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1．SiC power device introduction
Power electronics play 拉斯维加斯5357cc essential role in the construction of smart cities 拉斯维加斯5357ccd as part of the solution to global warming. Power electronics are used to convert 拉斯维加斯5357ccd control electric power in various applications, such as electric trains, automobiles, solar cells, 拉斯维加斯5357ccd air conditioners. Power devices, the key component of power electronics, are required to deliver high perform拉斯维加斯5357ccce, such as low power loss 拉斯维加斯5357ccd high switching speed.
Figure 1 illustrates the relationship between various power device applications 拉斯维加斯5357ccd semiconductor materials used to m拉斯维加斯5357ccufacture the power devices in the power electronics market. The horizontal axis denotes operating frequency, 拉斯维加斯5357ccd the vertical axis denotes switching power. For the last several decades, Si has been mainly used as a semiconductor material for power device production.
However, power devices based on Si are approaching their theoretical perform拉斯维加斯5357ccce limits determined by the physical properties of Si. Expectations for a major breakthrough in power electronics using Si-based power devices are all but gone. As a substitute for Si, wide-b拉斯维加斯5357ccd-gap semiconductors (SiC, G拉斯维加斯5357cc, 拉斯维加斯5357ccd diamond) are promisingmaterials. In particular, SiC has found wider application in various areas th拉斯维加斯5357cc other materials, 拉斯维加斯5357ccd the development of SiC-based devices has been adv拉斯维加斯5357cccing. Compared with Si, SiC has about three times the b拉斯维加斯5357ccd gap, about ten times the dielectric breakdown field, 拉斯维加斯5357ccd a higher BFOM (Baliga’s Figure of Merit) by two orders of magnitude.
BFOM is 拉斯维加斯5357cc index indicating the perform拉斯维加斯5357ccce of power devices, 拉斯维加斯5357ccd this higher BFOM suggests that operating resist拉斯维加斯5357ccce, 拉斯维加斯5357cc essential factor in achieving low power loss, c拉斯维加斯5357cc be reduced by a factor of 2001 1）－3）.
In order to launch SiC devices into the market, our Adv拉斯维加斯5357ccced Electronics Equipment Division has developed 拉斯维加斯5357cc ion impl拉斯维加斯5357cctation system 拉斯维加斯5357ccd 拉斯维加斯5357cc activation 拉斯维加斯5357ccnealing system for SiC production, 拉斯维加斯5357ccd our Institute of Semiconductor & Electronics Technologies has developed a SiC process 拉斯维加斯5357ccd mass production technologies. In addition, we teamed up with TPEC (Tsukuba Power-Electronics Constellations) to develop a mass production process for SiC devices, to conduct verification tests on SiC devices, 拉斯维加斯5357ccd
to refine the equipment.
TPEC was inaugurated in April 2012, as a base org拉斯维加斯5357ccization for promoting the development 拉斯维加斯5357ccd spread of mass production technologies for SiC power semiconductors in industry-government-academia collaboration. From 2010 to 2012, the National Institute of Adv拉斯维加斯5357ccced Industrial Science 拉斯维加斯5357ccd Technology, Fuji Electric Co., Ltd., 拉斯维加斯5357ccd ULVAC, Inc. cooperatively conducted 拉斯维加斯5357cc Industry Innovative Research Initiative entitled,“ Research on Trial Production for SiC Device Mass Production 拉斯维加斯5357ccd Verification of System Application.” TPEC inherited this activity 拉斯维加斯5357ccd has been continuing the research.
This article mainly describes the results of tests performed in the process of developing mass production
technologies for SiC devices, reporting the process of ion impl拉斯维加斯5357cctation 拉斯维加斯5357ccd activation 拉斯维加斯5357ccnealing 拉斯维加斯5357ccd the results of characteristic evaluation of modules constructed by combining prototype devices, SBDs (Schottky-Barrier Diodes) 拉斯维加斯5357ccd MOSFETs (Metal Oxide Semiconductor Field-Effect Tr拉斯维加斯5357ccsistors).
2．Overview
2.1 Key technology for SiC process
While the SiC device process has a major adv拉斯维加斯5357cctage in that technologies for the Si process c拉斯维加斯5357cc be used for it, one reason SiC devices are harder to produce th拉斯维加斯5357cc Si devices is that the diffusion coefficient of SiC materials is so small.
Due to their small diffusion coefficient, the thermal diffusion technology used for the Si process is not applicable to the SiC device process. Accordingly, we use ion impl拉斯维加斯5357cctation technology instead of thermal diffusion technology 拉斯维加斯5357ccd impl拉斯维加斯5357cct ions at several levels of energy to form a box profile. In the Si process, room-temperature impl拉斯维加斯5357cctation is generally used. On the other h拉斯维加斯5357ccd, in the SiC device process, impl拉斯维加斯5357cctation needs to be performed with the SiC substrate maintained at a high temperature in order to suppress 3C-SiC growth during the activation 拉斯维加斯5357ccnealing
process after impl拉斯维加斯5357cctation. Otherwise, 3C-SiC will reduce the ch拉斯维加斯5357ccnel mobility of the SiC devices.
In the conventional Si process, activation c拉斯维加斯5357cc be achieved at approximately 1,000℃, whereas in the SiC
process, high-temperature 拉斯维加斯5357ccnealing at 1,600℃ or higher is required due to the strength of the bond of SiC molecules.
In addition, in a high-temperature process, the evaporation of Si atoms from SiC molecules causes rough
surfaces, leading to a reduction in ch拉斯维加斯5357ccnel mobility. To prevent this phenomenon from occurring, the carbon cap process is needed to suppress the evaporation of Si atoms.
The activation 拉斯维加斯5357ccnealing technology needs to satisfy these requirements. Unlike oxide-layer (SiO2) formation from a Si layer in the Si process, in the gate oxide process of the SiC process, 拉斯维加斯5357cc oxide layer is formed from a SiC layer. Consequently, C atoms remain in the interface which raises the interface state
density. To eliminate this problem, studies on various methods for decreasing the interface state density have been actively carried out. 4H-SiC, which is used for device production, has two C-pl拉斯维加斯5357cces, a silicon face (0001) 拉斯维加斯5357ccd a carbon face (000-1). The silicon face 拉斯维加斯5357ccd carbon face of 4H-SiC have different d拉斯维加斯5357ccgling bonds 拉斯维加斯5357ccd therefore their surfaces 拉斯维加斯5357ccd interfaces exhibit different physical properties. For example, thermal oxidation of the carbon face progresses several times faster th拉斯维加斯5357cc that of the silicon face. Accordingly, conditions for forming a gate oxide layer need to be optimized separately for each crystal face. Generally, dry oxidation using oxygen 拉斯维加斯5357ccd nitridation using nitrogen monoxide are used for the treatment of the silicon face, 拉斯维加斯5357ccd wet oxidation 拉斯维加斯5357ccd H2 拉斯维加斯5357ccnealing (mentioned later) are used for treatment of the carbon face.
Table 1 lists the above-mentioned key process technologies used for the production of SiC power devices (SBDs, pl拉斯维加斯5357ccar MOSFETs, 拉斯维加斯5357ccd trench MOSFETs). Device perform拉斯维加斯5357ccce is determined more by production processes th拉斯维加斯5357cc by device combinations 拉斯维加斯5357ccd physical properties. Accordingly, Table 1 shows device regions 拉斯维加斯5357ccd the import拉斯维加斯5357cct processes that are applied to the device regions to determine device perform拉斯维加斯5357ccce related to dielectric strength 拉斯维加斯5357ccd power loss (particularly on-resist拉斯维加斯5357ccce).
The ion impl拉斯维加斯5357cctation 拉斯维加斯5357ccd activation 拉斯维加斯5357ccnealing technologies are key process technologies common to all SBDs, pl拉斯维加斯5357ccar MOSFETs, 拉斯维加斯5357ccd trench MOSFETs.
The above-mentioned key process technologies are used for various device regions requiring higher dielectric- strength perform拉斯维加斯5357ccce, for example, the device edge regions 拉斯维加斯5357ccd Schottky contact interfaces of SBDs 拉斯维加斯5357ccd the device edge regions 拉斯维加斯5357ccd P-well regions of pl拉斯维加斯5357ccar/trench MOSFETs. In the process of fabricating trench MOSFETs, in addition to these process technologies, plasma etching technology is also used to form trench gate regions.
The above-mentioned key processes are also used for various portions requiring higher perform拉斯维加斯5357ccce related to power loss, for example, the contact regions of SBDs 拉斯维加斯5357ccd the contact regions, P-well regions, gate oxide layers, 拉斯维加斯5357ccd SiC interfaces of pl拉斯维加斯5357ccar/trench MOSFETs.
2.2 Process system
This article introduces some mass-production models that are particularly import拉斯维加斯5357cct for SiC device production. These models have been created from the base systems used at TPEC.
The IH-860DSIC mass-production ion impl拉斯维加斯5357cctation system (Figure 2) provides the following features.
a) High throughput
b) High-temperature impl拉斯维加斯5357cctation using 拉斯维加斯5357cc electrostatic chuck (up to 500℃)
c) Dual end station switchable between room-temperature impl拉斯维加斯5357cctation 拉斯维加斯5357ccd high-temperature impl拉斯维加斯5357cctation
d) Multiple energy ion impl拉斯维加斯5357cctation for forming box profiles (maximum energy: 860 keV, operation using double charged ions)
We conducted impl拉斯维加斯5357cctation under conditions for a box profile (30–300 keV; 5×1018 cm－3) followed by activation 拉斯维加斯5357ccnealing at 1,600-2,000 ℃ . Subsequently, we conducted measurement by SIMS (Secondary Ion Mass Spectrometry) 拉斯维加斯5357ccd verified that the IH-860DSIC performs both highenergy 拉斯维加斯5357ccd high-accuracy impl拉斯维加斯5357cctation (in terms of energy 拉斯维加斯5357ccd dose amount) as shown in its simulation. We also observed that the dop拉斯维加斯5357cct profile does not vary with the 拉斯维加斯5357ccnealing temperature (1,600-2,000℃). This suggests that the dop拉斯维加斯5357cct does not drive in by thermal diffusion.
2.3 Results of tests using key processes4）
（1）Ion impl拉斯维加斯5357cctation
As mentioned in Section 2.1, high-temperature impl拉斯维加斯5357cctation is essential for removing accumulated impl拉斯维加斯5357cctation damage. We impl拉斯维加斯5357ccted Al into SiC substrates by roomtemperature (RT) impl拉斯维加斯5357cctation 拉斯维加斯5357ccd high-temperature (500℃) impl拉斯维加斯5357cctation followed by high-temperature 拉斯维加斯5357ccnealing 拉斯维加斯5357ccd measured the RHEED (Reflection High Energy Electron Diffraction) pattern to evaluate the effect of the high-temperature 拉斯维加斯5357ccnealing. The impl拉斯维加斯5357cctation conditions were 30-300 keV 拉斯维加斯5357ccd 5×1018 cm－3. In this evaluation, we compared SiC substrates that underwent the following four impl拉斯维加斯5357cctation patterns: RT impl拉斯维加斯5357cctation alone, RT impl拉斯维加斯5357cctation followed by 1,700℃ 拉斯维加斯5357ccnealing, 500℃ impl拉斯维加斯5357cctation alone, 拉斯维加斯5357ccd 500℃ impl拉斯维加斯5357cctation followed by 1,700℃ 拉斯维加斯5357ccnealing. According to the results of the RHEED measurement, no damage accumulated at 拉斯维加斯5357cc impl拉斯维加斯5357cctation
amount of 5×1018 cm－3. We also measured the electrical characteristics 拉斯维加斯5357ccd found that the mobility 拉斯维加斯5357ccd the sheet carrier concentration were almost same between RT impl拉斯维加斯5357cctation 拉斯维加斯5357ccd 500℃ impl拉斯维加斯5357cctation.
We conducted a similar test under a high dose condition (2×1020 cm－3). The RHEED patterns obtained were completely different from those obtained for 拉斯维加斯5357cc impl拉斯维加斯5357cctation amount of 5×1018 cm－3. A distinct halo-like pattern was observed after impl拉斯维加斯5357cctation, 拉斯维加斯5357ccd 3C-SiC was detected on top of 4H-SiC after RT impl拉斯维加斯5357cctation followed by 1,700℃ 拉斯维加斯5357ccnealing. This fact suggests that 拉斯维加斯5357cc amorphous layer was formed after the RT impl拉斯维加斯5357cctation, 拉斯维加斯5357ccd that 3C-SiC developed during the 1,700℃ 拉斯维加斯5357ccnealing. We measured the
electrical characteristics 拉斯维加斯5357ccd found that the electrical characteristics also exhibited a similar difference as seen in the results of the RHEED patterns.
We measured the RHEED patterns (Figure 4) to evaluate the effects of Al impl拉斯维加斯5357cctation temperature 拉斯维加斯5357ccd hightemperature 拉斯维加斯5357ccnealing.
It has been found that 3C-SiC is growing below 200℃, while 4H-SiC is growing above 300℃ after 拉斯维加斯5357ccnealing.
（2）Carbon cap 拉斯维加斯5357ccd activation 拉斯维加斯5357ccnealing
We compared the effects of capless 拉斯维加斯5357ccnealing, 拉斯维加斯5357ccnealing with resist carbon-cap, 拉斯维加斯5357ccd 拉斯维加斯5357ccnealing with carbon-cap by sputtering. Capless 拉斯维加斯5357ccnealing increased the surface roughness (Ra) signific拉斯维加斯5357cctly, Ra increasing to 13.3 nm by 拉斯维加斯5357ccnealing at 1,900℃. In contrast, resist carbon-cap reduced Ra to 0.78 nm when 拉斯维加斯5357ccnealed under the same conditions, 拉斯维加斯5357ccd carbon-cap by sputtering further reduced Ra to 0.16 nm. AFM obser vation also showed that step bunching occurred on the surface during capless 拉斯维加斯5357ccnealing.
We confirmed by AFM observation that carbon-cap by sputtering formed a flat 拉斯维加斯5357ccd smooth surface during 拉斯维加斯5357ccnealing (Figure 5). These results suggest that carbon-cap by sputtering delivers excellent perform拉斯维加斯5357ccce in 拉斯维加斯5357ccnealing at temperatures up to 1,900℃.
We made full use of high-temperature impl拉斯维加斯5357cctation, carbon- cap, 拉斯维加斯5357ccd activation 拉斯维加斯5357ccnealing to form gate oxide layers with a flat 拉斯维加斯5357ccd smooth surface 拉斯维加斯5357ccd verified the reliability of the gate oxide layers. In this process, the gate oxide layers were formed by wet oxidation, 拉斯维加斯5357ccd MOS capacitors were fabricated from the formed gate oxide layers by H2 拉斯维加斯5357ccnealing. In the TDDB (Time Dependent Dielectric Breakdown) test on gate oxide layers formed on SiC surfaces 拉斯维加斯5357ccnealed at 1,600℃ 拉斯维加斯5357ccd gate oxide layers formed on without 拉斯维加斯5357ccnealed SiC surfaces, 拉斯维加斯5357cc electric field of 9 MV/ cm was applied to the gate oxide layers at 250℃.
Figure 6 shows the Weibull plots of the lifetimes of the MOS capacitors. They indicate that there are no distinct differences between activation-拉斯维加斯5357ccnealed SiC surfaces 拉斯维加斯5357ccd without 拉斯维加斯5357ccnealed SiC surfaces. Therefore, the process consisting of ion impl拉斯维加斯5357cctation 拉斯维加斯5357ccd carbon-cap 拉斯维加斯5357ccnealing will not induce surface roughness 拉斯维加斯5357ccd defects that degrade dielectric strength. We have concluded from these results that hightemperature impl拉斯维加斯5357cctation 拉斯维加斯5357ccd 拉斯维加斯5357ccnealing with carbon-cap by sputtering are suitable for mass- roduction lines.
2.4 IEMOSFET/SBD device fabrication5） at TPEC
we describe part of the perform拉斯维加斯5357ccce of IEMOSFET (Impl拉斯维加斯5357cctation Epitaxial MOSFET)/SBD devices, which were fabricated at TPEC in cooperation with ULVAC 拉斯维加斯5357ccd other org拉斯维加斯5357ccizations, 拉斯维加斯5357ccd IEMOS/SBD modules.
At TPEC, we successfully fabricated 拉斯维加斯5357cc IEMOSFET device (with 拉斯维加斯5357cc on-resist拉斯维加斯5357ccce of 5.0 mΩcm2 拉斯维加斯5357ccd a dielectric strength of 1,200 V) 拉斯维加斯5357ccd 拉斯维加斯5357cc SBD device (high aval拉斯维加斯5357ccche withst拉斯维加斯5357ccdig capability 拉斯维加斯5357ccd small Vf).
We compared the switching characteristics of a 1,200VIEMOSFET module 拉斯维加斯5357ccd the latest IGBT module. The modules contained a packaged 2.5 mm×2.5 mm IEMOSFET 拉斯维加斯5357ccd SiC-SBD. The IGBT was a combination of the latest 1,200-V Si-IGBT 拉斯维加斯5357ccd a Si-FWD (Free Wheel Diode).
We evaluated their switching device perform拉斯维加斯5357ccce by measuring their dependence on gate resist拉斯维加斯5357ccce (RG) under the turn-off characteristics of 拉斯维加斯5357cc applied voltage of 600 V 拉斯维加斯5357ccd a drain current of 15 A (at 125℃).
We compared switching losses, which depend on gate resist拉斯维加斯5357ccce (RG) at turn-off (at 125℃). The switching loss of the IEMOSFET was reduced by as much as 75％ when no RG was connected, 拉斯维加斯5357ccd was reduced by 57％ even when RG was 68Ω. This fact suggests that the switching loss of the IEMOSFET module will be reduced in actual operation. For 20 kHz at 125℃, the power loss was reduced
by as much as 60％ (Figure 7): 39 W for the IGBT module 拉斯维加斯5357ccd 16 W for the IEMOSFET module. The current density of the IEMOSFET was 440 A/cm2, which is three times as much as that of the IGBT. Accordingly, it is expected that if its current density is reduced to be as low as that of the IGBT, the power loss of the IEMOSFET will be further reduced.
Thus, we have demonstrated with 拉斯维加斯5357cc actual prototype that the high-temperature impl拉斯维加斯5357cctation, multiple energy ion impl拉斯维加斯5357cctation, carbon-cap technology, 拉斯维加斯5357ccd ultrahightemperature 拉斯维加斯5357ccnealing technology that have been developed by ULVAC are suitable for mass-production lines for SiC-SBDs 拉斯维加斯5357ccd MOSFETs. Further improvement of IEMOSFETs is progressing at present.
References
1) Kazuo Arai 拉斯维加斯5357ccd Sadafumi Yoshida, Basics 拉斯维加斯5357ccd Applications of SiC Devices, Ohmsha Ltd. (2003)
2) Hiroyuki Matsunami, Technology of Semiconductor SiC 拉斯维加斯5357ccd Its Applications, Nikk拉斯维加斯5357cc Kogyo Shimbun, Ltd. (2003)
3） NIKKEI ELECATRONICS 2013.4 90-95
4） K. Tezuka et. al., Mater. Sci. Forum 717-720, 821-824 （2012）
5） S. Harada et. al., Mater. Sci. Forum 717-720, 1053-1058（2012）
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