Paper MS-ThA4
Thickness/Composition Metrology of Ultra-thin Lanthanum Oxide Cap Layer for CMOS Metal Gate Work Function Tuning

Thursday, November 12, 2009, 3:00 pm, Room C3

Beyond the 45 nm node CMOS application, metal gate and high-k dielectric are used and many new thin film materials are developed. In order to reduce the threshold voltage of the CMOS gate, the matching of the metal gate work function with the silicon band position is important. For NMOS work function tuning, lanthanum oxide (LaO_x) thin film is used. Device performance demands the use of less than 10 Å thick LaO_x and the stringent control of thickness and composition uniformity (1 to 2% 1σ) on 300 mm wafers. However, metrology of this new material is very difficult. First the LaO_x cap layer is so thin, only ellipsometry, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence spectroscopy (XRF) have the required sensitivity. Second, the composition of LaO_x changes with air exposure time due to its reaction with ambient moisture; such behavior renders ellipsometey and XPS ineffective, because the composition result and the thickness result from both techniques are strongly correlated in the case of ultra-thin film measurements. This leaves XRF to be the only candidate for both composition and thickness measurements. The main advantage of XRF is that the X-ray signals from the sample are proportional to the surface doses (atoms/cm³) of the elements in the ultra-thin film; hence, it is simple to calibrate the tool and develop measurement recipes. To satisfy the new metrology needs, wavelength dispersive XRF (WD-XRF) and energy dispersive XRF (ED-XRF) techniques were developed.

For tool calibration and drift monitoring, a LaO_x thin film standard with known thickness, density and composition had to be prepared. Due to high reactivity of LaO_x in air, it was difficult to prepare a stable standard and to determine all its attributes. To overcome this problem, a 200 Å thick LaO_x thin film on silicon standard capped with a thin TiN layer was prepared, its thickness and density were measured by X-ray reflectivity and its composition was derived by the XRF measurement itself by an ingenuity method. Due to the good sensitivity of WD-XRF to both La and O signals, recipe was developed to monitor the composition of freshly deposited PVD LaO_x thin films and their change with ambient exposure time. The results showed that the LaO_x ultra-thin film deposited with lanthanum oxide target was more stable in air than those deposited with a La metal target. ED-XRF recipe was developed to monitor the LaOx thickness uniformity on 300 mm wafers. The main advantage of ED-XRF was its small X-ray spot size that afforded 3 mm edge exclusion measurements. Study showed ED-XRF had < 1% (1σ) precision with good throughput of < 60 seconds per data point.