Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Heterojunction formed between semiconductor materials which...
Reexamination Certificate
1998-06-11
2001-05-01
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Heterojunction formed between semiconductor materials which...
C257S190000, C257S077000, C257S621000, C257S774000, C257S777000
Reexamination Certificate
active
06225651
ABSTRACT:
TECHNOLOGICAL FIELD
This invention relates to a structure with a microelectronic component made of a semi-conductor material difficult to etch and with metallized holes.
More precisely, it relates to the field of producing integrated circuits on semi-conductor materials with a large forbidden band, such as silicon carbide and gallium nitride. It is mainly concerned with structures or electronic devices that require metal electrodes to be carried on the back face of the components. In a more particular way, it relates to the creation of metallized holes that pass through an active layer of semi-conductor components.
Silicon carbide and gallium nitride are semiconductor materials with a large forbidden band which have physical and electrical properties that make them suitable for
hyperfrequency power electronic applications
electronic applications operating at high temperatures,
optronic transmission or detection applications working in the ultra-violet range.
The development of these materials is at present in the development phase. In the case of silicon carbide, it is noted that at present there is a great difference in the crystalline quality between thin films made epitaxial on the same semi-conductor (homo-structure) and thin films produced on another material (hetero-structure). The thin films obtained in accordance with the second method are generally polycrystalline or granular mono-crystalline or may also include numerous defects and dislocations. This has, as a consequence, a significant general degradation of the electrical characteristics of the electronic components produced on this structure. In addition, the crystallographic arrangements of this semi-conductor are such that they limit the possibility of controlling whether one crystalline polytype is obtained rather than another. By way of example, the cubic kind of polytype called 3C—SiC or &bgr;SiC is the only crystalline polytype that can be produced on silicon (cubic system). However, because of its electrical properties, this polytype is not the best candidate for the majority of electronic devices, the polytypes of the &agr;-SiC family of the hexagonal kind called 4H and 6H—SiC performing better in this case. These are at present developed on massive substrates of the same kind and cannot be made epitaxial on silicon. This method of development does not permit the production of structures where a thin film of silicon carbide, of very good crystalline quality is arranged on other insulating and/or semiconductor materials such as silicon oxide and silicon respectively. The production of an insulator embedded in a homo-structure is possible using ionic implantation methods but this method generally leads to films being obtained that include a high level of defects that are harmful to the production of semiconductor components.
Only the present-day methods of transferring thin films by sticking allow the creation of structures that include a thin film that can be used for the development of modern semi-conductor components. This sticking method can be associated, either with a prior implantation of species that generate a fracture zone, or with a method of removal by etching, honing or polishing which leads to the isolation of a thin film.
Silicon carbide is a material which is very stable both chemically and physically within the temperature range of 0 to 1000° C. The hardness and the density of this material are such that etching of this material is very difficult. Etching by a wet route is carried out in molten salt baths at temperatures above 500° C.
Methods of etching silicon carbide by a dry route require numerous masking studies to be made to find the correct etching conditions. In a general way, Reactive Ion Etching (RIE) equipment enables etching speeds less than 0.2 &mgr;m/min to be achieved. Systems employing high flux densities (plasma assisted by micro-waves) are the only ones at present that give speeds greater than previous speeds. The different types of masks studied are of the organic or the mineral type. Contrary to masks of the organic type (photosensitive resins) which permit very small depths of etched areas (less than 0.5 &mgr;m), masks of the mineral type (nickel-aluminium) generate very high etching selectivities. These selectivities allow etched depths of the order of a few micro-metres to be achieved, a performance obtained together with “reasonable” etching speed. The best etching results by a dry route are currently speeds of the order of 0.3 &mgr;m/min, the depths of the etched zones being of the order of a few micro-meters. These results are given in the following articles:
J. B. Casady et al. “Reactive Ion Etching of 6H—SiC using NF
3
”, International Conference on Silicon Carbide and Related Materials 1995, (Kyoto, Japan), Inst. Phys. Conf. Ser. No. 142, Chapter 3;
F. Lanois et al. “Angle Etch Control for Silicon Power Devices” Applied Physics Letters, Vol. 69, 8th July 1996).
The thickness of present-day SiC substrates being between 200 and 400 &mgr;m, the etching times to create holes passing right through cannot be contemplated for a massive silicon carbide structure (a homo-structure).
The development of gallium nitride, at the present time only in the form of thin films is carried out, in certain cases starting with a substrate made of silicon carbide. The problem is therefore identical for components produced on a thin film of gallium nitride.
Among the applications envisaged for these circuits developed on silicon carbide or gallium nitride one may mention hyperfrequency power applications and matrices and component networks.
For the first type of application, the components will operate at high frequencies (about 1 GHz and beyond) and will deal with high power. These components, usually produced in silicon and/or gallium arsenide, are then of the “field effect transistor” type. Such applications are in the course of development on silicon carbide or gallium nitride. The sequences that permit the production of such components are very similar to those currently developed for use on silicon or gallium arsenide.
Power systems operating at high frequencies demand the design of transistors having very precise dimensional characteristics. One should mention the very small gate lengths, less than a Am and the large gate widths, of the order of a millimeter. All these devices must be assembled on an insulating or semi-insulating support so as to get as close as possible to maximum accessible performance:high frequencies (f
T
, f
max
) and high added power efficiencies.
These transistors are designed by
numerous gate bars, structures that require the interdigitation and metallization of drains and sources. The same metallization step for the gate contacts and drain contacts forms two respective buses which through their interleaving excludes any contact being made onto the source areas situated in the centre. It is essential that the sources be connected to one another.
The current technology makes use of methods called air bridges, the metallizations being connected to one another through the front face. These methods require a large minimisation of all the parasitic elements (capacities and inductances) which these structures can create. Over and above certain critical values, the general electrical performance of the transistors is then limited by these elements. A second technique consists therefore of creating metallized holes that pass through the entire thickness of the semi-conductor material.
The interconnection of all the electrodes of common sources is then made through these metallized holes and lines or metallized planes situated on the rear face of the component. The component to be produced will therefore have to have available gate and drain contacts on the front face and source contacts on the rear face, the majority of the thickness of the substrate being of the insulating, semi-insulating or highly resistive type. Such vertical connections allow in addition, better thermal dissipation of the component.
At the present time, the production of hyperfrequency or op
Commissariat A l'Energie Atomique
Jackson, Jr. Jerome
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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