Abstract:
A zener diode is formed in a bipolar or BiCMOS fabrication process by modifying the existing masks that are used in the bipolar or BiCMOS fabrication process, thereby eliminating the need for a separate doping step. In addition, the reverse breakdown voltage of the zener diode is set to a desired value within a range of values by modifying the area of a new opening in one of existing masks.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for forming a zener diode and, more particularly, to a method of forming a zener diode in a npn and pnp bipolar process flow that requires no additional steps to set the reverse breakdown voltage of the zener diode to any voltage within a range of voltages. 
     2. Description of the Related Art 
     A zener diode is a p-n junction device that allows substantially no current flow through the diode when the diode is reverse biased (when the voltage on the n region of the diode is greater than the voltage on the p region of the diode.) However, when the n-to-p voltage difference is positive and exceeds a reverse breakdown voltage, a large breakdown current flows through the diode. 
     The magnitude of the reverse breakdown voltage, in turn, is a function of the relative doping concentrations of the n and p regions of the diode. Thus, by varying the relative doping concentrations, the reverse breakdown voltage can be set to any voltage within a range of voltages. 
     Zener diodes are commonly formed in a semiconductor process that utilizes a separate doping step to define the relative doping concentrations. The separate doping step typically forms the n or the p region of the diode, or adds a predefined amount of dopant to an existing n or p region, to thereby define the reverse breakdown voltage of the diode. 
     Zener diodes have a number of uses in semiconductor integrated circuits, including in an electrostatic discharge (ESD) protection circuit that provides ESD protection for variable power supply lines. FIG. 1 shows a circuit diagram that illustrates a conventional ESD protection circuit  100 . 
     As shown in FIG. 1, circuit  100  includes a zener clamp diode  110 , a npn transistor  112 , and a resistor  114 . Transistor  112  has a collector connected to a power supply line  116 , a base connected to diode  110  and resistor  114 , and an emitter connected to ground. Diode  110  also has a connection to power supply line  116 , while resistor  114  also has a connection to ground. 
     In operation, when the voltage on power supply line  116  is less than the reverse breakdown voltage, essentially no current flows through diode  110 . As a result, ground is on the base of transistor  112 , thereby turning off transistor  112 . When the voltage on power supply line  116  exceeds the reverse breakdown voltage, such as when a human body model (HBM) pulse is applied to line  116 , a current flows through diode  110  and resistor  114 , thereby placing a voltage on the base of transistor  112 . The voltage on the base of transistor  112 , in turn, turns on transistor  112 , thereby allowing a collector current IC to flow into the collector, and an emitter current IE to flow out of the emitter, of transistor  112 . 
     Although the separate doping step described above is conventionally utilized to form a zener diode, the need for a separate doping step increases the cost and complexity of the fabrication process. Thus, there is a need for a method of forming a zener diode that allows the reverse breakdown voltage to be set to any voltage within a range of voltages without requiring a separate doping step. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of forming a zener diode. The method allows the reverse breakdown voltage of the diode to be set by modifying the existing steps in a bipolar or BiCMOS fabrication process. The present method provides both a high tunable breakdown voltage range and a simple realization. 
     The method of the present invention begins with the step of forming a first zener region of a first conductivity type in a semiconductor material. The method also includes the steps of forming a second zener region of a second conductivity type in the substrate in the first zener region, and forming a layer of epitaxial material on the semiconductor material. The layer of epitaxial material has a surface, a first region that extends from the surface to the first zener region, and a second region that extends from the surface to the second zener region. The method also includes the step of doping the layer of epitaxial material so that the first region has the first conductivity type and the second region has the second conductivity type. 
     The method can also include the step of forming first and second transistor buried regions of the first and second conductivity types, respectively, in the semiconductor material. In addition, the method can form a third region of the first conductivity type in the epitaxial layer that extends from the surface to the first transistor buried region. 
     Further, a fourth region of the second conductivity type is formed in the epitaxial layer to extend from the surface to the second transistor buried region. The method can also form a first base of a second conductivity type that contacts the third region of the epitaxial layer; and a second base of a first conductivity type that contacts the fourth region of the epitaxial layer. 
     The present invention also provides a semiconductor device that includes a first zener region of a first conductivity type that is formed in a substrate, and a second zener region of the second conductivity type that is formed in the substrate in the first zener region. The semiconductor device also includes an epitaxial layer that is formed on the substrate. The epitaxial layer has a surface, a first region of the first conductivity type that extends from the surface to the first zener region, and a second region of the second conductivity type that extends from the surface to the second zener region. 
     The semiconductor device can also include a first isolation region of the first conductivity type that is formed in the substrate apart from the zener region. The semiconductor device can further include a first buried region of the first conductivity type that is formed in the substrate in the first isolation region, and a second buried region of the second conductivity type that is formed in the substrate. 
     The semiconductor device can additionally include, in the epitaxial layer, a third region of the first conductivity type that extends from the surface to the first buried region, and a fourth region of the second conductivity type that extends from the surface to the second buried region. In addition, a first base region of the second conductivity type contacts the third region of the epitaxial layer, and a second base region of the first conductivity type contacts the fourth region of the epitaxial layer. 
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating a conventional ESD protection circuit  100 . 
     FIG. 2 is a cross-sectional diagram illustrating an example of a semiconductor device  200  in accordance with the present invention. 
     FIGS. 3A-3M are cross-sectional drawings illustrating an example of a method of forming a semiconductor device in accordance with the present invention. 
     FIGS. 4A and 4B are plan views illustrating two of a number of shapes that zener opening  314  can have after mask  312  has been formed and patterned in accordance with the present invention. 
     FIG. 5 is a graph illustrating the reverse breakdown voltage of zener diode  430  vs. the current through zener diode  430  for four sizes of zener area  410  in accordance with the present invention. 
     FIG. 6 is a graph illustrating the reverse breakdown voltage of zener diode  430  vs. the size of opening  314  with and without deep zener trench in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 shows a cross-sectional diagram that illustrates an example of a semiconductor device  200  in accordance with the present invention. 
     As shown in FIG. 2, device  200 , which is formed on a conventionally-formed substrate  216 , includes a n-type zener region  220  that is formed in substrate  216 , a p-type isolation region  222  that is optionally formed in substrate  216 , and a n-type isolation region  224  that is formed in substrate  216 . (If substrate  216  has a p conductivity type of a sufficient dopant concentration, substrate  216  and region  222  can be considered the same.) As described in greater detail below, zener region  220  and isolation region  224  are formed at the same time and, therefore, have substantially the same dopant concentrations. 
     Device  200  additionally includes a p+ zener region  230  that is formed in n-type zener region  220 , and an optional n+ zener region  232  that is formed in zener region  220 . Device  200  further includes a n+ buried region  234  that is formed in p-type isolation region  222  (or substrate  216 ), and a p+ buried region  236  that is formed in n-type isolation region  224 . As described in greater detail below, p+ zener region  230  and p+ buried region  236  are formed at the same time and, therefore, have substantially the same dopant concentrations. 
     Device  200  also includes a n-type region  240  that is formed on substrate  216  over zener region  220  (or optional n+ region  232 ) and buried region  234 , and a p-type region  242  that is formed on substrate  216  over zener region  230  and buried region  236 . Region  240  over buried region  234  forms the collector of transistor  212 , while region  242  over buried region  236  forms the collector of transistor  214 . Device  200  further includes a p− base region  244  that contacts n-type collector region  240  over buried region  234 , and a n− base region  246  that contacts p-type collector region  242  over buried region  236 . 
     Device  200  additionally includes a n+ emitter region  250  that is formed in p− base region  244 , and a p+ emitter region  252  that is formed in a n− base region  246 . In addition, a n+ zener sinker  254  is optionally formed in region  240  to contact n-type zener region  220  or optional n+ region  232 , and a p+ zener sinker  256  is optionally formed in region  242  to contact p+ region  230 . Further, a n+ sinker  260  is formed in region  240  to contact n+ buried layer  234 , and a p+ sinker  262  is formed in region  242  to contact p+ buried layer  236 . 
     Sinkers  254 ,  256 ,  260 , and  262  reduce the series resistance to n-type zener region  220  (or n+ region  232 ), p+ region  230 , n+ buried region  234 , and p+ buried region  236 , respectively. Further, deep trench isolators  270  are formed between devices, shallow trench isolators  272  are formed in regions  240  and  242  over buried layers  234  and  236 , respectively, to define collector surface areas  274  and base/emitter surface areas  276 . In addition, a zener isolator  278  is optionally formed in the center of n-type zener region  220 . 
     FIGS. 3A-3M show cross-sectional drawings that illustrate an example of a method of forming a semiconductor device in accordance with the present invention. 
     Following this, a n-iso mask  304  is formed and patterned on oxide layer  302 . Mask  304  is patterned to have a zener opening  306  that exposes a zener surface  308  of substrate  300  (under oxide layer  302 ), and a pnp opening  310  that exposes a pnp surface  312  of substrate  300 . Zener opening  306 , in turn, has a zener area measured on the surface of a plane that passes through substantially all of the surface of mask  304 . 
     FIGS. 4A and 4B show plan views that illustrate two of a number of shapes that zener opening  306  can have after mask  304  has been formed and patterned in accordance with the present invention. As shown in FIG. 4A, mask  304  can be formed such that zener opening  306  has a square shape. In this case, a zener area  410  is defined by squaring a side-length L of opening  306 . 
     As shown in FIG. 4B, mask  304  can alternately be formed such that opening  306  has a multi-fingered shape. In this case, a zener area  412  is defined by the sum of the areas of each of the fingers. (A 5% breakdown voltage increase results for a square mask (represents a multiple cell layout) in comparison with a multiple finger layout.) 
     Once mask  304  has been patterned, zener surface  308  and pnp surface  312  in FIG. 3A are implanted with a dopant, such as phosphorous or arsenic, through overlying oxide layer  302 . The implant forms a first n-type zener implanted region in substrate  300  below zener opening  306 , and a first pnp implanted region in substrate  300  below opening  310 . Mask  304  is then stripped. 
     Following this, as shown in FIG. 3B, a p-iso mask  314  is optionally formed and patterned on oxide layer  302  (p-iso mask  314  and the subsequent boron implant are unnecessary if substrate  300  is formed with a p conductivity type and a sufficient dopant concentration). Mask  314  is patterned to have a npn opening  316  that exposes a npn surface  318  of substrate  300  (under oxide layer  302 ), and to protect zener surface  308  and pnp surface  312 . Once p-iso mask  314  has been patterned, npn surface  318  is implanted with a dopant, such as boron, through overlying oxide layer  302 . The implant forms a first npn implanted region in substrate  300  below opening  316 . P-iso mask  314  is then stripped. 
     After the implanted regions have been formed, as shown in FIG. 3C, substrate  300  is annealed in a neutral ambient, such as nitrogen. (Other ambients may alternately be used.) The annealing causes the dopants in the zener implanted region to diffuse and form an n-type zener region  320 -Z, the first npn implanted region to diffuse and form a p-type isolation region  320 -P, and the first pnp implanted region to diffuse and form a n-type isolation region  320 -N. 
     In accordance with the present invention, by varying the area of opening  306  in mask  304  (FIG.  3 A), the amount of dopant that is introduced into substrate  300  can be varied. By varying the amount of dopant that is introduced into substrate  300 , the doping profile in the center of n-type zener region  320 -Z can be varied. 
     Bipolar and BiCMOS fabrication processes typically include a number of long high-temperature cycles that cause dopant diffusion. The doping profile that results is a function of the total amount of dopant that is available for diffusion which, in turn, is a function of the area of opening  306  in mask  304 . 
     Thus, when the area of opening  306  is relatively small, a first doping profile results, and when the area of opening  306  is relatively large, a second doping profile results. Thus, varying the area of opening  306  varies the amount of dopant available for diffusion which, in turn, varies the doping profile. 
     Varying the doping profile allows the breakdown voltage of the diode to be varied. As described in greater detail below, a p-type region with a known dopant concentration is subsequently formed to define a p-n junction. Since the p-type region has a known dopant concentration, the reverse breakdown voltage of the diode can be set to any voltage within a range of voltages by varying the doping profile of the n-type region. 
     Returning to FIG. 3C, a first buried layer mask  322  is next formed on oxide layer  302 . Mask  322  is patterned to expose npn surface  318  under oxide layer  302 . Following this, npn surface  318  is implanted with a dopant, such as phosphorous or arsenic, through overlying oxide layer  302 . The implant forms a second npn implanted region at the surface of isolation region  320 -P (or in substrate  300  if region  320 -P is not formed). 
     Mask  322  can also be patterned to expose a first surface portion  324 -A of zener surface  308 . When mask  322  is patterned to expose portion  324 -A, the implant also forms a second zener implanted region at the surface of zener region  320 -Z. Mask  322  is then stripped. 
     As shown in FIG. 3D, a second buried layer mask  326  is formed on oxide layer  302  after mask  322  has been removed. Mask  326  is patterned to expose a second portion  324 -B of zener surface  308  and pnp surface  312  under oxide layer  302 , and protect first portion  324 -A of zener surface  308  and npn surface  318 . 
     Following this, second portion  324 -B of zener surface  308  and pnp surface  312  are implanted with a dopant, such as boron, through oxide layer  302 . The implant forms a third zener implanted region at the surface of zener region  320 -Z, and a second pnp implanted region at the surface of isolation region  320 -N. Mask  326  is then stripped. 
     After the implanted regions have been formed, as shown in FIG. 3E, substrate  300  is again annealed in a neutral ambient. (Other ambients may also be used.) This annealing step causes the dopants in the third zener implanted region to diffuse and form a p+ region  328 -A in zener region  320 -Z. In addition, if a second zener implanted region was formed at the surface of zener region  320 -Z, then the anneal causes the dopants in the second zener implanted region to diffuse and form a n+ region  328 -B in zener region  320 -Z. 
     The step also causes the dopants in the second npn implanted region to diffuse and form a n+ buried region  330  in isolation region  320 -P (or substrate  300  if region  320 -P is not present), and the second pnp implanted region to diffuse and form a p+ buried region  332  in isolation region  320 -N. This annealing step is shorter than the prior annealing step and, as a result, causes less diffusion. 
     Following this, sacrificial oxide layer  302  is removed, and a n-type epitaxial layer  334  is grown on substrate  300  using conventional epitaxial preparation and growth steps. After region  334  has been formed, a layer of sacrificial oxide  336  is formed on n− region  334 . Next, a mask  338  is formed and patterned on oxide layer  336 . 
     Mask  338  is patterned to expose the area of n− region  334  (under oxide layer  336 ) that overlies pnp surface  312 , and protect the areas of region  334  that overlie zener surface  308  and npn surface  318 . Mask  338  can also be patterned to expose the area of region  334  (under oxide layer  336 ) that overlies p+ region  328 -A. The area overlying pnp surface  312  is then implanted with a dopant, such as boron, through oxide layer  336  to form a p-type implanted region. 
     When mask  338  is also patterned to expose the area of region  334  that overlies p+ region  328 -A, the area overlying the surface of p+ region  328 -A is also implanted to form a p-type implanted region. Mask  338  is then removed. Substrate  300  is then annealed in a neutral ambient (other ambients may also be used), thereby causing the dopants in the p-type implanted regions to diffuse and form a p− region  340 . The area of region  334  formed over n+ buried region  330  forms the collector of transistor  212 , while the area of region  340  formed over p+ buried region  332  forms the collector of transistor  214 . 
     Next, as shown in FIG. 3F, a layer of nitride  342  is formed on oxide layer  336 . After this, a deep trench mask  344  is formed and patterned on nitride layer  342 . Mask  344  is patterned to expose a trench surface  346  on n− collector region  334 . In addition, mask  344  can also be patterned to expose a zener surface region  348  over the surface junction of p+ region  328 -A, and zener surface  308  or n+ region  328 -B when formed. 
     As shown in FIG. 3G, once mask  344  has been patterned, the exposed regions of nitride layer  342  and the underlying oxide layer  336  and substrate  300  are etched for a predetermined period of time to form deep trenches  350 . When mask  344  is patterned to expose the surface junction, the etch also forms deep zener trench  352 . Mask  344  is then stripped. 
     After this, as shown in FIG. 3H, a shallow trench mask  354  is formed and patterned on nitride layer  342 . Mask  354  is patterned to expose shallow trench regions over deep trenches  350 , a shallow trench region over n− collector region  334 , and a shallow trench region over p-type collector region  340 . Mask  354  can also be patterned to expose a shallow trench region over deep zener trench  352 . Once mask  354  has been patterned, the exposed regions are etched for a predetermined period of time to form shallow trenches  356  and, when the pattern is present, a shallow zener trench  358  over deep zener trench  352 . (The etch also enlarges the size of trenches  350  and  352 .) Mask  354  is then stripped. 
     Following this, as shown in FIG. 3I, a layer of liner oxide  360  is grown in trenches  350 ,  352 ,  356 , and  358 . Next, a layer of oxide is formed on nitride layer  342  to fill up trenches  350 ,  352 ,  356 , and  358 . The oxide layer is then planarized using conventional techniques, such as chemical-mechanical-polishing, to remove the oxide layer from the surface of nitride layer  342 , and form deep isolation regions  362  and shallow isolation regions  364 . If zener trenches  352  and  358  were formed, the planarization also forms zener isolation region  368 . 
     As shown in FIG. 3J, after the planarization, nitride layer  342  is removed. Next, a n+ sinker mask  370  is formed and patterned on oxide layer  366 . Mask  370  is patterned to expose the area of collector region  334  that is formed over a collector surface  372  between deep isolation region  362  and shallow isolation region  364  over n+ buried layer  330 . Mask  370  is also patterned to expose the area of collector region  334  formed over zener surface  308  or n+ region  328 -B if formed. 
     Once mask  370  has been patterned, the exposed regions of oxide layer  336  are implanted with a dopant, such as phosphorous or arsenic, to form a first collector implanted region in n-type region  334  over n+ buried layer  330 . The implant also forms a first zener implanted region in n− region  334  over zener surface  308 , or n+ region  328 -B if formed. Mask  370  is then removed. 
     As shown in FIG. 3K, a p+ sinker mask  374  is formed and patterned on oxide layer  336 . Mask  374  is patterned to expose the area of region  340  that is formed over a collector surface  376  between deep isolation region  362  and shallow isolation region  364  over p+ buried layer  332 . Mask  374  is also patterned to expose the area of region  340  that is formed over p+ region  328 -A. 
     Once mask  374  has been patterned, the exposed regions of oxide layer  336  are implanted with a dopant, such as boron, to form a second collector implanted region in p-type region  340  over p+ buried layer  332 . The implant also forms a second zener implanted region in p-type region  340  over p+ region  328 -A. Mask  374  is then removed. 
     After the implanted regions have been formed, as shown in FIG. 3L, substrate  300  is annealed in a neutral ambient, such as nitrogen. (Other ambients can also be used.) The annealing causes the dopants in the first zener implanted region to diffuse and form a n+ zener sinker region  378 , and the second zener implanted region to diffuse and form a p+ zener sinker region  380 . The annealing also causes the first collector implanted region to diffuse and form a n+ bipolar sinker region  382 , and the second collector implanted region to diffuse and form a p+ bipolar sinker region  384 . 
     Following this, the process continues with conventional steps. As shown in FIG. 3M, these steps include the formation of a p− base region  386  in collector region  334  over n+ buried layer  330 , and a n− base region  388  in collector region  340  over p+ buried layer  332 . Although FIG. 3M shows base regions  386  and  388  formed in collector region  334  and collector region  340 , respectively, the present method applies equally well to other base structures, including reduced-size base structures, grown base structures, and extrinsic base structures. 
     These conventional steps also include the formation of a n+ emitter region  390  in p− base region  386 , and a p+ emitter region  392  in n− base region  388 . Although FIG. 3L shows emitter regions  390  and  392  formed in base layers  386  and  388 , respectively, the present method applies equally well to other emitter structures, including single and double poly extrinsic emitter structures. 
     Thus, a method has been shown for forming a zener diode  394 , such as diode  210 , a npn bipolar transistor  396 , such as transistor  212 , and a pnp bipolar transistor  398 , such as transistor  214 . Zener diode  394  includes zener region  320 -Z, p+ region  328 -A, n+ region  328 -B, n+region  378 , and p+ region  380 . 
     Npn bipolar transistor  396  includes isolation region  320 -P, n+ buried region  330 , collector region  334 , sinker  382 , p− base  386 , and n+ emitter  390 . Pnp bipolar transistor  398  includes isolation region  320 -N, p+ buried region  332 , collector region  340 , sinker  384 , n− base  388 , and p+ emitter  392 . 
     In accordance with the present invention, the reverse breakdown voltage of zener diode  394  is set by varying the size of the zener area, such as zener area  410  or  412 . FIG. 5 shows a graph that illustrates the reverse breakdown voltage of zener diode  394  vs. the current through zener diode  394  for four sizes of zener area  410  in accordance with the present invention. 
     As shown in FIG. 5, a 5 um by 5 um opening  306  in n-iso mask  304 , represented by line A, has a breakdown voltage of approximately 28V, while a 4 um by 4 um opening  306 , represented by line B, has a breakdown voltage of approximately 30V. In addition, a 1.5 um by 1.5 um opening  306  in mask  312 , represented by line C, has a breakdown voltage of approximately 55V, while a 1 um by 1 um opening  306 , represented by line D, has a breakdown voltage of approximately 75V. 
     FIG. 6 shows a graph that illustrates the reverse breakdown voltage of zener diode  394  vs. the size of opening  306  with and without deep zener isolation  368  in accordance with the present invention. As shown in FIG. 6, the breakdown voltage of varies from approximately 80V down to 20V based on the area of the mask opening, and then becomes substantially constant when the size of opening  306  exceeds a 10 um by 10 um sized opening. 
     In addition, the value of the breakdown voltage is largely independent of the presence of zener trench isolation  368 . As further shown in FIG. 6, with trench isolation  368 , a 10 um by 10 um sized opening  306  produces a reverse breakdown voltage of approximately 27V while a 30 um by 30 um sized opening  306  produces a reverse breakdown voltage of the same 27V. 
     On the other hand, when zener trench isolation  368  is absent, a 10 um by 10 um sized opening  306  produces a reverse breakdown voltage of approximately 27V while a 30 um by 30 um sized opening  306  produces a reverse breakdown voltage of approximately 22V. Overall, a square-shaped opening  306  with a side length ranging from 100 um to 1 um has a reverse breakdown voltage range from approximately 20V to 80V. 
     In the present invention, after regions  332 ,  334 , and  336  have been formed, each subsequent high temperature step, including the formation of collector region  334  and collector region  340 , causes the dopants in isolation regions  320 -N and  320 -P to further diffuse into substrate  300 . For example, in a 0.18-micron fabrication process, regions  320 -N and  320 -P can have depths D of approximately 12 um when the fabrication process is complete. 
     It should be understood that various alternatives to the method of the invention described herein may be employed in practicing the invention. For example, rather than continuing to reuse a layer of gate oxide, the layer can be removed and replaced by a new layer. Further, the present method can also be incorporated into a BiCMOS process. 
     Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.