US3923358A

US3923358A - Method for manufacturing an image pickup tube

Info

Publication number: US3923358A
Application number: US407579A
Authority: US
Inventors: Shoichi Miyashiro; Shunji Shirouzu; Kazuo Shimizu; Okio Yoshida
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1970-01-16
Filing date: 1973-10-18
Publication date: 1975-12-02
Anticipated expiration: 1992-12-02

Abstract

An image pickup tube is manufactured by setting a PN junction type semiconductor target having a resistive sea made of cadmium telluride in a glass envelope and heating the envelope at 120*C to 400*C to effect evacuation.

Description

Unite States aiem 1191 1111 3,923,358

Miyashiro et a1. 1 1 Dec. 2, 1975 METHOD FOR MANUFACTURING AN 29/472.7; 316/17, 18, 19, 20; 317/235 NA; IMAGE PICKUP TUBE 220/21 R, 2.1 A. 2.3; 357/6 [75] Inventors: Shoichi Miyashiro, Yokohama;

Shunji Shirouzu, Ayase; Kazuo 1 1 References Cited Shimizu; Okio Yoshida, both of UNlTED STATES PATENTS Yokohama of Japan 3,253,331 5/1966 Limunsky 29/4727 73 A i Tokyo Shibaura Electric Co Ltd 3,331,998 7/1967 Zuleeg 317/235 NA Kawasaki Japan 3,419,746 12/1968 Crowell ct a1.... 317/235 NA 3,574,143 4/1971 Vratny 317/235 NA [22] Filed: Oct. 18, 1973 3,664,895 5/1972 Schaefer ct 211.. 317/235 NA 3,747,173 71973 L' d 29 25.13 [211 App]. No.: 407,579 m Related US. Application Data Primary E.raminerRoy Lake [60] Continuation-impart of Ser. No. 329,576. Feb. 5, Assistant Examinerjames Davie 1973, abandoned, which is a division of S81. N0. A y, g r FirmFlynn & Fri h f 105,823, Jan. 12, 1971, abandoned.

[57] ABSTRACT [30] Foreign Application Priority Data A k t b f t d h {f PN 1 n1mage p1c up u e 15 manu ac ure y se mg a i jf junction type semiconductor target having a resistive u y dpan sea made of cadmium telluride in a lass cnvelo c and U 8 Cl 357/6 heating the envelope at 120C to 400C to effect evac- [58] Field of Search 29/2513, 25.14, 25.15, 4 Claims, 4 Drawing Figures US. Patent Dec. 2, 1975 FIG.1

FIG.2

ELECTRON BEAM LIGHT W. M mA E B E LIGHT METHOD FOR MANUFACTURING AN IMAGE PICKUP TUBE CROSS-REFERENCE TO RELATED APPLICATION This is Continuation-impart of U.S. Pat. application Ser. No. 329,576, filed on Feb. 5 1973, now abandoned, which in turn is a Division of U.S. Pat. application Ser. No. 105,823, filed Jan. 12, 1971, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method for manufacturing an image pickup tube having a semiconductor target with a plurality of PN junctions.

An image pickup tube such as a vidicon and electron multiplication type camera tube generally includes a semiconductor target to convert photoimage signals to electric image signals. Such target comprises an N-type silicon substrate and a plurality of island regions arranged on that side of the substrate which is scanned by electron beams emitted from an electron gun. That surface of the substrate subjected to scanning is coated, to the exclusion of the island regions, with a protective layer made of insulating materiaLfor example, silicon dioxide. Further on the protective layer is formed a semi-insulating layer having a discharge time constant longer than one frame period of television scanning and shorter than the discharge time constant of the insulating layer, thereby enabling any charge on the film to leak sidewise so as to prevent the film from being unnecessarily charged when it is exposed to light. Such a semi-insulating layer is usually made of antimony triselenide or antimony trisulfide. When an envelope containing a semiconductor target formed of such substance is heated to be more evacuated, then the semiconductor layer has its resistivity prominently varied by said heat treatment, failing to present desired characteristics relative to, for example, dark current and resolution. Furthermore, when heated to above 350C the above-mentioned substance is revolatilized to be rendered unusable and found inadapted to be applied to 'an image pickup tube, for example, an electron multiplication type camera tube, whose photoelectric surface requires high temperature treatment.

In view of the problem involved in the heat treatment, it may be contemplated to prepare the semiinsulating layer from cadmium selenide or cadmium sulfide. However, each substance has a relatively low resistivity of about to 10' Q-cm and indicates too low a resolution capacity for practical application.

When the insulating layer is formed from the abovementioned material, its resistivity is greatly changed due to a slight change in evaporation conditions and, therefore, difficulty is presented in obtaining image pickup tubes having substantially equal characteristics, thus resulting in a poor yield.

SUMMARY OF THE INVENTION It isaccordingly the object of this invention to provide a method for manufacturing an image pickup tube equipped with a semiconductor target having a semiinsulating layer less liable to be changed in its characteristics even on heat treatment.

In an aspect of this invention there is provided a method for manufacturing an image pickup tube comprising the steps of preparing a semiconductor target including a semiconductor substrate of one conductivity type, a plurality of island regions of the opposite type formed on one side of the substrate, an insulating layer coated on said one side of the substrate to the exclusion of the island regions thereof, and'a semi-insulating layer of cadmium telluride coated at-least on the insulating layer; introducing the semiconductor target into a glass envelope; and evacuating the glass envelope while heating it in a range of to 400C.

The semiconductor layer made of cadmium telluride has a resistivity of the order of 10 Q-cm' at room temperature. The resistivity of the semiconductor layer is gradually increased with heating temperature and when the heating temperature reaches about 120C, then the resistivity of the semiconductor layer is decreased. Even when about 400C, a limit of the heat treatment of the image pickup tube, is attained, the layer still has a resistivity close to 10 Q-cm? Consequently, the image pickup tube according to this invention is satisfied in its resolution independently of the heat treatment.

It is commonly required to heat a semiconductor target at a temperature of 400C for 60 minutes in an H atmosphere before the formation of asemi-insulating layer. This heat treatment is intended to correct the state of boundary between the semiconductor substrate and the insulating layer to prevent a current leakage along the boundary which would cause resolution to be reduced. Generally, an envelope is formed from a glass having a softening point of about 400C. Use of glass having a softening point of above 400C makes it difficult to, for example, partially fuse or bore an envelope in an attempt to fixedly attach metal leads and sealing metal ring to the envelope. On the other hand, when use is made of glass having a softening point of below 400C, an envelope causes deformation or damage during heat treatment, since it is lower than the temperature at which the envelope is heat treated.

It is preferred that the lower limit of the aforementioned heat treatment be about 120C at which a best resolution is attained i.e. a highest resistivity of the cadmium telluride layer is attained. With a temperature close to 120C there is no fear that an indium seal will be melted by heat treatment. When the temperature of heat treatment is prominently raised from this lower temperature limit so as to improve a getter effect, then it is preferred that an aluminum seal be used, since the indium seal may be melted. Where attention is paid only to the getter effect, it is preferable that heat treatment be of the order of 250C.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross section of a vidicon as manufactured according to this invention;

FIG. 2 is a cross section of an electron multiplication tube according to this invention;

FIGS. 3 and 4 are cross sections showing different semiconductor target as used in the above-mentioned image pickup tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT There will now be described, by reference to accompanying drawings, a method for manufacturing an image pickup tube, especially a vidicon.

Referring to FIG. 3 there is shown a semiconductor target 7 as used in a vidicon. A reference numeral 1 denotes an N-type silicon substrate about 20 microns thick. On one side of the substrate 1 are provided in'a matrix arrangement a plurality of a P-type island regions 2 which, together with the substrate, define PN junctions therebetween. The island regions 2 are formed by diffusing boron in the substrate 1 so as to have a diameter of, for example, 6 microns and be spaced from each other at a pitch of l microns. On the aforementioned side of the substrate 1 including the exposed parts of the PN junctions excluding the island regions 2 is formed an insulating layer 3 of silicon dioxide about 0.5 micron thick. This insulating layer may be constituted by the silicon dioxide layer as it is which is used as making material in forming the island regions. Of course, use may be made of an insulating layer made of other material, for example, silicon monoxide or silicon nitride. The substrate 1 coated with the insulating layer 3 is heated at 400C for 60 minutes in an H atmosphere to decrease the charge density of the surface of the substrate so that a surface current flowing in the boundary area between the substrate and insulating layer may be effectively decreased. On the aforementioned side of the substrate is coated a semi-insulating layer 4 of cadmium telluride about 500 A thick so as to cover all the insulating layer 3 and island regions 2. Said cadmium telluride layer 4 may be formed by the ordinary film depositing technique, for example, a vacuum evaporation process. The periphery of the opposite side of the substrate 1 is formed thick to increase its mechanical strength. On said side is formed an N type layer 5. An aluminum electrode 6 is deposited on the peripheral portion of the N type layer 5.

The modified semiconductor target shown in FIG. 4 is only different from the preceding one in that the semi-insulating layer 4 covers the protective insulating 4 seal (i.e., of the type shown in US. Pat. No. 2,876,596 Kessler, Jr.) is used instead of the indium seal, it is possible to heat treat the envelope at higher temperature ranging 250C to 400C.

The vidicon so manufactured is characterized in that it is sufficiently practicable in respect of dark current, resolution etc., since the resistive sea of the semiconductor target is made of cadmium telluride and the envelope is heated at to least 120C during the evacuation.

An electron multiplication tube as shown in FIG. 2 is constituted by an image section 10a and a scanning section 10b. Since the scanning section has substantially the same construction as that of the vidicon, its explanation is omitted and only the image section will be described.

The forward section or image section 10a of the envelope 10 includes a photosensitive surface 17 for emitting electrons according to an incident light, and an electrode 18 for accelerating emitted electrons to conduct them to the semiconductor target 7.

Where the metal seal 16 of the electron multiplication tube is made of aluminum, the envelope can be heat treated at about 250C.

For reference, Table 1 shows changes in physical properties involved in the heat treatment of a prior art semiconductor target including a semi-insulating layer formed of antimony triselenide (Sb Se antimony trisulfide (Sb S cadmium selenide (CdSe) and cadmium sulfide (CdS) respectively, in comparison with those of a semiconductor target whose insulating layer is made of cadmium telluride according to this invention.

Table 1 Without heat Treated in Treated in Treated in Treated in Component treatment vacuum for vacuum for vacuum for vacuum for 3 hrs. at 3 hrs. at 3 hrs. at 3 hrs. at 120C 250C 350C 400C Sb=Se= 1.7 X 10" Same as 3.3 X l0 Revola- Revela- Q-cm' left fl-cm" tilized tilized (Solidified) Sb S 1.8 X l0" 4.4 X 10" fl-cm' fl-cm" (Solidified CdTe 2.2 X 10" 5.3 X l0" 3.8 X 10" 6.0 X10 fl-cm" O-cm" O-cm" fl-cm" CdSe 7.2 X 10' 9.4 X 10' 8.6 X lo 4.5 X 10' fl-cm" O-cm" n-cm" 6 -cm" CdS 6.5 X l0" 8.2 X 10'' 7.5 X 10" 3.7 X10 n-em" n-cm" n-cm n-cm" layer 3 alone and is not deposited on the P-type island regions.

Reference is invited to FIG. 1. An envelope 10 made of glass having a softening point of about 400C is prepared, and a mesh electrode 1 l, accelerating electrode 12 and electron gun 13 are mounted in the envelope by a known manner. The aforesaid semiconductor target 7 is mounted through a transparent electrode 14 to a face plate 15 in a manner to permit the electrode 6 of the semiconductor target to be brought into contact with the transparent electrode 14. Then, the face plate 15 is attached through an indium seal 16 to the opening of the envelope 10 to close it, thereby sealing the target 7 in the envelope 10.

The envelope 10 accommodating these members is heated at a temperature of about 120C for vacuum evacuation to complete a vidicon. When an aluminum As apparent from Table 1 above, a semi-insulating layer of Sb,Se or Sb,S,, when treated at higher temperatures than about 250C, substantially failed to be put to practical use, whereas a semi-insulating layer made of cadmium compounds, when treated even at 400C, only slightly varied in resistance and was not solidified or revolatilized, proving that heat treatment at such level of temperature did not harmfully affect its performance.

Referring to Table 1 above a temperature of C is the level at which heat treatment is applied in the manufacture of a general vidicon. However, if heating is applied in evacuating a glass bulb at as high temperatures as it is not softened, it will expel the larger amounts of gas deposited on the inner bulb walls or electrodes placed therein, thereby improving the performance and life of a vidicon. Since it is desired to conduct evacuation at as high a temperature as 400C, a limit point at which the glass material is not softened, the target of the present invention offers great advantage because its properties are not deteriorated even by such high temperature treatment.

What we claim is: l. A method for manufacturing an image pickup tube comprising steps of:

fabricating a semiconductor target including preparing a semiconductor substrate having one conductivity type, forming a plurality of island regions of the opposite conductivity type on one side of the substrate, forming an insulating layer on said one side of the substrate exclusive of the island regions thereof, and coating a semi-insulating layer of cadmium telluride at least on the insulating layer;

mounting said semiconductor target in a glass envelope; and

then heating the envelope at a temperature of from 250 to 400C for evacuation.

2. The method according to claim 1 comprising sealing the envelope with an aluminum seal after mounting of said target in said envelope and before heating of said envelope.

3. The method according to claim 1 further comprising mounting in the envelope an accelerating electrode, mesh electrode and electron gun.

4. The method according to claim 1 further comprising mounting in the envelope accelerating electrodes, a mesh electrode, an electron gun and a photosensitive surface.

Claims

1. A METHOD FOR MANUFACTURING AN IMAGE PICKUP TUBE COMPRISING STEPS OF: FABRICATING A SEMICONDUCTOR TARGET INCLUDING PREPARING A SEMICONDUCTOR SUBSTRATE HAVING ONE CONDUCTIVITY TYPE, FORMING A PLURALITY OF ISLAND REGIONS OF THE OPPOSITE CONDUCTIVITY TYPE ON ONE SIDE OF THE SUBSTRATED, FORMING AN INSULATING LAYER ON SAID ONE SIDE OF THE SUBSTRATE EXCLUSIVE OF THE ISLAND REGIONS THEREOF, AND COATING A SEMI-INSULATING LAYER OF CADMIUM TELLURIDE AT LEAST ON THE INSULATING LAYER, MOUNTING SAID SEMICONDUCTOR TARGET IN A GLASS ENVELOPE, AND THEN HEATING THE ENEVELOPE AT A TEMPERATURE OF FROM 250* TO 400*C FOR EVACUATION.