Cheryl Hartfield

Interface reliability assessments for copper/low-k products

Multiple new materials are being adopted by the semiconductor industry at a rapid rate for both semiconductor devices and packages. These advances are driving significant investigation into the impact of these materials on device and package reliability. Active investigation is focused on the impact of back-end-of-line (BEOL) processing on Cu/low-k reliability. This paper discusses Cu/low-k BEOL interfacial reliability issues and relates key items from the assembly process and packaging viewpoint that should be managed in order to prevent adverse assembly impact on BEOL interfacial reliability. Reliability failure mechanisms discussed include interface diffusion-controlled events such as the well-known example of Cu electromigration (EM), as well as stress-migration voiding. Interface defectivity impact on dielectric breakdown and leakage is discussed. Lastly, assessments of assembly impact on these Cu/low-k interfacial concerns are highlighted.

In situ laser processing in a scanning electron microscope

Laser delivery probes using multimode fiber optic delivery and bulk focusing optics have been constructed and used for performing materials processing experiments within scanning electron microscope/focused ion beam instruments. Controlling the current driving a 915-nm semiconductor diode laser module enables continuous or pulsed operation down to sub-microsecond durations, and with spot sizes on the order of 50 μm diameter, achieving irradiances at a sample surface exceeding 1 MW/cm2. Localized laser heating has been used to demonstrate laser chemical vapor deposition of Pt, surface melting of silicon, enhanced purity, and resistivity via laser annealing of Au deposits formed by electron beam induced deposition, and in situ secondary electron imaging of laser induced dewetting of Au metal films on SiOx.

Mechanical characterization of low-K dielectric materials

The implementation of materials in device interconnect structure is being driven by shrinking device geometries. In order to meet customer demands for increasing electrical performance, the industry is adopting a solution that provides both lower resistance and lower capacitance. Lower resistance is accomplished by switching from Al(Cu) to Cu interconnect and the capacitance is reduced by replacing SiO2 in the inter-level and inter-metal dielectric layers with lower dielectric constant materials (low-K materials) [1,2]. A change in materials in a process as complex as IC manufacturing is inherently accompanied by an increase in reliability risk. A thorough understanding of the low-K dielectric candidates is necessary for selection of the best candidate that has sufficient mechanical integrity to survive thermal stresses, CMP, packaging, and test, as well as allows for maximum extendibility to next generation devices. Towards this end, the industry has adopted methods and tools to measure mechanical proper...

Probing and wire bonding of aluminum capped copper pads

Microelectronics manufacturing has started to develop and, in some cases, ramp wafer fabrication processes using copper interconnects as the preferred metallurgy. Since bare copper wire bonding has not been accepted as a reliable and high throughput process, integrated circuit manufacturers are applying an aluminum cap on top of the copper to facilitate bonding. Past studies carried out on aluminum-based silicon wafers concluded that scrub damage on bond pads due to multiprobe testing should be minimized in order to maintain high assembly yields and a robust bond process. Texas Instruments has carried out an experimental study that compares wire-bonding characteristics of probed aluminum capped and etched aluminum bond pads. Wafers were probed multiple times to generate pads with measured damage ranging from 10-45% of total pad area. Analyses on bonded units include percent of Au-Al intermetallics formed, ball shear, wire pull, and underlying pad damage. The paper will highlight the differences found between the two methodologies and recommend basic changes that will enhance the assembly process of Al-capped copper bond pads.

Making the practically impossible “Merely difficult”—Cryogenic FIB lift-out for “Damage free” soft matter imaging

The preparation of thinned lamellae from bulk samples for transmission electron microscopy (TEM) analysis has been possible in the focussed ion beam scanning electron microscope (FIB‐SEM) for over 20 years via the in situ lift‐out method. Lift‐out offers a fast and site specific preparation method for TEM analysis, typically in the field of materials science. More recently it has been applied to a low‐water content biological sample (Rubino 2012). This work presents the successful lift‐out of high‐water content lamellae, under cryogenic conditions (cryo‐FIB lift‐out) and using a nanomanipulator retaining its full range of motion, which are advances on the work previously done by Rubino (2012). Strategies are explored for maintaining cryogenic conditions, grid attachment using cryo‐condensation of water and protection of the lamella when transferring to the TEM. Microsc. Res. Tech. 79:298–303, 2016.

Cryogenic FIB Lift-out as a Preparation Method for Damage-Free Soft Matter TEM Imaging

The removal of a thinned lamella from a bulk sample for Transmission Electron Microscopy (TEM) analysis has been possible in the Focused Ion Beam Scanning Electron Microscope (FIB-SEM) for over 20 years either via in situ (by use of a micromanipulator) or ex situ lift-out approaches [1]. Both offer swift, site specific preparation for TEM analysis, particularly in light of advancements in corrected TEM. These techniques, however, are currently only applied to samples at room temperature, typically from the materials sector. The majority of biological samples contain a high degree of water, which will be removed under vacuum, leading to the shrinking and rearrangement of the sample. To overcome this, samples can be prepared by either critical point drying, fixation and resin impregnation (often combined with heavy metal staining) or cryogenic fixation. For both fixed and cryo-preserved samples, the preparation of thin-sections has always typically been prepared with a microtome, which yield samples of 60100 nm. However, these commonly suffer from compression artifacts and/ or knife marks, which distort data. There is also a desire to move away from staining and methods which dehydrate or allow / permit structural or chemical re-arrangement.

Dielectric Integrity Test for Flip-Chip Devices with Cu/Low-k Interconnects

Migration to low-k dielectric materials in wafer-fab backend-of-line (BEOL) interconnect schemes is necessary for improved electrical performance of smaller and tighter geometries enabled by advanced silicon technology nodes. Unfortunately, low-k dielectrics are mechanically weaker compared to previous generation materials. Concurrent technology and market driven changes such as tighter bump pitch and replacing SnPb with Pb-free solder bump composition are leading to increased stress and risk to damage of the inherently weaker dielectrics in flip-chip packages. It is, hence, critical to characterize the structural integrity of the BEOL interconnect schemes containing low-k dielectrics. Traditional environmental reliability stress testing, while effective, is time-consuming and expensive, and not conducive for fast learning cycles required during early development phases. This paper demonstrates the utility of a test that enables rapid and accurate assessment of the mechanical integrity of low-K dielectrics. Silicon dies with Cu/low-k interconnect are assembled on to an organic substrate through flip-chip bumps, and cooled. The mismatch in coefficient of thermal expansion (CTE) between the silicon die and organic substrate can induce failure if the resulting thermal residual stress exceeds the strength of the structure. Since the level of thermal residual stress depends on the temperature, the magnitude of induced stress can be controlled by subjecting the sample to various degrees of cooling. Consequently, the strength of the BEOL structures can be determined, in conjunction with thermo-mechanical stress analyses results from finite element simulation of the test.

Advancing Materials Characterization in the FIB-SEM with Transmission Kikuchi Diffraction

SEM-based Transmission Kikuchi Diffraction (SEM-TKD) [1,2] is a technique variation of EBSD, applied here using an unmodified Oxford Instruments AZtec EBSD system. The advantage of SEMTKD is the collection of point and mapping data, from particles and electron transparent samples, at spatial resolutions which exceed those of traditional EBSD. This improvement is primarily a consequence of examining thin, electron transparent TEM samples in transmission, which reduces the effective diffraction and escape volume, as well as the use of zero to low sample tilts, reducing anisotropic beam spreading effects. Like EBSD, TKD may be used in conjunction with EDS for singlepoint, chemistry and crystallography-based phase identification. This is readily applicable to freestanding particles since the high spatial resolution is combined with a large solid angle of diffraction pattern capture, enabling automated indexing for any crystal orientation, for particle sizes down to the deep submicron scale. The technique is also used to produce 2D datasets from flat, thin sample areas, for microstructural analysis including crystallographic orientation, grain size, phase distribution, and grain boundary character and distribution

In-Situ Quantification of TEM Lamella Thickness and Ga Implantation in the FIB

Focused Ion beam (FIB) based tools have become the preferred method to prepare TEM lamellas, largely due to their high resolution imaging capabilities used to identify the site of interest. The quality and thickness of samples has become paramount in order to take full advantage of the ever increasing resolution in aberration corrected TEMs and accurately controlling the lamella thickness at the same time as minimising any amorphisation caused by ion implantation is challenging. For instruments combining a focused ion beam with an electron beam methods based on either back scattered electron contrast [1] or transmissivity of electrons [2] have been demonstrated. However, these methods only work on homogenous samples without compositional variations and require for the contrast to be calibrated using the same material. They also don’t provide any information on ion implantation or surface amorphisation and can greatly affect the quality of the TEM image obtainable from the lamella.

Local thickness and composition analysis of TEM lamellae in the FIB

Abstract High-resolution TEM image quality is greatly impacted by the thickness of the TEM sample (lamella) and the presence of any surface damage layer created during FIB–SEM sample preparation. Here we present a new technique that enables measurement of the local thickness and composition of TEM lamellae and discuss its application to the failure analysis of semiconductor devices. The local thickness in different device regions is accurately measured based on the X-ray emission excited by the electron beam in the FIB–SEM. Examples using this method to guide FIB–SEM preparation of high quality lamellae and to characterise redeposition are shown for Si and III–V semiconductor devices.