Transition Mode Reactive Sputtering by

Plasma Emission Monitor Control

Craig Outten, Ph.D.

Outline of Presentation

• Sputtering Review

• Plasma Emission Monitoring Overview

• Process & Film Example

Sputtering Review

substrate

Sputtered atom hits substrate & forms a film.

Ar+ ion

target

Sputtering Mechanics

In its most basic form, sputtering begins with the creation of an inert gas ion (Argon) in a vacuum chamber. The Ar+ ion is accelerated into a target made out of the source materials to be deposited. The impact of the Ar+ ion drives an atom of the source material off the target and onto the substrate. This is momentum transfer (billiard ball scattering)- not evaporation.

DC Magnetron Sputtering A conductive target is now mounted to a specially designed magnetron cathode. The target material is a typically a high purity metal disk. Permanent magnets in the cathode apply a magnetic field parallel to the target’s surface. Free electrons which otherwise would be attracted to the substrate are now trapped near the target’s surface and create a dense plasma. This trapping effect enables the cathode to be operated at low pressures, 2- 10 mTorr range.

DC Metallic Sputtering

Now… Metal atoms and argon ions are transported from the target to the substrate unimpeded. There are few Argon atoms to bump along their path. Dense metal films can be deposited at high deposition rates. The deposition rate is affected by the type of target material (it’s sputter yield), distance between the target and substrate (referred to as the source to substrate distance), the cathode power, and the Argon gas pressure.

metal target

RF Sputtering

DC sputtering does not work with dielectric materials. In this case, the target material is an insulator that will build up charge as Ar+ ions impact- this reduces the voltage potential between the anode and the cathode and the glow discharge can’t be maintained. Using AC power (typically at 13.56 MHz) enables electrons and ions in the plasma to follow the switching of the cathode between negative and negative states. Sputtering takes place when the cathode is biased in the negative state. Typically higher voltages required than DC sputtering combined with more challenges in system electrical design and fabrication. More expensive and complex than DC

sputtering; higher target costs; system cost.

Insulating target

cathode

substrate

Sputtered metal and reactive gas atoms hit substrate & form an oxide or nitride film. Ar+ ion

target

Reactive Sputtering Basics Reactive sputtering enables the deposition of dielectric films (oxides & nitrides) from a metal target. A small amount of reactive gas, typically either oxygen (O2) or nitrogen (N2), is mixed with Ar gas. The reactive gas is ionized and excited by the plasma discharge near the target surface Reactive gas atoms chemically bond with metal atoms to form a compound material. Ar+ ion sputters the compound material onto the substrate.

Reactive gas atom

Controlled chemical reactions takes place on the target surface.

DC Pulsed Sputtering

RF sputtering involves many design trade-offs including higher voltages and more complex power supplies. From a process standpoint, DC pulsed sputtering, a more recent development, sends an alternating voltage pulse through the cathode-anode assembly. DC pulsed sputtering typically lower cost to implement than RF sputtering- power supplies are less complex , and system design need not address possible RF leakage onto other components. Reactive sputtering can be performed with less costly metal targets. Metal or reactive sputtering can be accomplished from the same metal target.

Advantages of DC Pulsed Sputtering over RF Sputtering

DC Pulsed power supplies are much less expensive than RF generators and matching networks. Deposition of dielectric films by RF sputtering requires expensive and fragile ceramic targets. DC Pulsed reactive sputtering is performed with less metal targets. RF sputtering of dielectrics does not require a reactive process control. That is it’s advantage. However sputter rates are very low. DC pulsed reactive sputtering is order of magnitude higher. A control system is required for DC Pulsed reactive sputtering. The value proposition of Denton Vacuum Plasma Emission Monitoring (DV-PEM) Platform is that it dramatically reduces the cost of closed loop control to less than RF sputtering.

RF Sputtering DC Pulsed Sputtering

Power Supply Cost

Target Cost

Reactive control system

Dielectric Sputter Rate

Metal Sputter Rate

DV-PEM provides reactive gas control at an affordable price (less than RF sputtering).

Plasma Emission Monitoring Overview

Denton Vacuum Plasma Emission Monitoring (DV-PEM) Platform

S N S

UV-VIS Spectrometer

PEM PID Controller

Fast response MFC

Denton Vacuum Plasma Emission Monitoring (DV-PEM) is a closed loop control system for sputtering metal oxides and nitrides at high rate. An in-vacuum fiber optic sensor collects light from the plasma emission species above the target material. The plasma emission spectra is measured by a UV-VIS spectrometer and relayed to a PEM PID control software platform. The feedback at a specific wavelength is used to control the reactive gas flow through a fast response mass flow controller (MFC). The DV-PEM platform is fully automated and integrated into the equipment software.

Plasma Optical fiber

Reactive gas

Substrate

Substrate Holder

Deposition Chamber

Denton’s confocal design enables mounting of multiple cathodes and targets around a rotating single stage (frequently heated & with RF biasing capability) that holds the substrate. Can support multiple material research projects on a single system. Each cathode can be configured with a DV-PEM. The DV-PEM opens up the potential to deposit wide varieties of metal/dielectric and dielectric/dielectric multilayer coatings in a single deposition run. The confocal geometry provides extremely good uniformities (≤± 3%) along with well formed, dense films.

Actuator to open and close

shutter over the face of the

cathode

DV-PEM Hardware Configuration

Rotating wafer fixture with substrate heating and RF biasing

PEM sensor head and fiber optic

Plasma Emission Spectra

A typical PEM feedback signal collected by the in-vacuum optical fiber sensor and broken into a spectral curve by the UV-VIS spectrometer. This specific emission spectra is for a Titanium Dioxide (TiO2) process. The UV-VIS spectrometer detector signal is displayed as a plot of the relative intensity of elemental plasma emission species (spectral counts) with respect to wavelength (nm).

Tabulated Atomic Spectra

Denton Vacuum provides users with a look up table of the most prominent spectral wavelengths for commonly used metal targets, reactive gases, and Argon. The selected wavelength is then monitored by the PEM controller and used to control the reactive sputtering process. System offers the flexibility to use a metal plasma emission lines or gas plasma emission line, ex, O2, if it provides better feedback control for a specific material.

Hysteresis Curve Typically, a pre-sputter step is performed in pure Argon plasma with the shutter closed to condition the target and establish the Metallic Region. In this example, the light intensity of a metal line (wavelength) is monitored to generate a Hysteresis Curve. The intensity of the metal line is proportional to the sputter rate. Next, reactive gas is introduced to the chamber in small, precise increments, usually 1 sccm, through the fast response MFC. The target voltage is monitored and increases as a compound layer forms on the target surface. The reactive gas is ramped up until the target is fully reacted (i.e. oxidized or nitrided) which is referred to as the Poisoned Region. The reactive gas is then ramped down until the target surface becomes a pure metal again.

Metallic

Poisoned Region

Metallic Region

Region

Deposition Rate vs. PEM Set Point

The Hysteresis Curve is used to select the PEM set point. The ideal operating range occurs in the Transition Region. A % PEM Set Point is selected in the Operating Range. This is known as Transition Mode Sputtering. Films deposited in the Transition Region exhibit excellent dielectric properties. Sputter Rates in the Transition Region are much higher than in the Poisoned Region, 2-8X, depending on the dielectric material being deposited. In addition to depositing fully reacted materials, it is possible to controllably deposit sub-stoichiometric films which is not possible with RF sputtering.

Metallic Region

Transition Region

Poisoned Region

Process & Film Example

TiO2 Process Demonstration To demonstrate the capability of the DV-PEM, TiO2 was selected as a candidate material. TiO2 challenging to reactively sputter since it is an good electrical insulator. The anode of the cathode gets coated and becomes insulating. Electrons cannot find a ground path and charge build up leads to arcing. Arcing can result in poor film quality due to generation of macro-particles and instability of the power supply. This phenomena is known as the “disappearing anode effect”. Three sample sets were deposited with DC Pulsed power supply at different % PEM set points. The physical thickness of the films was measured on a Dektak 150 profilometer as a function of radial position. The refractive index was measured with a Perkin-Elmer Lambda 750 spectrophotometer.

Parameter Numerical Value

Base pressure 8.0 x 10-7

Torr

Power density 13 W/cm2

Pulse frequency range 80-100 kHz

Pulse time 1-2µs

Deposition pressure 5 mTorr

Deposition Conditions:

TiO2 Thickness Uniformity

Film thickness uniformities were well within ≤± 3% target. Film thickness vary because sputter rate is higher for higher % PEM set points.

25% % PEM

30% % PEM

20% % PEM

TiO2 Refractive Index Uniformity

The film refractive index should not change over this PEM Set Point Range. The lack of variability indicates a wide, stable process window.

25% % PEM

30% % PEM

20% % PEM

Conclusion

Concluding Remarks

• The Denton Vacuum Plasma Emission Monitoring closed loop control system delivers high-rate transition mode sputtering with a excellent process stability and film uniformity.

• High-quality dielectric optical thin films were deposited at much higher deposition rates possible with RF sputtering and with less expensive metal targets and a DC Pulsed power supply.

• The Denton Vacuum Plasma Emission Monitoring platform enables systems to be configured with multiple cathodes for deposition of a variety of different materials- metals, dielectrics, and alloys.

Thank you

• Thank you for your kind attention during my presentation.

• Questions?