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Thin is In Solving the Challenges of Thin Film Metrology at 65 nm and Beyond Arun R. Srivatsa, KLA-Tencor
Shrinking process windows and a variety of new materials and processes associated with the transition to 90 and 65 nm technologies, as well as the stringent requirements and costs associated with 300-mm manufacturing, place new demands on thin film process control and necessitate the migration to increasing on-product metrology. This article reviews both recent advancements in spectroscopic ellipsometry, as well as new techniques that enable semiconductor manufacturers to meet the thin films process control challenges of 65 nm and beyond. Introduction
With each node, windows for critical processes such as anti-reflective coatings (ARCs) and gate dielectric thickness continue to shrink. A rule of thumb is that the metrology error should not exceed 10 percent of the process window; consequently, metrology windows continue to shrink, increasing the demands on tool matching and repeatability. At the 90 nm and 65 nm nodes, we also see the introduction of a variety of new materials1-4 and processes, placing new demands on thin film process control. These include the introduction of siliconon-insulator (SOI) and silicon germanium (SiGe) based substrates, high-k-based capacitor dielectric stacks, heavily nitrided oxides for gate dielectric, low-k dielectrics, and copper metallization in the back end. The cost of 300 mm wafers, the desire for “lights-out” fabs, and chemical-mechanical planarization (CMP)-driven requirements necessitate a migration to increasing on-product metrology. On-product metrology implies more multi-layer, multi-parameter measurements without compromises in the metrology window for each layer. Spectroscopic ellipsometry (SE) is the “workhorse” of the semiconductor industry to control a variety of deposition, etch, and CMP processes in both the front end and the back end. This article reviews both recent advancements Winter 2005
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addition, the use of SOI substrates makes the measurement of gate dielectrics on SOI much more challenging. Optical measurements using fixed angle, single wavelength ellipsometry (SWE), typically done with helium-neon (HeNe) lasers, are no longer viable. The superficial Si is transparent at HeNe wavelength (633 nm), making this a multi-parameter measurement (measuring gate oxide, superficial Si, and buried oxide simultaneously) that is not possible with standard SWE. Controlling the strained Si process requires measurement of both thickness and strain in the layer. There are multiple approaches being pursued for the introduction of strain in the Si channel, and process control methods vary depending on the path used. Gate oxides are becoming more heavily
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A variety of new materials are also being introduced in the 90 and 65 nm nodes (see Figure 1). Many of these materials introduce new challenges to metrology and process control. For example, there is a gradual migration from silicon (Si) substrates to siliconon-insulator (SOI), strained Si, and strained silicon/silicon-germanium (SiGe) on insulator (sSOI). These generate new requirements: SOI substrates require monitoring of the thin superficial silicon layer and buried oxide thickness and uniformity. In
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With each technology node, process windows are shrinking for several critical front end and back end processes. Shrinking process windows require tighter metrology windows for viable process control. As stated above, a general rule of thumb is that the total metrology window should be less than 10 percent of the process window. There are different approaches used to quantify the metrology error. These approaches estimate the measurement uncertainty using one or more of the following factors: matching (difference in measurements) between multiple tools, short term (usually three to five days) repeatability and long term (30 days or more) repeatability of a measurement on a wafer on a single tool. The most rigorous interpretation defines the metrology uncertainty as the sum of the metrology errors associated with matching between multiple metrology tools and the repeatability of a single tool. At the 65 nm node, the metrology requirements for these parameters are much tighter compared to the 90 nm node. A good example of this is the considerably tighter metrology window for monitoring the critical poly ARC process. At 90 nm, the requirement for tool matching of both the refractive index (n) and extinction coefficient
(k) of ARC films is typically around 0.005. At 65 nm, the same requirement is typically around 0.0006 to 0.0008; an effective tightening of the metrology window by about six to eight times.
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in this production technology, as well as new techniques that enable semiconductor manufacturers to meet the thin films process control challenges of 65 nm and beyond. Applications discussed include the use of the desorber and its impact on gate oxide metrology, multi-layer, multi-parameter measurements, use of deep ultraviolet (DUV) wavelengths and how they enable metrology for some challenging filmstacks, and the measurement of dielectric films on patterned metal lines.
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nitrided, requiring control of nitrogen in the oxide. We are seeing the introduction of bilayer and nanolaminate-based high-k material stacks for capacitors. While there are a lot of challenges in the front end, we also see some challenges in the back end with the introduction of low-k and Cu.
Saving monitor wafer costs The introduction of 300 mm technology and the higher cost of 300 mm wafers provide an added impetus to measure directly on the product, thereby saving monitor wafer costs. Product wafer measurements have usually been carried out on large pads in the scribe line. With shrinking geometries, for some critical processes like STI, there is a marked lack of correlation between CMP rates on pads in the scribe line and CMP rates in the die. For process control in STI, in-die measurements of oxide and nitride filmstacks are required. Until now, with pads, one could use a homogenous film model to describe the substrate. For measurements of films directly on large areas with submicron patterned features, the assumption of substrate homogeneity is no longer valid. To measure films on these structures, new algorithms have been developed. Some fabs have also begun to implement design rules that do not permit the presence of large metrology pads anywhere on the wafer. Metrology measurements have to be carried out on tiled structures. While the bulk of the measurements today still remains on monitor wafers or on large metrology pads on product wafers, there is a steadily increasing need for in-die measurements or measurements on tiled structures in the scribe line.
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there are continuous advancements in films metrology hardware, algorithms, and applications development. Tool matching is usually the most difficult specification to achieve in a production environment. There is an increased emphasis on getting the hardware systems to be as identical as possible to meet the considerably tighter matching requirements. Since SE is the workhorse for thin film measurements, a lot of the focus has been on improving the SE hardware and extending its capability by complementing it with other techniques. In SE, one measures the standard functions tan (psi) and cos (delta) as a function of wavelength. A filmstack model is used to fit the spectra and derive thickness and the optical dispersion (plot of n and k as a function of wavelength) for the different films in the stack. The advances made in spectral matching on multiple systems are illustrated in Figure 2. Here, the residual spectral errors (difference between theory and model at each wavelength) for a thin oxide film are plotted as a function of alpha and beta (alpha and beta can be derived from tan (psi) and cos (delta) over wavelength for multiple systems). The magnitude of the errors is an estimate of the spectral fidelity
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of the system. Thin oxide films are used to evaluate the spectral fidelity, since the optical dispersion of thin silicon dioxide is well known and a standard model of tabulated values for n and k as a function of wavelength can be used. The plots show the residual errors from five previous-generation KLA-Tencor SE systems (ASET-F5x) on the left and five latest-generation SE systems (SpectraFx 100) on the right. The errors are plotted on the same scale in both sets of graphs. It is seen that the residual errors on the SpectraFx 100 systems are considerably smaller across all the wavelengths and close to zero. The magnitude of the errors on these production tools was found to be comparable to 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 SpectraFx100 is virtually identical from one system to another. From a spectral standpoint, the measurement hardware is intrinsically matched. Such good spectral fidelity and system-to-system spectral matching are key to meeting the extremely tight matching requirements on the most challenging film applications.
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Tool matching To meet the changing requirements outlined in the previous section,
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Multi-layer, multi-parameter measurements An example of the type of measurements that can be achieved with such systems and robust algorithms is shown in Table 1. This shows the measurement of a six-layer, low-kbased BEOL filmstack. A sevenwafer design of experiment (DoE) was carried out to evaluate the robustness of the measurement in predicting the introduced changes correctly with a single recipe. A total of 16 parameters was measured simultaneously: thickness, and n and k for all the layers except the top oxide layer, where only the thickness was measured. The refractive index was not measured for the oxide layer on top since this is usually well controlled. It is seen that with a single recipe we can correctly predict the various changes simultaneously
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ment is more complicated since one needs to measure more parameters simultaneously in a filmstack. Spectral fidelity and tool-to-tool spectral matching become more critical for multi-layered films. The example described above for the measurement of multiple parameters in a six-layer stack illustrates the evolution of this capability.
The migration of metrology from monitor wafers to product wafers is being accelerated with the introduction of 300 mm wafers. On monitor wafers, it is easier to keep the metrology simpler and monitor individual films or processes. On product wafers we often see the requirement to monitor the same films and processes in multi-layered stacks. The metrology requirements for the individual films and processes are unchanged though the measure-
Thin gate oxide measurements
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The requirements for thickness control of thin gate oxides are getting exceedingly small. We routinely see requirements for metrology errors on thin oxide measurements of 0.1 Ă… or less! To keep things in perspective, a monolayer of silicon is about 3 Ă… thick. With such stringent requirements, the stability of the film and its equilibration in the local fab environment play an important role in getting a handle on the metrology wafer 11
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1048.5
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0.7075
0.4418
0.4509
0.2986
0.5018
0.2292
653.4
577.3
613.6
647.2
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593.9
649.4
0.5883
1.6292
0.4666
0.7617
0.4200
0.7036
0.3338
1.7161
1.7224
1.7370
1.7075
1.7193
1.7210
1.7095
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0.5979
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1.3640
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1.3785
1.3713
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0.0861
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introduced in this seven-wafer DoE. The circles in different colors outline missing layers, double-deposited layers, half-deposited layers, and layers with a random variation in thickness. The measurements of these 16 parameters were also carried out with very good repeatability as evidenced by the standard deviation reported for each parameter.
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Table 1. Robust measurement of six-layer, low-k stack across seven wafer DoE. Robustness was tested by introducing missing layers, double deposited layers, half deposited layers, and other variations randomly in the filmstack.
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Extending SE to DUV and VUV wavelengths Thin oxide/nitride/oxide (ONO) filmstacks 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 around 50 Å, and may go down as low as 30 Å for the 65 nm node. This is a very challenging measurement due to extremely high correlation 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: correlation increases significantly as the nitride gets thinner. Since the nitride film has increased absorption characteristics at shorter wavelengths (Figure 4a), the use of shorter wavelengths increases the “contrast” between the top and bottom oxides. To enable these measurements SE has to be extended to wavelengths down to DUV wavelengths (190 nm) for ONOs with the nitride at 50 Å and down to VUV (150nm) for ONOs with nitrides at 30 Å. Figure 4b shows results on measurements done across a nitride thickness DoE. In this DoE, the top and bottom oxide thicknesses were kept fixed, while the nitride thickness was varied. The capability of both 190 SE and 150 SE systems to accurately track the process changes introduced was monitored. It is seen that both the 190 SE and 150 SE systems track the nitride thickness accurately. The 190 SE system shows a flat response for the top and bottom oxide thickness down to nitride thickness of
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Airborne Molecular Contamination (AMC): Effects on thin film measurements and how to control it using laser-based desorption
What is AMC ? AMC is short for airborne molecular contamination. This describes any species that is physically or chemically adsorbed on the wafer surface as it stabilizes in the local fab environment. The growth of AMC is relatively rapid initially with a gradual stabilization over time. The accumulation of this AMC layer is not uniform across a wafer and also varies from wafer to wafer depending on location within the cassette. Time-of-flight secondary ion mass spectrometry (TOFSIMS) analysis has shown that for a freshly processed wafer with a thin gate oxide, AMC is largely composed of water vapor and hydrocarbons. For wafers that are “aged” over several months in a cassette, hydrocarbons and water vapor are still the dominant species; however, we see a significant increase in the presence of large molecular weight species like plasticizers or siloxanes.
What is the effect of AMC on thin film measurements ? In ellipsometry (spectroscopic or single wavelength), we measure the optical thickness of films on substrates like silicon. The presence of AMC adds to the optical thickness of the film measured and leads to errors in the estimation of the film thickness. As shown in Figure 3a, for a thin silicon dioxide film of about 20 Å, the process window is usually around 2 Å. For good process control, the metrology error should be less than 10% of the process window, ie., 0.2 Å. However, the buildup of AMC on freshly grown oxide can exceed 0.5 Å within two hours, thereby introducing large errors in the measurement. Specifically, AMC causes the following: a) Queue time requirements for gate oxide measurements in fabs: To reduce the error due to AMC, many fabs implement strict time windows within which the gate oxide measurement is done. b) Difficulty in establishing fab-tofab matching for thin film metrology: The local environment in any two fabs is not identical. This leads to differences in AMC buildup as the stable wafers in fab A re-stabilize in the local environment in fab B; the difference in AMC optical Winter 2005
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errors. Airborne molecular contamination and its control are extremely important for thin oxide measurements (see sidebar on Airborne Molecular Contamination for a detailed description of the problem and its solution). Measurement systems need to be equipped with desorbers to enable meeting the stringent metrology requirements.
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thickness between fabs can easily exceed the metrology requirements for tool matching for films like thin gate oxides. This can easily lead to ambiguity in determining whether the metrology tools are not matched between fabs or the gate oxide being measured in the different fabs is not optically identical due to differences in AMC. c) AMC can cause large errors in estimation of within-wafer (WIW) uniformity: both in magnitude of the WIW uniformity and the contour. The AMC buildup is not uniform across the wafer and also varies depending on the location of the wafer in the cassette. The effect of this is clearly seen from the thickness contour maps in Figure 3b. As seen, the WIW uniformity is substantially smaller when measured following AMC removal; moreover the thickness map changes shape from thicker at the edges to thinner at the edges. Incorrect estimation of the WIW uniformity can lead to incorrect tuning of the oxide deposition process, which can further exacerbate the true WIW uniformity.
How can we control AMC? Since AMC is predominantly water vapor and hydrocarbons, the preferred method of control is to apply a desorption process prior to measurement. Desorption conditions are usually chosen to repeatably bring the wafer back to the same state by removing most of the AMC. Desorption is achieved by heating either the entire wafer (using a hot plate for instance) or by applying localized heating using a heat source like a laser. Upon desorption, the wafer will begin to restabilize in the local environment by re-adsorption of AMC. To minimize AMC-introduced errors, it is, therefore, preferable to minimize the time interval between desorption and measurement. This is achieved using localized laser-based desorption. Using this procedure, a stable measurement of the film thickness can be achieved. Figure 3c illustrates the stability of an SWE measurement with and without laser desorption. It is seen that the use of the desorber leads to a stable measurement over time.
50 Å, but begins to show deviations and correlations between the oxides when the nitride thickness is lower. The 150 SE system, on the other hand, shows a flat response for the top and bottom oxide thickness for
the entire DoE, per the design. So, 150 SE capability is required to monitor the process for thin ONO stacks with the nitride thickness below 50 Å.
We are also seeing the introduction of high-k-based materials for memory applications. Both high-k-based multilayers and nanolaminates are being studied. High-k materials are also being studied for gate dielectrics, though the introduction for that application appears more likely at 45 nm. As with nitride films, high-k films show increased absorption at lower wavelengths. Consequently, the use of 150 SE for these applications increases the contrast for the high-k multilayers, showing better viability to meet the stringent requirements on thickness and composition control. Composition, in this case, can be monitored by correlation to refractive index variations.
Patterned metrology Patterned metrology requiring the measurement of films on top of 1D or 2D structures is becoming increasingly important. The challenges posed by these requirements and an example of such measurements are described in the sidebar section on this topic.
Stress metrology
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At 300 mm we have also seen a migration of film stress measurements from dedicated tools to measurements enabled by a) b) Avg. T3 Nitride: k vs λ (nm) 1.2 hardware options on optical +4 150SE+SE +2 190SE+SE film measurement tools. This nom -2 1.0 is usually determined by -4 -6 measuring the bow of the -8 0.8 1 2 3 4 5 wafer before and after film Wafer Avg. T2 deposition. The differential 80 0.6 Oxide (T3) 150SE+SE 70 bow can then be used to cal190SE+SE 60 Nitride (T2) culate the stress in the film. 0.4 50 40 Oxide (T1) Initially stress was described 30 by a single global stress num0.2 20 Si 2 3 4 5 Wafer 1 ber for the entire wafer. A Avg. T1 0.0 single stress number for the +4 150 200 250 300 350 400 450 500 +2 entire wafer is inadequate for nom many processes. Stress in the -2 150SE+SE -4 film can be better controlled 190SE+SE -6 with a more accurate 2D 1 2 3 4 5 Wafer measurement map across the Figure 4. a) Variation of nitride dispersion with wavelength b) Tracking ONO DoEs with 190SE and 150SE wafer. A better understanding (Courtesy: Simona Spadoni and Davide Lodi, ST Microelectronics). 26
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of the variation across the wafer makes it possible to alter the process conditions for uniformity. Film measurement tools like the SpectraFx 200 now offer 2D stress measurement capability as an option. The spatial resolution of the measurement is on the order of millimeters. High-density mapping can be used in developmental mode to determine the variation of stress and identification and control of regions with high localized stress. In production mode, a userdefined map can be used with more data points in regions of the wafer which are more prone to higher levels of stress for a given process. An example of a line scan and a stress map across an oxide wafer are shown in Figure 6. From the variation in the stress profile across the wafer, it is evident that describing the stress in the wafer with a single global number is inadequate. Conclusion
With shrinking nodes we are seeing a greater number of metrology steps to maintain high yields. This is motivated by several reasons, including the introduction of a variety of materials and processes at a faster rate compared to earlier technology nodes. 300 mm wafer costs are leading to a greater number of measurements directly on product. The lack of correlation between measurements on proxy structures is leading to a migration of metrology to in-die measurements for critical applications like STI. In this article we have reviewed many of these challenges and the emerging metrology solutions to address these issues.
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Patterned Metrology: Challenges and Solutions
What is Patterned Metrology? The bulk of metrology measurements are carried out on monitor wafers or solid pads in scribe lines on product wafers. Motivated by changes in design rules and/or a lack of correlation between process variations (such as CMP rates) of films on pads and actual variations in device areas, there is a push for measurements of films either in-die or on patterned features mimicking device geometries. For example, for the STI process, at the 90 nm node, there is a lack of correlation between CMP rates on pads and in actual device areas, requiring measurements in-die. Patterned metrology refers to such types of film measurements on both 1D and 2D patterned structures.
What is the impact of these new requirements? The typical measurement feature size for SE is around 50 microns, while it is between 10 and 60 microns for reflectometry. Measurement pads in the scribe lines are usually bigger than 50 microns, enabling the assumption of homogenous substrates in film modeling. With the requirement to measure films deposited on patterned features mimicking device geometries (usually micron or sub-micron), these measurements have to be carried out on 1D or 2D patterned structures. The “substrate” is no longer homogenous, but typically consists of alternating amounts of two or more materials. A simple example of such a structure is a 1D metal grating with alternating “lines” of metal and dielectric, shown in Figure 5a. Depending on the thickness of these layers forming the “substrate” and the optical properties, there may also be additional interaction between the light beam and the layers below. The complex interaction of the light beam with these features and resulting reflections needs to be accounted for with appropriate algorithms. So, the requirement to measure in these structures makes film modeling more complicated, requiring more powerful algorithms to model the reflections from such structures.
How do we measure films on such patterned structures ? Measurements on patterned structures can be carried out using appropriate algorithms, with either SE or reflectometry. These algorithms enable modeling of the reflections from the patterned “substrate”, thereby allowing accurate measurement of the films on top of the patterned structures. An example of such measurements is shown in Figure 5b. This shows the measurement of a dielectric film stack on titanium nitride (TiN) on a patterned 1D aluminum array. The patterned metal array consists of alternating lines of aluminum and embedded dielectric. The SE measurements were compared to cross-sectional results using scanning electron microscope (SEM). It is seen that there is good correlation between the SE results and the SEM results; moreover, the SE results are seen to be closer to the target thickness.
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Acknowledgements
The author thanks several individuals for detailed technical discussions on numerous metrology topics that have helped develop his ongoing understanding of metrology and
Oxide Nitride
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Figure 5. a) Schematic of dielectric films on 1D patterned metal lines b) Excellent correlation between optical measurements and nominal thickness for a filmstack with oxide and nitride on patterned metal lines (Courtesy: Larr y Camilletti, Jazz Semiconductor).
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References
solutions. They include Carlos Ygartua, Torsten Kaack, John Fielden, Shankar Krishnan and Leonid Poslavsky from KLA-Tencor and Larry Camilleti from Jazz Semiconductor.
1. http://www.itrs.net 2. Y.C. Yeo, et al, Proceedings of the International Electron Devices Meeting (IEDM), p. 753 (2000).
3. H. van Meer and Kristin De Meyer, 2002 Symposium on VLSI Technology, Digest of Technical Papers, p. 170 (2002). 4. H . S . P. Wo n g , I B M J o u r n a l o f Research and Developemnt, V46, N2/3 (2002).
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