Semiconductor device

Abstract: A semiconductor device has a substrate with regions (54, 56) of first and second conductivity types forming a junction (58) extending to a surface (60) of the substrate, an oxide layer (66, 68) on the surface and covering the junction and a charged ion region in the oxide layer extending from adjacent the junction at the surface of the substrate over part of the region (54) of first conductivity type, the polarity of the ions in the ion region being that of the region of first conductivity type, whereby the breakdown voltage of the device is increased.

Description: This invention relates to a semiconductor device. In a semiconductor device of the type including a planar junction which extends to the surface of the device substrate, it is desirable in many environments to provide a high breakdown voltage of the junction (i.e. the level of reverse bias voltage which results in avalanche breakdown). A number of approaches have been developed in an attempt to accomplish that goal. In discussing those approaches, reference is made to Figures 1-5 of the accompanying drawings which show various prior art techniques. Referring to Figure 1, a typical device 10 of this type is shown, having a substrate 12 including a first region 14 of first (for example P) conductivity type, and a second region 16 of second (for example N) conductivity type. Upon application of a certain level of reverse bias voltage to the device 10, avalanche breakdown will occur. As is well known, the radius of curvature r of the PN junction 18 is very important in determining the breakdown voltage of the device 10. As pointed out in "Physics of Semiconductor Devices", 2nd Edition, authored by S. M. Sze, (1981) at pages 73 and 106-108, the cylindrical and/or spherical regions 20 of the junction 18 have a higher field intensity, so that the level of reverse bias resulting in avalanche breakdown is determined by the radius of curvature of those regions 20. As the radius of curvature of such a cylindrical and/or spherical region 20 is decreased, the breakdown voltage of the device decreases. In order to increase the radius r, deep diffused junctions 22 (Figure 2) have been undertaken. While this method has proven effective in increasing the radius r and in turn the breakdown voltage of the device, such an approach requires a long diffusion time and consumes valuable wafer space. Another well known approach (Figure 3) for increasing device breakdown voltage is to include a field plate 24 as an extension of contact 26 on region 16b. The field plate 24 extends beyond the junction 28b at the surface 30 of the substrate 12b and over a portion 32 of the substrate 12b by an oxide layer 34. This has the effect of increasing the dimension of the depletion region along the surface 30 of the substrate 12b when reverse bias voltage is applied to field plate 24 relative to substrate 12b (see dotted line 36 and dimension 38) which in turn increases the breakdown voltage of the device. This approach is described in "Surface Breakdown in Silicon Planar Diodes equipped with Field Plate", by Conti & Conti, published in Solid-State Electronics, 1972, Volume 15 pages 93-105. While such a method has proven effective in increasing the breakdown voltage of the device, again, similar to the previous method, a large amount of area is taken up, and also, such a method is applicable only over a certain range of resistivities and oxide thicknesses, as field induced breakdown may occur. Yet another approach for increasing breakdown voltage of such a device is shown in Figure 4. That approach includes provision of field limiting rings 40 of a conductivity type the same as that of the region 16c. Provision of such rings 40 results in a depletion region which is indicated by the dotted line 42 when reverse bias voltage is applied. Thus, the dimension 44 of the depletion region 42 along the surface 48 of the substrate 12c is increased, in turn resulting in a higher breakdown voltage. This phenomenon is described in "Blocking Capability of Planar Devices with Field Limiting Rings", by Brieger, Gerlach, and Pelka, published in Solid-State Electronics, Volume 26, Number 8, pages 739-745 (1983). While this system also has proven effective in increasing breakdown voltage, it too has the problem of using up a large surface area. Referring to Figure 5, a semiconductor device 50 is shown in cross-section. Such device 50 includes a substrate 52 having a region 54 of first conductivity type (in this example, P conductivity type), and a region 56 of second conductivity type (in this example N conductivity type), formed in the substrate 52 by means of, for example, diffusion. These regions 54, 56 together form a junction 58 which extends to the substantially flat surface 60 of the substrate 52 at 62 and 64. Oxide layer portions 66, 68 are shown on the surface 60 of the substrate 52. The oxide layer portion 66 covers the junction portion 62 adjacent surface 60, while the oxide layer portion 68 covers the junction portion 64 adjacent surface 60. In fact, these oxide layer portions 66, 68 are used as a mask during the diffusion forming the region 56. Charged ions 70, 72 are implanted into the oxide layer portions 66, 68 as shown. These charged ions 70, 72 are implanted at a certain energy level, so that they generally adopt the positions shown in Figure 5, with the ions 70 extending from generally over and adjacent to the junction portion 62 at the surface 60 of the substrate 52, over a portion 55 of the region 54, while the ions 72 extend from generally over and adjacent to the junction 64 portion at the surface 60 of the substrate 52, over a portion 57 of the region 54. Because the charged ions 70, 72 are all implanted at a certain energy level they are all positioned roughly the same distance from the surface 60 of the substrate 52. These charged ions 70, 72 are of the same polarity as the polarity of the region 54, and in this particular embodiment are cesium ions. A contact element 74 is subsequently formed by masking and application of metal as is well known, to contact the region 56. Through the use of the implanted ions 70, 72 the depletion region will take the form shown by the dotted line 76 in Figure 5 when reverse bias is applied, similar to that shown in Figure 3 when a field plate is used. This increased size of the depletion region 76 (particularly dimensions 78 and 80 along surface 60 of substrate 52) in turn results in an increase in the breakdown voltage of the device 50. Moreover, there is disclosed in IEEE Transactions on Nuclear Science NS-30, pages 4116-4121 (1983), a semiconductor device with a substrate having a region of first conductivity type and a region of second conductivity type together forming a junction which extends to a surface of the substrate, an oxide layer on the surface of the substrate and covering the junction at the surface and a charged ion region in the oxide layer extending generally from over and adjacent to the junction at the surface of the substrate over a portion of the region of first conductivity type, the polarity of the ions in the ion region being the same as the polarity of the region of first conductivity. A semiconductor device of this kind can be formed by a method comprising the steps of providing a semiconductor substrate having a region of first conductivity type and a region of second conductivity type, the regions forming a junction extending to a surface of the substrate, providing an oxide layer over a portion of the substrate and covering the junction at the surface of the substrate, and providing charged ions in a charged ion region of the oxide layer extending from over and adjacent the junction over a portion of the region of first conductivity type of the substrate. Nothing in the prior art suggests anything but a laterally uniform distribution of the ions in such a charged ion region in the oxide layer. The present invention however provides an improvement for a semiconductor device of the kind just described in that there is provided a semiconductor device comprising a substrate having a region (54a;54d) of first conductivity type and a region (56a;56d) of second conductivity type together forming a junction (58a) which extends to a surface (60a) of the substrate, an oxide layer (66a;68a) on the surface of the substrate and covering the junction at the surface, and a charged ion region in the oxide layer extending from over a portion of the region of second conductivity type, over the junction, to over a portion of the region of first conductivity type, the polarity of the ions (82,86;84,88) in the ion region being the same as the polarity of the region of first conductivity type, characterised in that the charged ion region in the oxide layer (66a;68a) is of greater concentration generally adjacent to the junction (58a) than over the portion of the region (54a) of first conductivity type. The semiconductor device thus has an extended depletion region as compared with the prior art. The invention also provides a semiconductor device of the kind described characterised in that the charged ion region is positioned so that the portion thereof generally adjacent to the junction is closer to the surface of the substrate than the portion thereof over the portion of the region of first conductivity type. The invention thus permits the depletion region of each device to be tailored as chosen to meet the operating parameters needed. The invention also provides a method of forming a semiconductor device comprising the steps of providing a semiconductor substrate having a region (54) of first conductivity type and a region (56) of second conductivity type, the regions forming a junction (58) extending to a surface of the substrate, providing an oxide layer (66,68) over a portion of the substrate and covering the junction at the surface of the substrate, and providing charged ions in a charged ion region of the oxide layer (66,68), extending from over a portion of the region of second conductivity type, over the junction (58), to over a portion of the region, (54) of first conductivity type, characterised in that the step of providing charged ions in the oxide layer comprises providing charged ions at a greater concentration generally adjacent the junction at the surface of the substrate than over the portion of the region of first conductivity type. The present invention thus utilises charged ions in an oxide layer which covers a portion of the substrate, and also covers the junction at the surface of the substrate. The charged ions are arranged so that they extend from generally over that junction at the substrate to over a portion of the substrate itself, the ions being of the same polarity as the conductivity type of that portion of the substrate. The invention accordingly provides a method of and an apparatus for increasing the breakdown voltage of a planar junction of a semiconductor device, which is effective in practice and use, whilst ensuring efficient layout of semiconductor wafer area. The invention is further described below with reference to the remaining Figures of the drawings, in which: Figure 6 shows a device in cross section illustrating a first embodiment of the invention; Figure 7 shows a device in cross section illustrating a second embodiment of the invention; Figure 8 shows a device in cross section illustrating a third embodiment of the invention; and Figure 9 shows a device in cross section illustrating a fourth embodiment of the invention. Referring to Figure 6, an embodiment of the present invention is shown. Certain charged ions 82, 84 of the same polarity type as region 54a, are implanted adjacent to the junction portions 62a, 64a, respectively, at a relatively high energy level, so as to be positioned quite close to the surface 60a of the substrate 52a, the ions 82, 94 extending from the junction portions 62a, 64a only a limited distance over the regions 55a, 57a of the substrate 52a. Subsequent to the implantation of those charged ions 82, 84, further charged ions 86, 88 are implanted at an energy level less than the original energy level, so that the implanted ions 86, 88 are positioned a greater distance from the surface 60a of the substrate 52a than the implanted ions 82, 84, with the second implantation being undertaken so that those implanted ions 86, 88 extend also from generally adjacent the junction portions 62a, 64a to a distance further over the regions 55a, 57a than the implanted ions 82, 84. This results in a depletion region 89 extending further in to the regions 55a, 57a than, for example, the depletion region of the Figure 1 device. Figure 7 shows a variation of the invention wherein the oxide layer portions 90, 92 on either side of the region 56b are stepped, that is, each oxide layer portion is thinnest at its area 90a, 92a adjacent the junction portions 62b, 64b at the surface 60b with the oxide layer areas 90b, 92b being thicker further from the junction portions 62b, 64b and over the regions 55b, 576b, and even thicker further on at 90c, 92c. Charged ions 94a, 94b, 94c, 94d, 94e, 94f (of the same conductivity type as region 54b) are then implanted, all simultaneously and at a certain energy level, so that they position themselves as shown in Figure 7. Thus, the charged ions 94a, 94b adjacent to the region 56b are closest to the surface 60b of the substrate 52b, while the charged ions 94c, 94d are somewhat further from the surface of the substrate 52b and the charged ions 94e, 94f are even further from the surface 60b of the substrate 52b. This results in a depletion region as indicated by the dotted line at 95 when reverse bias voltage is applied. It will thus be seen that through the above-cited techniques, the depletion region of each device can be tailored as chosen to meet the operating parameters needed. Referring to Figure 8, the implanting of charged ions at 100, 102 as described above is combined with field plates 104, 106 to define depletion regions which may be tailored to the specific case. This is also the situation in Figure 9, wherein the implanting of charged ions at 108, 110 as described above is combined with guard rings 112, 114 to define depletion regions which may be specifically configured to meet chosen parameters. It will therefore be readily seen that herein are provided a simple method and structure for increasing the breakdown voltage of a planar junction of a semiconductor device, meanwhile being efficient in use of wafer area, and being applicable to a variety of needs, including a wide range of resistivities and oxide thicknesses. Furthermore, there is no need for lengthy diffusion time for forming the junction, as was a problem in the prior art. It will be understood that the conductivity types of all elements as disclosed can be reversed. In such cases, where the region 54 of any embodiment is N type and the region 56 of any embodiment is P type, the implanted ions can be iodine, chlorine, or boron. Furthermore, multiple doses of charged ions can be used so that the concentrations thereof will vary in the oxide layers as chosen. For example, a structure generally similar to that shown in Figure 6 can be achieved by applying a heavy dose of charged ions in the areas adjacent the junction portions 62a, 64a, with a lighter dose of implanted ions adjacent thereto but extending further therefrom over the regions 55a, 57a. It is to be understood that the scope of the invention is not to be determined by the above description, but only by the following claims.

Claims: A semiconductor device comprising a substrate having a region (54a;54d) of first conductivity type and a region (56a;56d) of second conductivity type together forming a junction (58a) which extends to a surface (60a) of the substrate, an oxide layer (66a;68a) on the surface of the substrate and covering the junction at the surface, and a charged ion region in the oxide layer extending from over a portion of the region of second conductivity type, over the junction, to over a portion of the region of first conductivity type, the polarity of the ions (82,86;84,88) in the ion region being the same as the polarity of the region of first conductivity type, characterised in that the charged ion region in the oxide layer (66a;68a) is of greater concentration generally adjacent to the junction (58a) than over the portion of the region (54a) of first conductivity type. A semiconductor device comprising a substrate having a region (54b) of first conductivity type and a region (56b) of second conductivity type together forming a junction (58b) which extends to a surface (60b) of the substrate, an oxide layer (90'92) on the surface of the substrate and covering the junction at the surface, and a charged ion region in the oxide layer extending from over a portion of the region of second conductivity type, over the junction, to over a portion of the region of first conductivity type, the polarity of the ions (94a-94f) in the ion region being the same as the polarity of the region of first conductivity type, characterised in that the charged ion region is positioned so that the portion thereof generally adjacent to the junction (58b) is closer to the surface of the substrate than the portion thereof over the portion of the region (54b) of first conductivity type. A semiconductor device as claimed in claim 2, wherein the thickness of the oxide layer (90;92) over the charged ion region decreases in the direction from the region (54b) of first conductivity type to the junction (58b). A semiconductor device as claimed in claim 3, wherein the thickness of the oxide layer is decreased by being stepped. A semiconductor device as claimed in any preceding claim, wherein the first conductivity type is P conductivity type, the second conductivity type is N conductivity type, and the charged ions are cesium ions. A semiconductor device as claimed in any one of claims 1-4, wherein the first conductivity type is N conductivity type, the second conductivity type is P conductivity type, and the charged ions are iodine ions. A semiconductor device as claimed in any one of claims 1-4, wherein the first conductivity type is N conductivity type, the second conductivity type is P conductivity type, and the charged ions are chlorine ions. A semiconductor device as claimed in any one of claims 1-4, wherein the first conductivity type is N conductivity type, the second conductivity type is P conductivity type, and the charged ions are boron ions. A semiconductor device as claimed in any preceding claim, wherein a field plate (104;106) are provided above the charged ion region. A semiconductor device as claimed in any one of claims 1-8, wherein guard rings (112,114) of the second conductivity type extend into the region (54d) of first conductivity type adjacent to the region (56d) of second conductivity type." A method of forming a semiconductor device comprising the steps of providing a semiconductor substrate having a region (54) of first conductivity type and a region (56) of second conductivity type, the regions forming a junction (58) extending to a surface of the substrate, providing an oxide layer (66,68) over a portion of the substrate and covering the junction at the surface of the substrate, and providing charged ions in a charged ion region of the oxide layer (66,68), extending from over a portion of the region of second conductivity type, over the junction (58), to over a portion of the region (54) of first conductivity type, characterised in that the step of providing charged ions in the oxide layer comprises providing charged ions at a greater concentration generally adjacent the junction at the surface of the substrate than over the portion of the region of first conductivity type. A method as claimed in claim 11, wherein the step of providing the charged ions comprises implanting a first plurality of charged ions (82,84) at a first energy level into an area close to the surface of the substrate and adjacent the junction (58a) at the surface of the substrate, and implanting a second plurality of ions (86,88) at a second, lower energy level to provide ions spaced from the surface of the substrate further than the ions implanted at the first energy level, the second plurality of ions extending from and adjacent the junction (58a) at the surface of the substrate further therefrom over a portion of the region of first conductivity than said further plurality of ions. A method of forming a semiconductor device comprising the steps of providing a semiconductor substrate having a region (54) of first conductivity type and a region (56) of second conductivity type, the regions forming a junction (58) extending to a surface of the substrate, providing an oxide layer (66,68) over a portion of the substrate and covering the junction at the surface of the substrate, and providing charged ions in a charged ion region of the oxide layer (66,68), extending from over a portion of the region of second conductivity type, over to the junction (58), to over a portion of the region (54) of first conductivity type, characterised in that the step of providing the oxide layer over a portion of the substrate comprises providing a stepped oxide layer (90,92) of a first thickness adjacent the junction (58b) at the surface of the substrate and of a second, greater thickness over the portion of the region (54b) of first conductivity type, and in that the step of providing charged ions in the oxide layer comprises implanting charged ions at a certain energy level into the first and second oxide layer portions.

Inventors: BLANCHARD RICHARD A, COGAN ADRIAN I, BLANCHARD, RICHARD A., COGAN, ADRIAN I.
Applicants: SILICONIX INC, SILICONIX INCORPORATED
Application year: 1988
Main classification: H01L29
IPC codes: H01L29