The Ionized Physical Vapour Deposition

In the conventional magnetron discharge only a small fraction of sputtered atoms are ionised. The Ionized Physical Vapor Deposition (IPVD) presents the further development of the magnetron sputtering techniques that provide a high degree of ionization of the sputtered atoms. The initial development of IPVD was driven by the need to deposit metal layers and diffusion barriers into deep trenches of integrated circuit structures. Since then the approach has found numerous additional applications. Ionising the sputtered vapor opens the opportunity to control the bombardment energy by applying a bias voltage to the substrate. This opens several advantages for the quality of the deposited film: the directionality of the deposited flows, density and adhesion

  • directionality of the deposited flows
  • adhesion and density (especially important for substrates of complex shape)
  • density
  • control of the reactivity
  • lower deposition temperature
  • guiding of the deposition material to the desired areas of the substrate
The basic concept of IPVD is based on the same basic DC magnetron discharge, or some modification of it, though operated in a different parameter regime. For instance, larger plasma pressure is typically used, by the factor of ten, $p\sim 4~Pa$. Plasma volume is also larger. As a result the mean free path of the sputtered atoms is small comparing to the distance between the target and substrate. The multiple scattering results in the enhanced live-time of the spattered atoms in the volume. Together with a high background plasma density it results in the high ionisation degree of the sputtered species. Different approaches are utilized to increase the ionisation of the background gas and produce high density plasma everywhere in the volume. They include
  • utilisation of the supplementary plasma discharge, such as an electron cyclotron resonance, inductive, or hollow cathode discharge
  • shaping the target in a particular way
  • high power impulse operation of the magnetron discharge

Simple global models can often be used to describe the basic physical principles and understand the parameter scaling, assuming certain given discharge parameters. The discharge typically has the cylindrical geometry (radius $R$, length $l$). The metal atoms are sputtered at one end wall of the cylinder by the magnetron discharge due to an bombardment by background gas ions (typically Argon) with the energy $V_{DC} = 600~V$. $n_m$ and $n_{Ar}$ designate the densities of sputtered metal atoms and background argon atoms, respectively. Typically $n_m \ll n_{Ar}$.

One of the feature of the IPVD is that the substrate, being negatively biased, pulls significant ion current. This current causes a substantial re-sputtering of an already deposited metal ions. The density of the metal atoms in the background plasma is primarily determined by the atoms sputtered from the substrate. The contribution of the flux of sputtered atoms from the target is insignificant. In that sense the IPVD discharge constantly works in the recycling regime, continuously depositing and re-sputtering meal atoms to and from the substrate. The total ion current to the substrate is then \begin{equation} \label{I_substrate} I_{T} = e n_e u_B S, \end{equation} where $S$ designate the substrate area.

The current of metal atoms sputtered from the substrate is \begin{equation} \label{I_m} I_{m} = \gamma_{sput} I_{T}, \end{equation} The confinement time of sputtered metal atoms in fully collisional environment is determined by the diffusion rather than free-fall. \begin{equation} \label{taum} \tau_{m} = \dfrac{L^2}{D_m}, \end{equation} where $L$ designate the typical spatial scale of the plasma and $D_m$ is the diffusion coefficient. We can write the particle balance as follows \begin{equation} \label{part_balance} \gamma_{sput} n_e e u_B S = \dfrac{n_m}{\tau_{m}} V \end{equation} where $V$ designate the plasma volume. This results in the following expression for the metal atom density in the volume \begin{equation} \label{Nm} n_{m} = \dfrac{\gamma_{sput} e u_B S \tau_{m} }{V} n_e \end{equation} The metal ions are primarily produced by the direct ionisation of metal atoms in the plasma volume. The particle balance for metal ions is following \begin{equation} \label{Nm_ion} K^{i}_m n_e n_m V = n_{m+} u_B S, \end{equation} where $K^{i}_m$ designate the ionisation coefficient of metal atoms. Solving for $n_{m+}$ yields \begin{equation} \label{Nmplus} n_{m+} = K^{i}_m \gamma_{sput} e \tau_{m} n^2_e, \end{equation} One can see that $n_m$ scales as $n_e$ and $n_{m+}$ scales as $n^2_e$.

Leave a Reply