Titanium nitride (TiN) is a material with low resistivity which is suitable for use with gate dielectrics, and is therefore valued as a metal gate in the design of complementary metal-oxide-semiconductor (CMOS) devices.1 It is also useful for producing coatings against wear, or as a barrier layer for copper diffusion, due to its resilience to chemical and thermal exposure.2 Conventionally, TiN deposition was performed using physical vapor deposition methods. The issue with these techniques is that step coverage is low in deep contacts and via trenches due to the shadowing effects that are observable in high aspect ratio structures.
A thin film deposition technique known as atomic layer deposition, or ALD, ensures the film thickness can be controlled at Å-level, as well as conferring superior evenness of application and conformal coating for features which have a high aspect ratio, is explored in this article. Thin film deposition with TiN using ALD has been found to work using different precursor molecules like titanium tetrachloride (TiCl4), or tetrakis (dimethylamino) titanium (Ti(N(CH3)2)4, or TDMAT. 2,4,5
However, it is often necessary to optimize film growth as well as its electrical characteristics and uniformity. Ideally, it is possible to produce a high-quality final film by making sure that the thickness and electrical uniformity over the whole surface (where film deposition has taken place) is analyzed using non-destructive techniques. This article discusses one such non-destructive electrical method of characterizing the ALD-mediated plasma atomic layer of TiN on wafers of 200 mm diameter via THz spectroscopy.
Equipment and Process
In this experiment, plasma-enhanced ALD (PEALD) was used to lay down layers of TiN, alternating the doses of TDMAT and a mixed plasma containing N2H2, the instrument used is the Oxford Instruments FlexAL remote ICP plasma ALD platform shown in Figure 1. The TiN films were laid down on Si(100)wafers 200 mm across at a temperature of 350 oC. The precursor TDMAT molecules entered the reaction chamber at 70 oC.
Figure 1. Oxford Instruments FlexAL® remote ICP plasma ALD system.
Characterizing Uniformity of Thickness
The thickness and refractive index uniformity of the TiN film was assessed with an ex-situ spectroscopic ellipsometer (Woollam M-2000) that could produce spectra that are visible, up to the near infra-red region of 245 nm to 1700 nm. This confirmed good uniformity of the TiN film to within +/- 3% over the whole Si wafer measuring 200 mm across, as shown in Figure 2.
Figure 2. Thickness uniformity of ±3% measured for a 22 nm TiN film deposited by PEALD.
Mapping Electrical Resistivity Using No-Contact Methods
The 4 point-probe technique (4PP) has become a widely used method in the electronics industry. It evaluates the resistivity of all types of materials coated on wafers, including thin films. This technique uses contact to take local measurements. However, contact can damage the wafer surface samples. Local measurement values correlate with the path of least resistance across the electrodes inside the area of the four probes. Thus, using 4PP as a scaled technique to map the whole of the surface of large-scale wafers, particularly in an environment of production, is not recommended.
In this article the technology used to measure resistivity is the revolutionary Terahertz wave technology, which overcomes all of the hurdles posed by the 4PP techniques. As shown in Figure 3, the das Nano Both sturdy and adapted for industrial use, the Onyx platform is a Terahertz time-domain spectroscopy system that has been developed in collaboration with the Terahertz group at the Fraunhofer Institute for Industrial Mathematics (ITWM) to provide a dependable and precise system that is simple to use, while requiring very little operator training and no sample preparation.
Figure 3. Onyx materials quality inspector system comprising the Terahertz generation and control unit, the 3D positioning system, and the Terahertz measuring head mounted on top of the 3D positioning system.
The Onxy system has been tested on various types of substrate materials, such as soda-lime glass, quartz, silicon, SiO2, SiC, sapphire, paper, cellulose, PET, AZO, NbC, Ge and Sb, as well as a range of materials including graphene in various forms, monolayer, bilayer, flakes, oxide and stacked forms, ZnO, PEDOT, ITO, CNT, GaN, photoresins, and silver inks. It can also help to characterize novel substrates while storing data in its database which can be customized to user specifications.
Figure 4. Resistivity map across a 200 mm substrate for a 22 mm TiN film deposited by PEALD. A uniformity of 4% is measured. Spatial resolution of the map is 1 mm but resolutions down to 50 um are possible with Onyx.
In Figure 4, the resistivity map obtained with the Onyx system for a 22 nm TiN layer, which was deposited by PEALD, Oxford Instruments, is observable. The average resistivity in this case is 310 µΩ cm while the variation over the whole wafer is 4%. The TiN film thickness uniformity of +/- 3% corresponds well to the 4% uniformity value for resistivity and ellipsometry measurements taken with the Onyx platform. The higher values of resistivity come from the 4PP single point measurements and are thought to be due to the effect of grain boundaries.6 Overall, the use of the Onyx platform leads to rapid inspection of how uniform the resistivity is over the sample.
To summarize, this article shows the deposition of conductive TiN layers using PEALD over a wafer of 200 mm with superior uniformity of thickness and resistivity values. This was assessed using non-contact characterization methods, namely, ellipsometry and Onyx THz spectroscopy. This kind of non-destructive analytic techniques allows ALD to be carried out rapidly and automatically, which is essential for process development and for applications that involve high throughputs.
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- Tadjer et al., ECS Journal of Solid State Science and Technology 6, 165, (2017)
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- Boggild et al., 2D Mater 436, 4, 042003 (2017)
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
For more information on this source, please visit Oxford Instruments Plasma Technology.