below is described a case where said application is conducted easily at a high aligning accuracy.

As shown in FIG. 11, when UV application is made, via a metal mask 29, onto a protective sheet 22 fixed to a frame 25 for protective sheet, to form protective areas at the surface of the protective sheet 22, aligning holes 37 and 36 are beforehand formed in the frame 25 and the metal mask 29, respectively; the aligning pins 38 of a jig 39 are inserted into the holes 37 and 36; and UV irradiation is conducted in that state. This is a case where UV irradiation is made only to part of the adhesive layer of protective sheet to form protective areas at the protective sheet surface, and this practice is applicable also to later Examples. In the above, the jig 39 having registration pins 38 was used. Alternatively, aligning pins may be formed in the frame 25 or the mask 29, and holes corresponding to the pins may be formed in the mask 29 or the frame 25; or, based on outer side of frame 25, base means to which the jig 39 is applied are formed. It is advisable to form aligning means so that they can be used up to when the protective sheet is applied onto a wafer.

Next, the protective sheet 22 is applied onto a wafer 21. As shown in FIG. 12, alignment is conducted by inserting the aligning pins 38 of a jig 39 into the aligning holes 40 of a frame 27 attached to a dicing sheet 26 holding a wafer 21 and also into the aligning holes 37 of a frame 25 fixed to a protective sheet 22 having protective areas formed at the surface; thereby, the emitter areas of the wafer 21 and the protective areas of the protective sheet 22 are matched with each other, and application of protective s