Introduction
HOYA is the first manufacturer to have successfully developed a technology for mass-producing large monocrystal silicon carbide (SiC) substrates for use in high-performance semiconductor devices. This SiC substrate is made of so called "cubic SiC" (3C-SiC). With this development, it is now possible to manufacture large, defect-free 3C-SiC substrates at low cost, comparable to those involved in the manufacture of Si, a widely used semiconductor substrate material. Semiconductor devices using this 3C-SiC substrate are expected to significantly reduce power losses in electrical devices, industrial equipment, home appliances, and other devices that are subject to increasingly stringent power-conservation requirements. Additionally, electric vehicles and high-speed information processing are potentially promising applications for SiC-based semiconductors.

Features of SiC
SiC is a crystal consisting of one-to-one silicon-carbon covalent bonds. It is highly regarded as a material for wide-band-gap semiconductors. SiC semiconductors can handle higher voltages and greater power with less power loss than conventional Si-based semiconductors. Furthermore, the thermal conductivity of SiC is at least three times that of Si; thus, its electrical performance remains stable even at temperatures exceeding 300[C. Moreover, since the power loss in SiC-based devices is less than half that of Si-based semiconductors, SiC is expected to become a key energy-saving material. Finally, the recently developed technology for mass-producing 3C-SiC is expected to lead to widespread use of SiC-based semiconductor devices.

Features of HOYA SiC
The SiC substrate developed by HOYA has the following features:

- Monocrystal 3C-SiC (cubic): SiC has a variety of crystalline polytypes, each depending on the combination of stacking orders in the atomic-plane. Of these polytypes, 3C-SiC allows the highest speed of electron transport within the crystal (the saturated velocity of electrons is 2.7 times that of Si). Due to its cubic crystalline structure, which is similar to that of more conventional Si, 3C-SiC is the best substrate material for high-speed, high-efficiency, highly integrated devices [1].

Low density of crystalline defects (less than or equal to 10/cm2): Hetero-epitaxial growth using chemical vapor deposition (CVD) is most often employed to form 3C-SiC monocrystal on a Si substrate. Due to differences in the lattice constants (i.e., the unit size of each crystal) of Si and 3C-SiC, however, a discontinuous atomic-bond layer is formed at the boundary between these two materials. This is described as a "lattice mismatch" Planar defects resulting from this mismatch penetrate the 3C-SiC layers, lowering the electron mobility in the 3C-SiC, and causing undesired leakage of electrical signals. Due to these drawbacks, this material was thought to be ill-suited for use as a semiconductor material. However, HOYA has now devised a defect-cancellation mechanism to eliminate planar defects within the structure of 3C-SiC. This mechanism is based on the principle that if ridges are formed on the Si surface in a given direction, the crystalline defects will collide with each other during CVD and cancel each other out. This defect-cancellation mechanism enables us to reduce the defect density in 3C-SiC to below 10 cm-2 While crystals of the other polytypes of SiC, such as 6H-SiC and 4H-SiC, have micropores called "micropipes" running through them, HOYA's 3C-SiC has, in principle, no such micropipes. Therefore, large semiconductor devices can be fabricated with high production yields.

- Large monocrystal substrate (monocrystal 3C-SiC wafers six inches in size or larger): To produce SiC polytypes other than 3C-SiC, sublimation at temperatures higher than 2000?[C is usually employed. However, it is difficult to control the temperature gradient and material supply using this method, limiting the manufacturable size to a maximum diameter of four inches. By contrast, 3C-SiC requires a lower fabrication temperature than any other SiC polytype (1500?[C or less). Thus, CVD can be used to form 3C-SiC on a Si substrate. Although CVD enables formation of 3C-SiC on an area as large as the Si substrate (at least six inches in diameter) at a low production cost, a two-layer structure, where several micro-meter sections of 3C-SiC are deposited on a Si substrate, has been commonly used because of low 3C-SiC deposition rates. HOYA has succeeded in raising the SiC growth rate to more than 50 times the conventional rate, using a newly developed SiC fabrication process. With this new process, large monocrystal 3C-SiC substrates (at least 200 micro-meter thick after removing the Si base layer) can be manufactured. These can then be handled using the same processes that are applicable to conventional Si substrates. Thus, HOYA's 3C-SiC substrate has the same geometry as typical Si wafers, and can be used in conventional Si semiconductor device production lines without hardware modifications.

Fig. 1. Free-standing, monocrystal 3C-SiC substrate (200 mm thick)

- Homogeneity and low price (area fluctuations less than or equal to 10%, 1/100 price per unit area): By holding fluctuations in thickness and the electrical properties along the basal plane at 10% or lower, the large substrate (currently 6") will reduce the volume-manufacturing costs of semiconductor devices. It is expected to be a more cost-competitive semiconductor substrate than the SOI (silicon on insulator) substrate that previously attracted attention as the next-generation semiconductor substrate for high-temperature, highly integrated devices. This substrate will be offered at a price 1/100 (per unit area) that of conventional SiC products currently available on the market.

-Wide variable range of resistivity (between 0.01-100 Ohms-cm): For effective use in semiconductor devices, SiC must allow for a wide range of resistivity control, via the regulated addition of impurities (doping). HOYA has developed an effective N-doping method that allows for a range of resistivity control that covers four orders of magnitude, 0.01-100 Ohms-cm (impurity concentration 1x1015-1x1019/cm3).

Fig. 2. Change in resistivity as a function of carrier (donor) density

The physical and geometrical features that result from the use of HOYA's 3C-SiC substrate in semiconductor devices provide the advantages set forth in the following table.

Table 1. Features of HOYA's 3C-SiC substrate

Applications and Merits of HOYA's 3C-SiC

Substrate for power devices: In power devices (e.g., rectifiers and inverters), there is a strong demand for energy conservation, and the breakdown voltage of these devices has been improved year after year. The demands for higher efficiency and faster operation speeds continue to grow. Though widely used for years, Si has reached its physical limits in terms of these demands. By contrast, the advantages of SiC over Si as a material for power devices have become clear in recent years. SiC has a forbidden band gap that is at least twice as large as that of Si; it also exhibits at least three times the heat conductivity. Therefore, using SiC in the substrate material for power devices results in a higher breakdown voltage, lower power consumption, and successful operation at higher temperatures. The power loss in semiconductor devices using SiC substrate should be half that of devices using Si. Such SiC semiconductors will first be used as rectifiers, and will later be applied to a wide range of uses, from PC-mounted switching regulators to motor drives in electric vehicles. With HOYA's 3C-SiC, high-performance SiC-based power devices should soon be manufactured in high-production runs, without requiring significant modifications to conventional production lines or processing plants.

LSI:The design rule stipulating scales of 0.1 micro-meter or less for LSI circuits poses serious problems, such as current leakage and excessive heat generation. Such problems arise from the physical limitations of Si. The wide band gap and high saturation velocity of electrons in 3C-SiC make it a promising alternative that is expected to resolve problems related to dense circuits. For example, the electron velocity in 3C-SiC is not impaired even when a high electric field is applied. This will enable operations on Gigahertz order, since 3C-SiC has a saturated velocity of electrons 2.7 times than that of Si. Additionally, the production of thermally generated electrons and holes is prevented in 3C-SiC; therefore, the problems of current leakage and heat generation that are present in Si-based semiconductors do not arise when using SiC. Since the 3C-SiC substrate that HOYA has developed has a particularly high applicability to the conventional Si-based semiconductor manufacturing process, it is feasible to manufacture 3C-SiC using existing facilities. The use of SiC is expected to enable manufacturers to surpass the limitations of Si-based LSI, allowing for closely packed circuits at gate-lengths of 0.1 micro-meter or less, and GHz-level high-speed operations.

Substrate for light-emitting devices: SiC has a lattice constant consistent with that of gallium nitride (GaN), which has attracted attention as a promising blue-light emitting device. Furthermore, since SiC is a conductor, it can be used in the back-side electrode of light-emitting devices, whereas sapphire, the conventional substrate material, is an insulator, which limits the structure and fabrication of GaN-based light-emitting devices. The structural and physical merits of SiC have already led to its adoption for use in connection with blue light-emitting devices, and the use of SiC as the substrate material for GaN light-emitting devices has been found effective in improving device performance. However, the high price of SiC has prevented mass production of such high-performance GaN light-emitting devices. HOYA's large, low-priced 3C-SiC substrate will reduce manufacturing costs and broaden the potential applications of GaN-based light-emitting devices.

Sensor devices: SiC is considered to be a promising material for sensors, such as pressure, temperature, UV, and infrared sensors. When SiC is used as a sensor material, it provides the advantages of a large operating-temperature range and reduced thermal fluctuation. Indeed, 3C-SiC-based pressure sensors have a higher sensitivity than Si-based sensors when used at temperatures exceeding 200[C. SiC is also mechanically and chemically stable, so it can be used in the manufacture of sensors for use in extreme environments, such as high-temperature or corrosive environments. Since the electrical properties of HOYA's large, low-cost 3C-SiC do not fluctuate independently of its crystal orientation, it can justifiably be considered an optimum sensor material that meets the requirements for operational stability and cost efficiency.

Product Impact