It is well known to use a focused ion beam (FIB) for exposing conductors on an integrated circuit (IC) to aid in debug, or failure analysis, and editing of the integrated circuit. See for instance U.S. Pat. No. 6,225,626, issued May 1, 2001, to Talbot et al.; U.S. Pat. No. 5,140,164, issued Aug. 18, 1992 to Talbot et al.; and U.S. Pat. No. 5,616,921, issued Apr. 1, 1997 to Talbot et al., all incorporated herein by reference in their entireties. U.S. Pat. No. 6,225,626 discloses methods for exposing a selected feature of an IC such as a selective conductor, from the backside of the IC substrate without disturbing adjacent features of the device, such as the active (semiconductor) regions. The method includes determining a region of the IC in which the selected feature is located; obtaining from the backside of the IC substrate a near IR (infra-red) optical microscope image of the region; aligning the optical microscope image with a coordinate system of a milling (FIB) system; and using structures visible in the near IR microscope image as a guide (also described in U.S. application Ser. No. 10/159,527 by Madhumita Sengupta and Mamta Sinha), operating the milling system to expose the selected feature from the backside of the IC without disturbing adjacent features.
Other aspects of this approach include forming the trench through the backside of the substrate where the trench may be stepped in cross-section, and milling down most of the way through the substrate from the backside surface thereof to just above the substrate circuit elements. Following this an access hole is milled to to expose ILD0 and the metal interconnections (circuitry) typically formed overlying the principal surface of the substrate. The focused ion beam is then used to cut or reform various portions of these metal layers in order to reconnect the transistors formed within the substrate. This operation requires the local isolation of the routed interconnect prior to its construction by means of the deposition of a dielectric layer, as is described in U.S. Pat. No. 5,357,116 by Talbot et al. issued Oct. 18, 1994. This is used typically for failure analysis, debug, and edit (rework) of ICs. It is most typically used in the research and development and manufacturing engineering stages, but is applicable to the reworking or discretionary rewiring of actual production ICs. This is described in Crawford and PE Kudirka, “Electron Microbeam Testing for Large Microcircuit Arrays” IEEE-Proceedings of the Symposium on Electron, Ion and Laser Beam Technology (1971) 131–140. Credence, assignee of this application and of the above-referenced patents, produces commercially available FIB systems, such as the IDS P3Xd instrument and the IDS OptiFIB instrument. A FIB system typically includes a source of ions, such as gallium ions, suitable magnetic and/or electric lenses for focusing the ion beam, and also a chemical source, injected to enhance milling, such as xenon difluoride (XeF2). The OptiFIB includes the focused ion beam column coaxial with a light microscope for optical observation of the milling process when using near infra-red (as described in US Patent Publication number U.S. 20030102436A1), and for navigation (finding specific locations) across the IC die.
It has been recognized by the present inventors that prior to performing the final circuit editing operations (cutting and/or filling in of the metal layers), the edit is generally more successful if the access trench itself which is formed on the backside surface of the substrate is etched precisely so that the remaining silicon thickness is uniform across the trench floor. In most applications the trench does not extend all the way through the substrate; instead, it extends to within, for instance, 2 to 5 μm of the principal surface of the substrate, leaving an intervening thin layer of, e.g., silicon. The reason for this is that in advanced ICs, the transistor density on the principal surface leaves no room for very large trenching operations. The actual circuit editing is generally performed through this thin layer by the FIB. For instance, a 30 KeV gallium ion beam, assisted by a chemistry such as xenon difluoride (XeF2), or alternatively with a chemistry less aggressive to silicon such as chemicals containing iodine, bromine, or chlorine, can mill a small via (cavity) through the trench floor by locally removing the last few microns of silicon, and can then add dielectric and conductor, thereby performing the circuit (metal layer) edit.
Since this silicon layer is the floor of the trench formed from the backside surface-of the substrate, the present inventors have determined that the floor of the trench should be nearly perfectly flat (smooth) and parallel to the principal surface of the substrate; i.e., it is preferable if during the trenching process the remaining silicon has uniform thickness. A non-uniform trench floor may locally enhance the spontaneous etching, which may result in penetration of the ion beam through the silicon into the device active areas and lead to inaccuracy in the circuit edit. Operations such as the automation of edit steps (see for instance U.S. Pat. No. 6,031,229 by Keckley et al., issued Feb. 29, 2000) would depend on this uniformity. Additionally, non-uniformity affects the aspect ratio of the access hole milled through the remaining silicon, and may ultimately influence the success or yield of the attempted edit. Non-uniformity may occur, by way of example, due to the fast silicon removal rate by the assist chemistry which is typically injected into the FIB milling (etching) region. Often the detected non-uniformity is the result of a tilting of the floor of the trench due to the nature of the FIB chemical gas delivery injector.
It is believed that others in the field have not adequately recognized the significance of having such a uniform trench floor or at least of having its non-uniformity determined. This has been found by the present inventors to be a significant shortcoming of FIB trench etching processes. In addition to its importance in circuit editing accuracy, thickness uniformity of the trench floor is also critical for enabling accurate navigation by methods such as IR imaging navigation (as described by Talbot et al in U.S. Pat. No. 6,518,571) or voltage/materials contrast navigation, as described in U.S. patent application Ser. No. 10/789,336. In particular, voltage contrast navigation requires etching quite close to the active regions: therefore uniformity of the trench bottom to the die surface is even more important, so as not to damage the active regions. Potentially, as dimensions continue to decrease, milling down to the actual diffusions may be required to achieve sufficient navigation and editing accuracies, which will place even a more stringent requirement on uniformity of remaining substrate thickness.