A silicon wafer bonding process is described in which only thermally grown oxide is present between wafer pairs. Bonding occurs after insertion into an oxidizing ambient. It is proposed the wafers are drawn into intimate contact as a result of the gaseous oxygen between them being consumed by oxidation, thus producing a partial vacuum. The proposed bonding mechanism is polymerization of silanol bonds between wafer pairs. Silicon on insulator (SOI) is produced by etching away all but a few microns of one of the bonded pair. Capacitor measurements show a 27 μs minority-carrier lifetime and no degradation of the SOI-insulator interface. In addition, there is negligible charge at the bonding interface making the technique attractive for three-dimensional as well as planar SOI applications.
Boron diffusion in ion-implanted and annealed single-crystal and amorphized Si is compared to determine the effect of amorphization on the initial transient boron motion reported for single crystal. The boron was implanted at 20 keV and at doses of 1×1015 and 3×1015cm−2. The Si was either preamorphized or postamorphized to a depth of 320 nm by implantation of Si ions at three different energies. In the amorphized samples the entire boron profile was always contained within this distance. The samples were annealed by furnace or rapid thermal annealing to 900–1100 °C with or without a preanneal at 600 °C. The initial rapid diffusion transient in the tail region of the boron profile was observed in all the crystal samples. This transient was totally absent in the amorphized samples. This is manifest by careful comparison of boron concentration profiles determined by secondary ion mass spectrometry of single-crystal and amorphized samples after annealing. For anneals where significant motion occurs, the profiles of the amorphized samples could be fit with a computational model that did not include anomalous transient effects. It is proposed that excess interstitials cause the transient diffusion in the case of the crystalline samples. The source of interstitials is believed to be provided by the thermal dissolution of small clusters that are formed by the implantation process. They exist for only a short time, during which they enhance the boron diffusion. Since there is no enhanced diffusion in the amorphous region that regrows to single crystal, apparently interstitial clusters are neither produced by nor do they survive the regrowth process in that region. In addition, the interstitials generated by the damage beyond the amorphous-crystalline boundary are prevented from entering the regrown region by the dislocation loops formed at that boundary which act as a sink consuming the interstitials diffusing toward the surface.
The development of DRAM at IBM produced many novel processes and sophisticated analysis methods. Improvements in lithography and innovative process features reduced the cell size by a factor of 18.8 in the time between the 4Mb and 256Mb generations. The original substrate piate trench cell used in the 4Mb chip is still the basis of the 256Mb technology being developed today. This paper describes some of the more important and interesting innovations introduced in IBM CMOS DRAMs. Among them, shallow-trench isolation, 1-line and deep-UV (DUV) lithography, titanium salicidation, tungsten stud contacts, retrograde n-well, and planarized bacl(-end-ofline (BEOL) technology are core elements of current state-of-the-art logic technology described In other papers in this issue. The DRAM specific features described are borderless contacts, the trench capacitor, trench-isolated cell devices, and the "strap." Finally, the methods for study and control of leakage mechanisms which degrade DRAM retention time are described.^Copyright 1995 by International Business Machines Corporation. Copying in printed form for private use is permitted witiiout payment of royalty provided tliat (1) each reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract, but no other portions, of this paper may be copied or distributed royalty free without further permission by computer-based and other information-service systems. Permission to republish any other portion of this paper must be obtained from the Editor.
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