WO1998025100A1

WO1998025100A1 - Ceramic substrate electric igniter with nitrided tantalum bridge

Info

Publication number: WO1998025100A1
Application number: PCT/US1997/020595
Authority: WO
Inventors: Laurence W. Bos; Aaron W. Edwards, Jr.; William L. Wagner
Original assignee: International Resistive Company, Inc.
Priority date: 1996-12-05
Filing date: 1997-11-10
Publication date: 1998-06-11
Also published as: AU5435098A

Abstract

A device for igniting explosive and energetic material comprising an electrically non-conductive substrate, first and second conductive arms located on the substrate, and a tantalum nitride bridge element electrically interconnecting the conductive arms. Igniters constructed according to the present invention have a fast response time, can operate from low voltage power supplies, and are able to withstand unintended currents.

Description

cerami c subsrate el ectri c i gni ter wi th ni tri ded tantal um bri dge

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

₅ The present invention relates generally to devices for igniting explosive materials and, more particularly, to igniters for activating vehicle air bag inflators. The invention more especially concerns electrothermal devices for activating air bag inflators which operate at voltages employed in automotive electrical systems.

o 2. DESCRIPTION OF THE RELATED ART

Electrothermal devices have been used in the past to ignite or detonate explosive material. These devices are typically required to attain a sufficiently high temperature to ignite explosive material in a very short time after activation. The igniter element in such devices must continue to conduct electrical current and sustain a high temperature for long s enough to reliably ignite the explosive material. Some igniters have used metal filaments or "bridgewires" as the heating element. When an electrical current is passed through them, the bridgewire becomes sufficiently hot to ignite explosive material packed next to the device. A problem has existed with such bridgewires in that it is difficult to manufacture bridgewires with uniform electrical and thermal properties. Other devices have used a semiconductor o bridge as the heating element. While these devices overcome the problem of manufacturing bridgewires with uniform thermal properties, the manufacturing process for semiconductor bridge is complex and expensive.

Vehicle air bag systems commonly use a small amount of explosive material to ignite a propellant which inflates the air bag. An igniter for use in such air bag systems is subject to 5 additional constraints and requirements. The igniter must operate quickly and reliably at low activation currents, as the voltage available to power igniters used in vehicles is typically only 6-12 volts, supplied from the vehicle's 12 volt battery. Not only must the igniter operate quickly and reliably at low currents, but the igniter must also avoid unwanted activation caused by unintended electrical currents. The igniter must also be resistant to corrosion and 0 the effects of aging, as the air bag system may be installed for many years before it is eventually required to operate. In addition, the igniter must be sufficiently robust to survive the rigors of everyday use of the vehicle, as well as the impact of a vehicle collision that will cause the air bag system to deploy.

SUMMARY OF THE INVENTION

The present invention addresses the problems described above. In one broad aspect the invention comprises an electrothermal igniter which includes nitrided tantalum as a bridge element positioned on an electrically non-conducting substrate. A pair of electrical conductors are also located on the substrate and interconnected to the bridge element. The invention in another broad aspect concerns a method of making the igniters of the invention, and a method of using nitrided tantalum elements to ignite explosive materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be best appreciated upon reference to the following detailed description and the accompanying drawings, in which:

FIG. 1 is a plan view of an illustrative embodiment of the invention showing details of the igniter bridge element.

FIG. 2 is a cross-sectional view of the embodiment of FIG. 1.

FIG. 3 is a plan view of the embodiment of FIG. 1 showing the general layout of the igniter.

FIG. 4 is a graphical representation of the temperature rise of a tantalum nitride bridge element.

FIG. 5 is a graphical representation of the temperature profile through the thickness of a aluminum oxide substrate given a 500°C temperature rise at the substrate's surface. FIG. 6 is a block diagram showing the components of a system for inflating an vehicle air bag using an igniter.

FIG. 7 is a cross-sectional view showing an arrangement of an igniter and vehicle air bag head assembly. While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

1. PHYSICAL CONSTRUCTION Turning now to the drawings and referring initially to FIG. 1 , a plan view is given showing the details of the bridge element and surrounding components of an igniter 10. The igniter is shown with substrate 12, having various other components of the igniter formed on the substrate's surface. Bridge element 14 is positioned in the center of the substrate, and conductive arms 16a and 16b extend outward from the bridge element. Connection pads 18a and 18b connect to the conductive arms, and barrier members 20a and 20b overlay the boundaries between the conductive arms and the connection pads.

Substrate 12 provides support for the various components of igniter 10. The substrate preferably comprises a material that is robust and electrically non-conductive, that can be manufactured with a smooth and uniform surface, that is able to withstand high temperatures during manufacture and operation, and that is inexpensive to manufacture. A ceramic material, such as aluminum oxide, is a suitable material having these properties. High purity aluminum oxide may be used to achieve a very smooth and uniform surface, which is desirable for accurate and secure deposition of layers of material on the substrate's surface to form the other components of the igniter. Aluminum oxide having a purity of about 99.5% or above is preferred, yielding a grain size of about 80 to 400 millionths of an inch, which permits a very smooth surface to be achieved. The substrate may also be made from other materials having suitable properties.

Bridge element 14 consists of an electrically-conductive material that generates heat when electrical current is passed through it. The material used for the bridge element preferably has a low resistance, is robust, is resistant to corrosion and the effects of aging, and has a low temperature coefficient of resistance. Nitrided tantalum has been found to be especially effective and to have these properties. The tantalum used for the bridge element preferably has a very low nitrogen content, typically between about 5 and 20 atoms of nitrogen per 100 atoms of tantalum, and preferably about 10 atoms of nitrogen per 100 atoms of tantalum. The amount of nitrogen determines the resistivity and temperature coefficient of resistance of the electrothermal igniter. These characteristics must be carefully controlled to achieve the desired time-temperature relationship needed for ignition. A temperature coefficient of resistance between about +500 and -500 parts per million per degree celsius, and a sheet resistance between about 0.5 and 5 ohms is preferred.

The physical dimensions of the bridge element also affect the resistance of the bridge element. A suitable size for the bridge element is 0.002 inch wide, 0.003 inch long, resulting in a surface area of 6 x 10^" inch squared, and approximately 5,000 Angstroms thick. This provides a relatively thick and robust bridge element having a resistance of about 2 ohms when tantalum with a nitrogen content of about 10 nitrogen atoms per 100 atoms of tantalum is used. This results in a bridge element having a resistance which enables the igniter to conduct sufficient electrical current to operate at low voltages, typically from 6 to 12 volts for vehicle restraint systems. The physical dimensions of the bridge element may be varied widely to achieve the desired resistance for a particular application. Conductive arms 16a and 16b are electrically connected to the bridge element and provide means for conducting electrical current to the bridge element. The conductive arms may be composed of any high conductivity material that is robust, resistant to corrosion and the effects of aging, able to withstand high temperatures during manufacturing of the igniter, and capable of accurate deposition on the substrate. Gold is a suitable material having these properties. Especially preferred conductive arms are in the form of a layered structure of suitable materials with intermediate layers providing good adhesion, high conductivity, and high structural integrity. A preferred structure comprises layers of nitrided tantalum, titanium, palladium, and gold. A titanium-tungsten alloy, copper, nickel, platinum and solder are other materials that may be used, although other materials may also be found suitable. Connection pads 18a and 18b are electrically connected to the conductive arms and provide a convenient means to connect the igniter into an electrical circuit. The bridge element 14 and conductive arms 16a and 16b may be constructed with physical dimensions that are very small compared to the dimensions of the electrical wires, pins or other parts that may be used to connect the igniter to a control circuit. The conducting pads may provide a larger surface area to which electrical connections between the igniter and a control circuit can be made. The connecting pads may be composed of any high conductivity material that is robust, resistant to corrosion, and suitable for convenient mechanical and electrical connection to an electrical circuit. A very effective layered structure useful as a connecting pad comprises a bottom copper layer, a nickel layer deposited on the copper layer, and a top gold layer deposited on the nickel layer. This combination of layers provides the desired properties for the connection pads, although they may be comprised of titanium, palladium, titanium-tungsten, platinum or solder, or combinations of these materials. Other materials and combinations of materials having suitable properties may also be used. Barrier members 20a and 20b are formed over the transition area between the conductive arms and the connection pads. Solder may be used to make the connection between the connecting pads 18a and 18b and the igniter control circuit. The barrier members prevent the solder from flowing from the connection pads onto the conducting arms or bridge element. The barrier members may be made from epoxy or any other suitable material.

The overall layout of an igniter is shown in plan view in FIG. 3. The connection pads 18a and 18b are shown with through holes 22a and 22b which penetrate the connecting pads and substrate 12. The through holes receive connecting pins which may be used to mechanically and electrically connect the igniter to a head assembly and igniter control circuit, as shown in FIG. 7.

2. MANUFACTURING PROCESS

FIG. 2 is a cross-sectional view along line A-A of FIG. 1, showing the layers of material comprising the various components of an illustrative embodiment of the invention. A tantalum nitride layer 30 is deposited in a film over the entire surface of substrate 12. A sputtering technique may be used to form a thin layer of tantalum nitride, which permits the thickness of the film to be closely controlled. Titanium layers 32a and 32b are then deposited on top of the tantalum nitride layer 30 as an adhesive layer, and palladium layers 34a and 34b are deposited on the titanium layers as a second adhesive layer to provide a suitable material for the gold layers 36a and 36b to adhere to. A sputtering technique may be used for the titanium and palladium layers to permit thin films of these materials to be accurately formed. Gold layers 36a and 36b are formed on the palladium layers to form the conductive arms of the igniter (shown as 16a and 16b in FIG. 1). The gold layers may be formed using photo resist techniques to limit the area of gold plating to the desired shape of the conductive arms and to accurately define the gap between the connecting arms. This gap defines the length, and thus the resistance, of the bridge element of the igniter.

Copper layers 38a and 38b are formed on the underlying gold layers 36a and 36b. Nickel layers 40a and 40b are formed on the copper layers, and second gold layers 42a and 42b are formed on the nickel layers. The copper, nickel, and gold layers may be formed using photo resist techniques to limit the area of plating of the materials to the desired shape of the connection pads (shown as 18a and 18b in FIG. 1). After the materials for the conductive arms and the connection pads are deposited, the palladium and titanium layers are etched to remove excess material, being the region not covered by the materials forming the conductive arms and the connection pads. The tantalum nitride layer is then etched to remove excess material, being the region of the tantalum nitride layer that is not covered by the material forming the conductive arms and the connection pads, except for the area that forms the bridge element (shown as 14 on FIG. 1). Thus the resulting structure includes layers of material forming the conductive arms and the connection pads, and a thin strip or bridge of exposed tantalum nitride bridging the gap between the conductive arms.

In order to accurately control the resistance of the bridge element, its dimensions must be precisely controlled. The width of the tantalum nitride bridge element may be trimmed to achieve the desired physical dimensions, thus producing the desired resistance for the bridge element. A laser may be used to perform this trimming step, although other techniques may be used. Epoxy layers 44a and 44b are then formed at the boundaries of the conductive arms and the connection pads.

To reduce the cost of manufacture, many igniters may be formed simultaneously on a single large substrate. When the process for forming the components of the igniters is complete, the substrate is cut into many small pieces, each piece being an individual igniter as described above.

3. OPERATION OF THE IGNITER

When an electrical voltage is applied across the connecting pads of the igniter, the bridge element conducts an electrical current and its temperature rises. FIG. 4 shows the calculated temperature rise of a 0.002 inch wide by 0.003 inch long tantalum nitride resistive element as an electrical current passes through the element. In a typical automobile air bag system operating at 12 volts, it is generally considered that an igniter of the invention should reach a surface temperature of about 500°C in less than about 500 microseconds with a current of about 1.2 amperes or more. As shown in the figure, a temperature of 500°C, sufficient to reliably ignite explosive materials suitable for vehicle air bag inflation, can be reached in about 33 microseconds at an ignition current of 1.2 amperes. Other time- temperature relationships may be required for alternate explosive materials. In addition, the tantalum nitride bridge element can withstand a steady electrical current of about 0.4 amperes for 10 seconds without igniting the explosive material, providing protection against accidental air bag inflation caused by unintended currents.

When the bridge element's temperature rises during activation of the igniter, the substrate conducts heat away from the bridge element deposited on the substrate's surface. If the thermal conductivity of the substrate is too high, it may delay or prevent the bridge element from attaining a sufficiently high temperature to ignite the explosive. FIG. 5 shows the calculated temperature profile through the thickness of an aluminum oxide substrate given a 500°C temperature rise at the substrate's surface. The temperature profile is shown for 1.2 amperes bridge current after 33 microseconds and 1.7 amperes after 8 microseconds, corresponding to the time taken for the bridge element to reach the 500°C ignition temperature. As shown in FIG. 5, the heat dissipation within the substrate is not significant, and does not prevent or unduly delay the bridge element from reaching ignition temperature.

FIG. 6 is a block diagram showing the components of a system for inflating an vehicle air bag using an igniter. A collision sensor 50 detects a vehicle collision of sufficient severity to require deployment of at least one of the vehicle's air bags. Control circuit 52 receives an input signal from the collision sensor and completes an electrical circuit from power supply 54 to igniter 10 when the collision sensor detects a collision. Igniter 10 conducts electric current through the igniter bridge element (not shown) which ignites primary explosive 58 positioned in close proximity to the igniter. The primary explosive ignites secondary explosive or propellant 60 which causes air bag 62 to inflate.

FIG. 7 is a cross-sectional view showing an illustrative arrangement of an igniter mounted to an air bag inflator head assembly. A head assembly 70 is shown for connecting various components of a vehicle air bag inflator. The head assembly includes head casing 72. Igniter 10 is connected to the head casing via pins 74a and 74b. These pins also provide an electrical connection between the igniter and control circuit 52. A primary explosive 58 is tightly packed into the upper portion of the head casing, so that the primary explosive is in close proximity to the igniter. Secondary explosive or propellant 60 is located adjacent to the primary explosive.

Many modifications and variations may be made in the techniques and structures described and illustrated herein without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the methods and apparatus described herein are illustrative only and are not limiting upon the scope of the present invention.

Claims

CLAIMS:

1. An electrothermal device for igniting explosive and energetic materials, which comprises:

(a) an electrically non-conductive substrate; (b) first and second conductive arms located on the substrate in spaced relation; and

(c) a nitrided tantalum bridge located on the substrate and electrically interconnecting the first and second conductive arms.

2. A device as defined in claim 1 wherein the substrate comprises a ceramic material.

3. A device as defined in claim 1 wherein the substrate comprises aluminum oxide.

4. A device as defined in claim 3 wherein the aluminum oxide has a purity of about 99.5 percent.

5. A device as defined in claim 1 wherein the first and second conductive arms each comprise a layer comprising gold.

6. A device as defined in claim 5 which further comprises an electrically conductive metal layer underlying each gold layer.

7. A device as defined in claim 1 wherein the first and second conductive arms each comprise a layer comprising a metal selected from the group consisting of titanium, palladium, titanium-tungsten, copper, nickel, platinum, and solder.

8. A device as defined in claim 1 wherein the tantalum contains sufficient nitrogen to provide the tantalum with a temperature coefficient of resistance between about +500 and

-500 parts per million per degree Celsius.

9. A device as defined in claim 1 wherein the bridge has a sheet resistance between 0.5 and 5 ohms.

10. A device as defined in claim 2 wherein the first and second conductive arms each comprise a layer comprising gold.

11. A device as defined in claim 1 which further comprises:

(d) first and second connectors for applying electrical energy to the bridge via the first and second conductive arms.

12. A device as defined in claim 1 1 wherein the first and second connectors each comprise a layer comprising gold.

13. A device as defined in claim 12 which further comprises an electrically conductive metal layer underlying each gold layer of the first and second connectors.

14. A device as defined in claim 1 1 wherein the first and second connectors each comprise a layer comprising a metal selected from the group consisting of titanium, palladium, titanium-tungsten, copper, nickel, platinum, and solder.

15. A device for igniting explosive and energetic materials, which comprises: (a) a substrate; (b) an electrically non-conductive layer disposed on the substrate;

(c) first and second conductive arms located in spaced relation on the non- conductive layer; and

(d) a nitrided tantalum bridge disposed on the non-conductive layer, the bridge electrically interconnecting the first and second conductive arms.

16. A device for igniting explosive and energetic materials, which comprises:

(a) a ceramic substrate;

(b) first and second conductive arms located on the substrate in spaced relation; and

(c) a thin film metallic bridge deposited on the substrate, the bridge electrically interconnecting the first and second conductive arms.

17. A device for aiding in the rapid creation of a gas, which comprises:

(a) an electrically non-conductive substrate;

(b) first and second conductive arms located on the substrate, in spaced relation;

(c) a nitrided tantalum bridge located on the substrate, the bridge electrically connecting the first conductive arm to the second conductive arm; and

(d) an explosive material located in close proximity to the bridge.

18. An electrothermal device produced by the process consisting of:

(a) forming an electrically non-conductive substrate;

(b) depositing a nitrided tantalum bridge on the substrate; and (c) depositing a first and a second conductive arms on the nitrided tantalum.

19. A method of igniting explosive material comprising:

(a) providing an explosive material;

(b) providing a nitrided tantalum element; (c) positioning the nitrided tantalum element adjacent to the explosive material; and

(d) igniting the explosive material by applying electrical energy to the nitrided tantalum element.

20. A method of inflating automobile air bags comprising: (a) providing a deflated automobile air bag;

(b) providing an explosive material suitable for inflating the air bag;

(c) providing a nitrided tantalum element;

(d) positioning the explosive material adjacent to the air bag so that the air bag is inflated when the explosive material is ignited; (e) positioning the nitrided tantalum element adjacent to the explosive material;

(f) igniting the explosive material by applying electrical energy to the nitrided tantalum element.