Pushing probing to the limits
From temperatures of 4K to over 200°C, pressures of 0.0000001 mbar to 50 bar and alien gas environments, it's all possible at any place but the surface of our planet. Before you start rocketing your devices to Venus for testing under extreme temperature and pressure, consider the fact that on-wafer MEMS test systems, offer a powerful solutions for testing under these extremes right here on Earth.
Before we talk about simulating the conditions of deep space in a laboratory, the benefits of early, on-wafer testing need to be clear. The MEMS industry is growing rapidly yet early testing is still a much neglected area. Early testing is, however, an area that cannot and should not be ignored. There are many reasons why testing prior to packaging is beneficial.
Performing functional tests as well as process development / monitoring and failure analysis at an early stage of the MEMS production process is vital for the commercialisation of the microsystems as it will reduce production costs and time to market. Further, any small decrease in the test cost (in R&D and in production) leads to a further increase in profitability of MEMS manufacturers and/or the decrease in the price of a device.
The packaging process is costly. Failed devices that are tested after final packaging, waste not only money but R&D, process utilisation and foundry time. Testing at the earlier stages of design and development when yields are typically lower results in cost savings that may exceed 15%.
However beneficial early testing may be, standard equipment cannot meet the demands of the dies. In addition to electrical stimulation, devices may require testing using sound, light, vibration, fluidics, pressure, temperature, chemical or force stimulation. As a result of this input stimulation, the test engineer may need to measure any of these categories in addition to detecting and measuring mechanical, optical or electrical signals generated as a result of this stimulation. The devices may require testing in a controlled atmosphere to protect them from environmental damage or to correctly simulate the environment the device will operate in once packaged. This last requirement has been the focus of a lot of developments in the MEMS field, with micromirror arrays, pressure sensors, RF MEMS devices and microbolometers becoming more commonplace. Each one of these devices must be tested in an extreme environment. For these devices, a number of unique solutions have been designed specifically to enable these devices to be tested. This article will provide a brief overview of MEMS testing requirements and then focus on the solutions for extreme testing.
MEMS Testing Requirements
There are three phases in the life cycle of a MEMS device: the R&D phase, the pilot production phase and the volume production phase. Each of these phases has unique goals and requirements for testing. In the R&D phase, for example, the test system must be very flexible for performing a broad set of measurements over a wide range of operating parameters. In the volume production phase, however, all aspects of the test system are optimised for throughput; multiple devices need to be handled and tested in parallel. Additionally, the test system must be able to interface with the measurement equipment. A VNA needs to be integrated into the system when testing RF MEMS devices, and a motion analyser is necessary when testing micromirrors. Complete flexibility with different vendors and setups is key to any successful test setup. Only by reducing the complexity and length of these phases as well as providing a smooth transition from one phase to the next will a MEMS device become market worthy. Unique on-wafer test solutions exist for each phase of the development cycle. These solutions reduce the time of the R&D phase by providing the engineer a highly flexible and customisable tool that would otherwise need to be built from the ground up. The pilot production phase is shorter because the device does not have to be packaged and then tested, resulting in a long feedback loop. Rather, the device is tested at wafer level and characterisation and reliability data are available much earlier and without the expense of packaging bad devices. The volume production phase is made more efficient by preventing bad devices from being packaged through early stage wafer level test. Last but not least, the transition between phases is made easier when devices that were tested using test systems from one supplier during the R&D phase are also tested using equipment from the same supplier in the volume production phase. Furthermore, many systems can grow or be upgraded according to throughput requirements, making the transition between the pilot and volume production phases smooth and efficient and saving time and money.
Extreme Testing Solutions
Unique solutions are available for testing MEMS devices in several different extreme environments: high pressure, nearvacuum and at cryogenic temperatures. These systems can be manual, semiautomatic or fully automated and accommodate a number of different sizes of substrates.
Extreme: Vacuum
More and more MEMS devices today require vacuums to operate, such as RF MEMS, resonators and microbolometers, or must operate in near vacuums such as in space. Testing these devices has been done generally after they are packaged – a very expensive process which accounts for 60 - 80 percent of the total cost of the device. Testing these devices at wafer level has proved to be an extremely challenging task, but very rewarding if possible, as the payoffs are large.
This potential for cost saving is very significant, and SUSS MicroTec has a solution for testing devices in a high vacuum. The vacuum probe system is unique in that it offers the accuracy and functionality of a probe system in a high vacuum. This system can work in temperatures from -65°C to +200°C, ensuring a temperature uniformity of ±0.8 K and a stability of ±0.5 K. Vacuum levels up to 1 x 10-7 mbar (high vacuum) can be precisely controlled by an upstream vacuum regulation system, and devices may also be tested in user-defined inert gas atmospheres up to atmospheric pressure.
Most importantly, this system from SUSS MicroTec operates using standard probing procedures and control software. This means that the interface to test instrumentation used on standard probe systems is immediately available for use in these systems. The system is fully compatible with industry accepted test instrumentation, and uses industry standard probing procedures.
Extreme: Cryogenic Temperatures
The cutting-edge is getting colder. Superconducting materials are at the forefront of research, and as semiconductor devices get smaller and gates shrink, the characteristics of these devices are not completely explained with the current theory. Therefore, much of the fundamental research undertaken today involves testing these new elements at cryogenic temperatures. These elements include high electron mobility transistors (HEMT), infrared focal plane arrays, and superconductors and will likely become a part of next-generation MEMS devices.
There is a large opportunity to provide an enabling technology for testing these new elements. Building off of the experience from the vacuum probe system, SUSS MicroTec designed a unique cryogenic probe system using a similar chamber concept.
To achieve ice-free, completely clean cryogenic temperatures, the DUT and probing environment must be fully evacuated. The vacuum isolates the test set-up, avoids in-chamber heat transfer by convection and is necessary to keep the test environment at a constant temperature. All test accessories within the chamber must be impervious to extreme cold and vacuum conditions. Special cables ensure that there is no out-gassing during the test process or deformation of the cable from within. At cryogenic temperatures, cables become very brittle which requires all cables to be firmly mounted within the chamber. Movement must be confined to the portion of the cable that remains flexible outside the super cold environment. The probe tips which will contact the DUT need to be cooled to nearly the same temperature otherwise the probe tips will heat the device above the desired temperature making the test results invalid. Using these techniques, the cryogenic probe system is able to achieve temperatures down to 4K. Furthermore, this system can be built to function with a closed-loop cooling system. The process for cooling the chamber takes longer, but the system uses less liquid gas and therefore saves costs.
In addition to the probe system, SUSS MicroTec developed several accessories specifically for RF and microwave testing at cryogenic temperatures. The IZI Probe and Dual IZI Probe can be used at cryogenic temperatures and still retain all of the desired features of such a probe- long lifetime, better contact and less overtravel. The CSR family of calibration substrates from SUSS MicroTec are also optimised for cryogenic temperatures. Studies have shown that the CSR calibration substrate demonstrates superb load resistance stability over a wide range of temperatures. This means that the system calibration is extremely accurate at any temperature.
Extreme: High Pressure
The solution for testing devices in absolute pressure is also founded on the renowned chamber concept. The system supplies absolute pressure from 100 mbar to 50 bar, and also supports testing in a controlled gas atmosphere combined with controlled humidity. The massive, CE-certified chamber has a volume for accommodating substrates up to 200 mm with plenty of space for manipulators and other accessories. A topside view-port is provided for the microscope, along with four additional view-ports. Hundreds of feedthroughs are provided for measurement cables and all control lines to the probe system. Additionally, five fluidic feedthroughs are built in for supplying pressure to a sensor or coolant to a temperature chuck for testing from -40°C to +125°C. The compressor, booster and vacuum pump are located outside the test floor in a noise protection container, and the controller is integrated into the electronics rack with the probe system control electronics and the PC. This system enables testing of pressure sensors and any devices that may need to operate in a different gas atmosphere or at a different humidity. Instead of packaging the device and then testing for bad dies, the pressure probe system tests at wafer level before packaging. This reduces manufacturing costs and makes MEMS devices more market worthy. All of these extreme solutions are also complete solutions. SUSS MicroTec works with several partners supplying test equipment to provide a totally integrated test system that will meet exactly the requirements specified.
Conclusion
What used to be a challenge – testing MEMS devices in extreme environments – is now simple with the solutions presented here. However, the engineers at SUSS realise that the demands of MEMS testing are changing over time. In order to meet the challenges of the future, SUSS MicroTec and several other companies and research institutes founded MEMUNITY, the MEMS testing community made up of experts from industry, research institutions and academia. These partnerships mean that the application experts will always find the right MEMS-test solution, even for the most extreme of environments.
