Vertical high-voltage transistor

Field of the Invention

0001The present invention relates to semiconductor devices fabricated in a silicon substrate. More specifically, the present invention relates to field-effect semiconductor transistor structures capable of withstanding high voltages.

Background of the Invention

0002High-voltage, field-effect transistors (HVFETs) are well known in the semiconductor arts. Most often, HVFETs comprise a device structure that includes an extended drain region that supports the applied high-voltage when the device is in the "off" state. HVFETs of this type are commonly used in power conversion applications such as AC/DC converters for offline power supplies, motor controls, and so on. These devices can be switched at high voltages and achieve a high blocking voltage in the off state while minimizing the resistance to current flow in the "on" state. The blocking or breakdown voltage is generally denoted as Vbd. The acronym Rsp refers to the product of the resistance and surface area, and is generally used to describe the on-state performance of the device. An example of a prior art HVFET having an extended drain region with a top layer of a conductivity type opposite that of the extended drain region is found in <patcit id="pcit0001" dnum="US4811075A"><text>U.S. Patent No. 4,811,075</text></patcit>.

0003In a conventional HVFET the extended drain region is usually lightly doped to support high voltages applied to the drain when the device is off. The length of the extended drain region is also increased to spread the electric field over a larger area so the device can sustain higher voltages. However, when the device is on (i.e., conducting) current flows through the extended drain region. The combined decrease in doping and increase length of the extended drain region therefore have the deleterious effect on the on-state performance of the device, as both cause an increase in on-state resistance. In other words, conventional high-voltage FET designs are characterized by a trade-off between Vbd and Rsp.

0004To provide a quantitative example, a typical prior art vertical HVFET (NMOS-type) may have a Vbd of 600V with a Rsp of about 16 ohm-mm². Increasing the length of the extended drain would affect device performance by increasing Vbd beyond 600V at the expense of a higher Rsp value. Conversely, reducing the length of the extended drain would improve the on-state resistance to a value below 16 ohm-mm², but such a change in the device structure would also cause Vbd to be reduced to less than 600V.

0005A device structure for supporting higher Vbd voltages with a low Rsp value is disclosed in <patcit id="pcit0002" dnum="US4754310A"><text>U.S. Patent Nos. 4,754,310</text></patcit>, <patcit id="pcit0003" dnum="US5438215A"><text>5,438,215</text></patcit>, and also in the article entitled, <nplcit id="ncit0001" npl-type="s"><text>"Theory of Semiconductor Superjunction Devices" by T. Fujihira, Jpn. J. Appl. Phys., Vol. 36, pp. 6254-6262, Oct. 1977</text></nplcit>. In this device structure the extended drain region comprises alternating layers of semiconductor material having opposite conductivity types, e.g., PNPNP.... As high voltage is applied to the layers of one conductivity type, all of the layers are mutually depleted of charge carriers. This permits a high Vbd at much higher conducting layer doping concentrations as compared to single layer devices. The higher doping concentrations, of course, advantageously lower the Rsp of the transistor device. For example, in the article entitled, <nplcit id="ncit0002" npl-type="s"><text>"A new generation of high voltage MOSFETs breaks the limit line of silicon" by G. Deboy et al., IEDM tech. Digest, pp. 683-685, 1998</text></nplcit>, the authors report a vertical NMOS device with a Vbd of 600V and a Rsp of about 4 ohm-mm².

0006Another approach to the problem of achieving high-voltage capability is disclosed in the paper, "<nplcit id="ncit0003" npl-type="s"><text>Realization of High Breakdown Voltage in Thin SOI Devices" by S. Merchant et al., Proc. Intl. Symp. on Power Devices and ICs, pp. 31-30 35, 1991</text></nplcit>. This paper teaches an extended drain region that comprises a thin layer of silicon situated on top of a buried oxide layer disposed on top of a semiconductor substrate. In operation, the underlying silicon substrate depletes charge from the thin silicon layer at high voltages. The authors claim that high values of Vbd are obtained as long as the top silicon layer is sufficiently thin and the buried oxide layer is sufficiently thick. For instance, a lateral NMOS device with Vbd of 600V and Rsp of about 8 ohm- mm2 is obtained using this approach.

0007Other background references of possible interest to those skilled in the art include <patcit id="pcit0004" dnum="US6184555B"><text>U.S. Patent No. 6,184,555</text></patcit>, <patcit id="pcit0005" dnum="US6191447B"><text>6,191,447</text></patcit>, <patcit id="pcit0006" dnum="US6075259A"><text>6,075,259</text></patcit>, <patcit id="pcit0007" dnum="US5998833A"><text>5,998,833</text></patcit>, <patcit id="pcit0008" dnum="US5637898A"><text>5,637,898</text></patcit>, International Application No. PCT/IB98/02060 (<patcit id="pcit0009" dnum="WO9934449A"><text>International Publication No. WO 99/34449</text></patcit>), and the article, "<nplcit id="ncit0004" npl-type="s"><text>High Performance 600V Smart Power Technology Based on Thin Layer Silicon-on-Insulator" by T. Letavic et al., Proc. ISPSD, pp. 49-52, 1997</text></nplcit>.

0008<patcit id="pcit0010" dnum="JP2001085688A"><text>Japanese Patent Publication No. 2001-085688A</text></patcit> describes that a gate electrode is formed, via a gate insulating film, on an n-type semiconductor substrate as a drain region, and p-type impurity diffusion layers as base regions and n+ type impurity diffusion layers as source regions are formed in the semiconductor substrate in such a way that they are situated partly in the lower part of the gate electrode. Then, trenches are formed on both sides of the gate electrode in such a way that pass the p-type impurity diffusion layers and the n+ type impurity diffusion layers. Conductive members are buried, via insulating films, into the trenches. Source electrodes are formed on the conductive members.

0009<patcit id="pcit0011" dnum="JP2000349288A"><text>Japanese Patent Publication No. 2000-349288</text></patcit> describes a vertical MOSFET that has p-type well regions formed in the surface layer of an n-type drift area on an n+-type drain layer, n-type source regions in the well regions, and a gate electrode provided on the surfaces of the well regions sandwiched between the source regions and drift region via a gate insulating film. The MOSFET also has source electrodes, which are commonly brought into contact with the source regions and well regions. In the MOSFET, the impurity concentration distribution in the drift region is adjusted, so that the concentration becomes gradually linearly higher with the depth. In addition, trenches are dug in the MOSFET from the surfaces of the well regions, and polycrystalline silicon which is shortcircuited to the source electrodes is buried in the trenches through thick insulating films.

0010The document entitled "<nplcit id="ncit0005" npl-type="s"><text>Oxide-bypassed VDMOS(OBVDMOS): an alternative to superjunction high voltage MOS power devices" by Liang, Y.C., et al. (Electron Device Letters, IEEE , vol.22, no.8, pp.407-409, Aug. 2001</text></nplcit>) proposes that the superjunction concept be used to overcome the ideal silicon MOSFET limit. A novel device structure termed oxide-bypassed VDMOS (OBVDMOS) that requires the well-established oxide thickness control instead of the difficult doping control in translating the limit to a higher blocking voltage is described.

0011<patcit id="pcit0012" dnum="WO9735346A"><text>International Patent Publication WO 97/35346</text></patcit> describes a field effect-controllable semiconductor component that has a drain zone of the first conductivity type and also at least one gate electrode which is composed of polycrystalline silicon and is insulated from the drain zone. A source region of the second conductivity type is introduced in the drain zone. In addition, there is formed in the drain zone a trench structure, which reach from the surface of the epitaxial layer down into the substrate layer. An additional field plate made of polysilicon and embedded in an oxide layer is introduced in the trench structure. The thickness of the oxide surrounding the field plate increases down in a direction towards the drain.

0012Although the device structures described above achieve high Vbd with relatively low on-state resistance as compared to earlier designs, there is still an unsatisfied need for a high-voltage transistor structure that can support still high voltages while achieving a much lower on-state resistance.

0013A vertical high-voltage transistor in accordance with the invention is set out in claim 1. Specific embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

0014The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, wherein: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> is a cross-sectional side view of a vertical high-voltage, field-effect transistor (HVFET) device structure in accordance with one embodiment of the present invention.</li><li><figref idref="f0002">Figure 2</figref> is a cross-sectional side view of a lateral HVFET not part of the present invention.</li><li><figref idref="f0003">Figure 3A</figref> is a top view of lateral HVFET not part of of the present invention.</li><li><figref idref="f0003">Figure 3B</figref> is a cross-sectional side view of the lateral HVFET shown in <figref idref="f0003">Figure 3A</figref>, taken along cut lines A-A'.</li><li><figref idref="f0004">Figure 4</figref> is a cross-sectional side view of a vertical HVFET device structure not part of the present invention.</li><li><figref idref="f0005 f0006 f0007 f0008">Figures 5A-5K</figref> are cross-sectional side views of a vertical HVFET device structure taken at various stages in a fabrication process not part of the present invention.</li><li><figref idref="f0009">Figure 6</figref> is a cross-sectional side view of still another vertical HVFET device structure not part of the present invention.</li></ul>

DETAILED DESCRIPTION

0015Hight-voltage field-effect transistors having an extended drain or drift region and a method for making a high-voltage field-effect transistor are described. The HVFETs have a low specific on-state resistance and supports high voltage in the off-state. In the following description, numerous specific details are set forth, such as material types, doping levels, structural features, processing steps, etc., in order to provide a thorough understanding of the present invention. Practitioners having ordinary skill in the semiconductor arts will understand that the invention may be practiced without many of these details. In other instances, well-known elements, techniques, and processing steps have not been described in detail to avoid obscuring the invention.

0016<figref idref="f0001">Figure 1</figref> is a cross-sectional side view of a vertical n-channel (i.e., NMOS) HVFET 20 in accordance with one embodiment of the present invention. It should be understood that the elements in the figures are representational, and are not drawn to scale in the interest of clarity. It is also appreciated that a p-channel transistor may be realized by utilizing the opposite conductivity types for all of the illustrated diffusion/doped regions. Furthermore, although the figure appears to show two separate devices, those of skill will understand that such transistor structures are commonly fabricated in an annular, interdigitated, or otherwise replicated manner.

0017The device structure of <figref idref="f0001">Figure 1</figref> includes an insulated-gate, field-effect transistor (IGFET) having a gate 30 (comprised, for example, of polysilicon), and a gate-insulating layer 29 that insulates gate 30 from the underlying semiconductor regions. Gate-insulating layer 29 may comprise ordinary silicon dioxide or another appropriate dielectric insulating material. The extended drain region of vertical HVFET 20 comprises two or more parallel N-type drift regions 22 situated between p-type body regions 26 and extending down to the N+ substrate 21. For instance, <figref idref="f0001">Figure 1</figref> shows drift region 22a extending from beneath gate oxide 29a between P-body regions 26a &amp; 26b down to N+ substrate 21. Similarly, drift region 22b extends from gate oxide 29b between P-body regions 26c &amp; 26d down to N+ substrate 21.

0018Source electrode 32 is electrically connected to N+ source regions 27, which are disposed in respective P-body regions 26. For example, N+ source region 27a is disposed in P-body region 26a; N+ region 27b is disposed in P-body region 27b, and so on. It is appreciated that a variety of alternative source electrode connections are also possible. The area of the P-body regions directly beneath gate 30 (between N+ source regions 27 and drift regions 22) comprises the IGFET channel region of the transistor. In this particular embodiment, the gate region is a metal-oxide semiconductor (MOS), and the IGFET is a NMOS transistor. Thus, the channel regions of HVFET 20 are defined at one end by N+ source regions 27 and at the other end by N-type drift regions 22, which extend vertically from gate oxide 29 down to the N+ substrate 21. Insulating layers 33 separate gate 30 from source electrode 32.

0019The n-type extended drain or drift regions 22 are separated laterally by insulating regions or dielectric layers 28. In the embodiment of <figref idref="f0001">Figure 1</figref>, dielectric layers 28 extend vertically from beneath P-body regions 26 down to N+ substrate 21 along the full vertical length of the drift regions 22 thereby defining T-shaped semiconductor regions. By way of example, dielectric layers 28 may comprise silicon dioxide, but other insulating materials, such as silicon nitride, may also be used. Disposed within each of the dielectric layers 28, and fully insulated from the semiconductor substrate 21 and drift regions 22, is a field plate member 24. Field plate members 24 comprise a conducting layer of material such as heavily doped polysilicon, metal, metal alloys, etc. As shown in the embodiment of <figref idref="f0001">Figure 1</figref>, each of the field plate members 24 is electrically connected to source electrode 32. Alternatively, the field plate members may be connected to a separate electrode. Gates 30 are also connected to a separate electrode (not shown). Drain electrode 31 provides electrical connection to the bottom of N+ substrate 21.

0020The extended drain region of vertical NMOS high-voltage transistor 20 of <figref idref="f0001">Figure 1</figref> consists of a plurality of laterally interleaved layers of doped semiconductor material (e.g., n-type), insulating material (e.g., silicon dioxide), and conducting material (e.g., heavily-doped polysilicon). In the on state, a sufficient voltage is applied to the gate such that a channel of electrons is formed along the surface of the P-body regions 26. This provides a path for electron current flow from source electrode 32, N+ source regions 27, through the channel regions formed in P-body regions 26, down through the N-type drift regions 22, through the N+ substrate 21, to drain electrode 31.

0021Practitioners in the semiconductor arts will note that in a conventional vertical HVNMOS transistor, the N-type drift region is normally very thick (i.e., long) and lightly doped; both of which contribute to high on state resistance. In the device structure of <figref idref="f0001">Figure 1</figref>, on the other hand, the doping in the N-type drift regions may be considerably higher, such that the on-state resistance is dramatically lowered. Lowering the on-state resistance is achieved in HVFET 20 by the use of multiple, parallel-arranged extended drain or drift regions.

0022In the off state, a high voltage (e.g., 200V-1200V) is applied across the respective drain and source electrodes 31 and 32. As the voltage increases, the presence of field plate regions 24 on opposite sides of drift regions 22 cause the N-type drift regions to become depleted of free carriers. Ideally, the doping profile in the drift regions 22 is tailored such that the resulting electric field is approximately constant along the path from the drain to the source. For example, the doping concentration may be highest near the N+ substrate 21, lowest the near the P-body regions 26, and linearly graded in between.

0023The thickness of both the N-type drift regions 22 and oxide layers 28 should be designed so as to guard against prevent premature avalanche breakdown. Avalanche breakdown can be avoided by making the drift region relatively narrow, which reduces the ionization path and thereby increases the critical electric field at which avalanche occurs. In the same regard, making oxide layers 28 relatively wide allows the device structure to support a larger voltage for a given electric field.

0024By way of example, a device manufactured in accordance with <figref idref="f0001">Figure 1</figref> having a drift region that is about 50 µm high and about 0.4 - 0.8 µm wide, with an oxide layer width in the approximate range of 3.0 - 4.0 µm is capable of supporting about 800V. In such a device, the doping in the drift region may be linearly graded from about 5 x 10¹⁵ cm^-3 near the P-body regions to about 1 x 10¹⁷ cm^-3 near the N+ substrate. The on-state resistance of such a device is about 1.0 ohm-mm².

0025Practitioners in the art will appreciate that the device performance for HVFET 20 may be improved when manufactured as a smaller total cell pitch (i.e., combined width of field plate, oxide layer and drift regions) because the contribution of each drift region is fairly constant.

0026Referring now to <figref idref="f0002">Figure 2</figref>, there is shown a lateral NMOS high-voltage transistor 40 not part of the present invention. HVFET 40 of <figref idref="f0002">Figure 2</figref> operates according to the same principles discussed in connection with the transistor structure of <figref idref="f0001">Figure 1</figref>, except that current flows laterally, as opposed to vertically, through the drift regions. Note that in the HVFET of <figref idref="f0002">Figure 2</figref>, field plate members 44 are fully insulated from the semiconductor material by oxide layers 49.

0027In this example, field plate member 44a is disposed within oxide layer 49a just below the source and drain electrodes 46 and 45, respectively. Field plate member 44b is disposed within oxide layer 49b below N-type drift region 42a and above N-type drift region 42b. The field plate members may be connected to a field plate electrode at a certain location out of the plane of the figure. The N-type drift region, which comprises the extended drain of the transistor, extends laterally from beneath P-body region 48 across to N+ drain region 43. N+ drain region 43 connects both drift regions 42a &amp; 42b with drain electrode 45.

0028An N+ source region 47, which is electrically connected to source electrode 46, is disposed adjacent P-body region 48. The HVFET 40 utilizes a vertical MOS gate structure 12 that comprises a gate electrode 56 that connects to gate 55. In this example, gate 55 comprises a layer of polysilicon that extends vertically from gate electrode 56. Gate 55 extends below the P-body region, and may extend down to oxide layer 50, as shown. Gate 55 is insulated from N+ source region 47, P-body region 48, and N-type drift region 42 by gate oxide 53. An oxide region 58 separates gate electrode 56 from source electrode 46.

0029Oxide layer 50 insulates N+ substrate 41 from gate 55, N-type drift region 42, and N+ drain region 43. As can be seen, oxide layer 50 extends laterally over N+ substrate 41 beneath each of the regions 42, 43, and 55. Substrate electrode 57 provides electrical connection to the bottom of N+ substrate 41. The substrate may serve as the bottom field plate for drift region 42b.

0030The on-state and off-state operations of HVFET 40 are similar to those described for the embodiment of <figref idref="f0001">Figure 1</figref>. In this case, however, the source and drain electrodes are located on the top surface. This means that electrons flows down through N+ source region 47, across the channel region formed in P-body region 48 adjacent to gate oxide 53, laterally across the N-type drift regions 42, and up through the N+ drain region 43 before reaching the drain electrode.

0031Note that even though <figref idref="f0002">Figure 2</figref> shows a trench gate structure, planar gate structures could also be used. Additionally, a trench drain structure could also be used in an alternative implementation. Furthermore, although the HVFET of <figref idref="f0002">Figure 2</figref> shows the extended drain region comprising two laterally extending, parallel N-type drift regions 42a and 42b, other HVFETs may utilize more than two parallel drift regions. In other words, the HVFET of <figref idref="f0002">Figure 2</figref> is not limited to just two drift regions, but could include any number of layers of drift, oxide, and field plate regions within manufacturing limits.

0032<figref idref="f0003">Figures 3A &amp; 3B</figref> illustrate another lateral HVFET not part of the present invention. <figref idref="f0003">Figure 3A</figref> is a top view of a lateral HVNMOS transistor 60, and <figref idref="f0003">Figure 3B</figref> is a cross-sectional side view of the same device, taken along cut lines A-A', which extends through drift region 62a. (Note that the source electrode 66, drain electrode 65, gate 75, gate oxide 73 and oxide layer 79 are not depicted in <figref idref="f0003">Figure 3A</figref> to avoid confusion. These elements are shown in the cross-sectional side view of <figref idref="f0003">Figure 3B</figref>.)

0033The lateral device structure of <figref idref="f0003">Figure 3</figref> is similar to that shown in <figref idref="f0002">Figure 2</figref>. But rather than orient the drift, oxide, and field plate layered regions on top of one another (vertically), the HVFET of <figref idref="f0003">Figure 3</figref> has these regions oriented side-by-side. Unlike the HVFET of <figref idref="f0002">Figure 2</figref>, each of the N-type drift regions 62, oxide layers 69, and field plate members 64 extend from underlying insulating layer 70 toward the upper substrate surface. Each of the N-type drift regions 62 and field plate members 64 are insulated from N+ substrate 61 by insulating layer 70. In one example, layer 70 comprises silicon dioxide. An additional electrode 77 provides electrical connection to the bottom of N+ substrate 61.

0034The planar gate and drain configurations of HVNMOS transistor 60 are illustrated in the side view of <figref idref="f0003">Figure 3B</figref>. Alternatively, a trench drain structure and/or a trench gate structure may be utilized. In this example, a gate member 75 is disposed above P-body region 68 and is insulated from the semiconductor substrate by a gate oxide 73. Source electrode 66 contacts N+ source region 67, which is disposed in P-body region 68. P-body region 68 is itself shown disposed in N-type drift region 62.

0035N+ drain region 63 is disposed at the opposite end of the N-type drift region 62 and is electrically connected to drain electrode 65.

0036The HVFETs of <figref idref="f0002">Figures 2</figref> and <figref idref="f0003">3</figref> show the field plate members being coupled to the lowest chip potential, e.g., ground. The source may be tied to the field plate members (at the lowest chip potential), or, alternatively, the source region may be left floating. In other words, the HVFETs of <figref idref="f0001 f0002 f0003">Figures 1-3</figref> are not limited to a source follower configuration. Each of the transistor structures of the present invention may be implemented as a four-terminal device, wherein the drain, source, field plate members, and insulated gate members are connected to a separate circuit terminal. In another example, the field plate and insulated gate members may be connected together.

0037With reference now to <figref idref="f0004">Figure 4</figref>, there is shown a cross-sectional side view of another vertical HVNMOS transistor 80 not part of the present invention. The device structure shown in <figref idref="f0004">Figure 4</figref> is similar to that of <figref idref="f0001">Figure 1</figref>, except that the planar gate has been replaced by a trench gate structure. As in the vertical device structure of <figref idref="f0001">Figure 1</figref>, transistor 80 comprises a plurality of parallel-arranged N-type drift regions 82 that extend vertically from P-body regions 86 down to the N+ substrate 81. Each of the drift regions 82 is adjoined on both sides by an oxide layer 88. For example, N-type drift region 82a is bounded on one side by oxide layer 88a and on the opposite side by oxide layer 88b.

0038Disposed within each of the oxide layers 88, and fully insulated from the drift region and substrate semiconductor materials, is a field plate member 84 that may be electrically connected to source electrode 92. The N-type drift regions 82, oxide layers 88, and field plate members 84 collectively comprise a parallel layered structure that extends in a lateral direction, which is perpendicular to the direction of current flow in the on-state. When transistor 80 is in the on-state, current flows vertically from the drain electrode 91 through the parallel N-type drift regions 82, through the MOS channel formed on the sidewalls of the P-body region, to the source electrode 92.

0039The trench gate structure of vertical HVNMOS transistor 80 comprises gate members 90 disposed between field plate members 84 and P-body regions 86. In the example of <figref idref="f0004">Figure 4</figref>, a pair of N+ source regions 87 is disposed in each of P-body regions 86 on opposite sides. Each P-body region 86 is located at one end of a corresponding N-type drift region 82. A thin gate-insulating layer 89 (e.g., oxide) insulates each of gate members 90 (e.g., polysilicon) from the P-body semiconductor material.

0040For example, <figref idref="f0004">Figure 4</figref> shows gate members 90a &amp; 90b disposed along opposite sides of P-body region 86a. N+ source regions 87a &amp; 87b are disposed in P-body region 86a at opposite sides adjacent to the gate members; both regions 87a &amp; 87b are electrically connected to source electrode 92. P-body region 86a adjoins the source electrode at one end and drift region 82a at the other end. When transistor 80 is in the on-state conducting channel regions are formed along the sides of P-body region 86a such that current flows from source electrode 92, through N+ regions 87, across P-body 86, down through N-type drift regions 82 and N+ substrate 81, to drain electrode 91.

0041Practitioners in the art will appreciate that the pair of N+ source regions 87 shown disposed in each P-body region 86 of <figref idref="f0004">Figure 4</figref> may alternatively be replaced by a single N+ region that extends across the full width of region 86 adjacent to source electrode 92. In this case, the P-body region may be connected to the source electrode at various points (dimensionally into the page of the figure.) In one example, source electrode 92 may protrude through N+ source 87 to contact the underlying P-body region 86 (see <figref idref="f0008">Figure 5K</figref>).

0042The trench gate structure of the transistor of <figref idref="f0004">Figure 4</figref> potentially offers an advantage of a simplified manufacturing process, due to the elimination of the T-shaped semiconductor regions shown in <figref idref="f0001">Figure 1</figref>. Also, the vertical HVNMOS structure of transistor 80 may provide lower on-resistance due to the elimination of the JFET structure formed between the P-body regions.

0043<figref idref="f0005 f0006 f0007 f0008">Figures 5A-5K</figref> illustrate the various processing steps that may be employed to fabricate a vertical high-voltage transistor not part of the present invention. The described fabrication method may be used not only to form the device of <figref idref="f0008">Figure 5K</figref>, but also the vertical device structure shown in <figref idref="f0004">Figure 4</figref>.

0044<figref idref="f0005">Figure 5A</figref> shows a vertical high-voltage transistor after the initial processing step of forming an epitaxial layer 101 of n-type semiconductor material on an N+ substrate 100. To support applied voltages in the range of 200V to 1000V the device structure should have an epitaxial layer that is about 15µm to 120µm thick. By way of example, the epitaxial layer of the device shown in <figref idref="f0005 f0006 f0007 f0008">Figure 5</figref> is 40µm thick. The N+ substrate 100 is heavily doped to minimize its resistance to current flowing through to the drain electrode, which is located on the bottom of the substrate in the completed device. Substrate 100 may be thinned, for example, by grinding or etching, and metal may be deposited on its bottom surface to further reduce the on-resistance of the transistor. Most often, these processing steps would be performed after the topside processing has been completed.

0045The thickness and doping of epitaxial layer 101 largely determine the Vbd of the device. The doping may be carried out as the epitaxial layer is being formed. The optimal doping profile is linearly graded from the drain (at the bottom, adjacent to N+ substrate 100) to the source (at the top). Tailoring the doping concentration so that it is heavier near the substrate 100 results in a more uniform electric-field distribution. Linear grading may stop at some point below the top surface of the epitaxial layer 101. By way of example, for the example shown in <figref idref="f0005 f0006 f0007 f0008">Figure 5</figref> the doping concentration is approximately 2 x 10¹⁵ cm^-3 near the P-body region to about 6 x 10¹⁶ cm^-3 near the N+ substrate 100.

0046After the epitaxial layer 101 has been formed, the top surface of layer 101 is appropriately masked and deep trenches are then etched into, or alternatively completely through, the epitaxial layer. <figref idref="f0005">Figure 5B</figref> shows a cross-sectional view of the device structure following etching of epitaxial layer 101 and part of substrate 100. Note that the lateral width of the etched trenches is determined by the combined thickness of the dielectric and conductive refill layers, as described below.

0047Spacing between adjacent trenches is determined by the required thickness of the remaining mesa of epitaxial layer material, which, in turn, is governed by the breakdown voltage requirements of the device. It is this mesa of epitaxial material that eventually forms the N-type drift region of the device structure. It should be understood that this mesa of material might extend a considerable lateral distance in an orthogonal direction (into the page). Although the example of <figref idref="f0005 f0006 f0007 f0008">Figure 5</figref> illustrates a device having an extended drain region that comprises a single N-type drift region, it is appreciated that the vertical high-voltage transistor of <figref idref="f0005 f0006 f0007 f0008">Figure 5</figref> may be constructed with a plurality of parallel-arranged N-type drift regions. Ideally, it is desired to make the lateral thickness (i.e., width) of the N-type drift region(s) as narrow as can be reliably manufactured in order to achieve a very high Vbd with a low Rsp. Of course, a larger lateral thickness is easier to manufacture, but the specific on-resistance of the device suffers with a larger lateral thickness since the current is required to flow across a larger silicon area. In one implementation, the thickness is in the approximate range of 0.4 to 1.2 µm. In this example, the thickness of the mesa is about 1µm.

0048<figref idref="f0005">Figure 5C</figref> shows the device structure of <figref idref="f0005">Figure 5B</figref> after partial filling of the etched trenches with a dielectric material, e.g., silicon dioxide. As shown, in the example of <figref idref="f0005 f0006 f0007 f0008">Figure 5</figref> oxide region 102a covers one side of etched epitaxial region 101, while oxide region 102b covers the other side of epitaxial region 101. Oxide region 102 also covers the top surface of N+ substrate 100 in each of the trenches.

0049The dielectric material may be introduced into the trenches using a variety of well-known methods. For instance, regions 102 may be grown thermally, deposited by chemical vapor deposition, and/or spun on in liquid form. For a given lateral thickness of epitaxial layer material 101, the thickness of the dielectric layer may be set to provide a required breakdown voltage, with thicker dielectric layers providing a higher Vbd. However, thicker dielectric layers increase the cell pitch of the transistor structure and result in higher specific on-resistance. In one implementation, the device structure of <figref idref="f0005 f0006 f0007 f0008">Figure 5</figref> has an oxide layer lateral thickness of 4µm. For devices with other V_bd performance, this thickness may be in the range of about 2µm - 5µm.

0050<figref idref="f0005">Figure 5D</figref> illustrates the device structure of <figref idref="f0005">Figure 5C</figref> following the steps of filling the remaining portions of the trenches with a conductive material and planarizing the surface to form field plate regions 103. For example, the conductive material may comprise a heavily doped polysilicon, a metal (or metal alloys), and/or silicide. Conductor regions 103a and 103b form the field plate members of the device. In most cases, field plate members 103a and 103b should be made as narrow as can be reliably manufactured, since the field plate members occupy silicon area without directly contributing to device conductivity or breakdown voltage characteristics. In one example, the lateral thickness of field plate members 103 is approximately 0.5µm - 1.0µm. The planarization of the surface may be performed by conventional techniques such as chemical-mechanical polishing.

0051At this point in the process, fabrication of the extended drain region of the device is essentially complete. The remaining processing steps may be adapted to produce a stand-alone, high-voltage, depletion-mode MOSFET device structure (as shown in <figref idref="f0006">Figure 5G</figref> and <figref idref="f0009">Figure 6</figref>) or a high-voltage FET that incorporates a low-voltage MOSFET structure (e.g., <figref idref="f0008">Figure 5K</figref>), or other high-voltage devices.

0052<figref idref="f0006">Figure 5E</figref> is a cross-sectional side view of the device structure of <figref idref="f0005">Figure 5D</figref> after the introduction of an N+ source region 105 at the top surface of epitaxial layer 101. Source region 105 may be formed using ordinary deposition, diffusion, and/or implantation processing techniques.

0053After formation of the N+ source region 105 an interlevel dielectric layer 106 if formed over the device. In the example of <figref idref="f0005 f0006 f0007 f0008">Figure 5</figref>, interlevel dielectric layer 106 comprises ordinary silicon dioxide that may be deposited and patterned by conventional methods. Openings are formed in dielectric layer 106 and a conductive layer of material (e.g., metal, silicide, etc.) is deposited and patterned to produce the structure shown in <figref idref="f0006">Figure 5F</figref>. In this cross-sectional view, source electrode 109 provides electrical connection to N+ source region 105, and electrodes 110a and 110b provide electrical connection to field plate members 103a and 103b, respectively.

0054<figref idref="f0006">Figure 5G</figref> shows the device structure of <figref idref="f0006">Figure 5F</figref> following formation of a drain electrode 111 on the bottom of N+ substrate 100. For example, drain electrode 111 may be formed using the conventional technique of metal sputtering. As described earlier, the bottom of the substrate may first be subjected to grinding, implanting, etc., to lower the drain contact resistance.

0055The device of <figref idref="f0006">Figure 5G</figref> represents a completed high-voltage transistor having a stand-alone drift region; that is, the device of <figref idref="f0006">Figure 5G</figref> does not include a low-voltage, series MOSFET structure at the top of the epitaxial layer. Instead, the extended drift region formed by the epitaxial layer, itself, performs the function of the MOSFET without the inclusion of a P-body region. Practitioners in the arts will note that in this device structure current cannot be completely turned-off, since there exists a continuous n-type path for electrons to flow from source electrode 109 to drain electrode 111. Current flow in the device structure of <figref idref="f0006">Figure 5G</figref>, however, does saturate when the mesa-like epitaxial layer 101 is pinched-off at high drain voltages.

0056The device structure of <figref idref="f0009">Figure 6</figref> achieves pinch-off of the extended drain region at lower voltages than the device of <figref idref="f0006">Figure 5G</figref>. This is achieved by reducing the spacing between the field plate members 103 and epitaxial layer 101 near the top of the N-type drift region, thereby increasing the capacitance to pinch-off the vertical drift region at a relatively low voltage. <figref idref="f0009">Figure 6</figref> shows a multi-tiered field plate structure extending laterally into oxide regions 102a &amp; 102b to control the pinch-off voltage and, therefore, the saturation current. Alternatively, the field plate members may comprise a single step, a linearly graded lateral extension, or some other profile shape designed to achieve the same result.

0057Those skilled in the arts will appreciated that for certain circuit applications it may be advantageous to utilize the stand-alone transistor structure of <figref idref="f0006">Figure 5G</figref> (or <figref idref="f0009">Figure 6</figref>) in series with an ordinary external, low-voltage switching MOSFET. In such an application the low-voltage (e.g., 40V) MOSFET could be used for switching purposes in order to completely turn off current flow in the high-voltage (e.g., 700V) transistor device.

0058Referring now to <figref idref="f0007 f0008">Figures 5H-5K</figref>, there is shown an alternative processing sequence that may be used to fabricate a vertical HVNMOS transistor that includes an insulated gate MOS structure.

0059Trenches 112a and 112b are formed in respective dielectric layers 102a and 102b on opposite sides of epitaxial layer 101 to accommodate the formation of the insulated gate structure. The depth of trenches 112a and 112b extends from the surface of N+ source region 105 to a depth governed by the intended MOSFET channel length and field plating considerations. In this example, the trench depth is about 1-5µm. By way of example, trenches 112 may be formed by appropriate application of a patterned masking layer to the semiconductor substrate followed by conventional dry or wet etching techniques into oxide layer 102.

0060<figref idref="f0008">Figure 5J</figref> shows the device after formation of gate dielectric layers 116 and gate members 113 within trenches 112. The gate dielectric layers 116a &amp; 116b may be formed by growing or depositing oxide on the sidewalls of the stacked N+ source, P-body, and epitaxial regions. The device threshold voltage determines the thickness of layers 116. In one example, layers 116 comprise silicon dioxide having a thickness on the order of 25-100 nm (250 - 1000 angstroms).

0061In the example shown, a portion of dielectric layers 112 isolates field plate members 103 from gate members 113. Alternatively, trenches 112 may expose the top portion of field plate 103 and the same processing steps used to create layers 116 may also be used to form dielectric layers on the sidewalls of the field plates to isolate the field plates from the gate members.

0062Once dielectric layers 116 have been formed on the sidewalls of trenches 112, a conductive material, such as doped polysilicon, may be deposited to fill the remaining portions of the trenches. In this implementation, the doped polysilicon forms the gate members 113a and 113b of the MOS transistor structure. <figref idref="f0008">Figure 5J</figref> shows the device after introduction of a P-body region 107 and a N+ source region 105 at the top surface of epitaxial region 101. Regions 107 and 105 may be formed using standard implantation, deposition, and/or thermal diffusion processing steps. In the completed device, application of a sufficient voltage to gate members 113 causes a conductive channel to be formed along the sidewall portions of P-body region 107 between N+ source region 105 and epitaxial region 101. The channel length is therefore determined by the thickness of P-body region 107, which, for the particular example shown, may be approximately 0.5µm - 3.0µm, with the N+ source region 105 in the range of about 0.1-0.5µm. A shorter channel length results in a lower channel resistance, which likewise reduces the on-resistance of the device. It should be understood, however, that a too short channel would cause punch-through problems.

0063<figref idref="f0008">Figure 5K</figref> shows the completed HVFET device structure following formation of an interlevel dielectric layer 106 (e.g., silicon dioxide, silicon nitride, etc.). This layer may be deposited and patterned to form contact openings. In the embodiment shown, the etching of layer 106 is followed by etching of the field plates, gate members, N+ and P-body regions. This is followed by deposition and patterning of a conductive layer (e.g., metal, silicide, etc.) to create source electrode 109, gate electrodes 115, and field plate electrodes 110, which provide electrical connection to the respective regions of the device. The optional etching step described above allows the source electrode to contact the P-body region without patterning the N+ source region, thus simplifying the process. A conductive layer may also be applied to the bottom of substrate 100 (after optional treatment by grinding, etching, implanting, etc.) to form the drain electrode 111.

0064Note that while source electrode 109 is shown extending down to P-body 107 in the cross-sectional view of <figref idref="f0008">Figure 5K</figref>, in other examples electrode may only extend to the upper surface of source region 105. It should also be appreciated that electrode 109 does not separate region 105 into two separate source regions in the illustration of <figref idref="f0008">Figure 5K</figref>. Rather, electrode 109 is fabricated in the form of a plug that is surrounded by N+ material that comprises region 105.

0065Thus there is provided a high-voltage transistor with a low specific on-state resistance and that supports high voltage in the off-state includes one or more source regions disposed adjacent to a multi-layered extended drain structure which comprises extended drift regions separated from field plate members by one or more dielectric layers. With the field plate members at the lowest circuit potential, the transistor supports high voltages applied to the drain in the off-state.