Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Gettering of semiconductor substrate
Reexamination Certificate
1999-08-31
2001-10-30
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Gettering of semiconductor substrate
C438S471000, C438S475000
Reexamination Certificate
active
06309938
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to a transistor fabrication and, more specifically, to a bipolar transistor in which the base-emitter junction therein is exposed to deuterium while the transistor is being fabricated and a method of manufacture of such a transistor.
BACKGROUND OF THE INVENTION
The use of silicon in devices, such as n-p-n bipolar transistors, is well known. Equally well known is the time dependent degradation of these devices, which is caused by reverse-bias stress of the emitter-base junction of the n-p-n transistors. Reverse-bias stress results in the degradation of the common-emitter current gain (H
FE
=&Dgr;I
C
/&Dgr;I
B
). However, it is thought that the collector current (I
C
) is not affected by the stress, but an increase in the recombination component of the base current at low V
BE
has been observed. In BICMOS and BINMOS circuits this reverse-biasing will result in a long term performance degradation and eventually circuit failure. It is believed that the nature of the damage mechanism is that hot carriers generated in the reverse-biased base-emitter junction create interface trap states on the base oxide by breaking silicon/hydrogen, often referred to as the hot carrier degradation effect. More specifically, it is believed that the base-oxide damage (interface trap generation) is caused by the interaction of hot electrons with the Si/SiO
2
interface. Electrons that are generated by band-to-band (from the valence band to the bottom of the conduction band) tunneling at the base-emitter junction are subsequently accelerated (heated) by the junction electric field. In most cases, the substrate, as well as other structures within the device, comprises silicon, and the defects are thought to be caused by dangling bonds (i.e., unsaturated silicon bonds) that introduce states in the energy gap, which remove charge carriers or add unwanted charge carriers in the device, depending in part on the applied bias. While dangling bonds occur primarily at surfaces or interfaces in the device, they also are thought to occur in the bulk oxide. To alleviate the problems caused by such dangling bonds, a hydrogen passivation process has been adopted and has become a well-known and established practice in the fabrication of such devices.
In the hydrogen passivation process, it is thought that the defects that affect the operation of semiconductor devices are removed when the hydrogen bonds with the silicon at the dangling bond sites. While the hydrogen passivation process eliminates the immediate problem associated with these dangling bonds, it does not eliminate degradation permanently because the hydrogen atoms that are added by the passivation process can be “desorbed” or removed from the previous dangling bond sites by the hot carrier effect.
A hot carrier is an electron or hole that has a high kinetic energy, which is imparted to it when voltages are applied to electrodes of the device. Under such operating conditions, the hydrogen atoms, which were added by the hydrogen passivation process, are knocked off by the hot electrons. This hydrogen desorption results in aging or degradation of the device's performance. This hot carrier effect is particularly of concern with respect to smaller devices, such as a bipolar transistors.
Accordingly, what is needed in the art is a bipolar transistor device and a method of manufacture therefore that does experience the level of efficiency degradation experienced by the devices that are passivated with conventional hydrogen passivation processes. The present invention addresses these needs.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides a bipolar transistor and a method of manufacturing the transistor. The transistor includes: (1) a substrate having a base region, an emitter region and a base-emitter junction between said base and emitter regions and (2) a substantial concentration of an isotope of hydrogen located in the vicinity of the base-emitter junction.
The present invention therefore introduces the broad concept of employing, in lieu of hydrogen, an isotope of hydrogen to passivate material in the base-emitter junction in a bipolar transistor. For purposes of the present invention, “substantial concentration” is defined as a concentration of at least 10
16
cm
−3
of isotopic hydrogen.
In one embodiment of the present invention, the isotope is deuterium. The principles of the present invention may be applied to heavier isotopes of hydrogen including tritium and any later-discovered isotopes.
In one embodiment of the present invention, the transistor further comprises a collector region in said substrate and a base-collector junction between said base and collector regions. Those who are skilled in the art may recognize benefits in deuterating the base-collector junction, as well, although such is not necessary to the present invention.
In another embodiment of the present invention, the base-emitter junction is capable of transmitting reverse-bias electrical currents. As mentioned above, reverse-bias currents can be injurious to bipolar transistors not having deuterated base-emitter junctions.
In one embodiment of the present invention, the emitter region is at least partially composed of polysilicon that has a substantial concentration of a hydrogen isotope located or incorporated therein. Those who are skilled in the art are familiar with polysilicon structures that may be deposited on a substrate to form poles of a transistor.
In yet another embodiment of the present invention, the transistor further comprises a dielectric layer located over the life substrate. In preferred embodiments, the dielectric also has a substantial concentration of a hydrogen isotope located or incorporated therein. The present invention is fully compatible with current nonisotopic hydrogen passivation techniques, which can work with dielectric layers.
In another embodiment of the present invention, the isotope is covalently bonded to material in the base-emitter junction. Alternative bonding structures may exist, however. The present invention is independent of the type of bond between the isotope and the material in the base-emitter junction.
In yet another embodiment of the present invention, the substrate is generally planar and the isotope is concentrated in portions of a planar surface of the substrate. Thus, the isotope may be patterned.
In another embodiment of the present invention, the substrate is composed at least partially of silicon. However, other conventional or later-discovered substrate materials are within the scope of the present invention.
In one embodiment of the present invention, the transistor further comprises at least one electrical conductor that carries electrical current to the transistor. Those who are skilled in the art will recognize that the present invention allows, but does not require, electrical conductors to be associated with the substrate.
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Agere Systems Guardian Corp.
Bowers Charles
Smoot Stephen W.
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