Impedans's RF Expert - Paul Scullin

Position at Impedans: Head of Product Development

Could you give us a short overview on how you got involved with the plasma industry?
I started working with a company called Scientific Systems in 1998, as an electrical engineer. At that time the company’s main product was a Langmuir probe. One of my first tasks was to improve the functionality of and fix bugs in the Langmuir probe software, which required reading and understanding the algorithms used to analyse the Langmuir probe data. I was hooked by the mix of engineering and physics.

How has the plasma processing industry changed since you first joined Impedans?
I joined Impedans in 2007. I had developed radio-frequency probes at my previous company and then led the Octiv voltage/current probe product development at Impedans. I noticed the adoption of frequency tuned matching was becoming much wider. This challenged the way we had previously approached signal locking for RF VI sensors. Previous approaches used a synchronous sub-sampling approach based on a PLL lock, we designed the Octiv to require no hardware signal locking, the “lock” is determined retrospectively from the sampled data.
Another change that is now common across the industry is the use of pulsed RF power in plasma applications. I remember our first experience working with pulsed plasma was with a university research lab in 2008, to provide an RF VI Probe to record the profile of the RF pulse providing harmonic and impedance data during the pulse to facilitate their study.

Impedans used the tagline “Plasma Measurement”, can you give us a brief overview of the types of technology you have developed for measuring plasmas.
The Langmuir Probe System is used to measure the bulk plasma properties. It uses a probe that is placed into the plasma chamber. The probe is electrically biased to attract or repel charged particles in the plasma, and this is used to build an “IV characteristic” (i.e. the amount of charge collected at each bias voltage). The characteristic is analysed to report the plasma’s ion flux density, electron flux density and electron temperature. The probe tip can also be moved inside the chamber allowing each property to be plotted against position, giving insight into the uniformity or confinement of the bulk plasma.

The Semion RFEA System is used to measure the properties of the plasma at the substrate surface. The sensor is usually placed on a biased electrode in place of the substrate. The sensor is typically used to measure the energy and concentration of the ions hitting the surface. It does this using a series of grids, the first repel electrons so that all the collected charge is due to ions. The next grid sets an ion energy threshold repelling ions below the threshold, this grid is adjusted to build a Ion energy distribution, which shows the concentration of ions versus ion energy.

The Quantum RFEA System is used to measure the deposition rate at the substrate position. A quartz crystal microbalance is used to detect change of mass on the sensor collector. Like the Semion product, the Quantum sensor can repel the charged particles so a ratio of ion deposition to neutral deposition can be measured.

The Plato Probe System measures bulk plasma properties like the Langmuir, but uses an AC measurement technique to measure the “IV-characteristic” and can operate through an insulation layer on the probe tip allowing the sensor to operate in plasma that deposits a lot of insulating material.

Octiv is an RF voltage and current sensing platform, which is placed in series with the plasma chamber’s RF power delivery. The Octiv uses non-contact RF sensors to detect the voltage and current signals. The time-varying analog signals are sampled simultaneously with high speed ADC’s and transformed to voltage and current amplitudes and phases at the fundamental and harmonic frequencies. If there were no transmission line effects between the RF match and the electrode we would have a nice predictable sine wave voltage on the electrode and it would be easy to see the plasma properties such as electron temperature would drive current harmonics, however due to transmission line interactions, the electrode voltage gets distorted which in turn complicates how the plasma drives the current. Having said that, and regardless of the complex interaction, the voltage and current harmonics are still very sensitive to changes in plasma properties.

The Alfven sensor is an RF voltage-current sensor like the Octiv but is aimed at monitoring the voltage and current to detect short duration changes. The purpose is to capture and report these events. The events can be categorized into size and duration. In some cases a small number of events is normal and an increase can indicate a need to perform preventative maintenance on the tool. Due to the high speed of the Alfven it is also used in pulsed plasmas and reports pulse frequency, average voltage and current during on-time of the pulse as well as some other pulse statistics.

What is Impedans’ role in the semiconductor and the broader plasma processing industry?
I believe Impedans’ role is to provide solutions to give customers insight into the important plasma parameters and a means to measure them directly. Plasma processing tools are built to provide the right environmental conditions to create the desired plasma properties. Langmuir, Semion, Vertex and Quantum instruments are excellent at measuring the plasma properties directly. The Langmuir instrument is more for the bulk plasma properties. The Semion, Vertex and Quantum focus on the ion properties at the target surface. These instruments are not convenient for the production environment as most plasma production tools can not accommodate these instruments during production. For the production environment Impedans provide sensors that give indirect insight into the state of the plasma. The Octiv RFVI Sensor and the Moduli sensor cannot directly measure the plasma parameters but are both very sensitive to small changes in plasma properties. Robust statistical solutions are built around the sensitivity of these sensors to give customers an indication of how well the current running process matches the desired known good run that has already been validated.
We are currently developing sensors that measure the surface plasma properties much like the Semion built on a wireless instrumented wafer. These sensors will be able to measure important plasma surface properties directly, to validate the tool and also used as a sanity check against the indirect statistical indicators. They will be used as often as needed, but I believe for real-time production measurements the indirect statistical indicators are required.

Given the transition to industry 4.0, IoT and the smart manufacturing revolution, how does Impedans’ technology align with this roadmap?
I think the semiconductor industry has had a high digital connectivity around the fab for some time, well before the emphasis on industry 4.0 became popular. The big difference now is that smart sensors are required to provide information on the health and performance of the plasma tools. This is enabling smarter maintenance scheduling processes (predictive maintenance) and smart analytics are helping to improve the overall throughput of the fab. Impedans are uniquely positioned to bring the measurements from plasma sensors into the big data set. There are industries outside the semiconductor industry that use our products for monitoring their plasmas processes also. Our sensors provide standard industrial interfaces or a web interface based on the REST model interface, which is a very popular internet API model that in some cases can be easier to connect to than industrial interfaces, particularly when there are no existing industrial communication networks on site.

Companies like TSMC are starting to manufacture at the 3nm node. What are your thoughts?
I’m always amazed at how the lithography tool makers can produce solutions that work at these smaller and smaller nodes!

How have advances in technology helped Impedans make higher performance and more advanced plasma measurement products?
We are finding that technology advances targeted for low power wearable devices are a good fit for the new wireless sensors we have in development, where we’re trying to reduce power consumption and increase battery lifetime.

Covid-19 has affected most businesses in the world, but the semiconductor industry has seen a 4.9% increase in sales this year. What are your thoughts on this?
I think we have been lucky that the semiconductor industry was not hit negatively by the pandemic. In fact, reports are that the current problem at the leading semiconductor fabs is a lack of capacity to facilitate more orders.
Most businesses have had to change the way they work, at Impedans a large percentage of our employees are working remotely and all businesses that can do this are doing it. I believe the internet infrastructure has allowed many to work remotely almost seamlessly.