March 13, 2013
Semiconductor manufacturing test engineers face increased challenges related to broadband millimeter wave (MMW) on-wafer device characterization testing. In the past, the need for higher frequencies was based on capturing harmonic content. Today, many MMW applications are driving on-wafer testing to frequencies exceeding 100 GHz, requiring engineers to measure from near DC to 100+ GHz in order to develop accurate models.
Achieving accurate, stable measurements over extended time periods is a challenge for foundries and fab-less semiconductor companies requiring extensive testing of on-wafer devices. These measurements are typically performed using a vector network analyzer (VNA). To ensure accurate measurements, VNA re-calibration has often been required as frequently as every hour. This consumes valuable test time and reduces throughput during production. Systems such as the ME7838A Broadband VectorStar® VNA from Anritsu (figure 1) can significantly reduce calibration time and improve efficiency.
Typically, semiconductor manufacturers characterize and test transistors, such as those used for power amplifiers, as well as more complicated integrated systems during three stages. Each is discussed below.
Device Characterization – A primary data collection set is required for “first wafers” to help determine the specifications for the future product. Test engineers measure full s-parameter data on multiple devices across a broad frequency range at multiple bias conditions, power levels, and in some cases temperatures. These measurements are often more manual in nature, requiring the probes to be physically moved around the wafer. It is important that any variation from device to device across the wafer can be accurately determined and not caused by test equipment drift. Hence there is a need to ensure that the VNA system is correctly calibrated at intervals frequent enough to ensure consistent measurements.
The manual nature of measurement during this phase impacts the calibration, as well as adds time dealing with probe contact issues such as oxidation. While in some cases the calibration standards can be included on the wafer for easier calibration, the trade-off is the space these standards occupy. Often times, a separate calibration substrate is used, adding calibration time. In this situation, a 20-minute calibration may have to be performed every hour, meaning calibration alone can consume almost one third of the test engineer’s time during the device characterization stage!
Evaluation – During this stage, all device characterization data is used to evaluate device performance and determine product specifications. While measurement and calibration times are not big factors during this stage, confidence in the device characterization data plays a critical role. Accurate measurements allow for smaller measurement uncertainties, which can then be used to confidently make tradeoffs between tighter specifications and manufacturing yields. Tighter specs allow for products to be better positioned in today’s highly competitive marketplace.
Production – High volume automated test equipment is used at this stage to collect reduced data sets to verify the devices are meeting their set specifications. It is not uncommon for devices to be tested in a matter of seconds. Calibration is part of the automated process, as well. During automated production testing, calibration time is reduced, but may still be required every hour. With devices being measured every few seconds, time lost to calibration can be even more critical!
The ME7838A VectorStar Broadband VNA system features innovative broadband frequency extension modules with industry-leading dynamic range and stability in a miniaturized package that does not require large probe station systems. These advantages help reduce calibration times and improve efficiency at each stage.
Anritsu has prepared an app note that discusses how modern VNA test solutions improve on-wafer measurements. You can read it in its entirety here.