New Era for Metrology

Faster, smaller and cheaper ... such are the demands being made on consumer electronic products, and thus, the semiconductor devices at the heart of these products. With technology nodes at 90 nm for some devices ― and with 65 nm not to far in the future ― traditional methods for the metrology of layers on wafers, such as optical and opto-acoustic technologies, are being challenged to meet current and future requirements. Determining the thickness of the layer is not the only essential parameter at these smaller nodes as the atomic structure within the layers can have a significant impact on performance of the device. In searching for a solution, manufacturers are realising the benefits of X-ray diffraction (XRD) and High – Resolution X-ray Diffraction (HRXRD). Figure 1 shows a metrology system that utilises HRXRD, enabling absolute measurements traceable to accepted standards for thickness, composition and relaxation for multiple layers. XRD and X-ray reflectivity (XRR) are also available on the same platform.

Why HRXRD?

In wafer metrology a number of technologies are available and the choice of one over another typically depends on the parameters to be measured and relevant performance requirements (measurement area, repeatability, accuracy, throughput, etc.). The main metrology methods for the technology nodes exceeding 90 nm include ellipsometry, optical reflectometry, opto-acoustic methods and secondary ion mass spectroscopy (SIMS). While the various methods are non-destructive, with the exception of SIMS, all have certain limitations and are of limited suitability for analysing thin epitaxial films, especially at and beyond the 90 nm technology node.
Then there are the non-diffraction based X-ray methods, namely: X-ray reflectivity (XRR), X-ray fluorescence (XRF), X-ray photo-emission spectroscopy (XPS) and low energy electron-induced X-ray emission spectrometry (LEXES). With XRR, interference patterns result from the interaction of grazing incidence X-rays reflecting from films at very small incident angles. Thickness, density, surface, and interface roughness for either crystalline or amorphous layers can be derived from such patterns. XRF utilises a polychromatic X-ray beam that interacts directly with atoms to excite electrons, resulting in secondary X-ray emissions. The process is an elemental analysis technique used primarily for thickness control of thicker metal layers and for contamination control. XPS is a surface technique, detecting the photo-electric electrons emitted by the film on stimulation using X-rays. The spectrum is element specific, dependent on the binding energies of the electrons within the atom. XPS is extremely surface sensitive due to the short range of the photo-electrons in the solid. LEXES is the inverse of XPS: electrons are fired at the surface and stimulated X-rays are detected. Again the spectrum is element specific. However, the bombarding of electrons at a film may cause some modification to the film. All have a role to play in metrology and wafer inspection, though XPS and LEXES suffer from the fact that the measurements must be performed under high vacuum.

XRD is based on the diffraction (coherent scattering from the periodic arrangement of atoms in a crystal) of X-rays when directed onto crystalline material, such as a semiconductor substrate or a crystalline film deposited thereon. To obtain X-ray diffraction, the crystal planes are oriented with respect to the incident X-ray beam, such that the Bragg condition for diffraction is satisfied, namely:
2dsinB = λ,
where d is the spacing of the diffracting planes of atoms, B is the Bragg angle and λ is the X-ray wavelength.

XRD is used for the control of phase, texture and other properties of polycrystalline materials. For epitaxial layers, where the crystalline quality is inherently far higher than polycrystalline films, a high resolution version of XRD is needed. High resolution X-ray diffraction (HRXRD) increases the resolution of the incident beam by using X-ray conditioning optics to provide a highly parallel and monochromatic beam more suited to the characterisation of epitaxial layers (see Figure 2). For the metrology of epitaxial films HRXRD provides unique capabilities for 90 nm requirements for volume manufacturing and has been used for more than two decades by the compound semiconductor industry where hetroepitaxial layers are commonplace. Perhaps most significantly, the technology can provide an analysis of the atomic structure within the layer, an essential capability in the example below.

Controlling Strain
Prior to the 90 nm technology node, speed improvements within the transistors were obtained by scaling of the transistor itself. To obtain an extra improvement in the device performance, the strain is introduced within the device. Inducing strain in the channel of a CMOS transistor enhances the semiconductor properties to achieve increases of as much as 50% in hole mobility and linear drive current, and thus transistor switching speed. However, this requires careful attention to the atomic nature of the Si channel as changes in the atomic nature directly change the strain in the layer and hence the performance of the device.

An example of inducing and controlling strain in an epilayer is well represented by the selective deposition of SiGe on a wafer to form the source and drain of the transistor. As a SiGe epitaxial layer is deposited, its structure in the plane of the layer replicates that of the Si substrate, forcing the out-of-plane lattice spacing to be greater than the Si around it. The atoms in the silicon alongside the SiGe areas move to replicate the perpendicular lattice spacing of the epilayer, thus uniaxially compressing the atoms in the gate channel between the source and the drain. Figure 3 provides a transmission electron microscopy (TEM) image of compressive stress in a silicon substrate for a pMOS (positive channel MOS) device. The compressive stress ― essentially a force being applied to the silicon by displacement of the atoms from their equilibrium lattice positions ― thus induces a measurable amount of strain in the material, and as a result, increased hole mobility. As the SiGe thickness and Ge content increases, or subsequent processing is performed on the wafer, the internal strain within the atomic structure may become too high and the tendency is for the SiGe layer to start to relax, i.e. Si and Ge atoms start re-arranging themselves to form the natural SiGe structure, rather than that imposed on them by the Si substrate. If this relaxation process occurs, this causes a reduction in strain of the Si channel and hence a reduction in mobility. Typically the relaxation of the SiGe layer must be controlled to within 1%. HRXRD is the ideal technique for such a required sensitivity, along with the non-destructiveness and repeatability required for in-line monitoring.
In general, the processes to achieve the desired strain in semiconductors for enhanced performance are complex, with manufacturers taking different approaches in transistor design. At 65 nm and 45 nm nodes, significant mobility enhancement in nMOS transistors has been shown using a similar raised source and drain approach, but with SiC replacing the SiGe in order to introduce tensile strain into the channel. Further improvements to raised source and drain transistors have been suggested, for example by incorporating an additional SiN overlayer. However, whatever the method of inducing strain, the control of the strain in the Si channel is critical for device performance and can be monitored using HRXRD.

Measuring Thin Epitaxial Layers with HRXRD

In measuring epitaxial layers with the system shown in Figure 1, X-ray diffraction peaks result from the interference caused by scattering of the monochromatic beam at specific angles by each set of lattice planes. The peak intensities are attributable to the atomic structure within the lattice planes. Thus, the X-ray diffraction recorded provides a direct indication of the atomic arrangement in the material being examined, such as in the above cases, Si and SiGe.

Figure 4 shows a symmetric HRXRD scan from a single SiGe layer grown on an (001) oriented silicon substrate and capped with a silicon layer. The scan represents the intensity measured by the detector as the sample and the detector are moved in a 1:2 ratio. The most intense peak is signal from the Si substrate. The next most intense peak, at a lower angle, is from the SiGe epilayer. The angular separation of the SiGe layer peak from the silicon peak provides a direct measure of the perpendicular strain in the layer. The absolute angular position of this layer peak is a direct and absolute measurement of the epilayer's perpendicular lattice parameter.

The system shown in Figure 1 incorporates a Bede® Microsource® X-ray generator and an optic and Ge conditioning crystal, which together deliver a high intensity beam for wafer sizes from 50 mm to 300 mm. This enables HRXRD to be performed directly on sub-100 µm measurement pads without the surrounding areas influencing the measurement. Figure 5 shows an HRXRD scan and a relaxation measurement on a SiGe test pad. The SiGe epilayer peak and thickness fringes can be clearly resolved, thus enabling an accurate determination of thickness and composition. A clear, sharp relaxation peak can be observed at 0 percent in the relaxation scan, thus indicating a fully strained layer.

Conclusions
While optical and other methods of inspecting and measuring wafer layers remain useful systems in semiconductor manufacturing, technology nodes of 90 nm and beyond will require HRXRD for advanced metrology (deposition chamber qualification and process monitoring). This is especially true for applications in which the atoms in the already exceedingly thin layers are being strained to achieve enhanced transistor performance. HRXRD enables the non-destructive determination of not only such critical measurements as composition, relaxation, and thickness, but also the crystalline quality of the layer.

With the system shown in Figure 1, precision is enhanced by a design in which the wafer positioning and scan axes are servo driven using high-resolution optical encoders to ensure positional tolerances necessary for subatomic tool matching and measurements on wafers without the need for calibration wafers. As such, the system enables absolute measurement of key parameters for sub 90 nm applications, while meeting fab line volume manufacturing objectives.

Figure 1. BedeMetrix™-F X-ray Metrology System

Figure 2. Typical set-up for a HRXRD system

Figure 3. Compressive Strain in Silicon Substrate for pMOS Device Due to Deposition of SiGe Epitaxial Layer (Ref: K. Mistry et al, Symp. VLSI Tech. Dig., 2004, pp50-51)

Figure 4. HRXRD Scan (blue) of a Box Structure of SiGe Showing Interference Fringes, and the Best-fit Simulation (red) obtained by Automatic Data-fitting (Sample Courtesy of Dr Annelena Thilderquist, Applied Materials)

Figure 5. Measurement of HRXRD (004 reflection) and Bede "relaxation scan" from a measurement test pad.

References
Sir William W. H. Bragg and his son, Sir William W. L. Bragg, developed the equation discussed above to explain why the cleavage faces of crystals reflect X-ray beams at certain determinable angles of incidence.