Accelerator-based electron beam technologies for modification of bipolar semiconductor devices

Radiation processing technologies for static and dynamic parameters modification of silicon bipolar semiconductor devices implemented. Devices of different classes with wide range of operating currents (from a few mA to tens kA) and voltages (from a few volts to 8 kV) were processed in large scale including power diodes and thyristors, high-frequency bipolar and IGBT transistors, fast recovery diodes, pulsed switching diodes, precise temperature- compensated Zener diodes (in general more than fifty 50 device types), produced by different enterprises. The necessary changes in electrical parameters and characteristics of devices caused by formation in the device structures of electrically active and stable in the operating temperature range sub-nanoscale recombination centres. Technologies implemented in the air with high efficiency and controllability, and are an alternative to diffusion doping of Au or Pt, γ-ray, proton and low-Z ion irradiation.


Introduction
The development of physics theory and experimental equipment and methods in radiation material science during the second half of the 20th century, as well as actively seeking of new technology tools for solid-state electronics and semiconductor devices have led to the formation and application of radiation technologies in semiconductor manufacturing process due to their high efficiency. Design and creation of high-performance, high-reliable, easy-to-use and maintain electron beam accelerators with energies up to 10 MeV, fundamental improvement of individual functional units [1][2][3][4][5][6], established leading position of radiation technologies based on electron accelerators in today's semiconductor industry in comparison with other methods of radiation treatment. The effectiveness of accelerated electrons able to replace the atoms from crystal lattice sites was shown by different research teams in relation to devices based on silicon and other materials. 6 To whom any correspondence should be addressed.
Recombination statistics based upon a single dominant level have been used to predict the relative characteristics of gold-diffused, platinum-diffused, and electron-irradiated silicon power devices [7]. The measured characteristics of power rectifiers are shown to be in good agreement with these calculations. The results are also shown to be applicable to power thyristors. In [8] the impact of the electron and proton beam density onto bipolar transistors parameters modification rate was revealed. Achievement of fast recovery characteristics for silicon power diodes and optimization of pulse power thyristors on industrial batches level was shown in [9][10][11][12][13]. In [14,15] shown that to fulfil the requirements of dynamic and static characteristics of bipolar static induction transistor, the irradiation/anneal processing is crucial. Defect distributions features and silicon power devices parameters modification under electron beam irradiation in a wide energy range studied in [16][17][18]. Extended capacitance relaxation research method for radiation defects parameters measurements in semiconductors device structures is given in [19].

Main features and advantages
Main features and advantages of electron beam technology for the modification of semiconductor devices as follows:  possibility of processing in air,  electron flux density adjustment by beam current and distance from the output window due to scattering in air;  providing a large processing area due to operation in the scanning mode;  very ample opportunities for the design and construction of under-beam equipment, including beam control systems and a variety of holders, automated sample delivery systems, freestanding vacuum/environment chamber with a beam input window for special investigations;  simultaneous pass-through processing of several semiconductor wafers, arranged one behind the other due to deep penetrating ability of accelerated electrons (range in Si about 1-2 cm);  samples heating by beam and temperature control if necessary;  small footprint and favorable cost of equipment, easy to use and maintain. Mentioned advantages make it possible to do a preliminary experiment quickly and realize large volume technology to provide needed combination of electrical parameters for specific device.

Brief critical analysis of alternative processes
For comparison, we present the main disadvantages of alternative methods. So gold doping process results in a negative growth of reverse currents via p-n junctions and characterizes by low concentration reproducibility from wafer to wafer and within the same wafer. The platinum doping process allows to minimize leakage currents, but does not achieve the maximum rate of recombination values and also characterizes by low reproducibility.
Sources of -particles provide defects formation at shallow depths (no more than 30 microns), have a small processing area and intensity that limits the range of technological tasks solved and is suitable for low volume processing of small-area wafers or some laboratory experiments.
Widely used for different tasks gamma-ray Co 60 source creates displacements in semiconductors transferring small amounts of energy; needs a long time and associates with some organizational difficulties. Most sub-nanoscale recombination centers formed have relatively low annealing temperatures and not optimum electrophysical properties.
Proton-based (several MeV-level) technology requires very complex, expensive and energy consuming equipment, which casts doubt on the cost-effectiveness of this tool today yet, due to the difficulty of high volume wafer processing as well. In addition, hydrogen doping can cause different time and current instabilities in some devices. Nowadays real application in device technology is in-air proton irradiation with an initial (in vacuum) energy of 25 MeV [20,21]  automatic feeding system, which allows to process with wide (about 60 microns) struggle mechanically strong extended silicon-on-molybdenum power device structures.

Experimental results
This section contains recent results obtained by the authors for the devices of different classes that demonstrate possibilities of accelerator-based electron beam technologies. Fig. 1 shows experimental dependence of forward voltage drop U F as a function of electron fluence Ф e (after stabilizing annealing) for silicon low voltage (6,4 V) diode structure. Volt-ampere characteristics evolution after electron irradiation is shown in Fig. 2. This structure uses as a compensated one for precise thermally compensated Zener diode. Needed value of U F = 0,54 V could be provided in preliminary Au-doped structures at fluences about 3•10 17 cm -2 .    High power electrical energy converting systems require the use of high-voltage and high-current thyristors, adapted for reliable operation in series and/or parallel connection. Thyristors for parallel and serial connection need the highest identity on-state current-voltage and reverse recovery characteristics respectively. For this purpose technology of precise parameters control of HV HC silicon thyristors by splitting semiconductor elements into specific groups with different initial parameters and control the reverse recovery time of diode-satellites during irradiation. Test diode chips and thyristor elements were made from high resistance NTD silicon with close characteristics to provide high stability of lifetime reduction coefficient values. Test diode structures were 4x4 mm 2 in size and were cut from Al-metallized wafer after anode/cathode diffusion that used for power diode manufacturing. During irradiation test diode structure fixed at the center of the target using contact mandrel that connected with accelerator control unit via a coaxial cable. Lifetime control scheme and control equipment for precise modification of thyristors are shown in Fig. 5 and 6. Typical results of precise modification of thyristor parameters by 8 MeV electron irradiation using the considered above lifetime control hardware are presented. Fig. 7 shows the modification of reverse recovery charge spread for silicon high-voltage thyristors (type T273-1250-44, 1250A, 4400V) after electron-beam treatment. Fig. 8 shows the statistical distribution of on-state voltage drop for the same thyristors.     Represented above and some other obtained experimental results useful for real industrial applications summarized in the Table 1. Thus, the accelerator-based electron beam technologies could be effectively applied for manufacturing of competitive traditional and innovative solid state devices, elaboration new approaches in culling methods and radiation hardness investigation.