The new final test

As micro-electromechanical systems (MEMS) expand rapidly into many new areas improved, scalable test strategies are needed to handle transition from engineering development to high-volume production. Andreas Nagy, Product Manager In Stip MEMS, Multitest describes the new InMEMS standard platform to address performance sensor calibrations at strip level for automotive MEMS production.

With relatively few years in its history, MEMS has now arrived at a maturity level that nobody would dare consider a ‘play-ground’ for over-funded scientists. The majority of semiconductor companies have now become involved in MEMS and are trying to secure their share of this fast growing market. With annual growth at 17 percent, this market segment has convinced even the most conservative shareholders and businessmen that it will become one of the main contributors to the overall business plan.

The potential of final MEMS applications is exploding, driven by the fact that cost and size of the sensors are decreasing by a factor of hundreds compared to traditional sensor types. Realising sensor on silicon technology, and even integrating the sensor controllers on an ASIC within the same package, is increasing cost saving potential and reducing space allocation issues traditional sensors would naturally pose. This trend is now heating up the imagination of creative marketing departments. Today sensors can be located in areas where nobody would expect them. Similar to the standard semiconductor market, a clear trend is becoming obvious; package size reduction to the smallest size possible. Also the cost structure is reduced to a level that is inexpensive for consumer products affordable to include a MEMS package to support fancy applications.

A few examples for typical automotive applications are tyre pressure sensors, pressure sensors to actuate airbags when the door gets crushed by an accident, high g accelerometers to actuate airbags, low g accelerometers to detect if the car is rolling over and to actuate the right side airbag, yaw rate sensors for the navigation system or ESP (electronic stabilisation program) controllers and many others.

Consumer electronics applications are also skyrocketing, for example 3 axis log g accelerometers are used to detect if a laptop or any product that requires a hard drive is being dropped. The sensor would detect the device falling and would immediately secure the hard drive head to prevent a disk crash when it hits the floor. Just recently a Japanese game console provider announced its new platform where a 3-axis low g accelerometer would sense the players’ motion and this way drive the speed and orientation of game actors and features within the game. This opens the door for a complete new generation of games. MEMS microphones are also introducing new opportunities for the cell phone market and other businesses providing high-end audio quality at the smallest size. Pressure MEMS are used as altimeters in consumer products like watches.

While MEMS in the automotive industry primarily has to fulfill the highest-level of functionality and robustness the consumer market has more relaxed specifications. Both industries have the same demand to provide products at the lowest cost possible given the fact that more and more players are entering the arena.

Together with the need to reduce package size, has led to a new trend, this being the requirement to merge multiple sensors into one package. The low g accelerometers were the first MEMS applications, where 3 individual sensors in orthogonal orientations were put together in one package to monitor the orientation with respect to gravity fields. Some tyre pressure systems also require an accelerometer to wake up the sleeping system when the tyre is spinning to insure the battery can last for life. If these two MEMS types could be integrated into just one package other applications might follow.

Some 10 to 30 % of standard semiconductor manufacturing costs are driven by final tests. For MEMS, besides the regular ASIC test, the sensor needs to be calibrated, increasing this share even higher. The MEMS device needs to be stimulated during final test by some means – an accelerometer needs to be shaken; a pressure sensor needs to get pressurised; and, a yaw rate sensor needs to be turned. This additional parameter increases the complexity of a test handler because the stimulation for calibration has to be controlled at a higher level than the specification calls for.

In the past, custom tailored test handlers were developed just to fit the current need of a new MEMS development. With shorter life cycles for new MEMS, this is becoming costlier for manufactures because of limited scalability.

For over six years, Multitest has been involved in MEMS testing and has developed a wide portfolio of various MEMS test and calibration applications for:
● Accelerometer test up to 50 g for various axis (x,y,z,45deg,x+y)
● Accelerometer test for three axis low g MEMS
● Yaw rate sensor test with constant velocity or sinusoidal stimulus
● Pressure sensor test with up to five pressure set point
● Optical sensor test
● Electromagnetic sensor test

A modular MEMS test cart can receive two exchangeable stimulus boxes for the different sensor types. The stimulus boxes are kit-able for different package types and provide two contact sites each, which add up to a quad site test.

The cart can be used in an engineering environment as a standalone module with manual loading of the individual MEMS devices. Test program development and engineering can be done with a low investment this way. For high-volume production the MEMS test cart will be docked to a standard production gravity test handler. This platform would then load the devices in a fully automated manner with up to four contact sites parallel conditioned from –55°C up to +155°C. The devices can be tracked by optical character recognition or dot matrix code reading and in this way correlated to the calibration data at different temperature levels.

High throughput is guaranteed due to the maturity of the standard test platform even for new MEMS test applications. The scalability of the concept provides the highest flexibility to the customer giving a good return on investment.

For over a year Multitest has added MEMS test applications to its new generation standard test handler ‘InStrip’, which are thus called ‘InMEMS modules’.

The InStrip test handler supports semiconductor testing while the devices are still integrated to the leadframe or substrate strip. Highly parallel test can be supported without any restrictions from the handler site. The tester capability is the only bottleneck left. Strip handling is the perfect answer for decreasing package sizes where singulated handling has problems with positioning accuracy. The strips are typically serialised with a dot matrix code and the bin results are collected to an electronic map file, which will enable a laser marker after final test to label only the good devices according to their bin category. After singulation the devices can be sorted by simple mark inspection just before final packaging. With the Multitest InMark Laser marker and the standard SECS/GEM G84 strip mapping interface Multitest is one of the few companies that provides equipment to support the whole strip test process.

Especially for MEMS test, the highly parallel test capability of this platform is a key customer benefit. Due to the long test times for the MEMS calibration, and also the need to correlate the calibration data between the different tests, strip test seems to be the right process for MEMS.

For that reason Multitest developed its first application on the new platform using the same concepts and strategies that were successfully previously introduced for singulated test handlers:
● The InMEMS test concept is modular, flexible and scalable
● InMEMS test modules are kit-able for various packages and strips (leadframe or substrate strips)
● Highly parallel test is standard (for example 36 contact sites and higher)
● InMEMS test modules are provided standalone for engineering purposes on a bench with manual loading at tri-temp conditions
● InMEMS test modules hooked up to the InStrip standard test handler will support fully automated high-volume production at tri-temp conditions

With the scalable approach customers can start a project with a low-key investment only procuring an InMEMS module on a bench with manual loading of the strips for engineering and even lowvolume production. For high volume, they can then utilise the same investment using the InMEMS module on the standard InStrip test handler. The other option would be to continue to use the bench module for engineering tasks in Europe or in the US to support production sites in Asia where the fully equipped InMEMS and InStrip systems are running. The same InStrip test handler can later be converted for different MEMS test applications.

So far Multitest has launched an InMEMS test application for 3-axis low g accelerometer test and calibration called the InFlip Module. The InFlip module has a contact site with multiple contact sockets that can index along the strip and in this way test the strip row by row. The strip and contact sites are mounted into a cardanic frame that can rotate the strip ±185° around two orthogonal axes to any desired position with respect to the gravity field. Depending on the strip orientation, the 3-axis accelerometer gives different signals from –1g to +1g for the individual axis. This way all three axes can be calibrated in only one operation for multiple contact sites whereas before it would have needed to be handled multiple times for a shaker test of each individual sensor axis and device.

Some 600 signal lines are routed along the two frame axes to the tester. Therefore the device under test (DUT) board sits right next to the strip inside the core unit to improve the sensible signals by passives or actives. Position accuracy of ±0.1° is standard with even a much better repeatability. The module is designed for ambient and tri-temp tests from -40°C to +125°C.