Laser CVD and Plasma CVD of CrO.sub.2 Films and Cobalt Doped CrO.sub.2 Films Using Organometallic Precursors

This invention relates to magnetic thin films and is more especially directed to deposition of thin films of chromium dioxide onto all or specific portions of the surface of a substrate.

Chrome oxide, i.e., CrO.sub.2, has become highly favored as a magnetic recording medium because of its desirable ferromagnetic properties. Currently, magnetic recording material is typically produced by sputtering chromium dioxide onto a substrate such as tape or disk material. Sputtering has a very high failure rate, and this raises the average cost of producing the magnetic medium.

Deposition of thin films of CrO.sub.2 has been studied but no suitable or reliable means to produce chrome oxide recording media has been found, at least selectively on the micron scale.

The decomposition of metal hexacarbonyls to form metallic thin films has been studied, but there has been a propensity for the films formed from decomposition of hexacarbonyls to incorporate contaminants.

It is an object of this invention to provide an effective, reliable process for depositing a magnetic film, to wit CrO.sub.2 on a substrate.

It is another object of this invention to provide a process whereby the magnetic film can be deposited at room temperature.

It is a further object of the invention to provide a process in which the magnetic film can be deposited only on predetermined portions of the surface of a substrate, or else uniformly over the entire substrate surface, as required.

According to an aspect of this invention an organometallic material, to wit, chromium hexacarbonyl, is decomposed in a vacuum chamber by a laser assisted or a plasma assisted chemical vapor deposition technique. A suitable substrate, which can be a semiconductor wafer or a glass or plastic sheet, is placed into a chamber which is evacuated down to a typical background pressure of about 10.sup.-5 Torr, i.e., 10.sup.-2 milliTorr. The organometallic material is introduced as a vapor into the chamber, providing a partial pressure of about 10 milliTorr. This pressure is not especially critical. Oxygen or another oxidizing agent such as NO is introduced at a suitable pressure, e.g. 15 to 100 milliTorr. The chromium hexacarbonyl is subjected to photolytic or plasma decomposition. In the case of photolytic deposition, a laser beam is directed at the portion of the substrate on which the chrome oxide is to be deposited. The substrate is kept near or below room temperature, for example, using a copper block that can be kept cool with chilled water. The decomposition of the hexacarbonyl is surface mediated, and occurs only where the laser beam is incident onto the substrate. The laser beam wavelength is in the ultraviolet range, preferably near 230 nm, but acceptable results were produced using a nitrogen laser having a wavelength of 337 nm. Plasma assisted deposition can employ an RF plasma field e.g., 13.56 MHz, at 200 watts. In this case, the substrate can be heated by a cartridge heater to a temperature of 150.degree. C. to 300.degree. C. (323 K to 573 K). In either case, the O.sub.2 or NO partial pressure should be above about 15 milliTorr so that the film is formed as CrO.sub.2, rather than Cr.sub.2 0.sub.3.

For application of molydenum oxide or tungsten oxide films, the organometallic material can be molydenum hexacarbonyl or tungsten hexacarbonyl.

Selective deposition of CrO.sub.2 thin films, which can be accomplished with this invention, is of great interest because CrO.sub.2 is a ferromagnetic compound at room temperature with a Curie temperature of 118.degree. C. (391 K). These CrO.sub.2 films can be magnetized and demagnetized with low input of energy, making them ideal for microminiature magnetic memory applications. The films also have pronounced magneto-optic properties in the visible spectrum, including a substantial Kerr effect.

We have produced chromium dioxide films by photolytic and plasma deposition using Cr(CO).sub.6 and oxygen. Other materials have been employed satisfactorily as well. Chromacene (C.sub.5 H.sub.5).sub.2 Cr, CrO.sub.3 and benzene chromium Cr(C.sub.6 H.sub.6).sub.2 have been successfully used in place of the chromium hexacarbonyl. Nitrogen monoxide NO or ozone 0.sub.3 can be used as the oxidizing agent in place of oxygen 0.sub.2.

A typical vacuum system as is commonly used for vapor deposition can be employed here, and thus need not be described in any detail. In an experiment, a silicon substrate was placed in a solid brass substrate holder inside the vacuum chamber, and the holder was cooled by a chilled water system. Background pressure was about 10.sup.-5 Torr. The chromium hexacarbonyl was introduced into the vapor system by sublimation of the crystalline solid, at a pressure of about 10 milliTorr. This material has a known pressure relation given by the relation

where P is the pressure in Torr and T is temperature of the hexacarbonyl in degrees Kelvin.

Oxygen was introduced into the chamber at sufficient pressure to produce the desired CrO.sub.2, rather than Cr.sub.2 0.sub.3. Preferrably the oxygen should have partial pressure exceeding 15 milliTorr, up to or exceeding 100 milliTorr.

The holder was maintained at temperature between 288 K and 294 K, although this was not critical. At higher temperatures, e.g., up to 315 K, the film surface became somewhat rough.

Photolytic decomposition of the Cr(CO).sub.6 was carried out with a commercial pulsed nitrogen laser in the near ultraviolet (337 nm). The laser beam had a power of 330 mw, which was 4.5 mJ per pulse. The beam dimension was 6 mm.times.32 mm and was focused through a quartz window onto the silicon substrate. Photolytic deposition times ranged from 3 hours to 48 hours.

The chrome oxide coatings were examined in place on the silicon substrate, using scanning electron microscopy (SEM), x-ray emission spectroscopy (XES), and Auger electron spectroscopy (AES). Magneto-optic Kerr effect (MOKE) measurements were undertaken with a 5 mw He-Ne laser, two crossed polarizers, and a chopper wheel run at 800 Hz. All measurements were performed in air. The applied magnetic field was changed at a rate of 15 Oerstads per second, to a maximum of 1,500 oe. The reflected light was detected using a photodiode and transduced to a voltage, the signal being obtained using a lock-in amplifier.

XES spectra of the deposited films provided clear evidence of chromium deposition on temperature of the substrate. For example there was found to be a three-fold increase in the rate of deposition with six degrees drop, with evidence to indicate that the decomposition is a surface process. At lower temperatures, the surface residence time of the organometallic species increases, increasing the probability of removing a carbonyl ligand before desorption and reevaporation.

The cross-section of gaseous Cr(CO).sub.6 is greater at short wavelengths near 230 nm, although the deposition rate of chromium has been observed to be substantial at longer ultraviolet wave lengths, e.g. 337 nm.

Formation of CrO.sub.2 from photolysis of chromium hexacarbonyl results in relatively smooth films for deposition at substrate temperatures up to 294 K. At higher substrate temperatures, e.g. 315 K and above, the film surface becomes somewhat uneven.

At higher temperatures, some interdiffusion of both oxygen and chromium into the silicon substrate was observed. This can lead to non-uniform composition at the interface of the chromium dioxide and the silicon. However, this is not a problem at higher deposition rates as very rapid film growth leads to far more uniform films Also, to prevent diffusion into the silicon, a diffusion barrier can be applied as a sublayer below the chrome oxide.

For very rapid thin film growth where selective area processing is not required, plasma assisted deposition of chromium oxides can be undertaken. In this case, the substrate is placed in any typical plasma reactor whose chamber is evacuated to a background vacuum of e.g. 10.sup.-5 Torr. The sample is maintained at a given temperature, e.g. 242.degree. C. or 515 K, and a Cr(CO).sub.6 partial pressure of 10 mTorr is maintained. With a partial pressure of oxygen between 3 mTorr and 15 mTorr, the film composition is uniformly Cr.sub.2 O.sub.3, as determined by Auger depth profiling. Fabrication of CrO.sub.2 requires oxygen partial pressure in excess of 15 mTorr.

The deposited chrome oxide layers from this process are surprisingly free of contamination. Very little carbonyl incorporation has been observed to occur, despite the high rate of film growth. The rate of film growth under these operating conditions was approximately 3.0 nm/minute.

MOKE studies of these CrO.sub.2 films showed characteristics of ferromagnetism. It was also clear that antiferromagnetic chrome oxide layers can also be formed.

Because of the low-to-moderate temperatures employed in either method, it is clear that chromium dioxide ferromagnetic coatings can be applied to plastic substrates (e.g. Mylar) or other substrates commonly used in magnetic storage devices.

Cobalt can be doped into these CrO.sub.2 films e.g. by introducing a vapor of cobaltocene during or after deposition of the CrO.sub.2, Cr0.sub.3. Also, instead of chromium hexacarbonyl, chromacene (C.sub.5 H.sub.5).sub.2 Cr or benzene chromium (C.sub.6 H.sub.6).sub.2 Cr can be employed as the chromium-contributing organometallic material. It has also been found that films of oxides of other group VIB metals, i.e. WO.sub.2 and MoO.sub.2 can be deposited using tungsten hexacarbonyl and molybdenum hexacarbonyl as the organometallic material.

While the invention has been described with reference to preferred embodiments, it is clear that the invention is not limited only to these illustrative embodiments. Rather, many modifications and variations will present themselves to those skilled in the art without departure form the scope and spirit of this invention, as defined in the appended claims.