Inline Optical Spectroscopy for Advanced Gate Stacks

The incorporation of hafnium-based high dielectric constant (high-k) gate insulator films to control leakage currents and enable continued scaling of the equivalent oxide thickness (EOT) of complex advanced gate stack systems requires accurate process monitoring and control. Slight variations in the thickness and composition of the thin hafnium-based film and the silicon dioxide (SiO₂)-like interfacial layer (IL) between the high-k gate dielectric and silicon substrate may significantly affect the resultant transistor's electrical performance. Fast and reliable inline monitoring of the physical characteristics of complex gate stack film systems, therefore, offers a critical metrology challenge as devices continue to scale at the rapid pace of Moore's Law.

Optical spectroscopy has been a film metrology mainstay in semiconductor device manufacturing, achieving increasing relevance as a process enabler. The amplified complexity of the metrology platform, however, along with fundamental optical constraints, limited the capability of contemporary measurement systems for advanced technology nodes until the emergence of vacuum ultraviolet spectroscopic reflectometry (VUVSR). Indeed, conventional metrology systems are limited in their capability to simultaneously measure high-k films and ILs. Optical systems such as spectroscopic ellipsometers are unable to resolve wide bandgap films, such as the SiO₂-based IL between the hafnium-based high-k layer and silicon substrate because of intrinsic hardware limitations of the low wavelength range.¹

Adding to these challenges, additional complexity with structural changes (crystallinity) in high-k films, as a result of post-deposition thermal cycles, can lead to critical device (CD) performance degradation from leakage effects. Fortunately, VUVSR provides a non-destructive thin-film measurement solution for complex advanced gate stack film systems. It can resolve multilayer films in a single broadband (800 nm down to 120 nm) spectral measurement. The success of VUVSR capabilities derives from strong dispersion in the films at smaller wavelengths (particularly <150 nm), thereby achieving high-resolution disparate thickness and composition within thin-film systems that remains unattainable with existing ultraviolet (UV) optical and X-ray metrology methods.

Experiment

Wafers comprised of IL and high-k film thickness and composition variations were fabricated at Sematech to demonstrate the VUVSR measurement capability of discretely and simultaneously resolving physical variations corresponding to each layer.² An in situ steam generated (ISSG) SiO₂ film of ~20 Å was grown on 200 mm silicon (100) substrates followed by a wet spin etch of the wafer's outer half, designed to produce 10-Å-thick SiO₂ in a band from 100 to 50 mm radius to form a two-tiered oxide layer. This IL was capped with a conformal (HfO₂)_x (SiO₂)_1-x layer (x=1–0.8) with a targeted thickness range of 15–30 Å (Fig. 1).

1. A conformal high-k film on a two-tier in situ steam-generated interfacial oxide.

Additional samples were exposed to anneal processing with increasing high-temperature increments (T1–T4) to characterize post-deposition anneal (PDA) effects. All wafers were measured using a Metrosol VUVSR system. The IL thickness and film crystallinity was verified with high-resolution transmission electron microscopy (TEM) images and illustrated for as-deposited samples (Fig. 2, left) and following high-temperature anneal (Fig. 2, right). A clear transition of the high-k film from amorphous to crystalline can be observed between the as-deposited and annealed sample, along with the high-resolution TEM extracted thicknesses for each film. Corona-oxide-silicon (COS) measurements of EOT and X-ray diffraction (XRD) crystallinity measurements were also used to validate the VUVSR measurements.

2. High-resolution TEMs of non-annealed high-k/IL dielectric stacks (left) and high-temperature-annealed high-k/IL dielectric stacks. Note the onset of crystallization in the high-k layer (right).

Application highlights

Subtle changes in spectral features are directly related to the interaction of the optical properties and thickness variations of the measured films. Actual VUVSR measurements, therefore, rely on optical models to decouple contributions arising from the strong absorption features and thickness to quantify the parameters in question with a high degree of precision. Figure 3 illustrates the VUVSR capability of simultaneously resolving dissimilar ultrathin films. The brown curves characterize the ~20 Å SiO₂ IL from the center half of wafers below the high-k film, and the dark green curves represent the outer ~10 Å SiO₂ IL region. Both annealed (crystalline) and as-deposited (amorphous) hafnium-based high-k samples are shown. This VUVSR capability derives from capturing spectral features in the 120–150 nm VUV wavelength region. Distinguishing unique film contributions to the measured absorption peaks (extinction coefficient-k) results from realizing that high-k films dominate the optical properties below 135 nm wavelength, while the SiO₂ IL features reside at a wavelength around 120 nm (Fig. 3).

3. VUV reflectance spectra simulation of non-annealed and annealed high-k/IL film stack (top). Optical properties of the high-k and ILs dominate below 150 nm, which results in the VUV’s unique spectral response (bottom).

VUVSR measurements were used to extract the individual layer thickness and hafnium silicate composition for all as-deposited and annealed samples. Samples were treated as multiple layer films, with an effective media approximation (EMA) model applied to track changes in optical properties caused by silicon content in hafnium-based material. The model used three free parameters: high-k thickness, high-k EMA fraction and underlying IL oxide thickness. The EMA fraction was correlated to the nominal values of silicate content (x=1, 0.9 and 0.8) for the as-deposited samples, and to an annealed hafnium oxide (HfO₂) component for the annealed samples. The IL was modeled with a single set of optical properties.

Figure 4 illustrates one example of a 61-site measurement (thickness, composition) across a wafer's X axis, from data acquired in only a few minutes, and modeled using optical properties (Fig. 3b). These results indicate that VUVSR can simultaneously (and independently) determine three critical process metrics of the gate stack system. Both the composition (x=0.82, 18% SiO₂) and the high-k thickness (~31 Å) appear to be relatively constant across the wafer, indicating a conformal and uniform atomic layer deposition (ALD) process. Additionally, results accurately identify the IL thickness profile across the wafer extending from the ~10 Å plateau at the edge to the ~20 Å inner region.

4. VUV-resolved dielectric stack with high-k layer and IL measured across a non-annealed wafer with an IL mesa.

Significantly, the accurate detection characterization of the IL thickness step function buried beneath the high-k film further demonstrates the VUVSR technique's sensitivity. The VUVSR measurements were also correlated with electrical measurements by plotting all results as a normalized EOT expression³ of:

t_eq = t_IL + t_high-k (3.9/k_high-k)

Although the COS measurements do not distinguish the high-k from the IL contribution to the composite EOT as bulk electrical measurements of the stack, a linear correlation was obtained with the VUVSR results.

5. VUVSR spectra of as-deposited and annealed films indicating strong dispersion in the VUV as a function of PDA temperature.

VUV reflectance spectra for the annealed samples are illustrated in Figure 5. The sensitivity to anneal temperatures is exemplified only in the VUV region of 8.26–10.3 eV (150–120 nm), where a clear progression is indicated in the spectra as a function of temperature. The optical properties described in Figure 3b are also responsible for the spectral progression and unique shapes associated with the annealed samples. The crossover and pivot in the spectra going from room temperature to T1°C — about 9.3 eV (133 nm) in Figure 6 — are a result of changes in both the high-k and IL. Subsequent spectral shifts from T2°C-T4°C are dominant around ~9 eV (138 nm), which is close to the peak of the high-k anneal component in the optical property EMA.^4,5

6. Grazing incidence XRD measurements of as-deposited and annealed high-k films.

The lack of any further pivoting in the spectra with further annealing, or significant changes at either the higher and lower energies/wavelengths, points primarily to structural changes in the high-k film. The Table provides a quantification summary of the VUV graphical representation in Figure 5, where the effective medium volume fraction of the non-annealed HfO₂ is normalized to the as-deposited reference sample. These results offer valuable insight into the evolving microstructure of the high-k films as a function of temperature. For example, VUVSR results for the annealed samples were compared with XRD measurements of the 10% silicate (x=0.9) annealed films (Fig. 6). Because of the relatively long range-order sensitivity of conventional XRD, very little difference is evident with increasing anneal temperature relative to the same samples clearly distinguished by VUVSR in Figure 5.

VUVSR thickness and composition measurements of high-k and IL showed strong correlation to electrical COS-generated EOT results. The VUV spectra have been shown to group into distinct sets as a function of high-k thickness and silicate composition, while accurately quantify incremental steps in the buried IL thickness. Furthermore, within these groups, spectra are observed to respond uniquely as a function of anneal temperature, where high-resolution TEM images illustrate morphological transitions corresponding to crystalline order in the high-k films.

This degree of inline measurement sensitivity is currently being extended to include sample sets like those described above capped with various novel metal electrode layers of various thickness to provide rapid solutions for future technology nodes. Coupled with a lower cost of ownership, using the high throughput (>60 wph) VUVSR system for product and development wafers enables advanced process control with rapid, non-destructive feedback for both process control and process optimization.