Radiant energy – Inspection of solids or liquids by charged particles – Methods
Reexamination Certificate
1998-10-19
2001-06-26
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Methods
C250S310000
Reexamination Certificate
active
06252227
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to manufacturing a semiconductor wafer. More specifically, it relates to a method for preparing a semiconductor wafer for viewing with an SEM at a selected site on the wafer
INTRODUCTION
A semiconductor wafer is often tested by sectioning the wafer and then observing the section with a scanning electron microscope (SEM). The SEM produces a beam of electrons that is focused on a point in the section, and electrons that are reflected from the point are detected by an electron detector.
The electrons reflect differently from different materials, primarily according to whether a material is a conductor or non-conductor, and in this way the electrons provide an image of the point where they strike the section. The electron beam is scanned across the section in a way that is analogous scanning by a television camera, and a cathode ray tube (CRT) displays the detector signal in synchronism with the SEM beam.
Commonly this sectioning and scanning is used to observe a test wafer after one of the steps in processing the wafer. The resulting image shows for example the layers of metal and oxide, and this information helps developers to understand the preceding steps of the semiconductor manufacturing process.
A focused ion beam apparatus (FIB) can be used for cutting the wafer section. A beam of metal ions is directed at the surface of the silicon and the beam cuts away atoms in its path. The ion beam is located normal to the surface of the wafer to form a cut with vertical walls. Secondary electrons are produced when the ions strike the silicon, and these secondary electrons are used to form an image in a way that is analogous to the way the image is produced with an SEM. The ion beam is swept across the surface of the wafer to cut a desired shape, and the secondary electrons are detected and the detector signal is displayed on a cathode ray tube. The view on the CRT guides the person making the section cut with the FIB apparatus.
Using FIB apparatus has the advantage that the section can be made as a small hole in the wafer. The SEM and FIB are combined so that the section can be viewed as soon as the section has been cut. The wafer process can then be continued and a different section can be formed to view the effect of a later process step.
THE PRIOR ART
U.S. pat. No. 5,093,572 teaches the system described in the introduction to this specification.
SUMMARY OF THE INVENTION
One object of this invention is to provide a new and improved method for sectioning a wafer that is to be viewed with an SEM. A more specific object is to improve the image formed by the FIB sectioning apparatus so that an operator is less likely to over cut the wafer beyond the line where the wafer is to be viewed.
Wafers commonly have a layer of silicon dioxide (called oxide) located directly over the substrate. Oxide is also used to fill between conducting lines or nodes to isolate these conductors.
We have found that the ion beam splatters this oxide and that deposits of the oxide are left on the wafer and interfere with the image during the step of cutting the section with the FIB apparatus.
Our new method can be summarized in four steps.
First, the wafer is capped with a layer of a passivation, preferably polysilicon or silicon nitride (Si3N4).
In a second step, a small hole is cut where the sectioning cut with the FIB apparatus is to be made later. This hole is cut from the upper surface of the wafer to the oxide overlying the substrate. This step exposes the oxide so that it can be etched away in step three. Preferably, a laser is used to make this cut.
In step three, the oxide is etched with an SiO2 etcher. This step exposes the silicon substrate underlaying the oxide.
The fourth step is the sectioning cut with FIB apparatus. During this step, the view of the operator is much clearer than in the prior art and the cut can be made more accurately. As one feature of this step, the FIB apparatus is operated with a low current.
Other advantages and features of the invention will become apparent from the drawing and the description of a preferred embodiment of our invention.
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Chen At Chuan
Chen Huey Ling
Chen Mei Fun
Tseng Fouriers
Ackerman Stephen B.
Berman Jack
Robertson William
Saile George O.
Smith II Johnnie L
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