JP3131173B2 - Method for manufacturing field electron emission type surge absorbing element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for protecting an electronic circuit of an electronic device from an overvoltage such as a surge that enters through a power line or a communication line, or between lines or between each line and a ground. More particularly, the present invention relates to a method for manufacturing a surge absorbing element using a field electron emission phenomenon.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 16, between a line L1 and L2 such as a power supply line and a communication line leading to an electronic circuit 70 of an electronic device,
Alternatively, a surge absorbing element 72 is connected between each line and GND (ground) to protect the electronic circuit 70 from a surge such as an induced lightning. That is, when a surge voltage higher than the rating of the surge absorbing element 72 is applied between the lines L1 and L2 or between the lines L1 and L2 and GND, the surge absorbing element 72 conducts and bypasses the surge. This is a mechanism for protecting the electronic circuit 70.

As such a surge absorbing element 72, a gas arrester utilizing a discharge phenomenon in a discharge gap, a varistor which is a high-resistance element having a nonlinear voltage characteristic, or an avalanche (electron avalanche) of a pn junction type semiconductor is used. There are various types such as silicon surge absorbers utilizing the effect, but recently a surge absorbing element using a field electron emission phenomenon has been newly added.

[0004] A surge absorbing element using this field electron emission phenomenon is described in Japanese Patent Application No. 8-132728. FIG.
As shown in FIG. 7, a field electron emission type surge absorbing element 74 disclosed therein is a first substrate member made of an n-type semiconductor.
The second substrate member 14 and the second substrate member 14 are opposed to each other at a predetermined distance, and the inner peripheral edge of both members is a frame member 16 also serving as a spacer.
And an envelope 18 formed by hermetically sealing the inside of the envelope, and the inside of the envelope 18 is 10 ^-6 to 10 ^-8 Torr.
Is maintained in a high vacuum state. On the inner surface of the first substrate member 12, a large number of emitter cones 20 made of an n-type semiconductor are protruded at predetermined intervals. The emitter cone 20 has a pointed cone or pyramid shape, with the tip facing the inner surface of the second substrate member 14. The inner surface of the first substrate member 12 including the surface of the emitter cone 20 and the inner surface of the second substrate member 14
b, W, Mo, Cr, Ti, Th, Si, Ni, La,
A thin film made of Ge, Al, etc., a two-layer structure of W and Zr,
Alternatively, a protective film 76 made of a carbide, oxide, nitride, or inorganic compound containing at least one of the above substances is covered. First external electrodes 24 are provided on the outer surfaces of the first substrate member 12 and the second substrate member 14, respectively.
And a second external electrode 26, and a cathode terminal 28 and an anode terminal 30 are connected to each external electrode. By connecting the terminals 28, 30 to the lines L1, L2 or GND, the above-mentioned field emission type surge absorbing element 74 is connected.
Is inserted and connected between the lines L1 and L2 or between the lines L1 and L2 and GND, as shown in FIG.

The line L1 and L2 or the line L
1. When an overvoltage exceeding a rating such as a surge is applied between L2 and GND, and strong electric field concentration occurs at the tip 20a of the emitter cone on the cathode side, electrons in the n-type semiconductor are caused by a quantum mechanical tunnel effect. A so-called field electron emission phenomenon that is emitted into a vacuum beyond the potential barrier occurs. The emitted electrons are captured on the anode side of the high potential, that is, on the inner surface of the second substrate member 14, so that a precursor discharge in which a current flows between the second substrate member 14 and the first substrate member 12 is generated. The precursor discharge then shifts to a vacuum spark discharge (vacuum arc discharge). The following is conceivable as a mechanism in which the precursor discharge shifts to a vacuum spark discharge. That is, due to the action of thermal energy generated by the increase in current density at the tip of the emitter cone due to electron emission during the precursor discharge, metal vapor is formed from the metal constituting the protective film 76 covering the surface of the emitter cone. Occur,
The metal energy of the protective film 76 covering the inner surface of the second substrate member 14 also generates metal vapor due to thermal energy generated as a result of electrons colliding with the anode side due to the precursor discharge, and these charged metal vapors generate current. And a vacuum spark discharge is generated. Further, it is practically difficult to make the inside of the envelope 18 a complete vacuum, and gas molecules are slightly adsorbed or adhered to the surface of the material constituting the discharge space. Is released into the space by the impact of the precursor discharge, and the ionized gas molecules serve as elements for forming a current. This is also a factor promoting vacuum spark discharge.

The above-mentioned field electron emission phenomenon occurs only when the electric field intensity concentrated on the emitter cone 20 becomes higher than a predetermined value. This occurs when a voltage higher than a predetermined value is applied between both electrodes. Only means that a current flows between both electrodes. In other words, since a nonlinear relationship appears between the value of the voltage applied between the two electrodes and the flowing current, surge absorption occurs only when an overvoltage exceeding the rated voltage is applied and the overvoltage is bypassed. The function can be exerted.

In addition, compared with the speed of the electrons in the semiconductor, the electrons in the vacuum travel without being scattered, so that the field electron emission type surge absorbing element 74 can operate at an extremely high speed.

FIG. 18 shows the surge absorption characteristics of such a field emission type surge absorbing element 74.
It is a graph which shows the surge absorption waveform with respect to the original surge waveform whose peak voltage value is 3 kV. As shown in the figure, when a surge voltage is applied, a precursor discharge having a peak of about 2.32 kV is instantaneously generated.
A state in which a stable surge absorption waveform of 0 V is obtained is shown.

The emitter cone 20, which is the most important component of the field emission type surge absorbing element 74,
It was formed as follows. First, as shown in FIG. 19, an n-type Si substrate 42 is oxidized in an oxidizing atmosphere, and a surface of about 150 to 3000 angstroms thick
An O ₂ thin film 44 is formed. Then, a photoresist process, about the diameter on the surface of the SiO ₂ thin film 44 3 to 10 [mu] m
A circular oxide film mask 52 is formed by selectively etching the SiO ₂ thin film 44 by BHF (wet etching using buffered hydrofluoric acid) as shown in FIG.

Next, as shown in FIG.
Is subjected to anisotropic wet etching to erode the surface other than the oxide film mask 52, and this etching is stopped immediately before the oxide film mask 52 is separated. This results in an undercut 54 and a prototype emitter cone with a flat tip. Since the tip of the emitter cone cannot be sharpened by this method alone, as shown in FIG. 22, the surface is oxidized again to form the SiO ₂ thin film 44, and then wet etching is performed to remove the SiO ₂ thin film 44, and the tip 2 is removed.
0a forms a sharp emitter cone 20 (FIG. 2).
3).

[0011]

The operating voltage (rated voltage) of the field electron emission type surge absorbing element 74 depends on the degree of vacuum in the envelope 18 and the tip 20a of the emitter cone.
Distance between the inner surface of the substrate member 14 and the emitter cone 20
Surface work function or emitter cone tip 20a
Is determined by the sharpness of That is, when it is necessary to set the operating voltage of the field electron emission type surge absorbing element 74 relatively low, a method of increasing the degree of vacuum in the envelope 18,
A method for further reducing the distance between the tip end portion 20a of the emitter cone and the inner surface of the second substrate member 14,
Theoretically, a method of reducing the work function of the surface or a method of sharpening the tip 20a of the emitter cone can be considered. However, there is a certain limit in improving the degree of vacuum in the envelope 18, and it is extremely difficult to control the positional relationship between the tip 20a of the emitter cone and the second substrate member 14 on the order of μm. is there. In addition, in the conventional method in which the surface of the substrate member is subjected to an etching process and the emitter cone 20 is formed by using an undercut by over-etching, there is a certain limit in increasing the sharpness of the emitter cone tip portion 20a. there were. Further, since the emitter cone 20 is formed integrally with the substrate member, it is not suitable to greatly reduce the work function of the emitter cone 20 itself. Therefore, it is necessary to select a protective film covering the surface of the emitter cone that has the smallest possible work function and good discharge characteristics in order to reduce the firing voltage of the field emission type surge absorbing element. This is a more realistic method.

The present invention has been devised in view of the above-mentioned conventional problems, and an object thereof is to devise a constituent material of a protective film covering the surface of an emitter cone. Another object of the present invention is to realize a method of manufacturing a field electron emission type surge absorbing element in which an operating voltage can be set relatively low.

[0013]

In order to achieve the above object, a method of manufacturing a field emission type surge absorbing element according to the present invention comprises a semiconductor having a plurality of emitter cones integrally formed on one surface. A first substrate member provided with an electron emission portion, and a second substrate member provided with a flat portion on one surface,
The distal end portion of the emitter cone of the first substrate member and the plane portion of the second substrate member are arranged so as to face each other at a predetermined distance, and the outer edges of both substrate members are hermetically sealed. Forming an envelope, forming the inside of the envelope in a high vacuum state, forming external electrodes on the outer surfaces of both substrate members, respectively,
A method for manufacturing a field electron emission type surge absorbing element comprising forming a protective film made of a diamond thin film on the surface of the emitter cone, wherein a number of emitter cones are integrally formed on a surface of a substrate member made of a semiconductor, After forming a large number of fine scratches on the surface of the substrate member, the substrate member is placed in a vacuum system heated to a predetermined temperature, and a diamond material mixed with an impurity gas on the surface of the substrate member. The diamond thin film is formed by introducing a gas, activating the source gas, growing a diamond thin film which has been converted into a semiconductor on the surface of the emitter cone, and adjusting the concentration and pressure of the source gas of the diamond. controls sharpness of diamond crystals, also, the diamond
Operation by adjusting the amount of impurity added to
It is characterized in that the voltage is controlled .

In another aspect of the present invention, there is provided a method of manufacturing a field electron emission type surge absorbing element, comprising: an electron emission portion made of a semiconductor having a plurality of emitter cones integrally formed on one surface; and a flat portion having no emitter cone formed. A second substrate member having a semiconductor substrate having a plurality of emitter cones integrally formed on one surface, and a planar portion having no emitter cone formed thereon. The tip of the emitter cone of one of the substrate members and the plane portion of the other substrate member are arranged so as to face each other at a predetermined distance, and the outer edges of both substrate members are hermetically sealed to form an envelope. And forming the inside of the envelope into a high vacuum state, forming external electrodes on the outer surfaces of both substrate members, and further forming a protective film made of a diamond thin film on the surface of the emitter cone. A method for manufacturing a field electron emission type surge absorbing element, wherein a number of emitter cones are integrally formed on a surface of a substrate member made of a semiconductor, and a number of fine scratches are formed on the surface of the substrate member. The substrate member is arranged in a vacuum system heated to a temperature of, and a source gas of diamond mixed with an impurity gas is led to the surface of the substrate member, and the source gas is activated to activate the surface of the emitter cone. By growing a diamond thin film that has been made into a semiconductor, adjusting the concentration and pressure of the source gas of the diamond, the sharpness of the diamond crystal constituting the diamond thin film is controlled , and the diamond
The operating voltage can be adjusted by adjusting the amount of impurities added to the thin film.
Is controlled .

The diamond crystal has a negative electron affinity,
That is, since the band is bent so that the femil level on the surface coincides with the bottom of the conduction band, it has a property that the work function is small. For this reason, if a predetermined voltage or more is applied between both external electrodes of the field electron emission type surge absorbing element manufactured by the above method, and the electric field concentration occurs at the tip of the emitter cone, the tunnel effect is not required. Electrons are emitted from the diamond crystal by applying a relatively low voltage, and a high current density is obtained. This means that it is possible to set the operating voltage of the field emission type surge absorbing element to a relatively low value without reducing the distance between the electrodes or increasing the degree of vacuum in the envelope. means. Alternatively, if it is not necessary to set the operating voltage to a very low value, the conventional emitter / emitter may be used because the work function of the protective film covering the surface of the emitter cone is low.
The distance between the electrodes can be set to be larger than in the case where the cone and the protective film are used, and the manufacture of the field emission type surge absorbing element can be facilitated.

Diamond has excellent high-temperature stability,
Can withstand high temperatures up to 100 degrees. In addition, since it is the hardest substance on the earth, it has excellent spatter resistance. Therefore, by covering the surface of the emitter cone with a protective film made of a diamond thin film, it is possible to dramatically improve the life characteristics of the field emission type surge absorbing element.

In the present invention, since a diamond thin film is formed by the so-called chemical vapor deposition (CVD) as described above, a specific crystal plane of polycrystalline diamond can be selectively grown. For example, the negative electron affinity described above is particularly remarkable on the <111> plane of diamond, but during the above-mentioned manufacturing method, the <111> plane is selectively grown by adjusting the concentration of the source gas and the synthesis time. It can be done. In addition, the sharpness of the diamond crystal constituting the diamond thin film can be controlled by adjusting the concentration and pressure of the source gas of diamond.

Although an undoped diamond thin film containing no impurities has intrinsic semiconductivity, as described above, by mixing a predetermined impurity gas into a diamond source gas during the film formation process, its semiconductor properties are enhanced. be able to. For example, by mixing B (boron), a p-type semiconductor is formed, and N (nitrogen), P (phosphorus), C (carbon)
Can be made into an n-type semiconductor.

Further, by adjusting the amount of each impurity to be added, it becomes possible to control the resistivity of the diamond crystal which has been converted into a semiconductor. Furthermore, as the amount of impurities added increases, the diamond formed has a higher femil level, resulting in a lower work function and a lower firing voltage. This means that the operating voltage of the field electron emission type surge absorbing element can be controlled by adjusting the amount of impurity added.

In the emitter cone, a step of forming a large number of oxide masks on the surface of the substrate member, and performing a reactive ion etching on the surface of the substrate member so as to cover a portion not covered by the oxide mask. Erode,
Thus, a step of forming a columnar protrusion serving as a prototype of the emitter cone below the oxide film mask, and performing anisotropic wet etching on the protrusion to erode the surface to sharpen the tip. It is desirable to form it through a sharpening process.

Reactive ion etching (hereinafter abbreviated as "RIE") is classified as so-called dry etching.
A method in which a sample is placed between two parallel electrodes, a predetermined gas medium is filled in a vacuum chamber, and high-frequency plasma discharge is generated between the two electrodes to perform etching. And etching by a chemical reaction are superimposed to realize processing of the sample. For this reason, in addition to the characteristics that the etching rate is high and the substance to be etched can be selected, the etching direction is concentrated in the ion bombardment direction, and therefore, there is an advantage that anisotropy is excellent.

In wet etching, a sample is immersed in a predetermined chemical and etching is performed by utilizing the chemical reaction. Generally, the wet etching is excellent in the selectivity of a substance to be etched, but has strong isotropy. That is, it is characterized in that it is difficult to control the etching direction and overetching easily occurs. On the other hand, in the anisotropic wet etching, by devising the selection of an etchant, the isotropic property is reduced and the etching direction is easily controlled while the wet etching is performed.

In the above-described conventional method of forming an emitter cone, anisotropic wet etching in which the etching direction is relatively easy to control is used because wet etching is performed from the beginning on a flat substrate member. However, if the erosion is performed for a long time, the erosion in the width direction proceeds before the height of the emitter cone is formed by over-etching, so that an emitter cone having a sufficient height and width cannot be formed. For this reason, conventionally, the anisotropic wet etching cannot be continued until the tip of the emitter cone obtains a sufficient sharpness, and it has to be rounded up in an appropriate time. So, the emitter
After oxidizing the cone surface again, the tip was sharpened by removing this oxide film, but since the oxide film itself is only formed very thinly on the surface of the emitter cone, Removing this did not make the tip much sharper.

On the other hand, by using RIE which is more excellent in anisotropy, firstly, a columnar projection is formed on a plate-like substrate member to secure the height and width of the emitter cone to a certain extent. By using the undercut by etching to sharpen this projection, the height and width of the emitter cone can be reduced even if anisotropic wet etching is performed until the tip of the projection is sufficiently sharpened. It can be maintained at the required level. Actually, in the conventional forming method, the height of the emitter cone can be formed only to about 1 to 1.3 μm at most, but by using the forming method according to the present invention, the height of the emitter cone is reduced to 5 μm. Now you can set it.

In short, it is extremely difficult to sharpen the tip of a completely flat substrate member while securing the height and width simultaneously by anisotropic wet etching. If the anisotropic wet etching is performed after forming an emitter cone prototype having a width and a width, the tip can be sufficiently sharpened in a relatively short time. As described above, the sharpness of the tip of the emitter cone can be further increased as compared with the conventional method of forming the emitter cone. It is possible to further reduce the operating voltage of the device.

As a method of forming a large number of fine scratches on the surface of the above-mentioned substrate member, for example, a substrate member integrally formed with an emitter cone is immersed in a solution containing fine diamond particles, and then immersed in the solution. By applying ultrasonic vibration, it is possible to cause diamond particles to violently collide with the surface of the substrate member including the emitter cone.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first field emission type surge absorbing element 10 according to the present invention, as shown in FIG. 1, places a first substrate member 12 and a second substrate member 14 at a predetermined distance. The outer peripheral portions of the two substrate members are hermetically sealed with a frame member 16 also serving as a spacer interposed therebetween.
The envelope 18 is formed, and the inside of the envelope 18 is maintained in a high vacuum state of 10 ^-6 to 10 ^-8 Torr.

A recess 19 having a depth of several μm is formed on the inner surface of the first substrate member 12. On the bottom surface of the concave portion 19, a large number of emitter cones 20 are provided so as to project over substantially the entire surface at a predetermined interval. Although FIG. 1 is a cross-sectional view, only one row of emitter cones 20 is shown. However, actually, the emitter cones 20 are arranged vertically and horizontally at regular intervals. The first substrate member 12 is formed of an n-type semiconductor in which impurities such as P and As are mixed in Si. The emitter cone 20 is also made of an n-type semiconductor and is formed integrally with the first substrate member 12.
The emitter cone 20 has a pointed cone or pyramid shape, and the tip 20 a faces the inner surface of the second substrate member 14. However, the tip of the emitter cone 20
A predetermined gap is maintained between 20a and the inner surface of the second substrate member 14. The height of the emitter cone 20 is about 5
μm, the diameter of the bottom surface is set to about 3 to 10 μm, and the pitch between the emitter cones 20 is set to about 7.5 to 15 μm.

The first including the surface of the emitter cone 20
The inner surface of the substrate member 12 is coated with a first protective film 22. The first protective film 22 is composed of a diamond thin film, and as shown in FIG.
On the surface of 20, a large number of fine diamond crystals 23 are formed so as to be folded. Most of the diamond crystals 23 are constituted by <111> planes having particularly remarkable negative electron affinity and excellent electron emission characteristics.

On the outer surface of the first substrate member 12, a first layer 24a formed by evaporating Al or Cr, and a second layer 24 formed by evaporating Ni on the surface of the first layer 24a. A first external electrode 24 made of the layer 24b is formed. Al or Cr constituting the first layer 24a and Ni constituting the second layer 24b have good ohmic contact.
Was selected to realize

The second substrate member 14 is made of an n-type semiconductor similarly to the first substrate member 12, and the inner surface thereof is covered with a second protective film 25. This second protective film 25 is a thin film made of W, Mo, Ni, La, Al, etc.
Alternatively, it is composed of a carbide, oxide, nitride, or inorganic compound containing at least one of the above substances.
Further, on the outer surface of the second substrate member 14, Al or Cr
Is formed on the surface of the first layer 26a, and a second external electrode 26 is formed of a second layer 26b formed by evaporating Ni on the surface of the first layer 26a.

As is clear from the above, in the first field emission type surge absorbing element 10, the entire inner surface of the first substrate member 12 constitutes an electron emission portion, and The entire inner surface of the substrate member 14 constitutes a flat portion. As a constituent material of the second substrate member 14, other than the n-type semiconductor, other substances having substantially the same coefficient of thermal expansion as the first substrate member 12 can be used.
Corresponds to this.

The first external electrode 24 and the second external electrode
26 need not necessarily have a two-layer structure, and may be formed by evaporating only Ni. As the material of the frame member 16, Pyrex glass or the like having a thermal expansion coefficient close to that of the n-type semiconductor is used.

The first external electrode 24 has a cathode terminal 28
The anode terminal 30 is connected to the second external electrode 26. Each terminal is connected to a line L in the same manner as shown in FIG.
By connecting to 1, L2 or GND, the first field emission type surge absorbing element 10 is inserted and connected between the lines L1, L2 or between the lines L1, L2 and GND.
Thus, the line L1, L2 or the line L1, L2-GN
When a surge voltage higher than the rated voltage is applied across D, a strong electric field concentration occurs at the tip portion 20a of the emitter cone 20, and electrons are emitted into a vacuum through a potential barrier on the surface. Electrons generated by this so-called field electron emission phenomenon are captured on the inner surface of the second substrate member 14, so that a precursor discharge in which a current flows between the second substrate member 14 and the first substrate member 12 is generated, The transition of the precursor discharge to the vacuum spark discharge realizes the absorption of the surge.

As described above, on the surface of the emitter cone 20, the first protective film 22 having a large number of diamond crystals having excellent electron emission characteristics and excellent sputter resistance is formed. Not only can the discharge starting voltage of the first field electron emission type surge absorbing element 10 be easily set to a relatively low value, but also its life characteristics can be improved.

Although the size of the emitter cone 20 is emphasized in FIG. 1 for convenience of illustration, the emitter cone 20 is actually a size of μm as described above. On the other hand, the first substrate member 12 has a size of mm unit (for example, 2 to 6 mm square), and tens to hundreds of thousands of emitter cones 20 are formed. Incidentally, the overall size of the first field emission type surge absorbing element 10 is 6 mm.
The thickness is about 0.6 mm at the corner.

FIG. 3 shows a second field emission type surge absorbing element 32 according to the present invention. In this second field emission type surge absorbing element 32, a first substrate member 12 and a second substrate member 14 made of an n-type semiconductor are arranged facing each other at a predetermined distance, and the inner peripheral edges of both members are arranged. An envelope 18 is formed by hermetically sealing with a frame member 16 interposed therebetween, and the internal space of the envelope 18 is in a high vacuum state of 10 ^-6 to 10 ^-8 Torr.

A concave portion 19 having a depth of several μm is formed on the inner surface of the first substrate member 12.
A first electron-emitting portion 34 protrudingly arranged in a matrix,
The emitter cone 20 is divided into a first flat portion 35 which is not formed and maintains a flat shape. A concave portion 19 having a depth of several μm is also formed on the inner surface of the second substrate member 14, and the bottom surface of the concave portion 19 has a large number of emitter cones 20.
Are divided into a second electron emitting portion 36 protrudingly arranged in a dot matrix at a predetermined distance, and a second flat portion 37 maintaining a flat shape without forming an emitter cone. Have been. As shown, the first electron emitting portion 34 of the first substrate member 12 faces the second flat portion 37 of the second substrate member 14 and the second electron The two substrate members are positioned so that the emission portion 36 faces the first flat portion 35 of the first substrate member 12. A predetermined gap is maintained between the tip 20a of the emitter cone 20 and each of the flat portions 35 and 37.

On the surfaces of the first and second electron-emitting portions 34 and 36, a first protective film 22 made of the same diamond thin film as described above is formed. As shown in FIG. 2, a large number of fine diamond crystals 23 are formed so as to be folded.
Many of the diamond crystals 23 are composed of <111> planes having a remarkable negative electron affinity and excellent electron emission characteristics. Further, the first flat portion 35 and the second flat portion 37
Is coated with a second protective film 25 made of W, Mo, Ni, La, Al or the like.

On the outer surface of the first substrate member 12, a first layer 24a of Al or Cr and a second layer 24 of Ni
A first external electrode 24 having a two-layer structure b is formed, and a first external terminal 38 is connected to the first external electrode 24. On the outer surface of the second substrate member 14, a second external electrode 26 having a two-layer structure of a first layer 26a made of Al or Cr and a second layer 26b made of Ni is formed. A second external terminal 40 is connected to the second external electrode 26.

This second field electron emission type surge absorbing element
32 is provided between the first external electrode 24 and the second external electrode 26 by providing the emitter cone 20 on the first external terminal 38 side and the second external terminal 40 side as described above. It is characterized in that it has a structure capable of emitting electrons in both directions, and has the advantages that it can be used without concern for polarity and can cope with a case where an overvoltage is applied in the reverse direction for some reason.

Referring to FIGS. 4 to 15, the emitter cone 20 and the most important elements in manufacturing the first field emission type surge absorbing element 10 and the second field emission type surge absorbing element 32 will be described. A method for forming the first protective film 22 will be described. First, an n-type Si substrate 42 having a resistivity of 0.01 to 5 (Ω · cm) is oxidized in an oxidizing atmosphere,
An SiO ₂ thin film 44 having a thickness of 0 nm is formed (FIG. 4). Next,
A photoresist 46 is uniformly applied to the entire surface of the SiO ₂ thin film 44 by using a spinner, and a photomask 50 having a circular light-shielding pattern 48 formed thereon is covered above the photoresist 46. (FIG. 5). As a result, the surface of the photoresist 46 is exposed except for the portion where the ultraviolet rays are blocked by the pattern 48. Next, the exposed portion of the photoresist 46 is removed using a predetermined chemical, and a circular photoresist mask 51 is formed on the surface of the SiO ₂ thin film 44 (FIG. 6). Next, B
A photoresist mask 51 is formed in the SiO ₂ thin film 44 by HF (wet etching using buffered hydrofluoric acid).
After removing the portions not covered with the photoresist mask 51, the circular oxide film mask is removed.
52 are formed (FIG. 7). Although only one oxide film mask 52 is shown in FIG.
Are formed corresponding to the number of the emitter cones 20, and are actually formed in large numbers in a dot matrix at intervals of 15 μm. The diameter of the oxide film mask 52 is set to about 10 μm.

Next, the surface of the Si substrate 42 is subjected to RIE (reactive ion etching) to erode portions not covered by the oxide film mask 52 (FIG. 8). RI
E places a sample (Si substrate 42) between two parallel electrodes installed in a vacuum chamber, fills a predetermined gas medium into the vacuum chamber, and applies an electric power of about 150 W between both electrodes. In addition, a high-frequency plasma discharge is generated, and etching is performed using a chemical action of the plasma and a physical action of ion bombardment. In this case, it is desirable to use reactive O ₂ and SF ₆ as a gas medium, and the volume ratio between them is, for example, SF ₆ : O 2 under a pressure of 2.2 × 10 ⁻¹ Torr.
₂ = 9: 1 is set. Since RIE is excellent in anisotropy, the degree of undercut in which etching proceeds on the back side of the oxide film mask 52 is relatively low, and a quadrangular pyramid-shaped projection 53 having a flat tip is formed. (FIG. 9). This RIE is performed, for example, for 15 minutes under the above conditions.

Next, the surface of the Si substrate 42 is subjected to anisotropic wet etching to erode the surface of the protrusion 53 to sharpen the tip (FIG. 10). The etching solution used for the anisotropic wet etching is, for example, a mixed aqueous solution of KOH and H ₂ O, and the solution temperature is set to about 50 degrees Celsius. The mixing ratio of the two is, for example, mixing 50 g of KOH with 100 ml of H ₂ O.

When the above-described anisotropic wet etching proceeds and a certain degree of undercut 54 occurs, and the tip of the projection 53 becomes extremely sharp, the oxide film mask 52 loses stability and falls (FIGS. 11 and 11). 12). At this stage, the angle of the tip of the projection 53 is sharpened to 25 to 30 degrees, and the anisotropic wet etching is stopped because the emitter cone 20 is completed. The time required for the anisotropic wet etching up to here is expected to be about 8 minutes. Note that the tip of this emitter cone
20a still maintains the quadrangular pyramid state, but the foot portion is in the octagonal pyramid state.

Next, as shown in FIG.
The Si substrate on which the cone 20 is formed is placed in the ultrasonic vibration tank 55. The inside of the ultrasonic vibration tank 55 is filled with a solution such as ethanol or isopropyl alcohol to which a large number of diamond particles having a particle size on the order of 50 angstroms are added. Also, this ultrasonic vibration tank
On the back side of the bottom of 55, an ultrasonic oscillation member 56 made of a piezoelectric element
Is connected. When the ultrasonic oscillation member 56 is driven, the surface of the Si substrate 42 is polished by diamond particles, very fine scratches are formed at high density, and a part of the diamond particles adhere to the surface of the Si substrate 42. I do.

Next, the surface of the Si substrate 42 is subjected to a cleaning process using pure water after a cleaning process using ethanol or isopropyl alcohol, and then a first protective film made of a diamond thin film is formed on the surface on which the emitter cone 20 is formed. A film 22 is formed. At present, various methods for forming a diamond thin film under low pressure have been proposed, and in particular, examples using typical hot filament CVD and microwave plasma CVD will be described.

First, the hot filament CVD method is shown in FIG.
As shown in FIG. 4, a tungsten filament 58 and a substrate holder 60 are placed inside a quartz tube 57 placed in a vacuum vessel (not shown), and after the Si substrate 42 is placed on the surface of the substrate holder 60, the filament While heating to more than 2000 degrees Celsius by 58 and 600 to 1000 degrees Celsius by electric furnace 62 arranged around quartz tube 57, C
When a mixed gas of H ₄ (methane) and H ₂ is introduced, these are decomposed and excited (activated) by heating by the filament 58, and radical groups (CH ₃ , CH, C ₂ ) and atomic hydrogen (H) are produced. Is generated. These carbon radicals grow on the Si substrate 42 as a diamond thin film. At this time, the hydrogen atoms play a role in suppressing the growth of graphite (graphite). In addition, in the above mixed gas of CH ₄ and H ₂ ,
By mixing an impurity gas such as PH ₃ , NH ₃ and N ₂ , the diamond thin film can be made into an n-type semiconductor. The pressure in the vacuum container is set to several k to several tens kPa. When this hot filament CVD method is used, the growth rate of the diamond thin film is about several μm / hour.

According to the hot filament CVD method, the shape of the diamond particles to be formed can be controlled by the concentration of the introduced CH ₄ gas. For example, C
When the H ₄ concentration is set to 1% or less, the diamond crystal can be grown in an original angular shape of diamond. However, when the CH ₄ concentration is increased to about 2%, the crystal becomes a corner-shaped ball-shaped crystal. That is, the sharpness of each diamond crystal can be made variable. The shape of the diamond particles and the thickness of the diamond thin film can also be controlled by adjusting the pressure in the vacuum vessel within the range of several k to several tens kPa. In addition, CH ₄
By adjusting the gas concentration and the overall synthesis time, it is also possible to preferentially grow the <111> plane having a particularly strong negative electron affinity.

In addition to the mixed gas of CH ₄ and H ₂ , for example, a mixed gas of CO and H ₂ , a mixed gas of C ₂ H ₂ and H ₂ ,
Or those obtained by mixing vaporized alcohol or acetone and H ₂ can be used as a raw material for diamond film formation.

Next, as shown in FIG. 15, the microwave plasma CVD method is to install a substrate holder 60 inside a quartz tube 57 placed in a vacuum vessel (not shown), Although the Si substrate 42 is placed and a mixed gas of CH ₄ and H ₂ is introduced onto the surface of the Si substrate 42, it is common to the hot filament CVD method, but an electric furnace 62 and a hot filament 58 are used. Instead, waveguide 66
A 2.45 GHz microwave supplied from a microwave power supply 68 into the quartz tube 57 through a microwave power source generates an electrodeless plasma discharge, which is used to decompose methane and hydrogen using the plasma.
It is characterized in that it is excited. At this time, the generation of the standing wave is controlled by the action of the plunger 69, and a matching device (not shown) is adjusted so that the plasma is not spread over the entirety of the quartz tube 57 but is concentrated near the Si substrate 42. It is necessary.

In the above-mentioned hot filament CVD method, a part of the filament 58 evaporates and enters the inside of the diamond particles, which may lower the purity of the generated diamond particles.
Since the method D uses an electrodeless discharge action, there is no such danger. Also in this microwave plasma CVD method, the shape (sharpness) of the formed diamond particles and the thickness of the diamond thin film can be controlled by adjusting the CH ₄ concentration, the pressure in the vacuum vessel, and the synthesis time. 1
It is possible to selectively grow a specific crystal plane such as an 11> plane. Also, as in the hot filament CVD method, by mixing an impurity gas such as PH ₃ , NH ₃ , or N ₂ into the raw material gas, it is possible to convert the generated diamond thin film into an n-type semiconductor. The growth rate of diamond by the microwave plasma CVD method is 0.1 μm / hour.
The disadvantage is that it is slower than the D method. Again, CH ₄
And in addition to the mixed gas of H _2, for example, mixing or gas of CO and H _2, C ₂ H ₂ mixed gas of H ₂ or a vaporized alcohol or acetone and H ₂ that are mixed, Can also be used as a raw material for diamond formation.

As a method of forming a diamond thin film,
In addition to the above, for example, an electron impact CVD method, a magnetic field microwave plasma CVD method, a DC discharge plasma CVD method, a combustion flame CVD method, or the like can be used.

Prior to the formation of the diamond thin film as described above, the Si substrate 42 is immersed in a solution containing fine diamond particles and subjected to ultrasonic vibration to obtain
The step of forming many fine scratches on the surface of the i-substrate 42 is provided for the following reason. First, when the surface of the Si substrate 42 is close to a perfect smooth surface, the crystal nuclei are unlikely to remain on the substrate surface even when a gas serving as a diamond crystal material is guided to the surface of the Si substrate 42, and the diamond thin film is formed to a uniform thickness. Cannot be formed. Further, when the scratches formed on the surface of the Si substrate 42 are relatively rough, the nucleation density of diamond becomes low and the number of diamond crystals formed becomes small, contributing to the reduction of the firing voltage. > It is difficult to secure the absolute amount of the surface. On the other hand, if the Si substrate 42 is immersed in a solution containing fine diamond particles of the order of 50 angstroms and subjected to ultrasonic vibration, extremely fine flaws are formed on the surface of the Si substrate 42 including the emitter cone 20. A large number is formed, and diamond particles having a fine particle diameter adhere. For this reason, when a gas serving as a diamond crystal material is introduced to the surface of the Si substrate 42 by the hot filament CVD method, the microwave plasma CVD method, or the like, the nucleation density of diamond increases, and the electron emission characteristics are accordingly improved. The number of 111> faces increases. This means that the discharge start voltage of the field electron emission type surge absorbing element using the Si substrate 42 can be further reduced.

After forming the protective film 22 made of a diamond thin film on the surface of the emitter cone 20 as described above,
On the surface of the Si substrate 42, ie, the first substrate member 12,
The second substrate member 14 is overlapped with a frame member 16 therebetween,
The first field electron emission type surge absorbing element 10 is completed by joining the peripheral edges of the opposing surfaces of both substrate members 12 and 14 and both end surfaces of the frame member 16 in a vacuum atmosphere (FIG. 1). When forming the second field emission type surge absorbing element 32, the emitter cone is formed on a part of the inner surfaces of the first substrate member 12 and the second substrate member 14 in the same manner as described above. 20
And a first protective film 22 made of a diamond thin film and a second protective film 25 formed on the remaining portion.
The peripheral edges of the opposing surfaces of both substrate members and both end surfaces of the frame member 16 may be joined in a vacuum atmosphere. In addition, as a method for joining the two substrate members 12, 14 and the frame member 16, there are a method of fusing using a low melting point glass such as frit glass, a method of bonding using a polyimide-based adhesive, Materials containing mobile ions such as ⁺ and H ⁺ (for example, Pyrex glass)
The frame member 16 may be formed by using an anodic bonding method in which a DC voltage is applied between the substrate members 12 and 14 and the frame member 16 in a heating environment to bond them together.

In the above, an example in which an n-type semiconductor is used as the material of the first substrate member 12 and the second substrate member 14 has been described. However, the present invention is not limited to this.
Of course, each of the substrate members 12 and 14 can be made of a p-type semiconductor in which an impurity such as B (boron) is mixed in i. In this case, it is necessary to mix impurities such as B (boron) into the diamond thin film constituting the first protective film 22 to make it into a p-type semiconductor.

[0057]

As described above, in the field electron emission type surge absorbing element manufactured by the method according to the present invention, the surface of the emitter cone is covered with the protective film made of a diamond thin film having a negative electron affinity. Therefore, there is an advantage that the operating voltage can be easily set to a relatively low value.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a first field emission type surge absorbing element according to the present invention.

FIG. 2 is an explanatory diagram showing an image of a diamond thin film formed on the surface of an emitter cone.

FIG. 3 is a sectional view showing a second field-emission-type surge absorbing element according to the present invention.

FIG. 4 is a partial cross-sectional view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 5 is a partial sectional view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 6 is a partial cross-sectional view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 7 is a partial cross-sectional view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 8 is a partial cross-sectional view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 9 is a partial perspective view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 10 is a partial cross-sectional view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 11 is a partial cross-sectional view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 12 is a partial perspective view showing a process of forming an emitter cone on the surface of a Si substrate.

FIG. 13 is a cross-sectional view showing a state in which a Si substrate is arranged in an ultrasonic vibration tank.

FIG. 14 is a schematic view showing a hot filament CVD method.

FIG. 15 is a schematic view showing a microwave plasma CVD method.

FIG. 16 is a circuit diagram showing a usage example of a surge absorbing element.

FIG. 17 is a cross-sectional view showing a conventional field emission type surge absorbing element.

FIG. 18 is a waveform chart showing surge absorption characteristics of the field emission type surge absorbing element.

FIG. 19 is a partial cross-sectional view showing a process of forming a conventional emitter cone.

FIG. 20 is a partial cross-sectional view showing a process of forming a conventional emitter cone.

FIG. 21 is a partial cross-sectional view showing a process of forming a conventional emitter cone.

FIG. 22 is a partial cross-sectional view showing a process of forming a conventional emitter cone.

FIG. 23 is a partial cross-sectional view showing a process of forming a conventional emitter cone.

[Explanation of symbols]

 10 First Field Electron Emission Surge Absorber 12 First Substrate Member 14 Second Substrate Member 16 Frame Member 18 Enclosure 20 Emitter Cone 20a Tip of Emitter Cone 22 First Protective Film 24 First External electrode 26 second external electrode 32 second field electron emission type surge absorbing element 34 first electron emitting section 35 first plane section 36 second electron emitting section 37 second plane section 52 oxide mask 53 Projection 55 Ultrasonic vibration tank 56 Ultrasonic oscillation member

Continuation of the front page (56) References JP-A-2-278681 (JP, A) JP-A-6-260262 (JP, A) JP-A-8-264862 (JP, A) JP-A-8-212908 (JP) JP-A-7-320635 (JP, A) JP-A-7-29483 (JP, A) JP-A-5-205617 (JP, A) JP-A 8-195165 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01T 1/00-4/20

Claims (4)

(57) [Claims] 1. A first substrate member provided with an electron emitting portion made of a semiconductor having a plurality of emitter cones integrally formed on one surface thereof, and a second substrate member provided with a flat surface portion on one surface thereof. The tip of the emitter cone of the substrate member and the second
Are arranged so that the flat portions of the substrate members face each other with a predetermined distance therebetween, and the peripheral edges of both substrate members are hermetically sealed to form an envelope, and the inside of the envelope is brought into a high vacuum state. A method of manufacturing a field electron emission type surge absorbing element comprising forming external electrodes on the outer surfaces of both substrate members, respectively, and further forming a protective film made of a diamond thin film on the surface of the emitter cone. A large number of emitter cones are integrally formed on the surface of a substrate member made of, and after forming a large number of fine scratches on the surface of the substrate member, the substrate member is placed in a vacuum system heated to a predetermined temperature. , the surface of the substrate member, lead to the raw material gas of the diamond obtained by mixing an impurity gas, a raw material gas by activating grown diamond film obtained by the semiconductor to the surface of the emitter cone, the diamond The concentration of de material gas, by adjusting the pressure to control the sharpness of diamond crystals constituting the diamond film, also, the diamond
The operating voltage can be adjusted by adjusting the amount of impurities added to the thin film.
A method for manufacturing a field electron emission type surge absorbing element, characterized by controlling the following . 2. A first substrate member having an electron emission portion made of a semiconductor having a plurality of emitter cones integrally formed on one surface, a first substrate member having a flat portion on which no emitter cone is formed, and a plurality of emitter cones formed on the same surface. A second substrate member provided with an electron emission portion made of a semiconductor integrally formed with a cone and a flat portion on which no emitter cone is formed is formed by combining the tip of the emitter cone of one substrate member and the other substrate member. A flat portion is disposed so as to face at a predetermined distance, and the outer edges of both substrate members are hermetically sealed to form an envelope, and the inside of the envelope is brought into a high vacuum state. A method for manufacturing a field electron emission type surge absorption element, comprising forming an external electrode on an outer surface of each member and further forming a protective film made of a diamond thin film on the surface of the emitter cone, comprising: A large number of emitters
After integrally forming a cone and forming a large number of fine scratches on the surface of the substrate member, the substrate member is placed in a vacuum system heated to a predetermined temperature, and an impurity is formed on the surface of the substrate member.
A source gas of diamond mixed with a source gas, activates the source gas, and applies the source gas to the surface of the emitter cone.
By growing a diamond thin film which has been converted into a semiconductor and adjusting the concentration and pressure of the source gas for the diamond, the sharpness of the diamond crystal constituting the diamond thin film is controlled, and the impurity of the diamond thin film is also controlled.
A method for manufacturing a field-emission-type surge absorbing element, wherein an operating voltage is controlled by adjusting an amount of an additive . 3. The substrate of claim 1, wherein said emitter cone is provided on said substrate member.
Forming a large number of oxide masks on the surface of the substrate,
Reactive ion etching on the surface of the material
To cover the area not covered by the oxide mask.
So that the emitter cone is located below the oxide mask.
Forming a columnar protrusion serving as a prototype;
Anisotropic wet etching is applied to the
The method for manufacturing a field electron emission type surge absorbing element according to claim 1 or 2, wherein the method is formed through a step of sharpening the tip by eroding . 4. A base in which said emitter cone is integrally formed.
Place the plate member in a solution containing fine diamond particles.
After immersion, apply ultrasonic vibration to this.
4. The method for manufacturing a field emission type surge absorbing element according to claim 1 , wherein fine scratches are formed on the surface of the substrate member .