Abstract:
A semiconductor device including a bipolar transistor having an emitter layer consisting of a semiconductor containing indium, and a protective insulating film containing silicon and oxygen which is formed on the surface of the guard ring of the emitter layer, wherein the protective insulating film has a density of oxygen of less than 7×10 22  cm −3 . This semiconductor device prevents performance deterioration and ensures high performance in a power amplifier.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device and a power amplifier using the same. 
     In recent years, with the rapid growth in demand for mobile communication equipment, research and development of compound semiconductor devices for power amplifiers used in mobile communication equipment has been actively conducted. As one of such compound semiconductor devices, a hetero junction bipolar transistor (hereinafter called “HBT”) which has a high current drivability has been used. 
     AlGaAs has been widely used as a material for HBT emitter layers. However, there is an increasing trend to develop HBTs which use InGaP instead of AlGaAs because the former is more reliable in use over a long period that the latter. An example of an HBT which uses an InGaP emitter layer has been disclosed, for example, in. Japanese Patent Laid-Open Publication No. 07-106343. 
     This prior art is illustrated in FIG.  15 . An n-type GaAs emitter protective layer  306  is formed on an n-type InGaP emitter layer  305 ; an SiO 2  side wall  313  is formed on the area of the n-type GaAs emitter protective layer  306  which corresponds to the guard ring  312  of the n-type InGaP emitter layer  305 . The n-type GaAs emitter protective layer  306  prevents direct contact between the n-type InGaP emitter layer  305  and the SiO 2  side wall  313 , thereby avoiding an increase in a leakage current. 
     SUMMARY OF THE INVENTION 
     The present invention has an object to provide a semiconductor device in which, in a bipolar transistor having an emitter layer consisting of a semiconductor containing indium, a GaAs emitter protective layer is not used as a protective layer for preventing an increase in a leakage current between the emitter and base, and also provide a power amplifier using the same. 
     The above-said object can be achieved by covering the emitter layer guard ring surface of the bipolar transistor having an emitter layer consisting of a semiconductor containing indium, with a protective insulating film which contains silicon (Si) and oxygen (O) and has a density of oxygen of less than 7×10 22  cm −3 . 
     It is also acceptable that the density of oxygen of the protective insulating film is 3×10 22  cm −3  is or less, or 8×10 21  cm −3  or less. 
     Also, the protective insulating film may further contain nitrogen (N) or hydrogen (H) as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail based on the followings, wherein: 
     FIG. 1 is a sectional view illustrating a semiconductor device according to the present invention; 
     FIG. 2 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention; 
     FIG. 3 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention; 
     FIG. 4 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention; 
     FIG. 5 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention; 
     FIG. 6 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention; 
     FIG. 7 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention; 
     FIG. 8 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention, 
     FIG. 9 is a sectional view illustrating the manufacturing method for an embodiment of a semiconductor device according to the present invention; 
     FIG. 10 is a sectional view of an embodiment of a semiconductor device according to the present invention; 
     FIG. 11 is a graph of reverse current against the density of oxygen in the protective insulating film; 
     FIG. 12 is a circuit diagram for a power amplifier; 
     FIG. 13 is a graph of a reverse current Ib against the reverse voltage Vbe in the insulating film formed under forming condition 4 shown in Table 1; 
     FIG. 14 is a graph of reverse current Ib against the reverse voltage Vbe in the insulating film formed under forming condition 2 shown in Table 1; and 
     FIG. 15 is a sectional view illustrating a conventional semiconductor device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a sectional view illustrating an HBT which uses InGaP as an emitter layer material. In the figure, reference numeral  50  represents a GaAs base layer;  51  an InGaP emitter layer;  52  a GaAs ballast layer;  53  an InGaAs emitter contact layer;  54  a base electrode;  55  an emitter electrode;  80  a protective insulating film; and  90  an emitter wiring. The collector layer, the sub-collector layer and the collector electrode in the HBT are omitted in FIG. 1 for simpler illustration. The base electrode  54  contacts the base layer  50  due to diffusion of its material into the emitter layer  51  so as to make an Ohmic contact. Although the base electrode  54  contacts the emitter layer  51 , it is not necessary to take into consideration the electric current path from the base electrode  54  through the emitter layer  51 , the ballast layer  52  and the emitter contact layer  53  to the emitter electrode  55 . This is because the region (guard ring)  61 , which does not form a junction with the ballast layer  52 , of the InGaP emitter layer  51  is depleted and has a high resistance, and also because the interface  65  between the guard ring  61  and the base electrode  54  is a Schottky junction and has a high resistance. The surface  70  of the guard ring  61  is covered with a protective insulating film  80  to prevent progress in natural oxidation and any etchant infiltration failure in the manufacturing process. 
     FIG. 12 shows an example of a power amplifier for mobile communication equipment which has, as basic elements, HBTs using InGaP as an emitter layer material. In this power amplifier, a signal inputted through a signal input terminal  120  is sequentially amplified by HBTs  100 ,  101  and  102  connected through matching networks  110 ,  111 ,  112  and  113  before being outputted from a signal output terminal  125 . In this figure, reference numerals  141  to  146  represent choke inductors,  150  a collector wiring and  160  a base wiring. 
     When the power amplifier as shown in FIG. 12 was incorporated in a 1-GHz class system such as a GSM (Global System for Mobile Communication) and operated at a high power (several watts), its amplification factor gradually decreased. 
     In order to find the cause of this problem, the DC characteristics of the HBTs used in the power amplifier were investigated. It has been found that the amplification factors for HBTs  100 ,  101  and  102  decreased and, among them, the final stage HBT  102  showed a particularly remarkable decrease in amplification factor. Further, an examination of DC characteristics between terminals in the final stage HBT  102  has revealed that regarding the characteristics of the reverse current between the emitter and base, the emitter-base leakage current after amplification factor decrease (characteristic curve  202 ) is larger than that before amplification factor decrease (characteristic curve  201 ) as shown in FIG.  13 . 
     Then, in order to find the cause of the increase in leakage current between the emitter and the base, an investigation was also made as to how the reverse current between the emitter and base (per square centimeter emitter area) changes as the density of oxygen in the protective insulating film  80  is varied. Here, −5V was applied as the reverse voltage. 
     The density of oxygen in the protective insulating film was calculated from its constituent element atomic ratio and atomic density measured by the RBS (Rutherford Back Scattering) and HFS (Hydrogen Forward Scattering) methods. Supposing that the constituent elements of the protective insulating film are Si, O, N, and H (ingredients of the film material) and its composition is Si l O p N q Hr, constituent element atomic ratio l, p, q, r, as well as constituent element atomic density n all  can be calculated n all  represents a total number of Si, O, N and H atoms contained per cubic centimeter. The density of oxygen n o  can be obtained from the equation n o =n all ×p/(l+p+q+r). 
     FIG. 11 shows the result of the investigation: if the density of oxygen is in the range from 0 to 3×10 22  cm −3 , when −5V reverse voltage is applied, the reverse current is almost constant (10 −3  A/cm 2  or so). If the density of oxygen is larger than that, the reverse current sharply increases. FIG. 11 is a graph in which current values are plotted for densities of oxygen: −0, 8×10 21 , 7×10 22  and 1×10 23  cm −3 . These protective insulating film forming conditions are summarized in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Forming 
                 Forming 
                 Forming 
                 Forming 
               
               
                   
                 Condition 1 
                 Condition 2 
                 Condition 3 
                 Condition 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Density of oxygen 
                 −0 
                 8 × 10 21  cm −3   
                 7 × 10 22  cm −3   
                 1 × 10 23  cm −3   
               
               
                 Reverse 
                 −10 −3  A/cm 2   
                 −10 −3  A/cm 2   
                 10 3  A/cm 2   
                 10 4  A/cm 2   
               
               
                 Current 
                 (1) 4% SiH 4   
                 (1) 4% SiH 4   
                 (1) 5% SiH 4   
                 (1) 4% SiH 4   
               
               
                 (Vbe = 5V) 
                 (diluted 
                 (diluted 
                 (diluted 
                 (diluted 
               
               
                 Gas used 
                 with N 2 ; 
                 with N 2 ; 
                 with He; 
                 with N 2 ; 
               
               
                 (flow rate) 
                 500 sccm) 
                 450 sccm) 
                 100 sccm) 
                 500 sccm) 
               
               
                   
                 (2) NH 3   
                 (2) N 2 O 
                 (2) N 2 O 
                 (2) O 2   
               
               
                   
                 (30 sccm) 
                 (200 sccm) 
                 (220 sccm) 
                 (3500 sccm) 
               
               
                   
                   
                   
                   
                 (3) N 2   
               
               
                   
                   
                   
                   
                 (2000 sccm) 
               
               
                 Degree of vacuum 
                 49 Pa 
                 49 Pa 
                 40 Pa 
                 Atmospheric 
               
               
                   
                   
                   
                   
                 Pressure 
               
               
                 Substrate 
                 250° C. 
                 250° C. 
                 250° C. 
                 390° C. 
               
               
                 Temperature 
               
               
                 Supply 
                 RF power 
                 RF power 
                 RF power 
                 Heat 
               
               
                 Energy 
                 150 W 
                 150 W 
                 50 W 
               
               
                   
               
             
          
         
       
     
     Under the forming condition 1, a trace of oxygen actually remains in the insulating film forming device though no compound gas (which generates oxygen) is introduced as a material into the device. 
     When the protective insulating film forming condition 1 or 2 in Table 1 was used, the reverse current as shown in FIG. 11 was 10 −3  A/cm 2  or so, which demonstrates an improvement, or a decrease by 7 digits, as compared with the case in which the forming condition 4 was used. Even when forming condition 3 (plasma chemical vapor deposition) was used, the reverse current was 10 A/cm , which is smaller than 10 4  A/cm 2  in the case of the forming condition 4 (thermal chemical vapor deposition). 
     When HBTs which have an insulating film (made under the forming condition 2) were used for HBTs  100 ,  101  and  102  as shown in FIG. 12, no current amplification factor deterioration was observed. The initial DC characteristic of the reverse current between the emitter and base is shown as a characteristic curve in FIG.  14 . This characteristic remained almost unchanged even after operation at high power (several watts). The reason for this may be that, since the protective insulating film used has a low density of oxygen (3×10 22  cm −3  or less), generation of much indium oxide did not occur on the surface of the InGaP emitter layer and thus the initial reverse current was small and with no current leakage path proliferation. 
     Embodiment 1 
     A semiconductor device according to one embodiment of the present invention is explained below referring to FIG.  10 . Here, an HBT consists of the following layers formed on the main side surface of a semi-insulating GaAs substrate  1  one upon another in order: n type GaAs sub-collector layer  2 A (thickness: 600 nm; dopant: silicon; impurity concentration: 5×10 18  cm −3 ); n type GaAs collector layer  3 A (thickness: 800 nm; dopant: silicon; impurity concentration: 1×10 16  cm −3 ) p type GaAs base layer  4 A (thickness: 70 nm; dopant: carbon; impurity concentration: 3×10 19  cm −3 ); n type In x Ga 1−x P emitter layer  5 A (x: 0.5 ; thickness: 30 nm; dopant: silicon; impurity concentration: 3×10 17  cm −3 ); n type In Y Ga 1−y  As emitter contact layer  6 A (y: 0 around the junction with the n type In x Ga 1−x P emitter layer  5 A, 0.5 around the area adjacent to the emitter electrode  10 ; thickness:; 400 nm; dopant: silicon; impurity concentration: 3×10 17  cm −3  around the junction with. the n type In x Ga 1−x P emitter layer  5 A, 5×10 18  cm −3  around the area adjacent to the emitter electrode  10 ); first emitter electrode  10  and second emitter electrode  11 A; base electrode  11 ; and collector electrode  12 .  21 A denotes a protective insulating film (made under forming condition  2  as shown in Table 1) with a density of oxygen of 8×10 21  cm −3  which protects the main side surface of the emitter layer  5 A.  22 A denotes an insulating film which protects the junction between the emitter layer  5 A and base layer  4 A and the surface of the collector layer  3 A.  23  represents an insulating film including an insulator coating (SOG) made to prevent defects such as discontinuity and short-circuits in making collector wiring  30  and base wiring, while  24  represents an insulating film including an SOG made to prevent defects such as discontinuity and short-circuits in making emitter wiring  31 . Here, the base wiring, which forms an electrical junction with the base electrode  11  in a cross section other than that shown in FIG. 10, is not shown in the figure. 
     The method for manufacturing this semiconductor device is explained below by reference to FIGS. 2 to  9 . First, an epitaxial film is formed on the main side surface of a semi-insulating GaAs substrate  1  by placing the following layers one upon another in the order of mention using the MOCVD method: n type GaAs sub-collector layer  2  (thickness: 600 nm; dopant: silicon; impurity concentration: 5×10 18  cm −3  ); n type GaAs collector layer  3  (thickness: 800 nm; dopant; silicon; impurity concentration: 1×10 16  cm −3 ); p type GaAs base layer  4  (thickness: 70 nm; dopant: carbon; impurity concentration: 3×10 19  cm −3 ); n type In x Ga 1−x P emitter layer  5  (x: 0.5; thickness: 30 nm; dopant: silicon; impurity concentration: 3×10 17  cm −3 ); n type In y GA l-y  As emitter contact layer  6  (y: 0 around the junction with the n type In x Ga 1−x P layer  5 , 0.5 around the area adjacent to the emitter electrode  10 ; thickness: 400 nm; dopant: silicon; impurity concentration: 3×10 17  cm −3  around the junction with the n type In x Ga 1−x P layer  5 , 5×10 18  cm −3  around the area adjacent to the emitter electrode  10 ). Then, as shown in FIG. 2, WSi z  (z: approx.0.3; thickness: 300 nm) is formed by sputtering and then patterning is done by photolithography, followed by plasma etching to make a first layer emitter electrode  10 . 
     Then, as shown in FIG. 3, using the first layer emitter electrode  10  as a mask, then n type In y Ga 1−y As layer  6  is etched with phosphoric acid etching solution (H 3 PO 4  (85 weight %): H 2 O 2  (30 weight %): H 2 O=1:2:40) to make it an emitter contact layer  6 A, followed by forming an insulating film  20  all over by the thermal CVD method. The forming condition of the thermal CVD method used for forming the insulating film  20  is as follows: three types of gas are used (4% SiH 4  diluted with N 2 , and O 2  and N 2  and their flow rates are 500 sccm, 3,500 sccm and 2,000 sccm, respectively); atmospheric pressure and 390° C. substrate temperature. 
     Then, a resist pattern  40  is made by photolithography and the insulating film  20  is processed by plasma etching with a hydrofluoric acid solution to make it  20 A, as shown in FIG.  4 . 
     Next, Pt (20 nm), Ti (10 nm), Mo (30 nm), Ti (50 nm),.Pt (50 nm), and Au (120 nm) are deposited one upon another in the order of mention by evaporation before forming a base electrode  11  using the lift-off method. An Ohmic contact between the base layer  4  and the base electrode  11  is made by a thermal sintering process based at a later process. The base electrode  11  is formed on the first layer emitter electrode  10  by self-alignment, so also formed on the emitter electrode  10  is a second layer emitter electrode  11 A which consists of Pt, Ti, Mo, Ti, Pt and Au deposited one upon another in the order of mention (FIG.  5 ). 
     Then, a protective insulating film  21  which protects the surface of the n type In x Ga 1−x P emitter layer  5  is formed all over using the plasma CVD, method under forming condition 2 in Table 1 (two types of gas, 4% SiH 4  diluted with N 2  and N 2 O whose flow rates are 450 sccm and 200 sccm, respectively, are used; degree of vacuum 49 Pa; substrate temperature 250° C.; supply energy RF power 150W) (FIG.  6 ). 
     Then, patterning is done by photolithography and then insulating films  21  and  20 A are processed by plasma etching. At this stage, the insulating film  20 A is completely removed. Next, the n type In x Ga 1−x P emitter layer  5  is etched with hydrochloric acid, and the p type GaAs base layer  4  and n type GaAs collector layer  4  are etched with phosphoric acid etching solution (H 3 PO 4  (85 weight %): H 2 O 2  (30 weight %): H 2 O =1:2:40) to make them an emitter layer base layer  4 A, and a collector layer  3 A, respectively (FIG.  7 ). 
     Next, using the thermal CVD method, the whole surface is covered with an insulating film under the following forming condition: three types of gas are used (4% SiH 4  diluted with N 2 , and O 2  and N 2  whose flow rates are 500 sccm, 3,500 sccm and 2,000 sccm, respectively) atmospheric pressure; and 390° C. substrate temperature. Then, after patterning by photolithography, this insulating film is processed by plasma etching to make it 22A. Using this insulating film  22 A as a mask, a channel which reaches the n type GaAs sub-collector layer  2  is formed using a phosphoric acid etching solution as mentioned above to make a collector electrode  12  by depositing AuGe (60 nm), W (10 nm), Ni (10 nm), and Au (300 nm) one upon another by evaporation in the order of-mention (FIG.  8 ). An Ohmic contact of the collector electrode  12  with the n type GaAs sub-collector layer  2  is made by alloying at about 390° C. 
     Then an insulating film is formed all over the surface using the plasma CVD method under the following forming condition: two types of gas, 4% SiH 4  diluted with N 2  and N 2 O whose flow rates are 450 sccm and 200 sccm, respectively, are used; degree of vacuum 49 Pa; substrate temperature 250° C.; and supply energy RF power 150W, then to smoothen the surface to prevent such defects as discontinuity and short-circuits, rotary coating with SOG is done. Further, using the plasma CVD method (forming condition: two types of gas, 4% SiH diluted with N 2  and N 2 O whose flow rates are 450 sccm and 200 sccm respectively, are used; degree of vacuum 49 Pa; substrate temperature 250° C.; and supply energy RF power 150W), an insulating film is formed all over the surface to make an insulating film  23  which contains SOG; then an opening is made by plasma etching; Mo (50 nm) and Au (800 nm) are deposited on it by evaporation in the order of mention before a collector wiring  30  and a base wiring (not shown in the figure) are made by photolithography (FIG.  9 ). 
     Then, an insulating film is formed all over the surface using the plasma CVD method (forming condition: two types of gas, 4% SiH 4  diluted with N 2  and N 2 O whose flow rates are 450 sccm and 200 sccm respectively are used; degree of vacuum 49 Pa; substrate temperature 250° C.; and supply energy RF power 150W), then, rotary coating of the whole film surface with SOG is done to smoothen the surface to prevent such defects as discontinuity and short-circuits. Further, using the plasma CVD method (forming condition: two types of gas, 4% SiH 4  diluted with N 2  and N 2 O whose flow rates are 450 sccm and 200 sccm respectively, are used; degree of vacuum 49 Pa; substrate temperature 250° C.; and supply energy RF power 150W), an insulating film is formed all over the surface to make an insulating film 24 which contains SOG; then an opening is made by plasma etching; Mo (50 nm) and Au (800 nm) are deposited on it by evaporation in the order of mention before an emitter wiring  31  is made by photolithography to complete a semiconductor device as shown in FIG.  10 . 
     Embodiment 2 
     FIG. 12 is a circuit diagram for a power amplifier based on a semiconductor device according to the present invention. In the figure, reference numerals  100 ,  101  and  102  represent HBTs connected in parallel which each uses a semiconductor device according to the present invention.  110 ,  111 ,  112  and  113  represent matching networks,  120  a signal input terminal,  125  a signal output terminal,  141 ,  142 ,  143 ,  144 ,  145  and  146  represent choke inductors,  150  a collector wiring, and  160  a base wiring. 
     The circuit shown in FIG. 12 is the same as that for the power amplifier whose amplification factor has decreased due to high power operation as mentioned earlier except that HBTs  100 ,  101  and  102  each uses a semiconductor device according to the present invention. Therefore, in this circuit, the phenomenon of an amplification factor decrease caused by high power operation was not observed. This demonstrates that according to the present invention, in a power amplifier, performance deterioration can be prevented and thus high performance can be ensured.