Semiconductor device manufacturing: process – With measuring or testing
Reexamination Certificate
1998-04-20
2001-08-14
Nguyen, Ngoc-Yen (Department: 1754)
Semiconductor device manufacturing: process
With measuring or testing
C438S462000, C438S712000, C438S735000, C438S975000, C438S719000
Reexamination Certificate
active
06274393
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to a method for measuring submicron images which are currently not measurable or resolvable by any known prior art technique.
As semiconductor features continue to shrink and the subtleties of pattern densities become apparent (e.g. photoresist thickness variations across a chip), the ability to determine the quality of the photoresist features, especially for vias and contacts, becomes increasingly difficult. Is the image open at the bottom? What is the size of the image at the bottom? These are the important questions which are answered by the present invention. As semiconductor images shrink to the sub-micron regime, the ability to determine masking image quality becomes increasingly difficult.
2. Prior Art
Photoresist patterns on semiconductor wafers are typically about 1.0 micron thick. As horizontal images drop below 0.5 microns, this creates a submicron aspect ratio of 2:1 or higher. An important factor in the success of transferring the photoresist image into the semiconductor lies in the ability to determine the image size at the interface between the photoresist and the semiconductor (the “bottom” of the photoresist image).
Currently, low voltage scanning electron microscopes (SEMS) are used to measure these images. Unfortunately, submicron high aspect ratio images in the photoresist pattern create structures which do not allow repeatable accurate measurements using this technique.
Standard practice includes running a wafer through an etch step and a resist removal step, and then measuring the resulting etched image. Downsides of this procedure include 1) the time lost while the wafer is processed, 2) if the images are out of specification the wafer must be discarded, and 3) it does not accurately determine if the photoresist image is completely open.
An alternative method is disclosed in Tiro-Lira et al. U.S. Pat. No. 5,493,116 wherein an SEM is modified to include two detectors which work in conjunction to provide an improved depth of field using backscatter emission. Although this technique may work with 0.5 micron images in 2 micron thick resist (as described in the patent), as images reduce even further this technique may soon become too limited.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide a method for measuring submicron (below one micron) images.
A further object of the subject invention is the provision of a method which qualitatively indicates whether an image is “open” at the bottom thereof, as well as a structure which allows accurate measurements of currently unresolvable images.
The present invention provides several advantages relative to the standard prior art practice of running a wafer through an etch step and a resist removal step, and then measuring the resulting etched image. The present invention provides advantages of eliminating time lost while a defective wafer is processed, eliminating the waste of discarded wafers if the images are out of specification, and also accurately determines if the photoresist image is completely open.
In accordance with the teachings herein, the present invention provides a method of processing a semiconductor wafer for measuring submicron images thereon. A mask layer having openings therein is deposited onto the wafer, and a test region of the wafer is marked through the openings in the mask layer. The test region is subsequently inspected to detect for the presence of the markings thereon.
In a preferred embodiment, the mask layer is a photoresist layer, although in alternative embodiments the mask layer could be provided as an insulator mask layer or a metal mask layer.
In a preferred embodiment, the marking transfers an image of the bottom of the mask layer into the substrate by etching, such as by rastering a focused ion beam over openings in the mask layer in the presence of an etchant gas in a nonactive region (kerf) of the wafer. This method provides an etched mark in the wafer defined by the passage of the focused ion beam through the mask opening. The etchant is chosen to provide high selectivity between the substrate material and the mask material, thus perturbing the mask material as little as possible. In alternative embodiments, the marking could be performed by transferring an image of the bottom of the mask layer into the substrate by staining or dyeing.
The marking preferably comprises a directional marking, such as provided by the focused ion beam, to provide a measurement of the size of the opening in the mask layer. The marking is preferably performed in a region of the wafer which is not otherwise used to form active a circuit chips in the wafer, and also is preferably performed in several different test areas of the wafer.
In a preferred embodiment, following the marking step, the mask layer is removed from the test region to facilitate inspection of the wafer while leaving the mask layer in place over remaining portions of the wafer. The mask layer can be removed from the test region by a focused ion beam or by laser ablating, to facilitate a measurement of the dimensions of the transferred image. In one disclosed embodiment, the mask layer is removed by a focused ion beam in the presence of water vapor to enhance removal of the photoresist.
In one disclosed embodiment, a Focused Ion Beam (FIB) in the presence of an etchant gas such as Br/XeF2, is rastered over a submicron image in the kerf, to transfer an image of the bottom of the photoresist pattern into a silicon wafer substrate. Subsequent ablation of the photoresist in the rastered area facilitates a measurement of the transferred image. This inspection process can be performed on a product without sacrificing the wafer. Alternative methods of marking the wafer include staining or dyeing, for example adding a stain to the developing solution. Whatever method is selected, the marking transferred to the wafer must be unaffected by the subsequent process of removing the photoresist.
The present invention provides a novel physical transference of an image of the bottom of the photoresist pattern by an etchant technique or by dyeing or staining, such that the transferred image can be subsequently seen in a particle beam (FIB, SEM, etc). A localized area of the photoresist pattern may need to be removed (as by FIB, laser ablation, etc.) in order to provide for the subsequent inspection/measurement step.
The present invention provides a method for inspecting a photoresist pattern on a semiconductor wafer which comprises providing a photoresist mask layer on a wafer having images therein, marking at least one test region of the wafer through the photoresist mask layer in a portion of the wafer which is not otherwise used in a circuit formed in the semiconductor wafer, then removing the mask layer from the test region while leaving the mask layer in place over remaining portions of the wafer, and inspecting the test region to determine whether the wafer was actually marked to ensure that the photoresist mask images are properly formed and open to the bottoms thereof.
In greater detail, the mask layer can be a photoresist, and the images can include contacts. The etching step can be performed in a FIB tool. The mask can be removed with ions in the presence of water to enhance the removal of photoresist with respect to other layers. The etch detecting step measures the image transferred to ensure that the mask layer is properly opened.
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patent: 5711848 (1998-01-01), Iturralde
International Business Machines - Corporation
Leas James M.
Nguyen Ngoc-Yen
Scully Scott Murphy & Presser
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