WO1991014288A1

WO1991014288A1 - Magnetoresistor structure and operating method

Info

Publication number: WO1991014288A1
Application number: PCT/US1991/000753
Authority: WO
Inventors: Robert E. Eck; Ronald M. Hoendervoogt
Original assignee: Santa Barbara Research Center
Priority date: 1990-03-07
Filing date: 1991-02-05
Publication date: 1991-09-19

Abstract

A magnetoresistor structure has a pattern of transverse Hall voltage shorting strips (4) of multilayer construction. Each shorting strip includes a low impedance ohmic contact layer (16) which establishes a direct ohmic contact with the magnetoresistor material (2), a conductive layer (20) having a resistance low enough to short circuit the Hall voltage, and a blocking layer (18) between the ohmic contact (16) and conductive layers (20) which resist diffusion of the conductive layer material into the magnetoresistor (2). The blocking and ohmic contact layers complete a conductive path between the conductive layer and the underlying magnetoresistor material, and have dimensions which provide a good adherence of the shorting strips to the magnetoresistor.

Description

MAGNETORESISTOR STRUCTURE AND OPERATING METHOD

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to magnetoresistors of the type which includes a pattern of shorting strips to shunt Hall effect electric fields, and to an associated operating method.

Description of the Related Art

Magnetoresistors exhibit a resistance which varies in accordance with an applied perpendicular magnetic field. Most magnetoresistors exhibit positive magnetoresistance, with the resistance increasing in response to increases in magnetic field strength. The basis of the magneto- resistance is the Lorentz force, which causes the electrons to move in curved paths between collisions. This increases the effective electron path length, and is reflected as an increase in resistance.

As the electrons are deflected towards one side of the magnetoresistor (MR) , the accumulation of electrons along that side produces an internal transverse electric field which opposes and tends to negate the effect of the extern¬ ally applied magnetic field. This phenomenon is referred to as the Hall effect, and is described for example in Van Nostrand'ε Scientific Encyclopedia, 7th Edition, ed. by D. M. Considine, Van Nostrand Reinhold, 1989, page 1398.

By shorting the Hall electric field, one can obtain a agnetoresistance which does not saturate. This has been accomplished in practical MRs by forming a pattern of transverse shorting strips across an elongate MR. The con¬ ductive shorting strips form contacts with the underlying MR material, and in effect short circuit the Hall fields that would otherwise be built up. Examples of such Hall shorting strips are given in Wang et al. , "Semiconductive Magnetoresistors", MRL Bulletin of Research and Develop¬ ment, Vol. 2, No. 2, September 1988, and in U.S. Patent Nos. 3,772,100, 3,852,103 and 3,898,359. These references disclose the use of indium antimonide, both in its bulk form and otherwise, and indium arsenide as the underlying MR material, and conductive indium shorting strips.

The choice of materials for the shorting strips is quite limited, since the shorting material should form an effective ohmic (low impedance) contact with the underlying MR, and it should also not degrade the MR material by dif¬ fusing into it. One limitation of presently available MR structures is that the contact between the shorting strips and the underlying MR material is less than ideal, creating a tendency for the shorting strips to be dislodged during operation.

Various metallization techniques have been developed in fields other than MRs. One metallization scheme for integrated circuit infrared (IR) detectors involves a multilayer metallization for bringing in a lead to contact one side of an IR detecting p-n junction. In this applica¬ tion the primary purpose of the multilayer structure is to obtain good adherence to the underlying substrate, rather than to minimize the lead resistance. Since the IR detect- ors exhibit a junction impedance on the order of 10¹⁰ ohms at typical cryogenic operating temperatures, the resistance of the leads are dwarfed by this junction impedance so that the achievement of a very low resistance lead is immateri¬ al. This directly contrasts with MRs, in which the total resistance along the length of the MR is typically on the order of a kiloh . Since the transverse dimension can be several hundred times less than the MR elongate dimension, the provision of very low impedance transverse shorting strips is critical for satisfactory MR operation. The referenced IR detector lead contacts consist of a thin layer of palladium, typically about 200 Angstroms thick, to establish an ohmic contact with the p-n junction in the underlying indium antimonide detector material; an ohmic rather than a junction contact is desirable to avoid distorting the signal. A titanium layer about 600-800 Ang¬ stroms thick is provided over the palladium ohmic contact layer to assure good adhesion for the overlying conductive portion of the lead, which primarily carries the signal. The conductive portion is typically a layer of gold which is thick enough to obtain good lead continuity, on the order of about 1,000 Angstroms.

While this IR detector lead contact structure achieves the good adherence desired for MR shorting strips, any shorting along the surface of the IR detector is neither necessary nor intended, and is merely incidental and of no operational importance. In fact, the resistance of such IR detector leads is too high for effective use as an MR shorting strip. Furthermore, while the IR detectors have to withstand temperatures of up to 100*C during packaging, they are operated within a cryogenic temperature range of about 50*-120^*K. This is a far colder operating regime than typical applications for an MR, which include rota¬ tional timing devices and non-contact potentiometers used in automobiles, with operating temperature requirements covering a range of -40"C to 240*C. The diffusion of shorting strip material into the underlying MR material is much more of a concern at this higher operating temperature range.

Another problem with present MRs is that charge tends to accumulate on the surface of the MR material between shorting strips. If this accumulation continues far enough, it can shunt out the MR and render it ineffective.

SUMMARY OF THE INVENTION The present invention seeks to provide an MR structure having Hall effect shorting strips which have a greater and more reliable adherence than in the past, which establish an effective low impedance ohmic contact with the underly¬ ing MR material and a very low resistance path, and which prevent diffusion of the conductive material into the un¬ derlying MR material even at the higher automotive operat¬ ing temperatures. Another goal of the invention is to pro¬ vide an MR structure in which the accumulation of surface charge between shorting strips is inhibited, thereby pre- venting the MR from being shunted.

The new MR structure employs a modification of the IR detector lead described above as a shorting strip, but with entirely different operating criteria and for a much dif¬ ferent purpose. Starting with an MR which is preferably indium antimonide, multilayer shorting strips are formed with an ohmic contact layer, preferably palladium, nickel or platinum, in direct contact with the underlying MR material. A diffusion blocking layer of titanium and/or nickel on top of the ohmic contact layer prevents diffusion of an upper conductive layer into the MR material. An upper layer is preferably formed from gold or aluminum, to a thickness on the order of 0.5-1.5 microns, to establish a low resistance for each strip which effectively short circuits the Hall voltage. The ohmic contact and blocking layers are kept thin enough to prevent the shorting strips from peeling off, but are thick enough to effectively per¬ form their respective functions. These layers are prefer¬ ably about 200 and 600-800 Angstroms thick, respectively. To prevent the accumulation of charge on the MR sur- face between shorting strips, that surface is passivated with a layer of SiO_z or SiN. The resulting MR can be effec¬ tively operated in automotive environments at temperatures in the range of -40' to 240'C.

These and other features and advantages of the inven- tion will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of an MR structure constructed in accordance with the invention;

FIG. 2 is a sectional view taken along the lines 2-2 of FIG. 1; and

FIGs. 3a-3d are fragmentary sectional views showing the MR structure at successive stages in its fabrication.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a magnetoresistor (MR) 2 is shown in a raster pattern with Hall effect shorting strips. The MR is illustrated as a serpentine filament which achieves a lengthy longitudinal dimension for a given area, but many other geometric configurations are possible. Also, while a raster pattern of shorting strips is the primary applica¬ tion, the invention can also be applied to other shorting strip arrays.

The MR filament is typically about 80 microns wide. Spaced along its upper surface is a series of spaced con¬ ductive transverse shorting strips 4 which short out the Hall effect electric fields. The shorting strips have a special construction, described below, which give them ex¬ cellent adherence to the underlying MR material, establish a good low impedance contact with the MR along the entire lengths of the strips and provide effective short circuits. The shorting strips 4 are typically about 20 microns wide, with 20 micron εpacings between adjacent strips. Indium antimonide is preferably selected for the MR, but other materials such as indium arsenide, indium phos¬ phide and gallium arsenide might also be used. The MR material is extended at each end of the filament to form enlarged contact pads 6, which are also coated with the shorting strip material to provide access for external con¬ tacts to the MR structure. Several hundred shorting strips will typically be employed for a given MR.

A sectional view of the MR structure, taken between shorting strips, is provided in FIG. 2. The MR is formed on a dielectric substrate 8 which is capable of transmit¬ ting a magnetic flux. Sapphire is preferred for the sub¬ strate because of its rigidity. Oxide coated silicon, quartz or ceramics are other candidates for the substrate material. The MR material 2 is attached to the substrate by means of an epoxy glue 10. The epoxy layer should be as thin as possible consistent with good adhesion, and is typ¬ ically about 0.5-8 microns thick. If the epoxy reacts with the MR material, the underside of the MR can be passivated with a layer of SiO_z (a combination of SiO and Si0₂) or SiN, generally about 0.1 microns thick.

The MR material 2 is typically on the order of 1-10 microns thick. As mentioned above, it is possible for sur¬ face charge to accumulate on the sections of the MR between shorting strips, to the extent that the device can be shorted out. To overcome this problem, a layer 12 of pass¬ iveting material is deposited over the MR between shorting strips to inhibit the accumulation of surface charge. The passivating layer 12 is formed in a manner similar to pass- ivations used for integrated circuit applications, but not heretofore employed in connection with MRs. The passiva¬ tion layer 12 is ideally pure Si0₂, deposited by chemical vapor deposition. In practice, some SiO will also enter the passivation Si0₂ layer, so that the layer is actually a combination of the two which can be designated Si0_x. Alter- nately, SiN can be used for the passivation. The passiva¬ tion layer is preferably on the order of 0.1 microns thick. The novel MR shorting strips and a preferred fabrica¬ tion technique therefor are illustrated in FIGs. 3a-3d. To prepare the MR filament, a block of MR material is first sawed or machined flat, then lapped or milled and then etched with a lactic acid/nitric acid combination to remove crystalline damage. At this point the MR block is about 500-600 microns thick. It is then glued to the substrate and reduced to a thickness of about 6-12 microns by a grit lap followed by a turn polish, or by a diamond milling pro¬ cess. This is followed by another etch to obtain a pre¬ ferred thickness of about 5-10 microns. The serpentine shape and contact pads are then patterned by a photoresist mask. The unmasked material is etched away (chemical or dry etching) to obtain the desired serpentine pattern, and the photoresist dissolved away. Another etch of approxi¬ mately 0.5 microns is then performed to clean up the MR surface. An alternative approach to the use of bulk MR material is to deposit by evaporation or epitaxial deposi¬ tion a layer or MR material in the 1 to 2 micron range. In this case the epoxy and bottom passivation layer are not present.

At this point the passivation layer 12 is applied to the entire top surface of the MR by chemical vapor deposi¬ tion. It has been discovered that, whereas integrated cir¬ cuit passivation is conventionally performed at about 450^β- 500*C, the passivation for the MR can be conducted at a much lower temperature, on the order of 120'-200^. A pattern of photoresist 14 is next laid down over the MR areas between the intended locations of the shorting strips. This results in the structure shown in FIG. 3a. The photoresist sections 14 are separated by a distance corresponding to the shorting section width, which is pre- ferably about 20 microns. The passivation layer 12 is then etched away from the MR surface in the areas not covered by photoresist, leaving these areas bare to receive shorting strips, as shown in FIG. 3b.

The next step in the fabrication, illustrated in FIG. 3c, is to deposit a thin layer of a contact material 16 over the entire exposed structure. This material should be selected to provide a good ohmic (low impedance) contact with the underlying MR material, and to not diffuse into that material. Palladium is preferred for this purpose in connection with an indium antimonide MR, but nickel or platinum could also be used. The ohmic contact layer 16 can be electroplated onto the MR structure to a preferred thickness of about 200 Angstroms.

A blocking layer 18 is deposited over the ohmic con- tact layer 16, followed by the deposit of a conductive layer 20 on top of the blocking layer 18, preferably by evaporation. The blocking layer material is selected to obtain a good adhesion of the conducting layer, and to block the conducting layer material from diffusing into the MR material. For a conducting layer formed from gold or aluminum, either titanium or nickel could be used for the blocking layer 18. If it is kept thin enough, preferably to a thickness of about 600-800 Angstroms in conjunction with an ohmic contact layer about 200 Angstroms thick, the blocking layer will not be easily removed and will assure a good adhesion of the shorting strip to the MR material. The blocking layer can be deposited by vacuum deposition.

Gold is the preferred material at present for the con¬ ductive layer 20. In contrast to the leads used with prior IR detectors, the conductive layer 20 is made thick enough to offer a very low resistance across the length of the shorting strip, and thereby effectively short out Hall voltages in the underlying MR material. However, care should be taken that the conductive layer is not so thick as to create a tendency to be peeled off. A gold layer with a thickness in the approximate range of 0.5-1.5 microns, and preferably about 0.6-1.0 microns, has been found to satisfy these criteria. In addition to being a good conductor, gold has the advantage of being malleable enough to avoid unduly stressing as it heats. Aluminum might be used as a substitute for gold, since it can be fabricated thick enough to bond to the MR structure, but not so thick as to cause significant thermal expansion problems. Indium is another candidate conductor material, but is limited to relatively low temperatures since it melts above about 150*C.

The resulting structure, shown in FIG. 3c, has short¬ ing strips both directly in contact with the MR material in the intended shorting strip locations, and also over the photoresist 14 which masks the passivation sections 12. The photoresist is now dissolved to leave a finished MR structure, as shown in FIG. 3d. The shorting strips 4 are strongly adhered to the underlying MR material, while the conductive layers 20 provide effective Hall voltage short circuits via the conductive blocking and ohmic contact layers. At the same time, surface charge accumulation is effectively inhibited by the passivation layer 12 between conductive strips. While only a short segment of the over¬ all MR is shown in FIGs. 3a-3d, it should be understood that in general several hundred alternating shorting strips and passivating layers would be employed.

Other fabrication techniques are also available. For example, in the fabrication described above the passivation is performed first because the subsequent metallization process is more reliable, so that defects due to passiva¬ tion can be discarded or remedied before going through the metallization process. However, the shorting strip metal¬ lization may be deposited first if desired by masking the passivation sites with photoresist, laying down the triple- layer metallization over the entire structure, dissolving away the photoresist to lift off the metallization over the passivation areas, and then sealing the entire structure with a passivation layer.

While several illustrative embodiments of the inven- tion have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art without departing from the spirit and scope of the in¬ vention. Accordingly, it is intended that within the scope of the appended claims, the present invention may be prac- ticed otherwise than as specifically described.

Claims

WE C AIM:

1. A magnetoresistor (MR) structure, comprising: a member formed from an MR material, and a shorting strip array arranged in a pattern on said MR member, said shorting strip array comprising: an ohmic contact layer in direct contact with the MR material along the length of the shorting strip and selected from a material which establishes a low imped¬ ance ohmic contact with the MR material with no substantial diffusion into the MR material, a conductive layer over the ohmic contact layer, the conductive layer being formed from a conductive material and having dimensions to establish substantially a short circuit along the length of the shorting strip, and a blocking layer interposed between the oh - ic contact and conductive layers, said blocking layer formed from a conductive material which resists diffusion of the conductive layer material into the MR material and completes a conductive path between the conductive layer and the underlying MR material.

2. The MR structure of claim 1, wherein said MR material comprises bulk indium antimonide.

3. The MR structure of claim 1, wherein said ohmic contact layer is formed from palladium, nickel or platinum.

4. The MR structure of claim 1, wherein said block¬ ing layer is formed from titanium or nickel.

5. The MR structure of claim 1, wherein said conduc¬ tive layer is formed from gold or aluminum.

6. The MR structure of claim 5, wherein the thick¬ ness of said conductive layer is in the approximate range of 0.5-1.5 microns.

7. The MR structure of claim 1, wherein said ohmic contact and blocking layers are thin enough to resist peel¬ ing off from the MR material.

8. The MR structure of claim 7, wherein said ohmic contact layer is approximately 200 Angstroms thick, and the thickness of said blocking layer is in the approximate range of 600-800 Angstroms.

9. The MR structure of claim 1, said MR material comprising indium antimonide, said ohmic contact layer com¬ prising palladium, nickel or platinum, said blocking layer comprising titanium or nickel, and said conductive layer comprising gold or aluminum.

10. The MR structure of claim 9, wherein the thick¬ ness of said conductive layer is in the approximate range of 0.5-1.5 microns.

11. The MR structure of claim 10, further comprising means passivating the surface of the MR material not occu¬ pied by said shorting strip array to inhibit the accumula¬ tion of charge at said MR material surface.

12. The MR structure of claim 11, said passivating means comprising a layer of SiO_z or SiN.

13. The MR structure of claim 1, further comprising means passivating the surface of the MR material not occu¬ pied by said shorting strip array to inhibit the accumula¬ tion of charge at said MR material surface.

14. The MR structure of claim 13, said passivating means comprising a layer of SiO_z or SiN.

15. The MR structure of claim 1, said shorting strip array comprising a plurality of spaced shorting strips ar¬ ranged in a raster pattern on said MR member.

16. A magnetoresistor (MR) structure, comprising: an elongate member formed from a MR material, a plurality of spaced shorting strips arranged in a pattern on the elongate member for shorting out Hall ef- feet electric fields within the MR material, and a layer of passivating material over the MR material between said shorting strips for inhibiting the accumulation of charge along the surface of said MR material.

17. The MR structure of claim 16, said MR material comprising indium antimonide, and said passivating material comprising SiO_z or SiN.

18. The MR structure of claim 17, said layer of pass¬ ivating material being on the order of 0.1 microns thick.

19. A magnetoresistor (MR) operating method, compris¬ ing: a) providing an MR structure which comprises:

1) an elongate member formed from a MR material,

2) a plurality of spaced shorting strips arranged in a pattern on the elongate member, each strip comprising: i) an ohmic contact layer in direct contact with the MR material along the length of the short¬ ing strip and selected from a material which establishes a low impedance ohmic contact with the MR material with no substantial diffusion into the MR material, ii) a conductive layer over the ohmic contact layer, the conductive being formed from a conduct¬ ive material and having dimensions to establish substan¬ tially a short circuit along the length of the shorting strip, and iii) a blocking layer interposed be- tween the ohmic contact and conductive layers, said block¬ ing layer formed from a conductive material which resists diffusion of the conductive layer material into the MR material and completes a conductive path between the con¬ ductive layer and the underlying MR material, and b) operating said MR structure at a temperature within the approximate range of -40*C to 240*C.

20. The MR operating method of claim 19, said MR material comprising indium antimonide, said ohmic contact layer comprising palladium, nickel or platinum, said block¬ ing layer comprising titanium or nickel, and said conduct- ive layer comprising gold or aluminum.

21. The MR structure of claim 20, wherein the thick¬ ness of said conductive layer is in the approximate range of 0.5-1.5 microns.

22. The MR structure of claim 19, further comprising means passivating the surface of the MR material between said shorting strips to inhibit the accumulation of charge at said MR material surface.

23. The MR structure of claim 22, said passivating means comprising a layer of SiO_z or SiN.