Initial Phase Sample Preparation Thinning Processes
The three methods in the initial phase sample preparation thinning are the manual pre thinning, FIB Lift out and the Automated Pre thinning. All these three create a thick section that is ready for final thinning to electron transparency. The first method is the manual pre thinning technique. Manual preparation is relatively slow, typically requiring 4 hours to prepare a FIB-ready sample. It is said to require a very little capital investment, but this cost savings is offset somewhat by the expense for a skilled technician.
In the FIB lift out process, the FIB first cuts a relatively thick section from the full wafer. The section is attached to a probe nee a micromanipulator, then transported and attached to a inside the FIB chamber using the FIB cutting and metal capabilities. The entire process takes about half an hour. There is opportunity to reduce this time through automation though tin of it is consumed by milling and deposition processes that earn made significantly faster. FIB time is expensive. FIB systems re significant initial investment of one to two million dollars and ongoing support costs The automated pre-thinning process prepares mounted and specific thick-sections ready for final FIB thinning. It can make both the cross-sectional and plan-view samples. Cutting is done with dry, cryocooled diamond sawing that is independent of the crystal orientation of the underlying wafer. This allows it to accommodate angled pattern alignments and non-crystalline substrates. The system uses an automated stage and a high resolution optical microscope for target identification. It accepts up to 300 mm wafers and it can navigate automatically to predefined target coordinates. In the finished sample the target is located within a ridge 10-20 um thick and approximately 100 um tall, supported on a somewhat thicker base section that is glued to a TEM compatible grid.
Mechanical cutting and polishing procedures, whether manually and automatically controlled, pose a risk of damaging the sample through chipping, cracking or delamination. The automated system described here reduces this risk by eliminating wet polishing, and by using a special detachment procedure, it is able to isolate the target region from its surroundings prior to sawing. Once targets have been designated the entire process proceeds with minimal need for operator intervention. The need for high-resolution imaging and analysis to support semiconductor manufacturing is increasing the demand for STEM and TEM capability. Preparation of the ultra-thin samples required for these techniques introduces significant cost and delay into the analytical process. Manual techniques are slow, and their low initial investment is offset by higher ongoing expenses for the technicians labor. Each thinning technique has its share of advantages and disadvantages. The method of FIB lift out is expensive. Automated pre thinning sample preparation is less expensive than liftout. It is considered faster too for multiple sample jobs. By freeing the FIB to perform tasks for which it is uniquely capable, automated pre-thinning optimizes the allocation of resources in the analytical laboratory. Higher throughput and reduced costs provide a direct benefit in improved laboratory productivity.