Process for fabrication of a SIMOX substrate

Details Process for fabrication of a SIMOX substrate Process for fabrication of a SIMOX substrate

2002-11-26

2004-05-25

Elms, Richard (Department: 2824)

Semiconductor device manufacturing: process

Formation of electrically isolated lateral semiconductive...

Total dielectric isolation

C438S404000

Reexamination Certificate

active

06740565

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an SOI substrate comprising an oxide layer buried near the surface of a silicon substrate, with a single crystal silicon layer (hereinafter referred to as SOI (Silicon-On-Insulator) layer) formed thereover. More specifically, it relates to a process for fabrication of an SOI layer by SIMOX (Separation by Implanted Oxygen) technology.
BACKGROUND ART
SIMOX wafers and bonded SOI wafers are known as the primary types of SOI wafers that have single crystal silicon layers formed on insulators such as silicon oxide. SIMOX wafers are SOI wafers obtained by implanting oxygen ions into a single crystal silicon substrate by implantation of oxygen ions, and subsequently performing annealing treatment for chemical reaction between the oxygen ions and silicon atoms to form a buried oxide layer. Bonded SOI wafers, on the other hand, are SOI wafers obtained by bonding two single crystal silicon substrates, one of which is oxidized but the other is not, and thinning one of the two wafers.
Among such SOI wafers, SIMOX wafers are characterized by especially excellent layer thickness uniformity because the SOI layer thickness can be controlled by the oxygen ion implantation depth. In a SIMOX wafer, the SOI layer can be formed to a thickness of 0.3 &mgr;m or smaller, and satisfactory thickness control can even be achieved with SOI layers of approximately 0.1 &mgr;m thickness or smaller. In particular, SOI layers with thicknesses of 0.1 &mgr;m and smaller are generally applied for formation of fully depleted MOS-LSIs, where the film thickness of the SOI layer itself is directly proportional to the threshold voltage for the MOSFET operation; the SOI layer thickness uniformity is therefore an important quality for high-yield fabrication of high-performance devices. From this standpoint, SIMOX wafers with excellent SOI layer thickness uniformity are promising as next generation MOSFET wafers.
Since a MOS-LSI fabricated on a SOI substrate is electrically insulated from the substrate body in the device-formed region by a buried oxide layer as the insulator, it is possible to achieve excellent characteristics including improved radiation resistance or latch-up resistance, low power consumption operation and super-high speed operation. As disclosed in Japanese Unexamined Patent Publication HEI No. 9-64320, electrically separating the device-formed region from the substrate can reduce the effect of the junction capacitance between the device-formed region and the substrate, thereby providing an advantage for high-speed devices such as baseband processors.
Japanese Unexamined Patent Publication HEI No. 9-64320 proposes increasing the resistivity value of the support substrate of the SOI substrate to 50 &OHgr;cm or greater, as a technique for further improving the characteristics of a super-high frequency device formed on the SOI substrate. It is explained that the SOI substrate is produced by a “bonding method” whereby a high-resistance single crystal silicon substrate manufactured by a floating zone method is used on the support substrate side and a low-resistance single crystal silicon substrate manufactured by the Czochralski process is used on the device-formed side, and bonding is performed after thermally oxidizing each of the surfaces to form oxide films of the desired thickness, after which the silicon substrate on the device-formed side is polished to a desired thin thickness.
Although the high-resistance support substrate for the bonded SOI substrate proposed in Japanese Unexamined Patent Publication HEI No. 9-64320 can be easily manufactured by the aforementioned floating zone system, this type silicon substrates contain no oxygen, and therefore their mechanical strength is insufficient, slip tends to occur, and it is difficult to fabricate large-diameter substrates with diameters of 8 inches (200 mm) or greater.
Moreover, polishing of the silicon layer to serve as the device-formed section in an SOI substrate obtained by a bonding method results in deterioration of the film thickness uniformity, such that silicon layer thicknesses of about 0.1 &mgr;m cannot be formed with satisfactory layer thickness uniformity.
In attempting to solve the former problem, utilization of single crystal silicon substrates by the Czochralski process can be considered. A single crystal silicon substrate obtained by the Czochralski process has excellent mechanical strength due to an oxygen concentration of approximately 10
18
cm
−3
, while large-diameter substrates of 8 inches (200 mm) and larger can also be manufactured. Furthermore, by adjusting the impurity addition during crystal growth it is possible to manufacture a single crystal silicon substrate with a resistivity of 100 &OHgr;cm or greater. (For discussion on the relationship between resistivity and impurity concentration in silicon substrates, see, for example, “Physics of Semiconductor Devices (2nd Edition)” ed. by S. M. Sze, (1981), John Wiley & Sons, Inc., p.32.)
In attempting to solve the latter problem, fabrication of SOI substrates by the SIMOX process has been considered. In the SIMOX process, the SOI structure is formed by oxygen ion implantation and high-temperature heat treatment, but since the oxygen implantation depth can be controlled by the ion acceleration energy, it is characterized by giving excellent layer thickness uniformity of the SOI structure obtained after high-temperature heat treatment, thereby allowing manufacture of silicon layers with satisfactory layer thickness uniformity even with thicknesses of around 0.1 &mgr;m.
However, since single-crystal silicon substrates obtained by the Czochralski process contain trace amounts of oxygen, heat treatment at relatively low temperatures of about 500° C., which is employed for sintering or the like after Al interconnect formation in device fabrication steps, results in generation of a thermal donor (or oxygen donor) and lowers the resistivity of the substrate to a level of about 10 &OHgr;cm. (Thermal donors are discussed, for example, in “Silicon Science”, Chapter 7, Section 3, edited by the USC Japan Semiconductor Technology Research Society.) In order to reduce this effect it is necessary to reduce the oxygen content of the silicon crystals, but because the molten silicon melt is retained in a quartz crucible in the Czochralski process, it is impossible to completely avoid dissolution of oxygen from quartz into the silicon melt, such that a natural limit exists for the range of reducing the oxygen concentration in the crystals. Consequently, when using a silicon substrate according to the ordinary Czochralski process, a reduction in the resistivity occurs due to the thermal donor after heat treatment at low temperature even if the resistivity is high immediately after fabrication of the substrate, such that the high resistivity cannot be maintained.
Moreover, since oxidation, or heat treatment corresponding to oxidation, is carried out at a high temperature of 1300° C. or greater in most cases in the SIMOX process, the oxygen solubility limit in the silicon substrate increases in response to the treatment temperature. Thus, the oxygen concentration of the substrate after SIMOX fabrication increases even when the oxygen concentration of the substrate material is drastically lowered, creating a problem in that reduction in the resistivity due to thermal donor generation cannot be avoided during low-temperature heat treatment in the device manufacturing steps.
The present invention overcomes these problems associated with SOI substrates with high-resistivity silicon substrates and their manufacturing methods, by providing higher quality high-resistivity SOI substrates.
SUMMARY OF THE INVENTION
The present inventors have newly discovered that by carrying out a step of maintaining a temperature of from 800° C. to 1250° C. for a minimal predetermined time in the final stage of high-temperature heat treatment following oxygen ion implantation during fabrication of an SOI substrate by the SIMOX process using a single crystal silicon subst

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