Radiation Hardening of Mos Devices by Boron

OF THE DISCLOSURE A novel technique is disclosed for radiation hardening of MOS devices and specifically for stabilizing the gate threshold potential at room temperature of a radiation subjected MOS field-effect device of the type having a semiconductor substrate, an insulating layer of oxide on the substrate, and a gate electrode disposed on the insulating layer. In the preferred embodiment, the novel inventive technique contemplates the introduction of boron into the insulating oxide, the boron being introduced within a layer of the oxide of about 100 A-300 A thickness immediately adjacent the semiconductor-insulator interface. The concentration of boron in the oxide layer is preferably maintained on the order of 10 atoms/cm.. The novel technique serves to reduce and substantially annihilate radiation induced positive gate charge accumulations, which accumulations, if not eliminated, would cause shifting of the gate threshold potential of a radiation subjected MOS device, and thus render the device unstable and/or inoperative. ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the government for governmental purposes without the payment of any royalties thereon or therefor. BACKGROUND OF THE INVENTION This invention generally relates to MOS devices and particularly concerns a technique whereby the gate threshold potential, generally at room temperature, of a radiation subjected MOS field-effect device is stabilized. A MOS (metal-oxide-semiconductor) device, as is known, includes a semiconductor substrate, an insulating layer of oxide on the substrate, and a gate electrode disposed on the insulating layer. With a MOS field-effect transistor, for example, additional source and drain electrodes are disposed to either side of the gate electrode and a lateral current may be caused to flow between the source and drain electrodes through application of proper bias potential to the gate electrode. Specifically, and in the so-called "enhancement" mode, application of a biased potential to the gate produces a conducting layer beneath the metal oxide allowing lateral current flow between the source and drain electrodes. In a second mode of operation such as the so-called "depletion" mode, application of a bias potential to the gate electrodes produces an insulating region between the source and drain electrodes which serves to decrease current conduction. With the usual MOS devices, several volts, such as three or four volts of negative potential is necessary to be applied to the gate in the p-channel enhancement mode of the MOS device whereas, with a n-channel device, a few volts of positive potential applied to the gate is needed. By way of definition, the gate voltage at which approximately ten microamperes of drain current flows is commonly defined as the gate threshold potential of any particular MOS device. Such MOS devices, when subjected or exposed to ionizing radiation such as would occur in a space environment, suffer radiation damage in the form of charge trapped in the oxide and/or at the oxide-semiconductor 5 interface and undergo various changes in the electrical characteristics thereof. Such damage is not always permanent, but can oftentimes be "healed" or reduced through a time and/or temperature annealing process. One particular dominant and harmful effect on MOS jO devices due to their exposure to radiation has been a shift in the above-described gate threshold potential, these shifts commonly occurring toward the more negative gate voltages. As a result of these shifts in the gate threshold potentials, and at sufficient enough doses of ionizing radia15 tion, the devices and the circuits in which they are placed become unstable and in some instances are actually ren• dered inoperative. In order to safeguard the operation of electronic circuits which contain MOS devices such as on board a 20 spacecraft, for example, the MOS devices are normally shielded against the space ionizing radiation, the shielding normally comprising a heavy material designed to absorb most of the ionizing radiation from space and also radiation emanating from any other sources that might be 25 present on board the spacecraft itself, such as a nuclear power source or the like. As can be appreciated, however, the utilization of such shielding materials greatly increases the weight of a spacecraft and, as such, may pose severe disadvantages particularly for deep space missions 30 where weight limitations are severe. Furthermore, even if the radiation shielding is sufficient, the danger always exists that the shielding itself might be improperly designed or that an unexpected source of radiation may appear, or that the probability of malfuction of the various 35 circuits may increase due to even small changes in the operating characteristics of the MOS devices. SUMMARY OF THE INVENTION ^O It is a primary object of the instant invention to provide a novel technique whereby MOS devices of the type described can be "hardened" against radiation without requiring the utilization of external shielding. A more specific object of the instant invention concerns 45 the provision of a radiation hardening technique for MOS devices whereby the gate threshold potential, at room temperature, of a radiation subjected MOS field-effect device is stabilized against radiation-induced shifting thereof. These important objects, as well as others which will 50 become apparent as the description proceeds, are implemented by the instant invention which broadly can be described as comprising a novel technique for radiation hardening of MOS devices and specifically for stabilizing the gate threshold potential at room temperature of a 55 radiation subjected MOS field-effect device of the type having a semiconductor substrate, and a gate electrode disposed on the insulating layer. In the preferred embodiment, the novel inventive technique contemplates the introduction of boron or other 60 elements having so-called "acceptor" properties into the insulating oxide, the boron being introduced with a layer of the oxide of about 100 A-300 A. thickness immediately adjacent the semiconductor-insulator interface. The concentration of boron in the oxide layer is preferably main65 tained on the order of 10 atoms/cm.. The novel technique serves to reduce and substantially annihilate radiation induced positive gate charge accumulations, which accumulations, if not eliminated, would cause shifting of the gate threshold potential of a radiation subjected MOS 70 device, and thus render the device as well as the associated circuitry unstable and/or inoperative.