ANISOTROPIC RECTIFIER AND METHOD FOR FABRICATING SAME

Abstract: A rectifier is fabricated from a P-N junction having a P-type semiconductor layer and an adjacent N-type semi­ conductor layer. A mesa structure is formed in at least one of said layers. Impurities are deposited at the top of the mesa to form a high concentration region in the surface thereof. The impurities are diffused from the top surface of the mesa toward the P-N junction, whereby the mesa geometry causes the diffusion to take on a generally concave shape as it penetrates into the mesa. The distance between the perimeter of the high concentration region and the wafer substrate is therefore greater than the distance between the central portion of said region and the wafer substrate, providing improved breakdown voltage characteristics and a lower surface field. Breakdown voltage can be measured during device fabrication and precisely controlled by additional diffusions to drive the high concentration region to the required depth.

Description: BACKGROUND OF THE INVENTION The present invention relates to rectifiers and more particularly, to a rectifier having a mesa structure and concave doping profile. The structure of the rectifier results in a reduction of the electric field on the surface thereof and a corresponding improvement in breakdown voltage characteristics. Semiconductor rectifiers or "diodes" are used in virtually all electronic products to provide directional control of current flow in a circuit. Diodes are fabricated from a P-N junction which, fundamentally, is a structure containing a P-type semiconductor in contact with an N-type semiconductor. In such a device, current will easily flow in the direction from the P-type material to the N-type material. Current does not, however, flow easily in the opposite direction (i.e. from the N-type semiconductor to the P-type semiconductor). Various methods have been devised to fabricate semiconductor diodes. For example, a P-type semiconductor material can be brought into intimate physical contact with a separate N-type semiconductor material. In practice, however, P-N junctions are usually fabricated by chemically modifying an otherwise uniform crystal to create an abrupt change of doping (i.e., impurity levels) between two portions of the crystal. A semiconductor material frequently used to produce rectifiers is silicon. Silicon P-N junctions will normally break down at the surface of the device, and not in the bulk silicon as desired, unless special precautions well-known in the art are taken. With higher voltage breakdowns, more care has to be taken to prevent surface breakdown than with lower voltage breakdowns. If sufficient precautions are taken, avalanche or "zener" breakdown will occur in the direction opposite to normal current flow (the "reverse bias" direction), and even large electrical energy pulses in the reverse bias direction will not damage the diode. Silicon diodes operated in the zener breakdown mode are useful in maintaining constant voltages (e.g., power supply output voltages) from several volts to several hundred volts with power ratings up to 50 watts or more. At high voltages, it is important for rectifiers to have a surface field strength (voltage per unit area) that is a least one-third to one-half the breakdown voltage of the device. It is known that with proper surface contouring, the peak surface field of P-N junctions can be reduced. See, for example, the article entitled "Control Of Electric Field At Surface Of P-N Junctions", R. L. Davies and F. E. Gentry, IEEE Transactions on Electron Devices , July 1964, pp. 313-323. Pratical problems have arisen, however, in producing such P-N junctions. In practice, diodes are typically manufactured using heavily doped P-type and N-type regions with a high ohmic region in between. The width of the high ohmic region is critical in defining the breakdown voltage of the device. However, in previously known structures, including the contoured structures proposed by Davies and Gentry, a tradeoff occurred because as the width of the high ohmic region was decreased, the breakdown voltage at the surface of the device also decreased to unacceptable levels. It would be advantageous to provide a rectifier having improved voltage breakdown characteristics together with a method for manufacturing such rectifiers wherein precise control of the breakdown voltage is achieved. It would be further advantageous to provide such a rectifier having a surface field that is much lower than the electric field in the bulk semiconductor from which the device is fabricated. A rectifier having a surface field much lower than the electric field in the bulk semiconductor from which the device is fabricated is already known from DE-A-1 489 937. The present invention aims a rectifier structure which can be easily mass produced and have exceptional high temperature reverse bias stability. The invention further aims a rectifier that is very highly passivated. SUMMARY OF THE INVENTION In accordance with the present invention, a method is provided for fabricating a rectifier comprising a P-N junction having a P-type semiconductor layer and an adjacent N-type semiconductor layer with at least one of said semiconductor layers residing in a mesa structure having sidewalls that slope outwardly from a top of said mesa toward the P-N junction. In a preferred embodiment, the mesa will have a frustum shape. Impurities are deposited at the top of the mesa to form a high concentration region sufficient to extend entirely across the top of said mesa structure. The impurities are then diffused from the top surface deeper into the mesa toward the P-N junction, whereby the mesa geometry causes the diffusion to take on a generally concave shape as it penetrates into the mesa, with the distance between the perimeter of the high concentration region and the P-N junction being greater than the distance between the central portion of said region and the P-N junction. The method can comprise the further steps of measuring the breakdown voltage of the P-N junction after the diffusion step, and diffusing the impurities deeper into the mesa to achieve a lower breakdown voltage if the measured voltage is too high. The measuring and diffusing steps can be repeated until a desired breakdown voltage is achieved. The P-N junction for the rectifier can be formed in an epitaxial wafer. Where the wafer has a <100> crystal orientation, the mesa can be anisotropically etched therein. The shape of the resultant mesa is determined by the particular masking pattern used to define the etch. The method of fabricating the rectifier can comprise the further step of applying a passivation layer to the sidewall of the mesa. A rectifier in accordance with the present invention comprises a P-N junction formed from a layer of P-type semiconductor material and an adjacent layer of N-type semiconductor material with at least one of the layers residing in a mesa structure having a sidewall that slopes outwardly from a top of said mesa toward the P-N junction. A high concentration region extends from the top of the mesa toward the P-N junction, the high concentration region having a curved shape adjacent the mesa sidewall, whereby the distance between the perimeter of the high concentration region and the P-N junction is greater than the distance between the central portion of said region and the P-N junction. The rectifier can further comprise a metal layer overlaying at least part of the mesa sidewall to provide a field shield. The depth of the high concentration region can be set to establish a desired breakdown voltage for the rectifier. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of an epitaxial wafer having an N- epitaxial layer on a P+ substrate; Figure 2 is a cross-sectional view of the wafer of Figure 1 on which impurities have been deposited to form an N+ high concentration region; Figure 3 is a cross-sectional view of the wafer of Figure 2 on which a silicon nitride layer has been deposited; Figure 4 is a cross-sectional view of the wafer of Figure 3 after the silicon nitride layer has been selectively removed to define mesa locations; Figure 5 is a perspective view of the wafer after mesas have been etched therein; Figure 6 is a cross-sectional view of the mesas shown in Figure 5 after an oxide passivation layer has been grown on the mesa sidewalls; Figure 7 is a cross-sectional view of a mesa after the N+ high concentration region has been diffused deeper into the mesa toward the P-N junction; and Figure 8 is a cross-sectional view of the mesa of Figure 7 after a metalization layer has been applied to the tops and a portion of the sidewalls thereof. DETAILED DESCRIPTION OF THE INVENTION The rectifier of the present invention utilizes a mesa structure to obtain improved breakdown voltage characteristics and particularly a higher breakdown voltage at the surface of the device. This result is accomplished by providing a high ohmic region in between heavily doped P-type and N-type regions wherein the width of the high ohmic region is greater at its edges than in the central portion thereof. Since the high ohmic portion is wider along the sloped side wall of the mesa than in the bulk semiconductor material, a high breakdown voltage can be achieved at the surface of the device. The rectifier device is fabricated from a P-N junction 10 as shown in Figure 1 having a layer of P-type semiconductor material 12 and an adjacent layer of N-type semiconductor material 14. P-N junction 10 can comprise an epitaxial silicon wafer having a <100> crystal orientation. The N- epitaxial layer 14 is grown on the P+ substrate 12. Impurities are deposited at the top surface of the epitaxial layer 14 to form a high concentration N+ region 16 as shown in Figure 2. Then, as shown in Figure 3, a silicon nitride layer 18 is deposited on top of the high concentration region 16. Conventional techniques (e.g., masking and etching) are used to define a selective pattern in the silicon nitride layer 18. As shown in Figure 4, the pattern in the silicon nitride layer 18 results from the removal of silicon nitride in the areas designated 20, so that the silicon nitride 18 remains only in the areas shown. The resulting pattern defines mesa islands which are subsequently anisotropically etched (using the silicon nitride layer 18 as a mask) to provide mesas 22 as shown in Figure 5. In a preferred embodiment, the mesas are frustum shaped. By using an epitaxial wafer of <100> crystal orientation, the mesa wall angles are consistently reproducible by well-known anisotropic etching techniques. The cross-sectional shape of the mesa is dictated by the mask pattern used. If the pattern of the silicon nitride mask 18 is a circle as shown in Figure 5, then a frustum shaped mesa having a circular cross-section will result. If a square mask pattern is used, a mesa having generally square cross-section with rounded corners will result. Other shapes will be apparent to those skilled in the art. The resulting structure, shown in Figure 5, comprises silicon nitride layer 18 on top of the mesa structure, followed by the high concentration N+ region 16, the high ohmic N- region 14, and the heavily doped P+ region 12 at the base of the mesa. At this point, a thick oxide layer (SiO₂) 26 is grown on the device. The oxide layer grows only where there is no silicon nitride, e.g., on the side wall 24 of each mesa 22. The silicon nitride thereby serves the dual purpose of providing a mask for use in etching the mesa and providing a mask for use in applying the oxide layer. The oxide layer passivates the device. The passivation layer ensures a stable high voltage junction, rendering the surface of the device stable electrically and also rendering the passivated portions impervious to harmful environmental effects. After the silicon dioxide passivation layer is grown, the high concentration region 16 is driven, via conventional diffusion techniques, deeper into the mesa toward the P-N junction. As illustrated in Figure 7, the mesa geometry causes the diffusion to take on a generally concave shape as it penetrates into mesa 22. The concave shape is indicated by a dashed line having perimeter edges 30 and a central portion 28. The distance between the perimeter edges 30 of the high concentration region 16 and the wafer substrate 12 is greater than the distance between the central portion 28 and wafer substrate 12. Accordingly, the high ohmic N- region 14 is wider along the side wall surface 24 of mesa 22 than it is within the central portion of the mesa, resulting in a higher breakdown at the surface of the device. After diffusing the high concentration region into the mesa as shown in Figure 7, the breakdown voltage of the rectifier device can be measured by placing measurement probes across the junction and electrically breaking through the silicon nitride layer 18. If the breakdown voltage is found to be too high, then additional diffusion can be performed to drive the impurities forming the high concentration region even deeper into the mesa. The measuring and diffusing steps can be repeated as many times as necessary until a desired breakdown voltage is achieved. In this manner, it is possible to construct rectifiers having a very precise breakdown voltage, even if the epitaxial layer thickness and resistivity of the ohmic region vary from their intended specifications. This advantage is very helpful in the fabrication of zener diodes, because it is important to provide a precise zener breakdown voltage. In the past, it was often necessary to discard zener diodes during manufacture if they did not meet the rigid breakdown voltage requirements. The present invention obviates the need to waste such devices, since the devices can be optimized for precise voltages during the manufacturing process. After the high concentration region has been driven to the required depth, the silicon nitride layer is removed and metalization layers 32, 34 are deposited on the top and bottom surface of the wafer. Photoresist techniques are used to define required metal patterns on the wafer's surface which will enable leads to be attached when the P-N junctions are packaged as individual rectifiers. The metalization layer 32 can overlay all or part of the mesa slope to act as a field shield as shown in Figure 8. Alternatively, metalization layer 32 can overlay only the top of the mesa, in which case no field shield is provided. After metalization, the individual diodes on each wafer are tested, marked, and the wafer is cut and broken into individual diodes. Each individual diode can then be packaged in a conventional manner. It will be appreciated that the method described above can be modified in various respects without departing from the scope of the present invention. For example, the P-type and N-type materials shown in the drawings can be reversed. Thus, a semiconductor wafer having an N+ substrate with a P-epitaxial layer can be used. In such instance, the high concentration region would be fabricated using a P+ diffusion. The process can also be modified to perform the deposition of the high concentration region after the mesa etch and oxidation. Additional diffusions can be performed to make more sophisticated devices on the top of the mesa. Further, additional diffusions can be performed at different points in the process to lower contact resistance. Other fabrication techniques can be used to form the mesa structure. For example, the mesa can be mechanically cut into the wafer. Regardless of how the mesa is formed, an important aspect of the present invention is to drive the high concentration region toward the P-N junction after the mesa has been formed. In this manner, the diffusion will have the necessary concave shape and the resulting high ohmic region will be wider along its perimeter than in the central portion thereof.

Claims: A method for fabricating a rectifier comprising a P-N junction having a P-type semiconductor layer and an adjacent N-type semiconductor layer with at least one of said semiconductor layers residing in a mesa structure (22) having sidewalls that slope outwardly from a top of said mesa toward the P-N junction and impurities forming a high concentration region (16) in the entire top of said mesa structure (22), wherein the impurities are diffused from the top of said mesa deeper into said mesa toward the P-N junction, whereby this mesa geometry causes the diffusion to take on a curved shape adjacent the mesa sidewalls as it penetrates into the mesa, with the distance between the perimeter of the high concentration region (16) and the P-N junction being greater than the distance between the central portion of said region and the P-N junction. The method of claim 1 comprising the further steps of: measuring the breakdown voltage of the P-N junction after said diffusion step; and diffusing the impurities deeper into said mesa to achieve a lower breakdown voltage if the measured voltage is too high. The method of claim 2 comprising the further step of: repeating said measuring and diffusing steps until a desired breakdown voltage is achieved. The method of claim 1 wherein said P-N junction is formed in an epitaxial wafer. The method of claim 1 wherein said mesa has a frustum shape. The method of claim 1 wherein said mesa is anisotropically etched in a semiconductor material having a <100> crystal orientation. The method of claim 1 comprising the further step of applying a passivation layer to the sidewall of said mesa. The method of claim 1, wherein: the mesa structure is formed in at least the epitaxial layer (14) of an epitaxially grown P-N wafer (12, 14) with the top of the mesa at the top surface of the epitaxial layer and the side wall of said mesa sloping outwardly from the top of the mesa toward the wafer substrate (12); the high concentration semiconductor region (16) is formed in the entire top surface of the mesa; and said high concentration region (16) is driven from the top of said mesa deeper into said epitaxial layer. The method of claim 8 wherein said wafer has a <100> crystal orientation and the mesa shape is formed by the step of: forming a protective masking layer (18) in a selective pattern on top of said epitaxial layer; and anisotropically etching the masked structure in the areas not protected by the masking layer to form the desired mesa shape. The method of claim 8 or 9, comprising the further step of applying a passivation layer (26) on the mesa side wall after the etching step. The method of claim 10 wherein said high concentration semiconductor region (16) is formed after the formation of the mesa shape and the application of said passivation layer. The method of claims 10 and 9 wherein said protective masking layer (18) is left to remain on the top of said mesa (22) after formation of the mesa shape to serve as a mask during the application of said passivation layer (26). The method of claim 8 wherein the mesa is formed into a frustum shape. The method of claim 1 wherein said high concentration semiconductor region (16) is formed prior to the formation of the mesa shape. The method of claim 8 comprising the further step of: depositing a metallization layer (32, 34) on the top and bottom surfaces of said wafer, said metal overlaying at least part of the mesa side wall to provide a field shield. The method of claim 8 wherein a plurality of mesa shapes are formed in the wafer to fabricate a plurality of rectifiers simultaneously, said method comprising the further step of breaking the wafer into individual rectifiers after the fabrication thereof. The method of claim 8 comprising the further steps of: measuring the breakdown voltage of the P-N junction between the top of the mesa and the wafer substrate after said driving step; and driving said high concentration region deeper into said epitaxial layer to achieve a lower breakdown voltage if the measured voltage is too high. The method of claim 17 comprising the further step of repeating said measuring and driving steps until a desired breakdown voltage is achieved. A rectifier comprising a P-N junction formed from a P-type semiconductor layer (12) and an adjacent N-type semiconductor layer (14) with at least one of said semiconductor layers residing in a mesa structure (22) having sidewalls that slope outwardly from a top of said mesa toward the P-N junction, characterized by: a high concentration region (16) extending from the entire top surface of said mesa toward the P-N junction, the high concentration region having a curved shape adjacent the mesa sidewall, whereby the distance between the perimeter of the high concentration region and the P-N junction is greater than the distance between the central portion of said region and the P-N junction. The rectifier of claim 19 further characterized by a metal layer (32) overlaying at least part of the mesa side wall to provide a field shield. The rectifier of claim 19 wherein said mesa (22) is in the shape of a frustum. The rectifier of claim 19 wherein the depth of said high concentration region (16) is set to establish a desired breakdown voltage for the rectifier.

Inventors: EINTHOVEN, WILLEM G., MITCHELL, MUNI M.
Applicants: GENERAL INSTRUMENT CORPORATION
Application year: 1986
Main classification: H01L29
IPC codes: H01L29, H01L21