IRRADIATION
Heavy ion testing was conducted at the Texas A&M University Cyclotron facility [54] in both 2016 and 2018. The following chapter includes details about the 1200 V MOSFETs and JBS diodes used, the cyclotron beam characteristics for the various tests, the equipment used for each test, the general test procedures, and finally a section on minor observations made during testing that are not covered in the following chapters.
6.1 Devices under Test
The SiC JBS power diodes tested were 4th generation CPW-1200-S020B bare die, model number C4D020120A for a commercially packaged version [55]. These devices have a 1200 V blocking voltage and are rated for 20 A maximum current. The diodes were made of 4H SiC with an epitaxial layer approximately 10 µm thick.
The SiC power MOSFETs used in this experiment were 2nd generation CPM2-1200-0080B bare die from Wolfspeed [56]. The MOSFETs’ specifications are 1200 V, 80 mΩ, and 36 A for the blocking voltage, drain to source on resistance, and maximum drain to source current, respectively.
The devices are enhancement-mode MOSFETS. The devices are fabricated on 4H SiC and consist of vertical MOSFETs symmetric along one axis while repeating along another, as shown in Fig. 5. The stripes are repeated with a pitch of approximately 10 µm, determined by optical examination of the metal layers. The channel width is known to be approximately 1 µm and the epitaxial thickness is known to be approximately 10 µm.
All devices were packaged in open-cavity TO-247 packages with a conformal coating applied to prevent arcing between the die, bond wires, and package at high voltages. The conformal coating
26 used was Parylene-C, applied by Paratronix Inc [57].
6.2 Experimental Setup – Cyclotron Facility Information
Ions were used from both a 15 MeV/amu tune and 25 MeV/amu tune, meaning the energy of particles normalized to the atomic mass of each particle was either 15 MeV/amu or 25 MeV/amu.
The ions and relevant properties are indicated in Table I. Additional information regarding the various ions is available online [58]. Devices were mounted to the testing frame using a custom board which placed die 3 cm from the aramica window, shown in Fig. 12. Though current during testing was expected to remain below a few milliamps, the high breakdown voltages motivated a test board that only held one device at a time. No heat sinking was used for any of the irradiation runs as device power dissipation rarely exceeded 0.5 W for more than a couple seconds.
TABLE I. OVERVIEW OF ION LETS USED AND THEIR PROPERTIES
Ion Energy (MeV) Range in Si (µm) LET after 3cm air (MeV*cm2/mg)
N 350 402 0.9
Ne 300 293 3.5
Ar 1000 480 5.7
Ar 600 212 8.1
Cu 1187 155 18.7
Kr 2095 315 19.8
Kr 1259 150 28.6
Xe 1934 128 56.2
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Fig. 12. Circuit board with device mounted in front of the beam window. Photograph includes the structure the facility provides to mount circuit boards vertically.
6.3 Testing Procedures
During the first test trip, the Keithley K2410 source measuring unit (SMU), which can provide power to a device and measure the voltage and current provided at the same time, was used both to provide power to each device and measure the resulting leakage current during bias. The SMU was interfaced over a general purpose interface bus (GPIB) connected to an Agilent e5810 gateway. Both devices were placed in the irradiation chamber next to the device, and control was provided by a computer in the control room sent over local area network (LAN) to the e5810.
An Agilent B1505A power device analyzer was used to measure each device’s current-voltage (IV) characteristics prior to heavy ion testing. A few of the devices were also characterized after heavy-ion testing in an effort to gain information about potential failure mechanisms.
During the second test trip, an Agilent B1505A was instead controlled over LAN and GPIB
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through an Agilent e5810 to provide IV characteristics between each test run as well as to reverse bias the device and measure the development of leakage current during irradiation runs.
During both experiments, devices were biased at a fixed voltage during heavy-ion irradiation. A device was replaced if the device was destroyed due to SEB or the leakage current exceeded the power compliance of the measurement instrument. In most instances, each irradiation run consisted of a total fluence of 10,000 ions/cm2 using a flux of approximately 100 ions/(s*cm2). This flux was chosen as the lowest flux available which still remained stable over time. If the device still met the above criteria to be considered functioning, the bias was increased for the next run. Some additional test runs were completed on one device returning to a lower bias to determine whether or not existing leakage current influenced the magnitude of new leakage current increases. In total, 123 runs were completed using 28 diodes, and 75 runs were completed using 16 MOSFETs.
6.4 General Observations
Initial experiments determined the relationship between ion LET and reverse bias for the onset of leakage current degradation. After establishing a lower boundary for degradation events, devices were biased above this boundary but below the experimentally-observed threshold for SEB for these devices.
Fig. 13 shows a typical example of degradation of leakage current due to heavy-ion exposure.
Degradation consisted of discrete increases in leakage current of varying magnitude as previously reported in [9], [10]. At a given bias, the average increase in leakage current with respect to total fluence converged to a constant value as total fluence per run increased. This observation held true regardless of the amount of prior degradation in the device, which was tested by taking a couple of devices and irradiating them at 300 V, then 400 V, and then again at 300 V to compare the average
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increase in leakage current before and after the 400 V irradiation.
Fig. 13. Example strip chart showing discrete increases in leakage current increases over time for a single device during irradiation. This data was taken from a test run of a 1200V Wolfspeed MOSFET.
One test using the 600 MeV Ar beam was completed with the diode angled 30° from normal incidence. This results in an increase in the effective LET of roughly 15%, which, if LET were the sole variable in the radiation influencing SELC, would result in increased average leakage. However, the results were the opposite, with average leakage reduced by roughly 3-4 decades. This result has been since confirmed with more detail in Javanainen et al. [59]. Additionally, use of an inline resistor to prevent SEB, which is effective for silicon MOSFETs, proved as equally ineffective for SiC MOSFETs as previously shown for SiC diodes [9].
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