Patent Publication Number: US-2022216310-A1

Title: Semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of application Ser. No. 17/163,626 filed on Feb. 1, 2021, which claims priority to prior Japanese Patent Application No. 2020-157789 filed with the Japan Patent Office on Sep. 18, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The disclosure relates to a semiconductor device, or more specifically, to a power semiconductor device including a metal-oxide semiconductor field-effect transistor (MOSFET) capable of controlling soft recovery. 
     Japanese Patent Application Publication No. 2015-018913 (Patent Document 1) discloses a super junction MOSFET which is capable of reducing a reverse recovery current and a reverse recovery period by relaxing a hard recovery waveform during a reverse recovery operation, thereby achieving high-speed switching and a low reverse recovery loss. In this super junction MOSFET, a second buffer layer having a higher concentration than an n-type drift layer  4   a  of a parallel pn layer is formed under a first buffer layer. Carrier lifetime of each of the first buffer layer and the parallel pn layer is adjusted to be shorter than carrier lifetime of the second buffer layer. Thus, a rise of the hard recovery waveform is made gentle to form a soft recovery waveform (paragraph 0031 of Patent Document 1). 
     Japanese Patent Application Publication No. 2017-011001 (Patent Document 2) discloses a semiconductor device which is configured to reduce a forward voltage drop and suppress waveform oscillation during reverse recovery, and has soft recovery characteristics. In this semiconductor device, a width in a lateral direction of an n+ cathode region is set shorter than a width in a lateral direction of a FWD anode part. Thus, the above described configuration promotes hole injection from a portion of a p+ collector region, which faces the FWD anode part across an n −  drift layer, to the n −  drift layer. In this way, a carrier concentration on the n +  cathode region side of the n −  drift layer is increased. Accordingly, the forward voltage drop of the FWD can be reduced and the FWD is more likely to be turned on. As a consequence, it is possible to achieve the soft recovery (lowering a peak of a reverse recovery current If) and suppression of a waveform oscillation (lowering a peak of a voltage surge Vak) during the reverse recovery of the FWD (paragraph 0065 of Patent Document 2). 
     In general, when a switching element in a power semiconductor device transitions from an on-state to an off-state, for instance, a signal may fluctuate for a certain period on and before the switching element transitions completely to the off-state. This period from the start to the end of the fluctuation of the signal, or in other words, the period from a time point when the signal is once set to the off-state (with no current) to a time point when the signal completely becomes the off-state after the fluctuation of this signal in transition of the switching element from the on-state to the off-state is defined as reverse recovery time (T RR ). Short reverse recovery time is generally preferred in light of stabilization of operations of the semiconductor device and reduction in power consumption. In addition, a sharp change in current may bring about an adverse impact such as a breakdown on the semiconductor device. In this regard, a soft recovery technique is known as a technique for shortening the reverse recovery time while reducing the sharp change in current during the transition of the switching element from the on-state to the off-state. 
     SUMMARY 
     A semiconductor device according to one or more embodiments may include: a sub-layer with first conductivity type; a drift layer with first conductivity type, a base region with second conductivity type positioned on the drift layer, a source region in contact with the base region, a source electrode, a plurality of trenches, each trench in contact with the drift layer, the base region, and the source region, a plurality of insulating regions, each insulating region positioned inside of each trench, a plurality of gate electrodes, each gate electrode positioned inside of each trench, and a plurality of field plates, each field plate electrically connected to the source electrode and positioned in the insulating region in the trench. The field plate may comprise high-resistance polysilicon. In one or more embodiments, the resistance between one of the field plates inside of a trench and the source electrode may be 50 kΩ or more to 800 kΩ or less. In one or more embodiments, the resistance between one of the field plates and the source electrode may be 58 kΩ or more to 254 kΩ or less. In one or more embodiments, the resistance between one of the field plates inside of a trench and the source electrode may be 25 kΩ or more to 300 kΩ or less. In one or more embodiments, the resistance between one of the field plates and the source electrode may be 55 kΩ or more to 260 kΩ or less. In the disclosed embodiments, the drift layer may comprise impurities, a pinch-off state does not substantially occur at the impurity concentration. In disclosed embodiments, the gate electrode may be positioned closer to the sub-layer side than the base region. In disclosed embodiments, portion of the gate electrode having a shortest distance from the sub layer may be positioned such that the shortest distance is shorter than a distance of the sub-layer from the portion of the base region in contact with the trench of the gate electrode. In disclosed embodiments, portion of the gate electrode having the shortest distance from the sub layer may be positioned in a range of 0.1 μm or more and 0.5 μm or less than the distance of the sub-layer from the portion of the base region in contact with the trench of the gate electrode. 
     One or more additional or alternative embodiments may include a shallow region positioned inside the base region between the trenches, and positioned below the contact digging structure, and below the silicon contact. In disclosed embodiments, the shallow region may include a second conductivity type. In disclosed embodiments, the shallow region may include a higher impurity concentration than the base region. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a cross-sectional view of a semiconductor device according to one or more embodiments; 
         FIG. 2  is a circuit diagram illustrating an equivalent circuit of a semiconductor device, such as is illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a cross-sectional view of a semiconductor device according to one or more embodiments; 
         FIG. 4  is a circuit diagram illustrating an equivalent of a semiconductor device, such as is illustrated in  FIG. 3 ; 
         FIG. 5  is a diagram illustrating a graph for explaining a damping factor; 
         FIG. 6  is a diagram illustrating a top plan view of a semiconductor device according to one or more embodiments; 
         FIGS. 7A, 7B, 7C, 7D, 7E, 7F, 7G, and 7H  are diagrams illustrating cross-sectional views of a manufacturing process of a semiconductor device according to one or more embodiments; and 
         FIG. 8  is a graph diagram illustrating a relationship between drain-source voltage Vds and recovery current Id during a recovery period. 
     
    
    
     DETAILED DESCRIPTION 
     One or more embodiments are described in detail with reference to the drawings. In the following description of the drawings, identical or similar portions may be denoted by identical or similar reference numerals. The description of the drawings is schematic. Relations of thicknesses and dimensions, ratios of thicknesses of layers, and the like are mere examples and do not intend to limit the technical ideas of the invention. Dimensional relations or ratios may vary among the drawings. The following embodiments will describe an example in which a first conductivity type is an n-type and a second conductivity type is a p-type. However, there is also a case where it is possible to select a reverse relation of the conductivity types where the first conductivity type is the p-type and the second conductivity type is the n-type. When positional relations among components are discussed in the following description, explanations including an “upper side”, a “lower side”, a “right side”, a “left side”, and so forth are used as appropriate based on directions in the drawings to be referred to. Nonetheless, these directions do not limit the technical ideas of the invention. Meanwhile, the explanations including the “upper side”, the “lower side”, the “right side”, the “left side”, and so forth may be used even when relevant components are not in contact. On the other hand, an X axis and a Y axis may be illustrated when explaining the directions. Here, mainly in a case of a cross-sectional view, a “lateral direction” or a “longitudinal direction” may represent an X direction or an opposite direction to the X direction in the illustrated drawings. Meanwhile, a “height direction” may represent a Y direction in the illustrated drawing. In the meantime, a “depth direction” may represent an opposite direction to the Y direction in the illustrated drawing. 
       FIG. 1  is a diagram illustrating a cross-sectional view of a semiconductor device  100  according to one or more embodiments. The semiconductor device  100  includes a sub-layer  101 , a drift layer  103  positioned above the sub-layer  101 , a base region  105  positioned above the drift layer  103 , shallow regions  107 , an alloy layer  109 , a metal layer  111 , a first insulating region  113 , a metal region  115 , a source electrode  117 , trenches  121 , second insulating regions  123 , field plates  125 , gate electrodes  127 , and a source region  129 . In the semiconductor device  100 , the trenches  121  are positioned in a depth direction in the drift layer  103 , and each trench  121  is filled a member corresponding to the second insulating region  123 . The field plate  125  and the gate electrode  127  are positioned inside each second insulating region  123 . While the embodiment or embodiments of  FIG. 1  illustrates two trenches, the invention is not limited only to the above described configuration. The semiconductor device  100  of the embodiment or embodiments may include one, three, four, or more trenches instead. 
     The drift layer  103  is positioned above the sub-layer  101 . Each of the sub-layer  101  and the drift layer  103  may be of the first conductivity type, and the sub-layer  101  may have a higher impurity concentration than that of the drift layer  103 . The drift layer  103  may be formed by epitaxial growth and its impurity concentration is preferably set to such a concentration that does not cause a pinch-off. Here, a dosage amount of the impurity in the drift layer  103  is set preferably in a range from about 2.0 e 16  cm −3  to  9 . 0  e 16  cm −3  in the case of the 40-V withstand voltage class. Meanwhile, the dosage amount of the impurity in the drift layer  103  is set preferably in a range from about 1.3 e 16  cm −3  to 2.3 e 16  cm −3  in the case of the 100-V withstand voltage class. The drift layer  103  of the semiconductor device  100  may be set to a single impurity concentration. However, the invention is not limited to the foregoing. For example, as illustrated in  FIG. 1 , the drift layer  103  may include a drift layer  103 A that has a certain impurity concentration and a drift layer  103 B that has an impurity concentration different from that of the drift layer  103 A. In this case, an electric field intensity of the gate electrode  127  can be relaxed by controlling the concentration and the thickness of the drift layer  103 B near the gate electrode  127 . For example, the impurity concentration in drift layer  103 A is higher than that in drift layer  103 B. 
     The base region  105  is positioned above the drift layer  103 . The base region  105  may be of the second conductivity type. 
     Each shallow region  107  is positioned in the base region  105 . In the meantime, the shallow region  107  may be positioned below an excavated contact structure and below a silicon contact as illustrated in  FIG. 1 . The shallow region  107  may be of the second conductivity type and may have a higher impurity concentration than that of the base region  105 . Here, a breakdown is brought about at a p-n junction part below the silicon contact by providing the shallow region  107  making it possible to increase the electric field intensity only at a rise of a recovery current without extending a depletion layer toward the base region  105 . 
     The alloy layer  109  is positioned between the metal layer  111  and the base region  105  that includes the shallow regions  107 . The metal layer  111  may contain titanium. In general, when silicon is directly bonded to a metal (such as aluminum), interdiffusion of the metal (such as aluminum) and silicon may develop an aluminum spike. The alloy layer  109  is positioned in order to relax the aluminum spike. A silicide layer may be formed by subjecting the metal layer  111  to a thermal treatment. When the metal layer  111  contains titanium, a titanium silicide layer is formed by the thermal treatment. 
     The first insulating region  113  is positioned in the metal layer  111 . The first insulating region  113  may contain silicon dioxide (SiO 2 ). Alternatively, the first insulating region  113  may contain the same material as the second insulating region  123 . 
     The metal region  115  is positioned above the metal layer  111 . The metal region  115  may contain tungsten. The source electrode  117  is positioned above the metal region  115 . The source electrode  117  may be made of an aluminum alloy or an aluminum-copper alloy. 
     Each trench  121  is positioned inside the drift layer  103  in such a way as to extend in a depth direction of the semiconductor device  100 , or in other word, from the source electrode  117  side to the sub-layer  101  side. Here, a shape of an outer wall of the trench  121  may be set parallel to the depth direction of the semiconductor device  100  or gradually tapered in the depth direction. In the case of the tapered shape, the shape is preferably set to an elevation angle equal to or above 80 degrees but below 90 degrees when viewed from a bottom surface of the semiconductor device  100 , or more preferably set to the elevation angle in a range from 83 degrees to 87 degrees. 
     Each second insulating region  123  is positioned in the trench  121 . The field plate  125  and the gate electrode  127  are positioned in the second insulating region  123 . The field plate  125  is electrically coupled to the source electrode  117 . By providing the field plate  125  in the trench  121 , it is possible to achieve a high withstanding voltage while relaxing electric field concentration. 
     Each field plate  125  may contain a polycrystalline semiconductor material such as polycrystalline silicon. In the embodiment or embodiments, the field plate  125  contains high-resistance polycrystalline silicon. The field plate  125  of the embodiment or embodiments is electrically coupled to the source electrode  117  via this high-resistance polycrystalline silicon. In other words, the field plate  125  of the embodiment or embodiments includes high-resistance polycrystalline silicon and is electrically coupled to the source electrode  117 . Since the field plate  125  contains high-resistance polycrystalline silicon, an electric resistance Rfp between the field plate  125  and the source electrode  117  becomes high. Here, it is known to be effective to set a lower resistance between the field plate  125  and the source electrode  117  in order to reduce an effect of a displacement current. The embodiment or embodiments controls the displacement current by adjusting the resistance between the field plate  125  and the source electrode  117 . By controlling the displacement current, the soft recovery of the semiconductor device  100  is controlled. This resistance value between the field plate  125  and the source electrode  117  may be set in a range from about 50 kΩ to 800 kΩ inclusive per trench, or preferably in a range from about 58 kΩ to 254 kΩ inclusive. Meanwhile, a sheet resistance may be set to about 25 kΩ/sq or preferably equal to or above 29.7 kΩ. On the other hand, the value of Rfp may be set to about 5 Ω/sq. In this way, it is possible to improve a soft recovery characteristic of the semiconductor device  100 . In one or more embodiments, the resistance value between the field plate  125  and the source electrode  117  may be set in a range from about 25 kΩ to 300 kΩ inclusive per trench, or preferably in a range from about 55 kΩ to 260 kΩ inclusive.  FIG. 8  is a graph diagram illustrating the relationship between drain-source voltage Vds and recovery current Id during the recovery period. In  FIG. 8 , the results of the drain source voltage Vds and the recovery current Id during the recovery period are shown when the resistance (Rfp) value is small and the resistance (Rfp) value is large. When the resistance (Rfp) value is large, the voltage Vds is reduced only during the recovery period, and the magnitude and slope of the recovery current Id are reduced. 
     In the meantime, the gate electrode  127  is preferably positioned on the depth direction side relative to the base region  105 , or in other words, on the sub-layer  101  side. A portion of the gate electrode  127  having the shortest distance from the sub-layer  101  may be positioned closer to the sub-layer  101  than a portion of the base region  105  in contact with the trench  121  corresponding to the gate electrode  127  by an amount in a range from about 0.1 μm to 0.5 μm. In this way, the distance between the gate electrode  127  and the sub-layer  101  can be reduced. Here, the electric field intensity can be increased when the distance between the portion of the gate electrode  127  located closest to the sub-layer  101  and the portion of the base region  105  in contact with the trench  121  corresponding to the gate electrode  127  is short. A bottom portion of the gate electrode  127 , that is, an end of its surface close to the sub-layer  101  may be tapered. Alternatively, the end of the surface close to the sub-layer  101  may be rounded. In this way, it is possible to achieve the high withstanding voltage while relaxing the electric field concentration. 
     The source region  129  is positioned above the base region  105  and on upper side surfaces of the trenches  121 . The source region  129  may be of the first conductivity type. An outer wall of the source region  129 , or in other words, a wall where the alloy layer  109  and the metal layer  111  come into with the metal region  115  may be set substantially perpendicular in view of the bottom surface of the semiconductor device  100  or may be tapered as illustrated in  FIG. 1 . In this case, the outer wall is preferably set to an elevation angle equal to or above 80 degrees but below 90 degrees when viewed from the bottom surface of the semiconductor device 100, or more preferably set to the elevation angle in a range from 83 degrees to 87 degrees. By forming the outer surface into the tapered shape as described above, it is possible to reduce the electric field intensity while relaxing the electric field concentration. 
       FIG. 2  illustrates an equivalent circuit diagram of the semiconductor device  100  depicted in  FIG. 1 , which represents a case where just one trench  121  is positioned for the purpose of simplification. 
       FIG. 3  is a cross-sectional view illustrating another one or more embodiments of the semiconductor device  100 . In the embodiment or embodiments, the semiconductor device  100  includes the trenches  121 A,  121 B,  121 C, and  121 D, and the field plates  125 A,  125 B,  125 C, and  125 D are positioned inside the trenches  121 , respectively. Here, the resistance values Rfp between the field plates  125  and the source electrode  117  may be different from one another. In the semiconductor device  100  illustrated in  FIG. 3 , the resistance between a field plate  125 A and the source electrode  117  is set to Rfp_ 1  and the resistance between a field plate  125 B,  125 C, or  125 D and the source electrode  117  is set to Rfp_ 2 . Provision of such different resistance values Rfp makes it possible to deal with a change in displacement current more appropriately and to obtain a stable reverse recovery time (T RR ) characteristic even in the case of facing a sharp change in withstand voltage. 
       FIG. 4  illustrates an equivalent circuit diagram of the semiconductor device depicted in  FIG. 3 , which represents a case where just the trench  121 A and the trench  121 B are positioned for the purpose of simplification. 
       FIG. 5  is a diagram illustrating a relation between the resistance Rfp and a damping factor. The damping factor generally represents a braking coefficient for damping oscillation. The higher the damping factor is, the higher the ability of the soft recovery is achieved. As a consequence of analyzing the damping factor of the embodiment or embodiments, it turns out that the damping factor reaches a maximum at a certain value of the resistance Rfp and then reduces thereafter in some cases. From this finding, the inventor has obtained the knowledge that the performance of the soft recovery can be improved by obtaining the high damping factor as a consequence of controlling the resistance (Rfp) between the field plate  125  and the source electrode  117 . 
     In one or more embodiments, the shallow region  107  of the semiconductor device  100  is formed at a position deeper than the source region  129 . Further, the impurity concentration in the drift layer  103 A is higher than the impurity concentration in the drift layer  103 B, which is positioned between the drift layer  103 A and the base region. The drift layer  103 B comprises such impurity concentration that a pinch-off state does not substantially occur, which makes it difficult for the depletion layer to extend to the shallow region  107  and the drift layer  103 B during the period when the recovery current rises. 
     The electric field in the region below the shallow region  107  is further increased at a voltage lower than the product withstand voltage only during the recovery period, and the electric field breaks down in or near that region. As a result, the breakdown voltage drops temporarily. The shallow region  107  may be provided by engraving the upper surface of the semiconductor so as to be arranged under the contact digging structure and under the silicon contact allowing for more breakdown below or near that area. 
     Further, when the electric field in the region below the shallow region  107  increases, the potential of the field plate  125  rises. Here, the resistance Rfp between the field plate  125  of the semiconductor device  100  and the source electrode  117  according to one or more embodiments is high resistance. As a result, the maximum voltage at the start of recovery of the semiconductor device  100  is reduced. As described above, the soft recovery characteristics of the recovery period of the semiconductor device  100  may be improved. 
     The depletion layer extends during the steady state of recovery. The breakdown point of the semiconductor device  100  is arranged, for example, at the bottom of the trench  121 , and the breakdown is occurred at a voltage equal to or higher than the product withstand voltage. At the startup of recovery, since the concentration of impurities in the drift layer  103 A is low, the depletion layer tends to extend, and most of the recovery current flows through the capacitor generated between the drift layer  103 B and the field plate  125 . During the steady state of recovery, part of the recovery current flows through the capacitor generated between the drift layer  103 A and the field plate  125 . 
       FIG. 6  is a top plan view of the semiconductor device  100  of another embodiment or embodiments. With reference to  FIG. 6 , a description will be given of the control of the resistance (Rfp) between the field plate  125  and the source electrode  117 . As illustrated in  FIG. 6 , the semiconductor device  100  includes the trenches  121 . Two ends of each trench  121  are a positive electrode and a negative electrode. Here, a trench length LTR of a portion sandwiched by the positive electrode and the negative electrode may be adjusted in order to adjust the resistance Rfp. Meanwhile, in a manufacturing process, the resistance Rfp may be adjusted by controlling the dosage amount of the impurity in polycrystalline silicon. In addition, the resistance Rfp may be controlled by regulating a sheet resistance of polycrystalline silicon included in the field plate  125 . As mentioned above, it is possible to set the optimum resistance Rfp by using the trench length LTR, the dosage amount of the impurity in polycrystalline silicon, or a combination thereof. 
     In order to set the resistance Rfp between the field plate  125  and the source electrode  117  to the high resistance, it is also possible to set a contact resistance to a high resistance in addition to the field plate  125  and the trench  121 , for example. In  FIG. 6 , for instance, an electrode  131  to be electrically coupled to the source electrode is electrically coupled to the field plate  125  via a contact opening  133 . The resistance Rfp between the field plate  125  and the source electrode  117  can be set to the high resistance by changing the size of this contact opening  133 . 
       FIGS. 7A, 7B, 7C, 7D, 7E, 7F, 7G, and 7H  are diagrams for explaining a process of forming the shallow region  107 . As an example,  FIGS. 7A, 7B, 7C, 7D, 7E, 7F, 7G, and 7H  illustrate cross-sectional views of a portion between the trenches  121  depicted in  FIG. 1 , which are cross-sectional views that depict part of the portion between the trench  121 A and the trench  121 B for the convenience of explanations.  FIG. 7A  shows a process after the respective regions mentioned above are formed above the sub-layer  101  and the drift layer  103 .  FIG. 7B  shows a process after coating a photoresist  151  on a surface of this work-in-progress semiconductor device, the photoresist  151  is subjected to selective exposure by irradiating a photomask and then the photoresist  151  is selectively removed. In the embodiment or embodiments, a portion of the photoresist  151  located between the trench  121 A and the trench  121 B is removed so as to expose the first insulating region  113  at that portion.  FIG. 7C  shows a process after the first insulating region  113  is etched. This etching may be continued until reaching the source region  129 .  FIG. 7D  shows a process after the source region  129  is etched so as to reach the inside of the base region  105 . In the course of this etching, the source region  129  and the base region  105  may be tapered as illustrated in  FIG. 7D .  FIG. 7E  shows a process after the base region  105  is subjected to ion implantation. In this ion implantation, the impurity of the second conductivity type may be ion implanted.  FIG. 7F  shows a process after the photoresist coated on the surface of the semiconductor device  100  is removed.  FIG. 7G  shows a process after the shallow region  107  is formed in the base region  105  by diffusing the ion implanted impurity by a thermal treatment process such as rapid thermal annealing (RTA).  FIG. 7H  shows a process after the alloy layer  109 , the metal layer  111 , the metal region  115 , and the source electrode  117  are formed as needed. The semiconductor device  100  including the shallow regions  107  may be manufactured. 
     A conventional semiconductor device requires an additional circuit such as a snubber circuit in order to improve its soft recovery characteristic. In the meantime, the conventional semiconductor device faces a difficulty in controlling its structure and can hardly obtain a stable recovery characteristic as a consequence. The semiconductor device according to the one or more embodiments described above has an improved soft recovery capability. 
     The above-described aspects may be combined with each other as practicable within the contemplated scope of embodiments. The above described embodiments are to be considered in all respects as illustrative, and not restrictive. The illustrated and described embodiments may be extended to encompass other embodiments in addition to those specifically described above without departing from the intended scope of the invention. The scope of the invention is to be determined by the appended claims when read in light of the specification including equivalents, rather than solely by the foregoing description. Thus, all configurations including configurations that fall within equivalent arrangements of the claims are intended to be embraced in the invention.