Evaluation of the thermal stability of TiW/Cu heterojunctions using a combined SXPS and HAXPES approach

Abstract

Power semiconductor device architectures require the inclusion of a diffusion barrier to suppress or at best prevent the interdiffusion between the copper metallization interconnects and the surrounding silicon substructure. The binary pseudo-alloy of titanium–tungsten (TiW), with >70 at. % W, is a well-established copper diffusion barrier but is prone to degradation via the out-diffusion of titanium when exposed to high temperatures ([Formula: see text]400 [Formula: see text]C). Here, the thermal stability of physical vapor deposited TiW/Cu bilayer thin films in Si/SiO[Formula: see text](50 nm)/TiW(300 nm)/Cu(25 nm) stacks were characterized in response to annealing at 400 [Formula: see text]C for 0.5 h and 5 h, using a combination of soft and hard x-ray photoelectron spectroscopy and transmission electron microscopy. Results show that annealing promoted the segregation of titanium out of the TiW and interdiffusion into the copper metallization. Titanium was shown to be driven toward the free copper surface, accumulating there and forming a titanium oxide overlayer upon exposure to air. Annealing for longer timescales promoted a greater out-diffusion of titanium and a thicker oxide layer to grow on the copper surface. However, interface measurements suggest that the diffusion is not significant enough to compromise the barrier integrity, and the TiW/Cu interface remains stable even after 5 h of annealing.

Novel Approach for Assessing Cyclic Thermomechanical Behavior of Multilayered Structures

“…[ 41 ] These temperatures typically occur during the manufacturing of microelectronic devices as well as during short circuits and represent the maximum temperature such devices should be able to endure. [ 25 ] In regular temperature intervals, the temperature was held constant to take detailed images of the deflecting multilayer beam, with magnifications of 1000× and 5000× for curvature measurements, respectively. This leads to low heating rates with a maximum of 50 °C min −1 between heating steps and approximately 4 °C min −1 on average over the course of one heating cycle (an example heating cycle can be found in Figure ).…”

“…[8,9,23,24] Furthermore, the WTi layer has been examined regarding its residual stresses and thermal stability. [10,20,25] More recently, attention was placed on the properties of the interfaces, especially during fracture toughness experiments and cyclic heating. [26][27][28] The present work expands this bulk of knowledge by assessing the actual occurring internal stresses during thermomechanical cycling.…”

Capturing the dynamics of Ti diffusion across Ti$_x$W$_{1-x}$/Cu heterostructures using X-ray photoelectron spectroscopy

“…TiW is compatible with various metallisations (Al, Au, Ag, In and Cu) and has remarkable thermal stability at elevated temperatures (≤850 • C). [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Consequently, TiW diffusion barriers are now being widely implemented in next-generation SiC-based power semiconductor technologies with copper metallisation schemes, [20][21][22] and more recently within electrodes for GaAs photoconductive semiconductor switches (PCSSs), [23] and gate metal stacks in GaN-based high electron mobility transistor (HEMT) devices. [24] Diffusion barriers are needed as Cu and Si readily react at relatively low temperatures to form intermetallic copper-silicide compounds at the interface, which seriously hamper the performance and reliability of devices.…”

“…Fugger et al cite that this out-diffusion process is an "essential factor" in the failure of this barrier, [16] and others have also documented the segregation of Ti during high-temperature annealing. [12,12,19,20,30,31] Given the importance of the TiW barrier to the overall device performance, reliability and its application in future SiC technologies and beyond, this Ti diffusion degradation process must be better understood, including how it impacts the stability of the TiW/Cu structure. The common thread across the vast majority of past experimental studies on TiW and diffusion barriers in general, including the present authors' previous work, [19,32] is that ex-situ samples are used to track the evolution of the diffusion process and to determine the temperature at which the barrier fails.…”

“…[12,12,19,20,30,31] Given the importance of the TiW barrier to the overall device performance, reliability and its application in future SiC technologies and beyond, this Ti diffusion degradation process must be better understood, including how it impacts the stability of the TiW/Cu structure. The common thread across the vast majority of past experimental studies on TiW and diffusion barriers in general, including the present authors' previous work, [19,32] is that ex-situ samples are used to track the evolution of the diffusion process and to determine the temperature at which the barrier fails. Such studies also often focus on one Ti concentration and are therefore unable to address the effect of the titanium concentration of the film on the degradation mechanism.…”