Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-03-19
2001-01-09
Saadat, Mahshid (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S428000, C257S436000
Reexamination Certificate
active
06172408
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a radiation-sensitive semiconductor device comprising a semiconductor body including a substrate carrying a semiconductor layer, which is thin relative to the substrate and which comprises a radiation-sensitive semiconductor material, in which semiconductor layer a semiconductor element having electrical connections is formed which is sensitive to electromagnetic radiation incident on the surface of the semiconductor body, and radiation-reflecting means being provided between the substrate and the semiconductor layer. The invention also relates to a method of manufacturing such a device.
Such a device can very suitably be used as a detector in various optical systems, such as an optical glass fiber communication system or an optical disc system. By virtue of the small thickness of the semiconductor layer and the presence of the reflection means, the radiation-sensitive element can be of the so-called resonant-cavity type, that is the radiation to be detected traverses the radiation-sensitive layer a number of times. As a result, also radiation which is only very little absorbed by the radiation-sensitive semiconductor material can be satisfactorily detected. By virtue of the small thickness of the semiconductor layer, the semiconductor element formed therein still has a high response rate. The response of such a device exhibits a periodical variation in the wavelength domain. This enables the response to be accurately attuned to the wavelength of the radiation to be detected. If this wavelength is situated in or near the visible part of the spectrum, the device is hardly sensitive to (visible) background radiation. Consequently, the device can be regarded as a monolithic integration of a detector and a filter.
Such a device and method are known from U.S. Pat. No. 5,525,828, published on Jun. 11, 1996. Said known device comprises (see for example
FIG. 20
) a photodiode which is formed in a relatively thin semiconductor layer of silicon which is situated on a silicon substrate and separated therefrom by a first electrically insulating layer of silicon dioxide which serves as a mirror for the radiation incident on the semiconductor layer. At the upper side of the thin semiconductor layer, i.e. where the radiation to be detected is incident, there is also an electrically insulating layer, which is also made of silicon dioxide and which also serves as a (partly pervious) mirror. Said known device comprises two semiconductor regions which are recessed in the surface and situated at some distance from each other, said semiconductor regions being, respectively, of the n-conductivity type and the p-conductivity type and provided with electrical connections.
A drawback of the known device is that the wavelength selectivity thereof is insufficient for certain applications. Besides, it is also difficult to attune the properties of the device to a desired wavelength.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a device having a great wavelength selectivity and properties which can be set accurately and reproducibly. The invention also aims at providing a method by means of which the desired device can be manufactured in a simple manner.
To achieve this, a device of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the radiation-reflecting means comprise a metal layer bordering on the semiconductor layer, and the semiconductor layer provided with a metal layer is secured to the substrate by means of an adhesive layer. The invention is first of all based on the recognition that the desired properties of the device, such as the wavelength selectivity, depend upon the reflectivities of both mirrors and that, first of all, the reflectivity of the first mirror, that is the buried first insulating layer, must be determined by means of ellipsometry and reflection measurements in order to determine which reflectivity the second insulating layer should possess in order to realize, for example, a desired wavelength selectivity. The invention is further based on the recognition that if the reflectivity of the lower mirror is at least substantially equal to one, the desired value of the reflectivity of the upper mirror can be determined in a much simpler manner, namely by computing. By using a metal layer as the lower mirror, it is achieved in a simple manner that the reflectivity thereof is at least substantially equal to one. However, a metal layer buried in the semiconductor material cannot be obtained by means of a customary manufacturing process. The invention is further based on the recognition that the substrate does not necessarily have to consist of a semiconductor material, and that a relatively simple method of manufacturing a device in accordance with the invention is possible if the semiconductor layer provided with a metal layer is secured to the substrate by means of an adhesive layer. With respect to the known device, the functions of both mirrors have to be exchanged: the buried mirror will serve as a (semi-pervious) mirror so that the radiation to be detected is incident in the device, while the upper mirror is formed by providing the semiconductor layer with a metal layer instead of an insulating layer. Subsequently, the device is secured, on the side of the metal layer, to a substrate by means of an adhesive layer, which substrate may or may not be a semiconductor substrate. Subsequently, the original substrate is removed by means of selective etching, in which process the (buried) insulating layer serves as the etch-stop layer. Such a device has a very good (=great) wavelength selectivity and can be manufactured in an accurate and reproducible manner. In addition, the manufacture is relatively simple.
In a preferred embodiment of a device in accordance with the invention, further reflection means are situated at the surface of the semiconductor body and the thickness of the semiconductor layer is a small number of times the half wavelength of the electromagnetic radiation. Said reflection means may simply be formed by the transition from the semiconducting layer to air. In this case, the (first) insulating layer is removed after the removal of the semiconductor substrate, for example by etching. The reflectivity of the semiconductor-air transition is fixed and amounts to approximately 30%. By leaving the insulating layer, the reflectivity of said reflection means may be provided with another value, if desired. This can alternatively be achieved in an advantageous manner by providing said insulating layer with a further insulating layer of a material having a refractive index which differs from that of said insulating layer. By selecting the thickness of the semiconducting layer so as to be an integral number of times the half wavelength of the radiation to be detected, the device in accordance with the invention meets the most important condition created by a resonant cavity.
In a very favorable embodiment, the radiation-sensitive element comprises a photodiode having a first semiconductor region of a first conductivity type with a high doping concentration, and a second semiconductor region of a second conductivity type, which is opposite to the first conductivity type, and with a high doping concentration, and preferably, a third semiconductor region, situated between the first and the second semiconductor regions, which is of the first or the second conductivity type and has a low doping concentration, and the first and second semiconductor regions being provided with electrical connections. In an important variant of this embodiment, the semiconductor layer comprises a stack of epitaxial semiconductor layers which successively constitute the first, the third and the second semiconductor region. This variant has the important advantage that it is easy to manufacture because the layers of the diode with a high doping concentration can be provided together with the layer with a low doping concentration in a single epitaxial s
De Jager Stienke
Maas Henricus G. R.
Seto Myron W. L.
Eason Leroy
Saadat Mahshid
U.S. Philips Corporation
Wilson Allan R.
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