Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Complementary bipolar transistors
Reexamination Certificate
1998-12-24
2001-07-24
Nguyen, Tuan H. (Department: 2813)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Complementary bipolar transistors
C438S335000, C438S236000
Reexamination Certificate
active
06265277
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority from French App'n 94-00951, filed Jan. 28, 1994, which is hereby incorporated by reference. However, the content of the present application is not necessarily identical to that of the priority application.
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a method for making a bipolar transistor providing protection against electrostatic discharges in an integrated circuit.
Integrated circuits are generally highly sensitive to electrostatic discharges. Indeed, these discharges result in excess voltages that are short-lived but have high values that may go up to several kilovolts in peak voltage, and induce instantaneous dissipated power values of several tens of Watts. These electrostatic discharges are likely to cause deterioration in the working of the integrated circuits and may even destroy these circuits, for example by excessive heating. It is therefore vitally necessary to take steps against this type of phenomenon by placing protective devices at the input of the integrated circuits.
A first type of protection consists of the connection of limiter diodes between input points of integrated circuits and supply terminals of these integrated circuits. This approach notably has the drawback of being restrictive for the threshold voltages of these diodes limits the range of the values permitted at these input points in normal operation.
Increasingly, therefore, a second protective device using bipolar transistors is being resorted to.
Let it be assumed, for example, that the protective device relates to an integrated circuit that is powered between a positive voltage and a ground and has one input, and that it is sought to protect this integrated circuit against electrostatic discharges of positive voltages.
The integrated circuit is protected by connecting an NPN type bipolar transistor between its input and the ground. The collector is connected to the input of the integrated circuit. The base and the emitter are connected to the ground. The transistor is therefore off in the absence of any electrostatic discharge. For, it is assumed that the voltage values permitted by the manufacturer at input of the circuit are lower than the avalanche voltage value of the protective bipolar transistor. In the event of electrostatic discharge, the protective transistor limits the excess voltage characterizing this discharge. Indeed, the excess voltage prompts an avalanche phenomenon in the protective transistor.
From the technological point of view, the integrated circuits are made notably by metal-oxide-semiconductor (MOS) technology.
With regard to the making of bipolar transistors for the protection of MOS type integrated circuits, it is advantageous to make bipolar transistors without superimposed layers. Indeed, the making of this type of structure requires more steps to be performed than the making of MOS type circuits. It is therefore desirable to integrate the manufacturing of the protective bipolar transistors into the process of manufacturing the MOS type circuits that they protect. This makes it possible to avoid having to add diffusion steps to those already required for the manufacture of a MOS type circuit. Consequently, lateral type bipolar transistors are preferably made for this use.
The method of manufacturing a lateral bipolar transistor of an NPN type in the example chosen typically includes the following basic operations:
the preparation of a P type substrate,
the creation, on the surface of the substrate, of a field oxide layer by thermal oxidation, this layer demarcating two source/drain type implantation zones,
the implantation of N type dopants in the implantation zones and the diffusion of these dopants in the underlying semiconductor substrate,
the vapor phase deposition of an insulator layer on the circuit,
the opening of contacts in the implantation zones,
the metallization of the contacts.
The implantation zones are zones known as active zones. An active zone is a surface zone of the substrate covered with thin oxide. An active zone such as this is therefore capable of being doped by implantation. Hereinafter, the term “implantation zone” will characterize the active zones that are actually implanted during the manufacturing process. In the example, the implantation or active zones, with their underlying N type diffusion, form the emitter and the collector (hereinafter called electrodes) of the bipolar protective transistor.
The base of the bipolar transistor is constituted by the substrate, the latter being connected to the ground. In practice, the useful base of the transistor is that part of the substrate that is located between the N type diffusions of the emitter and of the collector. The term “useful base” designates the base zone that comes into play in the transistor effect.
The implantation zones are demarcated by the field oxide on the surface of the substrate. Indeed, the field oxide counters the passage of the dopant particles. It therefore insulates the substrate during the doping operations. The implantation zones are characterized by the fact that they are not covered with field oxide.
The making of the implantation and of the diffusion of N type dopants in the implantation zone is integrated into the process for making a MOS type circuit. There is no stacking of three layers with successive N, P and N type doping.
It is possible, before the creation of the field oxide, to carry out a P type implantation and diffusion in the zones that will be covered by the field oxide. Thus, by diffusion, insulator walls known as channel stops are created between the emitter and the collector on the one hand, and between these electrodes and the rest of the circuit on the other hand. Since the substrate is initially a P type substrate, the concentrations of impurities in the insulation wells are naturally greater than that of the substrate.
The insulation wells enable the prevention of a piercing of the transistor, namely a passage of leakage current between the electrodes. Furthermore, these insulator diffusions diminish the formation of parasitic MOS type transistors whose gates would be formed by interconnections passing over the implantation zones.
This method has a drawback. Indeed, it has been observed that, after an electrostatic discharge, leakage currents could appear, in normal operation, between the collector and the base. The term “normal operation” is understood to mean operation that induces no conduction in the protective bipolar transistor.
It has been discovered that these leakage currents are due to the injection of hot carriers into the field oxide between the electrodes, similarly to the phenomenon of ageing of the MOS type transistors. This injection occurs in a region of the field oxide known as the “bird's beak”. A bird's beak is a particular feature, shaped like a spike, located at the boundary of a field oxide and a surface of a thermally non-oxidized substrate. It is caused by the lateral oxidation of this substrate.
A hot carrier is the name given to a carrier (in this example an electron) accelerated by the field resulting from an electrostatic discharge and capable of acquiring sufficient energy to overcome the potential barrier of the field oxide. This injection occurs at the bird's beak placed at the boundary of the field oxide between the electrodes, and of the collector of the transistor. This injection remains limited as the carriers are hampered by the field oxide. An electrical field is created at the surface of the field oxide, on the collector side, and induces a leakage current between the collector and the substrate of the transistor.
In view of the above, the aim of the present invention is to propose a method for the making of a protective lateral bipolar transistor that can be used to eliminate the leakage currents that appear in normal operation, after a discharge.
The approach proposed by the invention is that of eliminating the field oxide zone which the injection of hot carriers occurs into, inducing leak
Galanthay Theodore E.
Morris James H.
Nguyen Tuan H.
SGS-Thomson Microelectronics S.A
Wolf Greenfield & Sacks P.C.
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