The Creation and Potential Cell Structures of SiC Devices

Aly Mashaly, Manager Power Systems Department at Rohm Semiconductor
Mineo Miura, SiC Power Device Engineer at Rohm Semiconductor

The first part of this article series described the creation of Silicon Carbide (SiC) substrate wafers, starting with the production of the raw material (SiC powder) to the so-called epi-ready SiC substrate wafers. 

The second part deals with the potential structures of SiC devices, focusing on different structures including SiC Schottky Barrier Diodes (SBDs), planar SiC MOSFETs and double trench MOSFETs, showing that cell structures significantly variate the physical properties and the performance of the final product. Finally, production testing procedures followed by Rohm for quality assurance purposes are discussed. 

Silicon (Si) and Silicon Carbide (SiC) Semiconductor Comparisons

AC/DC, DC/AC, DC/DC and AC/AC converters are the most popular power electronic systems. The efficiency of conventional power electronics technologies usually varies between 85 and 95 percent. In other words, approximately 10% of electrical energy is dissipated in form of heat at each power conversion stage. Generally speaking, the performance of power semiconductors is considered to be the main limiting factor for the efficiency of power electronics. Therefore, developing high-voltage and low-loss power semiconductors is a prerequisite for building the power grids of the future. 

Compared to silicon (Si) semiconductors, the electrical field strength of SiC is almost ten times higher (2.8MV/cm vs. 0.3MV/cm). The increased field strength of SiC material enables the deposition of a thinner layer structure, which is known as epitaxial layers on the SiC substrate. Its thickness is about one-tenth of that of Si epitaxial layers. At the same breakdown voltage, the doping concentration of SiC can be two orders of magnitude higher than of its Si equivalent. This reduces the device’s specific on resistance (RonA), resulting in a significant reduction of its conduction losses (Figure 1). 

Material property comparison between Si and SiC

Figure 1: The thinner layer stack enabled by the increased breakdown field strength of SiC leads to lower conduction losses

According to laws of semiconductor physics, the specific on resistance (RonA) increases dramatically with the breakdown voltage. Due to the above-mentioned properties of SiC, the RonA value of SiC will be 100 times lower at high voltages compared to Si. Because of its low RonA characteristics at high voltage, there is no need for using minority carrier device structure normally used in silicon high voltage devices like IGBTs and FRDs. In SiC power devices, majority of carrier devices like MOSFETs and SBDs are used for 600 to 3.3kV voltage range. Due to the absence of minority carriers in current conduction, the switching speed of SiC is dramatically improved, which lead to a dramatic reduction in switching losses. The mentioned features make SiC a very promising material for high-voltage applications, where thermal management is particularly important. 

on resistance of SiC

Figure 2: Specific on resistance (RonA) vs. breakdown voltage

Specific on resistance (RonA) vs. breakdown voltage

Figure 3: Many physical properties make SiC an attractive proposition for power electronics applications

Challenges facing power electronic systems have increased significantly in recent years. Requirements such as weight and efficiency play a predominant role. Furthermore, total system costs and efforts must be low during the production phase, without degrading the quality and robustness of the end product. Thanks to its physical properties, SiC has an enormous potential to meet the requirements of these challenges and the associated market trends. In power electronic systems, thermal design plays an important role in achieving systems featuring high power density and compact size. SiC is perfectly suited for these applications, as it offers three times better thermal conductivity than Si semiconductors. In addition, SiC supports higher operating temperatures (even over 250° C is in principle possible) compared to Si semiconductors because of its wider band gap (three times of Si). 

Development and Production of a SiC Devices

The SiC substrate wafer was described in detail in part 1 of this article series. These substrate wafers act as the base material for the subsequent production of SiC devices. Specific structures consisting of epitaxial layers, doping processes and metallization finally produce a SiC device, which can be a SiC diode, a SiC MOSFET or even a SiC IGBT depending on the specific structures. 

During the development of SiC devices, a highly precise epitaxial growth process is an essential prerequisite for producing drift layers with the desired thickness and optimum doping concentration. Using process optimization and adequate cleaning and conditioning of polished surface of substrate, it is possible to achieve extremely high purity along with high-quality SiC epitaxial layers. 

Activation annealing of the selectively doped area formed by ion-implantation at temperatures of more than 1600°C is among the challenges posed by the production of SiC devices. This has to do with the high stability of the material. Gate oxidation represents another challenge. Due to the remaining carbon clusters in the MOS interface (SiC + O2 => SiO2 + ↑CO2 +↑ CO + C), the channel mobility of SiC MOSFETs is very low compared to Si, leading to elevated channel resistances even at high gate voltages (Vgs) of i.e. 20V. Thus, the specific on resistance (RonA) of commercial MOSFETs is higher than the expected ideal values. Furthermore, this interface occasionally leads to unstable Vth values or poor Qbd values. Using a proprietary gate oxidation technology, Rohm managed to present its SiC MOSFETs to the market, featuring stable Vth values and high Qbd levels equivalent to Si MOSFETs. 

The device manufacturing process results in a so-called SiC-device wafer (Figure 4). In the subsequent processing steps, wafer is sawn and the devices are picked-up from this wafer for use in the final products (discrete packages or power modules). 

SiC-device wafer from ROHM Semiconductor

Figure 4: SiC-device wafer from ROHM Semiconductor

Power Applications for SiC Diodes

Compared to Si diodes, SiC SBDs are much more attractive for power electronic applications especially at voltages of 600V and beyond. SiC SBDs feature much better efficiency due to their lower switching losses and the elimination of the so-called reverse recovery current during turn-off (see Figure 5). The EMI performance of the entire system is improved because EMI emissions are reduced accordingly. 

SiC SBDs feature better switching behavior than standard Si FRDs

Figure 5: SiC SBDs feature better switching behavior than standard Si FRDs

SiC SBDs feature much lower reverse recovery currents and shorter reverse recovery times, which reduces the relevant energy losses significantly. 

ROHM has introduced its technological advances into the market with its second-generation SiC SBDs. The cross section of a SiC SBD is depicted in Figure 6. 

Structure of Rohm’s second-generation SiC SBDs

Figure 6: Structure of Rohm’s second-generation SiC SBDs

Rohm diodes feature the lowest forward voltage worldwide (Figure 7). At the same time, they provide low leakage currents thanks to the precise manufacturing processes. 

SiC SBD forward voltage at Tj = 125°C

Figure 7: SiC SBD forward voltage at Tj = 125°C

Rohm’s portfolio of second-generation SiC SBDs currently includes 650V products for 5A to 100A as well as 1200V and 1700V devices for a current level up to 50A. Automotive qualified SiC-SBDs from Rohm are also available. They are widely used in On-board charger Systems. 

Third-Generation SiC Diodes

Development of Third Generation SiC SBDs

In applications like switched-mode power supplies (SMPS), SiC SBDs are now well known to be the better alternative to Si-based FRDs (Fast Recovery Diodes) in PFC stages (power factor correction). In this application, a large inrush current occurs at the starting phase because the intermediate circuit capacitor (D.C. Link Capacitor) is not charged before turn-on. Due to the lower surge-current capability (IFSM) of Rohm’s second-generation SiC SBDs, it is recommended to use bypass diodes in such SMPS applications. To support continuously downsizing requirements from the market Rohm designed its third-generation SiC SBDs meeting requirements of high surge current capability IFSM of the market (Figure 8). Initial products up to 10A are already available. 

Structure of Rohm’s new JBS SiC diode

Figure 8: Structure of Rohm’s new JBS SiC diode

With the development of the Junction Barrier Schottky structure (JBS), Rohm managed to combine all advantages of SiC diodes in a single device. In this approach, P+ region are embedded underneath the Schottky barrier with optimum spacing in order to increase the diode’s robustness while keeping the low Vf. 

The new JBS diode combines the advantages of SBDs and PN diodes

Figure 9: The new JBS diode combines the advantages of SBDs and PN diodes

Due to the PN structure within the diode and the injection of minority carriers, the resistance of the epitaxial layer decreases with increasing temperature. On the other hand, the resistance of the epitaxial layer increases with the temperature of the SBD structure (Figure 9). 

SiC MOS Planar Structures

Rohm’s Second-Generation Planar Structure

The planar structure, which is among the most well-known structures of the semiconductor industry, also lends itself to SiC-MOS devices for high-voltage applications. Rohm is among leading suppliers that implemented planar structure in their products. It is a well-known fact that a parasitic diode (the so-called body diode) is formed between the P layer and the N drift layer of a MOSFET (Fig. 10). In the history of SiC device development, so-called “bipolar degradation” has been one of the most critical issues. The device’s on-resistance increases when a current is flowing in the body diode. Therefore, a stable behavior of the body diode is critical for the reliability of a SiC MOSFET in the final application. To ensure the reliability of their systems, power electronic engineers expect that the behavior of the body diode does not degrade. 

ROHM SiC MOSFET 2nd. Generation is based on a planar structure

Figure 10: ROHM SiC MOSFET 2nd. Generation is based on a planar structure

Bipolar Degradation of Body Diodes

Both crystal defects and manufacturing process of SiC MOSFETs have a great influence on the stability of the body diode. By acquiring the energy of hole-electron recombination when forward current flows, a certain type of a crystal dislocation changes its type from linear to planer shape. That can lead to a degradation of the on resistance of the body diode and the MOSFET. Based on its expertise in different manufacturing processes at the substrate, epitaxial growth and device level Rohm managed to prevent the degradation of the body diode. 
Figures 11 and 12 illustrate the results of comparative measurements between Rohm MOSFETs and planar SiC MOSFETs from other manufacturer. In particular, 4 planar MOSFETs from other supplier were compared to 22 planar MOSFETs from Rohm. All the evaluated MOSFETs feature a breakdown voltage of 1200V. Typical on resistance is 0.08Ω. Source current of 8A was conducted through body-diode. 

Rohm’s SiC MOSFETs exhibit no on-resistance degradation

Figure 11: Rohm’s SiC MOSFETs exhibit no on-resistance degradation

After 24 hours of continuous current flow, the on-resistance of the planar MOSFETs of other supplier had dramatically increased and blocking capability was lost. While Rohm’s planar MOSFETs exhibited no performance degradation even after 1000 hours. 

Rohm’s SiC MOSFETs exhibit no blocking voltage degradation

Figure 12: Rohm’s SiC MOSFETs exhibit no blocking voltage degradation

Exclusively from Rohm: The Third-Generation Trench Structure

For several decades, the trench gate structure has been a proven approach in low voltage Si-MOSFETs and Si IGBTs. This technology turned out to be beneficial for many power electronic applications. As the gate electrode is embedded into the drift layer, width of unit cell can be shrinked which enables higher current density. Exploring the conventional trench gate technology used for SiC MOSFETs, ROHM’s designers found interesting facts. 

As SiC features higher electrical field strengths than Si IGBTs, using the conventional trench gate structure would result in the following problem. In the off-state of a SiC device, a strong electrical field of approximately 2.66MV/cm occurs at the gate trench. The excessive stress applied to the gate oxide would degrade the reliability and lifetime of the devices significantly. Therefore, ROHM’s double trench SiC MOSFET structure was designed to suppress this strong electrical field. In this approach, the source and gate electrodes are embedded into the drift layer (the source electrode is cut deeper than the gate electrode). As a result, the gate oxide is exposed to electrical field strength of less than 1.66MV/cm (Figure 13). Deeper source electrodes therefore prevent the concentration of electrical fields at the bottom of the gate. 

As an additional advantage of the trench structure, the on-resistance (Rdson) is reduced by 50% at the same die size, which contributes to a significant reduction of the conduction losses (Figure 14). Input capacity is also reduced by 35%, resulting in lower switching losses and a substantial reduction of the total energy losses. This structure is therefore considered to be an important step towards even more efficient modules featuring increased power density. Furthermore, the reliability of the gate oxide is improved by the reduction of the electrical field strength. ROHM started its volume production of third-generation SiC MOS products with discrete SiC devices and full-SiC modules based on its proprietary double trench technology. This expands the existing MOSFET product lineup and contributes to the design of highly efficient and highly reliable power electronics. 

Comparison of electrical fields in single trench and double trench structures

Figure 13: Comparison of electrical fields in single trench and double trench structures

Trench SiC MOS devices feature 50% lower Rdson values

Figure 14: Trench SiC MOS devices feature 50% lower Rdson values

Quality Assurance Measures for Rohm’s SiC Devices 

SiC is a promising wide-band-gap material for industrial and automotive applications. Naturally, technology maturity and product quality are important factors for convincing the market that SiC can in fact meet the reliability and lifetime requirements of their systems. Quite often, however, it must still be determined how a semiconductor manufacturer like ROHM can ensure the quality of its SiC manufacturing process. Based on many years of experience in development and production of SiC and Si and big investments into its manufacturing sites, ROHM manages to meet and even exceed the reliability requirements. 

Possible Defects During SiC Wafer Production 
As reported in the first part of this article series and in numerous publications of research institutes and universities, SiC crystals can exhibit various defects including: 

  • Micro pipes
  • Threading screw dislocation (TSD)
  • Threading edge dislocation (TED)
  • Dislocations in the planes perpendicular to the crystallographic main axis

Most defects in the substrate result in damages to the layers during the epitaxial growth phase. On the other hand, other defects can also occur during the epitaxial growth phase, the ion implantation and dry etching processes. These spot defects usually emerge independent of the substrate’s quality. 

All kinds of macroscopic defects occurring during the epitaxial growth phase result in a significant increase of the leakage current and a degradation of the breakdown voltage which both influence the reliability of the SiC device. 

For these reasons, it is essential to understand the physical properties of the materials used in order to retrace the defects that can occur during the manufacturing process. This enables a continuous improvement of the manufacturing process. 

Furthermore, Rohm conducts various tests during the manufacturing process in order to screen defective parts and to ensure full control over each processing step. This enables ROHM to ensure the delivery of sustainable products for high-volume markets. 

Rohm’s Production Tests 

Quality assurance during Rohm’s SiC manufacturing process

Figure 15: Quality assurance during Rohm’s SiC manufacturing process

Rohm’s quality control is based on 100% optical inspections and electrical tests. In addition, special inspections are made during the manufacturing process of SiC devices. SiC devices with visible defects generally fail during the electrical tests (gate-source or drain-source shorts). Nonetheless, Rohm makes an optical inspection at the beginning of device production to screen any substrate and epi-layer defects. 

In addition to visible defects, there may also be invisible faults including minor crystal defects in the substrate. This can even be more critical because devices featuring these invisible defects can operate flawlessly for an indefinite time but fail in the field, thereby degrading the reliability of the system. To prevent this, Rohm uses its unique screening technologies to detect invisible defects before delivery to the customer. The technical parameters of the devices are checked by electrical characterization at the end of the manufacturing process. For traceability reasons, all steps are documented for every single device. 

More information: Rohm Semiconductor    Source: Bodo's Power Systems, February 2017