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Wafer tester monitors RF chip quality in realtime

Posted: 18 Mar 2005 ?? ?Print Version ?Bookmark and Share

Keywords:rf? rfic? wlan? rfid? Keithley?

Chips for RF applications are everywhere. RF chips are used in cellphones, WLANs and RFID (identification) systems, to name just a few. Semiconductor producers know that wafer-level RF measurements are needed to develop and produce advanced ICs for these applications. To a certain degree, this flies in the face of the 2003 recommendations by the ITRS Technical Working Group for Modeling and Simulation. It states: "The parameter extraction for RF compact models preferably tries to minimize RF measurements. Parameters should be extracted from standard I-V and C-V measurements with supporting simulations, if needed."

The problem is that standard I-V and C-V measurements make the direct extraction of Cox impossible for ultra-thin dielectrics, due to high leakage currents and non-linearities.

Those ITRS recommendations for minimizing the use of RF techniques are predicated on the assumption that they can't be made effectively, particularly in a production environment. Yet, accurate parameter extraction for circuit modeling at frequencies as high as 40GHz, and for RF compact model verification, has become essential.

This challenge is increasing for high-performance low-cost digital, RF, and analog/mixed-signal devices as the industry progresses towards 65nm feature sizes and smaller.

A response
In response, test-and-measurement supplier Keithley Instruments is now introducing a third-generation on-wafer RF measurement capability with some very promising features that address the conundrum.

Designed for parametric production process control, Keithley's new RF Option offers continuous, automatic realtime monitoring of measurement quality. The company claims its RF Option will now give you the highest throughput, lowest cost of operation, and easiest use of any competitive IC wafer tester.

Measurement integrity

Automatic continuous monitoring works by having the tester automatically detect events that would invalidate RF calibration, such as an unattended recalibration (changes in system configuration, such as a probe card change, can be set up to trigger an unattended recalibration). The RF Option then automatically triggers corrective action. There are also time-based trigger events, such as time-expired RF calibration.

Another class of trigger events is measurement-based. For example, while a test system prober is indexing to the next site, in the background the tester automatically verifies the quality of the probe contact, triggering automated probe cleaning if required.

"Our system now eliminates the need for an RF specialist to manually inspect raw data curves for anomalies that would suggest questionable RF measurements," says Keithley principal marketing consultant Carl Scharrer. "It also lowers the high cost of re-probe and re-work, making it operationally compatible with highly efficient so-called lights-out 300mm fabs."

Simultaneous DC and RF
Scharrer says that the RF Option, together with Keithley's Model S680 SimulTest option (and appropriately-designed test structures and probe cards), is the only RF test system on the market that can make simultaneous DC and RF measurements in parallel within the same probe touchdown. "This yields substantially higher throughput than methods that perform sequential DC measurements followed by RF measurements," says Scharrer.

RF parameters can also be extracted and de-embedded (for the first time in realtime) from measured s-parameters. "That's done using what we feel is the industry's largest RF parametric extraction library," adds Scharrer. "It's accomplished without the need for any post-processing, either, also saving time and increasing throughput."

Measurements at 40GHz
Combined with the company's Model S680 parametric test system, Keithley also offers a direct-dock capability for making measurements at 40GHz. Scharrer emphasizes that competitors Teradyne and Agilent Technologies are limited to about 6-GHz with their testers. "We're at 40GHz with production throughput levels."

"Competitive products also require a manual probe-card change by an RF specialist using a torque wrench followed by manual recalibration," points out Scharrer. "Our RF Option completes an automated RF probe card change in a fraction of the time it takes for a manual change."

Automated probe-card changes also increase measurement integrity by eliminating variability caused by human intervention in system-to-system variation in RF measurement results. And, because an RF specialist is no longer needed, it lowers the cost of operation. Scharrer says Keithley proved that different technicians using the same system in a production application can get the same results. "In the past, this wasn't even been possible in the lab. This product is extendable, in that it's suitable for both the lab and the fab, with excellent correlation between measurements made in both areas."

This automated probe card change capability is a new feature offered in Keithley's RF Option. "Overall, this VNA-based technique delivers better throughput and improved repeatability compared to ENA-based or PNA-based approaches," contends Scharrer.

Keithley's SofTouch option also maximizes probe-card lifetime by minimizing probe overdrive, while meeting measured quality criteria for probe contact. As an example of the efficacy of the technique, one fab's estimate was for a 100X lifetime increase for their RF probe card touchdown count, reducing probe-card costs by the same amount.

In use, Keithley offers a Probe Card Manager option that integrates with RF probe cards for continuous and automatic monitoring of probe touchdowns and remaining probe-card lifetime, to further reduce probe-card costs.

Use existing probers
The 40GHz RF Option also works with a fab's existing automated DC production probers. Keithley contends that can reduce initial acquisition costs by as much as two-thirds. It can also lower maintenance and operator costs compared to products that require purchasing a specialized RF prober.

Operational costs are also decreased because DC and RF tests can be run in a single-insertion mode on the same equipment. That eliminates the need to move wafer lots between a DC-only tester and an RF-only tester.

In addition, unlike competitive products that offer only a single RF probe-card supplier, Keithley's system gives you a choice of three suppliers. This minimizes probe card costs. Three vendors also means you can enjoy a more predictable and stable supply chain, to ensure confidence during production ramp-up.

Software supports many apps
The RF Option measurement capability also includes a software suite that lets you rapidly obtain large quantities of quality RF data. Keithley says it's built upon a mature operational GUI (graphical user interface).

What's more, for production parametric process control of high-k transistors, Keithley's RF-CV methodology replaces conventional CV or MFCV measurements. That lets production fabs more tightly control electrical gate thickness on sub-65-nm logic ICs.

For analog or RF ICs, the RF Option is claimed to cut RF circuit-model verification times to two weeks or less, compared to two months or more using other RF measurement options. That can give you a significant competitive advantage by shortening time-to-market.

RF IC companies can also use the RF Option for parametric production process control of BiCMOS standard figures of merit, such as ft or fmax. Likewise, organizations making SoCs can use the RF measurement capability for functional testing of benchmark circuits and figures of merit, such as LNA (low noise amplifiers) and cellphone filters.

Depth of RF support
These products are now supported by the company's RF experience in fabs around the world. Scharrer points out that Keithley application engineers have experience in RF test structure design for production environments, including transistor gate capacitors for RF-CV measurements for high-performance logic, and inductors and filters for high-performance analog applications. He notes that the company's semiconductor physicists can also help you interpret measurement results and apply them to better control processes.

Keithley's RF Option measurement capability is available now for both new Model S680 and existing Model S470 systems. It's also available as a field-retrofit option to existing Model S600 Series and Model S400 Series units already installed during the past 15 years. Pricing begins at about $150,000.

- Alex Mendelsohn

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