Abstract:
A bi-directional transient voltage suppression (“TVS”) device ( 101 ) includes a semiconductor die ( 201 ) that has a first avalanche diode ( 103 ) in series with a first rectifier diode ( 104 ) connected cathode to cathode, electrically coupled in an anti-parallel configuration with a second avalanche diode ( 105 ) in series with a second rectifier diode ( 106 ) also connected cathode to cathode. All the diodes of the TVS device are on a single semiconductor substrate ( 301 ). The die has a low resistivity buried diffused layer ( 303 ) having a first conductivity type disposed between a semiconductor substrate ( 301 ) having the opposite conductivity type and a high resistivity epitaxial layer ( 305 ) having the first conductivity type. The buried diffused layer shunts most of a transient current away from a portion of the epitaxial layer between the first avalanche diode and the first rectifier diode, thereby reducing the clamping voltage relative to the breakdown voltage. The TVS device is packaged as a flip chip ( 202 ) that has four solder bump pads ( 211 - 214 ). The abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims pursuant to 37 C.F.R. §1.72( b ).

Description:
RELATED APPLICATION 
   This is a divisional application of U.S. patent application Ser. No. 11/008,416, filed Dec. 9, 2004 now U.S. Pat. No. 7,361,942, which is a continuation application of U.S. patent application Ser. No. 10/635,088, filed Aug. 5, 2003 now U.S. Pat. No. 6,867,436, hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to active solid-state devices, and more particularly to a transient voltage suppression device having one or more avalanche diodes. 
   2. Description of the Related Art 
   Transient voltage suppression (“TVS”) devices comprising an avalanche diode are well known. As the reverse avalanche voltage is made smaller, a depletion region of the avalanche diode narrows, resulting in a higher internal capacitance of the avalanche diode. As operating frequencies become higher, the internal capacitance of the avalanche diode becomes problematic. A known solution to the capacitance problem is to add a rectifier diode in series with the avalanche diode, with either the anodes or the cathodes of the diodes connected together. A rectifier diode has a smaller capacitance than an avalanche diode has, and the total capacitance of a pair of such diodes in series is less than the sum of the two capacitances. 
   TVS devices having both diodes of such pair on a single die are also known. For example, U.S. Pat. No. 6,392,266 entitled TRANSIENT SUPPRESSING DEVICE AND METHOD, issued May 21, 2002, to Robb et al., discloses two transient voltage suppressors that are housed in a single semiconductor package, each transient voltage suppressor comprising two serially coupled diodes on one die. A TVS device comprising such a pair of diodes is a unidirectional device, in that the TVS device provides protection against voltage spikes or surges in one direction only. 
   Bi-directional TVS devices comprising two such pairs of diodes in an anti-parallel configuration are also known. Known bi-directional TVS devices comprise at least two die, wire bonded together inside a single semiconductor package. One example of such a TVS device is the Model No. PSLCO3 through PSLC24C family of TVS devices manufactured by ProTek Devices of Phoenix, Ariz., which includes four die inside a single semiconductor package. Such TVS devices work well for their intended uses, but when a very small bi-directional TVS device is required, a TVS device comprising a single die is preferred. 
   The reverse avalanche voltage, or breakdown voltage, is defined as the voltage at which the avalanche diode goes into avalanche mode, measured at a relatively low current such as one milliamp. The breakdown voltage is controlled by the doping level of an N+ diffusion layer relative to the doping level of a P+ diffusion layer of the avalanche diode. The clamping voltage is defined as the maximum voltage across the TVS device when a maximum surge current is flowing through it. The clamping voltage is typically measured at a relatively high current such as one amp. As a result, the clamping voltage is normally higher than the breakdown voltage. The clamping voltage of a TVS device is directly, although not necessarily linearly, proportional to the breakdown voltage of the avalanche diode. The amount by which the clamping voltage is greater than the breakdown voltage is directly proportional to the geometry of the PN junction and to the diffusion depth of the avalanche diode. A higher background resistivity of a doped epitaxial region of the die results in a higher clamping voltage relative to the breakdown voltage. 
   As electronic devices, especially battery-operated portable electronic devices such as cellular telephones become smaller, there is a need for a smaller TVS device. It is desirable that a TVS device has as low a clamping voltage as possible. When the TVS device reaches its clamping voltage, the TVS device prevents the electronic device under protection from exposure to any higher voltage than the clamping voltage. The clamping voltage of a prior art TVS device would disadvantageously rise if the avalanche diode were simply made smaller because a smaller PN junction area has a higher resistance. 
   OBJECTS OF THE INVENTION 
   It is therefore an object of the present invention to provide a semiconductor die that overcomes the disadvantages of the prior art, and in particular, to provide a semiconductor die that has a low clamping voltage. 
   It is another object of the present invention to provide a TVS device that overcomes the disadvantages of the prior art, and in particular, to provide a TVS device that has a low clamping voltage. 
   It is still another object of the present invention to provide a flip chip that overcomes the disadvantages of the prior art, and in particular, to provide a flip chip that has a low clamping voltage. 
   These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds. 
   SUMMARY OF THE INVENTION 
   Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to a semiconductor die that includes a semiconductor substrate diffused with a first material to give the substrate a first conductivity type. The substrate has a substrate surface. A buried layer is selectively formed in the substrate surface and is diffused with a second material to give the buried layer the opposite conductivity type as the substrate. An epitaxial layer is formed on the substrate surface and on the buried layer. The epitaxial layer is of the opposite conductivity type as the substrate. The epitaxial layer has an epitaxial surface distal from the substrate surface. A first diffused region is selectively formed on the epitaxial surface. The first diffused region is of the same conductivity type as the epitaxial layer. The first diffused region has a first surface distal from the substrate surface. A second diffused region is selectively formed on the first surface. The second diffused region is of the opposite conductivity type as the first diffused region. The first diffused region and the second diffused region combine to form a first semiconductor junction. A third diffused region is selectively formed on the epitaxial surface remote from the first diffused region. The third diffused region is of the opposite conductivity type as the epitaxial layer. The epitaxial layer and the third diffused region combine to form a second semiconductor junction. 
   Another aspect of the present invention relates to a transient voltage suppression device, which includes a semiconductor die that includes a semiconductor substrate diffused with a first material to give the substrate a first conductivity type. The substrate has a substrate surface. A buried layer is selectively formed in the substrate surface and is diffused with a second material to give the buried layer the opposite conductivity type as the substrate. An epitaxial layer is formed on the substrate surface and on the buried layer. The epitaxial layer is of the opposite conductivity type as the substrate. The epitaxial layer has an epitaxial surface distal from the substrate surface. A first diffused region is selectively formed on the epitaxial surface. The first diffused region is of the same conductivity type as the epitaxial layer. The first diffused region has a first surface distal from the substrate surface. A second diffused region is selectively formed on the first surface. The second diffused region is of the opposite conductivity type as the first diffused region. The first diffused region and the second diffused region combine to form a first semiconductor junction. A third diffused region is selectively formed on the epitaxial surface remote from the first diffused region. The third diffused region is of the opposite conductivity type as the epitaxial layer. The epitaxial layer and the third diffused region combine to form a second semiconductor junction. 
   A further aspect of the present invention relates to a flip chip that includes a transient voltage suppression device, which includes a semiconductor die that includes a semiconductor substrate diffused with a first material to give the substrate a first conductivity type. The substrate has a substrate surface. A buried layer is selectively formed in the substrate surface and is diffused with a second material to give the buried layer the opposite conductivity type as the substrate. An epitaxial layer is formed on the substrate surface and on the buried layer. The epitaxial layer is of the opposite conductivity type as the substrate. The epitaxial layer has an epitaxial surface distal from the substrate surface. A first diffused region is selectively formed on the epitaxial surface. The first diffused region is of the same conductivity type as the epitaxial layer. The first diffused region has a first surface distal from the substrate surface. A second diffused region is selectively formed on the first surface. The second diffused region is of the opposite conductivity type as the first diffused region. The first diffused region and the second diffused region combine to form a first semiconductor junction. A third diffused region is selectively formed on the epitaxial surface remote from the first diffused region. The third diffused region is of the opposite conductivity type as the epitaxial layer. The epitaxial layer and the third diffused region combine to form a second semiconductor junction. 
   Yet another aspect of the present invention relates to a bi-directional transient voltage suppression device, formed on one monolithic semiconductor die, which includes a first single-directional, transient voltage suppression circuit electrically coupled to a second single-directional, transient voltage suppression circuit in an anti-parallel configuration. The first single-directional, transient voltage suppression circuit includes a P+ substrate that has a substrate surface. A first N+ buried layer is selectively formed in the substrate surface. An N+ epitaxial layer is formed on the substrate surface and on the first N+ buried layer. The N+ epitaxial layer has an epitaxial surface distal from the substrate surface. An N+ first diffused region is selectively formed on the epitaxial surface and has a first surface distal from the substrate surface. A P+ second diffused region is selectively formed on the first surface of the N+ first diffused region. The N+ first diffused region and the P+ second diffused region combine to form a first semiconductor junction. A P+ third diffused region is selectively formed on the epitaxial surface remote from the N+ first diffused region. The N+ epitaxial layer and the P+ third diffused region combine to form a second semiconductor junction. The second single-directional, transient voltage suppression circuit includes the P+ substrate, a second N+buried layer selectively formed in the substrate surface, and the N+ epitaxial layer. An N+ fourth diffused region is selectively formed on the epitaxial surface and has a fourth surface distal from the substrate surface. A P+ fifth diffused region is selectively formed on the fourth surface of the N+ fourth diffused region. The N+ fourth diffused region and the P+ fifth diffused region combine to form a third semiconductor junction. A P+ sixth diffused region is selectively formed on the epitaxial surface remote from the N+ fourth diffused region. The N+ epitaxial layer and the P+ sixth diffused region combine to form a fourth semiconductor junction. 
   Still another aspect of the present invention relates to a method of manufacturing a transient voltage suppression device on a single, monolithic semiconductor die that has a top surface. The die has a first avalanche diode in series with a first rectifier diode, connected cathode-to-cathode, which is electrically coupled in an anti-parallel configuration with a second avalanche diode in series with a second rectifier diode, also connected cathode-to-cathode. The method includes the steps of: (a) applying an aluminum metalization layer to the entire top surface of the die; and (b) removing the aluminum metalization layer except for selected portions thereof to form a first aluminum region at one end of the die and a second aluminum region an opposite end of the die, such that the first aluminum region electrically couples the anode of the first avalanche diode to the anode of the second rectifier diode, and such that the second aluminum region electrically couples the anode of the first rectifier diode to the anode of the second avalanche diode, thereby forming a bi-directional transient voltage suppression device for providing protection against voltage spikes or surges, in two directions. 
   Still another aspect of the present invention relates to a method of manufacturing a flip chip on a single, monolithic semiconductor die that has a top surface. The die has a first avalanche diode in series with a first rectifier diode, connected cathode-to-cathode, which is electrically coupled in an anti-parallel configuration with a second avalanche diode in series with a second rectifier diode, also connected cathode-to-cathode. The method includes the steps of: (a) applying an aluminum metalization layer to the entire top surface of the die; (b) removing the aluminum metalization layer except for selected portions thereof to form a first aluminum region at one end of the die and a second aluminum region an opposite end of the die, such that the first aluminum region electrically couples the anode of the first avalanche diode to the anode of the second rectifier diode, and such that the second aluminum region electrically couples the anode of the first rectifier diode to the anode of the second avalanche diode; (c) applying a passivation layer to the entire top surface of the die; and (d) removing selected portions of the passivation layer such that a first solder bump pad and a second solder bump pad are opened over the first aluminum region, and such that a third solder bump pad and a fourth solder bump pad are opened over the second aluminum region. 
   Other aspects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described with greater specificity and clarity with reference to the following drawings, in which: 
       FIG. 1  is a schematic electrical diagram of a TVS device in accordance with the invention; 
       FIG. 2  is a simplified plan view of a semiconductor die of the TVS device in accordance with the invention; 
       FIG. 3  is a simplified cross-sectional view of the semiconductor die of  FIG. 2  through cut line  3 - 3 ; 
       FIG. 4  is a simplified right side view of the semiconductor die of  FIG. 2 ; 
       FIG. 5  is a simplified cross-sectional view of an alternate embodiment of the semiconductor die of  FIG. 2  through cut line  3 - 3 ; and 
       FIGS. 6-12  are simplified representations of masks used to manufacture the TVS device in accordance with the invention. 
   

   For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques are omitted to avoid unnecessarily obscuring the invention. Furthermore, elements in the drawing figures are not necessarily drawn to scale. 
   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The embodiments discussed below are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality, e.g., one die, two die. The terms first, second, and the like, in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms top, front, side, and the like, in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing relative positions. 
     FIG. 1  is a schematic electrical diagram of a TVS device  101  in accordance with the invention. The TVS device  101  comprises a first avalanche diode  103  in series with a first rectifier diode  104  connected cathode to cathode (“first pair”), electrically coupled in an anti-parallel configuration with a second avalanche diode  105  in series with a second rectifier diode  106  also connected cathode to cathode (“second pair”). The anode of the first avalanche diode  103  and the anode of the second rectifier diode  106  are connected to a same first node  110 . The anode of the first rectifier diode  104  and the anode of the second avalanche diode  105  are connected to a same second node  112 . In a typical use of the TVS device  101  in a common mode configuration, the first node  110  is connected to a line, and the second node  112  is connected to a ground. However, the TVS device  101  is symmetrical, and the first node  110  and the second node  112  are interchangeable. 
   The TVS device  101  in accordance with the invention is a bi-directional device in that the TVS device provides protection against voltage transients, spikes and surges in both directions. During normal operation, voltage swings are lower than the breakdown voltage of the avalanche diodes  103  and  105 ; therefore, current does not flow through the TVS device  101 . If a negative transient signal occurs that is greater than the breakdown voltage, the first avalanche diode  103  breaks down, thereby routing a surge current, through both the first avalanche diode  103  operating in an avalanche mode and the first rectifier diode  104  operating in a forward conducting mode, to ground. Concurrently, the second rectifier diode  106  operates in a rectifying mode. If a positive transient signal occurs that is greater than the breakdown voltage, the second avalanche diode  105  breaks down, thereby routing a surge current, through both the second avalanche diode  105  operating in the avalanche mode and the second rectifier diode  106  operating in the forward conducting mode, to ground. Concurrently, the first rectifier diode  104  operates in the rectifying mode. As a result, the voltage is limited to the clamping voltage of the TVS device  101 . 
   When no transient signal is present, the TVS device  101  produces a load with a high impedance at its intended operating frequency, as a result of the low capacitance of the rectifier diodes  104  and  106 . The capacitance of the TVS device  101  in accordance with the invention is less than approximately 10 ρF. The intended operating frequency of the TVS device  101  is approximately 500 MHz. The clamping voltage of the TVS device  101  is preselected to be in the range of approximately 8-30 volts. 
     FIG. 2  is a simplified plan view of the TVS device  101  in accordance with the invention. The TVS device  101  comprises one monolithic semiconductor die, or die,  201 . The one die  201  comprises the first avalanche diode  103  in series with the first rectifier diode  104  connected cathode to cathode, electrically coupled in anti-parallel configuration with the second avalanche diode  105  in series with the second rectifier diode  106  also connected cathode to cathode. Preferably, the TVS device  101  is packaged as a flip chip  202 . The flip chip  202  has four solder bump pads  211 - 214 . Solder bump pads  211  and  214  are electrically coupled to the first node  110 , and solder bump pads  212  and  213  are electrically coupled to the second node  112 . A solder bump (not shown) is placed at each solder bump pad  211 - 214 . Only one of the solder bump pads  211  and  214  at the first node  110 , and only one of the solder bump pads  212  and  213  at the second node  112  are required for electrical operation because the TVS device  101  is a two terminal device. However, a solder bump is placed at each of the four solder bump pads  211 - 214  for mechanical stability. 
     FIG. 3  is a simplified cross-sectional view of the die  201  through cut line  3 - 3 . The die  201  comprises a P+ semiconductor substrate, or substrate,  301 . The P+ substrate  301  has a substrate surface  302 . An N+ buried layer  303  is disposed on a portion of the substrate surface  302 . An N-type epitaxial (“N-EPI”) layer  305  is grown on the substrate surface  302 , including over the portion of the substrate surface having the N+ buried layer  303 . The N-EPI layer  305  has an epitaxial surface  306  distal from the substrate surface. There is a P+ isolation diffusion region  307  around the perimeter region of the die  201 . This perimeter region prevents an increase in leakage current that might occur as a result of damage to the edges of the die  201  that usually occurs when the die is separated from its wafer. The leakage current might cause the die to test “bad”, and might cause some undesirable interaction between the first pair and the second pair. A portion  408  (see  FIG. 4 ) of the P+ isolation diffusion region  307  extends between the first pair and the second pair. The portion  408  isolates the first pair from the second pair. The portion  408  of the P+ isolation diffusion region  307  electrically isolates the two anti-parallel circuits on the die  201 , and allows the two anti-parallel circuits to be on one die without interfering with one another. 
   A heavily doped, N+ first diffused region  311  is disposed on the epitaxial surface  306  of the N-EPI layer  305 . The N+ first diffused region  311  is the cathode of the first avalanche diode  103 . The N+ first diffused region  311  has a first surface  312  distal from the substrate surface  302 . A P+ second diffused region  313  is disposed on the first surface  312  of the N+ first diffused region  311 . The P+ second diffused region  313  is the anode of the first avalanche diode  103 . The P+ second diffused region  313  has a second surface  314  distal from the substrate surface  302 . The first avalanche diode  103  is formed by a PN, or semiconductor, junction between the N+ first diffused region  311  and the P+ second diffused region  313  of die  201 . 
   A P+ third diffused region  315  is disposed on the epitaxial surface  306  of the N-EPI layer  305 . The P+ third diffused region  315  is the anode of the first rectifier diode  104 . The P+ third diffused region  315  has a third surface  316  distal from the substrate surface  302 . The first rectifier diode  104  is formed by a semiconductor junction between the N-EPI layer  305  and the P+ third diffused region  315  of the die  201 . The second avalanche diode  105  and the second rectifier diode  106  of the second pair are similar to the first avalanche diode  103  and the first rectifier diode  104  of the first pair, respectively, and, therefore, are not described in detail. 
   During a transient over-voltage event, the N+ buried layer  303  reduces the clamping voltage of the TVS device  101 . The clamping voltage is controlled by the geometry and doping of the N+ buried layer  303 . Referring back to  FIG. 2 , the N+ buried layer  303  has a length  215  and a width  216 . The N+ buried layer  303  advantageously reduces the clamping resistance of the TVS device  101  when the TVS device is in an avalanche mode. The clamping voltage of the TVS device  101  in accordance with the invention is improved, i.e., lowered, by about 20% over prior art TVS devices. For example, a typical prior art TVS device has a clamping voltage of 9.5 volts, whereas, the TVS device  101  in accordance with the invention has a clamping voltage of only 7.6 volts. 
   The N-EPI layer  305  on the silicon P+ substrate  301  is a high resistivity (approximately 70 ohm-cm) N-type material. The resistivity of the N+ buried layer  303  is an order of magnitude lower than the resistivity of the N-EPI layer  305 . The doping levels and the thickness of the layers obtained through diffusion and the lateral dimensions for each element are well known to a person skilled in the art of semiconductors. The high resistivity of the N-EPI layer  305  is needed to produce a high reverse voltage for the rectifier diodes  104  and  106 . The high reverse voltage for the rectifier diodes  104  and  106  is needed so that the first rectifier diode  104  of the first pair is not in avalanche mode when the second pair is conducting during a transient event, and vice versa. Referring again to  FIG. 3 , the N-EPI layer  305  forms a high resistivity conduction path between the first avalanche diode  103  and the first rectifier diode  104  for conducting the surge current during a voltage surge. A distance  317  between the first avalanche diode  103  and the first rectifier diode  104  is approximately 150 μm. In a prior art TVS device lacking the N+ buried layer  303 , a surge current would travel the distance  317  between the first avalanche diode  103  and the first rectifier diode  104  through the high resistivity N-EPI layer  305 . In a prior art TVS device, the relatively long distance  317  would cause a large voltage drop between the first avalanche diode  103  and the first rectifier diode  104  when the first avalanche diode is in avalanche mode and conduction occurs between the first avalanche diode and the first rectifier diode. A distance  318  between the first avalanche diode  103  and the N+ buried layer  303  is approximately 10 μm. A distance  319  between the first rectifier diode  104  and the N+ buried layer  303  is approximately 15 μm. 
   With the N+ buried layer  303  in accordance with the invention, the surge current goes through the N+ buried layer  303  rather than going solely through the N-EPI layer  305  because of the much lower resistivity of the N+ buried layer relative to the N-EPI layer. The N+ buried layer  303  reduces the effective length of the high resistivity conduction path (through the N-EPI layer  305 ) from about 150 μm to about 25 μm, thereby reducing the resistance seen by the surge current, and consequently reducing the clamping voltage relative to the breakdown voltage. The N+ buried layer  303  significantly reduces the voltage drop between the first avalanche diode diffusion area and the first rectifier diode diffusion area. Advantageously, the N+ buried layer  303  shunts most of the surge current away from the portion of the high resistivity N-EPI layer  305  between the first avalanche diode  103  and the first rectifier diode  104 . The N+ buried layer  303  acts as a low resistance region for conduction between the first avalanche diode  103  and the first rectifier diode  104 . Without the N+ buried layer  303 , the clamping voltage is higher and the possibility of damaging electronics beyond the TVS device  101  is greater. Advantageously, the N+ buried layer  303  does not increase the capacitance of the TVS device  101 . The N+ buried layer  303  has no effect on the breakdown voltage of the first avalanche diode  103 , and, at low currents, has no effect on the forward bias voltage of the first rectifier diode  104 . The operation of the second pair is similar to the operation of the first pair, and, therefore, is not described in detail. 
   Although the N+ buried layer  303  advantageously reduces the clamping voltage relative to the breakdown voltage, the N+ buried layer has no effect on the breakdown voltage itself. For example, the breakdown voltage of the avalanche diodes  103  and  105  in accordance with the invention is approximately seven (7) volts; whereas, the clamping voltage of the avalanche diodes is advantageously only slightly higher at approximately eight (8) volts. 
     FIG. 4  is a simplified right side view of the die  201 . 
     FIG. 5  is a simplified cross-sectional view of an alternate embodiment  500  of the die  201  through cut line  3 - 3 , showing a larger N+ first diffused region  502  of the first avalanche diode  103 . The larger N+ first diffused region  502  extends to the N+ buried layer  303 . Although the alternative embodiment  500  takes longer to manufacture because a much longer N+ diffusion time is required, the advantage of the alternate embodiment is a further reduction of the clamping voltage relative to the breakdown voltage. 
     FIGS. 6-12  are simplified representations of masks  600 - 1200  used to manufacture the TVS device  101  in accordance with the invention. A method of manufacturing a small, low capacitance flip chip  202  that has bi-directional transient voltage protection and a low clamping voltage, comprises the following steps: 
   (1) Start with the P+ substrate  301 . It should be noted that the P+ substrate  301  is semiconductor material with a very high doping level for reduced resistivity, and is different from the P-type material of the PN junction. 
   (2) Grow a thermally deposited diffusion oxide, preferably SiO 2 , on the P+ substrate  301  and pattern the oxide in the shape of mask A  600 , to open two windows  601  and  602  for N+ diffusion. The larger the area of the two windows  601  and  602  in mask A  600 , the lower is the resistance of the two avalanche diodes  103  and  105 . 
   (3) Perform N+ diffusion to a depth  320  of approximately five (5) μm at the portions of the P+ substrate exposed by the two windows  601  and  602 . Upon completion of this N+ diffusion, the N+ buried layer  303  in accordance with the invention is formed. The greater the depth of the N+ buried layer  303 , the lower is the resistance. The higher the doping level of the N+ buried layer  303 , the lower is the resistance. The doping level is controlled by temperature, diffusion time and concentration of dopant on the surface. 
   (4) Remove the thermally grown diffusion oxide that was grown in step two. 
   (5) Grow a high resistivity N-EPI layer  305  of approximately 25 μm thickness on the same side of the P+ substrate  301  that was subjected to the N+ diffusion of step two. 
   (6) Grow a diffusion oxide  325  on the N-EPI layer  305  and pattern the oxide in the shape of mask B  700  for P+ diffusion. 
   (7) Perform P+ diffusion on the portions of the N-EPI layer  305  exposed by mask B, such that the diffusion penetrates to the P+ substrate  301 . Upon completion of the diffusion, these portions become the P+ isolation diffusion region  307  and  408 . 
   (8) Apply mask C over the existing diffusion oxide. Mask C  800  has two windows  801  and  802 . The larger the area of these windows  801  and  802 , the greater is the current-carrying capability of the resulting avalanche diodes  103  and  105 . 
   (9) Diffuse the N+ regions in the N-EPI layer  305  to form the N+ first diffused region  311  of the TVS device  101 . The N+ first diffused region  311  is used to fix the breakdown voltage of the TVS device  101 . The depth of the N+ first diffused region  311  is selected to produce a preselected breakdown voltage. A greater depth results in a higher breakdown voltage, which, in turn, results in a higher clamping voltage. At the same time, re-grow the thermally deposited diffusion oxide  325  and apply mask D  900  over the diffusion oxide. Mask D  900  has four windows  901 - 904 . 
   (10) Diffuse the P+ second diffused region  313  and P+ third diffused region  315  in both the N+ first diffused region  311  and in a region in the N-EPI layer  305  over the N+ buried layer  303  and adjacent to, but not in contact with, the N+ first diffused region. The P+ diffusion of this step is selected such that the breakdown voltage will be controlled to a given specification in the N+ first diffused region  311 . The P+ third diffused region  315  (anode) on the N-EPI layer and the N-EPI layer  305  (cathode) form the first rectifier diode junction. The high resistivity of the N-EPI layer  305  and the small size of the junction of the first rectifier diode  104  are preselected to provide a specific low value of junction capacitance. The P+ second diffused region  313  (anode) on the N+ first diffused region  311 , and the diffused N+ first region (cathode) form a first avalanche diode junction. The high doping level (and low resistivity) of the N+ first diffused region  311 , which is required for the desired avalanche breakdown voltage, results in a high internal capacitance of the first avalanche diode  103 . At the same time, re-grow the thermally deposited diffusion oxide  325  and apply mask E  1000  over the oxide. Mask E  1000  has four windows  1001 - 1004 . 
   (11) Apply an aluminum metalization layer to the entire top surface of the die  201  distal from the substrate surface  302  to provide a first external electrical contact to the second surface  314  of the P+ second diffused region  313 , and a second external electrical contact to the third surface  316  of the P+ third diffused region  315 , where exposed by windows  1001 - 1004  of mask E  1000 . 
   (12) Using mask F  1100 , remove the aluminum metalization layer except for portions  1101  and  1102  to form first and second aluminum regions  219  and  220  (see  FIG. 2 ) at each end of the die  201 , which electrically couple the anode of the first avalanche diode  103  to the anode of the second rectifier diode  106  and the anode of the first rectifier diode  104  to the anode of the second avalanche diode  105 , respectively. The aluminum region  219  also electrically couples the external electrical contact at the second surface  314  to solder bump pads  211  and  214 . The aluminum region  220  also electrically couples the external electrical contact at the third surface  316  to solder bump pads  212  and  213 . 
   (13) Apply a low temperature, chemically vapor deposited (“CVD”) SiO 2  layer  333  over the entire surface of the die  201  as a passivation layer. Alternatively, a nitride or another oxide is used as the passivation layer. 
   (14) Apply mask G  1200 , and pattern the CVD SiO 2  layer  333  such that two windows  1201 - 1202  and  1203 - 1204  are opened in the CVD SiO 2  layer to the aluminum metalization layer at each end of the die  201 . 
   (15) Apply underbump metallurgy in the open windows comprising nickel with a flash of gold as a passivant on the nickel surface. 
   (16) Screen print a solder paste over the underbump metallurgy and reflow the solder to construct the solder bumps. 
   The description of the method of manufacturing refers primarily to the first avalanche diode  103  and the first rectifier diode  104 , i.e., the first pair, for succinctness; however, the description also applies to the second pair. 
   The TVS device  101  and the flip chip  202  have the small dimensions of 0.02 inch width by 0.04 inch length by 0.02 inch height, and have a low clamping voltage of 8-30 volts and a low capacitance of 10 ρF or less. The clamping voltage and the capacitance of the TVS device  101  and the flip chip  202  are improvements over prior art TVS devices and flip chips of similar physical dimensions. Known prior art TVS devices and flip chips of similar physical dimensions have a clamping voltage of 9-36 volts and a capacitance of 30 ρF. 
   While the present invention has been described with respect to preferred embodiments thereof, such description is for illustrative purposes only, and is not to be construed as limiting the scope of the invention. Various modifications and changes may be made to the described embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               LIST OF REFERENCE NUMERALS 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               101 
               Transient Voltage Suppression (“TVS”) Device 
             
             
                 
               103 
               First Avalanche Diode 
             
             
                 
               104 
               First Rectifier Diode 
             
             
                 
               105 
               Second Avalanche Diode 
             
             
                 
               106 
               Second Rectifier Diode 
             
             
                 
               110 
               First Node 
             
             
                 
               112 
               Second Node 
             
             
                 
               201 
               Semiconductor Die, or Die 
             
             
                 
               202 
               Flip Chip 
             
             
                 
               211-214 
               Solder Bump Pads 
             
             
                 
               215 
               Length 
             
             
                 
               216 
               Width 
             
             
                 
               219-220 
               Aluminum Regions 
             
             
                 
               301 
               P+ Semiconductor Substrate, or Substrate 
             
             
                 
               302 
               Substrate Surface 
             
             
                 
               303 
               N+ Buried Layer 
             
             
                 
               305 
               N-Type Epitaxial (“N-EPI”) Layer 
             
             
                 
               306 
               Epitaxial Surface 
             
             
                 
               307 
               P+ Isolation Diffusion Region 
             
             
                 
               311 
               N+ First Diffused Region 
             
             
                 
               312 
               First Surface 
             
             
                 
               313 
               P+ Second Diffused Region 
             
             
                 
               314 
               Second Surface 
             
             
                 
               315 
               P+ Third Diffused Region 
             
             
                 
               316 
               Third Surface 
             
             
                 
               317 
               Distance 
             
             
                 
               318 
               Distance 
             
             
                 
               319 
               Distance 
             
             
                 
               320 
               Depth 
             
             
                 
               325 
               Thermally Deposited Diffusion Oxide 
             
             
                 
               333 
               Chemically Vapor Deposited (“CVD”) SiO 2  Layer 
             
             
                 
               408 
               Portion of the P+ Isolation Diffusion Region 
             
             
                 
               500 
               Alternate Embodiment of the TVS Device 
             
             
                 
               502 
               Larger N+ First Diffused Region 
             
             
                 
               600 
               Mask A 
             
             
                 
               601-602 
               Windows 
             
             
                 
               700 
               Mask B 
             
             
                 
               800 
               Mask C 
             
             
                 
               801-802 
               Windows 
             
             
                 
               900 
               Mask D 
             
             
                 
               901-904 
               Windows 
             
             
                 
               1000 
               Mask E 
             
             
                 
               1001-1004 
               Windows 
             
             
                 
               1100 
               Mask F 
             
             
                 
               1101-1102 
               Portions 
             
             
                 
               1200 
               Mask G 
             
             
                 
               1201-1204 
               Windows