Conductive structure for integrated circuits.

0001The present invention relates generally to semiconductor integrated circuits, and more specifically to a technique for forming metallic conductive structures for integrated circuits.

0002Highly conductive structures are often needed to connect different parts of semiconductor integrated circuits. Various metals are typically used for at least some of these interconnecting structures due to their low resistivity. Aluminum and aluminum alloys are popularly used for this purpose because of their high conductivity, processing of aluminum is easily controllable, and because of aluminum's good adhesion properties to underlying layers such as silicon dioxide.

0003One drawback to the use of aluminum as an interconnect for integrated circuits is that moderate to high current densities tend to cause electromigration of the aluminum. Current densities sufficient to cause electromigration are often found in the very small geometry semiconductor devices now being made, even with the use of lower operating voltages. Electromigration causes aluminum to physically move, so that gaps can be formed in conductive lines. If such gaps, or even significantly thinner than usual regions, are formed, the integrated circuit device is generally rendered inoperable.

0004It would be desirable to provide a metal signal line structure which is resistant to electromigration. Such a structure would preferably be highly conductive, and easy to include in standard semiconductor integrated circuit process flows.

0005It is therefore an object of the present invention to provide a method for forming a metal conductive structure for semiconductor integrated circuits.

0006It is another object of the present invention to provide such a method which results in a highly conductive structure.

0007It is a further object of the present invention to provide such a method which is compatible with standard integrated circuit manufacturing process flows.

0008Therefore, according to the present invention, an aluminum layer is formed over a semiconductor integrated circuit. A metal layer is formed over the aluminum layer, and both are etched to define a conductive signal line. Another metal layer is deposited and etched back to form metal sidewall regions, so that the aluminum in the conductive signal line is surrounded on the top and sides. The metallic layer used to form the sidewall regions can be selectively deposited to minimize the amount of etchback required. In the resulting structure, the metal side and top layers contain the aluminum in the signal line, and therefore help prevent problems caused by aluminum electromigration.

0009The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

0010<b>Figures 1 - 4</b> illustrate a preferred method for forming metal conductive structures according to the present invention.

0011The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention.

0012Referring to <b>Figure 1</b> two adjacent signal lines are to be formed on a portion of an integrated circuit. The signal lines illustrated, for purposes of example, as being formed over a field oxide region <b>10</b>. As will be apparent to those skilled in the art, the described technique will be used to form metallic conductive structures over various underlying features of the integrated circuit device. However, for purposes of simplicity, a portion of two signal lines passing over the field oxide region <b>10</b> is illustrated in the figures.

0013An aluminum layer <b>12</b> is formed over the surface of the device, followed by formation of a titanium nitride layer <b>14</b>. If desired, an optional barrier layer (not shown) can be formed over the device before the aluminum layer <b>12</b> is formed. The aluminum layer 12 is deposited to a thickness which is desired for the metal interconnect layer, and can be, for example, approximately 4,000 to 12,000 angstroms thick. The titanium nitride layer <b>14</b> functions as an antireflective coating, and will later perform the additional function of providing a cap layer to aluminum interconnect lines. The titanium nitride layer <b>14</b> is preferably deposited to a thickness of between approximately 200 and 500 angstroms to for the antireflective coating.

0014If desired, other metal layers may be used in place of titanium nitride. For purposes of the present invention, it is important that the metal used in layer <b>14</b> is capable of inhibiting hillock formation and electromigration in the underlying aluminum. Therefore, metals which do not function as an antireflective coating can be used in place of the titanium nitride. However, the use of titanium nitride provides both such inhibition of hillock formation and an antireflective coating, thus being beneficial in two respects and therefore preferred.

0015A layer of photoresist <b>16</b> is spun on and patterned to define interconnect lines. Two such lines are shown in <b>Figure 1</b>, as defined by the photoresist region <b>16</b>.

0016Referring to <b>Figure 2</b>, the titanium nitride <b>14</b> and aluminum <b>12</b> layers are anisotropically etched using the photoresist region <b>16</b> as a mask. This results in the formation of two aluminum signal lines <b>18</b>, <b>20</b>. Each signal line has a cap layer of titanium nitride <b>22</b>, <b>24</b> respectively. The photoresist <b>16</b> is then removed, resulting in the structure shown in <b>Figure 2</b>.

0017Referring to <b>Figure</b><b>3</b>, a layer of tungsten <b>26</b> is selectively grown over the surface of the integrated circuit. As is known in the art, selective tungsten deposition causes a tungsten layer <b>26</b> to be formed on conductive regions, such as aluminum interconnect <b>18, 20</b> and titanium nitride cap layers <b>22,</b><b>24</b>. Tungsten forms in these regions in preference to formation on insulating regions such as field oxide layer <b>10</b>. This causes the complete encapsulation of the aluminum interconnect and titanium nitride cap layers by tungsten <b>26.</b>

0018Referring to <b>Figure 4</b>, the tungsten <b>26</b> is anisotropically etched to form tungsten sidewall regions <b>28</b>, <b>30</b>, <b>32</b>, <b>34</b>. Sidewalls <b>28</b>, <b>30</b> completely encapsulate the sides of aluminum signal line <b>18</b>, while sidewalls <b>32</b>, <b>34</b> completely encapsulate the sides of aluminum signal line <b>20</b>. The sidewall regions <b>28 - 34</b>, in combination with the titanium nitride cap layers <b>22</b>, <b>24</b>, completely encapsulate tthe top and sides of the aluminum signal lines <b>18</b>, <b>20</b>.

0019An important advantage of the method just described is related to the techniques used to selectively deposit tungsten. If encapsulation of the aluminum interconnect line <b>18, 20</b> is attempted by simply selectively depositing tungsten, a highly selective deposition process must be performed. Such highly selective processes are known to attack aluminum to some degree, degrading the quality of the aluminum signal line <b>18</b>, <b>20</b>. Less selective processes which do not attack the aluminum in the signal line typically deposit some tungsten on insulator surfaces. This can result in a tungsten bridge between adjacent signal lines, causing an electrical short which interferes with the functioning of the device.

0020With the technique described above, a partially selective tungsten deposition process can be used. This prevents damage to the aluminum signal lines <b>18, 20</b>. The anisotropic etch used to form spacers <b>28 - 34</b> will remove any small amounts of tungsten which may be deposited on the surface of the field oxide layer <b>10</b>. This prevents any electrical shorting between signal lines. The result is that there is no problem caused by shorting adjacent signal lines through tungsten bridges, and high quality aluminum signal lines are retained.

0021The combination of a titanium nitride cap layer <b>22</b>, <b>24</b>, with the tungsten sidewalls <b>28 -</b><b>34</b> greatly inhibits electromigration of the aluminum during operation of the integrated circuit. Hillock formation during subsequent processing steps is also inhibited. This helps prevent this common failure mode of integrated circuit devices, which can greatly extend the average operating lifetime of integrated circuits fabricated using the described technique.

0022The titanium nitride layer <b>14</b> acts as an antireflective coating, so that signal line definition is improved during the exposure and patterning of photoresist layer <b>16</b>. No additional photomasking is required beyond the single masking layer already required to define the aluminum signal lines <b>18</b>, <b>20</b>. All of the remaining steps are self-aligning.

0023As is known in the art, tungsten etches easily in fluorine based chemistries. These chemistries are preferred for the anisotropic etching of the tungsten layer <b>26</b> because aluminum is not affected by these chemistries. Titanium nitride etches in fluorine based chemistries to some extent, but significant etching occurs only in the presence of significant amounts of free fluorine or under a high power ionic bombardment. If a low power anisotropic etch is used, the titanium nitride cap layers <b>22</b>, <b>24</b> will not etch significantly, so that the aluminum signal lines <b>18</b>, <b>20</b> will remain fully encapsulated.

0024While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.