Scanning transmission electron microscopy of gate stacks with HfO2 dielectrics and TiN electrodes

Abstract

High-angle annular dark-field (HAADF) imaging and electron energy-loss spectroscopy (EELS) in scanning transmission electron microscopy were used to investigate HfO2 gate dielectrics grown by atomic layer deposition on Si substrates, and their interfaces with TiN electrodes and silicon, as a function of annealing temperature. Annealing at high temperatures (900°C) caused significant roughening of both bottom (substrate) and top (electrode) interface. At the bottom interface, HAADF images showed clusters of Hf atoms that protruded into the interfacial SiO2 layer. Low-loss EELS established that even crystalline HfO2 films exposed to relative high temperatures (700°C) exhibited significant differences in their electronic structure relative to bulk HfO2. Further annealing caused the electronic structure to more closely resemble that of bulk HfO2, with the most significant change due to annealing with the TiN electrode.

A new analytical method for characterising the bonding environment at rough interfaces in high-k gate stacks using electron energy loss spectroscopy

“…Multiple scattering also cause background subtraction problems for the Si-L 2,3 edge in the presence of TiN as noted above. The change in thickness also results in errors in the values of t 0 predicted by equation (4). In addition, it changes the elastic scattering leading to a corresponding change in the intensity of the spectrum entering the spectrometer.…”

“…If the HAADF signals I poly-Si and I TiN are recorded in the bulk materials in regions of thickness t poly-Si and t TiN respectively, equation (4) can be re-written in terms of t(x) and t o (x) at position 'x' giving:…”

“…Since the interfacial layer thickness is generally much smaller than the combined thickness of the TiN and poly-Si phases we have: Equation (4) can be used to calculate 't o ' for all infinitesimal columns within the interface region and thereby estimate A o , the cross-sectional area of TiN within the analysis region 'Lt'. Spreading of the electron probe within the specimen means that equation (4) gives only an approximate value of t o but, as will be shown below, the effects of beam spreading can largely be neglected since the parameter of interest A o is determined by integrating the calculated value of 't o ' over the interfacial region. Furthermore since the equation for t o involves subtracting and dividing HAADF intensities it is not required to correct for the black level and gain of the HAADF photomultiplier tube.…”

“…Interface roughening can be caused by crystallisation, phase separation, interdiffusion, chemical reaction or may be due to a columnar grain structure. Electron energy loss spectroscopy (EELS) and energy dispersive X-ray (EDX) spectroscopy in a scanning transmission electron microscope (STEM) have been used previously to characterise the microstructure of high-k gate stacks, particularly at the interfaces [1][2][3][4][5][6][7]. For dielectrics involving Hf-Si-O with poly-Si gate electrodes, a silica (SiO 2 )-type layer has been observed at the dielectric/ gate electrode interface [1,3,5].…”

Detection and mobility of hafnium in SiO2

“…However, only high-angle annular dark-field imaging (HAADF) in scanning transmission electron microscopy (STEM) is capable of atomic spatial resolution with (near) single atom sensitivity [8]. Recently Hf atoms or clusters have been imaged in SiO 2 by HAADF/STEM [9,10]. In this letter we apply HAADF/STEM to study SiO 2 layers doped with Hf by ion implantation.…”