The Total Release Method for FIB In-situ TEM Sample Preparation

Abstract

T.M. Moore Omniprobe, Inc., Dallas, TX moore@omniprobe.com In 1965, Gordon Moore forecast that the microprocessor industry would continually scale to smaller feature sizes and the number of transistors would double every 18 months. Scaling below the 100nm node, combined with the implementation of copper and low dielectric constant insulators to increase the processor speed, has produced the situation in which SEM inspection no longer offers suitable resolution to image key artifacts and structures. The transmission electron microscope (TEM), once considered more of a development tool, is now in the forefront for process control and failure analysis, especially for measurements such as the thickness of semiconductor device non-planar barrier and seed layers. The use of focused ion beam (FIB) microscopes has become the method of choice for site-specific TEM sample preparation. Originally, the FIB was used as a final thinning step for mechanically prepared ribbons of semiconductor material adhered to modified TEM grids, known as the “H-Bar” method. More recently, the method for performing the entire TEM sample preparation process within the FIB is known as “in-situ lift-out” and is based on the use of a chamber-mounted nanomanipulator and beam-induced material deposition.[1-5] The use of the FIB offers advantages over conventional mechanical TEM sample preparation. The dual-beam FIB offers the ability to locate the lift-out site with SEM resolution and then use the ion beam to excise the sample without sacrificing the wafer, followed by thinning the extracted sample to the thickness required for TEM inspection. This is especially attractive for 300 mm processing where the value of each wafer in the flow can exceed $100,000. The risk to the quality and reliability of the process wafer due to gallium contamination from the ion beam is considered manageable.[6] In-situ lift-out also enables the return of the mostly abandoned practice of including informative test die on product wafers. The Total Release method for in-situ lift-out is designed to maximize throughput of a TEM sample preparation process.[4-5] The method can be simplified into three successive steps (see Figure 1). The first is the excision of the lift-out sample using FIB milling and extraction of the sample from its trench with two rapid ion milling steps, or “cuts”. The first cut is “U”-shaped and partially surrounds the target. The second is a straight cut that intersects the first cut beneath the target and produces a wedge-shaped sample. Then the probe is fixed to the released sample, typically with ionbeam metal deposition, and the sample is removed from the wafer by the nanomanipulator. The second step is the “holder-attach” step, during which the wedge is translated on the probe tip to the TEM sample holder (the lift-out grid). Then the sample is attached to the TEM holder (again, typically with ion beam-induced metal deposition) and later detached from the probe tip point using FIB milling. The third and final step is the thinning of the wedge into an electron-transparent thin section using FIB milling. The use of a simple probe tip for lift-out in the FIB has throughput and efficiency advantages over alternative methods. For example, lift-out can also be accomplished by a method referred to as ex-situ lift-out, in which ion beam-thinned samples, still attached to the wafer, are removed from the FIB and then detached from the wafer with a statically charged glass needle. These samples are then permanently deposited onto the suspended areas on a polymer membrane-coated TEM grid. Ex-situ lift-out can be faster than the in-situ method, and it requires less time in the FIB. However, in-situ lift-out offers a much higher overall throughput through