Spectroscopic Ellipsometry Film Metrology Braces for 45 nm and Beyond

With the advent of new materials and structures at the 65 and 45 nm nodes, demands on thin-film metrology are increasing in complexity while metrology budgets get tighter. In several key processes, it is no longer sufficient to monitor just thickness and refractive index for process control. One must measure or infer composition, porosity and other parameters. Using the systematic variation of optical properties with these parameters, recent advances in the application of spectroscopic ellipsometry (SE) have led to the successful adoption of this technique in R&D and production for monitoring composition in varied materials like high-k gate dielectrics, nitrided gate oxide and boron-doped silicon germanium (SiGe:B). There are important process control challenges and requirements for handling new materials and complex structures, and new applications data and potential solutions using optical thin-film metrology will be discussed.

Multiple-front challenges

There is an almost universal agreement that, at the 65 and 45 nm nodes, film metrology is getting more complex and intensive. Along with the usual tightening of process windows and metrology budgets (a general rule of thumb is that the total films’ metrology budget should be <10% of the process budget), this is driven by two other factors: the introduction of many new materials and innovative structures^1-5 in both the front and back end, and the migration of metrology from proxy measurements of films on monitor wafers to measurements on product wafers.

At the front end, many new materials introduce new challenges for metrology and process control. The challenges begin with the gradual migration from silicon to silicon on insulator (SOI) substrates. These changes generate new requirements: SOI substrates require monitoring of the thickness and uniformity of the thin superficial silicon layer and buried oxide. The use of SOI substrates also makes it much more difficult to measure gate dielectrics and multilayer structures. The superficial silicon is transparent at HeNe wavelength (633 nm) makes this a multi-parameter measurement (simultaneously measuring gate oxide, superficial silicon and buried oxide), which is impossible with standard fixed-angle, single-wavelength ellipsometry (SWE).

Spectroscopic ellipsometry moves into films composition metrology. This wafer map image shows the N profile in a thin SiOxNy film mapped using ellipsometry. (Source: KLA-Tencor)

While there are many more challenges arising in the front end, the use of low-k materials and copper also brings significant challenges to the back end. The use of low-k carbon-doped oxides (CDOs) with the associated barrier and etch-stop layers demands tighter metrology control with more complex stacks. Porous low-k dielectrics add complexity since, while it appears at this time that pore size and pore distribution are parameters that may not be required for production monitoring, an estimate of porosity and/or dielectric constant is required for production control.

1. Many new and highly complex materials are being introduced at a faster rate compared with previous technology nodes. (Source: KLA-Tencor)

The trend toward the inspection of product wafers is largely driven by a desire to eliminate monitor wafers, especially at 300 mm. In some instances, in-die measurements are required for process control because of the lack of correlation between variations in the die and larger features in the scribe lanes.⁶ Product wafer measurements are usually done on large pads in the scribe lane. As geometries shrink, many critical processes are affected. In shallow trench isolation (STI), for example, there is a marked lack of correlation between CMP rates on pads in the scribe lane and CMP rates in the die. For process control in STI, in-die measurements of oxide and nitride film stacks are required.

Solving film metrology problems

Optical thin-film metrology, largely based on SE, is used extensively for process control throughout the fab. SE is a rapid, non-destructive technique used for both monitor and product wafer measurements. The SE technique comprises two key ingredients: hardware with good spectral fidelity to extract information from the films, and applications expertise to create viable solutions using the spectral information and algorithm tools. Recent advances on both fronts have led to viable SE-based solutions for applications such as compositional monitoring of complex films in both R&D and production environments.

The primary improvements in hardware include improved optics, leading to better spectral fidelity, and extension of SE to deep ultraviolet (DUV) wavelengths (down to 150 nm). Combined, these two factors are important because the extension to DUV wavelengths enables the extraction of more information from the thin dielectric films that have greater absorption at these wavelengths, while spectral fidelity gives better resolution and minimizes the metrology error bars, helping to satisfy increasingly stringent requirements.

The quality of spectral fidelity can be easily determined by evaluating the spectral errors (difference between measured spectra and theoretical spectra) from a thin oxide film. As an example using our tools, the spectral quality of two generations of production SE systems was examined (Fig. 2 ). The errors are plotted on the same scale in both sets of graphs. It is seen that the residual errors on the newer SE systems are considerably smaller across all wavelengths and close to zero. The magnitude of the errors on these production tools was found to be comparable with errors from a research-grade system using a similar test. Equally important, it is seen that the “signature” of the remaining small residual errors on the latest SE tools is virtually identical from one system to another. From a spectral standpoint, the measurement hardware is intrinsically matched. High spectral fidelity and system-to-system matching are key factors for meeting the extremely tight requirements on the most challenging film applications.

2. Residual spectral errors are close to zero across all wavelengths, and the residual error signature is repeatable in newer generations of SE systems. (Source: KLA-Tencor)

Any discussion of optical monitoring of thin-gate dielectric films must address the issue of airborne molecular contamination (AMC). Detailed discussions are available.⁶ Using ellipsometric techniques and a desorber to address AMC, a viable production-proven solution has been formulated for monitoring the thickness and nitrogen concentration (%N) in thin SiON gate dielectrics. The solution has repeatedly demonstrated good correlation between the measured SE parameter and %N baseline data across a wide design of experiments (DoE). This type of optical solution is currently implemented successfully at several fabs worldwide.⁷

High-k gate optical metrology

Candidate materials are largely hafnium-based oxides or silicates and include HfO₂, HfSiO_x and HfSiO_xN_y. With these materials, there is typically an interfacial layer ~5-10 Å thick between the 20-40 Å high-k dielectric and silicon. The interlayer has a lower dielectric constant than the bulk high-k material. Process control schemes typically rely on thickness and composition monitoring of bulk high-k dielectric, coupled with electrical monitoring of the interface between the high-k dielectric and silicon. These high-k materials’ optical properties vary systematically with composition. At lower wavelengths, especially in DUV down to 150 nm, there is increased sensitivity to these materials because of increased absorption. Using this information, and by leveraging recent advances in the hardware, algorithms and applications methodologies, SE can simultaneously monitor two compositional parameters.

Figure 3 shows examples of optical measurements of composition in high-k films in a development fab, with results across an HfSiO_x DoE. In this case, SE was used to map and output %SiO₂ in the HfSiO_x films. A wide range of compositions, nearly 50% SiO₂ variation in the HfSiO_x, was sampled across a DoE with multiple wafers. X-ray photoelectron spectroscopy (XPS) was used as the reference technique. Measurements at 21 sites were carried out across each wafer (from center to edge) in the DoE using both XPS and SE. DUV wavelengths down to 150 nm were used to build up the optical models. The results show strong correlation between the SE output for composition and the XPS baseline across the DoE and within each wafer in the DoE. For the HfSiO_xN_y films, a recently developed algorithmic model was used to simultaneously compute both %SiO₂ and %N in the film. As with the HfSiO_x films, 21 site measurements were carried out across each wafer in the DoE to verify capability to track compositional variation within each wafer across the wide range of compositions in the DoE. Again, there is good correlation with the baseline across the wide range of compositions sampled in the DoE.

3. The tracking of composition in HfSiOx films with SE, with simultaneous determination of two compositions in HfSiON films with SE. (Source: KLA-Tencor)

Monitoring bilayer structures

As with the high-k materials, there is a systematic variation in optical properties of SiGe with increasing germanium concentration. The presence of boron at high dopant concentrations has a secondary effect on the optical properties. Using a DoE with relatively constant boron concentration (with some variation) and a systematic variation in germanium concentration, an SE-based optical solution was formulated to measure both single-layer SiGe:B and bilayer silicon-cap/SiGe:B/Si structures using the same recipe. The SiGe:B and silicon-cap layers’ thickness were simultaneously measured along with germanium concentration in the SiGe:B layer. Here, X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS) were used as baseline techniques. As with the other applications described earlier, excellent correlation was achieved between the optical measurement of germanium concentration and baseline techniques.

The ability to track multiple parameters simultaneously in a production environment can be seen from the results in Figure 4 . Results from a four-wafer DoE with roughly similar SiGe:B and silicon-cap thicknesses, but varying germanium concentration, are plotted. Measurements were carried out from center to the edge of the wafer using a standard nine-site Prometrix pattern. The nominal thickness of the SiGe:B layer was >1000 Å, with a thin silicon-cap layer. Within the nine-site pattern, the signature of the reactor is reproduced for the SiGe:B and silicon-cap thicknesses at varying germanium concentrations. The data from three tools in a production environment also shows that the results for the different parameters are well matched. Such tool-tool matching is possible because of the spectral fidelity described earlier.

4. Simultaneous measurement of thickness of silicon-cap and thickness and composition of SiGe:B layer across germanium concentration DoE with SE. Good tool-tool matching in a production environment in especially important. (Source: KLA-Tencor)

Ultrathin ONO film stack metrology

Thin oxide/nitride/oxide (ONO) film stacks are used in both DRAM and flash memory stacks. At the 90 nm node, the target for the nitride thickness for floating-gate flash is ~50 Å (and may be low as 30 Å for 65 nm). This is a challenging measurement caused by extremely high correlation demands between the top and bottom oxide layers. The extent of the correlation is driven by the thickness of the nitride layer separating the two oxides, since correlation increases significantly as the nitride gets thinner. Because the nitride film has increased absorption characteristics at shorter wavelengths, use of shorter wavelengths increases the contrast between the top and bottom oxides. To enable these measurements, SE technology must be extended down to DUV wavelengths (190 nm) for ONO stacks with the nitride at 50 Å and down to vacuum ultraviolet (VUV, 150 nm) for stacks with nitrides down to 30 Å.

The capability of both 190SE and 150SE systems to accurately track the introduced process changes was monitored. Both systems accurately track the nitride thickness. The 190SE system shows a flat response for the top and bottom oxide thickness down to nitride thickness of 50 Å, but begins to show deviations and correlations between the oxides when the nitride thickness is lower. The 150SE system shows a flat response for the top and bottom oxide thickness for the entire DoE, per the design. So for thin ONO stacks with the nitride thickness <50 Å, 150SE capability is recommended to monitor the process.

Multilayer, multi-parameter measurements

The Table shows an example of the type of measurements achieved using advanced systems with high spectral fidelity and robust algorithms. In this measurement of a six-layer, low-k back-end-of-line (BEOL) film stack, a seven-wafer DoE was carried out to evaluate the measurement’s robustness in correctly predicting the introduced changes with a single recipe. A total of 16 parameters were measured simultaneously: thickness, n and k for all the layers except the top oxide layer, where only the thickness was measured. It is seen that — with a single recipe — the various changes simultaneously introduced in this seven-wafer DoE can be correctly predicted. The circles in different colors outline missing layers, double-deposited layers, half-deposited layers and layers with a random variation in thickness.

The migration of metrology from monitor wafers to product wafers is being accelerated with 300 mm wafers. On monitor wafers, it is easier to keep the metrology simple and monitor individual films or processes. Product wafers require monitoring the same films and processes in multilayered stacks. Metrology requirements for individual films and processes are unchanged, though the measurement is more complicated because more parameters must be simultaneously measured in a film stack. Spectral fidelity and tool-tool spectral matching become more critical for multilayered films. The example for the measurement of multiple parameters in a six-layer stack illustrates the evolution of this capability. It must be noted, though, that in a typical production environment, one does not measure so many parameters simultaneously.

SE into the future

SE continues to be the technology of choice for production monitoring of films in today’s fabs. The continual advancements in spectral fidelity, extension of SE to lower wavelengths, and improvements in hardware, algorithms and applications capabilities are enabling the use of SE technology to report additional parameters like compositions in very thin to thick films, potentially satisfying the increasingly complex metrology requirements at the 65 and 45 nm nodes. Optical film metrology solutions based on SE are currently being adopted to monitor composition in several complex processes involving nitrided oxides and SiGe:B, and in the development of high-k materials. Recent technology advances on multiple fronts are facilitating the accelerated migration to product wafer metrology and multi-parameter, multilayer measurements. With these continued advances, SE-based film metrology could continue to be the workhorse technology for production metrology at 45 nm and beyond.

Acknowledgements

The author thanks several colleagues for detailed technical discussions and for making available many of the figures used. They include Arun Chatterjee, Torsten Kaack, Zhengquan Tan, Sungchul Yoo and Shankar Krishnan from KLA-Tencor; and Simona Spadoni, Rosella Piage and Davide Lodi from STMicroelectronics.