SiC, everyone is "digging holes"

It is obvious to all that the development speed of SiC power semiconductors in recent years has exceeded almost everyone’s expectations. Among them, SiC MOSFET has received special attention due to its potential to replace existing silicon superjunction (SJ) transistor and IGBT technologies.

Old rivals and new players in the industry are rushing in, doubling down on this emerging market.

In fact, the development history of SiC MOSFET is quite long. John Palmour, one of the founders of Cree, the predecessor of Wolfspeed, the world leader in SiC industry, applied for a structure involving the generation of MOS capacitors on SiC substrates as early as 1987. This project The patent was later regarded as the key to the birth of SiC MOSFET.

However, due to problems such as substrate yield and manufacturing process, SiC MOSFET was not officially commercialized until around 2010.

At that time, Cree launched the first SiC MOSFET on the market, the CMF20120D with a planar gate structure (other reports say that Rohm took the lead in launching the first planar SiC MOSFET in 2010). By 2015, ROHM was the first to achieve mass production of trench gate structure SiC MOSFETs. This structure can better utilize the characteristics of SiC materials and has a more complex process.

After nearly 10 years of development, trench SiC MOSFET is currently considered to be the more advantageous technical route and development direction in the technical route of SiC MOSFET.

In the debate over the technical route of SiC MOSFET, there have always been two different structural types: planar gate and trench gate.

Both planar gate and trench gate are vertical conductivity MOSFETs. They are similar in structure. The source is on the top layer and the drain is on the bottom layer. The difference between the two is the gate electrode.

Planar gate SiC MOSFET structure: means that the gate electrode and source electrode present a "planar" distribution on the same horizontal plane, and the channel is parallel to the substrate. The characteristics of the planar gate structure are simple process, good unit consistency, and relatively high avalanche energy. However, due to the presence of the JFET region in planar gate SiC MOSFET devices, the input capacitance is large, which increases the on-state resistance and reduces the current capability of the device.

Trench SiC MOSFET structure: The gate is located below the source, forming a "trench" in the semiconductor material. The channel and gate in the trench gate structure are perpendicular to the substrate, which is also different from the planar gate. A significant difference in structure. Despite its complex process, cell consistency is worse than planar structures. However, since the trench structure has no JFET effect and has a higher channel density, and the SiC crystal plane where the channel is located has a higher channel mobility, it can achieve lower specific on-resistance and achieve greater current. turn-on and wider switching speeds.

Therefore, the new generation of SiC MOSFET mainly studies and adopts this structure.

SiC power MOSFET device structure

Relatively speaking, the process complexity of planar gate SiC MOSFET is not that high, and the development history is relatively long. Related products at home and abroad have been mass-produced earlier. Driven by many car companies such as Tesla and BYD, planar gate SiC MOSFET power The module has entered the main drive inverter since 2018.

However, in the process of shrinking chip size and thereby increasing yields, the lateral topology of planar gate SiC MOSFETs places limits on how much they can ultimately be shrunk.

In contrast, trench SiC MOSFET devices have the following outstanding advantages due to their trench gate structure:

• The conductive channel is changed from horizontal to vertical, which effectively saves the device area and greatly increases the power density;

• The trench structure almost eliminates the JFET area, greatly reducing the input capacitance of the device, increasing the switching speed and reducing switching losses;

• The resistance in the JFET area is also eliminated, and the device Rdson can be further improved with lower current capabilities.

Compared with planar gate SiC MOSFET devices, trench SiC MOSFET has higher power density, faster switching speed, lower on-resistance and lower loss, so it has attracted great attention from enterprises in the industry.

In layman's terms, trench gate SiC MOSFET can be understood as "digging holes" on a flat surface. International SiC manufacturers are using trench gates to maximize the potential of SiC. However, although each company is "digging holes", the methods are slightly different. Looking at it, some manufacturers dig one hole, some dig two holes, and some dig diagonally. Various technical structures are emerging one after another, and a hundred flowers are blooming.

Schematic diagram of several trench gate SiC MOSFETs in the industry

To this end, SiC chip suppliers, especially major international manufacturers, are using their respective capabilities to explore trench SiC MOSFETs.

Among the leading suppliers of SiC devices, they have basically begun to lay out trench gate MOSFETs.

ROHM and Infineon are the first companies to move to trench SiC MOSFETs. According to a Yole report, the trench SiC MOSFET camp has expanded from the original ROHM and Infineon to a number of leading manufacturers, such as Sumitomo Electric, Mitsubishi Electric, Denso, Qorvo (UnitedSiC), ST, Wolfspeed, ON Semiconductor Semiconductors, etc., are all transforming from planar structure MOSFETs to trench structures.

In 2015, ROHM developed and mass-produced the world's first trench structure SiC MOSFET, and it was a dual-trench structure. Up to now, ROHM's trench SiC MOSFET has developed to the fourth generation. The dual trench structure has both source trench and gate trench.

ROHM double trench SiC MOSFET structure

(Photo source: Roma)

In a typical single-trench structure, the electric field is concentrated at the bottom of the gate trench, so long-term reliability has been an issue. The double-trench structure developed by Rohm also has a trench structure in the source area, which alleviates the electric field concentration at the bottom of the gate trench. This structure successfully reduces the electric field and prevents oxidation of the gate trench. layer destruction, ensuring long-term reliability and improving device performance.

It is understood that in the fourth-generation SiC MOSFET, Rohm has further improved the dual-trench structure and successfully reduced the on-resistance by about 40% compared to the third-generation product on the premise of improving the short-circuit withstand time; at the same time, it has significantly improved the Reducing the gate-to-leak capacitance has successfully reduced switching losses by approximately 50% compared to third-generation products.

Comparison of on-resistance and switching loss between the 4th generation SiC MOSFET and the 3rd generation (Source: ROHM)

Rohm predicts that the fourth-generation SiC MOSFET will gradually increase its proportion in its sales structure from 2023 until it becomes the main sales force in 2024-2025.

Compared with other competitors that are still challenging to produce the first mass-produced trench gate products, Rohm is already several positions ahead. According to its product roadmap, the on-resistance of the fifth-generation and sixth-generation products expected to be launched in 2025 and 2028 will be reduced by another 30% respectively.

ROHM’s SiC MOSFET technology roadmap

As we all know, "digging holes" is Infineon's ancestral craft.

In the era of silicon-based products, Infineon's trench IGBTs and trench MOSFETs are unique in the world. With the advent of the SiC era, most SiC MOSFETs on the market are planar cells, while Infineon still continues the trench structure route.

Schematic diagram of the structure of Infineon’s half-pack trench SiC MOSFET

2017年，英飞凌报道了采用半边导通结构的沟槽型SiC MOSFET器件，在栅极沟槽的一边形成导电沟道。从上图看到，参杂毗邻沟槽中的区域是不对称的，沟槽的左侧壁包含了MOS沟道，它被对准到a-plane面，以实现最佳的沟道迁移率，沟槽底部的大部分被嵌入到沟槽底部下方的p型区域中。

该结构可保护沟槽拐角不受电场峰值影响，提高器件可靠性，同时能进一步提升器件耐压，使得开关控制良好，动态损耗非常低。特别是，该特性对于抑制使用半桥的拓扑中寄生导通引起的额外损耗至关重要。

英飞凌的CoolSiC MOSFET沟槽分立器件系列，采用英飞凌独特的沟槽的方式，为其系统设计带来了许多好处，包括高可靠性、效率提高、实现高开关频率和高功率密度，降低系统复杂性和总系统成本。

英飞凌在2016年推出了第一代CoolSiC系列SiC MOSFET，并在2022年更新了第二代产品，相比第一代增强了25%-30%的载电流能力。

产能方面，英飞凌目前主要通过特有的“冷切割”技术，减少晶锭切割过程中材料的浪费，未来可以在相同晶锭中获得多一倍的碳化硅衬底来增加产能。另一方面，英飞凌去年宣布投资超过20亿欧元，对位于马来西亚的晶圆厂进行扩建，专门针对碳化硅晶圆进行扩产。

据Yole数据统计，全球碳化硅功率器件市场份额最高的厂商就是意法半导体（ST），同时凭借与特斯拉的合作，ST的SiC MOSFET产品也是最早在电动汽车上被大规模应用的，自Model3车型开始，特斯拉就一直大规模采用ST供应的碳化硅模块。

在芯片设计上，意法半导体持续深挖平面设计碳化硅MOSFET的技术潜力，2022年推出了第4代平面栅SiC MOSFET。相比上一代产品，第4代平面栅碳化硅的性能有所进步，包括导通电阻减少15%，工作频率增加一倍至1MHz。

而之前规划的沟槽栅产品则顺延成为意法半导体的第5代SiC MOSFET，目前应该在研发阶段，预计2025年量产。

意法半导体SiC MOSFET路线图

（图源：ST）

相比于平面型SiC MOSFET，沟槽型SiC MOSFET可以具有较小的导通电阻，寄生电容较小的同时开关性能更强。

产能方面，ST此前计划在2022财年投入21亿美元来扩大产能，包括扩建原有的6英寸碳化硅晶圆厂、2022投入运营的新加坡6英寸碳化硅晶圆厂。同时，2019年ST收购的瑞典碳化硅衬底生产商Norstel也开始进行8英寸碳化硅材料的测试，预计会在2025年前后可以在新加坡8英寸产线中应用。

2021年第3季度，随着收购衬底供应商GTAT的通过，安森美搭建了从碳化硅晶锭、衬底、器件生产到模块封装的垂直整合模式。

虽然其中一些项目的技术实力与各领域领先企业还有所差距，但其整体实力却更为均衡：与衬底龙头Wolfspeed相比，安森美的模块封测和量产经验略胜一筹；与器件设计实力超群的英飞凌相比，安森美又有来自GTAT碳化硅材料的加成。

从产品结构来看，安森美的第1代碳化硅MOSFET技术（M1）采用平面设计，耐压等级为1200V。之后从中衍生出900V和750V耐压的规格，微观结构也改为Hex Cell设计，这两个改动相叠加使得碳化硅MOSFET的导通电阻降低了35%左右。目前安森美推出的大部分碳化硅产品均基于M1与其衍生出的M2平台。

目前最新的一代碳化硅技术（M3）仍然采用平面技术，但是改为受专利保护的Strip Cell设计，导通性能较上一代衍生版本再提高了16%。这一代产品将逐渐成为公司的主力车规碳化硅平台，在电压规格上覆盖电动汽车主流的400V和800V平台。

据了解，安森美的下一代技术平台M4则会从平面结构升级为沟槽结构。与初代碳化硅技术相比，沟槽结构的SiC MOSFET在相同载电流的要求下可以减少相当的芯片面积。如果再加上M4平台可能采用8英寸晶圆生产，预期M4的成本较之前将显著降低。

事实上，安森美在沟槽栅方面已经研究了很多年，也有很多样品在进行内部测试，其认为唯一的问题在于，过早的推出沟槽栅产品在可靠性方面还有一定的风险。所以安森美正在进行可靠性优化，提升沟槽栅的利用率。

同时，在提升可靠性方面，安森美也在对沟槽栅进行摸底，在标准测试的基础上加一些认为有风险的测试点，力图将风险搞清楚。

另外，从封装角度讲，安森美提供各种不同的封装选项，还将推出下一代设计很强的封装，通过封装的不断迭代来适配不同的需求。

2019年，三菱电机也开发出了一种沟槽的SiC MOSFET，为了解决沟槽型的栅极绝缘膜在高电压下的断裂问题，三菱电机基于在结构设计阶段进行的先进模拟，开发了一种独特的电场限制结构，将应用于栅绝缘薄膜的电场减小到常规平面型水平，使栅绝缘薄膜在高电压下获得更高的可靠性。

三菱电机的新型沟槽型SiC MOSFET结构示意图

（图源：三菱电机）

三菱电机利用独特的电场限制结构确保器件可靠性，通过注入铝和氮来改变半导体层的电气特性，从而保护栅极绝缘膜。

具体来看，在垂直沟槽方向注入铝元素，使沟槽底部形成电场限制层，再通过其新技术斜向注入铝，形成连接电场场限制层和源极的侧接地，并斜向注入氮元素，再局部形成更容易导电的高浓度掺杂层。电场限制层将施加在栅极绝缘膜上的电场降低到传统平面结构水平，保证耐压的同时，提高器件的可靠性。连接电场限制层和源极的侧接地，实现了高速开关动作，减少开关损耗。

与平面结构相比，沟槽型器件Cell pitch更小，所以功率器件能排列更多的元胞。元胞高密度排列使得流动的电流变多，但各栅极的之间的间隔太小就会导致路径变窄，电流流动困难。将氮元素斜向注入，在局部形成更容易导电的高浓度掺杂层，使电流路径上的电流变得容易传输，从而降低电流通路的电阻。与没用高浓度层相比，电阻率降低了约25%。

作为一家在SiC行业中浸润了超过30年的企业，Wolfspeed及其前身Cree在1991年就推出了第一片量产碳化硅衬底。深厚的经验积累和历史沉淀让Wolfspeed的碳化硅衬底性能和质量独占鳌头，就连意法半导体、英飞凌和安森美等同行业竞争对手都不得不花费上亿美元向其采购。因此，Wolfspeed的碳化硅产品获得了至关重要的先发优势，成为了整个碳化硅行业的风向标。

In terms of design, Wolfspeed's silicon carbide MOSFET adopts a planar design and is currently in the 3rd generation, covering multiple voltage specifications between 650V and 1200V. Compared with the previous two generations of products, Gen3 planar MOSFET adopts a hexagonal unit cell micro design, and its on-resistance is reduced by 16% compared with the previous generation Strip Cell.

Wolfspeed Gen3 SiC MOSFET uses Hex Cell’s planar technology (Source: Wolfspeed)

It is understood that Wolfspeed's next generation product will be a trench gate design. Gen4 trench gate MOSFET is still under development, and the specific mass production time has not been disclosed.

Although trench structures are also being laid out, Wolfspeed, which has been committed to the development of silicon carbide diodes and MOSFETs from the beginning, believes that the technical advantages of planar gate SiC MOSFETs are far from exhausted.

Wolfspeed co-founder John Palmour once said: "Because trench MOSFETs have better on-resistance, it is a key performance indicator. As long as we are far ahead of trench SiC MOSFETs in on-resistance, I see no reason to change this , not to mention that we will continue to improve planar SiC MOSFET. Customers should not care whether it is a planar MOSFET or a trench MOSFET. What matters is the specific on-resistance. In fact, we don’t care which technology route, we only care about which design can. Bring maximum benefits to customers.”

In short, there is still room for deep exploration in planar structures, and there is also a market for reliability.

Back in 2016, Fuji Electric developed a 1200V SiC trench MOSFET for full SiC modules, achieving a low specific resistance of 3.5mΩcm2 and a threshold voltage of 5V while maintaining a high "channel" for turning on and off current. reliability.

As a result, the resistivity was successfully reduced by more than 50% compared to the previous planar structure. In addition, Fuji Electric has also developed a high-current density dedicated SiC module using a unique pin connection structure, which fully utilizes the advantages of SiC devices. Fuji Electric has already implemented All-SiC modules using this equipment.

In 2016, Sumitomo developed a V-groove SiC MOSFET device sample with a thick bottom oxide layer, further improving the gate oxide reliability and threshold stability of the device.

Sumitomo Electric’s SiC VMOSFET cross-sectional view

(Source: Sumitomo Electric)

Sumitomo Electric has newly developed a V-shaped trench MOSFET using a unique crystal surface. V-MOSFET has superior characteristics such as high efficiency, high blocking voltage, and high stability in harsh environments. It can achieve large current (single chip 200A) and is suitable for EVs and HEVs. In addition, Sumitomo Electric is working with the National Institute of Advanced Industrial Science and Technology to develop next-generation V-MOSFETs with the world's lowest on-resistance.

In March 2023, DENSO announced that it had developed the first inverter using SiC semiconductors.

Among them, Denso's unique trench-type MOS structure adopts trench gate semiconductor devices with its patented electric field relaxation technology, which improves the output of each chip because they reduce power loss caused by heat generation. The unique structure achieves high voltage and low on-resistance operation.

Denso’s trench gate structure (Source: Denso)

Some data show that Denso is similar to Sumitomo's trench structure, but changed to a U-shaped trench.

Source: Songge Power

Qorvo's SiC technology mainly comes from UnitedSiC, which was acquired in 2021. Now SiC is also the top priority for Qorvo's future development.

It is understood that, unlike the traditional SiC MOSFET design, Qorvo has taken a new approach. Its SiC MOSFET adopts a high-density trench SiC JFET structure. The channel resistance Rchannel in the SiC MOSFET is replaced by the resistance of the low-voltage silicon MOSFET in the SiC FET. The latter has much better electron mobility in the inversion layer, achieving ultra-low on-resistance per unit area and therefore lower losses. This structure is co-packaged with a low-voltage Si MOSFET, and the die area of ​​the SiC FET is also relatively small.

Comparison of SiC MOSFET (left) and Qorvo’s SiC FET (right) architecture (Source: Qorvo)

Qorvo expands its 1200V product line, extending its breakthrough fourth-generation SiC FET technology to higher voltage applications, with product specifications ranging from 23mΩ-70mΩ, targeting 800V electric vehicle on-board chargers (OBCs) and DC converters, etc. Application market.

It is understood that Renesas Electronics has just applied for a patent in 2023 and is preparing to study the silicon carbide trench structure, referred to as "periodic connection, variant dual-level trench MOSFET".

Source: Silicon carbide chip study notes

All in all, several important indicators for improving the performance of SiC MOSFETs, including smaller cell spacing, lower specific on-resistance, lower switching losses, and better gate oxygen protection, almost all point to the trench gate structure.

From the perspective of the industry as a whole, the current mass production of trench SiC MOSFETs is mainly international SiC manufacturers in Europe, the United States and Japan. Judging from the layout of international manufacturers, trench gate SiC MOSFET will be a more competitive solution in the future.

Nearly nine years have passed since the first mass-produced trench-gate SiC MOSFET product was launched in 2015. Many companies are developing trench-gate products, but currently there are not many manufacturers on the market that can launch mass-produced products.

Of course, designing and manufacturing high-performance trench gate SiC MOSFETs is also a top priority for the development of domestic SiC power devices. Some companies have shifted their research focus to trench gate SiC MOSFETs. However, it should be noted that international SiC giants have been in the SiC MOSFET field for many years and have accumulated many patents. The high patent barrier of trench structures is also a hurdle for domestic manufacturers to overcome.

According to the author of "Silicon Carbide Chip Study Notes": "Trench SiC MOSFET complete set of process and structural IP is the ticket to silicon carbide competition in the next ten years!" In the current period when the overall SiC market continues to grow at a rapid pace, it is appropriate to lay out in advance Only by following the technical route can we have the opportunity to take the lead in new application markets in the future.

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