Accurate measurement
Device characterization is an area that can have a tremendous impact on yield and time to market. Understanding such characterization is based on a history of device behaviour and a library of knowledge. With increased complexity comes an increased need for accuracy in characterization measurements. Joshua Preston, Marketing Group Manager at SUSS MicroTec, discusses the growing importance of exact accuracy in device characterization measurements.
Semiconductor device manufacturers face a challenging marketplace; consumers demand higher performance, better reliability and lower prices for their electronic devices. To meet this challenge, new technologies are being developed using advanced and/or hybrid designs, new materials and innovative manufacturing processes. To realize a return on the investment in these technologies, device manufacturers must keep yield high and time to market short.
One area in which significant gains in efficiency can be realised is in the device characterization process. In this process, it is necessary that accurate measurements are obtained quickly because the manufacturer benefits from more efficient model extraction, faster model turnaround and fewer design iterations, reduced time to market and ultimately a higher return on investment.
The device characterization process is critical for developing a well-designed component, but it is becoming increasingly more complex. The driver behind this is, as mentioned, the higher performance, better reliability and lower cost demanded by the end user, which results in devices that require challenging measurement setups to extract accurate parameters that are needed to verify the device model. It is not uncommon today that a measurement takes several hours (or even days) to set up. Additionally, the accuracy of the measurement cannot be stressed enough. Test engineers need to have full confidence in their results: they must be repeatable, understandable and most importantly real and true. If the results are flawed, then the models and ultimately the devices will be flawed as well, which necessitates another iteration of the entire process. This is further complicated by communication delays that result from the fact that device characterization is commonly done in a different location because it is in a different department or has been outsourced to a test house, both of which may be in another country.
To meet these challenges, test system manufacturers must provide test engineers with the tools necessary for accurate, efficient device characterization. This must be a holistic approach and starts with the wafer probe system, as the platform for the wafer-level measurements, and includes probes, cables and of course the measurement equipment. The ideal test system maximizes measurement accuracy, positioning accuracy and what I will call “human accuracy”, that is, the ergonomics and usability of the system. This article will focus on the improvements that can be made in these three areas and give concrete examples of how the device characterization process can be improved, thus increasing the return on investment in new devices.
Measurement Accuracy
At the basis of any device characterization and reliability test process is the accurate measurement of parameters from the device under test (DUT). These parameters include measurements of current-voltage (I-V), capacitance-voltage (C-V), lowfrequency (1/f) flicker noise, HF noise and scattering (S-) parameters, and are highly sensitive to interference from external sources of noise. Therefore, the most basic requirement for accurate parameter measurements is a test system featuring a shield based on sound electromagnetic design principles, such as ProbeShield Technology from SUSS MicroTec, to prevent unwanted EMI and RFI as well as light from influencing measurements. This is especially critical when sensitive measurements must be taken, and if implemented properly, the noise floor is kept low and results are more accurate (see fig. 1).
A well-designed EMI/RFI shield also allows measurement equipment to be integrated into the shielded environment of the system. For very sensitive measurements, elements like signal preamplifiers are used to capture the signal and amplify it before transmitting it to the main measurement unit such as a parameter analyzer. When these elements are placed inside the shield, the EMI/RFI and motion-sensitive elements like triaxial cables are kept in a protected environment and any exterior signals are kept from interfering with the equipment. A simple experiment as shown in fig. 2 confirms the benefit of measurement equipment integration. Furthermore, additional shielded rooms are no longer necessary if the equipment is integrated into the shielding of the test system. This reduces capital outlay and the overall cost required for test processes.
Integrating elements of the measurement setup inside the shielded environment has another major advantage. Cable lengths, obvious limitations in measurement accuracy, can be minimized. Shorter cables ensure a better measurement dynamic range and thus more accurate measurements. When all of these issues are taken into account, an optimal environment is created for extracting the most accurate measurements from the DUT.
Measurement integration also means that suppliers of the various elements of the device characterization system work together to ensure that the entire system provides value to the user. That is, the system should not just be a mixture of measurement equipment, probe station, probes and cables. Each piece must be correctly integrated and tuned to operate seamlessly with the other elements in the system. As such, a system with various elements that are simply thrown together runs the risk of operating inefficiently and driving up the cost of test to a point where it no longer provides value to the user.
Positioning Accuracy
The need for a faster feedback loop in the manufacturing process and faster classification of process improvements has increased the importance of wafer-level reliability. As a result, contact pads for reliability test are moving into the wafer kerfs alongside the pads used for device characterization. To accommodate all these pads, their size as well as the space between them, the pitch, is shrinking. Contacting small pads, sometimes as small as 40 x 40 µm, is already an issue for some device manufacturers and will become more pronounced in the near future. If stable, repeatable contact is not achieved, measurement results are compromised and time is wasted running test procedures over and over again.
As pads get smaller and more numerous and advanced packaging technologies like wafer bumping become ubiquitous, vertical probe cards must be used to contact the DUT. Vertical probe cards cannot be aligned like a traditional cantilever probe card because there is no viewing port for probe-to-pad alignment. To solve this challenge, probe station manufacturers have developed technologies that assist the operator in aligning probe cards to the pads. SUSS MicroTec’s MicroAlign Technology, for example, automates the probe-to-pad alignment process by using a self-correlating camera system and a unique software package. The process of aligning the probe card to the pads takes less than five minutes, after which test routines can be run.
Since many vertical probe cards have thousands of contact tips and can exert an extreme amount of force, the chuck stage must be able to handle such extreme forces. A well-designed stage minimizes deflection and therefore increases contact stability and repeatability while reducing damage done to wafers and probe cards.
Human Accuracy
The high rate of turnover of operators and technicians in the semiconductor industry is a significant cost to manufacturers, especially as measurement setups, tasks and equipment become more complicated. A test system with a highly ergonomic, safe and intelligent design provides the optimum the learning curve for new operators and technicians. An easy-to-use, intelligent system also helps safeguard investments made in measurement equipment, probe cards and wafers.
An example of an innovative new safeguard in wafer probe systems is SUSS’ patented ContactView system. This is an integrated, side-looking camera that provides an additional dimension for viewing the wafer and probe tips as they approach each other; an example is shown in fig. 3. This assists the operator in safely setting contact and prevents damage to probe cards and wafers.
Increasing the automation in the test system also improves human accuracy. Automated systems take the guesswork out of operation as well as assisting the operator in carrying out tasks. In addition, the system can be kept running overnight and on weekends without the need for human intervention. An example of such a system is SUSS’ unique ReAlign Technology, which automatically compensates for thermal drift after temperature change. Most test routines are run at various temperatures, presenting a significant challenge to the operator because she must adjust the alignment of the probe tips to the pads after each change in temperature to accommodate for thermal drift. ReAlign Technology automates this process based on the alignment algorithms from the MicroAlign system. The process requires no operator intervention and can be called from the test executive, which means lengthy tests at several different temperatures can be run overnight or on the weekends.
Soft Assistance
Of course, the accompanying software is also a means by which the human accuracy of a system can be enhanced. There are several software tools available today that help users configure wafer-level calibration for RF and microwave test systems as well as tools to find the correct test frequency when using Sparameter measurements to characterize gate oxides. Simple-touse software, when designed properly, guides operators through the setup and provides a system of checks to ensure that the calibration and test routines run safely. The software can also alert the operator or text executive when a recalibration or realignment is necessary. This is a considerable productivity improvement since the time needed to setup and run the routines is significantly reduced when compared to manual, unassisted setup. Furthermore, there are software tools available that support test engineers around the world when working to develop new technologies. A typical example is when the team in Laboratory A measures the DUT and obtains a result that is different than the team in Laboratory B. SussCert software, for example, generates confidence intervals to allow these teams to quickly judge the performance of the measurement system and easily compare results.
It can clearly be shown that there are significant gains to be realized from enhancing the device characterization process. Maximizing measurement accuracy by using a well-designed probe system with an advanced EMI/RFI shield results in the benefits of faster model turnaround and fewer design iterations, both of which are preconditions for reducing time to market and higher return on investment.
The system also needs to be able to keep up with the trends of shrinking pad-sizes and new technologies such as high-pin-count vertical probe cards by providing precise positioning accuracy. And of course, the entire system must be designed with the human interface in mind.
A strong focus on enhanced human accuracy increases the return on training investments and leads to gains in productivity. In addition, the use of intelligent hardware and software systems is the enabler for unattended test routines for automated generation of modeling and reliability data. Altogether, a welldesigned and integrated device characterization system helps test engineers add value to the semiconductor design and manufacturing process by overcoming current and new challenges with the highest accuracy.
