Magnetic Film Memory Design

PROCEEDINGS OF THE IRE

Magnetic Film Memory Design*J. I. RAFFELt, MEMBER, IRE, T.

A. H. ANDERSONt, MEMBER,

Summary-Thin magnetic films of permalloy have character-istics ideal for high-speed digital storage. A simple rotational modelmodified to include the effects of wall switching and dispersion ofthe preferred direction of magnetization provides a basis for describ-ing properties of engineering interest. A selection system has beenchosen which allows great latitude in film uniformity. Production offilms with magnetic properties uniform to within ± 10 per cent isreadily achieved. Specifications for operation in a destructive modecan easily be met by existing film arrays; the nondestructive mode isconsiderably more stringent unless very small signals can betolerated. The first film memory has been in reliable operation sincethe summer of 1959. It has 32 ten-bit words and has been operatedwith a minimum cycle time of 0.4 ,usec.

Higher speed and larger capacities will require higher bit densi-ties and improved techniques to minimize undesirable coupling be-tween drive and sense lines. The use of 1OX60 mil rectangles, bal-anced sense windings, and longer words will hopefully permitmemories of about 200,000 bits with cycle time under 0.2 MAsec.

I. INTRODUCTIONS INCE 1955, when Blois' succeeded in preparing

oriented films of permalloy there has been consid-erable study of their fundamental magnetic prop-

erties and their possible engineering application as digi-tal storage elements.The primary motivation for much of this work derives

from the uniique promise of films to provide extremelyfast, random-access storage systemns at relatively lowcost, based on the following considerations:

1) The switching time of films is about 1 nanosecond(nsec) for fields of approximately 2 oersteds.2"

2) The low uniaxial magnetic anisotropy permits re-versible or nondestructive readout with the applicationof modest transverse fields. For destructive readout itallows a word selection system which permits wide lati-tude in film properties.

3) The preparation of extremely thin layers (by eitherevaporation or plating) provides low flux which reducesthe voltage drive required and eliminates heating athigh frequencies.

4) The simple planar substrate-supported film mnakespossible extremiely high bit-densities and allows the useof low-impedance strip transmission lines for high-fre-quency pulse distribution, factors of utmost importance

* Received by the IRE, August 2, 1960.t Mass. Inst. of Tech. Lincoln Lab., Lexington, Mass.1 M. S. Blois, Jr., "Preparation of thin magnetic films and their

properties," J. Appl. Phys., vol. 26, pp. 975-980; August, 1955.2 D. 0. Smith and G. P. Weiss, "Steady-state and pulse measure-

ment-techniques for thin magnetic films in VHF-UHF range," J.Appl. Phys., vol. 29, pp. 290-291; March, 1958.

3 V. Dietrich and W. E. Proebster, "Millimicrosecond magnetiza-tioIn reversal in thini magnetic films," J. Appl. Phys., vol. 31, pp.281S-282S; May, 1960.

S. CROWTHERt, MEMBER, IRE,IRE, AND T. O. HERNDONt

in determining the compatibility of high speed and largecapacity.

5) Film preparation permits the fabrication, hani-dling, and testing of entire arrays rather than indi-vidual components.

6) The open-flux structure eliminates the need forthreading holes and permits the use of evaporated oretched wire techniques.

Having stated the considerations which have stimu-lated the study of films for storage, a more detailedoutline of their properties and a discussion of the practi-cal engineering problems are given below. This is fol-lowed by a description of the first operating magneticfilm memory and a brief estimate of future approachesand goals.

II. FILM PHYSICS

Permalloy films are normally prepared by eithervacuum deposition or by electroplating in a magneticfield. The resultant film exhibits a uniaxial magneticanisotropy, i.e., develops a preferred or easy axis.' Ifan anisotropy energy K sin2 0 is assumed, where 0 is theangle the magnetization makes with the easy axis,torque and energy calculations give the ideal static anddynamic magnetization characteristics.4'5 For combina-tions of applied transverse and longitudinal fields, Fig.1 gives the family of curves of equilibrium angle be-

90° 8 =1350tH T

° EASY AXIS

ORIGINAL89 HLSTATE

H' HLHW LHK

FOR T=O0

HT =- HL TANO +SIN8HK HK

Fig. 1-Equilibrium angle of magnetization vector forapplied transverse and longitudinal fields.

4D. 0. Smith, "The static and dynamic behavior of thin perm-alloy films," J. Appl. Phys. vol. 29, pp. 264-273; March, 1958.

5 C. D. Olson and A. V. Pohm, "Flux reversal in thin films of 82ccNi, 18% Fe," J. Appl. Phys., vol. 29, pp. 274-282; March, 1958.

1961 155

PROCEEDINGS OF TIHE IRE

tween the magnietization vector and the original easy-axis direction. Equilibrium occurs when the torque T,due to the applied fields, is zero. Values of 0 less than90° correspond to reversible rotation, values greaterthan 900, irreversible rotation. These plots assume acoherent rotation of the magnetization throughoutthe film, i.e., all the spins are aligned and rotate simul-taneously. The threshold curve marking the boundaryl)etween reversible and irreversible rotation is alsoshown in Fig. 1 and contains almost all the infornmationinecessary for describing the various selection systems(i.e., multicoordinate drive-field combinations) whichmav be used to read and write information in a singlefilml element in an array. From these curves it is easilyshown that a plot of transverse flux kT vs transversefield I1T (the transverse loop) will be liniear up to Hk,where saturation occurs. I'he longitudinial loop (longi-tuclinlal flux OL vs longitudinial field HL) is square witha threshold which corresponds to I7,. These loops areshown in Fig. 2(a).So far an ideal uniaxial anisotropy has been assumed

and the rotational threshold curves are derived directlyfromii this model. Experimental hysteresis loops areconitrasted with those predicted by theory in Fig. 2.IThe knee of the longitudinal loop H,0 may be consider-ablv below HF. This occurs because the longitudinialloop is not a rotational loop as expected, but rather theresult of domain-wall switching (i.e., a sequential piece-wise switching of small areas of material) similar to thatoccurring in bulk toroids. This process is inherenitlyslower than rotationi and provides an undesirablenethod for changing informationi.To a first order, the values of Hk and II,. will specify

the operating currents. The fields needed for rotationalswitching vary directly with H6 while the maxiiimumiilongitudinial field which can be applied without caus-ing wall 11ot;ion is IL,,.Another departure from the model is that the tramis-

verse loop opens slightly at high fields and in somlecases mnav be almost square, suggesting a poorly definedeasy axis. Experimentally it is found that films do ex-hibit an apparent dispersion of the easy axis.6'7 Fig. 3indicates how angular dispersion may be measured.

In interpreting this measurement it is convenienit toassuLmne that each individual crystallite or other bacicunit of uniclirectionial anisotropy, whatever its scale,switches independently of neighboring regions accord-ing to the rotational model. The first step in making themeasurement is to align the film in a hysteresigraph asshown in Fig. 3(a) with HL=0. An ac transverse fieldgreater than It will cause the magnetic vector of thoseregions with their hard axes in quadrants II and IV torotate in a counter-clockwise direction. For those re-

6 R. G. Alexander, "Anisotropy field measurements on Ni-Fethin films," J. Appl. Phys. vol. 30, pp. 266S-367S; April, 1959.

7 T. S. Crowther, "Anguilar Dispersion of the Easy Axis of IThinFerro-Magnetic Filt;s," M.I.T. Lincoln Lab. Group Rept. 51-2;February 24, 1959; ie6ised Malrch 30, 1960.

LONGITUDINAL LOOP

IDEALI

HL

TRANSVERSE LOOP

MODEL

HTHK

(a)

(b)Fig. 2-(a) Ideal magiletic-film B-1l loops for rotational

model; (b) experimenital B-H loops.

6 HT (a-C)

HL (d-c)

(0)

(b)Fig. 3-(a) Experimental technique; diagram for easy-axis dispersion

measurement. (b) B-H loops of longitudinal flux as a function oftransverse field for longitudinal dc field values of +0.35, +0.14,0, -0.14, and -0.35 oersted, reading from top to bottom.

156 Janiiary

Raffel, et al.: Mlagnetic lFilm Memory Design

gions with their hard axes in quadrants I and III [Fig.3(a)], rotation will be clockwise. When the two rotat-ing components are equal, L = 0, and a null is obtained.For zero dispersion, the application of an infinitesimalpositive HL dc field will cause the magnetization of theentire film to oscillate back and forth only in quadrantsI and IV. For finite dispersion the field required to keepall of the flux in quadrants I and IV can be used to cal-culate the dispersion. For a single region with skew athe dc field required will be given by

HL = Hk sin a.

That is, the dc field must be larger than the longitudinalcomponent of the ac field when it has the value Hk.

In addition, it is often found that there is a grossskew of the easy axis upon which the dispersion is super-posed. The consequences of both effects is an equivalentrotation of the threshold curve (Fig. 4), which will varyfor different regions of material. Therefore, small sec-tions will tend to switch somewhat below the thresholdcurve as plotted for the nominal easy axis. This causesincoherent rotation and reduces the maximum allow-able field for reversible switching which is of greatimportance for nondestructive readout.

incidence. For a small source such as a crucible thismeans delimiting the substrate area and keeping thesource as far from the substrate as is practical. In prac-tice, one square, 1.6 inchesX1.6 inches, is evaporatedat a time, at a distance of about 12 inches.

Composition: Both Hk and magnetostriction areknown to vary with composition.9 For all practicalpurposes the zero-magnetostrictive composition, 81 percent Ni-19 per cent Fe, is the most desirable becauseit eliminates most of the skew problem due to stresses,while providing low Hk and low dispersion. It should bepointed out, however, that films are compositionallyinhomogeneous with random local variations of at least1 to 2 per cent common in almost all films.'0 The extentto which this influences dispersion is not clear but itmeans that any measuremenit of magnetostriction rep-resents an average value only.

1Thickness: Wall coercive force for an infinite planarfilm is found to vary inversely with thickness accordingto Hw==t-n where n usually varies between 1/2 and 3/2depending upon the substrate surface and method offilm preparation.'1"2

Spot Shape: Spots may be formed by evaporationthrough a mask or by etching from a continuous film us-ing standard photoresist techniques.The lower limit for spot size and the upper limit for

thickness are imposed by the self-demagnetizing fieldresulting from free poles at the film edges. The demag-netizing field is proportional to thickness and inverselyproportional to spot diameter. At the center of a circu-lar spot it is given approximately by

HD ; 1/32 oersted

x

Fig. 4-Limit of transverse field HR, for reversiblerotation of a film with skew of easy axis a.

III. FILM PREPARATION

EvaporationThere are several parameters which are of first-order

magnetic significance in preparing films by evaporation.Angle of Incidence: By inclining the substrate at large

angles to the incident vapor beam it is possible to pro-duce films whose easy axes are perpendicular to the in-cident beam and have anisotropy values of hunidreds ofoersteds. This is thought to be produced by orientedvoids resulting from self-shadowing.8 The incident vaporbeam should be maintained normal to the surface in

order to prevent skewing of the easy axis due to oblique

8 D. 0. Smith, M. S. Cohen, and G. P. Weiss, "Oblique-incidenceanisotropy in evaporated permalloy films," J. Appl. Phys., vol. 31,pp. 1755-1762; October, 1960.

thickness (in thousands of angstroms)

diameter(in inches)

HD must be small compared to H, to prevent reversedomains from forming with resultant loss of informa-tion. For a 1/16-inch diameter spot 1000 A thick, thisgives a demagnetizing field of 2 oersted, a value per-haps - to 4 the typical wall-coercive force. For asym-metrical shapes such as rectangles, the contribution toHk because of shape anisotropy must be taken into ac-count. This will again inicrease with demagnetizingfield and hence depenid uponl thickness.

Dispersion is found to increase with decreasing spot

9 D. 0. Smith, "Anisotropy in permalloy films," J. Appl. Phys.,vol. 30, pp. 264S-265S; April, 1959.20 W. W. L. Chu, J. E. Wolfe, and B. C. Wagnier, "Somiie observa-tions on evaporated permalloy filns," J. Appl. Phys., vol. 30, pp.272S-273S; April, 1959.

"1 K. H. Behrndt and F. S. Maddocks, "Influence of suLbstrateprocessing on the magnetic properties and reproducibility of evap-orated nickel-iron films," J. Appl. Phys., vol. 30, pp. 276S-277S;April, 1959.

12 I. W. Wolf, H. W. Katz and A. E. Brain, "Magnletic Propertiesof Electrodeposited Thin Films," presented at the Electronic Com-ponents Conf., Philadelphia, Pa.; May 6-8, 1959. This work was doneat the G.E. Co., Syracuse, N. Y., under a subcontract from M.I.T.Lincoln Lab.

1961 157

PROCEEDINGS OF THIE IRE

size, probably due to the inifluenice of the demagnietizin-igfield and edge domiiailns.

Substrate Surface: In order to produce films which arewell orielited it is niecessary to deposit upon a locally,smooth surface such as fire-polished glass or mica. Thepresence of oil stainis, fingerprints, anid other contaml-iniation leads to poor reproducibility, especially in Hw.To provide a uniiform surface the substrate is normallycleanied ultrasomically, placed in the bell jar and a layerof SiO evaporated just prior to the permalloy.

Substrate Temperatutre: The usual practice is to heatthe substrate to about 400°C before and durinig deposi-tioii. This tenids to drive gases fromn the substrate andprovides anl anniieal which generally results in substani-tially, lower II/, values thani for films miade at roomtemnperatture.

Rate of Deposition: For evaporated filmus the rate ofdepositioni hals a direct effect oni filmii structure and, ingenieral, higher evaporation rates produce more uImi-formi filmiis.

Pressutre: The exact effect of residual gases in thevacuum system is inot entirely understood, but the pres-enice of oxygeIn in various amounts is known to play aconisiderable role in determining magnetic character-istics of bulk materials.13 Evaporation at convenitionialpressures of 10-5 imm Hg yields satisfactory filnms forimost purposes. The inmprovements to be obtainied fromoperation at ultrahigh vacuum remain to be deter-mined.

Electroplating

Electrodeposited permalloy films comparable in imag-nietic properties to evaporated films have been preparedby \Wolf, et al.'2 These films are plated on a coating ofsputtered gold which must be kept quite thin (about100 A) in order to obtain well-defiined anisotropy. Inadditioin, conitrol of sputtering voltage and plating-cur-rent density are very important. Voltages of 2 kv anidcurrent denisities of 3 ma/cm2 are typical.The high resistanice of the thin gold layer can lead to

severe thickiness gradients in the permalloy, because ofthe voltage drop from the electrode clip to the bottomof the slide. The substrate is usually reversed durinigthe run atnd clipped at the other edge to average out aniygradients.

In general, plated films seem to be somewhat higherinl Ilk thain the best evaporated films, but there is muchless variationi in Hk with composition. Values of 3-4oersteds are most conmmon as conmpared to approxi-miiately 2 oersteds for evaporated filmiis. In genieral, re-producibility seems to be somewhat better for platedfilms, especially from runi to ruin, although the differ-ence is tnot large.

13 R. H. Heidenreich and F. W. Reynolds, "Uniaxial magneticaiiisotropy and microstructure of ferromagnetic metal films," in"Structure and Properties of Thin Films," C. A. Neugebauer, J. B.Newkirk, and D. A. Vermilyea, Eds., John Wiley and Sons, Inc.,New York, N. Y., pp. 402-409; 1959.

IV. ENGINEERING DiJsiGNSelection SystemsThe m11ost iml)ortant paramiieter in the designi ol a film

memory is the choice of selectioni systemii, silnce this willdetermiiine the requiremeiits for fillll uLnliformiiity, thelocation in timne anid space of ptulse transienits wxhich maymask the signial, the requiremiienits of peripheral cir-cuitry, anid the arranigemiienit of signial anid drive linies.A niumnber of selection systemiis halve beeni proposed

which have varied widely in effectiveness.14-6 Tlhe oniewNhich will be described was emlployed inl the desigii ofthe TX-2 imiemiiory at the M.I.T. Lincoln Laboratory."With occasionial variatiois'l7'8 it seemiis, at the presenttimiie, to be generally accepte(l as the m1ostt desirable byworklers in the field.

TIhe mienmory is word-organiized with word-linles sup-ply-inig a field in the trainsverse direction sufficienit to ro-tate the imiagnietization 900. A currenit pulse of eitherpolarity oIn the digit linle supplies a field along the easyaxis which will cause the muagnietizationi to fall to theONE or ZERO directioni upoII the termination of thetranisverse pulse. Readout occurs oni the rise of thetranisverse pulse. The digit pulse starts at the middle of,anid enids after the comiipletioni of, the transverse pulse.In the TX-2 memory a slightly differenit miiodle is usedin order to elimninate the nieed for both at ONE andZERO digit driver. A dce currenit in the ZERO directionflows continuously and a ONEl pulse twice as large over-rides the dc for writing ONE's. The only differenice be-tweeni the two imiodes is that the ONE_ and ZERO sig-nals are equal but opposite in polarity for the first,while in the second the dc bias durinig read timiie in-creases the ONE anid decreases the ZERO signial. Thepulse sequenices for both mlodes are showni in Fig. 5.To providle nondestruCtive readout in the first miiode

of this svstemi, it is onvly iecessary that the tranisverse-read pulse be sufficiently below IH. to stay withiii thelimiiits of rotational reversibility. For writinig, the largetransverse is used as above. The amlounit of reversiblerotationi which cani be obtained will determinie the aiml-plitude of the signial. In geiieral. the sigiial will be de-creacsed by a factor of 2 to 5 fromii the destructive case.Note that unilike m10ost notndestructive readout memorysystenms there is nio nieed to clear before writing. Also,the samiie word equipimienit may be shared for nionidestruc-tive reading anid for writinig since there is onily at dif-ferenice in pulse heiglht betweeni the two.

Additionial advanitages of this selection systemii are

14 A. V. Pohm and S. M. Rubens, 'A compact coincident-currenitmemory," Proc. EJCC, pp. 120-124; December, 1959.

16 J I. Raffel, "Operating characteristics of a thin film memiiory,"J. Appl. Phys., vol. 30, pp. 608-618; April, 1959.

16 E. E. Bittman, "Using thin films in high-speed memories,"Electronics, vol. 32, pp. 55-57; June, 1959.

17 Remingtoni Rand UNIVAC, Proj. Lightning, Second Phase,First Quarter Rept., NObsr, pp. 45-47; February 28, 1959.

18 E. M. Bradley, "Making reproducible magnetic-film mlemnories,"Electronics, vol. 33, pp. 78-81; Septemiiber, 1960.

i58 Janu.arv

Raffel, et al.: Magnetic Film Memory Design

TRANSVERSEFIELD(WORD) 0

LONGITUDINALFIELD

-

(DIGIT)

FILM OUTPUTAT 0

READ TIME

(a)

0 H-

WRITE (1)r- - --\

-q ) -.

WRITE (0)

0-

0

vFKLWRITE (1)r-----

I I

WRITE (O)4

(b)Fig. 5-Pulse sequenices for two modes of destructive readout

memory operation. (a) Mode uIsinlg pulsed write zero; (b) modeusiiig dc read and write zero.

1) Only a unidirectional pulse is required on the wordline.

2) The difference in polarity between ONE's andZERO's nmeans that the effect of variations in filmoutput is reduced.

3) The word line is orthogonal to the sense line, re-

sulting in low capacitive and inductive couplingat signal time.

4) All drive wiindings may be straight lines, thus pro-

viding simple fabrication and good transmissioncharacteristics.

5) Wide variation in film characteristics can be tol-erated.

Film SpecificationsGeneral conclusions about the film uniformity prob-

lem may be extremnely misleading unless they are basedupon the constraints imposed by the selection systemwhich is used.

Fig. 6 shows the operating locus for HL and HT withvariations in I1W, Hk, anid skew angle a. For the destruc-tive system described above, the following conditionshave to be met assuming Hk, HW, and a all vary:

HT > Hk max

HL > [Hk sin a]max

HL < H1V min-

The tranisverse field must be larger than the maximumvalue of Hk anid the longitudiinal field must be less thanthe minimum wall coercive force but greater than themaximum effective longitudinal field produced by thenominally tran-isverse word current.

For the nonidestructive system, the above conditionshold for writing with the added proviso that for read-inlg

HIT < HR min

(where HR is shown in Fig. 4 for a single region). Thenominially transverse field, then, must be smaller than

HL ,

Fig. 6-Drive-current operatinig loci for variationi of Hw, Hk, anid a.

the lowest reversible limit field, HR min, over the entirefilm.

TestingMaginetic film characteristics are conveniienitly mneas-

ured with a hysteresigraph and a pulse tester. Low-fre-quency (1000-) composite characteristics of arrays ofspots are nmeasured with the hysteresigraph. Two pairsof Helmholtz coils can be used to generate mnutually per-

pendicular drive fields. The substrate is held below one

sense coil and may be rotated in its plane. A seconid coilconniected in opposition to the sense coil provides can-

cellation of air-flux coupling. The integrated signal isdisplayed on the Y axis of an oscilloscope while the X-axis displacement is proportional to the aimplitude ofthe drive field. Figs. 2(b) and 3(b) are displays fromii thehysteresigraph. In order to achieve high sensitivity a

number of inoise-bucking methods must be employedand careful initegrator design is required.19'20The pulse tester is used to measure characteristics of

spots within an array uinder pulse-switching conditions.2'In order to inisure identical coniditions for all spots andto minimize the equipmnent required, the pulse testeruses onily onie set of drive and sense circuitry. A mechan-ical manipulator indexes the array under a set of crosseddrive Iines and a senise coil.

Three pulse tests are giveni to every elemeint in eacharray. The first is a miieasuremiient of the ONE ampli-tude for the norinal destructive read memory cycle.This test is performed twice, the second time withread and write polarities interchanged. A difference be-

19 E. C Crittenden, Jr., A. A. Hudimac, and R. I. Strough, "Mag-netization hysteresis loop tracer for long specimens of extremelysmall cross section," Rev. Sci. Instr., vol. 22, pp. 872-877; December,1951.

20 D. D. Strassberg, "An Audio-Frequency Hysteresigraph forThin Magnetic Filmiis," M.I.T. Linicoln Lab. Group Rept. 51-10;October 16, 1959.

21 A. H. Ancterson and T. S. Crowther, "Techniiques for PulseTesting Maginetic Film Memory Elemiienits," M.I.T. Lincoln Lab.Group Rept. 51-1 1; January 29, 1960.

TO 0

WORD FIELDLIMITS

HK MAX -

1961 159

1

---vI,l,o


tween ONE amplitudes for the two digit polarities in-dicates skew, of the easy axis. If both ONE amplitudesare low, the reason may be high Hk, or read-disturbing(i.e., the longitudinial field exceeding the wall-coerciveforce). A second pulse sequence determines whether ornot read-disturbing is the cause of any low outputs. Thefinal test checks the film for operation in the nonde-structive read mode.

Film UniformityFor both evaporated and electroplated22 films, the

variation of H,, and H. from array to array is typicallyabout 10 per cent. A group of twenty evaporated16X 16 arrays were selected for pulse testing after meet-ing hysteresigraph measurement specifications of H.>1.0 oersted and H1.<4.0 oersteds. A digit field of 0.67oersted was chosen which fixes the maximum allowableskew to less than 100. An acceptable array was definedas one in which all the elemenits had a ONE output of Fig. 7-Magnetic film memory in the TX-2 computer1 mv under normiial miemiiory-operating conditions. Of at the M.I.T. Lincoln Laboratory.these twenty arrays, only four had any elements whichfailed to meet specifications for reasons other thanfaulty etching. These four imperfect films all failed for +10 +10the same reason --excessive skew. Data on plated films 220 43 K 5.6 Kshow about the same results. Yields for nondestructive Iqf 2memory operation are much lower. Only two of the 330twenty evaporated arrays had elements which could all INPUT 2NI472be read reversibly for transverse fields of 3 Hk. Thisvalue of drive results ill an output about one third that POSITIVE "AND 40GATES MAY BE 20~

for a drive field gre<tter thani HI. ADDED AT THIS 1OOhyPOINT FOR 200

36

DECODING IAf rN279The TX-2 Thin Film Memory 220 a

The first film memory, shown in Fig. 7, has been inloperation in the control elemenit of the TX-2 computer -10 -18since the summer of 1959. It has 32 ten-bit words and TO DRIVEuses destructive readout. Its operating speed in the LINEmachine is 0.8 ,usec and it has been bench-tested at a (a)0.4 ,usec cycle time. The memory uses two 16X 16 arraysof circular spots 116 inich in diameter anid about 700 A STAGE I STAGE H STAGE mthick. Conventionial round magniet wire is cemented to ?+10V

plastic boards to form the wirilg mats which sandwich 0t 1 NTROL 1600 2000thie film, formiillg closed loops for low impedance and 60f + 60fS 60ofmiiiiiiiiium crosstalk. The word and digit lines each have 60II loo6± 60two series turns, while the sense windiiig is a single turncrossed over in the center of the array for cancellation of 2251coupling to the parallel digit line. Driving currents are INPUT100 _250 ma on the word line and 200 ma on the digit line _ I 1

(superposed oin a negative bias of 100 ma). Input aiid fand ~~~~~14hlh 15$h 1.5 1.51h 1.5jhoutput circuits are shown in Fig. 8. The current driver X,hwhich supplies a 250-ma pulse with a rise timie of 20 100 100 100 100 200 200

nsec is used for both the digit lines and a two-coordinateferrite-core switch matrix. The switch matrix supplies -3V 220current pulses to the word lines which are terminated in ALL TRANSISTORS

ARE 2N501-10V

221 W. Wolf and T. S. Crowther, "Reproducibility of Electro- (b)deposited Thin Film Memory Arrays," presented at the Electronic Fig. 8-(a) TX-2 film-memory driver; (b) TX-2Components Conf, Washingtoni, D. C.; May 10-12, 1960. film-memory sense-amplifier.

160 January


diodes for suppression of the core recovery pulse andhalf-select switch-core outputs. The three-stage differ-cn!tial senise amiiplifier has a voltage gain of 1000 and arise time of 50 nsec.

V. DISCUSSIONGeneral ConsiderationsThe combiniation of latitude in film requirements and

iifTprovements inl film characteristics have reduced theuniformiiity problem to seconidary importance for the de-structive readout memory. \Vhile improvements incharacteristics will always be welcome, they are not acrucial issue. However, for the nondestructive memoryanyv inicrease in reversible limuit anid reduction of skew isextremely important.

Probably the most important engineering problem istoThhieve the optimum combination of capacity andspeed attainable (for a single memnory) within the limita-tioins imposed by transnmissioni delays, crosstalk, anidtranisistor-circuit bandwidth. Of course, it is always pos-sible to uitilize a multitude of small memories and thusobviate the nieed for large arrays in which problemsarisinig from high speed are ititensified. Economic con-siderationis, however, require that the number of bitssharing conmmiion drive anid seiise circuitry be high.

Dri've and Sense ConfigurationsThere are two fundamiienital sources of noise in the

fornm of crosstalk. A transienit induced in the sense wind-inlg due to coupling to the digit winding to which it isp'arallel occurs durinig wNriting. This prevents squeezingthe cycle tim-le because of interferenice with the nextreadout. The crossover of the sense winding is initenidedto minimiize the effect, but for long lines and very highbanidwtidth, canicellation is poor and considerable noiseresults. The second source of nioise is due to capacitivecouplitng betweeni the transverse linle and sense or digitw-inHdiings. This results in charging currents which inducespurious voltages in the sense windinig during read timewhich can mlask the signal.The choice of a senise-digit winiding combination

wlhich is balanced to ground and completely sym-mletrical with respect to all coupling is essential at veryhigh frequenicies. Two such configurations are shown inFig. 9. The planar sense suffers the disadvantage thatits imlpedatnce is relatively high and it requires an extralayer of fine wiring. The bridge circuit, on the otherhanid, requires that curreint be fed into the dummy line,thus doubling the digit current required.

Onie mleasure of the efficienicy of anly senise-digit com-biniation is the degree of coupling between the film andwindings. This will vary with the spacing between con-ductors and is limited by the thickness of the film sub-strate. A rough quantitative measure of the efficiency ofthe couplinig is the ratio of signal current Is to digit cur-

rent ID. (This is the reciprocal of the current gain re-quired in the digit-sense channel for destructive readoutoperation.)

DIGIT CURRENTs- DIGIT LINE R R SOURCE

- - -~----SENSE/ ~- DIGIT TO

RETURN DIFFERENCEFILM AMPLIFIER

(a) (b)Fig. 9-(a) A balanced planar-sense linie; (b) comnbinied

digit-sense line bridge circuit.

Is Vs/ZoID ID

This ratio will increase as switching speed inicreases, aindas Zo (or linie separation) decreases, producing higheroutptit voltage L's. At present this ratio is of the order of

1 mv/100 ohms- ~~~~~10-s.1 ampere-turn

For 10-nsec switchinig and 1-ohm impedance level thiswould be increased to 10-2. Perfect couplinig (efficienicyof 1) is not desirable either, because this would imiplyideal transformiier action which would result in heavyloadinig on the filmii. Probably somewhere between 10-'anid 10-2 would be anl upper limit providinig a range ofthree orders of miiagnitude of possible imnprovemiienltw"Thich requires considerable exploration. As the hiuie inm-pedance is lowered it becomes necessary to take ad-vantage of the lower imnpedance by properly matchinigto a sense amplifier for maximumii energy transfer. Fortransistors with input imupedanice in the range of a fewhundred ohnms this requires a transformiier input.The lower limit to Zo, without resorting to evaporated

lines or deposition of the film on a conductor, is imposedby substrate thickness, which cani be reduced to perhaps2 mil by usin1g mica. For a 60-mil width this gives anlimpedance of about 3 ohms. To obtain lower values it isprobably necessary to use evaporated linles. Here thelimitation of frequency cutoff due to losses in the lineimposes a lower limit on conductor thickiness. It can beshown that for (RC)2>LC,

coc C- 22\/(RC)2- LC

and for (RC)2<LC,

1961 161

(c)c = O° I


where R=total line resistance, C=total line capaci-tance, L = total line inductance, and xc = cutoff fre-quency of the line.The point at which losses become significant is where

(RC)2= LC

or

R/Zo= 1.

For copper,

2X 10-6 DRIZo = ~ X-X

377 ST

where T=conductor thickness in cm, S = line spacing incm, and D=line length in cm.

If it is assumed that S= T, then for

R/ZO 1

/ 2 X 10-6S= T = D

377

- 7300 A -/D.

For equal insulator and conductor thickness, layers of7300 A are required for only 1 cm of sense line. For 10cm 23,000 A of each would be required. At a frequencyof 1 kMc the skin depth limits the effective conductorthickness to about 10,000 A so that the insulator spac-ing must be increased to about 55,000 A for a 10-cmline. However, this is nearly the thickness of the micasubstrate upon which we are trying to improve in thefirst place.

In other words, at high frequencies for even moderatesense line lengths, we cannot employ insulation spacingless than the thinnest bulk substrate available withoutbeginning to run into rise-time degeneration. In addi-tion, of course, this assumes that thick conductors canbe evaporated without running into stress problems, in-sulators free of pinholes, and that neither of these affectthe magnetic-film characteristics adversely.

Note that the sense line length is crucial in deter-minling whether or not evaporated lines are desirable. Afurther problem with evaporated lines is that of inter-connection. Until it is possible to raise the bit densityhigh enough so that an entire memory can be fabricatedin a single small area it will be difficult to make use ofevaporated lines.

Bit Density

The choice of spot size is crucial in determiningmemory performance even with normal constructionpractices. Input currents are proportional to spot diam-eter, and signal decreases more than linearly with de-creasing diameter because the film must be made thinnerin order to prevent demagnetizing. Delays are also pro-portional to spot size. With a spacing of 10 spots perinch as in the TX-2 memory, a sense winding linking1000 bits would be over 8 feet long. For a dielectric

constant of 2 this would provide a delay of about 12nsec.

If one attempts to reduce spot size in order to shortenline lengths it is desirable to resort to thicker films inorder to maintain the signal level. It is possible toeliminate the consequent high demagnetizing field byusing paired films to provide an almost closed flux path.Fig. 10(b) shows the effect, as observed with a Kerrmagneto-optic apparatus, of reduction in demagnetizingforce resulting from pairing two spots around a driveline (light and dark regions indicate regions of oppositemagnetization). The spots are ' inch in diameter,

PAIRED

(a)

(b)Fig. 10-(a) Diagram of paired film experiment. (b) Kerr magneto-

optic demonstration of effect of film pairing. Light and darkregions are of opposite magnetization. Upper film is single, lowerfilm one of a pair.

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about 3400 A thick; the easy axis is horizontal. The ar-rangement of spots and drive line is shown in Fig. 10(a).The "sandwich" spacing is two mils. Following a satu-rating longitudinal field the single spot is broken up be-cause of the demagnetizing field, while the paired oneremainis nearly saturated. (In the photograph [Fig.10(b)], the spots appear elliptical because they werephotographed at an angle.) One of the difficulties withsuch a configurationi is that the spacing between filmsmust be kept small compared to spot diameter and yetaccommodate the word, digit, and sense windings. Also,the currents required are twice that for an ordinary striplie around a single film.

Since in mlost memiiories the number of words is con-siderably greater than the number of digits, it is reason-able to attempt to shorteni the sense winding and pro-vide a more nearly square array. One such method usesrectangles whose easy axes are along the long dimensioni.WVith 10X60-mil rectangles it is possible to raise thepacking density to 50 per inch along the sense windinigwhere it is needed most, while retaining the 10 per inchspacing along the word. The demagnietizinig factor forsuch a spot is slightly less than for a 60-mil circle sothere is no need to decrease the thickness.

Another approach to providing a square array is toextend word length, i.e., to provide multiple-word ac-cess. Word-organization and the low flux level of filmielements make extended word length extremely attrac-tive. For example, a 1000-word switch driving a word200 bits long would have a number of advantages overa 4000-word switch driving 50 bits. In addition to re-ducing sense winding length, fewer word drivers, lessword decoding, and fewer diodes would be required andparallel access to four words would be obtained. Thecost would be an increase by a factor of four in digitequipment (driving and sensing). The efficiency of thedigit-sense channel (i.e., the degree of coupling) whichdetermines the complexity and cost of the digit circuitrywould be extremely important in such a system.

Memory SpeedThe limitation on cycle time at present arises primar-

ily from the noise occurring during the write transientinterferiing with the signal in the following cycle. Whilethe bridge circuit is intended to minimize this transientit does not eliminate it entirely. It is possible to reducethe effect by reducing the number of times a write isfollowed directly by a read. Nondestructive readout ac-complishes this by eliminatinig rewriting. An alterna-tive method is to use a buffer of a few words to permitstrings of reads before rewriting. The effect can also bereduced by usinig a slightly modified mode of memoryoperation. Instead of turning off the digit drivers afterwriting (Fig. 5), each would be left in its present stateuntil the next write period. This eliminates the tran-sient caused by terminationi of the digit pulse. The resultis a reduction of cycle tine by approximately the widthof the write pulse of Fig. 5. This mode of operation pro-

duces either large ONES and small ZEROS, or smallONES and large ZEROS depending upon the polarityof the digit current. The state of the digit-driver is thenused to interpret the signal which is read out.

Since a diode per line is required for the core switch inthe TX-2 memory and only a single nonlinearity is re-quired, it is possible to use the diodes alone for switch-ing word lines and to eliminate the cores. A schematic ofa diode switch matrix is shown in Fig. 11. The switch is

LOAD DRIVERS

U)

>

w

a:

V)

Fig. 11-Diagram of a 4X4 diode word-drive matrix. Onedrive line with films is shown at lower right.

Fig. 12-An experimental diode matrix and 256-word memory using lOX60-mil films.

1961 163


capable of gating 20-nsec rise-time pulses. Note, how-ever, that although the cores were redunidant they iso-lated the word lines from the driver thus allowing theword line to be grounded. This is extremely importantat high frequencies in reducing the voltage coupled tothe sense winding. Therefore, it may be necessary to usethe diode matrix for selection, and to couple to eachdrive line with a linear transformer.

Present experiments center on a test memory whichis shown in Fig. 12. This shows a 256-word diode switch(32X8) connected to word lines spaced on 20-mil ceni-ters for driving 10X60 mil rectangles. The bridge digit-sense circuit is used. This prototype is intended to be theforerunner of a full-scale memory with 1000 two hun-

dred-bit words. The present goal is to attain a speed ofbetween 0.1- and 0.2-,sec cycle time.

AcKNOWLEDGMENTThe authors wish to express their thanks to Dr. D. 0.

Smith and colleagues, whose work oIn the physics offilms provided the sound basis without which anl ein-gineering program of the type presented here wouldhave been impossible. The authors are indebted, also, toD. Strassberg and H. Blatt for their work on instru-mentation and circuits. In addition, thanks are due tothe many people who conitributed to the preparationand testing of films aiid to the fabrication of wiring ar-rays.

The Development of the Flexible-DiskMagnetic Recorder*

R. T. PEARSONt

Summary-A brief history of the development of the flexible-diskmagnetic recorder is presented. Principles of the aeroelastic be-havior of the disk and the results of a mathematical analysis of theequilibrium operating conditions are discussed. Experimental resultsare given which illustrate the effects of operating parameters on diskdynamics. These results indicate the design considerations necessaryto produce a wide range of new storage devices. The characteristicsof some of the newly developed models are presented.

INTRODUCTIONT HE maintenance of a small and constant separa-

tion between a recording head and its associatedmedium has been a major problem in the develop-

ment of magnetic recording devices. A large numberof successful solutions to this problem have beenachieved for specific applications. The IBM RAMACdisk store,' for example, utilizes an air-floating head,while the H.D. File Drum manufactured by the Lab-oratory for Electronics, Inc., utilizes a read-write headsupported hydrodynamically at a constant separationof 200 microinches from the drum surface.2 Solutionssuch as these have provided excellent single-purpose de-

* Received by the IRE, July 29, 1960; revised manuscript re-ceived, October 25, 1960.

t Storage Device Group, Computer Div., Lab. for Electronics,Inc., Boston, Mass.

I T. Noyes and W. E. Dickinson, "Engineering design of a mag-netic-disk random-access memory," Proc. WJCC, pp. 42-44; 1956.

2 H. W. Fuller, S. P. Woodsum, and R. R. Evans, "Design andsystems aspect of the HD file drum," Proc. WJCC, Los Angeles,Calif., May 6-8, 1958, pp. 197-203; March, 1959.

vices, but they do not yield a good general approach topresent and ever-changing data storage requirements.The Storage Device Group of the Computer Products

Division began a program two years ago aimed at thedevelopment of a new class of magnetic recording de-vices. The initial objective of this program was to solvethe fundamental problem of reliable head-to-mediumseparation and thereby furnish the computer industrywith a replacement for the conventional high-speed,medium-capacity drums and disks that are used quiteuniversally at the present time. A separation means wassought which would be simple and inexpensive as well asreliable. Consideration was given to drum and disk ge-ometries and to methods of achieving fixed-out-of-con-tact operation or fluid-bearing head operation. Theseapproaches were discarded for reasons of cost and com-plexity. It appeared that initial studies should considera radical design approach. The result of the design re-view was a study objective which gave first considera-tion to a system wherein the read-write heads would befixed and the recording medium would in some mannerbe made to dynamically approach the heads.

EXPLORATORY EXPERIMENTSThe starting point of the experimental work was the

concept of a recording medium dynamically approach-ing fixed heads, and a simple laboratory experiment wasdesigned to illustrate the principle. A thin disk of mylarrecording tape (2 mils in thickness) was attached to the

164 January

Magnetic Film Memory Design

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