Patent Publication Number: US-6220164-B1

Title: Semiconductor igniter

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This application claims the priority of German Application No. 198 15 928.5, filed Apr. 9, 1998, the disclosure of which is expressly incorporated by reference herein. 
     The invention relates to a semiconductor igniter, particularly for the gas generator of a vehicle occupant protection system and, more particularly, to a semiconductor igniter having a semiconductor layer which is arranged on a carrier with the insertion of a thermal insulation layer and is connected at an end side to electric contact areas such that during the current passage in the ignition path range it heats up in an ignition-triggering manner. 
     Semiconductor igniters of this type, which, in contrast to hot wire igniters, are becoming more prevalent mainly because of their significantly lower interference susceptibility, are known from European Patent document EP 0 762 073 A1 or from U.S. Pat. No. 5,309,841. The semiconductor igniters consist of a considerably p-doped or n-doped semiconductor layer which is arranged between end-side contact pieces on an electrically insulated or non-conducting carrier. During the current passage, while generating an ionized semiconductor plasma, the igniter abruptly heats up and evaporates and, as a result, triggers the ignition—mostly by way of a primary ignition charge. For reasons of a high ignition efficiency, it is required to insert a thermal insulation layer between the semiconductor layer and the carrier. However, as a result, the mechanical bond from the semiconductor layer to the carrier is impaired, and there is the danger that the semiconductor layer may detach under the effect of thermal or dynamic loads as they occur particularly in an application in a motor vehicle, and the semiconductor igniter therefore becomes inoperative. 
     It is an object of the invention to construct a semiconductor igniter of the above-mentioned type such that a high constructive stability is achieved in a manner which is simple with respect to the manufacturing of the igniter while maintaining a high ignition efficiency. 
     According to the invention, this object is achieved by a semiconductor igniter, particularly for a gas generator of a vehicle occupant protection system, having a semiconductor layer which is arranged on a carrier with the insertion of a thermal insulation layer and is connected at an end side to electric contact areas such that during the current passage in the ignition path range it heats up in an ignition-triggering manner. The thermal insulation layer is limited to the ignition path range of the semiconductor layer and, on its end sections kept free of the thermal insulation layer. The semiconductor layer is fixedly connected with the carrier. 
     According to the invention, as a result of the spatial limiting of the thermal insulation layer to the ignition path range of the semiconductor layer in conjunction with a linking of the bridge end sections directly to the support, which link is of the same material and is therefore correspondingly firm, a support of the semiconductor layer is ensured which is extremely stable with respect to the occurring loads. Further, the operational reliability of the semiconductor igniter is significantly improved without additional high-expenditure measures. Nevertheless, the thermal shielding of the ignition path range, which is required for a high ignition efficiency, is fully maintained. 
     In a particularly preferred further embodiment of the invention, the semiconductor layer is molded on the end sections in one piece to the carrier, whereby an even more secure bond is ensured between the semiconductor layer and the carrier. 
     For further increasing the stability with a simultaneously high thermal protection effect, it is recommended to produce the thermal insulation layer of a porous material which supports the semiconductor layer in the ignition path range, specifically in a manner which is simple with respect to manufacturing. Here, the carrier material itself is locally made porous, for example, electrochemically. In this case, the material, which is made porous, is preferably oxidized in order to further reduce the thermal conductivity of the insulation layer. 
     However, optionally, it is also possible to construct the semiconductor layer preferably as a bridge structure which is free-standing in the ignition path range; specifically expediently such that the insulation material, which was first made porous, is removed by etching so that a hollow space is formed as the thermal insulation layer which reaches under the ignition path range. The hollow space is filled with air and can be evacuated as desired. This still further reduces the thermal ignition energy losses. 
     In a particularly preferred manner, the semiconductor layer is surrounded in the ignition path range by an ignition intensifying medium which burns in an explosive manner when heated, whereby, after a relatively low temperature level has been reached, non-electrically generated heat is made available to the ignition process. According to the invention, the ignition intensifying medium is expediently applied to the semiconductor layer in the form of a coating which is thin with a view to obtaining a short ignition delay. However, when using a porous insulation layer, it is, optionally or additionally, also possible to impregnate the porous insulation material with a gaseous or metal-containing ignition intensifying medium for intensifying the ignition pulse. 
     According to a preferred embodiment of the invention, the semiconductor layer is divided into several mutually parallel bridge-type webs which are thermally insulated with respect to one another and with respect to the carrier. As a result, in the case of a comparatively large bridge width which is advantageous for creating large contact surfaces for the ignition charge situated above the semiconductor layer, by way of the spaces existing between the bridge-type webs, a thermal insulation layer can be constructed without any problem on the bridge underside. 
     In a particularly preferable manner, the semiconductor layer is constructed as a semiconductor element which is operated in the blocking direction, heats up in an ignition-triggering manner when the switching voltage is exceeded and has at least a p-n transition junction, thus approximately as a pair of diodes connected antiparallel. This further reduces the interference susceptibility of the semiconductor igniter and produces a pronounced short, sharp ignition pulse. 
     According to the present invention, the carrier and the semiconductor layer are preferably produced from differently doped silicon, for example, in the form of a silicon wafer. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic, enlarged top view of a semiconductor igniter according to the invention; 
     FIG. 2 is a sectional view of the semiconductor igniter according to FIG. 1 along Line I—I; 
     FIG. 3 is a schematic view of a second embodiment of a semiconductor igniter with a semiconductor bridge, which is molded on in one piece, in a representation corresponding to FIG. 2; and 
     FIG. 4 is a schematic top view of another embodiment of a semiconductor igniter with a multi-part semiconductor bridge. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The semiconductor igniter illustrated in FIGS. 1 and 2 contains a carrier  2  in the form of a slightly p-doped silicon wafer, a thermal insulation layer  4  formed in the carrier  2  in the shape of a trough; a semiconductor bridge  6  which is also made of silicon but is highly n-doped and which, in the ignition path range  8 , is supported on the thermal insulation layer  4  and is applied on the bridge end sections  10 ,  12  by means of a mechanically fixed link made of the same material, directly to the carrier  2 ; as well as electric contact pieces  14 ,  16 , which cover the bridge end sections  10 ,  12  over a large surface area and which are connected with the electronic ignition system (not shown) by way of connection elements  18 ,  20 . 
     The thermal insulation layer  4  is produced from the carrier material itself such that the carrier  2  is made porous electrochemically or photochemically in a zone which is locally limited to the ignition path range  8  of the semiconductor bridge  6 . During the current passage through the semiconductor bridge  6 , the thermal insulation layer  4  provides for the electrically generated heat to be largely converted to ignition energy so that the ignition path material heats up abruptly and thereby triggers the ignition in the primary ignition charge (not shown) arranged above the semiconductor bridge  6 . At the end sections  10 ,  12  of the bridge  6 , which in contrast are kept free of the thermal insulation layer  4 , the semiconductor bridge  6  is securely anchored on the carrier  2  with respect to the occurring thermal and mechanical loads. For improving the thermal protection effect, the porous silicon layer  4  can be oxidized at least on the surface areas adjoining the ignition path range  8 . 
     In order to intensify the ignition pulse, the porous insulation layer  4  is filled with an explosive gas or gas mixture which, when the ignition path  8  is heated, burns abruptly and thereby provides additional thermal energy for the ignition process. Instead, the porous surfaces of the insulation layer  4  can also be coated with a thin ignition-intensifying metal-containing coating which may be deposited by means of the so-called sol-gel process and consists, for example, of Al, Mg, titanium hydride or the like. 
     In the case of the semiconductor igniter according to FIG. 3, where the elements corresponding to the first embodiment are indicated by a reference number increased by 100, the semiconductor bridge  106  is molded at its end sections  110  and  112  in one piece to the carrier  102 . The semiconductor bridge  106  is delimited from the carrier  102  by a different doping, specifically on the semiconductor bridge  106 , by a high n-silicon doping and, in the area of the carrier  102 , by a low p-silicon doping. Another difference is the fact that, in this embodiment, the thermal insulation layer consists of a hollow space  104  which is filled with air, can be evacuated as desired, and is worked into the carrier material in a trough-shaped manner. For this purpose, the carrier material below the later ignition path range  108  is first made porous again electrochemically or photochemically. Subsequently, the porous silicon material is removed by undercutting so that the hollow space  104  is created. The hollow space  104  reaches under the ignition path range  108  and extends to the bridge end sections  110 ,  112 . As an alternative, the hollow space  104  can also be formed directly by means of a plasma-type etching attack. For intensifying the ignition, a thin metallic coating  22  is again provided, which in this case is applied to the ignition path range  108  and is made of Al, Mg, titanium hydride or the like. The construction and method of operation of the semiconductor igniter according to FIG. 3 is otherwise the same as in the first embodiment. 
     In the case of the semiconductor igniter according to FIG. 4, where the elements corresponding to the previous embodiments are indicated by a reference number increased by 200, the carrier  202  and the semiconductor bridge  206  are made in the same manner as according to FIG. 3 in one piece from a silicon wafer. However, here the silicon material, which is made porous, is not etched away below the ignition path range  208  but remains as a thermal insulation layer  204 . Furthermore, the semiconductor bridge  206  is divided in the ignition path range  208  into several mutually parallel bridge webs  24 . This is done in order to, in the case of a large bridge width which is advantageous for a large-surface initiation of the primary ignition charge situated above the semiconductor bridge  206 , be able to carry out the electrochemical etching process for making the insulation layer  204  porous. The etching process is carried out by way of the spaces between the bridge webs  24  without any problem, that is, without an excessively high driving-in depth and therefore thickness of the insulation layer  206 . Instead of being divided into parallel bridge webs  24 , the semiconductor bridge  206  may also be provided with a plurality of etching holes or etching slots by way of which the etching process is then carried out for manufacturing the thermal insulation layer  204 . 
     According to FIG. 4, the semiconductor bridge  206  is constructed on its bridge webs  24  in the manner of a semiconductor element provided with several p-n transitions (junctions), thus approximately—as illustrated—as an antipolar-connected pair of diodes  26  which are operated in the blocking direction and, when the turnover voltage is exceeded, heats up in an ignition-pulse-generating manner. This further reduces the interference susceptibility of the semiconductor igniter and results in an even steeper ignition pulse. 
     Typically, the semiconductor bridge has a wall thickness between 1 and 10 μm; a length between 20 and 1,000 μm; and a width between 20 and 300 μm (according to FIG. 4, the bridge length amounts to approximately 100 μm and the bridge width amounts to approximately 200 μm). The thickness of the thermal insulation layer corresponds to approximately half the bridge width or web width and amounts to approximately 30 μm; that of the metallic ignition intensifying layer  22  amounts to approximately 0.5 μm; and the semiconductor igniter has an overall height of approximately 500 μm. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.