Hideki Nishida

Extraordinary Bubbles in Epitaxial Garnet Films

A method of producing extraordinary bubbles (EBs) in epitaxial garnet films is presented together with their static and dynamic properties. EBs can be produced by applying a number of pulse fields to strip domains through a loop conductor. The collapse fields of bubbles cut from these strip domains are distributed between the collapse field of ordinary bubbles (Ho) and a higher value of up to 1.4Ho for samples of composition (EuEr)3(GaFe)5O12. Also presented is an analysis of a domain wall structure model for EBs, in which a cylindrical domain wall is assumed to consist of N pairs of right‐ and left‐handed Bloch‐type sections connected with Neel‐type sections. By choosing suitable values for N, the model reproduces the static properties of EBs quite well.

Planar process for 8-µm period bubble devices using polyimide resin

A new and simple planar process using polyimide resin, polyimide iso-indroquinazolinedione (PIQ®), has been developed for 8-μm period bubble devices. PIQ is strongly heat resistant. This material is used as an interlaminar dielectric between conductor and Permalloy patterns. The PIQ layer effectively covers the steep steps of Au/Mo conductor patterns, thereby eliminating the problem of Permalloy step coverage at conductor edges. A 256-kbit memory device with an 8-μm period fabricated by the planar process exhibits an overall bias margin of more than 10 percent over the temperature range 15°C to 80°C for 300-kHz operation at a drive field of 55 Oe. PIQ does not introduce constraints on other process parameters or chip designs. Reproducibility and reliability of bubble devices fabricated by the PIQ process have been good.

Disturb properties of exchange-coupled composite plated wires

Disturb properties of four-layered films of Permalloy and Co-Ni alternately electroplated onto 0.1-mm diameter wires are reported. NDRO behaviors with various kinds of disturbs are shown. It is found that word disturbs intervening between digit disturbs are very influential. The mechanism in this case is different from that of wall creeping previously reported. Static changes of magnetization or hysteresis by intervening word pulses play an important role in the former case, although in the latter case dynamic changes of magnetization do. Some strange disturb behaviors are also found. These composite films have a wide operating region for NDRO.

Chemistry of photoresist reclamation

We have studied the chemical changes that result in photoresist used for LCD device manufacturing due to various stresses that the photoresist is subjected to during the recovery and reclamation process. (1) When the roll coater recovery photoresist is kept open at room temperature for 50 days, needle-shaped crystals formed in the recovery photoresist. Examination by X-ray diffraction revealed that those crystals are ammonium zinc sulfate hexahydrate that was grown owing to the dissolution of the roll material of coater. It is possible to avoid this problem by appropriate selection of the material that comes in contact with photoresist. (2) When new, tightly sealed photoresist was subjected to thermal stress at 40 degrees Celsius for 100 days, a bead-shaped coagulated material appeared. NMR analysis showed that the coagulated material was mainly a photosensitive ester tetramer formed by a thermal change. Furthermore, HPLC analysis showed that the tetramer consisted of mainly tetraesters, but contained diesters, triesters, even monoesters. (3) In the case of the spin coating, although there is virtually no damage from physical stress, water content of about 1% is absorbed from the air during the spin-coating process, and the photo-active compound is decomposed by the thermal stress on the photoresist, lowering the photosensitivity. This change in the photosensitivity of the photoresist can be virtually eliminated if the photoresist contains now water so that the photo-active compound chemical change does not occur during the distillation process. (4) In the reclamation of photoresist, (a) the photoresist is coated in a dry nitrogen atmosphere with no water content, and next (b) the photoresist in a coater cup is dissolved by a photoresist solvent and recovered, and then (c) the viscosity of the recovered photoresist is adjusted by distillation, and it is filtered at the end. By adding fresh photoresist, this type of reclamation can be continuously performed.

New single-mask approach to bubble device fabrication

Permalloy devices with two separate levels of fine permalloy and conductor patterns have been fabricated by the use of a single-mask process. This process makes possible the production of devices having an essentially conventional design in addition to a completely planar structure. The features of this new process include 1) preparation of a reticle on which patterns for both permalloy and conductor layers are superimposed, 2) use of SiO 2 , Mo/Au/Mo, SiO 2 , permalloy, and TiO 2 thin films deposited sequentially on the bubble materials, and 3) simultaneous delineation of both photoresist patterns with two different thicknesses and of the desired patterns by CF 4 plasma etching and ion-milling.

Abstract: A new single level masking approach to the fabrication of a magnetic bubble memory chip

We have developed a new method for the fabrication of the magnetic bubble memory chip which enables the realization of a memory chip with two separate levels of fine overlay patterns (Permalloy propagation and Al‐Cu conductor patterns) by employing a single level masking process. To fabricate a memory chip, four levels of thin films (SiO2, Al‐Cu, SiO2 and Permalloy) were coated on the bubble material first; then the photoresist patterns were delineated on top of these films. (AIP)

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.

PFC emission-reduction strategy for the LCD industry

— The perfluorocarbons (PFCs) used as etching-process and chamber-cleaning gases in the manufacture of LCD devices have a high global-warming potential and a long atmospheric lifetime. Thus, to voluntarily reduce these environmentally harmful PFCs, in 2001 the Japanese LCD association established the World LCD Industry Cooperation Committee (WLICC), together with its counterpart associations in the Republic of Korea and Taiwan. Since that time involving many discussions, the WLICC reached a consensus on reducing the aggregate absolute PFC emissions to the equivalent of less than 0.82 million metric tons of carbon by 2010. Each association has been taking whatever emission reduction steps it considers best to achieve this goal. Thanks to these measures, the PFC emissions from the Japanese LCD industry have remained at the same level, or less, than those of the year 2000, in spite of the fact that PFC purchases have continually increased. In the interim, the Japanese LCD association has been obtaining experimental data for the 2006 IPCC Inventory Guidelines, which will give the emission factors needed to estimate the PFC emissions. As a result, some data are quite different from emission factors given in the current 2000 Inventory Guidelines. All the data obtained have been submitted to the IPCC to be adopted in the new 2006 Guidelines.

New photoresist stripper and a system for recycling it

Although it is hard to strip away the photoresist layer damaged during the etching process in manufacturing liquid crystal display devices, monoethynoloamine (MEA) was found to have excellent photoresist stripping characteristics. Crosslinked novolak resin is easily dissolved in a MEA solution because MEA has a large cohesive energy, a strong affinity for novolak, and a small molecular weight. Pure MEA can be recycled with high yield by using a simple system that has short multiple-pipe distillation columns that have high heat-transfer efficiency. The pipes in our distillation system are made of 304 stainless steel because this material was found to resist corrosion in an aqueous MEA solution better than 316L stainless or Cu. This recycling system has been operating in production lines for more than two years without showing any signs of trouble.

Chemistry of photoresist recycling: II

We have been researching chemical changes that occur in positive-type photoresist within a recycling process. In this process, spin-coated photoresist is first collected together with the thinner used for washing the coated cup, and the diluted resists is then distilled. Next, by adding an appropriate amount of solvent, the viscosity of the resist is returned to its initial state. The resist is finally filtered and returned for use. Simulation experiments were performed using a spin coated. It was found that the photoresist takes on about 1 percent water content by absorbing moisture from the atmosphere during the spin- coating process, and that diazonaphthoquinone decomposes due to thermal stress, which is connected with a drop in photo- sensitivity. Accordingly, if the photoresist should happen to be free of water, and if heat should not happen to cause chemical changes in diazonaphthoquinone during refinement, sensitivity should not show a change in the extracted photoresist. With this in mind, it was decided that extracted photoresist must be refined in low-moisture conditions at low temperature. This photoresist recycling is therefore performed by refining photoresist at room temperature using equipment that can distill under reduced pressure. By adding fresh photoresist, this type of recycling can be continuously performed.