Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Chemical analysis
Reexamination Certificate
1999-02-05
2002-06-25
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Chemical analysis
C438S486000, C438S488000, C700S123000
Reexamination Certificate
active
06411906
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a method and system for inspecting a polycrystalline semiconductor film. More specifically, the invention relates to an inspecting method and system for measuring grain sizes of semiconductor grains forming a polycrystalline semiconductor film.
In general, a thin film transistor (TFT) using a polycrystalline silicon (poly-Si) has an advantage in that the crystal has a mobility 10 through 100 times as high as that of a TFT using an amorphous silicon (a-Si). Therefore, there has been studied and developed a drive circuit integrated thin film transistor-liquid crystal display (TFT-LCD) wherein a thin film transistor of a polycrystalline silicon is not only used as a pixel switching element for the liquid crystal display (LCD), but it is also used as a peripheral drive circuit for the liquid crystal display, to form the thin film transistors for the pixel and the drive circuit on the same substrate. In such a drive circuit integrated TFT-LCD, there is a correlation that the mobility of crystals of a thin film transistor of a polycrystalline silicon increases as the crystal grain size increases, as can be seen from a characteristic diagram of
FIG. 1
which shows the relationship between the crystal grain size and mobility. For that reason, there is an important problem of how to measure the crystal grain size.
Conventionally, the crystal grain size is measured by a scanning electron beam microscope (SEM) or the like after a grain boundary is selectively removed by an etching, such as SECO etching, or a cross section taken along a thickness direction of a substrate is observed by a transmission electron beam microscope (TEM). However, in such conventional inspection methods, it takes at least 2 hours to observe the grain size. In order to shorten the measuring time, it has been proposed to use an atomic beam frequency microscope (AFM). Although the grain size can be observed and measured by the AFM, it takes about 30 minutes to observe a one point to analyze the grain size.
In addition, there is well known a basic method for observing a grain size by means of an optical microscope having a magnification of 500 thorough 1000 by using the variation in irregularity on the surface of a film as an index of the grain size. However, since this method is greatly relied on the human naked eye, the measured results are easily influenced by the difference among individuals, so that there are problems in that quite accurate results can not be obtained and the measured results are not quantitative.
Moreover, there may be considered a grain size analysis using an ellipsometer which can measure a grain size in a non-destructive and non-contact manner in a short measuring time of 5 seconds per one point This conventional measuring method using the ellipsometer is used for analyzing an object having a flat surface, such as a silicon oxide film on a single crystal. However, if this method is used for analyzing a polycrystalline silicon, there is a problem in that it is difficult to construct an analysis model, e.g., it is difficult to quantify the grain size, mobility and so forth. Particularly in the case of the conventional ellipsometer measuring method, a polycrystalline silicon produced by the excimer laser annealing (ELA) method has irregularities on the surface even if the thickness thereof is, e.g., about 50 nm, so that it is particularly difficult to quantify the grain size, mobility and so forth. On the other hand, in a polycrystalline semiconductor film inspecting method according to the present invention, a polycrystalline semiconductor film having surface irregularities produced by the ELA is used as a sample, and a polycrystalline semiconductor film is also used as a reference sample.
For example, when 12-inch and 15-inch or more drive circuit integrated thin film transistor-liquid crystal displays are produced, the mean grain sizes of the polycrystalline silicon are suitably 0.25 &mgr;m and 0.45 &mgr;m or more, respectively. Because, in the case of a display size of 12 inches, the mobility of crystals in n-channel lightly doped drain-thin film transistors (n-ch LDD-TFT) having a grain size of 0.25 &mgr;m or less is 100 cm
2
/Vs or less, so that it is difficult to drive a 12-inch class LCD. In addition, because, in the case of a display size of 15 inches, the mobility of crystals in n-channel lightly doped drain-thin film transistors (n-ch LDD-TFT) having a grain size of 0.45 &mgr;m or less is 120 cm
2
/Vs or less, so that it is difficult to drive a 15-inch class or more LCD. Therefore, the polycrystalline silicon films of the drive circuit integrated thin film transistor-liquid crystal displays are preferably prepared so as to have grain sizes of about 0.25 &mgr;m and about 0.45 &mgr;m, respectively. However, it is impossible to accurately measure the polycrystalline silicon films having this range of grain size in a short time.
In conventional methods, e.g., in a method utilizing the relationship between mean grain sizes of a polycrystalline silicon and proportions of compositions of the polycrystalline silicon when the polycrystalline silicon is represented as a mixture of an amorphous silicon and a crystalline silicon (c-Si), as shown in
FIG. 2
, or in a method utilizing the relationship between mean grain sizes of a polycrystalline silicon and proportions of compositions of the polycrystalline silicon when the polycrystalline silicon is represented as a mixture of an amorphous silicon, a polycrystalline silicon and a crystalline silicon, as shown in
FIG. 3
, there is a problem in that there is no repeatability in a sample having a thickness difference of ±5%, so that it is not possible to achieve accuracy which can be utilzed in the measurement of grain sizes. Because both of the thickness and quality of a film are simultaneously calculated as parameters during analysis. That is, if only the thickness of the film is determined before the calculation of the quality of the film, the quality of the film is different from that of an actual sample. Moreover, if the quality of the film is calculated using the determined thickness of the film, it is required to calculate the thickness again using the calculated quality of the film.
In addition, the surface of polycrystalline silicon is coated with a natural oxide film, and a polycrystalline silicon film formed by the ELA method has surface irregularities as an inherent property, so that there is a problem in that conventional analyzing methods for use in polycrystalline silicon film having a flat surface can not be used. An example of such conventional polycrystalline silicon inspecting methods is disclosed in 1992 American Institute of Physics, J. Appl. Phys. 72(8), Oct. 15, 1992, “Comparative study of thin poly-Si films grown by ion implantation and annealing with spectroscopic ellipsometry, raman spectroscopy, and electron microscopy”. It is reported in this literature that the peak width at half height obtained by differentiating, two times, the peak near 4 eV appeared with a dielectric constant (∈) of a polycrystalline silicon is used as a parameter.
However, according to the above described conventional inspecting method, there is a problem in that the grain size of a polycrystalline silicon of 0.8 &mgr;m is estimated as 0.05 &mgr;m. In addition, an object to be estimated must have a grain size of 0.15 &mgr;m or less, and this has a low mobility, so that there is a problem in that this can not be used for forming a TFT channel for a liquid crystal display.
As described above, in any conventional inspection methods, it is difficult to quantify the grain size, mobility and so forth of crystals in analysis of the grain size, and it is difficult to accurately measure the crystal grain size in a short time.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a method and system for inspecting a polycrystalline semiconductor film, which can accurately measure the grain size o
Assouad Patrick
Kabushiki Kaisha Toshiba
Pillsbury & Winthrop LLP
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