Stacked ESD clamp with reduced variation in clamp voltage

An integrated circuit containing a stacked bipolar transistor which includes two bipolar transistors connected in series is disclosed. Each bipolar transistor includes a breakdown inducing feature. The breakdown inducing features have reflection symmetry with respect to each other. A process for forming an integrated circuit containing a stacked bipolar transistor which includes two bipolar transistors connected in series, with breakdown inducing features having reflection symmetry, is also disclosed.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuits. More particularly, this invention relates to bipolar transistors in integrated circuits.

BACKGROUND OF THE INVENTION

Bipolar transistors in integrated circuits may exhibit breakdown between bases and collectors of the bipolar transistors when voltages are applied to the collectors. Breakdown voltages may be sensitive to variations in locations of breakdown related elements such as collectors of the bipolar transistors with respect to the bases. Variations in the locations of the breakdown related elements between instances of the bipolar transistors may be due to alignment variations of photolithographic patterns for forming the bases and the breakdown related elements during fabrication of the integrated circuit.

SUMMARY OF THE INVENTION

An integrated circuit may contain a stacked bipolar transistor which includes two bipolar transistors electrically connected in series. In each bipolar transistor, a breakdown inducing feature is located such that collector-base breakdown is induced to occur in areas which have reflection symmetry with respect to bases of their respective bipolar transistors. A process of forming the integrated circuit containing the stacked bipolar transistor is also disclosed.

DETAILED DESCRIPTION

An integrated circuit may contain a stacked bipolar transistor which includes two bipolar transistors which have reflection symmetry with respect to each other, and are electrically connected in series. Each bipolar transistor contains a breakdown inducing feature which includes a collector and a base of the bipolar transistor, and which may induce collector-base breakdown in a breakdown region of the transistor. The breakdown inducing features and the breakdown regions have reflection symmetry with respect to bases of their respective bipolar transistors. Alignment variations during fabrication of the integrated circuit which may cause variations in locations of the breakdown inducing features with respect to the bases and collectors of their respective bipolar transistors may lead to variations in breakdown voltages of the bipolar transistors. Forming the breakdown inducing features and the breakdown regions to have reflection symmetry with respect to their respective bases may provide offsetting variations of breakdown voltages between the two bipolar transistors in the stacked bipolar transistor, and so may advantageously provide more uniformity of breakdown voltage among instances of the stacked bipolar transistor.

FIG. 1AthroughFIG. 1Eare cross-sections of an integrated circuit containing a stacked bipolar transistor formed according to a first embodiment, depicted in successive stages of fabrication. Referring toFIG. 1A, the integrated circuit1000is formed in and on a semiconductor substrate1002. The substrate1002may be a single crystal silicon wafer, a silicon-on-insulator (SOI) wafer, a hybrid orientation technology (HOT) wafer with regions of different crystal orientations, or other material appropriate for fabrication of the integrated circuit1000. A first buried collector layer1004is formed in the substrate1002in a first bipolar transistor area1006. A second buried collector layer1008is formed in the substrate1002in a second bipolar transistor area1010. In the instant embodiment, the substrate1002in the first bipolar transistor area1006and in the second bipolar transistor area1010is p-type, while the buried collectors1004and1008are n-type. In one realization of the instant embodiment, the buried collectors1004and1008may be formed by diffusing or ion implanting n-type dopants such as phosphorus, arsenic or antimony in areas defined for the buried collectors1004and1008and growing epitaxial semiconductor material over the implanted regions. In an alternate realization of the instant embodiment, the buried collectors1004and1008may be formed by ion implanting n-type dopants into the substrate1002with sufficient energy to form n-type layers at a desired depth below a top surface of the substrate1002. Other processes of forming the buried collectors1004and1008are within the scope of the instant embodiment.

Referring toFIG. 1B, a first collector right vertical member1012is formed in the substrate1002, extending from a top surface of the substrate1002to the first buried collector layer1004. A first collector left vertical member1014is formed in the substrate1002, also extending from a top surface of the substrate1002to the first buried collector layer1004. The first collector left vertical member1014is laterally separated from the first collector right vertical member1012. The first collector right vertical member1012and the first collector left vertical member1014have a same conductivity type as the first buried collector layer1004.

A second collector left vertical member1016and a second collector right vertical member1018are formed in the substrate1002, also extending from a top surface of the substrate1002to the second buried collector layer1008. The second collector left vertical member1016and the second collector right vertical member1018are located so as to have reflection symmetry with respect to the first collector right vertical member1012and the first collector left vertical member1014, respectively; that is, a lateral direction from the first collector right vertical member1012to the first collector left vertical member1014is opposite a lateral direction from the second collector left vertical member1016to the second collector right vertical member1018. A plane of the reflection symmetry is perpendicular to the top surface of the substrate1002and perpendicular to the plane ofFIG. 1B. In the instant embodiment, a lateral separation between the second collector left vertical member1016and the second collector right vertical member1018is substantially equal to a lateral separation between the first collector right vertical member1012and the first collector left vertical member1014.

An optional first well1020may be formed between the first collector right vertical member1012and the first collector left vertical member1014, above the first buried collector layer1004extending to the top surface of the substrate1002. A conductivity type of the first well1020if formed is opposite from the conductivity type of the first buried collector layer1004. An optional second well1022may be formed between the second collector left vertical member1016and the second collector right vertical member1018, above the second buried collector layer1008extending to the top surface of the substrate1002. A conductivity type of the second well1022if formed is opposite from the conductivity type of the second buried collector layer1008.

Referring toFIG. 1C, a first base right diffused region1024is formed in the substrate1002, extending to the top surface of the substrate1002, between the first collector right vertical member1012and the first collector left vertical member1014. The first base right diffused region1024is located closer to the first collector right vertical member1012than to the first collector left vertical member1014.

A first base left diffused region1026is formed in the substrate1002, extending to the top surface of the substrate1002, between the first collector right vertical member1012and the first collector left vertical member1014. The first base left diffused region1026is located closer to the first collector left vertical member1014than to the first collector right vertical member1012. In the instant embodiment, a lateral separation between the first base right diffused region1024and the first collector right vertical member1012is less than a lateral separation between the first base left diffused region1026and the first collector left vertical member1014. In one realization of the instant embodiment, the lateral separation between the first base right diffused region1024and the first collector right vertical member1012is between 0.75 and 0.85 microns less than the lateral separation between the first base left diffused region1026and the first collector left vertical member1014.

A second base left diffused region1028and a second base right diffused region1030are formed in the substrate1002, extending to the top surface of the substrate1002, between the second collector left vertical member1016and the second collector right vertical member1018. The second base left diffused region1028is located closer to the second collector left vertical member1016than to the second collector right vertical member1018. The second base right diffused region1030is located closer to the second collector right vertical member1018than to the second collector left vertical member1016. In the instant embodiment, a lateral separation between the second base left diffused region1028and the second collector left vertical member1016is less than a lateral separation between the second base right diffused region1030and the second collector right vertical member1018. in the instant embodiment, the lateral separation between the second base left diffused region1028and the second collector left vertical member1016is substantially equal to the lateral separation between the first base right diffused region1024and the first collector right vertical member1012.

The base diffused regions1024,1026,1028and1030have a conductivity type opposite from the collector vertical members1012,1014,1016and1018. The base diffused regions1024,1026,1028and1030may be formed for example by ion implanting dopants into the substrate1002in areas defined for the base diffused regions1024,1026,1028and1030by a base diffused region photoresist pattern (not shown), and subsequently annealing the substrate1002.

Referring toFIG. 1D, a first emitter diffused region1032is formed in the substrate1002extending to the top surface of the substrate1002between the first base right diffused region1024and the first base left diffused region1026. In the instant embodiment, the first emitter diffused region1032is located substantially equidistant from the first base right diffused region1024and the first base left diffused region1026.

A second emitter diffused region1034is formed in the substrate1002extending to the top surface of the substrate1002between the second base left diffused region1028and the second base right diffused region1030. In the instant embodiment, the second emitter diffused region1034is located substantially equidistant from the second base left diffused region1028and the second base right diffused region1030.

The emitter diffused regions1032and1034have a same conductivity type as the collector vertical members1012,1014,1016and1018. The emitter diffused regions1032and1034may be formed for example by ion implanting dopants into the substrate1002in areas defined for the emitter diffused regions1032and1034by an emitter diffused region photoresist pattern (not shown), and subsequently annealing the substrate1002. Regions of the substrate1002between the emitter diffused regions1032and1034and the first bipolar transistor area1006and the second bipolar transistor area1010may be further modified by introducing additional dopants, for example by ion implanting dopants followed by one or more subsequent anneal processes.

The first buried collector layer1004, the first collector right vertical member1012and the first collector left vertical member1014are included in a collector of the first bipolar transistor. The first base right diffused region1024and the first base left diffused region1026are included in a base of the first bipolar transistor. The first emitter diffused region1032is included in an emitter of the first bipolar transistor. Similarly, the second buried collector layer1008, the second collector left vertical member1016and the second collector right vertical member1018are included in a collector of the second bipolar transistor. The second base left diffused region1028and the second base right diffused region1030are included in a base of the second bipolar transistor. The second emitter diffused region1034is included in an emitter of the second bipolar transistor.

In the instant embodiment, the first bipolar transistor and the second bipolar transistor may be electrically coupled in series as depicted inFIG. 1Eto form the stacked bipolar transistor. The first collector right vertical member1012and the first collector left vertical member1014may be coupled to a first circuit node1036. The first base right diffused region1024and the first base left diffused region1026may be coupled to the first emitter diffused region1032, possibly through optional first resistors1038. The first emitter diffused region1032may be coupled to the second collector left vertical member1016and the second collector right vertical member1018for example through interconnect elements of the integrated circuit1000as depicted schematically by transistor connection1040. The second base left diffused region1028and the second base right diffused region1030may be coupled to the second emitter diffused region1034, possibly through optional second resistors1042. The second emitter diffused region1034may be coupled to a second circuit node1044.

During operation of the integrated circuit1000, a voltage may be applied to the first circuit node1036, for example an electro-static discharge (ESD) event, which causes collector-base breakdown in the stacked bipolar transistor. Forming the first collector right vertical member1012, the first collector left vertical member1014, the first base right diffused region1024and the first base left diffused region1026so that the lateral separation between the first base right diffused region1024and the first collector right vertical member1012is less than the lateral separation between the first base left diffused region1026and the first collector left vertical member1014may cause collector-base breakdown in the first bipolar transistor to be induced in a region between the first base right diffused region1024and the first collector right vertical member1012. The configuration of the first base right diffused region1024and the first collector right vertical member1012is a breakdown inducing feature of the first bipolar transistor. Similarly, forming the second collector left vertical member1016, the second collector right vertical member1018, the second base left diffused region1028and the second base right diffused region1030so that the lateral separation between the second base left diffused region1028and the second collector left vertical member1016is less than the lateral separation between the second base right diffused region1030and the second collector right vertical member1018may cause breakdown in the second bipolar transistor to be induced in a region between the second base left diffused region1028and the second collector left vertical member1016. The configuration of the second base left diffused region1028and the second collector left vertical member1016is a breakdown inducing feature of the second bipolar transistor. Forming the first bipolar transistor and the second bipolar transistor as described in reference toFIG. 1AthroughFIG. 1Dresults in the breakdown inducing feature of the second bipolar transistor having reflection symmetry with respect to the breakdown inducing feature of the first bipolar transistor. A plane of the reflection symmetry is perpendicular to the top surface of the substrate1002and perpendicular to the planes ofFIG. 1AthroughFIG. 1D.

A first collector-base breakdown voltage of the collector-base breakdown between the first base right diffused region1024and the first collector right vertical member1012may be an increasing function of the lateral separation between the first base right diffused region1024and the first collector right vertical member1012, that is, larger instances of the lateral separation between the first base right diffused region1024and the first collector right vertical member1012provide larger values of the first collector-base breakdown voltage. Instances of the lateral separation between the first base right diffused region1024and the first collector right vertical member1012may include unintentional alignment variations, commonly known as misalignments, in instances of the stacked bipolar transistor. Similarly, a second collector-base breakdown voltage of the collector-base breakdown between the second base left diffused region1028and the second collector left vertical member1016may be an increasing function of the lateral separation between the second base left diffused region1028and the second collector left vertical member1016. The lateral separation between the second base left diffused region1028and the second collector left vertical member1016may include the same misalignment as the lateral separation between the first base right diffused region1024and the first collector right vertical member1012. In such a case, a misalignment which increases the lateral separation between the first base right diffused region1024and the first collector right vertical member1012and so increases the first collector-base breakdown voltage, may decrease the lateral separation between the second base left diffused region1028and the second collector left vertical member1016by substantially a same lateral distance and so may decrease the second collector-base breakdown voltage. A collector-base breakdown voltage of the stacked bipolar transistor is substantially a sum of the first collector-base breakdown voltage and the second collector-base breakdown voltage. Thus, forming the breakdown inducing feature of the second bipolar transistor to have reflection symmetry with respect to the breakdown inducing feature of the first bipolar transistor may reduce a sensitivity of the collector-base breakdown voltage of the stacked bipolar transistor to the misalignments.

It will be recognized that a stacked bipolar transistor of opposite polarity to that described in reference toFIG. 1AthroughFIG. 1Emay be formed by appropriate changes in polarities of dopants and conductivity types. A stacked bipolar transistor of opposite polarity may have a reduced sensitivity of a collector-base breakdown voltage of the stacked bipolar transistor to misalignments of collector vertical members and base diffused regions, as described in reference toFIG. 1E.

In one realization of the instant embodiment, the first collector left vertical member1014and the second collector right vertical member1018may be omitted. In one realization of the instant embodiment, the first base left diffused region1026and the second base right diffused region1030may be omitted.

FIG. 2is a cross-section of an integrated circuit containing a stacked bipolar transistor formed according to a second embodiment. The integrated circuit2000is formed in and on a semiconductor substrate2002, as described in reference toFIG. 1A. An optional first buried collector right lobe2004and an optional first buried collector left lobe2006may be formed in the substrate2002as described in reference to the first buried collector layer1004inFIG. 1A. A first collector right vertical member2008and a first collector left vertical member2010are formed in the substrate2002and extend from a top surface of the substrate2002to the first buried collector right lobe2004and the first buried collector left lobe2006, if present, respectively. The first collector right vertical member2008and the first collector left vertical member2010are formed as described in reference toFIG. 1B. A first well2012is formed between the first collector right vertical member2008and the first collector left vertical member2010, above the first buried collector right lobe2004and the first buried collector left lobe2006if present, extending to the top surface of the substrate2002. A conductivity type of the first well2012is opposite from a conductivity type of the first collector right vertical member2008and the first collector left vertical member2010.

A first base right diffused region2014and a first base left diffused region2016are formed in the substrate2002between the first collector right vertical member2008and the first collector left vertical member2010, using processes as described in reference toFIG. 1C. The first base right diffused region2014is located closer to the first collector right vertical member2008than to the first collector left vertical member2010. The first base left diffused region2016is located closer to the first collector left vertical member2010than to the first collector right vertical member2008. In the instant embodiment, a lateral separation between the first base right diffused region2014and the first collector right vertical member2008is less than a lateral separation between the first base left diffused region2016and the first collector left vertical member2010. In one realization of the instant embodiment, the lateral separation between the first base right diffused region2014and the first collector right vertical member2008is between 0.75 and 0.85 microns less than the lateral separation between the first base left diffused region2016and the first collector left vertical member2010.

A first emitter diffused region2018is formed in the substrate2002extending to the top surface of the substrate2002between the first base right diffused region2014and the first base left diffused region2016. In the instant embodiment, the first emitter diffused region2018is located substantially equidistant from the first base right diffused region2014and the first base left diffused region2016. The first buried collector right lobe2004, the first buried collector left lobe2006, the first collector right vertical member2008, the first collector left vertical member2010, the first base right diffused region2014, the first base left diffused region2016and the first emitter diffused region2018are included in a first bipolar transistor2020of the stacked bipolar transistor.

A second bipolar transistor2022of the stacked bipolar transistor includes a second buried collector left lobe2024and a second buried collector right lobe2026if the optional first buried collector lobes2004and2006are present, and includes a second collector left vertical member2028, a second collector right vertical member2030, a second well2032, a second base left diffused region2034, a second base right diffused region2036and a second emitter diffused region2038. The second buried collector left lobe2024and the second buried collector right lobe2026, if present, are located so as to have reflection symmetry with respect to the first buried collector right lobe2004and the first buried collector left lobe2006, respectively. Similarly, the second collector left vertical member2028and the second collector right vertical member2030are located so as to have reflection symmetry with respect to the first collector right vertical member2008and the first collector left vertical member2010, respectively. Similarly, the second base left diffused region2034and the second base right diffused region2036are located so as to have reflection symmetry with respect to the first base right diffused region2014and the first base left diffused region2016, respectively. A plane of the reflection symmetry is perpendicular to the top surface of the substrate2002and perpendicular to the plane ofFIG. 2. A lateral separation between the second base left diffused region2034and the second collector left vertical member2028is substantially equal to the lateral separation between the first base right diffused region2014and the first collector right vertical member2008.

The optional first buried collector right lobe2004if present, the optional first buried collector left lobe2006if present, the first collector right vertical member2008and the first collector left vertical member2010are included in a collector of the first bipolar transistor2020. The first base right diffused region2014and the first base left diffused region2016are included in a base of the first bipolar transistor2020. The first emitter diffused region2018is included in an emitter of the first bipolar transistor2020. Similarly, the second buried collector left lobe2024if present, the second buried collector right lobe2026if present, the second collector left vertical member2028and the second collector right vertical member2030are included in a collector of the second bipolar transistor2022. The second base left diffused region2034and the second base right diffused region2036are included in a base of the second bipolar transistor2022. The second emitter diffused region2038is included in an emitter of the second bipolar transistor2022.

The first bipolar transistor2020and the second bipolar transistor2022may be electrically coupled in series by interconnect elements2040as described in reference toFIG. 1E. The configuration of the first base right diffused region2014and the first collector right vertical member2008is a breakdown inducing feature of the first bipolar transistor. Similarly, the configuration of the second base left diffused region2034and the second collector left vertical member2028is a breakdown inducing feature of the second bipolar transistor. Forming the breakdown inducing feature of the second bipolar transistor2022to have reflection symmetry with respect to the breakdown inducing feature of the first bipolar transistor2020may reduce a sensitivity of a collector-base breakdown voltage of the stacked bipolar transistor to misalignments of the collector vertical members2008,2010,2028and2030and the base diffused regions2014,2016,2032and2034, as described in reference toFIG. 1E. In the instant embodiment, locations of the first buried collector lobes2004and2006if present may be adjusted to provide balanced bipolar current density in the first bipolar transistor2020in view of the difference in lateral separations between the first base diffused regions2014and2016and the first collector vertical members2008and2010. Similarly, locations of the second buried collector lobes2024and2026if present may be adjusted to provide balanced bipolar current density in the second bipolar transistor2022in view of the difference in lateral separations between the second base diffused regions2032and2034and the second collector vertical members2028and2030.

It will be recognized that a stacked bipolar transistor of opposite polarity to that described in reference toFIG. 2may be formed by appropriate changes in polarities of dopants and conductivity types. A stacked bipolar transistor of opposite polarity may have a reduced sensitivity of a collector-base breakdown voltage of the stacked bipolar transistor to misalignments of collector vertical members and base diffused regions, as described in reference toFIG. 2.

In one realization of the instant embodiment, the first collector left vertical member2010, the first buried collector left lobe2006, the second collector right vertical member2030and the second buried collector right lobe2026may be omitted. In one realization of the instant embodiment, the first base left diffused region2016and the second base right diffused region2036may be omitted.

FIG. 3is a cross-section of an integrated circuit containing a stacked bipolar transistor formed according to a third embodiment. The integrated circuit3000is formed in and on a semiconductor substrate3002, as described in reference toFIG. 1A. An optional first buried collector layer3004may be formed in the substrate3002as described in reference toFIG. 1A. A first collector right vertical member3006and a first collector left vertical member3008are formed in the substrate3002and extend to the first buried collector layer3004if present. The first collector right vertical member3006and the first collector left vertical member3008are formed as described in reference toFIG. 1B.

A first collector right shallow well3010is formed in the substrate3002, extending from a top surface of the substrate3002to the first collector right vertical member3006. Similarly, a first collector left shallow well3012is formed in the substrate3002, extending from the top surface of the substrate3002to the first collector left vertical member3008. The first collector right shallow well3010and the first collector left shallow well3012have a same conductivity type as the first collector right vertical member3006. A lateral boundary of the first collector right shallow well3010on a side closest to the first collector left shallow well3012extends past a corresponding lateral boundary of the first collector right vertical member3006. A lateral boundary of the first collector left shallow well3012on a side closest to the first collector right shallow well3010extends past a corresponding lateral boundary of the first collector left vertical member3008. An optional first well3014may be formed between the first collector right vertical member3006and the first collector left vertical member3008, above the first buried collector layer3004if present and extending to the top surface of the substrate3002. A conductivity type of the first well3014if formed is opposite from a conductivity type of the first collector right shallow well3010and the first collector left shallow well3012.

A first base right diffused region3016and a first base left diffused region3018are formed in the substrate3002between the first collector right shallow well3010and the first collector left shallow well3012, using processes as described in reference toFIG. 1C. The first base right diffused region3016is located closer to the first collector right shallow well3010than to the first collector left shallow well3012. The first base left diffused region3018is located closer to the first collector left shallow well3012than to the first collector right shallow well3010. In the instant embodiment, a lateral separation between the first base right diffused region3016and the first collector right shallow well3010is less than a lateral separation between the first base left diffused region3018and the first collector left shallow well3012. In one realization of the instant embodiment, the lateral separation between the first base right diffused region3016and the first collector right shallow well3010is between 0.75 and 0.85 microns less than the lateral separation between the first base left diffused region3018and the first collector left shallow well3012.

A first emitter diffused region3020is formed in the substrate3002extending to the top surface of the substrate3002between the first base right diffused region3016and the first base left diffused region3018. In the instant embodiment, the first emitter diffused region3020is located substantially equidistant from the first base right diffused region3016and the first base left diffused region3018. The optional buried collector layer3004if present, the first collector right vertical member3006, the first collector left vertical member3008, the first collector right shallow well3010, the first collector left shallow well3012, the first base right diffused region3016, the first base left diffused region3018and the first emitter diffused region3020are included in a first bipolar transistor3022of the stacked bipolar transistor.

A second bipolar transistor3024of the stacked bipolar transistor includes a second buried collector layer3026if the optional first buried collector layer3004is present, and includes a second collector left vertical member3028, a second collector right vertical member3030, a second collector left shallow well3032, a second collector right shallow well3034, an optional second well3036, a second base left diffused region3038, a second base right diffused region3040and a second emitter diffused region3042. The second collector left shallow well3032and the second collector right shallow well3034are located so as to have reflection symmetry with respect to the first collector right shallow well3010and the first collector left shallow well3012, respectively. Similarly, the second base left diffused region3038and the second base right diffused region3040are located so as to have reflection symmetry with respect to the first base right diffused region3016and the first base left diffused region3018, respectively. A plane of the reflection symmetry is perpendicular to the top surface of the substrate3002and perpendicular to the plane ofFIG. 3. A lateral separation between the second base left diffused region3038and the second collector left shallow well3032is substantially equal to the lateral separation between the first base right diffused region3016and the first collector right shallow well3010. The first bipolar transistor3022and the second bipolar transistor3024may be electrically coupled in series by interconnect elements3044as described in reference toFIG. 1E.

The optional first buried collector layer3004if present, the first collector right vertical member3006, the first collector left vertical member3008, the first collector right shallow well3010and first collector left shallow well3012are included in a collector of the first bipolar transistor3022. The first base right diffused region3016and the first base left diffused region3018are included in a base of the first bipolar transistor3022. The first emitter diffused region3020is included in an emitter of the first bipolar transistor3022. Similarly, the second buried collector layer3026if present, the second collector left vertical member3028, the second collector right vertical member3030, the second collector left shallow well3032and the second collector right shallow well3034are included in a collector of the second bipolar transistor3024. The second base left diffused region3038and the second base right diffused region3040are included in a base of the second bipolar transistor3024. The second emitter diffused region3042is included in an emitter of the second bipolar transistor3024.

Forming the first collector right shallow well3010, the first collector left shallow well3012, the first base right diffused region3016and the first base left diffused region3018so that the lateral separation between the first base right diffused region3016and the first collector right shallow well3010is less than the lateral separation between the first base left diffused region3018and the first collector left shallow well3012may cause collector-base breakdown in the first bipolar transistor3022to be induced in a region between the first base right diffused region3016and the first collector right shallow well3010. The configuration of the first base right diffused region3016and the first collector right shallow well3010is a breakdown inducing feature of the first bipolar transistor. Similarly, forming the second collector left shallow well3032, the second collector right shallow well3034, the second base left diffused region3038and the second base right diffused region3040so that the lateral separation between the second base left diffused region3038and the second collector left shallow well3032is less than the lateral separation between the second base right diffused region3040and the second collector right shallow well3034may cause breakdown in the second bipolar transistor3024to be induced in a region between the second base left diffused region3038and the second collector left shallow well3032. The configuration of the second base left diffused region3038and the second collector left shallow well3032is a breakdown inducing feature of the second bipolar transistor.

A first collector-base breakdown voltage of the collector-base breakdown between the first base right diffused region3016and the first collector right shallow well3010may be an increasing function of the lateral separation between the first base right diffused region3016and the first collector right shallow well3010. Instances of the lateral separation between the first base right diffused region3016and the first collector right shallow well3010may include misalignments in instances of the stacked bipolar transistor. Similarly, a second collector-base breakdown voltage of the collector-base breakdown between the second base left diffused region3038and the second collector left shallow well3032may be an increasing function of the lateral separation between the second base left diffused region3038and the second collector left shallow well3032. The lateral separation between the second base left diffused region3038and the second collector left shallow well3032may include the same misalignment as the lateral separation between the first base right diffused region3016and the first collector right shallow well3010. In such a case, a misalignment which increases the lateral separation between the first base right diffused region3016and the first collector right shallow well3010and so increases the first collector-base breakdown voltage, may decrease the lateral separation between the second base left diffused region3038and the second collector left shallow well3032by substantially a same lateral distance and so may decrease the second collector-base breakdown voltage. A collector-base breakdown voltage of the stacked bipolar transistor is substantially a sum of the first collector-base breakdown voltage and the second collector-base breakdown voltage. Thus, forming the breakdown inducing feature of the second bipolar transistor3024to have reflection symmetry with respect to the breakdown inducing feature of the first bipolar transistor3022may reduce a sensitivity of the collector-base breakdown voltage of the stacked bipolar transistor to the misalignments.

It will be recognized that a stacked bipolar transistor of opposite polarity to that described in reference toFIG. 3may be formed by appropriate changes in polarities of dopants and conductivity types. A stacked bipolar transistor of opposite polarity may have a reduced sensitivity of a collector-base breakdown voltage of the stacked bipolar transistor to misalignments of collector shallow wells and base diffused regions, as described in reference toFIG. 3.

In one realization of the instant embodiment, the first collector left vertical member3008, the first collector left shallow well3012, the second collector right vertical member3030and the second collector right shallow well3034may be omitted. In one realization of the instant embodiment, the first base left diffused region3018and the second base right diffused region3040may be omitted.

FIG. 4is a cross-section of an integrated circuit containing a stacked bipolar transistor formed according to a fourth embodiment. The integrated circuit4000is formed in and on a semiconductor substrate4002, as described in reference toFIG. 1A. An optional first buried collector layer4004may be formed in the substrate4002as described in reference toFIG. 1A. A first collector right vertical member4006and a first collector left vertical member4008are formed in the substrate4002and extend from a top surface of the substrate4002to the first buried collector layer4004if present. The first collector right vertical member4006and the first collector left vertical member4008are formed as described in reference toFIG. 1B. An optional first well4010may be formed between the first collector right vertical member4006and the first collector left vertical member4008, above the first buried collector layer4004and extending to the top surface of the substrate4002. A conductivity type of the first well4010if formed is opposite from a conductivity type of the first collector right vertical member4006and the first collector left vertical member4008.

A first base right diffused region4012and a first base left diffused region4014are formed in the substrate4002between the first collector right vertical member4006and the first collector left vertical member4008, using processes as described in reference toFIG. 1C. The first base right diffused region4012is located closer to the first collector right vertical member4006than to the first collector left vertical member4008. The first base left diffused region4014is located closer to the first collector left vertical member4008than to the first collector right vertical member4006. In the instant embodiment, a lateral separation between the first base right diffused region4012and the first collector right vertical member4006is substantially equal to a lateral separation between the first base left diffused region4014and the first collector left vertical member4008.

A first emitter diffused region4016is formed in the substrate4002extending to the top surface of the substrate4002between the first base right diffused region4012and the first base left diffused region4014. In the instant embodiment, the first emitter diffused region4016is located substantially equidistant from the first base right diffused region4012and the first base left diffused region4014.

A first transistor right field plate4018is formed over the substrate4002between the first base right diffused region4012and the first collector right vertical member4006. A first transistor left field plate4020is formed over the substrate4002between the first base left diffused region4014and the first collector left vertical member4008. The first transistor field plates4018and4020may be formed of polycrystalline silicon, commonly known as polysilicon, a combination of polysilicon and metal silicide, a metal such as tungsten, titanium nitride, aluminum or copper, or other electrically conductive material. The first transistor field plates4018and4020may be formed over a dielectric layer to provide electrical isolation from the substrate4002.

The optional first buried collector layer4004if present, the first collector right vertical member4006, the first collector left vertical member4008, the first base right diffused region4012, the first base left diffused region4014, the first emitter diffused region4016, the first transistor right field plate4018and the first transistor left field plate4020are included in a first bipolar transistor4022of the stacked bipolar transistor. The optional first buried collector layer4004if present, the first collector right vertical member4006and the first collector left vertical member4008are included in a collector of the first bipolar transistor4022. The first base right diffused region4012and the first base left diffused region4014are included in a base of the first bipolar transistor4022. The first emitter diffused region4016is included in an emitter of the first bipolar transistor4022.

The first transistor right field plate4018and the first transistor left field plate4020are formed so that collector-base breakdown in the first bipolar transistor4022may be induced under the first transistor right field plate4018. The configuration of the first collector right vertical member4006, the first base right diffused region4012and the first transistor right field plate4018is a breakdown inducing feature of the first bipolar transistor. In one realization of the instant embodiment, a length of the first transistor right field plate4018may be greater than a length of the first transistor left field plate4020. In one realization of the instant embodiment, a distance of the first transistor right field plate4018from the first collector right vertical member4006may be less than a distance of the first transistor left field plate4020from the first collector left vertical member4008.

A second bipolar transistor4024of the stacked bipolar transistor includes a second buried collector layer4026if the optional first buried collector layer4004is present, and includes a second collector left vertical member4028, a second collector right vertical member4030, an optional second well4032, a second base left diffused region4034, a second base right diffused region4036, a second emitter diffused region4038, a second transistor left field plate4040and a second transistor right field plate4042.

The second buried collector layer4026if present, second collector left vertical member4028and the second collector right vertical member4030are included in a collector of the second bipolar transistor4024. The second base left diffused region4034and the second base right diffused region4036are included in a base of the second bipolar transistor4024. The second emitter diffused region4038is included in an emitter of the second bipolar transistor4024.

The second transistor left field plate4040and the second transistor right field plate4042are located so as to have reflection symmetry with respect to the first transistor right field plate4018and the first transistor left field plate4020, respectively. Similarly, the second base left diffused region4034and the second base right diffused region4036are located so as to have reflection symmetry with respect to the first base right diffused region4012and the first base left diffused region4014, respectively. A spatial configuration of the second base left diffused region4034, the second transistor left field plate4040and the second collector left vertical member4028has reflection symmetry to a spatial configuration of the first base right diffused region4012, the first transistor right field plate4018and the first collector right vertical member4006. A plane of the reflection symmetry is perpendicular to the top surface of the substrate4002and perpendicular to the plane ofFIG. 4. Forming the second base left diffused region4034, the second transistor left field plate4040and the second collector left vertical member4028to have a spatial configuration with reflection symmetry of the spatial configuration of the first base right diffused region4012, the first transistor right field plate4018and the first collector right vertical member4006may induce collector-base breakdown in the second bipolar transistor4024under the second transistor left field plate4040. The configuration of the second collector left vertical member4028, the second base left diffused region4034and the second transistor left field plate4040is a breakdown inducing feature of the second bipolar transistor. The first bipolar transistor4022and the second bipolar transistor4024may be electrically coupled in series by interconnect elements4044as described in reference toFIG. 1E.

A first collector-base breakdown voltage of the collector-base breakdown between the first base right diffused region4012and the first collector right vertical member4006may be a function of the spatial configuration of the first base right diffused region4012, the first transistor right field plate4018and the first collector right vertical member4006. Instances of the spatial configuration of the first base right diffused region4012, the first transistor right field plate4018and the first collector right vertical member4006may include misalignments in instances of the stacked bipolar transistor. Similarly, a second collector-base breakdown voltage of the collector-base breakdown between the second base left diffused region4034and the second collector left vertical member4028may be a function of the spatial configuration of the second base left diffused region4034, the second transistor left field plate4040and the second collector left vertical member4028, such that instances of the misalignments which increase the first collector-base breakdown voltage may decrease the second collector-base breakdown voltage, and vice versa. A collector-base breakdown voltage of the stacked bipolar transistor is substantially a sum of the first collector-base breakdown voltage and the second collector-base breakdown voltage. Thus, forming the breakdown inducing feature of the second bipolar transistor4024to have reflection symmetry with respect to the breakdown inducing feature of the first bipolar transistor4022may reduce a sensitivity of the collector-base breakdown voltage of the stacked bipolar transistor to the misalignments.

It will be recognized that a stacked bipolar transistor of opposite polarity to that described in reference toFIG. 4may be formed by appropriate changes in polarities of dopants and conductivity types. A stacked bipolar transistor of opposite polarity may have a reduced sensitivity of a collector-base breakdown voltage of the stacked bipolar transistor to misalignments, as described in reference toFIG. 4.

In one realization of the instant embodiment, the first collector left vertical member4008and the second collector right vertical member4030may be omitted. In one realization of the instant embodiment, the first transistor left field plate4020and the second transistor right field plate4042may be omitted. In one realization of the instant embodiment, the first base left diffused region4014and the second base right diffused region4036may be omitted.

FIG. 5is a cross-section of an integrated circuit containing a stacked bipolar transistor formed according to a fifth embodiment. The integrated circuit5000is formed in and on a semiconductor substrate5002, as described in reference toFIG. 1A. A first buried collector right lobe5004and a first buried collector left lobe5006are formed in the substrate5002as described in reference to the first buried collector layer1004inFIG. 1A. A first collector right vertical member5008and a first collector left vertical member5010are formed in the substrate5002and extend from a top surface of the substrate5002to the first buried collector right lobe5004and the first buried collector left lobe5006, respectively. The first collector right vertical member5008and the first collector left vertical member5010are formed as described in reference toFIG. 1B. A first well5012is formed between the first collector right vertical member5008and the first collector left vertical member5010, above the first buried collector right lobe5004and the first buried collector left lobe5006, extending to the top surface of the substrate5002. A conductivity type of the first well5012is opposite a conductivity type of the first buried collector right lobe5004and the first buried collector left lobe5006.

A first base right diffused region5014and a first base left diffused region5016are formed in the substrate5002between the first collector right vertical member5008and the first collector left vertical member5010, using processes as described in reference toFIG. 1C. The first base right diffused region5014is located closer to the first collector right vertical member5008than to the first collector left vertical member5010. The first base left diffused region5016is located closer to the first collector left vertical member5010than to the first collector right vertical member5008.

A first emitter diffused region5018is formed in the substrate5002extending to the top surface of the substrate5002between the first base right diffused region5014and the first base left diffused region5016. In the instant embodiment, the first emitter diffused region5018is located substantially equidistant from the first base right diffused region5014and the first base left diffused region5016. The first buried collector right lobe5004, the first buried collector left lobe5006, the first collector right vertical member5008, the first collector left vertical member5010, the first base right diffused region5014, the first base left diffused region5016and the first emitter diffused region5018are included in a first bipolar transistor5020of the stacked bipolar transistor. The first buried collector right lobe5004, the first buried collector left lobe5006, the first collector right vertical member5008and the first collector left vertical member5010are included in a collector of the first bipolar transistor5020. The first base right diffused region5014and the first base left diffused region5016are included in a base of the first bipolar transistor5020. The first emitter diffused region5018is included in an emitter of the first bipolar transistor5020. In the instant embodiment, an underlap of the first buried collector right lobe5004under the first base right diffused region5014is more than an underlap of the first buried collector left lobe5006under the first base left diffused region5016.

A second bipolar transistor5022of the stacked bipolar transistor includes a second buried collector left lobe5024, a second buried collector right lobe5026, a second collector left vertical member5028, a second collector right vertical member5030, a second well5032, a second base left diffused region5034, a second base right diffused region5036and a second emitter diffused region5038. The second buried collector left lobe5024and the second buried collector right lobe5026, are formed so as to have reflection symmetry with respect to the first buried collector right lobe5004and the first buried collector left lobe5006, respectively. Similarly, the second collector left vertical member5028and the second collector right vertical member5030are located so as to have reflection symmetry with respect to the first collector right vertical member5008and the first collector left vertical member5010, respectively. Similarly, the second base left diffused region5034and the second base right diffused region5036are located so as to have reflection symmetry with respect to the first base right diffused region5014and the first base left diffused region5016, respectively. A plane of the reflection symmetry is perpendicular to the top surface of the substrate5002and perpendicular to the plane ofFIG. 2. An underlap of the second buried collector left lobe5024under the second base left diffused region5034is substantially equal to the underlap of the first buried collector right lobe5004under the first base right diffused region5014.

The second buried collector left lobe5024, the second buried collector right lobe5026, the second collector left vertical member5028and the second collector right vertical member5030are included in a collector of the second bipolar transistor5022. The second base left diffused region5034and the second base right diffused region5036are included in a base of the second bipolar transistor5022. The second emitter diffused region5038is included in an emitter of the second bipolar transistor5022. The first bipolar transistor5020and the second bipolar transistor5022may be electrically coupled in series by interconnect elements5040as described in reference toFIG. 1E.

Forming the first buried collector right lobe5004, the first buried collector left lobe5006, the first base right diffused region5014and the first base left diffused region5016so that the underlap of the first buried collector right lobe5004under the first base right diffused region5014is greater than the underlap of the first buried collector left lobe5006under the first base left diffused region5016may cause collector-base breakdown in the first bipolar transistor5020to be induced in a region between the first base right diffused region5014and the first buried collector right lobe5004. The configuration of the first buried collector right lobe5004and the first base right diffused region5014is a breakdown inducing feature of the first bipolar transistor. Similarly, forming the second buried collector left lobe5024, the second buried collector right lobe5026, the second base left diffused region5034and the second base right diffused region5036so that the underlap of the second buried collector left lobe5024under the second base left diffused region5034is greater than the underlap of the second buried collector right lobe5026under the second base right diffused region5036may cause breakdown in the second bipolar transistor5022to be induced in a region between the second base left diffused region5034and the second buried collector left lobe5024. The configuration of the second base left diffused region5034and the second buried collector left lobe5024is a breakdown inducing feature of the second bipolar transistor.

A first collector-base breakdown voltage of the collector-base breakdown between the first base right diffused region5014and the first buried collector right lobe5004may be an increasing function of the underlap of the first buried collector right lobe5004under the first base right diffused region5014. Instances of the underlap of the first buried collector right lobe5004under the first base right diffused region5014may include misalignments in instances of the stacked bipolar transistor. Similarly, a second collector-base breakdown voltage of the collector-base breakdown between the second base left diffused region5034and the second buried collector left lobe5024may be an increasing function of the underlap of the second buried collector left lobe5024under the second base left diffused region5034. The underlap of the second buried collector left lobe5024under the second base left diffused region5034may include the same misalignment as the underlap of the first buried collector right lobe5004under the first base right diffused region5014. In such a case, a misalignment which increases the underlap of the first buried collector right lobe5004under the first base right diffused region5014and so increases the first collector-base breakdown voltage, may decrease the underlap of the second buried collector left lobe5024under the second base left diffused region5034by substantially a same distance and so may decrease the second collector-base breakdown voltage. A collector-base breakdown voltage of the stacked bipolar transistor is substantially a sum of the first collector-base breakdown voltage and the second collector-base breakdown voltage. Thus, forming the breakdown inducing feature of the second bipolar transistor5022to have reflection symmetry with respect to the breakdown inducing feature of the first bipolar transistor5020may reduce a sensitivity of the collector-base breakdown voltage of the stacked bipolar transistor to the misalignments.

It will be recognized that a stacked bipolar transistor of opposite polarity to that described in reference toFIG. 5may be formed by appropriate changes in polarities of dopants and conductivity types. A stacked bipolar transistor of opposite polarity may have a reduced sensitivity of a collector-base breakdown voltage of the stacked bipolar transistor to misalignments, as described in reference toFIG. 5.

In one realization of the instant embodiment, the first collector left vertical member5010and the second collector right vertical member5030may be omitted. In one realization of the instant embodiment, the first buried collector left lobe5006and the second buried collector right lobe5026may be omitted. In one realization of the instant embodiment, the first base left diffused region5016and the second base right diffused region5036may be omitted.

FIG. 6is a cross-section of an integrated circuit containing a stacked bipolar transistor formed according to a sixth embodiment. The integrated circuit6000is formed in and on a semiconductor substrate6002, as described in reference toFIG. 1A. An optional first buried collector layer6004may be formed in the substrate6002as described in reference toFIG. 1A. A first collector right vertical member6006and a first collector left vertical member6008are formed in the substrate6002and extend from a top surface of the substrate6002to the first buried collector layer6004if present. The first collector right vertical member6006and the first collector left vertical member6008are formed as described in reference toFIG. 1B.

A first base right diffused region6010and a first base left diffused region6012are formed in the substrate6002between the first collector right vertical member6006and the first collector left vertical member6008, using processes as described in reference toFIG. 1C. The first base right diffused region6010is located closer to the first collector right vertical member6006than to the first collector left vertical member6008. The first base left diffused region6012is located closer to the first collector left vertical member6008than to the first collector right vertical member6006. In the instant embodiment, a lateral separation between the first base right diffused region6010and the first collector right vertical member6006is substantially equal to a lateral separation between the first base left diffused region6012and the first collector left vertical member6008.

A first emitter diffused region6014is formed in the substrate6002extending to the top surface of the substrate6002between the first base right diffused region6010and the first base left diffused region6012. In the instant embodiment, the first emitter diffused region6014is located substantially equidistant from the first base right diffused region6010and the first base left diffused region6012.

A first transistor right implanted region6016having a same conductivity type as the first base right diffused region6010is formed in the substrate6002between the first base right diffused region6010and the first collector right vertical member6006. A first transistor left implanted region6018having the same conductivity type as the first base left diffused region6012is formed in the substrate6002between the first base left diffused region6012and the first collector left vertical member6008. The first transistor implanted regions6016and6018may be formed by ion implanting dopants followed by an anneal process.

The optional first buried collector layer6004if present, the first collector right vertical member6006, the first collector left vertical member6008, the first base right diffused region6010, the first base left diffused region6012, the first emitter diffused region6014, the first transistor right implanted region6016and the first transistor left implanted region6018are included in a first bipolar transistor6020of the stacked bipolar transistor. The optional first buried collector layer6004if present, the first collector right vertical member6006and the first collector left vertical member6008are included in a collector of the first bipolar transistor6020. The first base right diffused region6010and the first base left diffused region6012are included in a base of the first bipolar transistor6020. The first emitter diffused region6014is included in an emitter of the first bipolar transistor6020.

The first transistor right implanted region6016and the first transistor left implanted region6018are formed so that collector-base breakdown in the first bipolar transistor6020may be induced between the first collector right vertical member6006and the first base right diffused region6010. The configuration of the first base right diffused region6010, first transistor right implanted region6016and the first collector right vertical member6006is a breakdown inducing feature of the first bipolar transistor. In one realization of the instant embodiment, a length of the first transistor right implanted region6016may be greater than a length of the first transistor left implanted region6018. In one realization of the instant embodiment, a doping density of the first transistor right implanted region6016may be greater than a doping density of the first transistor left implanted region6018.

A second bipolar transistor6022of the stacked bipolar transistor includes a second buried collector layer6024if the optional first buried collector layer6004is present, and includes a second collector left vertical member6026, a second collector right vertical member6028, a second base left diffused region6030, a second base right diffused region6032, a second emitter diffused region6034, a second transistor left implanted region6036having the same conductivity type as the second base left diffused region6030and a second transistor right implanted region6038having the same conductivity type as the second base right diffused region6032. The second buried collector layer6024if present, the second collector left vertical member6026and the second collector right vertical member6028are included in a collector of the second bipolar transistor6022. The second base left diffused region6030and the second base right diffused region6032are included in a base of the second bipolar transistor6022. The second emitter diffused region6034is included in an emitter of the second bipolar transistor6022. The second transistor left implanted region6036and the second transistor right implanted region6038are located so as to have reflection symmetry with respect to the first transistor right implanted region6016and the first transistor left implanted region6018, respectively. Similarly, the second base left diffused region6030and the second base right diffused region6032are located so as to have reflection symmetry with respect to the first base right diffused region6010and the first base left diffused region6012, respectively. A spatial configuration of the second base left diffused region6030, the second transistor left implanted region6036and the second collector left vertical member6026has reflection symmetry to a spatial configuration of the first base right diffused region6010, the first transistor right implanted region6016and the first collector right vertical member6006. A plane of the reflection symmetry is perpendicular to the top surface of the substrate6002and perpendicular to the plane ofFIG. 6. Forming the second base left diffused region6030, the second transistor left implanted region6036and the second collector left vertical member6026to have a spatial configuration with reflection symmetry of the spatial configuration of the first base right diffused region6010, the first transistor right implanted region6016and the first collector right vertical member6006may induce collector-base breakdown in the second bipolar transistor6022under the second transistor left implanted region6036. The configuration of the second base left diffused region6030, second transistor left implanted region6036and the second collector left vertical member6026is a breakdown inducing feature of the second bipolar transistor. The first bipolar transistor6020and the second bipolar transistor6022may be electrically coupled in series by interconnect elements6040as described in reference toFIG. 1E.

A first collector-base breakdown voltage of the collector-base breakdown between the first base right diffused region6010and the first collector right vertical member6006may be a function of the spatial configuration of the first base right diffused region6010, the first transistor right implanted region6016and the first collector right vertical member6006. Instances of the spatial configuration of the first base right diffused region6010, the first transistor right implanted region6016and the first collector right vertical member6006may include misalignments in instances of the stacked bipolar transistor. Similarly, a second collector-base breakdown voltage of the collector-base breakdown between the second base left diffused region6030and the second collector left vertical member6026may be a function of the spatial configuration of the second base left diffused region6030, the second transistor left implanted region6036and the second collector left vertical member6026, such that instances of the misalignments which increase the first collector-base breakdown voltage may decrease the second collector-base breakdown voltage, and vice versa. A collector-base breakdown voltage of the stacked bipolar transistor is substantially a sum of the first collector-base breakdown voltage and the second collector-base breakdown voltage. Thus, forming the breakdown inducing feature of the second bipolar transistor6022to have reflection symmetry with respect to the breakdown inducing feature of the first bipolar transistor6020may reduce a sensitivity of the collector-base breakdown voltage of the stacked bipolar transistor to the misalignments.

It will be recognized that a stacked bipolar transistor of opposite polarity to that described in reference toFIG. 6may be formed by appropriate changes in polarities of dopants and conductivity types. A stacked bipolar transistor of opposite polarity may have a reduced sensitivity of a collector-base breakdown voltage of the stacked bipolar transistor to misalignments, as described in reference toFIG. 6.

In one realization of the instant embodiment, the first collector left vertical member6008and the second collector right vertical member6028may be omitted. In one realization of the instant embodiment, the first transistor left implanted region6018and the second transistor right implanted region6038may be omitted. In one realization of the instant embodiment, the first base left diffused region6012and the second base right diffused region6032may be omitted.