Radiant energy – Luminophor irradiation
Reexamination Certificate
2002-11-19
2004-02-17
Gagliaroi, Albert (Department: 2878)
Radiant energy
Luminophor irradiation
C250S459100
Reexamination Certificate
active
06693286
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for evaluating the quality of a semiconductor substrate represented by a silicon wafer such as an epitaxial wafer or the like, namely, for quantitatively evaluating impurities, defects and the like existing in a semiconductor substrate.
2. Description of the Related Art
Up to now, as an evaluation method of this kind, there has been disclosed a method for evaluating an epitaxial wafer used for a light emitting device, which method irradiates an epitaxial wafer for a light emitting device being a compound semiconductor with an excitation light, detects a photoluminescence light generated by excitation of carriers in an active layer of this wafer, and derives a non-radiative life time from a speed of variation in intensity of a photoluminescence light when the speed of variation in intensity of the photoluminescence light comes to be a fixed value (Japanese Patent Laid-Open Publication No. 2000-101,145).
In a method for evaluating an epitaxial wafer used for a light emitting device, composed in such a manner, since a non-radiative life time is a physical property value independent of an excited-carrier density, a good correlation with a luminous efficiency is kept with respect to a high-brightness LED having a high excited-carrier density. As the result, since it is possible to accurately and easily measure a non-radiative life time in an active layer without depending upon an excited-carrier density, it is possible to surely select an epitaxial wafer having a high luminous efficiency and improve the yield rate of manufacturing epitaxial wafers.
And there has been disclosed a method for evaluating a semiconductor device by measuring the decay time constant of a photoluminescence light on the basis of the photoluminescence light obtained by irradiating a semiconductor layer with a pulse light as applying a bias voltage in the forward direction between a p-type clad layer and an n-type clad layer, said semiconductor layer having a smaller band gap than the p-type clad layer and the n-type clad layer and being interposed between the p-type clad layer and the n-type clad layer (Japanese Patent Laid-Open Publication No. Hei 10-135,291 (1998/135,291)). This evaluation method computes the decay time constant of said photoluminescence light by subtracting the intensity of luminescence obtained by applying a bias voltage without irradiating an excitation light from the intensity of said photoluminescence light.
A semiconductor device evaluating method composed in such a manner is suitable for a semiconductor device having a pn junction, particularly a light emitting device such as an LED, a compound semiconductor laser and the like, and obtains the decay time constant of a photoluminescence light of a light emitting device by subtracting the intensity of a photoluminescence light reduced in influence of the inclination of an energy band of a semiconductor layer by applying a bias voltage in the forward direction to a pn junction without being irradiated with an excitation light from the intensity of a photoluminescence light when being irradiated with an excitation light. As the result, since even when the intensity of excitation varies, the inclination of an energy band and the decay time constant are little varied and the decay time constant can be measured more accurately, it is possible to improve the inspection of a light emitting device in accuracy and earlier detect the cause of a defect.
On the other hand, there has been disclosed a method for evaluating the life time of a semiconductor surface, said method evaluating the life time of a semiconductor thin layer or an area near it from the intensity of a light having a specific wavelength generated by recombination of electron-hole pairs generated near the surface of the semiconductor thin layer formed on the main surface of a semiconductor substrate by means of an excitation light having a larger energy than the band gap of the semiconductor to be inspected (Japanese Patent Laid-Open Publication No. Hei 8-139,146 (1996/139,146)); In this life time evaluating method, said light having a specific wavelength emitted by recombination of electron-hole pairs is a band-edge recombination and the area of depth in which electron-hole pairs are generated can be selectively changed by selection of the wavelength of said excitation light. And as its semiconductor substrate, a crystal of 0.1 &OHgr;cm or less in resistivity is used in order to make the diffusion length of carriers comparatively short and the intensity of band-edge recombination stronger.
In a method for evaluating the life time of a semiconductor surface composed in such a way, since the area of depth in which electron-hole pairs are generated can be selectively changed by selecting the wavelength of an excitation light, it is possible to selectively evaluate only the life time of a semiconductor thin layer or the life times of both a semiconductor thin layer and a semiconductor substrate.
However, said existing method for evaluating an epitaxial wafer for a light emitting device disclosed in Japanese Patent Laid-Open Publication No. 2000-101,145 has a disadvantage that although a light emitting device is conceived to emit light in the irradiation domain of an excitation light due to a sufficiently short life time in the order of nanoseconds of an epitaxial wafer (compound semiconductor) used in the light emitting device, in a semiconductor substrate such as an indirect band gap silicon substrate or the like having a long life time in the order of microseconds, an accurate life time cannot be measured without considering the diffusion of carriers excited in a semiconductor substrate by irradiating it with an excitation light.
And said existing method for evaluating a semiconductor device disclosed in Japanese Patent Laid-Open Publication No.Hei 10-135,291 (1998/135,291) has a problem that since an object of measurement is a compound semiconductor having a double hetero-structure having a short decay time constant, although the decay time constant can be obtained with a comparative accuracy by applying a bias voltage in the forward direction to a pn junction and thereby reducing the influence of the inclination of an energy band of a semiconductor layer, in a semiconductor substrate such as an indirect band gap silicon substrate or the like having a long life time a decay time constant cannot be accurately measured without considering the diffusion of carriers excited in the semiconductor substrate.
Further, said existing method for evaluating the life time of a semiconductor surface disclosed in Japanese Patent Laid-Open Publication No.Hei 8-139,146(1996/139,146) has a disadvantage that a domain of depth in which electron-hole pairs are generated cannot be controlled even by changing the wavelength of an excitation light.
That is to say, electron-hole pairs generated in a semiconductor thin layer by being irradiated with an excitation light have a finite life time, they sometimes diffuse and recombine and thereby emit light outside the area irradiated with an excitation light. As the result, even when changing the wavelength of an excitation light, only the area irradiated with an excitation light does not necessarily emit light, and the domain of depth in which electron-hole pairs are generated cannot be controlled.
An object of the present invention is to provide a method and an apparatus for evaluating the quality of a semiconductor substrate, which can accurately evaluate impurities, defects and the like in a semiconductor substrate by obtaining quantitatively the life time of a semiconductor substrate having a long life time without breaking and touching the semiconductor substrate.
Another object of the present invention is to provide a method for evaluating the quality of a semiconductor substrate, which can obtain a photoluminescence light intensity having a positive correlation with the life time of a thin film layer or a bulk substrate and can accurately
Hasegawa Takeshi
Ito Terumi
Shiraki Hiroyuki
Gagliaroi Albert
Mitsubishi Materials Silicon Corporation
Reed Smith LLP
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