SEMICONDUCTOR DEVICE PROVIDED WITH A METALLIZATION WITH A BARRIER LAYER COMPRISING AT LEAST TITANIUM, TUNGSTEN, OR NITROGEN, AND METHOD OF MANUFACTURING SAME

Abstract: A semiconductor device with a silicon substrate (1, 20) in which semiconductor elements are formed with semiconductor zones (2, 3, 21, 22) which adjoin a surface (4, 25) of the substrate. The surface is provided with a silicon oxide layer (7) with contact windows (8) within which the semiconductor zones are exposed. On the silicon oxide layer, a metallization (9) connected to the semiconductor zones is formed in a layer system (10) with a barrier layer (11) which comprises at least titanium, tungsten, or nitrogen, and with a layer (12) of conductive material. An aluminum layer (13) is provided on the silicon oxide layer (7) below the metallization (9). In spite of the presence of the barrier layer, surface states which may be present at the surface of the silicon substrate are passivated by hydrogen which is released upon reaction between the aluminum layer (13) and hydroxyl groups of the silicon oxide layer.

Description: Semiconductor device provided with a metallization with a barrier layer comprising at least titanium, tungsten, or nitrogen, and method of manufacturing same. The invention relates to a semiconductor device with a silicon substrate in which semiconductor elements are formed with semiconductor zones adjoining a surface of the substrate, which surface is provided with a layer of silicon oxide having contact windows within which the semiconductor zones are exposed, while on the layer of silicon oxide a metallization is formed in a layer system with a barrier layer comprising at least titanium, tungsten, or nitrogen and with a layer of conductive material lying on the barrier layer, which metallization is connected to the semiconductor zones. The invention also relates to a method of manufacturing such a device. The semiconductor elements may be, for example, bipolar transistors or MOS transistors. The semiconductor zones adjoining the substrate surface then are emitter, base, and collector zones of the bipolar transistors or source and drain zones of the MOS transistors. The semiconductor device may comprise a few to very many of such semiconductor elements. The device is called integrated circuit in the latter case. Said metallization then forms a first wiring layer on which in practice one or several further wiring layers are provided. These may be constructed in a similar layer system as the first wiring layer, with a barrier layer comprising titanium, tungsten, or nitrogen and with a layer of conductive material lying on the barrier layer. After all wiring layers have been provided, a passivating layer is finally applied, usually a layer of silicon oxide or silicon nitride which is deposited in a PECVD process (Plasma Enhanced Chemical Vapor Deposition). The barrier layer may be a layer of titanium nitride (TiN) or of a titanium-tungsten alloy (TiW) comprising 10 to 30 at. % titanium. Some 15 to 40 at. % nitrogen may also be added to the titanium-tungsten alloy (TiW(N)). The layer of conductive material lying on the barrier layer may be an aluminum layer or a tungsten layer. In the former case, the barrier layer prevents the aluminum from reacting with the silicon exposed in the contact window, in the latter case the barrier layer protects the exposed silicon during the deposition of the tungsten. A semiconductor device of the kind mentioned in the opening paragraph is known from US-5, 420,070. It is found in practice, when the semiconductor elements are bipolar transistors, that these elements may have gain factors which are dependent on the current flowing through the collector of the transistor. The gain factors are lower at low collector currents than at high collector currents. If the semiconductor elements are MOS transistors, they often show different threshold voltages. ' The invention is based on the recognition that the undesirable effects mentioned above are caused by the fact that surface states arising at the interface between the silicon substrate and the silicon oxide layer cannot be sufficiently passivated during the formation of the silicon oxide layer. These surface states may give rise to leakage currents between base and emitter in the bipolar transistors. This base leakage current has the result that still a comparatively strong base current flows at low collector currents. The surface states may capture charge during operation of the MOS transistor, so that the threshold voltage of a MOS transistor can be affected. These surface states would be passivated by hydrogen if the barrier layer comprising titanium, tungsten, or nitrogen were not used. This hydrogen is released on heating, for example from a passivating layer deposited in a plasma enhanced chemical vapor deposition process. Since said barrier layer is impermeable to hydrogen, however, the hydrogen cannot reach said surface states, or only with difficulty. Passivation accordingly does not take place, or not completely. The invention has for its object to counteract the above disadvantages by ensuring that the surface states can be passivated also in the case in which a barrier layer impermeable to hydrogen is used. The semiconductor device is for this purpose characterized in that a layer of aluminum is provided on the silicon oxide layer below the metallization. It is found that the aluminum layer when heated reacts with the hydroxyl groups present at the surface of the silicon oxide layer, whereby aluminum oxide and atomic hydrogen are formed. This atomic hydrogen diffuses through the silicon oxide layer and passivates said surface states. The bipolar transistors show a gain factor which is not dependent on the value of the collector current, while the MOS transistors all have the same threshold voltage in one and the same semiconductor device. During manufacture of the semiconductor device mentioned in the opening paragraph, the surface of the silicon substrate is first provided with semiconductor elements formed with semiconductor zones adjoining a surface of the substrate, and is subsequently provided with a silicon oxide layer in which contact windows are formed within which the semiconductor zones are exposed, whereupon a layer system with a barrier layer comprising at least titanium, tungsten, or nitrogen and a superimposed layer of conductive material are deposited on the silicon oxide layer, and a metallization system connected to the semiconductor zones is subsequently formed in said layer system. A layer of aluminium or of aluminium comprising a small quantity of silicon (aluminium-silicon) is provided on the silicon oxide layer below the metallization. This may be done in that the aluminum or aluminium-silicon layer is provided on the silicon oxide layer before the contact windows are formed. These contact windows are then etched into the aluminum layer and into the silicon oxide layer. The subsequent metallization layers are then deposited on the aluminum layer and in the contact windows. Preferably, however, the aluminum layer is not provided until after the contact windows have been formed in the silicon oxide layer. All layers in which the metallization is formed can then be deposited in one process step at a reduced pressure. The aluminum layer in that case is also provided on the silicon which lies exposed within the contact windows. The aluminum layer is preferably 5 to 50 nm thick. When an aluminum layer of such a small thickness is used, problems which may arise from the dissolution of silicon in aluminum are strongly counteracted. If silicon is dissolved in an aluminum layer deposited thereon, pits may arise in the silicon which are filled with aluminum from the layer. When thicker layers are used, these pits may become so deep that they short-circuit the pn junctions present in the silicon. The use of the small thickness mentioned above reduces the quantity of aluminum in which silicon can be dissolved. The quantity of aluminum in which silicon can be dissolved is further limited, and the formation of said pits is accordingly further counteracted, when the aluminum layer is so deposited that deposition of aluminum on the walls of the windows is counteracted. This may be readily achieved in a deposition process which shows a small step coverage during deposition of such thin layers. This is indeed the case in usual vapor and sputter deposition processes. The invention will be explained in more detail below by way of example with reference to a drawing, in which: Fig. 1 shows a bipolar transistor provided with a metallization according to the invention diagrammatically and in cross-section, Fig. 2 shows a MOS transistor provided with a metallization according to the invention diagrammatically and in cross-section, Figs. 3 to 5 show a few stages in the manufacture of a semiconductor device by a first embodiment of the method according to the invention, diagrammatically and in cross-section, Figs. 6 and 7 show a few stages in the manufacture of a semiconductor device by a second embodiment of the method according to the invention, diagrammatically and in cross-section, and Fig. 8 shows a semiconductor device provided with a further embodiment of a metallization according to the invention diagrammatically and in cross-section. Fig. 1 diagrammatically and in cross-section shows a bipolar transistor formed in a silicon substrate 1. This substrate comprises semiconductor zones 2, 3, in this case a base zone 2 and an emitter zone 3 which adjoin a surface 4 of the substrate 1. The substrate further comprises a collector zone 5 which is contacted by means of a buried zone 6 in a usual manner. The surface 4 is provided with a silicon oxide layer 7 with contact windows 8 within which the semiconductor zones 2, 3 are exposed. A metallization 9 connected to the semiconductor zones 2, 3 is formed on the silicon oxide layer 7. This metallization is formed in a layer system 10 with a barrier layer 11 and a layer 12 of conductive material, in this case aluminum to which a few percents of silicon and copper were added, which lies on the barrier layer 11. Fig. 2 diagrammatically and in cross-section shows a MOS transistor which is formed in a silicon substrate 20. This substrate comprises semiconductor zones 21, 22, in this case a source zone 21 and a drain zone 22. The substrate between these zones forms the channel region 24 of the MOS transistor. The MOS transistor lies enclosed between field insulation regions 23. The zones 21, 22 adjoin a surface 25 of the substrate 20. Above the channel region 24 there is a gate electrode 27 lying on a gate oxide 26. The surface 25 is provided with a silicon oxide layer 7 with contact windows 8 within which the semiconductor zones 21, 22 are exposed, similar to the case of the bipolar transistor. As in the bipolar transistor, a metallization 9 connected to the semiconductor zones 21, 22 is formed on the silicon oxide layer 7. This metallization is formed in a layer system 10 with a barrier layer 11 and a layer 12 of conductive material, in this case aluminum to which a few percents of silicon and copper were added, which lies on the barrier layer 11. The barrier layer 11 may be a layer of titanium nitride (TiN) or of a titanium-tungsten alloy (TiW) which comprises 10 to 30 at. % of titanium. An additional 15 to 40 at. % of nitrogen may have been added to the titanium-tungsten alloy (TiW(N)) . Only a single bipolar transistor and a single MOS transistor are shown for simplicity's sake. The semiconductor device, however, may comprise very many such semiconductor elements. The metallization 9 then forms a first wiring layer on which one or several further wiring layers are provided in practice. These layers may each be constructed in a layer system of the same kind as the first wiring layer, with a barrier layer and a layer of conductive material lying on the barrier layer. After all wiring layers have been provided, finally, a passivating layer is provided, usually a silicon oxide or silicon nitride layer deposited in a PECVD process (Plasma Enhanced Chemical Vapor Deposition). An aluminum layer 13 is formed on the silicon oxide layer 7 below the metallization 9. Upon heating, the aluminum layer 9 reacts with hydroxyl groups which are always present at the surface of the silicon oxide layer 7, whereby aluminum oxide and atomic hydrogen are formed. This atomic hydrogen diffuses through the silicon oxide layer 7 and passivates surface states which may be present at the surface 4, 25 of the silicon substrate. These surface states may give rise to leakage currents between base and emitter in bipolar transistors. This base leakage current has the result that a comparatively strong base current still flows in the case of small collector currents. The surface states can also capture charge during operation of the MOS transistors, so that the threshold voltage in a MOS transistor cannot be influenced by such a charge. These surface states would have been passivated by hydrogen if the barrier layer 11, which comprises at least titanium, tungsten, or nitrogen, were not used. This hydrogen is released upon heating from the passivating layer deposited in a plasma enhanced chemical vapor deposition process. Since said barrier layer is impermeable to hydrogen, however, this hydrogen cannot reach said surface states, or only with difficulty. Passivation accordingly does not take place, or not completely. The use of the aluminum layer 13 achieves that the bipolar transistors show a gain factor which is not dependent on the value of the collector current, and that the MOS transistors all show the same threshold voltage in one and the same semiconductor device. In the manufacture of the semiconductor device as shown in Figs. 3 to 6, where the same reference numerals are used as in Fig. 1 , the surface 4 of the silicon substrate 1 is provided with a silicon oxide layer 7 after semiconductor elements have been formed in this substrate, with semiconductor zones 2 adjoining the surface 4. An aluminum layer 13 is then deposited on the silicon oxide layer 7. A photoresist mask 30 is formed thereon, which leaves the aluminum layer uncovered at the areas of the contact windows 8 to be formed. These contact windows 8 are subsequently formed in the aluminum layer 13 and in the silicon oxide layer 7. The semiconductor zones 2 are exposed within these windows 8. Then a layer system 10 comprising a barrier layer 11 and a superimposed layer of conductive material 12 is deposited on the silicon oxide layer 7, in which layer system subsequently a metallization 9 connected to the semiconductor zones 2 is formed. An aluminum layer 13 is then provided on the silicon oxide layer 7 below the metallization 9, in this embodiment in that the aluminum layer 13 is provided on the silicon oxide layer 7 before the contact windows 8 are formed. The contact windows 8 are etched into the aluminum layer 13 and into the silicon oxide layer 7. The subsequent layers of the metallization 10 are then deposited on the aluminum layer 13 and in the contact windows 8. Preferably, however, as shown in Figs. 6 and 7, the aluminum layer 13 is not provided until after the contact windows 8 have been formed in the silicon oxide layer 7. The deposition of all layers 13, 11 , 12 in which the metallization 9 is formed can then take place at a reduced pressure in one process step. The aluminum layer 13 is thus also present on the silicon exposed at the surface 4 within the contact windows 8. The aluminum layer 13 preferably has a thickness of 5 to 50 nm. When an aluminum layer of such small thickness is used, problems which may arise owing to the dissolution of silicon in aluminum are strongly counteracted. When silicon is dissolved in an aluminum layer deposited thereon, pits may arise in the silicon which are filled with aluminum from the layer. When thicker layers are used, these pits may become so deep that they short-circuit pn junctions present in the silicon. The use of the small thickness mentioned above reduces the quantity of aluminum in which silicon can become dissolved. The aluminum layer 13 is preferably so deposited that deposition of aluminum on the walls 31 of the windows 8 is counteracted. This may be readily achieved in a deposition process which shows a small step coverage in the deposition of such thin layers. This is indeed the case in usual vapor and sputter deposition processes. The quantity of aluminum in which silicon can be dissolved is further reduced thereby, so that the formation of said pits is further counteracted. Fig. 8 finally shows a semiconductor device provided with a further embodiment of a metallization according to the invention diagrammatically and in cross- section. The layer of conductive material 12 lying on the barrier layer 11 was an aluminum layer in the preceding examples, but it may alternatively be a tungsten layer. In the former case, the barrier layer prevents the aluminum from reacting with the silicon exposed in the contact window, in the latter case the barrier layer protects the exposed silicon during the deposition of the tungsten. In the embodiment shown in Fig. 8, an aluminum layer 13 and a barrier layer 11 are deposited after the formation of the contact windows 8. Then the contact window is filled with tungsten 32 in that a thick layer of this material is deposited and this thick layer is subsequently subjected to an etching treatment until the barrier layer 13 on the silicon oxide layer 7 has become exposed again. Then the conductive aluminum layer 12 is deposited, and finally the metallization 9 is formed.

Claims: CLAIMS: 1. A semiconductor device with a silicon substrate in which semiconductor elements are formed with semiconductor zones adjoining a surface of the substrate, which surface is provided with a layer of silicon oxide having contact windows within which the semiconductor zones are exposed, while on the layer of silicon oxide a metallization is formed in a layer system with a barrier layer comprising at least titanium, tungsten, or nitrogen and with a layer of conductive material lying on the barrier layer, which metallization is connected to the semiconductor zones, characterized in that a layer of aluminum is provided on the silicon oxide layer below the metallization. 2. A semiconductor device as claimed in Claim 1 , characterized in that the aluminum layer is also provided on the silicon exposed within the contact windows. 3. A semiconductor device as claimed in Claim 1 or 2, characterized in that the aluminum layer has a thickness of 10 to 50 nm. 4. A method of manufacturing a semiconductor device with a silicon substrate in which semiconductor elements are formed with semiconductor zones adjoining a surface of the substrate, which surface is provided with a silicon oxide layer in which contact windows are formed within which the semiconductor zones are exposed, whereupon on the silicon oxide a layer system is deposited with a barrier layer comprising at least titanium, tungsten, or nitrogen, and a layer of conductive material lying thereon, in which subsequently a metallization connected to the semiconductor zones is formed, characterized in that an aluminum layer is provided on the silicon oxide layer below the metallization. 5. A method as claimed in Claim 4, characterized in that the aluminum layer is deposited after the windows have been formed in the silicon oxide layer. 6. A method as claimed in Claim 5, characterized in that the aluminum layer is deposited such that deposition of aluminum on the walls of the windows is counteracted.

Inventors: DEKKER RONALD, MAAS HENRICUS G F, VAN DEN EINDEN WILHELMUS T A J, DEKKER, RONALD, MAAS, HENRICUS, G., F., VAN DEN EINDEN, WILHELMUS, T., A., J.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1998
Main classification: H01L29
IPC codes: H01L21, H01L23, H01L29