FIELD-EFFECT TRANSISTOR WITH SELF-ALIGNED GATE AND METHOD FOR ITS MANUFACTURE

Abstract: A semiconductor device comprising a compound semi­ conductor substrate (10) whose surface is provided with a source region (6), a drain region (7) and an interventing channel region (5); a source electrode (8) formed on said source region (6); a drain electrode (9) mounted on said drain region (7); and a 3-ply gate electrode (1) formed on said channel region (5) and consisting of a high melting metal layer (4), a barrier metal layer (3) and a gold layer (2) in that order.

Description: This invention relates to a semiconductor device comprising a gate electrode of high melting metal provided in a self-alignment fashion, and, more particularly, to a super high frequency transistor and the method of manufacturing the same. When a super high frequency semiconductor device is manufactured, mask alignment should be effected with a precision of ±1 µm in the formation of the various regions of a substrate and an electrode. However, the ordinary photoengraving process has the drawbacks that variations arise in the channel length and source resistance, which are parameters in defining the property of a semiconductor device, because mask alignment is carried out with low precision; the device itself presents variations in its property and has a low yield; and, moreover, the manufacture of said device involves complex processing. For the resolution of the above-mentioned difficulties, the so-called self-alignment method has been proposed, this method comprising the steps of: forming on a substrate a gate electrode prepared from such a high melting metal as can withstand the high temperature heat treatment required for the activation of an ion-implanted layer; and ion-implanting an impurity into the substrate using said gate electrode as a mask. In the above-mentioned proposed method, the gate electrode is prepared from a high melting metal in place of, for example, a conventional low melting metal such as aluminium. Therefore, the proposed method offers the advantages that no difficulties arise even when a gate electrode is already formed at the time of the heat treatment required to activate an ion-implanted layer, said gate electrode being used in this case as a mask in ion implantation, thereby ensuring the precise formation of an impurity region. However, the proposed self-alignment method is accompanied with the drawback that the high melting metal has a high resistance; thus imposing limitations on the improvement of the high frequency property of an super high frequency transistor and, in fact, disallowing the attainment of any appreciable improvement. Apart from the proposed self-alignment method, attempts have also been made to deposit gold on the high-melting metal in order to reduce gate resistance. However, this attempt has also been accompanied with difficulties, such that, when annealing was carried out in a furnace for the activation of an ion-implanted layer, the gold penetrated the high melting metal and reacted with the substrate, thus destroying the Schottky contact. Therefore, only a high melting metal having high electric resistance must be used as a gate electrode. As a result, it is difficult to provide an super high frequency transistor. JP-A-57-153 475 describes a multi-layer electrode on a Ga As substrate. The electrode has a first layer of tungsten, a barrier layer of tungsten nitride and a third layer of gold. The tungsten nitride layer prevents reaction between the gold and tungsten layers and between the gold layer and the Ga As substrate. It is, accordingly, an object of this invention to provide a semiconductor device which is free of the drawbacks accompanying any conventional device, and which has so low a noise level as to be suitably applied in super high frequency uses. Another object of the invention is to provide a method of manufacturing the above-mentioned semiconductor device. To attain this first object, the present invention provides a semiconductor device which comprises a semiconductor device, comprising: a GaAs substrate whose surface is provided with a source region, a drain region and an intervening channel region; a source electrode formed on said source region; a drain electrode formed on said drain region; and a 3-ply gate electrode mounted on said channel region and consisting of a high melting point metal layer, a barrier metal layer and a gold layer, characterized in that said high metal melting point metal layer is formed of a material selected from the group consisting of TiW and WSi, and said barrier metal layer is formed of molybdenum. It is preferred that the high melting metal layer be prepared with a thickness ranging between 5nm (50Å) and 1,000Å; the barrier metal layer be prepared with a thickness ranging between 20nm (200Å) and 100nm (1,000Å); and the Au layer be prepared with a thickness ranging between 200nm (2,000Å) and 600nm (6,000Å). The gate electrode of the semiconductor device of this invention is prepared from a high melting metal. Consequently, the source and drain regions can be very precisely formed in self-alignment, with the gate electrode used as a mask, thereby ensuring the manufacture of a semiconductor device free from property variations. The gate electrode provided with a gold layer of low resistivity can be reduced in resistivity as a whole, thus making it possible to produce a field effect transistor adapted for use in high frequency applications. Since a barrier metal layer is interposed between the high melting metal layer and the gold layer, the unwanted penetration of gold through the high melting metal layer into the substrate, which might otherwise arise during annealing for the activation of ion implanted regions, resulting in reaction between the gold and the substrate, and giving rise to the destruction of the Schottky contact, is eliminated. Another embodiment of this invention provides a method of manufacturing a semiconductor device which comprises the steps of: forming a high melting metal layer, a barrier metal layer and a gold layer on an active layer mounted on a compound semiconductor substrate in the order mentioned, said high melting point metal layer being formed of a material selected from the group consisting of TiW and WSi and said barrier metal layer being formed of molybdenum; patterning said high melting metal layer, barrier metal layer and gold layer in the same pattern to provide a 3-ply gate electrode; forming an insulating film over the entire surface of said substrate that includes the surface of said 3-ply gate electrode; ion-implanting, in said substrate, an impurity having the same conductivity type as said active layer using said gate electrode and insulating films formed on both side walls of said gate electrode as masks; and mounting a source electrode on said source region and a drain electrode on said drain region. The insulating film, used in the present invention can be prepared from, for example, CVD-SiO₂. It is possible to subject the insulating film to anisotropic etching prior to the ion implantation step to retain the insulating film on only the side wall of said gate electrode. It is preferred that annealing for the activation of the ion-implanted region be carried out after forming an out-diffusion-preventing film, for example, PSG film over the entire surface of said ion-implanted region. The deposition of the source and drain electrodes can be effected, for example, by the lift-off method. As described above, with the method of this invention, since an impurity is implanted in the substrate to provide the source and drain regions by applying a mask consisting of the gate electrode and the insulating film deposited on the side wall of said gate electrode, the source and drain regions can be formed with high precision. Since the gate electrode is a 3-ply type consisting of a high melting metal layer, a barrier metal layer and a gold layer, the gold is prevented from penetrating through the high melting metal layer into the substrate during annealing, and the resistance of the gate electrode is reduced. Further, infrared rapid thermal annealing (IRTA) ensures the full activation of an impurity without a harmful affect on the gate electrode. This invention can be more fuly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view of a metal semiconductor field effect transistor embodying this invention; and Figs. 2A to 2F are sectional views indicating the sequential steps of manufacturing the field effect transistor of Fig. 1. Description may now be made, with reference to the accompanying drawings, of a metal semiconductor field effect transistor embodying this invention. The gate electrode 1 is a 3-ply type consisting of a gold layer 2, an Mo layer 3 acting as a barrier metal layer, and a TiW layer 4 functioning as a high melting metal layer. Said gate electrode 1 is mounted on an N type channel region 5 formed on the surface of a GaAs substrate 10. Provided on both sides of said channel region 5 are a source region 6 and a drain region 7, both of N⁺ type. A source electrode 8 and a drain electrode 9 are mounted on the corresponding source region 6 and drain region 7. Description may now be made, with reference to Figs. 2A to 2F, of the sequential steps of manufacturing the subject metal semiconductor field effect transistor. A TiW layer 4 (thickness, 100nm (1,000Å)) acting as a high melting metal layer, an Mo layer 3 (thickness, 100nm (1,000Å)) functioning as a barrier metal, and a gold layer 2 (thickness 500nm (5,000Å)) is formed on an N type active layer 5 to be used as a channel region which is formed on a GaAs substrate 10. The above-mentioned metal layers are selectively removed by means of anisotropic etching, for example, reactive ion etching (RIE), to form a gate electrode 1 on said N type active layer (channel region) 5. Then (Fig. 2B) a SiO₂ film 11 is provided with a thickness of 0.5 microns by means of plasma CVD over the entire surface of said gate electrode 1 and GaAs substrate 10. Next (Fig. 2C), an anisotropic etching, for example, RIE is applied to said SiO₂ film 11, thereby causing an SiO₂ film 11a to be retained on both side walls of said gate electrode 1. Thereafter (Fig. 2D), an N type impurity, for example, ₂₉Si is ion-implanted in the substrate 10 with the gate electrode 1 and SiO₂ film 11a used as masks to produce N⁺ type regions 6 and 7. Thereafter (Fig. 2E) a cap film 12 consisting of phosphosilicate glass (PSG) is formed over the entire surface of the structure to suppress out-diffusion. Later, annealing is applied by means of infrared ray irradiation for several to 10 seconds at a temperature of 700 to 1,000°C to activate said N⁺ type regions, thereby providing source and drain regions 6 and 7. Last (Fig. 2F), those portions of said PSG film which are formed in the prescribed section of said source region 6 and drain region 7 are selectively removed. Then an Au-Ge layer is mounted on the whole surface of the remaining mass. The PSG film 12 is etched off and the overlying Au-Ge layer is lifted off to provide the source electrode 8 and drain electrode 9, thus finishing the subject metal semiconductor field effect transistor. Referring to the above-mentioned process of manufacturing the semiconductor device, the anisotropic etching of the SiO₂ film 11 is not always required. It is possible to shift from the step of Fig. 2B in which the SiO₂ film 11 is formed on the entire surface of the fabricated structure, to the step of Fig 2D in which the N type impurity is ion implanted. In such a case, the SiO₂ film 11 has to be removed in the succeeding step. The thickness of the SiO₂ film 11 is controlled so as to prevent overlapping between the gate electrode 1 and a region defined between the source and drain regions 6 and 7, to isolate said gate electrode 1 from said intervening region. The SiO₂ film 11a retained on both side walls of the gate electrode 1 may eventually be removed or left in situ.

Claims: A semiconductor device, comprising: a GaAs substrate (10) whose surface is provided with a source region (6), a drain region (7) and an intervening channel region (8); a source electrode (8) formed on said source region (6); a drain electrode (9) formed on said drain region (7); and a 3-ply gate electrode (1) mounted on said channel region (5) and consisting of a high melting point metal layer (4), a barrier metal layer (3) and a gold layer (2), characterized in that said high metal melting point metal layer is formed of a material selected from the group consisting of TiW and WSi, and said barrier metal layer is formed of molybdenum. The semiconductor device according to claim 1, characterized in that said high melting metal (4) forms a Schottky contact with said compound semiconductor (10). The semiconductor device according to claim 1, characterized in that said channel region (5) is an N type impurity region, and said source and drain regions (6,7) are N⁺ type impurity regions. The semiconductor device according to claim 1, characterized in that said source and drain electrodes (8,9) are prepared from an Au-Ge alloy. A method of manufacturing a semiconductor device characterized by comprising the steps of: forming a high melting metal layer (4), a barrier metal layer (3) and a gold layer (2) on an active layer (5) mounted on a compound semiconductor substrate (10) in the order mentioned, said high melting point metal layer being formed of a material selected from the group consisting of TiW and WSi, and said barrier metal layer being formed of molybdenum; patterning said high melting metal layer (4), barrier metal layer (3) and gold layer (2) in the same pattern to provide a 3-ply gate electrode (1); forming an insulating film (11) over the entire surface of said substrate (10) that includes the surface of said 3-ply gate electrode (1); ion-implanting, in said substrate (10), an impurity having the same conductivity type as said active layer (5) using said gate electrode (1) and insulating films (11a) formed on both side walls of said gate electrode (1) as masks; and mounting a source electrode (8) on said source region (6), and a drain electrode (9) on said drain region (7). The method according to claim 5, characterized in that said insulating film (11) is prepared from CVD SiO₂. The method according to claim 5, characterized by comprising the step of subjecting said insulating film (11) to anisotropic etching, thereby causing an insulating film (11a) to be retained only on both side walls of said gate electrode (1). The method according to claim 7, characterized by comprising the step of forming an out-diffusion-preventing layer (12) over the whole surface of the resulted structure after said ion-implantation step. The method according to claim 5, characterized in that after said ion-implanting annealing is carried out by the irradiation of infrared rays. The method according to claim 5, characterized in that said source and drain electrodes (8,9) are formed by the lift-off method.

Inventors: FURUKAWA, MOTOKI C/O PATENT DIVISION, KISHITA, YOSHIHIRO C/O PATENT DIVISION, MITANI, TATSURO C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: H01L29
IPC codes: H01L29, H01L21