JPS5815446B2 - Method for forming zinc oxide film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a zinc oxide film used in surface wave devices and the like.

Piezoelectric materials used as surface wave elements include piezoelectric single crystals, piezoelectric ceramics, and piezoelectric thin films, but piezoelectric thin films are considered to be the best in terms of characteristics and cost, and among these piezoelectric thin films, zinc oxide is the most suitable. Used a lot.

Methods for forming zinc oxide films include sputtering, ion plating, I, C, B, and D methods, but the sputtering method is most commonly used due to its crystal axis orientation and resistivity. There is.

However, this sputtering method has major problems in that the deposition rate of the film is slow and that when the film thickness reaches about 20 μm, it becomes cloudy and the stress on the film becomes large, causing damage.

In order to solve the above-mentioned drawbacks of the sputtering method, the inventor has already proposed a method for forming a zinc oxide film by reactive beam evaporation and has conducted repeated experiments.

The present invention relates to a method for forming a zinc oxide film using this reactive beam evaporation method.

The equipment used for the reactive beam evaporation method includes an evaporation system, a reaction system,
It is composed of a film growth system.

The evaporation system consists of a crucible and a heating heater, and the zinc is heated and vaporized in the crucible, and then injected into the bell jar as an injection beam from the nozzle of the crucible.

The reaction system includes a high-frequency discharge excitation coil and a magnet to activate the atmospheric gas and enhance the reaction between oxygen and zinc.

The film growth system includes a substrate and a heater that heats the substrate.

If this reactive beam evaporation method is used, the amount of zinc evaporated can be increased, and the high-frequency plasma is densified and reactivity is increased, so that the growth rate of the film can be significantly accelerated.

However, when a zinc oxide film is formed on a substrate using this reactive beam evaporation method, the substrate may warp so that the film surface becomes concave, causing cracks within the film or peeling of the film from the substrate. Sometimes it happened.

This indicates that the stress within the film is tensile stress.

The present invention aims to solve the above problem by changing the warpage of the substrate so that the film surface becomes convex or warped.

The present invention was made based on an experimental result that the warpage of a substrate is related to the bias applied to the substrate, and the above object is achieved by applying a high frequency bias to the substrate.

A method for forming a zinc oxide film according to the present invention will be described below with reference to the drawings.

The figure is a front sectional view showing an example of an apparatus for forming a zinc oxide film according to the present invention.

A crucible 11 and a heating heater 12 are provided as a steam source in a high-vacuum bell jar 10, and zinc in the crucible 11 is heated.

The heated zinc is transferred from the nozzle of crucible 11 to bell jar 10.
Injected inside.

Oxygen gas is introduced into the bell jar 10 from an intake pipe 13.

A high frequency electric field is applied from a high frequency power source 15 to the high frequency discharge excitation coil 14 to generate a high frequency discharge, activating oxygen in the atmosphere and promoting reaction with zinc.

Note that in order to trap this high-frequency plasma and increase the density in the discharge region, the magnet 16 is connected to the coil 14.
, so that magnetism is generated in a direction perpendicular to the electric field of the coil 14.

The substrate 11 is heated by the heater 18, and zinc oxide is deposited on its surface to form a film.

Normally, the substrate 11 has a positive DC potential difference with respect to the bell jar due to the attachment of ions.

A high frequency bias is applied to the substrate 17 .

In order to apply the high frequency bias, a high frequency electric field excited in the substrate 17 by high frequency plasma generated in the bell jar 10 by the high frequency discharge excitation coil 14 is used as the high frequency bias.

Therefore, a DC bias and a high frequency bias are applied in a superimposed manner.

To this end, the substrate 17 is covered with a metal plate 19, and an inductance element 20 and a capacitance element 21 are connected to this metal plate 19 to form a matching circuit, and in this case, the bias applied to the capacitance element can be adjusted as variable. It is.

When the matching circuit is adjusted to resonate with the high frequency electric field generated in the substrate 17, the electric field of the substrate 17 is grounded through the matching circuit, and the DC potential of the substrate 17 becomes the lowest.

As the resonance state is removed, the matching circuit acts as an impedance, so the potential of the substrate 17 becomes higher.

While adjusting the matching circuit as described above, the high frequency bias of the substrate 17 is adjusted.
Controlling the potential (voltage) of the substrate 17 was effective when the voltage generated on the substrate 17 was in the range of +10 to -70V, and when it exceeded this, the crystallinity of the zinc oxide film deteriorated and the piezoelectric properties deteriorated.

As described above, the method for forming a zinc oxide film according to the present invention includes:
The high-frequency electric field induced by the high-frequency electric field excited in high-frequency plasma is used as a high-frequency bias, and the DC potential of the substrate is adjusted while adjusting the high-frequency bias using a matching circuit.

Although the matching circuit is constructed so that the potential of the substrate is in the range of 110 to -70 V, the matching circuit is not limited to the LC series resonant circuit as described above.

Further, the inductance element may be made variable.

When a high frequency bias was applied to the substrate and the potential of the substrate was adjusted, the warp of the substrate changed to a convex surface as shown in the following table, and the film stress changed to compressive stress, thus solving the above problem.

However, the voltage generated on the substrate is suitably in the range of +10 to -70V, and if it is below -70V, the crystallinity will deteriorate.

According to the present invention, the amount of warpage of the substrate can be adjusted by applying a high frequency bias to the substrate, so that
Even when forming a relatively thick zinc oxide film, cracking and peeling of the film can be prevented.

There is no need to use special equipment for applying the bias, and only slight modifications to conventional equipment are required.

As described above, the present invention makes it possible to efficiently manufacture a surface acoustic wave device in which a zinc oxide film is formed on a glass substrate, and to improve the yield.

BRIEF DESCRIPTION OF THE DRAWINGS The figure shows a front sectional view of an example of an apparatus for carrying out the invention. 11... Crucible, 14... Coil for high frequency discharge excitation, 17... Substrate, 19...
・Metal plate, 20...A, Ductance element, 21
...Capacitive element.

Claims (1)

[Scope of Claims] 1. A method for forming a zinc oxide film in which zinc is vaporized by heating and reacted with oxygen in the atmosphere in a high frequency discharge region to deposit zinc oxide on the substrate surface. A method for forming a zinc oxide film, comprising: connecting a resonant circuit for adjusting an induced high-frequency electric field to the substrate; and controlling the voltage generated on the substrate depending on the resonant state of the resonant circuit. 2. The method of forming a zinc oxide film according to claim 1, wherein an inductance element and a capacitance element are used in the resonant circuit, and at least one of them is made variable.