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edfas.org 5 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 4 effective, thus, requiring new techniques andmethods to be developed in order to overcome the challenges. This work expands on the sample preparation work thatwas conductedon the 130 nmSPI discussed inKimura et al. [5] by looking at the techniques and processes that were developed for a 45 nm SPI module. A review of the common sample preparation approacheswill be provided as well as a deeper investigation into the process recipes developed for the 45 nm SPI. METHODS FOR INTEGRATED CIRCUIT MATERIAL REMOVAL For delayering integrated circuits (ICs), there are severalmethods for removingmaterial. Each is considered for use with regards to the material of interest, material thickness, the dimensions of the region of interest, and other materials present on the IC. Consideration is also given to maintaining planarity upon material removal and the removal rate of a given technique. This section will examine the capabilities and limitations of chemical- mechanical polishing, wet chemical etching, dry chemi- cal etching, and vapor etching as delayering methods. Combined with metrology techniques described in the next section, thesemethodswere used todelayer samples in an iterative fashion, detailed in Fig. 1. Samples are processed with a material removal technique, and the end point is determinedwithmetrology. If an end point is reached, the sample is mosaic imaged at the target layer in the scanning electron microscope (SEM). CHEMICAL-MECHANICAL POLISHING Chemical-mechanical polishing (CMP) is amechanical polishing method that removes material with abrasive films and pads in conjunction with consumables such as lubricants and polishing suspensions. CMP tools are used to mechanically polish mounted ICs against an abrasive. Samples are mounted to tool compatible mounting chucks with wax, tape, or epoxy. The abrasive surface rotates on a platen during polishing to remove material while the sample chuck also rotates to remove material evenly, compensating for any error in the geometry of the system. Other variables such as sample oscillation and sample rotation can be modulated to control the opera- tion. Consumables are chosen to produce a controllable removal rate. Lubricants prevent frictional heat transfer onto samples and reduce surface damage from polished material while polishing suspensions allow for partially selective material removal rates. [6] The primary limitation of CMP lies in the challenges of maintaining sample planarity during polishing. [7] In this context, planarity is defined as the degree of parallelism between the IC surface and the polishing platen. Ideally, the sample should be perfectly parallel to the polishing plane, allowing for even surface removal. However, real world polishing systems do not offer this degree of align- ment precision, and biased material removal across the surface of the IC is unavoidable. Edge rounding is one such example of a defect caused by non-planarity, where material near the corners and edges polish faster than the center of an IC. [7] These effects are mitigated by improv- ing the IC mounting process to ensure bond lines are as flat as possible. [7] Overcoming the challenge of planarity becomes critical in advanced technology node devices with thinner layers. CMP also introduces unwanted artifacts on the polish- ing surface. Colloidal silica suspensions used for polish- ing soft metals commonly adsorb to the IC surface and can obstruct features during imaging. [8] Dust and other ambient particulate can also adhere to the polishing pad or film and can cause undesirable damage. Solvents and other IC cleaning methods can be employed to minimize these effects. [9] WET CHEMICAL ETCHING Wet chemical etching is a technique that involves using solutions and chemical reactions to remove mate- rial. This class of techniques can be highly selective depending on the etchant and material of interest. [10] Temperature and chemical concentration strongly affect chemical etch rates. [10] Limitations towet etching includematerial limitations, etch isotropy, and safety. Selectivity is a concern if there are no etch stops in the sample to prevent over-etching. Most chemical wet etches are isotropic, meaning they etch equally in all directions. [10] This can create the problem of undercutting, where the chemical etches material laterally. Submerging an IC in isotropic etchants will etch material from the sides as well as on the surface, destroy- ing features of interest onmultiple layers near edges. Wet etching also risks redeposition of etched materials and ambient particulates. DRY CHEMICAL ETCHING Dry chemical etching, or dry etching, includes plasma- based methods for material removal. Two methods are considered for their utility in this 45 nm node SPI decom- position: reactive ion etching (RIE) and plasma focused ion beam (P-FIB) milling. Reactive ion etching is a technique that exposes the sampleof interest toaplasmaof reactive ion species inside