Patent Publication Number: US-2019181251-A1

Title: Mesh structure for heterojunction bipolar transistors for rf applications

Description:
BACKGROUND 
     Field 
     Aspects of the present disclosure relate generally to a heterojunction bipolar transistor, and more particularly, to manufacturing methods and arrangement of the emitter mesa, base mesa, and collector mesa of the heterojunction bipolar transistor for RF applications. 
     Background 
     The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) that uses different semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that the HBT can handle signals of very high frequencies, up to several hundred GHz. The HBT is commonly used in modern ultrafast circuits, mostly radio-frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in cellular phones. 
     Conventional heterojunction bipolar transistor layout arranges the emitter in stripes. However, an HBT using such a structure faces a few challenges. For any given emitter mesa area (set by the required output RF power), the base mesa occupies a very large area. A typical ratio of the base mesa to emitter mesa area on a conventional HBT unit cell is around 2.4. An HBT&#39;s base-collector junction capacitance (Cbc) is a very key limiter of device performance, such as power gain, particularly at a high frequency. The large Cbc from the large base mesa area compromises the device&#39;s power gain and efficiency. An HBT with a stripe layout also occupies a large footprint to accommodate the emitter mesa area required to deliver a given output power, leading to large die size and high manufacturing cost. 
     Accordingly, it would be beneficial to provide an improved HBT structure and an improved manufacturing method that reduce area and improve the device performance. 
     SUMMARY 
     The following presents a simplified summary of one or more implementations to provide a basic understanding of such implementations. This summary is not an extensive overview of all contemplated implementations, and is intended to neither identify key nor critical elements of all implementations nor delineate the scope of any or all implementations. The sole purpose of the summary is to present concepts relate to one or more implementations in a simplified form as a prelude to a more detailed description that is presented later. 
     In one aspect, a heterojunction bipolar transistor (HBT) comprises a collector mesa, a base mesa on the collector mesa, and an emitter mesa on the base mesa. The emitter mesa has a plurality of openings. The HBT further comprises a plurality of base metals in the plurality of openings connected to the base mesa. 
     In another aspect, a method comprises providing a wafer with a collector mesa stack, a base mesa stack, and an emitter mesa stack; patterning the emitter mesa stack to define an emitter mesa having a plurality of openings; providing a plurality of base metals in the plurality of openings connected to the base mesa stack; and patterning the base mesa stack to define a base mesa. 
     To accomplish the foregoing and related ends, one or more implementations include the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects of the one or more implementations. These aspects are indicative, however, of but a few of the various ways in which the principles of various implementations may be employed and the described implementations are intended to include all such aspects and their equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a top-down view of an example HBT with a stripe layout. 
         FIG. 2  illustrates an exemplary cross-section of  FIG. 1  along line A-A′. 
         FIG. 3  illustrate another exemplary cross-section of  FIG. 1  along line A-A′. 
         FIG. 4  illustrates an exemplary implementation of an HBT with the emitter mesa arranged in a mesh structure according to certain aspects of the present disclosure. 
         FIG. 5  illustrates still another exemplary implementation of an HBT with the emitter mesa arranged in a mesh structure according to certain aspects of the present disclosure. 
         FIG. 6  illustrates an exemplary cross-section of  FIG. 5  along line B-B′ according to certain aspects of the present disclosure. 
         FIG. 7  illustrates still another exemplary implementation of an HBT with the emitter mesa arranged in a mesh structure according to certain aspects of the present disclosure. 
         FIGS. 8 a -8 g    illustrate an exemplary process flow of making an HBT according to certain aspects of the present disclosure. 
         FIG. 9  illustrates an exemplary method for manufacturing an HBT with the emitter mesa arranged in a mesh structure according to certain aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below, in connection with the appended drawings, is intended as a description of various aspects and is not intended to represent the only aspects in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing an understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts. 
     An HBT&#39;s base-collector capacitance (Cbc) is a very key limiter of its power gain, particularly at high frequencies. A conventional HBT often arranges the emitter mesa in stripes, which results in high Cbc.  FIG. 1  illustrates a top-down view of an example HBT with the stripe layout. The HBT  100  comprises a collector mesa  102  and a base mesa  104  on the collector mesa  102 . The HBT  100  further comprises a stripe of base metal  114  on the base mesa  104  to provide the connection to the base. An emitter mesa composed of a plurality of stripes  106  is on the base mesa  104 . To accommodate more base metals or larger emitter mesa, more base metals  114  may be placed interleaved with the emitter mesa stripes  106 . In addition, the HBT  100  also comprises a plurality of emitter metals  116  on the plurality of emitter mesa stripes  106  to provide electrical connection to the emitter. One or more collector metals  112  are placed on the collector mesa  102  to provide electrical connection to the collector. 
       FIG. 2  illustrates an exemplary cross-section of  FIG. 1  along line A-A′. The cross-section  200  comprises the collector mesa  102 , the base mesa  104  on the collector mesa  102 , and the emitter mesas  106  on the base mesa  104 . One or more stripes of the base metal  114 , one or more stripes of emitter metals  116 , and one or more stripes of collector metal  112  are placed (e.g., by deposition process) on the base mesa  104 , the emitter mesas  106 , and the collector mesa  102 , respectively. 
     Although each of the collector mesa, the base mesa, and the emitter mesa is illustrated as a single layer in the cross-section  200 , one should understand that each layer could include multiple sub-layers.  FIG. 3  illustrates an exemplary cross-section of an NPN HBT. The NPN HBT  300  comprises a collector mesa  302 , a base mesa  304 , and an emitter mesa  306 . The collector mesa comprises two sub-layers in this example: a semi insulating GaAs substrate  302 A and an N+ GaAs sub-collector  302 B. Similarly, the base mesa  304  also comprises multiple sub-layers in this example: a first InGaP etch stop layer  304 A, an N− GaAs collector  304 B, a P+ GaAs base  304 C, and a second InGaP etch stop layer  304 D. The N+ GaAs sub-collector  302 B, the first InGaP etch stop layer  304 A, and the N− GaAs collector  304 B forms the collector of the HBT  300 . The NPN HBT  300  further comprises one or more stripes of the base metal  314 , one or more stripes of emitter metals  316 , and one or more stripes of collector metal  312  placed (e.g., by deposition process) on the base mesa  304 , the emitter mesas  306 , and the collector mesa  302 , respectively. 
     The layout and structure illustrated in  FIG. 1  suffer from large base-collector junction area for any given emitter mesa area (set by the required current output RF power). The resulting large Cbc compromises the HBT&#39;s power gain and efficiency. According to certain aspects of the present disclosure, to reduce the base-collector junction area and Cbc, an emitter mesa may be arranged in a mesh structure, along with associated emitter metal. The openings of the mesh can be shaped in rectangular or hexagon or other suitable fashions. Metal pickups for the HBT base are arranged inside the openings of the mesh. The structure may further include an optional base metal donut surrounding the emitter mesh to further lower the base resistance. Optional base metal provides additional optimization space, trading off base resistance (Rb) with Cbc. The optional base metal donut is interconnected with the base metal dots inside the emitter mesh openings. The structure reduces the base mesa area/emitter mesa area ratio to be under 1.8. In addition, the structure achieves over 25% performance improvement over structures illustrated in  FIG. 1 . 
       FIG. 4  illustrates an exemplary implementation of an HBT with the emitter mesa arranged in a mesh structure according to certain aspects of the present disclosure. An HBT  400  comprises a collector mesa  402 , a base mesa  404  on the collector mesa  402 , and an emitter mesa  406  on the base mesa  404 . The emitter mesa  406  is arranged in a mesh like structure. The emitter mesa  406  has a plurality of openings  410 . The plurality of openings  410  provides windows for a plurality of base metals  414  to be placed and connected to the base mesa  404 . The plurality of base metals  414  are connected through another layer (or layers) of metal (not shown) and are electrically coupled to each other. 
     The plurality of openings  410  may be in any shape, such as square (as illustrated in  FIG. 4 ), rectangular, hexagon, etc. The size and/or the shape for each of the plurality of openings  410  may be different. The plurality of openings  410  may have same size and/or same shape for ease of the design and/or for high packing density. Each of the plurality of openings  410  is big enough to accommodate base metals  414  inside the opening, including the size of each of the plurality of base metals  414  itself and the necessary spacing between each of the plurality of base metals  414  and the emitter mesa  406 . Thus, the minimum size of the plurality of openings  410  is limited by the process technology used. Similarly, the spacing between one of the plurality of openings  410  to the neighboring one of the plurality of openings  410  is also a design choice with the minimum spacing limited by the process technology used. However, the spacing may be any size that is larger than or equal to the minimum allowed by the process technology. 
     Different sizes of HBTs are needed for different applications. For example, if an HBT is used as a power amplifier, the size of the HBT is chosen to meet a particular output power requirement. The mesh like emitter mesa structure provides flexibility in choosing the size of an HBT and the arrangement of the collector, base, and emitter. The number of openings  310  may be varied and can be any integer. For example, there may be four openings arranged in a 2×2 array. There can be more or less than 4 openings, including 1 opening. The arrangement of the plurality of openings  310  is flexible and is not limited to the square array. Other array is possible, such as 2×2, 3×3, or 3×1 array, just to give a few examples. By arranging HBT&#39;s emitter mesa in mesh structure (e.g., having plurality of openings), the packing density is improved. The base mesa area/emitter mesa area ratio may be reduced to be lower than 1.8. 
     The HBT  400  further comprises one or more emitter metal (not shown) on the emitter mesa  406 . The emitter metal may fully or partially cover the emitter mesa  406 . The HBT  400  also comprises one or more collector metals  412  on the collector mesa  402  to provide connection to the collector of the HBT  400 . 
     To lower the base resistance further, an optional base metal may be provided surrounding the emitter mesa.  FIG. 5  illustrates an exemplary implementation of an HBT with its emitter mesa arranged in a mesh structure and with an optional base metal surrounding the emitter mesa. Like the HBT  400 , an HBT  500  comprises a collector mesa  502 , a base mesa  504  on the collector mesa  502 , and an emitter mesa  506  on the base mesa  504 . The emitter mesa  506  is arranged in a mesh like structure. The emitter mesa  506  has a plurality of openings  510 . The plurality of openings  510  provides windows for a plurality of base metals  514  to be placed and connected to the base mesa  504 . The plurality of base metals  514  are connected through another layer (or layers) of metal (not shown) and are electrically coupled to each other. The emitter metal (not shown) is on the emitter mesa  506 . The emitter metal may fully or partially cover the emitter mesa  506 . The HBT  500  also comprises one or more collector metals  512  on the collector mesa  502  to provide connection to the collector of the HBT  500 . 
     In addition, the HBT  500  further comprises an optional base metal  524  surrounding the emitter mesa  506 . The optional base metal  524  may be in donut shape (as illustrated in  FIG. 5 ) or may be one or more stripes of metals (not illustrated). The optional base metal  524  is an outer base metal that is outside of the emitter mesa mesh. The optional base metal  524  is connected to the plurality of base metals  514  through another layer (or layers) of metal (not shown) so that the optional base metal  524  and the plurality of base metals  514  are electrically coupled. The optional base metal  524  yields a lower base resistance (Rb) but may increase Cbc. This provides an additional optimization space, trading off Rb with Cbc. 
       FIG. 6  illustrates an exemplary cross-section of  FIG. 5  along line B-B′ according to certain aspects of the present disclosure. The cross-section  600  comprises the collector mesa  502 , the base mesa  504  on the collector mesa  502 , and the emitter mesa  506  on the base mesa  504 . The cross-section  600  also includes the optional base metal  524 . 
     Although each of the collector mesa, the base mesa, and the emitter mesa is illustrated as a single layer in the cross-section  600 , one should understand that each layer could include multiple sub-layers, similar to the cross-section  300  in  FIG. 3 . For example, in an NPN HBT, the collector mesa  502  may comprise an intrinsic or lightly doped GaAs substrate and an N+ GaAs sub-collector. The collector metal may connect to the N+ GaAs sub-collector and electrically couple to the collector of the HBT. The emitter mesa may comprise an intrinsic InGaAs sub-layer, followed by a lightly N doped (e.g., 5E17) InGaP layer and a high N+ doped (e.g., 1E19) InGaAs layer. 
       FIG. 7  illustrates another exemplary implementation of an HBT with its emitter mesa arranged in a mesh structure according to certain aspects of the present disclosure. The HBT  700  is similar to the HBT  300  but with a different emitter mesa mesh structure. The HBT  700  comprises a collector mesa  702 , a base mesa  704  on the collector mesa  702 , and an emitter mesa  706  on the base mesa  704 . The emitter mesa  706  is arranged in a mesh like structure. The emitter mesa  706  has a plurality of openings  710 . The plurality of openings  710  provides windows for a plurality of base metals  714  to be placed and connected to the base mesa  704 . The plurality of base metals  714  are connected through another layer (or layers) of metal (not shown) and electrically coupled to each other. The emitter metal (not shown) is on the emitter mesa  706 . The emitter metal may fully or partially cover the emitter mesa  706 . The HBT  700  also comprises one or more collector metals  712  on the collector mesa  702  to provide connection to the collector of the HBT  700 . 
     Unlike the emitter mesa  400  whose plurality of openings  410  are in square shape, the plurality of openings  710  are in hexagon shape. The hexagon shape provides higher packing density than the square shape, resulting in smaller area for an HBT under same output power. In addition to the hexagon shape openings, the plurality of base metals  714  may be in hexagon shape to maximize the connection to the base and to reduce the base resistance. 
     Similar to the HBT in  FIGS. 5 and 6 , the HBT  700  may comprise an optional base metal (not shown) surrounding the emitter mesa  706 . The optional base metal may be in donut shape (as illustrated in  FIG. 5 ) or may include one or more stripes of metals. The optional base metal is connected to the plurality of base metals  714  through another layer (or layers) of metal (not shown) so that the optional base metal and the plurality of base metals  714  are electrically coupled. 
       FIGS. 8 a -8 g    illustrate an exemplary process flow of making an HBT.  FIG. 8 a    shows a starting wafer with required epi stacks. The wafer comprises a collector mesa stack  852 , a base mesa stack  854 , and an emitter mesa stack  856 . The collector mesa stack  852 , the base mesa stack  854 , and the emitter mesa stack  856  are so defined as they are the starting stacks for the collector mesa, base mesa, and emitter mesa of an HBT, respectively. Each of the collector mesa stack  852 , the base mesa stack  854 , and the emitter mesa stack  856  may comprises multiple sub-layers. For example, the collector mesa stack  852  includes a layer of semi-insulating substrate  802 A (e.g., comprising intrinsic GaAs) and a layer of sub-collector  802 B (e.g., comprising N+ GaAs). The base mesa stack  854  includes a first etch stop layer  804 A (e.g., comprising InGaP), a collector layer  804 B (e.g. comprising N− GaAs), a base layer  804 C (e.g., comprising P+ GaAs), and a second etch stop layer  804 D (e.g., comprising InGaP).  FIG. 8 b    illustrates part of the wafer after the placement of the emitter metal of the HBT. One or more emitter metal  816  on the emitter mesa stack  856  are patterned and defined (such as lithographic patterning and etching).  FIG. 8 c    illustrates part of the wafer after the patterning of emitter mesa by etching the emitter mesa stack  856 . The emitter metal stack  856  is patterned and etched to form a desired pattern as an emitter mesa  806 . The emitter mesa  806  may be formed in a variety of shapes, including the shapes illustrated in  FIGS. 4, 5, and 7 . In  FIG. 8 d   , the base metal  814  is patterned and defined on the base mesa stack  854 . The second etch stop layer  804 D is patterned and etched so that the base metal  814  contacts the collector layer  804 C.  FIG. 8 e    illustrates the structure after formation of base mesa. The base mesa stack  854  is patterned and etched to form the base mesa  804 , including patterning and etching layers  804 A- 804 D. In  FIG. 8 f   , one or more collector metals  812  are patterned and defined on the collector mesa stack  852 . Finally, as illustrated in  FIG. 8 g   , an implant isolation ring  822  may surround the HBT. The implant isolation ring defines the collector mesa  802  and forms the boundary of the HBT. 
       FIG. 9  illustrates an exemplary method for manufacturing an HBT with its emitter mesa arranged in a mesh structure according to certain aspects of the present disclosure. The description of method  900  below and the process flow diagrams provided in  FIG. 9  are merely as illustrative examples and are not intended to require or imply that the operations of the various aspects must be performed in the order presented 
     The HBT manufacturing method  900  starts with a wafer with required epi stacks. At  902 , a wafer with required epi stacks, including a collector mesa stack (e.g., the collector mesa stack  852 ), a base mesa stack (e.g., the base mesa stack  854 ), and an emitter mesa stack (e.g., the emitter mesa stack  856 ) is provided. Each mesa stack may comprise multiple sub-layers. For example, for an NPN HBT, the collector mesa stack may include a layer of intrinsic GaAs semi-insulating substrate (e.g., the semi-insulating substrate  802 A) and a layer of N+ GaAs sub-collector (e.g., the sub-collector  802 B). The base mesa stack may include a first InGaP etch stop layer (e.g., the etch stop layer  804 A)), an N− GaAs collector layer (e.g., the collector layer  804 B), a P+ GaAs base layer (e.g., the base layer  804 C), and a second InGaP etch stop layer (e.g., the etch stop layer  804 D). 
     At  904 , one or more emitter metals (e.g., the emitter metals  516  or  816 ) are placed on the emitter mesa stack. 
     At  906 , the emitter mesa is patterned and formed through a suitable process such as etching. The emitter mesa comprises a plurality of openings (e.g., the plurality of openings  410 ,  510 , or  710 ). The plurality of openings may be in any shape, such as square (as illustrated in  FIG. 4 ), rectangular, hexagon (as illustrated in  FIG. 7 ), etc. The size and/or the shape for each of the plurality of openings may be different or may be same. Each of the plurality of openings is big enough to accommodate a base metal (e.g., the base metal  414 ,  514 , or  714 ), including the size of the base metal itself and the necessary spacing between the base metal and the emitter mesa. Thus, the minimum size of the plurality of openings is limited by the process technology used. Similarly, the spacing between one opening to the neighboring opening is also a design choice and the minimum is limited by the process technology used. 
     At  908 , a plurality of base metals (e.g., the plurality of base metals  414 ,  514 , or  714 ) is provided in the plurality of openings. The plurality of base metals is on the base mesa stack and provides connection to the base of the HBT. The plurality of base metals may be with the same shape as the plurality of openings. The plurality of base metals is connected through another layer (or layers) of metal and is electrically coupled to each other. 
     At  910 , an optional base metal (outer base metal) (e.g., the base metal  524 ) may be placed on the base mesa stack and connected to the base metals in the plurality of openings. The optional base metal surrounds the emitter mesa and may yield a low base resistance. The optional base metal is electrically coupled to the plurality of base metals through another layer (or layers) of metal. 
     At  912 , the base mesa (e.g., the base mesa  404 ,  504 ,  704 , or  804 ) is patterned and formed through process such as etching. 
     At  914 , one or more collector metals (e.g., the collector metals  412 ,  512 ,  712 , or  812  are placed on the collector mesa stack. 
     Furthermore, a collector mesa may be further defined by placing isolation ring in the collector mesa stack. The isolation ring also forms the boundary of the HBT. 
     The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.