Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1998-10-09
2001-07-31
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
Reexamination Certificate
active
06268608
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to chemically enhanced ion beam etching, and in particular, to using a focused ion beam to selectively etch inter layer dielectrics deposited during integrated circuit fabrication.
Integrated circuits are fabricated by growing, depositing, diffusing, and etching thin layers of conductors, insulators, and semiconductors onto a substrate of a semiconductor material, such as silicon or gallium arsenide wafer. To keep the fabrication processes operating properly, or to diagnose and correct the process when a defect does occur, process engineers must be able to quickly examine the various processed layers.
A primary tool used for examining, analyzing, and repairing processing layers is a focused ion beam (FIB) system. FIB systems improve manufacturing yields by identifying and analyzing defects on in-process wafers, allowing the source of defects to be located and corrected. For example, layers can be sputter-etched by an FIB system to expose underlying layers for observation and testing, or cross sections can be cut to expose the edges of multiple layers to observe layer thickness, uniformity, and inclusions. FIB systems can also form images of micrcscopic features and can be used to repair or test integrated circuits by depositing conductive or insulating material.
The processing layers exposed by the removal of covering material using FIB etching can be examined either using the imaging capability of the FIB system, or using a scanning electron microscope (SEM). The electron beam of an SEM causes less sample damage than does the ion beam of an FIB system, and the SEM is typically capable of forming a higher resolution image. SEMs are often available within the same vacuum chamber as an FIB system, such as in the DualBeam™ family of FIB Systems from FEI Company, the assignee of the present invention. In such a system, a cross section of the processing layers can be milled and then observed within the same vacuum chamber, with little or no movement of the sample. Such a system is particularly well suited to process control applications, where specimens must be analyzed quickly to provide feedback to a production line.
Many of the layers in an integrated circuit are composed of relatively non-conductive materials that are used to separate conductive layers or as passivation and protection Layers for the chip. Such layers are known as interlayer dielectrics (ILDs). ILDs include deposited oxides of various densities, thermal oxides, spun on glass, and nitrides. When ILDs are cross-sectioned with a focused ion beam and viewed, it is often impossible to distinguish among them. Thus, individual layer thickness cannot be determined and process engineers cannot isolate defects to a particular layer.
To distinguish between different ILD layers, it has been necessary to remove the specimen from the vacuum chamber and etch it in a bath of wet chemicals, such as ammonium fluoride (NH
4
F) and hydrofluoric acid (HF), or a combination of NH
4
F, HF, and acetic acid. The wet etching process etches the various layers slightly differently, so that upon rinsing, cleaning, and re-inserting into a vacuum chamber, the different layers can be viewed. Unfortunately, the time required to perform the multitude of steps involved in this process makes it unsuitable for real-time process control. Moreover, the etching of a chemical bath cannot practically be limited to the area of interest; the entire wafer must be etched to increase the contrast in a cross section of a single device of interest.
It has also been found that plasma etching using gases such as CF
4
and C
4
F
8
, enhances the contrast between the layers that were exposed by focused ion beam milling. Plasma etching is performed in a plasma chamber associated with a plasma-generating device. As in the wet chemical process described above, it is necessary to remove the specimen from the FIB vacuum chamber, place it in the plasma chamber for etching, and then place it in another high vacuum imaging instrument, such as a scanning electron microscope, for observation. The time required to switch between machines makes the plasma etching process for contrast enhancement unsuitable for production support when process engineers need answers quickly to keep a fabrication line running smoothly.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved method and apparatus to selectively etch materials using a charged particle ion beam.
It is an object of the present invention to provide an improved method and apparatus to selectively etch ILD materials using a charged particle beam.
It is another object of the invention to provide additional compounds for charged particle beam etching by modifying etchant compounds to increase their adsorption onto the surface.
It is a further object of the invention to provide an improved method and apparatus to distinguish ILD layers in an integrated circuit cross section milled by a focused ion beam.
It is yet another object of the present invention to provide such a method and apparatus that does not require the specimen to be removed from the vacuum chamber.
It is still another object of the present invention to provide an improved method and apparatus for defect analyses in semiconductor integrated circuits.
It is yet a further another object of the present invention to provide an improved method and apparatus for process control in integrated circuit semiconductor manufacturing.
It is still a further object of the invention to provide rapid analysis of semiconductor processing steps by selectively delineating or removing dielectric layers.
In accordance with the invention, molecules of an etch-assisting gaseous compound are adsorbed onto the surface of a specimen in a charged particle beam system. The gas causes different materials on the specimen to be etched at different rates in the presence of the charged particle beam. Such selective etching provides an observer with a sharp, clean cross section that allows the various layers in the cross section to be distinguished by an observer. The selective etching also allows the removal of some materials without significantly affecting other materials on a sample.
A molecule of the etch-assisting gaseous compound preferably includes an etching portion and a functional group to increase the stickiness of the molecule and enhance adsorption. It is believed that the gas is adsorbed onto the surface of the exposed layers and the charged particle bombardment provides energy to initiate a reaction of the adsorbed gas molecule with the surface material to be etched. The reaction produces volatile products that dissipate in the vacuum chamber, thereby removing material from or etching the specimen.
The etch rate is thought to vary for different materials because the strength of the etch reaction may vary with different materials, the sticking coefficient of the gas may be different for different materials, and the reaction products may be different and have different degrees of volatility. The gas may inhibit the etching of some materials by producing a reaction product that is not volatile and that forms a protective film over the second layer.
A preferred gaseous compound for practicing the invention comprises a halogenated hydrocarbon with an added functional group to enhance adsorption. For example, 2,2,2-trifluoroacetamide selectively etches ILD layers so that they can be distinguished by an observer using SEM or FIB imaging, yet forms a protective film that inhibits further etching on silicon, either single- or poly-crystalline, and metallic layers.
In one preferred application, a cross section of the various layers of an integrated circuit is exposed using a liquid metal gallium focused ion beam. After the cross section is exposed, the specimen is tilted and the exposed cross section is ion-beam etched in a second etch step while a gas, such as 2,2,2-trifluoroacetamide, is directed at the surface. The gas preferentially assists the ion beam etching, thereby increasing the con
FEI Company
Nguyen Kiet T.
Scheinberg Michael O.
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