GB2178064A

GB2178064A - Producing a thin film by reactive evaporation

Info

Publication number: GB2178064A
Application number: GB08617904A
Authority: GB
Inventors: Masashi Kasaya
Original assignee: Diesel Kiki Co Ltd
Current assignee: Bosch Corp
Priority date: 1985-07-22
Filing date: 1986-07-22
Publication date: 1987-02-04
Also published as: KR920000590B1; KR870001326A; DE3624772A1; JPH0467724B2; GB8617904D0; JPS6222314A; DE3624772C2

Abstract

In a thin insulating film producing method by means of an evaporating method, an ionized vapor of a specific metal (Zr) and a reaction gas (40) (O2, N2, C2H2) which reacts with the specific metal to form a predetermined insulating compound are reacted in a bell jar (31) in which a member (8) to be coated is placed, and at this time, the pressure of the reaction gas (40) is controlled so as to gradually vary the ratio between the partial pressure of the ionized vapor of the metal (Zr) and the partial pressure of the reaction gas in order to gradually vary the composition of the thin film in the direction of the thickness of the thin film while the level of the voltage applied to the member (8) is gradually lowered as the formation of the insulating compound progresses, whereby a high quality insulation compound can be formed on the uppermost surface of the thin film due to no punctures occurring. <IMAGE>

Description

SPECIFICATION Method for producing a thin film Background of the invention The present invention relates to a thin film producing method suitable for coating and forming a thin insulation film on the surface of a desired member by a physical evaporating method, such as an ion-plating method or a sputtering 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.

In the case of the formation of a thin film by a physical evaporating method such as an ion-plating method, when the level of the voltage applied to the base metal on which the thin film is coated is increased, the collision energy of ion is increased, so that the thin film is strongly adhered to the base metal.

However, in a case where a thin insulation film is to be coated, a high quality insulation film cannot be formed due to a puncture occurring on the surface of the base metal when the applied voltage is increased, thereby it is difficult to form a thin insulation layer with good adherence to its desired base metal.

Summary of the invention It is an object of the present invention to provide an improved method for producing a thin insulation film by means of a physical evaporating method.

It is another object of the present invention to provide a method for producing a thin film which is capable of forming a thin insulation film having good peeling resistance on the surface of the base metal.

According to the present invention, in a thin film producing method for providing a thin insulating film on the surface of a desired material by an evaporating method, an ionized vapor of a specific metal and a reaction gas which reacts with the specific metal to form a predetermined insulating compound are reacted in a reaction chamber in which the member to be coated is placed. At this time, the pressure of the reaction gas is controlled so as to gradually vary the ratio between the partial pressure of the ionized vapor of the metal and the partial pressure of the reaction gas while the level of the voltage applied to the member to be coated in order to coat the ionized material in the reaction chamber on the member being coated by means of an evaporating method, is controlled so as to lower as the formation of the insulating compound progresses.

The ionized vapor of the metal can be obtained by ionizing the vapor of the metal by an appropriate ionizing means, whereby at least one part of the vapor of the metal will be ionized in the reaction chamber. A material which has good adherence with the associated coating is selected as the specific metal and a metal layer is firstly formed on the surface of the member being coated by reducing the partial pressure of the reaction gas to zero.

In this case, the voltage applied to the member being coated is set comparatively high, and the ionized metal collides with a high energy level to the surface of the member being coated, so that a metal layer is formed on the surface of the member being coated with high adherence. In this case, an ion-implantation effect due to the high voltage application can be expected.

After this, the partial pressure of the reaction gas is gradually increased in order to form a non-stoichiometric compound obtained by the reaction between the vapor of metal and the reactive gas on top of the metal layer formed as a ground layer. Finally, the partial pressure of the reaction gas is increased to a pressure sufficient for the formation of the desired insulation compound obtained by the chemical combination of the metal and the reaction gas. At the same time as the partial pressure of the reaction gas is increased, the level of the voltage applied to the member being coated is gradually lowered, so that the insulation compound is adhered to the member being coated by the metal layer at a low voltage level.

As a result, the metal layer as the ground layer is strongly adhered to the member being coated by the application of a high voltage, and the desired insulation compound is formed on the metal layer through the non-stoichiometric compound by the application of a comparatively low voltage. Therefore, a thin insulation film can be formed without losing the quality of the insulation compound while maintaining the high adherence of the insulation compound to the member being coated.

The invention will be better understood and 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 partial cross-sectional view showing an embodiment of a fuel injection valve having a valve member on a sliding surface of which an insulation film is formed in accordance with 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 apparatus used for forming the thin film shown in Figure 1.

Description of the preferred embodiment Figure 1 is a partial cross-sectional view showing an embodiment of a fuel injection valve having a valve member on a sliding surface of which an insulation film is formed in accordance with the present invention. A 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 body 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 an 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 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 neddle 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 is formed by a forming method according to the present invention.

In this embodiment, the thin film 26 is a composition represented by ZrO2~x, 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 which 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 ZrO2 in the region II from t t2 to t = t0 at the outer surface.

Between the regions I and II is a transition region Ill defined by t, < t < t2. In the region Ill the thin film 26 is composed of a non-stoichiometric compound represented by Zero2 x, where x varies from 2 to 0. As a result, the electrical resistance of the layer 26 becomes progressively higher with increasing distance from the needle valve 8 and increasing proximity to the wall of the guide hole 7.

When the thin film 26 is made to have the structure shown in Figure 2, the region I, i.e. the metal layer, adheres strongly 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 11, 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 III. 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, a coefficient of thermal expansion of the transition region ill assumes an intermediary value between those of the regions I and II, and gradually varies along the direction of thickness. Therefore, it is advantageous in that it has an excellent resistance to peeling against thermal shock caused by heating.

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

The needle valve 8 is disposed within a bell jar 31 and is 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. An ionizing electrode 41 for regulating the speed of a plating operation is located between the evaporation vessel 34 and the valve member 8 to be coated and is connected to the negative electrode of a variable d.c. voltage source 42 having a positive electrode which is grounded. Therefore, when the voltage of the variable d.c. voltage source 42 is regulated during the zirconium (Zr) in the evaporation vessel 34 is fused and evaporated by bombardment with electrons from an electron gun 35, it is possible to regulate the speed of the ion-plating operation by the operation of the ionizing electrode 41. The bell jar 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 bell jar 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 bell jar 31. After cleaning is finished, the Zr is vaporized and the resulting Zr ions are made to plate 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 formed. Although not illustrated, ionization of the Zr is expedited by the high frequency method or the thermionic method.

In order that the region I, i.e. metal layer, adheres strongly to the needle valve 8, during the region I is formed, the variable high d.c. voltage source 32 is regulated so as to increase its output level during when the region I is formed. Consequently, Zr ion collides with high energy with the desired outer surface of the needle valve 8, so that the Zr metal layer can be effectively adhered to the needle valve 8.

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 bell jar 31 from the cylinder 38. By this operation, the transition region Ill indicated by Zero2 x begins to be formed on the region I. The partial pressure of the reaction gas within the bell jar 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 Figure 2. As the partial pressure rate of the oxygen gas increases, the physical properties of the compound formed changes from one which is electrically conductive to one which provides insulation.As previously stated, in the case where the evaporating matter has an insulating property, the quality of the material as an insulation film of the thin film to be coated is impaired, due to the puncture on the surface of the member to be coated when the applied voltage is high.

In order to avoid this, each output of the variable high d.c. voltage sources 32 and 42 are regulated in such a way that the output level is gradually decreased as the partial pressure rate of the oxygen gas increases, so that no puncture arises on the surface of the needle valve 8.

Finally, the condition for forming the thin film is brought to a condition in which ZrO2 can be formed on the needle valve 8 without the occurrence of the puncture on the surface of the needle valve 8. This operation is continued until finally the composition of the deposited material becomes ZrO2, whereby the region II is formed 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 Figure 2 by the use of the conventional ion-plating method. Furthermore, since the level of high voltage applied to the needle valve 8 is controlled so as to vary as described above at the same time as the control of the partial pressure of the reaction gas, it is possible to obtain a high quality insulating region as the surface portion of the thin film 26. As a result, an insulating thin film with durability and high abrasion can be obtained.

According to the method mentioned above, the interior of the thin film 26 is a compound including only a little oxygen or a pure metal both of which have a tendency to strongly adhere to metal. As a result, the thin film can strongly adhere to the surface of a member to be coated due to the metal layer portion (Zr layer portion) thereof. Further more, since the high quality insulating layer is strongly adhered to the metal layer through the transit layer, the high quality thin insulation film can be coated with high adherence on the surface of the needle valve 8.

In the foregoing embodiment, Zr is used as the evaporation material while O2 is used as the reaction gas. It is however apparent that the materials for the disposed layer are not limited to these and other non-organic 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.

However, it is necessary to avoid the use of evaporating metals that would rapidly change in property together with the metal of the evaporation material and the associated coating member so as to form a metallic compound. Further, the combination must be avoided between a metal and a reaction gas that forms a compound that acts with the evaporation material and which would change suddenly in property with either one or the other of the metal of the evaporation material or the compound formed.

Furthermore, although in the above-mentioned embodiment a construction which supplies a metallic gas from an evaporation vessel 34 located within the vacuum chamber 31 is shown, it is also possible to introduce the metallic gas from outside the vacuum chamber 31 to within the vacuum chamber 31. Besides the ion-plating method, other physical evaporating methods such as the sputtering method can be used.

When the layer 26 is formed by the ion-plating method as described, the proceossing 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 layer 26, is neither strained nor 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.

Claims

1. A method for producing a thin film for forming a thin insulating film on the surface of a desired material by an evaporation method, said method comprising the steps of: in a reacting chamber in which said desired material 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 insulation compound, the ratio of the partial pressure of said metal vapor to that of said reaction gas being gradually changed during the reaction; and controlling the level of the voltage applied to said desired material to be coated so as to lower as the formation of the insulating compound progresses in order to coat the ionized material in the reaction chamber on said desired material by means of an evaporating 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 inner-most 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 insulation compound obtained by reacting the specific metal and the specific reaction gas.

5. A method as claimed in Claim 4 wherein the level of the voltage is gradually lowered from a predetermined high level as the formation of the thin film progresses in such a way that the puncture does not occur on the surface of the insulation compound formed during the formation of the complete insulation compound.

6. 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.

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

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

9. A method as claimed in Claim 3 wherein a transition region whose electric resistance varies gradually 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.

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

11. A method as claimed in Claim 10 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.

12. A method as claimed in Claim 9 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 insulation compound obtained by reacting the specific metal and the specific reaction gas.

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

14. A method of producing a thin film substantially as described herein with reference to, and as shown in, the accompanying drawings.