US3775120A

US3775120A - Vertical resistor

Info

Publication number: US3775120A
Application number: US00238272A
Authority: US
Inventors: J Black; H Gurev
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1971-04-02
Filing date: 1972-03-27
Publication date: 1973-11-27
Anticipated expiration: 1990-11-27

Abstract

There is disclosed a high valued vertical resistor whose high value is a result of contacing the resistive film 10 through pinholes 16 in an insulating film 15 on top of the resistive film. The value is controlled by the areal density of pinholes through the thin overlaying insulating film such that the higher the areal density the lower the resistance value for the particular resistor. The resistor is completed by providing a metal layer 20 over the pinholed film such that the metal extends through the pinholes to the resistive layer making contact thereto at a multiplicity of points. The number and size of the contacts made to the resistive layer as well as the resistivity and thickness of the resistive layer controls the total resistance value of the resistor. The holes in the thin insulating film are formed and the hole size and number controlled by one of three methods involving the use of opaque particles, metal particles and porous photorresists as masks for etching the thin insulating film. In one embodiment metallization fills the pinholes in the thin insulating layer so as to provide a multiplicity of contacts to the resistive layer which are spaced and insulated one from another. The structure in this case can be used as a sensing device for determinging a planar contact area of an electrically conducting structure contiguous to the top surface of the resistor, the contact area varying in an inverse manner with the resistance of the resistor. The structure fabricated without continuous overlaying metallization can also be used as a sensing device for sensing the contact area of a resilient structure having a conductive film on the outside thereof. The greatest utility of the vertical resistor thus formed is in electrical circuits in which a high but accurate resistive value for the resistor must be obtained. The resistance value of the resistor is provided by altering the areal density of the pinholes in the thin insulating film.

Description

United States Patent [1 1 Black et al. I

[ 4] VERTICAL RESISTOR [75] Inventors: ,laniesR. Black, Phoenix; HaroldS.

3,680,028. 521' us. (:1 96136.2, 96/36338/195, I 33s/30s,1-17/215 s11 Int.Cl..... G03c5/00 [58] Field ofSearch 96/362, 36; 338/13,

338/38, 327, 328, 195, 308; 117/215; 73/80; 324/65 P, 65 R; 178/2 T [56] References Clted UNITED STATES PATENTS 7 3,669,661 6/1972 Page et al'. 96/362: 3,618,201 1 1/1971 Makimoto et a1. 96/362 3,522,085 7/1970 \Vatanabe 96/362 2,289,791 7/1942 Loftis 338/327- X 2,482,316 9/1942 Bocking..... 338/328 X 3,049,001 8/1962 Mackay 73/80 Black et' a1 117/215 Primary Examiner-Norman G. Torchin Assistant ExaminerEdward C. Kimlin Attorney-Ronald J. Clark et al.

[57] ABSTRACT There is disclosed a high valued vertical resistor whose high value is a result of contacing the resistive film 10 through pinholes 16 in an insulating film 15 on top'of [4 1 Nov. 27,1973

the resistive film. The value is controlled by the areal density of pinholes through the't-hin overlaying insulating film such that the higher the areal density the lower the resistance value for the particular resistor. The resistor is completed by providing a metal layer 20 over the pinholed film such that the metal extends through the pinholes to the resistive layer making contact thereto at a multiplicity of points. The number and size of the contacts made to the resistive layer as well as the resistivity and thickness of the resistive layer controls the total resistance value of the resistor. The holes in the thin insulating film are formedand the'hole size and number controlled by one of three -methods involving the use of opaque particles, metal particles and porous photorresists as masks for etching the thin insulating film.

In one embodiment metallization fills the pinholes in the thin insulating layer so as to provide a multiplicity of contacts to the resistive layer which are spaced and the contact area varying in an inverse manner with the resistance of the resistor. The structure fabricated without continuous overlaying metallization can also be used as a sensing'device for sensing the contact 7 area of a resilient structure having a conductive film on the outside thereof.

The greatest utility of the vertical resistor thus formed is in electrical circuits in which a high but accurate resistive .value for the resistor must be obtained. The

resistance value of the resistor is provided by altering the areal density of the pinholes in the thin insulating 6 Claims, 4 Drawing Figures APATENTEDNDJV 21 I915 CONTACT, 2o

. 'r 'RESISTIVE FILM so x v ,CONTACT,l2

INSULATING FILM, l5

INSULATING FILM,I5

, 1 VERTICAL RESISTOR This is a division of application, Ser. No. 130,610,

While thin film vertical resistors are known in the art, there has been some difficulty experienced providing a so as to leave the aforementioned mask. It will be applying the photoresist solution to the thin insulating metal particles are then leached out of the photoresist preciated that the metal particles themselves are opaque to the radiation. The-radiation causesv crosslinking in the photoresist filrriadjacent and inbetween resistors with controllable high values. In the prior art, I

the value of the resistor is controlled by the resistivity, size and shape of the resistive layer as well as its thick ness. However design parameters preclude the fabrication of ultra-high vertical resistors from known resistor materials because of the relatively low resistivity of these materials. 1 a

The subject'resistor is a vertical thin film resistor in which the resistance is controlled and increased or augmented by contacting only small spaced-apart portions of the resistive film. While the resistivity augmentation total effective resistance The multiplicity of paths is increased by providing a multiplicity of contacts to the top surface of the resistive film through pinholes in a thin dielectric or insulating film deposited on top of the resistive film. This is accomplished 'by preferentially the resistive film and by forming a metal Iayerthereacross. During metallization the metallic contact material descends into the holes in the thin dielectric material so as to contact the resistive layer at the aforementioned multiplicity of points-By controlling the number and-size of the holes in the thin insulating film, the number of contacts and thus the number of parallel resistive pathsmay be controlled by the areal density of the holes in the thin insulating film.

The areal density of the holesin the thin insulating film is in turn provided by one of three photoresist masking techniques. The first .of these techniques in volves the use of a negative photoresist film which is deposited on the thin insulating. film which is then baked. Prior to exposure,,a predetermined number of these metal particles'such that the photoresist film is made chemically resistive in the illuminated areas. Such a photoresist film is said to be a negative photoresist. Leaching the metal particles out leaves holes in the photoresist through which the etchant is transmitted to the thin insulating film so as to etch it in these locations down to the resistive layer.

The third method involves the use of a photoresist solution with an excess of. sensitizer. It is known that controlled exposure of such a photoresist film will yield a film of very fine porosity. The film itself will have holes in it corresponding to the amount of the excess of the sensitizer. This film is then used as an etch mask for the thin insulating film.

Altemately the patterned photoresist in any of the above methods can be fabricated on a transparent sup-' port such as a glass slide and used as a master light mask to, form holes in a-conventional photoresist layer on applied directly to the insulating layer on the resistor. lnaddition to forming a thin film vertical resistor with controllable characteristics, the subject process 7 also leaves an intermediate product which can be used to sense the geometric area of a conducting substance contacting the surface of the pinholed thin dielectric area. If each of the pinholes is filled with a conducting metal and a conductor is placed on top of the resistor etching the thin insulating film downto the surface of opaque particles whose diameter corresponds to the hole size desired, are spread over the photoresist film.

to the above mentioned particles. It will thusbe appreciated the areal density of the particles on the photore- 'sist film corresponds to the areal density of theholes in the thin insulating film.

A second method involves mixing the required percentage of particles, which in this case constitute a fine metal powder, into the photoresist solution. After apascertained with the ocular fluid providing thecontact to the resistive layer through the pinholes in the thin insulating layer. As such a resistor formed in this manner can be utilized as a sensor in an opthalmic'in'strument.

SUMMARY OF THE INVENTlON It is therefore an object of this invention to provide a vertical resistor whose resistance value is variedby the areal density of apertures in an insulating film applied on top of the resistive film.

It is a further object of this invention to provide a method for controlling the resistance value of a vertical resistor incorporating a resistive film and an insulating film thereon, with the insulating film being provided with a multiplicity of apertures such that the resistance of the resistive element is proportional to the size and number of these apertures.

It is a further object of this invention to provide a method for ascertaining the contact area of a conductive member in which the conductive member is placed against one surface of a resistor, the resistance value of the resistor being proportional to the amount of surface contacted by the member.

It is a still further object of this invention to provide an improved vertical resistor fabricated in a manner so I BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of the subject resistor showing a conductive substrate, a resistive film and an insulating film which is provided with a multiplicity of apertures so as to permit contact to be made to the resistive film through'these apertures by the overlying metal film shown. v

FIG. 2 is a cross-sectional diagram of a vertical resistor provided with a conductive substrate, a'resistive film and an insulating film which is provided with the aforementioned apertures in which each of the apertures is filled in with a conductive material such that the structure shown in FIG. 2 can be used to ascertain the contact area of a conducting member placed on-top of. the resistor. e

FIG. 3 is a vertical resistor which is provided with an insulating film and a multiplicity of apertures therethrough such that the device is capable of ascertaining the area of a conductive liquid ontop thereof.

FIG. 4 is a diagram showing the use of the structure shown in FIG. 3 to measure the-contact area'of the human eye.

BRIEF DESCRIPTION OF THE INVENTION There is disclosed a vertical resistor whose value is controlled by the areal density of pinholes through a thin overlying insulating film such that the higherthe areal density the lower the resistance value for the particular resistor. The resistor is completed by providing a metal layer over the pinholed film. The metal in the layer extends through the pinholes to the resistive layer making contact theretoat a multiplicity of points. The

number and 'size of the contacts made to the resistive layer controls the total resistance value of the resistor. The size and number of holes in the thin insulating film are controlled by one of three methods. The first resistive film in the areas covered by the'particles. A

second metal involves 'mixing of the required percentage of fine metal powder in a photoresist solution which is then exposed and developed with the metal being leached out so as to provide the required holes in the photoresist. Etchant is then applied so as to etch the silicon dioxide layer as in the first method. The third method involves the use of a photoresist solution with an excess of sensitizer. is known that controlled exposure of such a photoresist will yield films of very fine porosity. Such a film can be used as an etch mask for the silicon dioxide.

It is the use of a pinholed photoresist film which en ables selective etching of the thin insulating layer so as to provide for the required number and size of the holes therethrough. In one embodiment after a continuous metal layer is deposited over-the pinholeddielectric layer, the metallization is etched down to the surface of the thin dielectric layer so as to provide a multiplicity of contacts to the resistive layer which are spaced and insulated one from another. The structure in this case can be used as asensing device for determining a planar contact area of an electrically conducting structure contiguous to the top surface of the resistor, the contact area being proportionalto the resistance of the resistor. The structure with the pinholed dielectric layer and absent any metallization can also be used as a sensing device for sensing the contact area of a conductive liquid or a resilient structure having a conductive film on the outside thereof.

The greatest utility of the vertical resistor thus formed is in the electrical circuits in which a high but accurate resistive value for the resistor must be obtained. The resistance value of the resistor is provided by altering-the areal density of the pinholes in the thin insulating film.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there are four main advantages in the fabrication of a vertical resistor in the subject manner.

The first is the increase or augmentation of the resistance values for conventional thin film resistors having resistivities in the 10" to 10 ohm centimeter range. The effective resistivity of these films is'increased by as much as four orders of magnitude by the subject contact. In the prior art fabrication of resistors having Secondly, high resistance devices can be fabricated" with relatively low temperature coefficients of resistivity. In the past and with the use of leaky dielectrics or indeed any other type of material, the temperature 00'- efficients of resistivity vary widely in the ultra-high resistance range. The temperature coefficients of resistiv- I ity (TCR) is defined as l/AT [R Ri/Rfl 10. R is the value of the resistor at room temperature and R is usually taken as the value of the resistor at C such that AT= 125C room temperature. The TCR of a resistor is usually measured in parts per million per degree centigrade which refers to the change in resistance per degree centigrade. In the prior art ultra-high resistance devices, TCRs of 5,000 parts per million per degree centigrade are common. If the resistor is fabricated with the aforementioned pinholed thin film insulator, effective resistivities on the order of 10 ohm centimeters can be achieved with TCR close to zero. Thus, the use of the pinhole structure can increase the resistivity of relatively low TCR materials while at the same time maintaining the TCR of the original material.

This brings aboutithe third advantage of the subject resistor which is that-by .use-of the pinhole thin'film the resistivity of already known material can :be augmented or increased. This has particular application in Nichrome, (nickel-chromium), cobalt-chrome, nitrid'ed I of'the thin film in a predictable'manneri It has been known that the smaller theco ntact area of a thinfilm resistor, the higher the: resistance. Unfortunately, if v a current crowding.iRather than making .a verysmall resistor, of for instance "Nichrome, the; subject system utilizes a layer of Nichrome, .on which' is deposited an insulating layer having amultiplicity of aperturesthere I with some nitrogen dispersed therein, andcobalt- .20 single small resistor is made, itwill burn out duetothe .current density at its contacts. This isreferred to as through. Contact to the Nichrome layer is made" through these apertures such that a multiplicity of very small. resistors result. The current density is, however,

spread acrossthis multiplicity of very small resistors such thatthetotal device can carryconsiderable current. The resistance of the resistivethin film layer is increased because'it is in-effect'a multiplicity of: parallel connected very small vertical resistors. By controlling the number and the size of theholes orapertures in'the thin film, the total resistance of the device can be easily controlledas w'ell'as augmented. Fourthly, and. as aby-product of thefabrication o the subject transistor,it was found that the pinhole insulating structure on top of the resistivethin film'provided for anewni'ethod'of'measuring contact area of chromium alloys. These materials usually have a resistivity' oflOf ohm centimeters. Additionally, cermet type resistorssuch as 'A1-Al2O whi'ch noljrnally have .a resistivity ch16"? 516 .655; centimeters can be utilized as the resistive film 10.

The resistivity of any v of the films utilized can be augv -mented inaccordan ce with the ratio of the area of the aforementioned insulator to the area of the apertures .or holes therein. This insulator, is shown as insulating film 315 which is first deposited on the resistive film l and thenetched to provide for the aforementioned apertures shown here at 16.

In the configuration shown in FIG. 1, a metallization layer forming contact :20 is deposited over the insulating film 15 such that the metal utilized inthe insulating layer fills the pinholesor apertures through the insulating film '15 and contacts the resistive film at the locations where the apertures are cut through to the resistive film. -As mentioned'previously, there are several ways of providing'that the insulatingfilm "have apredeterv -mined areal density of apertures. Although this text will specify only three of the many possible methods of providing these apertures it will be understood that any appropriate method of providing these apertures is within the scope of this invention.

- The insulating film 15, in one embodiment, is a one micron layer of silicon dioxide which-is provided with a negativephotoresist film (not shown). After thepho tore'sistfilm 'isapplied to the insulating film 15, it is baked. Priorto exposure, the photoresist film is provided with adispersion of ultraviolet opaque particles whose. diameters correspond. to the-aperture sizes desired and are in general in the micron andsub-micron range.The amount of material is adjusted such that the areal density of the apertures to-beprovided by-the a conducting bodyfWhen the conducting body 'is placed on top of the -subject resistor which does not havea'continuous metalylayer, and contact is made from-the conducting body through the pinhole to the resistive film, the resistance measured is dependent on the contact area. The larger the area contacted the more smallv resistors in paralleland'thus the lower the resistance. By fillingeach of the pinholes with a conductive material suchthat aplanar'top surface results in which the filled pinholes are isolatedone from .an- 7 other, the mere placing of .the conductive body on top of the resistorv such thatcertain of the ifilled'pinholes are contacted, results in asensingdevice which can quiteaccurately measure the area of a contacting body.

Referring to FIG. l,.a resistive film 10'isshown deposited on a conductivesubstrate 1 1. Depending on the application, the conductive substrate can :either be a metal or a semiconductor substrate which isqdoped heavilyv enough to form an ohmic contact with the resistive thin film. The conductive substrateflnone embodiment, mayrest ona further contact 12 which is of any normally usedmetal such asaluminum. As mentioned hereinbefore, the resistive film may vary substantially in thicknessandin resistivity and may be formed from any one of the conventional materials utilized for thin films.-These include nickel-chromium alloys, tantalum number and size of vtheypar'ticles is at the required value. The structure is then exposed toultraviolet light.

, and the photoresist film is then developedi' lhose areas of-the/photoresist film which were covered by theparticles during exposure are washed away by the developer leavingapertures in :the photoresist film corresponding in size and number to the size and number of particles utilized. An etchant is then deposited over the photoresist which etches the silicon dioxide insulating film' layer down to the resistive layer such that theinsulating film layer 15 is provided with pinholes beneath each particle. The particles which can be utilized include rsete l s. 2!!5i-9thi.9@@i9- A second method forproviding the pinhole structure for the insulating film 15 is accomplished by mixing the required percentage of a fine metal powderina photoresist solution. The photoresist is then exposed and developed in such a manner that cross-linking of the photoresist occurs in the areas intermediate the particles of the fine metal powder. These cross-linked areas remain after developing along with the metal particles. It will be appreciated that the metal particles have diameters which are roughly equivalent to the thickness of the photoresist film and therefore have top surfaces which are exposed for leaching. The metal particles are then leached out of the photoresist so as to leave holes in the photoresist corresponding to the size of the metal particles. An etching step follows similar to that referred to in thelfirst method such that pinholes are provided in the insulating film l5.

A third method of providing the appropriate pinholes inthe insulating film 15 involves the use of a photoresist solution in which an excess of sensitizer isprovided. It is known that controlled exposure of such a photoresist film will yield a film of very fine porosity whose porosity can be controlled by the amount of sensitizer and Y the amount of exposure. Such a film can then be used as an etch mask for the silicon dioxide insulating film 15. It will, however, be appreciated that other techniques for making a pinholed photoresist film are known and that these techniques are clearly within the scope of this invention. I

In a typical case, the apertures 16 provided in the insulating film 15 are on the order of V; micron i'n diameter and can vary in depth from one micron to micron depending on the thickness of the insulatingfilrn 15. In calculating the resistivity of the resistive film 10, it will be appreciated that the resistance R pt/A Typically, thickness of the thin resistive film, t, is on theorder of 10 centimeters. The area, A, in one case was 7 X l cm with a resulting resistance, R, of 25 kilohms. p in this case can be calculated tobe on the order of 1.75 X 10 ohm centimeters. A in this equation represents the total contact area to the resistive film. It is therefore this A which is varied by the number and size of the apertures in the insulating film 15. Thus,"the resistance is proportional to the areal density of the apertures which is in turn equal to the area of the apertures divided by the area of the surface of the resistive film As mentioned .before, the temperature coefficient of resistivity (TCR) of the device is of critical importance in ultra-high resistivity resistors. There are materials such as nickel-chromium, cobalt-chromium and tantalum which have a close to zero TCR. This zero TCR is not altered by the provision of the insulating film 15 in order to boost the resistivity of these films from 10" ohm centimeters to l to 10 ohm centimeters by the subject method. In general, those materials having a resistivity of l to 10 ohm centimeters usually have associated with them a TCR of 500 to 1 ,OOO. This represents a percent change in resistivity with a 100C change in temperature. It will thus be appreciated that by utilizing nickel-chromium, cobalt-chromium or tantalum materials, the same effective resistivity, i.e. 1 to 10 ohm centimeters, may be achieved by very close to zero TCR. When ultrahigh resistivity resistors are required having effective resistivities on the order of 10 ohm centimeters, aluminum-A1 0 cermets which normally have a resistivity of 10 to 10 ohm centimeters I can be provided with the pinholed insulating film so ohm centimeters resistivity resistor can be fabricated from the aforementioned cermet with a TCR of approximately 500.

Referring now to FIG. 2, if the conductive substrate 11 is provided with a resistive film 10 similar to that shown in FIG. 1, and is further provided with an insulating film 15 having the aforementioned pinholes 16 already formed therein, and if these pinholes are then filled with a conducting material, such as gold or any metal as shown at the areas 25, then the resistor thus formed can be utilized to measure the contact area denoted by the arrow 30 of a conductive material 31 which is pressed against the top side of the resistor at the surface 35. If a potential is applied between the I conductingmember 31 and the conductive substrate 11, the voltage drop thereacro'ss is a function of the contact area 30. This voltage drop. is measured as shown at 37 when the potential developed by battery 36 is supplied as shown. I I

The member 31 may be resilient or may be rigid as long as it is somewhat conductive such that a certain number of the filled apertures are shorted thereto. It will thus be appreciated that the resistance of the device will be proportional to the number of the metal filled apertures 25 contacted by the member 31. I

Referring to FIG. 3, a structure is shown in which the insulating film is provided with apertures 16 which are in this case left open and not filled with a conducting substance. This particular configuration has application in the measurement of thecontact area of a conducting liquid or of a member covered by a conducting film or liquid. Such a'member is thehuman eye which is in general covered by a layer of salty conductive fluid. The resistor therefore becomes a contact area sensor for the human eye insofar as the conductive liq-' tures 16 making contact with the resistive film 10. If as shown in FIG. 4 a potential 36 is applied between the human body and the conductive substrate 11, of the sensor 50, and the contact sensor 50 is pressed against the eye 40 such that the eye makes contact with the top I surface of the sensor, then for a given contact pressure the area of the eye contacted can be ascertained by the resistance of thedevice. Altemately, the pressure between the sensor and the eye can be increased until a standard present resistivity is obtained. The pressure necessary to obtain this resistivity is therefore correlated or correlatable with the pressure of the ocularv fluid within the eye; Thus the subject resistor can be utilized as a sensitive device for measuring the pressure of the ocular fluid. It will be appreciated, however, that the use of the subject resistor asa sensing device for contact area is not limited for use with the human eye and that any. member 31, whether fully conductive or only partially conductive, canv have its contact area measured by systems incorporating the subject resistor.

' Additionally, any non-conductive member surrounded by an electrically conductive liquid film can have its insulating layer having an areal density inversely A proportional to the resistivity of the completed resistor, forming a metal layer on top of said insulating layer such that said metal layer extends into said apertures and contacts that portion of said resistive film therebeneath whereby the resistivity of the resistive film in the completed resistor is increased by con-- 'tacting the top surface of said resistive film at a multiplicityof spaced locations corresponding to said apertures and whereby said resistivity is controlled by controlling the areal density of said apertures.

developing said photoresistive material whereby apertures for exposing spaced portions of said resistive film are formed therein corresponding in size and position to said particles, and etching said insulating layer through the apertures provided in saidphotoresistive material, whereby said insulating layer is provided with apertures corresponding in size and location to the size and location of said particles; 3. The method as recited in claim 2 wherein said particles are deposited on the top surface of saidvphotore sistive material.

4. The method as recited in claim 2 wherein said'particles are located within said photoresistive material, and are leached out during said developing step so as to form the apertures in said photoresistive material.

' said multiplicity of apertures for exposingspaced por- 10 5. The method as recited in claim 1 wherein forming tions of said resistive film includes depositing a powder made up of opaque particles to be illuminated on the surface of a photosensitive materialon said insulating i film, said particles forming said mask and the areal density of said particles corresponding to the areal density of said small apertures.

6. The method as recited in claim 1 wherein forming said multiplicity of apertures includes:

dispersing in a photosensitive material on said insulating layer a quantity of metal particles which are opaque to radiation and which have diameters roughly equal to the thickness of said photosensitive film, Y exposing said photosensitive material to radiation which causes a chemical change in that portion of said film intermediate said particles, developing said photosensitive material so as to further strengthen those portions of said photosensitive material intermediate said metal particles and make them inert to a leaching process, and leaching out said metal particles, whereby apertures are formed in said photosensitive material corresponding in size and position to the size and positionof said metal particles.

Claims

2. The method as recited in claim 1 wherein said apertures are formed photolithographically by covering said insulating layer with a photoresistive material, masking said photoresistive material by arranging opaque particles between the light source for said photolithographic process and said photosensitive material, the size of said particles and their locations corresponding to the size and locations of said apertures, whereby the areal density of said apertures is controLled by the areal density of said particles, developing said photoresistive material whereby apertures for exposing spaced portions of said resistive film are formed therein corresponding in size and position to said particles, and etching said insulating layer through the apertures provided in said photoresistive material, whereby said insulating layer is provided with apertures corresponding in size and location to the size and location of said particles.
3. The method as recited in claim 2 wherein said particles are deposited on the top surface of said photoresistive material.
4. The method as recited in claim 2 wherein said particles are located within said photoresistive material, and are leached out during said developing step so as to form the apertures in said photoresistive material.
5. The method as recited in claim 1 wherein forming said multiplicity of apertures for exposing spaced portions of said resistive film includes depositing a powder made up of opaque particles to be illuminated on the surface of a photosensitive material on said insulating film, said particles forming said mask and the areal density of said particles corresponding to the areal density of said small apertures.
6. The method as recited in claim 1 wherein forming said multiplicity of apertures includes: dispersing in a photosensitive material on said insulating layer a quantity of metal particles which are opaque to radiation and which have diameters roughly equal to the thickness of said photosensitive film, exposing said photosensitive material to radiation which causes a chemical change in that portion of said film intermediate said particles, developing said photosensitive material so as to further strengthen those portions of said photosensitive material intermediate said metal particles and make them inert to a leaching process, and leaching out said metal particles, whereby apertures are formed in said photosensitive material corresponding in size and position to the size and position of said metal particles.