US 3680028 A
There is disclosed a high valued vertical resistor whose high value is a result of contacting the resistive film through pinholes in an insulating film 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 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 photoresists as masks for etching the thin insulating film.
Description (OCR text may contain errors)
United States Patent Black et a1.
 VERTICAL RESISTOR  Inventors: James R. Black, Phoenix; Harold S. Gu-
rev, Paradise Valley, both of Ariz.
 Assignee: Motorola, Inc., Franklin Park, Ill.  Filed: April 2, 1971  Appl. No.: 130,610
 U.S. Cl ..338/308, 338/13, 338/38,
338/328, 73/80, 324/65 R, 178/2 T, 117/215  Int. Cl ..l-l0lc 7/00  Field of Search ..338/1 3, 38, 327, 328,195,
338/308; 117/215; 73/80; 324/65 P, 65 R; 178/2 T Primary Examiner-E. A. Goldberg Attorney-Mueller & Aichele 57 ABSTRACT [151 3,680,028 [451 July 25,1972
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 photoresists 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 determining 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.
9 Claims, 4 Drawing Figures CONTACT, 2O
INSULATING F|LM,|5 RESIST1VE FILM, IO
CONDUC TIVE /SUBSTRATE,H
PATENTEDJuL25 m2 3 Q 680. 028
mm W f REsssnvE FILM no CONDUCTIVE F7 */SUBSTRATE,H
INSULATING FILM, l5
Hg. 3 RESISTIVE CONDUCTIVE SUBSTRATE,
INVENTOR James R. Black BY Harold S. Gurev ATTY'S.
VERTICAL RESISTOR BACKGROUND This invention relates to vertical thin film resistors and more particularly to a resistor whose value is controlled by the areal density of pinholes in a thin dielectric layer deposited on top of the resistive layer.
While thin film'vertical resistors are known in the art, there has been some difficulty experienced in providing resistors with controllable high values. In the prior art, the value of the resistor is controlled by the resistively, size and shape of the resistive layer as well as its thickness. However design parameters preclude the fabrication of ultra-high vertical resistors from known resistor materials because of the relatively low resistivity of these materials.
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 is accomplished by effectively providing approximately l very small resistors, the final value of the resistor is controlled by the area of the resistive film contacted. As the number and size of the small spaced-apart portions are increased, the resistance of the completed device is decreased due to an increase in parallel resistive paths through the resistive layer. The increase in resistive paths effectively places more resistive segments of the resistive layer in parallel thus reducing the 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 etching the thin insulating film down to the surface of the resistive film and by forming a metal layer thereacross. 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 paths may be controlled by the areal density ofthe holes in the thin insulating film.
The areal density of the holes in the thin insulating film is in turn provided by one of three photoresist masking techniques. The first of these techniques involves the use of a negative photoresist film which is deposited on the thin insulating film and which is then baked. Prior to exposure, a predetermined number of opaque particles whose diameter corresponds to the whose size desired, are spread over the photoresist film. The photoresist film is then exposed to ultraviolet light and is developed. This leaves the photoresist with pinholes at the locations of the individual particles. This pinholed photoresist film is then used as a mask for etching the thin insulating layer such that the holes in the thin insulating layer correspond in location and size to the above mentioned particles. It will thus be appreciated the areal density of the particles on the photoresist film corresponds to the areal density of the holes 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 applying the photoresist solution to the thin insulating film, the entire structure is exposed and developed. The metal particles are then leached out of the photoresist so as to leave the aforementioned mask. It will be appreciated that the metal particles themselves are opaque to the radiation. The radiation causes cross-linking in the photoresist film adjacent and inbetween 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 support such as a glass slide and used as a master light mask to form holes in a conventional photoresist layer applied directly to the insulating layer on the resistor.
In addition to forming a thin film vertical resistor with controllable characteristics, the subject process 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 such that the conducting metal contacts only a portion of the metal filled pinholes, then the resistor will have a resistance value proportional to the amount of metal-filled pinholes contacted by the conductive material. The resistance is therefore proportional to the area of the resistor contacted by the conductive material. In a further application, the pinholes are not filled with a metallic conductor but rather are filled with a conductive fluid, such as that which exists on the surface of the human eye. By pressing the sensor against the human eye the surface contacted for a given pressure may be ascertained with the ocular fluid providing the contact 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 instrument.
SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a vertical resistor whose resistance value is varied by 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 the 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 as to control the resistance value of the resistor accurately in the higher resistance ranges.
It is still another object of this invention to provide a vertical resistor which prevents or reduces current crowding in a com tact area.
Other objects of this invention will be better understood upon reading the following descriptions in connection with the accompanying drawings.
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.
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.
BRIEF DESCRIPTION OF THE INVENTION There is disclosed a vertical resistor whose value is controlled by the areal density of pinholes through a 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 over the pinholed film. The metal in the layer 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 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 method involves the formation of a resistive thin film over a conductive substrate. This resistive thin film is covered with thin film of an insulating material such as silicon dioxide. On top of the silicon dioxide layer is formed a layer of photoresist. On top of the photoresist is deposited opaque particles whose diameter corresponds to the aperture size desired and which has areal density equal to the required value. The structure is then exposed to radiation, the photoresist developed and the silicon t dioxide layer then etched down to the resistive film in the areas covered by the particles. A second method 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. It 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 enables 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 pinholed dielectric 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 on from another. The structure in this case can be used as a sensing device for determining a planar contact area of an electrically conducting structure contiguous to the top surface of the resistor, the contact area being proportional to 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. I, 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 resistivitiesin the 10- to 10 ohm centimeter range. The effective remagnitude uncertainty in the resistance of the final product. By use of the subject pinhole insulator, the resistance of thin film devices can be controlled to within 10 percent without the necessity of utilizing leaky di-electric materials for the re sistance element. A;
Secondly, high resistancejidevices 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 coefficients of resistivity vary widely in the ultra-high resistance range. The temperature coefficients of resistivity (TCR) is defined as In the prior art ultra-high resistance devices, TCRs of 5,000
parts per million per degree centigrade are common. If the re-' sistor is fabricated with the aforementioned pinholed thin film I insulator, effective resistivities on the order of 10" ohm centimeters can be achieved with a 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 about the 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, nitrided tantalum and other such thin film resistors which normally have a resistivity on the order of 10 ohm centimeters. The resistivity of these materials can be increased by the subject system to l to l0 ohm centimeters by the use of the aforementioned pinhole insulating film. Thus, the subject system can augment the resistivity of low TCR materials making the devices fabricated in this manner suitable for emitter ballasts and indeed for any integrated circuit or transistor configuration.
Central to the concept of controllability, high resistance and low TCR is the idea of limiting the contact area to the thin film in order to increase the resistivity of the thin film in a predictable manner. It has been known that the smaller the contact area of a thin film resistor, the higher the resistance. Unfortunately, if a single small resistor is made, it willburn out due to the current density at its contacts. This is referred to as current crowding. Rather than making a very small resistor, of for instance Nichrome, the subject system utilizes a layer of Nichrome, on which is deposited an insulating layer having a multiplicity of apertures therethrough. 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 across this multiplicity of very small resistors'such that the total device can carry considerable current. The resistance of the resistive thin 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 the holes or apertures in the thin film, the total resistance of the device can be easily controlled as well as'augmented.
Fourthly, and as a by-product of the fabrication of the subject transistor, it was found that the pinhole insulating structure on top of the resistive thin film provided for a new method of measuring contact area of a conducting body. When the conducting body is placed on top of the subject resistor which does not have a continuous metal layer, and contact is made from the conducting body through the pinhole to lated one from another, the mere placing of the conductive body on top of the resistor such that certain of the filled pinholes are contacted, results in a sensing device which can quite accurately measure the area of a contacting body.
Referring to FIG. I, a resistive film is shown deposited on a conductive substrate 11. Depending on the application, the conductive substrate can either be a metal or a semiconductor substrate which is doped heavily enough to form an ohmic contact with the resistive thin film. The conductive substrate, in one embodiment, may rest on a further contact 12 which is any normally used metal such as aluminum. As mentioned hereinbefore, the resistive film may very substantially in thickness and in resistivity and may be formed from any one of the conventional materials utilized for thin films. These include nickel-chromium alloys, tantalum with some nitrogen dispersed therein, and cobalt-chromium alloys. These materials usually have a resistivity of l0- ohm centimeters. Additionally, cermet type resistors such as Al AL O; which normally have a resistivity of 10" to 10 ohm centimeters can be utilized as the resistive film 10.
The resistivity of any of the films utilized can be augmented in accordance 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 15 which is first deposited on the resistive film l0 and then etched to provide for the aforementioned apertures shown here at 16.
In the configuration shown in FIG. I, a metallization layer forming contact is deposited over the insulating film 15 such that the metal utilized in the insulating layer fills the pinholes or apertures through the insulating film 15 and contacts the resistive film 10 at the locations where the apertures are cut through to the resistive film.
As mentioned previously, there are several ways of providing that the insulating film 15 have a predetermined 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 negative photoresist film (not shown). After the photoresist film is applied to the insulating film 15, it is baked. Prior to exposure, the photoresist film is provided with a dispersion of ultraviolet opaque particles whose diameters correspond to the aperture sizes desired and are in general in the micron and sub-micron range. The amount of material is adjusted such that the areal density of the apertures to be provided by the number and size of the particles is at the required value. The structure is then exposed to ultraviolet light and the photoresist film is then developed. Those areas of the photoresist film which were covered by the particles during exposure are washed away by the developer leaving apertures 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 the insulating film layer 15 is provided with pinholes beneath each particle. The particles which can be utilized include powders of A1 0 glass and other ceramics.
A second method for providing the pinhole structure for the insulating film 15 is accomplished by mixing the required percentage of a fine metal powder in a 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 crosslinked 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 the first method such that pinholes are provided in the insulating film 15.
A third method of providing the appropriate pinholes in the insulating film l5 involves the use of a photoresist solution in which an excess of sensitizer is provided. 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 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.
In a typical case, the apertures 16 provided in the insulating film 15 are on the order of one-fourth micron in diameter and can vary in depth from one micron to one-fourth micron depending on the thickness of the insulating film 15. In calculating the resistivity of the resistive film 10, it will be appreciated that the resistance R pl/A. Typically, thickness of the thin resistive film, t, is on the order of 10- centimeters. The area, A, in one case was 7 X l0' cm with a resulting resistance, R, of 25 kilohms. p in this case can be calculated to be 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 10,
As mentioned before, the temperature coefficient of resistivity (TCR) of the device is of critical importance in ultrahigh resistivity resistors. There are materials such as nickelchromium, 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 1 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 1,000. This represents a 5 l0 percent change in resistivity with a C change in temperature. it will thus be appreciated that by utilizing nickelchromium, cobalt-chromium or tantalum materials, the same effective resistivity, i.e. l to 10 ohm centimeters, may be achieved at very close to zero TCR. When ultra-high 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 can be provided with the pinholed insulating film 15 so as to achieve the 10" ohm centimeters effective resistivity. Concomitant with this increase, however, there is no increase in the TCR of the cermet such that a l0 ohm centimeters resistivity resistor can be fabricated from the aforementioned cermet with a TCR of approximately 500.
Referring now to H6. 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 conducting member 31 and the conductive substrate 11, the voltage drop thereacross is a function of the contact area 30. This voltage drop is measured as shown at 37 when the potential developed by batter 36 is applied as shown.
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 25 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 3].
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 the contact area of a conducting liquid or of a member covered by a conducting film or liquid. Such a member is the human 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 liquid on the surface of the human eye seeps into the apertures 16 making contact with the resistive film 10. If as shown in H6. 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 surface of the sensor, then for a given contact pressure the area of the eye contacted can be ascertained by the resistance of the device. Alternately, 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 ocular 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 as a sensing device for contact area is notlimited for use with the human eye and that any member 3!, whether fully conductive or only partially conductive, can 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 contact area determined by the subject device.
What is claimed is: l. A vertical resistor comprising: a conductive substrate having an upper surface; a resistive film positioned on said upper surface; an insulating film positioned on said resistive film, said insulating film being formed with a multiplicity of apertures for exposing spaced portions of said resistive film therethrough; and contacting means positioned on top of said insulating film and extending into at least a plurality of said apertures so as to contact an equal plurality of those portions of said resistive film exposed by said plurality of apertures, the resistance value of the resistor being proportional in an inverse manner to the areal density of said plurality of apertures containing said contacting means and the resistance value of said insulating film, whereby the effective resistivity of said resistive film is augmented by said multiple contacting means.
2. The resistor as recited in claim 1 wherein said resistive film is selected from the group consisting of cermets, nickelchromium alloys, cobalt-chromium alloys, and nitrogen-rich tantalum.
3. The resistor as recited in claim 1 wherein said resistive film is an Al-Al O; cermet having a resistivity of between l0 and 10' ohm centimeters.
4. The resistor as recited in claim 1 wherein the resistance value of the resistor is given by:
R pr/A where R is the resistance in ohms p is the effective resistivity of the resistive film in the completed device,
I is the thickness of the resistive film in centimeters,
A is the total contact area on the resistive film exposed through said apertures. 5. The resistor as recited inclaim 1 wherein said resistive film as a thickness on the order of l0 centimeters.
6. The resistor as recited in claim 1 wherein the diameters of 1 said apertures are in the sub-micron range.
7. The resistor as recited in claim I. wherein said contacting means is positioned on top of a portion of said insulating film and extends into a portion of said apertures, whereby the area of said contacting means is proportional to the value of resistance measured between said contacting means and said substrate.
8. In combination: a conductive substrate having an upper surface;