ICMCTF2000 Session F1/E4-2: Mechanical Properties and Adhesion

In the rapidly expanding field of integrated circuits device fabrication, thin electroplated copper films are being deposited onto silicon wafer replacing the traditional aluminum as the interconnect material. As a result the adhesion of the copper film with the underlying seed and barrier layers have become of paramount importance. This study focused on the characterization of such films in term of scratch resistance and adhesion strength. Copper films were electroplated on thin layers of W/TiN/Si(100) and Cu/Ta/Si(100) substrates and the properties of these films were then characterized with a micro scratch tester using a diamond indenter of different sizes.

For the electroplated copper film on W seed, using a 200 µm tip, with an increased loading, minor delamination corresponding to damage along the sides of the scratch path was observed initially, followed by cracking at centre of the scratch. Finally a complete delamination of the film, causing tear of the film from the underlying substrate, was observed at a critical load of 16 N.

In the case of the film on Cu seed, no delamination was observed within the loading range up to 22 N under the 200 µm tip. When a 50 µm indenter was performed, a minor delamination was seen initially at a force of 4 N, and this delamination subsequently increased from 5 N onwards, however no full delamination was found within the loading range up to 22 N. The full delamination of the film on Cu seed occurred at 3 N when the scratch testing was performed using a 20 µm diamond tip. The failure phenomenon for the film on Cu seed is different from that on W seed. The initial failure caused by radial cracking in the film, which turned into chipping of the film as the loading force increased. The film then completely delaminated, causing substantial debris to be strewn along the scratch path.

Today's research activities covering the field of improving the properties of cutting tools are concentrated on optimizing manufacturing technologies and tool geometry as well as improved alloying of special cutting materials and coating of tools. Especially in the area of coated cutting inserts a high potential to enhance the wear resistance is still existing.

As a result of the reduced machinability of new cutting tool materials high mechanical and thermal loads during grinding influence the subsurface properties heavily. Hence, strong gradient residual stresses are induced in the subsurface of the tools during manufacturing. Even with optimized coating parameters deposited PVD-coatings fail due to insufficient subsurface properties of the substrates.

In this paper influences of residual stress distribution in subsurface layers on interface strength of PVD-coated carbides were investigated. The investigations were carried out with WC-based cemented carbides coated by PVD-deposited (TiAl)N layers. Considered topics are the influence of grinding and water peening on the subsurface residual stress state of the cemented carbide. Dependencies between stress distribution in subsurface layers and interface strength are highlighted.

X-ray measurements were carried out by using a new method for evaluating residual stress gradients. This stress gradients in the subsurface layers of the substrate were determined using the non-linear distribution of the diffraction angle versus the tilt angle in units of sin²Psi. Depth profiles of residual stress of ground and ground and waterpeened substrates before coating of the cemented carbide were determined. The results were compared with the well known sin²Psi-method using different lattice planes which correspond to different penetration depths. The adhesion and the wear behavior of the deposited coatings were analyzed in final cutting test during dry machining.

Mechanical applications of Physical Vapour Deposited coatings are numerous. In many cases, the thickness of material deposited is not large enough to avoid transmission to the substrate of a significant part of applied stress during traditional tribological tests. The consequence is that coating removal generally corresponds to a failure of the substrate under the interface. It is therefore essential to investigate mechanical behaviour of the coating of its own.

To study damage mechanisms of the film a microtribometer was installed in the vacuum chamber of a scanning electron microscope. It was mainly used with ball on disc configuration but other are available like diamond on disc or cylinder on disc. During the tests friction coefficient is measured by strain gauges and recorded on a computer. A link can be directly established between this measure and mechanisms observed on the microscope screen. The image can be transmitted to a video recorder which allows afterward comparison of several tests. This technique equally allows to study friction behaviour of a coating under vacuum conditions.

During service life of a piece used for mechanical application, the superficial film generally encounters fatigue type sollicitation. It radically differs from regularly increasing load that is applied on classical scratch test apparatus. Using the sames hardness diamond indenters as on scratch test device, we performed tests on a pin on disc tribometer. During tests, the diamond undergo nearly no damage, which means that contact pressure remains constant. Another advantage is that the highest shear stress is located in the layer.

These techniques were tested on several types of hard layers produced by PVD (DLC, TiN, TiBN...) and brought numerous informations concerning their mechanical behaviour.