TRENCH GATE POWER SWITCH WITH DOPED REGIONS TO INDUCE BREAKDOWN AT SELECTED AREAS

Abstract

A power device is divided into an active area, an active area perimeter, and a termination region. An array of insulated gates formed in trenches form cells in a p-well body, where n+ source regions are formed in the top surface of the silicon wafer and surround the tops of the trenches. A top cathode electrode contacts the source regions, and an anode electrode is on the bottom of the die. A sufficiently high reverse voltage causes a breakdown current to flow between the anode and cathode electrodes. To ensure that a reverse breakdown voltage current occurs away from the gate oxide and/or the termination region, the active area and the active area perimeter of the p-well are additionally doped with p-type dopants to form deep p+ regions in selected areas that extend below the trenches. The deep p+ regions channel the breakdown current away from active cells and the termination region.

Claims

1. An insulated trench gate device comprising: a first layer of a first conductivity type, the first layer having a first concentration of dopants of the first conductivity type; trenches formed in the first layer so as to terminate in the first layer; a gate oxide along sidewalls of the trenches; a first conductive material at least partially filling the trenches to form gates; first regions of a second conductivity type adjacent to and near the tops of at least some of the trenches in an active area of the device; a top first electrode electrically contacting the first regions; a second layer of the second conductivity type below the first layer; a third layer of the first conductivity type below the second layer; and second regions of the first conductivity type formed in the first layer that extend below the trenches in the active area, the second regions being electrically connected to the first electrode, the second regions having a second dopant concentration that is higher than the first dopant concentration.

2. The device of claim 1 wherein the first layer is a well formed in the second layer.

3. The device of claim 1 wherein the second regions conduct a breakdown voltage current when a sufficiently high reverse voltage is applied across the device.

4. The device of claim 1 wherein the second regions are formed in inactive cells of the device where no first regions are adjacent to the trenches.

5. The device of claim 1 wherein the device includes the active area that conducts current when the device is on, an active area perimeter, and a termination region, wherein the second regions are formed in both the active area and the active area perimeter.

6. The device of claim 5 wherein the first layer is a well formed in the second layer and where the second regions are formed at or near an outer perimeter of the well.

7. The device of claim 6 wherein the second regions formed at or near the outer perimeter of the well surround the active area.

8. The device of claim 7, wherein the second regions are formed in segments that surround the active area.

9. The device of claim 8 wherein the segments form concentric rings around the active area.

10. The device of claim 9 wherein the segments in one ring are staggered with respect to the segments in another ring.

11. The device of claim 1 wherein the second regions are distributed around the active area.

12. The device of claim 1 where the active area comprises cells, each cell having a gate, where cells having the second regions are inactive cells, and where the inactive cells make up less than 10 percent of the cells in the active area.

13. The device of claim 1 wherein the trenches are totally formed in the first layer.

14. The device of claim 1 wherein the second regions extend below the first layer.

15. The device of claim 14 further comprising a third layer of the first conductivity type below the second layer of the second conductivity type.

16. The device of claim 1 wherein the second regions are formed in an active area of the device and in an active area perimeter of the device, wherein cells in the active area perimeter are all inactive, and wherein a gate electrode electrically contacts the first conductive material in the trenches in an area outside of all the second regions in the active area perimeter.

17. The device of claim 1 wherein the first layer is a p-type well, the second layer is an n-type epitaxial layer, the first regions are highly doped n+ type regions formed in a surface of the p-well, and the second regions are highly doped p+ regions extending through the p-well.

18. The device of claim 1 wherein the device comprises the active area, containing active cells, and active area perimeter, containing inactive cells, and a termination region, wherein the second regions cause a breakdown voltage current to occur away from the termination region.

19. The device of claim 18 wherein the second regions also cause the breakdown voltage current to occur away from the gate oxide in active cells in the active region.

20. An insulated trench gate device formed in a die comprising: active cells in an active area of the device; inactive cells in an active area perimeter surrounding the active area; a termination region between the active area perimeter and an edge of the die; a first layer of a first conductivity type, the first layer having a first concentration of dopants of the first conductivity type; trenches formed in the first layer; a gate oxide along sidewalls of the trenches; a first conductive material at least partially filling the trenches to form gates; first regions of a second conductivity type adjacent to and near tops of at least some of the trenches in the active area of the device; a top first electrode electrically contacting the first regions; a second layer of the second conductivity type below the first layer; and second regions of the first conductivity type formed in the first layer that extend below the trenches in the active area perimeter, the second regions being electrically connected to the first electrode, the second regions having a second dopant concentration that is higher than the first dopant concentration.

21. The device of claim 20 wherein the trenches are formed so as to terminate in the first layer.

22. The device of claim 21 further comprising a third layer of the first conductivity type below the second layer.

23. The device of claim 20 wherein the second regions are also formed in the active area.

24. The device of claim 20 wherein the second regions for concentric rings around the active area.

25. The device of claim 20 wherein the concentric rings are formed by segments, wherein the segments in one ring are staggered with respect to segments in another ring.

26. The device of claim 20 wherein the second regions conduct a breakdown voltage current when a sufficiently high reverse voltage is applied across the device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is copied from Applicant's U.S. Pat. No. 9,391,184, except a sample depletion region boundary 19 has been added. FIG. 1 is a cross-section of a vertical power device having trench gates connected in parallel. This device uses an n-MOS to turn on devices in the cells.

[0029] FIG. 2 is copied from U.S. Pat. No. 5,998,836 to Williams and shows a vertical MOSFET with added clamp diodes distributed in the active area of the MOSFET for clamping a reverse voltage.

[0030] FIG. 3 illustrates a modification to the device of FIG. 1, where deep p+ regions have been distributed in the active area and deep p+ regions are formed in the active area perimeter to substantially surround the active area. Breakdown only occurs at one or more of the deep p+ regions.

[0031] FIG. 4 is a top down view of an array of cells in an active area of the power device, where active cells (current carrying cells) have a trench gate adjacent to an n+ source region, and inactive cells, where breakdown occurs, replace the n+ source region with a deep p+ region.

[0032] FIG. 5 is a top down view of an active area perimeter, where all the cells are inactive, since all the trenches are adjacent to deep p+ regions where breakdown occurs.

[0033] FIG. 6 is similar to FIG. 5 but, instead of the deep p+ regions being arranged side by side (surrounding the active area), the deep p+ regions are staggered to improve performance.

[0034] FIG. 7 is a cross-sectional view of the active area perimeter and the termination region of a power device that may be similar to that of FIG. 1, where the device has been modified to add deep p+ regions in the active area perimeter.

[0035] Elements that are the same or equivalent in the various figures may be labeled with the same numeral.

DETAILED DESCRIPTION

[0036] Although the techniques of the present invention can be used for various applications, a few examples will be given with reference to power devices that have trench gates formed in a p-well, where the p-well contains the active area. The conductivity types may be reversed in all embodiments.

[0037] FIG. 3 illustrates a modification to the power device 10 of FIG. 1 to improve the ruggedness of the device to breakdown occurrences.

[0038] In the power device 40, masks for the n-source implantation are modified to block the implantation of n-type dopants into certain cells in the active area 42. Those certain cells are inactive and will be where breakdown is more likely to occur.

[0039] A p-dopant implant mask then only exposes the silicon in the cells where deep p+ regions 44 are to be formed. P-type dopants, such as boron, are then implanted and annealed or diffused to cause the resulting p+ regions 44 to extend at least below the trenches 15 in the active area 42. In the example, the p+ regions 44 extend below the p-well 14.

[0040] FIG. 3 also includes an n+ EQR (equipotential ring) 47 at the outer edge of the die. FIG. 1 would also include a similar EQR 47. The EQR 47 may be connected to a floating metal.

[0041] FIG. 4 is a top down view of an array of cells 48 in the active area 42 of the power device 40, where active cells (current carrying cells) have a trench gate (doped polysilicon 50) adjacent to an n+ source region 18 and may include distributed p+ body contact regions 51. The active area 42 also includes inactive cells, where breakdown occurs, where the n+ source region 18 is replaced with a deep p+ region 44. The distribution of the deep p+ regions 44 is selected to ensure that breakdown occurs through the p+ regions 44, yet there is not a significant decrease in forward current due to the sacrifice of active cell areas. Simulation may be used to select the optimal distribution of the deep p+ regions 44. In the example, 1 in 5 cells in the horizontal direction is converted, and 1 in 3 cells in the vertical direction are converted, so about one out of 15 cells are converted to the inactive cells, but the percentage can be more or less. In one embodiment, the converted (inactive) cells make up less than one-tenth of the cells in the active area. The selection of which cells to convert to the deep p+ regions depends on the maximum allowable current to be withstood during the breakdown period.

[0042] To prevent breakdown in the termination region 60 (FIG. 3), a deep p+ region 54 is formed near or at the edge of the p-well 14 in the active area perimeter 56. The deep p+ region 54 essentially surrounds the active area 42. The deep p+ region 54 is contacted by the same cathode electrode 20 that contacts the deep p+ regions 44 in the active area 42. Multiple rings of the deep p+ regions 54 may be used. The deep p+ regions 54 in the active area perimeter 56 may be continuous or segmented. Although FIG. 3 shows the deep p+ region 54 at the end of the p-well 14, the deep p+ region 54 can be located between the end of the p-well 14 and the gate contact trench.

[0043] Due to the p+ regions 44 and 54, the depletion region boundary (in the n-epi layer 32 or n buffer layer 35) under those regions bulges downward toward the p+ substrate 30 in the event of a reverse voltage. Breakdown generally occurs at the depletion region areas that are closest to the p+ substrate 30. Therefore, the areas in which breakdown occurs can be selected by the locations of the deep p+ regions.

[0044] FIG. 5 is a top down view of the active area perimeter 56, where all the cells are inactive, since all the trenches (filled with insulated polysilicon 50) are adjacent to deep p+ regions 54 where breakdown occurs. The deep p+ regions 54 are connected to the cathode electrode 20 via openings 58 in a patterned dielectric layer. The deep p+ regions 54 are shown arranged in rows and columns. The polysilicon 50 in all the trenches is electrically connected together. A metal gate electrode may contact the gate polysilicon in the trenches within a contact area 57 identified with a dashed line. Therefore, the gate electrode is outside of the cathode electrode portion that contacts the deep p+ regions 54.

[0045] FIG. 6 is similar to FIG. 5 but the deep p+ regions 54 are staggered to improve the likelihood that breakdown will occur via one or more of the deep p+ regions 54 rather than in the termination region 60 (FIG. 3).

[0046] Alternatively, the deep p+ region 54 in the perimeter 56 can be continuous around the active area 42.

[0047] FIG. 7 is a cross-sectional view of the active area perimeter 56 and the termination region 60 of a power device that may be similar to that of FIG. 1 or FIG. 3, where the cross-section cuts across three deep p+ regions 54 in the active area perimeter 56 as well as the polysilicon 50 in a trench 62 that serves to electrically connect all the polysilicon in the device to a gate electrode 64. The gate electrode 64 may be formed during the same metallization step that forms the cathode electrode 20, so there is no need for two levels of metal for the gate electrode and cathode electrode.

[0048] The number and spacing of the guard rings 29 (or field limiting rings) result in a breakdown voltage in the termination area 60 that is higher than the breakdown voltage through the deep p+ regions 44 and 54, to ensure the breakdown does not occur in the termination region 60.

[0049] An n+ region 68 may be contacted by a floating metal to provide an EQR at the die perimeter.

[0050] In another embodiment, the “bottom” anode electrode may instead be formed on the top of the die and electrically connects to a deep buried p+ region that laterally conducts current to a p+ sinker connected to the anode electrode. Or, the sinker may extend down to the p+ substrate. Thus, the present invention applies to both vertical and lateral devices.

[0051] The various concepts described can be applied to any type of trench-gate device to improve the ruggedness of the device in response to a breakdown (includes breakover) condition.

[0052] Various features disclosed may be combined to achieve a desired result.

[0053] While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.