US20090243101A1

US20090243101A1 - Method for forming interconnection levels of an integrated circuit

Info

Publication number: US20090243101A1
Application number: US12/411,944
Authority: US
Inventors: Patrick Vannier
Original assignee: STMicroelectronics Crolles 2 SAS
Current assignee: STMicroelectronics Crolles 2 SAS
Priority date: 2008-03-28
Filing date: 2009-03-26
Publication date: 2009-10-01
Also published as: EP2105959A2; US8492264B2; US20120040525A1; EP2105959A3

Abstract

A method for forming interconnection levels of an integrated circuit, including the steps of: (a) forming an interconnection level comprising conductive tracks and vias separated by a porous dielectric material; (b) forming, on the interconnection level, a layer of a non-porous insulating material, said layer comprising openings above portions of porous dielectric material; (c) repeating steps (a) and (b) to obtain the adequate number of interconnection levels; and (d) annealing the structure.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of French patent application number 08/52035, filed on Mar. 28, 2008, entitled “METHOD FOR FORMING INTERCONNECTION LEVELS OF AN INTEGRATED CIRCUIT,” which is hereby incorporated by reference to the maximum extent allowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an integrated circuit and, more specifically, to a method for forming interconnection levels of an integrated circuit.
2. Discussion of the Related Art
Integrated circuits are comprised of a large number of electronic components which are formed in and on a semiconductor wafer. To properly connect these components, several interconnection levels form the upper portion of the integrated circuits. Each interconnection level comprises conductive tracks. Vias are formed to connect conductive tracks of different interconnection levels.
FIG. 1 is a cross-section view of an example of the stack of several interconnection levels (N_j, N_i+1, N_i+2. . . ) of an integrated circuit, level N₁being the interconnection level closest to the electronic components.
Each interconnection level N_icomprises a portion M_iin which are formed conductive tracks 10, located above a portion V_iin which are formed vias 12 of contact between tracks of adjacent levels (currently, the vias of interconnection level N₁are of a different nature than the vias of the other levels). In this drawing, the cross-section plane is such that the tracks are cut widthwise, so that conductive tracks 10 appear to be of same cross-section area as vias 12. Vias 12 enable properly connecting two conductive tracks 10 located in two neighboring interconnection levels. As an example, tracks 10 and vias 12 may be made of copper. A dielectric material 14 separates tracks 10 from one another and vias 12 from one another.
Nowadays, electronic components formed in integrated circuits operate at higher and higher frequencies. The frequency increase results in an increase in the values of the stray capacitances which form between the different conductive portions. Further, the continuous miniaturization of electronic components results in a decrease in the size of conductive tracks and a decrease in distances between tracks and between vias, which also increases the values of stray capacitances. Stray capacitances may disturb significantly the operation of a circuit. It is thus desired to decrease as much as possible such stray capacitances and, for this purpose, so-called “low-k” dielectric materials having very low relative permittivities, typically smaller than 3, are used between the different conductive portions.
However, the porosity of dielectric material 14 poses various problems. Especially, the copper of conductive tracks 10 diffuses more easily into porous dielectric materials than into non-porous dielectric materials. To limit such a diffusion, it is particularly useful to form, between two neighboring interconnection levels, a layer 16 which, conventionally, stops the diffusion of conductive material from an interconnection level to the dielectric material of the upper interconnection level and which forms an etch stop layer. Vias 12 cross layer 16. As an example, layer 16 may be made of silicon-carbon nitride (SiCN). It has also been provided to form a barrier layer (not shown) around the conductive tracks and the vias, this layer being made of a conductive material capable of avoiding the diffusion of the conductive material present in an interconnection level towards the porous dielectric material of the same interconnection level. This barrier layer is, for example, formed of tantalum and of tantalum nitride.
Further, on manufacturing of the stack of interconnection levels, various etch and/or polishing and cleaning operations are carried out in liquid or gas phase. Contaminating products may thus penetrate into the pores of the porous dielectric material during these operations. This may cause an alteration of the porous material or an increase in its relative permittivity, which limits the advantage of using such a porous material.
A way to restore the characteristics of the porous material comprises performing, after having formed each interconnection level, an anneal to eliminate the contaminating products present in the porous dielectric material.
FIG. 2 is a cross-section view illustrating a stack of two interconnection levels N_iand N_i+1. This drawing illustrates the result obtained after having carried out a chem./mech. polishing step (CMP) on the structure and an anneal step aiming at eliminating the contaminating products present in interconnection level N_i+1. The conductive tracks of the two interconnection levels are shown lengthwise in cross-section view.
Interconnection level N_icomprises conductive tracks 20 surrounded with a porous dielectric material 22. The bottom and the walls of conductive tracks 20 are covered with a thin barrier layer 24 of a material avoiding the diffusion of conductive material from conductive tracks 20 to porous dielectric material 22. A thin layer 26 of a material avoiding the diffusion of conductive material from conductive tracks 20 to interconnection level N_i+1, for example, made of SiCN, extends above interconnection level N_i. Interconnection level N_i+1, which comprises conductive tracks 28 connected by vias 30 to conductive tracks 20 of interconnection level N_iis formed above thin layer 26. A porous dielectric material 32 separates conductive tracks 28 from one another and vias 30 from one another. The walls and the bottom of conductive tracks 28 and of vias 30 are covered with a thin barrier layer 34 of a conductive material. Interconnection levels N_iand N_i+1may be obtained by different known methods.
On forming of interconnection level N_i+1, the etch and/or polishing and cleaning steps cause the contamination of porous dielectric material 32. An additional step, where an anneal of the structure is performed to enable evaporation of the contaminants, is then carried out. As an example, this anneal step may be carried out at a temperature of approximately 300° C. for approximately 30 minutes. This anneal needs to be performed before deposition of a layer homologous to layer 26 which would create a barrier against the evaporation of contaminants.
In FIG. 2, arrows 36 illustrate the evacuation, during the anneal, of the contaminating products present in porous dielectric material 32. Although the anneal enables eliminating the contaminating products present in porous dielectric material 32, it should be noted that it also causes the expansion of the conductive material of conductive tracks 28. This expansion modifies the upper surface of conductive tracks 28 and makes it rough. Problems, for example, in terms of reliability, may then arise when another interconnection level is desired to be formed on the upper surface of interconnection level N_i+1. Further, since porous dielectric material 32 is not protected on its upper surface, it is contaminated again by the contact with the air, especially by water vapor, when the structure is taken out of the furnace in which the anneal has been performed. This recontamination is illustrated in FIG. 2 by arrows 38.
To limit the expansion of the conductive material, the anneal temperature is may be decreased. However, a decrease in the anneal temperature causes an increase in the duration of this anneal and decreases its efficiency.

SUMMARY OF THE INVENTION

At least one embodiment of the present invention aims at providing a method for forming interconnection levels of an integrated circuit enabling avoiding at least some of the problems of prior art methods.
Thus, an embodiment of the present invention provides a method for forming interconnection levels of an integrated circuit, comprising the steps of:
(a) forming an interconnection level comprising conductive tracks and vias separated by a porous dielectric material;
(b) forming, on the interconnection level, a layer of a non-porous insulating material, said layer comprising openings above portions of porous dielectric material;
(c) repeating steps (a) and (b) to obtain the adequate number of interconnection levels; and
(d) annealing the structure.
According to an embodiment of the present invention, an anneal step is performed before each repetition at step (c).
According to an embodiment of the present invention, step (a) of formation of an interconnection level comprises the steps of
forming a layer of a porous dielectric material;
forming an oxide layer, then a titanium nitride layer on the layer of porous dielectric material;
forming openings in the titanium nitride layer and in an upper portion of the oxide layer at the level of the desired conductive tracks;
forming holes in the oxide layer and in an upper portion of the layer of porous dielectric material at the level of the desired vias;
etching, outside the areas covered with the titanium nitride layer, until the bottom of the holes reaches the conductive tracks of the lower interconnection level;
forming a conductive material in the etched portion; and
removing the materials located above the layer of porous dielectric material.
According to an embodiment of the present invention, the step of forming holes and the step of etching outside the areas covered by the titanium nitride layer are etch steps in the presence of argon and of C₄F₈.
According to an embodiment of the present invention, the removal of the materials located above the layer of porous dielectric material is performed by chem./mech. polishing (CMP).
According to an embodiment of the present invention, the layers of non porous insulating material are made of silicon-carbon nitride (SiCN) and the conductive tracks and the vias are made of copper.
An embodiment of the present invention provides an integrated circuit comprising a stack of interconnection levels, each interconnection level comprising conductive tracks, conductive tracks of different interconnection levels capable of being connected by vias, the conductive tracks and the vias being separated by porous dielectric materials, non-porous insulating layers crossed by the vias being formed on the different interconnection levels, said non-porous insulating layers comprising openings located on portions of porous dielectric materials.
According to an embodiment of the present invention, the porous dielectric materials have thicknesses ranging between 100 and 250 nm.
According to an embodiment of the present invention, the layers of non-porous insulating material are made of silicon-carbon nitride (SiCN) and the conductive tracks and the vias are made of copper.
According to an embodiment of the present invention, the openings in the non-porous insulating layers have dimensions greater than 70 nm.
The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, previously described, is a cross-section view of several interconnection levels of an integrated circuit;

FIG. 2, previously described, illustrates the result obtained after having performed an anneal on an interconnection level N_i+1; and

FIGS. 3A to 3I are cross-section views illustrating steps of a method for manufacturing a stacking of interconnection levels according to an embodiment of the present invention.

DETAILED DESCRIPTION

For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale.
FIGS. 3A to 3I are cross-section views illustrating results of steps of a method for manufacturing a stack of interconnection levels according to an embodiment of the present invention. FIGS. 3A to 3G are drawn along a first cross-section plane and FIGS. 3H and 3I along a second cross-section plane.
In FIG. 3A, it is started from a structure in which an interconnection level N_ihas already been formed. Interconnection level N_icomprises conductive tracks 40, two of these tracks being shown lengthwise in cross-section view in FIG. 3A. As an example, conductive tracks 40 may be made of copper. Vias (not shown) may also be formed to connect conductive tracks 40 to tracks of lower level. Conductive tracks 40 are separated by a porous dielectric material 42. The walls and the bottom of conductive tracks 40 are covered with a thin barrier layer 44 of a conductive material which prevents the diffusion of copper from conductive tracks 40 to porous dielectric material 42. Above interconnection level N_iis formed a non-porous thin insulating layer 46, for example, made of SiCN, which prevents the diffusion of copper from tracks 40 to interconnection level N_i+1which will be formed above insulating layer 46.
According to an aspect of the present invention, non-porous insulating layer 46 comprises openings 48 located above portions of porous dielectric material 42, a single one of openings 48 being shown in FIG. 3A. Openings 48 are formed above portions of interconnection level N_ihaving a low density of conductive tracks 40.
At the step illustrated in FIG. 3B, a thicker layer of porous dielectric material 50 has been formed on thin non-porous insulating layer 46. As an example, layer 50 of porous dielectric material may be obtained by introducing a pore-forming agent into a thick layer of non-porous dielectric material, then reacting the pore-forming agent, for example, by anneal, to eliminate the pore-forming agent and form the pores of the porous dielectric material. On top of layer 50 of porous dielectric material is formed a stack of two layers 52 and 54 which behave as masks in subsequent steps. As an example, layer 52 is a deposited silicon oxide layer and layer 54 is a titanium nitride layer (TiN).
At the step illustrated in FIG. 3C, openings 56 have been formed in titanium nitride layer 54, these openings extending slightly into oxide layer 52. The contour of openings 56 defines the contour of the conductive tracks which will be formed in interconnection level N_i+1. As an example, openings 56 may be formed by depositing a resist on titanium nitride layer 54, by appropriately insolating and etching this resist, and by etching layers 52 and 54. Layer 52 is only partially etched to avoid any direct contact between the resist and porous dielectric material 50 during the next step.
At the step illustrated in FIG. 3D, holes 58 which cross oxide layer 52 and an upper portion of layer 50 of porous dielectric material have been formed in openings 56. Holes 58 define the contour of the vias which will be formed in s interconnection layer N_i+1. Holes 58 may be obtained, by means of an adapted mask, by a physico-chemical etching performed in the presence of argon and of C₄F₈. Further, a hydrofluoric acid (HF) cleaning step is carried out after the etching. During the etching and the cleaning, contaminating products (for example, fluorine) penetrate into the pores of porous dielectric material 50, as lo illustrated in FIG. 3D by arrows 60.
At the step illustrated in FIG. 3E, an etching of the portion of oxide layer 52 and of layer 50 of porous dielectric material which are not protected by titanium nitride layer 54 has been performed. As an example, this etching may again be a physico-chemical etching in the presence of argon and of C₄F₈, followed by a cleaning with hydrofluoric acid. This etch step enables forming the contour of the conductive tracks and of the vias of interconnection level N_i+1. It is performed so that holes 58 cross thin SiGN layer 46 and that they reach conductive tracks 40 of interconnection level N_i. In the same way as in the previous etch step, contaminating products penetrate into porous dielectric material 50, during the etching and the cleaning, as indicated by arrows 62 in FIG. 3E.
At the step illustrated in FIG. 3F, the space created in the previous etch step has been filled with a conductive material to form conductive tracks 64 and vias 65 of interconnection level N_i+1. The conductive material of conductive tracks 64 and of vias 65 may be copper, and the metallization is carried out so that the copper fills the spaces contacting conductive track inductive material avoiding the diffusion of copper from conductive layers 64 and vias 65 to the neighboring porous dielectric material 50 may be formed before the metallization.
At the step illustrated in FIG. 3G, a chem./mech. polishing (CMP) for removing the excess copper and tantalum nitride 66 located above layer 50 of porous dielectric material, as well as titanium nitride layer 54 and oxide layer 52, has been carried out. In the same way as in the etch steps, during the polishing step and the subsequent cleaning step, contaminating products may penetrate into porous dielectric material 50, as illustrated in FIG. 3G by arrows 68.
FIGS. 3H to 3I illustrate subsequent steps of the manufacturing method according to an embodiment of the present invention, in a cross-section plane different from that of FIGS. 3A to 3G. In these drawings, all conductive tracks appear lengthwise in cross-section view and the different barrier layers (especially 44 and 66) have not been shown for the simplification.
FIG. 3H illustrates a structure substantially identical to that of FIG. 3G. In this drawing, interconnection levels N_iand N_i+1comprise several conductive tracks 40, 64 and several vias 65. Above interconnection level N_i+1is formed a non-porous insulating layer 70, for example, made of SiGN.
According to an aspect of the present invention, non-porous insulating layer 70 comprises openings 72 above portions of porous dielectric material 50. In FIG. 3H, a single one of openings 72 is shown. Openings 72 are formed above portions of interconnection level N_i+1having a low density of conductive tracks 64.
At the step of FIG. 3I, an interconnection level N_i+2has been formed on thin SiCN layer 70. Interconnection level N_i+2may be formed in the same way as interconnection level N_i+1. Interconnection level N_i+2comprises conductive tracks 74 and vias 76, the tracks and vias being separated by a porous dielectric material 78.
On top of interconnection level N_i+2is formed a thin non-porous insulating layer 80, for example, made of SiCN, which comprises openings 82 above portions of porous dielectric material 78. In FIG. 3I, a single one of openings 82 has been shown. In the same way as for openings 48 and 72, openings 82 are formed above portions of interconnection level N_i+2with a low density of conductive tracks.
Preferably, after the forming of each opening 48, 72, and 82 in SiCN layers 46, 70, and 80 on interconnection levels N_i, N_i+1, and N_i+2, an anneal of the structure enabling evaporation of the contaminating products present in the porous dielectric materials, respectively 42, 50 and 78, of these levels, is performed.
In FIG. 3I, the circulation of contaminating products during an anneal intended to eliminate contaminating products from interconnection level N_i+2has been shown. The contaminating products present in layer 78 of porous dielectric material tend to evaporate and to come out through opening 82, as shown by arrows 84. It should be noted that the contaminating products go round vias 76 of interconnection level N_i+2. Further, this anneal also allows for contaminating products present in the lower levels to migrate upwards in the structure, from level to level, via openings 48, 72 formed in non-porous insulating layers 46, 70, as shown by arrows 86, and to escape from the structure through openings 82 of non-porous insulating layer 80.
Non-porous insulating layers 46, 70, and 80 covering conductive tracks 40, 64, and 74 prevent the expansion of the conductive material of these tracks. This enables annealing at temperatures higher than those currently used and thus enables better evacuation of contaminating products. Further, the recontamination of the porous dielectric material after the anneal steps only occurs in regions with a low density of conductive materials, which does not increase stray capacitances in remote regions with a high density of conductive tracks.
Non-porous insulating layers 46, 70, and 80 may be made of any non-porous insulating material, but they will preferably be made of silicon-carbon nitride SiCN, this material stopping the passing of contaminating products and also avoiding diffusion of the material of conductive tracks 40, 64, and 74 towards the porous dielectric material of the upper levels.
As an example, layers 42, 50, 78 of porous dielectric material have thicknesses ranging between 100 and 250 nm. As an example also, the openings may have dimensions, sides or diameters greater than 70 nm.
Specific embodiments of the present invention have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, it should be understood that the anneal steps may be carried out after having formed several interconnection levels. Two interconnection levels or more may for example be formed before performing an anneal to evacuate the contaminating products from these two levels. A longer anneal step may also be provided once all interconnection levels have been formed to enable evaporation of the contaminating products remaining in the different interconnection levels.
Openings 48, 72, and 82 may be formed above one another or in shifted fashion, as shown in FIG. 3I.
Further, a specific method for forming an interconnection level comprising tracks and vias has been described, in which the conductive material of the tracks and vias is formed in a single step. It should be understood that the tracks and vias of each interconnection level may be formed separately and by any known method.
As an example, porous dielectric material 42, 50, 78, may be “BDIIx”, a material sold by Applied Materials.
Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.

Claims

1. A method for forming a stack of interconnection levels of an integrated circuit, comprising the steps of:

(a) forming an interconnection level comprising conductive tracks formed above conductive vias, the tracks and the vias being laterally separated by a porous dielectric material;

(b) forming, on the interconnection level, a layer of a non-porous insulating material, said layer comprising openings formed only above portions of porous dielectric material;

(c) repeating steps (a) and (b) to obtain the adequate number of interconnection levels of the stack, the conductive vias of an interconnection level of the stack having another interconnection level below contacting the conductive tracks of the interconnection level below; and

(d) annealing the structure.

2. The method of claim 1, wherein an anneal step is performed before each repetition at step (c).

3. The method of claim 1, wherein step (a) of formation of an interconnection level comprises the steps of:

forming a layer of a porous dielectric material;

forming a silicon oxide layer, then a titanium nitride layer on the layer of porous dielectric material;

forming openings in the titanium nitride layer and in an upper portion of the oxide layer at the level of the desired conductive tracks;

forming, at the bottom of the openings, holes in the oxide layer and in an upper portion of the layer of porous dielectric material at the level of the desired conductive vias;

etching, outside the areas covered with the titanium nitride layer, until the bottom of the holes reaches the conductive tracks of the lower interconnection level;

forming a conductive material in the etched portion; and

removing the materials located above the layer of porous dielectric material.

4. The method of claim 3, wherein the step of forming holes and the step of etching outside the areas covered by the titanium nitride layer are etch steps in the presence of argon and of C₄F₈.

5. The method of claim 3, wherein the removal of the materials located above the layer of porous dielectric material is performed by chem./mech. polishing.

6. The method of claim 1, wherein the layers of non porous insulating material are made of silicon-carbon nitride and the conductive tracks and the conductive vias are made of copper.

7. An integrated circuit comprising a stack of interconnection levels, each interconnection level of the stack comprising conductive tracks and conductive vias, conductive tracks of different interconnection levels being adapted to be connected by the vias, the tracks and the vias of a same interconnection level being laterally separated by porous dielectric materials, non-porous insulating layers crossed by the vias being formed on each interconnection level, said non-porous insulating layers comprising openings located only on portions of porous dielectric materials.

8. The integrated circuit of claim 7, wherein the porous dielectric materials have thicknesses ranging between 100 and 250 nm.

9. The integrated circuit of claim 7, wherein the layers of non-porous insulating material are made of silicon-carbon nitride and the conductive tracks and the conductive vias are made of copper.

10. The integrated circuit of claim 7, wherein the openings in the non-porous insulating layers have dimensions greater than 70 nm.