Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2000-06-01
2002-07-16
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S252100, C250S306000, C250S492100, C156S922000
Reexamination Certificate
active
06420703
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method for forming a calibration standard and standard formed and more particularly, relate to a method for forming a critical dimension scanning electron microscope (SEM) calibration standard by a focused ion beam (FIB) technique and the standard formed.
BACKGROUND OF THE INVENTION
In the study of electronic materials and processes for fabricating such materials into an electronic structure, a specimen of the electronic structure is frequently used for microscopic examination for purposes of failure analysis and device validation. For instance, a specimen of an electronic structure such as a silicon wafer is frequently analyzed in scanning electron microscope (SEM) and transmission electron microscope (TEM) to study a specific characteristic feature in the wafer. Such characteristic feature may include the circuit fabricated and the defect formed during the fabrication process. An electron microscope is one of the most useful equipment for analyzing the microscopic structure of semiconductor devices.
In preparing specimens of an electronic structure for electron microscopic examination, various polishing and milling processes can be used to section the structure until a specific characteristic feature is exposed. As device dimensions are continuously reduced to the sub-half-micron level, the techniques for preparing specimens for study in an electron microscope have become more important. The conventional methods for studying structures by an optical microscope cannot be used to study features in a modern electronic structure due to its unacceptable resolution.
In a focused ion beam (FIB) technique, focused ion beams are used to either locally deposit or remove materials. When the cluster impacts the surface of an electronic structure, the cluster disintegrates into atoms which are then scattered over the surface to remove a surface layer of the material. Typical ion beams have ea focused spot size of smaller than 100 nm when produced by a high intensity source. Sources of such high intensity ions can be either liquid metal ion sources or gas field ion sources. Both of these sources have a needle type form that relies on field ionization or evaporation to produce the ion beam. After the ion beam is produced, it is deflected in a high vacuum and directed to a desired surface area. The focused ion beams can be suitably used in the semiconductor processing industry in a cutting or attaching method to perform a circuit repair, a mask repair or a micromachining process. A cutting process is normally performed by locally sputtering a surface with a forced ion beam.
In an ion beam milling process, when a material is selectively etched by a beam of ions such as Ga+ focused to a sub-micron diameter, the technique is often referred to as focused ion beam etching or milling. FIG milling ins a very useful technique for restructuring a pattern on a mask or an integrated circuit, anc for diagnostic cross-sectioning of microstructures. In a typical FIB etching process, a beam of ions such as Ga+ is incident onto a surface to be etched and the beam can be deflected to produce a desirable pattern. The focused ion beam can be used to bombard a specimen surface such that a cavity can be formed on the surface of an electronic surface to review a characteristic feature of the structure for electron microscopic examination.
The FIB technique utilizes a primary beam of ions for removing a layer of material at a high current, and for observing the surface that was newly formed at a low current. The observation of the surface is made by detecting the secondary electrons emitted from the sample surface when the surface is bombarded by the ions. A detector is used to receive the secondary electrons emitted from the surface to form an image. The FIB method, even though can not produce an image of a high resolution like that obtainable in a SEM/TEM, can be used to sufficiently identify a newly formed cross-sectional surface which may contain the characteristic feature to be examined. The capability of the FIB technique for making observations down to a resolution of 5~10 nm enables the cutting of a precise plane in an electronic structure such that it may later examined by a SEM or TEM technique at a higher resolution than that capable with FIB.
In modern ULSI semiconductor devices, particularly in sub-0.18 nm devices, metal lines used for connecting devices on a chip becomes extremely thin such that the use of SEM for analysis is frequently required. When a SEM is utilized for analyzing a high density IC device, it can be used for either measurement or for analysis. A SEM that is used for measurements of critical line width is frequently called CD-SEM wherein CD stands for critical dimension. The major functions for a CD-SEM is to perform a critical dimension measurement of important material layers such as SiN, polysilicon, contact window and metal connecting lines. This is because the line width of these layers has a great influence on the properties of the IC device. Frequently, In-line CD-SEM utilizes field emission electron gun and operates under low acceleration voltage, i.e. lower than 1 kV, such that the electrical properties of the IC device is not damaged. A CD-SEM apparatus is constructed in a complex manner to fulfill its high accuracy and measurement stability. As a result, the cost of a CD-SEM apparatus is significantly higher than a traditional SEM apparatus.
Before a critical dimension on a semiconductor device can be determined by the CD-SEM technique, the CD-SEM apparatus must first be calibrated by a calibration standard of known line width. Such calibration is frequently carried out by using polysilicon lines deposited of a known width and thickness on a semiconductor substrate. In a conventional calibration standard of polysilicon lines, the standard is produced by first sputter depositing a polysilicon layer on the substrate, then photo-masking in a photolithography process defining the lines. The polysilicon lines are then etched in a dry etching or a wet etching process and the photomask is then removed. A typical calibration standard using polysilicon lines is shown in FIG.
1
.
The polysilicon lines
12
which are deposited on a substrate
10
have a width of approximately 0.3 nm and a thickness of approximately 0.5 nm. Since the etching process, regardless a dry etching or a wet etching process, never produces a vertical sidewall, the white lines
14
indicate a tapered sidewall. In the formation of the polysilicon lines
12
, two major problems are observed. First, a poor line width uniformity is normally obtained. For instance, as shown in
FIG. 1
, a line width uniformity of about 14 nm is observed in a single line, and a line width uniformity of about 0.01 nm with a 3 sigma is observed in 20 measurements. It is therefore possible to obtain a line width that has better uniformity.
A second problem encountered in forming a CD-SEM calibration standard with polysilicon lines is the roughness of the line edge obtained. For instance, as shown in
FIG. 1
, the polysilicon line
12
has a typical line edge roughness of about 20 nm in a 0.5 &mgr;m length. The cause of the line edge roughness is the large grain size of the photomask utilized which limits the line edge obtainable. It is therefore clear that, as long as the photomasking step of using large grain photomask is necessary, it is not possible to obtain line edge roughness of smaller than 20 nm in a 0.5 &mgr;m length.
It is therefore an object of the present invention to provide a CD-SEM calibration standard that does not have the drawbacks or shortcomings of the conventional calibration standard utilizing polysilicon lines formed by a photolithographic method.
It is another object of the present invention to provide a CD-SEM calibration standard that can be fabricated without using a photolithographic method.
It is a further object of the present invention to provide a CD-SEM calibration standard by a focused ion beam deposition technique.
It is
Chou Ming-Chun
Wu Chia-Fang
Lee John R.
Taiwan Semiconductor Manufacturing Co. Ltd
Tung Randy W.
Vanore David
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