The present invention relates to a power semiconductor device suitable for a power switching element or the like and a method for manufacturing the power semiconductor device.
In the field of power semiconductor devices, conventionally power switches such as insulated gate bipolar transistors (IGBTs) and metal oxide semiconductor field-effect transistors (MOSFETs) using Si have been widely used, but it is said that their performance is approaching its limit and becoming insufficient to clear an energy efficiency target toward a carbon-free society. On the other hand, in recent years, development of alternative techniques using new crystal materials, which are called wide band gaps, is expected, but there are many technical problems in terms of performance, cost, and, reliability, and therefore mass production is not easy, and improvement thereof is strongly desired.
Patent Literature 1: U.S. Pat. No. 5,506,421
Patent Literature 2: U.S. Pat. No. 5,396,085
Patent Literature 3: U.S. Pat. No. 5,323,040
Patent Literature 4: U.S. Pat. No. 5,614,749
Patent Literature 5: U.S. Pat. No. 5,610,492
Patent Literature 6: Japanese Patent Application Laid-open No. 2013-243333
Patent Literature 7: Japanese Patent Application Laid-open No. 2015-153893
Non-Patent Literature 1: “Lateral n-channel inversion mode 4H—SiC MOSFETs,” S. Sridenvan et al. IEEE Electron Device Letters, (Volume: 19, Issue: 7, Jul. 1998)
Non-Patent Literature 2: Determination of intrinsic phonon-limited mobility and carrier transport property extraction of 4H—SiC MOSFET, Noguchi, et al. 9.3.1 IEDM17, pp 219-222. (2017)
Non-Patent Literature 3: “1800V Bipolar mode MOSFETs: a first application of Silicon wafer Direct Bonding (SDB) technique to a power device,” A. Nakagawa, IEDM86, 5.6, pp 122-125
Non-Patent Literature 4: “Bonding of Dissimilar Semiconductor Materials for Energy-Harvesting and Energy-Saving Devices,” Naoteru Shigekawa, J. Vac. Soc. Jpn. Vol. 60, No. 11, (2017) 421-427
Non-Patent Literature 5: “Impact Ionization In Silicon: A REVIEW AND UPDATE,” W. MAES, et al. Solid State Electronics Vol. 33, No. 6 pp 705-718, 1990
Non-Patent Literature 6: “Effects of thermal annealing process on the electrical properties of p+-Si/n-SiC heterojunctions,” J. Liang, S. Nishida, M. Arai, and N. Shigekawa, Applied Physics Letters 104, 161604 (2014)
Transistors that have been mainstream as power transistors are IGBTs and MOSFETs made of silicon materials (hereinafter referred to as Si). Since the IGBTs are bipolar devices, resistance of a low concentration n-type drift layer on a p-type collector layer can be reduced due to a conductivity modulation effect of carriers, and thus a high breakdown voltage and a large current can be realized. However, during a turn-off operation, high-level injected electrons and holes require carrier disappearance time due to recombination, and since it takes time to fall down, the switching operation becomes slow, and thus the turn-off loss increases. On the other hand, since MOSFETs made of Si (hereinafter referred to as Si-MOSFETs) are unipolar devices, the recombination speed does not cause a problem in a low concentration drain diffusion layer (hereinafter referred to as a drift layer) on a high concentration drain diffusion layer, and thus the advantage of reducing turn-off loss is elicited. However, in a case in which the drift layer is thickened to increase a breakdown voltage for high-voltage applications, on-resistance thereof will increase, and there is a drawback that an on-voltage rises and an energy loss increases, which makes it unsuitable for high-voltage applications.
Due to technical limitations of Si devices as described above, wide bandgap semiconductors have been expected as a replaceable next-generation power semiconductor material in recent years. Among them, silicon carbide (hereinafter referred to as SiC) and gallium nitride (hereinafter referred to as GaN) both have a band gap of about 3 times that of Si, and also have a dielectric breakdown voltage of about 10 times that of Si, while their mobility is equivalent to that of Si. Due to such advantages, wafers can be thinned and used even for use in high voltage applications, on-resistance can be lowered, and thus high voltage and high speed operations are possible. Further, high temperature operations are possible, cooling becomes easier since high thermal conductivity and excellent heat dissipation are achieved, and thus they are highly anticipated as power semiconductor device materials for high-speed and high-power uses. However, there are problems in manufacturing that SiC has a high wafer cost and requires heat treatment at a high temperature in a wafer process as compared with Si.
Although a long period of time has passed since research on MOSFETs made of SiC crystals (hereinafter referred to as conventional SiC-MOSFETs) started (see, for example, Non-Patent Literature 1), a cause of the problem of low channel mobility of conduction electrons (about two orders of magnitude smaller than Si) due to quality of an interface of a gate thermal oxide film has not yet been fully identified. The cause is presumed to be that channel mobility decreases because there are many defects (interface state density) at an interface between a gate insulating film and SiC (see, for example, Non-Patent Literature 2). That is, it is known that although electron mobility in bulk crystals of SiC is 800-1000 cm/V·sec, surface channel mobility in SiC-MOSFETs is extremely low at 10 cm/V·sec (Si surface).
For that reason, although resistance of a SiC drift layer formed on a substrate drain diffusion layer is sufficiently low during a device operation, channel resistance of a MOSFET is too high, which is a serious problem. As a result, on-voltage rises and energy loss increases, and thus advantages of SiC cannot be fully utilized.
In order to solve the problem in the conventional examples, Patent Literature 2 has devised a structure in which a Si-MOSFET is stacked on SiC.
On the other hand, in the patent, lower ends of the side wall type trench gates 50 in
In the present invention, by stacking a drift layer made of SiC with high bulk mobility on a Si-MOSFET with high channel mobility, a new device structure that can achieve high performance and its manufacturing method are devised, thereby solving the problems of the above existing devices. Specific means for solving the problems will be exemplified below.
In a power semiconductor device according to the present invention, a first conductivity type SiC substrate that has a drain electrode, a first conductivity type drain layer, and a first conductivity type drift layer in order from a bottom surface side thereof, and a second conductivity type Si substrate thereon, which is the opposite conductivity type of the first conductivity type, are directly bonded together. Grooves (recess grooves) extending from a Si surface side through the Si substrate to a part of the SiC substrate are provided, and polycrystalline Si is embedded therein through respective gate oxide films, thereby forming trench gates. Here, bottom surface positions of the trench gates are lower than a bonding position between the SiC and the Si. On the other hand, it is characterized in that a second conductivity type impurity region (hereinafter referred to as a block layer) having a higher concentration than the Si substrate is formed in the vicinity of a bottom side between the trench gates in a horizontal direction in the second conductivity type Si substrate, and a gate electrode wiring and a source electrode wiring are provided on an upper portion on a front surface side of the second conductivity type Si substrate, whereby a MOSFET is manufactured.
Lower end positions of the trench gates are at or deeper than a heterojunction position between the drift layer of the first conductivity type SiC and the second conductivity type Si substrate, and are, for example, 0.2 μm or deeper, preferably 0.5 μm to 0.7 μm. If it is 0.7 μm or more, a feedback capacitance Crss, which is a parasitic capacitance between a gate electrode and a drain electrode, increases, which is not desirable. By setting the above lower end positions, an electric field effect is exerted on a MOS interface of the SiC drift layer in an on state, so that conduction electrons can easily overcome an energy step of a Si/SiC conduction band heterojunction to smoothly flow a drain current. On the other hand, in an off state, a reverse bias state occurs, and thus a high electric field from the first conductivity type SiC drift layer must be attenuated in the second conductivity type Si substrate, but by disposing the SiC drift layer and the Si substrate interface in an inward direction between the trench gates, they become walls of the electric field, so that intrusion of the electric field into Si can be relaxed. Further, if corners of bottom portions of the trench gates are sharp, dielectric breakdown may occur due to electric field concentration, and thus it is necessary to round the corners. For example, radii of curvature thereof may be 0.05 μm or more. This is because the electric field strength is inversely proportional to the radius of curvature of the gate, and it is desirable that the curvature be half the width of the gate if possible.
In the power semiconductor device according to the present invention, the electric field on the second conductivity type Si substrate must be significantly more relaxed in the off state. Preferably, it is effective to set the interval to be narrower such that a potential distribution in Si is spatially modulated and lowered due to a narrow channel effect generated between the trench gates to weaken the electric field strength. In addition to that, a measure by which the intrusion of the electric field into Si can be further prevented by providing a second conductivity type block region at a bottom portion of the second conductivity type Si substrate has been devised. By providing the block layer, a depletion layer width can be limited to the off state, and thus it is not necessary to rely on excessively narrowing only the interval between the trench gates. Further, since the block layer determines a channel surface potential and determines a substantial channel length even in the on state, resistance to avalanche breakdown associated with impact ionization increases as compared to Patent Literature 7 mentioned above, and it is possible to supply a stable current regardless of a thickness of the Si substrate. As described above, in the present invention, it is possible to stably achieve high performance in the on state and the off state by introducing the SiC recess grooves of the trenches, the narrow channel effect between the gates, and the block layer formation.
In a method for manufacturing a power semiconductor device according to the present invention, a SiC wafer on which a first conductivity type drift layer is formed and a second conductivity type Si wafer are integrated using a surface activated bonding technique (see, for example, Non-Patent Literature 3 and 4), which is then polished and formed into thin film, and then trench grooves are dug from a Si side to penetrate the Si substrate and a part (recess groove) of the SiC substrate. Then, after forming respective gate oxide films therein, gate electrodes are formed by filling polycrystalline Si into the trenches. It is characterized in that a second conductivity type block layer is formed on a bottom portion of the second conductivity type Si substrate between the trench gates, and a first conductivity type diffusion layer and a second conductivity type diffusion layer for electrical bonding with a source region and the substrates, and electrodes in contact with the first conductivity type diffusion layer and the second conductivity type diffusion layer are formed.
The MOSFET with which low loss and high breakdown voltage can be achieved has been devised on the basis of the device structure and the method for manufacturing suitable for this according to the present invention. The problem that needed to be solved of deterioration of channel mobility due to defects in the interface of the SiC oxide film in the on state operation can be avoided by converting the channel to a MOS channel using Si crystals. In particular, by sufficiently overlapping the trench gates on the SiC (recess groove) side with respect to the SiC/Si heterojunction portion, electrons are caused to easily overcome the conduction band energy step in the on state, so that a large drain current can be taken out. In addition, by providing the second conductivity type block layer on the bottom portion of the second conductivity type Si substrate, the actual channel length is determined, and thus a stable current can always flow even when the thickness of the substrate varies slightly. With this structure, high conductivity can be stably maintained over the entire conductive region made of Si and SiC from the source to the drain, and thus a low on voltage can be achieved. In addition, since the Si-MOSFET structure is adopted for the input, a gate input pulse amplitude thereof can be lower than in conventional SiC-MOSFETs, and thus oscillation noise and radiation can also be reduced.
On the other hand, there is no measure other than thickening the drift layer to increase the breakdown voltage in the conventional Si-MOSFET in the off state, but particularly in the present invention, since SiC is used for the drift layer, the performance can be significantly improved. As a measure to prevent a strong electric field from the drift layer of SiC from entering the Si region, the narrow channel effect caused by narrowing the space between the trench gates, and the formation of the second conductivity type block layer formed in the Si surface region, which enhances the narrow channel effect, can effectively prevent the penetration of the electric field into Si. Further, forming the trench gates up to the SiC region (recess grooves) is also effective in inhibiting the electric field from entering the second conductivity type Si substrate. By properly combining these measures, it is possible to realize low on-resistance and inhibit avalanche breakdown in the Si substrate in a high voltage off state and thus high device performance that could not be achieved with conventional Si-MOSFETs, Si-IGBTs, and SiC-MOSFETs is achieved.
The present invention proposes a new power MOSFET structure in which a MOSFET portion is made of Si and a drift layer portion is made of SiC, and proposes, as a manufacturing method thereof, a manufacturing process in which a Si substrate and a SiC substrate can be integrated using a method of direct bonding.
A first embodiment will be described in detail below with reference to
Regarding the junction thus manufactured, an operation of the MOSFET of the first embodiment will be described from the viewpoint of energy band.
On the other hand,
On the other hand, as a second measure, by forming the p-type block layer 14 having a higher impurity concentration than the p-type Si bulk layer 13 in a bottom region of the p-type Si bulk layer 13 with respect to the penetration of the electric field from the n−-type SiC drift layer 16, it is possible to effectively prevent the penetration of the electric field into the p-type Si bulk layer 13. A difference between presence and absence of the p-type block layer 14 is shown in
In order to predict effects of these two electric field relaxation methods, an electric field distribution in a depth direction in the off state is shown in
In addition, providing the overlapping region (recess grooves) between the trench gate electrode 23 and the n−-type SiC drift layer 16 shown in B of
Next, a method of manufacturing the MOSFET shown in the first embodiment, in which the element structure is formed after the SiC wafer and the Si wafer are directly bonded and integrated, will be described.
Direct wafer bonding has preferably been used for manufacturing power semiconductors (see, for example, Non-Patent Literature 3), and as a method of directly bonding such different semiconductor wafers, a solid bonding method performed by a surface activated bonding technique (hereinafter referred to as SAB) that maintains integrity of crystals by irradiating a surface with neutral atomic beam, removing a natural oxide film, making a surface layer in an amorphous state, and pressing it has been realized, and thus heterojunction devices are realized (see, for example, Non-Patent Literature 4). In addition, an amorphized portion is recrystallized by post-annealing after pressurization to obtain a continuous crystal interface. Non-Patent Literature 5 shows a bonding example of n+-type Si/n-type SiC (4H crystal structure) and diode characteristics improved by annealing. Further, an interface state present on a wafer bonding surface is significantly improved by annealing treatment after SAB bonding.
An example of the manufacturing method according to the first embodiment will be described in detail below with reference to
Next, in the step of
Next, a second embodiment of the present invention will be described with reference to
According to the present invention, a power semiconductor having high power, high efficiency, and low noise is realized, which contributes to reduction of total power consumption in social infrastructure and also contributes to improvement of environmental problems such as global warming.
4 p-type SiC block layer
9 p+-type substrate contact layer
10 n+-type source region
11 Gate extraction electrode
12 Source electrode
13 p-type Si bulk layer
14 p-type block layer
15 Si/SiC heterojunction interface
16 n−-type SiC drift layer
17 n+-type SiC drain layer
18 Drain electrode
19 Coating insulating film
22 Trench
23 Trench gate electrode
25 Gate oxide film
27 Potential distribution contour line
29 Trench gate bottom oxide film
30 n-type source diffusion layer
31, 32 Gate electrode
33 p-type SiC region
36 n-type drift region
37 n-type drain diffusion region
38 Drain electrode
39 Insulating protective film
42 Device of known example of Patent Literature 1
48 SiC substrate
49 Sidewall gate
52 Source electrode
54 Drain electrode
56′ n-type layer SiC drift layer/n-type Si layer interface
57 n-type layer Si diffusion layer
58 n-type Si source diffusion layer
59 n-type layer SiC drain diffusion layer
60 p-type Si substrate
61 Gate oxide film
62 Schottky junction electrode
71 Interface state
72 Drain diffusion layer
73 Trench corner gate oxide film
Number | Date | Country | Kind |
---|---|---|---|
2018-113277 | Jun 2018 | JP | national |
2018-153351 | Aug 2018 | JP | national |
2018-213516 | Nov 2018 | JP | national |
This application is a continuation application of International Application Number PCT/JP2019/005376 filed on Feb. 14, 2019, now pending, herein incorporated by reference. Further, this application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2018-113277, filed on Jun. 14, 2018, the prior Japanese Patent Application No. 2018-213516, filed on Nov. 14, 2018 and the prior Japanese Patent Application No. 2018-153351, filed on Aug. 17, 2018, entire contents of which are incorporated herein by reference.
Number | Date | Country | |
---|---|---|---|
Parent | PCT/JP2019/005376 | Feb 2019 | US |
Child | 17119123 | US |