Deposition of Tantalum Nitride and Copper films using Atomic Layer DepostionTianna Hankins, Scott Merry, Fred Newman PhD, Liam Bradshaw PhD, Michael Khbeis PhD

University of Washington, Washington Nanofabrication Facility, Seattle, WA

Introduction

Atomic layer deposition (ALD) is a fabrication tool used to create thin films and is widely used in the fabrication of semiconductor devices. ALD methodology produces very thin and conformal films and has great control over the film thickness and composition due deposition at the atomic level. Copper (Cu) is widely used in semiconductor devices due to its conducting properties and low resistivity. Tantalum Nitride (TaN) is used as a barrier layer to prevent copper from diffusing into surrounding materials and thus degrading their properties.

ALD is a Chemical Vapor Deposition Technique that uses gas phase reactions and surface reactions to grow thin films. The precursor molecules are separately and sequentially pulsed from a reactor to the sample surface and react in a self-limiting way so that the reaction terminates once all the reactive sites on the surface are consumed (Figure 1). The precursor molecules are mass transported through the inlet of the machine to the deposition zone on the sample surface. Adsorption of the precursor in the deposition zone leads to nucleation and migration to the growth sites. A heterogenous film growth is desired due to more stability of the film, lower defects and good adhesion of the precursors to the substrate. In this process, thermal heat is used as the energy to drive the reactions. ALD uses low pressure to increase the mean free path of the precursor molecules. Increasing the mean free path allows the molecules to have extra kinetic energy to allow good step coverage, uniform coating of the substrate and conformal coverage.

Copper has been used in the semiconductor industry as an interconnect due to its high electrical conductivity and low resistivity. The metallic elements have electropositive atoms that donate their valence electrons to form a “sea” of electrons surrounding the atom. Because the valence electrons are not in a fixed position, when a voltage is applied, the valence electrons move, causing a current to flow. The resistivity of a material quantifies how strongly it opposes the flow of electrical current. Due to these two properties, copper, and other metals, are the perfect material for the conduction of current in semiconductors.

Tantalum Nitride is an inorganic chemical compound that is sometimes used as a diffusion barrier in semiconductor devices. It is chemically inert, oxidation resistant and hard.

The ALD process of creating copper and tantalum nitride thin films is being used to create through silicon vias (TSV). Through silicon vias are vertical electrical connections that pass completely through a silicon wafer. They are used as high performance interconnect techniques to create 3D packages and 3D integrated circuits due to the high density of the vias and the shorter connections lengths.To create these TSV’s, there are a multitude of fabrication techniques involved, with ALD being one of them.

AcknowledgementsDr. Michael Khbeis, Washington Nanofabrication Facility, University of Washington

Fred Newman, Washington Nanofabrication Facility, University of Washington

Liam Bradshaw, Molecular Analysis Facility, University of Washington

Experimental

To create the TSV’s, the silicon wafers are first put through a series of photolithography fabrication techniques to create the vias. In order to create the interconnect devices, by coating the vias with thin films of silicon dioxide, tantalum nitride and copper, another series of fabrication techniques takes place. Plasma Enhanced Chemical Vapor Deposition (PECVD) is used to create thin film of silicon dioxide. Silicon dioxide is used for a multitude of reasons. It is an excellent electrical insulator, a diffusion barrier and capable of forming electrical interfaces with its substrate. Using the ALD, sequential films of TaN and Cu are then deposited. In order to ensure the repeatability of the thin films using the ALD fabrication technique, the films need to be characterized. Characterizing the thin films allows the technician to see the material, since the material is on the nanoscale and cannot be seen with the naked eye. In this experiment, a 4-Point Probe was used to check the resistivity of the material. (Figure 2). By checking the resistivity, the technician can tell two things: if there has been a uniform layer deposited and if the material is an insulator or a conductor. Another characterization technique being used was the Filmetrics (Figure 3). The Filmetrics measures thin film thickness by analysing how the film reflects light.

ResultsUsing the 4 Point Probe and the Filmetrics, we were able to tell that the ALD was indeed depositing uniform layers of TaN and Cu with repeatability. The 4 Point Probe not only allowed us to see that there was uniformity of the deposited layers between runs, but also showed us that the resistivity of the Cu had conducting properties (Figures 4 & 5) and the TaN had insulating properties (Figures 6 & 7). The Filmetrics showed us that the SiO2 was also being deposited in a uniform way (Figures 8 & 9). Knowing this, we were then comfortable in assuming that the ALD was depositing the desired material and able to start mass producing the TSV’s.

Figure 1: Schematic of Atomic Layer Deposition Process

ReferencesAskeland, D. R., & Wright, W. J. (2014). Chapter 2-Atomic Bonding. In Essentials of Materials Science and Engineering (pp. 30-31). Stamford, CT: Cengage Learning.

Cao, G. (2004). Chapter 5- Chemical Vapor Depoition. In Nanostructures & Materials: Synthesis, Properties & Applications (pp. 189-194). London: Imperial College Press.

Enigmatics: SiO2 Properties and Compounds. (n.d.). Retrieved May 17, 2016, from http://www.enigmatic-consulting.com/semiconductor_processing/CVD_Fundamentals/films/SiO2_properties

Tow, B. W., McNeill, D. W., & Gamble, H. S. (2005). Investigation of copper layers deposited by CVD using Cu(I)hfac(TMVS) precursor. Journal of Materials Science: Materials in Electronics, 16(7), 437-443. Retrieved May 18, 2016, from http://link.springer.com/article/10.1007/s10854-005-2311-7

Figure 2: A picture of the 4-Point Probe

Figure 3: A picture of the Filmetrics

Figure 4: 4 Point Probe contour map showing the resistivity of Cu using ALD

Figure 5: 4 Point Probe contour map showing the resistivity of Cu using ALD

Figure 7: 4 Point Probe contour map showing the resistivity of TaN using ALD

Figure 6: 4 Point Probe contour map showing the resistivity of TaN using ALD

Figure 8: Filmetrics contour map showing film thickness of SiO2 using PECVD

Figure 9: Filmetrics contour map showing film thickness of SiO2 using PECVD