FR2530383A1 - Monolithic integrated circuit comprising a Schottky logic section and a programmable memory with fuses - Google Patents

Abstract

In this integrated circuit, the same layer 5, serving as a diffusion barrier with respect to a layer of metallisation 4 for a Schottky contact 3, serves at fuse locations 6. According to the invention, the stop layer is a layer of tungsten titanium sprayed in the presence of nitrogen.

Description

MONOLITHIC INTEGRATED CIRCUIT COMPRISING A LOGICAL PART
SCHOTTKY AND A PROGRAMMABLE FUSE MEMORY.

The present invention relates to the field of monolithic integrated circuits with programmable memory, and more particularly to such circuits in which the logic part is with Schottky contact, for example circuits of the TTL family.
LS (Transistor Transistor Logic Low Power Schottky).

 Usually, programmable fuse memories (PROM) contain nickel chromium-based elements as fuse elements. Thus, if one wants to combine in monolithic form a logic circuit and a PROM with fuses, it is necessary, in addition to the metallurgical steps specific to the formation of contacts of the logic circuit, to provide additional metallurgical operations to form layers of nickel chrome of desired configuration. This considerably complicates the manufacturing processes and eventually leads to solutions that are difficult to metallurgically compatible (incompatible manufacturing temperatures, creation of stack effects, corrosion of metals, etc.).

 Thus, an object of the present invention is to provide a monolithic integrated circuit combining a logic circuit and a fused PROM without requiring an additional metallurgical step compared to those required for the manufacture of the logic circuit alone.

 On the other hand, it is desirable that the fusible elements be thermally insulated as well as possible. To do this, it is proposed to arrange in the air the portion of fuse brought to high temperature.

 Thus, another object of the present invention is to provide a method of manufacturing a particular structure for the arrangement in the air of a fuse in monolithic integrated structures where there are provided layers of polycrystalline silicon.

To achieve these objects, the invention consists in using portions of the barrier layer existing between a Schottky metallization and a contact metallization as fusible elements. For this, the present invention provides for choosing a barrier layer consisting of a titanium pseudo-alloy
nitrogen-doped tungsten. The term “doped with nitrogen” is understood to mean the fact that the layer of tungsten titanium comprises inclusions of nitrogen atoms as a result of the spraying process of this metallization of tungsten titanium in the presence of nitrogen, and preferably in an atmosphere of 'argon and nitrogen. This doped layer will be called TiW-N layer.

 According to another aspect, the present invention consists of a method of manufacturing an integrated circuit comprising a layer of TiW-N resting on a layer of polycrystalline silicon cut by chemical attack according to the profile of the layer of TiW-N, this attack chemical being continued so that there occurs an additional attack of polycrystalline silicon under the TiW-N so that the narrowed parts of the layer of TiW-N, corresponding to fuse zones, are in the air.

These objects, characteristics and advantages as well as others of the present invention will be explained in more detail in the following description of particular embodiments made in relation to the attached figures, among which
FIG. 1 is a sectional view of a first embodiment of the present invention, and
- Figures 2A to 2F are sectional and perspective views of successive stages of manufacture of a second embodiment of the present invention.

Figure 1 shows in its left part (I) a con
tact Schottky on a semiconductor wafer and in its right part (II) a fuse element.

The left part (I) of figure 1 represents
typical metallurgical layers linked to the existence of structures
to Schottky contact. Reference 1 designates a sili substrate
cium. Reference 2 designates an oxide layer. In a window formed in the oxide layer, a layer of metal 3 intended to establish Schottky contact is deposited and optionally combined with a lightly doped portion of the silicon wafer 1. This layer 3 can for example be consisting of a platinum / silicon alloy (PtSi) or any other metal capable of forming a silicide such as Pd, Ti, W, Mo, Cr, V. It is then a question of establishing a connection with the Schottky contact layer 3.If this connection is made directly with an aluminum layer 4, aluminum being one of the most commonly used materials in integrated circuit connections, diffusion phenomena occur from aluminum to the silicon through the PtSi layer and from the silicon to the aluminum. This results in rapid degradation of contact
Schottky don # t the characteristics change. Thus, it is usual to provide between the Schottky contact layer 3 and the connection layer 4 a stop layer 5 avoiding metallic diffusions.

 Among the stop layers 5 which can be used, there are in previous embodiments ~ TiW layers (usually 10% by weight of titanium and 90% by weight of tungsten deposited by sputtering under an argon atmosphere). Such layers, in order to have a satisfactory metallic diffusion blocking capacity, must have a thickness at least equal to approximately 1500 angstroms. This thickness is too large, given the resistivity of the TiW (60 to 90 microhmscentimeters), to allow the manufacture of a fuse which can be destroyed by a current of reasonable intensity.

 On the other hand, it has been proposed by certain authors (Nowicki et al, Thin Solid Films, 53 <19.78) pages 195-205), to improve the stopping characteristics of the TiW layer by spraying in the presence argon and nitrogen, which introduces nitrogen inclusions in the TiW layer.

However, a disadvantage of this technique with regard to Schottky diodes is that the resistivity of TiW increases considerably and passes to values of the order of 100 to 500 microhms-centimeters, which increases the value of the series resistance of the diode. . The present invention aims precisely to take advantage of this drawback by using a more resistive TiW-N layer to form a fuse as shown in the right part of FIG. 1. In addition, this TiW-N layer can be thinner that the layers of
TiW classics. It will be noted that the TiW-N layer can be etched by chemical attack before the deposition of the aluminum layer 4 but, it is preferred according to the present invention to uniformly deposit the TiR 5 layer and then the layer of aluminum 4, to etch these two layers together to form aluminum strips resting on TiW and then only to etch, at locations 6 where it is desired to form the fuses, the aluminum layer by means of a selective product with respect to TiW and Si02.

In top view, the portion of fuse 6 may for example have a length of the order of 2 microns for a width of 1 micron, that is to say that the strips of aluminum and
TiW will be engraved to a twist width of one micron and that the cuts in aluminum alone will have widths of the order of 2 microns.

 FIGS. 2A to 2F represent successive stages in the manufacture of a particular embodiment of the present invention #in which one aims to manufacture a monolithic integrated circuit comprising on at least one of its faces zones where one establishes ohmic contact with the semiconductor, areas where a Schottky contact is established with this semiconductor, and fuses.

 FIG. 2A very schematically represents a portion of the upper surface of a monolithic integrated circuit. By way of example, a layer 10 of type N is represented in the figure as a substrate, but generally corresponding in fact to an epitaxial layer formed on a substrate of type P. In this layer 10, zones are formed P-type 11 and 12, for example by diffusion. Conventionally, in bipolar integrated circuits, deep diffusions 13 of the P + type are provided intended to join the substrate on which the epitaxial layer is formed to form isolation walls. The semiconductor surface is covered with a layer of silica (SiO2) 14 open at certain locations 15.

 -As shown in FIG. 2B, a layer of polycrystalline silicon 16 is deposited on the wafer being treated, which rests on the layer of SiO2 14 and comes into contact with the semiconductor at locations 15 where this layer of SiO2 has been opened. The layer of polycrystalline silicon is generally covered with a thin layer of silica 17 and is localized by photogravure and chemical attack or any other known means (lift off ...). This polycrystalline silicon layer 16 can be heavily doped with the Nf type and serve as a source of dopant to form in the semiconductor wafer zones 18 of the type at the level of the openings 15.

 As shown in FIG. 2C, Schottky contacts can then be formed in certain selected locations, for example by depositing and alloying a layer of platinum with the semiconductor surface. In the case of the figure, this layer of platinum silicide 19 is formed at the boundary between a layer 12 of type P and layer 10 of type N. It is thus possible to obtain an ohmic contact with a base and a Schottky contact with a collector, the N-type layer 10 being lightly doped.

During the steps, the result of which is illustrated in FIG. 2D, the layer of silica 17 formed on the polycrystalline silicon layer 16 is first removed and a layer of TiW 20 is deposited uniformly. As has been explained above, this layer can actually be TiW-N which has better barrier layer characteristics in the diode areas
Schottky and better resistivity characteristics in areas where it is desired that this layer be used as a fuse. In addition, an aluminum layer is deposited and cut, three bands 21, 22 and 23 of which are shown in the figure.

 Then, as shown in FIG. 2E, the layer 20 of TiW-N is etched in particular to form narrowed locations 25 at the level of which the fuse is liable to melt. In the figure, this fuse zone 25 is located between the aluminum strips 21 and 22.

 Finally, in a last step illustrated in FIG. 2F, the polycrystalline silicon is etched using the TiW-N layer as a mask. This etching is continued long enough to partially attack the polycrystalline silicon under the edges of the TiW-N layer. Thus, at the level of the shrinkage 25 of the layer 20 of TiW-N, the underlying polycrystalline silicon is completely removed. Consequently, the narrowed zone is in the air and forms a bridge between two portions of polycrystalline silicon 16.

This structure has many advantages. On the one hand, the fact that the narrowed part serving as a fuse for the Ti-NW layer is in the air ensures better thermal insulation of this portion and promotes its melting when desired. On the other hand, at the other locations, the layer of
TiW-N rests on a layer of polycrystalline silicon and thus we take advantage of the conductivity of polycrystalline silicon which is better than that of TiW-N for current supply to the fusible zone, in particular if we take into account practical orders of magnitude layer thicknesses which are of the order of 2000 angstroms for the polycrystalline silicon layer, and 1000 angstroms for the TiW-N layer. The width of the narrowed area serving as a fuse can be of the order of 1 to 2 microns.

 Thus, TiW-N plays a double role of barrier layer and fuse and, similarly, polycrystalline silicon plays a triple role, namely of diffusion source for the formation of N + type zones, of ohmic contact on the zones to which an ohmic contact such as zone 18 of FIG. 2F is desired, and of a conduction layer for bringing the current to the fuse zones 25.

 The present invention is not limited to the embodiments which have been explicitly described above; it includes the various variants and generalizations thereof included in the field of claims below.

Claims (3)

 1. Integrated circuit comprising Schottky contacts separated from an aluminum connection metallization by a metallization of tungsten titanium (TiW) serving as a stop layer, characterized in that the layer of tungsten titanium contains nitrogen and in this that it extends on the one hand to the contacts Schottky, on the other hand on isolated parts of the surface of the wafer where it is used to form fuses.  2. Integrated circuit according to claim 1 characterized in that the Schottky contacts consist of platinum silicide formed on lightly doped silicon and in that the connection layer with respect to which the stop layer serves as a barrier diffusion consists of aluminum.  3. Integrated circuit according to one of claims 1 or 2, characterized in that the layer of tungsten titanium doped with nitrogen rests on a layer of highly doped polycrystalline silicon except at the locations where this layer of TiW-N is shrunk to form a fuse, locations where this layer of TiW-N is in the air.