GB2158104A

GB2158104A - Method for producing a thin film

Info

Publication number: GB2158104A
Application number: GB08510576A
Authority: GB
Inventors: Masashi Kasaya; Tatsijolp Abe
Original assignee: Diesel Kiki Co Ltd
Current assignee: Bosch Corp
Priority date: 1984-05-04
Filing date: 1985-04-25
Publication date: 1985-11-06
Also published as: JPH0237426B2; GB8510576D0; KR850007987A; DE3515807A1; KR900009101B1; JPS60234965A

Abstract

A method for producing a thin film on the surface of a member, in which the ionized vapor of a specific metal, such as Zirconium, is reacted with a specific reaction gas to form a predetermined compound is characterised in that the ratio of the partial pressure of the metal vapor to that of the reaction gas is gradually changed, so that the composition of the thin film (26) is gradually varied in the direction of the thickness of the thin film. An electric potential is applied to said member to cause the ionized material in the ionizing chamber to deposit on said member.

Description

SPECIFICATION Method for producing a thin film The present invention relates to a thin film producing method for coating and forming a thin film on the surface of a desired member by a physical or chemical evaporating method, such as an ion-plating method, a sputtering method or a chemical vapor deposition (CVD) method.

It is, for example, sometimes required to form a thin film, such as a thin insulation layer, on the surface of metal. As a conventional method for producing such a thin film layer there can be mentioned British Patent Application Disclosure No.

2123441, which discloses a method for forming a metal-glass compound film on the surface of metal by a reactive coating means. However, it is difficult to realize a tightly bonded state between the base metal and the thin film layer thereon by the disclosed method. That is, there is a disadvantage that the thin film layer is liable to peel off due to the stress produced between the base metal and the coated thin film owing to the difference in their coefficients of thermal expansion or due to mechanical force applied from outside.

It is therefore an object of the present invention to provide an improved method for producing a thin film by which the thin film can be tightly bonded on the surface of the base material.

It is a preferred objective of the present invention to provide a thin film producing method which is capable of tightly bonding a thin film made of an electrically conductive or an electrically non-conductive material on the surface of a member made of metal or other material.

According to the present invention, in a thin film producing method for providing a thin film on the surface of a desired material by an evaporating method, a vapor of a desired metal and a reaction gas which reacts with the metal to form a predetermined compound are introduced into a reaction chamber in which the member to be coated is placed and at least the portion of the vaporized metal is ionized. A predetermined electric potential is applied to the member to be coated, so that the surface of the member is coated with the ionized metal or the ionized compound of the metal and the reaction gas.At this time, the pressure of the reaction gas is controlled in such a way as to gradually vary the ratio between the partial pressure of the vapor of the metal and the partial pressure of the reaction gas, so that the composition of the thin film is gradually varied in the direction of the thickness of the thin film.

In the case that the member to be coated by the thin film is metal, at first, the partial pressure of the reaction gas is set at zero, and the metal layer is formed on the surface of the member. After this, the partial pressure of the reaction gas is gradually increased to form a non-stoichiometric compound region made of a metal vapor and the reaction gas on the metal layer. Finally, the pressure of the reaction gas is increased to a sufficient value for forming a desired compound made of the metal and the reaction gas. As a result, it is possible to obtain a thin film consisting of metal in the vicinity of the inner surface thereof, a predetermined metal compound in the vicinity of the outer surface thereof and a non-stoichiometric compound therebetween whose composition depends upon the partial pressure of the reaction gas during formation.

When, for example, the metal compound is an insulation material, it is possible to form an insulation layer on the metal member.

On the other hand, the present invention can also be applied for the formation of a thin metal film on a member made of the same material as that of the metal compound. In this case, at the start of the formation of the thin film, the partial pressure of the reaction gas is set at large value, under which condition the desired metal compound can be formed. After this, the partial pressure of the reaction gas is gradually decreased, whereby it becomes possible to form a metal in the vicinity of the outer surface of the formed thin film.

The thin film can be readily formed by a physical evaporation method such as an ion-plating method wherein an ionized metal such as Zr, Cr or Al vaporized from a vapor source is reacted with a reaction gas such as 02, N2 or C2H2 and the resulting compound is deposited on the surface of the member to be coated while the concentration of the reaction gas is simultaneously controlled so as to gradually increase or decrease the concentration of the reaction gas.

For instance, when zirconium (Zr) is selected as the metal and 02 is selected as the reaction gas, the thin film can be formed as follows. The member is disposed in an evaporation chamber which is then evacuated prior to the formation of the thin film. Next, in accordance with the ion-plating method, Zr is evaporated and the resulting Zr ions are deposited on the member to form a metal (Zr) layer. Subsequently, 2 is introduced into the chamber and the 02 concentration in the chamber is gradually increased at a prescribed rate. As a result there is further formed a layer made of a nonstoichiometric compound representable as ZrO,, on the outer surface of the metal (Zr) layer, and finally, the compound in vicinity of the outer surface of the desired thin film becomes stoichiometric ZrO2.The amount of oxygen of the layer thus increases from the inner surface of the thin film toward the outer surface thereof.

Since the inner side of the resulting thin film is a metal (Zr) layer containing little or no oxygen which adheres very tightly to the surface of the metal member, excellent adherence is obtained between the thin film and the member to be coated.

On the other hand, since the outer surface of the thin film can, if necessary, be made to constitute a hard insulating material, desired electrical insulation can be obtained. It is thus possible to realize a thin film that is excellent in both insulating property and resistance to abrasion and peeling.

The thin film can be formed by the conventional evaporating method modified only to permit con trol of the reaction gas concentration, making it easy to produce a thin film having excellent hardness and durability.

The invention will be better understood and the other objects and advantages thereof will be more apparent from the following detailed description of preferred embodiments with reference to the accompanying drawings.

Brief description of the drawings Figure 1 is a cross sectional view showing a fuel injection valve, the needle valve of which has been coated with a thin film in accordance with an embodiment of the present invention; Figure 2 is a graph indicating the composition of the thin film formed on the valve shown in Figure 1; and Figure 3 is a schematic view of the ion-plating unit used for forming the thin film shown in Figure 1.

Description of the preferred embodiments Figure 1 shows a fuel injection valve 1 constructed to function not only as a fuel injection valve but also as an electric switch. The fuel injection valve 1 has a nozzle holder 2, a plate member 3 and a nozzle 4, which are threaded into a sleeve nut 5. The nozzle 4 is composed of a nozzle body 6 and a needle valve 8 received in a guide hole 7 so as to be smoothly slidable therein. A conical member 9 which serves as a valve member is formed at the end portion of the needle valve 8 and a valve seat 10 the shape of which matches the conical member 9 is defined in the nozzle body 6. A chamber 11 is defined in the nozzle body 6 adjacent to the valve seat 10 and the chamber 11 is communicated with a fuel path 12.

The needle valve 8 is made of steel and is electrically connected to a conductive spring seat 14 through a conductive pin 13 when the fuel injection valve 1 is in closed condition.

A coil spring 16 is received in a spring chamber 15 defined in the nozzle holder 2, and one end portion of the coil spring 16 is supported by a shoulder portion 20 formed in the spring chamber 15 via a disc portion 19 formed at the lower end of an electrode 18 inserted into an insulation sleeve 17 in a force- fit condition while the other end of the coil spring 16 is supported by the spring seat 14. The insulation sleeve 17 is provided for insulating the conductive nozzle holder 2 from the electrode 18 and may be inserted into a hole 21 of the nozzle holder 2 snugly or with some clearance. Reference numerals 22 and 23 denote O-rings for maintaining oil-tight condition.

The coil spring 16 is also made from a suitable electrically conductive material such as steel, so that the electrode 18 and the needle valve 8 are in electrically connected condition through the pin 13, the spring seat 14 and the coil spring 16. To prevent the coil spring 16 from coming into electrical connection with the nozzle holder 2, there is provided an insulation sleeve 24, which is especially necessary in a small size fuel injection valve because of the small distance between the coil spring 16 and the wall surface of the spring chamber 15.

The nozzle body 6, the plate member 3, the sleeve nut 5 and the nozzle holder 2 are also made from electrically conductive materials.

In order to maintain the electrical insulation between the outer surface 8a of the larger diameter portion of the needle valve 8 and the inner surface of the guide hole 7 of the nozzle body 6, the needle valve 8 is coated with a thin film 26 which can be formed by the method according to the present invention.

In this embodiment, the thin film 26 is a composition represented by ZrO2,, wherein x varies from zero in the vicinity of the outer surface thereof to 2 in the vicinity of needle valve 8. That is to say, the thin film 26 is made of zirconium oxide (ZrO2) in the vicinity of the outer surface thereof, is formed of a Zr compound whose oxygen content 6 gradually decreases inwardly in the intermediate region thereof, and is formed solely of Zr in the vicinity of the needle valve 8. This is graphically represented in Figure 2 which shows the thin film 26 to be composed of only metal (Zr) in the region I from t = 0 at the surface of the needle valve 8 to t = t1, and of Zr2 in the region II from t = t2 to t = to at the outer surface.

Between the regions I and II is a transition region III defined by t1 > t > t2. In the region Ill, the thin film 26 is composed of a non-stoichiometric compound represented by ZrO2 x, where x varies from 2 to 0. As a result, the electrical resistance of the thin film 26 becomes progressively higher with increasing distance from the needle valve 8 and increasing proximity to the wall surface of the guide hole 7.

When the thin film 26 is made to have the structure shown in Fig. 2, the region I, i.e. the metal layer, adheres tightly to the metal of the needle valve 8, while high insulation between the needle valve 8 and the nozzle body 6 and excellent resistance to abrasion are guaranteed by the region II, i.e. the ZrO2 region. Moreover, the regions I and II, which are of different nature, are strongly bonded with each other by the transition region Ill. Consequently, the thin film 26 as a whole has excellent resistance to peeling and abrasion so that there can be realized a fuel injection valve having a switch with excellent durability.

Furthermore, the coefficient of thermal expansion of the transition region Ill is between those of the regions I and II and gradually changes in the thickness direction. Consequently the resistance of the thin film to peeling caused by thermal shock is remarkably improved.

Now, the method of forming a thin film 26 of the cross sectional structure shown in Fig. 2 on the surface of the needle valve 8 will be described with reference to Fig. 3.

The needle valve 8 is disposed a vacuum chamber 31 and connected through a switch SW to the negative electrode of a high voltage d.c. source 32.

An evaporation source or evaporation vessel 34 is disposed on a partition 33 and connected to the positive electrode of the high voltage d.c. source 32. Within the evaporation vessel 34 is disposed a quantity of Zr which is fused and evaporated by bombardment with electrons from an electron gun 35. The chamber 31 is evacuated and maintained at a prescribed vacuum pressure by a vacuum pump 36.

After the prescribed degree of vacuum has been attained in the vacuum chamber 31, Ar gas is introduced from a cylinder 40 through a valve 39.

The switch SW is closed to apply the d.c. voltage between the needle valve 8 and the evaporation vessel 34, causing a glow discharge for cleaning the interior of the chamber 31. After cleaning is fin ished, the Zr is vaporized and the resulting Zr ions are made to deposit on the surface of the needle valve 8 by the high negative voltage applied to the needle valve 8 at this time.

As a result, the region I is formed. Although not illustrated, ionization of the Zr is expedited by the high frequency method or the thermionic method.

When the region I has been formed to the prescribed thickness, a valve 37 is opened and oxygen (the reaction gas) is gradually introduced into the vacuum chamber 31 from the cylinder 38. By this operation, the transition region lli indicated by ZrO2x begins to be formed on the region I. The partial pressure of the reaction gas within the vacuum chamber 31 is controlled to increase gradually over time so as to form a transition region Ill having a gradient of oxygen content as illustrated in Fig. 2.

This operation is continued until finally the compositon of the deposited material becomes ZrO2, whereafter the region II is formed to a predetermined thickness on the transition region ill.

In the manner described above, mere control of the partial pressure of the reaction gas enables formation of a thin film 26 having the structure shown in Fig. 2 by the use of the conventional ionplating method.

In the foregoing embodiment, Zr is used as the evaporation material while O2 is used the reaction gas but it is apparent that the materials for the disposed layer are not limited to these and other nonorganic insulating materials may be used instead.

Accordingly, Al, Cr, Si or the like may be used as the evaporation material while N2, C2H2 or the like may be used as the reaction gas.

It is, however, necessary to avoid the use of an evaporation material that together with the metal of the member to be coated forms an intermetallic compound which rapidly changes in nature, and also to avoid combinations of evaporation material and reaction gas which produce a compound that interacts with the evaporation material to deteriorate the physical properties of either the evaporation material or the compound.

When the thin film 26 is formed by the ion-plating method as described, the processing temperature during the deposition can be lowered, e.g. to less than 550 C, so that the needle valve, which has been heat treated prior to formation of the thin film 26, does not develop strain and is not tempered. In addition, the present invention has an outstanding advantage in that it entails no danger of environmental contamination since the coating process is carried out by the dry system within the vacuum chamber.

In the embodiment described above, although the present invention is explained for the case where the thin film is formed on the outer surface of the metal and the outer surface of the thin film is formed as an insulation portion, the thin film producing method according to the present invention is not limited to the embodiment described above. That is, the present invention can be simi larly applied to the case where it is required to de posit the evaporated metal on the surface of a compound produced by the evaporated material and the reaction gas. Effects similar to those de scribed above will be obtained in this case, too.

Furthermore, in the embodiment described above, there is shown an arrangement wherein the evaporated metal is supplied from the evaporation source 34 placed in the vacuum chamber 31. However, the evaporated metal may be supplied into the vacuum chamber 31 from the outside.

The thin film producing method according to the present invention is not limited to use of the ionplating method but may also be worked using the sputtering method, CVD method or any other physical or chemical evaporating method.

Claims

1. A method for producing a thin film on the surface of a member to be coated, said method comprising the steps of: in a reacting chamber in which said member to be coated is placed, reacting the ionized vapor of a specific metal with a specific reaction gas which reacts with the specific metal to form a predetermined compound, the ratio of the partial pressure of said metal vapor to that of said reaction gas being gradually changed during the reaction; and applying a predetermined electric potential to said member to be coated to cause the ionized material in the reaction chamber to deposit on said member to be coated by an evaporation method.

2. A method as claimed in Claim 1 wherein the evaporation method is an ion-plating method.

3. A method as claimed in Claim 1 wherein the partial pressure of the specific reaction gas is controlled so as to gradually increase from zero, whereby the innermost surface of said thin film is formed of the specific metal.

4. A method as claimed in Claim 3 wherein the partial pressure of the specific reaction gas is controlled in such a way that the outermost surface of said thin film is formed of the complete compound obtained by reacting the specific metal and the specific reaction gas.

5. A method as claimed in Claim 4 wherein the intermediate portion of said thin film between its outer and inner surfaces is formed of non-stoichiometric compound of the specific metal and the specific reaction gas.

6. A method as claimed in Claim 1 wherein said metal is one member selected from the group consisting of Zr, Cr and Al.

7. A method as claimed in Claim 1 or 6 wherein said specific reaction gas is one member selected from the group consisting Oi;2, N2 and C2H2.

8. A method as claimed in Claim 3 wherein a transition region whose electric resistance varies progressively in the direction of thickness and a metal region of prescribed thickness formed of the specific metal are formed as a part of said thin film.

9. A method as claimed in Claim 8 wherein the evaporation method is an ion-plating method.

10. A method as claimed in Claim 9 wherein the concentration of the specific reaction gas is controlled during the formation of said thin film, whereby the partial pressure of the reaction gas is controlled.

11. A method as claimed in Claim 8 wherein an insulating region of a prescribed thickness is further formed on the outermost surface of the transition region, said insulation region being formed of the complete compound obtained by reacting the specific metal and the specific reaction gas.

12. A method as claimed in Claim 11 wherein the evaporation method is an ion-plating method.

13. A method as claimed in Claim 1 wherein the partial pressure of the specific reaction gas is controlled so as to gradually decrease from a predetermined high level, whereby the innermost surface of said thin film is formed of the complete compound obtained by reacting the specific metal and the specific reaction gas.

14. A method as claimed in Claim 13 wherein the outermost surface of said thin film is formed of the specific metal.

15. A method as claimed in Claim 14 wherein the intermediate portion of said thin film between its outer and inner surfaces is formed of non-stoichiometric compound of the specific metal and the specific reaction gas.

16. A method as claimed in Claim 1 and substantially as hereinbefore described.