Neil J. Bassom

Copper device editing: Strategy for focused ion beam milling of copper

Focused ion beam (FIB) methodologies for successfully milling copper (U.S. Patent No. 6,322,672 B1) have been demonstrated. Approaches to milling copper (Cu) are required because standard FIB mill procedures produce rough, uneven cuts that are unsuitable for circuit edits, a principal FIB function. Efforts to develop gas assisted etching (GAE) processes which would smoothly mill Cu failed because Cu halides are not volatile and remain on the substrate as corrosive electrically conductive debris. Single crystal studies show that Cu grains with different crystal orientations vary in mill rate by as much as 4×. Moreover, the (110) crystal orientation, which mills most slowly, forms a Cu3Ga phase when milled with a focused Ga ion beam. This phase is particularly resistant to milling and, in polycrystalline Cu, propagates during the milling operation, contributing to the uneven trench profiles. CoppeRx, a novel scan strategy, cleanly and uniformly removes polycrystalline Cu with minimal damage to the underlyin...

Focused ion beam induced deposition of low-resistivity copper material

Focused ion beam (FIB) induced processes for material etching and deposition have proven successful in integrated circuit device modification applications. Current FIB metal deposition processes are typically limited to resistivities in the range of 150–200μΩcm due to included impurities; however, today’s high-frequency devices require very low interconnect resistivity. The organometallic precursor material copper (I) hexafluoroacetylacetonate trimethylvinylsilane, or Cu(hfac)TMVS for FIB-assisted metal deposition was investigated. 50kV Ga+ ions were scanned over a defined area of an Al∕SiO2 resistivity test substrate in the presence of the precursor vapor, using two different 50kV FIB column designs with beam currents from 49to2070pA and current densities of 13–36Acm−2. Resistivity was measured by the four-point probe method. This study verifies prior reported resistivities of ⩽50μΩcm at room T across all deposition parameters for film growth yields ⩽0.18μm3nC−1 ion dose. Depositing on a heated substrate...

X-ray zone plate fabrication using a focused ion beam

An x-ray zone plate was fabricated using the novel approach of focused ion beam (FIB) milling. The FIB technique was developed in recent years, it has been successfully used for transmission electron microscopy (TEM) sample preparation, lithographic mask repair, and failure analysis of semiconductor devices. During FIB milling, material is removed by the physical sputtering action of ion bombardment. The sputter yield is high enough to remove a substantial amount of material, therefore FIB can perform a direct patterning with submicron accuracy. We succeeded in fabricating an x-ray phase zone plate using the Micrion 9500HT FIB station, which as a 50 kV Ga+ column. Circular Fresnel zones were milled in a 1.0-micrometer-thick TaSiN film deposited on a silicon wafer. The outermost zone width of the zone plate is 170 nm at a radius of 60 micrometer. An achieved aspect ratio was 6:1.

Development of void-free focused ion beam-assisted metal deposition process for subhalf-micrometer high aspect ratio vias

Low resistance metal deposition in deep submicron vias is required for circuit rewiring in focused ion beam (FIB)-based integrated circuit modification. Voids in high aspect ratio deposition, associated with the application of traditional FIB process to tungsten deposition in vias with aspect ratios beyond 10:1 contribute substantially to the resistance of the via. Pinch off of the via aperture is frequently observed. The dynamics of tungsten deposition within vias was studied through a series of via cross sections with variable deposition dose, and revealed accelerated deposition growth on the walls at the top of the vias. Accelerated deposition on the sidewalls, where the primary beam interacts with the substrate at a glancing angle, suggested that the deposition growth is initiated by secondary charged particles generated at the point of primary beam impact rather than by the primary beam itself. The results are in agreement with mechanisms previously proposed and confirmed by experiments. In order to ...

Electrical breakthrough effect for end pointing in 90 and 45nm node circuit edit

The interaction between high-energy Ga+ ions and condensed matter is studied for circuit edit applications. A new “electrical breakthrough effect” due to charging of, and Ga+ penetration/doping into, dielectrics is discovered. This new effect is proposed for end pointing in 90 and 45nm node circuit edits where integrated circuit device dimensions are of a few hundred nanometers. This new end point approach is very sensitive, reliable, and precise. Most importantly, it is not sensitive to device dimensions. A series of circuit edits involving milling holes of high aspect ratio (5–30) and small cross-section area (0.01–0.25μm2) on real chips has been successfully performed using the electrical breakthrough effect as the end point method.

End point of silicon milling using an optical beam induced current signal for controlled access to integrated circuits for backside circuit editing

Integrated devices increasingly use flip chip packaging, which complicates modification of the circuitry using focused ion beam (FIB) systems, as the thickness of the silicon must be reduced to a few micrometers before work can begin. A technique for end pointing the milling of bulk silicon from the backside of flip chip devices, using laser illumination to produce an optical beam induced current (OBIC), is described. System design and operation are described and results shown. The OBIC signal varies strongly with the thickness of the silicon above the bulk to well junction—by monitoring the OBIC signal in real time and end pointing on a characteristic feature the technique can reliably end point the bulk-well interface. It is shown that this system represents a significant advance over previous techniques and provides a robust and reliable way of end pointing. Theory of operation and reasons for the improved performance are discussed.

Focused ion beam in-situ cross sectioning and metrology in lithography

We demonstrate an approach to cross-section and measure sub- 0.25 micrometer photoresist profiles in both a manual and an automated fashion. This approach includes the use of a focused ion beam (FIB) system to cut small trenches through photoresist lines, leaving a clean, vertical face to measure. We demonstrate the advantage of using this process over existing techniques in the semiconductor industry. A FIB can locally cross-section the photoresist, resulting in a side- wall that is comparable to that of a mechanical cleave. It can then measure the profile of the photoresist at multiple points using a 5 nm gallium probe. The system accomplishes the entire process inside one vacuum chamber with a limited number of steps. In contrast, when using a SEM to measure profiles, the sample must be mechanically cleaved outside of the vacuum chamber, potentially destroying the entire part and leaving a slightly distorted viewing face. Also, a SEM probe can cause swelling of the photoresist due to higher currents and penetration depths than a FIB probe and must therefore be used at low accelerating voltages. When operated at these low accelerating voltages, the SEM has degraded resolution with a spot size near 10 nm. A scanning probe microscope (SPM), on the other hand, can non-destructively measure profiles, but it is slow and less automated than the FIB or SEM. Unlike a FIB, the SPM lacks the ability to image the material transition directly beneath the photoresist. We also address concerns of sample damage, gallium contamination, and image quality.