Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D8/00—Diodes
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D8/00—Diodes
  • H10D8/01—Manufacture or treatment
  • H10D8/041—Manufacture or treatment of multilayer diodes
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
  • H10D8/00—Diodes
  • H10D8/825—Diodes having bulk potential barriers, e.g. Camel diodes, planar doped barrier diodes or graded bandgap diodes

Definitions

• the present invention relates to semiconductor devices. More particularly, the present invention relates to low-voltage punch-through bidirectional transient-voltage suppression devices having symmetric current- voltage characteristics.
• One traditional device for overvoltage protection is the reversed biased p+n+ Zener diode. These devices perform well at higher voltages, but run into problems, specifically large leakage currents and high capacitance, at low breakdown voltages. For example, as breakdown voltages are reduced from 12 volts to 6.8 volts, leakage currents for these devices dramatically increase from about 1 ⁇ A to about 1 mA.
• n+p-p+n+ devices such as those described in U.S. Patent No. 5,880, 11 have current-voltage characteristics that are not symmetric.
• Semtech proposes a circuit of two of their transient-voltage suppressors in anti-parallel. Obviously, this arrangement adds expense in that it requires more than one device to achieve its intended function.
• a bi-directional transient voltage suppression device with symmetric current-voltage characteristics comprises: (a) a lower semiconductor layer of first conductivity type; (b) an upper semiconductor layer of first conductivity type; and (c) a middle semiconductor layer adjacent to and disposed between the lower and upper layers.
• the middle layer has a second conductivity type opposite the first conductivity type, such that upper and lower p-n junctions are formed.
• the middle layer has a net doping concentration that is highest at a midpoint between the junctions.
• the doping profile along a line normal to the lower, middle and upper layers is such that, within the lower, middle and upper layers, the doping profile on one side of a centerplane of the middle layer mirrors the doping profile on an opposite side of the centerplane.
• an integral of the net doping concentration of the middle layer taken over the distance between the junctions is such that breakdown, when it occurs, is punch through breakdown, rather than avalanche breakdown.
• the first conductivity type is p-type conductivity and the second conductivity type is n-type conductivity.
• Phosphorous is preferably used as an n-type dopant
• boron is used as a p-type dopant.
• the bidirectional transient voltage suppression device comprises a p++ semiconductor substrate, a first epitaxial p+ layer deposited on the p-H- substrate, an epitaxial n layer deposited on the first epitaxial p+ layer, and a second epitaxial p+ layer deposited on the epitaxial n layer.
• a p++ ohmic contact is typically formed at an upper surface of the second epitaxial p+ layer.
• the peak net doping concentration of each of the first and second epitaxial p+ layers ranges from 5 to 20 times the peak net doping concentration of the epitaxial n layer.
• the peak net doping concentration of the epitaxial n layer ranges from 2 x 10 I ⁇ cm “3 to 2 x 10 17 cm “3
• the peak net doping concentration of the first and second epitaxial p+ layers range from 2 x 10 17 cm “3 to 2 x 10 18 cm “3
• the integral of the middle layer net doping concentration taken over the distance between the junctions preferably ranges from 2xl0 12 to lxl0 13 cm "2 .
• the preferred distance between the upper and lower junctions ranges from 0.2 to 1.5 microns.
• the first and second epitaxial p+ layers are preferably sufficiently thick to more evenly distribute electrical current throughout the device. It is also preferred that the minority lifetime, punch through breakdown voltage, and theoretical avalanche breakdown voltage be selected so as to produce a Vceo having negative dynamic resistance that compensates for at least a portion of the positive dynamic resistance of the device in the on state.
• the first conductivity type is n-type conductivity and the second conductivity type is p- type conductivity.
• the bi-directional transient voltage suppression device preferably comprises an n++ substrate, a first epitaxial n+ layer deposited on the n++ substrate, an epitaxial p layer deposited on the first epitaxial n+ layer, and a second epitaxial n+ layer deposited on the epitaxial p layer.
• a method of making a bi-directional transient voltage suppression device is provided.
• the method comprises the following: (a) providing a semiconductor substrate of a first conductivity type; (b) depositing a lower epitaxial layer of the first conductivity type on the substrate; (c) depositing a middle epitaxial layer having a second conductivity type opposite the first conductivity type on the lower epitaxial layer, such that the lower layer and the middle layer form a lower p-n junction; (d) depositing an upper epitaxial layer of the first conductivity type on the middle epitaxial layer, such that the middle layer and the upper layer form an upper p-n junction; and (e) heating the substrate, the lower epitaxial layer, the middle epitaxial layer and the upper epitaxial layer.
• the middle layer is provided with a net carrier concentration that is highest at a midpoint between the junctions
• a doping profile along a line normal to the lower, middle and upper epitaxial layers is established in which, within the lower, middle and upper layers, the doping profile on one side of a centerplane of the middle layer mirrors the doping profile one an opposite side of the centerplane
• an integral of the middle layer net doping concentration taken over the distance between the junctions is such that breakdown, when it occurs, is punch through breakdown, rather than avalanche breakdown.
• the n epitaxial layer is grown to a thickness ranging from 1 to 4 microns, with the distance between junctions ranging from 0.2 to 1.5 microns after heating.
• the doping concentration of the upper p+ epitaxial layer upon deposition is preferably between 1% and 8% less than the doping of the lower p+ epitaxial layer upon deposition.
• One advantage of the present invention is that a low-voltage bidirectional transient- oltage suppressor is provided that has a low leakage current.
• a further advantage of the present invention is that a low- voltage bi- directional transient- voltage suppressor is provided that has a lower capacitance than the Zener transient-voltage suppression device with the same breakdown voltage.
• Yet another advantage of the present invention is that a low- voltage bidirectional transient- voltage suppressor is provided which has symmetric current- voltage characteristics. This is in contrast to, for example, the n+p-p+n+ devices described in U.S. Patent No. 5,880,511.
• Yet another advantage of the present invention is that a low-voltage bidirectional transient-voltage suppressor is provided which has acceptable clamping characteristics at high currents. More specifically, as noted above, U.S. Patent No. 5,880,511 claims that n+pn+ uniform-base punch-through devices suffer from poor clamping characteristics at high currents. A base with a uniform carrier concentration is indeed in danger of becoming intrinsic at temperatures that are lower than most other constructions. High temperature protection is important, for example, during power surges in which the region bordering the junction can rise by several hundred °C within milliseconds.
• a base with a high-doped portion and a low-doped portion will perform better than a uniformly doped base of intermediate doping concentration, because the high-doped portion will become intrinsic at a higher temperature.
• One approach is to put a high-doped portion on one side of the base, as proposed in U.S. Patent No. 5,880,511.
• the devices of the present invention take another approach by placing the high-doped portion in the center of the base. In this way, the device of the present invention does not give up current- voltage symmetry, while being able to provide a base with a peak doping concentration that is higher (and hence an intrinsic temperature that is higher) than that found in a uniform-base device.
• a base with these characteristics is achieved with a single epitaxial layer in preferred embodiments of the invention, other options are available.
• a base layer containing three epitaxial sub-layers, each with a homogeneous concentration is contemplated.
• the center base sublayer of such a device could occupy approximately 10% of the width of the total base and have ten times the concentration of the outer base sub-layers, which would divide the rest of the base width equally.
• Another advantage of the present invention is that low-voltage bidirectional transient-voltage suppressors are provided that offer protection against surface breakdown. In the punch-through devices of the present invention, this means ensuring that the depletion layer does not reach the opposing junction at the surface before reaching it in the bulk.
• Figure 1 is a cross-sectional view (not to scale) of a triple epitaxial structure for a low-voltage bi-directional transient-voltage suppressor device according to an embodiment of the present invention.
• Figure 2 is a cross-sectional view (not to scale) of the triple epitaxial structure according to Figure 1 after formation of a mesa structure.
• Figure 3 is a plot of acceptor (boron) concentration (indicated by diamonds) and net donor concentration (indicated by squares) as a function of thickness for a structure in accordance with the present invention after growth of the epitaxial layers.
• Figure 4 is an expanded view (with the horizontal scale more than 10 times enlarged) of a portion of Figure 3.
• acceptor (boron) concentration is indicated by diamonds
• donor (phosphorous) concentration is indicated by squares
• net donor (donor-acceptor) concentration is indicated by triangles.
• Figure 5 illustrates acceptor (boron) concentration (indicated by diamonds), donor (phosphorous) concentration (indicated by squares), and net donor concentration (indicated by triangles) as a function of thickness for the device of Figure 4 after a certain amount of diffusion of both the boron and the phosphorous atoms.
• Figure 6, like Figure 2 is a cross-sectional view (not to scale) of a triple epitaxial structure, but with a silicon oxide sidewall provided, according to an embodiment of the present invention.
• Figure 7 is an expanded view (not to scale) of region A of Figure 6, illustrating how the junctions curve away from one another.
• Figures 8A-8C are cross-sectional views (not to scale) illustrating a process for making a triple epitaxial device with a silicon oxide sidewall according to an embodiment of the present invention.
• Figures 9A and 9B are current- voltage traces illustrating the bidirectional breakdown characteristics for a bi-directional transient-voltage suppression device of the present invention (curve b) and a commercially available bi-directional transient-voltage suppressor (curve a).
• the current scale is 2mA/step.
• the vertical (current) scale is expanded to 200 ⁇ A/step.
• a p++p+np+ triple-epitaxial punch-through bidirectional transient-voltage suppressor 10 is shown schematically in cross sectional view.
• the device of the present invention is formed on a ⁇ ++ semiconductor substrate 12.
• three regions are epitaxially grown, preferably in one continuous process.
• a first epitaxial p+ region 14 is initially formed on the upper surface of p++ region 12.
• An epitaxial n region 16 is then formed on the upper surface of p+ region 14, and a second epitaxial p+ region 18 is formed on the upper surface of n region 16.
• a p++ ohmic contact (not shown) is typically provided on the upper surface of p+ region 18.
• Such a device contains two junctions: (1) the junction formed at the interface of epitaxially grown p+ region 14 and epitaxially grown n region 16, and (2) the junction formed at the interface of epitaxially grown n region 16 and epitaxially grown p+ region 18.
• the bi-directional transient- voltage suppressor 10 of FIG. 1 is typically provided with a mesa structure for junction termination.
• a structure like that shown in Figs. 1 and 2 is advantageous for several reasons. First, because the epitaxial layers can be grown in one continuous process, from the same raw materials, the p+ resistivity on both sides of the n layer can be matched to a much higher precision than would be the case if the first p+ layer would be replaced by a p+ substrate with, officially, the same resistivity. As a result, a much more symmetric breakdown voltage can be so established for both junctions with a triple epitaxial scheme.
• An n++n+pn+ triple-epitaxial punch-through bi-directional transient- voltage suppressor is also contemplated in connection with the present invention.
• pnp-type devices are preferred over npn-type devices for the following reasons: (1) An n base has a maximum resistivity as a function of temperature that occurs at a higher temperature than that observed for a p base having the same doping concentration. As a result, hot spot formation will set in at higher temperatures with an n base than with a p base. (2) The p layers outside the n base of the pnp-type device can be doped heavier than the n layers outside the p base of the npn-type device, while still having the same distributing resistance.
• the breakdown voltage associated with the bottom grown p+ region 14 is frequently greater (typically about 2% greater) than that associated with the upper grown p+ region 18, largely due to diffusion that occurs from the p+ region 14 into the n region 16 during growth of the n region 16.
• the doping level of the p+ region 18 can be adjusted to compensate for this effect. For example, this doping level can be diminished by about 2% to achieve a relatively good match between the breakdown voltages associated with the two p layers.
• thermal treatment during further processing should be kept fixed from batch to batch.
• further diffusion at high temperature results in a diminishing n region 16 width and lower punch-through breakdown.
• the amount of diffusion should be kept constant within smaller tolerances than the amount of diffusion associated with standard diodes.
• avalanche breakdown is caused by impact ionization, which leads to carrier multiplication.
• Punch through is caused by the depletion region of one junction of the device of the present invention reaching the opposing forward-biased junction. For a given breakdown voltage, the depletion region generally associated with punch through is wider than that associated with avalanche breakdown.
• punch through is expected to have less capacitance, less tunneling, and hence less leakage current, than that associated with avalanche breakdown.
• the theoretical avalanche breakdown voltage of the p-n junction in this case, the avalanche breakdown voltage where the second p region is replaced with an n++ region) is greater than the voltage at which punch through occurs.
• the n epitaxial region width is preferably about or greater than about 0.4 microns in thickness. (If this is not possible, for instance for very low voltages of about 2 V, the width should as large as possible under the circumstances.)
• the resistivity of this region is preferably about 0.3 to 0.08 ohm-cm.
• the n epitaxial layer 16 is preferably grown to a greater thickness than that discussed above, more preferably 1 to 4 microns, and most preferably about 2 microns. Diffusion during the processing to follow (beginning with epitaxial growth of the second p+ region 18 and continuing with subsequent processing) will then narrow the thickness of the n region of epitaxial layer 16, and will decrease the doping on both sides of both p-n junctions (for example, compare FIGS. 4 and 5 discussed below). If desired, the wafer can be tested after the final phase of thermal processing.
• n region widths, after diffusion are 0.2 to 1.5 microns, more preferably about 0.4 microns.
• the n- region is typically doped to about 2xl0 ! to about 2xl0 17 atoms/cm 3 during epitaxial growth.
• the product of the net doping concentration of the n region multiplied by its thickness, and more preferably the integral of the net doping concentration over the thickness, after diffusion be on the order of 2xl0 12 to lxlO 13 atoms/cm 2 .
• the p+ layers are doped to higher levels than the n region 16.
• boron a p-type dopant
• phosphorous an n-type dopant
• the doping levels of the p+ regions 14, 18 are preferably about 10 times higher than those of n region 16.
• the dopant concentration be selected to provide a resistivity in the p+ regions that ranges from about 0.02 to 0.2 ohm-cm. Typically, this corresponds to a doping level of 2x10 17 to about 2x10 18 atoms/cm 3 during epitaxial growth.
• the thickness of the two p+ regions can be adjusted to provide the desired overall resistance. Typical thicknesses are 10 to 50 microns.
• FIG. 3 is a plot of computer-simulated boron (acceptor) and phosphorus (donor) concentrations, as a function of thickness, for a three epitaxial layer p++p+np+ device according to an earlier test of the invention after epitaxial growth. This earlier test was performed before preferred numbers were established, so the concentrations for the n and p+ layers are lower in this figure than those of the presently preferred structure. These numbers, nonetheless, are sufficient to form working devices.
• the p++ region is on the right-hand side of the figure.
• Peak acceptor concentration m the p-H- region is 2x10 cm "
• peak acceptor concentration in the p+ regions is 2x10 16 cm “3
• peak donor concentration in the n region is 2xl0 15 cm “3 .
• FIG. 4 represents an enlargement of the plot of FIG. 3 in the vicinity of the n region and illustrates phosphorous (donor) concentrations, boron (acceptor) concentration and net donor (donor minus acceptor) concentration.
• FIG 5 shows the same region after diffusion. Note that the base region (i.e., the region having a net donor concentration) is reduced in size from about 2 microns to about 1.6 microns. Moreover, regions adjacent the base region having a net donor concentration before diffusion are shown to have a net acceptor concentration after diffusion that is greater in magnitude than the net donor concentration before diffusion.
• U.S. Patent No. 4,980,3105 the entire disclosure of which is hereby incorporated by reference, describes a process wherein an n layer having a relatively high concentration is diffused into a p wafer having a relatively low concentration.
• the wafer is etched to yield a plurality of mesa semiconductor structures, each having a p-n junction intersecting a sidewall of the mesa structure.
• a layer of oxide is grown on the sidewalls of the mesas, which oxide layer passivates the device.
• the oxidizing step curves the p-n junction toward the p layer in the vicinity of the oxide layer.
• the p-n junction is diffused deeper into the p layer with a diffusion front, which tends to curve the p-n junction back toward the n layer in the vicinity of the oxide layer.
• This diffusion is carried out to such an extent as to compensate for the curvature caused by the oxidizing step and thereby substantially flatten the p-n junction.
• the patent teaches that a plurality of successive oxidation/diffusion steps can be undertaken to further flatten the junction adjacent the mesa sidewall.
• the resultant p-n junction has a greater avalanche breakdown voltage in the vicinity of the oxide layer due to the substantial flatness of the p-n junction and the reduction of both p and n concentration near the surface.
• the p-n junction curves toward the p layer in the vicinity of the oxide layer and the width of the n region in this embodiment of the invention increases in the vicinity of the oxide, bending the junctions away for the n region and toward the adjacent p+ regions.
• FIG. 6 a bi-directional transient-voltage suppression device of the present invention is shown having p++ semiconductor substrate 12, p+ region 14, n region 16 and p+ region 18.
• a mesa structure is shown, the sides of which are covered with a layer of grown silicon oxide 19.
• FIG. 7 is an enlarged view of region "A" shown in FIG. 6.
• p-n junctions 17a and 17b bend away from n region 16 as the silicon oxide layer 19 is approached.
• the base region i.e., the n region
• this portion of the transistor has a higher punch through voltage than the bulk region, protecting the device against surface breakdown.
• punch- through breakdown voltage current begins to flow through the breakdown region. Since breakdown occurs in the bulk, the breakdown region constitutes a large percentage (typically more than 98%) of the junction area. Because the breakdown current flows over a large area, heat is likewise dissipated over a large area.
• each p-n junction has an associated depletion region that widens with increasing reverse bias. Assuming avalanche breakdown does not occur, as voltage is increased, the depletion region under reverse bias reaches further and further into the n region until the p-n junction at the other side of the n region is reached. At this point, a current path is provided between the first and second p+ regions and punch through occurs. Near the silicon oxide interface, the p-n junctions curve away from one another. As a result, the depletion region near the oxide layer interface is still some distance away from the opposing junction (which curves away from the depletion region) at the point where the depletion region in the bulk reaches the opposing junction. In this way, punch through occurs in the bulk, rather than at the surface.
• One result of the increase in donor (phosphorous) doping just under the oxide layer is an increase in the slope of the electric field at this region.
• This has both an advantage and a disadvantage.
• the advantage is that this step will narrow the depletion layer even further, helping to prevent surface breakdown.
• the disadvantage is that higher fields can lead to avalanche breakdown.
• the small increase in the peak field at the surface caused by dopant redistribution typically will not cause difficulties, if the peak field at punch through is made sufficiently lower than the peak field at avalanche breakdown.
• the peak field at which punch through occurs may be desirable for the peak field at which punch through occurs to approach, as closely as possible, the peak field at which avalanche breakdown occurs, for example, so that the Vceo of the transistor, with its negative dynamic resistance, reduces the positive dynamic resistance of the device.
• a compensation diffusion step can be added, after oxidation, to flatten the curvature of the junction somewhat, for example, as is set forth in U.S. Patent No. 4,980,315.
• the increased donor (phosphorous) concentration at the oxide layer will spread out. Nonetheless, since the total excess number of donor atoms near the oxide layer will remain roughly the same, the surface will continue to be protected from punch-through breakdown.
• the bi-directional transient- voltage suppressors of the present invention can be manufactured using standard silicon wafer fabrication techniques. A typical process flow is shown below with reference to FIGS. 8 A to 8C. Those of ordinary skill in the art will readily appreciate that the process flow disclosed herein is in no way meant to be restrictive as there are numerous alternative ways to create the bidirectional transient-voltage suppressors.
• the starting substrate material 12 for the bidirectional transient-voltage suppression device of the present invention is p-type (p++) silicon having a resistivity that is as low as possible, typically from 0.01 to 0.002 ohm-cm.
• n-type (n) epitaxial layer 16 having a doping concentration in the range of from about 2x10 to about 2xl0 17 atoms/cm 3 (with lower concentration being desired for higher breakdown voltages) is then grown to a thickness of between about 1 and about 4 ⁇ m (with greater thicknesses being desired for higher breakdown voltages and longer diffusion times) on p-type epitaxial layer 14, also using conventional epitaxial growth techniques.
• a p-type (p+) epitaxial layer 18 having the same doping concentration and thickness as layer 14 is grown on n-type epitaxial layer 16, again using conventional epitaxial growth techniques.
• These layers 14, 16 and 18 are preferably grown in one continuous process, without the wafers being exposed to air in between.
• a p-type (p++) region 20 is then formed in p-type epitaxial layer 18, either by deposition and diffusion, with a high enough surface concentration to form an ohmic contact, or by using other conventional methods such as aluminum alloying.
• a silicon nitride layer 22 is then deposited on the entire surface using conventional techniques, such as low-pressure chemical vapor deposition.
• a conventional photoresist masking and etching process is used to form a desired pattern in the silicon nitride layer 22.
• Moat trenches 23 are then formed using the patterned silicon nitride layer 22 as a mask using standard chemical etching techniques. The trenches 23 extend for a sufficient depth into the substrate (i.e., well beyond both junctions) to provide isolation and create a mesa structure.
• FIG. 8B shows the structure resulting after completing the silicon nitride masking and trench etching steps.
• a thick, passifying silicon oxide layer 19, preferably about 1/2 micron thick is grown on the structure of FIG. 8B.
• Grown oxide layers are preferable to deposited layers because the dopants are re-distributed during oxide growth, because grown oxide layers are more dense, and because the steam (where wet oxidation is employed) cleans by burning or oxidizing a significant part of the submicroscopic dust on the surface.
• the wafer is preferably subjected to steam at
• FIG. 8C shows a silicon dioxide layer
• Three epitaxial layers were grown in one continuous process step.
• the wafers were not exposed to air and not cooled down during the continuous growth of the three layers.
• the p-H- substrates had resistivities ranging from 0.005 to 0.002 ohm-cm.
• the first p+ epitaxial layer was 10 microns in thickness and had a resistivity of 0.5 ohm-cm.
• the n epitaxial layer was 2.5 microns in thickness and had a resistivity of 2.5 ohm-cm.
• the second p+ epitaxial layer had a thickness of 20 micron and a resistivity of 0.5 ohm-cm.
• a boron deposition step was performed for 1 hour at 1100 °C, with a slow temperature ramp up and ramp down. This deposition is performed on both sides of the wafer in a single step, creating ohmic contacts (p++ regions).
• a silicon nitride layer having a thickness of 200 nm was then deposited using conventional techniques.
• a patterned photoresist layer was then applied to the structure forming a mesa mask (the mesa moat region is the region not covered by the photoresist).
• the mesa moat was then etched using an etching medium of HF, HN0 3 and acetic acid, as is known in the art.
• each wafer was subjected to a diffusion time ranging from 0 to 8 hours at 1100 °C to achieve a variety of desired breakdown voltages.
• the nitride layer was then removed (for contact openings) in a plasma- etching step.
• the device was finished in a standard manner, including glassing, nickel plating, wafer testing, wafer sawing and assembly into individual devices.
• Two wafers subjected to relatively short diffusion times i.e., on the order of 2 hours or less) produced bi-directional, triple epitaxial, transient- voltage suppression devices of high quality in the desired voltage range of 4 to 7V.
• FIG. 9 A and 9B Current- voltage traces illustrating the bi-directional breakdown characteristics of one of these devices, along with a standard P6KE6.8CA (General Semiconductor Corporation bi-directional transient-voltage suppressor) Zener device are shown in Figs. 9 A and 9B.
• the horizontal axes in these figures correspond to voltage and the vertical axes correspond to current.
• the (horizontal) voltage scale is at 2V per step.
• the (vertical) current scale is at 2 mA per step for Fig. 9A; and expanded ten times, to 200 uA per step for Fig. 9B.

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