1. Field of the Invention
The invention relates to the field of semiconductor metrology, and in particular, to thin film composition determination including phase identification in multilayer structures.
2. Related Art
As modern semiconductor device geometries continue to shrink, performing metrology (i.e., determining material composition properties such as elemental distribution or molecular structure (phase)) on those devices becomes increasingly important and difficult. Currently, some of the most problematic metrology operations involve the analysis of multilayer stacks (e.g., a thin film on a substrate or multiple thin films formed on top of each other) that share one or more common elements. Conventional metrology techniques can have difficulty determining the material composition properties of such “shared element” structures since conventional techniques cannot readily distinguish between signals from the common element that originate from the different layers in the stack.
For example, to improve CMOS performance, a silicide contact layer can be created to improve electrical conductivity between transistors and metal lines (interconnects). The silicide layer is formed by depositing a metal (e.g., titanium (Ti), cobalt (Co), or nickel (Ni)) onto the polycrystalline silicon (polysilicon) gate and/or source/drain regions of a transistor, and applying an elevated temperature to form a surface layer of refractory metal silicide. The contact layer lowers the resistance of the polysilicon-interconnect interface, thereby enabling faster device performance.
Determining the phase of a silicide contact layer is of significance because phase can affect resistivity in a silicide. For example, cobalt can form at least two stable silicide phases: CoSi and CoSi2. For maximum performance benefit in a silicide contact layer, it is desirable to form the CoSi2 phase, which has a lower resistivity than the CoSi phase. Nickel exhibits three common phases: Ni2Si, NiSi, and NiSi2. Of the three, the NiSi phase has the lowest resistivity, and is therefore typically the desirable phase. Thus, the ability of metrology tools to determine the actual phase of a silicide layer is critical for the proper tuning of high performance semiconductor processes.
Traditionally, phase determinations have been made using x-ray fluorescence (XRF), x-ray diffraction (XRD), or transmission electron microscopy (TEM). XRF involves the application of a probe x-ray beam to a test sample to cause emission of characteristic x-rays from the test sample. The characteristic x-rays from the different atoms in the test sample can then be used to determine the concentrations of the different elements in the test sample. Unfortunately, XRF is a relatively slow technique (processing only a few wafers per hour at best), and is therefore not ideal for use in production line environments. Furthermore, the probe x-ray beam used in an XRF system generally produces a measurement spot size (i.e., the size of the probe beam incident upon the test sample) that is too large (>50 um) to measure high performance devices.
Finally, XRF can have problems with shared element structures. For example, a silicide contact layer over a polysilicon gate forms a multilayer structure in which both layers include silicon. Therefore, because an XRF probe x-ray beam cannot be “tuned” to only penetrate the top layer (particularly for very thin layers such as contact layers), characteristic silicon x-rays will be generated from both the silicide layer and the polysilicon layer, thereby making determination of the phase of the silicide layer impossible. Other x-ray techniques, such as x-ray diffraction (XRD), face similar problems (slow, large spot size, difficulty targeting just the layer of interest).
TEM involves sending a high-energy electron beam through a prepared test sample to determine atomic-level information about the test sample. TEM can generate accurate phase information, but is a destructive technique, due to the need to slice and thin the test sample so that sufficient electron transmission is provided. Therefore, TEM is not suitable for in-line metrology (i.e., measurements of production wafers). Other less common techniques for phase determination are generally unsuitable for in-line monitoring for similar reasons. For example, secondary ion mass spectrometry (SIMS) uses an ion beam to sputter the surface of a test sample, and then uses a mass spectrometer to analyze the sputtered particles. In this manner, an accurate determination of surface composition can be made. However, like TEM, SIMS is a destructive technique, and therefore cannot be used on production wafers.
Thus, it is desirable to provide a system and technique for performing thin film metrology on a shared element multilayer structure in a production line environment.