RU2395868C1 - METHOD FOR MANUFACTURING OF INTEGRATED SCHOTTKY-pn DIODES BASED ON SILICON CARBIDE - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method for manufacturing of integrated Schottky-pn diodes on the basis of silicon carbide includes development of p-areas with rectangular profile of alloying by implantation of boron ions with energies from the range of values (190÷700) keV and doses from the range of (1*10¹³÷1*10¹⁵) cm^-2 in weakly alloyed epitaxial layer of silicon carbide of n-type, arranged on intensely alloyed substrate of silicon carbide of n-type, at room temperature through photoresist mask for simultabneous formation of the main protective pn-transition, floating protective rings and subcontact p-areas. Implantation is carried out by means of photolithography with resolution from 0.5 micrometre to 1 micrometre, then annealing is performed in inertial atmosphere at the temperature from the range of (1500÷1600)°C for the time of (30÷90) minutes under layer of applied graphite. Further surface is oxidised, sputtering is carried out onto reverse side of nickel plate, as well as its burn-in to shape ohm contact, opening windows in oxide and sputtering of nickel Schottky contact with further annealing in inertial atmosphere.

EFFECT: invention makes it possible to produce integrated Schottky-pn diodes with increased breakthrough voltage by simpler and cheaper method with preservation of efficiency.

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Description

The invention relates to the field of electronic technology, and more specifically to a technology for creating high-voltage semiconductor diodes, and can be used to create integrated Schottky-pn silicon carbide diodes.

The widespread use of Schottky diodes (DShs) has always been constrained by the fact that during reverse bias, excessive leakages occur in them, and the voltage of the DSH breakdown is usually lower than that of pn junctions. In 4H-SiC (based on silicon carbide), at high reverse voltage, the electric field in the plane of the metal contact reaches ~ 10 ⁶ V / cm. At such large fields, volumetric leaks can occur due to field and / or thermal field emission of electrons from a metal into a semiconductor. To suppress (at least partial) such leaks, JBS structures (embedded under the Schottky p-region) were proposed and JBS diodes (integrated Schottky pn diodes) were created.

The leakage currents along the edge of the metal contact of the semiconductor device during reverse bias are explained by the concentration of electric field lines along the periphery of the contact. There are many ways to create edge contour protection: advanced metallization [J.N. Su and A.J.Steckl // Inst. Phys. Conf. Ser. No 142: Chapter 1, 1996, p.697]; epitaxial [K. Ueno, T. Urushidani et al. // Inst. Phys. Conf. Ser. No 142: Chapter 1, 1996, p.693] and implanted [Grekhov I.V., Ivanov P.A., Ilyinskaya N.D., Konkov O.I., Potapov A.S., Samsonova T.P. // FTP, 42 (2), 2008, p.211-214] pn-transition guard rings; floating guard rings [D.C. Sheridan, J.N. Merrett, J.D. Cressler et al. // Materials Science Forum. Vol.353-356, 2001, p.687] and others. From the point of view of work efficiency, floating guard rings are effective, allowing in some cases to almost completely get rid of edge effects [P.A. Ivanov, I.V. Grekhov, T.P. Samsonova // FTP FTP, 2005, 39 (12 ), p.1476].

A known method of creating JBS diodes based on silicon carbide [S.J. Kim, S. Kim, S. C. Kim, I. H. Kang, K. H. Lee and T. Matsuoka // Materials Science Forum. Vol.556-557, 2007, p.869] with the simultaneous implantation of guard rings and JBS structures. This method involves the hot implantation of aluminum through a mask of silicon oxide with ions of different energies to form a box profile of the implanted impurity. Postimplantation annealing is carried out in an argon atmosphere for 30 minutes at a temperature of 1650 ° C. After annealing, a nickel-titanium ohmic contact forms on the reverse side of the plate by sputtering, which is burned in a nitrogen atmosphere at a temperature of 900 ° C for 1.5 hours. Nickel Schottky-contact, thickened with aluminum, is sprayed on the upper side of the plate and does not burn.

However, this method has disadvantages. First, the need to use micron and even submicron photolithography in the process (due to the fact that diffusion of aluminum in silicon carbide at reasonable temperatures is negligible [F. Giannazzo, F. Roccaforte, D. Salinas, and V. Raineri. Mat. Science Forum, 600-603, (2009), p.603]), which makes such a method very expensive in industrial production. Secondly, the low switching speed of the diodes created by this technology, because Owing to the small ionization energy of aluminum, the concentration of free holes in the 4H-SiC <Al> layers is significantly higher, and with an increase in the hole concentration in the p-region, the injection coefficient of the pn junction increases.

The closest method (prototype) to the claimed invention in terms of features is a method of creating integrated Schottky-pn diodes based on silicon carbide (4H-SiC) with boron floating guard rings and JBS structures (contact pn junctions) [WO 2006122252, publ. 16-11-2006, US Application US 2006255423]. According to this method, on the upper side of an n-type 4H-SiC plate with an n-type epitaxial layer through a mask made of photoresist, with a resolution of 0.05 μm, boron ion implantation is carried out to a depth of 1 μm to simultaneously create guard rings and JBS structures. In the upper part of the rings, additional implantation creates lightly doped p-regions to increase speed. Postimplantation annealing is performed under a graphite layer in an Ar + SiH ₄ gas mixture at temperatures (1300 ÷ 1800) ° C for a short time (so that no appreciable boron diffusion is observed). After annealing, the implanted surface is oxidized. An ohmic contact is formed on the reverse side by sputtering and burning nickel at a temperature of (600 ÷ 1200) ° С in an inert atmosphere (argon or nitrogen). The windows opened in the oxide on the upper side of the plate are sprayed with a Schottky nickel contact, which is annealed at a temperature of (200 ÷ 800) ° С. The diodes created in this way are highly responsive.

The main disadvantages of the method are its complexity and high cost due to the need to use high-resolution photolithography to create JBS structures and a guard ring system located at close distances to increase the breakdown voltage, and to form additional lightly doped p-regions for high speed. Despite the complexity of the method, only 1200 V of the breakdown voltage of the diode was achieved in the final device (43% of the theoretical limit).

The present invention solves the problem of creating integrated Schottky-pn diodes with increased breakdown voltage in a simpler and cheaper way while maintaining performance.

The problem is solved by the method of manufacturing integrated Schottky-pn diodes based on silicon carbide, including the creation of p-regions with a rectangular doping profile by implantation of boron ions with energies from the interval of values (190 ÷ 700) keV and doses from the interval (1 × 10 ¹³ ÷ 1 × 10 ¹⁵ ) cm ^-2 into a lightly doped epitaxial layer of n-type silicon carbide located on a heavily doped n-type silicon carbide substrate at room temperature through a photoresist mask to simultaneously form the main protective pn junction, floating guards rings and contact p-regions, performed by photolithography with a resolution of 0.5 μm to 1 μm, annealing in an inert atmosphere at a temperature from the interval (1500 ÷ 1600) ° С for a time (30 ÷ 90) minutes under a layer of deposited graphite, surface oxidation sputtering on the reverse side of the nickel plate and burning it to form an ohmic contact, opening the windows in the oxide and spraying the nickel Schottky contact followed by annealing in an inert atmosphere.

The essence of the invention is explained by the fact that boron ions with high energy are implanted in a lightly doped epitaxial layer of n-type silicon carbide, which contributes to their deep penetration into the layer, and with prolonged annealing at high temperatures, boron diffuses both in the vertical and lateral directions, as a result, a smooth distribution profile of the implanted impurity is formed, forming extended regions of overcompensation, and as a result of these diffusion processes at a large penetration depth of boron under ntaktnye p-type region, the main protective pn-junction and the floating guard rings are formed on the necessary close distances from each other to ensure a high breakdown voltage, without the use of photolithography with high resolution, and also provides high speed without generating additional lightly doped regions.

The studies revealed that the efficiency of the guard rings and JBS structures is influenced by the depth of the guard pn junction, as well as the distribution profile of the impurity. Using secondary ion mass spectrometry, it was shown that during thermal annealing of 4H-SiC layers implanted with boron at room temperature, a noticeable diffusion acceleration of boron atoms introduced by implantation occurs even at temperatures (1500--1600) ° С. It was found that the mechanism of diffusion from the implanted source is substantially nonequilibrium (transient enhanced diffusion, TED), stimulated by radiation disturbances. Thanks to the TED mechanism, a pn junction implanted by boron in SiC can be formed at a depth greater than the range of boron ions. In addition, the obtained impurity distribution profile is a pn junction with an unsharp boundary, i.e., the concentration of acceptor impurities (boron) in the n layer gradually decreases into the epitaxial layer from the surface, forming an extended region of overcompensation. This allows one to form high-voltage planar pn junctions in silicon carbide and diodes based on them.

To achieve high performance integrated Schottky-pn diodes in implanted p-regions in the immediate vicinity of the Schottky metal and in the prototype and in the proposed method, lightly doped p-regions are created and a Schottky barrier is formed between the implanted layers and the Schottky metal. Due to this barrier, connected in series in the opposite direction with respect to the pn junctions of the JBS structures and guard rings, injection from areas with boron implantation is substantially suppressed, which makes such integrated Schottky-pn diodes comparable in switching speeds to conventional Schottky diodes. But in the proposed method, the lightly doped region near the Schottky metal, unlike the prototype, is created by deep boron implantation with its subsequent diffusion during prolonged annealing, which does not require additional operations and is technologically simpler and cheaper than in the prototype.

The authors found that under implantation regimes of boron ions with energies (190 ÷ 700) keV different from the prototype and similar doses (1 × 10 ¹³ ÷ 1 × 10 ¹⁵ ) cm ^-2 , at the greater depth than in the prototype, the main protective transition, floating guard rings and JBS structure, and in this case, in contrast to the prototype, it is possible to use photolithography with relatively low resolution - from 0.5 μm to 1 μm, because the subsequent annealing of the implanted layers proposed by the authors for (30 ÷ 90) min at temperatures (1500 ÷ 1600) ° С is accompanied by boron diffusion processes both in the vertical and lateral directions and brings together the created rings. In this case, as the authors revealed, a smooth distribution profile of the implanted impurity is formed with a concentration maximum located at a depth of (0.7 ÷ 1.0) μm to the surface, extending to a depth of (1 ÷ 3) μm, i.e., the concentration of acceptor impurities (boron) in n- the layer gradually decreases to the surface and deep into the epitaxial layer, forming extended regions of overcompensation.

The diodes formed in this way with guard rings and a JBS structure can achieve a breakdown voltage of up to 72% of the theoretically possible (up to 1800 V when creating a diode on a 4H-SiC film with an impurity concentration of 2 × 10 ¹⁵ cm ^-3 with a thickness of 12 μm) while maintaining speed ( like the prototype). Studies have shown that the integrated Schottky-pn diodes created by this method are not inferior to the Schottky diodes in terms of switching speed. This method is simpler and cheaper than the prototype method, because it is possible to use a less accurate and, as a result, simpler photolithography (accuracy up to 1 μm versus 0.05 μm in the prototype), and, in addition, in this method, the Schottky barrier between the depleted p-region and Schottky metal is created due to deeper implantation impurities, and not a separate implantation, as in the prototype.

The method is as follows.

On a n-type 4H-SiC wafer with an n-type 4H-SiC epitaxial layer, a photoresist mask (4 ÷ 5) μm thick is created using photolithography with a resolution of (0.5 ÷ 1.0) μm. Through the formed mask, cold implantation of boron ions with several energy values from the interval (190 ÷ 700) keV and doses from the interval (1 × 10 ¹³ ÷ 1 × 10 ¹⁵ ) cm ^{-2 is performed} to create p-regions with a rectangular doping profile. After removing the photoresist and encapsulating the irradiated surface with a layer of graphite, annealing is performed in an inert medium at a temperature from the range of (1500 ÷ 1600) ° С for (30 ÷ 90) minutes. After that, a protective layer of graphite is burned in an oxygen atmosphere and a thermal oxide is formed on the surface of the epitaxial layer. Further, nickel is sprayed onto the back side of the plate prepared by mechanical grinding, which is burned in vacuum, for example, at a temperature from the interval (900 ÷ 1050) ° С for a time (10 ÷ 60) minutes [T. Toda, Y.Ueda and M. Sawada // Mat. Science Forum, 338-342, (2000), p.989]. On the front side of the plate, the windows open in the oxide, into which the Schottky nickel contact is dusted, which is annealed at a temperature of (200 ÷ 400) ° С for an hour.

Example 1

According to the claims, a method was carried out in which an n-type silicon carbide plate with an n-type epitaxial film 12 μm thick and doping of 2 × 10 ¹⁵ cm ^-3 was taken. On the epitaxial layer, using photolithography with a resolution of 0.9 μm, performed on an AMK-2104-12 type alignment device, a mask was created from a photoresist type AZ1518 with a thickness of 5 μm with windows for implantation of the main security transition 50 μm wide, 4 floating guard rings 10 wide μm and an interval between them of 5 μm, a JBS structure with a width of 8 μm with an interval of 5 μm. Through the formed mask, the High Voltage Engineering Europe installation cold implanted boron ions with energies of 200 keV and 350 keV and doses of 3 × 10 ¹³ cm ^-2 and 6 × 10 ¹³ cm ^-2, respectively. After the photoresist was removed and the irradiated surface was encapsulated with a layer of graphite, annealing was performed in argon at a temperature of 1500 ° C for 60 minutes in an installation with internal resistive heating. As a result of annealing, pn diffusion junctions were formed at a depth of 2 μm, and the intervals between the guard rings and JBS structures decreased to 3 μm. After that, a protective layer of graphite was burned in a stream of dry oxygen (500 cm ³ / min) and a thermal oxide 300 nm thick was formed on the surface of the epitaxial layer. Nickel was sprayed onto the polished back side of the Balzers installation, which was burned in a vacuum at a temperature of 950 ° C for 15 minutes in a VUP-5 vacuum station. On the upper side in the oxide, windows were opened in which nickel was sprayed as a Schottky contact, which was annealed in vacuum at a temperature of 200 ° C for an hour. Thus, an integrated Schottky-pn diode with a breakdown voltage of 1800 V (72% of the theoretical limit) was created.

Example 2

The same as in example 1, but the implanted ion energies were 100 and 150 keV. As a result, the devices obtained had a breakdown voltage of about 1200 V, i.e. turned out to be no better than the prototype.

Example 3

Same as in example 1, but the implanted ion energies were 750 and 1050 keV. As a result, the resulting devices had a breakdown voltage of up to 1800 V, however, to achieve such energies at the High Voltage Engineering Europe installation, it is necessary to use 2- and 3-charged boron ions. To set the required dose of implantation, the irradiation process had to be carried out for two days, which led to a multiple rise in price of the created device.

Example 4

The same as in example 1, but the task was to create a photolithography resolution of 0.1 μm. As a result, at the combination installation AMK-2104-12 it was not possible to achieve such a resolution, the device was not created. To create such a resolution requires significantly more complex and expensive equipment.

Example 5

The same as in example 1, but the resolution is 2 microns. As a result, the guard rings were shorted together, the breakdown voltage was only 600 V.

Example 6

The same as in example 1, but the annealing time of 15 minutes. As a result, boron diffusion turned out to be insignificant, the device withstood a breakdown voltage of 1150 V.

Example 7

The same as in example 1, but the annealing time is 2 hours. As a result, the surface degradation of the epitaxial layer began. The created devices withstood a breakdown voltage of 800 V.

Claims (1)

A method of manufacturing integrated Schottky-pn diodes based on silicon carbide, including the creation of p-regions with a rectangular doping profile by implantation of boron ions with energies from the range of values (190 ÷ 700) keV and doses from the interval (1 · 10 ¹³ -1 · 10 ¹⁵ ) cm ^-2 in the lightly doped epitaxial layer of silicon carbide, n-type, disposed on a substrate heavily doped n-silicon carbide type, at room temperature through a photoresist mask to simultaneously form the primary protective pn-junction floating guard rings and podkon active p-regions, performed by photolithography with a resolution of 0.5 to 1 μm, annealing in an inert atmosphere at a temperature from the interval of 1500 ÷ 1600 ° C for a time of 30 ÷ 90 min under a layer of deposited graphite, surface oxidation, deposition on the back of the plate nickel and its burning to form an ohmic contact, opening windows in the oxide and deposition of nickel Schottky-contact with subsequent annealing in an inert atmosphere.