CA1313403C - Position sensor - Google Patents

Abstract

POSITION SENSOR

Abstract of the Disclosure For increased sensitivity an improved position sensor includes a magnetic circuit in which the stationary portion includes a permanent magnet whose width is optimally 1.5 times the tooth pitch of the exciter portion of the sensor and the magnet face proximate the exciter includes a thin layer of ferromagnetic material over which is centered a narrow magnetic sensing element, such as a magnetoresistor.
The sensing element has a width typically less than the tooth width. The sensing clement includes a thin film of a monocrystalline semiconductive material, preferably having only a moderate bulk mobility and a larger band gap, such as indium arsenide. Current carriers flow along the length of the thin film in a surface accumulation layer, effective to provide a significant apparent increase in mobility and conductivity of said semiconductive material, and an actual increase in magnetic sensitivity and temperature insensitivity. The flux density is typically applied by appropriate magnet thickness or choice of magnet material without the need of a flux guide.

Description

~313~03 Copending Patent Applications This patent application is related to the following Canadian patent applications, which are assigned to the same a~signee to which this patent application is assigned:
Canadian patent application Serial No.
604,137, which is based on United States Patent NoO
4,926,154 entitled, "Indium Arsenide Magnetoresistor,"
filed in the names of Jo&eph P. Heremans and Dale L.
Partin; and Canadian patent application Serial No.
604,.133, which i~ based on United States Patent No.
4,978,938 entitled, "Improved Magnetoreaistor," filed ln the names of Dale L. Partin, Joseph P. Heremans and Donald T. Morelli.
This patent application iE also related to the following earlier filed United States patent application, which also i6 assigned to th~ assignee of this invention;
Canadian patent application Serial No.
604,131, which is based on United States Patent No.
4,926,122 entitled, "Position Sensor," filed in the names of Thaddeus Schroeder and ~runo P. B. Lequesne.

Field of the Invsntion /-This invention relates to a position sensor and more particularly to an improved magnetic field sensing system having an improved magnetoresistive sensor for detectiny changes in magnetic flux passing sensor for detecting changes in magnetic 1ux passing through a magnetic flux sensitive element.

sackground of the Invention This invention is a further improvement on the improved magnetic field 6ensing system already being de~cribed and claimed in the a~ove-identified Canadian patent application Serial No. 604,131, filed in the names of Thaddeus Schroeder and Bruno P. ~.
Le~uesne and entitled, "Position Sensor. ~t The need for accurately and easily sensing position, speed or acceleration is growing, particularly in the automotive fiald. Anti lock braking ~ystems, traction control systems, electric power steering, four-wheel steering and throttle control are examples of functions that can use such sensing. Such applications not only reguire accuracy and precision, but frequently involve se~ere environments. Cost of such systems is an important factor, too.
~ or such applications, it is desira~le to have a position sen~or (speed and acceleration can he derived from a position signal) that is rugged and reliable, small and inexpensive, capable of low ~including zero) speed sensing and relatively immune to electromagnetic field interference from the other systems used in an automobile.
A well-known form of position sensor is a semiconductor magnetoresistive sensor. Such a sensor comprises a magnetic circuit that includes two basic parts. One of these parts, typically kept stationary, includes a semiconductive sensing element that is sensitive to the magnetic ~lux density passing through '.~

j, 1 3 1 3~03 its surface, and further includes a permanent ~agnet for creating a reference flux~ The other of the two parts, termed the exciter, includes a high magnetic permeability ele~ent with a series of ~eeth that ~oves with relation to the stationary ele~ent ~or changing the reluctan~e of the magnetic circuit and for cau~ing the ~agnetic flux through the ~ensing element to Yary in a f~shion corresponding to the position of the teeth.
Such a sensor is sensitive to the magnetic flux den~ity rather than to the rate of flux density change and so it does not have a lower speed limit.
Thi~ also makes it less sensitive to E.M.I. Moreover, its response is predictahly related to the distribution of flux density over the surface of the sensing element.
Typically, the stationary part includes a magnetoresistive element including a semiconductive element whose resistance varies with the magnetic flux density passing through it in controllable fashion so that an electrical output signal can be derived.
Moreover, when this magnetoresistor is produced ~rom a high electron mobility se~iconductor, such as compound 6emiconductors like indium antimonide or indium arsenide, a large electrical output signal can ~e available. If the output signal is sufficiently large, there is the possibility of providing an output signal that requires little or no further a~plification, B
factor of considerable advantage.
It is desirable to have a position sensor of high sensitivity so that a large electrical output signal can be produced efficiently and of easy 1313~03 ~anufacture so that it can be made reliably and at low c06t.
The magnitude of the flux variation~ in the ~en~ing ~l~ment for a given chang~ in position of the exciter i6 ~n i~portant factor ~n d~t~rmining the ~nsitivity of the ~en~or. Accordingly, a variety of d~ign6 have been atte~pted h~th~rto to m~xim~ze the change in the 1ux density through the ~ensor in respon~e to a given change in exciter po6ition.
Typically, th~6e ~ttemptc involved including a ~lux guid~ for the p~rmanent m~gnet included in the ~tatlonary part of the magnetic circuit to provide a r~turn path or the magnetlc ficld of the magn~t.
Additionally, sometime~ a field conc~ntrator of commen~urate ~ize has been provided contiguous to the magnetore~istive element to concentrate flux throu~h the magnetore~istive element.
~owever, for example, ~uch technique~ have typi~ally produced m~gnetic circuit 6ensitivitie~ no higher than about five percent for a typi~al exciter de~ign having a three milli~eter tooth pitch and one millimeter gap, where the ~en6itivity i~ defined ~ the differsnee between the maxi~um and mini~um flux den6ities 6ensed divided by the ~an flux ~ensity 6en~ed (half the ~um of the maximum and minimu~ ~lux den6ities ~ensed).
Two CoDIpanion Canadian patent application6 were concurrently filed her~lith, CSN 604,137 and CSN 604,133, which are more fully identified above.
CSN 604,137 and C~N 604,133 de~cribe the ~brication and prop~rtie~ of a new type of magnetore~i~tor thin film el~ment. CSN 604,137 detail~ the proce66 of growing a thin film of indium ar6~nide (InA~), a narrow-gap "~
,!,' semiconductor, on a semi-insulating indium phosphide ~InP) substrate, and shows that this device has a rathsr large sensitivity of electrical re~istance to magnetic field CSN 604,133 outlines various methods of enhancing the sensitivity of the device on the basi6 of the existence of a thin surface layer (known as an accumulation or inversion layer) of high density, high mobility electrons. Such electron accumulation or strong inversion layers can be induced in a variety of semiconductor thin films mater~als. While the devices described therein could be used in a wlde variety of magnetic field sensing applications without significant further development, the application of these magnetoresistors as position sensors in more ~tringent operating condition6 (such as those which exist in an automobile) requires interfacing the magnetoresistor with a suitable sensing system.
We have recognized that the Schroeder and Lequesne (CSN 604,131) type of mag~etic circuit i~ so effective in concentrating the mag~etic ~ield that lesser sensitive magnetoresistors may still work well enough to be useful at ~ome applications. In addition, we have recognized that some of the less sensitive magnetoresistor materials are magnetically #ensitive at higher temperatures. We have also recognized that the improved magnetoresistor concepts of CSN 604,137 and CSN 604,133 provide enhancement to lesser magnetically sensitive materials. We have thus recognized that the combination of all these concepts could provide e6pecially striking benefits. This patent application specifically describes and claims that combination.
There are several reasons why the improved ~ag-netoresistors described in CSN 604,137 and CSN 604,133 would be especially desirable for use in such a sensing systemO The reasons will not be mention~d in order of importance. ~irst, extreme compactness of these sensors make their use ideal in any sensing location, regardless of the space limitations. Secondly~ their improved sensitivity to magnetic field affords the designer a large amount of freedom in the placement of the sensor with respect to the exciter wheel. This means that the air ~ap between exciter and sensor can be larger than for a less sensitive de~ice without any diminution in magnitude of the electrical signal. This could prove to be important in applications where vibration and thermal expansion problems limit the degree of proximity o~ the sensor to the exciter wheel.
Also, the outstanding temperature stability o~ the ~ensitivity o~ the improved magnetore6i6tors will allow their application in extreme t~mperature environments, such as automotive anti-lock braking systems, in which temperatures can range from -50~C to ~200~C. Other applications may require operation at temperatures as high as +300C. We believe that the enhancement to system sensitivity afforded by the CSN 604,131 concepts and the enhancement to magnetoresistor sensitivity afforded by the C~N 604,137 and CSN 604,133 concepts, in combination, makes a wider group of semiconductor materials now available for use in magnetic field sensing. Materials that were previously considered as unacceptably now can be used, and will provide acceptable performance at much higher temperature.
This expands the range of applications where such sensing is practical, and provides other bene~its as well.

- J `~
;

1313~03 Accordingly, we think that the combination proposed in this patent application is especially attractive for automotive applications as part of linear or rotary position measurement systems. The sensitivity to magnetic field and high thermal stability of these sensors would be especia,lly beneficial.

Summary of the Invention The present invention is a novel magnetic circuit for use in a position senBor. It features a novel type of magnetoresistor that significantly improves the circuit. The combination is simple and planar in geometry, which makes it amenable for batch processing with a consequent saving in manufacturillg cost. Moreover, it makes possible attainment of sensitivities and/or sensing at higher temperatures appreciably higher than prior art structures.
In particular, the novel magnetic circui~
employs a stationary part that comprise6 a permanent magnet whose width is several times wider than that of the magnetic sensing element an,d, advantageously, at least about one and one half times the pitch of the exciter teeth. The sensing element is a magnetoresistor having an accumulation layer on its sensing area surface. Moreoverj in the preferred embodiment for ~urther improvement in the sensitivity, the surace o~
the magnet adjacent to which the teeth pass is provided with a thin layer of a magnetic material of high permeability. The magnetic sensing element advantageously is centered on this magnetic layer and is as described in CSN 604,137 or CSN 604,133.
Additionally, the width of the magnetic sensing element . .

1313~03 is desirably narrow for ~aximum sensitivity, but is wide enouyh to have a suitable resistance for good impedance matching with the electrical circuit used to det~ct the change in properties resulting from the magnetic flux being sensed. Preferably any flux guide or field concentrator is avoided by using a ~agnet of adequate ~trength.
It is characteristic of this magnetic csrcuit that the passing teeth of the exciter essentially vary only ~he ~patial distribution of the magnetic flux density along the width of the magnet for creating sharp local flux density variations that can been readily sensed by the sensing element, while the total flux density passing through the thin ferro~agnetic lS layer remains essentially constant. By way o contrast, in prior art magnetic circuits, the passing teeth of the exciter vary the circuit reluctance and consequently vary the total magnetic flux in the circuit.
The invention will be better understood from the followin~ more detailed description taken with the accompanying drawings.

Brief Description of the Drawings Figure lA is a schematic view of a magnetoresistor, showing its electrical current flow lines when no magnetic field is applied to it.
Figure lB is a sche~atic view of a magnetoresistor, showing how the electrical current flow lines are redirected in the plane of its major surface when a magnetic field is applied perpendicular to that surface.

Figure 2 is an isometric view showing a magnetoresistor having two integral sensing areas electrically in parallel.
Figure 3 is a three-dimensional or contour S plot showing the change of electrical resi~tance in a inqle elç~ent larger band gap ~emicollductor magnetoresi~tor with ehanges in temper~ture and magnetic field strength.
Figure 4 is a two-dimen6ional plot of the ~ fractional magnetoresistance over a wider temperature range than shown in Figure 3.
Figure 5 i6 a two-dimensional plot showing change in resistance with no magnetic field applied over a wider temperature range than shown in Figure 3.
Fi~ure 6 i~ an elevational view showing a semiconductor film in a pattern for providing a series connected plurality of sensing areas integrated in a ~ingle magnetoresistor.
Figure 7A is an elevational view showing a metallization pattern for superpositivn on the Figure 6 pattern~
Figure 7B is an elevational ~iew showing the Figure 7A metallization pattern superimposed on the Figure 6 semiconductor pattern to delineate the plurality of sensing areas.
Figure 8 is a three-dimensional or contour plot showing the change of electrical resistance of a multiple sensing area magnetoresistor such as show in Figure 7B.
Figures 9 and 10 are two-dimensional electron ~nergy to depth plots showing how electrons could be confined in an accumulation layer under special layers on surface of the sensing area of the magnetoresistor.

Figures llA, llB, and llC are schematic views showing a magnetoresistor having a gate electrode over each of a plurality of sensing area~ to electrically induce an accumulation layer in each sensing area. In Figures 7~ and 7C, the gate electrodes are electrically biased internally, by two diferent tischni~ues.
Figure 12 is a schematic vi~w showing a ~gnetoresistor having accumulativn layers not only in the sensing area , but also as conductors making electriGal contact to the edges of the sensing areas;
Figure 13 shows a typical magnetic circuit of a prior art position sensor o~ the type using a flux guide return path;
Figure 14 shows the magnetic circuit of a position sensor in accordance with a preferred embodiment of the present invention;
Figure 15 shows in more detail the stationary sensing portion of the magnetic circuit shown in Fi~ure 2;
Figures 16A and 16B show the magnetic circuit of Figure 2 for two different positions of its permanent magnet relative to the exciter; and Figures 17 and 18 are plots useful in discussing design considerations of the invention.

Description of the Preferred Embodiments As indicated above, a new approach to making magnetoresistors is described and claimed in CSN
604,137 and CSN 604,133. It was found that if an accumulation layer is induced in the surface of an extremely thin film of semiconductive material, the properties of the accumulation layer relevant to magnetic sensitivity can dominate over those of the remainder of the film.

Such accumulation layers can make higher band gap semiconductor materials useful in magneto~ensors.
Such materials can be used at higher operating temperatures than lower band gap se~iconduetiYe S material, such as indium anti~onide. ~owever, it may ev~n enhance the sensitiYity of indiuan antimonide enough to allcw it to be used at higher temperatures.
In this discussion the term accu~ulat:ion layer is uaed.
In this patent applica~ion, the term accumulation layer lQ is intended to also include an inversion layer, unless otherwise noted.
The accumulation layer i~ especially directed to use in magnetoresistors ~ade of higher band gap semiconductive materials. However, it is expected to be beneficial in magnetoresistors made of still other semiconductive material6.
A typical magnetoresistor element consists of a slab of semiconductor, typically rectangular in shape, through which a current is passed. Such a ma~neto resistor is described by S. ~ataoka in "Recent development of Magnetoresistive Devices and ~pplications~" Circulars of Electrotechnical La~oratory No. 182, Agency of Industrial Science and Technology, Tokyo (December 1974).
In the absence of magnetic field, the current lines go from one injecting electrode to the other in parallel lines ~see Figure lA). This flow is between electrodes along the top and bottom edges of the rectangle in Figure lA. The geometry (a rectangle in our example) is chosen so that an applied ~agnetic field, perpendi`cular to the slab, increases the current line trajectory (see Figure lA). The magnetic field perpendicular to the plane of the paper thus lengthens ~ 11 -the current flow lines~ The longer length leads to higher electrical resistance, so long as the resulting lateral voltage difference is electrically shorted, as ahown, by the top and bottom edge electrode~.
The best geometry for this effect to occur is one where the current injecting electrode6 are along the longest side of the reotangle, and the ratio of this dimension ~"width") to the shorte6t di~ension ("length"] is a~ large a~ possible. Such an optimal device geo~etry hence leads to a very low resi~tance.
K~taoka teaches that the magnetic field sensitivity of such devices is best when the devices are made out o~
semiconductors with as large a carrier mobility as possible. The resistivity of such devices is made Less temperature-dependent when the semiconductor material contains a large donor concentration, giving a large carrier density. These last two constraints i~ply that semiconductors with high electrical conductivity are best suited for practical applications.
Combined with the geometrical restriction~
described earlier, one can deduce that the final magnetoresistor element will have a low resistance.
This has a practical drawback. Under a constant ; voltage, the power dissipated by the device scales as the inverse of the resistance. To limit ohmic heating (which would limit the operational temperature range of the sensor, if not destroy the sensor itself~ while maintaining a large voltage output during sensor interrogation, it is desirable that a magnetoresistive element have a resistance around lkW~ We con~ider this to typically be equivalent to a resistance of about 300W-3kW. A number of ways have been proposed to achieve such resistances. For example, as Kataoka has 1313~03 pointed out, one can put a number of elementary devices in ~eries. Making a plurality of sensing areas as integral parts of a single element i5 shown in Figure 2. While only two ~ensing areas (i.e., devices) ~re ghown, on could make an element with tens or hundred6 of integral sensing areas i.e., devices).
If the ~etal-semiconductor (~agnetic-field independent) interfacial contact resistance of one ~uch elementary device is an appreciable fraction of the semiconductor resistance of this elementary device~ it will lower the sensitivity to a magnetic field. Thus, metals must be deposited which have a very low metal-semiconductor interfacial contact resistance to avoid this sensitivity degradation. In most cases we would prefer that the interfacial contact resistance between the sensing area and its electrodes be 10-100 times less than the resistance of the sensing area between those electrodes. Another option which alleviates the problem of low magnetoresistor de~ice resistance has been to use active layers that are as thin as possible. This has been done by thinning waers of indium antimonide ~InSb), which were sliced from bulk ingots, down to thicknes~es as small as 10 microns. The wafer thinning process is a very difficult process, since any residual damage from the thinning process will lower the electron mobility.
Reducing electron mobility will decrease the ~ensitivity to a magnetic field of devices made from this material.
Another approach has been to deposit fil~ of InSb onto an insulating substrate. On the other hand, in this latter case, the electron mobility of the resulting films is reduced to a fraction of that of ,~ ~
' 13 bulk InSb. This reduction occurs because of defects in the film. With typical mobilities of 20,000 cm2V lsec 1, these films produce devices with greatly reduced ~ensitivity to a ~agnetic fie].d compared to devices made from bulk In5b. The usuzll device 6tructure for the prior magnetoresi~tor~ made ~rom a i1m i~ schematically ~hown in Figure 2.
The great ma~ority of the prior work until now ha6 focused on InSb. This can be understood from th~ data in the following Table I.

TABLE I.
Potential Magnetoresistor Materials at 300~
15 Semiconductive Maximum Crystal Energy Material Electron L~ttice ~and MQb2li~y -1 Constant Gap (cm V sec ) (A) ~ eV) InSb 7B,000 6.478 0.17 Bil-xSbx (xC0.2) 32,000 6.429(Bi) 0-0.02 InAs 32,000 6.058 0.35 ; InO 53GaO.47 14,000 5.869 0.75 (on InP~

GaAs 8,000 5.654 1.4 GaSb 5,000 6O095 0.68 InP 4,500 5.869 1.27 Since the magnetoresistance effect is proportional to electron ~obility squared for small magnetic fields, InSb is highly preferable. However, the difficulty of growing co~pound semiconduc~ors in general, and the fact tha~ there is no suitable, lattice-matched, insula~ing sub~teate upon which it may be grown led us to try growing Bi fil~ns. Such work has 5 been previously reported by Partin e~ al. in Phy_ical Reviews B, 38, 3818-3824 tl988~ and by Hereman6 ~t al.
in Phy~ical Reviews B, 38, 10280-10284 5198~ u~ce~
wa~ obtained in growing epitaxial ~i thin fil~s~ with mobilities as high as 25,000 cm2V~1 sec~1 at 300 K ~and 27,000 cm2V lsec 1 for Bi1 XSbx ~t 300~).
Magnetoresistors made from these films had very low ~ensitivities. Modeling studies which we have just completed indicate that this is to our knowledge an unrecognized ~ffect of the fact that the energy band structure of Bi has several degenerate conduction band minima. Other high mobility materials shown in Table I
have a single, non-degenerate conduction band minimum.
InSb thin films (on semi-insulating GaAs substrates) were then grown using the metal organic chemical vapor deposition (M~CVD) growth techniques. After many m~nths of effort, films with electron mobilities of only 5,000 em2V~1sec 1 were produced.
Growth of Indium Arsenide (InAs) on semi-insulating GaAs, and also on semi-insulating InP
substrates, was tried. By semi-insulating we mean such high resistivity that they can be considered as substantially insulating. These latter substrate~ were made ~emi-insulating by doping the~ with Fe. They were tried in addition to Ga~s because there is less lattice mis~atch with InAs (see Table I ) . After some time, we were able to produce InAs films with a room temperature ~obility of 13,000 cm2V lsec 1 on InP substrates, and of lower mobility on GaAs substrates. The better InAs films were formed by the following process.
An MOCVD reactor manufactured by Emcore Corporation was used. InP substrates were heated to the growth temperature in an atmosphere of 40 torr of - high purity (palladium diffused3 hydrogen to which a ~oderate quantity of arsine was addedl (B0 9CCM, or ~tandard cubic centi~eters per minute ~ . This produced about 0.02 mole fraction of arsine. The arsine was used to retard thermal decomposition of the InP surface caused by loss of the more volatile phosphorus. The way in which arsine reduces the surface roughening during this process is not well understood. Phosphine would have been preferred, but was not available at the time in our reactor. After reaching a temperature o 600C., the arsine flow was reduced to 7 SCCM, and ethyl-dimethyl indium (EDMIn) was introduced to the growth chamber by bubbling high purity hydrogen (100 SCCM) through ~DMIn which was held at 40C. Higher or lower arsine flows during growth gave lower mobilities and worse surface morphologies. After 2.5 hours of InAs growth time, the EDMIn flow to the growth chamber was stopped and the samples were cooled to room temperature in an arsine-rich atmosphere ~as during heat up).
The thickness of the resulting InAs film was 2.3 mm. From conventional Hall effect measurements at 300 ~, the electron density was 1.4x1016 cm 3 and the electron mobility was 13,000 cm2V~1sec~1. These are effectively averages since the electron density and mobility may vary within a ilm. ~he fil~ was not intentionally doped. Even thou~h this is a very disappointing mobility, ~ crude magnetoresistor was -- 1313~03 made, since this required very little effort. A
rectangular ~ample was cleaved from the growth and In ~etal was hand soldered along two opposing sdges of the ~ample, and leads were connected to the In. The length, which is the vertical dimension in Figures lA and 1~, was 2 mm and the width, which was the horizontal dimension in Figures lA and lB, wafi 5 mm.
As expected, the resistance of the device was low (abou~ 50 W) since we did not have many ele~ent~ in serie60 However, the magnetoresistance effect was large. It is shown in Figure 3. Furthermore, the device resistance and magnetoresistanee were surprisingly stable with te~perature in the range shown in Figure 3, which is -50C. to ~lOO~C. A second, similar device was tested less thoroughly at temperatures a~ high as ~230C. The results of thi~
latter testing are shown in Figures 4 and 5. In Figure 4, the applied magnetic field was 0.4 Tesla. The fractional ~agnetoresistance is plotted as a function of temperature between E = O . 4 Tesla and ~ ~ 0. ~espite the fact that the indium metal used for contacts has a melting point of 156C., the magnetoresistor still functioned very surprisingly well at 230C., with the fractional increase in resistance for a given magnetic field (0.4 Tesla) reduced by less than one half compared to the response near room temperature (a~
~hown in Figure 4).
The device resistance in zero magnetic field, R(0), decreased over the same temperature range by a factor of 5 (as shown in Figure 5). Ne also fQund this to be surprisingly good, even ta~ing into account the relatively large energy gap of InAs.

~: 17 1 31 3~03 Our own detailed analysis of transport data from these films suggests that there are current carriers with two differen~ mobilities present. In retrospect, it looks like our result~ are related to an accumulation layer of electrons at the surface of the ~- ~en6ing layer. We have now recognized that Wieder has reported in Appl. Phys. ~etters, 25, 20Ç ~1974) that 6uch an aceu~ulation layer exi~ts just inside the InA~
near the air/In~s interface. $here appear o us to be some errors in the Weider report. However, we think that the basic conclusion that an electron accumulation layer exists is correct. These electrons are spatially separated from the positive charge at the air/InA6 interface. Thus, they are scattered relatively little by this charge, resulting in a higher mobility than would normally be the case. They al50 exist in a very high density in such an accumulation layer, so that as the temperature increases, the density of thermally generated carriers is a relatively small fraction of the density in the accumulation layer. This helps stabilize the resistance (at zero magnetic field) with te~perature. Thus, it appears that the relatively low measured electron mobility of 13,000 cm2V lsec~1 is an average for electrons in the accumulation layer and for those in the remainder of the thickness of the film.
Thus, normally one would want to grow a relatively thick layer of InAs to make a good ~agnetoresistor, since crystal quality ~and mobility) generally i~prove with thickness when growing on a lattice-mismatched substrate. However, the thicker the layer becomes, the greater its conductivity becomes and the less apparent the benefits or presence of a surface accumulation layer would be. Thus, our current lB

understanding of our devices suggests that relatively thinner layers are preferable, even if the average film mobility decreases somewhat, since this will make the conductivity of the surface accumulation layer a greater fraction of the total film conductivity. The exact relationships between film thickness, crystal quality and properties of the surface accumulation layer are currently under study. We currently prefer to use a nominally undoped layer of a thickness of approximately 1-3 micrometers.
Multi-element magnetoresistors were ~ubseguently made from this material using Au (or Sn) metallization.
First, conventional photolithography techniques were used to etch away unwanted areas of an Indium Arsenide (InAs) film from the surface of the Indium Phosphide (InP) substrate to delineate the pattern shown in Figure 6. A
dilute solution (0.5%) of bromine in methanol was used to etch the InAs. Then, a blanket layer of ~u metallization 1000 Angstroms thick was deposited using conventional vacuum evaporation techniques over the entire surface o the sample, after removing the photoresist. Conventional photolithography was then used to etch away unwanted areas of the Au film to delineate the gold pattern shown in Figure 7A. A dilute aqueous solution of KCN was used for this step. (~e think dissolved oxygen is helpful, which can diffuse into the solution from ambient air or be supplied in the form of a very small addition of hydrogen peroxide.) The resultant composite of the two patterns, with the gold pattern overlying the InAs film pattern, is shown in Figure 7~.
Leads were then attached by silver epoxy to the large AU end bonding pads. Leads could also be attached by normal and accepted filamentary wire bonding techniques. If so, and especially if a modern ., .:,, 1313~03 wire bonding apparatus were used, the bonding pads could easily be made much smaller. ~lso, many devices ~uch as ~hown in Figures 6, 7 and 7A could be made ~imultaneousl~ using conventional integrated circuit teohnology. The resulting devices typically have a resi6tance near 1 KW (typically ~ or - 20~) at room temperature in zero magnetic ~ield. Surprisingly, the ; magnetoresi~tance effect on the multisensing area device was much larger than the effect on a single sensing area device. For comparison, of these effects at a given magnetic field, see Figures 8 and 3. In the multi-element device (i.e., plural sensing area element), the sensing areas had a length to width ratio o~ 2/5. We do not understand why the multi-element device works b~tter since the length to width ratio of each element is 2/5, the same as for the single element device characterized in Figure 3, which was fabrlcated using part of the same InAs grown layer. Another multi-element magnetoresistor was made similarly to the one just described, but with a length to width ratio of 4/5. It had nearly as large a magnetoresistance as the one made according to the patterns in Figs. 4 and 5.
Again, we do not yet understand this, but the resulting devices work very well. Even a device with a length to width ratio of 6/5 works well.
The relative stability of these magnetoresistors with temperature also now appears to be increasingly important, since some automotive applications require operation from _50C. to as high as +170C. to ~200C., and there are known applications reyuiring even higher temperatures (to 300C.). There is reason to believe that our invention will provide . . ~: ~, ...

~ 3 1 3L~ 03 ~agnetoresistors operating at temperature as high as 300~C., and even higher.
A potential problem with InAs magnetoresistors made in accordance with thi~ invention ls the potential importance of the air/InAs interace, which ~ight cause the device characteristics to be sen~itive ko changes in ~he composition of ambient air, or cause the characteristics to slowly change with time or thermal history because of continued o~idation of the surface. Coating the surface~ of two d~vices with a particular epoxy made by Emerson and Cuming, a division of Grace Co. has been tried. The epoxy used was "Stycast," number 1267. Parts A and B were ~ixed, applied to the devices, and cured at 70C. ~or two hours. We did not observe any significant change~ in the device characteristics at room temperature as a result of this encapsulation process. We have not yet sy~tematically tested these devices at other temperatures, but we are encouraged by this preliminary result. We think other forms of encapsulants need to be explored, such as other epoxies and thin film dielectrics, such as SiO2 or Si3N4. Since exactly what occurs at the air/InAs interface which causes the accumulation layer is not yet known, intended for exploration is the depositing of a thin film of dielectric or high energy gap semiconductor (such as GaAs, In1 xGa~As, In1 xAlxAs, or AlSb) right after growth of the InAs is co~plete, and before expo~ure to air. We hope that this will still result in an accu~ulation layer at the interface between InAs and the dielectric or high energy gap semiconductor.
In order to still have a very low metal-semiconductor contact resistance between the InAs and the contact and shorting bar metallization, it ~ay be necessary to modify the processing sequence previously described in connection with Figures 6, 7A
and 7~. For example, with an inverse of ~he ma~k conte~plated in the previous discussilDn, the photoresist on the surface could then be used as a ~ask for wet etching (e.g~, by wet ch~mical~ or reactive ions, or ion beams) of the dielectric or high energy gap semiconductor layer to expose the InAs. Au or 1~ other ~etals could then be deposited by vacuu~
evaporation ~or by other conventional processes, such as sputtering, electroplating, etc.) and then the photoresist could be removed, resulting in lift-off of the undesired regions of metal. Alternatively, after etching through to the InAs, the photore~ist could be removed, Au or other metal could be deposited uniformly across the surface, and then after deposition of photoresist the mask pattern in Figure 7A could be aligned with the pattern etched into the dielectric and ~ the Au could be patterned as before.
; As an additional alternative, if a sufficiently thin layer (e.g., 200 Angstroms) of high ; energy gap semiconductor is present, the original processing sequence described could be modified by deposition of a low ~elting temperature eutectic alloy, such as Au-Ge, Au-Ge-Ni, Ag-Sn, etc., in place of Au.
After patterning similarly to the way Au was (or using the inverse of the mask in Figure 7 and lift-off), the sample is heated to a moderate temperature, typically to somewhere in the range of 360C. to 500C. for Au-Ge based alloys, thus allowing the liquid ~tal to locally dissolve the thin layer of high energy gap semiconductor, effectively contacting the InAs.

1313~03 In most recent work, the InAs growth procedures are changed somewhat. The procedures are the same as before, but the InP wafer iB heated to 460C in a larger arsine mole fraction (0.1). After 0.5 minute at 460~C, during which the native oxide on InP is believed to desorb, the ~emperature is lowered to 400C and 200 Angstroms of In~s thickness is grown.
The temperature is then raised to ths growth temperature of 625C (with the arsine mole fraction still 0.1), and then EDMIn is introduced while the arsine flow is abruptly reduced to 5 SCCM ~about 0.001 mole ~raction). The EDMIn is kept at 50C, and the high purity hydrogen is bubbling through it at a rate of 75 SCCM. Again, the arslne ~low of 5 SCCM seems near-optimal for these growth conditions. The re~ulting films have somewhat enhanced sensitivity to a magnetic field relative to those grown earlier.
While all of our recent work has concentrated on magnetoresistors fabricated from InAs films on semi-insulating (i.e., substantially electrically insulating) InP substrates, we think that a more mature growth capability will permit films of InAs with nearly comparable quality to be grown on semi-insulating GaAs substrates as well. In either case, other growth techniques such as molecular beam epitaxy liquid phase epitaxy or chloride-transport vapor phase epitaxy may also prove useful.
We are describing and claiming the above-mentioned Indium Arsenide (InAs) thin film devices,fabrication processes, and operating characteristics in a separate Canadian patent application Serial No. 604,137 entitled, "Indium ~rsenide Magneto-resistor," that is being si~ultaneously filed with this patent application in the names of J. P.
Hereman~ and D. L. Partin.
On the other hand, we think that the presence cf what may be a naturally occurring accumulation layer in the above-mentioned thin film In~s ~agnetore6istors i~ what ~akes th~m work so well, and which enabled production of a practical device. We believe that this fundamental concept is new to ~agnetore~istors, and that this thought can be expanded in a aultiplicity of ways, not only to Indium Arsenide but to other semiconductive materials as well. In this patent application we further describe and claim a variety of techniques by which an accumulatlon layer can be induced in the semiconductor layer, by other than a naturally occurrence or inherent occurrence as a result of the fabrication process.
The following discussion describes some of the other ways of inducing or enhancing an electron accumulation or inversion layer in InAs thin films and in other semioonductive materials in thin film form, to attain effective high mobilities. There are three basic advantages to the use of strong electron accumulation layers in magnetoresistor active regions.
It is repeated here that the term electron accumulation layer, as used in this patent application is also intended to include electron inversion layers.
First, electron accumulation layers or strong ele~tron inversion layers can contain a density of electrons significantly larger than the intrinsic den~ity at any given temperature. This must improve - the temperature stability, since the thermally excited carriers are a small fraction of the accumulated or strongly inverted ones.

Sesond, accumulation layers enhance the mobility of the carriers in the semiconductor. This effect has been experimentally observed in thin indium arsenide tInAs) fil~s, ~speci~lly at higher temperatures. They will enhance the ~;ensitlvity of ths ~agnetoresistor. One po6sible cause of thi6 ~ffect may be that in such accumulated or stzongly inverted layers large electron densities can be achieved without the presence of a large density of ionized impurities in the same spatial region, which would limit th~ carrier mobility. This efect is similar to the "modulation doping" of layers described by ~. Burns in Solid State Physics, pp. 726 747, ~cademic Press (1985). Such an effect is us~d in the fabrication of ~5 ~igh--Electron-Mobility-Transistors (HEMTs).
Third, accumulation or strong inversion layers are inherently close to the surface or interface of a semi~onductor. This makes it relatively easy to induce, enhance, or control these accumulation or strong inversion layers through the use of thin film structures deposited on top of the semiconductor, po~sibly in combination with voltage bia~es.
Accumulation layers have been used in silicon MOSFET Hall plates, and is described by H. P. Baltes et al. in Proc. IEEE, 74, pp. 1107-1132, especially pp.
1116-7, (1986). In the MOSFET Hall effect devices, a biased gate electrode in a Metal-Oxide-Semiconductor was used to generate a suitably thin electron layer close to the Se~iconductor-Oxide interface. Four electrodes were then used to eontact that layer: a ~ource and a drain through which current is passed, and two intermediate electrodes across which the Hall voltage is generated. Further, saltes et al. ibid.

1 3 1 3~03 also describe a split-drain MQSFET using an accumulation-layer based sensor with only four electrodes (one ~ource, two drains, and one gate)O One of the virtues of a magnetoresistor over a ~all efect device i~ that the magnetoresistor has ~nly two @lectrod~s. In order to preserve this in our improved magnetoresi~tor concept, we propose to use, in conjunction with a magnetoresistor layout such as described in Figure 2, a number of new ways to generate accumulation or inversion layers without using externally biased gate electrodes.
In a first embodiment, we make use of the fact that the natural interface between InAs and air is known to g2nerate an electron accumulation layer in InAs. A similar effect may exist ln InSb, and the technique may therefore be applicable to thin film magnetoresistors made with this semiconductor material~
We would, however, not expect such devices to work as well as InAs at very high temperatures. The very small energy gap of InSb (see Table I ) would cause thermal generation of carriers that would cause increased conductivity in the InSb film adjacent to the accumulation layer, making the conductivity of the accumulation layer a relatively small fraction of the total device conductivity. Thus, the benefits of the accumulation layer would be lost at a lower temperature in InSb than in the higher energy band gap InAs.~ We experimentally grew a 2.3 mm thick epitaxial layer of InAs on an insulating InP substrate using Metal Organic Chemical Vapor Deposition ~MOCVD). Hall and magnetoresistance measurements on the layer in the temperature range of 350K to 0.5K, and in magnetic fields up to 7 Tesla re~eal the presence of at least two "types" of carriers, in roughly equal concentrations, but with very di~ferent mobilities ~by a factor of 2 to 3). In retrospective view of the afore-mentioned Weider publication, it is reasonahle to 5 assume that one of them is the accumulation layer located near the air interface. We bui~t ~wo 2 ~
long, 5 mm wide magnetoresistor~ out of thi6 fil~ which develop a very u~able magnetic field sensitiv~ty, while ~aintaining good temperature ~tability (~ee Figure~ 3, 1~ 4, and 5~. We believe it is possible to preserve this sensitivity after covering the InAs surface with a suitable encapsulating coating (e.g., an epoxy or other dielectric).
In a second embodiment, a capping layer of large-gap semiconductor such as GaAs, InP, AlSb, or Inl yAlyAS can be grown on top of the narrow-gap active layer semiconductor (typically InAs or Inl xGaxAs with O<x<0.5, although a similar structure using InSk can be conceived). In this capping layer, we put donor-type impurities, such as Si, Te, Se, or S. These will release an electron, which will end up in the layer where it has minimum energy, i.e., the narrow-gap semiconductor. This leaves a layer of positively ionized donor-impurities in the large-gap capping layer; but they are spatially removed from the electrons in the active layer, and hence do not significantly scatter them.
In a third embodiment, we propose to deposit a layer of ~etal on top of the device active region 3~ with the purpose of creating a Schottky barrier. A
plot of the electron energy levels adjacent the metal-semiconductor interface in this third embodiment i5 shown in Figure 5. In referring to Figure 9, it 1313~03 can be seen that there will be a depletion of the top region of the active narrow-gap semiconductor~ If the acti~e layer is thin enough (1000-2000 Angstro~s), thi~
will confine electrons in the active layer toward6 the ~ubstrate, resultinq in electrical properties ~i~ilar to tho~e of an accumulation layer. ~Letals that generally form Schott~y barriers to III-V compound~, such as Au or Al ~ay be useful, although we have not adequately studied this ~tructure experimentally yet.
In a fourth embodiment, we propose to deposit on the active layer of a narrow-gap semiconductor a layer of large gap semiconductor, or of a dielectric such as SiO2 or Si3N4, and on top o that a gate electrode. An electron energy plot going through the layers at the relevant interfaces i6 shown in Figure 10. The metal of the gate electrode in Figure 10 can be chosen such as that it induces an accumulation region near the semiconductor-dielectric interface, by effect of the difference between the electron af~inity ; 20 in the semiconductor, and the work function in the metal. Conversely, a different metal with larger work function can be used to deplete the semiconductor-dielectric interface and electrostatically confine the electrons near the substrate, much as in the third embodiment mentioned above.
In a fifth embodiment, it is suggested that the gate electrodes described in the fourth embodiment be biased so as to generate accumulation layers in the semiconductor under them. Such a concept is schematically shown in Figures llA, llB, and llC. In Figure 11A, it can be seen that if desired, one could use one or more added contacts to separately bias the gate electrodes. This would not ordinarily be preferred but could be done. It would not be preferred because one of the advantages to a magnetoresistor resides in that it only has two contacts. We are only showing it here for complsteness. On the other hand, additional contacts are not actually necessary. The gate electrodes can be electrically biased by an ~nternal resistor circuit, examples of which are shown in Figures llB and llC.
1~ Reference is now specifically made to the fifth type of embodiments shown in Figures 11~ and llC.
Since the gate leakage currents are very minimal, a very high resistance (>lMW) circuit can be used for biasing. Afi a special case in Figure llB, reslstor R1 can be made extremely large (open circult), and the other resistors can all be made to have zero resistance (short circuit). Thus, the full positive bias applied to one external electrode ~relative to the other external electrode) is applied to all gates in this special case. An alternative is to connect the gates over each semiconductor region with the shorting bar between two other semiconductor regions located such that the potential difference between the gate (i.e., the shorting bar) and the active region induces an accumulation layer in the latter. This latter version of internal biasing of the gate electrodes is shown Figure llC. A special case of this configuration is one in which each gate is connected to ~he adjace~t ~horting bar. In this configuration, each element might be considered to be a MISFET transistor with gate and drain shorted.
In the five preceding embodiments, the accumulation layers were used only to enhance the desirable transport properties of the se~iconductor in the sensing area. The geometry of the magnetoresistor, i.e., the length over width ratlo of eaGh active element, was still defined by the use of ~etallic shorting bars. The structure o Figure llB can be extended to defi~e the geome~ry of the magne~ore~i~tive element~ the~selves, by modulating the carrier density and hence the conductivity, inside the semiconductor active layer. This forms a sixth embodiment of thi~
inYention. An example of such a structure is schematically shown in Figure 12. ~gain, an external (integrated into the chip) resistance network is u~ed in this sixth embodlment to bias a ~ucce~sion of gate electrode6 to create a series of ~trongly accumulated regions. These can be used instead o~ metallic shorting bars to create geometrical magnetoresistance.
Such a structure could potentially be superior to one in which metallic shorting bars are used, because ield-insensitive contact resistances between the metal and the semiconductor would be eli~inated.
Again, a special case can be considered for this sixth embodiment, as was considered in the fifth embodiment. In this special case of the sixth embodiment, the resistor R1 of Figure 12 i6 open-circuited and the other resistors (R2, R3 ...) are short circuited, so that the entire positive bias applied to one external electrode is also applied to each gate. Thus, the natural accumulation layer normally present on an InAs surface would exist between the gates as in Figure llA, but have a lower electron density. If desired, the gates could be biased negatively to eliminate the electron accumulation layers between the gates, or even to generate a strong inversion layer with carriers of the opposite type (holes). While the emphasis of this record of invention is on devices with only two external leads, the gates could be connected through a resi~tor network to a third external lead, making thi~ version of the magnetic field sensor externally controllable throu~h a voltage bias externally supplied to the gate lead. As hereinbefore indicated, a similar three terminal d~vice could be made with the device shown in Figure llA.
In a seventh embodiment, a lightly p-type film is grown (typically doped with Zn, Cd, Mg, se, or C). In the case of InAs, the surface would, we believe, still have a strongly degenerate electron layer, but it would be an inversion layer. Such an inversion laycr would have a large electron density near the surface, and then a relatively thick ~typically 0.1 mm to 1 mm or more, depending on dopant density) region of very low carrier density, similar to the ~pace charge region of an n+/p ~unction. This 2~ might be advantageously be used to reduce the conductivity of the film ad~acent to the electron strong inversion layer. At very high device operating temperatures, the intrinsic carrier density of narrow energy gap semiconductors like InAs would tend to defeat this strategy somewhat, and other, higher energy gap semiconductor6 such as In1_xGaxAs might be preferred (see Table I). InO 53GaO.47 case, since it can be lattice-matched to semi-in~ulating InP substrates. This makes it easier ; 30 to grow such films with high crystalline quality.
The acceptor dopants mentioned above (i,.e., Zn, Cd, Mg, Be, and C) have small activation energies in the III-V compounds of interest (see Table 1).

~ !

32 ~
However, ~here are other acceptor dopants with r~latively large activation energies, such ~e, in InO 53Gao 47AS. ThiS means that relatively large thermal energy is required to make the iron ionize and S contribute a hole to conduction. Hob~ever~ the iron will co~pen~ate a concentration of donor impuritie6 frequently present in the material so that they do not contribute electrons to the conduction bandO Thus, doping this material with iron, will make it tend to have a high resistivity, except in the electron rich accumulation layer. It would in this case be desirable to grow a thin undoped InO 53Gao 47As layer ~e.g., 0.1 micrometer thick, after correcting for iron diffusion efects) on top of the iron doped layer in order to obtain the highest possible electron mobility and density in the accumulation layer. It is recognized however, that findin~ suitable dopants with large activation energies may not be practical for ~maller band gap semiconductive materials.
Furthermore, the other embodiments discussed above could also be used in conjunction with this one advantageously to reduce the conductivity oE the film adjacent to the high electron density region.
The emphasis of the above discussion has been on electeon accumulation or inversion layers. Hole accumulation or inversion layers could also be used.
However, èlectrons are usually preferred as current carriers in magnetoresistors since they have higher mobilities in the materials shown in Table }.
With reference now to the drawings, Figure 13 shows a typical prior art form of position ~ensor 10 in which the magnetic circuit comprises an exciter portion 12 of ferromagnetic material made up of a succession of 1 31 3~03 teeth 12A spaced by gaps 12B and a stationary sensing portion comprising the permanent magnet 14 supporting on one surface the sensing element 16 and a flux guide 18 for providing a return path for the~ magnetic field.
As shown, the width of each tooth i~ ~bout equal to the width of the ~ag~et and of the sensing element.
Optionally, a field concentrator (not hown) ~ay be localized over the sensing element 16 in the form of a thin layer of a high per~eability erromasnetic material.
The exciter 12 typically is a plate with spaced teeth along one edge and is adapted to ~ove horizontally so that its teeth pass under the permanent magnet 14 and the sensing element 16 in accordance with the movement of a position that i6 being sensed.
Alternatively, the exciter may be a circular plate, with teeth around its circumference interspersed with 610ts, that rotates about a fixed center for varying the position of the teeth relative to the sensing ~0 element. The exciter is typically of a high permeability ferromagnetic ~aterial, such as iron.
The permanent magnet is polarized vertically in the plane of the paper, as indicated. The ~ensing element typically is a magnetoresistor, a two terminal element whose resistance increases with increasing magnetic flux passing vertically through its bulk and typically had nearly the same width as the magnet. The sensing element 16 is as hereinbefore de~cribed.
The flux guide 18 also is advantageously of a high permeability material, such as iron, and its presence can increase the flux density thrnugh the sensor by providing an efficient return path for the flux passing through the exciter. To this end, the center-to-center spacing of adjacent teeth o~ the exciter and the center-to center spacing of the magnetic path formed by the permanent magnet and the flux guide are made essentially equal, as shown. Such a flux guide, however, in fact adds little to the sensitivity and so is unnecessary if adequate flux density is provided, either by a magnet of sufficient thickness vr choice of magnet ~aterial.
Typical di~ensions might be about one millimeter both for the vertical thickness and for the horizontal width of the magnet 12, similarly about one ~illimeter for the height and width of each tooth 12A, about two millimeters for the width of a gap 12B, and about one millimeter for the separation between a tooth and the magnet in the position shown. The flux guide 18 typically would be of the same scale and would add about another millimeter to the height of the magnet path. The lateral dimension of the magnet normal to the plane of the drawing typically is wide enough to keep low any edge effects in the sensing element.
With a magnetic circuit of this kind, the maximum sensitivity that is obtained tends to be less than about five percent. Moreover, sensors are known in which the stationary part of the magnetic circuit includes a pair of magnetic sensing elements for use as ~eparate legs of a differential sensor. In such cases, the two sensing elements typically are so spaced that when one of the sensing elements is positioned directly opposite one tooth, the other 6ensing element is positioned directly opposite the center of the gap between adjacent teeth to maximize the difference of the outputs fro~ the time sensing element. Such sen~ors provide higher sensitivities but at the expense of greater complexity.
In Figure 14, there is ~hown a po5ition sensor 20 in accordance with a preferred e~bodiment of the present invention. Its ~agnetic circuit includes the exciter 12 that may be si~ilar to the exciter included in the position sensor 10 shown in Figure 13 and so the same reference number is used. ~he ~tationary portion of the magnetic oircuit is ~hown in greater detail in Figure 15. It includes a permanent magnet 22, magnetized vertically as shown, and on its bottom surface there is provided the sensing element 16 that may be similar to sensing element 16 in the po~ition sensor 10 of Figure 13. In accordance with a feature of the invention, intermediate between the ~ensing element 16 and the permanent magnet 22 there is included a layer 24 of high permeability magnetic material, such as iron, that covers the entire bottom surface of the permanent magnet 22. Additionally, to ensure that this layer does not electrically short the sensing element 16, there is included an insulating layer 26 intermediate between the sensing element 16 and the layer 24. If the layer 24 were of a non-conducting material, such as high permeability ferrite, the insulating layer 26 would become unnecessary and so might be omitted.
In sensor 20, in accordance with a ~eature of the invention, for increased sensitivity the width W of the permanent magnet 22 is considerably wider than the 3a typical width of the prior art sensor 10 shown in Figure 13. Advantageously, the width of the permanent magnet is made to be the sum of the width of one tooth and two gaps of the exciter, as shown, as seen in Figure 14, and so about one and one half times the pitch of the teeth of the exciter. By way of contra~t, in the sensor shown in Figure 13, the width of the permanent magnet 14 essentially matche~ that of a tooth 12~ of the exciter. Moreover, the improvement in ~ensitivity provided by thi~ increase in magnet width i8 ~urther augmented by the presence of the magnetic layer lB.
For ~aximum magnetic ~ensitivity, in our design it is another feature that the width of the ~ensing element is desirably as narrow as is convenient. However, for electrical cireuit efficiency, it is desirable that the element have a 6ufficiently high resistance, for example, at least 100 lS ohms, which imposes practical limits on how narrow the element may be. Also the sensing element needs to be wide enough to have adequate power dissipation capabilities. Nevertheless, the sensing element typically would be significantly narrower than the tooth element unless the exciter design involved unusually narrow teeth. As shown, the sensing element 16 i~ provided at opposite ends with electrodes 16A and 16B by means of which it may be connected into an appropriate electrical circuit. These are typically metallic platings deposited on the insulating layer 26.
The ferromagnetic layer illustratively can be about 0.1 millimeters thick and of a material such as low carbon steel 1008. rrhe result is a geometry made up of a series of planar layers that is easy to manufacture.
The sensing element 16 typically is cho~en in aceordance with the particular application intended. A
magnetoresistor is preferred, for reason apparent ~rom the foregoing. We prefer that the magne~ic field be 1 31 3~03 applied perpendicularly to the major Eace of the sensing area in the sensing element.
Figures 16A and 16B illustrate the conditions for ~axi~um and mini~um ~lu~ through the s~nsing ~lement 16 respectively for the po~ition sensor 20 6hown in Figure 14. As seen in Figure 16~, when the ~ensing element 16 is directly opposite a tooth 12A of the exciter, the flux density represented by lines 30 through sensing element 16 is comparatively high.
However, when the exciter has moved so that the sensing element 16 is opposite the center of a gap 12B between teeth, the flux density through the sensing element 16 i5 comparatively less. Typically, the maximum 1ux density may be 0.2 ~esla and the mlnimum flux 0.15 Tesla for a 2 millimeter thick MQ2 magnet. MQ2 magnet material is an NdFeB alloy that has an energy product between 13 -15 MGOe, is isotropic and 100 percent dense and i5 a trademarked product of General Motors Corporation.
The role of the ferromagnetic layer 24 makes it easier for the flux to travel towards or away from the sensing element 16, thus increasing the ~aximum flux and decreasing the minimum flux that passes through the sensing element, and thereby increasing the sensitivity, which is dependent on the difference between the maximum and minimum fluxes sensed.
In particular, the movement of the exciter teeth little affects the total flux density but does vary the spatial distribution of the flux density along the width of the magnet, crea~ing sharp local flux den ity variations ~hat can be sensed by a localized sensing element, such as a ~agnetoresistor. The ferromagnetic layer permits the flux density to be distributed along the magnet width in a way that reflects the profile of the air gap between the stationary portion of the magnetic circuit and the exciter. Where this air gap is narrow, the ~lux densi~y is high, where this gap is wide, the flux den~ity is low. Since this air "gap" i~ narrowest alongside a tooth of the exciter, the ~lux density there will be highest and this den~ity peak will follow the tooth ~ovement along the width of the magnet. In particular, our tests have shown that the addition of the thin ferromagnetic layer 24 in the manner described can essentially double the sensitivity of a sensor with an already optimum width magnet. The optimum thickness of the ferromagnetic layer is determined by the maximum flux density it is desired to guide without saturation.
Layers even as thin as five microns have proven to be useful for a sensed ~aximum flux density of about O.12T. For this flux density improvement tends to level off when the thickness reaches about 25 microns.
The magnetic layer 24 can be provided simply as a thin metallic foil attached to the sur~ace of the permanent magnet 22 using conventional adhesives.
Alternatively, magnets manufactured by compressing and/or sinterinq magnetic powder, such as MQ2 2S previously described, can produce a ferromagnetic layer as an integral part of the permanent magnet. To this end there is introduced into the die cavity an appropriate amount of iron powder, before or after the magnetic powder is introduced, and then the powders are compressed together. Moreover, the planar geometry ~akes feasible batch-processing whereby hundred~ of magnetoresistors may be deposited simultaneously on a relatively thin unmagnetized permanent magnet wafer having a ferromagnetic layer and an insulating layer.
The wafer would then be cut into separate sensors, the sensors packaged, and the permanent magnets magnetizedO
It appears that the increase in sen6itivity is ~chieved at the expense of a lowering of the ~ean flux density. If this is of eoncern Eor effective modulation of the par~icular magnetoresistor being used, the mean flux density can be increased to the desired level with little effeet on the sensitivity by increasing the thickness o the magne~ and/or the magnet type, thereby maintaining the desired planarity of the sensor and avoiding the need for a flux guicle to improve flux density. ~owever, in special instances where neither of these expedients is adequate, a flux guide may be induced to improve the flux density involving teeth further along the exciter.
In order to translate optimally the high magnetic sensitivity of the magnetic circuit described into high electrical sensitivity, the sensing element needs to be appropriately positioned on the ~agnet.
Figure 17 shows a typical envelope of maximum attainable sensitivity plotted against the normalized distance d/W of the sensing element where d is the distance from the midpoint of the magnet of width W.
It can be seen that the peak attainable sensitivity is at the midpoint of the magnet (d=0) and at a minimum at each end of the magnet (d/W~0.5). Accordinglyt the optimu~ location of the sensing element is at the midpoint of the magnet.
It is also important to have a proper width for the sensing element, particularly when the element i~ a magnetoresistor that produces an electrical output signal eorresponding to the average of the flux density across its surface.
The flux density distribution along the length of the magnetoresistor, however, ~an be assumed to be constant. Thus, one is required to consider the flux density or ~ensitivity distribution~ only along the magnetore~istor width. Because-of that, the e~fective electrical sensitivity will be directly related to the average magnetic sensitivity as lQ det~rmined by integrating the magnetic 6ensitivity distribution given in Figure 18 over the magnetoresistor width WMR. Figure 18 shows how the sensitivity varies along the magnet width for the alignment shown in Figures 16A and 16B. Looking at the sensitivity distribution, one would tend to maximi~e the electrical sensitivity by attempting to make WMR as small as possible. Small size, however, would lower the resistance and power dissipation capability of the ~agnetoresistor, and in turn lead to a lower output signal. The selection of WMR has to be a compromise which takes into account several conflicting requirements such as the practical limitations on the magnetoresistor length, the best possible sensitivity, sufficiently large resistance and power dissipation, the lowest possible magnetoresistor cost (smaller magnetoresistors are generally less expensive), etc.
Considering previously available magnetoresistor technology, the minimum practical value of WMR for the exciter design that has been discussed appear~ to be about O.3mm which amounts to d/W - O.033 and yields an effective magnetic sensitivity SM of about 28 percent.
We do not know at this time how this is affected by the i~proved magnetoresistor contemplated in this invention. A 0. 6mm width would stilL provide sensitivity of about 26 perrent. The width WMR in any case desirably should be less than the width of the teeth in ths usual design. The height of the ~ensing ele~ent may be small, typically tens of ~icrons, whereby the planarity of the associated fiurface is little disturbed by its presence.
It is also found in our design that the ratio of tooth width T to tooth pitch P also af$ect sensitivity. It has been found that the sensitivity tends to be maximum for T/P ratios of about 0.25 but to remain relatively flat over the range between 0.17 and 0.37.
It is also found in our design that the tooth pitch affects sen~itivity and in particular that increasing the tooth pitch can appreciably increase the ~ensitivity. For example, for the design discussed, a change in pitch from 3mm to 5mm can increase the maximum sensitivity to a~out 5~ percent when conditions are optimized. Since sensitivity decreases with increasing air gap size between the exciter and the magnet, increasing the tooth pitch offers a way to compensate for larger air gap sizes and offers a designer an ability to trade off between air gap width ~5 and tooth pitch.
In addition, it is found that the ~tationary portion of a sensor of the kind des~ri~ed can be used effectively with a broad ran~e of exciter wheel tooth pitch sizes. This feature offers a considerable cost 6aving potential, for example, for applications such a~
ABS designs that employ widely differing tooth pitch sizes. If a sensor of a particular stationary design is intended to operate with wheels having different tooth pitch sizes, the maynet width preferably should be cho~en to optimize the sensor for the smalle~t tooth pitch 6ize so that the lowest sensitivity, encountered - when using the exciter wheel of ~mallest tooth pitch ~ize, will be as high as possible~ As previou~ly di6cu6sed, the optimum magnet width is about 1.5 times the tooth pitch size.
It is to be understood that the specific e~bodi~0nts de~sibed are merely illustrative o the general principles of the invention and various modifications may be devi6ed without departing from the spirit and scope of the invention. For example, it i6 feasible to reverse the role~ of the stationAry portion and the movable portion of the position sensor.
Additionally, the various dimensions and materials mentioned are merely illustrative o~ a typical de~i~n and other designs could necessitate other dimensions and materials.

Claims (22)

1. A position sensor providing unamplified electrical output changes of the order of on volt in response to changes in applied magnetic field over a temperature range of several hundred degrees Centigrade;
said position sensor including a magnetic circuit that comprises;
an exciter portion including a series of teeth spaced apart by gaps for defining a tooth pitch, and a sensing portion for relative movement therebetween;
said sensing portion including a permanent magnet having a width at least several times wider than the width of an exciter tooth and supporting a magnetoresistive sensing element;
said magnetoresistive sensing element including a thin film of monocrystalline indium arsenide having inner and outer surfaces, with the inner surface being supported on a substantially electrically insulating monocrystalline indium phosphide substrate;
said indium arsenide film including a substantially rectangular sensing area having long and short edges and an electrical conductor extending along the length of each long edge;
said exciter tooth having a width that is greater than dimensions of said sensing area; and said thin film of indium arsenide being approximately 1 to 3 micrometers thick and nominally undoped, and having an average electron density of the order of 1016 electrons per cubic centimeter or lower and an average electron mobility of at least about 13,000 cm2volt-1second-1 but which exhibits a magnetic sensitivity and temperature insensitivity as if the indium arsenide film were at least an order of magnitude thinner, had an electron density at least an order of magnitude greater, and a significantly higher mobility.

2. A position sensor providing unamplified electrical output changes of the order of one volt in response to changes in applied magnetic field over a temperature range of several hundred degrees Centigrade;
said position sensor including a magnetic circuit that comprises:
an exciter portion including a series of teeth spaced apart by gaps for defining a tooth pitch, and a sensing portion for relative movement therebetween;
said sensing portion including a permanent magnet having a width at least several times wider than the width of an exciter tooth and supporting a magnetoresistive sensing element;
said magnetoresistive sensing element including a thin film of monocrystalline indium arsenide having inner and outer surfaces, with the inner surface being supported on a substantially electrically insulating monocrystalline indium phosphide substrate;
said indium arsenide film including a substantially rectangular sensing area having long and short edges and an electrical conductor extending along the length of each long edge of the rectangular sensing area;
said exciter tooth having a width that is greater than dimensions of said sensing area; and said thin film of indium arsenide being about 1 to 3 micrometers thick and nominally undoped; and having an average electron density of the order of 1016 electrons per cubic centimeter or lower and an average electron mobility of at least about 13,000 cm2volt-1second-1; and an electron accumulation layer adjacent the outer surface of said indium arsenide thin film and extending entirely across the sensing area between the conductors contacting its long edges, which accumulation layer has an electron density at least an order of magnitude higher than said average electron density and an electron mobility significantly greater than said average electron mobility, effective to provide a magnetic sensitivity and range of operating temperature as if the indium arsenide thin film were much thinner and had a much higher electron density and electron mobility.

3. The position sensor of claim 1 in which the magnet width is about 1.5 times the tooth pitch, the magnetoresistive sensing element includes a plurality of said thin film sensing areas, said plurality of areas are electrically in series, and said plurality of sensing areas are disposed in a combined area having a maximum dimension less than the width of said exciter tooth.

4. In a position sensor that includes a magnetic circuit characterized by an exciter portion including teeth spaced apart by gaps and a sensing portion for relative movement therebetween, wherein the sensing portion includes a permanent magnet having one surface approximate the exciter portion and being relatively wide compared to the width of an exciter tooth, a layer of high permeability magnetic material over said one surface, and a magnetic sensing element on said layer positioned along a limited portion intermediate between the two ends of the permanent magnet, the improvement wherein:
the magnetic sensing element includes a thin film of a monocrystalline semiconductive material having only a moderate average current carrier density and moderate average current carrier mobility, and a band gap of at least about 0.35 electron volt;
a sensing area in said thin film; and an accumulation layer in said thin film extending across said sensing area and disposed adjacent a surface of said thin film, where said current carriers can preferentially flow between conductive portions contacting opposed edges of said sensing area effective to provide an apparent increase in carrier mobility and concentration in said semiconductive material, an apparent reduction in thickness of said film, and an actual improvement in thickness of said film, and an actual improvement in the magnetic sensitivity of said film and in temperature insensitivity of the magnetic sensitivity of said film.

5. The position sensor of claim 4 in which the magnetic layer is coextensive with said one surface of the permanent magnet.

6. The position sensor of claim 5 in which said sensing element is substantially centered between the two ends of the permanent magnet and includes a plurality of said thin film sensing areas, said plurality of sensing areas are electrically in series, and said plurality of sensing areas are disposed in a combined area having a maximum dimension less than the width of said exciter tooth.

7. The position sensor of claim 5 in which the width of the sensing element is less than the width of a tooth.

8. The position sensor of claim 6 in which the width of said permanent magnet is approximately 1.5 times the tooth pitch of the exciter portion.

9. The position sensor of claim 8 in which the tooth width is between about 0.17 and 0.37 the tooth pitch.

10. The position sensor of claim 9 in which the tooth width is about 0.25 the tooth pitch.

11. In a position sensor that includes a magnetic circuit characterized by an exciter portion including teeth spaced apart by gaps and a sensing portion for relative movement therebetween, wherein the sensing portion includes a permanent magnet having one surface approximate the exciter portion and being relatively wide compared to the width of an excitor tooth, a layer of high permeability magnetic material over said one surface, and a magnetic sensing element on said layer positioned along a limited portion intermediate between the two ends of the permanent magnet, the improvement wherein:
the magnetic sensing element includes a thin film of a monocrystalline semiconductive material having only a moderate average current carrier density and moderate average current carrier mobility, and a band gap of at least about 0.35 electron volt;
a sensing area in said thin film;
an accumulation layer in said thin film extending across said sensing area and disposed adjacent a surface of said thin film, where said current carriers can preferentially flow between conductive portions contacting opposed edges of said sensing area; and means for maintaining said accumulation layer in said film during use of the position sensor without requiring more than two electrical contacts to said sensing element effective to provide an apparent increase in carrier mobility and concentration in said semiconductive material, an apparent reduction in thickness of said film, and an actual improvement in the magnetic sensitivity of said film and in temperature insensitivity of the magnetic sensitivity of said film.

12. The position sensor of claim 11 wherein the semiconductive material of the thin film is indium arsenide, and the means for maintaining the accumulation layer in the thin film is a dielectric coating on the indium aresnide thin film.

13. The position sensor of claim 11 in which the width of said magnet is approximately one and one-half times the tooth pitch of the exciter portion, the magnetoresistive sensing element includes a plurality of said thin film sensing areas, said plurality of areas are electrically in series, said plurality of sensing areas are disposed in a combined area having a maximum dimension less than the width of said exciter tooth, and the means for maintaining said accumulation layer in said film during use of said sensing element is a coating that creates the accumulation layer in the thin film.

14. The position sensor of claim 13 in which the coating includes a conductive electrode layer for inducing an electric field in the thin film to create the accumulation layer, and the means for maintaining the accumulation layer further includes electrical biasing means interconnecting the conductive electrode layer with said conductive portions contacting opposed edges of said sensing area.

15. The position sensor of claim 13 in which the tooth width is between about 0.17 and 0.37 the tooth pitch.

16. A position sensor comprising:
a stationary portion and an exciter portion adapted to move past said stationary portion;
said stationary portion including a permanent magnet having a planar major surface normal to the polarization of the magnet, a ferromagnetic layer overlying said major surface, and a sensing element centered along the width of said surface over said layer, the width of said sensing element being substantially less than the width of said magnet;
said exciter portion including a succession of teeth spaced apart by gaps, the width of each of said teeth being less than the width of each of said gaps and more than the width of said sensing element;
said magnetoresistive sensing element including a thin film of monocrystalline nominally undoped indium arsenide supported on a substantially electrically insulating monocrystalline indium phosphide substrate;
said thin film having a thickness of less than about 3 micrometers and a generally rectangular sensing area, substantially parallel conductors contacting said thin film for injecting current carriers into opposed edges of said sensing area;
said sensing area having a dimension in a direct path between said conductors that is significantly shorter than its dimension parallel to said conductors, and said sensing area dimensions are less than the width of said exciter tooth.

17. The position sensor of claim 16 in which said sensing element has a plurality of sensing areas, the plurality of sensing areas are successively disposed in an elongated portion of the thin film, with each sensing area extending across the entire width of the elongated portion, and a conductor extends across the entire width of the elongated portion on opposite sides of each sensing area, wherein the length of the elongated portion of the film has a dimension less than the width of said exciter tooth.

18. The position sensor of claim 4 in which the semiconductor material is an arsenide or antimonide of indium.

19. The position sensor of claim 18 in which the semiconductor film is of a thickness no greater than 3 micrometers.

20. The position sensor of claim 19 in which the areal electron density of the accumulation layer is substantially larger than the areal electron density of the bulk of the film.

21. The position sensor of claim 20 in which the average electron density of the bulk of the film is in the order of 1016 electronic per cubic centimeter.

22. The position sensor of claim 21 in which the film has an average electron mobility of at least about 13,000 cm2volt-1second-1.