EP0687354B1

EP0687354B1 - Improved semiconductor bridge explosive device

Info

Publication number: EP0687354B1
Application number: EP94909624A
Authority: EP
Inventors: Kenneth E. Willis; Martin G. Richman; William David Fahey; John G. Richards
Original assignee: Quantic Industries Inc
Current assignee: Quantic Industries Inc
Priority date: 1993-02-26
Filing date: 1994-02-23
Publication date: 1999-12-08
Anticipated expiration: 2014-02-23
Also published as: EP0687354A4; JP3701286B2; KR960701351A; JP3484517B2; DE69422026T2; AU6240194A; JP2004077117A; JPH10504634A; CA2156190A1; EP0687354A1; ATE187551T1; DE69422026D1; WO1994019661A1

Abstract

This invention discloses a method of fabricating an electroexplosive device which utilizes a semiconductor bridge (292) as an ignition element. The semiconductor bridge (292) is electrically connected to a metal header (100) by a small, low resistance contact to the extension of bridge material and through an insulating silicon substrate (270) to a eutectic bond (260) created by gold plating (350) on the metal header (100) and the silicon. The second electrode (360) of the bridge circuit is connected via wire bonds (130) to one or two conducting pins (110) which penetrate the metal header (100) and are insulated by surrounding glass (120). The design allows the use of standard semiconductor assembly methods. Since small pads of electrical material are used for electrical contact, the die size is small. A redundant connection via two conducting pins (110) insulated from the header (100) to one electrode (360) of the semiconductor bridge allows a post assembly test of the integrity of the wire bonds.

Description

Technical Field

The present invention relates to a method for producing electroexplosive devices which utilize a semiconductor bridge (SCB) as the ignition element.

Background

Military weapons systems and automotive air bag systems are typically activated by an electroexplosive device (EED). The EED usually employs a small metal bridgewire to ignite a contained explosive mixture. An electric current typically in the range of from about 1 amps to about 7 amps is passed through the bridgewire. Internal resistance heats the bridgewire to a temperature in excess of about 900° K. The hot bridgewire ignites an energetic powder, triggering the primer which in turn ignites the propellant or explosive in the system. The system may incorporate a pyrotechnic mixture, a propellant or an explosive powder.
A problem with the bridgewire type EED is a sensitivity to externally generated electric currents. High levels of electromagnetic energy from sources such as radio waves, static electricity, lightning or radar may induce an electric current within the bridgewire sufficient to cause an undesired, premature ignition.
The invention of the semiconductor bridge for electroexplosive devices was disclosed in U.S. Patent No. 3,366,055 by Hollander, Jr. Several embodiments were described by Hollander which encompass all current materials used to fabricate SCBs. A semiconductor bridge circuit as described by Hollander, Jr. will initiate the explosive reaction within the primer when a current is applied. The SCB circuit is significantly less susceptible to induced electric currents and the resultant possibility of accidental or premature ignition is reduced.
A semiconductor bridge circuit comprises a circuit formed on a semiconductor material such as silicon. A heavily doped silicon region of an n-type dopant such as phosphorous is vaporized when a current of sufficient amperage is applied. The silicon vapor is electrically heated and permeates the adjacent energetic powder mixture. Through localized convection and condensation, the energetic powder is heated to its ignition temperature leading to the desired explosive reaction being initiated.
Figure 1 shows in cross-sectional representation an EED 10 for a semiconductor bridge circuit 12 as known in the prior art. The housing 20 encases a semiconductor device 12 formed from a semiconductor material such as silicon. The SCB device includes a heavily doped bridge 13 which vaporizes when a threshold current is applied. The primer housing 20 positions the bridge 13 in close proximity to a charge 14 of an energetic powder such as lead azide. The EED 10 comprises a pair of metallic feed through leads 16 which pass through a ceramic header 18. A conventional glass to metal seal bonds the feed through leads 16 to the header 18. A metallic casing 20 made, for example, of aluminum surrounds the ceramic header 18 and a charge holder 22. Wire bonds 24 electrically interconnect the metal feed through leads 16 to bond pads 26 formed on opposite sides of the surface of the semiconductor bridge device 12, with one bonding pad located on each side of the bridge and connecting to the lead wire on the surface of the die. When a voltage is applied across feed through leads 16, current flows through the bridge 13. The bridge vaporizes forming a plasma cloud within the energetic powder 14. The electric current further heats the plasma vapor such that local convection and condensation heat the energetic powder 14 to ignition. The entire process from application of voltage to ignition takes place in less than about 20 micro-seconds.
A problem with the primer housing 10 of the prior art are (1) the ceramic header 18 is brittle and subject to fracture when the explosive device is handled roughly, and (2) the wire bonds 24 are in contact with the primer charge 14. The primer charge is compacted to maximize the explosive energy. Another problem is that compaction of the powder 14 applies stresses to the wire bonds 24 potentially leading to the wires either breaking or pulling loose from either the feed through leads 16 or from the bond pads 26. This package is not a preferred structure. Forming ceramic headers with metal feed-throughs is a relatively expensive process adding to the cost of the device. This is particularly true if the casing 20 must be hermetically sealed against the ceramic 18. Further, if large electrical pads are used to achieve low resistance connections, it increases the die 12 area and therefore the size and cost of the device.
The advantages of the SCB type initiator over the bridgewire include lower electrical energy requirements, less susceptibility to accidental or premature initiation and more rapid and precise firing times. However, methods used to attach the semiconductor bridge die to the EED header have demonstrated poor reliability and have been costly to produce. The SCB circuit is formed on a brittle semiconductor substrate. The package housing the device must provide both mechanical and environmental protection to the device. The components making up the electronic package must also be compatible with the SCB device, the energetic powder, and the attachment materials. The electrical connections to the die must withstand pressure from powder loading and consolidation.
Several patents have focused on methods for attaching the SCB to a header in order to lower cost and improve reliability. One method for fabricating the SCB to achieve efficient attachment to a header is disclosed in U.S. Patent No. 4,708,069 to Bickes, Jr., et al., and in Sandia National Labs Report No. SAND 86-2211 edited by Bickes, Jr., both of which are incorporated herein by reference. Bickes is distinguished from Hollander by using ".....a pair of spaced pads connected by a bridge, the area of each of said pads being much larger than the area of said bridge as shown in Figure 2. These large pads 30 are used to achieve electrical contact with a metalized layer 34 covering the pads. The large pad size described in Bickes was used to achieve a low resistance connection to the polysilicon bridge material 32. This low resistance contact allowed a low impedance bridge, typically about 1 ohm which is common in the art, to be used which further reduced susceptibility to RF energy.
U.S. Patent No. 3,292,537 discloses a detonator based upon a standard PN junction, for example, an ordinary junction transistor. Electrical contacts to the junction are made via pins which pass through a base of the detonator. The pins may be electrically isolated from the base using insulating material, and the pins may be electrically connected to the junction by lead wires.
U.S. Patent No. 5,029,529 (corresponding to the preamble of claim 1) describes a method of attaching an SCB die in an electrical primer housing which eliminates one lead wire to the die. This is shown in Figure 3 and is an apparent improvement over Bickes, et al. An electrically conducting die attach means 72 is used to attach the SCB die 52 to a copper alloy primer button 40. The electrical pulse to fire the bridge may follow a conductive path 74 through the silicon based device 52. In an alternative arrangement, the electrical pulse follows a conductive path through a shunt 76 attached to the side of the die 52. The attachment method disclosed uses an electrical primer 70 which is constructed from a cup 42 which forms one electrode for the external current source and a button 40 which forms the second electrode. This configuration requires an interposed insulator 54 and a conducting path from the cup 42 to the die 52 created by a wire 44 attached to the die and a conducting element 48 which is attached to the cup. This application thus requires a more complex assembly than conventional EEDs which may be acceptable for use in gun ammunition; it is relevant to this disclosure however only because of the method of achieving a conducting path through the silicon die 74 by doping the silicon. The described attachment method suffers from three disadvantages. First, the shunt 76 must be attached on the side of the die after the die is cut from the wafer; this process is not easily performed with standard semiconductor processing technology. Second, a single wire 44 connects the bridge to the conducting case, which is subject to failure. Third, the method utilizes the large pads of Bickes, et al. with the attendant disadvantages discussed above.

SUMMARY OF THE INVENTION

Therefore, in accordance with the invention, there is provided an electronic package incorporating a semiconductor bridge type initiator circuit which does not have the disadvantages of a ceramic header type package or large connecting pads. It is an advantage of the present invention that the package components are manufactured from a standard TO (transistor outline) package widely used in the semiconductor industry and available at low cost. It is another advantage of the invention that in one embodiment of the invention the lead wires are configured to minimize the potential for breakage and subsequent device failure. By using two wires connected at one end to one bonding pad on the die and at the opposite wire ends to separate and redundant pins insulated from the header, the device can be tested before and after loading the explosive powder. This test is accomplished by checking for the presence of a very low resistance between the redundant insulated pins. Any imperfection in the bonds or wire will increase the measured resistance so as to detect the flaw. Yet another advantage of the invention is that small pads of electrical material can be used to connect the bridge, as opposed to the large pads of Bickes', thereby reducing the amount of silicon per die which in turn produces higher yields, lower cost per die, increased structural rigidity, and resistance to fracture during powder pressing. It is another advantage of the invention that automated assembly methods developed for the semiconductor industry can be used in assembly, thereby improving reliability and reducing cost. It is another advantage of the invention that a eutectic bond between the bridge die and the metal header dissipates heat effectively, thereby reducing vulnerability to spurious induced currents in the bridge. This bonding method also provides more mechanical strength to resist fracture from pressing the explosive powder onto the header.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional side view of a prior art semiconductor bridge device.
Figure 2a is a top view of a second prior art semiconductor bridge device.
Figure 2b is a cross-sectional side view of the second prior art semiconductor bridge device shown in Figure 2a.
Figure 3 is a cross-sectional side view of a third prior art semiconductor bridge device.
Figure 4a is a top view of the present invention, showing the semiconductor bridge device, connecting wires, and package, but excluding the explosive material and top lid of the package.
Figure 4b is a side view of the invention taken along the cross-sectional line "A" of Figure 4a.
Figure 5 is a cross-sectional side view of the assembled apparatus of the present invention.
Figure 6 is a cross-sectional side view of the semiconductor bridge die of the present invention taken along the cross-sectional line "A" of Figure 7.
Figure 7 is a top view of the semiconductor bridge die of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Figure 4 illustrates an EED header assembly 200 adapted to house a semiconductor bridge device 150 in accordance with an embodiment of the invention.
The transistor outline (TO) header 100 is made of a steel alloy and is gold plated, as is common practice in the industry. The semiconductor bridge 150 is constructed in accordance with the methods of Hollander, but utilizes small pads of the electrical material which extend beyond the bridge. The electrical material is silicon which is doped so as to make it highly conductive. When assembled, the silicon in the die and gold plating on the header form a eutectic bond when heated, hence providing a good electrical, thermal and mechanical contact to the header, creating one side of the circuit through ground pin 140. The other side of the SCB circuit is redundantly connected to separate feed-through pins 110 by separate wire bonds 130. Feed-through pins 110 are isolated from the header body 100 and from each other by the glass insulators 120.
Since wire bonds 130 are the weakest element of the circuit, an advantage of this invention is that these bonds are redundant, and allow for nondestructive testing after assembly to confirm their integrity. After loading the explosive powder, the resistance between the redundant conductors 110 should remain very low if no damage to the wires or bonds have occurred. Thus, the slightest weakness, dislocation or breakage in the wire or the bonds can be detected by a small positive resistance measured during the test. In other words, one may connect the two leads of an ohmmeter to each of the pins 110. An open circuit or significant positive resistance indicates that one or both of the wire bonds 130 are damaged. A closed circuit indicates a functional device.
Figure 5 shows the rest of the EED assembly which is attached to the header assembly described above. A loading sleeve 170 is resistance welded to the header 100. The explosive powder 200 is then loaded and pressed into the sleeve 170. Finally, a cover 180 is welded over the entire EED to create a hermetic seal.
It is a significant advantage of the invention that all of the above assembly processes can be performed with automated equipment readily available in the semiconductor industry. In particular, the process of placing the die on the header, creating the eutectic bond between the die and the header, attaching the connecting wires between the die and the pins, and welding the load sleeve can be performed in a totally automated manner.
Figures 6 and 7 (not to scale) describe in greater detail the structure of the SCB die 150 and its attachment to the TO header 100. In this embodiment, the electrical material is heavily doped silicon which covers an area comparable to the bridge size. Therefore, the overall size of the die 270 can be small, approximately 50 mils by 50 mils or less. The substrate material 270 is approximately 5 mils thick and is intrinsic (relatively insulating) silicon with resistivity of approximately 100-200 ohm centimeters.
The SCB is fabricated by the following process. First, a field oxide insulating layer 280 is grown over the surface of the die. The edges of the field oxide 280 are approximately contiguous with the edge of the die 270.
Next, a masking step etches away the field oxide 280 to expose areas 292, 294 and 295, which will form the material of the bridge 292 and connecting pads 294 and 295 to the bridge 292. These exposed areas 292, 294 and 295 are doped with phosphorus to an approximate concentration of 10¹⁹ to 10²⁰ atoms/cc to yield a resistivity of approximately .8 milliohm-cm with a depth of dopant approximately 2 microns. This doping process forms the conducting region 300.
This bridge construction will yield a 1 ohm bridge, which is a standard in the art, if the W/L ratio is approximately 4. Similarly, a resistance of 2 ohms, which is common for automotive air bag initiators, is achieved when W/L is approximately 2. The length L of the bridge determines the voltage at which the bridge will function. For example, a length of 50 microns results in an operating voltage of about 20 volts. The top surface area of each of said pads 294 and 295 is relatively small compared to the bridge 292, preferably not more than twice the top surface area of the bridge 292.
Next, a metallization layer is deposited over pads 294 and 295. (A separate masking layer is used to expose pads 294 and 295.) In the preferred embodiment, the metallization layer comprises a first platinum silicide layer 330, followed by a titanium tungsten alloy 340 and an overplate of gold 350. The gold layer 350 provides for easy wire bonding to wire bonds 130.
In the preferred embodiment, platinum silicide layer 330 is approximately 600 Angstroms thick. This layer is created by the deposition of platinum on the silicon, then sintering for approximately 30 minutes at approximately 615 degrees centigrade. Finally, the remaining pure platinum is etched away leaving only the platinum silicide. The titanium/tungsten alloy layer 340 is approximately 1000 Angstroms thick and is about 85% tungsten and 15% titanium. It is vapor deposited.
Next, a contact (or via) hole 310 is etched through the silicon substrate 270. The back of the substrate is masked, and the contact hole is etched from the back of the substrate to the front. This hole will be 2-3 mils in diameter at the top and 4-5 mils in diameter at the bottom. As can be seen in Figure 7, the bridge 292 and pads 294 and 295 do not overlap the contact hole 310 and do not extend as far as the edge of the oxide 280 or substrate 270.
The final gold layer 350 is plated to a thickness of approximately 1.5-2 microns thick over the pads; it also completely fills the contact hole 310. A mask is first applied to the metallization layers 330, 340 and 350 to define separate bonding pads 355 and 360. Gold is then sputtered through the mask onto the front surface of the substrate, and also plated onto the front surface. Gold is also separately plated onto the back surface.
In this manner, one silicon pad 294 is connected to the bridge 292 and is also connected to the header 100 by metal pad 355. In other words, an electrical connection is made from pad 294 through the substrate to the header; metal pad 355 is the only electrical connection between that side of the doped silicon bridge material and the header. The other side of the doped silicon bridge layer is connected to metal pad 360, which is insulated from the substrate 270. Pad 360 is subsequently connected to wires 130.
Thereafter, the wafer is then etched to form individual SCB dies 150. The SCB die is attached to the header surface 100 through a eutectic bond 260 created by depositing a layer of gold 250 on top of the header and bonding the substrate 270 to the gold using conventional techniques, such as those described in the book VLSI Technology by S.M. Sze (2nd Edition). The die's small size and eutectic bond assures that the die will survive the pressure from pressing against the explosive powder. Wire bonds 130 are then attached to the die as described previously.

Claims

A semiconductor bridge explosive device in which said device is mounted on an electrically conducting header (100), said device comprising:

an electrically insulating substrate (270) mounted on said header (100),

first and second spaced pads (294, 295) separately connected to a bridge (292), the first pad being connected to the header by a conductive path through the substrate (270), and

an explosive material (200) in contact with said bridge (292),

characterised in that the first and second pads (294, 295) and the bridge (292) are defined by a heavily doped silicon layer disposed in a portion (300) of said insulating substrate,

a metallic electrode material (330, 340, 350) is disposed over said silicon layer and substrate (270), said electrode material (330, 340, 350) defining electrodes (355, 360),

a first of said electrodes (355) is in electrical contact with said header (100) and the first pad (294) of said silicon layer, and a second of said electrodes (360) is in electrical contact with the second pad (295) of said silicon layer,

said first electrode (355) defining the conductive path through the substrate and being the only electrical connection between said first pad (294) and the header (100),

and two conducting pins (110) extend through said header (100), said pins (110) being insulated from the header (100), and each of said pins (110) being connected by a separate wire conductor (130) to said second electrode (360).
A semiconductor bridge explosive device according to claim 1, wherein there is provided:

a contact hole (310) in a portion of said insulating substrate (270) extending from the surface of said substrate (270) to the bottom of said substrate,

and wherein said portion of the insulating substrate (270) does not include said contact hole (310) or the edges of said insulating substrate (270), and said first electrode (355) is in electrical contact with said header (100) through said contact hole (310).
A device according to claim 1 or 2, wherein the top surface area of each of said pads (294, 295) is not more than twice than the top surface area of said bridge (292).
A device according to any preceding claim, further comprising a third pin (140) in electrical contact with the header body (100).
A device according to any preceding claim, wherein said device is mounted within a transistor outline (TO) package.
A device according to any preceding claim, wherein the portion of said electrode material (330, 340, 350) in contact with said pads (294, 295) further comprises a bottom layer including a platinum silicide compound (330), a middle layer comprising an alloy including titanium and tungsten (340), and a top layer comprising a gold alloy (350).
A device according to any preceding claim, wherein said substrate is comprised of intrinsic silicon.