RF Power Sensing for Productivity and Calibration

Fabricating the nano-circuits employed in next-generation semiconductors demands processes that can repeatedly deliver finer geometries, increasingly complex high-aspect ratio (HAR) plasma-etch patterns, more layers and 3D structures. These processes are now so complex that multiple radio frequency (RF) power sources and matching networks are required to generate and maintain the plasma used for film deposition and the etching of the patterns on the wafers.

Addressing the technical fabrication challenges while maximizing throughput, yield and productivity – and keeping process power as low as possible – means that precise sensing and control of the RF power used in plasma processes has never been more important. This, in turn, demands sophisticated RF conversion technology and RF power sensing equipment that consistently deliver highly accurate measurements, often in real-time.

In addition, manufacturers and customers need the confidence that comes from systems that have been calibrated in line with internationally recognized standards.

As a result of these requirements, semiconductor manufacturers are now seeking advanced sensing solutions that are able to handle the significant demands of the latest RF power processes, and are accredited in accordance with key standards such as ISO/IEC 17025.

Originally developed to allow laboratories to demonstrate technical competence in testing and calibration services, ISO/IEC 17025 is a quality management system that can be applied to organizations and equipment that produce testing and calibration results. Accreditation in accordance with the standard not only gives semiconductor manufacturers and their customers the peace of mind that process measurements will deliver accurate measurements; it also addresses the growing demand for increased traceability.

RF Power Sensing

In many etch and deposition plasma power implementations built around RF power sources and matching networks, the RF generator has often been used as the basis for monitoring and recording the power supplied to the plasma chamber. However, this approach can present a number of issues – issues that become increasingly challenging as the need for more precise and higher power control grows. These include power fluctuations, the impact of temperature and recipe mix, problems caused by distance of the power source from the tool itself, as well as the ability to fine tune power at the point of delivery.

For instance, if the RF source is some distance away from the point of power usage, then there is the potential for significant insertion loss. This loss will impact the actual power that is provided. In theory, it is possible to calculate the loss and then adjust the power source settings accordingly. However, depending on the particular system set-up, insertion loss does not necessarily behave in a linear fashion. As powers increase it can be much harder to estimate the loss.

At the same time, the process can suffer as a result of quantization errors whereby the steps available to the user in the power command system do not necessarily correspond to the exact power delivery needed at the chamber.

These factors are among the reasons why semiconductor manufacturers are seeking calibrated equipment that can very precisely monitor plasma power. Unfortunately – whether it is available frequency bands or power ranges, accuracy levels or certification – the conventional power sensing technologies that have been around for decades are increasingly no longer capable of providing the functionality needed in the latest processes. As a result, we are seeing the emergence of new generations of much more accurate and highly integrated RF power measurement and calibration systems.

New Technologies for Precision RF Power Applications

Figure 1 shows a typical set up for an RF power system incorporating an RF generator, control system and matching network, as well as the latest generation of RF power sensing technology. Advanced Energy’s TEGAM^® GEMINI™ 5540A has been inserted into the system, enabling a continuous use, closed loop configuration providing direct control information back to the generator. 

Designed to improve the fidelity and consistency of leading-edge plasma-based processes, the unit’s calibration is certified in accordance with ISO 17025 and allows OEMs to accurately calibrate the system and monitor RF power in real time. Combining sensor and meter capability in a single instrument enables users to replace multiple tools, while wide frequency and power ranges from 0.2 to 200 MHz and 3 to 5,000 W respectively further support the use of fewer instruments.

"Power measurement from power sensors with ISO 17025 accredited calibrations supports precise power delivery"

Furthermore, thanks to excellent accuracy and an ISO 17025 compliant calibration, the same power meter can also be used for offline measurement and calibration during periodic system maintenance.  

The unit shown here measures with 2-sigma accuracy of ±0.5% + 0.5 W, which is up to four times more accurate than legacy technologies. A large, easy-to-read 6-digit OLED digital display simultaneously shows forward and reverse power, voltage standing wave ratio (VSWR) and frequency. Insertion loss is rated at less than 0.05 dB and, because there are multiple internal measurement channels, there is no need for system reconfiguration for forward and reverse power measurements. Meter calibration is required just once a year (compared to once every six months for conventional instruments) and accuracy can be calibrated for multiple, customer-specified frequencies. The unit is powered through a USB type B port, operating with a 5 VDC, 500 mA input, which can be supplied via a standard USB 2.0 port from a PC, an external battery pack, or AC power adaptor and off-the-shelf USB cable. 

In addition to taking measurements directly, the meter can also be connected to a PC via USB or Ethernet (optional configuration) for collection, storage and audit of data using real-time data visualization software. This provides a resource that significantly enhances process traceability, supports application trouble-shooting and helps users to identify ways to improve productivity and process efficiency.

The software - which automatically detects connected meters and can collect data from up to five meters simultaneously – can be used to graph forward and reverse power readings in real-time, zoom to points of interest, and save data to a user-defined location. In addition, an integrated simulation mode allows users to test an application when no meter is available and to see how the addition of one or more meters to an existing process could be used to improve measurement and control.

Summary

Driven by advanced semiconductor design and fabrication processes the demand for RF plasma power is growing rapidly. Analysts with Transparency Market Research¹, for example, estimate that the market will increase from $991 million in 2021 to $5.1 billion by the end of 2031 - a compound annual growth rate (CAGR) of 16.4% - and that the market will transform with the increased availability of systems that offer unique features for configuration and control.

In line with the growth, the transition from conventional measurement tools to next-generation, highly integrated RF power sensing and metering technology will play an important role in supporting these configuration and control requirements - both in terms of providing accurate and precise measurement of frequency and power and with respect to ensuring accurate system calibration in accordance with standards such as ISO 17025.