The service life of an integrated electronic circuit is may be determined by the reliability of the metal strip conductors of the entire metallization systems. Failures are mainly caused by electromigration during long-term operation, which result in a loss of cross-section of the strip conductor and, connected therewith, in an increase in the resistance of the same and, finally, in its burning out. Electromigration describes the effect of the migration of material in a conductor upon the occurrence of high current densities. A pulse transmission to ions in the conductor results in the case of very high current densities due to the directed movement of electrons, which, in the case of high current densities, may result in a “diffusion” of the ions and thus in a directed migration of material. Due to this, a loss of material is possibly caused on specific areas of the conductor, whereas at other points located downstream an accumulation of material may take place. Due to this, changed conditions for the current conduction result, which may result in a loss of power and, finally, in the complete failure of the metallization system and thus of the component. The current densities already occurring in integrated circuits during normal operation are relatively high, for instance a few kA per cm2 so that, in the case of a longer duration of operation of the components, the changes due to the electromigration must already be taken into consideration in the specification of the components in order to be able to indicate a guaranteed service life with a specified power behavior.
In order to determine and to control the reliability of metallization systems of microelectronic integrated circuits, accelerated tests with test structures especially provided for this purpose are carried out. A metallization test structure is heated for this and/or a current is passed through a test strip conductor of a specific length. Here, the current density is substantially higher than the current density permitted as a maximum during normal operation. E.g. the time is measured after which the measuring circuit leading across the test strip conductor to be tested is interrupted and/or an increase in resistance by a specific factor results.
As a rule, the metal strip conductors consist of thin, approx. 400 nm to 1000 nm thick metal layers. However, there are cases where substantially thicker metal strips, i.e. with a thickness being 2 to 5 times thicker, are required for the conducting of higher currents (current densities) in connection with the integration of power components. If several metallization planes are present, there are pads and/or electric junctions (vias), at which the strip conductors of metallization planes that are coated thereon, i.e. that are located thereon, are electrically connected in a vertical direction e.g. by means of truncated metal pieces (plugs) which are e.g. formed from aluminum or tungsten. These are also points at which special failures occur due to migration effects. This is a customary failure picture in the case of a connection of customary thin metal layers with thick metal conductor strips. AlCu and AlSiCu, copper, copper alloys, silver, etc. come e.g. into question as the metal for the conductor strips. The test structures are located on the process wafers and can be individually subjected to specific tests as wafer composites or separately and in housings provided with a cap.
Quite a number of electromigration test structures for the customary metallization systems (thin metal strip conductors) are known, e.g. from the JEDEC Standard No. 87, US-A 2004 026693 or DE 197 10 471 (Mitsubishi) or U.S. Pat. No. 5,712,510 (AMD) show other electromigration test structures.
The known test structures are not suited for testing the strip conductor systems with thick metal and the junctions from a thin to thick metallization system. There are several reasons for this. On the one hand, the test structures must be made in such a way that they are in conformity with the design rules for the production of integrated circuits, especially the metallization systems in order to detect failure mechanisms being as realistic as possible with the tests. I.e. the structural elements forming the test structure must correspond to the structural elements of the actual integrated circuit as regards their geometry and composition of material in order to be able to quantitatively determine the migration behavior of these actual structural elements on the basis of the measuring results of the test structure. If different widths for the conductors are e.g. used in the test structure and the actual metallization plane, the complex conditions are very different in the migration of material and, as a rule, the statements obtained therewith cannot be transferred to other conditions. Also, different geometrical ratios may result in different conditions in the production of the structural elements, whereby, in turn, other material properties, e.g. a different crystal structure, etc. may be caused.
The inner crystal structure of the metal strips which is connected with the type of production of the metal layers causes that a mere increase in the dimensions of known test structures changes the conditions in such a way that they are no longer representative of the material to be tested. The migration occurs at points of the test structure which are not intended for testing, due to which the implementation of the test is impossible.
If a junction area from a thin metal system to a thick system being located thereon—a realistic case—is concerned and if, apart from the thin strip conductor, the vertical connections and/or junctions (so-called vias) are also to be tested with respect to reliability, the current would be supplied via a piece of the lower thin strip conductor, which would be unproblematic in the case of customary amperages occurring during normal operation, however, in the case of an accelerated test, results in that, in this test, the width of the piece would have to be enlarged observing the customary dimensions in order to be able to transport the higher current for the test of the thick strip conductor without said piece being destroyed by overheating.
However, such thin metal strips as they are found in known test structures already have per se a grain structure that is unfavorable for electromigration effects, wherein the grains have a “diameter” that is substantially smaller than the width of the strip conductor, due to which there are many triple points (points, where three grain boundaries adjoin) at which electromigration especially attacks, since, as already mentioned, the migration of material must be understood as a directed diffusion so that the grain boundaries may act as preferred diffusion paths. If now such a path for the supply of the higher current for the testing of the junction area to the thick metal and for the test stretch of the thick metal conductor strip is widened, the situation regarding the electromigration is still aggravated, since more diffusion paths are created and a premature failure occurs just at a point which is not provided for the test, at all. This could e.g. result in an incorrectly ascertained service life for the actual test conductor strip so that, all in all, an incorrect specification for the inspected component could result. Thus, such a test structure does not detect the failure mechanism due to the high amperage or current density for the junction of thin to thick and the thick strip conductor.
The test current is typically a d-c current. If it flows through a test strip conductor piece with a diameter of equal size and a specific length, then a defective point such as a depletion of conductive material will occur in the case of an increase in the current density due to electromigration in a critical area after a specific period of time, which results in an increase in resistance or a failure of current conduction, which can be locally observed. The location depends on the current direction and the interaction of the various migration mechanisms, i.e. the location of the point on the strip conductor piece is not located in the center of the strip conductor under normal conditions, but is displaced in the direction of the electron movement with respect to the center. In the case of the pole reversal of the current direction the defective point would have to occur symmetrically towards the center of the strip conductor under the same conditions. If this is not the case further conclusion to the failure mechanism can be drawn herefrom.