Fabrication of bubble memory chips

Investigations have been made on the fabrication of accurate and uniform T-bar circuits. Chrome masks are preferable to emulsion masks, and furthermore, a minimum exposure and intimate contact have been demonstrated to be necessary for accurate and uniform pattern imaging on the AZ1350 resist. A newly developed chemical etchant, a nitric acid-base solution without ferric chloride, can almost eliminate undercutting of permalloy elements. Application of spin-on-glass prior to Permalloy evaporation can result in excellent step coverage at the places where T-bar circuits overlap conductors. Large memory chips having a capacity of 16 × 10³bits and a storage density of 10⁵bits/cm²have successfully been fabricated.     

A sensitive magnetoresistive power amplifier

A small, sensitive, low noise, high gain power amplifier, using the anisotropic magnetoresistance effect in thin film permalloy, has been designed and its characteristics calculated. The minimum detectable input current is determined by Johnson noise and hence by input resistance and desired bandwidth. An example of theoretical performance is as follows. For an amplifier unit with approximate dimensions of300 times 300 times 2 mum and with input and load resistances of 50 Ω each the calculated noise at room temperature is equivalent to 10^-8A for a bandwidth (BW) of 1MHz or to 10^-7A for a BW of 100 MHz. At the 10^-8A input current level, the calculated power gain issim 600,000corresponding to a current gain of 775. Power gain decreases with input current asI^{-4/3}, reaching unity atI = 2.1 times 10^{-4}A. Hence, for a BW of 1 MHz, at room temperature, the input current operating range for both amplification and signal-to-noise ratio greater than one is fromI=10^{-8}A to2.1 times 10^{-4}A. To achieve high gain, the amplifier is configured so that the magnetization of the permalloy is biased to lie nominally along the hard axis, the sensing current in the permalloy makes an angle of 45° with the nominal magnetization direction, and the input current produces a magnetic field along the easy axis. This microsize, low noise, silicon compatible power amplifier will be useful in digital and FM applications and possibly as an amplifier for crosstie and bubble memories.     

Magnetic bubble device scaling and density limits

Scaling of magnetic bubble devices to smaller bubble sizes and higher density is considered. Drive field requirements, materials requirements, fabrication requirements, current requirements, and detector signal-to-noise ratio are all calculated as a function of bubble size and related to practical limits imposed by bubble materials, fabrication techniques, and electromigration limits. It is concluded that "conventional" bubble devices using Permalloy bars can be made practical with 1-μm bubble domains (storage densitysim6 times 10^{6}bits/cm²). Although it may be possible to extend these Permalloy bar devices to even smaller bubbles, it seems more likely that other bubble devices such as contiguous disk devices or bubble lattice devices will in fact be used for densities greater than 6 × 10⁶bits/cm².     

Magnetooptical memory experiments on a rotating Mn-Cu-Bi disk medium

A thin film of a new magnetooptical memory medium, Mn-Cu-Bi, was prepared on a glass disk and their dynamic memory characteristics were examined with an experimental rotating disk exerciser. The Curie temperature of the medium was about 200°C. Curie-point writing was made on it. The written information was read by the Kerr effect. The Kerr rotation was ±1.3° at an Ar ion laser wavelength of 5145 Å, where the Kerr rotation of the medium was made maximum by an SiO overcoating. For track selection, a moving coil with a linear motion air bearing, whose shaft was rectangular in form, was used. By utilizing a laser for the light source of a moiré fringe measurement technique, an accurate track access was made possible. The accuracy was within 1 μm and the track-to-track transfer time was about 5 ms. Writing and reading were accomplished under computer control with the direct memory access mode. A 5.09 × 10^-6medium error rate without error correction was obtained for a 2.5 × 10⁵bit/cm²packing density at a data transfer rate of 0.5 MHz.     

Positive exchange bias in epitaxial permalloy/MgO integrated with Si (1 0 0)

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni_82.5Fe_17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (1 0 0).     

Plated-wire technology: A critical review

Electrodeposited cylindrical magnetic film memory technology is reviewed. Extensive references are made to previous work. New data is given in areas not previously reported. Special emphasis is given to the following items: 1) Applications: Plated-wire applications cover the spectrum from 100 to 1000 ns cycle time NDRO memories of 10⁵to 10⁷bits capacity in commercial and military markets. 2) Processing: Substrate preparation and electrochemical processing, as well as alternative magnetic plating solutions, and the control of pH, temperature, flow rate, Ni-Fe ratio, and other variables necessary for high yield plating are reviewed. 3) Aging: The conditions that stabilize the film and the test methods that predict a 10- to 100-year life are reviewed. 4) Testing: A practical approach to continuous production line testing is reviewed. 5) Bit Packing Density: Calculations on magnetization distribution along the wire are compared to experience. Although memory plane geometry is important, the wire characteristics dominate the achievable bit density. Wire diameter, film thickness Hk, and other factors are included in graphs useful for design purposes. 6) Memory Plane Construction: The relative advantages of several different forms are compared. The need for magnetic keepers and their advantages are reviewed. 7) Interactions: New data is presented on some of the effects of variations in word strap alignment and spacing as well as the sensitivity to variation in plated-wire spacing and bending.     

Uniaxial and cubic anisotropy constants in ion-implanted Sm,Tm,Ga:YIG bubble thin films

The effects of ion implantation into (SmTmY)3(GaFe)5O12garnet thin films have been studied by ferromagnetic resonance. He+ ions were used for implantation with doses ranging from 3 to 4 × 10¹⁵He+/ cm²and implantation energy ranging from 150 to 175 Kev. The uniaxial and cubic anisotropy constants have been studied as a function of temperature from 0°C to 100°C for both as-grown and ion-implanted films. The implantation has effectively changed an easy-axis anisotropy to that of an easy plane in the implanted layers of the films. A cubic anisotropy constant K1with values varying from 4 × 10³to 6 × 10³erg/cm³at room temperature has been observed in these films. The results also indicated that to the first order, the implanted region in the film was essentially magnetically uncoupled from the bulk of the film.     

Contiguous-disk bubble domain devices

Contiguous-disk bubble devices are an approach to higher bit density through the use of coarse overlay patterns in manipulating small bubbles to relax device lithography requirements. As a first step towards such an objective, a fully processed chip using ion-implanted devices has been tested, showing the feasibility of all required memory functions with 5-μm bubbles and 25-μm period overlay patterns. A critique of permalloy versus implanted contiguous-disk devices is made, pointing out their basic difference in magnetization reversal processes and explaining the superiority of the latter over the former in achieving a good edge affinity of bubbles. The requirements for a good implanted device are reviewed, including the selection of garnet material parameters (K1, λ111), of implantation parameters (ion energy and dosage) and of device pattern geometry (thickness and shape of implanted layer). An understanding of these requirements has made it possible to demonstrate 1-μm bubble propagation in several contiguous-disk type circuits with 4.5-μm periods, yielding an areal density of over 3 × 10⁷bit/in²made by conventional photolithography.     

Magnetic and magnetooptical properties of sputtered Fe5Si3films

Polycrystalline films of Fe5Si3have been prepared by RF sputtering from a sintered target composed of 62.5 at% Fe and 37.5 at% Si. The Faraday rotation and optical absorption were measured for these films in the visible and near infrared. At a wavelength of 6328Å, room-temperature absorption (corrected for reflection) and specific Faraday rotation are 3.7 × 10⁵cm^-1and 1.3 × 10⁵deg/cm, respectively. The rotation decreases with rising temperature and vanishes around 120°C, the Curie temperature of the films. From polar and in-plane hysteresis loop measurements the spontaneous magnetization is found to lie in the film plane. Square hysteresis loops are observed when the external field is applied in the plane of the film, and the in-plane coercive force decreases monotonically with temperature at an average rate of -2 Oe/°C as Tcis approached.     

Electron-beam fabrication of high-density amorphous bubble film devices

A low-temperature, all-vacuum process combined with electron-beam lithography suitable for single-level masking devices using 2-μm diameter amorphous bubble films has been developed. A test vehicle which uses 0.75-μm wide chevrons and 1-μm wide T.I bars in an 8,000- bit chip configuration, resulting in an areal density of 1×10⁷bits/in², was used. Important process features are found to be: (1) laminated NiFe films to obtain low Hcand high magneto-resistive effect when deposited at low substrate temperature, (2) maintenance of low surface temperature during metallization to preserve the integrity of exposed and developed electron-beam resist pattern, and (3) proper resist profile for ease of the lift-off process. Excellent bubble device operating characteristics have been obtained as a result of uniformity in materials and structure resulting from careful control of fabrication parameters.     

Holographic read-write memory and capacity enhancement by 3-D storage

The capacity and limitations of fast random-access holographic memory are discussed. Capacities such 10¹¹bits can be envisioned by superposition of holograms in a thick erasable storage material. The organization of the read-write holographic memory provides the possibility of varying the reference beam angle at any given point of the memory plane.     

Characterization of magnetic domain walls using electron magnetic chiral dichroism

Domain walls and spin states of permalloy were investigated by electron magnetic chiral dichroism (EMCD) technique in Lorentz imaging mode using a JEM-2100F transmission electron microscope. EMCD signals from both Fe and Ni L_3,2 edges were detected from the Bloch lines but not from the adjacent main wall. The magnetic polarity orientation of the circular Bloch line is opposite to that of the cross Bloch line. The orientations of Fe and Ni spins are parallel rather than antiparallel, both at the cross Bloch line and circular Bloch line.     

An ultra-high data rate mass storage system

This paper describes the mechanical, electronic, and signal processing capabilities of a special purpose instrumentation mass storage system developed by Bell & Howell Datatape Division. The system is presently operational and encompasses many of the features of computer storage systems including vacuum column tape handling and fast start-stop capability. Throughput rates of up to 450 MBS and total user storage capacity of 2.9 × 10¹¹bits are operational specifications of the system. Bit error rates of 1 × 10^-9are achievable using error detection and correction techniques which complement the ENRZ^TMsource encoding scheme. Search speeds of up to 1140 cm/sec (450 ips) provide worst case access times of approximately four minutes to any portion of the data.     

Magnetic bubble mass memory

An experimental magnetic bubble mass memory module complete with all control function and detection electronics has been built and operated. The module contains twenty-eight 16 448-bit mass memory chips and operates at a nominal rotating field frequency of 100 kHz. The module has an average access time of 2.7 ms, a read/write cycle time of 5.14 ms, and a data rate of 700 kbit/sec. A read error rate of <1.6 × 10¹²and error-free propagation in excess of 8.4 × 10¹⁵bubble cycles have been demonstrated.     

Magnetic recording on domain walls in a single crystal film

The existing magnetic data recording media employ polycrystalline tracks, on which data are encoded using domains with different orientations of magnetic moments. We have numerically simulated data recording on the track in a single crystal film, in which the domain structure is formed using an intrinsic magnetostatic field of the crystal and the Bloch domain walls play the role of information bits.     

Characterization of magnetic domain walls using electron magnetic chiral dichroism

Abstract

Domain walls and spin states of permalloy were investigated by electron magnetic chiral dichroism (EMCD) technique in Lorentz imaging mode using a JEM-2100F transmission electron microscope. EMCD signals from both Fe and Ni L_3,2 edges were detected from the Bloch lines but not from the adjacent main wall. The magnetic polarity orientation of the circular Bloch line is opposite to that of the cross Bloch line. The orientations of Fe and Ni spins are parallel rather than antiparallel, both at the cross Bloch line and circular Bloch line.     

Rotatable magnetic anisotropy in epitaxial ferrite layers

Magnetic domain structure and magnetic anisotropy were studied in monocrystalline epilayers of Mg0.9Mn0.3Fe1.8O4ferrite. The layers, several micrometers thick, were obtained by a CVD method on monocrystalline MgO substrates. Domain observations were performed by the Bitter's method. Magnetic anisotropy measurements were performed by torque and FMR methods. In the demagnetized state, a typical stripe structure of 2.0 to 2.8 μm period was observed. From the domains behavior in the in-plane magnetic fields it was found that in these epilayers the rotatable anisotropy was present. The existence of this anisotropy was confirmed by torque measurements in small in-plane fields. The magnetic parameters characterizing these layers are: 4ΠM = 3500 Gs, K1= - 2.2 × 10⁴ergs/cc, KN= 2.3 × 10⁵ergs/cc.     

Charged walls in thin magnetic films

Charged walls are domain walls which carry a net "magnetic charge" (div M) due to their orientation relative to the domain magnetizations. They differ from ordinary Bloch and Néel walls (which are uncharged) primarily in their much wider profile. In order to calculate such walls, a variational method was developed. It is based on the separation of that part of stray field energy which would be present even with an infinitely thin wall. The main results of the calculations are as follows. 1) Isolated charged walls do exist if exchange energy is taken into account, as opposed to the periodic solution known for the limit of negligible exchange energy. 2) Rotated, partially charged walls develop a Néel-wall-like narrow core region. Detailed results for the wall profiles, energies and widths as a function of wall angle, orientation, film thickness, and material parameters are presented. They are applied to two examples: the case of a Permalloy film in a domain tip propagation memory, and the case of the implanted layer on a contiguous disk bubble device.     

0.25 × 106bit/in2NDRO coupled film memory elements

The fabrication and operation of high-density (0.25 × 10⁶bit/in²) nondestructive readout (NDRO) memory elements are described. The high density is made possible by coupled films and Permalloy keeper. The NDRO is made possible by multiple-pulse WRITE or hard-direction bias field. Typical performance parameters areI_{w} = 60mA,I_{b} = 30mA, andV_{s} = 150μV/3 ns. The small signal is detectable by multiple-pulse READ. When such memory elements are to be fabricated with peripheral circuits on the same Si chip, a self-contained chip will be obtained. Such chips could enjoy the same advantages as semiconductor memory chips such as few leads, modularity, amenability to bit organization, and possibility for error-correction, but would be capable of higher storage density due to simpler planar configuration.     

High-speed creep effects in magnetic films

Creep measurements were made on single domain walls in thin magnetic films using high-speed pulses with variable rise and fall times (0.4 ns to>1 mus) and durations (<1 ns to 3 μs). Combinations of these pulses and dc fields were applied along the hard axis while simultaneously easy-axis dc fields were applied. The two basic measurements that were made were onset of creep and the distance the wall crept per pulse as a function of applied fields. Definite rise-time effects were found, the exact behavior depending on the domain wall structure. For Bloch walls, gyromagnetic effects of the total wall (similar to wall streaming) are present for rise timeslsim 20ns, whereas for longer rise times the Bloch to Néel wall transition appears to be responsible. For films thinner than 900 Å the existence of a cross-tie structure was found to be necessary for creep. For this wall structure the exact mechanism which causes creep enhancement for rise times <100 ns is unknown.