Platinum or Palladium/Cobalt Multilayer on a Zinc Oxide or Indium Oxide Layer for Magneto-Optical Recording

The Drawing consists of two Figures. FIG. 1 shows a comparison of the squareness of a hysteresis loop obtained with a Pt/Co multilayer sputtered onto a non-etched zinc oxide base layer which has been exposed to room atmosphere with that obtained with a Pt/Co multilayer sputtered onto a similar zinc oxide base layer which has been etched after the exposure to room atmosphere.

FIG. 2 shows a hysteresis loop obtained with a Pt/Co multilayer sputtered onto a non-etched zinc oxide base layer which has not been exposed to room atmosphere.

The instant invention provides a superior magneto-optical (MO) recording medium. The presence of a base layer of zinc oxide or indium oxide between the multilayer recording film and the substrate provides this improvement over MO recording medium of the prior art. Zinc oxide and indium oxide are transparent to the radiation used in recording and reading and have indices of refraction, n, of about 2. It has now been found that a base layer of zinc oxide or indium oxide not only enhances the Kerr effect, but also significantly increases the coercivity of the multilayer recording film. Tin oxide may also be useful as a base layer to enhance both the Kerr effect and the magnetic coercivity. These oxides are preferably deposited onto the substrate by sputtering. Si.sub.3 N.sub.4 is the most commonly used dielectric layer to enhance the Kerr effect, but no enhancement of the coercivity has been observed when it is used.

The oxide film can be deposited upon a variety of substrates, e. g., glass, quartz or a transparent plastic such as polycarbonate or polymethylmethacrylate. The substrate must be transparent to the radiation used to record on the multilayer recording film, as well as to the radiation used to read.

The multilayer recording film is deposited onto the oxide film by sputtering alternate layers of platinum and cobalt or palladium and cobalt. Argon, krypton, xenon or a mixture thereof can be used as the sputter gas when sputtering the multilayer, but it is preferred to use krypton, xenon or a mixture thereof as the sputter gas and to use a sputtering gas pressure of about 2 to about 12 mTorr (about 0.27 to about 1.6 Pa).

Pt/Co multilayers are preferred over Pd/Co multilayers because they exhibit a higher Kerr effect. As a result, the preferred magneto-optical recording medium of the invention is comprised of a substrate, a base layer of zinc oxide or indium oxide sputtered onto the substrate and a multilayer recording film sputtered onto the base layer, wherein the zinc oxide or indium oxide film is from about 200 A to about 4500 A (about 20 nm to about 450 nm) thick, preferably from about 200 A to about 2000 A (about 20 nm to about 200 nm) thick, the multilayer recording film is a platinum/cobalt multilayer film consisting of alternating layers of platinum and cobalt, and the substrate is transparent to the radiation used to record and read.

The platinum/cobalt multilayer film is deposited by sputtering alternate layers of platinum and cobalt. Preferably all cobalt layers within the multilayer film have substantially the same thickness d.sub.Co and all said platinum layers within the multilayer film have substantially the same thickness d.sub.Pt. Preferably d.sub.Co is less than about 12 A (1.2 nm), d.sub.Pt is less than about 24 A (2.4 nm) and the total thickness of the multilayer film is less than about 750 A (75 nm). Most preferably d.sub.Co is from about 2 to about 5 A (0.2 to about 0.5 nm) and d.sub.Pt /d.sub.Co is from about 1 to about 5. The preferred ranges set forth herein identify structures having the most suitable characteristics for magneto-optical recording.

DC or RF magnetron sputtering can be used to deposit the platinum/cobalt multilayer film. The substrates with the zinc oxide or indium oxide base layer are typically placed on a rotating table and exposed to the Pt and Co sputtering fluxes in alternate, repeating fashion. The motion of the table can be programmed to permit the table to pause for preselected periods during Pt and Co deposition. In this manner, the relative thicknesses of Pt and Co can be controlled. Preferably the Co and Pt targets are physically separated to eliminate the possiblity of overlap of their sputtered fluxes. While argon, krypton, xenon or a mixture thereof can be used as the sputter gas when sputtering the Pt/Co multilayer, it is preferred to use krypton, xenon or a mixture thereof as the sputter gas and to use a sputtering gas pressure of about 2 to about 12 mTorr (about 0.27 to about 1.6 Pa).

In these Examples of the invention, the zinc oxide thin films were sputtered onto glass substrates in a diffusion-pumped vacuum system whose background pressure was about 5.times.10.sup.31 6 Torr (6.7.times.10.sup.-4 Pa). Films were sputtered at a total argon plus oxygen pressure of 8 mTorr (1.1 Pa). An oxygen partial pressure of 1 mTorr (0.13 Pa) was used for Examples 1-9, 2 mTorr (0.27 Pa) was used for Examples 10-21, and 4 mTorr (0.53 Pa) was used for Examples 22-32. Zinc oxide film thicknesses ranged from 500 A to 4000 A (50 nm to 400 nm). X-ray diffraction results verified that all films were ZnO with the c-axis oriented perpendicular to the thin film plane. After the zinc oxide films were sputtered onto the glass substrates, the substrates were removed from this chamber, thereby exposing the zinc oxide film to the laboratory environment, and then transferred into another vacuum chamber for deposition of Pt/Co multilayers.

Several essentially identical zinc oxide films were prepared and prior to sputtering the multilayers, some of these zinc oxide films were sputter-etched under different, but mild, conditions in order to create a clean zinc oxide surface layer, i.e., to remove any contamination which may have occurred when the zinc oxide was exposed to the atmosphere during the transfer process.

Pt/Co multilayers were subsequently sputtered at 7 mTorr (0.94 Pa) of Kr on to uncoated and zinc oxide-coated glass and silicon substrates. All multilayer films were prepared by DC magnetron sputtering from separate 6.5" (16.5 cm) diameter Co and Pt targets. Forty watts was the sputtering power to each target. Substrates were on a rotatable table and were alternately coated by each metal. The substrate to target distance was about 3" (7.6 cm). There was no significant difference in results obtained with a glass substrate and those obtained with a polished silicon substrate. Glass substrates were used for the examples and experiments described herein. During deposition the rotating table was programmed to remain stationary under each target for a given time by controlling the substrate motion with a computer. Prior to deposition the vacuum chamber was pumped to a background pressure of about 2.times.10.sup.-7 Torr (2.7.times.1O.sup.-5 Pa) before admitting the sputtering gas.

A pair of adjacent layers of Pt and Co is referred to as a bilayer. The number of Pt layers and the number of Co layers are of course each equal to the number of bilayers. In all the Examples, 10 bilayers were used. The thickness d.sub.Pt of the Pt layers is 11.0 A (1.1 nm) and the thickness d.sub.Co of the Co layers is 3.5 A (0.35 nm).

The perpendicular magnetic hysteresis loops of films on zinc oxide-boated and uncoated glass substrates were measured and H.sub.c noted. All of these multilayer films have an easy axis of magnetization perpendicular to the film plane and a magnetic squareness equal to one, that is, the remanent magnetization at zero applied field equals the saturation value.

The composition of the sputter gas used in sputtering the zinc oxide films, the thickness of the zinc oxide films, the sputter cleaning conditions, i.e., the voltage and time of sputtering, used to etch the zinc oxide films and the H.sub.c are shown in Table I. For comparison, the same multilayer on a glass substrate with no base layer has an H.sub.c of about 850 Oe.

Multilayer films sputtered on zinc oxide had substantially larger coercivities than those sputtered directly on glass. The magnitude of Hc depends only weakly on the zinc oxide preparation conditions and the zinc oxide thickness, and more strongly on the sputter-etch or cleaning conditions. In general, when the zinc oxide had been exposed to the atmosphere, etching the zinc oxide layer improved the squareness of the magnetic hysteresis loop compared to that of unetched zinc oxide layers. FIG. 1 illustrates this improvement and shows a plot of the hysteresis loops obtained from Example 30 with no etching and that of Example 31 with etching before the deposition of the multilayer. Milder etching conditions only slightly affected the magnitude of H.sub.c, either increasing it or decreasing it slightly relative to the H.sub.c obtained with the unetched zinc oxide. Larger etch voltages and longer etch times always decreased H.sub.c, although they improved the square shape of the hysteresis loop. If the etching voltage is too large or the etching time too long, the zinc oxide surface may become roughened and this consequently decreases the coercivity. It is believed that the key role of etching is to clean and restore the zinc oxide surface to its condition prior to removing the film from the vacuum chamber in which it was deposited. Therefore depositing the zinc oxide layer and the Pt/Co multilayers in the same vacuum chamber may eliminate the need for sputter-etching to clean the zinc oxide surface. This is demonstrated in Example 33 in which essentially the same Pt/Co multilayer film sputtered in all the other Examples was deposited onto a 1000 A (100 nm) zinc oxide layer in the same vacuum chamber without exposing the zinc oxide to the atmosphere and without etching the zinc oxide. FIG. 2 shows the square hysteresis loop obtained. The H.sub.c is about 3100 Oe. Etching may also play a role in helping to nucleate a Pt/Co microstructure favorable for high coercivity.

In these Examples of the invention, the indium oxide thin films were sputtered onto glass substrates in a diffusion-pumped vacuum system whose background pressure was about 5.times.10.sup.-6 Torr (6.7.times.10.sup.-4 Pa). Films were sputtered at a total argon plus oxygen pressure of 8 mTorr (1.1 Pa). An oxygen partial pressure of 2 mTorr (0.27 Pa) was used. Indium film thicknesses ranged from 1000 A to 1800 A (100 nm to 180 nm). After the indium oxide films were sputtered onto the glass substrates, the substrates were removed from this chamber, thereby exposing the indium oxide film to the laboratory environment, and then transferred into another vacuum chamber for deposition of Pt/Co multilayers.

Several essentially identical indium oxide films were prepared and prior to sputtering the multilayers, some of these indium oxide films were sputter-etched under different, but mild, conditions in order to create a clean indium oxide surface layer, i.e., to remove any contamination which may have occurred when the indium oxide was exposed to the atmosphere during the transfer process.

Pt/Co multilayers were subsequently sputtered at 7 mTorr (0.94 Pa) of Kr on to indium oxide-coated glass and silicon substrates. All multilayer films were prepared by DC magnetron sputtering from separate 6.5" (16.5 cm) diameter Co and Pt targets. Forty watts was the sputtering power to each target. Substrates were on a rotatable table and were alternately coated by each metal. The substrate to target distance was about 3" (7.6 cm). There was no significant difference in results obtained with a glass substrate and those obtained with a polished silicon substrate. Glass substrates were used for the examples and experiments described herein. During deposition the rotating table was programmed to remain stationary under each target for a given time by controlling the substrate motion with a computer. Prior to deposition the vacuum chamber was pumped to a background pressure of about 2.times.10.sup.31 7 Torr (2.7.times.10.sup.-5 Pa) before admitting the sputtering gas.

In all the Examples, 10 bilayers were used. The thickness d.sub.Pt of the Pt layers is 11.0 A (1.1 nm) and the thickness d.sub.Co of the Co layers is 3.5 A (0.35 nm).

The perpendicular magnetic hysteresis loops of films indium oxide-coated were measured and H.sub.c noted. All of these multilayer films have an easy axis of magnetization perpendicular to the film plane and a magnetic squareness equal to one, that is, the remanent magnetization at zero applied field equals the saturation value.

The composition of the sputter gas used in sputtering the indium oxide films, the thickness of the indium oxide films, the sputter cleaning conditions, i.e., the voltage and time of sputtering, used to etch the indium oxide films and the H.sub.c are shown in Table II. For comparison, the same multilayer on a glass substrate with no base layer has an H.sub.c of about 850 Oe.

Multilayer films sputtered on indium oxide had substantially larger coercivities than those sputtered directly on glass. The magnitude of H.sub.c depends only weakly on the indium oxide preparation conditions and the indium oxide thickness, and more strongly on the sputter-etch or cleaning conditions. In general, when the indium oxide had been exposed to the atmosphere, etching the zinc oxide layer improved the squareness of the magnetic hysteresis loop compared to that of unetched indium oxide layers. Milder etching conditions had little effect on the magnitude of H.sub.c. Larger etch voltages and longer etch times reduce H.sub.c, although they improved the square shape of the hysteresis loop. If the etching voltage is too large or the etching time too long, the indium oxide surface may become roughened and this consequently decreases the coercivity. It is believed that the key role of etching is to clean and restore the indium oxide surface to its condition prior to removing the film from the vacuum chamber in which it was deposited. Therefore depositing the indium oxide layer and the Pt/Co multilayers in the same vacuum chamber may eliminate the need for sputter-etching to clean the indium oxide surface. Etching may also play a role in helping to nucleate a Pt/Co microstructure favorable for high coercivity.