Activation energy determination for recrystallization in electroplated-copper films using differential scanning calorimetry

Journal of Electronic Materials, Jun 2003 by Donthu, Suresh K, Vora, Mehul M, Lahiri, S K, Thompson, Carl V, Yi, S

The apparent activation energy for recrystallization during self-annealing of 1.5-[mu]m-thick electroplated copper films was determined using constant-heating-rate scans in a differential scanning calorimeter (DSC). The apparent-activation energy was measured to be 0.62 eV/atom. Ex-situ microscopy studies showed that self-annealing in electroplated Cu films is characterized by site-saturated nucleation and diffusion-limited, two-dimensional grain growth. For a transformation with these characteristics, the apparent activation energy measured using a differential scanning calorimeter corresponds to the activation energy for grain-boundary motion. The measured activation energy is reasonably close to values reported for grain boundary motion from in-situ microscopy studies. The value is also close to the activation energy for grain boundary diffusion in Cu. This work demonstrates the feasibility of using differential scanning calorimetry (DSC) as a relatively straightforward method to study the kinetics of the self-annealing process.

Key words: DSC, self-annealing, copper, electroplating

INTRODUCTION

The microelectronics industry is currently undergoing a major transition to replace Al alloys with Cu as the on-chip interconnect material. Copper has a lower resistivity and is anticipated to have a higher electromigration resistance compared to Al alloys.1 The patterning technique for Cu interconnects, known as the dual-damascene process, requires conformal deposition in trenches with depth-to-width ratios as high as six. Electroplating is increasingly recognized as the preferred method of Cu deposition in dual-damascene structures because of superior trench-filling capability and of economic advantages over physical- and chemical-vapor deposition techniques.2 However, as-plated Cu films are unstable at room temperature, exhibiting a property known as self-annealing. At room temperature, the grain size of the as-deposited films changes from a few tens of nanometers to a few microns in times of a few hours to a few days, depending on film-deposition parameters, such as current density, film thickness, and bath composition. During self-annealing, the grain size increases, stress in the Cu film decreases, and the resistivity of the Cu film drops.3 It is important to understand the characteristics of this transient behavior of electroplated films if this method is to be incorporated into production of future generation circuits based on deep submicron complementary metal-oxide semiconductor devices. A detailed understanding of the driving forces responsible for this transient behavior is still lacking.

Determination of the activation energy of any kinetic process is important for understanding the underlying atomic processes. Differential scanning calorimetry (DSC) is a very useful tool for obtaining the activation energies of kinetic processes in thin films because of the small sample size required and its very high sensitivity for measurement of small heat changes. In a typical non-isothermal DSC experiment, a sample and a reference with similar heat-capacity characteristics are heated at a preset constant rate, while their temperatures are kept equal. During a phase transformation in the sample, depending on whether it is exothermic or endothermic, more energy must be absorbed from or supplied to the sample to keep the sample and the reference at the same temperature. This results in a peak in the thermograph, which is a record of the heat flow versus temperature. The apparent activation energy of the transformation can be calculated from the shift in peak transformation temperature with varying scan rates.4

In the present work, nucleation and growth characteristics of self-annealing in plated Cu films are characterized using ion-induced secondary electron microscopy. It is then shown that for the transformation characteristics of self-annealing, the apparent activation energy measured using the DSC technique corresponds to the activation energy for the rate-controlling step for grain growth in plated Cu films.

EXPERIMENTAL DETAILS

Blanket Cu films of 1.5-[mu]m thickness were deposited on 200-mm-thick (100) Si substrates in an electroplating system using a commercial electrolyte. The electrolyte contained proprietary additives to improve trench filling. Prior to the deposition of Cu, 500 nm of SiO^sub 2^ was deposited onto Si substrates using plasma enhanced chemical vapor deposition (PECVD) followed by sputter deposition of a 50-nm-thick, Ta-barrier layer and a 200-nm-thick Cu-seed layer. The barrier and seed layers were deposited sequentially without breaking vacuum.

The DSC measurements were performed in a nitrogen ambient using a modulated DSC system. Rectangular samples, approximately 10 mm x 5 mm, were cut from the wafer and heated to 200[degrees]C at different heating rates. The Cu film was etched away from one of the heated samples using dilute nitric acid and used as the reference for each DSC run so that heat changes detected are only from structural changes.