Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2000-10-06
2003-06-03
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S311000, C250S306000, C250S492300, C324S754120, 43
Reexamination Certificate
active
06573499
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to microstructured pattern inspection method, particularly, to a method of inspecting the microstructured patterns, such as contact holes and linear patterns, that are formed on semiconductor wafers with the photolithography that uses an optical exposure apparatus such as a stepper.
In the manufacture of semiconductors, photolithography is used to form patterns on semiconductor wafers. The formation of these patterns most commonly uses the reduction projection alignment method that applies an apparatus in which a reticle formed by enlarging the circuit patterns for several chips is used for reduction projection alignment (hereinafter, this apparatus is referred to as the stepper). In the reduction projection alignment method using the stepper, a reduced image of the mask pattern of the reticle is exposed to light so as to be projected and formed on the photoresist coating of the wafer, with the result that a resist pattern, a copy of the reticle mask pattern, is formed on that wafer by processing chemically the photosensitized photoresist coating. The patterns for several chips that have been formed on the reticle can be copied with a single shot (exposure). This procedure is “stepped and repeated” to copy more such patterns on the wafer.
An example of forming contact holes on the insulating film of the wafer is described below. First, a photoresist coating is formed on the insulating film. Next, the photoresist coating undergoes exposure using a reticle provided with a pattern of contact holes of the design size and arrangement, and then undergoes chemical processing. After this, contact hole patterns passing through the insulating film can be formed on the wafer by performing processes such as etching, and in this etching process, the photoresist coating that has been created by copying the required pattern functions as a mask.
To ensure that the stepper forms patterns on the wafer as described above, the microstructured patterns on the dimensionally enlarged reticle must undergo reduction projection alignment on the wafer through projection optics. The surface and bottom of the exposed layer (photoresist coating) of the patterns that have been exposed to light in the reduction projection alignment process occasionally differ in size, shape, position, and other factors.
The first main cause of these differences is a combination of defects in the wafer material and defects in the workmanship of the substance exposed to light on the wafer, such as a resist. The warping, distortion, deflection, and the like, of the wafer itself can occur during its manufacture or according to the subsequent elapse of time or the particular ambient environmental conditions such as temperature, and these defects affect optical interference. The shapes of the patterns formed will also be affected by the nature of the substance to be used as a resist, and by the resist coating thickness, coating status, and other factors. Such deviations (from design specifications) in terms of the forming positions and dimensions at the exposed surface and bottom of the microstructured patterns due to the characteristics of the exposed substance (hereinafter, these deviations are collectively called “dislocations”) are usually distributed over a wide range in a specific area of the wafer, and with a fixed tendency.
The second main cause is such insufficiency in the performance of the optics used in the stepper as schematized in FIG.
2
. As shown in FIG.
2
(
a
), no problems occur in the vicinity of the reticle center. As shown in FIG.
2
(
b
), however, if light is emitted obliquely to the surface of the wafer, lens aberration, such as astigmatism or comatic aberration, will occur at the edges of the reticle. Dislocation due to such aberration mainly appears within a single-shot area, radially from its central position and with a fixed tendency.
The third main cause is a defect in the nature of the optics of the stepper, that is to say, a shift in focal position (defocusing), which arises from the fact that the lenses in the optics used for exposure suffer deformation due to the heat generated during exposure (this event is called “lens heating”).
The fourth main cause is a defect in the performance of the optics of the stepper. If the optics of the stepper has any inclined parts such as lens, since the emitted light enters laterally, the exposure pattern within a single-shot area skews in a fixed direction.
Differences between the design specifications and actually formed patterns are mainly caused by the four factors described above. The first problem resulting from these differences is that dislocation occurs between the patterns that were formed on the surface and bottom of the photoresist. Similarly, there also occurs misalignment with respect to the pattern in the lower layer or upper layer of the insulator, due to the axial and position offsets between the design specifications and actually exposed patterns. Axial or position offsets in contact hole patterns reduce the area of the hole, thus increasing electrical resistance, and finally leading to deteriorated semiconductor performance. In some cases, the semiconductor loses electroconductivity, which is a critical defect in the semiconductor device itself.
With respect to these problems, at present, exposure accuracy at the bottom area of the microstructured patterns is usually evaluated by calculating the area of the bottom. However, there is no established method for evaluating quantitatively the optics of the stepper, the wafer, or the like, from the quantity or direction of pattern dislocation or from these factors.
SUMMARY OF THE INVENTION
The present invention is therefore intended to provide a method of evaluating each microstructured pattern of a semiconductor by calculating as a dislocation vector the relationship in position between the surface and bottom of the photoresist on the microstructured pattern. The present invention is also intended to provide a method of evaluating exposure accuracy quantitatively on a single-shot, single-chip, or wafer-by-wafer basis, or a method of evaluating each section of the pattern exposure system, detecting abnormality, and issuing a related warning.
During microstructured pattern evaluation based on the present invention, the formation status of the patterns on the surface of the exposed layer (hereinafter, simply called the surface layer) and at the bottom of the exposed layer (hereinafter, simply called the bottom layer) and the relationship in position between the surface layer and the bottom. layer are analyzed, then the relative dislocation between both layers is calculated as a dislocation vector, and this vector is displayed on the screen of the corresponding apparatus. Also, a warning will be issued if the dislocation vector oversteps the dislocation tolerance that has been set beforehand. In addition, the exposure system, the wafer, and other targets can be evaluated by classifying calculated characteristic quantities according to the particular tendency and characterizing each single-shot, single-chip, or wafer area.
That is to say, according to the present invention, the microstructured pattern inspection method for inspecting the microstructured patterns formed on the thin coating of a substrate through pattern optical exposure is characterized in that said inspection method comprises a process for acquiring images of the microstructured patterns formed on said thin coating, a process for identifying both the shape of the microstructured pattern on the surface of said thin coating and the shape of the microstructured pattern at the bottom of said thin coating, from said images, and a process for detecting the dislocation between the two microstructured patterns that have been identified in the third process mentioned above. The shapes of the microstructured patterns can be identified by detecting the profiles of the patterns.
For circular microstructured patterns such as contact hole patterns, misalignment between the gravity center of t
Komuro Osamu
Mizuno Fumio
Sasajima Fumihiro
Anderson Bruce
Hashmi Zia R.
Hitachi , Ltd.
Kenyon & Kenyon
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