METHOD OF MAKING A HIGH-VOLTAGE TRANSISTOR WITH MULTIPLE LATERAL CONDUCTION LAYERS

Abstract: A method for making a high voltage insulated gate field-effect transistor having an insulated gate field-effect device structure with a source and a drain comprises the steps of forming the drain (19) with an extended well region (17) having one or more buried layers (18) of opposite conduction type sandwiched therein. The one or more buried layers (18) create an associated plurality of parallel JFET conduction channels (25) in the extended portion of the well region. A minimal number of processing steps are required to form the parallel JFET conduction channels which provide the HVFET with a low on-state resistance.

Description: METHOD OF MAKING A HIGH-VOLTAGE TRANSISTOR WITH MULTIPLE LATERAL CONDUCTION LAYERS RELATED APPLICATIONS The present application is a continuation-in-part of Serial No. 08/744,182, filed November 5, 1996. This application is also related to Serial No. 09/245,030, filed February 5, 1999, entitled, "High-Voltage Transistor With Multi-Layer Conduction Region". The related applications are assigned to the assignee of the present application. FIELD OF THE INVENTION The present invention relates to high voltage field-effect transistors. More specifically, the present invention relates to processes for fabricating high voltage field-effect transistor structures that include an insulated gate field-effect transistor in series with a junction field-effect transistor. BACKGROUND OF THE INVENTION It is conventional to construct a high-voltage, insulated-gate, field- effect transistor (HVFET) having a high breakdown voltage and a low "on- state" resistance. To accomplish this end, practitioners in the art have used an insulated gate field-effect transistor (IGFET) placed in series with a high- voltage junction field-effect transistor (JFET). Such a transistor is capable of switching at high voltages, has low values of on-state resistance, and has insulated-gate control. Moreover, the HVFET may advantageously be fabricated near low voltage logic transistors on a single integrated circuit chip to form what is commonly referred to as a power integrated circuit (PIC). One goal in the art is to produce a transistor with a high breakdown voltage (V bd ) using as small a surface area as possible. The HVFET must provide a V d that is above the minimum allowed for a given application. Realizing high V bd in a small area reduces the cost of the PIC. Traditional HVFET devices with a high breakdown voltage require large amounts of silicon area and are expensive to fabricate. It is also desirable to fabricate HVFETs that occupy as small a surface area as possible to realize a given on-state resistance. The figure of merit often used is known as specific on-resistance (R sp ), which is the product of on-state resistance and surface area. A lower R sp allows a smaller HVFET transistor to be used to meet the on-state resistance requirements of a given application, which reduces the area and, respectively, the cost of the PIC. Another goal in the art is to provide a highly manufacturable HVFET design that consistently delivers the required combination of V bd and R sp over a range of normal process variances. To realize this goal, the manufacturing process should introduce minimal variance in the critical device parameters, and the HVFET should exhibit minimal sensitivity to process variations. To try to achieve the aforementioned goals, researchers and engineers have experimented with a variety of different device structures. For example, a lateral HVFET, is disclosed in "High Voltage Thin Layer Devices (RESURF Devices)," by Appels and Vaes, IEDM Tech. Digest, pp. 238-241 , (1979). This device is fabricated in accordance with the Reduced Surface Field (RESURF) principal, in which an extended drain region is used to support the high off-state voltage. The RESURF principal, however, mandates that the charge in the extended drain region, which serves as the channel of a lateral junction field-effect transistor (JFET), be carefully controlled to obtain high V bd - To keep the maximum electric field below the critical field at which avalanche breakdown occurs, the amount of charge in the JFET channel is typically limited to a maximum of about 1 x 10 12 cm "2 . When the HVFET is in the "on" state, the resistance of the JFET channel constitutes a large portion of the on-state resistance of the HVFET. Therefore, the limitation on the maximum charge in the JFET channel also sets the minimum specific on-resistance of the device. A HVFET having an extended drain region with a top layer of a conductivity type opposite that of the extended drain region is disclosed in U.S. Patent No. 4,811 ,075. The '075 patent teaches that this structure approximately doubles the charge in the JFET channel of an HVFET, thereby lowering the R sp by about 50%. Because this top layer helps to deplete the extended drain when the extended drain is supporting a high voltage, a high breakdown voltage is maintained despite the increased charge density. A HVFET in which two JFET channels are arranged in parallel to increase charge and reduce R sp is described in U.S. Patent No. 5,313,082. This structure has several drawbacks. First, proper charge balance among the layers must be maintained in accordance with the RESURF principal discussed above. Secondly, according to the '082 patent the N-well region, the P-type buried region, and the upper N-type region are all diffused from the surface. This makes it very difficult to maintain adequate charge balance among the layers. In addition, the heavily doped p-n junction between the buried layer and drain diffusion region degrades the V d of the device. Thus, there still exists a need for an improved HVFET and a method of fabricating the same. The HVFET should exhibit a low specific on-state resistance, be easily integrated on the same chip along with low voltage logic devices, achieve the required minimum breakdown voltage in the smallest possible surface area, and be relatively inexpensive to manufacture. SUMMARY OF THE INVENTION In one embodiment, a method of fabricating a HVFET comprises implanting a dopant of first conductivity type into a substrate of a second conductivity type. This is followed by the step of forming a first region of the first conductivity type by diffusing the dopant into the substrate. A dopant of second conductivity type is then implanted into the first region to form a buried layer sandwiched therein. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, wherein: Figure 1 is a cross-sectional side view of one embodiment of a high- voltage, field-effect transistor (HVFET) device structure in accordance with the present invention. Figure 2 is a cross-sectional side view of another embodiment of a HVFET fabricated in accordance with the method of the present invention. Figure 3 is a cross-sectional side view of still another embodiment, which includes a plurality of buried layers. Figure 4 is a cross-sectional side view of an alternative embodiment, which also includes a plurality of buried layers. Figure 5 is a cross-sectional side view of still another embodiment of a HVFET fabricated in accordance with the method of the present invention. Figure 6 is a cross-sectional side view of still, another embodiment of a HVFET fabricated in accordance with the method of the present invention. Figure 7 is a top view of a HVFET having inter-digitated source and drain "fingertip" regions. Figure 8 is a cross-sectional side view of the HVFET shown in Figure 7 taken along cut line B:B. Figure 9 is a cross-sectional side view of the HVFET shown in Figure 7 taken along cut line C:C. Figure 10 is a cross-sectional side view of the HVFET shown in Figure 7 taken along cut line D:D. Figures 11a-11i are cross-sectional side views that illustrate various processing steps that may be used to fabricate a HVFET in accordance with the method of the present invention. Figure 12 is a plot of a typical impurity concentration profile within the laterally extended drain portion of a HVFET following the process steps illustrated in Figure 11c. Figure 13 is a plot illustrating net impurity concentration profile after compensation for an HVFET with five JFET channels according to one embodiment of the invention. Figure 14 is a cross-sectional side view of yet another embodiment of the present invention. DETAILED DESCRIPTION In the following description, numerous specific details are set forth, such as material types, doping levels, structures, 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. The present invention relates to a method for fabricating a high- voltage field-effect transistor that provides a low on-state resistance for a given breakdown voltage. While n-channel HVFETs are presented herein for illustrative purposes, a p-channel HVFET can be fabricated by appropriate reversal of the conductivity types associated with the various regions and layers. Device Structure Referring now to Figure 1 , an exemplary n-channel HVFET is shown in accordance with one embodiment of the present invention. It should be understood that the present invention equally contemplates an analogous p- channel HVFET. The p-channel transistor may be realized by utilizing the opposite conductivity types for all of the illustrated diffusion regions. Figure 1 illustrates an insulated-gate, field-effect transistor (IGFET) having a gate 12 (comprised, for example, of polysilicon), an insulating layer 20, comprised of silicon dioxide or another appropriate dielectric insulating material, and an underlying lightly-doped p-type substrate region 16. Gate 12, insulating layer 20 and substrate 16 together form the insulated gate region of the device. In one embodiment, the gate region is a metal-oxide semiconductor (MOS), and the IGFET is a MOS transistor. An optional p-type region 15 is disposed in substrate 16 spaced-apart from N-well region 17. Additionally, a p-type buried layer 35 may be included beneath P-well 15. A N+ source diffusion region 14 is shown formed in region 15. An IGFET channel region 28 is formed between N+ source diffusion region 14 and N-well region 17. A source electrode 10 provides an electrical connection to N+ source diffusion region 14. Similarly, a drain electrode 11 connects to a N + drain diffusion region 19. Source and drain electrodes 10 and 11 may comprise a number of widely used metals or metal alloys. Source electrode 10 is shown extending over an insulative layer 27 formed over gate 12 where it functions as a field plate. In the illustrative embodiment, a P+ diffusion region 13 is disposed adjacent to N+ source diffusion region 14. Diffusion region 13 increases the integrity of the source-to-substrate connection and reduces susceptibility of the device to parasitic bipolar effects. The HVFET of Figure 1 also includes an N-well region 17 having a laterally extended drain portion 23 with a lateral boundary 21 formed within substrate 16. Disposed within N-well region 17 is a p-type buried region 18, and drain diffusion region 19. Buried region 18 is sandwiched within N-well region 17 in the laterally extended drain portion 23. As can be seen, buried region 18 is surrounded above, below and laterally by N-weli region 17. The embodiment of Figure 1 also shows buried region 18 separated from N+ drain diffusion region 19 by a portion of the N-well region 17 to improve the breakdown voltage of the transistor. A drain electrode 11 provides an electrical connection to N+ drain diffusion region 19. Note that drain electrode 11 also connects to a field plate member 26, which is insulated from the substrate and is located adjacent to drain diffusion region 19 over N-well region 17. Like the extended portion of source electrode 10, field plate member 26 acts to reduce peaks in the localized electric field, thereby increasing the breakdown voltage of the transistor. When the HVFET is in the on-state, electron current flows from the source diffusion region 14 through the IGFET channel region 28, and then through dual, parallel JFET channels, formed by the N-above region 24 and the N-below region 25, and finally to drain diffusion region 19. As described below, the combined charge in the N-above and N-below regions 24 & 25 may be about 3 x 10 12 cm "2 , which is about three times higher than that of a conventional, single JFET channel device. Thus, the resistance of the extended drain region is reduced to about 1/3 that of a conventional device. As will be described below, other embodiments of the invention comprise additional JFET channels in the N-well region 17 formed by a plurality of p-type buried layers. Thus, the following discussion of features to the invention in which only a single P-buried region lies within the N-well region applies equally to embodiments possessing a plurality of P-buried regions in the N-well region. In the off-state, N-above region 24, buried region 18, N-below region 25, and a portion of the substrate 16 are mutually depleted of free carriers. In order to keep the electric field below the critical electric field at which avalanche breakdown occurs, the charge in each layer is balanced. For example, the charge concentration is approximately 1 x 10 12 cm "2 in N-above region 24, about 2 x 10 12 cm "2 in buried region 18, and about 2 x 10 12 cm "2 in N-below region 25. In one implementation, buried region 18 is not left floating (electrically), but instead is connected to substrate 16 or another region having substantially the same potential. Substrate 16 is typically connected to ground, which provides the double-sided JFET with enhanced switching characteristics. As discussed above, the HVFET of Figure 1 may include an additional region 15 into which the N+ source diffusion region 14 and the P+ diffusion region 13 are diffused. One function of region 15 is to reduce the susceptibility of the HVFET to drain-to-source punch-through. Another function is to provide an appropriate threshold voltage for the IGFET with less variance. Region 15 also lowers the base resistance of the parasitic NPN device and, thereby increases the safe operating area of the HVFET. The embodiment of Figure 1 may also include a p-type buried layer 35 underlying the N+ source diffusion region 14 and the P+ diffusion region 13. Note that this region can be formed with the same implant step as p-type buried region 18, so as to minimize the cost and complexity of the process. Buried layer 35 offers the same advantages as those described above for P- region 15. However, buried layer 35 can be more heavily doped than region 15 because it is removed from the IGFET channel region and, therefore, does not affect the threshold voltage of the IGFET. Being more heavily doped, this layer is also effective in preventing parasitic NPN action. Another embodiment of the invention is shown in Figure 2. This embodiment differs from that of Figure 1 only in the thickness of the oxide above the laterally extended portion 23 of N-well region 17. In Figure 1 , a uniform, thin film of oxide 36 is employed. The IGFET gate oxide may be used, which has a typical thickness of 200-1000 angstroms. One advantage of using such thin oxide is that it reduces the required energy for the p-type buried implant used to form region 18. In contrast, the device of Figure 2 shows a thick oxide layer 40 (typically 5000-15000 angstroms) above most of the laterally extended drift portion 23 of N-well region 17. One benefit of thick oxide layer 40 is that it provides an additional level when designing the source and drain field plates that extend from the source and drain electrodes. Thick oxide layer 40 may also provide higher yield and reliability. It should be understood, however, that the inclusion of the overlying oxide layer, or the thickness thereof, is not an essential aspect of the present invention. In certain embodiments, it may be eliminated entirely, or substituted with other suitable materials such as silicon nitride, etc. Figure 3 shows another embodiment of the invention in which a plurality of JFET conduction channels 41 are formed in the N-well region 17. Other aspects and features of this embodiment are similar to the embodiment with dual JFET channels illustrated in Figures 1 and 2. For example, drain electrode 11 connects to a drain diffusion region 19 and includes a field plate member 45 that covers part of laterally extended portion 23. Similarly, source electrode 11 is electrically connected to N+ source diffusion region 14 and P+ diffusion region 13. Source electrode also includes a metal field plate member that extends over the channel region of the IGFET. Surrounding N+ source diffusion region 14 and P+ diffusion region 13, a p-type region 15 is optionally included to prevent punch-though. Gate 12 controls current flow in the IGFET channel region formed between N+ source diffusion region 14 and N-well region 17. A thick oxide layer 40 may optionally overlie laterally extended portion 23 of N-well region 17. The drain field plate 45, which may be constructed from polysilicon or other suitable materials, is separated from an overlying portion of drain metal 11 by the inter-level dielectric layer 50. With continuing reference to Figure 3, two or more p-type buried regions 60 are disposed within N-well region 17. Regions 60 may be formed, for example, by high-energy ion implantation. This results in an N-well region 17 that is divided into multiple JFET conduction channels (e.g., Nι-N k+1 ) interleaved with the P-buried regions 60. The implant energies and doses may be chosen such that the maximum charge in the uppermost conduction channel (N is limited to about 1 X 10 12 /cm 2 , in order to keep the maximum electric field at the N 1 /PB- 1 junction below the critical electric field at which avalanche breakdown occurs. The maximum charge in each P-buried regions (PB-ι-PB k ) and each of the remaining JFET channels (N 2 -N k+ ι) is limited to about 2 X 10 12 /cm 2 in the embodiment of Figure 3. Those of ordinary skill in the art will appreciate that to construct a N- well region 17 with a plurality of JFET conduction channels, the doping and implant energy levels of the N-well and the plurality of P-buried regions may be chosen to approximate the above-described charge levels. Although the uppermost buried region 60 (labeled "PB1") is illustrated as lying below the upper surface of the N-well region 17, this particular region may also be disposed at the upper surface of the N-well region 17. With attention directed to Figure 4, an embodiment of the invention is illustrated in which p-type region 60 denoted PB1 is formed coincident with, and just below, the upper surface of the N-well region 17. In the embodiment of Figure 4 the number of JFET channels is equal to the number of P-buried regions 60. The charge in the uppermost P-buried layer PB1 is constrained to about 1x10 12 /cm 2 , while the charge in each of the remaining P-buried regions and the charge in each of the JFET channels 41 is constrained to about 2x10 12 /cm 2 . Because the resistance of the JFET channels 41 is inversely proportional to the total charge in these channels, each additional P-buried region 60 results in a reduction in on-resistance of the HVFET. For example, Figure 4 shows a plurality of k buried regions 60 implanted into N-well region 17. As a result, there exist k JFET conduction channels 41 formed in N-well region 17. Thus, the embodiments illustrated in Figures 3 and 4 may achieve a much lower on-resistance at the same breakdown voltage achieved by prior art devices. Figures 3 and 4 also show the optional inclusion of additional p-type buried regions 65 formed vertically spaced-apart from one another beneath the source diffusion regions 13 and 14. To avoid affecting the splitting of the current in the various JFET conduction channels, the additional buried regions 65 are spaced laterally from buried regions 60 formed in the extended portion 23 of N-well 17. The additional regions 65 counteract the penetration of drain potential into the IGFET channel region. This means that the source diffusion regions 13 and 14 may be fabricated closer to the N-well region 17, advantageously resulting in a HVFET with a reduced IGFET channel length. Another embodiment of the invention is illustrated in Figure 5. Similar to the previous embodiments, this structure comprises an HVFET having a drain electrode 11 connected to a field plate 26, a drain diffusion region 19, a source electrode 10 (also having an extended field plate), source diffusion regions 13 and 14, substrate 16, and a p-type buried region 18. The embodiment shown in Figure 5 differs from the previous embodiments in that it does not include an N-well region 17. Rather, the structure of Figure 5 comprises a n-type layer 106, which may be formed by epitaxial deposition onto substrate 16. Alternatively, ion implantation and diffusion may be utilized to form layer 106 in substrate 16. Layer 106, like previous embodiments, includes a laterally extended portion 23 into which is sandwiched a buried layer 18. A p-type diffusion region 110 is formed within the n-type layer 106 and is disposed under source diffusion regions 13 and 14. Region 110 provides an IGFET channel region 28 disposed under gate 12 between source diffusion region 14 and n-type layer 106. Region 110 also provides protection from the occurrence of drain-to-source punch-through. The P- buried region 18 disposed within n-type layer 106 acts as an effective gate for a parallel-configured JFET having dual current channels. In the case where layer 106 is formed by epitaxial deposition, the HVFET structure may be formed by a single high-energy implant to form region 18. A P+ isolation region 109 may be needed where layer 106 is formed by epitaxial deposition. On the other hand, in the case where n-type layer 106 is formed by implantation, P+ isolation region 109 may not be required. An additional p-type buried layer 35 may be implanted underneath the source diffusion regions 13 and 14 to prevent punch-through. This permits realization of an IGFET with reduced channel length; it also reduces the susceptibility of the device structure to parasitic NPN action. Figure 6 depicts a cross-sectional view of another embodiment of the present invention which includes n-type layer 106, which may be formed by epitaxial deposition onto, or implantation and diffusion into, p-type substrate 16. A p-type diffusion region 111 formed within n-type layer 106 serves as the IGFET channel region 120 and provides protection from the occurrence of drain-to-source punch-through. In this embodiment the IGFET channel region 120 is formed in a circular, rectilinear or hemispherical shape between regions 14 and 106. Dual JFET channels are provided for current flow through the N-above region 24 and the N-below region 25. The presence of the additional IGFET channel region in the embodiment of Figure 6 provides about twice the IGFET channel width for a given HVFET width compared to previous embodiments. It also has advantages of lower IGFET channel resistance and higher IGFET saturation current compared to other embodiments. While only a single p-type buried layer 18 is illustrated, additional p-type buried layers may be included as discussed previously. In an alternative embodiment of the present invention, the buried region may be formed with a plurality of openings that vertically connect the above conduction region to the below region, thereby permitting current to flow between the above and below region through the openings. Figure 14 is a cross-sectional side view of this embodiment, which shows a p-type buried layer 88 extending through N-well region 17 and under diffusion regions 13 and 14 of the device. Buried layer 88 includes openings 81 that connect the N-above region 72 to the N-below region 73 to form dual JFET conduction channels. Note that one of the openings is located adjacent to gate 12. Practitioners in the art will appreciate that the embodiment of Figure 14 advantageously permits the design of short IGFET channel lengths. In the embodiment of Figure 14, buried layer 88 may be connected to ground potential via substrate 16 to ensure optimal switching characteristics for the device. Also, it should be understood that the breakdown voltage of the HVFET can be increased by locating one of the openings 81 close to the drain diffusion region 19. The location, size, shape, and number of openings 81 may vary considerably in the embodiment of Figure 14. For example, openings 81 may be hexagonal, rectilinear, circular, triangular, or some other shape. Individual ones of the openings may also vary with respect to each other. Additionally, the location, size, shape and number of openings may be arranged to create a variety of patterns (e.g., checkerboard) in buried layer 88. Those of ordinary skill in the art will further understand that the effective charges in the N-above region 72, N-below region 73, and buried layer 88 can be adjusted such that buried layer 88 depletes in a three-dimensional fashion. For instance, a spherical p-type buried layer can deplete n-type charges around it in a three-dimensional fashion. Thus, a spherically or other shaped buried layer can be formed to take advantage of multi-dimensional depletions. With reference now to Figure 7, a top view of a HVFET 500 having inter-digitated source and drain "fingertip" regions is illustrated. Various cross-sectional side views of the device structure are shown in Figures 3, 8, 9 and 10. (Note that Figure 3 is a view taken along cut line A:A of Figure 7. Figures 1 , 2, 4, 5, and 6 also show other possible cross-sectional views taken through line A:A.) Figure 7 shows HVFET 500 having a source fingertip 505 that includes a source electrode 10. Disposed on either side of source electrode 510 are drain fingertips 515 and 520 included in drain electrode 11. Further disposed on either side of source electrode 10 are additional source electrodes 530 and 535. Gate 12, which may be constructed of polysilicon or other suitable materials, is located adjacent source electrode 10. Similarly, gates 545 and 550 are adjacent additional source electrodes 530 and 535. During operation of the HVFET 500, current flows from the source region to the drain region through the IGFET channel region and then through the plurality of parallel-arranged JFET conduction channels disposed within N-well region 17. The electric field in the inter-digitated HVFET 500 tends to be highest at the source fingertip 505 and drain fingertips 520 and 515 due to the small radius of each fingertip. To alleviate voltage breakdown in these regions, a source fingertip buffer region (or hole) 560 may be created in N-well region 17 surrounding the source fingertip 505. Buffer region 560 is similar in function to area 60 shown in Figure 3 of U.S. Patent No. 5,258,636 which patent is herein incorporated by reference. With continued reference to Figures 7-10, field plate extensions 553 and 555 counteract voltage breakdown at drain fingertips 515 and 520. Field plate extensions 553 and 555 overlay and are separated from the polysilicon drain field plate 45 by an inter-level dielectric layer 50 (see Figure 3). Note that along the sides of drain fingertips 515 and 520, drain field plate 11 has a substantially shorter extension beyond the polysilicon drain field plate 45 towards the source electrode 10. This is illustrated in, for example, the previously described cross-section taken along cut line A:A in Figure 3. Figure 8 is a cross-sectional view of HVFET 500 taken along line B:B of Figure 7. As can be seen, buffer region 560 surrounds the source fingertip 505 to mollify the high electric field in this area. Buffer region 560 provides increased separation between N-well 17 and source diffusion region 14 in the area surrounding the source fingertip 505. This buffer region 560 allows the source fingertip radius (defined from axis of rotation to closest end of gate 12) to be very small (less than five microns). In contrast, the radius defined by the distance between the axis of rotation and the lateral edge of the N-well 17 about buffer area 560 may be considerably larger. This larger radius is beneficial as it reduces the electric field strength in this area. Other than the buffer region 560 discussed above, the device structure at source fingertip 505 is similar to that illustrated in Figure 3. For example, a drain diffusion region 19 is disposed underneath drain electrode 11. Similarly, source diffusion regions 13 and 14 are disposed underneath source electrode 10. A p-type region 15 may optionally surround source diffusion regions to prevent punch-through. In addition, additional P-buried regions 65 may be formed beneath the source diffusion region, as explained with respect to Figure 3. Figure 9 is a cross-sectional side view taken along line C:C of Figure 8. Here, at drain fingertip 520, drain electrode 11 includes a drain field plate extension 555 to mollify the high electric field in this area. As can be seen, the drain electrode 11 has a portion that overlies the drain field plate 45 and extends laterally over the buried regions 60. In one implementation, field plate 555 extends laterally a distance (X) of approximately 20-80 microns past the end of drain field plate 45. This is a considerably larger extension than is found along line A:A of Figure 3, which may be, for example 10-20 microns. In this example the drain fingertip radii (defined from the axis of rotation to the farthest edge of drain diffusion region 19) may be 5 microns or less. Fabricating HVFET 500 with a small fingertip radius, of course, reduces the required silicon area for the transistor and thus lowers its cost. Figure 10 is a cross-sectional view taken along cut line D:D of Figure 7. This view shows a JFET tap 542, which provides an electrical connection to N-well 17, so that the drain voltage and/or current of the HVFET can be safely coupled to a control circuit. JFET tap 542 typically comprises a metal or metal alloy and extends down through inter-dielectric layer 50 to contact an N+ diffusion region 700. The N+ diffusion region 700 is located near a perimeter boundary of the N-well region 17. In this embodiment, JFET tap 542 is laterally separated from the active IGFET channel areas to avoid interfering with normal device operation. When HVFET 500 is in the off state, JFET tap 542 provides a convenient power source for control circuitry and the like. Despite voltages of up to 700 volts at the drain, JFET conduction channels 41 pinch-off and keep the voltage at JFET tap 542 from exceeding approximately 10-100 volts. When HVFET 500 is in the on state, JFET tap 542 can be used to sense the drain voltage. This connection is therefore useful in applications where current limiting or similar functions are important. Device Fabrication The processing steps and techniques described below may be appropriately employed to fabricate the various device structures disclosed above. Starting with an ordinary p-type substrate 121 , Figure 11a is a cross- sectional view of the substrate following formation of the N-well region 123. N-well 123 may be defined using conventional photolithography followed by implantation of a n-type dopant such as phosphorus. A typical implant dose is in the range of 5x10 12 /cm 2 to 5x10 13 /cm 2 and implant energy of 150keV. The dose is chosen to provide the proper amount of charge in each of the JFET channel regions. Therefore the dose selected for a particular implementation depends on the actual number of JFET channels to be formed. Following implantation, the dopant is driven into substrate 121 to a depth of approximately 5-15 μm. An optional step in the invented process is the formation of thick oxide layer 125 as shown in Figure 11b. Depending on the desired device structure, the laterally extended portion of the drain may either be entirely covered by oxide (as shown), partially covered, or completely free of oxide. By way of example, a typical thickness of oxide layer 125 is about 8000 angstroms. Next, definition of p-type buried layer 130 is achieved using ordinary photolithography steps and one or more ion implantation steps that introduce a p-type dopant such as boron into the N well region 123. The dose and energy for each of the ion implantations are chosen to provide the required amount of charge in each of the buried layers 130, and also in the corresponding JFET conduction channels. A cross-sectional view of the semiconductor substrate after formation of a single buried layer 130 is illustrated in Figure 11c. The buried layer 130 may be formed using an implant dose of about 4x10 12 /cm 2 with energy of about 1250keV. At this dose and energy, a top JFET conduction channel 122 is produced above buried layer 130. A bottom JFET conduction channel 124 is produced underneath buried layer 130. Another option is to form an additional p-type buried layer 132 within substrate 121 outside of the N-well region 123. The buried layer 132 may be formed using the same mask, and by the same ion implantation, as is used to form buried layer 130 within the N-well region 123. Thus, the formation of the additional buried layer 132 does not require an additional implantation step. Additional buried layer 132 provides device performance advantages such as reduced susceptibility to drain-to-source punch-through. As discussed earlier, formation of oxide layer 125 over the laterally extended portion of N-well region 123 is an optional step of the process of the present invention. Several benefits of not forming oxide layer 125 include reduced processing costs and a reduction in the energy required to implant the underlying buried layers. For example, without oxide layer 125 an implant energy level of about 800keV may be suitable to form a single buried layer 130. For a given implantation energy, the thickness of oxide layer 125 affects the depth of buried layer 130 within N-well region 123. This means that variations in the thickness of oxide layer 125 can be utilized to purposefully vary the depth of buried layer 130. Moreover, the thickness of oxide layer 125 may be varied either continuously (sloped) or discontinuously (abrupt). Figure 11d is a cross-sectional view that illustrates how discontinuous thickness variations in oxide layer 125 may be utilized to achieve a discontinuous buried layer 130 comprising multiple buried layer sections 130a & 130b disposed at different depths within N-well region 123. Using a single implantation step through a two-tiered oxide layer (comprising sections 125a and 125b) produces buried layer sections 130a formed at a relatively shallow depth, and buried layer sections 130b formed relatively deep within N- well region 123. In the areas where the oxide layer is relatively thin (125b) the underlying buried layer sections 130b are located deep. On the other hand, in the areas where the oxide layer is relatively thick (125a) the underlying buried layer sections 130a are located relatively shallow. Thus, by employing a single P-buried implant, multiple buried layer sections can be created at differing depths within N-well region 123. Figure 11e illustrates a cross-sectional view of the structure of Figure 11 b following high-energy ion implantation into N-well region 123 to create multiple buried layers 150 (PBι-PB k ). As can be seen, this produces an associated plurality of JFET conduction channels 160 (N N k+1 ) interleaved with buried layers 150. In an exemplary embodiment, the implant energies and doses are chosen such that the charge in the uppermost conduction channel 160 (Ni) is about 1x0 12 /cm 2 . This keeps the maximum electric field at the N-i/PB-i junction below the critical electric field at which avalanche breakdown occurs. By the same requirement, the charge in each underlying buried layer 150 (PB f -PB k ) and in each of the underlying JFET conduction channels 160 (N 2 -N k+ ι) is about 2x10 12 /cm 2 . As shown in Figure 3, and discussed previously, the same implant step that forms buried layers 150 may be used to simultaneously form additional buried layers 155 (PBι'-PB k ') in the substrate 121 beneath the source region. In other words, the same mask layer that is used to form buried layers 150 within the N-well region 123 can be used to form additional buried layers 155. Thus, the formation of the additional p-type buried layers 155 does not require additional implantation steps beyond those needed to form the p-type buried layers 150. Figure 11f illustrates a device structure similar to that of Figure 11 e, except that in Figure 11 e the uppermost buried layer 150 (PB1) is formed just under the surface of the N-well region 123. This is accomplished by appropriate reduction of the implant energies used to form each of the buried layers 150. Note that in this embodiment the number of JFET conduction channels 160 is identical to the number of buried layers 150. For example, the maximum charge in the uppermost P-buried layer 150 (PB1) is approximately 1x10 12 /cm 2 , while the maximum charge in each of the remaining P-buried layers 150 (and the charge in each of the JFET conduction channels 160) is approximately 2x10 12 /cm 2 . The remaining processing steps are similar regardless of whether the laterally extended portion of N-well region 123 is formed with a single buried layer, multiple buried layers, or regardless of the thickness of the overlying oxide layer. Furthermore, it should be understood that the laterally extended portion of N-well region 123 may also advantageously comprise the high- voltage portion of other lateral power devices. For example, high-power diodes, JFETs, LIGBTs, and so on may also be incorporated in the laterally extended portion of N-well region 123. In the method of manufacturing a high-voltage IGFET, the growth or deposition of a thin gate oxide layer 170 follows the previously described high-energy implantation step (or steps). After formation of gate oxide layer 170, polysilicon field plate and gate 126 may be deposited and patterned. Figure 11 g shows a cross-sectional view of the semiconductor substrate following completion of these steps. Next, the substrate surface is appropriately masked and a dopant such as arsenic or phosphorous is implanted to form N+ source diffusion region 128 and N+ drain diffusion region 131 , as shown in Figure 11 h. At this point in the process, an optional P+ region 135 may be created adjacent to the source diffusion region 128 by ion implantation. Practitioners in the semiconductor fabrication arts will understand that it may be desirable to also form a P+ region adjacent to drain diffusion region 131. Following formation of the source and drain diffusion regions, an inter- level dielectric layer 132 may be deposited (and then densified or reflowed, if necessary) as illustrated in Figure 11 i. By way of example, dielectric layer 132 may comprise a low-temperature oxide (LTO). Conventional photolithography and etching steps are employed to form contacts to the source and drain regions. A suitable conductive material, such as an aluminum or titanium alloy is commonly deposited and patterned to form the source and drain electrodes 134 and 135, respectively. Deposition of a passivation layer 136 and definition of pad openings complete the process. A cross-sectional view of the HVFET following the passivation step is shown in Figure 11 i. Figure 1 is a plot of a typical impurity concentration profile within the laterally extended drain portion of the HVFET following the process steps illustrated in Figure 11c. Figure 12 is for illustrative purposes only, and is not intended to limit the invention to the quantities described therein. The vertical axis of the graph represents the ion concentration represented as the logarithm of the number of ions per cubic centimeter. The horizontal axis of the graph represents the vertical distance (depth) into the semiconductor substrate 121. The depth is measured in microns from the semiconductor substrate surface. The uncompensated impurity profile produced by the implantation and diffusion of the n-type laterally extended portion of N-well region 123 is represented by line 201. The uncompensated impurity profile produced by the high-energy implantation and diffusion of the p-type buried region 130 sandwiched within N-well region 123 is represented by line 202. Line 203 represents the net concentration of impurities after compensation has occurred. As evidenced by the similarity of line 203 to lines 201 and 202, the net effect of compensation is minimal. In other words, the method of the present invention provides for charge matching at low N-well doping levels. This advantageously results in a reduced amount of carrier scattering, as well as reduction in other undesirable side effects associated with processes involving multiple diffusions from the surface. Figure 12 shows the peak concentration of buried layer 130 below the surface of the laterally extended portion of the drain region 123. It should be understood that the depth of the peak concentration is determined primarily by implant energy. The plot of Figure 12 also shows that buried region 130 is only about 1 urn wide, which is primarily a function of the straggle of the high- energy implant and the time/temperature of subsequent diffusion steps. Figure 12 also illustrates N-top region 122 (see Figure 11c) being formed in the region from about 0 to 1.0 μm below the surface of the substrate. The region between about 2.0 to 8.0 μm represents N-bottom region 124. The region between about 1.0 to 2.0 μm represents p-type buried region 130. According to the method of the present invention, the thickness of each region and the charge contained in each region may be selected independently by varying the energy and dose used to form N-well region 123 and buried region 130. Figure 13 is a plot illustrating the net impurity concentration profile after compensation for an HVFET with five JFET channels formed by four successive p-type buried implants. The vertical axis represents the ion log concentration and the horizontal axis represents the vertical distance into the semiconductor substrate as measured from the surface. The dose and energy of the N-well implant and each buried layer implant are chosen to provide the appropriate doping in each layer, as previously described. Although the processing steps in the foregoing description are for fabrication of a n-channel HVFET, it is appreciated that a p-channel HVFET can be realized by simple reversal of the conductivity types employed to form the various regions/layers. Many other modifications are also within the scope of the invention. For example, rather than forming the N-well region by implanting and diffusing as described above, this region may be formed by epitaxial deposition, followed by high-energy implantation of the p-type dopant used to form the buried layers. In another variation, rather than implanting p-type dopant into a N-well, the n-type JFET conduction channels of the N-well region may be formed by high-energy implantation into an appropriately doped p-type diffusion or substrate region. This produces n-type doping around p-type buried regions. Therefore, It should be understood that although the present invention has been described in conjunction with specific embodiments, numerous modifications and alterations are well within the scope of the present invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

Claims: We claim: 1. A method of fabricating an extended drain of a high-voltage field-effect transistor (HVFET) comprising the steps of: (a) forming a well region of a first conductivity type in a substrate of a second conductivity type, the well region having a laterally extended portion; (b) implanting a dopant of the second conductivity type into the laterally extended portion of the well region to form a buried region therein; forming a drain diffusion region of the first conductivity type in the well. 2. The method according to claim 1 wherein the buried region is disposed beneath a surface of the substrate. 3. The method according to claim 1 , wherein step (a) comprises the steps of: implanting a dopant of the first conductivity type into the substrate; diffusing the dopant in the substrate. 4. The method according to claim 1 wherein the first conductivity type is n- type and the second conductivity type is p-type. 5. The method according to claim 1 wherein the drain diffusion region is spaced-apart from the buried region. 6. The method according to claim 1 wherein the buried region is sandwiched within the well region such that dual JFET conduction channels are formed above and below the buried region. 7. The method according to claim 1 wherein step (b) is performed through a masking layer having a discontinuous thickness formed over the substrate such that the buried region comprises separate buried sections disposed at different depths within the well region. 8. A method of fabricating an extended drain of a high-voltage field-effect transistor (HVFET) comprising the steps of: (a) forming a well region of a first conductivity type in a substrate of a second conductivity type, the well region having a laterally extended portion; (b) implanting a first dopant of the second conductivity type into the laterally extended portion of the well region to form a first buried region therein; (c) implanting a second dopant of the second conductivity type into the laterally extended portion of the well region to form a second buried region therein, the second buried region being disposed at a different depth in the well region than the first buried region; and (d) forming a drain diffusion region of the first conductivity type in the well. 9. The method according to claim 8 wherein the first buried region is disposed approximately 0.5-2.0 μm beneath a surface of the substrate. 10. The method according to claim 8, wherein the first dopant is the same as the second dopant. 11. The method according to claim 8 wherein the first conductivity type is n- type and the second conductivity type is p-type. 12. The method according to claim 8 wherein the drain diffusion region is spaced-apart from both the first and second buried regions. 13. The method according to claim 8 wherein the first buried region is disposed above the second buried region. 14. A method of fabricating a high-voltage field-effect transistor (HVFET) comprising the steps of: (a) forming a well region of a first conductivity type in a substrate of a second conductivity type, the well region having a laterally extended portion with a lateral boundary; (b) implanting a dopant of the second conductivity type into the substrate to form a first buried region within the laterally extended portion of the well region; (c) forming a gate insulated from the substrate by a gate oxide layer, the gate extending over the substrate adjacent the well region; (d) implanting a dopant of the first conductivity type into the substrate to form a source diffusion region spaced-apart from the well region, and into the well region to form a drain diffusion region spaced-apart from the first buried region, a channel region being formed between the source diffusion region and the well region under the gate; and (e) forming source and drain electrodes connected to the source and drain diffusion regions, respectively. 15. The method according to claim 14 wherein step (b) also forms a second buried region in an area of the substrate that is beneath the source diffusion region. 16. The method according to claim 14 wherein step (b) is performed through a masking layer having a discontinuous thickness such that the first buried region comprises separate buried layer sections disposed at different depths within the well region. 17. The method according to claim 16 wherein the masking layer comprises an oxide. 18. The method according to claim 14 wherein the first buried region is disposed approximately 0.5-2.0 μm beneath a surface of the substrate. 19. The method according to claim 14 wherein the first conductivity type is n-type and the second conductivity type is p-type. 20. The method according to claim 15 wherein the second buried layer is spaced-apart from the lateral boundary by a region of the substrate beneath the channel region. 21. The method according to claim 14 wherein the first buried region is spaced-apart from the lateral boundary. 22. A method of fabricating a high-voltage field-effect transistor (HVFET) comprising the steps of: (a) forming a well region of a first conductivity type in a substrate of a second conductivity type, the well region having a laterally extended portion with a lateral boundary; (b) successively implanting a dopant of the second conductivity type into the substrate to form a plurality of buried layers within the laterally extended portion of the well region, each successive implant being performed at a different energy such that the plurality of buried layers are spaced-apart from one another thereby creating an associated plurality of JFET channels within the well region; (c) forming a gate insulated from the substrate by a gate oxide layer, the gate extending over the substrate adjacent the well region; (d) forming a source diffusion region in the substrate spaced-apart from the well region, a channel region being formed between the source diffusion region and the well region under the gate; and (e) forming a drain diffusion region in the well region. 23. The method according to claim 22 further comprising the step of: (f) forming source and drain electrodes connected to the source and drain diffusion regions, respectively. 24. The method according to claim 22 wherein a first buried layer of the plurality of buried layers is contiguous with a surface of the substrate. 25. The method according to claim 22 wherein an uppermost buried layer of the plurality of buried layers is disposed beneath a surface of the substrate. 26. The method according to claim 22 wherein an uppermost buried layer of the plurality of buried layers is disposed approximately 0.5-2.0 μm below a surface of the substrate. 27. The method according to claim 22 wherein the first conductivity type is n-type and the second conductivity type is p-type. 28. The method according to claim 22 wherein each of the plurality of buried layers is spaced-apart from the lateral boundary of the well region. 29. The method according to claim 22 wherein step (b) also forms a corresponding plurality of additional buried layers in an area of the substrate that is beneath the source diffusion region. 30. The method according to claim 22 wherein step (b) is performed through a masking layer having a discontinuous thickness such that a first buried layer comprises separate buried sections disposed at different depths within the well region. 31. The method according to claim 30 wherein the masking layer comprises an oxide. 32. A method of fabricating an extended drain of a high-voltage field-effect transistor (HVFET) comprising the steps of: (a) forming a well region of a first conductivity type in a substrate of a second conductivity type, the well region having a laterally extended portion; (b) forming a masking layer over the laterally extended portion of the well region, the masking layer having a varying thickness; and (c) implanting a dopant of the second conductivity type through the masking layer to form a buried region within the well region, the buried region comprising separate buried sections disposed at different depths within the well region corresponding to the varying thickness of the oxide layer. 33. A method of fabricating an extended drain of a high-voltage field-effect transistor (HVFET) comprising the steps of: (a) forming a drain diffusion region of a first conductivity type in a semiconductor material of a second conductivity type; (b) successively implanting a dopant of the second conductivity type into the semiconductor material to form a plurality of parallel-configured JFET channels each of which is formed at a different depth below a surface of the substrate, the JFET channels being connected to, and extending laterally from, the drain diffusion region. 34. The method according to claim 33 wherein an uppermost one of the JFET channels is disposed beneath the surface of the substrate. 35. The method according to claim 33 wherein the semiconductor material comprises an epitaxial layer formed over a substrate. 36. The method according to claim 33 wherein the semiconductor material comprises a substrate. 37. The method according to claim 33 wherein an uppermost one of the JFET channels is disposed approximately 0.5-2.0 μm below a surface of the substrate. 38. The method according to claim 33 wherein the first conductivity type is n-type and the second conductivity type is p-type. 39. The method according to claim 33 further comprising the step of: forming a drain electrode connected to the drain diffusion regions. 40. A method of fabricating a high-voltage field-effect transistor (HVFET) comprising the steps of: forming an epitaxial layer of a first conductivity type over a substrate of a second conductivity type; forming a first region of the second conductivity type in the epitaxial layer; forming an insulated gate over a portion of the first region; forming a source diffusion region in the first region spaced-apart from the epitaxial layer, a channel region being defined therebetween, the insulated gate being disposed over the channel region; implanting a dopant into the epitaxial layer to form a buried region of the second conductivity type disposed therein; and forming a drain diffusion region in the epitaxial layer spaced-apart from the buried region. 41. The method according to claim 40 wherein the buried region is spaced- apart from the channel region. 42. The method according to claim 40 wherein the implanting step is performed through a masking layer having a discontinuous thickness such that the buried region comprises separate buried sections disposed at different depths within the epitaxial layer. 43. The method according to claim 40 wherein the masking layer comprises an oxide. 44. The method according to claim 40 further comprising the step of: forming an isolation region connected to the substrate and the first region. 45. The method according to claim 40 wherein the first conductivity type is n-type and the second conductivity type is p-type. 46. The method according to claim 40 wherein the buried region comprises a plurality of buried layers disposed at different depths within the epitaxial layer. 47. The method according to claim 40 wherein the implanting step also forms an additional buried region underneath the source diffusion region.

Inventors: AJIT JANARDHANAN S, DISNEY DONALD R, RUMENNIK VLADIMIR, AJIT, JANARDHANAN, S., DISNEY, DONALD, R., RUMENNIK, VLADIMIR
Applicants: AJIT JANARDHANAN S, DISNEY DONALD R, POWER INTEGRATIONS INC, RUMENNIK VLADIMIR, AJIT, JANARDHANAN, S., DISNEY, DONALD, R., POWER INTEGRATIONS, INC., RUMENNIK, VLADIMIR
Application year: 2000
Main classification: H01L21
IPC codes: H01L21, H01L29