The technology nodes of 45nm and beyond pose aggressive requirements on copper metallization processes. Barrier and seed layers become thinner; aspect ratios become higher. Barrier and Cu seed films must offer electrical continuity for the electroplating process and provide optimal Cu orientation for minimum resistance, while maintaining conformality with smooth morphology and no overhang . These demands facilitate need for the thickness and structural characterization after seed/barrier manufacturing step. A solution is developed based on combination of x-ray reflectometry (XRR) and 2D wide angle x-ray diffraction (WA-XRD) techniques.
Asaf Kay, Alex Tokar, Jordan Valley Semiconductors, Ltd., Migdal Ha’Emek, Israel; Matthew Wormington, Jordan Valley Semiconductors, Ltd., Austin, TX USA
As the semiconductor industry continues to evolve, much smaller and higher aspect ratio features are required for high performance or low power devices. For such devices, copper has replaced aluminum as the main element used in interconnects from the first to the last metal layer. The advantage of copper is that it has the second highest electrical conductivity of any element, just after silver, but is much more common and hence less expensive. However, copper is very mobile in silicon and it readily diffuses into other layers and contaminates them. The solution for this problem is to deposit a barrier layer such as tantalum, yet again, however, there are issues in that tantalum can be deposited in more than one crystallographic phase, each with very different electrical resistivities. The α-Ta phase has a much lower resistivity compared to the β-phase and is therefore desirable.
Not only is the phase of the barrier important but so too is the orientation distribution of its grains, i.e., crystallographic texture. The tantalum layer is deposited in a (111) orientation and therefore exhibits low compressive stress and facilitates a strong preferred (111) texture in the subsequently deposited Cu seed layer, which is desirable since such a texture has been shown to have much better electromigration performance and hence lifetime. Since microstructure has such a profound influence on the performance and lifetime of Cu interconnects, it should be evident that there is great benefit in the metrology of microstructural parameters.
To be able to cope with all the challenges, one has to carefully monitor Cu metallization processes. We have combined two X-ray based techniques on a single fab-proven platform: one is X-ray reflectivity (XRR) and the other is wide-angle X-ray diffraction (WA-XRD).
X-ray reflectometry (XRR) is a new, but nonetheless firmly established technique [2,3]. XRR is one of the most accurate thin-film characterization methods available as it does not make use of material parameters that are not precisely known. Until recently, the XRR method had suffered from some limitations such as, long data collection times (typically tens of minutes), large spot size, and complex mechanics (moving sample and detectors) that resulted in low performance and high maintenance. The "fast XRR method" was specifically developed to overcome these issues to provide fast, reliable XRR measurements for high-volume production.
Wide-angle X-ray diffraction (WA-XRD) has been used for the lab-based characterization of thin, polycrystalline films for many years . However, the spot-size of the X-ray beam was often large and the measurement speed is slow, since scanning a point (0D) detector was commonly used. This has limited the use of the technique for in-line or patterned wafer metrology. To address these issues, we developed a WA-XRD measurement channel optimized for copper structure metallization processes.
X-ray reflectivity (XRR) is a non-destructive, standard-less technique for the measurement of multilayer properties such as thickness, density, and roughness. XRR measurements are highly sensitive to the electron density of sub-micron structures irrespective of their crystalline nature.
|Figure 1. Schematic diagrams of the Jordan Valley a) XRR and b) WA-XRD channels.|
Consider an X-ray beam illuminating the surface of a sample at low (1-2°) incidence angle as shown in Fig. 1a. The index of refraction for all materials in the hard X-ray wavelength region is slightly less than one, consequently, the X-ray beam is totally reflected if the incidence angle is a smaller than a certain critical angle. In this region, the penetration depth is only a few nanometers. At slightly higher angles, the X-rays start to penetrate and are reflected from the interfaces of thin-films resulting is a series of interference fringes.
|Figure 2. Measured XRR data (blue curve) and best-fit simulation (red curve) from a Cu seed / Ta barrier system. Typical acquisition times are a few seconds for scribe line measurements.|
Typical XRR data from a Cu/Ta film stack is shown in Fig. 2. These data show the critical angle at about 0.4°, whose position gives the average electron density. The fringe spacing is directly related to the film thickness. The short period fringes give the thickness of the Cu layer while the longer period fringes yield the thickness of the Ta barrier. A model fitting approach is used for data analysis. Simultaneous measurements of both Cu and barrier thickness are possible in only a few seconds while providing additional information on the density and roughness (from fringe decay and envelope).
As mentioned, the most commonly encountered XRR systems to date have been scanning systems that move the source/optics or sample and the detector over the range of measurement angles. These systems are mechanically complex and have low throughput due to the serial data acquisition. In order to cope with the new demands of the evolving semiconductors industry, there is a need for much more advanced XRR system.
WA-XRD is a nondestructive technique for characterizing the crystallographic microstructure of polycrystalline materials. In this technique, the intensity diffracted from films at comparatively high-angles is measured as a function in order to study and quantify such properties as crystallographic phase, grain-size and texture. According to Bragg’s law, the geometric condition for diffraction from atomic planes with spacing d is given as 2d sin q – 1. For a beam of X-rays with wavelength l similar to the atomic spacing, strong diffraction occurs at angles q that are a few tens of degrees. The intensity distribution can be measured and quantified to provide valuable insights into the properties of polycrystalline materials: typically phase from the position of the diffraction peaks, grain-size from the full-width half-max of the diffraction peaks, and texture from the relative intensity of diffraction peaks.
XRD is a well established method and has been used in many industries and in R&D labs, however the setups are not ideally suited for use in semiconductor fabs for many of the same reasons described above for the conventional XRR tools. When applying automated XRD analysis in fabs, there is a need to dramatically reduce acquisition times while providing adequate precision while using a small spot so as to allow measurements on patterned wafers. We have developed a WA-XRD channel that can be applied in fabs; it has a very short acquisition times (from a few seconds to a few tens of seconds) and is designed for high-volume production use. Both the hardware and software have been optimized for fab-based measurements.
The classic X-ray diffractometer again works by scanning of the sample and/or detector over very wide angular ranges. This leads to significant mechanical complexity due to the stability that is required for accurate and precision positioning of a small X-ray beam over a large wafer area. While such measurements are certainly possible, they can be very time consuming taking many minutes and, for some measurements, several hours. The JVX WA-XRD channel has no moving source/detector and acquires data over a wide range of diffraction angles with single-shot acquisitions (Fig. 1b). The channel was developed in such a way so as to be able to handle a number of applications.
The channel has a small, high intensity X-ray spot that allows patterned wafer measurements. A custom-designed area detector is used to provide parallel acquisition of the 2D diffraction pattern, which provides information about both the lattice spacings and orientations of the polycrystalline grains. The phase, texture and grain size of polycrystalline thin-films can be measured and mapped over an entire 300mm wafer (Fig. 3).
|Figure 3. 2D X-ray diffraction pattern from a) copper with strong 111 fiber texture and b) from randomly oriented copper with tantalum showing Debye rings.|
Using the 2D diffraction pattern one can extract quantitative parameters by integrating the data to produce 1D intensity distributions that can be fitted to analytical peak functions. If the diffracted intensity is integrated around the direction of the Debye rings (f direction) then one can get the familiar X-ray diffraction pattern – intensity as a function of 2q– from which phase and grain-size information can be extracted. The phase of the Ta(N) barrier is important since it influences the material’s resistivity and likewise for texture and grain-size in the overlying copper (Fig. 4).
|Figure 4. 1D integrated diffraction pattern from a) copper with strong 111 fiber texture and b) from randomly oriented copper with tantalum.|
If, however, one integrates the intensity of the diffraction over the 2q direction, then one will get intensity as a function of f. Analysis of such data provides a quantitative estimation about texture, which has been shown to influence CMP erosion rates and electromigration performance in copper interconnects.
The XRR and WA-XRD techniques and their application to advanced Cu metallization are briefly described in this article. Both are well established techniques for measuring thickness, density and roughness (through XRR) and grain size, texture and crystallographic phase (through WA-XRD). The difficulties associated with traditional X-ray systems in the context of automated, high-volume manufacturing were highlighted.
The 6200iRD tool combines two measurement channels: XRR and WA-XRD. Both channels are optimized for high-volume silicon manufacturing because of very low measurement times (a few seconds), small spot-size, and mechanical precision and stability. The combination of these two channels provides comprehensive metrology to the metallization processes: one can monitor the thickness and the density of the copper and barrier layers while also obtaining valuable microstructure information in terms of phase, grain-size, and texture.
1. P. H. Haumesser, et al., "Copper Deposition: Challenges at 32nm," Semiconductor Fabtech 29, (2006), 108-114.
2 .C. Wyon, "X-ray Metrology for Advanced Microelectronics," Eur. Phys. J. Appl. Phys. 49, (2010) 20101.
3. D. K. Bowen, B.K. Tanner, X-ray Metrology in Semiconductor Manufacturing, Taylor & Francis (2006).
4. M. Birkholz, Thin Film Analysis by X-Ray Scattering, Wiley-VCH (2006).
Matthew Wormington graduated with a BSc(Hons.) in physics from the U. of Birmingham, UK. He did graduate work at the U. of Warwick, UK and is a senior technologist for Jordan Valley Semiconductors Inc.
Alexander Tokar received an engineering degree from Steel and Alloys Institute, Moscow, majoring in X-ray diffraction, and received his PhD from the Israel Institute of Technology (IIT) in materials science. He is a manager, worldwide application support at Jordan Valley Semiconductors.