Forming of the last metallization level of an integrated circuit

An integrated circuit including one or several metallization levels, metal conductive strips and metal contact pads being formed on the last metallization level, the last level being covered with a passivation layer in which are formed openings above the contact pads. The thickness of the pads, at least at the level of their portions not covered by the passivation layer, is smaller than the thickness of said conductive strips.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuits and more specifically to input/output contact pads formed on the last metallization level of integrated circuits.

2. Discussion of the Related Art

FIGS. 1 and 2, respectively, are a partial top view and a corresponding cross-section view of the last metallization level of an exemplary integrated circuit. A coil1and a contact pad2are formed on a substrate3. Substrate3is an insulating layer covering a previous metallization level or a semiconductor substrate. Coil1is spiral-shaped in top view and five portions5,6,7,8and9are visible inFIG. 2. Contact pad2is square-shaped in top view as current in integrated circuits. A passivation layer10covers the coil and substrate3, as well as the edges of contact pad2. An opening11in passivation layer10exposes a central portion of contact pad2.

The forming of the last metallization level of an integrated circuit such as shown inFIGS. 1 and 2consists of covering substrate3with a metal layer, generally made of aluminum, then etching this metal layer to form coil1and contact pad2, and finally covering the entire structure with a passivation layer in which an opening is formed above contact pad2.

The development of integrated circuit manufacturing processes enables placing more and more elements on the same substrate surface area. On a given metallization level, the width of the conductive strips is smaller and smaller. Similarly, the size and the spacing of the contact pads are decreased to increase their number.

FIG. 3is a cross-section view of the contact pad ofFIG. 2after welding of a metal wire. The tip of the metal wire substantially has the shape of a ball20resting on contact pad2. Upon welding, ball20is laid on contact pad2, and a strong force, as well as ultrasound, are applied to form an intermetallic welding area21between ball20and contact pad2.

Since aluminum is a relatively flexible material, the application of a strong force and of ultrasound causes a penetration of ball20into contact pad2and results on the one hand in the forming of cracks22in passivation layer10, and on the other hand in the forming of aluminum projections23on either side of ball20, some of the projections rising above passivation layer10. The presence of cracks22and of aluminum projections23is likely to cause short-circuits between the input/output contact pads generally placed next to one another and this, becomes more likely as the pads get closer and closer. This results in reliability problems for the concerned integrated circuits.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an integrated circuit comprising on its last metallization level lightly-resistive conductive strips and small-size metal conductive pads.

Another object of the present invention is to provide a method for manufacturing an integrated circuit exhibiting on its last metallization level lightly-resistive conductive strips and small-size metal contact pads.

To achieve these and other objects, the present invention provides an integrated circuit comprising one or several metallization levels, metal conductive strips and metal contact pads being formed on the last metallization level, the last level being covered with a passivation layer in which are formed openings above the contact pads. The thickness of the pads, at least at the level of their portions not covered by the passivation layer, is smaller than the thickness of said conductive strips.

According to an embodiment of the present invention, at least one conductive strip forms a coil.

According to an embodiment of the present invention, several of said conductive strips form a supply network.

According to an embodiment of the present invention, the last metallization level is formed on an insulating layer, each contact pad being formed of a conductive layer covering an insulating portion laid on the insulating layer.

According to an embodiment of the present invention, the contact pads are made of aluminum.

The present invention also provides a method for forming the last metallization level of an integrated circuit comprising:

depositing a metal layer on a substrate;

etching the metal layer to form metal portions and said conductive strips;

covering the substrate, the conductive strips, and the metal portions with a passivation layer;

forming openings in the passivation layer above the metal portions; and partially etching the metal portions to decrease their thickness to obtain said contact pads.

The present invention also provides a method for forming the last metallization level of an integrated circuit comprising:

depositing a metal layer on a substrate;

etching the metal layer to form metal portions and said conductive strips;

covering the conductive strips with a protection layer;

partially etching the metal portions to decrease their thickness to obtain said contact pads;

removing, if necessary, the protection layer;

covering the substrate, the conductive strips, and the contact pads with a passivation layer; and

forming openings in the passivation layer above the contact pads.

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 drawings are not to scale.

The present inventor has determined the origin of the above-mentioned reliability problems for high-density circuits. They are due to an increase in the thickness-to-width ratio of the contact pads. Indeed, to reduce the surface area taken up by conductive strips, while maintaining a resistivity which is as small as possible, the thickness of the metal layer is increased. Further, the decrease in the contact pad width also contributes to increasing the thickness-to-width ratio, which enhances reliability problems.

A decrease in the metal layer thickness can only be envisaged with difficulty since this would increase the resistivity of the conductive strips. The use of a material more conductive than aluminum, such as copper, would however make welding operations more difficult.

To solve these problems, the present invention provides placing on the last metallization level of an integrated circuit “thick” metal conductive strips and “thin” contact pads.

FIG. 4is a partial cross-section view of an integrated circuit according to an embodiment of the present invention. The top view corresponding to the shown circuit portion is identical to that ofFIG. 1. A coil30and a metal contact pad31are laid above a substrate32. Five portions of coil30are visible. Contact pad31has, in this example, a substantially parallele-pipedal shape. Coil30and substrate32, as well as the sides and the edge of the upper surface of contact pad31are covered with a passivation layer33. An opening35in passivation layer33is provided above contact pad31.

As previously, substrate32is an insulating layer covering a previous metallization level or a semiconductor substrate in the case where the circuit only comprises a single metallization level. Contact pad31and coil30are formed of identical or different materials, the contact pads being currently made of aluminum.

According to the present invention, the thickness of contact pad31is smaller than the thickness of the metal forming coil30. In this example, the coil has a thickness which is substantially twice that of the contact pad.

FIG. 5is a partial cross-section view of an integrated circuit according to another embodiment of the present invention.

The top view corresponding to the shown circuit portion is identical to that ofFIG. 1. A coil40and a metal contact pad42are laid on a substrate41. Coil40, substrate41, and connection pad42are covered with a passivation layer44. An opening45in passivation layer44is also provided above connection pad42.

The contact pad substantially has the shape of a half-box, the portions of the contact pad covered with passivation layer44being thicker than the exposed portions.

According to the present invention, the thickness of the portion of conductive layer42exposed by opening45is smaller than the thickness of the metal forming coil40.

The two examples of integrated circuits shown inFIGS. 4 and 5comprise one coil only in addition to a contact pad. However, the conductive strips formed in the last metallization level of an integrated circuit according to the present invention may have other functions. The conductive strip(s) may for example form a passive component or a network of lightly-resistive conductive lines. Generally, in an integrated circuit according to the present invention, the portions of the contact pads which are not covered by the passivation layer have a thickness smaller than that of the conductive strips placed on the last metallization level of the integrated circuit.

FIG. 6is a cross-section view of the contact pad of the circuit ofFIG. 4after welding of a metal wire. The tip of the metal wire forms a metal ball50, conventionally made of gold, which rests on contact pad31. Contact pad31has been hollowed during the welding almost down to substrate32and small aluminum projections have formed between ball50and passivation layer33. The aluminum volume displaced in the welding process being relatively small, since the aluminum thickness is small, the pressure exerted on passivation layer33is small and causes no cracking. Further, the aluminum projections51,52obtained on either side of ball50have small sizes and do not rise above passivation layer33.

An advantage of the structure of an integrated circuit according to the present invention is that the passivation layer is not cracked at the level of the contact pads after welding of a connection wire.

Another advantage of the structure of an integrated circuit according to the present invention is that no aluminum splinter is formed at the welding of a connection wire on the contact pad.

The last metallization level of an integrated circuit according to the present invention may be formed in several ways. Two examples of methods are described hereafter.

In a first method example, a metal layer100is formed on a substrate101having an insulating upper portion, as illustrated inFIG. 7A.

At the next step, illustrated inFIG. 7B, metal layer100is etched to form in this example a coil110and a metal portion111. The top view of the obtained structure is identical to that ofFIG. 1. Five portions of coil110are visible above substrate101.

At the next step, illustrated inFIG. 7C, the entire structure is covered with a passivation layer120.

At the next step, illustrated inFIG. 7D, passivation layer120is etched to form an opening130above metal portion111.

At the next step, illustrated inFIG. 7E, metal portion111is etched to decrease its thickness. In this example, the etching is anisotropic and only the portion of metal portion111uncovered by passivation layer120is partially etched. In this example, the thickness of the exposed portion of portion111is decreased by half. It will be within the abilities of those skilled in the art to define the optimal etch process enabling maintaining an “ideal” metal thickness providing a good-quality welding.

An advantage of the previously-described method is that it requires no additional mask with respect to a conventional method for forming the last metallization level of an integrated circuit.

In another method example, a metal layer150is deposited, as previously and as shown inFIG. 8A, on a substrate151. Metal layer150is then etched, as illustrated inFIG. 8B, to form a coil160and a metal portion161. The top view of the obtained structure is identical to that inFIG. 1.

At the next step, illustrated inFIG. 8C, coil160is covered with a protection layer170. Generally, all the integrated circuit elements of which the thickness of the metal layer is desired to be kept are covered. Metal portion161intended to form a contact pad is then etched. The etch time is determined to obtain the desired thickness for the contact pads. In this example, the contact pad thickness is decreased by half.

At the next step, illustrated inFIG. 8D, protection layer170may be removed, after which the entire structure is covered with a passivation layer180. An opening190in passivation layer180is then formed, as illustrated inFIG. 8E, above metal portion161. The thickness of the metal contact pad thus formed can be set independently from the thickness of the passive components or of the conductive strip networks placed on the same metallization level of the integrated circuit.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the metal contact pads may have various shapes. Further, other methods for forming the last metallization level of an integrated circuit comprising contact pads and conductive strips of different thicknesses may be devised.