Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
2002-12-02
2004-12-21
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C257S076000, C257S192000, C257S288000, C438S105000, C438S285000
Reexamination Certificate
active
06833562
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device utilizing a silicon carbide (SiC) substrate and its manufacturing method.
2. Description of the Related Art
A silicon carbide semiconductor (hereinafter, abbreviated as SiC) is capable of forming a pn junction and a forbidden band width thereof is wide as compared with other semiconductor such as a Silicon (Si) or a Gallium Arsenide (GaAs). It is reported that the forbidden band width of 3C-(C denotes a cubic system as will be described later) SiC is 2.23 eV (electron Volt), that of 6H-(H denotes a hexagonal system as will be described later) SiC is 2.93 eV, and that of 4H-SiC is 3.26 eV. As is well known, there are trade-off relationships in principle prescribed by the forbidden band width between an on resistance of a power device and a reverse direction withstanding voltage thereof and between the on resistance thereof and a switching frequency (switching speed) thereof. Hence, it is difficult to obtain a high performance exceeding a limit determined by the forbidden band of Si from currently available Si power devices. However, since, if the power device is constituted by SiC with the wide forbidden band width, the above-described trade-off relationships are largely relieved, such a power device that the on resistance, the reverse direction blocking voltage, and the switching speed have remarkably or simultaneously been improved can be realized. Furthermore, since SiC is thermally, chemically, and mechanically stable and is superior in a radiation ray withstanding characteristic, it is expected that SiC can be realized not only as a high frequency device and the power device but also as an environment withstanding characteristic semiconductor device which operates under a strict condition such as a high ambient temperature, an erosion, and radiation ray irradiation.
In a MOS (Metal Oxide Semiconductor) capacitor, SiC power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) to control a large current, and an IGBT (Insulated Gate Bipolar Transistor) to control the large current especially from among SiC devices, it is an important problem to be solved for SiC devices to be put into practice that a contact resistance on a source and a drain (n type polarity) which provides causes of a thermal loss increase and of an operating speed reduction is reduced to a negligible level and highly reliable and high performance gate insulating film and MOS interface characteristic are realized.
A technology to obtain a low contact resistance in SiC singlecrystalline has been proposed. That is to say, the following method has been proposed. After a contact metallic film is formed on SiC through a vacuum deposition, a rapid thermal annealing (RTA) is carried out for several minutes at a high temperature heat process (so-called, a contact annealing) is carried out for several minutes at a high temperature equal to or higher than 950° C. under a vacuum or inactive gas atmosphere to form a reaction layer between SiC and the contact metal which provides a contact electrode. According to a Journal of Applied Physics, 77, page 1317 (1995) authored by J. Crofton et al., n type region of SiC substrate using an Ni (Nickel) film indicates the contact resistance of an extremely low practical level in an order of 10
−7
&OHgr;cm
2
. According to a book authored by J. Crofton called Solid-State Electronics, 41, page 1725 (1997), p type region of SiC substrate using an Al(Aluminum)-Ti(Titanium) alloy film indicates the contact resistance of the extremely low practical level in an order of 10
−6
&OHgr;cm
2
. In addition, in recent times, the low contact resistance in an order of 10
−7
&OHgr;cm
2
is also obtained in each of n type region and p type region of 4H—SiC substrate using a thin Ni and a Ti—Al laminated layer.
SUMMARY OF THE INVENTION
However, it has been determined that the well known RTA process described above (contact annealing) gives a harmful effect on the reliability of the gate insulating film and MOS interface characteristic if the RTA process is applied simply to the actual device. For example, a paper announced at 1999 of T. Takami at al. Extended Abstract of Symposium on Future Electron Devices 2000 (Tokyo), FED-169, page 127, (1999) has described a manufacturing method of the MOS capacitor in which, after the RTA process for one minute was carried out at 1000° C. under the vacuum atmosphere on a thermal oxide film of about 48 nm thickness formed on an n type 4H-SiC substrate having n type epitaxial growth layer, an Al (Aluminum) electrode was formed. Then, the paper has evaluated a current-voltage characteristic (I-V characteristic) of the manufactured MOS capacitor (refer to
FIGS. 1A and 1B
) and a high-frequency capacitance-DC bias voltage characteristic (C-V characteristic) thereof (refer to FIG.
2
). At this time, the following results were indicated as compared with a specimen to which no RTA process (without RTA) was applied. That is to say, the paper has indicated such specific data as described below and pointed out a seriousness of problem: (1) A withstanding voltage (a breakdown voltage) of the gate insulating film originally having about 40 volts was rapidly dropped to 40×⅛, viz., 5 volts or lower (refer to FIG.
1
A); (2) A leakage current of the gate insulating film was remarkably increased (refer to FIG.
1
A)); and (3) A flat-band voltage is shifted from an ordinary in proximity to 0 volts into a positive direction by 15 volts or higher (refer to FIG.
2
). There are many reports that have pointed out in the same way. It is of course that this problem places the same importance on the power MOSFET and IGBT having the same structure as the MOS capacitor.
As a solution of the problem described above, it can easily be conceived that the temperature of the RTA process (contact annealing) is reduced to, for example, 850° C. or lower. However, this method introduces another harmful effect such a special dislike effect on the power device as to increase the contact resistance on a source and a drain rapidly. Consequently, this method cannot be said any more a fundamental countermeasure of the above-described problem.
It is, therefore, an object of the present invention to provide a silicon carbide semiconductor device and its manufacturing method which can solve the problems of deteriorations of the gate insulating film and MOS interface characteristic caused by the RTA process during the formation of the contact on the SiC substrates without introduction of the increase in a contact resistance in an ohmic contact.
The above-described object can be achieved by providing a silicon carbide semiconductor device, comprising: a gate insulating film: an electrode member that is inactive to the gate insulating film; an insulating film that is inactive to the gate insulating film; and a singlecrystalline silicon carbide substrate, the gate insulating film being treated with a predetermined heat process after being enclosed with the electrode member, the insulating film, and the singlecrystalline silicon carbide substrate.
The above-described object can also be achieved by providing a silicon carbide semiconductor device, comprising: a gate insulating film: an electrode member that is inactive to the gate insulating film; a field insulating film that is inactive to the gate insulating film; and a singlecrystalline silicon carbide substrate, the gate insulating film being treated with a predetermined heat process after being enclosed with the electrode member, the field insulating film, and the singlecrystalline silicon carbide substrate.
The above-described object can also be achieved by providing a silicon carbide semiconductor device, comprising: comprising: a singlecrystalline silicon carbide substrate; a field insulating film formed on a surface of the substrate; a gate window opened in the field insulating film; a gate insulating film formed by a method including a thermal oxidization over the whole surface of the singlecrystalline
Okushi Hideyo
Tanimoto Satoshi
Forde Remmon R.
McDermott Will & Emery LLP
Nissan Motor Co,. Ltd.
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