Title of Invention

A METHOD OF PRODUCING SILICON POWER SEMICONDUCTOR THYRISTORS WITH NARROW REVERSE RECOVERY PARAMETERS

Abstract

The present invention relates to provides a method of controlling reverse recovery parameters of silicon power semiconductor thyristors through a two-step method comprising an initial coarse-correction using gold diffusion at wafer-processing stage followed by exposure of fully fabricated chips to gamma rays for fine tuning. As the second step of irradiation reversible and incremental dosages suffice. The above helps in lowering the manufacturing cycle time. This is proven to be a versatile method especially to meet stringent custom-specific requirements and has been adapted for industrial manufacturing.

Full Text

FIELD OF INVENTION The present invention relates to a method of fine-tuning reverse recovery parameters of silicon power semiconductor thyristors to improve manufacturing yield and cycle time during industrial manufacturing. BACKGROUND OF INVENTION Reverse recovery parameters of semiconductor devices such as diodes and thyristors depend strongly on the rate of recombination of electrons and holes. During switching from ON to OFF state, a finite time known as reverse recovery time [tRR] elapses before carriers recombine and are completely depleted from the base layer of semiconductor device. The charge and current during this period are termed as reverse recovery charge [QRR] and reverse recovery current [IRR] respectively. These recovery parameters are important especially in circuits where the devices are series connected as an stack. The series connection demands that the thyristors are within a narrow band with both upper and lower specification limits so as to enable them switch-off concurrently and share the stack voltage equally. Unequal sharing results in reverse voltage in excess of the rated values when the thyristors become electrically damaged. Generally, two alternative approaches are employed for control of minority carrier lifetime. One method uses diffusion of heavy metal atoms such as gold or platinum into silicon [1-3]. These create energy levels within band gap, which act as recombination centers that determine the reverse recovery charge and current. The carrier lifetime can be controlled by the concentration of gold solution, process temperature and rate of cooling. The other method creates recombination centers through lattice damages arising out of exposure to high-energy beam of neutron; portion electron or gamma particles [3,4]. The carrier lifetime,..in this case, is determined by the energy and dosage of radiation. Heavy metal diffusion using gold or platinum is essentially a high-temperature process (800-900 °C) that is carried out on silicon wafer during wafer processing stage. Thereafter, the wafer is processed through a series of relatively lower temperature process such as deposition of electrode layers, application of silicone-rubber, etc. to complete the chip fabrication. Hence, reprocessing through metal diffusion for any further tuning of recovery parameters is not possible, once the chip is fabricated and measured. Also the effect cannot be reversed by any other means. These limitations hamper realization of recovery parameters in a close band and with a reasonable manufacturing yield. OBJECTS OF INVENTION An object of present invention is to propose a method of fine-tuning the recovery parameters of fully fabricated chips, which eliminates the limitations of prior art method of single-step heavy metal diffusion. Another object of present invention is to propose a method of fine-tuning the recovery parameters of fully fabricated chips, which meets the narrow band specifications of reverse recovery parameters. A further object of present invention is to propose a method of fine-tuning the recovery parameters of fully fabricated chips, which is easily adaptable to an industrial manufacturing process. SUMMARY OF INVENTION Accordingly, there is provided a method of producing silicon power semiconductor thyristors with narrow reverse recovery parameters, the method comprising the steps of: (a) forming a convention p+ -n-p-n-n+ thyristors structure by diffusing silicon wafers with impurities; (b) subjecting the diffused wafers to a gold diffusion process; (c) joining the wafers to molybdenum plates by adapting an eutectic alloying process followed by beveling and passivation of p-n junctions terminating at the cylindrical edge; (d) subjecting the fully fabricated chips produced by step (c), to a plurality of electrical measurements that include forward voltage drop, and reverse recovery parameters, the forward conduction and reverse recovery parameters having an opposed relationship, thereby restricting the recovery parameters to a narrow band; (e) exposing the gold-diffused thyristors chips produced by step (c) to gamma rays for a duration that establishes a prescribed dose of gamma rays at calculated dose rate; and (f) subjecting the gamma-irradiated thyristor chips produced under step step (e) to a stabilization-annealing process in which the chips being annealed for about 12.00 hours at a temperature around 200°C to ensure stabilizing of the lattice damages, wherein the gold diffusion process is carried-out at a temperature of 800-830°C and soak time of 2-3 hrs followed by cooling to room temperature. The invention discloses a two-step method to employ gold diffusion as an initial coarse-correction process at wafer processing stage followed by a step of exposing the chips to gamma rays. The gamma ray is administered based on coarsely adjusted values of recovery parameters for fine-tuning. In case of inadequate correction, gamma irradiation is repeated in incremental steps. A suitable annealing process is employed to partially or totally reverse excessive radiation damages. The present invention aims at improving manufacturing yield and cycle-time in an industrial environment. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 Schematic diagram of reverse recovery characteristics of thyristor showing parameters VT, QRR, IRR, tRR and tq. Fig. 2 Histograms of reverse recovery charge [QRR] as obtained using gold diffusion and as shifted by gamma irradiation. QRR measured at ITRM=500A, di/dt=-0.5A/µs, Tj=90°C; Specification:0.6-0.9 mC. Fig. 3 Trade-off curve - Forward voltage drop [VT] versus reverse recovery charge [QRR] for a 3200V/1000A phase control thyristor used in High Voltage Direct Current (HVDC) power stations. QRR measured at ITRM=500A, di/dt=- 0.5A/µs, Tj=90°C; Specification:0.6-0.9 mC. VT measured at IT=1200A, Tj=25 °C. Specification Fig. 4 Histograms of reverse recovery current [IRR] as obtained using gold diffusion and as shifted by gamma irradiation. IRR measured at ITRM=1000A, di/dt=-1.5A/µs, Tj=120°C; Specification: 34-42 A. Table-1 Incremental correction of reverse recovery charge [QRR] using repeated gamma-irradiation steps. QRR measured at ITRM=500A, di/dt=-0.5A/us, Tj=90°C. Table-2 Reversal of gamma irradiation effect by annealing. QRR measured at ITRM=650A, di/dt=-5A/us, Tj=12u°C. DETAILED DESCRIPTION OF THE INVENTION Chip fabrication with single-step gold diffusion: Silicon wafers (Float zone, n-type, ϕ54-mm) of 140 Ohm-cm resistivity and 800- microns thick are diffused with p-type (Aluminium, Boron) and n-type (Phosphorus) impurities to form a conventional p+-n-p-n-n+ thyristor structure. The diffused wafers are subjected to gold diffusion process carried out at a temperature range of 800-830 °C and soak time of 2 to 3 hours followed by cooling to room temperature. Gold tetrachloride solution is used as dopant source. The wafers are joined to 1.5 to 2.5 mm thick molybdenum by means of a eutectic alloying process. Aluminium (20-microns thick) and silver (2-microns thick) layers are deposited using physical vapour deposition to form anode, cathode and gate electrode. P-n junctions terminating at the cylindrical edge are beveled and passivated to complete chip fabrication. The fully fabricated chips are subjected to various electrical measurements that include forward voltage drop (VT) and reverse recovery parameters QRR, IRR, trr and tq. These forward and reverse parameters have trade-off relationship with one-another. The recovery parameters need to be brought to a narrow custom- specified band. The forward voltage drop increases as the correction is applied to decrease the recovery parameters. The process control becomes a critical exercise, as the drop has an upper specification limit. Limitations of gold diffusions as a single-step control process: Non-conforming parameters cannot be corrected at this stage through re- diffusion of gold since wafers are now fully fabricated into chips. Also, the partial or complete reversal of gold recombination centers is not possible. These limitations of gold diffusion demand stringent process control measures to achieve minimum non-conformities. Gamma irradiation as secondary control process: After realizing the coarsely corrected values of the recovery parameters using gold diffusion, the secondary fine-correction is achieved by using gamma irradiation process to further lower and bring the values within the acceptable limits. Gold-diffused thyristor chips are exposed to gamma rays using BARC-make GC900 gamma chamber that uses 60Co isotope (half-life = 5.2714 years) as radioactive source. The dose administered is in terms of Mega-rads and is based on correlation data pre-constructed for a given type of semiconductor chip. Dose rate as applicable for the source at the date and time of irradiation is computed in Mega-rads/hour using the decay rate and original radioactivity (in Curies). The chips are exposed for a duration (hours) that establishes the prescribed dose at the calculated dose rate. Gamma irradiation is relatively inexpensive and dosage accurately controllable. Gamma chamber uses Cobalt isotope as radioactive source having certain half life. Using decay rate and original radioactivity for Cobalt isotope, chips are exposed to radiation. With different experimental settings and test results, we establish the dose rate. Stablilization-annealing process: The gamma irradiated thyristor chips are annealed for duration of 12 hours at 200 °C which is well above the maximum continuous operating junction temperature of 125 °C for thyristor in order to ensure that the lattice damages are stabilized and no changes in thyristor characteristics take place during testing and in real-time operation. Reversal-annealing process : An annealing method in the temperature range of 360-400 °C for duration of 60- 150 minutes is employed for reversing the recovery parameters in cases of excessive gamma-irradiation. Subsequent to reversal, re-irradiation is carried out to achieve the values within acceptable limits. EXAMPLES The above invention has been verified in two specific cases of manufacture of a capsule-type control thyristor of rating 3200V/1000A. In either case, the end- application uses series-connected thyristor to build stacks designed to support high voltage (several kilovolts). In the first example, the thyristors were manufactured against a customer specification for a narrow band of reverse recovery charge [QRR] for use in converter and inverter stations of High Voltage Direct Current [HVDC]. The target specification was QRR = 0.6 to 0.9 mC and VT≤ 1.70V. Fig-2 shows the histograms of QRR where the values are contained in the required specification band by applying a combination of both gold diffusion and gamma irradiation processes. Fig-3 shows the QRR-VT trade-off curve with the data realized in the specification window. The second case illustrates data on thyristors manufactured for VFD-application [Variable Frequency Drive] with a narrow band specification of reverse recovery current [IRR] in the range 34-42 A. Fig. 4 shows the histograms of IRR. The initial correction to a coarse level helps in cutting down manufacturing cycle time through relatively short fine-tuning steps. Manufacturing yield is improved compared to single-step gold diffusion process, which is neither correctible nor reversible. The data on incremental correction of QRR on fully fabricated thyristor chips is presented in Table-1. In this case, thyristors were subject to repeated gamma irradiation to realize the specified values. Fig.1: It describe the reverse recovery characteristic of thyristor showing parameters Vt, Irr, Qrr, trr and tq. The thyristor is turned off from conduction to blocking mode by withdrawal of anode current. As the response is not instantaneous, it is controlled by inductance in the load circuit. The current starts reducing by rate of di/dt and passes through zero but withdrawal continues at the same di/dt rate due to inductive effect and current reaches its peak (Irr). Further thyristor starts supporting reverse voltage. Now onwards, reverse current reduces exponentially and get influenced by recombination of carriers (holes & electrons). trr is defined as the time between when current passes through zero and the time where the current has dropped to 20% of Irr. When the thyristor is turned off, as the response is not instantaneous, a finite time is elapsed before switch is sufficiently off, that response time is called turned off (tq) time. Fig 2: It shows histograms of reverse recovery charge (Qrr). Our target for Qrr is 0.6-0.9 mC for a particular type of thyristor 3200 V/1000 A. Without gold diffusion & gamma irradiation we get Qrr in range of 1.8-2.2 mC. With the gold diffusion process a coarse correction is done so that we are getting Qrr in range of 1.0-1.90 mC. Further a fine correction is done by gamma irradiation so that we are getting Qrr in range of 0.6-0.9 mC which is our required range. Fig. 3: It is trade off curve for Forward voltage drop Vt versus reverse recovery charge. As we reduce the reverse recovery charge Qrr with the help of processes like gold diffusion and gamma irradiation, correspondingly forward voltage drop Vt is increased. For a particular type of phase control thyristor-3200 V/1000 A, without gold diffusion and gamma irradiation, the obtained window of Qrr & Vt is 1.2-1.8 mC and 1.40-1.50 V, with the process of gold diffusion and gamma irradiation, the available window is 0.6-0.9 mC and 1.50-1.70 V which is the required range. Table-2 shows reversal of QRR using an annealing procedure [360-400 °C, 60-150 minutes]. The annealed chips are re-irradiated, wherever necessary, so as to realize the target values. REFERENCES: 1. BJ.Baliga, Modren Power Devices, John Wiley & sons, 1987, pp.36-58, 380-387 and 410-413. 2. A.G.MILNES, Deep Impurities in Semiconductors, Wiley, New York, 1973. 3. BJ.Baliga and E.Sun, "comparison of gold, platinum and electron irradiation for controlling lifetime in power rectifiers', IEEE Transactions on Electron Devices, ED-24, 685-688 (1977). 4. R.O. Carlson, Y.S.Sun, and H.B. Assalit, "Lifetime control is silicon power devices by electron or gamma irradiation", IEEE Transactions on Electron Devices, ED-24, 1103-1108 (1977). 5. 'Method of controlling leakage currents and reverse recovery time of rectifiers by hot electron irradiation and post-annealing treatments,' US patent No. 4,137,099. 6. 'Non-uniform minority carrier lifetime distribution in high performance silicon power devices', US Patent No. 6,828,690. 7. 'Semiconductor switching device and method of controlling a carrier lifetime in a semiconductor switching device', US patent No. 6,465,871. 8. 'Semiconductor switching device having different carrier lifetimes between a first portion serving as a main current path and the remaining portion of the device,' US patent No. 6,100,575. 9. Thyristor manufacturing method and thyristor,' US patent No. 6,163,040. 10.'Multilayer semiconductor element', US patent No. 4,298,882. 11.'Method for producing large-area power semiconductor components', US patent No. 4,806,497. 12.'Method of annealing fully-fabricated, radiation damaged semiconductor devices,' US patent No. 5,017,508. a) Technical description on table 1: This table shows correction in Qrr in different stages so that target value of Qrr can be achieved-which has been shown in table. For 2 chips at sl no. 15 & 16, targetted value has been achieved in three stages of irradiation. b) Technical description on table 2: This table describes the reversal of gamma irradiation effect by annealing the chip at temp 360-400 deg C for duration of 60-150 min. Table shows that by combination of annealing temp & time, the gamma irradiation effect can be reversed by 33%-88%. WE CLAIM: 1. A method of producing silicon power semiconductor thyristors with narrow reverse recovery parameters, the method comprising the steps of: (a) forming a convention p+ -n-p-n-n+ thyristors structure by diffusing silicon wafers with impurities; (b) subjecting the diffused wafers to a gold diffusion process; (c) joining the wafers to molybdenum plates by adapting an eutectic alloying process followed by beveling and passivation of p-n junctions terminating at the cylindrical edge; (d) subjecting the fully-fabricated chips produced by step (c), to a plurality of electrical measurements that include forward voltage drop, and reverse recovery parameters, the forward conduction and reverse recovery parameters having an opposed relationship, thereby restricting the recovery parameters to a narrow band; (e) exposing the gold-diffused thyristors chips produced by step (c) to gamma rays for a duration that establishes a prescribed dose of gamma rays at calculated dose rate; and (f) subjecting the gamma-irradiated thyristor chips produced under step step (e) to a stabilization-annealing process in which the chips being annealed for about 12.00 hours at a temperature around 200°C to ensure stabilizing of the lattice damages, wherein the gold diffusion process is carried-out at a temperature of 800-830°C and soak time of 2-3 hrs followed by cooling to room temperature. 2. The method as claimed in claim 1, wherein the silicon wafers are n-type, 054mm of 140 ohm-cm resistivity and 800 microns thickness. 3. The method as claimed in claim 1 or 2, wherein the silicon wafers diffused with p-type and n-type impurities. 4. The method as claimed in claim 1, wherein the molybdenum plate is of thickness between 1.5 to 2.5 mm, and wherein the eutectic alloying step comprises depositing aluminium of 20-microns thick and silver of 2-microns thick layers using physical vapor deposition process to form anode, cathode and gate electrodes. 5. The method as claimed in claim 1, wherein the gamma rays are generated by using a gamma chamber that uses 60Co isotope, and wherein the prescribed dose administrated is selected in terms of Megarads and is based on correlation data pre-constructed for a given type of semiconductor chip. 6. The method as claimed in claim 1, comprising the step of reversing the recovery parameters in cases of excessive gamma-irradiation, wherein a temperature range of 360°C to 400°C for duration of 60-150 minutes is employed for reversing. ABSTRACT TITLE" A METHOD OF PRODUCING SILICON POWER SEMICONDUCTOR THYRISTORS WITH NARROW REVERSE RECOVERY PARAMETERS" The present invention relates to provides a method of controlling reverse recovery parameters of silicon power semiconductor thyristors through a two-step method comprising an initial coarse-correction using gold diffusion at wafer-processing stage followed by exposure of fully fabricated chips to gamma rays for fine tuning. As the second step of irradiation reversible and incremental dosages suffice. The above helps in lowering the manufacturing cycle time. This is proven to be a versatile method especially to meet stringent custom-specific requirements and has been adapted for industrial manufacturing.

Documents:

00279-kol-2007 correspondence-1.1.pdf

00279-kol-2007 form-18.pdf

0279-kol-2007-abstract.pdf

0279-kol-2007-assignment.pdf

0279-kol-2007-claims.pdf

0279-kol-2007-correspondence others.pdf

0279-kol-2007-description (complete).pdf

0279-kol-2007-drawings.pdf

0279-kol-2007-form1.pdf

0279-kol-2007-form2.pdf

0279-kol-2007-form3.pdf

279-KOL-2007-(02-05-2012)-CORRESPONDENCE.pdf

279-KOL-2007-(24-08-2012)-AMANDED CLAIMS.pdf

279-KOL-2007-(24-08-2012)-AMANDED PAGES OF SPECIFICATION.pdf

279-KOL-2007-(24-08-2012)-CORRESPONDENCE.pdf

279-KOL-2007-(24-08-2012)-FORM-13.pdf

279-KOL-2007-(30-12-2011)-ABSTRACT.pdf

279-KOL-2007-(30-12-2011)-AMANDED CLAIMS.pdf

279-KOL-2007-(30-12-2011)-DESCRIPTION (COMPLETE).pdf

279-KOL-2007-(30-12-2011)-DRAWINGS.pdf

279-KOL-2007-(30-12-2011)-EXAMINATION REPORT REPLY RECIEVED.PDF

279-KOL-2007-(30-12-2011)-FORM-2.pdf

279-KOL-2007-(30-12-2011)-FORM-5.pdf

279-KOL-2007-(30-12-2011)-OTHERS.pdf

279-KOL-2007-CANCELLED PAGES.pdf

279-KOL-2007-CORRESPONDENCE 1.1.pdf

279-KOL-2007-CORRESPONDENCE.pdf

279-KOL-2007-EXAMINATION REPORT.pdf

279-KOL-2007-FORM 13.pdf

279-KOL-2007-FORM 18.pdf

279-KOL-2007-GPA.pdf

279-KOL-2007-GRANTED-ABSTRACT.pdf

279-KOL-2007-GRANTED-CLAIMS.pdf

279-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

279-KOL-2007-GRANTED-DRAWINGS.pdf

279-KOL-2007-GRANTED-FORM 1.pdf

279-KOL-2007-GRANTED-FORM 2.pdf

279-KOL-2007-GRANTED-FORM 3.pdf

279-KOL-2007-GRANTED-FORM 5.pdf

279-KOL-2007-GRANTED-SPECIFICATION-COMPLETE.pdf

279-KOL-2007-REPLY TO EXAMINATION REPORT.pdf