Electrically resistive structure

Details Electrically resistive structure Electrically resistive structure

1997-03-10

2001-10-02

Lee, Eddie C. (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Combined with electrical contact or lead

Of specified material other than unalloyed aluminum

C257S767000, C257S536000, C257S537000

Reexamination Certificate

active

06297556

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an electrically resistive structure comprising a substrate which is provided on at least one side with a first and a second film of resistive material, the materials of the first and second films being mutually different.
An electrically resistive structure of this type is known from European Patent Specification EP-B 0 175 654, wherein an Al
2
O
3
substrate is consecutively provided with resistive films of cermet and NiCr. Since the sheet resistance of the NiCr film is significantly lower than that of the cermet film, such a structure may be viewed as an in-plane parallel arrangement of a high-ohmic resistor (cermet) and a low-ohmic shunt (NiCr).
When a structure of this type is embodied as a connecting double strip between two terminal points on the substrate, its in-plane electrical resistance between those points can be successively increased by, for example:
Increasing the path-length of the structure, or decreasing its width;
Etching away the low-ohmic shunt strip;
Increasing the path-length of the remaining high-ohmic resistor strip, or decreasing its width.
Such procedures can be controllably performed with the aid of well-known selective masking and etching techniques, such as elucidated for example in the book “The Chemistry of the Semiconductor Industry”, edited by S. J. Moss and A. Ledwith, ISBN
0-216-92005-1,
Blackie & Son, London (1987), in particular in chapters 9 and 11. In this manner, it is possible to produce a substrate having on its surface well-defined strips of resistive material which demonstrate a variety of accurately trimmed resistances. When provided with external electrical contacts, such strips serve as integrated resistors, so that it is possible to achieve an entire integrated thin film resistor network upon the substrate.
In the interest of maximising the range of possible resistance values which can be achieved on any given specimen substrate, it is advantageous if the sheet resistances of the materials of the first and second films differ by at least one order of magnitude (i.e. factor of ten), and preferably by several orders of magnitude (such as a factor of 1000). In addition, the achievement of well-defined resistance tolerances over a relatively wide temperature range requires the resistive structure to have a stabilised Temperature Coefficient of Resistance (TCR).
For purposes of clarity, the sheet resistance R
□
of a film of thickness t comprising a material of electrical resistivity &rgr; is here defined as R
□
=&rgr;/t.
The inventors have observed that the TCR of various resistive materials in a single-layer configuration can generally be significantly stabilised by subjecting those materials to an annealing step, typically performed at a temperature of about 350-550° C. in a gaseous atmosphere (comprising, for example, air, nitrogen or argon). In the case of a bilayer resistive structure, however, it has unfortunately been observed that subjection of the structure to such annealing treatment generally causes deterioration of the properties of at least one of the structure's component resistive materials. In particular, the TCR-value of at least one of the materials may change significantly from that which was originally intended. In addition, annealing can lead to a substantial reduction of the difference in sheet resistance between the first and second resistive films, particularly when this difference is originally large (e.g. factor 100-1000).
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electrically resistive structure as described in the opening paragraph, in which structure a sizeable magnitude-difference between the sheet resistances of the first and second resistive films can be achieved and maintained. It is a further object of the invention that the resultant TCR of this structure should be essentially stable and predictable. In particular, it is an object of the invention that these properties should remain well-preserved in the event that the resistive structure is subjected to an annealing treatment as hereabove set forth.
These and other objects are achieved in an electrically resistive structure according to the opening paragraph, characterised in that an anti-diffusion film (i.e. diffusion barrier) is disposed between the first and second resistive films.
The invention rests upon the inventors' observation that, in the known resistive structure, annealing treatment can induce significant material interdiffusion at the interface between the adjacent first and second resistive films. Since these films are typically thin (generally of the order of a few hundred nanometers), the migration of even a small quantity of metal ions from a low-resistance film (e.g. CuNi) into a bordering high-resistance film (e.g. CrSi) can cause a dramatic decrease in the sheet resistance of the latter film, whereby an initially sizeable magnitude-difference between the sheet resistances of the two films is consequently sharply reduced. Such migration effects also tend to significantly alter the TCR of the component films from its desired value. The presence of the inventive diffusion barrier stringently inhibits these unwanted effects.
Use of such an anti-diffusion film according to the invention allows considerable simplification and acceleration of the resistive structure's manufacture. This is because the entire structure can be annealed in one step, once the various films have been deposited in an unbroken deposition cycle. In the absence of the inventive diffusion barrier, any attempt at thermally-induced TCR stabilisation would have to be carried out on a tedious and generally less effective film-by-film basis, whereby the structure would have to be repeatedly annealed after deposition of each individual film.
The anti-diffusion film in accordance with the invention is preferably an electrical conductor, thereby ensuring uniform electrical contact between the lower and upper resistive films. Such electrical contact has the advantage that it allows the lower resistive film to be conveniently contacted via randomly placed electrical terminals on the surface of the upper resistive film.
However, the inventive diffusion barrier need not necessarily comprise electrically conductive material. In such a case, electrical contact with the lower resistive film cannot conveniently be made via the upper resistive film, but must instead be achieved separately, e.g. with the aid of bridging edge contacts, or exposure of part of the lower film by lithographic removal of overlying material.
In addition to its primary ability to hinder diffusion, the material of the diffusion barrier should favourably have a low TCR (less than or of the order of 50 ppm/K), and should preferably be such that it can conveniently be deposited by conventional industrial means such as, for example, sputtering or vapour deposition (physical or chemical).
In the light of such desirable properties, very satisfactory results have been obtained with resistive structures in which the inventive anti-diffusion film comprises a WTi alloy. In particular, a highly effective embodiment of the inventive structure is characterised in that the material of the anti-diffusion film is comprised of a WTiN alloy, and preferably contains at least 95 mol. % (W
x
Ti
1−x
)
y
N
1−y
wherein both x and y lie in the range 0.7-0.9 (the remaining 5 mol. % of the film being allowed to comprise other substances, present as additives or impurities). Such a WTiN film is electrically conductive, typically has a TCR of less than 30 ppm/K, and can be conveniently deposited by, for example, sputtering a WTi alloy target in an atmosphere containing nitrogen gas. A minimal diffusion barrier thickness of about 100 nm is generally sufficient to ensure its effective performance.
An example of a suitable non-conductive material for use in the inventive anti-diffusion barrier is SiO
2
.
Appropriate high-ohmic alloy materials for use in the inventive structure include, for example, CrSi,

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